High specific surface area iron phosphate, its preparation methods, and applications

Abstract

This invention belongs to the field of battery materials technology, and discloses high specific surface area iron phosphate, its preparation method, lithium iron phosphate, and batteries. The specific surface area of ​​the iron phosphate is greater than 8.5 m². 2 / g, with a primary particle size of 50~150nm. First, amorphous iron phosphate is prepared; an inorganic acid is added to the slurry of amorphous iron phosphate and water for pre-crystallization, followed by the addition of ammonium salt for crystallization; the crystallized slurry undergoes solid-liquid separation, and the resulting solid phase is washed, dried, and dehydrated to obtain iron phosphate with a high specific surface area. The iron phosphate provided by this invention has a high specific surface area, good crystallinity, and stable crystal form, which can ensure the stability of the structure of lithium iron phosphate obtained in subsequent production. Iron phosphate can be further prepared into lithium iron phosphate, and lithium iron phosphate can improve the electrical performance of batteries.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to iron phosphate, its preparation method, and its applications. Background Technology

[0002] Among lithium-ion battery cathode materials, lithium iron phosphate (LFP) has become a research hotspot due to its advantages such as non-toxicity, low cost, and good safety. With the development of new energy power generation and storage, and home energy storage applications, the cost and long-cycle performance advantages of LFP are becoming increasingly prominent, leading to its widespread use in automotive and large-scale energy storage applications. The particle size and morphology of LFP play a crucial role in its performance. Existing ammonia-based LFP preparation processes suffer from drawbacks such as large particle size, uneven particle distribution, and low specific surface area. The particle size and specific surface area of ​​LFP directly affect the specific surface area of ​​lithium iron phosphate, which in turn affects the battery's energy density, rate capability, and cycle performance. Cathode materials with a large specific surface area result in better rate characteristics. Therefore, researching LFP with uniform particle distribution and high specific surface area, and its preparation process, has significant application value and research importance. Summary of the Invention

[0003] In view of the problems existing in the prior art, the main objective of this invention is to provide a high specific surface area iron phosphate and its preparation method.

[0004] To achieve the above objectives, the present invention provides the following specific technical solutions.

[0005] First, the present invention provides a high specific surface area iron phosphate, wherein the specific surface area of ​​the iron phosphate is greater than 8.5 m². 2 / g, the primary particle size is 50~150nm, and the secondary particles are in the form of sticky rods or flakes.

[0006] In a further preferred embodiment, the crystallinity of the iron phosphate is 60-90%.

[0007] Secondly, this invention provides a method for preparing iron phosphate with a high specific surface area, comprising:

[0008] The phosphorus source solution and the iron source solution are mixed and reacted to obtain amorphous iron phosphate slurry;

[0009] Amorphous ferric phosphate slurry was subjected to solid-liquid separation to obtain amorphous ferric phosphate;

[0010] In the slurry re-pulped from amorphous ferric phosphate and water, inorganic acid is first added for pre-crystallization, and then ammonium salt is added for crystallization.

[0011] The crystallized slurry was subjected to solid-liquid separation treatment. The resulting solid phase was washed, dried and dehydrated to obtain ferric phosphate with a high specific surface area.

[0012] In a further preferred embodiment, the phosphorus source is at least one of phosphoric acid, monoammonium phosphate, diammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0013] In a further preferred embodiment, the iron source is at least one selected from ferrous sulfate, ferrous nitrate, iron powder, ferric chloride, and ferric oxide.

[0014] In a further preferred embodiment, the reaction temperature is 25~65℃ and the pH value is 1.4~2.2.

[0015] In a further preferred embodiment, the inorganic acid is one or both of phosphoric acid and sulfuric acid.

[0016] In a further preferred embodiment, an inorganic acid is added to the pulp re-pulped from amorphous ferric phosphate and water to adjust the pH of the system to 1.3 to 1.8.

[0017] In a further preferred embodiment, an inorganic acid is added and the system is heated to 65℃~85℃ for pre-crystallization.

[0018] In a further preferred embodiment, the ammonium salt is at least one of ammonium sulfate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0019] In a further preferred embodiment, the ammonium salt is ammonium sulfate; the amount of ammonium sulfate solution added is determined based on the molar ratio of iron in the slurry re-pulped from ammonium sulfate, amorphous ferric phosphate, and water being 0.01 to 0.1:1.

[0020] In a further preferred embodiment, the temperature at which ammonium salt is added for crystallization is higher than the pre-crystallization temperature, but does not exceed 100°C.

[0021] In a further preferred embodiment, the drying temperature is 60~180°C.

[0022] In a further preferred embodiment, the temperature of the dehydration treatment is 400~580℃.

[0023] Based on the same inventive concept, the present invention provides a lithium iron phosphate, which is prepared using the aforementioned high specific surface area iron phosphate.

[0024] The present invention also provides a battery comprising the aforementioned lithium iron phosphate.

