Nickel-rich layered materials LiNi 1-x-y Mn x Co y O 2 are promising candidates for high energy density lithium-ion battery cathodes. Unfortunately, they suffer from capacity fading upon cycling, especially with high voltage charging. It is critical to have mechanistic understanding of such fade. Herein, synchrotron-based techniques (including scattering, spectroscopy, and microcopy) and finite element analysis were utilized to understand the LiNi 0.6 Mn 0.2 Co 0.2 O 2 material from structural, chemical, morphological, and mechanical points of view. The lattice structural changes are shown to be relatively reversible during cycling, even when 4.9V charging was applied. However, local disorder and strain were induced by high voltage charging. Nano-resolution 3D transmission X-ray microscopy data analyzed by machine learning methodology reveals that high-voltage charging induced significant oxidation state inhomogeneities in the cycled particles. Regions at the surface have rock-salt type structure with lower oxidation state and build up the impedance while regions with higher oxidization state are scattered in the bulk and are likely deactivated during cycling. In addition, the development of micro-cracks is highly dependent on the pristine state morphology and cycling conditions. Hollow particles seem to be more robust against stress-induced cracks than the solid ones, suggesting that morphology engineering can be effective in mitigating the crack problem in these materials. The engineering support from D. Van Campen, V. Borzenets and D. Day for the TXM experiment at beamline 6-2C of SSRL is gratefully acknowledged. The work done at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under contract DE-SC0012704. This research used beamlines 7-BM and 28-ID-2 of the National Synchrotron Light Source II, a U.S.

Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium batteries. Herein, we report a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge under thermal abuse conditions. We observe that the mechanical breakdown of charged LiNiMnCoO materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles, i.e., intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, finite element modeling confirms our experimental observation that the crack formation is attributable to the formation of oxygen vacancies, oxygen release, and phase transformations. This study is designed to directly observe the chemomechanical behavior of layered oxide cathode materials and provides a chemical basis for strengthening primary and secondary particles by stabilizing the oxygen anions in the lattice.

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Complex chemomechanical interplay exists over a wide range of length scales within the hierarchically structured lithium-ion battery. At the mesoscale, the interdependent structural complexity and chemical heterogeneity collectively govern the local chemistry and, as a result, critically influence the cell level performance. Here we investigate the morphology and state of charge (SOC) inhomogeneity within secondary NCA particles that were cycled in solid polymer batteries. We observe substantial inhomogeneity in the nickel oxidation state (a proxy for SOC) and loss of structural integrity within secondary particles after only 20 cycles due to significant intergranular cracking. The formation of mesoscale cracks causes loss of ionic and electrical contact within cathode particles, triggering increases in local impedance and rearrangement of transport pathways for charge carriers. This can eventually lead to deactivation of sub-particle level domains in solid-state lithium-ion batteries. Our findings highlight the importance of proper mesoscale strain and defect management in polymer lithium-ion batteries.

Functional materials and devices are usually morphologically complex and chemically heterogeneous. Their structures are often designed to be hierarchical because of the desired functionalities, which usually require many different components to work together in a coherent manner. The lithium ion battery, as an energy storage device, is a very typical example of this kind of structure. In a lithium ion battery, the cathode, anode, and separator are soaked in a liquid electrolyte, facilitating the back and forward shuttling of the lithium ions for energy storage and release. The desired performance of a lithium ion battery has many different aspects that need to be engineered and balanced depending on the targeted applications. In most cases, the cathode material has become the limiting factor for further improvements and, thus, has attracted intense attention from the research community. While the improvement in the overall performance of the lithium ion battery is the ultimate goal of the research in this field, understanding the relationship between the microscopic properties and the macroscopic behaviors of the materials/devices can inform the design of better battery chemistries for practical applications. As a result, it is of great fundamental and practical importance to investigate the electrode materials using experimental probes that can provide good chemical sensitivity and sufficient spatial resolution, ideally, under operating conditions. With this motivation, our group has been focusing on the development of the nanoscale full-field X-ray spectro-microscopy, which has now become a well-recognized tool for imaging battery electrode materials at the particle level. With nanoscale spatial resolution, this technique can effectively and efficiently tackle the intrinsically complicated mesoscale chemistry. It allows us to monitor the particles' morphological and chemical evolution upon battery operation, providing valuable insights that can be incorporated into the design of new battery chemistries. In this Account, we review a series of our recent studies of battery electrode materials using nanoscale full-field X-ray spectro-microscopy. The materials that are the subjects of our studies, including layer-structured and spinel-structured oxide cathodes, are technically very important as they not only play an important role in today's devices but also possess promising potential for future developments. We discuss how the subparticle level compositional and state-of-charge heterogeneity can be visualized and linked to the bulk performance through systematic quantification of the imaging data. Subsequently, we highlight recent ex situ and in situ observations of the cathode particles' response to different reaction conditions, including the spontaneously adjusted reaction pathways and the morphological changes for the mechanical strain release. The important role of surface chemistry in the system is also discussed. While the microscopic investigation at the particle level provides useful insights, the degree...

Three-dimensional compositional heterogeneities can yield stable sodium layered oxide cathode materials.

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Low-dimensional polarization-sensitive photodetectors have captured everincreasing attention due to their unique applications in navigation, optical switching, and communication. However, to date, the study of dimensionality-dependent polarization-sensitive photodetectors has not received much attention. Herein, a double-faced polarization-sensitive photodetector based on an individual high-quality Sb 2 Se 3 microbelt is proposed, where both the wide and narrow side facets serve as irradiation targets. Polarization-sensitive measurements reveal the anisotropy in the photocurrents recorded parallel and perpendicular to the long axis of the Sb 2 Se 3 microbelt. Strikingly, the single Sb 2 Se 3 microbelt photodetector reveals the obvious perpendicular reversal with a 90° angle on polarization sensitivity from the wide surface to the narrow surface. With decreasing inclination angle, the anisotropic ratio varies from 1.18 to 0.87 gradually. Interestingly, the photodetector can lose its polarization sensitivity at a particular inclination angle. Angle-resolved polarized Raman spectroscopy confirms the asymmetry and anisotropic character of the Sb 2 Se 3 microbelt. The polarization sensitivity of the Sb 2 Se 3 microbelt photodetectors originates from the optical anisotropy induced by the crystal structure anisotropy, rather than one-dimensional geometry morphological anisotropy. The use of an anisotropic Sb 2 Se 3 microbelt in double-faced polarization-sensitive photodetection may contribute to new functionality in innovative optoelectronic applications.