Sodium vanadium phosphate composite positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of sodium ion positive electrode materials, in particular to a sodium vanadium phosphate composite positive electrode material and a preparation method thereof. The composite positive electrode material is Na < 3.1 > V2 (PO4) 2.9 (SiO4) 0.1 (at) NC-x, the sodium vanadium phosphate composite positive electrode material has a graded porous coralline structure, and mesopores and through pore channels are formed in the coralline structure. The sodium vanadium phosphate composite positive electrode material effectively widens the migration channel of sodium ions, enhances the structural stability of the material, constructs the nitrogen-doped amorphous carbon coating layer with moderate and uniform thickness, synergistically inhibits the corrosion of electrolyte and buffers the volume change, maintains high specific capacity, and also has a good application prospect. The problems of poor interface stability and ultra-high rate bottleneck are solved, and the electrochemical performance is good.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion positive electrode materials, and in particular to a sodium vanadium phosphate composite positive electrode material and a preparation method thereof. Background Art

[0002] With the continuous growth of global energy demand and the increasing emphasis on environmental protection, the development of efficient and sustainable energy storage technologies has become a key issue in the current new energy field. Although lithium-ion batteries have achieved great success in portable electronic devices and electric vehicles, their widespread application in large-scale energy storage is limited by resource scarcity and uneven distribution. In contrast, sodium-ion batteries are considered a highly promising alternative energy storage technology due to the abundance of sodium resources, low cost, and similar physical and chemical properties to lithium.

[0003] Sodium-ion batteries (NaIBs) consist of a positive electrode, a negative electrode, a current collector, an electrolyte, and a separator. The cathode material is the core factor determining NaIB performance. Therefore, developing high-performance NaIB cathode materials is crucial for advancing NaIB technology. Among the numerous NaIB cathode materials, sodium vanadium phosphate (Na₃V₂(PO₄)₃), abbreviated as NVP, has attracted considerable attention due to its unique sodium superion conductor (NASICON) structure. This structure possesses an open three-dimensional backbone, providing fast pathways for sodium ion migration and exhibiting excellent Na₂ conductivity. Furthermore, Na₃V₂(PO₄)₃ exhibits high operating voltage, theoretical specific capacity, and good thermal stability. However, Na₃V₂(PO₄)₃ has low intrinsic electronic conductivity and is susceptible to volume changes during Na₂ ion insertion and extraction, limiting its rate capability and cycling stability.

[0004] To overcome the aforementioned shortcomings of sodium vanadium phosphate, researchers have proposed a variety of modification strategies, primarily including surface coating, ion doping, and morphology manipulation. Traditional carbon coating strategies can improve the conductivity of sodium vanadium phosphate, but they can lead to incomplete or even uniform carbon coating of the sodium vanadium phosphate particles, thus affecting their ultimate performance. Furthermore, during high-temperature calcination, the sodium vanadium phosphate particles tend to agglomerate, resulting in an uneven carbon coating and impairing the efficiency of electron and ion transmission.

[0005] In addition, for sodium vanadium phosphate materials, ion doping mainly involves Na, V and PO3-4 sites. Specifically, the Na site can be replaced by monovalent alkali metal cations and divalent metal ions, such as Li + Mg 2+ , K +Although Na site doping can improve the electrochemical performance of sodium vanadium phosphate to a certain extent, these doping ions occupy the available Na sites, thereby reducing the active Na that can be inserted and removed. + , so the improvement of battery performance is limited. For V site doping, common doping ions are Al 3+ 、Fe 3+ 、Mn 3+ etc., but since V is the electrochemical active center, V 3+ / V 4+ The redox reaction of sodium vanadium phosphate is the main source of its capacity, and the doping of electrochemically inactive Al 3+ When Fe 3+ Although doping can participate in redox, its reaction potential (2.5V) is lower than V 3+ (3.4V), resulting in a decrease in the overall voltage platform and energy density; while doping with Mn 3+ Ions with large radii may cause local lattice expansion, increase cyclic stress, and lead to cracks in sodium vanadium phosphate particles.

[0006] In addition to the triangular structure of BO3-3, PO3-4 sites are usually doped with polyvalent anions such as SO2-4 and SiO4-4, which are more similar to PO3-4 structures. BO3-3 doping can form a surface passivation layer, effectively inhibiting the side reactions of the electrolyte. However, since BO3-3 is an inert group and its triangular structure is quite different from the tetrahedral structure of PO3-4, doping may change the V 3+ The coordination environment of SO2-4 inhibits its redox activity. However, SO2-4 and SiO4-4 can show better electrochemical performance than BO3-3 due to their structural compatibility. SO2-4 doping will slightly increase the 2+ / V 3+ 、V 3+ / V 4+ and V 4+ / V 5+ Redox potential, but V 2+ / V 3+ An increase in low voltage results in a decrease in the overall average voltage. SiO4-4 is redox-active, and replacing the inert PO3-4 with SiO4-4 can increase the capacity of the sodium vanadium phosphate material while maintaining its crystal structure. However, introducing too much or too little silicon content is detrimental to improving the electrochemical performance of the sodium vanadium phosphate. High silicon content can also lead to structural instability in the sodium vanadium phosphate. Therefore, it is necessary to control the silicon doping content to maintain the structural stability of the sodium vanadium phosphate while achieving optimal electrochemical performance.

