Aqueous lithium iron phosphate and preparation method thereof

Abstract

The present invention relates to the technical field of lithium-ion battery positive electrode materials, and more specifically to an aqueous lithium iron phosphate and a preparation method thereof. The aqueous lithium iron phosphate comprises a lithium iron phosphate core, a first carbon coating layer wrapped around the core, and a second carbon coating layer wrapped around the first carbon coating layer, wherein the raw material of the second carbon coating layer comprises an organic polymer containing a hydrophilic group. The mass of the second carbon coating layer is calculated as a percentage W of the mass of the aqueous lithium iron phosphate, and the particle sizes D10, D50, and D90 of the aqueous lithium iron phosphate satisfy the following inequalities: 0.05 ≤ W × (D90-D10) / D50 × 100 ≤ 10.00, and 0.05% ≤ W ≤ 1.0%. The aqueous lithium iron phosphate has a stable structure and can be dispersed using water as a solvent, avoiding the high cost and environmental pollution caused by the use of organic solvents.

Description

Technical Field

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

[0002] Currently, lithium-ion battery cathode slurries typically use fluorinated polymers such as polyvinylidene fluoride as a binder and organic solvents such as NMP (N-methylpyrrolidone) as a solvent. However, organic solvents are prone to environmental pollution and pose health risks to operators, necessitating additional equipment and operational procedures to recover the organic solvents during production. However, this recovery process cannot completely recycle the organic solvents. Even with secondary recovery, there are still cases where the recovery is incomplete and volatilization pollutes the environment. Furthermore, this recovery process consumes significant energy, increasing costs during the coating and drying process. Furthermore, NMP itself, as an industrial solvent, requires additional purchase, further increasing costs.

[0003] The lithium iron phosphate cathode materials currently used in industry are extremely hydrophobic due to their nanoparticle size and surface coating with elemental carbon. This leads to agglomeration in water, preventing uniform dispersion and degrading the material's electrical performance. This prevents the use of low-cost, low-pollution water as a solvent for lithium iron phosphate cathode materials. To achieve dispersion in water, existing aqueous lithium iron phosphates require the addition of large amounts of surfactants and thickeners during the preparation process, as well as specific high-temperature stirring steps to enhance the dispersion of the lithium iron phosphate material. This cumbersome preparation process does not fundamentally change the hydrophobic and difficult-to-disperse properties of lithium iron phosphate. Summary of the Invention

[0004] Therefore, the object of the present invention is to provide an aqueous lithium iron phosphate and a preparation method thereof, the purpose of which is to increase hydrophilic functional groups on the surface of the lithium iron phosphate through a second layer coating, fundamentally changing the hydrophobic properties of the material, so that the material can achieve the dispersion effect in the aqueous solvent as before in the oily solvent, without the need to add additional substances and steps to improve the dispersibility. When the aqueous lithium iron phosphate is used as a positive electrode material, water can be used as a solvent to disperse it for subsequent coating, avoiding the high cost and environmental pollution caused by the use of organic solvents.

[0005] To this end, the present invention provides the following technical solutions.

[0006] The aqueous lithium iron phosphate provided by the present invention includes a lithium iron phosphate core, a first carbon coating layer wrapped around the core, and a second carbon coating layer wrapped around the first carbon coating layer, wherein the raw material of the second carbon coating layer includes an organic polymer containing hydrophilic groups; wherein the mass of the second carbon coating layer is a percentage of the mass of the aqueous lithium iron phosphate, W, and the particle sizes D10, D50, and D90 of the aqueous lithium iron phosphate satisfy the following inequalities: 0.05≤W×(D90-D10) / D50×100≤10.00, and 0.05%≤W≤1.0%. The hydrophilic groups include, but are not limited to, one or more of hydroxyl groups, ether bonds, carboxyl groups, ester groups, and phosphate groups.

[0007] Optionally, the particle size of the aqueous lithium iron phosphate is: D10 is 0.1-1.0 μm, D50 is 0.2-2.0 μm, and D90 is 1.0-10.0 μm.

[0008] Optionally, the mass of the first carbon coating layer is 0.5% to 2% of the mass of the aqueous lithium iron phosphate. Typically, but not limiting, the testing method can be: using a carbon-sulfur tester to test the carbon content of the sample coated with the first carbon coating layer to obtain the mass of the first carbon coating layer; further, using a carbon-sulfur tester to test the carbon content of the sample coated with the second carbon coating layer, deducting the mass of the first carbon coating layer from the obtained result to obtain the mass of the second carbon coating layer; and finally calculating the ratio of the mass of the first carbon coating layer to the mass of the aqueous lithium iron phosphate and the ratio of the mass of the second carbon coating layer to the mass of the aqueous lithium iron phosphate.

