Spherical sodium ferric pyrophosphate-carbon composite positive electrode material, precursor and preparation method thereof, and sodium ion battery

Abstract

The application discloses a spherical sodium iron pyrophosphate-carbon composite positive electrode material, a precursor, a preparation method of the precursor and a sodium ion battery. The precursor is provided with a nanofibril as a spherical structure framework, and a precursor of the spherical sodium iron pyrophosphate is distributed in the pores of the spherical structure framework. The nanofibril is a nanofibril of a lignin fiber. The spherical precursor is not prone to structural collapse or hollow phenomenon due to water removal in the drying process. Further, after carbonization in an inert atmosphere, the nanofibril can form a three-dimensional conductive network, improve the electronic conductivity of the prepared material, and the prepared NFP@C has good electrochemical performance.

Description

Technical Field

[0001] This invention relates to a spherical sodium iron pyrophosphate carbon composite cathode material, a precursor thereof, a method for preparing the same, and a sodium-ion battery. Background Technology

[0002] Following the successful commercialization of LiFePO4, sodium iron phosphate (NaFePO4), based on sodium ions, has also attracted researchers' attention. Studies have shown that NaFePO4 exists in two structures: olivine and sodium phosphite. The sodium phosphite type NaFePO4 has been proven to be electrochemically inactive, while the olivine type NaFePO4 is thermodynamically unstable, and its complex synthesis process and low operating voltage hinder its practical application. The crystal structure of sodium phosphite-type sodium iron pyrophosphate (Na2FeP2O7) material can donate Na... + The migration provides a three-dimensional sawtooth channel and has a higher operating voltage (~3.4V vs. Na / Na). + In addition, Na2FeP2O7 exhibits superior thermal stability at 600℃, making it safer and more reliable.

[0003] Besides Na₂FeP₂O₇, Na has a non-integer stoichiometric ratio. 4-α Fe 2+α / 2 (P₂O₇)₂ type materials also exhibit good electrochemical performance. Non-integer stoichiometric materials have non-integer elemental proportions in their compound composition, which allows for greater flexibility in adjusting composition and performance. The most typical example is Na. 3.12 Fe 2.44 (P₂O₇)₂(NFP), with Na₂FeP₂O₇ (theoretical capacity is 97 mAh g) -1 Compared to Na, 3.12 Fe 2.44 (P₂O₇)₂ is based on a redox reaction involving 2.44 electrons, giving it a theoretical capacity of 117.6 mAh g⁻¹. -1 .

[0004] While pyrophosphate-type polyanionic cathode materials provide excellent diffusion channels for sodium ions, their extremely poor electronic conductivity severely limits their rate performance. To improve the electronic conductivity of these materials, it is typically necessary to coat the material surface with carbon materials, or to prepare them as nanoparticles and then coat them with carbon materials to enhance their rate performance. For example, Chinese Patent CN116344772A discloses a method that uses a nano-iron phosphate precursor mixed with a sodium source, dopants of cerium and niobium, and a carbon source, followed by sintering under a protective atmosphere to achieve carbon coating and improve rate performance. Another example is Chinese Patent CN116230923A, which discloses a method where the raw materials and dispersing solvent for cathode materials are ground and dried to obtain an iron phosphate nano-cathode material precursor. This precursor is then sintered under an inert atmosphere to obtain a carbon-coated iron phosphate pyrophosphate nano-cathode material. The raw materials for the cathode material include iron phosphate, an external phosphorus source, a sodium source, and an organic carbon source. However, these treatment methods often lead to a decrease in the tap density of the material, which is not conducive to improving the energy density of sodium-ion batteries.

[0005] Spray drying is a green and efficient drying method often used in the preparation of lithium / sodium-ion battery materials. It can adjust the particle size and morphology of sodium-ion battery cathode materials, typically yielding products with smaller particle sizes, no agglomeration, and a spherical morphology. In the spray drying process, the formation of atomized droplets is a key step, as the droplets act as nucleation centers, ultimately forming well-crystalled and dense particles. Materials prepared by this method have a multi-level structure; the primary particles are often small in size, narrow in distribution, and have a large specific surface area, while the secondary forming of spherical particles results in a compact structure, effectively improving the overall material's tap density, high electrochemical performance, and environmental friendliness. However, during spray drying, spherical particles are prone to collapse, forming incomplete spheres or spheres with voids.

[0006] Therefore, how to obtain pyrophosphate-type polyanionic cathode materials with high tap density is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the defect of low tap density of sodium iron pyrophosphate carbon composite cathode material in the prior art, and to provide a spherical sodium iron pyrophosphate carbon composite cathode material, precursor and preparation method thereof, and sodium-ion battery.

