Sn vs. Ge: Effects of Elastic and Plastic Deformation of LGPS-type Solid Electrolytes on Charge-Discharge Properties of Composite Cathodes for All-solid-state Batteries

Abstract

The charge-discharge properties of all-solid-state batteries are affected by both chemical and physical factors. Physical issues mainly arise from the microstructure of the composites and the mechanical properties of the solid electrolytes themselves. However, physical issues have been investigated by focusing on the microstructures of the composites rather than the mechanical properties of the solid electrolytes themselves. In this study, composite cathodes with similar microstructures were fabricated using LiCoO₂ as the active material and either Li_9.81Sn_0.81P_2.19S₁₂ or Li₁₀GeP₂S₁₂ as the solid electrolyte. The composite with Li_9.81Sn_0.81P_2.19S₁₂ exhibited higher capacity retention and coulombic efficiency with increasing C-rates at 1.9–3.6 V vs. In-Li than that with Li₁₀GeP₂S₁₂. Moreover, when charging–discharging at 1.9–3.8 V, where the expansion and shrinkage of LiCoO₂ were greater those at 1.9–3.6 V, the composite with Li_9.81Sn_0.81P_2.19S₁₂ exhibited a higher capacity, capacity retention, and Coulombic efficiency than those of the composite with Li₁₀GeP₂S₁₂. These results are attributed to the high elastic modulus, high yield stress, and volumetrically-large elastic-deformability, which enable Li_9.81Sn_0.81P_2.19S₁₂ to reversibly deform while maintaining contact with LiCoO₂, unlike Li₁₀GeP₂S₁₂. These results demonstrate that solid electrolytes with low elastic moduli are not absolutely suitable for all-solid-state batteries, and that a high yield stress and volumetrically-large elastic-deformability are especially significant for reversible deformation. These findings provide new insights for the development of composite electrodes for all-solid-state batteries.

1. Introduction

All-solid-state batteries (ASSBs) have attracted attention as new-generation rechargeable batteries since the discovery of super ionic conductors.^1–15 From the viewpoint of carbon neutrality, rechargeable batteries, including ASSBs, are required to generate high energies with high charge–discharge rates in large-scale applications, such as power plants and electric vehicles. The construction of ASSBs with high energy densities and rate capabilities requires knowledge of the materials and their combinations. For example, composites consisting of active materials and solid electrolytes have been utilized as electrodes to ensure sufficient ion-conductive pathways in the electrode layers. The theoretical capacity depends on the type of active material. However, the theoretical capacity of composites can only be achieved when the materials are ideally constructed without losses due to chemical and physical issues. Physical issues are especially significant for ASSBs because, unlike liquid electrolytes, solid electrolytes cannot penetrate voids and cracks among the particles of the active materials. For instance, when the active materials and solid electrolytes in composite electrodes are not uniformly mixed, the numbers of reaction sites and electron/ion-conductive pathways decrease. Therefore, electrons and Li ions must react and migrate via a small number of sites and pathways, even if the number of reacting electrons and Li ions increases owing to an increase in the charge–discharge rate (C-rate). Consequently, the active materials are not fully utilized for charge–discharge with increasing C-rates, even if they can be fully utilized at low C-rates. Thus, the practical values are likely to differ from the theoretical values with increasing C-rates, even in the absence of chemical issues. Consequently, enhancing the physical properties of composite electrodes is an important task.

The authors previously attempted to control the physical properties of composite cathodes and solid electrolyte layers by adjusting the particle size and its distribution in Li₁₀GeP₂S₁₂ (LGPS)-type solid electrolytes.^16–18 The Meyer hardness of pelletized LGPS can be increased by reducing the size of the particles and narrowing the size distribution, thereby stabilizing the charge-discharge cycle of Li-In | LGPS | In-Li symmetric cells.^17,18 In the case of composite cathodes comprising LiCoO₂ and LGPS (LCO/LGPS), reducing the size of the particles and narrowing the size distribution increased the uniformity in the composite.¹⁶ This generated three-dimensional pathways for electron- and ion-conduction; hence, the capacity and rate-capability were enhanced. These reports indicate that physical structures at the microscale (microstructure) contribute largely to the charge–discharge properties of ASSBs by affecting the mechanical properties. Other researchers have also attempted to improve the charge–discharge properties of ASSBs by enhancing the mechanical properties through microstructure control.^19–25 However, investigations of the charge–discharge properties of composite electrodes have not focused on the mechanical properties of the solid electrolytes or active materials themselves because other factors such as chemical issues, microstructures, and ionic conductivities were not excluded. The volumetric changes in the active materials may increase with increasing theoretical capacities. Therefore, when active materials with high theoretical capacities are used for composite electrodes, the mechanical properties of the solid electrolytes must be suitable to flexibly address volumetric changes. Therefore, solid electrolytes are expected to possess suitable mechanical properties for composite electrodes. The low elastic moduli of solid electrolytes have generally been the focus from the viewpoint of contact with the active materials.^13,19,20,23 However, previous reports have not directly investigated the isolated effects of the elastic moduli of the solid electrolytes on the charge–discharge properties. Therefore, investigating the direct relationships between the mechanical properties of the solid electrolytes themselves and the charge–discharge properties of composite electrodes, while excluding other factors such as chemical issues, microstructures, and ionic conductivities as much as possible, is important.