[0009] Optionally, the thickness of the second carbon coating layer is 1-10 nm; typically but not limitedly, it can be measured using TEM.

[0010] Optionally, the organic polymer containing a hydrophilic group includes one or more of polyvinyl alcohol, polyacrylic acid, polyacrylate, and polyvinyl ether, wherein polyacrylate includes polymethyl acrylate, polyethyl acrylate, and the like, and polyvinyl ether includes polyvinyl methyl ether, polyvinyl ethyl ether, and the like.

[0011] The present invention provides a method for preparing the above-mentioned aqueous lithium iron phosphate, comprising the following steps: S1: mixing a lithium source, an iron source, a phosphorus source, and a carbon source in a liquid medium to obtain a slurry, grinding, spray drying, sintering under a protective atmosphere, and crushing to obtain lithium iron phosphate particles having a first carbon coating layer; S2: mixing the lithium iron phosphate particles having the first carbon coating layer and the organic polymer containing a hydrophilic group, spray drying, sintering under a protective atmosphere, and crushing to obtain the aqueous lithium iron phosphate.

[0012] In this aqueous lithium iron phosphate preparation method, typically, but not exclusively, the quality of the second carbon coating layer can be controlled by conducting thermogravimetric analysis of the hydrophilic group-containing organic polymer at the corresponding temperature and atmosphere in advance to determine the mass percentage of the sintered residue. This can be used as the mass percentage of the second carbon coating layer to be produced, and the amount of the hydrophilic group-containing organic polymer used can be inferred based on the planned coating amount. The thickness of the second carbon coating layer can be controlled based on the planned coating amount.

[0013] In the preparation method of the aqueous lithium iron phosphate, typically but not limitedly, a centrifugal sprayer is used for spray drying, the temperature in the spray tower is controlled to be 240°C ± 20°C, the hot air temperature at the air inlet is 280±20°C, and the hot air temperature at the air outlet is 200±20°C. The tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center values.

[0014] Optionally, in S1, the sintering temperature is 740-800° C. and the sintering time is 6-12 hours.

[0015] Optionally, in S1, the molar ratio of lithium element, iron element and phosphorus element in the lithium source, iron source and phosphorus source is 1.00-1.10:1.00:1.00-1.10.

[0016] Optionally, in S1, the mass of the carbon source is 1% to 10% of the theoretical mass of the lithium iron phosphate; wherein the theoretical mass of the lithium iron phosphate is calculated based on the amount of iron element used, and the molar ratio of the iron element to the theoretical lithium iron phosphate is 1:1.

[0017] Optionally, in S1, the mass of the liquid medium is 5 to 10 times the sum of the masses of the lithium source, the iron source, the phosphorus source, and the carbon source.

[0018] Optionally, the lithium iron phosphate particles having the first carbon coating layer have a particle size D10 of 0.1 to 1.0 μm, a D50 of 0.2 to 2.0 μm, and a D90 of 1.0 to 10.0 μm.

[0019] Optionally, in S1, the method of mixing the lithium source, iron source, phosphorus source and carbon source in the liquid medium includes mixing them using a stirrer; optionally, the mixing time is 0.5 to 1 hour.

[0020] Optionally, in S1, the grinding is to grind the D50 of the particles in the slurry to 0.2 to 1.5 μm.

[0021] Optionally, in S1, the equipment used for grinding includes a sand mill.

[0022] Optionally, in S2, the sintering temperature is 400-480° C. and the sintering time is 3-8 hours.

[0023] Optionally, in S2, the organic polymer containing a hydrophilic group is prepared into a solution before mixing; optionally, the solvent in the organic polymer solution containing a hydrophilic group includes one or more of water, methanol, ethanol, and acetone. Since the function of the solution is to ensure that the organic polymer is evenly dispersed on the surface of the material after the solvent is completely evaporated by spray drying, it is only necessary to ensure that the organic polymer is completely dissolved in the solution; the concentration of the solution does not have an impact.

[0024] Optionally, the lithium source includes one or more of lithium dihydrogen phosphate, lithium monohydrogen phosphate, lithium nitrate, lithium carbonate, lithium phosphate, and lithium hydroxide.

[0025] Optionally, the iron source includes one or more of ferric phosphate, ferric oxide, ferrous oxalate, iron, and ferric nitrate.

[0026] Optionally, the phosphorus source includes one or more of iron phosphate, ammonium dihydrogen phosphate, ammonium phosphate, lithium phosphate, and phosphoric acid.