[0007] CN114864905A discloses a composite material of graphene-composite silicon-doped sodium vanadium phosphate and its preparation method and application. The patent uses graphene-composite silicon-doped sodium vanadium phosphate. After synthesizing a silicon-doped sodium vanadium phosphate precursor gel by hydrothermal method, the gel is mechanically stirred and mixed with graphene and then calcined. The preparation process requires multiple stirring, drying, grinding and calcining steps, which easily lead to the stacking of graphene sheets. Then, the carbon layer cannot be evenly coated during the composite process, causing the sodium vanadium phosphate active particles to agglomerate, affecting the structural integrity of the composite material. In addition, the graphene composite layer relies on physical adsorption or van der Waals force to adhere to the surface of the active particles. Since graphene is a rigid two-dimensional crystal, it is difficult to adapt to the volume change caused by the repeated deintercalation of sodium ions in the sodium vanadium phosphate during the charge and discharge process. Therefore, the graphene composite layer is prone to interfacial separation with the sodium vanadium phosphate particles during the cycle process, resulting in interruption of the electron conduction path, accelerated capacity decay, reduced rate performance and cycle efficiency of the composite material, and is not conducive to improving the electrochemical performance of the positive electrode material. This patent uses graphene composite silicon doped sodium vanadium phosphate. The carbon layer of the graphene composite mainly relies on the conductivity of the graphene itself to provide an electron path, does not participate in the redox reaction, and cannot provide additional capacity and electrochemical active sites. In addition, the two-dimensional structure of the graphene sheets is easily curled and stacked through π-π bond interactions to form a dense barrier, thereby hindering the diffusion path of sodium ions. The specific surface area of ​​the composite material is reduced, and the active sites are reduced, which affects the electrochemical properties of the material. Especially under high-rate conditions, the electrochemical performance is not ideal.

[0008] CN118289729B discloses a sodium vanadium phosphate cathode material based on a porous coral structure induced by phosphomolybdic acid and its preparation method. The patent uses phosphomolybdic acid to induce a porous coral structure, which stimulates V 4+ / V 5+ Redox reaction, with a high voltage platform of 3.9V, but V 5+ It is easy to react with the electrolyte at high potential, resulting in rapid capacity decay during the cycle. In addition, research has found that the nitrogen-doped carbon coating constructed by the method described will lead to insufficient buffering of the volume change of the sodium vanadium phosphate lattice during the repeated insertion and extraction of sodium ions, which can easily cause structural collapse and accelerate capacity decay. In addition, during the high-temperature calcination process, the metal oxide coating introduced by molybdenum doping may increase the complexity of the material. Although the MoO3 coating can improve conductivity, since this rigid oxide cannot adapt to the cyclic strain during the charge and discharge process, it may cause cracks at the interface, affecting the battery life.

[0009] This patent synthesizes sodium vanadium phosphate positive electrode material with phosphomolybdic acid-induced porous coral structure through a hydrothermal method. Although it can also form a porous coral morphology, the hydrothermal method needs to be carried out under high temperature and high pressure conditions, and has high energy consumption. The hydrothermal reaction needs to be maintained at 160-180°C for 8-10 hours. Under high temperature conditions, the volume of the reaction gas in the closed reactor will expand, thereby generating huge pressure, and this pressure is uncontrollable, which not only brings serious safety hazards, but also puts extremely high demands on production equipment and operators. The control of hydrothermal reaction conditions becomes extremely complicated. Slight changes in parameters such as reaction time, temperature and pressure may have a significant impact on the reaction results, resulting in uneven carbon coating and pore structure of the final product. Summary of the Invention

[0010] The object of the present invention is to provide a sodium vanadium phosphate composite cathode material and a preparation method thereof. The sodium vanadium phosphate composite cathode material effectively broadens the migration channel of sodium ions, enhances the structural stability of the material, and simultaneously constructs a nitrogen-doped amorphous carbon coating layer of moderate thickness and uniformity, synergistically inhibiting electrolyte corrosion and buffering volume changes. While maintaining a high specific capacity, it solves the problems of poor interface stability and ultra-high rate bottlenecks, and has good electrochemical performance.

[0011] The technical solution of the present invention is: a sodium vanadium phosphate composite positive electrode material is Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x; where x is the nitrogen source and Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 The molar ratio is 1≤x≤3.

[0012] In the sodium vanadium phosphate composite positive electrode material, a nitrogen-doped amorphous carbon coating layer with a thickness of 8-12 nm is uniformly coated on the surface of silicon-doped sodium vanadium phosphate particles.

