Carbon-doped iron phosphate and preparation method thereof and preparation method of lithium iron phosphate material

Abstract

A preparation method of carbon-doped iron phosphate, comprising the following steps: (1) mixing ammonium ferrocyanide, a phosphorus source and pyridine oxide to obtain an iron-phosphorus mixed solution; (2) performing spray pyrolysis on the iron-phosphorus mixed solution obtained in step (1) to obtain solid particles; (3) crushing the solid particles obtained in step (2) to obtain carbon-doped iron phosphate; and further applying the carbon-doped iron phosphate obtained by the above preparation method to prepare lithium iron phosphate.

Description

Technical Field

[0001] This disclosure relates to the field of lithium-ion battery technology, specifically to a carbon-doped iron phosphate and its preparation method, and a method for preparing lithium iron phosphate materials. Background Technology

[0002] In the fields of energy storage and new energy, lithium iron phosphate batteries have become one of the most promising lithium-ion batteries due to their advantages such as good rate performance, long cycle life, safety and environmental friendliness.

[0003] In the existing process, the main bottleneck affecting the application of lithium iron phosphate is the low energy density. The main reasons include: (1) Although the carbon coating process of lithium iron phosphate currently used can increase conductivity, the conductivity between individual lithium iron phosphate particles has not been significantly improved; (2) Since the coating carbon is flocculent amorphous carbon, it affects the compaction density of lithium iron phosphate; (3) The size of lithium iron phosphate particles is too large, resulting in a long migration distance of lithium ions and poor battery capacity.

[0004] Ferric phosphate is a precursor for lithium iron phosphate (LFP). Currently, the main process for preparing ferric phosphate involves adding purified ferrous sulfate or ferrous phosphate (a byproduct of titanium dioxide production), along with ammonia, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate, and using a one-step aging method for controlled crystallization to obtain the final product. However, this process often results in insufficient oxidation precipitation, leading to larger primary ferric phosphate particles and a smaller specific surface area, which is detrimental to subsequent LFP processing. For example, CN111115606A discloses a method for preparing ultrafine spherical ferric phosphate using liquid-phase precipitation combined with spray drying. This method uses a ferric salt solution and phosphoric acid as raw materials, adjusting the pH of the reaction with ammonia and phosphoric acid. While this method can produce ultrafine spherical ferric phosphate, the use of ferric salts such as ferric sulfate results in the production of ammonium sulfate as a byproduct. Furthermore, a large amount of sulfate ions are easily adsorbed on the surface of the ferric phosphate, making it difficult to wash off, thus leading to excessively high sulfur content and affecting the performance of LFP.

[0005] Therefore, how to prepare carbon-doped iron phosphate with small particle size, uniform size, high tap density and purity, and how to further prepare lithium iron phosphate with excellent electrical properties are problems that need to be solved. Summary of the Invention

[0006] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of the claims.

[0007] To address the above issues, the purpose of this disclosure is to provide a carbon-doped iron phosphate and its preparation method, as well as a method for preparing lithium iron phosphate materials. Compared with the prior art, the carbon-doped iron phosphate provided by this disclosure has higher purity and tap density, which is beneficial to improving the performance and quality of lithium iron phosphate materials. The preparation method of the carbon-doped iron phosphate avoids the use of pH adjusters, reduces the generation of by-products and cleaning steps, greatly simplifies the production process, and has high industrial application value.

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

[0009] In a first aspect, this disclosure provides a method for preparing carbon-doped iron phosphate, the method comprising the following steps:

[0010] (1) Mix ammonium ferrocyanide, phosphorus source and pyridine oxide to obtain iron-phosphorus mixture;

[0011] (2) The iron-phosphorus mixture obtained in step (1) is subjected to spray pyrolysis to obtain solid particles;

[0012] (3) The solid particles obtained in step (2) are crushed to obtain carbon-doped iron phosphate.

[0013] This disclosure uses ammonium ferrocyanide as the iron source, achieving a homogeneous iron-phosphorus mixture without the need for pH adjustment, and the ammonium ferrocyanide is oxidized to ammonium ferricyanide. Furthermore, this disclosure controls the product dispersion state through a spray pyrolysis process, reducing agglomeration. During spray pyrolysis, ammonium ferricyanide reacts with the phosphorus source to produce ferric phosphate, hydrogen cyanide gas, and ammonia gas, with no other byproducts. Therefore, the obtained ferric phosphate has high purity, avoiding multiple washing steps, simplifying the process, and facilitating industrial application. Additionally, in this disclosure, pyridine oxide acts as both an oxidant and a surfactant, oxidizing ferrous ions to ferric ions. It also controls the surface tension of the spray droplets, preventing excessive porosity in the solid particles obtained from spray pyrolysis, increasing the tap density of the solid particles, and resulting in smaller, more uniform particle sizes. Simultaneously, pyridine oxide carbonizes after spray pyrolysis, forming carbon doping in the ferric phosphate, thereby significantly improving conductivity.

