Intercalation reactions

The similarity of M0S2 to graphite has been noted. Like elemental carbon, which has been found to form nanotubular stmctures, M0S2 has also been found to form nested stmctures upon exposure to the electron beam in an electron microscope (23). Moreover, M0S2 displays a variety of intercalation reactions typical of layered materials. Single-layer M0S2 has been successfully prepared and manipulated (22). 

Zirconium monochloride reacts with sodium ethoxide to form additional adducts which hydrolyze in water. The monochloride does not react with benzene in a Friedel-Crafts reaction, and does not enter into intercalation reactions similar to those of zirconium disulfide. Both monohaUdes add hydrogen reversibly up to a limiting composition of ZrXH (131). 

The quality and quantity of sites which are capable of reversible lithium accommodation depend in a complex manner on the crystallinity, the texture, the (mi-cro)structure, and the (micro)morphology of the carbonaceous host material [7, 19, 22, 40-57]. The type of carbon determines the current/potential characteristics of the electrochemical intercalation reaction and also potential side-reactions. Carbonaceous materials suitable for lithium intercalation are commercially available in many types and qualities [19, 43, 58-61], Many exotic carbons have been specially synthesized on a laboratory scale by pyrolysis of various precursors, e.g., carbons with a remarkably high lithium storage capacity (see Secs. 

Apart from reactions with the electrolyte at the carbon surface, the irreversible specific charge is furthermore strongly affected by the possible co-intercalation of polar solvent molecules between the graphene layers of highly graphitic matrices [139]. This so-called "solvated intercalation reaction" depends (i) on the crystallinity and the morphology of the parent carbonaceous material, which will be discussed in Sec. 

The mobility of lithium ions in cells based on cation intercalation reactions in clearly a crucial factor in terms of fast and/or deep discharge, energy density, and cycle number. This is especially true for polymer electrolytes. There are numerous techniques available to measure transport... 

Numerous intercalation reactions are known in which one reactant enters the lattice of the other. Such behaviour is conveniently illustrated by reference to two recent studies. Lithium undergoes a low temperature (298 K) topochemical reversible reaction with transition metal compounds (e.g. TiS2, NbSe3) [1211] in which the host lattice structure may be partially retained (e.g. in Li TiS2, LijNbSe3). The reaction [1212]... 

The Li-Ion system was developed to eliminate problems of lithium metal deposition. On charge, lithium metal electrodes deposit moss-like or dendrite-like metallic lithium on the surface of the metal anode. Once such metallic lithium is deposited, the battery is vulnerable to internal shorting, which may cause dangerous thermal run away. The use of carbonaceous material as the anode active material can completely prevent such dangerous phenomenon. Carbon materials can intercalate lithium into their structure (up to LiCe). The intercalation reaction is very reversible and the intercalated carbons have a potential about 50mV from the lithium metal potential. As a result, no lithium metal is found in the Li-Ion cell. The electrochemical reactions at the surface insert the lithium atoms formed at the electrode surface directly into the carbon anode matrix (Li insertion). There is no lithium metal, only lithium ions in the cell (this is the reason why Li-Ion batteries are named). Therefore, carbonaceous material is the key material for Li-Ion batteries. Carbonaceous anode materials are the key to their ever-increasing capacity. No other proposed anode material has proven to perform as well. The carbon materials have demonstrated lower initial irreversible capacities, higher cycle-ability and faster mobility of Li in the solid phase. 

In the intercalation reactions, ions (anions X or cations M+) penetrate into the van der Waals gaps between the ordered carbon layers resulting in the enlargement of their inter-layer distance [23,24]. The corresponding charges are conducted by carbon and accepted into the carbon host lattice. 

Intercalation reactions (14.1) represent the ideal case there is an increase in the inter-layer distance while the carbon atom arrangement within the layers remains unchanged. However, during intercalation of cations from polymer [25] and solid [26] electrolytes, ternary phases (M (solv)yC /C (solv)yX ) are produced because the solvent from the electrolyte is also accepted into the carbon lattice. 

