Surface Lattice Modulation Stabilizes K₂Mn[Fe(CN)₆] Cathode for High‐Energy K‐Ion Batteries

Potassium‐ion batteries (PIBs) are considered as competitive candidates for energy storage applications due to their abundant resources and low cost. K 2 Mn[Fe(CN) 6 ] (KMnF) is an ideal cathode for PIBs because of its high theoretical specific energy (≈600 Wh kg −1 ). However, it suffers from severe Mn dissolution and complex phase transitions caused by Jahn–Teller distortion, resulting in rapid capacity decay. Here, a simple, controllable and universal “transition metal (TM 2+ ) ion exchange” strategy is proposed to modulate the surface lattice of KMnF, not only stabilizing the structure but also maintaining its inherent high capacity. The surface Mn 2+ is substituted by TM 2+ , including Fe 2+ , Ni 2+ , Cu 2+ , or Co 2+ , forming heterogeneous protection layer. Especially when the surface Mn 2+ is modified by redox‐active Fe 2+ , it exhibits a capacity as high as 144 mAh g −1 and considerable energy density of 560 Wh kg −1 and a remarkable capacity retention (86% after 1,000 cycles at 50 mA g −1 , and 71% after 5,000 cycles at 1,000 mA g −1 , respectively). The surface‐modified KMnF cathode proved to be effective in stabilizing the structure by preventing the Mn dissolution and formation of tetragonal phase caused by Jahn–Teller distortion. This work provides a simple and universal strategy for stabilizing high‐energy Mn‐based cathode.