Method for preparing composite sodium iron phosphate positive electrode material based on biomass grinding aid

Abstract

The application discloses a method for preparing a composite sodium iron phosphate positive electrode material based on a biomass grinding aid. The method comprises the following steps: stirring a yam with water to obtain yam homogenate, and filtering the yam homogenate to obtain filtrate; weighing sodium pyrophosphate, ferrous oxalate, iron phosphate and sodium carbonate according to stoichiometric ratios, adding water, a carbon source and the filtrate, and stirring uniformly to obtain a mixed slurry; grinding the mixed slurry, and performing spray drying to obtain a composite sodium iron phosphate precursor powder; and calcining and crushing the composite sodium iron phosphate precursor powder to obtain a composite sodium iron phosphate positive electrode material. The yam biomass colloid is used as the grinding aid, and the yam biomass colloid has a wide source, low cost and biodegradability, can reduce particle agglomeration and improve grinding efficiency, and can improve the electrochemical performance and cycle capacity retention rate of the finished product; the process is compatible with existing equipment and is easy to industrialize.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode material preparation technology, specifically relating to a method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid. Background Technology

[0002] Sodium-ion batteries, due to the abundant and widely distributed nature of sodium resources and their low cost, have broad application prospects in large-scale energy storage and low-speed electric vehicles. Composite sodium iron phosphate cathode materials, characterized by high theoretical specific capacity, good structural stability, and excellent cycle performance, are currently one of the research hotspots for sodium-ion battery cathode materials. At present, the mainstream process routes for composite sodium iron phosphate cathode materials are mainly divided into three technical routes based on the iron source: iron phosphate route, ferrous oxalate route, and iron oxide red route. The preparation processes of these three mainstream routes typically include four core steps: material preparation, sand milling, spray drying, and high-temperature sintering. The sand milling process is a crucial link between raw material pretreatment and subsequent sintering, and its core function revolves around addressing two core needs: low intrinsic conductivity and compatibility with the sintering process. Sand milling, through high-energy mechanical grinding (high-speed collision and shearing between the grinding media and raw material particles), breaks insoluble or slightly soluble raw material particles down to the nanoscale (typically 100~400nm), which helps improve the conductivity of the cathode material. Raw material particles containing elements such as Na, Fe, and P are milled to form a uniform slurry in the liquid phase, which provides a guarantee for subsequent homogenization during sintering and avoids the formation of impurity phases caused by local Na-rich or Fe-poor areas.

[0003] Iron sources such as ferric phosphate, ferrous oxalate, and iron oxide red are all insoluble or slightly soluble powders. During the sand milling process, due to the nano-sized raw material particles and increased specific surface area, their surfaces become charged due to bond breakage. This leads to particle adsorption and agglomeration, reducing grinding efficiency, increasing energy consumption, and causing uneven particle size distribution after grinding. Consequently, the electrochemical performance of the finished cathode material after sintering is reduced. To avoid agglomeration during the grinding process, the industry currently mainly uses industrially produced chemically synthesized grinding aids such as polyethylene glycol, polymeric polyols, and polymeric alkanolamines. There is almost no precedent for using biomass-based grinding aids.

[0004] Chemically synthesized grinding aids are costly, and their stability is affected by the pH and temperature of the material system. Furthermore, some grinding aids may generate harmful gases during decomposition, polluting the environment and contradicting the trend of green chemistry. Therefore, developing a widely available, low-cost, environmentally friendly grinding aid with excellent grinding performance to replace traditional chemical grinding aids in the preparation of composite sodium iron phosphate is of great significance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the main objective of this invention is to provide a method for preparing composite sodium iron phosphate cathode materials based on biomass grinding aids.

[0006] This invention provides a method for preparing composite sodium iron phosphate cathode materials based on biomass grinding aids, comprising the following steps: Take yam, add water and stir to obtain yam homogenate, filter and collect the filtrate; Sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate were weighed according to the stoichiometric ratio, and water, carbon source and the filtrate were added. After stirring evenly, a mixed slurry was obtained. The mixed slurry is ground and then spray-dried to obtain a composite sodium iron phosphate precursor powder. The composite sodium iron phosphate precursor powder is then calcined and pulverized to obtain the composite sodium iron phosphate cathode material.

