Iron(III) phosphate, preparation method therefor, and use thereof

Abstract

The present disclosure provides iron(III) phosphate, a preparation method therefor, and a use thereof. The preparation method comprises: using dendritic poly(propylene imine) as a medium to replace alkaline substances in the prior art to adjust pH, so that iron(III) phosphate is produced, and at the same time the iron(III) phosphate can precipitate and grow on a dendritic structure of the dendritic poly(propylene imine) by means of a coordination effect; and finally, removing the poly(propylene imine) at a high temperature to obtain the iron(III) phosphate containing a three-dimensional dendritic network channel. The preparation method does not introduce other impurities, effectively reduces the presence of alkalis in coprecipitation wastewater, and reduces the wastewater treatment cost; moreover, the three-dimensional dendritic network channel makes the iron(III) phosphate easier to grind, and the channel can shorten the lithium-ion transmission distance during the subsequent preparation for lithium iron phosphate, thereby improving the electrical performance of the lithium iron phosphate.

Description

Ferric phosphate and its preparation method and use Technical Field

[0001] The present invention relates to the field of cathode material preparation, and relates to iron phosphate and a preparation method and application thereof. Background Art

[0002] With the rapid development of new energy industries, electric vehicles, and the smart Internet of Things, market demand for lithium-ion batteries is rapidly increasing. Lithium-ion batteries are categorized into various types based on their cathode materials, including lithium cobalt oxide, lithium manganese oxide, ternary materials, and lithium iron phosphate. Among these, lithium iron phosphate batteries hold the greatest potential due to their excellent cycle performance and low production cost.

[0003] Iron phosphate is a common raw material used in the preparation of lithium iron phosphate cathode materials. Even slight changes in its microstructure and chemical composition can significantly impact its performance. Currently, the primary method for synthesizing iron phosphate is co-precipitation, which involves introducing a large amount of alkaline material into an acidic environment to adjust the solution's pH to produce a precipitated iron phosphate.

[0004] The above-mentioned co-precipitation method has some problems that need to be solved. For example, the commonly used alkaline substances for adjusting pH are hydroxides, such as sodium hydroxide, potassium hydroxide, sodium bicarbonate, ammonia water, etc. The introduction of alkaline metal compounds will also cause impurity ions to be wrapped in the iron phosphate precipitate, which will require a lot of water washing costs to remove later. The use of ammonia water leads to the presence of a large amount of ammonium ions in the co-precipitate, increasing the cost of wastewater treatment.

[0005] CN112340719B uses chloride salt instead of the alkaline solution required for conventional reactions. Phosphoric acid is then added to allow hydrogen ions and chloride ions to escape from the system in the form of hydrogen chloride, thereby increasing the pH of the solution system and generating iron phosphate precipitation. However, this method still inevitably introduces impurity metal cations in the chloride salt, which must be removed by extensive washing steps.

[0006] Therefore, it is of great practical significance to seek a method for preparing iron phosphate that has high ion utilization, can fully react to adjust the pH of the solution, and is not affected by impurities.

[0007] Summary of the Invention

[0008] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0009] In view of the problems existing in the prior art, the purpose of the present disclosure is to provide an iron phosphate and its preparation method and use. The preparation method uses a tree-shaped polypropylene imine as a medium, replacing the alkaline substances in the prior art to adjust the pH, thereby generating iron phosphate and simultaneously allowing the iron phosphate to precipitate and grow on its tree-shaped structure through coordination. Finally, after removing the polypropylene imine at high temperature, the iron phosphate containing a three-dimensional tree-shaped network channel can be obtained. The preparation method does not introduce other impurities, effectively reduces the presence of alkali in the co-precipitation wastewater, and reduces the cost of wastewater treatment. In addition, the three-dimensional tree-shaped network channel makes the iron phosphate easier to grind and process. In the subsequent preparation of lithium iron phosphate, the channel can shorten the lithium ion transmission distance, thereby improving the electrical properties of the lithium iron phosphate.

[0010] To achieve this goal, the present disclosure adopts the following technical solutions:

[0011] In a first aspect, the present disclosure provides a method for preparing ferric phosphate, the preparation method comprising:

[0012] The tree-like polypropylene imine is mixed with the iron phosphate raw material, and a generation reaction is carried out to obtain primary iron phosphate containing the tree-like polypropylene imine, which is then calcined to remove the tree-like polypropylene imine and obtain iron phosphate.

