In Situ Substitution Strategy with High‐Valence Cation for Ultra‐Stable Aqueous K‐Ion Batteries

ABSTRACT Aqueous K‐ion batteries (AKIBs) are promising for large‐scale energy storage due to their high safety and low cost. However, the manganese‐based Prussian blue analogue (KMnPBA) cathode suffers from severe Mn dissolution and Jahn‐Teller distortion during cycling, triggering crystal‐structure collapse and rapid capacity decay. Herein, we report a high‐valence cation in situ substitution strategy to stabilize KMnPBA by adding Al(OTF) 3 to a highly concentrated KOTF electrolyte. During discharge, Al 3+ ions fill Mn dissolution vacancies and form robust Al‐N coordination bonds, generating a gradient structure characterized by a stable surface and high‐capacity interior. As a result, the KMnPBA cathode in optimized electrolyte delivers a high initial discharge capacity of 158.2 mAh g −1 at 0.2 A g −1 , with 97.1% capacity retention after 500 cycles, while maintaining 90.3% retention even after 30000 cycles at 6 A g −1 . Additionally, the KMnPBA||PTCDI pouch cell achieves an energy density of 88.3 Wh kg −1 , retaining 87.8% capacity after 6000 cycles and stably operating from −20°C to 60°C. This study provides an effective approach to address the issue of capacity decay arising from the dissolution of manganese‐based cathode materials, paving the way for AKIBs with ultra‐long cycle stability.