Preparation method of high-dispersion gradient crystallized ferric sodium pyrophosphate

Abstract

The invention relates to a preparation method of high-dispersion gradient crystallized ferric sodium pyrophosphate, and belongs to the technical field of sodium ion batteries. The preparation method comprises the following steps: firstly, uniformly stirring iron phosphate, a sodium source, a carbon source and water to obtain initial slurry; then grinding the initial slurry under the action of sodium polycarboxylate, polyvinyl alcohol and polyethylene glycol to obtain fine ground slurry; and carrying out spray drying and sintering on the finely ground slurry to obtain the high-dispersion gradient crystallized ferric sodium pyrophosphate. A staged sanding process is adopted, different dispersants are added in each stage, the viscosity and particle size uniformity of the slurry are accurately regulated and controlled, meanwhile, a spray drying process is optimized to inhibit secondary agglomeration of particles after spray drying, and a multi-stage rotary kiln is further utilized subsequently, so that the secondary agglomeration of the particles after spray drying is inhibited. And the uniformity of the sintering process and the integrity of crystal form growth are guaranteed through atmosphere regulation and control and accurate setting of the platform temperature, and finally the electrical property of the material is effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and particularly relates to a method for preparing highly dispersed gradient crystallized iron sodium pyrophosphate. Background Technology

[0002] Polyanionic materials have shown great application prospects and market potential in the field of sodium-ion batteries due to their high chemical stability, efficient electrochemical performance and structural composability.

[0003] However, this type of material still has significant technical defects in processing and technology; patent CN119528107A discloses a sodium iron pyrophosphate cathode material and its preparation method and battery. Although it adopts a hydrothermal reaction combined with a segmented sintering strategy to improve the material structure and crystal form, the sand milling process uses a single process, without controlling the degree of particle refinement, and only optimizes the coating effect through two carbon source additions and doping, without involving the precise use of dispersants, making it difficult to solve the problems of particle agglomeration and viscosity imbalance in the sand milling stage; patent CN115196610A discloses A sodium-ion battery cathode material, sodium iron pyrophosphate / carbon, and its synthesis method were developed, along with a sodium-ion battery. The process involves a two-stage high-temperature sintering process using an added platform, with organophosphonic acid providing phosphorus and carbon sources to simplify the process. However, sand milling is only used as an auxiliary step after pre-sintering, failing to address the issue of material sintering uniformity during the sintering process. Furthermore, a gradient refinement system was not formed, and rotary kiln-type drum sintering was not used to ensure sintering uniformity. Additionally, only traditional nitrogen protection was employed, without real-time adjustment based on the furnace atmosphere and temperature, making it difficult to achieve precise control of the product.

[0004] Sodium iron pyrophosphate (NFPP) is often produced using sol-gel methods and spray drying. While the stability of hollow spherical NFPP can be improved by controlling the inlet and outlet temperatures during spray drying, differentiated treatment schemes for coarse and fine particles in the slurry are not developed during the pretreatment sand milling stage. This indicates that current research generally focuses on carbon coating and material morphology control, lacking comprehensive control over the entire process, especially in the sand milling stage where precise control of particle size morphology and viscosity is lacking. Furthermore, existing research also has significant shortcomings in the selection and dosage control of dispersants during the sand milling stage. For example, adding only the carbon source during sand milling without selecting a suitable dispersant based on the actual particle size results in excessively high slurry viscosity. Moreover, traditional static sintering during the sintering process can easily lead to poor crystal morphology of the precursors at the top and bottom of the sagger, thus affecting the overall quality of the material.

[0005] In summary, current mainstream NFPP material preparation processes suffer from numerous technical problems: traditionally synthesized NFPP materials exhibit severe particle agglomeration, and single-stage, single-system sand milling processes struggle to achieve uniform particle refinement, easily leading to agglomeration. This results in large agglomerated particles failing to fully contact the electrolyte, trapping sodium ion deintercalation sites, and hindering the realization of theoretical capacity. During sand milling, as particle size decreases, slurry viscosity increases dramatically, easily causing equipment blockage and damage, exacerbating uneven particle breakage, and causing particles to be crushed rather than uniformly ground, leading to lattice defects. Furthermore, particles are prone to breakage during drying. Secondary agglomeration leads to the formation of large-sized agglomerates with poor particle size uniformity. Excessive large particles can affect subsequent cell coating and processing, resulting in uneven electrode thickness, hindered ion diffusion in thick areas, and overcharging in thin areas, thus affecting the overall battery performance. Traditional sintering processes often employ single-stage sintering and static sintering in roller kilns, resulting in uneven temperature field distribution. This can easily lead to insufficient and uneven crystal growth of the material between the upper and lower layers of the sagger, as well as between the periphery and the center, affecting the overall material quality. Precursors that are only spray-treated before sintering may have uneven particle size distribution, and insufficient phase purity during sintering can affect the material's subsequent processing performance.

[0006] Therefore, it is of great significance to develop a method for preparing NFPP materials that can ensure the uniformity and good dispersibility of slurry particle size during the sand milling stage, and can ensure the balanced growth of crystal form through multi-platform temperature and atmosphere synergistic control and rotary kiln sintering during the sintering stage. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a method for preparing highly dispersed gradient crystallized sodium iron pyrophosphate. The method employs a staged sand milling process, adding different dispersants at each stage to precisely control the slurry viscosity and particle size uniformity. Simultaneously, the spray drying process is optimized to suppress secondary agglomeration of particles after spray drying. Subsequently, a multi-stage rotary kiln is utilized, with precise atmosphere control and platform temperature settings ensuring the uniformity of the sintering process and the integrity of crystal growth, ultimately achieving an effective improvement in the material's electrical properties.