[0027] Optionally, the carbon source includes one or more of citric acid, glucose, sucrose, and polyethylene glycol.

[0028] Optionally, the liquid medium includes one or more of water, methanol, ethanol, and acetone.

[0029] Optionally, the protective atmosphere includes at least one of nitrogen, inert gas, and carbon dioxide.

[0030] Optionally, the pulverization method includes air flow pulverization.

[0031] The present invention also provides a positive electrode plate, comprising the aqueous lithium iron phosphate or the aqueous lithium iron phosphate prepared by the above preparation method.

[0032] The present invention also provides a lithium-ion battery, comprising the above-mentioned positive electrode sheet, a negative electrode sheet, a separator and an electrolyte.

[0033] In summary, the beneficial effects achieved by the present invention are:

[0034] The aqueous lithium iron phosphate provided by the present invention includes a lithium iron phosphate core, a first carbon coating layer wrapped around the core, and a second carbon coating layer wrapped around the first carbon coating layer, wherein the raw material of the second carbon coating layer includes an organic polymer containing a hydrophilic group; wherein the percentage of the mass of the second carbon coating layer to the mass of the aqueous lithium iron phosphate is calculated as W, and the particle sizes D10, D50, and D90 of the aqueous lithium iron phosphate satisfy the inequality: 0.05≤W×(D90-D10) / D50×100≤10.00, 0.05%≤W≤1.0%. The aqueous lithium iron phosphate has a stable structure, and the raw material of the second carbon coating layer includes an organic polymer containing a hydrophilic group. At the same time, the percentage of the mass of the second carbon coating layer to the mass of the aqueous lithium iron phosphate, W, is controlled within the range of 0.05≤W×(D90-D10) / D50×100≤10.00 and 0.05%≤W≤1.0%. This allows the aqueous lithium iron phosphate to be dispersed using water as a solvent for subsequent coating when used as a positive electrode material. Moreover, when the aqueous lithium iron phosphate provided by the present invention is dispersed using water as a solvent, the solid content of the slurry can reach above 55%, the upper limit of the coating surface density can reach above 18.5 mg / cm2, and the viscosity increase after standing for 6 hours is small, making it convenient for processing and use. Furthermore, the positive electrode sheet produced by dispersing the aqueous lithium iron phosphate provided by the present invention using water as a solvent can achieve a 0.1C material gram capacity of 157.7 mAh / g or higher, and a 1C material gram capacity of 146.4 mAh / g or higher after assembly into a battery. The capacity retention rate can reach 97.9% or higher after 200 cycles at 1C, demonstrating excellent electrical performance. The aqueous lithium iron phosphate provided by the present invention avoids the high costs and environmental pollution caused by the use of organic solvents during battery preparation.

[0035] The present invention provides a method for preparing aqueous lithium iron phosphate, comprising the following steps: S1: mixing a lithium source, an iron source, a phosphorus source, and a carbon source in a liquid medium to obtain a slurry, grinding, spray drying, sintering under a protective atmosphere, and pulverizing to obtain lithium iron phosphate particles having a first carbon coating layer; S2: mixing the lithium iron phosphate particles having the first carbon coating layer with an organic polymer containing a hydrophilic group, spray drying, sintering under a protective atmosphere, and pulverizing to obtain the aqueous lithium iron phosphate. The preparation method is simple, does not require a harsh preparation environment, and is suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0037] Figure 1 Schematic diagram of the preparation method of the present invention. DETAILED DESCRIPTION

[0038] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0039] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0040] Example 1

[0041] This embodiment provides an aqueous lithium iron phosphate and a preparation method thereof. The flow chart of the preparation method is as follows: Figure 1 As shown, the specific steps include:

[0042] (1) Weigh 2.45 g of lithium hydroxide, 15.00 g of solid iron phosphate, and 1.0 g of glucose and pour them into 150 g of deionized water in turn, stir them on an automatic stirrer for 1 h to mix evenly, pour them into a sand mill and grind them for 1 h. The particle size D50 of the slurry obtained is tested to be 0.4 μm, spray dried (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240°C ± 20°C, the air inlet hot air temperature is 280 ± 20°C, the air outlet hot air temperature is 200 ± 20°C, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value), and then transferred to a nitrogen atmosphere and sintered at 760°C for 8 h. After cooling, the mixture is air flow crushed, and the crushed particle size D10 is 0.2 μm, D50 is 0.5 μm, and D90 is 2.0 μm, thereby obtaining lithium iron phosphate particles (LFP-1) with a first carbon coating layer.

