In-situ fabrication of an interface-integrated asymmetric bilayer composite solid electrolyte for high-performance lithium metal batteries

Asymmetric composite solid electrolytes (CSEs) represent an emerging class of solid-state electrolytes for advanced lithium metal batteries (LMBs). However, asymmetric CSEs still face challenges including increased interfacial impedance between dissimilar layers, complex fabrication processes, and potential delamination under prolonged cycling. In this study, a precursor solution containing trifluoroethyl methacrylate (TFEMA), vinyl ethylene carbonate (VEC), N , N ′-methylenebisacrylamide (MBA), Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO), succinonitrile (SN), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is injected into the battery to in-situ construct an asymmetric CSEs (MVFS-CSE). LLZTO gravitationally settles and deposits on the lithium metal anode (LMA), forming a mechanically robust ionic conductive barrier that resists dendrite intrusion. In-situ thermal polymerization of double-bonded monomers generates an oxidation-resistant polymer layer laminated to the high-voltage LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathode, simultaneously eliminating distinct interlayer boundaries. MVFS-CSE demonstrates a high ionic conductivity of 5.02 × 10 −4 S cm −1 and a lithium-ion transference number of 0.68 at ambient temperature. Li/MVFS-CSE/Li symmetric cell achieves stable cycling for over 1600 h at 0.1 mA cm −2 , and NCM811/MVFS-CSE/Li cell delivers an initial discharge capacity of 169 mAh g −1 at 0.5C with 87.5% capacity retention after 500 cycles. This work demonstrates the outstanding performance of interface-integrated asymmetric CSEs, providing an adaptable approach for developing safe solid-state batteries with high energy density.