In situ epitaxial growth and electrochemical conversion of LiNi _0.5 Mn _1.5 O ₄ thin layer on Ni-rich cathode materials for high voltage lithium-ion batteries

Ni-rich LiNi_xCo_yMn_1-x-yO₂ (0.5 < x < 1) cathode materials have attracted considerable interest due to their high energy density and low cost. However, they are subject to capacity fading during cycling, such as structural degradation and irreversible oxygen release, especially under high voltage. Herein, we report an in situ epitaxial growth strategy to construct a thin layer of LiNi_0.25Mn_0.75O₂ on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811). Both of them share the same crystal structure. Interestingly, the LiNi_0.25Mn_0.75O₂ layer can be electrochemically converted into a stable spinel LiNi_0.5Mn_1.5O₄ (LNM) due to the Jahn-Teller effect under high voltage cycling. The derived LNM protective layer can effectively alleviate the harmful side reactions between the electrode and electrolyte and suppress oxygen release as well. Furthermore, the coating LNM layer can enhance Li⁺ ion diffusion due to its three-dimensional channels for Li⁺ ion transport. When used as half-cells with lithium as the anode, NCM811@LNM-1% realizes a large reversible capacity of 202.4 mA h g^-1 at 0.5 C, with high capacity retention of 86.52% at 0.5 C and 82.78% at 1 C, respectively, after 200 cycles in the voltage range of 2.8-4.5 V. Moreover, the assembled pouch full-cell with NCM811@LNM-1% as cathode and commercial graphite as an anode can deliver 11.63 mA h capacity with a high capacity retention of 80.05% after 139 cycles in the same voltage range. This work demonstrates a facile approach to the fabrication of NCM811@LNM cathode materials for enhancing performance in lithium-ion batteries under high voltage, rendering its promising applications.