Nano single crystal lithium iron phosphate and its preparation method and application

Abstract

The present invention belongs to the technical field of positive electrode active materials, and specifically relates to a nano single crystal lithium iron phosphate and its preparation method and application. The preparation method comprises the following steps: after ball milling a mixture containing a lithium source, an iron source and a phosphorus source in a supercritical fluid, roasting, removing impurities and drying to obtain lithium iron phosphate; after dispersing the lithium iron phosphate in propylene glycol and sintering, a nano single crystal lithium iron phosphate is obtained. The preparation method of the nano single crystal lithium iron phosphate of the present invention combines supercritical fluid technology with secondary sintering, so that the carbon coating on the surface of the lithium iron phosphate is uniform and the grain size of the lithium iron phosphate is refined, thereby improving the electron transmission rate and cycle stability of the lithium iron phosphate.

Description

Technical Field

[0001] The invention belongs to the technical field of positive electrode active materials, and specifically relates to a nano single crystal 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 make 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 obtained by sintering is severely agglomerated and the particle size of the product particles is relatively large. Summary of the invention

[0004] The present invention provides a nanometer single crystal lithium iron phosphate and a preparation method and application thereof. The preparation method solves the technical problems of serious lithium iron phosphate agglomeration and large product particle size in a secondary 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 nano single crystal lithium iron phosphate, comprising the following steps:

[0007] After ball milling a mixture containing a lithium source, an iron source and a phosphorus source in a supercritical fluid, the mixture is calcined, impurity-removed and dried to obtain lithium iron phosphate;

[0008] Dispersing lithium iron phosphate in propylene glycol and then sintering to obtain nano single crystal lithium iron phosphate; wherein the sintering temperature is 250°C to 300°C;

[0009] The sintering comprises the following steps:

[0010] Heat to 190°C~220°C at a heating rate of 8°C / min~10°C / min, and keep warm for 8h~12h;

[0011] Heat from 190°C~220°C to 230°C at a heating rate of 15°C / min~20°C / min, and keep warm for 8h~12h;

[0012] Raise the temperature from 230°C to 250°C~300°C at a heating rate of 25°C / min~30°C / min and keep warm for 12h~18h.

[0013] 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.

[0014] In some possible implementations, in the calcination step, the calcination atmosphere is hydrogen, the calcination temperature is 600° C. to 900° C., and the calcination time is 12 h to 16 h.

[0015] In some possible implementations, the average particle size of the lithium iron phosphate is 0.7 μm to 0.95 μm.

[0016] In some possible implementations, the mass ratio of the lithium iron phosphate to propylene glycol is (1-3):1.

[0017] In some possible implementations, the supercritical fluid includes one of carbon dioxide, ethane, propylene, and propane.

[0018] In some possible implementations, in the ball milling step, 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).

[0019] In a second aspect, the present invention provides nano single crystal lithium iron phosphate prepared by the method for preparing nano single crystal lithium iron phosphate provided by the present invention.

[0020] In a third aspect, the present invention provides an application of the nano single crystal lithium iron phosphate provided by the present invention in household appliances, electric vehicles, ships, satellites or energy storage systems.

[0021] The nano single crystal 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:

[0022] (1) The method for preparing nano-single crystal lithium iron phosphate of the present invention combines supercritical fluid technology and non-Newtonian fluid process 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, thereby reducing the particle size of the lithium iron phosphate product.

[0023] (2) The method for preparing nano single crystal 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.

[0024] (3) In the preparation method of nano single crystal lithium iron phosphate of the present invention, a staged heating and heat preservation method is adopted in the sintering step, which not only prevents the direct dissipation of propylene glycol due to excessively rapid heating and inability to decompose, but also allows the propylene glycol to be directionally decomposed to finally form a stable carbon coating layer of lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] 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.

