Operando Electrochemical Kinetics in Particulate Porous Electrodes by Quantifying the Mesoscale Spatiotemporal Heterogeneities

Abstract

heterogeneities in individual particles detected by synchrotron X-ray provide deep insights on the possible degradation mechanisms, the evolutions of the heterogeneities among hundreds of particles sitting in realistic surroundings are critical for the understanding of the true local electrochemical kinetics that dictate the real-time performance. Recent breakthroughs in a few synchrotron facilities have enabled in situ imaging of large number of particles in realistic battery electrodes. [18,19,24-27] However, achieving very high spatial and temporal resolutions at the same time is still very challenging. Yet more accessible testing platforms that can enable the economical and systematic verifications of new mathematical models to achieve the comprehensive understanding of dynamic heterogeneities in relevant electrochemical systems are critically needed. The immediate consequence of the spatiotemporal heterogeneities is that the actual reacting interfacial area at any instant, i.e., area of the operando (local and working) electrochemical interface, is only a small portion of the total available interfacial area usually obtained from the Brunauer-Emmett-Teller (BET) method. Given that existing electroanalytical techniques [28,29] rely on the square-law scaling D Li ∼ [I(t)/S] 2 to extract the diffusion coefficient D Li from the total current I(t) and the assumed constant total interfacial area S, the electrochemical kinetics in systems with strong heterogeneities may have been misinterpreted due to the smaller operando interfacial area. As one of the most widely used electrodes for both the nonaqueous [15,30] and aqueous [31] batteries, graphite electrodes are known to have strong reaction heterogeneities [13,15] reflected by its particle-by-particle reaction mechanism, [15,22] during the phase transformation between ordered stages [32] upon ion intercalation. Depending on the choices of electrode area, e.g., BET or geometric, the lithium-ion diffusion coefficient in graphite (D Li) extracted by the classic electroanalytical methods varies by about 8 orders of magnitude in the literature. [33-41] Still, D Li obtained for SOC ranges with phase transformation were always about 2 orders of magnitude lower than the average. [34,40] The discrepancy has long been doubted as the inaccuracy of the interfacial area, [42] but conclusive evidence is still missing. Similar orders-of-magnitude discrepancies also exist in other porous electrodes composed of phase-transforming [29] or solidsolution particles, [43-45] missing satisfactory explanations. The Electrochemical energy systems rely on particulate porous electrodes to store or convert energies. While the three-dimensional (3D) porous structures are introduced to maximize the interfacial area for better overall performance of the system, spatiotemporal heterogeneities arising from materials thermodynamics are localizing the charge transfer processes onto a limited portion of the available interfaces. Here, a simple but precise method is demonstrated to directly track an...

Asynchronous-to-Synchronous Transition of Li Reactions in Solid-Solution Cathodes

“…The easy access, low cost, and nondestructiveness of optical microscopy make it a suitable technique for conducting exhaustive and exploratory studies in Li-ion batteries and has been gaining popularity in recent years. − In this work, the change in the reflected light intensity from an NMC cathode is tracked continuously over 3 days and used to infer the local composition change during slow charging over cycles, starting with the formation cycle. The dependence of the reflected light intensity on the Li content has been previously verified in a variety of active materials, including graphite, , silicon, , and LiCoO 2 . Our operando experiments reveal that NMC particles react asynchronously during the first charge of the formation cycle but transition toward uniform (synchronous) reactions in subsequent (dis)­charge sequences.…”

“…13−15 In this work, the change in the reflected light intensity from an NMC cathode is tracked continuously over 3 days and used to infer the local composition change during slow charging over cycles, starting with the formation cycle. The dependence of the reflected light intensity on the Li content has been previously verified in a variety of active materials, including graphite, 16,17 silicon, 8,18 and LiCoO 2 . 19 Our operando experiments reveal that NMC particles react asynchronously during the first The numerical simulations show that solid-state diffusion alone cannot explain these observations and the phenomenon requires a spatially varying electric potential across the electrode surface.…”

Understanding the Phase Equilibrium and Kinetics of Electrochemically Driven Phase Transition in CoOxHy during Electrocatalytic Reactions

“…These fraction-time relaxation curves shown in Figure d track the phase transition process and therefore can represent the phase transition kinetics. We use the Kolmogorov–Johnson–Mehl–Avrami (KJMA) model to model these relaxation curves, which takes the form below − …”

Visualization of Electrochemically Accessible Sites in Flow-through Mode for Maximizing Available Active Area toward Superior Electrocatalytic Ammonia Oxidation

“…These electrodes are typically assessed using macroscopic properties that aggregate or average the behavior of a large number of microsurfaces located in different local microenvironments . Unfortunately, the complex pore structure of porous electrodes can significantly exacerbate the variability of these local microenvironments compared to planar electrodes, which further localizes the charge transfer process onto the electrochemically accessible portion of the overall interface . It is essential to understand the spatial distribution of electrochemically accessible surface areas, which helps to guide the design and operation of the FTEs.…”

“…17 Unfortunately, the complex pore structure of porous electrodes can significantly exacerbate the variability of these local microenvironments compared to planar electrodes, which further localizes the charge transfer process onto the electrochemically accessible portion of the overall interface. 18 It is essential to understand the spatial distribution of electrochemically accessible surface areas, which helps to guide the design and operation of the FTEs. But it is also challenging for the lack of easily accessible testing platforms to simultaneously achieve very high spatial and temporal resolutions of observations.…”