Electrochemistry of Vacancy-Decorated α–MnO₂: Improved Ion Diffusion and Capacity Retention via Li₂O Incorporation

One-dimensional tunnel manganese oxides (TuMOs) such as hollandite α–MnO₂ are attracting considerable interest in the context of intercalation batteries because its unique structure induces a robust framework for ion insertion and deinsertion. However, practical deployment of TuMOs still encounters critical challenges such as poor rate capability due to long-distance 1D diffusion and performance degradation induced by structural instability during charge/discharge cycling. Through an integration of experiment and simulation, we show that Mn and O vacancy complexes can be effectively introduced in the tunnel wall through elaborated acid leaching and open additional pathways to enhance ion diffusion. Through first-principles calculations, it is found that the cross-tunnel diffusion barrier for Li⁺ ions through Mn-O vacancy complexes is significantly lower than that by crossing the intact wall without vacancies. Moreover, we show that the vacancy-enriched hollandite α–MnO₂ can be effectively stabilized via Li₂O incorporation to significantly improve its capacity retention. Experimentally, Li₂O incorporation into α–MnO₂ structure is achieved via wet mixing of α-MnO₂ with LiOH in methanol, followed by drying and heat treatment. After 100 intercalation/extraction cycles, the Li₂O-stabilized α–MnO₂ electrode exhibits 53% capacity retention, compared to 17% and 40% shown by the acid-leached only and pristine α–MnO2. Computationally, atomic structural evolutions during intercalation/deintercalation cycles are comprehensively studied via first-principles calculations to provide fundamental understandings of the observed capacity retention improvements.