Operando electrochemical Kerr Gated Raman Spectroscopy to Probe the High States of Charge in Graphite Electrodes for Li-Ion Batteries
Authors: Neale Alex R. , Milan David Costa , Braga Filipe , Sazanovich Igor , Hardwick Laurence J. 
Published: July 2022

In order to improve the lifetime of lithium ion (Li-ion) cells, methods to reliably probe the cell state of charge are essential to track lithium inventory loss and understand the relation to key degradation processes in the cell. NMR and X-ray/neutron scattering techniques have been demonstrated to probe the full range of intercalation from pure graphite through to stage 1 LiC6.1-3 While Raman spectroscopy/microscopy has been proven a powerful tool for the probing the earlier stages 4 and 3 of Li intercalation into graphite electrodes, the process is plagued by significant growth in competing fluorescence/emission signals as the intercalation proceeds through the lower stages between Li0.5C6 and LiC6. Coupled with a loss of Raman scattering intensity due to increased conductivities of the lithiated graphite,4, 5 the graphite bands also become swamped by the growth of overlapping fluorescence/emission signals, becoming difficult to reliably observe/assign. Consequently, diagnosing the state of charge of the highly lithiated graphite electrode by Raman spectroscopy is challenging.
To overcome this, we report in this work the use of operando electrochemical Kerr gated Raman spectroscopy to track the critical changes in the graphitic bands during the Li intercalation process into a graphite negative electrode. Kerr gated Raman spectroscopy is a fluorescence suppression technique that exploits the varied time domains of the Raman scattering and fluorescence/emission processes. We have previously reported on the use of Kerr gated Raman spectroscopy as an effective tool to measure the Raman spectra of highly fluorescing, degraded/aged battery electrolyte materials based on the lithium hexafluorophosphate salt.6 In this work, a dedicated operando measurement cell was designed to permit observation of the graphite working electrode in the Li|graphite half-cell during electrochemical Li intercalation. Therein, the Kerr gated Raman spectra are compared with conventional continuous wave Raman spectroscopy, outlining the benefits and limitations of both methodologies.
Importantly, owing to the efficacy of the Kerr gate in filtering out a significant portion of the problematic emission signals, the graphitic Raman bands could be observed even during the transitions through intercalation stages 2 and 1 (i.e., through Li0.5C6 to LiC6) with much greater clarity than has been achieved by conventional Raman techniques.

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