6-7Li NMR

In this paper, new highlights are proposed to interpret the reversible and irreversible capacities of nanostructured carbons in lithium batteries. A proportional dependence between the irreversible capacity and the active surface area (ASA) of carbon materials will be demonstrated, showing the ASA concept more universal than any of the other parameters which were previously considered. In-situ 7Li NMR will be also presented as a means to... 

IN-SITU 7LI-NMR STUDY OF THE REVERSIBLE LITHIUM INSERTION MECHANISM... 

In the following part of this paper, in-situ 7Li-NMR is used to better know the state(s) of lithium reversibly stored in disordered carbons. 

The HRTEM observation of the cross section of a coated fiber showed that the core is constituted of aromatic layers highly misoriented, whereas they are preferentially oriented in parallel for the thin coating pairs of stacked layers form mainly Basic Structural Units (BSUs) in which the average interlayer distance is smaller than between the aromatic layers in the bulk of the fiber. Since the nanotexture is more dense for the pyrolytic carbon than for the fiber itself, it acts as a barrier which prevents the diffusion of the large solvated lithium ions to the core of the fiber, allowing the passivation layer to be less developed after this treatment. Hence, the major amount of lithium inserted is involved in the reversible contribution therefore this composite material is extremely interesting for the in-situ 7Li NMR study of the reversible insertion. 

Figure 5. 7Li NMR spectra recorded during 3 cycles of reversible lithium insertion-deinsertion in the composite carbon electrode. The peak at 0ppm is due to ionic lithium (Li+PF6 and passivation layer). The peak of lithium at 263 ppm is not shown.

Figure 7. Schematic model based on the TEM image analysis and on in situ 7Li-NMR during galvanostatic reduction/oxidation of the carbon composite. During insertion, ionic lithium penetrates at first in the smallest interlayer spacings, then it diffuses in the slit-shaped pores where quasi-metallic clusters are formed.

During the past few decades, it has been shown that multi-nuclear NMR is a very powerful experimental technique to study alkali complexes particularly in non-aqueous solutions. The experimental method used throughout this work was 7Li NMR. This method possesses features that make it convenient for this kind of investigation. Particular attention was given to high-pressure 7Li NMR, which includes the first example reported for... 

It could be shown (Fig. 2) that the Li+ ion in DMSO is coordinated by four solvent molecules. y-Butyrolactone was found to be a convenient diluent in this kind of study. Contrary to DMSO (DN — 29.8), it is a rather weak donor (DN = 18.0) which appreciably dissolves alkali metal salts. Its interaction with DMSO is much weaker than that between DMSO and a metal cation. By plotting the chemical shift of the 7Li NMR signal vs. 

Fig. 2. 7Li NMR chemical-shift variation as a function of the DMSO/ LiC104 molar ratio in y-butyrolactone solution at 25°C (92).

Fig. 3. 7Li NMR shift measured for 0.05 M LiC104 in acetonitrile— nitromethane solvent mixtures at 25°C (93).

Fig. 4. 7Li NMR spectra of L1CIO4 in H2O-DMSO mixtures of different compositions at 25°C (92).

Figure 2. Dy(P30io)2 is a lanthanide shift reagent commonly used in biological 7Li NMR experiments. The Dy3+ ion has a coordination number of nine with two P3O10 moieties, acting as tetradentate ligands, and one molecule of H2O coordinated in the first coordination sphere up to seven Li+ ions can bind in the second coordination sphere.

Figure 3. Monitoring the uptake of Li+ into human erythrocytes after incubation in media containing 2 mM Li+ using 7Li NMR spectroscopy. The signals corresponding to the intra-, and extracellular Li+ are separated by the presence of the paramagnetic shift reagent, Dy(P30io)2, the extracellular medium [34].

Figure 4. Comparison of the uptake of Li+ into erythrocytes from rat and rabbit, determined by 7Li NMR spectroscopy [34],...

The CP/MAS-29Si solid state NMR spectrum of 6a proves the presence of six crystallographically (and chemically) different 29Si nuclei, of which two sorts coincide (Fig. 15). It is noteworthy that the 7Li NMR spectrum of 6a in solution (Fig. 16) showed the expected distinction of four Li sorts The two singlets at S = 1.78 and 1.56 correspond... 

Fig. 28. (Left) Molecular structure of 42. 2 C6H6Me. (Right) Dynamic carousel process and the 7Li NMR spectrum of 42. [Reprinted with permission from (70). Copyright 1997, Royal Society of Chemistry.]... 

The solid-state hexamers (2)—(4) at first appeared to dissolve intact in benzene (94). Cryoscopic rmm measurements over a range of concentrations (0.03-0.09 M, molarity expressed relative to the empirical formula mass) implied n values of 5.9-6.1. Furthermore, their room-temperature 7Li NMR spectra in c/8-toluene each consisted of broad singlets within the narrow chemical shift (6) range of + 0.6 to -0.2 ppm (relative to external phenyllithium in the same solvent). However, variations in temperature and concentration affected the 7Li NMR spectra of (2) and, in particular, of (4) (95). Figure 18a shows these spectra for three d8-toluene solutions of (4) at -100°C. The most concentrated solution has a dominant signal at 8 -+0.7, though five or six other signals (indicated by asterisks) are apparent. On dilution,... 

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