In the area of solid state chemistry, graphite intercalation compounds (GICs) are a family of materials prepared from graphite. In particular, the sheets of carbon that comprise graphite can be pried apart by the insertion (intercalation) of ions. The graphite is viewed as a host and the inserted ions as guests. The materials have the formula (guest)Cn where n ≥ 6. The insertion of the guests increases the distance between the carbon sheets. Common guests are reducing agents such as alkali metals. Strong oxidants also intercalate into graphite. Intercalation involves electron transfer into or out of the carbon sheets. So, in some sense, graphite intercalation compounds are salts. Intercalation is often reversible: the inserted ions can be removed and the sheets of carbon collapse to a graphite-like structure. The properties of graphite intercalation compounds differ from those of the parent graphite.
Preparation and structure
These materials are prepared by treating graphite with a strong oxidant or a strong reducing agent:
C + m X → CXm 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.
Examples
Lithium graphite intercalation compound The lithium graphite intercalation compound is the anode active material for nearly all lithium-ion batteries produced today. Jürgen Otto Besenhard demonstrated in the late 1970's that lithium could be intercalated into graphite as an anode material. Soon after, Samar Basu demonstrated the preparation of lithiated graphite by chemical means. Then, in 1983, Rachid Yazami demonstrated the reversible electrochemical intercalation of lithium in graphite at room temperature using polyethylene oxide solvent. Later, graphite was integrated with cathode materials to form a high voltage lithium-ion battery; see History of the lithium-ion battery for more details.
Graphite undergoes a series of staging corresponding to ordered variations of lithium intercalation into its layers. The LiC6 GIC, corresponding to alternating Li and graphene sheets, has a theoretical capacity of 372 mAh/g. This staging behavior is also associated with a change in the color - as Li is intercalated, the color changes from blue, to red, to gold.
Other alkali and alkaline earth derivatives
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:
3 KC8 → KC24 + 2 K 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 CaC6 is obtained by immersing highly oriented pyrolytic graphite in liquid Li–Ca alloy for 10 days at 350 °C (662 °F). The crystal structure of CaC6 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.
With barium and ammonia, the cations are solvated, giving the stoichiometry (Ba(NH3)2.5C10.9 (stage 1)) or those with caesium, hydrogen and potassium (CsC8·K2H4/3C8 (stage 1)). In situ adsorption on free-standing graphene and intercalation in bilayer graphene of the alkali metals K, Cs, and Li was observed by means of low-energy electron microscopy. Different from other alkali metals, the amount of Na intercalation is very small. Quantum-mechanical calculations show that this originates from a quite general phenomenon: among the alkali and alkaline earth metals, Na and Mg generally have the weakest chemical binding to a given substrate, compared with the other elements in the same group of the periodic table. The phenomenon arises from the competition between trends in the ionization energy and the ion–substrate coupling, down the columns of the periodic table. However, considerable Na intercalation into graphite can occur in cases when the ion is wrapped in a solvent shell through the process of co-intercalation. A complex magnesium(I) species has also been intercalated into graphite.
Graphite bisulfate, perchlorate, hexafluoroarsenate: oxidized carbons The intercalation compounds graphite bisulfate and graphite perchlorate can be prepared by treating graphite with strong oxidizing agents in the presence of strong acids. In contrast to the potassium and calcium graphites, the carbon layers are oxidized in this process:
48 C + O + 6 H2SO4 → 2 ([C24]+[HSO4]−·2H2SO4) + H2O In graphite perchlorate, planar layers of carbon atoms are 794 picometers apart, separated by ClO−4 ions. Cathodic reduction of graphite perchlorate is analogous to heating KC8, which leads to a sequential elimination of HClO4. Both graphite bisulfate and graphite perchlorate are better conductors as compared to graphite, as predicted by using a positive-hole mechanism. Reaction of graphite with [O2]+[AsF6]− affords the salt [C8]+[AsF6]−.
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