Research On Performance Of S@TiO₂/MXene As Composite Cathodes For Lithium-sulfur Batteries

In recent years,lithium-ion batteries are increasingly difficult to fulfill the energy demand for portable electronic devices and electric vehicles due to their lower energy density.Lithium-sulfur(Li-S)batteries have been considered as one of the most promising solutions for high energy electrical sources,owing to their higher theoretical specific capacity and energy density(more than five times of lithium-ion batteries).In addition,sulfur can be a type of ideal electrode material with its other advantages,such as abundant content in crust,low cost and non-toxicity,etc.However,several problems with lithium cathode must be addressed before their large-scale market applications,for example,the shuttle effect,volume expansion,or low electrical conductivity etc.In this thesis,the electrochemical properties of a series of composite cathodes with different compositions and microstructures were studied by using of chemical synthesis methods to control the structure of materials.The cathode of lithium-sulfur batteries comprised of sublimed sulfur with one or two materials of TiO2 hollow sphere,MXene-Ti2C,or biological porous carbon.A sulfur cathode with excellent electrochemical performances was proposed by the encapsulation of TiO2 hollow spheres,the relax of volume expansion of Ti2C layered structure,and the improvement of conductivity of carbon or Ti2C.The detail contents mainly include the following aspects:(1)Resorcinol-formaldehyde(RF)resin microspheres were synthesized by the hydrothermal method.TiO2 hollow spheres were obtained by in situ precipitation and calcination at high temperature using RF resin microspheres as templates.S@TiO2 composite cathodes were prepared by melting sublimed sulfur into TiO2 hollow spheres.The S@TiO2 composite possessed an initial specific discharge capacity of up to 1325.5 mAh g-1 and preserved 675.2 mAh g-1 after 100 cycles at 0.2 C.(2)As a raw material,MXene-Th2AlC was etched by HF acid to obtain layer Ti2C.By testing the electrochemical properties,Ti2C/S composite cathodes showed the specific discharge capacity of 258.9 mAh g-1 after 300 cycles at 0.2 C.The electrochemical impedance spectroscopy measurement of Ti2C/S composite showed significantly lower impedance than that of S@TiO2.(3)Kelp was used as a porous carbon source by carbonization and KOH solution activation.The influence of the weight ratio of carbon and alkali solution(C:K)on porousity of bio-carbon was analyzed.In the case of C:K=1:1,batteries with kelp-derived carbon displayed the best electrochemical performance with the specific discharge capacity of 662.9 mAh g-1 after 100 cycles at 0.2 C.(4)The as-prepared TiO2 hollow spheres,kelp-derived porous carbon and elemental sulfur were combined to form S@TiO2/C composites,in which TiO2 was to encapsulate sulfur for alleviating the volume expansion of polysulfides and porous carbon was to enhance the conductivity of cathode.The electrochemical properties of S@TiO2/C composite electrode with different sulfur ratio were studied in this thesis,.The S@TiO2/C composite with 60%sulfur content exhibited a better electrochemical performance.In case of 0.5 C,the specific discharge capacity still maintained 303.8 mAh g-1 after 400 cycles.(5)In this thesis,a new structure of S@TiO2/Ti2C composite was proposed to improve the electrochemical performance of lithium-sulfur batteries.The hollow sphere structure of TiO2 provides space for the volume change of sulfur cathode during charge and discharge.The layered Ti2C has a good conductivity and the interlayer space for further reducing the influence of volume expansion.At the same time,the adsorption of polysulfide ions by TiO2 and Ti2C can effectively retard the shuttle effect during charge and discharge.The S@TiO2/Ti2C composite electrode showed an excellent cyclability at high current densities.The specific capacity at 2 C and 5 C maintained 464.0 mAh g-1 and 227.3 mAh g-1,respectively,after cycling 200 cycles.Based on the studies in this thesis,sulfur was encapsulated by TiO2 hollow spheres and then embedded into interspaces of Ti2C to form S@TiO2/Ti2C composite.The lithium-sulfur batteries with S@TiO2/Ti2C cathodes exhibited an excellent cyclability,especially at higher charge and discharge current densities,which provided a novel route for the development of lithium-sulfur batteries.