[0013] During the research and design process of the present invention, in order to solve the problem that the ultra-high rate performance of existing sodium vanadium phosphate positive electrode materials is limited and cannot be effectively balanced with the structural stability of sodium vanadium phosphate, an innovative dual modification strategy is proposed through the synergistic effect of silicon-doped sodium vanadium phosphate and nitrogen-doped carbon layer surface coating. The surface of the silicon-doped sodium vanadium phosphate particles is uniformly coated with a nitrogen-doped amorphous carbon coating layer with a thickness of 8-12nm to form a sodium vanadium phosphate composite positive electrode material with a hierarchical porous coral-like structure.

[0014] An appropriate amount of redox-active silicate groups (SiO4-4) are used to replace some phosphate groups (PO3-4) in the sodium vanadium phosphate lattice with isoelectronics to stabilize V 3+ / V 4+The 3.4V platform is used to avoid electrolyte side reactions at high potentials, and the crystal structure of sodium vanadium phosphate is optimized, which can effectively broaden the migration channel of sodium ions and enhance the structural stability of the material. At the same time, a nitrogen-doped amorphous carbon coating layer with uniform and moderate thickness is constructed on its surface to improve the conductivity of the material, synergistically inhibit the corrosion of the electrolyte and buffer the volume change. While maintaining a high specific capacity, it solves the bottleneck of the balance between interface stability and ultra-high rate. At a high rate of 60C, the capacity can reach 80-83mAh / g. When the current density is restored from 60C to 1C, the capacity can be restored to 115.6mAh g -1 The structure has good reversibility, and the capacity retention rate can reach 90-92% after 200 cycles at a 1C rate, with good electrochemical performance.

[0015] The nitrogen source and silicon source are simultaneously introduced into the sodium vanadium phosphate system. The introduction of the silicon source improves the rate performance of the sodium vanadium phosphate, while the nitrogen source can introduce additional active sites, improve the interfacial sodium ion storage capacity, expand the contact area between the active material and the electrolyte, and accelerate the diffusion rate of sodium ions between different phases, which significantly and synergistically improves the electrochemical performance of the sodium vanadium phosphate positive electrode.

[0016] At the same time, the nitrogen atoms in the nitrogen-doped amorphous carbon coating layer chemically bond with the vanadium and oxygen atoms on the surface of the sodium vanadium phosphate, so that the carbon layer and the sodium vanadium phosphate are tightly combined at the interface, which can avoid the phenomenon of sodium vanadium phosphate particles separating from the interface and agglomerating due to the shedding of the coating layer. The carbon layer is more evenly coated on the dispersed sodium vanadium phosphate particles, and the integrity of the coating can be maintained even at high temperatures. The structural stability is good, which effectively improves the cyclic stability of the material.

[0017] More importantly, the present inventors discovered during the research process that the nitrogen-doped amorphous carbon coating plays a crucial role in the electrochemical performance of the sodium vanadium phosphate cathode material, having a buffering effect on alleviating the volume change of the sodium vanadium phosphate cathode material during the charge and discharge process. However, an excessively thick nitrogen-doped amorphous carbon coating may hinder the mobility of sodium ions, resulting in poor conductivity, while an excessively thin nitrogen-doped amorphous carbon coating fails to protect the crystal structure of the sodium vanadium phosphate. During the repeated insertion and extraction of sodium ions, this will lead to insufficient buffering of the sodium vanadium phosphate lattice volume change, thereby causing structural collapse and accelerating capacity decay.

[0018] When the thickness of the nitrogen-doped amorphous carbon coating on the surface of silicon-doped sodium vanadium phosphate particles is controlled at 8-12nm, the thickness within this nanoscale range can form a continuous conductive network, while allowing sodium ions to be rapidly transported through surface defects or pores. This not only inhibits the corrosion of the electrolyte, buffers the volume change of sodium vanadium phosphate during charge and discharge, effectively improves the cyclic stability of the material, but also avoids the mass burden caused by the introduction of excessive carbon. When this sodium vanadium phosphate composite positive electrode material is used in sodium ion batteries, after 200 cycles at a current density of 1C, the capacity retention rate is still 91.2%, which has excellent electrochemical performance.

[0019] The sodium vanadium phosphate composite positive electrode material has a hierarchical porous coral-like structure, wherein the coral-like structure has mesopores and through pores.

[0020] The sodium vanadium phosphate composite positive electrode material contains a hierarchical porous coral-like structure with mesopores and through-pores, which effectively expands the contact area between the active material and the electrolyte, accelerates the diffusion rate of sodium ions between different phases, improves the capacity and rate performance of the battery, and thus has excellent electrochemical performance.

[0021] In the present invention, the sodium vanadium phosphate composite positive electrode material has a particle size (particle diameter) of 1-5 μm.

[0022] In the present invention, the sodium vanadium phosphate composite cathode material is subjected to specific surface area and pore size analysis by nitrogen adsorption / desorption method, and the specific surface area is measured to be 31.2-48.3m 2 / g, pore size is 6.5-20nm.

[0023] The preparation method of the sodium vanadium phosphate composite positive electrode material comprises the following steps:

[0024] (1) Preparation of Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x precursor:

[0025] Dissolve the vanadium source and carbon source in deionized water and heat in a water bath at 30-60°C for 30-90 minutes. Use 20-40 mL of deionized water and stir magnetically during the reaction.

