Insights on the synthesis, properties, and performance of high-nickel layered oxide cathodes



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Lithium-ion batteries have had a profound impact on the modern world; the explosion of portable electronics shaped the ways we communicate and interface within a global society. With heavy commercial investment and governmental policies directed towards electric vehicle (EV) adoption, lithium-ion batteries (LIBs) are poised to revolutionize our transport infrastructure as well. Widespread vehicle electrification necessitates higher-energy-density, longer-cycle-life LIBs, and of the available paths forward, advancements in cathode material are the most attainable in the near future. Among the established cathode materials, high-nickel layered oxides, LiNiₓM₁₋ₓO₂ (M = Mn, Al, Co, etc.), with greater than 90% Ni-content, are highly regarded for their unmatched energy density. However, high-Ni cathodes possess severe reactivity and stability concerns, casting doubt on their practical viability. This dissertation aims to address these challenges by leveraging fundamental understanding of the layered oxide synthesis to facilitate the rational design of stabilized high-nickel oxide cathodes with high energy density and cycle stability. First, the influence of elevated oxygen pressure during the synthesis of LiNiO₂ (LNO) is investigated. Increasing O₂ pressure increases the oxygen chemical potential, reducing the formation of oxygen vacancies within the layered oxide lattice. Consequently, the LNO structure possesses lower cation disordering and achieves a near three-fold improvement in cycle life, demonstrating the readily achievable benefits of systematic synthesis optimization. Second, a molten salt synthesis is developed to produce single-crystalline high-Ni cathodes. A combination of oxidizing additives, facile lithiation sources, and evaporatively stable molten salts enables the production of single-crystalline LiNiO₂ with distinct primary morphology and excellent phase quality. The molten salt synthesis provides a new avenue for tailoring the properties and morphology of high-nickel layered oxides while avoiding deleterious increases to synthesis temperature. Third, a robust evaluation of the effect of single-crystalline morphology on the electrochemical performance of LiNiO₂ is performed. It is revealed that the main barrier limiting single-crystal performance is poor diffusion kinetics. However, the elimination of intergranular boundaries elicits surprising phase transition behavior; despite sluggish diffusion, nucleation-controlled transformations are maintained to much higher current draws due to the larger contiguous grain size. Finally, the directed design of a Te doped single-crystal cathode is performed. Incorporation of Te alleviates some of the kinetic hindrance observed in single-crystal LNO by substantially increasing ion diffusivity. By further addressing surface reactivity using a stabilized electrolyte, a high-capacity single crystal cathode is obtained with high cycling stability and improved rate capability.


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