[0014] In one embodiment, the ammonium ferrocyanide in step (1) is mixed in the form of an ammonium ferrocyanide solution.

[0015] In one embodiment, the concentration of iron ions in the ammonium ferrocyanide solution is 50 g / L, 60 g / L, 80 g / L, 100 g / L, 120 g / L, 140 g / L, 160 g / L, 180 g / L, or 200 g / L, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0016] In one embodiment, the phosphorus source includes any one or a combination of at least two of phosphoric acid, diammonium hydrogen phosphate, or ammonium dihydrogen phosphate.

[0017] In this disclosure, phosphoric acid, diammonium hydrogen phosphate, or ammonium dihydrogen phosphate reacts with a phosphorus source during spray pyrolysis to generate iron phosphate, hydrogen cyanide, and ammonia. Furthermore, the carbonized pyridine oxide causes the iron phosphate to become carbon-doped. Taking diammonium hydrogen phosphate as an example, the main reaction is as follows:

[0018] (NH4)2HPO4+(NH4)3[Fe(CN)6]→FePO4+6HCN+5NH3.

[0019] In one embodiment, the molar ratio of iron to phosphorus in the iron-phosphorus mixture is (1-1.05):1, for example, it can be 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1 or 1.05:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] In one embodiment, the molar ratio of iron ions in the pyridine oxide and iron-phosphorus mixture is (1-1.5):1, for example, it can be 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1 or 1.5:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0021] In this disclosure, by selecting and controlling the molar ratio of iron ions in the pyridine oxide and iron-phosphorus mixture within a specific range, excessive carbon doping or incomplete oxidation can be avoided, thereby ensuring that the lithium iron phosphate material prepared from carbon-doped iron phosphate has good capacity and cycle performance.

[0022] In one embodiment, the carrier gas used in step (2) of the spray pyrolysis includes air.

[0023] In one embodiment, the flow rate of the carrier gas is 20-30 L / min, for example, it can be 20 L / min, 22 L / min, 24 L / min, 26 L / min, 28 L / min or 30 L / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0024] In one embodiment, the flow rate of the iron-phosphorus mixture in the spray pyrolysis is 1.5-1.8 mL / min, for example, it can be 1.5 mL / min, 1.6 mL / min, 1.7 mL / min or 1.8 mL / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] In one embodiment, the average particle size of the atomized droplets of the iron-phosphorus mixture in the spray pyrolysis is 5-15 μm, for example, it can be 5 μm, 6 μm, 10 μm, 12 μm, 14 μm or 15 μm, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0026] In this disclosure, by selecting and controlling the flow rate of the iron-phosphorus mixture and the average particle size of the atomized droplets within a specific range, the particle size of the resulting solid particles can be further controlled.

[0027] In one embodiment, the temperature of the spray pyrolysis is 650-750°C, for example, it can be 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C or 750°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0028] In this disclosure, by selecting and controlling the temperature of spray pyrolysis within a specific range, it is possible to promote the formation of carbon-doped iron phosphate and promote the volatilization of other substances besides carbon-doped iron phosphate, thereby reducing the content of impurities in carbon-doped iron phosphate and obtaining high-purity carbon-doped iron phosphate.

[0029] In one embodiment, the average particle size of the solid particles in step (2) is 1-3 μm, for example, it can be 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, 2.2 μm, 2.5 μm, 2.8 μm or 3 μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] In one embodiment, the smoke and dust after spray pyrolysis in step (2) are subjected to gas-solid separation to obtain solid powder and tail gas; the tail gas is absorbed by an absorbent to obtain an absorbent liquid, and the absorbent liquid and ferrous salt are mixed to obtain regenerated ammonium ferrocyanide, which is reused in step (1) to prepare iron-phosphorus mixture; the solid powder and the solid particles in step (3) are mixed and then crushed.

[0031] In this disclosure, there is no particular limitation on the type of ferrous salt, which can be any ferrous salt commonly used in the art, such as any one or a combination of at least two of ferrous sulfate, ferrous chloride or ferrous nitrate.

[0032] In one embodiment, the absorbent comprises an ammonium carbonate solution.

[0033] In one embodiment, the mass concentration of the ammonium carbonate solution is 10-40%, for example, it can be 10%, 12%, 14%, 16%, 18%, 20%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, or 40%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] In this disclosure, an absorbent is used to absorb cyanide ions, and then a ferrous salt is added to the absorbent solution to react and generate ammonium ferrocyanide, thereby achieving material recycling. Taking ferrous sulfate as an example, the reaction formula for absorption and ammonium ferrocyanide regeneration is as follows:

[0035] (NH4)2CO3+2HCN=2NH4CN+H2O+CO2;

[0036] FeSO4+6NH4CN=(NH4)4[Fe(CN)6]+(NH4)2SO4.

