Synthesis And Performance Of TiO₂-based Nanocomposite Electrode Materials For Li-air Batteries

With the urgent demands of large-scale energy devices,such as electric vehicles?EVs?and hybrid electric vehicles?HEVs?,the attention of next-generation battery has focused on lithium-air?Li-air?batteries because of their remarkably high energy density(3600 Wh·kg^-1).However,the Li-air technology is still remained at an infancy stage of development at this moment,there are still many challenges,such as the low achieved capacity and energy density,lack of high-rate discharge ability,insufficient capacity sustainability and low round-trip efficiency.The cathode electrode,including materials and structures,has been identified to play the most critical role in improving the performance of Li-air batteries.Particularly,high performance electrocatalysts which can decrease the charge/discharge overpotential and improve the cycle ability are eager to be developed.The transition metal oxides were very attractive to scientific community due to their easily preparation process,low cost and good catalytic performances.TiO₂ is good catalysts for Li-air batteries because of its superior catalytic properties,excellent stability,and environmental friendliness,therefore it is has a great potential for application.In this work,based on the advantages of TiO₂,the catalytic activity was further improvd by means of doping and compound.The fabrications of superior catalytic activity nanocomposite cathode materials and the influence of its composition,morphology and structure on the performance of battery were discussed.The results obtained provides theoretical basis and technical support for such research.?1?The Fe-TiO₂/N-doped graphene was prepared by sol-gel method.After nitrogen dopiong,graphene has better physical and chemical properties than the pristine graphene.By introduction N-doped graphene can not only improve the composite conductivity but also prevent the TiO₂agglomeration.The magnified FESEM image reveals that a large number of Fe-TiO₂ nanoparticles with a small particle size of³0 nm are uniformly dispersed on N-doped graphene.Combine XPS with Raman can properly testify that Fe³⁺ions can incorporate into the lattice to from Ti-O-Fe bonds,which will enhance the catalytic and improve the conductivity.The Fe-TiO₂/N-doped graphene electrode delivers a discharge capacity of 13,500 mAh g^-11 and a cyclic retention of 45cycles,which are much better compared to the pristine counterpart.?2?The amorphous oxygen-deficient TiO_2-x-x thin layer coated carbon nanotubes?CNTs?are synthesized by atomic layer deposition?ALD?and employed as cathode materials for lithium-air battery.The cathode demonstrates high electrocatalytic activity towards electrode reactions,resulting from the introduction of oxygen-deficient TiO_2-x into the nanocomposites.It is found by XPS that the intrinsically isotropic nature of amorphous TiO₂ which have certain amount of Ti^3.5+and Ti³⁺.Consequently,the battery with the corresponding CNTs@TiO_2-x cathode shows high discharge/charge capacities and good cycling performance,which delivers a discharge capacity of11,000 mAh g^-11 and the cyclic retention of more than 90 cycles are achieved,while with the pristine CNTs only 50 cycles are obtained.?3?Nitrogen-doped carbon nanofibers?N-CNFs?prepared by electrospinning are used as flexible substrate and an amorphous TiO₂ layer is coated by ALD technique and then decorated with Ru nanoparticles.The Ru/N-CNFs@TiO₂ composite is directly used as a free-standing electrode for Li-O₂ batteries and the electrode delivers a high specific capacity,improved round-trip efficiency and good cycling ability.The superior electrochemical performance can be attributed to the amorphous TiO₂ protecting layer and superior catalytic activity of Ru nanoparticles.Based on density functional theory?DFT?calculations from first principles,the carbon electrode after coating with TiO₂ is more stable during discharge/charge process.The analysis of Li₂O₂ on three different interfaces?Li₂O₂/N-CNFs,Li₂O₂/TiO₂,and Li₂O₂/Ru?indicates that the electron transport capacity was higher on Ru and TiO₂ compared with N-CNFs,therefore,Li₂O₂ could be formed and decomposed more easily on the Ru/N-CNFs@TiO₂ cathode.The reversibility of the electrode reaction is improved.