Research Of Sulfur/Carbon Composites With Net-work Structure As Cathode Materials For Lithium Sulfur Batteries

Lithium-sulfur battery (Li-S) is regarded as one of the most promising systems fornext generation batteries due to sulfurs high theoretical capacity of1675mAh g-1and highenergy density of2600Wh kg-1. However, several problems are challenging the practicalrealization of Li-S batteries. The insulating nature of sulfur and lithium sulfide causes lowactive material utilization. A shuttle phenomenon resulted from the dissolution ofpolysulfides in the electrolyte leads to low specific capacity and poor cyclability of Li-Sbatteries. In this thesis, three carbon/sulfur (C/S) composites with net-work structure havebeen successfully synthesized and explored as cathode materials for Li-S batteries to pursueimproved electrochemical performance.A high S content (82wt%S) P-AB@S composite with core–shell structure has beensuccessfully synthesized by in situ depositing S onto the surface of electric conductiveacetylene black (AB) with polymer (polyethylene glycol, PEG) chains grafting. The PEGchains are inserted into the sulfur-layer to reinforce material structural stability. Moreimportantly, with a cathode containing66wt%sulfur and approximately3mg cm-2sulfurloading on electrode, the Li-S cell with P-AB@S composite delivers a specific capacity of577mAh g1after500cycles at100mA g1between1.5V and2.8V. Moreover, thepreparation method is facile, cost-efective and is easily subjected to mass production.A carbon fiber cloth/sulfur (CFC/S) composite has been synthesized containing highsulfur loading (6.7mg cm-2). High sulfur loading and high capacity per area (>7mAh cm-2)of composite cathode are beneficial for achieving high energy density of Li-S batteries. It isproved that in-situ forming polysulfides in electrolyte can suppress the further dissolutionof polysulfides and improve the cycle performance of Li-S batteries. The CFC/S compositemaintains a reversible capacity of1100.4mAh g-1after56cycles with a retention rate of96%. Moreover, the low-cost CFC/S composite can be easily produced in large scale,showing its great potential in the practical application of high energy density Li-S batteries.A hollow spherical HSCW@S composite with net-work structure is synthesized byusing carbon nanotubes (CNTs) as conductive framework. The surface of CNTs is modifiedby PEG. Sulfur has been deposited on the surface of carbon nanotubes byoxidation-reduction method in the solution. Then the sulfur-deposited CNTs twine to form a hollow spherical structure. This template-free method is a simple, cheap. The Li-S cell withHSCW@S composite maintains a reversible capacity of887.5mAh g-1after40cycles at100mA g-1current density. Moreover, reversible capacities of618.2and578.1mAh g-1areobtained at500and1000mA g-1, respectively. The excellent performance is ascribed to thegood electric conductivity of CNTs and the unique hollow spherical structure with enoughspace for confining sulfur and trapping polysulfides.