Preparation method and application of lithium iron phosphate positive electrode material

Abstract

The present invention provides a preparation method and application of a lithium iron phosphate positive electrode material. The preparation method comprises the following steps: (1) after a first mixing of an iron source and pure water, oxidation is sequentially performed, a metal ion doping source is added for a second mixing, and a phosphorus source is added for a third mixing to obtain a pure water system mixed solution, an alcohol solution is added to the pure water system mixed solution for a fourth mixing, and then the mixture is allowed to stand, and an iron phosphate precursor is obtained after centrifugal washing; (2) after a first grinding of the iron phosphate precursor and a lithium source in step (1), a carbon source is added for a second grinding to obtain a mixed powder, the mixed powder is sequentially subjected to a first sintering to obtain a first carbon-coated lithium iron phosphate, the first carbon-coated lithium iron phosphate is subjected to a second sintering, and the lithium iron phosphate positive electrode material is obtained after screening. The preparation method of the present invention has the advantages of low cost and high preparation purity.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion batteries, and in particular to a preparation method and application of a lithium iron phosphate positive electrode material. Background Art

[0002] At present, LiFePO4 has been widely used, and the mainstream production methods are: ferrous oxalate process, iron red process, full wet process, and iron phosphate process.

[0003] (1) Ferrous oxalate process: FeC2O2, (NH4)2HPO4, and Li2CO3 are mixed and ground, and then sintered under an inert atmosphere to produce lithium iron phosphate positive electrode material. For example, CN105226282A discloses a preparation process for lithium iron phosphate material, which uses ferrous oxalate, a phosphorus source, and a carbon source to mix and sinter, and then prepare lithium iron phosphate under a thermal reduction state. This can effectively increase the specific surface area of ​​the material. This process requires multiple sinterings, and a large amount of waste gas is emitted during the pre-sintering process, which has high costs and environmental risks.

[0004] (2) Iron red process: Fe2O3, LiH2PO4 and carbon source are mixed and ground, and LiFePO4 positive electrode material is synthesized by carbon thermal reduction. This process has high cost of LiH2PO4, and the particle size of the lithium iron phosphate material is difficult to control, and the performance is slightly poor.

[0005] (3) Full wet process: FeSO4, LiOH and H3PO4 are synthesized into lithium iron phosphate material through hydrothermal or solvent thermal reaction under high temperature and high pressure conditions in a reactor. This process requires high temperature and high pressure equipment, excessive lithium source requirements, and high production costs.

[0006] (4) Iron phosphate process: CN110021747A discloses a method for preparing lithium iron phosphate cathode material using pickled iron red as raw material. The method comprises mixing a lithium source, an iron source, pickled iron red and a carbon source to obtain a mixture, and sintering the mixture under protective gas to obtain a carbon-coated lithium iron phosphate cathode material. The lithium iron phosphate cathode material is obtained by mixing and grinding the iron phosphate precursor with a carbon source and a lithium source, and then carbon thermal reduction is performed. This process has certain production cost advantages, but the iron source in the preparation process of the iron phosphate precursor is mostly iron sulfate or nitrate, and the precipitant is mainly ammonia water or sodium hydroxide. This process will produce a series of by-products, such as (NH4)2SO4, Na2SO4, NH4NO3, and NaNO3, which will affect the performance of the lithium iron phosphate cathode material.

[0007] Although relevant technicians have done a lot of work, there are still problems such as high preparation cost, high impurity content, and poor product performance. It is crucial to develop a low-cost, high-performance lithium iron phosphate positive electrode material. Summary of the Invention

[0008] The purpose of the present invention is to provide a preparation method of a lithium iron phosphate positive electrode material and its application.

[0009] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0010] One of the objectives of the present invention is to provide a method for preparing a lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0011] (1) After the iron source and pure water are first mixed, oxidation is sequentially performed, a metal ion doping source is added for a second mixing, and a phosphorus source is added for a third mixing to obtain a pure water system mixed solution, an alcohol solution is added to the pure water system mixed solution for a fourth mixing, and then the mixture is allowed to stand, and then centrifuged and washed to obtain an iron phosphate precursor;

[0012] (2) The iron phosphate precursor and the lithium source described in step (1) are subjected to a first grinding, and then a carbon source is added and subjected to a second grinding to obtain a mixed powder. The mixed powder is subjected to a first sintering in sequence to obtain a first carbon-coated lithium iron phosphate. The first carbon-coated lithium iron phosphate is subjected to a second sintering, and the lithium iron phosphate positive electrode material is obtained after screening.

