Multi-Functional Na_3-3xY_xPO₄ Coating for High-Performance O3-Type Single-Crystal Layered Oxide Cathodes in Sodium-Ion Batteries

O3-type layered oxide cathodes have emerged as one of the most promising candidates for high performance sodium-ion batteries, attributed to their high specific capacity and facile synthesis process. However, the irreversible multiphase transition, severe interfacial side reactions, and surface residual alkali issues lead to inferior cycling stability and rate performance of O3-type cathodes. Herein, a multi-functional modification integrating a Na_3-3xY_xPO₄ (NYP) coating, surface residual alkali removal, and bulk Y doping was proposed in O3-NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) single-crystal particles. The NYP coating layer functions not only as a protective barrier to suppress interfacial side reactions but also as a rapid channel for Na⁺ transport, thereby effectively mitigating interfacial deterioration and optimizing Na⁺ diffusion kinetics. Simultaneously, the residual alkali on the cathode material is consumed to construct the NYP coating layer, eliminating its detrimental effect. Additionally, Y incorporation effectively reinforces the lattice structure through the formation of robust Y-O bonds, significantly suppressing structural degradation during cycling. Benefiting from the synergistic effects of surface and bulk modifications, the modified cathode (NYP-0.5@NFM) exhibits superior electrochemical performance, with a capacity retention of 91.67 % after 100 cycles at 1C, and a discharge capacity of 106 mAh g⁻¹ at 5C, significantly outperforming the pristine NFM cathode.