Advanced phosphate materials as positive electrodes for Na-ion batteries

Advanced phosphate materials as positive electrodes for Na-ion batteries Carlos Manuel Berlanga Cruzado October 2025 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 (cc) 2025 Carlos Manuel Berlanga Cruzado (cc by-nc 4.0) REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 Advanced phosphate materials as positive electrodes for Na-ion batteries Departamento de Química Orgánica e Inorgánica CIC Energigune A dissertation submitted to the University of the Basque Country in partial fulfillments of the requirements for the degree of Ph.D. By Carlos Manuel Berlanga Cruzado Thesis supervisors: Dra. Montse Galcerán Mestres Prof. Teófilo Rojo Tutor: Prof. Teófilo Rojo October 2025 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xi The scientist has begun to realize his social responsibility… J. D. Bernal REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xii REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xiii Resumen Las baterías de ion-sodio (NIBs, por sus siglas en inglés) han emergido en los últimos años como una de las alternativas más prometedoras a las baterías de ion-litio (LIBs), impulsadas por la creciente preocupación sobre la disponibilidad limitada del litio, su coste y los problemas de sostenibilidad asociados a su extracción. Aunque las baterías de sodio presentan una densidad energética inferior a la de las de litio, ofrecen ventajas significativas en términos de coste, abundancia de materia prima y menor impacto ambiental. Estas características las convierten en candidatas ideales para aplicaciones de almacenamiento estacionario de energía y en aquellas donde la alta densidad energética no resulta prioritaria. La presente tesis doctoral se centra en el desarrollo de métodos de síntesis sostenibles, de bajo coste y responsables con el medio ambiente para la obtención de materiales catódicos basados en fosfatos aplicables a baterías de ion-sodio. El trabajo busca promover el uso de materias primas abundantes y no tóxicas, reduciendo el impacto económico y ecológico de los procesos de producción, sin comprometer las prestaciones electroquímicas de los materiales resultantes. El proyecto aborda tres objetivos fundamentales: 1. Establecer nuevos paradigmas de síntesis verde para materiales catódicos fosfatados de sodio. 2. Profundizar en la comprensión estructural, morfológica y electroquímica de dichos materiales. 3. Sentar las bases para su escalado industrial, consolidando las NIBs como alternativas viables a las LIBs. La investigación combina enfoques experimentales directos e indirectos. Por un lado, se exploran rutas de síntesis indirecta mediante sodiación de materiales litiados (LiMn1-yFeyPO4), con recuperación del litio como subproducto de valor. Por otro, se REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xiv desarrollan procesos de síntesis directa de fases Nax(TM)y(PO4)z (donde TM = Mn, Fe o combinaciones de ambos), analizando cómo parámetros como el pH o la relación Mn:Fe influyen en la formación de distintas fases cristalinas. El estudio recurre a técnicas avanzadas de caracterización estructural y espectroscópica (difracción de neutrones, espectroscopías Mössbauer y de absorción de rayos X, entre otras) para identificar y estudiar diferentes polimorfos como trifilita, xenofilita y maricita, y comprender sus mecanismos de inserción y extracción de sodio durante el ciclado electroquímico. En conjunto, los resultados de esta tesis contribuyen al conocimiento fundamental sobre los fosfatos de sodio como materiales catódicos y aportan información clave para el diseño de materiales de almacenamiento energético de próxima generación, sostenibles y económicamente competitivos. Capítulo 1. Introducción Este capítulo contextualiza la presente tesis doctoral. En primer lugar, se ofrece una descripción general que justifica el interés científico y tecnológico en el desarrollo de baterías de ion-sodio como alternativa sostenible a las de ion-litio. A continuación, se explica el principio de funcionamiento de las baterías recargables y los fundamentos electroquímicos que las rigen. Finalmente, se presenta una visión general del estado del arte en la investigación sobre materiales para baterías de sodio, poniendo especial énfasis en los materiales polianiónicos como cátodos, debido a su estabilidad estructural, seguridad y potencial para aplicaciones a gran escala. Capítulo 2 – Técnicas experimentales y de caracterización El segundo capítulo describe de manera detallada las técnicas experimentales empleadas en la investigación: REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xv •Técnicas estructurales y morfológicas, incluyendo difracción de rayos X (XRD), microscopía electrónica de barrido (SEM) y transmisión (TEM), y difracción de neutrones. •Técnicas físico-químicas, como la espectroscopía Mössbauer, el análisis térmico (TGA/DSC) y la espectroscopía de absorción de rayos X (XAS). •Técnicas electroquímicas, que abarcan desde la voltamperometría cíclica hasta las pruebas de carga-descarga galvanostática. Estas metodologías permiten correlacionar la estructura cristalina y la composición química con el rendimiento electroquímico de los materiales sintetizados, aportando una visión integral de los fenómenos que determinan su funcionalidad como cátodos en baterías de ion-sodio. Capítulo 3 – Síntesis indirecta de trifilita-NaMn1-yFeyPO4 y recuperación de litio En este capítulo se presenta un método de síntesis indirecta para la obtención de la fase trifilita NaMn1-yFeyPO4 a partir de precursores comerciales de LiMn1-yFeyPO4 (proporcionados por Aleees), con recuperación simultánea del litio. El proceso propuesto sustituye los reactivos peligrosos y costosos empleados tradicionalmente (como NO2BF4 o acetonitrilo bajo atmósfera inerte) por reactivos económicos y no tóxicos (Na2S2O8, Na2S2O3 y agua), reduciendo drásticamente la huella ambiental del proceso. Los resultados demuestran que la delitiación-sodiación controlada permite obtener materiales de estructura trifilita con buenas propiedades electroquímicas, al tiempo que se recupera litio de forma eficiente. Esta aproximación constituye un avance significativo hacia procesos industriales circulares y sostenibles para la fabricación de cátodos de sodio. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xvi Capítulo 4 – Síntesis directa de polimorfos Nax(TM)y(PO4)z y efecto de las condiciones experimentales El cuarto capítulo se dedica al estudio de la síntesis directa de diferentes polimorfos de fosfatos sódicos, con la fórmula general Nax(TM)y(PO4)z (TM = Mn, Fe o mezclas de ambos). Se analizan las condiciones experimentales que determinan la formación de cada fase, en especial el pH de la solución y la naturaleza del precursor sódico (NaOH, Na2SO4 o acetato de sodio). El objetivo principal es lograr la síntesis directa de la fase trifilita NaTM(PO4), cuya obtención se ha considerado históricamente difícil, dado que la fase maricita suele formarse preferentemente. El estudio sistemático de las variables de síntesis permitió identificar condiciones que favorecen la estabilización de distintas fases y proporcionó información fundamental sobre los mecanismos de nucleación y crecimiento en medios acuosos. Capítulo 5 – Caracterización avanzada de la fase trifilita NaMn1-yFeyPO4 En el quinto capítulo se presenta un análisis estructural y electroquímico detallado de la fase trifilita NaMn1-yFeyPO4 obtenida mediante síntesis hidrotermal directa. A través de técnicas avanzadas como la difracción de neutrones y la espectroscopía Mössbauer, se determinaron con precisión los parámetros de celda unitaria, la proporción Mn:Fe en la red cristalina y la distribución de estados de oxidación (Fe2+/Fe3+). Asimismo, se llevó a cabo una evaluación exhaustiva de la morfología, estabilidad térmica y comportamiento electroquímico, revelando cómo la sustitución parcial de Mn por Fe modifica la capacidad y la cinética de inserción de sodio. Las medidas XAS in situ y ex situ permitieron seguir la evolución del entorno local de los metales de transición durante el ciclado, aportando una comprensión profunda de los mecanismos redox implicados. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxiii Contents List of figures xxix List of tables xli Acronyms xliv 1. Introduction 1.1. Overview of energy demand ......................................... 3 1.2. Batteries .................................................................... 4 1.3. Na-ion batteries .......................................................... 6 1.3.1. Anodes for Na-ion Batteries .................................... 9 1.3.1.1. Hard Carbon .................................................. 10 1.3.1.2. Alloy-Based Anodes ........................................ 11 1.3.1.3. Metal Oxides and Sulfides ............................... 11 1.3.2. Electrolyte Materials for Na-ion Batteries ................. 11 1.3.3.1. Organic Liquid Electrolytes .............................. 12 1.3.3.2. Ceramic Solid-State Electrolytes ...................... 13 1.3.3.3. Gel Polymer Solid-State Electrolytes ................. 13 1.3.3. Cathode for Na-ion batteries .................................. 13 1.3.3.1. Layered oxides .............................................. 14 1.3.3.2. Prussian blue analogues .................................. 15 1.3.3.3. Polyanionic materials ...................................... 17 1.4. Objectives and goals ................................................... 26 1.5. References ................................................................ 28 2. Experimental Techniques ........................................... 35 2.1. X-Ray Powder Diffraction techniques ............................. 36 2.2. Electron microscopy technique ..................................... 38 2.2.1. Scanning Electron Microscopy ................................ 39 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxiv 2.2.2. Transmission Electron Microscopy ........................... 41 2.3. Thermal analysis couple with Mass Spectrometry technique ................................................................................ 41 2.4. Inductively Coupled Plasma Atomic Emission Spectrometry technique ........................................................................... 43 2.5. Raman Spectroscopy technique .................................... 44 2.6. X-Ray Absorption Spectroscopy technique ..................... 45 2.7. Mössbauer Spectroscopy technique............................... 46 2.8. Solid State Nuclear Magnetic Resonance technique ......... 47 2.9. Electrochemical techniques .......................................... 48 2.9.1. Electrode preparation ........................................... 48 2.9.2. Cell assembly ...................................................... 49 2.9.3. Galvanostatic experiments .................................... 50 2.9.4. Cyclic voltammetry ............................................... 51 2.9.5. Potentiostatic Intermittent Titration Technique ......... 51 2.10. References ................................................................ 52 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization .................................................................. 53 3.1. Introduction............................................................... 54 3.2. Synthesis and phase identification ................................ 54 3.2.1. Chemical oxidation of LiFePO4 ................................ 55 3.2.2. Chemical oxidation of LiMn0.65Fe0.35PO4 .................... 56 3.2.3. Chemical reduction of Mn1-yFeyPO4 .......................... 57 3.2.4. Li-Recovery and Synthesis of LiFePO4 ..................... 67 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxv 3.3. Structural, morphological and physicochemical characterization ................................................................... 69 3.3.1. (Li/Na)FePO4 materials characterization .................. 69 3.3.2. (Li/Na)Mn0.65Fe0.35PO4 materials characterization ...... 77 3.3.3. Recovered LiFePO4 material characterization ............ 80 3.4. Electrochemical characterization of (Li,Na)Mn1-yFeyPO4 .... 82 3.4.1. Electrochemical properties of synthesized NaMn1yFeyPO4 82 3.4.2. Electrochemical properties of regenerated LiFePO4.... 92 3.5. Summary and Conclusions ........................................... 93 3.6. References ................................................................ 95 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) .......................... 99 4.1. Introduction............................................................. 100 4.2. Density functional theory........................................... 100 4.3. Solubility analysis and selection of the precursors ........ 103 4.4. Synthesis and phase identification of NaxMn1-yFeyPO4 of (0 ≤ y ≤ 1) .............................................................................. 106 4.4.1. Synthesis and phase identification of natrophilite NaMnPO4 ....................................................................... 106 4.4.2. Synthesis and phase identification of triphylite NaMn1yFeyPO4 (y = 0.25, 0.50, and 0.75) .................................... 107 4.4.3. Effect of the pH on triphylite NaMn0.25Fe0.75PO4 ....... 108 4.4.4. Synthesis of NaMn1-yFeyPO4 substituting Mn with Fe progressively .................................................................. 112 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxvi 4.5. Synthesis of Xenophyllite Na4TM7(PO4)6 (TM = Fe, Mn).... 118 4.6. Correlation of pH with obtained polymorphs. ................ 120 4.7. Monitoring of the hydrothermal synthesis via in-situ Synchrotron XRD ............................................................... 122 4.8. Summary and Conclusions ......................................... 126 4.9. References .............................................................. 128 5. Characterization of triphylite NaMn1-yFeyPO4 (0≤y≤30) 127 5.1. Introduction............................................................. 128 5.2. Structural, morphological, and physicochemical characterization ................................................................. 128 5.3. Thermal stability ...................................................... 137 5.4. Electrochemistry Characterization ............................... 138 5.5. Reaction mechanism: X-Ray absorption spectroscopy ... 144 5.6. Summary and Conclusions ......................................... 148 5.7. References .............................................................. 149 6. Characterization of xenophyllite Na4TM7(PO4)6 (TM = Fe, Mn) and its derivates ......................................................... 153 6.1. Introduction............................................................. 