Revealing ZnMn3O7 as an advanced cathode material for Zn-ion batteries

Abstract

Rechargeable aqueous Zn-Mn batteries have emerged as a promising candidate for grid-scale energy storage application, offering high specific energy, cost-effectiveness, environmental sustainability, and superior safety characteristics. ZnMn₃O₇ (ZMO) has recently gained attention as a potential cathode for aqueous energy storage systems, attributed to its layered structure, abundant manganese redox centers, and intrinsic vacancy sites that enable efficient ion diffusion. However, direct synthesis of ZMO remains challenging, as it preferentially transforms into the Zn-deficient spinel structure (Zn_0.75Mn_0.25)Mn₂O₄. In this study, we approach a synthesis method for ZMO via chemical ion-exchange method, employing Na₂Mn₃O₇ (NMO) as the starting precursor. The process involves a chemical ion-exchange reaction facilitated by 5 M ZnSO₄ as the ionic solution, enabling efficient cation exchange at the vacancy sites of Na₂Mn₃O₇. Hydrated ZnMn₃O₇.3H₂O was prepared and subjected to controlled calcination within a temperature range of 100–600 °C to study its phase transitions and structural evolution. This investigation provided valuable insights into its thermal stability and the transformation mechanisms responsible for forming the anhydrous ZnMn₃O₇ phase. The ion-exchange mechanism was systematically studied through structural and morphological characterizations at different calcination stages. Electrochemical testing of ZMO with 1 M Zn(CF₃SO₃)₂ + 0.1 M MnSO₄ as the electrolyte demonstrated outstanding cycling stability, delivering a reversible discharge capacity of around 140 mAh g⁻¹ and 99% Coulombic efficiency over 100 cycles at a 1 C rate. These findings highlight the material's promise as a high-performance cathode for advanced energy storage applications.