Iron phosphate graded lithium iron phosphate material and preparation method thereof

Abstract

This invention discloses a graded lithium iron phosphate material and its preparation method, belonging to the field of positive electrode active materials for lithium-ion batteries. The method involves preparing two types of iron phosphate using a co-precipitation method, with at least one type being a doped iron phosphate. The two types of iron phosphate are coarsely ground and mixed in deionized water or methanol to obtain a uniform iron phosphate slurry. A lithium source, carbon source, and additives are added to the iron phosphate slurry, along with deionized water or methanol, and the mixture is finely ground and mixed to obtain a uniform mixed slurry. The mixed slurry is granulated using a spray granulation device to obtain a dry powder. Under nitrogen protection, the powder is sintered in a kiln, and the sintered material is obtained after furnace cooling. The sintered material is then crushed, graded, and sieved to obtain the graded lithium iron phosphate material. The lithium iron phosphate material prepared by this invention can be used as a positive electrode active material for automotive power lithium batteries, providing better power performance.

Description

Technical Field

[0001] This invention belongs to the field of positive electrode active materials for lithium-ion batteries, and relates to a lithium iron phosphate material with iron phosphate gradation and its preparation method. Background Technology

[0002] Currently, the most widely used positive electrode active material for automotive power lithium batteries is lithium iron phosphate (LiFePO4) with an olivine structure. This material has usable energy density, low production cost, and environmentally friendly properties. To meet the further demands of the electric vehicle market—namely, to enable lithium batteries to provide longer driving range and be fully charged in a shorter time—positive electrode active materials need to have higher power performance. This means that LiFePO4 per unit mass or unit volume needs to store or release more energy at high rates (mainly 1.0C).

[0003] There are two main directions for improving the power performance of LiFePO4: one is to increase the compaction density of the powder, which can increase the energy density per unit volume in the battery; the other is to increase the discharge specific capacity and voltage plateau of the material at high rates. This increases the energy density per unit mass of the material itself, while the voltage plateau is determined by the crystal characteristics of the material and has a very small adjustable range. Therefore, it actually increases the discharge specific capacity.

[0004] Grading is the most commonly used method to improve the compaction density of powder, and there are various grading methods. For example, BTR (Tianjin) Nanomaterials Manufacturing Co., Ltd. proposed in patent CN113562714A that using iron phosphate with mixed large and small particles as raw material, the graded lithium iron phosphate has a high powder compaction density. This grading method has a simple process, but it has special requirements on the particle size of the raw material. Another example is Changzhou Lithium Source New Energy Technology Co., Ltd., which proposed in patent CN114314550A that adjusting the flow rate of a centrifugal spray peristaltic pump and a two-fluid spray peristaltic pump can prepare a precursor with mixed large and small particles, which is then sintered to prepare graded lithium iron phosphate. This grading method has very high requirements for the control of process parameters in the spray drying process.

[0005] Doping is the most common method to improve discharge specific capacity, and there are many types of doping elements and methods. For example, Guangdong Bangpu Recycling Technology Co., Ltd. proposed in patent CN115367725A to first prepare Mn, Co, Ni, Al, and Ti-doped iron phosphate using a co-precipitation method, and then use this raw material to prepare doped lithium iron phosphate. Another example is Sichuan Wanpeng Times Technology Co., Ltd., which proposed in patent CN115806283A to add additives containing Sr, B, Sn, Mg, Ca, Ti, Nb, and V during mixing, and then prepare doped lithium iron phosphate. Even with the same element types and doping amounts, doping in the precursor and doping during mixing have different effects on the performance of lithium iron phosphate because the preparation process also affects the doping effect. Summary of the Invention

[0006] The purpose of this invention is to provide a lithium iron phosphate material with iron phosphate gradation and its preparation method. This lithium iron phosphate material can be used as a positive electrode active material for automotive power lithium batteries and can provide better power performance.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing a graded lithium iron phosphate material includes the following steps:

[0009] (1) Two types of iron phosphate were prepared by co-precipitation method, wherein at least one of the iron phosphates was a doped iron phosphate, and the doping element was one or more of Mg, Al, Ti, Zr, Mn, Co, Ni, V, and Nb; if both iron phosphates were doped iron phosphates, then the types of doping elements and / or the total mass of doping elements of the two iron phosphates were different.

