Preparation And Modification Of LiNi_1-x-yCo_xMn_yO₂ Cathode Material For Lithium Battery

High nickel content cathode materials are one of the most promising alternative material for portable electronic equipments,large energy storage power stations and electric vehicles?EVs?due to their high discharge capacity,high energy density and excellent cycle life.Particularly interesting for Ni-rich layered LiNi_0.6Co_0.2Mn_0.2O₂?NCM622?,the material has the best comprehensive performance,with a higher discharge capacity compared with LiCoO₂ and LiMn₂O₄,stable cycle performance than LiNi_0.8Co_0.1Mn_0.1O₂ and LiNi_0.8Co_0.15Al_-0.05O₂.However,a common problem with high nickel materials is that multiple phase transitions occur during charging and discharging.Unstable structure can cause battery performance to decrease over time.The similar radius of Ni²⁺ ions?0.69??and Li⁺ ions?0.76??,the occupancy of Ni²⁺ in Li⁺ is the main factor affecting the deterioration of electrochemical properties of the material.Layered NCM622 cathode materials were synthesized by mixing stoichiometric of LiNi_0.6CO_0.2Mn_0.2?OH?₂ and 5wt% excess LiOH.Under the process conditions of calcination temperature of 900?,holding time of 12 h and oxygen flow of 150ml·min^-1,the specific discharge capacity was 176 mAh·g^-1 at 0.1C.Modification research based on the process of synthesizing NCM622,the Li[Ni_0.6Co_0.2Mn_-0.2]_1-xV_x]O₂ cathode material with vanadium-doped transition metal layer was synthesized.The XRD pattern of samples was Rietveld refinement to prove that vanadium entered the transition metal layer site,and the Ni occupancy of the Li lattice position decreased from 0.0392 to 0.0130.XPS and EDS Mapping tests also demonstrated the presence of vanadium element,and samples with a doping content of 0.5% exhibited the highest electrochemical performance.The initial specific discharge capacity of V0.5 was 189.9mAh·g^-1 at 0.1C,and the capacity retention was 95.33% after 100 cycles at 1C.Mainly due to the decrease of charge transfer impedance and the increase of lithium ion diffusion coefficient,which contributes to the deintercalation of lithium-ions.In order to obtain more excellent electrochemical performance of the battery,Li_1-xNa_x[Ni_0.6Co_0.2Mn_0.2]_0.995V_0.005]O₂ cathode material was synthesized by sodium doping on the basis of vanadium modified NCM622 material.The XRD pattern of samples was Rietveld refinement to prove that vanadium entered the transition lithium ion lattice site.The sodium-doped lithium potential can effectively enlarge the lithium layer spacing,exhibiting a low charge transfer resistance Rct of 46?,and a lithium ion diffusion coefficient of 7.64×10-13 cm²·s^-1,which is advantageous for lithium ion deintercalation,conducive to the deintercalation of lithium ions.When the doping amount of sodium is 1%,the battery exhibits higher discharge specific capacity and rate performance.The discharge specific capacity is 201.8mAh·g^-1 at 0.1C,and the discharge specific capacity is 157.1 mAh·g^-1 at 5C.