Modelling electro-chemically induced stresses in all-solid-state batteries: Screening electrolyte and cathode materials in composite cathodes

All-solid-state lithium batteries (ASSBs) are gaining significant attention worldwide as one of the most promising alternatives to lithium-ion batteries due to their superior safety and potentially higher energy density. However, one of the main problems of known ASSBs remains rapid capacity degradation, which needs to be addressed before their large-scale market introduction. One important degradation mechanism is the mechanical fatigue of the cathode layer due to the volume change of the cathode active material (CAM) during cycling. Quasi-zero-strain CAMs such as LixNi0.3Co0.6Mn0.1O2 (NCM361) and LixNi0.2Co0.7Mn0.1O2 (NCM 271) could solve this problem, but their use in ASSBs has not been investigated yet. We theoretically investigate the suitability of these CAMs in composite cathodes with various solid electrolytes such as poly(ethylene oxide) (PEO), Li3PS4 (LPS), Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li7La3Zr2O12 (LLZO) with respect to the mechanical stresses occurring at microscopic grain level and compare them with LixNi0.9Co0.05Mn0.05O2 (NCM955) and LiCoO2 (LCO). Although the quasi-zero-strain materials develop stresses in the GPa range during cycling, they still exhibit the lowest stresses of all the CAMs studied and could be of particular interest when using stiff electrolytes such as LATP or LLZO. High-capacity NCMs such as NCM955 exhibit a large volume change and should preferably be used together with electrolytes with bulk modulus less than 25 GPa such as PEO and LPS. While for soft electrolytes such as PEO and LPS the difference between the lattice strains along the different axes of the active material determines the stresses, for stiff electrolytes such as LATP and LLZO the total volume change is more important. Finally, a method is introduced to determine the stresses quickly from the free macroscopic strain mismatch without stress simulations.