[0025] The above-described one or more technical solutions of the present invention can achieve at least one of the following beneficial effects:

[0026] The iron phosphate particles provided by this invention are small, uniformly distributed, and have a high specific surface area, which is beneficial for obtaining lithium iron phosphate with a high specific surface area.

[0027] The iron phosphate provided by this invention has a high specific surface area, good crystallinity, and stable crystal form, which can ensure the stability of the lithium iron phosphate structure obtained in subsequent production.

[0028] The method for preparing high specific surface area iron phosphate according to the present invention is suitable for industrial mass production, and is simple to operate and highly adaptable to existing equipment and production lines.

[0029] The lithium iron phosphate prepared from iron phosphate provided by this invention is beneficial for improving the rate performance of batteries. Attached Figure Description

[0030] Figures 1-6 The images shown are SEM images of the iron phosphate obtained in Examples 1-6, respectively.

[0031] Figures 7-11 The images are SEM images of iron phosphate obtained from Comparative Examples 1 to 5.

[0032] Figures 12-15 The images shown are SEM images of ferric phosphate obtained in Examples 7-10. Detailed Implementation

[0033] Some embodiments of the present invention provide ferric phosphate with a high specific surface area, wherein the specific surface area of ​​the ferric phosphate is greater than 8.5 m². 2 / g, the primary particle size is 50~150nm, and the secondary particles are in the form of sticky rods or flakes.

[0034] Lithium iron phosphate (LiFePO4) has a high specific surface area, and lithium iron phosphate prepared using it as a precursor material also has a high specific surface area. The high specific surface area of ​​LiFePO4 materials improves the energy density, rate performance, and cycle performance of batteries.

[0035] The smaller the primary particle size of iron phosphate, the higher the grinding efficiency during the preparation of lithium iron phosphate. At the same time, the smaller the primary particles of iron phosphate, the smaller the primary particles of lithium iron phosphate prepared from it as a precursor material will also be. During battery operation, the lithium ion transport path becomes shorter, thereby improving ion transport efficiency and enhancing the battery's rate performance and low-temperature performance.

[0036] In some specific embodiments of the present invention, the crystallinity of the iron phosphate is 60-90%. Higher crystallinity results in a more stable iron phosphate crystal form and a more stable structure of the resulting lithium iron phosphate. The iron phosphate provided by the present invention has a specific surface area greater than 8.5 m². 2 / g, with a crystallinity of 60~90%, taking into account both specific surface area and crystallinity, the resulting product has good performance.

[0037] Currently, the main production process for ferric phosphate is a two-step method. First, a neutralization reaction between a phosphorus source and a ferric source produces amorphous ferric phosphate with low crystallinity, resulting in spherical primary particles. Then, phosphoric acid is added to induce crystal transformation and remove excess impurities, forming dihydrate ferric phosphate. Finally, dehydration produces anhydrous ferric phosphate, with spherical primary particles. However, during sintering, these particles agglomerate, forming porous aggregates with no specific morphology. This process typically results in severe material agglomeration, large particles, low specific surface area, and significant difficulty in subsequent crushing. In addition, there are methods to increase specific surface area by using surfactants during crystallization to inhibit particle agglomeration, and research on creating porous morphologies in ferric phosphate through artificial pores to improve specific surface area. However, the introduction of surfactants impacts subsequent wastewater treatment, and the artificial pore method is difficult to scale up to production volumes.

[0038] The commonly used process for synthesizing iron phosphate involves first generating amorphous iron phosphate yellow material, and then adding phosphoric acid for crystallization. Since phosphoric acid itself has a high viscosity, as phosphate ions are continuously added, the viscosity of the crystallization slurry gradually increases, resulting in poor dispersion of the generated iron phosphate crystal nuclei and larger aggregates of iron phosphate crystals. This easily leads to the formation of iron phosphate with large primary particles, uneven distribution, and low specific surface area.

[0039] To address the aforementioned deficiencies, the present invention proposes the following specific technical solutions.

[0040] Some embodiments of the present invention provide a method for preparing high specific surface area iron phosphate, comprising:

[0041] The phosphorus source solution and the iron source solution are mixed and reacted to obtain amorphous iron phosphate slurry;

[0042] Amorphous ferric phosphate slurry was subjected to solid-liquid separation to obtain amorphous ferric phosphate;

[0043] In the slurry re-pulped from amorphous ferric phosphate and water, inorganic acid is first added for pre-crystallization, and then ammonium salt is added for crystallization.

[0044] The crystallized slurry was subjected to solid-liquid separation treatment, and the resulting solid phase was washed, dried and dehydrated to obtain ferric phosphate with a high specific surface area.

