In recent work the ionic transportation properties of organic electrolyte in Li-ion batteries has been described in detail by the present authors, taking into account ionic diffusion and migration processes. Advanced battery electrolytes may, however, be composed of various salts. Therefore the ionic transport properties of such complex electrolytes have been investigated from a theoretical point of view. Detailed information about transient and steady-state behavior of the electrolyte has been simulated, including potential gradients and the diffusion and migration fluxes for all ions. It was found that supportive electrolytes are an effective way to reduce the electric field and, consequently, the migration overpotential. Simultaneously, the diffusion overpotential, in general, increases. Nethertheless, supportive salts reduce the total overpotential across the electrolyte, especially when high currents are applied for short periods of time.