The Effect of Mechanical Cycling Rate, Operating Temperature, and Solvent on the Mechanics and Electronic Resistivity of Composite Electrodes

Introduction Solid-state batteries (SSBs) are next-generation batteries that are expected to exhibit higher energy density, power density, and safety than current lithium-ion batteries. In composite SSB electrodes, randomly distributed particles of active material (AM) and solid electrolyte (SE), and voids form three-dimensionally complicated ion/electron conduction paths and AM/SE interfaces. Such complex mass transport paths and limited AM/SE interfaces can locally impede the ion/electron supply to AM particles particularly under the rapid (dis)charging, potentially leading to a three-dimensional (3D) inhomogeneous reaction within each AM particle, as well as between particles. The occurrence of such inhomogeneous reaction can severely deteriorate the capacity, power output, and cycle life of SSBs. Therefore, it is essential to understand the mechanism of the occurrence of the inhomogeneous reaction and to design electrodes that enable uniform reaction progression. The most effective approach for this is the direct observation of the 3D inhomogeneous reaction within individual AM particles in actual composite SSB electrodes. However, most of existing methods enables only one- or two-dimensional observation of the inhomogeneous reaction1, 2, offering limited insight into the 3D reaction inhomogeneity in AM particles in SSB electrodes. With this background, in this work, we developed a 3D visualization technique of inhomogeneous reaction within individual AM particles in a composite SSB electrode using X-ray nano computed tomography technique combined with X-ray absorption near-edge structure spectroscopy (nano CT-XANES). Using this technique, we three-dimensionally observed the inhomogeneous reaction in each of thousands of AM particles in composite SSB electrodes. Furthermore, based on the obtained information, we discussed the optimal AM particle parameters (e.g., size, shape, particle distribution) that can alleviate the inhomogeneous reaction within and between AM particles, thereby maximizing electrode performance. Experimental The composite cathodes to be observed were prepared by mixing LiNi1/3Mn1/3Co1/3O2 (NMC) primary particle powders and Li2.2C0.8B0.2O3 (LCBO) solid electrolyte powders in a 1:1 weight ratio. The model SSBs were fabricated with the composite cathode (~50 μm thickness), LCBO solid electrolyte, poly (ethylene oxide) (PEO)-based polymer electrolyte, and Li metal anode. The model SSBs were charged to 100 mAh/g at 0.1~0.2 C, and the inhomogeneous reactions within AM particles in the composite cathodes after charging were visualized using nano CT-XANES. In the nano CT-XANES measurements, CT measurements were conducted near the Ni K-edge (8345.9-8352.4 eV) by incrementing the X-ray energy in 0.2 eV steps, with the entire set of measurements requiring approximately 25 minutes. The state-of-charge (SOC) of NMC (x in Li x Ni1/3Mn1/3Co1/3O2) in each voxel in the 3D CT images was evaluated based on the peak top energy shift of the Ni K-edge XANES spectrum in the respective voxels. Results and discussion Figures 1(a) and (b) show 3D SOC maps of the composite cathode after charging to 100 mAh/g at 0.2 C viewed from two different angles, and the corresponding charging curve, respectively. The observation area was approximately 34 × 47 μm in the in-plane direction of the electrode and approximately 34 μm thick with the voxel size of approximately 0.3 μm. The red/blue colored regions represent the charged (x = 0.45)/uncharged (x = 1.0) AM particles, respectively, while the transparent regions correspond to the SE or voids. As shown in this figure, the SOC of individual AM particles after charging varied significantly among the particles. While some particles, such as particle A, were charged to a Li content corresponding to the charging capacity of the entire electrode of 100 mAh/g (x = 0.64), there were also insufficiently charged particles (particle B) and excessively charged ones (particle C) relative to the average capacity of the entire electrode. These results indicates that the charge reactions proceeded inhomogeneously between AM particles. Furthermore, as shown in the 3D SOC maps of representative AM particles in Fig. 1(c), the reaction proceeded inhomogeneously within a particle. While the reaction progressed relatively uniformly within particles 1 and 2, the reaction proceeded quite inhomogeneously within particles 3 and 4. As described above, the charging state and its intra-particle inhomogeneity varied significantly among AM particles. To identify the characteristics of AM particles that determine the charging state and its inhomogeneity in each particle, in the presentation, we will statistically investigate the correlation between reaction states of each particle and morphological characteristics such as size and shape, for thousands of AM particles in the observation area.

