A phosphate positive electrode material and its preparation method and application

Abstract

The present invention discloses a phosphate positive electrode material, a preparation method thereof, and an application thereof. The preparation method comprises the following steps: preparing a mixed slurry of at least two types of iron phosphates with different iron-phosphorus ratios, a lithium source, a phosphorus source, and a carbon source, grinding the mixture and spray-drying the mixture to obtain a precursor; and calcining the precursor to obtain a phosphate positive electrode material. The method of the present invention not only improves the conductivity of the material by carbon coating, but also achieves the effect of large and small particle grading, thereby increasing the compaction density of the material and reducing the specific surface area of ​​the material. The above factors ensure that the phosphate material prepared by the method of the present invention not only has a high capacity, but also has excellent processing properties such as homogenization, which is conducive to the practical application of the material.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to a phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] In recent years, China has strongly supported the development of the electric vehicle industry. Automakers are placing increasing demands on battery manufacturers, with an urgent need for high-energy-density, long-life, and low-cost batteries. Lithium-ion batteries are favored by battery manufacturers for their high operating voltage, high energy density, long cycle life, and low pollution levels. The performance of lithium-ion batteries is crucially determined by the properties of the cathode material.

[0003] Phosphate cathode materials have attracted considerable attention as potential alternatives to commercial layered cathode materials due to their advantages in structural stability, cost-effectiveness, and environmental friendliness. Since 1997, olivine-structured LiFePO4 has attracted widespread attention and been extensively studied due to its relatively high theoretical capacity (170 mAh / g), high safety, and low cost. However, its low battery voltage (3.4 V vs. Li / Li) + ) related energy density limits its use in electric vehicles. LiMnPO4 has a higher energy density than LiFePO4 because it has a higher working voltage Mn 3+ / Mn 2+ (4.1Vvs.Li / Li+), compared with Fe 3+ / Fe 2+ However, LiMnPO4 is an insulator with a band gap of about 2eV, and the John-Teller effect is inevitable because the interface of LiMnPO4 / MnPO4 during the charge / discharge process creates a large kinetic barrier for ion and electron transport, resulting in low electronic conductivity. LiFePO4 is considered to be a semiconductor with a band gap of about 0.3eV. Compared with LiFePO4 (electronic conductivity of 10 -8 S / cm) compared to LiMnPO4 (electronic conductivity <10 -10 S / cm) is much lower, resulting in poorer electrochemical performance.

[0004] In recent years, people have reduced the particle size, coated with electronic conductive agents, or doped with Mg 2+ 、Fe 2+ 、Cu 2+ 、Co 2+ 、Ni 2+ , Ca 2 +Cations such as etc. are used to improve the electronic conductivity of LiMnPO4. Reducing the particle size can shorten the transport distance of electrons and lithium ions, thereby improving the rate performance. Coating with an electron conductive agent can increase the surface conductivity. Cation doping is also considered an important method because it can introduce defects into the lattice, thus greatly improving the conductivity of the material.

[0005] Currently, the solid solution LiFe x Mn<0000​​​​​​​​​​​​​​​​​​​​​​​​

[0012] preparing a mixed slurry of at least two iron phosphates with different iron-to-phosphorus ratios, a lithium source, a phosphorus source, and a carbon source, grinding the mixture and spray drying the mixture to obtain a precursor;

[0013] The precursor is calcined to obtain a phosphate positive electrode material.

[0014] In the present invention, the iron-phosphorus ratio refers to the molar ratio of the Fe element to the P element in ferric phosphate.

[0015] The method of the present invention utilizes the difference in crystallinity of iron phosphates with different iron-phosphorus ratios. At least two iron phosphates with different iron-phosphorus ratios are mixed with a carbon source and other raw materials, ground, and spray-dried. The mixture is then calcined at the same temperature. The resulting particles grow to different sizes, achieving a gradation effect of large and small particles. This is because: the smaller the iron-phosphorus ratio, the lower the crystallinity of the iron phosphate, making it easier to form lithium iron manganese phosphate at the same calcination temperature, resulting in larger particle size. Conversely, the larger the iron-phosphorus ratio, the higher the crystallinity of the iron phosphate, making it less likely to form lithium iron manganese phosphate at the same calcination temperature, resulting in smaller particle size.

[0016] Therefore, the method of the present invention not only improves the conductivity of the material through carbon coating, but also achieves the effect of large and small particle gradation, increasing the material's compaction density and reducing the material's specific surface area. These factors make the phosphate material prepared using the method of the present invention not only high in capacity but also has excellent processing properties such as homogenization, which is conducive to the material's practical application.

[0017] The preparation method of the invention is simple and suitable for industrial production.

[0018] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0019] Preferably, the iron-phosphorus ratio of the iron phosphate is within the range of 0.9-1.03. For example, the iron-phosphorus ratio of the iron phosphate can be 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, 1.01, 1.02 or 1.03, etc. The iron-to-lithium ratios of at least two iron phosphates with different iron-phosphorus ratios are both within this range and different.

[0020] In the present invention, the raw materials used to prepare the mixed slurry do not have a single iron-phosphorus ratio of the iron phosphate. For example, two iron phosphates with different iron-phosphorus ratios are used as raw materials for example: the iron-phosphorus ratio of one iron phosphate can be 0.92, while the iron-phosphorus ratio of the other iron phosphate is 1.01; or, the iron-phosphorus ratio of one iron phosphate can be 0.95, while the iron-phosphorus ratio of the other iron phosphate is 1.03; or, the iron-phosphorus ratio of one iron phosphate can be 0.94, while the iron-phosphorus ratio of the other iron phosphate is 0.98. The above are merely illustrative examples and do not constitute a limitation of the present invention.

