Study On Surface Modification And Electrochemical Properties Of Layered Cathode Materials

New energy vehicles with zero emission are the strategic choice for the country to achieve the goals of"carbon neutralization"and"carbon peak".However,at present,power batteries have the problem of insufficient energy density,resulting in short driving mileage,which hinders the promotion and use of new energy vehicles.Increasing the cut-off voltage of ternary layered cathode material（NCM）and developing high voltage cobalt free lithium rich manganese-based materials（LLOs）with superior capacity can effectively improve the energy density of lithium-ion batteries and alleviate users’mileage anxiety.So,it has been a hot research direction of many scientific researchers.However,when the cut-off voltage exceeds4.4 V,the ternary material is more likely to react with the electrolyte,destroy the electrode structure and affect the cycle stability of the material.At the same time,although cobalt free lithium rich manganese-based materials have high energy density,they still face commercialization problems such as low coulomb efficiency,poor rate performance,short cycle life and serious voltage attenuation.Therefore,this thesis significantly improves the structural stability of ternary materials under high voltage and promotes the commercialization of cobalt free lithium rich manganese-based materials by coating and modifying the material surface,phosphorylation,constructing the surface integrated structure.The main work is summarized as follows:1.Raw NCM523 materials were synthesized by hydrothermal reaction and solid phase method,and then the materials were wet treated with ammonium fluoride and various nitrates to prepare NCM523 composites coated by different fluoride.Electrochemical performance tests show that they present excellent cycle stability at high voltage.The cathodes were further determined by XRD,Raman,TEM and XPS which reveals the mechanism of fluoride coating in improving the stability of the materials under high voltage from the sight of surface morphology,valence state and component changes of the materials.What’s more,starting from the three factors of compound valence,metal ion radius and pH value of fluoride,the inhibitory effect of different fluoride on material interface reaction is explored,and the laws are concluded and summarized.2.Cobalt free lithium rich manganese-based cathodes were fabricated by solid state method under high temperature.The as-prepared materials were phosphorylated by NH₄H₂PO₄ to adjust the surface component and promote the activation of monoclinic phase.The phosphorylation degree was controlled by adjusting the treated pH value,and the influence of the phosphorylation degree on the structure and properties of the materials was investigated by the in-depth analysis of the pristine and pH=6.5 samples.STEM,Raman and XPS etc.were employed to explore the effect of phosphorylation on the material structure.And rate capacity,cyclic stability,EIS and CV were tested to reveal the improvement effect and mechanism of phosphorylation.The results suggest that the structural stability and anionic redox activity of the materials are significantly improved after phosphorylation.3.Raw cobalt free lithium rich manganese-based materials with high capacity were synthesized by molten salt method.When oxide was uniformly coated on the surface of commercial carbonate precursor in advance,and then calcined at high temperature with molten salt,surface integrated modified materials doped with metal ions could be prepared.The mechanism of surface integrated structure inhibiting interface reaction and structure evolution was revealed by STEM,Raman and XPS,while the systematic electrochemical performance test verified the effectiveness and universality of the surface integrated structure constructed by different nitrate treatments.Finally,the rules are summarized by DFT calculation,which provides a theoretical basis for the selection of cations in surface integrated structures for the first time.