High-compaction lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a high-compaction lithium iron phosphate positive electrode material and a preparation method thereof, and particularly belongs to the technical field of lithium ion batteries. The preparation method of the high-compaction lithium iron phosphate positive electrode material comprises the following steps: carrying out pre-reaction on a lithium source, a phosphorus source, an iron source and a cationic dispersant, then selecting a proper silane coupling agent and a proper carbon source, carrying out ball milling and blending, and finally sintering to obtain the high-compaction lithium iron phosphate positive electrode material. The prepared lithium iron phosphate positive electrode material has excellent electrical performance indexes, and shows excellent effects in the aspects of compaction density, low-rate discharge specific capacity, first charge-discharge efficiency, high-rate discharge specific capacity and cycling stability. The lithium iron phosphate positive electrode material provided by the invention is simple in preparation process, easy to implement and suitable for large-scale production, and meanwhile, proper types of dispersants and carbon sources are selectively added in the preparation process, so that a collaborative optimization effect is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of lithium-ion batteries, and particularly relates to a high-compaction lithium iron phosphate cathode material and a preparation method thereof. Background Art

[0002] Due to its advantages such as high energy density, long cycle life, and environmental friendliness, lithium-ion batteries have become the mainstream power sources in fields such as electric vehicles, energy storage systems, and portable electronic devices. As the core component of lithium-ion batteries, the performance of the cathode material directly affects the energy density, rate performance, and cycle stability of the battery.

[0003] As a typical olivine-structured cathode material, lithium iron phosphate (LiFePO4) has advantages such as high safety, low cost, environmental friendliness, and good thermal stability. However, its low electronic conductivity and lithium-ion diffusion rate limit its application under high-rate charge and discharge and high-compaction density conditions.

[0004] To improve the electrochemical performance of lithium iron phosphate, researchers have proposed various modification strategies, including: Nanonization: By reducing the particle size to shorten the lithium-ion diffusion path, but the nanoparticles are prone to agglomeration, resulting in a decrease in the compaction density. Carbon coating: Coating a conductive carbon layer on the material surface to improve the electronic conductivity, but traditional carbon sources (such as glucose, sucrose) are prone to produce an uneven carbon layer during the sintering process, affecting the performance. Doping modification: By introducing metal ions (such as Nb 5+ 、Mg 2+ ) or non-metallic elements (such as S, N) to optimize the lattice structure, but too high doping amount may lead to a decrease in the structural stability of the material. Surfactant-assisted synthesis: Using surfactants to control the morphology of precursor particles, but the anionic surfactants repel the surface charge of lithium iron phosphate, resulting in poor dispersion effects.

[0005] Although the above methods have improved the performance of lithium iron phosphate to a certain extent, there are still the following problems: It is difficult to balance nanonization and high-compaction density; the uniformity and conductivity of the carbon coating layer are insufficient; the selection and addition timing of surfactants have a significant impact on the material performance, but there is a lack of systematic research; the insufficient interfacial bonding force leads to a decrease in the cycle stability.

[0006] Therefore, developing a preparation method for a lithium iron phosphate cathode material that can simultaneously achieve high-compaction density, high conductivity, and excellent cycle stability has important application value. Summary of the Invention

[0007] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a lithium iron phosphate cathode material and a preparation method thereof that can simultaneously achieve high-compaction density, high specific capacity, and excellent cycle stability.

[0008] To achieve the above object, the present invention discloses the following technical solutions:

[0009] In a first aspect, the present invention provides a preparation method of a high-compact lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0010] (1) Under a nitrogen atmosphere, a lithium source, a phosphorus source, and an iron source are added to deionized water according to a molar ratio of lithium element, phosphorus element, and iron element of (1-2):(0.8-1):1, stirred evenly, and then a cationic dispersant is added and mixed evenly to obtain a mixed solution;

[0011] (2) The mixed solution obtained in step (1) is transferred to a high-pressure reaction vessel, reacted at 160-180 °C for 4-5 h to obtain a reaction slurry, the reaction slurry is centrifuged, the liquid is removed and the residue is retained to obtain a reaction product, and the reaction product is washed with a lithium salt solution having a concentration of 0.05 wt%, and the washed reaction product is dried to obtain a powder;

