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. K2Mn[Fe(CN)(6)] (KMnF) is an ideal cathode for PIBs because of its high theoretical specific energy (approximate to 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 (TM2+) 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 Mn2+ is substituted by TM2+, including Fe2+, Ni2+, Cu2+, or Co2+, forming heterogeneous protection layer. Especially when the surface Mn2+ is modified by redox-active Fe2+, 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.