Design,Preparation And Structure-property Relationship Of Modified Li-rich Manganese-based Cathode Materials

Li-ion batteries have become the preferred devices in large-scale energy storage systems such as electric vehicles and smart grids.High energy density is the prerequisite for large-scale applications of these devices.In a Li-ion battery,the energy density is largely determined by the cathode materials.The0.5Li₂Mn O₃·0.5Li MO₂ series（M=Ni,Co,Mn）has attracted much attention for its high specific discharge capacity during cycling.The ultrahigh capacity in Li-rich manganese-based cathode materials is mainly sourced from the irreversible oxidation of lattice oxygen during the electrochemical reaction.However,this irreversible oxidation process usually degrades the electrochemical performance,causing capacity loss and voltage fade.These problems have seriously restricted the applicability of Li-rich manganese-based cathode materials.Therefore,the Li-rich manganese-based cathode materials were investigated by the combination of the theoretical calculations and experiments in the paper.The related research on the oxidation of the lattice oxygen,the formation of oxygen vacancy,the reduction of the transition metal,the transition of layer to spinel,and the two-phase reaction was first carried out,which aimed at the problems of structural basis and reaction mechanism of the material.A series of application exploration on theoretical designs,preparation,and modification were subsequently conducted for the problems of low initial coulombic efficiency,voltage attenuation,serious capacity fadeout,and poor rate performance.The specific work is as follows:Firstly,the reaction conditions of Ni–Co–Mn-based precursors synthesized by the solvothermal method were investigated.High-angle annular dark field–scanning transmission electron microscopy（HAADF–STEM）analysis of the Li-rich manganese-based cathode material synthesized under the optimized conditions revealed a two-phase composite structure of hexagonal Li MO₂ and monoclinic Li₂Mn O₃,with space groups of R-3m and C2/m,respectively.The theoretical calculations of the two-phase structure proved why the poor rate performance of the Li-rich manganese-based cathode materials and the voltage must be raised to activate the Li₂Mn O₃ component.Subsequently,the precursor of the Ni–Co binary cathode material was synthesized by the solvothermal method under the optimal reaction conditions,and Li-rich cathode materials containing different proportions of the Li₂Mn O₃ and Li Ni_1/3Co_1/3Mn_1/3O₂ phases were synthesized by the high-temperature solid-phase method.The relevant characteristics of the materials were evaluated to explain the related reaction mechanism of Li-rich manganese-based cathode materials.Finally,the first-principles calculations proved that Mn and Co in the Li-rich manganese-based cathode materials were reduced by the oxidation-reduction reaction of the lattice oxygen,leading to voltage attenuation.Therefore,the physicochemical properties of the Li-rich manganese-based cathode materials were improved by controlling the release of oxygen to regulate the local structure during the electrochemical reaction,which was achieved by surface coating and lattice doping.Secondly,Nb-doped Li-rich manganese-based cathode materials were designed and prepared to explore the mechanism of improving the electrochemical properties of the materials.Density functional theory calculations based on first principles demonstrated a stronger bond energy in Nb–O than in Mn–O.After calculating the correlation parameters of pristine and Nb-doped materials,the Nb-doped material was found to have a smaller Li-ion migration barrier energy and a greater oxygen-vacancy formation enthalpy than the pristine material,indicating that Nb doping accelerated the migration of Li⁺and stabilized the material structure.The pristine and Nb-doped Li-rich manganese-based cathode materials were then synthesized by the high-temperature solid-phase method.The effects of Nb doping on the morphology,crystal structure,surface element valence,and electrochemical properties of the materials were characterized by various techniques.In electrochemical tests,the electrochemical performance（especially the cycle performance）was significantly enhanced by Nb doping.The cycle retention rate of Nb-0.02（The sample with the Nb doping amount of 2 mol%）reached 98.50%after 300 cycles at 0.2 C,which was higher than that of the pristine material（86.68%）under the same parameters.Thirdly,the Li-rich manganese-based cathode materials were modified by the mixed coatings（Al₂O₃ and Li Al O₂）to regulate their local structure,and the related mechanism of improving the physical and chemical properties of the materials was investigated.In DFT calculations,the energy barriers of the Li ions were lower in Li Al O₂ than in the Al₂O₃ buffer layer and the pure pristine surface.Meanwhile,the oxygen-vacancy formation energy was lower at the interface of the Li Al O₂ buffer layer than on the pure pristine surface,indicating that at the modified Li Al O₂ interface,the Li⁺migration was accelerated and more interfacial oxygen vacancies were formed.The stabilization of O atoms in the interface structure reduced the release of oxygen from the material.In addition,the Al₂O₃ coating prevented the direct corrosion of the material surface by HF in the electrolyte,thus protecting the whole material.To confirm this finding,the conditions for synthesizing Li-rich manganese-based cathode materials by the sol–gel method were discussed.The pristine materials were modified by a wet chemical method using aluminum isopropoxide as the aluminum source.The resulting Li-rich manganese-based cathode materials were coated with a mixed Al₂O₃and Li Al O₂ layer.The influences of the mixed-coating layer on the morphology,crystal structure,and electronic state of the Li-rich manganese-based cathode materials were investigated by various characterization methods.Electrochemical tests indicated that the mixed-coating layer greatly improves the electrochemical performance of Li-rich manganese-based cathode materials.When coated with mixed-coating layer,the cathode materials achieved a specific discharge capacity and initial coulombic efficiency of 272.7 m Ah g^-1 and 88.55%at 0.1C,respectively,whereas the pristine material reached only 246.7 m Ah g^-1 and 75.5%at 0.1C,respectively.The capacity retention of the modified material was 83.55%after 300cycles at 0.5C.Under the same parameters,the cycle retention rate of pristine materials was 53.96%.Finally,considering the common advantages of lattice doping and surface coating,the Li-rich manganese-based cathode materials were modified with Na₃VO₄,which exerts a triple modification effect.Theoretical calculations revealed that when modified with Na₃VO₄,Li-rich manganese-based cathode materials can dope their Li sites with Na atoms and their transition metal sites with V atoms,thus stabilizing the material structure and accelerating the Li⁺migration.Besides,the V element formed Li₃VO₄ on the surface of the materials when entered the crystal lattice,thereby reducing the formation of residual lithium compounds on the surface.As a lithium-ion conductor,Li₃VO₄ can also accelerate the migration of Li⁺ions and protect the cathode material from corrosion of by-products such as HF.To investigate the triple modification effect mechanism,the influence of the synthesis conditions on the physical and chemical properties of the material were first discussed.Pristine and Na₃VO₄-modified Li-rich manganese-based cathode materials were then synthesized by the solid-phase method at an appropriate calcination.The morphology,crystal structure,element valence,and electrochemical properties of the materials were investigated by various testing methods.Electrochemical tests confirmed that Na₃VO₄doping greatly promotes the electrochemical performance of Li-rich manganese-based cathode materials.The initial specific discharge capacity and coulombic efficiency of the modified materials reached 315.2 m Ah g^-1 and 89.37%at 0.1 C,respectively,while those of the pristine materials were 280.3 m Ah g^-1 and 83.14%under the same conditions.Meanwhile,the rate performances of the modified and pristine cathode materials were 145.2 m Ah g^-1 and 104.4 m Ah g^-1 at 8 C,respectively.In conclusion,Nb doping,mixed-coating modification,and Na₃VO₄ modification contribute specific advantages and solve many problems simultaneously through their synergistic effect,thus improving the electrochemical performance of materials.In addition,the research on the structural basis,reaction mechanism,and application exploration of the Li-rich manganese-based cathode materials is of great significance to guide its theoretical design,optimization,and application.