Unveiling intrinsic oxidation resilience of standard-concentration ether electrolyte via solvent-anion synergy for 4.5 V anode-free sodium metal batteries

Abstract Electrolytes play a pivotal role in determining the electrochemical performance of anode-free sodium metal batteries. Ether-based electrolytes exhibit superior compatibility with sodium metal anode, while their poor oxidation stability has historically restricted their application in high-voltage systems. Although high concentration and localized high concentration strategies have been employed to improve the oxidation resistance, their high costs and complexity remain significant barriers. Herein, a low-cost, standard-concentration (1 M) ether-based electrolyte that achieved exceptional high-voltage stability was developed. The extended ether chains endow tetraethylene glycol dimethyl ether with intrinsically enhanced oxidation resistance by lowering its highest occupied molecular orbital energy level, while maintaining excellent reduction stability. Furthermore, the BF4− anions preferentially decompose to form a robust boride- and fluoride-rich interphase at the cathode surface. This synergistic effect between the solvent and anion enables an anode-free Al@C||Na2Fe2(SO4)3 battery to deliver 500 stable cycles at a high charging cut-off voltage of 4.5 V, with an average discharging voltage of 3.8 V. This work not only demonstrates the feasibility of high-voltage ether-based electrolytes at standard concentrations, but also provides critical insights and references for the development of advanced electrolytes for next-generation batteries.