Delineating the interfacial interactions and gas evolution in low-cobalt, high-energy density lithium-based batteries

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Sim, Richard, Ph. D.

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Layered oxides (LiNi₁₋ [subscript x] M [subscript x] O₂; M = Co, Mn, Al) are promising cathode materials that can achieve key performance metrics with their high energy densities and rate performance. However, these materials suffer from a series of surface and bulk degradations during charge-discharge that may compromise the performance and safety of the battery. This dissertation focuses on elucidating the surface and bulk degradation mechanisms that result from electrode-electrolyte incompatibility for high-energy layered-oxide cathodes and lithium-metal anodes. Extensive discussions are provided on the set-up of specialized instrumentation for advanced gas detection and application of advanced characterization tools to elucidate the surface interfacial reactions at the electrode-electrolyte interface for the development of sustainable, safe, and long-lasting lithium-based batteries. An evaluation of the influence of calendering on cobalt-free LiNi₀.₉Mn₀.₀₅Al₀.₀₅O₂ (NMA-90) cathodes is explored as a facile method to minimize the degree of electrode-level effects on cell performance for future projects. Calendered NMA-90 | graphite full cells exhibit lower cell impedance, enhanced cyclability, and pulse-power performance. Overall, calendering is demonstrated to be a crucial post-processing technique for commercial high-nickel cathodes. Gas evolution from the cathode at high voltages remains a pervasive issue for practical batteries. The details of the design and set-up of an online electrochemical mass spectrometry system (OEMS) that operates well for cells with lean and volatile electrolytes is provided. This system is applied to a broad series of cathode-electrolyte systems and charging conditions to elucidate the factors leading to gas release from the cathode. Important topics to gas evolution from the layered-oxide cathode, including voltage, state of charge, cathode composition, electrolytes, and surface coatings are discussed. A separate pathway to achieving high energy densities with the cathode is to increase the upper cut-off voltage of the battery to 4.6 V from the standard 4.4 V vs Li/Li⁺. A localized saturated electrolyte (LSE) is employed to enable the stable cycling of a cobalt-free, low-nickel LiNi₀.₇Mn₀.₂₅Al₀.₀₅O₂ in lithium-metal half-cells. It is found that the improvements to the cycling performance are due to a beneficial interphase layer, reduced degree of rock-salt phase formation, and reduced gas evolution from the cathode. Transition-metal dissolution from the cathode is exacerbated at high voltages, which will influence solid-electrolyte interphase (SEI) formation at the anode. Secondary-ion mass spectrometry (SIMS) is employed to track the distributions of common products found in the SEI. Spatial correlations between deposited transition-metals, organic/inorganic electrolyte decomposition products, and lithium metal on the lithium-metal anode are established and quantified for a variety of anodes cycled with different cobalt-free cathodes and electrolytes. Critical insights into the effects of spatial heterogeneity and SEI localization on cell cyclability are provided.



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