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

Reversible three-electron redox of Cr /Cr 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 , reversible three-electron redox of Cr /Cr is realized successfully in NaCr V O (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 and Cr 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 V O cathode exhibits a high reversible capacity of 130 mAh g with promising cycle stability and rate capability. The strategy opens new opportunity for developing high-capacity cathode materials for potassium-ion batteries.