[0045] During crystallization, phosphoric acid is added first, followed by ammonium salt. Phosphoric acid is adsorbed onto the surface of the amorphous particles. The addition of ammonium salt causes ammonium ions to complex with iron between the amorphous particles. The ammonium salt is at least one of ammonium sulfate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, with ammonium sulfate being preferred. The sulfate ions in ammonium sulfate can replace some of the phosphate ions, reducing the iron-to-phosphorus ratio and decreasing particle aggregation. During dehydration, sulfate and ammonium ions are burned off as gas, creating space that increases the specific surface area of ​​ferric phosphate. The positive pressure of the gas also helps to create pores, reducing particle aggregation and resulting in ferric phosphate with small, uniformly distributed particles and a high specific surface area.

[0046] In some specific embodiments, the phosphorus source is at least one selected from phosphoric acid, monoammonium phosphate, diammonium dihydrogen phosphate, and diammonium hydrogen phosphate. Those skilled in the art may also choose other phosphorus sources for synthesizing iron phosphate.

[0047] In some specific embodiments, the iron source is at least one selected from ferrous sulfate, ferrous nitrate, iron powder, ferric chloride, and ferric oxide. Those skilled in the art can also choose other iron sources for synthesizing ferric phosphate.

[0048] The concentrations of the phosphorus source solution and the iron source solution can be selected from the conventional concentrations, such as 3%~7% for the phosphorus source solution and 4%~7% for the iron source solution.

[0049] In some specific embodiments, the reaction temperature is 25~50℃, and the pH value is 1.5~2.1. When the reaction temperature increases, the movement rate of molecules or ions in the reaction system will accelerate, and various materials can be mixed more uniformly. However, if the synthesis temperature is too high, the activation energy on the surface of the synthesized amorphous iron phosphate particles will increase, and the particles will be more prone to agglomeration, which is not conducive to uniform particle growth. Furthermore, if the temperature is too high and the time is too long, the synthesized amorphous iron phosphate will directly transform into crystalline iron phosphate, resulting in iron phosphate with a high impurity content.

[0050] In some specific embodiments, the inorganic acid is one or both of phosphoric acid and sulfuric acid.

[0051] In some specific embodiments, the ammonium salt is at least one of ammonium sulfate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, and is more preferably ammonium sulfate.

[0052] In some specific embodiments, an inorganic acid is added to the slurry re-pulped from amorphous ferric phosphate and water to bring the pH of the system to 1.3-1.8. As the pH decreases, the system's supersaturation increases, accelerating crystal nucleation, but slowing crystal growth, resulting in smaller product particle size. If the pH is too low, some of the generated amorphous ferric phosphate will dissolve, increasing iron and phosphorus loss. If the pH is too high, the system's supersaturation decreases, reducing the crystal nucleation rate, resulting in larger product particle size, and the formation of basic ferric phosphate or some ferric hydroxide, leading to poor impurity levels in the final ferric phosphate product.

[0053] In some specific embodiments, the inorganic acid is added for more than 10 minutes, and then the system is heated to 65-85°C for pre-crystallization. If the inorganic acid is added for too short a time, there may be uneven bonding between phosphate and amorphous iron phosphate. The heating is to promote the reaction between phosphate and amorphous iron phosphate particles for pre-crystallization, but the temperature should not be too high, otherwise the slurry will crystallize and turn white. Ammonium sulfate is added before the slurry turns white and transforms into crystals.

[0054] In some specific embodiments, the amount of ammonium sulfate added is determined based on the molar ratio of iron in the slurry re-mixed with amorphous ferric phosphate and water, which is 0.01~0.1:1. As the amount of ammonium sulfate added increases, the viscosity of the slurry increases. Excessive ammonium sulfate addition leads to severe particle aggregation due to the increased viscosity affecting the dispersibility of amorphous ferric phosphate in the slurry. This hinders the reformation of the ferric phosphate crystal structure during crystallization, preventing normal whitening during crystallization. The product surface exhibits severe adhesion / agglomeration, resulting in a reduced specific surface area, significantly decreased crystallinity, and poor crystal structure stability. Some embodiments of the present invention achieve a relatively balanced state of specific surface area and crystallinity in ferric phosphate by controlling the amount of ammonium sulfate added.

[0055] In some specific embodiments, the crystallization temperature for adding ammonium sulfate is higher than the pre-crystallization temperature, but not higher than 100°C. When the crystallization temperature is too low, the movement rate of molecules or ions in the system is very slow, the average kinetic energy is small, the crystallization and whitening time is long, the crystal nuclei form slowly and in small numbers, resulting in large crystal grain size.

[0056] In some specific embodiments, the drying temperature is 60~180℃.

[0057] In some specific embodiments, the dehydration treatment temperature is 400~580℃. The dehydration temperature must ensure control of the moisture content in the ferric phosphate, and it should also be conducive to the formation of sulfur dioxide and the release of ammonia from sulfate and ammonium ions.

[0058] Furthermore, the present invention also provides lithium iron phosphate prepared by the aforementioned iron phosphate, and a battery using the obtained lithium iron phosphate material as the positive electrode active material.

