Cation-Deficient Spinel ZnMn₂O₄ Cathode in Zn(CF₃SO₃)₂ Electrolyte for Rechargeable Aqueous Zn-Ion Battery

Rechargeable aqueous Zn-ion batteries are attractive cheap, safe and green energy storage technologies but are bottlenecked by limitation in high-capacity cathode and compatible electrolyte to achieve satisfactory cyclability. Here we report the application of nonstoichiometric ZnMn₂O₄/carbon composite as a new Zn-insertion cathode material in aqueous Zn(CF₃SO₃)₂ electrolyte. In 3 M Zn(CF₃SO₃)₂ solution that enables ∼100% Zn plating/stripping efficiency with long-term stability and suppresses Mn dissolution, the spinel/carbon hybrid exhibits a reversible capacity of 150 mAh g^-1 and a capacity retention of 94% over 500 cycles at a high rate of 500 mA g^-1. The remarkable electrode performance results from the facile charge transfer and Zn insertion in the structurally robust spinel featuring small particle size and abundant cation vacancies, as evidenced by combined electrochemical measurements, XRD, Raman, synchrotron X-ray absorption spectroscopy, FTIR, and NMR analysis. The results would enlighten and promote the use of cation-defective spinel compounds and trifluoromethanesulfonic electrolyte to develop high-performance rechargeable zinc batteries.