A ferric phosphate material, its preparation method and application

Abstract

This disclosure provides an iron phosphate material, its preparation method, and its applications. The method includes the following steps: mixing a solution of a temperature-sensitive polymer having an upper critical dissolution temperature, a solution containing an iron source, and a solution containing a phosphorus source to obtain a mixed solution; gelling the mixed solution, then adding an alkaline solution to adjust the pH to acidic, and heating to obtain a precipitate; aging the precipitate with a phosphoric acid solution to obtain a precursor material; and calcining the precursor material to obtain the iron phosphate material. This disclosure introduces a temperature-sensitive polymer to prepare a thermally responsive reversible sol-gel as a medium to subdivide and divert the alkaline precipitant, thereby achieving uniform diffusion of the precipitant, avoiding problems such as excessive local concentration and poor product uniformity, and inhibiting the movement of nanoparticles, thus preventing the aggregation of iron phosphate particles.

Description

Technical Field

[0001] This disclosure belongs to the field of iron phosphate materials technology, specifically relating to an iron phosphate material, its preparation method, and its application. Background Technology

[0002] Lithium-ion secondary batteries, as energy storage devices, possess advantages such as high capacity, high voltage, high cycle performance, and high energy density, thus finding widespread application in electric vehicles, smart grids, and portable electronic products. In the past two years, lithium iron phosphate (LFP) cathode materials have experienced rapid development and frequent upgrades, leading to a corresponding increase in the price of iron phosphate, a key raw material in their production. As one of the precursor materials for preparing LFP cathode materials, the morphology and quality of iron phosphate synthesis significantly affect the electrochemical performance of LFP battery systems.

[0003] Currently, the commonly used method for preparing ferric phosphate is coprecipitation, which belongs to the liquid-phase method. Coprecipitation not only has a short reaction time but also ensures uniform mixing of raw materials. Specifically, it involves adding a precipitant to completely precipitate the desired product precursor in the mixed solution. Most existing ferric phosphate preparation processes utilize alkaline solutions as precipitants to adjust the pH of the system. However, the addition of alkaline solutions often results in excessively high local concentrations and instantaneous nucleation, leading to excessively rapid crystal growth. Simultaneously, it can encapsulate surrounding impurity ions, causing them to precipitate as well, resulting in poor product uniformity.

[0004] To address the aforementioned issues, CN116216677A discloses a β-cyclodextrin / NaOH saturated solution. This solution utilizes the characteristic of slow and stable release of hydroxide ions, effectively avoiding instantaneous localized high concentrations and poor product uniformity. However, the above preparation process still requires freshly prepared saturated solution each time the pH is adjusted, thus extending the process time and increasing costs.

[0005] Therefore, there is an urgent need in this field to develop a sustainable and regenerative method to solve the problem of excessively high local concentrations of precipitants in coprecipitation methods. Summary of the Invention

[0006] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of the claims.

[0007] To address the shortcomings of existing technologies, the purpose of this disclosure is to provide an iron phosphate material, its preparation method, and its applications. This disclosure provides a method to solve the problem of excessively high local concentrations caused by the addition of a precipitant during the co-precipitation method for preparing iron phosphate materials. By introducing a temperature-sensitive polymer, a thermally responsive reversible sol-gel is prepared as a medium to finely separate and distribute the alkaline precipitant, thereby achieving uniform diffusion of the precipitant and avoiding the problems of excessively high local concentrations and poor product uniformity. Furthermore, it can inhibit the movement of nanoparticles, thus preventing the aggregation of iron phosphate particles.

[0008] To achieve this objective, the present disclosure adopts the following technical solution:

[0009] In a first aspect, this disclosure provides a method for preparing iron phosphate materials, the method comprising the following steps:

[0010] A solution of a thermosensitive polymer with an upper critical dissolution temperature, a solution containing an iron source, and a solution containing a phosphorus source are mixed to obtain a mixture.

