Study On Modification And Electrochemical Performance Of Layered Structure Ni-Rich Ternary Cathode Material

As carbon emissions and related climate issues receive more and more attention,new energy vehicles account for an increasing proportion of the automotive market.One of the main obstacles to the development of new energy vehicles is the problem of battery life,which is the energy density of the vehicle's power battery.At present,the energy density of batteries is mainly determined by the cathode material.Among the many cathode materials,Ni-rich ternary materials(LiNi1-x-yCoxMnyO2,1-x-y>0.6)is one of the most potential cathode materials,with high energy density,high platform potential,good rate performance,and cost.However,their large-scale application is severely limited due to their poor cycle performance,difficult storage,and low safety.The poor cycle performance of Nirich materials is closely related to their structure and surface properties.In order to solve the problem of rapid capacity decay of Ni-rich materials,this article is dedicated to stabilizing the structure and protecting the surface.The research content and main conclusions of this article are as follows:(1)Co-precipitation method is used to coat(Ni0.4Co0.2Mn0.4)1-xTix(OH)2+0.2x on the surface of the Ni-rich precursor.After sintering,the surface-coated and bulkdoped cathode electrode material is completed at the same time.The structure and surface morphology of the modified material have not been significantly changed.The cycle performance of Ni-rich materials has been significantly improved.In the charge and discharge test at 25?,2.8-4.5 V,1C,the material with the best performance has an initial specific capacity of 197.0 mAh g-1.After 200 cycles,the capacity retention is as high as 89.9%.At 50?,the specific capacity can reach 241.3 mAh g-1,and the capacity retention after 100 cycles is 69.9%.The reason for the obvious modification effect is that part of the Ti is enriched on the particle surface,which improves the Young's modulus,fracture toughness and other mechanical properties of the particle surface.The side reactions of the interface are also inhibited.Secondly,the change of the unit cell parameters during the charge and discharge process is reduced due to the doping of Ti in the crystal lattice;in addition,the Ti enriched at the grain boundary plays a role in strengthening the grain boundary.Compared with TiO2 doping,the core of the co-precipitation modified material has smaller grains and some voids,which helps to relieve the internal stress during the cycles.(2)Two types of electrolytes,Li7La3Zr2O12(LLZO)and Li0.33La0.557TiO3(LLTO)are selected as the matrix.The transition group elements are doped by ball milling to improve their electronic conductivity.Two types of mixed conductor materials are obtained as Ni-rich ternary positive electrode surface coating material.The garnet-structured electrolyte LLZO doped with Fe,Co,Ni and the impure phase is found.The grain size,porosity,and type of impure phase are affected by doping.The electronic conductivity can be increased by 1 to 3 orders of magnitude by doping elements.The electronic conductivity of the Co-doped sample is 2.75×10-6 S cm-1.The perovskite structure electrolyte LLTO is doped with Fe,Co,Ni,and Cr.The samples doped with Fe and Co are found impurity phases,and the doped Ni and Cr almost do not find impurity phases.The electronic conductivity of LLTO is 5.59×10-10 S cm-1.The electronic conductivity of the Co-doped sample reaches 6.66×10-7 S cm-1.(3)Co-precipitation method is used to coat the mixed conductor hydroxide La0.557Ti0.9Co0.1(OH)5.571(La3Zr1.5Co0.5(OH)16.5)on the surface of the precursor of Ni-rich materials.After high temperature sintering,Ni-rich ternary material doped with Ti(Zr)bulk phase and coated on the surface of La4NiLiO8 is formed.The initial specific capacity of the La and Ti modified samples is 195.5 mAh g-1 at 25?,2.8-4.5 V,and 1C.The capacity after 200 cycles is 170.1 mAh g-1.The capacity retention is as high as 87.0%.The specific capacity at 5C is 178.3 mAh g-1.The capacity retention of La and Zr modified samples is 81.4%.The cycle stability of Ni-rich materials is greatly improved through the dual effects of doping and coating.The doping of Ti(Zr)plays a role in stabilizing the crystal structure.The surfaceenriched Ti(Zr)and the generated La4NiLiO8 nanoparticles protect the surface of the Ni-rich material from being corroded by the electrolyte and inhibit the interface side reaction during the cycles.The La4NiLiO8 on the surface have good conductivity,which is helpful to the improvement of rate performance.(4)The nano-scale garnet-type mixed conductor Li7.5La3Zr1.5Co0.5O12(LLZCO)powder is prepared by ball milling.After mechanical mixing and annealing treatment,the LLZCO mixed conductor is coated on the surface of the Ni-rich ternary cathode material.The initial specific capacity of the coated sample is 196.2 mAh g-1 at 25?,2.8-4.5 V voltage range and 1C charge and discharge conditions.The capacity retention after 200 cycles is as high as 85.8%.When discharged at 5C,the specific capacity of the coated sample is 180.9 mAh g-1.The coated samples have an excellent capacity retention.The reason is that the variation range of the lattice constant during the cycle is suppressed by the LLZCO coating layer to prevent cracks on the particle surface.Surface phase transformation and voltage drop are suppressed due to the coating layer.The rate performance of the Ni-rich material is improved due to the better lithium ion diffusion coefficient of the modified material.