Method for preparing lithium iron phosphate, lithium iron phosphate, positive electrode sheet, and lithium ion battery

Abstract

The application relates to the technical field of lithium ion batteries, and specifically provides a preparation method of lithium iron phosphate, lithium iron phosphate, a positive electrode sheet and a lithium ion battery. The preparation method of the lithium iron phosphate provided by the application comprises the following steps: providing Ti-MOF / polymer composite nanofibers; mixing the Ti-MOF / polymer composite nanofibers with a lithium source and a phosphorus-iron source to obtain a precursor mixture; and sintering the precursor mixture under an inert atmosphere. The method provided by the application not only avoids a large amount of loss and dust pollution caused by directly using nanometer powder, but also retains the characteristics of a large specific surface area and high activity of the nanofiber, which is helpful to diffusion into the crystal interior at a low sintering temperature and improvement of the ion conductivity of LFP; the carbonized fiber skeleton after calcination can construct a conductive network, further improving the electronic conductivity of LFP, and being helpful to reduction of the use of a conductive agent in the positive electrode sheet.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically providing a method for preparing lithium iron phosphate, lithium iron phosphate, a positive electrode sheet, and a lithium-ion battery. Background Technology

[0002] In recent years, lithium-ion batteries have received widespread attention and application in the energy storage field. Among them, lithium iron phosphate (LFP), as the positive electrode active material, holds an absolute advantage in the energy storage field due to its advantages such as stable structure, low cost, and long cycle life. Energy efficiency is one of the key indicators for evaluating the performance of lithium iron phosphate in energy storage cells. However, as a semiconductor-like material, lithium iron phosphate has relatively low ionic and electronic conductivity, so elemental doping or surface coating modification is required to improve its kinetic performance. Currently, the most common methods on the market are Ti doping and carbon coating. However, due to the difficulty of solid-phase diffusion during the synthesis process, the Ti doping effect is often poor.

[0003] CN118610442A improves the material interface and enhances its electrical properties by spraying a series of metal-organic framework (MOF) materials onto the surface of lithium-rich manganese-based materials. CN116829768A modifies lithium extraction electrode materials from salt lakes by combining lithium manganese oxide electrodes with Ti-MOFs to suppress the dissolution of Mn during the use of lithium manganese oxide electrodes, thereby improving the electrochemical lithium extraction rate and the purity of lithium.

[0004] The above-mentioned solutions mainly modify the electrode materials through simple spraying or compositing, failing to fully utilize the advantages of MOF materials such as large specific surface area and high reactivity. At the same time, spraying or compositing makes it difficult to ensure the uniformity of coating on the material surface.

[0005] Accordingly, a new technical solution is needed in this field to solve the above-mentioned technical problems. Summary of the Invention

[0006] The present invention aims to solve the above-mentioned technical problems, namely, to solve the problem that the modification of electrode materials by simple spraying or compositing in the prior art fails to fully utilize the advantages of MOF materials such as large specific surface area and high reactivity, and that spraying or compositing is difficult to ensure the uniformity of coating on the material surface.

[0007] In a first aspect, the present invention provides a method for preparing lithium iron phosphate, wherein the preparation method includes:

[0008] Provides Ti-MOF / polymer composite nanofibers;

[0009] Ti-MOF / polymer composite nanofibers were mixed with lithium source and iron phosphorus source to obtain precursor mixture;

[0010] The precursor mixture is sintered under an inert atmosphere to obtain the final product.

[0011] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the amount of Ti-MOF / polymer composite nanofibers accounts for 0.2%-2% of the total mass of the lithium source and the iron phosphate source.

[0012] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the ratio of the amount of lithium source to the amount of iron phosphate source is 1.01-1.08:1, based on the molar ratio of Li to Fe.

[0013] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the sintering temperature is 650-750℃.

[0014] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the Ti-MOF / polymer composite nanofibers are provided by the following method:

[0015] Provide spinning solutions containing carboxylic acid aromatic hydrocarbons and polymers;

[0016] The spinning solution is spun into composite nanofibers;

[0017] The composite nanofibers were mixed with an organic solvent solution of titanate to obtain a mixed system;

[0018] The mixture was reacted at a preset temperature for a preset time to obtain Ti-MOF / polymer composite nanofibers; the spinning solution was prepared by the following method:

[0019] Aromatic carboxylic acids are dispersed in a polar aprotic solvent to obtain a dispersion.

