In situ 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 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.

There are reports on the use of both NMR and ESR for the study of electrode materials [83-85] and bulk products of electrochemical processes [86,87], For instance, 7Li NMR may be found to be very useful for the study of Li intercalation into carbonaceous materials [88], A major advantage of these techniques is that they are applied in situ. The electrochemical cell is, in fact, an NMR tube in which the studied electrode is mounted so that it can be placed within the magnet s cavity. While NMR provides information on the environment of the element studied, within the electrode measured, ESR provides information on the formation and stability of radical ions when formed during the course of an electrochemical process. 

Group 1. - 7Li NMR spectroscopy was used to characterise a lithium/ graphite intercalation compound with a formula close to LiC 1 In situ solid-state 7Li NMR data were reported for lithium inserted into disordered carbon. There was evidence for both Lis+ and metallic lithium species.2 7Li and 13C solid-state NMR spectra were used to characterise a mesoporous tantalum oxide lithium fulleride (Ceo) composite material.3... 

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