A lithium iron phosphate composite material and preparation method thereof

Abstract

This article discloses a lithium iron phosphate composite material and its preparation method, which are related to the field of lithium-ion batteries. The preparation method described herein effectively addresses the existing performance shortcomings of lithium iron phosphate by designing the morphology of lithium iron phosphate in the product system, introducing titanium dioxide nanotubes, and constructing a specially structured cross-linked carbon conductive network coating. When used as a positive electrode material in lithium-ion batteries, it exhibits excellent rate performance. Therefore, the prepared lithium iron phosphate composite material is highly suitable for the full-chain integrated production of power batteries.

Description

Technical Field

[0001] This article relates to the field of lithium-ion batteries, and specifically to a lithium iron phosphate composite material and a preparation method thereof. Background Art

[0002] Lithium iron phosphate (LIFP) has become a popular choice for cathode materials in mainstream new energy lithium-ion batteries due to its high cycle performance and safety. However, due to its lower tap and compacted densities compared to ternary materials, the energy density of lithium-ion batteries produced using LFP is far lower than that of ternary materials, resulting in insufficient rate performance and significantly limiting its use in the automotive sector. While existing technologies often employ carbon doping or nano-particle size reduction to improve the rate performance of LFP-based materials, such approaches can reduce the overall material specific capacity due to material doping or lead to product agglomeration, ultimately failing to achieve the desired effect. Summary of the Invention

[0003] Based on the defects of related technologies, the purpose of this article is to provide a method for preparing lithium iron phosphate composite materials. By designing the morphology of lithium iron phosphate in the product system, introducing titanium dioxide nanotubes and constructing a cross-linked carbon conductive network coating with a special structure, the existing performance defects of lithium iron phosphate are effectively solved, and it can exhibit excellent rate performance when used as a positive electrode material for lithium-ion batteries.

[0004] In order to achieve the above objectives, the technical solutions adopted in this paper are:

[0005] A method for preparing a lithium iron phosphate composite material comprises the following steps:

[0006] Add nano titanium dioxide powder to water at 80-90° C. and disperse evenly to obtain a dispersed aqueous solution of nano titanium dioxide powder;

[0007] Adding an iron source, a phosphorus source, and a lithium source to a dispersed aqueous solution of nano-titanium dioxide powder and mixing them uniformly to obtain a composite dispersed solution;

[0008] Adding a polymerizable monomer to a composite dispersed solution, then adding a crosslinking agent and an initiator and heating to 80-90° C. and mixing to obtain a gel precursor; the polymerizable monomer is at least one of hydroxymethyl acrylamide, acrylamide, and ethyl acetoacetate methacrylate;

[0009] After the gel precursor is dried, it is placed in an air atmosphere and kept at 300-450° C. for 5-8 hours, and then kept at 600-850° C. for 9-12 hours in a protective atmosphere to obtain a lithium iron phosphate composite material.

[0010] In one embodiment, the dispersion temperature of the aqueous solution of the nano-titanium dioxide powder is the same as the heating temperature of the gel precursor.

[0011] In the preparation method of the lithium iron phosphate composite material described in this article, nano-titanium dioxide powder is pre-mixed into the lithium iron phosphate preparation raw material before preparing the gel precursor. This component can form titanium dioxide nanotubes under the specific calcination conditions described in this article. The nanotubes have good chemical stability and thermal stability, as well as good mechanical structure stability and large specific surface area. They can effectively provide attachment sites when lithium iron phosphate is generated, and have high lithium ion transmission efficiency in the subsequent electrochemical lithium deintercalation process. The subsequent introduction of the gel precursor and the conversion of the carbon material can make the finally formed porous microspheres of lithium iron phosphate embedded with titanium dioxide nanotubes and covered with a carbon cross-linked conductive network, providing it with rich lithium ion transmission channels. In addition, in the preparation method described in this article, compared with the conventional sol-gel method for constructing the carbon layer, a polymer monomer is used to construct a high molecular three-dimensional network precursor. Even after subsequent calcination, this precursor will not significantly change its structure and still maintain its spatial effect. The final product can achieve excellent rate performance.

