Self-Discharge Mechanism of High-Voltage KVPO4F for K-Ion Batteries

Abstract

Current performances of Li-, Na-, or K-ion batteries are mainly limited by the specific capacity of the positive electrode. Therefore, it is important to reach the highest capacity possible for a given electrode material. Here, we investigate the performance limitation of KVPO4F, a prospective material for K-ion batteries, which can deliver only 80% of its theoretical capacity. We discover that the capacity limitation of KVPO4F is related to a kinetic competition between K+deinsertion and side reactions ascribed to the electrolyte degradation at high potentials. Homeotypic VPO4F can be obtained from KVPO4F through a chemical deintercalation process, which disproves a possible structural limitation or instability. The deintercalated compound was characterized by electron and X-ray diffraction, X-ray absorption spectroscopy, and nuclear magnetic resonance spectroscopy. Despite the structural stability, a spontaneous reaction occurs between the deintercalated KxVPO4F (x < 0.5) and the electrolyte (0.8 M KPF6in ethylene carbonate/diethylene carbonate), with an electron transfer to vanadium compensated by K+intercalation. This reaction leads to self-discharge until the open circuit potential is lower than 4.7 V versus K+/K, corresponding to the K0.5VPO4F composition.