Robust asymmetric poly(ionic liquid)-based electrolyte with a compatible cathode interface enabling 4.5 V solid-state lithium metal batteries

The high energy density of Ni-rich layered oxides renders them promising cathodes for advanced solid-state lithium metal batteries. However, interfacial instability significantly hinders their widespread application, especially at high cut-off potentials. Herein, a robust polyethylene supporting poly(ionic liquid)-based asymmetric electrolyte (PEACE4-4) with excellent cathode and anode interfacial compatibility is developed. This is achieved by using tris(pentafluorophenyl)borane as a functional additive and an asymmetric polyethylene membrane, modified with polymethyl methacrylate and Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 , as the substrate. The electrolyte exhibits a tensile strength of 24 MPa and an elongation of 80%, achieving the balance between mechanical and electrochemical properties. Theoretical calculations indicate that tris(pentafluorophenyl)boron exhibits a higher adsorption capacity on the LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) surface, leading to a cathode electrolyte interphase rich in LiF and Li 2 CO 3 , while its solvation regulation simultaneously suppresses reactions on the anode side. The optimized electrolytes enable stable operation of Li/NCM811 cells at a high voltage of 4.5 V. Remarkably, the cell exhibits excellent cycling stability with capacity retentions of 78.4% after 300 cycles and performs effectively within a range from −20 to 60 °C. This asymmetric strategy provides a practical and effective approach for enhancing polymer electrolyte performance and holds promise for all-climate solid-state lithium metal batteries.