Towards High-Performance All-Solid-State Lithium Batteries: from Polymer Electrolytes to Solid Halide Electrolytes

Jing Luo, The University of Western Ontario

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 Li₃HoBr_xCl_6-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.