Versatile Molecular Engineering of <i>In Situ</i> Cross-Linked Multifunctional Electrolytes for Long-Lifetime and Safe Semisolid Lithium Metal Batteries

The practical application of semisolid lithium metal batteries is impeded by inadequate ionic conductivity, suboptimal oxidation/reduction stability, and safety concerns of the electrolyte. Herein, a versatile molecular engineering strategy is proposed to construct a robust polymer framework for semisolid electrolytes, which creates highly compatible cross-linked networks by the in situ gelation of concentrated succinonitrile-based plastic crystal electrolytes and multifunctional nitrogen- and fluorine-rich monomers. This strategy allows the electrolyte to promote rapid Li-ion transpsort through weak coordination with the polymer segments. Meanwhile, the strong interactions between the polymer matrix and succinonitrile enhance their mutual solubility, reduce the crystallinity of succinonitrile, and establish fast ion-conductive pathways. The resultant electrolyte induces the formation of LiF/Li₃N-rich solid electrolyte interphases and achieves uniform lithium deposition behaviors. Moreover, it mitigates fire risks by cothermally decomposing to produce fire-extinguishing gases (CO₂ and NH₃) and leveraging the nonflammability of succinonitrile. Significant improvements in electrochemical performance have been observed in Li symmetric, Li||LiFePO₄, and Li||LiNi_0.8Co_0.1Mn_0.1O₂ cells both at room temperature and high temperature (60 °C). As a demonstration model, this molecular engineering strategy has been successfully applied to enhance thermal stability and safety in Li||LiNi_0.8Co_0.1Mn_0.1O₂ pouch cells, offering a promising solution for semisolid lithium metal batteries under extreme conditions.