A long cycle life lithium iron phosphate positive electrode material and preparation method

Abstract

The present invention discloses a lithium iron phosphate cathode material with a long cycle life and a preparation method thereof. The method comprises steps S1: mixing a doped metal powder with a silane coupling agent, adding graphene to the mixed solution, stirring, and collecting a precipitate to obtain a graphene-coated doped metal powder; S2: mixing iron phosphate, a lithium source, and a pore-forming agent, drying the mixture, and placing the mixture in a sintering furnace for primary sintering under an inert atmosphere to obtain a lithium iron phosphate precursor; and S3: mixing the lithium iron phosphate precursor with the doped metal powder and performing a secondary sintering to obtain a lithium iron phosphate cathode material. The present invention incorporates the graphene-coated doped metal powder into the preparation process of the lithium iron phosphate. After the secondary sintering, the expansion properties of the graphene are utilized to make the doped metal and lithium more reasonable in microscopic morphology. In addition to shortening the diffusion path of lithium ions, the expanded graphene can also be utilized to inhibit the expansion of the lithium iron phosphate material, shortening the ion migration path, thereby improving the cycle performance of the material.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery positive electrode materials, and in particular to a lithium iron phosphate positive electrode material with a long cycle life and a preparation method thereof. Background Art

[0002] Lithium-ion batteries, with their high energy density, low self-discharge, long lifespan, and high operating voltage, have gradually become a leading choice for high-power power batteries and large-capacity energy storage batteries. Improvements in the performance of lithium-ion batteries are closely linked to the performance of electrode materials.

[0003] Currently, the main cathode materials used in lithium-ion batteries include lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, nickel-cobalt-manganese ternary cathode materials, and lithium iron phosphate. Lithium iron phosphate, due to its advantages such as excellent safety, stable charge and discharge performance, environmental friendliness, and wide availability of raw materials, aligns with the development trend of commercial lithium-ion batteries and has become the preferred cathode material for energy storage power stations. In lithium iron phosphate (LiFePO4), phosphorus and iron atoms share an oxygen atom, which they bond to, weakening the oxygen-iron bond.

[0004] The polyanion groups enhance LiFePO4's stability. Iron ions are released during use, boosting redox capacity and increasing electrode potential. During charge and discharge, LiFePO4 interacts with iron-phosphorus oxide, preventing significant changes in the distance between them. The volume of the two atoms changes during charge and discharge, offsetting the distance between them. Their structure and atomic arrangement are also relatively close, preventing overlap during the arrangement process, resulting in excellent cycling performance. Furthermore, covalent bonds form between the lithium and oxygen atoms in LiFePO4, making it relatively stable. Even at high temperatures, oxygen is not released, contributing to enhanced thermal stability. While LiFePO4 offers many advantages, its octahedral and tetrahedral structures present limitations, particularly during charge and discharge, which cause volume changes that allow lithium ions to intercalate and deintercalate, hindering its ability to diffuse. LiFePO4 cannot form a continuous iron oxide network, hindering its full potential as an electron conductor. Electron conduction occurs only through the interaction of iron and oxygen atoms, resulting in low electron mobility and limited lithium ion diffusion. Furthermore, the electrochemical performance of LiFePO4 will also show a significant attenuation in low temperature environments. Based on this, how to modify LiFePO4 materials to improve their electrochemical performance in low temperature environments has become a technical problem that needs to be solved urgently in the field. To solve this problem, several modification schemes for LiFePO4 positive electrode materials have emerged in the prior art, such as references:

[0005] Reference 1: Chinese patent document with publication number CN110255522A

[0006] Reference 1 records a method for preparing multi-stage porous lithium iron phosphate, which includes the following steps: preparation of primary porous ammonium ferrous phosphate, preparation of secondary porous lithium iron phosphate and preparation of tertiary porous lithium iron phosphate, and finally synthesis of multi-stage porous lithium iron phosphate; the multi-stage porous lithium iron phosphate prepared by the present invention has the technical effect of high battery capacity and excellent cycle performance, which is much better than the lithium iron phosphate prepared by the prior art.