[0037] In one embodiment, the crushing pressure in step (3) is 0.2-0.6 MPa, for example, it can be 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa or 0.6 MPa, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0038] As an optional technical solution of the first aspect of this disclosure, the method for preparing carbon-doped iron phosphate includes the following steps:

[0039] (1) A ferrocyanide ammonium solution with a concentration of 50-200 g / L, a phosphorus source, and pyridine oxide are mixed to obtain an iron-phosphorus mixture. The molar ratio of iron to phosphorus in the iron-phosphorus mixture is (1-1.05):1. The phosphorus source includes any one or a combination of at least two of phosphoric acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate. The molar ratio of pyridine oxide to the iron ions contained in the iron-phosphorus mixture is (1-1.5):1.

[0040] (2) The iron-phosphorus mixture obtained in step (1) is sprayed at a flow rate of 1.5-1.8 mL / min and air with a flow rate of 20-30 L / min is used as the carrier gas. The spray pyrolysis is carried out at a temperature of 650-750℃. The average particle size of the atomized droplets of the iron-phosphorus mixture in the spray pyrolysis is 5-15 μm, and solid particles are obtained. The average particle size of the solid particles is 1-3 μm.

[0041] The smoke and dust after spray pyrolysis are subjected to gas-solid separation to obtain solid powder and tail gas. The tail gas is absorbed by ammonium carbonate solution with a mass concentration of 10-40% to obtain absorbent. The absorbent and ferrous salt are mixed to obtain regenerated ammonium ferrocyanide. The regenerated ammonium ferrocyanide is reused in step (1) to prepare iron-phosphorus mixture.

[0042] (3) The solid particles and solid powder obtained in step (2) are mixed and crushed at 0.2-0.6 MPa to obtain carbon-doped iron phosphate.

[0043] Secondly, this disclosure provides a carbon-doped iron phosphate, which is obtained by the preparation method of carbon-doped iron phosphate described in the first aspect of this disclosure.

[0044] In one embodiment, the purity of the carbon-doped iron phosphate is 99.98-99.99%, for example, it can be 99.98% or 99.99%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0045] In one embodiment, the tap density of the carbon-doped iron phosphate is 1.26-1.29 g / cm³. 3 For example, it could be 1.26 g / cm³ 3 1.27g / cm 3 1.28g / cm 3 Or 1.29g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0046] In one embodiment, the D50 particle size of the carbon-doped iron phosphate is 0.2-0.5 μm, for example, it can be 0.2 μm, 0.3 μm, 0.4 μm or 0.5 μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] Thirdly, this disclosure provides a method for preparing lithium iron phosphate material, wherein the method for preparing lithium iron phosphate material uses carbon-doped iron phosphate obtained in the second aspect of this disclosure;

[0048] The preparation method of the lithium iron phosphate material includes the following steps:

[0049] (a) A mixture of carbon-doped iron phosphate, lithium source and organic carbon source was mixed and then ball-milled to obtain a mixed powder;

[0050] (b) The mixed powder obtained in step (a) is calcined to obtain lithium iron phosphate material.

[0051] The carbon-doped iron phosphate disclosed herein is used to prepare lithium iron phosphate materials, which can improve the specific capacity and capacity retention of lithium iron phosphate materials and have good electrical properties.

[0052] In one embodiment, the molar ratio of lithium, iron and phosphorus in the mixed powder in step (a) is (1-1.2):1:1, for example, it can be 1:1:1, 1.02:1:1, 1.04:1:1, 1.06:1:1, 1.08:1:1, 1.1:1:1, 1.12:1:1, 1.14:1:1, 1.16:1:1, 1.18:1:1 or 1.2:1:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0053] In one embodiment, the organic carbon source accounts for 5-20% of the mass of carbon-doped iron phosphate, for example, 5%, 6%, 8%, 10%, 12%, 14%, 16%, 18% or 20%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0054] In this disclosure, the lithium source is not specifically limited and any lithium source commonly used in the art for preparing lithium iron phosphate materials can be used, such as lithium carbonate and / or lithium hydroxide.

[0055] In this disclosure, the organic carbon source is not specifically limited and can be any organic carbon source commonly used in the art for preparing lithium iron phosphate materials, such as any one or a combination of at least two of glucose, sucrose, starch or citric acid.

[0056] In this disclosure, the ball milling medium is not particularly limited and any ball milling medium commonly used in the art can be used, such as ethanol. The purpose of ball milling in this disclosure is to mix the raw materials uniformly. The ball milling time is generally 2-6 hours, and the average particle size after ball milling is generally 0.1-0.5 μm.

