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.