Structural Design,Preparation Modification And Electrochemical Performance Of Carbon/Sulfur Composite Cathode Materials For Li-S Batteries

In order to meet the demands for the rapid development of the electric portable tools with high power and the electric vehicles,it's an urgent problem to exploit a new generation of energy storage system with high energy and high power.Lithium-sulfur batteries have gradually attracted more attention due to its high theoretical capacity of 2600 wh kg-1,rich resources,low cost and environmental friendliness.However,its low utilization,low recycle-stability,low multiplying power,and low security performance restrained Li-S battery from a large-scale use.The solution is to improve conductivity of the active material and suppress effect of polysulfide dissolution on the shuttle.Therefore,the success of this research and development lies in the better design and modification of composite cathode materials which can prevent the dissolution of lithium polysulfide in the electrolyte and improve electrical conductivity of sulfur cathode.A series of carbon/sulfur composites were designed and modified and their electrochemical characteristics were systematically investigated.The main innovations of this dissertation are as follows:Carbon-sphere/sulfur composites with hollow structure were prepared by simple hydrothermal method.The pore structure of the obtained hollow carbon spheres was activated and controlled by KOH immersion and hydrothermal process.It was found that the vacuum treatment can significantly increase the sulfur content for hollow carbon spheres.The hollow carbon spheres can provide enough space to accommodate the sulfur,but also achieve effective conductive networks.The internal space of the carbon sphere can limit the volume expansion of the sulfur.The resulting polysulfides are confined to avoid the "shuttle effect",increase sulfur utilization and enhance electrochemical performance.Activated HCS-3 by hydrothermal process reveals abundant pore structures and HCS-3/S sulfur content is 68%.The HCS-3/S exhibits the initial discharge specific capacity of 1171.5 mAh g-1 at 0.1 C rates.It still maintained of 1016.2 mAh g-1 and the capacity retention of 86.7%cycled after 100 cycles.Carbon materials derived from bean dregs with rich pore structures and high surface areas were obtained.The influences of hydrothermal activation and solid-phase activation treatment on the morphology and pore structure were investigated.The ratio of micropores increases with the activation increase,expecially for the DZC-5 material showing a fine micro-nano structure.The initial discharge capacity of DZC/S-5 material revesls 1238.9 mAh g-1(0.1 C),and the specific capacity of 871.3 mAh g-1 cycled at 0.2 C after 100 cycles.The activated carbon derived from bean dregs was activated by hydrothermal method using KOH,and further nitrogen-doped by adding formaldehyde coupled with spray deposition.The addition of formaldehyde can promote the formation of order struction for carbon materials.The order rod-shaped carbon arrays diaplays a rich interstices and hollow pore structure,with the high specific surface area of 1625 m2 g-1.Carbon arrays doped with the nitrogen could effectively restrain the migration of polysulfide by chemical adsorption.The modified N-OCNA/S composites exhibited remarkable cycle stability and rate capability with discharge specific capacity of 1146 mAh g-1 at 0.1 C,and the capacity maintainance of 1192 mAh g-1 and the capacity retention of 82.4%at 0.2 C after 100 cycles.A composite material(CNT@MnO2/S)which MnO2 nanoparticles were in situ coated on the carbon nanotube via hydrothermal reaction.When it was used as sulfur matrix,the CNT@MnO2/S electrode showed excellent cycling stability.At the same time,when it was used CNT@MnO2 as a coating layer on the commercial separators and combined with S electrode to assemble the cell,it exhibit the discharge specific capacity of 1255.9 mAh g-1,and the capacity still maintain 1061.3 mAh g-1 after 100 cycles at 0.2 C.Compares with the electrochemical performance of cell which used the separator without the CNT@MnO2 layer,the functions of CNT@MnO2 layer in electrode for promoting the electrochemical performance of Li-S batteries were preliminary researched.A wrapped carbon nanotube microspheres were designed and prepared by ultrasonic atomization,which could effectively relieve the volume expansion of sulfur during the reaction process and significantly increase the conductivity of the material.Moreover the microspheres are doped with nitrogen by the addition of ethylenediamine.The nitrogen-doped carbon microspheres with the high content of 72.1 wt%,It exhibit the discharge specific capacity of 1241.3 mAh g-1,and the capacity retention rate of 86.4%at 0.2 C after 200 cycles.the specific discharge capacity of NSCNT/S decreased from 1022.7 to 396.9 mAh g-1 with the current density increased form 0.1 C to 1 C.But their Coulombic efficiency was maintained about 97%after 600 cycles.