[0007] Preferably, in the filtrate preparation step, the yam is first washed, peeled, and cut into 1-2 cm pieces, then water is added and stirred at 1000-2000 rpm for 30-60 minutes.

[0008] Preferably, in the filtrate preparation step, the mass ratio of yam to water is 1:2~4.

[0009] Preferably, in the preparation step of the filtrate, the specific operation of filtration is as follows: the yam homogenate is passed through a 300-500 mesh sieve to remove solid residue.

[0010] Preferably, in the preparation step of the mixed slurry, the molar ratio of sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate is 1:1:2:0.025.

[0011] Preferably, in the preparation step of the mixed slurry, the amount of carbon source added is 2 to 5 wt% of the total amount of sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate.

[0012] Preferably, the carbon source is at least one of glucose, sucrose, and fructose.

[0013] Preferably, in the preparation step of the mixed slurry, the total mass ratio of water to sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate is 1:1.5~3.

[0014] Preferably, in the preparation step of the mixed slurry, the amount of filtrate added is 20-30 wt% of the total amount of sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate.

[0015] Preferably, in the preparation step of the composite sodium iron phosphate precursor powder, the grinding uses zirconia balls with a particle size of 0.2 mm as the grinding medium, the filling rate of the grinding medium is 60~80%, and the particle size of the slurry after grinding is D50=200nm.

[0016] Preferably, in the preparation step of the composite sodium iron phosphate precursor powder, the spray drying parameters include: an inlet air temperature of 250°C, an outlet air temperature of 100°C, and the atomizer used for spray drying is a gear-driven high-speed centrifugal atomizer with a rotation speed of 10,000~12,000 rpm.

[0017] Preferably, in the preparation step of the composite sodium iron phosphate cathode material, the calcination atmosphere is an inert gas atmosphere, the calcination temperature is 520~600℃, and the calcination time is 6~8h.

[0018] Preferably, in the preparation step of the composite sodium iron phosphate cathode material, the pulverization is performed by air jet milling, and the particle size of the pulverized powder is D50=8μm.

[0019] Compared with the prior art, the beneficial effects of the present invention include: (1) This invention is the first to use yam mucus, a biomass material, as a grinding aid in the preparation of composite sodium iron phosphate, replacing the traditional chemically synthesized polyol grinding aid. Yam is widely available and inexpensive (the price is only 1 / 5 to 1 / 3 of polyethylene glycol). Yam mucus is completely biodegradable and can be decomposed into harmless substances such as CO2 and H2O during high-temperature sintering, leaving no organic impurities. This solves the problem of high energy consumption and high pollution in the production process of chemically synthesized grinding aids in the traditional chemical industry.

[0020] (2) Biomass grinding aids and glucose, etc., are used as mixed carbon sources. They are uniformly coated on the surface of the material by spray drying. The mixed carbon sources are sintered under an inert atmosphere. The carbon sources are decomposed into conductive carbon layers and uniformly coated on the surface of the material, which improves the electrical conductivity and electrochemical performance of the material. At the same time, it provides an effective buffer for the volume change of the material during the charge and discharge process. The prepared material has excellent rate performance and cycle performance. Biomass grinding aids can reduce the amount of carbon sources such as glucose (the price is only 1 / 3 to 1 / 2 of that of polyethylene glycol), further reducing the cost of raw materials.

[0021] (3) In some embodiments, when glucose is used as a single carbon source, the crushing process can easily cause the carbon coating layer to fall off or be damaged. Mannan in the biomass grinding aid can increase the adhesion and bonding of the carbon coating layer formed on the surface of the positive electrode material by the composite carbon source due to its excellent adhesion and self-polymerization properties. Glucose and biomass grinding aid are more closely and densely coated on the surface of the positive electrode material, so that it is not easy to fall off during the crushing process and packaging and transportation process, which is beneficial to improving the conductivity and electrochemical performance of the positive electrode material.

