Abstract
Since their commercialization in the 1990s, lithium-ion batteries (LIBs) have become the predominant energy source for portable electronic devices, electric vehicles, and energy storage systems, renowned for their high energy density, long cycle life, and low self-discharge rate. However, as the demand for high-performance batteries increases, traditional LIBs face significant challenges, including capacity degradation, limited cycle life, and safety concerns. These issues are primarily due to volumetric changes in electrode materials and side reactions of electrolytes during charge and discharge cycles. To address these challenges, this thesis investigates novel anode materials, focusing on silicon anodes and lithium metal anodes.Silicon anodes, with a theoretical capacity of approximately 4200 mAh/g, offer much higher energy density compared to conventional graphite anodes. However, the volumetric expansion of silicon during lithiation, which can reach up to 300%, leads to structural degradation and instability of the solid electrolyte interphase (SEI) layer, severely impacting the cycle life and stability of the battery. Various strategies have been proposed to mitigate this challenge, including nano-structuring of silicon particles, development of silicon-carbon composites, and optimization of electrolyte additives to improve SEI stability.
Lithium metal anodes, in their body-centered cubic phase (LiBCC), are considered the ultimate anode for rechargeable batteries due to their high specific capacity (3860 mAh/g) and low redox potential (−3.040 V vs. standard hydrogen electrode). The SEI layer plays a critical role in determining the stability of the anode, acting as a self-passivating layer that electronically insulates the electrolyte from the free electrons in the anode while remaining conductive to Li+ cations. This study employs advanced characterization techniques such as cryo-electron microscopy (cryo-EM) and high-resolution transmission electron microscopy (HRTEM) to investigate the instability of lithium carbonate (Li2CO3) in the SEI of lithium metal anodes and the influence of electrolyte additives like ethylene sulfate (DTD) and 13-propanesulfonate (PS) on SEI formation and stability. The research reveals that Li2CO3 is thermodynamically unstable in contact with LiBCC, leading to its decomposition and poor SEI performance. In contrast, sulfur-containing additives show superior performance, forming electronically insulating layers that enhance SEI stability. This thesis also explores the SEI structures formed on different anode surfaces, including silicon, lithium metal, artificial graphite, and hard carbon, in the presence of fluorine-containing electrolyte additives. Using cryo-EM, detailed analyses of SEI composition and morphology highlight the differences in SEI formation mechanisms across various anode materials and the beneficial effects of fluorine-containing additives.
Furthermore, the study examines the impact of different electrolyte additives, such as ethylene sulfite (ES), fluoroethylene carbonate (FEC), and lithium difluorophosphate (LiPO2F2), on the SEI formation on nano-silicon anodes under low discharge voltage conditions. Cryo-TEM analyses demonstrate that these additives significantly improve the structural and chemical stability of the SEI layer, enhancing the cycling performance and longevity of nano-silicon anodes. The research findings suggest that fluorine-containing electrolytes form more stable SEI layers due to the formation of LiF and other stable compounds, which prevent further electrolyte decomposition and improve battery performance.
In addition to lithium-ion batteries, this thesis addresses the development of dendrite-free zinc deposition in zinc batteries, which are gaining attention for their high capacity, safety, and cost-effectiveness. The study introduces a novel electrolyte additive, cesium sulfate, which effectively inhibits zinc dendrite growth by forming a self-healing electrostatic shielding layer. This mechanism ensures uniform zinc ion deposition, preventing dendrite formation and enhancing the cycle life and stability of zinc anodes. The research demonstrates that the optimal concentration of cesium sulfate in the electrolyte not only prevents dendrite growth but also mitigates corrosion and passivation of the zinc metal anode, improving the overall electrochemical performance of zinc batteries.
Overall, this work provides valuable insights and practical guidelines for the design and optimization of high-capacity, durable, and safe lithium-ion and zinc batteries. The findings contribute to the development of next-generation batteries with enhanced performance and stability, addressing the growing demand for high-energy-density, reliable energy storage solutions in various applications.
| Date of Award | 19 Aug 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Zhiyuan ZENG (Supervisor) & Meng Gu (External Supervisor) |