Thermal stability of graphite anode with electrolyte in lithium-ion cells

Thermal stability of electrochemically lithiated graphite with 1 M LiPF6/EC + DMC and PVdF-binder has been investigated. DSC measurements using an airtight sample case reveal a mild heat generation started from 130 degreesC with a small peak at 140 degreesC. The mild heat generation continued until a sharp exothermic peak appeared at 280 degreesC. The heat evolved in the small peak at 140 degreesC decreased by storage of the lithiated graphite with PVdF and the electrolyte at 50 degreesC for 3 days before the DSC measurements. The lithiated graphite with the electrolyte without PVdF-binder did not show the small peak at 140 degreesC. The peak at 140 degreesC seems to be caused by the reaction (the Solid Electrolyte Interphase (SEI) formation) of the electrolyte and lithiated graphite, whose surface is covered by poly(vinylidene flouride) (PVdF)-binder without formation of SEI at a lower temperature. The mild heat generation from 140 to 280 degreesC is the reaction of the lithiated graphite and the electrolyte through SEI (SEI formation), because there was no such mild heat generation when non-lithiated graphite was used. The peak at 280 degreesC is probably a direct reaction of lithiated graphite and electrolyte by a breakdown of SEI. (C) 2002 Elsevier Science B.V. All rights reserved.