K-Rich Spinel Interface of Air-Stable Layered Oxide Cathodes for Potassium-Ion Batteries

Potassium-containing transition metal layered oxides (K_xTmO₂), although possessing high energy density and suitable operating voltage, suffer from severe hygroscopic properties due to their two dimensional (2D) layered structure. Their air sensitivity compromises structural stability during prolonged air exposure, therefore increasing the cost. The common sense for designing air-stable layered cathode materials is to avoid contact with H₂O molecules. In this study, it is surprisingly found that P3-type K_xTmO₂ forms an ultra-thin, potassium-rich spinel phase wrapping layer after simply water immersion, remarkedly reduces the reaction activity of the material's surface with air. Combined with Density Function Theory (DFT) calculations, this spinel phase is found to be able to effectively withstand air deterioration and preserving the crystal structure. Consequently, the water-treated material, when exposed to air, can largely maintain its good electrochemical performance, with capacity retention up to 99.15% compared to the fresh samples. Such an in situ surface phase transformation mechanism is also corroborated in other K_xTmO₂, underscoring its effectiveness in enhancing the air stability of P3-type layered oxides for K⁺ storage.