[0020] The polymer is added to the dispersion to form a spinning solution;

[0021] in:

[0022] The carboxylic acid aromatic hydrocarbons are selected from terephthalic acid or triphenylcarboxylic acid.

[0023] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the carboxylic acid aromatic hydrocarbon or the polymer satisfies at least one of the following conditions:

[0024] The carboxylic acid aromatic hydrocarbon is selected from terephthalic acid;

[0025] The polymer is a nitrogen-containing polymer;

[0026] The polymer is selected from polyimide and / or polyacrylonitrile.

[0027] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the molar volume concentration of the dispersion is 0.033-0.1 mmol / mL.

[0028] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the molar volume concentration of the organic solvent solution of the titanate is 0.033-0.1 mmol / mL.

[0029] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the preset temperature is 100-150℃; and / or the preset time is 12-48h.

[0030] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the Ti-MOF / polymer composite nanofibers are dried before being mixed with the lithium source and the iron phosphate source.

[0031] In the preferred embodiment of the above-mentioned method for preparing lithium iron phosphate, the lithium source is selected from lithium carbonate; and / or, the iron phosphate source is selected from iron phosphate.

[0032] In a second aspect, the present invention provides a lithium iron phosphate, wherein the interior of the lithium iron phosphate crystal is bulk-doped with titanium, and its surface is coated with a carbon nanofiber network, wherein the titanium and the carbon nanofiber network are derived from Ti-MOF / polymer composite nanofibers.

[0033] The present invention also provides another lithium iron phosphate, wherein the lithium iron phosphate is prepared by the preparation method described in the first aspect above.

[0034] In a third aspect, the present invention provides a positive electrode sheet, wherein the positive electrode sheet comprises the aforementioned lithium iron phosphate.

[0035] In the preferred embodiment of the above-mentioned positive electrode sheet, the positive electrode sheet further includes a conductive agent, the content of which is configured to be reduced based on the conductivity of the carbon nanofiber network in the lithium iron phosphate.

[0036] In a fourth aspect, the present invention provides a lithium-ion battery, wherein the lithium-ion battery includes the positive electrode sheet described in the third aspect above.

[0037] The technical solution of this application has the following technical effects:

[0038] 1. The method for preparing lithium iron phosphate provided by this invention involves sintering a mixture of Ti-MOF / polymer composite nanofibers with a lithium source and an iron phosphate source. The Ti-MOF / polymer composite nanofibers simultaneously serve as both a titanium source and a carbon source (i.e., a "bifunctional precursor"), allowing for the preparation of modified lithium iron phosphate in a single sintering step. This method incorporates Ti elements into the LFP crystal, and the nanofibers are carbonized on the LFP surface, achieving a carbon coating effect. This not only avoids the significant losses and dust pollution caused by directly using nanopowders, but also allows the nanofibers to retain their large specific surface area and high activity, facilitating diffusion into the crystal at low sintering temperatures and improving the ionic conductivity of LFP. The calcined carbonized fiber skeleton can construct a conductive network, further enhancing the electronic conductivity of LFP and helping to reduce the use of conductive agents in the positive electrode.

[0039] 2. The method of the present invention can significantly reduce the content of magnetic materials in lithium iron phosphate materials, thereby improving the safety, performance and reliability of the battery. Attached Figure Description

[0040] The preferred embodiments of the present invention are described below with reference to the accompanying drawings, in which:

[0041] Figure 1 This is a process flow diagram of the method for preparing lithium iron phosphate according to the present invention;

[0042] Figure 2 This is a flowchart illustrating the preparation process of Ti-MOF / polymer composite nanofibers in the lithium iron phosphate preparation method of the present invention. Detailed Implementation

[0043] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0044] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0045] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0046] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0047] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0048] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass described in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0049] The terms "first" and "second" are used only to describe purpose, to distinguish purposes such as substances from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature.

[0050] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used in the following examples are commercially available unless otherwise specified.

[0051] As noted in the background section, existing technologies that modify electrode materials through simple spraying or lamination fail to fully utilize the advantages of MOF materials, such as their large specific surface area and high reactivity. Furthermore, spraying or lamination methods struggle to ensure uniform coating on the material surface. The method of this invention not only avoids the significant losses and dust pollution caused by directly using nanopowders, but also allows the nanofibers to retain their large specific surface area and high activity, facilitating diffusion into the crystal interior at low sintering temperatures and improving the ionic conductivity of LFP. The calcined carbonized fiber skeleton can construct a conductive network, further enhancing the electronic conductivity of LFP and helping to reduce the use of conductive agents in the positive electrode.