[0021] For another example, using three iron phosphates with different iron-phosphorus ratios as raw materials is exemplified: the iron-phosphorus ratio of the first iron phosphate can be 0.92, the iron-phosphorus ratio of the second iron phosphate can be 0.97, and the iron-phosphorus ratio of the third iron phosphate can be 1.01; or the iron-phosphorus ratio of the first iron phosphate can be 0.91, the iron-phosphorus ratio of the second iron phosphate can be 0.96, and the iron-phosphorus ratio of the third iron phosphate can be 1.03; or the iron-phosphorus ratio of the first iron phosphate can be 0.93, the iron-phosphorus ratio of the second iron phosphate can be 0.97, and the iron-phosphorus ratio of the third iron phosphate can be 1.02. The above are merely illustrative examples and do not constitute a limitation of the present invention.

[0022] Preferably, the iron phosphate includes a first iron phosphate, a second iron phosphate and a third iron phosphate, wherein the iron-phosphorus ratio of the first iron phosphate is n1, the iron-phosphorus ratio of the second iron phosphate is n2, the iron-phosphorus ratio of the third iron phosphate is n3, and n1 <n2<n3。

[0023] Preferably, 0.90≤n1<0.95, for example, n1 can be 0.90, 0.91, 0.92, 0.93 or 0.94, etc.

[0024] Preferably, 0.95≤n2<0.99, for example, n2 can be 0.95, 0.96, 0.97 or 0.98, etc.

[0025] Preferably, 0.99≤n3<1.03, for example, n3 can be 0.99, 1.00, 1.01 or 1.02, etc.

[0026] As a preferred technical solution of the method for preparing the phosphate positive electrode material of the present invention, the amount of the first iron phosphate is a, the amount of the second iron phosphate is b, and the amount of the third iron phosphate is c, a / 64:b / 8:c=k1:k2:k1, 3≤k2 / k1≤4. For example, k2 / k1 can be 3, 3.2, 3.4, 3.5, 3.6, 3.8, or 4, etc.

[0027] Preferably, the lithium source includes at least one of lithium carbonate, lithium hydroxide, dilithium hydrogen phosphate and lithium dihydrogen phosphate.

[0028] Preferably, the phosphorus source includes at least one of dilithium hydrogen phosphate and lithium dihydrogen phosphate.

[0029] Preferably, in the mixed slurry, the molar ratio of Li:Fe:P is 1:1:1.

[0030] Preferably, the raw materials used to prepare the mixed slurry also include a manganese source.

[0031] Preferably, the manganese source comprises at least one of manganese carbonate, manganese nitrate and manganese oxalate;

[0032] Preferably, when the raw materials used to prepare the mixed slurry include a manganese source, the molar ratio of Li:Mn:Fe:P elements in the mixed slurry is 1:x:(1-x):1, wherein 0.5≤x≤0.8, for example, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75 or 0.8, etc.

[0033] In the preparation method of the present invention, the raw materials used to prepare the mixed slurry include iron phosphate, a lithium source and a carbon source, by which carbon-coated lithium iron phosphate can be prepared. Furthermore, on this basis, the raw materials also include a manganese source, by which carbon-coated lithium manganese iron phosphate can be prepared.

[0034] As a preferred technical solution of the method for preparing the phosphate cathode material of the present invention, the carbon source includes at least one of glucose, sucrose, fructose, citric acid, ascorbic acid, asphalt and epoxy resin;

[0035] Preferably, the amount of the carbon source is δ(a+b+c), 0.3≤δ≤0.4, such as 0.3, 0.32, 0.35, 0.38 or 0.4.

[0036] Those skilled in the art should understand that the preparation of the mixed slurry requires not only the use of corresponding raw materials but also a solvent. The present invention does not specifically limit the type of the solvent, and it can be water, for example.

[0037] Preferably, the solid content of the mixed slurry is 30%-40%, for example, 30%, 32%, 34%, 35%, 37%, 38% or 40%.

[0038] Preferably, the grinding method includes sand milling or ball milling.

[0039] Preferably, the grinding is stopped until the slurry particle size D50 = 70nm-150nm, and D50 can be, for example, 70nm, 80nm, 85nm, 90nm, 95nm, 100nm, 105nm, 110nm, 115nm, 120nm, 125nm, 130nm, 135nm, 140nm or 150nm.

[0040] As a preferred technical solution of the preparation method of the phosphate positive electrode material of the present invention, the calcination temperature is 780℃-820℃, for example, 780℃, 785℃, 790℃, 795℃, 800℃, 805℃, 810℃, 815℃ or 820℃, etc.

[0041] Preferably, the heating rate of the calcination is 1.5°C / min-2°C / min, such as 1.5°C / min, 1.7°C / min, 1.8°C / min or 2°C / min, etc. Within this range, the gradation of large and small particles can be better ensured, and the particle size distribution is better.

[0042] Preferably, the calcination holding time is 8 h to 12 h, for example, 8 h, 8.5 h, 9 h, 9.5 h, 10 h, 10.5 h, 11 h or 12 h.