[0059] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0060] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0061] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0062] Example 1

[0063] (1) Preparation: Weigh 59.26g of monoammonium phosphate and add it to a beaker. Add 166.41g of water and stir to dissolve, thus preparing a 7% phosphorus content monoammonium phosphate solution. Weigh 700g of ferrous sulfate solid and add 800g of deionized water. Stir for 10 minutes to pre-dissolve. Then add 5% of the total weight of concentrated sulfuric acid to prepare a solution with an iron concentration of about 7%. Then add 43.27g of 25% hydrogen peroxide to oxidize and obtain a ferric sulfate solution.

[0064] (2) Synthesis: 50g of water was added to the flask as the base liquid, and a rapid stirring was started at a stirring frequency of 1200r / min. Ferric sulfate solution, monoammonium phosphate solution and ammonia water were added to the flask in parallel using a peristaltic pump, with the addition time controlled at 30min. The synthesis temperature was 45℃ and the pH value was 1.8. The reaction was then continued for another 30min. After the reaction was completed, a yellow material was obtained. The yellow material was subjected to pressure filtration to wash the solid phase, and the washed yellow material was obtained.

[0065] (3) Crystallization: The washed yellow material is re-watered and slurried to disperse it. The dispersed slurry is added to a flask and a rapid stirring is started at a frequency of 1200 r / min. 5.19 g of phosphoric acid is added and the temperature is raised to 65°C. Stirring is continued for 15 min, and the pH value is measured and recorded. The pH is controlled within the range of 1.3-1.8. Then, 0.02 times the molar amount of iron in the slurry is added to ammonium sulfate solid. The temperature is raised to 85°C and stirring is continued. After the slurry turns white, the stirring frequency is adjusted to 500 r / min and the aging is continued for three hours. The reaction is then complete.

[0066] (4) After aging, the slurry is filtered, and the cake is washed twice with 750g of water. The filter cake is dried in an oven at 100℃ for 10h to obtain ferric phosphate dihydrate. The prepared ferric phosphate dihydrate is placed in a muffle furnace at 530℃ for 3h to dehydrate and obtain anhydrous ferric phosphate.

[0067] Example 2

[0068] The only difference between Example 2 and Example 1 is that:

[0069] In step (3) crystallization, the amount of ammonium sulfate added is 0.04 times the molar amount of iron in the slurry.

[0070] Example 3

[0071] The only difference between Example 3 and Example 1 is that:

[0072] In step (3) crystallization, the amount of ammonium sulfate added is 0.08 times the molar amount of iron in the slurry.

[0073] Example 4

[0074] The only difference between Example 4 and Example 1 is that:

[0075] In step (3) crystallization, the amount of ammonium sulfate added is 0.10 times the molar amount of iron in the slurry.

[0076] Example 5

[0077] The only difference between Example 5 and Example 1 is that:

[0078] In step (3) the crystallization step, ammonium dihydrogen phosphate is added.

[0079] Example 6

[0080] The only difference between Example 6 and Example 1 is that:

[0081] In step (3) the crystallization step, diammonium hydrogen phosphate is added.

[0082] Comparative Example 1

[0083] The only difference between Comparative Example 1 and Example 1 is that:

[0084] In step (3) of the crystallization process, no ammonium sulfate was added.

[0085] Comparative Example 2

[0086] The only difference between Comparative Example 2 and Example 1 is that:

[0087] In step (3) crystallization, the amount of ammonium sulfate added is 0.50 times the molar amount of iron in the slurry.

[0088] Comparative Example 3

[0089] The only difference between Comparative Example 3 and Example 1 is the crystallization step (3), which is as follows:

[0090] (3) Crystallization: The washed yellow material is re-watered and dispersed. The dispersed slurry is added to a flask and a rapid stirring is started at a frequency of 1200 r / min. Then, ammonium sulfate is added at a molar amount of iron in the slurry. The mixture is stirred for 10 min, and 5.19 g of phosphoric acid is added. The mixture is stirred for another 5 min. A sample is taken to measure and record the pH value. The pH is controlled within the range of 1.3-1.8. The mixture is stirred and the temperature is raised to 85℃. After the slurry turns white, the stirring frequency is adjusted to 500 r / min. The mixture is kept warm and aged for three hours. The reaction is then complete.

[0091] Comparative Example 4

[0092] The only difference between Comparative Example 4 and Example 1 is the crystallization step (3), which is as follows:

[0093] (3) Crystallization: The washed yellow material is re-watered and dispersed. The dispersed slurry is added to a flask and a rapid stirring is started at a frequency of 1200 r / min. Then, ammonium sulfate and 5.19 g of phosphoric acid, which are 0.02 times the molar amount of iron in the slurry, are added simultaneously. The pH value is measured and recorded after stirring for 5 min. The pH is controlled within the range of 1.3-1.8. Stirring is continued, and the temperature is raised to 85℃. After the slurry turns white, the stirring frequency is adjusted to 500 r / min and the aging is continued for three hours. The reaction is then complete.

