Preparation method of cactus-like sodium ion battery cathode material

Abstract

The present invention belongs to the field of sodium ion positive electrode materials, and relates to a method for preparing a cactus-shaped sodium ion battery positive electrode material, comprising the following steps: S1: wet-grinding a mixture of a sodium source, an iron source, a phosphorus source, gelatin and carbon nanotubes to obtain a slurry. S2: spray-drying the slurry to obtain a precursor powder. S3: calcining the precursor powder in an oxygen-free environment in one step to obtain a cactus-shaped sodium iron phosphate pyrophosphate positive electrode material with a nitrogen-doped carbon material coated on the surface. The present invention obtains a cactus-shaped sodium iron phosphate pyrophosphate positive electrode material with a nitrogen-doped carbon material coated on the surface by wet grinding, spray drying and one-step calcination. The cactus-shaped morphology can significantly increase the specific surface area, interlayer spacing and porosity of the positive electrode material, increase the contact area between the positive electrode material and the electrolyte, and can achieve a comprehensive improvement in conductivity, capacity, low temperature performance, cycle performance and high rate performance.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion cathode materials, and particularly relates to a preparation method of a cactus-like sodium ion battery cathode material. Background Art

[0002] Based on years of technological development and optimization, since commercialization in the 1990s of the 20th century, lithium ion batteries have become the representative of secondary batteries with the most excellent comprehensive performance and have a mature battery technology route. However, due to the continuous development of the new energy industry and the abundance limit of lithium elements in the earth's crust, lithium ion batteries may gradually be difficult to support the current growing energy storage demand. The working principle of sodium ion batteries is similar to that of lithium ion batteries, but the reserves of sodium salt resources are more abundant and the mining is simpler, which has broad prospects in the large-scale application in the energy storage field.

[0003] A sodium ion battery includes a positive electrode, a negative electrode, an electrolyte, a separator and accessory components, etc. Among them, the performance of the positive electrode material and the negative electrode material plays a decisive role in the overall performance of the sodium ion battery, especially the positive electrode material. Sodium iron pyrophosphate phosphate as a sodium ion cathode material has the advantages of low cost, environmental friendliness and stable structure.

[0004] However, the existing sodium iron pyrophosphate phosphate cathode material still has the following disadvantages:

[0005] First, poor electrical conductivity: Sodium iron pyrophosphate phosphate usually shows low electronic conductivity and sodium ion diffusion rate, which will lead to an increase in the internal resistance of the battery during charge and discharge, an aggravation of the polarization phenomenon, and affect the overall efficiency and power output of the battery.

[0006] Second, low capacity: Although sodium iron pyrophosphate phosphate has a certain theoretical specific capacity, its actually available capacity is often low.

[0007] Third, poor low-temperature performance: In a low-temperature environment, the ion diffusion rate of sodium iron pyrophosphate phosphate is significantly reduced, and the electronic conductivity is also affected, resulting in an increase in the internal resistance of the battery and an increase in the voltage drop, thereby affecting the charge and discharge performance of the battery.

[0008] Fourth, poor cycling performance: After long-term cyclic use, the capacity decay accelerates, which will affect the service life of the battery.

[0009] Fifth, poor high-rate performance: Due to the insufficient electronic conductivity and sodium ion diffusivity, the existing cathode materials perform poorly during high-rate charge and discharge.

[0010] Therefore, it is necessary to effectively improve the electrical conductivity, capacity, low-temperature performance, cycling performance and high-rate performance of the sodium iron pyrophosphate phosphate cathode material. Summary of the Invention

[0011] (1) Technical problems to be solved

[0012] In view of the above technical problems, the present invention provides a method for preparing a cactus-like sodium-ion battery cathode material, which can effectively improve the electrical conductivity, capacity, low-temperature performance, cycling performance, and high-rate performance of the sodium iron pyrophosphate cathode material.

[0013] (2) Technical solutions

[0014] To achieve the above object, the main technical solutions adopted by the present invention include:

[0015] A method for preparing a cactus-like sodium-ion battery cathode material, comprising the following steps:

[0016] S1: Wet-mill a mixture of a sodium source, an iron source, a phosphorus source, gelatin, and carbon nanotubes to obtain a slurry;

[0017] S2: Perform spray drying on the slurry to obtain a precursor powder;

[0018] S3: Perform one-step calcination on the precursor powder obtained in step S2 in an anaerobic environment to obtain a sodium iron pyrophosphate cathode material with a cactus-like surface coated with a nitrogen-doped carbon material.

[0019] In the method for preparing the cactus-like sodium-ion battery cathode material as described above, preferably, in step S1, the molar ratio of sodium, iron, and phosphorus elements in the sodium source, iron source, and phosphorus source is 4:(2.8 - 3):4;

[0020] The carbon element in the gelatin accounts for 1 - 5% of the total mass of the mixture of the sodium source, iron source, phosphorus source, gelatin, and carbon nanotubes;

[0021] The carbon nanotubes account for 5 - 15% of the mass of the gelatin.

