Research On Modified Cathode, Structural Design And Electrochemical Performance For Lithium-sulfur Battery

Abstract:Lithium-sulfur battery is one of the most promising candidates for high energy density secondary lithium batteries. Due to its large theoretical specific capacity, low cost and environmental friendliness, etc., elemental sulfur is expected to become the cathode material of the next generation of high-energy lithium battery. However, there are many disadvantages of sulfur cathode:(1) sulfur and its reduced products are electron insulating at room temperature;(2) the shuttling effect of the intermediate products lithium polysulfides in organic liquid electrolytes during charging and discharging processes;(3) sulfur cathode has considerable volume effects during the charge/discharge cycle. These factors contribute to the low utilization of the active material of lithium-sulfur batteries, poor cycling performance, poor rate performance, which hinder the application of lithium-sulfur battery.In order to solve the major problem of sulfur electrode, this paper started from designing and building the structure of the sulfur composite and electrode, carried out extensive research work aming at improving the performance of sulfur cathode. This paper systematically studied the effects of different carbon materials on constructing the structure and properties of sulfur composite. Spray pyrolysis method was used to build a new carbon-sulfur composite cathode material for lithium-sulfur batteries. The electrochemical properties of the lithium-sulfur batteries in which multifunctional carbon paper used as sulfur cathode current collector was studied. Magnetron sputtering and electrochemical deposition of conductive layer on sulfur cathode to modify cathode was investigated. A new type of lithium-sulfur battery cathode structure and conductive barrier was designed and fabricated, and its unique electrochemical performance and mechanism were studied. Specific results are as follows:Carbon nanotubes/sulfur (CNT/S), carbon fiber/sulfur (CNF/S), active carbon/sulfur (AC/S) and the conductive carbon black/S (SP/S) composite were prepared by the method of liquid phase in situ deposition. By comparing the four kinds of materials, we can find that the structure and morphology of the carbon material is the key factor of the improving effect. Porous carbon materials, incomparison to non-porous and less-porous carbon materials, are more effective in improving the cycle stability and utilization of the active material of sulfur cathode. CNT/S and AC/S composites exhibit better electrochemical performance. CNT can improve the conductivity of composites, but results in poor uniformity and low sulfur loading. AC can absorb the active substance sulfur effectively, but has poor electrical conductivity.Spray pyrolysis method with SiO2as the template was used to synthesis mesopores carbon spheres (SPC) with the surface area of1133m2g-1and total pore volume of2.75cm3g-1.The as-prepared SPC was then used as a sulfur-loading conductive substrate to prepare mesoporous carbon spheres/sulfur (SPC/S) composite. Electrochemical studies show that the unique structure can effectively enhance the cycling stability of the composite, the capacity can reach637mAh g-1after50cycles at0.2C rate, and the capacity retention rate can reach62.9%. Mesopores inside the mesoporous carbon spheres are beneficial the nanocrystallization of sulfur, and can also confine and capture the active substance and shorten ion diffusion path. All of the advantages are conducive to enhancing the rate capability of the composite, the capacity can maintain470mAh g-1at1C rate.For preparation of sulfur positive electrode, commercial carbon papers were used as the positive collector. Electrochemical studies show that the carbon paper collector can significantly improve the cycle stability of the sulfur positive electrode. It can deliver a high capacity of786mAh g-1after100cycles at0.2C rate, and the cycle capacity retention rate is89.6%. The carbon paper can both act as a current collector and an active substance sulfur loading substrate. Moreover, it can also catch and confine the dissolved polysulfides, showing multiple functions in lithium sulfur battery. This significant improvement can be attributed to the excellent electrical conductivity and porous network skeleton structure of carbon paper.For modifying the positive electrode, magnetron sputtering of conductive carbon was used to modify the SPC/S composite cathode. The carbon-coated SPC/S composite cathode shows an initial discharge capacity of956mAh g-1and642mAh g-1after50cycles at0.5C rate, the capacity retention rate increase to67.2%. Electrochemical deposition of polyaniline (PANI) conductive film on positive electrode was used to modify the pure S cathode.The PANI-coated sulfur cathode shows an initial discharge capacity of1094mAh g-1and725mAh g-1after100cycles at0.2C rate, the capacity retention rate also increase66.3%. Research results shows that magnetron sputtering of carbon can effectively enhance the conductivity of the electrode, reducing the irreversible loss of active substance in carbon-sulfur composite. Electrochemical deposition of PANI can help to form conductive nano network structure, which can effectively bound active substance sulfur and reduce the dissolution and diffusion of polysulfide.For the structure design of positive electrode, we designed and prepared two kinds of interlayer placed between the cathode and membrane in lithium-sulfur battery. A simple carbonization process of filter paper was used to get the carbon paper with excellent performance of conductive. A simple pressing process of commercial nickel mesh was used to obtain the structural stable conductive nickel mesh. These two kinds of interlayer were added to a conventional lithium-sulfur battery respectively and were proved to obtain a significant improvement in the electrochemical performance. At0.2C rate, the cell with cabon paper can deliver the discharge capacity of810mAh g-1after50cycles, and the cell with nickel mesh can deliver the discharge capacity of640mAh g-1after80cycles. The improvement effect of the carbon paper and the nickel mesh spacer are due to their excellent conductivity and porous network structure, which can also act as the conductive support for the insulation product and mitigate the volume change and adsorb polysulfide.