How to efficiently intercalate alkaline-earth metals into graphite using LiCl-KCl molten salts: An overview on the case of barium

In this paper, we present the three first stages currently known Ba-based Graphite Intercalation Compounds (GIC). They are synthesized using the LiCl-KCl molten salts method, which is especially efficient in order to help the intercalation of barium into the 2D graphitic galleries. According to the chosen reaction conditions, it is possible to obtain either the BaC6 binary compound or one of the two quaternary phases whose intercalated sheets contain simultaneously lithium, potassium and barium. By combining X-ray diffraction measurements and ion beam analyses for these compounds, their chemical formula is well established: Li0.2K0.6Ba0.35C6 for the α-phase, Li0.2K0.75Ba0.6C6 for the β one and BaC6 for the binary.The intercalated sheets of this last compound are single-layered. Its repeat distance reaches 529 pm and its 2D unit cell is hexagonal and commensurate with the graphene one with a parameter of 430.6 pm However, α- and β-phases are much more complex. For α- compound, the intercalated sheets are three-layered with an interplanar distance of 650 pm, while for β-phase, they are six-layered and the interplanar distance reaches 950 pm Both 2D unit cells are hexagonal, with a planar lattice parameter a of 1074 pm and 1280 pm respectively.Finally, a model of reaction mechanism is proposed, which breaks down the intercalation process into several steps: intercalation of lithium (LiC6), then replacement by barium (BaC6), next, formation of the β-phase, and ultimately its elimination to the benefit of the α one. Thus, this latter appears as the most stable GIC in this LiCl-KCl molten medium.