A myriad of layered transition metal cathodes have been developed and a wide range of LiNi_xMn_yCo_zO₂ (NMC) chemistries are currently used in lithium-ion batteries (LiBs). They have been deployed in electric and hybrid-electric vehicles and are amongst the most promising LiB cathodes because their layered structures provide higher reversible capacities and more favourable rate capabilities than previously commercialised LiCoO₂ (LCO) cathodes to enable longer-range operation and faster-charging ^[1]. Most NMC electrodes consist of anisotropic single crystal primary particles aggregated to form polycrystalline secondary particles. Electrodes composed of polycrystalline NMC particles have large gravimetric capacity and good rate capabilities but do not perform as well as single crystal equivalents in terms of volumetric energy density, cycling stability and production adaptability. This has prompted research into well-dispersed single-crystal NMC products as an alternative solution for high-energy-density batteries.

In recent years 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 increased in popularity extremely rapidly, with electrochemical acoustic time-of-flight (EA-ToF) spectroscopy being used to study various degradation phenomena in LiBs such as severe gas formation^[2], cell expansion ^[3], and cathode dissolution^[4]. In addition, studies have also used EA-ToF spectroscopy in tandem with other characterisation tools such as X-ray diffraction (XRD)^[5] and X-ray computed tomography (X-ray CT) ^[2] to further enhance our understanding of the structural changes taking place in the materials that make up electrochemical devices. There are multiple degradation mechanisms at play in the ageing of a LiB and learning how to predict which mechanism - or combination of mechanisms - is most critical in particular conditions, is vital to understanding and mitigating degradation, especially for ostensibly similar cells. For instance, progress in EA-ToF investigations of LiB dilation/contraction can inform understanding of structural changes during charge/discharge and a greater understanding of these LiB dynamics is required to overcome limitations in current density, battery lifetime and capacity fade.

In this presentation, 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. Here, 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 SC-NMC811 electrodes achieved higher capacities than cells composed of PC-NMC811 electrodes with the cells containing PC-NMC811 electrodes undergoing more expansion during constant current cycling at different C-rates. In addition, X-ray computed tomography was used to confirm significant differences in morphology between SC-NMC811 and PC-NMC811 electrodes such as particle sphericity and equivalent 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 due to the difference in crystallinity of the NMC811 electrodes as opposed to noticeable intrinsic physical properties since all other materials remained the same.

References

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[2] M. T. M. Pham, J. J. Darst, D. P. Finegan, J. B. Robinson, T. M. M. Heenan, M. D. R. Kok, F. Iacoviello, R. Owen, W. Q. Walker, O. V. Magdysyuk, T. Connolley, E. Darcy, G. Hinds, D. J. L. Brett, P. R. Shearing, J. Power Sources 2020, 470, 228039.

[3] C. Bommier, W. Chang, J. Li, S. Biswas, G. Davies, J. Nanda, D. Steingart, J. Electrochem. Soc. 2020, 167, 020517.

[4] J. B. Robinson, R. E. Owen, M. D. R. Kok, M. Maier, J. Majasan, M. Braglia, R. Stocker, T. Amietszajew, A. J. Roberts, R. Bhagat, D. Billsson, J. Z. Olson, J. Park, G. Hinds, A. Ahlberg Tidblad, D. J. L. Brett, P. R. Shearing, J. Electrochem. Soc. 2020, 167, 120530.

[5] C. Bommier, W. Chang, Y. Lu, J. Yeung, G. Davies, R. Mohr, M. Williams, D. Steingart, Cell Reports Phys. Sci. 2020, 1, 100035.