Modified lithium iron phosphate and its preparation method and application

Abstract

The present invention relates to the technical field of positive electrode active materials, and specifically to a modified lithium iron phosphate and a preparation method and application thereof. The preparation method comprises the steps of: after ball milling the reaction solution in a supercritical fluid, performing a first calcination, removing impurities and drying treatment to obtain lithium iron phosphate; the reaction solution contains a lithium source, an iron source and a phosphorus source; the supercritical fluid comprises one of carbon dioxide, ethane, propylene and propane; the lithium iron phosphate, starch and water are made into a non-Newtonian fluid mixture and then a second calcination is performed to obtain modified lithium iron phosphate. The preparation method of the modified lithium iron phosphate of the present invention combines supercritical fluid technology and non-Newtonian fluid technology to prepare the modified lithium iron phosphate. The supercritical fluid technology can make the lithium source, iron source and phosphorus source uniformly dispersed during the preparation process, reduce the particle size of the lithium iron phosphate product, and the non-Newtonian fluid can make the surface of the lithium iron phosphate uniformly carbon-coated, thereby improving the electron transfer rate (conductivity).

Description

Technical Field

[0001] The present invention relates to the technical field of positive electrode active materials, and in particular to a modified lithium iron phosphate and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate (LiFePO4), the active material for the positive electrode of lithium batteries, has the characteristics of high thermal stability and long cycle life, but its electronic conductivity is low and its ion diffusion performance is poor. The existing methods for improving the conductivity of lithium iron phosphate mainly include: ① doping with other materials with higher conductivity; ② surface coating with carbon and other materials; ③ improving the preparation method of lithium iron phosphate to control the size and morphology of the product particles. However, for ①, doping with other materials with higher conductivity, too high a doping amount will greatly reduce the cycle stability of the lithium iron phosphate material, and too low a doping amount will not meet the demand; for ②, the thickness of the carbon material cannot be accurately controlled, and the yield is low; for ③, in the preparation method, whether the solid phase method or the liquid phase method is used, only one sintering is performed to produce lithium iron phosphate. However, one-time sintering generally results in problems such as uneven quality and large particles, which leads to low conductivity of lithium iron phosphate products.

[0003] In the existing secondary sintering method, lithium iron phosphate is prepared by the first sintering, and the conductive material is doped or coated on the surface by the second sintering. However, the lithium iron phosphate produced by the traditional sintering method still has the problems of uneven quality and large particles. Summary of the invention

[0004] The present invention provides a modified lithium iron phosphate and a preparation method and application thereof, so as to solve the problems of uneven quality and large particles of lithium iron phosphate prepared by a traditional sintering method.

[0005] The present invention is achieved through the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing modified lithium iron phosphate, comprising the following steps:

[0007] S10. After the reaction solution is ball-milled in a supercritical fluid, a first calcination, impurity removal and drying treatment is performed to obtain lithium iron phosphate;

[0008] The reaction solution contains a lithium source, an iron source and a phosphorus source;

[0009] The supercritical fluid includes one of carbon dioxide, ethane, propylene and propane;

[0010] S20. calcining the non-Newtonian fluid mixture containing the lithium iron phosphate, starch and water for the second time to obtain modified lithium iron phosphate.

[0011] In some possible implementations, the molar ratio of lithium element, iron element and phosphorus element in the lithium source, the iron source and the phosphorus source is 0.99-1.05:0.99-1.05:1.

[0012] In some possible implementations, the mass ratio of the lithium iron phosphate to the starch is 1:0.2-0.3.

[0013] In some possible implementations, preparing the non-Newtonian fluid mixture comprises the following steps:

[0014] After mixing the lithium iron phosphate and the starch, water is added under stirring conditions at a rotation speed of 100 rpm to 200 rpm;

[0015] After the water is added, continue stirring at a speed of 50 rpm to 90 rpm for 10 min to 20 min;

[0016] The stirring is continued for 5 to 8 minutes at a rotation speed of 20 to 40 rpm to obtain the non-Newtonian fluid mixture.

[0017] In some possible implementations, in the step of preparing the non-Newtonian fluid mixture, the rate of adding water is 10 ml / min to 20 ml / min.

[0018] In some possible implementations, in the ball milling step, the ball milling time is 12 h to 24 h, and the ball milling speed is 300 rpm to 500 rpm.

[0019] In some possible implementations, in the first calcination step, the calcination atmosphere is an inert gas, the calcination temperature is 600° C. to 900° C., and the calcination time is 6 h to 12 h.

[0020] In some possible implementations, in the second calcination step, the calcination temperature is 600° C. to 700° C., and the calcination time is 6 h to 12 h.

[0021] In a second aspect, the present invention provides a modified lithium iron phosphate obtained by the preparation method of the modified lithium iron phosphate provided by the present invention, wherein the average particle size of the modified lithium iron phosphate is 0.6 μm to 1.12 μm.