[0010] (2) Add the two types of ferric phosphate to deionized water or methanol and coarsely grind and mix to obtain a uniform ferric phosphate slurry;

[0011] (3) Add lithium source, carbon source, and additives to the above iron phosphate slurry, and also add deionized water or methanol, and grind and mix them to obtain a uniform mixed slurry.

[0012] (4) The above-mentioned mixed slurry is granulated using a spray granulation device to obtain a dry powder material;

[0013] (5) The above powder material is sintered in a kiln under nitrogen protection and then cooled in the kiln to obtain the sintered material.

[0014] (6) The sintered material is crushed, graded and sieved to obtain iron phosphate graded lithium iron phosphate material.

[0015] Furthermore, in step (1), the total mass of doped elements in one type of ferric phosphate accounts for 0.3 ≤ a ≤ 1.0% of the total mass of ferric phosphate, while the total mass of doped elements in the other type of ferric phosphate accounts for 0 ≤ b ≤ 0.5% of the total mass of ferric phosphate.

[0016] Furthermore, in step (1), the Fe / P molar ratio in the two types of ferric phosphate is 0.93:1 to 0.97:1.

[0017] Furthermore, in step (2), the mass ratio of the two types of iron phosphate is 4:1 to 1:4.

[0018] Furthermore, the particle size of the solid particles in the ferric phosphate slurry obtained in step (2) is 1 to 2 μm.

[0019] Furthermore, the solid content of the ferric phosphate slurry obtained in step (2) is 25% to 55%.

[0020] Furthermore, in step (3), the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium hydroxide monohydrate, lithium acetate, and lithium oxalate.

[0021] Furthermore, the lithium-phosphorus molar ratio Li / P of the mixed slurry obtained in step (3) is 0.99:1 to 1.01:1.

[0022] Furthermore, in step (3), the carbon source is one or more of glucose, fructose, sucrose, maltodextrin, cyclodextrin, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), carbon nanotubes (CNT), and graphite sheets.

[0023] Furthermore, in step (3), the carbon source accounts for 12% to 18% of the total mass of the two iron phosphates.

[0024] Furthermore, the additive in step (3) contains one or more of the doping elements Mg, Al, Ti, Zr, Mn, Co, Ni, V, and Nb.

[0025] Furthermore, in step (3), the total mass of the dopant elements contained in the additive accounts for 0 to 1.0% of the total mass of the two types of iron phosphate.

[0026] Furthermore, the nanoparticle size of the solid particles in the mixed slurry obtained in step (3) is 250–450 nm.

[0027] Furthermore, the solid content of the mixed slurry obtained in step (3) is 25% to 55%.

[0028] Furthermore, the particle size of the powder material obtained in step (4) is 2≤D50≤15μm, D100<40μm.

[0029] Furthermore, the sintering process in step (5) is divided into two stages: the sintering temperature of the first stage is 300-650℃ and the holding time is 2-6 hours; the sintering temperature of the second stage is 700-800℃ and the holding time is 6-12 hours.

[0030] A lithium iron phosphate material with iron phosphate gradation is prepared by the above-described preparation method. The particle size distribution of this lithium iron phosphate material is 0.8 ≤ D50 ≤ 1.5 μm, D100 ≤ 10 μm, with a carbon content of 1.2–2.0 wt.%, and a compaction density (PD) of the dried powder at 20 MPa ≥ 2.54 g / cm³. 3 At room temperature (1.0C), the discharge specific capacity reaches over 140 mAh / g.

[0031] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0032] 1. Two Ferric Phosphate Gradients: This invention uses two ferric phosphate gradients, at least one of which is a doped ferric phosphate. Both ferric phosphates are prepared by co-precipitation, where the dopant element is incorporated and uniformly distributed within the ferric phosphate. If both ferric phosphates are doped, then at least one type of dopant element and the total mass of dopant elements will differ between the two types. This difference allows the two doped ferric phosphates to exert different modifying effects during gradation. For example, using two ferric phosphate gradients with different types of dopant elements but the same total mass of dopant elements to prepare lithium iron phosphate materials, the Co-doped ferric phosphate produces lithium iron phosphate with higher electronic conductivity, while the Ti-doped ferric phosphate produces lithium iron phosphate with a higher Li diffusion coefficient. Both can improve the discharge specific capacity and thus increase the energy density. For example, lithium iron phosphate materials can be prepared using two iron phosphate gradations with the same type of doping elements but different total masses of doping elements. The iron phosphate with a larger total mass of doping elements produces small-particle lithium iron phosphate, which improves the discharge specific capacity. The iron phosphate with a smaller total mass of doping elements produces small-particle lithium iron phosphate and some large-particle lithium iron phosphate, which improves the powder compaction density and also improves the energy density.

