High-density lithium iron phosphate and preparation method thereof

Abstract

The application provides a high-compaction-density lithium iron phosphate and a preparation method thereof, and relates to the technical field of lithium batteries. The preparation method comprises the following steps: adding an iron source, phosphoric acid and fumaric acid into an aqueous system; under the condition that the pH is less than 2, making the iron ions dissolved from the iron source and the fumaric acid perform a coordination reaction; adjusting the pH of the aqueous system to 3-5 to make the iron ions react with the phosphate to generate a lithium iron phosphate (MIL-88A / FePO4) composite material. The method realizes the pre-construction of the MIL-88A framework and the in-situ precipitation of the lithium iron phosphate through two-step pH control, and prepares an organic-inorganic uniform composite material; the template effect of the framework solves the passivation problem of the traditional reaction, so that the reaction is more complete; and the organic matter can be used as an internal carbon source precursor and is uniformly dispersed, which lays a structural foundation for the subsequent formation of an excellent conductive network.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium batteries, in particular to a high-compaction-density lithium iron phosphate and a preparation method thereof. BACKGROUND

[0002] With the increasing emphasis on clean energy and sustainable development worldwide, secondary battery technology represented by lithium-ion batteries has become the core driving force for the development of electric vehicles, consumer electronics and large-scale energy storage systems. In the composition of lithium-ion batteries, the positive electrode material is a key component that determines the energy density, power density, cycle life and safety performance of the battery. Among the many positive electrode material systems, lithium iron phosphate (LiFePO4) material with olivine structure has been widely commercialized in the fields of power batteries and energy storage due to its stable chemical structure, excellent thermal stability, long cycle life, environmental friendliness and low cost.

[0003] Although lithium iron phosphate material has many performance advantages, its low intrinsic electrical conductivity and ion diffusion rate, as well as relatively low compaction density (usually less than 2.4 g / cm 3 ), to some extent, limits the further improvement of the volumetric energy density of lithium-ion batteries. In order to overcome these limitations, various modification strategies have been developed in the prior art. Common physical modification methods include adjusting the temperature profile of the sintering process, fine grinding of the material to control the particle size distribution, or using particles of different sizes for slurry grading, in order to achieve tighter particle packing. In addition, chemical modification methods have also been widely studied, for example, a patent document discloses a technical solution that combines bulk and surface co-doping with a multi-step sintering process to control the grain, aiming to improve the capacity of the material without sacrificing the compaction density. Another disclosed technical solution adopts a one-step sintering and two-step sintering combined process to prepare lithium iron phosphate material with high compaction density and low magnetic foreign matter content by precisely controlling the sintering atmosphere and temperature.

[0004] However, the above prior art still faces many challenges in practical application. By adjusting the sintering temperature or the physical means such as grinding particle size, although the compaction density can be improved to some extent, the electrochemical performance of the material is often sacrificed, for example, the capacity decay or the rate performance may be reduced. In particular, the high-temperature sintering process not only significantly increases the production energy consumption, but also easily generates magnetic impurities such as iron phosphide due to the reduction of iron, which affects the final safety performance of the battery, and the batch stability of the material at high temperature is also difficult to control. For technical solutions that use complex processes such as multi-step sintering and doping, although the performance is improved, the process flow is long and the control links are many, which leads to increased production cost and improved process complexity. Even the disclosed patent technology needs to be improved, for example, the above-mentioned technical solution using secondary sintering process still needs to optimize the process parameters to further control the magnetic foreign matter and improve the compaction density; and the technical solution using doping modification faces the problem of how to further optimize the types and contents of doping elements to maximize the electronic conductivity and ion mobility of the material.

[0005] In summary, although the prior art has made some progress in improving the compaction density of lithium iron phosphate material, there is a common problem that the performance is difficult to be optimized simultaneously. How to effectively improve the compaction density of the material on the basis of ensuring high specific capacity and excellent cycle performance, while simplifying the preparation process, reducing the production cost, avoiding the side effects such as excessive magnetic impurities and unstable performance caused by high-temperature sintering, has become a technical problem to be solved in the field of lithium iron phosphate cathode material. Therefore, developing a new preparation method of lithium iron phosphate material which can realize high compaction density, high capacity and stable and controllable process simultaneously has important practical significance for promoting the improvement of energy density of lithium ion battery.

