Activating electrochemically inert ZnMn₂O₄ via a synergistic effect of cationic and anionic dual vacancies

MnO₂ species are inexpensive and attractive green cathode materials for aqueous zinc-ion batteries (AZIBs). Still, they usually degrade into electrochemically inert ZnMn₂O₄ (ZMO) due to the sluggish diffusion ability of Zn²⁺, which blocks the internal lattice diffusion channels, significantly hindering the cycle stability of AZIBs. Although Zn vacancy engineering is expected to enhance the electrochemical performance of ZMO, there is an urgent need to develop defect structures that can effectively inhibit the further generation of inert ZMO. Herein, to activate the electrochemical performance of the spent ZMO, cationic and anionic dual vacancies are introduced into ZMO through an acid treatment. The acid treatment removes Zn²⁺ and forms H-ZMO with Mn cationic vacancies and O anionic vacancies. The dual vacancy structure increases the diffusion energy barrier of the octahedron-tetragon-octahedron transition for Zn²⁺, thereby inhibiting the transition of MnO₂ to ZMO. Moreover, we reveal that the vacancy structure can promote the Grotthuss hopping ability of H⁺ as the dominant carrier, thereby significantly improving the electrochemical properties of H-ZMO. This article provides universal principles and methods for recycling and activating the electrochemical performance of ZMO, which is conducive to large-scale industrialization processes of AZIBs cathode.