[0094] Comparative Example 5

[0095] The difference between Comparative Example 5 and Example 1 is that ammonium sulfate was added during the synthesis of the yellow pigment. Specifically, steps (2) and (3) are different:

[0096] (2) Synthesis: 50g of water was added to the flask as the base liquid, and a rapid stirrer was started at a frequency of 1200r / min. Then, ammonium sulfate was added as in Example 1. Ferric sulfate solution, monoammonium phosphate solution, and ammonia water were added to the flask in parallel using a peristaltic pump, with the addition time controlled at 30min. The reaction was then continued for another 30min. After the reaction was completed, a yellow material was obtained. The yellow material was subjected to pressure filtration to wash the solid phase, and the washed yellow material was obtained.

[0097] (3) Crystallization: The washed yellow material is re-watered and dispersed. The dispersed slurry is added to a flask and a rapid stirring is started at a frequency of 1200 r / min. Then, 5.19 g of phosphoric acid is added and stirred for 5 min. The pH value is measured and recorded. The pH is controlled within the range of 1.3-1.8. Stirring continues, and the temperature is raised to 85℃. After the slurry turns white, the stirring frequency is adjusted to 500 r / min and the aging is continued for three hours. The reaction is then complete.

[0098] Figure 1-6 The images shown are SEM images of the iron phosphate obtained in Examples 1-6, respectively.

[0099] Figure 7-11 The images are SEM images of iron phosphate obtained from Comparative Examples 1-5.

[0100] The particle size of the primary ferric phosphate particles can be determined from the SEM images. Since the primary particles have a rod-like morphology, the particle size here refers to the longitudinal dimension of the particles.

[0101] The physicochemical characteristics of the obtained iron phosphate were further determined by the following methods.

[0102] Specific surface area: Referencing GB / T 13390-2008 Determination of specific surface area of ​​metal powders - Nitrogen adsorption method, the specific surface area was measured using a specific surface area analyzer.

[0103] Particle size distribution: Refer to HG / T 4701 Determination of particle size of iron phosphate for batteries 5.11; Mix the sample well, weigh 0.1g of the sample, place it in a 100mL clean beaker containing 50mL of water, sonicate for 15min, and then use a Bettersize2600 laser particle size analyzer wet method system for detection.

[0104] Crystallinity: After the sample was tested by an X-ray diffractometer, the XRD pattern was exported, and then the crystallinity data was obtained by processing it according to MDI Jade software.

[0105] pH value: Referencing GB / T 9724-2007 General Rules for pH Value Determination of Chemical Reagents, weigh 6g of sample, accurately place it in a beaker, add 54g of water, sonicate for 15min, and measure the pH value using a Mettler pH meter.

[0106] The results are shown in Table 1.

[0107] Table 1

[0108]

[0109] Without the addition of ammonium salts during crystallization (as shown in Comparative Example 1), the resulting ferric phosphate had a small specific surface area and high crystallinity. When a certain amount of ammonium sulfate / ammonium dihydrogen phosphate / diammonium hydrogen phosphate was added during crystallization, the specific surface area of ​​the resulting ferric phosphate was greatly increased, but the crystallinity decreased. Furthermore, with increasing amounts of ammonium sulfate (as shown in Examples 1-4 and Comparative Example 2), the specific surface area and crystallinity of the ferric phosphate showed a gradual decreasing trend. In other words, the appropriate amount of ammonium salt added is crucial for obtaining ferric phosphate with both high specific surface area and suitable crystallinity.

[0110] In the crystallization process, ammonium sulfate was added first, followed by phosphoric acid (as shown in Comparative Example 3). The resulting ferric phosphate had a smaller specific surface area, lower crystallinity, and uneven particle size distribution. This may be because: when ammonium sulfate is added first, the ammonium ions directly complex with the iron, and the effect of the subsequently added phosphoric acid is limited, preventing an increase in specific surface area and further increasing particle size. Furthermore, the SEM image of the ferric phosphate obtained in Comparative Example 3 shows particle agglomeration, with even larger secondary particles.

[0111] When ammonium sulfate and phosphoric acid are added simultaneously during crystallization (as shown in Comparative Example 4), the resulting ferric phosphate has a smaller specific surface area, little change in crystallinity, and uneven particle size distribution. This may be because: when phosphoric acid and ammonium sulfate are added to the slurry simultaneously, ammonium ions undergo a complexation reaction with iron, and sulfate ions are also adsorbed on the surface of amorphous ferric phosphate, reducing the binding sites between phosphate ions and amorphous ferric phosphate. Furthermore, since ammonium sulfate binds to amorphous ferric phosphate, after the amorphous ferric phosphate transforms into monoclinic dihydrate ferric phosphate, the ammonium ions are detached, meaning that ammonium sulfate does not play a role, and the specific surface area of ​​ferric phosphate cannot be increased.

