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

Abstract

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.