Intercalation hosts

In the rechargeable lithium-ion batteries, in which current-generating reactions are based on a "rocking chair" mechanism, the ions of lithium shift back and forth between the intercalation hosts of the cathode and anode. The... 

These materials are known as insertion or intercalation hosts. The overall electrochemical process of a lithium battery is illustrated schematically in Fig. 7.2. During discharge it involves the dissolution of lithium ions at the anode, their migration across the electrolyte and their insertion within the crystal structure of the host compound, while the compensating electrons travel in the external circuit to be injected into the electronic band structure of the same host. The charging process is the reverse and the cell reaction may be written as ... 

Fig. 7.3 Schematic diagram showing the discharge of a LijrCnD )-A B> cell in which both positive and negative electrodes are based on intercalation hosts. (By permission of Chtm. hid, 1995, 77, 285.)...

Molybdenum disulphide is another layered intercalation host, similar to titanium disulphide. This material occurs naturally and formed the basis of the positive electrode for the first high production cylindrical AA-sized cell, manufactured by Moli Energy Ltd in Canada in the 1980s. Cycle life of 100-300 was achieved in practical cells with average discharge voltages of 1.8 V for low rates, giving a theoretical density of approximately 300 Wh/kg. 

Optimum behaviour was obtained with a cathode mixture comprising the intercalation host with electrolyte and graphite which, respectively, reduced contact polarization and enhanced the electronic conductance. 

Wang X, Liu H, Jin Y, Chen C. Polymer-functionalized multiwalled carbon nanotubes as lithium intercalation hosts. J Phys Chem B 2006 110 10236-10240. 

Zirconium phosphates have been extensively investigated as intercalation hosts (531. and the catalytic properties of some pillared zirconium phosphates has been reviewed by Clearfield (541. 

Various advances in this area during the past decade have been recently reviewed, and highlights are summarized here. The layered LiM02 structure adopted by LiCo02 described in Section 3.6.3 (a-NaFe02 type, or 03 ), is a versatile intercalation host. Isostructural alternatives that target less costly materials were initially based on substitution of M = Co with ions (Ni, Mn, and... 

Remarkably, a large number of electronically conducting intercalation hosts are accessible today, including numerous transition-metal oxides with framework lattices, such as vanadium pentoxide and nickel oxide, and organic solids with molecular lattices. Additionally, a large variety of ionic guest species have been studied. 

Kawata. S. Kitagawa. S. Kumagai, H. Ishiyama, T. Honda. K. Tobita. H. Adachi, K. Katada. M. Novel intercalation host systems based on transition metal (Fe, Co". Mn )-chloranilate coordination polymers. Single... 

Carbon materials have long been incorporated into the electrodes of energy-storage devices as electro-conductive additives supports for active materials electron transfer catalysts intercalation hosts substrates for... 

One of the typical examples is NASCION compound (Fig. 9.1), an acronym for A/d mper /on conductor. Before attracting interest as a cathode-active material, NASICON has a long research history of over 20 years as a solid electrolyte. Since Dehnas reported - that while LiTi2(PO )3 works as a lithium intercalation host and NaTi3(PO )3 works as a Na intercalation host, many attractive cathodes have been proposed based on the NASICON material group. 

At this point the fundamental difference between intercalation and alloying type materials becomes obvious. With the intercalation hosts, studied over the past decades, the researchers simply follow" nature that is, the crystal lattice that is slightly modified in order to make it more favorable with respect to particular properties of interest. 

Unlike intercalation hosts, alloying type materials always exhibit large volumetric variations. Therefore, they are intrinsically unstable and the research is performed against nature, i.e., the electrode should be stable despite the unavoidable volumetric variations. Therefore, in this case there is no obvious strategy to follow, except for keeping the active cores as small as possible as explained above (cf Fig. 11.3). 

Dimensionally stable intercalation hosts allow easy electrode preparation with predictable properties that completely rely on the properties of their crystal lattices. Crystal lattice and its appropriate modification in this case is the natural and reliable starting point of the research. 

If the electrochemical potential of pure Li metal is constant, the cell potential at equilibrium is a function of the concentration of intercalant species in the host. The composition of the guest in the intercalated host is given by X = n/N, where n is the number of moles of intercalant in the host and N is the total number of sites in the host. Assuming the number of host sites, N, is constant and the host lattice expands freely during intercalation, the free energy change of an intercalation process is a function of composition and temperature. 

Intercalation into lamellar metal oxides and oxide chlorides has also received some attention [353, 354] in particular because of the capacity to intercalate ferrocene. Thus, FeOCl intercalation complexes with ferrocene [355-357] and substituted ferrocenes [358, 359], chromocene [355], cobaltocene [355, 360], ruthenocene [358], and intercalation host-guest complexes of cobaltocene into TiOCl and VOCl [361] have been described and investigated in some detail. Intercalation of some organo-tin compounds into FeOCl has also been reported [362]. 

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