[0022] In the method for preparing the cactus-like sodium-ion battery cathode material as described above, preferably, in step S1, after mixing the sodium source, iron source, phosphorus source, gelatin, and carbon nanotubes, water is added to obtain a mixed material, and then wet-milling is performed;

[0023] The sodium source is one or more of sodium carbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium oxalate, and sodium pyrophosphate;

[0024] The iron source is one or more of iron phosphate, ferrous citrate, ferrous oxalate, ferrous acetylacetonate, iron nitrate, iron chloride, and ferrous sulfate;

[0025] In the method for preparing the cactus-like sodium-ion battery cathode material as described above, preferably, in step S1, after mixing the sodium source, iron source, phosphorus source, gelatin, and carbon nanotubes, water is added to obtain a mixed material, and then wet-milling is performed;

[0026] Among them, the rotation speed of wet grinding is 2000 - 3000 r / min, the flow rate of the mixed material through the wet grinding equipment during wet grinding is 50 - 100 L / h, the duration of wet grinding is 0.5 - 10 h, the particle size of the grinding medium used for wet grinding is 0.03 - 0.3 mm, and the grinding medium accounts for 70 - 85% of the volume of the grinding chamber.

[0027] For the preparation method of the cactus-like sodium-ion battery cathode material as described above, preferably, in the slurry obtained in step S1, the solid content is 15 - 40 wt%, and the particle size of the solid particles is 50 - 200 nm.

[0028] For the preparation method of the cactus-like sodium-ion battery cathode material as described above, preferably, in step S2, the feeding rate of spray drying is 60 - 90 mL / min, the pressure of the spray gun is 0.2 - 0.4 MPa, the feeding temperature is 190 - 240 °C, and the discharging temperature is 80 - 130 °C.

[0029] For the preparation method of the cactus-like sodium-ion battery cathode material as described above, preferably, in step S3, the precursor powder obtained in step S2 is subjected to one-step calcination treatment under a mixed atmosphere of nitrogen and ammonia to obtain a sodium iron pyrophosphate cathode material with a cactus-like surface coated with nitrogen-doped carbon material;

[0030] In the mixed atmosphere, the volume ratio of ammonia is 10 - 20%.

[0031] For the preparation method of the cactus-like sodium-ion battery cathode material as described above, preferably, in step S3, the precursor powder is heated to 400 - 600 °C at a heating rate of 2 - 8 °C / min, calcined for 5 - 16 h, and then cooled.

[0032] For the preparation method of the cactus-like sodium-ion battery cathode material as described above, preferably, in step S3, the cooling time is 10 - 60 min.

[0033] For the preparation method of the cactus-like sodium-ion battery cathode material as described above, preferably, in step S3, the structural general formula of the sodium iron pyrophosphate cathode material with a cactus-like surface coated with nitrogen-doped carbon material is Na 4 Fe x (PO 4 ) 2 P 2 O 7 @C-N, 2.8 < x ≤ 3.

[0034] (III) Beneficial effects

[0035] Through wet grinding, spray drying, and one-step calcination treatment, the present invention enables the carbide of carbon nanotubes combined with gelatin to form a burr-like nitrogen-doped material on the surface of the sodium iron pyrophosphate phosphate cathode material, thereby obtaining a sodium iron pyrophosphate phosphate cathode material with a cactus-like spherical surface coated with a nitrogen-doped carbon material.

[0036] The functions of carbon nanotubes are as follows: First, carbon nanotubes contribute to constructing a uniform and efficient conductive network, significantly improving the electron conduction ability of the cathode material, reducing internal resistance, and enhancing the charge and discharge performance of the battery. Second, carbon nanotubes are one of the key factors for forming the burr-like structure, which helps to construct cactus-like spherical particles, can increase the specific surface area and the number of active sites of the material, and further enhance the electrochemical reaction activity of the cathode material. Third, the presence of carbon nanotubes can also achieve a balance between the density and porosity of the cathode material, ensuring both high energy density and facilitating the penetration of the electrolyte and the progress of electrochemical reactions.

[0037] Gelatin is rich in carbon and nitrogen elements and is crucial for forming the nitrogen-doped carbon material. The atomic radius of carbon atoms is similar to that of nitrogen atoms. Nitrogen doping can change the atomic and electronic structure of the carbon layer matrix, increase the conductivity of the cathode material, and can also relieve the lattice distortion of the composite material, improve the wettability of sodium iron pyrophosphate phosphate with the electrolyte, and further enhance its conductivity. In addition, gelatin has excellent adhesiveness and stability, which can effectively improve the agglomeration phenomenon of the precursor powder and ensure the uniform dispersion of the material.

