On the thermal behavior of lithium intercalated graphites

The thermal behavior and structural changes of representative types of lithiated graphitic materials were investigated in solutions comprising ethylene carbonate (EC), ethylmethyl carbonate (EMC), dimethyl carbonate (DMC) and LiPF₆. We show that the protective films formed on intercalated graphite electrodes upon cathodic polarization are stable in these electrolyte solutions up to ∼80°C, in DSC experiments. Upon increasing the temperature, between 80 and 120°C, reactions of the surface films with solution species take place and the level of the graphite lithiation is reduced. After destruction of the surface films on lithiated graphite at higher temperatures 120°C, solvent molecules diffuse into the graphite particles and interact with Li ions therein thus forming reduction products. The later decompose upon further heating at temperatures ≥ 200°C, with the formation of gaseous products. This results in an internal pressure within the graphite particles that causes their partial exfoliation in an endothermic process. While the protective surface films on the lithiated graphite are removed from the particles in the course of the thermal reactions, we have indications that surface films on fully delithiated graphite do not react with the electrolyte solution at least up to 200°C.