[0012] However, in the dispersion of titanium dioxide nanotubes and the construction of the three-dimensional conductive network carbon coating layer, it is difficult to achieve the expected technical effect if the specific processing conditions mentioned above are not adopted.

[0013] In one embodiment, the dispersion rate of the nano-titanium dioxide powder when uniformly dispersed in water is 200-300 rpm.

[0014] In one embodiment, the ratio of the total mass of the iron source, phosphorus source, and lithium source to the mass of the nano-titanium dioxide powder is m(iron source+phosphorus source+lithium source):m(nano-titanium dioxide powder)=1:(0.05-0.25).

[0015] Furthermore, the ratio of the total mass of the iron source, phosphorus source and lithium source to the mass of the nano-titanium dioxide powder is m(iron source+phosphorus source+lithium source):m(nano-titanium dioxide powder)=1:(0.15-0.25).

[0016] As the content of titanium dioxide nanotubes in the final product gradually increases, the Li-O bond formed by lithium iron phosphate gradually grows and reaches a peak, and then begins to shorten. The longer length of this bond is beneficial to improving the diffusion of lithium ions in the crystal lattice, thereby improving the lithium transmission efficiency of lithium iron phosphate. Therefore, when within the above range, the rate effect of the prepared product is better.

[0017] In one embodiment, the iron source is at least one of ferric nitrate, ferric sulfate, ferrous oxalate, ferric carbonate, ferric chloride, ferric oxide, and ferrous fluoride; the phosphorus source is at least one of phosphoric acid and ammonium dihydrogen phosphate; and the lithium source is at least one of lithium dihydrogen phosphate, lithium acetate, lithium carbonate, lithium hydride, and lithium chloride.

[0018] In one embodiment, the ratio of the total mass of the iron source, phosphorus source, and lithium source to the mass of water in the dispersed aqueous solution of titanium dioxide nanotubes is m(iron source+phosphorus source+lithium source):m(water in the dispersed aqueous solution of titanium dioxide nanotubes)=1:(1-2).

[0019] In one embodiment, the ratio of the total mass of the iron source, phosphorus source, and lithium source to the mass of the polymerized monomer is m(iron source+phosphorus source+lithium source):m(polymerized monomer)=1:(0.04-0.1).

[0020] In one embodiment, the cross-linking agent is at least one of epichlorohydrin, N,N-methylenebisacrylamide, dimethylaminopropylamine, and diethylaminopropylamine; and the initiator is at least one of potassium persulfate, sodium persulfate, ammonium persulfate, benzoyl peroxide, and azobisisobutyronitrile.

[0021] By adding a crosslinking agent and an initiator to the polymer monomer, the initiator decomposes under heat to produce SO4 2- The active excitation factors such as hydroxybenzoic acid and hydroxybenzoic acid react with the polymerization monomers, and the double bonds of the polymerization monomers are opened to form monomer free radicals. The monomer free radicals can quickly react with other polymerization monomers to carry out chain growth reactions. In addition, the required free radical polymerization reaction of the monomers and the active bond cross-linking reaction of the cross-linking agent form a stable and permanent three-dimensional network structure. This network structure will not collapse after calcination and has high stability.

[0022] In one embodiment, the mass ratio of the polymerizable monomer to the cross-linking agent is (3-4):1.

[0023] In one embodiment, the mass ratio of the initiator to the sum of the mass of the polymerizable monomer and the cross-linking agent is m(initiator):m(polymerizable monomer+cross-linking agent)=(0.05-0.15):1.

[0024] Another purpose of this article is to provide the iron phosphate composite material prepared by the preparation method of the lithium iron phosphate composite material.

[0025] In one embodiment, the particle size D of the lithium iron phosphate composite material is 50 It is 30 to 60 nm.

[0026] Another object of this invention is to provide a lithium-ion battery, wherein the positive electrode plate of the lithium-ion battery comprises the lithium iron phosphate composite material described herein.