In this study, Li_9.81Sn_0.81P_2.19S₁₂ (denoted as LSnPS) with an LGPS-type structure is evaluated as a solid electrolyte. LSnPS has an ionic conductivity of 5 × 10⁻³ S cm⁻¹,^15,26,27 which is relatively close to that of LGPS (1.2 × 10⁻² S cm⁻¹). It has also been reported that the decomposition energy of LSnPS is the same as that of LGPS.²⁸ This is consistent with the similarity of the cyclic voltammogram (potential window) of Au | LSnPS | Li to that of Au | LGPS | Li (0–5 V vs. Li anode).^26,29 Therefore, when the microstructures in the composite electrodes are controlled to be similar, LSnPS can be compared with LGPS to investigate the effects of the mechanical properties of the solid electrolytes themselves on the charge–discharge properties.

Herein, composites consisting of LCO and LSnPS (LCO/LSnPS), which possess a microstructure similar to that of microstructurally controlled LCO/LGPS, are fabricated by adjusting the particle size and its distribution of LSnPS. The charge–discharge properties of LCO/LSnPS are investigated and compared with those of LCO/LGPS, focusing on the elastic and plastic deformability of LSnPS and LGPS. The results show that materials with a low elastic modulus are not absolutely suitable for accommodating the charge–discharge of composite electrodes, and that a high yield stress and volumetrically-large elastic-deformability are especially significant for reversible deformation.

2. Experimental

2.1 Sample preparation

LGPS and LSnPS were synthesized by solid-state reactions. Li₂S (Mitsuwa Chemical; >99.9 %), P₂S₅ (Sigma-Aldrich; >99.9 %), GeS₂ (Kojundo Chemical; >99.9 %), and SnS₂ (Kojundo Chemical; >99.9 %) were used as the starting materials. The precursors were ground in an agate mortar for 15 min. The mixtures were then milled using a vibrating mill (CMT; TI100). The milled precursors were heated in an electric furnace at 823 K for 8 h under an Ar flow, affording the pristine samples (as-prepared: AP). The temperature was increased to 823 K at a rate of 5 K min⁻¹. The obtained AP-LGPS and AP-LSnPS powders were additionally ball-milled in wet conditions (wet-milling: WM).¹⁷ The AP samples were milled at 300 rpm for 80 min in ZrO₂ pots using a planetary mill containing ZrO₂ balls (ϕ 5–10 mm) and anhydrous heptane (Wako Pure Chemicals, 99.0 %). 8 and 3 g of the ZrO₂ balls are used for LGPS and LSnPS, respectively. The ball-milled samples were then dried at 353 K for 12 h under vacuum.

Au | solid electrolyte | Au symmetric cells were fabricated for the alternating current (AC) electrochemical impedance spectroscopy (EIS) analyses. First, 50 mg of the obtained solid electrolyte was pelletized in ϕ 5 mm at a pressure of at 182 MPa for 1 min. Au powder (12.5 mg) was added to the top and bottom faces of the pellets, which were then pressed at 255 MPa for 1 min.

LCO/LGPS and LCO/LSnPS composite cathodes were fabricated as per a previous report.³⁰ LCO (coated with amorphous LiNbO₃ (LNO/LCO)) and the obtained solid electrolytes were mixed with agate balls (ϕ 3 mm) in glass vials for 10–20 min at 140 rpm using pot-mill rotators (Nitto; ANZ-10S). The weight ratio of LCO to the solid electrolyte was 7 : 3, which is equal to 5 : 5 in volume.

