Design,Preparation And Applications Of Titanium-based Metal Oxide Nanomaterials In Lithium-sulfur Batteries

In order to meet the urgent needs of large-scale energy storage and portable power for industrialization,informationization,electrification and urbanization in China,the development of cheap,high-performance electrical energy storage technology and key materials has become one of the key points of China’s strategic development,and is also one of the frontiers and research hotspots of the world wide scientific and technological development.In recent years,lithium-sulfur batteries（LSBs）have become a new generation of high-performance and inexpensive candidate energy storage systems after lithium-ion batteries due to their higher energy density,cheap and abundant sulfur resources.However,the development and application of lithium-sulfur batteries are also limited by the existence of a series of key issues to be solved,mainly including the following two categories of problems:（1）Dissolution of lithium polysulfide formed in the cathode electrode side and its"shuttle effect"during the charge and discharge processes;（2）Large volume expansion and electrical insulation of the sulfur cathode electrode.For the problem（1）,scholars mainly adopted two strategies of"prevent"and"block",by introducing sulfur carriers that have chemical and physical adsorption in the cathode electrode,the soluble polysulfide is adsorbed on the surface as much as possible,prevent it from detaching the carrier and entering the electrolyte.Meanwhile,multifunctional selective penetration separator with a denser structure and strong chemical adsorption capability to soluble lithium polysulfides have been developed,to block and to intercept the polysulfide entered into the electrolyte,and finally avoid the shuttle of polysulfides to the surface of the lithium metal anode.Therefore,the energy density and cycle stability of the lithium-sulfur battery can be effectively improved.For the problem（2）,scholars mainly choose three-dimensional nanomaterials with better conductivity and larger pore volume as sulfur carriers,to improve the rate performance and cycle stability of lithium-sulfur batteries.Based on the above background,combined with the rich resources of titanium-based nanomaterials and its excellent chemical adsorption properties to polysulfides,a series of titanium-based oxide nanomaterials with different morphologies and microstructures were synthesized,including nanowires,nanowires/sheets hybrid and nanowire clusters,nanoparticle clusters.Subsequently,according to the different functions requirements of the cathode and separator for lithium-sulfur batteries to"prevent"and to"block"lithium polysulfides,the nanowires/sheets hybrid and nanowire clustered structures were used to modify the separator and as the cathode host material for lithium-sulfur batteries,respectively.The main research works done in this dissertation are as follows:（1）Based on the functional requirements of the separator for lithium-sulfur batteries,a sandwich structured modification layer composed of titanium dioxide nanowires and sheets was designed and applied on the cathode side surface of the traditional polypropylene（PP）separator.Resulted from the synergistic effect of the unique sandwiched structure of the modified layer and the strong chemical adsorption to lithium polysulfide on the polar surface of titanium dioxides,the channel for the polysulfides shuttle is effectively"blocked"without affecting the normal transportation of lithium ions,and thus,the cycle stability of lithium-sulfur batteries is obviously enhanced.Herein,the Ti O₂ nanowires and the nanowires/sheets hybrid（STO-W/S）are prepared directly by an optimized hydrothermally assisted low-temperature annealing process,Being dispersed in ethanol,the Ti O₂ nanowires and the nanowires/sheets hybrid（STO-W/S）were prepared on the surface of the PP separators by flow-directed self-assembly process,The obtained modification layer are about 4microns.The testing results on the electrochemical performance studies show that the modified PP separators gain significant improvements on the wettability,ion mobility and polysulfide shuttle suppression effect over neat PP separators.The lithium-sulfur battery assembled with the modified PP separators exhibits high capacity and long cyclic stability,namely,813 and 541 m Ah g^-1 at 1 C for the 1^st and 500^th cycles,respectively,with a capacity fading rate of～0.067%for each cycle.The results observed above further manifest that the newly proposed strategy for the development of multi-functional separator for high-performance lithium-sulfur batteries are feasible and effective.（2）In terms of the sulfur fixing capability of the cathode electrode host materials to sulfur and its compounds,a clustered titanium dioxide nanowires structure is designed and synthesized and then used as the host material for the sulfur.Benefited from its highly porous structure,larger specific surface area,and strong chemisorption,the soluble polysulfide formed during the charge and discharge process can be effectively"prevented"from dissolving and detaching from the clustered titanium dioxide nanowires host structure.Herein,the Ti O₂ nanowire clusters were prepared directly by optimized hydrothermally assisted low-temperature annealing.Subsequently,the cathode material of S@Ti O₂ nanowire clusters was further prepared by melt-diffusion method.The loading amount of sulfur is about 69 wt.%.The results of electrochemical testes indicate that the charge-discharge capacity performance and the capacity fading rate of lithium-sulfur batteries using S@Ti O₂ nanowire clusters as cathode electrodes are better than that of lithium-sulfur batteries with Ti N nanoparticle clusters and porous carbon black combined with sulfur at different current densities（0.1 C,0.2 C,0.5 C,1 C,1.5 C and 2 C）.And it can still provide a capacity of about 522m Ah g^-1 at a high rate of 2 C after 400 cycles.The capacity fading rate of each cycle is only～0.086%.In summary,the rational design of the nanostructure of the sulfur host materials and the reasonable modification of the separators can effectively"prevent"and"block"the dissolution and shuttle of polysulfide generated during the charging and discharging processes of the lithium-sulfur battery,However,it still needs further optimize the morphology,structure and composition of the titanium-based nanomaterials from the perspective of the synthesis process,and enhance the"prevent"and"block"effectiveness of the materials against polysulfides.