High-Performance Na-Ion Conducting Polymer Gel Membrane for Supercapacitor Applications

The scarcity of Li along with its uneven distribution in earth has become a great challenge in the field of electrochemical energy storage. Replacing Li with Na may address the above issues, as Na and Li are very similar in electrochemical characteristics, and Na is abundant in nature with low cost. Herein, the Na-ion conducting gel polymer membranes comprising a 2-HEC/EMITf/NaTf system are synthesized by employing the solution-casting method. The effects of NaTf salt and EMITf ionic liquid on the electrical and electrochemical properties of membranes are investigated. It is demonstrated that the salt is dissolved in the polymer matrix, thereby raising its ionic conductivity (IC), which is further raised by ionic liquid incorporation. The optimized membrane (2-HEC:EMITf:NaTf = 1:0.6:0.15 in mass, named as GPE/Na-3) exhibits a high room-temperature IC of 1.11× 10^–3 S cm^–1 along with a wide electrochemical stability window of 4.8 V. The membrane also displays sound tensile strength and breaking strain (4.5 MPa and 94.4%, respectively). As the electrolyte, the optimized membrane is combined with graphene electrodes to fabricate a supercapacitor. The supercapacitor presents a high capacitive behavior with an appealing cyclic stability. As evident from the above performance, it is believed that the Na-ion conducting gel polymer membrane possesses potential applications in future Na-ion energy storage devices.