[0006] Therefore, the present application is proposed. SUMMARY

[0007] The purpose of the present application is to provide a high compaction density lithium iron phosphate and a preparation method thereof. The preparation method of the MIL-88A / FePO4 composite material is prepared by a two-step pH-controlled in-situ synthesis method, which uses a pre-constructed MIL-88A framework as a template to precipitate iron phosphate, which not only solves the problem of reaction passivation, but also obtains a composite material with highly uniform dispersion of built-in carbon source precursors.

[0008] In order to achieve the above-mentioned purpose of the present application, the following technical scheme is adopted:

[0009] In a first aspect, the present application provides a preparation method of a MIL-88A / FePO4 composite material, comprising:

[0010] adding an iron source, phosphoric acid and fumaric acid into an aqueous system;

[0011] The iron ions dissolved from the iron source and the fumaric acid are subjected to a coordination reaction under the condition that the pH is less than 2; wherein the molar ratio of the iron source to fumaric acid is (6-14):1.

[0012] The pH of the aqueous system is adjusted to 3-5, so that the iron ions react with the phosphate ions to generate iron phosphate precipitate, thereby generating the MIL-88A / FePO4 composite material in situ.

[0013] In an optional embodiment, the iron source comprises at least one of ferrous oxide and ferric oxide.

[0014] In an optional embodiment, the aqueous system is subjected to stirring and mixing during the coordination reaction.

[0015] In an optional embodiment, the stirring and mixing is performed for not less than 12 hours; and / or, the stirring and mixing is performed at a rotation speed of not less than 700 rpm / min.

[0016] In a second aspect, the present application provides a MIL-88A / FePO4 composite material, which is prepared by the method for preparing the MIL-88A / FePO4 composite material according to any one of the preceding embodiments.

[0017] In a third aspect, the present application provides a method for preparing high-compact-density lithium iron phosphate, comprising:

[0018] The MIL-88A / FePO4 composite material according to the preceding embodiments is mixed with a lithium source and then subjected to high-temperature sintering, thereby obtaining the high-compact-density lithium iron phosphate.

[0019] In a fourth aspect, the present application provides high-compact-density lithium iron phosphate, which is prepared by the method for preparing the high-compact-density lithium iron phosphate according to the preceding embodiments.

[0020] In a fifth aspect, the present application provides a positive electrode, which comprises the high-compact-density lithium iron phosphate according to the preceding embodiments.

[0021] In a sixth aspect, the present application provides a battery, which comprises the positive electrode according to the preceding embodiments.

[0022] In a seventh aspect, the present application provides an electrical equipment, which comprises the battery according to the preceding embodiments.

[0023] The application provides high compaction density lithium iron phosphate and a preparation method thereof. The preparation method of the MIL-88A / FePO4 composite material is realized by unique two-step pH partition control, and the selectivity of the reactants is precisely controlled. Under the initial strong acid condition (pH less than 2), the coordination reaction of iron ions and fumaric acid is preferentially promoted, and the metal organic framework (MIL-88A) structure is preliminarily constructed. The separation in time sequence avoids the disorderly competition of phosphate and fumaric acid for iron ions, and ensures the preferential formation of the target framework structure, laying an ordered foundation for the subsequent construction of the composite material.

[0024] Secondly, by subsequently increasing the pH to the interval of 3-5, the precipitation process of iron phosphate is triggered. Since the iron phosphate is generated in situ in the presence of the MIL-88A framework structure which has been formed, the high specific surface area and porous structure of the MIL-88A play the role of a template and a dispersant. This effectively solves the "passivation" problem in the preparation of traditional iron phosphate, that is, the generated compact iron phosphate layer covers the surface of the raw material, hindering the further contact and conversion of the internal reactants, thereby helping to realize more complete and uniform reaction.

[0025] Finally, the process prepares a new composite material, in which the organic framework component MIL-88A and the inorganic iron phosphate component are uniformly and closely combined at the molecular or nanometer scale. This structure enables the organic ligand (fumaric acid) to act as an inherent and highly dispersed carbon source precursor. In the subsequent heat treatment process for preparing the final product, it can be in-situ decomposed to form a uniformly coated conductive carbon network. Compared with the traditional mechanical mixing of external carbon sources, the distribution of the in-situ carbon source is more uniform and the contact is closer, laying a structural foundation for improving the performance of the final material. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the specific embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0027] Figure 1 The flowchart of the preparation method of the MIL-88A / FePO4 composite material provided in the embodiments of the present application.

[0028] Figure 2 The XRD graph of the MIL-88A / FePO4 prepared in Example 1.

[0029] Figure 3SEM image of lithium iron phosphate prepared for Example 1.

