Iron phosphate, method of preparation and lithium iron phosphate

Abstract

The application discloses iron phosphate, a preparation method and lithium iron phosphate, and relates to the technical field of lithium ion batteries, and in particular to a preparation method of iron phosphate, which comprises the following steps: performing ultrasonic treatment on a mixed solution comprising a phosphorus source, an iron source, a doping element source and a first surfactant to obtain a pretreated solution; mixing the pretreated solution with seed crystals and a structure directing agent to obtain a to-be-reacted solution; sequentially performing a low-temperature aging reaction and a high-temperature crystallization reaction on the to-be-reacted solution to obtain a post-reaction solution; the high-temperature crystallization reaction is performed under microwave conditions; and sequentially performing washing, drying and heat treatment on a solid phase separated from the post-reaction solution to obtain the iron phosphate. The application promotes uniform nucleation, doping dispersion and complex coordination through ultrasonic cavitation; the low-temperature aging, seed crystals and the directing agent are combined to realize the guidance of the spherical configuration; the high-temperature microwave crystallization controls the preferred growth, and the iron phosphate with high sphericity, a narrow distribution, high tap density, uniform elements and good crystallization is obtained through the cooperation of the steps, and the cycle performance of the lithium battery positive electrode is significantly improved.

Description

Technical Field

[0001] This invention relates to the field of inorganic non-metallic materials technology, and more specifically, to iron phosphate, its preparation method, and lithium iron phosphate. Background Technology

[0002] As an important precursor for lithium iron phosphate cathode materials, the morphology, particle size distribution, crystallinity, and elemental composition of iron phosphate directly affect the final cathode material's compaction density, electrochemical performance, and processing performance. An ideal iron phosphate precursor should possess the following characteristics: regular spherical morphology, suitable particle size distribution range, high purity, and performance optimization achievable through controllable doping.

[0003] Currently, the industrial preparation of spherical iron phosphate mainly employs precipitation and hydrothermal / solvothermal methods. However, existing preparation technologies generally suffer from several problems that urgently need to be addressed, including low tap density, uneven distribution of dopant ions, and insufficient crystallinity.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide iron phosphate, a preparation method, and lithium iron phosphate, providing an iron phosphate material with high tap density, uniform elemental distribution, and high crystallinity, so as to improve the performance of lithium-ion battery cathode materials using it.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing ferric phosphate, comprising: A pretreated solution is obtained by ultrasonic treatment of a mixture containing a phosphorus source, an iron source, a dopant element source, and a first surfactant. The pretreatment solution is mixed with seed crystals and a structure guiding agent to obtain a reaction solution. The reaction solution is then subjected to a low-temperature aging reaction and a high-temperature crystallization reaction in sequence to obtain a post-reaction solution. The high-temperature crystallization reaction is carried out under microwave conditions. The solid phase separated from the reaction liquid is sequentially washed, dried, and heat-treated to obtain the iron phosphate.

[0007] In an optional embodiment, the molar ratio of Fe to P in the mixture is (0.9-1.1):1; And / or, the concentration of iron in the mixture is 0.5-2.0 mol / L; And / or, the concentration of phosphorus in the mixture is 0.5-2.5 mol / L; And / or, the doping element is selected from at least two of magnesium, aluminum, titanium and manganese; And / or, the dopant element source is selected from at least two of magnesium nitrate, magnesium acetate, aluminum nitrate, aluminum sulfate, titanium oxysulfate, tetrabutyl titanate, manganese sulfate, and manganese acetate; And / or, the molar ratio of the dopant element to the iron element is 0.5%-3%; And / or, the first surfactant is selected from at least one of anionic surfactants and nonionic surfactants; And / or, the mass fraction of the first surfactant in the mixture is 0.1%-2%.

[0008] In an optional embodiment, the ultrasonic treatment has a power of 500w-1500w, a frequency of 20kHz-40kHz, a duration of 10-60min, and a temperature of 30-60℃. And / or, the first surfactant is selected from at least one of sodium dodecyl sulfate and polyethylene glycol.

[0009] In an optional embodiment, the seed crystal is selected from at least one of ferric phosphate dihydrate and nano-sized amorphous ferric phosphate; And / or, the amount of seed crystals added is 0.5%-3% of the theoretical iron phosphate yield; And / or, the structure-directing agent is selected from cationic surfactants; And / or, the mass fraction of the structure-directing agent in the reaction solution is 0.05%-0.7%.

[0010] In an optional embodiment, the low-temperature aging reaction is carried out at a temperature of 90-120°C for a time of 30-90 minutes. And / or, the heating rate of the low-temperature aging reaction is 8-12℃ / min; And / or, the high-temperature crystallization reaction is carried out at a temperature of 150℃-200℃ for a time of 1-6 hours; And / or, the heating rate of the high-temperature crystallization reaction is 5-6℃ / min.

[0011] In an optional implementation, the microwave power is 300W-800W; And / or, the structure directing agent is selected from at least one of hexadecyltrimethylammonium bromide and octadecyldimethylbenzylammonium chloride.

