Design And Synthesis Of High Sulfur-Loading Cathode Materials And High-Performance Separator Coating For Lithium Sulfur Batteries

With the development of advanced portable electronic devices and zero-emission electric vehicles,the demand for batteries with long cycle life,large capacity,and low self-discharge rate is increasing.Sulfur is abundant in the crust.It endows a high theoretical specific energy of 2600 Wh kg-1 and a specific capacity of 1675 mAh g-1 as the cathode of lithium sulfur batteries.For packaged lithium-sulfur batteries,the actual specific energy can reach the range of 400-600 Wh kg-1.Lithium-sulfur battery has become an extremely promising energy storage system.However,sulfur and its discharge products are almost electronic insulators,and the intermediate lithium polysulfides are easily dissolved into the electrolyte and shuttled to the anode to react with metallic lithium,resulting in loss of capacity and poor cycling performance.In this paper,combined with the reviewed global research progress of lithium-sulfur batteries at present,the design and synthesis of a novel structural and functional porous carbon material have been proposed.It is used as the host material for sulfur cathode and the modified coating layer on the cathode side of separator.The effects and related mechanism of the components,structure,and morphology of the prepared functional porous carbon on the electrochemical properties have been investigated.Finally,a high performance sulfur cathode is obtained.A novel structural one dimensional porous carbon hybrid with carbon nanotube as core and nitrogen/oxygen co-doped porous carbon as coating layer coated is prepared by the template method which uses phenolic resin as precursor,CNTs as additives and introduces of nitrogen and the residual oxygen in the resin.This carbon material possesses the high conductivity,pore volume and specific surface area.When it is used as the host material for sulfur cathode,the carbon material can assist a high loading of sulfur and is beneficial to store sulfur and further adsorb the soluble polysulfides.Therefore,it presents a superior sulfur confining ability and facilitates the obtained sulfur cathode possessing superior comprehensive electrochemical performance.Under a high sulfur loading of 75 wt%,the sulfur cathode achieves a discharged capacity of 684 mAh g-1 and the capacity retention of 61.5%after 200 cycles at 0.2 C.The introduced nitrogen/oxygen co-doping enhances the polarity and improves the affinity of the carbon matrix material towards lithium polysulfide.It successfully suppresses the shuttle effect of the lithium polysulfide shuttle to the negative electrode and improves the cycle stability as well as the capacity retention.In addition,a porous carbon with TiN nano particles around 20-50 nm loading on the surface are obtained.The introduction of TiN can enhance the adsorption towards lithium polysulfide to some extent.The further prepared sulfur cathode achieves a specific capacity over 500 mAh g'1 after 500 cycles at 2 C.But the mesopores of the synthesized carbon fiber are distributed at 2-10 nm.The ability of the relatively large pores to confine sulfur and its discharge products is weak,resulting in a gradually increased polarization during the charging process.Furthermore,the one-dimensional nitrogen and oxygen co-doped porous carbon nanofiber is used as a separator coating layer,which presents enhanced suppression on the shuttle effect of soluble polysulfides and improves the lithium ion diffusion rate obviously.The cathode without any modification presents outstanding electrochemical performance when it couples with the modified separator.The sulfur cathode maintains the discharge capacity of 1032 mAh g-1 and the capacity retention of 81.8%after 100 cycles at 0.2 C.In addition,the LiNO3 in the electrolyte partially oxidizes lithium polysulfide to a high-valent thiol group and self-reduced to insoluble LiNO2.which deposits on the separator and further blocks the shuttle movement of lithium polysulfide.In this case,it can not only reduce the corrosion of the lithium metal anode but increase the discharge capacity of the sulfur cathode.When the sulfur areal density reaches at 6.0 mg cm-2,the sulfur cathode can obtain an areal specific capacity of approximately 5.0 mAh cm-2 when coupled with modified separator.