[0030] Figure 4 SEM image of lithium iron phosphate prepared for Example 1. DETAILED DESCRIPTION

[0031] The embodiments of the present application will be described in detail below with examples, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. The specific conditions are not specified in the examples, and the conventional conditions or the conditions recommended by the manufacturer are used. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained by purchase.

[0032] REFERENCE Figure 1 In the embodiments of the present application, a preparation method of MIL-88A / FePO4 composite material is provided, comprising:

[0033] Step S1, iron source, phosphoric acid and fumaric acid are added into an aqueous system.

[0034] This step is the starting step of the preparation process, i.e. preparation and mixing. The core is to put all the necessary chemical reactants, (1) "iron source" providing iron element, (2) "phosphoric acid" providing phosphate radical, and (3) "fumaric acid" as an organic ligand, into a reaction environment with water as the main solvent. After this step, an aqueous mixture or suspension containing all the reactants is obtained, which creates the basic conditions for the subsequent chemical reaction.

[0035] Using an aqueous system as the reaction medium has the advantages of low cost, environmental friendliness, safe operation, and easy control of reactant concentration and pH value, and is a commonly used method in chemical synthesis.

[0036] In actual operation, a certain amount of phosphoric acid can be mixed with water to prepare a phosphoric acid solution, and then the solid iron source (such as ferric oxide) and fumaric acid are added to the phosphoric acid solution in a reaction kettle with a stirring device.

[0037] Among them, the molar ratio of iron source to fumaric acid is controlled to be (6-14):1, for example, it can be 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, etc. The reason is that too high will result in too little content of product MIL-88A, affecting the subsequent carbon reduction; too low will result in too high content of product MIL-88A, making the subsequent carbon layer too thick, affecting the compaction.

[0038] Step S2, under the condition that the pH is less than 2, the iron ions dissolved from the iron source are allowed to carry out coordination reaction with the fumaric acid.

[0039] This step is the first key control step. It requires the reaction to be carried out in a strongly acidic environment (pH < 2). Under such conditions, two main processes occur:

[0040] (1) Iron source dissolution: The solid iron source dissolves under the action of strong acid, releasing iron ions (Fe 3+ ) into the solution.

[0041] (2) Selective coordination reaction: Iron ions in the solution undergo a "coordination reaction" with fumaric acid molecules, forming a specific structure composed of metal ions (iron) and organic ligands (fumaric acid).

[0042] This step mainly generates a metal-organic framework (MIL-88A) structure formed by the coordination of iron ions and fumaric acid. At this time, phosphate ions have been severely protonated (mainly in the form of H3PO4 molecules) in strong acid and have weak coordination ability, so they do not participate in or rarely participate in coordination reactions.

[0043] The advantage of this step is to provide and promote the selectivity of the reaction, that is, by strictly controlling the pH value below 2, the reactivity of phosphate ions can be inhibited, allowing iron ions to react with fumaric acid first. This ensures the pre-ordered construction of the MIL-88A framework structure, rather than a chaotic process of unordered generation of various products. This lays the foundation for the subsequent preparation of structured composite materials.

[0044] To achieve a pH of less than 2, in actual operation, acidic substances (solutions) can be used, such as high-concentration phosphoric acid solutions (e.g., molar concentration greater than or equal to 3 mol / L), and if necessary, a small amount of other strong acids (such as dilute nitric acid or dilute hydrochloric acid) can be added to accurately adjust and maintain the pH value.

[0045] In addition, to ensure sufficient reaction, long-time, high-intensity stirring is usually required.

[0046] Step S3: Adjusting the pH of the aqueous system to 3-5 to allow iron ions to react with phosphate ions to form iron phosphate precipitates, thereby in-situ preparing the MIL-88A / FePO4 composite material.

[0047] This step is the second control step. After completing the previous step, the acidity of the entire system is reduced by adding a basic substance, allowing the pH value to increase from less than 2 to the range of 3-5. For example, the pH can be 3, 3.5, 4, 4.5, 5, etc.

[0048] When the pH value increases, the degree of protonation of phosphoric acid decreases, and the concentration and reactivity of phosphate ions in the solution increase significantly. At this time, phosphate ions will rapidly react with the remaining iron ions in the solution to form water-insoluble iron phosphate (FePO4) precipitate. Since this precipitation process occurs in a system that already contains a large amount of MIL-88A framework, the generated iron phosphate will adhere to and deposit on the inner and outer surfaces of MIL-88A, and the two are tightly combined, ultimately obtaining a "in situ" generated, uniformly composite MIL-88A / FePO4 composite material.

