Preparation And Application Of Pine Needle Carbon Nanotubes/Carbon Fiber Composites For Lithium Sulfur Battery

The demand for high energy density rechargeable battery makes the lithium-sulfur battery become one of the most promising candidates,due to its various advantages,such as high theoretical specific capacity,low cost,environmental friendliness.However,the practical use faces with a variety of problems such as low specific capacity and short cycle life because of the insulating nature of sulfur and solid discharge products,the solubility of intermediary products and volume change when cycling.The researchers found that the lack of stability of electronic conductor is a crucial reason for poor cycle stability.Meanwhile we consider that the the microscopic location of the electrochemical reaction of the sulfur electrode in the discharge process of the battery is pointed out at the interface between the electron conductor and the electrolyte.In order to decrease the volume expansion of the sulfur cathode and confirm the micro space position of electrochemical reactions,we have presented a binder-free three-dimensional porous material as conductive material which was composited with the sulfur.We carried out a series of researches about the preparation of different surface composition,the microstructure and optimization of performance around the integral cathode.The main works and results are summarized as follows:?1?Preparation and characterization of pine needle carbon nanotubes /carbon fiber composites.In this paper,we used carbon papers with low manufacturing cost and high conductvity as substrate,ferrocene was acted as a producer for Fe catalyst particles and xylene was selected as carbon source,growed pine needle carbon nanotubes/carbon fiber composites were treated by catalyst precursor preset method through chemical vapor deposition.Sample morphology and composition profiles were characterized by scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and X-ray diffraction?XRD?.These characterization results showed that the microstructure of composites were pine needle carbon nanotubes/carbon fiber?CNTs/CF?.Moreover,the CNTs which had structures of high-distributed,diameter-uniformed nanotubes possessing high surface area.TEM images showed that the CNTs with a uniform diameter 24 nm and the length 60 ?m had a multi-walled structure and the inter-layer space was about 0.3 nm.?2?Preparation and electrochemical properties of sulfur/pine needle carbon nanotubes/carbon fiber compositesThe CNTs/CF was used as a three-dimensional porous electron conductor,and the sulfur cathode was prepared by the solvent method.The cathode consisting of the S/CNTs/CF was utilized for button cells.The battery was tested by charging and discharging,EIS and CV at the optimal conditions of sealing pressure?5.0 MPa?and the sulfur cathode's molding pressure?3.0 MPa?.The results showed that the initial discharge specific capacity of an elemental sulfur positive electrode with S/CNTs/CF was 1213.6 mAh/g and the utilization of electrochemically active sulfur was 72.45%.The specific capacity remained as high as 798.4 mAh/g after 55 cycles.With the increase of the numbers of cycles,the peak value of the reduction peak and the peak of the oxidation peak had a small change and the transferring resistance of the interface between the cell andelectrolyte changed a little.Compared with S/C?activated carbon?cells,the electrochemical performance of S/CNTs/CF cells was improved significantly showing that the S/CNTs/CF cathode material has a stable structure and the contact region between the cathode material and the electrolyte in the process of charging and discharging changes a little,which hinder the destruction of the three-dimensional conductive porous network structure,finally the problem of poor stability in the process of lithium sulfur battery was resolved.?3?The influence of different surface compositions of pine needle carbon nanotubes/carbon fiber composites on performance of lithium-sulfur batteryBy dipping method prepared NTs_HNO3/CF and CNTs _H2O2/CF,we have studies the influence on the performance of lithium sulfur battery.Compared with CNTs,we found that the graphite structure of NTs_HNO3 and CNTs _H2O2 materials was not destroyed,but the C?Fe₃C and Fe₂O₃ crystalline patterns have changed by XRD characterization.We used S/CNTs/CF,S/NTs_HNO3/CF and S/CNTs _H2O2/CF composites as cathode materials.The battery was tested by charging and discharging,EIS and CV.It showed that the electrochemical properties of the three composites have changed significantly.The cycle stability of S/CNTs/CF battery is the best,the cycle stability of S/NTs_HNO3/CF battery is the worst,the reduction of capability of S/CNTs _H2O2/CF battery is the most serious;There are two reduction peaks and one oxidation peak in the cyclic voltammetry curve of S/CNTs/CF battery.There are three reduction peaks and one oxidation peak on the first cycle cyclic voltammetry curve of S/NTs_HNO3/CF battery and S/CNTs _H2O2/CF battery.It showed that the chemical properties of the surface of the conductive material has obvious influence on the electrochemical properties of the battery.