[0043] As a preferred technical solution of the preparation method of the phosphate positive electrode material of the present invention, the preparation method comprises the following steps:

[0044] (1) adding a mol of a first iron phosphate, b mol of a second iron phosphate, c mol of a third iron phosphate, d(a+b+c) mol of manganese carbonate, 1 / 2(a+b+c) mol of lithium carbonate, d(a+b+c) mol of lithium dihydrogen phosphate, and e(a+b+c) mol of glucose into water to prepare a mixed slurry having a solid content of 30%-40%;

[0045] The iron-phosphorus ratio of the first ferric phosphate is greater than or equal to 0.90 and less than 0.95, the iron-phosphorus ratio of the second ferric phosphate is greater than or equal to 0.95 and less than 0.99, the iron-phosphorus ratio of the third ferric phosphate is greater than or equal to 0.99 and less than 1.03, and a, b, c, d, and e satisfy the relationship a:b / 8:c / 64=k1:k2:k1, 3≤k2 / k1≤4, 1≤d1≤4, 1≤d2≤4, and 0.3≤δ≤0.4;

[0046] (2) sand-milling the mixed slurry until the slurry particle size D50 is 70 nm-150 nm to obtain a lithium manganese iron phosphate precursor slurry;

[0047] (3) After the lithium manganese iron phosphate precursor slurry is spray-dried, the material precursor is collected, and the collected material precursor is heated to 780°C-820°C at a heating rate of 1.5°C / min-2°C / min under nitrogen atmosphere, kept warm for 8h-10h, and naturally cooled to obtain the phosphate positive electrode material.

[0048] In this preferred technical solution, d1 can be, for example, 1, 1.2, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.2, 2.5, 2.6, 2.8, 3.0, 3.3, 3.5, 3.6, 3.8 or 4, and d2 can be, for example, 1, 1.2, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.2, 2.5, 2.6, 2.8, 3.0, 3.3, 3.5, 3.6, 3.8 or 4, etc.

[0049] In a second aspect, the present invention provides a phosphate positive electrode material, which is prepared by the method described in the first aspect. The phosphate positive electrode material includes large-particle phosphate and small-particle phosphate, and both the large-particle phosphate and the small-particle phosphate have a carbon coating layer.

[0050] In the present invention, the phosphate positive electrode material may be a lithium iron phosphate positive electrode material or a lithium iron manganese phosphate positive electrode material.

[0051] In one embodiment, the chemical formula of lithium manganese iron phosphate is LiMn x Fe 1-x PO4, wherein 0.5≤x≤0.8, such as 0.5, 0.55, 0.6, 0.65, 0.7, 0.75 or 0.8, etc.

[0052] Preferably, the particle size of the large-particle phosphate is greater than 400 nm, such as 410 nm, 430 nm, 450 nm, 470 nm, 480 nm or 500 nm; the particle size of the small-particle phosphate is less than 200 nm, such as 190 nm, 180 nm, 160 nm, 150 nm, 135 nm, 120 nm, 100 nm, 80 nm or 50 nm.

[0053] Small-particle phosphates (e.g., particle size less than 200 nm) have high capacity, large specific surface area, low compaction density, and are difficult to process. Large-particle phosphates (e.g., particle size greater than 400 nm) have a small specific surface area and are easy to process, but have low capacity. Therefore, the phosphate cathode material of the present invention combines the advantages of both types of phosphates, ensuring the high capacity of the phosphate material while reducing the material's specific surface area, improving powder compaction, and exhibiting good processing properties.

[0054] Preferably, the particle size of the large particle phosphate is above 300 nm (for example, 300 nm, 325 nm, 350 nm, 370 nm, 390 nm, 410 nm, 430 nm, 450 nm, 470 nm, 480 nm or 500 nm, etc.), the particle size of the small particle phosphate is below 100 nm (for example, 100 nm, 90 nm, 80 nm, 70 nm, 60 nm or 50 nm, etc.), and the phosphate positive electrode material also includes a medium particle phosphate, the medium particle phosphate has a carbon coating layer, and the particle size of the medium particle phosphate is greater than 100 nm and less than 300 nm (for example, 110 nm, 120 nm, 130 nm, 145 nm, The phosphate particles may be selected from the group consisting of 160nm, 180nm, 200nm, 215nm, 225nm, 240nm, 260nm, 280nm or 290nm, etc.), the small-particle phosphate accounts for 10%-20% (for example, 10%, 12%, 13%, 14%, 15%, 16%, 18% or 20%, etc.), the large-particle phosphate accounts for 10%-20% (for example, 10%, 12%, 13%, 14%, 15%, 16%, 18% or 20%, etc.), and the medium-particle phosphate accounts for 60%-80% (for example, 60%, 62%, 63%, 64%, 66%, 68%, 70%, 73%, 76%, 78% or 80%, etc.). The proportion here refers to the proportion in number. Taking the proportion of large-particle phosphate as 10% as an example, it refers to the ratio of phosphate particles with a particle size of more than 300nm to the total number of phosphate particles.

[0055] In this preferred technical solution, the phosphate positive electrode material includes large-particle phosphate, small-particle phosphate and medium-particle phosphate of a certain size and satisfies the above-mentioned proportions. The particle size is approximately in a normally distributed gradation, making the prepared phosphate positive electrode material (such as lithium manganese iron phosphate positive electrode material) more significant in terms of low specific surface area, high compaction density, high capacity and easy processing.