ASSBs were fabricated using the obtained composites as cathodes, LGPS ground in an agate mortar for 30 min (hand-ground LGPS: HG-LGPS) as the solid electrolyte layer, and In-Li as the anode. HG-LGPS (80 mg), as the solid electrolyte layer, was first pelletized to ϕ 10 mm by pressing at 110 MPa for 1 min. The composites and Al meshes for the current collectors were added to the top faces of the pelletized HG-LGPS and pressed at 530 MPa for 1 min. In and Li foils were placed on the faces opposite the composite layers. The Cu meshes of the current collectors were also set on In and Li foils. A pressure of 210 MPa was applied to the pellets for 3 s, resulting in Al | composite | HG-LGPS | In-Li | Cu cells. For cross-sectional observations, the ASSB pellets were cut using a slicer (JASCO; Multi-Angle Slicer HW-1) to expose their cross-sections. The cross-sections were flattened for 2 h using a focused Ar-ion beam (Hitachi High-Tech; IM4000) at an acceleration voltage of 6 kV while cooling at 183 K.

2.2 Characterization

The obtained solid electrolytes were identified by X-ray diffraction (XRD, Rigaku, SmartLab) at 10–35° with CuKα₁ irradiation from an X-ray tube set to a current of 200 mA and voltage of 45 kV. The particle-size distributions of the solid electrolytes were investigated using a laser-scattering particle size distribution analyzer (PSA, HORIBA; Partica LA-960). The microstructures of the composite cathodes were observed using a scanning electron microscope equipped with an energy-dispersive X-ray spectroscope (SEM-EDX). The mechanical properties were evaluated via indentation measurements (A&D Company; RTF-1250).³¹ In an Ar glove box, LGPS and LSnPS pellets were fixed on a tungsten carbide test stage (E = 600 GPa). The samples were then immersed in silicone oil (Shin-Etsu Chemical; KF-968-100CS) and transferred from the glove-box to the testing apparatus under atmospheric conditions. A spherical indenter (ϕ 12 mm) was used to apply a maximum load of 120 N, at a rate of 0.03 mm min⁻¹. This indentation test was repeated for 100 cycles to obtain a consistent P-h curve.^18,31 After the cyclic indentation measurements, a pyramid-shaped and spherical indenter (ϕ 1.2 mm) was used to evaluate the Mayer hardness and yield stress. For determination of the Meyer hardness using the Vickers indenter, the Meyer hardness, H_M, was defined as the ratio of the peak load, P_max, to the projected area, A_r, of the residual indentation impression. With the approximation of A_r ≈ A_c, where A_c is the projected contact area, an alternative expression for H_M is given by:³²

  

\begin{equation} H_{M} = P_{\text{max}}/A_{c} = P_{\text{max}}/gh_{c}{}^{2} = \gamma_{1}{}^{2}k_{1}/g \end{equation} (1)

where the geometrical factor, g, is 24.5° for the Vickers indenter.

First, k₁ was calculated using a well-defined quadratic relationship that describes the loading paths, as follows:

  

\begin{equation} P = k_{1}h^{2} \end{equation} (2)

P is the load defined by the elastoplastic constant k₁ and residual depth h. This equation can be rearranged as follows by taking the square root of both sides:

  

\begin{equation} P^{1/2} = k_{1}{}^{1/2}h \end{equation} (3)

Equation 2 indicates that k₁ can be calculated by plotting the P-h curve as the square root of P on the vertical axis and h on the horizontal axis.

Secondly, γ₁ was calculated using the equations described below. The relative residual depth, ξ_r, is defined by h_r of the residual impression normalized by the maximum penetration depth, h_max, as follows:

  

\begin{equation} \xi_{\mathrm{r}} = h_{r}/h_{\textit{max}} \end{equation} (4)

The surface profile parameter, γ₁, is defined by the ratio of h_max to h_c, using the contact depth, h_c, induced at P_max. The empirical formula represents the correlation between γ₁ and ξ_r.

  

\begin{equation} \gamma_{1} = \pi/2\ (1 - 0.43\xi_{\text{r}}{}^{1/2}) \end{equation} (5)

To calculate the elastic modulus, the relationship between P and h in the linear elastic region is expressed as follows:

  

\begin{equation} P = 4/3 E^{*} R^{1/2}h^{3/2} \end{equation} (6)

R and E are the radius of the spherical indenter and elastic modulus, respectively. Using the approximation a_c² ≈ 2Rh_c (a_c: contact radius) for a_c ≪ R, the mean contact pressure P_m is given as follows:

  

\begin{equation} P_{m} = \frac{P}{\pi ac^{2}} = \frac{P}{\pi Rh} = \frac{4E}{3\pi}\left(\frac{h}{R}\right)^{\frac{1}{2}} \end{equation} (7)

The P-h curves, as raw data, were converted to the P_m curves (P_m − h/R); the slope was calculated using linear fitting, and the elastic modulus was calculated using Eq. 7. In addition, the yield point was determined as the point that gradually deviated from the straight line in the P_m curve.