[0026] Figure 1 This is a schematic flow chart of the method for preparing nano single crystal lithium iron phosphate in Example 1 of the present invention;

[0027] Figure 2 This is a SEM image of nano single crystal lithium iron phosphate in Example 1 of the present invention.

[0028] 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

[0029] 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.

[0030] 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.

[0031] 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 items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all represent: a, b, c, a~b (i.e., a and b), a~c, b~c, or a~b~c, where a, b, and c can be single or multiple, respectively.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] Nano single crystals are single crystal materials with a grain size of no more than 100 nm. They are composed of countless nanoscale single crystal particles and have high-density interfaces and excellent performance.

[0041] In order to obtain nano single crystal lithium iron phosphate, the embodiment of the present invention provides a nano single crystal lithium iron phosphate and a preparation method thereof. The preparation method uses supercritical fluid to prepare lithium iron phosphate, which 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 thus prepare lithium iron phosphate with extremely fine grains, which is mixed with propylene glycol during secondary sintering to improve the electron transmission rate (conductivity / resistivity).

[0042] The following is a detailed description of a nano single crystal lithium iron phosphate and a preparation method thereof according to an embodiment of the present invention.

[0043] The first aspect of the present invention provides a method for preparing nano single crystal lithium iron phosphate, such as Figure 1 As shown, the following steps are included:

[0044] S10. After ball-milling a mixture containing a lithium source, an iron source and a phosphorus source in a supercritical fluid, the mixture is calcined, impurity-removed and dried to obtain lithium iron phosphate.

[0045] S20. Dispersing lithium iron phosphate in propylene glycol and sintering to obtain nano single crystal lithium iron phosphate; wherein the sintering temperature is 250°C to 300°C;

[0046] Sintering includes the following steps:

[0047] Heat to 190°C~220°C at a heating rate of 8°C / min~10°C / min, and keep warm for 8h~12h;

[0048] Heat from 190°C~220°C to 230°C at a heating rate of 15°C / min~20°C / min, and keep warm for 8h~12h;

[0049] Raise the temperature from 230°C to 250°C~300°C at a heating rate of 25°C / min~30°C / min and keep warm for 12h~18h.

[0050] The method for preparing nano single crystal lithium iron phosphate of the embodiment of the present invention is to place the mixture in a supercritical fluid for ball milling. 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, and the particle size of the lithium iron phosphate product can be reduced, thereby preparing lithium iron phosphate with extremely fine grains at the nano level; in addition, the supercritical fluid is also used as a carbon source to perform in-situ carbon doping on the lithium iron phosphate, and the gas generated during roasting can further refine the particle size of the lithium iron phosphate. The prepared lithium iron phosphate is evenly dispersed in propylene glycol, and when sintering, the temperature is raised and kept in stages, which not only prevents the direct escape and inability to decompose due to excessively fast temperature rise, but also allows the propylene glycol to be directionally decomposed to finally form a stable carbon coating layer of lithium iron phosphate. The temperature was raised to 190°C~220°C at a heating rate of 8°C / min~10°C / min and kept for 8h~12h, propylene glycol was dehydrated and reconstructed to generate small molecular organic substances such as 2-propenal, acetaldehyde and formaldehyde; the temperature was raised from 190°C~220°C to 230°C at a heating rate of 15°C / min~20°C / min and kept for 8h~12h, the small molecular organic substances would be further decomposed into CC and C=C bonds, thereby gradually forming carbon cores; the temperature was raised from 230°C to 250°C~300°C at a heating rate of 25°C / min~30°C / min and kept for 12h~18h, the carbon cores were further polymerized and reorganized to form a stable carbonaceous material coating lithium iron phosphate; propylene glycol decomposed at high temperature to form a uniform carbon material, which was adsorbed on the surface of lithium iron phosphate particles in situ, further reducing the grain size of lithium iron phosphate to form nano single crystal lithium iron phosphate. The method for preparing nano-single-crystal lithium iron phosphate in an embodiment of the present invention combines supercritical fluid technology with secondary sintering to make the carbon coating on the surface of lithium iron phosphate uniform and refine the particle size of lithium iron phosphate grains, thereby improving the electron transfer rate and cycle stability of lithium iron phosphate.

