Li Chemical Potential Driven Phase Stability of NCM–Oxide Electrolyte Co‐Sintered Composite Cathodes

All‐solid‐state batteries (ASSBs) employing oxide electrolytes and Ni‐rich cathodes are seen as a promising candidate for next‐generation energy storage systems due to their superior safety and high energy density. However, the chemical compatibility between cathode materials and oxide electrolytes during co‐sintering for fabricating integrated ASSBs remains largely underexplored. Herein, the chemical compatibility between the LiN i0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathode and three representative oxide electrolytes, namely Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP), Li 0.33 La 0.56 TiO 3 (LLTO), and Ta‐doped Li 7 La 3 Zr 2 O 12 (LLZTO), was systematically studied. It is found that NCM811 undergoes much more severe side reactions with LATP and LLTO than with LLZTO during co‐sintering. Upon co‐sintering with LATP/LLTO, NCM811 suffers from lithium loss and undergoes a phase transition from a layered structure to spinel and even rock‐salt phases at high temperatures, accompanied by Li/Ni cation mixing. In contrast, NCM811 maintains favorable phase stability when co‐sintered with LLZTO, which exhibits a higher Li chemical potential than NCM811. The Li chemical potential is identified as a thermodynamic descriptor for evaluating the phase stability of oxide‐based composite cathodes during co‐sintering, which provides rational guidelines for oxide electrolyte selection in composite cathode design. Moreover, feasible optimization strategies are proposed to guide future research and industrialization, thereby accelerating the practical application of oxide‐based ASSBs.