Ferric phosphate and its preparation method and use

Abstract

The present disclosure provides an iron phosphate and a preparation method and use thereof. The preparation method uses tree-shaped polypropylene imine as a medium to replace the alkaline substance of 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, iron phosphate containing a three-dimensional tree-shaped network channel can be obtained. The preparation method does not bring 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. When lithium iron phosphate is prepared subsequently, the channel can shorten the lithium ion transmission distance, thereby improving the electrical properties of lithium iron phosphate.

Description

Technical Field

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

[0002] At present, with the rapid development of new energy industry, electric vehicles and smart Internet of Things, the market demand for lithium-ion batteries is growing rapidly. According to the different positive electrode materials, lithium-ion batteries are divided into lithium cobalt oxide, lithium manganese oxide, ternary material system and lithium iron phosphate. Among them, lithium iron phosphate battery has become the most promising lithium-ion battery with its excellent cycle performance and low preparation cost.

[0003] The raw materials commonly used in preparing lithium iron phosphate positive electrode materials include iron phosphate. Slight changes in the microstructure and chemical composition of iron phosphate will have a huge impact on the performance of lithium iron phosphate. At present, the synthesis method of iron phosphate is mainly co-precipitation, that is, a large amount of alkaline substances are introduced into an acidic environment to adjust the pH of the solution to obtain iron phosphate precipitation.

[0004] The above 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 cost 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-precipitation, which increases the cost of wastewater treatment.

[0005] CN112340719B uses chloride salt instead of the alkaline solution required for the conventional reaction, and then adds phosphoric acid 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 impure metal cations in the chloride salt, and a large number of washing steps are still required to remove them in the later stage.

[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. Summary of the invention

[0007] 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.

[0008] In view of the problems existing in the prior art, the purpose of the present disclosure is to provide an iron phosphate and a preparation method and use thereof. The preparation method uses tree-shaped polypropylene imine as a medium to replace the alkaline substance of the prior art to adjust the pH, thereby generating iron phosphate and simultaneously allowing the iron phosphate to precipitate and grow on its tree structure through coordination. Finally, after removing the polypropylene imine at high temperature, iron phosphate containing a three-dimensional tree network channel can be obtained. The preparation method does not bring 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 network channel makes the iron phosphate easier to grind and process. When lithium iron phosphate is prepared subsequently, the channel can shorten the lithium ion transmission distance, thereby improving the electrical properties of lithium iron phosphate.

[0009] To achieve this purpose, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present disclosure provides a method for preparing iron phosphate, the preparation method comprising:

[0011] The dendritic polypropylene imine and the ferric phosphate raw material are mixed and reacted to obtain primary ferric phosphate containing the dendritic polypropylene imine, which is then calcined to remove the dendritic polypropylene imine and obtain ferric phosphate.

[0012] The present disclosure uses dendritic polypropylene imine as a medium to replace the alkaline substances in the prior art to adjust the pH, because a large number of primary and tertiary amines in polypropylene imine are alkaline. The present disclosure finds that replacing conventional precipitants such as sodium hydroxide, ammonia water or other inorganic alkaline substances with dendritic polypropylene imine can reduce the introduction of impurities during the coprecipitation of iron phosphate, reduce the treatment cost of wastewater (such as the presence of ammonium ions) after the precipitation process, and can successfully prepare iron phosphate. For the large number of amine groups possessed by polypropylene imine, the lone pair of 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.

[0013] Polypropylene imine can form a more complex tree-like structure by repeatedly branching monomers. When the number of branching generations is low, an open amorphous structure is formed. When the number of branching generations is high, a spheroid with a tight outside and a loose inside is formed after three generations. Therefore, the number of branching generations of polyethylene imine can be controlled to synthesize iron phosphates with different morphologies. Iron phosphate grows on polypropylene imine under the action of adsorption, which can accelerate the crystallization of primary particles. When the primary iron phosphate containing tree-shaped polypropylene imine is treated at high temperature to dehydrate and the carbon, hydrogen and nitrogen elements in the polypropylene imine are volatilized in the form of gas, a three-dimensional tree-like cavity channel can be formed inside the iron phosphate. This unique iron phosphate is easy to grind and crush for the subsequent synthesis of lithium iron phosphate, and the three-dimensional network channel is also beneficial to the diffusion of lithium ions, thereby improving the electrical properties of the obtained lithium iron phosphate cathode material.