[0022] In a third aspect, the present invention provides an application of the modified lithium iron phosphate in household appliances, electric vehicles, ships, satellites or energy storage systems.

[0023] The modified lithium iron phosphate and the preparation method thereof provided by the present invention have at least the following beneficial technical effects compared with the prior art:

[0024] (1) The preparation method of the modified lithium iron phosphate of the present invention combines supercritical fluid technology and non-Newtonian fluid technology to prepare the modified lithium iron phosphate. The supercritical fluid technology can make the lithium source, iron source and phosphorus source uniformly dispersed during the preparation process, reducing the particle size of the lithium iron phosphate product. The non-Newtonian fluid can make the surface of the lithium iron phosphate uniformly carbon-coated, thereby improving the electron transfer rate (conductivity).

[0025] (2) The preparation method of the modified lithium iron phosphate of the present invention comprises ball milling the reaction solution in a supercritical fluid. Due to the advantages of the supercritical fluid such as strong solubility, good diffusion performance and low surface tension, the lithium source, iron source and phosphorus source can be evenly dispersed during the ball milling process, thereby improving the quality uniformity of the product. In addition, the supercritical fluid (one of carbon dioxide, ethane, propylene and propane) is used as a carbon source to carry out in-situ carbon doping of the lithium iron phosphate, and the gas generated during calcination can further refine the particle size of the lithium iron phosphate.

[0026] (3) The preparation method of the modified lithium iron phosphate of the present invention comprises preparing a non-Newtonian fluid mixture of lithium iron phosphate, starch and water, wherein the lithium iron phosphate can be uniformly dispersed in the starch, and the lithium iron phosphate and the starch will not show sedimentation and stratification for a long time; during calcination, the lithium iron phosphate can be in-situ and uniformly carbon-coated. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present drawings or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present drawings. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0028] Figure 1 Schematic diagram of the process of preparing modified lithium iron phosphate in Example 1 of the present invention;

[0029] Figure 2 This is a SEM image of the modified lithium iron phosphate in Example 1 of the present invention.

[0030] The purpose, features and advantages of this figure will be further described in conjunction with the embodiments with reference to the accompanying drawings. DETAILED DESCRIPTION

[0031] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0032] In the present invention, the term "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects before and after are in an "or" relationship.

[0033] In the present invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, a~b (i.e. a and b), a~c, b~c, or a~b~c, where a, b, c can be single or multiple, respectively.

[0034] The terms "first" and "second" are used only for descriptive purposes to distinguish objects such as substances from each other, and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. For example, without departing from the scope of the implementation regulations of the present invention, the first XX may also be referred to as the second XX, and similarly, the second XX may also be referred to as the first XX. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0035] The terms used in the embodiments of the present invention are only for the purpose of describing specific implementation rules, and are not intended to limit the present invention. The singular forms "a", "said" and "the" used in the implementation rules of the present invention and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0036] It should be understood that in various embodiments of the present invention, the size of the serial numbers of the above-mentioned processes does not mean the order of execution, some or all of the steps can be executed in parallel or one after the other, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the implementation regulations of the present invention.

[0037] The weight of the relevant components mentioned in the description of the embodiments of the present invention may not only refer to the specific content of each component, but also indicate the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the description of the embodiments of the present invention, it is within the scope disclosed in the description of the embodiments of the present invention. Specifically, the mass described in the description of the embodiments of the present invention may be a mass unit known in the chemical industry such as μg, mg, g, kg, etc.

[0038] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0039] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0040] The cycle stability of lithium iron phosphate materials is one of the main reasons that limit its application field. The existing methods for improving lithium iron phosphate are generally to dope other materials into it, coat the surface with carbon and other materials to improve its conductivity or cycle stability. But no matter which method is used, the preparation method has a greater impact on the particle size, morphology, etc. of the product. The particle size, morphology, etc. of the product obtained under different preparation conditions with the same doping element are different. Among them, problems such as uneven quality, large particle size, and low yield caused by uneven dispersion of lithium source, iron source, and phosphorus source in the reaction system are particularly common. In addition, when coating the surface of lithium iron phosphate, the uneven dispersion of the coating source will also cause uneven quality and large particle size of the coated product.

[0041] Supercritical fluid is a fluid in a special state. The density of supercritical fluid is similar to that of liquid, and it can dissolve a large amount of solute like liquid; the viscosity is similar to that of gas, and it has a lower viscosity, which is conducive to the diffusion and mass transfer of solutes in it; the diffusion coefficient is between that of gas and liquid, which allows it to diffuse as quickly as gas and maintain a high solubility like liquid. This makes it possible for supercritical fluid to be used as a solvent, and the reaction system of lithium source, iron source and phosphorus source as a solute, so that the reaction system of lithium source, iron source and phosphorus source can fully diffuse and transfer mass in the supercritical fluid, thereby improving the quality uniformity and yield of lithium iron phosphate.