[0057] In one embodiment, the calcination temperature in step (b) is 750-850°C, for example, 750°C, 760°C, 780°C, 800°C, 820°C, 840°C or 850°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0058] In one embodiment, the calcination time is 12-24 hours, for example, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours or 24 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0059] Compared with the prior art, this disclosure has the following beneficial effects:

[0060] (1) The method for preparing carbon-doped iron phosphate provided in this disclosure uses ammonium ferrous hydride as the iron source and combines it with spray pyrolysis process. This method can not only obtain carbon-doped iron phosphate with uniform morphology, small particle size and high tap density, but also avoid the use of pH adjuster, avoid the introduction of acid radical ions and impurity cations, and produce no by-products, thus effectively improving the purity of carbon-doped iron phosphate.

[0061] (2) The carbon-doped iron phosphate preparation method provided in this disclosure uses pyridine oxide as both an oxidant and a surfactant. It can not only oxidize ferrous ions to ferric ions, but also control the surface tension of the spray droplets, avoid excessive porosity of the solid particles obtained by spray pyrolysis, improve the tap density of the solid particles, and make the solid particles smaller and more uniform in size. At the same time, after the pyrolysis of pyridine oxide, carbonization occurs, forming carbon doping in iron phosphate, thereby significantly improving conductivity.

[0062] (3) The method for preparing carbon-doped iron phosphate provided in this disclosure does not require multiple washing processes, which simplifies the process flow and enables the recycling of materials.

[0063] (4) The carbon-doped iron phosphate provided in this disclosure is used to prepare lithium iron phosphate materials, which can improve the specific capacity and capacity retention of lithium iron phosphate, thereby improving the electrochemical performance.

[0064] (5) The carbon-doped iron phosphate provided in this disclosure has a purity of over 99.96% and a tap density of 1.21 g / cm³. 3 The D50 particle size reaches below 0.8 μm; under optimal conditions, the purity of carbon-doped iron phosphate can reach over 99.98%, and the tap density reaches 1.26 g / cm³. 3 The D50 particle size reaches below 0.5 μm; the lithium iron phosphate material prepared by carbon doping iron phosphate as described in this disclosure has an initial charge capacity of over 160.9 mAh / g and a capacity retention rate of over 94.3% after 100 cycles; under optimal conditions, the initial charge capacity reaches over 163.5 mAh / g and the capacity retention rate after 100 cycles reaches over 95.1%.

[0065] After reading and understanding the detailed description, other aspects can be understood. Detailed Implementation

[0066] The technical solutions of this disclosure will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of this disclosure and should not be construed as specific limitations thereof.

[0067] Example 1

[0068] This embodiment provides a method for preparing carbon-doped iron phosphate, which includes the following steps:

[0069] (1) A 100 g / L ammonium ferrocyanide solution, diammonium hydrogen phosphate and pyridine oxide are mixed to obtain an iron-phosphorus mixture. The molar ratio of iron to phosphorus in the iron-phosphorus mixture is 1.02:1, and the molar ratio of pyridine oxide to iron ions in the iron-phosphorus mixture is 1.2:1.

[0070] (2) The iron-phosphorus mixture obtained in step (1) is sprayed at a flow rate of 1.6 mL / min and air with a flow rate of 25 L / min is used as the carrier gas. The average particle size of the atomized droplets of the iron-phosphorus mixture in the spray pyrolysis is 10 μm, and solid particles are obtained with an average particle size of 1.5 μm.

[0071] The smoke and dust after spray pyrolysis are subjected to gas-solid separation to obtain solid powder and tail gas. The tail gas is absorbed by ammonium carbonate solution with a mass concentration of 20% to obtain absorbent. The absorbent and ferrous sulfate are mixed according to the stoichiometric ratio to obtain regenerated ammonium ferrocyanide. The regenerated ammonium ferrocyanide is reused in step (1) to prepare iron-phosphorus mixture.

[0072] (3) The solid particles and solid powder obtained in step (2) are mixed and crushed at 0.4 MPa to obtain carbon-doped iron phosphate.

[0073] This embodiment also provides a carbon-doped iron phosphate, which is obtained by the above-described method for preparing carbon-doped iron phosphate.

[0074] This embodiment also provides a method for preparing lithium iron phosphate material, the method comprising the following steps:

[0075] (a) The carbon-doped iron phosphate, lithium carbonate and starch prepared above are mixed and then ball-milled for 4 hours using ethanol as the medium to obtain a mixed powder. The molar ratio of lithium, iron and phosphorus in the mixed powder is 1.1:1:1. The mass of starch accounts for 10% of the mass of carbon-doped iron phosphate. After drying, the powder is placed in a muffle furnace.

[0076] (b) The mixed powder obtained in step (a) is calcined at 800°C for 18 hours to obtain lithium iron phosphate material.