1. Purpose Solid-state lithium-ion batteries (SSLIBs) have attracted extensive attentions because of their potentials to improve safety and to achieve higher power and energy densities compared with the conventional lithium-ion batteries. In composite electrodes for SSLIBs, particles of active material (AM) and solid electrolyte (SE) are three-dimensionally distributed and form complicated Li-ion and electron pathways. Then, especially during high rate charging/discharging, the inhomogeneous reaction distribution may be formed in the electrode. The reaction distribution can substantially deteriorate the battery performances of SSLIBs such as capacity, power output, rate capability, and cyclability. Therefore, understanding how the reaction distribution affects the battery performances is crucial for the development of high-performance SSLIBs. In this study, we performed operando three-dimensional (3D) observation of reaction distributions in composite electrodes for SSLIBs using CT-XAFS1 to elucidate the influence of reaction distributions on the battery performances of SSLIBs. 2. Experimental An SSLIB cell with a configuration of Li x CoO2 (LCO)-Li2.2C0.8B0.2O3 (LCBO) composite electrode|LCBO electrolyte|poly (ethylene oxide) (PEO)-based polymer electrolyte|Li metal electrode was fabricated based on the literature2. The composite electrode comprised 8 mg of LCO and 2 mg of LCBO. The SSLIB cell was charged to 4.5V and discharged to 2.0 V with a current of 240 μA (0.3 C) for 3 cycles at 100 °C. The reaction distribution in the composite electrode was observed every 40 minutes during charging and discharging using operando CT-XAFS. The CT-XAFS measurements were carried out in the energy range between 7725.5-7730 eV with an energy step of 0.1 eV under an exposure time of 20 ms per energy and projection angle from -65 to 65° with an angle step of 0.1°. The observation area was 517 × 517 × 49 μm. The spatial and time resolutions were 3.1 μm and 40 min., respectively. 3. Results and Discussion Figure 1(a) shows the charge-discharge curves of the SSLIB cell. The SSLIB cell exhibited the initial charge and discharge capacities of 84 and 46 mAh·g−1, respectively. Those degraded to 24 and 13 mAh·g−1 in the 2nd cycle, and 12 and 12 mAh·g−1 in the 3rd cycle, respectively. Figure 1(b) shows the 3D charging state (Li content) maps of the composite electrode and the representative 2D cross-sectional charging state maps in the in-plane direction at the end of 1st, 2nd, and 3rd charge. Colored/uncolored areas stand for the regions where the AMs exist or not, respectively. The red/blue areas stand for the charged (x = 0.6) /discharged (x = 1.0) AMs, respectively. After the 1st charge, the AMs generally showed a high charging state (red). However, the regions with high charging state significantly decreased after the 2nd charge, and most of the AMs showed a charging state corresponding to x = 0.7 ~ 0.8 (green). The regions with high charging state further decreased after the 3rd charge, and AMs with a low charging state were increased (blue). To more precisely investigate where the less-charged AMs existed after the 2nd and 3rd charge, we subtracted the charging state map after the 1st discharge from that after the 2nd charge, and also the charging state map after the 2nd discharge from that after the 3rd charge (Fig. 1(c)). The 2D maps in Fig. 1(c) thus indicate to what extent the charge reaction progressed during the 2nd or 3rd charge compared with the end of the 1st or 2nd discharge, respectively. The red/blue areas in Fig. 1(c) represent the AM regions where the reaction progressed (Δx = 0.4) or not (Δx = 0) from the end of the 1st or 2nd discharge, respectively. As shown in the left map in Fig. 1(c), the inner parts of the aggregated AMs were significantly less charged compared to the regions near AM/SE interfaces during the 2nd charge. Similarly to the 2nd charge, the charge reaction preferentially progressed near the AM/SE interfaces, while the reaction insufficiently progressed at the inner parts of the aggregated AMs during the 3rd charge. Such a variation in the charging state distribution during cycling could be explained by considering the microstructural change due to the expansion/shrinkage of AM particles. The above results suggest that the expansion/shrinkage of AM particles reduced the connections between AM particles but did not reduce the AM/SE interfaces. We further discuss the mechanism of the capacity degradation during cycling in the presentation.

Designing composite electrodes with optimal microstructure, composition, and choice of active material (AM) as well as solid electrolyte (SE) is critically important for the development of high-performance solid-state batteries (SSBs). To optimize AM loading, which includes loading amount, composition, and dispersion state, and to maximize AM utilization in composite SSB electrodes, we need to precisely understand how the AM loading affects electrochemical reactions taking place in the electrodes. Here, using computed tomography combined with X-ray absorption near edge structure spectroscopy (CT-XANES), we performed operando three-dimensional (3D) observations of electrochemical reactions in composite SSB electrodes with different AM loading amounts to understand the influence of the AM loading on the electrochemical reactions. In the composite electrode with higher AM loading amount, the lower reacted regions were mainly found at the inner parts of the aggregated AM regions. It was suggested that such a reaction distribution resulted from the slow intergranular ion transport between AM particles. In the composite electrode with lower AM loading amount, the electrochemical reaction progressed more homogeneously compared to the one with higher loading. This is probably because the lower AM loading mitigated the AM aggregation and decreased the number of high-resistance AM–AM interfaces that Li ions must pass through. Such a reaction distribution formation due to the slow ion transport between the AM particles can be a serious restriction in composite SSB electrodes. This is in marked contrast to conventional liquid-based lithium ion battery (LIB) electrodes, in which a majority of AM particles can directly exchange Li ions with the surrounding liquid electrolyte. Therefore, the optimal design for composite SSB electrodes can significantly differ from that for liquid-based LIBs. Our analysis technique can provide valuable information to rationally design optimal composite electrodes, and hence we expect that this technique contributes to the further development of high-performance SSBs.