[0014] The following are optional technical solutions of the present disclosure, but are not intended to be limitations of 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.

[0015] 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.

[0016] The present disclosure uses dendritic polypropylene imine as an alkalinity regulator, so the method of adding it can be similar to that of a conventional alkaline solution, such as ammonia water, and it is better to add it alone into a solution containing the ferric phosphate raw material.

[0017] In one embodiment, the pH of the formation reaction is 1.5-2.2, 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, and other values ​​not listed within the above range are also applicable.

[0018] 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.

[0019] In one embodiment, in the solution A, the concentration of iron ions 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, and other values ​​not listed within the above numerical range are also applicable.

[0020] In one embodiment, in the solution A, the molar ratio of iron ions 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.

[0021] In one embodiment, in the solution B, the concentration of the dendritic polypropylene imine is 2 to 5 wt%, for example, 2wt%, 2.3wt%, 2.5wt%, 2.8wt%, 3wt%, 3.2wt%, 3.5wt%, 3.8wt%, 4wt%, 4.2wt%, 4.4wt%, 4.8wt% or 5wt%, etc., but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0022] Solution B has a lower concentration and is less protonated, allowing more amine groups to act as bases to adjust pH.

[0023] As an optional technical solution of the present disclosure, the ferric phosphate raw material includes an iron source and a phosphorus source.

[0024] The present application does not specifically limit the iron source and phosphorus source of the iron phosphate raw material used. Any iron source and phosphorus source that can obtain iron phosphate by coprecipitation method are applicable to the present disclosure.

[0025] In one embodiment, the iron source includes 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.

[0026] In one embodiment, when the iron source comprises pure iron, the ferric phosphate raw material further comprises an oxidant.

[0027] When pure iron is used, it generates ferrous substances in the reaction. For example, pure iron and phosphoric acid react to generate ferrous dihydrogen phosphate. At this time, an oxidant such as hydrogen peroxide is added to oxidize it into trivalent iron, which is adsorbed on the dendritic polyimide and further forms ferric phosphate.

[0028] In one embodiment, the phosphorus source includes any one of diammonium 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 diammonium 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 diammonium phosphate and phosphoric acid.

[0029] As an optional technical solution of the present disclosure, the preparation method further comprises, after the primary iron phosphate with tree-shaped polypropylene imine is generated, heat preservation and aging.

[0030] In one embodiment, the heat preservation and aging method includes stirring at 60-90°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C, for 4-12h, for example, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h or 12h, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0031] In one embodiment, the preparation method further comprises, before the calcination, washing and drying.

[0032] In one embodiment, the washing cleaning solution comprises water and / or ethanol.

[0033] In one embodiment, the drying temperature is 60-120°C, for example 60°C, 70°C, 80°C, 90°C, 100°C, 110°C or 120°C, and the drying time is 4-10h, for example 4h, 5h, 6h, 7h, 8h, 9h or 10h, but is not limited to the listed values, and other values ​​not listed within the above numerical range are also applicable.

[0034] In one embodiment, the calcining is performed under an oxygen-containing atmosphere.

[0035] The carbon, hydrogen and oxygen in the organic polypropylene imine can react to generate gas under 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.

[0036] 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 unlisted values ​​within the above numerical range are also applicable.

[0037] As an optional technical solution of the present disclosure, the method for preparing the tree-shaped polypropylene imine comprises:

[0038] Mixing ethylenediamine, acrylonitrile, a first catalyst and a first solvent to perform a first catalytic reaction to obtain a dendritic polypropyleneimine precursor;

[0039] The obtained dendritic polypropylene imine precursor, the second catalyst and the second solvent are mixed, and a second catalytic reaction is carried out under hydrogen to obtain the first generation dendritic polypropylene imine;

[0040] The nth generation of 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 of dendritic polypropylene imine, where n is a natural number.

