Co-intercalation

It was also shown in 1983 [11] that lithium can be reversibly inserted into graphite at room temperatures when a polymeric electrolyte is used. Prior experiments with liquid electrolytes were unsuccessful due to co-intercalation of species from the organic electrolytes that were used at that time. This problem has been subsequently solved by the use of other electrolytes. 

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. 

Figure 10. Schematic model showing the influence of the thickness of a graphite flake on the extent of co-intercalation of solvent molecules in the internal van der Waals gaps of graphite, (a) Thick graphite flakes (b) thin graphite flakes. Prepared with data from Ref. [169].

Thirdly, strong solvent co-intercalation, in particular into internal van der Waals gaps, can only be expected for kinetically stable ternary compounds Li solv) Cn. For example, comparison of DMC and DEC with dimethoxyethane (DME), shows that the kinetic stability of Li>.(DME)yCn can be considered much higher than that of LiJr(DMC)vC and LiJt(DEC)yC and of course Liy(EC)yCn [169]. With EC/DME, solvent co-intercalation proceeds on a macroscopic scale, i.e., the external van der Waals gaps and some internal ones can participate in the solvent co-... 

Numerous research activities have focused on the improvement of the protective films and the suppression of solvent cointercalation. Beside ethylene carbonate, significant improvements have been achieved with other film-forming electrolyte components such as C02 [156, 169-177], N20 [170, 177], S02 [155, 169, 177-179], S/ [170, 177, 180, 181], ethyl propyl carbonate [182], ethyl methyl carbonate [183, 184], and other asymmetric alkyl methyl carbonates [185], vinylpropylene carbonate [186], ethylene sulfite [187], S,S-dialkyl dithiocarbonates [188], vinylene carbonate [189], and chloroethylene carbonate [190-194] (which evolves C02 during reduction [195]). In many cases the suppression of solvent co-intercalation is due to the fact that the electrolyte components form effective SEI films already at potential which are positive relative to the potentials of solvent co-intercalation. An excess of DMC or DEC in the electrolyte inhibits PC co-intercalation into graphite, too [183]. 

Furthermore, the molecular size of the Li+ -solvating solvents may affect the tendency for solvent co-intercalation. Crown ethers [19, 152-154, 196, 197] and other bulky electrolyte additives [196] are assumed to coordinate Li+ ions in solution in such a way that solvent co-intercalation is suppressed. The electrochemical formation of binary lithiated graphites Li tC6 was also reported for the reduction... 

Graphitic anodes which have been "prefilmed" in an electrolyte "A" containing effective film-forming components before they are used in a different electrolyte "B" with less effective film-forming properties show lower irreversible charge losses and/or a decreased tendency to solvent co-intercalation [155, 201, 202], However, sufficient insolubility of the pre-formed films in the electrolyte "B" is required to ascertain long-term operation of the anode. 

LiC104 / PC electrolyte (Fig. 6), a process which was considered to be responsible for the continuing solvent decomposition when graphite is charged in PC-based electrolytes. This showed that, even in EC-based electrolytes, some degree of solvent co-intercalation exists but does not prevent formation of a stable SEI. It is clear that... 

The smaller ion may intercalate faster into the graphite galleries. Reaction (5) may be the rate-determining step for the solvent co-intercalation process, and if so, molecules that form large and stable solvated lithium cations will have a smaller tendency for co-intercalation into the graphite. 

The chemical composition of the SEI formed on carbonaceous anodes is, in general, similar to that formed on metallic lithium or inert electrodes. However some differences are expected as a result of the variety of chemical compositions and morphologies of carbon surfaces, each of which can affect the i() value for the various reduction reactions differently. Another factor, when dealing with graphite, is solvent co-intercalation. Assuming Li2C03 to be a major SEI building material, the thickness of the SEI was estimated to be about 45 A [711. 

At the prismatic surfaces, the primary reduction reaction is the reduction of those solvent(s) co-intercalated inside graphite, whereas on the basal plane graphite sites, reduction reactions may proceed in an excess of... 

Figure 16. Model In PC based electrolytes, solvent co-intercalation, gas formation and crevice formation in polycrystalline graphite materials are inter-related reactions. In fact, there is a subsequence of reactions (1) PC co-intercalation, (2) gas formation, (3) crevice formation ultimately resulting in exfoliation and macroscopic destruction of graphite [40],...

Also presented were data on carbon-coating of graphite powder using a propylene gas thermal decomposition processes. High weight percent amorphous carbon-coatings are possible with this method, and the process appears uniquely suited to materials that are reductively stable to 700°C. The coated materials work better in the 30% PC electrolyte solutions, thus showing better resistance to solvent co-intercalation problems versus uncoated types. 

STABILIZATION OF GRAPHITE NITRATE VIA CO-INTERCALATION OF ORGANIC COMPOUNDS... 

Graphite nitrate stabilization co-intercalation exfoliation proton... 

Holmstrom, S.C., Patil, A.J., Butler, M. and Mann, S. (2007) Influence of polymer co-intercalation on guest release from aminopropyl-functionalized magnesium phyllosilicate mesolamellar nanocomposites. Journal of Materials Chemistry, 17, 3894—3900. 

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