[0049] The formation mechanism and technical advantages of the MIL-88A / FePO4 composite material prepared in this example are mainly based on the following principles:

[0050] First, the formation of the composite material precursor is based on the principle of ligand selective coordination controlled by pH value. In the initial stage of the preparation process, the system is controlled in a strong acidic environment with a pH value less than 2. Under this condition, phosphoric acid almost completely exists in the form of protonated H3PO4 molecules, and its pKa1 value is about 2.14, resulting in its extremely weak coordination ability with Fe 3+ ions. At the same time, although fumaric acid also mainly maintains a molecular form, the carboxyl oxygen atoms in its structure still have the ability to coordinate with Fe 3+ ions, thereby preferentially forming iron-fumaric acid coordination structures, i.e., MIL-88A framework. When the pH value of the system is adjusted to the weakly acidic range of 3-5, the concentration of phosphate ions (PO4 3- ) increases significantly, and its coordination ability becomes dominant, combining with Fe 3+ ions in the system to generate iron phosphate (FePO4) precipitate. Since this process occurs in situ in the formed MIL-88A framework, it solves the technical problem that in the conventional reaction of phosphoric acid with iron oxide, the generated dense iron phosphate passivation layer hinders the continuous progress of the reaction. The high specific surface area and porous structure of MIL-88A can effectively adsorb and disperse raw material particles, avoiding rapid formation of passivation layers and ensuring full and uniform reaction.

[0051] Secondly, the composite precursor exhibits a unique structural advantage in the subsequent high-temperature sintering process for preparing lithium iron phosphate. As a crystalline porous material, the organic ligand (fumaric acid) of the MIL-88A component will be converted into a carbon skeleton with high specific surface area and uniform mesoporous structure in situ during pyrolysis. This in-situ generated carbon skeleton, as an ideal dispersion carrier, can uniformly coat the surface of active material particles. This unique "core-shell" structure can effectively prevent the excessive growth and hard agglomeration of lithium iron phosphate grains during high-temperature sintering, so that the particle size of the final product is more uniform, mainly at the nanoscale. Finally, this low interfacial resistance structure formed by nano-sized particles and porous carbon is extremely beneficial to the close packing of particles, thereby significantly improving the tap density of the lithium iron phosphate positive electrode material. At the same time, MIL-88A as an in-situ carbon source also reduces the amount of additional carbon source added.

[0052] In addition, in-situ compounding is achieved in the method, where "in-situ" generation means that the two components are uniformly and closely combined at the nanometer or molecular scale, rather than simply physically mixed, which is beneficial to the formation of a material with uniform performance; and, reaction passivation is avoided. According to mechanism analysis, the pre-formed MIL-88A framework has a high specific surface area and can adsorb and disperse raw material particles, avoiding the complete wrapping of unreacted iron source by the dense iron phosphate layer generated during the reaction, thereby solving the "passivation" problem that hinders the continuous progress of the reaction, and making the reaction more complete.

[0053] The adjustment of the pH value can be achieved by adding an alkaline solution drop by drop, for example, sodium hydroxide (NaOH) solution is used in the examples. The pH value of the system needs to be continuously monitored during the addition process to ensure that it is stable in the target interval of 3-5.

[0054] It should be noted that the embodiment provides an in-situ preparation method of a MIL-88A / FePO4 composite material. Unlike the conventional method, which first prepares each component material and then physically mixes or compiles through an additional step, the core of the method adopted in the embodiment is that the in-situ formation of the final composite material has been completed during the preparation of MIL-88A.

[0055] The reason is that the present application ingeniously utilizes the significant difference in coordination ability of different ligands (fumaric acid and phosphate) with iron ions under different pH environments. By pre-adding all core reactants (iron source, phosphoric acid, fumaric acid) into the same aqueous reaction system, first under strong acid conditions with pH less than 2, the characteristic of inhibited coordination ability of phosphate is utilized to selectively induce the reaction of iron ions with fumaric acid, thereby pre-constructing the framework structure of MIL-88A. Subsequently, only by adjusting the pH value of the system to 3-5, the reaction advantage of phosphate can be changed to trigger the in-situ precipitation and growth of iron phosphate on the formed MIL-88A framework, directly obtaining the final product composite.

[0056] This design is essentially equivalent to a "one-step" or "one-pot" synthesis strategy. The entire composite formation process, from the initial raw materials to the final product, is sequentially completed in the same reactor through the phased regulation of pH value. This process does not require the separation of intermediate products in the middle, nor does it need to add additional reactants, functional substances or specific solvents to promote the composite. This brings significant technical advantages:

[0057] (1) First, the process flow is greatly simplified. The traditional method may involve multiple feeding, separation, purification, and re-mixing steps, which are cumbersome. This makes the overall operation convenient, simple, and fast, thereby significantly shortening the production cycle, achieving efficient production, and greatly reducing equipment, energy consumption, and labor costs.

