Solid polymer electrolytes in all-solid-state lithium metal batteries: From microstructures to properties

All-solid-state lithium (Li) metal batteries (ASSLMBs) are considered one of the most promising secondary batteries due to their high theoretical capacity and high safety performance. However, low room-temperature ionic conductivity and poor interfacial stability are two key factors affecting the practical application of ASSLMBs, and our understanding of the mechanisms behind these key problems from microscopic perspective is still limited. In this review, the mechanisms and advanced characterization techniques of ASSLMBs are summarized to correlate the microstructures and properties. Firstly, we summarize the challenges faced by solid polymer electrolytes (SPEs) in ASSLMBs, such as the low room-temperature ionic conductivity and the poor interfacial stability. Secondly, several typical improvement methods of polymer ASSLMBs are discussed, including composite SPEs, ultra-thin SPEs, SPEs surface modification and Li anode surface modification. Finally, we conclude the characterizations for correlating the microstructures and the properties of SPEs, with emphasis on the use of emerging advanced techniques (e.g., cryo-transmission electron microscopy) for in-depth analyzing ASSLMBs. The influence of the microstructures on the properties is very important. Until now, it has been difficult for us to understand the microstructures of batteries. However, some recent studies have demonstrated that we have a better understanding of the microstructures of batteries. Then we suggest that in situ characterization, nondestructive characterization and sub-angstrom resolution are the key technologies to help us further understand the batteries’ microstructures and promote the development of batteries. And potential investigations to understand the microstructures evolution and the batteries behaviors are also prospected to expect further reasonable theoretical guidance for the design of ASSLMBs with ideal performance.