(680f) 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) [1], [2], [3], [4]. Available imaging techniques for visualizing LIB electrodes are limited to nanoscale, single particle resolutions using sophisticated synchrotron X-ray or electron microscopy [5], [6], [7], [8], [9]. Optical microscopy offers the correct spatial resolution for visualizing the submicron connection of active particles to CBD, but is limited by colorimetry of electrode materials- with graphite being the exception due to its visible color change during lithiation [10], [11]. Tremendous progress has been made to visualize changes in phase separation, particle dynamics, and intercalation behavior within battery materials using advanced operando spectroscopic or combinatorial microscopic techniques, such as operando optical microscopy and modeling [10] ,scattering[12], or reflectance microscopy [13]. However, due to the colorimetric constraints of battery electrodes, these microscopy approaches risk skewed results due to optical artefacts and difficulty differentiating objects beyond intensity of signal- especially in multi-composite systems such as the battery electrode [9]. Beyond this, these measurements are limited to extremely specialized synchrotron facilities which are not available to most researchers. The urgency of advancing and discovering new battery electrode materials requires a high throughput technique not limited by the sensitivity, speed, or precision localization of its equipment [9]. 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 our technique, we visualize commonly used industry grade LIB electrodes (carbon content 1-4%, regimented processing), including NMC (LiNi₅Mn₃Co₂O₂), LFP (LiFePO₄), and LCO (LiCoO₂), against formulaically similar in-house made LIB electrodes (< 3.5% carbon content) as a proof of concept. To test method efficacy, a manual defect is created on commercial NMC to demonstrate that areas of electronic discontinuities can in fact be represented by non-fluorescence (Fig.1.) We demonstrate that compared to commercial grade, poorly processed electrodes, those which are fabricated in house, reveal isolated active particles, agglomerated carbon islands, and dead zones, undiscernible by brightfield imaging (Fig. 2.)

We hypothesize that electrode heterogeneity, as measured by fluorescent signal, is correlated to electrode performance. We have a library of high- and low-performing electrodes, as measured by coin-cell testing [1], [2], for which complete imaging is performed using EFM. Using a physics-based approach, we identify dark spots of low electronic accessibility using a priori knowledge of active particle properties such as size and circularity. A simple image segmentation approach is applied to processed binarized images through connected component analysis, informed by particle properties. We correlate the average number of inactive particles imaged within each electrode type to discharge capacity at 1C. The regression coefficient for detected inactive particles was -0.1437 (95% CI: -0.022 to 0.336, p = 0.0487) (Fig. 3.) A p-value of 0.0487 indicates that the relationship between disconnected particles detected and discharge capacity is statistically significant at the 0.05 level (two-tailed test). This association demonstrates the method’s efficacy and future potential as a predictive screening tool for battery electrodes. In summary, this quick (<20 min), 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.

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