[0058] (2) Second, the product purity is high. Since the introduction of additional substances and the complex separation and purification process are avoided, the risk of byproduct generation and process impurity introduction is effectively reduced, ensuring the high purity of the obtained composite material.

[0059] (3) Third, the composite effect is excellent. The in-situ growth method ensures the uniform composite and close contact of MIL-88A and FePO4 at the nanoscale, obtaining a uniform microstructure that is difficult to achieve by traditional mechanical mixing methods, laying a solid structural foundation for the final performance of the material.

[0060] In some embodiments, the iron source includes at least one of ferrous oxide and ferric oxide.

[0061] In some embodiments, the aqueous system is stirred and mixed during the coordination reaction process.

[0062] In some embodiments, the stirring and mixing time is not less than 12 hours.

[0063] In the embodiment, the stirring process needs to be continued for a long enough time. In the solid-liquid reaction system, the dissolution of the iron source (such as ferric oxide) and the subsequent coordination reaction of the iron ions and the fumaric acid need a certain time to reach equilibrium or be completed. The stirring time of no less than 12 hours is set to ensure that the solid-phase iron source has enough time to dissolve, release sufficient iron ions to participate in the reaction, and provide sufficient time for the diffusion, collision and orientation of the iron ions and the fumaric acid molecules, so as to form a MIL-88A framework with regular structure and high crystallinity.

[0064] In some embodiments, the stirring speed is no less than 700 rpm / min.

[0065] The above stirring process should have sufficient strength. Higher stirring speed is mainly used to overcome the mass transfer resistance in the reaction system. The stirring speed of no less than 700 rpm / min is set to ensure the uniformity of the system. High speed can effectively suspend the solid iron source particles in the whole solution, prevent them from settling at the bottom of the reactor, and thus increase the solid-liquid contact area; and the high efficiency of mass transfer. High-intensity stirring can reduce the thickness of the diffusion boundary layer on the particle surface, accelerate the dissolution rate of the iron source and the macroscopic and microscopic mixing of the dissolved iron ions and the fumaric acid in the solution, and avoid the problem of uneven reaction caused by local concentration unevenness.

[0066] In the embodiment, a MIL-88A / FePO4 composite material is provided, which is prepared by the preparation method of the MIL-88A / FePO4 composite material according to any one of the preceding embodiments.

[0067] In the embodiment, a preparation method of high-compact-density lithium iron phosphate is provided, which comprises:

[0068] The MIL-88A / FePO4 composite material according to the preceding embodiments is mixed with a lithium source and then sintered at high temperature to obtain the high-compact-density lithium iron phosphate.

[0069] The lithium source can include but is not limited to lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate and the like.

[0070] By the two-step pH control method described above, in-situ precipitation and growth of FePO4 on the pre-formed MIL-88A framework is achieved. This process enables highly uniform composite and intimate interface contact between MIL-88A component and FePO4 component at nanometer or molecular scale, forming a unique microstructure that is difficult to obtain by traditional physical mixing methods. This unique structure, such as the interface state between components, pore structure, and uniformity of organic components (as precursors for subsequent carbon source) in the inorganic matrix, is directly determined by its unique preparation method.

[0071] Therefore, the novelty and creativity of the composite material not only lies in its chemical components, but also in the inherent structural characteristics and excellent performance brought by the specific preparation method.

[0072] The high-temperature sintering described above, for example, can be: after the MIL-88A / FePO4 composite material is uniformly mixed with lithium carbonate and other lithium sources, sintering is carried out at a temperature of about 700°C to about 830°C under the protection of an inert atmosphere.

[0073] In the embodiments of the present application, a high-density lithium iron phosphate is provided, which is prepared by the preparation method of high-density lithium iron phosphate as described in the foregoing embodiments.

[0074] In the embodiments of the present application, a positive electrode is provided, which comprises the high-density lithium iron phosphate as described in the foregoing embodiments.

[0075] The positive electrode provided in the embodiments contains high-density lithium iron phosphate as a positive electrode active material. By using the high-density characteristics of the active material, higher active material loading can be achieved on the positive electrode sheet, thereby significantly improving the volume energy density of the final battery. In specific implementation, the positive electrode is usually prepared by mixing the high-density lithium iron phosphate active material described above with a conductive agent (such as conductive carbon black) for improving electronic conductivity and a binder (such as PVDF) for providing mechanical adhesion, forming a uniform slurry, and then coating the slurry on an aluminum foil as a current collector, and performing processes such as drying and rolling. In addition, the positive electrode can also include, but is not limited to, conductive agents, binders, current collectors, and the like.

