Solid-state lithium batteries (SSLBs) with high safety have emerged to meet the increasing energy density demands of electric vehicles, hybrid electric vehicles, and portable electronic devices. However, the dendrite formation, high interfacial resistance, and deleterious interfacial reactions caused by solid–solid contact between electrode and electrolyte have hindered the commercialization of SSLBs. Thus, in this review, the state-of-the-art developments in the rational design of solid-state electrolyte and their progression toward practical applications are reviewed. First, the origin of interface instability and the sluggish charge carrier transportation in solid–solid interface are presented. Second, various strategies toward stabilizing interfacial stability (reducing interfacial resistance, suppressing lithium dendrites, and side reactions) are summarized from the physical and chemical perspective, including building protective layer, constructing 3D and gradient structures, etc. Finally, the remaining challenges and future development trends of solid-state electrolyte are prospected. This review provides a deep insight into solving the interfacial instability issues and promising solutions to enable practical high-energy-density lithium metal batteries.

Separators or electrolyte membranes are recognized as the key components to guarantee ion transport in rechargeable batteries. However, the ever-growing applications of the battery systems for diverse working environments bring new challenges, which require advanced battery membranes with high thermal stability, excellent mechanical strength, high voltage tolerance, etc. Therefore, it is highly desirable to design novel methods/concepts to solve the current challenges for battery membranes through understanding the mechanism of novel phenomena and electrochemical reactions in battery systems working under unconventional conditions. Recently, the new emerging Janus separators or electrolyte membranes with two or more distinct chemical/physical properties arising from their asymmetric structure and composition, are promising to address the above challenges via rational design of their targeted functionalities. To this end, in this review, we first briefly cover the current challenges of the traditional battery membrane for battery devices working in unconventional conditions. Then, the state-of-art developments of the rational design of Janus membranes to overcome the above challenges for diverse battery applications are summarized. Finally, we outline these latest developments, challenges, and future potential directions of the Janus membrane. Our review is aimed to provide basic guidance for developing functional separators or electrolyte membranes for advanced batteries.