Surface Modification Of LiCoO₂by MgF₂though Various Ways As Cathode Material For Lithium-ion Batteries

High energy density is offered by Li-ion batteries, so it has been widely used in portable electronicdevices, and it is important in electric vehicles and hybrid electric vehicles. Ease of production, excellentcycling performance and rate capability are the reasons that commercialized Li-ion batteries practicallyadopt LiCoO2as positive electrode materials. LiCoO2delivers a discharge capacity of140mAh/g,although it has a high theoretical capacity of274mAh/g when it is charged up to4.2V. In order to increasethe reversible capacity of the LiCoO2, it needs to be charged above4.2V. However, the capacity fade ofLiCoO2is quite pronounced with increasing cut off voltage. Some research group reported that the mostplausible reason for the capacity deterioration of LiCoO2is structural phase transition occurring near4.55V, dissolution of highly oxidized Co is increasing, and impedance is growing with cycling.To enhance the electrochemical performance of LiCoO2at high voltage, partial substitution of Co byAl, Mg, Mn, B, and Zn in LiCoO2have been extensively studied, the substitution is quite efficient toimprove cycling performance at high voltage, however, such substitutions were usually done forelectrochemically active element such as Co, extensive studies over the past ten years have been carriedout practical coating species and coating methods, electrochemical properties have been shown to besignificantly enhanced by Al2O3and ZrO2coating when it is charged to4.5V, the reason is not due to thechange of materials structure but due to the suppression of reaction between positive electrode andelectrolyte, which significantly reduces the impedance layer on the surface of LiCoO2. Some resultssuggest that the Al2O3coating layer is not endurable from the HF attack in the electrolyte after extendedcycling (more than1000cycles), Some of the Al2O3coating layer converted to AlF3. Recently, the effect ofAlF3coating amount on LiCoO2is effective in enhancing the cycling performance and rate capability. Based on previous theoretical and experimental works, LiCoO2is synthesized by sol-gel method inthis work. The cycling performance of LiCoO2is enhanced by surface modified by MgF2through severalways and their effects are compared. The main work in this thesis is as follows:Firstly, LiCoO2cathode material is synthesized by sol-gel method and it is modified by differentcontents of MgF2via chemical deposition method. The physical properties and electrochemicalperformance of pristine and MgF2-coated LiCoO2are investigated by X-ray Diffraction (XRD), FieldEmission Scanning Electron Microscopy (FESEM), High Resolution Transmission Electron Microscopy(HRTEM), Fourier Transform Infrared (FTIR) and so on. It is found that the MgF2surface modificationdoes not change the bulk structure of LiCoO2. The effects of the electrochemical behaviors of LiCoO2andMgF2-coated LiCoO2are studied at high voltage of4.5V.1wt.%MgF2-coated LiCoO2exhibits the bestcycling and ratio performances, CV results confirm that MgF2surface coating decreases the electrodepolarization; the results of EIS and FTIR for pristine and1wt.%MgF2-coated LiCoO2furtherdemonstrates that modification layer suppresses the growth of SEI film; DSC tests show that MgF2coatingalso improves the thermal stability of LiCoO2electrode.In conclusion,1wt.%MgF2-coated LiCoO2exhibits the best electrochemical and thermalperformances.Secondly, surface modification of LiCoO2through chemical deposition method, solid phase mixingmethod, modified materials mixed with electrolyte. The results show that the cycling and the ratioperformance are enhanced under the high cut-off voltage of4.5V, but1wt.%MgF2-coated LiCoO2material shows the best electrochemical performance through chemical deposition method. EIS and FTIRmodification effectively inhibit the growth of the interfacial film impedance. DSC result shows that thebest way to improve the thermal stability of LiCoO2is also through chemical deposition method. In conclusion, the best way to exhibit the electrochemical and thermal performances is throughchemical deposition method.