[0011] The mixture was gelled, then an alkaline solution was added to adjust the pH to acidic, and a precipitate was obtained after heating.

[0012] The precipitate and phosphoric acid solution are aged to obtain a precursor material; the precursor material is then calcined to obtain the iron phosphate material.

[0013] This disclosure utilizes a temperature-sensitive polymer with an upper critical solution temperature (UCST) to rapidly prepare highly dispersible iron phosphate materials by combining sol-gel and co-precipitation methods. The UCST-type temperature-sensitive polymer exhibits reversible transitions between sol and gel at different temperatures; below the UCST temperature, it is a semi-solid gel, while above the UCST temperature, it becomes a highly fluid sol-liquid. When the iron phosphate preparation raw materials and the temperature-sensitive polymer are uniformly mixed to form a gel below the UCST temperature, the polymer possesses a porous structure and framework characteristics. The added alkaline precipitant can be subdivided and dispersed by the pores on the gel surface, thus achieving uniform diffusion of the precipitant and avoiding the formation of impurities due to excessively high local concentrations caused by the addition of precipitant in traditional methods. Upon further temperature increases, the gel transforms into a sol-liquid, and iron phosphate begins to co-precipitate.

[0014] Furthermore, the thermosensitive polymer exhibits increased deassociation and swelling upon heating, and the resulting sol-liquid polymer inhibits the movement of nanoparticles, thus preventing the aggregation of ferric phosphate particles. Existing sol-gel methods can prepare highly dispersed ferric phosphate, but require lengthy gel drying and subsequent calcination, thus prolonging the process time. Existing co-precipitation methods can quickly obtain ferric phosphate products but are prone to aggregation, and the addition of alkali can easily lead to excessively high local concentrations, resulting in uneven distribution and affecting the consistency and stability of the product morphology. Based on these considerations, this disclosure combines the advantages of both methods, and the thermosensitive polymer, due to the reversibility of the sol-gel process, can be recycled and reused, thereby reducing production costs.

[0015] In one embodiment, the thermosensitive polymer having an upper critical dissolution temperature includes an acrylic acid-acrylamide copolymer.

[0016] In one embodiment, the molar percentage of acrylamide structural units in the acrylic acid-acrylamide copolymer is 40%-45%, for example, it can be 40%, 41%, 42%, 43%, 44%, 45%, etc.

[0017] In this disclosure, by controlling the molar percentage of acrylamide structural units in the acrylic acid-acrylamide copolymer, the acrylic acid-acrylamide copolymer has a suitable UCST temperature range. If the content is too low, the UCST temperature of the acrylic acid-acrylamide will be low, which will increase the difficulty of gelation treatment, such as requiring it to be kept at a low temperature for a long time, and vice versa.

[0018] In one embodiment, the number-average molecular weight of the acrylamide structural units in the acrylic acid-acrylamide copolymer is 2 × 10⁻⁶. 4 -10×10 6 g / mol, for example, could be 2 × 10 4 g / mol, 3×10 4 g / mol, 4×10 4 g / mol, 5×10 4 g / mol, 6×10 4 g / mol, 7×10 4 g / mol, 8×10 4 g / mol, 9×10 4 g / mol, 1×10 5 g / mol, 2×10 5 g / mol, 5×10 5 g / mol, 8×10 5 g / mol, 1×10 6 g / mol, 2×106 g / mol, 5×10 6 g / mol, 8×10 6 g / mol, 10×10 6 g / mol, etc.; further options include 5×10 g / mol. 5 -5×10 6 g / mol.

[0019] In this disclosure, by controlling the number-average molecular weight of the acrylamide structural units in the acrylic acid-acrylamide copolymer, it is made to have suitable solubility and reversible sol-gel transition temperature. If the number-average molecular weight is too low, the reversible sol-gel transition temperature will be low, and if it is too high, the acrylic acid-acrylamide copolymer will be difficult to dissolve.

