Research Of Sulfur Cathode Material Based On Sepiolite For Lithium-sulfur Battery

Lithium-sulfur battery,is a new type of secondary battery system with sulfur as cathode material,and characterized by high specific capacity,low price,environment friendliness and low density.Its energy density is 5 times higher than that of the traditional lithium-ion battery,and has becoming one research hotspot.However,the commercialization of lithium-sulfur batteries has been limited,due to its poor conductivity of sulfur,the dissolution of intermediate lithium polysulfides in electrolyte and huge volume change accompanying with the electrochemical reaction.In this dissertation,an experimental study based on absorption,enhanced conductivity,structural fixation for cathode materials of lithium-sulfur battery is carried out.Sepiolite with good absorbability and cheap price is used as the cathode material of lithium-sulfur battery.A simple vacuum heat treatment method is used to combine sepiolite?Sep?with elemental sulfur?S?,conductive carbon materials including carbon nanotube?CNT?and graphene oxide?GO?are added to enhance the electrical conductivity and structural stability of the composite respectively.Then,the conductive polymer polyaniline?PANI?is used to further stabilize the structure of the composites,constructing a good support structure and conductive network.Owing to the adsorption,enhanced conductivity,structural fixation and other multiple effects,dissolution of the polysulfide in the electrolyte is effectively restrained,the electrode structure is stabilized and the conversion efficiency of the cathode materials is improved,finally obtaining a high-performance lithium-sulfur battery cathode materials.The main research findings are summarized as follows:1.Owing to polysulfide is easily dissolved during the electrochemical reaction of lithium-sulfur battery,sepiolite with strong absorption for polar materials including polysulfide is used to synthesize the sepiolite/S composite with sulfur content about 40wt%,by simple vacuum melting and heat treatment methods.The first discharge specific capacity of the battery is 1436 mAh g^-1 at 0.2 C(1C=1675 mA g^-1)rate,and the discharge specific capacity remains at 1187 mAh g^-1 after 100 cycles.The coulombic efficiency reaches 96.7%.The electrochemical performance of the composite at low rate is good,indicating that porous structure can load more sulfur and adsorption of sepiolite can effectively inhibit the polysulfide dissolution and reduce the shuttle effect.However,as the conductivity of the natural sepiolite material is poor,and the specific surface area and adsorption performance are obviously affected by impurities in sepiolite,the coulombic efficiency of the battery at high current rates is low.2.In order to enhance the adsorption of sepiolite and load more sulfur,the majority impurities in sepiolite is removed by using the acid modification technique.The specific surface area of sepiolite is increased from 62.09 m² g^-1 to 101.94 m² g^-1.The average pore size is increased from 6.68 nm to 7.75 nm.The acid-modified sepiolite/S composite exhibits 1472 mAh g^-11 discharge specific capacity after the first cycle at 0.2 C rate,and remains 1201 mAh g^-1 after 100 cycles corresponding coulombic efficiency reaches96.3%.At 1 C rate,the first discharge specific capacity is 1285 mAh g^-1,and the discharge specific capacity remains around 1001 mAh g^-1after 100 cycles.Compared with natural sepiolite,the electrochemical performance of the acid-modified composite at low current rates is improved.However,after several cycles,the structure of the electrode material is destroyed in a certain extent which results in decreased stability of the electrode and poor conductivity of the composite,finally reduces the cycle life and coulombic efficiency at high current rates.3.On account of the poor conductivity and low coulombic efficiency of sepiolite/S composite during charge and discharge process at high current rates,Sep/CNT/S composite with good conductivity and Sep/GO/S composite with stable structure are prepared by adding a small amount of CNT and GO carbon materials.With gradual increasing CNT content,a good three-dimensional conductive network is constructed and the structure is stabilized,which enhance the overall conductivity of the electrode.The first discharge specific capacity of Sep/CNT/S composite reaches 810 mAh g^-1at 1 C current rate,the discharge specific capacity is maintained at 710 mAh g^-11 after 100 cycles,and the coulombic efficiency remains above 92%.The surface roughness of the electrode reduces after cycling,which indicates that the volume expansion of the active material is effectively inhibited.In addition,the conducive three-dimensional structure is beneficial to the infiltration of electrolyte,which effectively improves the coulombic efficiency of the battery at high current rate and promotes the stability of the cycle.At 2 C current rate,Sep/GO/S composite's discharge specific capacity reaches 972 mAh g^-11 after the first cycle and maintains at 521 mAh g^-1 after 500 cycles,showing better cycling performance.The surface functional groups of GO can immobilize sulfur.Combining the adsorption properties of sepiolite,the active material and the polysulfide can be double-adsorbed,and the lamellar structure of the GO can wrap portions of S and sepiolite.At high current rates GO can stabilize the electrode structure,inhibit the adverse effect of shuttle effect on the battery and improve the cycle reversibility.Finally,Sep/GO/S composite show good cycle performance at high current rates.4.In order to further enhance the cyclic stability of sepiolite-based composite at high currentrates,Sep/CNT/S@PANIandSep/GO/S@PANIcompositeswith three-dimensional structure are synthesized by chemical oxidation.The electrochemical tests show that Sep/CNT/S@PANI composite has the discharge specific capacity 1200mAh g^-1 after the first cycle at 2 C current rate,which keeps 650 mAh g^-1 after 300 cycles.The coulombic efficiency reaches 93%.The discharge specific capacity of Sep/GO/S@PANI composite at 2 C current rate is1402 mAh g^-1 after the first cycle,and maintains at 674 mAh g^-1 after 500 cycles,corresponding coulombic efficiency is over95%.Both two composites exhibit good cyclic performance and rate performance.It reveals that the three-dimensional material enveloped by polyaniline can effectively improve the structural stability of the composite,reduce the shuttle effect during the electrochemical reaction,and enhance the cycling performance of the battery at high current rates.