Achievement of high durability of δ-MnO2 based pseudocapacitive electrode enabled by Zn doping induced reattachment

As a traditional pseudocapacitive electrode, layered δ-MnO2 is famous for its good cycling stability due to the H+ non-favorable mechanism. However, its durability is still unsatisfied for practical use owning to the crystal structure change during long-term cycling. In the work here, superior cycling performance with high capacitance of ∼281 F g−1 at 0.2 A g−1 is achieved for Zn doped δ-MnO2 with a high mass loading of 5.4 mg cm−2. Periodic oscillation of the capacity is found for the Zn-doped samples, while monotonous capacity decline accompanied by continuous mass loss is observed for the pure δ-MnO2 during long-term cycling. For the optimized doping sample of Zn-3-MnO2 prepared with a Zn:Mn feed molar ratio of 3:100, the oscillation period is ∼5500 cycles at 10 A g−1. Its specific capacitance is increased to 117% of the initial value (∼188 F g−1 at 10 A g−1) in the first 500 cycles and then declines to about 107% in the following ∼5000 cycles, which is accompanied by a color change of the electrolyte, clear and then yellow. 105% of the initial capacitance is remained after 20,000 cycles cycling. Similar capacity oscillation can be observed for Zn-1-MnO2 and Zn-5-MnO2, while their oscillation periods are different and the enhancement is smaller than the decline in each period, resulting in obvious capacity decay after long-term cycling. Through careful characterization to the samples before and after cycling, an orientated reattachment mechanism is proposed to explain the capacity change along cycling, where Zn doping is supposed to play a very important role to this process. [Display omitted] •Zn doped δ-MnO2 is prepared through a facile hydrothermal procedure.•Optimized doping sample Zn-3 exhibits high capacity and superior durability with a periodic oscillation for the capacity.•A Zn-induced orientated reattachment mechanism is proposed to explain the capacity change along long-term cycling.