[0008] During the research and development process, the inventors discovered that when spray drying is used to process materials for secondary granulation, the surface of the droplets first dries and solidifies, and the internal moisture then diffuses from the inside to the outside under the influence of heat and eventually evaporates. If the process conditions are not properly controlled, the spherical particles formed after spraying can easily collapse during drying, forming incomplete spheres or spheres with voids. Furthermore, sodium iron pyrophosphate cathode materials have poor conductivity, requiring morphological design to improve their conductivity. However, simple nano-sizing can increase the specific surface area of ​​the prepared material, leading to increased side reactions, and reduce tap density, limiting electrode loading; all of these factors are detrimental to improving the electrochemical performance of the battery.

[0009] Based on this, in the preparation of the spray slurry, a certain amount of lignocellulose is added, and the lignocellulose is fully fiberized into nanoscale filaments through ball milling. These filaments are then thoroughly mixed with sodium iron pyrophosphate to form a uniform slurry. The slurry is further spray-dried to form NFP@C precursor spheres with nanofibers as the spherical structural framework. These precursor spheres, formed during spray drying, are less prone to structural collapse or hollowing due to moisture removal. Furthermore, after carbonization in an inert atmosphere, the nanofibers can form a three-dimensional conductive network, improving the electronic conductivity of the prepared material, resulting in NFP@C exhibiting good electrochemical performance.

[0010] This invention designs a multi-level particle size spherical Na 4-α Fe 2+α / 2 (P₂O₇)₂(NFP@C) sodium ion cathode material, such as Na 3.12 Fe 2.44 (P2O7)2@C is first processed by ball milling to fully fiberize lignin fibers into nanoscale filaments, which are then fully mixed with sodium iron pyrophosphate raw material to form a uniform slurry.

[0011] This invention provides a precursor sphere for a spherical sodium iron pyrophosphate carbon composite cathode material. The precursor sphere has a spherical structural framework of nanofibers, and sodium iron pyrophosphate precursors are distributed in the pores of the spherical structural framework.

[0012] The nanofibers are lignin fiber nanofibers.

[0013] In this invention, the chemical composition of the sodium ferric pyrophosphate can be Na. 4-α Fe 2+α / 2 The value of α in (P2O7)2 can be 0.5-1.0, for example, 0.66-0.88.

[0014] In this invention, the chemical composition of the sodium pyrophosphate can be sodium iron pyrophosphate of the sodium phosphate type, Na2FeP2O7.

[0015] In this invention, the sodium iron pyrophosphate precursor can be a conventional sodium iron pyrophosphate precursor in the art. For example, the sodium iron pyrophosphate precursor contains an iron source, a phosphorus source, and a sodium source. Alternatively, the sodium iron pyrophosphate precursor may contain an iron-phosphorus compound and a sodium source.

[0016] The iron source can be a conventional iron source in the art, such as Fe2O3, Fe3O4, Fe(NO3)3, FeC2O4 or NH4Fe(SO4)2.

[0017] The phosphorus source can be a conventional phosphorus source in the art, such as NaH2PO4, Na2HPO4, NH4H2PO4 or H3PO4.

[0018] The sodium source can be a conventional sodium source in the art, such as Na2CO3, NaOH, Na2C2O4 or CH3COONa.

[0019] The proportions of the iron source, the phosphorus source, and the sodium source can be determined according to the Na... 4-α Fe 2+α / 2 The stoichiometry of (P₂O₇)₂ is used for proportioning, and the value of α can range from 0.5 to 1.0, for example, 0.66 to 0.88. Generally, the sodium source is in excess by 2-5%, for example, 3%.

[0020] The proportions of the iron source, the phosphorus source, and the sodium source can also be determined according to the stoichiometry of Na2FeP2O7.

[0021] The iron-phosphorus compound may be a conventional iron-phosphorus compound in the art, such as FePO4.

[0022] The iron-phosphorus compound can be obtained by reacting the iron source and the phosphorus source. For example, FePO4 is obtained by reacting NH4Fe(SO4)2 and H3PO4.

[0023] In this invention, the lignin fiber can be a conventional lignin fiber in the art, generally an organic flocculent fibrous material obtained from natural renewable wood through chemical treatment and mechanical processing.

[0024] In this invention, the raw material for the lignin fiber can be selected from industrial wood fibers.

[0025] In this invention, the nanofibers can be made from lignin fibers with a fiber length of not less than 1 mm.

[0026] In this invention, the nanofibers can be prepared by a circulating sand milling method.