[0042] Non-Newtonian fluids have unique rheological properties. The viscosity is not fixed, but changes with the change of shear stress or shear rate. This change can be linear or nonlinear. It has a certain shape and strength when it is still, but it will flow only when the shear stress exceeds its yield stress. Therefore, the rheological properties of non-Newtonian fluids can provide a better stable environment for solutes and prevent solutes from precipitating or agglomerating.

[0043] In order to realize the use of the above-mentioned supercritical fluid and non-Newtonian fluid in the lithium iron phosphate preparation process, an embodiment of the present invention provides a modified lithium iron phosphate and a preparation method thereof. The preparation method combines supercritical fluid technology and non-Newtonian fluid technology to prepare modified lithium iron phosphate. Supercritical fluid technology can make the lithium source, iron source and phosphorus source uniformly dispersed during the preparation process, reducing the particle size of the lithium iron phosphate product. Non-Newtonian fluid can make the surface of lithium iron phosphate uniformly carbon-coated, improving the electron transfer rate (conductivity / resistivity).

[0044] The following is a detailed description of a modified lithium iron phosphate and a preparation method thereof according to an embodiment of the present invention.

[0045] A first aspect of an embodiment of the present invention provides a method for preparing modified lithium iron phosphate, such as Figure 1 As shown, the following steps are included:

[0046] S10. After the reaction solution is ball-milled in a supercritical fluid, a first calcination, impurity removal and drying treatment is performed to obtain lithium iron phosphate;

[0047] The reaction solution contains a lithium source, an iron source and a phosphorus source;

[0048] The supercritical fluid includes one of carbon dioxide, ethane, propylene, and propane.

[0049] S20. Prepare a non-Newtonian fluid mixture of lithium iron phosphate, starch and water and then perform a second calcination to obtain modified lithium iron phosphate.

[0050] The preparation method of the modified lithium iron phosphate of the embodiment of the present invention is to ball-mill the reaction solution in a supercritical fluid. Due to the advantages of the supercritical fluid such as strong solubility, good diffusion performance and low surface tension, the uniform dispersion of the lithium source, iron source and phosphorus source during the ball milling process can be improved, thereby improving the uniformity of the quality of the product; the supercritical fluid is used as a carbon source to perform in-situ carbon doping on the lithium iron phosphate, and the gas generated during calcination can further refine the particle size of the lithium iron phosphate. The lithium iron phosphate, starch and water are made into a non-Newtonian fluid mixture, and the lithium iron phosphate can be evenly dispersed in the starch, and the lithium iron phosphate and starch will not settle and stratify for a long time; during calcination, the lithium iron phosphate can be carbon-coated in situ and evenly. The preparation method of the modified lithium iron phosphate of the embodiment of the present invention combines supercritical fluid technology and non-Newtonian fluid technology to prepare modified lithium iron phosphate. The supercritical fluid technology can make the lithium source, iron source and phosphorus source uniformly dispersed during the preparation process, reduce the particle size of the lithium iron phosphate product, and the non-Newtonian fluid can make the surface of the lithium iron phosphate uniformly carbon-coated, thereby improving the electron transfer rate (conductivity).

[0051] As a supercritical fluid, carbon dioxide has a critical temperature of 31°C to 32°C and a critical pressure of 7.3MPa to 7.4MPa.

[0052] As a supercritical fluid, ethane has a critical temperature of 30°C to 32.25°C and a critical pressure of 4.88MPa.

[0053] As a supercritical fluid, propylene has a critical temperature of 91.9°C and a critical pressure of 4.762MPa.

[0054] As a supercritical fluid, propane has a critical temperature of 96.8°C and a critical pressure of 4.3975MPa.

[0055] In some embodiments, in the above step S10, the method of ball milling the reaction solution in a supercritical fluid comprises the following steps:

[0056] After the reaction solution and the grinding balls are mixed, a supercritical fluid source is introduced under the critical temperature of the supercritical fluid and vacuum until the critical pressure of the supercritical fluid is reached, followed by ball milling.

[0057] In some specific embodiments, the reaction solution also contains a mixture of ethanol and water, and the volume ratio of ethanol to water is (2-3): 1. In the exemplary embodiment, it can be a typical but non-limiting volume ratio such as 2: 1, 2.5: 1, 3: 1, or a range between any two volume ratios. In this case, the mixture of ethanol and water is used as a ball milling dispersion medium to further promote the dispersion of the lithium source, iron source, and phosphorus source, so that the generated product has a fine particle size and uniform quality, and improves the quality and yield of the product. It should be noted that the mixture of ethanol and water is used as a dispersion medium for ball milling, and the amount added is logically the same as the amount added during conventional ball milling, and is not particularly limited in the embodiments of the present invention.

[0058] In some specific embodiments, the grinding balls are at least one of zirconium oxide grinding balls, stainless steel grinding balls, and agate.

[0059] In some embodiments, the grinding balls include large-diameter grinding balls, medium-diameter grinding balls, and small-diameter grinding balls, and the number ratio is 2:(10-12):(20-25). In this case, the large-diameter grinding balls, medium-diameter grinding balls, and small-diameter grinding balls fully cooperate to promote the generation and refinement of lithium iron phosphate particles.