[0112] Adding ammonium sulfate during the synthesis of ferric phosphate (as shown in Comparative Example 5) increases the specific surface area of ​​the resulting ferric phosphate to some extent compared to Comparative Example 1, but it is still relatively smaller than that of Examples 1-6. Adding ammonium sulfate during the synthesis of ferric phosphate increases the amount of ammonium in the mixture. The introduced sulfate ions are lost with byproducts during washing. During crystallization, because the ammonium ions are already attached to the surface of the amorphous ferric phosphate, the size of the seed crystals increases. The particle size does not decrease after crystallization. During sintering, the ammonium ions are burned off as gas, thus increasing the specific surface area, but the increase is limited.

[0113] In Examples 1-6, phosphoric acid was added first during crystallization, followed by ammonium sulfate / ammonium dihydrogen phosphate / diammonium hydrogen phosphate. Phosphoric acid initially adsorbs onto the surface of the amorphous particles. The subsequent addition of ammonium sulfate / ammonium dihydrogen phosphate / diammonium hydrogen phosphate causes ammonium ions to complex with iron between the nanoparticles. For ammonium sulfate, sulfate ions can also replace some phosphate ions, reducing the iron-to-phosphorus ratio and decreasing particle aggregation. During dehydration, sulfate and ammonium ions are burned off as gas, creating space that increases the specific surface area of ​​ferric phosphate. Furthermore, the positive pressure of the gas acts as a pore-forming agent, reducing particle aggregation and resulting in ferric phosphate with small, uniformly distributed particles and a high specific surface area.

[0114] Example 7

[0115] (1) Preparation: Weigh 68.03g of diammonium hydrogen phosphate and add it to a beaker. Add 159.88g of water and stir to dissolve, thus preparing a 7% phosphorus content diammonium phosphate solution. Weigh 700g of ferrous sulfate solid and add 800g of deionized water. Stir for 10 minutes to pre-dissolve. Then add 5% of the total weight of concentrated sulfuric acid to prepare a solution with an iron concentration of about 7%. Then add 43.27g of 25% hydrogen peroxide to oxidize and obtain a ferric sulfate solution.

[0116] (2) Synthesis: 50g of water was added to the flask as the base liquid, and a rapid stirring was started at a stirring frequency of 1200r / min. Ferric sulfate solution, diammonium phosphate solution and ammonia water were added to the flask in parallel using a peristaltic pump, with the addition time controlled at 30min. The synthesis temperature was 35℃ and the pH value was 1.9. The reaction was then continued for another 30min. After the reaction was completed, a yellow material was obtained. The yellow material was subjected to pressure filtration and the solid phase was washed to obtain the washed yellow material.

[0117] (3) Crystallization: The washed yellow material is re-watered and slurried to disperse it. The dispersed slurry is added to a flask and a rapid stirring is started at a frequency of 1200 r / min. 5.19 g of phosphoric acid is added, the temperature is raised to 70℃, and stirring is continued for 15 min. A sample is taken to measure and record the pH value. The pH is controlled within the range of 1.3-1.8. Then, ammonium sulfate is added at 0.01 times the molar amount of iron in the slurry. The temperature is raised to 90℃ and stirring is continued. After the slurry turns white, the stirring frequency is adjusted to 500 r / min and the aging is continued for three hours. The reaction is then complete.

[0118] (4) After aging, the slurry is filtered, and the filter cake is washed twice with 750g of water. The filter cake is dried in an oven at 120℃ for 9h to obtain ferric phosphate dihydrate. The prepared ferric phosphate dihydrate is placed in a muffle furnace at 540℃ for 3h to dehydrate and obtain anhydrous ferric phosphate.

[0119] Example 8

[0120] (1) Preparation: Weigh 59.26g of monoammonium phosphate and add it to a beaker. Add 260g of water and stir to dissolve, thus preparing a 5% monoammonium phosphate solution. Weigh 700g of ferrous sulfate solid and add 800g of deionized water. Stir for 10 minutes to pre-dissolve. Then add 5% of the total weight of concentrated sulfuric acid to prepare a solution with an iron concentration of about 7%. Then add 43.27g of 25% hydrogen peroxide to oxidize and obtain a ferric sulfate solution.

[0121] (2) Synthesis: 50g of water was added to the flask as the base liquid, and a rapid stirring was started at a stirring frequency of 1200r / min. Ferric sulfate solution, monoammonium phosphate solution and ammonia water were added to the flask in parallel using a peristaltic pump, with the addition time controlled at 30min, the synthesis temperature at 30℃, and the pH value at 2.0. The reaction was then continued for another 30min. After the reaction was completed, a yellow material was obtained. The yellow material was subjected to pressure filtration to wash the solid phase, and the washed yellow material was obtained.

[0122] (3) Crystallization: The washed yellow material is re-watered and slurried to disperse it. The dispersed slurry is added to a flask and a rapid stirring is started at a frequency of 1200 r / min. 5.19 g of phosphoric acid is added, the temperature is raised to 80℃, and stirring is continued for 15 min. A sample is taken to measure and record the pH value. The pH is controlled within the range of 1.3-1.8. Then, ammonium sulfate is added at 0.01 times the molar amount of iron in the slurry. The temperature is raised to 95℃ and stirring is continued. After the slurry turns white, the stirring frequency is adjusted to 500 r / min and the aging is continued for three hours. The reaction is then complete.

