Polymorphic Effects on Electrochemical Performance of Conversion‐Based MnO₂ Anode Materials for Next‐Generation Li Batteries

Abstract In this study, four different MnO₂ polymorphs are synthesized with a controlled morphology of hollow porous structures to systematically investigate the influences of polymorphs in conversion‐based material. As the structure of these materials transforms into nanosized metal and maintains an extremely low‐crystalline phase during cell operation, the effects of polymorphs are overlooked as compared to the case of insertion‐based materials. Thus, differences in the ion storage behaviors among various MnO₂ polymorphs are not well identified. Herein, the structural changes, charge storage reaction, and electrochemical performance of the different MnO₂ polymorphs are investigated in detail. The experimental results demonstrate that the charge storage reactions, as part of which spinel‐phased MnO₂ formation is observed after lithiation and delithiation instead of recovery of the original phases, are similar for all the samples. However, the electrochemical performance varies depending on the initial crystal structure. Among the four polymorphs, the spinel‐type λ‐MnO₂ delivers the highest reversible capacity of ≈1270 mAh g⁻¹. The structural similarity between the cycled and pristine states of λ‐MnO₂ induces faster kinetics, resulting in the better electrochemical performance. These findings suggest that polymorphs are another important factor to consider when designing high‐performance materials for next‐generation rechargeable batteries.