[0077] Example 2

[0078] This embodiment provides a method for preparing carbon-doped iron phosphate, which includes the following steps:

[0079] (1) A 50 g / L ammonium ferrocyanide solution, diammonium hydrogen phosphate and pyridine oxide are mixed to obtain an iron-phosphorus mixture. The molar ratio of iron to phosphorus in the iron-phosphorus mixture is 1:1, and the molar ratio of pyridine oxide to iron ions in the iron-phosphorus mixture is 1:1.

[0080] (2) The iron-phosphorus mixture obtained in step (1) is sprayed at a flow rate of 1.5 mL / min and air with a flow rate of 20 L / min is used as the carrier gas. The average particle size of the atomized droplets of the iron-phosphorus mixture in the spray pyrolysis is 5 μm, and solid particles are obtained with an average particle size of 1 μm.

[0081] The smoke and dust after spray pyrolysis are subjected to gas-solid separation to obtain solid powder and tail gas. The tail gas is absorbed by ammonium carbonate solution with a mass concentration of 10% to obtain absorbent. The absorbent and ferrous sulfate are mixed according to the stoichiometric ratio to obtain regenerated ammonium ferrocyanide. The regenerated ammonium ferrocyanide is reused in step (1) to prepare iron-phosphorus mixture.

[0082] (3) The solid particles and solid powder obtained in step (2) are mixed and crushed at 0.2 MPa to obtain carbon-doped iron phosphate.

[0083] This embodiment also provides a carbon-doped iron phosphate, which is obtained by the above-described method for preparing carbon-doped iron phosphate.

[0084] This embodiment also provides a method for preparing lithium iron phosphate material, the method comprising the following steps:

[0085] (a) The carbon-doped iron phosphate, lithium carbonate and starch prepared above are mixed and then ball-milled for 4 hours using ethanol as the medium to obtain a mixed powder. The molar ratio of lithium, iron and phosphorus in the mixed powder is 1:1:1. The mass of starch accounts for 10% of the mass of carbon-doped iron phosphate. After drying, the powder is placed in a muffle furnace.

[0086] (b) The mixed powder obtained in step (a) is calcined at 750°C for 24 hours to obtain lithium iron phosphate material.

[0087] Example 3

[0088] This embodiment provides a method for preparing carbon-doped iron phosphate, which includes the following steps:

[0089] (1) A 200 g / L ammonium ferrocyanide solution, diammonium hydrogen phosphate and pyridine oxide are mixed to obtain an iron-phosphorus mixture. The molar ratio of iron to phosphorus in the iron-phosphorus mixture is 1.05:1, and the molar ratio of pyridine oxide to iron ions in the iron-phosphorus mixture is 1.5:1.

[0090] (2) The iron-phosphorus mixture obtained in step (1) is sprayed at a flow rate of 1.8 mL / min and air with a flow rate of 30 L / min is used as the carrier gas. The average particle size of the atomized droplets of the iron-phosphorus mixture during the spray pyrolysis is 15 μm, and solid particles are obtained with an average particle size of 2 μm.

[0091] The smoke and dust after spray pyrolysis are subjected to gas-solid separation to obtain solid powder and tail gas. The tail gas is absorbed by ammonium carbonate solution with a mass concentration of 40% to obtain absorbent. The absorbent and ferrous sulfate are mixed according to the stoichiometric ratio to obtain regenerated ammonium ferrocyanide. The regenerated ammonium ferrocyanide is reused in step (1) to prepare iron-phosphorus mixture.

[0092] (3) The solid particles and solid powder obtained in step (2) are mixed and crushed at 0.6 MPa to obtain carbon-doped iron phosphate.

[0093] This embodiment also provides a carbon-doped iron phosphate, which is obtained by the above-described method for preparing carbon-doped iron phosphate.

[0094] This embodiment also provides a method for preparing lithium iron phosphate material, the method comprising the following steps:

[0095] (a) The carbon-doped iron phosphate, lithium carbonate and starch prepared above are mixed and then ball-milled for 4 hours using ethanol as the medium to obtain a mixed powder. The molar ratio of lithium, iron and phosphorus in the mixed powder is 1.2:1:1. The mass of starch accounts for 10% of the mass of carbon-doped iron phosphate. After drying, the powder is placed in a muffle furnace.

[0096] (b) The mixed powder obtained in step (a) is calcined at 850°C for 12 hours to obtain lithium iron phosphate material.

[0097] Example 4

[0098] This embodiment provides a method for preparing carbon-doped iron phosphate. The only difference between this method and Example 1 is that the molar ratio of iron ions in the pyridine oxide and iron-phosphorus mixture is 0.8:1.

[0099] This embodiment also provides a carbon-doped iron phosphate, which is obtained by the above-described method for preparing carbon-doped iron phosphate.

[0100] This embodiment also provides a method for preparing lithium iron phosphate material. The difference between the preparation method of lithium iron phosphate material and that of Example 1 is that the carbon-doped iron phosphate used is obtained in this embodiment.

