VPO₄ F Fluorophosphates Polyanion Cathodes for High-Voltage Proton Storage

Proton batteries are emerging in electrochemical energy storage because of the associated fast kinetics, low cost and high safety. However, their development is hindered by the relatively low energy density due to the limited choice of cathode materials. Herein, metal phosphate polyanion cathodes are proposed as the proton cathode for the first time. Combining experimental results and theoretical simulations, a universal criterion for the proton cathode was put forward. Vanadium fluorophosphate (VPO₄ F) was demonstrated as a promising high-voltage proton cathode material with a specific capacity of 116 mAh g^-1 at a high potential of 1.0 V (vs. SHE). The proton insertion/extraction mechanism in the VPO₄ F electrode was also verified through X-ray diffraction (XRD) and photoelectron spectroscopy (XPS). Furthermore, the stability of VPO₄ F was investigated in various electrolytes and the optimized electrolyte enabled the stable operation of VPO₄ F for 300 cycles. This work provides new inspiration in the exploitation of new electrode materials for electrochemical proton storage devices.