[0043] (2) Weigh 10.00 g of LFP-1 prepared in (1) and 100 g of polyvinyl alcohol solution (the solvent is water and the solute polyvinyl alcohol is 0.1 g), mix them evenly, and spray dry them (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240°C ± 20°C, the hot air temperature at the air inlet is 280±20°C, the hot air temperature at the air outlet is 200±20°C, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value). Then, sinter them at 460°C for 5 h under a nitrogen atmosphere. After cooling, they are air flow crushed, and the crushed particle size D10 is 0.2 μm, D50 is 0.5 μm, and D90 is 2.0 μm to obtain aqueous lithium iron phosphate (LFP-A).

[0044] A portion of LFP-1 was tested for carbon content using a carbon-sulfur tester to determine the mass of the first carbon coating layer. A portion of LFP-A was tested for carbon content using a carbon-sulfur tester. The mass of the second carbon coating layer was calculated by deducting the mass of the first carbon coating layer from the result. The mass of the first carbon coating layer was calculated to be 1.33% of the mass of the aqueous LFP-A, and the percentage W of the mass of the second carbon coating layer to the mass of the aqueous LFP-A was 0.62%, where W × (D90 - D10) / D50 × 100 = 2.23. Transmission electron microscopy (TEM) determined the thickness of the second carbon coating layer to be 7 nm.

[0045] Example 2

[0046] This embodiment provides an aqueous lithium iron phosphate and a preparation method thereof. The flow chart of the preparation method is as follows: Figure 1 As shown, the specific steps include:

[0047] (1) Weigh 3.87 g of lithium carbonate, 15.00 g of iron phosphate solid, and 1.2 g of sucrose ammonium solid and pour them into 180 g of 50% ethanol solution in turn, stir on an automatic stirrer for 0.5 h to mix evenly, pour into a sand mill and grind for 0.6 h. The particle size D50 of the slurry obtained is tested to be 0.6 μm, spray dried (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240°C ± 20°C, the air inlet hot air temperature is 280±20°C, the air outlet hot air temperature is 200±20°C, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value), and then transferred to an argon atmosphere and sintered at 780°C for 6 h. After cooling, the mixture is air flow crushed, and the crushed particle size D10 is 0.3 μm, D50 is 0.8 μm, and D90 is 5 μm, thereby obtaining lithium iron phosphate particles (LFP-2) with a first carbon coating layer.

[0048] (2) Weigh 10.00 g of LFP-2 prepared in (1) and 80 g of polyvinyl methyl ether solution (the solvent is methanol and the solute polyvinyl methyl ether is 0.08 g), mix them evenly, and spray dry them (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240°C ± 20°C, the hot air temperature at the air inlet is 280±20°C, the hot air temperature at the air outlet is 200±20°C, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value). Then, they are sintered at 480°C for 3 hours under an argon atmosphere. After cooling, the coated material is subjected to air flow pulverization, and the pulverized particle size D10 is 0.3 μm, D50 is 0.8 μm, and D90 is 5 μm, thereby obtaining aqueous lithium iron phosphate (LFP-B).

[0049] A portion of LFP-2 was tested for carbon content using a carbon-sulfur tester to determine the mass of the first carbon coating layer. A portion of LFP-B was tested for carbon content using a carbon-sulfur tester. The mass of the second carbon coating layer was calculated by deducting the mass of the first carbon coating layer from the result. The mass of the first carbon coating layer was calculated to be 1.60% of the mass of the aqueous LFP-B, and the percentage W of the mass of the second carbon coating layer to the mass of the aqueous LFP-B was 0.74%, with W × (D90 - D10) / D50 × 100 = 4.35. The thickness of the second carbon coating layer was determined by transmission electron microscopy (TEM) to be 8 nm.

[0050] Example 3

[0051] This embodiment provides an aqueous lithium iron phosphate and a preparation method thereof. The flow chart of the preparation method is as follows: Figure 1 As shown, the specific steps include:

[0052] (1) Weigh 10.39 g of lithium dihydrogen phosphate, 14.30 g of ferrous oxalate solid, and 0.5 g of citric acid and pour them into 170 g of 40% methanol solution in sequence, stir on an automatic stirrer for 0.8 h to mix evenly, pour into a sand mill and grind for 0.5 h. The particle size D50 of the slurry obtained is tested to be 0.3 μm, spray dried (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240°C ± 20°C, the air inlet hot air temperature is 280±20°C, the air outlet hot air temperature is 200±20°C, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value), and then transferred to a nitrogen atmosphere and sintered at 780°C for 10 h. After cooling, the mixture is subjected to air flow pulverization, and the pulverized particle size D10 is 0.5 μm, D50 is 1.0 μm, and D90 is 2.0 μm, thereby obtaining lithium iron phosphate particles (LFP-3) with a first carbon coating layer.

