Study On The Preparation Of Conductive Polyaniline And Its Application For Lithium/Sulfur Battery

Lithium Sulfur battery?Li/S battery?is attracting increased attention,for its high energy density,low costs and environmentally friendly.However polysulfide anions(S_n^2-,4?n?8),the intermediate products generated during charge-discharge process of lithium sulfur battery have high solubility in organic electrolytes.Once dissolving in electrolyte,these anions could diffuse to lithium anode drived by concentration gradient and directly react with Li metal continuously,resulting in an internal shuttle phenomenon and meanwhile poor cycle life and low coulombic efficiency of Li-S battery.These drawbacks have seriously retarded industrial development of Li/S battery.In this paper,synthesis of conductive polyaniline was optimized,on the basis of which the performance of the polyaniline modified sulfur cathode was studied.In order to solve the problem mentioned above,The feasibility of using the reversible anion doping/dedoping reaction during the electrochemical oxidation-reduction process of polyaniline is discussed to suppress the dissolution and migration of S_n^2-?4?n?8?in the electrolyte.The optimized polymerization conditions for aniline chemically oxided by?NH₄?₂S₂O₈ were as followed:with as when[S₂O₈^2-]/[An]=1:1,[HCl]=0.8 mol/L,T=0?,t=2 h,Conductivity of prepared polyaniline was 3.15 S/cm,and the yield was94.16%;EDS analysis was used to detect the element composition of PAn,which showed that both the contents of S and Cl in doped PAn were about 4 wt.%,the doping proportion of the polyaniline was about 21 at.%,indicating that average 4.76 aniline units was doped by one Cl^-or SO₄^2-group and the higher the doping proportion is,the higher the conductivity is.Both the cycle life and the coulombic efficiency of lithium sulfur batteries modified by conductive polyaniline were improved significantly,the discharge capacity being 1105 mAh/g in the first cycle and the capacity retention rate being 69%after 200cycles.By contrast,the discharge capacity was only 935 mAh/g and the capacity retention rate is 62.8%after 200 cycles for the lithium sulfur batteries using common S/C cathode.GO/PAn/S/C cathode Li-S battery has a higher coulombic efficiency and better cycle life,the discharge capacity being 1185.6 mAh/g and the capacity after 100cycles being 905.5 mAh/g respectively.The modified sulfur cathode modified by PAn dispersed in molecular scale showed higer coulombic efficiency and better cycle life.The electrode reaction of PAn showed a good reversibility in a range of 1.7 V³.0V in charge-discharge process.The anion doping behavior occurred during the charge process,in contrary,the dedoping behavior occurred during the discharge process.Results also showed that the element composition of the cathode changed due to the anion doping behavior in charge process.It is worth noting that,the physical adsorption also occurred during the charge process,leading to the decrease of the specific surface area and the pore volume of PAn/C to some extent.Study of the modification mechanism showed that PAn can be reversiblely doped and dedoped by different anion?sulfonimide anion,polysulfide anion?in the electrolyte during the charge-discharge process.The oxidation peak changed to some degree with the doping anionchanged.Dynamic behavior of PAn/S/C system during discharge process was changed because of steply dedoping of polysulfide anion.In summary,the modified sulfur cathode with polyaniline as an additive was successfully prepared,and the electrochemical doping/dedoping behavior of PAn was feasiblely used to restrain the transfer of the S_n^2-?4?n?8?in the lithium sulfur battery.