Carbon-coated lithium iron phosphate anode material and preparation method and application thereof

Abstract

This invention provides a carbon-coated lithium iron phosphate cathode material, its preparation method, and its application. The carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K = (b - 6) × 10 4 +(1-λ)×10 3 +d×10, and K<100, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer. The carbon-coated lithium iron phosphate cathode material of this invention establishes a synergistic mechanism between the carbon coating layer and the bulk structure characteristics, which can guide whether lithium iron phosphate materials possess excellent carbon coating layers and electrochemical performance. Materials that satisfy this synergistic relationship have thinner, denser, and more uniform carbon coating layers, excellent compaction density performance, and shorter Li-axis lengths within the bulk structure. + The diffusion pathway also results in superior electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a carbon-coated lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Lithium iron phosphate (LiFePO4) cathode materials have attracted widespread attention in the lithium-ion battery field due to their advantages such as high theoretical capacity (170 mAh / g), stable voltage platform, wide availability of raw materials, environmental friendliness, and low cost. Especially in recent years, with the accelerated development of the new energy vehicle market, reducing the cost of lithium-ion batteries and improving battery safety have become even more crucial focuses in the new energy sector. However, due to the inherent performance limitations of LiFePO4 materials, its poor electronic conductivity and low ion diffusion rate adversely affect its rate performance and low-temperature performance. Its lower operating voltage and tap density also result in a persistent gap in energy density compared to ternary cathode materials.

[0003] With advancements in existing technologies, LiFePO4 materials have gradually developed modification strategies such as particle nano-sizing, elemental doping, and surface carbon coating. Among these, coating lithium iron phosphate particles with highly conductive carbon materials not only improves their ion mobility and surface electronic conductivity but also, to some extent, inhibits excessive particle size growth and shortens the LiFePO4 oxidative stress curve. + The embedding pathway can significantly enhance the rate performance and low-temperature performance of LiFePO4. However, carbon-coated LiFePO4 materials still suffer from low compaction density, which leads to lower battery energy density and shorter driving range, limiting their application in the power battery field. Therefore, it is necessary to further study the carbon layer coating on the surface of lithium iron phosphate particles to prepare a carbon coating layer with appropriate thickness, uniform and coherent distribution, and density, thereby obtaining a LiFePO4 cathode material with excellent compaction density and electrochemical performance.

[0004] Existing technologies primarily improve the uniformity and coherence of carbon layers by experimenting with different carbon sources and surfactants. For example, CN115692637A combines glucose with surfactants alkyl glucoside, polyoxyethylene alkylphenol ether, and hexadecyltrimethylammonium bromide, and then assists with yttrium oxide doping to obtain a LiFePO4 cathode material with higher discharge specific capacity. Another example is CN11365130A, which uses one or two of citric acid, glucose, and pitch as carbon sources, concluding that the combination of citric acid and glucose is optimal, exhibiting the best battery discharge specific capacity when sintered at 700℃.

[0005] However, existing technologies have not studied the state of the carbon coating layer itself. They have not focused on the effect of the carbon coating layer on the compaction density of LiFePO4 material, nor have they explained the mechanism of the influence of carbon coating and dopants on its electrochemical performance. All of these will ultimately affect the energy density of lithium iron phosphate batteries.

[0006] Based on the above research, there is a need to provide a carbon-coated lithium iron phosphate cathode material, which can elucidate the influence mechanism of the carbon coating layer and dopants on its physicochemical properties. Summary of the Invention

[0007] The purpose of this invention is to provide a carbon-coated lithium iron phosphate cathode material, its preparation method, and its applications. This carbon-coated lithium iron phosphate cathode material establishes a synergistic mechanism between the carbon coating layer and the bulk structure characteristics, which can guide whether lithium iron phosphate materials possess excellent carbon coating layers and electrochemical performance. Materials satisfying this synergistic relationship have thinner, denser, and more uniform carbon coating layers, and shorter Li₂ atoms within the bulk structure. + The diffusion pathway results in superior electrochemical performance.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship:

[0010] K = (b - 6) × 10 4 +(1-λ)×10 3 +d×10, and K<100;

[0011] Where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer. The Raman characteristic peak G is at 1580 cm⁻¹. -1 Nearby, stretching vibration modes belonging to in-plane bonds of carbon atoms are related to the degree of graphitization. Their peak width and intensity are also related to defects, and their position is related to the morphology of carbon materials; for example, in carbon nanotubes, this peak shifts towards lower wavenumbers. The Raman characteristic peak D is located at 1350 cm⁻¹. -1 Nearby, the boundary vibration modes belonging to the disorder-induced hexagonal Brillouin zone are used for defect characterization. The higher the peak intensity ratio λ of the Raman characteristic peak G to the Raman characteristic peak D, the higher the degree of graphitization.