[0053] (2) Weigh 10.00 g of LFP-3 prepared in (1) and 120 g of polyacrylic acid solution (the solvent is ethanol and the solute polyacrylic acid is 0.02 g), mix them evenly, and spray dry them (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240°C ± 20°C, the hot air temperature at the air inlet is 280±20°C, the hot air temperature at the air outlet is 200±20°C, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value). Then, they are sintered at 440°C for 6 h under an argon atmosphere. After cooling, the coated material is subjected to air flow pulverization, and the pulverized particle size D10 is 0.5 μm, D50 is 1.0 μm, and D90 is 2.0 μm to obtain aqueous lithium iron phosphate (LFP-C).

[0054] A portion of LFP-3 was tested for carbon content using a carbon-sulfur tester to determine the mass of the first carbon coating layer. A portion of LFP-C was also tested for carbon content using a carbon-sulfur tester. The mass of the second carbon coating layer was calculated by deducting the mass of the first carbon coating layer from the result. The mass of the first carbon coating layer was calculated to be 1.14% of the mass of the aqueous LFP-C, and the percentage W of the mass of the second carbon coating layer to the mass of the aqueous LFP-C was 0.12%, where W × (D90 - D10) / D50 × 100 = 0.18. Transmission electron microscopy (TEM) determined the thickness of the second carbon coating layer to be 2 nm.

[0055] Example 4

[0056] This embodiment provides an aqueous lithium iron phosphate and a preparation method thereof. The flow chart of the preparation method is as follows: Figure 1 As shown, the specific steps include:

[0057] (1) Weigh 2.45g of lithium hydroxide, 15.00g of solid iron phosphate, and 1.0g of glucose and pour them into 150g of deionized water in turn, stir them on an automatic stirrer for 1h to mix evenly, pour them into a sand mill and grind them for 2h, test the particle size D50 of the slurry obtained and find it is 0.15μm, spray dry (using a centrifugal sprayer, the temperature in the spray tower is controlled at 240℃±20℃, the air inlet hot air temperature is 280±20℃, the air outlet hot air temperature is 200±20℃, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value), then transfer to a nitrogen atmosphere and sinter at 740℃ for 12h, cool, and perform air flow crushing, the crushed particle size D10 is 0.1μm, D50 is 0.2μm, and D90 is 2.2μm, to obtain lithium iron phosphate particles (LFP-4) with a first carbon coating layer.

[0058] (2) Weigh 10.00 g of LFP-4 prepared in (1) and 100 g of polyvinyl alcohol solution (the solvent is water and the solute polyvinyl alcohol is 0.15 g), mix them evenly, and spray dry them (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240°C ± 20°C, the hot air temperature at the air inlet is 280±20°C, the hot air temperature at the air outlet is 200±20°C, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value). Then, sinter them at 400°C for 8 h in a nitrogen atmosphere. After cooling, they are air flow crushed, and the crushed particle size D10 is 0.1 μm, D50 is 0.2 μm, and D90 is 2.2 μm to obtain aqueous lithium iron phosphate (LFP-D).

[0059] A portion of LFP-4 was tested for carbon content using a carbon-sulfur tester to determine the mass of the first carbon coating layer. A portion of LFP-D was tested for carbon content using a carbon-sulfur tester. The mass of the second carbon coating layer was calculated by deducting the mass of the first carbon coating layer from the result. The mass of the first carbon coating layer was calculated to be 1.29% of the mass of the aqueous LFP-D, and the percentage W of the mass of the second carbon coating layer to the mass of the aqueous LFP-D was 0.94%, with W × (D90 - D10) / D50 × 100 = 9.87. The thickness of the second carbon coating layer was determined to be 10 nm using transmission electron microscopy (TEM).

[0060] Example 5

[0061] This embodiment provides an aqueous lithium iron phosphate and a preparation method thereof. The flow chart of the preparation method is as follows: Figure 1 As shown, the specific steps include:

[0062] (1) Weigh 2.45g of lithium hydroxide, 15.00g of solid iron phosphate, and 1.0g of glucose and pour them into 150g of deionized water in turn, stir them on an automatic stirrer for 1h to mix evenly, pour them into a sand mill and grind them for 0.3h. The particle size D50 of the slurry obtained is tested to be 1.5μm, spray dried (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240℃±20℃, the air inlet hot air temperature is 280±20℃, the air outlet hot air temperature is 200±20℃, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value), and then transferred to a nitrogen atmosphere and sintered at 800℃ for 6h. After cooling, the mixture is air flow crushed, and the crushed particle size D10 is 0.8μm, D50 is 1.9μm, and D90 is 3.1μm, thereby obtaining lithium iron phosphate particles (LFP-5) with a first carbon coating layer.

