Li_1.2Ni_0.15Co_0.10Mn_0.55O₂

The development of low cost,high energy density and good cycle stability of lithium-ion batteries has become the focus of attention.Lithium-rich cathode materials have a large specific capacity(?250 m Ah g)^-1),high discharge voltage(3.8V)and wide range of charge and discharge voltage(2?4.6V),its theoretical energy density up to 900 Wh/kg,is one of the ideal cathode materials for power batteries in the future.However,this material also has the disadvantages of low first coulomb efficiency,voltage and capacity attenuation during long cycles,poor rate performance and high cost.To solve this problem,based on the design concept of"high manganese"and"low cobalt",this paper adopts first-principles calculation of density functional theory to study Li_1.2Ni_0.15Co_0.10Mn_0.55O_2.The crystal structure,mechanical stability,thermodynamic properties and electronic structure of a series of cathode materials,as well as the synergistic mechanism of the cyclic stability of O-vacancy and Mg doped modified cathode materials.The reaction mechanism of Li_1.2Ni_0.15Co_0.10Mn_0.55O₂ precursor and the influence mechanism of preparation process(sintering temperature,sintering time and heating rate)on the structure,morphology and electrochemical properties of the product were further discussed.In addition,oxide Mg O and phosphate Alpo are also used₄The surface modification method can improve the problems such as large loss of first discharge capacity,poor cycle performance and low rate of power of lithium manganese base cathode materials.The following research conclusions are obtained:(1)The crystal structures and binding energies of Li Mn O₂,Li Ni_1/3Co_1/3Mn_1/3O₂,Li7/6Ni1/3Co1/6Mn1/3O2,Li7/6Ni1/3Co1/3 Mn1/6O2,Li7/6Ni1/6Co1/3Mn1/3O2,Li7/6Ni1/6Co1/6Mn1/2O2were calculated by CASTEP and VASP.The energy of Li_7/6Ni_1/6Co_1/6Mn_1/2O₂ system is-2.04e V,which has the advantages of high discharge capacity,stable structure and good safety,but its electronic conductivity is inferior to Li Ni_1/3Co_1/3Mn_1/3O₂.(2)In order to solve the problem of insufficient conductivity of Li_7/6Ni_1/6Co_1/6Mn_1/2O₂materials,the co modification strategy of O vacancy and Mg O coating is proposed theoretically.It is found that the band gap eg of the materials with oxygen vacancy is 0.24 e V,while the Mg O coated materials have good metallicity,and the oxygen vacancy reduces the diffusion barrier of Mg atoms,stabilizes the lattice structure and improves the comprehensive performance,It provides a theoretical basis for further improving the conductivity of the material.(3)Based on thermodynamic calculation and design of lithium-rich Mn-based cathode materials,the precipitation mechanism and crystallization process of the precursors were discussed.It was found that when p H=8.0,the concentrations of the three metal ions in the solution tended to be the lowest,and the loss of each ion in the solution was the least,so the expected homogeneous precursors could be obtained.At the same time,sodium carbonate was also found to be the precipitant with a concentration of 0.3 mol L^-1.Under the conditions of stirring speed of 600r/min,reaction temperature of 55?and reaction time of 8 h,spherical Ni with good morphology,high vibration density and excellent dispersion can be obtained Ni_0.1875Co_0.125Mn_0.6875CO₃precursor.(4)The influence of sintering parameters on the crystal growth of Li _1.2Ni_0.15Co_0.10Mn_0.55O₂was elucidated,and it was found that the optimal pre-sintering temperature was 550°C,the pre-sintering time was 5 h,the heating rate was 20°C/min,the calcination temperature was850°C,and the calcination time was 14 h.In addition,the regulation law of the heating rate on the grain morphology was also found,namely,the heating rate If the rate of sintering is too fast,the rate of densification process reaction at the early stage of sintering is too large,the pores of solute are closed in advance,and the gas loses the channel for pores to escape and cannot be completely discharged from the crystal,and partly stays inside the grain,which leads to poor crystallinity of the material and deteriorates the comprehensive serviceability of the product.(5)The effects of different Mg O and Al PO₄ coating contents on the crystal structure,microstructure and electrochemical properties of Li_1.2Ni_0.15Co_0.10Mn_0.55O₂ were investigated.The mechanism of oxygen vacancy improving the conductivity,crystal structure stability and safety of the material provides a technical reference for the industrial application of the material.