154 6.2. Structural, morphological and physicochemical characterization ................................................................. 154 6.3. Electrochemistry ...................................................... 167 6.4. Reaction mechanism: Mössbauer spectroscopy ............ 168 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxvii 6.5. Summary and Conclusions ......................................... 173 6.6 References .............................................................. 174 7. Reaction mechanism of activated maricite NaFePO4 . 177 7.1. Introduction............................................................. 178 7.2. Synthesis and characterization of pristine compound .... 178 7.2.1. Hydrothermal synthesis ...................................... 178 7.2.2. Structural, morphological, and physicochemical characterization of the synthesised maricite NaFePO4 .......... 179 7.2.3. Electrochemical characterization of pristine maricite NaFePO4 ........................................................................ 182 7.3. Electrochemical evaluation of mechanically activated maricite NaFePO4 ............................................................... 183 7.4. Effects of mechanical activation on maricite NaFePO4 by ball milling: structure, electrochemistry, and local environments ... 186 7.4.1. Structural and morphological characterization following mechanical activation ...................................................... 186 7.4.2. Operando X-Ray Diffraction characterization .......... 197 7.4.3. Operando and ex-situ spectroscopic characterization ... ........................................................................ 200 7.5. Summary and Conclusions ......................................... 209 7.6. References .............................................................. 210 8. General conclusions ................................................. 213 List of contributions .......................................................... 217 Publications ....................................................................... 217 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxviii Conferences and exhibitions ................................................ 218 Projects ............................................................................ 218 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxix List of figures Figure 1.1. Cumulative installed capacity (in GW) and the forecast. Source: EIU, IEA. ............................................................... 3 Figure 1.2. Map of lithium resource availability and geostrategic impacts (base map by Daniel Dalet). .................................... 4 Figure 1.3. The number of scientific publications on Na-ion battery since 2015. ....................................................................... 6 Figure 1.4. Structural patterns of a) O3-type layered oxides, b) P2type layered oxides, c) O2-type layered oxides. The capital letters (A, B, C) are used to describe the packing patterns of the oxygenion frameworks. Grey, red and purple colors correspond to Na, O and M element respectively. ............................................... 13 Figure 1.5. Common types of structural complexity in PBAs: (a) octahedral tilts, (b) correlated ‘slides’ of A-site cations, (c) Jahn– Teller distortions, (d) A-site occupancy (dis)order, (e) hexacyanometallate vacancies and the resulting connected porenetwork structure, and (f) framework hydration. ................... 15 Figure 1.6. Crystal structures of: a) olivine NaMPO4, b) NASICON Na3V2(PO 4)3, c) triclinic Na2MP2O7, d) orthorhombic Na2MP2O7, e) orthorhombic Na4M3(PO4)2P2O7, f) orthorhombic Na2MPO4F, g) monoclinic Na2MPO4F, and h) tetragonal Na3M2(PO4)2F3 (M represents transition metals). ............................................. 17 Figure 1.7. The crystal structure of olivine-group M2XO4. Octahedra in M1 and M2 sites and tetrahedra in X site are illustrated. ..... 19 Figure 1.8. Polyhedral representation of the olivine-type structures (a,b) and their conversion to the maricite (c-e) through the introduction of Na to the M2 site (c) and the shift of the P position (d). Red spheres designate oxygen atoms, a, b, c are the unit cell directions. ........................................................................ 19 Figure 1.9. Galvanostatic charge and discharge curves at C/20 of mNFP (green) and at C/10 t-NFP (blue-orange) and the REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxx corresponding derivative dq/dV plot of t-NFP which highlights the redox peaks. .................................................................... 22 Figure 1.10. (a) Voltage versus time plot of the XRD in situ experiment. (b) 2θ versus time plot of the XRD patterns comprising a full cycle (charge and discharge). The level of grey indicates the relative intensity (the darker the more intense). Horizontal bars at the right indicate the position of the Bragg peaks for each of the phases involved. (c) Sum of the integrated intensity of the (020) and (211) reflections for each of the phases involved versus time. ........................................................ 23 Figure 2.1. Bruker D8 Discover diffractometer deployed in this work. ...................................................................................... 37 Figure 2.2. Bragg-Brentano geometry in XRD setup for powder measurements. ................................................................. 37 Figure 2.3. Scanning Electron Microscope Quanta 200 FEG (FEI) available at CIC energiGUNE. ............................................. 40 Figure 2.4. Thermogravimetric analyzer TGA NETZSCH STA 449 F3 Jupiter coupled to a Mass Spectrometer QMS 403 Aëolos Quadro available at CIC energiGUNE. ............................................. 42 Figure 2.5. Ultima 2 Optical Emission Spectrometer used in this work. ...................................................................................... 43 Figure 3.1. General proposed synthesis approached for the indirect synthesis of NaMn1-yFeyPO4. ............................................... 54 Figure 3.2. XRD pattern of synthesized DL_01 compared to FePO4 from the literature (ICSD code: 92199) for phase identification. ...................................................................................... 55 Figure 3.3. XRD patterns of the LiMn0.65Fe0.35PO4 pristine and the obtained compounds after different chemical oxidation conditions compared to FePO4 from the literature as reference with similar XRD pattern (ICSD code: 92199) for phase identification. ...... 57 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxxi Figure 3.4. XRD pattern of the synthesized SD_A compared to FePO4 from the database (ICSD code: 92199) for phase identification. ...................................................................................... 59 Figure 3.5. XRD patterns of the synthesized SD_B compared with Fe1.375PO4(OH) (ICSD code: 170854) and S8 (ICSD code: 870) from the database for phase identification. ........................... 60 Figure 3.6. XRD pattern of the synthesized SD_C compared with different NaxFePO4 phases (ICSD code: 169118, 169119, 92199) and S8 (ICSD code: 870) for phase identification. ................. 61 Figure 3.7. XRD pattern of SD_D compared with Na0.7FePO4 (ICSD code: 169119) and S8 (ICSD code: 870) from the database for phase identification. .......................................................... 62 Figure 3.8. XRD pattern of the synthesized SD_E. ...................... 63 Figure 3.9. XRD pattern of the synthesized SD_F compared with NaFePO4 (ICSD code: 169118) from the database for phase identification. ................................................................... 64 Figure 3.10. Rounded flask of 5 L during the synthesis of a batch of triphylite C/NaFePO4 using the indirect synthesis method. ...... 64 Figure 3.11. XRD patterns of synthesized C/FePO4 and C/NaFePO4 compared to the commercial C/LiFePO4 used as precursor. ..... 65 Figure 3.12. Scheme of the different chemical reduction approaches for obtaining NaFePO4. ....................................................... 66 Figure 3.13. XRD patterns of synthesized C/Fe0.35Mn0.65PO4 and C/NaFe0.35Mn0.65PO4 compared to the commercial C/LiFe0.35Mn0.65PO4 used as precursor. ................................. 67 Figure 3.14. General scheme of the synthesis of C/NaFePO4, including the recovery and recycling of lithium procedure. ................... 68 Figure 3.15. Rietveld refinement of the XRD pattern of the commercial C/LiFePO4. Experimental pattern (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions (vertical bars in green). (RBragg = 2.54% and χ2 = 4.87). ...................................................... 70 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxxii Figure 3.16. Rietveld refinement of the XRD pattern of the synthesized C/FePO4. Experimental pattern (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions (vertical bars in green). (RBragg = 4.35 % and χ2 = 4.06). ............................................................. 70 Figure 3.17. Rietveld refinement of the XRD pattern of the synthesized C/NaFePO4. Experimental pattern (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions (vertical bars in green). (RBragg = 3.91 % and χ2 = 1.59). ............................................................. 71 Figure 3.18. Raman spectra of the as-synthesized C/NaFePO4, C/FePO4 and the precursor C/LiFePO4 in (a) air and (b) argon atmosphere. ..................................................................... 72 Figure 3.19. (a-c) SEM and (d-f) TEM images of (a,d) C-LiFePO4, (b,e) C/FePO4 and (c,f) C/NaFePO4. ............................................. 73 Figure 3.20. TEM images of C/NaFePO4 particles in which the cracks are highlighted with circles. ................................................ 74 Figure 3.21. Thermogravimetric analysis of C/LiFePO4, C/FePO4 and C/NaFePO4 in air. .............................................................. 75 Figure 3.22. Thermogravimetric analysis and analysis of the outlet gases (H2O and CO2) detected by mass spectrometry of C/FePO4 in air. .............................................................................. 76 Figure 3.23. Mass spectrometry with the outlet of gases (H2O and CO2) of C/FePO4 in air. ....................................................... 76 Figure 3.24. Le Bail refinement of the XRD pattern of the commercial precursor C/LiMn0.65Fe0.35PO4. Experimental pattern (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions (vertical bars in green). (RBragg = 39.4% and χ2 =5.54)............................................ 77 Figure 3.25. Le Bail refinement of the XRD pattern of the commercial precursor C/Mn0.65Fe0.35PO4. Experimental pattern (red), calculated profile (black), difference between experimental and REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xxxix Figure 7.9. XRD patterns of NaFePO4/C obtained by hydrothermal synthesis (NFP_0h) and at different ball milling times (NFP_Xh). .................................................................................... 187 Figure 7.10. SEM images of the NaFePO4/C after different ball milling times (a) 3h (NFP_3h), (b) 6h (NFP_6h), (c) 9h (NFP_9h) and (d) 12h (NFP_12h). .............................................................. 188 Figure 7.11. Raman spectra for sample after ball milling for different time periods. .................................................................. 190 Figure 7.12. 23Na solid state NMR spectra of pristine NFP_0 and at 3h and 12h ball milling time. ................................................. 192 Figure 7.13. 31P solid state NMR spectrum of pristine NFP_0h and at 3h and 12h ball milling time. ............................................ 192 Figure 7.14. Mössbauer spectra at different ball milling times (3h, 6h, 9h, 12h). ....................................................................... 193 Figure 7.15. XAS of pristine maricite NFP_0h and NFP_12h. XAS characterization at O K-edges (a, b), Fe L-edges (c), and Na Kedges (d, e). Both electron (TEY) and fluorescence (FY) modes of detection were shown. Fe L3 suffered self-absorption in fluorescence detection mode and is not shown. The dotted line corresponds to the spectra of the pristine powder (NFP_0h). 196 Figure 7.16. Schematic illustration of the ball milling effect on crystalline stick shaped NFP. The ball milling with C65 leads to a crystalline NFP with smaller size and the formation of new more disordered species possibly located at the surface. .............. 197 Figure 7.17. (a) Voltage versus time curve of operando synchrotron XRD experiment of NFP_12h and (b) selected region of 2D-plot of 2θ versus time of the XRD patterns comprising the first full charge and discharge (warmer color indicates the more intense peaks). .......................................................................... 198 Figure 7.18. (a) Voltage versus time curve of operando synchrotron XRD experiment of NFP_12h and and (b) evolution of the unit cell parameters and occupancy along the full cycle. .................. 199 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xl Figure 7.19. (a) Voltage versus time curves of operando Mössbauer experiment of NFP_12h, (b) 2D-plot of the evolution of the Mössbauer spectra during cycling, (c) evolution of the percentage of Fe2+ and Fe3+, (d) evolution of the different components of Fe during cycling measured by operando Mössbauer Spectroscopy. Note: Fe2+: equivalent to M1 in maricite NaxFePO4, Fe2+ (B): equivalent to M2 in triphylite NaxFePO4, Fe3+ (a): equivalent to M1 in maricite NaxFePO4; Fe3+(B): equivalent to M2 in triphylite NaxFePO4 or heterosite FePO4. .......................................... 203 Figure 7.20. (a) Galvanostatic charge and discharge curves of NFP_12h and (b) both electron and fluorescence modes of O Kedges, Fe L2,3-edges, and Na K-edges. Fe L3 suffered selfabsorption in fluorescence detection mode and is not shown. The dotted line corresponds to the spectra of NFP_12h at point 1 (OCV). ........................................................................... 206 Figure 7.21. (a) Galvanostatic discharge and charge curve of NFP_12h and (b) both electron and fluorescence modes of O Kedges, Fe L2,3-edges, and Na K-edges. Fe L3 suffered selfabsorption in fluorescence detection mode and is not shown. The dotted line corresponds to the spectra of NFP_12h at point 1 (OCV). ........................................................................... 207 Figure 7.22. General scheme of the reaction mechanism of activated maricite NaFePO4. ........................................................... 