A Novel Dual-Ion Capacitive Deionization System Design with Ultrahigh Desalination Performance

Capacitive deionization is an emerging desalination technology with mild operation conditions and high energy efficiency. However, its application is limited due to the low deionization capacity of traditional capacitive electrodes. Herein, we report a novel dual-ion capacitive deionization system with a lithium-ion battery cathode LiMn₂O₄/C and a sodium-ion battery anode NaTi₂(PO₄)₃/C. Lithium ions could enhance the charge transfer during CDI desalination, while NaTi₂(PO₄)₃/C provided direct intercalation sites for sodium ions. The electrochemical capacities of the battery electrodes fitted well, which was favorable for the optimization of the desalination capacity. The low potential of the redox couple Ti³⁺/Ti⁴⁺ (-0.8 V versus Ag/AgCl) and intercalation/deintercalation behaviors of sodium ions that suppressed hydrogen evolution could enlarge the voltage window of the CDI process to 1.8 V. The novel CDI cell achieved an ultrahigh desalination capacity of 140.03 mg·g^-1 at 1.8 V with an initial salinity of 20 mM, revealing a new direction for the CDI performance enhancement.