Cathode materials for high performance lithium-sulfur batteries

Since the late 20th century, energy crises have acquired worldwide attention. In the last two decades a lot of renewable energy sources have been fully developed and used, including solar energy, wind energy, tide energy and so on. However, the application of these energy sources are hindered by time and space restrictions. For example, solar energy can only be used at day time with relatively clear whether. To make full use of these energy resources, a variety of energy storage devices have been developed. Among them, lithium-ion batteries (LIBs) are the most successful commercialized energy storage devices and are widely used in our daily life, including phones, computers, electric vehicles and so on. However, the energy density of LIBs is hindered by the theoretical specific capacity of the lithium transition metal oxide cathode. Lithium-sulfur batteries (LSBs) with a theoretical specific capacity of 1675 mA h g-1 are regarded as the most promising next generation energy storage devices. But several obstacles, including the low conductivity of S and Li2S, the big volume change of S during charge and discharge and the notorious shuttle effect, stand in the road of commercialization of LSBs. In the thesis, two different strategies have been applied to solve these problems. First, ZIF-67, one kind of metal-organic framework (MOF), was used as a template to synthesis porous carbon frameworks. The carbon frameworks were used as a S host to accommodate the volume change of S and improve the conductivity of the electrode. What's more, the Co centers in ZIF- 67 transferred into cobalt phosphide and cobalt sulphides, based on the detailed experiment condition. Cobalt phosphide and cobalt sulphides with high catalyst activity accelerate the reactions in the electrodes and alleviated the shuttle effect and thus improved the electrochemical performance. Second, sulfurized poly acrylonitrile (SPAN) was used as a source of S for LSBs. The covalent C-S bonds in SPAN alleviated the shuttle effect through reducing the formation of lithium polysulfides. Carbon nanotubes (CNTs) and Se-doping further improved the electrochemical performance of SPAN through improving the conductivity and accelerating the reactions. Samples with different levels of Se-doping were synthesized and characterized to find the best conditions. Meanwhile, the structure of the as-synthesized SPAN samples was characterized by a variety of methods to gain some insight about structure of SPAN, which is a subject of debate among researchers. Through these two strategies, the shuttle effect in LSBs was reduced and the performance of LSBs were improved. A higher specific capacity and a better cyclic stability were achieved. At the same time, a better understanding of the mechanism of LSBs was gained.