[0033] 2. Synergistic Effect of Two Particle Sizes: This invention uses two types of iron phosphate with different total mass of doped elements as raw materials to prepare lithium iron phosphate materials with particle size distribution. During sintering, iron phosphate with a larger total mass of doped elements forms small lithium iron phosphate particles (D50 of 100–500 nm) because the large amount of doped elements can hinder grain or particle growth; iron phosphate with a smaller total mass of doped elements forms large lithium iron phosphate particles (D50 of 1.0–1.5 μm) because the small amount of doped elements cannot completely hinder particle growth. This lithium iron phosphate material with particle size distribution has both high powder compaction density and a 1.0C discharge specific capacity. In this type of graded lithium iron phosphate material, the majority of small particles provide excellent electrical performance because their electronic conductivity and Li diffusion coefficient are higher than those of large particles; the minority of large particles provide high powder compaction density without significantly reducing electrical performance. Typically, if three regions of this type of graded lithium iron phosphate material are randomly selected, the number of large particles will not exceed 10 in a 5k magnification SEM image, and will not exceed 3 in a 10k magnification SEM image. This is not only due to the low number of large particles, but also because elemental doping plays a modifying role.

[0034] 3. Role of Doping Elements: Most of the doping elements used in this invention have limited solid solution in the lithium iron phosphate (LiFePO4) lattice, and the total mass of Fe or Li that can be replaced in the lattice is very small. Only the three transition metal elements Mn, Co, and Ni can be completely dissolved and replace Fe in any proportion. However, according to actual modification needs, the total mass of these three transition metals will not exceed the design upper limit of 1.0%. Large lithium iron phosphate particles require more element doping inside the lattice to simultaneously improve the electronic conductivity and Li diffusion coefficient inside the particles during room temperature / low temperature charge and discharge. During sintering, some lithium iron phosphate particles with fewer doping elements will be doped inside the lattice along with the formation of lithium iron phosphate, and some will be enriched at the grain boundaries inside the large particles. Small lithium iron phosphate particles require more element doping on the grains or particle surfaces to prevent grain or particle adhesion and growth during sintering, thereby improving the discharge specific capacity. In iron phosphate with a high doping element content, a small portion is doped into the crystal lattice along with the formation of lithium iron phosphate, while most of it is enriched on the surface of the grains or small particles. This can also reduce the amount of Fe leaching during high-temperature cycling or storage.

[0035] 4. Role of additives: Since different doping processes can affect element distribution and doping effect, in addition to using at least one doped iron phosphate, the preparation method of this invention also adopts a mixed doping process. In the mixed process of preparing lithium iron phosphate, doping elements are also added by means of additives. The main role of these doping elements is to prevent the adhesion and agglomeration between large and small particles during sintering, and to ensure that the D100 of the graded lithium iron phosphate material is ≤10μm. Attached Figure Description

[0036] Figure 1 This is a SEM image of the particle morphology of the lithium iron phosphate material of the iron phosphate graded type in Example 2.

[0037] Figure 2 This is a SEM image of the particle morphology of lithium iron phosphate material in Comparative Example 2.

[0038] Figure 3 These are the 1.0C discharge curves of the two lithium iron phosphate materials in Example 2 and Comparative Example 2.

[0039] Figure 4 This is a SEM image of the particle morphology of the lithium iron phosphate material of the iron phosphate graded type in Example 3.

[0040] Figure 5 This is a SEM image of the particle morphology of lithium iron phosphate material in Comparative Example 3.

[0041] Figure 6 These are the 1.0C discharge curves of the two lithium iron phosphate materials in Example 3 and Comparative Example 3.

