Polymer composite lithium iron phosphate material and preparation method thereof

Abstract

The application discloses a polymer composite lithium iron phosphate material and a preparation method thereof, relates to the technical field of lithium iron phosphate positive electrode materials, and comprises the following steps: S1, ultrasonicating bacterial cellulose in a lithium polyacrylate aqueous solution to obtain a fiber dispersion liquid; S2, adding the fiber dispersion liquid into lithium iron phosphate slurry for mixing and dispersing, then adding PEDOT:PSS for crosslinking, and finally performing spray granulation and gradient annealing to obtain the polymer composite lithium iron phosphate material. The polymer coated on the surface of the lithium iron phosphate makes the positive electrode material have more stable conductivity and cycle stability, and solves the problems of discontinuous conductive network, many interface side reactions and high cost in traditional modification.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium iron phosphate positive electrode materials, and in particular relates to a polymer composite lithium iron phosphate material and a preparation method thereof. Background Art

[0002] Lithium iron phosphate (LiFePO4) is a popular choice for power batteries due to its safety and low cost as a cathode material. However, intrinsic defects of LiFePO4, such as low electronic conductivity and slow lithium ion diffusion, limit its high-rate performance and low-temperature performance.

[0003] Existing technologies mostly modify lithium iron phosphate through carbon coating, metal doping and conductive polymer composites. Conductive polymers mostly use polypyrrole, polyaniline, cellulose, etc. These celluloses are prone to volume expansion or decomposition during long-term circulation, resulting in a decrease in interface stability.

[0004] PEDOT:PSS has been tried for lithium iron phosphate modification due to its high conductivity and flexibility, but its interfacial compatibility with lithium iron phosphate is poor, making it unable to evenly coat the particles, and volume changes during charging and discharging can cause the coating to rupture.

[0005] Bacterial cellulose was used in the hydrothermal preparation of lithium iron phosphate in patent document CN109256528A to improve graphene coating. This method is to mix it with active materials after carbonization, but it fails to fully utilize its surface functional groups (such as hydroxyl groups) for chemical modification, resulting in poor fiber dispersion and weak bonding with the matrix. Summary of the Invention

[0006] In order to overcome the above technical problems, the present invention provides a polymer composite lithium iron phosphate material and a preparation method thereof. The present invention coats the surface of lithium iron phosphate with a polymer to make the positive electrode material have more stable conductivity and cycle stability, thereby solving the problems of discontinuous conductive network, multiple interface side reactions and high cost in traditional modification.

[0007] The present invention solves the above technical problems through the following technical solutions.

[0008] A method for preparing a polymer composite lithium iron phosphate material comprises the following steps:

[0009] S1. Add bacterial cellulose (BC) to a lithium polyacrylate (LiPAA) aqueous solution and ultrasonicate to obtain a fiber dispersion.

[0010] S2. Add the fiber dispersion to the lithium iron phosphate slurry for mixing and dispersion, then add PEDOT:PSS [poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid), CAS No.: 155090-83-8] for cross-linking, spray granulation, and gradient annealing to obtain a polymer composite lithium iron phosphate material.

[0011] According to some embodiments of the present invention, the bacterial cellulose needs to be pretreated, and the pretreatment method includes: crushing the bacterial cellulose, alkali washing and water washing.

[0012] According to some embodiments of the present invention, the bacterial cellulose has a diameter of 50-100 nm.

[0013] According to some embodiments of the present invention, the surface functional groups of the bacterial cellulose are hydroxyl groups.

[0014] According to some embodiments of the present invention, the alkali solution for alkali washing is a 3-5 wt % NaOH aqueous solution.

[0015] According to some embodiments of the present invention, the temperature of the alkali washing is 60-80°C.

[0016] According to some embodiments of the present invention, the alkali washing time is 1 to 3 hours.

[0017] According to some embodiments of the present invention, the water washing ends after the pH value is 6.5-7.5.

[0018] According to some embodiments of the present invention, the diameter of the bacterial fibers after the pretreatment is reduced to ≤30 nm, preferably ≤20 nm.

[0019] According to some embodiments of the present invention, the ultrasound duration is 0.5 to 2 hours.

[0020] According to some embodiments of the present invention, the fiber dispersion comprises the following raw materials in parts by mass: 100 parts bacterial cellulose, 2-5 parts lithium polyacrylate, and 100-150 parts water;

[0021] Preferably, the fiber dispersion comprises the following raw materials in parts by mass: 100 parts of bacterial cellulose, 2-4 parts of lithium polyacrylate and 100-130 parts of water.

