Introduction of KPF₆ in Diluted KFSI-Based Ether Electrolyte for High-Voltage K-Ion Batteries

Owing to the enhanced ion dissociation properties of potassium bis(fluorosulfonyl)imide (KFSI)-based electrolytes, these exhibits superior ionic conductivity as compared with KPF6. However, KFSI electrolytes are prone to Al corrosion at high potentials above 3.5 V vs K/K+, limiting their application in 4 V-class potassium-ion batteries (PIBs). This work demonstrates that a dual-salt KFSI/KPF6 electrolyte achieves excellent oxidative stability and effective Al corrosion suppression at 4.2 V, enabling the stable and reversible operation of a K0.4V2O5 cathode. The incorporation of KFSI and KPF6 plays a pivotal role in stabilizing the electrode/electrolyte interface by promoting the formation of a robust, passivating cathode–electrolyte interphase enriched in KF and phosphate-derived species. These species suppress Al current collector corrosion, typically exacerbated by aggressive oxidative decomposition of salts and solvents under high-voltage conditions. The electrochemical impedance spectroscopy results reveal substantially reduced interfacial resistance in the cosalt system, particularly at the optimized composition of 0.5 mol kg–1 KFSI + 0.2 mol kg–1 KPF6, which also delivers an impressive discharge capacity of ∼100 mAh g–1 with high Coulombic efficiency (>94%) over 100 cycles. The galvanostatic profiles show minimal voltage hysteresis and superior retention compared with those of cells without the cosalt. This work provides a clear and rational pathway toward achieving high-voltage and long-life PIBs through targeted electrolyte engineering that mitigates corrosion and interfacial instability.