A method for preparing high-capacity lithium iron phosphate cathode material with high pressure and high density

Abstract

This invention provides a method for preparing a high-compact, high-capacity lithium iron phosphate cathode material. The preparation method includes the following steps: (1) mixing ferric sulfate, lithium phosphate, glucose, ethanol, and an emulsifier, stirring, heating to evaporate the ethanol, and sintering under a nitrogen atmosphere at a temperature of 700-800℃ for 6-7 hours to obtain a primary lithium iron phosphate cathode material; (2) mixing the primary lithium iron phosphate cathode material, modified phenolic resin, and ethanol, stirring, heating to evaporate the ethanol, and sintering under a nitrogen atmosphere at a temperature of 700-800℃ for 9-10 hours to obtain a secondary-coated lithium iron phosphate cathode material; (3) pulverizing the secondary-coated lithium iron phosphate cathode material and sieving it to obtain a lithium iron phosphate cathode material. The lithium iron phosphate cathode material prepared by this invention has a high compaction density, resulting in a large charge / discharge capacity and high cycle stability in button batteries.

Description

Technical Field

[0001] This invention belongs to the field of battery cathode material technology, specifically relating to a method for preparing a high-pressure, high-capacity lithium iron phosphate cathode material. Background Technology

[0002] Lithium-ion batteries, as a new type of green energy, are widely used in electric vehicles, mobile phones, computers, and other fields. The cathode material of lithium-ion batteries directly affects key performance characteristics such as battery safety, energy density, and processability. Lithium iron phosphate (LFP) is a common cathode material due to its numerous advantages, including good safety, low cost, stable structure, and long cycle life. It has gradually become the most promising cathode material in the industry and is expected to be applied in energy storage, electric vehicles, and other fields.

[0003] With the continuous expansion of the lithium-ion battery market, people are placing increasingly higher demands on battery energy density. The cathode material of lithium iron phosphate batteries is a mixture of lithium iron phosphate, electrolyte, and carbon source, with the carbon source being a crucial component. Carbon coating is one of the simplest and most effective methods to improve conductivity and enhance electrochemical performance. Currently, carbon sources used for coating include glucose, sucrose, starch, polymer alcohols, phenolic resins, dilute acids, and amides. However, existing carbon-coated lithium iron phosphate cathode materials cannot simultaneously improve the two key indicators of lithium iron phosphate batteries: capacity and compaction density.

[0004] Therefore, achieving breakthroughs in both electrical performance and compaction density based on existing technologies is a key technical challenge in the development of lithium iron phosphate cathode materials. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a high-compaction, high-capacity lithium iron phosphate cathode material with high compaction density. The lithium iron phosphate cathode material, when used to make a button battery, has a large charge-discharge capacity and high cycle stability.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing a high-pressure, high-capacity lithium iron phosphate cathode material, the method comprising the following steps:

[0008] (1) Mix iron sulfate, lithium phosphate, glucose, ethanol and emulsifier in a mass ratio of 14-16:18-20:1:300-350:0.03-0.05, stir at 400-500 rpm for 1-2 hours, heat to evaporate ethanol, sinter under nitrogen atmosphere at 700-800℃ for 6-7 hours to obtain primary lithium iron phosphate cathode material;

[0009] (2) Mix primary lithium iron phosphate cathode material, modified phenolic resin and ethanol in a mass ratio of 1:0.2-0.3:10, stir at 400-500 rpm for 3-4 hours, heat to evaporate ethanol, sinter in a nitrogen atmosphere at a sintering temperature of 700-800℃ for 9-10 hours to obtain secondary coated lithium iron phosphate cathode material;

[0010] (3) The secondary coated lithium iron phosphate cathode material is crushed and sieved to obtain lithium iron phosphate cathode material.

[0011] Furthermore, the emulsifier is selected from one or more of octanol polyoxyethylene ether, Dow TERGITOL® TMN-6, and dodecylphenol polyoxyethylene ether.

[0012] Furthermore, the emulsifier is a compound of octanol polyoxyethylene ether, Dow TERGITOL® TMN-6 and dodecylphenol polyoxyethylene ether in a mass ratio of 2-3:1:0.5-0.8.