[0007] Reference 2: Chinese patent document with publication number CN114122406A

[0008] Reference 2 records that the present invention discloses a method for preparing graphene-modified lithium iron phosphate and a lithium iron phosphate battery, wherein the lithium iron phosphate prepared by the method for preparing lithium iron phosphate is prepared by graphene and Li7La3Zr20 12 The lithium iron phosphate battery cell is made of a positive electrode sheet, a first separator, a negative electrode sheet and a second separator stacked and wound in a "Z" shape. The positive electrode sheet is made of a positive electrode slurry and a carbon-coated aluminum foil, and the negative electrode sheet is made of a negative electrode slurry and a carbon-coated copper foil. The present invention utilizes graphene and Li7La3Zr20 12 It improves the shortcomings of poor electronic conductivity and poor ionic conductivity of lithium iron phosphate materials, improves the material's charge and discharge capabilities under large currents, reduces battery internal resistance, and improves battery rate and cycle performance, which can meet the market's development needs for high-energy, high-power lithium-ion batteries.

[0009] The modification methods described in the two aforementioned patent documents are both capable of improving the electrochemical performance of lithium iron phosphate materials. However, the modification methods that can improve the electrochemical performance of lithium iron phosphate materials are not limited to the two methods mentioned above. Based on this, the applicant has proposed a method for modifying lithium iron phosphate materials that is different from the existing technology. The modification process is relatively simple, and the resulting lithium iron phosphate material has significantly improved cycle performance in low-temperature environments. Summary of the Invention

[0010] The purpose of the present invention is to enrich the technical route of lithium iron phosphate positive electrode material modification and provide a lithium iron phosphate positive electrode material with a long cycle life and a preparation method different from the prior art.

[0011] In order to solve the above-mentioned technical problems, the present invention adopts a technical solution: a method for preparing a lithium iron phosphate positive electrode material with a long cycle life, comprising the following steps:

[0012] S1. Mixing a doped metal powder with a silane coupling agent, filtering and washing the mixture, and then dispersing the mixture in water to obtain a mixed solution. Adding graphene to the mixed solution, stirring the mixture under magnetic stirring, filtering the precipitate, washing the precipitate, and drying the precipitate to obtain a doped metal powder wrapped with graphene.

[0013] S2, mixing the iron phosphate, the lithium source and the pore-forming agent, placing the mixture in a sintering furnace after drying, and sintering the mixture once under an inert atmosphere to obtain a lithium iron phosphate precursor;

[0014] S3. The lithium iron phosphate precursor is mixed with the doped metal powder prepared in step S1, and after mechanical stirring, the mixture is transferred into a sintering furnace and subjected to secondary sintering under an inert atmosphere to obtain a lithium iron phosphate positive electrode material.

[0015] As a further optimization of the preparation method of a lithium iron phosphate positive electrode material with a long cycle life of the present invention: the doped metal powder is Co powder, Mg powder or Zn powder, and the particle size of the doped metal powder is 50-100 μm.

[0016] As a further optimization of the preparation method of a lithium iron phosphate positive electrode material with a long cycle life of the present invention: the weight ratio of the added doped metal powder and graphene in step S1 is 10:0.01-0.5.

[0017] As a further optimization of the preparation method of a lithium iron phosphate positive electrode material with a long cycle life of the present invention: the silane coupling agent is aminotriethoxysilane, vinyltriethoxysilane or γ-(2,3-epoxypropoxy)propyltrimethoxysilane.

[0018] As a further optimization of the preparation method of a lithium iron phosphate positive electrode material with a long cycle life of the present invention: in the step S2, the weight ratio of the added iron phosphate, lithium source and pore-forming agent is 10:10-20:1-2.

[0019] As a further optimization of the preparation method of a lithium iron phosphate positive electrode material with a long cycle life of the present invention: the lithium source is lithium carbonate or lithium hydroxide, and the pore-forming agent is 2-camphenone or p-dichlorobenzene.

[0020] As a further optimization of the method for preparing a lithium iron phosphate positive electrode material with a long cycle life of the present invention: in step S3, the weight ratio of the lithium iron phosphate precursor to the doped metal powder prepared in step S1 is 10:0.01-0.1.