[0038] One-step calcination treatment is also one of the key steps for forming the cactus-like spherical morphology. The cactus-like spherical morphology can significantly increase the specific surface area, layer spacing, and porosity of the cathode material, and increase the contact area between the cathode material and the electrolyte.

[0039] In summary, the nitrogen-doped carbon material and the efficient conductive network formed by it can significantly improve the electron conduction ability of the cathode material and enhance the conductivity. Based on the effects of the efficient conductive network and nitrogen doping, as well as the large specific surface area, rich pore structure, and large layer spacing of the cactus-like spherical particles, the contact area between the cathode material and the electrolyte increases, the capacity of the cathode material is improved, sodium ions can diffuse rapidly under low-temperature conditions, thereby improving the low-temperature performance of the cathode material, and at the same time, the cycle performance and high-rate performance of the cathode material can also be enhanced. Description of the Drawings

[0040] Figure 1 SEM image of the sodium iron pyrophosphate phosphate cathode material with a cactus-like spherical surface coated with a nitrogen-doped carbon material prepared in Example 1;

[0041] Figure 2 SEM image of the sodium iron pyrophosphate phosphate cathode material with a cactus-like spherical surface coated with a nitrogen-doped carbon material prepared in Example 2;

[0042] Figure 3 SEM image of the sodium iron pyrophosphate phosphate cathode material prepared as Comparative Example 1;

[0043] Figure 4 First charge-discharge curve of the battery composed of the sodium iron pyrophosphate phosphate cathode material prepared in Example 1. Detailed implementation mode

[0044] To better explain the present invention for easy understanding, the present invention will be described in detail below in conjunction with the drawings and specific implementation modes.

[0045] The present invention provides a preparation method for a cactus-shaped sodium ion battery cathode material, comprising the following steps:

[0046] S1: Wet-mill a mixture of a sodium source, an iron source, a phosphorus source, gelatin, and carbon nanotubes to obtain a slurry.

[0047] S2: Spray-dry the slurry to obtain a precursor powder.

[0048] S3: Perform a one-step calcination treatment on the precursor powder obtained in step S2 in an oxygen-free environment to obtain a sodium iron pyrophosphate phosphate cathode material with a cactus-shaped surface coated with a nitrogen-doped carbon material.

[0049] In the present invention, through wet-milling, spray-drying, and one-step calcination treatment, the carbide of carbon nanotubes combined with gelatin forms a burr-shaped nitrogen-doped material on the surface of the sodium iron pyrophosphate phosphate cathode material, and then a sodium iron pyrophosphate phosphate cathode material with a cactus-shaped surface coated with a nitrogen-doped carbon material is prepared. This special cactus / burr-shaped morphology can significantly increase the specific surface area and electrical transport performance of the cathode material, and can also balance a relatively high battery energy density and an appropriate porosity, which helps the electrolyte to penetrate into the entire electrode interior.

[0050] In the present invention, the functions of carbon nanotubes (CNT) include the following:

[0051] First, carbon nanotubes help to construct a uniform and efficient conductive network, significantly improve the electron conduction ability of the cathode material, reduce the internal resistance, and enhance the charge-discharge performance of the battery.

[0052] Second, carbon nanotubes are one of the key factors for forming the burr-shaped structure. The burr-shaped structure helps to construct cactus-shaped particles, which can increase the specific surface area and the number of active sites of the material, and further improve the electrochemical reaction activity of the cathode material.

[0053] Third, the presence of carbon nanotubes can also balance the density and porosity of the cathode material, ensuring both a high energy density and facilitating the penetration of the electrolyte and the progress of the electrochemical reaction.

[0054] If only gelatin is used without adding carbon nanotubes, it will be impossible to form a burr-like structure, and a cactus-like morphology cannot be formed on the surface of the cathode material, thereby resulting in insignificant improvement in the various properties of the cathode material.

[0055] Gelatin is rich in carbon and nitrogen elements, which is the key to forming nitrogen-doped carbon materials. The atomic radius of carbon atoms is similar to that of nitrogen atoms. Nitrogen doping can change the atomic and electronic structure of the carbon layer matrix, increase the conductivity of the cathode material, and can also relieve the lattice distortion of the composite material, improve the wettability of sodium iron pyrophosphate with the electrolyte, and further improve its conductivity.

[0056] In addition, gelatin has excellent adhesiveness and stability, and can also effectively improve the agglomeration phenomenon of the precursor powder to ensure uniform dispersion of the material.

[0057] One-step calcination treatment is also one of the key steps to form a cactus-like morphology. The cactus-like morphology can significantly increase the specific surface area, layer spacing and porosity of the cathode material, and increase the contact area between the cathode material and the electrolyte. If a two-step calcination treatment in stages is adopted in step S3, a burr-like structure cannot be formed on the surface of the prepared cathode material, and a cactus-like morphology cannot be formed on the surface of the cathode material.