[0056] Preferably, the specific surface area of ​​the phosphate cathode material is 15 m 2 / g-18m 2 / g, for example 15m 2 / g、16m 2 / g, 16.5m 2 / g、17m 2 / g, 17.5m 2 / g or 18m 2 / g, etc.

[0057] Preferably, the compaction density of the phosphate cathode material is 2.15 g / cm 3 -2.3g / cm 3 , for example 2.15g / cm 3, 2.16g / cm 3 , 2.18g / cm 3 , 2.2g / cm 3 , 2.22g / cm 3 , 2.24g / cm 3 , 2.25g / cm 3 , 2.27g / cm 3 or 2.3g / cm 3 wait.

[0058] In a third aspect, the present invention provides a positive electrode, which includes the phosphate positive electrode material as described in the second aspect.

[0059] In a fourth aspect, the present invention provides a lithium-ion battery, comprising the positive electrode as described in the third aspect.

[0060] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] (1) The method of the present invention utilizes the difference in crystallinity of iron phosphates with different iron-phosphorus ratios. At least two iron phosphates with different iron-phosphorus ratios are mixed with a carbon source and other raw materials, ground, and spray-dried, and then calcined at the same temperature. The material particles grow to different sizes, which not only improves the conductivity of the material by carbon coating, but also achieves the effect of large and small particle gradation, increases the compaction density of the material, and reduces the specific surface area of ​​the material. The above factors make the phosphate material prepared by the method of the present invention not only have high capacity, but also have excellent processing properties such as homogenization, which is conducive to the practical application of the material.

[0063] (2) The preparation method of the present invention is simple and suitable for industrial production. DETAILED DESCRIPTION

[0064] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0065] In the present invention, the iron-to-phosphorus ratio is abbreviated as Fe / P.

[0066] In the embodiments of the present invention, iron phosphates with different iron-phosphorus ratios were purchased from Huanggang Linli New Energy Technology Co., Ltd.

[0067] Example 1

[0068] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material comprises the following steps:

[0069] (1) 0.22 mol of a first iron phosphate (Fe / P=0.94), 5.39 mol of a second iron phosphate (Fe / P=0.98), 14.38 mol of a third iron phosphate (Fe / P=1.02), 30 mol of manganese carbonate, 10 mol of lithium carbonate, 30 mol of lithium dihydrogen phosphate and 6 mol of glucose were added to water to prepare a mixed slurry with a solid content of 30%.

[0070] (2) The mixed slurry is sand-milled until the slurry particle size D50 is 100 nm to obtain lithium manganese iron phosphate precursor slurry.

[0071] (3) After the lithium manganese iron phosphate precursor slurry is spray-dried, the material precursor is collected and heated to 780°C at a heating rate of 1.5°C / min under nitrogen atmosphere, kept warm for 10 hours, and cooled naturally to obtain a carbon-coated lithium manganese iron phosphate positive electrode material, wherein the chemical formula of lithium manganese iron phosphate is LiMn 0.6 Fe 0.4 PO4.

[0072] The carbon-coated lithium iron manganese phosphate positive electrode material prepared in this embodiment includes large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate. The large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate all have a carbon coating layer. The particle size of the large-particle lithium iron manganese phosphate is in the range of 300nm to 500nm, the particle size of the small-particle lithium iron manganese phosphate is in the range of 50nm to 100nm, and the particle size of the medium-particle lithium iron manganese phosphate is greater than 100nm and less than 300nm. The proportion of the small-particle lithium iron manganese phosphate is 10%, the proportion of the large-particle lithium iron manganese phosphate is 10%, and the proportion of the medium-particle lithium iron manganese phosphate is 80%.

[0073] Example 2

[0074] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material comprises the following steps:

[0075] 0.21 mol of the first iron phosphate (Fe / P=0.93), 6.60 mol of the second iron phosphate (Fe / P=0.97), 13.20 mol of the third iron phosphate (Fe / P=1.01), 30 mol of manganese carbonate, 10 mol of lithium carbonate, 30 mol of lithium dihydrogen phosphate and 7 mol of glucose were added to water to prepare a mixed slurry with a solid content of 40%.

[0076] (2) The mixed slurry is sand-milled until the slurry particle size D50 is 120 nm to obtain lithium manganese iron phosphate precursor slurry.

[0077] (3) After the lithium manganese iron phosphate precursor slurry is spray-dried, the material precursor is collected and heated to 780°C at a heating rate of 1.5°C / min under nitrogen atmosphere, kept warm for 10 hours, and cooled naturally to obtain a carbon-coated lithium manganese iron phosphate positive electrode material, wherein the chemical formula of lithium manganese iron phosphate is LiMn 0.6 Fe 0.4 PO4.

[0078] The carbon-coated lithium iron manganese phosphate positive electrode material prepared in this embodiment includes large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate. The large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate all have a carbon coating layer. The particle size of the large-particle lithium iron manganese phosphate is in the range of 300nm to 500nm, the particle size of the small-particle lithium iron manganese phosphate is in the range of 50nm to 100nm, and the particle size of the medium-particle lithium iron manganese phosphate is greater than 100nm and less than 300nm. The proportion of the small-particle lithium iron manganese phosphate is 12.5%, the proportion of the large-particle lithium iron manganese phosphate is 12.5%, and the proportion of the medium-particle lithium iron manganese phosphate is 75%.

