Understanding the chemistry in halide superionic conductor solid electrolytes for all-solid-state batteries

Recently, halide superionic conductors have been considered as emerging solid electrolytes for all-solid-state batteries, due to their high Li+ conductivity, electrochemical stability, and enhanced mechanical properties in comparison with the typical sulfide and various oxide based solid electrolytes. In this chapter, a comprehensive understanding of halide-based solid electrolytes is presented for all-solid-state batteries. We begin with an overview of the fundamental understanding, mechanism, characterization techniques, and elemental substitutions to enhance the performance of halide-based solid electrolytes. This chapter addresses theoretical knowledge of electrochemical and interfacial compatibility of halide-based solid electrolytes. In addition, it critically examines the key challenges to their practical realization, such as material cost, environmental stability (air and moisture sensitivity), and processability in aqueous and non-aqueous slurries. Finally, a perspective for future research opportunities is provided, which includes further enlargement of the chemical space of halide electrolytes and the creation of scalable, stable, and cost-effective solutions for the next generation of solid-state battery technologies.