Intercalation staging

This section looks at two other aspects of intercalation staging, a particular type of ordering in layered compounds and cointercalation, where more than one type of guest is intercalated in the same compound. 

The surface films formed are not sufficiently passivating, and their formation involves partial exfoliation of the graphite. Thick and resistive surface films are formed. The irreversible capacity loss is pronounced, and the electrode cannot reach all the Li-graphite intercalation stages (MF, DMC, and ether solutions). 

Figure 25 Typical chronoamperograms and schematic view of the structure of lithiated graphite electrodes in three classes of electrolyte solutions (as indicated). 1. Reversible behavior 2. partially reversible behavior, low capacity (x < 1 in LijC6 3. irreversible behavior, the electrode is deactivated and partially exfoliated before reaching intercalation stages. Note that in reality the graphite particles are usually flakes, the electrode s structure is porous and the surface films are also formed inside the electrode among the particles, and thus have aporous structure [87]. (With copyrights from Elsevier Science Ltd., 1998.)...

Figure 26 A comparison of the chemical diffusion coefficient of Li into graphite (calculated from PITT), the intensity of the major XRD peaks (e.g., 002, 004) during intercalation (vs. E), and a completed slow scan rate cycling voltammogram of a thin (10 pm) composite graphite electrode (KS-6 from Lonza) in EC-DMC/LiAsF6 solution. Note that as the electrode is thinner and the particles are more oriented (with their basal planes parallel to the current collector), the scan rate is slower and the CV peaks are sharper and better resolved. The various phases (intercalation stages) are indicated [87]. (With copyrights from Elsevier Science Ltd., 1998.)...

Figure 3. Defined intercalation stages from cathodic reduction of MoSj in dimethylsulfoxide electrolyte solutions containing ions (after ref. 3).

Figure 13 shows a typical electrochemical response of graphite and disordered carbon electrodes (a, b, respectively), related to the diffusion and accumulation of hthium in the bulk carbon particles. The differential capacitance of these electrodes is nicely reflected by slow scan cyclic voltammetry. As already discussed in detail [105-107], the peaks in the CV of Figure 13a (4 sets of redox peaks) reflect phase transition tetween Li-graphite intercalation stages (indicated in the figure), and they correspond to the plateaus in Figure 11a Their shape depends on the resolution of these experiments. The resolution of the voltammetric response of these electrodes depends on the thickness of the electrode, the resistance of the surface films, and the potential scan rate [108]. The best resolution in electrochemical studies of these systems is obtained in experiments with single particles [109-110]. Such experiments, however, are difficult and require special apparatus. Using composite electrodes, a condition for meaningful results, is a situation in which the electrodes are thin and the solution reaches the entire active mass, and, in fact, aU the particles interact in parallel with both the current collector and solution species. In such a situation, the composite electrodes can be considered as an array of microelectrodes, and then toe resolution of the measurements and their reliability are high.

VS. E (the differential capacity increases as the potential is lower), reflects a Li adsorption-type mechanism and the formation of a solid Li-C solution [96]. The chemical diffusion coefficient (Z J of Li into the carbons could be calculated by both PUT or EIS (the Warburg -type, low frequency domain) [105-108]. Djj vs. " is a peak-shaped function for graphitic materials, with sharp minima at the peak potentials (i.e., the potentials of phase transition between intercalation stages), while Du vs. E for disordered carbons is a function with maxima, as clearly seen in Figure 13. 

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