Flexible PVA/BMIMOTf/LLZTO composite electrolyte with liquid-comparable ionic conductivity for solid-state lithium metal battery

Poly(vinyl alcohol) (PVA)/1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIMOTf)/Li6.4La3Zr1.4Ta0.6O12 (LLZTO) solid-state composite electrolyte (SSCE) membranes were synthesized for solid-state lithium metal battery application. The garnet-type LLZTO nanoparticles were surface–coated with the polydopamine layer of 8–10 nm thickness to enhance the dispersion status of LLZTO particles in the PVA matrix. The hydrophilic BMIMOTf ionic liquid (IL) was added along with LLZTO nanoparticles to enhance the ionic conductivity and electrochemical stability of the SSCE membranes. The synthesized composite electrolyte membrane containing 7 wt% of LLZTO and 60 wt% of BMIMOTf showed the outstanding Li+ conductivity of 2 × 10−3 S cm−1 and the lithium transference number of 0.76 at room temperature in the firm and flexible solid state with the tensile strength of 8 MPa. Such a high single ion conduction characteristic led to the quite low interfacial resistance of 39 Ω between the composite electrolyte and the lithium anode. Owing to these superior properties of composite membranes, the LiFePO4|SSCE|Li cell exhibited an excellent discharge capacity of 165 mAh g−1 at 0.2 C, maintaining the coulombic efficiency of 98% after 100 cycles at room temperature.