Lithium iron phosphate material and preparation method thereof, positive electrode material, positive electrode plate and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion battery positive electrode materials, in particular to a lithium iron phosphate material and a preparation method thereof, a positive electrode material, a positive electrode plate and a lithium ion battery. The method aims at solving the problem that more iron is dissolved out in the charging and discharging process of the lithium iron phosphate positive electrode material due to the fact that iron phosphide is generated in an existing method for preparing the lithium iron phosphate positive electrode material by introducing liquid polyacrylonitrile as a carbon source and mixing the liquid polyacrylonitrile with a lithium iron phosphate precursor. The lithium iron phosphate material provided by the invention comprises a LiFePO4 inner core and a coating layer coating the LiFePO4 inner core, wherein the coating layer is formed by cyclized polyacrylonitrile. When the lithium iron phosphate material is applied to a lithium ion battery as a positive electrode material, not only can the electronic conductivity of lithium iron phosphate be effectively improved, but also Fe dissolution can be obviously inhibited, the problem of iron dissolution of lithium iron phosphate can be effectively improved, and synergistic improvement of electrochemical performance and structural stability can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for lithium-ion batteries, and particularly to a lithium iron phosphate material and a preparation method thereof, a cathode material, a cathode electrode sheet and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries (LIBs) have brought revolutionary changes in the fields of electronic products, electric vehicles, etc. due to their advantages such as high energy density, high efficiency, light weight and portability.

[0003] Olivine-type lithium iron phosphate (LiFePO 4 ) cathode material is considered to be one of the most promising cathode materials due to its appropriate theoretical capacity (~170 mAh·g -1 ), low cost and high safety. However, olivine LiFePO 4 (LFP) exhibits low electronic conductivity (10 -9 ~10 -10 S·cm -1 ) and poor lithium ion diffusion coefficient (10 -14 ~10 -16 cm 2 ·s -1 ), which limits its further development.

[0004] Currently, in the fields of power and energy storage, the electrochemical performance of LiFePO 4 is mainly improved by carbon coating to enhance its rate performance and cycle life. Carbon coating on LiFePO 4 can not only enhance its conductivity, inhibit the growth and agglomeration of grains, reduce the polarization effect of the electrode, but also provide an electron channel for Li 4 in LiFePO + , making the insertion and extraction of lithium ions easier.

[0005] In the prior art, glucose or sucrose is usually used as the carbon source. Research shows that when the temperature reaches the melting point of glucose or sucrose, they will adhere to the surface of the lithium iron phosphate precursor. During high-temperature sintering, the carbon source inhibits the grain growth of lithium iron phosphate and forms a carbon coating layer at the same time. This method can effectively improve the conductivity of lithium iron phosphate and is beneficial to the infiltration of the electrolyte. However, after excessive amorphous carbon is compounded with lithium iron phosphate, the tap density will be greatly reduced, resulting in a decrease in the energy density of the battery.

[0006] CN116495715A discloses a lithium iron phosphate cathode material, its preparation method and application. The method is as follows: mix iron phosphate, a lithium source and a carbon source, and then sinter to obtain the lithium iron phosphate cathode material; the carbon source includes liquid polyacrylonitrile; the sintering temperature is 750 - 810 °C and not 750 °C. By introducing liquid polyacrylonitrile as the carbon source, iron phosphide can be generated at a relatively low sintering temperature, and then a lithium iron phosphate cathode material containing iron phosphide can be prepared; since iron phosphide has a relatively high conductivity, to a certain extent, the amount of the carbon source can be reduced to decrease the content of amorphous carbon in the lithium iron phosphate material, thereby increasing the tap density of the lithium iron phosphate material, and further enhancing its electrochemical specific capacity and the energy density of the battery.

[0007] However, due to LiFePO 4 In an electrolyte system with a specific lithium salt (such as LiPF 6 ), a relatively obvious iron dissolution reaction will occur. The iron ions dissolved from the cathode migrate through the electrolyte to the anode (such as graphite) and will be reductively deposited on the surface and inside of the anode. The iron deposited on the anode will: (1) occupy the ion transport channels, hinder the intercalation and deintercalation of lithium ions in graphite, resulting in the attenuation of the battery rate performance and cycle performance; (2) thicken the SEI film, increase the interfacial impedance, and also cause the loss of active lithium; (3) catalyze the decomposition of the electrolyte to generate CO 2 , CH 4 and other gases; (4) induce the uneven deposition of metallic lithium, pierce the separator, trigger an internal short circuit, and bring serious impacts on the battery safety performance.

