Passive-Film Formation on Metal Substrates in IM LiPF 6 /EC-DMC Solutions
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the impedance of surface films formed on Ni at low (0.2 V) potentials."2 These workers also examined the films on Li surfaces using Fourier-transform infrared spectroscopy (FTIR), but no FTIR data were reported for the case of Ni. They modeled their impedance data to calculate film thicknesses that varied from 5 A to over 150 A (15 nm) in LiPF6 in propylene carbonate (PC). This is comparable to the values reported by Peled.6 However, these films were not stable over long-time storage and tended to dissolve after formation in this system. Subsequent work on film formation on a Au substrate was done using an electrochemical quartz crystal microbalance.'3 Some cyclic voltammograms (CVs) were reported by Ein-Eli et al. for a Pt electrode in IM LiAsF6 solutions in PC and DMC, but the intent was to study the effects of SO 2 as an additive, and not film formation.' 4 For the most part, much of the previous work has focused on the formation and nature of the film in direct contact with the highly energetic negative electrode materials, namely lithium metal and the carbon intercalation compounds. Our interest was in being able to separate the contribution and role of the solvent and salt in this film-formation process, removed from the highly reducing electrode material. In addition, we were also interested in determining to what extent the metal substrates employed as the current collectors could contribute to the initial firstcycle loss and inefficiencies. To obtain a better understanding of the role that the passive film plays in Li-ion cells, the reduction processes were studied using a number of metal and carbon materials as substrates. In this paper, we report on metal substrates of Cu, 304 stainless steel (SS), and Mo substrates. The Cu and 304 SS were of particular interest since Cu is used in most commercial cells and both metals are used in our test cells for the characterization of carbon anodes. Film formation was examined as a function of applied potential, down to voltages where Li deposition was observed. The films were then examined ex situ using secondary ion mass spectroscopy (SIMS) and, in preliminary tests, by FTIR and Raman spectroscopy. Complex-impedance spectroscopy was used as a complementary analytical technique. EXPERIMENTAL Electrochemical Procedures Three-electrode tee cells were used for testing the 1/2"-dia. sample discs. The cells were assembled in the dry room (RH
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