Gradient Sodium Deficiency Optimization in O3-Type Cathode Materials for Superior Performance and Air Stability.

O3-type layered oxides are promising cathode materials for sodium-ion batteries due to their easy synthesis and high sodium content. However, complex phase transitions and poor air stability limit their practical applications. Introducing sodium deficiency suppresses reactions with air and improves phase stability, but often at the cost of significantly compromising the sodium storage capacity. Herein, we present a hierarchical composition regulation strategy to achieve radial concentration control of sodium in the O3-type layered oxides, constructing radially distributed sodium gradients. The gradient Na content structure not only can alleviate the volume changes caused by the O3-P3 phase transition, which minimizes the degradation of electrochemical performance during cycling, but also suppresses Na/H exchange. This ensures enhanced air stability, improved kinetic performance, and cycling stability. The modified cathode material exhibits a capacity retention rate of 93.37% after 400 cycles at 5 C. When exposed to 82% relative humidity, CO concentration of 3044 ppm for 10 h, it still maintains a specific capacity of 84.9 mA h g after 300 cycles at 1 C, with a capacity retention rate of 77.27%. This work provides a strategy for radial sodium concentration control, contributing to the development of high-performance and air-stable O3-type sodium-ion battery cathode materials.