[0012] This invention establishes a synergistic effect between the physicochemical properties of the LiFePO4 carbon coating and the bulk structural characteristics of LiFePO4 by studying the physicochemical properties of the carbon coating and the bulk structural characteristics of LiFePO4. This synergistic mechanism reflects, on the one hand, the degree of graphitization of the carbon coating on the surface of LiFePO4 particles, thereby reflecting the thickness, density, and uniformity of the carbon coating; on the other hand, the size of the cell parameters can represent the lithium-ion diffusion distance and the lithium-ion diffusion rate. Therefore, this invention introduces b, λ, and d. The specific formula satisfied by b, λ, and d can represent the coating effect of the carbon coating, as well as the compaction density and electrochemical performance of the material, thus providing guidance for whether lithium iron phosphate materials possess excellent carbon coating and electrochemical performance.

[0013] Furthermore, since the K value calculated by this invention is less than 100, the carbon-coated lithium iron phosphate cathode material can achieve a higher compaction density and possess excellent electrochemical performance, making it well-suited for high-energy-density lithium-ion power batteries. This solves the problems of poor conductivity and low energy density of LiFePO4 cathode material.

[0014] The unit of b in this invention is The unit of d is nm, and K = (b-6) × 10 4 +(1-λ)×10 3 The validity of +d×10 only applies to the values ​​on both sides of the equation.

[0015] The value K < 100 can be, for example, 99, 90, 80, 70, 60, 50, 40 or 30, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable. Preferably, 20 < K < 100, and more preferably, K < 80.

[0016] Preferably, the carbon content in the carbon-coated lithium iron phosphate cathode material ranges from 1.15wt% to 1.35wt%, for example, it can be 1.15wt%, 1.2wt%, 1.25wt%, 1.30wt%, or 1.35wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] Preferably, the For example, it could be or However, this does not apply to all unlisted values ​​within the range of values.

[0018] Preferably, 1.00≤λ≤1.10, for example, it can be 1, 1.03, 1.05, 1.07 or 1.1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0019] Preferably, 2.0nm≤d≤4.0nm, for example, it can be 2.0nm, 2.5nm, 3.0nm, 3.5nm or 4.0nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] The values ​​of b, λ, and d described in this invention are within the above-mentioned range, which is beneficial to make the calculated K value < 100, thus obtaining a high-performance lithium iron phosphate cathode material.

[0021] Preferably, the compaction density of the carbon-coated lithium iron phosphate cathode material is 2.55 g / cm³. 3 -2.61g / cm 3 For example, it could be 2.55 g / cm³ 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.60g / cm 3 Or 2.61 g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0022] In a second aspect, the present invention provides a method for preparing a carbon-coated lithium iron phosphate cathode material as described in the first aspect, the method comprising the following steps:

[0023] The lithium source, iron source, phosphorus source, carbon source, dopant, surfactant and solvent are mixed, ground, dried and sintered to obtain the carbon-coated lithium iron phosphate cathode material.

[0024] This invention employs a specific preparation method, first mixing, grinding, drying, and sintering the raw materials. During preparation, specific dopants and surfactants are added, which enables a synergistic effect between the carbon coating layer and the internal crystal structure, resulting in a carbon-coated lithium iron phosphate cathode material with a K value <100, uniform carbon coating layer, and excellent performance.

[0025] Preferably, the surfactant comprises any one or a combination of at least two of 9-fluorenone, Triton X-100, β-cyclodextrin, glyceryl glucoside, polystyrene sulfonic acid or dodecylbenzene sulfonic acid, and preferably 9-fluorenone.

