Lithium iron phosphate electrode material, preparation method thereof, positive plate and lithium ion battery

Abstract

The invention belongs to the technical field of batteries, and relates to a lithium iron phosphate electrode material, a preparation method thereof, a positive plate and a lithium ion battery, during preparation of the lithium iron phosphate electrode material, two different carbon sources are used, and one of the carbon sources is alkane with more than 5 main chain carbon atoms. The alkane has a cross-linking characteristic and can form a cross-linked network in a roasting process, so that the conductivity of the lithium iron phosphate electrode material is improved; the carbon residue after roasting cracking still keeps the crosslinking characteristic, a uniform and compact carbon coating layer can be formed on the surfaces of lithium iron phosphate particles, and the electronic conductivity is remarkably improved; wherein the second carbon source can be uniformly dispersed on the surfaces of the lithium iron phosphate particles by using the organic solvent, so that the problem of non-uniform distribution of the carbon source is avoided; therefore, the use of the first carbon source, the second carbon source and the organic solvent synergistically improves the electrochemical performance of the lithium iron phosphate electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, relates to an electrode material, and particularly relates to a lithium iron phosphate electrode material, a preparation method thereof, a positive electrode sheet and a lithium ion battery. Background Art

[0002] Lithium ion batteries are widely used in technical fields such as electric vehicles and energy storage due to their advantages of high energy density, long cycle life and environmental friendliness. Among them, lithium iron phosphate (LiFePO4) has the advantages of rich raw materials, low price and good safety performance, and is considered to be one of the most promising positive electrode materials for lithium ion batteries. However, the low electronic conductivity of lithium iron phosphate limits its application under high current charge and discharge conditions. To solve this technical problem, carbon coating can be used to improve the electronic conductivity of lithium iron phosphate.

[0003] Existing carbon coating methods include high-temperature pyrolysis of organic carbon sources such as glucose and sucrose to form a carbon coating layer on the surface of lithium iron phosphate, thereby improving its electronic conductivity. In addition, chemical vapor deposition method and sol-gel method can also be used for carbon coating. The effect of carbon coating is highly correlated with the carbon source. The thickness and uniformity of the carbon coating layer affect the performance of the lithium iron phosphate electrode material, and the uniformity of the carbon coating layer in the existing technology needs to be further improved.

[0004] Therefore, how to perform carbon coating to improve the conductive performance and compaction density of lithium iron phosphate is a technical problem urgently to be solved in this field. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a lithium iron phosphate electrode material, a preparation method thereof, a positive electrode sheet and a lithium ion battery. The preparation method can improve the uniformity of the carbon coating layer of lithium iron phosphate, thereby improving the compaction density and conductive performance of the lithium iron phosphate electrode material and realizing the improvement of the electrochemical performance of the lithium iron phosphate electrode material.

[0006] To achieve the purpose of this invention, the following technical solutions are adopted in the present invention:

[0007] In the first aspect, the present invention provides a preparation method of a lithium iron phosphate electrode material, and the preparation method includes the following steps:

[0008] Mix the first slurry and the second slurry and then dry them to obtain an intermediate product of lithium iron phosphate; the intermediate product of lithium iron phosphate is calcined to obtain the lithium iron phosphate electrode material;

[0009] The first slurry includes iron phosphate, a lithium source and a first carbon source;

[0010] The second slurry includes iron phosphate, a lithium source, a second carbon source and an organic solvent;

[0011] The first carbon source includes non-alkane organic carbon sources;

[0012] The second carbon source is an alkane with 5 or more carbon atoms in the main chain.

[0013] The preparation method provided by the present invention uses two different carbon sources to achieve the preparation of the surface coating layer of lithium iron phosphate. The first carbon source is a conventional carbon source, but the second carbon source used in combination is an alkane with 5 or more carbon atoms in the main chain. This carbon source has cross-linking properties and can form a cross-linked network during the calcination process, improving the conductivity of the lithium iron phosphate electrode material; moreover, the residual carbon after calcination and cracking still retains the cross-linking properties and can form a uniform and dense carbon coating layer on the surface of the lithium iron phosphate particles, significantly improving the electronic conductivity; the use of organic solvents can achieve the uniform dispersion of the second carbon source on the surface of the lithium iron phosphate particles, avoiding the problem of uneven distribution of the carbon source; therefore, the use of the first carbon source, the second carbon source, and the organic solvent synergistically improves the electrochemical performance of the lithium iron phosphate electrode material.

[0014] The more carbon atoms in the main chain of the alkane, the more difficult it is to dissolve in the organic solvent, which is not conducive to the formation of a uniform coating layer; when the number of carbon atoms in the main chain is small, the cross-linking performance will deteriorate, which is also not conducive to the formation of a uniform coating layer.