[0013] Octyl alcohol polyoxyethylene ether, purchased from Jining Tangyi Chemical Co., Ltd., specifically n-octyl alcohol polyoxyethylene ether.

[0014] Dodecylphenol polyoxyethylene ether was purchased from Jining Tangyi Chemical Co., Ltd.

[0015] Dow Chemical's TERGITOL® TMN-6, whose main component is polyethylene glycol trimethyl nonyl ether.

[0016] In existing technologies, the preparation of lithium iron phosphate and its coating with carbon sources often involves mixing and stirring followed by ball milling. The aim is to achieve more complete particle development, a smoother particle surface, and better surface uniformity, thereby increasing the compaction density of the cathode material. However, in actual production, ball milling has been found to have high equipment requirements, significant quality control challenges, and high costs. The inventors attempted to improve particle morphology through the action of emulsifiers. Through extensive experiments, they discovered that adding specific proportions of octanol polyoxyethylene ether, Dow's TERGITOL® TMN-6, and dodecylphenol polyoxyethylene ether can increase the compaction density of the cathode material. It is speculated that under these conditions, the emulsifier regulates the surface tension between particles, resulting in more uniform particles. This also helps prevent particle agglomeration and accumulation, maintaining uniform particle dispersion.

[0017] Furthermore, the modified phenolic resin is prepared by:

[0018] (1) Phenol, sodium carboxymethyl cellulose and formaldehyde are mixed in a mass ratio of 1.2-1.5:1:4-5, the pH is adjusted to 11 with 15-20wt% sodium hydroxide aqueous solution, the temperature is raised to 100℃ and reacted for 80-100 min, and dehydrated under reduced pressure to obtain phenolic resin modified with sodium carboxymethyl cellulose.

[0019] (2) Dissolve 1 part by weight of sodium carboxymethyl cellulose modified phenolic resin in 6-10 parts by weight of ethanol, add 0.02-0.04 parts by weight of modified carbon nanotubes, stir at 300-400 rpm for 30-40 min, heat to 60-70℃ and react for 1-2 h to obtain modified phenolic resin.

[0020] The sodium carboxymethyl cellulose salt was purchased from Merck, model C4888.

[0021] Further, the preparation method of the modified carbon nanotubes is as follows: (1) tert-butanol peroxide and sodium hydroxide are mixed in a molar ratio of 3-5:1, and reacted at 0℃ for 4-6 hours under a nitrogen atmosphere. After filtration, a solid phase is obtained, washed, and dried to obtain sodium tert-butanol peroxide; (2) sodium tert-butanol peroxide, chloromethylethoxydimethylsilane, and toluene are mixed in a molar ratio of 6-8:1:20-30, and reacted at 10℃-20℃ for 15 hours under a nitrogen atmosphere. 20h, remove toluene by rotary evaporation under reduced pressure to obtain crude product; (3) add extractant to crude product, store at 2-4℃ for 12-15h, filter, and rotary evaporate the extract to obtain coupling agent; (4) dissolve 1 part by weight of coupling agent in 300-500 parts by weight of 25wt% ethanol aqueous solution, continue to add 80-90 parts by weight of carbon nanotubes, stir at 70-80℃ for 20-30min, filter and dry to obtain modified carbon nanotubes.

[0022] Furthermore, the carbon nanotubes are aminated multi-walled carbon nanotubes with an amino content of 0.45 wt%, an outer diameter of 8-15 nm, an inner diameter of 2-4 nm, a length of 50-60 μm, and a specific surface area of ​​200-250 m². 2 / g. Purchased from Beijing Deco Island Gold Technology Co., Ltd.