[0027] The lithium iron phosphate composite material described in this article is small in size. At the same time, it not only inherits the high safety and high cycle stability of lithium iron phosphate itself, but also exhibits a high discharge specific capacity at a large rate and excellent rate performance, and can be effectively used in the field of power batteries for new energy vehicles.

[0028] The beneficial effect of this article is that it provides a preparation method for lithium iron phosphate composite materials. By designing the morphology of lithium iron phosphate in the product system, introducing titanium dioxide nanotubes and constructing a cross-linked carbon conductive network coating layer with a special structure, it effectively solves the existing performance defects of lithium iron phosphate and can exhibit excellent rate performance when used as a positive electrode material for lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a scanning electron microscope image of the lithium iron phosphate composite material prepared in Example 1 of this article. DETAILED DESCRIPTION

[0030] To better illustrate the purpose, technical solutions, and advantages of this article, the following will be further described in conjunction with specific examples and comparative examples. The purpose is to provide a detailed understanding of the content of this article, rather than to limit it. The experimental reagents and instruments involved in the implementation of this article, unless otherwise specified, are all commonly used ordinary reagents and instruments. The nano titanium dioxide powder used in each example and comparative example herein is anatase-type titanium dioxide with a particle size of 30 to 100 nm.

[0031] Example 1

[0032] An embodiment of a lithium iron phosphate composite material and a preparation method thereof described herein includes the following steps:

[0033] (1) adding nano-titanium dioxide powder to water at 85° C. and stirring and dispersing at a rate of 200 rpm for 0.5 h until uniform, to obtain a dispersed aqueous solution of nano-titanium dioxide powder;

[0034] (2) adding ferrous oxalate, ammonium dihydrogen phosphate, and lithium acetate to the dispersed aqueous solution of nano-titanium dioxide powder according to the stoichiometric ratio of lithium iron phosphate and mixing them uniformly to obtain a composite dispersed solution;

[0035] The sum of the mass of ferrous oxalate, ammonium dihydrogen phosphate, and lithium acetate is m1, m1: the mass of water in the dispersed aqueous solution of nano-titanium dioxide powder = 1:1; m1: the mass of nano-titanium dioxide powder = 1:0.05;

[0036] (3) adding acrylamide as a polymer monomer to the composite dispersion solution, followed by adding a crosslinking agent N,N-methylenebisacrylamide and an initiator ammonium persulfate and continuing to heat and mix at 85° C. to obtain a gel precursor;

[0037] Wherein, m1: mass of polymerized monomer = 1:0.06; the mass ratio of polymerized monomer to cross-linking agent is 3:1, and the mass ratio of initiator to the sum of the mass of polymerized monomer and cross-linking agent is 0.1:1;

[0038] (4) After the gel precursor was dried in a drying oven for 14 h, it was placed in a muffle furnace in an air atmosphere and kept at 300°C for 5 h, and then kept at 600°C in a box furnace in a nitrogen atmosphere for 9 h to obtain a lithium iron phosphate composite material.

[0039] Example 2

[0040] An embodiment of a lithium iron phosphate composite material and a preparation method thereof described herein includes the following steps:

[0041] (1) Add nano-titanium dioxide powder to water at 85° C. and stir and disperse at a rate of 200 rpm for 0.5 h until uniform, to obtain a dispersed aqueous solution of nano-titanium dioxide powder;

[0042] (2) adding ferric chloride, phosphoric acid, and lithium carbonate to the dispersed aqueous solution of nano-titanium dioxide powder according to the stoichiometric ratio of lithium iron phosphate and mixing them uniformly to obtain a composite dispersed solution;

[0043] The mass of ferrous oxalate, ammonium dihydrogen phosphate, and lithium acetate is m1, m1: the mass of water in the dispersed aqueous solution of nano-titanium dioxide powder = 1:1; m1: the mass of nano-titanium dioxide powder = 1:0.05;