The emergence of solid lithium superionic conductors exhibiting conductivity surpassing that of liquid electrolytes has positioned solid-state batteries (SSBs) as one of the most promising and realistic next-generation energy storage devices. SSBs are expected to demonstrate superior safety, as well as higher capacity and power density compared to conventional lithium-ion batteries. In practice, however, SSBs often fail to achieve the high performance expected from the superior properties of their individual constituent materials. This is partly because the performance of SSBs is not solely dependent on the constituent materials but also heavily influenced by the design of each battery component. For example, for the electrode of SSBs, composite electrodes consisting of active materials and solid electrolytes (and conductive additives, if necessary) are often employed. Within these composite electrodes, particles of active materials and solid electrolytes, along with voids, are randomly interspersed, forming a complex microstructure. This intricate microstructure creates highly tortuous electron and ion conduction pathways and complex heterogeneous material interfaces within the electrode. Consequently, particularly under high current charging/discharging, local stagnation of ion/electron transport within the electrode can arise, leading to spatially inhomogeneous electrochemical reactions throughout the entire electrode or within individual active material particles. Such inhomogeneous electrochemical reactions hinder the materials from fully exhibiting their inherent performance, potentially leading to severe degradation of battery capacity, power output, as well as the impairment of cycle life due to localized overcharging/overdischarging. To suppress the occurrence of such inhomogeneous reactions and maximize battery performance and lifespan, it is necessary to directly observe the electrochemical reaction within composite electrodes during (dis)charging and establish appropriate design guidelines based on these observations. However, conventional characterization techniques have been unable to directly observe the time-varying electrochemical reactions that take place three-dimensionally and inhomogeneously within electrodes at both the particle scale (~few micron) and electrode scale (~hundreds of microns) during (dis)charging. Therefore, previous studies have been limited to obtaining averaged information on the overall electrochemical reactions within electrodes using conventional electrochemical measurements, estimating the electrochemical reactions in electrodes through numerical simulations, or conducting lower-dimensional (1D or 2D) observations of the 3D electrochemical reaction using X-ray or electron probe imaging techniques. Against this background, we have developed operando 3D imaging techniques for probing electrochemical reactions in SSB electrodes based on computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES)1–4. These techniques can track the spatiotemporal evolution of the electrochemical reaction within composite electrodes during (dis)charge cycles, thereby allowing for the analysis of electrochemical reactions in five dimensions (5D), encompassing 3D spatial coordinates, time, and chemical state information. As shown in Fig. 1a1, our technique based on micro CT-XANES enables the operando 3D imaging of the evolution of the electrochemical reactions at the electrode scale with a spatial resolution of a few micrometers and a temporal resolution of approximately 30 minutes. This allows for the evaluation of the state of charge (SOC) distribution at any arbitrary cross-section in the thickness and in-plane directions within the same bulk region in electrodes at any given time. Consequently, it enables the quantitative analysis of when, where, how, and why low SOC regions appear within the electrode1,2,4, and allows for uncovering the correlation between local capacity degradation during cycling and electrode microstructures or the reaction history in previous (dis)charge cycles3. Furthermore, our advanced technique based on nano CT-XANES enables the operando, 3D, and simultaneous observation of the intra- and inter-particle reaction distributions among thousands of active material particles within SSB electrodes with a spatial resolution of sub-micrometers and a temporal resolution of approximately 30 minutes, as illustrated in Fig. 1b. The obtained information allows for the quantitative and statistical analysis of the correlations between the reaction characteristics of each active particle and its morphological features, facilitating the data-driven determination of optimal active material particle parameters, such as particle size, shape, arrangement, and size distribution. In the presentation, we will demonstrate how our 5D electrochemical reaction analysis techniques can provide useful insights for battery research. References Y. Kimura et al., J. Phys. Chem. Lett., 11, 3629–3636 (2020).Y. Kimura et al., ACS Appl. Energy Mater., 3, 7782–7793 (2020).Y. Kimura et al., Small Methods, 7, 2300310 (2023).S. Huang et al., J. Phys. Chem. C (2024) https://doi.org/10.1021/acs.jpcc.4c00318. Acknowledgements This work was supported by JST PRESTO Grant Number JPMJPR23J3, JST Mirai Program Grant Number JPMJMI21G3, and JST GteX Grant Number JPMJGX23S2, Japan. Figure 1

All-solid-state batteries show higher safety with low risk of leakage and explosion because of nonflammable inorganic solid electrolytes, which are alternative to conventional organic liquid electrolytes. Bulk-type all-solid-state batteries are constructed by pressing positive and negative electrode layers and a solid electrolyte layer at room temperature. The batteries are capable of having high energy density by adding large amounts of active materials into composite electrodes, which are composed of solid electrolyte particles and active material particles. We have investigated electrochemical performances of bulk-type all-solid-state cells using a LiCoO2 composite positive electrode and a Li2S-P2S5 solid electrolyte.1 Composite positive electrodes have many solid-solid interfaces, and electrochemical reactions at the interfaces have not been studied well. Inhomogeneous reaction may occur in the composite electrodes when solid-solid contact areas are insufficient. To improve cell performance, investigation of state-of-charge (SOC) distributions in the electrodes and fabrication of electrodes showing uniform reaction distributions are important. Raman spectroscopy is a suitable technique for investigating SOC of active materials because Raman spectra are sensitive to the structural changes of active materials during charge-discharge tests. Moreover, Raman mapping technique enables us to obtain SOC distribution maps of composite electrodes. In this study, Raman mapping was carried out for charged and discharged LiCoO2 composite positive electrodes in all-solid-state cells to investigate SOC distributions. A 75Li2S·25P2S5 (mol%) glass and indium foil were used as a solid electrolyte and a negative electrode, respectively. A composite positive electrode was prepared by mixing LiCoO2 particles and 75Li2S·25P2S5 glass particles with 80:20 in weight ratio. All-solid-state cells were charged and discharged with a cut-off voltage of 2.6-4.2 V (vs. Li+/Li) at 25oC under a current density of 0.064 mA cm-2. Raman mapping was conducted for surface parts of composite positive electrodes prepared by an Ar ion-milling technique. There are two strong Raman bands at 486 and 596 cm-1, originating from the E g and A 1g modes corresponding to O-Co-O bending and Co-O stretching, respectively.2 Those peaks of E g and A 1g modes shifted to 470 cm-1 and 582 cm-1 respectively, when the cell was charged to 4.2 V (vs. Li+/Li) and returned to original peak positions after the discharging process. To evaluate SOC of the composite positive electrode, A 1g peak positions were investigated in detail. The mapping image showed that charge-discharge reactions did not proceed uniformly, and inhomogeneous reaction distributions existed at the areas of insufficient contacts between LiCoO2 particles and solid electrolyte particles.3 To achieve uniform electrochemical reactions, a composite positive electrode having sufficient interfaces between LiCoO2 and solid electrolyte particles should be prepared. To increase contact points between them, a composite positive electrode with solid electrolytes with a smaller particle size was fabricated. Raman mapping images of the charged and discharged electrodes suggest that uniform electrochemical reactions are achieved. It is noteworthy that the use of smaller solid electrolyte particles is effective way to obtain uniform reaction distributions for composite positive electrodes in all-solid-state lithium batteries. Acknowledgement This research was financially supported by JST, ALCA-SPRING.

