Design And Synthesis Of TiO₂ Loaded Sulfur Composites And Their Enhanced Performances For Lithium-sulfur Battery

With the growth of automobile production day and night,the gradual exhaustion of oil resources and the increasing seriousness of environmental pollution,it is extremely anxious to exploit renewable energy instead of non renewable energy,especially the electrochemical energy storage system.Lithium sulfur battery is one of the lithium batteries which takes sulfur element as the positive electrode and lithiummetal as the negative electrode.Both of the theoretical capacity and specific energy are high,which are 1675 mAh/g and 2600 Wh/kg,respectively.When comparing with lithium ion batteries,lithium sulfur batteries have the advantages of light weight and small volume in case of the same energy storage.And the elemental sulfur has lots ofadvantages such as rich earth reserves,environmental friend and so on.It can be seenthat the active material of the lithium sulfur battery is an excellent energy storage material.Lithium sulfur battery has bright prospects in the new energy,and it will play an important role in alleviating the usage of fossil fuel and reducing the greenhouse effect.But there are several problems to be solved:firstly,the conductivity problem(the conductivity of sulfur species are extremely poor),which seriously affects the high rate performance of the batteries.Then,the volume problem(the largely volumetric expansion/shrinkage of sulfur during the discharge/charge processes),leading to the damage of sulfur cathode.Last but not least,solubility problems(lithium polysulfides are easy to dissolve in electrolyte),which will results in shuttle effect.In this paper,in order to solve these issues,we design and synthesize two types of titanium dioxide loaded sulfur composites(hollow carbon sphere-titanium dioxide/sulfur composite and titanium dioxide-titanium carbide/sulfur composite),which can be used as the sulfur positive electrode.These two composites have the advantages of good conductivity,porous channel,strong chemical interaction with polysulfides and so on,which can solve the poor conductivity of sulfur species,the largely volumetric expansion/shrinkage of sulfur element,the serious shuttle effect of lithium polysufides and so on.The main research results as follows:(1)Hollow carbon sphere-titanium dioxide/sulfur(HC-TiO2/S)compositeWe combine the template and melting method to prepare the HC-TiO2/S composite.Firstly,we prepare silica.Then,HC-TiO2 is prepared by template method.Finally,HC-TiO2/S composite is prepared by melting method.The results of XRD,SEM,TEM,XPS indicat that we have successfully prepared the designed HC-TiO2/S composite.Hollow carbon spheres and titanium dioxide have synergistic effect,which can effectively solve the above mentioned problems.On one hand,hollow carbon spheres have good conductivity,which can compensate for the non-conductivity of sulfur.On the other hand,hollow carbon spheres have enough space to store sulfur and sulfur compound which produced in the reaction process,and then weaken the volume change and shuttle effect.Titanium dioxide has strong chemisorption with polysulfides,which can effectively inhibit the shuttle effect.The test results show that the lithium sulfur batteries have enhanced performance,such as good cycle stability and excellent rate performance.HC-TiO2/S cathode can deliver a discharge specific capacity of 671 mAh/g at 1 C after 300 cycles.When cycled at 0.05,0.1,0.2,0.5,1,2 and 4 C,the reversible capacity of the HC-TiO2/S cathode were 1227,1050,922,809,716,621 and 527 mAh g-1,respectively.And when the current rate come back to 0.1 C after discharge-charge several times at a series of different rate,the discharge capacity can be recovered to 1042 mAh/g.(2)Titanium dioxide-titanium carbide/sulfur(TiO2-TiC/S)compositesTiO2-TiC/S composite are prepared by water bath oxidation-reduction reaction and melting-permeation method.Firstly,porous TiO2-TiC was prepared by water bath oxidation-reduction reaction.Then,TiO2-TiC/S composite was prepared by melting method.The results of XRD,SEM,TEM,XPS tindicate that we have successfully obtained the TiO2-TiC/S composite.Among them,titanium carbide and titanium dioxide can play great roles in solving the problem of lithium sulfur batteries.TiC has extremely excellent conductivity,which can solve the problem of sulfur species' poor conductivity.TiC and TiO2 have strong chemisorption of lithium polysulfides,which can effectively inhibit the shuttle effect.Moreover,porous TiO2-TiC has enough space to store sulfur and their reaction products,which can inhibit the effect of volume change and shuttle effect on battery consumption.The electrochemical performance test shows that the TiO2-TiC/S electrode has relatively good electrochemical properties.The CV test shows that the electrochemical reversibility and cycle stability are good.EIS test shows that the electrical conductivity is extremely excellent.The cyclic performance test shows when the battery is cycled at 1 C rate,the discharge capacity can still stabilized at 591 mAh/g after 200 cycles.