[0022] (4) The mannan-protein complex in yam biomass colloid has unique hydration and surface adsorption properties, and its anti-agglomeration effect is better than that of traditional polyethylene glycol grinding aids. Under the same grinding conditions, the grinding aid of the present invention can shorten the grinding time by 5% to 9%, significantly improve the grinding efficiency, and reduce energy consumption.

[0023] (5) Due to the uniform dispersion of the material and the absence of residual organic impurities, the composite sodium iron phosphate cathode material obtained after sintering has a more complete crystal structure, a spherical morphology, and is relatively regular. The combination of large and small particles helps to improve the compaction density of the material. Test results show that, compared with Comparative Example 1, the initial discharge specific capacity of this cathode material is increased by 1%~2%, the 0.2C coulombic efficiency is increased by 1.6~1.8%, and the capacity retention rate after 500 cycles at 1C is increased by 3%~4%.

[0024] (6) The grinding aid addition process of the present invention can be directly integrated into the existing preparation process of composite sodium iron phosphate without modifying the existing production equipment. It has strong process compatibility and is easy to industrialize. Moreover, the preparation process of biomass grinding aid is simple and can achieve continuous production, which is suitable for large-scale industrial application. Attached Figure Description

[0025] Figure 1 This is a scanning electron microscope image of the composite sodium iron phosphate cathode material prepared in Example 1 of the present invention.

[0026] Figure 2 The XRD patterns are those of the composite sodium iron phosphate cathode materials prepared in Example 3 and Comparative Example 1. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] Composite sodium iron phosphate cathode materials have the characteristics of high theoretical specific capacity, good structural stability and excellent cycle performance, and are one of the current research hotspots for sodium-ion battery cathode materials. Currently, composite sodium iron phosphate cathode materials are mainly produced through three mainstream processes based on different iron sources: iron phosphate, ferrous oxalate, and iron oxide red. The core processes of all three involve four steps: batching, sand milling, spray drying, and high-temperature sintering. Sand milling, as a crucial link between pretreatment and sintering, aims to break the raw materials to the nanoscale (100-400 nm) through high-energy mechanical grinding to improve conductivity and ensure uniform elemental distribution, preventing the formation of impurities during sintering. However, due to the increased specific surface area and surface charging effect after the iron source powder is nano-sized, agglomeration is easily triggered, reducing grinding efficiency and impairing the final electrochemical performance. To address this problem, the industry currently widely uses chemically synthesized grinding aids such as polyethylene glycol, but there are few precedents for using biomass-based grinding aids. Existing chemical grinding aids suffer from high costs, poor stability due to environmental factors (pH, temperature), and the potential to decompose and produce harmful gases, making them insufficient to meet the development trend of green chemistry.

[0029] In view of this, the present invention provides a method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid, comprising the following steps: Take yam, add water and stir to obtain yam homogenate, filter and collect the filtrate; Sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate were weighed according to the stoichiometric ratio, and water, carbon source and the filtrate were added. After stirring evenly, a mixed slurry was obtained. The mixed slurry is ground and then spray-dried to obtain a composite sodium iron phosphate precursor powder. The composite sodium iron phosphate precursor powder is then calcined and pulverized to obtain the composite sodium iron phosphate cathode material.

[0030] In this invention, yam mucilage is a biocolloid primarily composed of a mannan-protein complex. These two components are tightly linked by glycopeptide bonds, forming a high-molecular-weight complex (with a molecular weight reaching hundreds of thousands of Da). This complex is rich in polar hydroxyl groups, forming a hydrated gel network. During the grinding process, it can rapidly adsorb onto the surface of raw material particles, reducing their surface energy. Simultaneously, the polar groups interact strongly with water molecules, creating a stable hydrated layer on the particle surface, hindering particle collision and aggregation. Furthermore, the mannan-protein complex exhibits excellent viscoelasticity and shear-thinning properties, improving the fluidity of the slurry and reducing energy loss during grinding, thereby significantly improving grinding efficiency.