[0008] The above method of introducing liquid polyacrylonitrile as the carbon source to mix with the lithium iron phosphate precursor to prepare the lithium iron phosphate cathode material can generate iron phosphide at a relatively low sintering temperature to reduce the amount of the carbon source and decrease the content of amorphous carbon in the lithium iron phosphate material, thereby increasing the tap density and electrochemical specific capacity of the lithium iron phosphate material. However, as a magnetic impurity, iron phosphide will cause a decline in the overcharge safety of lithium iron phosphate; in addition, iron phosphide will also lead to more iron dissolution during the charge and discharge process of the lithium iron phosphate cathode material.

[0009] Correspondingly, a new technical solution is needed in the art to solve the above technical problems. Summary of the Invention

[0010] The present invention aims to solve the above technical problems, that is, to solve the problems that the existing method of introducing liquid polyacrylonitrile as the carbon source to mix with the lithium iron phosphate precursor to prepare the lithium iron phosphate cathode material will generate iron phosphide, which causes a decline in the overcharge safety of lithium iron phosphate and leads to more iron dissolution during the charge and discharge process of the lithium iron phosphate cathode material.

[0011] In a first aspect, the present invention provides a lithium iron phosphate material, wherein the lithium iron phosphate material comprises a LiFePO 4 core and a coating layer coating the LiFePO 4 core, and the coating layer is formed of cyclized polyacrylonitrile.

[0012] In a preferred technical solution of the above lithium iron phosphate material, the cyclized polyacrylonitrile has graphitic nitrogen and pyridine nitrogen.

[0013] In a preferred technical solution of the above lithium iron phosphate material, the cyclized polyacrylonitrile has conjugated large π bonds.

[0014] In a preferred technical solution of the above lithium iron phosphate material, the cyclized polyacrylonitrile has conjugated units, and the conjugated units exist in the cyclized polyacrylonitrile in linear and planar forms, wherein nitrogen atoms participate in the conjugated system to form conjugated large π bonds.

[0015] In a preferred technical solution of the above lithium iron phosphate material, the LiFePO 4 core is formed of lithium iron phosphate, and the cyclized polyacrylonitrile is formed by cyclizing polyacrylonitrile.

[0016] In a preferred technical solution of the above lithium iron phosphate material, the mass ratio of the lithium iron phosphate to the polyacrylonitrile is 1:(0.02 - 0.06).

[0017] In a second aspect, the present invention provides a preparation method of the above lithium iron phosphate material, wherein the preparation method comprises:

[0018] providing a mixture of lithium iron phosphate and polyacrylonitrile;

[0019] calcining the mixture in an inert atmosphere to cause the polyacrylonitrile to undergo a cyclization reaction, thus obtaining the product.

[0020] In a preferred technical solution of the above preparation method, the calcining is carried out at a temperature of 300 - 600 °C;

[0021] and / or, the calcining time is 2 - 6 h.

[0022] In a preferred technical solution of the above preparation method, the mass ratio of the lithium iron phosphate to the polyacrylonitrile is 1:(0.02 - 0.06).

[0023] In a third aspect, the present invention provides a cathode material, and the cathode material is the above lithium iron phosphate material or the lithium iron phosphate material prepared by the above preparation method.

[0024] In a fourth aspect, the present invention provides a cathode plate, wherein the cathode plate comprises the above cathode material.

[0025] In a fifth aspect, the present invention provides a lithium-ion battery, wherein the lithium-ion battery includes the positive electrode sheet described above.

[0026] The lithium iron phosphate material, preparation method, positive electrode material, positive electrode sheet and lithium-ion battery of the present application have the following technical effects:

[0027] 1. When the lithium iron phosphate material of the present invention is used as a positive electrode material in a lithium-ion battery, it can not only effectively improve the electronic conductivity of lithium iron phosphate, but also significantly inhibit Fe dissolution, effectively improving the problem of iron dissolution in lithium iron phosphate, and achieving a synergistic improvement in electrochemical performance and structural stability.

