Development of metal&metal oxides decorated graphene-based electrode materials for next generation LI-ION LI-O2 batteries

Official URL: http://risc01.sabanciuniv.edu/record=b2486480_(Table of contents)

Batteries are the global solution for the future energy crisis emerging from depleting fossil fuels and environmental issues. Even though lithium-ion batteries are widely commercialized for powering portable electronics, materials development for their electrodes has never stopped. In this Ph.D. thesis, metal and metal oxides decorated graphene-based electrode materials were developed to sustain long term operation and enhance Li-ion storage capacity. Moreover, Li-O2 as the next-generation batteries were studied to compensate for immense energy demand in the automotive and aerospace industry. A new catalyst material was developed to be used as their porous air cathode partaking in oxygen evolution reactions (OER) and oxygen reduction reactions (ORR). The graphene oxide (GO) utilized in this study was synthesized by the improved Hummers’ method. Then a straightforward, one-step thermal route has been established to fabricate reduced- (rGO) and nitrogen-doped reduced graphene oxide (NrGO) electrodes with remarkable lithium-ion storage properties. The electrochemical properties of the rGO and NrGO electrodes have been extensively compared in a Li-ion half-cell. The NrGO electrodes exhibited a reversible capacity of 240 mAhg-1 at a high current of 10 Ag-1 after 500 cycles of operation with 90 % capacity retention. Further, we have investigated the synergistic effect of NrGO and nanotubular TiO2 to achieve high rate capabilities with high discharge capacities through a simple, one-step and scalable method. First, hydrogen titanate nanotubes were hydrothermally grown on the surface of NrGO sheets and then converted to a mixed phase of TiO2-B and anatase by thermal annealing. The prepared anode showed a stable discharge capacity of 150 mAhg -1 at 1C current rate after 50 cycles. Moreover, we introduced a simple and cost-effective spray-drying method to fabricate a layered (sandwich-like) anode structure using Si nanoparticles (NPs) and rGO. The Si NPs were synthesized by the magnesiothermic reduction of SiO2 nanoparticles. By a scalable and straightforward spraying/drying method, we embedded Si NPs between two layers of rGO sheets. The sandwich-like structure, which successfully contains the expansion of Si particles, protected the anode from detrimental conditions. With this new and uncomplicated production technique, the rGO-Si-rGO anode after 50 cycles, showed a high specific capacity of 1089 mAhg-1 at 1C with 97% coulombic efficiency and a stable cycling performance at current densities up to 5C. Lastly, cerium (IV) oxide (CeO2) nanorods were synthesized by hydrothermal treatment and supported on NrGO by another hydrothermal step. Herein, CeO2/NrGO catalyst materials were studied as a Li-O2 cathode using an aprotic electrolyte, which includes lithium iodide (LiI) as a redox mediator. The results showed that the novel catalyst hybrid of CeO2 and NrGO with LiI directly increased the electrochemical performance of Li-O2 battery. Their synergetic effect improved the kinetics of OER and ORR. The impact of LiI on CeO2/NrGO by comparing bare NrGO air cathode was investigated for the first time in this study. The addition of LiI decreased the overpotential up to 0.78 V in CeO2/NrGO air cathode. CeO2/NrGO were tested at the different current densities and revealed a maximum capacity of 5040 mAhg-1 at 25 mAg-1 current density.