Solid electrolytes are ion conductors that should have a high ionic conductivity, low electronic conductivity, and low activation energy. Solid electrolytes with high ionic conductivity can be used as separators for solid-state batteries, which will greatly improve the safety of electric vehicles (EVs) and power-storage stations since they can use lithium-ion batteries as power sources. Currently, solid electrolytes can be divided into two main categories: inorganic and organic. Organic solid electrolytes are also called solid polymer electrolytes, which are widely regarded as a kind of electrolyte that contains polymeric materials and allows the movement of ions [1]. Gel polymer electrolytes represent an intermediate type between liquid and solid electrolyte, and will be discussed in Chapter 11. In this chapter, solid or dry polymer electrolytes and inorganic solid electrolytes will be discussed.