[0013] The present disclosure uses dendrimer polypropylene imine as a medium to replace the alkaline substances of the prior art to adjust the pH, because a large number of primary and tertiary amines in polypropylene imine are alkaline. The present disclosure has found that replacing conventional precipitants such as sodium hydroxide, ammonia or other inorganic alkaline substances can reduce the introduction of impurities in the co-precipitation process of ferric phosphate, reduce the treatment cost of wastewater (such as ammonium ions) after the precipitation process, and can successfully prepare ferric phosphate. For the large number of amine groups possessed by polypropylene imine, the lone pair electrons it possesses can not only bind H + Protonation reduces the pH of the solution (R-NH2+H + →R-NH3 + ), and can also coordinate with iron sources, especially trivalent iron ions, and then adsorb phosphate, and then co-precipitate iron phosphate on the dendritic polypropylene imine, using the tree structure as a matrix and support to form a three-dimensional tree network channel inside the iron phosphate.

[0014] Polypropylene imine can form a more complex tree-like structure through repeated branching of monomers. Low branching generations form an open, amorphous structure, while higher branching generations, after three generations, form a spherical structure with a tight exterior and loose interior. Therefore, the branching generation of polyethylene imine can be controlled to synthesize iron phosphates with different morphologies. Iron phosphate grows on polypropylene imine under adsorption, accelerating primary particle crystallization. When the primary iron phosphate containing the tree-like polypropylene imine is subjected to high-temperature dehydration and the carbon, hydrogen, and nitrogen elements in the polypropylene imine are volatilized as gases, three-dimensional tree-like cavity channels are formed within the iron phosphate. This unique iron phosphate is easily milled and crushed for subsequent lithium iron phosphate synthesis. The three-dimensional network of channels also facilitates lithium ion diffusion, thereby improving the electrical properties of the resulting lithium iron phosphate cathode material.

[0015] The following are optional technical solutions of the present disclosure, but are not intended to limit the technical solutions provided by the present disclosure. Through the following technical solutions, the technical objectives and beneficial effects of the present disclosure can be better achieved and realized.

[0016] As an optional technical solution of the present disclosure, the preparation method includes first preparing the ferric phosphate raw material into solution A, preparing the dendritic polypropylene imine into solution B, and adding solution B into solution A until the pH of the generating reaction is reached.

[0017] The present disclosure uses dendritic polypropylene imine as an alkalinity regulator, so the method of adding it can be similar to that of conventional alkaline solutions, such as ammonia water, and it is better to add it separately into the solution containing the ferric phosphate raw material.

[0018] In one embodiment, the pH of the formation reaction is 1.5 to 2.2, such as 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1 or 2.2, but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0019] Dendritic polypropylene imine is used as an alkali solution to adjust the pH. If the pH is adjusted to too high, not only will the amount of dendritic polypropylene imine used increase, but it will also easily generate iron hydroxide colloid and bring impurities; if the pH is adjusted to too low, it will affect the coprecipitation rate.

[0020] In one embodiment, the concentration of iron ions in solution A is 0.1 to 2 mol / L, for example, 0.1 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.4 mol / L, 1.6 mol / L, 1.8 mol / L or 2 mol / L, but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0021] In one embodiment, in the solution A, the molar ratio of iron ion to phosphate is 1:(1-1.1), for example, 1:1, 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09 or 1:1, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0022] In one embodiment, in the solution B, the concentration of the dendritic polypropylene imine is 2 to 5 wt%, for example, 2 wt%, 2.3 wt%, 2.5 wt%, 2.8 wt%, 3 wt%, 3.2 wt%, 3.5 wt%, 3.8 wt%, 4 wt%, 4.2 wt%, 4.4 wt%, 4.8 wt% or 5 wt%, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0023] Solution B has a lower concentration and is less protonated, allowing more amine groups to act as bases to adjust pH.

[0024] As an optional technical solution of the present disclosure, the ferric phosphate raw material includes an iron source and a phosphorus source.

[0025] The present application does not specifically limit the iron source and phosphorus source of the ferric phosphate raw material used. Any iron source and phosphorus source that can be used to obtain ferric phosphate by co-precipitation is applicable to the present disclosure.

