Spatially Confined MnO<inf>2</inf> 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<inf>2</inf> 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<inf>2</inf> electrode using SiO<inf>2</inf>-confined nanostructure is demonstrated. Remarkably, an exceptionally good electrochemical stability under large negative polarization in aqueous (Li<inf>2</inf>SO<inf>4</inf>) electrolyte, usually unattainable for MnO<inf>2</inf>-based electrode, is achieved. Even more interestingly, this MnO<inf>2</inf>-SiO<inf>2</inf> 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. The possibility of stabilizing MnO <inf>2</inf> electrode using a SiO <inf>2</inf> -confined nanostructure, which exhibits exceptionally good electrochemical stability under large negative polarization in aqueous electrolyte, is demonstrated. The nanostructured composite exhibits unique mixed pseudocapacitance-battery behaviors involving consecutive reversible charge transfer from Mn(IV) to Mn(II). A strategy to design and stabilize electrochemical materials that are composed of intrinsically unstable but high-performing component materials is suggested. ? 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.