Fluoride-Rich Solid-Electrolyte-Interface Enabling Stable Sodium Metal Batteries in High-Safe Electrolytes

Sodium metal batteries (SMBs) are promising for large scale energy storage due to the remarkable capacity of sodium metal anode (SMA) and the natural abundance of Na-containing resources. However, multiple challenges exist with regards to the usage of SMBs, including dendritic Na growth, poor cyclability of SMA, and severe safety hazards stemming from the employment of the highly flammable liquid electrolytes. Herein, by introducing two functional fluorinated solvents, 1,1,2,2-tetra-fluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) and fluoroethylene carbonate (FEC) into trimethyl phosphate (TMP)-based electrolyte, a SMA-compatible flame-retardant electrolyte is enabled, in which Na/Na symmetrical cells can cycle for 800 h at 1.0 mA cm^?2 or 3.0 mAh cm^?2. Specifically, the non-solvating HFE plays a critical role in increasing the local electrolyte concentration and reducing the unfavorable decomposition of TMP molecules. By introducing FEC as the co-solvent simultaneously, its preferential defluorination induces a fluoride-rich solid-electrolyte interphase that prevents Na metal surface against the continuous parasitic reactions. More importantly, the designed electrolyte is endowed with an intrinsic non-flammability, which manifests a prerequisite for the real-life application of SMBs.