Crystalline Porous Materials and Their Applications in Electrochemical Energy Storage Devices: Lithium Batteries and Li-Sulfur Batteries

Monday, 29 May 2017: 15:20
Grand Salon C - Section 15 (Hilton New Orleans Riverside)
Z. Zhang (Nagoya University), K. Awaga (RCMS Nagoya Univ.), and Y. Wu (Nagoya University)
The development of new electrochemically active materials towards the application of battery electrodes is still one of the most intriguing topics in energy-related science. A good electrode material requires not only a large energy storage density, but also high permeability to electrolytes and robustness during the electrochemical process. From the perspective of synthetic material science, we chose highly crystalline porous materials, including metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) as the target of research. The crystallinity of materials guaranteed the stability upon the electrochemcial treatment, while the porosity allowed the penetration of electrolytes and a high mobility of ions in the materials. Towards Li-ion batteries, we could achieve the electron accepting ability on both metal cation clusters and organic linkers of MOFs with proper design strategies and a total capacity that is comparable with major commercialized batteries has been obtained. A series of physical measurements were performed and resulted an insight to the mechanism of the electrochemical processes, hence the discovery of a "bipolar charging" mechanism that has not been observed for traditional Li-ion batteries. Such mechanism utilized the insertion of both Li-ions and anions from the electrolyte, which alternates the doping feature and the ionic conductivity of MOFs in electrochemical steps. We also examined the potential of these porous materials as the substrate of sulfur in the cathode of Li-S batteries, as they can easily separate sulfur into nanoparticles through a simple adsorption approach. We performed studies on a series of isoreticular MOFs, UiO-66-68, which adopt same structural topology and the only difference is pore size and pore volume. The C-rate dependent electrochemcial behavior of sulfur impregnated MOFs were analyzed and suggested the importance of pore size. Furthermore, we prepared MOFs and COFs with functionalized sulfur binding groups, and the linkage between the sulfur and porous frameworks generated a new type of polymer, namely MOF/COF-graft-polysulfides. These new type of polymers were applied as the cathode materials of Li-S batteries, and the binding groups sucessfully increased the cyclic performance of Li-S batteries referencing with non-functionalized MOFs/COFs.