[0076] For example, in the preparation of the positive electrode, the positive electrode active material, the conductive agent, and the binder described above can be mixed in a certain mass ratio in a specific solvent (such as N-methyl pyrrolidone, NMP, used to dissolve PVDF) to form a uniform slurry, and then the slurry is uniformly coated on an aluminum foil current collector, and then subjected to processes such as drying, rolling (to achieve the target density), and cutting, to finally form a positive electrode sheet.

[0077] In the embodiments of the present application, a battery is provided, which comprises the positive electrode as described in the foregoing embodiments.

[0078] The battery described above, wherein the positive electrode comprises the lithium iron phosphate with high tap density as the active material. A complete battery can be formed by isolating the positive electrode and a negative electrode (such as a graphite negative electrode) by a porous diaphragm, and together infiltrating in an electrolyte as an ion transmission medium, and finally packaging the core components in a battery shell.

[0079] In the embodiments of the present application, an electrical equipment is provided, which comprises the battery as described in the foregoing embodiments.

[0080] The electrical equipment provided in the embodiments comprises the battery described above. Since the battery adopts the lithium iron phosphate positive electrode with high tap density of the present application, the electrical equipment equipped with the battery can also benefit from the advantages of high energy density, long cycle life and high safety of the battery. The electrical equipment can include but is not limited to: new energy vehicles such as pure electric vehicles, hybrid electric vehicles, electric buses, electric trucks, electric ships, electric bicycles, etc.; energy storage systems such as grid-level energy storage stations, household energy storage systems, uninterruptible power supplies (UPS), communication base station backup power supplies, solar / wind power generation matching energy storage devices, etc.; portable electronic devices and tools such as portable mobile energy storage power supplies, electric tools, unmanned aerial vehicles, mobile medical devices, and other portable or fixed electronic devices with high requirements for safety and cycle life.

[0081] The present application will be further described in the following specific embodiments. It should be understood that these embodiments are only used for a more detailed description, and should not be understood as limiting the present application in any form.

[0082] Embodiment 1

[0083] In this embodiment, the preparation of lithium iron phosphate material is carried out.

[0084] Experimental method:

[0085] (1) A 3L phosphoric acid solution with a molar concentration of 3mol / L is prepared, and 658.6g of pure fumaric acid with a purity of 97% is added at room temperature. 50 At 30μm of ferric oxide, the pH of the system is controlled to be less than 2 (using dilute nitric acid or dilute hydrochloric acid), and stirring is carried out for 1h (stirring rate 300rpm / min);

[0086] (2) Then 116.1g of fumaric acid is added to the system, and the molar ratio of the iron source to the fumaric acid is 8, and stirring is carried out for 12h (750rpm / min) to ensure that the Fe 3+ and fumaric acid are fully positioned to form MIL-88A, and the pH of the system is controlled to be less than 2 during the period;

[0087] (3) Subsequently, adjust the pH between 3-5 (with sodium hydroxide solution), the remaining Fe 3+ and phosphate form iron phosphate precipitate, and then washed and dried to remove excess phosphate and fumaric acid, and finally MIL-88A / FePO4 material is prepared.

[0088] (4) MIL-88A / FePO4 is mixed with lithium carbonate and sintered at high temperature to prepare lithium iron phosphate material.

[0089] Example 2

[0090] In this embodiment, the preparation of lithium iron phosphate material is carried out.

[0091] Experimental method:

[0092] (1) 3L of phosphoric acid solution with a molar concentration of 5mol / L is prepared, and 2305.3g of 97% pure D 50 Fe2O3 of 10μm, control system pH<2 (with dilute nitric acid or dilute hydrochloric acid), stirring for 0.5h (stirring rate 200rpm / min);

[0093] (2) Then 422.3g of fumaric acid is added to the system, the molar ratio of iron source to fumaric acid is 10.5, and stirring is carried out for 12h (800rpm / min) to ensure that Fe 3+ and fumaric acid are fully coordinated to form MIL-88A, and the system pH is controlled to be less than 2 during the period;

[0094] (3) Subsequently, adjust the pH between 3-5 (with sodium hydroxide solution), the remaining Fe 3+ and phosphate form iron phosphate precipitate, and then washed and dried to remove excess phosphate and fumaric acid, and finally MIL-88A / FePO4 material is prepared.

