Synthesis Of Novel Cathode Materials Of Lithium-Sulfur Batteries And Their Properties

Lithium-sulfur batteries(LSBs)have received widespread concern in recent years due to their advantages of high energy density,environmental friendly,low cost of raw materials and so on.LSBs are expected to replace the current lithium-ion batteries to be a new generation of commercial batteries to meet the requirements of the ever-changing electronic products and electric vehicles in battery life and energy density.However,the long-cycle stability of LSBs is still limited by the poor conductivity of elemental sulfur in positive electrode,the dissolution of intermediate product polysulfide,the expansion of the volume caused by the different density between charge/discharge products and active material and the shuttle effect.In order to improve the long-term stability of LSBs,this dissertation fabricated the composites of TiO2 and sulfur based on the fact that the metal oxide TiO2 can chemically adsorb polysulfides and provide storage space for the elemental sulfur.By synthesizing two types of porous TiO2 and the related TiO2/S composites,the cycle stability of the devices had been significantly improved.The research of this thesis can be divided into the following two sections:(1)Using P123 as a surfactant,mesoporous SiO2 was synthesized and used as a hard template for fabricating mesoporous TiO2.The synthesized mesoporous TiO2 was used as the host material of sulfur,and the TiO2/S composites were fabricated via the melt-diffusion method.The as-prepared complex was used as the electrode material of lithium-sulfur battery,and displayed good electrochemical properties.As a result,the specific discharge capacity of the first cycle is 444.4 mAh g-1,and the specific charge capacity is 409.9 mAh g-1,the Coulomb efficiency is as high as 92.2%.After 200 cycles,it still maintained a capacity of 343.5 mAh g-1,with a small average capacity decay rate of 0.12%and nearly 100%of Coulomb efficiency per lap.(2)The commercialized P25 was used as raw material,and the titanate nanowires precursor was synthesized by hydrothermal method.The target product rutile TiO2 mesocrystals was prepared by stirring under acidic condition.The TiO2 and sulfur composites(denoted as RTMs/S)were synthesized by mixing TiO2 mesocrystals with sulfur via a melt-diffusion method so that elemental sulfur was introduced into the nanopores of rutile TiO2 successfully.Making the electrodes with composites,as a result,the RTMs/S composite electrode exhibited obvious advantages in electrochemical properties,with a specific discharge capacity of 667.7 mAh g'1 in the first cycle,significantly higher than that of the P25/S composite electrode(453.4 mAh g-1)and titanate/S composite electrode(345.7 mAh g-1).The specific charge capacity of the first cycle is 623.0 mAh g-1 and the Coulomb efficiency is as high as 93.3%.After 300 cycles,RTMs/S composite electrode still maintained a specific capacity of 427.4 mAh g-1,with a small average capacity decay rate of 0.12%per cycle,and the Coulomb efficiency is about 100%.