[0008] The purpose of this invention is to provide a method for preparing highly dispersed gradient crystallized iron pyrophosphate sodium, comprising the following steps: S1. Mix ferric phosphate, sodium source, carbon source and water evenly to obtain the initial slurry; S2. Under the action of sodium polycarboxylate, the initial slurry described in S1 is coarsely ground to obtain a coarsely ground slurry; S3. Under the action of polyvinyl alcohol, the coarse grinding slurry described in S2 is subjected to transition grinding to obtain a transition grinding slurry; S4. Under the action of polyethylene glycol, the transition slurry described in S3 is finely ground to obtain a finely ground slurry with a viscosity of 60 mPa·s-110 mPa·s; the molecular weight of the polyethylene glycol is 500-800. S5. Spray dry the finely ground slurry described in S4 to obtain the NFPP spray precursor; S6. The NFPP spray precursor described in S5 is sintered in a multi-stage rotary kiln to obtain the highly dispersed gradient crystallized sodium iron pyrophosphate.

[0009] In one embodiment of the present invention, in S1, the sodium source is selected from one or more of sodium carbonate, sodium dihydrogen phosphate, sodium nitrate and sodium bicarbonate; And / or, the carbon source is selected from one or more of sucrose, polyethylene glycol, polyacrylic acid, glucose, fructose, melamine, acetylene black and carbon nanotubes; And / or, the mass ratio of the iron phosphate, sodium source, carbon source and water is (8-13):(1.2-1.8):(0.8-1.4):32.

[0010] In one embodiment of the present invention, in S2, the process parameters for coarse grinding are: the grinding media are zirconia beads with a diameter of 0.8 mm to 1.2 mm, the rotation speed is 700 r / min to 950 r / min, and the time is 50 min to 90 min.

[0011] In one embodiment of the present invention, in S2, the sodium polycarboxylate is added 5-20 minutes after the start of coarse grinding, and the amount added is 0.3wt%-0.5wt% of the initial slurry. Applying sodium polycarboxylate to the coarse grinding stage can quickly disperse large particle agglomerates. It can quickly hydrolyze and release carboxylate anions in water, which adsorb onto the surface of material particles to form a negative charge layer. Electrostatic repulsion ensures the dispersibility between particles, and the hydrophilic segments in the molecule can form a solvation layer, providing additional steric hindrance and further improving the slurry's carrying capacity.

[0012] In one embodiment of the present invention, in S3, the process parameters of the transition mill are: the grinding media are zirconia beads with a diameter of 0.4 mm to 0.6 mm, the rotation speed is 1100 r / min to 1400 r / min, and the time is 30 min to 40 min.

[0013] In one embodiment of the present invention, in S3, the polyvinyl alcohol is added 8-12 minutes after the start of the transition mill, and the amount added is 0.08wt%-0.12wt% of the initial slurry. Polyvinyl alcohol is used in the transition mill stage. In response to the agglomerates that are easily formed by the increased surface energy of the particles in this stage, a large number of hydroxyl groups on its molecular chain can form hydrogen bonds with the hydroxyl groups and phosphate groups on the surface of the polyanionic particles and be firmly adsorbed onto the particle surface, preventing the particles from loosely agglomerating due to van der Waals forces, avoiding the situation where large particles are not broken and small particles are not differentiated. Moreover, when polyvinyl alcohol is heated to above 250°C, it can be completely decomposed into water and carbon dioxide, with no ash residue, and has little impact on the material properties.

[0014] In one embodiment of the present invention, in S4, the process parameters for fine grinding are: the grinding media are zirconia beads with a diameter of 0.15 mm to 0.25 mm, the rotation speed is 1550 r / min to 1750 r / min, and the time is 80 min to 100 min.

[0015] In one embodiment of the present invention, in S4, the polyethylene glycol is added 10-20 minutes after the start of fine grinding, and the amount added is 0.06wt%-0.1wt% of the initial slurry. Polyethylene glycol is used in the fine grinding stage. After fine grinding, the particle size is significantly reduced, the specific surface area increases sharply, and the surface energy further increases, leading to a significant enhancement of van der Waals forces and hydrogen bonds between particles, making it very easy to form fine particle agglomerates. Polyethylene glycol with a molecular weight of 500-800 belongs to a short-chain polyether structure and has extremely high fluidity. It does not cause an increase in slurry viscosity due to molecular chain entanglement. Under the high shear environment of fine grinding, it can quickly diffuse to the surface of each ultrafine particle, effectively avoiding the entanglement and uneven adsorption problems that are prone to occur with long-chain molecules (such as PVA and long-chain polycarboxylate). At the same time, its short-chain structure is not easily broken or desorbed under high shear force, and can continuously maintain adsorption stability, ensuring that the dispersion effect does not decrease throughout the fine grinding process.

[0016] In one embodiment of the present invention, in S5, the process parameters of the spray drying are: inlet air temperature of 220℃-250℃, outlet air temperature of 95℃-105℃, rotation speed of 19000r / min-21000r / min, and feed rate of 95mL / min-105mL / min. By optimizing the spray drying process, the particle size can be precisely controlled by adjusting the inlet air temperature and rotation speed, effectively suppressing secondary agglomeration after spraying. The inlet air temperature can quickly remove moisture from the slurry, avoiding particle agglomeration caused by slow drying rate due to low temperature. At the same time, the rotation speed can keep the particle D50 at about 15μm-20μm, which is not only conducive to enhancing the stability of the subsequent sintering process, but also further narrows the particle size distribution and improves particle uniformity.

