Water–Solid Interface Engineering Stabilizes K‐Birnessite Cathode

Abstract Crystal water mediated ion hydration and transport is a fundamental physicochemical process in a wide range of applications and natural processes, such as crystal water containing battery materials. In this context, K‐Birnessite with interlaminar H 2 O (K 0.21 MnO 2 ⋅0.31H 2 O) is reported for nonaqueous potassium‐ion storage with enhanced capacity and rate performance. It is clarified that the water–solid interface plays an important role in building and stabilizing the K‐Birnessite layered structure and facilitating the K + diffusion for the K‐ion battery system. In addition to the enlarged ionic channel dimensions and effective shielding of the electrostatic interaction with K + , the concerted diffusion of K + /H 2 O with the rotation of H 2 O molecules is further revealed to account for the quite low activation energies by first‐principles simulations. Moreover, the interlayer H 2 O exerts on the electronic structures, and thus on the electrochemical voltage to result in the competition between the split of crystal field (“∇ JT ”) and the electronic superexchange interaction (“O w ‐K‐O”) in the K‐Birnessite structure. The results provide new insights into hydrated ion kinetics and open an exciting direction for battery material design.