Study On Modification Of Separator And Cathode For Li-S Batteries

Compared with the traditional lithium-ion batteries,rechargeable lithium sulfur batteries share broader economic interests and development prospect due to their high theoretical specific capacity?1675 mAh/g?,energy density?2600 Wh/kg?,low cost and environmental compatibility.However,lithium sulfur batteries are far from practical application on account of some physical and chemical nature: i)the electrical insulating properties of elemental sulfur and discharge product Li₂S/Li₂S₂;ii)the difference in density of sulfur and Li₂S/Li₂S₂ during cycling results in severe volume expansion and iii)the dissolution and shuttling of lithium polysulfides.Herein,KB@Ir-modified separator was applied in Li-S batteries with improved electrochemical performance through the polar adsorption and catalytic properties of Ir nanoparticles.And a superior electrochemical properties were obtained by using KB@Cu composite as host material.Moreover,the higher capacity and sulfur loading were achieved by optimizing the binder.On the basis of present research we successfully manufactured soft package Li-S batteries.KB@Ir composite was prepared by a microwave-assisted reduction process with Ir nanoparticles uniform deposition on activated KB.The adsorption and catalytic properties was extensively investigated by adsorption experiment,XPS,CV and Tafel test.The iridium nanoparticles not only exhibit strong chemical interaction with the polysulfides,but also efficiently accelerate the kinetic process for polysulfides conversion.The admirable electrochemical properties were achieved with KB@Ir-modified separator.A high initial capacity of 1508 mAh/g can be achieved under a charge/discharge rate of 0.2 C,and there still remains a reversible capacity of 653 mAh/g even at 2 C.Also the cell showes a low capacity decay rate of 0.105 % per cycle with 452 mAh/g capacity over 500 cycles at 1.0 C.KB@Cu composite was synthesized by high temperature solid phase reduction method,and an improved electrochemical performances were obtained with KB@Cu composite as host material.The metal Cu polarity material have the effect of immobilizing the sulfur and polysulfides,metal Cu could react with sulfur to form CuSx during heat treatment,and the strong bonds between metal Cu and sulfur allow the CuSx to directly form Li2 S and metal Cu without forming soluble high-order polysulfides during lithiation,meanwhile the metal Cu could improve the electronic conductivity of electrode.The KB@Cu/S composite delivers an initial discharge capacity of 1003 mAh/g at 0.2 C,and still retains a discharge capacity of 508 mAh/g after 100 cycles.The cell shows a discharge capacity of 499 mAh/g at high current rate of 2 C.We analyzed the influence on the electrochemical performance of Li-S batteries with PVDF,PEO and GA as binder,and found that the cell with GA as binder shows the better cycle performance.This is because the excellent mechanical properties of GA endow the sulfur cathode with high binding strength and suitable ductility to buffer volume change,furthermore,the abundant oxygen-containing functional groups chemically and physically confine sulfur species within the cathode to inhibit the shuttling effect of polysulfides.On this basis,the electrode with 3.4 mg/cm² sulfur loading was successfully prepared taking advantage of the excellent bonding capacity of GA.In addition,2 Ah soft package Li-S batteries were successfully fabricated on the basis of existing work with an initial discharge capacity of 1784 mAh,and the cell could continue working for more than 7000 min.The feasibility of commercialized Li-S battery is preliminarily explored.