Lithium iron phosphate with hollow yolk-shell structure, and preparation method and use thereof

Abstract

The application belongs to the technical field of electrode material preparation, and provides a lithium iron phosphate with a hollow egg yolk shell structure, a preparation method and application thereof.The lithium iron phosphate has a specific hollow egg yolk shell structure, and compared with irregular bulk materials, the material with the structure has a higher specific surface area and abundant electrochemical reaction active sites, thereby being beneficial to the infiltration of electrolyte, shortening the diffusion path of lithium ions and accelerating the deintercalation rate of lithium ions, effectively enhancing the transmission and transfer of charges, and further improving the rate performance of the lithium iron phosphate battery.In addition, the hollow egg yolk shell structure can effectively adjust the volume change and stress change of the electrode material in the charging and discharging process, thereby improving the cycle stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrode material preparation and relates to a lithium iron phosphate with a hollow yolk shell structure and a preparation method and application thereof. BACKGROUND

[0002] The lithium iron phosphate material (LiFePO4) is a new type of lithium ion battery cathode material, has high capacity, excellent cycle performance and safety, and has been widely used in electric vehicles and energy storage systems and other fields.

[0003] However, the lithium iron phosphate has defects of poor electronic conductivity and low lithium ion diffusion rate, which will make its rate performance worse and limit its further application in power batteries.

[0004] At present, one of the common methods for preparing the lithium iron phosphate material is to uniformly mix the iron phosphate material with a lithium source and a carbon source, and then to obtain the lithium iron phosphate by high-temperature calcination. As one of the precursor raw materials, the iron phosphate has crystal structure, particle size and morphology, which will directly affect the various performances of the synthesized lithium iron phosphate. Therefore, the iron phosphate material is often modified, such as ion doping and carbon coating, or the morphology and specific surface area thereof are adjusted, to improve the electrochemical performance of the lithium iron phosphate cathode material.

[0005] The existing preparation method of the iron phosphate mostly uses phosphoric acid and an iron source as the reaction raw materials, obtains iron phosphate dihydrate through a heating coprecipitation reaction, and then obtains the iron phosphate material through calcination. For example, CN108264031A discloses a preparation method of iron phosphate and lithium iron phosphate and iron phosphate and lithium iron phosphate materials. The preparation method obtains nanodisc-shaped iron phosphate through hydrothermal reaction by mixing an iron salt solution and a phosphate solution. The product obtained by this method mostly has problems of serious agglomeration, large particle size and small specific surface area. In addition, the obtained iron phosphate has no special structure, which is not conducive to improving the diffusion and transmission of lithium ions and is not conducive to further improving the electrical performance of the lithium iron phosphate.

[0006] Therefore, it is of great significance to develop a simple and economical scheme to prepare the iron phosphate which is beneficial to the diffusion of lithium ions and the penetration of the interface electrolyte, so as to obtain the lithium iron phosphate material with excellent performance. SUMMARY

[0007] In view of the problems in the prior art, the purpose of the present application is to provide a lithium iron phosphate with a hollow yolk shell structure and a preparation method and application thereof. The specific hollow yolk shell structure is formed by the iron phosphate and the carbon coating layer, so that the lithium iron phosphate further formed with the lithium source can inherit and obtain this specific hollow yolk shell structure, thereby promoting the rapid diffusion of lithium ions and electrolyte, effectively improving the electrical conductivity and improving the overall electrochemical performance.

[0008] To achieve the above object, the present application adopts the following technical solutions:

[0009] In a first aspect, the present application provides a lithium iron phosphate with a hollow egg-yolk shell structure, comprising:

[0010] a shell part, the shell part comprising a lithium iron phosphate shell, and an outer carbon layer covering the lithium iron phosphate shell;

[0011] a core part inside the shell part, the core part comprising a lithium iron phosphate core, and an inner carbon layer covering the lithium iron phosphate core;

[0012] a hollow part between the core part and the shell part.

[0013] The lithium iron phosphate material prepared by the present application has a specific hollow egg-yolk shell structure. Compared with irregular bulk materials, the material with the structure has a higher specific surface area and abundant electrochemical reaction active sites, which is beneficial to the infiltration of electrolyte, shortens the diffusion path of lithium ions and accelerates the de-intercalation rate, and also helps to enhance the transmission and transfer of electric charge, thereby improving the rate performance of the lithium iron phosphate battery. In addition, the hollow egg-yolk shell structure can effectively adjust the volume change and stress change of the electrode material during the charging and discharging process, thereby improving the cycle stability.

[0014] It should be noted that the hollow egg-yolk shell structure means that the shell part and the core part are completely separated by the hollow part, i.e. the core part completely exists inside the shell part without a fixed connection part; if the core part and the shell part have a certain fixed connection part, i.e. only an intermittent hollow part is generated inside the shell part, the performance of the product will be greatly lost.

[0015] The following is a preferred technical solution of the present application, but not as a limitation of the technical solutions provided by the present application. Through the following technical solutions, the technical purpose and beneficial effects of the present application can be better achieved and realized.

[0016] As a preferred technical solution of the present application, the specific surface area of the lithium iron phosphate with a hollow egg-yolk shell structure is 13-16 m 2 / g, for example, it can be 13.3 m 2 / g, 13.6 m 2 / g, 14.0 m 2 / g, 14.5 m 2 / g, 15.2 m 2 / g, 15.4 m 2 / g, 15.8 m 2 / g or 16 m 2 / g, etc., but is not limited to the listed values, and other values not listed in the above range are also applicable.

