Reversible and High-rate Hard Carbon Negative Electrodes in a Fluorine-free Sodium-salt Electrolyte

Hard carbon is widely studied as a promising negative electrode in sodium-ion batteries. To achieve its stable charge-discharge reaction, a fluorine-rich passivation film arising from a fluorinated salt or solvent in an electrolyte was demonstrated to be effective, but its essential role remained unclear. Here, we report a sodium tetraphenylborate (NaBPh4)/1,2-dimethoxyethane (DME) electrolyte that is free from fluorine but enables the highly stable and high-rate charge-discharge cycling of hard carbon electrodes as compared to other combinations of Na salts and solvents. Surface analysis of the cycled electrode shows that the NaBPh4 is not decomposed during the cycle and that solid electrolyte interphase (SEI) is derived from DME. Hence, fluorine-based components are not indispensable to stabilize the hard carbon/electrolyte interface. The DME-derived SEI, though containing no F component, can highly stabilize the interface to enable the reversible and high-rate cycling of hard carbon.