Layered lithium transition metal oxides, in particular, NMCs (LiNi_xCo_yMn_zO₂) represent a family of prominent lithium ion battery cathode materials with the potential to increase energy densities and lifetime, reduce costs, and improve safety for electric vehicles and grid storage. Our work has focused on various strategies to improve performance and to understand the limitations to these strategies, which include altering compositions, utilizing cation substitutions, and charging to higher than usual potentials in cells. Understanding the effects of these strategies on surface and bulk behavior and correlating structure-performance relationships advance our understanding of NMC materials. This also provides information relevant to the efficacy of various approaches toward ensuring reliable operation of these materials in batteries intended for demanding traction and grid storage applications. In this Account, we start by comparing NMCs to the isostructural LiCoO₂ cathode, which is widely used in consumer batteries. Effects of changing the metal content (Ni, Mn, Co) upon structure and performance of NMCs are briefly discussed. Our early work on the effects of partial substitution of Al, Fe, and Ti for Co on the electrochemical and bulk structural properties is then covered. The original aim of this work was to reduce the Co content (and thus the raw materials cost) and to determine the effect of the substitutions on the electrochemical and bulk structural properties. More recently, we have turned to the application of synchrotron and advanced microscopy techniques to understand both bulk and surface characteristics of the NMCs. Via nanoscale-to-macroscale spectroscopy and atomically resolved imaging techniques, we were able to determine that the surfaces of NMC undergo heterogeneous reconstruction from a layered structure to rock salt under a variety of conditions. Interestingly, formation of rock salt also occurs under abuse conditions. The surface structural and chemical changes affect the charge distribution, the charge compensation mechanisms, and ultimately, the battery performance. Surface reconstruction, cathode/electrolyte interface layer formation, and oxygen loss are intimately related, making it difficult to disentangle the effects of each of these phenomena. They are driven by the different redox activities of Ni and O on the surface and in the bulk; there is a greater tendency for charge compensation to occur on oxygen anions at particle surfaces rather than on Ni, whereas the Ni in the bulk is more redox active than on the surface. Finally, our latest research efforts are directed toward understanding the thermal properties of NMCs, which is highly relevant to their safety in operating cells.

Nanoscale full-field (FF) transmission X-ray microscopy (TXM) and ensemble-averaged soft X-ray absorption spectroscopy (soft XAS) were used to investigate state-of-charge (SOC) heterogeneities in electrochemically charged or discharged and chemically oxidized samples of LiNi0.6Mn0.2Co0.2O2 cathode materials. We observed considerable and similar non-uniformities in terms of Ni oxidation states (and, by proxy, lithium distributions) for all the samples in the bulk. Therefore, the chemically delithiated samples are similar to the electrochemically charged samples in terms of mesoscale charge heterogeneity in large polycrystalline particle ensembles. However, the gradient oxidation states of transition metals on the surface, which is partly responsible for the electrode degradation mechanism known as surface reconstruction, is much less apparent in chemically delithiated samples. Chemical delithiation is extensively used in preparation of charged cathodes free of inactive components for fundamental battery studies. The surface chemistry and bulk lithium distribution of chemically delithiated samples may not accurately represent the electrochemically charged materials in a real battery. In our work, bulk and surface-sensitive characterization techniques were used to investigate the differences between chemically and electrochemically delithiated layered cathode materials. The two types of materials show similar bulk lithium distributions but distinctly different surface chemistries. The present study not only strongly implies that the electrolytic solution is an active participant in the reactions that lead to the surface reconstruction to a rock salt structure observed in layered oxide cathode materials, but also develops a chemical basis for understanding and improving charge homogeneity across the length scale of secondary particles. Both chemical and electrochemical delithiation methods lead to state-of-charge non-uniformity in the bulk of layered transition metal oxide cathode particles. Less surface reconstruction to rock salt is observed for chemically delithiated samples compared with electrochemically delithiated ones. This indicates that electrolytic solution plays a significant role in the surface reconstruction phenomenon.

The cycling performance of nickel-rich lithium nickel cobalt manganese oxide (NMC) electrodes in Li-ion batteries (LIBs) partially depends on the control of the kinetics of degradation processes that result in impedance rise. The impedance contribution from surface film formation at the NMC/electrolyte interfaces is highly dependent on the initial chemical composition and the structure of the NMC surfaces. Through comparison of film quantity and electrochemical performance of composite electrodes made of pristine- and surface treated-NMC materials, we are able to demonstrate that a simple surface treatment suppressed the subsequent film formation and reduced impedance rise of the Li/NMC half-cells during cycling. Detailed modelling of factors affecting cell impedance provide further insights to index individual interphase resistance, highlighting the underlying positive effects of the proposed surface treatment, and demonstrating the importance of homogeneous, electronically conducting matrices throughout the composite electrode.

Understanding how structural and chemical transformations take place in particles under thermal conditions can inform designing thermally robust electrode materials. Such a study necessitates the use of diagnostic techniques that are capable of probing the transformations at multiple length scales and at different states of charge (SOC). In this study, the thermal behavior of LiNi_0.6Mn_0.2Co_0.2O₂ (NMC-622) was examined as a function of SOC, using an array of bulk and surface-sensitive techniques. In general, thermal stability decreases as lithium content is lowered and conversion in the bulk to progressively reduced metal oxides (spinels, rock salt) occurs as the temperature is raised. Hard X-ray absorption spectroscopy (XAS) and X-ray Raman spectroscopy (XRS) experiments, which probe the bulk, reveal that Ni and Co are eventually reduced when partially delithiated samples (regardless of the SOC) are heated, although Mn is not. Surface-sensitive synchrotron techniques, such as soft XAS and transmission X-ray microscopy (TXM), however, reveal that for 50% delithiated samples, apparent oxidation of nickel occurs at particle surfaces under some circumstances. This is partially compensated by reduction of cobalt but may also be a consequence of redistribution of lithium ions upon heating. TXM results indicate the movement of reduced nickel ions into particle interiors or oxidized nickel ions to the surface or both. These experiments illustrate the complexity of the thermal behavior of NMC cathode materials. The study also informs the importance of investigating the surface and bulk difference as a function of SOC when studying the thermal behaviors of battery materials.