[0017] Preferably, the hollow yolk shell structure lithium iron phosphate has a median particle size of 500-600 nm, for example, it can be 500 nm, 510 nm, 520 nm, 530 nm, 540 nm, 550 nm, 560 nm, 570 nm, 580 nm, 590 nm or 600 nm, etc., but not only limited to the listed values, other values not listed in the above value range are also applicable.

[0018] Preferably, the thickness of the outer carbon layer and the inner carbon layer is 3-6 nm, for example, it can be 3 nm, 3.2 nm, 3.5 nm, 3.8 nm, 4 nm, 4.3 nm, 4.5 nm, 4.8 nm or 5 nm, etc., but not only limited to the listed values, other values not listed in the above value range are also applicable.

[0019] In a second aspect, the present application provides a preparation method of the hollow yolk shell structure lithium iron phosphate of the first aspect, the preparation method comprising:

[0020] (1) mixing an organic iron salt, a polyamide and a solvent to obtain a precursor solution; spray drying the precursor solution to obtain a precursor powder; annealing the precursor powder, controlling the heating rate of the annealing to be 8-10 ℃ / min, to obtain hollow yolk shell structure iron oxide microspheres;

[0021] (2) mixing the iron oxide microspheres, dopamine and a buffer to perform a polymerization reaction, to obtain pre-coated microspheres; carbonizing the pre-coated microspheres to form an outer carbon layer and an inner carbon layer, to obtain carbon-coated microspheres; mixing urea, phosphoric acid and the carbon-coated microspheres to perform a hydrothermal reaction, to obtain hollow yolk shell structure carbon-coated iron phosphate;

[0022] (3) mixing the obtained hollow yolk shell structure carbon-coated iron phosphate and a lithium source to perform sintering, to obtain the hollow yolk shell structure lithium iron phosphate.

[0023] The preparation method of the present application first forms a precursor solution of an organic iron salt and a polyamide powder, and then forms a carbon template wrapped Fe 3+The precursor solution containing Fe2O3 is dispersed into fine droplets under the atomization of spray drying, and the solvent is rapidly vaporized under high temperature, and the carbon template of polyamide is formed, and the thermal shrinkage effect of the carbon template can tightly wrap the Fe2O3 and finally form a dense and compact spherical precursor particle. In the subsequent high-temperature calcination process of annealing, the higher heating rate causes a larger temperature gradient difference between the surface and the inside of the microsphere, the surface temperature of the sphere is high, the carbon template of polyamide is heated and reacts with oxygen, the Fe2O3 in the carbon template also combines with oxygen and solidifies and crystallizes, and a rigid shell of Fe2O3 is gradually formed; the temperature gradient difference between the inside and the outside of the sphere causes the carbon template of polyamide wrapped with Fe2O3 to gradually separate from the rigid shell, and the polyamide has good thermal shrinkage, thereby forming a hollow part and a core, the core continues to oxidize under heat and is finally converted into a Fe2O3 core sphere, and the carbon template in the Fe2O3 core sphere forms a pore structure in the process of being completely oxidized, and the hollow yolk shell structure iron oxide microsphere material is prepared after annealing.

[0024] Specifically, when the precursor solution containing Fe2O3 is atomized under the atomization of spray drying, the solvent is rapidly vaporized under high temperature, and the carbon template of polyamide is formed, and the thermal shrinkage effect of the carbon template can tightly wrap the Fe2O3 and finally form a dense and compact spherical precursor particle. 3+ 3+ In the subsequent high-temperature calcination process of annealing, the higher heating rate causes a larger temperature gradient difference between the surface and the inside of the microsphere, the surface temperature of the sphere is high, the carbon template of polyamide is heated and reacts with oxygen, the Fe2O3 in the carbon template also combines with oxygen and solidifies and crystallizes, and a rigid shell of Fe2O3 is gradually formed; the temperature gradient difference between the inside and the outside of the sphere causes the carbon template of polyamide wrapped with Fe2O3 to gradually separate from the rigid shell, and the polyamide has good thermal shrinkage, thereby forming a hollow part and a core, the core continues to oxidize under heat and is finally converted into a Fe2O3 core sphere, and the carbon template in the Fe2O3 core sphere forms a pore structure in the process of being completely oxidized, and the hollow yolk shell structure iron oxide microsphere material is prepared after annealing. 3+ 3+

[0025] ​​​Subsequently, dopamine is polymerized in-situ on the surface of the iron oxide microspheres and penetrates into the interior, and high-temperature carbonization is performed to form an outer carbon layer and an inner carbon layer after carbonization of the polydopamine, the carbon layer not only provides amorphous carbon to improve the electronic / ion conductivity of the electrode material, but also supports the specific hollow yolk shell structure form; subsequently, the carbon-coated microspheres are mixed with phosphoric acid, so that the phosphoric acid can fully react with the iron oxide shell covered by the outer carbon layer and the iron oxide sphere wrapped by the inner carbon layer to generate iron phosphate, and finally inherit the hollow yolk shell structure. The amorphous carbon layer can also be used as a carbon source when synthesizing lithium iron phosphate from iron phosphate and a lithium source, without the need to add other carbon sources. Finally, the carbon-coated iron phosphate with a hollow yolk shell structure is reacted with a lithium source, so that the lithium iron phosphate inherits the hollow yolk shell structure. Preparing the iron phosphate first and then reacting it with the lithium source is conducive to the maintenance and inheritance of the specific hollow yolk shell structure, and selecting to directly generate lithium iron phosphate in one step by mixing phosphoric acid and a lithium source at the same time will cause the destruction of the hollow yolk shell structure, and even the lithium iron phosphate cannot inherit a stable hollow yolk shell structure.

