Preprint of "Tracking Structural Evolution in Lithium-ion Batteries via In-Operando Scanning Electron Microscopy"

Tracking Structural Evolution in Lithium-ion Batteries via In-Operando Scanning Electron Microscopy Ondřej Klvača,b*, David Trochtaa,b, Libor Nováka, Peter Priecela, Manuel Bornhöffta, and Tomáš Kazdab, Zhao Liua* a Thermo Fisher Scientific, 5350 NE Dawson Creek Dr, Hillsboro, OR 97124, USA b Department of Electrical and Electronic Technology, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technická 10, 616 00 Brno, Czech Republic * Corresponding author Abstract: The study of lithium-ion cells is paramount for optimizing their performance, safety, and longevity, which are critical for applications such as portable electronics, electric vehicles, and renewable energy storage systems. Scanning Electron Microscopy (SEM) is instrumental in the examination of lithium-ion batteries, offering high-resolution imaging and detailed insights into the microstructure and morphology of the cells. In-situ/operando analyses are invaluable as they provide precise descriptions of the dynamic phenomena and their temporal evolution within the battery. However, the application of SEM for insitu/operando analyses is hindered by the challenging preparation of samples that can function within the SEM environment. Therefore, we introduces a universally applicable methodology that integrates sample preparation using a broad ion beam polisher with the transfer of samples to the SEM in a specially designed holder under an inert atmosphere. The efficacy of this system and workflow is demonstrated on Nickel Manganese Cobalt Oxide (NMC) and Lithium Titanate Oxide (LTO) battery cells, revealing grain cracking and expansion, as well as electrode expansion and contraction. Additionally, a graphite-metal lithium system is analyzed, where expansion and cracking of graphite grains were observed. The study delineates a procedure enabling the investigation of submicron changes at the granular level and at the scale of entire electrodes or larger cell components, applicable across various chemistries. We posit that this work provides significant insights for both fundamental research of electroactive materials and the optimization of the manufacturing processes for complete cells. Keywords: Li-ion batteries, electrochemical cycling, SEM, BIB, in-situ, operando, structural evolution Highlights: • Integration of broad ion beam polisher and operando SEM • Operando SEM analysis of cross-sectional electrode materials • Examination of submicron and whole-cell changes 1. Introduction With advancements in the technology field, decarbonization efforts, and ecological demands, the need for advanced electrochemical power sources is increasing. However, large-scale production faces challenges connected with limited amount of raw materials, rising costs, and environmental impacts. Research in this field focuses on enhancing cell capacity, lifespan, and possible usage of electroactive materials with a lower environmental burden. Operando techniques offer real-time insights into the kinetics of processes and mechanisms occurring within a battery during operation. Unlike ex-situ or post-mortem methods, these techniques enable the precise identification of when a phenomenon occurs, leading to a clearer understanding of its underlying cause. A variety of operando techniques are available, each suited to different types of analysis.[1][2][3][4] X-ray techniques, such as computed tomography (CT) [5][6][7][8][9][10][11][12][13][14][15], are widely used in operando studies of lithium-ion batteries, enabling non-destructive 3D analysis during operation. These methods allow data collection across various scales, from the entire cell down to microstructural details, providing valuable insights into electrode aging and associated morphological changes. However, a key drawback of CT analysis is its lower spatial resolution compared to electron microscopy, as well as the extended time required to complete each scan. To overcome the resolution limitations of CT, X-ray diffraction (XRD) [16][17][18][19][20][21][22] can be employed. Unlike CT, XRD offers resolution at the inter-atomic scale, though its diffraction data still originates from a larger sample region. In comparison to X-ray techniques, electron microscopy allows investigation of the microstructure down to the level of individual atoms. Together with complementary methods such as energy-dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD) and focused ion beam (FIB) that can be used for site-specific sample preparation, the electron microscopy can be used to investigate many of the important phenomena in battery behaviour such as solid electrolyte interphase (SEI) formation, Li plating, phase changes, or formation of defects in the electrodes[1][2]. Transmission electron microscopy (TEM) provides atomic-scale resolution, enabling detailed insights into electrochemical processes. However, its application is limited by challenging sample preparation and the small volume of material that can be analyzed. As a result, TEM studies are often restricted to model systems, such as miniaturized battery setups or half-cells created on dedicated TEM biasing holders [23][24][25]. Additionally, TEM studies may be affected by radiation damage and local sample overheating. Despite these challenges, TEM remains a powerful tool for investigating electrochemical processes at a fundamental level, making it well-suited for basic research.