[0123] (4) Filter the aged slurry, wash the cake with 1000g of water twice; put the filter cake into an oven at 100℃ / 12h to dry it, and obtain ferric phosphate dihydrate. Put the prepared ferric phosphate dihydrate into a muffle furnace at 560℃ for 2.5h to dehydrate it, and obtain anhydrous ferric phosphate.

[0124] Example 9

[0125] (1) Preparation: Weigh 118.52g of monoammonium phosphate and add it to a beaker. Add 332.82g of water and stir to dissolve, thus preparing a 7% phosphorus content monoammonium phosphate solution. Weigh 1400g of ferrous sulfate solid and add 2240g of deionized water. Stir for 10min to pre-dissolve. Then add 9% of the total weight of concentrated sulfuric acid to prepare a solution with an iron concentration of about 6%. Then add 86.54g of 25% hydrogen peroxide to oxidize and obtain a ferric sulfate solution.

[0126] (2) Synthesis: 100g of water was added to the flask as the base liquid, and a rapid stirring was started at a stirring frequency of 1200r / min. Ferric sulfate solution, monoammonium phosphate solution and ammonia water were added to the flask in parallel using a peristaltic pump, with the addition time controlled at 30min, the synthesis temperature at 50℃, and the pH value at 1.7. The reaction was then continued for 45min. After the reaction was completed, a yellow material was obtained. The yellow material was subjected to pressure filtration, and the solid phase was washed to obtain the washed yellow material.

[0127] (3) Crystallization: The washed yellow material is re-watered and dispersed. The dispersed slurry is added to a flask and a rapid stirring is started at a frequency of 1200 r / min. 10.38 g of phosphoric acid is added, the temperature is raised to 85℃, and stirring is continued for 15 min. A sample is taken to measure and record the pH value. The pH is controlled within the range of 1.3-1.8. Then, ammonium sulfate is added at 0.01 times the molar amount of iron in the slurry. The temperature is raised to 90℃ and stirring is continued. After the slurry turns white, the stirring frequency is adjusted to 500 r / min and the aging is continued for three hours. The reaction is then complete.

[0128] (4) After aging, the slurry is filtered, and the cake is washed twice with 1200g of water. The filter cake is dried in an oven at 90℃ for 12h to obtain ferric phosphate dihydrate. The prepared ferric phosphate dihydrate is placed in a muffle furnace at 570℃ for 2h to dehydrate and obtain anhydrous ferric phosphate.

[0129] Example 10

[0130] (1) Preparation: Weigh 50.48g of phosphoric acid and add it to a beaker. Add 177.41g of water and stir to dissolve, thus preparing a 7% phosphorus content monoammonium phosphate solution. Weigh 89.92g of ferrous nitrate solid and add it to 420g of deionized water. Stir for 10 minutes to pre-dissolve. Then add 5% of the total weight of concentrated sulfuric acid to prepare a solution with an iron concentration of about 5%. Then add 43.27g of 25% hydrogen peroxide to oxidize and obtain a ferric sulfate solution.

[0131] (2) Synthesis: 50g of water was added to the flask as the base liquid, and a rapid stirring was started at a stirring frequency of 1200r / min. Ferric nitrate solution, phosphoric acid solution and ammonia water were added to the flask in parallel using a peristaltic pump, with the addition time controlled at 30min. The synthesis temperature was 55℃ and the pH value was 1.6. The reaction was then continued for another 30min. After the reaction was completed, a yellow material was obtained. The yellow material was subjected to pressure filtration to wash the solid phase, and the washed yellow material was obtained.

[0132] (3) Crystallization: The washed yellow material is re-watered and slurried to disperse it. The dispersed slurry is added to a flask and a rapid stirring is started at a frequency of 1200 r / min. 5.91 g of phosphoric acid is added, the temperature is raised to 75°C, and stirring is continued for 15 min. A sample is taken to measure and record the pH value. The pH is controlled at 1.6. Then, 0.015 times the molar amount of iron in the slurry is added to ammonium sulfate, the temperature is raised to 85°C, and stirring is continued. After the slurry turns white, the stirring frequency is adjusted to 500 r / min, and the aging is continued for 2 hours. The reaction is then complete.

[0133] (4) After aging, the slurry is filtered, and the cake is washed twice with 1000g of water. The filter cake is dried in an oven at 130℃ for 12h to obtain ferric phosphate dihydrate. The prepared ferric phosphate dihydrate is placed in a muffle furnace at 550℃ for 3h to dehydrate and obtain anhydrous ferric phosphate.

[0134] Figure 12-15 The images shown are SEM images of ferric phosphate obtained in Examples 7-10.

[0135] The relevant physicochemical properties of iron phosphate in Examples 7-10 are shown in Table 2.