[0012] In an optional embodiment, the washing includes alternating water washing and ethanol washing; And / or, the drying temperature is 80-120℃ and the time is 6-12h; And / or, the heat treatment temperature is 400℃-600℃, and the time is 2-6h; And / or, the heating rate of the heat treatment is 5-6 °C / min.

[0013] Secondly, the present invention provides ferric phosphate prepared by the method for preparing ferric phosphate described in the foregoing embodiments, wherein the ferric phosphate comprises spherical or near-spherical secondary particles.

[0014] In an optional embodiment, the particle size of the secondary particles is in the range of 5-10 μm; And / or, the tap density of the iron phosphate is ≥1.0 g / cm³. 3 ; And / or, the doping elements in the iron phosphate are present in the iron phosphate lattice in a substitutional or interstitial manner.

[0015] Thirdly, the present invention provides a lithium iron phosphate, which is prepared from the iron phosphate precursor described in the foregoing embodiments.

[0016] The present invention has the following beneficial effects: The method for preparing iron phosphate in this application achieves the following objectives through the local high temperature and pressure and strong shear force generated by ultrasonic cavitation effect: (1) breaking up agglomerated colloids or crystal nuclei, making the initial nucleation points smaller and more uniform; (2) strongly dispersing dopant ions and surfactant molecules, so that they achieve uniform distribution at the molecular level in the solution, laying the foundation for subsequent uniform doping and adsorption; (3) strengthening the mass transfer process, accelerating the coordination between iron source, phosphorus source and dopant ions, and forming a more stable precursor complex. In the low-temperature aging stage, the synergistic effect of seed crystal and structure guiding agent promotes the orderly assembly and epitaxial growth of surfactant on the surface of crystal nuclei, laying the foundation for spherical configuration. In the high-temperature crystallization stage, the microwave heating characteristics are used to achieve rapid and uniform heating, and the non-thermal effect is used to regulate the preferential growth of crystal faces. Combined with the selective adsorption of crystal faces by structure guiding agent, it drives the formation of dense and uniform spherical secondary particles. The steps in this application work together to ensure that the iron phosphate product has high sphericity, narrow particle size distribution, high tap density, uniform element distribution and good crystallinity, which significantly improves the performance of lithium-ion battery cathode materials, especially the cycle performance. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 The image shows the XRD pattern of the iron phosphate prepared in Example 1. Figure 2 Here is a SEM image of the iron phosphate prepared in Example 1; Figure 3 The XRD pattern of the iron phosphate prepared in Comparative Example 2; Figure 4 The image shows a SEM image of the iron phosphate prepared in Comparative Example 2. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0020] This invention provides a method for preparing ferric phosphate, comprising: A pretreated solution is obtained by ultrasonic treatment of a mixture containing a phosphorus source, an iron source, a dopant element source, and a first surfactant. The pretreatment solution is mixed with seed crystals and a structure guiding agent to obtain a reaction solution. The reaction solution is then subjected to a low-temperature aging reaction and a high-temperature crystallization reaction in sequence to obtain a post-reaction solution. The high-temperature crystallization reaction is carried out under microwave conditions. The solid phase separated from the reaction liquid is sequentially washed, dried, and heat-treated to obtain the iron phosphate.

[0021] In the preparation method of iron phosphate in this application, the local high temperature and high pressure and strong shear force generated by ultrasonic cavitation effect achieve the following objectives: (1) to break up agglomerated colloids or crystal nuclei, making the initial nucleation points smaller and more uniform; (2) to strongly disperse dopant ions and surfactant molecules, so that they achieve uniform distribution at the molecular level in the solution, laying the foundation for subsequent uniform doping and adsorption; (3) to strengthen the mass transfer process, accelerate the coordination between iron source, phosphorus source and dopant ions, and form a more stable precursor complex.

[0022] In the low-temperature aging stage, the synergistic effect of seed crystals and structure-directing agents promotes the orderly assembly and epitaxial growth of surfactants on the crystal nucleus surface, laying the foundation for spherical configuration. In the high-temperature crystallization stage, the heating characteristics of microwaves are used to achieve rapid and uniform heating of the reaction system, and the non-thermal effect of microwaves is used to regulate the preferential growth of crystal faces. Combined with the selective adsorption of crystal faces by structure-directing agents, dense and uniform spherical secondary particles are formed.

[0023] The coordinated steps in this application ensure that the iron phosphate product has high sphericity, narrow particle size distribution, high tap density, uniform elemental distribution, and good crystallinity, which can improve the performance of lithium-ion battery cathode materials using it, especially the improvement of cycle performance.

[0024] In an optional embodiment, the molar ratio of Fe to P in the mixture is (0.9-1.1):1, for example, 0.90:1, 0.94:1, 0.98:1, 1.02:1, 1.06:1, 1.10:1, 1.14:1, 1.18:1, 1.22:1, 1.26:1, or 1.30:1; this is close to the stoichiometric ratio, which helps to ensure the purity of the main phase and the integrity of the crystal structure of the product, and suppresses the formation of impurity phases.

