Mechanical Insights into the Electrochemical Properties of Thornlike Micro-/Nanostructures of PDA@MnO2@NMC Composites in Aqueous Zn Ion Batteries

As emerging energy storage devices, aqueous zinc ion batteries (AZIBs) with outstanding advantages of high safety, high energy density, and environmental friendliness have attracted much research interest. Herein, the favorable thornlike MnO2 micro-/nanostructures (PDA@MnO2@NMC) are rationally constructed by the incorporation of both carbon substrates (NMC) and polydopamine (PDA) surface modifications. Ex situ X-ray diffraction and Raman characteristics show the formation of MnOOH and ZnMn2O4 products, corresponding to H+ and Zn2+ insertions in two discharge platforms. Density functional theory (DFT) calculations also demonstrate that PDA can firmly anchor onto MnO2 surfaces and prevent the dissolution of MnOOH. In addition, PDA with more hydrophilic groups can capture more H+ together with the increased surface capacitance and the extension of the first discharge platform, while the NMC carbon substrate can provide abundant active sites for the overgrown MnO2 nanowires, improve the conductivity, and promote fast ion and electron transportations. Further, electrochemical impedance spectroscopy (EIS) and GITT results show that the ohmic resistance of PDA@MnO2@NMC decreases to almost half and, in particular, the ion diffusion coefficient increases more than 30 times of pure MnO2. As such, PDA@MnO2@NMC in the AZIB cathode exhibits excellent electrochemical performance compared to the pure MnO2, which is expected to have favorable competitiveness in energy storage devices.