2.3 Electrochemical measurements

EIS measurements were performed using Au | solid electrolyte | Au symmetric cells and a frequency resonance analyzer (Solartron Analytical; 1260). The measurement conditions were set to an amplitude of 10 mV, frequency range of 0.1 Hz to 7 MHz, temperature of 175–300 K, and pressure of 100 MPa. At low temperatures, 2 semicircles appeared in the high- and low-frequency regions with capacitances of 1.2–2.8 × 10⁻¹¹ and 1.2–2.0 × 10⁻¹⁰ F, respectively. Consequently, the semicircles in the high- and low-frequency regions are assigned to the bulk and grain boundary resistance, respectively.³³

The charge-discharge properties of the obtained ASSBs were measured using a potentio/galvanostat (TOYO SYSTEM; TOSCAT-3100) at voltages of 1.9–3.6, 3.8, 4.0 V vs. In-Li anode (≈2.5–4.2, 4.4, 4.6 V vs. Li⁺/Li). The constant-current + constant-voltage (CCCV) and constant-current (CC) modes were applied for the charge and discharge measurements, respectively. During charge-discharge tests, the ASSBs were pressed approximately at 30 MPa, which was adjusted by a bolt screwed with the torque of 20 Nm.

3. Results and Discussion

Figure 1 shows the XRD patterns of the AP- and WM-LGPS and LSnPS samples. The XRD pattern of LGPS changed negligibly after WM, indicating that the crystallinity of LGPS was maintained. The XRD pattern of AP-LSnPS also exhibited sharp peaks, indicating high crystallinity. The XRD pattern and peak width of WM-LSnPS were almost the same as those of AP-LSnPS, indicating that the crystallinity of LSnPS was maintained after WM. Consequently, WM-LSnPS was used for comparison with WM-LGPS.

Figure 1.

XRD patterns of (a) LGPS and (b) LSnPS. The original figure is registered in J-STAGE data. All figures below are be registered in the same way.^D1

The mechanical properties of pelletized powders are affected by the particle size and its distribution.^17,18 Therefore, the particle-size distributions of AP-LGPS, WM-LGPS, AP-LSnPS, and WM-LSnPS were evaluated using a PSA, as shown in Fig. 2. The median size (D₅₀) of the LGPS samples followed the order: AP > WM, but the differences in D₅₀ were small. AP- and WM-LGPS contained large particles (>10 m). The D₅₀ of LSnPS clearly decreased after WM, compared with that of AP-LSnPS. However, the width of the particle-size distribution of WM-LSnPS was similar to that of AP-LSnPS. The particle-size distributions of LGPS and LSnPS were confirmed using also SEM (Fig. S1).

Figure 2.

Particle-size distribution of (a) LGPS and (b) LSnPS.

Differences in the particle-size distribution affect the total ionic conductivity, even if the bulk conductivity does not change, because of differences in the number of grain boundaries.^16,17 Therefore, the ionic conductivity of AP-LGPS, WM-LGPS, AP-LSnPS, and WM-LSnPS was investigated using EIS (Figs. 3 and S2). The ionic conductivity of the LGPS samples followed the order: AP (7.0 mS cm⁻¹) > WM (2.4 mS cm⁻¹). This order agrees with that of the D₅₀ values, suggesting that the order of the ionic conductivities is derived from the grain-boundary resistance. The authors previously reported that down-sizing the particles increased the grain boundary resistance in WM-LGPS by increasing the number of grain boundaries.¹⁷ In the case of LSnPS, the ionic conductivity followed the order: AP (2.6 mS cm⁻¹) > WM (1.1 mS cm⁻¹). The difference between AP- and WM-LSnPS was also due to the difference in the grain boundaries. In fact, the Nyquist plots at 200 K indicate that the grain boundary resistance, determined from the semicircle in the low-frequency region, increased after WM (Fig. S3). However, the bulk resistance, determined from the semicircles in the high-frequency region, did not change significantly with WM. Thus, the particle size of LSnPS was successfully controlled without the loss of ionic conductivity in the bulk.

Figure 3.

Nyquist plots of LSnPS. The measurements were conducted using Au | solid-electrolyte | Au symmetric cells under 100 MPa of pressure with an amplitude of 10 mV, frequency range of 0.1 Hz to 7 MHz, and temperature of 278 K.

