Date of Graduation

5-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry (PhD)

Degree Level

Graduate

Department

Chemistry & Biochemistry

Advisor/Mentor

Xiao, Jie

Committee Member

Chen, Jingyi

Second Committee Member

Coridan, Robert H.

Third Committee Member

Tian, Z. Ryan

Keywords

Battery Chemistry; Energy Storage Materials; In situ Characterization

Abstract

Since the first generation of lithium-ion batteries featured lithium cobalt oxide cathode and carbon anode commercialized in the 1990s, the high-capacity materials with lower cost are in demand to further increase the battery energy density. Lithium metal and silicon anode are promising high-capacity anode materials to achieve next-generation lithium batteries. However, both the materials actively react in electrolytes and suffer from dramatic volume change. Therefore, a reliable passivation layer at the electrolyte/electrode interphase (i.e., solid electrolyte interphase, or “SEI”) is required to support the long-term cycling of both materials. Cetrimonium hydro fluoride (CTAHF2) has been proposed and synthesized as an electrolyte additive, which has the unique advantages of increasing the electrolyte wettability and introducing more LiF content in the electrode surface layer. By incorporating 4 M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane (DME) electrolyte, the cycling life has been increased for both lithium metal and silicon anode. To understand the origination and evolution of SEI layers in energy storage systems, an integrated microscopic study has been applied to explore the interfacial reactions initiated on the surfaces of different electrode materials. Specifically, atomic force microscopy (AFM), combined with the scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS), is employed to probe the properties of SEI layers formed on different electrode surfaces. A custom-designed electrochemical cell has been proposed to allow the monitoring of SEI layers by using in situ AFM in a “living” cell. Layered LiNi0.8Mn0.1Co0.1O2 (NMC811) is one of the most promising cathode materials for modern lithium-ion batteries with respect to its high reversible capacity. Whereas the redox reactions of Ni2+/Ni3+ and Ni3+/Ni4+ contribute the majority of reversible capacity, the highly reactive Ni-rich surface also encourages the growth of surface impurity species, which causes the irreversible capacity loss and degradation of cycle life. In this work, the residual lithium compounds induced cell failure based on NMC811 was investigated. An acid-base titration method is employed to quantify the carbonate species generated during ambient storage. Finally, a feasible coating method with ethylene carbonate as the coating material has been proposed and helps to maintain the chemical and structural stability of the materials during the ambient environment storage. In comparison to the non-treated extended air-storage samples, the coating treated samples effectively alleviate the initial capacity loss and cycle life degradation. The surface chemical and structural changes and their relevance to electrochemical performances are further discussed in this work.

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