The complex electrochemistry of graphite electrodes in lithium-ion batteries

This paper discusses the interrelated phenomena of solid electrolyte interp hase (SEI) formation and the irreversible charge consumption which occurs d uring the first cycle of a graphite electrode, as well as their relevance t o the cycling stability of lithium-ion batteries. Thus, results from releva nt characterization methods, namely, in situ mass spectrometry, in situ inf rared spectroscopy, in situ Raman and video microscopy, in situ scanning pr obe microscopy, in situ quartz crystal microbalance, and differential scann ing calorimetry were combined for a more thorough understanding of observat ions made in cycling experiments. From electrochemical cycling tests, we ha ve learned that a high specific charge (similar to 360 Ah/kg of carbon), sa tisfactory cycle life of the graphite electrodes (1000 deep cycles), and an irreversible charge of <7% during SEI formation can only be obtained when water contamination of the cell is avoided. Under such conditions, a good-q uality SEI film is formed on the carbon surface. We conclude that during SE I film formation, at first the carbonate solvent(s) are reduced, forming et hylene gas, organic radicals, oligomers, and polymers. Then a SEI film is p recipitated on the surface via a nucleation and growth mechanism. The irrev ersible charge consumption due to SEI formation is proportional to the BET specific surface area of the graphite and rapidly increases with increasing water content in the cell. (C) 2001 Elsevier Science B.V. All rights reser ved.