[0044] (3) adding the polymerizable monomer hydroxymethyl acrylamide to the composite dispersion solution, then adding the crosslinking agent N,N-methylenebisacrylamide and the initiator ammonium persulfate and heating to 85° C. and mixing to obtain a gel precursor;

[0045] Wherein, m1: mass of polymerized monomer = 1:0.06; the mass ratio of polymerized monomer to cross-linking agent is 4:1, and the mass ratio of initiator to the sum of the mass of polymerized monomer and cross-linking agent is 0.1:1;

[0046] (4) After the gel precursor was dried in a drying oven for 14 h, it was placed in a muffle furnace in an air atmosphere and kept at 400 ° C for 6 h, and then kept at 700 ° C for 10 h in a box furnace in a nitrogen atmosphere to obtain a lithium iron phosphate composite material.

[0047] Example 3

[0048] An embodiment of a lithium iron phosphate composite material and a preparation method thereof described herein differs from Example 1 only in that m1:mass of nano-titanium dioxide powder = 1:0.1.

[0049] Example 4

[0050] An embodiment of a lithium iron phosphate composite material and a preparation method thereof described herein differs from Example 1 only in that m1:mass of nano-titanium dioxide powder = 1:0.15.

[0051] Example 5

[0052] An embodiment of a lithium iron phosphate composite material and a preparation method thereof described herein differs from embodiment 1 only in that m1:mass of nano-titanium dioxide powder = 1:0.2.

[0053] Example 6

[0054] An embodiment of a lithium iron phosphate composite material and a preparation method thereof described herein differs from Example 1 only in that m1:mass of nano-titanium dioxide powder = 1:0.25.

[0055] Comparative Example 1

[0056] A lithium iron phosphate composite material and a preparation method thereof, the preparation method comprising the following steps:

[0057] (1) Add nano-titanium dioxide powder to water at 85° C. and stir and disperse at a rate of 200 rpm for 0.5 h until uniform, to obtain a dispersed aqueous solution of nano-titanium dioxide powder;

[0058] (2) adding ferrous oxalate, ammonium dihydrogen phosphate, and lithium acetate to the dispersed aqueous solution of nano-titanium dioxide powder according to the stoichiometric ratio of lithium iron phosphate and mixing them uniformly to obtain a composite dispersed solution;

[0059] The mass of ferrous oxalate, ammonium dihydrogen phosphate, and lithium acetate is m1, m1: the mass of water in the dispersed aqueous solution of nano-titanium dioxide powder = 1:1; m1: the mass of nano-titanium dioxide powder = 1:0.2;

[0060] (3) Citric acid was added to the composite dispersion solution, and then heated at 85°C and stirred at 200 rpm until a sol-gel was formed. After drying in a drying oven for 14 h, the solution was kept at 600°C in a box furnace in a nitrogen atmosphere for 9 h to obtain a lithium iron phosphate composite material; m1: the mass of citric acid = 1:0.06.

[0061] Comparative Example 2

[0062] An embodiment of a lithium iron phosphate composite material and a preparation method thereof, which differs from Example 1 only in that the preparation method of the composite dispersed solution is: ferrous oxalate, ammonium dihydrogen phosphate, and lithium acetate are added to water in a stoichiometric ratio of lithium iron phosphate at 85°C and mixed uniformly to obtain a composite dispersed solution; the mass of ferrous oxalate, ammonium dihydrogen phosphate, and lithium acetate is m1, and the mass of m1:water = 1:1.

[0063] Comparative Example 3

[0064] An embodiment of a lithium iron phosphate composite material and a preparation method thereof, which differs from embodiment 5 only in that the temperature when the nano titanium dioxide powder is added to water in step (1) is 75°C, and the temperature of heating and mixing in step (3) is 75°C.

[0065] Comparative Example 4

[0066] An embodiment of a lithium iron phosphate composite material and a preparation method thereof, which differs from embodiment 5 only in that the temperature when the nano titanium dioxide powder is added to water in step (1) is 95°C, and the temperature of heating and mixing in step (3) is 95°C.