[0042] Figure 7 This is a SEM image of the particle morphology of the lithium iron phosphate material of the iron phosphate graded type in Example 5.

[0043] Figure 8 This is a SEM image of the particle morphology of lithium iron phosphate material in Comparative Example 5.

[0044] Figure 9 These are the 1.0C discharge curves of the two lithium iron phosphate materials in Example 5 and Comparative Example 5. Detailed Implementation

[0045] To make the various technical features, advantages, or effects of the present invention more apparent and understandable, a detailed description is provided below in conjunction with the accompanying drawings.

[0046] Example 1

[0047] Two types of ferric phosphate (Fe / P molar ratio between 0.93 and 0.97, the same below) were prepared by co-precipitation. Ferric phosphate A contained 1.0 wt.% Nb, and ferric phosphate B contained 0 wt.% Nb. The two ferric phosphates were added to deionized water at a mass ratio of A:B = 4:1, with a solid content of 25%. The mixture was coarsely ground to 1 μm using a basket mill. Lithium carbonate, glucose, PEG, and deionized water were added to the ferric phosphate slurry, with a Li / P ratio of 0.99 and a total carbon source mass accounting for 18% of the total ferric phosphate mass, adjusting the slurry's solid content to 30%. The mixture was then finely ground to 250 nm using a horizontal sand mill. Granulation was performed using a centrifugal spray dryer, followed by calcination of the dried powder. The first sintering temperature was 300℃, with a holding time of 6 hours; the second sintering temperature was 800℃, with a holding time of 6 hours. After cooling, the material was crushed, graded, and sieved to obtain ferric phosphate graded lithium iron phosphate.

[0048] Example 2

[0049] Two types of ferric phosphate were prepared by co-precipitation. Ferric phosphate A contained 0.8 wt.% Ti, and ferric phosphate B contained 0.2 wt.% Ti. The two ferric phosphates were added to deionized water at a mass ratio of A:B = 2:1, with a solid content of 30%. The mixture was coarsely ground using a basket mill to a particle size of 1.2 μm. Lithium carbonate, cyclodextrin, PVP, and deionized water were added to the ferric phosphate slurry, with a Li / P ratio of 0.995 and a total carbon source mass of 16% of the total ferric phosphate mass, adjusting the slurry's solid content to 35%. The mixture was then finely ground using a horizontal mill to a particle size of 300 nm. The granules were granulated using a centrifugal spray dryer, and then the dried powder was calcined. The first sintering temperature was 400℃ for 5 hours, and the second sintering temperature was 775℃ for 8 hours. After cooling, the material was crushed, graded, and sieved to obtain ferric phosphate graded lithium iron phosphate.

[0050] Example 3

[0051] Two types of Mg / Ti co-doped iron phosphate were prepared by co-precipitation. Iron phosphate A contained 0.3 wt.% Mg and 0.6 wt.% Ti, while iron phosphate B contained 0.1 wt.% Mg and 0.2 wt.% Ti. The two types of Mg / Ti co-doped iron phosphate were added to methanol at a mass ratio of A:B = 1:1, with a solid content of 35%. The mixture was coarsely ground to 1.5 μm using a basket mill. Lithium hydroxide, fructose, CNTs, nano-titanium dioxide, and methanol were added to the iron phosphate slurry, with a Li / P ratio of 1.000. The total mass of the carbon source accounted for 15% of the total mass of iron phosphate, and the Ti content of the nano-titanium dioxide was 0.2 wt.% of the total mass of iron phosphate. The solid content of the slurry was adjusted to 40%. The mixture was finely ground to 350 nm using a horizontal mill. The granules were then granulated using a centrifugal spray dryer, and the dried powder was calcined. The first stage of sintering was carried out at a temperature of 500℃ for 4 hours, and the second stage was carried out at a temperature of 750℃ for 9 hours. After cooling, the material was crushed, graded, and sieved to obtain lithium iron phosphate graded lithium iron phosphate.

