Study On The Phase Transition Mechanism Of C/Ti_nO_2n-1 Nanocomposites And The Performance Of Lithium-sulfur Batteries

With the ever-increasing demand for portable electronic devices,electric vehicles,smart grid and renewable energy,batteries are required to have the advantages such as light weight,high power density and long cycle life.Lithium-sulfur(Li-S)batteries are regarded as the most potential next-generation energy storage systems due to their high theoretical discharge capacity(1675 mAh/g),energy density(2600 Wh/kg),abundant reserves of sulfur,low cost and environmental friendly.However,due to the poor conductivity of sulfur cathode,volume expansion and the "shuttling effect" of lithium polysulfides during cycling strongly hinder the commercial application of Li-S battery.Therefore,developing the new sulfur hostr materials to overcome the above mentioned problems have great practical significance for improving the performance of Li-S batteries and prolonging their cycle life.TinO2n-1(4≤n≤10)is a series of non-stoichiometric titanium oxides with specific crystal structures,excellent electrical conductivity,chemical stability and strong adsorption ability on lithium polysulfides.The use of TinO2n-1 as sulfur hosts can significantly inhibit the "shuttling effect" and improve the cycling performance of Li-S batteires.Therefore,it is of great significance to carry out the research on the synthesis,preparation and application of TinO2n-1 in Li-S batteries which can provide new research ideas for the design and development of energy storage materials.In this paper,carbon black and polydopamine(PDA)were mixed with TiO2 by ball milling and chemical coating method,respectively.TinO2n-1 phases with different n values were obtained by the carbothermal reduction reaction(CRR)at different calcination temperatures.The results of XRD and TEM show that the n value of TinO2n-1 phases decreases with the increase of calcination temperature.The formation sequence of the TinO2n-1 phases during the CRR was TiO2→Ti20O39→TinO2n-1(n=4-10)→Ti3O5→Ti2O3→TiO.The surface encapsulation of PDA on TiO2 precursor can effectively reduce the reaction temperature and inhibit the coarsening of grains during the formation of TinO2n-1 phase with the same n value when compared with that prepared by ball mill method.Two types of interface structures between different TinO2n-1 phases are further studied by means of TEM.The results showed that coherent interface was formed between Ti5O9/Ti4O7 through phase transition,while the incoherent interface generated between Ti4O7/Ti3O5 through oriented attachment(OA)growth mechanism.Thermodynamic calculation results of the CRR suggested that the reaction of Ti4o7+CO=Ti3O5+CO2 cannot process spontaneously in the temperature range of 298-1598 K,which further proved the OA growth mechanism of Ti4O7/Ti3O5 interface.Based on the above results,the formation mechanisms of the two types of interfaces were summarized at both atomic level and nanoscales.The mesoporous hollow carbon sphere(HCS)fabricated by hard template method was used as template to design and synthesize the HCS@Ti4O7 nanocomposites with excellent conductivity and large specific surface area(512 m2/g).Such material was used as sulfur hosts of Li-S batteries,which can achieve 70%high sulfur loading content.The first cycle discharge capacity can reach 1427 mAh·g-1 at the current density of 0.1 C.When the current density increased to 0.5 C,the first cycle discharge capacity can reach up to 1168 mAh·g-1 and remain at 601 nAh·g-1 after 800 cycles with the capacity decay rate of only 0.06%per cycle,indicating the favorable cycling stability.Furthermore,the morphology and structure of the HCS@Ti4O7/S electrode after cycle were analyzed by TEM.It was found that the morphology and composition distribution of the electrode material after 800 cycles was nearly the same with the fresh material,suggesting the excellent structural stability of such electrode materials.By carrying out the XPS analyzes on the HCS@Ti4O7 sulfur hosts,it was found that the Ti 2p spin splitting peaks shifted 0.2 eV toward low energy direction after the adsorption of Li2S6.Meanwhile,the characteristic peak of Ti-S bond appeared in the spectrum.The results indicated that HCS@Ti4O7 had strong chemical adsorption on lithium polysulfides.