Tetrahedral Occupied V Ions Enabling Reversible Three-Electron Redox of Cr³⁺/Cr⁶⁺ in Layered Cathode Materials for Potassium-Ion Batteries

Reversible three-electron redox of Cr3+/Cr6+ in layered cathode materials for rechargeable batteries is very attractive in layered cathode materials, which leads to high capacity and energy density for rechargeable batteries. However, the poor reversibility and Cr-ion migration make it very challenging. In this work, by introducing V ions into tetrahedral sites of layer-structured NaCrO2, reversible three-electron redox of Cr3+/Cr6+ is realized successfully in NaCr0.92V0.05O2 (NCV05) cathode for potassium-ion batteries with a cut-off voltage of 4.0 V. V ions can weaken the attraction of Cr to electrons, leading to enhanced valence change of Cr ions. On the other hand, V in tetrahedral sites can facilitate the reversible migration of Cr between octahedral and tetrahedral sites via coulombic repulsion to realize the reversible redox between Cr3+ and Cr6+ during charge and discharge processes. In addition, V ions can inhibit the phase transition from O-3 phase to O'3 phase during the charge process by adjusting the crystal lattices. As a result, the NaCr0.92V0.05O2 cathode exhibits a high reversible capacity of 130 mAh g(-1) with promising cycle stability and rate capability. The strategy opens new opportunity for developing high-capacity cathode materials for potassium-ion batteries.