Mg Substitution Improves the Surface Stability of High‐Voltage Spinel Cathodes under Accelerated Aging Conditions

Abstract LiNi _0.5 Mn _1.5 O ₄ (LNMO) cathodes offer a cobalt‐free, high‐voltage alternative to current state‐of‐the‐art Li‐ion battery cathodes, and are particularly well‐suited for high‐power applications due to their 3D lithium‐ion pathways and structural stability. However, degradation of commercial electrolytes at high voltages exacerbates capacity decay, as instability at the cathode surface causes active material loss, surface reconstructions, thickening surface layers, and increases in internal cell resistance. Cationic substitution has been proposed to enhance surface stability, thus limiting capacity decay. Here, we demonstrate the stabilizing effect of Mg on the LNMO cathode surface, which is most evident during the early stages of cycling. This study indicates that improved O 2 p ‐TM 3 d hybridization in Mg‐substituted LNMO, facilitated by Li‐site defects, leads to the formation of a stable surface layer that is corrosion‐resistant at high voltage. Examination of Fe‐substituted and unsubstituted LNMO further confirms that the surface stability is uniquely enabled by Mg substitution. This work offers valuable insights into surface design for reducing degradation in high‐voltage spinel cathodes.