[0026] In one embodiment, the iron source comprises any one of pure iron, ferric chloride or ferric chloride, or a combination of at least two of them. Typical but non-limiting examples of the combination include a combination of pure iron and ferric chloride, a combination of pure iron and ferric chloride, or a combination of ferric chloride and ferric chloride.

[0027] In one embodiment, when the iron source comprises pure iron, the ferric phosphate raw material further comprises an oxidant.

[0028] When pure iron is used, it generates ferrous substances in the reaction. For example, pure iron is mixed with phosphoric acid to generate ferrous dihydrogen phosphate. At this time, by adding an oxidant such as hydrogen peroxide, it is oxidized to trivalent iron, which is then adsorbed on the dendritic polyimide and further forms ferric phosphate.

[0029] In one embodiment, the phosphorus source includes any one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate or sodium hydrogen phosphate, or a combination of at least two thereof. Typical but non-limiting examples of the combination include a combination of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, a combination of ammonium phosphate and phosphoric acid, a combination of sodium dihydrogen phosphate and sodium hydrogen phosphate, or a combination of ammonium dihydrogen phosphate and phosphoric acid.

[0030] As an optional technical solution of the present disclosure, the preparation method further includes, after the primary ferric phosphate with tree-shaped polypropylene imine is generated, heat preservation and aging.

[0031] In one embodiment, the heat preservation and aging method includes stirring at 60 to 90°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C, for 4 to 12 hours, for example, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours or 12 hours, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0032] In one embodiment, the preparation method further comprises washing and drying before calcining.

[0033] In one embodiment, the washing solution comprises water and / or ethanol.

[0034] In one embodiment, the drying temperature is 60 to 120° C., for example, 60° C., 70° C., 80° C., 90° C., 100° C., 110° C., or 120° C., and the drying time is 4 to 10 h, for example, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, or 10 h, but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0035] In one embodiment, the calcining is performed under an oxygen-containing atmosphere.

[0036] The carbon, hydrogen and oxygen in the organic polypropylene imine can react to generate gas in the air, so the calcination is carried out in an oxygen-containing environment. Air can be used for the purpose of low cost and simple operation.

[0037] In one embodiment, the calcination temperature is 500-750°C, for example, 500°C, 530°C, 550°C, 580°C, 600°C, 630°C, 650°C, 700°C, 730°C or 750°C, and the calcination time is 2-8h, for example, 2h, 3h, 4h, 5h, 6h, 7h or 8h, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0038] As an optional technical solution of the present disclosure, the method for preparing the tree-shaped polypropylene imine includes:

[0039] Mixing ethylenediamine, acrylonitrile, a first catalyst and a first solvent to perform a first catalytic reaction to obtain a dendritic polypropyleneimine precursor;

[0040] The obtained dendrimer polypropylene imine precursor, a second catalyst, and a second solvent are mixed and subjected to a second catalytic reaction under hydrogen to obtain a first-generation dendrimer polypropylene imine;

[0041] The nth generation dendritic polypropylene imine is used to replace the dendritic polypropylene imine precursor to carry out a second catalytic reaction to obtain the n+1th generation dendritic polypropylene imine, where n is a natural number.

[0042] As an optional technical solution of the present disclosure, the method for preparing the tree-shaped polypropylene imine further includes:

[0043] Ethylenediamine and a first solvent are mixed, acrylonitrile is added to carry out a low-temperature reaction, and then a first catalyst is added to carry out a high-temperature reaction. After cooling, the first solvent is removed to obtain residual organic matter, which is dissolved in a third solvent and then cooled and crystallized to obtain a dendritic polypropyleneimine precursor.

[0044] In one embodiment, the first solvent comprises water.

[0045] In one embodiment, the mass ratio of the ethylenediamine to the first solvent is 1:(3-5), for example, 1:3, 1:3.3, 1:3.5, 1:3.8, 1:4, 1:4.3, 1:4.7 or 1:5, etc., but is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

[0046] In one embodiment, the amount of acrylonitrile used is equal to the mass of the first solvent.

[0047] In one embodiment, the temperature of the low-temperature reaction is -5°C to 5°C, for example, -5°C, -4°C, -3°C, -2°C, -1°C, 0°C, 1°C, 2°C, 3°C, 4°C or 5°C, etc., but is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

[0048] In one embodiment, the first catalyst comprises acetic acid.

