Block copolymer electrolytes for lithium metal batteries: Strategies to boost both ionic conductivity and mechanical strength

The mechanically hard phase and ionically conductive phase endow suitably designed block copolymer electrolytes (BCPEs) with the "Janus" property, thus providing the opportunity to decouple the tradeoff between mechanical strength and ionic conductivity by controlling the phase-separated structures. The conductivity of BCPEs is predominantly determined by the molecular structure of block copolymers and the type and concentration of additives, while the manipulation of phase-separated structures helps strengthen their mechanical support and ion transport. This review article presents an overview of BCPEs and focuses on the "molecular structure-phase structure-property" relationship. Ideally, BCPE membranes should have high-throughput and aligned ion transport channels perpendicular to electrodes. First, given the desired attributes of polymer electrolytes, i.e., high ionic conductivity, high strength, low thickness, and high limiting current density, we summarize the research status and optimization strategies for BCPEs. Second, we present a summary of methods that control the phase behavior of BCPEs based on the phase separation mechanism. Third, BCPEs are classified into dual-ion conductor and single-ion conductor, whose advantages and disadvantages are analyzed. Furthermore, we propose a design rationale for high-performance quasi-solid-state BCPEs. We elaborate polymerization methods for the regulation of molecular and phase structure. These aspects are believed to collectively contribute to BCPE membranes with both high ion-conductivity and high mechanical strength, further boosting the development of safe and high-energy solid-state lithium metal batteries.