Development of K4Fe3(PO4)2(P2O7) as a novel Fe-based cathode with high energy densities and excellent cyclability in rechargeable potassium batteries

We introduce K4Fe3(PO4)2(P2O7) as a novel cathode material with superior electrochemical performance for K-ion batteries. First-principles calculation is used to predict the theoretical properties and detailed K+ storage mechanism of K4Fe3(PO4)2(P2O7), which are consistent with experimental results. K4Fe3(PO4)2(P2O7) exhibits a large specific discharge capacity of ~118 mAh g−1, approaching the theoretical capacity, at C/20 (1C ​= ​120 ​mA ​g−1) in the voltage range of 2.1–4.1V (vs. K+/K), allowing ~3 ​mol of K+ de/intercalation per formula unit with a small volume change of ~4% during charge/discharge. Even at 5C, up to ~70% of its theoretical specific capacity is retained, and this outstanding power-capability is related to the low activation barrier energy for K+ diffusion, as verified through first-principles calculations. Furthermore, K4Fe3(PO4)2(P2O7) exhibits excellent cyclability with retention of ~82% of the initial capacity after 500 cycles at 5C. The above theoretical and experimental results suggest the feasibility of using K4Fe3(PO4)2(P2O7) as a cathode material for rechargeable potassium batteries.