Compound intercalation

In the particular case of lithiation or delithiation of cathode materials used in lithium secondary batteries, the calculation of the electrochemical equivalent involves an additional parameter related to the reaction of intercalation of lithium cations into the crystal lattice of the host cathode materials. Consider the theoretical reversible reaction of intercalation of lithium into a crystal lattice of a solid host material (e.g., oxide, sulfide)  

Hence during the lithiation reaction (i.e., charge), x moles of lithium cations are reduced and intercalated intoy moles of the solid host material, and a quantity of electricity, xF, must be supplied to the cell. Conversely, during dehthiation (i.e., discharge), x moles of lithium cations are produced and a quantity of electricity, xP, is supplied to the external circuit of the cell. Therefore, the quantity of electricity, Q, expressed in coulombs (Ah), delivered following the deintercalation of lithium from a mass, of the solid host material or a mass, of the final intercalated compound is given by the two following relations  

From the above equation it is possible to define two types of electrochemical equivalents. The first electrochemical equivalent, denoted EJ A), is the quantity of electricity consumed per unit mass of the solid host material, M, during the lithiation reaction (i.e., charging) and is defined by the following equation  

The two electrochemical equivalents of some selected solid host cathode materials and corresponding intercalated compounds used in rechargeable lithium batteries are presented in Table 9.13. 

It is possible to insert additional atoms or molecules into the inter-lamellar gap of many layer-lattice materials, including molybdenum disulphide, creating what are called intercalation compounds. The intercalated substances may be alkali or alkalyne-earth metals (sodium, potassium, rubidium, caesium, calcium, strontium), salts or organic bases such as ethylene diamine or pyridine . 

Many layer-lattice compounds can intercalate additional metal atoms of the same element as comprised in the original structure (e.g. niobium in niobium diselenide), but molybdenum disulphide will not do so. The behaviour may be determined by the availability of electrons suitably oriented to form bonds with the additional metal atoms, although it seems unlikely that this single factor applies to all intercalation effects. 

The effect of intercalating like metal atoms is of course to change the atomic ratios, and for example it has been reported that niobium diselenide can intercalate additional niobium atoms to a composition of Nb, jSej There will also be corresponding changes in the crystal lattice parameters, and these are discussed in relation to lubrication properties in Chapter 14. 

The physics and chemistry of molybdenum disulphide intercalation compounds have been reviewed by Woollam and Somoano . Perhaps the most interesting of these properties is superconductivity below 6.9°K, ° obtained with either organic bases or alkali metals. Some of the intercalation compounds show high alkali ion diffusivity, and this has led to them being considered for use in electrodes for high energy-density batteries . 

What might perhaps be considered as an extreme intercalation is the storage of hydrogen in atomic form in a strong magnetic field in exfoliated molybdenum 

Submitted by S. KIKKAWA, F. KANAMARU, and M. KOIZUMI Checked by SUZANNE M. RICHt and ALLAN JACOBSONt 

The layered compound iron(lll) chloride oxide FeOCl absorbs pyridine derivative molecules into its van der Waals gap forming the intercalation compounds FeOCl (pyridine derivative), . Iron(III) chloride oxide and the pyridine derivatives act, respectively, as a Lewis acid and base in the reaction with a partial transfer of the pyridine derivative electrons to the host FeOCl layer. Electrical resistivity of the host FeOCl decreases from 107 2-cm to 103 J2-cm with intercalation, and the interlayer distance expands to almost twice the original value. 

Methanol and ethylene glycol, which are not directly intercalated into FeOCl itself, will enter the expanded interlayer region of FeOCl(4-amino pyridine). These molecules attack the chloride ions which are weakly bound to the previously intercalated 4-aminopyridine (APy) and cause the elimination of the 4-aminopyridine. They substitute for Cl in a FeOCl layer and are grafted to the FeO layer without the reconstruction of the host FeO layer. 

The layered oxide KTiNbOs will form intercalation compounds, although it does not intercalate organic amines directly. If its interlayer potassium is replaced by protons through treatment with HC1, the product, HTiNbOs, will intercalate organic amine molecules. 

