Two intercalation compounds

In the lithium-ion approach, the metallic lithium anode is replaced by a lithium intercalation material. Then, two intercalation compound hosts, with high reversibility, are used as electrodes. The structures of the two electrode hosts are not significantly altered as the cell is cycled. Therefore the surface area of both electrodes can be kept small and constant. In a practical cell, the surface area of the powders used to make up the electrodes is normally in the 1 m2/g range and does not increase with cycle number [4]. This means the safety problems of AA and larger size cells can be solved. 

Two intercalation compounds, CsF Br2 and 2CsF-Br2, were isolated from the reaction of CsF with Br2. In the I I compound. X-ray analysis revealed eclipsed Cs- F layers (Cs+ positioned above Cs+), whereas the layers in the 2 1 compound are staggered. In both compounds, the Br-Br distance is larger than that for free gaseous bromine, suggesting some charge transfer from fluoride to bromine. ... 

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

Pandya et al. have used extended X-ray ascription fine structure (EXAFS) to study both cathodically deposited -Ni(OH)2 and chemically prepared / -Ni(OH)2 [44], Measurements were done at both 77 and 297 K. The results for / -Ni(OH)2 are in agreement with the neutron diffraction data [22]. In the case of -Ni(OH)2 they found a contraction in the first Ni-Ni bond distance in the basal plane. The value was 3.13A for / -Ni(OH)2 and 3.08A for a-Ni(OH)2. The fact that a similar significant contraction of 0.05A was seen at both 77 and 297K when using two reference compounds (NiO and / -Ni(OH)2) led them to conclude that the contraction was a real effect and not an artifact due to structural disorder. They speculate that the contraction may be due to hydrogen bonding of OH groups in the brucite planes with intercalated water molecules. These ex-situ results on a - Ni(OH)2 were compared with in-situ results in I mol L"1 KOH. In the ex-situ experiments the a - Ni(OH)2 was prepared electrochemi-cally, washed with water and dried in vac-... 

Fig. 14.4 Schematic diagram showing the three-dimensional and two-dimensional solvated and unsolvated cationic and anionic intercalation compounds S = Solvent [23].

In alkali metal intercalation compounds, the guest is ionised in the host, donating its outer s electron to the host s electronic energy levels. Thus there are two aspects to consider, the sites where the ion resides, and the energy levels or bands that the electron occupies. Guests such as water that remain neutral will only be discussed in the section on cointercalation. In some hosts, notably graphite, some guests accept electrons from the... 

Fig. 7.2 Classification of intercalation compounds (a) host of chains weakly bonded together (b) three-dimensional host with one-dimensional lattice of sites for guest ions (c) layered host (two-dimensional host and two-dimensional lattice of sites) (d) three-dimensional host with three-dimensional lattice of sites.

In various forms, lattice-gas models permeate statistical mechanics. Consider a lattice in which each site has two states. If we interpret the states as full or empty , we have a lattice-gas model, and an obvious model for an intercalation compound. If the states are spin up and spin down , we have an Ising model for a magnetic system if the states are Atom A and Atom B , we have a model for a binary alloy. Many different approximation techniques have been derived for such models, and many lattices and interactions have been considered. 

The electrons, if they are separated from the ions, will also contribute to the entropy, and one might naively expect an expression similar to Eqn (7.8). Then the chemical potential for an atom would be the sum of two terms like Eqn (7.10), one from ions and one from electrons, and so the entropy term would be doubled. This is not so, however, in metallic intercalation compounds. In metals, the entropy of electrons is small. Electrons added by intercalation do not have a choice of all the empty states in a band, but only those within kT of the Fermi energy. If the Fermi energy is expressed as a temperature Tp and is measured from the bottom of the band, the change in entropy with the number n of electrons, dS/dn, is of order kTfTj (Kittel, 1971), not of order k like Eqn (7.8) for... 

Omloo and Jellinek7 have described the synthesis and characterization of intercalation compounds of alkali metals with the group V layered transition metal dichalcogenides. Typically, these types of intercalation complexes are sensitive to moisture and must be handled in dry argon or nitrogen atmospheres. The alkali metal atoms occupy either octahedral or trigonal prismatic holes between X-M—X slabs. There are two principal means by which these compounds may be prepared. 

DiSalvo et al.9 have carried out a systematic survey of intercalation compounds of 2H(a)-TaS2 with post-transition metals. In particular, the system SnxTaS2 was found to exist in two composition domains, 0 < x < /3 and x = 1. The following discussion briefly describes the techniques used by DiSalvo to synthesize the compound SnTaS2. Syntheses of other transition and post-transition metal intercalation complexes with the layered transition metal dichalcogenides are discussed in References 9 and 20-24. 

Makovicky Hyde (1981) have reviewed incommensurate misfit structures in graphite intercalation compounds, brucite-type compounds, sulphides and related layered systems. A simple two-dimensional incommensurate system is provided by graphite with adsorbed rare gas monolayers. At low densities and high temperatures. 

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