[0028] 2. The method of the present invention can avoid the decrease in overcharge safety of lithium iron phosphate caused by the formation of iron phosphide and prevent more iron dissolution problems from occurring during the charge and discharge process of the lithium iron phosphate positive electrode material. Description of the Drawings

[0029] Figure 1 XPS spectrum of N1s of the coating layer in Example 1. Detailed Embodiments

[0030] The preferred embodiments of the present invention are described below. Those skilled in the art should understand that these embodiments are only used to explain the technical principle of the present invention and are not intended to limit the protection scope of the present invention.

[0031] In the present application, the term "and / or" describes the association relationship of associated objects and indicates that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist simultaneously, or B exists alone. Where A and B may be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after.

[0032] In the present application, "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or its similar expression means any combination of these items, including any combination of single item (item) or plural items (items). For example, "at least one (item) of a, b, or c", or, "at least one (item) of a, b, and c" can both represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0033] It should be understood that in various embodiments of the present application, the magnitudes of the serial numbers of the above processes do not mean the order of execution. Some or all steps can 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 to the implementation process of the embodiments of the present application.

[0034] The terms used in the embodiments of the present application are for the purpose of describing specific embodiments only and are not intended to limit the present application. The singular forms "a", "the", and "said" used in the embodiments of the present application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0035] The weight of the relevant components mentioned in the specification of the embodiments of the present application not only can refer to the specific content of each component, but also can represent the proportional relationship of the weights between the components. Therefore, as long as the content of the relevant components in the specification of the embodiments of the present application is enlarged or reduced in proportion, it is within the scope disclosed in the specification of the embodiments of the present application. Specifically, the mass described in the specification of the embodiments of the present application can be mass units well-known in the chemical industry such as μg, mg, g, kg, etc.

[0036] The terms "first" and "second" are only used for descriptive purposes to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX can also be referred to as the second XX, and similarly, the second XX can also be referred to as the first XX. Thus, the features defined with "first" and "second" can explicitly or implicitly include one or more of such features.

[0037] The experimental methods in the following embodiments are all conventional methods unless otherwise specified, and the materials, reagents, etc. used in the following embodiments can all be obtained from commercial channels unless otherwise specified.

[0038] Based on the problems pointed out in the background technology that the existing method of introducing liquid polyacrylonitrile as a carbon source and mixing it with a lithium iron phosphate precursor to prepare a lithium iron phosphate cathode material will generate iron phosphide, which causes a decrease in the overcharge safety of lithium iron phosphate and leads to more iron dissolution during the charge and discharge process of the lithium iron phosphate cathode material, the present invention provides a lithium iron phosphate material, a preparation method, a cathode material, a cathode electrode sheet, and a lithium ion battery. The method of the present invention can avoid the problems of a decrease in the overcharge safety of lithium iron phosphate caused by the generation of iron phosphide and more iron dissolution during the charge and discharge process of the lithium iron phosphate cathode material. When the provided lithium iron phosphate material is used as a cathode material in a lithium ion battery, it can not only effectively improve the electronic conductivity of lithium iron phosphate, but also significantly inhibit Fe dissolution, effectively improve the problem of iron dissolution in lithium iron phosphate, and achieve a synergistic improvement in electrochemical performance and structural stability.

[0039] Specifically, in the first aspect, the present invention provides a lithium iron phosphate material, wherein the lithium iron phosphate material includes LiFePO 4 core and a coating of LiFePO 4The coating layer of the core, and the coating layer is formed of cyclized polyacrylonitrile.

[0040] Conventional glucose and sucrose coatings are only simple carbothermal reduction coatings, lacking a spatially extended π-bond structure, and only improving the carrier mobility to a certain extent. At the same time, there is a lack of a stable bonding effect between the coating layer and the main material, which will form a certain amount of free carbon and cannot effectively inhibit the dissolution of Fe during charge and discharge.

