During Li metal electrodeposition from a 0.5 M LiPF6-PC electrolyte solution onto a horizontal Li metal electrode, the refractive index profile corresponding to the concentration profile of Li+ ion near the cathode was measured in situ by holographic interferometry. The Li+ concentration gradient around the rapidly growing dendrite arms is steeper than at the cathode plane, clearly reflecting the local current density convergence at the dendrite tips and arms. As in a LiClO4-PC solution, an incubation period was observed between the start of current passage and the onset of the refractive index fringe shift. It increases with decreasing applied current density. The incubation period in LiPF6-PC is shorter than that in LiClO4-PC at current densities greater than 1.0 mA cm(-2); however, at 0.5 mA cm-2 in LiPF6-PC electrolyte solution it is appreciably longer than in LiClO4-PC. This complicated behavior is apparently due to the solution chemistry of LiPF6-PC electrolyte, which produces HF and oxyfluoride impurities that are lacking in the LiClO4-PC system. Thus, the different current dependence of the incubation time in these two systems may yield clues for an elucidation of the dynamics of solid electrolyte interphase formation. (c) 2007 The Electrochemical Society.