Issues and Advances in Scaling up Sulfide-Based All-Solid-State Batteries

ConspectusAll-solid-state batteries (ASSBs) are considered to be a next-generation energy storage concept that offers enhanced safety and potentially high energy density. The identification of solid electrolytes (SEs) with high ionic conductivity was the stepping-stone that enabled the recent surge in activity in this research area. Among the various types of SEs, including those based on oxides, sulfides, polymers, and hybrids thereof, sulfide-based SEs have gained discernible attention owing to their exceptional room temperature ionic conductivity comparable even to those of their liquid electrolyte counterparts. Moreover, the good deformability of sulfide SEs renders them suitable for reducing the interfacial resistance between particles, thereby obviating the need for high-temperature sintering. Nevertheless, sulfide-based ASSB technology still remains at the research stage without any manufacturing schemes having been established. This state of affairs originates from the complex challenges presented by various aspects of these SEs: their weak stability in air, questions surrounding the exact combination of slurry solvent and polymeric binder for solution-based electrode fabrication, their high interfacial resistance resulting from solid particle contacts, and limited scalability with respect to electrode fabrication and cell assembly. In this Account, we review recent developments in which these issues were addressed by starting with the materials and moving on to processing, focusing on new trials. As for enhancing the air stability of sulfide SEs, strengthening the metal-sulfur bond based on the hard-soft acid-base (HSAB) theory has yielded the most notable results, although the resulting sacrificed energy density and weakened anode interface stability would need to be resolved. Novel electrode fabrication techniques that endeavor to overcome the critical issues originating from the use of sulfide SEs are subsequently introduced. The wet chemical coating process can take advantage of the know-how and facilities inherited from the more established lithium-ion batteries (LIBs). However, the dilemmatic matter of contention relating to the polarity mismatch among the slurry solvent, SE, and binder requires attention. Recent solutions to these problems involved the exploration of various emerging concepts, such as polarity switching during electrode fabrication, fine polarity tuning by accurate grafting, and infiltration of the electrode voids by a solution of the SE. The process of using a dry film with a fibrous binder has also raised interest, motivated by lowering the manufacturing cost, maintaining the environment, and boosting the volumetric energy density. Finally, optimization of the cell assembly and operation is reviewed. In particular, the application of external pressure to each unit cell has been universally adopted both in the fabrication step and during cell operation to realize high cell performance. The effect of pressurization is discussed by correlating it with the interface stability and robust interparticle contacts. Based on the significant progress that has been made thus far, we aim to encourage the battery community to engage their wide-ranging expertise toward advancing sulfide-based ASSBs that are practically feasible.