[0015] Therefore, as a preferred technical solution, the second carbon source includes any one or a combination of at least two of n-pentane, n-hexane, n-heptane, n-nonane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane, n-octadecane, n-eicosane, or n-dotriacontane.

[0016] Preferably, the organic solvent includes alcohol-based organic solvents.

[0017] By using the organic solvent, the present invention realizes the uniform dispersion of the second carbon source on the surface of the lithium iron phosphate particles, avoiding the problem of uneven distribution of the carbon source in the traditional method, thereby significantly improving the performance stability and consistency of the material.

[0018] Preferably, the alcohol-based organic solvents include any one or a combination of at least two of ethanol, propanol, or butanol. Typical but non-limiting combinations include the combination of ethanol and propanol, the combination of propanol and butanol, the combination of ethanol and butanol, or the combination of ethanol, propanol, and butanol.

[0019] When the amount of the second carbon source is small, the carbon coating of the lithium iron phosphate electrode material will be too thin and the conductivity will be poor; when the amount of the second carbon source is large, the carbon coating layer will be too thick, affecting the tap density.

[0020] Preferably, the second carbon source is 3wt%-10wt% of the total mass of iron phosphate and lithium source in the second slurry, and can be, for example, 3wt%, 5wt%, 6wt%, 8wt% or 10wt%, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0021] Preferably, the mass ratio of the organic solvent to the second carbon source in the second slurry is 8:1-12:1, and can be, for example, 8:1, 9:1, 10:1, 11:1 or 12:1, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0022] Preferably, the second slurry further includes an additive, and the additive is 3wt%-10wt% of the total mass of iron phosphate and lithium source in the second slurry, and can be, for example, 3wt%, 5wt%, 8wt%, 9wt% or 10wt%, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0023] As a further preferred technical solution, let the total mass of the additive and the second carbon source in the second slurry be M1, and the total mass of iron phosphate and lithium source in the second slurry be M2, then M1 is 7wt%-15wt% of M2, and can be, for example, 7wt%, 8wt%, 10wt%, 12wt% or 15wt%, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0024] Preferably, the second slurry further includes an additive, and the additive includes polyethylene glycol (PEG) and / or polyvinyl alcohol (PVA).

[0025] The additive in the second slurry of the present invention can act as a carbon source and play a dispersing effect.

[0026] The first carbon source of the preparation method provided by the present invention is a conventional carbon source in the art. By cooperating with a specific second carbon source, the electrochemical performance of the lithium iron phosphate electrode material is synergistically improved. When only the first carbon source or only the second carbon source is used, the technical effect of synergistically using the first carbon source and the second carbon source cannot be achieved.

[0027] Preferably, the first carbon source includes any one or at least two combinations of glucose, sucrose, starch or Tween. Typical but non-limiting combinations include the combination of glucose and sucrose, the combination of sucrose and starch, the combination of starch and Tween, the combination of glucose, sucrose and starch, the combination of sucrose, starch and Tween, or the combination of glucose, sucrose, starch and Tween.

[0028] Preferably, the first carbon source is 3wt%-10wt% of the total mass of iron phosphate and lithium source in the first slurry. For example, it can be 3wt%, 5wt%, 6wt%, 8wt% or 10wt%, but is not limited to the listed values. The other unlisted values within the numerical range are equally applicable.

[0029] Preferably, the first slurry further includes an additive, and the additive is 3wt%-10wt% of the total mass of iron phosphate and lithium source in the first slurry. For example, it can be 3wt%, 5wt%, 6wt%, 8wt% or 10wt%, but is not limited to the listed values. The other unlisted values within the numerical range are equally applicable.

[0030] As a further preferred technical solution, let the total mass of the additive and the second carbon source in the first slurry be M3, and the total mass of iron phosphate and lithium source in the first slurry be M4. Then M3 is 7wt%-15wt% of M4. For example, it can be 7wt%, 8wt%, 10wt%, 12wt% or 15wt%, but is not limited to the listed values. The other unlisted values within the numerical range are equally applicable.

[0031] Preferably, the additive in the first slurry includes polyethylene glycol and / or polyvinyl alcohol.

[0032] The additive in the second slurry of the present invention can act as a carbon source and play a dispersing effect.

[0033] Preferably, the median particle size D50 of iron phosphate in the first slurry is 2μm - 6μm, and the specific surface area BET is 6m 2 / g - 10m 2 / g.

[0034] The median particle size D50 of iron phosphate in the first slurry is 2μm - 6μm. For example, it can be 2μm, 3μm, 4μm, 5μm or 6μm, but is not limited to the listed values. The other unlisted values within the numerical range are equally applicable.

