Strain regulating and kinetics accelerating of micro-sized silicon anodes via dual-size hollow graphitic carbons conductive additives

www.small-journal.com 2205284 (1 of 11) © 2022 The Authors. Small published by Wiley-VCH GmbH Strain Regulating and Kinetics Accelerating of Micro-Sized Silicon Anodes via Dual-Size Hollow Graphitic Carbons Conductive Additives Qitao Shi, Yuanhao Cheng, Jiaqi Wang, Junhua Zhou, Huy Quang Ta, Xueyu Lian, Klaudia Kurtyka, Barbara Trzebicka, Thomas Gemming, and Mark H. Rümmeli* DOI: 10.1002/smll.202205284 1. Introduction Rechargeable secondary batteries are playing an increasingly significant role in modern society. There is a great demand for lithium-ion batteries for use in applications such as consumer Micro-sized silicon (µSi) anode features fewer interfacial side reactions and lower costs compared to nanosized silicon, and has higher commercial value when applied as a lithium-ion battery (LIB) anode. However, the high localized stress generated during (de)lithiation causes electrode breakdown and performance deterioration of the µSi anode. In this work, hollow graphitic carbons with tailored dual sizes are employed as conductive additives for the µSi anode to overcome electrode failure. The dual-size hollow graphitic carbons (HGC) additives consist of particles with micrometer size similar to the µSi particles; these additives are used for strain regulation. Additionally, nanometer-size particles similar to commercial carbon black Spheron (SP) are used mainly for kinetics acceleration. In addition to building an efficient conductive network, the dual-size hollow graphitic carbon conductive additive prevents the fracture of the electrode by reducing local stress and alleviating volume expansion. The µSi anode with dual-size hollow graphitic carbons as conductive additives achieves an impressive capacity of 651.4mAhg−1 after 500cycles at a high current density of 2Ag−1. These findings suggest that dual-size hollow graphitic carbons are expected to be superior conductive additives for micro-sized alloy anodes similar to µSi. ReseaRch aRticle Q. Shi, Y. Cheng, J. Wang, J. Zhou, H. Q. Ta, X. Lian, M. H. Rümmeli Soochow Institute for Energy and Materials InnovationS College of Energy Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province Key Laboratory of Core Technology of High Specific Energy Battery and Key Materials for Petroleum and Chemical Industry Soochow University Suzhou 215006, Suzhou 215006, China E-mail: [email protected] Q. Shi, Y. Cheng, J. Wang, J. Zhou, H. Q. Ta, X. Lian, M. H. Rümmeli Jiangsu Key Laboratory of Advanced Negative Carbon Technologies Soochow University Suzhou, Jiangsu 215123, P. R. China H. Q. Ta, M. H. Rümmeli Institute for Complex Materials IFW Dresden 20 Helmholtz Strasse, 01069 Dresden, Germany K. Kurtyka, B. Trzebicka, T. Gemming, M. H. Rümmeli Centre of Polymer and Carbon Materials Polish Academy of Sciences M. Curie-Sklodowskiej 34, Zabrze 41–819, Poland M. H. Rümmeli Institute of Environmental Technology VSB-Technical University of Ostrava 17. Listopadu 15, Ostrava 70833, Czech Republic The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202205284. © 2022 The Authors. Small published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. electronics, portable laptops, and renewable energy vehicles.[1–3] Unfortunately, the commercial graphite anode with a theoretical capacity of 372mAh g−1 cannot meet the growing demands placed on lithiumion batteries (LIB). Therefore, the development of next-generation anode materials with higher capacity has become imperative. Compared to commercial graphite, alloy anode materials possess higher theoretical lithium storage capacity (2–10 times higher) and lower working potential. The emerging alloy anodes include Si, Sn, Ge, and Al.[4–6] Among the alternative anode materials, Si has attracted the most attention in both academia and industry due to its extremely high theoretical capacity of 3759 mAh g−1 (at room temperature), as well as its abundant resources, relatively low Li uptake voltage, low cost, and environmental friendliness.[7–10] Despite the attractive advantages mentioned above, the commercialization of Si anode still faces numerous difficulties that are mainly caused by the dramatic volume change and intrinsic sluggish dynamics during the electrochemical process.[11] These negative effects are greatly magnified with the increase in the material size.