The Formation of Residual Lithium Compounds on Ni‐Rich NCM Oxides: Their Impact on the Electrochemical Performance of Sulfide‐Based ASSBs

Abstract Residual lithium compounds (RLCs) are known to form on the surface of nickel‐rich LiNi_1‐x‐yCo_xMn_yO₂ (NCM) oxides during synthesis and storage. In this study, the impact of RLCs on cathode performance in sulfide‐based all‐solid‐state batteries (ASSBs) is investigated by employing practically relevant approaches to generate (or remove) RLCs on (or from) NCM single crystal particles. It is revealed that Li₂CO₃ is the predominant component in samples exposed to air. Surprisingly, heat treatment at high temperatures does not remove RLCs but increases the overall RLC content, accompanied by the partial transformation of existing RLCs into Li₂O. These samples exhibit compromised electrochemical performance due to asymmetric overpotential increase during cell discharge. However, it is possible to recover performance through controlled ambient air storage which enables the conversion of existing Li₂O into Li₂CO₃ and formation of fresh Li₂CO₃ on the surface. Notably, the beneficial effects are not replicated with pure CO₂ or moisturized air storage, emphasizing the significance of storage conditions and reaction pathways for Li₂CO₃ formation. This study demonstrates that removal of Li₂O residuals through the formation of Li₂CO₃ under controlled ambient air exposure proves to be advantageous for sulfide‐based ASSBs, thereby offering valuable guidance for the development of optimized NCM‐based ASSB systems.