Research On Sulfur Cathode Materials And Electrolytes For Lithium-Sulfur Batteries

ABSTRACT:Lithium-sulfur battery, considered as one of the most potential secondary battery system, is expected to achieve a wide range of application in the field of high energy density demands, such as portable electronic communication, energy storage equipments and electric vehicles, due to the advantages of high energy density, abundant resources, low cost and environmental friendliness. However, the low utilization of the active material, poor cycle life, low rate performance and high self-discharge etc., which are derived from the limitation of current techniques, storage mechanism and defects of the electrode materials, severely restricts the development of lithium-sulfur battery.In view of above problems existed in lithium-sulfur batteries, the researches on both electrode materials and electrolytes were carried out in order to improve the battery performance and reveal the internal fading mechanism. In this paper, the effects of the performance by adding an organic solvent to ionic liquid based electrolyte were studied; the components of ternary electrolyte system were optimized; the feasibility of its application in lithium/sulfur batteries was evaluated. Meanwhile, optimized liquid phase precipitation process was choosed to make nanomaterials; the performances of sulfur and sulfur-carbon nanomaterials in the ternary electrolyte were characterized, the different performances caused by the different combination type between sulfur and nano-carbon black were analyzed as well. Also, the relationship among components, morphology, structure and electrochemical properties of sulfur/hollow carbon fiber composite materials prepared under different conditions was studied; the role of hollow carbon fiber on improving battery performance was analyzed. At last, the effects of different types of electrolyte additives on battery performance were explored; the inhibition mechanisms of different additives on the shuttle behavior were discussed. Here come the main conclusions:(1) Although the thermal stability of the PYR14TFSI based electrolyte was reduced by the addition of PEGDME, it helps improving the conductivity of electrolyte, reducing the viscosity of the system, and maintaining the reactivity and reversibility of electrode. Besides, the more content of PEGDME were added, the greater improvement it got. The ionic conductivity of optimized LiTFSI+PYRi4TFSI+PEGDME mixture can reach up to4.4×10-3S/cm at room temperature and the stable electrochemical window is0-4.1V(vs.Li+/Li). The ternary electrolyte mixture had good compatibility with lithium electrode, and ensured good electrochemical stability of Li/Li symmetric cell during long-term cycles, especially under proper elevated temperature, which is suitable for being used in the lithium-sulfur battery.(2) Spherical sulfur nanomaterial with size in range of100nm was successfully synthesized by proper liquid phase precipitation process. The initial specific capacity of the nanosulfur electrode can reach to1050mAh/g at a rate of0.02C, the coulombic efficiency is almost100%, but the capacity fades rapidly. The sulfur/carbon black composite prepared via liquid phase precipitation method, realized uniform distribution and close contact of the sulfur and carbon, which is beneficial to the electrode conductivity. The sulfur/carbon composite electrode exhibited an initial specific capacity of1200mAh/g at a rate of0.02C, and the capacity retained a700mAh/g after50cycles. Compared to the element sulfur nanomaterial, sulfur/carbon black composite enhanced the conductivity and structral stability of the electrode, increased the utilization of active material, and improved the cycle performance of the cell.(3) Sulfur/hollow carbon fiber composite material prepared by high temperature processing method prone to agglomerate severely, and the distribution of sulfur/carbon was not uniform. Good distribution of sulfur/carbon particles and small sulfur particles could be ensured via liquid phase precipitation method. Sulfur and carbon particles could be contacted closely when load of sulfur content is suitable, some sulfur particles could insert into the hollow path of the carbon fiber. The initial specific capacity of the composite got to1165mAh/g at a rate of0.1C, the capacity remained520mAh/g after40cycles. The capacity retention rate reached to75.7%compared to the2nd cycle, and cycle stability at high rate was pretty good. The stable three-dimensional conductive network with abundant holes constructed by the hollow carbon fiber, promoted the fast delivery of the charge and ion, realized more load of active material, enhanced the contact of electrode and electrolyte, reduced the loss of active material which was attributed to the precipitation of insoluble products, and restrained the destruction of the electrode from the repeated volume changes. Thus, it improved the specific capacity, cycle performance and rate performance of the composite material.(4) Shuttle effect during the charge process was particularly significant; the capacity of the high potential platform of the charge curve was closely related to the shuttle effect, meanwhile, the electrochemical oxidation process of the polysulfide competed with the reduction process of shuttle effect. The self-consumption of LiNO3for film-forming, changed the components status of lithium metal surface and improved the structural stability of SEI film on the anode, which effectively blocked the reaction between polysulfide and the anode and restricted the shuttle behavior. The initial specific capacity reached up to1135mAh/g at a rate of0.1C under a suitable content of LiNO3, the capacity remained580mAh/g after40cycles; the coulombic efficiency was closely to100%. Hence, it obviously restricts the overcharge and improves the performance of the cell. Moreover, Low solubility of polysulfide in PYR14TFSI and high viscosity of PYR14TFSI, which inhibited dissolving and diffusing of polysulfide in the electrolyte, actually suppressed the shuttle effect. However, the general effects of adding PYR14TFSI were still not very good compared with adding LiNO3.