Mesocrystal MnO cubes as anode for Li-ion capacitors

Abstract

Mesocrystal MnO cubes consisted of nanocrystals (~16 nm) with percolated nanopores (~10.5 nm) and a porosity of 45% were synthesized through polyvinyl alcohol (PVA) assisted hydrothermal growth and homoepitaxial aggregation. The resulting MnO crystal makes up of appreciable amount of Mn3+, ~11.8%, ascribed to incomplete reduction during the hydrothermal growth, and the presence of such an appreciable amount of Mn3+ is thought to lead to the enhancement of Li ion diffusion coefficient from 6.96×10-14 cm2/s to 3.33×10-13 cm2/s. Carbon coating derived from polyvinyl alcohol and homoepitaxial connection of nanocrystals provides excellent charge and mass transfer pathways. Such mesocrystal MnO cubes enhanced the contact of electrolyte and electrode materials, at the same time the active nanocrystals with trivalent manganese ions and cationic vacancies would promote the conversion reaction for lithium-ion insertion and extraction, leading to a high capacity of 637 mA h/g at 100 mA/g in a relatively smaller voltage range of 0.05-2.50 V, as compared with a voltage window of 0.01-3 V used by other research groups. The high voltage (4 V) Li-ion capacitor, a full cell with mesocrystal MnO cubes as anode and activated carbon as cathode, demonstrated excellent cycling performance with the degradation rate of 0.002% per cycle, and the achieved maximum energy in full capacitor reached 227 W h kg-1 that calculated on the total weight of the active materials in both electrodes.