[0079] Example 3

[0080] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material comprises the following steps:

[0081] (1) 0.25 mol of a first iron phosphate (Fe / P=0.92), 6.75 mol of a second iron phosphate (Fe / P=0.96), 16.00 mol of a third iron phosphate (Fe / P=1.00), 46 mol of manganese carbonate, 11.5 mol of lithium carbonate, 46 mol of lithium dihydrogen phosphate and 9 mol of glucose were added to water to prepare a mixed slurry with a solid content of 35%.

[0082] (2) The mixed slurry is sand-milled until the slurry particle size D50 is 135 nm to obtain lithium manganese iron phosphate precursor slurry.

[0083] (3) After the lithium manganese iron phosphate precursor slurry is spray-dried, the material precursor is collected and heated to 800°C at a heating rate of 2°C / min under nitrogen atmosphere, kept warm for 8 hours, and cooled naturally to obtain a carbon-coated lithium manganese iron phosphate positive electrode material, wherein the chemical formula of lithium manganese iron phosphate is LiMn 0.67 Fe 0.32 PO4.

[0084] The carbon-coated lithium iron manganese phosphate positive electrode material prepared in this embodiment includes large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate. The large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate all have a carbon coating layer. The particle size of the large-particle lithium iron manganese phosphate is in the range of 300nm to 500nm, the particle size of the small-particle lithium iron manganese phosphate is in the range of 50nm to 100nm, and the particle size of the medium-particle lithium iron manganese phosphate is greater than 100nm and less than 300nm. The small-particle lithium iron manganese phosphate accounts for 17.5%, the large-particle lithium iron manganese phosphate accounts for 17.5%, and the medium-particle lithium iron manganese phosphate accounts for 65%.

[0085] Example 4

[0086] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material comprises the following steps:

[0087] (1) 10 mol of a first iron phosphate (Fe / P=0.94), 10 mol of a third iron phosphate (Fe / P=1.02), 30 mol of manganese carbonate, 10 mol of lithium carbonate, 30 mol of lithium dihydrogen phosphate, and 6 mol of glucose were added to water to prepare a mixed slurry with a solid content of 30%.

[0088] (2) The mixed slurry is sand-milled until the slurry particle size D50 is 100 nm to obtain lithium manganese iron phosphate precursor slurry.

[0089] (3) After the lithium manganese iron phosphate precursor slurry is spray-dried, the material precursor is collected and heated to 780°C at a heating rate of 1.5°C / min under nitrogen atmosphere, kept warm for 10 hours, and cooled naturally to obtain a carbon-coated lithium manganese iron phosphate positive electrode material, wherein the chemical formula of lithium manganese iron phosphate is LiMn 0.6 Fe 0.4 PO4.

[0090] The carbon-coated lithium manganese iron phosphate positive electrode material prepared in this embodiment includes large-particle lithium manganese iron phosphate and small-particle lithium manganese iron phosphate. Both the large-particle lithium manganese iron phosphate and the small-particle lithium manganese iron phosphate have a carbon coating layer. The particle size of the large-particle lithium manganese iron phosphate is greater than 400nm and less than or equal to 500nm, and the particle size of the small-particle lithium manganese iron phosphate is greater than or equal to 50nm and less than 200nm. The small-particle lithium manganese iron phosphate accounts for 50%, and the large-particle lithium manganese iron phosphate accounts for 50%.

[0091] Example 5

[0092] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material comprises the following steps:

[0093] (1) 0.22 mol of the first iron phosphate (Fe / P=0.92), 5.39 mol of the second iron phosphate (Fe / P=0.99), 14.38 mol of the third iron phosphate (Fe / P=1.02), 10 mol of lithium carbonate and 6 mol of glucose were added to water to prepare a mixed slurry with a solid content of 30%.

[0094] (2) The mixed slurry is sand-milled until the slurry particle size D50 is 125 nm to obtain lithium iron phosphate precursor slurry.

[0095] (3) After the lithium iron phosphate precursor slurry is spray-dried, the material precursor is collected and heated to 810°C at a heating rate of 1.5°C / min under nitrogen atmosphere, kept warm for 9 hours, and cooled naturally to obtain a carbon-coated lithium iron phosphate positive electrode material, wherein the chemical formula of lithium manganese iron phosphate is LiFePO4.

[0096] The carbon-coated lithium iron phosphate positive electrode material prepared in this embodiment includes large-particle lithium iron phosphate, small-particle lithium iron phosphate and medium-particle lithium iron phosphate. The large-particle lithium iron phosphate, small-particle lithium iron phosphate and medium-particle lithium iron phosphate all have a carbon coating layer. The particle size of the large-particle phosphate is in the range of 300nm to 500nm, and the particle size of the small-particle phosphate is in the range of 50nm to 100nm. The phosphate positive electrode material also includes medium-particle phosphate, and the particle size of the medium-particle phosphate is greater than 100nm and less than 300nm. The small-particle phosphate accounts for 10%, the large-particle phosphate accounts for 10%, and the medium-particle phosphate accounts for 80%.

[0097] Example 6

[0098] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material is different from that of Example 1 in that the material is ground to a particle size D50 of 180 nm.