[0020] In one embodiment, the method for preparing the acrylic acid-acrylamide copolymer includes mixing acrylic acid, acrylamide and water, then adding an initiator, and reacting to obtain the acrylic acid-acrylamide copolymer.

[0021] In one embodiment, the molar ratio of acrylic acid to acrylamide is (1-1.5):1, for example, it can be 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, etc.

[0022] In one embodiment, the initiator comprises a combination of 2-((butylthio)-carbonylthio)thio-2-methylpropionic acid and 4,4'-azobis(4-cyanovaleric acid).

[0023] In one embodiment, the molar ratio of 2-((butyl thio)-carbonyl thio)thio-2-methylpropionic acid, 4,4'-azobis(4-cyanovaleric acid) and acrylic acid-acrylamide copolymer is (10-12):(0.2-0.3):(400-450), for example, it can be 10:0.2:400, 10:0.25:450, 10:0.3:450, 10.2:0.22:410, 10.5:0.25:415, 10.8:0.28:420, 11:0.3:430, 11.2:0.2:440, 11.5:0.25:450, 12:0.2:450, 12:0.3:400, etc.

[0024] In this disclosure, by adjusting the molar ratio of 2-((butyl thio)-carbonyl thio)thio-2-methylpropionic acid, 4,4'-azobis(4-cyanovaleric acid), and acrylic acid-acrylamide copolymer, the initiator plays a moderate crosslinking role. If the molar ratio is too low, there will be fewer crosslinking points and a lower degree of crosslinking, and the resulting network structure will be larger, which will not be able to play the role of precipitant diversion. Conversely, a higher molar ratio will result in a higher degree of crosslinking, a smaller spatial network, and a higher gel strength, which is not conducive to the conversion into a dilute sol for coprecipitation reaction.

[0025] In one embodiment, the reaction is further preceded by a nitrogen-based foaming treatment.

[0026] In this disclosure, the time for nitrogen foaming treatment is 2-5 hours, for example, 2 hours, 3 hours, 4 hours, 5 hours, etc.

[0027] In one embodiment, the reaction temperature is 70-85°C, for example, 70°C, 75°C, 80°C, 85°C, etc.; the reaction time is 60-80h, for example, 60h, 65h, 70h, 75h, 80h, etc.

[0028] In one embodiment, the upper critical dissolution temperature of the thermosensitive polymer having an upper critical dissolution temperature is 30-40°C, for example, it can be 30°C, 33°C, 36°C, 38°C, 40°C, etc.

[0029] In one embodiment, the mass concentration of the solution of the thermosensitive polymer having an upper critical dissolution temperature is 8wt%-15wt%, for example, it can be 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, etc.

[0030] In this disclosure, by controlling the mass concentration of a solution of a thermosensitive polymer with an upper critical dissolution temperature, the reversible sol-gel generated can respond rapidly to temperature stimulation. If the mass concentration is too low, gelation will not occur, and if it is too high, it will not be able to transform into a dilute sol.

[0031] In one embodiment, the iron source includes any one or a combination of at least two of ferrous chloride, ferrous sulfate, ferric chloride, ferric nitrate, or ferric perchlorate.

[0032] In one embodiment, when the iron source includes ferrous chloride and / or ferrous sulfate, an oxidant is used to oxidize the ferrous chloride and / or ferrous sulfate to convert ferrous iron into ferric iron.

[0033] In this disclosure, oxidizing agents include, but are not limited to, hydrogen peroxide.

[0034] In one embodiment, the phosphorus source includes any one or a combination of at least two of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, or disodium hydrogen phosphate.

[0035] In one embodiment, the molar ratio of the iron source to the phosphorus source is 1:(1-1.1), for example, it can be 1:1, 1:1.02, 1:1.05, 1:1.08, 1:1.1, etc.

[0036] In one embodiment, the concentration of the iron source solution is 0.1-1 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, etc.

[0037] In one embodiment, the concentration of the phosphorus source solution is 0.1-1 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, etc.

