Dual Regulation of Surface Residual Alkali and Sodium Layer Spacing for Enhanced Air Stability and Sodium-Ion Kinetics in O3-Type Cathodes

Cost-effective sodium-ion batteries (SIBs) are an important complementary technology to lithium-ion batteries for large-scale energy storage. O3-type layered oxides are promising cathode materials for sodium-ion batteries, but their development is constrained by slow Na+ transfer kinetics and air sensitivity. Herein, to address these issues, the effects of surface residual alkali and sodium layer spacing on the electrochemical performance and air stability of O3-NaxCu1/9Ni2/9Fe1/3Mn1/3O2 (x = 1.00, 0.95, 0.93, 0.91 and 0.89) were investigated. It is found that reducing the Na content could reduce the surface residual alkali content and lower the Na+ diffusion barrier. However, reducing the Na content also led to an expansion of the sodium layer spacing, making active Na+ ions more prone to spontaneous extraction. A moderate reduction in Na content led to significant suppression of surface residual alkali and a slight expansion in Na layer spacing, as exemplified by the Na0.93CNFM sample, enabling improved air stability, rate performance, and cycling stability. We have uncovered the influence of surface residual alkali and Na layer spacing on the air stability of O3-type layered NaxCu1/9Ni2/9Fe1/3Mn1/3O2. Our findings shed light on the underlying mechanism that affects the air sensitivity of the layered cathode materials for SIBs.