[0023] Due to the poor charge / discharge capacity of lithium iron phosphate (LFP) materials, this invention employs a secondary carbon coating method to improve the material's charge / discharge capacity. Commercially available phenolic resins, through carbothermal decomposition, form a non-uniform and discontinuous carbon coating layer, exhibiting many carbon-rich regions (thick carbon layers) and carbon-poor regions (thin carbon layers). This results in uneven electron transport capabilities in all directions, leading to poor charge / discharge capacity. This invention attempts to add sodium carboxymethyl cellulose (CMC) salt during the preparation of phenolic resin, obtaining a CMC-modified phenolic resin, which is then further modified with multi-walled carbon nanotubes. The aim is to improve the charge / discharge capacity of the cathode material by introducing multiple functional groups. However, the compatibility of commercially available carbon nanotubes and crosslinking agents with CMC-modified phenolic resin is not ideal, resulting in poor battery cycle stability. The inventors further improved the cycle stability of the cathode material by selecting amminated carbon nanotubes of a specific size and modifying them with a self-made coupling agent. After coating LFP with the modified carbon nanotubes and CMC-modified phenolic resin, the cycle stability of the resulting cathode material was also improved. It is speculated that the phenolic resin prepared under these conditions has a higher dispersion in the solvent, resulting in a more uniform coating on the lithium iron phosphate surface and better carbonization. Simultaneously, the more uniform secondary coating leads to more even heating in the sagger during sintering, resulting in less residual carbon that does not contribute to the coating and thus improving the material's compaction density.

[0024] Furthermore, the extractant is a mixture of n-hexane and ethyl acetate in a volume ratio of 3:1.

[0025] Furthermore, the volume of the extractant is 5-7 times that of the crude product.

[0026] Furthermore, the secondary coated lithium iron phosphate cathode material is transported to an air jet mill for pulverization.

[0027] Furthermore, the pulverization conditions are: pulverization frequency 40-45 Hz, grading frequency 35-40 Hz, and air source pressure 1.0-1.5 MPa.

[0028] Furthermore, after crushing, it is passed through a 400-500 mesh sieve.

[0029] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0030] 1. This invention provides a method for preparing a high-capacity lithium iron phosphate cathode material with high compaction density. The lithium iron phosphate cathode material, when used to make a button battery, exhibits high charge / discharge capacity, high compaction density, and high cycle stability.

[0031] 2. Through numerous experiments, it was found that adding specific proportions of octanol polyoxyethylene ether, Dow TERGITOL® TMN-6, and dodecylphenol polyoxyethylene ether can improve the compaction density of the cathode material.

[0032] 3. Due to the poor charge / discharge capacity of lithium iron phosphate materials, this invention uses secondary carbon coating to improve the charge / discharge capacity of the material. This invention selects ammoniated carbon nanotubes of a specific size and modifies them using a self-made coupling agent. The resulting modified carbon nanotubes are then coated with phenolic resin modified with sodium carboxymethyl cellulose to obtain phenolic resin. This process improves the cycle stability of the resulting cathode material. Simultaneously, the secondary coating is more uniform, resulting in more even heating in the sintering crucible during sintering, and less residual carbon that does not contribute to the coating, which helps improve the compaction density of the material. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1

[0035] This embodiment provides a method for preparing a high-pressure, high-capacity lithium iron phosphate cathode material, the method comprising the following steps:

[0036] (1) Mix iron sulfate, lithium phosphate, glucose, ethanol and emulsifier in a mass ratio of 15:19:1:320:0.04, stir at 450 rpm for 1.5 h, heat to evaporate ethanol, sinter under nitrogen atmosphere at 750 °C for 6.5 h to obtain primary lithium iron phosphate cathode material.

[0037] (2) The primary lithium iron phosphate cathode material, modified phenolic resin and ethanol were mixed in a mass ratio of 1:0.25:10, stirred at 450 rpm for 3.5 h, heated to evaporate the ethanol, and sintered in a nitrogen atmosphere at a sintering temperature of 750 °C for 9.5 h to obtain the secondary coated lithium iron phosphate cathode material.

[0038] (3) The secondary coated lithium iron phosphate cathode material is transported to an air jet mill for pulverization. The pulverization conditions are: pulverization frequency 43HZ, grading frequency 38HZ, air source pressure 1.2MPa, and after pulverization, it is passed through a 400-mesh sieve to obtain lithium iron phosphate cathode material.

