Fluorinated Ether Based Electrolyte Enabling Sodium-Metal Batteries with Exceptional Cycling Stability.

Sodium-metal batteries with conventional organic liquid electrolytes have disadvantages including dendrite deposition and safety concern. In this work, we report a low-flammable electrolyte (NaPF6-FRE) consisting of 1 M NaPF6 in 1,2-dimethoxyethane (DME), fluoroethylene carbonate (FEC), and 1,1,1,3,3,3-hexafluoroisopropylmethyl ether (HFPM) (2:1:2, in volume ratio). The symmetric Na and Na||Cu cells with a 1 M NaPF6-DME electrolyte absorbed in a porous separator, such as the porous glass-fiber, show very poor cycling performance. In addition, the cell with a Na3V2(PO4)3 (NVP) cathode and 1 M NaPF6-DME electrolyte shows low Coulombic efficiency. FEC was added into the NaPF6-DME-based electrolyte to reduce the irreversible capacity of the NVP cathode and improve the Coulombic efficiency of the cell. However, the high reactivity of FEC with the Na electrode leads to formation of an unstable solid electrolyte interphase (SEI) and large interfacial resistance, and HFPM was further added to stabilize the Na electrode surface by forming a new fluorine-containing organic layer. The new prepared low-flammable electrolyte (NaPF6-FRE) with 1 M NaPF6 in DME, FEC, and HFPM (2:1:2, in volume ratio) shows a wide electrochemical window of 5.2 V. The Na symmetric cells with this low-flammable electrolyte show superior cycling performance for 800 h with a stable voltage profile at 0.5 mA cm-2, 0.5 mA h cm-2 and 1 mA cm-2, 1 mA h cm-2, respectively. The NVP||Na cells show an excellent capacity retention of 94% after 2000 cycles and superior Coulombic efficiency of 99.9% on average at 5 C.