[0041] In the present invention, the provided lithium iron phosphate material includes LiFePO 4 Core and coated LiFePO 4 The coating layer of the core, wherein the coating layer is formed of cyclized polyacrylonitrile. When used as a cathode material in a lithium ion battery, it can not only effectively improve the electronic conductivity of lithium iron phosphate, but also significantly inhibit the dissolution of Fe, effectively improve the problem of iron dissolution in lithium iron phosphate, and achieve a synergistic improvement in electrochemical performance and structural stability.

[0042] According to the technical solution of the present invention, the cyclized polyacrylonitrile has graphitic nitrogen and pyridine nitrogen.

[0043] In the present invention, the cyclized polyacrylonitrile has two structures, one with graphitic nitrogen and the other with pyridine nitrogen. Among them, graphitic nitrogen is connected to three carbon atoms, and pyridine nitrogen has a lone pair of electrons. When used as a coating layer to coat the surface of LiFePO 4 core, the presence of pyridine nitrogen and graphitic nitrogen significantly improves the electronic conductivity of the material, effectively reduces the internal resistance and polarization of the system, reduces the voltage difference of the charge and discharge platform of the energy storage lithium ion battery, and correspondingly improves the energy efficiency; while pyridine nitrogen, due to having a lone pair of electrons, can form a stable coordination bond with Fe, thereby effectively inhibiting the dissolution of Fe during charge and discharge and significantly improving the problem of iron dissolution in lithium iron phosphate.

[0044] In some specific embodiments, the cyclized polyacrylonitrile has a conjugated large π-bond.

[0045] In the present invention, the cyclized polyacrylonitrile has a spatially extended π-bond structure, exhibits high electronic conductivity, and has stable chemical properties. When used as a coating layer to coat the surface of LiFePO 4 core, it improves the electronic conductivity of the material, effectively reduces the internal resistance and polarization of the system, reduces the voltage difference of the charge and discharge platform of the energy storage lithium ion battery, and correspondingly improves the energy efficiency.

[0046] Specifically, the cyclized polyacrylonitrile has conjugated units, and the conjugated units exist in the cyclized polyacrylonitrile in the form of lines and planes, where nitrogen atoms participate in the conjugated system to form a conjugated large π-bond.

[0047] In some specific embodiments, the LiFePO 4 core is formed of lithium iron phosphate, and the cyclized polyacrylonitrile is formed by cyclizing polyacrylonitrile.

[0048] In some specific embodiments, the mass ratio of the lithium iron phosphate to the polyacrylonitrile is 1:(0.02 - 0.06).

[0049] In a second aspect of the present invention, there is provided a method for preparing the above lithium iron phosphate material, wherein the preparation method includes:

[0050] S1. Provide a mixture of lithium iron phosphate and polyacrylonitrile;

[0051] S2. Calcinate the mixture in an inert atmosphere to cause the polyacrylonitrile to undergo a cyclization reaction, thus obtaining the product.

[0052] In the existing methods, liquid polyacrylonitrile is introduced as a carbon source to prepare the lithium iron phosphate cathode material. Although it is possible to produce iron phosphide at a lower sintering temperature, reduce the amount of the carbon source to reduce the content of amorphous carbon in the lithium iron phosphate material, and further improve the tap density and electrochemical specific capacity of the lithium iron phosphate material, iron phosphide, as a magnetic impurity, will cause a decrease in the overcharge safety of the lithium iron phosphate; in addition, iron phosphide will cause more iron dissolution during the charge and discharge process of the lithium iron phosphate cathode material.

[0053] In the present invention, directly mixing the lithium iron phosphate and the polyacrylonitrile and then calcining can not only avoid the problem of generating iron phosphide magnetic impurities during the synthesis of the lithium iron phosphate, but also the polyacrylonitrile undergoes the following cyclization reaction during the calcination process:

[0054] The cyclization reaction yields two structures of cyclized polyacrylonitrile, which then coat the lithium iron phosphate to obtain the cyclized polyacrylonitrile-coated lithium iron phosphate material. In this material, the coating material cyclized polyacrylonitrile has two structures. One structure has graphitic nitrogen, and the graphitic nitrogen is connected to three carbon atoms; one structure has pyridinic nitrogen, and the pyridinic nitrogen has a lone pair of electrons. The presence of graphitic nitrogen and pyridinic nitrogen significantly improves the electronic conductivity of the material, effectively reduces the internal resistance and polarization of the system, reduces the voltage difference of the charge and discharge platform of the energy storage lithium-ion battery, and correspondingly improves the energy efficiency; and pyridinic nitrogen, due to having a lone pair of electrons, can form a stable coordination bond with Fe, thereby effectively inhibiting the dissolution of Fe during the charge and discharge process and significantly improving the problem of iron dissolution of the lithium iron phosphate.