[0013] The lithium iron phosphate material of the present invention is not only low in preparation cost, but also has better discharge performance and cycle performance. The step (1) of the present invention adopts an alcohol-water system, which avoids the large amount of Na2CO3 introduced by ammonia water or sodium hydroxide in the traditional precipitation method. + or NH4 + Impurity ions, these ions will have an adverse effect on the material properties. While improving the purity of iron phosphate, the amount of phosphoric acid and hydrogen peroxide used is reduced, and a low-cost iron source is used. At the same time, the alcohol water can be separated and purified and reused, which reduces raw material consumption and reduces production costs. In step (1) of the present invention, the metal ion doping source is added before step (2) to prepare lithium iron phosphate, and the metal cations are doped in the liquid phase, thereby improving the uniformity of the metal cation distribution. At the same time, liquid phase doping also avoids the disadvantage of high energy consumption of high-temperature doping, thereby reducing production costs.

[0014] In step (2) of the present invention, a lithium iron phosphate positive electrode material is prepared by two coating and sintering steps. The first coating is characterized in that the carbon coating amount is small and agglomeration is not easy during the sintering process. The second coating is a gas phase coating, which is not easy to agglomerate during the sintering process and the carbon coating layer is uniform, and can be directly sieved to produce a finished product. At the same time, the first coating evenly disperses the carbon source among the lithium iron phosphate materials, enhancing local ionic conductivity. The second coating coats the surface of the lithium iron phosphate material with a layer of conductive amorphous carbon network, which not only improves electronic conductivity but also inhibits grain growth, reduces energy consumption and prevents the agglomeration and growth of lithium iron phosphate particles at high temperatures, thereby significantly improving the discharge performance and cycle performance of the lithium iron phosphate positive electrode material.

[0015] As a preferred technical solution of the present invention, the iron source in step (1) includes any one of ferrous sulfate, ferric sulfate, ferric oxide, ferric chloride, ferrous nitrate or ferric nitrate, or a combination of at least two thereof, wherein typical but non-limiting examples of the combination include: a combination of ferrous sulfate and ferric sulfate, a combination of ferric sulfate and ferric oxide, a combination of ferric oxide and ferric chloride, a combination of ferric chloride and ferrous nitrate, or a combination of ferrous nitrate and ferric nitrate, etc.

[0016] Preferably, the oxidizing agent in step (1) comprises hydrogen peroxide.

[0017] Preferably, the metal ion doping source in step (1) comprises any one or a combination of at least two of compounds of manganese, aluminum, magnesium, chromium, antimony, zirconium, niobium, titanium, zinc, cobalt, molybdenum or vanadium, wherein typical but non-limiting examples of the combination include: a combination of manganese and aluminum, a combination of magnesium and chromium, a combination of antimony and zirconium, a combination of niobium and titanium, a combination of zinc and cobalt, or a combination of molybdenum and vanadium, etc.

[0018] Preferably, the phosphorus source in step (1) includes phosphoric acid.

[0019] Preferably, the alcohol solution in step (1) comprises any one of methanol, ethanol, acetonitrile, acetone or isopropanol, or a combination of at least two of them, wherein typical but non-limiting examples of the combination include: a combination of methanol and ethanol, a combination of ethanol and acetonitrile, a combination of acetonitrile and acetone, or a combination of acetone and isopropanol, etc.

[0020] As a preferred technical solution of the present invention, the iron source, pure water, hydrogen peroxide, metal ion doping source, phosphoric acid and alcohol solution in step (1) are prepared in a molar ratio of iron element: pure water: hydrogen peroxide: metal element: phosphoric acid: alcohol of (1.0-1.5): (350-500): (0.1 ~1.5):(0.01~0.03):(1.0~1.5):(100~200) for feeding, wherein the molar ratio can be 1.0:350:0.1:0.01:1.0:100, 1.2:400:0.5:0.02:1.2:150, 1.5:500:1.5:0.03:1.5:200, 1.0:500:1.5:0.03:1.5:200, or 1.5:350:1.5:0.01:1.0:150, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0021] As a preferred technical solution of the present invention, in step (1), the first mixing, the second mixing, the third mixing and the fourth mixing are all performed under a vacuum environment.

[0022] Preferably, the first mixing in step (1) comprises heating the iron source and pure water to a constant temperature.

[0023] Preferably, the constant temperature is 60~90℃, and the constant temperature can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃ or 90℃, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, and it is further preferably 70~80℃.

[0024] Preferably, the time for the third mixing in step (1) is 0.5 to 1.5 h, wherein the time can be 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h, 1.0 h, 1.1 h, 1.2 h, 1.3 h, 1.4 h or 1.5 h, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0025] Preferably, the addition rate of the alcohol solution to the pure water system mixture in step (1) is 20~60 L / min, wherein the rate can be 20 L / min, 25 L / min, 30 L / min, 35 L / min, 40 L / min, 45 L / min, 50 L / min, 55 L / min or 60 L / min, etc., but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable, and more preferably 32~38 L / min.

