Competition‐Coupling Trade‐Off of Supramolecular Interactions in Janus Composite Quasi‐Solid Electrolytes Enables High‐Safety and Long‐Life Lithium Metal Batteries

The Li_1+xAl_xTi_2-x(PO₄)₃ (LATP)-polymer composite solid electrolyte offers environmental stability and safety for high-energy lithium metal batteries (LMBs), yet suffers from interfacial instability and high interfacial resistance. Herein, a Janus self-supporting skeleton (J-SSK) is engineered via multi-scale coupling of poly(vinylidene fluoride-trifluorethylene) (PVDF-TrFE), LATP, 2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl) ureido) ethyl methacrylate (UPyMA) monomer, where intermolecular multiple hydrogen bonds reinforce mechanical robustness while the Janus structure isolates LATP from direct Li contact. In situ copolymerizing vinylene carbonate (VC) and UPyMA monomer in J-SSK to construct Janus composite quasi-solid electrolyte (J-CQSE) achieves seamless integration of electrode/electrolyte interfaces and establishes hierarchical coupling across J-SSK, polymer matrix, and lithium salts. The resultant J-CQSE demonstrates exceptional flame retardancy, high room-temperature ionic conductivity (1.2 mS cm^-1), and superior Li⁺ transference number (0.75). The trade-off of its multi-scale coupling and competitive hydrogen bonding is realized, contributing to stable LiF/Li₃N-rich solid electrolyte interphase (SEI) and Li plating/stripping with persistent dendrite suppression. After 1000 cycles at room temperature, the LiFePO₄/Li full cell delivers capacity retentions of 89.5% at 1C, 98.9% at 4C, and 75.2% at 10C, respectively. The LiNi_0.8Co_0.1Mn_0.1O₂/Li cell retains 72.9% capacity over 100 cycles at 0.2C. Importantly, the LFP/J-CQSE/Li pouch cell passes rigorous mechanical/thermal abuse and combustion tests, validating its practical viability for advanced safe LMBs.