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

Abstract

The present disclosure relates to a lithium iron phosphate material and a preparation method thereof, and a lithium ion battery, wherein a precursor is prepared by using hydroxy iron phosphate and hydroxy iron phosphate containing metal doping elements as phosphorus iron sources, respectively, the compaction density of the lithium iron phosphate material is effectively improved based on a four-stage particle size grading design, and meanwhile, the capacity is efficiently exerted. The lithium iron phosphate material is used for preparing a lithium ion battery, the energy density, rate performance and cycle life of the battery can be significantly improved, the development needs of high-end application markets such as long endurance of electric vehicles and high efficiency of energy storage systems are met, and the lithium iron phosphate material has a good industrial application prospect.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a lithium iron phosphate material, a preparation method thereof, and a lithium ion battery. BACKGROUND

[0002] Lithium iron phosphate (LiFePO4) is a mainstream lithium ion battery cathode material, which has high theoretical capacity, good thermal stability, and low cost, and is widely used in power batteries and energy storage fields. By improving the compaction density of the cathode material, the volumetric energy density of the lithium ion battery can be effectively improved to meet the demand for long endurance mileage in terminal applications. Existing preparation processes mainly synthesize lithium iron phosphate materials with high compaction density through solid-phase method, liquid-phase method, and carbon thermal reduction method. However, the lithium iron phosphate materials prepared by the existing processes still have problems such as insufficient capacity, high resistivity, and poor cycle life, which seriously restrict the performance upgrade of the battery. SUMMARY

[0003] The purpose of the present disclosure is to provide a lithium iron phosphate material with improved electrical performance, a preparation method thereof, and a lithium ion battery.

[0004] To achieve the above-mentioned purpose, the first aspect of the present disclosure provides a preparation method of a lithium iron phosphate material, which comprises:

[0005] wet grinding, drying, and then first sintering to obtain a first precursor;

[0006] wet grinding, drying, and then second sintering to obtain a second precursor;

[0007] wet grinding to obtain a first slurry with a first particle size;

[0008] wet grinding to obtain a second slurry with a second particle size;

[0009] wet grinding to obtain a third slurry with a third particle size;

[0010] wet grinding to obtain a fourth slurry with a fourth particle size;

[0011] drying, and then third sintering to obtain a lithium iron phosphate material;

[0012] The first iron-phosphorus source is iron hydroxyphosphate, and the second iron-phosphorus source is iron hydroxyphosphate containing a metal doping element; the first particle size, the second particle size, the third particle size, and the fourth particle size are sequentially decreasing.

[0013] Optionally, the iron-phosphorus molar ratio of the first iron-phosphorus source is 1.40-1.46, and the BET specific surface area is 4-50 m 2 / g.

[0014] Optionally, the iron-phosphorus molar ratio of the second iron-phosphorus source is 1.44-1.50, and the BET specific surface area is 30-100 m 2 / g.

[0015] Optionally, the metal doping element includes at least one of Ti, Nb, V, Mn, Mg, Ni, and Zr.

[0016] Optionally, the content of the metal doping element in the second iron-phosphorus source is 0.1-0.4 wt% based on the total weight of the second iron-phosphorus source.

[0017] Optionally, the first lithium-phosphorus source and the second lithium-phosphorus source each independently include at least one of lithium phosphate, di-lithium hydrogen phosphate, and dihydrogen lithium phosphate;

[0018] The first metal dopant includes at least one of a manganese dopant, a vanadium dopant, a nickel dopant, and a zirconium dopant;

[0019] The second metal dopant includes at least one of a titanium dopant, a niobium dopant, and a magnesium dopant;

[0020] The first supplement and the second supplement each independently include at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate;

[0021] The first carbon source, the second carbon source, the third carbon source, the fourth carbon source, the fifth carbon source, and the sixth carbon source each independently include at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinylpyrrolidone, and starch.

[0022] Optionally, the first metal dopant includes at least one of manganese acetate, ammonium metavanadate, nickel oxide, and zirconium oxide.

[0023] The second metal dopant includes at least one of titanium dioxide, niobium oxide, and magnesium acetate.

[0024] Optionally, the first mixture further contains a first supplement, and the second mixture further contains a second supplement.

[0025] The first supplement and the second supplement each independently comprise at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate.

[0026] Optionally, the carbon content of the first precursor is 0.1-0.3 wt% and the metal doping content is 0.1-0.5 wt% based on the total weight of the first precursor.

[0027] The carbon content of the second precursor is 0.1-0.3 wt% and the metal doping content is 0.1-0.7 wt% based on the total weight of the second precursor.

[0028] Optionally, the weight ratio of the first precursor to the third carbon source in the third mixture is 1000: (30-60).

[0029] The weight ratio of the first precursor to the fourth carbon source in the fourth mixture is 1000: (30-60).

[0030] The weight ratio of the second precursor to the fifth carbon source in the fifth mixture is 1000: (30-60).

[0031] The weight ratio of the second precursor to the sixth carbon source in the sixth mixture is 1000: (30-60).

[0032] Optionally, the first particle size is 1.0-2.0 μm, the second particle size is 0.5-1.0 μm, the third particle size is 0.2-0.5 μm, and the fourth particle size is 0.1-0.2 μm; and / or,

[0033] The weight ratio of the first slurry, the second slurry, the third slurry, and the fourth slurry in the seventh mixture is 1: (0.5-1): (0.25-1): (0.25-1).

[0034] Optionally, the first sintering condition comprises a temperature of 800-840 °C and a time of 4-10 h.

[0035] The second sintering condition comprises a temperature of 600-700 °C and a time of 3-5 h.

[0036] The third sintering condition comprises a temperature of 750-800 °C and a time of 4-8 h.

[0037] In a second aspect of the present disclosure, a lithium iron phosphate material prepared by the method of the first aspect of the present disclosure is provided.

[0038] In a third aspect of the present disclosure, a lithium ion battery comprising the lithium iron phosphate material of the second aspect of the present disclosure is provided.

[0039] By the above technical solution, the present disclosure respectively uses hydroxy iron phosphate and hydroxy iron phosphate containing metal doping elements as phosphorus iron sources to prepare two kinds of precursors, improves the electronic conductivity and lithium ion transmission efficiency of the lithium iron phosphate material, and can inhibit inter-particle melting during secondary sintering, effectively improves the compaction density of the lithium iron phosphate material based on the four-level particle size grading design, and at the same time ensures the efficient capacity play. The lithium ion battery prepared by the lithium iron phosphate material can significantly improve the energy density, rate performance and cycle life of the battery, meet the development needs of the high-end application market such as long endurance of electric vehicles and high efficiency of energy storage systems, and has good industrialization application prospect.