[0051] In the embodiment, the sintering temperature is 250° C. to 300° C., which may be a typical temperature such as 250° C. or 300° C., or a range between any two temperatures. In this case, glycerol is decomposed into a stable carbonaceous material, which coats the lithium iron phosphate to form a carbon coating.

[0052] In some embodiments, in the above step S10, the supercritical fluid includes one of carbon dioxide, ethane, propylene, and propane. These small molecule carbon sources can be made into supercritical fluids under mild conditions, and the mixture can be evenly dispersed therein.

[0053] Carbon dioxide, the critical temperature is 31℃~32℃, and the critical pressure is 7.3MPa~7.4MPa.

[0054] Ethane, the critical temperature is 30℃~32.25℃, and the critical pressure is 4.88MPa.

[0055] Propylene, critical temperature is 91.9℃, critical pressure is 4.762MPa.

[0056] Propane, critical temperature is 96.8℃, critical pressure is 4.3975MPa.

[0057] In some embodiments, in the above step S10, the molar ratio of lithium, iron and phosphorus in the lithium source, iron source and phosphorus source is (1-1.15): (0.95-1.05): 1. In the exemplary embodiment, it can be a typical molar ratio such as 1:1:1 or a range between any two molar ratios. It should be noted that the molar ratio of the lithium source, iron source and phosphorus source is a ratio known in the art, and those skilled in the art can adjust it as needed. It is only used as an example in the embodiments of the present invention.

[0058] In some embodiments, in the above step S10, the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium acetate.

[0059] In some embodiments, in the above step S10, the iron source includes at least one of ferrous sulfate, ferrous chloride, and ferrous oxalate.

[0060] In some embodiments, in the above step S10, the phosphorus source includes phosphoric acid.

[0061] In some embodiments, in the above step S10, ball milling includes the following steps:

[0062] After the mixed material and the grinding balls are mixed, a supercritical fluid source is introduced to the critical pressure of the supercritical fluid under the critical temperature of the supercritical fluid and vacuum to form a supercritical fluid and then ball milled.

[0063] The supercritical fluid source refers to the substance that is made into the supercritical fluid.

[0064] In some specific embodiments, at the critical temperature of the supercritical fluid and under vacuum, the implementation principle of introducing a supercritical fluid source to the critical pressure of the supercritical fluid is as follows:

[0065] The mixed material and grinding balls are placed in a ball mill, heated to the critical temperature of the supercritical fluid, and then a supercritical fluid source is introduced into the supercritical fluid. The ball mill monitors the pressure therein in real time. When the critical pressure of the supercritical fluid is reached, the introduction of the supercritical fluid source is stopped;

[0066] The temperature and pressure changes are controlled during the ball milling process. When the pressure is lower than the critical pressure, a supercritical fluid source is introduced in time to supplement the pressure.

[0067] In some specific embodiments, in the ball mill, the ball milling method adopts a stirring ball mill, that is, the stirring rod is driven by a motor to drive the grinding balls to rotate to achieve the effect of ball milling. The sealing of the ball mill jar is achieved by a built-in rubber gasket and an external fastener. It can withstand a pressure of up to 15 MPa in a sealed state. The ball mill jar is heated by an electric heating sleeve. A sealing tube is opened on the top of the ball mill jar to place a thermocouple for temperature control. Three gas valves are set on the top of the jar, one for the air inlet valve and the other for the air outlet valve. The remaining valves are used as safety valves for standby. The top cover side opening is connected to a pressure gauge for pressure testing. There is also a control system to control the heating, ball milling speed and stripping time of the device. It should be noted that the working temperature and working pressure of the ball mill in this field can be installed and set as needed, which is a conventional operation in this field and does not belong to the scope of protection required by the embodiments of the present invention.

