Charge Transfer and Electrochemistry in Graphite Intercalation Compounds
Charge transfer in graphite intercalation compounds (GICs) refers to the exchange of electrons between the graphitic carbon (C) and intercalate (I) layers [1,2]. It is an important quantity which affects directly the electrical, thermal and lattice dynamical properties. If electrons are “donated” to the carbon layers from the intercalate layers, the GIC is referred to as “donor-type”, on the other hand, if electrons are transferred from the C layers and “accepted” by the I layers, the GIC is termed “acceptor-type”. The position of the Fermi level (EF) in the carbon it bands must, of course, be consistent with the degree of charge transfer. From the point of view of the carbon sublattice, charge transfer can be defined by the quantity fc, which is the hole (electron) concentration expressed as the number of free carriers per carbon atom. Charge transfer is also expressed in the literature from the point of view of the intercalant. For example, in KC24, fK≅1, indicates one electron per K atom is donated from the K atom. For charge neutrality in this latter example, we would have 24fc=fK, if charge is exchanged between the K(4s) and carbon it states, and no other states are involved. In stage 2 KC24 this is thought to be the case, however in other GICs the situation may be more complex. For example, (1) localized carbon states which can accept charge may be formed as a result of intercalation or (2) several charged intercalate species may coexist in the intercalate layer. Charge transfer in GICs is studied using a variety of experimental techniques [1,2] which may be sensitive to the mobile cariers in the carbon n bands (i.e. sensitive to fc), or sensitive to the excess charge localized on particular intercalated molecules or atoms (i.e. sensitive to fI, where fI is the excess charge residing on the intercalated species I). For reasons such as those given above, caution must be exercised when comparing values obtained for fc and fI. If these values are not consistent, it may indicate that all the states participating in the charge transfer were not taken into account.
KeywordsCharge Transfer Propylene Carbonate Excess Charge Graphite Intercalation Compound Charge Transfer Effect
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