Enabling High‐Performance Potassium‐Ion Batteries by Manipulating Interfacial Chemistry

Abstract 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.