[0041] As an optional technical solution of the present disclosure, the method for preparing the tree-shaped polypropylene imine also includes:

[0042] Ethylenediamine and a first solvent are mixed, acrylonitrile is added to react at low temperature, a first catalyst is added to react at high temperature, the first solvent is removed after cooling to obtain residual organic matter, a third solvent is added to dissolve the organic matter, and then the organic matter is cooled and crystallized to obtain a dendritic polypropylene imine precursor.

[0043] In one embodiment, the first solvent comprises water.

[0044] 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.

[0045] In one embodiment, the amount of acrylonitrile used is equal to the mass of the first solvent.

[0046] 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, but is not limited to the listed values, and other unlisted values ​​within the above numerical range are also applicable.

[0047] In one embodiment, the first catalyst comprises acetic acid.

[0048] 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, and other unlisted values ​​within the above numerical range are also applicable.

[0049] In one embodiment, the third solvent comprises methanol.

[0050] In one embodiment, the method of removing the first solvent comprises distillation under reduced pressure.

[0051] In one embodiment, the second solvent comprises 1,4-dioxane.

[0052] In one embodiment, the second catalyst comprises a Urushihara cobalt catalyst.

[0053] 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, and other unlisted values ​​within the above numerical range are also applicable.

[0054] As an optional technical solution of the present disclosure, the tree-shaped polypropylene imine used is any one of the first generation tree-shaped polypropylene imine to the fifth generation tree-shaped polypropylene imine or a mixture of at least two of them.

[0055] In one embodiment, the molecular weight of the dendritic 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.

[0056] In order to achieve different morphological characteristics, the higher the generation number of the tree-like polypropylene imine, the more obvious the tree-like structure is, and the more spherical it is, which is loose inside and tight outside. In this way, iron phosphates of different sizes and morphologies can be prepared.

[0057] As an optional technical solution of the present disclosure, the preparation method comprises:

[0058] Ethylenediamine and 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 under ice bath conditions, reacting at low temperature for 2-4 hours, adding acetic acid as a first catalyst, heating the mixture to 70-90° C. and refluxing for 4-6 hours to react at high temperature, then cooling to room temperature to remove the first solvent water phase, performing reduced pressure distillation to obtain the remaining organic matter, then using the third solvent methanol to heat and dissolve, cooling and crystallizing, suction filtering, and washing with water to remove by-products to obtain a dendritic polypropylene imine precursor;

[0059] A dendritic polypropylene imine precursor, a second solvent 1,4-dioxane, and a second catalyst cobalt are added to an autoclave, hydrogen is introduced to heat the autoclave to 50-70° C. for a second catalytic reaction, and when the hydrogen pressure no longer decreases, the reaction is stopped, the second catalyst is filtered, and the second solvent is removed by cooling to obtain a first-generation polypropylene imine dendrimer; the nth-generation dendritic polypropylene imine is used to replace the dendritic polypropylene imine precursor for a second catalytic reaction to obtain an n+1th-generation dendritic polypropylene imine; a mixture of the first-generation dendritic polypropylene imine to the fifth-generation dendritic polypropylene imine having a molecular weight of 300-50,000 is obtained;

[0060] The iron source and the phosphorus source are prepared into solution A, and the concentration of the iron ion in solution A is controlled to be 0.1-2 mol / L, and the molar ratio of the iron ion to the phosphate is 1:(1-1.1); the mixture of the dendritic polypropylene imine is prepared into solution B, and the concentration of the dendritic polypropylene imine in solution B is controlled to be 2-5 wt%; the solution A is placed in a reaction kettle, and the solution B is slowly added until the pH value is 1.5-2.2, and the solution is heated to perform a generation reaction to generate a precipitate, that is, to generate primary iron phosphate containing the dendritic polypropylene imine, and then stirred at 60-90° C. for 4-12 hours and kept warm for a period of time for aging;

[0061] The precipitated material is filtered, washed with pure water and ethanol until the upper centrifuged liquid is colorless and clear, dried in a vacuum drying oven at 60-120°C for 4-10 hours, and then placed in a muffle furnace under an air atmosphere, heated to 500-750°C for calcination for 2-8 hours, and kept warm to remove crystal water, and the tree-shaped polypropylene imine is converted into gas and removed from the iron phosphate. After crushing, anhydrous iron phosphate containing three-dimensional tree-shaped network channels is obtained.

