Electrolytes which can effectively enhance the smoothness of deposited films during electroplating process are provided. The electrolyte contains a solvent, a metal (M1) salt containing a metal to be deposited, and an additive metal (M2) salt. The cations of the additive salt can be preferentially adsorbed, but cannot be deposited on the protruded region of the deposited film, therefore forms a positively charged electrical shield which covers the protruded region. This self-assembled electrical shield (SAES) will prevent further deposition of metal (M1) in the protruded region so metal (M1) will be preferentially deposited onto the non-protruded region. This self-smoothing process will effectively improve the smoothness of deposited films during electroplating process.