[0058] Preferably, in the above step S1, the molar ratio of sodium element, iron element and phosphorus element in the sodium source, iron source and phosphorus source is 4:(2.8-3):4. The carbon element in gelatin accounts for 1-5% of the total mass of the mixture of sodium source, iron source, phosphorus source, gelatin and carbon nanotubes, and the carbon nanotubes account for 5-15% of the mass of gelatin.

[0059] The above sodium source can be one or more of sodium carbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium oxalate and sodium pyrophosphate, the iron source can be one or more of iron phosphate, ferrous citrate, ferrous oxalate, ferrous acetylacetonate, iron nitrate, iron chloride and ferrous sulfate, and the phosphorus source can be one or more of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, iron phosphate, iron pyrophosphate and sodium dihydrogen phosphate.

[0060] The specific operation of step S1 is as follows: After mixing the sodium source, iron source, phosphorus source, gelatin and carbon nanotubes, water is added to obtain a mixed material, and then wet grinding is carried out. The rotation speed of the wet grinding is 2000-3000 r / min, the flow rate of the mixed material through the wet grinding equipment during the wet grinding process is 50-100 L / h, the duration of the wet grinding is 0.5-10 h, the particle size of the grinding medium used for wet grinding is 0.03-0.3 mm, and the grinding medium accounts for 70-85% of the volume of the grinding chamber.

[0061] After step S1 is completed, the solid content of the obtained slurry is 15-40 wt%, and the particle size of the solid particles in the slurry is 50-200 nm.

[0062] Preferably, in the above step S2, the slurry can enter the spray drying chamber through a spray gun. The feeding rate of spray drying is 60 - 90 mL / min, the pressure of the spray gun for sample injection is 0.2 - 0.4 MPa, the feeding temperature is 190 - 240 °C, and the discharging temperature is 80 - 130 °C.

[0063] Preferably, in the above step S3, the precursor powder obtained in step S2 can be subjected to a one-step calcination treatment in a mixed atmosphere of nitrogen and ammonia to obtain a sodium iron pyrophosphate phosphate cathode material with a cactus-like surface coated with nitrogen-doped carbon material. In the mixed atmosphere, the volume ratio of ammonia can be 10 - 20%.

[0064] In the mixed atmosphere formed by nitrogen and ammonia, nitrogen can be used as a protective gas to prevent the oxidation of particulate materials, and ammonia can provide active nitrogen during the calcination process, further enhancing the level of nitrogen doping on the basis of gelatin. A small amount of nitrogen doping can improve the electrochemical performance of the coating material on the surface of the cathode material. The specific principle is as follows:

[0065] 1. Nitrogen doping can introduce electron donors and improve the electron conduction ability of carbon materials, because higher conductivity can bring better charge and discharge performance.

[0066] 2. The doping of nitrogen element can provide additional redox active sites, which helps to enhance the pseudocapacitance characteristics of the cathode material, thereby improving the capacity and rate performance of the cathode material.

[0067] 3. Nitrogen doping can change the surface chemical properties of carbon materials, making their surface more hydrophilic and conducive to the infiltration of electrolytes.

[0068] 4. Nitrogen doping can increase more active sites, which can promote the adsorption and desorption of ions, thereby improving the capacity and cycle stability of the cathode material.

[0069] Further preferably, in step S3, the precursor powder can be heated to 400 - 600 °C at a heating rate of 2 - 8 °C / min, then calcined for 5 - 16 h, and finally cooled rapidly to obtain a sodium iron pyrophosphate phosphate cathode material with a cactus-like surface coated with nitrogen-doped carbon material. The cooling time needs to be maintained at 10 - 60 min. In addition to the above one-step calcination, rapid cooling will also affect the morphology of the cathode material. Rapid cooling can cause the expanded cathode material to shrink rapidly. Under the combined action of one-step calcination and rapid cooling, a cathode material with a cactus-like morphology is finally obtained.

[0070] In addition, the structural general formula of the sodium iron pyrophosphate phosphate cathode material with a cactus-like surface coated with nitrogen-doped carbon material prepared in step S3 can be expressed as Na 4 Fe x (PO4 ) 2 P 2 O 7 @C-N, where 2.8 < x ≤ 3.

[0071] In summary, the nitrogen-doped carbon material and the highly conductive network formed thereby can significantly improve the electron conduction ability of the cathode material and enhance the conductivity. Based on the effects of the highly conductive network and nitrogen doping, as well as the large specific surface area, rich pore structure, and large interlayer spacing of the cactus-like particles, the contact area between the cathode material and the electrolyte increases, the capacity of the cathode material is improved, sodium ions can rapidly diffuse under low-temperature conditions, thereby improving the low-temperature performance of the cathode material, and at the same time, the cycle performance and high-rate performance of the cathode material can also be improved.

[0072] To further clarify the solution of the present invention and its technological progressiveness, the following will be described in conjunction with specific examples and technical effects.