[0049] In one embodiment, the temperature of the high temperature reaction is 70-90°C, for example, 70°C, 74°C, 78°C, 82°C, 86°C or 90°C, but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0050] In one embodiment, the third solvent comprises methanol.

[0051] In one embodiment, the method of removing the first solvent comprises distillation under reduced pressure.

[0052] In one embodiment, the second solvent comprises 1,4-dioxane.

[0053] In one embodiment, the second catalyst comprises a Urushihara cobalt catalyst.

[0054] In one embodiment, the temperature of the second catalytic reaction is 50-70°C, for example, 50°C, 54°C, 58°C, 62°C, 66°C or 70°C, but is not limited to the listed values. Other values ​​not listed within the above numerical range are also applicable.

[0055] As an optional technical solution of the present disclosure, the dendrimer polypropylene imine used is any one of the first generation to the fifth generation dendrimer polypropylene imine or a mixture of at least two of them.

[0056] In one embodiment, the molecular weight of the dendrimer polypropyleneimine used in the preparation method is 300 to 50,000, for example, 3,000, 4,000, 6,000, 8,000, 10,000, 13,000, 16,000, 20,000, 23,000, 25,000, 28,000, 30,000, 34,000, 38,000, 42,000, 46,000 or 50,000, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0057] In order to achieve different morphological characteristics, the higher the generation number of the tree-like polypropylene imine, the more obvious the tree structure is, and the more spherical it is with a loose inside and a tight outside, so that iron phosphates of different sizes and morphologies can be prepared.

[0058] As an optional technical solution of the present disclosure, the preparation method includes:

[0059] Ethylenediamine and a first solvent, water, are uniformly mixed in a mass ratio of 1:(3-5), acrylonitrile of the same mass as water is slowly added dropwise in an ice bath, and the mixture is reacted at low temperature for 2-4 hours. Acetic acid is added as a first catalyst, and the mixture is heated to 70-90° C. and refluxed for 4-6 hours to carry out a high-temperature reaction. The mixture is then cooled to room temperature to remove the first solvent, aqueous phase, and vacuum distilled to obtain the remaining organic matter. The third solvent, methanol, is then heated and dissolved, and by-products are removed by cooling, crystallization, suction filtration, and washing with water to obtain a dendritic polypropyleneimine precursor.

[0060] A dendrimer polypropylene imine precursor, a second solvent 1,4-dioxane, and a second catalyst lacquer raw cobalt are added to an autoclave, hydrogen is introduced to heat the autoclave to 50-70° C. for a second catalytic reaction, the reaction is stopped when the hydrogen pressure no longer decreases, the second catalyst is filtered, the second solvent is removed by cooling, and a first-generation polypropylene imine dendrimer is obtained; the nth-generation dendrimer polypropylene imine is substituted for the dendrimer polypropylene imine precursor for a second catalytic reaction to obtain an n+1th-generation dendrimer polypropylene imine; and a mixture of the first-generation to fifth-generation dendrimer polypropylene imines having a molecular weight of 300-50,000 is obtained.

[0061] An iron source and a phosphorus source are prepared into solution A, wherein the concentration of iron ions in solution A is controlled to be 0.1 to 2 mol / L, and the molar ratio of iron ions to phosphate is controlled to be 1:(1 to 1.1); a mixture of dendritic polypropylene imine is prepared into solution B, wherein the concentration of dendritic polypropylene imine in solution B is controlled to be 2 to 5 wt %; solution A is placed in a reactor, solution B is slowly added until the pH reaches 1.5 to 2.2, and the reaction is heated to generate a precipitate, i.e., primary iron phosphate containing dendritic polypropylene imine, and the mixture is stirred at 60 to 90° C. for 4 to 12 hours and then kept warm for aging;

[0062] The precipitated material is filtered, washed with pure water and ethanol until the upper centrifuged liquid becomes colorless and clear, dried in a vacuum drying oven at 60-120°C for 4-10 hours, then placed in a muffle furnace under an air atmosphere, heated to 500-750°C and calcined for 2-8 hours, kept warm to remove crystal water, and the tree-shaped polypropylene imine is converted into gas and removed from the ferric phosphate. After crushing, anhydrous ferric phosphate containing three-dimensional tree-shaped network channels is obtained.

[0063] In a second aspect, the present disclosure provides an iron phosphate, which is obtained using the preparation method described in the first aspect.