[0026] It should be noted that the present application uses organic iron salt, which is easier to mix uniformly with polyamide in an organic solution. If inorganic iron salt is used, the mixing uniformity of polyamide is poor, which greatly affects the morphology and performance of iron phosphate and lithium iron phosphate.

[0027] It should also be noted that the annealing heating rate is 8-10℃ / min, for example, 8℃ / min, 8.2℃ / min, 8.5℃ / min, 8.8℃ / min, 9℃ / min, 9.3℃ / min, 9.5℃ / min, 9.8℃ / min or 10℃ / min, etc., but not limited to the listed values, other values not listed in the above value range are also applicable.

[0028] In the present application, the annealing heating rate is too low, which is not suitable for generating a large temperature gradient on the surface and inside the microspheres, thereby affecting the generation of the hollow yolk shell structure. When the heating rate is too fast, the crystal will grow excessively, and the specific surface area will be too small, thereby affecting the activity of the product.

[0029] As a preferred technical solution of the present application, the organic iron salt contains a trivalent iron element, and the organic iron salt includes iron salicylate and / or iron gluconate.

[0030] Preferably, the solvent includes N-methylpyrrolidone and / or trifluoromethanesulfonamide.

[0031] Preferably, the molar ratio of the organic iron salt to the polyamide is 1:(1.5-2.5), such as 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4 or 1:2.5, etc., but not only limited to the listed values, other values not listed in the above range are also applicable.

[0032] In the present application, the amount of polyamide as a carbon template is too low, which is not conducive to the formation of hollow yolk shell structure; the amount is too high, which will make the stress on the shell part too large, and further cause the breakage of the shell part.

[0033] As a preferred technical solution of the present application, the inlet temperature of the spray drying is 230-370℃, such as 230℃, 250℃, 270℃, 290℃, 310℃, 330℃, 350℃ or 370℃, etc., but not only limited to the listed values, other values not listed in the above range are also applicable.

[0034] In the present application, the inlet temperature of the spray drying needs to be higher than the boiling point of the solvent in the precursor solution, so that the precursor solution can be rapidly vaporized.

[0035] Preferably, the liquid inlet speed of the spray drying is 2-4 mL / min, such as 2 mL / min, 2.2 mL / min, 2.4 mL / min, 2.6 mL / min, 2.8 mL / min, 3 mL / min, 3.2 mL / min, 3.4 mL / min, 3.6 mL / min or 3.8 mL / min, etc., but not only limited to the listed values, other values not listed in the above range are also applicable.

[0036] Preferably, the gas pressure of the working gas of the spray drying is 0.08-0.12 MPa, such as 0.08 MPa, 0.09 MPa, 0.1 MPa, 0.11 MPa or 0.12 MPa, etc., but not only limited to the listed values, other values not listed in the above range are also applicable.

[0037] Preferably, the holding temperature of the annealing is 500-600℃, such as 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃ or 600℃, etc., and the holding time is 4-6 h, such as 4 h, 4.3 h, 4.5 h, 4.8 h, 5 h, 5.3 h, 5.5 h, 5.8 h or 6 h, etc., but not only limited to the listed values, other values not listed in the above range are also applicable.

[0038] As a preferred technical solution of the present application, the buffer solution comprises tris(hydroxymethyl)aminomethane buffer solution, and the pH is 8-9, for example, 8, 8.2, 8.4, 8.6, 8.8 or 9, but is not limited to the listed values, and other values not listed in the above value range are also applicable.

[0039] Preferably, the concentration of dopamine in the buffer solution is 0.6-1 g / L, for example, 0.6 g / L, 0.7 g / L, 0.8 g / L, 0.9 g / L or 1 g / L, but is not limited to the listed values, and other values not listed in the above value range are also applicable.

[0040] In the present application, the role of dopamine is to coat a carbon layer on the surface of the iron oxide microspheres and the surface of the core part inside the iron oxide microspheres, and dopamine can be polymerized and coated at alkaline room temperature; while other substances such as pyrrole and aniline are also used to form carbon coating, but their formation conditions need to be in an acidic environment or need to add an oxidizing agent and need to be heated, which will affect the iron oxide microspheres and their structure. The reaction conditions of dopamine are more mild and suitable for iron oxide microspheres, so dopamine is selected for carbon coating.

[0041] Preferably, the holding temperature of carbonization is 350-450℃, for example, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, 410℃, 420℃, 430℃, 440℃ or 450℃, and the holding time is 1-3 h, for example, 1 h, 1.3 h, 1.5 h, 1.8 h, 2 h, 2.3 h, 2.5 h, 2.8 h or 3 h, but is not limited to the listed values, and other values not listed in the above value range are also applicable.

[0042] Preferably, the concentration of phosphoric acid is 0.4-0.8 mol / L, for example, 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, 0.55 mol / L, 0.6 mol / L, 0.65 mol / L, 0.7 mol / L, 0.75 mol / L or 0.8 mol / L, but is not limited to the listed values, and other values not listed in the above value range are also applicable.

[0043] Preferably, the molar ratio of urea, phosphorus element and iron element is (1-3):1:1, for example, 1:1:1, 1.3:1:1, 1.5:1:1, 1.8:1:1, 2:1:1, 2.3:1:1, 2.5:1:1, 2.8:1:1 or 3:1:1, and the use amount of the urea, phosphoric acid and the carbon-coated microspheres is controlled, but is not limited to the listed values, and other values not listed in the above value range are also applicable.