208 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xli List of tables Table 1-1. Comparison of different aspects between Li-ion and Naion batteries. ..................................................................... 7 Table 1-2. Assignation of M1, M2 and X sites depending on the structure and representative formula. .................................. 24 Table 3-1. Strategies for the synthesis of C/Mn0.65Fe0.35PO4 from C/LiMn0.65Fe0.35PO4. (modifications included in bold). ............. 56 Table 3-2. Different strategies for the synthesis of C/NaFePO4 from C/FePO4 (modifications included in bold). ............................. 58 Table 3-3. Rietveld refined cell parameters of C/LiFePO4, C/FePO4 and C/NaFePO4. ...................................................................... 69 Table 3-4. Unit cell parameters of C/LiMn0.65Fe0.35PO4, C/Mn0.65Fe0.35PO4 and C/NaMn0.65Fe0.35PO4 obtained by refinement. ...................................................................... 79 Table 3-5. Comparative table summarizing the electrochemical performance of the reported t-NFP (triphylite NaFePO4) and mNFP (maricite NaFePO4) with our t-NFP and t-NMFP studied in this work................................................................................ 87 Table 4-1. List of precursors used for the hydrothermal synthesis .................................................................................... 100 Table 4-2. List of the possible species formed during the synthesis .................................................................................... 104 Table 4-3. Equilibrium constants of the possible species formed during the synthesis. ....................................................... 105 Table 4-4. Reagents ratio and obtained phase for the triphylite targeted compositions decreasing the pH. .......................... 110 Table 4-5. Reagents ratio and obtained phase for the triphylite targeted compositions increasing the pH. ........................... 112 Table 4-6. Reagents ratio and obtained phase for the different triphylite targeted compositions replacing progressively Mn for Fe. ................................................................................ 114 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xlii Table 4-7. Unit cell parameters of the Reagents ratio and obtained phase for the different triphylite targeted compositions replacing progressively Mn for Fe .................................................... 117 Table 5-1. Crystallographic data (space group, cell parameters, refinement agreement factors, cell volume, atomic positions, isotropic thermal factors Biso and site occupancies) of NaMnPO4 obtained from the Rietveld refinement of its NPD pattern. .... 132 Table 5-2. Crystallographic data (space group, cell parameters, refinement agreement factors, cell volume, atomic positions, isotropic thermal factors Biso and site occupancies) of NaMn0.90Fe0.10PO4 obtained from the Rietveld refinement of its NPD pattern. .................................................................. 132 Table 5-3. Crystallographic data (space group, cell parameters, refinement agreement factors, cell volume, atomic positions, isotropic thermal factors Biso and site occupancies) of NaMn0.80Fe0.20PO4 obtained from the Rietveld refinement of its NPD pattern. .................................................................. 133 Table 5-4. Crystallographic data (space group, cell parameters, refinement agreement factors, cell volume, atomic positions, isotropic thermal factors Biso and site occupancies) of NaMn0.70Fe0.30PO4 obtained from the Rietveld refinement of its NPD pattern. .................................................................. 133 Table 5-5. Lattice parameters of the obtained triphylite NaMn1yFeyPO4 samples and its real composition. .......................... 134 Table 6-1. Refined lattice parameters of the obtained xenophyllite Na4Fe7(PO4)6 and Na4Mn7(PO4)6. ....................................... 155 Table 6-2. Average atomic percentage composition of Na4Mn7(PO4)6, labeled as Na4Mn7(PO4)6, measured by EDX. ....................... 158 Table 6-3. Average atomic percentage composition of xenophyllite Na4Fe7(PO4)6, measured by EDX. ...................................... 159 Table 6-4. Average atomic percentage composition of xenophyllite Na4Fe7(PO4)6 after ball milling measured by EDX. ................ 162 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xliii Table 6-5. Room temperature 57Fe Mössbauer spectroscopy parameters of as synthesized Na4Fe7(PO4)6 and ball milled HT_xenFe@C, the isomer shift (IS) are given with respect to αFe standard at 300K. ....................................................... 166 Table 6-6. Mössbauer hyperfine parameters of the MCR-ALS pure components. .................................................................. 169 Table 6-7. Operando Mössbauer spectroscopy parameters of six subspectra identified for HT_xenFe@C and its predominance depending on the state of charge. ..................................... 172 Table 7-1. Summary of first charge and discharge capacity of the NaFePO4/C after different ball milling times 0h (NFP_0h) 3h (NFP_3h), 6h (NFP_6h), 9h (NFP_9h) and 12h (NFP_12h). ... 186 Table 7-2. Summary of the unit cell parameters and average crystallite size obtained from the Le Bail refinement of the XRD patterns of the 5 samples depending on the ball-milling time. .................................................................................... 188 Table 7-3. Summary of valence of the iron contained in the samples as determined from Mössbauer spectroscopy. ..................... 194 Table 7-4. Mössbauer hyperfine parameters of the MCR-ALS pure components. .................................................................. 203 Table 7-5. Summary of the in situ Mössbauer spectroscopy data. Evolution of the voltage capacity, time and valence of the iron. .................................................................................... 204 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xliv Acronyms AM Active Material Ar Argon Co Cobalt CP-MAS Cross Polarization Magic Angle Spinning Cu Copper CSA Chemical Shift Anisotropy CV Cyclic Voltametry DEC Diethyl Carbonate DFT Density Functional Theory DMC Dimethyl Carbonate DTA Differential Thermal Analysis EC Ethylene Carbonate EDX Energy Dispersive X-ray Spectroscopy EIU Economist Intelligence Unit EOC End Of Charge EOD End Of Discharge Fe Iron FEC Fluoroethylene Carbonate FEG Field Emission Gun GFID Gas Flame Ionization Detector HAADF High-Angle Annular Dark Field HRXRD High-Resolution X-Ray Diffraction ICP-AES Inductively Coupled Plasma - Atomic Emission Spectroscopy ICSD Inorganic Crystal Structure Database IEA International Energy Agency I Intensity REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xlv IS Isomer Shift λ Wavelenght Li Lithium LIBs Lithium-Ion Batteries LW Linewidth MAS Magic Angle Spinning MCR-ALS Multivatiate Curve Resolution - Alternating Least Squares Mn Manganese m-NFP Maricite NaFePO4 (Sodium Iron phosphate) MQMAS Multiple-Quantum Magic Angle Spinning MSPD Materials Science Powder Diffraction Na Sodium NFP NaFePO4 (Sodium Iron Phosphate) NFPP Na2FeP2O7 (Sodium Iron Pyrophosphate) NIBs Sodium-Ion Batteries NMP N-Methyl-2-Pyrrolydone NPD Neutron Powder Diffraction O Oxygen OCV Open Circuit Voltage P Phosphorus PBAs Prussian Blue Analogues PC Propylene Carbonate PCA Principal Component Analysis PEO Polyethylene Oxide PITT Potentiostatic Intermittent Titration Technique PVDF Polyvinylidene Fluoride QMS Quadrupole Mass Spectrometry QS Quadrupole Splitting REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 xlvi Rh Rhodium SEI Solid Electrolyte Interphase SEM Scanning Electron Microscopy Sb Antimony Si Silicon Sn Tin SLI Starter-Lighting-Ignition ssNMR Solid-State Nuclear Magnetic Resonance SXRD Synchrotron X-Ray Diffraction TEM Transmission Electron Microscopy TEY Total Electron Yield TFY Total Fluorescence Yield TGA Thermogravimetric Analysis TM Transition Metal t-NFP Triphylite NaFePO4 (Sodium Iron Phosphate) VC Vinylene Carbonate XAS X-Ray Absorption Spectroscopy XRD X-Ray Diffraction REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1 1. Introduction 1.1. Overview of energy demand ......................................... 2 1.2. Batteries .................................................................... 3 1.3. Na-ion batteries .......................................................... 5 1.3.1. Anodes for Na-ion Batteries .................................... 8 1.3.1.1. Hard Carbon ................................................... 9 1.3.1.2. Alloy-Based Anodes ........................................ 10 1.3.1.3. Metal Oxides and Sulfides ............................... 10 1.3.2. Electrolyte Materials for Na-ion Batteries ................. 10 1.3.3.1. Organic Liquid Electrolytes .............................. 11 1.3.3.2. Ceramic Solid-State Electrolytes ...................... 12 1.3.3.3. Gel Polymer Solid-State Electrolytes ................. 12 1.3.3. Cathode for Na-ion batteries .................................. 12 1.3.3.1. Layered oxides .............................................. 13 1.3.3.2. Prussian blue analogues .................................. 14 1.3.3.3. Polyanionic materials ...................................... 16 1.4. Objectives and goals ................................................... 25 1.5. References ................................................................ 27 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 2 1.1. Overview of energy demand The post-pandemic recovery in energy consumption accelerated to 1.8% growth in 2024, up from just 1.2% in 2023.1 This was supported by strong demand in Asia, where consumption was expected to expand by 3.1%, despite the clouds over China’s economic outlook. Growth in energy consumption has also been strong in the Middle East, particularly if the climate there continues to warm, bolstering demand for air conditioning. However, it is expected that energy demand in Europe will record its third consecutive year of decline as the region continues to battle against high energy prices and limited gas supply. Nevertheless, the situation in Europe has improved from 2022-23. Growth was also only marginal in North America and Latin America, with soft economic growth. The need to strengthen energy security in the wake of the energy crisis, in addition to decarbonization efforts, will drive many governments to push ahead even faster with the deployment of renewable energy. Renewable energy continued expanding quickly in 2024, with combined solar and wind energy consumption growing by about 11% year on year (Figure 1.1). Capacity additions set a record high of about 400 GW in 2023 and to grew even more in 2024. The increasing integration of renewable energy sources, such as solar and wind, into the power grid necessitates the use of efficient and reliable energy storage solutions, primarily batteries. These energy sources are intermittent, with production fluctuations due to weather conditions and time of day, creating a mismatch between energy supply and demand. Batteries help bridge this gap by storing excess energy generated during peak production times and releasing it when production is low, thus ensuring a stable and continuous energy supply. Additionally, batteries enhance grid resilience, reduce the REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 9 1.3.1.1. Hard Carbon Hard carbon is one of the most promising anode materials for NIBs due to its high capacity, good cycling stability, and relatively low cost. Unlike graphite, which is used in LIBs, sodium does not intercalate into graphite efficiently using conventional carbonate-based electrolytes, posing a significant challenge for developing effective NIBs. Hard carbon, with its disordered structure and high surface area, provides suitable sites for sodium ion storage, overcoming this challenge. Its microstructure comprises a combination of both graphitic nanodomains and amorphous regions, which facilitate the reversible insertion of sodium ions. Recent studies have shown that hard carbon can achieve capacities up to 300 mAh g⁻¹, making it a competitive candidate for commercial applications. Furthermore, hard carbon can be synthesized from a variety of low-cost precursors, including biomass, which adds to its economic attractiveness. The synthesis process typically involves pyrolysis at high temperatures, leading to the development of a porous structure that enhances sodium storage capacity. In addition to its inherent properties, research has focused on optimizing the synthesis parameters and post-treatment processes to further enhance its electrochemical performance, specially their limited first initial coulombic efficiency. Electrochemical characterization of hard carbon anodes has demonstrated stable performance over numerous charge-discharge cycles, with minimal capacity degradation, which is crucial for the longevity of NIBs. The ongoing research aims to address the challenges of initial irreversible capacity loss and improve the overall efficiency and performance of hard carbon anodes.29 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 10 1.3.1.2. Alloy-Based Anodes Although Silicon (Si) is a promising anode material for LIBs, it is not suitable for NIBs due to the lack of stable Na-Si alloy formation. As a result, research has focused on other alloy-forming elements that can reversibly react with sodium. Alloy-based anodes, such as tin (Sn), antimony (Sb), and phosphorus (P), offer high theoretical capacities. For example, sodium can alloy with tin to form Na15Sn4, providing a high theoretical capacity of 847 mAh g-1. However, these materials often suffer from significant volume changes during cycling, leading to mechanical degradation and reduced cycle life. Strategies such as nanostructuring and the use of composites have been employed to mitigate these issues.30 1.3.1.3. Metal Oxides and Sulfides Transition metal oxides and sulfides, such as Fe2O3, Co3O4, and MoS2, have been investigated as anode materials for NIBs. These materials undergo conversion reactions with sodium, providing high capacities. However, they also face challenges related to volume expansion and poor electrical conductivity. Research is ongoing to improve their performance through material engineering and the incorporation of conductive additives.31 1.3.2. Electrolyte Materials for Na-ion Batteries Electrolytes play a vital role in the performance, safety, and stability of NIBs. They provide the medium for sodium ion transport between the anode and cathode during charging and discharging cycles. Several types of electrolytes have been developed for NIBs. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 11 1.3.3.1. Organic Liquid Electrolytes Liquid electrolytes, typically composed of sodium salts dissolved in organic solvents, as used in Li-ion technology, are the most commonly used in NIBs. These electrolytes play a crucial role in facilitating the movement of sodium ions between the anode and cathode during charge and discharge cycles. Sodium hexafluorophosphate (NaPF6) in carbonate solvents is a popular choice due to its good ionic conductivity and stability, which are essential for efficient battery operation. The carbonate solvents often include a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), which provide a balance between high dielectric constant and low viscosity, enhancing the overall ionic mobility within the electrolyte. Despite their advantages, organic liquid electrolytes for NIBs face several challenges. One significant issue is the flammability of organic solvents, which poses safety risks, especially under high temperatures or in the event of a short circuit. Additionally, electrolyte decomposition can occur during cycling, leading to the formation of detrimental byproducts that can degrade the performance of the battery. This decomposition is often accelerated at higher voltages, limiting the operating range of the battery. Furthermore, compatibility with electrode materials is another critical factor. The electrolyte must form a stable solid-electrolyte interphase (SEI) layer on the anode to prevent further decomposition and ensure long-term cycling stability. To address these issues, researchers are exploring various strategies. Additives such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are commonly added to the electrolyte formulation to improve the formation and stability of the SEI layer. These additives help to enhance the overall performance and safety of the NIBs by reducing electrolyte decomposition and improving the compatibility REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 12 with electrode materials. Moreover, optimizing the ratio of solvents and salts can also lead to better performance. For instance, increasing the concentration of the sodium salt can improve ionic conductivity and reduce the occurrence of side reactions.32 1.3.3.2. Ceramic Solid-State Electrolytes Solid-state electrolytes offer the promise of enhanced safety and stability compared to liquid electrolytes. They include inorganic ceramics, such as NASICON-type (Na3Zr2Si2PO12), β-alumina, and sulfide-based electrolytes (Na3PS4), as well as polymer electrolytes. These materials provide good ionic conductivity and are nonflammable. However, challenges related to interfacial resistance, mechanical properties, and processing costs must be overcome for practical applications. Recent advancements in material design and engineering are showing promise in addressing these issues.33 1.3.3.3. Gel Polymer Solid-State Electrolytes Gel polymer electrolytes combine the benefits of liquid and solid-state electrolytes. They consist of a polymer matrix swollen with a liquid electrolyte, offering high ionic conductivity and improved safety. Common polymers used include polyethylene oxide (PEO) and polyvinylidene fluoride (PVDF). These electrolytes provide flexibility and ease of processing but still face challenges related to mechanical strength and long-term stability.34 1.3.3. Cathode for Na-ion batteries A critical component of NIBs is the cathode material, which significantly influences their energy density, cycling stability, and rate performance. The development of advanced cathode materials with REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 13 improved structural stability, electronic conductivity, and sodium ion diffusion pathways is essential for enhancing the performance and commercialization potential of NIBs. 1.3.3.1. Layered oxides Na-ion layered oxides typically adopt a layered crystal structure similar to that of Li-ion counterpart, generally represented by the formula NaxTMO2, where TM is a transition metal such as Fe, Ni, Co, Mn, etc. or a combination thereof. These materials crystallize in either O3 or P2-type structures, differentiated by the stacking sequence of Na and MO2 layers along the c-axis. They also differ in their sodium content: O3-type structures typically accommodate a higher sodium content (Na1MO2), while P2-type structures generally have a lower sodium content (Na2/3MO2). In O3-type materials, Na layers alternate with octahedral MO2 layers, while P2-type materials feature prismatic coordination of Na ions (Figure 1.4). The choice of transition metal and the stacking type significantly influence the structural stability and electrochemical properties of these cathodes.35 Figure 1.4. Structural patterns of a) O3-type layered oxides, b) P2-type layered oxides, c) O2-type layered oxides. The capital letters (A, B, C) are used to describe the packing patterns of the oxygen-ion frameworks. Grey, red and purple colors correspond to Na, O and M element respectively.35 The electrochemical performance of Na-ion layered oxides is closely tied to their structural characteristics. Key performance indicators REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 14 include specific capacity, cycling stability, and rate capability. For instance, O3-type NaxNi0.5Mn0.5O2 can deliver a specific capacity up to 200 mAh g-1, with stable cycling at moderate current densities. On the other hand, P2-type Na2/3Mn0.5Fe0.5O2 exhibits excellent rate capabilities and structural stability upon cycling but typically offers lower specific capacities.36–38 In 2024, layered oxides dominated the market share but faced issues such as poor high-temperature stability and reliance on nickel resources. They can reach an energy density of 160 Wh kg-1, but typically has a cycle life of 3,000 cycles and a 30% capacity decay at -20 ºC.39 1.3.3.2. Prussian blue analogues Prussian Blue Analogues (PBAs) represent a class of open-framework materials characterized by their cyanide-bridged transition metal complexes. Known for their versatility and unique properties in ion exchange and electrochromism, PBAs have emerged as promising cathode materials for NIBs. Their open-framework structure facilitates rapid and reversible insertion/extraction of Na+ ions, which is critical for high-performance batteries. The generic formula for Prussian Blue Analogues is AxTMy[Fe(CN)6]z·nH2O, where TM is a transition metal such as Fe, Co, Ni, or Mn, and A is a monovalent cation, typically Na+. The transition metals are linked by cyanide groups forming [TM-Fe(CN)6] frameworks (Figure 1.5). The color ─and, hence, the name─ of the PBA compounds tend to vary depending on the sodium content, being typically green for the desodiated state (Berlin Green, BG), blue for partially sodiated state (Prussian Blue, PB), and white for fully sodiated state (Prussian White, PW).40 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 15 Figure 1.5. Common types of structural complexity in PBAs: (a) octahedral tilts, (b) correlated ‘slides’ of A-site cations, (c) Jahn–Teller distortions, (d) A-site occupancy (dis)order, (e) hexacyanometallate vacancies and the resulting connected pore-network structure, and (f) framework hydration.41 The unique aspect of PBAs is the presence of large voids within the crystal structure, which can accommodate a variety of cations and water molecules. This porosity is crucial for the rapid diffusion of Na+ ions during battery operation. PBAs are known for their low-cost, non-toxicity, and especially their robust redox activity. The electrochemical behavior of PBAs is primarily governed by the redox reactions of the transition metals in the cyanide framework. These materials can exhibit high theoretical capacities, typically ranging from 100 to 170 mAh g-1. Moreover, their open-framework structure contributes to excellent rate capabilities and cycling stability, as the large channels allow for quick and efficient Na+ transport.41 Despite their advantageous properties, PBAs face several challenges. One significant issue is the potential for cyanide leaching, which raises environmental and safety concerns. Moreover, the hydration water present in the crystal structure can be variable, which affects the electrochemical consistency and stability of the material.40 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 16 To address these issues, recent research has focused on modifying the chemical composition and synthesis methods of PBAs. Strategies such as partial substitution of cyanide with other ligands or coating the PBA particles with protective layers have been explored to enhance stability and safety. Additionally, the synthesis of water-free PBAs could potentially improve the electrochemical performance and address the variability caused by hydration. 1.3.3.3. Polyanionic materials Polyanionic compounds, characterized by their polyanion groups such as phosphates (PO4), sulfates (SO4), and silicates (SiO4), have attracted attention in the field of NIBs due to their excellent thermal stability and safety profiles. These materials offer a diverse range of structures and properties that are critical for developing advanced cathode materials. This section delves into the structural and electrochemical characteristics of polyanionic compounds and discusses their significance in the context of Na-ion battery technology. Polyanionic compounds feature a robust three-dimensional framework composed of metal ions coordinated by polyanionic groups. This framework results in strong covalent bonding within the structure, which significantly enhances the thermal and chemical stability of the material. An additional benefit of these compounds is the positive inductive effect of the polyanionic groups, which raises the working potential of the material by stabilizing the redox couple and increasing the energy density. Common examples include NASICON-like structures (e.g., Na3V2(PO4)3) and olivine-type phosphates (e.g., NaFePO4) (Figure 1.6). These frameworks allow for stable cycling, a key challenge in battery material stability.35 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 17 Figure 1.6. Crystal structures of: a) olivine NaMPO4, b) NASICON Na3V2(PO 4)3, c) triclinic Na2MP2O7, d) orthorhombic Na2MP2O7, e) orthorhombic Na4M3(PO4)2P2O7, f) orthorhombic Na2MPO4F, g) monoclinic Na2MPO4F, and h) tetragonal Na3M2(PO4)2F3 (M represents transition metals).35 The electrochemical performance of polyanionic compounds is marked by their stable voltage plateaus, which are conducive to predictable and steady energy output. Materials like Na3V2(PO4)3 can offer good specific capacities (up to 120 mAh g-1) and exhibit excellent cycling stability with low-capacity fade over many cycles. This stability is due to the strong covalent bonds in the polyanion groups, which help maintain the integrity of the crystal structure during Na+ insertion and extraction.35,42 Additionally, the polyanionic framework tends to lower the environmental risk associated with battery materials, as they are more resistant to releasing harmful substances upon decomposition. Despite their advantages, polyanionic compounds face several challenges that need addressing to optimize their utility in NIBs. One REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 18 significant issue is their inherently lower electronic and ionic conductivity, which can limit ion transport and reduce the overall rate performance. Emerging research has focused on addressing these limitations through various strategies. Doping with different metal ions or creating composite materials are common approaches to enhance electrical conductivity. Another promising strategy involves optimizing the synthesis process to produce nanostructured materials, which can offer shorter ion diffusion pathways and larger surface areas for improved rate capabilities.43 Olivine is a silicate mineral with an olive-green hue, typically represented by the approximate composition (Mg,Fe)2SiO4 has been a subject of intense investigation among geoscientists for years. It is widely regarded as the predominant mineral in the Earth's upper mantle and is pivotal in regulating a wide range of petrogenetic processes and the Earth's physical properties.44–46 The geochemical and thermobarometric data derived from olivine provide a valuable resource for understanding rock formation, lithospheric mantle geodynamics, and the genesis and exploration of magmatic ores and deep-origin minerals such as pyroxenes and garnets. Currently, the plural term olivines is broadly used to describe a group of minerals conforming to the general formula M2XO4 (where M can be an s-, p-, or d-block metal or a vacancy, and X is typically Al, Si, or P) (Figure 1.7), all of which exhibit the fundamental olivine-type structure. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 25 1.4. Objectives and goals The primary objective of this thesis is to develop and optimize new, cost-effective synthesis methods for various phosphate cathode materials suitable for sodium-ion batteries. The focus is on creating environmentally friendly synthesis routes that utilize abundant and non-toxic components while minimizing production costs. A comprehensive understanding of the synthesis and electrochemical mechanisms is crucial for advancing the application of phosphate cathodes in sodium-ion batteries, with an emphasis on affordability and material efficiency. The overarching goals of this PhD research are to: 1. Establish a new paradigm for the synthesis of phosphate cathode materials that prioritizes environmental sustainability without compromising material performance or costeffectiveness. 2. Enhance the theoretical and practical understanding of phosphate materials as cathodes in sodium-ion batteries, setting a foundation for their future industrial application. 3. Contribute to the global effort in advancing sodium-ion battery technologies by providing viable alternatives to current lithium-ion systems, particularly for applications where cost, availability, and environmental impact are critical considerations. By achieving these objectives and goals, this research aims to address some of the critical challenges in the field of energy storage materials and pave the way for the next generation of sodium-ion batteries. The outcomes are expected not only to enrich the academic understanding of phosphate-based cathode materials but also to affect (influence) the broader landscape of energy storage technologies in a sustainable and cost-effective manner. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 26 The thesis will be structured, apart from this initial introduction chapter, of the following chapters: Chapter 2, where the experimental characterization techniques will be explained, including structural/morphological, physico-chemical and electrochemical techniques. In chapter 3, an indirect synthesis of triphylite-NaMn1-yFeyPO4 will be detailed starting from commercial LiMn1-yFeyPO4 and with a method that enables the recovery of lithium in the process. Chapter 4 is dedicated to the study of the direct synthesis of different polymorphs with the formula Nax(TM)y(PO4)z (TM = Mn, Fe or combination of both) and understanding the difference of the various experimental conditions (as the pH) for obtaining different phases. Chapters 5 and 6 consist of the deeper characterizations of the synthesized materials, including the use of advanced techniques such as XAS and Mössbauer spectroscopy, of the triphylite and xenophyllite respectively obtained in chapter 4. Chapter 7 details the reaction mechanism of the maricite NaFePO4 using various advanced characterization techniques following its activation by ball milling with carbon. Chapter 8 summarizes the general conclusions of this work. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 27 1.5. References (1) Energy Outlook 2024. (2) Darlington Eze Ekechukwu; Peter Simpa. 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Carbon Energy 2022, 4 (5), 878– 900. https://doi.org/10.1002/cey2.187. (67) Hunger, K.; Mischke, P.; Rieper, W. Azo Dyes, 1. General. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH, Ed.; Wiley, 2011. https://doi.org/10.1002/14356007.a03_245.pub2. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 1. Introduction 34 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 41 2.2.2. Transmission Electron Microscopy A Transmission Electron Microscope (TEM) has a high voltage of 100 KeV or higher (up to 1 MeV) and provides both image and diffraction information about the samples. In a TEM the electron beam emitted by a cathode is focused on a specimen using several condenser lenses causing an enlarged version to appear on a fluorescent screen. This electron beam carries information about the specimen. Unlike SEM, the electron beam crosses the sample and, therefore, the key requirement is a low thickness of the sample. Besides the higher resolution compared with SEM imagining, one of the main advantages of TEM imaging is the possibility to do local electron diffraction patterns, which gives information about the crystalline structure of the sample. TEM measurements have been done in a FEI Tecnai G2 operated at 200kV field emission gun (FEG) high resolution and equipped with High Angle Annular Dark Field (HAADF) detector and X-ray Dispersive Energy (EDX) spectrometer. The samples were ultrasonicated under acetone and the resulting dispersion of the powder was transferred to a quantifoil carbon film fixed on a 3 mm copper grid (200 mesh). 2.3. Thermal analysis couple with Mass Spectrometry technique Thermal analysis is a group of techniques that study the properties of materials as they change with the temperature. Thermogravimetric analysis is an analytical technique used to determine the thermal stability of the sample. It measures changes in the weight of a material as a function of the temperature. The changes observed in the mass are related to chemical or physical processes that occur upon heating the sample. Differential thermal analysis is an analytical technique that monitors the difference in temperature between the sample and a reference REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 42 material against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed. The observed peaks in the temperature difference curve are related to endothermic or exothermic processes, such a phase transitions that don’t imply any weight variation. For this purpose, TGA NETZSCH STA 449 F3 Jupiter couple with the Mass Spectrometer QMS 403 Aëolos Quadro was used to collect the thermogravimetric curve and the DTA curve (Figure 2.4). Mass spectrometry is an analytical technique used to measure the masses of molecules, identify the chemical structures of compounds within a sample, and quantify the amount of these compounds. The sample is ionized, which means the molecules are converted into charged particles (ions). The generated ions are then separated based on their mass-to-charge ratio (m/z) using an analyzer. After separation, the ions are detected, usually by a detector that counts ion hits and measures their intensity, which correlates to the abundance of each ion. The data collected by the detector is used to generate a mass spectrum. Figure 2.4. Thermogravimetric analyzer TGA NETZSCH STA 449 F3 Jupiter coupled to a Mass Spectrometer QMS 403 Aëolos Quadro available at CIC energiGUNE. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 43 2.4. Inductively Coupled Plasma Atomic Emission Spectrometry technique The inductively coupled plasma atomic emission spectroscopy (ICPAES) is an analytical technique used for elemental determinations. It is a type of mass spectrometry that uses inductively coupled plasma to ionize the sample, creating atomic and small polyatomic ions, which emit electromagnetic radiation at wavelengths characteristic of a particular element. The concentration of the element within the sample is indicated by the intensity of this emission. The ICP-AES equipment used was an Ultima 2 Horiba Yobin Yvon (Figure 2.5). Since the samples have to be liquid prior to being analyzed, the powder samples are previously digested in an acid solution of HNO3 and then heated at 190 °C for 15 minutes. Figure 2.5. Ultima 2 Optical Emission Spectrometer used in this work. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 44 2.5. Raman Spectroscopy technique Raman spectroscopy is a non-destructive vibrational spectroscopic technique used in chemistry to collect a spectrum with a unique fingerprint of molecules. Each molecule has a different set of vibrational energy levels, and the emitted photons have unique wavelength shifts, allowing the identification of what is in a sample. When a photon (electromagnetic radiation of specific frequency ν) reaches a molecule, it interacts with the polarizable electron density and the bonds of the molecule, exciting the molecule to a so-called virtual energy state for a short period of time. Then, the photon of frequency ν (Rayleigh scattering or elastic scattering) as well as that of frequency v ± vi (“Raman scattering” or inelastic scattering), where vi represents a vibrational frequency of the molecule, are scattered. Raman scattering is much weaker than Rayleigh scattering, thus only a very small fraction of molecules undergoes inelastic scattering. The resulting inelastically scattered photon can be of either lower or higher energy than the incoming photon. In case the scattered photon exhibited higher frequency than the incident radiation, a blue shift is observed, generating what is known as anti-Stokes lines. On the contrary, if the scattered photon exhibits a lower frequency than the incident radiation, a red shift occurs, and these bands are referred to as Stokes lines. In the spectrum, the different peaks correspond to different Raman excitations. Raman analyses were conducted in the range 100 - 4,000 cm-1 using a Renishaw InVia spectrometer equipped with a 532 nm green laser operated at a power of 0.5 mW available at CIC energiGUNE. The laser spot size was approximately 1 mm in diameter. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 45 2.6. X-Ray Absorption Spectroscopy technique The X-Ray Absorption Spectroscopy (XAS) technique uses a wide range of tunable X-rays provided by synchrotron facilities to identify the oxidation state of ions in different samples. Incident photon energies irradiating the samples are scanned across an absorption edge creating a core hole state. The area underneath an absorption spectrum represents the partial density of unoccupied states above the Fermi level. The spectra can be separated into three different regions: the pre-edge region describes core electron excitation to unoccupied states close to the Fermi level, the main-edge probes the upper quasi-bound states, and the post-edge which leads to excitation above the ionization threshold. The response to each incident photon energy can be detected as either transmitted X-rays, fluorescence or electron yield. There are direct and indirect detection modes. Transmission mode is a direct detection mode, which identifies first-order absorption processes. Due to the short attenuation lengths of soft X-rays, XAS measurements in transmission mode are only relevant for thin samples, and not for common battery materials. Therefore, indirect detection modes are more suitable for battery analysis and rely on secondary processes which are separated into radiative and nonradiative relaxation. In the latter case, the generated core hole state relaxes back to the ground state by filling the core-hole with an electron from a higher energy level via the emission of an Auger electron. The Auger electron can then scatter inelastically and create low energy secondary electrons which leave the sample. The generated number of core holes is proportional to the drain current to replace the lost photoelectrons, which is measured by a high precision ammeter. This detection mode is surface sensitive, with probing depths of 2-10 nm. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 46 due to the short attenuation length of the scattered electrons and is called the total electron yield (TEY). Another mode of indirect detection is total fluorescence yield (TFY). The excited system relaxes back to the ground state by filling the core hole with a valence level electron via the emission of a photon with the energy difference of the two energy levels. The TFY detection is more bulk sensitive (up to 100 nm), because photons have a longer attenuation length than electrons. Measurements were performed at BL29-BOREAS beamtime in ALBA (Barcelona). XAS data were collected in both total electron and fluorescence yield mode. Signals from TEY mode correspond to the chemical state of the surface layer of the material, whereas FY corresponds to approximately 100 nm into the particle, thus having a notable contribution from the bulk crystal structure, especially considering the particle size of the materials. 2.7. Mössbauer Spectroscopy technique The Mössbauer Spectroscopy is a technique which is based on the energy level transitions that nuclei undergo in atoms. These energy levels transitions can provide information about the local environment of an atom as they depend on the electronic and magnetic environment. The Mössbauer effect consists of the recoilfree emission of a gamma-ray from a nucleus (source) in the excited state and the resonant absorption by an identical nucleus (absorber or sample). This effect is only observed in a limited number of isotopes, being 57Fe the most commonly studied. In 57Fe Mossbauer spectroscopy, a 57Co source is used. This source decays to an excited state of 57Fe by electron capture and then, it subsequently decays to a ground state emitting the desired gammaray (of 14.4 keV), that can be absorbed by a 57Fe nucleus in the REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 47 sample. In order to tune the energy, the source is moved in a range of velocities using a linear motor, producing the Doppler effect.4 In the resulting spectra, gamma-ray intensity is plotted as a function of the source velocity. At velocities corresponding to the resonant energy levels of the sample, a fraction of the gamma-rays is absorbed, resulting in a drop in the measured intensity and a corresponding dip in the spectrum. 57Fe Mössbauer spectra were measured at room temperature (293 K) with a 57Co:Rh source and a Kr gas proportional counter. The spectrometer was operated with a triangular velocity waveform, and the spectra were fitted with superpositions of appropriate sets of the Lorentzian lines using NORMOS program for powder measurement and PC-MOS computer program for operando measurements.5,6 Isomer shift values are given relative to α-Fe metal. In the operando Mössbauer spectroscopy measurements, the cell was cycled between 4.5 and 1.5 V vs. Na+/Na. 2.8. Solid State Nuclear Magnetic Resonance technique Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is a valuable tool for characterizing local environments in complex materials, particularly those containing nuclei such as 23Na and 31P. The 23Na nucleus, with a spin of 3/2, is quadrupolar and thus highly sensitive to local electric field gradients and coordination symmetry. Its spectra often exhibit broad features due to quadrupolar interactions, but these can be significantly narrowed using magicangle spinning (MAS), quadrupolar echo, or multiple-quantum MAS (MQMAS) techniques. These methods enable the resolution of distinct sodium sites, providing insights into structural order and local chemical environments in materials such as ion conductors, glasses, and biological systems. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 48 In contrast, 31P is a spin-1/2 nucleus with 100% natural abundance and no quadrupolar interaction, resulting in sharp, well-resolved spectra under MAS. It is widely employed in the study of phosphatebased systems, including inorganic solids, biominerals, and lipid membranes. The use of cross-polarization (CP-MAS) enhances sensitivity, particularly in systems where phosphorus is present at low concentrations. Analysis of chemical shift anisotropy (CSA) and dipolar couplings in 31P ssNMR further provides detailed information on molecular orientation, local geometry, and dynamic processes. Together, 23Na and 31P ssNMR offer complementary insights into the structure and function of complex solid-state materials. Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) experiments were performed at 52.9 MHz, on a Bruker-300 spectrometer charged to a field of 4.7 T using a 1.3 mm MAS probe at rotor spinning speed of 50 kHz. A rotor synchronized spin−echo pulse sequence (90°−τ−180°−τ1−acquisition) was used with typical 90° and 180° pulses of 1.2 and 2.4 μs for 23Na and 1.5 and 4 μs, for 31P respectively. Recycle day of 1s was used for all experiments. The number of scans was set to 4k for 23Na and 6k for 31P experiments. 