[0067] Comparative Example 5

[0068] An embodiment of a lithium iron phosphate composite material and a preparation method thereof, which differs from embodiment 5 only in that in step (4), the gel precursor is directly heated at 600° C. in a box furnace in a nitrogen atmosphere for 9 hours after drying to obtain the lithium iron phosphate composite material.

[0069] Comparative Example 6

[0070] An embodiment of a lithium iron phosphate composite material and a preparation method thereof differs from embodiment 5 only in that m1:mass of nano-titanium dioxide powder = 1:0.35.

[0071] Effect Example 1

[0072] In order to verify the performance of the products of each embodiment of this article, the particle size D50 of each product was tested using the LPS wet test (EV sampler) standard operating procedure, and then it was used as the positive electrode material of the lithium-ion battery. The lithium iron phosphate composite material, the conductive agent, and the binder were mixed in a ratio of 8:1:1 to prepare a slurry and a positive electrode sheet. The negative electrode sheet was prepared with commercial graphite, and then assembled into a button battery. The battery was charged and discharged at a rate of 0.1C at a voltage of 4.3V, and then at a rate of 5C. The discharge specific capacity and the first coulombic efficiency under the first charge and discharge were calculated, and the discharge specific capacity at 5C was then calculated, and the capacity ratio T at the large rate of 5C and the small rate of 0.1C were calculated.

[0073] The test results are shown in Table 1.

[0074] Table 1

[0075]

[0076] As can be seen from Table 1, the particle size of the products prepared in each embodiment of this invention is relatively small, and can reach a particle size of D 50 In the range of 30 to 60 nm, such as Figure 1As shown, the dispersion is high, with the product in Example 5 having the smallest and highest dispersion. As the doping level of the formed titanium dioxide nanotubes increases, their size first decreases and then increases. In contrast, the product in Comparative Example 1 uses a conventional sol-gel method to construct a carbon cross-linked network. This structure collapses during the subsequent calcination process, and therefore cannot maintain its smaller size compared to the product in Example 5. The product in Comparative Example 2 does not incorporate titanium dioxide nanotubes, and therefore has a larger size than Example 1, which has the lowest doping level of titanium dioxide nanotubes formed from nano-titanium dioxide powder among the various examples. On the other hand, in the preparation of the products in Comparative Examples 3 and 4, the amount of nano-titanium dioxide powder added was the same as in Example 4, but both excessively high and low temperatures during dispersion and gel preparation resulted in larger product sizes, indicating that product size is not solely determined by the amount of raw materials added. The product in Comparative Example 5 did not undergo low-temperature preheating during preparation, and the monomers, crosslinking agents, and other substances contained in its gel precursor could not be effectively degraded, resulting in the presence of impurities that may affect dispersibility, significant local agglomeration, and a larger final product size. Corresponding to its size, the product in Example 5 exhibited a discharge capacity of over 160 mAh / g at 0.1C in electrochemical testing, close to the theoretical capacity of lithium iron phosphate. At the same time, the initial efficiency was as high as 97.2%, and the discharge capacity at a high rate of 5C could still reach over 140 mAh / g, with a capacity ratio of up to 84.4. In contrast, the products in the comparative examples failed to perform as expected, especially the product in comparative example 5, whose discharge capacity at 0.1C was less than 100 mAh / g and only 34 mAh / g at 5C. According to Comparative Example 2, Example 1, Examples 3 to 6, and Comparative Example 6, it can be seen that the titanium dioxide nanotubes in the product play a vital role in the high-rate performance of the product. Without the introduction of this component, the 5C discharge capacity of the product of Comparative Example 2 is only 13 mAh / g, and the capacity ratio T is only 0.107. With the introduction of nano-titanium dioxide powder and the gradual increase in content, the electrochemical performance of the product gradually improves, but after reaching a peak, its performance begins to gradually decline. As shown in Comparative Example 6, when the amount of nano-titanium dioxide powder added is too much, the electrochemical performance of the product is poor.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this document and are not intended to limit the scope of protection of this document.

