Electrochemical CO2 discount response (ECO2RR) pushed by renewable electrical energy holds promise to retailer intermittent vitality in chemical bonds whereas producing value-added chemical substances and fuels sustainably. Sadly, it stays a grand problem to concurrently obtain excessive Faradaic effectivity (FE), low overpotential, and excessive present density of ECO2RR. Herein, we report the synthesis of heterostructured Bi-Cu2S nanocrystals through a one-pot solution-phase technique. The epitaxial progress of Cu2S on Bi results in considerable interfacial websites and the resultant heterostructured Bi-Cu2S nanocrystals allow extremely environment friendly ECO2RR with largely decreased overpotential (240 mV decrease than Bi), near-unity FE (>98%) for formate manufacturing, and excessive partial present density (2.4- and 5.2-fold increased JHCOO- than Cu2S and Bi at -1.0 V vs. reversible hydrogen electrode, RHE). Density purposeful concept (DFT) calculations present that the electron switch from Bi to Cu2S on the interface results in the preferential stabilization of the formate-evolution intermediate (*OCHO).