Vanadium aluminum phosphate electrode material and preparation method thereof

Abstract

The invention provides a vanadium aluminum phosphate electrode material and a preparation method thereof. In target product components, an aluminum source, a vanadium source and a phosphorus source are respectively aluminum nitrate or other aluminum sources, vanadium pentoxide or ammonium vanadate, ammonium dihydrogen phosphate, phosphoric acid or other phosphorus sources, and a nitrogen source of nitrogen-doped carbon is from polyvinylpyrrolidone or other nitrogen sources of a gel component; the carbon source is selected from polyvinyl alcohol and glycerol. By adopting an exogenous plasticizing physical crosslinking sol-gel method, in the whole reaction system, uniform distribution of reaction ions and intermediates is maintained, and finally, gel is networked, so that the problem of capacity fading caused by an unstable structure of an existing aluminum ion battery positive electrode material in a circulating process is solved, and the service life of the aluminum ion battery positive electrode material is prolonged. And the problems of high reaction system selection requirement, long preparation period and complex process of the existing endogenous covalent polycondensation sol-gel method for preparing the electrode material are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical energy storage. Background Art

[0002] As a new type of secondary battery, aluminum-ion batteries are regarded as a potential alternative to lithium-ion batteries due to the high theoretical capacity (2980 mAh / g), low cost (abundant aluminum resources), and high safety of aluminum metal anodes, and have important application prospects especially in the field of large-scale energy storage.

[0003] Compared with lithium-ion batteries, due to the slow charge-discharge electrochemical reaction kinetics of the cathode materials, aluminum-ion batteries face problems such as low charge-discharge rate, low specific capacity, low energy density, and low output voltage. In corrosive electrolytes, most cathodes have unstable host structures, resulting in poor cycle performance of aluminum-ion batteries.

[0004] Current research hotspots in the cathode material system of aluminum-ion batteries focus on carbon-based materials, transition metal oxides / sulfides, organic electrode materials, and metal-organic frameworks (MOFs), etc. Among them, transition metal oxides and sulfides have attracted much attention due to their advantages such as rich reserves, low cost, simple synthesis process, and strong tunability of crystal structure. However, such materials generally have dissolution phenomena in strongly corrosive electrolyte systems, leading to irreversible loss of electrode active substances and structural collapse, seriously restricting the cycle life and capacity retention rate of aluminum-ion batteries.

[0005] Research in the field of sodium / lithium-ion batteries shows that phosphate systems (such as sodium vanadium phosphate, lithium vanadium phosphate) are more suitable for ion insertion / extraction due to their stable three-dimensional framework structures. Traditional preparation methods include: (1) High-temperature solid-state method: Mix vanadium source, lithium source, and phosphorus source according to stoichiometric ratios, fully mix them through grinding or ball milling, and then perform high-temperature calcination under inert gas protection. The calcination temperature is generally above 700 °C, and the calcination time is usually several hours. This method is simple to operate and has low cost, but has problems such as uneven particle size and agglomeration. (2) Hydrothermal method: Dissolve vanadium source, lithium source, and phosphorus source in a solvent according to a certain ratio, fully stir and then put it into a high-pressure reaction kettle, react at a certain temperature and pressure, and obtain the product after washing, drying, etc. The powders prepared by the hydrothermal method have the advantages of complete crystal grain development, small and uniform particle size, and less particle agglomeration, but have high equipment requirements and are difficult to produce on a large scale. (3) Sol-gel method: Dissolve raw materials in a suitable solvent, form a gel through hydrolysis, polycondensation and other reactions, and then obtain the product through drying, calcination and other steps. This method can fully mix raw materials at the molecular or ionic level, which is beneficial to preparing materials with uniform properties, but the process is complex and the cycle is long. Summary of the Invention

[0006] The object of the present invention is to overcome the problem of capacity attenuation caused by unstable structure during the cycle of the existing cathode materials for aluminum-ion batteries, and to solve the problems of high requirements for the reaction system selection, long preparation period and complex process in the preparation of electrode materials by the existing endogenous covalent polycondensation sol-gel method. The present invention provides a vanadium aluminum phosphate electrode material with high structural stability and its preparation method.

