How dynamic is the SEI?

The surface chemistry of graphite and intermetallic AlSb has been studied by XPS (X-ray photoelectron spectroscopy) in a Li-ion battery context using LiPF6 in EC/DEC as electrolyte. The main results for graphite are as follows: the SEI (solid electrolyte interphase) is different for the lithiated state after 3 cycles (0.01 V) compared to the delithiated state (1.5 V); after 50 cycles the SEI is thicker; there are more Li2CO3 or semi-carbonates on the surface of the delithiated sample (1.5 V) than on the lithiated sample (0.01 V); LiF is continuously formed during the first cycles but a steady state is reached after 50 cycles; a new peak in the C Is spectra indicating a fluorine-containing compound is found at high photon energies (292 eV). The main results for AlSb are as follows: the SEI is different for the lithiated state (0.01 V) compared to the delithiated state (1.2 V) after 3 cycles; after 50 cycles the surface layer thickness is slightly larger but significantly thinner than for graphite; contrary to graphite, more Li2CO3 or semi-carbonates are found on the surface of the lithiated sample; also here a new peak indicating a fluorine-containing compound is found in the C 1s spectra at 292 eV The general result is that the SEI has many similar features between graphite and AlSb but also important differences. The carbonaceous layer is dynamically shifting in chemical composition during cycling for both samples. (c) 2007 Elsevier B.V. All rights reserved.