[0025] In an optional embodiment, the concentration of iron in the mixture is 0.5-2.0 mol / L, for example 0.5 mol / L, 0.7 mol / L, 0.9 mol / L, 1.1 mol / L, 1.3 mol / L, 1.5 mol / L, 1.7 mol / L, 1.9 mol / L, 2.1 mol / L, 2.3 mol / L, or 2.5 mol / L.

[0026] In an optional embodiment, the concentration of phosphorus in the mixture is 0.5-2.5 mol / L, for example, 0.5 mol / L, 0.7 mol / L, 0.9 mol / L, 1.1 mol / L, 1.3 mol / L, 1.5 mol / L, 1.7 mol / L, 1.9 mol / L, 2.1 mol / L, 2.3 mol / L, and 2.5 mol / L. Regulating the concentrations of iron and phosphorus sources promotes uniform nucleation while avoiding localized and severe precipitation or aggregation due to excessively high concentrations.

[0027] In an optional embodiment, the doping element is selected from at least two of magnesium, aluminum, titanium and manganese.

[0028] In an optional embodiment, the dopant element source is selected from at least two of magnesium nitrate, magnesium acetate, aluminum nitrate, aluminum sulfate, titanium oxysulfate, tetrabutyl titanate, manganese sulfate, and manganese acetate.

[0029] Multi-element co-doping with at least two different metal elements allows dopant ions to be doped in the crystal lattice in a substitution or interstitial manner under the synergistic effect of ultrasonic pre-dispersion and microwave uniform heating, thereby synergistically controlling the crystal defect concentration, electron / ion transport channels and structural stability.

[0030] In an optional embodiment, the molar ratio of the dopant element to the iron element is 0.5%-3%, for example, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, or 3.0%; while ensuring that the dopant element effectively enters the iron phosphate lattice and plays a modifying role, excessive doping is avoided to prevent phase separation, increased lattice distortion, or precipitation of impurity phases.

[0031] In an optional embodiment, the first surfactant is selected from at least one of anionic surfactants such as sodium dodecyl sulfate and nonionic surfactants such as polyethylene glycol.

[0032] In an optional embodiment, the mass fraction of the first surfactant in the mixture is 0.1%-2%, for example 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.9%, or 2.0%.

[0033] By limiting the type and concentration range of the first surfactant and coordinating it with the ultrasonic treatment step, efficient dispersion of the reaction system can be achieved in the initial stage of the reaction.

[0034] In an optional embodiment, the power of the ultrasonic treatment is 500W-1500W, for example 500 W, 611 W, 722 W, 833 W, 944 W, 1056 W, 1167 W, 1278 W, 1389 W, 1500 W; the frequency is 20kHz-40kHz, for example 20.0 kHz, 22.2 kHz, 24.4 kHz, 26.7 kHz, 28.9 kHz, 31.1 kHz, 33.3 kHz, 35.6 kHz, 37.8 kHz, 40.0 kHz; the time is 10-60 min, for example 10 min, 16 min, 22 min, 28 min, 34 min, 40 min, 46 min, 52 min, 60 min; the temperature is 30-60℃, for example 30℃, 33℃, 36℃, 39℃, 42℃, 45℃, 48℃. ℃, 51 ℃, 54 ℃, 57 ℃, 60 ℃.

[0035] Under these conditions, ultrasonic treatment can generate moderate and stable local high temperature and high pressure and strong shear force through ultrasonic cavitation effect. This is sufficient to break the initial agglomerates and dissociate the weakly bound crystal nuclei, while avoiding excessive cavitation that could lead to precursor decomposition or destruction of ion coordination structure. The doped ions and surfactants achieve full dispersion and dynamic equilibrium in the solution, forming a homogeneous precursor complex system, and can maintain the stability of the reaction system.

[0036] In an optional embodiment, the seed crystal is selected from at least one of ferric phosphate dihydrate and nanoscale amorphous ferric phosphate.

[0037] In an optional embodiment, the amount of seed crystal added is 0.5%-3% of the theoretical iron phosphate yield, for example 0.5%, 0.8%, 1.1%, 1.4%, 1.7%, 2.0%, 2.3%, 2.6%, 2.9%, 3.0%.

[0038] It provides structure-matched and size-adapted crystallization templates, which can effectively guide epitaxial growth and suppress random nucleation.

[0039] In an optional embodiment, the structure directing agent is selected from cationic surfactants, such as hexadecyltrimethylammonium bromide and octadecyldimethylbenzylammonium chloride; the positively charged hydrophilic groups of the cationic surfactant can preferentially adsorb onto the negatively charged iron phosphate crystal nuclei to form a oriented and ordered interface layer, thereby selectively inhibiting the rapid growth of high-energy crystal planes, guiding isotropic stacking, and promoting the formation of spherical secondary particles.