[12] Nanostructured Si has been proven to be effective in alleviating volume expansion and enhancing electrode Small 2023, 19, 2205284 16136829, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202205284 by Technical University Ostrava, Wiley Online Library on [06/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com 2205284 (2 of 11) © 2022 The Authors. Small published by Wiley-VCH GmbH conductivity.[13] Nevertheless, the troublesome interfacial side reactions caused by the large surface area and the high cost of the synthesis and manufacturing of nanosized Si prevent its use in practical applications. By contrast, micro-sized Si (µSi) benefits from its fewer electrochemical side reactions and lower production cost when considered for use as an anode material for lithium ion batteries.[14] However, the volume change of µSi will result in rapid electrode cracking. To date, various strategies have been proposed to solve the above-mentioned problems, including material structure design,[15–17] carbon compositing,[14,18–20] electrolyte modification,[21,22] and functional binder exploitation.[23–26] Exploration and improvement of conductive additives have been barely mentioned in the previous research on µSi anodes. The reported conductive additives in lithium secondary battery area in recent years can be generally divided into the following categories: carbon materials, such as 0D carbon black spheron (SP),[27,28] 1D carbon nanotubes,[29,30] 2D graphitic carbons[31–33] and 3D graphite,[34] and other kinds of materials such as Ti4O7[35] and Cu nanowires.[36] All of the above conductive additives are expected to affect the µSi electrode solely by proving better electrical connections. Hollow graphitic carbons have been extensively reported as anode materials for alkali metal ion batteries.[37–39] However, the hollow architecture will lead to a low volumetric energy density, which is unfavorable for practical applications. To rationally take advantage of hollow graphitic carbons and compensate for the inadequacy of research on conductive additives, we propose novel hollow graphitic carbons (HGC) with tailored dual sizes as functional conductive additives. The tailored dual sizes are the micrometer size which is similar to the size of the µSi particles and the nanometer size which is similar to the size of the particles of the commercial carbon black SP conductive additive. The micro-sized hollow graphitic carbons (µHGC) act to realize a rational distribution of µSi particles and alleviate local space expansion due to their dimensional similarity and natural elasticity. On the other hand, the nanosized hollow graphitic carbons (nHGC) provide abundant contact points to enhance the overall conductivity of the µSi electrode due to superior electron transport. With the assistance of the dual-size hollow graphitic carbons conductive additives (µHGC + nHGC), µSi anode shows stable electron and ion transport and enhanced lithium storage and exhibits a satisfactory capacity of 651.4 mAh g−1 after 500cycles at a high current density of 2Ag−1, which is far better than that of the µSi electrode with SP as the conductive additive. Exsitu electrochemical impedance spectra (EIS), Raman spectra, and electron microscopy were employed to elucidate the advanced functions of the dual-size HGC additives in µSi electrodes, confirming its suitability and superiority for use as a conductive additive for µSi (alloy) anodes. 2. Results and Discussion 2.1. Materials Characterization The morphologies of the as-purchased µSi particles, SP, the as-synthesized micro-sized hollow graphitic carbons, and nanosized graphitic carbons were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure S1a (Supporting Information) shows the microsized bulk structures of the Si particles and their size distributions are displayed in Figure S1b (Supporting Information). An examination of Figure 1a confirms that the size of µHGC is similar to that of Si. The morphologies of nHGC and SP are exhibited in Figure 1b,c, in which the difference between the hollow structure and the solid structure is shown visually. It is speculated that with the same weight ratio in the electrode, nHGC will occupy more space in the electrode and will construct a more effective conductive network than SP. The TEM image of nHGC vividly depicts the 3D hollow structure with thin graphitic layers (Figure 1e) that promotes elasticity. HRTEM image of SP shows the graphitic lattice with a solid structure (Figure1f). Figure1g–i show the size distributions of the above-mentioned µHGC, nHGC, and