Studies On Lithium Sulfur Batteries Eabled By Super-aligned Carbon Nanotubes

Lithium-sulfur(Li-S) batteries are extensively studied as extremely promising candidates for high capacity energy storage system, with their high theoretical capacity(1675 m Ah/g), high energy density(2600 Wh/kg), low cost and environmentally friendly nature. Despite of the immense potential of Li-S batteries, a series of bottlenecks must be solved befor e its large-scale production, including:(1) the insulating nature of sulfur and the final discharge product;(2) the high solubility of the intermediate polysulfides, which resulted in shuttle effect;(3) the volume variation and structure failure during cycling.To solve the above problems, a sulfur/carbon composite based on super-aligned carbon nanotubes(SACNTs) was designed and synthesized, and the electrochemical effect of sulfur content and surfactant was examined. For battery improvement, a novel hierarchical cathode structure of sulfur/nano-graphite/graphene/SACNT was p roposed and synthesized, followed by the analysis of effects of these carbon materials. Here come the main conclusions:(1)A novel, flexible sulfur/SACNT cathode was synthesized by the method of in-situ deposition in liquid phase and vacuum filtration. In this structure, SACNT provided a 3D conductive network as well as a mechanical supporting frame, which trapped the sulfur particles and intermediate polysulfieds, while the open structure facilitated electrolyte ingress throughout the whole electrode.(2)Surfactant and sulfur content significantly influenced the electrochemical performance of sulfur/SACNT cathode. The sample with 35% sulfur loading delivered high initial discharge capacities, that were ~1589 m Ah/g at 0.5C, ~1294 m Ah/g at 1C, ~1007 m Ah/g at 2C and ~742 m Ah/g at 5C, respectively. After 200 cycles at 1C, the sample could still maintain a capacity of ~438 m Ah/g.(3)A novel, hierarchical structure of sulfur/ nano-graphite/graphene/SACNT was designed and obtained. Nano-graphite offered a nano-scaled conducting path, while trapping sulfur and polysulfides by its pores and large specific surface area. The SACNT offered a micron-scaled conducting path, with nano-graphite and sulfur anchored on the tube surface, combining the nano- and micron-scaled conducting network. And the graphene provided an encapsulation structure by wrapping the SACNTs, nano-graphites and sulfur particles, reducing the solubility of the polysulfide. Lastly, a layer-by-layer structure was observed in the vertical direction, accomm odating the volume change during the cycle test.(4) The hierarchical structured sulfur/ nano-graphite/graphene/SACNT showed excellent ultra-long cycling performance, with an initial discharge capacity of ~951 m Ah/g at 1C, and ~381 m Ah/g after 1000 cycles, resulting in an extremely low decay rate(0.0599% per cycle). Thus such hierarchical structured composite is of great promise for high-rate, ultra-long-cycle-life rechargeable batteries.