A transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material and its preparation method and application

Abstract

The present application provides a preparation method of a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material, comprising the following steps: using a solution gel method to uniformly dissolve a sodium source, a vanadium source, a transition metal source, a phosphorus source, a fluorine source, and a reducing agent in a preset stoichiometric ratio in DIW, and continuously heating and stirring in a water bath until the mixture is in a gel state; drying the gel, grinding it into powder, and obtaining a sodium vanadium fluorophosphate precursor powder; ball milling a mixed material A obtained by mixing the precursor powder with dopamine hydrochloride and carbon nanotubes in a preset ratio; heat-treating the material obtained by ball milling in an inert gas in a tube furnace, and naturally cooling it to room temperature, and finally obtaining a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material, and providing a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material and an application thereof; the present application is simple, efficient, economical, and environmentally friendly, and has reference significance for the synthesis and modification of positive electrode materials for lithium and sodium ion batteries.

Description

Technical Field

[0001] The present application belongs to the technical field of electrode material preparation, and specifically relates to a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material and a preparation method and application thereof. Background Art

[0002] With the intensification of environmental pollution and the scarcity of resources, new clean energy has achieved unprecedented development, but these energy sources cannot be fully utilized due to the constraints of time and regional conditions, so the demand for large-scale energy storage devices is growing rapidly. Among them, sodium-ion batteries have received great attention due to their advantages such as high abundance of sodium in the earth's crust and low cost. The capacity of sodium-ion batteries is restricted by the positive electrode material, so the development of high-performance positive electrode materials is the key to achieving large-scale application of sodium-ion batteries. Sodium vanadium fluorophosphate belongs to polyanionic compounds and has been widely studied because of its advantages such as high voltage platform, stable structure and good thermal stability. However, due to the poor electronic conductivity of the material itself, slow Na+ diffusion kinetics and large volume expansion, its development is restricted. If it is not modified, its capacity and energy density are difficult to meet the requirements of sodium-ion batteries as energy storage devices.

[0003] At present, the existing material synthesis methods include hydrothermal method, solid phase sintering method, solution gel method, etc. Among them, the hydrothermal method requires a high temperature and high pressure working environment, and has high requirements for equipment. Secondly, the hydrothermal process requires fine and complex process and low output, which is not suitable for large-scale industrial development. For the solid phase sintering method, the material needs to be calcined at high temperature, which has high energy consumption and is easy to pollute the environment. At the same time, solid phase sintering is easy to cause secondary crystallization of the material, causing the material to agglomerate. The modification method for the material synthesized by the traditional solution gel method is generally a one-step doping coating method, that is, the doping coating is directly achieved in one step by mixing the raw materials. First, the particle size of the material produced by the one-step doping coating is uncontrollable, the particle size is large and extremely uneven, and the morphology is irregular, which has a great impact on the circulation and rate performance of the material. Secondly, the carbon layer coated in the aqueous solution is looser than the carbon layer tightly coated by the secondary ball milling, so that the conductivity of the material will also be affected to a certain extent. At the same time, for the common single carbon layer coating modification, the improvement of the conductivity of NVPF is limited, and the conductivity between the particles is not effectively improved. Therefore, there are still many problems in improving material properties using traditional modification methods.

[0004] In view of this, this application is hereby filed. Summary of the invention

[0005] In order to solve one of the above technical defects, a method for preparing a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material is provided in an embodiment of the present application.

[0006] The technical solution of the present application provides a method for preparing a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material, comprising the following steps:

[0007] The sodium source, vanadium source, transition metal source, phosphorus source, fluorine source and reducing agent of preset stoichiometric ratio are uniformly dissolved in DIW by the solution gel method, and the mixture is continuously heated and stirred in a water bath until it becomes a gel state;

[0008] The gel is dried and ground into powder to obtain sodium vanadium fluorophosphate precursor powder;

[0009] ball milling a mixed material A obtained by mixing the precursor powder with dopamine hydrochloride and carbon nanotubes in a preset ratio;

[0010] The material obtained by ball milling is heat treated in an inert gas in a tube furnace and naturally cooled to room temperature to finally obtain a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material.

[0011] Preferably, the molar ratio of the sodium source, vanadium source, transition metal source, phosphorus source, fluorine source and reducing agent is 3:1:1:2:3:3;

[0012] The mass ratio of the precursor powder to dopamine hydrochloride and carbon nanotubes is 5:1:1.

[0013] Preferably, the transition metal source is one of V2O5, Fe2O3, Co2O3, Nb2O5, Cu2O, or a combination of any two of them.

[0014] Preferably, the mass ratio of the mixed material A to the ball loading is 5:1.

[0015] Preferably, the ball milling speed of the ball mill is 400-600 r / min, the ball milling time is 10 hours, and the ball milling working mode is intermittent.

