Spinel-Structured Nickel Manganese Oxides for Electrochemical Magnesium Recovery

Recent years have witnessed the importance of securing the supply of metallic resources. Magnesium is the lightest structured metal on earth and is widely adopted in weight-sensitive applications and next-generation energy storage techniques. However, magnesium is currently produced by an energy-intensive process or by heavy use of chemicals, and there is a need to develop a new route to address these issues. In this study, electrochemical separation processes employing spinel-structured oxide electrodes are investigated. Delithiated LiMn2O4 (LMO) and LiNixMn2–xO4 (LNMO) showed high selectivity toward Mg2+ among the cations abundant in aqueous environments (Na+, Mg2+, K+, and Ca2+). Specifically, the LNMO electrode achieved high selectivity with separation factors of 10.47 for Mg2+/Na+, 13.59 for Mg2+/K+, and 6.40 for Mg2+/Ca2+. Although LMO manifested high magnesium-uptake capacity, a rapid drop in performance was observed during cyclic operations. Meanwhile, LNMO exhibited significantly enhanced long-term stability, with around 3 times larger capacity than that of LMO up to 100 cycles, suggesting the feasibility of the electrochemical process in magnesium separation and production. Moreover, the physicochemical behaviors of the spinel oxide electrodes in magnesium recovery are unveiled, and the strategies for further improvement of performance are discussed.