Investigating the structural change and degradation mechanism of MnO₂ for lithium-ion batteries

Tunnel-type manganese dioxide (MnO₂) is widely used as a cathode material for commercial lithium-metal primary batteries due to its low cost, non-toxicity, and high capacity of 308 mAh g⁻¹. However, it has not been used in rechargeable batteries because it showed poor cycle stability in the past. In this study, we systematically investigate the electrochemical behavior of β-MnO₂ as a cathode for lithium-ion batteries, identifying key degradation mechanisms and proposing a surface modification strategy to enhance its performance. Specifically, β-MnO₂ delivers a reversible capacity of approximately 220 mAh g⁻¹ at 30 mA g⁻¹. During first cycle, β-MnO₂ transforms irreversibly to Li_xMnO₂ with an orthorhombic phase as shown by in-situ X-ray diffraction, which causes some Li to be trapped in the structure. The emergence of a spinel-like LiMn₂O₄ phase upon further cycling contributes to capacity fading. The cycle stability of MnO₂ is significantly influenced by the electrolyte composition, with higher capacity fading observed in electrolytes containing larger amount of ethylene carbonate which correlates with increased manganese dissolution. To mitigate these issues, a 2 wt% Li₃PO₄ surface coating was applied to the β-MnO₂ particles, which improves its capacity retention and Coulombic efficiency, particularly at higher cut-off voltages, by reducing electrolyte decomposition. © 2025 Elsevier B.V.