[0055] In some specific embodiments, the calcination is carried out at a temperature of 300 - 600 °C.

[0056] In some specific embodiments, the calcination temperature is 300 °C, 400 °C, 500 °C, 600 °C or any value within the above calcination temperature range.

[0057] In a preferred embodiment, the calcination is carried out at a temperature of 500 °C.

[0058] In some specific embodiments, the calcination time is 2 - 6 h.

[0059] In some specific embodiments, the calcination time is 2 h, 4 h, 6 h or any value within the above calcination time range.

[0060] In a preferred embodiment, the calcination time is 4 h.

[0061] In some specific embodiments, the mass ratio of lithium iron phosphate to polyacrylonitrile is 1:(0.02 - 0.06). Specifically, it can be 1:0.02, 1:0.04, 1:0.05, 1:0.06 or any value within the above mass ratio range.

[0062] In some specific embodiments, the mixture of lithium iron phosphate and polyacrylonitrile is provided by ball milling.

[0063] In some specific embodiments, the rotation speed during ball milling is 600 r / min and the ball milling time is 8 h.

[0064] In some specific embodiments, the inert atmosphere can be, for example, an argon atmosphere.

[0065] Further, in the third aspect of the present invention, a cathode material is provided, and the cathode material is the above lithium iron phosphate material or the lithium iron phosphate material prepared by the above preparation method.

[0066] Further, in the fourth aspect of the present invention, a cathode electrode sheet is provided, wherein the cathode electrode sheet includes the above cathode material.

[0067] Further, in the fifth aspect of the present invention, a lithium ion battery is provided, wherein the lithium ion battery includes the above cathode electrode sheet.

[0068] The lithium iron phosphate material provided by the present invention has a cyclized polyacrylonitrile coating layer. When it is used as a cathode material in a lithium ion battery, since the coating material cyclized polyacrylonitrile has two structures, one with graphitic nitrogen and the other with pyridinic nitrogen, it can not only improve the electronic conductivity of the material, but also effectively inhibit the dissolution of Fe during charge and discharge, thereby effectively improving the problem of iron dissolution in lithium iron phosphate.

[0069] The following will detail the lithium iron phosphate material and its preparation method, cathode material, cathode electrode sheet and lithium ion battery of the present application through several specific embodiments.

[0070] In the following examples and comparative examples, the lithium iron phosphate raw materials used can be directly purchased from the market or prepared according to existing methods, such as being prepared according to the following method:

[0071] First, stoichiometric amounts of LiOH·H 2 O and FePO 4 ·2H 2 O are used as raw materials, and sucrose (20 wt% of LiFePO 4 ) is used as the carbon source and reducing agent. They are mixed and ball-milled using a stirred ball mill with zirconia as the medium. Ball-milling is carried out at a speed of 3000 rmp in ethanol for 2.5 h to obtain a precursor with a spherical powder structure. After the precursor mixture is dried in air at 60 °C, it is sintered in a nitrogen atmosphere at 650 °C for 8.5 h to obtain the LiFePO 4 / C composite material.

[0072] In the following examples and comparative examples, the preparation of the positive electrode, the preparation of the negative electrode, and the assembly of the battery are all carried out according to the following steps:

[0073] 1. Positive electrode plate

[0074] The lithium iron phosphate materials prepared in each example and comparative example are used as the positive electrode materials respectively. The positive electrode materials are mixed with the conductive agent carbon nanotubes and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 92:5:3 with N-methylpyrrolidone (NMP) to obtain a positive electrode slurry; the positive electrode slurry is uniformly coated on the current collector aluminum foil, and then dried in a vacuum drying oven at 60 °C for 20 hours; then it is pressed into a positive electrode plate with a diameter of 11 mm and a thickness of 54 μm under a pressure of 100 MPa, and the positive electrode plate is placed in a vacuum drying oven at 120 °C and dried for 12 h.