[0027] The grinding time can be 1-3 hours, for example 2 hours.

[0028] In this invention, the mass percentage of the nanofibers relative to the spherical sodium iron pyrophosphate precursor can be 0.1-1.0%, for example 0.2-0.8%, or even 0.3-0.5%.

[0029] In this invention, the pores of the spherical structural framework or the precursor of the spherical sodium iron pyrophosphate may also contain a carbon source.

[0030] The carbon source may be a conventional carbon source in the art, such as one or more of H2C2O4, citric acid, glucose, conductive carbon black (Super-p, SP), carbon black (CB), carbon nanotubes (CNTs) and graphene, and also such as citric acid, SP or graphene.

[0031] The mass percentage of the carbon source relative to the spherical sodium iron pyrophosphate precursor can be 0.1-2.0%, for example 1.0%.

[0032] The present invention also provides a method for preparing the precursor spheres of the spherical sodium iron pyrophosphate carbon composite cathode material, which includes the following steps: spray drying a slurry containing the sodium iron pyrophosphate precursor and the nanofibers to obtain the precursor spheres;

[0033] When the precursor spheres also contain a carbon source, the slurry containing the sodium iron pyrophosphate precursor, the carbon source, and the nanofibers is obtained by spray drying.

[0034] In this invention, the solid content in the slurry can be 5-15%, for example 10%, where the percentage refers to the mass percentage.

[0035] In this invention, the slurry can be prepared by the following method: mixing the sodium iron pyrophosphate precursor, lignocellulose and water to obtain a mixture A, and milling the mixture until the lignocellulose forms nanofibers, thus obtaining the slurry.

[0036] During the sand milling process, lignocellulose gels under the action of friction, which can increase the viscosity of the spray-dried slurry without changing the solid content.

[0037] The process may include stirring before grinding, stirring until a uniform suspension is formed, thus obtaining the product.

[0038] The stirring time can be 1-2 hours, for example, 1 hour.

[0039] In this invention, the spray drying can be carried out using conventional spray drying equipment in the art.

[0040] In this invention, during the spray drying process, the slurry feeding rate can be 1.0-1.6 L / h, for example 1.2 L / h.

[0041] In this invention, during the spray drying process, the air inlet temperature can be 160-220℃, for example, 200℃.

[0042] In this invention, Na with a non-integer stoichiometric ratio was prepared by spray drying. 4-α Fe 2+α / 2 (P₂O₇)₂ type materials (e.g., Na) 3.12 Fe 2.44 In the composite cathode material precursor of (P2O7)2 and carbon materials, lignocellulose is added during the raw material sand milling and mixing process before spray drying granulation. During the sand milling and mixing process, the lignocellulose generates nanofibers under the shear force of the sand mill. In the subsequent spray drying process, the network structure formed by the nanofibers provides a supporting framework for the spherical droplets formed by the spray, effectively preventing them from becoming hollow or collapsing during drying. Further high-temperature carbonization of the nanofibers can form a conductive network, which helps to improve the conductivity of the material.

[0043] This invention also provides a method for preparing a spherical sodium iron pyrophosphate carbon composite cathode material, which includes the following steps:

[0044] In an inert atmosphere, the precursor spheres of the spherical sodium iron pyrophosphate carbon composite cathode material described above are carbonized by heat treatment to obtain the desired product.

[0045] The heat treatment temperature is 500-800℃.

[0046] In this invention, during the heat treatment process at a temperature of 500-800℃, the sodium iron pyrophosphate electrode material forms a crystal structure, and the nanofibers undergo graphitization.

[0047] In this invention, the inert atmosphere can be a conventional inert atmosphere in the art, such as an Ar atmosphere, or an Ar / H2 (5% H2) atmosphere. The H2 in the inert atmosphere can provide additional reducing effect.

[0048] In this invention, the temperature of the heat treatment can be 500-700℃, for example 600℃.

[0049] In this invention, the heat treatment time can be 6-20 hours, for example, 10 hours.

[0050] In this invention, a preheating process may be included before the heat treatment. During the preheating process, the nanofibers also undergo a certain degree of carbonization.

[0051] The temperature of the preheating treatment can be 250-400℃, for example, 300℃.

[0052] The preheating treatment time can be 2-4 hours.

[0053] The present invention also provides a spherical sodium iron pyrophosphate carbon composite cathode material, which is prepared by the above method.

[0054] In this invention, the 0.1C reversible capacity of the spherical iron sodium carbon pyrophosphate composite cathode material can be ≥88.30%, for example 88.87%, 89.17%, 88.97%, 90.08%, 88.47%, or 88.37%.