[0064] In a third aspect, the present disclosure provides a positive electrode material, wherein the positive electrode material is prepared using the iron phosphate described in the second aspect. The positive electrode material comprises lithium iron phosphate and lithium iron manganese phosphate.

[0065] Compared with the existing technical solutions, the present disclosure has at least the following beneficial effects:

[0066] This disclosure provides a method for addressing the low utilization rate of large amounts of alkaline substances during the production of iron phosphate. By introducing a tree-like structure of polypropylene imine as a base to adjust the pH of the solution, the iron phosphate is simultaneously precipitated and grown on the tree-like structure through coordination. Finally, the iron phosphate is removed at high temperature to produce an iron phosphate containing a three-dimensional tree-like network of channels. This disclosure can effectively reduce the presence of alkali in co-precipitation wastewater and lower wastewater treatment costs. The three-dimensional tree-like network of channels makes the iron phosphate easier to grind and process. In the later preparation of lithium iron phosphate, these channels can shorten the lithium ion transmission distance, thereby improving the electrical properties of the lithium iron phosphate.

[0067] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0069] FIG1 is a SEM image of anhydrous ferric phosphate obtained in Example 1. DETAILED DESCRIPTION

[0070] The technical solution of the present disclosure is further illustrated below through specific implementation methods.

[0071] It should be apparent to those skilled in the art that the embodiments are only intended to help understand the present disclosure and should not be considered as specific limitations of the present disclosure.

[0072] The dendritic polyimide used in the following examples and comparative examples was prepared by the following method:

[0073] Ethylenediamine and water were mixed in a 1:4 mass ratio. Acrylonitrile (amount of water) was slowly added dropwise in an ice bath. The mixture was reacted at low temperature for 3 hours. Acetic acid was added as a catalyst, and the mixture was heated to 80°C and refluxed for 5 hours. The mixture was cooled to room temperature, the aqueous phase was removed, and the remaining organic matter was distilled under reduced pressure. The remaining organic matter was dissolved in methanol, cooled, crystallized, and filtered. The byproducts were washed with water to obtain a dendrimer polypropylene imine precursor. The dendrimer polypropylene imine precursor, 1,4-dioxane solvent, and Urushihara cobalt catalyst were added to an autoclave. The reaction was heated to 60°C and maintained until the hydrogen pressure ceased to decrease. The catalyst was then filtered, the mixture was cooled, and the solvent was evaporated to obtain the first-generation dendrimer polypropylene imine. The first-generation dendrimer polypropylene imine was used to replace the dendrimer polypropylene imine precursor and reacted again with 1,4-dioxane solvent and Urushihara cobalt catalyst under hydrogen to obtain the second-generation dendrimer polypropylene imine. This process was repeated to obtain dendrimer polypropylene imines of generations 1 to 7.

[0074] Example 1

[0075] This embodiment provides a method for preparing iron phosphate, which comprises:

[0076] (1) Ferric chloride and ammonium dihydrogen phosphate were prepared to contain 0.5 mol / L of Fe 3+ solution and 0.5 mol / L PO4 3- Solution A (molar ratio Fe 3+ :PO4 3- =1:1); second-generation and fourth-generation dendrimer polypropylene imine were prepared into a 3 wt% solution B at a molar ratio of 1:1; solution A was uniformly mixed in a reactor with a stirring speed of 700 rpm, and then solution B was slowly added to the reactor. The pH of the system was adjusted to 1.8 to form a precipitate, and the mixture was heated to 85° C. in a water bath for 4 hours for aging;

[0077] (2) The precipitate was filtered out and washed alternately with deionized water and ethanol. After the upper centrifuged liquid became colorless and clear, the washed precipitate was placed in a drying oven and dried at 90°C for 8 h to obtain FePO4·xH2O material. The prepared FePO4·xH2O was then placed in a muffle furnace in an air atmosphere, heated to 600°C and calcined for 6 h. After crushing, anhydrous iron phosphate containing a three-dimensional tree-shaped network channel was obtained.

[0078] FIG1 is a SEM image of the anhydrous ferric phosphate obtained in Example 1. It can be seen from the image that the ferric phosphate is agglomerated from flaky primary particles into secondary particles, and the agglomerated secondary particles are very loose and uniform in size.