[0060] In some embodiments, the diameter of the large-diameter grinding ball is 80 mm to 100 mm, the diameter of the medium-diameter grinding ball is 50 mm to 70 mm, and the diameter of the small-diameter grinding ball is 20 mm to 25 mm.

[0061] In some embodiments, in the ball milling step, the ball milling time is 12 h to 24 h, and the ball milling speed is 300 rpm to 500 rpm.

[0062] In some embodiments, in the above step S10, the molar ratio of lithium element, iron element and phosphorus element in the lithium source, iron source and phosphorus source is (0.99-1.05):(0.99-1.05):1. In the exemplary embodiment, it can be a typical but non-limiting molar ratio such as 1:1:1, 0.99:1.05:1, 1.05:0.99:1, or a range between any two molar ratios.

[0063] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium acetate.

[0064] In some embodiments, the iron source includes at least one of ferrous sulfate, ferrous chloride, and ferrous oxalate.

[0065] In some embodiments, the phosphorus source includes phosphoric acid.

[0066] In some embodiments, in the above step S10, in the first calcination step, the calcination atmosphere is an inert gas, the calcination temperature is 600° C. to 900° C., and the calcination time is 6 h to 12 h.

[0067] In some specific embodiments, the inert gas is at least one of helium, neon, and argon.

[0068] In some embodiments, in the above step S10, the impurity removal includes the steps of: dispersing the first calcined product in water and then centrifuging to obtain a precipitated product. In this case, the unreacted lithium source, iron source and phosphorus source in the reaction solution are removed.

[0069] In some embodiments, in the above step S10, the drying temperature is 100°C to 120°C. In the exemplary embodiment, it can be a typical but non-limiting temperature such as 100°C, 110°C, 120°C, or a range between any two temperatures. It should be noted that the drying time needs to be adjusted according to the actual output, as long as the moisture can be removed, and is not particularly limited in the embodiments of the present invention.

[0070] In some embodiments, the drying method is spray drying or oven drying.

[0071] In some specific embodiments, drying comprises the steps of: drying at a temperature of 100° C. to 120° C. and then grinding to obtain lithium iron phosphate.

[0072] In some embodiments, in the above step S10, the average particle size of lithium iron phosphate is 0.7μm to 0.95μm. In the exemplary embodiment, it can be a typical but non-limiting average particle size such as 0.7μm, 0.81μm, 0.85μm, 0.89μm, 0.92μm, 0.95μm, or a range between any two average particle sizes.

[0073] In some embodiments, in the above step S20, the mass ratio of lithium iron phosphate and starch is 1:(0.2-0.3). In the exemplary embodiment, it can be a typical but non-limiting average particle size such as 1:0.2, 1:0.25, 1:0.3, or a range between any two average particle sizes. It should be noted that the volume ratio of starch and water can be adjusted according to the actual preparation, as long as the lithium iron phosphate, starch and water can be made into a non-Newtonian fluid state. However, as an example, the volume ratio of starch and water is (1.95-2.05):1, which can be 1.95:1, 2:1, 2.05:1, etc.

[0074] In some embodiments, in the above step S20, the starch includes at least one of wheat starch, sweet potato starch, corn starch and potato starch.

[0075] In some embodiments, in the above step S20, in this case, lithium iron phosphate, starch and water can be made into a non-Newtonian fluid state.

[0076] In some embodiments, in the above step S20, preparing the non-Newtonian fluid mixture comprises the following steps:

[0077] After mixing lithium iron phosphate and starch, water is added under stirring conditions at a speed of 100 rpm to 200 rpm;

[0078] After the water is added, continue stirring at a speed of 50 rpm to 90 rpm for 10 min to 20 min;

[0079] The stirring is continued for 5 to 8 minutes at a rotation speed of 20 to 40 rpm to obtain a non-Newtonian fluid mixture.

[0080] In the above preparation of the non-Newtonian fluid mixture, water is added to the mixture of lithium iron phosphate and starch under the stirring condition of a rotation speed of 100rpm to 200rpm, so that the starch particles and water can be quickly dispersed; stirring is continued under the stirring condition of a rotation speed of 50rpm to 90rpm, so that the starch particles are fully dissolved and evenly distributed, agglomeration and precipitation are avoided, and a non-Newtonian fluid is formed; stirring is continued under the stirring condition of a rotation speed of 20rpm to 40rpm to maintain the uniformity and stability of the non-Newtonian fluid mixture.

[0081] In some specific embodiments, in the step of preparing the non-Newtonian fluid mixture, the rate of adding water is 10 ml / min to 20 ml / min.

[0082] In some embodiments, in the second calcination step, the calcination temperature is 600° C. to 700° C., and the calcination time is 6 h to 12 h.