[0012] (3) Under a nitrogen atmosphere, the powder obtained in step (2), a silane coupling agent, and a carbon source are placed in a ball mill according to a mass ratio of 100:(0.6-1.5):(1-6) and wet ball milled for 3 h to obtain a ball mill product, and the ball mill product is spray dried to obtain a precursor, wherein the carbon source is composed of carbon black, sorbitol, and poly(4-vinylbenzoic acid sodium salt) with a mass ratio of 1:(0.8-1.5):(3-10);

[0013] (4) The precursor is placed in a nitrogen atmosphere and sintered at 650-720 °C for 7-12 h. After the sintered product is subjected to air crushing treatment, a high-compact lithium iron phosphate cathode material is obtained.

[0014] Preferably, the lithium source is selected from at least one of lithium hydroxide, lithium carbonate, and lithium nitrate.

[0015] Preferably, the phosphorus source is selected from phosphoric acid.

[0016] Preferably, the iron source is selected from at least one of ferrous chloride, ferrous sulfate heptahydrate, and ferrous nitrate.

[0017] Preferably, the cationic dispersant is selected from at least one of dodecyltrimethylammonium bromide, octadecyltrimethylammonium bromide, and cetyltrimethylammonium bromide.

[0018] Further preferably, the addition amount of the cationic dispersant is 4-5% of the total mass of the lithium source, the phosphorus source, and the iron source.

[0019] Preferably, the silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane.

[0020] Preferably, in step (2), the washed reaction product is vacuum dried at 120-140°C for 1-2h.

[0021] In a second aspect, the present invention provides a high tap density lithium iron phosphate cathode material, which is prepared by the preparation method described in the first aspect.

[0022] In a third aspect, the present invention provides the application of the high tap density lithium iron phosphate cathode material described in the second aspect in the preparation of a lithium iron phosphate battery material with high tap density, high capacity and long life.

[0023] Advantages of the present invention:

[0024] 1. The lithium iron phosphate cathode material provided by the present invention has excellent electrical performance indexes, and shows excellent performance effects in tap density, low-rate discharge specific capacity, first charge-discharge efficiency, high-rate discharge specific capacity and cycle stability;

[0025] 2. The preparation process of the lithium iron phosphate cathode material provided by the present invention is simple and easy to implement, suitable for large-scale production, and at the same time, the dispersant and carbon source selected and added in the preparation process have a synergistic optimization effect. Specific embodiments

[0026] The technical solutions in the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0027] The technical solutions of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not used to limit the present invention.

[0028] In the present invention:

[0029] Poly(4-vinylbenzoic acid sodium salt) is purchased from Hangzhou Yuhao Chemical Technology Co., Ltd., and the weight average molecular weight is selected to be 4000-5000Da.

[0030] Preparation of high tap density lithium iron phosphate cathode material:

[0031] Preparation of Example 1

[0032] (1) Under a nitrogen atmosphere, lithium hydroxide, phosphoric acid, and ferrous chloride were added to deionized water according to a molar ratio of lithium element, phosphorus element, and iron element of 1:1:1 and stirred evenly. Then, dodecyltrimethylammonium bromide was added and mixed evenly to obtain a mixed solution. Among them, the addition amount of dodecyltrimethylammonium bromide was 4% of the total mass of lithium hydroxide, phosphoric acid, and ferrous chloride;

[0033] (2) The mixed solution obtained in step (1) was transferred to a high-pressure reaction kettle and reacted at 180 °C for 4 h to obtain a reaction slurry. The reaction slurry was centrifuged to remove the liquid and retain the residue to obtain a reaction product. The reaction product was washed with a lithium chloride solution with a concentration of 0.05 wt%, and the washed reaction product was vacuum-dried at 130 °C for 1.5 h to obtain a powder;

[0034] (3) Under a nitrogen atmosphere, the powder obtained in step (2), 3-aminopropyltriethoxysilane, and a carbon source were placed in a ball mill according to a mass ratio of 100:0.6:1 and wet ball-milled for 3 h to obtain a ball-milled product. The ball-milled product was spray-dried to obtain a precursor. Among them, the carbon source was composed of carbon black, sorbitol, and poly(4-vinylbenzoic acid sodium salt) with a mass ratio of 1:0.8:3;

[0035] (4) The precursor was placed in a nitrogen atmosphere and sintered at 720 °C for 7 h. After the sintered product was subjected to air crushing treatment, a high-compact lithium iron phosphate cathode material was obtained.

