Fluoride-Free Surface Passivation Enables Low-Concentration Nonflammable Electrolytes for K-Ion Batteries

Here, we show that a low-concentration (1 M) KPF6 electrolyte combining ethylene carbonate, propylene carbonate, and triethyl phosphate (TEP) achieves stable cycling with graphite anodes, retains high ionic conductivity, and is nonflammable. Surface characterization of the solid–electrolyte interphase (SEI) reveals that KPF6 does not reduce to KF. Instead, TEP results in the accumulation of inorganic phosphates that passivate against reductive decomposition of carbonate solvents and lessen resistance to K-ion transport through the SEI. Overall, carbonate/phosphate solvent mixtures serve as a nonflammable baseline for future engineering of the SEI layer that eliminates ionically insulating fluorinated phases.