[0099] The carbon-coated lithium iron manganese phosphate positive electrode material prepared in this embodiment includes large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate. The large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate all have a carbon coating layer. The particle size of the large-particle lithium iron manganese phosphate is in the range of 350nm to 550nm, the particle size of the small-particle lithium iron manganese phosphate is in the range of 100nm to 150nm, and the particle size of the medium-particle lithium iron manganese phosphate is greater than 150nm and less than 350nm. The proportion of the small-particle phosphate is 10%, the proportion of the large-particle phosphate is 10%, and the proportion of the medium-particle phosphate is 80%.

[0100] Example 7

[0101] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material is different from that of Example 1 in that the heating rate of the calcination step is 7° C. / min.

[0102] The carbon-coated lithium iron manganese phosphate positive electrode material prepared in this embodiment includes large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate. The large-particle lithium iron manganese phosphate, small-particle lithium iron manganese phosphate and medium-particle lithium iron manganese phosphate all have a carbon coating layer. The particle size of the large-particle lithium iron manganese phosphate is in the range of 600nm to 800nm, the particle size of the small-particle lithium iron manganese phosphate is in the range of 350nm to 400nm, the particle size of the medium-particle phosphate is greater than 400nm and less than 600nm, the proportion of the small-particle phosphate is 10%, the proportion of the large-particle phosphate is 10%, and the proportion of the medium-particle phosphate is 80%.

[0103] Comparative Example 1

[0104] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material is different from that of Example 1 in that the second iron phosphate and the third iron phosphate are replaced by an equimolar amount of the first iron phosphate.

[0105] The carbon-coated lithium manganese iron phosphate positive electrode material prepared in this comparative example is large-particle lithium manganese iron phosphate, which has a carbon coating layer. The particle size of the large-particle phosphate is in the range of 300nm to 500nm, and the proportion of the large-particle phosphate is 100%.

[0106] Comparative Example 2

[0107] A method for preparing a carbon-coated lithium manganese iron phosphate positive electrode material is different from Example 1 in that the first iron phosphate and the second iron phosphate are replaced by an equimolar amount of the third iron phosphate.

[0108] The carbon-coated lithium manganese iron phosphate positive electrode material prepared in this comparative example is small-particle lithium manganese iron phosphate, which has a carbon coating layer. The particle size of the small-particle phosphate is in the range of 50nm to 100nm, and the proportion of the small-particle phosphate is 100%.

[0109] Application Example 1

[0110] A lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte.

[0111] The method for preparing a lithium-ion battery provided in this application example includes the following steps:

[0112] ① Preparation of positive electrode sheet:

[0113] Homogenize the positive electrode material (carbon-coated lithium manganese iron phosphate positive electrode material of Example 1) with a mass ratio of Super P (SP) to polyvinylidene fluoride (PVDF) of 90:5:5, using nitrogen methyl pyrrolidone (NMP) as the solvent to prepare a positive electrode slurry with a solid content of 50%;

[0114] The positive electrode slurry was coated on a 20 μm thick aluminum foil, dried and rolled to obtain a positive electrode sheet with an area density of 8 mg / cm 2 ;

[0115] ② Using a polyethylene (PE) separator, add 80 μL of an electrolyte solution. The electrolyte consists of lithium hexafluorophosphate (LiPF6) and a solvent. The LiPF6 concentration is 1 mol / L. The solvent is composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). The volume ratio of EC, EMC, and DMC is 1:1:1. A lithium sheet is used as the negative electrode, and an R2032 button cell shell is used for button assembly to obtain a lithium-ion battery.

[0116] Application Example 2-7

[0117] A lithium ion battery differs from Application Example 1 in that the positive electrode material of Example 1 is replaced by the positive electrode materials of Examples 2 to 7 respectively.

[0118] Comparative Application Examples 1-2

[0119] A lithium ion battery differs from Application Example 1 in that the positive electrode material of Example 1 is replaced by the positive electrode materials of Comparative Examples 1-2 respectively.

[0120] Performance testing:

[0121] (1) Powder compaction density test:

[0122] Add 1g of positive electrode material to the mold of the compaction density test instrument, pressurize the mold to 2T, maintain for 1min, measure the height of the mold, and the compaction density = 1g / (mold height-mold height without material) / bottom area of ​​the mold pit.

[0123] (2) Specific surface area test:

[0124] The test was performed using a surface area analyzer.

[0125] The cathode materials of Examples 1-7 and Comparative Examples 1-2 were tested for powder compaction density and specific surface area. The results are shown in Table 1.

[0126] Table 1

[0127] sample <![CDATA[Powder Compaction Density (g / cm 3 )]]> <![CDATA[Specific surface area (m 2 / g)]]> Example 1 2.23 17.6 Example 2 2.21 17.3 Example 3 2.20 17.5 Example 4 2.15 18.0 Example 5 2.42 13.4 Example 6 2.15 16.5 Example 7 2.19 15.5 Comparative Example 1 2.18 15.6 Comparative Example 2 1.89 26.8

[0128] As shown in Table 1, the phosphate cathode material prepared by the method of the present invention has a lower specific surface area and a higher compaction density. 2 / g or less, compacted density is 2.15g / cm 3 The above effectively improves the processing performance of the material. However, Comparative Examples 1 and 2 are not graded, so they have large specific surface areas but low compaction densities and poor processing performance.

[0129] At the same time, by comparing Example 1 with Example 4, it can be seen that the grading of three particle sizes, large, medium and small, is more conducive to forming a normally distributed grading effect of large and small particles, compared with the grading of two particle sizes, thereby improving the compaction density of the material.