[0017] In one embodiment of the present invention, in S6, the sintering rotation speed is 0.5 r / min-2 r / min, and four temperature zones are set, with the following process parameters for each temperature zone: The temperature of the first temperature zone is 160℃-200℃, the holding time is 30min-40min, the nitrogen atmosphere is 110L / min-130L / min; The temperature in the second temperature zone is 300℃-350℃, the holding time is 380min-450min, the nitrogen atmosphere is 110L / min-130L / min; The temperature in the third temperature zone is 540-590℃, the holding time is 520min-600min, the atmosphere is argon, and the argon flow rate is 150L / min-170L / min; The fourth temperature zone involves cooling to below 100℃ for 200-300 minutes under a nitrogen atmosphere at a flow rate of 95-105 L / min. Through the application of a multi-stage rotary kiln and dynamic control of the inert atmosphere, gradient heating and slow cooling of the NFPP spray precursor were achieved. On one hand, the rotary kiln employs a drum-type structure and rotates at a speed of 0.5-2 r / min for feeding, promoting more complete crystal growth. On the other hand, the gradient settings of the four temperature zones and the precise control of the corresponding inert atmosphere facilitate crystal growth. The system provides a stable guarantee: the first temperature zone (preheating zone) can efficiently remove residual free water molecules in the NFPP spray precursor; the second temperature zone (low temperature zone) introduces nitrogen gas, which can effectively remove residual bound water and other organic impurities, and also improve the conductivity of the material through nitrogen doping; the third temperature zone (high temperature zone) uses an argon atmosphere to achieve thorough protection of the material, ensure the stability of the crystal structure, and avoid the introduction of impurity phases during high-temperature crystallization; the fourth temperature zone slowly cools down to below 100°C in a nitrogen atmosphere to further consolidate the integrity of the crystal structure and ensure the overall crystal quality.

[0018] The technical solution of the present invention has the following advantages compared with the prior art: (1) The preparation method of the present invention uses a staged sand milling process of coarse grinding, intermediate grinding and fine grinding, and adds three dispersants, sodium polycarboxylate, polyvinyl alcohol and polyethylene glycol, in corresponding stages. This not only achieves a gradient change in the sand milling particle size from coarse to fine, but also effectively avoids the problem of the viscosity of the slurry increasing as the particle size decreases. This significantly improves the slurry dispersibility, increases the sand milling efficiency, prevents the blockage of the sand milling equipment pipeline and internal parts, and greatly shortens the sand milling time.

[0019] (2) The preparation method described in this invention firstly solves the problem of particle agglomeration from the source by processing in stages and using a special dispersant in the sand milling stage; in the coarse milling stage, sodium polycarboxylate hydrolyzes in water to generate carboxylate anions, which adsorb onto the particle surface to form a negative charge layer, while its hydrophilic chain segments construct a solubilization layer, which quickly disperses large particle agglomerates; in the intermediate milling stage, a large number of hydroxyl groups on the polyvinyl alcohol molecular chain form hydrogen bonds with the particle surface groups, which are firmly adsorbed onto the particle surface, effectively inhibiting the loose agglomeration that is prone to occur due to the increase of particle surface energy in this stage, and avoiding the situation where large particles are not fully broken and small particles agglomerate in advance; in the fine milling stage, polyethylene glycol, with its strong fluidity and stable adsorption performance brought by the short-chain polyether structure, firmly locks the dispersion state of ultrafine particles, and finally obtains a high-quality slurry with low viscosity and uniform particle size, realizing the gradient refinement of particles from coarse to fine. Following spray drying, rapid dehydration is achieved by controlling the inlet air temperature, precise granulation is accomplished with appropriate rotation speed, and a stable feed rate ensures the processing is completed, effectively suppressing secondary particle agglomeration during the drying stage and producing regular spray-dried particles. Simultaneously, optimization of process parameters further narrows the particle size distribution range, maintaining particle consistency. The entire process utilizes the synergistic cooperation of sand milling and spray drying. On one hand, the combination of segmented sand milling and a dedicated dispersant precisely controls the particle size and morphology; on the other hand, optimization of spray drying parameters compensates for any possible minor particle size deviations. Ultimately, particle uniformity control is achieved throughout the entire process from sand milling to drying, resulting in precursor particles that are uniform in size and have excellent dispersibility. This also shortens the sodium ion diffusion path, creating a crucial prerequisite for simultaneous crystallization in the subsequent multi-stage rotary kiln sintering stage.

[0020] (3) The preparation method described in this invention achieves uniform crystallization of NFPP spray precursors through the synergistic effect of multi-stage rotary kiln and dynamic atmosphere control: the preceding process has ensured that the NFPP spray precursors are highly consistent in size and composition, while the multi-stage rotary kiln feeds the material in a rotating manner, ensuring that the contact time of each particle with the temperature field and atmosphere is exactly the same, allowing the material crystal growth to be more complete and full; in terms of atmosphere control, the scientific combination of nitrogen and argon is precisely adapted to the impurity residue characteristics of the precursors. Although the preceding drying process has removed most of the moisture, trace amounts of bound water and organic impurities may still remain. The low-temperature zone is circulated The introduced nitrogen gas can not only completely remove these residual impurities, but also improve the conductivity of the material through nitrogen doping. In the high-temperature zone, argon gas with stronger inertness is used for protection to prevent the uniform particles from generating impurity phases due to oxidation during high-temperature crystallization. These impurity phases are superimposed on the low-impurity phase base formed in the previous stage, further improving the purity of the material. The gradient heating design of the multi-stage temperature zone can buffer the reaction stress of the particles. The particles refined in the previous stage have a relatively large specific surface area. Gradient heating can avoid lattice defects caused by rapid heating. Combined with the rotation of the rotary kiln, the particle crystallization process is more stable, further enhancing the structural stability of the material.

