Insight into the Extreme Side Reaction between LiNi0.5Co0.2Mn0.3O2 and Li1.3Al0.3Ti1.7(PO4)3 during Cosintering for All-Solid-State Batteries

All-solid-sate batteries (ASSBs) with a NASICON-type solid-state electrolyte (SSE) of Li1.3Al0.3Ti1.7(PO4)3 (LATP) can be accepted as a promising candidate to significantly improve safety and energy density due to their high oxidation potential and high ionic conductivity. However, thermodynamic instability between the cathode and LATP is scarcely investigated during cosintering preparation for the integrated configuration of ASSBs. Herein, the structural compatibility between commercially layered LiNi0.5Co0.2Mn0.3O2 (NCM523) and LATP SSE was systematically investigated by cosintering at 600 °C. It is noticeable that an extreme side reaction between Li from NCM523 and phosphate from LATP happens during its cosintering process, leading to a severe phase transition from a layered to a spinel structure with high Li/Ni mixing. Consequently, the capacity of NCM523 is lost during the preparation of the NCM523–LATP composite cathode. Based on this, we suggested that the interface modification of the NCM523/LATP interface is valued significantly to inhibit this extreme side reaction, quickening the application of LATP-based ASSBs.