[0035] The specific surface area BET of iron phosphate in the first slurry is 6m 2 / g - 10m 2 / g. For example, it can be 6m 2 / g, 7m 2 / g, 8m 2 / g, 9m 2 / g or 10m 2 / g, but is not limited to the listed values. The other unlisted values within the numerical range are equally applicable.

[0036] Preferably, the molar ratio of Fe, P, and Li in the first slurry is 1:(1.02 - 1.045):(1.01 - 1.07).

[0037] The higher the iron-to-phosphorus ratio in the first slurry, the better the electrochemical performance of the lithium iron phosphate electrode material, but the tap density will be on the low side. Therefore, in the present invention, the molar ratio of Fe to P in the first slurry is 1:(1.02 - 1.045), for example, it can be 1:1.02, 1:1.03, 1:1.04 or 1:1.045, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0038] Preferably, the molar ratio of Fe and P in the first slurry is controlled by the composition of iron phosphate in the first slurry.

[0039] Preferably, the median particle size D50 of the materials in the first slurry is 0.3 μm - 0.6 μm, for example, it can be 0.3 μm, 0.4 μm, 0.5 μm or 0.6 μm, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0040] Preferably, the median particle size D50 of iron phosphate in the second slurry is 2 μm - 6 μm, and the specific surface area BET is 6 m 2 / g - 10 m 2 / g.

[0041] The median particle size D50 of iron phosphate in the second slurry is 2 μm - 6 μm, for example, it can be 2 μm, 3 μm, 4 μm, 5 μm or 6 μm, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0042] The specific surface area BET of iron phosphate in the second slurry is 6 m 2 / g - 10 m 2 / g, for example, it can be 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g or 10 m 2 / g, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0043] Preferably, the molar ratio of Fe, P, and Li in the second slurry is 1:(1.02 - 1.045):(1.01 - 1.07).

[0044] The higher the iron-to-phosphorus ratio in the second slurry, the better the electrochemical performance of the lithium iron phosphate electrode material, but the tap density will be on the low side. Therefore, in the present invention, the molar ratio of Fe to P in the second slurry is 1:(1.02 - 1.045), for example, it can be 1:1.02, 1:1.03, 1:1.04 or 1:1.045, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0045] Preferably, the molar ratio of Fe to P in the second slurry is controlled by the composition of iron phosphate in the second slurry.

[0046] Preferably, in order to facilitate the mixing of the first slurry and the second slurry and subsequent drying, the solid contents of the first slurry and the second slurry are independently 34%-40%, for example, they can be 34%, 35%, 36%, 38%, 39% or 40%, but are not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0047] Preferably, the median particle size D50 of the materials in the second slurry is 1 μm - 1.5 μm, for example, it can be 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm or 1.5 μm, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0048] Preferably, the mass ratio of the first slurry to the second slurry is 2.2:1 - 3.8:1, for example, it can be 2.2:1, 2.7:1, 2.8:1, 3:1, 3.2:1, 3.3:1 or 3.8:1, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0049] Preferably, the drying method includes spray drying.

[0050] Preferably, the roasting includes: heating up to 500°C - 750°C at a heating rate of 5°C / min - 15°C / min and holding for 6h - 10h; then heating up to 800°C - 880°C at a heating rate of 10°C / min - 30°C / min and holding for 10h - 20h.

[0051] The roasting in the present invention includes two stages. The heating rate of the first-stage roasting is 5°C / min - 15°C / min, for example, it can be 5°C / min, 8°C / min, 10°C / min, 12°C / min or 15°C / min, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0052] The temperature of the first-stage roasting is 500°C - 750°C, for example, it can be 500°C, 550°C, 600°C, 650°C, 700°C or 750°C, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0053] The holding time of the first-stage roasting is 6h - 10h, for example, it can be 6h, 7h, 8h, 9h or 10h, but is not limited to the listed values, and the other unlisted values within the numerical range are equally applicable.

[0054] The heating rate of the second-stage roasting is 10°C / min - 30°C / min. For example, it can be 10°C / min, 15°C / min, 20°C / min, 25°C / min, or 30°C / min, but is not limited to the listed values. The other unlisted values within the numerical range are equally applicable.

[0055] The preparation method provided by the present invention improves the conductivity of the lithium iron phosphate electrode material through the use of the second carbon source, and also increases the sintering tolerance temperature of lithium iron phosphate. It can perform the second-stage roasting at a temperature as high as 880°C, avoiding the defect of iron phosphide caused by too high sintering temperature, and has a significant promoting effect on conductivity and tap density.

[0056] The temperature of the second-stage roasting is 800°C - 880°C. For example, it can be 800°C, 810°C, 830°C, 850°C, 860°C, or 880°C, but is not limited to the listed values. The other unlisted values within the numerical range are equally applicable.