[0007] The technical solution adopted by the present invention to achieve the above object is: a preparation method of a vanadium aluminum phosphate electrode material, comprising the following steps:

[0008] S1: The first solution: 1-3 parts of NH4H2PO4 are dissolved in 30 parts of water, and then 1-3 parts of Al(NO3)3·9H2O are added and dissolved to form a homogeneous solution;

[0009] S2: The second solution: 1-3 parts of oxalic acid, 0.7-2.1 parts of V2O5, 30 parts of water, stirred in a water bath at 80℃-85℃ for 3-5 hours, and V2O5 is completely reduced to form a dark blue solution;

[0010] S3: Mix the first solution and the second solution, add 0.2-1 part of polyvinylpyrrolidone, add 3-9 parts of polyvinyl alcohol, heat up to 90℃-92℃, after complete dissolution, add 3-9 parts of glycerol, stir evenly, and cool to form a gel;

[0011] S4: Dry in an oven at 60℃-70℃ to obtain a precursor, grind the precursor, calcine in an argon atmosphere at 300℃-400℃ for 4-5 hours, and then calcine in an argon atmosphere at 800℃-850℃ for 7-9 hours.

[0012] Further optimization: In the first solution, 1.3 parts of NH4H2PO4 are dissolved in 30 parts of water, and then 1.4 parts of Al(NO3)3·9H2O are added and dissolved to form a homogeneous solution.

[0013] Further optimization: In the second solution, 1 part of oxalic acid, 0.7 part of V2O5, 30 parts of water, stirred in a water bath at 80℃ for 3 hours, and V2O5 is completely reduced to form a dark blue solution;

[0014] Further optimization: In step S3, mix the first and second solutions, add 0.2 part of polyvinylpyrrolidone, add 6 parts of polyvinyl alcohol, heat up to 90℃, after complete dissolution, add 9 parts of glycerol, stir evenly, and cool to form a gel;

[0015] Further optimization: In step S4, dry in an oven at 60℃ to obtain a precursor. Grind the precursor, calcine in an argon atmosphere at 350℃ for 4 hours, and then calcine in an argon atmosphere at 800℃ for 8 hours.

[0016] An aluminum vanadium phosphate electrode material, an aluminum vanadium phosphate electrode material prepared by the method described in claims 1-5.

[0017] An aluminum vanadium phosphate electrode material and a preparation method thereof according to the present invention adopt an exogenous plasticized physical cross-linking sol-gel method. In the whole reaction system, the reaction ions and intermediates are kept uniformly distributed, and finally gel networking is achieved, overcoming the problem of capacity attenuation caused by unstable structure during the cycle of the existing cathode materials for aluminum ion batteries, and solving the problems of high requirements for the reaction system selection, long preparation period and complex process in the preparation of electrode materials by the existing endogenous covalent polycondensation sol-gel method. Description of the Drawings

[0018] Figure 1 It is a high-resolution transmission electron microscope image of the AVP@NC electrode material.

[0019] Figure 2 It is an XRD spectrum of the AVP@NC electrode material.

[0020] Figure 3 It is the Coulomb efficiency-specific capacity curve of the Al / / AVP@NC battery.

[0021] Figure 4 It is the voltage-specific capacity curve of the Al / / AVP@NC battery.

[0022] Figure 5 It is the differential capacity-voltage curve of the Al / / AVP@NC battery.

[0023] Figure 6 It is the preparation flow chart of the AVP@NC electrode material. Detailed Description of the Invention

[0024] Preparation of nitrogen-doped carbon (NC) coated aluminum vanadium phosphate (AlV2(PO4)3, AVP@NC) electrode material:

[0025] The aluminum source, vanadium source, and phosphorus source in the target product composition are aluminum nitrate or other aluminum sources, vanadium pentoxide or ammonium vanadate, ammonium dihydrogen phosphate, phosphoric acid or other phosphorus sources respectively. The nitrogen source of the nitrogen-doped carbon comes from polyvinylpyrrolidone or other nitrogen sources in the gel composition; the carbon source comes from polyvinyl alcohol and glycerol.