[26][27][28][29][30][31][32] The large specimen chamber of a Scanning Electron Microscope offers the possibility for operando studies of whole batteries at the cost of lower image resolution compared to TEM, still reaching spatial resolution of units of nanometers on sample surface. At the same time SEM allows us to monitor a real-size battery in large-scale, capturing structural changes in all the parts and interfaces. SEM operando studies can be applied to batteries at different levels of development, including quality control of samples resulting from the manufacturing process. [33] For scanning electron microscopy (SEM) imaging, the battery must be partially opened on one side to allow electron access to the surface of interest. Over the past decades, various approaches have been explored to achieve this geometry [25][34]. Additionally, since the battery is exposed to the vacuum of the SEM chamber, a vacuum-compatible electrolyte is required—one that remains stable and does not evaporate at pressures as low as 10-6 Pa. One of the earliest operando SEM experiments was conducted by Baudry in 1988 [35], utilizing a solid-state electrolyte. Since then, various vacuum-compatible solid-state electrolytes have been employed in subsequent operando SEM studies [35][36][37][38][39][40][41]. The lower ionic conductivity of solid-state electrolytes can be mitigated by heating the sample within the SEM chamber [38][41][37][35]. Additionally, the design of the SEM holder must account for the compression of the solid-state battery both during operando analysis and throughout the transfer process. Depending on the type and size of the solid-state battery, this can result in high forces to be handled by the SEM biasing holder. Various types of ionic liquids with low vapor pressure are commonly used as electrolytes in SEM operando studies [42][43][44][45][46][47][48][49][50][51][52]. However, replacing a conventional electrolyte with an ionic liquid can alter certain battery properties. For instance, the composition of the solid electrolyte interphase (SEI) layer may differ due to the absence of organic components [53]. A cooled hybrid polymer-based electrolyte can also be used in SEM studies [54]. When employing an electrolyte with a vapor pressure exceeding the SEM vacuum threshold is unavoidable, encapsulation within an electron-transparent membrane can prevent evaporation. Silicon nitride windows, typically several tens of nanometers thick, are commonly used for this purpose[55]. Alternatively, slight evaporation of a conventional electrolyte can be managed without full encapsulation by utilizing environmental SEM (ESEM) mode, where the chamber pressure is maintained at several hundred pascals [56]. However, both the membrane encapsulation and ESEM approaches result in reduced SEM resolution due to electron scattering within the membrane, gas, or the electrolyte layer that may form on the battery surface. In a quasi-in-situ approach, the battery is cycled under standard high-pressure conditions and subsequently exposed to the electron beam for imaging at selected states of the charge-discharge cycles [57]. However, this method necessitates repeated battery disassembly and reassembly, which can introduce reproducibility challenges. For direct SEM imaging of the battery surface, the battery can be opened from the side to enable planar imaging of one electrode. Alternatively, a perpendicular cross-section can be prepared, allowing SEM imaging of all internal interfaces within the battery. In a planar geometry that enables imaging of the outer surface of one electrode, an electron beam-transparent current collector is typically used to allow SEM imaging of the electrochemically active material through the modified collector. This is most commonly achieved by depositing the active material onto a metal grid [56][42][43][44][46][48][52] or by creating a small hole in a metal foil serving as the collector [36]. However, in this configuration, SEM imaging is limited to the outer surface of one electrode and may not accurately represent bulk behavior. For instance, it does not allow for the assessment of variations in lithiation depth as a function of distance from the counter electrode. For the cross-sectional geometry presented in this study, careful surface preparation is essential to prevent artificial features in SEM images caused by surface topography. The battery can be opened and polished using various methods, with mechanical cutting commonly employed as an initial step [54][37]. To enhance surface quality after rough cutting, ion beam polishing techniques—such as broad ion beam (BIB) polishing or FIB milling in a FIB-SEM system—are typically used. Alternatively, cryomicrotomy [47][41] or laser milling [58] can be utilized for cross-section preparation. Cross-sectioning must be performed carefully to avoid