In Situ Synthesis Of Sulfur Cathode Materials,Study Of Their Electrochemical Performances And Activation Mechanism

Lithium sulfur battery has many advantages such as high capacity,low cost and environmental benignity,making it promising battery system in future.However,pure sulfur has low conductivity and its intermediate products easily dissolve in electrolyte during charge/discharge cycles.The shortcomings heavily restrict its application and popularization.The presented paper prepared hollow polyaniline/sulfur composite?hPANI/S?,graphene/sulfur/polyaniline composite?G/S/PANI?and graphene/sulfur/silica composite?G/S/SiO₂?,and studied their microstructures,electrochemical performances and potential improvement mechanisms.The detailed results are following:1.Hollow polyaniline/sulfur composite?hPANI/S?was prepared via template,in situ polymerization and other chemical reactions.TG-DSC reveals that sulfur content in the composite is 54.6 wt%.The hPANI/S composite has decreased charge/discharge capacities firstly and then has stable ones in the followed cycles.The composite still has discharge capacity of 601.2 mAh g-1 in the 100 th cycle,which is higher than that of pure sulfur at the same conditions.From microstruture results,hPANI/S composite has smaller sulfur particle size than pure sulfur prepared at similar conditions.The composite has lower cathodic reduction peak than pure sulfur from CV results,indicating the conversion between S and Li2 S occurring easily in the hPANI/S composite.EIS results show that hPANI/S composite has higher conductivity than pure sulfur.In addition,the hollow structure in hPANI/S composite benefits to accommodate polysulfides,and effectively decreases the polysulfides dissolution in the electrolyte and enhances the utilization of the active substance.2.Graphene/sulfur/polyaniline composite?G/S/PANI?was synthesized via in situ deposition and polymerization method.TG-DSC results show that sulfur content in G/S/PANI composite is 75.0 wt%.XRD and SEM results indicate that G/S/PANI composite has smaller sulfur particle size than pure sulfur prepared at the same conditions.The G/S/PANI composite has initial discharge capacity of 371.6 mAh g-1 at 0.1 C.The discharge capacity increases to 756.8 mAh g-1 in the 30 th cycle and presents a stable value in the subsequent cycles.The discharge capacity of G/S/PANI composite is 703.2 mAh g-1 in the 150 th cycle,which is higher than those of pure sulfur and G/S composite at the same conditions.CV and EIS results show that G/S/PANI composite has better electrochemical performance than pure sulfur and G/S composite,confirming the synergetic effect of graphene and polyaniline.TEM results show that sulfur particle of G/S/PANI composite changes to sulfur membrane of one after 30 chare/discharge cycles,which indicates sulfur diffusion occurring on graphene surface during charge/discharge cycles.So the contact area of sulfur and graphene increases enhance the conductivity and electrochemical performance of G/S/PANI composite thereby.3.Graphene/sulfur/silica composite?G/S/SiO₂?was synthesized via in situ deposition and St?ber method.Sulfur and silica content in the composite is calculated to be 4.0 wt% and 4.0 wt%,respectively.The G/S/SiO₂ composite in the 150 th charge/discharge cycle shows good cycle stability and its discharge capacity is 681.0 mAh g-1.The composite has high coulombic efficiency higher than 95.0% and presents excellent C-rate performance.The G/S/SiO₂ composite has increased charge/discharge capacities during the first 30 cycles,which is considered as activation process.The phenomenon is relative to changes in microstructure and EIS of the composite during charge/discharge cycles.The sulfur in G/S/SiO₂ composite is changed from particle to membrane,caused by the heat released,the sulfur structure collapsed and the intermediate product diffused during charge/discharge cycles,which results in sulfur uniform distribution on graphene surface.EIS result reveals that G/S/SiO₂ composite has gradually improved conductivity with charge/discharge cycle increase and presents a stable value in the subsequent ones.