[0039] The emulsifier is a compound of octanol polyoxyethylene ether, Dow TERGITOL® TMN-6 and dodecylphenol polyoxyethylene ether in a mass ratio of 2.5:1:0.7.

[0040] The modified phenolic resin is prepared by:

[0041] (1) Phenol, sodium carboxymethyl cellulose and formaldehyde were mixed in a mass ratio of 1.4:1:4.5, the pH was adjusted to 11 with 17wt% sodium hydroxide aqueous solution, the temperature was raised to 100℃ and reacted for 90 min, and dehydrated under reduced pressure to obtain phenolic resin modified with sodium carboxymethyl cellulose.

[0042] (2) Dissolve 1 part by weight of sodium carboxymethyl cellulose modified phenolic resin in 8 parts by weight of ethanol, add 0.03 parts by weight of modified carbon nanotubes, stir at 350 rpm for 35 min, heat to 65℃ and react for 1.5 h to obtain modified phenolic resin.

[0043] The modified carbon nanotubes are prepared as follows: (1) tert-butanol peroxide and sodium hydroxide are mixed in a molar ratio of 4:1 and reacted at 0°C for 5 hours under a nitrogen atmosphere. After filtration, a solid phase is obtained, washed, and dried to obtain sodium tert-butanol peroxide; (2) sodium tert-butanol peroxide, chloromethylethoxydimethylsilane and toluene are mixed in a molar ratio of 7:1:25 and reacted at 15°C for 17 hours under a nitrogen atmosphere. Toluene is removed by rotary evaporation under reduced pressure to obtain a crude product; (3) an extractant is added to the crude product and stored at 3°C ​​for 14 hours. After filtration, the extract is rotary evaporated to obtain a coupling agent; (4) 1 part by weight of the coupling agent is dissolved in 400 parts by weight of 25 wt% ethanol aqueous solution, and 85 parts by weight of carbon nanotubes are added. The mixture is stirred at 75°C for 25 minutes, filtered, and dried to obtain modified carbon nanotubes.

[0044] The carbon nanotubes are aminated multi-walled carbon nanotubes with an amino content of 0.45 wt%, an outer diameter of 8-15 nm, an inner diameter of 2-4 nm, a length of 50-60 μm, and a specific surface area of ​​200-250 m² / g. 2 / g. Purchased from Beijing Deco Island Gold Technology Co., Ltd.

[0045] The extractant is a mixture of n-hexane and ethyl acetate in a volume ratio of 3:1.

[0046] The volume of the extractant is 6 times that of the crude product.

[0047] The sodium carboxymethyl cellulose salt was purchased from Merck, model C4888.

[0048] Example 2

[0049] This embodiment provides a method for preparing a high-pressure, high-capacity lithium iron phosphate cathode material, the method comprising the following steps:

[0050] (1) Mix iron sulfate, lithium phosphate, glucose, ethanol and emulsifier in a mass ratio of 14:20:1:300:0.05, stir at 400 rpm for 2 h, heat to evaporate ethanol, sinter under nitrogen atmosphere at 700 °C for 6 h to obtain primary lithium iron phosphate cathode material.

[0051] (2) The primary lithium iron phosphate cathode material, modified phenolic resin and ethanol were mixed in a mass ratio of 1:0.2:10, stirred at 500 rpm for 3 h, heated to evaporate the ethanol, and sintered in a nitrogen atmosphere at a sintering temperature of 800℃ for 9 h to obtain the secondary coated lithium iron phosphate cathode material.

[0052] (3) The secondary coated lithium iron phosphate cathode material is transported to an air jet mill for pulverization. The pulverization conditions are: pulverization frequency 40 Hz, grading frequency 40 Hz, air source pressure 1.5 MPa, and after pulverization, it is passed through a 500-mesh sieve to obtain lithium iron phosphate cathode material.

[0053] The emulsifier is a compound of octanol polyoxyethylene ether, Dow TERGITOL® TMN-6 and dodecylphenol polyoxyethylene ether in a mass ratio of 2:1:0.8.

