Study On The Modification And Electrochemical Performance Of TiO₂/S Cathode Material For Li-S Batteries

In recent years,with the rapid growth of demand for the high-performance energy storage devices,such as intelligent electronic devices and electric vehicles.People began to develop a variety of new energy storage devices with high specific energy.Lithium-Sulfur batteries are considered to be one of the most promising candidates for next-generation high energy secondary batteries which has a high theoretical specific capacity of 1675 mA h·g-1 and energy density of 2500 Wh·kg-1.Sulfur is abundant element on earth with low cost,low toxicity and environment friendly.Unfortunately,there are several challenges toward the commercialization of a Li-S battery system.Such as almost the intrinsic insulating nature of S,the dissolution of polysulfides intermediate species(Li2Sx,4?x?8)and the resulting shuttling effect which bring about the low utilization of active meterial,poor cycling stability and low rate performance.This work has briefly illustrated the working mechanism and existing defects of Li-S batteries.Moreover,summarized the study progress in recent years.In view of above problems existed in Li-S batteries,we carried out a study on modifying of sulfur cathode materials.By the introduction of metal oxides TiO2,to improve the performance of lithium sulfur batteries.However,many oxides usually possess low electrical conductivity.Black TiO2,with the presence of oxygen vacancies and Ti3+ species,is a very good electronic conductor which has strong chemical bonds with S specieswe introduce the absence of oxygen by reducing the TiO2,thus improve the electronic conductivity.Based on this,Application of metal oxide with the presence of oxygen vacancies and high conductivity polymer materials that combine the attractive properties of the two components is viewed as an effective project to tune the behavior of the resulting materials for enhanced Li-S batteries performance.This kind of material can effectively improve the electron conductivity,suppress the dissolution of polysulfides intermediate species as well as the shuttle effect,which increase the utilization of active materials and cycle stability of Li-S batteries to a large extent.Specific work is as follows:(1)Titanium glycolate spheres were prepared via sol-gel method,then amorphous porous titanium dioxide spherical particles were obtianed through thermal hydrolysis.The B-TiO2 species with the presence of oxygen vacancies and Ti3+ are obtained after annealing through in a reducing gas atmosphere.B-TiO2/S composites was prepared by heat treatment using TiO2 and sulfur.The B-TiO2/S composites exhibit a prominent improvement from the first discharge specific capacity of 1047 mA h·g-1to a reversible capacity of 740 m A h·g-1after 50 cycles.(2)We prepared PAS@R-TiO2 materials using porous TiO2 spherical particles as precursor and phenolic formaldehyde(PF)as carbon source.A thin layer of the mixture solution of PF is uniformly coated on the surface of TiO2 precursors through a cooperative assembly-directed strategy in isopropanol.After annealing at 700°C,the oxygen vacancy appeared in porous black TiO2.phenolic formaldehyde formed high conductive polyacenes PAS polymer via pyrolysis method.The hierarchically nanoporous PAS@R-TiO2 spheres are obtained via an in situ carbothermal reduction.The PEG was removed after annealing.The phase and morphology were characterized by XRD,XPS and SEM.PAS@R-TiO2 spheres with diameter of 400-500 nm were prepared,and The average thickness of PAS outer shell is around 10 nm.Lastly,Sulfur is encapsulate into the pores through a melting diffusion method,which contained PAS@R-TiO2/S composite.The PAS@R-TiO2/S electrode exhibits a prominent improvement from the first discharge specific capacity of 1 066 mA h·g-1 to a reversible capacity of 829 mA h·g-1after 100 cycles,corresponding to capacity retention of77.8 %.Rate capability of three electrodes were also measured at different current density from 0.2 C to 2 C.After returning back to 0.2 C rate,a reversible capacity of 976mAh·g-1was achieved with the PAS@R-TiO2/S electrode,meaning a good rate performance and high cycling stability.