[0068] In some specific embodiments, the mixture 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 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 amount of ethanol and water added as a dispersion medium for ball milling is the same as the amount added during conventional ball milling, and is not particularly limited in the embodiments of the present invention.

[0069] In some specific embodiments, the grinding balls are at least one of zirconium oxide grinding balls, stainless steel grinding balls, and agate.

[0070] 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, the medium-diameter grinding balls, and the small-diameter grinding balls fully cooperate to promote the generation and refinement of lithium iron phosphate particles.

[0071] 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.

[0072] 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.

[0073] In some embodiments, in the above step S10, in the calcination step, the calcination atmosphere is hydrogen, the calcination temperature is 600° C. to 900° C., and the calcination time is 12 h to 16 h.

[0074] In some embodiments, in the above step S10, impurity removal includes the steps of: dispersing the calcined product in water and centrifuging to obtain a precipitated product. In this case, unreacted lithium source, iron source and phosphorus source in the reaction solution are removed. It should be noted that the speed and water volume of the centrifuge can be adjusted as needed, which is a conventional technical operation for those skilled in the art and does not need to be particularly limited in the embodiments of the present invention.

[0075] 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 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.

[0076] In some embodiments, the drying method is spray drying or oven drying.

[0077] 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.

[0078] In some embodiments, in the above step S10, the average particle size of the prepared lithium iron phosphate is 0.7μm~0.95μm. In the exemplary embodiment, it can be a typical 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.

[0079] In some embodiments, in the above step S20, the mass ratio of lithium iron phosphate to glycerol is (1-3):1. In the exemplary embodiment, it can be a typical mass ratio such as 1:1, 2:1, 3:1, or a range between any two mass ratios. In this case, lithium iron phosphate can be evenly dispersed in glycerol to form a uniform dispersion.

[0080] In some embodiments, in the above step S20, the method of dispersing lithium iron phosphate in propylene glycol includes: adding lithium iron phosphate to propylene glycol in batches (each adding amount is one tenth of the total amount of lithium iron phosphate) at a stirring speed of 50rpm~150rpm, and continuing to stir for 5min~10min after the addition is completed.

[0081] In some embodiments, a method for preparing nano single crystal lithium iron phosphate is provided, the steps are as follows:

[0082] S11. Obtain raw materials of lithium source, iron source and phosphorus source with a molar ratio of lithium element, iron element and phosphorus element in the lithium source, iron source and phosphorus source of (1-1.15):(0.95-1.05):1; mix the lithium source, iron source, phosphorus source, ethanol and water (the volume ratio of ethanol to water is 2-3:1) to obtain a mixture.

[0083] S21. Select grinding balls: the ratio of large diameter grinding balls (diameter is 80mm~100mm), medium diameter grinding balls (diameter is 50mm~70mm), and small diameter grinding balls (diameter is 20mm~25mm) is 2: (10~12): (20~25); the supercritical fluid includes one of carbon dioxide, ethane, propylene, and propane.

[0084] S31. After mixing the mixture and the grinding balls, introduce a supercritical fluid source to the critical pressure of the supercritical fluid under the critical temperature of the supercritical fluid source and vacuum conditions, and then ball mill. The ball milling time is 12h~24h, and the ball milling speed is 300rpm~500rpm.

[0085] S41. In a hydrogen atmosphere, the ball-milled product is calcined at a temperature of 600°C to 900°C for a time of 12h to 16h.

[0086] S51. Disperse the calcined product in water and centrifuge to obtain a precipitated product.

[0087] S61. Dry the precipitated product at 100°C to 120°C to remove moisture, and grind to obtain lithium iron phosphate.

[0088] S71. Add lithium iron phosphate to propylene glycol in batches (each addition amount is one tenth of the total amount of lithium iron phosphate) at a stirring speed of 50 rpm to 150 rpm. After the addition is completed, continue stirring for 5 min to 10 min to obtain a lithium iron phosphate dispersion.

