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 their practical implementation. This work conducts a comparative analysis of two electrolyte solutions based on weakly coordinating solvents, consisting of potassium bis(fluorosulfonyl)imide (KFSI) in tetrahydrofuran (THF) and 2-methyltetrahydrofuran (MeTHF). The physical and electrochemical properties of these electrolyte solutions, as well as the plating and stripping behavior of potassium metal on aluminum current collectors, were investigated. Furthermore, this study attempts to demonstrate how solvent structure modulates solvation behavior, which governs SEI formation and thereby determines electrochemical performance. 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. 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 electrolyte solutions in potassium metal batteries.