Locally Superconcentrated Electrolytes for Ultra-Fast-Charging Lithium Metal Batteries with High-Voltage Cathodes

Electric vehicles capable of recharging in the same time it takes to refuel a gasoline-powered car require electrolytes that maximize areal ion flux to enable electrochemical reactions to proceed at the same rate that current is passed through the external circuit. While strategies for increasing ionic charge carrier concentration in electrolytes are well-established, enhancements are made at the expense of carrier mobility, placing a ceiling on areal ion flux below the requirement for fast-charge. Here, we explore locally superconcentrated electrolytes, which employ a noncoordinating diluent to reduce viscosity, for delivering an 80% change in state-of-charge in Li|NMC622 batteries in 5-15 min. We investigate the effects of concentration, viscosity, ionic conductivity, and solvation on lowering fast-charge overpotentials and extending cycle life. We identify divergent failure mechanisms that occur on different time scales when cycling the cells at different charge rates and depths of discharge, which has implications for future electrolyte designs.