[0026] The surfactant used in this invention is preferably 9-fluorenone. Since the molecular structure of 9-fluorenone contains two benzene rings, it is easier to form a graphite-structured carbon layer during surface carbon coating, which is beneficial to improving the graphitization degree of the carbon coating layer.

[0027] Preferably, the amount of surfactant added is 0.1-1 wt% of the iron source mass, for example, it can be 0.1 wt%, 0.3 wt%, 0.5 wt%, 0.8 wt% or 1 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 0.2-0.6 wt%.

[0028] The amount of surfactant added in this invention affects the thickness and density of the coated carbon layer. Within a specific range, the amount of surfactant added can balance the compaction density and electrochemical performance of the LiFePO4 material itself, thereby giving it the optimal energy density.

[0029] Preferably, the carbon source includes any one or a combination of at least two of glucose, sucrose, polyethylene glycol, or citric acid.

[0030] Preferably, the amount of carbon source added is 10-12 wt% of the mass of the iron source, for example, it can be 10 wt%, 11 wt% or 12 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 10.5-11.5 wt%.

[0031] Preferably, the dopant includes any one or a combination of at least two of titanium dioxide, ammonium titanium oxalate, vanadium pentoxide, or ammonium metavanadate.

[0032] Preferably, the amount of dopant added is 0.3-0.5 wt% of the iron source mass, for example, it can be 0.3 wt%, 0.35 wt%, 0.4 wt% or 0.5 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] The doping amount of the dopant described in this invention not only affects the b-value of the LiFePO4 material's cell parameter, but also inhibits abnormal LiFePO4 growth, ensures the uniformity of the carbon coating thickness, and shortens the Li... + The migration distance within its bulk structure means that the addition of dopants affects the K value, which in turn affects the material's compaction density and electrochemical performance.

[0034] Preferably, the molar ratio of lithium in the lithium source to iron in the iron source is (1.02-1.05):1, for example, it can be 1.02:1, 1.03:1, 1.04:1 or 1.05:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, the molar ratio of iron in the iron source to phosphorus in the phosphorus source is (0.95-0.99):1, for example, it can be 0.95:1, 0.96:1, 0.97:1, 0.98:1 or 0.99:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] Preferably, the lithium source includes any one or a combination of at least two of lithium hydroxide, lithium carbonate, or lithium dihydrogen phosphate.

[0037] Preferably, the iron source includes any one or a combination of at least two of ferric phosphate, ferric nitrate, or ferric oxide.

[0038] Preferably, the phosphorus source includes any one or a combination of at least two of ferric phosphate, phosphoric acid, or ammonium dihydrogen phosphate.

[0039] Preferably, the solvent includes water.

[0040] Preferably, the grinding includes coarse grinding and fine grinding performed sequentially. The coarse grinding is performed until the particle size D50 is 1.0-1.5 μm, for example, it can be 1.0 μm, 1.2 μm, 1.4 μm or 1.5 μm. The fine grinding is performed until the particle size D50 is 0.30-0.50 μm, for example, it can be 0.3 μm, 0.4 μm or 0.5 μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0041] Preferably, the drying method includes spray drying.

[0042] Preferably, the inlet air temperature of the spray dryer is 200-300℃, for example, 200℃, 250℃ or 300℃, and the outlet air temperature is 80-100℃, for example, 80℃, 90℃ or 100℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0043] Preferably, the sintering temperature is 770-800℃, for example, 770℃, 780℃, 790℃ or 800℃, and the holding time is 10-20h, for example, 10h, 15h or 20h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0044] Preferably, the sintering atmosphere includes any one or a combination of at least two of nitrogen, argon, or helium.

[0045] Preferably, the material is further pulverized after sintering.

[0046] Preferably, the particle size D50 of the pulverized particles is 0.8-2μm, for example, it can be 0.8μm, 1.0μm, 1.5μm or 2μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] Thirdly, the present invention provides a lithium-ion battery comprising a carbon-coated lithium iron phosphate cathode material as described in the first aspect.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] This invention establishes a synergistic effect between the carbon coating layer and the bulk structure of LiFePO4 by studying their physicochemical properties. When the carbon-coated lithium iron phosphate cathode material satisfies the specific relationship and the calculated K value is less than 100, it indicates that the carbon coating layer has excellent coating effect, and the carbon-coated lithium iron phosphate cathode material has excellent compaction density and electrochemical performance. This provides guidance on whether lithium iron phosphate materials have excellent carbon coating and electrochemical performance, and can also solve problems such as poor conductivity and low energy density of LiFePO4 cathode materials. Attached Figure Description

[0050] Figure 1 This is a transmission electron microscope (TEM) image of the carbon-coated lithium iron phosphate cathode material described in Example 1 of the present invention.