[0089] S81. Raise the temperature to 190°C~220°C at a heating rate of 8°C / min~10°C / min, and keep warm for 8h~12h;

[0090] Heat from 190°C~220°C to 230°C at a heating rate of 15°C / min~20°C / min, and keep warm for 8h~12h;

[0091] The temperature is raised from 230°C to 250°C~300°C at a heating rate of 25°C / min~30°C / min and kept at this temperature for 12h~18h to obtain nano single crystal lithium iron phosphate.

[0092] A second aspect of the embodiments of the present invention provides nano single crystal lithium iron phosphate prepared by the method for preparing nano single crystal lithium iron phosphate provided by the embodiments of the present invention.

[0093] The following is a description of the specific embodiments. In the following specific embodiments, the ball milling in the supercritical fluid is carried out in a high-pressure ball mill; the phosphoric acid is concentrated phosphoric acid with a concentration of 86%.

[0094] Example 1

[0095] Example 1 provides a method for preparing nano single crystal lithium iron phosphate, the steps are as follows:

[0096] (1) obtaining raw materials of lithium source, iron source and phosphorus source with a molar ratio of lithium element, iron element and phosphorus element of 1:1:1; mixing lithium hydroxide, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2-3:1) to obtain a mixture.

[0097] (2) Select grinding balls: the 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 2:11:23;

[0098] The source of supercritical fluid is carbon dioxide.

[0099] (3) After the mixed material and the grinding balls are mixed, carbon dioxide is introduced to the critical pressure of the supercritical fluid to form a supercritical fluid under the critical temperature of carbon dioxide and vacuum conditions, and then ball milling is performed. The ball milling time is 18 hours and the ball milling speed is 400 rpm.

[0100] (4) The ball-milled product was calcined in a hydrogen atmosphere at a temperature of 700°C for 15 h.

[0101] (5) The calcined product is dispersed in water and centrifuged to obtain a precipitated product.

[0102] (6) The precipitated product is dried at 100°C to remove moisture, and then ground to obtain lithium iron phosphate.

[0103] (7) Add lithium iron phosphate to propylene glycol in batches (each adding amount is one tenth of the total amount of lithium iron phosphate) at a stirring speed of 100 rpm. After the addition is completed, continue stirring for 8 minutes to obtain a lithium iron phosphate dispersion.

[0104] (8) Heating at a rate of 10°C / min to 220°C and keeping at that temperature for 8 h; Heating at a rate of 18°C / min from 220°C to 230°C and keeping at that temperature for 10 h;

[0105] The temperature was raised from 230°C to 270°C at a heating rate of 25°C / min and kept at this temperature for 16 hours to obtain nano single crystal lithium iron phosphate.

[0106] Example 2

[0107] Example 2 provides a method for preparing nano single crystal lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0108] (8), heating to 200 °C at a heating rate of 8 °C / min and keeping at this temperature for 9 h;

[0109] The temperature was raised from 200°C to 230°C at a heating rate of 19°C / min and kept at this temperature for 12 h;

[0110] The temperature was raised from 230°C to 300°C at a heating rate of 30°C / min and kept at this temperature for 12 hours to obtain nano single crystal lithium iron phosphate.

[0111] Example 3

[0112] Example 3 provides a method for preparing nano single crystal lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0113] (8), heating to 190 °C at a heating rate of 9 °C / min and keeping at this temperature for 12 h;

[0114] The temperature was raised from 190°C to 230°C at a heating rate of 20°C / min and kept at this temperature for 10 h;

[0115] The temperature was raised from 230°C to 300°C at a heating rate of 30°C / min and kept at this temperature for 12 hours to obtain nano single crystal lithium iron phosphate.