[0052] Specifically, in its first aspect, the present invention provides a method for preparing lithium iron phosphate; please refer to [link to relevant documentation]. Figure 1 The preparation method includes:

[0053] S1. Provides Ti-MOF / polymer composite nanofibers;

[0054] S2. Mix Ti-MOF / polymer composite nanofibers with lithium source and iron phosphorus source to obtain precursor mixture;

[0055] S3. Sinter the precursor mixture under an inert atmosphere to obtain the final product.

[0056] The method for preparing lithium iron phosphate (LFP) provided by this invention involves sintering a mixture of Ti-MOF / polymer composite nanofibers with a lithium source and an iron phosphate source. The Ti-MOF / polymer composite nanofibers play a dual role during sintering: firstly, as a titanium source, enabling bulk doping of Ti within the LFP crystal; secondly, as a carbon source, forming a conductive carbon nanofiber network on the LFP surface after carbonization. This method not only avoids the significant losses and dust pollution caused by directly using nanopowders, but also allows the nanofibers to retain their large specific surface area and high activity, facilitating diffusion into the crystal at low sintering temperatures and improving the ionic conductivity of LFP. The calcined carbonized fiber skeleton can construct a conductive network, further enhancing the electronic conductivity of LFP and helping to reduce the use of conductive agents in the positive electrode.

[0057] In some specific embodiments, in step S2, the amount of the Ti-MOF / polymer composite nanofiber accounts for 0.2-2% of the total mass of the lithium source and the iron phosphorus source.

[0058] If the amount of Ti-MOF / polymer composite nanofiber is too low, its effect cannot be fully realized; if it is too high, it may lead to the formation of impurity phases or structural disorder. The present invention selects this range to improve performance while taking into account material cost and preparation stability.

[0059] In some exemplary embodiments, in step S2, the amount of the Ti-MOF / polymer composite nanofiber is 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2% of the total mass of the lithium source and the iron-phosphorus source, or any value within the range.

[0060] In some preferred embodiments, in step S2, the ratio of the lithium source to the iron phosphate source is 1.01-1.08:1, based on the molar ratio of Li to Fe.

[0061] A slight excess of lithium source can compensate for the loss of lithium volatilization during sintering, ensure the reaction proceeds fully, improve the purity of lithium iron phosphate in the product, and reduce the adverse effects of unreacted impurities on electrochemical performance.

[0062] In some specific embodiments, the sintering temperature in step S3 is 650-750℃.

[0063] The present invention selects a sintering temperature of 650-750℃. This range can ensure that lithium iron phosphate crystallizes fully and forms a stable structure, while avoiding excessive particle growth (reducing specific surface area) due to excessive temperature or incomplete crystallization (affecting conductivity) due to excessively low temperature, thus balancing the structural integrity and electrochemical activity of the material.

[0064] In some exemplary embodiments, in step S3, the sintering temperature is 650°C, 680°C, 700°C, 720°C, 750°C, or any value within the range.

[0065] For some specific implementation methods, please refer to Figure 2 In step S1, the Ti-MOF / polymer composite nanofibers are provided using a method comprising the following steps:

[0066] S11. Provide spinning solution containing carboxylic acid aromatic hydrocarbons and polymers;

[0067] S12. The spinning solution is spun into composite nanofibers;

[0068] S13. The composite nanofibers are mixed with an organic solvent solution of titanate to obtain a mixed system;

[0069] S14. React the mixture at a preset temperature for a preset time to obtain Ti-MOF / polymer composite nanofibers;

[0070] In step S11, the spinning solution is prepared by the following method:

[0071] S111. Disperse carboxylic acid aromatic hydrocarbons in a polar aprotic solvent to obtain a dispersion;

[0072] S112. Add the polymer to the dispersion to form a spinning solution.

[0073] This invention involves preparing composite nanofibers from spinning solution and reacting them with titanate esters (such as TTIP). This enables uniform loading of Ti elements on the nanofibers and controllable construction of MOF structures. The resulting Ti-MOF / polymer composite nanofibers have high specific surface area, uniform nanoscale, and good dispersibility, and can more efficiently modify lithium iron phosphate.

