Enhanced Reaction Kinetics of N-MnO2 Nanosheets with Oxygen Vacancies via Mild NH3middotH2O Bath Treatment for Advanced Aqueous Supercapacitors

: Suffering from the sluggish kinetics resulting from the inferior conductivity of manganese dioxide (MnO2), many endeavors have been devoted to promoting the electrochemical performance of MnO2. Doping heteroatoms into host materials has emerged as an effective strategy to generate lattice vacancies for fast diffusion of ions/electrons in MnO2-based materials for supercapacitors (SCs). To eliminate the use of dangerous solvents and the severe reaction conditions used in previous reports, in this work, we first obtain N-doped MnO2 (N-MnO2) nanosheets with abundant oxygen vacancies simultaneously by a mild hydrothermal reaction. Thus, thanks to the mutual effect of N doping and oxygen vacancies, the capability of ion diffusion for N-MnO2/NGCF (NGCF represents N-doped single-wall carbon nanotube cross-linked graphene composite film) has increased an order of magnitude (1.68 x 10-11 cm2 s-1 compared with 8.42 x 10-13 cm2 s-1 for MnO2/NGCF) and shows superior rate capability, where at 30 A g-1 a capacitance retention of 51.7% remains, i.e., 185.1 F g-1, compared with 44% at 15 A g-1 for MnO2/NGCF. In addition, we apply ex situ X-ray diffraction (XRD) and scanning electron microscopy (SEM) to verify the fact that nitrogen doping with abundant vacancies will benefit reversible intercalation/stripping of Na+ by triggering the phase change from MnO2 to MnOOH for N-MnO2 compared with MnO2 with adsorption/desorption during the charge-discharge process, which has never been discussed before. Excitingly, an asymmetric supercapacitor (ASC), fabricated by N-MnO2/NGCF as a cathode and MnO2/NGCF as an anode, delivers a maximum energy and power density of 75.3 Wh kg-1 and 18.1 x 103 W kg-1, respectively. This work provides an approach to design self-standing advanced N-doped MnO2 and a deeper insight into the mode of reaction of MnO2 in neutral electrolyte for aqueous SCs.