[0026] After the solution turns clear blue, add sodium source, phosphorus source, silicon source, and nitrogen source in sequence, raise the temperature to 80-100°C, and continue stirring for 4-6 hours until a gel is formed;

[0027] Then place the gel at 80-120℃ and vacuum dry for 12-24h, take it out and get Na 3.1 V2(PO4) 2.9 (SiO4)0.1 @NC-x precursor.

[0028] (2) Preparation of Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x composite materials:

[0029] The Na obtained in step (1) 3.1 V2(PO4) 2.9 (SiO4) 0.1 The NC-x precursor is ground into powder to obtain precursor powder;

[0030] The precursor powder was placed in a horizontal tube furnace with argon as the protective gas at a gas flow rate of 50-100 mL / min;

[0031] Use staged calcination: first pre-sinter at low temperature 300-500℃ for 3-5h;

[0032] Then, the final sintering is carried out at 600-800℃ for 6-12h, with a heating rate of 3-5℃ / min;

[0033] After cooling naturally to room temperature, take out and get Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x composite materials.

[0034] In the present invention, the molar ratio of the sodium source: the vanadium source: the phosphorus source: the silicon source: the carbon source in the preparation method of the sodium vanadium phosphate composite positive electrode material is 3.1:2:2.9:0.1:5.

[0035] In the present invention, the vanadium source in the preparation method of the sodium vanadium phosphate composite positive electrode material is at least one of ammonium metavanadate, sodium metavanadate or vanadium pentoxide.

[0036] In the present invention, the sodium source in the preparation method of the sodium vanadium phosphate composite positive electrode material is at least one of sodium acetate, sodium oxalate, sodium carbonate or sodium hydroxide.

[0037] In the present invention, the phosphorus source in the preparation method of the sodium vanadium phosphate composite positive electrode material is at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, sodium phosphate or phosphoric acid.

[0038] In the present invention, the silicon source in the preparation method of the sodium vanadium phosphate composite positive electrode material is at least one of silicon dioxide, tetraethyl silicate or sodium silicate.

[0039] In the present invention, the carbon source in the preparation method of the sodium vanadium phosphate composite positive electrode material is at least one of glucose, citric acid monohydrate, oxalic acid or sucrose; the nitrogen source is at least one of urea, melamine, polyvinyl pyrrolidone or dopamine. After high-temperature calcination, the carbon source not only serves as a carbon coating layer on the outside of the sodium vanadium phosphate particles, but also acts as a reducing agent and complexing agent to promote the formation of V in the sol-gel process. 5+ complex reaction to form a stable complex with vanadium ions.

[0040] The preparation method of the present invention introduces nitrogen source and a certain amount of silicon source into the traditional sodium vanadium phosphate synthesis system (containing pentavalent vanadium source, carbon source, sodium source and phosphorus source) through sol-gel method. First, the vanadium source and carbon source are added into deionized water and stirred. The carbon source acts as a reducing agent and complexing agent. The vanadium source and the carbon source are mixed with the vanadium phosphate in a water bath at 30-60℃. 5+ After sufficient reaction, a stable complex is formed, and then the temperature is raised to 80-100°C and the remaining raw materials are added. The complex is cross-linked with the hydrolysis product of the silicon source to form a porous gel skeleton. The solvent evaporates during the drying process of the wet gel, causing the gel network to shrink and form through-holes.

[0041] The next step is a staged calcination process. During the first low-temperature pre-sintering (300-500°C), the nitrogen source not only dopes the carbon but also acts as a pore-forming agent. The CO2 and NH3 bubbles generated by its pyrolysis guide the sodium vanadium phosphate nanoparticles to grow around the bubbles, forming a porous coral-like structure. During the subsequent high-temperature calcination (600-800°C), the nitrogen-containing carbon source pyrolyzes to form a nitrogen-doped amorphous carbon layer that covers the surface of the sodium vanadium phosphate, fixing the coral-like morphology.

[0042] The nitrogen source and carbon source are pyrolyzed to produce gases during the high-temperature calcination process. The pyrolysis gas of the nitrogen source guides the growth of sodium vanadium phosphate particles to form a coral structure, while the gas (such as CO2 and H2O) produced by the pyrolysis of the carbon source further expands the pores and diffuses to form a pore structure inside the sodium vanadium phosphate. The pore size of this pore is less than 50nm, which is consistent with the characteristics of mesopores.

[0043] The high bond energy and thermal stability of the Si-O bond in the incorporated SiO4-4 inhibit the collapse of pores during high-temperature calcination, maintaining the porous coral structure of sodium vanadium phosphate, and ultimately forming a high-performance composite positive electrode material with a hierarchical porous structure.

[0044] Furthermore, the preparation method has mild reaction conditions, does not require high temperature and high pressure conditions, has high safety and controllability, low energy consumption, and good reproducibility, making it suitable for large-scale production. The raw materials in the preparation method are mixed and reacted at the molecular level, resulting in a material with higher uniformity and consistency.

