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An intrinsically non-flammable electrolyte for high performance potassium batteries.

Potassium-ion batteries are promising for low-cost and large-scale energy storage applications, but the major obstacle to their application is the lack of safe and effective electrolytes. Here, we demonstrate that a phosphate-based fire retardant such as triethyl phosphate works as a single solvent with potassium bis(fluorosulfonyl)imide at 0.9 M, which is different from our previous understanding, based on Li and Na systems where phosphates cannot work at low concentrations. Moreover, our electrolyte is optimized at 2 M, where it exhibits the synergistic advantages of low cost, low viscosity, and high conductivity, as well as the formation of a uniform and robust salt-derived solid-electrolyte interphase layer, leading to non-dendritic K-metal plating/stripping with Coulombic efficiency of 99.6%, along with a highly reversible graphite anode. The successful demonstration of using non-flammable phosphate as the sole solvent at low concentrations opens up an avenue for electrolyte design towards safe, low-cost, and high-performance large-scale batteries.