GICs and Batteries

Intercalation can be defined as the process of inserting atoms or molecules (guest chemical species) between layers in a host material with layered structure such as graphite. Intercalation can be achieved using a solid, liquid, or gaseous intercalate reagent, as discussed in Chapter 2. However, preparation from the vapor is the most common. When graphite is used as a host material, a high degree of three-dimensional (3D) structural ordering is generally desired. The intercalation rate and the resulting intercalate concentration are strongly dependent on the intercalation conditions, such as pressure, temperature difference between the host graphite material and the intercalate, the physical dimensions of the sample, the degree of crystalline order, and the defect density within the host graphite material. The most important factors for controlling the physicochemical properties of GICs are the host material and the types of intercalate. Compared with other host materials, fibrous materials, including vapor-grown carbon fibers (VGCFs) (Dresselhaus et al., 1988), have shown particular suitability for GIC from the viewpoint of practical applications. Fiber hosts are normally intercalated using techniques similar to those considered for HOPG-based GICs, though the specific intercalation conditions may be different with regard to intercalation temperature, time, and other conditions. It is noteworthy that the intercalation of chemical species within fiber hosts is successful at lower temperature ranges than for bulk graphite or HOPG host materials (Meschi, 1988; Meschi et al., 1986). Because of the small size of the fibrous hosts, with diameter around 10 μm , the intercalation time tends to be shorter. With regard to the kinetics, the intercalation of fibers is initiated at the free edges of the fibers and then proceeds along the fiber length, thus depending on the macroscopic structure or morphology of the host fibers (Shioya et al., 1986). Fibers prepared from polymeric precursors can be intercalated in the radial direction (Goldberg and Kalnin, 1981). However, for the case of low crystalline fibrous carbon such as PAN-based carbon fiber, it is very difficult to fully form intercalated materials. On the other hand, covalent GICs such as fluorinated graphite and graphite oxide can be synthesized (see Sections 2.3.4 and 9.1.8).