[0045] The beneficial effects of the present invention are as follows: the sodium vanadium phosphate composite positive electrode material described in the present invention utilizes SiO4-4 to replace part of PO3-4 in the sodium vanadium phosphate bulk phase to regulate the crystal structure of the polyanionic compound Na3V2(PO4)3. Since the doped silicon element has a larger ionic radius, it can effectively broaden the migration channel of sodium ions and enhance the structural stability of the material.

[0046] Specifically, relative to P 5+ For example, Si 4+ The radius is larger (Si 4+ :0.024nm,P 5+ :0.017nm), lower electronegativity (Si 4+ :1.90,P 5+ : 2.19), making the Si-O bond stronger than the PO bond, thereby enhancing the structural stability of sodium vanadium phosphate. In addition, based on the traditional carbon layer coating, a strategy of nitrogen atom doping and modification of the carbon layer was proposed. A nitrogen-containing carbon coating layer of uniform thickness was formed on the outside of the sodium vanadium phosphate. The nitrogen atom doping modified the surface of the carbon layer, thereby introducing defect structures and additional active sites, effectively improving the electronic conductivity of the amorphous carbon layer, promoting the rapid conduction of electrons between sodium vanadium phosphate particles, and significantly improving the rate performance and long cycle performance of the sodium vanadium phosphate positive electrode.

[0047] Compared with pure sodium vanadium phosphate positive electrode, the sodium ion battery prepared with the positive electrode material has significantly improved rate performance and long cycle performance. In a constant current charge and discharge test at a 1C rate, the first discharge specific capacity is 108.2-115.3mAh / g, and the capacity retention rate after 200 cycles at 1C is 90%-92%. The rate performance and cycle performance are both higher than those of pure sodium vanadium phosphate (87.2mAh / g, 63.9%).

[0048] The preparation method uses a simple sol-gel method to form a porous coral-like structure, expanding the contact area between the active material and the electrolyte, thereby accelerating the diffusion of sodium ions between the different phases. The experimental synthesis process is simple, using a carbon source as a complexing agent to enhance the gel's dissolution effect. It eliminates the need for complex processes and high costs, has a short synthesis cycle, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 The sodium vanadium phosphate composite positive electrode material Na prepared in Example 1 of the present invention 3.1 V2(PO4) 2.9 (SiO4) 0.1 X-ray diffraction pattern of @NC-2.

[0050] Figure 2 The sodium vanadium phosphate composite positive electrode material Na prepared in Example 1 of the present invention3.1 V2(PO4) 2.9 (SiO4) 0.1 Scanning electron microscope image of @NC-2.

[0051] Figure 3 The sodium vanadium phosphate composite positive electrode material Na prepared in Example 1 of the present invention 3.1 V2(PO4) 2.9 (SiO4) 0.1 High-magnification transmission electron microscope image of @NC-2.

[0052] Figure 4 The sodium vanadium phosphate composite positive electrode material Na prepared in Example 1 of the present invention 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2 EDS mapping diagram.

[0053] Figure 5 The sodium vanadium phosphate composite positive electrode material Na prepared in Example 1 of the present invention 3.1 V2(PO4) 2.9 (SiO4) 0.1 Comparison of battery cycle curves of 200 cycles at 1C when the undoped sodium vanadium phosphate positive electrode material NVP / C prepared in @NC-2 and Comparative Example 1 is used in sodium-ion batteries.

[0054] Figure 6 The sodium vanadium phosphate composite positive electrode material Na prepared in Examples 1, 2, and 3 of the present invention 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2、Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-1、Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-3 and the undoped sodium vanadium phosphate positive electrode material NVP / C prepared in Comparative Example 1 are used as positive electrode materials for sodium ion batteries. Comparison of rate performance.

[0055] Figure 7 The sodium vanadium phosphate composite positive electrode material Na prepared in Example 1 of the present invention 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2 and the first cycle charge-discharge curve of the undoped sodium vanadium phosphate positive electrode material NVP / C prepared in Comparative Example 1 at 0.1C when used as a positive electrode material in a sodium ion battery. DETAILED DESCRIPTION

[0056] The technical solution of the present invention is described in detail below with reference to the accompanying drawings.

[0057] The raw materials used in the following examples and comparative examples were all commercially available. Unless otherwise specified, the equipment and processes used were conventional laboratory instruments and methods, which are well known to those skilled in the art.

[0058] 1. The sodium vanadium phosphate composite cathode material of the present invention is used as the active material of the sodium ion cathode material, and the assembly of the sodium ion button half-cell comprises the following steps:

[0059] (1) Preparation of half-cell positive electrode sheet:

[0060] The Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x was used as the positive electrode active material. It was added to an agate mortar with a conductive additive (acetylene black) and a binder (polyvinylidene fluoride) in a mass ratio of 7:2:1 and ground for 20 minutes. After grinding evenly, 20wt% N-methylpyrrolidone (NMP) was added and grinding continued for 10 minutes to make a uniform slurry. The resulting slurry was then coated on the surface of the current collector (aluminum foil) with a thickness of 150μm. It was then dried in a vacuum oven for 10 hours to obtain a working electrode, and cut into circular electrode pieces with a diameter of 12mm using a cutting machine.