[0074] The principle of the above-mentioned scheme of the present invention lies in the following: In the preparation process of the Ti-MOF / polymer composite nanofibers, carboxylic acid aromatic hydrocarbons (such as terephthalic acid) act as organic ligands (i.e., organic framework units constituting the MOF skeleton), and their carboxyl groups (-COOH) are used to react with titanium ions (Ti ions) released by the hydrolysis or reaction of titanate esters. 4+ The titanium ion coordinates with the carbon nanofiber to form the framework structure of Ti-MOF. The polymer (such as polyimide), as the fiber matrix material, acts as a template carrier, providing a template for the in-situ growth of Ti-MOF, inducing and controlling its crystal growth. Furthermore, during subsequent sintering, the polymer carbonizes to form a nitrogen-doped carbon nanofiber network, significantly improving the electronic conductivity of the final product, lithium iron phosphate. In short, Ti-MOF formation is achieved through an in-situ coordination reaction on a polymer fiber template, with titanium ions as the metal center and carboxylic acid aromatic hydrocarbons as organic ligands.

[0075] In some specific embodiments, the carboxylic acid aromatic hydrocarbon is selected from terephthalic acid or triphenylcarboxylic acid.

[0076] In a preferred embodiment, the carboxylic acid aromatic hydrocarbon is selected from terephthalic acid.

[0077] In some preferred embodiments, the polymer is a nitrogen-containing polymer.

[0078] In some specific embodiments, the polymer is selected from polyimide or polyacrylonitrile.

[0079] In a preferred embodiment, the polymer is selected from polyimide.

[0080] It should be noted that although terephthalic acid is used as a carboxylic acid aromatic hydrocarbon and polyimide as a nitrogen-containing polymer only by way of example in the specific embodiments of the present invention, the scope of protection of the technical solution of the present invention is not limited thereto. Those skilled in the art will understand that terephthalic acid also contains carboxyl functional groups that can coordinate with titanium ions, and when used as an organic ligand, it can also react with titanate esters to form a Ti-MOF structure; while polyacrylonitrile, as a common polymer material that also contains nitrogen, can also serve as a fiber template and form a nitrogen-doped carbon network after sintering. Therefore, the mechanism of action and final effect of applying other listed carboxylic acid aromatic hydrocarbons (such as terephthalic acid) and nitrogen-containing polymers (such as polyacrylonitrile) to the present invention are reasonably foreseeable and all fall within the scope of protection of the present invention.

[0081] In some specific embodiments, in step S111, the molar volume concentration of the dispersion is 0.033-0.1 mmol / mL.

[0082] The concentration selected in this invention ensures that the spinning solution has suitable viscosity and stability, which is beneficial for the subsequent formation of nanofibers. At the same time, it provides sufficient organic ligands for the formation of MOF structure, ensuring the integrity of Ti-MOF structure.

[0083] In this invention, during the preparation of Ti-MOF / polymer composite nanofibers, the polar aprotic solvent must be able to readily dissolve carboxylic acid aromatic hydrocarbons (such as terephthalic acid) and nitrogen-containing polymers (such as polyimide), and be suitable for electrospinning processes. Solvents meeting these conditions include, but are not limited to, at least one of N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF).

[0084] It should be noted that the above list of polar aprotic solvents is exemplary and is only intended to help understand the present invention, and is not intended to limit the present invention. Any polar aprotic solvent that can well dissolve carboxylic acid aromatic hydrocarbons (such as terephthalic acid) and nitrogen-containing polymers (such as polyimide) and is suitable for electrospinning processes is applicable to the present invention.

[0085] In some preferred embodiments, the polar aprotic solvent is DMF.

[0086] It is understood that when a nitrogen-containing polymer (such as polyimide, PI) is added to the dispersion to prepare a spinning solution in this invention, the amount of the nitrogen-containing polymer (such as polyimide) is such that the spinning solution has a viscosity suitable for electrospinning. For example, in one specific embodiment, the nitrogen-containing polymer is polyimide (PI), and its mass concentration in the spinning solution can be 10wt% to 20wt%, preferably 15wt% to 18wt%. In some specific embodiments, this invention has achieved good spinning results using a concentration of 16wt%.

[0087] In some specific embodiments, the molar volume concentration of the organic solvent solution of the titanate is 0.033-0.1 mmol / mL.

[0088] In this invention, the titanate ester serves as the metal source for forming Ti-MOF. Its core function is to provide titanium ions, enabling them to undergo coordination reactions with organic ligands (such as terephthalic acid) on the surface of nanofibers, thereby generating Ti-MOF structures in situ on the fibers.

