Operando Quantification of the Mesoscale Spatiotemporal Heterogeneities in Particulate Porous Electrode
Authors: Agrawal Shubham , Bai Peng 
Published: July 2021

Lithium-ion batteries consisting of the particulate porous electrodes, continue to be dominant energy storage devices. While the highly porous structures were introduced in the battery materials to maximize the interfacial area for better overall performance of the system, the appearance of spatiotemporal heterogeneities arising from the material thermodynamics leads to the localization of charge-transfer processes onto a limited portion of the available interfaces. The past investigations of the heterogeneities in the battery materials rely on the synchrotron X-ray[1]–[4], which is a specialized technique and not easily accessible. Instead, in this study, we have fabricated an operando benchtop setup under optical microscope which provides several advantages while investigating the true electrochemical kinetics in graphite electrodes, such as: (i) the setup houses practical porous graphite electrodes under realistic electrochemical surroundings; (ii) the visible light from the optical microscope does not cause any damage to the material; (iii) the method can analyze hundreds of particles, statistically representing the entire composite electrode; and (iv) the setup can capture snapshots every two seconds, thus revealing high-fidelity real-time electrochemical performance. We use this simple but precision method to track and analyze the operando (i.e., local and working) interfaces at the mesoscale in the graphite electrode to obtain the true local current density, which was found to be two orders of magnitude higher than the globally-averaged current density adopted by the existing studies.
With our new kinetic theory for the dynamics of the phase evolution, we were able to achieve self-consistent precision understandings of the true local electrochemical kinetics, which have long been inferred only from globally-averaged electrode responses. The kinetic parameters, especially the diffusion coefficient, obtained from analyzing the operando interfacial current density are four orders of magnitude higher than the existing reports, but are much closer to and consistent with the ab initio predictions[5]. Our results therefore resolve the long-standing discrepancy between diffusion coefficients obtained from the traditional experiments and the first principle calculations. Contrary to prevailing beliefs, the electrochemical dynamics is not controlled by the solid-state diffusion process once the spatiotemporal reaction heterogeneities emerge. Traditional electroanalytical models based on planar electrodes that assume diffusion-limited process need to be modified to account for other rate-governing processes. Here, we advocate more careful examination of the phase transformation behaviors at both the micro and meso scales, for which the methods developed here are critical.

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Figure 1