[0021] (4) The preparation method described in this invention achieves particle uniformity through the combination of staged sand milling and a special dispersant. The crystal structure is ensured by the use of a multi-stage rotary kiln and dynamic atmosphere control. The uniform particle size can shorten the sodium ion diffusion path, and the complete crystal structure can reduce ion transport resistance. Combined with the improvement of conductivity by nitrogen doping, the charging and discharging speed is significantly improved. Staged sand milling and spray drying effectively reduce particle agglomeration and lattice defects. The multi-stage rotary kiln sintering process further suppresses the generation of impurity phases, giving the material low impurity phase characteristics and a stable structure. This makes the volume expansion and contraction of the material more coordinated during charging and discharging, avoiding particle cracking and pulverization, thereby greatly extending the cycle life. Attached Figure Description

[0022] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein: Figure 1 This is a peak intensity diagram of highly dispersed gradient crystallized iron pyrophosphate sodium prepared in Example 1 of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0024] In this invention, unless otherwise stated, the sodium polycarboxylate used in the embodiments of this invention is of the type Tamol 850.

[0025] In this invention, unless otherwise stated, the polyvinyl alcohol used in the embodiments of this invention is PVA1788.

[0026] In this invention, unless otherwise stated, the polyethylene glycol used in the embodiments of this invention is PEG600.

[0027] In this invention, unless otherwise stated, the multi-stage rotary kiln used in the embodiments of this invention adopts a drum structure and is fed in a rotating manner. Example 1

[0028] The preparation method of highly dispersed gradient crystallized iron pyrophosphate sodium in this embodiment specifically includes the following steps: S1. Preparation of initial slurry: Weigh 1032g of ferric phosphate, 154g of sodium carbonate, 101g of glucose and 3200g of deionized water, mix them and put them into a stirring tank, and stir at a speed of 500r / min to obtain the initial slurry. S2. Preparation of coarse grinding slurry: A horizontal sand mill was used, with the rotation speed set to 800 r / min and 1 mm zirconia beads filled in. The initial slurry was transferred to the horizontal sand mill for coarse grinding for 70 min. 17.94 g of sodium polycarboxylate was added 10 min after the start of coarse grinding to obtain coarse grinding slurry. S3. Preparation of transition slurry: A sand mill was used with a rotation speed of 1200 r / min and 0.5 mm zirconia beads filled in. The sand mill slurry was transferred to the sand mill for transition milling for 35 min. 4.5 g of polyvinyl alcohol was added 10 min after the start of the transition milling to obtain the transition slurry. S4. Preparation of fine grinding slurry: Continue grinding in the sand mill, adjust the speed to 1600 r / min, and replace the grinding media with 0.2 mm zirconia beads. Grind the transition slurry for 90 min, and add 3.4 g of polyethylene glycol 15 min after the start of fine grinding to obtain fine grinding slurry.

[0029] S5. Preparation of NFPP spray precursor: Finely ground slurry is fed into a centrifugal spray dryer by a peristaltic pump. The inlet air temperature is controlled at 235℃, the outlet air temperature at 100℃, the rotation speed at 20000r / min, and the feed rate at 100mL / min. After spray drying, an NFPP spray precursor with a particle size D50 of about 15μm-20μm is obtained. S6. Preparation of highly dispersed gradient crystallized sodium ferric pyrophosphate: NFPP spray precursor was fed into a multi-stage rotary kiln for sintering. The rotary kiln speed was controlled at 1 r / min, and four temperature zones were set. The specific parameters for each temperature zone were as follows: Zone 1: temperature 180℃, holding time 35 min, nitrogen atmosphere, nitrogen flow rate 120 L / min; Zone 2: temperature 320℃, holding time 400 min, nitrogen atmosphere, nitrogen flow rate 120 L / min; Zone 3: temperature 550℃, holding time 540 min, argon atmosphere, argon flow rate 160 L / min; Zone 4: cooling to below 100℃, cooling time 240 min, nitrogen atmosphere, nitrogen flow rate 100 L / min. After sintering, highly dispersed gradient crystallized sodium ferric pyrophosphate was obtained by sieving. Example 2

[0030] The basic structure is the same as in Example 1, except that sodium carbonate is replaced with sodium carbonate and sodium nitrate in a mass ratio of 1:1; and glucose is replaced with sucrose and glucose in a mass ratio of 1:1.5. Example 3

[0031] The process is basically the same as in Example 1, except that the rotation speed in S2 is adjusted to 900 r / min, the rotation speed in S3 is adjusted to 1300 r / min, the rotation speed in S4 is adjusted to 1700 r / min, and the temperature in the three temperature zones in S6 is adjusted to 580°C. Comparative Example 1

[0032] The process is basically the same as in Example 1, except that the rotation speed is 900 r / min in the coarse grinding stage, the intermediate grinding stage, and the fine grinding stage. Comparative Example 2

[0033] The process is basically the same as in Example 1, except that the dispersant used in the coarse grinding stage, the intermediate grinding stage, and the fine grinding stage is sodium polycarboxylate. Comparative Example 3

[0034] The process is basically the same as in Example 1, except that the inlet air temperature for spray drying is 200°C and the rotation speed is 12000 r / min. Comparative Example 4

[0035] The process is basically the same as in Example 1, except that a roller kiln is used for sintering. The saggers are 150mm×150mm×100mm in size and are arranged in two rows and two columns. Each sagger is filled with 2kg of material and then placed in the roller kiln for sintering. Comparative Example 5

[0036] The process is basically the same as in Example 1, except that the atmosphere in the third temperature zone is nitrogen. Comparative Example 6

[0037] It is basically the same as Example 1, except that: no first temperature zone and second temperature zone are set. Comparative Example 7

[0038] The method is basically the same as in Example 1, except that the amount of sodium polycarboxylate added is 45g (1%). Comparative Example 8

[0039] The basic structure is the same as in Example 1, except that sodium polycarboxylate is replaced with magnesium stearate. Comparative Example 9

[0040] The basic structure is the same as in Example 1, except that sodium polycarboxylate is replaced with polyethylene glycol, polyvinyl alcohol is replaced with sodium polycarboxylate, and polyethylene glycol is replaced with polyvinyl alcohol. Test Example 1

