Combustion Synthesis And Coating Modification Of Rare Earth Doped Lithium-rich Ternary Cathode Materials

Low-cost,safe and high energy density lithium-ion batteries are an urgent demand in the current society,however,which is limited by cathode materials.Li(Li_xTM_1-x)O₂ has a discharge capacity of more than 300 m Ah g^-1 and an energy density of more than 900 Wh kg^-1,which is a good choice for lithium-ion batteries at present.However,it has many shortcomings,such as first irreversible capacity loss,bad rate performance,high voltage decay,and fast specific energy decay,so it has not yet reached commercial maturity.Therefore,it is important to develop high stability Li-rich cathode materials by a new method and modification.In this paper,under the guidance of first-principles calculation,a new and efficient combustion process is used to prepare a commercial Li-rich cathode material with high performance through the construction of oxygen vacancy(Vo)on the material surface,doping of rare earth elements and design of high conductivity 3D structure.Moreover,Vo action,voltage decay mechanism and the kinetics of electrode reaction process in Li-rich cathode materials are studied.The new combustion method was used to successfully produce Li_1.2Ni_0.133Co_0.133Mn_0.533O₂ material.The effects of the concentration of solution,the amount of combustion-supporting agent,the ignition temperature and the heat treatment time on the properties of the materials were investigated by orthogonal experiment.The principle of combustion synthesis is systematically studied by thermodynamic calculation.Urea can induce Vo of different concentrations in Li-rich cathode materials.DFT shows that with the increase of Vo,Both the overall conductivity and the electrochemical activity of Li are increased,which is beneficial to improve the electrochemical performance.U1.5@LLMO material has the lowest Li⁺/Ni²⁺mixture,the highest Vo concentration and the highest Li⁺diffusion coefficient(1.346×10^-12cm²s^-1).The average discharge capacity and capacity retention rate of 100 cycles are respectively 185 m Ah g^-1,94.33%.The 13 kinds of rare earth elements(La,Ce,Pr,Nd,Sm,Eu,Gd,Dy,Er,Tm,Yb,Lu,Y)were used for doping Li_1.2Ni_0.133Co_0.133Mn_0.533O₂.The doping sites,formation energy and doping mechanism are systematically studied by DFT,which shows that rare earth elements tend to occupy Co site in Li MO₂ phase.The doping effect of rare earth element Yb is the most stable,the Fermi energy level is in the strong electron peak,and the electrical conductivity is greatly improved.And Yb-doped materials significantly enhance the hybridization between Mn-O and Ni-O,and increase the effective charge of O,and reduce the generation of O₂ in the process of lithium de-intercalation.Through experimental verification,Yb and other doped rare earth elements with low formation energy can reduce the degree of Li⁺/Ni²⁺mixing,expand the crystal cell,and reduce the apparent activation energy of the material.The first discharge capacity of Yb-doped material is 342.5 m Ah g^-1,which is 73.4 m Ah g^-1higher than pure material.A multi-scale coating modification method for the preparation of high conductivity Li-rich cathode materials was proposed.DFT indicates that a large number of Vo caused by charging the material to 4.8 V is the major reason for voltage decay.The transition metal ions near the Vo will spontaneously move to Li layer,and the resistance of return on thermodynamics is extremely big.This will limit the transition metal ions in Li layers and cause voltage decay.Then a high conductivity structure was designed using COMSOL to suppress voltage decay.The fast ionic conductor Li₄Si O₄ and high electronic conductor graphene was used to modify Yb-doped Li_1.2Ni_0.133Co_0.123Mn_0.533Yb_0.01O₂ material.After ball mill and granulation,the original large pieces of the poor electrical conductivity of the back turned into good electrical conductivity compact unit comprised of small conductive particles.After the modification,the electron and ion transport rates of the high conductivity cathode material are greatly enhanced,the ability to resist electrolyte erosion is enhanced,the decay of voltage and capacity is alleviated,the surface oxygen reactivity is reduced,and the corrosion resistance of electrolyte is enhanced.Through the in-depth analysis of the cyclic voltammetry curves,the constant current intermittent potential titration curves and the non-in-situ impedance of the electrode material,it is found that the range of 4.6-4.8 V is the main reason for the change of impedance.The continuous escape of lattice oxygen at high potential results in the increase of R_ct and R_sl,which makes the transition metal redox more difficult and represents by voltage decay.GO-LS@RLLMO material has excellent thermal stability and ability to prevent the dissolution of metal ions.The voltage decay of 200 cycles in the range of 2.8-4.6 V is only 0.10 V,and the decay rate is 2.83%.The electrochemical performance is excellent.In this paper,the rare earth doped material Li_1.2Ni_0.133Co_0.133Mn_0.533O₂ were prepared by combustion method and modified by coating.The multiscale structure modified Li-rich ternary cathode material has the characteristics of low cost,no pollution,high energy density and low voltage decay,which make the material industrialization and commercialization.