[0089] Based on this technical principle, the present invention does not have any special restrictions on the specific type of titanate, and can be selected from, but is not limited to, at least one of: methyl titanate, ethyl titanate, tetraisopropyl titanate (TTIP), n-butyl titanate, etc.

[0090] It is understood that selecting titanates with different carbon chain lengths may have a slight impact on the reaction rate and the crystallinity of the MOF, but this is all within the scope of the present invention. Among them, tetraisopropyl titanate (TTIP) is used in some preferred embodiments due to its moderate reactivity, ease of control, and reasonable price.

[0091] In this invention, the organic solvent of the titanate must be able to dissolve the titanate and participate in or not hinder the formation reaction of MOF. Typically, alcohol solvents with 1-4 carbon atoms, such as methanol, ethanol, isopropanol, n-butanol, etc., can be selected.

[0092] In some preferred embodiments, the organic solvent is methanol.

[0093] The above-mentioned molar volume concentration of the present invention can ensure that titanate (such as TTIP) is uniformly dissolved in methanol, avoid uneven distribution of Ti element caused by excessively high local concentration, make Ti loading on nanofiber more uniform, and improve the modification effect of Ti-MOF / polymer composite nanofiber.

[0094] In some specific embodiments, the preset temperature is 100-150℃; and / or the preset time is 12-48h.

[0095] The reaction conditions described above in this invention can promote the full reaction between titanate (such as TTIP) and the surface of nanofibers, ensuring the complete growth of the MOF crystal structure, while avoiding fiber structure damage caused by excessively high temperature or incomplete reaction caused by excessively low temperature, thus ensuring the quality stability of Ti-MOF / polymer composite nanofibers.

[0096] In some exemplary embodiments, the preset temperature is 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, or any value within the range.

[0097] In some preferred embodiments, the mixture is subjected to ultrasonic treatment before reacting at a preset temperature.

[0098] In some exemplary embodiments, the conditions for the ultrasonic treatment are: power 200-400W, frequency 20-40kHz, and time 0.5-2h.

[0099] For example, in some embodiments, the conditions for the ultrasonic treatment are: power 300W, frequency 30kHz, and time 1 hour.

[0100] In some specific embodiments, in step S2, the Ti-MOF / polymer composite nanofibers are dried before being mixed with the lithium source and the iron-phosphorus source.

[0101] Drying the Ti-MOF / polymer composite nanofibers before mixing removes adsorbed moisture and volatile impurities from their surface, preventing these impurities from generating gases (such as H2O and CO2) during sintering that could lead to pores or structural defects in the product, thus improving the density and structural stability of lithium iron phosphate.

[0102] It should be noted that the drying conditions are not specifically limited in this invention, and those skilled in the art can adjust the drying conditions according to the actual situation. For example, in some specific embodiments, the drying process is drying at 80°C for 8-24 hours, and the drying time can be 8 hours, 10 hours, 12 hours, 15 hours, 18 hours, 20 hours, 24 hours, or any value within the range.

[0103] The drying conditions of this invention can efficiently remove moisture without damaging the structure of Ti-MOF / polymer composite nanofibers (avoiding the collapse of the MOF skeleton due to high temperature), ensuring the drying effect while protecting the modified function of the fiber.

[0104] In some specific embodiments, the lithium source is selected from lithium carbonate (Li2CO3); and / or, the iron phosphorus source is selected from iron phosphate (FePO4).

[0105] In the preparation method described in this invention, the lithium source and the iron phosphorus source are conventional choices for preparing lithium iron phosphate in the art. The lithium source can be selected from, but is not limited to, at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium nitrate; the iron phosphorus source can be selected from, but is not limited to, at least one of iron phosphate, iron oxalate, iron nitrate, iron oxide, and combinations of phosphoric acid or phosphates. Those skilled in the art will understand that any compound that can be converted into lithium iron phosphate after high-temperature sintering without introducing interfering impurities falls within the scope of this invention. To obtain optimal reaction efficiency and product purity, in some preferred embodiments, the lithium source is selected from Li₂CO₃, and the iron phosphorus source is selected from FePO₄.

[0106] In a second aspect, the present invention provides a lithium iron phosphate, wherein the interior of the lithium iron phosphate crystal is bulk-doped with titanium, and its surface is coated with a carbon nanofiber network, wherein the titanium and the carbon nanofiber network are derived from Ti-MOF / polymer composite nanofibers.