[0057] The heat preservation time of the second-stage roasting is 10h - 20h. For example, it can be 10h, 12h, 15h, 16h, 18h, or 20h, but is not limited to the listed values. The other unlisted values within the numerical range are equally applicable.

[0058] The roasting conditions affect the growth of lithium iron phosphate crystals, the tap density, and the electrochemical performance of the lithium iron phosphate electrode material. The present invention adopts two-stage heating roasting, which can not only ensure the sufficient growth of lithium iron phosphate crystals but also avoid the negative impact of traditional one-stage high-temperature roasting on the capacity of the material. Therefore, the tap density of the lithium iron phosphate electrode material is increased, and its high-capacity characteristics are maintained.

[0059] The roasting in the present invention is carried out in a protective atmosphere, and the gas used in the protective atmosphere includes nitrogen and / or inert gas.

[0060] In the second aspect, the present invention provides a lithium iron phosphate electrode material, which is prepared by the preparation method described in the first aspect.

[0061] The lithium iron phosphate electrode material prepared by the preparation method described in the first aspect of the present invention has a tap density of 2.55g / cc - 2.71g / cc, and a discharge capacity of 155mAh / g - 161mAh / g under the conditions of a voltage of 3.75V and a current of 0.1C.

[0062] In the third aspect, the present invention provides a positive electrode sheet, which includes the lithium iron phosphate electrode material described in the second aspect.

[0063] In the fourth aspect, the present invention provides a lithium-ion battery, which includes the lithium iron phosphate electrode material described in the second aspect, or includes the positive electrode sheet described in the third aspect.

[0064] The numerical ranges described in the present invention include not only the point values exemplified above, but also any point values between the above numerical ranges that are not exemplified. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the described ranges.

[0065] Compared with the prior art, the present invention has the following beneficial effects:

[0066] The preparation method provided by the present invention uses two different carbon sources to achieve the preparation of the surface coating layer of lithium iron phosphate. The first carbon source is a conventional carbon source, and the second carbon source used in combination is an alkane with more than 5 main-chain carbon atoms. This carbon source has a cross-linking property and can form a cross-linking network during the roasting process, improving the conductivity of the lithium iron phosphate electrode material; moreover, the residual carbon after roasting and pyrolysis still retains the cross-linking property and can form a uniform and dense carbon coating layer on the surface of the lithium iron phosphate particles, significantly improving the electronic conductivity; the use of organic solvents can achieve the uniform dispersion of the second carbon source on the surface of the lithium iron phosphate particles, avoiding the problem of uneven distribution of the carbon source; therefore, the use of the first carbon source, the second carbon source, and the organic solvent synergistically improves the electrochemical performance of the lithium iron phosphate electrode material. Description of the Drawings

[0067] Figure 1 SEM diagram of the lithium iron phosphate electrode material obtained in Example 1;

[0068] Figure 2 XRD diagram of the lithium iron phosphate electrode material obtained in Example 1. Detailed Embodiments

[0069] The technical solution of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0070] Example 1

[0071] This example provides a preparation method of a lithium iron phosphate electrode material, and the preparation method includes the following steps:

[0072] (1) Prepare the first slurry: 75 kg of iron phosphate (Fe / P molar ratio is 0.97, median particle size D50 is 4 μm, specific surface area is 8 m 2 / g), 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol (PEG-400) are mixed with 181.5 kg of pure water, and after grinding, a first slurry with a solid content of 36 wt% and a median particle size D50 of 0.5 μm of the material is obtained;

[0073] (2) Prepare the second slurry: First, dissolve 1.645 kg of n-dodecane in 16.45 kg of ethanol, and then mix it with 25 kg of iron phosphate (Fe / P molar ratio is 0.97, median particle size D50 is 4 μm, specific surface area is 8 m 2 / g), 6.25 kg of lithium carbonate, 1.125 kg of polyethylene glycol (PEG-400) and 44.05 kg of pure water, and grind to obtain the second slurry with a solid content of 36 wt% and a median particle size D50 of 1.2 μm of the material;

[0074] (3) Prepare the lithium iron phosphate intermediate: After the first slurry and the second slurry are uniformly mixed according to a mass ratio of 3:1, spray drying is carried out to obtain the lithium iron phosphate intermediate;

[0075] (4) Prepare the lithium iron phosphate electrode material: The lithium iron phosphate intermediate is calcined in a nitrogen atmosphere. First, it is heated to 700 °C at a heating rate of 10 °C / min and kept warm for 8 h, and then it is heated to 850 °C at a heating rate of 20 °C / min and kept warm for 16 h. After the calcination is completed, crushing, classification and screening are carried out to obtain the lithium iron phosphate electrode material with a particle size D50 of 0.97 μm.

