DEMS study of gas evolution at thick graphite electrodes for lithium-ion batteries: the effect of gamma-butyrolactone

Differential electrochemical mass spectrometry (DEMS) was used to study the reductive decomposition of an electrolyte based on ethylene carbonate/dime thyl carbonate (EC/DMC), as well as the formation of a solid electrolyte in terphase (SEI) in this electrolyte, at thick (75-100 mum) porous graphite c omposite electrodes. A number of graphite electrodes differing in their ele ctrochemical lithium intercalation properties were investigated in potentia l-sweep experiments. They proved to be similar with respect to the evolutio n of ethylene and hydrogen gas during the first two charge/discharge cycles . Due to an incomplete coulombic conversion, a high irreversible capacity, as well as slow diffusion kinetics and an enhanced ohmic resistance of the electrodes, SEI formation on these thick electrodes was not yet complete af ter the first charge/discharge cycle. Undesired gas evolution can be reduce d by adding gamma -butyrolactone (GBL) as an electrolyte co-solvent. The am ount of ethylene and hydrogen gas evolved decreases with increasing percent ages of GBL in an EC/DMC electrolyte, indicating that the SEI layer is buil t up from GBL rather than from EC decomposition products. (C) 2001 Elsevier Science B.V. All rights reserved.