[0054] The modified phenolic resin is prepared by:

[0055] (1) Phenol, sodium carboxymethyl cellulose and formaldehyde were mixed in a mass ratio of 1.2:1:5. The pH was adjusted to 11 using a 15wt% sodium hydroxide aqueous solution. The mixture was heated to 100℃ and reacted for 100 min. The mixture was then dehydrated under reduced pressure to obtain phenolic resin modified with sodium carboxymethyl cellulose.

[0056] (2) Dissolve 1 part by weight of sodium carboxymethyl cellulose modified phenolic resin in 10 parts by weight of ethanol, add 0.02 parts by weight of modified carbon nanotubes, stir at 400 rpm for 30 min, heat to 70℃ and react for 1 h to obtain modified phenolic resin.

[0057] The modified carbon nanotubes are prepared as follows: (1) tert-butanol peroxide and sodium hydroxide are mixed in a molar ratio of 5:1 and reacted at 0°C for 4 hours under a nitrogen atmosphere. After filtration, a solid phase is obtained, washed, and dried to obtain sodium tert-butanol peroxide; (2) sodium tert-butanol peroxide, chloromethylethoxydimethylsilane and toluene are mixed in a molar ratio of 8:1:20 and reacted at 10°C for 20 hours under a nitrogen atmosphere. Toluene is removed by rotary evaporation under reduced pressure to obtain a crude product; (3) an extractant is added to the crude product, stored at 2°C for 15 hours, filtered, and the extract is rotary evaporated to obtain a coupling agent; (4) 1 part by weight of the coupling agent is dissolved in 300 parts by weight of 25 wt% ethanol aqueous solution, and 90 parts by weight of carbon nanotubes are added. The mixture is stirred at 70°C for 30 minutes, filtered, and dried to obtain modified carbon nanotubes.

[0058] The carbon nanotubes are aminated multi-walled carbon nanotubes with an amino content of 0.45 wt%, an outer diameter of 8-15 nm, an inner diameter of 2-4 nm, a length of 50-60 μm, and a specific surface area of ​​200-250 m² / g. 2 / g. Purchased from Beijing Deco Island Gold Technology Co., Ltd.

[0059] The extractant is a mixture of n-hexane and ethyl acetate in a volume ratio of 3:1.

[0060] The volume of the extractant is 5 times that of the crude product.

[0061] The sodium carboxymethyl cellulose salt was purchased from Merck, model C4888.

[0062] Example 3

[0063] This embodiment provides a method for preparing a high-pressure, high-capacity lithium iron phosphate cathode material, the method comprising the following steps:

[0064] (1) Mix iron sulfate, lithium phosphate, glucose, ethanol and emulsifier in a mass ratio of 16:18:1:350:0.03, stir at 500 rpm for 2 h, heat to evaporate ethanol, sinter under nitrogen atmosphere at 700℃ for 7 h to obtain primary lithium iron phosphate cathode material.

[0065] (2) The primary lithium iron phosphate cathode material, modified phenolic resin and ethanol were mixed in a mass ratio of 1:0.3:10, stirred at 400 rpm for 4 h, heated to evaporate the ethanol, and sintered in a nitrogen atmosphere at a sintering temperature of 8700℃ for 10 h to obtain the secondary coated lithium iron phosphate cathode material.

[0066] (3) The secondary coated lithium iron phosphate cathode material is transported to an air jet mill for pulverization. The pulverization conditions are: pulverization frequency 405HZ, grading frequency 35HZ, air source pressure 1MPa, and after pulverization, it is passed through a 400-mesh sieve to obtain lithium iron phosphate cathode material.

[0067] The emulsifier is a compound of octanol polyoxyethylene ether, Dow TERGITOL® TMN-6 and dodecylphenol polyoxyethylene ether in a mass ratio of 3:1:0.5.

[0068] The modified phenolic resin is prepared by:

[0069] (1) Phenol, sodium carboxymethyl cellulose and formaldehyde were mixed in a mass ratio of 1.5:1:4. The pH was adjusted to 11 using a 20wt% sodium hydroxide aqueous solution. The mixture was heated to 100℃ and reacted for 80 min. The mixture was then dehydrated under reduced pressure to obtain phenolic resin modified with sodium carboxymethyl cellulose.