[0051] Figure 2 This is a transmission electron microscope (TEM) image of the carbon-coated lithium iron phosphate cathode material described in Comparative Example 1 of the present invention.

[0052] Figure 3 The image shows the Raman spectrum of the carbon-coated lithium iron phosphate cathode material described in Example 1 of this invention.

[0053] Figure 4 The image shows the Raman spectrum of the carbon-coated lithium iron phosphate cathode material described in Comparative Example 1 of this invention. Detailed Implementation

[0054] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0055] Example 1

[0056] This embodiment provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3+d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0057] The preparation method of the carbon-coated lithium iron phosphate cathode material includes the following steps:

[0058] (1) Weigh 498g of lithium carbonate and 2000g of ferric phosphate for later use; then weigh 170g of glucose and 50g of polyethylene glycol, the total mass of which is 220g; finally weigh 6g of 9-fluorenone and 8g of titanium dioxide.

[0059] The mass of 9-fluorenone is 0.3 wt% of the mass of ferric phosphate, the total mass of glucose and polyethylene glycol is 11 wt% of the mass of ferric phosphate, and the mass of titanium dioxide is 0.4 wt% of the mass of ferric phosphate.

[0060] (2) The raw material weighed in step (1) is mixed with 2700g of pure water and slurried. The slurried material is then ground in a sand mill until the particle size D50 is 1.2μm, and then ground again until the particle size D50 is 0.4μm.

[0061] (3) Spray dry the slurry after grinding in step (2), with the spray inlet temperature at 220°C and the outlet temperature at 90°C, to obtain powder.

[0062] (4) The powder obtained above was placed in a nitrogen atmosphere and sintered at 400°C for 1 hour, followed by sintering at 790°C for 10 hours. After the temperature dropped to room temperature, the sintered material was pulverized until the particle size D50 was 1.2 μm to obtain the carbon-coated lithium iron phosphate cathode material. The transmission electron microscope image of the carbon-coated lithium iron phosphate cathode material is shown below. Figure 1 As shown, by Figure 1 It can be seen that the carbon coating layer is only 2-3 nm in size, and its distribution is very uniform, coherent, and dense. The Raman spectrum of the carbon-coated lithium iron phosphate cathode material is shown below. Figure 3 As shown, by Figure 3 The Raman G / D peak intensity ratio is 1.06, indicating that a carbon layer with a good degree of graphitization was obtained.

[0063] Example 2

[0064] This embodiment provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3+d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0065] The preparation method of the carbon-coated lithium iron phosphate cathode material includes the following steps:

[0066] (1) Weigh 498g of lithium carbonate and 2000g of ferric phosphate for later use; then weigh 160g of glucose and 50g of polyethylene glycol, the total mass of which is 210g; finally weigh 4g of 9-fluorenone, 6g of titanium dioxide and 4g of ammonium metavanadate.

[0067] The mass of 9-fluorenone is 0.2 wt% of the mass of ferric phosphate, the total mass of glucose and polyethylene glycol is 10.5 wt% of the mass of ferric phosphate, and the total mass of titanium dioxide and ammonium metavanadate is 0.5 wt% of the mass of ferric phosphate.

[0068] (2) The raw material weighed in step 1 is mixed with 2700g of pure water and slurried. The slurried material is then ground in a sand mill until the particle size D50 is 1.0μm, and then ground again until the particle size D50 is 0.3μm.

[0069] (3) Spray dry the slurry after grinding in step (2), with the spray inlet temperature at 300°C and the outlet temperature at 100°C, to obtain powder.

[0070] (4) The powder obtained above is placed in a nitrogen atmosphere and sintered at 400°C for 1 hour, then sintered at 800°C for 10 hours. After the temperature drops to room temperature, the sintered material is crushed until the particle size D50 is 0.8 μm to obtain the carbon-coated lithium iron phosphate cathode material.

