Deep Eutectic Solvent-Based Gel Electrolyte with Fast Na Ions Transport in Sodium-Metal Batteries.

Deep eutectic electrolyte (DEE) with intrinsic nonflammability is deemed as a promising electrolyte candidate for high safety sodium-metal batteries (SMBs). Nevertheless, the leakage risk and inferior electrode compatibility impede their further application toward high-performance SMBs. Herein, a novel DEE based on sodium bis(fluorosulfonyl)imide (NaFSi) and ethyl-2-cyano-3-ethoxyacrylate (ECE) is developed through their Lewis acid-base interaction and the further ECE-based deep eutectic gel electrolyte (GE) is constructed with cross-linked ploy(vinylene carbonate) (PVC) as matrix network. It is demonstrated that incorporating electron-withdrawing PVC polymer into the ECE-based DEE changes the coordination environment of Na ions, thus facilitating the Na-ions desolvation and transport and leading to a high transference number of Na-ions (0.7). Moreover, the strong interaction between PVC chains and Na-ions can induce the preferential reduction of PVC to form O-rich inorganic solid electrolyte interphase (SEI), thus promoting the electrode/electrolyte interfacial stability with superior long-term cycling stability. Benefiting from the stable SEI layer, the NaV(PO)//Na cell with GE displays outstanding cycling (capacity retention of 94.5% after 300 cycles at 1C) and rate (high capacity of 75.5 mAh g at 2 °C) performance than that of ECE-based DEE cell, showcasing the promising application of advanced deep eutectic GE in high performance SMBs.