Low-cost　copper　(Cu)-based　electrocatalysts　have　been　widely　established　with　the　special　capability　of　generating　C2+　products　from　the　CO2　reduction　reaction　(CO2RR).　However,　efficient　formate　production　has　been　rarely　achieved　due　to　the　instant　reduction　of　most　reported　Cu-based　catalysts　upon　CO2RR,　and　the　derived　metallic　Cu　compromises　the　C-1　selectivity.　Herein,　we　demonstrate　that　the　incorporation　of　alkali　metal　ions　is　intrinsically　effective　for　stabilizing　the　Cu(I)-S　bonds　by　forming　ternary　copper　sulfides　(M-Cu(I)-S,　M　=　Na,　K,　and　Rb).　The　strengthened　Cu-S　bonds　can　be　well　preserved　in　M-Cu(I)-S　during　the　CO2RR,　contributing　to　the　protonation　effect　and　thus　highly　efficient　production　of　formate.　Moreover,　the　M-Cu(I)-S　catalysts　also　exhibit　enhanced　electrical　conductivity　relative　to　that　of　Cu2S,　favorably　promoting　the　reaction　kinetics.　Accordingly,　the　RbCu7S4　as　a　representative　catalyst　achieves　a　Faradaic　efficiency　of　90.4　+/-　1.3%　for　formate　at　only　-0.7　V　versus　reversible　hydrogen　electrode　(V-RHE),　with　a　high　partial　current　density　of　272.1　mA　cm(-2)　and　stable　operation　over　72　h.　This　study　could　provide　a　different　series　of　Cu-based　electrocatalysts　for　efficient　formate　production　on　an　industrial　scale　from　the　CO2RR.