Characterization of Gel Polymer Electrolytes Based on Poly (Ionic Liquid)-Glyme Mixtures for Lithium Metal Batteries

Abstract

Solid polymer electrolytes (SPE) have generated an extensive and sustained interest for its application in lithium metal batteries.[ 1 ] Compared with liquid electrolytes, solid polymer electrolytes have higher safety and thermal stability, since they can provide a physical barrier layer to prevent efficiently lithium dendrite growth and avoid thermal runaway under high temperature or impact. Despite the substantial benefits, some limitations remain to be improved, such as a low ionic conductivity at room temperature and low transference number. Several studies are being conducted to overcome these weaknesses and develop new generation of solid polymer electrolyte lithium metal batteries.[ 2 ] Most of the research on SPEs is focused on polyethylene oxide (PEO) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and its derivative. In general, PEO-based solid electrolytes have shown good dimensional stability, good safety, and can mechanically prevent dendritic growth. However, they exhibit low ionic conductivity and rather low cationic transference number (t+) of ca. 0.15. To achieve sufficient conductivity at low temperatures, polymers that are difficult to crystallize are proposed. Ionic liquids (IL) are poor crystal formers; therefore, polymers of IL are potentially good alternatives to PEO for lower temperature conductivity. An Iongel membrane based on the poly(dimethyldiallylammonium) polyDADMA-TFSI poly(ionic liquid) has gained attention in polymer metal batteries due to its favorable mechanical properties and stability against Li-metal.[ 3 ] The present study provides an insight into the properties of free-standing membranes using poly (ionic liquid)-glyme mixtures compared to typical polymer salt blends (PEO-LiTFSI) and PDADMA-LiTFSI. The benefit of tetraethylene glycol dimethyl ether (G4) is highlighted and appears to be the key component to obtain higher ionic conductivity regardless on the type of polymer used. The impact of the increase in LiG4TFSI/poly ionic liquid ratio within the electrolyte on the transference number, ionic conductivity and mechanical properties was further analyzed. For 33-66 wt% composition for PDADMAT-LiG4TFSI and PEO-LiG4TFSI membranes, a good compromise between the physicochemical and electrochemical properties was achieved.