Studies On MF_x（M=La,Zr;X=3,4） Surface Modification Of High Voltage Cathode Material LiNi_0.5Mn_1.5O₄

Batteries have been widely used to provide energy for all walks of life.Among them,lithium ion batteries are considered to be one of the best energy storage materials,due to its high energy storage density,negligible memory effect and long life expectancy and so on,which has been widely used in portable electronic products.The industrial structure of lithium-ion batteries market has changed greatly in recent years.Application in the field of electric vehicles has grown by leaps and bounds,and further promotes the development of lithium batteries.New lithium battery products continue to emerge,pushing lithium-ion battery market to expand continuously.However,so far,improving the lithium ion battery energy density become the bottleneck of emerging market share continues to expand except that increasing the requirement of the security and service life.An effective way is to increase the voltage of the positive electrode,since the anode material is close to the potential of lithium metal.In the cathode materials,LiNi0.5Mn1.5O4 material not only has high working voltage(~ 4.7 V vs.Li/Li+),high energy density(650 Wh kg-1),environmental friendly and low-cost,become very promising anode material in lithium-ion batteries field.However,LiNi0.5Mn1.5O4 material still faces from many problems that have not been satisfactorily solved in practical applications: 1)The capacity declines rapidly in the electrochemical cycle and the rate performance is poor 2)The transition metal ions of the material on the surface dissolve in electrolytes,especially in high temperature,resulting the structure damage of LiNi0.5Mn1.5O4 material 3)Under high voltage,the electrolyte can be easily oxidized and decomposed,causing serious side reaction.In order to improve its electrochemical properties,through adding additives in the electrolyte or replacing the solvent of electrolyte,the electrolyte becomes stable and resists high voltage.As well as the other scheme is to modify LNMO materials,mainly including the following methods: morphology design,ion doping and surface modification.Among them,the surface modification is a simple and low cost method.The chemical environment of LiNi0.5Mn1.5O4 electrode surface is slightly affected by matching the appropriate coating material.The stability and electrochemical properties of the active materials are improved through the physical barrier effect of the coating layer,thus avoiding the influence of the material structure.In this work,LiNi0.5Mn1.5O4 materials of disordered phase were synthesized by a sol gel method and the surface of LiNi0.5Mn1.5O4 material was modified for the first time with LaF3 and ZrF4.The main contents of this thesis include the following two parts:1.LaF3 material with high ionic conductivity and stable chemical properties was coated the LiNi0.5Mn1.5O4 material surface.It has been found that the electrochemical and rate performances are improved after surface modification by charge-discharge of a constant current.In particular,4 wt.% coated LiNi0.5Mn1.5O4 shows the best electrochemical performance compared with the original material.In order to explore the change of composition,morphology and microstructure of the material before and after the coating,the characterization and test were carried: X-ray powder diffraction(XRD),laser Raman spectroscopy(Raman),field emission scanning electron microscopy(FESEM),high-resolution transmission electron microscope(HRTEM),electron diffraction spectroscopy(EDS)and X-ray photoelectron spectroscopy(XPS).The bulk structure of the LiNi0.5Mn1.5O4 materials remained unchanged after surface modification.Thermal stability of the LiNi0.5Mn1.5O4 materials is tested through the differential scanning calorimetry(DSC).The results indicate that the thermal stability is also effectively improved after surface modification.In order to further explore its modification mechanism,the electrochemical impedance spectroscopy(EIS)and the fourier transform infrared spectroscopy(FTIR)are studied.The results manifest that the electrode material successfully suppresses the growth of the solid electrolyte interface(SEI)film after coating.A series test of high temperature aging is explored to further study,demonstrating that LaF3 material with the moderate amount could effectively restrain the growth of SEI film and continuous dissolution of transition metal(TM).At the same time,the surface structure of materials is more stable.2.ZrF4 thin layer with different content was successfully coated on the surface of LiNi0.5Mn1.5O4 material with spinel structure by simple coprecipitation method.Through XRD,Raman,FESEM,EDS and HRTEM,the morphology and structure of the materials were studied.The results show that the host structure of the material is invariant after the ZrF4 surface modification.Electrochemical measurement results show that the cycle performance is improved with the working voltage range 3.5 ~ 4.9 V after ZrF4 modification of material,the electrochemical properties of 2 wt.% ZrF4 coating material is the best.The discharge capacity and capacity retention 117.1 m Ah g-1 and 95.5% respectively after 120 th cycles,and the discharge capacity and capacity retention of original LiNi0.5Mn1.5O4 material only 99.9 m Ah g-1 and 80.1%.According to rate capability test,rate performance of 2 wt.% ZrF4 coated LiNi0.5Mn1.5O4 samples(105 m Ah g-1)is also improved compared with the original material(77 m Ah g-1)in 2 C current densities.EIS and FTIR were studied to understand the intrinsic mechanism.The results show that the ZrF4 thin layer can avoid direct contact between the LiNi0.5Mn1.5O4 materials and electrolyte and inhibit surface side reaction.Therefore,the ZrF4 coating layer enhances the interface stability between electrode material and the electrolyte,thus improving the electrochemical performance of LiNi0.5Mn1.5O4 material.