Construction of δ-MnO2/rGO cathodes via in situ oil–water interface reaction for enhancing electrochemical performance of aqueous zinc-ion batteries

Abstract

MnO₂-based cathodes typically suffer from poor electrical conductivity, severe dissolution, and agglomeration in aqueous zinc-ion batteries (AZIBs), and rGO species are utilized to resolve these issues due to high surface area and favorable electrical conductivity. Nevertheless, the undesired composite structures of MnO₂ and rGO significantly restrain the Zn²⁺ storage capability of cathode materials, which is mainly determined by the synthesis method. Herein, we proposed an innovative oil–water interface reaction to in situ grow δ-MnO₂ on GO substrates, and a subsequent thermal reduction was applied to obtain the δ-MnO₂/rGO composites (in-MG-2). Compared with samples prepared by simple physical blending and in situ growth of δ-MnO₂ on rGO substrates, the as-prepared in-MG-2 samples demonstrate favorable features, including well-dispersed δ-MnO₂ nanosheets, expanded (001) crystal planes, and the formation of Mn–O–C bonds, which can enhance the diffusion rates of Zn²⁺ ions, accelerate interfacial charge transfer and reduce de-solvation energy barrier. Therefore, the as-assembled Zn//in-MG-2 cell exhibits a higher capacity of 331.11 mAh g⁻¹ at 0.2 A g⁻¹ and 142.8 mAh g⁻¹ at 4.0 A g⁻¹ and an enhanced stability of 97.6% capacity retention after 500 cycles at 1.0 A g⁻¹. The Zn//in-MG-2 pouch cell can also power a smartphone and exhibit superior cycling stability. Moreover, the storage mechanism involving the cooperative insertion of H⁺/Zn²⁺ was revealed in the in-MG-2 cathode. This work not only provides a promising cathode for AZIBs but also exploits a new oil–water interface reaction to grow other transition metal compounds on GO substrates.