Development of nanofiller reinforced composite membranes for lithium sulfur batteries

The widespread use of electric vehicles has required the development of high energy density energy storage systems with suitable size and driving range. Among the emerging battery systems, lithium sulfur (Li-S) batteries attract attention especially for applications requiring large capacity and long battery life, as they have 5 times the theoretical energy density than state-of-the-art Li-ion batteries. One of the most important factors that cause capacity loss in Li-S batteries is the dissolution of the polysulfides in the liquid electrolyte and their shuttle effect between the anode and the cathode. For this reason, recently, the use of ion exchange membranes instead of liquid electrolyte in order to ensure capacity retention in Li-S batteries has been the subject of research. Nafion and Aquivion, which are proton exchange membranes, can be used for this purpose. These polymers consist of perfluoroether side chains containing sulfonic acid groups attached to the perfluoroethylene backbone. By lithiation of these polymers, it is possible to use as Li+ ion exchange electrolyte and a separator in batteries. In this work, lithiated Nafion and lithiated Aquivion based, Al2O3 nanofiller reinforced hybrid composite ion-exchange membranes have been developed. In the studies, firstly, Nafion and Aquivion ionomers in solution were lithiated and produced in powder form. Using these powders, Li-Naf, Li-Aqu and by mixing these ionomers in certain proportions Li-Naf:Li-Aqu hybrid membranes were prepared in film form. Later, hybrid composite membranes were produced by adding Al2O3 nanofiller to Li-Naf:Li-Aqu/1:2 composition, which has the highest conductivity and electrochemical performance as a result of the structural and electrochemical characterizations carried out. The ion exchange capacity, polysulfide transition and swelling behavior of the produced membranes were examined and the structural characterization was carried out by tensile test, FTIR, XRD, TGA and FESEM. CR2032 half cells were designed in order to perform electrochemical analysis. Electrochemical performances of membranes were examined by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge tests. At the end of the studies, it was observed that the hybrid membrane obtained by using lithiated Nafion and Aquivion at a ratio of 1: 2 offers higher electrochemical performance. The best electrochemical performance was achieved when this membrane was reinforced with %1 Al2O3 nanofiller.