Enhancing the cycling performance of manganese oxides through pre-sodiation for aqueous Zn-ion batteries

Although Li-ion batteries have dominated the portable electronics market due to their high energy density and long cycle-life, it is essential to find alternative energy storage devices that can be cost-effective and environmentally friendly because of the low abundance and high cost of Li. In this regard, aqueous Zn-ion batteries are promising, because of their high abundance, low cost, high gravimetric capacity of the Zn anode and the environmental friendliness of aqueous electrolytes compared to flammable and costly organic electrolytes. Manganese oxides are the preferred cathode materials for aqueous Zn-ion batteries because of their specific capacity above 200 mA h g⁻¹, low cost and environmental friendliness. However, they suffer from capacity fading due to manganese dissolution and structural transformation upon cycling. In this study, pre-sodiated manganese oxide Na_0.6MnO₂ is synthesized by a hydrothermal method, followed by annealing at 900 °C, and its performance was tested for Zn-ion batteries by means of cyclic voltammetry, galvanostatic charge-discharge cycling and electrochemical impedance spectroscopy. Interestingly, Na_0.6MnO₂ delivers an initial specific capacity of 165 mA h g⁻¹, showing approximately 77% capacity retention over 150 cycles when cycled at 0.2 A g⁻¹ in the voltage domain of 1.0-2.0 V vs. Zn in 1.0 M ZnSO₄ + 0.1 M MnSO₄. On the other hand, Mn₂O₃ (without pre-sodiation) exhibits a high specific capacity above 200 mA h g⁻¹; however, it undergoes severe capacity fading and retains only 26% capacity after 150 cycles. The ex situ XRD analysis shows the formation of a major spinel ZnMn₂O₄ phase along with a ZnMn₃O₇ phase, thus confirming the intercalation of the zinc-ion during the discharge process. Thus, this study enlightens the importance of pre-sodiation of manganese oxides in aqueous Zn-ion batteries for maintaining a stable specific capacity upon prolonged cycling.