Study On Modification Of The Doped Li₄Ti₅O₁₂ Anode Material For Lithium-ion Battery

Spinel Li4_Ti₅O₁₂ has drawn extensive attention as one of the most promising anode material candidates for rechargeable lithium-ion batteries, due to its easy preparation, low raw material cost, environmental friendliness, excellent safety and reliability compared with those of carbon electrodes. However, the key limitation of Li4_Ti₅O₁₂ is its low ionic and electronic conductivity, calculated to be 10-9 S/cm at room temperature, which really hinders its using for commercial application such as HEV (hybrid electric vehicle) and power storage batteries. In this paper, Li4_Ti₅O_12-xXx(X=F, Cl, Br; 0≤x≤0.3) were prepared by high temperature solid state reaction, and doping with halogen anions improves their own ionic and electronic conductivity to enhance the electrochemical properties.Spinel Li4_Ti₅O_12-xXx(X=F, Cl, Br; 0≤x≤0.3) anode materials doped by halogen anions were prepared by a high temperature solid-state method using TiO2-anatase and LiOH? H2O as raw materials. The as-prepared samples are blocky-shaped particles, which have a high purity, good crystallinity, and homogeneous size distribution with a scale of 1-10μm. The content of this research is as follows:The anode materials Li4_Ti₅O_12-xFx (x=0, 0.1, 0.2, 0.3) with a trace amount of F doping were synthesized by solid-state method. The results of X-ray diffraction showed that the F-doped Li4_Ti₅O₁₂ have a fine characteristic phase of spinel Li4_Ti₅O₁₂. With increasing F content of Li4_Ti₅O_12-xFx, there is an increase in the lattice constant, and the peaks shift to higher angels. The investigation of SEM showed that the homogeneous size distribution with a scale of 1-10μm. Although the discharge capacities of F-doped samples were decreased, the cycle stability of Li4_Ti₅O_12-xFx was enhanced obviously at current density of 2C. This indicates that the structural stability of Li4_Ti₅O₁₂ with F-doping can be further improved. In addition, F-doping can raise the ion conductivity of Li4_Ti₅O₁₂, which may be one of main reasons for improving the cycle stability of materials at high current density.Li4_Ti₅O_12-xClx (0≤x≤0.4) anode materials were successfully synthesized by high temperature solid-state method. The diffraction peaks of XRD for all samples corresponded to a single phase spinel structure and no other impure peaks were detected. With increasing Cl content of Li4_Ti₅O_12-xClx, there is an increase in the lattice constants. From the scanning electron micrographic (SEM) photographs, it is found that all of the as-prepared particles had the similar blocky-shaped morphology with the particle size distribution in a scale of 1-10μm. Electrochemical evaluation presented that the discharge capacity were notably enhanced by Cl-doping,with the discharge capacity of 120 mAh/g at 2 C for Li4_Ti₅O_12-xClx (x = 0.2), which was nearly 33 mAh/g higher than that of undoped sample (the discharge capacity of Li4_Ti₅O₁₂87.3 mAh/g).It indicates that Cl-doping can effectively increase the discharge capacity of Li4_Ti₅O₁₂ at high current density. But the discharge capacity of Li4_Ti₅O_12-xClx decreased when xï¹¥0.2.However, doping with Cl- can raise the ion conductivity of Li4_Ti₅O₁₂ by increase in the amount of Ti3+ for Li4_Ti₅O_12-xClx.Br-doped Li4_Ti₅O₁₂ in the form of Li4_Ti₅O_12-xBrx (0≤x≤0.3) compounds were successfully synthesized via high temperature solid state reaction. The morphology, structure and electrochemical properties of the spinel Li4_Ti₅O_12-xBrx (0≤x≤0.3) materials were investigated. SEM photographs and XRD of samples Li4_Ti₅O_12-xBrx are very similar with that of Li4_Ti₅O_12-xClx. Electrochemical performance tests show that the sample Li4_Ti₅O_12-xBrx (x = 0.2) presents the best discharge capacity among all the samples, and shows better reversibility and higher cyclic stability compared with pristine Li4_Ti₅O₁₂, especially at high current rates. When the discharge rate was 0.5 C, the Li4_Ti₅O_12-xBrx (x = 0.2) sample presented the excellent discharge capacity of 172 mAhg-1, which was very close to its theoretical capacity (175 mAhg-1), while that of the pristine Li4_Ti₅O₁₂ was 123.2 mAhg-1 only.