Graphite intercalation compounds
Graphite intercalation compounds (GICs) are complex materials having a formula CXm where the ion Xn+ or Xn− is inserted (intercalated) between the oppositely charged carbon layers. Typically m is much less than 1. These materials are deeply colored solids that exhibit a range of electrical and redox properties of potential applications.
These materials are prepared by treating graphite with a strong oxidant or a strong reducing agent:
The reaction is reversible.
The host (graphite) and the guest X interact by charge transfer. An analogous process is the basis of commercial lithium-ion batteries.
In a graphite intercalation compound not every layer is necessarily occupied by guests. In so-called stage 1 compounds, graphite layers and intercalated layers alternate and in stage 2 compounds, two graphite layers with no guest material in between alternate with an intercalated layer. The actual composition may vary and therefore these compounds are an example of non-stoichiometric compounds. It is customary to specify the composition together with the stage. The layers are pushed apart upon incorporation of the guest ions.
One of the best studied graphite intercalation compounds, KC8, is prepared by melting potassium over graphite powder. The potassium is absorbed into the graphite and the material changes color from black to bronze. The resulting solid is pyrophoric. The composition is explained by assuming that the potassium to potassium distance is twice the distance between hexagons in the carbon framework. The bond between anionic graphite layers and potassium cations is ionic. The electrical conductivity of the material is greater than that of α-graphite. KC8 is a superconductor with a very low critical temperature Tc = 0.14 K. Heating KC8 leads to the formation of a series of decomposition products as the K atoms are eliminated:
Via the intermediates KC24 (blue in color), KC36, KC48, ultimately the compound KC60 results.
The stoichiometry MC8 is observed for M = K, Rb and Cs. For smaller ions M = Li+, Sr2+, Ba2+, Eu2+, Yb3+, and Ca2+, the limiting stoichiometry is MC6. Calcium graphite CaC
6 is obtained by immersing highly oriented pyrolytic graphite in liquid Li–Ca alloy for 10 days at 350 °C. The crystal structure of CaC
6 belongs to the R3m space group. The graphite interlayer distance increases upon Ca intercalation from 3.35 to 4.524 Å, and the carbon-carbon distance increases from 1.42 to 1.444 Å.
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