[0063] (2) Weigh 10.00 g of LFP-5 prepared in (1) and 100 g of polyvinyl alcohol solution (the solvent is water and the solute polyvinyl alcohol is 0.01 g), mix them evenly, and spray dry them (using a centrifugal sprayer, the temperature in the spray tower is controlled to 240°C ± 20°C, the hot air temperature at the air inlet is 280±20°C, the hot air temperature at the air outlet is 200±20°C, the tolerance is the maximum acceptable equipment accuracy, and the equipment setting parameters are the center value). Then, they are sintered at 450°C for 6 h in a nitrogen atmosphere. After cooling, they are air flow crushed. The crushed particle size D10 is 0.8 μm, D50 is 1.9 μm, and D90 is 3.1 μm, thereby obtaining aqueous lithium iron phosphate (LFP-E).

[0064] A portion of LFP-5 was tested for carbon content using a carbon-sulfur tester to determine the mass of the first carbon coating layer. A portion of LFP-E was also tested for carbon content using a carbon-sulfur tester. The mass of the second carbon coating layer was calculated by deducting the mass of the first carbon coating layer from the result. The mass of the first carbon coating layer was calculated to be 1.35% of the mass of the aqueous LFP-E, and the percentage W of the mass of the second carbon coating layer to the mass of the aqueous LFP-E was 0.06%, where W × (D90 - D10) / D50 × 100 = 0.07. The thickness of the second carbon coating layer was determined to be 1 nm using transmission electron microscopy (TEM).

[0065] Comparative Examples 1 to 3

[0066] The lithium iron phosphate provided by Comparative Examples 1 to 3 are the lithium iron phosphate particles with the first carbon coating layer prepared in step (1) of Examples 1 to 3, respectively. Comparative Examples 1 to 3 correspond to Examples 1 to 3, respectively.

[0067] Comparative Example 4

[0068] This comparative example provides an aqueous lithium iron phosphate and a preparation method thereof, which is different from Example 2 only in that the mass of the solute polyethylene ether in the polyethylene ether solution used in step (2) is 0.24 g.

[0069] The first carbon coating layer was measured to be 1.6% of the mass of the aqueous LFP-F, while the second carbon coating layer's mass as a percentage of the aqueous LFP-F was 2.22%, with W × (D90 - D10) / D50 × 100 = 13.05. Transmission electron microscopy (TEM) measurements revealed a thickness of 21 nm for the second carbon coating layer.

[0070] Comparative Example 5

[0071] This comparative example provides an aqueous lithium iron phosphate and a preparation method thereof, which is different from Example 3 only in that the mass of the solute polyacrylic acid in the polyacrylic acid solution used in step (2) is 0.005 g.

[0072] The first carbon coating layer was measured and calculated to be 1.14% of the mass of the aqueous LFP-G, while the second carbon coating layer's mass as a percentage of the aqueous LFP-G mass, W, was 0.03% (W x (D90 - D10) / D50 x 100 = 0.045). Transmission electron microscopy (TEM) measurements revealed a thickness of 0.5 nm for the second carbon coating layer.

[0073] Test Example 1 (Processing Performance)

[0074] The final products obtained in each embodiment and comparative example were mixed according to the mass ratio of LFP (lithium iron phosphate): CMC (carboxymethyl cellulose): SBR (styrene-butadiene rubber) = 96.5:1.0:2.5, and homogenized with water as the solvent. The amount of solvent water added was dynamically added according to the situation to ensure that the final discharge viscosity was within 5000-10000 cps (corresponding to the slurry viscosity value required for actual production of the enterprise). In specific operations, when the solid content of comparative examples 1, 2, and 3 was similar to that in the embodiments, the stirring was stuck and the product could not be rotated due to poor dispersibility in water. Therefore, solvent water was continued to be added to reduce the solid content until it could be stirred smoothly, and then the solid content was recorded and subsequent operations such as viscosity testing were performed.

[0075] The initial viscosity and the viscosity after standing for 6 hours were tested, and the test results are shown in Table 1. The viscosity was directly measured by a viscosity tester, and the solid content was directly calculated from the amount of material and liquid components added.