[0040] In an optional embodiment, the mass fraction of the structure-directing agent in the reaction solution is 0.05%-0.7%, for example, 0.05%, 0.12%, 0.19%, 0.26%, 0.33%, 0.40%, 0.47%, 0.54%, 0.61%, 0.68%, or 0.70%. This is sufficient to form an effective coating on the crystal nucleus surface and play a role in crystal plane regulation, while avoiding excessive addition that could lead to excessive micelle solubilization, an excessively thick interfacial layer, or excessive residual carbon impurities.

[0041] In an optional embodiment, the temperature of the low-temperature aging reaction is 90-120℃, for example 90℃, 93℃, 96℃, 99℃, 102℃, 105℃, 108℃, 111℃, 114℃, 117℃, 120℃; and the time is 30-90min, for example 30 min, 37 min, 44 min, 51 min, 58 min, 65 min, 72 min, 79 min, 86 min, 90 min.

[0042] In an optional embodiment, the heating rate of the low-temperature aging reaction is 8-12℃ / min, for example 8.0℃ / min, 8.4℃ / min, 8.9℃ / min, 9.3℃ / min, 9.8℃ / min, 10.2℃ / min, 10.7℃ / min, 11.1℃ / min, 11.6℃ / min, or 12.0℃ / min.

[0043] The low-temperature aging stage promotes the activation of the seed crystal surface and the orderly assembly of the structure-directing agent, thus constructing a crystallization template with matching structure and stable interface for subsequent spherical growth.

[0044] In an optional embodiment, the temperature of the high-temperature crystallization reaction is 150℃-200℃, for example 150℃, 156℃, 162℃, 168℃, 174℃, 180℃, 186℃, 192℃, 198℃, 200℃; and the time is 1-6h, for example 1.0h, 1.6h, 2.2h, 2.8h, 3.4h, 4.0h, 4.6h, 5.2h, 6.0h.

[0045] In an optional embodiment, the heating rate of the high-temperature crystallization reaction is 5-6 °C / min, for example 5.0 °C / min, 5.1 °C / min, 5.2 °C / min, 5.3 °C / min, 5.4 °C / min, 5.5 °C / min, 5.6 °C / min, 5.7 °C / min, 5.8 °C / min, 5.9 °C / min, or 6.0 °C / min.

[0046] The high-temperature crystallization stage achieves full rearrangement of grains, densification of the lattice, and uniform growth of secondary particles under the drive of a microwave field, taking into account both crystal integrity and morphological regularity. The gradient design of the two-stage temperature control parameters forms a stepwise crystallization path of "configuration first, densification later", which is conducive to ensuring that iron phosphate has the characteristics of high tap density, uniform element distribution and high crystallinity.

[0047] In an optional implementation, the microwave power is 300W-800W, for example 300W, 356W, 411W, 467W, 522W, 578W, 633W, 689W, 744W, or 800W. This ensures that the reaction system receives sufficient and controllable energy input to achieve rapid and uniform bulk heating, promoting synchronous growth of crystal nuclei and orderly reconstruction of the crystal lattice. At the same time, it avoids excessive power from causing local overheating, violent boiling, or unexpected side reactions. This ensures that the non-thermal effects of microwaves (such as molecular polarization orientation and bond vibration activation) and thermal effects work synergistically for preferential growth of crystal faces and construction of spherical structures, improving process stability and product uniformity. This overcomes the problems of high energy consumption, long reaction time, large temperature gradient, and poor batch consistency of traditional heating methods (such as muffle furnaces, rotary kilns, and oil baths).

[0048] In an optional embodiment, the washing includes alternating water washing and ethanol washing.

[0049] In an optional embodiment, the drying temperature is 80-120℃, for example 80℃, 84℃, 88℃, 92℃, 96℃, 100℃, 104℃, 108℃, 112℃, 116℃, 120℃; and the time is 6-12h, for example 6.0h, 6.7h, 7.3h, 8.0h, 8.7h, 9.3h, 10.0h, 10.7h, 11.3h, 12.0h.

[0050] In an optional embodiment, the heat treatment temperature is 400℃-600℃, for example 400℃, 422℃, 444℃, 467℃, 489℃, 511℃, 533℃, 556℃, 578℃, 600℃; and the time is 2-6h, for example 2.0h, 2.4h, 2.9h, 3.3h, 3.8h, 4.2h, 4.7h, 5.1h, 5.6h, 6.0h.

[0051] In an optional embodiment, the heating rate of the heat treatment is 5-6℃ / min, for example 5.0℃ / min, 5.1℃ / min, 5.2℃ / min, 5.3℃ / min, 5.4℃ / min, 5.5℃ / min, 5.6℃ / min, 5.7℃ / min, 5.8℃ / min, 5.9℃ / min, or 6.0℃ / min.

[0052] Limitations on washing, drying, and heat treatment parameters facilitate efficient removal of impurities, stability of the ferric phosphate structure, purification of the crystal phase, and reduction of defects.