All-solid-state lithium ion secondary battery is a promising next generation one in terms of safety, energy density and flexibility in shape. In the all-solid-state lithium ion secondary battery, composite electrodes are organized by particles of active material, solid electrolyte, conductive additive material and binder forming three-dimensional ionic and electronic conduction pass. It is recognized the suitable ratio of active material is an important factor against high volumetric energy density and rate performance. Inhomogeneity of reaction in composite electrodes derived from their morphology affects the battery performance. Therefore, it is necessary to understand the relationship between morphology of composite electrode and reaction distribution phenomenon. Nevertheless, there are few studies from such a point of view both for the liquid and solid electrolyte system, 1-4 and it has never been reported observing the phenomenon experimentally in solid-state battery. In this study, we developed operando 2D-imaging X-ray Absorption Spectroscopic (2D-XAS) technique to investigate the reaction distribution within composite electrodes of in-plane and simulated cross sectional direction. We discuss here the correlation between electrochemical behavior and electrode morphology along two kinds of electrodes composed by different binder material. Composite electrodes were prepared by mixing active material (LiNi1/3Co1/3Mn1/3O2 (NCM)), solid electrolyte (75Li2S-25P2S5 glass (SE)), conductive additive (Acetylene Black (AB)) and binder with some solvent in the ratio of NCM:SE:AB:binder = 70:30:3:3 [wt%]. Styrene butadiene copolymer rubber (SBR) or styrene ethylene butylene styrene copolymer (SEBS) was used as a binder material in each electrochemical cell. The composite electrodes and SE sheets were pressed together under 300 MPa uniaxially, and then placed in laminate-type layered cells with counter electrodes (Li metal foil). All the process was done in Ar-filled grove-box. These cells were designed to observe the reaction distribution in not only the in-plane but also the cross sectional direction by the operando 2D-XAS measurements. The operando 2D-XAS measurements were performed at BL37XU, SPring-8, Japan. Ni K-edge XAS spectra of the NCM electrodes were collected every 20 minutes using CMOS 2D detector with a spatial resolution of 1.3 square micrometer, under running 1/40 C rate charging program. Hereafter, we refer to the two kinds of electrochemical cells with SBR binder and with SEBS as SBR-cell and SEBS-cell, respectively. In the same electrochemical measurement condition, the SBR-cell showed higher capacity at 10th cycle than the SEBS-cell. Also, it was observed that the SEBS-cell had low density, and that active material and SE distributed not homogeneously over the whole electrode from a cross section image through Scanning Electron Microscope.During the 2D-XAS measurements, galvanostatic charge tests were performed for both of the SBR- and the SEBS-cell with the State Of Charge (SOC) range of 0-22% under stable voltage. The x-ray absorption image obtained from the 2D-XAS measurements, means the existence ratio of the active material in the composite electrode (containing information along thickness dimension), reflecting the dispersiveness of the active material. The active material in the SBR-cell dispersed homogeneously, while that in the SEBS-cell localized. This suggests that dispersiveness of the active material in the composite electrode was influenced by characteristics of the binder material. Concerning the reaction distribution behavior in in-plane, the SBR-cell almost homogeneously reacted across the electrode from the initial of the charging process. In contrast, some specific regions reacted preferentially in the SEBS-cell even at the early stage of the charging process, which generated the distribution clearly through the measurement. Comparing existence ratio of Ni with reaction mapping in the both composite electrodes, it suggests that the reaction occurs preferentially in specific region containing a lot of active material particles along thickness dimension when they are greatly localized. In terms of cross sectional study of the composite electrodes, we observed that the electrode reaction proceeded isotropically from near the electrolyte to the current collector side in the case of charging. This result indicates that the limiting factor here is the ionic conductivity.

