Ultrafine Amorphous MnO<sub><i>x</i></sub> Nanoparticles for a High Mass Loading Electrode

High electrochemically active manganese\ndioxide (MnO<sub>2</sub>) has been deemed to be a promising active\nmaterial for a high mass loading electrode. Here, an amorphous MnO<sub><i>x</i></sub> nanostructure constituted by ultrafine MnO<sub><i>x</i></sub> nanoparticles was fabricated by a simple,\ncontrollable, and cost-effective strategy. The electrochemical properties\nof the as-prepared MnO<sub><i>x</i></sub> nanostructures\nwere significantly improved due to their amorphous crystalline structure.\nFor a high mass loading electrode, an optimized content of amorphous\nMnO<sub><i>x</i></sub>, carbon black, and carbon nanotubes\nwas measured to be 79.1%, 13.5%, and 7.4%, respectively. A high mass\nloading electrode of 13.5 mg·cm<sup>–2</sup> was fabricated\nwith a capacitance of 371.3 F·g<sup>–1</sup> calculated\nbased on the weight of pure amorphous MnO<sub><i>x</i></sub>. Hence, a high mass loading asymmetric supercapacitor assembled\nby amorphous MnO<sub><i>x</i></sub> (12.6 mg·cm<sup>–2</sup>) and activated porous carbon (APC, 22.4 mg·cm<sup>–2</sup>) delivers an ultrahigh areal energy density of 780.3\nμW h·cm<sup>–2</sup>. Furthermore, a good cycling\nstability with 98% of the initial capacitance retention was achieved\nafter 8000 cycles, meeting the requirements of a commercial energy\nstorage device.