[0053] The present invention also provides an iron phosphate prepared by the iron phosphate preparation method described in the foregoing embodiments, wherein the iron phosphate comprises spherical or near-spherical secondary particles, which is beneficial to improving powder flowability, bulk density and subsequent electrode processing performance.

[0054] In an optional embodiment, the secondary particles have a particle size in the range of 5-10 μm and a narrow particle size distribution.

[0055] In an optional embodiment, the tap density of the iron phosphate is ≥1.0 g / cm³. 3 For example, 1.0 g / cm³, 1.2 g / cm³, 1.4 g / cm³, 1.6 g / cm³, and 1.8 g / cm³ reflect that the primary units inside the secondary particles are tightly packed and have moderate porosity.

[0056] In an optional embodiment, the doping elements in the iron phosphate exist in the iron phosphate lattice in a substitutional or interstitial manner, which helps to ensure the modification effect and improve structural stability.

[0057] The present invention also provides a lithium iron phosphate, which is prepared from the iron phosphate precursor described in the foregoing embodiments.

[0058] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0059] Example 1 This embodiment provides a method for preparing ferric phosphate, specifically including the following steps: 1. Prepare 500 ml of a 1.0 mol / L ferric nitrate nonahydrate solution. Prepare 500 ml of a 1.0 mol / L phosphoric acid solution. Slowly add the phosphoric acid solution dropwise to the ferric nitrate solution while stirring to form a basic suspension.

[0060] 2. Add a mixed aqueous solution of 0.01 mol magnesium nitrate (Mg doping amount is about 1% of Fe, doping amount is based on molar amount, the same below) and 0.005 mol titanium oxysulfate (Ti doping amount is about 0.5% of Fe) to the suspension, then add 1.0 g sodium dodecyl sulfate (SDS) and stir for 30 min.

[0061] 3. Place the above mixture in an ultrasonic treatment bath at 1000W power and 40KHz frequency, and ultrasonically treat for 30 minutes in a 50℃ water bath.

[0062] 4. After ultrasonication, transfer the mixture into the inner liner of a 2L microwave hydrothermal reactor, add 2.0g of iron phosphate dihydrate seed crystals and 0.5g of cetyltrimethylammonium bromide (CTAB), stir well and then seal.

[0063] 5. Place the reactor into the microwave synthesizer. Set the program: increase to 100℃ at 10℃ / min, hold for 60 minutes; then increase to 180℃ at 5℃ / min, microwave power 500W, hold for 240 minutes.

[0064] 6. After the reaction is complete and the mixture is cooled, centrifuge to collect the precipitate, wash it three times each with deionized water and ethanol, and dry it at 100℃ for 12 hours.

[0065] 7. The dried powder is heated to 500°C in air at a rate of 5°C / min and held for 4 hours. It is then cooled in the furnace to obtain the final product. The XRD pattern is shown below. Figure 1 As shown, the SEM image is as follows: Figure 2 As shown.

[0066] Example 2 This embodiment provides a method for preparing ferric phosphate, specifically including the following steps: 1. Same as step 1 in Example 1 2. Add a mixed aqueous solution of 0.015 mol aluminum nitrate nonahydrate (Al doping amount is about 1.5% of Fe) and 0.01 mol manganese acetate (Mn doping amount is about 1% of Fe) to the basic suspension, then add 2.0 g polyethylene glycol (PEG-1000) and stir for 30 min.

[0067] 3. Sonicate at 800W power and 30KHz frequency in a 40℃ water bath for 45 minutes.

[0068] 4. Add 1.5g of amorphous iron phosphate nanoparticles as seed crystals and 0.3g of octadecyl dimethyl benzyl ammonium chloride.

[0069] 5. Microwave hydrothermal program: First, raise the temperature to 90℃ at 10℃ / min and hold for 90min, then raise the temperature to 160℃ at 5℃ / min and hold for 180min. Microwave power: 400W.

[0070] 6. Post-processing is the same as in Example 1.

[0071] 7. Heat treatment conditions: Nitrogen atmosphere, heat to 550℃ at 5℃ / min and hold for 3 hours, then cool with the furnace to obtain the final product.

[0072] Example 3 This embodiment provides a method for preparing ferric phosphate, specifically including the following steps: 1. Prepare 500 ml of a 0.55 mol / L ferric nitrate nonahydrate solution. Prepare 500 ml of a 0.5 mol / L phosphoric acid solution. Slowly add the phosphoric acid solution dropwise to the ferric nitrate solution while stirring to form a basic suspension.

[0073] 2. Add a mixed aqueous solution of 0.01 mol magnesium nitrate (Mg doping amount is about 0.1% of Fe) and 0.005 mol titanium oxysulfate (Ti doping amount is about 0.4% of Fe) to the suspension, then add sodium dodecyl sulfate (SDS) to make the concentration 0.1%, and stir for 30 min.

