Novel design and facile synthesis of porous carbon nanocomposites for lithium sulfur battery

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Rechargeable energy storage devices are being seen as having a crucial role in the powering of myriad portable electronic devices and hybrid electrical vehicles. The composition, morphology, structure and preparation method can affect the electrochemical performance. The properties of electrode materials are of extreme significance for the electrochemical performances of lithium-sulfur batteries. Lithium-sulfur battery based on sulfur cathode has the advantages of high specific capacity, high energy density, low lost and natural abundance of sulfur. These advantages over conventional lithium-ion batteries have driven researchers to make a lot of efforts to understand the redox mechanisms and improve the electrode performance. In order to fully realize the potential of lithium-sulfur battery and to approach commercialization, there are still many problems to overcome. Among them are i) low conductivity nature of sulfur and the discharge product, ii) lithium polysulfide intermediates dissolution and shuttle phenomenon, iii) volumetric expansion upon discharge and iv) lithium metal dendrite formation on anode side. In this thesis, the work is mostly focused on the cathode materials in order to address the first two problems. In the first part of the thesis, we report a rational design and synthesis of 3D hybrid–porous carbon with a hierarchical pore architecture for high performance supercapacitors. It contains nanopores (< 2 nm diameter) and mesopores (2 – 4 nm), derived from carbonization of unique porous metal organic frameworks (MOFs). Owning to the synergistic effect of micropores and mesopores, the hybrid-porous carbon has exceptionally high ion–accessible surface area and low ion diffusion resistance, which is desired for high performance Li-S battery. The initial and 50th cycle discharge capacity 3D hybrid–C/S sulfur cathode are as high as 1343 mAh g⁻¹ and 540 mAh g⁻¹ at a high current rate of 0.5 C. 3D hybrid–C@graphene nanocomposites were prepared by a hydrothermal and chemical vapor deposition method. When applied as an cathode material in lithium-sulfur batteries. 3D hybrid–C@graphene/S nanocomposite exhibited a high lithium storage capacity of 907 mAh g⁻¹. The materials also demonstrated an excellent high rate capacity and a stable cycle performance. 3D porous MXene/S sulfur composite was successfully prepared that demonstrated high reversible capacity, high Coulombic efficiency and excellent cyclability. MXene@graphene/S hybrid sulfur composite were synthesized using a chemical hydrothermal method, exhibiting a high specific capacity of 1284 mAh·g⁻¹ with a superior cycling stability and high rate capabilities. The synthesis of 3D hybrid–C@MXene encapsulated sulfur composites were employed as cathode materials for Li-S batteries. The sulfur composite cathodes delivered a high specific capacity of 1378 mAh/g at 0.1 C current rate and exhibited a stable cycling performance. A rational synthesis of the novel porous N-Ti₃C₂Tₓ nanosheets via the electrostatic self-assembly and effective nitrogen doping process, achieving the porous-modification and nitrogen group surface decoration for porous N-Ti₃C₂Tₓ nanosheets, synchronously. The porous N-doped MXene nanosheets/sulfur composites electrodes with a high sulfur loading exhibits low polarization, stable cycling performance, and excellent rate capability, compared with mixed Ti₃C₂Tₓ/S electrodes showed electrochemical performance, including a high reversible capacity of 1144 mAh g⁻¹ at 0.2 C rate, a high level of capacity retention of 950 mAh g⁻¹ after 200 cycles and good cycling stability with a high sulfur loading of 5.1 mg cm⁻² and high current rate of 2 C rate.
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