[0107] The present invention also provides another lithium iron phosphate, wherein the lithium iron phosphate is prepared by the preparation method described in the first aspect above.

[0108] In a third aspect, the present invention provides a positive electrode sheet, wherein the positive electrode sheet comprises lithium iron phosphate as described in the second aspect above.

[0109] In some embodiments, the positive electrode further includes a conductive agent, the content of which is configured to be reduced based on the conductivity of the carbon nanofiber network in the lithium iron phosphate.

[0110] Based on the carbon nanofiber conductive network constructed on the surface of the lithium iron phosphate material, the positive electrode sheet can significantly reduce or even completely eliminate the need for additional conductive agents.

[0111] The Ti-MOF / polymer composite nanofibers prepared in this invention, after sintering, simultaneously achieve bulk titanium doping and surface coating of a carbon nanofiber network. This unique structure not only enhances the ionic / electronic conductivity of the material bulk, but the carbon nanofiber network on its surface can also directly serve as a conductive framework within the electrode, thereby significantly reducing or even completely replacing the conductive agent in the positive electrode. Ultimately, this leads to a comprehensive improvement in battery energy efficiency.

[0112] It is understood that the positive electrode sheet can be prepared and used according to conventional methods known in the art.

[0113] In a fourth aspect, the present invention provides a lithium-ion battery, wherein the lithium-ion battery includes the positive electrode sheet described in the third aspect above.

[0114] It is understood that the lithium-ion battery can be prepared and used according to conventional methods known in the art.

[0115] In this invention, to characterize the intrinsic electrochemical performance of the lithium iron phosphate material prepared by this invention, the materials of the examples and comparative examples were assembled into coin cells for testing. The coin cells used lithium metal sheets as the counter electrode, and the test results effectively reflected the capacity, efficiency, and kinetic performance of the material itself. Those skilled in the art will understand that materials exhibiting high capacity and high energy efficiency in half-cells can also achieve excellent overall performance when applied to full cells containing components such as a negative electrode (e.g., graphite) and electrolyte.

[0116] The following detailed embodiments illustrate the preparation method of lithium iron phosphate, lithium iron phosphate, positive electrode sheet, and lithium-ion battery of this application.

[0117] Example 1

[0118] This embodiment provides a lithium iron phosphate, wherein the internal structure of the lithium iron phosphate crystal is doped with bulk titanium, and its surface is coated with a carbon nanofiber network.

[0119] The method for preparing lithium iron phosphate includes the following steps:

[0120] S1. Provides Ti-MOF / polymer composite nanofibers;

[0121] Specifically, it includes:

[0122] S11. Provide spinning solution containing carboxylic acid aromatic hydrocarbons and polymers;

[0123] S111. Disperse 1 mmol of H2BDC uniformly in 15 mL of DMF solvent to obtain a dispersion.

[0124] S112. A certain amount of soluble polyimide (PI) is added to the dispersion to prepare an electrospinning solution with a concentration of 16 wt%.

[0125] S12. The above spinning solution is made into composite nanofibers using an electrospinning device at a voltage of 18kV.

[0126] S13. Place the prepared composite nanofibers in a high-pressure reactor and slowly add 15 mL of TTIP (1 mmol) methanol solution, mix, and obtain a mixed system.

[0127] S14. The mixture is ultrasonically treated (power 300 W, frequency 30 kHz, time 1 hour) and then reacted at 120℃ for 24 hours to obtain Ti-MOF / polymer composite nanofibers.

[0128] S2. The Ti-MOF / polymer composite nanofibers prepared above are dried at 80°C for 12 hours. Then, Li2CO3 and FePO4 are mixed at a ratio of 1.03:1 (based on the molar ratio of Li and Fe elements) and 0.5wt% of Ti-MOF / polymer composite nanofibers are added and ball-milled to obtain a precursor mixture.

[0129] S3. The precursor mixture is sintered at 680°C for 8 hours in a nitrogen-filled sintering furnace, and then pulverized by airflow to obtain lithium iron phosphate (LFP) material with high energy efficiency.

[0130] This embodiment also provides a positive electrode sheet, the preparation method of which is as follows:

[0131] The prepared LFP material and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 98%:2% using N-methylpyrrolidone (NMP) as a solvent. After being stirred and mixed evenly, the mixture was coated onto aluminum foil. After being completely dried, it was rolled into an electrode sheet using a roller press to obtain the positive electrode sheet.

