Novel Alkaline Zn/Na_0.44MnO₂ Dual-Ion Battery with a High Capacity and Long Cycle Lifespan

A rechargeable aqueous Zn/Mn battery is a promising device for large-scale energy storage because of its abundant resources, low cost, and high safety. However, its application is plagued by a poor life cycle because of the electrochemical instability of MnO2 in aqueous electrolytes. Here, an alkaline Zn–Na0.44MnO2 dual-ion battery (denoted AZMDIB) is developed for the first time using Na0.44MnO2 as the cathode, a zinc metal sheet as the anode, and a 6 M NaOH aqueous solution as the electrolyte. When the discharge cutoff voltage is lowered to 0.3 V (vs Zn/Zn2+), the Na0.44MnO2 cathode delivers a high capacity of 345.5 mA h g–1 but with a poor cycling performance. The charge–discharge mechanism and structural evolution of the Na0.44MnO2 cathode in an extended potential window (1.95–0.3 V) are also explored. The Na0.44MnO2 electrode experiences two different electrochemical processes: Na+ ions insert/extract reversibly in the potential range of 1.95–1.1 V, and a phase transition occurs from Na0.559MnO2 to Mn(OH)2 below 1.1 V. The latter irreversible reaction is probably due to proton insertion, leading to a severe capacity fade. Nevertheless, in a narrower voltage range (2.0–1.1 V), the AZMDIB full cell exhibits a high reversible capacity (80.2 mA h g–1 at 0.5 C), high rate capability (32 mA h g–1 at 50 C), and excellent cycling stability (73% capacity retention over 1000 cycles at 10 C). Benefiting from the merits of environmental friendliness, cost-effectiveness, and high electrochemical performance, the rechargeable AZMDIB is a promising contender for grid-scale energy storage applications.