[0021] As a further optimization of the preparation method of a lithium iron phosphate positive electrode material with a long cycle life of the present invention: the primary sintering process in step S2 is specifically: heating to 300-400°C at a heating rate of 5°C / min and keeping warm for 2-5h, then heating to 600-800°C at a heating rate of 8-10°C / min and keeping warm for 3-6h, and finally cooling to room temperature at a cooling rate of 5°C / min.

[0022] As a further optimization of the preparation method of a lithium iron phosphate positive electrode material with a long cycle life of the present invention: the secondary sintering process in step S3 is specifically: heating to 200-320°C at a heating rate of 1-2°C / min and keeping warm for 1-2 hours, then heating to 500-600°C at a heating rate of 8-10°C / min and keeping warm for 1-2 hours, and finally naturally cooling to room temperature.

[0023] A lithium iron phosphate positive electrode material with a long cycle life is prepared by the above method.

[0024] The present invention has the following beneficial effects: during the preparation process of lithium iron phosphate, doped metal powder wrapped in graphene is incorporated into the present invention. The doping of metal elements can increase the defects of the lithium iron phosphate crystal structure, thereby reducing the polarization effect of the structure and improving the transmission speed of lithium ions during the charging and discharging process. At the same time, the metal powder and graphene are first modified so that the metal can be dispersed and located on the surface of the graphene. After mixing with the lithium iron phosphate precursor, secondary sintering is performed, and the expansion properties of the graphene are utilized to make the doped metal and lithium more compact in microscopic morphology. This not only shortens the diffusion path of lithium ions, but also utilizes the expanded graphene to inhibit the expansion of the lithium iron phosphate material, thereby improving the cycle performance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is an electron microscope photograph of the lithium iron phosphate positive electrode material prepared in Example 1. DETAILED DESCRIPTION

[0026] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with examples, but the content of the present invention is not limited to the following examples.

[0027] A method for preparing a lithium iron phosphate cathode material with a long cycle life:

[0028] The doped metal powder is mixed with a silane coupling agent, filtered and washed, and then dispersed in water to obtain a mixed solution. Graphene is added to the mixed solution, and after magnetic stirring, the precipitate is filtered, washed, and dried to obtain the doped metal powder wrapped with graphene.

[0029] The doped metal powder is Co powder, Mg powder or Zn powder, the particle size of the doped metal powder is 50-100 μm, and the weight ratio of the doped metal powder to the graphene is 10:0.01-0.5.

[0030] Wherein, the silane coupling agent is aminotriethoxysilane, vinyltriethoxysilane or γ-(2,3-epoxypropoxy)propyltrimethoxysilane.

[0031] The iron phosphate, lithium source, and pore-forming agent are mixed, dried, and placed in a sintering furnace for a primary sintering process under an inert atmosphere. The primary sintering process is as follows: heating to 300-400°C at a heating rate of 5°C / min and holding for 2-5 hours, then heating to 600-800°C at a heating rate of 8-10°C / min and holding for 3-6 hours, and finally cooling to room temperature at a cooling rate of 5°C / min. After sintering, a lithium iron phosphate precursor is obtained.

[0032] The weight ratio of the iron phosphate, lithium source and pore-forming agent is 10:10-20:1-2.

[0033] Wherein, the lithium source is lithium carbonate or lithium hydroxide.

[0034] Wherein, the pore-forming agent is 2-camphor or p-dichlorobenzene.

[0035] The lithium iron phosphate precursor is mixed with the doped metal powder prepared in step S1, mechanically stirred, and then transferred to a sintering furnace for secondary sintering under an inert atmosphere. The secondary sintering process is specifically as follows: heating to 200-320°C at a heating rate of 1-2°C / min and holding at this temperature for 1-2 hours, then heating to 500-600°C at a heating rate of 8-10°C / min and holding at this temperature for 1-2 hours, and finally naturally cooling to room temperature. After sintering, the lithium iron phosphate positive electrode material is obtained.

