Study On Synthesis And Modification Of High Voltage Cathode Materials For Lithium Ion Batteries

High-voltage cathode material LiNi0.5Mn1.5O4 has been considered as one of the most promising cathode materials for lithium ion battery in the recent years owing to its high capacity,high operating voltage, and abundant raw materials, good safety performance, environment-friendliness. However, due to its preparation conditions, it also has shortcomings, such as high cost, poor cycling stability, which restrict its scale development.To optimize process parameters and enhance the electrochemical performance, this thesis synthesized LiNi0.5Mn1.5O4 cathode material by solid-phase method, in which Li2CO3, MnO2 and NiO was used as raw materials. And the materials’crystal structure, surface morphology, electrochemical properties were also studied through X-ray diffraction (XRD), Scanning Electron Microscopy(SEM), Transmission Electron Microscopy (TEM), Charge/Discharge performance and Electrochemical Impedance Spectroscopy(EIS).The effects of different calcination temperature (T=750℃,800℃,850℃,900℃), different holding time (t=6h,12h,18h,24h), different lithium excess (X=0,0.02,0.05,0.08) and different milling conditions (V=250r/min,300r/min, 350r/min,400r/min) on the electrochemical properties of LiNi0.5Mn1.5O4 cathode material were studied. As the results showed:The sample which was obtained by the preparation conditions (X=0.05,V=350r/min,T=90O℃,t=12h) had the best electrochemical properties. Its initial specific discharge capacity was 125.3mAh/g (0.2C), after 100 charge-discharge cycles, the specific discharge capacity was 118.7 mAh/g, with a capacity retain ratio up to 94.7%. The electrolyte decomposed easily due to its 4.7V high voltage, so we used three different electrolyte to test the materials. The results showed that electrolyte had a great influence on the electrical properties of the material. When we used LiPF6/(EC+DMC+EMC) as an electrolyte, the material exhibited good cycle performance.After obtaining the optimum parameters, in order to study the effects of chromium doping and ferrum doping on the electrochemical properties of LiNi0.5Mn1.5O4 cathode material.LiNi0.5Mn1.5-xMxO4/LiNi0.5-xMxMn1.5O4(M=Fe、Cr, x=0、0.05) were synthesized by solid-phase method at the optimum parameters. The study found that a small amount of Fe doping and Cr doping did not destroy the spinel structure of LiNi0.5Mn1.5O4 cathode material,and the sample remained an intact cubic spinel structure and having high crystallinity. LiNi0.5Mn1.45Fe0.05O4 exhibited better electrochemical performance than LiNi0.5Mn1.45Cr0.05O4. When Fe doping was applied on LiNi0.5Mn1.5O4, not only was the stability of electrode structure improved, but also its electric conductivity and cycle performance were excellent.At 0.2C rate, the specific discharge capacity of LiNi0.5Mn1.45Fe0.05O4 was 124.5mAh/g, after 20 charge-discharge cycles, the discharge capacity was 120.1mAh/g, with a capacity retain ratio 96.5%.Then Ni positions were doped with iron and chromium, LiNi0.45Mn1.5Fe0.05O4/LiNi0.45Mn1.5Cr0.05O4 were also synthesized by solid-phase method at the optimum parameters. Results showed that both samples exhibited excellent electrochemical performance. At 0.2C rates, the specific discharge capacity of LiNi0.45Mn1.5Fe0.05O4 and LiNi0.45Mn1.5Cr0.05O4 were 132.3mAh/g and 133.8mAh/g, after 20 charge-discharge cycles the two samples’discharge capacity became 132.5 mAh/g and 133.8mAh/g respectively. Both of them exhibited excellent cycle performance. The above experiments showed that suitable ion doping did not reduce the discharge capacity of the materials, rather it improved the electrical conductivity and reversibility of materials in the insertion-extraction process. Besides, being ion-doped,the cycling performance of materials had been improved. Comparison showed that the effect of Fe ion doping was better than Mn ion doping,when the doping position are same. But the high rate discharge performance of the materials was still not good enough.