[0055] In this invention, the 1C reversible capacity of the spherical iron sodium carbon pyrophosphate composite cathode material can be ≥76.00%, for example 76.71%, 78.39%, 76.4%, 76.5%, 77.55%, and 76.5%.

[0056] In this invention, the capacity retention rate of the spherical sodium iron pyrophosphate carbon composite cathode material after 200 cycles at 1C rate can be ≥80.00%, for example 85.90%, 87.84%, 85.38%, 86.00%, 84.67%, and 84.77%.

[0057] The present invention also provides an application of the spherical sodium iron pyrophosphate carbon composite cathode material as described above as a cathode material in sodium-ion batteries.

[0058] The present invention also provides a sodium-ion battery comprising the spherical sodium iron pyrophosphate carbon composite cathode material as described above.

[0059] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0060] The reagents and raw materials used in this invention are all commercially available.

[0061] The positive and progressive effects of this invention are as follows:

[0062] (1) This invention employs a spray drying process to obtain NFP@C precursor spheres with nanofibers as the spherical structural framework. The spheres formed during the spray drying process are less prone to structural collapse or hollowing due to moisture removal. Their structure is stable, and the spherical structure formed through secondary granulation can improve their tap density, which is beneficial for increasing the energy density of sodium-ion batteries.

[0063] (2) In this invention, the NFP@C precursor spheres with nanofibers as the spherical structural framework are carbonized under an inert atmosphere. The nanofibers can form a three-dimensional conductive network, and can achieve in-situ carbon coating or mixed carbon coating. This solves the problem of poor conductivity of sodium iron pyrophosphate cathode materials and improves the electronic conductivity of the prepared materials. Finally, a multi-level spherical Na... 3.12 Fe 2.44 (P2O7)2@C sodium ion cathode material (NFP@C) has good electrochemical performance.

[0064] (3) This invention relates to inexpensive and readily available raw materials, which are easy to industrialize and have prospects for industrial application. Detailed Implementation

[0065] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0066] In the following examples and comparative examples, all temperatures refer to degrees Celsius (°C).

[0067] In the following examples and comparative examples, the actual amount of sodium source added is calculated as follows: sodium source added according to stoichiometric ratio * (1 + excess percentage); for example, when the sodium source excess is 3%, the actual amount of sodium source added = sodium source added according to stoichiometric ratio * 1.03.

[0068] In the following examples, the lignocellulose raw material is industrial lignocellulose, white, of superior grade, with a fiber length of not less than 1 mm.

[0069] In the following examples and comparative examples, according to Na 3.12 Fe 2.44 The stoichiometric ratio of sodium source, iron source, and phosphorus source for (P2O7)2 is determined by weighing out the stoichiometric ratio.

[0070] In the following examples and comparative examples, 5% H2 in Ar / H2(5%H2) refers to 5% H2 by volume.

[0071] Example 1

[0072] 1. Weigh the iron source (Fe(NO3)3·9H2O), phosphorus source (NH4H2PO4), sodium source (Na2CO3) and carbon source (citric acid) according to the stoichiometric ratio, and keep the sodium source in excess by 3%. Then add 0.5% of the total mass of the above materials as lignocellulose.

[0073] 2. Add deionized water to the above raw materials, maintain a solid content of 10%, and stir vigorously for 1 hour until the raw materials and deionized water form a uniform suspension.

[0074] 3. Transfer the above suspension to a sand mill and perform circulating sand milling for 2 hours. This process allows the lignocellulose in the suspension to further fibrillate, forming nanofibers. The suspension will then transform into a uniform and stable slurry as the nanofibers are formed.

[0075] 4. The uniform slurry is introduced into a spray dryer for drying to form NFP@C precursor spheres with nanofibers as the spherical structural framework. During the spray drying process, the peristaltic pump feed rate is 1.2 L / h, and the inlet air temperature is 200℃.

[0076] 5. The material obtained in the above drying steps is placed in an alumina crucible, compacted, and then placed in a tube furnace for high-temperature treatment under an inert atmosphere. The treatment temperature regime is as follows: first, treatment at 300℃ for 4 hours, then increasing to 600℃ for 10 hours. After naturally cooling to room temperature under an inert atmosphere, the product is removed, quickly sealed, and stored in a desiccator. The product is Na. 3.12 Fe 2.44 (P2O7)2@C-1 is denoted as NFP@C-1.

[0077] 6. Assemble button half-cells to test and evaluate the electrochemical performance of the prepared materials.