Claims

1. A method for preparing a lithium iron phosphate composite material, characterized in that: The following steps are involved: Add nano titanium dioxide powder to water at 80-90° C. and disperse evenly to obtain a dispersed aqueous solution of nano titanium dioxide powder; Adding an iron source, a phosphorus source, and a lithium source to a dispersed aqueous solution of nano-titanium dioxide powder and mixing them uniformly to obtain a composite dispersed solution; Adding a polymerizable monomer to a composite dispersed solution, then adding a crosslinking agent and an initiator and heating to 80-90° C. and mixing to obtain a gel precursor; the polymerizable monomer is at least one of hydroxymethyl acrylamide, acrylamide, and ethyl acetoacetate methacrylate; After the gel precursor is dried, it is placed in an air atmosphere and kept at 300-450° C. for 5-8 hours, and then kept at 600-850° C. for 9-12 hours in a protective atmosphere to obtain a lithium iron phosphate composite material.

2. The method for preparing the lithium iron phosphate composite material according to claim 1, wherein: The ratio of the total mass of the iron source, phosphorus source and lithium source to the mass of the nano-titanium dioxide powder is m(iron source+phosphorus source+lithium source):m(nano-titanium dioxide powder)=1:(0.05-0.25).

3. The method for preparing the lithium iron phosphate composite material according to claim 2, wherein: The ratio of the total mass of the iron source, phosphorus source and lithium source to the mass of the nano-titanium dioxide powder is m(iron source+phosphorus source+lithium source):m(nano-titanium dioxide powder)=1:(0.15-0.25).

4. The method for preparing the lithium iron phosphate composite material according to claim 1, wherein: The iron source is at least one of ferric nitrate, ferric sulfate, ferrous oxalate, ferric carbonate, ferric chloride, ferric oxide, and ferrous fluoride; the phosphorus source is at least one of phosphoric acid and ammonium dihydrogen phosphate; and the lithium source is at least one of lithium dihydrogen phosphate, lithium acetate, lithium carbonate, lithium hydride, and lithium chloride.

5. The method for preparing the lithium iron phosphate composite material according to claim 1, wherein: The ratio of the total mass of the iron source, phosphorus source and lithium source to the mass of water in the dispersed aqueous solution of nano-titanium dioxide powder is m(iron source+phosphorus source+lithium source):m(water in the dispersed aqueous solution of nano-titanium dioxide powder)=1:(1-2).

6. The method for preparing the lithium iron phosphate composite material according to claim 1, wherein: The ratio of the total mass of the iron source, phosphorus source and lithium source to the mass of the polymerized monomer is m(iron source+phosphorus source+lithium source):m(polymerized monomer)=1:(0.04-0.1).

7. The method for preparing the lithium iron phosphate composite material according to claim 1, wherein: The cross-linking agent is at least one of epichlorohydrin, N,N-methylenebisacrylamide, dimethylaminopropylamine, and diethylaminopropylamine; the initiator is at least one of potassium persulfate, sodium persulfate, ammonium persulfate, benzoyl peroxide, and azobisisobutyronitrile.

8. The method for preparing the lithium iron phosphate composite material according to claim 1, wherein: The mass ratio of the polymerizable monomer to the cross-linking agent is (3-4):

1.

9. The method for preparing the lithium iron phosphate composite material according to claim 1, wherein: The mass ratio of the initiator to the sum of the masses of the polymerization monomer and the cross-linking agent is m(initiator):m(polymerization monomer+cross-linking agent)=(0.05-0.15):

1.

10. The iron phosphate composite material prepared by the method for preparing the lithium iron phosphate composite material according to any one of claims 1 to 9.

11. The lithium iron phosphate composite material according to claim 10, wherein The particle size D of the lithium iron phosphate composite material 50 It is 30 to 60 nm.

12. A lithium ion battery, characterized in that: The positive electrode plate of the lithium-ion battery comprises the lithium iron phosphate composite material according to claim 10 or 11.

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