SP additives which are in accordance with our design. X-ray diffraction spectra were used to analyze the graphitic structure. The XRD patterns of the µHGC, nHGC, and SP additives exhibit broad peaks at ≈21.6° (Figure 2a), corresponding to the (0 0 2) plane of graphitic carbon. The difference between their peak intensities implies that µHGC, nHGC has more disordered areas than SP, which is beneficial for electrolyte penetration and transport. The Raman spectra presented in Figure2b show the structural information of µHGC, nHGC, and SP and exhibit two peaks centered at 1346and 1579cm−1 that is indexed to the D band (defect feature) and G band (graphitic feature), respectively. The D/G intensity ratio of SP (1.24) clearly exceeds that of µHGC (1.13) and nHGC (1.08), suggesting that µHGC and nHGC possess a higher degree of graphitization, which is conducive to fast electron transport. The XRD pattern and Raman spectrum of µSi are also provided in the supplementary materials (Figure S2, Supporting Information). The N2 adsorption/desorption isotherm curves of µHGC, nHGC, and SP additives display a type-IV shape (Figure 2c), indicating the existence of mesopores and macropores. Figure2d shows the pore size distributions of the above carbon additives. The mesopores with diameters in the 1–10nm range account for most of the pores in the three samples. µHGC has more pores with diameters >100 nm than nHGC and SP because it is synthesized on a micro-sized MgO template (Figure S3, Supporting Information). For the solid SP, the pore size varies from 1–110nm, possibly due to the agglomeration of the SP nanoparticles (Figure S4, Supporting Information). µHGC and nHGC have specific surface areas of 308.4 and 590.3m2 g−1, respectively, which are much larger than that of SP (62.7m2 g−1). The high surface area can promote electrolyte penetration as extensively reported. 2.2. Electrode Evaluation Figure 3a plots the particles configurations of the µSi, µHGC and nHGC additives in the designed µHGC + nHGC@µSi electrode (note that the electrode is named as conductive additives@µSi, for example, µHGC+nHGC@µSi means µSi electrode with µHGC and nHGC as conductive additives), where the µHGC particles are dispersed between the µSi particles to reduce the local agglomeration of µSi, and nHGC particles are Small 2023, 19, 2205284 16136829, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202205284 by Technical University Ostrava, Wiley Online Library on [06/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com 2205284 (3 of 11) © 2022 The Authors. Small published by Wiley-VCH GmbH located at the gaps between the micro-sized particles to build a highly-interconnected conductive network. Figure3d depicts the distributions of the µSi and SP particles in the SP@µSi electrode, where µSi particles are dispersed more densely and tend to cause more severe strain during lithiation. A comparison of the SEM images of the µHGC + nHGC@µSi and SP@µSi electrodes (Figure 3b,e) shows that the µSi particles are distributed to a greater extent in the µHGC+nHGC@µSi electrode, confirming that µHGC additives effectively regulate the µSi configuration within the binder network. We further measured the EDS mapping of the Si element (purple area) to better show the Si distributions in both electrodes. Consistent with the assumed configuration in Figure 3a,d, we proposed that µSi in the mapping area shows less agglomeration in the µHGC + nHGC@µSi electrode, as more no-Si regions are located between µSi; these can be either µHGC or pores. This is not found in the SP@µSi electrode. To validate our design and evaluate the advantages of dualsize conductive additives, we first performed EIS measurements for both electrodes in the fresh state. The Nyquist plots for both electrodes are shown in Figure 4a. The semicircle in the high-frequency region normally represents the charge transfer resistance (Rct) that is associated with Li-ion migration through the electrolyte, separator, and solid electrolyte interface. The µHGC + nHGC@µSi anode in the fresh state exhibit a semicircle with a smaller radius than that of the fresh SP@µSi anode, pointing to the weaker Rct enabled by dual-size HGC additives. The oblique line in the low-frequency range represents the Li-ion diffusion impedance within the electrode materials, known as the Warburg impedance (Ws). Furthermore, the Li-ion diffusion coefficient (DLi+) can be calculated using the following equations,[40,41] 2 Li 22 24422 σ =+ D RT AnFC (1) // ρρ () == ==C n V mM V VM VM (2) where R is the gas constant (8.314JK−1 mol−1), T is room temperature (298K), A is the electrode surface area (1.98cm2), n is the number of electrons transferred during the reaction, F is the Faraday constant (96500C mol−1), C is the molar concentration of Li+ in the electrode that can be calculated from Small 2023, 19, 2205284 Figure 1. SEM images of a) micro-sized hollow graphitic carbons (µHGC). b) nano-sized hollow graphitic carbons (nHGC) and c) commercial conductive additives SP. TEM images of d) µHGC, e) nHGC and f) SP. size distributions of g) µHGC, h) nHGC and i) SP. 16136829, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202205284 by Technical University Ostrava, Wiley Online Library on [06/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com 2205284 (4 of 11) © 2022 The Authors. Small published by Wiley-VCH GmbH Equation(2), and σ is the Warburg coefficient that is equal to the slope of the Z≈ω−1/2 plot presented in Figure4b. In Equation(2), ρ and M are the molar mass of the electrode material (28g mol−1 for Si) and the tap density (0.43g cm−3). Considering that the dual-size hollow graphitic carbons are only used as conductive additives, the active material in electrodes should only be that of silicon. In this work, we merely replace the SP additives in the SP@µSi anode with the dual-size HGC additives, with the exception of the Warburg coefficient σ, the variables in the DLi+ formula are identical for both electrodes. Thus, the difference between the DLi+ values of the two anodes depends on σ. The DLi+ of the µHGC+ nHGC@µSi anode is 8.19× 10−15 cm2 s−1, which is three times higher than that of the SP@µSi anode (1.76× 10−15cm2 s−1). As a control, we also Small 2023, 19, 2205284 Figure 3. a) Schematic of the µSi, µHGC and nHGC particles’ distributions in the µHGC+nHGC@µSi electrode. d) Schematic of the µSi and SP particles’ distributions in the SP@µSi electrode. SEM images of b) the µHGC+ nHGC@µSi electrode and e) the SP@µSi electrode. Si element EDS mapping of c) the µHGC+nHGC@µSi electrode and f) the SP@µSi electrode. Figure 2. a) XRD measurements, b) Raman spectra, c) BET-specific area measurements and d) pore volume distributions of µHGC, nHGC, and SP. 16136829, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202205284 by Technical University Ostrava, Wiley Online Library on [06/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com 2205284 (5 of 11) © 2022 The Authors. Small published by Wiley-VCH GmbH prepared a µSi electrode with µHGC or nHGC as conductive additives. The results of the EIS measurements of the µHGC@ µSi anode and the nHGC@µSi anode are plotted in Figure S5 (Supporting Information). It is observed that in a fresh cell, nHGC additives are more effective for reducing the impedance because of their nano size that allows them to fill more gaps and create more contact points between the µSi particles. Furthermore, EIS measurements of the cells after 3-day calendar aging were carried out. It was found that the impedance of the µHGC + nHGC@µSi anode shows a smaller increase than the SP@µSi anode. The Warburg coefficients of the aged µHGC+nHGC@µSi anode and the SP@µSi anode increase to 140.5 and 382.70, corresponding to the Li+ diffusion coefficients decline to 4.180× 10−15cm2 s−1 and 0.549× 10−15 cm2 s−1, respectively. The increase in the electrode resistance and the deceleration of Li+ diffusion can be ascribed to the formation of the SEI layer that blocks electron and ion transport. Moreover, the kinetics of the µHGC + nHGC@µSi anode shows less decay because the dual-size HGC builds a more effective conductive network and absorbs electrolyte preferentially due to its high specific surface. Thus, dual-size HGC conductive additive successfully improves the electrode capability against calendar aging, suggesting its potential for enabling prolonged service life. 2.3. Electrochemical Performance The electrochemical properties of a µSi anode with dual-size HGC and SP as conductive additives were evaluated as shown in Figure 5. Cyclic voltammetry for the initial three cycles in the voltage window of 0.01 to 2 V at a scan rate of 0.1mVs−1 was applied to examine the electrochemical reaction of the µHGC+nHGC@µSi anode (Figure5a). An irreversible reduction peak at below 0.1V was observed during the first cathodic scanning but disappeared in the subsequent cycles, which is attributed to the lithiation of the crystalline Si to generate a Li-Si alloy.