[0071] Example 3

[0072] This embodiment provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3 +d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0073] The preparation method of the carbon-coated lithium iron phosphate cathode material includes the following steps:

[0074] (1) Weigh 498g of lithium carbonate and 2000g of ferric phosphate for later use; then weigh 180g of glucose and 50g of polyethylene glycol, the total mass of which is 230g; finally weigh 20g of 9-fluorenone and 4g of titanium dioxide.

[0075] The mass of 9-fluorenone is 1 wt% of the mass of ferric phosphate, the total mass of glucose and polyethylene glycol is 11.5 wt% of the mass of ferric phosphate, and the mass of titanium dioxide is 0.2 wt% of the mass of ferric phosphate.

[0076] (2) Mix the raw material weighed in step 1 with 2700g of pure water to form a slurry. The slurry material is then ground in a sand mill until the particle size D50 is 1.5μm, and then ground again until the particle size D50 is 0.5μm.

[0077] (3) Spray dry the slurry after grinding in step (2), with the spray inlet temperature at 200°C and the outlet temperature at 80°C, to obtain powder.

[0078] (4) The powder obtained above is placed in a nitrogen atmosphere and sintered at 400°C for 1 hour, then sintered at 770°C for 20 hours. After the temperature drops to room temperature, the sintered material is crushed until the particle size D50 is 2μm to obtain the carbon-coated lithium iron phosphate cathode material.

[0079] Example 4

[0080] This embodiment provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3 +d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0081] The preparation method of the carbon-coated lithium iron phosphate cathode material is the same as that in Example 1, except that 9-fluorenone is replaced by glycerol glucoside.

[0082] Example 5

[0083] This embodiment provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3+d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0084] The preparation method of the carbon-coated lithium iron phosphate cathode material is the same as that in Example 1, except that 9-fluorenone is replaced by Triton X-100 by mass.

[0085] Example 6

[0086] This embodiment provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3 +d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0087] The preparation method of the carbon-coated lithium iron phosphate cathode material is the same as that in Example 1, except that 9-fluorenone is replaced by polystyrene sulfonic acid.

[0088] Example 7

[0089] This embodiment provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3 +d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0090] The preparation method of the carbon-coated lithium iron phosphate cathode material is the same as in Example 1, except that the mass of 9-fluorenone is adjusted to 20g and the mass of 9-fluorenone is 1wt% of the mass of the iron phosphate.

[0091] Example 8

[0092] This embodiment provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3+d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0093] The preparation method of the carbon-coated lithium iron phosphate cathode material is the same as in Example 1, except that the mass of 9-fluorenone is 2g and the mass of 9-fluorenone is 0.1wt% of the mass of iron phosphate.

[0094] Comparative Example 1

[0095] This comparative example provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3 +d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0096] The preparation method of the carbon-coated lithium iron phosphate cathode material is the same as that in Example 1, except that 9-fluorenone is not added. The transmission electron microscope image of the carbon-coated lithium iron phosphate cathode material is shown below. Figure 2 As shown, by Figure 2 The carbon coating layer is 7–9 nm thick, unevenly distributed, loosely packed, and relatively thick. The Raman spectrum of the carbon-coated lithium iron phosphate cathode material is shown below. Figure 4 As shown, by Figure 4 It can be seen that the Raman G / D peak intensity ratio is 0.97, indicating that the degree of graphitization of its carbon layer is worse than that of Example 1.

[0097] Comparative Example 2

[0098] This comparative example provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3 +d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0099] The preparation method of the carbon-coated lithium iron phosphate cathode material is the same as that in Example 1, except that titanium dioxide is not added.

[0100] Comparative Example 3

[0101] This comparative example provides a carbon-coated lithium iron phosphate cathode material, wherein the carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K=(b-6)×10 4 +(1-λ)×10 3 +d×10, where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

[0102] The preparation method of the carbon-coated lithium iron phosphate cathode material is the same as that in Example 1, except that 9-fluorenone and titanium dioxide are not added.

