Ion Conduction in Composite Polymer Electrolytes: Potential Electrolytes for Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) are expected to become alternatives to lithium‐ion batteries (LIBs) as next‐generation rechargeable batteries, owing to abundant sodium sources and low cost. However, SIBs still use liquid organic electrolytes (LOEs), which are highly flammable and have the tendency to leak. Although inorganic solid electrolytes (ISEs) and solid polymer electrolytes (SPEs) have been investigated for many years, given their higher safety level, neither of them is likely to be commercialized because of the rigidity of ISEs and the low room‐temperature ionic conductivity of SPEs. During the last decade, composite polymer electrolytes (CPEs), composed of ISEs and SPEs, exhibiting both relatively high ionic conductivity and flexibility, have gained much attention and are considered as promising electrolytes. However, the ionic conductivities of CPEs are still unsatisfactory for practical application. Hence, this Review focuses on the principle of sodium ion conductors and particularly on recent investigations and development of CPEs. Best of both worlds: Solid‐state inorganic and polymer electrolytes for sodium‐ion batteries are safer than commercial liquid organic electrolytes but suffer from rigidity and low room‐temperature ionic conductivities, respectively. This Review focuses on composite polymer electrolytes made up of inorganic solid electrolytes and solid polymer electrolytes, which can realize both high ionic conductivity and flexibility.