[0076] To test the upper limit of coating surface density, the specific operation is as follows: first apply a thin layer by scraping, and then test its surface density after drying at 90℃. Then, increase the scraper spacing, apply the layer again, and then test its surface density after drying at 90℃. Repeat the above operation until cracks appear on the surface of the product after drying at 90℃. The surface density obtained by the operation before cracking is used as the surface density of the corresponding product.

[0077] Table 1

[0078]

[0079] Test Example 2 (Electrode Moisture Residual and Electrical Performance)

[0080] The final products obtained in the examples and comparative examples were coated to a surface density of 14 mg / cm 2 The positive electrode sheet used was a nano 12um aluminum foil as the current collector. Among them, the sample examples 1 to 5, comparative examples 3-1, and comparative examples 4 to 5 were dispersed using water as the solvent, using the formula and method given in test example 1; the sample comparative example 3-2 was dispersed using NMP as the solvent, and the slurry solid content was controlled to 62%. The coated positive electrode sheet was coated with 2.2mg / cm 3 The positive electrode was rolled to a desired compaction density and then vacuum-baked at 100°C for 10 hours. The residual moisture in the electrode was then measured using a Karl Fischer moisture meter at 170°C. A pair of lithium button batteries (negative electrode: φ16mm×0.6mm lithium sheet, electrolyte: New A60, separator: uncoated Celgard 2075) was assembled using a 2032 button battery housing.

[0081] The performance of the above lithium button battery was tested using the Blue Electric test system: 0.1C material gram capacity: at a room temperature of 25°C, charge to 3.75V at 0.1C, then discharge to 2.0V, repeat two cycles, and calculate its gram capacity based on the second cycle data; 1C material gram capacity: at a room temperature of 25°C, charge to 3.75V at 1C, then discharge to 2.0V, calculate its gram capacity; 1C cycle 200-week capacity retention rate: at a room temperature of 25°C, charge to 3.75V at 1C, then discharge to 2.0V, cycle 200 weeks, and measure the material cycle capacity retention rate. The test results are shown in Table 2. Since the data obtained in Comparative Examples 1 and 2 are similar to those in Comparative Example 3 regardless of whether water or NMP is used as the solvent for dispersion, they are not given here.

[0082] Table 2

[0083]

[0084] As can be seen from Tables 1 and 2, when water is used as the dispersion solvent, the dispersibility (solid content) of the final product slurry obtained in the embodiment, the rebound (rise) amount of static viscosity after 6 hours, and the upper limit of the coating surface density are significantly better than those of Comparative Examples 1, 2, 3, and 5. This shows that the aqueous lithium iron phosphate material proposed in this application can be completely replaced by NMP for slurry coating with water as a solvent, which reduces the cost of battery production and reduces the harm of NMP to personnel and the environment. In addition, the residual moisture content of the electrode sheet of the aqueous lithium iron phosphate positive electrode material prepared in Examples 1 to 3, the 0.1C and 1C capacity after deduction, and the 1C cycle retention rate are all comparable to those of Comparative Example 3-2 using NMP as a dispersion solvent, indicating that the presence of the second carbon coating layer in the final product obtained in the embodiment and the use of water as a solvent for dispersion will not affect the electrical properties of the material. However, the residual moisture content and various electrical properties of the electrode sheet of Comparative Example 3-1 using water as a dispersion solvent are far worse than those of the embodiment, further proving that the product obtained in Comparative Example 3 cannot use water as a dispersion solvent. In the final product obtained in Comparative Example 4, the raw material of the second carbon coating layer includes an organic polymer containing a hydrophilic group, but W×(D90-D10) / D50×100 is greater than 10. Although the solid content and 6h static viscosity rebound (rise) data in Table 1 are good, according to Table 2, after the battery is made, the residual moisture content of the electrode and the subsequent electrical performance test are significantly deteriorated, and it cannot be actually applied to production. For Comparative Example 5, in which the raw material of the second carbon coating layer includes an organic polymer containing a hydrophilic group, but W×(D90-D10) / D50×100 is less than 0.05, it can be seen that the dispersion (solid content) of the final product slurry, the rebound (rise) amount of the 6h static viscosity, and the coating surface density are all better than those of Comparative Examples 1, 2, and 3 without the second carbon coating layer; however, the improvement in electrical performance is not large, far less than the improvement in the corresponding performance in the embodiment, and cannot meet the needs of actual production. Moreover, under aqueous homogenization, the residual moisture content and electrical performance of the electrode are seriously affected.

[0085] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications derived therefrom remain within the scope of protection of the present invention.

