Fully lithiated graphite

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.)...

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.)...

Fig. 14.9 DSC spectra of fully lithiated graphite and overcharged LiFeP04 with traces of 1.2 mol LiFFg in EC-EMC (3 7) electrolyte at 10 °C min ...

Among possible alternative alloying elements, silicon is the most attractive and widely investigated [3, 4] because of its high gravimetric and volumetric capacities and abundance in the natural environment. Silicon in the fully lithiated form of Li4.4Si provides a theoretical specific capacity of 4,212 mAh/g which is 10 times more than the capacity of graphite. The specific capacities and volume changes of the different electrochemically active elements are shown in Table 15.1. 

Figure 26 DSC thermogramms of fully lithiated grajAite-electrolyte samples (the modified and the pristine samples were shifted by —25 W/g and —75 W/g respectively). The electrolyte/graphite ratio is 5 iL/ 2 mg. The heat flow values are in units of Watt per gram of graphite. Sample exploded. Reproduced from [123] by permission of the J. Solid State Electrochem.

Table 14.2 Elow of enthalpy deduced from the DSC spectra of the fully delithiated and overcharged carbon-coated LiFeP04 and the fully lithiated carbon-coated graphite (see Eig. 14.8) that of the overcharged cathode elements investigated (see Fig. 14.9) are reported in the three last columns...

In-situ x-ray diffraction (XRD) was performed on a coin type cell with a 4x6 mm Kapton window coated with conductive thin copper layer. The graphite electrode was pressed against the Kapton window so as to be reached by the x-ray beam. After several lithiation/delithiation cycles under a C/10 rate between 1.5 and 0V, the cell was fully delithiated up to 1.5V. The cycle capacity achieved with the graphite electrode is about 360mAh/g. The cell was then re-lithiated under a slower rate of C/20. XRD patterns were taken for about five minutes every hour while the cell is under continuous discharge. As result the lithium composition x in LixC6 was incremented by 0.05 between two successive XRD scans. 

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