Li-vacant topotactic subsurface Pathways: A Key to stable Li-ion storage and migration in LiNi_0.5Mn_1.5O₄ Cathodes

High-voltage LiNi_0.5Mn_1.5O₄ (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.