Vanadium-Based Pyrophosphate Material K₂(VO)₃(P₂O₇)₂ as a High Voltage Cathode for Potassium Ion Batteries

Vanadium-based polyanionic cathode materials are attractive in secondary batteries for their high working platform, abundant redox centers, robust framework, and high safety. Here we report for the first time a vanadium-based polyanionic compound K-2(VO)(3)(P2O7)(2) as a potential potassium storage material. The material exhibits a high average discharge potential of 3.6 V (vs K+/K), and it can deliver a specific discharge capacity of 84 mAh g(-1), which is close to its 2 K theoretical capacity value of 85 mAh g(-1), involving a two-electron-transfer V5+/V4+ redox potential located at 4.0 V (vs K/K+). By ex situ XRD and XPS analyses, the K-storage mechanism in K-2(VO)(3)(P2O7)(2) is identified as a solid solution single-phase reaction involving V4+/V5+ redox with a tiny volume change (only 1.6%).