Structural Optimization And Sodium Storage Mechanism Of Na₃V₂(PO₄)₃@C Cathode Materia

Utilization of clean energy is a feasible solution to the current energy crisis and environmental problems,and the use of these sustainable resources is closely related to large-scale energy storage technology.Compared with limited lithium resources,sodium-ion batteries have a brighter application prospect in the field of large-scale energy storage due to their resource and cost advantages.However,due to the large atomic mass and radius of sodium ions,the energy density and cycle stability of the deintercalation type cathode material are not ideal.Polyanionic compounds usually show a higher voltage plateau and good thermodynamic stability due to the strong induction effect and stability of the anionic group.The fast ion conductor NASICON Na₃V₂（PO₄）₃ has attracted wide attention of researchers due to its wide ion diffusion channel,excellent thermodynamic stability,and high energy density.However,its low electronic conductivity has seriously hindered its practical application.In this thesis,NASICON-type Na₃V₂（PO₄）₃ is selected as the research object,and the electrochemical performance of Na₃V₂（PO₄）₃ is improved by optimizing synthesis parameters,carbon modification and bulk doping.The main research content and conclusions are as follows:（1）Study the influence of sodium salt type and sintering temperature on properties of Na₃V₂（PO₄）₃.Choose different sodium salts（sodium hydroxide and sodium carbonate）to synthesize Na₃V₂（PO₄）₃@C by solid-phase method,and compare and analyze the crystallinity,morphology evolution and electrochemistry performance of Na₃V₂（PO₄）₃@C prepared from different sodium salts.The relevant research results show that sodium vanadium phosphate can be obtained from both sodium salts as raw materials,owing to the reaction among the different raw materials,inducing the morphology and performance difference of Na₃V₂（PO₄）₃@C products.Among them,Na₃V₂（PO₄）₃@C prepared with sodium hydroxide as the raw material display serious agglomeration and there is no capacity observed at 20 C,while the material prepared with sodium carbonate as the raw material has a smaller particle size and a carbon coating layer of about 8nm on the surface.Compared with the sodium storage performance,Na₃V₂（PO₄）₃@C synthesized with sodium carbonate as raw material has higher specific capacity(the capacity at 20 C is 71.6 mAh g^-1)and better cycling stability（the capacity retention rate cycled at 5C after 300 cycles of circulation is 93.5%）.Based on choosing sodium carbonate as the optimized sodium salt,the influence of calcination temperature（650℃,750℃and 850℃）on the physical and chemical properties and electrochemical properties of Na₃V₂（PO₄）₃@C was studied.It could be found that with the increase of the calcination temperature,the crystallinity of the material is improved,and the agglomeration behavior of the particles is intensified.Appropriate calcination temperature should simultaneously obtain products with good crystallinity and smaller particle size.Comprehensive analysis of electrochemical test results,750℃is the optimized calcination temperature.（2）In order to further reduce the cost,cheap and easily available carbohydrates were used as carbon sources to compare the effects of carbon sources with different polymerization degrees（glucose,sucrose and soluble starch）on the resulting Na₃V₂（PO₄）₃@C.Studies have shown that the carbon matrix of organic matter cracking has little effect on the crystallization of Na₃V₂（PO₄）₃,and sodium vanadium phosphate nanoparticles will be embedded in the formed carbon network.Carbohydrate compounds with different degrees of polymerization affect the final performance by affecting the form of carbon composite,and polymers originated from its intrinsic combination properties tends to obtain high conductive carbon matrix in the whole integrity.It is easy to obtain single-particle carbon-coated nanoparticles with glucose as the carbon source,and sucrose as the carbon source tends to obtain flaky particles,and soluble starch as the carbon-coated raw material can form a film on the surface of the precursor particles during the mechanical mixing stage.In the subsequent high-temperature sintering process,a uniform carbon coating layer with higher electronic conductivity is formed,which effectively inhibits the crystal growth of particles during the high-temperature sintering process and buffers the volumetric strain caused by the deintercalation of sodium ions.Half-cell and full-cell performance test results show that as the degree of polymerization of sugar compounds increases,the electrochemical performance gradually increases.Among them,Na₃V₂（PO₄）₃@C synthesized from soluble starch shows the best sodium storage performance:a specific discharge capacity of 72 m A h g^-1 at 40 C,and can maintain 82.8%of initial capacity after 1000 cycles at 1 C.And the capacity retention rate of a full battery matched with hard carbon is 80.1%after 100 cycles at 1 C.The analysis of sodium ion diffusion kinetics further supports this rule,and the electrochemical reaction of these type of materials is controlled by the diffusion process.