Lithium-ion batteries consisting of the particulate porous electrodes, are the dominant energy storage devices, however, they suffer from random failures and safety-related accidents. Such arbitrary incidents arise from the spatiotemporal heterogeneities occurring due to the material thermodynamics
[1] which results to local hot and cold spots and early battery failures. The past investigations of the heterogeneities in the battery materials rely on the sophisticated synchrotron X-ray methods
[2]–[4], which are not easily accessible. Instead, in this study, we employ Raman spectroscopy on an
in situ benchtop setup which provides several advantages while investigating the true electrochemical kinetics in graphite electrodes, such as: (i) the setup houses practical porous NMC532 cathodes under realistic electrochemical surroundings; (ii) the high resolution of 1-2 µm provides sufficient information on the larger cathode particles; (iii) the method can analyze hundreds of particles, statistically representing the entire composite electrode; and (iv) the spectra can be quantified to reveal the actual reaction area, thus revealing high-fidelity real-time electrochemical performance. We use this simple but precision method to track and analyze the active reaction regions at the mesoscale in the NMC532 electrode to obtain the true local current density, which was found to be an order of magnitude higher than the globally-averaged current density adopted by the existing studies.
Raman spectroscopy can distinguish the vibrational modes of NMC532 with varying Li ion concentration. Since NMC532 is a solid-solution material, the Raman spectrum at each selected location can be deconvoluted into three distinct phases: empty, completely lithiated and partially lithiated. The partially lithiated regions participate in the surface reaction and can be identified to reveal the active reaction area. Thus, we quantified the active area fraction under slow galvanostatic conditions and generated a map of active regions with varying global Li ion fractions. Our results show that only 18% – 50% of the total particle area is active at any instant leading to higher local current densities than the globally-averaged electrode responses. The methods developed in this study are critical to estimate the kinetic parameters using true electrochemical responses and further aid in the design of the particulate porous electrodes.
References
[1] S. Agrawal, P. Bai, Cell Reports Phys. Sci. 2022, 3, 1.
[2] S.J. Harris, A. Timmons, D.R. Baker, C. Monroe, Chem. Phys. Lett. 2010, 485, 265.
[3] C. Tian, Y. Xu, D. Nordlund, F. Lin, J. Liu, Z. Sun, Y. Liu, M. Doeff, Joule 2018, 2, 464.
[4] S. Fang, M. Yan, R.J. Hamers, J. Power Sources 2017, 352, 18.
