Mg-inserted δ-MnO2 nanosheet assembly for enhanced energy storage

Supercapacitors based on aqueous electrolytes containing multivalent metal ions have attracted much interest due to their higher capacitances than those containing monovalent metal ions. In this paper, divalent metal ions-inserted δ-MnO2 nanosheet assemblies have been synthesized hydrothermally by introducing metal precursors in the synthetic systems. The thickness of MnO2-based nanosheet and the morphology of the assemblies were varied by adjusting the metal ions. Pure MnO2 nanosheet assembly shows the highest capacitance of 210.5 F g−1 at 0.2 A g−1 in aqueous Na2SO4 electrolyte, slightly higher than Mg-inserted and Fe-involved assemblies and the least for Ni-involved assembly. When aqueous MgSO4 electrolyte was used, Mg-inserted assembly shows the highest capacitance of about 301.8 F g−1 much higher than pure MnO2 or Fe/Ni-involved assemblies at 0.2 A g−1. Mg-inserted MnO2 sample maintains 90 % of the initial capacitance after 3000 cycles in the divalent electrolyte, while 50 % for pure MnO2 assembly. Based on the experimental data, the proposed difference between Mg-inserted and Ni-/Fe-involved assemblies combined with the charge nature in electrolytes is suggested to be responsible for their electrocapacitive behavior.