[0041] (1) The particle size of the finely ground slurry prepared in the examples and comparative examples was tested using a Malvern 3000 laser particle size analyzer. Deionized water was used as the solvent, and sodium pyrophosphate with a concentration of 5% was added to it. Then about 12 drops of the sample to be tested were added. The mixture was placed in an ultrasonic homogenizer and ultrasonically treated for 3 minutes. After ultrasonic treatment, the relevant index was immediately tested using a laser particle size analyzer, and the particle size and particle size span of the sample were recorded. (2) Viscosity was tested using an NDJ-8S rotational viscometer. Probe No. 3 was selected, and the rotation speed was set to 60 r / min. 50 mL of the sample to be tested was placed in a beaker. The probe was connected to the rotating device of the viscometer and then placed vertically into the beaker. After the instrument showed that the test was completed, the viscosity value was recorded. The specific test results are shown in Table 1: Table 1

[0042] As shown in Table 1, the finely ground slurries of Examples 1-3 all exhibited excellent performance, with D50 concentrated between 0.262 μm and 0.305 μm, DMAX not exceeding 1.288 μm, viscosity between 68 mPa·s and 109 mPa·s, and particle size range ≤1.44. This result is attributed to the staged sand milling process of coarse grinding, intermediate grinding, and fine grinding, as well as the addition of three specific dispersants—sodium polycarboxylate, polyvinyl alcohol, and polyethylene glycol—at the corresponding stages. This not only achieved a gradient refinement of particles from coarse to fine but also effectively avoided the problem of viscosity increasing as particle size decreases, significantly improving the dispersibility and particle size uniformity of the slurry. In contrast, the indicators of each comparative example showed varying degrees of deterioration. Among them, the comparative example... 1. Because a single rotation speed is used in all three stages of coarse grinding, intermediate grinding, and fine grinding, it cannot adapt to the particle refinement requirements of different stages. In the coarse grinding stage, large-diameter zirconium beads require a lower rotation speed to avoid excessive material temperature leading to evaporation of dispersant or moisture. In the fine grinding stage, small-diameter zirconium beads require a higher rotation speed to improve grinding efficiency. Under a single rotation speed, not only are large particles not fully broken down, but the fine grinding efficiency is also significantly insufficient, ultimately resulting in a D50 of 0.883 μm, a DMAX of 6.612 μm, a viscosity of 340 mPa·s, a particle size range of 66.2, and a deterioration in dispersion. Comparative Example 2 uses only sodium polycarboxylate throughout the process. Although this dispersant can quickly disperse large particles in the coarse grinding stage by forming an electrostatic repulsion layer through hydrolysis to generate carboxylate anions, it cannot adapt to the intermediate grinding and fine grinding stages. The increased surface energy and smaller particle size during the grinding stage make it difficult to suppress secondary agglomeration caused by increased attraction between particles in the later stages using only electrostatic repulsion. This resulted in a DMAX of 16.463 μm, a viscosity soaring to 680 mPa·s, and a particle size range of 53.2. In Comparative Example 7, the excessive addition of sodium polycarboxylate, far exceeding the adsorption limit of the particle surface, led to micellar bridging and a coexistence of soft and hard agglomeration. Although the D50 decreased to 0.145 μm, the DMAX increased to 5.550 μm, and the particle size range soared to 157.1, disrupting particle uniformity. In Comparative Example 8, sodium polycarboxylate was replaced with magnesium stearate. This dispersant, as an oil-soluble fatty acid salt, has low solubility and uneven dispersion in the water-based sand milling system. The steric hindrance of fatty acid chains prevents the formation of an effective electrostatic repulsion layer, easily leading to "water-in-oil" agglomeration. This results in a D50 of 3.468 μm, a DMAX of 31.762 μm, a sudden increase in viscosity to 832 mPa·s, a particle size range of 38.87, and extremely poor dispersion. Comparative Example 9 disrupted the order of addition of the three dispersants, causing sodium polycarboxylate to fail to rapidly disperse large particles in the coarse grinding stage, polyvinyl alcohol to fail to effectively inhibit loose agglomeration in the transition grinding stage, and polyethylene glycol to fail to stabilize ultrafine particles in the fine grinding stage. Ultimately, this resulted in a D50 of 0.624 μm, a DMAX of 6.589 μm, a viscosity of 433 mPa·s, a particle size range of 32.5, and a significant decrease in slurry dispersibility and particle size uniformity. Test Example 2

[0043] The NFPP spray precursors prepared in the examples and comparative examples were subjected to particle size testing according to Test Example 1, and the moisture content was tested using a rapid moisture analyzer. For the moisture content test, approximately 4.5 g of the sample was weighed and placed in the instrument tray, the lid was closed, and the temperature was set to 120°C for 10 min. The relevant test results are shown in Table 2. Table 2

[0044] As shown in Table 2, the NFPP spray precursors prepared in Example 1 and Comparative Example 3 exhibit significant differences in particle size distribution and moisture content. This is mainly due to the different spray drying process parameters. Comparative Example 3 uses a low inlet air temperature of 200℃ and a low atomizing disc speed of 12000r / min, resulting in a slow slurry drying rate and inability to quickly remove moisture. This not only significantly increases the moisture content of the precursor powder but also easily triggers secondary particle agglomeration, leading to an overall larger particle size and uneven distribution. In contrast, Example 1 uses a high inlet air temperature and a high atomizing speed, which can quickly remove moisture from the slurry, effectively reducing the moisture content of the precursor powder. Furthermore, the high speed can spin the slurry into smaller particles, achieving precise granulation and significantly narrowing the particle size distribution range. This results in all particle size indicators of the precursor powder being far superior to those of Comparative Example 3, ensuring particle uniformity and creating favorable conditions for the uniform growth of the material crystal form during subsequent sintering. Test Example 3