[0095] (4) MIL-88A / FePO4 is mixed with lithium carbonate and sintered at high temperature to prepare lithium iron phosphate material.

[0096] Example 3

[0097] In this embodiment, the preparation of lithium iron phosphate material is carried out.

[0098] Experimental method:

[0099] (1) 3L of phosphoric acid solution with a molar concentration of 5mol / L is prepared, and 2305.3g of 97% pure D 50 Fe2O3 of 10μm, control system pH<2 (with dilute nitric acid or dilute hydrochloric acid), stirring for 0.5h (stirring rate 200rpm / min);

[0100] (2) Then 86.4 g of fumaric acid was added into the system, the molar ratio of iron source to fumaric acid was 10.5, and stirring was performed for 12 h (800 rpm / min) to ensure that Fe 3+ was fully coordinated with fumaric acid to form MIL-88A, and the pH of the system was controlled to be less than 2 during the process;

[0101] (3) Then the pH was adjusted to be between 3 and 5, the remaining Fe3+ was coordinated with phosphate to form iron phosphate precipitate, and then washing and drying were performed to remove the excess phosphate and fumaric acid, and finally the MIL-88A / FePO4 material was prepared.

[0102] (4) The MIL-88A / FePO4 was mixed with lithium carbonate and sintered at high temperature to prepare a lithium iron phosphate material.

[0103] Comparative Example 1

[0104] In this comparative example, a lithium iron phosphate material was prepared.

[0105] Experimental method:

[0106] (1) A 5L phosphoric acid solution with a molar concentration of 0.05 mol / L was prepared, and 41.5 g of Fe2O3 with a purity of 97% and a D50 of 30 μm was added at room temperature, and stirring was performed for 2 h (stirring rate 200 rpm / min);

[0107] (2) Then 12.7 g of fumaric acid was added into the system, and stirring was performed for 12 h (800 rpm / min), and the pH of the system was controlled to be less than 2 (dilute nitric acid or dilute hydrochloric acid) during the process, and then the pH was adjusted to be between 3 and 5, and the remaining Fe 3+ was coordinated with phosphate to form iron phosphate precipitate, and then washing and drying were performed to remove the excess phosphate and fumaric acid, and finally the material was prepared;

[0108] (3) Then lithium carbonate was mixed and sintered at high temperature to prepare a lithium iron phosphate material.

[0109] Comparative Example 2

[0110] In this comparative example, a lithium iron phosphate material was prepared.

[0111] Experimental method:

[0112] (1) A 3L phosphoric acid solution with a molar concentration of 3 mol / L was prepared, and 658.6 g of Fe2O3 with a purity of 97% and a D 50 of 30 μm was added at room temperature, and the pH was adjusted to be between 3 and 5 (dilute nitric acid or dilute hydrochloric acid);

[0113] (2) Then 58.3 g of commercially available MIL-88A material was added, and stirring was performed for 5 h;

[0114] (3) Subsequently, washing and drying are performed, and finally the MIL-88A / FePO4 material is prepared.

[0115] (4) The MIL-88A / FePO4 is mixed with lithium carbonate and sintered at high temperature to prepare the lithium iron phosphate material.

[0116] Comparative Example 3

[0117] In this comparative example, the lithium iron phosphate material is prepared.

[0118] The experimental method is basically the same as in Example 1, except that in step (1) the initial pH is controlled at 3-5.

[0119] Comparative Example 4

[0120] In this comparative example, the lithium iron phosphate material is prepared.

[0121] The experimental method is basically the same as in Example 1, except that in step (1) the initial pH is controlled at neutral.

[0122] Comparative Example 5

[0123] In this comparative example, the lithium iron phosphate material is prepared.

[0124] The experimental method is basically the same as in Example 1, except that in step (2) the stirring time after adding the fumaric acid is 2 h.

[0125] Comparative Example 6

[0126] In this comparative example, the lithium iron phosphate material is prepared.

[0127] The experimental method is basically the same as in Example 1, except that in step (2) the stirring rate after adding the fumaric acid is 200 rpm / min.

[0128] Comparative Example 7

[0129] In this comparative example, the lithium iron phosphate material is prepared.

[0130] The experimental method is basically the same as in Example 1, except that the amount of fumaric acid added is 464.4 g, and the molar ratio of the iron source to the fumaric acid is 2.

[0131] Comparative Example 8

[0132] In this comparative example, the lithium iron phosphate material is prepared.