[0076] The SEM image of the lithium iron phosphate electrode material obtained in this example is as shown in Figure 1 shown. It can be seen from Figure 1 that the obtained lithium iron phosphate electrode material is spherical-like particles, small particles are filled in the gaps between large particles, the grading effect is obvious, and there is no floating carbon on the particle surface.

[0077] The XRD pattern of the lithium iron phosphate electrode material obtained in this example is as shown in Figure 2 shown. It can be seen from Figure 2 that the obtained lithium iron phosphate electrode material is of the PDF#29-0715 crystal form, and no new crystal form or impurity peaks are generated, indicating that a pure-phase lithium iron phosphate electrode material is formed.

[0078] Example 2

[0079] This example provides a method for preparing a lithium iron phosphate electrode material, and the preparation method includes the following steps:

[0080] (1) Prepare the first slurry: Mix 75 kg of iron phosphate (Fe / P molar ratio is 0.96, median particle size D50 is 2 μm, specific surface area is 10 m 2 / g), 18.75 kg of lithium carbonate, 2.813 kg of glucose, 3.75 kg of polyethylene glycol (PEG-400) and 178.4 kg of pure water, and grind to obtain the first slurry with a solid content of 36 wt% and a median particle size D50 of 0.3 μm of the material;

[0081] (2) Prepare the second slurry: First, dissolve 0.938 kg of n-dodecane in 7.504 kg of ethanol, and then mix it with 25 kg of iron phosphate (Fe / P molar ratio is 0.96, median particle size D50 is 2 μm, specific surface area is 10 m 2 / g), 6.25 kg of lithium carbonate, 1.25 kg of polyethylene glycol (PEG-400) and 51.94 kg of pure water, and grind to obtain a second slurry with a solid content of 36 wt% and a median particle size D50 of 1.2 μm for the material;

[0082] (3) Prepare the lithium iron phosphate intermediate: After the first slurry and the second slurry are uniformly mixed according to a mass ratio of 2.2:1, spray drying is carried out to obtain the lithium iron phosphate intermediate;

[0083] (4) Prepare the lithium iron phosphate electrode material: The lithium iron phosphate intermediate is calcined in a nitrogen atmosphere. First, it is heated to 500 °C at a heating rate of 5 °C / min and held for 10 h, and then heated to 800 °C at a heating rate of 10 °C / min and held for 20 h. After the calcination is completed, crushing, classification and screening are carried out to obtain a lithium iron phosphate electrode material with a particle size D50 of 1.18 μm.

[0084] Example 3

[0085] This example provides a method for preparing a lithium iron phosphate electrode material, and the preparation method includes the following steps:

[0086] (1) Prepare the first slurry: Mix 75 kg of iron phosphate (Fe / P molar ratio is 0.98, median particle size D50 is 6 μm, specific surface area is 6 m 2 / g), 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol (PEG-400) and 181.5 kg of pure water, and grind to obtain a first slurry with a solid content of 36 wt% and a median particle size D50 of 0.3 μm for the material;

[0087] (2) Prepare the second slurry: First, dissolve 3.125 kg of n-dodecane in 37.5 kg of ethanol, and then mix it with 25 kg of iron phosphate (Fe / P molar ratio is 0.98, median particle size D50 is 6 μm, specific surface area is 6 m 2 / g), 6.25 kg of lithium carbonate, 1.562 kg of polyethylene glycol (PEG-400) and 25.83 kg of pure water, and grind to obtain a second slurry with a solid content of 36.2 wt% and a median particle size D50 of 1.2 μm for the material;

[0088] (3) Prepare the lithium iron phosphate intermediate: After the first slurry and the second slurry are uniformly mixed according to a mass ratio of 3.8:1, spray drying is carried out to obtain the lithium iron phosphate intermediate;

[0089] (4) Preparation of lithium iron phosphate electrode material: The intermediate product of lithium iron phosphate is calcined in a nitrogen atmosphere. First, it is heated to 750 °C at a heating rate of 15 °C / min and held for 6 h, then heated to 880 °C at a heating rate of 30 °C / min and held for 10 h. After the calcination is completed, it is crushed, classified, and screened to obtain a lithium iron phosphate electrode material with a D50 particle size of 0.81 μm.

[0090] Example 4

[0091] This example provides a method for preparing a lithium iron phosphate electrode material. Except for the process parameters for preparing the first slurry being different from those in Example 1, the rest are the same as those in Example 1.

[0092] The process parameters for preparing the first slurry in this example are as follows: 75 kg of iron phosphate (Fe / P molar ratio is 0.97, median particle size D50 is 4 μm, specific surface area is 8 m 2 / g), 18.75 kg of lithium carbonate, 2.813 kg of glucose, 3.375 kg of polyethylene glycol (PEG - 400), and 179.5 kg of pure water are mixed and ground to obtain a first slurry with a solid content of 36 wt% and a median particle size D50 of 0.5 μm for the material.