[0031] The chemical reaction involved in this invention is as follows: 2Na4P2O7+2FeC2O4+4FePO4+0.025Na2CO3+C6H 12 O6 (using glucose as a carbon source) + biomass grinding aid = 2Na4.05 Fe3(PO4)2P2O7 / C+CO2+H2O In this process, glucose and biomass grinding aids decompose into elemental carbon under anaerobic conditions (inert atmosphere) as temperature increases. The elemental carbon then undergoes a redox reaction with ferric iron, promoting the oxidation of Na+. + Na is embedded in the crystal structure of FePO4, forming an olivine structure coated with elemental carbon. 4.05 Fe3(PO4)2P2O7 cathode material.

[0032] Most of the C, H, and O elements in glucose and biomass grinding aids are converted into CO2 and H2O, while only some of the C elements are converted into elemental carbon, which coats the surface of the cathode material to improve its conductivity.

[0033] In addition, since some sodium volatilizes during the high-temperature sintering process, a sodium supplement (sodium carbonate) is usually added to replenish the sodium volatilized during the high-temperature sintering process, generally by an additional 5% molar ratio. The carbonate ions of the added sodium carbonate are converted into CO2, and the sodium is basically completely volatilized.

[0034] Preferably, in the filtrate preparation step, the yam is first washed, peeled, and cut into 1-2 cm pieces, then water is added and stirred at 1000-2000 rpm for 30-60 minutes.

[0035] In some embodiments of the present invention, when preparing the filtrate, the yam is washed, peeled, cut into small pieces, and then water is added and stirred. This is because the yam mucilage is mainly found in the root and stem. When the yam is cut or peeled, the mucilage stored in the internal tissue will flow out. Furthermore, in order to obtain this mucilage more efficiently, the usual operation is to add water and stir at high speed until uniform, so that the mucilage in the tissue can be fully extracted.

[0036] Preferably, in the filtrate preparation step, the mass ratio of yam to water is 1:2~4.

[0037] In some embodiments of the present invention, the mass ratio of yam to water is 1:2 to 4; for example, it can be any one of 1:2, 1:3, and 1:4, or any range between two of them. This ensures efficient extraction of the yam mucilage without excessive dilution of the mucilage.

[0038] Preferably, in the preparation step of the filtrate, the specific operation of filtration is as follows: the yam homogenate is passed through a 300-500 mesh sieve to remove solid residue.

[0039] In some embodiments of the present invention, in order to obtain pure yam mucilage, the yam homogenate needs to be filtered; among them, sieve filtration is the simplest, has high filtration efficiency, and good effect.

[0040] Preferably, in the preparation step of the mixed slurry, the molar ratio of sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate is 1:1:2:0.025.

[0041] Preferably, in the preparation step of the mixed slurry, the amount of carbon source added is 2 to 5 wt% of the total amount of sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate.

[0042] In some embodiments of the present invention, the carbon source is coated on the surface of the cathode material during the thickness calcination process, thereby improving the conductivity of the material. However, the amount of carbon source added needs to be moderate. Too little carbon source will result in uneven coating and poor local conductivity, while too much carbon source will not only waste materials but also cause carbon particles to agglomerate, block ion channels, and increase ion diffusion resistance.

[0043] Preferably, the carbon source is at least one of glucose, sucrose, and fructose.

[0044] In some embodiments of the present invention, glucose, sucrose and fructose are readily available carbon sources, and their calcination products are elemental carbon, water and carbon dioxide, wherein the water and carbon dioxide will evaporate or escape during the addition reaction, leaving no organic impurities.

[0045] Preferably, in the preparation step of the mixed slurry, the total mass ratio of water to sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate is 1:1.5~3.

[0046] Preferably, in the preparation step of the mixed slurry, the amount of filtrate added is 20-30 wt% of the total amount of sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate.

[0047] In some embodiments of the present invention, the filtrate is used as a grinding aid in the grinding process, and its dosage is 20-30 wt% of the total amount of sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate; for example, it can be any one or any combination of 20%, 25%, and 30%. This ensures efficient grinding even with a relatively small dosage.

[0048] Preferably, in the preparation step of the composite sodium iron phosphate precursor powder, the grinding uses zirconia balls with a particle size of 0.2 mm as the grinding medium, the filling rate of the grinding medium is 60~80%, and the particle size of the slurry after grinding is D50=200nm.

