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-Na_<i>x</i>Cu_1/9Ni_2/9Fe_1/3Mn_1/3O₂ (<i>x</i> = 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 Na_0.93CNFM 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 Na_<i>x</i>Cu_1/9Ni_2/9Fe_1/3Mn_1/3O₂. Our findings shed light on the underlying mechanism that affects the air sensitivity of the layered cathode materials for SIBs.