Transition Metal Substitution of Hollandite α-MnO₂: Enhanced Potential and Structural Stability on Lithiation from First-Principles Calculation

Abstract

Hollandite α-MnO2, consisting of manganese–oxygen octahedra, has recently attracted attention due to its high theoretical capacity, yet it suffers capacity degradation during repeated (de)lithiation. Here we use a new conceptual approach to substitute one of the Mn in the tunnel wall via the form of Mn0.875M0.125O2 (M = Ti, V, Cr, Nb, Ru), aiming to increase the lithiation potential and attain the theoretical capacity via the enhanced structural stability, with the ultimate goal of improved capacity retention upon repeated (de)lithiation. A bottom-up screening using density functional theory (DFT) was performed to identify the effect of the transition metal substitution on lithiation of α-MnO2. The calculations reveal that substitution with electron-accepting Cr ions results in a more significant increase in the lithiation potential of MnO2 than the other substituents. In terms of structural stability, both Cr and Nb are capable of effectively stabilizing the tunnel structure of α-MnO2 under increased levels of lithiation, thus providing the opportunity for significant increases in the cyclability and delivered capacity. Our study not only discovers the new lithiation pathway and intermediates at the atomic level but also develops the key concepts to optimize the lithiation potential and structural durability for future α-MnO2-based materials. This approach opens a new avenue for materials design of 1D tunnel structured materials for use as stable host frameworks for electrochemical ion (de)insertion.