[0049] Preferably, in the preparation step of the composite sodium iron phosphate precursor powder, the spray drying parameters include: an inlet air temperature of 250°C, an outlet air temperature of 100°C, and the atomizer used for spray drying is a gear-driven high-speed centrifugal atomizer with a rotation speed of 10,000~12,000 rpm.

[0050] Preferably, in the preparation step of the composite sodium iron phosphate cathode material, the calcination atmosphere is an inert gas atmosphere, the calcination temperature is 520~600℃, and the calcination time is 6~8h.

[0051] Preferably, in the preparation step of the composite sodium iron phosphate cathode material, the pulverization is performed by air jet milling, and the particle size of the pulverized powder is D50=8μm.

[0052] Example 1 A method for preparing composite sodium iron phosphate cathode material based on biomass grinding aids, the specific steps of which are as follows: (1) Select fresh yam, wash it, peel it and cut it into small pieces of 1-2cm; add deionized water at a mass ratio of 1:2 of yam to deionized water, stir it with a high-speed mixer at a speed of 1000rpm, and stir for 60min to obtain yam homogenate; filter the homogenate through a 300-mesh sieve to remove solid residue, and collect the filtrate as biomass grinding aid; (2) Weigh sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate in a molar ratio of 1:1:2:0.025 and add them to a premixing tank. Sodium carbonate is added to compensate for sodium loss during the sintering process. Add glucose at 5% of the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Add deionized water at a ratio of 1:1.5 to the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Add the above biomass grinding aid at 20% of the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Stir at 1000 rpm for 60 min to obtain a uniform mixed slurry. (3) Pump the mixed slurry into the sand mill, select zirconia beads with a particle size of 0.2 mm as the grinding media, and fill the grinding media of the sand mill at 80% until the particle size in the slurry is ground to D50 of 200 nm. (4) The slurry after sand milling is fed into a spray dryer, and the inlet temperature is controlled at 250℃ and the outlet temperature at 100℃. The atomizer used for spray drying is a gear-driven high-speed centrifugal atomizer with a rotation speed of 10,000 rpm. The composite sodium iron phosphate precursor powder with good flowability is obtained. The precursor powder is placed in an atmosphere sintering furnace and heated to 520℃ at a heating rate of 5℃ / min under an inert gas atmosphere, and held for 8 hours. Then it is cooled to room temperature at a cooling rate of 3℃ / min, and then pulverized by airflow to obtain the finished composite sodium iron phosphate cathode material (D50=8μm).

[0053] Figure 1 This is a scanning electron microscope (SEM) image of the composite sodium iron phosphate cathode material prepared in Example 1 of this invention. Figure 1We can see that the morphology of the prepared sample is spherical and relatively regular, and the combination of large and small particles helps to improve the compaction density of the material.

[0054] Example 2 A method for preparing composite sodium iron phosphate cathode material based on biomass grinding aids, the specific steps of which are as follows: (1) Select fresh yam, wash it, peel it and cut it into small pieces of 1-2cm; add deionized water at a mass ratio of 1:4 between yam and deionized water, and stir it with a high-speed mixer at a speed of 2000rpm for 30 minutes to obtain yam homogenate; filter the homogenate through a 500-mesh sieve to remove solid residue, and collect the filtrate as biomass grinding aid; (2) Weigh sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate in a molar ratio of 1:1:2:0.025 and add them to a premixing tank. Sodium carbonate is added to compensate for sodium loss during the sintering process. Add glucose at 2% of the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Add deionized water at a ratio of 1:3 to the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Add the above biomass grinding aid at 30% of the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Stir at 3000 rpm for 30 minutes to obtain a uniform mixed slurry. (3) Pump the mixed slurry into the sand mill, select zirconia beads with a particle size of 0.2 mm as the grinding medium, and fill the grinding medium of the sand mill with 60% until the particle size of the slurry is ground to D50 of 200 nm. (4) The slurry after sand milling is fed into a spray dryer, and the inlet temperature is controlled at 250℃ and the outlet temperature at 100℃. The atomizer used for spray drying is a gear-driven high-speed centrifugal atomizer with a rotation speed of 12000 rpm. The composite sodium iron phosphate precursor powder with good flowability is obtained. The precursor powder is placed in an atmosphere sintering furnace and heated to 600℃ at a heating rate of 5℃ / min under an inert gas atmosphere, and held for 6h. Then it is cooled to room temperature at a cooling rate of 3℃ / min, and then pulverized by airflow to obtain the finished composite sodium iron phosphate cathode material (D50=8μm).