[0026] Preferably, the fourth mixing time in step (1) is 2 to 3 hours, wherein the time can be 2 hours, 2.1 hours, 2.2 hours, 2.3 hours, 2.4 hours, 2.5 hours, 2.6 hours, 2.7 hours, 2.8 hours, 2.9 hours or 3 hours, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0027] Preferably, the standing time in step (1) is 0.5 to 1.5 h, wherein the time can be 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h, 1.0 h, 1.1 h, 1.2 h, 1.3 h, 1.4 h or 1.5 h, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0028] Preferably, the centrifugal washing in step (1) includes isothermal distillation or azeotropic distillation.

[0029] The alcohol solution is washed, purified and separated to obtain pure water and alcohol solution, which effectively avoids the large consumption of raw materials in the traditional precipitation method, greatly reduces the production cost and meets the requirements of green and environmentally friendly production.

[0030] As a preferred technical solution of the present invention, the molar ratio of the iron phosphate precursor, the lithium source and the carbon source in step (2) is (0.9~1.1):(0.9~1.1):(0.01~0.1), wherein the molar ratio can be 0.9:0.9:0.01, 1.0:1.0:0.05, 1.1:1.1:0.1, 0.9:1.1:0.01, 0.9:1.1:0.1, 1.1:1.1:0.01 or 1.1:1.0:0.05, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0031] Preferably, the lithium source includes any one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium nitrate, lithium oxalate, lithium acetate or lithium citrate, or a combination of at least two thereof, wherein typical but non-limiting examples of the combination include: a combination of lithium carbonate and lithium hydroxide, a combination of lithium hydroxide and lithium dihydrogen phosphate, a combination of lithium dihydrogen phosphate and lithium nitrate, a combination of lithium nitrate and lithium oxalate, a combination of lithium oxalate and lithium acetate, or a combination of lithium acetate and lithium citrate, etc.

[0032] Preferably, the carbon source comprises any one of glucose, sucrose, citric acid or crushed asphalt, or a combination of at least two of them, wherein typical but non-limiting examples of the combination include: a combination of glucose and sucrose, a combination of sucrose and citric acid, or a combination of citric acid and crushed asphalt, etc.

[0033] As a preferred technical solution of the present invention, the time for the first grinding in step (2) is 8 to 16 hours, wherein the time can be 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours or 16 hours, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0034] Preferably, the second grinding time in step (2) is 0.1~12h, wherein the time can be 0.1h, 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h or 12h, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0035] Preferably, the first sintering in step (2) is performed in an inert atmosphere.

[0036] Preferably, the inert atmosphere comprises 5% hydrogen-argon mixed gas and / or nitrogen.

[0037] Preferably, the temperature of the first sintering in step (2) is 600-700°C, wherein the temperature can be 600°C, 620°C, 640°C, 660°C, 680°C or 700°C, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0038] Preferably, the first sintering time in step (2) is 6 to 8 hours, wherein the time can be 6 hours, 6.2 hours, 6.4 hours, 6.6 hours, 6.8 hours, 7 hours, 7.2 hours, 7.4 hours, 7.6 hours, 7.8 hours or 8 hours, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, and more preferably 7.5 to 8 hours.

[0039] The second sintering can achieve the best sintering effect within the preferred range, avoiding the growth of lithium iron phosphate particles caused by too long sintering time.

[0040] Preferably, in step (2), the first sintering is followed by air flow milling to obtain the first carbon-coated lithium iron phosphate.

[0041] Preferably, the coating amount of the first carbon-coated lithium iron phosphate is 2~4%, wherein the coating amount can be 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8% or 4%, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0042] Preferably, the thickness of the carbon layer of the first carbon-coated lithium iron phosphate is 2~5nm, where the thickness can be 2nm, 2.5nm, 3nm, 3.5nm, 4nm, 4.5nm or 5nm, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0043] As a preferred technical solution of the present invention, the second sintering in step (2) includes sintering the first carbon-coated lithium iron phosphate in a protective atmosphere using a gaseous carbon source.

[0044] Preferably, the mass ratio of the first carbon-coated lithium iron phosphate and the gaseous carbon source is (0.9~1.1):(0.01~0.1), wherein the mass ratio can be 0.9:0.01, 0.9:0.05, 0.9:0.1, 1:0.01, 1:0.05, 1:0.1, 1.1:0.01, 1.1:0.05 or 1.1:0.1, etc., but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0045] Preferably, the gaseous carbon source includes any one of methane, ethane, acetylene or propane, or a combination of at least two of them, wherein typical but non-limiting examples of the combination include: a combination of methane and ethane, a combination of ethane and acetylene, or a combination of acetylene and propane.

[0046] Preferably, the protective atmosphere comprises a nitrogen atmosphere.

[0047] As a preferred technical solution of the present invention, the temperature of the second sintering in step (2) is 700~800℃, wherein the temperature can be 700℃, 720℃, 740℃, 760℃, 780℃ or 800℃, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable.

[0048] Preferably, the second sintering time in step (2) is 6 to 10 hours, wherein the time can be 6 hours, 7 hours, 8 hours, 9 hours or 10 hours, etc., but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, and more preferably 7 to 9 hours.