[0074] 3. Place the above mixture in an ultrasonic treatment chamber and ultrasonically treat it for 10 minutes in a 30°C water bath at a power of 1500W and a frequency of 20KHz.

[0075] 4. After ultrasonication, transfer the mixture into the inner liner of a 2L microwave hydrothermal reactor, add 0.5% iron phosphate dihydrate seed crystals (based on the theoretical preparation of iron phosphate) and 0.5% hexadecyltrimethylammonium bromide (CTAB) (based on the total mass of the reaction system), stir evenly, and then seal.

[0076] 5. Place the reactor into the microwave synthesizer. Set the program: increase to 90°C at 8°C / min, hold for 90 minutes; then increase to 150°C at 5°C / min, microwave power 300W, hold for 360 minutes.

[0077] 6. After the reaction is complete and the mixture is cooled, centrifuge to collect the precipitate, wash it three times each with deionized water and ethanol, and dry it at 80°C for 12 hours.

[0078] 7. The dried powder is heated to 400°C in air at a rate of 5°C / min, held at that temperature for 6 hours, and then cooled in the furnace to obtain the final product.

[0079] Example 4 This embodiment provides a method for preparing ferric phosphate, specifically including the following steps: 1. Prepare 500 ml of a 2 mol / L ferric nitrate nonahydrate solution. Prepare 500 ml of a 2.2 mol / L phosphoric acid solution. Slowly add the phosphoric acid solution dropwise to the ferric nitrate solution while stirring to form a basic suspension.

[0080] 2. Add a mixed aqueous solution of 0.01 mol magnesium nitrate (Mg doping amount is about 1% of Fe) and 0.005 mol titanium oxysulfate (Ti doping amount is about 2% of Fe) to the suspension, then add sodium dodecyl sulfate (SDS) to make its concentration 1%, and stir for 30 min.

[0081] 3. Place the above mixture in an ultrasonic treatment chamber and ultrasonically treat it for 60 minutes at 500W power and 30KHz frequency in a 60℃ water bath.

[0082] 4. After ultrasonication, transfer the mixture into the inner liner of a 2L microwave hydrothermal reactor, add 3% iron phosphate dihydrate seed crystals (based on the theoretical preparation of iron phosphate) and 0.05% cetyltrimethylammonium bromide (CTAB) (based on the total mass of the reaction system), stir evenly, and then seal.

[0083] 5. Place the reactor into the microwave synthesizer. Set the program: increase to 120℃ at 12℃ / min, hold for 30 minutes; then increase to 200℃ at 5℃ / min, microwave power 800W, hold for 60 minutes.

[0084] 6. After the reaction is complete and the mixture is cooled, centrifuge to collect the precipitate, wash it three times each with deionized water and ethanol, and dry it at 120°C for 6 hours.

[0085] 7. The dried powder is heated to 600°C in air at a rate of 5°C / min, held for 2 hours, and then cooled in the furnace to obtain the final product.

[0086] Comparative Example 1 This embodiment provides a method for preparing iron phosphate that does not use ultrasound, microwave, or introduce doping elements, specifically including the following steps: 1. Prepare the same basic suspension as in Example 1.

[0087] 2. Add only 1.0 g SDS to the basic suspension, without adding any dopant, and stir for 30 min.

[0088] 3. No ultrasonic treatment is performed; mechanical stirring is carried out in a 50°C water bath for 1 hour.

[0089] 4. Transfer the mixture to a regular stainless steel high-pressure reactor without adding any special seed crystals or cationic surfactants.

[0090] 5. Place the reactor in an oven and statically heat it at 180°C for 12 hours.

[0091] 6. Post-processing is the same as in Example 1.

[0092] 7. The drying steps are the same as in Example 1.

[0093] Comparative Example 2 This comparative example provides a method for preparing ferric phosphate, which differs from Example 1 only in that the ultrasonic treatment in step 3 is omitted, and CTAB is not added in step 4. The XRD pattern of the prepared ferric phosphate is shown below. Figure 3 As shown, the SEM image is as follows: Figure 4 As shown.

[0094] Comparative Example 3 This comparative example provides a method for preparing ferric phosphate, which differs from Example 1 only in that ferric phosphate is not doped, and magnesium nitrate, titanium oxysulfate, and anionic surfactant are not added in step 2.

[0095] Comparative Example 4 This comparative example provides a method for preparing ferric phosphate, which differs from Example 1 only in that the ultrasonic treatment in step 3 is omitted and the ferric phosphate dihydrate seed crystals in step 4 are not added.

[0096] Comparative Example 5 This comparative example provides a method for preparing ferric phosphate, which differs from Example 1 only in that CTAB and ferric phosphate dihydrate seed crystals are not added in step 4.

[0097] Comparative Example 6 This comparative example provides a method for preparing iron phosphate, which differs from Example 1 only in that the high-temperature crystallization reaction in step 5 is not carried out under microwave conditions.

[0098] The test methods and results of the composition and some parameters of the iron phosphate prepared in the above embodiments and comparative examples are shown in Table 1.

