Spatially Confined MnO2 Nanostructure Enabling Consecutive Reversible Charge Transfer from Mn(IV) to Mn(II) in a Mixed Pseudocapacitor-Battery Electrode

Mn oxides are highly important electrode materials for aqueous electrochemical energy storage devices, including batteries and supercapacitors. Although MnO2 is a promising pseudocapacitor material because of its outstanding rate and capacity performance, its electrochemical instability in aqueous electrolyte prevents its use at low electrochemical potential. Here, the possibility of stabilizing MnO2 electrode using SiO2-confined nanostructure is demonstrated. Remarkably, an exceptionally good electrochemical stability under large negative polarization in aqueous (Li2SO4) electrolyte, usually unattainable for MnO2-based electrode, is achieved. Even more interestingly, this MnO2-SiO2 nanostructured composite exhibits unique mixed pseudocapacitance-battery behaviors involving consecutive reversible charge transfer from Mn(IV) to Mn(II), which enable simultaneous high-capacity and high-rate characteristics, via different charge-transfer kinetic mechanisms. This suggests a strategy to design and stabilize electrochemical materials that are comprised of intrinsically unstable but high-performing component materials.