Loss of oxygen in layered transition-metal oxides is a major reason for the structural degradation and thus the fade of electrochemical performance in the cathodes for Li-ion batteries. Via in situ transmission electron microscopy observations of LiNi 0.80 Co 0.15 Al 0.05 O 2 (NCA), we found that the oxygen loss in the layered cathode is a twostage process with distinct release rates. The initial rapid oxygen loss generates a high concentration of oxygen vacancies, which results in the formation of an amorphized, vacancy-containing rock-salt layer in the surface. In the second stage, the slower rate of oxygen loss allows recrystallization of this defective phase via coalescing of atomic oxygen vacancies, which results in the formation of a cavity-containing surface layer with a crystalline rock-salt structure over the layered phase in the bulk. Comparison of the in situ results with electrochemically cycled NCA cathodes confirms this two-stage process of oxygen loss. These results provide unprecedented microscopic details regarding the structural degradation of layered oxides arising from oxygen loss and have broader implications in manipulating the oxygen activity in the electrode.

Unlocking the full performance capabilities of battery materials will require a thorough understanding of the underlying electrochemical mechanisms at a variety of length scales. A broad arsenal of X-ray microscopy and mapping techniques is now available to probe these processes down to the nanoscale. The tunable nature of X-ray sources allows for the extraction of chemical states through spectromicroscopy. The addition of phase contrast imaging can retrieve the complex-valued refraction of the material, giving an even more nuanced chemical picture. Tomography and coherent Bragg diffraction imaging provide a reconstructed three-dimensional volume of the specimen, as well as internal strain information from the latter. Many recent insights into battery materials have been achieved through the creative use of these, and similar, methods. Experiments performed while the battery is being actively cycled reveal behavior that differs significantly from what is observed at equilibrium and metastable conditions. Planned improvements to X-ray source brightness and coherence will extend these techniques by alleviating the current trade-off in time, chemical, and spatial resolution.

The surface configuration of pristine layered oxide cathode particles for Li-ion batteries significantly affects the electrochemical behavior, which is generally considered to be a thin rock-salt layer in the surface. Unfortunately, aside from its thin nature and spatial location on the surface, the true structural nature of this surface rocksalt layer remains largely unknown, creating the need to understand its configuration and the underlying mechanisms of formation. Using scanning transmission electron microscopy, we have found a correlation between the surface rock-salt formation and the crystal facets on pristine LiNi 0.80 Co 0.15 Al 0.05 O 2 primary particles. It is found that the originally (014̅ ) and ( 003) surfaces of the layered phase result in two kinds of rock-salt reconstructions: the (002) and (111) rock-salt surfaces, respectively. Stepped surface configurations are generated for both reconstructions. The (002) configuration is relatively flat with monatomic steps while the (111) configuration shows significant surface roughening. Both reconstructions reduce the ionic and electronic conductivity of the cathode, leading to a reduced electrochemical performance.

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The synthesis of glycogen in bacteria and starch in plants is allosterically controlled by the production of ADP-glucose by ADP-glucose pyrophosphorylase. Using computational studies, site-directed mutagenesis, and kinetic characterization, we found a critical region for transmitting the allosteric signal in the Escherichia coli ADP-glucose pyrophosphorylase. Molecular dynamics simulations and structural comparisons with other ADP-glucose pyrophosphorylases provided information to hypothesize that a Pro103-Arg115 loop is part of an activation path. It had strongly correlated movements with regions of the enzyme associated with regulation and ATP binding, and a network analysis showed that the optimal network pathways linking ATP and the activator binding Lys39 mainly involved residues of this loop. This hypothesis was biochemically tested by mutagenesis. We found that several alanine mutants of the Pro103-Arg115 loop had altered activation profiles for fructose-1,6-bisphosphate. Mutants P103A, Q106A, R107A, W113A, Y114A, and R115A had the most altered kinetic profiles, primarily characterized by a lack of response to fructose-1,6-bisphosphate. This loop is a distinct insertional element present only in allosterically regulated sugar nucleotide pyrophosphorylases that could have been acquired to build a triggering mechanism to link proto-allosteric and catalytic sites.

Side reactions involving surface reduction play a critical role in the failure of LiNi 0.8 Co 0.15 Al 0.05 O 2 to reach its theoretical capacity as a cathode material for Li-ion batteries. While macroscopic consequences are known, the underlying nanoscopic mechanisms are not fully elucidated. By coupling Xray spectroscopy with several X-ray microscopy modalities, we have spatially resolved the extent of Ni oxidation at several states of charge and uncovered heterogeneity that is hidden when considering ensemble measurements alone. The use of morphologically controlled particles enabled high-resolution imaging of these materials, uncovering gradients of Ni oxidation states within individual primary particles. At high states of charge, these gradients revealed regions of possible oxygen deficiency extending deeper into the particle than previously observed. Surface-sensitive X-ray coupled scanning tunneling microscopy allows oxidation states to be measured at the material's surface, showing predominantly Ni II in the first atomic layer and mixtures of Ni II with Ni III /Ni IV already appearing 1.5 nm into the particle. These results reveal the subtle interplay between irreversible surface transformations and the bulk reactions that ultimately define function, which will refine strategies of surface passivation that are key to overcoming current performance limitations.

LiNi 0.80 Co 0.15 Al 0.05 O 2 , a cathode material of the high energy density layered oxide family, was synthesized at various time and temperature values to form single particles. The result of each synthesis condition was comprehensively characterized, demonstrating differing purity, morphology, and electrochemical properties, expanding the current knowledge of the LiNi 0.80 Co 0.15 Al 0.05 O 2 system. 800 • C was found to be the temperature in which particle growth was reliably observed. Particle size was shown to correlate with annealing time, concurrent to Li/Ni site mixing. Morphology was also dependent on annealing time. Shorter anneals yielded particles of platelet morphology, while longer anneals yielded more three-dimensional structures. Samples that had longer total annealing times resulted in higher specific capacity values.

Redox-driven phase transformations in solids determine the performance of lithium-ion batteries, crucial in the technological transition from fossil fuels. Couplings between chemistry and strain define reversibility and fatigue of an electrode. The accurate definition of all phases in the transformation, their energetics, and nanoscale location within a particle produces fundamental understanding of these couplings needed to design materials with ultimate performance. Here we demonstrate that scanning X-ray diffraction microscopy (SXDM) extends our ability to image battery processes in single particles. In LiFePO crystals equilibrated after delithiation, SXDM revealed the existence of domains of miscibility between LiFePO and LiFePO. These solid solutions are conventionally thought to be metastable, and were previously undetected by spectromicroscopy. The observation provides experimental verification of predictions that the LiFePO-FePO phase diagram can be altered by coherency strain under certain interfacial orientations. It enriches our understanding of the interaction between diffusion, chemistry, and mechanics in solid state transformations.