![]() The understanding of chemomechanical interplay has remained at the descriptive level, thus, the quantification or model to fingerprint these processes is highly desired. The mechanical stress, caused by anisotropic structural, chemical and state of charge (SOC) heterogeneities, is released through crack formation, undermining the continuous diffusion pathways of electrons and ions and creating fresh surfaces for electrode–electrolyte side reactions. The development of these morphological defects entails the multiscale chemo-mechanical coupling associated with internal mechanical stress. Morphological defects contribute to chronic and acute failures of batteries. Finally, our latest research efforts are directed toward understanding the thermal properties of NMCs, which is highly relevant to their safety in operating cells. 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. 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. The surface structural and chemical changes affect the charge distribution, the charge compensation mechanisms, and ultimately, the battery performance. Interestingly, formation of rock salt also occurs under abuse conditions. 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. More recently, we have turned to the application of synchrotron and advanced microscopy techniques to understand both bulk and surface characteristics of the NMCs. 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. 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. Effects of changing the metal content (Ni, Mn, Co) upon structure and performance of NMCs are briefly discussed. This also provides information relevant to more » 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 LiCoO 2 cathode, which is widely used in consumer batteries. Understanding the effects of these strategies on surface and bulk behavior and correlating structure-performance relationships advance our understanding of NMC materials. 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. Layered lithium transition metal oxides, in particular, NMCs (LiNi xCo yMn zO 2) 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. Engineering Ni/Mn/Co distribution in NMC particles may provide a path toward controlling the charge distribution and thus chemomechanical properties of polycrystalline battery = , The resulting material delivers excellent reversible capacity, rate capability, and cycle life at high operating voltages. The local Mn and Ni concentrations in individual NMC particles are positively and negatively correlated with the electrochemically induced Ni oxidation, respectively, whereas the Co concentration does not impose a clear effect on the Ni oxidation. These NMC particles exhibit a broad, continuous distribution of local Ni/Mn/Co stoichiometry, which does not compromise the global layeredness. In this work, we develop heterogeneous compositional distributions in polycrystalline LiNi 1–x–yMn xCo yO 2 (NMC) particles to investigate the interplay between local stoichiometry and charge distribution. However, little is known about how the local TM stoichiometry influences the charging behavior of battery particles thus impacting battery performance. The isostructural nature of Li-layered cathodes allows for accommodating multiple transition metals (TMs).
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