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Transformation of Spinel Zn2Mn4O8·H2O to Layered δ‐MnO2‐Based Composite Nanosheets with Enhanced Capacitance in Aqueous Electrolyte

Transition from the layered phase to spinel phase of active materials usually occurs in lithium secondary batteries with organic electrolytes, whereas the reverse conversion is rarely observed during electrochemical measurements. Herein, spinel Zn2Mn4O8·H2O nanoparticles (NPs) are synthesized at room temperature and are transformed into large spinel ZnMn2O4 particles after calcination at 800 °C. Cyclic voltammetry of the Zn2Mn4O8·H2O NPs‐based electrodes leads to the formation of δ‐MnO2‐based composite nanosheets at the potential window of 0−1.2 V, and the latter shows the capacitances of 187 and 327 F g−1 at 0.1 A g−1 in aqueous Na2SO4 and MgSO4 electrolytes, respectively. However, the spinel‐to‐layered phase transition is not observed for Zn2Mn4O8 particles, which show a much lower capacitance even in MgSO4 solution. The presence of crystal water in Zn2Mn4O8·H2O NPs, combined with the existence of Na+ or Mg2+, not Al3+ ions in electrolytes, is helpful for both the release of Zn (II) ions and movement of Mn (III) ions during the spinel‐to‐layered phase transition driven by electrochemistry. The unique transition of Zn2Mn4O8·H2O NPs to δ‐MnO2‐based composite nanosheets and their structure–property relationship are discussed and analyzed based on the experimental results.