[0026] Preparation of Al x V y (PO4) z (AVP@NC) material, in which the molar ratio of Al / V / P in the molecular formula is x / y / z. According to this molar ratio and the theoretical yield, the specific molar amounts or masses of the raw materials NH4H2PO4, V2O5, and aluminum nitrate are determined. Oxalic acid is used as a reducing agent for V2O5, and the molar ratio of V2O5 / oxalic acid = 1 / 3.

[0027] The specific implementation method is as follows Figure 6 as shown

[0028] S1: The first solution: 1.3 parts of NH4H2PO4 are dissolved in 30 parts of water, and then 1.4 parts of Al(NO3)3·9H2O are added and dissolved to form a homogeneous solution;

[0029] S2: The second solution: 1 part of oxalic acid, 0.7 part of V2O5, 30 parts of water, stirred in a water bath at 80 °C for 3 hours, and V2O5 is completely reduced to a dark blue solution;

[0030] S3: Mix the first and second solutions, add 0.2 part of polyvinylpyrrolidone (PVP), add 6 parts of polyvinyl alcohol, heat up to 90 °C, after complete dissolution, add 9 parts of glycerol, stir evenly, and cool to form a gel;

[0031] S4: Dry in an oven at 60 °C to obtain a precursor. The precursor is ground and calcined in an argon atmosphere at 350 °C for 4 hours, and then calcined in an argon atmosphere at 800 °C for 8 hours.

[0032] Figure 1 is the high-resolution transmission electron microscope of the AVP@NC material. The material shows an obvious coating structure, and AVP is evenly distributed in the carbon layer. Figure 2 is the XRD test result of the material. The peak positions and peak intensities are consistent with the standard card of PDF#45-0319, Na3Fe2(PO4)3. The crystal structure of the sample is monoclinic, the space group symbol is Cc(9), and the unit cell parameters The included angle is 90.0°×125.141°×90.0°. Figure 3 Further proves the stability of the electrode material during the cycling process. After 65 cycles, the capacity retention rate is above 95%, the Coulomb efficiency is stable at 95%-96%, the electrode reaction is single, and only Al 3+ is involved in the insertion / extraction of the material. Although the phase change of AVP is also involved, due to the structural stability of AVP, the battery cycles stably. Figure 4 is the voltage-capacity curve of the battery. There are obvious voltage plateaus at about 1.9 V in the charging curve and about 1.7 V in the discharging curve, indicating that there are rapid deintercalation / insertion electrochemical reactions here. Figure 5 The redox peaks in the measured differential capacity voltage curve are sharp. The oxidation peak appears at 1.88 V and the reduction peak appears at 1.68 V, indicating that the ion diffusion and electron transfer processes of the electrode material are faster and the reaction kinetics are better.

[0033] The traditional endogenous covalent polycondensation sol-gel method uses hydrolyzable metal alkoxides or metal salts. After hydrolysis, sol formation, gel networking, aging, drying, and calcination in sequence, the target product is formed. Among them, hydrolysis, sol formation, and gel networking involve the formation of new chemical bonds and are called the endogenous covalent polycondensation sol-gel method. According to the characteristics of the sol-gel method, the present invention adopts an exogenous plasticized physical cross-linking sol-gel method, that is, after all the chemical reactions in the system are completed and a homogeneous solution is formed, polyvinyl alcohol / glycerol is added to form an exogenous physical cross-linking sol-gel network. In the entire reaction system, the reaction ions and intermediates are kept uniformly distributed, and finally gel networking occurs.

