The influence of in-situ induced and MnO2 polymorph-dependent structural changes to the energy storage and mechanism of supercapacitors

Abstract

Due to its economic, environmental, and theoretical advantages, manganese dioxide (MnO2) is a promising cathode material for supercapacitors. However, the commercial application of MnO2 still faces challenges such as low intrinsic conductivity, slow ion diffusion, and structural instability. In this study, four crystalline forms of MnO2 powder electrodes (α-, β-, γ-, and δ-MnO2) were prepared, and results showed that with a specific capacitance of 218.2 F∙g−1, δ-MnO2 exhibited the best electrochemical performance. This was primarily due to the fact that δ-MnO2 possesses a high specific surface area, rich oxygen vacancy, and fast electron-ion transfer speed. A self-supported electrode (δ-MnO2/NF) without binder addition was further prepared using an in-situ-induced technique. The results revealed that the specific capacitance of δ-MnO2/NF was as high as 809.4 F∙g−1 and that the capacity retention rate and coulombic efficiency after 5000 cycles were 92.8 % and 99.2 %, respectively, which were superior to those of δ-MnO2 (80.3 % and 93.5 %). In-situ induction not only amplifies the structural advantages of δ-MnO2 but also generates more electrochemical active sites due to the intense interaction between NF and δ-MnO2. In addition, ultrathin nanosheet arrays formed on the NF provide a three-dimensional conductive network for the transport of ions and electrons. Density functional theory calculations were performed to verify the excellent electronic conductivity and ion diffusivity of δ-MnO2 and showed that transferring electrons from metal Ni to δ-MnO2 is the basis for producing numerous oxygen vacancies in δ-MnO2/NF.