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

Abstract

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.