Hybrid Electrolyte Engineering Enables Safe and Wide-Temperature Redox Flow Batteries.

Electrolyte is an important component in redox flow batteries (RFBs) that determines the current capability, potential window, and safety, but both aqueous and nonaqueous electrolytes have their intrinsic limits. Here, we develop the proof-of-concept hybrid electrolyte chemistry to enable the design of safe and wide-temperature RFBs. In addition to the non-flammable characteristics, the hybrid electrolyte also inherits the high electrochemical stability and wide operational temperature range. It can show a potential window of 2.5 V and maintain high ion conductivities at low temperatures. It also enables LiI to achieve high Coulombic efficiencies of >99.9 %, showing long cycling stability over 800 cycles. Moreover, it enables the successful operation of Zn/LiI RFBs at -20 °C for 150 cycles with nearly no capacity loss. This study highlights the great potential of hybrid electrolyte chemistry for the approach of safe and high-performance large-scale energy storage systems in wide temperature ranges.