[0101] Example 5

[0102] This embodiment provides a method for preparing carbon-doped iron phosphate. The only difference between this method and Example 1 is that the molar ratio of iron ions in the pyridine oxide and iron-phosphorus mixture is 1.7:1.

[0103] This embodiment also provides a carbon-doped iron phosphate, which is obtained by the above-described method for preparing carbon-doped iron phosphate.

[0104] This embodiment also provides a method for preparing lithium iron phosphate material. The difference between the preparation method of lithium iron phosphate material and that of Example 1 is that the carbon-doped iron phosphate used is obtained in this embodiment.

[0105] Comparative Example 1

[0106] This comparative example provides a method for preparing carbon-doped iron phosphate, which differs from Example 1 only in that it uses ferrous sulfate with an equivalent iron ion concentration as the iron source.

[0107] This comparative example also provides a carbon-doped iron phosphate, which is obtained by the above-described method for preparing carbon-doped iron phosphate.

[0108] This comparative example also provides a method for preparing lithium iron phosphate material. The difference between this method and Example 1 is that the carbon-doped iron phosphate used is obtained by this comparative example.

[0109] Comparative Example 2

[0110] This comparative example provides a method for preparing iron phosphate. The only difference from Example 1 is that pyridine oxide is not added to the iron-phosphate mixture, and ammonium ferricyanide solution is used as the iron source. The concentration of iron in the ammonium ferricyanide solution is the same as that in the ammonium ferricyanide solution in Example 1.

[0111] This comparative example also provides an iron phosphate, which is obtained by the above-described method for preparing iron phosphate.

[0112] This comparative example also provides a method for preparing lithium iron phosphate material. The difference between this method and Example 1 is that the carbon-doped iron phosphate used is replaced with the iron phosphate prepared in this comparative example.

[0113] Comparative Example 3

[0114] This comparative example provides a method for preparing iron phosphate. The only difference from Example 1 is that pyridine oxide is not added to the iron-phosphate mixture, and iron sulfate solution is used as the iron source. The iron sulfate solution has the same iron concentration as the ammonium ferrocyanide solution in Example 1.

[0115] This comparative example also provides an iron phosphate, which is obtained by the above-described method for preparing iron phosphate.

[0116] This comparative example also provides a method for preparing lithium iron phosphate material. The difference between this method and Example 1 is that the carbon-doped iron phosphate used is replaced with the iron phosphate prepared in this comparative example.

[0117] Comparative Example 4

[0118] This comparative example provides a method for preparing ferric phosphate, which differs from Example 1 only in that pyridine oxide is replaced with hydrogen peroxide.

[0119] This comparative example also provides an iron phosphate, which is obtained by the above-described method for preparing iron phosphate.

[0120] This comparative example also provides a method for preparing lithium iron phosphate material. The difference between this method and Example 1 is that the carbon-doped iron phosphate used is replaced with the iron phosphate prepared in this comparative example.

[0121] The purity of carbon-doped iron phosphate obtained in Examples 1-5 and Comparative Example 1, and iron phosphate obtained in Comparative Examples 2-4 were detected by X-ray fluorescence spectrometry. The tap density and specific surface area of ​​carbon-doped iron phosphate obtained in Examples 1-5 and Comparative Example 1, and iron phosphate obtained in Comparative Examples 2-4 were tested by BET method. The results are shown in Table 1.

[0122] The sulfur content of carbon-doped iron phosphate obtained in Examples 1-5 and Comparative Example 1, and iron phosphate obtained in Comparative Examples 2-4 were detected using inductively coupled plasma atomic emission spectrometry (ICP-AES). The results are shown in Table 1.

[0123] The D50 particle size of carbon-doped iron phosphate obtained in Examples 1-5 and Comparative Example 1, and iron phosphate obtained in Comparative Examples 2-4 were detected using a laser particle size analyzer. The results are shown in Table 1.

[0124] Electrical performance testing: The lithium iron phosphate materials obtained in Examples 1-5 and Comparative Examples 1-4, the conductive agent (Super P), and the binder (polyvinylidene fluoride, PVDF) were mixed at a mass ratio of 90:5:5. The mixture was then added to the solvent N-methylpyrrolidone (NMP) and stirred to obtain a lithium iron phosphate slurry. This slurry was coated onto the positive electrode current collector aluminum foil and dried to obtain the positive electrode sheet. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a mass ratio of 1:1:1 to prepare an electrolyte with a LiPF6 concentration of 1.0 mol / L. A 2025 coin cell was assembled using a polyethylene film (PE) as the separator and a lithium metal sheet as the negative electrode, and its electrochemical performance was tested.

[0125] Charge-discharge tests were conducted at 2-3.75V and 25℃ to obtain the initial charge capacity and capacity retention rate after 100 cycles. The results are shown in Table 1. The compaction density of the lithium iron phosphate material in Table 1 was measured using a PRCD compaction density meter.

