A Lithium Iron Phosphate Cathode Material, Its Preparation Method and Application

Abstract

The present invention discloses a lithium iron phosphate cathode material, a preparation method thereof and an application. The lithium iron phosphate cathode material has a core-shell structure, including a lithium iron phosphate as the inner core, a lithium iron phosphate inner shell layer grown along the surface of the inner core, and a carbon-coated outer shell layer coated outside the inner shell layer; wherein, the primary particle size of the lithium iron phosphate inner core is less than 150 nm. The present invention first prepares a small-sized lithium iron phosphate inner core by a hydrothermal method, and then uses the ferrous oxalate process to grow and crystallize the lithium iron phosphate inner shell layer along the interface of the inner core, thereby obtaining a lithium iron phosphate cathode material with a highly consistent core-shell structure; since the inner core of the cathode material is also lithium iron phosphate, it will not reduce the overall capacity of the material; the present invention uses the ferrous oxalate process route to prepare the lithium iron phosphate cathode material with a core-shell structure, which only requires a small amount of hydrothermal method to prepare the lithium iron phosphate inner core on the basis of easy control of the ingredients and simple process, which is beneficial to the control of the manufacturing cost of the material.

Description

Technical Field

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

[0002] With the large-scale application of lithium-ion batteries, positive electrode materials are the core of positive electrode materials both in terms of performance impact and cost share. Lithium iron phosphate has become the most widely used positive electrode material for lithium-ion batteries due to its advantages of long cycle, high safety and low cost.

[0003] The ferrous oxalate process is one of the process routes for preparing lithium iron phosphate materials by the high-temperature solid phase method. It uses ferrous oxalate as the iron source and lithium carbonate / lithium hydroxide as the lithium source. After ball milling and granulation, it is calcined at high temperature in a reducing atmosphere to prepare lithium iron phosphate positive electrode materials. It has the advantages of simple process, easy control of ingredients, and high compaction density of the finished material. However, the lithium iron phosphate positive electrode material prepared by this method has an uneven particle size distribution, irregular morphology, and the particle size and morphology are not easy to control. The poor consistency of the primary particle size and morphology of the lithium iron phosphate positive electrode material will lead to serious polarization during the cycle, affecting the performance of the long cycle performance of lithium iron phosphate. The regular and smooth particle surface of the positive electrode material and the uniform particle size distribution are the guarantee for achieving the ultra-long cycle performance of the positive electrode material, but the existing ferrous oxalate process route is difficult to achieve this effect. Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention provides a lithium iron phosphate positive electrode material and a preparation method thereof. The lithium iron phosphate positive electrode material is prepared by adopting a ferrous oxalate process to obtain a lithium iron phosphate positive electrode material with highly consistent primary particle size and morphology.

[0005] In order to achieve the above object, the present invention adopts the following technical solution.

[0006] A lithium iron phosphate positive electrode material, the lithium iron phosphate positive electrode material has a core-shell structure, including lithium iron phosphate as an inner core, a lithium iron phosphate inner shell layer (i.e., an intermediate layer) grown along the surface of the inner core, and a carbon coating layer outer shell coated outside the inner shell layer; wherein the primary particle size of the lithium iron phosphate inner core is less than 150nm.

[0007] As a preferred embodiment, the lithium iron phosphate positive electrode material has a primary particle size of 70-150 nm in the lithium iron phosphate core.

[0008] As a preferred embodiment, the lithium iron phosphate positive electrode material has a diameter d of lithium iron phosphate particles of 0.55±0.21 μm.

[0009] The present invention also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material, which adopts the following technical scheme.

[0010] A method for preparing the above-mentioned lithium iron phosphate positive electrode material comprises the following steps:

[0011] (1) preparing a lithium iron phosphate core by a hydrothermal method;

[0012] (2) Preparation of lithium iron phosphate positive electrode material by ferrous oxalate method: comprising the following steps:

[0013] S2-1: weighing a lithium source, ferrous oxalate, and a phosphorus source, adding them into a ball mill, and adding acetylene black and the lithium iron phosphate core prepared in step (1) to obtain a mixture; wherein the amount of the lithium iron phosphate core added is 3 to 5% (for example, 3.5%, 4%, 4.5%, 4.8%) of the total mass of the lithium source, ferrous oxalate, and phosphorus source;

