The evolution of the passivating layer (during the first reduction/reoxidation cycle, in function of the number of reduction/reoxidation cycles and with the storage of the electrode in the electrolyte) formed at the surface of a lithiated carbon electrode in LiCF3SO3/carbonates mixture electrolyte has been followed by using 1 MeV 4He+ induced Rutherford Backscattering Spectrometry (RBS). Two RBS simulation codes "RUMP" and "PERM" , whose approaches are different, have been applied to the treatment of the RBS data and have allowed to obtain informations about the structure (thickness and atomic elemental composition) of the passivating layer. These two codes provide similar results. The passivating layer has a thickness in the range 20-30 nm. Its structure appears to be complex with the presence of two different sublayers respectively composed of the reduction products of the salt in the inner sublayer (with the main presence of fluorine and sulphur indicating the possible existence of LiF and Li2S) and reduction products of the solvents in the outer sublayer (with a high concentration in oxygen indicating the presence of Li2CO3 and RCO3Li where R is an alkyl radical).