Defect regulated spinel Mn3O4 obtained by glycerol–assisted method for high–energy–density aqueous zinc–ion batteries

Rechargeable aqueous zinc-ion batteries (RAZIBs) show great potential as a competitive candidate for reliable energy storage by virtue of cost-effectiveness, high safety, and environmental friendliness. However, unsatisfactory cycle stability of cathode material impedes the development of high-performance RAZIBs. This study reveals a strategic polyol-mediated process by using glycerol as the solvent for solvothermal reaction. After heat treatment in air, Mn-deficient Mn3O4 spinel (D-Mn3O4) can be obtained with rich Mn valence states (Mn2+/Mn3+/Mn4+), expanded crystal structure, high surface area, and good electrolyte compatability. Compared to well-crystallized Mn3O4, the presence of manganese vacancies in D-Mn3O4 enables lower charge-transfer resistance (86.0 vs 196.5 Ω), reduced activation energy for ion insertion (30.9 vs 50.4 kJ mol-1), and boosted solid-state ion diffusivity (9.45 × 10-12 vs 4.61 × 10-14 cm2 s-1). Therefore, D-Mn3O4 exhibits promising electrochemical performance with high capacity (284 mAh g-1), high specific energy (388.5 Wh kg-1) and stable cycle retention (87% after 200 cyclesat 0.3 A g-1). On the contrary, the well-crystallized Mn3O4 sample suffers from severe capacity fading with only 48% capacity retention. Moreover, the specific energies obtained after 200 cycles are 336.1 and 166.0 Wh kg-1 for D-Mn3O4 and Mn3O4, respectively. The drastic differences between the electrochemical performance of D-Mn3O4 and Mn3O4 manifest that the existing manganese vacancies in Mn3O4 spinel structure enhance energy storage capability.