Tetrahedral Occupied V Ions Enabling Reversible Three-Electron Redox of Cr 3+ /Cr 6+ in Layered Cathode Materials for Potassium-Ion Batteries.

Reversible three-electron redox of Cr 3+ /Cr 6+ 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 NaCrO 2 , reversible three-electron redox of Cr 3+ /Cr 6+ is realized successfully in NaCr 0.92 V 0.05 O 2 (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 Cr 3+ and Cr 6+ during charge and discharge processes. In addition, V ions can inhibit the phase transition from O3 phase to O'3 phase during the charge process by adjusting the crystal lattices. As a result, the NaCr 0.92 V 0.05 O 2 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.