[0038] In one embodiment, the mass concentration of the thermosensitive polymer in the mixture is not less than 3 wt%, and can be selected from 3 wt% to 6 wt%, for example, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, 5.5 wt%, 6 wt%, etc.

[0039] In this disclosure, by adjusting the mass concentration of the thermosensitive polymer in the mixed solution, the reversible sol-gel generated can respond rapidly to temperature stimulation. If the mass concentration is too low, gelation will not occur, and vice versa.

[0040] In one embodiment, the gelation process includes natural cooling.

[0041] In one embodiment, the pH value is 1.7-2.2, for example, it can be 1.7, 1.8, 2, 2.1, 2.2, etc.

[0042] In one embodiment, the heating temperature is 60-85°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, etc.

[0043] In one embodiment, the concentration of the phosphoric acid solution is 0.2-2 mol / L, for example, it can be 0.2 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, etc.

[0044] In one embodiment, the aging reaction temperature is 80-100℃, for example, 80℃, 85℃, 90℃, 95℃, 100℃, etc.; the time is 2-5h, for example, 2h, 3h, 4h, 5h, etc.

[0045] In one embodiment, the aging reaction is followed by filtration, washing, and drying processes in sequence.

[0046] In this disclosure, the washing process involves washing with deionized water and ethanol until the upper liquid is clear.

[0047] In this disclosure, the drying process can be exemplarily performed in a drying oven at 70-110°C.

[0048] In one embodiment, the calcination treatment temperature is 500-650℃, for example, 500℃, 550℃, 600℃, 650℃, etc.; the time is 3-5h, for example, 3h, 4h, 5h, etc.

[0049] In this disclosure, the equipment for the calcination treatment includes, but is not limited to, a muffle furnace, which removes water of crystallization from the crystal structure through calcination.

[0050] In a second aspect, this disclosure provides an iron phosphate material, which is prepared by the method for preparing iron phosphate materials according to the first aspect.

[0051] Thirdly, this disclosure provides a lithium iron phosphate cathode material, wherein the raw materials for preparing the lithium iron phosphate cathode material include the iron phosphate material according to the second aspect.

[0052] Fourthly, this disclosure provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the material of the positive electrode comprises the lithium iron phosphate positive electrode material according to the third aspect.

[0053] Compared with the prior art, this disclosure has the following beneficial effects:

[0054] This disclosure provides a method for preparing ferric phosphate materials, which rapidly prepares highly dispersible ferric phosphate materials by introducing a temperature-sensitive polymer with an upper critical solution temperature (UCST) and combining sol-gel and co-precipitation methods. The UCST-type temperature-sensitive polymer can achieve reversible transitions between sol and gel at different temperatures; below the UCST temperature, it is a semi-solid gel, and above the UCST temperature, it is a highly fluid sol-liquid. When the raw materials for ferric phosphate preparation and the temperature-sensitive polymer are uniformly mixed to form a gel below the UCST temperature, the polymer possesses a porous structure and framework characteristics. The added alkaline precipitant can be subdivided and distributed by the pores on the gel surface, thereby achieving uniform diffusion of the precipitant and avoiding the formation of impurities due to excessively high local concentrations caused by the addition of precipitant in traditional methods. Upon further temperature increases, the gel transforms into a sol-liquid, and ferric phosphate begins to co-precipitate.

[0055] Furthermore, the thermosensitive polymer exhibits increased deassociation and swelling upon heating, and the resulting sol-liquid polymer can inhibit the movement of nanoparticles, thus preventing the aggregation of ferric phosphate particles. Existing sol-gel methods can prepare highly dispersed ferric phosphate, but require lengthy gel drying and subsequent calcination, thus prolonging the process time. Existing co-precipitation methods can quickly obtain ferric phosphate products but are prone to aggregation, and the addition of alkali can easily cause localized excessive concentrations and uneven distribution, affecting the consistency and stability of the product morphology. Based on these considerations, this disclosure combines the advantages of both methods, and the thermosensitive polymer, due to the reversibility of the sol-gel process, can be recycled and reused, thereby reducing production costs.