[0132] This embodiment further provides a lithium-ion battery, the preparation method of which is as follows:

[0133] Negative electrode sheet: 85g of carboxymethyl cellulose (CMC) powder was dissolved in 4.6kg of deionized water to prepare a slurry with a solid content of 1.8wt%. 8kg of artificial graphite and 0.21kg of conductive carbon black were dry-mixed and stirred evenly. Then, an appropriate amount of CMC slurry and styrene-butadiene rubber (SBR) were added and stirred evenly. The slurry was adjusted to a suitable viscosity and solid content for coating, and then coated onto a 6μm thick copper foil current collector to obtain the negative electrode sheet.

[0134] Lithium-ion battery assembly process: Take the prepared positive and negative electrode sheets and assemble them into a coin cell in the following order: negative electrode shell, negative electrode sheet, separator, positive electrode sheet, gasket, spring sheet, and positive electrode shell. The separator is made of polyethylene (PE), and the electrolyte is a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 5:5, with an electrolyte addition of 80 μL. After assembly, the cells are sealed using a coin cell press to obtain the lithium-ion battery.

[0135] Example 2

[0136] The steps are the same as in Example 1, except that the amount of H2BDC used in step S111 is adjusted to 1.5 mmol.

[0137] Example 3

[0138] The steps are the same as in Example 1, except that the amount of H2BDC used in step S111 is adjusted to 0.5 mmol.

[0139] Example 4

[0140] The steps are the same as in Example 1, except that the amount of TTIP in step S13 is adjusted to 1.5 mmol.

[0141] Example 5

[0142] The steps are the same as in Example 1, except that the amount of TTIP in step S13 is adjusted to 0.5 mmol.

[0143] Example 6

[0144] The steps are the same as in Example 1, except that the reaction temperature in step S14 is adjusted to 150°C.

[0145] Example 7

[0146] The steps are the same as in Example 1, except that the reaction temperature in step S14 is adjusted to 100°C.

[0147] Example 8

[0148] The steps are the same as in Example 1, except that the amount of Ti-MOF / polymer composite nanofibers in step S2 is adjusted to 2wt%.

[0149] Example 9

[0150] The steps are the same as in Example 1, except that the amount of Ti-MOF / polymer composite nanofibers in step S2 is adjusted to 0.2wt%.

[0151] Example 10

[0152] The steps are the same as in Example 1, except that the sintering temperature in step S3 is adjusted to 750°C.

[0153] Example 11

[0154] The steps are the same as in Example 1, except that the sintering temperature in step S3 is adjusted to 650°C.

[0155] Example 12

[0156] The steps are the same as in Example 1, except that the ratio of Li2CO3 to FePO4 in step S2 (based on the molar ratio of Li and Fe elements) is adjusted to 1.01:1.

[0157] Example 13

[0158] The steps are the same as in Example 1, except that the ratio of Li2CO3 to FePO4 in step S2 (based on the molar ratio of Li and Fe elements) is adjusted to 1.08:1.

[0159] Example 14

[0160] The steps are the same as in Example 1, except that the reaction time of the mixed system after ultrasonic treatment in step S14 at 120°C is adjusted to 12 hours.

[0161] Example 15

[0162] The steps are the same as in Example 1, except that the reaction time of the mixed system after ultrasonic treatment in step S14 at 120°C is adjusted to 48 hours.

[0163] Comparative Example

[0164] Preparation of conventional LFP: Li2CO3 and FePO4 were mixed in a ratio of 1.03:1 (based on the molar ratio of Li and Fe elements), and 0.5wt% titanium dioxide and 1.2wt% glucose were added. Water was added and stirred to form an aqueous solution. The aqueous solution was spray-dried and sintered at 680℃ for 8 hours. Finally, LFP powder was obtained by air crushing.

[0165] Experimental Example 1: Electrochemical performance testing of materials

[0166] I. Magnetic Material Content Test

[0167] The lithium iron phosphate materials prepared in Examples 1-15 and the comparative examples were tested for their magnetic material content by magnetic separation-inductively coupled plasma (ICP) method, and the results are recorded in Table 1.

[0168] II. Electrochemical Performance Testing

[0169] To evaluate the intrinsic electrochemical performance of the lithium iron phosphate material prepared in this invention, the materials obtained in Examples 1-15 and the comparative examples were used to prepare coin half-cells for testing. Specifically, LFP and PVDF were mixed at a mass ratio of 98%:2%, using NMP as a solvent. After thorough mixing, a slurry was prepared and coated onto aluminum foil. After complete drying, the slurry was rolled into an electrode sheet using a roller press to obtain the positive electrode sheet for testing.

