Cyclic ether-based electrolyte solutions for potassium metal batteries

Abstract

Potassium metal batteries (KMBs) are promising candidates for large-scale energy storage due to the high abundance of potassium and its very low reduction potential. However, poor reversibility during plating and stripping due to the high reactivity of the potassium metal anode and the formation of an unstable solid electrolyte interphase (SEI) hinders its practical implementation. In this work, the physical and electrochemical properties of two new electrolyte solutions using weakly coordinating solvents, consisting of potassium bis(fluorosulfonyl)imide (KFSI) in tetrahydrofuran (THF) and 2-methyltetrahydrofuran (MeTHF) are reported. Both electrolyte solutions enabled reversible potassium metal plating/stripping, achieving a Coulombic efficiency (CE) of about 99% for 500 cycles at 0.5 mA cm‾2 and 1 mAh cm‾2. Various characterization techniques were used to analyze the influence of the solvation structure on the chemical composition of the SEI formed in the electrolyte solutions. Spectroscopic analysis unveiled the formation of contact ion pairs (CIPs) and aggregates (AGGs), resulting in the formation of an anion-derived SEI. Cycling in full cells with an organic cathode material gave the proof of concept for the application of these new electrolytes in potassium metal batteries.