[0014] S2-2: adding the mixture and anhydrous ethanol into a planetary ball mill for ball milling, and filtering and drying to obtain a block product after the ball milling is completed;

[0015] S2-3: Grinding the block product in step S2-2 to obtain a powder precursor, and adding glucose to mix evenly to obtain a lithium iron phosphate positive electrode material precursor;

[0016] S2-4: calcining the lithium iron phosphate positive electrode material precursor at 600-800° C. in an inert atmosphere, obtaining a calcined product after cooling, and crushing the calcined product to obtain the lithium iron phosphate positive electrode material.

[0017] In the above preparation method, as a preferred embodiment, in step (1), preparing the lithium iron phosphate core by a hydrothermal method comprises the following steps:

[0018] S1-1: Weigh the lithium source, iron source and phosphorus source respectively;

[0019] S1-2: prepare a phosphorus source aqueous solution, then add ascorbic acid and a weighed iron source thereto in sequence, stir to fully dissolve them, and the resulting solution is recorded as solution A;

[0020] S1-3: Prepare a lithium source aqueous solution, referred to as solution B;

[0021] S1-4: after uniformly mixing solution A and solution B, perform a hydrothermal reaction, the temperature of the hydrothermal reaction is 120 to 180° C. (for example, 130° C., 150° C., 160° C., 170° C.);

[0022] S1-5: Wash the hydrothermal reaction product with deionized water, dry it, and then calcine it in an inert atmosphere at 450 - 650 °C (for example, 460 °C, 480 °C, 500 °C, 550 °C, 580 °C, 600 °C, 620 °C, 640 °C) to obtain lithium iron phosphate solid. After grinding, the lithium iron phosphate core is prepared.

[0023] In the above preparation method, as a preferred embodiment, in the step S1-1, the lithium source is selected from one or more of lithium oxide, lithium carbonate, lithium phosphate, and lithium hydroxide; the iron source is selected from one or more of ferrous sulfate, ferrous oxalate, ferrous chloride, and ferrous glycinate; the phosphorus source is selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium phosphate.

[0024] In the above preparation method, as a preferred embodiment, in the step S1-2, the mass concentration of the phosphorus source aqueous solution is 15 - 25 wt% (for example, 16 wt%, 18 wt%, 20 wt%, 22 wt%, 24 wt%).

[0025] In the above preparation method, as a preferred embodiment, in the step S1-2, the mass concentration of ascorbic acid in the phosphorus source aqueous solution is 0.1 - 0.3 wt% (for example, 0.2%).

[0026] In the above preparation method, as a preferred embodiment, in the step S1-3, the mass concentration of the lithium source aqueous solution is 10% - 12% (for example, 10.5%, 11%, 11.5%, 10.8%).

[0027] In the above preparation method, as a preferred embodiment, in the step S1-4, the reaction time of the hydrothermal reaction is 4 - 8 h (for example, 5 h, 6 h, 7 h).

[0028] In the above preparation method, as a preferred embodiment, in the step S1-5, the calcination time is 2 - 4 h (for example, 2.5 h, 3 h, 3.5 h).

[0029] In the above preparation method, as a preferred embodiment, in the step S2-1, the lithium source is selected from one or more of lithium oxide, lithium carbonate, lithium phosphate, and lithium hydroxide; the phosphorus source is selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium phosphate.

[0030] In the above preparation method, as a preferred embodiment, in the step S2-1, the lithium source, ferrous oxalate, and phosphorus source are weighed according to the molar ratio of elements Li, Fe, and P in the lithium source, ferrous oxalate, and phosphorus source being 1:1:1.

[0031] In the above preparation method, as a preferred embodiment, in the step S2-1, the addition amount of acetylene black is 0.1%-0.3% (for example, 0.15%, 0.2%, 0.25%) of the total mass of the lithium source, ferrous oxalate and phosphorus source.

[0032] In the above preparation method, as a preferred embodiment, in the step S2-2, the mass ratio between the mixture and absolute ethanol is 1∶1.1-1.3 (for example, 1∶1.15, 1∶1.2, 1∶1.25).