[0049] Preferably, the pressure of the gaseous carbon source is 0.1~0.3Mpa, wherein the pressure can be 0.1 Mpa, 0.15 Mpa, 0.2 Mpa, 0.25 Mpa or 0.3 Mpa, etc., but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable, and more preferably 0.15~0.25Mpa.

[0050] Preferably, the introduction rate of the gaseous carbon source is 150-300 mL / min, wherein the introduction rate can be 150 mL / min, 180 mL / min, 210 mL / min, 240 mL / min, 270 mL / min or 300 mL / min, etc., but is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable, and more preferably 180-220 mL / min;

[0051] Preferably, in step (2), the newly added coating amount of lithium iron phosphate in the second sintering is 1-3%;

[0052] Preferably, the thickness of the newly added carbon layer of lithium iron phosphate in the second sintering in step (2) is 1-3 nm.

[0053] As a preferred technical solution of the present invention, the preparation method comprises the following steps:

[0054] (1) heating the iron source and pure water to 60-90° C., performing a first mixing, sequentially adding an oxidant for oxidation, adding a metal ion doping source for a second mixing, and adding a phosphorus source for a third mixing for 0.5-1.5 h to obtain a pure water system mixed solution, adding an alcohol solution to the pure water system mixed solution at a rate of 20-60 L / min for a fourth mixing for 2-3 h, and then standing for 0.5-1.5 h, and centrifuging and washing to obtain an iron phosphate precursor;

[0055] (2) The iron phosphate precursor and the lithium source described in step (1) are subjected to a first grinding for 8 to 16 hours, and then a carbon source is added and subjected to a second grinding for 0.1 to 12 hours to obtain a mixed powder. The mixed powder is subjected to a first sintering at a temperature of 600 to 700° C. and a time of 6 to 8 hours to obtain a first carbon-coated lithium iron phosphate with a coating amount of 2 to 4% and a coating thickness of 2 to 5 nm. The first carbon-coated lithium iron phosphate is subjected to a second sintering at a temperature of 700 to 800° C. and a time of 6 to 10 hours, and the lithium iron phosphate positive electrode material is obtained after screening.

[0056] A second object of the present invention is to provide an application of the method for preparing the lithium iron phosphate positive electrode material as described in the first object, wherein the preparation method is applied to the field of lithium ion batteries.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] (1) The low-cost, high-purity ferric phosphate material prepared by the present invention adopts an alcohol-water system, which avoids the large amount of Na2CO3 introduced by ammonia or sodium hydroxide in the traditional precipitation method. + or NH4 + Impurity ions improve the purity of iron phosphate while reducing the amount of phosphoric acid and hydrogen peroxide, using a low-cost iron source. At the same time, alcohol water can be separated and purified and reused, reducing raw material consumption and lowering production costs.

[0059] (2) The present invention prepares multi-element doped iron phosphate materials through an alcohol-water system, which is characterized in that the traditional doping process is advanced from the lithium iron phosphate stage to the iron phosphate stage, and metal cations are doped in the liquid phase, thereby improving the uniformity of the metal cation distribution. At the same time, liquid phase doping also avoids the disadvantage of high energy consumption of high-temperature doping, thereby reducing production costs.

[0060] (3) The present invention forms a lithium iron phosphate positive electrode material by coating twice, which is characterized in that the first coating has a small amount of carbon coating and is not easy to agglomerate during the sintering process. The second coating adopts gas phase coating, which is not easy to agglomerate during the sintering process and the carbon coating layer is uniform, and can be directly sieved to form a finished product. At the same time, the first coating evenly disperses the carbon source between the lithium iron phosphate materials, enhancing the local ionic conductivity. The second coating coats the surface of the lithium iron phosphate material with a layer of conductive amorphous carbon network, which not only improves the electronic conductivity but also inhibits the growth of grains, reduces energy consumption and avoids the agglomeration and growth of lithium iron phosphate particles at high temperatures, thereby significantly improving the discharge performance and cycle performance of the lithium iron phosphate positive electrode material. Among them, the discharge performance can reach a 1C discharge capacity of more than 145mAh / g, and the discharge capacity retention rate at -25℃ can reach more than 81.54% at 1C / 2h, the discharge capacity retention rate at 0℃ can reach more than 98.69%, and the discharge capacity retention rate at 25℃ can reach more than 100%. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 This is an SEM image of the lithium iron phosphate positive electrode sheet in Example 1 of the present invention.

[0062] Figure 2 45°C & 3C / 3C cycle curves of Examples 1-4 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0063] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0064] Example 1

[0065] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material:

[0066] (1) 400 kg of FeSO4 and 10 cubic meters of pure water were put into a 30 cubic meter vacuum reactor, heated to 75 °C, and 170 kg of 30% hydrogen peroxide was added. Then, titanium solution and niobium solution were added. After thorough stirring, phosphoric acid was added and stirred for 1 hour. 9 cubic meters of alcohol solution was added at 35 L / min, stirred for 2.5 hours, and then allowed to stand for 1 hour. The iron phosphate precursor was obtained by centrifugation and washing.