[0052] Example 4

[0053] Two types of Nb-doped ferric phosphate were prepared by co-precipitation: ferric phosphate A contained 0.6 wt.% Nb, and ferric phosphate B contained 0.3 wt.% Nb. The two types of Nb-doped ferric phosphate were added to methanol at a mass ratio of A:B = 1:2, with a solid content of 40%. The mixture was coarsely ground to 1.8 μm using a basket mill. Lithium hydroxide, sucrose, graphite flakes, tetrabutyl titanate, ammonium metavanadate, and methanol were added to the ferric phosphate slurry, where the Li / P ratio was 1.005, the total carbon source mass accounted for 14% of the total ferric phosphate mass, the Ti content of tetrabutyl titanate was 0.1 wt.% of the total ferric phosphate mass, and the V content of ammonium metavanadate was 0.4 wt.% of the total ferric phosphate mass. The solid content of the slurry was adjusted to 45%. The mixture was finely ground to 400 nm using a horizontal sand mill. The granules were then granulated using a centrifugal spray dryer, and the dried powder was calcined. The first stage of sintering was carried out at a temperature of 600℃ for 3 hours, and the second stage was carried out at a temperature of 725℃ for 10 hours. After cooling, the material was crushed, graded, and sieved to obtain lithium iron phosphate graded lithium iron phosphate.

[0054] Example 5

[0055] Two types of Co / Nb co-doped iron phosphate were prepared by co-precipitation. Iron phosphate A contained 0.1 wt% Co and 0.2 wt% Nb, while iron phosphate B contained 0.1 wt% Co and 0.1 wt% Nb. The two types of Co / Nb co-doped iron phosphate were added to methanol at a mass ratio of A:B = 1:4, with a solid content of 50%. The mixture was coarsely ground to 2 μm using a basket mill. Lithium hydroxide monohydrate, maltobiose, PEG, PVP, hydrated titanium dioxide, vanadium pentoxide, and methanol were added to the iron phosphate slurry, with a Li / P ratio of 1.01. The total mass of carbon sources accounted for 12% of the total mass of iron phosphate. The Ti content of the hydrated titanium dioxide was 0.5 wt% of the total mass of iron phosphate, and the V content of the vanadium pentoxide was 0.5 wt% of the total mass of iron phosphate. The solid content of the slurry was adjusted to 55%. The mixture was finely ground to 450 nm using a horizontal mill. The material was granulated using a centrifugal spray dryer, and then the dried powder was calcined. The first stage sintering temperature was 650℃, and the holding time was 2 hours. The second stage sintering temperature was 700℃, and the holding time was 12 hours. After cooling, the material was crushed, graded, and sieved to obtain lithium iron phosphate graded material.

[0056] Example 6

[0057] Two types of doped ferric phosphate were prepared by co-precipitation. Ferric phosphate A contained 0.8 wt.% Nb, while ferric phosphate B contained 0.2 wt.% Mg and 0.6 wt.% Ti. The two doped ferric phosphates were added to deionized water at a mass ratio of A:B = 1:3, with a solid content of 55%. The mixture was coarsely ground to 1.4 μm using a basket mill. Lithium hydroxide monohydrate, lithium oxalate, cyclodextrin, PEG, PVP, nano-titanium dioxide, and deionized water were added to the ferric phosphate slurry, where the Li / P ratio was 0.993, the total carbon source mass accounted for 13% of the total ferric phosphate mass, and the Ti content of the nano-titanium dioxide was 0.3 wt.% of the total ferric phosphate mass. The solid content of the slurry was adjusted to 35%. The mixture was finely ground to 280 nm using a horizontal sand mill. The granules were then granulated using a centrifugal spray dryer, and the dried powder was calcined. The first stage sintering temperature is 550℃, and the holding time is 3 hours. The second stage sintering temperature is 780℃, and the holding time is 10 hours. After cooling, the material is crushed, graded, and sieved to obtain lithium iron phosphate graded lithium iron phosphate.

