Electrolytes which can effectively enhance the smoothness of deposited films during overcharging process of metal ion batteries are provided. The electrolyte contains a solvent, a metal (M1) salt containing the M1 cations to be intercalated or alloyed into the anion material of the metal ion battery, and an additive metal (M2) salt containing the M2 cations not to be intercalated or alloyed into the anode material. The M2 cations of the additive salt can be preferentially adsorbed, but cannot be deposited on the protruded region of the anode surface, therefore forms a positively charged electrical shield (PAES) which covers the protruded region. During overcharging or other charging processes under extreme conditions, the un-intercalated or un-alloyed M1 cations will be prevented to deposit in the protruded region of the anode by this self-assembled electrical shield (SAES), instead they will be preferentially deposited onto the non-protruded region. This self-smoothing process will effectively improve the smoothness of the deposited M1 film during overcharging of a metal ion battery, thus significantly improve the safety of the metal ion battery.