[0067] (2) Iron phosphate and lithium carbonate were mixed at a molar ratio of 1:1.05 and ground for 12 hours. Glucose was added at 3% of the total mass of the mixture and ground for 8 hours. The mixture was then pressed into blocks and sintered in a kiln under a 5% hydrogen and argon mixed atmosphere at a sintering temperature of 700°C for 8 hours. The first sintered material was subjected to air flow milling to obtain a first carbon-coated lithium iron phosphate. The first carbon-coated lithium iron phosphate was subjected to a second sintering in a rotary kiln using acetylene as a carbon source and argon as a protective gas at a sintering temperature of 800°C for 8 hours, an acetylene pressure of 0.2 MPa, and an acetylene introduction rate of 200 mL / min. The second sintered material was sieved to obtain the lithium iron phosphate positive electrode material.

[0068] The SEM of the lithium iron phosphate positive electrode sheet prepared in this embodiment is as follows Figure 1 shown.

[0069] Example 2

[0070] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material:

[0071] (1) 400 kg of FeSO4 and 10 cubic meters of pure water were put into a 30 cubic meter vacuum reactor, heated to 60 °C, and 190 kg of 30% hydrogen peroxide was added. Then, titanium solution and niobium solution were added. After thorough stirring, phosphoric acid was added and stirred for 1 hour. 8 cubic meters of alcohol solution was added at 20 L / min, stirred for 2.5 hours, and then allowed to stand for 1 hour. The iron phosphate precursor was obtained by centrifugation and washing.

[0072] (2) Iron phosphate and lithium carbonate were mixed at a molar ratio of 1:1.1 and ground for 12 hours. Glucose was added at 3% of the total mass of the mixture and ground for 8 hours. The mixture was then pressed into blocks and sintered in a kiln under a 5% hydrogen and argon mixed atmosphere at a sintering temperature of 700°C for 8 hours. The first sintered material was subjected to air flow milling to obtain a first carbon-coated lithium iron phosphate. The first carbon-coated lithium iron phosphate was subjected to a second sintering in a rotary kiln using acetylene as a carbon source and argon as a protective gas at a sintering temperature of 700°C for 8 hours, an acetylene pressure of 0.3 MPa, and an acetylene introduction rate of 300 mL / min. The second sintered material was sieved to obtain the lithium iron phosphate positive electrode material.

[0073] Example 3

[0074] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material:

[0075] (1) 400 kg of FeSO4 and 10 cubic meters of pure water were put into a 30 cubic meter vacuum reactor, heated to 85 °C, and 150 kg of 30% hydrogen peroxide was added. Then, titanium solution and niobium solution were added. After thorough stirring, phosphoric acid was added and stirred for 1 hour. 10 cubic meters of alcohol solution was added at 50 L / min, stirred for 2.5 hours, and then allowed to stand for 1 hour. The iron phosphate precursor was obtained by centrifugation and washing.

[0076] (2) Iron phosphate and lithium carbonate were mixed at a molar ratio of 1:1.1 and ground for 12 hours. Glucose was added at 3% of the total mass of the mixture and ground for 8 hours. The mixture was then pressed into blocks and sintered in a kiln under a 5% hydrogen and argon mixed atmosphere at a sintering temperature of 700°C for 8 hours. The first sintered material was subjected to air flow milling to obtain a first carbon-coated lithium iron phosphate. The first carbon-coated lithium iron phosphate was subjected to a second sintering in a rotary kiln using acetylene as a carbon source and argon as a protective gas at a sintering temperature of 700°C for 8 hours, an acetylene pressure of 0.2 MPa, and an acetylene introduction rate of 200 mL / min. The second sintered material was sieved to obtain the lithium iron phosphate positive electrode material.

[0077] Example 4

[0078] In this embodiment, except that the glucose in step (2) is changed to the crushed asphalt, the other conditions are the same as those in embodiment 1.

[0079] Example 5

[0080] In this embodiment, except that heating to 75°C in step (1) is replaced by heating to 55°C, other conditions are the same as those in Example 1.

[0081] Example 6

[0082] In this embodiment, except that heating to 95°C is replaced with heating to 75°C in step (1), other conditions are the same as those in Example 1.

[0083] Example 7

[0084] In this embodiment, except that the addition of 9 cubic meters of alcohol solution at 35 L / min in step (1) is replaced by the addition of 9 cubic meters of alcohol solution at 15 L / min, other conditions are the same as those in Example 1.

[0085] Example 8

[0086] In this embodiment, except that the addition of 9 cubic meters of alcohol solution at 35 L / min in step (1) is replaced by the addition of 9 cubic meters of alcohol solution at 65 L / min, other conditions are the same as those in Example 1.