[0022] According to some embodiments of the present invention, the lithium iron phosphate slurry includes the following raw materials in parts by mass: 100 parts of lithium iron phosphate raw material, 0.1-0.3 parts of ammonium persulfate and 50-70 parts of solvent;

[0023] Wherein, the solvent is ethanol and water; preferably, the ethanol:water ratio in the solvent is (80-90:10-20);

[0024] Preferably, the lithium iron phosphate slurry comprises the following raw materials in parts by mass: 100 parts of lithium iron phosphate raw material, 0.1-0.2 parts of ammonium persulfate and 50-60 parts of solvent.

[0025] According to some embodiments of the present invention, the D50 of the lithium iron phosphate raw material is 0.2-2 μm, preferably 0.5-1.5 μm.

[0026] According to some embodiments of the present invention, the lithium iron phosphate raw material is conventional in the art and can be prepared by a high-temperature solid-phase method, a hydrothermal method or a co-precipitation method; preferably, it is prepared by a high-temperature solid-phase method.

[0027] According to some embodiments of the present invention, the PEDOT:PSS needs to be adjusted to a pH of 3.5-4.0 with aqueous ammonia before use.

[0028] According to some embodiments of the present invention, the mass ratio of the lithium iron phosphate slurry: the fiber dispersion: the PEDOT:PSS is 100:1~5:2~8; preferably, the mass ratio of the lithium iron phosphate slurry: the fiber dispersion: the PEDOT:PSS is 100:1~3:3~5.

[0029] According to some embodiments of the present invention, the particle size after spray granulation is controlled to be ≤5 μm, preferably ≤3 μm.

[0030] According to some embodiments of the present invention, the gradient annealing is divided into a low temperature stage, a medium temperature stage and a high temperature stage.

[0031] The low temperature stage is heating at 110-130°C for 1.5-2.5h under nitrogen protection;

[0032] The medium temperature stage is heating at 240-260° C. for 1-1.5 hours under a nitrogen / hydrogen atmosphere; the volume ratio of nitrogen / hydrogen is 90-95 / 10-5;

[0033] The high temperature stage is performed under an argon atmosphere at 380-420° C. for 20-40 min.

[0034] According to some embodiments of the present invention, the preparation method of lithium polyacrylate comprises the following steps: reacting polyacrylic acid and lithium hydroxide at 40-60°C; preferably, the preparation method of lithium polyacrylate comprises the following steps: dispersing polyacrylic acid in water, adding lithium hydroxide dropwise, reacting at 40-60°C to a pH of 7.0±0.2, and then placing in a dialysis bag and washing with deionized water until the conductivity is ≤10μS / cm.

[0035] The polyacrylic acid has an Mw of 2000-4500, preferably 2000-3000.

[0036] The present invention also provides a polymer composite lithium iron phosphate material, which is prepared by the above-mentioned preparation method of the polymer composite lithium iron phosphate material.

[0037] On the basis of conforming to the common sense in this field, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present invention.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] The core steps of the polymer composite lithium iron phosphate material of the present invention are first adding BC-LiPAA to construct a skeleton and then introducing PEDOT:PSS to fill the pores.

[0040] Lithium polyacrylate acts as a binder and interface modifier to improve the structural stability of the positive electrode material; bacterial cellulose nanofibers are interwoven into a porous skeleton, and the residual hydroxyl groups on their surface interact with Li + Form weak coordination to accelerate ion transport; the conductive network BC-LiPAA constructed by the two.

[0041] PEDOT:PSS can provide a conjugated π electron system and high conductivity. In collaboration with the conductive network BC-LiPAA, it forms a uniform coating layer on the LiFePO4 surface through in situ polymerization, constructing a continuous and stable electron / ion dual pathway.

[0042] The polymer composite lithium iron phosphate material of the present invention has high discharge specific capacity at low temperature and good cycle capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is an SEM image of the positive electrode piece prepared from the polymer composite lithium iron phosphate material of Example 1.

[0044] Figure 2 This is an SEM image of the positive electrode sheet prepared from the polymer composite lithium iron phosphate material of Comparative Example 1.