[0044] In the present application, the role of urea is to adjust the acid environment during the hydrothermal reaction of phosphoric acid and ferroferric oxide microspheres, so that the reaction system is not too acidic, which is not conducive to the formation of iron phosphate.

[0045] Preferably, the holding temperature of the hydrothermal reaction is 120-150°C, such as 120°C, 125°C, 130°C, 135°C, 140°C, 145°C or 150°C, etc., and the holding time is 16-24h, such as 16h, 18h, 20h, 22h or 24h, etc., but not limited to the listed values, other values not listed within the above-mentioned value range are also applicable.

[0046] As a preferred technical solution of the present application, step (2) further comprises dehydrating the obtained hollow yolk shell structure carbon-coated iron phosphate to obtain a hollow yolk shell structure carbon-coated anhydrous iron phosphate.

[0047] Preferably, the dehydration method comprises holding at 500-750°C, such as 500°C, 530°C, 550°C, 580°C, 600°C, 630°C, 650°C, 680°C, 700°C, 720°C or 750°C, etc., for 4-6h, such as 4h, 4.3h, 4.5h, 4.8h, 5h, 5.3h, 5.5h, 5.8h or 6h, etc., but not limited to the listed values, other values not listed within the above-mentioned value range are also applicable.

[0048] As a preferred technical solution of the present application, the mixing method of step (3) comprises ball milling for 2-5h, such as 2h, 2.3h, 2.5h, 2.8h, 3h, 3.3h, 3.5h, 3.8h, 4h, 4.3h, 4.5h, 4.8h or 5h, etc., but not limited to the listed values, other values not listed within the above-mentioned value range are also applicable.

[0049] Preferably, the lithium source comprises at least one of lithium carbonate, lithium hydroxide or lithium acetate.

[0050] Preferably, the amount of hollow yolk shell structure carbon-coated iron phosphate and lithium source is controlled according to the molar ratio of iron element to lithium element as 1:(1-1.02), such as 1:1, 1:1.01 or 1:1.02, etc., but not limited to the listed values, other values not listed within the above-mentioned value range are also applicable.

[0051] Preferably, the sintering method comprises pre-sintering at 400-550°C, for example 400°C, 420°C, 450°C, 480°C, 500°C, 520°C or 550°C, etc., for 3-5h, for example 3h, 3.3h, 3.5h, 3.8h, 4h, 4.3h, 4.5h, 4.8h or 5h, etc., and then secondary sintering at 650-750°C, for example 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C or 750°C, etc., for 6-10h, for example 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h or 10h, etc., but not limited to the listed values, and other values not listed in the above value range are also applicable.

[0052] As a preferred technical solution of the present application, the preparation method comprises:

[0053] (1) Preparation of hollow egg yolk shell structure iron oxide microspheres:

[0054] The organic iron salt containing trivalent iron and the polyamide powder are uniformly dispersed in the solvent at a molar ratio of 1:(1.5-2.5), and magnetically stirred for 0.5-1h to form a clear and transparent precursor solution; then the precursor solution is fed into a spray dryer, the inlet temperature of the spray gun is set to 230-370°C, the liquid feeding speed is controlled to 2-4mL / min, the gas pressure of the working gas is 0.08-0.12MPa, and the equipment is started to make the precursor solution rapidly dry into a precursor powder with a polyamide carbon template wrapped around Fe 3+ under the action of hot nitrogen gas flow; then the precursor powder is collected in a collector and placed in a muffle furnace for annealing in an air atmosphere at 500-600°C for 4-6h at a heating rate of 8-10°C / min, to obtain hollow egg yolk shell structure iron oxide microspheres;

[0055] (2) Preparation of hollow egg yolk shell structure carbon-coated iron phosphate:

[0056] A Tris buffer solution with pH of 8-9 is prepared, dopamine is added into the buffer solution, the concentration of dopamine in the buffer solution is controlled to be 0.6-1 g / L, the hollow yolk shell structured iron oxide microspheres obtained in step (1) are added into the dopamine buffer solution after stirring for 5-15 min, a polymerization reaction is carried out by continuously stirring at room temperature for 16-24 h, dopamine is polymerized in situ, the product is centrifuged, washed and vacuum dried after the reaction is completed, and the pre-coated microspheres are obtained; then, the pre-coated microspheres are placed in a muffle furnace and carbonized at 350-450 ℃ in a high-purity nitrogen atmosphere for 1-3 h to form an outer carbon layer and an inner carbon layer, the product is collected after being naturally cooled to room temperature, and the carbon-coated microspheres are obtained; the carbon-coated microspheres are dispersed in deionized water, then urea is added, and after being uniformly stirred, a 0.4-0.8 mol / L phosphoric acid solution is added as phosphoric acid, the molar ratio of urea, phosphorus element and iron element is (1-3):1:1, the amounts of the urea, phosphoric acid and the carbon-coated microspheres are controlled, and after stirring at room temperature for a period of time, the mixture is transferred to a stainless steel autoclave and placed in an oven for hydrothermal reaction at 120-150 ℃ for 16-24 h; after being cooled to room temperature, the product is centrifuged, washed, vacuum dried and obtained as the hollow yolk shell structured carbon-coated iron phosphate; then, the hollow yolk shell structured carbon-coated iron phosphate is placed in a muffle furnace, heated to 500-750 ℃ and kept for 4-6 h to remove the crystal water, and the hollow yolk shell structured carbon-coated anhydrous iron phosphate is obtained.