[0062] In a second aspect, the present disclosure provides an iron phosphate, which is obtained using the preparation method described in the first aspect.

[0063] 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.

[0064] Compared with the prior art solutions, the present invention has at least the following beneficial effects:

[0065] The present disclosure provides a method for solving the problem of low utilization rate of a large amount of alkaline substances in the production process of iron phosphate, by introducing polypropylene imine with a tree structure as a base to adjust the pH of the solution, and by coordination, the iron phosphate is precipitated and grown on its tree structure, and finally removed at high temperature to generate iron phosphate containing a three-dimensional tree network channel. The present disclosure can effectively reduce the presence of alkali in co-precipitation wastewater and reduce the cost of wastewater treatment. The three-dimensional tree network channel makes the iron phosphate easier to grind and process. When preparing lithium iron phosphate in the later stage, the channel can shorten the lithium ion transmission distance, thereby improving the electrical properties of lithium iron phosphate.

[0066] Other aspects will be apparent upon reading and understanding the drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The accompanying drawings are used to provide further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of the present 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.

[0068] Figure 1This is a SEM image of the anhydrous ferric phosphate obtained in Example 1. DETAILED DESCRIPTION

[0069] The technical solution of the present disclosure is further illustrated below through specific implementation methods.

[0070] 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.

[0071] The dendritic polyimide used in the following examples and comparative examples was prepared by the following method:

[0072] Ethylenediamine and water are mixed evenly in a mass ratio of 1:4, and acrylonitrile of the same mass as water is slowly added dropwise under ice bath conditions, reacted at low temperature for 3 hours, acetic acid is added as a catalyst, the mixture is heated to 80°C and refluxed for 5 hours, cooled to room temperature to remove the water phase, and the remaining organic matter is distilled under reduced pressure, heated and dissolved with methanol, cooled and crystallized, filtered, and washed with water to remove by-products to obtain a dendritic polypropylene imine precursor. Dendritic polypropylene imine precursor, 1,4-dioxane solvent, and Urushihara cobalt catalyst are added to the autoclave, hydrogen is introduced to heat the reactor to 60°C for reaction, the reaction is maintained until the hydrogen pressure no longer decreases, the catalyst is filtered, and the solvent is cooled and evaporated to obtain the first generation of dendritic polypropylene imine. The first generation of dendritic polypropylene imine replaces the dendritic polypropylene imine precursor and reacts with 1,4-dioxane solvent and Urushihara cobalt catalyst again under hydrogen to obtain the second generation of dendritic polypropylene imine; this process is repeated to obtain the 1st to 7th generation of dendritic polypropylene imine.

[0073] Example 1

[0074] This embodiment provides a method for preparing iron phosphate, the preparation method comprising:

[0075] (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); preparing a solution B with a concentration of 3 wt% by molar ratio of the second and fourth generation dendritic polypropylene imine at 1:1; uniformly mixing the solution A in a reactor and setting the stirring speed to 700 rpm, then slowly adding the solution B to the reactor, adjusting the pH in the system to 1.8, generating a precipitate, and heating the water bath to 85°C for 4 hours for aging;

[0076] (2) The precipitate is filtered out and washed alternately with deionized water and ethanol. When the upper centrifuged liquid becomes colorless and clear, the washed precipitate is placed in a drying oven and dried at 90°C for 8 h to obtain FePO4·xH2O material. The prepared FePO4·xH2O is then placed in a muffle furnace in an air atmosphere and heated to 600°C for calcination for 6 h. After crushing, anhydrous iron phosphate containing a three-dimensional tree-shaped network channel is obtained.

[0077] Figure 1 : is a SEM picture of the anhydrous ferric phosphate obtained in Example 1. It can be seen from the picture 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.

[0078] Example 2

[0079] This embodiment provides a method for preparing ferric phosphate. The preparation method uses a dendritic polypropylene imine precursor to replace the second generation and fourth generation dendritic polypropylene imine in Example 1. Other conditions are exactly the same as those in Example 1.

