Preparation And Performances Reseach Of High-voltage Lini_0.5Mn_1.5O₄Cathode Material For Lithium-ion Batteries

As a newly developing, green and high energy battery, lithium ion battery has come into widespread attention of the people for its large specific capacity, high operating voltage, low self discharge rate, good environmental protection, without memory effect and so on. Lithium nickel manganate (LiNi0.5Mn1.5O4) is one of the emphasis anode materials due to possessing the higher charge-discharge platform (4.7V). However, its low specific conductance will be in urgent need of improving.In the present thesis, the LiNi0.5Mn1.5O4was prepared by co-precipitation process using LiCl, NiCl2·6H2O and MnCl2·4H2O as raw materials, oxalate as a precipitant. To obtain the optimal solid reaction conditions, the prepared precursor was presintered at the400℃for three hours, and then calcined at700℃,730℃and750℃for ten hours respectively. The experimental results of X Ray Diffraction (XRD) and the Scanning Electron Microscope (SEM) demonstrated that all the samples respectively calcined at the three different temperatures had the cubic spinel structure of LiNi0.5Mn1.5O4and the better crystallinity. However, for the sample calcined at700℃, the particle size was the minimum compared to the other two samples. Moreover, the analysis Fourier Transform Infrared spectroscopy (FT-IR) indicated this sample was composed of Fd3m and P4332phases. Furthermore, this sample displayed better electrical performance which may be attributed to the right phase proportion between Fd3m and P4332.On the other hand, the powders properties had been improved by using polyacrylic acid (PAA) as a dispersant agent in the oxalate co-precipitation process. The calcined powders still remained the cubic spinel structure of LiNi0.5Mn1.5O4without any undesired impurities. According to the SEM, the powders had the uniform particle size and their aggregation was obviously reduced because of the dispersive effect of polyacrylic acid. Meanwhile, the electrochemical performances tests indicated that all the samples synthesized by polymer-assisted method possessed better cyclic stability and performance ratio compared to the samples without PAA addition. The discharge capacity for the sample added3wt%PAA was significantly enhanced about87.5%at1C,39.4%at2C,43.5%at5C and1.5%at IOC with respect to the samples without PAA. The great enhancement could be attributed to effectively shortening the Li-ion diffusion path. In order to improve the bulk conductivity, the ions of ruthenium or chromium were doped to LiNi0.5Mn1.5O4respctively. The experimental results displayed that the samples still remained the spinel structure and the size of paticles had a slight decrease. Compared with the non-doping samples, the peaks of LixNi1-xO as impurity showed in the XRD patterns were distinctly restrained even disappeared. This showed that the dopant of ruthenium or chromium effectively prevented the generation of LixNi1-xO impurity. Therefore, the electrochemical performances for the doped LiNi0.5Mn1.5O4were improved, especially at the high C-rate.