[0061] (2) Assembly of button half-cell:

[0062] The positive electrode sheet prepared in step (1) is used as the positive electrode, the sodium sheet is used as the counter electrode material, the Whatman diaphragm is used as the diaphragm between the positive and negative electrodes, and the electrolyte is NaClO4+EC / DEC+5%FEC; wherein NaClO4, EC, DEC and FEC represent sodium perchlorate, ethylene carbonate, diethyl carbonate and fluoroethylene carbonate, respectively (1M NaClO4 is dissolved in an EC / DEC system with a volume ratio of 1:1, and 5wt% FEC is added to prepare the electrolyte), and the positive electrode shell-positive electrode sheet-diaphragm and the electrolyte-sodium sheet-gasket-spring-negative electrode shell are placed in sequence to assemble into a CR2032 button battery.

[0063] Example 1

[0064] The sodium vanadium phosphate composite positive electrode material is Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2; among them, Na 3.1 V2(PO4) 2.9 (SiO4)0.1 The molar ratio of nitrogen source is 1:2.

[0065] The particle size of the composite positive electrode material is 4 μm; the thickness of the nitrogen-doped amorphous carbon coating layer is 9 nm.

[0066] The specific surface area of ​​the composite cathode material is 45.6 m 2 / g, and the average pore diameter is 12.4nm.

[0067] The preparation method of the sodium vanadium phosphate composite positive electrode material specifically comprises the following steps:

[0068] (1) Preparation of Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2 precursor:

[0069] Dissolve 0.822 g of vanadium pentoxide and 2.034 g of oxalic acid in 40 mL of deionized water, heat in a water bath at 60°C, and stir magnetically for 90 min.

[0070] After the solution turns clear blue, 0.742 g of sodium carbonate, 1.506 g of ammonium dihydrogen phosphate, 100 μL of tetraethyl silicate, and 0.542 g of urea are added in sequence, the temperature is raised to 80° C., and stirring is continued for 5 h until a gel is formed.

[0071] Then the gel was placed at 120 ° C and vacuum dried for 12 h, and then taken out to obtain Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2 precursor.

[0072] (2) Preparation of Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2 composite material:

[0073] The Na obtained in step (1) 3.1 V2(PO4) 2.9 (SiO4) 0.1 The @NC-2 precursor is ground into powder to obtain precursor powder.

[0074] The precursor powder was placed in a horizontal tube furnace with argon as the protective gas at a gas flow rate of 50 mL / min.

[0075] The calcination was carried out in stages: first, pre-sintering was carried out at a low temperature of 300°C for 3 hours.

[0076] Then the final sintering was carried out at 800 °C for 6 h, with a heating rate of 3 °C / min;

[0077] After cooling naturally to room temperature, take out and get Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2 composite material.

[0078] Depend on Figure 1 It can be seen that Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 There are no obvious impurity peaks in the @NC-2 sample, and the position and intensity of its diffraction peaks match those of sodium vanadium phosphate with a standard rhombohedral NASICON structure, indicating that the crystal structure of sodium vanadium phosphate has not changed after being coated with a nitrogen-doped carbon layer and combined with silicon doping.

[0079] Depend on Figure 2 It can be clearly seen that the porous coral-like structure of sodium vanadium phosphate is induced by the hydrothermal-assisted sol-gel method under high temperature and high pressure conditions. This porous coral-like structure can effectively expand the contact area between the active material and the electrolyte, accelerate the diffusion rate of sodium ions between different phases, and improve the utilization rate of the positive electrode material of the sodium ion battery.

[0080] Depend on Figure 3 It can be seen that a nitrogen-containing amorphous carbon coating layer of about 9 nm is formed on the outside of the sodium vanadium phosphate composite material, and the thickness of the carbon layer modified by nitrogen doping is uniform.

[0081] Depend on Figure 4 It can be seen that Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 The @NC-2 sample successfully introduced Si and N elements, and the elements were evenly distributed on the detected particles.

[0082] The Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2 composite material was used as the positive electrode active material. After the sodium ion button half-cell was assembled according to the aforementioned assembly method, it was left aside for 12 hours, and then the battery was subjected to constant current charge and discharge tests at room temperature in the voltage range of 2.3-4.1V.

[0083] like Figure 5-7 The electrochemical test results show that the first discharge capacity of the assembled half-cell at 0.1C can reach 120.7 mAh g -1 , after 200 cycles at 1C current density, the capacity retention rate is still 91.2%.

[0084] In addition, Na3.1 V2(PO4) 2.9 (SiO4) 0.1 The discharge specific capacities of the @NC-2 electrode at current densities of 0.5, 1, 2, 5, 10, 20, 40, and 60 C are 117.4, 115.3, 110.5, 105.9, 100.4, 94.1, 85.4, and 80.3 mAh·g, respectively. -1 When the current density is restored from 60C to 1C, the capacity can be restored to 115.6mAh·g -1 , indicating that the modified material has improved structural stability and excellent electrochemical properties.