[0056] After reading and understanding the accompanying diagrams and detailed descriptions, the other aspects can be understood. Attached Figure Description

[0057] The accompanying drawings are used to provide a further understanding of the technical solutions in this paper and form part of the specification. They are used together with the embodiments of this application to explain the technical solutions in this paper and do not constitute a limitation on the technical solutions in this paper.

[0058] Figure 1 The image shows a SEM image of the iron phosphate prepared in Example 1. Detailed Implementation

[0059] The technical solutions of this disclosure will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of this disclosure and should not be construed as specific limitations thereof.

[0060] Example 1

[0061] This embodiment provides a method such as Figure 1The illustrated iron phosphate material (FePO4) and its preparation method include the following steps:

[0062] Acrylamide and acrylic acid were dissolved in deionized water at a molar ratio of 0.42:0.58 to form a monomer solution. Then, 2-((butylthio)-carbonylthio)thio-2-methylpropionic acid initiator and 4,4'-azobis(4-cyanovaleric acid) free radical initiator were added to the monomer solution to obtain a reaction solution. The molar ratio of 2-((butylthio)-carbonylthio)thio-2-methylpropionic acid initiator, 4,4'-azobis(4-cyanovaleric acid) free radical initiator, and acrylamide-acrylic acid copolymer was 11:0.2:420. After bubbling the reaction solution with nitrogen for 3 hours, it was placed in a distilled water bath (75°C) and soaked for 3 days to obtain the acrylamide-acrylic acid copolymer.

[0063] Ferric chloride and ammonium dihydrogen phosphate were prepared into a 0.5 mol / L solution using water as the solvent. A certain amount of acrylamide-acrylic acid copolymer (abbreviated as P(AA-co-AM)s) was taken, with a molar percentage content of 42% of acrylamide structural units and a number average molecular weight of 1.3 × 10⁻⁶. 5 (g / mol, upper critical dissolution temperature 37℃) and water are mixed, heated in a water bath to above 35℃ and stirred to dissolve, to prepare a 10wt% acrylamide-acrylic acid copolymer solution. Ferric chloride solution and ammonium dihydrogen phosphate solution are then mixed with Fe... 3+ :PO4 3- =1:1 molar ratio added to the acrylamide-acrylic acid copolymer solution, so that the mass concentration of P(AA-co-AM)s in the acrylamide-acrylic acid copolymer solution in the mixed solution is 4wt%. After mixing evenly, a mixed solution is obtained.

[0064] The temperature of the mixture was lowered to below 35°C for gelation treatment, and a 30% ammonia solution was slowly added to allow it to diffuse evenly. After adjusting the pH to 1.7, the temperature was raised to 65°C, causing the mixture to change from a semi-gel state to a sol liquid. Ferric phosphate precipitate began to appear. After waiting for 30 minutes, the mixture was filtered and removed.

[0065] Ferric phosphate precipitate was placed in a phosphoric acid solution (1 mol / L) and aged at 80°C for 4 hours. The precursor material was filtered out, and the supernatant was washed with deionized water and ethanol until clear. The washed precursor material was dried in a drying oven at 70°C to obtain FePO4·xH2O material. The prepared FePO4·xH2O material was then placed in a muffle furnace and heated to 600°C at a rate of 4°C and held for 4 hours to remove water of crystallization and other impurities, thus obtaining anhydrous FePO4 material.

[0066] Example 2

[0067] The difference between this embodiment and Example 1 is that the molar ratio between the 2-((butyl thio)-carbonyl thio)thio-2-methylpropionic acid initiator, the 4,4'-azobis(4-cyanovaleric acid) radical initiator, and the acrylamide-acrylic acid copolymer is 10:0.2:420, while the other conditions are exactly the same as in Example 1.

