Study On Modification Of LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Materials For Lithium Ion Batteries

Layered nickel-cobalt-manganese oxides LiNi_0.5Co_0.2Mn_0.3O₂ are one of the most widely used cathode materials on a commercial scale for lithium ion batteries.Because of the prominent advantages of high capacity,low cost and good cycle stability,the LiNi_0.5Co_0.2Mn_0.3O₂ has attracted much attention.However,there are some drawbacks associated with LiNi_0.5Co_0.2Mn_0.3O₂,such as capacity fade in the long term cycling at high voltage and high initial irreversible capacity loss during the first charge-discharge cycle.In this paper,the material was modified by forming the composite and coating structure via the addition of the second phase.The influence of different coating/composite materials and different charge-discharge processes on the electrochemical performance have been discussed.In this paper,MoO₃/LiNi_0.5Co_0.2Mn_0.3O₂ composite cathode materials were prepared by ball milling method.The irreversible capacity loss decreases from23.6mAh/g to-5mAh/g and the initial coulombic efficiency increases from 86.9%to103.1%as the content of MoO₃ increasing from 0wt.%to 10wt.%in voltage range between 2.0-4.2V.When the discharge cutoff voltage increases from 2.0V to 2.8V,the initial coulombic efficiency reduces from 103.1%to 86.6%and the capacity retention rate increases from 70.9%to 94.7%after 50 cycles as the content of MoO₃increasing to 10wt.%,indicating that the discharge cutoff voltage has a significant effect on the electrochemical performance of the material.The coating/blending structure of Cr₈O₂₁/LiNi_0.5Co_0.2Mn_0.3O₂ materials were constructed by ball milling method.The irreversible capacity loss decreases from23.9mAh/g to-2.4mAh/g and the initial coulombic efficiency increases from 86.6%to 101.4%as the content of Cr₈O₂₁ increasing from 0wt.%to 7.5wt.%in the composite cathode materials in the voltage range between 2.8-4.2V.After 50 cycles,the capacity retention rate increases from 90%?0wt.%?to 94%?2.5wt.%?.The sample with 2.5wt.%Cr₈O₂₁1 shows the best cyclability.As the content of Cr₈O₂₁increases from 0 wt.%to 2.5 wt.%,the capacity retention rate are 83.4%and 93.6%,respectively after 50 cycles in the voltage range of 2.8-4.35V.Electrochemical impedance spectroscopy?EIS?results imply that the increase of capacity retention could be mainly attributed to the coating layer,which suppresses the increase in impedance upon cycling by inhibiting the electrolyte/electrode parasitic reaction.LaNiO₃ coated LiNi_0.5Co_0.2Mn_0.3O₂ materials were synthesized by sol-gel method.No obvious change on dicharege capacity is observed with the increase of the coating content of LaNiO₃ in the voltage range of 2.8-4.2V.The capacity retention rate are 90%,98.6%,99.1%and 96.2%with different coating content?0wt.%,1wt.%,3wt.%and 5wt.%?after 50 cycles.When the coating content is3wt.%,the sample exhibits the best cycle stability.For the high charge cut-off voltages of 4.5V and 4.7V,the capacity retention rate of the sample with 3wt.%LaNiO₃ reaches to 96.3%and 88%,respectively,while the one of the sample without LaNiO₃ is only 82.4%and 83%,respectively after 50 cycles.The sample with 3wt.%LaNiO₃ shows the best cycle stability.The improvement of the cyclic stability should contribute to the LaNiO₃ coating layer,which inhibits the parasitic reaction between the electrolyte and electrode,thus reducing the interface impedance and the charge transfer impedance.