[0126] Table 1

[0127]

[0128] The following points can be observed from Table 1:

[0129] (1) As can be seen from the data in Examples 1-5, the purity of the carbon-doped iron phosphate provided in this disclosure can reach over 99.96%, and the tap density can reach 1.21 g / cm³. 3 The D50 particle size reaches below 0.8 μm; under optimal conditions, the purity of carbon-doped iron phosphate can reach over 99.98%, and the tap density reaches 1.26 g / cm³. 3 The D50 particle size reaches below 0.5 μm; the lithium iron phosphate material prepared by carbon doping iron phosphate as described in this disclosure has an initial charge capacity of over 160.9 mAh / g and a capacity retention rate of over 94.3% after 100 cycles; under optimal conditions, the initial charge capacity reaches over 163.5 mAh / g and the capacity retention rate after 100 cycles reaches over 95.1%.

[0130] (2) A comprehensive comparison of the data from Examples 1 and 4-5 shows that the only difference between Examples 4-5 and Example 1 is that the molar ratio of iron ions in the pyridine oxide and iron-phosphorus mixture is not within the range selectable by this disclosure. The tap density in Example 1 is significantly higher than that in Examples 4-5, the D50 particle size in Example 1 is significantly smaller than that in Examples 4-5, and the first charge capacity and 100-cycle capacity retention rate of the lithium iron phosphate material prepared by carbon doped iron phosphate obtained in Example 1 are significantly higher than those in Examples 4-5. It can be seen that by controlling the molar ratio of iron ions in the pyridine oxide and iron-phosphorus mixture, this disclosure can further improve the tap density of carbon doped lithium iron phosphate material and reduce the particle size, thereby further improving the performance of the obtained lithium iron phosphate material.

[0131] (3) A comprehensive comparison of the data from Example 1 and Comparative Examples 1-3 shows that the only difference between Comparative Example 1 and Example 1 is the use of ferrous sulfate as the iron source; the only difference between Comparative Example 2 and Example 1 is the use of ammonium ferricyanide as the iron source instead of pyridine oxide; and the only difference between Comparative Example 3 and Example 1 is the use of ferric sulfate instead of pyridine oxide. The performance of carbon-doped iron phosphate in Example 1 is significantly better than that in Comparative Examples 1-3, and the performance of lithium iron phosphate material prepared using carbon-doped iron phosphate obtained in Example 1 is significantly better than that in Comparative Examples 1-3. Therefore, it can be seen that using ammonium ferrocyanide as the iron source in this disclosure can avoid excessive sulfur content in carbon-doped iron phosphate and improve the purity of carbon-doped iron phosphate. At the same time, using pyridine oxide as an oxidant in this disclosure can oxidize ferrous ions to iron ions and can also act as a surfactant to reduce the surface tension of the spray droplets, avoid excessive porosity of the solid particles obtained by spray pyrolysis, and improve the tap density of the solid particles. In addition, after spray pyrolysis, pyridine oxide carbonizes to form carbon-doped iron phosphate, which significantly improves the conductivity of carbon-doped iron phosphate and ultimately improves the capacity and cycle performance of lithium iron phosphate materials.

[0132] (4) A comprehensive comparison of the data of Example 1 and Comparative Example 4 shows that the only difference between Comparative Example 4 and Example 1 is that pyridine oxide is replaced with hydrogen peroxide. The performance of carbon-doped iron phosphate in Example 1 is significantly better than that of Comparative Example 4. Furthermore, the performance of lithium iron phosphate material prepared using carbon-doped iron phosphate obtained in Example 1 is significantly better than that of Comparative Example 4. Thus, it can be seen that the present disclosure uses pyridine oxide as an oxidant to obtain carbon-doped iron phosphate and improve the capacity and cycle performance of lithium iron phosphate material.

[0133] In summary, the carbon-doped iron phosphate preparation method provided in this disclosure can improve the tap density of solid particles, resulting in smaller particle size, more uniform size, and higher purity. This is beneficial for improving the performance and quality of lithium iron phosphate materials, greatly simplifies the production process, and has high industrial application value.

