Unlocking exceptional sodium ion storage capability of manganese dioxide via controllable incorporation of cation vacancies

Abstract

Defect engineering has been widely considered as a promising structure modulation strategy to realize enhanced Na⁺ storage in MnO₂; however, incorporation of cation vacancies in a controllable manner is still in urgent need. In this study, a novel “Ion doping + Selective etching” strategy has been proposed that enables facile and controllable creation of cation vacancies in α/δ-MnO₂ nanostructures. More importantly, the effects of cation vacancies on Na⁺ storage are found to be strongly correlated with the crystal structures of MnO₂. 3D ion diffusion channels are formed in α-MnO₂ with the formation of cation vacancies, which facilitate ion (de)intercalation under high rate conditions. In δ-MnO₂, the cation vacancies mainly promote charge transfer and provide abundant Na⁺ insertion sites, and therefore result in dramatically increased specific capacitance at low current densities. The reported work paves the way for rational design of defect structures in a metal oxide framework towards enhanced charge storage capability.