The reaction of precursors containing both nitrogen and oxygen atoms with Ni II under 500 8 8Cc an generate aN /O mixing coordinated Ni-N 3 Os ingle-atom catalyst (SAC) in which the oxygen atom can be gradually removed under high temperature due to the weaker NiÀOi nteraction, resulting in avacancy-defect Ni-N 3 -V SACatNisite under 800 8 8C. Forthe reaction of Ni II with the precursor simply containing nitrogen atoms,o nly an o-vacancy-defect Ni-N 4 SACw as obtained. Experimental and DFT calculations reveal that the presence of av acancy-defect in Ni-N 3 -V SACc an dramatically boost the electrocatalytic activity for CO 2 reduction, with extremely high CO 2 reduction current density of 65 mA cm À2 and high Faradaic efficiency over 90 %a tÀ0.9 Vv s. RHE, as well as arecordhigh turnover frequency of 1.35 10 5 h À1 ,muchhigher than those of Ni-N 4 SAC, and being one of the best reported electrocatalysts for CO 2 -to-CO conversion to date.