[0034] By using the exogenous plasticized physical cross-linking sol-gel method, the preparation efficiency of the material can be improved and the production process can be simplified. The vanadium source and oxalic acid that undergo chemical reactions are used as the first group of solutions. V 5+ is reduced to V 3+ , oxalic acid is oxidized to CO2, and other ion interferences are excluded. The phosphorus source and aluminum source that do not undergo chemical reactions are used as the second group of solutions and are dissolved evenly. After the reaction of the first group of solutions is completed, the first two groups of solutions with uniform solution components are mixed into the third group of solutions. In the third group of solutions, polyethanol / glycerol is added. Polyvinyl alcohol (PVA) forms a gel network skeleton and forms a three-dimensional structure through hydrogen bonds and physical entanglement, with a concentration of 5–15 wt% (the higher the concentration, the greater the gel strength, but the fluidity decreases). Glycerol is used as a plasticizer to break some of the hydrogen bonds between PVA molecules, reduce the brittleness of the gel, and improve the flexibility, with a concentration of 5–15 wt% (too high will cause the gel to soften or even lose its shape).

[0035] The exogenous plasticized physical cross-linking sol-gel method solves the drawback of the narrow raw material selection range of the endogenous covalent polycondensation sol-gel method, that is, the metal salt needs to be able to hydrolyze and form a gel network, and expands the application scope of the sol-gel method. The exogenous plasticized physical cross-linking sol-gel method forms a sol-gel network in a homogeneous solution through external intervention. Each ion preferentially occupies the position with the largest lattice energy (or the position with the lowest total energy of the system), and the gel network structure also avoids the problem of different phases precipitating successively due to solvent evaporation and supersaturation.

[0036] For the solvent evaporation in the sol-gel method, vacuum drying, freeze drying, or supercritical CO2 drying can be selected to avoid sample aggregation and achieve porosity. Polyvinyl alcohol / glycerol and polyvinylpyrrolidone in the sol-gel become the source of nitrogen-doped carbon after high-temperature calcination, enhancing the conductivity of the material. The polar surface of the nitrogen-doped carbon enhances the wettability with the electrolyte and reduces the ion transport energy barrier. Cetyltrimethylammonium bromide (CTAB) and F127 block copolymer are added to the solution as soft templates, and mesopores are formed in the material after high-temperature calcination. During the high-temperature calcination process, the carbon formed by polyvinyl alcohol / glycerol acts as a reducing agent to further reduce the high-valent transition metal elements.

[0037] The exogenous plasticized physical cross-linking sol-gel method has the following effects: the three-dimensional network structure of the gel restricts the free diffusion of ions through physical barrier effects, reducing stepwise crystallization caused by solubility differences; before gelation, each ion is uniformly dispersed, and the initial ratio is maintained after network curing, avoiding local concentration fluctuations; the nanopores of the gel slow down the solvent evaporation rate, making the system closer to the equilibrium state and reducing the supersaturation gradient; each component reaches the supersaturation critical point simultaneously, promoting coprecipitation rather than stepwise crystallization.

[0038] In the present invention, according to whether a chemical reaction occurs between the reaction raw materials, the solution is divided into different groups. After obtaining a homogeneous solution for each group, they are then mixed into a homogeneous solution. This avoids the problems of non-uniform chemical reactions and slow ion diffusion caused by the physical barrier of the exogenous gel network structure.

[0039] The exogenous plasticized physical cross-linking sol-gel method utilizes the principle of the lowest energy dominated by thermodynamics followed in material synthesis to achieve the preparation of the target product. Moreover, the two-step high-temperature calcination process promotes the migration of ions to thermodynamically stable sites, and different ions enter the accurate positions in the crystal lattice.

