Research Progress on Halide Solid Electrolytes

With the growing demand for large-scale energy storage systems, electric vehicles, and smart devices, the development of rechargeable batteries with a high energy density, a fast-charging capability, and a long cycle life becomes critically important. Although the existing lithium-ion batteries dominate the market, the organic liquid electrolytes used are flammable and pose a risk of thermal runaway, restricting their further application. In contrast, all-solid-state lithium batteries offer a highly promising solution to this challenge. To replace flammable liquid electrolytes with solid-state electrolytes (SSEs), all-solid-state lithium batteries significantly enhance thermal stability and enable the use of high-voltage cathode materials to increase energy density due to the wider electrochemical stability window of SSEs. Among the various solid-state electrolyte materials, halide solid-state electrolytes (HSSEs) have attracted much attention due to their unique combination of properties. They typically exhibit a high ionic conductivity, reaching 10^–3 S·cm^–1 to 10^–4 S·cm^–1 at room temperature, which is comparable to some liquid electrolytes and sulfides. Furthermore, they possess a wide electrochemical window (i.e., 3.0–5.0 V vs. Li⁺/Li), making them compatible with high-voltage cathode materials, and demonstrate a good mechanical deformability, which facilitates an intimate solid-solid interfacial contact. This review summarizes the key progress in understanding HSSEs, focusing on their classification, ion transport mechanisms, synthesis methods, and performance optimization strategies, while also outlining the current challenges and future directions.

Firstly, for material systems, researchers propose a classification system based on the valence state of the central metal ion, dividing halide electrolytes into three categories, i.e., 1) divalent metal-based electrolytes (e.g., Li₂MX₄), which typically exhibit spinel-type structures but generally suffer from low ionic conductivity (<10^–4 S·cm^–1), 2) trivalent metal-based electrolytes (e.g., Li₃MX₆), the most widely studied class, whose conductivity is highly sensitive to crystal structure and lithium-ion/vacancy distribution, and 3) tetravalent metal-based electrolytes (e.g., Li₂MCl₆, M = Zr, Hf), represented by Li₂ZrCl₆ (0.81×10^–3 S·cm^–1), which combine good stability and cost-effectiveness, showing a great potential for practical application. In terms of ion transport mechanisms, research reveals the decisive influence of the anion framework on lithium-ion diffusion behavior. Cation doping (e.g., introducing Zr⁴⁺, In³⁺) introduces additional lithium sites and vacancies, thereby reducing migration barriers and constructing interconnected three-dimensional diffusion networks. Anion doping also proves highly effective, i.e., oxygen doping can induce amorphization, creating a more disordered local structure that facilitates rapid lithium-ion hopping, as seen in the superionic amorphous xLi₂O–TaCl₅ system, which achieves an ionic conductivity of 6.6×10^–3 S·cm^–1; fluorine doping can form stable, passivating interfacial layers, thereby enhancing high-voltage stability. The existing research on amorphous halide electrolytes emerges as a new frontier. These materials can possess lower activation energies and more efficient percolative ion transport due to their lack of long-range order. To further enhance the comprehensive performance of halide electrolytes, researchers also develop various modification strategies. Chemical doping, which modulates the lattice structure and defect chemistry by introducing heteroatoms, becomes a primary approach of improving ionic conductivity. Composite electrolyte design achieves synergistic effects via combining halide electrolytes with other materials.

HSSEs emerge as a highly promising material for next-generation all-solid-state lithium batteries. Their unique combination of high ionic conductivity, wide electrochemical window, and good mechanical properties makes them key materials for achieving high energy density and safe energy storage. Significant progress is made in understanding the classification of crystalline and amorphous halide materials, elucidating their ion transport mechanisms, and developing diverse synthesis and modification strategies to tailor their properties.

Despite this significant progress, several challenges are addressed to fully realize their potential. 1) Improving the ionic conductivity of fluoride-based electrolytes: Although fluorides offer the advantage of high-voltage stability, their ionic conductivity remains rather low. Future research must focus on overcoming this bottleneck through novel structural design, doping strategies, or the development of amorphous fluoride phases; 2) Enhancing electrochemical stability against lithium metal anodes: When halide solid-state electrolytes come into contact with lithium metal anodes, their metal cations are susceptible to reduction, leading to interfacial instability and capacity decay. Therefore, the rational design of stable artificial solid-electrolyte interphase layers that can both protect the electrolyte from reduction and ensure smooth lithium-ion transport is crucial; 3) Deepening the understanding of amorphous transport mechanisms: Although amorphous halides exhibit a superior conductivity, the fundamental principles governing ion transport in these disordered structures are not yet fully understood. Advanced characterization techniques and theoretical simulations are needed to elucidate the roles of dynamic structural relaxation and local chemical environments; and 4) Developing scalable and sustainable synthesis methods: Moving beyond laboratory-scale synthesis to develop cost-effective, environmentally friendly, and scalable production methods is key to commercialization. This includes exploring and optimizing water-based or solvent-based synthesis routes for a wider range of materials. The focused research will pave a way for the practical application of halide solid-state electrolytes. Their successful integration into all-solid-state lithium batteries holds a potential to revolutionize energy storage, providing safer, more powerful, and longer-lasting batteries for a wide range of applications, from consumer electronics to electric vehicles and grid-scale energy storage.