All-solid-state batteries (ASSBs), which incorporate nonflammable solid electrolytes, are expected to be the next generation batteries due to their high safety. However, the practical application of ASSBs requires an improvement in battery performance. From the perspective of the charge transfer pathway, understanding the solid-to-solid contact between battery materials is crucial for enhancing battery performance. Especially, quantifying the change in solid-to-solid contact between active material (AM) and solid electrolyte (SE) depending on various AM shapes is necessary. In this study, the effect of AM shapes on the contact characteristics inside electrode layers was investigated using the discrete element method (DEM).Three-dimensional DEM code developed in our group was used [1]. DEM is a method for analyzing powder behavior by calculating the motion of solid particles based on Newton’s second law for their time evolution. Electrode structures filled with either spherical or plate-like AM particles and spherical SE particles were generated. AM particles were constructed as aggregates (d p = 7 µm) composed of primary particles (d p = 1 µm). SE particles were considered as single spherical particles (d p = 1 µm). The volume ratio of AM to SE was set to 6:4. The electrodes were compressed at the specified pressure and then unloaded. The contact angle and stress between the AM and SE particles were calculated to analyze contact characteristics.The results showed that the contact characteristics differed depending on the particle shapes. In the case of spherical AM particles, the contact angle frequency indicated isotropy. Meanwhile, in the case of plate-like AM particles, the contact angle frequency was anisotropic because thickness direction had many contacts. For single crystals, the ionic diffusion coefficient is anisotropic [2]. Therefore, contact anisotropy has a significant effect on battery performance. The distribution of stress per particle was also evaluated. The stress distribution in the spherical AM condition was smaller than that in the plate-like AM condition. These results showed the importance of optimizing the packing structure depending on particle shape.In this study, contact characteristics and stress distribution in electrode layers with different AM particle shapes were evaluated by numerical simulation. The findings of this study are that contact characteristics and stress distribution vary with particle shape. These results emphasized that optimization of the packing structure is important for improving the battery performance of ASSBs. In the future, these studies will be expanded to investigate the effect of particle shape on battery performance. Acknowledgment This study was supported by JST-Mirai Program Grant Number JPMJMI24G1, Japan.

Silicon-graphite (Si-Gr) composite electrodes, which are based on high-capacity silicon mixed with conductive graphite with low volume expansion, have garnered significant interest due to their excellent electrochemical performance and stability. However, increasing the silicon content in the composite electrodes accelerates degradation, leading to a limitation in achieving higher energy density. In addition, Si-Gr composite electrodes undergo multi-stage reactions of silicon and graphite, resulting in complex reaction kinetics and intricate volume change behaviors. Therefore, it is challenging to analyze the impact of each active material's reaction on the electrode degradation. Furthermore, as the degradation of composite electrodes has primarily been understood and studied in the context of silicon's significant volume expansion and degradation, there is a lack of research evaluating the influence of graphite reactions. Despite having a low volume expansion upon lithiation, graphite occupies a significant volume fraction in the composite electrode and exhibits a relatively consistent arrangement. Therefore, it is anticipated that graphite would also influence electrode structural changes and the consequent electrode defect formation.In this study, we observe the average stress corresponding to the electrochemical reactions of the charge/discharge processes of Si-Gr composite electrodes in situ. The average stress can be understood as the internal pressure build-up within the electrode due to the volume expansion of the reactions. The stress and the reaction capacity were differentiated with respect to potential to analyze the contributions of each reaction on the volume change and the internal force evolution. Interestingly, graphite reactions generally induced higher stress than those of silicon during the lithiation process, and their stress per capacity increased as the State of Charge (SoC) increased. These results indicate that graphite can significantly contribute to the stress inside the electrode even though it exhibits a much lower expansion rate than silicon. Moreover, stress behavior that contradicts the silicon volume contraction was also observed during silicon delithiation. This result can be attributed to the stress-relieving phenomenon of the binder, highlighting the importance of the interaction between the active materials and binders. To observe the initial degradation of the composite electrode, repeated charge-discharge cycles were conducted, revealing intervals of unstable stress variation. Subsequent examination using ex-situ SEM showed that debonding and fracture at the interface between graphite and silicon/binder were the primary causes. Furthermore, EDS, XPS, and indentation analysis showed that such structural defects were caused by changes in the silicon-binder structure due to SEI formation. As confirmed by the micro indentation test, the viscoelastic binder and silicon were transformed into a brittle silicon-binder structure. Consequently, it failed to sufficiently dampen the forces generated by the active materials’ volume change resulting in the defects. These research findings contribute to a fundamental understanding of the stability of Si-Gr composite electrodes and our method can be expanded to various energy electrodes of different materials and compositions to enhance understanding of their behavior and contribute to their stability improvement. Choi, J., G. Kim, and S.Y. Kim, Silicon Graphite Composite Anode Degradation: Effects of Silicon Ratio, Current Density, and Temperature. Energy Technology, 2023. Ghamlouche, A., M. Müller, F. Jeschull, and J. Maibach, Degradation Phenomena in Silicon/Graphite Electrodes with Varying Silicon Content. Journal of The Electrochemical Society, 2022. 169(2). Wetjen, M., et al., Differentiating the Degradation Phenomena in Silicon-Graphite Electrodes for Lithium-Ion Batteries. Journal of The Electrochemical Society, 2017. 164(12): p. A2840-A2852. Berhaut, C.L., et al., Multiscale Multiphase Lithiation and Delithiation Mechanisms in a Composite Electrode Unraveled by Simultaneous Operando Small-Angle and Wide-Angle X-Ray Scattering. ACS Nano, 2019. 13(10): p. 11538-11551. Yao, K.P.C., et al., Operando Quantification of (De)Lithiation Behavior of Silicon–Graphite Blended Electrodes for Lithium ‐Ion Batteries. Advanced Energy Materials, 2019. 9(8). Sethuraman, V.A., et al., Real-time stress measurements in lithium-ion battery negative-electrodes. Journal of Power Sources, 2012. 206: p. 334-342. Schweidler, S., et al., Volume Changes of Graphite Anodes Revisited: A Combined Operando X-ray Diffraction and In Situ Pressure Analysis Study. The Journal of Physical Chemistry C, 2018. 122(16): p. 8829-8835. Liu, X.H., et al., Size-dependent fracture of silicon nanoparticles during lithiation. ACS Nano, 2012. 6(2): p. 1522-31. Lee, H.-J., et al., Lithiation Pathway Mechanism of Si-C Composite Anode Revealed by the Role of Nanopore using In Situ Lithiation. ACS Energy Letters, 2022. 7(8): p. 2469-2476. Ogata, K., et al., Revealing lithium–silicide phase transformations in nano-structured silicon-based lithium ion batteries via in situ NMR spectroscopy. Nature Communications, 2014. 5(1).