[0073] Example 1

[0074] This example provides a method for preparing a cactus-like sodium-ion battery cathode material, including the following steps:

[0075] S1: Mix sodium carbonate, iron phosphate, sodium dihydrogen phosphate, gelatin, and carbon nanotubes, add water to obtain a mixed material, and then perform wet milling to obtain a slurry with a solid content of 16 wt% and an average particle size of 150 nm for the solid particles.

[0076] In step S1, sodium carbonate and sodium dihydrogen phosphate are sodium sources, iron phosphate is an iron source, and sodium dihydrogen phosphate and iron phosphate are phosphorus sources. The molar ratio of sodium, iron, and phosphorus elements in the sodium source, iron source, and phosphorus source is 4:2.91:4. The carbon element in gelatin accounts for 4% of the total mass of the mixture of the sodium source, iron source, phosphorus source, gelatin, and carbon nanotubes, and the carbon nanotubes account for 10% of the mass of gelatin. The rotation speed of wet milling is 2500 r / min, the flow rate of the mixed material through the wet milling equipment during wet milling is 80 L / h, the duration of wet milling is 5 h, the particle size of the grinding medium used for wet milling is 0.1 mm, and the grinding medium accounts for 78% of the volume of the grinding chamber.

[0077] S2: Perform spray drying on the slurry. The feeding rate of spray drying is 75 mL / min, the pressure of the spray gun is 0.3 MPa, the feeding temperature is 230 °C, and the discharging temperature is 110 °C to obtain a precursor powder.

[0078] S3: Under a mixed atmosphere of nitrogen and ammonia, heat the precursor powder to 500 °C at a heating rate of 2 °C / min, then calcine for 10 h, and then cool down for 30 min to obtain a sodium iron pyrophosphate cathode material with a cactus-like surface coated with nitrogen-doped carbon material. The volume fraction of ammonia in the mixed atmosphere is 15%.

[0079] The structural formula of the sodium iron pyrophosphate phosphate cathode material with a cactus-like spherical surface coated with nitrogen-doped carbon material prepared in this example is Na 4 Fe 2.91 (PO 4 ) 2 P 2 O 7 @C-N.

[0080] Example 2

[0081] This example provides a method for preparing a cactus-like spherical sodium-ion battery cathode material, including the following steps:

[0082] S1: After mixing ferrous oxalate, sodium dihydrogen phosphate, gelatin, and carbon nanotubes, add water to obtain a mixed material, and then perform wet grinding to obtain a slurry with a solid content of 15 wt% and an average particle size of 50 nm for the solid particles.

[0083] In step S1, sodium dihydrogen phosphate is used as the sodium source and phosphorus source, and ferrous oxalate is used as the iron source. The molar ratio of sodium, iron, and phosphorus elements in the sodium source, iron source, and phosphorus source is 4:3:4. The carbon element in the gelatin accounts for 5% of the total mass of the mixture of the sodium source, iron source, phosphorus source, gelatin, and carbon nanotubes, and the carbon nanotubes account for 5% of the mass of the gelatin. The rotation speed of the wet grinding is 2000 r / min, the flow rate of the mixed material through the wet grinding equipment during the wet grinding process is 50 L / h, the duration of the wet grinding is 0.5 h, the particle size of the grinding medium used for the wet grinding is 0.03 mm, and the grinding medium accounts for 85% of the volume of the grinding chamber.

[0084] S2: Perform spray drying on the slurry. The feeding rate of the spray drying is 60 mL / min, the pressure of the spray gun is 0.2 MPa, the feeding temperature is 240 °C, and the discharging temperature is 80 °C to obtain a precursor powder.

[0085] S3: Under a mixed atmosphere of nitrogen and ammonia, heat the precursor powder to 550 °C at a heating rate of 5 °C / min, then calcine for 8 h, and then cool down for 20 min to obtain a sodium iron pyrophosphate phosphate cathode material with a cactus-like spherical surface coated with nitrogen-doped carbon material. The volume fraction of ammonia in the mixed atmosphere is 10%.

[0086] The structural formula of the sodium iron pyrophosphate phosphate cathode material with a cactus-like spherical surface coated with nitrogen-doped carbon material prepared in this example is Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 @C-N.

[0087] Example 3

[0088] This embodiment provides a method for preparing a cactus-like sodium-ion battery cathode material, comprising the following steps:

[0089] S1: After mixing ferrous citrate, iron phosphate, sodium pyrophosphate, gelatin, and carbon nanotubes, water is added to obtain a mixed material, which is then wet-milled to obtain a slurry with a solid content of 40 wt% and an average particle size of 200 nm for the solid particles.