[0093] Example 5

[0094] This example provides a method for preparing a lithium iron phosphate electrode material. Except for the process parameters for preparing the first slurry being different from those in Example 1, the rest are the same as those in Example 1.

[0095] The process parameters for preparing the first slurry in this example are as follows: 75 kg of iron phosphate (Fe / P molar ratio is 0.97, median particle size D50 is 4 μm, specific surface area is 8 m 2 / g), 18.75 kg of lithium carbonate, 9.375 kg of glucose, 9.375 kg of polyethylene glycol (PEG - 400), and 200 kg of pure water are mixed and ground to obtain a first slurry with a solid content of 36 wt% and a median particle size D50 of 0.5 μm for the material.

[0096] Example 6

[0097] This example provides a method for preparing a lithium iron phosphate electrode material. Except for the process parameters for preparing the second slurry being different from those in Example 1, the rest are the same as those in Example 1.

[0098] The process parameters for preparing the second slurry in this example are as follows: First, 0.938 kg of n - dodecane is dissolved in 9.38 kg of ethanol, and then it is mixed with 25 kg of iron phosphate (Fe / P molar ratio is 0.97, median particle size D50 is 4 μm, specific surface area is 8 m 2 / g), 6.25 kg of lithium carbonate, 1.125 kg of polyethylene glycol (PEG-400) and 49.84 kg of pure water were mixed and ground to obtain a second slurry with a solid content of 36 wt% and a median particle size D50 of 1.2 μm for the material.

[0099] Example 7

[0100] This example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 in all respects except that the process parameters for preparing the second slurry are different from those in Example 1.

[0101] The process parameters for preparing the second slurry in this example were as follows: First, 3.125 kg of n-dodecane was dissolved in 31.25 kg of ethanol, and then it was mixed with 25 kg of iron phosphate (Fe / P molar ratio of 0.97, median particle size D50 of 4 μm, specific surface area of 8 m 2 / g), 6.25 kg of lithium carbonate, 3.125 kg of polyethylene glycol (PEG-400) and 35.42 kg of pure water were mixed and ground to obtain a second slurry with a solid content of 36 wt% and a median particle size D50 of 1.2 μm for the material.

[0102] Example 8

[0103] This example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 in all respects except that the n-dodecane in the second slurry was replaced with an equal mass of n-hexadecane.

[0104] Example 9

[0105] This example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 in all respects except that the n-dodecane in the second slurry was replaced with an equal mass of n-octadecane.

[0106] Example 10

[0107] This example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 in all respects except that the n-dodecane in the second slurry was replaced with an equal mass of n-eicosane.

[0108] Example 11

[0109] This example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 in all respects except that the n-dodecane in the second slurry was replaced with an equal mass of n-dotriacontane.

[0110] Example 12

[0111] This example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 in all respects except that the n-dodecane in the second slurry was replaced with an equal mass of n-pentane.

[0112] Example 13

[0113] This example provides a method for preparing a lithium iron phosphate electrode material. Except that polyethylene glycol is not used in both the first slurry and the second slurry, the rest are the same as in Example 1.

[0114] Example 14

[0115] This example provides a method for preparing a lithium iron phosphate electrode material. Except that the maximum calcination temperature is 950 °C, the rest are the same as in Example 1.

[0116] Example 15

[0117] This example provides a method for preparing a lithium iron phosphate electrode material. The preparation method includes the following steps:

[0118] (1) Prepare the first slurry: 75 kg of iron phosphate (Fe / P molar ratio is 0.97, median particle size D50 is 4 μm, specific surface area is 8 m 2 / g), 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol (PEG-400) and 181.5 kg of pure water are mixed and ground to obtain a first slurry with a solid content of 36 wt% and a median particle size D50 of 0.5 μm for the material;

[0119] (2) Prepare the second slurry: First, dissolve 1.645 kg of n-dodecane in 16.45 kg of ethanol, and then mix it with 25 kg of iron phosphate (Fe / P molar ratio is 0.97, median particle size D50 is 4 μm, specific surface area is 8 m 2 / g), 6.25 kg of lithium carbonate, 1.125 kg of polyethylene glycol (PEG-400) and 44.05 kg of pure water, and grind to obtain a second slurry with a solid content of 36 wt% and a median particle size D50 of 1.2 μm for the material;

[0120] (3) Prepare the lithium iron phosphate intermediate: After the first slurry obtained in step (1) and the second slurry obtained in step (2) are uniformly mixed, spray drying is carried out to obtain the lithium iron phosphate intermediate;

[0121] (4) Prepare the lithium iron phosphate electrode material: The lithium iron phosphate intermediate is calcined in a nitrogen atmosphere, heated to 850 °C at a heating rate of 10 °C / min, held for 24 h, and after the calcination is completed, it is crushed, classified and screened to obtain a lithium iron phosphate electrode material with a particle size D50 of 0.93 μm.