[0040] Other features and advantages of the present disclosure will be described in detail in the following specific embodiments section. BRIEF DESCRIPTION OF DRAWINGS

[0041] The accompanying drawings are included to provide a further understanding of the present disclosure and constitute a part of the specification, and together with the following specific embodiments, serve to explain the present disclosure but do not constitute a limitation on the present disclosure. In the drawings:

[0042] Figure 1 is an electron microscope photo of the lithium iron phosphate material prepared in Example 1. DETAILED DESCRIPTION

[0043] The specific embodiments of the present disclosure are described in detail below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure, and do not limit the present disclosure.

[0044] In a first aspect of the present disclosure, a preparation method of a lithium iron phosphate material is provided, which comprises the following steps S1-S7:

[0045] S1, wet grinding, drying, and then first sintering of a first mixed material containing a first phosphorus iron source, a first phosphorus lithium source, a first metal dopant and a first carbon source are performed to obtain a first precursor;

[0046] S2, wet grinding, drying, and then second sintering of a second mixed material containing a second phosphorus iron source, a second phosphorus lithium source, a second metal dopant and a second carbon source and an optional second supplement are performed to obtain a second precursor;

[0047] S3, wet grinding of a third mixed material containing the first precursor and a third carbon source is performed to obtain a first slurry with a first particle size;

[0048] S3, wet grinding of a fourth mixed material containing the first precursor and a fourth carbon source is performed to obtain a second slurry with a second particle size;

[0049] S5, wet grinding the fifth mixture containing the second precursor and a fifth carbon source to obtain a third slurry with a third particle size;

[0050] S6, wet grinding the sixth mixture containing the second precursor and a sixth carbon source to obtain a fourth slurry with a fourth particle size;

[0051] S7, drying the seventh mixture containing the first slurry, the second slurry, the third slurry and the fourth slurry, and then performing a third sintering to obtain a lithium iron phosphate material;

[0052] The first phosphorus-iron source is iron hydroxyl phosphate, and the second phosphorus-iron source is iron hydroxyl phosphate containing a metal-doped element. The first particle size, the second particle size, the third particle size and the fourth particle size decrease in turn.

[0053] The present disclosure uses two different doped systems of phosphorus-iron sources as raw materials to prepare the precursor, which is beneficial to improve the electronic conductivity and lithium ion transmission efficiency of the lithium iron phosphate material, and can inhibit the inter-particle fusion during the secondary sintering. Based on the four-level particle size grading design, the compaction density of the lithium iron phosphate material is effectively improved, while ensuring the efficient capacity.

[0054] In step S1, iron hydroxyl phosphate is used as the first phosphorus-iron source to prepare the first precursor, which is used to prepare the first slurry with the largest particle size (i.e. the first particle size) and the second slurry with the medium particle size (i.e. the second particle size) in the subsequent steps.

[0055] The chemical formula of the iron hydroxyl phosphate can be Fe x (PO4)(OH)·nH2O, and x is 1.4-1.5. In one specific embodiment, the iron-phosphorus molar ratio (Fe / P) of the first phosphorus-iron source can be 1.40-1.46, and the BET specific surface area can be 4-50 m 2 / g.

[0056] The iron hydroxyl phosphate can be prepared by a co-precipitation method. In one specific embodiment, the preparation steps of the iron hydroxyl phosphate can include:

[0057] (1) mixing titanium white by-product ferrous sulfate, a phosphorus source and a precipitating agent for pressure filtration purification to obtain a ferrous sulfate solution;

[0058] (2) adding an appropriate amount of phosphoric acid to the ferrous sulfate solution to reduce the pH value of the ferrous sulfate solution to 2-2.5;

[0059] (3) adding hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution and ammonia water to the ferrous sulfate solution, and then reacting at 25-70°C for 2-6h to form a mixed slurry, and then washing and filtering multiple times to obtain an iron hydroxyl phosphate precursor;

[0060] (4) The above iron hydroxyphosphate precursor is subjected to flash drying under the conditions of an air inlet temperature of 220±20°C and an air outlet temperature of 110±10°C, and sintering for 4-5h under an air atmosphere at 530-560°C to obtain iron hydroxyphosphate.

[0061] The first phosphorus-lithium source is a compound containing phosphorus and lithium elements, and the lithium-phosphorus molar ratio (Li / P) of the first phosphorus-lithium source can be 1-3. Specifically, the first phosphorus-lithium source can include at least one of lithium phosphate (Li3PO4), lithium dihydrogen phosphate (LiH2PO4), and lithium hydrogen phosphate (Li2HPO4). The amount of the first phosphorus-lithium source can be adjusted within a certain range, and specifically, the molar ratio of the first phosphorus-lithium source calculated based on Li to the first phosphorus-iron source calculated based on Fe can be 1.00-1.05, preferably 1.01-1.05. The first phosphorus-lithium source preferably has a high purity (e.g., more than 98%) to improve the quality of the lithium iron phosphate material.

[0062] The first metal dopant can include at least one of a manganese dopant, a vanadium dopant, a nickel dopant, and a zirconium dopant. The introduction of the above-mentioned metal dopants is beneficial to further improve the discharge platform and rate performance of large particles and medium particles in the lithium iron phosphate material, and is also beneficial to improving the roundness of large particles and medium particles after secondary sintering, so as to further improve the tap density of the lithium iron phosphate material. Specifically, the first metal dopant can include at least one of manganese acetate (Mn(CH3COO)2), ammonium metavanadate (NH4VO3), nickel oxide (NiO), and zirconium oxide (ZrO2). The amount of the first metal dopant can be adjusted within a certain range to make the first precursor have a suitable first metal dopant content. Specifically, the metal dopant content (including the metal dopant content introduced by the first metal dopant) of the first precursor can be 0.1-0.5wt%, preferably 0.3-0.4wt%, based on the total weight of the first precursor.

[0063] The first carbon source can include at least one of glucose, sucrose, polyethylene glycol (PEG), citric acid, polyvinylpyrrolidone (PVP), and starch. A lower amount of the first carbon source is beneficial to obtaining a high-quality precursor with higher crystallinity, more regular morphology, and more concentrated size distribution. The obtained first precursor has a lower carbon content, and specifically, the carbon content of the first precursor can be 0.1-0.3wt%, based on the total weight of the first precursor.

[0064] The first mixture can further comprise a first supplement, which can include at least one of a phosphorus-containing compound and a lithium-containing compound, for adjusting the elemental ratio in the finished product and improving the purity of the material. Specifically, the first supplement can include at least one of phosphoric acid (H3PO4), ammonium dihydrogen phosphate (NH4H2PO4), lithium carbonate (Li2CO3), lithium hydroxide (LiOH), lithium acetate (CH3COOLi), lithium oxalate (Li2C2O4), and preferably at least one of phosphoric acid and lithium carbonate. The amount of the first supplement can be adjusted as needed to obtain a first precursor with a suitable phosphorus-iron-lithium molar ratio (P / Fe / Li), which can be 1:(0.950-0.970):(0.960-1.019).

