Symmetrical Molecular Topology Enables Ultrathin Solid Polymer Electrolytes for Stable Lithium‐Metal Batteries

Solid polymer electrolytes (SPEs) have emerged as promising candidates for lithium‐metal batteries owing to their advantages in safety, flexibility, and processability. However, ultrathin SPEs (<10 µm) still face challenges in practical applications, including structural inhomogeneity, sluggish ion transport, and lithium dendrite penetration. This study breaks through the conventional paradigm of compositional modulation and proposes a symmetrical molecular topology design strategy based on 2,2‐Bis(4‐allyloxy‐3,5‐dibromophenyl)propane (BADBP) polymerization network. The diallyloxy symmetric structure of BADBP bridges and constructs a 3D crosslinked network, effectively repairing the pore defects in the poly(vinylidene fluoride‐co‐hexafluoropropylene) matrix, achieving an ultrathin thickness of 6 µm with high mechanical robustness and uniform ion channels. The bromophenyl groups in BADBP reduce the crystallinity of the matrix via steric hindrance effects, while the high bond energy of C─Br bonds endows the electrolyte with exceptional thermal stability. Moreover, bromine atoms electrostatically anchor TFSI⁻ anions, promoting lithium salt dissociation and forming a LiF/LiBr‐rich interphase layer. As a result, the modified Li||LiNi0.8Co0.1Mn0.1O2 cells demonstrate stable cycling at both room temperature and 60 °C, along with 5C fast‐charging capability. The pouch cell further passes nail penetration and high‐temperature safety tests. This work establishes a design paradigm for designing high‐performance ultrathin SPEs in lithium‐metal batteries.