Gel polymer electrolytes (GPEs), consisting of polymer matrix, organic solvents and salts, are an appealing choice of electrolytes for advanced energy storage systems as they combine high ionic conductivity, low cost, improved safety, and ease of fabrication into desired shape and size. Nevertheless, it should be pointed out that even with immense efforts directing towards GPEs, the conductivity of GPEs is still limited because the evaporation of the organic solvents in GPEs reduces the volume of electrolyte; thus decreasing the contact area across the electrode and reducing the conductivity. Cross-linked GPE can be a good candidate to remedy this drawback.

In this research, the fiber-reinforced cross-linked GPEs, in which the fibers are held together to form a meshed sheet or web. This 3D scaffold is then loaded with an appropriate electrolyte to create an electrochemically and mechanically stable electrolyte layer for battery application. The combination of polymer and fibers forms a fiber-reinforced polymer composite which possesses a unique blend of electrical and mechanical properties to enhance the performance of these ionic conductors. Reinforcing the polymer material with inert fibers which are not subject to chemical attack improves their shear strength and stiffness.

The fiber reinforcement of GPEs will preliminarily employ a non-woven polyester fabric as a substrate.  It has high porosity and good chemical resistance to organic liquids while maintaining good contact with the polymer; the holes in the cloth also increase the surface area to offer the open spaces for ion transfer. The fiber-reinforced gel polymer based on cross-linked PMMA is fabricated by free radical polymerization. The prepared organic solution and polyester cloth are placed together between two pieces of 0.25” thick optical lenses with heating at 75^oC. The sketch of the sample is shown in Fig. 1a. Upon completion of the fiber-reinforced GPE, the samples are subjected to comprehensive electrochemical testing in order to evaluate the properties of this novel ion conducting electrolyte. The conductivity will be evaluated using potentiostatic impendence spectroscopy based on testing protocols available in the Gamry Reference-600 potentiostat. For sodium-air battery application, the electrolyte is used as the separator between the anode and the cathode as shown in Fig. 1b. The sodium sheet is used as the anode, the reference electrode Ag/AgCl is placed next to the anode while the platinum wire used as the cathode exposes to the oxygen environment. The open circuit potential and cyclic voltammetry tests are adopted for characterizing the electrochemical performance of fiber-reinforced cross-linked GPE in sodium-air battery.

This research has successfully developed fiber-reinforced GPE based on cross-linked PMMA capable of delivering ionic conductivity comparable to that of cross-linked GPE (10^-3 S/cm), while also being a robust separator capable of withstanding the pressure generated while under long term use in sodium-air batteries. The ion transfer thermodynamics are also investigated by activation energy measurement. The electrochemical behavior of sodium-air battery accommodated this light-weight, mechanically stable fiber-reinforced cross-linked GPE indicate that the increased energy density could help the metal-air battery technique to take the leap from bench top to commercial application.