Salt Concentration-Regulated Desalination Mechanism Evolution in Battery Deionization for Freshwater

Battery deionization (BDI) based on the Faradaic process featuring with large ion storage capacity and a high energy efficiency holds great prospects for the next-generation desalination application. Owing to the intrinsic salt ion insertion and the electrochemical redox nature of BDI, the ion insertion dynamics within the electrode during the electrochemical redox processes could be strongly influenced by the properties of the electrolyte, especially the salt concentration. Although tremendous effort has been devoted to developing the BDI electrode materials, the relationship between the detailed desalination dynamics and salt concentration in electrolytes has never been systematically studied since the invention of this technology. To unveil the underlying behaviors, we have employed the NaTi2(PO4)3@C electrode as the representative BDI electrode and systematically investigated the electrochemical desalination dynamics under different concentration NaCl solutions. For the first time, standard freshwater has been obtained from NaCl solution via the BDI technique. Interestingly, the intrinsic desalination mechanism evolves from a diffusion-controlled charge contribution process into the capacitive charge contribution-dominated process as the NaCl concentration in water decreases from normal saline water into freshwater. The present study demonstrated the possibility of achieving freshwater with BDI. The insights obtained here on the ion storage dynamics paves the way for developing high-efficiency BDI electrodes toward practical desalination applications for freshwater.