[0065] The liquid medium used in the wet milling can include at least one of deionized water and anhydrous ethanol. Specifically, the conditions for the wet milling can include a temperature of 10-45°C, a sand milling particle size D50 of 0.7-0.9 μm, and a solid content of 30-50 wt%.

[0066] The drying can be spray drying, and the conditions for the drying can include an inlet temperature of 180-240°C, an outlet temperature of 80-140°C, a particle size D50 of 20-60 μm, and a moisture content of ≤1.5 wt%.

[0067] The first sintering can be performed in the presence of a protective gas, which can include at least one of nitrogen and argon. The conditions for the first sintering can include a temperature of 800-840°C and a time of 4-10 h. Further, the first precursor particles can be obtained by a gas flow crushing step after the first sintering.

[0068] In step S2, a second precursor is prepared using iron hydroxyphosphate containing a metal-doped element as a second iron-phosphorus source, which is used to prepare a third slurry with a sub-medium particle size (i.e., a third particle size) and a fourth slurry with a minimum particle size (i.e., a fourth particle size) in subsequent steps.

[0069] The metal-doped iron hydroxyl phosphate can be an in-situ interstitially doped iron hydroxyl phosphate, which is a composite iron phosphate source formed by embedding metal-doped elements in the form of interstitial atoms or substitution atoms in the crystal lattice during the crystallization process of the iron hydroxyl phosphate matrix. The metal-doped iron hydroxyl phosphate is used as the second iron phosphate source, which can on the one hand construct an ordered and stable doped metal-iron-phosphorus oxygen skeleton in the precursor stage, improve the electronic conductivity and lithium ion transmission efficiency of the lithium iron phosphate material, and inhibit lithium-iron ion mixing defects in the secondary sintering process, so as to realize grain refinement and control grain growth morphology, and on the other hand, due to the different doping systems of the first and second iron phosphate sources, the inter-particle fusion can be inhibited during the secondary sintering, so that the final product can still maintain the independence of the particles and the preset particle size distribution while realizing high crystallinity, thereby solving the performance degradation problem of traditional high compaction density materials caused by serious particle sintering.

[0070] The second iron phosphate source can have a high iron-phosphorus molar ratio and specific surface area to prepare secondary medium particles and small particles with a high iron-phosphorus molar ratio, thereby improving the theoretical specific capacity of the material. Specifically, the iron-phosphorus molar ratio (Fe / P) of the second iron phosphate source can be 1.44-1.50, and the BET specific surface area can be 30-100 m 2 / g. The metal-doped elements can include at least one of Ti, Nb, Mn, Mg, Ni, and Zr, and preferably at least one of Ti and Nb. Further, the content of the metal-doped elements in the second iron phosphate source is 0.1-0.4 wt% based on the total weight of the second iron phosphate source.

[0071] The metal-doped iron hydroxyl phosphate can be prepared by a co-precipitation method. In one specific embodiment, the preparation steps of the metal-doped iron hydroxyl phosphate can include:

[0072] (1) mixing titanium dioxide by-product ferrous sulfate, a phosphorus source, and a precipitating agent for pressure filtration purification to obtain a ferrous sulfate solution;

[0073] (2) adding an appropriate amount of phosphoric acid to the ferrous sulfate solution to reduce the pH value of the ferrous sulfate solution to 2-2.5;

[0074] (3) adding doped metal oxides, hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate solution, and ammonia water to the ferrous sulfate solution, and then reacting at 25-70°C for 2-6h to form a mixed slurry, and then washing and pressure filtering multiple times to obtain a metal element-doped iron hydroxyl phosphate precursor;

[0075] (4) flash drying the above iron hydroxyl phosphate precursor at an air inlet temperature of 220±20°C and an air outlet temperature of 110±10°C, and sintering at 530-560°C in an air atmosphere for 4-5h to obtain the metal-doped iron hydroxyl phosphate.

[0076] The lithium-phosphorus molar ratio (Li / P) of the second phosphorus-lithium source can be 1-3. Specifically, the second phosphorus-lithium source can include at least one of lithium phosphate, lithium hydrogen phosphate, lithium dihydrogen phosphate, and preferably at least one of lithium phosphate and lithium hydrogen phosphate. The amount of the second phosphorus-lithium source can be adjusted within a certain range. Specifically, the molar ratio of the second phosphorus-lithium source (calculated in terms of Li) to the second phosphorus-iron source (calculated in terms of Fe) can be 1.0-1.1, and preferably 1.03-1.08. The second phosphorus-lithium source preferably has a high purity (e.g., 98% or higher) to improve the quality of the lithium iron phosphate material.

[0077] The second metal dopant can include at least one of a titanium dopant, a niobium dopant, and a magnesium dopant. The introduction of the above-mentioned types of metal dopants is beneficial to refining the crystal grains and inhibiting the excessive growth of the crystal grains during the secondary sintering process. Specifically, the second metal dopant can include at least one of titanium dioxide (TiO2), niobium oxide (Nb2O5), and magnesium acetate (Mg(CH3COO)2). The amount of the second metal dopant can be adjusted within a certain range to enable the second precursor to have a suitable second metal dopant content. Specifically, the metal dopant content (including the total metal dopant content introduced by the second phosphorus-iron source and the second metal dopant) of the second precursor can be 0.1-0.7wt% based on the total weight of the second precursor, and preferably 0.5-0.6wt%.

[0078] The second carbon source can include at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinylpyrrolidone, and starch. The amount of the second carbon source can be adjusted within a lower range to enable the obtained second precursor to have a lower carbon content. Specifically, the carbon content of the second precursor can be 0.1-0.3wt% based on the total weight of the second precursor.

[0079] The second mixture can also contain a second supplement. The second supplement can include at least one of a phosphorus-containing compound and a lithium-containing compound. Specifically, the second supplement can include at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium carbonate, lithium hydroxide, lithium acetate, and lithium oxalate, and preferably at least one of phosphoric acid and lithium carbonate. The amount of the second supplement can be adjusted as needed to enable the obtained second precursor to have a suitable phosphorus-iron-lithium molar ratio. Specifically, the phosphorus-iron-lithium molar ratio (P / Fe / Li) of the second precursor can be 1:(0.970-1.000):(0.999-1.080).

[0080] The liquid medium used in wet grinding can include at least one of deionized water and anhydrous ethanol. Specifically, the conditions for wet grinding can include a temperature of 10-45°C, a sand grinding particle size D50 of 0.2-0.4μm, and a solid content of 30-50wt%.

