Improving the intrinsic conductivity of δ-MnO2 by indium doping for high-performance neutral aqueous sodium-ion supercapacitors with commercial-level mass-loading

Abstract

Due to their potential use in energy storage and next-generation water purification devices, neutral aqueous supercapacitors (NASCs) have attracted significant attention from researchers. δ-MnO2 is a well-known cathode material for supercapacitors due to its 2D structure and high capacitance. However, its poor conductivity limits the mass loading on the electrode. Moreover, the dissolution of δ-MnO2 hampers its cycling stability and applicability in water purification. Here, indium is introduced to δ-MnO2 with the aid of iron. The crystal water and conductivity of the δ-MnO2 are significantly influenced by indium. After optimization of the concentration of indium in δ-MnO2, the gravimetric capacitance of δ-MnO2 increases to 1302 F g−1 (95% of the theoretical limit). At a mass loading of 10.6 mg cm−2, 180 F g−1, 1.9 F cm−2 and 54 F cm−3 at 10 mA cm−2 can be obtained. A NASC (In doped δ-MnO2//active carbon) exhibits a maximum areal energy density of 0.55 mW h cm−2 and a volumetric energy density of 18.7 mW h cm−3 at an areal power density of 1.0 mW cm−2. 90% capacitance retention after 13 000 cycles arises from the prevention of Jahn-Teller distortion. First principles calculations and experimental results demonstrate that In doping is effective in enhancing the intrinsic properties of δ-MnO2. These findings open up a new path towards high-performance supercapacitors.