[0016] Preferably, the calcination conditions in the tube furnace are: in an argon atmosphere, 400°C-600°C for 4-8 hours; gas flow rate: 100-200 sccm, heating rate: 5°C / min).

[0017] The technical solution of the present application also provides an application of a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material for preparing a positive electrode material.

[0018] The technical solution of the present application also provides a transition metal-doped double-carbon-coated modified sodium vanadium fluorophosphate material, the particles of which are connected by a 3D conductive network to provide a continuous and stable electron / ion transport framework.

[0019] Beneficial effects of this application:

[0020] 1. This application innovatively adopts the modification strategy of transition metal ion doping carbon coating and synthesizes NVPF electrode materials by solution gel method-ball milling method. First, by XRD before and after doping ( Figure 3 ) comparison shows that the characteristic peak shifts to the left after doping, proving that ions with a larger radius than vanadium ions will cause the material to expand its lattice after doping, making the diffusion channel of Na+ larger, effectively improving the material's capacity and rate performance. Secondly, due to the difference in electronegativity after doping, there will be more oxygen vacancies ( Figure 4 ) is produced, thereby improving the electronic conductivity of the material itself. At the same time, since the doped ions have a large ionic radius, they can serve as structural pillars after doping, effectively alleviating the structural collapse caused by Na+ deintercalation, which is of great help to the structural stability and cycle stability of the material. Secondly, the material is compounded with the carbon material by ball milling. This method can make the contact between the two closer and make the coating more uniform, which plays an important role in improving the conductivity of the material. The present application is simple, efficient, economical and environmentally friendly, and has reference significance for the synthesis and modification of positive electrode materials for lithium and sodium ion batteries.

[0021] 2. This application uses a dual carbon composite of dopamine and carbon nanotubes. The two play their respective roles. The uniform carbon layer obtained by dopamine polymerization will inhibit the excessive growth and adverse deformation of crystal particles during the heat treatment process, thereby promoting the nano- and uniformization of particle size. Secondly, it will shorten the Na + The diffusion path of Na + Rapid deintercalation during the charge and discharge process improves the rate performance of the material. Carbon nanotubes form a three-dimensional composite structure with dopamine-coated particles, improving the high-speed electron conduction between particles and providing a continuous and stable electron / ion transmission framework. The synergistic effect of the two greatly improves the performance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0023] Figure 1 This is a scanning electron microscope image of the doped, coated and modified NVPF prepared in the examples of the present application;

[0024] Figure 2 This is a scanning electron microscope image of the original NVPF prepared in the embodiment of the present application;

[0025] Figure 3 XRD spectra of the modified NVPF and the original NVPF prepared in the examples of the present application;

[0026] Figure 4 EPR diagrams of the modified NVPF and the original NVPF prepared in the examples of the present application;

[0027] Figure 5 Long cycle diagram of NVPF prepared for the embodiment of the present application;

[0028] Figure 6 This is a ratio diagram of the NVPF prepared in the examples of the present application. DETAILED DESCRIPTION

[0029] In order to make the technical solutions and advantages in the embodiments of the present application more clearly understood, the exemplary embodiments of the present application are further described in detail below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present application, rather than an exhaustive list of all the embodiments. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0030] The embodiment of the present application provides a method for preparing a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material, and the specific steps are as follows:

[0031] S1: Dissolve V2O5, oxalic acid, NH4H2PO4 and NaF in DIW in a molar ratio of 19:1:60:40:60. Place the beaker containing the mixed solution on a heating stirring table and continue heating it in a water bath at 70°C and 300-500r / min. Then, add ammonia water drop by drop into the solution to adjust the pH value of the solution to 7. Continue heating and stirring the solution for 4-6 hours until gel appears.

[0032] S2: Dry the gel at 60° C. for 12 h, and then grind the obtained material thoroughly to obtain a NVPF precursor.

[0033] S3: Add the NVPF precursor obtained in step S2, dopamine hydrochloride and carbon nanotubes in a mass ratio of 5:1:1 into a ball mill, add an appropriate amount of Tris buffer, and then add zirconium oxide balls in a mass ratio of 5:1 between the ball mill and the material, the ball milling speed is 400-600r / min, and the ball milling time is 10 hours. The working mode of 20 minutes of work and 10 minutes of rest is specially set to prevent the material from sticking to the wall.

[0034] S4: Place the dried mixed material into a tube furnace, keep it at 400℃-600℃ for 4-8 hours (gas flow rate: 100-200sccm, heating rate: 5℃ / min) under argon atmosphere, and cool it naturally to room temperature to obtain high-purity sodium vanadium fluorophosphate electrode material. The reaction temperature, heating rate, gas flow rate and reaction time are all adjustable. The synthesized material is characterized by scanning electron microscopy, XRD spectrum and cycle rate performance.

[0035] like Figure 1 , Figure 2 Shown are scanning electron microscope images of the modified NVPF and the original NVPF, respectively; the SEM images show that the particle size of the material after doping and coating modification is smaller and more uniform, and the particles are connected by a conductive network.