[0087] Example 9

[0088] In this embodiment, except that the addition of 9 cubic meters of alcohol solution at 35 L / min in step (1) is replaced by the addition of 1.5 cubic meters of alcohol solution at 35 L / min, other conditions are the same as those in Example 1.

[0089] Example 10

[0090] In this embodiment, except that the addition of 9 cubic meters of alcohol solution at 35 L / min in step (1) is replaced by the addition of 20 cubic meters of alcohol solution at 35 L / min, other conditions are the same as those in Example 1.

[0091] Example 11

[0092] In this example, except that the acetylene pressure of the carbon source gas in step (2) was 0.3 MPa and the acetylene introduction rate was 200 mL / min, it was replaced with an acetylene pressure of 0.1 MPa and an acetylene introduction rate of 100 mL / min, all other conditions were the same as those in Example 1.

[0093] Example 12

[0094] In this embodiment, except that the first sintering time of 8 h in step (2) is replaced by the first sintering time of 12 h, other conditions are the same as those in embodiment 1.

[0095] Example 13

[0096] In this embodiment, except that the second sintering time of 8 h in step (2) is replaced by the second sintering time of 12 h, other conditions are the same as those in embodiment 1.

[0097] Comparative Example 1

[0098] This comparative example provides a method for preparing a lithium iron phosphate positive electrode material:

[0099] (1) 400 kg of FeSO4 and 10 cubic meters of pure water were put into a 30 cubic meter vacuum reactor, heated to 75 °C, and 170 kg of 30% hydrogen peroxide was added. Then, titanium solution and niobium solution were added. After thorough stirring, phosphoric acid was added and stirred for 1 hour. 9 cubic meters of alcohol solution was added at 35 L / min, stirred for 2.5 hours, and then allowed to stand for 1 hour. The iron phosphate precursor was obtained by centrifugation and washing.

[0100] (2) Iron phosphate and lithium carbonate were mixed at a molar ratio of 1:1.05 and ground for 12 hours. Glucose was added at 5% of the total mass of the mixture and ground for 8 hours. The mixture was then pressed into blocks and sintered in a kiln under a 5% hydrogen-argon mixed atmosphere at a sintering temperature of 700°C for 8 hours. The first sintered material was subjected to air flow milling to obtain a first carbon-coated lithium iron phosphate. The carbon coating amount was the same as in Example 1. The lithium iron phosphate positive electrode material was directly sieved to obtain the above-mentioned material.

[0101] Among them, the battery cycle performance of Examples 1-4 of the present invention and Comparative Example 1 is as follows: Figure 2 shown.

[0102] Comparative Example 2

[0103] In this comparative example, except that heating to 90°C is used instead of heating to 75°C in step (1), other conditions are the same as those in comparative example 1.

[0104] Comparative Example 3

[0105] In this comparative example, except that the addition rate of 9 squares of alcohol solution at 35 L / min in step (1) was replaced by the addition rate of alcohol solution at 50 L / min, all other conditions were the same as those in comparative example 1.

[0106] Comparative Example 4

[0107] In this comparative example, the titanium solution and the niobium solution in step (1) are not added, and a metal ion doping source is added in step (2) for co-sintering. Other conditions are the same as those in Example 1.

[0108] Comparative Example 5

[0109] In this comparative example, except that alcohol solution was not added to the system in step (1), other conditions were the same as those in Example 1.

[0110] The lithium iron phosphate cathode materials provided in Examples 1-13 and Comparative Examples 1-5 were used as the positive electrode material, metallic lithium was used as the negative electrode, the electrolyte solvent was EC:EMC:DMC = 1:1:1, and the solute was 1.0 mol / L LiPF6. The positive and negative electrode sheets, separators, and electrolyte were prepared in a nitrogen-protected glove box to form a C2025 battery. The battery was charged at 0.1C constant current and constant voltage at 2.5-3.65V, and discharged at 0.1C constant current to 2.5V (referred to as 0.1C charge and discharge); then charged at 1C constant current and constant voltage, and discharged at 1C constant current to 2.5V (referred to as 1C charge and discharge). The test results are shown in Table 1.

[0111] Table 1

[0112]

[0113] From the above results, it can be concluded that the 0.1C gram capacity of the lithium iron phosphate positive electrode materials of Examples 1-4 of the present invention are all above 150 mAh / g, and the 1C gram capacity is all above 130 mAh / g. The gram capacity of the positive electrode material is better. At the same time, the gram capacity of only one coating is lower than the gram capacity of the second coating under the same carbon coating amount, indicating that the combination of gas phase and solid phase coating can reduce the introduction of impurities, make the surface coating more uniform, and better improve the gram capacity and processing performance of the lithium iron phosphate positive electrode material.