[0036] <Example 1>

[0037] 100 g of Co powder (particle size of 60 μm) was mixed with 50 g of aminotriethoxysilane, filtered and washed, and then dispersed in water to obtain a mixed solution. 1 g of graphene was added to the mixed solution, and after magnetic stirring, the precipitate was filtered, washed, and dried to obtain a doped metal powder wrapped in graphene.

[0038] Take 1000g of iron phosphate, 1500g of lithium carbonate and 150g of p-dichlorobenzene, mix them, dry them and place them in a sintering furnace, and perform a sintering process under an inert atmosphere. The specific sintering process is: heat to 300℃ at a heating rate of 5℃ / min and keep it warm for 5h, then heat to 600℃ at a heating rate of 10℃ / min and keep it warm for 5h, and finally cool to room temperature at a cooling rate of 5℃ / min to obtain a lithium iron phosphate precursor.

[0039] Take 1000g of lithium iron phosphate precursor and mix it with 10g of doped metal powder. After mechanical stirring, transfer it into a sintering furnace and carry out secondary sintering under an inert atmosphere. The secondary sintering process is as follows: heat it to 200℃ at a heating rate of 1℃ / min and keep it warm for 2h, then heat it to 600℃ at a heating rate of 8℃ / min and keep it warm for 1h, and finally cool it naturally to room temperature. After sintering, the lithium iron phosphate positive electrode material is obtained. The electron microscope photo of the material is shown in the figure below. Figure 1 shown.

[0040] <Example 2>

[0041] 120 g of Mg powder (particle size of 100 μm) was mixed with 60 g of aminotriethoxysilane, filtered and washed, and then dispersed in water to obtain a mixed solution. 15 g of graphene was added to the mixed solution. After magnetic stirring, the precipitate was filtered, washed, and dried to obtain a doped metal powder wrapped in graphene.

[0042] 1000g of iron phosphate, 1000g of lithium hydroxide and 200g of 2-camphor were mixed, dried and placed in a sintering furnace for sintering under an inert atmosphere. The sintering process was as follows: heating to 400°C at a heating rate of 5°C / min and keeping warm for 2h, then heating to 800°C at a heating rate of 8°C / min and keeping warm for 3h, and finally cooling to room temperature at a cooling rate of 5°C / min to obtain a lithium iron phosphate precursor.

[0043] 1000g of lithium iron phosphate precursor was mixed with 20g of doped metal powder, mechanically stirred, and then transferred to a sintering furnace for secondary sintering under an inert atmosphere. The secondary sintering process was as follows: heating to 320°C at a heating rate of 2°C / min and holding for 1 hour, then heating to 550°C at a heating rate of 10°C / min and holding for 1.5 hours, and finally cooling naturally to room temperature. After sintering, the lithium iron phosphate cathode material was obtained.

[0044] <Example 3>

[0045] 100 g of Zn powder (particle size of 80 μm) was mixed with 50 g of vinyltriethoxysilane, filtered and washed, and then dispersed in water to obtain a mixed solution. 50 g of graphene was added to the mixed solution. After magnetic stirring, the precipitate was filtered, washed, and dried to obtain a doped metal powder wrapped in graphene.

[0046] Take 1000g of iron phosphate, 2000g of lithium carbonate and 200g of p-dichlorobenzene, mix them, dry them and place them in a sintering furnace, and perform a sintering process under an inert atmosphere. The specific sintering process is: heat to 350℃ at a heating rate of 5℃ / min and keep it warm for 3.5h, then heat to 650℃ at a heating rate of 9℃ / min and keep it warm for 4h, and finally cool to room temperature at a cooling rate of 5℃ / min to obtain a lithium iron phosphate precursor.

[0047] 1000g of lithium iron phosphate precursor was mixed with 50g of doped metal powder, mechanically stirred, and then transferred to a sintering furnace for secondary sintering under an inert atmosphere. The secondary sintering process was as follows: heating to 200°C at a heating rate of 1°C / min and holding for 2 hours, then heating to 600°C at a heating rate of 8°C / min and holding for 1 hour, and finally cooling naturally to room temperature. After sintering, the lithium iron phosphate cathode material was obtained.