[0045] Figure 3 This is an SEM image of the positive electrode sheet prepared from the polymer composite lithium iron phosphate material of Comparative Example 2.

[0046] Figure 4 This is an SEM image of the positive electrode sheet prepared from the polymer composite lithium iron phosphate material of Comparative Example 3. DETAILED DESCRIPTION

[0047] To facilitate understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0048] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0049] The "ranges" disclosed herein are defined in terms of lower and upper limits. A given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined in any manner, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values ​​listed are 1 and 2, and if the maximum range values ​​listed are 3, 4, and 5, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this disclosure, unless otherwise indicated, the numerical range "ab" is an abbreviation for any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely an abbreviation for these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0050] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0051] Unless otherwise specified, all technical features and optional technical features of the present invention can be combined with each other to form a new technical solution.

[0052] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, "the method includes steps (a) and (b)" indicates that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, "the method may further include step (c)" indicates that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0053] Unless otherwise specified, the terms "include" and "comprising" used in the present invention may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0054] Unless otherwise specified, the term "or" is inclusive in this disclosure. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0055] The raw material information used in the following examples is as follows:

[0056] Lithium iron phosphate is sourced from Hunan Yuneng New Energy Battery Materials Co., Ltd., with a D50 of 0.5 μm.

[0057] Bacterial cellulose was purchased from Nanjing Tianlu Nanotechnology Co., Ltd. The fiber diameter is 50-100 nm, the crystal structure is cellulose I type, and the surface contains hydroxyl functional groups. The specific structure is:

[0058]

[0059] PEDOT:PSS (1.5% aqueous solution) was purchased from Shanghai Yien Chemical Technology Co., Ltd. and adjusted to pH 4 with ammonia before use.

[0060] The Mw of polyacrylic acid is 3,000.

[0061] Example 1

[0062] The preparation method of the polymer composite lithium iron phosphate material of this embodiment is as follows:

[0063] Preparation of lithium polyacrylate: Control the temperature at 50°C, disperse polyacrylic acid in water, and slowly add lithium hydroxide dropwise until the pH reaches 7.0±0.2. Wash the product with deionized water in a dialysis bag to a conductivity of 10 μS / cm, and then vacuum dry the product to obtain lithium polyacrylate;

[0064] Pretreatment of bacterial cellulose: The bacterial cellulose was crushed by a high-speed shearing machine, then placed in a 3-5 wt% NaOH aqueous solution at 80°C for 2 hours, and then washed with water until the pH reached 7.0. After the washing was completed, the diameter of the bacterial cellulose after pretreatment was less than 20 nm.

[0065] S1. The pretreated bacterial cellulose was added to a lithium polyacrylate aqueous solution and ultrasonicated at 400W for 2h to obtain a fiber dispersion;

[0066] The fiber dispersion comprises the following raw materials in parts by weight: 100 parts of bacterial cellulose, 3 parts of sodium acrylate and 100 parts of water;

[0067] S2. The lithium iron phosphate slurry comprises the following raw materials in parts by mass: 100 parts of lithium iron phosphate raw material, 0.2 parts of ammonium persulfate and 50 parts of solvent (volume ratio of ethanol / water = 1:1);

[0068] The fiber dispersion is added to the lithium iron phosphate slurry for mixing and dispersion, and then PEDOT:PSS is added for cross-linking and spray granulation is performed to a particle size of ≤3 μm, and gradient annealing is performed to obtain a polymer composite lithium iron phosphate material; the gradient annealing is divided into a low temperature stage, a medium temperature stage and a high temperature stage;

[0069] The mass ratio of lithium iron phosphate slurry: fiber dispersion: PEDOT: PSS is 100:2:5;

[0070] Low temperature stage: heating at 150℃ for 2h under nitrogen protection;

[0071] Medium temperature stage: heating at 250°C for 1 h in a nitrogen / hydrogen atmosphere of 95:5 (volume ratio);

[0072] High temperature stage: heating at 400 °C for 30 min under argon atmosphere.

[0073] Example 2

[0074] The difference between this embodiment and embodiment 1 is that:

[0075] The bacterial cellulose in this example was not alkali-washed, but was crushed using a high-speed shearing machine and then washed with water before use.

[0076] Other raw materials, steps and parameters are the same as in Example 1.