[0085] Example 2

[0086] The sodium vanadium phosphate composite positive electrode material is Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-1, where Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 The molar ratio of nitrogen source is 1:1.

[0087] The difference between the preparation method and Example 1 is that the amount of urea added is 0.271 g, and the rest is the same as Example 1.

[0088] Will Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 When NC-1 is used as a cathode material for sodium-ion batteries, the rate performance at different rates is as follows: Figure 5 As shown, the material is used as the positive electrode material of the sodium ion battery. The discharge specific capacity of the assembled half-cell at current densities of 0.5, 1, 2, 5, 10, 20, 40, and 60C are 110.2, 104.5, 88.3, ​​78.7, 68.7, 57.9, 46.6, and 40.5 mAh·g, respectively. -1 When the current density is restored from 60C to 1C, the capacity can be restored to 104.6mAh·g -1 .

[0089] Example 3

[0090] The sodium vanadium phosphate composite positive electrode material is Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-3, where Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 The molar ratio of nitrogen source is 1:3.

[0091] The difference between the preparation method and Example 1 is that the amount of urea added is 0.813 g, and the rest is the same as Example 1.

[0092] Will Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 When NC-3 is used as a cathode material for sodium-ion batteries, the rate performance at different rates is as follows: Figure 5 As shown, the material is used as the positive electrode material of the sodium ion battery. The discharge specific capacity of the assembled half-cell at current densities of 0.5, 1, 2, 5, 10, 20, 40, and 60C are 107.1, 100.8, 80.2, 72.7, 64.8, 53.5, 40.1, and 34.4 mAh·g, respectively. -1 When the current density is restored from 60C to 1C, the capacity can be restored to 100.6mAh·g -1 .

[0093] Comparative Example 1

[0094] The preparation method of the undoped sodium vanadium phosphate positive electrode material Na3V2(PO4)3 / C (NVP / C) in this comparative example is different from that in Example 1 in that the sodium vanadium phosphate is not doped with a silicon source or a nitrogen source, and is only coated with an excess carbon source on the surface of the pure sodium vanadium phosphate.

[0095] First, 0.822g of vanadium pentoxide and 2.034g of oxalic acid were dissolved in 40mL of deionized water and magnetically stirred in a water bath at 60°C for 90min to allow for a full reaction. Then, after the solution turned clear blue, 0.742g of sodium carbonate and 1.506g of ammonium dihydrogen phosphate were added in sequence, the temperature was raised to 80°C, and stirring was continued for 5h until a gel was formed. The solution was then placed in a vacuum oven at 120°C for 12h to dry, then taken out and ground into powder in an agate mortar. The precursor powder was then placed in a horizontal tube furnace with argon as the protective gas at a gas flow rate of 50mL / min. It was pre-sintered at 300°C for 3h and then sintered at 800°C for 6h, with a heating rate of 3°C·min. -1 After cooling naturally to room temperature, the NVP / C material was obtained.

[0096] The preparation method of the positive electrode sheet and the assembly method of the button half-cell are the same as those in Example 1. After assembly, the battery is left for 12 hours, and then a constant current charge and discharge test is performed on the battery at room temperature in a voltage range of 2.3-4.1V.

[0097] When the sodium vanadium phosphate cathode material NVP / C prepared in this comparative example is used in sodium ion batteries, the battery cycle curve of 200 cycles at 1C is as follows: Figure 5As shown, the discharge capacity at different rates is as follows Figure 6 As shown, the first cycle charge and discharge curve at 0.1C is as follows Figure 7 shown.

[0098] like Figure 5-7 The electrochemical test shows that the material is used as the cathode material of the sodium ion battery. The first discharge capacity of the assembled half-cell at 0.1C is 101.4 mAh g -1 , after 200 cycles at a current density of 1C, the capacity retention rate is only 63.9%, and the cycle performance is poor.

[0099] In addition, the discharge specific capacities of the NVP / C electrode at current densities of 0.5, 1, 2, 5, 10, 20, 40, and 60 C are 100.7, 85.5, 70.8, 64.6, 54.9, 42.8, 38.4, and 20.8 mAh·g, respectively. -1 When the current density is restored from 60C to 1C, the capacity can only be restored to 84.5mAh·g -1 The rate performance is far inferior to that of the Na doped sodium vanadium phosphate in Examples 1, 2, and 3. 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-1、Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-2、Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-3.

[0100] Comparative Example 2

[0101] The preparation method of Comparative Example 2 is different from that of Example 1 in that the amount of oxalic acid added is 0.945 g, and the rest of the method is the same as that of Example 1.

[0102] The nitrogen-containing amorphous carbon coating layer formed on the outside of the sodium vanadium phosphate composite positive electrode material prepared in this comparative example is about 5 nm.

[0103] Electrochemical tests show that the material is used as the cathode material for sodium ion batteries. The first discharge capacity of the assembled half-cell at 1C is 99.75 mAh g -1 , after 200 cycles at 1C current density, the capacity retention rate is 78.7%.