[0122] Comparative Example 1

[0123] This comparative example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 except that dodecane in the second slurry is replaced with glucose in equal mass.

[0124] Comparative Example 2

[0125] This comparative example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 except that glucose in the first slurry is replaced with dodecane in equal mass.

[0126] Comparative Example 3

[0127] This comparative example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 except that ethanol in the second slurry is replaced with water in equal mass.

[0128] Comparative Example 4

[0129] This comparative example provides a method for preparing a lithium iron phosphate electrode material, which is the same as Example 1 except that half of the dodecane in the second slurry is replaced with glucose in equal mass.

[0130] Performance Characterization

[0131] The electrochemical performance of the above lithium iron phosphate electrode material was characterized by an electrochemical workstation:

[0132] The lithium iron phosphate cathode material, binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black were mixed at a mass ratio of 92:4:4, and N-methylpyrrolidone was added and stirred into the first cathode slurry; then the first cathode slurry was uniformly coated on the cathode current collector aluminum foil, dried and laminated to make a cathode electrode sheet; a metal lithium was used as the anode electrode sheet; 1 mol / L LiPF6 / (EC + DMC + EMC) was used as the electrolyte; the cathode electrode sheet, PE separator, and anode electrode sheet were stacked in sequence, packaged, left standing, and formed to obtain a lithium-ion battery, and then electrochemical tests were carried out in the voltage range of 2.0 V - 3.75 V.

[0133] Under the conditions of 25 °C, charge-discharge voltage of 2 V - 3.75 V, and first charge-discharge rate of 0.1 C, the first discharge specific capacity, first efficiency were tested, and the capacity retention rate and compaction density at 1 C for 200 cycles were also tested (using a powder compaction tester (UTM7305), according to the standard method GBT 24533-2019, to measure the powder compaction density of the synthetic materials in Examples and Comparative Examples under 3T pressure), and the obtained results are shown in Table 1.

[0134] Table 1

[0135]

[0136]

[0137] As can be seen from Table 1, compared with Example 1, the ratios of the total mass of the additive and the first carbon source to the total mass of iron phosphate and the lithium source in the first slurry of Example 4 and Example 5 are not within the range of 7 wt% - 15 wt%, resulting in a slight decrease in the tap density of the obtained lithium iron phosphate electrode material. Thus, it can be known that in order to obtain the technical effect of high tap density, it is preferably to control the ratio of the total mass of the additive and the first carbon source to the total mass of iron phosphate and the lithium source in the first slurry to be 7 wt% - 15 wt%.

[0138] The addition amounts of the second carbon source and the additive in the second slurry of Example 6 are relatively low; the addition amounts of the second carbon source and the additive in the second slurry of Example 7 are relatively high; such that the ratios of the total mass of the additive and the second carbon source to the total mass of iron phosphate and the lithium source in the second slurry are not within the range of 7 wt% - 15 wt%, resulting in a slight decrease in the tap density of the obtained lithium iron phosphate electrode material. Thus, it can be known that in order to obtain the technical effect of high tap density, it is preferably to control the ratio of the total mass of the additive and the second carbon source to the total mass of iron phosphate and the lithium source in the second slurry to be 7 wt% - 15 wt%.

[0139] As can be seen from the comparison between Example 1, Example 8, Example 9, Example 10, Example 11, Example 12 and Comparative Example 1, when the second carbon source is n - pentane, n - dodecane, n - hexadecane, n - octadecane, n - eicosane and n - dotriacontane, the lithium iron phosphate electrode material can all have a relatively high tap density; among them, when the second carbon source is n - octadecane or n - eicosane, the tap density and the electrochemical performance of the obtained lithium iron phosphate electrode material are the best.

[0140] As can be seen from the comparison between Example 13 and Example 1, when no additive is used in the first slurry and the second slurry, it will lead to a significant decrease in the tap density of the obtained lithium iron phosphate electrode material.

[0141] As can be seen from the comparison between Example 14 and Example 1, although further increasing the calcination temperature can increase the tap density, it will lead to a significant decline in the electrochemical performance. Therefore, it is necessary to preferably control the maximum temperature of the calcination temperature to be 880 °C.

[0142] As can be seen from the comparison between Comparative Example 1, Comparative Example 2, Comparative Example 4 and Example 1, in order to achieve the technical effect of the present invention and obtain a lithium iron phosphate electrode material with good tap density and electrochemical performance, it is necessary to use different first carbon sources and second carbon sources. When the second carbon source is not all alkanes with more than 5 main - chain carbon atoms, the electrochemical performance of the obtained lithium iron phosphate electrode material will be significantly reduced.

