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 °C with a small peak at 140 °C. The mild heat generation continued until a sharp exothermic peak appeared at 280 °C. The heat evolved in the small peak at 140 °C decreased by storage of the lithiated graphite with PVdF and the electrolyte at 50 °C for 3 days before the DSC measurements. The lithiated graphite with the electrolyte without PVdF-binder did not show the small peak at 140 °C. The peak at 140 °C 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 °C 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 °C is probably a direct reaction of lithiated graphite and electrolyte by a breakdown of SEI.