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Structure Design And Performance Study Of LiNixCOyMn1-x-yO2(NCM811) Cathode Material

Posted on:2023-01-31Degree:MasterType:Thesis
Country:ChinaCandidate:Q L TongFull Text:PDF
GTID:2531306836462814Subject:(degree of mechanical engineering)
Abstract/Summary:PDF Full Text Request
Lithium-ion batteries are widely used in portable electronic devices and electric vehicles due to their high energy density,long cycle life and low cost.Nickel-rich ternary material Li Ni0.8Co0.1Mn0.1O2(NCM811)has broad development prospects as a representative of high energy density lithium-ion battery cathode materials.However,problems such as poor cycling performance,unstable structure and interface side reaction restrict the large-scale application of NCM811 cathode materials.Therefore,the design of a low-cost,simple process and large-scale application of NCM811material modification method will greatly promote the industrial application of NCM811 material.In this paper,Mg2+doped NCM811 material and Ge coating modified NCM811electrode were prepared by designing and modifying the NCM811 cathode material and its electrode plate,and the mechanism analysis and design verification were carried out by combining theoretical calculation and experiment.Main work contents include:(1)NCM811 cathode material was synthesized by high temperature solid-state method,and its morphology and structure were characterized and electrochemical characterization was carried out,and the optimum technological parameters for the synthesis of NCM811 material were explored.The discharge capacity of NCM811material prepared according to the best technological parameters is 201 m Ah/g at 0.1C rate,the capacity retention rate of 100 cycles at 1C rate is 82%,and the capacity decay rate of each cycle is only 0.18%.The differential capacity curve(d Q/d V)showed that the materials prepared under the optimal process parameters had good reversibility of phase transformation during the cycling process,and the scanning electron microscope(SEM)and X-ray diffraction(XRD)tests also showed that the material had uniform particle size,lowest cation mixing degree and high crystallinity.(2)The electronic structure of NCM811 doped with La,Ce and Mg ions has been calculated by density Functional theory(DFT).The results show that the Mg2+doped NCM811 material has the lowest band gap and the highest diffusion rate of lithium ion compared with other ion-doped materials.Further experimental verification shows that0.5wt%Mg-doped NCM811 material has an ionic conductivity of 3.2×10-5 S/cm,a reversible specific capacity of 165.2 m Ah/g after 100 cycles at 1C rate,and a cycle retention rate of 86.9%.Further optimization of Mg doping ratio showed that 1.0wt%Mg doped NCM811 material had excellent electrochemical performance,with a capacity retention rate of 89.6%for 100 cycles at 1C,and a reversible capacity of 142.2m Ah/g at 10C.(3)NCM811-Mg@Ge electrode was prepared by magnetron sputtering coating on the surface of NCM811-Mg electrode,and the electronic structure of the electrode before and after modification was calculated by DFT.The calculated results show that the band gap of NCM811-Mg@Ge electrode decreases to 0.72 e V,and the electronic conductivity of the electrode increases from 8.1×10-5 S/cm to 9.6×10-5 S/cm.Further experimental verification shows that the Ge electrode prepared with 100W power deposited on the electrode surface for 5 min has the best electrochemical performance,and the capacity retention rate is 83.1%in 200 cycles of 1C.SEM test results show that some Ge atoms enter into the electrode along the electrode pores,forming a three-dimensional conductive network structure and improving the conductivity of the electrode.XPS test showed that the Ge coating significantly alleviated the corrosion of NCM811-Mg material by electrolyte and inhibited the irreversible surface phase transformation.The 18650 cylindrical battery assembled with NCM811-Mg@Ge electrode matched with commercial graphite negative electrode still has a discharge capacity of 2034 m Ah after 1000 cycles at 2C rate,with an average capacity decay rate of only 0.014%per cycle.
Keywords/Search Tags:NCM811 cathode, Mg doping, Surface Ge coating, Lithium-ion batteries
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