Surface oxygen-locked LiNi0.6Mn0.4O2: Towards stable cycling at 4.7 V

Abstract

The mid-nickel cobalt-free LiNi_0.6Mn_0.4O₂(NM64) material, which avoids the hexagonal 2 (H2) to hexagonal 3 (H3) transition at high voltages, is optimal cathode material known for its stable structure. Nevertheless, capacity degradation caused by the release of lattice oxygen on the electrode surface hinders its industrial application. Herein, a surface oxygen locking layer (LiAlO₂) is constructed to fix the oxygen at the surface of NM64 secondary particles. The in-situ construction of LiAlO₂ protective layer on the surface of NM64 secondary particles reduces the formation of surface oxygen vacancies and inhibits oxygen-induced electrolyte decomposition at high voltage. Consequently, it helps minimize side reactions at the electrolyte/cathode interface. Notably, NM64 with the addition of a protective layer exhibits long-cycle stability (80 % retention after 400 cycles) under 3.0–4.5 V. Furthermore, adding a protective layer enhances capacity retention from 59.7 % to 81.5 % after 300 cycles at ultra-high voltage 4.7 V. LiAlO₂ protective layer oxygen-locking strategy enhances the stability and capacity retention of NM64 at high voltages, which proposes a new insight into suppressing the lattice oxygen evolution under high voltage.