[0103] The b, λ, d, and K values ​​of the carbon-coated lithium iron phosphate cathode materials described in the above embodiments and comparative examples are shown in Table 1. The carbon content of the carbon-coated lithium iron phosphate cathode materials was measured by a carbon-sulfur analyzer. The compaction density method is as follows: 1g of carbon-coated lithium iron phosphate cathode material was weighed and placed into a compaction mold. The mold was then placed into a compaction device to begin the test. The powder compaction density result was taken under a pressure of 30KN. The graphitization degree method is as follows: An appropriate amount of carbon-coated lithium iron phosphate cathode material was weighed and subjected to Raman testing. The graphitization degree was determined by the G peak (1590cm) in the Raman spectrum. -1 (graphite structure carbon) and D peak (1350 cm⁻¹) -1 The ratio of G / D (disordered carbon) represents the degree of graphitization of carbon; the higher the G / D ratio, the higher the degree of graphitization of carbon.

[0104] The carbon-coated lithium iron phosphate cathode material is used to prepare a button cell battery. The preparation of the button cell battery includes: weighing the carbon-coated lithium iron phosphate cathode material and the binder polyvinylidene fluoride at a mass ratio of 95:3; dissolving the binder and the organic solvent N-methylpyrrolidone; adding approximately 2% of Super P by mass and stirring thoroughly with the lithium iron phosphate cathode material to form a slurry; coating the slurry onto the surface of carbon-coated aluminum foil; drying and rolling to obtain a battery cathode sheet; stamping the rolled cathode sheet into a cathode sheet with a diameter of 13 mm; accurately weighing and calculating the effective mass of the lithium iron phosphate cathode material based on the composition of the cathode sheet; and assembling the button cell battery in a glove box using the prepared cathode sheet, electrolyte, PE separator with a diameter of 18 mm, and lithium sheet with a diameter of 14 mm.

[0105] Electrochemical performance tests were conducted on the button batteries: the specific capacity test was performed using the Shenzhen Xinwei Battery Testing System at a test temperature of 25℃. Two cycles of the charge-discharge rate were performed at each of the following rates: 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C. Finally, two cycles of the test were performed at the 0.1C rate.

[0106] The carbon content, compaction density, 0.1C first discharge capacity, and 5C discharge capacity of the carbon-coated lithium iron phosphate cathode material are shown in Table 1.

[0107] Table 1

[0108]

[0109] As can be seen from Table 1:

[0110] As shown in Examples 1 and Comparative Examples 1-3, the carbon-coated lithium iron phosphate cathode material of the present invention satisfies a specific formula, and the calculated K value is <100, which ensures that the material has excellent compaction density and electrochemical performance. It can also be seen that the introduction of surfactants and dopants in the present invention enables a synergistic effect between the surface carbon coating layer and the internal crystal structure, thereby ensuring the material's performance. As shown in Examples 1 and Examples 4-6, the preferred surfactant in the present invention is 9-fluorenone, because its molecular structure contains two benzene rings, making it easier to form a graphitized carbon layer, thereby further reducing the K value and improving the material's compaction density and electrochemical performance. As shown in Examples 1-3 and Examples 7-8, the amount of surfactant added in the present invention affects the synergistic effect between the carbon coating layer and the internal crystal structure. The amount of surfactant added mainly affects the degree of graphitization λ of the carbon layer and the thickness d of the carbon coating layer, thereby affecting the material's K value, compaction density, and electrochemical performance. As shown in Example 1 and Comparative Examples 1-3, without the addition of surfactant, the graphitization degree λ of the carbon coating layer is significantly reduced, and the thickness d of the carbon layer is increased, resulting in a significant increase in the K value of the material, a significant decrease in the compaction density, and a deterioration in electrochemical performance. Without the addition of dopants, lithium iron phosphate particles grow abnormally. While large particles help improve the compaction density, they also make it difficult to coat the particle surface with a uniform carbon layer. Simultaneously, the cell parameter b in the material structure also increases significantly. + The longer diffusion path in the structure leads to a significant decrease in its electrochemical performance.

[0111] In summary, this invention provides a carbon-coated lithium iron phosphate cathode material, its preparation method, and its application. The carbon-coated lithium iron phosphate cathode material establishes a synergistic mechanism between the carbon coating layer and the bulk structure characteristics of the material. Materials satisfying this synergistic relationship have a thinner, denser, and more uniform carbon coating layer on their particle surface, and shorter Li₂ atoms within their bulk structure. + Diffusion pathways can significantly improve the compaction density and electrochemical performance of materials, providing guidance for the preparation of high-energy-density carbon-coated lithium iron phosphate cathode materials.