[0104] Comparative Example 3

[0105] The preparation method of Comparative Example 3 is different from that of Example 1 in that the amount of oxalic acid added is 3.66 g, and the rest of the method is the same as that of Example 1.

[0106] The nitrogen-containing amorphous carbon coating layer formed on the outside of the sodium vanadium phosphate composite positive electrode material prepared in this comparative example is about 16 nm.

[0107] Electrochemical tests show that the material is used as the cathode material for sodium ion batteries. The first discharge capacity of the assembled half-cell at 1C is 101.06 mAh g -1 , after 200 cycles at 1C current density, the capacity retention rate is 81.8%.

[0108] Comparative Example 4

[0109] The preparation method of Comparative Example 4 is different from that of Example 1 in that the pre-sintering temperature in step (2) is 600° C., and the rest of the method is the same as that of Example 1.

[0110] Electrochemical tests show that the material is used as the cathode material for sodium ion batteries. The first discharge capacity of the assembled half-cell at 1C is 88.18 mAh g -1 , after 200 cycles at 1C current density, the capacity retention rate is 65.5%.

[0111] Comparative Example 5

[0112] The preparation method of Comparative Example 5 is different from that of Example 1 in that the final sintering temperature in step (2) is 900° C., and the rest of the method is the same as that of Example 1.

[0113] Electrochemical tests show that the material is used as the cathode material for sodium ion batteries. The first discharge capacity of the assembled half-cell at 1C is 99.75 mAh g -1 , after 200 cycles at 1C current density, the capacity retention rate is 79.2%.

Claims

1. A sodium vanadium phosphate composite positive electrode material, characterized in that: The composite cathode material is Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x; where x is the nitrogen source and Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 The molar ratio of , 1≤x≤3; The sodium vanadium phosphate composite positive electrode material is a material in which the surface of silicon-doped sodium vanadium phosphate particles is uniformly coated with a nitrogen-doped amorphous carbon coating layer with a thickness of 8-12 nm; The sodium vanadium phosphate composite positive electrode material has a hierarchical porous coral-like structure, wherein the coral-like structure has mesopores and through pores.

2. The sodium vanadium phosphate composite cathode material according to claim 1, characterized in that The particle size of the composite positive electrode material is 1-5 μm.

3. The sodium vanadium phosphate composite cathode material according to claim 1, characterized in that The specific surface area of ​​the composite cathode material is 31.2-48.3m 2 / g, pore size is 6.5-20nm.

4. A method for preparing the sodium vanadium phosphate composite positive electrode material according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) Preparation of Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x precursor: Dissolve the vanadium source and carbon source in deionized water, and heat in a water bath at 30-60°C for 30-90 minutes to carry out complexation reaction; After the solution turns clear blue, add sodium source, phosphorus source, silicon source, and nitrogen source in sequence, raise the temperature to 80-100°C, and continue stirring for 4-6 hours until a gel is formed; Then place the gel at 80-120℃ and vacuum dry for 12-24h, take it out and get Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x precursor; (2) Preparation of Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x composite materials: The Na obtained in step (1) 3.1 V2(PO4) 2.9 (SiO4) 0.1 The NC-x precursor is ground into powder to obtain precursor powder; The precursor powder was placed in a horizontal tube furnace with argon as the protective gas at a gas flow rate of 50-100 mL / min; Use staged calcination: first pre-sinter at low temperature 300-500℃ for 3-5h; Then, the final sintering is carried out at 600-800℃ for 6-12h, with a heating rate of 3-5℃ / min; After cooling naturally to room temperature, take out and get Na 3.1 V2(PO4) 2.9 (SiO4) 0.1 @NC-x composite materials.

5. The method for preparing the sodium vanadium phosphate composite cathode material according to claim 4, characterized in that: The molar ratio of the sodium source: the vanadium source: the phosphorus source: the silicon source: the carbon source is 3.1:2:2.9:0.1:

5.

6. The method for preparing the sodium vanadium phosphate composite cathode material according to claim 4, characterized in that: The vanadium source is at least one of ammonium metavanadate, sodium metavanadate or vanadium pentoxide.

7. The method for preparing the sodium vanadium phosphate composite cathode material according to claim 4, characterized in that: The sodium source is at least one of sodium acetate, sodium oxalate, sodium carbonate or sodium hydroxide.

8. The method for preparing the sodium vanadium phosphate composite cathode material according to claim 4, characterized in that: The phosphorus source is at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, sodium phosphate or phosphoric acid.

9. The method for preparing the sodium vanadium phosphate composite cathode material according to claim 4, characterized in that: The silicon source is at least one of silicon dioxide, tetraethyl silicate or sodium silicate.

10. The method for preparing the sodium vanadium phosphate composite cathode material according to claim 4, characterized in that: The carbon source is at least one of glucose, citric acid monohydrate, oxalic acid or sucrose; and the nitrogen source is at least one of urea, melamine, polyvinyl pyrrolidone or dopamine.

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