[0045] XRD analysis was performed on the sodium ferric pyrophosphate prepared in the examples and comparative examples, and the results are as follows: Figure 1 As shown in Table 3: Table 3

[0046] from Figure 1As shown in Table 3, the impurity peak intensities in Example 1 were all low, while the main peak intensity was significantly higher, with impurity peak proportions all <5%. In contrast, all comparative examples, due to deviations from the optimized process parameters, exhibited varying degrees of increased impurity peak proportions and decreased main peak intensity, resulting in compromised crystal structure purity and integrity. Specifically, Comparative Example 1, employing a single rotation speed in the coarse grinding, intermediate grinding, and fine grinding stages, could not adapt to the particle refinement requirements of different stages, leading to uneven particle dispersion and insufficient crystallization. The impurity peak intensity rose to 42, while the main peak intensity dropped to 352, with an impurity peak proportion of 10.65%. Comparative Example 2, using only sodium polycarboxylate as a dispersant throughout the process, could not suppress the agglomeration caused by increased particle surface energy during the intermediate grinding and fine grinding stages. The impurity peak proportion was 12.64%, higher than in the examples. This was due to the low slurry dispersion, resulting in particles... Agglomeration prevents the exhaust gas from being fully discharged at the predetermined temperature plateau, leading to the formation of impurity phases at high temperatures, resulting in lattice defects or distortions. In Comparative Example 3, due to low inlet air temperature and insufficient rotation speed during spray drying, the precursor particles agglomerated again and had a high moisture content, affecting subsequent sintering and crystallization. The impurity peak accounted for 9.72%. In subsequent electrical performance tests, due to uneven particle size distribution and large specific surface area, it was difficult to mix evenly with conductive agents and binders such as carbon black and PVDF during coating. Powder and slag shedding occurred during the tack electrode test, resulting in poor electrical performance. Comparative Example 4 used static sintering in a roller kiln, resulting in uneven heating of the material and accumulation of waste gas. This prevented the complete removal of impurities and moisture from the surface sample, interfering with crystal growth and causing the proportion of impurity peaks to soar to 12.26%. Comparative Example 5 used nitrogen gas instead of argon in the high-temperature zone, resulting in insufficient inert protection and the introduction of impurity phases during high-temperature crystallization, with the proportion of impurity peaks reaching 12.27%. Comparative Example 6 did not have first and second temperature zones, and the incomplete removal of moisture and organic impurities directly affected crystal growth, with the proportion of impurity peaks reaching as high as 15.00%. Comparative Example 7 was affected by sodium polycarboxylate. Excessive addition, exceeding the adsorption limit on the particle surface, resulted in micellar bridging and a coexistence of soft and hard agglomerations, interfering with crystal growth and increasing the proportion of impurity peaks to 9.58%. In Comparative Example 8, sodium polycarboxylate was replaced with magnesium stearate. This dispersant has low solubility and uneven dispersion in the water-based sand milling system, and may leave residual magnesium ions after decomposition, generating impurity phases, with the proportion of impurity peaks reaching as high as 17.72%. In Comparative Example 9, the order of addition of the three dispersants was disrupted, leading to the failure of dispersion effects at each stage, severe particle agglomeration, and an impurity peak proportion of 13.17%. In summary, the staged sand milling process, the specific dispersant combination, the multi-stage rotary kiln sintering, and the dynamic atmosphere control scheme adopted in the examples can reduce particle agglomeration from the source, ensure the uniformity and protection of the sintering process, and thus significantly improve the crystal structure purity of sodium iron pyrophosphate, inhibit the formation of impurity phases, and effectively ensure the high purity and integrity of the crystal structure. Test Example 4

[0047] The sodium iron pyrophosphate prepared in the examples and comparative examples was used to make a coin cell. The specific steps included: sodium iron pyrophosphate, conductive agent Super P, and binder PVDF were mixed in a mass ratio of 8:1:1, and N-methylpyrrolidone (NMP) was added to prepare a uniform slurry. The slurry was uniformly coated on the surface of aluminum foil, dried under vacuum at 120°C for 5 hours, and then stamped to form a positive electrode sheet. Using sodium metal as the counter electrode, glass fiber as the separator, and 1 mol / L NaPF6-EC / DMC (volume ratio 1:1) as the electrolyte, a CR2032 type coin cell was assembled in an argon glove box.

[0048] (1) Charge-discharge specific capacity: In a constant temperature chamber at 25℃, first charge at a constant current of 0.1C to 3.8V, then charge at a constant voltage of 3.8V until the current is less than 0.02C and stop, then discharge at a constant current of 0.2C to 2.0V. Record the charge specific capacity, discharge specific capacity and efficiency of this cycle; in the second cycle, charge at a constant current of 0.5C to 3.8V, then charge at a constant voltage of 3.8V until the current is less than 0.02C and stop, then discharge at a constant current of 0.5C to 2.0V. Record the discharge specific capacity of this cycle; in the third cycle, charge at a constant current of 1.0C to 3.8V, then charge at a constant voltage of 3.8V until the current is less than 0.02C and stop, finally discharge at a constant current of 1.0C to 2.0V. Record the discharge specific capacity of this cycle; (2) Cyclic stability: In a 25℃ constant temperature chamber, the test conditions are 1.0C rate and voltage range of 2.0V-3.8V. The battery is subjected to 50 cycles of charge and discharge test. The charge and discharge process of each cycle refers to the charge and discharge operation of the corresponding rate (constant current charging to 3.8V, then constant voltage charging to the current less than 0.02C, and then constant current discharging to 2.0V). The capacity retention rate after 50 cycles is calculated by recording the change of the battery's discharge specific capacity during the cycle. Table 4 shows the final measured relevant performance: Table 4

[0049] As can be seen from Table 4, the electrochemical performance of the examples is excellent, with the initial charge capacity all above 117mAh / g, the initial discharge capacity not less than 107.5mAh / g, the 1C / 0.1C capacity ratio reaching 93.5%-96.5%, and the capacity retention rate after 50 cycles all ≥99.0%.

