| Recently,due to the trouble of energy crisis and environmental pollution,more and more supportation on electrical vehicle market were put forward by the national government.As one of the main components of new energy vehicles,Lithium-ion batteries have been the research emphasis.Compared with the traditional cathode materials,such as LiCoO2,LiFePO4,nickel-rich cathode materials possess higher specific capacity because Ni is active substance during charging and discharging.In addition,nickel-rich cathode materials has the advantage of low cost,non-toxic and abundant resources of nickel.Therefore,nickel-rich cathode material,as the future of lithium-ion battery alternative cathode material,has great potential for application.However,a series of security problems emerged during cycling for nickel-rich cathode material,such as poor cycle stability,bad safety performance on account of the side reaction between Ni4+ and electrolyte.Thence,the relationship between the composition and properties of Ni-Mn binary materials system was studied in this paper through a combination of first-principles calculation and experiments as well as the structure and electrochemical performance of LiNi0.7Mn0.3O2 cathode materials by doping Nb5+ or F-were investigated,too.Models of lithium nickel oxide doped with manganese was established by using the CASTEP(Cambridge Sequential Total Energy Package)code based on density functional theory,firstly.The simulation of the lithium ion deintercalation process for Ni-Mn binary materials LiNixMn1-xO2(x=0.6,0.7,0.8)with different components were calculated to research the changes of cell volume and lattice parameters.Results show that with the increase of x(0.6 to 0.8),the cell volume and average working voltage decreased from 105.154 ?3 to 103.337 ?3 and 3.574581 V to 3.523451 V,respectively,which was corresponding with the experiment(x=0.6~0.8,102.92 ?3 to 102.23 ?3).The bond populations show that valence of manganese remains constant but nickel approaches to +3 with the increase of x.The density of states of each material in full lithium ion and semi-full lithium ion state is overlap closely,while the overlap is weak when the lithium ions are completely removed,which indicates that the material structure becomes unstable.Ion doping is one of the most commonly used methods for the modification of cathode materials.However,currently ions selecting work is semi-empirical.In the fourth and fifth chapter,first-principles calculation was used to study the effects of Nb5+ and F-substitution on the structure and electrochemical properties of LiNi0.7Mn0.3O2 cathode materials.It turned out that the supercell volume and electron distribution above the Fermi level both increased compared to the none dopted one,which may affected its physical and chemical properties.Furthermore,the Gibbs free energy reduced while Nb dopant made it decreased from-44343.99 eV to-45244.41 eV and F dopant also had a declining of-44343.958 eV to-44794.402 eV,and the crystal structure may be more stable after doping.In order to verify the theoretical calculation results,Li[Ni0.7Mn0.3]1-xNbxO2(x=0,0.01,0.02,0.03,0.04)and LiNi0.7Mn0.3O2-xFx(x=0,0.01,0.02,0.03)cathode materials were synthesized by coprecipitation and solid state reaction method in the fourth and fifth chapters.XRD tests confirmed that the two powders both had a well-defined layer structure based on hexagonal a-NaFeO2 structure and the cell volume increased compared to the none dopant one.The ratio of c/a and I(003)/I(104)with Nb dopant increased slightly and the layered structure became more perfect.However,cationic mixing had kinder increased with F dopant.On account of the value of c/a was still above 4.899 and I(003)/I(104)>1.2,which showed the materials doping by F still had complete layered structure.SEM image showed that the primary particles of Nb dopant got more round and compact as well as increased the contact area with the electrolyte which was beneficial to the transmission of lithium ions.Since LiF is also a kind of sintering additives,the primary particles grew toward the direction of single crystal and LiNi0.7Mn0.3O2-xFx(x=0.01,0.02,0.03)materials had higher tap density.EIS test suggested that after doping Nb5+ and F-,the impedance spectrum of intermediate frequency area semicircle,which corresponded to electrochemical impedance Rct decreasing in the battery,contrasted with no-doped samples,were significantly decreased.The electrochemical performance test showed that the cycling performance and the rate properties of the composites were enhanced after Nb5+ and F-doping with the best performance by doping 2% Nb5+ and 1% F-.The capacity retention ratio for the battery at a current rate of 0.2 C with Nb5+ was promoted from 75 % to 91.5% after 50 cycles,while a retention ratio with F-dopant was rising from 75.64% to 90.12%.The capacity of the doped material remained unchanged when back to 0.2 C after charging and discharging cycle with the high rates.Finally,X-ray photoelectron spectroscopy(XPS)was respectively used to test the none doped and doped 2% Nb5+ and 1% mol F-samples,analyzing the valence states of the elements before and after doping.The results promoted that nickel was given priority to with three price and the Ni3+/Ni2+ area ratio was 7.7040 in none doped material,compared to the doped one that the Ni3+ was decreased(the ratio of Nb5+ doped material is 4.9575,the ratio of F-material doped was 3.2110),while the valence of other elements kept constant.This indicated that the Ni2+ content in the material both increased after doping with higher valence cation Nb5+ and lower valence anion F-,which was consistent with the chemical equilibrium theory and also proved that the dopant ions were successfully incorporated into the lattice of the material.And the theoretical calculation results showed that the average bond of Ni-O increased from 2.018895 ? to 2.028066 ? after mixing with Nb,indicating that nickel inclined to the low price;Ni-F bond length was 2.2589 ? after mixing F,stating that nickel prices got lower by binding to F.Theoretical calculation showed the average price of nickel was reduced after doping,which was consistent with the results of XPS. |
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