[42] The Li-Si alloy is delithiated to amorphous Si during the first anodic scanning, resulting in two oxidation peaks at ≈0.4 and 0.52V. In addition, the reduction peaks that shift to 0.2V in the subsequent cycles correspond to the transformation of amorphous Si to LixSi, indicating that crystalline Si cannot be regenerated from the Li-Si alloy during electrochemical oxidation. As a control, the SP@µSi anode shows similar electrochemical reactions in the first three cycles (Figure S6, Supporting Information). Figure 5b exhibits the galvanostatic charge/discharge profiles of both electrodes at the first, 10th, and 50th cycle, in which the curves for µHGC+nHGC@µSi anode are marked with hollow circles and the curves for SP@ µSi anode are marked with solid circles. The voltage platforms during charge/discharge are consistent with the observed CV profiles. In addition to the electrochemical properties revealed in the voltage profiles, the profiles also enable a clear evaluation of the electrode stability. Although the initial capacity of the µHGC+nHGC@µSi anode is slightly lower than that of the SP@µSi anode, the performance of the µHGC+nHGC@µSi anode quickly surpasses that of the SP@µSi anode at the 10th cycle and then shows a steadily increasing difference, showing a considerable advantage of ≈400mAhg−1 at the 50th cycle. The Small 2023, 19, 2205284 Figure 4. EIS measurements of µHGC+nHGC@µSi and SP@µSi anodes a) at the fresh state and c) after 3-day calendar aging. relationship between Z and ω−1/2 at low frequencies of the µHGC+nHGC@µSi and SP@µSi anodes b) at the fresh state and d) after 3-day calendar aging. 16136829, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202205284 by Technical University Ostrava, Wiley Online Library on [06/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com 2205284 (6 of 11) © 2022 The Authors. Small published by Wiley-VCH GmbH voltage profiles of the µHGC@Si and nHGC@Si anodes are shown in Figure S7 (Supporting Information) for comparison. As shown in voltage profiles, the capacities of µHGC@Si and nHGC@Si anodes both exceed that of SP@µSi at the 50th cycle. Besides, µHGC@Si anode shows less capacity decay than nHGC@Si, which could be elucidated that µHGC plays a more critical role in regulating the large strain than nHGC. Ultimately, we explored the long-term cycling performance of µHGC + nHGC@@µSi anode and compared the SP@ µSi anode at a current density of 0.8 A g−1 over 500 cycles (Figure5c). The µHGC+ nHGC@µSi anode displays a lower initial Coulombic efficiency (ICE) of 85.9% than the SP@µSi anode (93.5%), which is mainly attributed to the high specific surface area of the µHGC and nHGC additives. Surprisingly, the Coulombic efficiency of the µHGC + nHGC@µSi anode exceeds that of the SP@µSi anode merely at the 4th cycle (96.63–95.9%). This can be explained as due to the µSi particles in the SP@µSi electrode facing more severe bulk pulverization, leading to more fresh surfaces being exposed to electrolyte and continuous SEI formation. By contrast, the volume variation of the µSi particles in the µHGC+nHGC@µSi electrode is alleviated by the elastic hollow graphitic structure of the µHGC and nHGC additives, thus better maintaining bulk integrity and exposing less surface to the electrolyte. Both electrodes display fast capacity decay during the first 200 cycles. However, it is important to highlight that the µHGC+nHGC@µSi anode still maintains a relatively high capacity of 828.2mAhg−1, whereas the capacity of the SP@µSi anode declines to 348.6 mAh g−1 which is even worse than that of commercial graphite. After 500 cycles, µHGC + nHGC@µSi anode manifests an outstanding capacity of 633mAhg−1 that is far higher than that of the graphite anode, suggesting its commercial competitiveness. The inset in Figure 5c vividly shows the superiority of µHGC+ nHGC@µSi anode after 200cycles. By contrast, the capacity of the SP@µSi anode drops to 211 mAh g−1, below the value necessary for practical use. Figure5d shows the rate Small 2023, 19, 2205284 Figure 5. a) CV profile of the µHGC+nHGC@µSi anode. b) voltage profiles of the µHGC+nHGC@µSi and SP@µSi anodes at the 1st, 10th and 50th cycles. c) Long-term cycling performance at a current density of 0.8 Ag−1 d) rate capability of the µHGC+nHGC@µSi and SP@µSi anodes. e)Long-term cycling performance at a high current density of 2Ag−1 and f) long-term cycling performances of the µHGC+nHGC@µSi anodes with different mass loadings. 