| Due to its high energydensity, lithium ion batteries are widely used in all kinds of portable electronic products. Carbonaceous materials have been widely used as anode material, but carbonaceous anode exist crucial safety concerns, so it is necessary to develop new types of anode materials with higher energy density and better security. As a zero-strain insertion material, spinel Li4Ti5O12showed excellent cycle performance. Due to the security and stability, Li4Ti5O12was considered to be promising for lithium-ion battery. However, seriously affected by the morphology, crystal structure and the form of crystal plane, conductivity of lithium ions and electronic of Li4Ti5O12material interface and internal need to be further improved. Li4Ti5O12with different crystal structures were synthesised by hydrothermal method, and their structures were characterized by XRD and SEM, the electrochemical properties were characterized by EIS and charge-discharge tests. The influence of the crystal structure on the electrochemical properties of Li4Ti5O12was investigated, and relationship between LiTi5O12crystal structure and electrochemical properties was established. Furthermore, doping research were conducted for the poor high rate performance of Li4Ti5O12.The main results are as follows:1. controllable synthesis of Li4Ti5O12by hydrothermal methodThe influence of hydrothermal reaction conditions on the crystal structure and morphology of Li4Ti5O12was investigated.(1)The reaction conditions had a certain influence on purity of the material. When n(Li):n(Ti)=4.75:5, LiOH concentration was between3mol·L-1～5mol·L-1, hydrothermal reaction temperature was between120℃～180℃, reaction time was greater than24h and calcination temperature was higher than800℃, the synthesized samples were phase-pure Li4Ti5O12. The capacity of non-pure phase Li4Ti5O12was relatively lower.(2)The reaction conditions had a certain influence on crystals tructure of the material. With n(Li):n(Ti) increased, the proportion of (311) plane first increased and then decreased; with LiOH concentration increased, the proportion of (311) plane first increased and then decreased; between100℃～160℃, with hydrothermal reaction temperature increased, the proportion of (311) plane first increased and then decreased; with calcination temperature increased, the proportion of (311) plane decreased. With hydrothermal reaction time increased, Li4Ti5O12crystal changed from (111) plane dominanted to (400) plane dominanted.(3)The crystal structure of the sample had some influence on EIS test results and discharge capacity. When one condition of n(Li):n(Ti), LiOH solution concentration, hydrothermal reaction temperature and calcination temperature changed, with the proportion of (311) plane increased, the charge transfer resistance Rct, gradually reduced. Moreover, when the proportion of (311) plane was larger, the structure of the material was relatively stable, and its discharge capacity was higher.(4)With the increase of hydrothermal reaction temperature, hydrothermal reaction time, and calcination temperature, material particles increased gradually and agglomerated, so its rate performance deteriorated, while the proportion of the plane had no significant effect on the rate performance.(5)Addition of NH4F made XRD diffraction peaks shift to higher angle direction, and the crystal structure changed,(002),(200) and (-133) plane occured. Irregular morphology of the particles changed to hexahedron. With the amount of NH4F increased, the proportion of (200) plane increased, and the value of Rct, increased, charge-discharge performance was deteriorated.2. Hydrothermal synthesis of Y-doped Li4Ti5O12Y-doped Li4Ti5O12made XRD diffraction peaks of the (111) plane shift to higher angle direction, and lattice parameter was smaller. With the doping amount of Y increased, the discharge capacity of samples at0.1C gradually decreased. However Y-doped Li4Ti5O12showed better charge-discharge performance at high rate. |
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