[0075] 2. Negative electrode plate

[0076] A metal lithium sheet with a diameter of 15.6 mm and a thickness of 0.45 mm is used as the negative electrode plate.

[0077] 3. Separator

[0078] A polyethylene porous membrane with an alumina ceramic layer coated on the surface and a thickness of 25 μm is used as the separator.

[0079] 4. Electrolyte

[0080] LiPF 6 is dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1, and the molar concentration of LiPF 6 is 1 mol / L.

[0081] 5. Preparation of lithium-ion battery

[0082] The positive electrode plate, separator, negative electrode plate and electrolyte were assembled into a 2025-type button cell in an Ar glove box with water content and oxygen content both less than 5 ppm.

[0083] Example 1

[0084] The preparation method of the lithium iron phosphate material in this example includes the following steps:

[0085] S1. Mix lithium iron phosphate and polyacrylonitrile according to a mass ratio of 1:0.04, set the rotation speed at 600 r / min, and ball mill for 8 h;

[0086] S2. Calcinate the obtained mixture in an inert atmosphere (argon) at 500 °C for 4 h to carry out the cyclization reaction of PAN, and finally obtain the lithium iron phosphate material.

[0087] The obtained lithium iron phosphate material includes a LiFePO 4 formed LiFePO 4 core and a coating layer covering the LiFePO 4 core. The coating layer is formed by cyclized polyacrylonitrile obtained by cyclizing polyacrylonitrile.

[0088] Perform XPS spectral analysis of N 1s on the coating layer. The results are shown in Figure 1 . It can be seen from Figure 1 that in the XPS spectrum of N 1s, pyridine nitrogen with lone pair electrons is located at 398.8 eV, and graphitic nitrogen connected to three carbon atoms is located at 400.4 eV, which proves the existence of two structures of cyclized polyacrylonitrile (c-PAN).

[0089] Example 2

[0090] Referring to Example 1, the difference from Example 1 is that in step S1, the mass ratio of lithium iron phosphate to polyacrylonitrile is 1:0.02.

[0091] Example 3

[0092] Referring to Example 1, the difference from Example 1 is that in step S1, the mass ratio of lithium iron phosphate to polyacrylonitrile is 1:0.06.

[0093] Example 4

[0094] Referring to Example 1, the difference from Example 1 is that in step S2, the calcination temperature is 300 °C.

[0095] Example 5

[0096] Referring to Example 1, the difference from Example 1 is that in step S2, the calcination temperature is 400 °C.

[0097] Example 6

[0098] Referring to Example 1, the difference from Example 1 is that in step S2, the calcination temperature is 600 °C.

[0099] Example 7

[0100] Referring to Example 1, the difference from Example 1 is that in step S2, the calcination time is 2 h.

[0101] Example 8

[0102] Referring to Example 1, the difference from Example 1 is that in step S2, the calcination time is 6 h.

[0103] Comparative Example 1

[0104] This comparative example provides a lithium iron phosphate coated with a conventional carbon source glucose (G-LFP), which is prepared by a method comprising the following steps:

[0105] Step 1: Mix iron phosphate, lithium carbonate, glucose (mass ratio 1:0.25:0.11) with water and then transport it to a grinder for mixing and grinding to prepare a slurry with a solid content of 35%;

[0106] Step 2: Set the rotation speed of the sand mill to 2000 r / min, and by adjusting the sanding time, prepare a slurry with a particle size D 50 of 500 nm;

[0107] Step 3: Use spray drying to dry the slurry obtained in step 2, and by controlling the inlet / outlet temperature of the spray dryer, prepare a lithium iron phosphate precursor with a moisture content of 1.5%;

[0108] Step 4: Use a muffle furnace for sintering. Take the material obtained in step 3 with nitrogen as the protective gas, and perform high-temperature sintering at 780 °C for 10 h to obtain a lithium iron phosphate material coated with a conventional carbon source glucose (G-LFP).

