A λ-MnO2-based electrochemical system enables rapid and energy-efficient lithium recovery from desalination brine with high selectivity.

Given that a considerably large population suffers from shortage of water, there are numerous on‐going efforts to turn seawater into freshwater, and electrochemical desalination processes—particularly capacitive deionization (CDI)—have gained significant attention due to their high energy efficiency and reliable performance. Meanwhile, carbonaceous electrode materials, which are most commonly used in CDI systems, have poor long‐term stability due to unfavorable interactions with oxygen in saline water. Herein, rapid and vigorous inversion of surface charges in heteroatom‐doped carbon electrodes, which leads to a robust operation of CDI with high desalination capacity, is reported for the first time. By carbonization of coffee wastes, nitrogen‐ and sulfur‐codoped activated carbon with hierarchical micro/mesopores are prepared in an environmentally‐friendly manner, and this carbon results in a significantly higher inverted capacity than that of various activated carbon counterparts in long‐term CDI operations, without any sign of drop in performance. Investigations on the changes in physicochemical properties of the electrodes during the inversion disclose the favorable roles of nitrogen and sulfur dopants, which can be summarized as enlarging the difference between the surface charges of the two electrodes by chemical interactions with oxygen in the anode and carbon in the cathode.

Recently developed electrochemical lithium recovery systems, whose operation principle mimics that of lithium-ion battery, enable selective recovery of lithium from source waters with a wide range of lithium ions (Li + ) concentrations; however, physicochemical behaviors of the key componentLi + -selective electrodein realistic operation conditions have been poorly understood. Herein, we report an investigation on a λ-MnO 2 electrode during the electrochemical lithium recovery process with regards to the Li + concentration in source water and operation rate of the system. Three distinctive stages of λ-MnO 2 originating from different limiting factors for lithium recovery are defined with regard to the rate of Li + supply from the electrolyte: depleted, transition, and saturated regions. By characterization of λ-MnO 2 at different stages using diverse X-ray techniques, the importance of Li + concentration in the vicinity of the electrode surface is revealed. On the basis of this understanding, increasing the density of the electrode/electrolyte interface is suggested as a realistic and general route to enhance the overall lithium recovery performance and is experimentally corroborated at a wide range of operation environments.