[0143] From the comparison between Comparative Example 3 and Example 1, it can be seen that the second slurry needs to be prepared in an organic solvent; otherwise, it is impossible to obtain a lithium iron phosphate electrode material with a high tap density and electrochemical performance.

[0144] In summary, the preparation method provided by the present invention uses two different carbon sources to achieve the preparation of the surface coating layer of lithium iron phosphate. The first carbon source is a conventional carbon source, but the second carbon source used in combination is an alkane with more than 5 main-chain carbon atoms. This carbon source has a cross-linking property and can form a cross-linking network during the roasting process, improving the conductivity of the lithium iron phosphate electrode material. Moreover, the residual carbon after roasting and pyrolysis still retains the cross-linking property and can form a uniform and dense carbon coating layer on the surface of the lithium iron phosphate particles, significantly improving the electronic conductivity. The use of an organic solvent can achieve the uniform dispersion of the second carbon source on the surface of the lithium iron phosphate particles, avoiding the problem of uneven carbon source distribution. Therefore, the use of the first carbon source, the second carbon source, and the organic solvent synergistically improves the electrochemical performance of the lithium iron phosphate electrode material.

[0145] The applicant declares that 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 thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A preparation method of a lithium iron phosphate electrode material, characterized in that, The preparation method includes the following steps: The first slurry and the second slurry are mixed and then dried to obtain an intermediate product of lithium iron phosphate; the intermediate product of lithium iron phosphate is calcined to obtain the lithium iron phosphate electrode material; The first slurry includes iron phosphate, a lithium source, and a first carbon source; The second slurry includes iron phosphate, a lithium source, a second carbon source, and an organic solvent; The first carbon source includes a non-alkane organic carbon source; The second carbon source is an alkane with 5 or more main-chain carbon atoms.

2. The preparation method according to claim 1, wherein The second carbon source includes any one or a combination of at least two of n-pentane, n-hexane, n-heptane, n-nonane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane, n-octadecane, n-eicosane, or n-dotriacontane.

3. The preparation method according to claim 1, wherein The organic solvent includes an alcohol organic solvent.

4. The preparation method according to any one of claims 1 to 3, characterized in that, The second carbon source is 3 wt% - 10 wt% of the total mass of iron phosphate and the lithium source in the second slurry; and / or, the mass ratio of the organic solvent to the second carbon source in the second slurry is 8:1 - 12:1; and / or, the second slurry further includes an additive, and the additive is 3 wt% - 10 wt% of the total mass of iron phosphate and the lithium source in the second slurry; and / or, the second slurry further includes an additive, and the additive includes polyethylene glycol and / or polyvinyl alcohol.

5. The preparation method according to claim 1, characterized in that, The first carbon source includes any one or a combination of at least two of glucose, sucrose, starch, or Tween; and / or, the first carbon source is 3 wt% - 10 wt% of the total mass of iron phosphate and the lithium source in the first slurry; and / or, the first slurry further includes an additive, and the additive is 3 wt% - 10 wt% of the total mass of iron phosphate and the lithium source in the first slurry; and / or, the first slurry further includes an additive, and the additive includes polyethylene glycol and / or polyvinyl alcohol.

6. The preparation method according to claim 1, characterized in that, The median particle size D50 of iron phosphate in the first slurry is 2 μm - 6 μm, and the specific surface area BET is 6 m 2 / g - 10 m 2 / g; and / or, the molar ratio of Fe, P, and Li in the first slurry is 1:(1.02 - 1.045):(1.01 - 1.07); and / or, the median particle size D50 of the materials in the first slurry is 0.3 μm - 0.6 μm; And / or, the median particle size D50 of iron phosphate in the second slurry is 2 μm - 6 μm, and the specific surface area BET is 6 m 2 / g - 10 m 2 / g; and / or, the molar ratio of Fe, P, and Li in the second slurry is 1:(1.02 - 1.045):(1.01 - 1.07); and / or, the median particle size D50 of the materials in the second slurry is 1 μm - 1.5 μm; and / or, the mass ratio of the first slurry to the second slurry is 2.2:1 - 3.8:

1.

7. The preparation method according to claim 1, wherein The drying method includes spray drying; and / or, the calcination includes: heating at a heating rate of 5 °C / min - 15 °C / min to 500 °C - 750 °C and holding for 6 h - 10 h; then heating at a heating rate of 10 °C / min - 30 °C / min to 800 °C - 880 °C and holding for 10 h - 20 h.

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

9. A positive electrode sheet, characterized in that, The positive electrode sheet includes the lithium iron phosphate electrode material according to claim 8.

10. A lithium-ion battery, characterized in that, The lithium ion battery includes the lithium iron phosphate electrode material according to claim 8, or includes the positive electrode sheet according to claim 9.

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