Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a lithium iron phosphate positive electrode material and a preparation method thereof. The preparation method of the lithium iron phosphate positive electrode material comprises the following steps: (1) growing MIL-88A on the surface of graphene in situ; (2) carrying out carbonization treatment on the MIL-88A; and (3) embedding lithium iron phosphate into the GO-Fe-C three-dimensional conductive skeleton. The graphene has an extremely high specific surface area, so that a large amount of MOF can grow on the surface of the graphene in situ, the binding force between the graphene and the MOF is greatly improved, the carbonized MOF and the graphene form a three-dimensional conductive skeleton, the conductive efficiency is greatly improved, the carbonized MOF still retains the porous structure, lithium ion diffusion is accelerated, and the performance of the lithium ion battery is improved. And the graphene skeleton greatly improves electron conduction, and the electrochemical performance of the lithium iron phosphate electrode is improved through the mutual cooperation and synergistic effect of the two.

Description

Technical Field

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

[0002] In the field of lithium-ion battery cathode materials, lithium iron phosphate has attracted much attention due to its high safety, long cycle life and environmental friendliness, but its low electronic conductivity and ion diffusion rate limit its high rate performance and application range. To improve this problem, the existing technology mainly improves conductivity through amorphous carbon coating, graphene doping and other means.

[0003] Chinese patent CN 119480954 A discloses a carbon-coated lithium iron phosphate positive electrode material and its preparation method and application. The preparation method comprises: carbon-coating the lithium iron phosphate positive electrode material and the first carbon source to obtain the carbon-coated lithium iron phosphate positive electrode material; the carbon coating process conditions include: setting a heating section, a constant temperature section and a cooling section, wherein the temperature of the heating section is increased from 50°C to 750°C; the temperature of the constant temperature section is 730°C to 850°C. Compared with the existing roller kiln for carbon coating, the preparation method has lower energy consumption, higher production efficiency and shorter production time.

[0004] Chinese patent CN 119340354 A discloses a method for preparing a graphene composite lithium iron phosphate positive electrode material, comprising the following steps: S1, adding graphene oxide to deionized water, stirring to obtain solution I; S2, dissolving the iron salt in deionized water at a mass ratio of iron salt to graphene oxide of 1:0.05, adding solution I, and stirring to obtain solution II; S3, adding phosphoric acid, ethylene glycol, and lithium salt to solution II in sequence at a molar ratio of iron salt to phosphoric acid to lithium salt of 1:1:3, and stirring to obtain solution III; S4, placing solution III in a reactor, sealing it with a cover, heating it in a gradient water bath, and then soaking the product obtained in the water bath in deionized water, and drying it in a vacuum freeze dryer; S5, vacuum drying; optimizing the crystallinity and morphology of the product by precisely controlling the water bath temperature and time; forming a uniform and ordered crystal structure in stages by water bathing to improve the electrochemical performance of the product.

[0005] However, the above methods still have defects such as uneven coating, material agglomeration, complex process or high cost, and a more efficient and controllable modification strategy is urgently needed. Summary of the invention

[0006] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a lithium iron phosphate positive electrode material to overcome the defects of insufficient electrical conductivity and ion diffusion rate of lithium iron phosphate, improve the electrochemical performance of the positive electrode, and utilize the synergistic effect of graphene and MOF-derived porous carbon to improve its performance; the present invention also provides a preparation method thereof.

[0007] The purpose of the present invention is achieved through the following technical solutions: In one aspect, the present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps: (1) In situ growth of MIL-88A on graphene surface: S1: treating the surface of graphene: dispersing the graphene in a mixture of concentrated sulfuric acid and concentrated nitric acid, ultrasonically treating the graphene, washing the mixture by centrifugation until it becomes neutral, and vacuum drying the mixture in an oven to obtain the graphene after surface treatment; S2: dispersing the surface treated graphene in DMF, adding ferric chloride hexahydrate and fumaric acid, putting into a reactor, stirring evenly to react, and after the reaction is completed, centrifuging and washing, and drying in an oven to obtain in-situ grown MIL-88A graphene; (2) Carbonization treatment of MIL-88A: calcining the graphene grown in situ on MIL-88A in a nitrogen atmosphere and naturally cooling it to room temperature to form a GO-Fe@C three-dimensional conductive skeleton; (3) Embedding lithium iron phosphate into the GO-Fe@C three-dimensional conductive skeleton: The GO-Fe@C three-dimensional conductive skeleton is mixed with ferrous oxalate, Li2CO3, and NH4H2PO4 by ball milling, and calcined in a nitrogen atmosphere containing hydrogen to obtain a lithium iron phosphate positive electrode material.

[0008] Since graphene has an extremely high specific surface area, a large number of MOFs can be grown in situ on the graphene surface, which greatly improves the bonding force between graphene and MOF. After carbonization, MOF and graphene form a three-dimensional conductive skeleton, which greatly improves the conductivity efficiency. At the same time, the carbonized MOF still retains its porous structure to accelerate the diffusion of lithium ions, while the graphene skeleton greatly improves electron conduction. The two work together synergistically to improve the electrochemical performance of lithium iron phosphate electrodes.

[0009] Preferably, in step S1, the volume ratio of concentrated sulfuric acid to concentrated nitric acid is (2-4):1; and the drying temperature is 40-50°C.

[0010] Preferably, in step S2, the mass ratio of the surface-treated graphene to ferric chloride hexahydrate is 1:(1-3), and the molar ratio of ferric chloride hexahydrate to fumaric acid is 1:(3-5).

[0011] Preferably, in step S2, the reaction temperature is 100-120°C, the reaction time is 3-5h, and the drying temperature is 40-50°C.

[0012] Preferably, in step (2), the calcination temperature is 600-700° C. and the calcination time is 1-2 h.

[0013] Preferably, in step (3), ferrous oxalate, Li2CO3, and NH4H2PO4 are added in a molar ratio of Fe:Li:P=1:(1-1.1):(0.9-1.1), wherein the mass ratio of GO-Fe@C three-dimensional conductive skeleton to ferrous oxalate is 1:(10-12).

[0014] Preferably, in step (3), the ball-to-material ratio of ball milling is 1:(3-5), the ball milling time is 2-4 h, and the ball milling speed is 300-400 r / min.

[0015] Preferably, in step (3), in the nitrogen atmosphere containing hydrogen, the volume proportion of hydrogen is 3-5%.

[0016] Preferably, in step (3), the calcination temperature is 600-700° C. and the calcination time is 7-9 h.

[0017] On the other hand, the present invention provides a lithium iron phosphate positive electrode material, which is prepared by the above-mentioned method for preparing the lithium iron phosphate positive electrode material.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. The in-situ growth of MOF technology improves the binding force between MOF and graphene, improves the dispersion of graphene, and effectively prevents its agglomeration; 2. The three-dimensional network conductive skeleton is formed by graphene and MOF-derived carbon, which greatly improves the electron conduction rate; 3. After the iron-based MOF is carbonized, its iron element and ferrous oxalate are used as iron sources to synthesize lithium iron phosphate in a reducing gas. At the same time, the iron source provided by MOF can fix lithium iron phosphate, strengthen its close integration with the three-dimensional conductive network, and enhance its electrochemical performance; 4.MOF-derived porous carbon still has a very high specific surface area and porous structure, which can greatly improve the lithium ion diffusion efficiency, and synergize with the high electrical conductivity of graphene to improve the working efficiency of the electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a SEM image of lithium iron phosphate grown in situ in graphene and MOF-derived carbon in Example 1; Figure 2 is a SEM image of lithium iron phosphate grown in situ in graphene and MOF-derived carbon in Example 2; Figure 3 is a SEM image of lithium iron phosphate grown in situ in graphene and MOF-derived carbon in Example 3; Figure 4 The electrochemical cyclic voltammetry curves of Examples 1-3 and Comparative Examples 1-3 are shown; Figure 5The graph is an electrochemical charge-discharge curve (GCD) of the electrodes of Examples 1-3 and Comparative Examples 1-3 at a current of 0.5A. DETAILED DESCRIPTION

[0020] In order to enable persons skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0021] The mass concentration of concentrated sulfuric acid used in the embodiment is 98%, and the mass concentration of concentrated nitric acid is 70%.

[0022] The graphene used in the embodiment is high-grade pure graphene with a purity of 98% and a specific surface area of ​​100-200m 2 / g.

