Development of KPF₆/KFSA Binary-Salt Solutions for Long-Life and High-Voltage K-Ion Batteries

A series of binary-salt electrolytes of KPF₆/KN(SO₂F)₂ (KFSA) in carbonate ester solvents have been developed for high-voltage K-ion batteries by clarifying the effect of salt ratio and different solvents on the physical properties of the electrolyte solutions and electrochemical performance of K-ion batteries. The KPF₆/KFSA carbonate ester solutions, such as KPF₆/KFSA ethylene carbonate (EC)/diethyl carbonate (DEC), exhibit higher ionic conductivity than single-salt KPF₆ one, and higher KFSA content results in higher ionic conductivity. The KPF₆-rich binary-salt electrolytes with KPF₆/KFSA ratios of ≥3 (mol/mol) provide enough oxidation stability and passivation against Al corrosion at 4.6 V over 100 h, ensuring reversible operation of a 4 V class positive electrode, K₂Mn[Fe(CN)₆] in half-cell. Graphite negative electrodes exhibit higher Coulombic efficiency and better rate performance in 0.75 mol kg^-1 K(PF₆)_0.9(FSA)_0.1/EC/DEC and 1 mol kg^-1 K(PF₆)_0.75(FSA)_0.25/EC/DEC electrolytes than those in the KPF₆ one. Surface analysis by hard X-ray photoelectron spectroscopy reveals that the decomposition product of N(SO₂F)₂^- anion contributes to stabilizing solid electrolyte interphase on a graphite electrode. From comparing different solvents of EC/DEC, EC/ethyl methyl carbonate, and EC/propylene carbonate (PC), the K₂Mn[Fe(CN)₆] electrode demonstrates the highest Coulombic efficiency in the EC/PC binary electrolyte, while graphite electrodes exhibit no significant difference. Based on the half-cell tests, we successfully achieve the 3.6 V class full cell of graphite|K(PF₆)_0.75(FSA)_0.25/EC/PC|K₂Mn[Fe(CN)₆] showing excellent cyclability over 500 cycles, which is far superior to that of the conventional KPF₆/EC/DEC electrolyte cell.