Development of Advanced Halide-Based Solid-State Electrolytes and Cathode Materials for All-Solid-State Batteries

Jiamin Fu, Western University

Abstract

All-solid-state batteries (ASSBs) are widely recognized as the next frontier in energy storage systems, holding the potential to surpass the capabilities of current lithium-ion batteries (LIBs). Their unique feature of utilizing non-flammable solid-state electrolytes (SSEs) instead of liquid electrolytes (LEs) directly addresses safety concerns in power supply applications, particularly within the electric vehicle and aviation industries. However, the inherent rigidity of solid materials introduces a challenge in the form of sluggish ion diffusion behaviors, observable within SSEs, the cathode active materials (CAMs), and the interfaces between these components. This sluggish diffusion ultimately contributes to a reduction in the operational efficiency of ASSBs.

Within the scope of this thesis, a comprehensive investigation is undertaken into the intricacies influencing ion transport in halide materials. Factors such as bond covalency, long-range and short-range structures, and the amorphous state are meticulously examined. Regarding the bond covalency, the superionic conducting transition of Li₃InBr₆ was investigated by in situ characterization upon the transition temperature (T_c). It is revealed that this superionic transition correlates with the covalency of the In-Br bond. The superionic conducting transition behaviors can be further tuned through cation (Y and Ga) doping in Li₃InBr₆ via the modification of metal-ligand covalency. Regarding the crystal structures, a new class of zeolite-like halide frameworks was studied, SmCl₃ for example, in which 1-dimensional channels are enclosed by [SmCl₉]^6- tricapped trigonal prisms to provide a short jumping distance of 2.08 Å between two octahedrons for Li-ion hopping. Similar to zeolites, the SmCl₃ framework can be grafted with halide species to obtain mobile ions without altering the base structure, achieving an ionic conductivity over 10^-4 S cm^-1 at 30 °C with LiCl as the adsorbent. Regarding the design of heterogeneous structure electrolytes (HSEs), the structural variances between high- and low-coordination halide frameworks were exploited to develop a new class of Na⁺-conducting halide HSEs. The HSEs using UCl₃-type high-coordination frameworks achieved the highest Na-ion conductivity (2.7 mS cm^-1 at room temperature) among halides so far. The synergistic processes within halide HSEs were unrevealed, providing a comprehensive explanation of the amorphization effect.

Taking advancements a step further, an all-in-one cathode electrode solution is realized through the rational design of a halide cathode material, Li_1.3Fe_1.2Cl₄. All-in-one active materials can transport ions and electrons themselves, thus showing advantages in high active material content (theoretically 100%), low tortuosity for Li⁺/e^- diffusion, and eliminating cathode/SE interface reactions. The identification of halide materials, studies on diffusion mechanisms, and electrode engineering highlighted in this dissertation pave the way for the development of high-performance ASSBs.