[0070] (2) Dissolve 1 part by weight of sodium carboxymethyl cellulose modified phenolic resin in 6 parts by weight of ethanol, add 0.04 parts by weight of modified carbon nanotubes, stir at 300 rpm for 40 min, heat to 60℃ and react for 2 h to obtain modified phenolic resin.

[0071] The modified carbon nanotubes are prepared as follows: (1) tert-butanol peroxide and sodium hydroxide are mixed in a molar ratio of 3:1 and reacted at 0°C for 6 hours under a nitrogen atmosphere. After filtration, a solid phase is obtained, washed, and dried to obtain sodium tert-butanol peroxide; (2) sodium tert-butanol peroxide, chloromethylethoxydimethylsilane and toluene are mixed in a molar ratio of 6:1:30 and reacted at 20°C for 15 hours under a nitrogen atmosphere. Toluene is removed by rotary evaporation under reduced pressure to obtain a crude product; (3) an extractant is added to the crude product, stored at 4°C for 12 hours, filtered, and the extract is rotary evaporated to obtain a coupling agent; (4) 1 part by weight of the coupling agent is dissolved in 3500 parts by weight of 25wt% ethanol aqueous solution, and 80 parts by weight of carbon nanotubes are added. The mixture is stirred at 780°C for 20 minutes, filtered, and dried to obtain modified carbon nanotubes.

[0072] The carbon nanotubes are aminated multi-walled carbon nanotubes with an amino content of 0.45 wt%, an outer diameter of 8-15 nm, an inner diameter of 2-4 nm, a length of 50-60 μm, and a specific surface area of ​​200-250 m² / g. 2 / g. Purchased from Beijing Deco Island Gold Technology Co., Ltd.

[0073] The extractant is a mixture of n-hexane and ethyl acetate in a volume ratio of 3:1.

[0074] The volume of the extractant is 5 times that of the crude product.

[0075] The sodium carboxymethyl cellulose salt was purchased from Merck, model C4888.

[0076] Comparative Example 1

[0077] The difference between this comparative example and Example 1 is that this method for preparing a high-pressure, high-capacity lithium iron phosphate cathode material includes the following steps:

[0078] (1) Mix iron sulfate, lithium phosphate, glucose, ethanol and emulsifier in a mass ratio of 10:25:1:320:0.01, stir at 300 rpm for 1 h, heat to evaporate ethanol, sinter under nitrogen atmosphere at 700℃ for 4 h to obtain primary lithium iron phosphate cathode material.

[0079] (2) The primary lithium iron phosphate cathode material, modified phenolic resin and ethanol were mixed in a mass ratio of 1:0.1:10, stirred at 300 rpm for 2 h, heated to evaporate the ethanol, and sintered in a nitrogen atmosphere at a sintering temperature of 800℃ for 9 h to obtain the secondary coated lithium iron phosphate cathode material.

[0080] (3) The secondary coated lithium iron phosphate cathode material is crushed and sieved to obtain lithium iron phosphate cathode material.

[0081] Comparative Example 2

[0082] The difference between this comparative example and Example 1 is that the phenolic resin used is commercially available and was purchased from Hebei Zetian Chemical Co., Ltd.

[0083] Comparative Example 3

[0084] The difference between this comparative example and Example 1 is that the modified phenolic resin is prepared by mixing phenol, sodium carboxymethyl cellulose and formaldehyde in a mass ratio of 1.4:1:4.3, adjusting the pH to 11 with 20wt% sodium hydroxide aqueous solution, heating to 100℃ and reacting for 100 min, and dehydrating under reduced pressure to obtain the modified phenolic resin.

[0085] Comparative Example 4

[0086] The difference between this comparative example and Example 1 is that the modified carbon nanotubes are replaced with carbon nanotubes. The carbon nanotubes are aminated multi-walled carbon nanotubes with an amino content of 0.45 wt%, an outer diameter of 8-15 nm, an inner diameter of 2-4 nm, a length of 50-60 μm, and a specific surface area of ​​200-250 m². 2 / g. Purchased from Beijing Deco Island Gold Technology Co., Ltd.