[0058] Example 7

[0059] Two types of doped iron phosphate were prepared by co-precipitation. Iron phosphate A contained 0.4 wt.% Ti, while iron phosphate B contained 0.1 wt.% Co and 0.2 wt.% Nb. The two doped iron phosphates were added to methanol at a mass ratio of A:B = 3:1, with a solid content of 45%. The mixture was coarsely ground to 1.6 μm using a basket mill. Lithium hydroxide, lithium acetate, glucose, carbon nanotubes, graphite flakes, hydrated titanium dioxide, vanadium pentoxide, and methanol were added to the iron phosphate slurry, with a Li / P ratio of 1.003. The total mass of the carbon source accounted for 17% of the total mass of iron phosphate. The Ti content of the hydrated titanium dioxide was 0.4 wt.% of the total mass of iron phosphate, and the V content of the vanadium pentoxide was 0.1 wt.% of the total mass of iron phosphate. The solid content of the slurry was adjusted to 25%. The mixture was finely ground to 380 nm using a horizontal sand mill. The granules were then calcined after centrifugal spray drying. The first stage sintering temperature was 450℃, and the holding time was 4 hours. The second stage sintering temperature was 740℃, and the holding time was 12 hours. After cooling, the material was crushed, graded, and sieved to obtain lithium iron phosphate graded lithium iron phosphate.

[0060] Comparative Example 1

[0061] As a comparative example of Example 1, Nb-doped iron phosphate was prepared by co-precipitation, wherein the Nb content was 0.8 wt.%. The Nb-doped iron phosphate was added to deionized water with a solid content of 25%, and coarsely ground to 1 μm using a basket mill. Lithium carbonate, glucose, PEG, and deionized water were added to the iron phosphate slurry, wherein the Li / P ratio was 0.99, the total mass of the carbon source accounted for 18% of the total mass of iron phosphate, and the solid content of the slurry was adjusted to 30%. The mixture was then finely ground to 250 nm using a horizontal sand mill. Granulation was performed using a centrifugal spray dryer, followed by calcination of the dried powder. The first stage sintering temperature was 300°C, and the holding time was 6 hours; the second stage sintering temperature was 800°C, and the holding time was 6 hours. After cooling, the material was crushed, graded, and sieved to obtain lithium iron phosphate.

[0062] Comparative Example 2

[0063] As a comparative example of Example 2, Ti-doped iron phosphate was prepared by co-precipitation, wherein the Ti content was 0.6 wt.%. The Ti-doped iron phosphate was added to deionized water with a solid content of 30%, and coarsely ground to 1.2 μm using a basket mill. Lithium carbonate, cyclodextrin, PVP, and deionized water were added to the iron phosphate slurry, wherein the Li / P ratio was 0.995, the total mass of the carbon source was 16% of the total mass of iron phosphate, and the solid content of the slurry was adjusted to 35%. The mixture was then finely ground to 300 nm using a horizontal sand mill. Granulation was performed using a centrifugal spray dryer, followed by calcination of the dried powder. The first stage sintering temperature was 400°C, and the holding time was 5 hours; the second stage sintering temperature was 775°C, and the holding time was 8 hours. After cooling, the material was crushed, graded, and sieved to obtain lithium iron phosphate.

[0064] Comparative Example 3

[0065] As a comparative example of Example 3, Mg / Ti co-doped iron phosphate was prepared by co-precipitation, wherein the Mg content was 0.2 wt.% and the Ti content was 0.4 wt.%. The Mg / Ti co-doped iron phosphate was added to methanol, with a solid content of 35%, and coarsely ground to 1.5 μm using a basket mill. Lithium hydroxide, fructose, CNTs, nano-titanium dioxide, and methanol were added to the iron phosphate slurry, wherein the Li / P ratio was 1.000, the total mass of the carbon source accounted for 15% of the total mass of iron phosphate, and the Ti content of the nano-titanium dioxide was 0.2 wt.% of the total mass of iron phosphate, adjusting the solid content of the slurry to 40%. The mixture was finely ground to 350 nm using a horizontal mill. Granulation was performed using a centrifugal spray dryer, followed by calcination of the dried powder. The first stage sintering temperature was 500℃, and the holding time was 4 hours; the second stage sintering temperature was 750℃, and the holding time was 9 hours. After cooling, the material was crushed, graded, and sieved to obtain lithium iron phosphate.