[0036] Preparation of Example 2

[0037] (1) Under a nitrogen atmosphere, lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate were added to deionized water according to a molar ratio of lithium element, phosphorus element, and iron element of 2:1:1 and stirred evenly. Then, octadecyltrimethylammonium bromide was added and mixed evenly to obtain a mixed solution. Among them, the addition amount of octadecyltrimethylammonium bromide was 5% of the total mass of lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate;

[0038] (2) The mixed solution obtained in step (1) was transferred to a high-pressure reaction kettle and reacted at 170 °C for 4 h to obtain a reaction slurry. The reaction slurry was centrifuged to remove the liquid and retain the residue to obtain a reaction product. The reaction product was washed with a lithium chloride solution with a concentration of 0.05 wt%, and the washed reaction product was vacuum-dried at 120 °C for 2 h to obtain a powder;

[0039] (3) Under a nitrogen atmosphere, the powder obtained in step (2), 3-aminopropyltrimethoxysilane, and a carbon source were placed in a ball mill according to a mass ratio of 100:1.1:3 and wet ball-milled for 3 h to obtain a ball-milled product. The ball-milled product was spray-dried to obtain a precursor. Among them, the carbon source was composed of carbon black, sorbitol, and poly(4-vinylbenzoic acid sodium salt) with a mass ratio of 1:1.3:6;

[0040] (4) The precursor is placed in a nitrogen atmosphere and sintered at 700 °C for 8 h. After the sintered product is subjected to air crushing treatment, a high-density lithium iron phosphate cathode material is obtained.

[0041] Preparation of Example 3

[0042] (1) Under a nitrogen atmosphere, lithium hydroxide, phosphoric acid, and ferrous nitrate are added to deionized water according to a molar ratio of lithium element, phosphorus element, and iron element of 1:0.8:1 and stirred evenly. Then cetyltrimethylammonium bromide is added and mixed evenly to obtain a mixed solution. Among them, the addition amount of cetyltrimethylammonium bromide is 4% of the total mass of lithium hydroxide, phosphoric acid, and ferrous nitrate;

[0043] (2) The mixed solution obtained in step (1) is transferred to a high-pressure reactor and reacted at 160 °C for 5 h to obtain a reaction slurry. The reaction slurry is centrifuged to remove the liquid and retain the residue to obtain a reaction product. The reaction product is washed with a lithium chloride solution with a concentration of 0.05 wt%, and the washed reaction product is vacuum dried at 140 °C for 1 h to obtain a powder;

[0044] (3) Under a nitrogen atmosphere, the powder obtained in step (2), 3-aminopropyltrimethoxysilane, and a carbon source are placed in a ball mill according to a mass ratio of 100:1.5:6 and wet ball milled for 3 h to obtain a ball milled product. The ball milled product is spray dried to obtain a precursor. Among them, the carbon source is composed of carbon black, sorbitol, and poly(4-vinylbenzoic acid sodium salt) with a mass ratio of 1:1.5:10;

[0045] (4) The precursor is placed in a nitrogen atmosphere and sintered at 650 °C for 12 h. After the sintered product is subjected to air crushing treatment, a high-density lithium iron phosphate cathode material is obtained.

[0046] To verify the role of the key components of the present invention, based on Example 2, the key components are defaulted or replaced as follows:

[0047] Preparation of Comparative Example 1

[0048] (1) Under a nitrogen atmosphere, lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate are added to deionized water according to a molar ratio of lithium element, phosphorus element, and iron element of 2:1:1 and stirred evenly. Then sodium dodecylbenzenesulfonate is added and mixed evenly to obtain a mixed solution. Among them, the addition amount of sodium dodecylbenzenesulfonate is 5% of the total mass of lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate;

[0049] (2) Transfer the mixed solution obtained in step (1) to a high-pressure reactor, react at 170 °C for 4 h to obtain a reaction slurry, centrifuge the reaction slurry, remove the liquid and retain the residue to obtain a reaction product, wash the reaction product with a lithium chloride solution with a concentration of 0.05 wt%, and vacuum dry the washed reaction product at 120 °C for 2 h to obtain a powder;