[0057] (3) Preparation of the hollow yolk shell structured lithium iron phosphate:

[0058] Under a high-purity argon atmosphere, the hollow yolk shell structured carbon-coated anhydrous iron phosphate of step (2) is mixed with a lithium source at a molar ratio of iron element to lithium element of 1:(1-1.02) by ball milling for 2-5 h, dried, pre-sintered at 400-550 ℃ for 3-5 h, and then secondarily sintered at 650-750 ℃ for 6-10 h, and the hollow yolk shell structured lithium iron phosphate is obtained.

[0059] In a third aspect, the present application provides a lithium ion battery containing the hollow yolk shell structured lithium iron phosphate of the first aspect.

[0060] Compared with the prior art, the present application has at least the following beneficial effects:

[0061] (1) The lithium iron phosphate material prepared by the present application has a specific hollow yolk shell structure. Compared with irregular bulk materials, the material with the structure has a higher specific surface area and abundant electrochemical reaction active sites, which is beneficial to the infiltration of electrolyte, shortens the diffusion path of lithium ions and accelerates the de-intercalation rate, and also helps to enhance the transmission and transfer of electric charge, thereby improving the rate performance of the lithium iron phosphate battery. In addition, the hollow yolk shell structure can effectively adjust the volume change and stress change of the electrode material during the charging and discharging process, thereby improving the cycle stability.

[0062] (2) The preparation method of the present application first in-situ polymerizes dopamine on the surface of the hollow yolk shell structure of the iron oxide microspheres, and then carbonizes the polydopamine to form a carbon layer. The carbon layer not only provides amorphous carbon to improve the electronic / ion conductivity of the electrode material, but also supports the specific hollow yolk shell structure. After the carbon-coated microspheres are mixed with phosphoric acid, the phosphoric acid can fully react with the iron oxide shell covered by the outer carbon layer and the iron oxide sphere wrapped by the inner carbon layer to form carbon-coated lithium iron phosphate, and then react with lithium source to obtain lithium iron phosphate, thereby maintaining the inheritance and stability of the hollow yolk shell structure. In addition, the amorphous carbon layer can also act as a carbon source for the synthesis of lithium iron phosphate from iron phosphate and lithium source, without the need for additional addition of other carbon sources. BRIEF DESCRIPTION OF DRAWINGS

[0063] Figure 1 is a flowchart of the preparation method of the hollow yolk shell structure of lithium iron phosphate of Example 1;

[0064] Figure 2 is an XRD graph of the carbon-coated lithium iron phosphate of the hollow yolk shell structure of Example 1;

[0065] Figure 3 is an SEM graph of the lithium iron phosphate of the hollow yolk shell structure of Example 1;

[0066] Figure 4 is a TEM graph of the lithium iron phosphate of the hollow yolk shell structure of Example 1;

[0067] Figure 5 is a cycle stability test graph of the lithium iron phosphate of the hollow yolk shell structure of Example 1 and the lithium iron phosphate obtained in Comparative Example 1 at a current density of 0.1C;

[0068] Figure 6 is an SEM graph of the lithium iron phosphate of Comparative Example 2. DETAILED DESCRIPTION

[0069] The technical solutions of the present application will be further described below through specific embodiments.

[0070] Those skilled in the art should understand that the embodiments are only to help understand the present application and should not be considered as specific limitations of the present application.

[0071] Embodiment 1

[0072] The present embodiment provides a hollow yolk-shell structured lithium iron phosphate, comprising:

[0073] a shell part, the shell part comprising a lithium iron phosphate spherical shell, and an outer carbon layer coating the lithium iron phosphate spherical shell;

[0074] a core part located inside the shell part; the core part comprising a lithium iron phosphate sphere, and an inner carbon layer coating the lithium iron phosphate sphere;

[0075] a hollow part located between the core part and the shell part.

[0076] As shown in the following scheme, the preparation method of the hollow yolk-shell structured lithium iron phosphate comprises: Figure 1

[0077] (1) Preparation of hollow yolk-shell structured iron oxide microspheres:

[0078] According to a molar ratio of 1:2, 2 mmol of iron salicylate and 4 mmol of polyamide powder are uniformly dispersed into 100 mL of solvent N-methyl pyrrolidone, magnetically stirred for 0.5 h to form a clear and transparent precursor solution; then the precursor solution is fed into a spray dryer, the inlet temperature of the spray gun is set to 250°C, the liquid feeding speed is controlled to 3 mL / min, the gas pressure of the working gas is 0.1 MPa, and the equipment is started to make the precursor solution rapidly dry into a precursor powder of spherical Fe 3+ coated with a polyamide carbon template under the action of hot nitrogen gas flow; then the precursor powder is collected in a collector and placed in a muffle furnace to be annealed at 500°C for 5 h in an air atmosphere at a heating rate of 10°C / min, thereby obtaining hollow yolk-shell structured iron oxide microspheres;

[0079] (2) Preparation of hollow yolk-shell structured carbon-coated lithium iron phosphate:

