Design And Synthesis Of Carbon-Based Cathode Materials For High-performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (Li-S batteries) are very appealing rechargeable batteries for energy storage and powering electric vehicles with high specific capacity of 1672 mAh g-1 and high energy density of 2567 Wh kg-1. The merits such as low costs, non-toxicity and abundance in nature of sulfur bring extra benefits to their market potential. However, the practical use of Li-S batteries is still hindered by several issues including the low utilization of sulfur, poor rate capability and short Iifespan. The insulating nature of sulfur and solid-state discharge products and polysulphide dissolution in organic electrolyte are generally considered responsible for the above issues. To overcome the drawbacks of a sulfur cathode, on account of physical confinement and chemisorption,3D carbon frameworks with hierarchical porosity and hollow carbon coated manganese dioxide nanostructure were fabricated. Benefited from significantly enhance structural stability and kinetics for lithium storage, these materials exhibit excellent electrochemical performance when evaluated as the cathode materials in Li-S batteries. The details work includes:1) An efficient strategy has been developed for the fabrication of sulfur/hierarchical porous carbon framework nanocomposite with sulfur loading of 65 wt.% by metal-assisted polymer-blowing process, followed by acidic etching and sulfur infiltration. In virtue of the unique nanostructure, where the microporous/mesoporous acts as the sulfur reservoir and the nanoreactor for a more complete redox process, prevents the discharge products from escaping, the macropores facilitate transportation of electrolyte and solvated lithium ions during the cycling process. Meanwhile, the three-dimensional carbon structure provides continuous electronic access. When evaluated as cathode of Li-S batteries, they exhibit high durability and excellent rate capability. The initial discharge capacity up to 950 mAh g-1 at 0.5 C and 620-750 mAh g-1 at 2-5 C, respectively. Remarkably, high capacity retention of over 70% for as long as 500 cycles can be still remained.2) Hard template method were employed for nanocasting of hollow carbon coated manganese dioxide nanostructure combining polar metal oxide (MnO2) with highly conductive carbon material. In order to increase the sulfur loading, hollow nanostructure were introduced. At last, sulfur loading of the composite is up to 70 wt.%. The coated carbon prevents the discharge products from escaping the confined space, and the strongly covalent stabilization of sulfur and its discharge products on manganese dioxide, both synergistically reduces the loss of active material, and further enhance the cycling stability of the electrode. Therefore, the hybrid MnO2@C/S composite exhibit excellent lithium storage performance with a high capacity of 854.4 mAh g-1 at 0.1 C. At an higher current density of 1 C, high capacity retention of over 360 mAh g-1 can be still obtained after 500 cycles, which exhibits excellent cycling stability.