[0079] Example 2

[0080] This embodiment provides a method for preparing ferric phosphate. The preparation method uses a dendrimer polypropylene imine precursor to replace the second and fourth generation dendrimer polypropylene imine in Example 1. Other conditions are exactly the same as those in Example 1.

[0081] Example 3

[0082] This embodiment provides a method for preparing ferric phosphate. The preparation method uses the first-generation dendrimer polypropylene imine to replace the second-generation and fourth-generation dendrimer polypropylene imine in Example 1. Other conditions are exactly the same as those in Example 1.

[0083] Example 4

[0084] This embodiment provides a method for preparing ferric phosphate, wherein the preparation method uses only the fourth-generation dendrimer polypropylene imine, that is, the fourth-generation dendrimer polypropylene imine is used to replace the second-generation dendrimer polypropylene imine in Example 1. Other than this, the other conditions are exactly the same as those in Example 1.

[0085] Example 5

[0086] This embodiment provides a method for preparing ferric phosphate. The preparation method uses the fifth-generation dendrimer polypropylene imine to replace the second-generation dendrimer polypropylene imine in Example 1. Other conditions are exactly the same as those in Example 1.

[0087] Example 6

[0088] This embodiment provides a method for preparing ferric phosphate. In the preparation method, the 7th generation dendrimer polypropylene imine is used to replace the 2nd generation dendrimer polypropylene imine in Example 1. Other conditions are exactly the same as those in Example 1.

[0089] Example 7

[0090] This embodiment provides a method for preparing ferric phosphate. The preparation method adjusts the amount of dendrimer polypropyleneimine, that is, changes the pH by adjusting the amount of solution B, so that the pH is adjusted from 1.8 to 1.2. Other conditions are exactly the same as those in Example 1.

[0091] Example 8

[0092] This embodiment provides a method for preparing ferric phosphate. The preparation method adjusts the amount of dendrimer polypropyleneimine, that is, changes the pH by adjusting the amount of solution B, so that the pH is adjusted from 1.8 to 1.5. Other conditions are exactly the same as those in Example 1.

[0093] Example 9

[0094] This embodiment provides a method for preparing ferric phosphate. The preparation method adjusts the amount of dendrimer polypropyleneimine, that is, changes the pH by adjusting the amount of solution B, so that the pH is adjusted from 1.8 to 2.2. Other conditions are exactly the same as those in Example 1.

[0095] Example 10

[0096] This example provides a method for preparing ferric phosphate. The method adjusts the amount of dendrimer polypropyleneimine, that is, changes the pH by adjusting the amount of solution B, so that the pH is adjusted from 1.8 to 2.5. Other conditions are exactly the same as those in Example 1.

[0097] Example 11

[0098] This embodiment provides a method for preparing iron phosphate. In the preparation method, the calcination temperature is adjusted from 600° C. to 470° C. Other conditions are exactly the same as those in Example 1.

[0099] Example 12

[0100] This embodiment provides a method for preparing iron phosphate. In the preparation method, the calcination temperature is adjusted from 600° C. to 750° C. Other conditions are exactly the same as those in Example 1.

[0101] Example 13

[0102] This embodiment provides a method for preparing ferric phosphate. In the preparation method, the calcination time is adjusted from 6 hours to 3 hours. Other conditions are exactly the same as those in Example 1.

[0103] Example 14

[0104] This embodiment provides a method for preparing iron phosphate. In the preparation method, the calcination time is adjusted from 6 hours to 8 hours. Other conditions are exactly the same as those in Example 1.

[0105] Comparative Example 1

[0106] This comparative example provides a method for preparing ferric phosphate, in which ammonia water is used instead of dendritic polyimide for pH adjustment. Other conditions are exactly the same as those in Example 1.

[0107] The anhydrous ferric phosphate materials obtained in the Examples and Comparative Examples were uniformly dispersed in water with lithium carbonate and glucose. The mixture was ball-milled for 5 hours until uniformly mixed, with a stoichiometric molar ratio of 1:1.02:0.05 for iron, lithium, and carbon. The mixture was then spray-dried to obtain a precursor powder. The precursor powder was then heated at 350°C for 2 hours at a heating rate of 3°C / min under a nitrogen atmosphere, then calcined at 750°C for 8 hours to obtain the LiFePO4 / C cathode active material.