[0099] Crystallinity: Using the original XRD file, import it into Jade for processing and fitting. After smoothing and finding the peaks, perform peak fitting, click report, and select the three strong peaks Jade software to calculate crystallinity.

[0100] Specific surface area: The surface area was dried at 120℃ for 2 hours using the Bestar specific surface area testing instrument, with a reference method and an air pressure of 0.35mA. After the equipment was processed, the specific surface area was tested.

[0101] D50, D100: The Malvern laser particle size analyzer was set to anhydrous iron phosphate refractive index, and the ultrasonic test was set to 30 seconds. After adding the sample to the instrument's test range, the test was clicked to obtain the D50 / D100 data.

[0102] TD: Measure 100g of sample with a graduated cylinder, put it into a tap density meter, vibrate for 4-5 minutes, and calculate TD using mass / volume.

[0103] Sphericity: Imagej was used for processing. Electron microscopy images were imported, and the particles were separated from the background by adjusting the threshold. The sphericity was obtained by measuring the ratio of the major axis to the minor axis (average value of multiple particles).

[0104] Table 1: Composition of battery-grade iron phosphate prepared in the examples

[0105] As can be seen from Table 1, the examples are superior to the comparative examples in key physical properties such as crystallinity, specific surface area, D50 / D100 stability, tap density (TD), and sphericity. This indicates that the iron phosphate prepared by the examples is more stable, more porous (facilitating grinding and with better activity), has better uniformity, improved compaction, and is closer to the ideal spherical morphology.

[0106] in addition, Figure 1 The iron phosphate in the sample has a higher main peak intensity, higher crystallinity, and no impurity peaks, indicating that its doped ions have entered the interior of the crystal lattice and that the doping is uniform and stable. Figure 3 Compared to crystallinity Figure 1 Lower. (Through) Figure 2 and Figure 4 The comparison shows that Figure 2 The iron phosphate in it has higher sphericity, is not a single spherical particle, has high consistency, and the primary particle distribution is more uniform.

[0107] Lithium iron phosphate was prepared using the iron phosphate prepared in the above embodiments and comparative examples as a precursor, and the batteries were assembled and their electrical performance was tested. The preparation of lithium iron phosphate, battery assembly, and testing methods are as follows, and the test results are shown in Table 2: Preparation of lithium iron phosphate: Lithium source (lithium carbonate), iron-phosphorus source (iron phosphate prepared in each example and comparative example) were weighed according to stoichiometric ratio, and 15wt% carbon source (glucose) was added. The mixture was ball-milled with anhydrous ethanol as the medium and sintered in two stages under inert gas (decomposition of precursor at 380℃ and high-temperature crystallization at 700℃) to obtain lithium iron phosphate cathode material.

[0108] Battery assembly: The lithium iron phosphate positive electrode material is slurried, PVDF binder is added, coated and dried on aluminum foil, and then punched into small round pieces by a punching machine to obtain the positive electrode sheet. The battery is then encapsulated in an inert environment (assembly sequence: negative electrode - spring sheet - gasket - lithium sheet, 2-3 drops of electrolyte are added to cover the separator, and finally the positive electrode sheet is placed in---electrolyte 1 mol / L lithium hexafluorophosphate dissolved in a mixed solvent of EC and DMC (volume ratio 1:1).

[0109] Powder compaction density: Tested using a UTM7305 compaction density tester according to the requirements of the national standard GB / T 30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries".

[0110] 0.1C initial charge capacity, 0.1C initial efficiency, and 0.5C capacity retention: The LAND battery testing system was used to test the battery according to the national standard GB / T 42161-2022 "Test Method for Initial Discharge Specific Capacity and Initial Charge-Discharge Efficiency of Lithium Iron Phosphate Electrochemical Performance".

[0111] Table 2: Performance Test Results of Lithium Iron Phosphate

[0112] In summary, the iron phosphate and its preparation method in this application have the following advantages: 1. Deep synergistic effect of multiple technologies: Ultrasonic treatment endows the system with initial homogeneity and highly reactive precursor complexes; microwave radiation provides a rapid and homogeneous bulk heating field, precisely driving crystallization kinetics; ion doping regulates defect structure and intrinsic electrochemical properties from within the crystal lattice; and surfactants, under the synergy of ultrasonic dispersion and microwave field, efficiently achieve crystal facet selective adsorption, morphology guidance and aggregation inhibition—the four technologies complement each other, are sequentially connected, and are coupled by field effects.

[0113] 2. Excellent overall performance of the product: The obtained iron phosphate has a highly regular spherical or near-spherical secondary particle morphology, high tap density, moderate specific surface area, uniform element distribution and good crystallinity. As a precursor, it can significantly improve the processing performance (such as electrode tap density) and electrochemical performance (such as cycle stability and capacity retention) of lithium iron phosphate cathode materials.

