Sodium-Manganese Oxides in Faradaic Desalination: Achieving Long-Cycling Stability Through Morphological and Structural Optimization

Water scarcity, driven by climate change and population growth, necessitates innovative desalination technologies. Conventional methods for brackish water desalination are limited by high-energy demands, especially in the low salinity range, prompting the exploration of electrochemical approaches like faradaic deionization. Sodium-manganese oxides, traditionally used in sodium-ion batteries, show promise as faradaic deionization electrode materials due to their abundance, low toxicity, and cost-effectiveness. However, capacity fading during cycling, often caused by structural changes, volume expansion, or chemical transformations, remains a critical challenge. This study investigates the impact of morphology and crystal structure on the electrochemical performance of commercial and synthesized sodium-manganese oxides for faradaic deionization applications. Structural and electrochemical characterization in three-electrode cells with low-concentration electrolytes provided insights into the charge storage mechanisms. Rocking-chair full flow cell experiments demonstrated that the mixed-phase sodium-manganese oxide exhibited superior desalination performance, achieving a high salt removal capacity of 54.5 mg g⁻¹ and a mean value in the salt removal rate of 1.49 mg g⁻¹ min⁻¹. Notably, mixed-phase sodium-manganese oxide maintained 98% capacity retention over 870 cycles, one of the longest reported cycling experiments in this field, effectively mitigating the Jahn-Teller effect. These findings highlight the crucial role of sodium-manganese oxide structure and morphology in electrochemical performance, positioning mixed-phase sodium-manganese oxide as a strong candidate for sustainable water treatment technologies.