Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Sun, Xueliang

Abstract

Lithium-ion batteries (LIBs) are critical for the development of electric vehicles (EVs) because of their higher operating voltages compared to other energy storage technologies. However, the development of start-of-the-art LIBs touched the ceiling because of three main challenges: safety risks, limited energy density, and high cost. Accordingly, all-solid-state lithium-ion batteries (ASSLIBs) have recently emerged as promising alternative batteries for next-generation EVs because of their ability to overcome the drawbacks of conventional LIBs. Whether in conventional liquid LIBs or ASSLIBs, cathode materials are crucial in determining the overall performance. Hence, this thesis focuses on understanding the degradation mechanism of cathode interfaces and developing novel interfacial strategies in both liquid LIBs and ASSLIBs.

The first work in this thesis develops a hybrid Li3PO4-TiO2 coating layer by atomic layer deposition (ALD) to improve both interfacial ionic/electronic conductivities and stability for high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode in liquid LIBs.

In the second work, a dual-functional Li3PO4 (LPO) modification is designed for Ni-rich layered oxide cathodes, aiming to address both the interfacial side reactions and the microstructural cracks in sulfide-based ASSLIBs.

In the third work, the origin of cathode interface degradation in sulfide-based ASSLIBs is unveiled by the X-ray and electrochemical analyses. Residual lithium compounds on the surface of Ni-rich layered cathodes are proved as the main reason that triggers the oxidation of sulfide solid-state electrolytes (SSEs), therefore inducing severe side reactions at cathode interface and structural degradation of Ni-rich cathodes.

The fourth wok for the first time reports a controllable semi-conductive additive, poly(3,4-ethylenedioxythiophene) (PEDOT), in sulfide-based ASSLIBs, therefore realizing effective electron transfer at the cathode/SSE/additive three-phase interface along with a competitive rate capacity.

To realize fast-charging ASSLIBs, the fifth work investigates the interfacial evolution of Al foil current collector in all-climate environment. At room temperature, side reactions are the main challenge for interfacial stability. At low temperature, the low Li+ and electron transfer kinetics along with side reactions are the key limitations for rate capability. The challenges at both room temperature and low temperature can be addressed by the graphene modification on Al foil.

Summary for Lay Audience

Electric Vehicles (EVs), as an ideal solution to address the challenges of global climate change, developed rapidly in the past ten years. The performance of lithium-ion batteries (LIBs), particularly the cathodes in LIBs, is critical for the development of safe, long-range, and fast-charged EVs. However, the start-of-the-art cathodes still face some significant challenges that deteriorate the performance of LIBs, therefore limiting their application in EVs. In conventional liquid-based LIBs, high working voltage of cathode materials leads to severe side reactions with the liquid organic electrolytes. In solid-state batteries (SSBs), the unstable cathode interfaces also bring poor performance. Hence, this thesis develops multiple strategies to address the challenges of cathode interfaces in both liquid LIBs and SSBs as well as investigates the interfacial degradation mechanisms by advanced characterizations.

First of all, a hybrid coating material, Li3PO4-TiO2, is deposited on the surface of high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode by atomic layer deposition. The coating layer avoids the direct contact between LNMO and liquid electrolytes, therefore suppressing the side reactions. Furthermore, the interfacial ionic/electronic conductivities also be enhanced by the hybrid coating layer, resulting in the much improved performance. The second work in this thesis focuses on the cathode interface in sulfide-based SSBs. A dual-functional Li3PO4 modification is developed to address both the side reactions with sulfide electrolytes and microstructure evolution of Ni-rich cathodes. As a result, an excellent long-term cycling performance is achieved with an improved rate capability. In the third work, the degradation mechanism of cathode interface in sulfide-based SSBs is clearly unveiled by advanced X-ray characterizations. Residual lithium compounds on the surface of Ni-rich cathodes are the origin that triggering side reactions with sulfide electrolytes, therefore deteriorating the performance of SSBs. The fourth part for the first time develops a semi-conductive poly(3,4-ethylenedioxythiophene) as additive in sulfide SSBs, that realizing an excellent rate performance. The fifth work investigates the effect of Al foil current collector in halide-based SSBs. Side reactions between Al foil and halide electrolyte and limited charge transfer kinetics are two main reasons that limiting the performance of SSBs, which can be addressed by the modification of graphene coating.

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