[0170] The coin cell half-cell assembly process is as follows: The coin cell half-cell is assembled in the following order: positive electrode shell, lithium plate, separator, positive electrode, gasket, spring contact, and negative electrode shell. A polyethylene (PE) separator is used, and the electrolyte is a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 5:5, with an electrolyte addition volume of 80 μL. After assembly, the cells are sealed using a coin cell press and allowed to stand for 12 hours before electrochemical performance testing.

[0171] The test results are shown in Table 1:

[0172] Table 1. Electrochemical performance test results of the cells in the examples and comparative examples.

[0173]

[0174] As can be seen from the results in Table 1 above:

[0175] 1. In terms of material purity and safety, the battery assembled from the material prepared by the method of the present invention has a magnetic material content (e.g., 60 ppb in Example 1) that is orders of magnitude lower than that of the comparative example (650 ppb), which greatly improves the safety and reliability of the battery.

[0176] 2. In terms of electrochemical performance, the lithium iron phosphate material prepared by the method of the present invention exhibits significantly improved energy efficiency in coin half-cells (for example, up to 92.5% in Example 1, while the comparative example is only 87.3%).

[0177] The significant improvement in overall performance is attributed to Ti-MOF / polymer composite nanofibers as a bifunctional precursor, which enables uniform bulk doping of titanium and effective coating of carbon nanofiber networks.

[0178] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A method for preparing lithium iron phosphate, characterized in that, The preparation method includes: Provides Ti-MOF / polymer composite nanofibers; Ti-MOF / polymer composite nanofibers were mixed with lithium source and iron phosphorus source to obtain precursor mixture; The precursor mixture is sintered under an inert atmosphere to obtain the final product. The polymer is selected from polyimide.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The amount of the Ti-MOF / polymer composite nanofibers is 0.2-2% of the total mass of the lithium source and the iron phosphorus source.

3. The method for preparing lithium iron phosphate according to claim 2, characterized in that, The ratio of lithium source to iron phosphorus source is 1.01-1.08:1, based on the molar ratio of Li to Fe.

4. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The sintering temperature is 650-750℃.

5. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The Ti-MOF / polymer composite nanofibers are provided by the following method: Provide spinning solutions containing carboxylic acid aromatic hydrocarbons and polymers; The spinning solution is spun into composite nanofibers; The composite nanofibers were mixed with an organic solvent solution of titanate to obtain a mixed system; The mixture was reacted at a preset temperature for a preset time to obtain Ti-MOF / polymer composite nanofibers; the spinning solution was prepared by the following method: Aromatic carboxylic acids are dispersed in a polar aprotic solvent to obtain a dispersion. The polymer is added to the dispersion to form a spinning solution; in: The carboxylic acid aromatic hydrocarbons are selected from terephthalic acid or triphenylcarboxylic acid.

6. The method for preparing lithium iron phosphate according to claim 5, characterized in that, The carboxylic acid aromatic hydrocarbon is selected from terephthalic acid.

7. The method for preparing lithium iron phosphate according to claim 5, characterized in that, The molar volume concentration of the dispersion is 0.033-0.1 mmol / mL.

8. The method for preparing lithium iron phosphate according to claim 5, characterized in that, The molar volume concentration of the organic solvent solution of the titanate is 0.033-0.1 mmol / mL.

9. The method for preparing lithium iron phosphate according to claim 5, characterized in that, The preset temperature is 100-150℃; and / or the preset time is 12-48h.

10. The method for preparing lithium iron phosphate according to claim 1, characterized in that, The Ti-MOF / polymer composite nanofibers are dried before being mixed with the lithium source and the iron-phosphorus source.

11. The method for preparing lithium iron phosphate according to any one of claims 1-10, characterized in that, The lithium source is selected from lithium carbonate; and / or, the iron phosphorus source is selected from iron phosphate.

12. A lithium iron phosphate, characterized in that, The lithium iron phosphate is prepared by the method of any one of claims 1-11.

13. A positive electrode plate, characterized in that, The positive electrode includes the lithium iron phosphate as described in claim 12.

14. The positive electrode sheet according to claim 13, characterized in that, The positive electrode also includes a conductive agent, the content of which is configured to be reduced based on the conductivity of the carbon nanofiber network in the lithium iron phosphate.

15. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode sheet as described in claim 13 or 14.

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