Operando Electrochemical Fluorescent Microscopy to Predict Li-Ion Battery Performance

A real-time, high-throughput visualization technique for heterogeneity in lithium-ion battery (LIB) electrodes is necessary to understand battery performance on a mechanistic level. Our group, along with others, has seen that battery performance is governed by short range (<20 um) electron transfer between the active material and carbon binder domain (CBD) in electrodes 1–3. Available imaging techniques for visualizing the electronic connectivity of LIB electrodes are limited to nanoscale, single particle resolutions using sophisticated synchrotron X-ray or electron microscopy 4–6. Optical microscopy offers the correct spatial resolution for visualizing the submicron connections of active particles to CBD, but is limited by the colorimetry of electrode materials- with graphite being the exception due to its visible color change during lithiation 7. There is a rapidly growing body of research interested in studying the ionic diffusion pathways of Li-ion using operando optical microscopy, however none currently exist for studying the submicron electronic connectivity using an operando approach- especially under a industrially applicable lens 8. To the best of our knowledge, this study presents the first spatially- and time- resolved technique for visualizing the electronic connectivity of commercial LIB electrodes using operando electrochemical fluorescent microscopy (EFM). This technique relies on the principle of electrofluorochromism, which we use to our advantage for a simple electrochemical system involving heterogeneous electron transfer. This allows us to use fluorescence as a real-time tracker for electronic heterogeneity, where electronic ‘dead-zones’ present as non-fluorescent regions in 2D images. Using this technique, we visualize commonly used commercial LIB electrodes (carbon content 1-4%, regimented processing), including NMC (LiNi x Mn y Co 1-x-y O 2 ), LFP (LiFePO 4 ), and LCO (LiCoO 2 ) against formulaically similar in-house made LIB electrodes (< 3.5% carbon content) as a proof of concept. We first validate the efficacy of this technique by testing commercial NMC, with a flaked off piece of active material, to show that areas of electronic discontinuities can in fact be represented by non-fluorescence ( Fig.1. ) We also find that when compared to commercial electrodes, those which are fabricated in house reveal isolated active particles, agglomerated carbon islands, and dead zones, undiscernible by brightfield imaging ( Fig. 2. ) Global feature extraction is performed on post-processed images to quantify electrode topology and evaluate electronic connectivity of active particles of battery electrodes, as well as heterogeneous mapping to determine the heterogeneity index of electrodes imaged. Testing our technique on low- and high- performing electrodes from an established library of coin-cell data allows for a straightforward way of correlating performance metrics to heterogeneity features. This quick (<1hr), reproducible visualization technique is general enough to be used to study the electronic connectivity of emerging new battery electrodes, as well as verify commercially available ones. Using this approach to verify LIB electrodes prior to assembly could save months to years of battery testing by being used as an alternative to lengthy full cell testing. 1 S. L. Morelly, N. J. Alvarez, and M. H. Tang, “Short-range contacts govern the performance of industry-relevant battery cathodes,” Journal of Power Sources , vol. 387, pp. 49–56, May 2018, doi: 10.1016/j.jpowsour.2018.03.039. 2 R. M. Saraka, S. L. Morelly, M. H. Tang, and N. J. Alvarez, “Correlating Processing Conditions to Short- and Long-Range Order in Coating and Drying Lithium-Ion Batteries,” ACS Appl. Energy Mater. , vol. 3, no. 12, pp. 11681–11689, Dec. 2020, doi: 10.1021/acsaem.0c01305. 3 J. Entwistle, R. Ge, K. Pardikar, R. Smith, and D. Cumming, “Carbon binder domain networks and electrical conductivity in lithium-ion battery electrodes: A critical review,” Renewable and Sustainable Energy Reviews , vol. 166, p. 112624, Sep. 2022, doi: 10.1016/j.rser.2022.112624. 4 S. Li et al. , “Mutual modulation between surface chemistry and bulk microstructure within secondary particles of nickel-rich layered oxides,” Nat Commun , vol. 11, no. 1, Art. no. 1, Sep. 2020, doi: 10.1038/s41467-020-18278-y. 5 P.-C. Tsai et al. , “Single-particle measurements of electrochemical kinetics in NMC and NCA cathodes for Li-ion batteries,” Energy Environ. Sci. , vol. 11, no. 4, pp. 860–871, Apr. 2018, doi: 10.1039/C8EE00001H. 6 A. Singer et al. , “Nucleation of dislocations and their dynamics in layered oxide cathode materials during battery charging,” Nat Energy , vol. 3, no. 8, Art. no. 8, Aug. 2018, doi: 10.1038/s41560-018-0184-2. 7 Y. Yamagishi, H. Morita, Y. Nomura, and E. Igaki, “Visualizing Lithiation of Graphite Composite Anodes in All-Solid-State Batteries Using Operando Time-of-Flight Secondary Ion Mass Spectrometry,” J. Phys. Chem. Lett. , vol. 12, no. 19, pp. 4623–4627, May 2021, doi: 10.1021/acs.jpclett.1c01089. 8 A. J. Merryweather, C. Schnedermann, Q. Jacquet, C. P. Grey, and A. Rao, “Operando optical tracking of single-particle ion dynamics in batteries,” Nature , vol. 594, no. 7864, Art. no. 7864, Jun. 2021, doi: 10.1038/s41586-021-03584-2. Figure 1