[0083] A second aspect of an embodiment of the present invention provides modified lithium iron phosphate obtained by the preparation method of modified lithium iron phosphate provided by an embodiment of the present invention, wherein the average particle size of the modified lithium iron phosphate is 0.6 μm to 1.12 μm.

[0084] In some embodiments, the powder resistivity of the modified lithium iron phosphate is 2.8 Ω·cm to 15 Ω·cm.

[0085] The following is explained in conjunction with specific embodiments; in the following specific embodiments, ball milling in a supercritical fluid is carried out in a high-pressure ball mill.

[0086] Example 1

[0087] Example 1 provides a method for preparing modified lithium iron phosphate, the steps are as follows:

[0088] (1) Reaction liquid: lithium carbonate, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2:1); stainless steel grinding balls: the number ratio of large diameter grinding balls (diameter 90 mm), medium diameter grinding balls (diameter 60 mm), and small diameter grinding balls (diameter 20 mm) is 2:10:20; the supercritical fluid source is carbon dioxide.

[0089] After the reaction solution and grinding balls are mixed, carbon dioxide is introduced to the critical pressure of carbon dioxide (7.3MPa-7.4MPa) at the critical temperature of carbon dioxide (31°C-32°C) and vacuum conditions, and then ball milled. The ball milling time is 20 hours and the ball milling speed is 400rpm.

[0090] (2) The ball-milled product was calcined in an argon atmosphere at a temperature of 700° C. for 10 h.

[0091] (3) The calcined product is dispersed in water and centrifuged to obtain a precipitated product.

[0092] (4) The precipitated product was dried at 110° C. to remove moisture, and then ground to obtain lithium iron phosphate, wherein the average particle size of the lithium iron phosphate was 0.85 μm.

[0093] (5) Preparation of non-Newtonian fluid mixture:

[0094] ① Mix lithium iron phosphate and corn starch in a mass ratio of 1:0.25.

[0095] ② Add water at a rate of 15 ml / min while stirring at 150 rpm.

[0096] ③ After the water is added, stirring is continued for 18 minutes at a speed of 70 rpm to form a non-Newtonian fluid.

[0097] ④ Continue stirring for 7 minutes at a rotation speed of 30 rpm to obtain a stable non-Newtonian fluid mixture.

[0098] (6) The non-Newtonian fluid mixture was calcined at 650° C. for 10 h to obtain modified lithium iron phosphate.

[0099] Example 1 also provides an olefin-modified lithium iron phosphate, which is prepared by the preparation method of the modified lithium iron phosphate of this example. The average particle size of the modified lithium iron phosphate is 1.05 μm, and the powder resistivity is 9.8 Ω·cm.

[0100] Example 2

[0101] Example 2 provides a method for preparing modified lithium iron phosphate, the steps are as follows:

[0102] (1) Reaction liquid: lithium carbonate, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2:1); stainless steel grinding balls: the number ratio of large diameter grinding balls (diameter 90 mm), medium diameter grinding balls (diameter 60 mm), and small diameter grinding balls (diameter 20 mm) is 2:10:20; the supercritical fluid source is ethane.

[0103] After the reaction solution and the grinding balls are mixed, ethane is introduced to the critical pressure of ethane (4.88 MPa) at the critical temperature of ethane (30°C-32.25°C) and vacuum conditions, and then ball milling is performed. The ball milling time is 12 h and the ball milling speed is 300 rpm.

[0104] (2) The ball-milled product was calcined in an argon atmosphere at a temperature of 600° C. for 12 h.

[0105] (3) The calcined product is dispersed in water and centrifuged to obtain a precipitated product.

[0106] (4) The precipitated product was dried at 110° C. to remove moisture, and then ground to obtain lithium iron phosphate, wherein the average particle size of the lithium iron phosphate was 0.7 μm.

[0107] (5) Preparation of non-Newtonian fluid mixture:

[0108] ① Mix lithium iron phosphate and corn starch in a mass ratio of 1:0.25.

[0109] ② Add water at a rate of 12 ml / min while stirring at 100 rpm.

[0110] ③ After the water is added, continue stirring at a speed of 50 rpm for 20 minutes to form a non-Newtonian fluid.

[0111] ④ Continue stirring at a speed of 20 rpm for 8 min to obtain a stable non-Newtonian fluid mixture.

[0112] (6) calcining the non-Newtonian fluid mixture at 600° C. for 12 h to obtain modified lithium iron phosphate.

[0113] Example 2 also provides a modified lithium iron phosphate, which is prepared by the preparation method of this example: the average particle size of the modified lithium iron phosphate is 0.83 μm, and the powder resistivity is 12 Ω·cm.

[0114] Example 3

[0115] Example 3 provides a method for preparing modified lithium iron phosphate, the steps are as follows:

[0116] (1) Reaction liquid: lithium carbonate, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2:1); stainless steel grinding balls: the number ratio of large diameter grinding balls (diameter 90 mm), medium diameter grinding balls (diameter 60 mm), and small diameter grinding balls (diameter 20 mm) is 2:10:20; the supercritical fluid source is propane.

