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

Abstract

Mn oxides are highly important electrode materials for aqueous electrochemical energy storage devices, including batteries and supercapacitors. Although MnO 2 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 MnO 2 electrode using SiO 2 ‐confined nanostructure is demonstrated. Remarkably, an exceptionally good electrochemical stability under large negative polarization in aqueous (Li 2 SO 4 ) electrolyte, usually unattainable for MnO 2 ‐based electrode, is achieved. Even more interestingly, this MnO 2 –SiO 2 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.