[0078] Electrode slurry was prepared according to a mass ratio of active electrode material: conductive carbon black: binder of 85:8:7. The solvent addition was adjusted to control the slurry viscosity to 3000 mPa·s. The slurry was then coated onto aluminum foil using a doctor blade coating method. The aluminum foil was then dried in an oven at 80°C for 2 hours, followed by drying in a vacuum drying oven at 105°C for 12 hours. After compaction with a rolling mill, it was cut into 10mm diameter electrode discs, weighed, and transferred to a glove box for use as working electrode sheets. A CR 2032 coin cell was assembled in the order of positive electrode, separator, and negative electrode, with a sodium metal sheet as the negative electrode and Whatman glass fiber as the separator. The battery was connected to a battery testing system and charged / discharge tested within the 2V-4V range.

[0079] The electrode active material is the product NFP@C-1 obtained in step 5.

[0080] Example 2

[0081] 1. Weigh Fe(NO3)3·9H2O, NH4H2PO4, and Na2CO3 according to the stoichiometric ratio, and keep the sodium source in excess by 3%. Then add 1% SP (conductive carbon black) as a carbon source and 0.5% lignocellulose, which is the total mass of the above materials.

[0082] 2. Add deionized water to the above raw materials, maintain a solid content of 10% by mass, and stir vigorously for 1 hour until the raw materials and deionized water form a uniform suspension.

[0083] 3. Transfer the above suspension to a sand mill and perform circulating sand milling for 2 hours under circulating water cooling. During the sand milling process, the lignocellulose in the suspension further fibrillates, forming nanofibers. The suspension will transform into a uniform and stable slurry as nanofibers are generated.

[0084] 4. The above-mentioned uniform slurry is introduced into a spray drying device for drying to obtain spherical Na. 3.12 Fe 2.44 The precursor of (P2O7)2@C material is an NFP@C precursor sphere with nanofibers as the spherical structural framework. During spray drying, the peristaltic pump feed rate is 1.2 L / h, and the inlet air temperature is 200 degrees Celsius.

[0085] 5. The material obtained in the above drying steps is placed in an alumina crucible, compacted, and then placed in a tube furnace for high-temperature treatment under an Ar / H2 (5% H2) reducing inert atmosphere. The treatment temperature regime is as follows: first, treatment at 300°C for 2 hours, then increasing to 600°C for 10 hours. After naturally cooling to room temperature under an inert atmosphere, the product is removed, quickly sealed, and stored in a desiccator. The product is Na. 3.12 Fe 2.44 (P2O7)2@C-2 is denoted as NFP@C-2.

[0086] 6. The preparation and testing of the button cell are the same as in Example 1. The electrode active material is the product NFP@C-2 obtained in step 5.

[0087] Example 3

[0088] 1. Weigh FeC2O4·2H2O, NaH2PO4, and Na2CO3 according to the stoichiometric ratio, keeping the sodium source in 3% excess. Then add 1% citric acid as a carbon source and 0.5% lignocellulose, which is equal to the total mass of the above materials.

[0089] 2. Add deionized water to the above raw materials, maintain a solid content of 10% by mass, and stir vigorously for 1 hour until the raw materials and deionized water form a uniform suspension.

[0090] 3. Transfer the above suspension to a sand mill and perform circulating sand milling for 2 hours under circulating water cooling. During the sand milling process, the lignocellulose in the suspension further fibrillates, forming nanofibers. The suspension will transform into a uniform and stable slurry as nanofibers are generated.

[0091] 4. The above-mentioned uniform slurry is introduced into a spray drying device for drying to obtain spherical Na. 3.12 Fe 2.44The precursor of (P2O7)2@C material is an NFP@C precursor sphere with nanofibers as the spherical structural framework. During spray drying, the peristaltic pump feed rate is 1.2 L / h, and the inlet air temperature is 200 degrees Celsius.

[0092] 5. The material obtained in the above drying steps is placed in an alumina crucible, compacted, and then placed in a tube furnace for high-temperature treatment under an inert Ar atmosphere. The treatment temperature regime is as follows: first, treatment at 300°C for 4 hours, then increasing the temperature to 600°C for 10 hours. After naturally cooling to room temperature under an inert atmosphere, the product is removed, quickly sealed, and stored in a desiccator. The product is Na. 3.12 Fe 2.44 (P2O7)2@C-3 is denoted as NFP@C-3.

[0093] 6. The preparation and testing of the button cell are the same as in Example 1. The electrode active material is the product NFP@C-3 obtained in step 5.

