Tavorite-like orthorhombic AVPO4F (A = Li, Na) for novel high-voltage cathodes in rechargeable batteries

We theoretically propose novel orthorhombic phase of tavorite-like AxVPO4F cathodes (A = Li, Na and 0 ≤ x ≤ 1) applicable for Li-ion and Na-ion batteries. Phase stability, electronic structures, electrochemical properties, and diffusion mechanisms of A ions and vacancies of AxVPO4F are clarified using the density functional method. Both LiVPO4F and NaVPO4F would experience bi-phase reactions upon the A extractions for 0.125 ≤ x ≤ 0.5 and 0.5 ≤ x ≤ 1, and exhibit high voltages of 4.12 and 3.80 V and the high capacities of 156 and 143 mAh g−1, respectively. The effect of small polaron on the ion diffusion mechanism is carefully addressed. The activation energies of Na ion diffusion gain 323 and 530 meV whilst they lower to 95 and 240 meV for Li ion diffusion in the Na/Li-rich and poor phases, respectively. Therefore, the proposed cathodes can be expected to exhibit significantly faster Li/Na-ion diffusion than that in popular cathodes such as lithium olivine phosphate LiFePO4 (430 meV), high-rate triclinic LiVPO4F (418 meV) and NaVOPO4 (627 meV).