Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Prof. Xueliang (Andy) Sun

Abstract

The development of lithium-ion batteries (LIBs) has been hampered by the intrinsic limitations of the electrode materials. High-performance LIBs demand electrode materials with fast lithium/electron diffusion rate, stable surface chemistry and high specific capacity. Surface modification by atomic layer deposition (ALD) is an essential method to optimize the performance of the electrode materials. The research in this thesis aims at achieving high-performance LIBs via surface modification and understanding the mechanisms via synchrotron radiation.

Firstly, by applying ALD FePO4 on LiNi0.5Mn1.5O4 (LNMO), we successfully alleviated the electrolyte decomposition under high voltage by using the electrochemically active FePO4 as a buffer layer. By coating the high energy Li1.2Mn0.54Co0.13Ni0.13O2 (HENMC) with AlPO4, we demonstrated higher thermal resistance compared with the most widely used Al2O3 as the coating material.

The irreversible phase change in cathode materials is an intrinsic property that is difficult to be addressed by simple coating, therefore, we extended the practice of ALD to accurately control the surface composition by post annealing TiO2 coated LNMO. We demonstrated the effectiveness of creating a surface layer of spinel TiMn2O4-like structure and Ti-doped LNMO sub-surface, which protect the material surface from the electrolyte attack and prevent the intrinsic phase change simultaneously.

To understand how the structure evolves, we used synchrotron radiation to study the behavior of HENMC in the initial cycle and 450th cycle in an in-situ manner. The in-situ X-ray absorption (XAS) has been demonstrated to be an outstanding method to track the change of transition metals while the cell is under operation. We found that the Ni and Co have lost their electrochemical activity after long-term cycling due to the phase segregation.

We also studied the surface behaviors of graphene nanoribbons (GNRs) synthesized from chemically unzipped carbon nanotubes and the correlation with the electrochemical performance used as anode materials. We found that defects, surface area and surface functional groups introduced by the chemical treatment play pivotal roles.

In summary, the discoveries in this thesis provide important methods and unveil critical understandings to achieve high-performance LIBs.

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