[0108] The electrochemical performance of lithium-ion batteries was tested using button-type cells. The specific steps were as follows: the positive electrode material active material, the conductive agent acetylene black, and the adhesive polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a ratio of 92:4:4. The mixture was then coated on aluminum foil and dried in a vacuum drying oven. After drying, the battery was assembled in an argon glove box. The positive electrode was pressed using a tablet press. The negative electrode was a metal lithium sheet. The electrolyte was 1M LiPF6-EC:DMC (volume ratio 1:1). A polypropylene porous membrane served as the separator. The electrochemical performance was tested, and the results are shown in Table 1 below.

[0109] Table 1

[0110] As can be seen from Table 1:

[0111] By comparing Example 1 with Example 2-6, in the preparation process of the modified ferric phosphate described in the present disclosure, the algebra of the tree-shaped polypropylene imine will affect its performance. If the algebra of the added tree-shaped polypropylene imine is too high, the process of synthesizing the tree-shaped polypropylene imine will be more complicated, and defects will easily occur, affecting the characteristics of its synthetic structure, and the higher the algebra, the more spherical it is. After the synthesized ferric phosphate is calcined, a tree-shaped network conductive channel cannot be obtained; if the algebra is too low, the tree-shaped polypropylene imine is the first or second generation, and the tree-shaped structure is not obvious but the structure is stable, and the number of amino groups contained is moderate and easy to protonate. Therefore, when the modified ferric phosphate introduces the tree-shaped polypropylene imine, the effect brought by the mixture of low algebra and medium and high algebra is the best.

[0112] Comparing Examples 1 and 7-10, it is clear that adjusting the pH of the coprecipitation during the preparation of the modified ferric phosphate disclosed herein can affect its performance. Dendrimer polypropylene imine is used as an alkaline solution to adjust the pH. Adjusting the pH too high not only increases the amount of dendrimer polypropylene imine required but also easily generates ferric hydroxide colloids, which introduce impurities. Adjusting the pH too low can affect the coprecipitation rate.

[0113] Comparing Example 1 with Examples 11-15, it is clear that adjusting the calcination temperature and time during the preparation of the modified ferric phosphate disclosed herein can affect its performance. Because the dendritic polypropylene imine coprecipitates with the ferric phosphate due to its coordination effect, insufficient calcination temperature or heat treatment time prevents the dendritic polypropylene imine from being completely released from the ferric phosphate, resulting in a complete three-dimensional tree-like channel. Excessive calcination temperature or heat treatment time can damage the ferric phosphate's crystal structure.

Claims

1. A preparation method of iron phosphate, comprising: Mixing dendritic polypropylene imine with an iron phosphate raw material, carrying out a formation reaction to obtain primary iron phosphate containing dendritic polypropylene imine, and calcining to remove dendritic polypropylene imine to obtain iron phosphate.

2. The preparation method according to claim 1, wherein, the preparation method includes first formulating the iron phosphate raw material into solution A, formulating the dendritic polypropylene imine into solution B, and adding solution B to solution A until the pH of the formation reaction is reached.

3. The preparation method according to claim 2, wherein, the pH of the formation reaction is 1.5 - 2.

2.

4. The preparation method according to claim 2 or 3, wherein, in solution B, the concentration of the dendritic polypropylene imine is 2 - 5 wt%.

5. The preparation method according to any one of claims 1 - 4, wherein, the preparation method further includes, after generating the primary iron phosphate containing dendritic polypropylene imine, carrying out heat preservation and aging.

6. The preparation method according to claim 5, wherein, the method of heat preservation and aging includes stirring at 60 - 90 °C for 4 - 12 h.

7. The preparation method according to any one of claims 1 - 6, wherein, the calcination is carried out in an oxygen-containing atmosphere.

8. The preparation method according to any one of claims 1 - 7, wherein, the temperature of the calcination is 500 - 750 °C, and the time is 2 - 8 h.

9. The preparation method according to any one of claims 1 - 8, wherein, the method for preparing the dendritic polypropylene imine includes: Mixing ethylenediamine, acrylonitrile, a first catalyst and a first solvent, carrying out a first catalytic reaction to obtain a dendritic polypropylene imine precursor; Mixing the obtained dendritic polypropylene imine precursor, a second catalyst and a second solvent, and carrying out a second catalytic reaction under hydrogen to obtain the first generation of dendritic polypropylene imine; Using the nth generation of dendritic polypropylene imine to replace the dendritic polypropylene imine precursor for the second catalytic reaction to obtain the (n + 1)th generation of dendritic polypropylene imine, where n is a natural number.

