Disordered carbon electrodes

Extension of Cell-Impedance-Controlled Lithium Transport Concept to the Disordered Carbon Electrode... 

This chapter addresses several issues dealing with the mechanism of SEI formation on inert substrates, lithium, carbonaceous materials and tin-based alloys. Attention is currently focused on the correlation between the composition and morphology of the solid-electrolyte interphase forming on the different planes of highly ordered pyrolytic graphite (HOPG) and different types of disordered carbon electrodes in lithium-ion cells. 

Figure 22 Elemental depth profiles of the SEI on disordered carbon electrodes in UAsF, electrolyte. Reproduced from [84] by permission of Elsevier Science Ltd.

Sandf G., Gerald R. E., Scanlon L. G., Carrado K. A. and Winans R. E., Computational, electrochemical and 7Li NMR studies of lithiated disordered carbons electrodes in lithium ion cells. Materials Research Society Symposium Proceedings Materials for Electrochemical Energy Storage and Conversion II — Batteries, Capacitors and Fuel Cells) 496 (1998), 95-100. 

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.

When dealing with failure or stabilization of carbon electrodes, we can distinguish between factors that relate directly to surface reactions and those that relate to the nature of the Li insertion mechanism into the carbon. However, it should be noted that the factors related to the surface phenomena are also definitely connected to the 3D structure of the carbons. As already mentioned, Li insertion into either soft or hard disordered carbons, and the failure or stability of hthiated disordered carbon electrodes are much less dependent on their surface chemistry as compared with graphitic materials. This is due to the fact that graphites are much softer and weaker than disordered carbons. Hence, failure and stability of disordered carbon electrodes are mostly determined by structural factors, the type of Li insertion sites available, the number of C-H bonds, and the existence of sites to which Li is inserted irreversibly [81-85,94,95,97-99]. Discussion of these structural impacts on the behavior of the electrodes is beyond the scope of this chapter. 

Endo M, Kim C, Hiraoka T, Karaki T, Matthews MJ, Brown SDM, Dresselhaus MS. Li storage behavior in polyparaphenylene (PPP)-based disordered carbon as a negative electrode for Li ion batteries, Mol Cryst Liq Cryst 1998 310 353-358. 

Photocurrent-potential curves and photocurrent spectra have been reported for DLC electrodes [61, 62, 170], Under illumination, the DLC demonstrates properties of an intrinsic wide-gap semiconductor or insulator. By treating the spectra in the above-described way, the mobility gap in this disordered carbon material was estimated. 

When carbon electrodes are used, Li may be inserted/intercalated reversibly into the carbon at potentials as high as 1 V versus Li/Li+ (after the formation of surface films). In the case of disordered carbons, insertion may occur at even higher potentials. In the case of graphite (as described in the next section), the onset for lithium intercalation is around 0.3 V versus (Li/Li+). With glassy carbon, there is no considerable lithium insertion, and hence this electrode behavior depends solely on the solvent and anion used and their cathodic stability [28],... 

As discussed in the next section, lithiated carbon electrodes are covered with surface films that influence and, in some cases, determine their electrochemical behavior (in terms of stability and reversibility). They are formed during the first intercalation process of the pristine materials, and their formation involves an irreversible consumption of charge that depends on the surface area of the carbons. This irreversible loss of capacity during the first intercalation/deintercalation cycle is common to all carbonaceous materials. However, several hard, disordered carbons exhibit additional irreversibility during the first cycle, in addition to that related to surface reactions with solution species and film formation. This additional irreversibility relates to consumption of lithium at sites of the disordered carbon, from which it cannot be electrochemically removed [346-351],... 

Carbon and graphite are used in batteries as electrodes or as additives in order to enhance the electronic conductivity of the electrodes. As electrodes, graphites and disordered carbons reversibly insert lithium, and hence they may serve as the anode material in -> lithium batteries. Graphitic carbons intercalate lithium in a reversible multi-stage process up to LiC6 (a theoretical capacity of 372 mAh g-1) and are used as the main anode material in commercial rechargeable Li ion batteries. As additives, carbon and graphite can be found in most of... 

Spectral Photoresponses of Carbon-Doped Ti02 Film Electrodes. Raman spectra used to identify disordered carbon in the flame-formed samples in addition to lower nonstoichiometric titanium oxides identified by X-ray diffraction. 314... 

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