[0117] After the reaction solution and the grinding balls are mixed, propane is introduced to the critical pressure of propane (4.3975 MPa) at the critical temperature of propane (96.8°C) and vacuum conditions, and then ball milling is performed. The ball milling time is 22 hours and the ball milling speed is 500 rpm.

[0118] (2) The ball-milled product was calcined in an argon atmosphere at a temperature of 900° C. for 6 h.

[0119] (3) The calcined product is dispersed in water and centrifuged to obtain a precipitated product.

[0120] (4) The precipitated product was dried at 110° C. to remove moisture, and then ground to obtain lithium iron phosphate, wherein the average particle size of the lithium iron phosphate was 0.95 μm.

[0121] (5) Preparation of non-Newtonian fluid mixture:

[0122] ① Mix lithium iron phosphate and corn starch in a mass ratio of 1:0.25.

[0123] ② Add water at a rate of 20 ml / min while stirring at 180 rpm.

[0124] ③ After the water is added, continue stirring at 80 rpm for 15 minutes to form a non-Newtonian fluid.

[0125] ④ Continue stirring for 6 minutes at a rotation speed of 35 rpm to obtain a stable non-Newtonian fluid mixture.

[0126] (6) The non-Newtonian fluid mixture is calcined at 700° C. for 8 h to obtain modified lithium iron phosphate.

[0127] Example 3 also provides a modified lithium iron phosphate, which is prepared by the preparation method of this example: the average particle size of the modified lithium iron phosphate is 1.12 μm, and the powder resistivity is 3.8 Ω·cm.

[0128] Example 4

[0129] Example 4 provides a method for preparing modified lithium iron phosphate, the steps are as follows:

[0130] (1) Reaction liquid: lithium carbonate, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2:1); stainless steel grinding balls: the number ratio of large diameter grinding balls (diameter 90 mm), medium diameter grinding balls (diameter 60 mm), and small diameter grinding balls (diameter 20 mm) is 2:10:20; the supercritical fluid source is propylene.

[0131] After the reaction liquid and the grinding balls are mixed, propylene is introduced to the critical pressure of propylene (4.762 MPa) at the critical temperature of propylene (91.9°C) and vacuum conditions, and then ball milled. The ball milling time is 24 hours and the ball milling speed is 450 rpm.

[0132] (2) The ball-milled product was calcined in an argon atmosphere at a temperature of 850° C. for 8 h.

[0133] (3) The calcined product is dispersed in water and centrifuged to obtain a precipitated product.

[0134] (4) The precipitated product was dried at 110° C. to remove moisture, and then ground to obtain lithium iron phosphate, wherein the average particle size of the lithium iron phosphate was 0.89 μm.

[0135] (5) Preparation of non-Newtonian fluid mixture:

[0136] ① Mix lithium iron phosphate and corn starch in a mass ratio of 1:0.25.

[0137] ② Add water at a rate of 10 ml / min while stirring at 200 rpm.

[0138] ③ After the water is added, continue stirring at a speed of 90 rpm for 12 minutes to form a non-Newtonian fluid.

[0139] ④ Continue stirring at a speed of 40 rpm for 5 min to obtain a stable non-Newtonian fluid mixture.

[0140] (6) The non-Newtonian fluid mixture was calcined at 690° C. for 9 h to obtain modified lithium iron phosphate.

[0141] Example 4 also provides a modified lithium iron phosphate, which is prepared by the preparation method of this example: the average particle size of the modified lithium iron phosphate is 0.98 μm, and the powder resistivity is 4.2 Ω·cm.

[0142] Example 5

[0143] Example 5 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 3, except that:

[0144] In step (1), the ball milling time is 18 h and the ball milling speed is 300 rpm.

[0145] In step (2), the calcination temperature is 890° C. and the calcination time is 9 h.

[0146] In step (4), the average particle size of the obtained lithium iron phosphate is 0.81 μm.

[0147] Example 5 also provides a modified lithium iron phosphate, which is prepared by the preparation method of this example: the average particle size of the modified lithium iron phosphate is 0.95 μm, and the powder resistivity is 4.5 Ω·cm.

[0148] Example 6

[0149] Example 6 provides a method for preparing modified lithium iron phosphate, the steps are basically the same as those of Example 3, except that:

[0150] The average particle size of the lithium iron phosphate obtained in step (4) is 0.92 μm.

[0151] The content of step (5) is as follows:

[0152] ① Mix lithium iron phosphate and sweet potato starch in a mass ratio of 1:0.25.

[0153] ② Add water at a rate of 10 ml / min while stirring at 100 rpm.

[0154] ③ After the water is added, continue stirring at a speed of 60 rpm for 20 minutes to form a non-Newtonian fluid.

[0155] ④ Continue stirring at a speed of 40 rpm for 8 min to obtain a stable non-Newtonian fluid mixture.

