Synthesis and Intercalation Chemistry

Alkali metal GICs are the best known donor type GICs, since they are easily prepared and their brilliant gold color for stage-1 GICs has attracted scientists working in intercalation chemistry. They have therefore been targets of intensive and detailed studies of their solid-state properties on the basis of the employment of highly oriented pyrolytic graphite (HOPG). There are several intercalation methods, which are classified basically into vapor-phase reaction, reaction of the mixture of graphite and alkali metal, high pressure reaction, electrochemical reaction, and reaction in a solvent. Among these methods, vapor-phase intercalation reaction is the most popular. The two-zone method can easily give alkali metal GICs with well-defined single-stage phases. The vapor pressure of the alkali metal becomes high enough to obtain a satisfactory reaction rate for intercalation reaction in the temperature range 200-550°C, at which we can use a Pyrex glass tube as a reaction chamber. Figure 2.1 shows a typical two-zone method, where graphite and alkali metal are maintained at different temperatures, TG and TI, respectively, in a vacuum-sealed glass tube placed in a two-zone furnace. Changing TI controls the vapor pressure of the alkali metal. Figure 2.2 presents the conditions of intercalation reaction with potassium, where single-stage phase samples with stages 1 to 8 are obtained by changing the temperature difference TG — TK (Nishitani et al., 1983). Typical experimental conditions are given in Table 2.1 for the preparation of K, Rb, and Cs GICs (Dresselhaus and Dresselhaus, 1981). The intercalation reaction is again carried out by heating a mixture of graphite and alkali metal in a vacuum-sealed glass tube. In this case, the reaction becomes considerably more rapid owing to direct contact of molten alkali metal with graphite, although the reaction takes place in a similar manner to the vapor-phase reaction. A stainless steel tube is used for the intercalation of lithium since lithium vapor degrades a glass tube because of its high chemical activity. Alkali metal can be intercalated into graphite when the alkali metal is solvated in liquid ammonia or an organic solvent such as dimethylsulphoxide (DMSO).