Impact of Electrolyte Formulation on the Phase Behavior and Interphase Formation of Sb/Graphite Electrodes for K‐Ion Batteries

High capacity alloy‐type materials are attractive anodes for potassium‐ion batteries yet their practical use is hampered by extreme volumetric expansion that causes mechanical instability and rapid capacity fading. Here we show that the electrolyte formulation decisively governs both the electrochemical performance and structural integrity of a high‐capacity Sb/graphite composite anode (70:30 wt.%). A localized high‐concentration electrolyte (LHCE) comprising KFSI, glyme solvents, and 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl ether as diluent delivers markedly improved durability, sustaining over 300 cycles with 400 mAh g −1 (1.5 mAh cm −2 ), whereas a conventional carbonate‐based electrolyte (CBE) exhibits rapid degradation. Operando Raman spectroscopy, operando energy‐dispersive X‐ray diffraction, operando electrochemical dilatometry, and ex situ XPS and TEM reveal that the benefit arises from a two‐dimensional electrolyte effect on both surface and bulk electrode behavior. The CBE promotes crystalline multiphase K–Sb alloying together with pronounced graphite participation and forms a thick, organic‐rich SEI, leading to large, poorly reversible swelling and mechanical damage. In contrast, the LHCE favors predominantly amorphous K x Sb formation, suppresses deep K + intercalation into graphite, and forms a thin inorganic, KF‐rich interphase that mitigates internal strain. These insights link solvation, interphase chemistry, and chemo‐mechanics, guiding electrolyte design for stable alloy anodes.