Study On The Modification Of High Voltage Performance Of LiNi_0.6Co_0.2Mn_0.2O₂ As Cathode Material For Lithium Ion Batteries

Since the commercialization of lithium ion batteries?LIBs?,they have made significant progress in energy density,cycle performance and cost reduction.Compared with the commonly used LiCoO₂ and LiFePO₄ cathode materials,the Ni-rich layered LiNi_1-x-yCo_xMn_yO₂?NCM?cathode materials,has become one of the main materials of choice for Lithium-ions batteries due to the the high working voltage,high specific discharge capacity,good thermal stability and low cost.Due to the rapid development of portable electronics devices and hybrid electric vehicles?HEVs?,the demand for NCM cathode materials will also increase dramatically.In order to meet the growing demand for energy density,it can increase cut-off operating voltages to provide higher reversible capacity.Cation mixing occurs during the preparation process of the Ni-rich layered cathode material,and then the cation mixing degree will be aggravated during the long-term cycle process under high voltage.And the occurrence of side reaction on the surface of particles is accompanied by irreversible phase change,which leads to the reduction of structural stability and electrochemical properties of cathode materials.In order to improve the structural stability and electrochemical performance of the cathode material,the cathode material was modified by element doping and surface coating.The boracic polyanion doped Ni-rich LiNi_0.6Co_0.2Mn_0.2O₂ cathode material was synthesized by mechanical liquid phase stirring-high temperature sintering.The Rietveld refinement results reveal that boracic polyanion doping can inhibit the degree of cation mixing of LiNi_0.6Co_0.2Mn_0.2O₂ cathode material.The electrochemical testing showed that the boracic polyanion doped samples had better cycle performance and rate performance.The capacity retention of the 1 mol%boracic polyanion doped sample?B1-NCM622?sample after 100 cycles of 1 C was 76.07%.The electrochemical impedance spectroscopy?EIS?examination that boracic polyanion doped of LiNi_0.6Co_0.2Mn_0.2O₂ samples has lower solid electrolyte interface layer impedance(R_sf)and charge transfer impedance(R_ct),and the charge transfer resistance of the B1-NCM622 sample after 50 cycles was only 388.6?.The GITT confirms that boracic polyanion doping can improve the chemical diffusion coefficient of the NCM622cathode material.Simultaneously,the pole piece after 100 cycles was characterized,which showed that the sample after doping with boracic polyanion doping had better layered cathode material,and the spherical particle morphology was more complete.The lithium titanate coated LiNi_0.6Co_0.2Mn_0.2O₂ cathode material was synthesized by mechanical stirring,lithiation and heat treatment,and the crystal structure and electrochemical performances of NCM622 cathode material coated with lithium titante were studied.The lithium titanate surface coating can reduce the degree of cation mixing effectively,effectively slow the increase of electrochemical impedance and improve the electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ cathode material.It has been found that the best cycle performance for 2 wt.%lithium tianate coated sample.The T2-NCM622 samples exhibits a capacity retention of 87.02%at the voltage range of 2.8-4.5 V in 1 C after 100 cycles.The T2-NCM622 samples has better rate performance.The lithium titanate coating delivers the discharge capacities of 194.45,184.02,172.76,161.62,151.52,138.17 and 131.44 mAh g^-1 at current densities of0.1 C,0.2 C,0.5 C,1 C,2 C,4 C and 5 C,respectively.It has better discharge capacity than NCM622 samples at a large current density.Cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?examination indicate the lithium titanate coated LiNi_0.6Co_0.2Mn_0.2O₂ cathode material has lower electrochemical polarization and electrochemical impedance.The GITT proves that lithium titanate coated is beneficial to increase the lithium ion diffusion coefficient.