Synthesis and application of a highly fluorinated ether molecule as an electrolyte co-solvent for enhanced performance rechargeable lithium batteries

Wang, Ruo (2024). Synthesis and application of a highly fluorinated ether molecule as an electrolyte co-solvent for enhanced performance rechargeable lithium batteries. University of Birmingham. Ph.D.

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Abstract

The demand for high-performance energy storage systems has driven significant research efforts in the field of secondary lithium batteries. As one of the key components of these batteries, electrolytes play a crucial role in determining their electrochemical performance and stability. In this thesis, a novel electrolyte has been explored to address the challenges and limitations associated with conventional electrolyte systems, which are described in Chapter 1. These challenges and limitations include compatibility issues with high-voltage cathodes, lithium metal anode, and graphite anode, as well as the electrolyte's high-voltage tolerance and stability under high temperature. These issues are critical factors that need to be addressed in battery technology development, as they are essential for improving battery performance and safety. Given the critical role of the solid electrolyte interphase (SEI) and cathode electrolyte interface (CEI) on electrode surfaces in influencing battery cycling, this thesis endeavours to develop and design an electrolyte that fosters the creation of highly stable SEI and CEI layers. The primary goal is to render this electrolyte compatible with high-energy-density, high-voltage lithium metal batteries initially, and subsequently expand its applicability to a broader range of electrode materials and operating temperatures.
The methods and approaches utilized in the thesis are presented in Chapter 2.
Localized high-concentration electrolytes (LHCE) exhibit excellent compatibility with both the lithium metal anode and the high-voltage cathodes. In Chapter 3, the development of LHCE is delved into, utilizing the meticulously designed and synthesized molecule, TTME (1,1,1-trifluoro-2-[(2,2,2-trifluoroethoxy) methoxy] ethane). The symmetrical structure of TTME reduces the overall molecule's polarity, thereby endowing it with diluent-like properties akin to traditional localized high-concentration electrolytes. Additionally, the competitive interaction between the strong electronegativity of oxygen and the -CF3 group allows TTME to partially participate in the solvation structure of Li+ ions. The unique solvation structure of TTME facilitated the coordination of Li+ ions, providing insights into electrolyte design.
In Chapter 4, the performance of TTME-d electrolyte, consisting of 1.4 M LiFSI and DME-TTME (1:4 by volume of 1,2-Dimethoxyethane and TTME) was investigated and found to have excellent electrochemical performance. Lithium metal cells made using this electrolyte demonstrated excellent stability, forming a double-layer SEI structure on the lithium metal surface, which can inhibit dendrite growth and ensure good cycling performance of lithium metal batteries. Additionally, this electrolyte can also generate a more stable CEI film on the cathode side, which helps prevent cathode pulverization and inhibits the dissolution of transition metal ions, thereby extending the cycling capacity.
Chapter 5 explored the co-intercalation of DME with graphite in Li||graphite cells using the TTME-d electrolyte, leading to stabilized performance and capacity retention. The fluorinated ether electrolyte system exhibited superior stability compared to the conventional carbonate electrolyte, showing promise for high-voltage applications such as electric vehicles. Large-capacity NCM811||graphite pouch cells, utilizing the fluorinated ether electrolyte outperformed cells with a carbonate electrolyte under different temperature conditions were tested, and found to retain 91.7% capacity after 300 cycles.
Overall, the insights gained from this research provide valuable contributions to the field of secondary lithium batteries, particularly in the area of advanced electrolyte systems. The exceptional solvation structure of TTME, along with the formation of robust SEI on both anode and cathode, has opened new avenues for the design and optimization of electrolytes to meet the growing demand for high-performance energy storage technologies in various applications. These findings contribute to the development of more efficient and reliable secondary lithium batteries, bringing us closer to a sustainable and greener future.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):