2.9. Electrochemical techniques 2.9.1. Electrode preparation For the electrode preparation, active materials and additives have been mixed using N-Methyl-2-pyrrolidone (NMP) as solvent. In order to increase the adhesion of the electrode composite to the aluminum foil, Polyvinylidene fluoride (PVdF) has been used. First, PVdF has been dissolved in NMP. Afterwards, the active material and carbon black (Super C65), which has been used as conducting additive to increase the electronic conductivity, have been added and mixed with the magnetic stirrer for 1 hour until a homogeneous slurry is obtained. The slurries prepared for this work consist of a (active REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 49 material:conductive additive:binding polymer) ratio of 8:1:1 for Ccoated powders as C/t-NaFePO4 and 7:2:1 for non C-coated powders as t-NaMn1-yFeyPO4. The slurry has been poured on an aluminum foil (20 µm, Hohsen) and cast using a Doctor Blade coating machine setting a height of 150 µm. The laminates have been dried under vacuum overnight at 120 °C and electrodes discs with a diameter of 12 mm have been punched and pressed with 4 tons cm-2. Finally, before assembling the electrochemical cells, the electrodes has been dried under vacuum at 120°C overnight. 2.9.2. Cell assembly Half-cells have been assembled for the study of cathode material, where the cathode electrode is assembled versus a constant voltage material. In this work, metallic sodium has been used both as a counter electrode and reference electrode in the case of the three electrode cells. A Whatman glass-fiber separator (GF/D) is placed between the cathode material and metallic Na after being soaked with the electrolyte. 1M NaPF6 EC:PC 1:1 W.% (ethylene carbonate: propylene carbonate) has been used as standard electrolyte unless otherwise indicated. Galvanostatic data of coin cells CR2032 were typically collected with the Maccor Battery tester and the VMP3 potentiostat/galvanostat was used for collecting galvanostatic data, cyclic voltammograms and PITT data in the case of two-electrode and three-electrode Swagelok configuration cell. In the operando Mössbauer spectroscopy measurements, the Mössbauer absorbers were prepared as self-standing electrodes of the cathode active material (previously ball milled with C65) mixed with PTFE binder and mounted in a specifically designed in situ electrochemical cell.7 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 2. Experimental Techniques 50 In the operando XRD measurements, the patterns were collected at a wavelength λ = 0.825890 Å using our homemade operando electrochemical cell equipped with a Beryllium (Be) window as a current collector and using a transparent plunger also equipped with a Be window for transmission geometry.2 Electrode discs were prepared by mixing the cathode active material with carbon C65 (Imerys) in a ratio of 62: 8 using Whatman GF/D borosilicate glass as separator, a 1.0 M sodium hexafluorophosphate (NaPF6, SigmaAldrich) in a 1 : 1 vol. mixture of ethylene carbonate (EC, Acros) and dimethyl carbonate (DMC) as electrolyte and high purity Na metal (Sigma-Aldrich) as negative electrode. 2.9.3. Galvanostatic experiments The electrochemical performances of the cells have been evaluated with a galvanostatic cycling, where a constant current is applied with potential limitations. Therefore, the amount of exchanges charges is quantified and compared with the theoretical capacity. The voltage of a battery V0 depends on the chemical potentials of the positive and negative electrodes. In galvanostatic cyclic chargedischarge measurements, Voltage is potted against time. Nevertheless, in the battery field, the presentation of the results usually entails the conversion of time to specific capacity (mAh) or to gravimetric specific capacity (mAh g-1), being the last one the amount of electricity (mAh) that can 1 g of a material deliver in a single charge/discharge at a determined C-rate. C-rate (in A g-1 or mA g-1) is a measure of the rate at which a battery is charged or discharge related to its maximum capacity. At 1C means that the discharge current will discharge the battery com pletely in 1 hour. Galvanostatic cyclic charge-discharge experiments at a determined C-rate depending on the materials and C-rate capability rates have been carried out. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 57 Figure 3.3. XRD patterns of the LiMn0.65Fe0.35PO4 pristine and the obtained compounds after different chemical oxidation conditions compared to FePO4 from the literature as reference with similar XRD pattern (ICSD code: 92199) for phase identification. 3.2.3. Chemical reduction of Mn1-yFeyPO4 Firstly, to optimize the sodiation process, various synthesis approaches have been employed to successfully obtain the C/NaFePO₄ pure phase from the obtained C/FePO₄. For this chemical reduction, sodium thiosulfate (Na2S2O3) has been selected as the reducing agent due to its moderate reducing properties. This is expected to minimize damage to the carbon coating or decomposition into undesired products, since Na2S2O3 provides a milder reduction environment compared to stronger reductants. The strategies implemented (Table 3-2) include: REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 58 Table 3-2. Different strategies for the synthesis of C/NaFePO4 from C/FePO4 (modifications included in bold). Strategy Reducing agent Ratio Concentration (M) T (°C) Stirring time (h) a Na2S2O3 4:1 0.134 80 24 b Na2S2O3 4:1 0.532 80 24 c Na2S2O3 4:1 0.532 60 24 d Na2S2O3 + Na2SO4 4:1 0.532 60 24 e Na2S2O4 1:2 0.133 60 24 f Na2S2O3 6:1 0.8 60 24 a) Using diluted Na2S2O3 as reducing agent at 80 ºC The first trial was performed using diluted Na2S2O3 (0.134 M) in aqueous media, with a molar ratio (Na2S2O3:FePO4) of 4:1, at a temperature of 80 ºC for 24 hours. The temperature was increased with the idea of favoring the reaction kinetically. The powder is obtained as pure unreacted C/FePO4 (SD_A, see Figure 3.4), suggesting that the reaction time of Na2S2O3 can be very slow. The presence of small shoulders in the XRD pattern may suggest the onset of a slight sodiation process, indicating partial incorporation of sodium into the FePO4 framework. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 59 Figure 3.4. XRD pattern of the synthesized SD_A compared to FePO4 from the database (ICSD code: 92199) for phase identification. b) Increment of concentration The following test was carried out using a high concentration of Na2S2O3 (0.532 M), with a molar ratio (Na2S2O3:FePO4) of 4:1, at a temperature of 80 ºC for 24 hours. The obtained powder consists of a mixture of Fe1.375PO(OH), which is not the targeted product, and S8 (SD_B, Figure 3.5), showing that at this higher concentration of Na2S2O3 all the Fe is still in oxidation state (III) affecting somehow the C/FePO4. This suggest that at this temperature Fe2+ might not be stable and could be easily re-oxidized to Fe3+. The presence of S8 can be attributed to a common sideproduct of reactions where Na2S2O3 is involved. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 60 Figure 3.5. XRD patterns of the synthesized SD_B compared with Fe1.375PO4(OH) (ICSD code: 170854) and S8 (ICSD code: 870) from the database for phase identification. c) Decrease in temperature To analyze the effect of the temperature on this reaction, experiments at lower temperature (60 ºC) for 24 hours were carried out using an excess of Na2S2O3 with the same concentration (0.532 M), and a molar ratio (Na2S2O3:FePO4) of 4:1. The obtained powder, called SD_C is a mixture of NaFePO4, Na0.7FePO4, FePO4 and S8 (see Figure 3.6), suggesting that, although the targeted NaFePO4 is finally obtained, the presence of other impurities indicates that the reducing/sodiation reaction is not completely effective. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 61 Figure 3.6. XRD pattern of the synthesized SD_C compared with different NaxFePO4 phases (ICSD code: 169118, 169119, 92199) and S8 (ICSD code: 870) for phase identification. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 62 d) Addition of Na2SO4 as extra sodium salt To improve the oxidation process and compensate the sodium deficiency in the products, a similar experiment to the c) previously described was carried out with the addition of sodium sulphate, Na2SO4. In this case, the obtained powder, SD_D was a mixture of Na0.7FePO4 and S8 (see Figure 3.7), showing that alternative experimental conditions still need to be found. Figure 3.7. XRD pattern of SD_D compared with Na0.7FePO4 (ICSD code: 169119) and S8 (ICSD code: 870) from the database for phase identification. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 63 e) Using Na2S2O4 as reducing agent An alternative reducing agent, sodium dithionite, Na2S2O4, known to be a stronger reducing agent, was used in the new synthesis. The reaction was carried out using an aqueous solution of Na2S2O4 with a concentration of 0.133 M and a molar ratio (Na2S2O4:FePO4) of 1:2 at 25 ºC for 24 hours. However, in this case, the obtained phase, SD_E was found to be amorphous (see Figure 3.8). This result is likely due to the strong reducing character of Na2S2O4, which gives rise to the amorphization of the obtained compound. Consequently, Na2S2O4 was discarded as sodiation agent. Figure 3.8. XRD pattern of the synthesized SD_E. f) Improve the reducing process by using concentrated Na2S2O3, Based on the previous results, we decided to make use of Na2S2O3 instead of Na2S2O4. To improve the efficiency of its reducing activity, the molar ratio (Na2S2O3:FePO4) was increased from 4:1 to 6:1, and the concentration from 0.532 to 0.8 M, while maintaining the temperature and time parameters (60 ºC for 24 hours). To avoid the foreseeable presence of S8 in the final product, the powder was washed with toluene at 60ºC for 1 hour. Finally, the pure C/NaFePO4 phase henceforth: SD_F, was finally obtained by filtration under these conditions, as shown by the XRD pattern in Figure 3.9. The byproduct Na2S4O6 is soluble and would remain dissolved in the waterwater. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 64 Figure 3.9. XRD pattern of the synthesized SD_F compared with NaFePO4 (ICSD code: 169118) from the database for phase identification. Summarizing, the pure C/NaFePO4 phase was obtained by the chemical sodiation of C/FePO4 with a 0.8M Na2S2O3 aqueous (SigmaAldrich, 99%) solution reducing agent in a (Na2S2O3:FePO4) ratio of 6:1, at the temperature of 60 ºC for 24 h. (Figure 3.10). Using this approach, 150 grams of t-C/NaFePO4 can be obtained in one batch, with a yield of 91%. Figure 3.10. Rounded flask of 5 L during the synthesis of a batch of triphylite C/NaFePO4 using the indirect synthesis method. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 65 The XRD patterns sequence of the full indirect synthesis is shown in Figure 3.11 and a schematic description of the different approaches used to obtain the C/NaFePO4 pure phase from C/FePO4 is shown in Figure 3.12, which summarizes the experimental strategies showed in Table 3-2. Figure 3.11. XRD patterns of synthesized C/FePO4 and C/NaFePO4 compared to the commercial C/LiFePO4 used as precursor. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 66 Figure 3.12. Scheme of the different chemical reduction approaches for obtaining NaFePO4. To validate this strategy, the same chemical reduction procedure (strategy F, Figure 3.12) was employed to synthesize C/NaMn0.35Fe0.65O4 from the previously obtained phase C/Mn0.35Fe0.65PO4 phase, as detailed previously in Section 3.2.2. This process involved the selective reduction of Fe and Mn precursors within the carbon-coated framework, enabling the transformation while preserving the structural integrity of the material. The XRD patterns of the synthesized C/NaMn0.35Fe0.65O4 confirmed its successful formation as a pure phase, with no detectable secondary phases or impurities (Figure 3.13). This indicates that the reduction procedure is effective for tailoring the material's composition while maintaining its crystallinity. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 73 The morphology and size of the C/LiFePO4, C/FePO4 and C/NaFePO4 particles were characterized by SEM, revealing that all three compounds exhibit good homogeneity and a similar particle size distribution within 50-300 nm of diameter (Figure 3.19.a-c). In addition, TEM images show that the particles are uniformly carbon coated after the oxidation and reduction treatments (Figure 3.19.df), indicating that the oxidation/reduction reaction does not alter the sample morphology from surface to bulk. A higher magnification TEM image of C/NaFePO4 (Figure 3.20) clearly confirms the presence of cracks, which are highlighted in purple. This can be attributed to the extraction and insertion of Li+ and Na+ during the chemical synthesis, primarily due to the significant volume mismatch between FePO4/NaFePO4 (18%), compared to LiFePO4/FePO4 (7%).3 Figure 3.19. (a-c) SEM and (d-f) TEM images of (a,d) C-LiFePO4, (b,e) C/FePO4 and (c,f) C/NaFePO4. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 74 Figure 3.20. TEM images of C/NaFePO4 particles in which the cracks are highlighted with circles. The percentage of carbon in the resulting C/NaFePO4 were determined by TGA measurements in air (see Figure 3.21), expecting the combustion of carbon liberated as CO2. This is important because the percentage of carbon directly affects the accurate normalization of electrochemical capacity, ensuring that the reported performance reflects only the active LiFePO4 phase. However, at temperatures above 200 ºC, the heating of C/NaFePO4 gives rise to an increase in the mass that can be attributed to the decomposition of NaFePO4 into species such as Na3Fe2(PO4)3 and Fe2O3 due to the oxidation processes of Fe2+ to Fe3+. A similar phenomenon is observed with C/LiFePO4.8 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 75 Figure 3.21. Thermogravimetric analysis of C/LiFePO4, C/FePO4 and C/NaFePO4 in air. For this reason, to evaluate the percentage of carbon it was measured C/FePO4, where Fe is already in the oxidation state (III). When C/FePO4 is heated, a small fraction of humidity H2O is liberated in the range of RT-150ºC. Then, a weight loss of approximately 2% is observed in the range of 350-520 ºC (Figure 3.22), which is attributed to the Carbon-coating layer in the samples. This loss was corroborated by the measurements performed in the Mass Spectrometer coupled to the TGA and assigned to the liberation of CO2 (Figure 3.23). REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 76 Figure 3.22. Thermogravimetric analysis and analysis of the outlet gases (H2O and CO2) detected by mass spectrometry of C/FePO4 in air. Figure 3.23. Mass spectrometry with the outlet of gases (H2O and CO2) of C/FePO4 in air. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 77 3.3.2. (Li/Na)Mn0.65Fe0.35PO4 materials characterization Le Bail refinements of the C/LiMn0.65Fe0.35PO4, C/Mn0.65Fe0.35PO4 and C/NaMn0.65Fe0.35PO4 obtained after chemical delithiation and sodiation are shown in Figure 3.24, Figure 3.25 and Figure 3.26 respectively. It can be noticed that the C/Mn0.65Fe0.35PO4 peaks are shifted to a higher 2 compared to those of the C/LiMn0.65Fe0.35PO4 peaks, due to the cell contraction that results from deinsertion of lithium ions. However, the insertion of sodium ions into FePO4 to form NaFePO4 structure leads to larger unit cell parameters, and thus the peaks are shifted to lower 2. This effect is similar than the observed with (Li,Na)FePO4 componds. These compounds crystallize in the orthorhombic system with Pnma space group. The Le Bail refined cell parameters of C/LiMn0.65Fe0.35PO4, C/Mn0.65Fe0.35PO4 and C/NaMn0.65Fe0.35PO4 are shown in Table 3-4 and are in good agreement with the expected values.9 Figure 3.24. Le Bail refinement of the XRD pattern of the commercial precursor C/LiMn0.65Fe0.35PO4. Experimental pattern (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions (vertical bars in green). (RBragg = 39.4% and χ2 =5.54). REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 78 Figure 3.25. Le Bail refinement of the XRD pattern of the commercial precursor C/Mn0.65Fe0.35PO4. Experimental pattern (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions (vertical bars in green). (RBragg = 1.11% and χ2 = 4.04). Figure 3.26. Le Bail refinement of the XRD pattern of the commercial precursor C/NaMn0.65Fe0.35PO4. Experimental pattern (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions (vertical bars in green). (RBragg =3.60 % and χ2 =2.42). REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 79 Table 3-4. Unit cell parameters of C/LiMn0.65Fe0.35PO4, C/Mn0.65Fe0.35PO4 and C/NaMn0.65Fe0.35PO4 obtained by refinement. a (Å) b (Å) c (Å) V (Å3) C/LiMn0.65Fe0.35PO4 10.3936(4) 6.0646(2) 4.7290(2) 298.09(2) C/Mn0.65Fe0.35PO4 9.6973(3) 5.8870(2) 4.7788(1) 272.891(1) C/NaMn0.65Fe0.35PO4 10.4800(3) 6.2555(1) 4.9715(1) 325.92(1) The SEM images (Figure 3.27) depict the morphology of (a, b) CLiMn0.65Fe0.35PO4, (c, d) C/Mn0.65Fe0.35PO4, and (e, f) C/NaMn0.65Fe0.35PO4. Uniform spherical particles with varying sizes are observed, indicating successful formation of the materials. The morphology reveals a bimodal size distribution, comprising primary particle in the range of 1-5 µm and larger secondary particles in the range of 10-30 µm. The images at higher magnifications (b, d, f) reveal a smooth surface texture with some agglomeration. Figure 3.27. SEM images of (a),(b) C-LiMn0.65Fe0.35PO4, (c),(d) C/Mn0.65Fe0.35PO4 and (e),(f) C/NaMn0.65Fe0.35PO4. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 80 3.3.3. Recovered LiFePO4 material characterization A characterization of the recovered Li3PO4 and synthesized LiFePO4 (section 3.2.4) was performed to analyze their structural framework, morphological features, which are important for understanding their performance. XRD patterns of the recovered Li3PO4 and the synthesized LiFePO4 are shown in Figure 3.28 and Figure 3.29 respectively. Both compounds crystallize in the orthorhombic system, with a Pmn21 space group for the Li3PO4 and Pmna for LiFePO4. The unit cell parameters determined by Le Bail refinement are a = 6.1282(5) Å, b = 5.2499(4) Å, c = 4.88226(4) Å and a = 10.3309(3) Å, b = 6.00860(15) Å, c = 4.69534(11) Å, for Li3PO4 and LiFePO4, respectively, and are in good agreement with those reported in the literature.10,11 Figure 3.28. Le Bail Refinement of Li3PO4 obtained from Li+ containing wastewater (Section 3.2.4). Experimental patter (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions vertical bars in green). REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 81 Figure 3.29. Le Bail Refinement of LiFePO4 obtained from recycled Li3PO4. Experimental patter (red), calculated profile (black), difference between experimental and calculated (blue) and Bragg positions vertical bars in green). The morphology of the particles of the recovered Li3PO4 and the synthesized LiFePO4 was also characterized by scanning electron microscopy (SEM), which showed a homogeneous distribution of agglomerates (≈ 50µm) of particles (600-700 nm) in the case of Li3PO4 and a homogeneous particle size distribution (0.5-2 µm) in the case of the LiFePO4 (Figure 3.30). Figure 3.30. SEM imaged of (a) recycled Li3PO4 and (b) LiFePO4 obtained from recycled Li3PO4. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 82 3.4. Electrochemical characterization of (Li,Na)Mn1-yFeyPO4 3.4.1. Electrochemical properties of synthesized NaMn1-yFeyPO4 Following the synthesis and comprehensive structural, morphological, and physicochemical characterization of the synthesized C/NaFePO4 and C/NaMn0.65Fe0.35PO4 in section 3.2.3, it is essential to evaluate their electrochemical behavior under application-relevant conditions. For the electrochemical testing, the slurries prepared consisted of (C/NaMn1-yFeyPO4:C65:PVDF) with a ratio of 8:1:1. First, the electrochemical performance of C/NaFePO4 was evaluated in Na-half cells in the voltage window 1.5 – 4.0 V (vs. Na+/Na). Figure 3.31 illustrates the cyclability of C/NaFePO4 at C/10 rate for the first 50 cycles at 25ºC. The capacity at the end of the first discharge was 132 mAh g-1, which corresponds to 86% of the theoretical capacity (154 mAh g-1). Despite the initial charge-discharge discrepancy (only in the first cycle) and the significant volume mismatch between end members (NaFePO4 and FePO4), the material delivers a coulombic efficiency of 100% that was maintained after 50 cycles, demonstrating a good cycling stability (87% of capacity retention) (Figure 3.31). The charge and discharge profiles, depicted in Figure 3.32, were asymmetrical, displaying two distinct plateaus at approximately 3.1 and 2.95 V versus Na+/Na in charge, and only one plateau around 2.80 V in discharge. This behavior aligns with the different processes occurring, as previously described.2,3,12,13 Additionally, the polarization decreased from 0.42 V (first cycle) to 0.28 V in subsequent cycles. The higher initial polarization in the first cycle could be attributed to the formation of different species on the REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 89 Phase Synthesis method Reagents Disch. capacity (mAh g-1) Crate Comments Ref. t-NFP Electrochemical synthesis LiFePO4 and AlF3 as coating 117 0.1C Impossibility of scaling-up: no industrial interest. Active material:acetylene black:PTFE (80:10:10 wt%) Jeong et al.19 99 0.2C 89 0.5C 74 1C 54 2C t-NFP Electrochemical synthesis LiFePO4 125 0.5C Impossibility of scaling-up: no industrial interest. Active material:Carbon black:PVdF (wt% not specified) Heubner et al.20 t-NFP Redox reaction (solvent: acetonitrile) LiFePO4, NO2BF4 and NaI 141 0.07C NaFePO4 was wrapped with Polythiophene. NaFePO4:Carbon black:PVdF (80:10:10 wt%) Ali et al.6 128 0.16C 119 0.32C 93 0.65C 77 1.3C 48 1.95C t-NFP Redox reaction (solvent: acetonitrile) LiFePO4, NO2BF4 and NaI 89 0.1C Aqueous electrolyte C-NaFePO4:Super C65:PVdF (75:20:5 wt%) FernándezRopero et al.21 81 0.2C 51 1C REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 90 Phase Synthesis method Reagents Disch. capacity (mAh g-1) Crate Comments Ref. 39 2C 114 0.2C 106 0.5C 96 1C 84 2C 66 5C 46 10C t-NFP Solvothermal synthesis (solvent: ethanol) LiFePO4, Na2S2O8, and NaOH 125 0.1C Solvothermal temperatures between 160 and 180 °C C-NaFePO4:C65:PVdF (80:10:10 wt%) Xu et al.22 110 0.2C 90 0.5C 72 1C 55 2C 38 5C t-NFP Redox reaction in aqueous media LiFePO4, Na2S2O8 and Na2S2O3 132 0.1C Eco-friendly, low cost and scalable synthesis. Excellent performance at high C-rates (>C) C-NaFePO4:C65:PVdF (80:10:10 wt%) This work 114 0.2C 106 0.5C 96 1C 84 2C REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 91 Phase Synthesis method Reagents Disch. capacity (mAh g-1) Crate Comments Ref. 66 5C tNMFP Redox reaction in aqueous media LiMn0.65Fe0.35PO4, Na2S2O8 and Na2S2O3 108 0.1C Eco-friendly, low cost and scalable synthesis. C-NaMn0.65Fe0.35PO4:C65:PVdF (80:10:10 wt%) This work REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 92 3.4.2. Electrochemical properties of regenerated LiFePO4. The electrochemical performance of the regenerated LiFePO4 obtained from recovered Li3PO4 detailed in section 3.2.4 was also evaluated within the voltage window of 2.5 - 4.2 V (vs. Li+/Li) (Figure 3.35). In the first cycle, a discharge capacity of 143 mAh g-1 was observed which increased to approximately 160 mAh g-1 after five cycles. The initial coulombic efficiency was relatively low (90%), although, it rapidly improved, stabilizing to close to 100% after the third cycle. This behavior, together with the slopping profile observed at end of the discharge curve (Figure 3.35 inset), may suggest limited electronic conductivity of the LiFePO4 particles. Such a limitation can be attributed to the absence of a conductive carbon source during synthesis, which typically plays a crucial role in enhancing electron transport in olivine-type cathodes. The progressive improvement in capacity upon cycling may also indicate partial activation of the electrode material and improved interparticle contact over time. For comparison, commercial carbon-coated LiFePO4 from Aleees1 typically delivers capacities greater than 156 mAh g-1 and coulombic efficiencies higher than 95% in the first cycle at C/10. Figure 3.35. Discharge capacity vs. cycle number of the C/LiFePO4 obtained from recovered Li3PO4 in half cell at C/20. The charge/discharge curve corresponding to the second cycle is shown in the inset. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 93 3.5. Summary and Conclusions A cost-effective, waste-free, environmentally friendly, and potentially scalable indirect method for synthesizing triphylite-C/NaFePO4 and triphylite-C/NaMn0.35Fe0.65PO4 has been demonstrated and confirmed. The intermediate C/FePO4 was synthesized by chemical oxidation of commercial C/LiFePO4 using an aqueous solution Na2S2O8 as the oxidizing agent at room temperature. The successful synthesis of the intermediate NaMn0.35Fe0.65PO4 required doubling the amount of oxidizing agent and maintaining the reaction at 60 °C. The triphylite C/NaFePO4 and C/NaMn0.65Fe0.35PO4 were obtained by the chemical sodiation of C/FePO4 and C/Mn0.65Fe0.35PO4 with aqueous solution of the reducing agent Na2S2O3 in a ratio (Na2S2O3:FePO4) ratio of 6:1, at the temperature of 60 ºC. The morphological characterization confirmed that the Carbon Coating layer did not disappear during the synthesis process and that the topotactical reaction did not alter the sample morphology from surface to bulk. T-C/NaFePO4 demonstrates excellent electrochemical performance, delivering a capacity of 132 mAh g-1 at C/10 and retaining approximately 101 mAh g-1 with 100% coulombic efficiency of 100% after 200 cycles. Moreover, its markedly superior rate capability, achieving 84 mAh g-1 at 2C, surpasses previously reported values (39 to 54 mAh g-1), establishing it as one of the top-performing C/NaFePO4 to date. T-C/NaMn0.35Fe0.65PO4 shows a lower electrochemical performance compared to its analogue, with an initial capacity of 108 mAh g-1, and a retention of approximately 100 mAh g-1 after 10 cycles. This diminished performance is likely attributed to its intrinsically lower electronic conductivity and reduced structural stability caused by the Jahn-Teller distortion. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 94 The lithium extracted during the synthesis can be recovered from wastewater by synthesizing Li3PO4, which can then be further converted into a C/LiFePO4 product with outstanding electrochemical performance (approximately 155 mAh g-1, equivalent to 94% of the theoretical capacity). Consequently, the proposed synthesis method to get C/NaFePO4 and the excellent electrochemical performance can make this material to be considered as a promising candidate as cathode for future industrial applications in sodium-ion batteries. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 95 3.6. References (1) Aleees. https://www.aleees.com. (2) Moreau, P.; Guyomard, D.; Gaubicher, J.; Boucher, F. Structure and Stability of Sodium Intercalated Phases in Olivine FePO4. Chem. Mater. 2010, 22 (14), 4126–4128. https://doi.org/10.1021/cm101377h. (3) Casas-Cabanas, M.; Roddatis, V. V.; Saurel, D.; Kubiak, P.; Carretero-González, J.; Palomares, V.; Serras, P.; Rojo, T. Crystal Chemistry of Na Insertion/Deinsertion in FePO4– NaFePO4. J. Mater. Chem. 2012, 22 (34), 17421. https://doi.org/10.1039/c2jm33639a. (4) Zhu, Y.; Xu, Y.; Liu, Y.; Luo, C.; Wang, C. Comparison of Electrochemical Performances of Olivine NaFePO4 in Sodium-Ion Batteries and Olivine LiFePO4 in Lithium-Ion Batteries. Nanoscale 2013, 5 (2), 780–787. https://doi.org/10.1039/C2NR32758A. (5) Wongittharom, N.; Lee, T.-C.; Wang, C.-H.; Wang, Y.-C.; Chang, J.-K. Electrochemical Performance of Na/NaFePO4 Sodium-Ion Batteries with Ionic Liquid Electrolytes. J. Mater. Chem. A 2014, 2 (16), 5655. https://doi.org/10.1039/c3ta15273a. (6) Ali, G.; Lee, J.-H.; Susanto, D.; Choi, S.-W.; Cho, B. W.; Nam, K.-W.; Chung, K. Y. Polythiophene-Wrapped Olivine NaFePO4 as a Cathode for Na-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8 (24), 15422–15429. https://doi.org/10.1021/acsami.6b04014. (7) Lepage, D.; Sobh, F.; Kuss, C.; Liang, G.; Schougaard, S. B. Delithiation Kinetics Study of Carbon Coated and Carbon Free LiFePO4. J. Power Sources 2014, 256, 61–65. https://doi.org/10.1016/j.jpowsour.2013.12.054. (8) Ni, J.; Wang, Y. Temperature-Driven Structural Evolution of Carbon Modified LiFePO4 in Air. RSC Adv. 2015, 5 (39), 30537– 30541. https://doi.org/10.1039/C5RA04744G. (9) Ouaneche, T.; Stievano, L.; Monconduit, L.; Guéry, C.; Sougrati, M. T.; Recham, N. Olivine NaMn0.66 Fe0.34 PO4 as a Cathode Material for Advanced Sodium Ion Batteries. Batter. Supercaps 2024, e202400214. https://doi.org/10.1002/batt.202400214. (10) Keffer, C.; Mighell, A. D.; Mauer, F.; Swanson, H. E.; Block, S. Crystal Structure of Twinned Low-Temperature Lithium Phosphate. Inorg. Chem. 1967, 6 (1), 119–125. https://doi.org/10.1021/ic50047a027. (11) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho‐olivines as Positive‐Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1997, 144 (4), 1188– 1194. https://doi.org/10.1149/1.1837571. (12) Galceran, M.; Saurel, D.; Acebedo, B.; Roddatis, V. V.; Martin, E.; Rojo, T.; Casas-Cabanas, M. The Mechanism of NaFePO4 (de)Sodiation Determined by in Situ X-Ray Diffraction. Phys REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 96 Chem Chem Phys 2014, 16 (19), 8837–8842. https://doi.org/10.1039/C4CP01089B. (13) Whiteside, A.; Fisher, C. A. J.; Parker, S. C.; Saiful Islam, M. Particle Shapes and Surface Structures of Olivine NaFePO4 in Comparison to LiFePO4. Phys Chem Chem Phys 2014, 16 (39), 21788–21794. https://doi.org/10.1039/C4CP02356K. (14) Wang, D.; Wu, Y.; Lv, J.; Wang, R.; Xu, S. Carbon Encapsulated Maricite NaFePO4 Nanoparticles as Cathode Material for Sodium-Ion Batteries. Colloids Surf. Physicochem. Eng. Asp. 2019, 583, 123957. https://doi.org/10.1016/j.colsurfa.2019.123957. (15) Kim, J.; Seo, D.