Nickel-rich layered materials are emerging as cathodes of choice for next-generation high energy density lithium ion batteries intended for electric vehicles. This is because of their higher practical capacities compared to compositions with lower Ni content, as well as the potential for lower raw materials cost. The higher practical capacity of these materials comes at the expense of shorter cycle life, however, due to undesirable structure and chemical transformations, especially at particle surfaces. To understand these changes more fully, the charge compensation mechanism and bulk and surface structural changes of LiNi0.6Mn0.2Co0.2O2 were probed using synchrotron techniques and electron energy loss spectroscopy in this study. In the bulk, both the crystal and electronic structure changes are reversible upon cycling to high voltages, whereas particle surfaces undergo significant reduction and structural reconstruction. While Ni is the major contributor to charge compensation, Co and O (through transition metal-oxygen hybridization) are also redox active. An important finding from depth-dependent transition metal L-edge and O K-edge X-ray spectroscopy is that oxygen redox activity exhibits depth-dependent characteristics. This likely drives the structural and chemical transformations observed at particle surfaces in Ni-rich materials.

Redox phase transformations are relevant to a number of metrics pertaining to the electrochemical performance of batteries. These phase transformations deviate from and are more complicated than the conventional theory of phase nucleation and propagation, owing to simultaneous changes of cationic and anionic valence states as well as the polycrystalline nature of battery materials. Herein, we propose an integrative approach of mapping valence states and constructing chemical topographies to investigate the redox phase transformation in polycrystalline layered oxide cathode materials under thermal abuse conditions. We discover that, in addition to the three-dimensional heterogeneous phase transformation, there is a mesoscale evolution of local valence curvatures in valence state topographies. The relative probability of negative and positive local valence curvatures alternates during the layered-to-spinel/rocksalt phase transformation. The implementation of our method can potentially provide a universal approach to study phase transformation behaviors in battery materials and beyond.

Rechargeable battery technologies have ignited major breakthroughs in contemporary society, including but not limited to revolutions in transportation, electronics, and grid energy storage. The remarkable development of rechargeable batteries is largely attributed to in-depth efforts to improve battery electrode and electrolyte materials. There are, however, still intimidating challenges of lower cost, longer cycle and calendar life, higher energy density, and better safety for large scale energy storage and vehicular applications. Further progress with rechargeable batteries may require new chemistries (lithium ion batteries and beyond) and better understanding of materials electrochemistry in the various battery technologies. In the past decade, advancement of battery materials has been complemented by new analytical techniques that are capable of probing battery chemistries at various length and time scales. Synchrotron X-ray techniques stand out as one of the most effective methods that allow for nearly nondestructive probing of materials characteristics such as electronic and geometric structures with various depth sensitivities through spectroscopy, scattering, and imaging capabilities. This article begins with the discussion of various rechargeable batteries and associated important scientific questions in the field, followed by a review of synchrotron X-ray based analytical tools (scattering, spectroscopy, and imaging) and their successful applications (ex situ, in situ, and in operando) in gaining fundamental insights into these scientific questions. Furthermore, electron microscopy and spectroscopy complement the detection length scales of synchrotron X-ray tools and are also discussed toward the end. We highlight the importance of studying battery materials by combining analytical techniques with complementary length sensitivities, such as the combination of X-ray absorption spectroscopy and electron spectroscopy with spatial resolution, because a sole technique may lead to biased and inaccurate conclusions. We then discuss the current progress of experimental design for synchrotron experiments and methods to mitigate beam effects. Finally, a perspective is provided to elaborate how synchrotron techniques can impact the development of next-generation battery chemistries.

The first-ever demonstration of stabilized Si/lithium-manganese-rich full cells, capable of retaining >90% energy over early cycling and >90% capacity over more than 750 cycles at the 1C rate (100% depth-of-discharge), is made through the utilization of a modified ionic-liquid electrolyte capable of forming a favorable cathode-electrolyte interface.

Solar-driven photocatalytic conversion of CO₂ into fuels has attracted a lot of interest; however, developing active catalysts that can selectively convert CO₂ to fuels with desirable reaction products remains a grand challenge. For instance, complete suppression of the competing H₂ evolution during photocatalytic CO₂-to-CO conversion has not been achieved before. We design and synthesize a spongy nickel-organic heterogeneous photocatalyst via a photochemical route. The catalyst has a crystalline network architecture with a high concentration of defects. It is highly active in converting CO₂ to CO, with a production rate of ~1.6 × 10⁴ μmol hour^-1 g^-1. No measurable H₂ is generated during the reaction, leading to nearly 100% selective CO production over H₂ evolution. When the spongy Ni-organic catalyst is enriched with Rh or Ag nanocrystals, the controlled photocatalytic CO₂ reduction reactions generate formic acid and acetic acid. Achieving such a spongy nickel-organic photocatalyst is a critical step toward practical production of high-value multicarbon fuels using solar energy.