Electrochemical selective ion separation in capacitive deionization with sodium manganese oxide

Electrochemical selective ion separation via capacitive deionization, for example, separation of lithium resource from brine, using lithium ion batteries is proposed and demonstrated to have the potential for separating specific ions selectively from a solution containing diverse ions. This separation method is of great industrial concern because of applicability in various fields such as deionization, water softening, purification, heavy metal removal, and resource recovery. Nevertheless, besides the selectivity of materials for lithium ion batteries toward Li⁺, there is very little investigation on the selectivity of the materials for sodium ion batteries toward Na⁺. Here, the electrochemical selectivity of sodium manganese oxide (Na_0.44MnO₂), one of the most widely used material in sodium ion batteries, for Na⁺ and other cations (K⁺, Mg²⁺, and Ca²⁺) is investigated. Selective Na⁺ separation using the system consisting of Na_0.44MnO₂ and a Ag/AgCl electrode is successfully demonstrated from a solution containing diverse cations (Na⁺, K⁺, Mg²⁺, and Ca²⁺) via a two-step process that involves a capturing step (charging process) and a releasing step (discharging process). The results showed that Na_0.44-xMnO₂ has over 13 times higher selectivity for Na⁺ than for K⁺ and 6-8times higher selectivity for Na⁺ than for Mg²⁺ and Ca²⁺ in the electrolyte containing equal concentrations of the respective ions. Additionally, as a practical demonstration, Na⁺ was successfully separated from an industrial raw material used for pure KOH production (estimated ratio of Na⁺:K⁺=1:200).