Claims

1. A method for preparing carbon-doped ferriphosphate, comprising the following steps: (1) mixing ammonium ferrocyanide, a phosphorus source and pyridine oxide to obtain an iron-phosphorus mixture; (2) performing spray pyrolysis on the iron-phosphorus mixture obtained in step (1) to obtain solid particles; (3) crushing the solid particles obtained in step (2) to obtain carbon-doped ferriphosphate; in step (1), the ammonium ferrocyanide is mixed in the form of an ammonium ferrocyanide solution, and the concentration of iron ions in the ammonium ferrocyanide solution is 50-200 g / L; the molar ratio of iron to phosphorus in the iron-phosphorus mixture is (1-1.05) : 1; the molar ratio of pyridine oxide to iron ions contained in the iron-phosphorus mixture is (1-1.5) : 1; the phosphorus source comprises any one or a combination of at least two of phosphoric acid, diammonium hydrogen phosphate or ammonium dihydrogen phosphate; in step (2), the carrier gas used in the spray pyrolysis comprises air, and the flow rate of the carrier gas is 20-30 L / min; the flow rate of the iron-phosphorus mixture in the spray pyrolysis is 1.5-1.8 mL / min; the average particle size of the atomized droplets of the iron-phosphorus mixture in the spray pyrolysis is 5-15 μm; the temperature of the spray pyrolysis is 650-750 ℃; in step (2), the average particle size of the solid particles is 1-3 μm; in step (2), the fume after the spray pyrolysis is subjected to gas-solid separation to obtain solid powder and tail gas; the tail gas is absorbed by an absorbent to obtain an absorption liquid, the absorption liquid and a ferrous salt are mixed to obtain regenerated ammonium ferrocyanide, the regenerated ammonium ferrocyanide is used for preparing the iron-phosphorus mixture in step (1), and the solid powder and the solid particles in step (3) are mixed and then crushed; the absorbent comprises an ammonium carbonate solution, and the mass concentration of the ammonium carbonate solution is 10-40%; in step (3), the crushing pressure is 0.2-0.6 MPa; and the method for preparing carbon-doped ferriphosphate comprises the following steps: (1) mixing an ammonium ferrocyanide solution with a concentration of 50-200 g / L of iron ions, a phosphorus source and pyridine oxide to obtain an iron-phosphorus mixture, the molar ratio of iron to phosphorus in the iron-phosphorus mixture is (1-1.05) : 1, the phosphorus source comprises any one or a combination of at least two of phosphoric acid, diammonium hydrogen phosphate or ammonium dihydrogen phosphate, and the molar ratio of pyridine oxide to iron ions contained in the iron-phosphorus mixture is (1-1.5) : 1; (2) performing spray pyrolysis on the iron-phosphorus mixture obtained in step (1) at a flow rate of 1.5-1.8 mL / min and using air with a flow rate of 20-30 L / min as a carrier gas at a temperature of 650-750 ℃, the average particle size of the atomized droplets of the iron-phosphorus mixture in the spray pyrolysis is 5-15 μm, and solid particles with an average particle size of 1-3 μm are obtained. ​ ​ ​ ​ ​ ​ 2. The method of claim 1, wherein the carbon-doped iron phosphate is prepared by the steps of: ​ 3. The method of claim 1, wherein the carbon-doped iron phosphate is prepared by the steps of: ​ 4. The method of claim 1, wherein the carbon-doped iron phosphate is prepared by the steps of: ​ 5. The method of claim 1, wherein the carbon-doped iron phosphate is prepared by the steps of: ​ 6. The method of claim 1, wherein the carbon-doped iron phosphate is prepared by the steps of: ​ 7. The method for preparing carbon-doped iron phosphate according to claim 1, wherein, ​ 8. The method for preparing carbon-doped iron phosphate according to claim 1, wherein, ​ ​ 9. The method for preparing carbon-doped iron phosphate according to claim 1, wherein, ​ 10. The method for preparing carbon-doped iron phosphate according to claim 1, wherein, ​ ​ ​ The smoke dust after the spray pyrolysis is subjected to gas-solid separation to obtain solid powder and tail gas, the tail gas is absorbed by using an ammonium carbonate solution with a mass concentration of 10-40% to obtain an absorption liquid, the absorption liquid is mixed with a ferrous salt to obtain regenerated ferrous ammonium cyanide, and the regenerated ferrous ammonium cyanide is used in the preparation of the iron-phosphorus mixed solution in step (1); (3) the solid particles and the solid powder obtained in step (2) are mixed and crushed under a pressure of 0.2-0.6 MPa to obtain carbon-doped iron phosphate.

11. Carbon-doped iron phosphate prepared by the method according to any one of claims 1-10.

12. A method for preparing lithium iron phosphate material by using the carbon-doped iron phosphate according to claim 11, comprising the following steps: (a) mixing carbon-doped iron phosphate, a lithium source and an organic carbon source, and then performing ball milling to obtain mixed powder; (b) calcining the mixed powder obtained in step (a) to obtain lithium iron phosphate material.

13. The method for preparing lithium iron phosphate materials by carbon doping with iron phosphate according to claim 12, wherein, In step (a), the molar ratio of lithium, iron and phosphorus in the mixed powder is (1-1.2):1:1, and the mass of the organic carbon source accounts for 5-20% of the mass of the carbon-doped iron phosphate; In step (b), the calcination temperature is 750-850°C, and the calcination time is 12-24 h.

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