[0080] ​A Tris buffer solution with a pH of 8.5 was prepared in a volume of 50 mL, 30 mg of dopamine was added to the buffer solution to control the concentration of dopamine in the buffer solution at 0.6 g / L, and after stirring for 10 min, the hollow yolk shell structure of iron oxide microspheres obtained in step (1) was added to the dopamine buffer solution, and the polymerization reaction was carried out at room temperature for 24 h under continuous stirring to allow in-situ polymerization of dopamine. After the reaction was completed, the product was centrifuged and washed several times with deionized water and anhydrous ethanol, and then vacuum dried at 70°C to obtain pre-coated microspheres. Subsequently, the pre-coated microspheres were placed in a muffle furnace and carbonized at 400°C for 2 h in a high-purity nitrogen atmosphere to form an outer carbon layer and an inner carbon layer. After natural cooling to room temperature, the product was collected to obtain carbon-coated microspheres. The carbon-coated microspheres were dispersed in 50 mL of deionized water, then 4 mmol of urea was added, and after stirring uniformly, 4 mL of 0.5 mol / L phosphoric acid solution as phosphoric acid was added. The molar ratio of urea, phosphorus and iron was 2:1:1, and the amounts of urea, phosphoric acid and carbon-coated microspheres were controlled. After stirring at room temperature for a period of time, the mixture was transferred to a stainless steel autoclave and placed in an oven for hydrothermal reaction at 120°C for 16 h. After cooling to room temperature, the product was collected by centrifugation, washed, and vacuum dried to obtain carbon-coated iron phosphate with a hollow yolk shell structure. Subsequently, the carbon-coated iron phosphate with a hollow yolk shell structure was placed in a muffle furnace, heated to 500°C for 4 h to remove the crystallization water, thereby obtaining carbon-coated anhydrous iron phosphate with a hollow yolk shell structure.

[0081] (3) Preparation of lithium iron phosphate with a hollow yolk shell structure:

[0082] Under a high-purity argon atmosphere, the carbon-coated anhydrous iron phosphate with a hollow yolk shell structure obtained in step (2) was mixed with lithium carbonate as a lithium source at a molar ratio of iron to lithium of 1:1 and ball milled for 3 h. After drying, pre-sintering was first carried out at 400°C for 3 h at a heating rate of 5°C / min, and then secondary sintering was carried out at 700°C for 8 h to obtain lithium iron phosphate with a hollow yolk shell structure.

[0083] Example 2

[0084] This example provides a lithium iron phosphate with a hollow yolk shell structure, and the preparation method adjusts the annealing heating rate in step (1) from 10°C / min to 8°C / min, and the other conditions are exactly the same as in Example 1.

[0085] Example 3

[0086] This example provides a lithium iron phosphate with a hollow yolk shell structure, and the preparation method adjusts the annealing heating rate in step (1) from 10°C / min to 13°C / min, and the other conditions are exactly the same as in Example 1.

[0087] Example 4

[0088] This example provides a hollow yolk shell structure of lithium iron phosphate, the preparation method of which adjusts the concentration of dopamine in the buffer solution from 0.6 g / L to 0.4 g / L in step (2), and other conditions are exactly the same as those in Example 1.

[0089] Example 5

[0090] This example provides a hollow yolk shell structure of lithium iron phosphate, the preparation method of which adjusts the concentration of dopamine in the buffer solution from 0.6 g / L to 0.8 g / L in step (2), and other conditions are exactly the same as those in Example 1.

[0091] Example 6

[0092] This example provides a hollow yolk shell structure of lithium iron phosphate, the preparation method of which adjusts the concentration of dopamine in the buffer solution from 0.6 g / L to 1 g / L in step (2), and other conditions are exactly the same as those in Example 1.

[0093] Example 7

[0094] This example provides a hollow yolk shell structure of lithium iron phosphate, the preparation method of which adjusts the concentration of dopamine in the buffer solution from 0.6 g / L to 1.2 g / L in step (2), and other conditions are exactly the same as those in Example 1.

[0095] Example 8

[0096] This example provides a hollow yolk shell structure of lithium iron phosphate, the preparation method of which adjusts the temperature of hydrothermal reaction from 120℃ to 100℃ in step (2), and other conditions are exactly the same as those in Example 1.

[0097] Example 9

[0098] This example provides a hollow yolk shell structure of lithium iron phosphate, the preparation method of which adjusts the temperature of hydrothermal reaction from 120℃ to 140℃ in step (2), and other conditions are exactly the same as those in Example 1.

[0099] Example 10

[0100] This example provides a hollow yolk shell structure of lithium iron phosphate, the preparation method of which adjusts the temperature of hydrothermal reaction from 120℃ to 160℃ in step (2), and other conditions are exactly the same as those in Example 1.

[0101] Comparative Example 1

[0102] The comparative example 1 provides a lithium iron phosphate material, and a preparation method thereof comprises the following steps:

[0103] S1: 2 mmol of Fe2O3 was dispersed in 50 mL of deionized water, then 8 mmol of urea was added, and after stirring, 8 mL of 0.5 mol / L phosphoric acid solution was added to the above mixture. After stirring at room temperature for 30 min, the mixture was transferred to a 100 mL stainless steel autoclave and placed in an oven at 120°C for 16 h. After cooling to room temperature, the product was collected by centrifugation, washed, and vacuum dried to obtain a lithium iron phosphate material; then, the above product was placed in a muffle furnace and heated to 500°C for 4 h to remove the crystal water, thereby obtaining anhydrous lithium iron phosphate;

[0104] S2: lithium carbonate, anhydrous lithium iron phosphate and glucose were dispersed in anhydrous ethanol according to the molar ratio of 1:1:0.08, and ball-milled for 3 h to obtain a mixture. The mixture was then spray-dried to obtain a precursor powder; then, the precursor powder was heated to 400°C at a rate of 5°C / min under an argon atmosphere and kept for 3 h, and then heated to 700°C for calcination for 8 h to obtain a lithium iron phosphate material.