Figure 4 shows the charge-discharge properties of LCO/WM-LGPS | HG-LGPS | In-Li and LCO/WM-LSnPS | HG-LGPS | In-Li cells at 0.05–5C of the C-rates up to 3.6 V vs. In-Li. The 1st discharge capacity at 0.05C for the cell employing WM-LGPS was 130.2 mAh g⁻¹, where the value is comparable to the theoretical capacities of 137 mAh g⁻¹ (Fig. 4a). The capacity of the cell using WM-LGPS decreased with increasing C-rate. The cell with WM-LGPS maintained cycle stability and a high coulombic efficiency (≈100 %) up to 1C (Figs. 4c and 4d). Subsequently, the cycle stability and coulombic efficiency began to decrease from 2C. The capacity at 0.05C after operation at 5C was 98.7 % of the ratio to that before 0.1C and hardly changed (126.7 → 125.1 mAh g⁻¹), indicating that the decrease in the capacity and coulombic efficiency at high C-rates was not derived from (electro)chemical degradation. The 1st discharge capacity of the cell with WM-LSnPS at 0.05C was 125.3 mAh g⁻¹, where the value is similar to the theoretical values (Fig. 4b). This implies that the LSnPS/LCO interface seems to be not thermodynamically deactivated. The 1st discharge capacity of the cell employing WM-LSnPS was slightly lower than that of the cell employing WM-LGPS. This seems to be because the relative density (≈contact among particles) of LSnPS was slightly lower than that of LGPS (Fig. S4). Furthermore, the slightly lower 1st discharge capacities of the LSnPS-based cell suggest that interphases accelerating reactions seem to be not thermodynamically formed. The capacities of the cell using WM-LSnPS decreased with increasing C-rates, as demonstrated in the charge-discharge properties of the cell using WM-LGPS. However, the cell with WM-LSnPS showed high cycle stability up to 2C. The capacity of the cell with WM-LSnPS did not decrease significantly over five cycles, even at 5C although that of the cell with WM-LGPS started to decrease from 2C. The Coulombic efficiency of the cell using WM-LSnPS was also higher at 5C than those of the cell using WM-LGPS. The capacity of the cell employing WM-LSnPS at 0.05C after operation at 5C was 98.7 % of the ratio to that before 0.1C and hardly changed (123.4 → 121.8 mAh g⁻¹). The data show that WM-LSnPS seem to be not (electro)chemically deactivated. Furthermore, the 98.7 % of the ratio for WM-LSnPS is same as that for WM-LGPS. This suggests that there are not significant differences in the (electro)chemical stability between WM-LSnPS and -LGPS both thermodynamically and kinetically. WM-LSnPS was mixed with LCO in a similar uniform manner to WM-LGPS, as shown in Fig. 5, indicating that the LCO/WM-LSnPS composite possessed a microstructure similar to that of LCO/WM-LGPS. WM-LSnPS was not so advantageous also in the number of grain boundaries (Fig. 2) depending on particle size distribution, the relative density (Fig. S4) meaning strength of contacts, and the ionic conductivity (Figs. 3, S2, and S3), comparing with WM-LGPS. The differences between LSnPS and LGPS tended to appear at high C-rates, as mentioned above. This suggests that LSnPS more effectively addresses the rapid volume change in LCO. Thus, the ability of LSnPS to adapt to volumetric changes in LCO was further evaluated.

Figure 4.

(a, b) Charge-discharge curves at 1.9–3.6 V vs. In-Li for composite | HG-LGPS | In-Li cells. (a) WM-LGPS, (b) WM-LSnPS were utilized for the composite cathodes. Charge and discharge were performed in CCCV and CC mode, respectively. C-rates in the CC processes were 0.05–5C. (c) Discharge capacity retention and (d) coulombic efficiencies of the cells employing WM-LGPS and -LSnPS.

Figure 5.

Cross-sectional SEM-EDX images of (a) LCO/WM-LGPS and (b) LCO/WM-LSnPS composites.

