Water–Solid Interface Engineering Stabilizes K-Birnessite Cathode

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.