As a promising candidate for the flame-retardant electrolyte, triethyl phosphate (TEP)/potassium bis(fluorosulfonyl)amide (KFSI)-based electrolyte has drawn much attention in the K-ion battery community. Although the TEP/KFSI formula at a moderate main salt concentration (normally, <3 m) enables the compatibility of the reactive K metal anode, the long-standing oxidative instability of KFSI salt remains unsolved. Here, an additive strategy is reported to address the high-voltage issue in the TEP/KFSI electrolyte, and generalize it to the other KFSI-based electrolytes. The addition of potassium nitrate changes the surface charge distribution and effectively suppresses the decomposition of KFSI toward the high-voltage cathode. The nitrate-containing electrolyte enables superior stability of a 4.3 V-class K-ion battery, as evidenced by its 80% capacity retention over 2000 cycles (≈6 months) at the 1 C rate. Moreover, the long-cycling stability of the graphite-based full cell with Prussian Blue cathode is demonstrated.
