In Situ Polymerization as a Promising Strategy for Solid-State Sodium–Metal Batteries: Mechanisms, Advances, and Perspectives

Sodium-metal batteries (SMBs) are seen as a promising energy storage option due to their high energy density, abundant sodium resources, and cost-effectiveness. However, traditional organic liquid electrolytes (LEs) suffer from inherent safety risks, including leakage and flammability. Solid polymer electrolytes (SPEs) are regarded as a viable alternative to LEs for safer SMBs. However, the poor interfacial contact and insufficient ion transport associated with conventional ex situ prepared SPEs make it difficult for them to outperform LEs. The in situ polymerization technique, characterized by its process scalability and ability to form conformal interfaces, has proven effective in overcoming the obstacles associated with SPEs. In this review, we summarize recent research on in situ polymerization techniques for SMBs, including free-radical polymerization, ionic polymerization, electropolymerization, and related approaches. The importance of in situ polymerization in streamlining the preparation process, enhancing the compatibility of the electrode-electrolyte interface, bolstering battery safety, and optimizing electrochemical performance is highlighted. Furthermore, we systematically discuss the multiscale design principles of this technology, emphasizing precursor formulation, reaction kinetics, and targeted bulk-matrix properties. The advanced functional applications of in situ polymerization in SMBs, particularly in suppressing crosstalk effects, regulating solvation structures, and designing artificial interfacial layers and materials, are discussed. Finally, critical challenges, future prospects and pathways toward the practical, large-scale application of in situ polymerization are evaluated. This review aims to provide systematic insights to facilitate the rapid progress and implementation of in situ-polymerized electrolytes in SMBs.