[0050] (3) Under a nitrogen atmosphere, place the powder obtained in step (2), 3-aminopropyltrimethoxysilane, and a carbon source in a mass ratio of 100:1.1:3 in a ball mill and perform wet ball milling for 3 h to obtain a ball-milled product, and spray dry the ball-milled product to obtain a precursor. The carbon source consists of carbon black, sorbitol, and poly(4-vinylbenzoic acid sodium salt) in a mass ratio of 1:1.3:6;

[0051] (4) Place the precursor in a nitrogen atmosphere, sinter at 700 °C for 8 h, and after air-crushing treatment of the sintered product, obtain a high-compact lithium iron phosphate cathode material.

[0052] That is, compared with Example 2, replace the cationic dispersant octadecyltrimethylammonium bromide with an equal amount of the anionic dispersant sodium hexadecylbenzenesulfonate, and the rest is the same as in Example 2.

[0053] Preparation of Comparative Example 2

[0054] (1) Under a nitrogen atmosphere, add lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate to deionized water in a molar ratio of lithium element, phosphorus element, and iron element of 2:1:1, stir evenly, and then add octadecyltrimethylammonium bromide and mix evenly to obtain a mixed solution. The addition amount of octadecyltrimethylammonium bromide is 5% of the total mass of lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate;

[0055] (2) Transfer the mixed solution obtained in step (1) to a high-pressure reactor, react at 170 °C for 4 h to obtain a reaction slurry, centrifuge the reaction slurry, remove the liquid and retain the residue to obtain a reaction product, wash the reaction product with a lithium chloride solution with a concentration of 0.05 wt%, and vacuum dry the washed reaction product at 120 °C for 2 h to obtain a powder;

[0056] (3) Under a nitrogen atmosphere, place the powder obtained in step (2), 3-aminopropyltrimethoxysilane, and a carbon source in a mass ratio of 100:1.1:3 in a ball mill and perform wet ball milling for 3 h to obtain a ball-milled product, and spray dry the ball-milled product to obtain a precursor. The carbon source consists of sorbitol and poly(4-vinylbenzoic acid sodium salt) in a mass ratio of 1.3:6;

[0057] (4) Place the precursor in a nitrogen atmosphere and sinter it at 700 °C for 8 h. After subjecting the sintered product to air crushing treatment, a high-density lithium iron phosphate cathode material is obtained.

[0058] That is, compared with Example 2, carbon black is missing in the carbon source composition in step (3), and the rest is the same as in Example 2.

[0059] Preparation of Comparative Example 3

[0060] (1) Under a nitrogen atmosphere, add lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate to deionized water according to a molar ratio of lithium element, phosphorus element, and iron element of 2:1:1, stir evenly, and then add octadecyltrimethylammonium bromide and mix evenly to obtain a mixed solution. Among them, the addition amount of octadecyltrimethylammonium bromide is 5% of the total mass of lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate;

[0061] (2) Transfer the mixed solution obtained in step (1) to a high-pressure reaction kettle, react at 170 °C for 4 h to obtain a reaction slurry, centrifuge the reaction slurry, remove the liquid and retain the residue to obtain a reaction product, wash the reaction product with a lithium chloride solution with a concentration of 0.05 wt%, and vacuum-dry the washed reaction product at 120 °C for 2 h to obtain a powder;

[0062] (3) Under a nitrogen atmosphere, place the powder obtained in step (2) and 3-aminopropyltrimethoxysilane and a carbon source in a mass ratio of 100:1.1:3 in a ball mill for wet ball milling for 3 h to obtain a ball-milled product, and spray-dry the ball-milled product to obtain a precursor. Among them, the carbon source is composed of carbon black and poly(4-vinylbenzoate sodium salt) with a mass ratio of 1:6;

[0063] (4) Place the precursor in a nitrogen atmosphere and sinter it at 700 °C for 8 h. After subjecting the sintered product to air crushing treatment, a high-density lithium iron phosphate cathode material is obtained.

[0064] That is, compared with Example 2, sorbitol is missing in the carbon source composition in step (3), and the rest is the same as in Example 2.