[0068] Example 3

[0069] The difference between this embodiment and Example 1 is that the molar ratio between the 2-((butyl thio)-carbonyl thio)thio-2-methylpropionic acid initiator, the 4,4'-azobis(4-cyanovaleric acid) radical initiator, and the acrylamide-acrylic acid copolymer is 12:0.2:420, while the other conditions are exactly the same as in Example 1.

[0070] Example 4

[0071] The difference between this embodiment and Example 1 is that the molar ratio between the 2-((butyl thio)-carbonyl thio)thio-2-methylpropionic acid initiator, the 4,4'-azobis(4-cyanovaleric acid) radical initiator, and the acrylamide-acrylic acid copolymer is 10:0.25:450, while the other conditions are exactly the same as in Example 1.

[0072] Example 5

[0073] The difference between this embodiment and Example 1 is that the molar ratio between the 2-((butyl thio)-carbonyl thio)thio-2-methylpropionic acid initiator, the 4,4'-azobis(4-cyanovaleric acid) radical initiator, and the acrylamide-acrylic acid copolymer is 12:0.3:400, while the other conditions are exactly the same as in Example 1.

[0074] Example 6

[0075] The difference between this embodiment and Example 1 is that the acrylamide-acrylic acid copolymer has a molar percentage of 40% for acrylamide structural units and a number-average molecular weight of 2 × 10⁻⁶ for the acrylamide structural units. 4 g / mol, with other conditions exactly the same as in Example 1.

[0076] Example 7

[0077] The difference between this embodiment and Example 1 is that the acrylamide-acrylic acid copolymer has a molar percentage of 45% for acrylamide structural units and a number-average molecular weight of 7.9 × 10⁻⁶ for the acrylamide structural units. 4 g / mol, with other conditions exactly the same as in Example 1.

[0078] Example 8

[0079] The difference between this embodiment and Example 1 is that the molar percentage of acrylamide structural units in the acrylic acid-acrylamide copolymer is 35%, while all other aspects are the same as in Example 1.

[0080] Example 9

[0081] The difference between this embodiment and Example 1 is that the molar percentage of acrylamide structural units in the acrylic acid-acrylamide copolymer is 50%, while all other aspects are the same as in Example 1.

[0082] Example 10

[0083] The difference between this embodiment and Example 1 is that the concentration of the acrylamide-acrylic acid copolymer solution in the mixture is 3 wt%, while all other aspects are the same as in Example 1.

[0084] Example 11

[0085] The difference between this embodiment and Example 1 is that the concentration of the acrylamide-acrylic acid copolymer solution in the mixture is 6 wt%, while all other aspects are the same as in Example 1.

[0086] Example 12

[0087] The difference between this embodiment and Example 1 is that the mass concentration of the acrylamide-acrylic acid copolymer in the mixture is 1 wt%, while all other aspects are the same as in Example 1.

[0088] Example 13

[0089] The difference between this embodiment and Example 1 is that the acrylamide-acrylic acid copolymer is replaced with polyacrylamide, while everything else is the same as in Example 1.

[0090] Comparative Example 1

[0091] This comparative example provides a method for preparing an iron phosphate material, which includes the following steps:

[0092] (1) Ferric chloride and ammonium dihydrogen phosphate were respectively prepared into 0.5 mol / L Fe solutions. 3+ Solution and PO4 3- Solution, the two solutions are prepared according to Fe 3+ [PO4] 3- The mixture was prepared in a 1:1 molar ratio, heated to 65°C in a water bath, and ammonia was added to adjust the pH to 1.7, resulting in a precipitate. After 30 minutes, the precipitate was filtered out. The precipitate was then placed in a 1 mol / L phosphoric acid solution and aged at 80°C for 4 hours. The FePO4·xH2O precipitate was filtered out, and the supernatant was washed with deionized water and ethanol to obtain a clear solution.

[0093] (3) The washed precipitate was dried in a drying oven at 70°C to obtain FePO4·xH2O material. The FePO4·xH2O prepared above was then placed in a muffle furnace and heated to 600°C at a heating rate of 4°C and held for 4 hours to remove crystal water and other impurities, thus obtaining anhydrous FePO4 material.