[0033] In the above preparation method, as a preferred embodiment, in the step S2-2, ball milling is carried out at a speed of 250-400 r / min (for example, 260 r / min, 280 r / min, 300 r / min, 350 r / min, 370 r / min, 380 r / min) for 6-12 h (for example, 7 h, 8 h, 9 h, 10 h, 11 h).

[0034] In the above preparation method, as a preferred embodiment, in the step S2-3, the massive product in the step S2-2 is ground to obtain a powdery precursor, and 2%-4% (for example, 2.5%, 3%, 3.5%, 3.8%) of the mass of the powdery precursor is added and mixed evenly to obtain a precursor of the lithium iron phosphate cathode material.

[0035] In the above preparation method, as a preferred embodiment, in the step S2-4, the calcination time is 9-15 h.

[0036] In the above preparation method, as a preferred embodiment, in the step S1-5 or S2-4, the inert atmosphere is selected from one of nitrogen, helium, argon and neon.

[0037] The present invention also provides an application of the above lithium iron phosphate cathode material in a lithium battery.

[0038] In the present invention, without conflict, the above technical features can be freely combined to form a new technical solution.

[0039] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0040] (1) In the preferred embodiment of the present invention, small-sized lithium iron phosphate cores are first prepared by a hydrothermal method, and then the lithium iron phosphate inner shell layer is grown and crystallized along the inner core interface by a solid-phase method, thereby obtaining a lithium iron phosphate cathode material with a core-shell structure having high consistency; the high consistency of the size and morphology of the cathode material is beneficial to reducing the internal resistance and improving the long-cycle performance of the cathode material.

[0041] (2) The lithium iron phosphate cathode material with a core-shell structure prepared by adopting the technical scheme of the present invention will not reduce the overall capacity of the material because the core of the cathode material is also lithium iron phosphate.

[0042] (3) The present invention adopts an iron oxalate process route to prepare a lithium iron phosphate cathode material with a core-shell structure. On the basis of easy control of the ingredients and simple process, only a small amount of hydrothermal method is required to prepare the lithium iron phosphate core, which is beneficial to the control of the manufacturing cost of the material. Brief Description of the Drawings

[0043] Figure 1 It is a structural diagram of the lithium iron phosphate cathode material of the present invention.

[0044] Figure 2 It is an SEM image of the lithium iron phosphate cathode material in Example 1 of the present invention at a magnification of 20K.

[0045] Figure 3 It is an SEM image of the lithium iron phosphate cathode material in Comparative Example 3 at a magnification of 20K.

[0046] Figure 4 It is the charge-discharge curve of the lithium iron phosphate cathode material in Example 1 of the present invention at 0.1C.

[0047] Figure 5 It is the 1C / 1C normal temperature cycle retention rate of the lithium iron phosphate cathode material in Example 1 of the present invention. Detailed Description of the Embodiments

[0048] The present invention will be described in detail below in conjunction with the accompanying drawings of the specification and the embodiments of the present invention. Each example is provided by way of explanation of the present invention rather than limitation of the present invention. In fact, those skilled in the art will clearly understand that modifications and variations can be made to the present invention without departing from the scope or spirit of the present invention. For example, the features shown or described as part of one embodiment can be used in another embodiment to produce yet another embodiment. Therefore, it is desirable that the present invention includes such modifications and variations that fall within the scope of the appended claims and their equivalents.

[0049] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0050] The process parameters not specified under specific conditions in the following examples are usually in accordance with conventional conditions.

[0051] Such asFigure 1 As shown, the lithium iron phosphate cathode material according to the present invention includes a lithium iron phosphate core, a lithium iron phosphate inner shell layer grown along the core, and a carbon-coated outer shell layer coated outside the inner shell layer. Among them, the primary particle size of the lithium iron phosphate in the core is less than 150 nm; the particle diameter d of the lithium iron phosphate cathode material is 0.55 ± 0.21 μm (here, the particle diameter is the particle diameter of the target product). Here, the primary particle size is the grain size of the material, which is the true size of the material under the microscope, and is distinguished from the secondary particle size after agglomeration of the material measured by the D50 or D90 of the laser particle size analyzer.