[0116] Example 4

[0117] Example 4 provides a method for preparing nano single crystal lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0118] (8), heating to 210 °C at a heating rate of 10 °C / min and keeping at this temperature for 12 h;

[0119] The temperature was raised from 210°C to 230°C at a heating rate of 15°C / min and kept at this temperature for 8 h;

[0120] The temperature was raised from 230°C to 250°C at a heating rate of 25°C / min and kept at this temperature for 12h to 18h to obtain nano single crystal lithium iron phosphate.

[0121] Example 5

[0122] Example 5 provides a method for preparing nano single crystal lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0123] (2) The ratio of the number of selected grinding balls is 2:10:20: that of large diameter grinding balls (80 mm in diameter), medium diameter grinding balls (50 mm in diameter), and small diameter grinding balls (25 mm in diameter).

[0124] In (3), after the mixed material and the grinding balls are mixed, carbon dioxide is introduced to the critical pressure of the supercritical fluid to form a supercritical fluid under the critical temperature of carbon dioxide and vacuum conditions, and then ball milling is performed. The ball milling time is 12 hours and the ball milling speed is 500 rpm.

[0125] (4) The ball-milled product was calcined in a hydrogen atmosphere at a temperature of 890°C for 13 h.

[0126] Example 6

[0127] Example 6 provides a method for preparing nano single crystal lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0128] In (2), ethane is selected as the supercritical fluid source.

[0129] In (3), after the mixed material and the grinding balls are mixed, ethane is introduced to the critical pressure of ethane to form a supercritical fluid under the critical temperature of ethane and vacuum conditions, and then ball milling is performed.

[0130] (4) The ball-milled product was calcined in a hydrogen atmosphere at a temperature of 600°C for 16 h.

[0131] Example 7

[0132] Example 7 provides a method for preparing nano single crystal lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0133] In (2), propylene is selected as the supercritical fluid source.

[0134] In (3), after the mixed material and the grinding balls are mixed, propylene is introduced to the critical pressure of propylene to form a supercritical fluid under the critical temperature of propylene and vacuum conditions, and then ball milling is performed.

[0135] (4) The ball-milled product was calcined in a hydrogen atmosphere at a temperature of 650°C for 15 h.

[0136] Example 8

[0137] Example 8 provides a method for preparing nano single crystal lithium iron phosphate, the steps are basically the same as those of Example 1, except that:

[0138] In (2), propane is selected as the supercritical fluid source.

[0139] In (3), after the mixed material and the grinding balls are mixed, propane is introduced to the critical pressure of propane to form a supercritical fluid under the critical temperature of propane and vacuum conditions, and then ball milling is performed.

[0140] (4) The ball-milled product was calcined in a hydrogen atmosphere at a temperature of 800°C for 13 h.

[0141] Comparative Example 1

[0142] Comparative Example 1 provides a method for preparing lithium iron phosphate, the steps are as follows:

[0143] 1) obtaining raw materials of lithium source, iron source and phosphorus source with a molar ratio of lithium element, iron element and phosphorus element of 1:1:1; mixing lithium hydroxide, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2-3:1) to obtain a mixture.

[0144] 2) Select grinding balls: the ratio of large diameter grinding balls (diameter 90mm), medium diameter grinding balls (diameter 60mm), and small diameter grinding balls (diameter 20mm) is 2:11:23.

[0145] 3) The mixed material and grinding balls are mixed and then ball-milled for 18 h at a speed of 400 rpm.

[0146] 4) The ball-milled product was calcined in a hydrogen atmosphere at a temperature of 700°C for 15 h.

[0147] 5) Disperse the calcined product in water and centrifuge to obtain a precipitated product.

[0148] 6) Dry the precipitated product at 100°C to remove moisture, and grind it to obtain lithium iron phosphate.

[0149] Comparative Example 2

[0150] Comparative Example 2 provides a method for preparing lithium iron phosphate, the steps are as follows:

[0151] 1) obtaining raw materials of lithium source, iron source and phosphorus source with a molar ratio of lithium element, iron element and phosphorus element of 1:1:1; mixing lithium hydroxide, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2-3:1) to obtain a mixture.

