Polarity Gradient CEI Driven Rapid Desolvation for Extreme Fast-Charging Potassium-Ion Batteries

Potassium-ion batteries (PIBs) offer an opportunity for superior fast-charging compared to lithium-ion batteries, owing to their faster K⁺ transport in electrolyte. However, severe side reactions at the cathode electrolyte interphase (CEI), sluggish K⁺ transport, and cathode structural degradation hinder the development of fast-charging PIBs. Herein, we tailor-design a polarity gradient CEI via an electrolyte additives modification strategy. Specifically, the outer B-F/B-O species assist in withdrawing solvent molecules around K⁺ during the desolvation process, while abundant K₂CO₃ and KF throughout the CEI facilitate K⁺ transport and structural stability. Consequently, the KFeHCF/graphite full cell demonstrates improved charge transfer and diffusion kinetics, with suppressed Fe dissolution, enhancing stability of both the cathode bulk structure and interphase under fast-charging conditions. The full cell with optimized CEI delivers high reversible capacities of 126.5 mAh g^-1 at 0.02 A g^-1 and 95.8 mAh g^-1 at 1 A g^-1 (a charging time of 7.5 min for 80% of the capacity), and maintains 67 mAh g^-1 at 5 A g^-1 as well as good long cycle life. Moreover, it retains 85.1 mAh g^-1 and exhibits good rate performance even at -10 °C. Our work reveals the critical role of rationally regulating CEI components and structure for fast-charging PIBs.