Halide-Based Solid Electrolyte with High Ionic Conductivity for All-Solid-State Sodium Batteries

All-solid-state batteries (ASSBs) are becoming a promising energy storage technology as they bring the safety of state-of-the-art batteries to the next level by replacing flammable organic liquid electrolytes (OLEs) with nonflammable solid electrolytes (SEs). The good mechanical properties of SEs further allow the usage of metal anodes to achieve very high energy density. Thus, developing SEs with desired properties is crucial to the commercialization of ASSBs. Despite the tremendous progress that has been made in the past decades, the progress and application are still in the infancy, experiencing numerous challenges for sodium solid-state batteries due to inherently low room-temperature ionic conductivity, interface complications, and fabrication. Inorganic halide-based SEs have emerged as a game changer because of their fast-conducting characteristics, adequate thermodynamic stability, great deformability, and good oxidative stability. Moreover, halide-based SEs are generally stable against moisture owing to their positive hydrolysis reaction energy. However, most of the studied halide-based SEs are based on the Li-ion system, with only a few based on the Na-ion system. Moreover, despite halide-based Na SEs theoretically having high ionic conductivity, experimentally obtained samples have much lower ionic conductivities compared to their Li counterparts (usually < 0.1 mS cm^–1). Therefore, developing halide-based SEs with high Na⁺ ionic conductivity is crucial for Na ASSBs. We recently identified a new Na halide-based SE with a room temperature Na⁺ ionic conductivity over 1 mS cm^–1. The activation energy is also as low as 0.23 eV. Interestingly, the material shows a mixed amorphous and crystalline phase, and its ionic conductivity decreases with the increase of the crystalline phase. The ionic conductivity of > 1 mS cm^–1is already comparable to some organic liquid electrolytes and thus is sufficient for many practical applications. The developed strategy has the potential to be applied to other halide-based SEs to improve their ionic conductivity, promoting the development of SIBs.