[0077] Example 3

[0078] The difference between this embodiment and embodiment 1 is that:

[0079] The fiber dispersion comprises the following raw materials in parts by weight: 100 parts bacterial cellulose, 4 parts lithium polyacrylate and 100 parts water;

[0080] Other raw materials, steps and parameters are the same as in Example 1.

[0081] Example 4

[0082] The difference between this embodiment and embodiment 1 is that:

[0083] The fiber dispersion includes the following raw materials in parts by mass: 100 parts of bacterial cellulose, 7 parts of lithium polyacrylate and 100 parts of water.

[0084] Other raw materials, steps and parameters are the same as in Example 1.

[0085] Example 5

[0086] The difference between this embodiment and embodiment 1 is that:

[0087] The mass ratio of the lithium iron phosphate slurry: the fiber dispersion: the PEDOT:PSS is 100:3:3;

[0088] Other raw materials, steps and parameters are the same as in Example 1.

[0089] Example 6

[0090] The difference between this embodiment and embodiment 1 is that:

[0091] The gradient annealing is divided into a low temperature stage, a medium temperature stage and a high temperature stage.

[0092] The low temperature stage is heating at 130°C for 1 hour under nitrogen protection;

[0093] The medium temperature stage is heating at 240° C. for 1.5 h under a nitrogen / hydrogen atmosphere; the volume ratio of the nitrogen / hydrogen is 90 / 10.

[0094] Other raw materials, steps and parameters are the same as in Example 1.

[0095] Comparative Example 1

[0096] The difference between this comparative example and Example 1 is:

[0097] In this comparative example S1, lithium polyacrylate was not added, so the fiber dispersion did not contain lithium polyacrylate;

[0098] The fiber dispersion comprises the following raw materials in parts by weight: 100 parts of bacterial cellulose and 100-130 parts of water;

[0099] Other raw materials, steps and parameters are the same as in Example 1.

[0100] Comparative Example 2

[0101] The difference between this comparative example and Example 1 is:

[0102] The method of adding the polymer composite lithium iron phosphate material of this comparative example is as follows: adding polypropylene lithium solution to lithium iron phosphate slurry for mixing and dispersion, and then adding PEDOT:PSS for cross-linking;

[0103] The mass ratio of lithium iron phosphate slurry: polypropylene lithium solution (concentration 3wt%): PEDOT:PSS is 100:1~3:3~5;

[0104] Other raw materials, steps and parameters are the same as in Example 1.

[0105] Comparative Example 3

[0106] The difference between this comparative example and Example 1 is:

[0107] In S2 of this comparative example, PEDOT:PSS was not added;

[0108] Other raw materials, steps and parameters are the same as in Example 1.

[0109] Test Case

[0110] The steps for preparing the polymer composite lithium iron phosphate material obtained in the above examples and comparative examples into a button-type half-cell are as follows:

[0111] (1) Adding a polymer composite lithium iron phosphate material: acetylene black: polyvinylidene fluoride in a mass ratio of 8:1:1, a solvent N-methylpyrrolidone (NMP) was added to adjust the viscosity to 4000~8000mPa·s to prepare a positive electrode slurry;

[0112] (2) The positive electrode slurry is evenly coated on one surface (perpendicular to the thickness direction) of the aluminum foil (positive electrode current collector) and dried, and then rolled and sheared to obtain a positive electrode sheet containing a positive electrode film layer.

[0113] (3) Using lithium hexafluorophosphate-ethyl methyl carbonate as the electrolyte and polypropylene as the isolation membrane, the prepared positive electrode sheet and negative lithium sheet are combined to make a button half-cell.

[0114] The button half-cell was tested in the voltage range of 2.0V to 3.8V using a Blue Electric device, including 1.0C and 2.0C discharge specific capacity, as well as 100 cycles of capacity retention at 1C. The test results are shown in Table 1 below.

[0115]

[0116] According to the results in the above table combined with the SEM test results, we can know that:

[0117] The SEM image of the positive electrode of the polymer composite lithium iron phosphate material prepared in Example 1 is as follows: Figure 1 The surface of the positive electrode sheet made of the polymer composite lithium iron phosphate material of Example 1 is uniform and has no cracks.

[0118] The SEM image of the positive electrode of the polymer composite lithium iron phosphate material prepared in Comparative Example 1 is as follows: Figure 2 In Comparative Example 1, because no lithium polyacrylate was added, the bacterial fiber and lithium iron phosphate raw material particles were loosely bonded and easily fell off, and the PEDOT:PSS was unevenly distributed, resulting in incomplete pore filling. After the button battery was made, the interface side reaction was aggravated.

