Tremendous efforts have been made in developing highly selective catalysts for electrochemical CO 2 conversion to formate. However, the rapid deactivation resulting from structural reconstruction and phase transition poses considerable challenges to the system's durability, particularly at industrially relevant current densities. In this study, we develop a stable hexagonal phase ( g-In 2 S 3 ) catalyst, demonstrating exceptional selectivity toward formate production with a Faradaic efficiency exceeding 90% across a broad current range from - 100 to - 1,300 mA cm - 2 . Theoretical calculations suggest that the formation of sulfur (S) vacancy in g-In 2 S 3 is impeded under CO 2 reduction conditions, thereby contributing to enhanced electrode stability during electrolysis. Further mechanism studies reveal that the persistence of the S atom enables electron -enriched indium (In) -active sites, likely modifying the adsorption/desorption of the key intermediates for formate production. When coupled with a commercial GaInP/GaAs/Ge triple -junction solar cell, the solar -assisted CO 2 electrolysis reaches a benchmark solar -to -formate conversion efficiency of 14.3% under standard illumination.