Ultrasmall α‐MnO2 with Low Aspect Ratio: Applications to Electrochemical Multivalent‐Ion Intercalation Hosts and Aerobic Oxidation Catalysts

Abstract Hollandite‐type α ‐MnO 2 exhibits exceptional promise in current industrial applications and in advancing next‐generation green energy technologies, such as multivalent (Mg 2+ , Ca 2+ , and Zn 2+ ) ion battery cathodes and aerobic oxidation catalysts. Considering the slow diffusion of multivalent cations within α ‐MnO 2 tunnels and the catalytic activity at edge surfaces, ultrasmall α ‐MnO 2 particles with a lower aspect ratio are expected to unlock the full potential. In this study, ultrasmall α ‐MnO 2 (<10 nm) with a low aspect ratio ( c / a ≈ 2) is synthesized using a newly developed alcohol solution process. This material demonstrates exceptional performance across various multivalent battery systems, primarily due to the significantly reduced cation diffusion distance. Notably, an ultrasmall α ‐MnO 2 ‐graphene composite achieves high capacity with low overpotential when paired with an F‐free electrolyte in Ca battery. Regarding aerobic oxidation catalysis, the nanosizing of α ‐MnO 2 has a profound impact on aerobic oxidation catalysis. The increased efficiency of oxidative conversion reactions, such as the oxidation of 1‐phenylethanol, is attributed to the greatly expanded active surface area of the catalyst. The versatile functionality of ultrasmall α ‐MnO 2 underscores its potential to revolutionize energy storage and catalysis, offering broad applicability in next‐generation green energy technologies.