[0152] 2) Select grinding balls: the ratio of large diameter grinding balls (diameter 90mm), medium diameter grinding balls (diameter 60mm), and small diameter grinding balls (diameter 20mm) is 2:11:23;

[0153] The source of supercritical fluid is carbon dioxide.

[0154] 3) After the mixture and the grinding balls are mixed, carbon dioxide is introduced to the critical pressure of the supercritical fluid to form a supercritical fluid under the critical temperature of carbon dioxide and vacuum conditions, and then ball milling is performed. The ball milling time is 18 hours and the ball milling speed is 400 rpm.

[0155] 4) The ball-milled product was calcined in a hydrogen atmosphere at a temperature of 700°C for 15 h.

[0156] 5) Disperse the calcined product in water and centrifuge to obtain a precipitated product.

[0157] 6) Dry the precipitated product at 100°C to remove moisture, and grind it to obtain lithium iron phosphate.

[0158] Comparative Example 3

[0159] Comparative Example 3 provides a method for preparing modified lithium iron phosphate, the steps are as follows:

[0160] 1) obtaining raw materials of lithium source, iron source and phosphorus source with a molar ratio of lithium element, iron element and phosphorus element of 1:1:1; mixing lithium hydroxide, ferrous chloride, phosphoric acid, ethanol and water (the volume ratio of ethanol to water is 2-3:1) to obtain a mixture.

[0161] 2) Select grinding balls: the ratio of large diameter grinding balls (diameter 90mm), medium diameter grinding balls (diameter 60mm), and small diameter grinding balls (diameter 20mm) is 2:11:23.

[0162] 3) The mixed material and grinding balls are mixed and then ball-milled for 18 h at a speed of 400 rpm.

[0163] 4) The ball-milled product was calcined in a hydrogen atmosphere at a temperature of 700°C for 15 h.

[0164] 5) Disperse the calcined product in water and centrifuge to obtain a precipitated product.

[0165] 6) Dry the precipitated product at 100°C to remove moisture, and grind it to obtain lithium iron phosphate.

[0166] 7) Add lithium iron phosphate to propylene glycol in batches (each adding amount is one tenth of the total amount of lithium iron phosphate) at a stirring speed of 100 rpm. After the addition is completed, continue stirring for 8 minutes to obtain a lithium iron phosphate dispersion.

[0167] 8) Heating the temperature to 270°C at a rate of 30°C / min and maintaining the temperature for 16 hours to obtain modified lithium iron phosphate.

[0168] In order to verify the advancement of the nano single crystal lithium iron phosphate and the preparation method thereof provided by the embodiment of the present invention, firstly, taking embodiment 1 as an example, a scanning electron microscope is used to scan the nano single crystal lithium iron phosphate prepared in embodiment 1, and the corresponding Figure 2 In the SEM image shown, the particle size D1 is 335.83nm, D2 is 185.73nm, D3 is 72.28nm, D4 is 54.06nm, and D5 is 157.59nm. Secondly, the nano single crystal lithium iron phosphate or modified lithium iron phosphate prepared in the embodiment and the comparative example is made into a button battery, and then the button battery is 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~3.8V using a blue electric device. The test results are shown in Table 1 below.

[0169] The preparation steps of button cells are as follows:

[0170] (1) Preparation of positive electrode slurry: nano single crystal lithium iron phosphate / modified lithium iron phosphate: carbon nanotube: polyvinylidene fluoride are mixed in a mass ratio of 90:5:5, and solvent N-methylpyrrolidone (NMP) is added to adjust the viscosity to 4000 mPa·s~8000 mPa·s to prepare positive electrode slurry.

[0171] (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.

[0172] (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 make a button half-cell.