[0048] <Example 4>

[0049] 100 g of Co powder (particle size of 60 μm) was mixed with 50 g of aminotriethoxysilane, filtered and washed, and then dispersed in water to obtain a mixed solution. 1 g of graphene was added to the mixed solution, and after magnetic stirring, the precipitate was filtered, washed, and dried to obtain a doped metal powder wrapped in graphene.

[0050] Take 1000g of iron phosphate, 1200g of lithium hydroxide and 120g of p-dichlorobenzene, mix them, dry them and place them in a sintering furnace, and perform a sintering process under an inert atmosphere. The specific sintering process is: heat to 350℃ at a heating rate of 5℃ / min and keep it warm for 3.5h, then heat to 700℃ at a heating rate of 10℃ / min and keep it warm for 4.5h, and finally cool to room temperature at a cooling rate of 5℃ / min to obtain a lithium iron phosphate precursor.

[0051] 1000g of lithium iron phosphate precursor was mixed with 25g of doped metal powder, mechanically stirred, and then transferred to a sintering furnace for secondary sintering under an inert atmosphere. The secondary sintering process was as follows: heating to 200°C at a heating rate of 1.5°C / min and holding for 2 hours, then heating to 600°C at a heating rate of 8°C / min and holding for 1 hour, and finally cooling naturally to room temperature. After sintering, the lithium iron phosphate cathode material was obtained.

[0052] <Example 5>

[0053] 100 g of Co powder (particle size of 50 μm) was mixed with 50 g of aminotriethoxysilane, filtered and washed, and then dispersed in water to obtain a mixed solution. 10 g of graphene was added to the mixed solution, and after magnetic stirring, the precipitate was filtered, washed, and dried to obtain a doped metal powder wrapped in graphene.

[0054] Take 1000g of iron phosphate, 1500g of lithium carbonate and 150g of p-dichlorobenzene, mix them, dry them and place them in a sintering furnace, and sinter them once under an inert atmosphere. The specific sintering process is: heat to 380℃ at a heating rate of 5℃ / min and keep it warm for 5h, then heat to 750℃ at a heating rate of 10℃ / min and keep it warm for 5h, and finally cool to room temperature at a cooling rate of 5℃ / min to obtain a lithium iron phosphate precursor.

[0055] 1000g of lithium iron phosphate precursor was mixed with 20g of doped metal powder, mechanically stirred, and then transferred to a sintering furnace for secondary sintering under an inert atmosphere. The secondary sintering process was as follows: heating to 250°C at a heating rate of 1°C / min and holding for 1.5 hours, then heating to 500°C at a heating rate of 10°C / min and holding for 1 hour, and finally cooling naturally to room temperature. After sintering, the lithium iron phosphate cathode material was obtained.

[0056] Comparative Example 1

[0057] Take 1000g of iron phosphate, 1500g of lithium carbonate and 150g of p-dichlorobenzene, mix them, dry them and place them in a sintering furnace, and perform a sintering process under an inert atmosphere. The specific sintering process is: heat to 300℃ at a heating rate of 5℃ / min and keep it warm for 5h, then heat to 600℃ at a heating rate of 10℃ / min and keep it warm for 5h, and finally cool to room temperature at a cooling rate of 5℃ / min to obtain a lithium iron phosphate precursor.

[0058] 1000g of lithium iron phosphate precursor was subjected to secondary sintering in an inert atmosphere. The secondary sintering process was as follows: heating to 200°C at a heating rate of 1°C / min and holding for 2 hours, then heating to 600°C at a heating rate of 8°C / min and holding for 1 hour, and finally cooling naturally to room temperature. After sintering, the lithium iron phosphate cathode material was obtained.

[0059] Comparative Example 2

[0060] 100 g of Co powder (particle size of 60 μm) was mixed with 50 g of aminotriethoxysilane, filtered and washed, and then dispersed in water to obtain a mixed solution. 1 g of graphene was added to the mixed solution, and after magnetic stirring, the precipitate was filtered, washed, and dried to obtain a doped metal powder wrapped in graphene.

