Preparing Of Aqueous Binder And Carbon/Sulfur Composites As Cathode Materials For High-performance Rechargeable Lithium-sulfur Battery

With the rapid development of electric vehicles,traditional rechargeable Li ion batteries may not meet the requirements of the market.Lithium-sulfur(Li-S)batteries with high energy density(2600 Wh kg'1),are considered as one of the first choice for the next generation battery technology.However,the electrochemical properties of Li-S batteries are seriously limited by several inherent problems,including the low conductivity of sulfur and discharge product lithium polysulfide,the shuttle effect and the severe volume variation of sulfur electrode during cycling.In order to settle these problems,a multi-functional binder and new type of sulfur host material are prepared to optimize the structure of sulfur cathode,increasing the utilization of active material and cycle performance of the Li-S batteries.In this thesis,the low-cost natural polymer guar gum(GG)is successfully used for the first time to enhance the electrochemical performance of sulfur cathode.GG shows higher hardness and viscosity than those of commercial binder Polyvinylidene Fluoride(PVDF).These characters of GG provide the cathode with appropriate ductility,which could decrease the adverse influence from the volume changes.Moreover,GG binder delivers prominent chemical and electrochemical stability,i.e.,the swelling and soluble capacity of GG in the electrolyte is negligible.In addition,cyclic voltammetry(CV)curve of GG shows that no obvious electrochemical reaction appears in the scanning potential rang(1.8-2.6 V vs.Li/Li+),demonstrating that GG is electrochemically stable.The Fourier transform infrared(FTIR)results show that there are chemical interactions between lithium polysulfide and GG,which may trap the active material in the cathode material and then to a certain degree inhibit the shuttle effect.At 0.2 C,the S/GG cathode obtains reversible capacity of 777 mAh g-1 after 150 cycles,however,under the same conditions,S/PVDF cathode only processes a discharge capacity of 491 mAh g-1.Besides,cobalt@graphitic carbon-porous(Co@GC-PC)composite derived from bimetallic metal-organic frameworks is employed as the sulfur host cathode material for Li-S batteries.Co@GC-PC composite shows large surface area and an abundance of macro/meso pores.In situ UV/Vis analysis indicates that this host material presents strong poly sulfides absorbing capability during the discharge process.Density functional theory calculations indicate a strong interaction between Co and the Li2S which is the final product during the discharge process,suggesting the decomposition of Li2S on Co(111)is a fast process,which is beneficial for the Li2S oxidation in the following charge process.As a result,at 0.2 C,the discharge capacity of S/Co@GC-PC cathode is stabilized at 790 mAh g-1 after 220 cycles.