[0114] In Examples 5-6, the heating temperature was increased or decreased, and the battery discharge performance decreased compared to Example 1. In Examples 7-8, the rate of addition of the alcohol solution was increased or decreased, and the battery discharge performance decreased. In Examples 9-10, the ratio of the alcohol solution to pure water was increased or decreased, and the battery discharge performance decreased. The changes in battery performance in Examples 7-10 are because the ratio of the alcohol solution to pure water and the rate of addition of the alcohol solution are key to controlling the size of the iron phosphate particles. In Example 11, the discharge performance of the battery decreased slightly after the acetylene injection rate and pressure were changed. In Examples 12-13, the time of the first sintering and the second sintering was increased, and the battery discharge performance decreased slightly compared to Example 1.

[0115] In Comparative Example 1, only one coating was performed, and the discharge performance of the battery was reduced compared with that in Example 1; in Comparative Example 2, compared with Comparative Example 1, the heating temperature in step (1) was increased, and the performance of the battery was further reduced on the basis of Comparative Example 1; in Comparative Example 3, compared with Comparative Example 1, the addition rate of the alcohol solution was further reduced, and the discharge performance of the battery was further reduced compared with Comparative Example 1; in Comparative Example 4, compared with Comparative Example 1, the element doping was replaced from the iron phosphate section to the lithium iron phosphate section, and the discharge performance of the battery was reduced; in Comparative Example 5, compared with Comparative Example 1, no alcohol solution was added. The alcohol solution is an alkaline solution with a precipitation effect. If the alkaline solution is not added, the yield of the substance is reduced and the discharge performance of the battery is reduced.

[0116] To further evaluate the performance of lithium batteries made with the lithium iron phosphate positive electrode material in Example 1, finished batteries were produced from Examples 1-13 and Comparative Examples 1-5. Soft-pack lithium-ion batteries were produced using a soft-pack lamination process. The lithium iron phosphate positive electrode sheet, graphite negative electrode sheet, and PE+OBS separator were laminated, assembled, and baked until the moisture content was acceptable. Electrolyte was injected, and the battery was hot-pressed, placed at high temperature, and then packaged. After volume separation and room temperature placement, the finished battery was obtained. The lithium iron phosphate battery was subjected to discharge tests at different temperatures and high-temperature cycle tests. The test results are shown in Table 2.

[0117] The discharge test method at different temperatures is as follows: adjust the SOC of Examples 1-13 and Comparative Examples 1-5 to 100% at 25°C, place the batteries at different temperatures for different time periods (-25 / 0°C for 2 hours, 25°C for 1 hour), discharge at 0.1C / 1C / 3C (3.7V~2.5V), place them for 30 minutes, and then terminate the test.

[0118] Table 2

[0119]

[0120] The above results show that the battery in Example 1 has a high discharge capacity retention rate at -25~25°C, and its low-temperature rate discharge performance is significantly better than that of each comparative example. Moreover, after 1200 cycles at 45°C and 3C / 3C, Example 1 still has a capacity retention rate of 85%, showing excellent high-temperature and high-rate cycle performance. The rate discharge performance and high-temperature cycle performance of the soft-pack lithium iron phosphate battery prepared using the lithium iron phosphate positive electrode material in Example 1 are significantly higher than those of the batteries prepared using Examples 2-13 and Comparative Examples 1-5.

[0121] In Examples 5-6, the heating temperature was increased or decreased, and the battery capacity retention rate decreased compared to Example 1. In Examples 7-8, the rate of addition of the alcohol solution was increased or decreased, and the battery capacity retention rate decreased. In Examples 9-10, the ratio of the alcohol solution to pure water was increased or decreased, and the battery capacity retention rate decreased. The changes in battery performance in Examples 7-10 are because the ratio of the alcohol solution to pure water and the rate of addition of the alcohol solution are key to controlling the size of the iron phosphate particles. In Example 11, after changing the acetylene injection rate and pressure, the battery capacity retention rate decreased slightly. In Examples 12-13, the time of the first sintering and the second sintering was increased, and the overall battery capacity retention rate decreased slightly compared to Example 1.

[0122] In Comparative Example 1, only one coating is performed, and the capacity retention rate of the battery is lower than that of Example 1; in Comparative Example 2, compared with Comparative Example 1, the heating temperature of step (1) is increased, and the capacity retention rate performance of the battery is further reduced on the basis of Comparative Example 1; in Comparative Example 3, compared with Comparative Example 1, the addition rate of the alcohol solution is further reduced, and the capacity retention rate performance of the battery is further reduced compared with Comparative Example 1; in Comparative Example 4, compared with Comparative Example 1, the element doping is replaced from the iron phosphate section to the lithium iron phosphate section, and the capacity retention rate of the battery is reduced; in Comparative Example 5, compared with Comparative Example 1, no alcohol solution is added. The alcohol solution is an alkaline solution with a precipitation effect. If the alkaline solution is not added, the yield of the substance is reduced and the capacity retention rate of the battery is reduced.

