Phase-Modified Strongly Coupled δ/ε-MnO2 Homojunction Cathode for Kinetics-Enhanced Zinc-Ion Batteries

Rechargeable Zn-MnO2 batteries using mild water electrolytes have garnered significant interest owing to their impressive theoretical energy density and eco-friendly characteristics. However, MnO2 suffers from huge structural changes during the cycles, resulting in very poor stability at high charge–discharge depths. Briefly, the above problems are caused by slow kinetic processes and the dissolution of Mn atoms in the cycles. In this paper, a 2D homojunction electrode material (δ/ε-MnO2) based on δ-MnO2 and ε-MnO2 has been prepared by a two-step electrochemical deposition method. According to the DFT calculations, the charge transfer and bonding between interfaces result in the generation of electronic states near the Fermi surface, giving δ/ε-MnO2 a more continuous distribution of electron states and better conductivity, which is conducive to the rapid insertion/extraction of Zn2+ and H+. Moreover, the strongly coupled Mn–O–Mn interfacial bond can effectively impede dissolution of Mn atoms and thus maintain the structural integrity of δ/ε-MnO2 during the cycles. Accordingly, the δ/ε-MnO2 cathode exhibits high capacity (383 mAh g–1 at 0.1 A g–1), superior rate performance (150 mAh g–1 at 5 A g–1), and excellent cycling stability over 2000 cycles (91.3% at 3 A g–1). Profoundly, this unique homojunction provides a novel paradigm for reasonable selection of different components.