The composition and formation of the SEI which is formed on lithium, which has been electrochemically deposited on a Cu substrate, is investigated by Raman spectroscopy and mass spectrometry. Semicarbonates, Li2CO3 and Li2C2 are identified as SEI components. The existence of Li2C2 is further proven with mass spectrometry by analyzing the gases which are formed by hydrolysis. Raman mapping reveals that Li2CO3 is homogeneously distributed over the copper substrate, on which lithium was deposited. At contrast, Li2C2 is mainly located on the plated lithium. An in situ Raman spectroscopy method for SEI investigation was developed and shows that Li2C2 is not formed at a potential >0 V vs. Li/Li+.