High-performance biphasic NaxMnO2 electrodes for cost-effective and high-power aqueous sodium batteries and capacitors†

Aqueous sodium batteries and capacitors offer a low-cost and sustainable alternative to lithium-based energy storage systems, with their performance crucially dependant on the choice of electrode materials. Among the candidates commonly used in sodium-ion devices, various phase modifications of presodiated manganese oxide are considered promising. In this work, we synthesized biphasic (orthorhombic/monoclinic) NaMnO₂ using a cost-effective sol–gel technique and investigated its performance as an electrode material for aqueous sodium electrochemical systems. The performance was evaluated through cyclic voltammetry and galvanostatic charge–discharge measurements. The results demonstrated that NaMnO₂ electrodes were highly suitable for high-power energy devices, exhibiting a specific capacity of 103 mA h g⁻¹ and high capacity retention, even under high current conditions (82% capacity retention as the current increased from 1C to 20C). The superior electrochemical performance, especially under high discharge current conditions, was attributed to the optimal combination of different pseudocapacitive mechanisms associated with the biphasic monoclinic-orthorhombic phase structure, which ensured both high capacity and stability during cycling, as well as the morphology of the samples. These results paved the way for the development of high-power, stable, and cost-effective aqueous sodium-ion storage devices.