Effect of Composition of Low-Molecular-Weight Plasticizers on the Properties of Gel Polymer Electrolytes for Lithium Metal Batteries Based on Composite Membrane Nafion and Sulfonated Zirconia

Modern studies in the field of lithium metal batteries focus on the development of safe and stable electrolytes with high ionic conductivity and electrochemical stability. In this context, promising materials are gel polymer electrolytes based on perfluorosulfonic acid cation-exchange membranes modified with inorganic fillers and solvated with aprotic solvents. This study addresses Nafion® composite membranes containing surface-sulfonated zirconia (sZrO2) and solvated with binary mixtures of ethylene carbonate (EC) with dimethylacetamide (DMAc), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) at a 1 : 1 ratio by volume. The presented systems demonstrate the dependence of ionic conductivity on the nature of the solvent, following the series: EC–DMAc > EC–DMF > EC–DMSO, with the maximum conductivity (2.2 mS/cm at 25°C) being achieved for the Nafion@sZrO2–EC–DMAc composition. The resulting electrolytes are electrochemically stable up to 4.2 V (vs. to Li+/Li), which makes them compatible with cathode materials such as LiFePO4 (LFP). However, DMSO slowly interacts with lithium metal to form a low-conductivity SEI layer, whereas EC–DMAc and EC–DMF systems provide stable operation of Li|Li cells for >1000 h at a current density of 0.1 mA/cm2. Cyclic tests of Li|LFP cells with Nafion@sZrO2–EC–DMAc and Nafion@sZrO2–EC–DMF electrolytes show an initial discharge capacity of ∼155 mA h/g (0.1C) and high stability—capacity loss after 50 cycles at 0.5C is 4.7 and 2.0%, respectively. The obtained results confirm the potential of using Nafion®-based composite gel polymer electrolytes solvated with amide solvents in new generation lithium metal batteries.