Researchers from the Central Laser Facility (CLF) and the University of Liverpool have used Kerr gated Raman spectroscopy – a technique which was developed and first demonstrated in CLF in 1999 – to characterise Lithium ion batteries. The LiPF6 results indicate that Kerr gated Raman spectroscopy can detect decomposition compounds and surface film on pristine LiPF6 particles. Due to large background fluorescence emission interfering with the scattering signal, observing this would be impossible otherwise.
Raman spectroscopy, a technique that has been around since the late 1920's, is a type of vibrational spectroscopy utilizing visible light and is capable of assessing the chemical composition and structure of molecules and materials In Raman spectroscopy, the sample under study is illuminated by laser light. Incident photons can be scattered inelastically when molecules in the sample interact with incoming photons through its virtual energy level. Scattering can be conceived of as the absorption of one photon and the instantaneous emission of another photon. The energy difference between the incoming and scattered photons is related to vibrational and rotational transitions that occur in the molecule, providing useful information about the structure of the object under study. However, information from the sample is often drowned up by noise. Another phenomena that can also occur when the molecule is excited by an optical beam (light) is fluorescence. Fluorescence can be a form of “light pollution" when it interferes with the useful signal of Raman scattering.
This is where the Kerr gate comes in. Kerr gate is an optical gating technique which suppresses the unwanted fluorescence signals originating from the material under study, thus, allowing Raman scattering to reveal the useful information on the structure of the Lithium ion batteries. Kerr gate Raman benefits of the two key ideas: 1) the fluorescence background develops on a slower time scale than instant Raman scattering and 2) the Kerr gate, upon activation by laser, switches the polarisation of the Raman signal.
When the mixed (Raman + fluorescence) signal arrives at the Kerr gate, only the Raman signal will have its polarisation switched thank to careful adjustment of the activation time of Kerr gate. The polarisation of fluorescence background remains unswitched. Since, the output polariser allows through only the switched polarisation signal of Raman but not the unswitched polarisation as of fluorescence background, this technique effectively filters the useful Raman signal from the “light pollution" of the fluorescence background.
Using this technique, researchers at the University of Liverpool have characterised lithium ion battery electrolyte materials. For lithium-ion batteries, lithium hexafluorophosphate (LiPF6) is often used as the lithium salt in organic carbonate-based electrolytes. LiPF6 is unstable at temperatures above 60° C and it readily reacts with trace amounts of water due to its high hydrophilicity, leading to several decomposition reactions. The scientists used spectroscopic analysis of subtle changes in Raman peak shape associated with changes to solvent–ion interactions using Kerr gated Raman and this result highlights the potential to understand electrolyte ageing in batteries by conducting a series of systematic and quantifiable experiments. Moreover, the data reveals that fluorescence-causing species is present in the fresh electrolyte. It is an open question as to what role and relevance it could have in Li-ion battery chemistry. Interestingly the concentration of fluorescent species did not increase significantly within the confines of the initial ageing experiments as the fluorescence background signal was not observed to increase between fresh and aged electrolytes.
The Kerr gated approach has been demonstrated to be effective in the suppression of background fluorescence emission in the spectra collected for the battery components (salt and electrolyte), which offers possibilities for this technique to study electrode/electrolyte interfaces and to monitor degradation processes during Li-ion battery cycling and storage. By using a Kerr gate approach, we may be able to detect these trace compounds with greater sensitivity. The present study highlights Kerr gated Raman spectroscopy as a powerful tool that can be applied to investigate electrode/electrolyte interfaces and the speciation of the solid electrolyte interphase of battery systems.
To read the paper, go here: https://pubs.rsc.org/en/content/articlepdf/2019/cp/c9cp02430a