Rechargeable Na-ion batteries (NIBs) are attractive large-scale energy storage systems compared to Li-ion batteries due to the substantial reserve and low cost of sodium resources. The recent rapid development of NIBs will no doubt accelerate the commercialization process. As one of the indispensable components in current battery systems, organic liquid electrolytes are widely used for their high ionic conductivity and good wettability, but the low thermal stability, especially the easy flammability and leakage make them at risk of safety issues. The booming solid-state batteries with solid-state electrolytes (SSEs) show promise as alternatives to organic liquid systems due to their improved safety and higher energy density. However, several challenges including low ionic conductivity, poor wettability, low stability/incompatibility between electrodes and electrolytes, etc., may degrade performance, hindering the development of practical applications. In this review, an overview of Na-ion SSEs is first outlined according to the classification of solid polymer electrolytes, composite polymer electrolytes, inorganic solid electrolytes, etc. Furthermore, the current challenges and critical perspectives for the potential development of solid-state sodium batteries are discussed in detail.

Solid-State Sodium Batteries

A. Cationic Conductors 2406 1. Lithium Ion Conductors 2406 2. Proton Conductors 2408 3. Divalent Cation Conductors 2410 B. Anionic Conductors 2411 1. Chloride Ion Conductors 2411 2. Oxide Ion Conductors 2412 III. Solid Electrolytes with Rare Earths as Dopants 2415 A. Cationic Conductors 2415 1. Lithium Ion Conductors 2415 2. Sodium Ion Conductors 2416 3. Proton Conductors 2417 B. Anionic Conductors 2417 1. Zirconium Dioxides 2417 2. Bismuth Oxides 2418 3. Thorium Oxides 2419 4. Hafnium Oxides 2419 5. Apatites 2419 IV. Trivalent Cation Conductors 2420 A. Ln3+−â-aluminas and Ln3+−â′′-aluminas 2420 B. â-Alumina-Related Materials 2422 C. Perovskite-Type Structures 2422 D. Sc2(WO4)3-Type Solid Electrolytes 2423 E. NASICON-Type Structures 2425 V. Concluding Remarks 2427 VI. Acknowledgment 2427 VII. References 2427

B. E. Taylor

Papers

Ionic conductivity in sodium yttrium silicon oxide (Na5YSi4O12)-type silicates