[0090] In step S1, sodium pyrophosphate is the sodium source, ferrous citrate and iron phosphate are the iron sources, and iron phosphate and sodium pyrophosphate are the phosphorus sources. The molar ratio of sodium, iron, and phosphorus in the sodium source, iron source, and phosphorus source is 4:2.95:4. The carbon element in gelatin accounts for 1% of the total mass of the mixture of the sodium source, iron source, phosphorus source, gelatin, and carbon nanotubes, and the carbon nanotubes account for 15% of the mass of gelatin. The rotation speed of the wet milling is 3000 r / min, the flow rate of the mixed material through the wet milling equipment during the wet milling process is 100 L / h, the duration of the wet milling is 10 h, the particle size of the grinding medium used for the wet milling is 0.3 mm, and the grinding medium accounts for 70% of the volume of the grinding chamber.

[0091] S2: The slurry is subjected to spray drying treatment. The feeding rate of the spray drying is 90 mL / min, the pressure of the spray gun is 0.4 MPa, the feeding temperature is 190 °C, and the discharging temperature is 130 °C to obtain a precursor powder.

[0092] S3: Under a mixed atmosphere of nitrogen and ammonia, the precursor powder is heated to 600 °C at a heating rate of 8 °C / min, then calcined for 5 h, and then cooled for 60 min to obtain a cactus-like sodium iron pyrophosphate cathode material with a nitrogen-doped carbon material coated on the surface. The volume ratio of ammonia in the mixed atmosphere is 20%.

[0093] The structural formula of the cactus-like sodium iron pyrophosphate cathode material with a nitrogen-doped carbon material coated on the surface prepared in this embodiment is Na 4 Fe 2.95 (PO 4 ) 2 P 2 O 7 @C-N.

[0094] Example 4

[0095] This embodiment provides a method for preparing a cactus-like sodium-ion battery cathode material, comprising the following steps:

[0096] S1: After mixing disodium hydrogen phosphate, ferrous acetylacetonate, phosphoric acid, gelatin, and carbon nanotubes, water is added to obtain a mixed material, which is then wet-milled to obtain a slurry with a solid content of 20 wt% and an average particle size of 100 nm for the solid particles.

[0097] In step S1, disodium hydrogen phosphate is used as the sodium source, ferrous acetylacetonate is used as the iron source, and phosphoric acid is used as the phosphorus source. The molar ratio of sodium, iron, and phosphorus in the sodium source, iron source, and phosphorus source is 4:2.8:4. The carbon element in gelatin accounts for 3.5% of the total mass of the mixture of the sodium source, iron source, phosphorus source, gelatin, and carbon nanotubes, and the carbon nanotubes account for 8% of the mass of gelatin. The rotation speed of wet grinding is 2200 r / min, the flow rate of the mixed material through the wet grinding equipment during wet grinding is 60 L / h, the duration of wet grinding is 4 h, the particle size of the grinding medium used for wet grinding is 0.15 mm, and the grinding medium accounts for 81% of the volume of the grinding chamber.

[0098] S2: Spray-dry the slurry. The feeding rate of spray drying is 60 - 90 mL / min, the pressure of the spray gun is 0.3 MPa, the feeding temperature is 200 °C, and the discharging temperature is 100 °C to obtain the precursor powder.

[0099] S3: Under a mixed atmosphere of nitrogen and ammonia, heat the precursor powder to 400 °C at a heating rate of 3 °C / min, then calcine for 16 h, and then cool for 10 min to obtain a sodium iron pyrophosphate positive electrode material with a cactus-like spherical surface coated with nitrogen-doped carbon material. The volume fraction of ammonia in the mixed atmosphere is 18%.

[0100] The structural formula of the sodium iron pyrophosphate positive electrode material with a cactus-like spherical surface coated with nitrogen-doped carbon material prepared in this example is Na 4 Fe 2.8 (PO 4 ) 2 P 2 O 7 @C-N.

[0101] Example 5

[0102] This example provides a method for preparing a cactus-like spherical sodium-ion battery positive electrode material, which is different from Example 1 in that sodium oxalate is used as the sodium source, iron nitrate is used as the iron source, and ammonium monohydrogen phosphate is used as the phosphorus source.

[0103] Example 6

[0104] This example provides a method for preparing a cactus-like spherical sodium-ion battery positive electrode material, which is different from Example 1 in that sodium carbonate is used as the sodium source, ferric chloride is used as the iron source, and ammonium dihydrogen phosphate is used as the phosphorus source.

[0105] Example 7

[0106] This example provides a method for preparing a cactus-like spherical sodium-ion battery positive electrode material, which is different from Example 1 in that sodium oxalate is used as the sodium source, ferrous sulfate is used as the iron source, and iron pyrophosphate is used as the phosphorus source.

[0107] Comparative Example 1

[0108] This comparative example provides a method for preparing a positive electrode material for a sodium ion battery. The difference from Example 2 is that the precursor powder is heated to 300°C at a heating rate of 2°C / min in a nitrogen atmosphere and calcined for 3 hours, then heated to 600°C at a heating rate of 2°C / min and calcined for 10 hours, and then cooled naturally.