[0052] A preparation method of the above lithium iron phosphate cathode material includes the following steps:

[0053] (1) Preparing the lithium iron phosphate core by hydrothermal method

[0054] S1-1: Weigh lithium hydroxide monohydrate, ferrous sulfate heptahydrate and phosphoric acid; among them, the molar ratio of lithium hydroxide monohydrate, ferrous sulfate heptahydrate and phosphoric acid is 3∶1∶1;

[0055] S1-2: Prepare a phosphoric acid solution with a mass concentration of 15-25 wt%, and successively add ascorbic acid and the weighed ferrous sulfate heptahydrate thereto, and stir to dissolve it fully to form a solution denoted as solution A; the mass concentration of ascorbic acid in the phosphoric acid solution is 0.1-0.3 wt%;

[0056] S1-3: Prepare an aqueous lithium hydroxide solution with a mass concentration of 10-12%, denoted as solution B;

[0057] S1-4: Mix solution A and solution B evenly and pour them into a reaction kettle with a stainless steel inner lining for hydrothermal reaction. The temperature of the hydrothermal reaction is 120-180 °C, and the reaction time is 4-8 h;

[0058] S1-5: Wash the hydrothermal reaction product with deionized water, dry it, and then calcine it in an inert atmosphere at 450-650 °C for 2-4 h to obtain a white lithium iron phosphate solid, which is ground to obtain a lithium iron phosphate core with a primary particle size less than 150 nm.

[0059] The present invention uses the hydrothermal method to prepare the lithium iron phosphate core, and the reaction equation is as follows: FeSO4 + H3PO4 + 3LiOH → LiFePO4 + Li2SO4 + 3H2O,

[0060] In step S1-2, ascorbic acid is used to prevent the oxidation of ferrous ions.

[0061] (2) Preparing the lithium iron phosphate cathode material

[0062] S2-1: weighing a lithium source, ferrous oxalate, and a phosphorus source, adding them into a ball mill, and adding acetylene black and the lithium iron phosphate core prepared in step (1) to obtain a mixture; wherein the molar ratio of the lithium source, ferrous oxalate, and phosphorus source is 1:1:1, the amount of acetylene black added is 0.1-0.3% of the total mass of the lithium source, ferrous oxalate, and phosphorus source, and the amount of the lithium iron phosphate core added is 3-5% of the total mass of the lithium source, ferrous oxalate, and phosphorus source;

[0063] S2-2: Add the mixture and anhydrous ethanol into a planetary ball mill at a mass ratio of mixture: anhydrous ethanol = 1:1.2 and ball mill at a speed of 250-400 r / min for 6-12 hours. After the ball milling is completed, the product is filtered and dried to obtain a block product;

[0064] S2-3: Grind the block product in step S2-2 to obtain a powdered precursor, and add 2-4% of the precursor mass of glucose (organic carbon source, used to form a carbon coating layer) and mix evenly to obtain a lithium iron phosphate positive electrode material precursor.

[0065] In the present invention, acetylene black is used as an inorganic carbon source and acts as a conductive agent in the positive electrode material. Glucose is used as an organic carbon source and is used to form a carbon coating layer on the surface of the positive electrode material in the sintering step.

[0066] S2-4: calcining the lithium iron phosphate cathode material precursor at 600-800° C. in an inert atmosphere for 9-15 hours, then cooling to obtain a calcined product, and crushing the cooled calcined product to obtain a lithium iron phosphate cathode material having a core-shell structure; Figure 1 shown.

[0067] from Figure 1 It can be seen from the schematic diagram of the material structure that the core of the lithium iron phosphate positive electrode material is small particles of lithium iron phosphate, the inner shell is lithium iron phosphate grown along the core, and the outermost layer is a carbon coating shell.

[0068] Example 1

[0069] A method for preparing a lithium iron phosphate positive electrode material comprises the following steps.

