Thermodynamic stability of Li2C2 and LiC6

Highlights: •LiC6 is shown to be thermodynamically unstable above 330 °C. •No order–disorder transition in LiC6 was observed. •Limiting values for the enthalpy of formation of LiC6 are given. -- Abstract: Information about the thermodynamic stability of LiC6 in the literature is ambiguous at present. The thermodynamic stability of LiC6 and Li2C2 is investigated by an approach combining differential scanning calorimetry (DSC), X-ray diffraction (XRD) and long-time annealing experiments. It is found that LiC6 is thermodynamically unstable above 330 °C, where in the corresponding composition range Li2C2 and graphite are in thermodynamic equilibrium. During heating, a de-intercalation process is observed in LiC6, and eventually a transformation to Li2C2 and graphite. The enthalpy of formation of LiC6 is below zero as it forms from the pure components, the limits of the enthalpy of formation are set by the tangents to the enthalpies of formation of Li2C2 and graphite. An order–disorder transition in LiC6 was not observed