Study On The Preparation And Lithium Storage Mechanism Of Sulfurized Polyacrylonitrile Cathode Materials

With the rapid development of mobile electric devices and electric vehicles in recent years,the demand for rechargeable lithium batteries with high energy density and long cycle life becomes higher and higher.Among the new energy storage devices,lithium/sulfur battery is considered as one of the most promising next generation high-energy-density rechargeable lithium batteries owing to its high theoretical energy density of about 2600Wh/kg,and elemental sulfur is low cost and environmentally friendly.Among the cathodes of lithium/sulfur batteries,the most advantages of sulfurized polyacrylonitrile over elemental sulfur are nearly 100%coulombic efficiency?low self-discharge rate?good chemically compatible with the carbonate based electrolytes and stable capacity retention,therefore,sulfurized polyacrylonitrile has extensive application prospect in high-energy-density rechargeable lithium/sulfur batteries.However,at present the molecular structure and electrochemical reaction mechanism of sulfurized polyacrylonitrile are controversial,the intrinsic influencing factors on electrochemical performance are indefinite,and the sulfurized polyacrylonitrile with high specific capacity and excellent cycling performance is to be futher developed.Moreover,there are few reports on the related sulfurized polymer cathodes,so it is worth exploring whether there are other sulfide polymer materials with similar electrochemical reaction mechanism and excellent electrochemical properties.To address the above problems,this dissertation mainly focuses on expounding molecular structure and the electrochemical reaction mechanism of sulfurized polyacrylonitrile,the construction of sulfurized polyacrylonitrile with high electrochemical properties,and exploring the new nitrogen-doped carbyne polysulfide.The detail contents are listed as follows:?1?The molecular structure and electrochemical reaction mechanism of sulfurized polyacrylonitrile are illuminated in detail.Research indicates that sulfurized polyacrylonitrile is mainly composed of pyridine rings and the pyridine rings bonded to sulfur atoms.Through the analysis of MAS-NMR,it is found that the C=N and C=CH radicals in six-number rings will be break and react with lithium ions in first discharge process,besides sulfur,so the actual discharge specific capacity of sulfurized polyacrylonitrile exceeds the theoretical specific capacity of elemental sulfur.In the first charge process,a fraction of carbon atoms and nitrogen atoms do not take off the lithium ions,so there are residual lithium ions in the charging products,which lead to the first irreversible capacity loss.However,the residual lithium ions in the charging products are helpful to improve the electrical conductivity of materials,so the second discharge voltage is higher than the first discharge voltage.?2?The sulfurized polyacrylonitrile cathode materials have been prepared by controlling the reaction pressure in the heat treatment process.Research indicates that the molecular structure of the prepared sulfurized polyacrylonitrile under the appropriate high reaction pressure is more complete,which is beneficial to improve the conductivity of materials and reduce the electrode polarization impedance,and finally improve the battery performance.The SPAN-5 cathode prepared under 5MPa reaction pressure displays a high discharge specific capacity of 1357mAh/g at 200mA/g current density up to 100 cycles.?3?A NiS₂-SPAN composite is synthesized by adding NiCO3 in the heat treatment process of sulfurized polyacrylonitrile.Research indicates that the NiS2 in composite is helpful to reduce the charge transfer impedance and improve redox dynamics.In addition,in the discharge process,NiS2 can accerelate the reduction reaction of short chain sulfur into Li2S.Therefore,the NiS2-SPAN composite presents excellent electrochemical performance,after 100 cycles,the discharge specific capacity is 1533mAh/g at 200mA/g.?4?A Se_xS_1-x composite is synthesized by dissolution and re-precipitation,and then the Se_xS_1-xPAN cathode is prepared by co-heating the mixture of Se_xS_1-x and polyacrylonitrile.A moderate amount of selenium is not only helpful to reduce the electrode polarizationr impedance and improve redox dynamics,but also can improve cycling performance and average discharge voltage on the basis of not decreasing the discharge capacity.The Se0.05S0.95PAN electrode shows a discharge capacity of 642mAh/g-composite at 100mA/g current density up to 200 cycles,which is an excellent electrochemical performance.?5?A nitrogen-doped carbyne polysulfide carbon cathode material is successfully synthesized.According to the electrochemical reaction mechanism of sulfurized polyacrylonitrile,a nitrogen-doped carbyne is prepared through dehydrochlorination of 1,1,2-trichloroethane and used urea as nitrogen source,and then the nitrogen-doped carbyne polysulfide is prepared by co-heating the mixture of nitrogen-doped carbyne and sulfur.Research indicates that the nitrogen-doped carbyne polysulfide contains six-number rings with nitrogen atoms,and nitrogen doping is beneficial to improve electrical conductivity of materials and reduce the electrode polarization impedance.After 300 cycles,the nitrogen-doped carbyne polysulfide shows a high discharge capacity of 900mAh/g at the 200mA/g current density,which is a very good electrochemical performance.