Recent progress of asymmetric solid-state electrolytes for lithium/sodium-metal batteries

The huge market in electric road vehicles and portable electronic devices is boosting the development of high-energy-density solid-state alkali-metal batteries with high safety, including lithium-metal batteries and sodium-metal batteries. However, solid-state electrolytes (SSEs) are still the main barrier that hinders the development of solid-state alkali-metal batteries, because there is no such a single SSE that is compatible with both the highly reductive and chemically active alkali-metal anodes and oxidative high-voltage cathodes. Asymmetric solid-state electrolytes (denoted as ASEs) with more than one layer of SSE are reported to be able to effectively tackle such issues by constructing a multiple layered-like structure. In ASEs, each layer of SSE contains a different composition or morphology. SSEs with such an asymmetric structure exhibit Janus property, which not only satisfies the different stability requirements from the cathode and the anode respectively, but also compensates the disadvantages of the individual SSEs ingenuously. In this way, the advantages of each individual SSE are fully utilized and superior electrochemical performances of solid-state full cells are realized. This review focuses on discussing various original ASEs that have been developed recently, including design principles, synthetic methods of bilayer/tri-layer structured polymer/ceramic ASEs and asymmetric gel electrolytes, and the exhibited electrochemical properties of solid-state lithium/sodium-metal batteries. Finally, we provide perspectives and suggestions towards ASEs for future applications in solid-state batteries.