[0105] Comparative example 2

[0106] The comparative example 1 provides a lithium iron phosphate material, and a preparation method thereof comprises the following steps:

[0107] Comparative example 3

[0108] The comparative example 1 provides a lithium iron phosphate material, and a preparation method thereof comprises the following steps:

[0109] Comparative example 4

[0110] The comparative example 1 provides a lithium iron phosphate material, and a preparation method thereof comprises the following steps:

[0111] Characterization and testing:

[0112] I. Figure 2 , Figure 3 and Figure 4respectively, the XRD pattern of the carbon-coated iron phosphate of the hollow yolk-shell structure of Example 1 and the SEM and TEM images of the obtained lithium iron phosphate of the hollow yolk-shell structure can be seen from the figures, the XRD pattern of the iron phosphate material has the same characteristic peaks compared with the FePO4 standard card, no other impurity diffraction peak, good crystallinity; the SEM image shows that the lithium iron phosphate material is in spherical structure, the TEM image shows that the lithium iron phosphate material has obvious hollow yolk-shell structure. Figure 6 is the SEM image of the lithium iron phosphate of Comparative Example 2, compared with Example 1, the lithium iron phosphate material obtained from Comparative Example 2 has a cavity structure, but it does not have a complete yolk-shell structure.

[0113] II. In combination with SEM and TEM morphology characterization and specific surface area test, the specific surface area, median particle size and thickness of the outer carbon layer of the lithium iron phosphate of the hollow yolk-shell structure obtained from Example 1 were tested, and the lithium iron phosphate in the part of the examples which changed was also tested, and the obtained results are recorded in Table 1.

[0114] Table 1

[0115] Group Specific surface area (m 2 / g) Median particle diameter (nm) Thickness of outer carbon layer (nm) Example 1 14.5 530 3.5 Example 2 14.0 550 3.5 Example 5 15.4 505 4.0 Example 6 15.2 515 4.5 Example 9 15.8 500 3.5

[0116] III. The lithium iron phosphate obtained from the examples and comparative examples was assembled as a positive electrode material to form a button cell, and lithium ion battery electrochemical performance test was carried out (the charge and discharge voltage was controlled between 2.5-4.5V), and the results are shown in Table 2.

[0117] Table 2

[0118] Group 0.1 C discharge specific capacity (mAh / g) 0.5 C discharge specific capacity (mAh / g) First charge-discharge efficiency (%) Example 1 157.7 151.4 98.03 Example 2 156.9 150.6 97.37 Example 3 151.6 145.3 94.68 Example 4 155.7 149.4 96.43 Example 5 158.5 152.3 98.75 Example 6 158.9 152.6 98.93 Example 7 156.6 150.4 97.25 Example 8 156.8 150.7 97.32 Example 9 159.4 153.7 99.11 Example 10 157.2 150.7 97.72 Comparative Example 1 142.6 131.8 89.11 Comparative Example 2 152.4 144.3 95.75 Comparative Example 3 144.3 134.5 90.56 Comparative Example 4 148.5 140.4 92.32

[0119] From Table 1 and Table 2, it can be seen that:

[0120] As can be seen from the table, the electrochemical performance of the lithium iron phosphate product prepared in the examples is obviously better than that of Comparative Example 1, especially Example 9.

[0121] From the comparison of Example 1 and Example 2 and 3 and Comparative Example 4, it can be obtained that in the preparation process of the hollow yolk-shell structure lithium iron phosphate described in the application, the heating rate will affect its structure, if the heating rate of the furnace body is too large, not only the heating element will be damaged, but also the crystal will grow excessively, reducing its specific surface area, thereby affecting the activity of the product. If the heating rate is too low, it cannot produce a large temperature gradient on the surface and inside of the microspheres, thereby affecting the generation of the hollow yolk-shell structure.

[0122] As can be seen from the comparison between Example 1 and Examples 4-7, in the preparation process of the hollow yolk shell structure lithium iron phosphate of the present invention, the mass concentration of dopamine affects the thickness of the carbon coating layer. If the mass concentration of dopamine is too low, the thickness of the carbon coating layer will be too thin, which is not conducive to improving the conductivity of the cathode material and regulating the volume stress changes generated during charging and discharging. If the mass concentration of dopamine is too high, the thickness of the carbon coating layer will be too thick, which may hinder the migration of lithium ions and thus affect the performance of the battery.

[0123] A comparison of Examples 1 and 8-10 shows that during the preparation of the hollow yolk structure lithium iron phosphate of the present invention, the hydrothermal reaction temperature affects its structure and thus its performance. If the reaction temperature is too low, it may affect the phase transformation of the product particles and affect their crystallinity. If the reaction temperature is too high, the product particles are prone to agglomeration and the particle size may be too large, resulting in a reduction in the performance of the electrode material.

[0124] Compared to the hollow yolk-shell spherical structure of lithium iron phosphate prepared in Example 1, the products obtained in Comparative Examples 1 and 3 were solid, irregular lithium iron phosphate, while the product obtained in Comparative Example 4 was a solid spherical lithium iron phosphate material. Because the hollow yolk-shell spherical cathode material of Example 1 has a large specific surface area, a short and fast lithium-ion diffusion path, and low insertion / extraction resistance, therefore, in conjunction with Table 2 and... Figure 5 It can be seen that the lithium-ion battery using the hollow eggshell structure of lithium iron phosphate obtained in Example 1 has excellent electrochemical performance and stability.

