Thermal stabilities of Mn-based active materials in combination with the ceramic electrolyte LATP for ASSB bulk cathodes

Ternary composite bulk cathodes consisting of particulate active material (AM), solid-state electrolyte (SSE) and electrical conductor are essential to achieve competitive ceramic all solid-sate batteries (ASSBs). Firmly bonded contacts between AM and SSE as well as between the SSE particles are required to obtain fast Li-ion transfer and, thus, a good electrochemical performance. Consequently, sintering processes are unavoidable. However, decomposition processes, the formation of mixed phases and interdiffusion can take place during high temperature annealing, so that non-conductive or electrochemically inactive phases can be formed, which significantly reduce the ASSB performance. Consequently, a thermally stable material combination needs to be found. By understanding the thermodynamical processes, the selection of components can be simplified or the heat treatment can be optimized. For that purpose, this study investigates the thermal stabilities of the three Mn-based AMs LiMn2O4 (LMO-s), LiMnO2 (LMO-l) and LiMnPO4 (LMP) in combination with the ceramic electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP). It can be demonstrated, that LMO-s and LMO-l already decompose below 500 °C due to reduction of the Mn transition metal and the formation of oxygen gas. It results in a porous multicomponent composite, which is unusable for the application in ASSBs. In contrast, the powder mixture of LMP and LATP is thermally stable up to 800 °C in argon atmosphere and shows a dense microstructure. The addition of Ag as electrical conductor to LMP and LATP does not have an impact on the thermal stability, so that this material combination is promising for further ASSB bulk cathode development.