[0061] Take 1000g of iron phosphate, 1500g of lithium carbonate, 150g of p-dichlorobenzene and 10g of doped metal powder, mix them, place them in a sintering furnace after drying, and perform one sintering under an inert atmosphere. The specific sintering process is: heating to 300℃ at a heating rate of 5℃ / min and keeping warm for 5h, then heating to 600℃ at a heating rate of 10℃ / min and keeping warm for 5h, and finally cooling to room temperature at a cooling rate of 5℃ / min to obtain a lithium iron phosphate precursor.

[0062] Comparative Example 3

[0063] 100 g of Co powder (particle size of 60 μm) was mixed with 50 g of aminotriethoxysilane, filtered and washed, and then dispersed in water to obtain a mixed solution. 1 g of graphene was added to the mixed solution, and after magnetic stirring, the precipitate was filtered, washed, and dried to obtain a doped metal powder wrapped in graphene.

[0064] Take 1000g of iron phosphate, 1500g of lithium carbonate and 150g of p-dichlorobenzene, mix them, dry them and place them in a sintering furnace, and perform a sintering process under an inert atmosphere. The specific sintering process is: heat to 300℃ at a heating rate of 5℃ / min and keep it warm for 5h, then heat to 600℃ at a heating rate of 10℃ / min and keep it warm for 5h, and finally cool to room temperature at a cooling rate of 5℃ / min to obtain a lithium iron phosphate precursor.

[0065] 1000g of lithium iron phosphate precursor was mixed with 10g of doped metal powder, mechanically stirred, and then transferred to a sintering furnace for secondary sintering under an inert atmosphere. The secondary sintering process was as follows: heating to 200°C at a heating rate of 5°C / min and holding for 2 hours, then heating to 600°C at a heating rate of 8°C / min and holding for 1 hour, and finally cooling naturally to room temperature. After sintering, the lithium iron phosphate cathode material was obtained.

[0066] Comparative Example 4

[0067] 100 g of Co powder (particle size of 60 μm) was mixed with 50 g of aminotriethoxysilane, filtered and washed, and then dispersed in water to obtain a mixed solution. 1 g of graphene was added to the mixed solution, and after magnetic stirring, the precipitate was filtered, washed, and dried to obtain a doped metal powder wrapped in graphene.

[0068] Take 1000g of iron phosphate, 1500g of lithium carbonate and 150g of p-dichlorobenzene, mix them, dry them and place them in a sintering furnace, and perform a sintering process under an inert atmosphere. The specific sintering process is: heat to 300℃ at a heating rate of 5℃ / min and keep it warm for 5h, then heat to 600℃ at a heating rate of 10℃ / min and keep it warm for 5h, and finally cool to room temperature at a cooling rate of 5℃ / min to obtain a lithium iron phosphate precursor.

[0069] 1000g of lithium iron phosphate precursor was mixed with 10g of doped metal powder, mechanically stirred, and then transferred to a sintering furnace for secondary sintering under an inert atmosphere. The secondary sintering process was as follows: heating to 200°C at a heating rate of 10°C / min and holding for 2 hours, then heating to 600°C at a heating rate of 8°C / min and holding for 1 hour, and finally cooling naturally to room temperature. After sintering, the lithium iron phosphate cathode material was obtained.

[0070] <Electrochemical Performance>

[0071] The lithium iron phosphate positive electrode material prepared in Examples 1-5 of the present invention and Comparative Examples 1-4 was used as the positive electrode active material to prepare a lithium battery. Specifically, the lithium iron phosphate positive electrode material, the carbon black conductive agent SP, and the binder PVDF were mixed uniformly in a weight ratio of 94:4:2 to obtain a positive electrode slurry. The slurry was then coated on aluminum foil, vacuum dried and rolled to form a positive electrode sheet. A lithium metal sheet was used as the negative electrode. The sheet-like positive electrode material, lithium metal sheet, separator, and electrolyte were assembled into a button cell for low-temperature cycle performance testing. The test conditions were: test temperature -25°C, voltage 2.0V-3.75V, constant current and constant voltage charging mode to charge to 3.75V and discharge to 2.0V, with a charge and discharge current of 0.1C. The battery was then cycled 800 times with a charge and discharge current of 1C, and the cut-off voltage condition was the same as 0.1C. The test results are shown in the following table.

