Preparation And Properities Of Li₄Ti₅O₁₂ As Anode Material Of Lithium-ion Batteries

Lithium ion battery is a new generation green non-pollution battery which was introduced into the market in the 1990s. It is widely used in portable electron apparatus and cars due to its highlights, such as high voltage, low self-discharge rate, compass size, light weight and non-memory effect. The anode materials for lithium ion batteries are the key to constrain its whole performance. The lithium ion batteries generally use carbon/graphite as the anode material. But when the battery discharged for the first time, there is passivation film formed on the surface of carbon, which will cause capacity loss. Moreover, the potential of carbon material is very close to that of lithium, when overcharged, the metal lithium will be separated out on the surface of carbon, which is the intrinsic safety problem. While Li₄Ti₅O₁₂, when used as anode material for Li-ion battery, there is no such concerns, and there is almost no structure change during the charge/discharge process, that is why there is so small irreversible capacity loss. The potential of Li₄Ti₅O₁₂ is 1.55 V (vs. Li/Li⁺), when combined with 4 V cathode materials such as LiCoO₂, LiNiO₂ or LiMn₂O₄, the material Li₄Ti₅O₁₂ can construct a 2.5 V cell. So it attracts many researchers' interests.The lithium titanate composites were synthesized by high temperature solidstate method and doped monobasicly and dibasicly, and then the influences to the samples as the anode materials for lithium ion battery were studied with different doping ratio and doping elements. According to the ion diameter, the following doping plans were chose: Fe³⁺, Cr³⁺, Sn⁴⁺ monobasic doping and Sn⁴⁺, Cr³⁺ dibasic doping. The crystalline structure of the samples was analyzed by X-ray diffraction analysis, and the electrochemical proprieties of the samples were tested by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic cycling tests. The results were listed as follows:The XRD test indicated that, the spinel lithium titanate composite Li_4/3Ti_5/3O₄ was successfully synthesized by the traditional solid-state method at 900℃. The peaks of the sample were identical with that of the standard chart (26-1198) with some differences of the intensity and width of the peaks. Cation doping did not change the spinel structure of the sample, but when with larger doping amount, the spinel lithium titanate samples were not very pure.The un-doped lithium titanate sample as the anode material for lithium ion battery had an initial capacity of 172.62 mAh·g^-1, and a flat discharge platform around the voltage of 1.55 V. The discharge capacity of the platform took up 90% of the initial discharge capacity. The sample obtained a capacity of 144.9 mAh·g^-1 with a capacity retention of 90% after 50 cycles.The iron doping had effectively decreased the discharge voltage platform of the samples as the anode materials for lithium ion battery, but at the same time, the iron doping also decreased the electrochemical performances of the samples, and when the doping ratio was Fe∶Ti=2∶1 (molar ratio), the capacity retention of the sample was only 65.3%; the Cr doping decreased the discharge voltage with a wide and flat platform around 1.4 V. The Cr doping increased the capacity of the material, but when with a larger doping amount, the capacity retention of the sample decreased; Sn doped samples had a wide and flat discharge platform around 1.5 V, and the initial discharge capacity of the Sn doped samples was large. The doping of Sn element increased the capacity of the lithium titanate, but when doped too much, the capacity decreased.Sn, Cr dibasic doping: All the doping samples had distinct discharge platform between 1.35 V and 1.55 V, which indicated that there was no phase change during the discharge process. Along with the increase of the doping amount, the discharge voltage platform reduced and the capacity of the samples increased. Sn, Cr dibasic doping decreased the discharge voltage and increased the cyclic performance of the material at the same time, and was a feasible doping method.