Designing Electrolytes with Steric Hindrance and Film‐Forming Booster for High‐Voltage Potassium Metal Batteries

Abstract Potassium metal batteries coupling with high‐voltage manganese hexacyanoferrate (MnHCF) cathodes are promising candidates for energy storage devices. Ethers are the primary electrolyte solvent candidates for reversible potassium metal anodes, but their poor oxidative stability at high voltage restricts the application. Taking advantage of the steric hindrance, a dilute (1 m ) non‐fluorinated ether with extended alkyl groups is designed to reduce the solvation capability to K ions, thus suppressing the solvent decomposition and tailoring the interphase composition on both cathode and anode sides. The accompanying high viscosity associated with the long alkyl groups can be readily resolved by incorporating an S‐containing additive, which further boosts the oxidative stability to 4.6 V. Furthermore, the additive contributes to the S‐rich organic species among the interphases that inhibit the metal dissolution of the cathode and induce the homogenous K deposition through promoted kinetics. Benefiting from the stable interphases and restricted side reactions, the dilute non‐fluorinated ether‐based electrolyte enables the stable cycling of MnHCF||K cell for over 200 cycles with a high Coulombic efficiency of over 99.4% under a negative/positive capacity ratio of 4.