All-solid-state batteries (ASSBs) have garnered significant interest as a promising alternative to conventional lithium-ion batteries, offering improved safety, higher energy density, longer lifespan, faster charging and wider operating temperature range. A key advantage of ASSBs is the use of solid electrolytes (SE), which eliminate the risks associated with flammable liquid electrolytes. However, the complex electrochemical behavior and multi-scale nature of ASSBs pose challenges in designing and optimizing their performance. This study employs Discrete Element Method (DEM) particle-based models to better understand degradation behavior of ASSBs by considering electrochemical reaction, mass and charge transfer, intercalation expansion and material deformationOur simulations were developed in MATLAB and were performed on a half-cell configuration comprising a SE-layer and an anode electrode. The anode electrode was composed of a particle mixture containing active material (AM) and SE. In our simulations, Si was employed as AM, which experiences substantial intercalation expansion during charge cycling 1. Our previously developed DEM model2,3 resolved elastic and plastic deformation of both AM and SE particles which become compressed due to the intercalation expansion. The simulation of intercalation was achieved by integrating the Newman electrochemical model,4 into the DEM mechanical model. Building on our earlier study5, where the Li concentration was resolved within each AM particle, this study is also accounting for the Li-ion potential within each SE particle. Interparticle conduction and diffusion were computed by determining the contact area of AM-AM and SE-SE contacts, which were affected by the compression of the electrode. The electrochemical reaction was calculated at the AM-SE interface using the Butler-Volmer equation, and in addition the influence of compressive stress on the reaction6 was included.Figure 1(a) depicts the Li concentration and Li-ion potential distribution within the half-cell during charging at different states of charge (SOC), while Figure 1(b) provides a 3D view of the distributions at a SOC of 50%. A considerable expansion of the Si anode is evident. The gradients of Li concentration and Li-ion potential signify significant mass and charge transfer resistance during the charging process. Comparing the distribution of the particles, the anode becomes more porous after the first cycle for the same value of SOC. This effect is attributed to the extensive expansion of the Si anode, leading to plastic compression of the SE layer, which becomes more compact after each cycle. The growing porosity diminishes the contact area between particles, ultimately resulting in the degradation of the half-cell's capacity. In addition to SE-layer compaction, this study aims to investigate several degradation mechanisms and their impact on the performance of ASSBs, including the development of shell voids resulting from AM-SE delamination, crack propagation through SE, and fragmentation of AM.The results of this study provide valuable insights into understanding the effect of mechanical degradation on the performance of ASSBs. By improving our understanding of these mechanisms, we can work towards enhancing the durability and lifespan of ASSBs, ultimately contributing to the development of next-generation energy storage systems. Acknowledgment This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas, “Science on Interfacial Ion Dynamics for Solid State Ionics Devices, Computational & Data Science” (Gp-A03, grant number 19H05815); and MEXT “Program for Promoting Researches on the Supercomputer Fugaku” (Fugaku battery & Fuel Cell Project), grant number JPMXP1020200301. References M. N. Obrovac and L. Christensen, Electrochem. Solid-State Lett., 7, A93 (2004).M. So, G. Inoue, R. Hirate, K. Nunoshita, S. Ishikawa, and Y. Tsuge, J. Electrochem. Soc., 168, 030538 (2021).M. So, G. Inoue, R. Hirate, K. Nunoshita, S. Ishikawa, and Y. Tsuge, Journal of Power Sources, 508, 230344 (2021).J. Newman and K. E. Thomas-Alyea, Electrochemical Systems, 3rd edition., p. 672, Wiley-Interscience, Hoboken, N.J, (2004).M. So, S. Yano, A. Permatasari, T. D. Pham, K. Park, and G. Inoue, Journal of Power Sources, 546, 231956 (2022).G. Bucci, T. Swamy, S. Bishop, B. W. Sheldon, Y.-M. Chiang, and W. C. Carter, J. Electrochem. Soc., 164, A645 (2017). Figure 1

