“Water-in-Salt” Electrolyte Enabled High Voltage Aqueous Li-ion Chemistries

The application of Li-ion batteries for vehicle-electrification and grid-storage is deterred by their safety, environmental and cost concerns, which are mainly imparted from the non-aqueous electrolytes used therein. These obstacles could be circumvented by an aqueous alternative; however, narrow electrochemical stability window (1.23 V) of the latter, imposed by hydrogen and oxygen evolutions at anode and cathode, respectively, sets an intrinsic limit on the practical voltage and energy output of an aqueous Li-ion cell. Here, we report a new aqueous electrolyte, whose electrochemical stability window was expanded to ~3.0 V via super-concentration and concomitant interphasial chemistry. A full Li-ion battery of 2.3 V was demonstrated to cycle over 1000 times in this electrolyte, with nearly 100% Coulombic efficiency at both low (0.15 C) and high (4.5 C) rates. For the first time, breaking Pourbaix-limits makes it possible for an aqueous Li-ion chemistry to deliver energy density over 100Wh/kg.