（3）On the basis of improving the conductivity of the material through carbon coating,this section introduces the synergistic modification of high-valence Nb doping and carbon coating.Na₃V_2-xNb_x（PO₄）₃@C（x=0,0.1,0.2,0.3）was prepared by sol-gel method,and the effect of Nb⁵⁺doping on the crystal structure,element valence state composition,particle morphology,electrochemical properties,and sodium transfer dynamics were systematically studied.The unit cell parameters of Na₃V_2-xNb_x（PO₄）₃@C are slightly reduced and the content of V³⁺increases due to the result of charge balance due to the smaller ion radius and high valence of Nb⁵⁺.In addition,the introduction of Nb will slightly reduce the particle size,resulting in Na₃V_1.8Nb_0.2（PO₄）₃@C shows a large specific surface area(34.62 m² g^-1).The electrochemical test results show that,since the introduced Nb is an electrochemically inert element,it could act as a pillar atom in the structure.The Na₃V_2-xNb_x（PO₄）₃@C sample with the increase of Nb doping,the initial discharge specific capacity gradually decreased,but the rate performance and cycle stability under high current density gradually improved,of which Na₃V_1.8Nb_0.2（PO₄）₃@C sample showed a specific capacity of 81.6 m A h g^-1 at50 C.Diffusion kinetic analysis shows that the optimized Na₃V_1.8Nb_0.2（PO₄）₃@C material has a high sodium ion diffusion coefficient,and the electrochemical reaction process of this type of material is controlled by the diffusion behavior,and the modified sample has a high contribution value of pseudocapacitance.（4）On the basis of the single element doping modification Na₃V₂（PO₄）₃@C in previous chapter,this part combines three strategies of structure modification,morphology design and carbon layer graphitization adjustment at the same time.Ferrous oxalate was used as the doping source to synthesize Na₃V_2-xFe_x（PO₄）₃@C（x=0,0.05,0.15,0.25）,which possesses porous structure,improved electrical conductivity and electrochemical performance.This is mainly because the decomposition of Fe C₂O₄ can induce the generation of porous structure within a certain range,and the particle size of the sample modified by iron doping is significantly reduced.The solid-state NMR results show that some iron atoms enter the crystal structure,breaking the original regular atomic and rearrangement;some iron atoms would diffuse into the outer carbonized layer to adjust the degree of graphitization.The empirical formula and DFT theoretical calculations confirmed that the iron element should occupy the transition metal sites in the crystal lattice and adjust the bulk structure and the electron transport performance after doping is improved.Half-cell and full-cell test results show that the iron-doped Na₃V_1.85Fe_0.15（PO₄）₃@C shows good rate performance(it shows a specific capacity of 94.05 mAh g^-1 at 20 C),improved cycling stability（the capacity retention rate is 91.52%after 1200 cycles at 1C）and high capacity of full cell(initial capacity at 0.1C is 105.07 mAh g^-1).XRD patterns under different various voltages in process of charging/discharging confirm that the electrochemical process is highly reversible.After cycling,the crystal structure and porous structure of the material can still be maintained.In addition,sodium ion diffusion kinetics test confirms that the iron-doped sample has a small charge transfer resistance value,higher sodium ion diffusion coefficient and pseudocapacitance contribution value,and decreased sodium ion diffusion energy barrier.In short,this thesis chosen Na₃V₂（PO₄）₃@C cathode material of sodium-ion battery as research object,and carries out gradual progress for the basic synthesis of materials and various modification strategies such as carbon recombination,ion doping,and morphology control.The research involves the knowledge of the basic synthesis process,the optimal design of carbon composite materials,the basic modification principle of ion doping and the coordinated design of multiple effects,which would have guiding significance for the design and development of polyanionic compound cathode materials.