[0087] Comparative Example 5

[0088] The difference between this comparative example and Example 1 is that the carbon nanotubes are multi-walled carbon nanotubes with an outer diameter of 20-25 nm, an inner diameter of 5-7 nm, a length of 100-120 μm, and a specific surface area of ​​130-160 m². 2 / g. Purchased from Beijing Deco Island Gold Technology Co., Ltd.

[0089] Comparative Example 6

[0090] The difference between this comparative example and Example 1 is that the modified carbon nanotubes are prepared by dissolving 1 part by weight of silane coupling agent KH550 in 350 parts by weight of 25wt% ethanol aqueous solution, adding 80 parts by weight of carbon nanotubes, stirring at 80°C for 25 min, filtering and drying to obtain modified carbon nanotubes.

[0091] The silane coupling agent KH550 was purchased from Shandong Yuanjin New Materials Co., Ltd.

[0092] Comparative Example 7

[0093] The difference between this comparative example and Example 1 is that the emulsifier is a mixture of octanol polyoxyethylene ether, Dow TERGITOL® TMN-6 and dodecylphenol polyoxyethylene ether in a mass ratio of 1:1:1.

[0094] Comparative Example 8

[0095] The difference between this comparative example and Example 1 is that the emulsifier is a mixture of fatty alcohol polyoxyethylene ether AEO-9, Tween-60 and dodecylphenol polyoxyethylene ether in a mass ratio of 1:1:1.

[0096] Fatty alcohol polyoxyethylene ether AEO-9 and Tween-60 were purchased from Shandong Maifeng Biotechnology Co., Ltd.

[0097] Performance testing

[0098] 1. Button Cell Test: The lithium iron phosphate cathode material, PVDF solution, and carbon black prepared in the examples and comparative examples were weighed at a solid weight ratio of 86:7.5:6.5 and mixed evenly. The mixture was then coated onto a current collector, baked for 4 hours, and cut into small discs. Using lithium sheets as the negative electrode and 1 mol / L lithium hexafluorophosphate (ethylene carbonate: diethyl carbonate volume ratio 1:1) as the electrolyte, and a PE single-layer separator, button cells were assembled. A 1C (160mA) current test was used, with a voltage range of 2.0-3.7V. The initial discharge specific capacity was measured.

[0099] 2. Compaction density test: Weigh 5g of lithium iron phosphate cathode material and test the compaction density under a pressure of 16KN.

[0100] Table 1 Performance Test Results

[0101]

[0102] Examples 1-3 show that the cathode material prepared by this invention has the characteristics of high compaction density and high capacity, as well as high cycle stability. Comparative Example 1 shows that changing the composition of the raw materials and the preparation method both lead to a decrease in performance. Comparative Examples 2-6 show that changing the preparation method of the phenolic resin affects the initial discharge specific capacity and cycle stability of the battery, and also has varying degrees of impact on the compaction density. Comparative Examples 7 and 8 show that changing the type of emulsifier affects the compaction density.