[0050] Comparing Example 1 and Comparative Example 1, it can be seen that the initial charge capacity, initial discharge capacity 1C / 0.1C capacity ratio, and 50-cycle retention rate of Comparative Example 1 are all significantly lower than those of Example 1. This is because Comparative Example 1 uses a single rotation speed in the coarse grinding, intermediate grinding, and fine grinding stages, which cannot adapt to the particle refinement requirements of different stages. This results in large particles not being fully broken down and agglomeration, poor slurry dispersibility, a large particle size range, obstructed sodium ion diffusion paths, and numerous lattice defects, affecting the electrochemical activity and ion transport efficiency of the material, ultimately leading to a decline in various performance indicators.

[0051] Comparing Example 1 and Comparative Example 2, it can be seen that the initial charge capacity, initial discharge capacity, 1C / 0.1C capacity ratio, and 50-cycle retention rate of Comparative Example 2 are all lower than those of Example 1. This is because Comparative Example 2 uses only sodium polycarboxylate as a dispersant throughout the process. This dispersant cannot adapt to the characteristics of increased particle surface energy and smaller particle size in the transition grinding and fine grinding stages, making it difficult to suppress the agglomeration phenomenon in the corresponding stages. This leads to a surge in slurry viscosity, uneven particle distribution, low compaction density after electrode coating, insufficient electrolyte wetting, and hindered sodium ion intercalation and deintercalation, thereby affecting the electrochemical performance of the material.

[0052] Comparing Example 1 and Comparative Example 3, it can be seen that all indicators of Comparative Example 3 are significantly lower than those of Example 1. This is because the spray drying in Comparative Example 3 uses a low inlet air temperature and a low atomization speed, resulting in a slow drying rate of the slurry. Moisture cannot be removed quickly, and the moisture content of the precursor powder is as high as 5.68%. Furthermore, there is severe secondary agglomeration of particles and uneven particle size distribution. During subsequent sintering, moisture and agglomerated particles affect the sufficiency of crystal growth, increasing the proportion of impurity phases to 9.72%, reducing the proportion of active ingredients, and causing uneven interparticle spacing and hindered ion diffusion, ultimately leading to a significant decline in electrochemical performance.

[0053] Comparing Example 1 and Comparative Example 4, it can be seen that the initial charge capacity, initial discharge capacity, 1C / 0.1C capacity ratio, and 50-cycle retention rate of Comparative Example 4 are all lower than those of Example 1. This is because Comparative Example 4 uses static sintering in a roller kiln, resulting in uneven temperature distribution. The materials in the upper and lower layers of the sagger, as well as the center and surrounding areas, are heated unevenly, leading to incomplete crystallization and the formation of more impurities. At the same time, waste gas tends to accumulate in the upper part, and impurities and moisture cannot be fully removed, further affecting the integrity of the crystal form.

[0054] Comparing Example 1 and Comparative Example 5, it can be seen that the initial charge capacity, initial discharge capacity, 1C / 0.1C capacity ratio, and 50-cycle retention rate of Comparative Example 5 are all lower than those of Example 1. This is because Comparative Example 5 uses only a nitrogen atmosphere in the third sintering temperature zone, while argon has a higher inert strength than nitrogen. A single nitrogen atmosphere cannot form a dense protective film during the high-temperature crystallization stage, resulting in the formation of impurity phases (impurity peaks account for 12.27%) when the material comes into contact with oxygen, affecting the phase purity of the material.

[0055] Comparing Example 1 and Comparative Example 6, it can be seen that all indicators of Comparative Example 6 are significantly lower than those of Example 1. This is because Comparative Example 6 did not have a first and second temperature zone, making it impossible to fully remove free and bound water from the precursor, and also impossible to improve conductivity through nitrogen doping; at the same time, rapid heating led to an increase in lattice defects, with the impurity phase accounting for as high as 15.00%, resulting in poor material structural stability. During charge and discharge, the material's volume expansion and contraction were not coordinated, making the particles prone to cracking and pulverization, hindering ion transport, and ultimately leading to a significant decline in capacity and cycle performance.

[0056] Comparing Example 1 and Comparative Example 7, it can be seen that the initial charge capacity, initial discharge capacity, 1C / 0.1C capacity ratio, and 50-cycle retention rate of Comparative Example 7 are significantly lower than those of Example 1. This is because the amount of sodium polycarboxylate added in Comparative Example 7 is excessive, far exceeding the adsorption limit on the particle surface, resulting in micellar bridging effect and a phenomenon of "coexistence of soft and hard agglomeration," leading to uneven particle distribution. During sintering, excessive dispersant leads to an increase in carbon content, severely interfering with crystal growth, and the proportion of impurity peaks rises to 9.58%. The uneven electrode structure causes local stress concentration during charging and discharging, hindering ion transport and thus reducing the electrochemical performance of the material.

[0057] Comparing Example 1 and Comparative Example 8, it can be seen that the electrochemical performance of Comparative Example 8 is significantly degraded, both being far lower than that of Example 1. This is because Comparative Example 8 replaced sodium polycarboxylate with magnesium stearate. Magnesium stearate is an oil-soluble fatty acid salt, which has low solubility and uneven dispersion in a water-based sand milling system. The steric hindrance of the fatty acid chain alone cannot form an effective electrostatic repulsion layer, easily forming "water-in-oil" agglomerates with uneven particle distribution. Furthermore, its decomposition temperature is relatively high, leaving trace amounts of magnesium ions after decomposition. These ions react with phosphate ions to form an electrochemically inactive magnesium phosphate impurity phase, reducing the actual capacity and hindering sodium ion insertion / extraction, ultimately leading to the deterioration of electrochemical performance.

