Diminishing the migration resistance of zinc ions by cation vacancy engineering in a spinel-framework

Abstract

The three-dimensional framework structure makes the ZnMn₂O₄ spinel an excellent host material for zinc ion intercalation and deintercalation processes. However, the high migration energy barrier of zinc ions from the tetrahedron 8a site to the octahedron 16c site in the ZnMn₂O₄ spinel framework limits its practical application for zinc-ion batteries. The focus of this work is to reduce the migration resistance of zinc ions by constructing cationic zinc vacancies in ZnMn₂O₄ (named Zn_1−x□_xMn₂O₄). The reasonable structure adjusted by alkaline etching is conducive to the diffusion of zinc ions. Combining the electrochemical tests and theoretical investigations, it was found that the introduction of zinc vacancies in the Zn_1−x□_xMn₂O₄ spinel framework caused the diffusion barrier of zinc ions to be reduced by approximately 0.4 eV compared to ZnMn₂O₄. In addition, the optimal Zn_0.65□_0.35Mn₂O₄ with 35% zinc vacancies was cross-linked by conductive carbon nanotubes (CNTs) to establish an excellent conducting network. The as-prepared Zn_0.65□_0.35Mn₂O₄/CNT composite exhibits a high reversible capacity of 479.3 mA h g⁻¹ at 0.05 A g⁻¹ with a good energy density of 640 W h kg⁻¹ at 28 W kg⁻¹. Our work is to convert the electrochemically poorly active traditional ZnMn₂O₄ spinel into a functionally active electrode for zinc-ion batteries.