[0081] The drying can be spray drying, and specifically, the conditions of the drying can include: a feeding port temperature of 180-240 DEG C, a discharging port temperature of 80-140 DEG C, a particle size D50 of 20-60 pm, and a moisture content of ≤1.5 wt%.

[0082] The second sintering can be performed in the presence of a protective gas, which can include at least one of nitrogen and argon, and specifically, the conditions of the second sintering can include: a temperature of 600-700 DEG C and a time of 3-5 h. Further, the second precursor particles can be obtained through a gas flow crushing step after the second sintering.

[0083] In steps S3-S6, the two precursor particles obtained in the above steps are respectively wet ground with the carbon sources to prepare four slurries with specific target particle sizes.

[0084] The third carbon source, the fourth carbon source, the fifth carbon source, and the sixth carbon source can each independently include at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinylpyrrolidone, and starch; and preferably include two of glucose, sucrose, polyethylene glycol, and polyvinylpyrrolidone. The amount of each carbon source (i.e., the third carbon source to the sixth carbon source) can be higher than that in the preparation of the precursor, which not only forms a complete and dense carbon coating layer in the final sintering, but also is beneficial to maintaining the particle size distribution, improving the compaction density and capacity, constructing a multi-dimensional composite conductive network, and reducing the internal resistance.

[0085] Specifically, in the third mixture, the weight ratio of the first precursor and the third carbon source can be 1000:(30-60). In the fourth mixture, the weight ratio of the first precursor and the fourth carbon source can be 1000:(30-60). In the fifth mixture, the weight ratio of the second precursor and the fifth carbon source can be 1000:(30-60). In the sixth mixture, the weight ratio of the second precursor and the sixth carbon source can be 1000:(30-60).

[0086] The liquid medium used in the wet grinding can include at least one of deionized water and anhydrous ethanol, and specifically, the conditions of the wet grinding can include: a temperature of 10-45 DEG C and a solid content of 30-50 wt%.

[0087] The particle size of the solid phase material in the first slurry, the second slurry, the third slurry and the fourth slurry decreases in turn. In an embodiment, the first particle size can be 1.0-2.0 μm, preferably 1.0-1.4 μm; the second particle size can be 0.5-1.0 μm, preferably 0.5-0.7 μm; the third particle size can be 0.2-0.5 μm, preferably 0.3-0.4 μm; and the fourth particle size can be 0.1-0.2 μm. The above particle size grading scheme can significantly improve the bulk density and build an excellent electrochemically active network, achieving ultra-high volumetric energy density while ensuring high capacity performance, low internal resistance and good rate performance.

[0088] In step S7, the proportions of the first slurry, the second slurry, the third slurry and the fourth slurry can be adjusted within a certain range. In an embodiment, the weight ratio of the first slurry, the second slurry, the third slurry and the fourth slurry in the seventh mixture can be 1:(0.5-1):(0.25-1):(0.25-1), preferably 1:(0.5-0.75):(0.25-0.75):(0.25-0.75). The above slurry proportioning is conducive to further improving the electrical properties of the lithium iron phosphate material.

[0089] The drying can be spray drying. Specifically, the drying conditions can include: the temperature of the feed inlet is 180-240 °C, the temperature of the discharge outlet is 80-140 °C, the particle size D50 is 20-60 μm, and the moisture content is ≤1.5 wt%.

[0090] The third sintering can be performed in the presence of a protective gas, which can include at least one of nitrogen and argon. Specifically, the third sintering conditions can include: being performed in a nitrogen atmosphere, the temperature is 750-800 °C, and the time is 4-8 h. Further, the lithium iron phosphate material can be obtained by a gas flow crushing step after the third sintering.

[0091] In a second aspect, the present disclosure provides a lithium iron phosphate material prepared by the method of the first aspect of the present disclosure.

[0092] The lithium iron phosphate material includes four types of particles, i.e., large particles, medium particles, sub-medium particles, and small particles, and exhibits a relatively continuous particle size distribution and quantity proportion. Specifically, the quantity proportion of particles with a particle size distribution of 1 μm or more can be 1-3 %, the quantity proportion of particles with a particle size distribution of 500-1000 nm can be 4-10 %, the quantity proportion of particles with a particle size distribution of 200-500 μm can be 15-30 %, and the quantity proportion of particles with a particle size distribution of 200 nm or less can be 65-80 %. In particular, the particles with a particle size distribution of 200-1000 nm have higher particle roundness and completeness, which is beneficial to ensuring the stability of the grading structure, achieving a higher compaction density, gram capacity, and lower powder resistivity, improving the electrochemical performance and long-term cycle stability. Specifically, the compaction density of the lithium iron phosphate material can be 2.64 g / cm 3 The powder resistivity can be 15 Ω·cm or less, preferably 10 Ω·cm or less.

[0093] The lithium iron phosphate material is used to prepare a lithium ion battery, which can significantly improve the energy density, rate performance, and cycle life of the battery, meet the development needs of high-end application markets such as long-range electric vehicles and high-efficiency energy storage systems, and has good industrial application prospects.

[0094] In a third aspect, the present disclosure provides a lithium ion battery comprising the lithium iron phosphate material of the second aspect.

[0095] The present disclosure does not have special limitations on the specific structure, packaging form, and preparation method of the lithium ion battery. For example, the lithium ion battery can include a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The positive electrode sheet can include a positive electrode current collector and a positive electrode material, the positive electrode material includes the lithium iron phosphate material of the second aspect of the present disclosure, the negative electrode sheet can include a negative electrode current collector and a negative electrode material (such as graphite), the electrolyte can be a carbonate electrolyte, and the separator can be a polyolefin separator. The packaging form of the lithium ion battery can be a soft pack battery, a square aluminum shell battery, a blade aluminum shell battery, etc. The lithium ion battery has the same beneficial effects as described above, and will not be described here.

[0096] The present disclosure is further described in detail through the following examples, but is not used to limit the present disclosure. The raw materials used in the examples can be obtained by commercial channels unless otherwise specified.

[0097] In the examples, the iron hydroxy phosphate is prepared as follows:

[0098] (1) The titanium white powder by-product ferrous sulfate, phosphoric acid, and the precipitating agent sodium hydroxide are mixed in a weight ratio of 1:(0.001-0.005):(0.005-0.007) for pressure filtration purification to obtain a ferrous sulfate solution;

[0099] (2) adding an appropriate amount of phosphoric acid into the ferrous sulfate solution to reduce the pH value of the ferrous sulfate solution to 2-2.5;

[0100] (3) adding hydrogen peroxide (concentration of 30-60 wt %), phosphoric acid, ammonium dihydrogen phosphate solution (concentration of 30 wt %) and ammonia (concentration of 28-29 wt %) into the ferrous sulfate solution to form a mixed slurry under the condition of 25-70℃ for 2-6h, and then washing and filtering the mixed slurry multiple times to obtain the iron hydroxyphosphate precursor; the weight ratio of ferrous sulfate, hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate and ammonia is 1000:145:78:332:91;

[0101] (4) flash drying the iron hydroxyphosphate precursor under the condition of air temperature of 220±20℃ and air outlet temperature of 110±10℃, and sintering the iron hydroxyphosphate precursor under the condition of air at 530-560℃ for 4-5h to obtain the iron hydroxyphosphate.