[0080] Example 3

[0081] This embodiment provides a method for preparing ferric phosphate. The preparation method uses the first generation of dendritic polypropylene imine to replace the second generation and fourth generation of dendritic polypropylene imine in Example 1. Apart from this, other conditions are exactly the same as those in Example 1.

[0082] Example 4

[0083] This embodiment provides a method for preparing iron phosphate, wherein the preparation method uses only the 4th generation dendritic polypropylene imine, that is, the 4th generation dendritic polypropylene imine is used to replace the 2nd generation dendritic polypropylene imine in Example 1. Apart from this, other conditions are exactly the same as those in Example 1.

[0084] Example 5

[0085] This embodiment provides a method for preparing iron phosphate. The preparation method uses the fifth-generation dendritic polypropylene imine to replace the second-generation dendritic polypropylene imine in Example 1. Other conditions are exactly the same as those in Example 1.

[0086] Example 6

[0087] This embodiment provides a method for preparing iron phosphate. The preparation method uses the 7th generation dendritic polypropylene imine to replace the 2nd generation dendritic polypropylene imine in Example 1. Apart from this, other conditions are exactly the same as those in Example 1.

[0088] Example 7

[0089] This embodiment provides a method for preparing iron phosphate, wherein the preparation method adjusts the amount of dendritic polypropylene imine, that is, changes the pH by adjusting the amount of solution B, so that the pH is adjusted from 1.8 to 1.2. Apart from this, other conditions are exactly the same as those in Example 1.

[0090] Example 8

[0091] This embodiment provides a method for preparing ferric phosphate, wherein the preparation method adjusts the amount of dendritic polypropylene imine, that is, changes the pH by adjusting the amount of solution B, so that the pH is adjusted from 1.8 to 1.5. Apart from this, other conditions are exactly the same as those in Example 1.

[0092] Example 9

[0093] This embodiment provides a method for preparing iron phosphate, wherein the preparation method adjusts the amount of dendritic polypropylene imine, that is, changes the pH by adjusting the amount of solution B, so that the pH is adjusted from 1.8 to 2.2. Apart from this, other conditions are exactly the same as those in Example 1.

[0094] Example 10

[0095] This embodiment provides a method for preparing iron phosphate, wherein the preparation method adjusts the amount of dendritic polypropylene imine, that is, changes the pH by adjusting the amount of solution B, so that the pH is adjusted from 1.8 to 2.5. Apart from this, other conditions are exactly the same as those in Example 1.

[0096] Embodiment 11

[0097] This embodiment provides a method for preparing iron phosphate. In the preparation method, the calcination temperature is adjusted from 600° C. to 470° C. Apart from this, other conditions are exactly the same as those in Example 1.

[0098] Example 12

[0099] This embodiment provides a method for preparing iron phosphate. In the preparation method, the calcination temperature is adjusted from 600° C. to 750° C. Apart from this, other conditions are exactly the same as those in Example 1.

[0100] Example 13

[0101] This embodiment provides a method for preparing iron phosphate, in which the calcination time is adjusted from 6 hours to 3 hours. Apart from this, other conditions are exactly the same as those in Example 1.

[0102] Embodiment 14

[0103] This embodiment provides a method for preparing iron phosphate, in which the calcination time is adjusted from 6 hours to 8 hours. Apart from this, other conditions are exactly the same as those in Example 1.

[0104] Comparative Example 1

[0105] This comparative example provides a method for preparing ferric phosphate, wherein the preparation method uses ammonia water instead of dendritic polyimide for pH adjustment. Apart from this, other conditions are exactly the same as those in Example 1.

[0106] The anhydrous iron phosphate material obtained in the embodiment and the comparative example was uniformly dispersed and mixed with lithium carbonate and glucose in water, and the mixture was ball-milled for 5 hours until uniformly mixed according to the stoichiometric ratio of iron, lithium and carbon elements of 1:1.02:0.05, and then spray-dried to obtain a precursor powder. The precursor powder was kept at 350°C for 2 hours at a heating rate of 3°C / min under a nitrogen protective atmosphere, and then heated to 750°C and calcined at high temperature for 8 hours to obtain the LiFePO4 / C positive electrode material active substance.