[0065] Preparation of Comparative Example 4

[0066] (1) Under a nitrogen atmosphere, add lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate to deionized water according to a molar ratio of lithium element, phosphorus element, and iron element of 2:1:1, stir evenly, and then add octadecyltrimethylammonium bromide and mix evenly to obtain a mixed solution. Among them, the addition amount of octadecyltrimethylammonium bromide is 5% of the total mass of lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate;

[0067] (2) Transfer the mixed solution obtained in step (1) to a high-pressure reactor, react at 170 °C for 4 h to obtain a reaction slurry, centrifuge the reaction slurry, remove the liquid and retain the residue to obtain a reaction product, wash the reaction product with a lithium chloride solution with a concentration of 0.05 wt%, and vacuum-dry the washed reaction product at 120 °C for 2 h to obtain a powder;

[0068] (3) Under a nitrogen atmosphere, place the powder obtained in step (2), 3-aminopropyltrimethoxysilane, and a carbon source in a mass ratio of 100:1.1:3 in a ball mill and perform wet ball milling for 3 h to obtain a ball-milled product, and spray-dry the ball-milled product to obtain a precursor. The carbon source consists of carbon black and sorbitol with a mass ratio of 1:1.3;

[0069] (4) Place the precursor in a nitrogen atmosphere and sinter at 700 °C for 8 h. After the sintered product is subjected to air crushing treatment, a high-compact lithium iron phosphate cathode material is obtained.

[0070] That is, compared with Example 2, poly(4-vinylbenzoate sodium salt) is missing in the carbon source composition in step (3), and the rest is the same as in Example 2.

[0071] Preparation of Comparative Example 5

[0072] (1) Under a nitrogen atmosphere, add lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate to deionized water in a molar ratio of lithium element, phosphorus element, and iron element of 2:1:1, stir evenly, and then add octadecyltrimethylammonium bromide and mix evenly to obtain a mixed solution. The addition amount of octadecyltrimethylammonium bromide is 5% of the total mass of lithium hydroxide, phosphoric acid, and ferrous sulfate heptahydrate;

[0073] (2) Transfer the mixed solution obtained in step (1) to a high-pressure reactor, react at 170 °C for 4 h to obtain a reaction slurry, centrifuge the reaction slurry, remove the liquid and retain the residue to obtain a reaction product, wash the reaction product with a lithium chloride solution with a concentration of 0.05 wt%, and vacuum-dry the washed reaction product at 120 °C for 2 h to obtain a powder;

[0074] (3) Under a nitrogen atmosphere, place the powder obtained in step (2), 3-aminopropyltrimethoxysilane, and a carbon source in a mass ratio of 100:1.1:3 in a ball mill and perform wet ball milling for 3 h to obtain a ball-milled product, and spray-dry the ball-milled product to obtain a precursor. The carbon source consists of carbon black, sorbitol, and poly(4-vinylbenzoate sodium salt) with a mass ratio of 1:0.5:2;

[0075] (4) Place the precursor in a nitrogen atmosphere and sinter at 700 °C for 8 h. After the sintered product is subjected to air crushing treatment, a high-compact lithium iron phosphate cathode material is obtained.

[0076] That is, compared with Example 2, the mass ratio of carbon black, sorbitol and poly(4-vinylbenzenesulfonate) in step (3) is adjusted to 1:0.5:2, and the rest is the same as in Example 2.

[0077] Preparation of Comparative Example 6

[0078] (1) Under a nitrogen atmosphere, lithium hydroxide, phosphoric acid and ferrous sulfate heptahydrate were added to deionized water in a molar ratio of lithium element, phosphorus element and iron element of 2:1:1 and stirred evenly, and then octadecyltrimethylammonium bromide was added and mixed evenly to obtain a mixed solution. The addition amount of octadecyltrimethylammonium bromide is 5% of the total mass of lithium hydroxide, phosphoric acid and ferrous sulfate heptahydrate;

[0079] (2) Transfer the mixed solution obtained in step (1) to a high-pressure reaction kettle, react at 170 °C for 4 h to obtain a reaction slurry, centrifuge the reaction slurry, remove the liquid and leave the residue to obtain a reaction product, and wash the reaction product with a lithium chloride solution with a concentration of 0.05 wt%. The washed reaction product was vacuum dried at 120 °C for 2 h to obtain a powder;