[0109] Comparative Example 2

[0110] This comparative example provides a method for preparing a positive electrode material for a sodium ion battery, which is different from Example 2 in that the amount of carbon nanotubes added is 0.

[0111] Comparative Example 3

[0112] This comparative example provides a method for preparing a positive electrode material for a sodium ion battery, which differs from Example 2 in that gelatin is replaced by glucose.

[0113] The microscopic morphology of the sodium ion positive electrode materials prepared in Examples 1-7 and Comparative Examples 1-3 was recorded by electron microscope observation. Figure 1-2 As shown, the surface of the sodium ion battery positive electrode material prepared in Example 1-2 has a burr-like structure, presenting a cactus-like morphology. Similarly, the surface of the sodium ion battery positive electrode material of Example 3-7 also has a burr-like structure, presenting a cactus-like morphology.

[0114] like Figure 3 As shown, the sodium ion battery positive electrode material prepared in Comparative Example 1 is a spherical particle, and there is no burr-like structure on the surface, and it does not have a cactus-like morphology. Similarly, the sodium ion battery positive electrode materials of Comparative Examples 2-3 are also spherical particles, and there is no burr-like structure on the surface, and they do not present a cactus-like morphology.

[0115] Through the morphological comparison of the above-mentioned embodiments and comparative examples, it can be seen that the one-step calcination treatment is the key step in forming the cactus-like morphology, and the two-step calcination treatment method will directly lead to the inability to form the cactus-like morphological characteristics on the surface of the sodium ion positive electrode material. In addition, carbon nanotubes are also a key factor in the formation of burr-like structures and cactus-like morphologies on the surface of the sodium ion positive electrode material. Since glucose does not contain nitrogen, even in a mixed atmosphere of nitrogen and ammonia, the sodium ion positive electrode material of Comparative Example 3 is difficult to form a burr-like structure on the surface and cannot present a cactus-like morphology.

[0116] The sodium ion positive electrode material, carbon black conductive agent and polyvinylidene fluoride prepared in Examples 1-7 and Comparative Examples 1-3 were mixed in a mass ratio of 90:5:5 to obtain a homogenate, which was coated on an aluminum foil and dried in a vacuum drying oven at 110°C for 4 hours to obtain a positive electrode film. The positive electrode film was punched into a disc with a radius of 0.6 mm using a punching machine, and a metal sodium sheet was used as a counter electrode. NaClO with a concentration of 1 mol / L was added. 4 It was dissolved in a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) in a volume ratio of 1:1, and 5% by volume of fluoroethylene carbonate (FEC) was added as an additive to obtain an electrolyte. The separator was glass fiber and assembled into a CR2016 button battery in a glove box.

[0117] The electrochemical performance tests of the above button cells were carried out. Unless otherwise specified, all tests were carried out at room temperature. The details are as follows:

[0118] Carry out constant current charge and discharge test at a current density of 0.1C (1C = 129mAh / g), record and calculate the discharge capacity during the first discharge process, the unit is mAh / g, calculate the ratio of the actual amount of electricity discharged during the first charge and discharge process to the amount of electricity input during charging, and obtain the first cycle coulomb efficiency, expressed as a percentage. Figure 4 It can be seen that within the voltage range of 2-4 V, the first discharge capacity of the battery corresponding to Example 1 is 109 mAh / g, and the first cycle coulombic efficiency is 88.99%.

[0119] Continue to perform 100 complete charge and discharge cycles at a current density of 0.1C, record the discharge capacity for each cycle, and calculate the capacity retention rate after the 100th cycle, that is, the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity, expressed as a percentage.

[0120] Increase the current density to 5C, perform a complete charge and discharge cycle on the battery, and record and calculate the discharge specific capacity during this discharge process in mAh / g.

[0121] After the battery is placed in a -20°C environment and stabilized for a period of time, a complete discharge cycle is performed at a current density of 0.1 C. The discharge capacity at this low temperature is recorded and compared with the discharge capacity measured at room temperature to calculate the low-temperature discharge capacity retention rate, expressed as a percentage.

[0122] In addition, the specific surface area of ​​the positive electrode materials prepared in each embodiment and comparative example is calculated, and the unit is m 2 / g.

[0123] The above parameters are shown in Table 1.