[0173]

[0174] According to Table 1 above and the instructions attached Figure 2 At least the following conclusions can be drawn:

[0175] (1) Figure 2In the figure, the particle size D1 is 335.83 nm, D2 is 185.73 nm, D3 is 72.28 nm, D4 is 54.06 nm, and D5 is 157.59 nm. It can be seen that the preparation method of nano single crystal lithium iron phosphate in the embodiment of the present invention can refine the particle size of lithium iron phosphate grains to reach the nanometer level, thereby improving the electron transfer rate and cycle stability of lithium iron phosphate.

[0176] (2) In Table 1, the battery capacity of the nano single crystal lithium iron phosphate prepared by the method for preparing nano single crystal lithium iron phosphate in the embodiment of the present invention after 5C discharge can still reach above 140.31 mAh / g, indicating that the nano single crystal lithium iron phosphate prepared by the method for preparing nano single crystal lithium iron phosphate in the embodiment of the present invention has very high rate performance.

[0177] (3) In the method for preparing nano-single-crystal lithium iron phosphate of the embodiment of the present invention, the mixed material 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 in-situ carbon-dope the lithium iron phosphate, and the gas generated during calcination can further refine the particle size of the lithium iron phosphate.

[0178] (4) The method for preparing nano-single-crystalline lithium iron phosphate according to an embodiment of the present invention comprises uniformly dispersing the prepared lithium iron phosphate in propylene glycol and calcining the mixture in steps. The propylene glycol decomposes at high temperature to form uniform carbon nanoparticles, which are adsorbed on the surface of the lithium iron phosphate particles in situ, thereby further reducing the grain size of the lithium iron phosphate to form nano-single-crystalline lithium iron phosphate.

[0179] It should be noted that the present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present invention are all included in the technical scope of the present invention. In addition, without departing from 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 are also included in the scope of the present invention.

Claims

1. A method for preparing nano single crystal lithium iron phosphate, characterized in that: The steps include: After ball milling a mixture containing a lithium source, an iron source and a phosphorus source in a supercritical fluid, the mixture is calcined, impurity-removed and dried to obtain lithium iron phosphate; Dispersing lithium iron phosphate in propylene glycol and then sintering to obtain nano single crystal lithium iron phosphate; wherein the sintering temperature is 250°C to 300°C; The sintering comprises the following steps: Heat to 190°C~220°C at a heating rate of 8°C / min~10°C / min, and keep warm for 8h~12h; Heat from 190°C~220°C to 230°C at a heating rate of 15°C / min~20°C / min, and keep warm for 8h~12h; Raise the temperature from 230°C to 250°C~300°C at a heating rate of 25°C / min~30°C / min and keep warm for 12h~18h.

2. The method for preparing nano single crystal lithium iron phosphate according to claim 1, characterized in that: In the ball milling step, the ball milling time is 12h~24h, and the ball milling speed is 300rpm~500rpm.

3. The method for preparing nano single crystal lithium iron phosphate according to claim 1, characterized in that: In the calcination step, the calcination atmosphere is hydrogen, the calcination temperature is 600° C. to 900° C., and the calcination time is 12 h to 16 h.

4. The method for preparing nano single crystal lithium iron phosphate according to claim 1, characterized in that: The average particle size of the lithium iron phosphate is 0.7 μm to 0.95 μm.

5. The method for preparing nano single crystal lithium iron phosphate according to claim 1, characterized in that: The mass ratio of the lithium iron phosphate to propylene glycol is 1-3:

1.

6. The method for preparing nano single crystal lithium iron phosphate according to claim 1, characterized in that: The supercritical fluid includes one of carbon dioxide, ethane, propylene and propane.

7. The method for preparing nano single crystal lithium iron phosphate according to claim 1, characterized in that: In the ball milling step, 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.

8. Nano single crystal lithium iron phosphate prepared by the method for preparing nano single crystal lithium iron phosphate according to any one of claims 1 to 7.

9. Use of the nano single crystal lithium iron phosphate as claimed in claim 8 in household appliances, electric vehicles, ships, satellites or energy storage systems.

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