[0080] (3) Under a nitrogen atmosphere, the powder obtained in step (2), 3-aminopropyltrimethoxysilane and a carbon source were placed in a ball mill at a mass ratio of 100:1.1:3 and wet ball milled for 3 h to obtain a ball milled product. The ball milled product was spray dried to obtain a precursor, wherein the carbon source is composed of carbon black, sorbitol and poly(4-vinylbenzenesulfonate) with a mass ratio of 1:2:15;

[0081] (4) The precursor was placed in a nitrogen atmosphere and sintered at 700 °C for 8 h. After the sintered product was subjected to air crushing treatment, a high-compact lithium iron phosphate cathode material was obtained.

[0082] That is, compared with Example 2, the mass ratio of carbon black, sorbitol and poly(4-vinylbenzenesulfonate) in step (3) is adjusted to 1:2:15, and the rest is the same as in Example 2.

[0083] Performance Test

[0084] The tap density of the lithium iron phosphate cathode materials of Examples 1-3 and Comparative Examples 1-6 was tested by a tap density meter (TDDC-5); then the cathode materials prepared in the examples and comparative examples were made into coin cells, and the preparation steps are as follows:

[0085] 1. Preparation of the positive electrode slurry: The lithium iron phosphate cathode material, carbon black and polyvinylidene fluoride were mixed at a mass ratio of 15:1:1, and the solvent N-methylpyrrolidone was added to adjust the viscosity to 4000 mPa·s to 8000 mPa·s to obtain the positive electrode slurry;

[0086] 2. The positive electrode slurry is uniformly coated on one surface (perpendicular to the thickness direction) of the aluminum foil (positive electrode current collector) and dried, and then roll-pressed and sliced to obtain a positive electrode sheet containing a positive electrode film layer;

[0087] 3. Using lithium hexafluorophosphate - ethyl methyl carbonate as the electrolyte and polypropylene as the separator, a coin-type half-cell is fabricated by combining the prepared positive electrode sheet and the negative electrode lithium sheet.

[0088] The fabricated coin-type battery is subjected to electrical performance tests including 0.1C discharge specific capacity, 0.1C first charge-discharge efficiency, 5C discharge specific capacity, and 1C capacity retention rate after 500 cycles in the voltage range of 2.0V - 3.8V using a Neware tester. The specific test results are shown in Table 1.

[0089] Table 1

[0090]

[0091] Result analysis:

[0092] According to the data in Table 1, Examples 1 - 3 are significantly superior to Comparative Examples 1 - 6 in all performance indicators such as tap density, discharge specific capacity, first charge-discharge efficiency, high-rate performance, and cycle stability, indicating that the preparation process and the selection of additives provided by the present invention have a synergistic optimization effect.

[0093] An anionic dispersant was used in Comparative Example 1, while a cationic dispersant, namely octadecyltrimethylammonium bromide, was used in Example 2. According to the experimental results in Table 1, all indicators of Comparative Example 1 are lower than those of Example 2, especially the tap density decreased significantly, indicating that the type of dispersant has a great influence on the dispersion effect and structural compactness of the material. The cationic dispersant interacts better with the material surface, forming more uniform particles, thereby improving the density and electrochemical performance.

[0094] Compared with Example 2, Comparative Examples 2-4 lacked carbon black, sorbitol, and poly(4-vinylbenzoic acid sodium salt), respectively. Compared with Example 2, the tap density and discharge capacity of Comparative Examples 2-4 decreased. Comparative Example 2 lacked carbon black, and the tap density decreased from 2.72 to 2.45, and the discharge capacity also decreased. This shows that carbon black, as a conductive agent, improves the conductivity of the material, thereby enhancing the rate performance and capacity. Without carbon black, electron conduction is insufficient, affecting the performance. Comparative Example 3 lacked sorbitol, and the tap density dropped to 2.28, the lowest among all comparative examples, and the discharge capacity was also very low. This shows that sorbitol, as part of the carbon source, decomposes during high-temperature sintering to form a conductive carbon layer, and at the same time it plays a dispersing role during the ball milling process, helping to form a uniform carbon coating layer, improving the structural stability and conductivity of the material. Without sorbitol, the carbon layer coating is uneven, resulting in poor conductivity and poor contact between particles, affecting the tap density and capacity. Comparative Example 4 lacked poly(4-vinylbenzoic acid sodium salt), and the tap density and capacity also decreased. This polymer acts as a binder and structural support in the carbon source, helping to form a stable three-dimensional conductive network during the sintering process, and at the same time it also plays a role in preventing particle agglomeration, maintaining the structural integrity of the material. Without it, the carbon structure is not continuous enough, affecting electron transport and stability.

