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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.

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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.

Architecting grain crystallographic orientation can modulate charge distribution and chemomechanical properties for enhancing the performance of polycrystalline battery materials. However, probing the interplay between charge distribution, grain crystallographic orientation, and performance remains a daunting challenge. Herein, we elucidate the spatially resolved charge distribution in lithium layered oxides with different grain crystallographic arrangements and establish a model to quantify their charge distributions. While the holistic "surface-to-bulk" charge distribution prevails in polycrystalline particles, the crystallographic orientation-guided redox reaction governs the charge distribution in the local charged nanodomains. Compared to the randomly oriented grains, the radially aligned grains exhibit a lower cell polarization and higher capacity retention upon battery cycling. The radially aligned grains create less tortuous lithium ion pathways, thus improving the charge homogeneity as statistically quantified from over 20 million nanodomains in polycrystalline particles. This study provides an improved understanding of the charge distribution and chemomechanical properties of polycrystalline battery materials.