Mechanism of Internal Crack Formation in All-Solid-State Batteries Due to Expansion and Shrinkage and Its Effects
Authors: Eto Yuki , So Magnus , Yano Takeru , Inoue Gen 
Published: December 2024

Si anode, a high-capacity active material, is expected to be used in all-solid-state batteries, which are expected to be put into practical use as soon as possible in terms of safety and output characteristics. However, Si expands about three times during charging, which may cause cracks between the solid electrolyte (SE) and active material (AM). Cracks can cause a break in the Li+ conduction path and a decrease in the interfacial area and can also lead to Li metal precipitation (dendrites), which is a cause of short circuits (1). The mechanism of crack formation due to AM expansion and contraction during charging and discharging and its effects have not been fully elucidated, so in this study, we calculated the void formation in a particle-deposited structure using the discrete element method (DEM). The conductivity and interfacial area that change dynamically due to crack formation were determined, and their effects on battery performance were investigated quantitatively.
DEM was used to account for the elastoplastic deformation of sulfide-based SE. Simulations were performed using the equations from previous studies (2), (3). DEM is a method for calculating the forces acting between individual particles and the stress distribution in a particle-filled structure, assuming springs for elasticity and dashpots for viscous damping between contacting particles. SE is a single particle of 75Li2S-25P2S5 and AM is an aggregate consisting of several primary particles of Si with an expansion coefficient of 3.08. The particle diameters are 2 μm and 10 μm, respectively. The spring constants between contacting particles were calculated from the effective Young's modulus and effective radius of each particle. Ion conductivity and coverage changes during anode expansion and contraction were calculated for three phases of porous electrode layers: AM, SE, and void, and their frequency distributions were obtained. The calculations assume that the Si particles expand and contract uniformly. The periodic boundary condition was applied, and calculations were performed for the top and bottom surfaces under three conditions of constant confining pressure (5, 100, and 340 MPa) and a total of four conditions of constant thickness and varying confining pressure.
At a constant pressure, a void was formed during the process of AM expansion at the end of charging, while at a constant thickness, a void was formed during the process of AM contraction at the end of discharging. Although these two crack morphologies have been reported in previous studies (4), (5). the formation mechanism, which has not been clarified, can now be clarified by numerical calculations. Based on this structure, the relative conductivity of the SE phase and the coverage between SE and AM were calculated and frequency distributions were obtained. It was found that the pore formation mechanism differs depending on the restraint conditions, and that the effects on the relative conductivity and coverage are different.
The mechanism of crack initiation due to AM expansion and contraction during charging and discharging was clarified by DEM calculations for each constraint condition. The influence of cracks was replaced by relative conductivity and coverage, and battery performance could be predicted. In the future, it is necessary to improve the accuracy of the calculation prediction by comparing with the results of model experiments and actual measurements. Through these efforts, we aim to establish the basic technology for the early commercialization of all-solid-state batteries. Furthermore, this calculation technique should be applied not only to all-individual batteries but also to all lithium-ion batteries.
This study was supported by JST-Mirai Program (JPMJMI24G1), Japan.
References
(1) G. McConohy et al., Nature Energy (2023).
(2) M. So and G. Inoue et al., MethodsX, 9, 101857 (2022).
(3) M. So and G. Inoue et al., J. Power Sources, 546, 231956 (2022).
(4) Xiaohan Wu et al., Adv. Ene. Mate., 9, 1901547 (2019).
(5) Yamamoto et al., JPS., 473, 228595 (2020).