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‐y Co x Mn y O 2 (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 Li 2 CO 3 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 Li 2 O. 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 Li 2 O into Li 2 CO 3 and formation of fresh Li 2 CO 3 on the surface. Notably, the beneficial effects are not replicated with pure CO 2 or moisturized air storage, emphasizing the significance of storage conditions and reaction pathways for Li 2 CO 3 formation. This study demonstrates that removal of Li 2 O residuals through the formation of Li 2 CO 3 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.