Effects of volume variations under different compressive pressures on the performance and microstructure of all-solid-state batteries

Compressive pressure applied during the operation of all-solid-state batteries that employ a sulfide solid electrolyte (SE) assists particle contact, thereby maintaining the ionic and electronic conduction network. The relationship between compressive pressure and volume variation in active materials is critical for designing practical full-cells with enhanced cell-based energy densities. However, studies into these aspects are rare. Here, we systematically investigate the effects of volume change of active materials [silicon, graphite, and LiNi1/3Mn1/3Co1/3O2 (NMC)] at different pressures (75 and 50 MPa) on electrochemical performance, cell internal resistance, and microstructure of full-cells with a thin SE layer (approximately 75-μm-thick). Pressurization at 75 MPa and use of graphite with lower expansion ratios improves capacity and capacity retentions. Increasing variation in the negative electrode volume increases charge-transfer resistance and crack formation in the NMC-composite layer. This indicates that the buffering effect via the elastic deformation of the thin SE layer is insufficient. Pressure facilitates plastic deformation of LixSi and SE, resulting in their improved contact, while perpendicular cracks appear throughout the Si-composite layer, effectively alleviating stress derived from variations in the volume of Si. This study provides important mechanistic insights into the design of advanced active materials and batteries required for industrial applications.