[0066] Comparative Example 4

[0067] As a comparative example of Example 4, Nb-doped iron phosphate was prepared by co-precipitation, wherein the Nb content was 0.3 wt.%. The Nb-doped iron phosphate was added to methanol, with a solid content of 40%, and coarsely ground to 1.8 μm using a basket mill. Lithium hydroxide, sucrose, graphite flakes, tetrabutyl titanate, ammonium metavanadate, and methanol were added to the iron phosphate slurry, wherein the Li / P ratio was 1.005, the total mass of the carbon source accounted for 14% of the total mass of iron phosphate, the Ti content of tetrabutyl titanate was 0.1 wt.% of the total mass of iron phosphate, and the V content of ammonium metavanadate was 0.4 wt.% of the total mass of iron phosphate, adjusting the solid content of the slurry to 45%. The mixture was finely ground to 400 nm using a horizontal mill. Granulation was performed using a centrifugal spray dryer, followed by calcination of the dried powder. The sintering temperature for the first stage was 600°C, and the holding time was 3 hours; the sintering temperature for the second stage was 725°C, and the holding time was 10 hours. After the material is cooled, it is crushed, graded, and sieved to obtain lithium iron phosphate.

[0068] Comparative Example 5

[0069] As a comparative example of Example 5, Co / Nb co-doped iron phosphate was prepared by co-precipitation, wherein the Co content was 0.1 wt.% and the Nb content was 0.12 wt.%. The Co / Nb co-doped iron phosphate was added to methanol, with a solid content of 50%, and coarsely ground to 2 μm using a basket mill. Lithium hydroxide monohydrate, maltobiose, PEG, PVP, hydrated titanium dioxide, vanadium pentoxide, and methanol were added to the iron phosphate slurry, wherein the Li / P ratio was 1.01, the total mass of carbon sources accounted for 12% of the total mass of iron phosphate, the Ti content of hydrated titanium dioxide was 0.5 wt.% of the total mass of iron phosphate, and the V content of vanadium pentoxide was 0.5 wt.% of the total mass of iron phosphate, adjusting the solid content of the slurry to 55%. The mixture was finely ground to 450 nm using a horizontal mill. Granulation was performed using a centrifugal spray dryer, and then the dried powder was calcined. The first stage of sintering was carried out at a temperature of 650℃ for 2 hours, and the second stage was carried out at a temperature of 700℃ for 12 hours. After cooling, the material was crushed, graded, and sieved to obtain lithium iron phosphate.

[0070] Experimental Test

[0071] The lithium iron phosphate materials prepared in Examples 2 and 2, 3 and 3, and 5 and 5 were used as representatives to test the charge-discharge performance of batteries as positive electrode active materials. The specific tests are as follows:

[0072] Lithium iron phosphate material, conductive carbon material, and polyvinylidene chloride binder were mixed in a mass ratio of 90:5:5. N-methylpyrrolidone was added dropwise, and the mixture was ground into a paste. This paste was then coated onto aluminum foil, dried at 120°C, cut into sheets, and then seamlessly rolled to obtain the positive electrode test electrode. The active material loading on the positive electrode sheet was controlled at 9–10 mg, and the electrode compaction density was controlled at 2.35–2.40 g / cm³. 2 .

[0073] The coin cell model is CR2032. The counter electrode (reference electrode) is a lithium metal sheet, and the electrolyte is a product from Shenzhen Capchem Technology Co., Ltd. The coin cells were assembled in a glove box with controlled moisture and oxygen levels. The coin cells were placed in a 25℃ constant temperature chamber for 24 hours, and then connected to a blue-chip battery tester for charge-discharge testing. The coin cells were placed in a 25℃ constant temperature chamber for charge-discharge testing. The nominal capacity at 1C was set at 170mAh / g. Within the range of 2.0 to 3.75V, the cells were first activated at 0.1C for one week, then charged and discharged at 1.0C. At a constant voltage cutoff current of 0.02C at 3.75V, the charge-discharge curves were observed.

[0074] from Figure 1 and Figure 2 As can be seen, both the lithium iron phosphate materials in Example 2 and Comparative Example 2 exhibit a large and small particle size distribution morphology, but the large and small particles in Example 2 are larger, resulting in a higher powder compaction density. Figure 3 As can be seen from Table 1, the lithium iron phosphate materials of Example 2 and Comparative Example 2 have the same discharge specific capacity at 0.1C and 1.0C. Assuming that the discharge voltage is controlled at 3.38V in actual use, and combining the formulas for powder compaction density and energy density: Energy density per unit volume = Powder compaction density × Discharge specific capacity × Discharge voltage, it can be calculated that Example 2 can provide a higher volumetric energy density in the battery.

