Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Sun, Xueliang

Abstract

Compared to the conventional lithium-ion batteries (LIBs) using flammable liquid electrolytes, all-solid-state lithium batteries (ASSLBs) using solid-state electrolytes (SSEs) show advantages of high safety and energy density. The development of high-performance SSEs will be a key accelerator for the advancement of practical ASSLBs. Solid polymer electrolyte (SPEs) and halide-based SSEs two types of promising SSE candidates with different advantages and disadvantages. Common SPEs are flexible for full cell integration but have poor room-temperature performance and poor stability with high-voltage cathodes. On the other hand, the superionic halide SSEs show good compatibility with the favorable 4-V class oxide cathodes, but they are incompatible with the “holy grail” lithium metal anode. These challenges need to be addressed before application in ASSLBs.

In this thesis, the concerns of SPEs and halide SSEs are systematically studied and addressed. First, a poly(butylene oxide) (PBO) based SPE is fabricated by a solvent-free method. Pre-cycling at an elevated temperature is found to induce LiF- and Li2O-rich cathode electrolyte interface that enables feasible performance near room temperature and good compatibility with 4-V class cathodes. Next step, a new rapid, solvent-free, and in-situ crosslinking process via instant treatment of the terminal hydroxyl groups of PBO is proposed for constructing a robust PBO SPE interface on the superionic halide SSEs. Resulting compatibility with lithium metal anodes is demonstrated. To further understand the fundamental properties of the newly revived halide SSEs, the overlooked structural thermal stability is investigated using in-situ and ex-situ X-ray diffraction and X-ray absorption spectroscopy. Using the Li-Y-Cl SSEs as examples, the different starting structures and processing procedures are found to play an important role in structural thermal stability and ionic conductivity. As a strategy to tune the ionic conductivity of halide SSEs, a series of Li3HoBrxCl6-x (x = 0‒6) electrolyte is synthesized to study the effects of anionic halide mixing. A wide range of Li3MBr3Cl3 (M = Gd‒Lu, Y) electrolytes with a Li3MBr6-like structure also reach a high room-temperature conductivity of approximately 3 mS cm-1. These results on the synthesis of SPEs and halide SSEs, interfacial engineering, and mechanism studies contribute to the advancement of high-performance ASSLBs.

Summary for Lay Audience

All-solid-state lithium batteries (ASSLBs) are promising next-generation batteries due to their enhanced energy density and high safety. The development of practical solid-state electrolytes (SSEs) is essential for the transition from conventional lithium-ion batteries to ASSLBs. Solid electrolytes can be broadly divided into polymer-based and inorganic SSEs. The poly(ethylene oxide) (PEO) based solid polymer electrolytes (SPEs) have been successfully used in commercial models due to good compatibility with electrodes and good processability. However, drawbacks are the required operating temperature of 60‒80 oC and poor compatibility with high-voltage oxide cathodes. Comparatively, the inorganic SSEs show advantages of high ionic conductivity, inflammability, and good mechanical strength. Breakthroughs in ionic conductivity for halide SSEs since 2018 have led to the revival of halide SSEs, but challenges such as the compatibility issues with lithium metal anode, fundamental understanding of the SSE structural stability, and further improvement of ionic conductivity remained to be studied.

In this thesis, promising SSE candidates from SPEs to halide SSEs are systematically assessed. First, a poly(butylene oxide) (PBO) based SPE is developed with feasible performance near room temperature and good compatibility with 4-V class cathodes for application in ASSLBs. Next, a rapid and solvent-free crosslinking process is proposed for constructing a robust PBO SPE interface on the superionic halide SSEs to enable compatibility with lithium metal anodes. To further understand the properties of halide SSEs, the overlooked structural thermal stability is examined. The different starting crystal structures and processing procedures are found to be importantd. As a strategy to tune the ionic conductivity of halide SSEs, the effect of anionic halide mixing is studied. The improvement of ionic conductivity by halide mixing is versatile for a wide range of Li3M(Br/Cl)6 (M = Gd‒Lu, Y) SSEs. These results on the synthesis of SPEs and halide SSEs, interfacial engineering, and mechanism studies shall pave the way to achieve high-performance ASSLBs.

Available for download on Friday, January 26, 2024

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