[0072] Table 1 Low temperature cycle performance test results

[0073]

[0074] As can be seen from the above table, the lithium iron phosphate positive electrode materials prepared in Examples 1-5 exhibit excellent cycle performance in a low temperature environment. After 1C charge and discharge cycles to 800 times, the discharge capacity retention rate can reach more than 93%. However, after 1C charge and discharge cycles to 800 times, the discharge capacity retention rate of Comparative Example 1 is only 77.86%, which is significantly lower than that of Example 1. The applicant believes that this is due to the fact that no doping material that can improve the low-temperature cycle performance of the material is added to Comparative Example 1, while the discharge capacity retention rate of Comparative Example 2 is 80.45% after 1C charge and discharge cycles to 800 times. The applicant believes that although doping materials are added to Comparative Example 2, secondary sintering is not performed, so that the function of the doping material is not fully utilized. After 1C charge and discharge cycles to 800 times, the discharge capacity retention rate of Comparative Examples 3 and 4 is around 87%, which is slightly higher than that of Comparative Examples 1 and 2, but significantly lower than that of Examples 1-5. The applicant believes that this fully demonstrates that the heating rate of the secondary sintering process has a greater impact on the cycle performance of the final material.

[0075] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material with a long cycle life, characterized in that: include: S1, mixing the doped metal powder with the silane coupling agent, filtering, washing, and then dispersing in water to obtain a mixed solution, adding graphene to the mixed solution, magnetically stirring, and filtering, washing, and drying the precipitate to obtain the doped metal powder wrapped with graphene; S2, mixing the iron phosphate, the lithium source and the pore-forming agent, drying and sintering the mixture once in an inert atmosphere in a sintering furnace to obtain a lithium iron phosphate precursor; S3, mixing the lithium iron phosphate precursor and the graphene-wrapped doped metal powder, mechanically stirring, transferring the mixture into a sintering furnace, and performing secondary sintering under an inert atmosphere to obtain a lithium iron phosphate positive electrode material; The primary sintering in step S2 is as follows: heating to 300-400°C at a heating rate of 5°C / min and holding for 2-5 hours, then heating to 600-800°C at a heating rate of 8-10°C / min and holding for 3-6 hours, and cooling to room temperature at a cooling rate of 5°C / min; The secondary sintering in step S3 is as follows: heating to 200-320°C at a heating rate of 1-2°C / min and keeping warm for 1-2 hours, then heating to 500-600°C at a heating rate of 8-10°C / min and keeping warm for 1-2 hours, and naturally cooling to room temperature; the doped metal powder is Co powder, Mg powder or Zn powder, and the particle size of the doped metal powder is 50-100 μm.

2. The method for preparing a lithium iron phosphate cathode material with a long cycle life according to claim 1, wherein: In step S1, the weight ratio of the added doped metal powder to the graphene is 10:0.01-0.

5.

3. The method for preparing a lithium iron phosphate cathode material with a long cycle life according to claim 1, wherein: The silane coupling agent is aminotriethoxysilane, vinyltriethoxysilane or γ-(2,3-epoxypropoxy)propyltrimethoxysilane.

4. The method for preparing a lithium iron phosphate cathode material with a long cycle life according to claim 1, wherein: In the step S2, the weight ratio of the iron phosphate, the lithium source and the pore-forming agent is 10:10-20:1-2.

5. The method for preparing a lithium iron phosphate cathode material with a long cycle life according to claim 1, wherein: The lithium source is lithium carbonate or lithium hydroxide, and the pore-forming agent is 2-camphenone or p-dichlorobenzene.

6. The method for preparing a lithium iron phosphate cathode material with a long cycle life according to claim 1, wherein: In the step S3, the weight ratio of the lithium iron phosphate precursor to the graphene-wrapped doping metal powder prepared in the step S1 is 10:0.01-0.

1.

7. A lithium iron phosphate cathode material with a long cycle life, characterized by: The invention is prepared by the method according to any one of claims 1 to 6.

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