[0070] (1) Preparation of lithium iron phosphate core by hydrothermal method: comprising the following steps:

[0071] S1-1: Weigh lithium hydroxide monohydrate, ferrous sulfate heptahydrate and phosphoric acid; wherein the molar ratio of lithium hydroxide monohydrate, ferrous sulfate heptahydrate and phosphoric acid is 3:1:1;

[0072] S1-2: prepare a phosphoric acid solution with a mass concentration of 20wt%, add ascorbic acid and weighed ferrous sulfate heptahydrate thereto in sequence, stir to fully dissolve, and the resulting solution is recorded as solution A; the mass concentration of ascorbic acid in the phosphoric acid solution is 0.2wt%;

[0073] S1-3: preparing a lithium hydroxide aqueous solution with a mass concentration of 12%, denoted as solution B;

[0074] S1-4: Mix solution A and solution B evenly and pour them into a stainless steel-lined reactor for hydrothermal reaction. The temperature of the hydrothermal reaction is 150° C. and the reaction time is 8 h.

[0075] S1-5: The hydrothermal reaction product is washed with deionized water, dried, and calcined at 500° C. in an inert atmosphere for 3 hours to obtain a white lithium iron phosphate solid, which is ground to obtain a lithium iron phosphate core, wherein the primary particle size of the lithium iron phosphate in the core is less than 150 nm.

[0076] (2) Preparation of lithium iron phosphate positive electrode material by ferrous oxalate method: comprising the following steps:

[0077] S2-1: lithium carbonate (lithium source), ferrous oxalate dihydrate (iron source), and ammonium dihydrogen phosphate (phosphorus source) are weighed according to the molar ratio [n(Li)∶n(Fe)∶n(P)=1∶1∶1] and added into a ball mill, and acetylene black and the lithium iron phosphate core prepared in step (1) are added to obtain a mixture; wherein the amount of acetylene black added is 0.2% of the total mass of the lithium source, the iron source and the phosphorus source, and the amount of the lithium iron phosphate core added is 3% of the total mass of the lithium source, the iron source and the phosphorus source;

[0078] S2-2: Add the mixture and anhydrous ethanol into a planetary ball mill at a mass ratio of mixture: anhydrous ethanol = 1:1.2 and mill at a speed of 350 r / min for 8 hours. After the ball milling is completed, the product is filtered and dried to obtain a block product;

[0079] S2-3: Grind the block product in step S2-2 to obtain a powdered precursor, and add 3% of the precursor mass of glucose to mix evenly to obtain a lithium iron phosphate positive electrode material precursor.

[0080] S2-4: calcining the lithium iron phosphate cathode material precursor at 680° C. in an inert atmosphere for 12 h, then cooling to obtain a calcined product, and crushing the cooled calcined product to obtain a lithium iron phosphate cathode material with a core-shell structure.

[0081] Example 2

[0082] A method for preparing a lithium iron phosphate positive electrode material comprises the following steps.

[0083] (1) Preparation of lithium iron phosphate core by hydrothermal method: comprising the following steps:

[0084] S1-1: Weigh lithium hydroxide monohydrate, ferrous sulfate heptahydrate and phosphoric acid; wherein the molar ratio of lithium hydroxide monohydrate, ferrous sulfate heptahydrate and phosphoric acid is 3:1:1;

[0085] S1-2: preparing a phosphoric acid solution with a mass concentration of 20 wt%, adding ascorbic acid and weighed ferrous sulfate heptahydrate thereto in sequence, stirring to fully dissolve them, to form a solution A; the mass concentration of ascorbic acid in the phosphoric acid solution is 0.2 wt%;

[0086] S1-3: preparing a lithium hydroxide aqueous solution with a mass concentration of 12%, denoted as solution B;

[0087] S1-4: Mix solution A and solution B evenly and pour them into a stainless steel-lined reactor for hydrothermal reaction. The temperature of the hydrothermal reaction is 150° C. and the reaction time is 8 h.

[0088] S1-5: The hydrothermal reaction product is washed with deionized water, dried, and then calcined at 550° C. in an inert atmosphere for 2.5 hours to obtain a white lithium iron phosphate solid, which is ground to obtain a lithium iron phosphate core, wherein the primary particle size of the lithium iron phosphate in the core is less than 150 nm.

