Potassium-ion batteries (PIBs) have been lauded as the next-generation energy storage systems on account of their high voltage capabilities and low costs and the high abundance of potassium resources. However, the practical utility of PIBs has been heavily encumbered by severe K metal dendrite formation, safety issues, and insufficient electrochemical performance during operations─indeed critical issues that underpin the need for functional electrolytes with high thermal stability, robust solid-electrolyte interphase (SEI)-forming capabilities, and high electrochemical performance. In a bid to establish a knowledge framework for harnessing high rate capabilities and long cycle life from graphite negative electrodes, this study presents the physical properties and electrochemical behavior of a high K+ concentration inorganic ionic liquid (IL) electrolyte, K[FSA]-Cs[FSA] (FSA- = bis(fluorosulfonyl)amide) (54:46 in mol), at an intermediate temperature of 70 °C. This IL electrolyte demonstrates an ionic conductivity of 2.54 mS cm-1 and a wide electrochemical window of 5.82 V. Charge-discharge tests performed on a graphite negative electrode manifest a high discharge capacity of 278 mAh g-1 (0.5 C) at 70 °C, a high rate capability (106 mAh g-1 at 100 C), and a long cyclability (98.7% after 450 cycles). Stable interfacial properties observed by electrochemical impedance spectroscopy during cycling are attributed to the formation of sulfide-rich all-inorganic SEI, which was examined through X-ray photoelectron spectroscopy. The performance of the IL is collated with that of an N-methyl-N-propylpyrrolidinium-based organic IL to provide insight into the synergism between the highly concentrated K+ electrolyte at intermediate temperatures and the all-inorganic SEI during electrochemical operations of the graphite negative electrode.
Keywords: graphite negative electrode; high rate capability; inorganic ionic liquid electrolyte; intermediate temperature; potassium-ion batteries.