A Study Of Surface Modification And Electrochemical Performance Of High Voltage Cathode Materials Lithium Nickel Manganese Oxide For Lithium-ion Batteries

The spinel LiNi0.5Mn1.5O4(LNMO)is a promising cathode material for power battery due to its high voltage plateau and high energy density.However,LNMO has an advantage of rapid capacity decay,especially at elevated temperatures,which hinder its practical application.It is generally believed that the capacity decay of LNMO is caused by the dissolution of transition metal ions and the side reactions between the electrode and the electrolyte.At present,LNMO is mainly modified by two methods:bulk doping and surface coating.The surface coating can be divided into inorganic coating and organic coating.In this thesis,we use the organic coating method to modify LNMO.First,the raw material LNMO is synthesized by coprecipitation method and high temperature sintering method.Then,a conductive polymer layer is coated on the surface of LNMO by three different methods.The electrochemical properties of the composites have been significantly improved,especially the rate performance and the cycling performance at elevated temperatures.In the chaper 3,the cyclized polyacrylonitrile(cPAN)is coated on the surface of LNMO by a simple heat-treatment.The results of HR-TEM,EDS and XRD show that the cPAN layer is uniformally coated on the surface of LNMO and the thickness is between 2 and 3 nm.The process of coating cPAN does not destroy the structure of LNMO.The electrochemical test results show that the cycling performance and rate performance of cPAN-LNMO is significantly enhanced.The discharge capacity of cPAN-LNMO is up to 116 mAh g-1 at 10 C and that of LNMO is only 105.4 mAh g-1.After 100 cycles at 55? and 5 C,the discharge capacity of cPAN-LNMO is 112.9 mAh g-1 with the capacity retention of 95.2%,while the capacity retention of LNMO is only 87.8%.In the chapter 4,three different proportions of commercialized conducive polymer PEDOT:PSS are coated on the surface of LNMO by a simple wet method.The results show that the electrochemical performance of the composite LNMO-3 is the best when the coating amount is 3.0 wt%.After 100 cycles at 5 C,the discharge capacity of LNMO-3 is 116.0 mAh g-1 with the capacity rention closed to 100%.The discharge capacity of LNMO-3 is 6.1 and 5.8 mAh g-1 higher than LNMO at 5 C and 10 C,respectively.After 100 cycles at 55?and 5 C,the discharge capacity of LNMO is 115.6 mAh g'1 with the capacity retention of 91.6%.In the chapter 5,the conductive polymer PEDOT layer is coated on the surface of LNMO by a in-situ polymerization method.The results show that the PEDOT is homogeneously grown on the surface of LNMO and the thickness is between 3 and 5 nm.After 200 cycles at 1C,the discharge capacity of PEDOT-LNMO is maintained at 121.1 mAh g-1 with the capacity retention of 97.3%,while the discharge capacity of LNMO is reduced to 112.6 mAh g-1 with the capacity retention of 92.8%.The discharge capacity of PEDOT-LNMO is 8.1 and 13.9 mAh g'1 higher than that of LNMO at 5 C and 10 C.The improved electrochemical performance of LNMO coated with the conductive polymer is mainly due to two points:first,the conductive polymer can improve the conductivity of the composite and reduce the polarization of the cells,therefore improving the rate performance of LNMO.Second,the conductive polymer coating layer can effectively avoid the direct contact between the electrode and the eletrolyte,and reduce the production of the side reactions,thereby improving the cycling performance of the composite.