Study On Novel Binders For Sulfur Cathode Materials

Lithium-sulfur batteries are currently one of the most potential battery systems due to their large specific capacity?1675 mAh·g-1? and energy density?2600 Wh·kg-1?. In addition, elemental sulfur also benefits from advantages of natural abundance, low cost and environmental friendliness. However, the electrochemical performance of sulfur electrodes are restricted by?i?the low electronic conductivity of sulfur and its discharge products,?ii?the high solubility of intermediates in the electrolyte and?iii?electrode volume changes during cycling. In order to improve the specific capacity and cycling stability of lithium-sulfur batteries, different binders were used to strengthen the electrode structure and modified the electrode/electrolyte interface. Different effects of binders on the electrochemical performance of lithium-sulfur batteries were explored.In this paper, we first replaced the semi crystalline polymer polyvinylidene fluoride?PVDF? with water-soluble binder LA132. LA132 binder is a multi copolymer with strong polar bond-CN, and the bond strength is stronger than PVDF. After multiple cycles, the surface of the PVDF electrode was covered with the deposition film and the electrode structure was deteriorated. The LA132 electrode still kept the porous structure. Electrochemical tests showed that the LA132 electrode had a better cyclic stability, and the electrochemical performance of the electrodes was improved even when the binder content was reduced to 5 wt%.Based on the above experiments, polyacrylic acid?PAA? with strong polar bonds was used to prepare the conductive composite dispersion with graphene at high temperature. The strong hydrogen bond force formed in the composite relieves the internal forces in PAA and inhibits the stacking of graphene nanosheets. Electrochemical tests showed that the discharge specific capacity of G-PAA electrode was improved to 133% compared with that of PAA electrode. The enhanced electrochemical properties of the electrodes are related to the special structure of the active materials and the conductive binder. The graphene nanosheets were uniformly interspersed in the electrodes, which provide a substrate for electron transfer, and the formation of porous structure is beneficial to the transmission of Li-ion.In order to further improve the cyclic stability of the electrode, PAA and conductive polymer PEDOT:PSS with the adsorption ability of the lithium sulfide were mixed in different ratios. This mixed binder formulation combines the local improvement to the solvent system offered by PAA and the lithium?poly?sulfide-stabilising effect of PEDOT:PSS. When PAA and PEDOT:PSS were mixed at a ratio of 2:3 as binders, the electrode delivered a high initial discharge capacity of 1121 mAh·g-1 at 0.5 C. And after 80 cycles at the same rate, the reversible capacity remained at 833 mAh·g^-1.