[0036] like Figure 3 The XRD spectra of the modified NVPF and the original NVPF are shown in Figure 2. Figure 3 It can be seen that compared with the original NVPF, the characteristic peaks of the modified material shifted to the left as a whole, proving that lattice expansion occurred after modification and the diffusion channel of Na+ became larger.

[0037] like Figure 4 The figure shows the EPR comparison of the modified NVPF and the original NVPF, which proves that more oxygen vacancies are generated after doping to balance the charge.

[0038] like Figure 5 The figure shows the long cycle diagram of the modified NVPF; the material can still maintain a capacity of 110 mAh / g after 200 cycles and has an extremely high capacity retention rate.

[0039] like Figure 6 The figure shows the rate diagram of the modified NVPF. The material can still reach a capacity of 80 mAh / g under a high current of 10C, proving that the material still has good performance under high current.

[0040] In summary, the use of transition metal ions with larger radii (Fe, Nb, Co, Cu, etc.) to dope achieves the following three purposes: (a) Taking advantage of the difference in electronegativity between the doped ions and vanadium, after the doped ions partially replace vanadium, more oxygen vacancies will be generated in order to maintain charge balance. The generation of vacancies greatly improves the electronic conductivity of the material. (b) Taking advantage of the difference in ionic radius between the doped elements and vanadium, the lattice will expand after doping, thereby increasing the charge balance of the Na + The diffusion of Na+ provides a wider path, which effectively improves the capacity and rate performance of the material. (c) Since the doped ions have a larger ionic radius, they can serve as structural pillars after doping, effectively alleviating the Na+ + The structural collapse caused by deintercalation is of great help to the structural stability and cyclic stability of the material.

[0041] The ball milling method is used for carbon coating to achieve the following purposes: (a) The material is compounded with the carbon material by ball milling. This method can make the contact between the two closer and make the coating more uniform, which plays an important role in improving the conductivity of the material. (b) Dopamine and carbon nanotubes are used for dual carbon composites. The two play their respective roles. The uniform carbon layer obtained by dopamine polymerization will inhibit the excessive growth and adverse deformation of crystal particles during heat treatment, thereby promoting the nano-sizing and uniformization of particle size. Secondly, it will shorten the Na + The diffusion path of Na + Rapid deintercalation during the charge and discharge process improves the rate performance of the material. Carbon nanotubes form a three-dimensional composite structure with dopamine-coated particles, improving the high-speed electron conduction between particles and providing a continuous and stable electron / ion transmission framework. The synergistic effect of the two greatly improves the performance of the material.

[0042] It is worth noting that the present application uses a simple sol-gel method to synthesize a doped and modified precursor, and then composite-coates the precursor with a carbon material by ball milling, and then obtains a transition metal ion-doped carbon-coated sodium vanadium fluorophosphate positive electrode material by drying, calcining, and other means, wherein the synthesis method, modification strategy, etc. are protected by patents.

[0043] It is worth noting that the technical steps S1-S4, material proportions, ball milling time, ball milling speed, heat treatment temperature, air flow rate, etc. of the present application, including but not limited to the innovative details of the above parameters, are all protected by patents.

[0044] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0045] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A method for preparing a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material, characterized in that: The steps include: The sodium source, vanadium source, transition metal source, phosphorus source, fluorine source and reducing agent of preset stoichiometric ratio are uniformly dissolved in DIW by the solution gel method, and the mixture is continuously heated and stirred in a water bath until it becomes a gel state; The gel is dried and ground into powder to obtain sodium vanadium fluorophosphate precursor powder; ball milling a mixed material A obtained by mixing the precursor powder with dopamine hydrochloride and carbon nanotubes in a preset ratio; The ball-milled material is heat-treated in an inert gas in a tube furnace and naturally cooled to room temperature to finally obtain a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material; The molar ratio of the sodium source, vanadium source, transition metal source, phosphorus source, fluorine source and reducing agent is 3:1:1:2:3:3; The mass ratio of precursor powder to dopamine hydrochloride and carbon nanotubes is 5:1:1; The transition metal source is one of Fe2O3, Co2O3, Nb2O5, Cu2O or any combination thereof; the ball milling speed of the ball mill is 400-600r / min, and the ball milling time is 10 hours; the ball milling working mode is intermittent; the tubular furnace calcination conditions are: in an argon atmosphere, 400°C-600°C for 4-8 hours; the gas flow rate is 100-200 sccm, and the heating rate is 5°C / min.

2. The preparation method according to claim 1, characterized in that The mass ratio of mixed material A to ball loading is 5:

1.

3. Application of a transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material, characterized in that: The transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material is prepared by the preparation method according to any one of claims 1 to 2, and the transition metal-doped carbon-coated modified sodium vanadium fluorophosphate material is used to prepare a positive electrode material.

Images