Simulation of Optimization and Utilization for LiB with Multi Element Network Lithium ion batteries (LiBs) are used in electric vehicles (EVs), hybrid electric vehicles (HEVs) and so on. However, it is required to have more power density and more energy density. In order to develop the performance of LiB, not only material characteristics such as active materials and electrolytes [1][2], but also optimizing electrode structure is required[3]. Therefore, chasing the state in electrode layer using the numerical computation is a critical measure for the comprehension of phenomenon in the cell. However, in the relatively micro-scale system such as the electrode layer, a slight difference in structure affects the battery performance with a slight difference in structure. However, usual simulations demand the reactive interface area and the tortuosity factor which critically affect the cell performance by reasonableness or approximation, as it might overlook the phenomenon from minute structure of electrode layer. The purpose of this study is to develop a numerical computation technique that reflected the micro characteristic of electrode structure directly by building a Multi Element Network Model based on simulation of porous structures. This simulation first located active materials randomly in three-dimensional space. The active material particles are all spherical and overlap is not allowed. Next, assuming the space except active material particles as electrolyte phase, and located imaginary spheres with greatest diameter at positions fixed by 4 active material particles or more, which memorize electrolyte information such as ion electric potential at the position. After constructing electrode structure, the (active material) particle network and pore network were built as shown in Figure 1. The electronic conduction was calculated in particle network, as the ionic conduction and diffusion calculated with pore network, while the electrode reaction occurring at interface between active material particles and electrolyte. Finally, by applying this model to a galvanostatic discharge simulation based on the porous electrode theory [4][5], the state inside electrode layer was converged with iterative computation by taking the mass balance and electron balance of each particles and imaginary electrolyte spheres. LiCoO2 and graphite were employed as cathode and anode active materials, respectively. Volume ratio of active material and the thickness of both electrode were set to be 0.5 , 10 µm respectively. The particle size according to normal distribution, which have a median diameter 2 µm. Figure 2 shows a calculated result of potential distribution in the cell when discharging at 100C discharge rate. However, because the calculation load in a primary stage of iterative computation, this is a result in which potential profile converged before time advanced. Compared with the overvoltage depending on the reaction or transport resistance, the overvoltage due to electron transport was largest which means the main resistance being the electron transport resistance in this case. This result proves that Multi Element Network makes it available to assign the main factor in electrode layer, and more evaluation of various electrode structure will be reported in poster. Reference [1] M. Shui, S. Gao, J. Shu, W. Zheng, D. Xu, L. Chen, L. Feng, Y. Ren, Ionics, 19(2013), 41-46. [2] K. Ishikawa, T. Sugawara, Y. Munakata, K. Kanamura, the 56th Battery Symposium in Japan(2015), 3E16. [3] G. Inoue et al., ECS Trans., 25 (1), (2009), 1519-1527. [4] M. Doyle, T. F. Fuller, J. Newman, J. Electrochem. Soc., 140(1993), 1526-1533. [5] G. M. Goldin, A. M. Colclasure, A. H. Wiedemann, R. J. Kee, Electrochimica Acta, 64 (2012), 118-129. Figure 1

The understanding of electron and ion transfers in composite porous electrodes is essential to improve their electrochemical performance. Composite porous electrodes are generally made of an active material (AM), a carbon black conducting additive (CB) and a polymeric binder (B). At battery assembly, it is infiltrated by a liquid electrolyte (EL). An electrode has a double hierarchical structure: the first one consists of agglomerates of particles of AM and the second is of agglomerates of CB - B mixture. From the nanometric to the macroscopic level, this double structure is therefore a hierarchy of interfaces: AM/AM, CB/CB, CB/AM, EL/AM, EL/CB, electrode/current collector. The charge transports in composite electrodes are generally explained by considering the percolation and the tortuosity of the CB particles network for electrons on one hand, and of the pores network for ions on the other hand. The contributions of the different interfaces within composite electrodes are however scarcely studied. Indeed, this requires the simultaneous measurement of ionic and electronic conductivities. To do so, the development of an instrumentation that takes into account both the constraints imposed by the nature of the samples and the mobilities of mobile species (ions, electrons) is required. The in- and ex-situ dielectric spectroscopy enables this objective to be fulfilled over a wide frequency range from 40 Hz to 10 GHz between 200 and 300 K [1-5]. The nature and the quality of the interfaces depend on the shape and the dimensionality of the particles of the active material (for example, platelets, cuboids, etc.), which can strongly influence electronic transfers within the composite electrode [5]. Strong interactions between the liquid electrolyte and the active material, and between the electrolyte and the carbon black have been highlighted [3,4]. The binder (B) also affects the properties of these interfaces. Space charges, which are created on the surface of electronically conductive materials (AM and CB), generate electrical polarizations whose frequency responses are located in the range of radio frequencies and microwaves. The presence of an electrolyte modifies the intensities and dynamics of space charge polarizations because of ion-electron and dipole-electron Coulomb interactions [3,4]. On the other hand, ionic double-layer capacitances are created at the solid/liquid interfaces on the electrolyte side. The latter also generate electric polarizations at lower frequencies than those of electronic space charges because of the lower mobility of the ions. These different interactions modify the electronic mobility of the active material and the carbon black as well as the diffusion of the ions (cations and anions) of the confined electrolyte into the porous network of the electrode [4]. In this presentation, we will report our recent discoveries of the perturbations due to interfaces on electronic and ionic conductions within composite electrodes for Li-Ion batteries.

The impact of both various binders and carbon additives on the electrochemical behavior of intermetallics-based (FeSn2, NiSb2, TiSnSb) negative electrodesvs.Li was evaluated and accurately studied for FeSn2. The formulation of the composite electrode allowed enhancement of both the capacity retention as well as the rate capability. CMC/VGCF (carboxymethyl cellulose/vapor grown carbon fiber) used as a binder/conductive additive allows retention of 100% of the specific capacity during 35 cycles at 2C rate (2Li h−1), whereas the fading is dramatic after only a few cycles at a low rate with classical powdered electrodes. Notably, an extra capacity is observed with CMC/VGCF. In the case of FeSn2, it is shown that the improvement of performance achieved with CMC/VGCF results from better efficiency of the conductive additive to form an electronic percolation web around the active material (AM) particles rather than from the buffering of the volume variations of the AM particles, contrary to the case of Si-based electrodes. The improved cycle life is also likely due to the better ability of CMC to cover the particles compared to PVDF (polyvinylidene fluoride), resulting not only in stronger interparticle bonding but also in a better SEI layer. It is suggested that the growing of an insulating SEI layer by the degradation of the liquid electrolyte is an important factor in the fading mechanism of FeSn2 composite electrodes. Finally, the aqueous processing of the FeSn2, NiSb2, and TiSnSb intermetallics-based composite electrodes is feasible.