[0112] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A carbon-coated lithium iron phosphate cathode material, characterized in that, The carbon-coated lithium iron phosphate cathode material satisfies the following relationship: K = (b - 6) × 10 4 + (1 - λ) × 10 3 + d × 10, and K < 100; Where b is the b-axis length of the cell parameter of the carbon-coated lithium iron phosphate cathode material, λ is the intensity ratio of the Raman characteristic peak G to the Raman characteristic peak D of the carbon-coated lithium iron phosphate cathode material, and d is the average thickness of the carbon coating layer.

2. The carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, K < 80; Preferably, the carbon content in the carbon-coated lithium iron phosphate cathode material ranges from 1.15 wt% to 1.35 wt%. Preferably, Preferably, 1.00 ≤ λ ≤ 1.10; Preferably, 2.0nm ≤ d ≤ 4.0nm; Preferably, the compaction density of the carbon-coated lithium iron phosphate cathode material is 2.55 g / cm³. 3 -2.61g / cm 3 .

3. A method for preparing a carbon-coated lithium iron phosphate cathode material as described in claim 1 or 2, characterized in that, The preparation method includes the following steps: The lithium source, iron source, phosphorus source, carbon source, dopant, surfactant and solvent are mixed, ground, dried and sintered to obtain the carbon-coated lithium iron phosphate cathode material.

4. The preparation method according to claim 3, characterized in that, The surfactant comprises any one or a combination of at least two of 9-fluorenone, Triton X-100, β-cyclodextrin, glyceryl glucoside, polystyrene sulfonic acid or dodecylbenzene sulfonic acid, preferably 9-fluorenone; Preferably, the amount of surfactant added is 0.1-1 wt% of the mass of the iron source, more preferably 0.2-0.6 wt%.

5. The preparation method according to claim 3 or 4, characterized in that, The carbon source includes any one or a combination of at least two of glucose, sucrose, polyethylene glycol, or citric acid. Preferably, the amount of carbon source added is 10-12 wt% of the mass of the iron source, and more preferably 10.5-11.5 wt%. Preferably, the dopant includes any one or a combination of at least two of titanium dioxide, ammonium titanium oxalate, vanadium pentoxide, or ammonium metavanadate; Preferably, the amount of the dopant added is 0.3-0.5 wt% of the mass of the iron source.

6. The preparation method according to any one of claims 3-5, characterized in that, The molar ratio of lithium in the lithium source to iron in the iron source is (1.02-1.05):1; Preferably, the molar ratio of iron in the iron source to phosphorus in the phosphorus source is (0.95-0.99):1; Preferably, the lithium source includes any one or a combination of at least two of lithium hydroxide, lithium carbonate, or lithium dihydrogen phosphate; Preferably, the iron source includes any one or a combination of at least two of ferric phosphate, ferric nitrate, or ferric oxide; Preferably, the phosphorus source includes any one or a combination of at least two of ferric phosphate, phosphoric acid, or ammonium dihydrogen phosphate; Preferably, the solvent includes water.

7. The preparation method according to any one of claims 3-6, characterized in that, The grinding process includes coarse grinding and fine grinding performed sequentially. The coarse grinding is performed until the particle size D50 is 1.0-1.5 μm, and the fine grinding is performed until the particle size D50 is 0.3-0.5 μm. Preferably, the drying method includes spray drying; Preferably, the inlet air temperature of the spray dryer is 200-300℃, and the outlet air temperature is 80-100℃.

8. The preparation method according to any one of claims 3-7, characterized in that, The sintering temperature is 770-800℃, and the holding time is 10-20h; Preferably, the sintering atmosphere includes any one or a combination of at least two of nitrogen, argon, or helium.

9. The preparation method according to any one of claims 3-8, characterized in that, The material was further pulverized after sintering; Preferably, the particle size D50 of the pulverized particles is 0.8-2 μm.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the carbon-coated lithium iron phosphate cathode material as described in claim 1 or 2.

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