16136829, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202205284 by Technical University Ostrava, Wiley Online Library on [06/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com 2205284 (7 of 11) © 2022 The Authors. Small published by Wiley-VCH GmbH capabilities of both anodes at various current densities. It is observed that the capability of µHGC+ nHGC@µSi anode is comprehensively superior to that of the SP@µSi anode at all current densities. Furthermore, when tested at a high current density of 5 A g−1, the µHGC + nHGC@µSi anode still obtains a capacity of 753.4–1066.7mAh g−1 during five cycles, while the SP@µSi anode fails to work under such a high current. This huge difference can be ascribed to the µHGC and nHGC additives facilitating the construction of a more effective conductive network and a more stable electrode structure, as will be further discussed below. The cycling and rate performance of the µHGC@µSi and nHGC@µSi anodes are provided in Figure S8 (Supporting Information) for comparison. It shows that the µHGC+nHGC@µSi anode yields a higher reversible Li+ storage capacity than the µSi anode constructed with only µHGC or nHGC after 150 cycles. Moreover, the µHGC+nHGC@µSi anode is capable of working under a high current density (5Ag−1) and, again, provides excellent capacity when the current density returns to 0.8 A g−1. In stark contrast, the µHGC@µSi and nHGC@µSi anodes fail to sustain rate tests. The long-term cycling performance of the batteries at a current density of 2Ag−1 is shown in Figure5e to better confirm the superior rate capability of the µHGC+nHGC@ µSi anode. Upon discharge and charge test for 500 cycles, the Li+ storage capacity of the µHGC + nHGC@µSi anode first declines in the initial 200 cycles and then gradually attains a stable level of more than 651.4mAhg−1 until the 500th cycle. By contrast, the capacity of the control SP@µSi anode continuously drops from the initial cycle and declines to ≈0 at ≈the 300th cycle. Figure5f shows the performance of the µHGC+ nHGC@µSi electrodes with different mass loadings of 0.7, 1.5, and 2.5mgcm−2, respectively. Only slightly lower capacity decay is observed when the electrode mass loading increases from 0.7 to 1.5 mg cm−2, obtaining satisfactory capacities of 996.7and 856.3mAhg−1, respectively. When the mass loading increases to 2.5mgcm−2, the performance shows a clear deterioration with a capacity retention of barely 438mAhg−1, demonstrating the limitation of the dual-size HGC additives to some extent. However, summarizing the performance of the µHGC+nHGC@µSi anode tested by the above methods, it can be affirmed that the dual-size HGC successfully improves the Li+ storage performance of the µSi anode even by acting merely as conductive additives. 2.4. Structure and Kinetics Evolution A series of electrochemical characterizations have convincingly elucidated the applicability of dual-size HGC as conductive additives in the µSi-based anode. The remarkable performance can be simply explained by two aspects: 1) the strain generated during lithiation is regulated and alleviated by the compressible HGC additives; 2) µHGC and nHGC additives synergistically build a robust conductive network that can withstand the continuous volume change. It has been confirmed by in situ TEM on a particle level that the hollow graphitic structures can be pressed under external pressure and then return to their original structure when withdrawing the pressure.[14] Here, we conducted ex-situ SEM and Raman spectroscopy on the electrode level to gain in-depth insight into the essential function of strain regulation, namely the alleviation of the µSi particle pulverization induced by Li+ insertion and the reduction in the electrode expansion. Figure 6a,b shows the surface morphology of the µHGC+nHGC@µSi electrode in the fresh state and after 50 cycles, respectively. Only some slight cracks are observed in the aged electrode, illuminating the role of dual-size HGC in maintaining structural stability. By contrast, many huge cracks are observed in the aged SP@µSi electrode and a large area of active materials fell off the current collector and lost electrical connection during cycling tests as shown in Figure 6e,f. Furthermore, the thickness changes of the µHGC + nHGC@µSi electrode (Figures 6c,d) and the SP@µSi electrode (Figures 6g,h) were evaluated by cross-sectional imaging measurement for deep understanding. It was observed that the thickness variation of the dual-size-HGCassisted electrode is much smaller than that of the conventional SP-assisted electrode, strongly