Carbon-Based Sulfur-Containing Cathode Materials For Rechargeable Lithium-sulfur Batteries

Rechargeable lithium–sulfur batteries are promising candidates for advanced energy storage areas due to their high energy density and low cost. Nevertheless, commercial applications that utilize lithium–sulfur batteries have not been very successful because of the low electrical conductivity of sulfur, dissolution of lithium polysulfides in organic electrolyte, and volume expansion of sulfur during discharge. Carbon-based sulfur-containing materials prepared by carbon materials and sulfur can improve the performance of sulfur cathodes. However, their cycle stability and rate performance were not satisfactory. To obtain high-performance sulfur cathode materials, carbon-based sulfur-containing materials with various structure were designed and fabricated, the role of advanced carbon structure in the lithium-sulfur batteries has been studied, and the reduced graphene oxide(RGO) coating and amorphous carbon coating were designed to improve the performance of sulfur cathodes in this dissertation. The main content were summarized as follows:(1) A thermally exfoliated graphene nanosheet(TG) with a large specific surface area, high pore volume, excellent conductivity was used to prepare a TG–S stack-up nanocomposite. RGO was coated on the TG–S nanocomposite through a liquid process. In this rational design, the stack-up TG and RGO coating can provide effectively conductive network for sulfur and lithium polysulfides. The stack-up TG with high volume can provide enough space for sulfur expansion. Besides, the stack-up structure and RGO coating layers can also accommodate the lithiation-induced strain of sulfur. Furthermore, the RGO coating layers can efficiently entrap the lithium polysulfides that diffusion out of the stack-up TG. Thus, the as-prepared RGO-TG-S nanocomposite can serve as a nanoelectrochemical reaction chamber for sulfur cathodes. A reversible capacity of 667 m Ah g-1 was observed after 200 cycles at a high rate of 1.6 A g-1 and a high coulombic efficiency(96%) was achieved at the same rate.(2) A series of carbon nanotubes(CNTs) with different pore sizes were used to prepare the CNT-S nanocomposites. The results showed that the pore size of CNTs had important influence to the performance of CNT-S nanocomposite. The RGO coating layers were coated on the surface of CNT-S nanocomposites through a hydrothermal process. The RGO coating layers can improve the electrical conductivity of the sulfur on the surface of CNTs. In addition, the RGO coating layers can also restrain the outward diffusion of lithium polysulfides. The three-dimensional structure constituted by RGO layers and CNTs can provide enough space for sulfur expansion. Thus, the RGO-CNT-S materials have good lithium-sulfur battery performance. Results showed that the pore size of CNTs had important influence to the performance of RGO-CNT-S nanocomposites.(3) Commercial activated carbon(AC) with abundant micropores, excellent conductivity, high surface area, large volume, and strong adsorption property was used to prepare AC-S nanocomposite. The amorphous carbon derived from hydrothermal carbonization of glucose was used to improve the performance of AC-S nanocomposite. The carbon framework constituted by AC and amorphous carbon coating layers can provide an excellent conductive network for sulfur and lithium polysulfides. By combining the narrow channels of AC with amorphous carbon coating layers, the outward diffusion of lithium polysulfides was effectively suppressed. Furthermore, the amorphous carbon coating layers can also provide a buffer region in the amorphous carbon coated AC-S(C-AC–S) nanocomposite to accommodate stress and volume changes during the discharge and charge processes. The role of the glucose concentration in the performance of C-AC-S nancomposite had been studied. The C3.5-AC-S nanocomposite showed a reversible capacity of 590 m Ah g-1 after 300 cycles at a rate of 1.6 A g-1. Furthermore, A series of biological activated carbon were used to prepare carbon-sulfur nanocomposites. Results showed that the sisal fiber carbon-sulfur(SFC-S) nanocomposite had the most excellent performance. The SFC had the suitable microporous structure, ultrahigh surface area, and large pore volume, thereby the sulfur existing in the micropore of SFC and providing enough nanospace for sulfur expansion. In addition, the suitable microporous structure can provide excellent conductive network and control the lithium polysulfides diffusion.(4) CMK-3-S and CMK-8-S nanocomposites were prepared using ordered mesoporous carbon CMK-3(two-dimensional hexagonal structure) and CMK-8(three-dimensional cubic structure) as sulfur matrixes. Results showed that the three-dimensional cubic structure mesopore structure was advantageous for controlling the lithium polysulfides diffusion. Furthermore, the RGO coating layers and amorphous carbon coating layers were used to improve the performance of these CMK-S nanocomposites. Results showed that the CMK-3-S nanocomposite with amorphous carbon coating layers had the best performance. This nanocomposite delivered a reversible capacity of 621.3 m Ah g-1 after 200 cycles at a rate of 0.8 A g-1.(5) One-dimensional mesoporous carbon nanofiber(MCNF) with bimodal mesoprous structure, large volume, and high surface area was prepared by an easy self-template strategy. Comparing with the MCNF carbonized in lower temperature, the MCNF carbonized in high temperature had graphitic pore wall, which is advantageous for sulfur cathode performance. Furthermore, the RGO coating layers and amorphous carbon coating layers were employed to improve the performance of MCNF-S nanocomposite. Due to the one-dimesional structure was destroyed during coating, all the coating layers reduced the capacity. It is worth noting that C-MCNF-S nanocomposite with amorphous coating layers had excellent cycle performance. This nanocomposite demonstrated a reversible capacity of 583.7 m Ah g-1 after 300 cycles at a rate of 0.8 A g-1.