[0023] Example 1 The method for preparing the lithium iron phosphate positive electrode material of this embodiment comprises the following steps: (1) In situ growth of MIL-88A on graphene surface: S1 treats the graphene surface; Graphene was dispersed in a mixture of concentrated sulfuric acid and concentrated nitric acid solution with a volume ratio of 2:1 (the mass ratio of graphene to the mixture was 1:10), ultrasonically treated at 40kHz for 10min, stirred for 2h, centrifuged and washed until neutral, and then vacuum dried in an oven at 40°C; S2: 0.1 parts by mass of the surface-treated graphene is dispersed in 10 parts by mass of DMF, and then 0.1 parts by mass of ferric chloride hexahydrate and 0.13 parts by mass of fumaric acid (molar ratio of 1:3) are added, put into a reactor, stirred evenly, reacted at 100°C for 5 hours, then centrifuged and washed, dried in an oven at 40°C, and stored for later use; (2) Carbonization treatment of MIL-88A: calcining the in-situ grown MIL-88A graphene at 600 °C for 2 h in a nitrogen atmosphere, and then naturally cooling to room temperature to form a GO-Fe@C three-dimensional conductive skeleton; (3) Embedding lithium iron phosphate in the GO-Fe@C three-dimensional conductive skeleton: 0.1 parts by mass of GO-Fe@C, 1 part by mass of ferrous oxalate, 0.56 parts by mass of lithium carbonate, and 0.72 parts by mass of ammonium dihydrogen phosphate (molar ratio of Fe:Li:P=1:1.1:0.9) were ball-milled at 300 r / min for 4 h, with a ball-to-material ratio of 1:3. The mixture was calcined at 600 °C for 9 h in a nitrogen atmosphere containing 3% (volume percentage) hydrogen. After natural cooling, a high-performance lithium iron phosphate positive electrode material was obtained.

[0024] Example 2 The method for preparing the lithium iron phosphate positive electrode material of this embodiment comprises the following steps: (1) In situ growth of MIL-88A on graphene surface: S1 treats the graphene surface; Graphene was dispersed in a mixture of concentrated sulfuric acid and concentrated nitric acid solution with a volume ratio of 3:1 (the mass ratio of graphene to the mixture was 1:10), ultrasonically treated at 40kHz for 10min, stirred for 2h, centrifuged and washed until neutral, and then vacuum dried in an oven at 40°C; S2: 0.1 parts by mass of the surface-treated graphene is dispersed in 10 parts by mass of DMF, and then 0.2 parts by mass of ferric chloride hexahydrate and 0.34 parts by mass of fumaric acid (molar ratio of 1:4) are added, put into a reactor, stirred evenly, reacted at 110°C for 4 hours, then centrifuged and washed, dried in an oven at 45°C, and stored for later use; (2) Carbonization treatment of MIL-88A: In a nitrogen atmosphere, the in-situ grown MIL-88A graphene was calcined at 650 °C for 1.5 h and then naturally cooled to room temperature to form a GO-Fe@C three-dimensional conductive skeleton; (3) Embedding lithium iron phosphate in the GO-Fe@C three-dimensional conductive skeleton: 0.1 parts by mass of GO-Fe@C, 1.1 parts by mass of ferrous oxalate, 0.59 parts by mass of lithium carbonate, and 0.88 parts by mass of ammonium dihydrogen phosphate (molar ratio of Fe:Li:P=1:1.05:1) were ball-milled at 350 r / min for 3 h, with a ball-to-material ratio of 1:4. The mixture was calcined at 650°C for 8 h in a nitrogen atmosphere containing 4% (volume percentage) hydrogen. After natural cooling, a high-performance lithium iron phosphate positive electrode material was obtained.

[0025] Example 3 The method for preparing the lithium iron phosphate positive electrode material of this embodiment comprises the following steps: (1) In situ growth of MIL-88A on graphene surface: S1 treats the graphene surface; Graphene was dispersed in a mixture of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 4:1 (the mass ratio of graphene to the mixture was 1:10), ultrasonically treated at 40kHz for 10 minutes, stirred for 2 hours, centrifuged and washed until neutral, and then vacuum dried in an oven at 50°C; S2: 0.1 parts by mass of the surface-treated graphene is dispersed in 10 parts by mass of DMF, and then 0.3 parts by mass of ferric chloride hexahydrate and 0.64 parts by mass of fumaric acid (molar ratio of 1:5) are added, put into a reactor, stirred evenly, reacted at 120°C for 3h, then centrifuged and washed, dried in an oven at 50°C, and stored for later use; (2) Carbonization treatment of MIL-88A: In a nitrogen atmosphere, the in-situ grown MIL-88A graphene was calcined at 700 °C for 1 h, and then naturally cooled to room temperature to form a GO-Fe@C three-dimensional conductive skeleton; (3) Embedding lithium iron phosphate in the GO-Fe@C three-dimensional conductive skeleton: 0.1 parts by mass of GO-Fe@C, 1.2 parts by mass of ferrous oxalate, 0.62 parts by mass of lithium carbonate, and 1.05 parts by mass of ammonium dihydrogen phosphate (molar ratio of Fe:Li:P=1:1:1.1) were ball-milled at 400 r / min for 2 h, with a ball-to-material ratio of 1:5. The mixture was calcined at 700 °C for 7 h in a nitrogen atmosphere containing 5% (volume percentage) hydrogen. After natural cooling, a high-performance lithium iron phosphate positive electrode material was obtained.