[0107] The button cell was assembled to test the electrochemical performance of lithium-ion batteries. The specific steps were as follows: the positive electrode material active substance, the conductive agent acetylene black and the adhesive polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a ratio of 92:4:4, and then coated on aluminum foil and placed in a vacuum drying oven for drying. After drying, the battery was assembled in an argon glove box, and pressed into a positive electrode sheet by a sheet press. The negative electrode was a metal lithium sheet, the electrolyte was 1M LiPF6-EC:DMC (volume ratio 1:1), and a polypropylene porous membrane was used as a separator. The electrochemical performance was tested, and the results are shown in Table 1 below.

[0108] Table 1

[0109]

[0110]

[0111] It can be seen from Table 1 that:

[0112] By comparing Example 1 with Example 2-6, in the preparation process of the modified iron phosphate disclosed in the present invention, 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 cumbersome, and defects are prone to affect the characteristics of its synthetic structure, and the higher the algebra, the more spherical it is, and the dendritic network conductive channel cannot be obtained after the synthetic iron phosphate is calcined; if the algebra is too low, the tree-shaped polypropylene imine is the first or second generation when the tree-shaped polypropylene imine is not obvious but the structure is stable, and the number of amine groups contained is moderate and easy to protonate. Therefore, when the modified iron phosphate introduces the tree-shaped polypropylene imine, the effect brought by the mixture of low algebra and medium and high algebra is the best.

[0113] By comparing Example 1 with Examples 7-10, in the preparation process of the modified ferric phosphate disclosed in the present invention, adjusting the pH of the coprecipitation will affect its performance. Dendritic polypropylene imine is used as an alkali solution to adjust the pH. If the pH is adjusted to too high, not only the amount of dendritic polypropylene imine is increased, but also iron hydroxide colloid is easily generated to bring impurities; if the pH is adjusted to too low, the coprecipitation rate will be affected.

[0114] By comparing Example 1 with Examples 11-15, in the preparation process of the modified ferric phosphate disclosed in the present invention, adjusting the temperature and time during the calcination process will affect its performance. Because the tree-shaped polypropylene imine is co-precipitated with the ferric phosphate due to the coordination effect, if the calcination temperature is not enough or the heat treatment time is insufficient, the tree-shaped polypropylene imine cannot be completely separated from the ferric phosphate, thereby having a complete three-dimensional tree-shaped channel; if the calcination temperature is too high or the heat treatment time is too long, the crystal form of the ferric phosphate will be destroyed.

Claims

1. A method for preparing ferric phosphate, comprising: Mixing the tree-shaped polypropylene imine and the iron phosphate raw material to perform a generation reaction to obtain primary iron phosphate containing the tree-shaped polypropylene imine, and calcining to remove the tree-shaped polypropylene imine to obtain iron phosphate; The method for preparing the tree-shaped polypropylene imine comprises: Mixing ethylenediamine, acrylonitrile, a first catalyst and a first solvent to perform a first catalytic reaction to obtain a dendritic polypropyleneimine precursor; the first solvent comprises water; the first catalyst comprises acetic acid; The obtained dendritic polypropylene imine precursor, a second catalyst and a second solvent are mixed, and a second catalytic reaction is carried out under hydrogen to obtain a first generation dendritic polypropylene imine; the second solvent includes 1,4-dioxane; the second catalyst includes Urushihara cobalt catalyst; The nth generation of 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 of dendritic polypropylene imine, where n is a natural number.

2. The preparation method according to claim 1, wherein The preparation method comprises the following steps: firstly 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.

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

2.

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

5. The preparation method according to claim 1, wherein The preparation method further comprises: after the primary ferric phosphate with tree-shaped polypropylene imine is generated, heat preservation and aging is performed.

6. The preparation method according to claim 5, wherein: The heat preservation and aging method comprises stirring at 60 to 90° C. for 4 to 12 hours.

7. The preparation method according to claim 1, wherein The calcination is performed in an oxygen-containing atmosphere.

8. The preparation method according to claim 1, wherein The calcination temperature is 500-750° C. and the calcination time is 2-8 hours.

