Over-Reduction-Controlled Mixed-Valent Manganese Oxide with Tunable Mn2+/Mn3+ Ratio for High-Performance Asymmetric Supercapacitor with Enhanced Cycling Stability.

Manganese oxides composed of various valence states Mnx+ (x = 2, 3, and 4) have attracted wide attention as promising electrode materials for asymmetric supercapacitor. However, the poor electrical conductivity limited their performance and application. Appropriate regulation content of Mnx+ in mixed-valent manganese oxide can tune the electronic structure and further improve their conductivity and performance. Herein, we prepared manganese oxides with different Mn2+/Mn3+ ratios through an over-reduction (OR) strategy for tuning the internal electron structure of mixed-valent manganese, which could make these material oxides a good platform for researching the structure-property relationships. The Mn2+/Mn3+ ratio of manganese oxide could be precisely tuned from 0.6 to 1.7 by controlling the amount of reducing agent for manipulating the redox processes, where the manganese oxide electrode with the most appropriate Mn2+/Mn3+ ratio, as 1.65 (OR4) exhibits large capacitance (274 F g-1) and the assembling asymmetric supercapacitors by combining OR4 (positive) and the commercial activated carbon (as negative) achieved large 2.0 V voltage window and high energy density of 27.7 Wh kg-1 (power density of 500 W kg-1). The cycle lifespan of the OR4//AC could keep about 92.9% after 10 000-cycle tests owing to the Jahn-Teller distortion of the Mn(III)O6 octahedron, which is more competitive compared to other work. Moreover, a red-light-emitting diode (LED) can easily be lit for 15 min by two all-solid supercapacitor devices in a series.