[0136] Table 2

[0137]

[0138] Lithium iron phosphate materials were prepared by means of the iron phosphate obtained in Examples 1-10 and Comparative Examples 1-5 respectively:

[0139] (1) Weigh appropriate amounts of iron phosphate, lithium carbonate, glucose and PEG-2000 powder and add them to a sand mill jar. The molar ratio of iron phosphate and lithium carbonate is 1:1.029, and the mass of glucose and PEG-2000 is 8.5% and 2% of the mass of iron phosphate, respectively.

[0140] (2) Add an appropriate amount of deionized water and control the solid content to 50% for sand milling. The sand milling parameters are 2200 rpm for 3 hours.

[0141] (3) The slurry after sand milling is spray-dried, and the spray-dried powder is placed in a box furnace and sintered at 760°C for 8 hours under nitrogen atmosphere to obtain lithium iron phosphate powder.

[0142] The lithium iron phosphate materials obtained above are assembled into batteries in the following ways:

[0143] (1) The prepared lithium iron phosphate powder was added to a homogenizer along with PVDF and conductive carbon in a mass ratio of 8:1:1, and an appropriate amount of polyvinylpyrrolidone was added for homogenization.

[0144] (2) Apply the well-mixed slurry evenly onto the aluminum foil using a scraper, and then place it in a vacuum oven to dry overnight.

[0145] (3) The dried material is sliced ​​using a slicer to obtain a positive electrode sheet, which is then weighed and the loading of active material is calculated. The positive electrode shell, positive electrode sheet, UBE separator, lithium sheet, gasket, spring sheet and negative electrode shell are assembled in sequence, and an appropriate amount of TC-E201 electrolyte is added to them. The assembly is then placed in a tablet press for tablet pressing and encapsulation to obtain the button cell required for testing.

[0146] The assembled button cells were placed in a battery testing system for battery performance testing. The test voltage range was 2.5V to 3.75V, the test temperature was room temperature, and the charge and discharge capacity was tested at 0.2C, 0.5C, 1C, 2C, and 5C.

[0147] The performance of coin cells assembled from lithium iron phosphate prepared using the iron phosphates obtained in Examples 1-8 and Comparative Examples 1-5 is shown in Table 3 below. It can be seen that the lithium iron phosphate prepared using the pre-crystallization and crystallization processes of this invention can improve the battery's discharge specific capacity and rate performance.

[0148] Table 3

[0149]

[0150] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing high specific surface area iron phosphate, characterized in that, The specific surface area of ​​the iron phosphate is greater than 8.5 m². 2 / g, primary particles have a particle size of 50~150nm, and secondary particles are in the form of sticky rods or flakes; the crystallinity of the ferric phosphate is 60~90%; the preparation method of the ferric phosphate includes: Phosphorus source solution and iron source solution are mixed and reacted to obtain amorphous iron phosphate slurry; Amorphous ferric phosphate slurry was subjected to solid-liquid separation to obtain amorphous ferric phosphate; In the slurry re-pulped from amorphous ferric phosphate and water, inorganic acid is first added for pre-crystallization, and then ammonium salt is added for crystallization. The crystallized slurry was subjected to solid-liquid separation treatment. The resulting solid phase was washed, dried and dehydrated to obtain ferric phosphate with a high specific surface area.

2. The method for preparing high specific surface area iron phosphate as described in claim 1, characterized in that, The phosphorus source is at least one of phosphoric acid, monoammonium phosphate, diammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the iron source is at least one of ferrous sulfate, ferrous nitrate, iron powder, ferric chloride, and iron oxide; the inorganic acid is one or two of phosphoric acid and sulfuric acid; and the ammonium salt is at least one of ammonium sulfate, diammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

3. The method for preparing high specific surface area iron phosphate as described in claim 1, characterized in that, The reaction is carried out at a temperature of 25-65°C and a pH value of 1.4-2.

2.

4. The method for preparing high specific surface area iron phosphate as described in claim 1, characterized in that, Inorganic acid was added to the pulp of amorphous ferric phosphate and water to adjust the pH of the system to 1.3~1.8; after adding inorganic acid, the system was heated to 65℃~85℃ for pre-crystallization.

5. The method for preparing high specific surface area iron phosphate as described in claim 1, characterized in that, The ammonium salt is ammonium sulfate; the amount of ammonium sulfate added is determined based on the molar ratio of iron in the slurry re-pulped with ammonium sulfate, amorphous ferric phosphate, and water being 0.01 to 0.1:

1.

6. The method for preparing high specific surface area iron phosphate as described in claim 1 or 5, characterized in that, The temperature for crystallization by adding ammonium salt is higher than the pre-crystallization temperature, but not exceeding 100°C.

7. Ferric phosphate, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 6.

8. A lithium iron phosphate, characterized in that, It was prepared using the high specific surface area iron phosphate as described in claim 7 as a precursor.

9. A battery, characterized in that, Including the lithium iron phosphate as described in claim 8.

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