Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Prof. Xueliang Sun

2nd Supervisor

Prof. Tsun-Kong Sham

Co-Supervisor

Abstract

Lithium (Li) and Silicon (Si) have been considered as promising anode materials for the next-generation batteries due to high theoretical capacity (3860 mAh g-1 for Li and 3580 mAh g-1 for Si). However, they both face great challenges that obstruct their practical application, such as unstable solid-electrolyte interphase (SEI) and large volume change. Moreover, Li also suffers from the uncontrollable dendrite growth, whereas Si suffers from slow reaction kinetics. This thesis mainly focuses on the interphase engineering to tackle the challenges of Li and Si anode materials.

First, organic polyurea (PU) coating is deposited on Li by molecular layer deposition (MLD) as a protective film. The post-cycling investigation revealed that PU can suppress dendrite growth and stabilize SEI. This work demonstrates MLD thin film technology as an effective strategy for interphase engineering on anode material. Secondly, the following study shows that the mechanical property of MLD PU film can be tuned by inorganic aluminum crosslinker. The stiffness of PU is improved by the incorporation of crosslinker, and the cycling stability of Li is improved significantly owing to the enhanced mechanical property. Thirdly, the accurate control on the gradient distribution of inorganic component for the nano-scale thin film is achieved by MLD for the first time. The inorganic lithiophilic zinc sites can facilitate the Li nucleation on the inner side, while organic insulating PU on the top can confine Li deposition underneath. Owing to the elaborate design of ‘gradient coating’, the protected Li exhibits long cycling over 1500 hours in the next-generation Li-O2 battery. Both studies on mechanical property and gradient of inorganic component reveal the role of MLD thin film for Li and provide deep insight on the design of interphase engineering. Finally, the MLD PU strategy is extended to Si/C composite electrode to tackle the volume change and unstable SEI. The flexible PU film can enable a stable performance of Si/C anode with a high areal capacity over 3 mAh cm-2. The excellent cycling performance enabled by MLD strategy in this work contributes great potential to break through the bottleneck of energy density for Lithium-ion batteries (LIBs).

Summary for Lay Audience

The current Lithium-ion batteries (LIBs) have almost reached a bottleneck on energy density which is challenging to meet the increasing demand for energy storage. The commercial graphite anode delivers a limited capacity of 372 mAh g-1, so the research on alternative anode materials with higher capacity is urgently needed. Lithium (Li) and Silicon (Si) are promising anode materials with approximately 10-fold capacity than graphite. However, the practical application of Li and Si meets great challenges, including the large volume change under charge/discharge, and unstable interphase due to the side reaction between electrode and electrolyte. Moreover, Li metal anode suffers from serious dendrite growth. Dendrite can penetrate the separator and lead to fire hazard from short-circuiting. This thesis focuses on interphase modification to tackle the challenges for Li and Si anode materials.

First, molecular layer deposition (MLD) is utilized to deposit polyurea (PU) coating on Li to suppress dendrite. MLD is selected due to its conformal coverage and accurate thickness control. The polar groups in PU can regulate the ion flux and lead to unform Li deposition, while the good flexibility of PU can suppress dendrite growth. This work provides an effective strategy for interphase engineering realized by MLD technique. Secondly, inorganic aluminum crosslinker is found to improve the stiffness of PU. The PU coating with enhanced mechanical property can further improve the performance and extend cycling life. Thirdly, a novel ‘gradient coating’ is developed by MLD for Li metal. The gradient of inorganic component can facilitate uniform Li nucleation, while the organic component at top can confine Li deposition underneath. The protected Li enables next-generation Li-O2 battery a stable and long operation over 1500 hours. Both studies on mechanical property and gradient of inorganic component disclosed deep insights on how to design anode interphase. Finally, flexible PU coating is extended to Si/C anode to mitigate volume change and stabilize interphase, achieving a high areal capacity with stable performance. This work contributes great potential for the development of high energy density next-generation battery.

Available for download on Thursday, December 07, 2023

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