Preparation And Modification Of LiNi 0.5 Mn 0.5 O 2 Cathode Material

LiNi_0.5Mn_0.5O₂ cathode material of lithium ion battery is considered as one of the ideal substitutes for LiCoO2 due to the advantages of high specific capacity,low production cost,environmental friendly,etc.However,the layered structure of LiNi_0.5Mn_0.5O₂ cathode material still has the shortcoming of serious cation mixing,poor high rate performance,low cycle performance under high voltage and low temperature conditions.If the cycle performance under high voltage and low temperature conditions has been improved,it is of great significance to promote the commercialization of this material.In the paper,LiNi_0.5Mn_0.5O₂ cathode material was prepared by hydroxide co-precipitation combined with high temperature solid-state method.And its electrochemical performance can be improved by doping and modification.To get better electrochemical performance for LiNi_0.5Mn_0.5O₂,a simple and green modification method is adopted to obtain ZrO2-modification composite materials by directly milling ZrO2 and hydroxide precursors under the high temperature solid state conditions.The properties of the LiNi_0.5Mn_0.5O₂ cathode materials were characterized by X-ray diffraction,scaning electronmicroscopy,energy spectrum analysis,X-photoelectron spectroscopy analysis and electrochemical impedance spectroscopy,etc.?1?In the preparation process of LiNi_0.5Mn_0.5O₂ cathode material,effects of different sintering temperature,lithium contents and heating times on the electrochemical properties of LiNi_0.5Mn_0.5O₂ cathode material were investigated.The results show that LiNi_0.5Mn_0.5O₂ cathode material prepared with 1.08 of Li/?Ni+Mn?ratio and sintering temperature at 950?for 16 h exhibits the better layered structure,high particle spherical degree and uniform distribution.XPS analysis showed that the valence states of Ni and Mn for LiNi_0.5Mn_0.5O₂ were +2,+4,respectively.When cycled at 0.2C in the voltage rang of 2.75⁴.2V,the optimized LiNi_0.5Mn_0.5O₂ delivers an initial discharge specific capacity of 145.3mAh?g-1 with capacity retention of 98.4% after 100 cycles at 25?.But the cycle performance of this material is poor in a higher voltage rang of 2.75⁴.35 V and remains capacity retention of 81.8% after 100 cycles.And the discharge specific capacity for the corresponding electrode cycled at-20 oC is about 28.9% of its initial discharge specific capacity cycled at 25?.?2?Co-doped LiNi0.5Mn0.5-xCoxO2 cathode material was prepared.The results show that all of the doped samples have a typical a-NaFeO2 layer structure with Co doping the crystal lattice and substituting part of Mn.XPS analysis showed that oxygen deficiency in doped materials and the valence states of Ni,Mn and Co were mainly +2,+4 and +3,respectively.It has been found that the Co-doped LiNi_0.5Mn_0.5O₂ shows better cycling stability,rate capacity and low-temperature property than those of Li Ni0.5Mn0.5O2 without Co doping.when cycled at 25 oC with 0.2C in the voltage range of 2.75⁴.35 V,the LiNi0.5Mn0.5-xCoxO2 with x=0.12 delivered initial discharge specific capacity of 180.8 mAh/g and remained capacity retention rate of 92.3% after 100 cycles.The discharge specific capacity for the corresponding electrode cycled at-20 oC is about 66.3% of its initial discharge specific capacity cycled at 25 oC.?3?LiNi_0.5Mn_0.5O₂ was modified with different contents of ZrO2.The results show that ZrO2-modified materials still have a-NaFeO2 type layered structure by import small amount of ZrO2.The discharge specific capacity is reduced,but the cycle stability and low temperature performance of the cathode material have been improved by coating small amount of ZrO2.The cathode materials have impurity phase,the particle agglomeration is serious,and the electrochemical performance of the material is obviously decreased with increment of ZrO2 content.When cycled at 0.2C in the voltage rang of 2.75⁴.35 V,the content of ZrO2 with 0.01 molar ratio delivers an initial discharge specific capacity of 158.3mAh?g-1 with capacity retention of 97.2% after 100 cycles at 25?.And the discharge specific capacity for the corresponding electrode cycled at-20 oC is about 50% of its initial discharge specific capacity cycled at 25?.