Synthesis And Electrochemical Performance Of Spinel Lithium Titanate

Spinel Li4Ti5O12 is a promising candidate of negative electrode materials for lithium-ion batteries due to its excellent cycle performance and safety, low volume change and cost. It has been ranked as the secondary negative electrode material for lithium-ions power battery by DOE(Department of Energy, USA). A study on the synthesis, improvements, structural characteristics, electrochemical performance, theoretical capacity and intercalation mechanism of spinel Li4Ti5O12 were carried out systemically and in detail through test measures such as TG, XRD, XPS, SEM, CV, EIS, etc.Spinel Li4Ti5O12 with excellent performances was prepared via optimized solid state method and hydrolyzation method, respectively. Experiment results indicated that the ratio of raw materials Li/Ti, calcination temperature and reaction time played an important role in the electrochemical performances. The optimized condition for solid state method was: The raw materials Li/Ti with the ratio of 0.86 were calcinated at 800℃for 12h. The prepared sample at this developed condition possessed well-distributed morphology, high phase purity and degree of crystallinity, its reversible capacity and capacity retention after 50 cycles were 158mAh·g-1 and 97.5%, respectively. The hydrolyzation method could synthesize spinel Li4Ti5O12 at lower ratio of raw materials Li/Ti and calcination temperature and shorter reaction time for its lower-sized and amorphous TiO2 precursor. The optimized condition for hydrolyzation method was: The raw materials Li/Ti with the ratio of 0.83 were calcinated at 750℃for 8h. The prepared sample at this developed condition possessed well-distributed morphology, high phase purity and degree of crystallinity, its reversible capacity and capacity retentionafter 50 cycles were 169mAh·g-1 and 99.5%, respectively. Reducing the reactants'granularity and the actived energy of reaction could effectively lower the calcination temperature and shorten the reaction time, thus reducing the average size of the obtained samples, make excellent groundwork for electrode materials to possess better electrochemical performance.In order to improve the rate performance, the effect of 16d sites Li doping and bulk Ag doping on the structure and electrochemical performances of spinel Li4Ti5O12 were studied, respectively. Li doping could effectively increase the lithium-ion and electronic conductivity of Li4+xTi5-xO12-δ(0≤x≤0.2), evidently improve its rate performance. With the increasing of Li doping amount, lithium-ion and electronic conductivity of Li4+xTi5-xO12-δ(0≤x≤0.2) increased, however its cycling stability was depressed when the Li doping was of x=0.2 for the higher amount of oxygen vacancy. The Li doping of x=0.1, the appropriate Li doping amount, showed improved rate capability and better high rate performance comparing to undoped sample; Because the ionic radius of Ag+ was much bigger than that of the Ti4+, the Ag+ could be doped into the lattice of spinel Li4Ti5O12. Ag could increase the electronic conductivity of Li4Ti5O12/Ag remarkably, the electronic conductivity of Li4Ti5O12/Ag increased with the content of Ag increasing. Because metal Ag could not accommodate Li, the high Ag content would decrease the reversible capacity of Li4Ti5O12/Ag. Compared to simple hydrolyzation method, ultrasonic-assisted method could enhance the dispersity of Ag particles and reduce the particle size of Ag particles. It could evidently increase the electronic conductivity of Li4Ti5O12/Ag at lower Ag content, contributing to the better rate performance and higher reversible capacity of Li4Ti5O12/Ag.The electrochemical performances of spinel lithium titanate in the voltage of 2.0~1.0V were also deeply explored. The higher electronic conductivity and lithium-ions diffusion coefficient of the reduction product Li7Ti5O12 contributed to the excellent high power charge performance of spinel lithium titanate. According to this situation, the effect of working modes on the rate performances of spinel lithium titanate were stuied. Under low discharge and fast charge mode, the high rate performances of spinel Li4Ti5O12 were greatly improved. It only lost 9.6% of the reversible capacity of 0.5C even at 30C.In order to further empolder the reversible capacity and rate performances of spinel lithium titanates, we had stuied its electrochemical performances under low potential and the influence of acetylene back on its electrochemical performances. When discharge voltage of Li4Ti5O12 extended from 1.0V to 0.01V, its cycling stability was not affected and its reversible capacity was increased. Under 0.6V, acetylene back was both electronic conducting additive and lithium-ion conducting additive for Li4Ti5O12, which made the reaction area of electrode be inhanced markedly and contributd to the excellent high rate performances of spinel lithium titanate under 0.6V. Accordingly, both the reversible capacity and high rate performance of spinel lithium titanate were improved when the discharge voltage extended from 1.0V to 0.01V. At last, combining the electrochemical and XRD results with the crystal structure of spinel lithium tinitades, we demonstrated the corresponding reaction mechanism of the low-potential intercalation behavior of Li4Ti5O12 and modified the classical viewpoint on the theoretical capacity of Li4Ti5O12. Under 0.6V, lithium-ions could intercalate into the tetrahedral sites of Li7Ti5O12. The theoretical capacity of Li4Ti5O12 was limited by the number of tetravalent titanium, but not the octahedral or tetrahedral sites to accommodate lithium-ions in the voltage range of 2.0V to 0.01V, corresponding to 291.8mAh·g-1, but not 291.8mAh·g-1 or 291.8mAh·g-1. The shape of the discharge curves of spinel lithium titanate were totally different due to the different way of lithium-ions being accommodated in the interstitial positions in the corresponding voltage ranges of 2.0~0.6V and 0.6~0.01V.