Reducing the weight of the negative current collector is an effective strategy for lightweighting vehicle batteries. Herein, thin copper foil with high tensile strength was prepared through electrodeposition in Br−-based aqueous solution. The synergistic effect of additive Br−, polyethylene glycol (PEG), Bis-(sodium sulfopropyl)-disulfide (SPS) on the microstructure and tensile strength of the copper foils have been systematically explored. It was found that the addition of Br− modifies the microstructure of copper via grain refinement, thereby enhancing the mechanical properties of copper foil. The obtained copper foil reaches an enhanced tensile strength of 469.8 N/mm2 and an elongation of 2.2%. The results of electrochemical measurements confirm that Br− enhances electrochemical polarization of PEG and attenuates the acceleration of SPS. The improved mechanical properties of copper foils are attributed to the closer proximity and stronger adsorption energy between Br− and Cu atoms, which results in a synergy of grain refinement and nanotwinned grains associated with increased dislocation density as well as dislocations entangled around nanotwin GBs.