Study on Polyaniline Containing Cathode Material for Lithium Sulfur Battery

Author:Yao Yu Jie

Supervisor:wang xian hong


Degree Year:2019





A typical lithium sulfur battery is composed of sulfur cathode with high theoretical specific capacity(1675 mAh/g)and lithium anode,which provides very high energy density of 2600 Wh/kg.Despite the advantages,the insulating nature of sulfur and lithium sulfides lead to poor electrochemical energy conversion,while the"shuttle effect" due to dissolution and diffusion of polysulfides causes poor cycling performance.In addition,the large volume change during charging-discharging process may cause the collapse of cathode,and the poor low temperature performance of lithium sulfur battery also limits the practical application of Li-S battery.Polyaniline could contribute to solve the above issues,since it can improve the conductivity of cathode and act as an effective suppressor to trap polysulfides.In this thesis,polyaniline containing sulfur/carbon nanocomposites are designed and prepared to improve the capacity as well as cycle performance of lithium sulfur battery.The main results are summarized as follows:1.Extremely low content of polyaniline(<2 wt%)was uniformly covered on the surface of sulfur-wrapped carbon(SC)nanoparticles via dilute aniline polymerization.Even Ca.1 wt%of polyaniline loading can improve 46%capacity compared to sulfur-wrapped carbon composite after 100th at 0.5 C.Moreover,even after 500 cycles,the SC cathode material containing Ca.2 wt%polyaniline maintained coulombic efficiency above 95%with a high capacity retention rate of 55%(a decay of only 0.09%per cycle),in contrast,the cell without polyaniline showed a relatively faster capacity decay(0.246%per cycle).The reason was attributed to the specific property of polyaniline in that it can act as a very effective suppressor for diffusion and dissolution of polysulfides even under small amount of polyaniline loading,and it can also accelerate the sluggish charge transfer process.2.Dual-coated PANI-SC nano-composite was synthesized by dilute aniline polymerization and used as the cathode material in lithium sulfur battery.The obtained nanocomposite could reduce high overpotential performance in charging process exhibiting excellent rate performance.At 0.5 C,this nanocomposite displayed discharge capacity of 725 mAh/g after 100 cycles at high sulfur loading(2.1 mg/cm2),a significant increase in capacity by Ca.47%was realized compared with SC analog without polyaniline.3.A strategy was proposed using pyrene as redox mediator to improve the low temperature performance of polyaniline containing lithium sulfur battery.Since polyaniline could trap polysulfides into cathode region,subseqently the redox mediator reacted with sulfur or polysulfides in cathode region,which could accelerate polysulfides equilibration and prolong the high voltage plateau of lithium sulfur battery leading to significant improvement in sulfur utilization.Using pyrene as redox mediator,the 50th discharge capacity of the polyaniline containing SC cathode reached 626.1 mAh/g at 0℃,while that without addition of pyrene was only 509.8 mAh/g,Ca.23%increase was realized based on redox mediator strategy.Moreover,the 50th discharge capacity of the polyaniline containing SC cathode maintained 516.7 mAh/g at-15℃,once the pyrene was added as redox mediator,a Ca.25%increase was observed compare with that without addition of pyrene(412.9 mAh/g).Therefore,our result provides a new strategy to improve the low temperature performance of polyaniline containing lithium sulfur battery by regulating the electrochemical reaction of lithium sulfur battery through redox mediator.