Graphite lithiated

The first lithiated graphitic carbons (lithium-graphite intercalation compounds, abbreviated as Li-GIC s),... 

Figure 6. Simplified scheme showing the stage formation during electrochemical formation of lithiated graphite. Left schematic galvanosta-tic curve. Right schematic voltam-metric curve. Prepared with data from 192, 100, 104, 105, 110], For a more detailed discussion, see text.

The excess charge consumed in the first cycle is generally ascribed to SEI formation and corrosion-like reactions of Li C6[19, 66, 118-120]. Like metallic lithium and Li-rich Li alloys, lithiated graphites, and more generally lithiated carbons are thermodynamically unstable in all known electrolytes, and therefore the surfaces which are exposed to the electrolyte have to be kinetically protected by SEI films (see Chapter III, Sec.6). Neverthe-... 

Whereas the electrochemical decomposition of propylene carbonate (PC) on graphite electrodes at potentials between 1 and 0.8 V vs. Li/Li was already reported in 1970 [140], it took about four years to find out that this reaction is accompanied by a partially reversible electrochemical intercalation of solvated lithium ions, Li (solv)y, into the graphite host [64], In general, the intercalation of Li (and other alkali-metal) ions from electrolytes with organic donor solvents into fairly crystalline graphitic carbons quite often yields solvated (ternary) lithiated graphites, Li r(solv)yC 1 (Fig. 8) [7,24,26,65,66,141-146],... 

Figure 8. Schematic drawing of binary ( Li, C ) and ternary [ Lftsolv. C, ] lithiated graphites. Modified and redrawn from Ref. [26],...

Figure 9. Schematic model of the film-formation mechanism on/in graphite (a) the situation before reaction (b) formation of ternary lithiated graphite Lir(solv)vC , (c) film formation due to decomposition of Li t(solv)v. Prepared with data from Ref. [155],...

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... 

In the ideal situation of 100% utilization x = 1.0), the capacity corresponding to the above anode half reaction is 372 mA h g However, due to the low ion conductivity of the polymer electrolyte and the high interfacial impedance between it and the graphite electrode, this elegant example of electrochemical preparation of lithiated graphite is of limited practical significance. 

Because of the similar potentials between fully lithiated graphite and lithium metal, it has been suggested that the chemical nature of the SEIs in both cases should be similar. On the other hand, it has also been realized that for carbonaceous anodes this formation process is not expected to start until the potential of this anode is cathodically polarized (the discharge process in Figure 11) to a certain level, because the intrinsic potentials of such anode materials are much higher than the reduction potential for most of the solvents and salts. Indeed, this potential polarization process causes one of the most fundamental differences between the SEI on lithium metal and that on a carbonaceous anode. For lithium metal, the SEI forms instantaneously upon its contact with electrolytes, and the reduction of electrolyte components should be indiscriminate to all species possible,while, on a carbonaceous anode, the formation of the SEI should be stepwise and preferential reduction of certain electrolyte components is possible. 

Figure 28. DSC trace of the reactions occurring between a fully lithiated graphitic anode and electrolyte. Anode surfaces both rinsed with DMC and unrinsed were studied. (Reproduced with permission from ref 338 (Figure i). Copyright 1998 The Electrochemical Society.)...

This sharp decline in cell output at subzero temperatures is the combined consequence of the decreased capacity utilization and depressed cell potential at a given drain rate, and the possible causes have been attributed so far, under various conditions, to the retarded ion transport in bulk electrolyte solutions, ° ° - ° ° the increased resistance of the surface films at either the cathode/electrolyte inter-face506,507 Qj. anode/electrolyte interface, the resistance associated with charge-transfer processes at both cathode and anode interfaces, and the retarded diffusion coefficients of lithium ion in lithiated graphite anodes. - The efforts by different research teams have targeted those individual electrolyte-related properties to widen the temperature range of service for lithium ion cells. 

Figure 68. Nyquist plots of a charged lithium ion cell, a lithiated graphite/graphite cell, and a delithiated cathode/ cathode symmetrical cell. The inset is an equivalent circuit used for the interpretation of the impedance spectra. (Reproduced with permission from ref 512 (Figure 3). Copyright 2003 Elsevier.)...

Figure 74. Improved thermal stability of an electrolyte by flame retardant HMPN (a, left) DSC traces for baseline electrolyte with (1.68%) and without HMPN in the presence of a fully lithiated graphite anode (Reproduced with permission from ref 523 (Figure 5). Copyright 2000 The Electrochemical Society.) (b, right) SHR of baseline electrolyte with (10.0%) and without HMPN in the presence of metallic lithium. (Reproduced with permission from ref 523 (Figure 6). Copyright 2000 The Electrochemical Society.)...

All the above studies indicated clearly that reduction of solvent, salt, and additives (e.g., H20) by Li contribute together to the buildup of the surface films on lithium in solutions. It should be emphasized that XRD, XPS, and AES studies of Li electrodes, as well as the indirect identification of surface species from studies of reactions of lithiated graphite or Li/Hg amalgam with electrolyte solutions, could not provide specific enough information on the chemical composition of the surface films. Moreover, application of XPS for Li electrodes may induce secondary surface reactions. Visible changes appear on Li surfaces during XPS measurements. More specific information on the composition of the surface layers formed on Li could be obtained by surface-sensitive FTIR spectroscopy that was introduced into this field in the middle of 1985 by Yeager et al. [84,85,178], and which is a nondestructive technique. 

Due to its layered structure, graphite is the carbonaceous material most sensitive to the detrimental effects of interactions with solution species. Co-intercalation of solution species such as solvent molecules together with the Li ions may lead to exfoliation and destruction of their structure, as indeed happens in solutions of PC, y-BL, THF and other solvents [356-358], Lithiated graphite electrodes behave reversibly only in solutions in which highly passivating, stable surface layers are formed on the pristine material before any Li intercalation takes place. 

Figure 25 illustrates three cases of lithiated-graphite electrodes in solutions [87], (Typical chronopotentiograms are also presented.)... 

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.)...

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