[0133] The experimental method is basically the same as in Example 1, except that the amount of fumaric acid added is 46.44 g, and the molar ratio of the iron source to the fumaric acid is 20.

[0134] Test Experiment:

[0135] 1. Test method:

[0136] (1) The tap density of the lithium iron phosphate prepared in each example and the comparative example was tested, and the test method was as follows: a four-probe tester was used for testing, the powder was placed on a tablet press, a pressure of 30 MPa was applied, the pressure was maintained for 1 min, then the pressure relief was measured, and finally the mold height was measured. The results are shown in Table 1.

[0137] (2) The lithium ion battery was prepared as follows: lithium iron phosphate (the product prepared in the examples and the comparative example), conductive agent CNT and adhesive PVDF were mixed in a solvent (NMP) at a mass ratio of 80:15:5, stirred uniformly, and then uniformly coated on a current collector aluminum foil, film pressing and drying were performed, and then the aluminum foil was cut into a circular sheet with a diameter of 12 mm, and finally a positive electrode sheet of a button cell was obtained. Lithium sheet (thickness of 0.6 mm) was used as the negative electrode, 1 mol / L LiPF6-containing fluorine vinyl carbonate, dimethyl carbonate and ethylene carbonate (volume ratio of 10:45:45) were used as the electrolyte, and a purchased polypropylene film was cut as a separator. The button cell was assembled in an argon atmosphere in a glove box.

[0138] (3) Electrochemical and cycle performance of the lithium ion battery: the test voltage was set to 2.0-3.75 V, and the test temperature was strictly maintained at 25°C. The battery test system (LAND CT2001A) was used to test the initial discharge gram capacity of the button cell at 0.1C and the discharge gram capacity at 1C.

[0139] 2. Test results: The results are shown in Table 1.

[0140] Table 1, test results of examples and comparative examples

[0141]

[0142] 3. Analysis:

[0143] From the data in the table, it can be seen that example 1 can achieve a high tap density of nearly 2.6 g / cm 3 . Comparative example 1 has too low acid concentration, resulting in too little iron ion leaching, too little MIL-88A content, and too little carbon-coated final material, affecting the performance of the lithium iron phosphate material; Comparative example 2 uses a non-in situ method, the MIL-88A is not uniformly distributed and is not tightly combined with the lithium iron phosphate, affecting the tap density of the final material; Comparative examples 3, 4, 5, 6 and 8 have too high pH, too short stirring time, too slow stirring speed and too low fumaric acid addition amount, resulting in the failure of in situ formation of MIL-88A or too low content of MIL-88A, which makes the final material have too little carbon coating and the performance is reduced. Comparative example 7 has too much fumaric acid addition amount, resulting in too much MIL-88A, which makes the final material have too high carbon content and specific surface area, and the tap density is reduced.

[0144] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and are not intended to limit the present application; although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions recorded in the above embodiments can still be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a MIL-88A / FePO4 composite material, characterized in that, The application relates to a preparation method of a MIL-88A / FePO4 composite material. Iron source, phosphoric acid and fumaric acid are added into an aqueous system; The aqueous system is stirred and mixed under the condition that the pH is less than 2, so that iron ions dissolved from the iron source are subjected to a coordination reaction with the fumaric acid; wherein the molar ratio of the iron source to the fumaric acid is 6-14; the stirring and mixing time is not less than 12 hours; and the stirring and mixing speed is not less than 700 rpm / min; The pH of the aqueous system is adjusted to 3-5, so that the iron ions are reacted with the phosphate to generate iron phosphate precipitates, thereby in-situ preparing the MIL-88A / FePO4 composite material.

2. The method for preparing the MIL-88A / FePO4 composite material as described in claim 1, characterized in that, The iron source comprises at least one of ferrous oxide and magnetite containing trivalent iron.

3. A MIL-88A / FePO4 composite material, characterized in that, The MIL-88A / FePO4 composite material is prepared by the preparation method of claim 1 or 2.

4. A method of producing high tap density lithium iron phosphate, characterized by, The application relates to a preparation method of a MIL-88A / FePO4 composite material. The MIL-88A / FePO4 composite material is mixed with a lithium source and then subjected to high-temperature sintering, so that the high-compaction-density lithium iron phosphate is obtained.

5. A high tap density lithium iron phosphate characterized in that, The high-compaction-density lithium iron phosphate is prepared by the preparation method of claim 4.

6. A positive electrode, characterized by comprising: The application relates to a high-compaction-density lithium iron phosphate.

7. A battery, characterized by The application relates to a positive electrode.

8. An electrical device, comprising: The application relates to a battery.

Images