Atomic substituents effect on boosting desalination performances of Zn-doped NaxCoO2

The layered-transition-metal oxides NaxMO2 (M = Mn, Co, V, etc.) has been widely recognized as a sodium-storage electrode material for Faradaic deionization. However, the relatively low desalination capacity caused by the poor conductivity makes it still inadequate for the high performing requirements. Herein, Zn-doped NaxCoO2 has been developed for the enhanced capacitive deionization (CDI) properties. The partial Co3+ substituted by Zn2+ can induce the appearance of electronic holes and effectively improve the electrical conductivity, which consequently is beneficial to the enhancement of the desalination performance. Compared with the pristine material, the capacity retention of Na0.71Co0.99Zn0.01O2 improved from 88.6% to 98.3% over 50 cycles, indicating that the inactive Zn2+ ions can also prevent the irreversible interlayer-gliding from alleviating the structure destruction. Additionally, in situ Raman was employed for the first time to investigate the mechanism, demonstrating that Na+ ions were reversibly inserted/extracted into/out of interlayers, along with c-axis length decreased and expanded back completely.