Phase-separation-driven solid polymer electrolytes for high-performance lithium-metal batteries: A comprehensive review

Abstract

Solid-state lithium metal batteries (SSLMBs) offer high energy-density potential and improved thermal safety by reducing leakage-prone and flammable liquid components, making them promising candidates for high-safety, high-specific-energy batteries. Solid polymer electrolytes (SPEs) feature low density, good flexibility, structural tunability, facile processing, and scalability; however, it remains a great challenge to co‑optimize high ionic conductivity, mechanical strength, electrochemical stability, and interfacial compatibility. Current studies predominantly improve SPE performance by incorporating inorganic fillers or small-molecule plasticizers, whereas the phase evolution and structure-property relationships of multiphase systems containing a second polymer component remain insufficiently understood. Influenced by segmental interactions, component ratios, lithium-salt coordination, polymer crystallization, and processing history, uncontrolled macrophase separation can cause compositional heterogeneity, internal defects, disrupted ion-transport pathways, and deteriorated interfacial contact. In contrast, precise control over phase separation can generate bicontinuous soft-hard micro/nanostructures, in which the soft phase forms continuous ion-transport pathways and improves interfacial contact, while the hard phase provides mechanical support, thereby partially decoupling the trade-off between ionic conductivity and mechanical strength. Accordingly, this review focuses on phase-separation-driven solid polymer electrolytes, systematically elucidating their structural design principles and the relationships among phase structure, ion transport, mechanical properties, and interfacial behavior. It further summarizes the driving factors, regulation strategies, advanced characterization techniques, and theoretical simulation methods of phase separation, while discussing challenges related to morphological stability, interfacial regulation, and scalable manufacturing, thereby providing guidance for designing multiphase polymer electrolytes for high-performance SSLMBs.