High-energy-density materials that undergo conversion and/or alloying reactions hold promise for next-generation lithium (Li) ion batteries. However, these materials experience substantial volume change during electrochemical operation, which causes mechanical fracture of the material and structural disintegration of the electrode, leading to capacity loss. In this work, we use x-ray tomography during battery operation to visualize and quantify the origins and evolution of electrochemical and mechanical degradation. Tomography provides the time-resolved, three-dimensional chemical composition and morphology within individual particles and throughout the electrode. In the model material tin(II) oxide, we witness distributions in onset and rate of core-shell lithiation, crack initiation and growth along preexisting defects, and irreversible distortion of the electrode, highlighting tomography as a tool to guide the development of durable materials and strain-tolerant electrodes.

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We report the use of synchrotron radiation X-ray tomographic microscopy (SRXTM) to obtain statistically signifi cant volume ( ∼ 700 × 700 × 70 μ m 3 ) 3D reconstructions of porous electrode microstructures of transition metal oxide based electrodes. [ 1 ] We implement a segmentation algorithm that allows identifi cation of individual particles and validate it by showing that the calculated particle size distribution (PSD) is in agreement with experimentally determined PSD obtained with laser diffraction. We study the microstructure of LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC)-based cathodes, prepared with varying weight percent of carbon black and binder (2-5 wt%) and different compressions (0-2000 bar), and their electrochemical performance. Tomographic data (raw and processed with particles identifi ed and labeled) and the corresponding electrochemical data for 16 different cathodes is provided open source. [ 2 ] The microstructure datasets can be used to study electrode properties like porosity, tortuosity, electrode anisotropy, and homogeneity, or as realistic geometries for three dimensional (3D) electrochemical simulations. The electrochemical data is intended to aid in the verifi cation of simulation models. The large number of studied particles (approx. 7000-19000 per electrode) allows us to investigate spatially resolved PSD and shows that the vicinity of electrode boundaries is populated by smaller particles than the bulk electrode. In addition to insight into electrode morphology, we demonstrate that the technique is capable of resolving features on the sub-particle level such as particle fracture, which is observed here under high compression conditions. It is becoming increasingly clear that the development of next generation, higher performance lithium ion batteries (LIB) will require a concerted effort between experimentalists and simulation experts. In addition to the development of predictive tools for the selection of active materials, realization of LIBs with high C-rate capabilities and energy density will require the development of roadmaps for achieving favorable porous electrode microstructures. [ 3 , 4 ] However, due to the lack of publically available microstructural data on porous electrodes to date, there exists a disconnect between the experimental and simulation communities. The simulation community, despite significant advances in computation over the past decades, must rely on simplifi ed pictures of porous electrode microstructure or computer generated microstructures that bear an as-of-yet unquantifi ed relationship to real battery microstructures.While one-dimensional (1D) simulations are appealing for their simplicity and computational effi ciency, there are significant limitations. Newman-type models rely on the representation of the electrode's complexity by effective medium approximations. [ 5 ] For systems featuring a broad PSD, as often found in real LIB electrodes, the validity of the Bruggeman relation, which is widely used to estimate the electrode's tortuosity from porosity, ha...

The ability to engineer electrode microstructures to increase power and energy densities is critical to the development of high-energy density lithium-ion batteries. Because high tortuosities in porous electrodes are linked to lower delivered energy and power densities, in this paper, we experimentally and computationally study tortuosity and consider possible approaches to decrease it. We investigate the effect of electrode processing on the tortuosity of in-house fabricated porous electrodes, using three-dimensionally reconstructed microstructures obtained by synchrotron x-ray tomography. Computer-generated electrodes are used to understand the experimental findings and assess the impact of particle size distribution and particle packing on tortuosity and reactive area density. We highlight the limitations and tradeoffs of reducing tortuosity and develop a practical set of guidelines for active material manufacture and electrode preparation.

Tortuosity values of porous battery electrodes determined using electrochemical impedance spectroscopy in symmetric cells with a non-intercalating electrolyte are typically higher than those values based on numerical analysis of 3D tomographic reconstructions. The electrochemical approach assumes that the electronic resistance in the porous coating is negligible and that the tortuosity of the porous electrode can be calculated from the ionic resistance determined by fitting a transmission line equivalent circuit model to the experimental data. In this work, we validate the assumptions behind the electrochemical approach. First, we experimentally and theoretically investigate the influence of the electronic resistance of the porous electrode on the extracted ionic resistances using a general transmission line model, and provide a convenient method to determine whether the electronic resistance is sufficiently low for the model to be correctly applied. Second, using a macroscopic setup with known tortuosity, we prove that the ionic resistance quantified by the transmission line model indeed yields the true tortuosity of a porous medium. Based on our findings, we analyze the tortuosities of porous electrodes using both X-ray tomography and electrochemical impedance spectroscopy on electrodes from the same coating and conclude that the distribution of the polymeric binder phase, which is not imaged in most tomographic experiments, is a key reason for the underestimated tortuosity values calculated from 3D reconstructions of electrode microstructures. In commercially relevant lithium ion battery cells operating at high currents or low temperatures and/or cells with thick and low porosity electrodes (i.e., electrodes with high areal capacity and high volumetric energy density), the ionic transport in the electrolyte throughout the thickness of the porous electrode becomes limiting, leading to the buildup of excessive electrolyte concentration gradients across the thickness of the electrode. Concentration gradients not only lead to increased overpotentials and thus lower accessible capacities, but also play an important role in battery aging caused by lithium plating reactions at the graphite anode/separator interface.1 Along with the intrinsic transport parameters of the liquid electrolyte, the morphological properties of a porous electrode, quantified by the parameters porosity and tortuosity, are key to understanding the buildup of concentration gradients across the electrode thickness and the resulting performance limitations of porous electrodes.In the battery community, there are currently two commonly used approaches to obtain values for the tortuosity of porous electrodes; however, they yield different results. One is based on numerical diffusion simulations on 3D reconstructions of the electrode obtained using X-ray (XTM) or focused ion beam scanning electron microscopy (FIB SEM) tomography.2,3 The other approach is based on electrochemical impedance spectroscopy (EIS) measurements of the electrodes in a symme...