Intercalation into ordered carbon materials, especially graphite, is a long-standing activity of our group calling upon innovative synthesis routes, at the origin of the preparation of numerous graphite intercalation compounds (GICs), especially the LiC6 used in Li-ion batteries.
Recent works have been focused on the use of an alkali metal such as lithium or potassium as an intercalation vector. Indeed, a novel synthesis route using lithium-based alloys has allowed the preparation of several bulk compounds including the superconducting CaC6 and Li3Ca2C6 materials and the europium-based EuC6 and Li0.25Eu1.95C6 compounds which exhibit novel magnetic properties. Moreover, this synthesis method in a molten alloy medium has recently led to the successful intercalation of gold, an element widely studied at the nanometric scale.
The more classical intercalation methods such as vapour phase reactions involving alkali metals are applied to other low-dimensional carbon-based materials, such as carbon nanotubes (CNTs).
Graphite possesses a strongly anisotropic lamellar structure. Since the intercalation reactions are performed with pyrolytic graphite, the highly oriented structure of the host lattice permits detailed diffraction studies: (00l) analyses provide access to c-axis stacking order and in the case of organised intercalate layers, a wide range of commensurate and incommensurate phases have been investigated over the past years according to the chemical nature of the intercalate. Furthermore, the 3D structure of the bulk reagent is systematically compared to that of the 2D intercalated reagent. It is then possible to determine how the graphitic host lattice imposes its structure on the intercalate.
Physical properties of GICs strongly depend on the nature of the intercalate. CaC6 and Li3Ca2C6 become superconducting below 11.50 K and 11.15 K respectively, the highest measured critical temperatures for any GIC.
In the lithium-europium-graphite system, the original and complex magnetic properties of EuC6 and Li0.25Eu1.95C6 are studied in our Institute. Complementary experiments depending on the probed properties are carried out via collaboration with specialists in other fields (e.g. muon spin spectroscopy, electro-dynamic measurements, Mössbauer spectroscopy).
For all low dimensional carbon-based host lattices, intercalation is accompanied by electron transfer between the intercalate and the carbon material. This electron transfer can be advantageously used for efficient dispersion of CNTs as well as facilitating their handling. Since the dispersion process takes place near an alkali metal atom, the resulting debundled state of the CNT bundles reveals the localisation of the intercalated metal within the intercalation compound. Whatever the alkali metal (from Na to Cs), it has been shown that the interstitial sites are not fully occupied resulting in only a partial separation of the CNTs belonging to a given bundle. The as-obtained ramified CNT structures are of interest because they show an increased accessible surface area compared to that of pristine CNT bundles.