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 (Na0.44MnO2), 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 Na0.44MnO2 and a Ag/AgCI electrode is successfully demonstrated from a solution containing diverse cations (Na+, K+, Mg+, and Ca2+) via a two-step process that involves a capturing step (charging process) and a releasing step (discharging process). The results showed that Na0.44-X,MnO2 has over 13 times higher selectivity for Na+ than for K+ and 6-8 times 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). (C) 2017 Elsevier Inc. All rights reserved.