[0094] Comparative Example 2

[0095] The difference between this comparative example and Example 1 is that, after adding ferric chloride solution and ammonium dihydrogen phosphate solution to the acrylamide-acrylic acid copolymer solution and mixing them evenly, the temperature of the mixture is not lowered to below 35°C for gelation treatment. Instead, the temperature of the mixture is maintained at 35°C, and ammonia solution is added dropwise to adjust the pH value to 1.7. Everything else is the same as in Example 1.

[0096] Preparation Example

[0097] The anhydrous FePO4 materials provided in Examples 1 to 13 and Comparative Examples 1 to 2, along with lithium carbonate and glucose, were uniformly dispersed and mixed in water at a stoichiometric ratio of 1:1.02:0.06 for iron, lithium, and carbon. The mixture was ball-milled at 700 r / min for 4 h until homogeneous, and then spray-dried to obtain precursor powder. The precursor powder was heated to 300 °C at a heating rate of 3 °C / min and held for 2 h under a nitrogen atmosphere, followed by calcination at 750 °C for 7 h to obtain LiFePO4 / C cathode material.

[0098] The obtained LiFePO4 / C cathode material was assembled into a button cell, and the electrochemical performance of the lithium-ion battery was tested. The specific steps were as follows: the positive electrode material LiFePO4 / C active material, the conductive agent acetylene black, and the adhesive polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a mass ratio of 90:5:5, ground into a slurry, coated onto aluminum foil, and dried in a vacuum drying oven. After drying, the battery was assembled in an argon glove box, and pressed into a positive electrode sheet with a size of 12 mm using a pressing machine. The negative electrode was a lithium metal sheet. The electrolyte composition was 1 mol / L lithium hexafluorophosphate (LiPF6) lithium salt, and the solvent was a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1. A polypropylene porous membrane was used as the separator, and the button cell was assembled.

[0099] Test conditions

[0100] The button cells provided in Preparation Examples 1 to 13 and Comparative Preparation Examples 1 to 2 were tested. The voltage range of the tests was 2.4-4.6V, and the test rates were 0.1C and 1C, respectively, where 1C = 170mA / g.

[0101] The test results are shown in Table 1:

[0102] Table 1

[0103]

[0104] As can be seen from Table 1, the method for preparing ferric phosphate materials provided in Examples 1-7 of this disclosure introduces an acrylamide-acrylic acid copolymer with an upper critical solution temperature (UCST) to combine sol-gel and co-precipitation methods to rapidly prepare highly dispersible ferric phosphate materials. Furthermore, the content relationship of the components is optimized to avoid problems such as excessive local concentration and poor product uniformity.

[0105] A comparison of Preparation Example 1, Preparation Example 8 and Preparation Example 9 shows that by controlling the molar percentage of acrylamide structural units in the acrylic acid-acrylamide copolymer, the acrylic acid-acrylamide copolymer can have a higher suitable UCST temperature range.

[0106] A comparison of Preparation Example 1, Preparation Example 10, and Preparation Example 11 shows that by controlling the optimal concentration of the acrylamide-acrylic acid copolymer solution, the reversible sol-gel generated can respond rapidly to temperature stimulation.

[0107] A comparison of Preparation Example 1 and Preparation Example 12 shows that by adjusting the mass concentration of the thermosensitive polymer in the mixture, the reversible sol-gel generated can respond rapidly to temperature stimulation.

[0108] A comparison of Preparation Example 1 and Preparation Example 13 shows that replacing the acrylamide-acrylic acid copolymer with other types in the prior art does not achieve all the technical effects of this disclosure.

[0109] A comparison between Preparation Example 1 and Comparative Preparation Example 1 shows that the iron phosphate material prepared by the conventional co-precipitation method produces batteries with poor electrochemical performance.