[0102] The iron hydroxyphosphate doped with Ti element is prepared as follows:

[0103] (1) mixing the by-product ferrous sulfate of titanium white powder, phosphoric acid and the precipitator sodium hydroxide according to the weight ratio of 1:(0.001-0.005):(0.005-0.007) to perform pressure filtration and purification to obtain a ferrous sulfate solution;

[0104] (2) adding an appropriate amount of phosphoric acid into the ferrous sulfate solution to reduce the pH value of the ferrous sulfate solution to 2-2.5;

[0105] (3) adding TiO2, hydrogen peroxide (concentration of 30-60 wt %), phosphoric acid, ammonium dihydrogen phosphate solution (concentration of 30 wt %) and ammonia (concentration of 28-29 wt %) into the ferrous sulfate solution to form a mixed slurry under the condition of 25-70℃ for 2-6h, and then washing and filtering the mixed slurry multiple times to obtain the iron hydroxyphosphate precursor; the weight ratio of ferrous sulfate, TiO2, hydrogen peroxide, phosphoric acid, ammonium dihydrogen phosphate and ammonia is 1000:2:145:78:332:91;

[0106] (4) flash drying the iron hydroxyphosphate precursor under the condition of air temperature of 220±20℃ and air outlet temperature of 110±10℃, and sintering the iron hydroxyphosphate precursor under the condition of air at 530-560℃ for 4-5h to obtain the iron hydroxyphosphate doped with Ti element.

[0107] The Zeiss Sigma 360 scanning electron microscope equipment is used to detect the micro-morphology of the lithium iron phosphate material, and the detection condition is: voltage of 10kV and shooting of 20k times. The particle size distribution of the lithium iron phosphate material is counted by Nano Measurer; the carbon content and the metal element content are detected by a carbon and sulfur analyzer.

[0108] Example 1

[0109] S1, 4000 g of iron hydroxyl phosphate (Fe / P is 1.43, BET specific surface area is 30 m 2 / g), 1230 g of lithium phosphate and 3.1 g of lithium carbonate are mixed, while 43.3 g of manganese acetate, 7.56 g of ammonium metavanadate, 137.6 g of glucose, 91.7 g of polyethylene glycol and 8300 g of deionized water are added in a ball mill to form a dispersion; the dispersion is moved to a sand mill and grinded under the conditions of temperature 20℃, particle size D50 of 0.8 μm and solid content of 40 wt%; the grinded slurry is spray dried, the inlet air temperature is controlled at 210℃, the outlet air temperature is controlled at 110℃, the granulation D50 is 40 μm and the moisture is ≤1.5 wt%; the obtained precursor powder is calcined at 820℃, the sintering time is 7 h and the protective atmosphere is nitrogen; the sintered material is crushed to obtain a first precursor (P / Fe / Li is 1:0.96:0.989, the carbon content is 0.2 wt%, the manganese content is 0.3 wt% and the vanadium content is 0.1 wt%).

[0110] S2, 4000 g of iron hydroxyl phosphate doped with Ti element (Fe / P is 1.47, BET specific surface area is 65 m 2 / g, Ti content is 0.23 wt%), 1203.7 g of lithium phosphate and 61.7 g of lithium carbonate are mixed, while 30.6 g of titanium dioxide, 137.6 g of glucose, 91.7 g of polyethylene glycol and 8300 g of deionized water are added in a ball mill to form a dispersion; the dispersion is moved to a sand mill and grinded under the conditions of temperature 20℃, particle size D50 of 0.3 μm and solid content of 40 wt%; the grinded slurry is spray dried, the inlet air temperature is controlled at 210℃, the outlet air temperature is controlled at 110℃, the granulation D50 is 40 μm and the moisture is ≤1.5 wt%; the obtained precursor powder is calcined at 650℃, the sintering time is 4 h and the protective atmosphere is nitrogen; the sintered material is crushed to obtain a second precursor (P / Fe / Li is 1:0.985:1.034, the carbon content is 0.2 wt%, the titanium content is 0.6 wt%).

[0111] S3, 4000 g of the first precursor, 120 g of polyvinylpyrrolidone, 40 g of glucose and 6000 g of deionized water are mixed to form a dispersion; the dispersion is moved to a sand mill and grinded under the conditions of temperature 20℃, particle size D50 of 1.2 μm and solid content of 40 wt% to obtain a first slurry with particle size of 1.2 μm.

[0112] S4. Mix 4000g of the first precursor, 120g of polyvinylpyrrolidone, 40g of glucose and 6000g of deionized water to form a dispersion; transfer the dispersion to a sand mill and grind it at a temperature of 20℃, a particle size D50 of 0.6μm and a solid content of 40wt% to obtain a second slurry with a particle size of 0.6μm.

[0113] S5. Mix 4000g of the second precursor, 120g of polyvinylpyrrolidone, 40g of glucose and 6000g of deionized water to form a dispersion; transfer the dispersion to a sand mill and grind it at a temperature of 20℃, a particle size D50 of 0.35μm and a solid content of 40wt% to obtain a third slurry with a particle size of 0.35μm.

[0114] S6. Mix 4000g of the second precursor, 120g of polyvinylpyrrolidone, 40g of glucose and 6000g of deionized water to form a dispersion; transfer the dispersion to a sand mill and grind it at a temperature of 20℃, a particle size D50 of 0.15μm and a solid content of 40wt% to obtain a fourth slurry with a particle size of 0.15μm.

[0115] S7. Mix the first, second, third, and fourth slurries in a weight ratio of 3:3:2:2 for 1 hour. Spray dry the resulting slurry, controlling the inlet air temperature at 210℃ and the outlet air temperature at 110℃, achieving a granulation D50 of 40μm and a moisture content ≤1.5wt%. Calcinate the resulting powder at 780℃ for 6 hours under a nitrogen atmosphere. Pulverize the sintered material to obtain lithium iron phosphate material with a carbon content of 1.3wt%. See electron micrograph for details. Figure 1 As shown, the lithium iron phosphate material has a dense packing structure formed by four-fold particle size distribution particles, of which the number of particles with a particle size distribution of more than 1000 nm accounts for 2%, the number of particles with a particle size distribution of 500~1000 nm accounts for 7%, the number of particles with a particle size distribution of 200~500 nm accounts for 20%, and the number of particles with a particle size distribution of less than 200 nm accounts for 71%.