[0114] 3. High efficiency and energy saving: Microwave heating significantly shortens the hydrothermal crystallization and subsequent heat treatment cycle, improving energy utilization efficiency and unit production capacity; ultrasonic pretreatment enhances the homogeneity and reaction controllability of the system, reduces reaction process fluctuations, and significantly improves batch-to-batch consistency.

[0115] 4. High doping uniformity: Ultrasonic pre-dispersion ensures that doped ions are uniformly distributed at the molecular level at the beginning of the reaction; combined with the temperature field uniformity brought by microwaves, it effectively overcomes the concentration gradient problem under the traditional diffusion-dominated mechanism, fundamentally suppresses local enrichment and component segregation, and ensures the uniformity and stability of doping in space and structure.

[0116] 5. In-situ conductive coating effect: The surfactants and structure-directing agents used are all carbon-containing organic materials. During microwave heat treatment, they are simultaneously carbonized or converted into conductive polymers in situ, forming a uniform and shape-preserving conductive coating layer on the surface of iron phosphate particles. Compared with traditional external conductive agents or high-temperature sintering carbon coating processes, it has the advantages of coating integrity, interfacial compatibility and low energy consumption.

[0117] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing ferric phosphate, characterized in that, include: A pretreated solution is obtained by ultrasonic treatment of a mixture containing a phosphorus source, an iron source, a dopant element source, and a first surfactant. The pretreatment solution is mixed with seed crystals and a structure guiding agent to obtain a reaction solution. The reaction solution is then subjected to a low-temperature aging reaction and a high-temperature crystallization reaction in sequence to obtain a post-reaction solution. The high-temperature crystallization reaction is carried out under microwave conditions. The solid phase separated from the reaction liquid is sequentially washed, dried, and heat-treated to obtain the iron phosphate.

2. The method for preparing ferric phosphate according to claim 1, characterized in that, The molar ratio of Fe to P in the mixture is (0.9-1.1):1; And / or, the concentration of iron in the mixture is 0.5-2.0 mol / L; And / or, the concentration of phosphorus in the mixture is 0.5-2.5 mol / L; And / or, the doping element is selected from at least two of magnesium, aluminum, titanium and manganese; And / or, the dopant element source is selected from at least two of magnesium nitrate, magnesium acetate, aluminum nitrate, aluminum sulfate, titanium oxysulfate, tetrabutyl titanate, manganese sulfate, and manganese acetate; And / or, the molar ratio of the dopant element to the iron element is 0.5%-3%; And / or, the first surfactant is selected from at least one of anionic surfactants and nonionic surfactants; And / or, the mass fraction of the first surfactant in the mixture is 0.1%-2%.

3. The method for preparing ferric phosphate according to claim 1, characterized in that, The ultrasonic treatment has a power of 500w-1500w, a frequency of 20kHz-40kHz, a duration of 10-60min, and a temperature of 30-60℃. And / or, the first surfactant is selected from at least one of sodium dodecyl sulfate and polyethylene glycol.

4. The method for preparing ferric phosphate according to claim 1, characterized in that, The seed crystal is selected from at least one of ferric phosphate dihydrate and nano-sized amorphous ferric phosphate. And / or, the amount of seed crystals added is 0.5%-3% of the theoretical iron phosphate yield; And / or, the structure-directing agent is selected from cationic surfactants; And / or, the mass fraction of the structure-directing agent in the reaction solution is 0.05%-0.7%.

5. The method for preparing ferric phosphate according to claim 1, characterized in that, The low-temperature aging reaction is carried out at a temperature of 90-120℃ for a time of 30-90 minutes. And / or, the heating rate of the low-temperature aging reaction is 8-12℃ / min; And / or, the high-temperature crystallization reaction is carried out at a temperature of 150℃-200℃ for a time of 1-6 hours; And / or, the heating rate of the high-temperature crystallization reaction is 5-6℃ / min.

6. The method for preparing ferric phosphate according to claim 1, characterized in that, The microwave power is 300W-800W; And / or, the structure directing agent is selected from at least one of hexadecyltrimethylammonium bromide and octadecyldimethylbenzylammonium chloride.

7. The method for preparing ferric phosphate according to claim 1, characterized in that, The washing process includes alternating water washing and ethanol washing; And / or, the drying temperature is 80-120℃ and the time is 6-12h; And / or, the heat treatment temperature is 400℃-600℃, and the time is 2-6h; And / or, the heating rate of the heat treatment is 5-6 °C / min.

8. Ferric phosphate prepared by the method of any one of claims 1-7, characterized in that, The iron phosphate comprises spherical or near-spherical secondary particles.

9. The ferric phosphate according to claim 8, characterized in that, The particle size of the secondary particles is in the range of 5-10 μm; And / or, the tap density of the iron phosphate is ≥1.0 g / cm³. 3 ; And / or, the doping elements in the iron phosphate are present in the iron phosphate lattice in a substitutional or interstitial manner.

10. A lithium iron phosphate, characterized in that, It is prepared using the iron phosphate as a precursor as described in claim 8 or 9.

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