[0055] Example 3 A method for preparing composite sodium iron phosphate cathode material based on biomass grinding aids, the specific steps of which are as follows: (1) Select fresh yam, wash it, peel it and cut it into small pieces of 1-2cm; add deionized water at a mass ratio of 1:3 between yam and deionized water, and stir it with a high-speed mixer at a speed of 1500rpm for 40 minutes to obtain yam homogenate; filter the homogenate through a 400-mesh sieve to remove solid residue, and collect the filtrate as biomass grinding aid; (2) Weigh sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate in a molar ratio of 1:1:2:0.025 and add them to a premixing tank. Sodium carbonate is added to compensate for sodium loss during the sintering process. Add glucose at 3% of the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Add deionized water at a ratio of 1:3 to the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Add the above biomass grinding aid at 25% of the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate). Stir at 2000 rpm for 40 minutes to obtain a uniform mixed slurry. (3) Pump the mixed slurry into the sand mill, select zirconia beads with a particle size of 0.2 mm as the grinding media, and fill the sand mill with 70% grinding media until the particle size in the slurry is ground to D50 of 200 nm. (4) The slurry after sand milling is fed into a spray dryer, and the inlet temperature is controlled at 250℃ and the outlet temperature at 100℃. The atomizer used for spray drying is a gear-driven high-speed centrifugal atomizer with a rotation speed of 11000 rpm. The composite sodium iron phosphate precursor powder with good flowability is obtained. The precursor powder is placed in an atmosphere sintering furnace and heated to 550℃ at a heating rate of 5℃ / min under an inert gas atmosphere, and held for 7h. Then it is cooled to room temperature at a cooling rate of 3℃ / min, and then pulverized by airflow to obtain the finished composite sodium iron phosphate cathode material (D50=8μm).

[0056] Comparative Example 1 A method for preparing a composite sodium iron phosphate cathode material, the specific steps of which are as follows: (1) Weigh sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate in a molar ratio of 1:1:2:0.025 and add them to a premixing tank. Sodium carbonate is added to compensate for sodium loss during sintering. Add glucose at 3% of the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate), add deionized water at a ratio of 1:3 to the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate), add PEG-2000 at 2.5% of the total mass of the four raw materials (sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate), and stir at 2000 rpm for 40 min to obtain a uniform slurry. (2) Pump the mixed slurry into the sand mill, select zirconia beads with a particle size of 0.2 mm as the grinding media, and fill the sand mill with 70% grinding media until the particle size in the slurry is ground to D50 of 200 nm. (3) The slurry after sand milling is fed into a spray dryer, and the inlet temperature is controlled at 250℃ and the outlet temperature at 100℃. The atomizer used for spray drying is a gear-driven high-speed centrifugal atomizer with a rotation speed of 10000 rpm. The composite sodium iron phosphate precursor powder with good flowability is obtained. The precursor powder is placed in an atmosphere sintering furnace and heated to 550℃ at a heating rate of 5℃ / min under an inert gas atmosphere, and held for 7h. Then it is cooled to room temperature at a cooling rate of 3℃ / min, and then pulverized by airflow to obtain the finished composite sodium iron phosphate cathode material (D50=8μm).

[0057] Figure 2 The images show the XRD patterns of the composite sodium iron phosphate cathode materials prepared in Example 3 and Comparative Example 1. Figure 2 The characterization results show that all diffraction peaks of the sample prepared in Example 3 of this invention correspond one-to-one with the standard card (PDF#89-0579), indicating that it is a pure-phase composite sodium iron phosphate. Although the diffraction peaks of the Comparative Example 1 sample are in the same position, their diffraction peak intensities are significantly lower than those of Example 3, indicating that the crystallinity of the Comparative Example 1 sample is poor.