Li⁺ is progressively deintercalated from LiCoO₂ to form Li_0.5CoO₂, Li_0.3CoO₂, and Li_0.15CoO₂ upon charging at voltages of 3.6, 3.8, and 4.0 vs. In-Li (≈4.2, 4.4, and 4.6 vs. Li⁺/Li in Li-ion batteries), respectively.^34,35 The lattice constant, c, changed from 14.1 Å to 14.4, 14.3, and 13.8 Å owing to a change in the Li⁺ concentration, accompanied by a hexagonal → monoclinic → hexagonal phase change.^34,35 This means that the volume of LCO expands by up to 2 % and shrinks by up to −2 % along the c-axis even during charging only. Thus, the effect of the large volumetric change of LCO on the charge-discharge properties up to 3.8 and 4.0 vs. In-Li was investigated for the WM-LGPS and WM-LSnPS composites (Figs. 6 and S5). Although WM-LSnPS displayed slightly lower capacities at voltages up to 3.6 V compared to WM-LGPS (Fig. 4), the capacities of the cell with WM-LSnPS were obviously higher at voltages up to 3.8 V than those of the cell with WM-LGPS, over 50 cycles. The cell employing WM-LSnPS also exhibited a higher capacity retention rate (82.9 %) at the 50th cycle than that with WM-LGPS (67.7 %). Notably, WM-LSnPS retained approximately 99 % of its Coulombic efficiency over 5–50 cycles, whereas the Coulombic efficiency of WM-LGPS decreased unstably. The charge-discharge properties at voltages up to 4.0 V tended to be similar to those up to 3.8 V (Fig. S5). The ionic conductivity of WM-LSnPS (1.1 mS cm⁻¹) was similar to that of WM-LGPS (2.4 mS cm⁻¹), as shown in Figs. 3 and S2. In addition, electrochemical deactivation was negligible up to 3.6 V vs. In-Li (≈4.2 V vs. Li⁺/Li), as shown in Fig. 4. The authors previously reported that LCO itself is relatively stable at voltages up to 4.6 V vs. Li anode (≈4.0 V vs. In-Li)³⁶ and that LGPS and LSnPS are not drastically decomposed at voltages up to 5 V vs. Li anode (≈4.4 V vs. In-Li).^26,29 Moreover, the microstructure of the composite with WM-LSnPS was similar to that with WM-LGPS, as shown in Fig. 5. These points suggest that the composites with WM-LGPS and WM-LSnPS are similar from the viewpoints of the electrochemical and microstructural properties. Thus, the mechanical properties of WM-LSnPS and WM-LGPS may affect the charge-discharge properties of the composite cathodes.

Figure 6.

(a, b) Charge-discharge curves at 1.9–3.8 V vs. In-Li, (c) discharge capacity retention, and (d) coulombic efficiency of composite | HG-LGPS | In-Li cells. (a) WM-LGPS and (b) WM-LSnPS were utilized for the composite cathodes. Charge and discharge were performed in CCCV and CC mode, respectively. C-rate in the CC processes was 0.2C.