[0089] (2) Preparing a lithium iron phosphate positive electrode material precursor by a ferrous oxalate method: comprising the following steps:

[0090] S2-1: lithium carbonate (lithium source), ferrous oxalate dihydrate (iron source), and ammonium dihydrogen phosphate (phosphorus source) are weighed according to the molar ratio [n(Li)∶n(Fe)∶n(P)=1∶1∶1] and added into a ball mill, and acetylene black and the lithium iron phosphate core prepared in step (1) are added to obtain a mixture; wherein the amount of acetylene black added is 0.2% of the total mass of the lithium source, the iron source and the phosphorus source, and the amount of the lithium iron phosphate core added is 3% of the total mass of the lithium source, the iron source and the phosphorus source;

[0091] S2-2: Add the mixture and anhydrous ethanol into a planetary ball mill at a mass ratio of mixture: anhydrous ethanol = 1:1.2 and mill at a speed of 350 r / min for 8 hours. After the ball milling is completed, the product is filtered and dried to obtain a block product;

[0092] S2-3: Grind the block product in step S2-2 to obtain a powdered precursor, add 3% of the precursor mass of glucose and mix evenly to obtain a lithium iron phosphate positive electrode material precursor.

[0093] S2-4: calcining the lithium iron phosphate cathode material precursor at 750° C. in an inert atmosphere for 10 hours, then cooling to obtain a calcined product, and crushing the cooled calcined product to obtain a lithium iron phosphate cathode material with a core-shell structure.

[0094] Comparative Example 1

[0095] The difference from Example 1 lies in that the addition amount of the lithium iron phosphate core in step S2-1 is 1.5% of the total mass of the lithium source, iron source and phosphorus source. The primary particle size of the lithium iron phosphate in the core of the obtained core-shell structured lithium iron phosphate cathode material is less than 150 nm.

[0096] Comparative Example 2

[0097] The difference from Example 1 lies in that the addition amount of the lithium iron phosphate core in step S2-1 is 0% of the total mass of the lithium source, iron source and phosphorus source; that is, no lithium iron phosphate core is added.

[0098] Comparative Example 3

[0099] The difference from Example 1 lies in that the sintering temperature in step S1-5 is 750 °C and the time is 6 h; increasing the sintering temperature and prolonging the sintering time result in a large size of the lithium iron phosphate core, which does not meet the requirement that the primary particle size of all particles is less than 150 nm.

[0100] Comparative Example 4

[0101] The difference from Example 1 lies in that the sintering temperature in step S2-4 is 590 °C and the time is 8 h.

[0102] Comparative Example 5

[0103] The difference from Example 1 lies in that the sintering temperature in step S2-4 is 810 °C and the time is 16 h.

[0104] Microstructure test

[0105] The microstructure of the lithium iron phosphate cathode material of the present invention was tested by SEM. Figure 2 The SEM image of the lithium iron phosphate cathode material of Example 1 at a magnification of 20K is shown. Figure 3 The SEM image of the lithium iron phosphate cathode material of Comparative Example 3 at a magnification of 20K is shown.

[0106] It can be seen from Figure 2 that the core-shell structured lithium iron phosphate cathode material prepared in Example 1 has high consistency in particle size and morphology, and the particle diameter d of the target lithium iron phosphate cathode material prepared is 0.55 ± 0.21 μm. The high consistency in particle size is beneficial to reducing the internal resistance and improving the long cycle performance of the cathode material.

[0107] It can be seen from Figure 3 that the particle size consistency of the lithium iron phosphate particles prepared in Comparative Example 3 using a larger-sized lithium iron phosphate core becomes poor.

[0108] Assembly of the battery and electrochemical performance test

[0109] The lithium iron phosphate cathode materials obtained in Examples 1-2 and Comparative Examples 1-5 of the present invention were made into cathode sheets. Lithium sheets were used as the anode, a 1 mol / L LiPF6 ethylene carbonate / dimethyl carbonate (volume ratio 1:1) solution was used as the electrolyte, and a polypropylene microporous membrane was used as the battery separator to assemble a CR2032 type lithium ion battery for electrochemical performance testing at a voltage of 2.5-3.7 V and a temperature of 25 °C.

[0110] Table 1 lists the relevant performances of the lithium iron phosphate cathode materials in Examples 1-2 and Comparative Examples 1-5 when applied to batteries. Figure 4 This is the charge-discharge curve of the lithium iron phosphate cathode material of Example 1 of the present invention at 0.1C. Figure 5 This is the 1C / 1C normal temperature cycle retention rate of the lithium iron phosphate cathode material of Example 1 of the present invention.

