Preparation technology and challenges of sulfide electrolyte films

Lithium-ion batteries, serving as an efficient energy storage technology, address the instability of renewable energy sources and promote transportation electrification. However, their safety and energy density face significant challenges owing to the use of liquid organic electrolytes, which are flammable, prone to leakage, and incapable of inhibiting dendrite penetration. Solid electrolytes exhibit characteristics, such as non-flammability and high mechanical strength, to suppress dendrite penetration. Consequently, solid electrolytes are considered viable alternatives to liquid organic electrolytes to enhance the safety of lithium batteries. Notably, lithium metal anode is expected to be used in solid-state batteries to greatly promote the energy density and thus alleviate range anxiety, drivers’ concern about insufficient battery range, which remains a major barrier to the widespread adoption of electric vehicles. Solid electrolytes include polymers, oxides, sulfides, and halides. Among them, sulfide solid electrolytes, characterized by their exceptionally high ionic conductivity (exceeding 10‒2 S/cm at room temperature), excellent processability, and their capacity to reduce porosity and interfacial impedance via cold-pressing procedures, have emerged as the primary focus of research. Research efforts have primarily focused on developing novel high-ionic-conductivity solid-state electrolyte materials and elucidating the fundamental mechanisms of lithium-ion conduction. Additionally, substantial progress has been made in resolving the high air sensitivity of sulfide solid electrolytes, laying the foundation for mass production. Despite these advancements, the large-scale fabrication of sulfide solid-state batteries remains a challenge, as the lamination of solid electrolyte and electrode films is the primary technology for pouch-type sulfide solid-state lithium batteries. A lack of mature preparation methodologies and an insufficient understanding of various fabrication routes have significantly impeded the industrial-scale production of solid-state electrolyte films. In light of the current research status, this review aims to provide a comprehensive analysis of the engineering-scale preparation processes for sulfide solid-state electrolyte films. It first briefly summarizes the common environmental control factors in the production process of sulfide solid-state electrolyte films, including the production environment, corrosion of equipment, and personnel safety. Then, it classifies the preparation methods of sulfide solid-state electrolyte films into two categories: wet-based and dry-based. Wet-based preparation technology exhibits greater compatibility with existing lithium-ion battery production lines because of the universality of equipment, similarity of processes, and compatibility of film formation methods. In contrast, dry-based preparation technology exhibits significant advantages in terms of cost management by eliminating the need for solvents and handling equipment, simplifying the process flow, and increasing material utilization rate. Subsequently, this review undertakes an in-depth analysis and comparison between wet- and dry-based preparation technologies. It systematically examines wet-based methods from the perspectives of solvent selection, binder utilization, slurry preparation, and film-formation techniques, identifying key challenges and proposing corresponding countermeasures. Similarly, it explores key factors influencing dry-based methods, including melt extrusion/hot-pressing dry film-forming technology and binder fibrillation, and evaluates their progress. Furthermore, the study assesses progress in sulfide solid-state electrolyte film preparation technology and highlights three wet film-forming process routes, considering different substrates and their associated challenges. Finally, this review anticipates the role of artificial intelligence and machine learning in optimizing sulfide electrolyte film engineering. It also emphasizes the importance of environmentally friendly solvents and green preparation processes in overcoming the limitations of wet-process technology. By providing an in-depth discussion of the current research landscape, this review serves as a valuable reference for advancing the large-scale preparation and engineering application of ultra-thin sulfide electrolyte films in the field of solid-state batteries.