Lithium-ion batteries (LIBs) with high gravimetric and volumetric energy density are very important key devices for the establishment of the sustainable energy system which is consisted of solar cells, wind power generations, smart grid, and batteries. In addition, the LIBs are extensively expected for a power supply of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). Much higher energy density is needed to increase the mileage per charge. Current LIBs use the composite electrodes which are composed of active materials, binders, conductive agents, and current collector. The charging and discharging performance of the composite electrode must be affected by the “composite” state (kinds of materials, mixing ratio, thickness, and tap density). In addition to that, the materials which have large volume change during the charging and discharging are strongly affected by the state of the composite electrode. It is difficult to distinguish the electrochemical characteristics of only active materials from that of the composite electrode.However, the intrinsic properties of active materials are very important to evaluate the electrode performance and understand the mechanism of the charging and discharging reaction. In this study, the single particle measurement technique was used to study the electrochemical lithiation of silicon single particle. This technique is very useful to measure the intrinsic properties of single particle of the active materials in the actual liquid electrolyte. Our group has reported that some positive electrode active materials have very good rate characteristics [1,2]. The microelectrode with 20 mm diameter is used as the micro probe to contact and apply the electrochemical technique to the one active material particle. The electrodeposited Cu covers the tip of the micro probe because the Cu is stable against the electrode potential of the lithiation of silicon. Our previous study has reported that this technique is applicable to measure the volume expansion of silicon single particle during the lithiation [3]. Figure 1 shows the drastic volume expansion behavior of a silicon single particle during the lithiation. A silicon particle contacted with micro-probe was expanding with the lithiation. The apparent volume expansion ratio is larger than the theoretical expectation. So far, we estimate that the reasons are the aggregation state of the silicon particle, the emergence of amorphous phase of the lithiated silicon and anisotropic properties of volume change. In order to discuss the reason of larger volume expansion and volume change mechanism of the silicon particle more precisely, micro-tweezers system was used to move the silicon particle to SEM observation and Raman spectroscopy measurement after the 1st charging by the single particle measurement technique. Some researchers use an in-situ TEM technique in order to understand the mechanism of silicon lithiation and delithiation [4]. This excellent technique can observe the silicon volume change behavior accompanied with the lithiation and delithiation with very high magnification, but it needs high vacuum condition. The advantage of our technique is the electrochemical condition is very near to the actual LIBs. Therefore, it is important to combine their knowledge and our results to find the volume change mechanism of silicon electrode during the lithiation and delithiation. In this presentation, we would like to discuss the comparison.Furthermore, high capacity active materials which have volume change during charging and discharging need the way to restrict or accommodate the volume change for the usage to practical battery electrodes. From the viewpoint of such approach, the binder effect is very important. A small piece of composite electrode with 20~40 mm was picked up to measure the electrochemical characteristics in the single particle measurement system. Comparing with the pristine silicon particle measurement results, we would like to discuss the binder effect to volume change behavior. References (1) K. Dokko, N. Nakata, K. Kanamura, J. Power Sources 189, 783 (2009)(2) H. Munakata, B. Takemura, T. Saito, K. Kanamura, J. Power Sources 217, 444 (2012)(3) K. Nishikawa, H. Munakata, K. Kanamura, J. Power Sources 243,630 (2013) [4]. M. T. McDowell, S. W. Lee, J. T. Harris, B. A. Korgel, C. Wang, W. D. Nix, and Y. Cui, Nano Lett., 13, 758 (2013)Figure 1. Volume expansion behavior of silicon single particle during the first lithiation

Simulation of the compaction of an all-solid-state battery cathode with coated particles using the discrete element method

Microstructural characteristics of lithium‐ion battery cathodes determine their performance. Thus, modern simulation tools are increasingly important for the custom design of multiphase cathodes. This work presents a new method for generating virtual, yet realistic cathode microstructures. A precondition is a 3D template of a commercial cathode, reconstructed via focused ion beam/scanning electron microscopy (FIB/SEM) tomography and appropriate algorithms. The characteristically shaped micrometer‐sized active material (AM) particles and agglomerates of nano‐sized carbon‐binder (CB) particles are individually extracted from the voxel‐based templates. Thereby, a library of roughly 1100 AM particles and 20 CB agglomerates is created. Next, a virtual cathode microstructure is predefined, and representative sets of AM particles and CB agglomerates are built. The following re‐assembly of AM particles within a predefined volume box works using dropping and rolling algorithms. Thereby, one can generate cathodes with specified characteristics, such as the volume fraction of AM, CB and pore space, particle‐size distributions, and gradients thereof. Naturally, such a virtual twin is a promising starting point for physics‐based electrochemical performance models. The workflow from the commercial cathode microstructure through to a full virtual twin will be explained and assessed for a blend cathode made of the two AMs, LiNiCoAlO2 (NCA) and LiCoO2 (LCO).