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Further improvements in anode performance have been achieved through the inclusion of certain metal salts in the electrolyte, and more recently by dkect incorporation into the anode (92,96,97). Good anode performance has been shown to depend on the formation of carbon—fluorine intercalation compounds at the electrode surface (98). These intercalation compounds resist further oxidation by fluorine to form (CF ), have good electrical conductivity, and are wet by the electrolyte. The presence of certain metals enhance the formation of the intercalation compounds. Lithium, aluminum, or nickel fluoride appear to be the best salts for this purpose (92,98). 

It is used as a fluorinating reagent in semiconductor doping, to synthesi2e some hexafluoroarsenate compounds, and in the manufacture of graphite intercalated compounds (10) (see Semiconductors). AsF has been used to achieve >8% total area simulated air-mass 1 power conversion efficiencies in Si p-n junction solar cells (11) (see Solarenergy). It is commercially produced, but usage is estimated to be less than 100 kg/yr. 

Potassium Graphite. Potassium, mbidium, and cesium react with graphite and activated charcoal to form intercalation compounds CgM, C24M, C gM, C gM, and C qM (61,62). Potassium graphite [12081 -88-8] 8 P gold-colored flakes, is prepared by mixing molten potassium with graphite at 120—150°C. 

There is Htde evidence of the direct formation of sodium carbide from the elements (29,30), but sodium and graphite form lamellar intercalation compounds (16,31—33). At 500—700°C, sodium and sodium carbonate produce the carbide, Na2C2 above 700°C, free carbon is also formed (34). Sodium reacts with carbon monoxide to give sodium carbide (34), and with acetylene to give sodium acetyHde, NaHC2, and sodium carbide (disodium acetyHde), Na2C2 (see Carbides) (8). 

The alkah metal—graphite compounds formed by graphite absorption of the fused metals Na, K, Rb, and Cs, represent a special type of metal—carbon compound (6). These intercalation compounds having formulas MCg are brown MC are gray and MC q are strongly graphitic. 

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. 

In the lithium-ion approach, the metallic lithium anode is replaced by a lithium intercalation material. Then, tw O 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 elecftodes can be kept small and constant. In a practical cell, the surface area of the powders used to make up the elecftodes is nomrally in the 1 m /g range and does not increase with cycle number [4]. This means the safety problems of AA and larger size cells can be solved. 

Raman spectra have also been reported on ropes of SWCNTs doped with the alkali metals K and Rb and with the halogen Br2 [30]. It is found that the doping of CNTs with alkali metals and halogens yield Raman spectra that show spectral shifts of the modes near 1580 cm" associated with charge transfer. Upshifts in the mode frequencies are observed and are associated with the donation of electrons from the CNTs to the halogens in the case of acceptors, and downshifts are observed for electron charge transfer to the CNT from the alkali metal donors. These frequency shifts of the CNT Raman-active modes can in principle be u.sed to characterise the CNT-based intercalation compound for the amount of intercalate uptake that has occurred on the CNT wall. 

Issi, J. -P., Transport properties of metal chloride acceptor graphite intercalation compounds. In Graphite Intercalation Compounds,... 

Cj(K prepared — the first alkali metal-graphite intercalation compound. 

First metal halide-graphite intercalation compound made with FeCl.i. 

Figure 8.16 Layer-plane sequence along the c-axis for graphite in various stage I -5 of alkali-metal graphite intercalation compounds. Comparison with Fig. 8.15 shows that the horizontal planes are being viewed diagonally across the figure. /,. is the interlayer repeat distance along the c-axis.

A quite different sort of graphite intercalation compound is formed by the halides of many elements, particularly those halides which themselves have layer structures or weak intermolecular binding. The first such compound (1932) was with FeCl3 chlorides, in general, have been the most studied, but fluoride and bromide intercalates are also known. Halides which have been reported to intercalate include the following ... 

Carbon will react directly at high temperatures with many elements such as sulphur and iron. It also forms intercalation compounds in which a wide range of molecules enter the interlayer spacing of the graphite. This can lead to disruption of the material but also produces a whole new class of potentially useful materials. 

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