2:15 PM - EN05.16.04
Correlative Electrochemical Acoustic Time-of-Flight Spectroscopy and X-Ray Imaging to Monitor the Performance of Single-Crystal and Polycrystalline NMC811/Gr Lithium-Ion Batteries
Harry Michael1,2,Rhodri Owen1,2,James Robinson1,2,Tom Heenan1,2,Rhodri Jervis1,2,Dan Brett1,2,Paul Shearing1,2
University College London1,The Faraday Institution2
NMC electrodes typically consist of primary particles aggregated to form polycrystalline secondary particles. Electrodes composed of polycrystalline NMC particles have high gravimetric capacity and good rate capabilities but do not perform as well as single crystal equivalents in terms of volumetric energy density and cycling stability. This has prompted research into well-dispersed single-crystal NMC products as an alternative solution for high-energy-density batteries.
There has also been a move towards the application of new characterisation techniques to better understand the degradation processes and extend lifetime. Of these, acoustic techniques have rapidly increased in popularity, with electrochemical acoustic time-of-flight (EA-ToF) spectroscopy being used to study various degradation phenomena in LiBs such as severe gas formation, cell stiffness , and cathode dissolution. In addition, studies have commonly coupled EA-ToF spectroscopy with complementary characterisation tools including X-ray diffraction (XRD) and X-ray computed tomography (CT)  to further enhance understanding of the structural changes taking place in the materials and architectural changes at the device level. Progress in EA-ToF investigations of LiB thermal expansion can provide a greater understanding of these dynamic processes, something which will help to overcome limitations in current density, battery lifetime and capacity fade and ultimately minimise the total cost of battery systems. Furthermore, EA-ToF spectroscopy is a non-invasive, non-destructive technique that can be applied to batteries with minimal cost and high time-resolution results, making it an attractive proposition for detailed on-board diagnostics of operating batteries.
Here, we will discuss the underlying principles of EA-ToF spectroscopy and report EA-ToF spectrograms collected for pouch cells containing either single-crystal NMC811 (SC-NMC811) or polycrystalline NMC811 (PC-NMC811) electrodes. For the first time known to the authors, EA-ToF spectroscopy has been shown to be effective in distinguishing between LiBs composed of either SC-NMC811 or PC-NMC811 electrodes. Cells composed of PC-NMC811 electrodes had a higher degree of gas evolution compared to cells containing SC-NMC811 electrodes and underwent larger changes in the acoustic signal’s time-of-flight (ToF) during constant current cycling at a range of C-rates indicating expansion, fracture or dislocation of the reflective interfaces inside the cell. In addition, X-ray CT was used to confirm significant differences in morphology between SC-NMC811 and PC-NMC811 electrodes such as particle sphericity and particle diameter that may have a direct effect on acoustic signal interaction with these electrode interfaces. However, there was no noticeable difference in the internal architecture of the SC-NMC811/Gr cell and PC-NMC811/Gr cell, suggesting that the variation in acoustic response between the cells was principally due to the difference in crystallinity and morphology of the NMC811 electrodes.
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