Flujo de la experiencia: sistema de IDEACIÓN

The viability of this method was tested using ion chromatography (IC). IC works by separating molecules dependent on their interaction with the stationary phase. More in-depth information on the technique can be found here [184]. However, for this work it is sufficient to report that it is able to detect the concentration of specific chemical elements. For this study the quantity of fluorine was calibrated to the system, this is done by creating standards of known concentrations of fluorine and comparing the sample with these standards. This was done with fluoride ion standards of 0.01, 0.05, 0.1, 0.5 and 1 mg.l-1 solutions on an ICS 2500 ion chromatograph. This sample was then compared to purified and distilled tap water.

The Dionex software used only exported to PDF but screen prints of these are presented in Figure 34. It shows that the distilled and purified water contained 0.68 mg.l-1 of fluoride ions. Whereas, the fluoride ions present in an electrode washed sample after using this new method was only 0.06 mg.l-1. This is a difference of more than ten times less fluoride in the electrode washing sample than in purified water.

Figure 33. Illustration of the method to passivate LiPF6 and selectively remove sample species prior to analysis with GC-

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7.2.3. Discussion and conclusions

Using the method provided above the total quantity of F- ions in the electrode washing sample was reduced to below distilled water levels. Because the sample was exposed to excess quantities of water the very small quantities of F- _{ions present do not originate from either HF,} or LiPF6.

Previous to this work the only method of extracting LiPF6 from samples was with a liquid- liquid extraction method proposed by Petibon et al. [146]. Their method exposed the sample to moisture prior to analysis meaning that the sample may have been contaminated by production of HF. The method proposed in this work circumvents these problems by initially mixing the solvent with the sample in an inert atmosphere. This introduces a problem with vaporisation points of solvents within a glove box environment which meant that a higher vaporisation point solvent had to be used in the glove box. The problem is that this solvent is highly soluble in

Figure 34. Screen prints of the ion chromatography taken of distilled water and the electrode washing sample after processing with the method illustrated in Figure 33.

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water and therefore an additional step had to be added to change the solvent being used so that the precipitates could be removed with the liquid phase, this was done with a desiccator and two additions of water.

The biggest step change is the addition of CaCO3 to the sample prior to taking it out of the glove box and when it is hydrolysed. This means that any reaction which does occur prior to the sample being exposed to moisture creates Calcium difluoride (CaF2) in place of reacting with the sample. This method of forming CaF2 from Ca- containing compounds was established many years ago and is the basis for the use of fluoride within toothpaste [185]. The reaction between CaCO3 and HF is shown explicitly in the work of Yasui et al. [186]. Therefore, the method presented here uses the same principles as Petibon et al. [146] but improves it by ensuring that there is no reaction between HF and the sample.

However, because the surface films chemical composition is unknown it is not possible to guarantee that some of it is not lost in the aqueous phase or chemically changed through the sample evaporation process. Therefore, this method is of most use in studying the oligomer compounds present within the surface film and not in building a complete picture of the surface film’s composition with HPLC. XPS and other methods can be used to compliment this deficiency.

It should be noted that the spectra obtained for lithium-ion surface film samples is not typical of HPLC / GPC spectra (chapter eight); in that they are not well-resolved peaks. The only other spectra using HPLC on lithium-ion cells, also provides similarly unresolved peaks [187]. This means that that the results in this work are as optimised as they are likely to be for commercial samples. This is due to the variety of chemical products being resolved. This makes the technique no use for the absolute quantification of specific oligomers but is useful for the quantification of the sum of the polymeric components present.

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7.3.

Conclusions

XPS, FTIR, Raman and EDAX provide a means of analysing the chemical structure of the surface film. XPS was highlighted as being especially effective due to its high resolution in detecting chemical species in the <10nm range. However, analysis of the chemical structure only goes so far in describing the relationship between cell resistance or capacity and the surface film. Resistance of the cell can also increase due to an increase in the thickness of the surface film. Others have used traditional sputter-depth profile methods. However, this contains fundamental flaws for cell materials with inhomogeneous surfaces of unknown chemical composition. This work proposes a method of XPS analysis that sputters until a steady-state is reached in the material, at which point the relative concentrations can be used to determine the quantity of surface film relative to graphite. This comparison provides information on the thickness or quantity of surface film around each particle.

This innovative XPS method hinges on the assumption that the surface film components contain a greater quantity of non C-C environments than graphite, and that these non-graphite components are equivalent between samples. The problem is that XPS is not able to detect the length of oligomers within the surface film. Therefore, if oligomers are formed of different lengths, then it could appear as though the surface film had increased less than it has between different samples. This work proposes using HPLC to detect these polymeric species. However, LiPF6 is present in the sample washing and this forms HF in the presence of water. This work proposes a method that stops reaction of HF with the oligomer species and allows analysis of the sample with HPLC through a wet-chemistry method. This method uses CaCO3 to cause the HF to preferentially react with it, in place of the oligomers. However, the method has not been studied for its impact on non-polymeric species present in the surface film so should not be used for the study of these components with HPLC.

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Chapters six and seven contribute work to tackle the second objective which is to:

Determine the optimum post-mortem methods that allow connection between electrical performance data and material changes at the negative electrode. This was done by overcoming five challenges and the optimum method for post-mortem analysis is:

 Open the cell (using the new method for 18650-type cells) inside a glove box and never expose samples to air

 Analyse with XPS to determine chemical composition of the electrolyte deposits

 Sputter the sample until a steady-state is reached

 Analyse with XPS to determine chemical composition of the surface film

 Compare the total concentration of graphite to non-graphite from the XPS results to determine surface film thickness

 Where further clarification is required on functional group assignment or morphology investigate with IR or SEM.

 Determine changes in polymeric species with HPLC using the new method for sample preparation outlined in section 7.2

This approach should avoid chemical changes to the surface film as a consequence of the preparation and analysis processes. It should also provide information on the chemical composition of the surface film, deposits and electrode thickness with XPS. Where XPS is not able to determine changes in surface film, HPLC can now be used to investigate changes in the polymeric species present. It also uses a minimal number of techniques making characterisation simpler by reducing the time involved with multiple sample preparations.

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