[0119] The SEM image of the positive electrode of the polymer composite lithium iron phosphate material prepared in Comparative Example 2 is as follows: Figure 3As shown. Comparative Example 2, in which the raw materials did not contain bacterial cellulose, directly coating the particles with PEDOT:PSS resulted in dense packing. The resulting electrode exhibited cracking during testing and poor flexibility; capacity decayed rapidly during high-rate cycling. Comparative Example 2, in which the raw materials did not contain bacterial cellulose, directly coating the particles with PEDOT:PSS resulted in dense packing. The resulting electrode exhibited cracking during testing and poor flexibility; capacity decayed rapidly during high-rate cycling.

[0120] The SEM image of the positive electrode of the polymer composite lithium iron phosphate material prepared in Comparative Example 3 is shown in Figure 4. In Comparative Example 3, the polymer composite lithium iron phosphate material relies solely on the carbon coating layer for electron conduction, resulting in high resistance and low discharge specific capacity; the color of the prepared electrode is uneven.

[0121] Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in the present invention can be purchased from the market or prepared by existing methods. The specific embodiments described above further illustrate the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing a polymer composite lithium iron phosphate material, characterized in that: The following steps are involved: S1. adding bacterial cellulose to an aqueous solution of lithium polyacrylate and ultrasonicating to obtain a fiber dispersion; S2. Add fiber dispersion to lithium iron phosphate slurry for mixing and dispersion, then add PEDOT:PSS for cross-linking, spray granulation, and gradient annealing to obtain polymer composite lithium iron phosphate material.

2. The method for preparing the polymer composite lithium iron phosphate material according to claim 1, wherein: The bacterial cellulose satisfies at least one of the following conditions a to b: a. The bacterial cellulose has a diameter of 50 to 100 nm; b. The bacterial cellulose needs to be pretreated, and the pretreatment method includes crushing the bacterial cellulose, alkali washing and water washing.

3. The method for preparing the polymer composite lithium iron phosphate material according to claim 2, wherein: The pretreatment satisfies at least one of the following conditions a to d: a. The alkali solution for the alkali wash is a 3 to 5wt% aqueous solution of NaOH; b. The alkali washing temperature is 60 to 80 ° C; c. The alkali washing time is 1 to 3 hours; d. The water washing is completed after the pH is 6.5 to 7.

5.

4. The method for preparing the polymer composite lithium iron phosphate material according to claim 1, wherein: The fiber dispersion comprises the following raw materials in parts by mass: 100 parts of bacterial cellulose, 2 to 5 parts of lithium polyacrylate and 100 to 150 parts of water.

5. The method for preparing the polymer composite lithium iron phosphate material according to claim 1, wherein: The lithium iron phosphate slurry comprises the following raw materials in parts by mass: 100 parts of lithium iron phosphate raw material, 0.1-0.3 parts of ammonium persulfate and 50-70 parts of solvent; The D50 of the lithium iron phosphate raw material is 0.2~2μm.

6. The method for preparing the polymer composite lithium iron phosphate material according to claim 1, wherein: The mass ratio of the lithium iron phosphate slurry: the fiber dispersion: the PEDOT:PSS is 100:1-5:2-8.

7. The method for preparing the polymer composite lithium iron phosphate material according to claim 1, wherein: The PEDOT:PSS needs to be adjusted to a pH of 3.5-4.0 with ammonia water before use.

8. The method for preparing the polymer composite lithium iron phosphate material according to claim 7, wherein: The gradient annealing is divided into a low temperature stage, a medium temperature stage and a high temperature stage; The low temperature stage is heating at 110-130°C for 1.5-2.5h under nitrogen protection; The medium temperature stage is heating at 240-260°C for 1-1.5 hours under a nitrogen / hydrogen atmosphere; The high temperature stage is performed under an argon atmosphere at 380-420° C. for 20-40 min.

9. The method for preparing the polymer composite lithium iron phosphate material according to claim 7, wherein: The preparation method of lithium polyacrylate comprises the following steps: reacting polyacrylic acid and lithium hydroxide at 40-60°C.

10. A polymer composite lithium iron phosphate material, characterized in that: The polymer composite lithium iron phosphate material is prepared by the preparation method of any one of claims 1 to 9.

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