[0123] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The preparation method comprises the following steps: (1) After the iron source and pure water are first mixed, oxidation is carried out in sequence, a metal ion doping source is added for a second mixing, and a phosphorus source is added for a third mixing for 0.5 to 1.5 hours to obtain a pure water system mixed solution, an alcohol solution is added to the pure water system mixed solution for a fourth mixing for 2 to 3 hours, and then the mixture is allowed to stand for 0.5 to 1.5 hours, and an iron phosphate precursor is obtained after centrifugal washing; The oxidizing oxidant includes hydrogen peroxide; the metal ion doping source includes any one or a combination of at least two compounds of manganese, aluminum, magnesium, chromium, antimony, zirconium, niobium, titanium, zinc, cobalt, molybdenum or vanadium; the phosphorus source includes phosphoric acid; the iron source, pure water, hydrogen peroxide, metal ion doping source, phosphoric acid and alcohol solution are added according to the molar ratio of iron element: pure water: hydrogen peroxide: metal element: phosphoric acid: alcohol of (1.0-1.5): (350-500): (0.1-1.5): (0.1-0.03): (1.0-1.5): (100-200); the first mixing, the second mixing, the third mixing and the fourth mixing are all carried out under vacuum; the first mixing includes heating the iron source and pure water to a constant temperature of 70-80°C; The alcohol solution is added to the pure water system mixture at a rate of 32-38 L / min; (2) The iron phosphate precursor and the lithium source described in step (1) are subjected to a first grinding, and then a carbon source is added for a second grinding to obtain a mixed powder, the mixed powder is sequentially subjected to a first sintering at 600-700° C., the first sintering time is 6-8 hours, to obtain a first carbon-coated lithium iron phosphate, the first carbon-coated lithium iron phosphate is subjected to a second sintering at 700-800° C., the second sintering time is 6-10 hours, the second sintering includes sintering the first carbon-coated lithium iron phosphate using a gas carbon source under a protective atmosphere, and the lithium iron phosphate positive electrode material is obtained after screening treatment; The molar ratio of the iron phosphate precursor, the lithium source and the carbon source is (0.9-1.1): (0.9-1.1): (0.01-0.1); the first sintering is performed in an inert atmosphere; The introduction pressure of the gaseous carbon source is 0.15-0.25 MPa, and the introduction rate of the gaseous carbon source is 180-220 mL / min.

2. The preparation method according to claim 1, characterized in that The iron source in step (1) includes ferrous sulfate and / or ferrous nitrate.

3. The preparation method according to claim 1, characterized in that The alcohol solution in step (1) includes any one of methanol, ethanol, or isopropanol, or a combination of at least two of them.

4. The preparation method according to claim 1, characterized in that The lithium source includes any one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium nitrate, lithium oxalate, lithium acetate or lithium citrate, or a combination of at least two thereof.

5. The preparation method according to claim 1, characterized in that The carbon source includes any one of glucose, sucrose, citric acid or crushed asphalt, or a combination of at least two of them.

6. The preparation method according to claim 1, characterized in that The first grinding time in step (2) is 8 to 16 hours.

7. The preparation method according to claim 1, characterized in that The second grinding time in step (2) is 0.1 to 12 hours.

8. The preparation method according to claim 1, characterized in that The inert atmosphere includes 5% hydrogen-argon mixed gas and / or nitrogen.

9. The preparation method according to claim 1, characterized in that The first sintering time in step (2) is 7.5 to 8 hours.

10. The preparation method according to claim 1, characterized in that Step (2) After the first sintering, air flow milling is performed to obtain a first carbon-coated lithium iron phosphate.

11. The preparation method according to claim 10, characterized in that: The coating amount of the first carbon-coated lithium iron phosphate is 2-4%.

12. The preparation method according to claim 10, characterized in that The thickness of the carbon layer of the first carbon-coated lithium iron phosphate is 2-5 nm.

13. The preparation method according to claim 10, characterized in that The mass ratio of the first carbon-coated lithium iron phosphate to the gaseous carbon source is (0.9-1.1): (0.01-0.1).

14. The preparation method according to claim 1, characterized in that The gaseous carbon source includes any one of methane, ethane, acetylene or propane, or a combination of at least two of them.

15. The preparation method according to claim 1, characterized in that The protective atmosphere includes a nitrogen atmosphere.

16. The preparation method according to claim 1, characterized in that The second sintering time in step (2) is 7 to 9 hours.

17. The preparation method according to claim 1, characterized in that In step (2), the newly added coating amount of lithium iron phosphate in the second sintering is 1-3%.

18. The preparation method according to claim 1, characterized in that In step (2), the thickness of the newly added carbon layer of lithium iron phosphate in the second sintering is 1-3 nm.

19. The preparation method according to claim 1, characterized in that The first grinding time in step (2) is 8 to 16 hours, and the second grinding time is 0.1 to 12 hours; in step (2), the coating amount of the first carbon-coated lithium iron phosphate is 2 to 4%, and the coating thickness is 2 to 5 nm.

20. An application of the method for preparing a lithium iron phosphate positive electrode material according to any one of claims 1 to 19, characterized in that: The preparation method is applied to the field of lithium ion batteries.

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