[0111] Table 1 Relevant performances of the lithium iron phosphate cathode materials in Examples 1-2 and Comparative Examples 1-5 when applied to batteries

[0112]

[0113]

[0114] As can be seen from Table 1, by comparing Examples 1-2 with Comparative Examples 1-2, it can be known that with the addition of the lithium iron phosphate core, the tap density of the material increases, the powder resistivity decreases, and the cycle performance is improved; by comparing Example 1 with Comparative Example 2, it can be known that the addition of the lithium iron phosphate core does not have an adverse effect on the specific capacity of the material, but rather slightly increases it. This is because the presence of the lithium iron phosphate core provides crystal nuclei during the crystal growth process, eliminating the nucleation step and accelerating crystal growth.

[0115] In Comparative Example 3, larger-sized cores were obtained by increasing the sintering temperature and prolonging the sintering time during the preparation of the lithium iron phosphate core; Figure 2 With Figure 3 it can be seen that the addition of the large-sized cores in Comparative Example 3 will reduce the consistency of the particle size distribution, thereby reducing the specific capacity and affecting the cycle performance. This is because as the core size increases, the specific surface area decreases, and the inner shell layer of lithium iron phosphate cannot effectively grow along the core interface during the sintering process.

[0116] By comparing Examples 1-2 with Comparative Examples 4-5, it can be known that insufficient crystal growth is caused by a relatively low sintering temperature and insufficient sintering time during the synthesis of the cathode material; over-sintering will damage the crystal structure and cause abnormal crystal growth when the sintering temperature is relatively high and the sintering time is too long; both will result in the inability to exert the specific capacity of the material and affect the cycle performance of the material.

[0117] It should be understood that the use of these embodiments is only for illustrating the present invention rather than intending to limit the protection scope of the present invention. In addition, it should also be understood that after reading the technical content of the present invention, those skilled in the art can make various changes, modifications and / or variations to the present invention, and all these equivalent forms also fall within the protection scope defined by the appended claims of this application.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate positive electrode material has a core-shell structure, including lithium iron phosphate as a core, a lithium iron phosphate inner shell layer obtained by growing along the surface of the core, and a carbon coating layer outer shell coated outside the inner shell layer; wherein the primary particle size of the lithium iron phosphate core is less than 150nm, the diameter d of the lithium iron phosphate particles of the lithium iron phosphate positive electrode material is 0.55±0.21μm, and the preparation method of the lithium iron phosphate positive electrode material comprises the following steps: (1) preparing a lithium iron phosphate core by a hydrothermal method, and grinding to obtain a lithium iron phosphate core with a primary particle size of less than 150 nm; (2) Preparation of lithium iron phosphate positive electrode material by ferrous oxalate method: comprising the following steps: S2-1: weighing a lithium source, ferrous oxalate, and a phosphorus source, adding them into a ball mill, and adding acetylene black and the lithium iron phosphate core prepared in step (1) to obtain a mixture; wherein the amount of the lithium iron phosphate core added is 3 to 5% of the total mass of the lithium source, ferrous oxalate, and phosphorus source; S2-2: adding the mixture and anhydrous ethanol into a planetary ball mill for ball milling, and filtering and drying to obtain a block product after the ball milling is completed; S2-3: Grinding the block product in step S2-2 to obtain a powder precursor, and adding glucose to mix evenly to obtain a lithium iron phosphate positive electrode material precursor; S2-4: calcining the lithium iron phosphate positive electrode material precursor at 600-800° C. in an inert atmosphere, obtaining a calcined product after cooling, and crushing the calcined product to obtain the lithium iron phosphate positive electrode material.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The primary particle size of the lithium iron phosphate core is 70-150nm.

