rechargeable batteries. Efforts have been devoted to studying different battery components (e.g., cathode, anode, electrolyte, and binder), aiming to improve energy and power densities, enhance safety, prolong lifetime, and reduce cost. An important aspect of the battery research is to identify the fading pathways of battery particles and electrodes at multiple length/time scales under practical operating conditions. [1] Redox reactions in batteries commonly involve phase transformation, lattice volume change, stress buildup, grain boundary weakening, and particle fracturing. These processes intertwine at multiple length and time scales, are termed as the chemomechanical interplay, and contribute to the complex fading mechanisms of composite battery electrodes. Mapping the chemomechanical transformation of battery particles and particle ensembles represents a promising methodology to establish the relationship among all these processes. Such a study will potentially provide insights into designing materials and electrodes whereThe multiscale chemomechanical interplay in lithium-ion batteries builds up mechanical stress, provokes morphological breakdown, and leads to state of charge heterogeneity. Quantifying the interplay in complex composite electrodes with multiscale resolution constitutes a frontier challenge in precisely diagnosing the fading mechanism of batteries. In this study, hard X-ray phase contrast tomography, capable of nanoprobing thousands of active particles at once, enables an unprecedented statistical analysis of the chemomechanical transformation of composite electrodes under fast charging conditions. The damage heterogeneity is demonstrated to prevail at all length scales, which stems from the unbalanced electron conduction and ionic diffusion, and collectively leads to the nonuniform utilization of active particles spatially and temporally. This study highlights that the statistical mapping of the chemomechanical transformation offers a diagnostic method for the particles utilization and fading, hence could improve electrode formulation for fast-charging batteries.
Ni–Fe layered double hydroxides (LDHs) are promising for catalyzing the oxygen evolution reaction (OER) in alkaline media. However, the OER mechanism is highly debated, partially because of the lack of an ideal catalyst with 100% exposed active sites for unambiguous characterization. Herein, we develop an alcohol intercalation method to prepare ultrathin Ni–Fe LDH with a 1/3 unit-cell thickness and 100% exposed active sites. The ultrathin LDH catalyst exhibits an intrinsic activity similar to the bulk LDH and allows a direct and reliable characterization of the catalyst without any interference from “bulk” inactive species. Operando synchrotron X-ray analysis indicates that the metallic ions in ultrathin Ni–Fe LDH are fully oxidized into tetravalence states at low applied potentials and that the OER occurs on the tetravalent Ni and Fe ions following a decoupled proton/electron mechanism. Our findings demonstrate that a full oxidization of metal ions is crucial for highly active NiFe LDHs and that it can be accomplished by engineering ultrathin nanostructures.
Transmission x-ray microscopy (TXM), which can provide morphological and chemical structural information inside of battery component materials at tens of nanometer scale, has become a powerful tool in battery research. This article presents a short review of the TXM, including its instrumentation, battery research applications, and the practical sample preparation and data analysis in the TXM applications. A brief discussion on the challenges and opportunities in the TXM applications is presented at the end.
Single-crystal materials have played a unique role in the development of high-performance cathode materials for Li batteries due to their favorable chemomechanical stability. The molten salt synthesis method has become one of the most prominent techniques used to synthesize single-crystal layered and spinel materials. In this work, the molten salt synthesis method is used as a technique to tune both the morphology and Mn3+ content of high-voltage LiNi0.5Mn1.5O4 (LNMO) cathodes. The resulting materials are thoroughly characterized by a suite of analytical techniques, including synchrotron X-ray core-level spectroscopy, which are sensitive to the material properties on multiple length scales. The multidimensional characterization allows us to build a materials library according to the molten salt phase diagram as well as to establish the relationship among synthesis, material properties, and battery performance. The results of this work show that the Mn3+ content is primarily dependent on the synthesis temperature and increases as the temperature is increased. The particle morphology is mostly dependent on the composition of the molten salt flux, which can be tailored to obtain well-defined octahedrons enclosed by (111) facets, plates with predominant (112̅) facets, irregularly shaped particles, or mixtures of these. The electrochemical measurements indicate that the Mn3+ content has a larger contribution to the battery performance of LNMO than do morphological characteristics and that a significant amount of Mn3+ could become detrimental to the battery performance. However, with similar Mn3+ contents, morphology still plays a role in influencing the battery cycle life and rate performance. The insights of molten salt synthesis parameters on the formation of LNMO, with deconvolution of the roles of Mn3+ and morphology, are crucial to continuing studies in the rational design of LNMO cathode materials for high-energy Li batteries.
Spinel LiNi0.5Mn1.5O4 (LNMO) can adopt two crystallographic structures: an ordered P4332 structure and a disordered Fd3̅m structure. The disordered phase is associated with the reduction of a small amount of Mn4+ to Mn3+. LNMO single-crystals likely contain local regions of both ordered and disordered regions, which ensemble-averaged characterizations fail to distinguish. Herein, we employ high-spatial-resolution synchrotron X-ray nanodiffraction techniques to identify lattice distortions and structural defects in LNMO samples with octahedral and plate-like morphologies containing ∼6% and ∼22% of Mn3+, respectively. Differences in properties between the two particles give rise to different distributions of lattice variations, which may indicate differences in phase distributions. Bragg coherent diffraction is also used to observe phase heterogeneities in single grains. Lattice distortions and structural defects could shut down or open up local diffusion pathways for lithium ions, making lithium ion diffusion more complicated and potentially more tortuous than that in a perfect LNMO lattice.
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