Faradaic deionization (FDI) provides an effective solution to respond to the global water crisis. However, the ions intercalation/deintercalation process with multiple redox reactions leads to structural collapse and unstable cyclability. Here, we develop an asymmetrical FDI device assembled by Ca2+-decorated NaxCoO2 (x ≤ 0.71, y ≤ 0.05) as the Faradaic negative electrode and activated carbon as the positive electrode. Na0.27Ca0.03CoO2·0.6H2O was synthesized via a facile sol–gel and chemical oxidation method, which delivered a desalination capacity of 83.5 ± 2.4 mg g–1 and a charge efficiency close to 1, and an inappreciable capacity fading was observed after 50 cycles. It is found that the presence of Ca2+ residing in the face-sharing sites helps to maintain the layered structure and promotes efficient deintercalation of Na+ by anchoring the CoO2 slabs, which results in its unexpected desalination capacity and good cyclability. Moreover, electrochemical quartz-crystal microbalance (EQCM) was successfully used to reveal Faradaic intercalation mechanism.

https://pubs.acs.org/doi/10.1021/acssuschemeng.9b02157