9. The preparation method according to claim 1, wherein: The method for preparing the tree-shaped polypropylene imine also includes: Ethylenediamine and a first solvent are mixed, acrylonitrile is added to react at low temperature, a first catalyst is added to react at high temperature, the first solvent is removed after cooling to obtain residual organic matter, a third solvent is added to dissolve the organic matter, and then the organic matter is cooled and crystallized to obtain a dendritic polypropylene imine precursor.

10. The preparation method according to claim 1 or 9, wherein: The mass ratio of the ethylenediamine to the first solvent is 1:(3-5).

11. The preparation method according to claim 1 or 9, wherein: The amount of acrylonitrile used is equal to the mass of the first solvent.

12. The preparation method according to claim 9, wherein: The temperature of the low temperature reaction is -5°C to 5°C.

13. The preparation method according to claim 9, wherein: The temperature of the high temperature reaction is 70-90°C.

14. The preparation method according to claim 9, wherein: The third solvent includes methanol.

15. The preparation method according to claim 9, wherein: The method of removing the first solvent includes distillation under reduced pressure.

16. The preparation method according to claim 1 or 9, wherein: The temperature of the second catalytic reaction is 50-70°C.

17. The preparation method according to claim 1, wherein: The used dendritic polypropylene imine is any one of the first generation dendritic polypropylene imine to the fifth generation dendritic polypropylene imine or a mixture of at least two of them.

18. The preparation method according to claim 1, wherein: The molecular weight of the tree-shaped polypropylene imine used in the preparation method is 300-50000.

19. The preparation method according to claim 1, wherein: The preparation method comprises: Ethylenediamine and 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 under ice bath conditions, reacting at low temperature for 2-4 hours, adding acetic acid as a first catalyst, heating the mixture to 70-90° C. and refluxing for 4-6 hours to react at high temperature, then cooling to room temperature to remove the first solvent water phase, performing reduced pressure distillation to obtain the remaining organic matter, then using the third solvent methanol to heat and dissolve, cooling and crystallizing, suction filtering, and washing with water to remove by-products to obtain a dendritic polypropylene imine precursor; A dendritic polypropylene imine precursor, a second solvent 1,4-dioxane, and a second catalyst cobalt are added to an autoclave, hydrogen is introduced to heat the autoclave to 50-70° C. for a second catalytic reaction, and when the hydrogen pressure no longer decreases, the reaction is stopped, the second catalyst is filtered, and the second solvent is removed by cooling to obtain a first-generation polypropylene imine dendrimer; the nth-generation dendritic polypropylene imine is used to replace the dendritic polypropylene imine precursor for a second catalytic reaction to obtain an n+1th-generation dendritic polypropylene imine; a mixture of the first-generation dendritic polypropylene imine to the fifth-generation dendritic polypropylene imine having a molecular weight of 300-50,000 is obtained; The iron source and the phosphorus source are prepared into solution A, and the concentration of the iron ion in solution A is controlled to be 0.1-2 mol / L, and the molar ratio of the iron ion to the phosphate is 1:(1-1.1); the mixture of the dendritic polypropylene imine is prepared into solution B, and the concentration of the dendritic polypropylene imine in solution B is controlled to be 2-5 wt%; the solution A is placed in a reaction kettle, and the solution B is slowly added until the pH value is 1.5-2.2, and the solution is heated to perform a generation reaction to generate a precipitate, that is, to generate primary iron phosphate containing the dendritic polypropylene imine, and then stirred at 60-90° C. for 4-12 hours and kept warm for a period of time for aging; The precipitated material is filtered, washed with pure water and ethanol until the upper centrifuged liquid is colorless and clear, dried in a vacuum drying oven at 60-120°C for 4-10 hours, and then placed in a muffle furnace under an air atmosphere, heated to 500-750°C for calcination for 2-8 hours, and kept warm to remove crystal water, and the tree-shaped polypropylene imine is converted into gas and removed from the iron phosphate. After crushing, anhydrous iron phosphate containing three-dimensional tree-shaped network channels is obtained.

20. Ferric phosphate obtained by the preparation method according to any one of claims 1 to 19.

21. A positive electrode material made using the iron phosphate according to claim 20.

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