-H.; Kim, H.; Park, I.; Yoo, J.-K.; Jung, S.-K.; Park, Y.-U.; Goddard Iii, W. A.; Kang, K. Unexpected Discovery of Low-Cost Maricite NaFePO4 as a High-Performance Electrode for Na-Ion Batteries. Energy Environ. Sci. 2015, 8 (2), 540–545. https://doi.org/10.1039/C4EE03215B. (16) Liu, Y.; Zhang, N.; Wang, F.; Liu, X.; Jiao, L.; Fan, L. Approaching the Downsizing Limit of Maricite NaFePO4 toward High‐Performance Cathode for Sodium‐Ion Batteries. Adv. Funct. Mater. 2018, 28 (30), 1801917. https://doi.org/10.1002/adfm.201801917. (17) Hwang, J.; Matsumoto, K.; Orikasa, Y.; Katayama, M.; Inada, Y.; Nohira, T.; Hagiwara, R. Crystalline Maricite NaFePO4 as a Positive Electrode Material for Sodium Secondary Batteries Operating at Intermediate Temperature. J. Power Sources 2018, 377, 80–86. https://doi.org/10.1016/j.jpowsour.2017.12.003. (18) Ma, X.; Xia, J.; Wu, X.; Pan, Z.; Shen, P. K. Remarkable Enhancement in the Electrochemical Activity of Maricite NaFePO4 on High-Surface-Area Carbon Cloth for Sodium-Ion Batteries. Carbon 2019, 146, 78–87. https://doi.org/10.1016/j.carbon.2019.02.004. (19) Jeong, S.; Kim, B. H.; Park, Y. D.; Lee, C. Y.; Mun, J.; Tron, A. Artificially Coated NaFePO4 for Aqueous Rechargeable SodiumIon Batteries. J. Alloys Compd. 2019, 784, 720–726. https://doi.org/10.1016/j.jallcom.2019.01.046. (20) Heubner, C.; Heiden, S.; Schneider, M.; Michaelis, A. In-Situ Preparation and Electrochemical Characterization of Submicron Sized NaFePO4 Cathode Material for Sodium-Ion Batteries. Electrochimica Acta 2017, 233, 78–84. https://doi.org/10.1016/j.electacta.2017.02.107. (21) Fernández-Ropero, A. J.; Saurel, D.; Acebedo, B.; Rojo, T.; Casas-Cabanas, M. Electrochemical Characterization of NaFePO4 as Positive Electrode in Aqueous Sodium-Ion Batteries. J. Power Sources 2015, 291, 40–45. https://doi.org/10.1016/j.jpowsour.2015.05.006. (22) Xu, Y.; Dai, R.; Wang, X.; Qiao, Z.; Wen, H.; Ruan, D.; Wang, Y. High-Performance Triphylite-NaFePO4 Synthesized by REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 97 Solvothermal Sodium Insertion Process for Sodium-Ion Batteries. Chem. Phys. Lett. 2024, 834, 140983. https://doi.org/10.1016/j.cplett.2023.140983. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 3. Indirect synthesis of triphylite-NaMn1-yFeyPO4 and its characterization 98 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 105 Table 4-3. Equilibrium constants of the possible species formed during the synthesis. Chemical equilibrium Equilibrium constant (K) expression Value of K H2O ⇋H++OHKe=[H+]·[OH-] 10-14 H++SO4 2- ⇋HSO4 - K2=[HSO4 -] [H+]·[SO4 2-] 101.99 H++HSO4 -⇋ H2SO4 K3=[H2SO4] [H+]·[SO4 2-] 10-3 H++PO4 3- ⇋ HPO4 2K4=[HPO4 2-] [H+]·[PO4 3-] 1012.38 H++HPO4 2- ⇋ H2PO4 - K5=[H2PO4 -] [H+]·[HPO4 2-] 107.2 H++H2PO4 - ⇋ H3PO4 K6=[H3PO4] [H+]·[H2PO4 -] 102.15 OH-+Na+⇋ NaOH β7=[NaOH] [OH-]·[Na+] 10-0.2 SO4 2-+Na+⇋ NaSO4 - β8=[NaSO4 -] [SO4 2-]·[Na+] 100.7 CH3COO-+Na+⇋ NaCH3COOβ9=[NaCH3COO] [CH3COO-]·[Na+] 10−4.74 OH-+Mn2+⇋ MnOH+ β10=[MnOH+] [OH-]·[Mn2+] 104.6 Mn2++SO4 2- ⇋ MnSO4 β11=[MnSO4] [SO4 2-]·[Mn2+] 102.67 3Mn2++2PO4 3- ⇋ Mn3(PO4)2 β12=[Mn3(PO4)2] [PO4 3-]2·[Mn2+]3 10-25 Mn2++HPO4 2- ⇋ MnHPO4 β13=[MnHPO4] [HPO4 2-]·[Mn2+] 10-12.95 REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 106 4.4. Synthesis and phase identification of NaxMn1-yFeyPO4 of (0 ≤ y ≤ 1) 4.4.1. Synthesis and phase identification of natrophilite NaMnPO4 The hydrothermal synthesis of natrophilite NaMnPO4 was carried out by adapting the experimental conditions previously reported for the synthesis of the isostructural lithiophilite LiMnPO4.2 An aqueous solution was prepared by dissolving NaOH, MnSO4·H2O and H3PO4 in a molar ratio of 3:1:1, corresponding to 1.456 g of NaOH, 2.420 g of MnSO4·H2O and 1.384 g of H3PO4 in a total volume of 30 ml of H2O. The precursor solution was transferred to a 45 ml PTFE-liner that was then sealed within a PARR stainless steel autoclave. Then, the autoclave was placed into a muffle furnace and heated up to 180 ºC for 4h. After completion of the reaction, the autoclave was cooled to room temperature and the obtained solution was filtered and rinsed with distilled water to remove any residual soluble species. The powder obtained, hereafter referred to as HT_NaMn, was characterized by XRD as shown in Figure 4.2, confirming the successful formation of pure natrophilite NaMnPO4. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 107 Figure 4.2. XRD pattern of the synthesized HT_NaMn matching with the reported natrophilite NaMnPO4 (ICSD code: 36249). 4.4.2. Synthesis and phase identification of triphylite NaMn1-yFeyPO4 (y = 0.25, 0.50, and 0.75) The synthesis of triphylite NaMn1-yFeyPO4 (y = 0.25, 0.50 and 0.75) solid solution was prepared using the same hydrothermal synthesis protocol previously described for natrophilite NaMnPO4. For each target composition, the amount of MnSO4·H2O was partially replaced by FeSO4·7H2O in accordance with the desired Mn:Fe stoichiometry. The reaction conditions, including the molar ratio of reactants, total solvent volume, autoclave type, and heating temperature (180 °C), were kept constant to ensure comparability. The resulting materials were labeled as HT_tMn75Fe25 (y = 0.25), HT_tMn50Fe50 (y = 0.50), HT_tMn25Fe75 (y = 0.75). After careful analysis of the XRD patterns of the obtained compounds (see Figure 4.3), it was observed that in HT_tMn75Fe25 and HT_tMn50Fe50 the triphylite phase is obtained along with Fe3O4 impurities, while in HT_tMn25Fe75 an unidentified phase is REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 108 obtained instead. These findings will be further discussed later in section 4.5. Figure 4.3. XRD patterns of the synthesized samples HT_tMn75Fe25, HT_tMn50Fe50 and HT_tMn25Fe75 compared with the reported natrophilite NaMnPO4 (ICSD code: 36249). 4.4.3. Effect of the pH on triphylite NaMn0.25Fe0.75PO4 4.4.3.1. Effect of decreasing the pH In an effort to explore the influence of pH on the phase formation during hydrothermal synthesis, targeted modifications were made to the reaction mixture to promote the crystallization of the triphylite NaMn0.25Fe0.75PO4 phase. This strategy was designed to lower the pH of the solution to determine whether this adjustment could thermodynamically favor the formation of the targeted triphylite phase, particularly with high Fe content (such as NaMn0.25Fe0.75PO4). In the first approach, NaOH (used as the primary sodium source in previous syntheses) was replaced by Na2SO4. This modification REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 109 significantly reduced the pH of the reaction mixture (below 6), as Na2SO4 is a neutral salt compared to the strongly basic NaOH. To account for the difference in sodium content (Na2SO4 providing two Na equivalents per formula unit), the molar ration of Na2SO4:H3PO4 molar ratio was adjusted to 1.5:1 instead of 3:1 ration used in the original synthesis using NaOH. The obtained compound, labeled as HT75_Na2SO4 (where HT refers to hydrothermal synthesis, and “75” indicates the Fe content of 0.75), was characterized by XRD. The XRD pattern (Figure 4.4) indicated the formation of the alluaudite NaMn(Mn,Fe)2(PO4)3 phase, which crystallizes in the monoclinic system with C2/c space group. No trace nor evidence of the triphylite phase was observed. Figure 4.4. XRD pattern of synthesized HT75_Na2SO4 matching with the reported alluaudite NaMnFe2(PO4)3 (ICSD code: 261050). In the second approach, NaCH3COO as a weaker base compared NaOH, was used as a sodium soured. Given that NaCH3COO provides the same number of Na+ equivalents per unit formula as NaOH, the NaCH3COO:H3PO4 molar ratio was maintained at 3:1, consistent with the original experiment (1). The resulting product, labeled HT75_NaCH3COO, was also characterized by XRD. Similar to the REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 110 previous experiment with Na2SO4, the XRD pattern (Figure 4.5) also corresponded to the alluaudite NaMn(Mn,Fe)2(PO4)3 phase (monoclinic system with C2/c as a space group). A set of additional peaks corresponding to unidentified impurity phases were observed. Figure 4.5. XRD pattern of the synthesized HT75_NaCH3COO matching with the reported alluaudite NaMnFe2(PO4)3 (ICSD code: 261050). Summarizing, these results suggest that lowering the pH of the reaction mixture through substitution of the sodium source with either Na₂SO₄ or NaCH₃COO does not promote the formation of the triphylite phase at high Fe content. Instead, both approaches favor the crystallization of the alluaudite-type NaMn(Mn,Fe)2(PO4)3 structure. The experimental parameters and results of the two approaches are summarized in Table 4-4. Table 4-4. Reagents ratio and obtained phase for the triphylite targeted compositions decreasing the pH. Synthesis conde Targeted composition Na Source (ratio) MnSO4· H2O FeSO4· 7H2O H3PO4 Obtained phase HT_Na2SO 4 NaMn0.25Fe0.75PO4 Na2SO4 (1.5) 0.25 0.75 1 Alluaudite HT_NaCH3 COO NaMn0.25Fe0.75PO4 NaCH3COO (3) 0.25 0.75 1 Alluaudite REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 111 4.4.3.2. Effect of increasing the pH To encourage the formation of the triphylite NaMn1-yFeyPO4 phase, we systematically examined the effect of pH by adjusting the NaOH:H3PO4 molar ratio. An alkaline synthesis environment, such as those created by a NaOH:H3PO4 ratio of 4:1, is considered favorable for triphylite stabilization. The obtained samples were labeled according to their targeted composition: HT_tMn75Fe25pH10-12 for NaMn0.75Fe0.25PO4, HT_tMn50Fe50pH10-12 for NaMn0.50Fe0.50PO4 and HT_tMn25Fe75pH10-12 for NaMn0.25Fe0.75PO4. The final synthesized compounds were analyzed by XRD (Figure 4.6.) revealing the successful formation of triphylite polymorphs for the targeted compositions NaMn1-yFeyPO4 in the range 0.25 ≤ y ≤ 1 (Table 4-5). Figure 4.6. XRD patterns of the synthesized HT_tFe25pH10, HT_tFe50pH10 and HT_tFe75pH10, matching with the natrophilite/triphylite NaMnPO4 phase (ICSD code: 36249). REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 112 Table 4-5. Reagents ratio and obtained phase for the triphylite targeted compositions increasing the pH. Synthesis code Targeted composition NaOH MnSO4· H2O FeSO4 ·7H2O H3PO4 Obtained phase HT_tMn75Fe 25pH10-12 NaMn0.75Fe0.25PO4 4 0.75 0.25 1 Triphylite HT_tMn50Fe 50pH10-12 NaMn0.50Fe0.50PO4 4 0.5 0.5 1 Triphylite HT_tMn25Fe 75_pH10-12 NaMn0.25Fe0.75PO4 4 0.25 0.75 1 Triphylite Overall, these observations indicate that higher pH values favor the stabilization of the triphylite phase, a trend that remains evident even with increasing Fe content in the material. 4.4.4. Synthesis of NaMn1-yFeyPO4 substituting Mn with Fe progressively To systematically investigate the effect of progressive Fe substitution on the triphylite NaMn1-yFeyPO4 phase, a series of compositions with incremental Fe content (Δy = 0.05) was synthesized under the same hydrothermal conditions as described in section 4.4.2. The targeted compositions spanned the range 0.25 > y > 0.90. As Figure 4.7 shows, XRD analysis confirmed the formation of triphylite phase for targeted compositions in the range of 0.25 ≥ y ≥ 0.60. However, increasing Fe content, the presence of Fe3O4 peaks becomes progressively more prominent in the XRD patterns. This suggest incomplete incorporation of Fe into the triphylite NaMn1yFeyPO4 phase and implies that the actual Fe content in the synthesized phases is lower than the nominal target. Furthermore, for compositions with y > 0.60, the XRD patterns exhibit additional diffraction peaks that do not match any known triphylite or natrophilite phases, indicating the formation of an unidentified secondary phase. Table 4-6 summarizes all the results. REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 113 Figure 4.7. XRD patterns of NaMn1-yFeyPO4 samples substituting Mn with Fe progressively compared with natrophilite NaMnPO4 from the database (ICSD code: 36249). REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 4. Direct synthesis of transition metal phosphate-based polymorphs Nax(TM)y(PO4)z (TM = Mn, Fe) 114 Table 4-6. Reagents ratio and obtained phase for the different triphylite targeted compositions replacing progressively Mn for Fe. Synthesis code Targeted composition NaOH MnSO4·H2O FeSO4·7H2O H3PO4 Obtained phase HTprog_tNaMnPO4 NaMnPO4 3 1 0 1 Natrophilite HTprog_tMn75Fe25 NaMn0.75Fe0.25PO4 3 0.75 0.25 1 Triphylite, Fe2O3 HTprog_tMn70Fe30 NaMn0.70Fe0.30PO4 3 0.70 0.30 1 Triphylite, Fe2O3 HTprog_tMn65Fe35 NaMn0.65Fe0.35PO4 3 0.65 0.35 1 Triphylite, Fe2O3 HTprog_tMn60Fe40 NaMn0.60Fe0.40PO4 3 0.60 0.40 1 Triphylite, Fe2O3 HTprog_tMn55Fe45 NaMn0.55Fe0.45PO4 3 0.55 0.45 1 Triphylite, Fe2O3 HTprog_tMn50Fe50 NaMn0.50Fe0.50PO4 3 0.50 0.50 1 Triphylite, Fe2O3 HTprog_tMn45Fe55 NaMn0.45Fe0.55PO4 3 0.45 0.55 1 Triphylite, Fe2O3 HTprog_tMn40Fe60 NaMn0.40Fe0.60PO4 3 0.40 0.60 1 Triphylite, Fe2O3, Undetermined HTprog_tMn35Fe65 NaMn0.35Fe0.65PO4 3 0.35 0.65 1 Undetermined HTprog_tMn30Fe70 NaMn0.30Fe0.70PO4 3 0.30 0.70 1 Undetermined HTprog_tMn25Fe75 NaMn0.25Fe0.75PO4 3 0.25 0.75 1 Undetermined HTprog_tMn20Fe80 NaMn0.20Fe0.80PO4 3 0.20 0.80 1 Undetermined HTprog_tMn15Fe85 NaMn0.15Fe0.85PO4 3 0.15 0.85 1 Undetermined HTprog_tMn10Fe90 NaMn0.10Fe0.90PO4 3 0.10 0.90 1 Undetermined HTprog_tFe100 NaFePO4 3 0 1 1 Undetermined REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034 List of contributions 218 Conferences and exhibitions XXXVI Reunión Bienal de la Real Sociedad Española de Química (Sitges, 2017), with an Oral Communication: Síntesis fácil y económica de NaFePO4 olivino como electrodo positivo en baterías de ion sodio. Power our future 2017 (Vitoria-Gasteiz, 2017), with a poster Easy and low-cost synthesis and electrochemical performance of olivine NaFePO4 as positive electrode for sodium-ion batteries. E-MRS 2019 Spring Meeting - IUMRS-ICAM International (Nice, 2019) Conference on Advanced Materials, with an Oral Communication: A novel low-cost synthesis of olivine NaFePO4 as positive electrode for sodium-ion batteries. Power our future 2019 (Vitoria-Gasteiz, 2019), with a poster: Green and low-cost synthesis of olivine NaFePO4 as positive electrodes for sodium-ion batteries. CIRMAT (Madrid, 2022): with a poster: Sustainable synthesis of triphylite NaFePO4 as positive electrode for sodium ion batteries: from materials to recycling. The Battery Show 2023 (2023, Stuttgart) Advanced Automotive Battery Conference (2024, Strasbourg) Projects Iberdrola Project (2020-2021): Analysis of the Positive Impact of the Application of Sustainable Sodium-Ion Batteries in the Electric Grid. (P.I.: Carlos Manuel Berlanga Cruzado) REGISTRO TELEMÁTICO Sarreren Erregistro Orokorra / Registro General de Entradas 20/10/2025 18:50 EHU2025E050034