[0040] The endogenous covalent polycondensation sol-gel method requires that the reaction raw materials must have hydrolyzability and polycondensation ability. The exogenous plasticized physical cross-linking sol-gel method in the present invention constructs a gel network on the basis of a homogeneous solution, broadening the application scope of the sol-gel method. The expansion of this scheme is to prepare nanomaterials and porous materials on the basis of a homogeneous solution through the exogenous plasticized physical cross-linking sol-gel method (different from the endogenous covalent polycondensation sol-gel method generated by the hydrolysis and polycondensation of precursors such as metal salts or metal alkoxides inside the solution). The sources of exogenous gels are natural polymer gel materials such as gelatin, whey protein, agar, sodium alginate, carrageenan, pectin, etc.; synthetic polymer gel materials such as polyvinyl alcohol, polyacrylamide, poly(N-isopropylacrylamide), etc. The exogenous plasticized physical cross-linking sol-gel method not only includes forming a gel by adding exogenous sol-gel components on the basis of a homogeneous solution, but also includes that after adding exogenous components, a chemical reaction or physical cross-linking occurs with a certain component in the solution to generate a sol-gel. This scheme uses aluminum source, phosphorus source, and vanadium source as raw materials to prepare aluminum vanadium phosphate and carbon-coated aluminum vanadium phosphate through the exogenous plasticized physical cross-linking sol-gel method. The aluminum source can also be replaced with a magnesium source, calcium source, lithium source, etc. in this scheme, and the vanadium source can be replaced with other transition metal elements. After the elements in the target product are in a stoichiometric ratio and become a homogeneous solution, exogenous sol-gel raw materials are added to achieve plasticized physical cross-linking, and then the target product is obtained after drying and calcination. For the ion doping of vanadium sites and PO4 sites in vanadium phosphate electrode materials, a homogeneous solution is obtained by adjusting the pH value of the solution or adding a complexing agent, and the exogenous plasticized physical cross-linking sol-gel method is also applicable. This scheme is also applicable to materials other than aluminum vanadium phosphate, calcium vanadium phosphate, magnesium vanadium phosphate, zinc vanadium phosphate, lithium vanadium phosphate, etc. Forming a sol-gel network structure through exogenous plasticized physical cross-linking on the basis of a homogeneous solution is another option for preparing different types of nanomaterials and porous materials.

Claims

1. A preparation method of an aluminum vanadium phosphate electrode material, characterized in that: It includes the following steps: S1: The first solution: 1 - 3 parts of NH4H2PO4 are dissolved in 30 parts of water, and then 1 - 3 parts of Al(NO3)3·9H2O are added and dissolved to form a homogeneous solution; S2: The second solution: 1 - 3 parts of oxalic acid, 0.7 - 2.1 parts of V2O5, 30 parts of water, stirred in a water bath at 80℃ - 85℃ for 3 - 5 hours, and V2O5 is completely reduced to form a dark blue solution; S3: Mix the first solution and the second solution, add 0.2 - 1 part of polyvinylpyrrolidone, add 3 - 9 parts of polyvinyl alcohol, heat up to 90℃ - 92℃, after complete dissolution, add 3 - 9 parts of glycerol, stir evenly, and cool to form a gel; S4: Dry in an oven at 60℃ - 70℃ to obtain a precursor. Grind the precursor and calcine it in an argon atmosphere at 300℃ - 400℃ for 4 - 5 hours, and then calcine it in an argon atmosphere at 800℃ - 850℃ for 7 - 9 hours.

2. The preparation method of an aluminum vanadium phosphate electrode material according to claim 1, characterized in that: For further optimization, in the first solution: 1.3 parts of NH4H2PO4 are dissolved in 30 parts of water, and then 1.4 parts of Al(NO3)3·9H2O are added and dissolved to form a homogeneous solution.

3. The preparation method of an aluminum vanadium phosphate electrode material according to claim 1, wherein: For further optimization, in the second solution: 1 part of oxalic acid, 0.7 part of V2O5, 30 parts of water, stirred in a water bath at 80℃ for 3 hours, and V2O5 is completely reduced to form a dark blue solution; 4. The preparation method of an aluminum vanadium phosphate electrode material according to claim 1, characterized in that: For further optimization, in step S3, mix the first and second solutions, add 0.2 part of polyvinylpyrrolidone, add 6 parts of polyvinyl alcohol, heat up to 90℃, after complete dissolution, add 9 parts of glycerol, stir evenly, and cool to form a gel; 5. The preparation method of an aluminum vanadium phosphate electrode material according to claim 1, characterized in that: For further optimization, in step S4, dry in an oven at 60℃ to obtain a precursor. Grind the precursor and calcine it in an argon atmosphere at 350℃ for 4 hours, and then calcine it in an argon atmosphere at 800℃ for 8 hours.

6. An aluminum vanadium phosphate electrode material, characterized in that: The aluminum vanadium phosphate electrode material prepared by the method according to claims 1 - 5.

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