Preparation And Modification Of High Voltage Cathode Material For Lithium Ion Batteries

Currently, LiCoO₂, LiFePO₄, Li Mn₂O₄ have gradually become the positive electrode materials for the commercial lithium ion battery. However, it’s difficult for them to overcome their own shortcomings. The cost price of synthesizing LiCoO₂ is higher, and chemical synthesis pollutes the environment. The voltage which LiFePO₄ material provides for is only 3.6V. The capacity loss of Li Mn₂O₄ is very large. The preparation of LiNi_0.5Mn_1.5O₄ material costs little and is no toxicity or pollution. Its output voltage is above 4.5V, and fading rate of discharge capacity is small. LiNi_0.5Mn_1.5O₄ with its own advantages is successfully among the ranks of the positive electrode materials for the next generation of 5V high potential lithium ion battery.LiNi_0.5Mn_1.5O₄ as the positive electrode material was synthesized via sol-gel self-combustion reaction. According to the results of its physical properties and electric performance test, exploring what the calcination temperature has an influence up on macro performance. These consequences show that the best calcination temperature is at 850℃. The physical characterization shows that material presents octahedral geometry morphology, and average particle size was about 1.5μm. In 0.1C ratio test, the first discharge capacity could reaches to 116.34mAh/g. After 30 cycles, it still kept the discharge specific capacity of 96.73mAh/g.This paper made a primary research on the compatibility between LiNi_0.5Mn_1.5O₄ and different electrolytic liquid systems. Making up the different electrolytic liquid systems, which dissolving 1M LiPF6 in different proportions of EC（Ethylene carbonate）, DEC（diethyl carbonate）, EMC（ethyl methyl carbonate） organic solvent. Contrast the performances of LiNi_0.5Mn_1.5O₄ with different electrolytic liquid systems. The test results showed that at 0.1C rate, the first discharge specific capacity could be increased from 86.95mAh/g to 124.41mAh/g, with increasing the percentage content of DEC. The length of charge-discharge potential platforms extends nearly 45%. After 50 times cycle, the performance of charging-discharging has been improved. The average coulombic efficiency of material rose by just 3%.Two different space groups of LiNi_0.5Mn_1.5O₄ materials were synthesized via the same method. Through the testing of physical properties and electrical property, to analyze the different macro performances of Fd-3m and P4332 space group. On the geometry appearance, there is no significant difference between P4332 space group and Fd-3m space group. But, the particle size of P4332 space group is smaller 0.5μm than Fd-3m. The electrical property indicates that material of P4332 space group has only a high potential platform in 4.6⁴.85 V. Material of Fd-3m space group not only has main potential platform in 4.5⁴.8V, but also in 3.9⁴.0V has a short memory potential platform. After 20 cycles, the discharge specific capacity of Fd-3m space group decreases from 112.44mAh/g to 91.05mAh/g, and the discharge specific capacity of P4332 only reduce 9.16mAh/g. The molecular model of Fd-3m space group and molecular model of P4332 space group were systematically analyzed by computer simulation. These aspects contain geometric structure, density of states, atomic distribution, bonds layout and so on. Analyzing results of molecular model shows that density of states of P4332 space group is more obvious dispersion than Fd-3m space group. The conductivity of P4332 space group is not good. All bonds of P4332 space group are shorter than that of Fd-3m. Extra Mn-Mn bonds enhance the stability of crystal structure, but block the movement of the lithium ion.Finally, trying to manufacture LiNi_0.5Mn_1.5O₄ doped with small amounts of semiconductor material In₂O₃. LiNi_0.5Mn_1.5O₄ doped with In₂O₃ eliminates potential platform near 4.0V, and appears a section of potential platforms in the vicinity of 3.0V. In₂O₃ improves the first discharge specific capacity to 189.30mAh/g. After 10 cycles, the discharge specific capacity cuts short by 10mAh/g.