O3-Type NaCrO₂ as a Superior Cathode Material for Sodium/Potassium-Ion Batteries Ensured by High Structural...

O3-type NaCrO₂ is attracting increasing attention as potential cathode material for sodium-ion batteries (SIBs). Bare NaCrO₂ is usually synthesized by a solid-state reaction and suffers from serious capacity decay and poor power capability. Modification by coating is an effective method to improve the electrochemical properties, but it inevitably reduces the energy density. To avoid the decrease of energy density and optimize the electrochemical performance, a specific route, i.e., a freeze-drying-assisted sol-gel method, has been adopted to synthesize bare NaCrO₂ in this work. Three-phase coexistence during charging is confirmed for the first time, which contributes to delaying the disappearance of the O3 phase and then improving the structural reversibility, resulting in superior cycle stability (∼50% capacity retention after 3000 cycles at 5C). Meanwhile, as-synthesized NaCrO₂ delivers an outstanding rate capability (82.1 mAh g^-1 at 50C), which is attributed to the fast Na⁺ diffusivity and high electronic conductivity proved by density functional theory (DFT) calculations. It is worth mentioning that NaCrO₂ also exhibits excellent electrochemical properties when used as a cathode for potassium-ion batteries (PIBs). This work provides new perspectives on the structural evolution of NaCrO₂, and the results are expected to contribute to the development of SIBs and PIBs.