[0094] Example 4

[0095] 1. Weigh FeC2O4·2H2O, NaH2PO4, and Na2CO3 according to the stoichiometric ratio, keeping the sodium source in excess by 3%. Then add 1% graphene as a carbon source and 0.3% lignocellulose, which is the total mass of the above materials.

[0096] 2. Add deionized water to the above raw materials, maintain a solid content of 10% by mass, and stir vigorously for 1 hour until the raw materials and deionized water form a uniform suspension.

[0097] 3. Transfer the above suspension to a sand mill and perform circulating sand milling for 2 hours under circulating water cooling. During the sand milling process, the lignocellulose in the suspension further fibrillates, forming nanofibers. The suspension will transform into a uniform and stable slurry as nanofibers are generated.

[0098] 4. The above-mentioned uniform slurry is introduced into a spray drying device for drying to obtain spherical Na. 3.12 Fe 2.44 The precursor of (P2O7)2@C material is an NFP@C precursor sphere with nanofibers as the spherical structural framework. During spray drying, the peristaltic pump feed rate is 1.2 L / h, and the inlet air temperature is 200 degrees Celsius.

[0099] 5. The material obtained in the above drying steps is placed in an alumina crucible, compacted, and then placed in a tube furnace for high-temperature treatment under an inert Ar atmosphere. The treatment temperature regime is as follows: first, treatment at 300°C for 2 hours, then increasing the temperature to 600°C for 10 hours. After naturally cooling to room temperature under an inert atmosphere, the product is removed, quickly sealed, and stored in a desiccator. The product is Na. 3.12 Fe2.44 (P2O7)2@C-4 is denoted as NFP@C-4.

[0100] 6. The preparation and testing of the button cell are the same as in Example 1. The electrode active material is the product NFP@C-4 obtained in step 5.

[0101] Example 5

[0102] 1. Weigh Fe2O3, H3PO4, and Na2CO3 according to the stoichiometric ratio, keeping the sodium source in 3% excess. Then add 1% citric acid as a carbon source and 0.5% lignocellulose, which is the total mass of the above materials.

[0103] 2. Add deionized water to the above raw materials, maintain a solid content of 10% by mass, and stir vigorously for 1 hour until the raw materials and deionized water form a uniform suspension.

[0104] 3. Transfer the above suspension to a sand mill and perform circulating sand milling for 2 hours under circulating water cooling. During the sand milling process, the lignocellulose in the suspension further fibrillates, forming nanofibers. The suspension will transform into a uniform and stable slurry as nanofibers are generated.

[0105] 4. The above-mentioned uniform slurry is introduced into a spray drying device for drying to obtain spherical Na. 3.12 Fe 2.44 The precursor of (P2O7)2@C material is an NFP@C precursor sphere with nanofibers as the spherical structural framework. During spray drying, the peristaltic pump feed rate is 1.2 L / h, and the inlet air temperature is 200 degrees Celsius.

[0106] 5. The material obtained in the above drying steps is placed in an alumina crucible, compacted, and then placed in a tube furnace for high-temperature treatment under an Ar / H2 (5% H2) reducing inert atmosphere. The treatment temperature regime is as follows: first, treatment at 300°C for 4 hours, then increasing the temperature to 600°C for 10 hours. After naturally cooling to room temperature under an inert atmosphere, the product is removed, quickly sealed, and stored in a desiccator. The product is Na. 3.12 Fe 2.44 (P2O7)2@C-5 is denoted as NFP@C-5.

[0107] 6. The preparation and testing of the button cell are the same as in Example 1. The electrode active material is the product NFP@C-5 obtained in step 5.

[0108] Example 6

[0109] 1. Weigh out NH4Fe(SO4)2, H3PO4, and sodium carbonate according to the stoichiometric ratio. First, dissolve NH4Fe(SO4)2 in deionized water, then add H3PO4. Adjust the pH of the solution with ammonia. When precipitation begins to form, reduce the rate of ammonia addition and continue stirring until the precipitation stops. Centrifuge to separate the FePO4 precipitate. Wash the precipitate three times with deionized water to remove residual sulfate ions.

[0110] 2. Mix the above precipitate with Na2CO3 in stoichiometric ratio, keeping the sodium source in 3% excess, and then add 1% citric acid as a carbon source and 0.5% lignocellulose, which is the total mass of the above materials.

[0111] 3. Add deionized water to the above raw materials, maintain a solid content of 10% by mass, and stir vigorously for 1 hour until the raw materials and deionized water form a uniform suspension.

[0112] 4. Transfer the above suspension to a sand mill and perform circulating sand milling for 2 hours under circulating water cooling. During the sand milling process, the lignocellulose in the suspension further fibrillates, forming nanofibers. The suspension will transform into a uniform and stable slurry as nanofibers are generated.

