Enhanced pseudo-capacitance and rate performance of amorphous MnO2 for supercapacitor by high Na doping and structural water content

Owing to the advantages of high theoretical capacitance, low cost, and environmental friendliness, MnO2 shows a great potential as the electrode material for supercapacitors. However, its low conductivity and sluggish ion migration lead to the unsatisfied electrochemical performance especially at high rates. In this work, amorphous MnO2 with both high Na doping and structural water content (nw-MnO2) is synthesized via a simple redox reaction in the saturated sodium chloride solution. On the one hand, high Na doping can effectively improve the conductivity of MnO2; on the other hand, the charge shielding effect enabled by structural water facilitates the rapid ion migration in MnO2 lattice. In addition, the amorphous feature also provides more active sites and decreases the ion diffusion length. Benefitting from the above merits, the prepared nw-MnO2 electrode delivers superior rate performance (324.7 F g−1 at 0.5 A g−1 and 182.6 F g−1 even at an ultrahigh rate of 50 A g−1) and excellent cycle stability with 91% of capacitance retention over 10,000 cycles. Moreover, the assembled nw-MnO2//activated carbon asymmetric supercapacitor delivers a high energy density of 81.1 Wh kg−1 and superior capacity retention of 101% after 7000 cycles.