[0058] Performance testing The performance of the composite sodium iron phosphate cathode materials prepared in Examples 1, 2, 3, and Comparative Example 1 was tested. The test conditions were as follows: the prepared cathode material was used as the cathode, metallic sodium as the anode, 1 mol / L NaClO4 / EC-DMC (volume ratio 1:1) as the electrolyte, and a glass fiber membrane was used as the separator. The cells were assembled into CR2032 coin cells, and the test voltage was 3.6~2V. The grinding time, initial discharge specific capacity (0.2C), 1C discharge specific capacity, and capacity retention after 500 1C cycles were tested. The test results are shown in Table 1.

[0059] Table 1 Performance test results of the examples and comparative examples

[0060] Referring to Table 1, the test results show that the composite sodium iron phosphate cathode material prepared using biomass grinding aids in this invention can shorten the grinding time by 5% to 9%, significantly improving grinding efficiency and reducing energy consumption. Furthermore, the batteries prepared in the embodiments of this invention exhibit significantly better electrochemical performance than similar batteries prepared using traditional polyethylene glycol grinding aids, verifying the advanced nature and practicality of this invention.

[0061] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid, characterized in that, Includes the following steps: Take yam, add water and stir to obtain yam homogenate, filter and collect the filtrate; Sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate were weighed according to the stoichiometric ratio, and water, carbon source and the filtrate were added. After stirring evenly, a mixed slurry was obtained. The mixed slurry is ground and then spray-dried to obtain a composite sodium iron phosphate precursor powder. The composite sodium iron phosphate precursor powder is then calcined and pulverized to obtain the composite sodium iron phosphate cathode material.

2. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 1, characterized in that, In the filtrate preparation step, the yam is first washed, peeled, and cut into 1-2 cm pieces, then water is added and stirred at 1000-2000 rpm for 30-60 minutes.

3. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 1, characterized in that, In the filtrate preparation step, the mass ratio of yam to water is 1:2~4; and / or In the preparation step of the filtrate, the specific operation of filtration is as follows: the yam homogenate is passed through a 300-500 mesh sieve to remove solid residue.

4. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 1, characterized in that, In the preparation step of the mixed slurry, the molar ratio of sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate is 1:1:2:0.

025.

5. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 4, characterized in that, In the preparation step of the mixed slurry, the amount of carbon source added is 2-5 wt% of the total amount of sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate; and / or The carbon source is at least one of glucose, sucrose, and fructose.

6. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 4, characterized in that, In the preparation step of the mixed slurry, the total mass ratio of water to sodium pyrophosphate, ferrous oxalate, ferric phosphate, and sodium carbonate is 1:1.5~3; and / or In the preparation step of the mixed slurry, the amount of filtrate added is 20-30 wt% of the total amount of sodium pyrophosphate, ferrous oxalate, ferric phosphate and sodium carbonate.

7. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 1, characterized in that, In the preparation step of the composite sodium iron phosphate precursor powder, the grinding uses zirconia balls with a particle size of 0.2 mm as the grinding medium, the filling rate of the grinding medium is 60~80%, and the particle size of the slurry after grinding is D50=200nm.

8. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 1, characterized in that, In the preparation step of the composite sodium iron phosphate precursor powder, the parameters of the spray drying include: an inlet air temperature of 250°C, an outlet air temperature of 100°C, and the atomizer used for spray drying is a gear-driven high-speed centrifugal atomizer with a rotation speed of 10,000~12,000 rpm.

9. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 1, characterized in that, In the preparation steps of the composite sodium iron phosphate cathode material, the calcination atmosphere is an inert gas atmosphere, the calcination temperature is 520~600℃, and the calcination time is 6~8h.

10. The method for preparing composite sodium iron phosphate cathode material based on biomass grinding aid according to claim 1, characterized in that, In the preparation steps of the composite sodium iron phosphate cathode material, the pulverization is performed by air jet milling, and the particle size of the pulverized powder is D50=8μm.

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