[0116] Example 2

[0117] S1. 4000g of ferric hydroxyphosphate (Fe / P = 1.40, BET specific surface area = 4m²) 2 / g), 1221.6 g of lithium phosphate and 39.2 g of lithium carbonate are mixed, while 29.02 g of manganese acetate, 15.2 g of ammonium metavanadate, 184.3 g of polyethylene glycol, 115.2 g of citric acid and 5500 g of deionized water are added to a ball mill to form a dispersion; the dispersion is moved to a sand mill and ground at a temperature of 10°C, a particle size D50 of 0.9 μm and a solid content of 50 wt%; the obtained slurry is spray dried, with the inlet air temperature controlled at 180°C, the outlet air temperature at 80°C, the particle size D50 at 60 μm and the moisture content ≤1.5 wt%; the obtained precursor powder is calcined at 840°C, with the sintering time of 4 h and the protective atmosphere selected as nitrogen; the sintered material is crushed to obtain a first precursor (P / Fe / Li is 1:0.950:0.998, the carbon content is 0.1 wt%, the manganese content is 0.2 wt% and the vanadium content is 0.2 wt%).

[0118] S2, 4000 g of Ti-doped iron hydroxyphosphate (Fe / P is 1.50, the BET specific surface area is 100 m 2 / g, Ti content is 0.12 wt%), 1207.6 g of lithium phosphate and 6.4 g of phosphoric acid are mixed, while 38.6 g of titanium dioxide, 184.9 g of polyethylene glycol, 115.6 g of citric acid and 5500 g of deionized water are added to a ball mill to form a dispersion; the dispersion is moved to a sand mill and ground at a temperature of 10°C, a particle size D50 of 0.2 μm and a solid content of 50 wt%; the obtained slurry is spray dried, with the inlet air temperature controlled at 180°C, the outlet air temperature at 80°C, the particle size D50 at 60 μm and the moisture content ≤1.5 wt%; the obtained precursor powder is calcined at 700°C, with the sintering time of 3 h and the protective atmosphere selected as nitrogen; the sintered material is crushed to obtain a second precursor (P / Fe / Li is 1:1:1.03, the carbon content is 0.1 wt%, the titanium content is 0.6 wt%).

[0119] S3, 4000 g of the first precursor, 80 g of polyvinylpyrrolidone, 60 g of sucrose and 4000 g of deionized water are mixed to form a dispersion; the dispersion is moved to a sand mill and ground at a temperature of 10°C, a particle size D50 of 1.0 μm and a solid content of 50 wt% to obtain a first slurry with a particle size of 1.0 μm.

[0120] S4, 4000 g of the first precursor, 80 g of polyvinylpyrrolidone, 60 g of sucrose and 4000 g of deionized water are mixed to form a dispersion; the dispersion is moved to a sand mill and ground at a temperature of 10°C, a particle size D50 of 0.5 μm and a solid content of 50 wt% to obtain a second slurry with a particle size of 0.5 μm.

[0121] S5, 4000g of the second precursor, 80g of polyvinylpyrrolidone, 60g of sucrose and 4000g of deionized water are mixed to form a dispersion; the dispersion is moved to a sand mill and ground under the conditions of a temperature of 10℃, a particle size D50 of 0.3μm and a solid content of 50wt%, to obtain a third slurry with a particle size of 0.3μm.

[0122] S6, 4000g of the second precursor, 80g of polyvinylpyrrolidone, 60g of sucrose and 4000g of deionized water are mixed to form a dispersion; the dispersion is moved to a sand mill and ground under the conditions of a temperature of 10℃, a particle size D50 of 0.1μm and a solid content of 50wt%, to obtain a fourth slurry with a particle size of 0.1μm.

[0123] S7, the first slurry, the second slurry, the third slurry and the fourth slurry are mixed in a weight ratio of 4:3:2:1, the mixing time is 1h, the obtained mixed slurry is spray dried, the inlet air temperature is controlled to be 180℃, the outlet air temperature is controlled to be 80℃, the particle size D50 is 60μm, and the moisture content is ≤1.5wt%; the obtained powder is calcined at 800℃, the sintering time is 4h, and the protective atmosphere is selected to be nitrogen; the sintered material is crushed to obtain a lithium iron phosphate material, the carbon content of which is 1.1wt%, and the electron microscope photograph is similar to Figure 1 , wherein the number of particles with a particle size distribution of 1000nm or more accounts for 3%, the number of particles with a particle size distribution of 500-1000nm accounts for 7%, the number of particles with a particle size distribution of 200-500nm accounts for 22.5%, and the number of particles with a particle size distribution of 200nm accounts for 67.5%.

[0124] Example 3

[0125] S1, 4000g of iron hydroxyl phosphate (Fe / P is 1.46, BET specific surface area is 50m 2 / g), 1248.6g of lithium phosphate and 13.8g of lithium carbonate are mixed, 58.0g of manganese acetate, 138.2g of polyethylene glycol, 92.1g of sucrose and 12730g of deionized water are added at the same time to form a dispersion in a ball mill; the dispersion is moved to a sand mill and ground under the conditions of a temperature of 30℃, a particle size D50 of 0.7μm and a solid content of 30wt%; the obtained slurry is spray dried, the inlet air temperature is controlled to be 240℃, the outlet air temperature is controlled to be 140℃, the particle size D50 is 20μm, and the moisture content is ≤1.5wt%; the obtained precursor powder is calcined at 800℃, the sintering time is 10h, and the protective atmosphere is selected to be nitrogen; the sintered material is crushed to obtain a first precursor (P / Fe / Li is 1:0.970:1.019, the carbon content is 0.3wt%, and the manganese content is 0.4wt%).

[0126] S2, 4000 g of Ti-doped iron hydroxyl phosphate (Fe / P of 1.44, BET specific surface area of 30 m 2 / g, Ti content of 0.34 wt%), 1215.0 g of lithium phosphate, and 22.7 g of lithium carbonate were mixed, 23.1 g of titanium dioxide, 138.2 g of polyethylene glycol, 92.1 g of sucrose, and 12870 g of deionized water were added into a ball mill to form a dispersion; the dispersion was moved to a sand mill and ground under the conditions of a temperature of 30°C, a particle size D50 of 0.4 μm, and a solid content of 30 wt%; the obtained slurry was spray dried, the inlet air temperature was controlled to be 240°C, the outlet air temperature was controlled to be 140°C, the particle size D50 was controlled to be 20 μm, and the moisture content was controlled to be less than or equal to 1.5 wt%; the obtained precursor powder was calcined at 600°C, the sintering time was 5 h, and the protective atmosphere was nitrogen; and the sintered material was crushed to obtain a second precursor (P / Fe / Li of 1:0.970:1.047, carbon content of 0.3 wt%, and titanium content of 0.6 wt%).

