Interphase Stabilization of LiNi0.5Mn1.5O4 Cathode for 5 V‐Class All‐Solid‐State Batteries

Abstract Employing high voltage cobalt‐free spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) as a cathode is promising for high energy density and cost‐effectiveness, but it has challenges in all‐solid‐state batteries (ASSBs). Here, it is revealed that the limitation of lithium argyrodite sulfide solid electrolyte (Li 6 PS 5 Cl) with the LNMO cathode is due to the intrinsic chemical incompatibility and poor oxidative stability. Through a careful analysis of the interphase of LNMO, it is elucidated that even the halide solid electrolyte (Li 3 InCl 6 ) with high oxidative stability can be decomposed to form resistive interphase layers with LNMO in ASSBs. Interestingly, with Fe‐doping and a Li 3 PO 4 protective layer coating, LNMO with Li 3 InCl 6 displays stable cycle performance with a stabilized interphase at a high voltage (≈4.7 V) in ASSBs. The enhanced interfacial stability with the extended electrochemical stability window through doping and coating enables high electrochemical stability with LNMO in ASSBs. This work provides guidance for employing high‐voltage cathodes in ASSBs and highlights the importance of stable interphases to enable stable cycling in ASSBs.