Determining the kinetics and mechanisms of intercalation reactions is not trivial. Quenching studies (in which an aliquot of the reaction suspension is removed and the sohd product recovered through filtration) have frequently proved to be unreliable. The material isolated is often atypical of the reaction matrix as a whole, having been affected by the quenching process. Therefore, it is desirable to use a non-invasive probe to observe the reaction in situ, in real time. This allows the extraction of both qualitative and quantitative information on the kinetics of a process and the exact steps lying on the reaction pathway. A variety of techniques have previously been applied to monitor re-... 

The work described in this review involves the use of in-situ X-ray powder diffraction to investigate a variety of intercalation reactions involving LDHs. 

The values of the exponent n that can be obtained for an intercalation reaction into a layered host are therefore hmited. Possible values are highhghted in bold in Table 1. 

The desirability of using a non-invasive in-situ probe has already been discussed. There is, however, a problem, in that standard characterisation techniques are unable to penetrate bulky reaction vessels. As a result of this, little is known about the reaction dynamics or kinetics of intercalation reactions. A non-invasive probe which can interrogate a typical intercalation process is required. It is also necessary to employ short data collection times in order that kinetic information may be obtained. X-ray powder diffraction is a highly appropriate tool. It is non-invasive, and is a powerful characterisation technique when used in combination with ex-situ analyses. 

The change in intensity of the Bragg reflections can be monitored as a function of time, since data collection times can be as little as 10 s. This is well within the average reaction time for the intercalation reactions of LDHs. From these data, both qualitative and quantitative information regarding the mechanism and kinetics of a reaction can be obtained. A wide variety of reactions have been monitored using EDXRD [15,20-25], in addition to the intercalation reactions of LDHs. 

An experimental apparatus designed for the study of intercalation reactions by EDXRD has been developed by O Hare and co-workers. This is shown in Fig. 3 [26,27]. 

Fig. 3 The experimental apparatus used to monitor intercalation reactions in situ. Reproduced with permission from Chem Mater (2005) 17 2632-2640...

Staging is a method by which the energy barrier to an intercalation reaction may be reduced. However, the traditional models of staging [36] require the layers of the host material to bend, and therefore to be flexible. In contrast, LDHs have rigid layers, and are hence not expected to undergo staging processes. 

The phase with a d-spacing of 14.8 A is present in the reaction mixture for less than 15 min, while the 1,4-BDA reflection increases steadily in intensity, and at the end of the process is the sole phase present. This suggests that the same mechanism as for the LiAl - Cl selective intercalation reactions is operating here. That is, both anions are initially intercalated, followed by extrusion of the less favored isomer to give the thermodynamically favored product. 

The above has been a brief discussion of the use of in-situ, time-resolved EDXRD techniques to study the intercalation reactions of LDHs. A variety of other hosts such as metal dichalcogenides have also been investigated, but these fall outside the scope of this review. 

In general, it is possible to obtain good kinetic and mechanistic information for the LiAl-Cl and CaAl-NOs host materials. With MgAl-NOs, the intercalation reactions tend to be too rapid to observe. Other LDH hosts such as [Cu2Cr(OH)6]Cl H2O have also been investigated, but problems regarding the crystallinity of the host matrix have been encountered. 

This work has produced significant advances in our understanding of intercalation reactions. A wide range of mechanisms is observed, which can be rationalised on the basis of the vast array of factors governing an intercalation reaction. 

Choy et al. have also intercalated biological macromolecules such as DNA, ATP and nucleosides into Mg/Al-NOs LDHs [189,190,194,195], where the host lattice may protect relatively delicate biomolecules from degradation and also aid their transport to specific targets within the body, and hence the intercalation reactions lead to the formation of novel bioinorganic nanohybrids with potential practical significance, such as new DNA reservoirs or carriers for the delivery of genetic material to cells [189]. 

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