[0109] Comparative Example 2

[0110] This comparative example provides a lithium iron phosphate obtained by a method. The specific method is as follows:

[0111] Mix polyacrylonitrile and lithium iron phosphate in N,N-dimethylformamide at a mass ratio of 1:9, then place it in a quartz tube, sinter at 230 °C for 12 h under nitrogen protection, and collect it after cooling to room temperature to obtain the lithium iron phosphate material.

[0112] Comparative Example 3

[0113] This comparative example provides lithium iron phosphate obtained by another method. The specific method is as follows:

[0114] 1.4 kg of iron phosphate, 0.349 kg of lithium carbonate, 87 g of glucose, 122 g of liquid polyacrylonitrile and 4.1 L of deionized water were mixed, transferred to a sand mill and milled for 3 h. After spray drying at 220 °C for 2 h, sintering was carried out at 800 °C for 10 h under an argon atmosphere to obtain a lithium iron phosphate material.

[0115] Test Example 1

[0116] This test example examined the conductivity and iron dissolution amount of the lithium iron phosphate materials prepared in each example and comparative example of the present invention.

[0117] 1. Powder conductivity

[0118] Test method: The conductivity of the lithium iron phosphate cathode material was measured according to the method of the standard GB / T 30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium Ion Batteries".

[0119] 2. Iron dissolution

[0120] The battery cells prepared in each example and comparative example were subjected to constant current charge and discharge tests at a current density of 0.2C within a voltage range of 2.5 - 3.6 V. After 1000 cycles, the battery cells were disassembled for iron dissolution tests. The test method is as follows:

[0121] 1) Place a 50 ml beaker on an electronic balance with a ten-thousandth place, zero it, accurately weigh 5.0000 ± 0.005 g (aiming for close to 5.0000 g) of lithium iron phosphate sample, then measure 50.0 ml of pure water with a measuring cylinder, pour it into the beaker, seal it with plastic wrap, shake it by hand a few times and let it stand at room temperature for 70 h;

[0122] 2) After standing, place a clean centrifuge tube on 3 layers of medium-speed filter paper, pour the sample for filtration, and rinse the inner wall of the beaker with a wash bottle;

[0123] 3) Assemble the suction filtration device with 3 layers of 0.22 μm filter membrane, connect it to a vacuum pump, turn on the vacuum pump, pour the filtrate into the suction filtration cup for suction filtration, wash the inner walls of the centrifuge tube and the beaker thoroughly with a wash bottle. After complete suction filtration, volume-fix all the filtrate into a 100 ml volumetric flask, and then perform inductively coupled plasma testing (ICP testing).

[0124] The results are shown in Table 1:

[0125] Table 1. Test Results of Conductivity and Fe Dissolution of Lithium Iron Phosphate in Examples and Comparative Examples

[0126] As can be seen from the results in Table 1, compared with Comparative Examples 1 to 3, the lithium iron phosphate materials in Examples 1 to 8 of the present invention have higher conductivity and less iron dissolution after 1000 cycles. It can be seen that the lithium iron phosphate material with a cyclized polyacrylonitrile coating layer provided by the present invention can not only significantly improve the conductivity of lithium iron phosphate, but also significantly inhibit the dissolution of Fe in the lithium iron phosphate material during charge and discharge.

[0127] Test Example 2

[0128] This test example investigated the effect of different calcination temperatures on the Fe dissolution of the obtained polyacrylonitrile-coated lithium iron phosphate.

[0129] Test method: Mix lithium iron phosphate and polyacrylonitrile according to a mass ratio of 1:0.04, set the rotation speed at 600 r / min, and ball mill for 8 h; calcine the obtained mixture in an inert atmosphere at different calcination temperatures (see Table 2) for 4 h to obtain polyacrylonitrile-coated lithium iron phosphate. Use the lithium iron phosphate obtained at different calcination temperatures as the positive electrode active material, prepare the battery according to the aforementioned method, and then test the Fe dissolution amount after 1000 cycles according to the method of Test Example 1 to investigate the effect of different calcination temperatures on the Fe dissolution of polyacrylonitrile-coated lithium iron phosphate.

