Fluorinated Diluents Enable Crowded Solvation Environments to Form Anion‐Rich SEIs for High‐Performance Potassium‐Ion Batteries

Electrolytes play a crucial role in improving the cycling stability and lifespan of potassium‐ion batteries (PIBs). Nevertheless, conventional electrolytes often form a non‐uniform, low‐conductivity solid electrolyte interface (SEI), leading to uncontrolled potassium deposition and inactive potassium formation, thus falling short of the performance requirements for high‐energy‐density PIBs. Herein, 1,2‐difluorobenzene (1,2‐dfBen) is introduced as a crowding diluent to the conventional commercial electrolyte, which consists of potassium bis(fluorosulfonyl)imide (KFSI) and dimethyl carbonate (DMC). The combined effects of the low energy level of the lowest unoccupied molecular orbital (LUMO) and the pronounced fluorine‐donating capability of 1,2‐dfBen synergistically alter the solvation structure and interfacial chemistry. Consequently, the implementation of this straightforward electrolyte formulation results in exceptional stability for K//K symmetric cells, as evidenced by their ability to sustain 200 h. Furthermore, K//Cu cells exhibit a notably high average coulombic efficiency (CE) of 93.7%, and K//graphite cells demonstrate a charge capacity of 230 mAh g −1 with a CE of 97.5% after 100 cycles. As a demonstration of practical device applicability, the assembled potassium‐ion full cell (PTCDA//graphite) exhibits superior cycling stability. This research presents a feasible design for localized high‐concentration electrolyte (LHCE), paving the way for the development of high CE and long‐lasting PIBs.