supporting the above-mentioned role of strain regulation. Notably, the cross-sectional images also show that the active materials in the µHGC+nHGC@µSi electrode are still tightly attached to the current collector, while those in the SP@µSi electrode dramatically fall off. Figure S9 (Supporting Information) displayed the structure evolution of the µHGC@µSi electrode during the 50-cycle electrochemical test. The aged µHGC@µSi electrode shows very little cracks and quite small thickness change, confirming that µHGC manages to regulate the strain induced by lithiation. And Figure S10 (Supporting Information) showed the morphology of the nHGC@µSi electrode before and after cycling, in which serious thickness change is observed. Comparing the thickness evolution of the electrodes, it can be concluded that in dual-size HGC additives, µHGC works to regulate the strain of the electrode, and nHGC mainly works to build a more effective conductive network along with µHGC. Ex situ Raman spectroscopy was employed to detect the structure evolution of the active materials and conductive additives in both electrodes. Figure 6i shows the Raman spectra of the fresh µHGC+ nHGC@µSi and SP@µSi electrodes. A sharp and intense peak located at 520 cm−1 is observed in both electrodes, indicating the presence of crystalline Si. The Raman results for the carbonaceous additives in the electrodes contain a defect peak (1350cm−1) and characteristic peak (1580cm−1), and the intensity of the D/G peaks of the SP (1.28) additives is higher than that of the HGC additives (0.81), in agreement with the above findings. Figure6j displays the Raman spectra of the µHGC + nHGC@µSi and SP@µSi electrodes after 50cycles. It is observed that the characteristic peak of Si is considerably more intense in the aged µHGC+nHGC@µSi electrode than that in the aged SP@µSi electrode. This confirms that the dual-size HGC additives protect the µSi particle from pulverization and maintain its structural integrity, which cannot be realized by the commercial conductive additives SP. The peak at 520 cm−1 shifts to 475 cm−1 after 50 cycles, indicating that the crystalline Si was transformed to amorphous Si, in good agreement with the obtained CV profiles.[43] Moreover, the aged µHGC+nHGC@µSi electrode shows stronger peaks related to carbonaceous materials than the SP@µSi electrode, suggesting that HGC additives are more electrochemically and dynamically stable in the µSi electrode than the SP additives. In addition, Small 2023, 19, 2205284 16136829, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202205284 by Technical University Ostrava, Wiley Online Library on [06/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com 2205284 (8 of 11) © 2022 The Authors. Small published by Wiley-VCH GmbH Small 2023, 19, 2205284 the D/G intensity ratio of dual-size HGC increases from 0.81 to 0.93, indicating a good structural integrity. Various exsitu EIS measurements were carried out to illuminate the superior kinetics of the µHGC+nHGC@µSi anode. Figure 7a,b show the results of an ex situ EIS study of the µHGC+nHGC@µSi and SP@µSi anodes upon the first lithiation to 0.5, 0.2, 0.1, and 0V. Comparison of the Nyquist plots in Figure7a,b shows that the µHGC+nHGC@µSi anode exhibits smaller charger Rct at an arbitrary lithiated state than the SP@µSi anode. The relationship between Z and ω−1/2 associated with Figure7a,b at low frequencies is shown in FigureS11 (Supporting Information). The Warburg coefficients plotted in Figure S11 (Supporting Information) suggest that Li-ion transport is faster in the µHGC+nHGC@µSi electrode than in SP@µSi during the entire lithiation process. As a control, the results of the exsitu EIS measurements of the µHGC@µSi and nHGC@µSi anodes are plotted in Figure S12 (Supporting Information). The difference in the EIS results reveals that the Figure 6. Surface morphology of the µHGC+nHGC@µSi electrode a) in the fresh state and b) after 50cycles. Cross-section of the µHGC+ nHGC@ µSi electrode c) in the fresh state and d) after 50cycles. Surface morphology of the SP@µSi electrode e) in the fresh state and f) after 50cycles. Crosssection of the SP@µSi electrode g) in the fresh state and h) after 50cycles. Raman spectra of the µHGC+nHGC@µSi and SP@µSi electrodes i) in the fresh state and j) after 50cycles. 16136829, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202205284 by Technical University Ostrava, Wiley Online Library on [06/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License