| In recent years, as the environmental problems go serious and the electronic products become more and more portable, the study of lithium secondary batteries goes further. Because of the high price and toxicity of the industrialized LiCoO2 cathode material, more attention has been paid to the substitutes for it. Lithium manganese oxide proves one of the most promising cathode materials since it is inexpensive and environmental friendly. LiNi0.5Mn1.5O4 is a 5V cathode material. The charge and discharge plateau is between 4.6 and 4.8V vs. Li/Li+, and the theoretical capacity is 147mAh g-1, so it possesses higher energy density and is promising to be used in Li-ion power sources for electrical vehicles.In the present work, fluorine doping in LiNi0.5Mn1.5O4 and the effect on the electrochemical performances are studied. The result shows that sol-gel method to dope fluorine was better than the planetary ball milling. The precursor was first heated to 450℃in air for 5h to make the acetates decompos fully, thereafter ground thoroughly and pressed into pills, and then calcinated in oxygen at 850℃for 12h.The samples were characterized by XRD,SEM and BET and the electrochemical performances were tested by constant-current charge and discharge, cyclic voltammograms, AC impedance to optimize the different synthesize conditions, and different fluorine doping amount,and then tested the performances at high temperature.XRD results indicate that all the samples belong to spinels phase and there is no obvious difference between different fluorine doping amount samples, and SEM photographs show that the particles become larger and uniform as the fluorine amount increases. At the same time, the initial discharge capacity declines and the cyclability becomes better. LiNi0.5Mn1.5O3.9F0.1 is the best material. At high temperature storage, the cell potentials decline because of self-discharge via the lithium ion intercalation into the poor lithium phase. Fluorine doping reduces the impedance between the electrode and the electrolyte and is beneficial to the electrochemical performance at high temperature. Under high temperature, the electrolyte will decompose and influences the SEI membrane forming on the surface of the electrode, resulting in the failure of continuous charge and discharge. If the sample is activated at room temperature by charge and discharge several times, then it can charge and discharge at high temperature.At last, efforts have further been made to explore new electrolytes for the 5V cathode material, but BMIBF4 and PP14-TFSI can not meet the requirements. More work will be done to reach this goal. |