[0075] from Figure 4 and Figure 5 As can be seen, the lithium iron phosphate material in Comparative Example 3 contains only small particles, while the lithium iron phosphate graded material in Example 3 is mainly composed of small particles with a small amount of large particles, resulting in a higher powder compaction density. Figure 6 As can be seen from the table, the lithium iron phosphate materials of Comparative Example 3 and Example 3 have similar discharge specific capacities at 1.0C. Table 1 shows that Example 3 has a higher volumetric energy density.

[0076] from Figure 7 and Figure 8 As can be seen, both Example 5 and Comparative Example 5 contain only small particles of lithium iron phosphate material, but the particles in Example 5 have a larger particle size, resulting in a higher powder compaction density. Figure 9As can be seen from the table, the lithium iron phosphate materials of Example 5 and Comparative Example 5 have almost the same discharge specific capacity at 1.0C. Table 1 shows that Example 5 has a higher volumetric energy density.

[0077] Table 1. Powder compaction density and electrical properties of lithium iron phosphate materials

[0078]

[0079] Although the present invention has been disclosed above with reference to embodiments, it is not intended to limit the present invention. Appropriate modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention should be covered within the protection scope of the present invention, which is defined by the claims.

Claims

1. A method for preparing a graded lithium iron phosphate material, characterized in that, Includes the following steps: (1) Two types of iron phosphate were prepared by co-precipitation method, wherein at least one type of iron phosphate was doped iron phosphate, and the doping element was one or more of Mg, Al, Ti, Zr, Mn, Co, Ni, V, and Nb; if both types of iron phosphate were doped iron phosphate, then the types of doping elements and / or the total mass of doping elements were different; in the two types of iron phosphate, the total mass of doping elements in one type of iron phosphate accounted for 0.3≤a≤1.0% of the total mass of iron phosphate, and the total mass of doping elements in the other type of iron phosphate accounted for 0≤b≤0.5% of the total mass of iron phosphate; the Fe / P molar ratio in the two types of iron phosphate was 0.93:1~0.97:1; (2) Add the two types of ferric phosphate to deionized water or methanol and coarsely grind and mix to obtain a uniform ferric phosphate slurry; (3) Add lithium source, carbon source and additives to the above iron phosphate slurry. The additives contain one or more of the doping elements Mg, Al, Ti, Zr, Mn, Co, Ni, V and Nb. Also add deionized water or methanol, grind and mix finely to obtain a uniform mixed slurry. (4) The above-mentioned mixed slurry is granulated using a spray granulation device to obtain a dry powder material; (5) Under nitrogen protection, the above powder material is sintered in a kiln and cooled with the furnace to obtain the sintered material. (6) The sintered material is crushed, graded and sieved to obtain iron phosphate graded lithium iron phosphate material.

2. The preparation method according to claim 1, characterized in that, In step (2), the mass ratio of the two types of ferric phosphate is 4:1 to 1:4; the particle size of the solid particles in the obtained ferric phosphate slurry is 1 to 2 μm, and the solid content is 25% to 55%.

3. The preparation method according to claim 1, characterized in that, In step (3), the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium hydroxide monohydrate, lithium acetate, and lithium oxalate.

4. The preparation method according to claim 1, characterized in that, In step (3), the carbon source is one or more of glucose, fructose, sucrose, maltobiose, cyclodextrin, polyethylene glycol, polyvinylpyrrolidine, carbon nanotubes, and graphite sheets, and the mass of the carbon source accounts for 12% to 18% of the total mass of the two iron phosphates.

5. The preparation method according to claim 1, characterized in that, The nanoparticle size of the solid particles in the mixed slurry obtained in step (3) is 250~450nm, the solid content is 25%~55%, and the lithium-phosphorus molar ratio of the mixed slurry is Li / P=0.99:1~1.01:

1.

6. The preparation method according to claim 1, characterized in that, The particle size of the powder material obtained in step (4) is 2≤D50≤15μm, D100<40μm.

7. The preparation method according to claim 1, characterized in that, The sintering process in step (5) is divided into two stages: the sintering temperature of the first stage is 300~650℃ and the holding time is 2~6 hours; the sintering temperature of the second stage is 700~800℃ and the holding time is 6~12 hours.

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

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