Common electrolytes are thermodynamically unstable at the interface of metallic lithium and lithiated carbon anodes, as well as at the interface of high voltage cathodes, leading to the electrochemical decomposition of the same and the formation of a kinetic passivation layer, the so-called solid electrolyte interphase (SEI). It is generally accepted that the formed passivation layer at the anode surface is indispensable for the successful operation of lithium-ion batteries. The common understanding is that the SEI mainly consists of insoluble electrolyte decomposition products. With the appropriate knowledge of the SEI formation process and composition, it is possible to adapt its properties to any lithium-ion battery requirement in order to obtain the optimum performance and lifetime.