[0103] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a high compaction high capacity lithium iron phosphate cathode material, characterized in that, The preparation method comprises the following steps: (1) mixing iron sulfate, lithium phosphate, glucose, ethanol and emulsifier in a mass ratio of 14-16:18-20:1:300-350:0.03-0.05, stirring at 400-500 rpm for 1-2 h, evaporating ethanol by heating, sintering under a nitrogen atmosphere, sintering temperature 700-800 ℃, holding time 6-7 h, to obtain a primary lithium iron phosphate positive electrode material; (2) mixing the primary lithium iron phosphate positive electrode material, modified phenolic resin and ethanol in a mass ratio of 1:0.2-0.3:10, stirring at 400-500 rpm for 3-4 h, evaporating ethanol by heating, sintering under a nitrogen atmosphere, sintering temperature 700-800 ℃, sintering time 9-10 h, to obtain a secondary coated lithium iron phosphate positive electrode material; (3) crushing the secondary coated lithium iron phosphate positive electrode material and sieving, to obtain a lithium iron phosphate positive electrode material; The preparation method of the modified phenolic resin is as follows: (i) mixing phenol, carboxymethyl cellulose sodium salt and formaldehyde in a mass ratio of 1.2-1.5:1:4-5, adjusting pH to 11 using 15-20 wt% sodium hydroxide aqueous solution, heating to 100 ℃ for 80-100 min, and dehydrating under reduced pressure, to obtain a carboxymethyl cellulose sodium salt modified phenolic resin; (ii) dissolving 1 part by weight of the carboxymethyl cellulose sodium salt modified phenolic resin in 6-10 parts by weight of ethanol, continuously adding 0.02-0.04 parts by weight of modified carbon nanotubes, stirring at 300-400 rpm for 30-40 min, heating to 60-70 ℃ for 1-2 h, to obtain a modified phenolic resin; The preparation method of the modified carbon nanotube is as follows: (1) mixing tert-butyl hydroperoxide and sodium hydroxide in a molar ratio of 3-5:1, reacting at 0 ℃ for 4-6 h under a nitrogen atmosphere, filtering, washing and drying to obtain sodium tert-butyl hydroperoxide; (2) mixing sodium tert-butyl hydroperoxide, chloromethyl ethoxy dimethyl silane and toluene in a molar ratio of 6-8:1:20-30, reacting at 10-20 ℃ for 15-20 h under a nitrogen atmosphere, and removing toluene by rotary evaporation under reduced pressure to obtain a crude product; (3) adding an extracting agent to the crude product, storing at 2-4 ℃ for 12-15 h, filtering, and rotary evaporating the extracting liquid to obtain a coupling agent; (4) dissolving 1 part by weight of the coupling agent in 300-500 parts by weight of 25 wt% ethanol aqueous solution, continuously adding 80-90 parts by weight of carbon nanotubes, stirring at 70-80 ℃ for 20-30 min, filtering and drying to obtain modified carbon nanotubes; wherein the carbon nanotubes are amino-functionalized multi-walled carbon nanotubes.

2. The method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, The emulsifier is selected from one or more of octoxynol, Tergitol® TMN-6 of Dow Chemical Company, and dodecyl phenol polyoxyethylene ether.

3. The method of claim 2, wherein the high-density high-capacity lithium iron phosphate cathode material is prepared by the steps of: mixing a lithium source, an iron source, a phosphorus source, and a carbon source to form a mixture; and heating the mixture at a temperature of 600-800°C for 6-24 hours in a non-oxidizing atmosphere. The emulsifier is a mixture of octoxynol, Tergitol® TMN-6 of Dow Chemical Company and dodecyl phenol polyoxyethylene ether in a mass ratio of 2-3:1:0.5-0.

8.

4. The method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, Amino-functionalized multi-walled carbon nanotubes with 0.45 wt% amino content, outer diameter: 8-15 nm, inner diameter: 2-4 nm, length: 50-60 pm, specific surface area: 200-250 m2 / g. 2 / g.

5. The method for preparing a high-capacity, high-pressure lithium iron phosphate cathode material according to claim 1, characterized in that, The carboxymethyl cellulose sodium salt has a viscosity of 400-800 cP.

6. The method of claim 1, wherein the high-density high-capacity lithium iron phosphate cathode material is prepared by the steps of: mixing a lithium source, an iron source, and a phosphate source to form a mixture; and heating the mixture at a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. The twice-coated lithium iron phosphate positive electrode material is transported to an air flow pulverizer for pulverization.

7. The method of claim 6, wherein the high-density high-capacity lithium iron phosphate cathode material is prepared by the steps of: mixing a lithium source, an iron source, a phosphorus source, and a carbon source to form a mixture; and heating the mixture at a temperature of 600-800°C for 6-24 hours in a non-oxidizing atmosphere. The pulverization is performed at a pulverization frequency of 40-45 HZ, a classification frequency of 35-40 HZ, and an air source pressure of 1.0-1.5 MPa.

8. The method of claim 7, wherein the high-density high-capacity lithium iron phosphate cathode material is prepared by the steps of: mixing a lithium source, an iron source, and a phosphate source to form a mixture; and heating the mixture at a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. After the pulverization, the material is passed through a 400-500 mesh screen.

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