[0156] The calcination temperature in step (6) is 700° C. and the calcination time is 10 h.

[0157] Example 6 also provides a modified lithium iron phosphate, which is prepared by the preparation method of this example: the average particle size of the modified lithium iron phosphate is 0.95 μm, and the powder resistivity is 4.5 Ω·cm.

[0158] Comparative Example 1

[0159] Comparative Example 1 provides a method for preparing modified lithium iron phosphate, the steps are as follows:

[0160] 1) Reaction liquid: lithium carbonate, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2:1); stainless steel grinding balls: the number ratio of large diameter grinding balls (diameter is 90 mm), medium diameter grinding balls (diameter is 60 mm), and small diameter grinding balls (diameter is 20 mm) is 2:10:20; the supercritical fluid source is propane.

[0161] After the reaction solution and the grinding balls are mixed, propane is introduced to the critical pressure of propane (4.3975 MPa) at the critical temperature of propane (96.8°C) and vacuum conditions, and then ball milling is performed. The ball milling time is 22 hours and the ball milling speed is 500 rpm.

[0162] 2) The ball-milled product was calcined under an argon atmosphere at a temperature of 900° C. for 6 h.

[0163] 3) The calcined product is dispersed in water and centrifuged to obtain a precipitated product.

[0164] 4) Drying the precipitated product at 110° C. to remove moisture, and grinding to obtain lithium iron phosphate, wherein the average particle size of the lithium iron phosphate is 0.95 μm.

[0165] 5) The lithium iron phosphate and corn starch are mixed uniformly in a mass ratio of 1:0.25 to obtain a dry blend.

[0166] 6) Dry-mixing the materials at 700° C. for 8 hours to obtain modified lithium iron phosphate.

[0167] Comparative Example 1 also provides a modified lithium iron phosphate, which is prepared by the preparation method of this comparative example: the modified lithium iron phosphate has an average particle size of 2.35 μm and a powder resistivity of 25 Ω·cm.

[0168] Comparative Example 2

[0169] Comparative Example 2 provides a method for preparing modified lithium iron phosphate, the steps are as follows:

[0170] 1) Reaction liquid: lithium carbonate, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2:1); stainless steel grinding balls: the number ratio of large diameter grinding balls (diameter 90 mm), medium diameter grinding balls (diameter 60 mm), and small diameter grinding balls (diameter 20 mm) is 2:10:20;

[0171] The reaction solution and grinding balls were mixed and then ball-milled. The ball-milling time was 22 h and the ball-milling speed was 500 rpm.

[0172] 2) The ball-milled product was calcined under an argon atmosphere at a temperature of 900° C. for 6 h.

[0173] 3) The calcined product is dispersed in water and centrifuged to obtain a precipitated product.

[0174] 4) The precipitated product was dried at 110° C. to remove moisture, and then ground to obtain lithium iron phosphate, wherein the average particle size of the lithium iron phosphate was 1.25 μm.

[0175] (5) Preparation of non-Newtonian fluid mixture:

[0176] ① Mix lithium iron phosphate and corn starch in a mass ratio of 1:0.25.

[0177] ② Add water at a rate of 20 ml / min while stirring at 180 rpm.

[0178] ③ After the water is added, continue stirring at 80 rpm for 15 minutes to form a non-Newtonian fluid.

[0179] ④ Continue stirring for 6 minutes at a rotation speed of 35 rpm to obtain a stable non-Newtonian fluid mixture.

[0180] (6) The dry-mixed materials are mixed at a temperature of 700° C. for 8 hours to obtain modified lithium iron phosphate.

[0181] Comparative Example 2 also provides a modified lithium iron phosphate, which is prepared by the preparation method of this comparative example: the average particle size of the modified lithium iron phosphate is 3.01 μm, and the powder resistivity is 30 Ω·cm.

[0182] In order to verify the advancement of the modified lithium iron phosphate and the preparation method thereof provided in the embodiment of the present invention, firstly, taking embodiment 1 as an example, a scanning electron microscope is used to scan the modified lithium iron phosphate prepared in embodiment 1, and the corresponding Figure 2 Secondly, the modified lithium iron phosphate of the embodiment and the comparative example was made into button batteries, and then the button batteries were subjected to electrical performance tests including 0.1C discharge, initial charge and discharge efficiency, 5C discharge and 1C cycle 500 times capacity retention rate in the voltage range of 2.0V to 3.8V using a blue electric device. The test results are shown in Table 1 below.

[0183] The preparation steps of button cells are as follows:

[0184] (1) Preparation of positive electrode slurry: Modified lithium iron phosphate: carbon nanotube: polyvinylidene fluoride were mixed in a mass ratio of 90:5:5, and solvent N-methylpyrrolidone (NMP) was added to adjust the viscosity to about 6000 mPa·s to prepare positive electrode slurry.

[0185] (2) The positive electrode slurry is evenly coated on one surface (perpendicular to the thickness direction) of the aluminum foil (positive electrode current collector) and dried, and then rolled and sliced ​​to obtain a positive electrode sheet containing a positive electrode film layer.