WM-LGPS and WM-LSnPS pellets were subjected to indentation tests, according to the authors’ previous report on LGPS,¹⁸ aiming to clarify the differences in their mechanical properties. Figure 7 shows the P-h curves of WM-LGPS and WM-LSnPS. WM-LGPS was penetrated by approximately 13 µm in each cycle. The penetration depth did not revert to zero after unloading. This irreversibility led to a total penetration depth of 35.6 µm at the 100th cycle. These results indicate that WM-LGPS was deformed both elastically and plastically, which is consistent with the authors’ previous report.¹⁸ The penetration depth of WM-LSnPS at each cycle was more than 20 µm, which is deeper than that of WM-LGPS. However, the penetration depth of WM-LSnPS was only 30.6 µm in the 100th cycle, which is shallower than that of WM-LGPS. These results indicate that WM-LSnPS underwent greater elastic deformation than WM-LGPS. To further investigate the differences in the deformability, the elastic moduli, yield stresses, and Meyer hardness were estimated (Table 1). The Meyer hardness of WM-LSnPS (0.34 GPa) was slightly lower than that of WM-LGPS (0.51 GPa). In contrast, the yield stress of WM-LSnPS (0.85 GPa) was slightly higher than that of WM-LGPS (0.71 GPa). The differences in the Meyer hardness and yield stress of WM-LSnPS and WM-LGPS were small. However, WM-LSnPS exhibited a higher elastic modulus (27.9 GPa) than WM-LGPS (15.7 GPa). This result is reasonable because the relative density of LSnPS is a little lower than that of LGPS, regardless of the particle size (Fig. S4). However, the relative densities did not drastically differ from each other with particle sizes and material kinds. This indicates that the intrinsic mechanical properties of WM-LSnPS and WM-LGPS were investigated as functions of kinds of materials. Consequently, the differences in the elastic modulus, yield stress, and Meyer hardness seem to be derived from the difference between Sn⁴⁺ and Ge⁴⁺. Sn⁴⁺ tends to be covalently bonded.³⁷ The covalent characteristics of Sn⁴⁺ may increase the strength of the bonds with anions. In fact, the average bond length of Sn-S2 (2.1074(19) Å) and Sn-S3 (2.1597(17) Å) (ICSD 252040) is shorter than that of Ge-S2 (2.1440(15) Å) and Ge-S3 (2.1477(14) Å) (ICSD 188887), even though the ionic radius of Sn⁴⁺ (CN4: 0.69 Å) is larger than that of Ge⁴⁺ (CN4: 0.53 Å).³⁸ This suggests that compared with Ge⁴⁺, Sn⁴⁺ forms stronger bonds with S²⁻. Thus, the SnS₄ units are harder than the GeS₄ units. Additionally, the lattice constants a (= b = 8.73764) and c (= 12.71660) of LSnPS were only 0.2 and 0.6 % larger than those of LGPS (a = b = 8.71900, c = 12.63900), respectively, regardless of the 30 % difference in the ionic radii of Sn⁴⁺ and Ge⁴⁺. This indicates that the ions were more densely packed in the crystal lattice of LSnPS than in that of LGPS. These crystallographic characteristics may be the origin of the higher elastic modulus and yield stress of WM-LSnPS compared to those of WM-LGPS. In contrast, the hardness of WM-LSnPS, which indicates its plastic deformability, was slightly lower than that of WM-LGPS. The hardness of pelletized powder samples is affected not only by the plastic deformability of the bulk particles, but also by the flow (migration) of the particles.¹⁸ The flow of particles depends on the particle size, shape, relative density (void), and other factors. The relative density of LSnPS is lower than that of LGPS, as shown in Fig. S4. Consequently, the LSnPS particles seem to flow more easily than the LGPS particles, leading to the lower hardness of WM-LSnPS. In fact, WM-LSnPS exhibited a larger hysteresis, indicating plastic deformation, in the 1st cycle than WM-LGPS, although WM-LSnPS underwent greater elastic deformation up to the 100th cycle than WM-LGPS. Cracking has been reported as a representative physical issue in ASSBs.^17,39,40 This suggests that both elastic and plastic deformation occur in ASSBs by crossing the yield stress. Thus, the elastic moduli, yield stress, and Meyer hardness should influence the mechanical properties of ASSBs. The difference in the elastic modulus indicates that WM-LSnPS requires a higher stress for elastic deformation than WM-LGPS. However, the differences in the P-h curve and yield stress prove that WM-LSnPS can undergo elastic deformation in a larger volumetric range than WM-LGPS. Thus, WM-LSnPS exhibited greater reversible deformability than WM-LGPS. The reversible deformability influenced the charge-discharge properties of the cell employing WM-LSnPS, as shown in Fig. 8. When LCO undergoes rapid and/or large expansion during charging at high C-rates and/or high voltages, WM-LSnPS and WM-LGPS are subjected to high stress by LCO. WM-LSnPS undergoes greater elastic deformation than WM-LGPS because WM-LSnPS possesses a higher elastic modulus, higher yield stress, and volumetrically-larger elastic-deformability, which means that the allowed range for volumetric amount of elastic deformation is greater than that of WM-LGPS. Therefore, when LCO shrinks after expansion, the elastically deformed WM-LSnPS reverts to its pristine shape. Therefore, the contacts between LCO and WM-LSnPS remain, resulting in a high capacity, capacity retention, and coulombic efficiency. In contrast, WM-LGPS undergoes greater plastic deformation than WM-LSnPS during the expansion of LCO. The plastically deformed WM-LGPS cannot revert to its pristine shape during LCO shrinkage, leading to a loss of contact with LCO. Consequently, the LCO/WM-LGPS composite exhibited low capacity, capacity retention, and coulombic efficiency at high C-rates and voltages. In this study, the charge-discharge tests were performed under the external pressure of 30 MPa. Stresses inside composite electrodes owing to volumetric change of active materials should increase and decrease as the external pressure increases and decreases, respectively. When increasing the external pressure, solid electrolytes would get easy to exceed their yield stresses because of the increase in the inner stress. This causes irreversible deformation of the solid electrolytes. In contrast, when decreasing the external pressure, the solid electrolytes should be hard to deform both elastically and plastically because the inner stresses also decrease. Then, particles of the active materials and the solid electrolytes get easy to be detached in the composite electrodes, resulting in contact loss. Therefore, we will have to select materials with suitable mechanical properties, depending on required properties of ASSBs utilized in each situation.

Figure 7.

P-h curves of pelletized (a) WM-LGPS and (b) WM-LSnPS. A spherical indenter (ϕ 12 mm) was used to apply a maximum load of 120 N at a rate of 0.03 mm min⁻¹.

Table 1. Mechanical properties of pelletized WM-LGPS and WM-LSnPS.

Sample	Elastic modulus/GPa	Yield stress/GPa	Meyer hardness/GPa
WM-LGPS	15.7	0.71	0.51
WM-LSnPS	27.9	0.85	0.34

Figure 8.

Proposed deformation of WM-LGPS and WM-LSnPS in the composite cathodes during charge–discharge.

