Room Temperature to 150 °C Lithium Metal Batteries Enabled by a Rigid Molecular Ionic Composite Electrolyte

Molecular ionic composites (MICs), made from ionic liquids and a rigid-rod polymer poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT), are a new type of rigid gel electrolyte that combine fast ion transport with high thermal stability and mechanical strength. In this work, a MIC electrolyte membrane is prepared that is composed of PBDT, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr₁₄TFSI) in a mass ratio of 10:10:80. The ionic conductivity at 25 °C is 0.56 mS cm⁻¹ with no added flammable/volatile components. Although the polymer content is only 10 wt%, this MIC membrane is rigid with a tensile modulus of 410 MPa at room temperature. The MIC membrane remains stable and rigid at 200 °C with the shear storage modulus (G′) only slightly decreasing by 35%. Li/MIC/LiFePO₄ cells demonstrate stable cycling performance over a wide temperature range from 23 to 150 °C. The specific discharge capacity at 100 and 150 °C at 1 C rate exceeds 160 mAh g⁻¹. The discharge capacity retention is 99% after 50 cycles at 150 °C. This stable battery performance shows that this low polymer content MIC membrane qualifies for use as a solid electrolyte in lithium metal batteries operating over a wide temperature range.