short-circuiting or mechanically damaging the battery. Additionally, special attention should be given to preventing the detachment of individual particles, particularly from the electrode layers, during the cutting and polishing process. FIB technology can also be utilized for additional planar-view analysis [42][49][40], such as cutting through lithium dendrites formed during operando experiments in a FIB-SEM system [43]. A specific case is the usage of the FIB together with a micromanipulator. The micromanipulator can serve as a current collector for small amounts of electroactive material fixed mechanically and electrically by FIB-assisted deposition of conductive materials [50][51]. In this case, a very small amount of material is sufficient for characterization, and it is assured that measured electrochemical data corresponds to the investigated area. However, this single area may not always show the same behavior for various reasons. For example, the cycling currents of such a system are very small, which carries the risk of data distortion by electromagnetic interference [59]. BIB polishing enables the preparation of cross-sections of entire electrodes or even full battery cells in a relatively short time. Compared to mechanical methods, BIB polishing introduces minimal artifacts; for instance, curtaining artifacts can be significantly reduced by rocking the sample during the milling process [60][33]. We propose a workflow for operando battery characterization in SEM that combines the advantages of the previously discussed approaches. This method provides a robust testing environment, ensuring repeatable results while minimizing the time and effort required for battery sample preparation prior to SEM analysis. The cross-sectional view is achieved by placing the battery in a biasing holder within the SEM. The sample surface is prepared using a broad ion beam polisher [61] to ensure smooth cross-sections. To maintain sample integrity, all transfer steps—between the glove box and the CleanMill polisher, as well as between the glove box and the SEM—are facilitated by the CleanConnect (CC) transfer device [62]. The workflow is demonstrated on an examples of lithium nickel manganese cobalt oxide (NMC) – lithium-titanate oxide (LTO) and graphite – metallic lithium systems with a vacuum compatible ionic liquid electrolyte and can be applied to various vacuum compatible battery materials. Moreover, apart from ex-situ techniques typically analyzing pristine and cycled electrodes separately (Figure 1a), our technique can examine the same location continuously, providing a more consistent and detailed observation of the processes (Figure 1a). Data post-processing correlating the SEM image information with battery charge state in time allows for systematic characterization and direct comparison of individual battery samples. Figure 1: Comparison of data obtained during ex-situ (a) and in-situ (b) analysis. Ex-situ SEM analysis of NMC cathode cross-section, pristine and cycled electrode - due to the destructive nature of this analysis, it is not possible to examine the same region and its changes over time; Operando SEM analysis of selected NMC grain cross-section at different charging stages – observation of crack evolution during first formation cycle 5 μm 20 μm pristine cycled 0.13 V 2.31 V 2.32 V 2.80 V a b 2. .Experimental method 2. 1 Materials Commercially available electrode sheets from CustomCells were used in this study. We used NMC 111 coated on an aluminum current collector, LTO coated on an aluminum current collector, and graphite on a copper current collector. The capacity of the electrodes is specified in the datasheet as 1 mAh/cm². In the supplementary experiment, metallic lithium (99.9%; Sigma-Aldrich) was used as the counter electrode. A Whatman GF/C glass fiber filter with a thickness of 260 µm was used as the separator. For NMC – LTO systems, an ionic liquid electrolyte was prepared using a mixture of 1methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr₁₃-TFSI, >99%; SigmaAldrich) and lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI, >99%; Sigma-Aldrich) at a concentration of 0.5 M. For graphite – Li metal system, mixture of LiTFSI: 1-propyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI), 1:9 mol (Solvionic >99.5%) was used. 2.2. Cells assembly and placement into SEM In this publication, the results of three experiments are presented. In the first experiment, the thickness of the entire NMC electrode in the NMC-LTO system (denoted as sample 1) was measured. In the second experiment, the expansion and morphological changes of individual NMC grains in a newly constructed NMC-LTO system (sample 2) were analyzed. The third experiment (sample 3) serves as a supplementary verification of the functionality of system with different materials. Graphite vs Li metal was used. Firstly, pieces of approximately 5 mm x 8 mm were cut from the electrode sheets with scissors. A slightly larger piece of approximately 7 mm x 12 mm was cut from the separator with a razor blade. The investigated electrode was consequently moved to CleanMill polisher and a crosssection was prepared using Ar ion beam. For