10. The preparation method according to claim 9, wherein, the method for preparing the dendritic polypropylene imine further includes: Mixing ethylenediamine and a first solvent, adding acrylonitrile for a low-temperature reaction, then adding a first catalyst for a high-temperature reaction, cooling and removing the first solvent, obtaining the remaining organic matter, adding a third solvent to dissolve and then carrying out cooling crystallization to obtain the dendritic polypropylene imine precursor; Optionally, the first solvent includes water; Optionally, the mass ratio of ethylenediamine to the first solvent is 1:(3 - 5); Optionally, the dosage of acrylonitrile is equal to the mass of the first solvent; Optionally, the temperature of the low-temperature reaction is -5 °C to 5 °C; Optionally, the first catalyst includes acetic acid; Optionally, the temperature of the high-temperature reaction is 70 - 90 °C; Optionally, the third solvent includes methanol; Optionally, the method for removing the first solvent includes vacuum distillation; Optionally, the second solvent includes 1,4 - dioxane; Optionally, the second catalyst includes Umemoto cobalt catalyst; Optionally, the temperature of the second catalytic reaction is 50 - 70 °C.

11. The preparation method according to claim 9 or 10, wherein, the dendritic polypropyleneimine used is any one or a mixture composed of at least two of the 1st generation to the 5th generation dendritic polypropyleneimines.

12. The preparation method according to any one of claims 1-11, wherein, the molecular weight of the dendritic polypropyleneimine used in the preparation method is 300 to 50,000.

13. The preparation method according to any one of claims 9-12, wherein, the preparation method includes: Mix ethylenediamine and the first solvent water evenly at a mass ratio of 1:(3-5), slowly drop acrylonitrile with the same mass as water under ice bath conditions, carry out the reaction at low temperature for 2-4 h, add acetic acid as the first catalyst, raise the temperature of the mixture to 70-90 °C and reflux for 4-6 h to carry out the high-temperature reaction, then cool to room temperature to remove the first solvent water phase, carry out vacuum distillation to obtain the remaining organic matter, then dissolve it by heating with the third solvent methanol, and obtain the dendritic polypropyleneimine precursor after cooling crystallization, filtration, and washing with water to remove by-products; Add the dendritic polypropyleneimine precursor, the second solvent 1,4-dioxane, and the second catalyst Umemoto cobalt into an autoclave, introduce hydrogen to raise the temperature of the reaction kettle to 50-70 °C for the second catalytic reaction. When the hydrogen pressure no longer drops, stop the reaction, filter the second catalyst, cool and evaporate the second solvent to obtain the 1st generation polypropyleneimine dendrimer; use the nth generation dendritic polypropyleneimine to replace the dendritic polypropyleneimine precursor for the second catalytic reaction to obtain the (n + 1)th generation dendritic polypropyleneimine; obtain a mixture of the 1st generation to the 5th generation dendritic polypropyleneimines with a molecular weight of 300 to 50,000; Prepare solution A by mixing an iron source and a phosphorus source, controlling the concentration of iron ions in solution A to be 0.1-2 mol / L, and the molar ratio of iron ions to phosphate ions to be 1:(1-1.1); prepare solution B with the mixture of dendritic polypropyleneimines, controlling the concentration of dendritic polypropyleneimine in solution B to be 2-5 wt%; place solution A in a reaction kettle, slowly add solution B until the pH is 1.5-2.2, heat for the formation reaction to produce a precipitate, that is, produce primary iron phosphate containing dendritic polypropyleneimine, and then stir at 60-90 °C for 4-12 h and keep warm for a period of time for aging; Filter the precipitate, wash it with pure water and ethanol until the upper centrifuged liquid is colorless and clear, dry it in a vacuum drying oven at 60-120 °C for 4-10 h, then place it in a muffle furnace under an air atmosphere, raise the temperature to 500-750 °C and calcine for 2-8 h, keep warm to remove crystal water, and make the dendritic polypropyleneimine turn into gas and remove it from the iron phosphate. After crushing, anhydrous iron phosphate with a three-dimensional dendritic network channel inside is obtained.

14. An iron phosphate obtained by using the preparation method according to any one of claims 1-13.

15. A cathode material prepared by using the iron phosphate according to claim 14.

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