[0113] 5. The above-mentioned uniform slurry is introduced into a spray drying device for drying to obtain spherical Na. 3.12 Fe 2.44 The precursor of (P2O7)2@C material is an NFP@C precursor sphere with nanofibers as the spherical structural framework. During spray drying, the peristaltic pump feed rate is 1.2 L / h, and the inlet air temperature is 200 degrees Celsius.

[0114] 6. The material obtained in the above drying steps is placed in an alumina crucible, compacted, and then placed in a tube furnace for high-temperature treatment under an Ar / H2 (5% H2) reducing inert atmosphere. The treatment temperature regime is as follows: first, treatment at 300°C for 4 hours, then increasing the temperature to 600°C for 10 hours. After naturally cooling to room temperature under an inert atmosphere, the product is removed, quickly sealed, and stored in a desiccator. The product is Na. 3.12 Fe 2.44 (P2O7)2@C-6 is denoted as NFP@C-6.

[0115] 7. The button cell preparation and testing are the same as in Example 1. The electrode active material is the product NFP@C-6 obtained in step 6.

[0116] Comparative Example 1

[0117] The sample was prepared according to the steps in Example 1, but without the addition of lignocellulose. The final product was sodium phosphate rock (Na). 3.12 Fe 2.44(P2O7)2@C is denoted as NFP@C-D1. The button cell preparation and testing are the same as in Example 1.

[0118] Comparative Example 2

[0119] The sample was prepared according to the steps in Example 2, but without the addition of lignocellulose. The final product was sodium phosphate rock (Na). 3.12 Fe 2.44 (P2O7)2@C is denoted as NFP@C-D2. The button cell preparation and testing are the same as in Example 1.

[0120] Comparative Example 3

[0121] The sample was prepared according to the steps in Example 3, but without the addition of lignocellulose. The final product was sodium phosphate rock (Na). 3.12 Fe 2.44 (P2O7)2@C is denoted as NFP@C-D3. The button cell preparation and testing are the same as in Example 1.

[0122] Comparative Example 4

[0123] The sample was prepared according to the steps in Example 4, but without the addition of lignocellulose. The final product was sodium phosphate rock (Na). 3.12 Fe 2.44 (P2O7)2@C is denoted as NFP@C-D4. The button cell preparation and testing are the same as in Example 1.

[0124] The loading of active materials and electrochemical performance of the button cell electrodes in Examples 1-6 and Comparative Examples 1-4 are shown in Table 1 below.

[0125] Table 1

[0126]

[0127] As shown in Table 1:

[0128] (1) The NFP@C precursors with nanofibers as the spherical structural framework in Examples 1-6 are less likely to collapse or become hollow during the spray drying process due to the removal of moisture. Their structure is stable. The spherical structure formed by secondary granulation can improve the loading of active materials and has a high tap density, which is beneficial to improving the energy density of sodium-ion batteries.

[0129] (2) In Examples 1-6, the NFP@C precursor spheres with nanofibers as the spherical structural framework are carbonized in an inert atmosphere. The nanofibers can form a three-dimensional conductive network with good conductivity, weak electrode polarization, and higher reversible capacity.

[0130] (3) Compared with NFP@C in Examples 1-4, the spheres formed during the spray drying process are prone to structural collapse or hollowing due to the removal of moisture during the drying process. They have low tap density and poor conductivity.

Claims

1. A method for preparing a spherical sodium iron pyrophosphate carbon composite cathode material, characterized in that, It includes the following steps: In an inert atmosphere, the precursor spheres of the spherical iron-sodium-carbon pyrophosphate composite cathode material are obtained by preheating and heat treatment carbonization; the preheating temperature is 250-400℃, and the heat treatment temperature is 500-800℃; wherein: (1) The precursor sphere is a spherical structure framework with nanofibers as the framework, and sodium iron pyrophosphate precursor is distributed in the pores of the spherical structure framework. The nanofibers are lignin fiber nanofibers; (2) The preparation method of the precursor sphere of the spherical sodium iron pyrophosphate carbon composite cathode material includes the following steps: spray drying the slurry containing the sodium iron pyrophosphate precursor and the nanofibers to obtain the precursor sphere.

