Research On Preparation Of Sulfurized Polyaniline Cathode Materials And Improvement Of Rate Performance

With a theoretical energy density as high as 2600 Wh Kg^-1,lithium-sulfur batteries have outstanding merit.Generally speaking,the conventional elemental sulfur-based cathode materials will inevitably suffer from the dissolution of lithium polysulfides in ether-based electrolytes,which seriously affects the cycle performance of lithium-sulfur batteries.In contrast,sulfurized polyaniline（SPANI）,a new sulfurized polymer cathode material,can be perfectly adapted to commercial carbonate-based electrolytes and does not produce polysulfides during the discharge process,thus SPANI has better cycle performance than elemental sulfur-based cathode materials.In addition,SPANI has the advantages of simple synthesis and low price,which have the potential for industrialization.However,the poor electrical conductivity of SPANI and the low conductivity of the discharge product of lithium sulfide lead to the poor electrochemical performance of SPANI at a high current rate,which seriously hinders the application and development of SPANI.This thesis addresses the thorny issue of how to improve the electrochemical performance of SPANI at a high current rate and this study intends to carry out the research work from two aspects of improving the ionic conductivity or electronic conductivity of the cathode material.The details are as follows:（1）Spinel lithium titanate(Li₄Ti₅O₁₂,abbreviated as LTO)with the fast ionic conducting property was compounded with SPANI to improve the diffusion kinetics of lithium ions,thereby improving the rate performance of SPANI.The experimental results show that the rate performance of the SPANI/Li₄Ti₅O₁₂ material（No.SPANI-LTO-2）prepared with 0.2 g lithium titanate shows the most obvious enhancement compared with the pristine SPANI.SPANI-LTO-2 and SPANI exhibit reversible discharge-specific capacities of 811.04 m Ah g^-1 and 566.01 m Ah g^-1 at 0.5 C,respectively.After 100 cycles,the specific capacity of SPANI-LTO-2 is 643.04 m Ah g^-1 with a capacity retention rate of 79.29%,while the remaining specific capacity of SPANI is 422.28 m Ah g^-1 with a capacity retention rate of only 74.61%.It indicates that the rate performance of SPANI was significantly improved after being compounded with LTO.（2）A simple preparation method was used to synthesize SPANI/Co S₂ composites.The high electronic conductivity of Co S₂ was used to improve the rate performance of SPANI.The experimental results show that the presence of Co S₂ can effectively improve the rate performance of SPANI,in which the composite prepared by adding0.3 g Co CO₃（No.SPANI-Co S₂-3）has the most obvious effect on the improvement of rate performance.SPANI-Co S₂-3 and SPANI show 640.44 m Ah g^-1 and 449.60 m Ah g^-1 at 2 C,respectively.After 500 cycles,SPANI-Co S₂-3 still has a discharge capacity of347.78 m Ah g^-1 with a capacity retention rate of 54.30%,while SPANI has a discharge capacity of 160.77 m Ah g^-1 with a capacity retention rate of 35.76%.It demonstrates that the rate properties of SPANI were significantly improved after being compounded with Co S₂.（3）To obtain SPANI composites with both high-rate performance and excellent volume-specific capacity.The polyaniline with high tap density was first selected as the raw material and then mixed with elemental sulfur and Ni CO₃ to obtain SPANI/Ni S₂composites.Ni S₂ with high electronic conductivity was used to improve the rate performance of SPANI.The composite prepared by adding 0.2 g of anhydrous Ni CO₃（No.SPANI-Ni S₂-2）has the most obvious enhancement.SPANI-Ni S₂-2 and SPANI exhibit high reversible discharge specific capacities of 520.31 m Ah g^-1 and 336.99 m Ah g^-1 at a large rate of 4 C,respectively.After 2000 cycles,the SPANI-Ni S₂-2 still has a capacity of 234.50 m Ah g^-1,with a capacity retention rate of 45.07%.In contrast,the discharge-specific capacity of SPANI remains at 24.95 m Ah g^-1 and the capacity retention rate is only 7.40%.In addition,the tap densities of SPANI-LTO-2 and SPANI-Co S₂-3,which are prepared using nanoscale raw materials in the previous two chapters,show the values of 0.55 g cm^-3and 0.47 g cm^-3,respectively,and the volumetric discharge specific capacities are only 446.07 m Ah cm^-3 and 420.41 m Ah cm^-3.In contrast,the micron-scale spherical raw material used in this chapter can obtain a tap density as high as 0.93 g cm^-3 for SPANI-Ni S₂-2 and the volume discharge specific capacity at 0.5 C is 769.48 m Ah cm^-3.It is shown that the SPANI/Ni S₂ composites prepared by using the micron-scale spherical raw material and Ni CO₃ has a higher tap density and the nickel disulfide can significantly improve the rate performance of the polyaniline sulfide.