Tuning effect of vanadium substitution on the structural and electronic properties of potassium hollandite surfaces

Metal oxide surfaces possess unique properties that are crucial for a wide variety of applications. Herein, density functional theory calculations are performed to study surfaces of potassium hollandite, KMn₈O₁₆, a promising cathode material for electrochemical energy storage, and the vanadium-substituted analog KMn₇VO₁₆. The results show that there is a clear increase in the stability of KMn₈O₁₆ with (001) < (110) < (100) or (010), apt to adopt an elongated rod-like morphology. The vanadium (V)-substitution lowers the crystal symmetry and prefers to occupy the surface sites, resulting in electron redistribution and selective tuning of surface energy depending on the surface structures. In particular, the higher stability of substituted V⁴⁺ compared with Mn⁴⁺ ions leads to stabilization of the (001) surface due to the direct interaction of reduced Mn^δ+ ions on the surface, while such tuning effect decreases with the increase in surface stability, (110) > (100) and (010). As a result, the KMnO₁₆ rod is shortened upon V-substitution as observed experimentally, effectively facilitating the ion transport during discharge. The V substituents also introduce stabilization to the defect surfaces resulting from Mn²⁺ dissolution during cycling, thereby hindering further structural decay. Our study demonstrates the potential tuning effect of V-substitution to promote the ion transport and mitigate the capacity degradation of α-MnO₂-based materials.