Understanding the Performance of Sulfur Electrode Based on Polymer Binders, Sulfur Types and Electrolytes

Due to environmental concerns, it becomes more and more important to find alternative energy sources beyond fossil fuels with increasing demanding of energy. Lithium-ion-batteries (LIB) have drawn significant research attention due to their unique advantages of relatively high energy densities. Applications of LIB have been extended from consumer electronics to large-scale grid energy storage. However, the energy density of currently existing LIB remain insufficient to meet the requirements for applications in electric vehicles (EV) and hybrid electric vehicles (HEV), and thus it is desirable to develop much higher energy density rechargeable batteries with low cost. There has been significant progress in the development of high energy density anode materials such as Si-based alloy etc. over the recent years. However, the low capacity of cathode materials is still one of the major obstacles for developing high-performance batteries. Sulfur has been considered as one of the promising candidates of such cathode material due to its high theoretical capacity (1,675 mAh/g) and its natural abundance. Despite the above advantages, there are still many challenges in developing a practical lithium-sulfur battery for commercialization. As known so far that the problems of lithium-sulfur batteries include: 1) very poor conductivity of sulfur (10^-16 S/m at 293 K); 2) large volumetric expansion upon lithiation (80%); 3) dissolution of the intermediate polysulfides during charge-discharge processes. These problems result in fast capacity fading and low Coulombic efficiency and therefore poor cycle performance of lithium-sulfur cells. Considerable efforts have been devoted to solve the above mentioned problems in the Li-S battery system such as encapsulating sulfur particles with conductive carbon, grapheme oxide, or polymer binder, adding confinement for sulfur particles such as loading sulfur into carbon nanotubes and designing core-shell structure etc.

Conductive polymer, PEDOT, has been used in lithium-sulfur batteries to coat carbon/sulfur particles to minimize polysulfides dissolution and improve the electrochemical performance of Li-S batteries.The advantages of PEDOT with both good electrical conductivity and enhanced affinity for sulfur and polysulfides have been shown. In this study, we used PEDOT as polymer binder and commercial micrometric sulfur particles to study performance of Li-S batteries. The electrochemical performance of PEDOT based sulfur electrode was compared with that of conventional polymer binder of PVDF based sulfur electrode. Meanwhile, the impact of particle size of sulfur on the electrochemical performance was also studied by comparing commercial micrometric sulfur electrode and synthesized nanometric sulfur electrode. Finally, electrochemical performance of sulfur cells with high viscosity electrolyte and low viscosity electrolyte was compared to study the lithium ion diffusion impact on the polysulfides dissolution.

In this study, the electrochemical performance of lithium-sulfur cells with different polymer binder (PVDF vs. PEDOT), different particle sized sulfur powder (micrometric vs. nanometric), and different electrolyte (DOL/DME vs. PEGDME) was investigated. We have demonstrated that PEDOT conductive polymer with commercial micrometric sulfur effectively improved the performance of Li-S batteries due to the bifunction of PEDOT with both good electrical conductivity and enhanced affinity for sulfur and polysulfides. The better cycle performance of micrometric sulfur cell than synthesized colloidal nanometric sulfur cell could be ascribed to the relatively lower sulfur utilization and better prevention of polysulfides dissolution. Meanwhile, the better performance of cells with PEGDME electrolyte than that of DOL/DME electrolyte is due to the larger viscosity, lower particle mobility, and lower polysulfides dissolution. The increased conductivity by conductive polymer and the electrolytes play important roles in the better cycle performance.