Li-vacant topotactic subsurface Pathways: A Key to stable Li-ion storage and migration in LiNi0.5Mn1.5O4 Cathodes

High-voltage LiNi0.5Mn1.5O4 (LNMO) with a spinel structure holds great promise for enhancing the performance of Li-ion rechargeable batteries (LIBs) in the mobility industry. A critical challenge remains in stabilizing Li-ion storage and migration within these cathode materials. Surface engineering emerges as a pivotal technology, significantly influencing the chemical status of LNMO particle surfaces through the application of specific functional materials. In this study, we present a novel synthesis of disordered spinel LNMO cathode materials (space group: Fd-3 m) featuring a Li-vacant topotactic subsurface and an external surface modified with K2CO3, achieved via a KOH-assisted wet chemistry approach. The LNMO particles are categorized into three distinct regions based on two compositional boundaries: bulk, (inner) subsurface, and (external) surface. The delithiated subsurface exhibits an intensified ordered arrangement of Ni and Mn, minimizing the formation of detrimental impurity phases while promoting efficient Li-ion transport throughout the spinel lattice. Furthermore, the incorporation of K2CO3 provides chemical protection to the external surfaces of LNMO particles, effectively mitigating H2O adsorption and oxidative electrolyte decomposition. These synergistic effects culminate in remarkable electrochemical performance (reversible discharge capacity: -110 mA h/g at a current density of 0.2C; discharge capacity retention: -97 % after 100 cycles) and thermal stability of LNMO, offering significant insights for the advancement of superior high-voltage cathode materials for next-generation LIBs.