[0095] In Comparative Examples 5 and 6, the ratios of the three carbon sources were adjusted. The results showed that the performance of Comparative Example 5 was worse than that of Example 2, indicating that too low a proportion of sorbitol and poly(4-vinylbenzoic acid sodium salt) in the carbon source may not be able to form an effective conductive network; although the cycle retention rate of Comparative Example 6 was close to that of Example 2, the tap density and discharge capacity were still low, indicating that too high a proportion of sorbitol and poly(4-vinylbenzoic acid sodium salt) in the carbon source led to too much carbon content, affecting the tap density of the material, and at the same time causing the carbon layer to be too thick to hinder the diffusion of lithium ions.

[0096] Based on the comprehensive experimental data, carbon black provides a direct electron conduction path as a conductive agent; sorbitol decomposes during the sintering process to generate amorphous carbon, enhancing conductivity and promoting the connection between particles, improving the tap density; while poly(4-vinylbenzoic acid sodium salt) forms a continuous network structure after carbonization, and at the same time inhibits particle growth, maintaining an efficient ion and electron transport path for the material.

[0097] In summary, under the synergistic effect of the dispersant and the three carbon sources, the lithium iron phosphate cathode material of the present invention finally realizes a lithium iron phosphate cathode material with high tap density, high capacity, and long life.

[0098] Finally, it should be noted that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a high-density lithium iron phosphate positive electrode material, characterized in that: The preparation method comprises the following steps: (1) under a nitrogen atmosphere, a lithium source, a phosphorus source and an iron source are added to deionized water at a molar ratio of lithium element, phosphorus element and iron element of (1-2):(0.8-1):1, and stirred evenly, and then a cationic dispersant is added and mixed evenly to obtain a mixed solution; (2) transferring the mixed solution obtained in step (1) into a high-pressure reaction vessel, reacting at 160-180° C. for 4-5 hours to obtain a reaction slurry, centrifuging the reaction slurry to remove the liquid and retain the residue to obtain a reaction product, washing the reaction product with a lithium salt solution having a concentration of 0.05 wt%, and drying the washed reaction product to obtain a powder; (3) Under a nitrogen atmosphere, the powder obtained in step (2) is placed in a ball mill with a silane coupling agent and a carbon source in a mass ratio of 100:(0.6-1.5):(1-6) for wet ball milling for 3 hours to obtain a ball milled product, and the ball milled product is spray-dried to obtain a precursor, wherein the carbon source is composed of carbon black, sorbitol, and poly(4-vinylbenzoic acid sodium salt) in a mass ratio of 1:(0.8-1.5):(3-10); (4) placing the precursor in a nitrogen atmosphere, sintering at 650-720° C. for 7-12 hours, and subjecting the sintered product to gas crushing treatment to obtain a high-density lithium iron phosphate positive electrode material.

2. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: The lithium source is selected from at least one of lithium hydroxide, lithium carbonate and lithium nitrate.

3. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: The phosphorus source is selected from phosphoric acid.

4. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: The iron source is selected from at least one of ferrous chloride, ferrous sulfate heptahydrate and ferrous nitrate.

5. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: The cationic dispersant is selected from at least one of dodecyltrimethylammonium bromide, octadecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide.

6. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 3, characterized in that: The added amount of the cationic dispersant is 4-5% of the total mass of the lithium source, the phosphorus source and the iron source.

7. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: The silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane.

8. The method for preparing a high-density lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step (2), the washed reaction product is vacuum dried at 120-140° C. for 1-2 hours.

9. A high-density lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is prepared by the preparation method according to any one of claims 1 to 7.

10. Use of the high-density lithium iron phosphate positive electrode material according to claim 9 in preparing a lithium iron phosphate battery material with high density, high capacity and long life.

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