[0125] In contrast to Comparative Example 2, when polyamide was used in Example 1, due to the thermal shrinkage effect of polyamide, the carbon template formed during the spray drying process encapsulated the Fe. 3+ The spherical precursor particles are more compact and stable, resulting in a more complete hollow yolk shell structure in the subsequently prepared iron phosphate and lithium iron phosphate materials, and thus better electrical performance.

[0126] This invention illustrates the detailed process equipment and process flow through the above embodiments. However, this invention is not limited to the detailed process equipment and process flow described above, meaning that this invention does not necessarily depend on the detailed process equipment and process flow to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials for the product of this invention, addition of auxiliary components, and selection of specific methods, all fall within the protection scope and disclosure scope of this invention.

[0127] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0128] It should be further noted that any technically feasible combination of the various technical features described in the above embodiments is possible, provided that there is no contradiction, and the disclosure of the present application shall be deemed to include all such technically feasible combinations.

[0129] Furthermore, any combination of the various embodiments of the present application is possible, provided that there is no contradiction, and the disclosure of the present application shall be deemed to include all such technically feasible combinations.

Claims

1. A method of producing a lithium iron phosphate having a hollow yolk shell structure, characterized by, The preparation method comprises: (1) mixing an organic iron salt, a polyamide and a solvent to obtain a precursor solution; spray drying the precursor solution to obtain a precursor powder; annealing the precursor powder, and controlling the heating rate of the annealing to be 8-10 ℃ / min to obtain iron oxide microspheres with a hollow yolk-shell structure; (2) mixing the iron oxide microspheres, dopamine and a buffer solution, the concentration of the dopamine in the buffer solution being 0.6-1 g / L, to perform a polymerization reaction to obtain pre-coated microspheres; carbonizing the pre-coated microspheres to form an outer carbon layer and an inner carbon layer, and obtaining carbon-coated microspheres; mixing urea, phosphoric acid and the carbon-coated microspheres to perform a hydrothermal reaction, the holding temperature of the hydrothermal reaction being 120-150 ℃, to obtain carbon-coated iron phosphate with a hollow yolk-shell structure; (3) mixing the obtained carbon-coated iron phosphate with a hollow yolk-shell structure and a lithium source to perform sintering to obtain lithium iron phosphate with a hollow yolk-shell structure. The lithium iron phosphate with a hollow yolk-shell structure comprises: a shell part, the shell part comprising a lithium iron phosphate shell, and an outer carbon layer coating the lithium iron phosphate shell; a core part located inside the shell part, the core part comprising a lithium iron phosphate core, and an inner carbon layer coating the lithium iron phosphate core; a hollow part located between the core part and the shell part, and the shell part and the core part being completely separated by the hollow part in the lithium iron phosphate with a hollow yolk-shell structure.

2. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The specific surface area of the lithium iron phosphate of the hollow yolk shell structure is 13-16 m 2 / g.

3. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The median particle size of the lithium iron phosphate with a hollow yolk-shell structure is 500-600 nm.

4. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The thickness of the outer carbon layer and the inner carbon layer is 3-6 nm.

5. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The organic iron salt contains a trivalent iron element, and the organic iron salt comprises iron salicylate and / or iron gluconate.

6. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The solvent comprises N-methyl pyrrolidone and / or trifluoromethanesulfonamide.

7. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The molar ratio of the organic iron salt to the polyamide is 1:(1.5-2.5).

8. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The inlet temperature of the spray drying is 230-370 ℃.

9. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The liquid feeding speed of the spray drying is 2-4 mL / min.

10. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The gas pressure of the working gas of the spray drying is 0.08-0.12 MPa.

11. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The holding temperature of the annealing is 500-600 ℃, and the holding time is 4-6 h.

12. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The buffer solution comprises tris(hydroxymethyl)aminomethane buffer, and the pH is 8-9.

13. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The holding temperature of the carbonization is 350-450 ℃, and the holding time is 1-3 h.

14. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The concentration of the phosphoric acid is 0.4-0.8 mol / L.

15. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The molar ratio of urea, phosphorus element and iron element is (1-3):1:1, and the amounts of the urea, the phosphoric acid and the carbon-coated microspheres are controlled.

16. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The holding time of the hydrothermal reaction is 16-24 h.

17. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, Step (2) further comprises dehydrating the obtained carbon-coated iron phosphate with a hollow yolk-shell structure to obtain carbon-coated anhydrous iron phosphate with a hollow yolk-shell structure, and then mixing the carbon-coated anhydrous iron phosphate with the lithium source.

18. The method of claim 17, wherein the hollow yolk shell structure of lithium iron phosphate is prepared by a process comprising: The dehydration method comprises holding at 500-750 ℃ for 4-6 h.

19. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The mixing method of step (3) comprises ball milling for 2-5 h.

20. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The lithium source comprises at least one of lithium carbonate, lithium hydroxide or lithium acetate.

21. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The amount of the carbon-coated iron phosphate with hollow yolk shell structure and the lithium source is controlled according to a molar ratio of iron to lithium of 1: (1-1.02).

22. The method for preparing lithium iron phosphate with a hollow yolk shell structure according to claim 1, characterized in that, The sintering method comprises pre-sintering at 400-550 ℃ for 3-5 h and secondary sintering at 650-750 ℃ for 6-10 h.

23. A lithium-ion battery, characterized by, Lithium iron phosphate with hollow yolk shell structure obtained by the preparation method in any one of claims 1-22.

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