Generally, materials with low elastic moduli have been demonstrated to be suitable for accommodating the volumetric change of the active materials in ASSBs.^13,19,20,23 However, the present results indicate that solid electrolytes with low elastic moduli are not absolutely suitable for composite electrodes in ASSBs. This is the opposite of general approaches. The most significant characteristics for achieving reversible deformation of solid electrolytes are a high yield stress and volumetrically-large elastic-deformability. These findings provide new strategies for developing composite electrodes for ASSBs.

4. Conclusion

This study successfully demonstrated that the elastic and plastic deformability of LGPS-type solid electrolytes affect the charge-discharge properties of the electrolytes, especially at high C-rates and/or voltages, resulting in rapid and/or large changes in the LCO volume. The high elastic modulus, high yield stress, and volumetrically-large elastic-deformability enabled LSnPS to deform reversibly in the LCO/LSnPS composite while maintaining contact with LCO, resulting in a high capacity, capacity retention, and coulombic efficiency. The results suggest that solid electrolytes with low elastic moduli are not absolutely suitable for composite electrodes in ASSBs, which contrasts with the general approach. A high yield stress and volumetrically-large elastic-deformability are particularly important for the reversible deformation of solid electrolytes. These findings provide new insights for the development of composite electrodes for ASSBs.

Acknowledgments

This study was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 19H05793) from the Japan Society for the Promotion of Science (JSPS), and Green Technologies for Excellence (GteX) Program (No. JPMJGX23S5) provided by the Japan Science and Technology Agency (JST).

The authors were waived from the APC with the support of The Committee of Battery Technology, ECSJ.

Data Availability Statement

The data that support the findings of this study and high-resolution original figures are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.28787720.

• 1) The charge-discharge properties of all-solid-state batteries are affected by both chemical and physical factors. Physical issues mainly arise from the microstructure of the composites and the mechanical properties of the solid electrolytes themselves. However, physical issues have been investigated by focusing on the microstructures of the composites rather than the mechanical properties of the solid electrolytes themselves. In this study, composite cathodes with similar microstructures were fabricated using LiCoO₂ as the active material and either Li_9.81Sn_0.81P_2.19S₁₂ or Li₁₀GeP₂S₁₂ as the solid electrolyte. The composite with Li_9.81Sn_0.81P_2.19S₁₂ exhibited higher capacity retention and coulombic efficiency with increasing C-rates at 1.9–3.6 V vs. In-Li than that with Li₁₀GeP₂S₁₂. Moreover, when charging–discharging at 1.9–3.8 V, where the expansion and shrinkage of LiCoO₂ were greater those at 1.9–3.6 V, the composite with Li_9.81Sn_0.81P_2.19S₁₂ exhibited a higher capacity, capacity retention, and Coulombic efficiency than those of the composite with Li₁₀GeP₂S₁₂. These results are attributed to the high elastic modulus, high yield stress, and volumetrically-large elastic-deformability, which enable Li_9.81Sn_0.81P_2.19S₁₂ to reversibly deform while maintaining contact with LiCoO₂, unlike Li₁₀GeP₂S₁₂. These results demonstrate that solid electrolytes with low elastic moduli are not absolutely suitable for all-solid-state batteries, and that a high yield stress and volumetrically-large elastic-deformability are especially significant for reversible deformation. These findings provide new insights for the development of composite electrodes for all-solid-state batteries.

CRediT Authorship Contribution Statement

Kenta Watanabe: Conceptualization (Equal), Data curation (Lead), Formal analysis (Lead), Supervision (Equal), Writing – original draft (Lead), Writing – review & editing (Lead)

Hideaki Nakayama: Investigation (Lead)

Han-Seul Kim: Investigation (Supporting)

Kazuhiro Hikima: Data curation (Lead), Investigation (Lead), Writing – review & editing (Lead)

Naoki Matsui: Supervision (Supporting)

Kota Suzuki: Supervision (Supporting)

Satoshi Obokata: Investigation (Equal)

Hiroyuki Muto: Supervision (Supporting)

Atsunori Matsuda: Supervision (Equal)

Ryoji Kanno: Supervision (Equal)

Masaaki Hirayama: Conceptualization (Lead), Funding acquisition (Lead), Methodology (Equal), Project administration (Lead), Supervision (Lead), Writing – review & editing (Equal)

Conflict of Interest

The authors declare no conflict of interest.

Funding

Japan Society for the Promotion of Science: 19H05793

Japan Science and Technology Agency: JPMJGX23S5

Footnotes

Part of this paper was presented at the 65th Battery Symposium in Japan in 2024 (Presentation #1G20).

K. Watanabe, K. Hikima, N. Matsui, K. Suzuki, A. Matsuda, and M. Hirayama: ECSJ Active Members

R. Kanno: ECSJ Fellow

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