Journal
ELECTROCHIMICA ACTA
Volume 138, Issue -, Pages 430-436Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.electacta.2014.06.129
Keywords
graphite; SEI; XPS; HAXPES; SEI thickness
Categories
Funding
- European Community's Seventh Framework Programme (FP7) [226716]
- Swedish Hybrid Vehicle Centre
- Swedish Energy Agency
- FFI
- VINNOVA Green-car project
- StandUp for Energy
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In cycled Li-ion batteries, the carbon negative electrode is buried under a thin passivating layer referred to as the solid electrolyte interphase (SEI). In the present study, the increased depth sensitivity of hard X-ray photoelectron spectroscopy (HAXPES) as compared to conventional X-ray photoelectron spectroscopy (XPS) is used to study electrochemical changes at such a buried carbon/SEI. Samples from graphite/LiFePO4 cells cycled to specific potentials during the first four charge/discharge cycles were studied. The results show dynamic changes in the SEI during cycling. Reversible, state of charge (SOC) dependent changes in the SEI thickness as well as amounts of lithium oxide, lithium fluoride, lithium and carbon active material were discussed. Moreover, the results indicate lithium enrichment close to the carbon active material surface, which could not be explained by intercalation of lithium into carbon with LiC6 structure or by SEI formation at the surface. Potential dependent shifts in the binding energy of the carbon active material C1s feature showed the importance of internal energy calibration with an SEI feature rather than carbon active material. (C) 2014 Elsevier Ltd. All rights reserved.
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