Study On Preparation And Modification Of Cathode Materials With High-Nickel And Low-Cobalt

In recent years,lithium-ion batteries have been widely used in power batteries due to their excellent energy density.Currently,the mainstream materials such as Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）and Li Ni_0.8Co_0.15Al_0.05O₂（NCA）nickel-rich cathode materials have been regarded as the best choice for the next generation of power battery.However,with the increase of raw material cost and the battery energy density,Li Ni_xM_yO₂（x≥0.9）cathode material with high nickel and low cobalt has gradually appeared in the vision of researchers.（1）In order to seek breakthrough in material synthesis stage,this paper synthersized a small particle spherical precursor by solvothermal method with Ni（NO₃）₂·6H₂O and Co（NO₃）₂·6H₂O as raw materials of transition metals,isopropanol as solvent,and the synthesis process of precursor was confirmed as Ostwald ripening process.Based on this,the optimal condition of precursor synthesis is obtained as the reaction time of 9 h at 160°C.Combined with the morphology,structure and electrochemical properties of the materials,the optimal lithium ratio was1.05,and the optimal heat treatment schedule is sintered for 4 h at 500°C and then for 12 h at 710°C.（2）Problems such as lattice collapse,thermal runaway caused by irreversible phase transition and poor interface stability caused by microcracks have always existed in Ni-rich materials.In this paper,Ta₂O₅was selected as the dopant and Li Ni_0.9Co_0.1O₂ cathode material was synthesized by solid phase doping.In addition,through the low temperature condition of solid phase diffusion driving force will Ta⁵⁺successful introduction to lithium layer,alleviate the lithium ions in the embedded lithium layer in the process of collapse and the crystal defects such as the occurrence of irreversible phase transition.At the same time,adjust and control the growth of primary particle of material and dissipate the strain energy effectively generated when the lattice shrinks to stabilize the crystal structure.Electrochemical test results showed that 1.0wt.%Ta₂O₅ is the best doping amount,and the cycling retention reached 80.3%at 2.7-4.3 V voltage and 0.5 C.In particular,the cycling retention after200 cycles is as high as 86.9%under 5 C discharge condition,which is obviously improved compared with the pristine.（3）In order to solve the decomposition of residual lithium and electrolyte on the surface of positive electrode material and improve the stability of electrolyte/electrode interface,this paper introduced Al PO₄through solid phase method,and successfully constructed a Li Al O₂-Li₃PO₄ double coating layer on the surface of the material.This kind of lithium coating layer can reduce the residual lithium on the surface,form an effective protection against electrolyte and HF corrosion and promote Li⁺diffusion.The results showed that the introduction of0.4wt.%Al PO₄ improved the electrochemical performance of the sample to the best,the first discharge capacity of the sample was 198m Ah g^-1 at2.7-4.3 V,1 C,and the capacity retention was 79.9%after 100 cycles,the capacity retention at 5 C was also increased to 87.4%after 100 cycles.The dual regulation of Al PO₄ successfully solved the problem of poor interface stability of pristine.48 figures,12 tables and 125 references included in this thesis.