[0124] Table 1 Electrochemical Performance Data Sheet of Sodium-Ion Batteries Corresponding to Examples 1-7 and Comparative Examples 1-3

[0125]

[0126] As can be seen from Table 1, the first discharge capacities of the batteries assembled with the cathode materials of Examples 1-7 are in the range of 102-109 mAh / g at a current density of 0.1C, which is significantly higher than that of Comparative Examples 1-3. This indicates that the capacity and conductivity of the sodium iron pyrophosphate cathode material with a fairy ball-like surface coated with nitrogen-doped carbon material of the present invention have been improved. The first-cycle Coulombic efficiency of the batteries assembled with the cathode materials of Examples 1-7 is basically 90% or more at a current density of 0.1C, which is significantly higher than 80-85% of Comparative Examples 1-3. This shows that the sodium iron pyrophosphate cathode material with a fairy ball-like surface coated with nitrogen-doped carbon material of the present invention has good conductivity. The capacity retention rate of the batteries assembled with the cathode materials of Examples 1-7 after 100 cycles at a current density of 0.1C is above 99%, higher than 82-90% of Comparative Examples 1-3. This indicates that the cycling performance of the sodium iron pyrophosphate cathode material with a fairy ball-like surface coated with nitrogen-doped carbon material of the present invention has been significantly improved. The first discharge capacity of the batteries assembled with the cathode materials of Examples 1-7 at a current density of 5C can reach 80-87% of the first discharge capacity of the batteries at 0.1C, while Comparative Examples 1-3 can only maintain at about 72%. This shows that the sodium iron pyrophosphate cathode material with a fairy ball-like surface coated with nitrogen-doped carbon material of the present invention has good high-rate performance. The low-temperature discharge capacity retention rate of the batteries assembled with the cathode materials of Examples 1-7 at -20°C and a current density of 0.1C can maintain at about 81-86% compared with that at room temperature, while Comparative Examples 1-3 can only maintain at 74% or less. This indicates that the low-temperature performance of the sodium iron pyrophosphate cathode material with a fairy ball-like surface coated with nitrogen-doped carbon material of the present invention has been improved.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a cactus-shaped sodium ion battery positive electrode material, characterized in that: The steps include: S1: wet-grinding a mixture of a sodium source, an iron source, a phosphorus source, gelatin and carbon nanotubes to obtain a slurry; the molar ratio of sodium element, iron element and phosphorus element in the sodium source, iron source and phosphorus source is 4:(2.8-3):4; the carbon element in gelatin accounts for 1-5% of the total mass of the mixture of the sodium source, iron source, phosphorus source, gelatin and carbon nanotubes; and the carbon nanotubes account for 5-15% of the mass of the gelatin; S2: spray drying the slurry to obtain a precursor powder; S3: calcining the precursor powder obtained in step S2 in an oxygen-free environment to obtain a sodium iron phosphate pyrophosphate positive electrode material with a cactus-shaped surface coated with nitrogen-doped carbon material; In step S3, the precursor powder obtained in step S2 is subjected to a one-step calcination treatment in a mixed atmosphere of nitrogen and ammonia. The precursor powder is heated to 400-600°C at a heating rate of 2-8°C / min, calcined for 5-16h, and then cooled to obtain a sodium iron phosphate pyrophosphate positive electrode material with a cactus-shaped surface coated with nitrogen-doped carbon material; in the mixed atmosphere, the volume proportion of ammonia is 10-20%, and the cooling time is 10-60min.

2. The method for preparing the cactus-shaped sodium ion battery positive electrode material according to claim 1, characterized in that: In step S1, the sodium source is one or more of sodium carbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium oxalate and sodium pyrophosphate; The iron source is one or more of ferrous phosphate, ferrous citrate, ferrous oxalate, ferrous acetylacetonate, ferrous nitrate, ferric chloride and ferrous sulfate; The phosphorus source is one or more of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ferric phosphate, ferric pyrophosphate and sodium dihydrogen phosphate.

3. The method for preparing the cactus-shaped sodium ion battery positive electrode material according to claim 1, characterized in that: In step S1, a sodium source, an iron source, a phosphorus source, gelatin and carbon nanotubes are mixed, water is added to obtain a mixed material, and then wet-milling is performed; Among them, the rotation speed of wet grinding is 2000-3000r / min, the flow rate of the mixed material through the wet grinding equipment during wet grinding is 50-100L / h, the duration of wet grinding is 0.5-10h, the particle size of the grinding medium used in wet grinding is 0.03-0.3mm, and the grinding medium occupies 70-85% of the volume of the grinding chamber.

4. The method for preparing the cactus-shaped sodium ion battery positive electrode material according to claim 1, characterized in that: The slurry obtained in step S1 has a solid content of 15-40 wt % and a particle size of the solid particles of 50-200 nm.

5. The method for preparing the cactus-shaped sodium ion battery positive electrode material according to claim 1, characterized in that: In step S2, the feed rate of spray drying is 60-90 mL / min, the pressure of the spray gun is 0.2-0.4 MPa, the feed temperature is 190-240°C, and the discharge temperature is 80-130°C.

6. The method for preparing the cactus-shaped sodium ion battery positive electrode material according to claim 1, characterized in that: In step S3, the structure of the sodium iron phosphate pyrophosphate positive electrode material with a cactus-shaped surface coated with nitrogen-doped carbon material is Na4Fe x (PO4)2P2O7@CN, 2.8<x≤3.

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