Claims

1. An aqueous lithium iron phosphate, characterized in that: It comprises a lithium iron phosphate core, a first carbon coating layer wrapped around the core, and a second carbon coating layer wrapped around the first carbon coating layer, wherein the raw material of the second carbon coating layer comprises an organic polymer containing a hydrophilic group; The percentage of the mass of the second carbon coating layer to the mass of the aqueous lithium iron phosphate is calculated as W, and the particle sizes D10, D50, and D90 of the aqueous lithium iron phosphate satisfy the inequality: 0.05≤W×(D90-D10) / D50×100≤10.00, 0.05%≤W≤1.0%.

2. The aqueous lithium iron phosphate according to claim 1, characterized in that The particle size of the aqueous lithium iron phosphate is: D10 is 0.1-1.0 μm, D50 is 0.2-2.0 μm, and D90 is 1.0-10.0 μm; And / or, the mass of the first carbon coating layer is 0.5% to 2% of the mass of the aqueous lithium iron phosphate; And / or, the thickness of the second carbon coating layer is 1-10 nm; And / or, the organic polymer containing a hydrophilic group includes one or more of polyvinyl alcohol, polyacrylic acid, polyacrylate, and polyvinyl ether.

3. A method for preparing aqueous lithium iron phosphate according to claim 1 or 2, characterized in that: The steps include: S1: mixing a lithium source, an iron source, a phosphorus source, and a carbon source in a liquid medium to obtain a slurry, grinding, spray drying, sintering under a protective atmosphere, and crushing to obtain lithium iron phosphate particles having a first carbon coating layer; S2: mixing the lithium iron phosphate particles having the first carbon coating layer and the organic polymer containing the hydrophilic group, spray drying, sintering under a protective atmosphere, and crushing to obtain the aqueous lithium iron phosphate; In the step S2, the sintering temperature is 400-480°C and the sintering time is 3-8 hours.

4. The preparation method according to claim 3, characterized in that In S1, the sintering temperature is 740-800°C and the sintering time is 6-12 hours; And / or, in S1, the molar ratio of lithium element, iron element, and phosphorus element in the lithium source, iron source, and phosphorus source is 1.00-1.10:1.00:1.00-1.10; And / or, in S1, the mass of the carbon source is 1% to 10% of the theoretical mass of the lithium iron phosphate; And / or, in S1, the mass of the liquid medium is 5 to 10 times the sum of the masses of the lithium source, the iron source, the phosphorus source, and the carbon source; And / or, the lithium iron phosphate particles having the first carbon coating layer have a particle size D10 of 0.1-1.0 μm, a D50 of 0.2-2.0 μm, and a D90 of 1.0-10.0 μm.

5. The preparation method according to claim 3, characterized in that In said S1, the method of mixing the lithium source, iron source, phosphorus source and carbon source in the liquid medium comprises mixing them using a stirrer; And / or, in S1, grinding is grinding the particles in the slurry to a D50 of 0.2-1.5 μm; And / or, in said S1, the equipment used for grinding includes a sand mill.

6. The preparation method according to claim 5, characterized in that In S1, the mixing time is 0.5 to 1 hour.

7. The preparation method according to claim 3, characterized in that In S2, the organic polymer containing a hydrophilic group is prepared into a solution before mixing.

8. The preparation method according to claim 7, characterized in that In the organic polymer solution containing hydrophilic groups, the solvent includes one or more of water, methanol, ethanol, and acetone.

9. The preparation method according to any one of claims 3 to 8, characterized in that The lithium source includes one or more of lithium dihydrogen phosphate, lithium monohydrogen phosphate, lithium nitrate, lithium carbonate, lithium phosphate, and lithium hydroxide; and / or, the iron source comprises one or more of ferric phosphate, ferric oxide, ferrous oxalate, iron, and ferric nitrate; and / or, the phosphorus source comprises one or more of ferric phosphate, ammonium dihydrogen phosphate, ammonium phosphate, lithium phosphate, and phosphoric acid; And / or, the carbon source includes one or more of citric acid, glucose, sucrose, and polyethylene glycol; And / or, the liquid medium includes one or more of water, methanol, ethanol, and acetone; And / or, the protective atmosphere includes at least one of nitrogen, inert gas, and carbon dioxide; And / or, the pulverization method includes jet pulverization.

10. A positive electrode plate, characterized in that: The invention comprises the aqueous lithium iron phosphate as claimed in claim 1 or 2 or the aqueous lithium iron phosphate prepared by the preparation method according to any one of claims 3 to 9.

11. A lithium-ion battery, characterized in that: The invention comprises the positive electrode sheet according to claim 10, and also comprises a negative electrode sheet, a separator and an electrolyte.

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