Date of Graduation

7-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor

Xiangbo Meng

Committee Member

Steve Tung

Second Committee Member

Uchechukwu Wejinya

Third Committee Member

Jingyi Chen

Fourth Committee Member

Arun Nair

Keywords

Atomic layer deposition, Electrical energy storage (EES), Electrode performance, Lithium-ion batteries, Sodium-ion batteries, Solid-state lithium batteries

Abstract

As fossil fuels are depleting and causing environmental issues, it becomes imperative to widely implement renewable energies (e.g., solar and wind power). In this context, electrical energy storage (EES) systems are essential to accommodate the intermittent supply and uneven distribution of renewable energies. To this end, high-energy-density EES devices are highly demanded, such as rechargeable batteries. This dissertation presents my efforts in developing novel battery materials for lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and solidstate lithium batteries (SSLBs). These efforts include electrochemical evaluations of battery materials, surface modifications of electrodes via atomic layer deposition (ALD), and advanced characterizations of batteries materials.

In this dissertation, my first effort invested on the growth of ZnO nanofilms via ALD. ALD technique possesses some unrivaled merits, including atomic-scale controllability, excellent uniformity and conformality, and the accessibility of a large variety of materials. In the past decade, it has been widely used for surface-modifying battery electrodes. This study investigated growth characteristics and underlying mechanisms of ALD ZnO nanofilms using diethylzinc and water precursors. The temperature-controllable growth and crystal-orientation of ALD ZnO films were disclosed. This study offered key understanding and preliminary preparation for the following study to surface-modify electrode materials.

A subsequent effort was focused on developing a novel anode for SIBs. SIBs have attracted an ever-growing research interest, due to their cost-effectiveness as large-scale stationary EES devices. However, compared with LIBs, SIBs are significantly hindered by the lack of suitable anodes. In this study, Cu2S was chosen for high capacity and fast-charge capability. The nitrogendoped graphene (NG) wrapped Cu2S (Cu2S@NG) composite anode exhibited stable cyclabilityand excellent rate capability. The electrochemical mechanism was investigated using synchrotronbased X-ray techniques. Whereas the parasitic reactions at the electrode/electrolyte interface continuously deteriorated performance, which could be resolved by surface modifications via ALD. In the sequential effort to enhance interfacial stability of Cu2S@NG anode, the electrochemically inactive Al2O3 via ALD was selected as the coating material. A 6-nm ultrathin Al2O3 coating could improve the interfacial stability of the Cu2S@NG anode and thereby enhance its electrochemical performance with the highest capacity reported to date.

Additionally, this dissertation includes an effort in investigating a promising solid-state electrolyte (SSE), Li7La3Zr2O12 (LLZO). Conventionally, flammable liquid-organic electrolytes have been used in LIBs but induced some safety issues (e.g., fires and explosions). In this regard, SSEs are highly regarded to replace liquid organic electrolytes. LLZO is among the most promising SSEs with high ionic conductivity (~10-3 S cm-2 at room temperature) and high stability with Li metal anode. Nevertheless, LLZO’s stability in the ambient storage is challenging, causing deteriorated ionic conductivity and interfacial instability. This study investigated the structural and stoichiometric reversibility of air-aged LLZO during heat treatment. In addition, the correlation between restoration degree and dopant chemistry was unveiled.

In summary, the contributions in this dissertation provide critical understandings to electrochemical mechanisms and degradation causes of novel electrode and electrolyte materials and demonstrate the vital surface modification route via ALD for enhanced performance.

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