[0186] (5) Using lithium hexafluorophosphate-ethyl methyl carbonate as the electrolyte and polypropylene as the isolation membrane, the prepared positive electrode sheet and negative lithium sheet are combined to form a button half-cell.

[0187] Table 1

[0188]

[0189] From Table 1 above and the instructions attached Figure 2 At least the following conclusions can be drawn:

[0190] (1) The preparation method of the modified lithium iron phosphate of the embodiment of the present invention combines supercritical fluid technology and non-Newtonian fluid process to prepare the modified lithium iron phosphate. The supercritical fluid technology can make the lithium source, iron source and phosphorus source uniformly dispersed during the preparation process, reducing the particle size of the lithium iron phosphate product. The non-Newtonian fluid can make the surface of the lithium iron phosphate uniformly carbon-coated, thereby improving the electron transfer rate (conductivity).

[0191] (2) In the preparation method of the modified lithium iron phosphate of the embodiment of the present invention, the reaction solution is ball-milled in a supercritical fluid. Due to the advantages of the supercritical fluid such as strong solubility, good diffusion performance and low surface tension, the lithium source, iron source and phosphorus source can be evenly dispersed during the ball milling process, thereby improving the quality uniformity of the product; in addition, the supercritical fluid (at least one of carbon dioxide, ethane, propylene and propane) is used as a carbon source to carry out in-situ carbon doping of the lithium iron phosphate, and the gas generated during calcination can further refine the particle size of the lithium iron phosphate.

[0192] (3) The preparation method of the modified lithium iron phosphate of the embodiment of the present invention is to prepare a non-Newtonian fluid mixture of lithium iron phosphate, starch and water, so that the lithium iron phosphate can be evenly dispersed in the starch, and the lithium iron phosphate and the starch will not show sedimentation and stratification for a long time; during calcination, the lithium iron phosphate can be carbon-coated in situ and evenly.

[0193] (4) By Figure 2 It can be seen that the modified lithium iron phosphate prepared by the preparation method of the modified lithium iron phosphate in the embodiment of the present invention has a small particle size and good uniformity.

[0194] It should be noted that the present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and embodiments having substantially the same structure as the technical idea and exerting the same effects within the scope of the technical solution of the present invention are all included in the technical scope of the present invention. In addition, within the scope of the main purpose of the present invention, various modifications that can be thought of by those skilled in the art to the embodiments, and other methods of combining some of the constituent elements in the embodiments to construct are also included in the scope of the present invention.

Claims

1. A method for preparing modified lithium iron phosphate, characterized in that: The steps include: S10. After the reaction solution is ball-milled in a supercritical fluid, a first calcination, impurity removal and drying treatment is performed to obtain lithium iron phosphate; The reaction solution contains a lithium source, an iron source and a phosphorus source; The supercritical fluid includes one of carbon dioxide, ethane, propylene and propane; S20. The lithium iron phosphate, starch and water are made into a non-Newtonian fluid mixture and then subjected to a second calcination to obtain modified lithium iron phosphate.

2. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: The molar ratio of lithium element, iron element and phosphorus element in the lithium source, the iron source and the phosphorus source is 0.99-1.05:0.99-1.05:

1.

3. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: The mass ratio of the lithium iron phosphate to the starch is 1:0.2-0.

3.

4. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: The preparation of the non-Newtonian fluid mixture comprises the following steps: After mixing the lithium iron phosphate and the starch, water is added under stirring conditions at a rotation speed of 100 rpm to 200 rpm; After the water is added, continue stirring at a speed of 50 rpm to 90 rpm for 10 min to 20 min; The stirring is continued for 5 to 8 minutes at a rotation speed of 20 to 40 rpm to obtain the non-Newtonian fluid mixture.

5. The method for preparing modified lithium iron phosphate according to claim 4, characterized in that: In the step of preparing the non-Newtonian fluid mixture, the rate of adding water is 10 ml / min to 20 ml / min.

6. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: In the ball milling step, the ball milling time is 12 hours to 24 hours, and the ball milling speed is 300 rpm to 500 rpm.

7. The method for preparing modified lithium iron phosphate according to claim 6, characterized in that: In the first calcination step, the calcination atmosphere is an inert gas, the calcination temperature is 600° C. to 900° C., and the calcination time is 6 h to 12 h.

8. The method for preparing modified lithium iron phosphate according to claim 1, characterized in that: In the second calcination step, the calcination temperature is 600° C. to 700° C., and the calcination time is 6 h to 12 h.

9. A modified lithium iron phosphate obtained by the method for preparing modified lithium iron phosphate according to any one of claims 1 to 8, characterized in that: The average particle size of the modified lithium iron phosphate is 0.6 μm to 1.12 μm.

10. Use of the modified lithium iron phosphate according to claim 9 in household appliances, electric vehicles, ships, satellites or energy storage systems.

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