Date of Graduation

12-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Meng, Xiangbo

Committee Member

Zhou, Wenchao

Second Committee Member

Millett, Paul C.

Third Committee Member

Zhao, Yue

Fourth Committee Member

Coridan, Robert H.

Keywords

Lithium-ion batteries; NMC cathodes; atomic layer deposition; surface coating; metal oxides; metal sulfides; mechanical integrity; electrode/electrolyte interface; phase transition; oxygen release

Abstract

Layered nickel-rich cathodes LiNixMnyCozO2 (NMCs, x + y + z =1, x ≥ 0.6) are regarded as one of the most promising cathode materials for next-generation lithium-ion batteries (LIBs), given their remarkably reduced cost and increased capacity compared to the conventional LiCoO2 cathode. However, the deployment of these Ni-rich cathodes has been hindered by the continuous loss of practical capacity and reduction in average working voltage, inherently due to their interfacial, structural, and thermodynamic instability. To address these issues, interfacial engineering via surface modification has been well-recognized as an effective strategy and a large number of coating materials have been investigated. Different from surface coatings by wet-chemistry processes, we aim at accurately tuning the interface of NMCs using atomic layer deposition (ALD). ALD is a newly emerged technique, featuring its low process temperature (≤300 ℃ generally), unrivaled conformal and uniform deposition, surface-controlled growth with the atomic-scale preciseness. It is to date the only technique enabling surface coatings over prefabricated battery electrodes directly while also enabling surface modifications over powder-based electrode materials.

In my PhD project, we have investigated different ALD processes for various coatings and their effects on NMCs, categorized into two classes: (1) metal oxides, such as aluminum oxide (Al2O3) and (2) metal sulfides, such as lithium sulfide (Li2S). In the project, our studies have revealed that the conformal Al2O3 and Li2S coatings can well protect LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes. In addition, the Al2O3 and Li2S coatings can remarkably improve the rate capability of these cathodes. Our investigation reveals that all these beneficial effects of the ALD coatings lie in the following aspects: (i) reinforce the mechanical integrity of the electrode; (ii) stabilize the electrode/electrolyte interface; and (iii) suppress the phase transition of the cathode structure. In particular, we for the first time revealed that sulfides could be an important class of functional coating materials for LIBs. In this regard, our study has revealed that the Li2S coating has played some unique role. We disclosed that the Li2S layer have reacted with O2 released from the NMC structure, and thereby have mitigated electrolyte oxidation and electrode corrosion. In addition to the ALD coating on nickel-rich cathodes, we studied the ALD growth of zirconium oxide (ZrO2) thin films and explored the crystallinity and mechanical properties of ZrO2 films evolved with deposition temperatures. In our studies, we have employed a suite of advanced synchrotron-based techniques at Argonne National Laboratory and Brookhaven National Laboratory to explore the underlying mechanisms of different ALD coatings, including synchrotron-based X-ray diffraction (XRD), transmission electron microscopy (TEM), and transmission X-ray microscopy (TXM). The success of these research activities will advance our knowledge on NMC cathodes, help commercialize Ni-rich cathodes, and deliver advanced ALD coating technologies.

In summary, this dissertation has demonstrated that ALD is an important and effective tool for tackling various issues of LIBs through applying various coatings over electrodes. In the project, we have advanced some fundamental understandings of different ALD coatings on NMCs.

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