2. The preparation method according to claim 1, characterized in that, The chemical composition of sodium ferric pyrophosphate in the sodium pyrophosphate precursor is Na. 4-α Fe 2+α / 2 (P2O7)2, the value of α ranges from 0.5 to 1.0; And / or, the nanofibers are made from lignin fibers with a fiber length of not less than 1 mm; And / or, the nanofibers are milled using a circulating sand milling method, and the milling time is 1-3 hours; And / or, the sodium iron pyrophosphate precursor contains an iron source, a phosphorus source, and a sodium source; the iron source is Fe2O3, Fe3O4, Fe(NO3)3, FeC2O4, or NH4Fe(SO4)2; the phosphorus source is NaH2PO4, Na2HPO4, NH4H2PO4, or H3PO4; the sodium source is Na2CO3, NaOH, Na2C2O4, or CH3COONa; Alternatively, the sodium iron pyrophosphate precursor may contain an iron-phosphorus compound and a sodium source.

3. The preparation method according to claim 2, characterized in that, The value of α ranges from 0.66 to 0.

88.

4. The preparation method according to claim 2, characterized in that, The grinding time is 2 hours.

5. The preparation method according to claim 1 or 2, characterized in that, The nanofibers comprise 0.1-1.0% by mass relative to the spherical sodium iron pyrophosphate precursor. And / or, the pores of the spherical structural framework or the precursor of the spherical sodium iron pyrophosphate may also contain a carbon source; The carbon source is one or more of H2C2O4, citric acid, glucose, carbon black, carbon nanotubes, and graphene; when the precursor spheres also contain a carbon source, the slurry containing the sodium iron pyrophosphate precursor, the carbon source, and the nanofibers is spray-dried to obtain the product. The carbon source has a mass percentage of 0.1-2.0% relative to the spherical sodium pyrophosphate precursor.

6. The preparation method according to claim 5, characterized in that, The nanofibers comprise 0.2-0.8% by mass relative to the spherical sodium iron pyrophosphate precursor. And / or, the carbon black is conductive carbon black; The carbon source has a mass percentage of 1.0% relative to the spherical sodium pyrophosphate precursor.

7. The preparation method according to claim 5, characterized in that, The nanofibers account for 0.3-0.5% of the mass percentage relative to the spherical sodium iron pyrophosphate precursor.

8. The preparation method according to claim 1, characterized in that, The solid content in the slurry is 5-15%, and the percentage refers to the mass percentage. And / or, the slurry is prepared by the following method: mixing the sodium iron pyrophosphate precursor, lignocellulose and water to obtain mixture A, and milling the mixture until the lignocellulose forms nanofibers, thus obtaining the slurry; And / or, during the spray drying process, the feed rate of the slurry is 1.0-1.6 L / h; And / or, during the spray drying process, the air inlet temperature is 160-220℃.

9. The preparation method according to claim 8, characterized in that, The solid content in the slurry is 10%, and the percentage refers to the mass percentage. And / or, during the spray drying process, the feed rate of the slurry is 1.2 L / h; And / or, during the spray drying process, the air inlet temperature is 200°C.

10. The preparation method according to claim 1, characterized in that, The heat treatment temperature is 500-700℃; And / or, the heat treatment time is 6-20 hours; And / or, the preheating treatment time is 2-4 hours.

11. The preparation method according to claim 10, characterized in that, The heat treatment temperature is 600℃; And / or, the heat treatment time is 10 hours.

12. A spherical sodium iron pyrophosphate carbon composite cathode material, which is prepared by any one of the preparation methods described in any one of claims 1-11.

13. The spherical sodium iron pyrophosphate carbon composite cathode material as described in claim 12, wherein the 0.1C reversible capacity of the spherical sodium iron pyrophosphate carbon composite cathode material is ≥88.30%; And / or, the 1C reversible capacity of the spherical sodium iron pyrophosphate carbon composite cathode material is ≥76.00%; And / or, the capacity retention rate of the spherical sodium iron pyrophosphate carbon composite cathode material is ≥80.00% after 200 cycles at 1C rate.

14. The spherical sodium iron pyrophosphate carbon composite cathode material as described in claim 13, wherein the 0.1C reversible capacity of the spherical sodium iron pyrophosphate carbon composite cathode material is 88.87%, 89.17%, 88.97%, 90.08%, 88.47%, or 88.37%; And / or, the 1C reversible capacity of the spherical iron-sodium-carbon pyrophosphate composite cathode material is 76.71%, 78.39%, 76.4%, 76.5%, 77.55%, and 76.5%; And / or, the capacity retention of the spherical sodium iron pyrophosphate carbon composite cathode material after 200 cycles at 1C rate is 85.90%, 87.84%, 85.38%, 86.00%, 84.67%, and 84.77%.

15. A sodium-ion battery comprising a spherical sodium iron pyrophosphate carbon composite cathode material as described in any one of claims 12-14.

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