[0026] Example 1-3 SEM images of lithium iron phosphate grown in situ in graphene and MOF-derived carbon are shown in Figure 1-3 As shown, it can be seen that the lithium iron phosphate embedded and grown in the graphene and MIL-88A-derived carbon conductive network has a large number of pores, which come from the micropores retained after the carbonization of MIL-88A. This also shows that the surface of lithium iron phosphate is coated with graphene and MIL-88A-derived carbon conductive network. This porous structure combined with the three-dimensional conductive skeleton significantly improves the specific surface area and lithium ion diffusion rate of lithium iron phosphate, thereby improving its electrochemical performance.

[0027] Comparative Example 1 Only lithium iron phosphate is embedded in graphene, and related operations of MOF are cancelled.

[0028] The preparation method of the lithium iron phosphate positive electrode material is specifically as follows: 0.1 parts by mass of graphene, 1 part by mass of ferrous oxalate, 0.56 parts by mass of lithium carbonate, and 0.72 parts by mass of ammonium dihydrogen phosphate (molar ratio of Fe:Li:P=1:1.1:0.9) are ball-milled and mixed at 300 r / min for 4 hours, with a ball-to-material ratio of 1:3, and then calcined at 600°C for 9 hours in a nitrogen atmosphere containing 3% (volume percentage) hydrogen to obtain the lithium iron phosphate positive electrode material.

[0029] Comparative Example 2 The operation of in-situ growth of MOF on the graphene surface is cancelled, and it is replaced by direct synthesis of MOF and carbonization followed by embedding of lithium iron phosphate.

[0030] The method for preparing the lithium iron phosphate positive electrode material comprises the following steps: (1) Add 0.1 parts by mass of ferric chloride hexahydrate and 0.13 parts by mass of fumaric acid (molar ratio of 1:3) to 10 parts by mass of DMF, put into a reaction kettle, stir evenly, react at 100°C for 5 hours, then centrifuge and wash, and dry in an oven at 40°C to obtain MIL-88A; (2) Carbonization treatment of MIL-88A: calcining MIL-88A at 600°C for 2 h in a nitrogen atmosphere, and then naturally cooling to room temperature to form MIL-88A-derived carbon; (3) Graphene and MIL-88A composite modified lithium iron phosphate: 0.073 mass parts of graphene, 0.027 mass parts of MIL-88A derived carbon, 1 mass part of ferrous oxalate, 0.56 mass parts of lithium carbonate, and 0.72 mass parts of ammonium dihydrogen phosphate (molar ratio of Fe:Li:P=1:1.1:0.9) were ball-milled at 300 r / min for 4 h, with a ball-to-material ratio of 1:3, and then calcined at 600 °C for 9 h in a nitrogen atmosphere containing 3% (volume percentage) hydrogen, and naturally cooled to obtain lithium iron phosphate positive electrode material.

[0031] Comparative Example 3 The MOF-related operations were cancelled to generate lithium iron phosphate, and glucose with a content of 1% (as a percentage of the total mass of the prepared lithium iron phosphate material) was added and calcined together.

[0032] The preparation method of the lithium iron phosphate positive electrode material is specifically as follows: 0.073 parts by mass of graphene, 0.03 parts by mass of glucose, 1 part by mass of ferrous oxalate, 0.56 parts by mass of lithium carbonate, and 0.72 parts by mass of ammonium dihydrogen phosphate (molar ratio of Fe:Li:P=1:1.1:0.9) are ball-milled and mixed at 300 r / min for 4 hours, with a ball-to-material ratio of 1:3, and then calcined at 600°C for 9 hours in a nitrogen atmosphere containing 3% (volume percentage) of hydrogen to obtain the lithium iron phosphate positive electrode material.

[0033] The electrochemical performance of the lithium iron phosphate electrode materials prepared in Examples 1-3 and Comparative Examples 1-3 was tested.

[0034] Preparation of lithium iron phosphate electrode sheet: S1: dissolving PVDF in NMP to prepare a 5 wt.% PVDF solution; S2: mixing the prepared lithium iron phosphate cathode material and the PVDF solution at a mass ratio of 9:1 and stirring evenly; S3 uses a coater to coat the slurry on aluminum foil with a thickness of 60 μm and places it in an oven at 70°C for drying; S4 cuts the dried aluminum foil into 12 mm round pieces.