[0110] As can be seen from Preparation Example 1 and Comparative Preparation Example 2, if the temperature of the mixture is not lowered to below 35°C for gelation treatment, uniform diffusion of the precipitant cannot be achieved, resulting in poor battery performance.

Claims

1. A method for preparing iron phosphate materials, comprising the following steps: A solution of a thermosensitive polymer with an upper critical dissolution temperature, a solution containing an iron source, and a solution containing a phosphorus source are mixed to obtain a mixture. The mixture was gelled, then an alkaline solution was added to adjust the pH to acidic, and a precipitate was obtained after heating. The precipitate and phosphoric acid solution were aged to obtain the precursor material. The precursor material is calcined to obtain the iron phosphate material; The thermosensitive polymer with an upper critical dissolution temperature is an acrylic acid-acrylamide copolymer.

2. The method according to claim 1, wherein, The acrylic acid-acrylamide copolymer contains 40%-45% acrylamide structural units in its molar percentage.

3. The method according to claim 1, wherein, The number-average molecular weight of the acrylamide structural units in the acrylic acid-acrylamide copolymer is 2 × 10⁻⁶. 4 -10×10 6 g / mol.

4. The method according to claim 3, wherein, The number-average molecular weight of the acrylamide structural units in the acrylic acid-acrylamide copolymer is 5 × 10⁻⁶. 5 -5×10 6 g / mol.

5. The method according to claim 1, wherein, The preparation method of the acrylic acid-acrylamide copolymer includes mixing acrylic acid, acrylamide and water, then adding an initiator, and reacting to obtain the acrylic acid-acrylamide copolymer; The molar ratio of acrylic acid to acrylamide is (1-1.5):

1.

6. The method according to claim 5, wherein, The initiator comprises a combination of 2-((butylthio)-carbonylthio)thio-2-methylpropionic acid and 4,4'-azobis(4-cyanovaleric acid); The molar ratio of 2-((butyl thio)-carbonyl thio)thio-2-methylpropionic acid, 4,4'-azobis(4-cyanovaleric acid) and acrylic acid-acrylamide copolymer is (10-12):(0.2-0.3):(400-450); The reaction process also includes a nitrogen-based foaming treatment. The reaction is carried out at a temperature of 70-85℃ for 60-80 hours.

7. The method according to claim 1, wherein, The upper critical dissolution temperature of the thermosensitive polymer is 30-40℃.

8. The method according to claim 1, wherein, The mass concentration of the solution of the thermosensitive polymer having an upper critical dissolution temperature is 8wt%-15wt%.

9. The method according to claim 1, wherein, The iron source includes any one or a combination of at least two of ferrous chloride, ferrous sulfate, ferric chloride, ferric nitrate, or ferric perchlorate; The phosphorus source includes any one or a combination of at least two of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, or disodium hydrogen phosphate. The molar ratio of the iron source to the phosphorus source is 1:(1-1.1).

10. The method according to claim 1, wherein when the iron source comprises ferrous chloride and / or ferrous sulfate, the ferrous chloride and / or ferrous sulfate are oxidized using an oxidizing agent.

11. The method according to claim 1, wherein, The concentration of the iron-containing source solution is 0.1-1 mol / L; The concentration of the phosphorus source solution is 0.1-1 mol / L; The mass concentration of the thermosensitive polymer in the mixture is not less than 3 wt%.

12. The method according to claim 11, wherein, The mass concentration of the thermosensitive polymer in the mixture is 3wt%-6wt%.

13. The method according to claim 1, wherein, The gelation process includes natural cooling; The pH value is 1.7-2.2; The heating temperature is 60-85℃.

14. The method according to claim 1, wherein, The concentration of the phosphoric acid solution is 0.2-2 mol / L; The aging reaction is carried out at a temperature of 80-100℃ for 2-5 hours. The aging reaction is followed by filtration, washing and drying processes. The calcination treatment is carried out at a temperature of 500-650℃ for 3-5 hours.

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