New kinetical and thermodynamical data concerning the intercalation of lithium and calcium into graphite Original Research Article

It is possible to synthesise easily the first stage LiC6 compound by immersing a pyrographite platelet in liquid lithium. The same experiments carried out using liquid lithium–calcium alloys with a Li/Ca atomic ratio, respectively, of 1 and 2, at 350 °C for ten days give two first stage ternary graphite intercalation compounds containing five- and seven-layered intercalated sheets, respectively. The experiments carried out with a liquid alloy rich in lithium (Li/Ca atomic ratio of 3) at 350 °C for ten days, in order to be sure that the thermodynamical equilibrium is reached, show that the reaction product is the CaC6 binary compound, and not LiC6. This very unexpected result can be partially explained by kinetical and thermodynamical argumentation. Before the equilibrium, the experiments show that lithium intercalates first, leading to a mixture of several stages of graphite–lithium compounds with an increasing enrichment in LiC6. This compound disappears gradually to the benefit of CaC6, which remains alone in the system. In these conditions, it appears that CaC6 is more stable than LiC6. This particular situation in these systems is observed for the first time. The temperature has a strong influence on the reaction. The ternary compounds are mainly obtained at the lowest temperatures whereas the binary ones are obtained for higher values.