the NMC electrode, 16 kV and 3.5 mA were used for one hour; for the graphite electrode, 10 kV and 3.5 mA were used for 1.5 hours. Sample rocking +- 40° was used to reduce curtaining artifact. The resulting area size of the completely polished region was about 500 um. The similar method can be used to prepare a cross-section of both electrodes individually or the whole cell at once. The investigated electrode was then aligned together with counter electrode and separator. From our experience, it has been found to be advisable to place the investigated electrode in a few micrometers overlap above the separator to avoid flooding the inspected area with electrolyte during the SEM operando experiment. The alignment and mechanical fixation was done in our designed clamping tool. It contains two clamping plates, which can be pressed together using a screw. In addition to mechanical clamping, these plates also serve as an electrical contact to the pins on the edge of the clamping tool. The pre-prepared cell was then transferred to the Ar-filled glovebox. The specimen was left in the glovebox antechamber for several hours under vacuum to dry all parts of the system before subsequent application of the electrolyte. The electrolyte was applied to the separator edges with a pipette until the cell was soaked properly. The prepared cell was then transferred to the SEM chamber in the Ar atmosphere using the CleanConnect Sample Transfer System. In the SEM chamber, a stage with the contact pins was pre-installed, into which the pins on the clamping tool fit when inserted. These pins are wired through a vacuum feedthrough to a potentiostat outside the SEM chamber (Figure 2). A BioLogic SP-150 potentiostat was used for electrochemical measurements. The capacity of the system was estimated based on the electrode size, considering the datasheet value of 1 mAh/cm2. Experimental workflow scheme and clamping tool images are involved in supplementary data. 2.3 Sample imaging and data processing For sample imaging, a Quattro ESEM (Thermo Fisher Scientific) was utilized. An automated control script (Thermo Scientific AutoScript 4 Software) was developed to manage SEM imaging and stage movements, enabling the imaging multiple areas + different horizontal field width (HFW)/magnification/image grab settings that allowed us to take both detailed and overview images. The acquired images were then correlated with specific points on the electrochemical curve based on the data from potentiostat. By compositing these images, a video was produced, where the current frame is indicated by a cursor (red cross) on the curve. Further analysis of the images was conducted using Phenom ParticleMetric particle analysis software and Python automatic edge detection. For selected NMC grains, changes in size were examined. The Phenom software identifies grain boundaries and calculates the area of each grain. The real size of the particles can be calculated from the known field of view of SEM images. In the similar way electrode boundaries (edges) was detected using Python script to track whole electrode thickness changes. For both, changes are expressed as percentages relative to the initial image taken at the start of the measurement. Figure 2: Operando SEM experimental scheme 3. Results and discussion 3.1 Sample 1 – monitoring NMC electrode thickness The NMC-LTO battery was prepared as described in chapter 2, moved to SEM, and connected to a potentiostat. After two initial formation cycles conducted at 0.1C in a potential window of 1.3 – 2.8 V, 15 min rest phase, the battery was further cycled in constant current followed by constant voltage (CCCV) mode at 1.0C, 5 min rest phase, and imaged. Notably, the cycling continued beyond the usual operational window (0.8 – 3.3 V) to amplify the observed phenomena and evaluate the expansion of the entire NMC electrode (Figure 3). The thickness of the electroactive layer was continuously measured during cycling (Figure 3). This data indicates electrode expansion during battery charging (NMC electrode delithiation) and contraction during battery discharging (NMC electrode lithiation). Moreover, after expansion during the battery charging phase, the electrode does not return to its original thickness during battery discharging. Instead, it gradually expands with each subsequent cycle, leading to irreversible changes in the electrode. In the first cycle, the thickness was approximately 37.9 μm in the battery charged state and 37.0 μm in the discharged state. By the end of the 13th cycle, these values increased to 39.8 μm and 39.1 μm, respectively. Declaration of generative AI and AI-assisted technologies in the writing process: During the preparation of this work the authors used GPT-4o (OpenAI), NotebookLM (Google), and DeepL Translate in order to enhance text quality, fix grammar errors, and improve readability. After using these tools, the authors reviewed and edited the content as needed and take full responsibility for the content of the published article. References: [1] D. Liu, Z. Shadike, R. Lin, K. Qian, H. Li, K. Li, S. Wang, Q. Yu, M. Liu, S. Ganapathy, X. 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