Surface Modification And Electrochemical Study Of LiNi_0.4Co_0.2Mn_0.4O₂ Cathode Materials

The layer-structured Nickel,Cobalt,Manganese as lithium insertion compounds with a high capacity,structure and thermo-stability have been extensively studied as possible alternatives to LiCoO₂.With the aim of improving its cycling properties,in the present work, melting impregnation method and solid section method were used to coat metal oxides and LiAlO₂ on the surface of LiNi_0.4Co_0.2Mn_0.4O₂ cathode materials,respectively. Al₂O₃,ZnO and MgO coated LiNi_0.4Co_0.2Mn_0.4O₂ materials were synthesized by melting impregnation method using Al(NO₃)₃·6H₂O,Zn(NO₃)₂·6H₂O and Mg(NO₃)₂·6H₂O as coating resources.The effect of electrochemical properties with different coating amount of such materials has been studied.The results show that the surface of coated LiNi_0.4Co_0.2Mn_0.4O₂ particles are covered by a layer of asymmetric nano-oxides,the lattice parameters increased as well and this leads to a double effect of surface modification and partial doping;The 3 wt% Al₂O₃ coated samples have the largest lattice parameters but the 1 wt%Al₂O₃ coated ones have the best rate ability and cycling properties,which indicates that the stability of the crystal structure together with the thickness of the coating layer are important factors to the electrochemical properties of LiNi_0.4Co_0.2Mn_0.4O₂;Although decreasing the initial capacity for the coated ones,the rate ability and cycling stability has increased evidently,the 1 wt%Al₂O₃ coated ones have got a discharge capacity of 104 mAh·g^-1after 300 cycles at a current density of 160mA·g^-1,the capacity fading rate decreases from 0.139%per cycle to 0.108%per cycle for the coated ones;The characters of the coating material itself was the main factor to influence the electrochemical properties of LiNi_0.4Co_0.2Mn_0.4O₂ after comparing the cycling properties of the Al₂O₃,ZnO and MgO coated ones.Solid reaction method was used to coatα-LiAlO₂ on the surface of LiNi_0.4Co_0.2Mn_0.4O₂, a flocculent layer ofα-LiAlO₂ formed on the surface of LiNi_0.4Co_0.2Mn_0.4O₂ particles after baking with the mixture of LiNO₃ and Al(OH)₃.The sol-gel method can also get a homogeneous on the surface of LiNi_0.4Co_0.2Mn_0.4O₂ particles,but the former has a better electrochemical properties.The 3 wt%coated ones show an effective improvement of the rate ability and cycling performance of LiNi_0.4Co_0.2Mn_0.4O₂,we have got the capacity of 142 mAh·g^-1and 108 mAh·g^-1when cycling at 1C and 3C rate.The room temperature test at 1 C after 100 cycles shows that the capacity fading rate decreases from 0.19%per cycle to 0.14%, high temperature tests at 55℃show that the capacity fading rate decreases from 0.19%to 0.15 after cycling at 1C rate for 150 times.Experimental impedance spectra(EIS)analysis indicates that the existence of the coating layer leads to the increase of the SEI resistance,but the increase of the charge transfer resistance have been restrained effectively thus slow down the increase of the total resistance.The factors that influence the electrochemical properties of LiNi_0.4Co_0.2Mn_0.4O₂ have been discussed,the results show that the constant voltage step during charge process could restrict the capacity loss in the cycles thus prolong the cycle life of the cells;The discharge capacity of LiNi_0.4Co_0.2Mn_0.4O₂ increase linearly with the raise of the upper cut-off voltage limit,but the cycling stability decreased.The existence of the coating layer increases the surface resistance,however,it can absorb the stress and relax the volume change during the charge and discharge process thus restricts the polarization during the cycles and enhances the cyclic stability.After cycles,with the effect of the coating layers,cyclic voltammogram(CV) indicates that the remove of the anodic and cathodic peaks is small,EIS results shows that the SEI film resistance increased a little,but the total Z' decreased a lot;the SEI film resistance (R_SEI)and SEI film capacitance(Q_SEI)decrease linearly during the charge process which indicate the thickness of SEI film decreased;the charge transfer resistance(R_ct)first decreased and then increased,the lowest point at around 4 V indicates the smallest obstacle of delithiation.It could see from the composite electrodes that the contact of the LiNi_0.4Co_0.2Mn_0.4O₂ particles,the active particles and the conducting blinder are very well and the morphology change is small after cycles.