[0035] Battery Assembly: A button cell was assembled using a CR2032 battery case, a lithium sheet as the negative electrode material, polypropylene as the separator, and 1 mol / L LiPF6 as the electrolyte. The electrochemical performance of the dual-electrode system was tested at room temperature. The test results are shown in Table 1 and Figure 4-5 shown.

[0036] Table 1 Electrochemical test results

[0037] From Table 1 and Figure 5It can be seen that the specific capacity of the lithium iron phosphate positive electrode material prepared by the method of the present invention can reach up to 160 mAh / g or more, which is much higher than that of comparative examples 1-3. Figure 4 It can be seen that the polarization potential difference of Examples 1-3 is much smaller than that of Comparative Examples 1-3, and its peak shape is sharper and the current density is higher, indicating that the lithium ion embedding, extraction and redox reaction of iron ions in Examples 1-3 are easier to carry out during the charge and discharge process, and the electrochemical performance of the electrode is better. This is because the present invention improves the conductivity and ion diffusion properties of lithium iron phosphate by constructing a three-dimensional conductive network, in-situ growth of MOF on the graphene surface, and the synergistic effect of the porous structure of MOF porous carbon derivatives and graphene. A three-dimensional conductive network is constructed by combining graphene and MIL-88A derived carbon, which significantly improves the conductivity of lithium iron phosphate. The in-situ growth of MIL-88A on the graphene surface improves the bonding strength between graphene and MIL-88A, prevents the structure from being destroyed in subsequent processes and during charge and discharge, and improves the stability of the three-dimensional conductive network. Graphene provides high conductivity and accelerates the passage of electrons. The porous structure of the carbonized derivative of MIL-88A provides lithium ion diffusion channels and accelerates the diffusion of lithium ions. The two work synergistically to improve the conductivity and ion diffusion rate at the same time, making up for the disadvantages of lithium iron phosphate and significantly enhancing its electrochemical performance.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: (1) In situ growth of MIL-88A on graphene surface: S1: treating the surface of graphene: dispersing the graphene in a mixture of concentrated sulfuric acid and concentrated nitric acid, ultrasonically treating the graphene, washing the mixture by centrifugation until it becomes neutral, and vacuum drying the mixture in an oven to obtain the graphene after surface treatment; S2: dispersing the surface treated graphene in DMF, adding ferric chloride hexahydrate and fumaric acid, putting into a reactor, stirring evenly to react, and after the reaction is completed, centrifuging and washing, and drying in an oven to obtain in-situ grown MIL-88A graphene; (2) Carbonization treatment of MIL-88A: calcining the graphene grown in situ on MIL-88A in a nitrogen atmosphere and naturally cooling it to room temperature to form a GO-Fe@C three-dimensional conductive skeleton; (3) Embedding lithium iron phosphate into the GO-Fe@C three-dimensional conductive skeleton: The GO-Fe@C three-dimensional conductive skeleton is mixed with ferrous oxalate, Li2CO3, and NH4H2PO4 by ball milling, and calcined in a nitrogen atmosphere containing hydrogen to obtain a lithium iron phosphate positive electrode material.

2. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S1, the volume ratio of concentrated sulfuric acid to concentrated nitric acid is (2-4):1; and the drying temperature is 40-50°C.

3. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S2, the mass ratio of the surface-treated graphene to ferric chloride hexahydrate is 1:(1-3), and the molar ratio of ferric chloride hexahydrate to fumaric acid is 1:(3-5).

4. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S2, the reaction temperature is 100-120°C, the reaction time is 3-5h, and the drying temperature is 40-50°C.

5. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (2), the calcination temperature is 600-700°C and the calcination time is 1-2h.

6. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (3), ferrous oxalate, Li2CO3, and NH4H2PO4 are added in a molar ratio of Fe:Li:P=1:(1-1.1):(0.9-1.1), wherein the mass ratio of GO-Fe@C three-dimensional conductive skeleton to ferrous oxalate is 1:(10-12).

7. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (3), the ball-to-material ratio of ball milling is 1:(3-5), the ball milling time is 2-4 hours, and the ball milling speed is 300-400 r / min.

8. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (3), in the nitrogen atmosphere containing hydrogen, the volume proportion of hydrogen is 3-5%.

9. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step (3), the calcination temperature is 600-700°C and the calcination time is 7-9h.

10. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is prepared by the preparation method of the lithium iron phosphate positive electrode material according to any one of claims 1 to 9.

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