[0058] Comparing Example 1 and Comparative Example 9, it can be seen that the initial charge capacity, initial discharge capacity, 1C / 0.1C capacity ratio, and 50-cycle retention rate of Comparative Example 9 are all lower than those of Example 1. This is because Comparative Example 9 disrupted the order of addition of the three dispersants. Sodium polycarboxylate failed to quickly disperse large particles during the coarse grinding stage, polyvinyl alcohol failed to suppress the loose agglomeration during the transition grinding stage, and polyethylene glycol failed to stabilize the ultrafine particles during the fine grinding stage, resulting in poor slurry dispersibility, a large particle size range, and uneven particle distribution. Subsequently, after sintering, the proportion of impurity phase reached 13.17%, the electrode coating thickness was uneven, the electrolyte wetting was insufficient, and the sodium ion diffusion path was inconsistent, ultimately leading to a significant decrease in rate performance and cycle stability.

[0059] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing highly dispersed gradient crystallized sodium ferric pyrophosphate, characterized in that, Includes the following steps: S1. Mix ferric phosphate, sodium source, carbon source and water evenly to obtain the initial slurry; S2. Under the action of sodium polycarboxylate, the initial slurry described in S1 is coarsely ground to obtain a coarsely ground slurry; S3. Under the action of polyvinyl alcohol, the coarse grinding slurry described in S2 is subjected to transition grinding to obtain a transition grinding slurry; S4. Under the action of polyethylene glycol, the transition slurry described in S3 is finely ground to obtain a finely ground slurry with a viscosity of 60 mPa·s-110 mPa·s; the molecular weight of the polyethylene glycol is 500-800. S5. Spray dry the finely ground slurry described in S4 to obtain the NFPP spray precursor; S6. The NFPP spray precursor described in S5 is sintered in a multi-stage rotary kiln to obtain the highly dispersed gradient crystallized sodium iron pyrophosphate.

2. The method for preparing highly dispersed gradient crystallized iron sodium pyrophosphate according to claim 1, characterized in that, In S1, the sodium source is selected from one or more of sodium carbonate, sodium dihydrogen phosphate, sodium nitrate, and sodium bicarbonate; And / or, the carbon source is selected from one or more of sucrose, polyethylene glycol, polyacrylic acid, glucose, fructose, melamine, acetylene black and carbon nanotubes; And / or, the mass ratio of the iron phosphate, sodium source, carbon source and water is (8-13):(1.2-1.8):(0.8-1.4):

32.

3. The method for preparing highly dispersed gradient crystallized iron sodium pyrophosphate according to claim 1, characterized in that, In S2, the process parameters for coarse grinding are as follows: the grinding media are zirconia beads with a diameter of 0.8 mm to 1.2 mm, the rotation speed is 700 r / min to 950 r / min, and the time is 50 min to 90 min.

4. The method for preparing highly dispersed gradient crystallized iron sodium pyrophosphate according to claim 1, characterized in that, In S2, the sodium polycarboxylate is added 5-20 minutes after the start of coarse grinding, and the amount added is 0.3wt%-0.5wt% of the initial slurry.

5. The method for preparing highly dispersed gradient crystallized sodium ferric pyrophosphate according to claim 1, characterized in that, In S3, the process parameters for the transition mill are: the grinding media are zirconia beads with a diameter of 0.4 mm to 0.6 mm, the rotation speed is 1100 r / min to 1400 r / min, and the time is 30 min to 40 min.

6. The method for preparing highly dispersed gradient crystallized sodium ferric pyrophosphate according to claim 1, characterized in that, In S3, the polyvinyl alcohol is added 8-12 minutes after the start of the transition mill, and the amount added is 0.08wt%-0.12wt% of the initial slurry.

7. The method for preparing highly dispersed gradient crystallized sodium ferric pyrophosphate according to claim 1, characterized in that, In S4, the process parameters for fine grinding are as follows: the grinding media are zirconia beads with a diameter of 0.15mm-0.25mm, the rotation speed is 1550r / min-1750r / min, and the time is 80min-100min.

8. The method for preparing highly dispersed gradient crystallized iron pyrophosphate sodium according to claim 1, characterized in that, In S4, the polyethylene glycol is added 10-20 minutes after the start of fine grinding, and the amount added is 0.06wt%-0.1wt% of the initial slurry.

9. The method for preparing highly dispersed gradient crystallized iron sodium pyrophosphate according to claim 1, characterized in that, In S5, the process parameters for spray drying are: inlet air temperature of 220℃-250℃, outlet air temperature of 95℃-105℃, rotation speed of 19000r / min-21000r / min, and feed rate of 95mL / min-105mL / min.

10. The method for preparing highly dispersed gradient crystallized iron sodium pyrophosphate according to claim 1, characterized in that, In S6, the sintering speed is 0.5 r / min-2 r / min, and four temperature zones are set, with the following process parameters for each zone: The temperature of the first temperature zone is 160℃-200℃, the holding time is 30min-40min, the nitrogen atmosphere is 110L / min-130L / min; The temperature in the second temperature zone is 300℃-350℃, the holding time is 380min-450min, the nitrogen atmosphere is 110L / min-130L / min; The temperature in the third temperature zone is 540-590℃, the holding time is 520min-600min, the atmosphere is argon, and the argon flow rate is 150L / min-170L / min; The fourth temperature zone is for cooling down to below 100℃, with a cooling time of 200-300 minutes, under a nitrogen atmosphere, with a nitrogen flow rate of 95-105 L / min.

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