[0127] S3, 4000 g of the first precursor, 120 g of polyvinylpyrrolidone, 100 g of polyethylene glycol, and 9300 g of deionized water were mixed to form a dispersion; the dispersion was moved to a sand mill and ground under the conditions of a temperature of 10°C, a particle size D50 of 1.4 μm, and a solid content of 30 wt% to obtain a first slurry with a particle size of 1.4 μm.

[0128] S5, 4000 g of the second precursor, 120 g of polyvinylpyrrolidone, 100 g of polyethylene glycol, and 9300 g of deionized water were mixed to form a dispersion; the dispersion was moved to a sand mill and ground under the conditions of a temperature of 10°C, a particle size D50 of 0.7 μm, and a solid content of 30 wt% to obtain a second slurry with a particle size of 0.7 μm.

[0129] S6, 4000 g of the second precursor, 120 g of polyvinylpyrrolidone, 100 g of polyethylene glycol, and 9300 g of deionized water were mixed to form a dispersion; the dispersion was moved to a sand mill and ground under the conditions of a temperature of 10°C, a particle size D50 of 0.4 μm, and a solid content of 30 wt% to obtain a third slurry with a particle size of 0.4 μm.

[0130] S7, 4000 g of the third precursor, 120 g of polyvinylpyrrolidone, 100 g of polyethylene glycol, and 9300 g of deionized water were mixed to form a dispersion; the dispersion was moved to a sand mill and ground under the conditions of a temperature of 10°C, a particle size D50 of 0.2 μm, and a solid content of 30 wt% to obtain a fourth slurry with a particle size of 0.2 μm.

[0131] S8, the first slurry, the second slurry, the third slurry, the fourth slurry are mixed in a weight ratio of 4:2:2:2, the mixing time is 1h, the obtained mixed slurry is spray dried, the inlet air temperature is controlled to be 240℃, the outlet air temperature is controlled to be 140℃, the granulation D50 is 20pm, the moisture content is less than or equal to 1.5wt%, the obtained powder is calcined at 750℃, the sintering time is 8h, and the protective atmosphere is selected to be nitrogen; the sintered material is crushed to obtain a lithium iron phosphate material, the carbon content of which is 1.5wt%, and the electron microscope photograph is similar to Figure 1 The particle size distribution of the lithium iron phosphate material prepared by the method of example 1 is similar to that of the lithium iron phosphate material prepared by the method of example 2, wherein the number of particles with a particle size distribution of more than 1000nm accounts for 1.5%, the number of particles with a particle size distribution of 500-1000nm accounts for 5%, the number of particles with a particle size distribution of 200-500nm accounts for 18%, and the number of particles with a particle size distribution of less than 200nm accounts for 75.5%.

[0132] Example 4

[0133] The lithium iron phosphate material is prepared by the method of example 1, and the difference lies in that in step S1, 30.6g of titanium dioxide is used instead of manganese acetate and ammonium metavanadate, and in the prepared lithium iron phosphate material, the number of particles with a particle size distribution of more than 1000nm accounts for 1%, the number of particles with a particle size distribution of 500-1000nm accounts for 9%, the number of particles with a particle size distribution of 200-500nm accounts for 25%, and the number of particles with a particle size distribution of less than 200nm accounts for 65%.

[0134] Example 5

[0135] The lithium iron phosphate material is prepared by the method of example 1, and the difference lies in that in step S1, the manganese content of the first precursor is 0.1wt%, and the vanadium content is 0.1wt%, and in step S2, the titanium content of the second precursor is 0.1wt%, and in the prepared lithium iron phosphate material, the number of particles with a particle size distribution of more than 1000nm accounts for 3%, the number of particles with a particle size distribution of 500-1000nm accounts for 9%, the number of particles with a particle size distribution of 200-500nm accounts for 21%, and the number of particles with a particle size distribution of less than 200nm accounts for 67%.

[0136] Example 6

[0137] The lithium iron phosphate material is prepared by the method of example 1, and the difference lies in that in step S7, the weight ratio of the first slurry, the second slurry, the third slurry, and the fourth slurry is 2.5:2.5:2.5:2.5, and in the prepared lithium iron phosphate material, the number of particles with a particle size distribution of more than 1000nm accounts for 1.5%, the number of particles with a particle size distribution of 500-1000nm accounts for 5%, the number of particles with a particle size distribution of 200-500nm accounts for 18%, and the number of particles with a particle size distribution of less than 200nm accounts for 75.5%.

[0138] Comparative Example 1

[0139] The lithium iron phosphate material was prepared according to the method of Example 1, except that in step S2, 4000 g of iron hydroxyphosphate (Fe / P = 1.47, BET specific surface area = 65 m 2 / g) was used to replace the Ti-doped iron hydroxyphosphate. The number of particles with a particle size distribution of 1000 nm or more accounted for 2.5%, the number of particles with a particle size distribution of 500-1000 nm accounted for 13%, the number of particles with a particle size distribution of 200-500 nm accounted for 32%, and the number of particles with a particle size distribution of 200 nm or less accounted for 52.5% in the prepared lithium iron phosphate material.

[0140] Comparative Example 2

[0141] The lithium iron phosphate material was prepared according to the method of Example 1, except that in step S1, 4000 g of iron phosphate (Fe / P = 0.960, BET specific surface area = 10 m 2 / g) was used to replace the iron hydroxyphosphate, and in step S2, 4000 g of iron phosphate (Fe / P = 0.985, BET specific surface area = 20 m 2 / g) was used to replace the Ti-doped iron hydroxyphosphate. The number of particles with a particle size distribution of 1000 nm or more accounted for 2%, the number of particles with a particle size distribution of 500-1000 nm accounted for 9%, the number of particles with a particle size distribution of 200-500 nm accounted for 40%, and the number of particles with a particle size distribution of 200 nm or less accounted for 49% in the prepared lithium iron phosphate material.

[0142] Comparative Example 3

[0143] The lithium iron phosphate material was prepared according to the method of Example 1, except that in step S7, the first slurry, the third slurry, and the fourth slurry were mixed in a weight ratio of 6:2:2, i.e., without adding the second slurry. The number of particles with a particle size distribution of 1000 nm or more accounted for 7%, the number of particles with a particle size distribution of 500-1000 nm accounted for 6%, the number of particles with a particle size distribution of 200-500 nm accounted for 25%, and the number of particles with a particle size distribution of 200 nm or less accounted for 62% in the prepared lithium iron phosphate material.

[0144] Test Example

[0145] The lithium iron phosphate materials of the examples and comparative examples were tested for compaction density, and assembled into batteries to test the electrical performance, with the results shown in Table 1.

