Sulfur-Rich Hyperbranched Polymers:Synthesis And Application In Li-S Batteries

Li-S battery is a new kind of lithium ion battery, which consists of a lithium metal anode and a sulfur cathode. Li-S battery has outstanding merits such as the high theoretical capacity of 1672 mA·h·g-1, the high theoretical specific energy of 2600 W ·h·kg-1, the abundant reserves and low cost of sulfur. However, intermediate lithium polysulfides would dissolve in the liquid electrolytes and shuttle between the anode and cathode. The shuttle effect, along with the raised resistance of sulfur and low load of sulfur, intensively degraded the stability and increased pointless cost of sulfur-active materials. Therefore, polymers with high sulfur content, high stability and high processability are eagerly expected.In this thesis, sulfur-rich hyperbranched polymers (SRHPs) were synthesized via thiol-ene addition of polymeric sulfur (S8) radicals to 1,3-diisopropenylbenzene (DIB). Benefited from their branched molecular architecture, SRHPs presented good solubility in organic solvents with an ultrahigh concentration of 400 mg·mL-1. SRHPs have high molecular weight with the largest Mn of 5400 g·mol-1 and Mw of 2 3500 g· mol-1. After end-capping by sequential click chemistry of thiol-ene and Menschutkin-quaternization reactions, water soluble SRHPs were obtained.The sulfur-rich SRHPs were employed as solution-processible cathode-active materials for Li-S batteries, by facile fluid infiltration into conductive frameworks of graphene-based ultralight aerogels (GUAs). The SRHPs-based cells showed high initial specific capacities of 1247.6 mA·h·g-1 with 400 charge-discharge cycles. The cells also demonstrated an excellent rate capability and a considerable depression of shuttle effect with stable coulombic efficiency of around 100%. The electrochemical performance of SRHP in Li-S batteries overwhelmed the case of neat sulfur, due to the chemical fixation of sulfur. The combination of high solubility, structure flexibility, and superior electrochemical performance opens a door for the promising application of SRHPs.