[0130] (3) Slurry test:

[0131] The positive electrode materials from Examples 1-7 and Comparative Example 1 were homogenized at a mass ratio of positive electrode material: SP:PVDF = 90:5:5. NMP was used as the solvent to prepare a positive electrode slurry with a solid content of 50%. The particle size D50 of the slurry was measured using a particle size analyzer. The slurry was then stirred at 1000 rpm for 4 hours and allowed to stand. The viscosity was then measured using a viscometer at different standing times. The results are shown in Table 2.

[0132] Table 2

[0133]

[0134] As shown in Table 2, the phosphate cathode material prepared by the present invention has good processing performance. After the material is homogenized, the slurry viscosity can reach about 5000 mPa / s, and the viscosity rebound is very small after the slurry is allowed to stand, which is conducive to processing.

[0135] (4) Normal temperature cycle performance test:

[0136] Under 25°C conditions, a blue electric tester was used to place the button batteries of Application Examples 1-7 and Comparative Examples 1-2 in a high-temperature oven at 25°C for charge and discharge tests. The voltage range was 2.0V-4.35V, and 0.1C charge and discharge was activated for one cycle. Then, it was charged at 0.5C constant current and constant voltage, with a cut-off current of 0.05C. Constant current discharge was performed at 0.1C and 1C currents, respectively, and then cycled at 1C / 1C for 100 cycles to obtain data on 0.1C first discharge capacity, 1C first discharge capacity, first coulomb efficiency, 100th cycle discharge capacity and 100th cycle capacity retention rate, wherein the first coulomb efficiency = 0.1C first discharge capacity / 0.1C first charging capacity. The results are shown in Table 3.

[0137] Table 3

[0138]

[0139] As shown in Table 3, batteries assembled using the phosphate cathode material of the present invention exhibit high capacity, high initial efficiency, and excellent cycling performance. The 0.1C initial discharge capacity is above 150 mAh / g, the 1C initial discharge specific capacity is above 143.0 mAh / g, the initial efficiency is above 97.0%, and the capacity retention remains above 99.5% after 100 cycles at 25°C.

[0140] By comparing Application Examples 1-3 with Application Example 4, it can be seen that the grading of three particle sizes, large, medium and small, is more conducive to forming a normally distributed grading effect of large and small particles than the grading of two particle sizes, and is more conducive to improving the electrochemical performance.

[0141] By comparing Application Example 1 with Application Example 6, it can be seen that during the preparation process of the positive electrode material used in the battery of Application Example 6, the particle size D50 of the slurry after grinding is large, and the particle size of the particles formed after calcination is generally large, resulting in deviations in the electrical properties of the material compared to Application Example 1.

[0142] By comparing Example 1 with Application Example 7, it can be seen that during the preparation process of the positive electrode material used in the battery of Application Example 7, the calcination heating rate is too high, and the particle size formed after calcination is also generally large, resulting in deviations in the electrical properties of the material.

[0143] In summary, the phosphate positive electrode material prepared by the present invention utilizes different iron-phosphorus ratios of the raw material iron phosphate to prepare positive electrode materials with large and small particle sizes, which not only ensures that the positive electrode material has excellent electrochemical properties, but also has good processing performance, thereby improving the practical performance of the lithium manganese iron phosphate material and giving it a great market competitive advantage.

[0144] Although the slurry made from the positive electrode material of Comparative Example 1 has low viscosity and rebound strength, its capacity utilization is very low. Although the positive electrode material of Comparative Example 2 has good capacity, due to the large number of small particles and large specific surface area, side reactions increase and the cycle performance deteriorates. At the same time, the slurry viscosity of this material is high and the viscosity rebounds quickly, making it very difficult to process.

[0145] The applicant states that the present invention is intended to illustrate the detailed methods of the present invention through the above-described embodiments, but the present invention is not limited to the above-described detailed methods, that is, it does not mean that the present invention must rely on the above-described detailed methods in order to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., are all within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a phosphate positive electrode material, characterized in that: The preparation method comprises the following steps: Three iron phosphates with different iron-phosphorus ratios, a lithium source, a phosphorus source, and a carbon source are prepared into a mixed slurry, which is then ground and spray-dried to obtain a precursor. calcining the precursor to obtain a phosphate positive electrode material; The iron phosphate includes a first iron phosphate, a second iron phosphate and a third iron phosphate, wherein the iron-phosphorus ratio of the first iron phosphate is n1, the iron-phosphorus ratio of the second iron phosphate is n2, the iron-phosphorus ratio of the third iron phosphate is n3, and n1 <n2<n3;0.90≤n1<0.95;0.95≤n2<0.99;0.99≤n3<1.03; The phosphate positive electrode material includes large-particle phosphate and small-particle phosphate, and both the large-particle phosphate and the small-particle phosphate have a carbon coating layer; the particle size of the large-particle phosphate is greater than 300nm, and the particle size of the small-particle phosphate is less than 100nm. The phosphate positive electrode material also includes medium-particle phosphate, and the medium-particle phosphate has a carbon coating layer. The particle size of the medium-particle phosphate is greater than 100nm and less than 300nm. The small-particle phosphate accounts for 10%-20%, the large-particle phosphate accounts for 10%-20%, and the medium-particle phosphate accounts for 60%-80%.