[0146] The test method for compaction density was as follows: 1 g of lithium iron phosphate material sample was weighed in a compaction mold, and placed into a compaction device to start the test. The powder compaction density result was taken at a pressure of 30 KN.

[0147] The test method of the powder resistivity is the four-probe method.

[0148] The battery assembly method is as follows: the lithium iron phosphate material, Super-P and PVDF are dispersed in NMP according to the weight ratio of 80:10:10, uniformly ball-milled and dispersed, coated on an aluminum foil, vacuum dried, and an anode sheet is prepared, the electrolyte is 1 mol / L LiPF6, the volume ratio of the solvent is EC:DMC:EMC=1:1:1, the separator is a Celgard polypropylene film, the metal lithium sheet is the negative electrode, and the half-battery is assembled.

[0149] The first discharge capacity test method is as follows: the test voltage range is 2.0-3.75 V, the charging is performed in a constant current and constant voltage mode to 3.75 V, the discharging is performed in a constant current mode to 2.0 V, the charge and discharge current is 0.1 C for 1 cycle; and then the charge and discharge current is 1 C for 3 cycles, and the cutoff voltage condition is the same as that of 0.1 C.

[0150] Table 1

[0151]

[0152] As shown in Table 1, the lithium iron phosphate material prepared in the examples has higher compaction density and lower resistivity, and exhibits more excellent electrical performance for lithium ion batteries.

[0153] The preferred embodiments of the present disclosure are described in detail above with reference to the accompanying drawings, but the present disclosure is not limited to the specific details in the above-described embodiments, and various simple modifications can be made to the technical solutions of the present disclosure within the technical concept of the present disclosure, and these simple modifications all belong to the protection scope of the present disclosure.

[0154] In addition, it should be noted that each specific technical feature described in the above specific embodiments can be combined in any appropriate manner without contradiction, and in order to avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0155] In addition, various different embodiments of the present disclosure can also be combined in any manner, as long as they do not deviate from the idea of the present disclosure, and they should also be considered as disclosed by the present disclosure.

Claims

1. A method for producing a lithium iron phosphate material, characterized by, The method comprises: a first mixture containing a first phosphorus iron source, a first phosphorus lithium source, a first metal dopant and a first carbon source is wet ground, dried, and then first sintered to obtain a first precursor; a second mixture containing a second phosphorus iron source, a second phosphorus lithium source, a second metal dopant and a second carbon source is wet ground, dried, and then second sintered to obtain a second precursor; a third mixture containing the first precursor and a third carbon source is wet ground to obtain a first slurry with a first particle size; a fourth mixture containing the first precursor and a fourth carbon source is wet ground to obtain a second slurry with a second particle size; a fifth mixture containing the second precursor and a fifth carbon source is wet ground to obtain a third slurry with a third particle size; a sixth mixture containing the second precursor and a sixth carbon source is wet ground to obtain a fourth slurry with a fourth particle size; a seventh mixture containing the first slurry, the second slurry, the third slurry and the fourth slurry is dried, and then third sintered to obtain a lithium iron phosphate material; wherein the first phosphorus iron source is iron hydroxyphosphate, and the second phosphorus iron source is iron hydroxyphosphate containing a metal doping element; the first particle size, the second particle size, the third particle size and the fourth particle size decrease in turn.

2. The method of claim 1, wherein, The first phosphorus iron source has a molar ratio of iron to phosphorus of 1.40-1.46 and a BET specific surface area of 4-50 m 2 / g.

3. The method of claim 1, wherein, The iron-phosphorus molar ratio of the second phosphorus iron source is 1.44-1.50, and the BET specific surface area is 30-100 m 2 / g.

4. The method of claim 1, wherein, The metal doping element includes at least one of Ti, Nb, V, Mn, Mg, Ni and Zr.

5. The method of claim 1, wherein, The content of the metal doping element in the second phosphorus iron source is 0.1-0.4wt% based on the total weight of the second phosphorus iron source.

6. The method of claim 1, wherein, The first phosphorus lithium source and the second phosphorus lithium source each independently includes at least one of lithium phosphate, di-lithium hydrogen phosphate and dihydrogen lithium phosphate; The first metal dopant includes at least one of a manganese dopant, a vanadium dopant, a nickel dopant and a zirconium dopant; The second metal dopant includes at least one of a titanium dopant, a niobium dopant and a magnesium dopant; The first carbon source, the second carbon source, the third carbon source, the fourth carbon source, the fifth carbon source and the sixth carbon source each independently includes at least one of glucose, sucrose, polyethylene glycol, citric acid, polyvinylpyrrolidone and starch.

7. The method of claim 6, wherein, The first metal dopant includes at least one of manganese acetate, ammonium metavanadate, nickel oxide and zirconium oxide; The second metal dopant includes at least one of titanium dioxide, niobium oxide and magnesium acetate.

8. The method of claim 1, wherein, The first mixture further contains a first supplement, and the second mixture further contains a second supplement; The first supplement and the second supplement each independently includes at least one of phosphoric acid, ammonium dihydrogen phosphate, lithium carbonate, lithium hydroxide, lithium acetate and lithium oxalate.

9. The method of claim 1, wherein, The carbon content of the first precursor is 0.1-0.3wt% and the metal doping content is 0.1-0.5wt% based on the total weight of the first precursor; The carbon content of the second precursor is 0.1-0.3wt% and the metal doping content is 0.1-0.7wt% based on the total weight of the second precursor.

10. The method of claim 1, wherein, The weight ratio of the first precursor and the third carbon source in the third mixture is 1000: (30-60); The weight ratio of the first precursor and the fourth carbon source in the fourth mixture is 1000: (30-60); The weight ratio of the second precursor and the fifth carbon source in the fifth mixture is 1000: (30-60); The weight ratio of the second precursor and the sixth carbon source in the sixth mixture is 1000: (30-60).

11. The method of claim 1, wherein, The first particle size is 1.0-2.0 μm, the second particle size is 0.5-1.0 μm, the third particle size is 0.2-0.5 μm, and the fourth particle size is 0.1-0.2 μm; And / or, The weight ratio of the first slurry, the second slurry, the third slurry and the fourth slurry in the seventh mixture is 1: (0.5-1): (0.25-1): (0.25-1).

12. The method of claim 1, wherein, The first sintering condition includes a temperature of 800-840 ℃ and a time of 4-10 h; The second sintering condition includes a temperature of 600-700 ℃ and a time of 3-5 h; The third sintering condition includes a temperature of 750-800 ℃ and a time of 4-8 h.

13. A lithium iron phosphate material prepared by the method of any one of claims 1-12.

14. A lithium-ion battery, characterized by The lithium iron phosphate material of claim 13.

Images