[0130] The results are shown in Table 2:

[0131] Table 2. Effect of Different Calcination Temperatures on Fe Dissolution of Lithium Iron Phosphate Calcination Temperature (°C) Fe Dissolution after 1000 Cycles (ppm) 100 342 200 327 300 255 400 185 500 138 600 201 700 358 800 609

[0132] As can be seen from the results in Table 2, the calcination temperature has a certain effect on the Fe dissolution of the obtained polyacrylonitrile-coated lithium iron phosphate. When the calcination temperature is 300 - 600 °C, the Fe dissolution amount of the obtained polyacrylonitrile-coated lithium iron phosphate is less, and when the calcination temperature is 500 °C, the Fe dissolution amount of the obtained polyacrylonitrile-coated lithium iron phosphate is the least.

[0133] Test Example 3

[0134] This test example investigated the effect of different calcination times on the Fe dissolution of the obtained lithium iron phosphate.

[0135] Test method: Lithium iron phosphate and polyacrylonitrile were mixed at a mass ratio of 1:0.04, the rotation speed was set at 600 r / min, and the ball milling time was 8 h; the obtained mixture was calcined at 500 °C for different times (see Table 3) in an inert atmosphere to obtain polyacrylonitrile-coated lithium iron phosphate. The lithium iron phosphate obtained with different calcination times was used as the positive electrode active material, and the battery was prepared according to the aforementioned method, and then the Fe dissolution amount after 1000 cycles was tested according to the method of Test Example 1 to investigate the effect of different calcination times on the Fe dissolution of lithium iron phosphate.

[0136] The results are shown in Table 3:

[0137] Table 3. Effect of different calcination times on the Fe dissolution of lithium iron phosphate Calcination Time (h) Fe Dissolution after 1000 Cycles (ppm) 1 285 2 236 3 159 4 138 5 150 6 165 7 222

[0138] It can be seen from the results in Table 3 that the calcination time has a certain effect on the Fe dissolution of the obtained polyacrylonitrile-coated lithium iron phosphate. When the calcination time is 2 - 7 h, the Fe dissolution amount of the obtained polyacrylonitrile-coated lithium iron phosphate is less, but since the energy consumption for calcination for 7 h is relatively large, the present invention selects 2 - 6 h. And when the calcination time is 4 h, the Fe dissolution amount of the obtained polyacrylonitrile-coated lithium iron phosphate is the least. Therefore, the present invention takes 4 h of calcination as the most preferred scheme.

[0139] So far, the technical solution of the present invention has been described in conjunction with the shown preferred embodiments. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle 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 these changes or substitutions will all fall within the protection scope of the present invention.

Claims

1. A lithium iron phosphate material, characterized in that: The lithium iron phosphate material comprises a LiFePO4 core and a coating layer coating the LiFePO4 core, wherein the coating layer is formed by cyclized polyacrylonitrile.

2. The lithium iron phosphate material according to claim 1, characterized in that: The cyclized polyacrylonitrile has graphitic nitrogen and pyridinic nitrogen.

3. The lithium iron phosphate material according to claim 2, characterized in that: The LiFePO4 core is formed by lithium iron phosphate, and the cyclized polyacrylonitrile is formed by cyclization of polyacrylonitrile.

4. The lithium iron phosphate material according to claim 3, characterized in that: The mass ratio of the lithium iron phosphate to polyacrylonitrile is 1:(0.02-0.06).

5. A method for preparing a lithium iron phosphate material, characterized in that: The preparation method comprises: Providing a mixture of lithium iron phosphate and polyacrylonitrile; The mixture is calcined in an inert atmosphere to allow polyacrylonitrile to undergo a cyclization reaction to obtain the product.

6. The preparation method according to claim 5, characterized in that: The calcination is carried out at a temperature of 300 to 600° C. And / or, the calcination time is 2 to 6 hours.

7. The preparation method according to claim 5, characterized in that: The mass ratio of lithium iron phosphate to polyacrylonitrile is 1:(0.02-0.06).

8. A positive electrode material, characterized in that The positive electrode material is the lithium iron phosphate material described in any one of claims 1-4 or the lithium iron phosphate material prepared by the preparation method described in any one of claims 5-7.

9. A positive electrode sheet, characterized in that: The positive electrode plate comprises the positive electrode material according to claim 8.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet according to claim 9.

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