2. The method for preparing a phosphate cathode material according to claim 1, wherein: The amount of the first ferric phosphate is a, the amount of the second ferric phosphate is b, and the amount of the third ferric phosphate is c, a:b / 8:c / 64=k1:k2:k1, 3≤k2 / k1≤4.

3. The method for preparing a phosphate cathode material according to claim 1 or 2, characterized in that: The lithium source includes at least one of lithium carbonate, lithium hydroxide, dilithium hydrogen phosphate and lithium dihydrogen phosphate.

4. The method for preparing a phosphate cathode material according to claim 3, wherein: The phosphorus source includes at least one of dilithium hydrogen phosphate and lithium dihydrogen phosphate.

5. The method for preparing a phosphate cathode material according to claim 3, wherein: In the mixed slurry, the molar ratio of Li:Fe:P is 1:1:

1.

6. The method for preparing a phosphate cathode material according to claim 3, wherein: The raw materials used to prepare the mixed slurry also include a manganese source.

7. The method for preparing a phosphate cathode material according to claim 6, wherein: The manganese source includes at least one of manganese carbonate, manganese nitrate and manganese oxalate.

8. The method for preparing a phosphate cathode material according to claim 6, wherein: When the raw materials used to prepare the mixed slurry include a manganese source, the molar ratio of Li:Mn:Fe:P in the mixed slurry is 1:x:(1-x):1, wherein 0.5≤x≤0.

8.

9. The method for preparing a phosphate cathode material according to claim 1 or 2, characterized in that: The carbon source includes at least one of glucose, sucrose, fructose, citric acid, ascorbic acid, asphalt and epoxy resin.

10. The method for preparing a phosphate cathode material according to claim 9, wherein: The amount of the carbon source is δ(a+b+c), 0.3≤δ≤0.

4.

11. The method for preparing a phosphate cathode material according to claim 1 or 2, characterized in that: The solid content of the mixed slurry is 30%-40%.

12. The method for preparing a phosphate cathode material according to claim 11, wherein: The grinding method includes sand milling or ball milling.

13. The method for preparing a phosphate cathode material according to claim 11, wherein: The grinding is stopped when the slurry particle size D50 is 70nm-150nm.

14. The method for preparing a phosphate cathode material according to claim 1 or 2, characterized in that: The calcination temperature is 780°C-820°C. 15 . The method for preparing a phosphate positive electrode material according to claim 14 , wherein the heating rate of the calcination is 1.5° C. / min-2° C. / min. 16 . The method for preparing a phosphate positive electrode material according to claim 14 , wherein the calcination holding time is 8 h to 12 h.

17. The method for preparing a phosphate cathode material according to claim 1 or 2, characterized in that: The preparation method comprises the following steps: (1) adding a mol of a first iron phosphate, b mol of a second iron phosphate, c mol of a third iron phosphate, d1(a+b+c) mol of manganese carbonate, 1 / 2(a+b+c) mol of lithium carbonate, d2(a+b+c) mol of lithium dihydrogen phosphate, and δ(a+b+c) mol of glucose into water to prepare a mixed slurry having a solid content of 30%-40%; The iron-phosphorus ratio of the first ferric phosphate is greater than or equal to 0.90 and less than 0.95, the iron-phosphorus ratio of the second ferric phosphate is greater than or equal to 0.95 and less than 0.99, the iron-phosphorus ratio of the third ferric phosphate is greater than or equal to 0.99 and less than 1.03, and a, b, c, d, and e satisfy the relationship a:b / 8:c / 64=k1:k2:k1, 3≤k2 / k1≤4, 1≤d1≤4, 1≤d2≤4, and 0.3≤δ≤0.4; (2) sand-milling the mixed slurry until the slurry particle size D50 is 70 nm-150 nm to obtain a lithium manganese iron phosphate precursor slurry; (3) After the lithium manganese iron phosphate precursor slurry is spray-dried, the material precursor is collected, and the collected material precursor is heated to 780°C-820°C at a heating rate of 1.5°C / min-2°C / min under nitrogen atmosphere, kept warm for 8h-10h, and naturally cooled to obtain the phosphate positive electrode material.

18. A phosphate positive electrode material, characterized in that: The phosphate positive electrode material is prepared by the method according to any one of claims 1 to 17, and the phosphate positive electrode material includes large-particle phosphate and small-particle phosphate, and the large-particle phosphate and the small-particle phosphate both have a carbon coating layer; the particle size of the large-particle phosphate is greater than 300 nm, and the particle size of the small-particle phosphate is less than 100 nm. The phosphate positive electrode material also includes medium-particle phosphate, and the medium-particle phosphate has a carbon coating layer. The particle size of the medium-particle phosphate is greater than 100 nm and less than 300 nm. The small-particle phosphate accounts for 10%-20%, the large-particle phosphate accounts for 10%-20%, and the medium-particle phosphate accounts for 60%-80%.

19. The phosphate cathode material according to claim 18, wherein the specific surface area of ​​the phosphate cathode material is 15 m 2 / g-18m 2 / g.

20. The phosphate cathode material according to claim 18, wherein the compaction density of the phosphate cathode material under a pressure of 2T is 2.15 g / cm 3 -2.3g / cm 3 .

21. A positive electrode, characterized in that The positive electrode comprises the phosphate positive electrode material according to any one of claims 18 to 20.

22. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode according to claim 21.

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