3. A method for preparing the lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, The steps include: (1) preparing a lithium iron phosphate core by a hydrothermal method, and grinding to obtain a lithium iron phosphate core with a primary particle size of less than 150 nm; (2) Preparation of lithium iron phosphate positive electrode material by ferrous oxalate method: comprising the following steps: S2-1: weighing a lithium source, ferrous oxalate, and a phosphorus source, adding them into a ball mill, and adding acetylene black and the lithium iron phosphate core prepared in step (1) to obtain a mixture; wherein the amount of the lithium iron phosphate core added is 3 to 5% of the total mass of the lithium source, ferrous oxalate, and phosphorus source; S2-2: adding the mixture and anhydrous ethanol into a planetary ball mill for ball milling, and filtering and drying to obtain a block product after the ball milling is completed; S2-3: Grinding the block product in step S2-2 to obtain a powder precursor, and adding glucose to mix evenly to obtain a lithium iron phosphate positive electrode material precursor; S2-4: calcining the lithium iron phosphate positive electrode material precursor at 600-800° C. in an inert atmosphere, obtaining a calcined product after cooling, and crushing the calcined product to obtain the lithium iron phosphate positive electrode material.

4. The preparation method of the lithium iron phosphate cathode material according to claim 3, wherein, In the step (1), the hydrothermal method is used to prepare the lithium iron phosphate core, which comprises the following steps: S1-1: Weigh the lithium source, iron source and phosphorus source respectively; S1-2: prepare a phosphorus source aqueous solution, then add ascorbic acid and a weighed iron source thereto in sequence, stir to fully dissolve them, and the resulting solution is recorded as solution A; S1-3: Prepare a lithium source aqueous solution, referred to as solution B; S1-4: mixing solution A and solution B evenly and then performing a hydrothermal reaction, wherein the temperature of the hydrothermal reaction is 120 to 180° C.; S1-5: Wash the hydrothermal reaction product with deionized water, dry it, and then calcine it in an inert atmosphere at 450 - 650 °C to obtain lithium iron phosphate solid. After grinding, a lithium iron phosphate core is prepared.

5. The preparation method of the lithium iron phosphate cathode material according to claim 4, wherein In the step S1-1, the lithium source is selected from one or more of lithium oxide, lithium carbonate, lithium phosphate, and lithium hydroxide; the iron source is selected from one or more of ferrous sulfate, ferrous oxalate, ferrous chloride, and ferrous glycinate; the phosphorus source is selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium phosphate. And / or, in the step S1-2, the mass concentration of the phosphorus source aqueous solution is 15 - 25 wt%. And / or, in the step S1-2, the mass concentration of ascorbic acid in the phosphorus source aqueous solution is 0.1 - 0.3 wt%. And / or, in the step S1-3, the mass concentration of the lithium source aqueous solution is 10% - 12%. And / or, in the step S1-4, the reaction time of the hydrothermal reaction is 4 - 8 h. And / or, in the step S1-5, the calcination time is 2 - 4 h.

6. The preparation method of the lithium iron phosphate cathode material according to claim 4, wherein, In the step S2-1, the lithium source is selected from one or more of lithium oxide, lithium carbonate, lithium phosphate, and lithium hydroxide. And / or, the phosphorus source is selected from one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium phosphate.

7. The preparation method of the lithium iron phosphate cathode material according to claim 4 or 6, characterized in that, In the step S2-1, the lithium source, ferrous oxalate, and phosphorus source are weighed according to the molar ratio of elements Li, Fe, and P in the lithium source, ferrous oxalate, and phosphorus source being 1:1:

1. And / or, in the step S2-1, the addition amount of acetylene black is 0.1% - 0.3% of the total mass of the lithium source, ferrous oxalate, and phosphorus source.

8. The preparation method of the lithium iron phosphate cathode material according to claim 4, wherein, In the step S2-2, the mass ratio between the mixture and absolute ethanol is 1:1.1 - 1.

3. And / or, in the step S2-2, ball milling is performed at a speed of 250 - 400 r / min for 6 - 12 h. And / or, in the step S2-3, the massive product in the step S2-2 is ground to obtain a powdery precursor, and 2% - 4% of the mass of the powdery precursor of glucose is added and mixed evenly to obtain a lithium iron phosphate cathode material precursor. And / or, in the step S2-4, the calcination time is 9 - 15 h.

9. The preparation method of the lithium iron phosphate cathode material according to claim 4, characterized in that, In the step S1-5 or S2-4, the inert atmosphere is selected from one of nitrogen, helium, argon, and neon.

10. Application of the lithium iron phosphate cathode material according to claim 1 or 2 or prepared by the preparation method according to any one of claims 3 - 9 in a lithium battery.

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