Lithium iron phosphate cathode material and its preparation method

Abstract

The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a lithium iron phosphate cathode material and a preparation method thereof. The preparation method of the lithium iron phosphate cathode material comprises the following steps: (1) in-situ growth of MIL-88A on the surface of graphene; (2) carbonization treatment of MIL-88A; (3) embedding lithium iron phosphate into the GO-Fe@C three-dimensional conductive framework. Since graphene has an extremely high specific surface area, a large amount of MOF can be in-situ grown on the surface of graphene, which greatly improves the binding force between graphene and MOF. After carbonization, MOF and graphene form a three-dimensional conductive framework, greatly improving the conduction efficiency. At the same time, the carbonized MOF still retains its porous structure to accelerate the diffusion of lithium ions, while the graphene framework greatly improves the electron conduction. The two cooperate with each other and synergistically improve the electrochemical performance of the lithium iron phosphate electrode.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a lithium iron phosphate cathode 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. However, its low electronic conductivity and ion diffusion rate limit its high-rate performance and application scope. To improve this problem, in the prior art, the conductivity is mainly improved by means of amorphous carbon coating, graphene doping and the like.

[0003] Chinese Patent CN 119480954 A discloses a carbon-coated lithium iron phosphate cathode material, a preparation method and an application thereof. The preparation method includes: performing carbon coating on the lithium iron phosphate cathode material and a first carbon source to obtain the carbon-coated lithium iron phosphate cathode material; the carbon coating process conditions include: setting a heating section, an isothermal section and a cooling section, wherein the temperature in the heating section increases from 50°C to 750°C; the temperature in the isothermal section is 730°C - 850°C. This preparation method has lower energy consumption, higher production efficiency and shorter production time compared with the existing roller hearth kiln for carbon coating.

[0004] Chinese Patent CN 119340354 A discloses a preparation method of a graphene composite lithium iron phosphate cathode material, including the following steps: S1, adding graphene oxide into deionized water and stirring to obtain Solution I; S2, dissolving the iron salt in deionized water according to the mass ratio of iron salt:graphene oxide of 1:0.05, adding Solution I and stirring to obtain Solution II; S3, sequentially adding phosphoric acid, ethylene glycol and lithium salt into Solution II according to the molar ratio of iron salt:phosphoric acid:lithium salt of 1:1:3 and stirring to obtain Solution III; S4, placing Solution III in a reactor, covering and sealing it, heating it in a gradient water bath, then soaking the product obtained from the water bath with deionized water and drying it in a vacuum freeze dryer; S5, vacuum drying; by precisely controlling the water bath temperature and time, optimizing the crystallinity and morphology of the product; forming a uniform and ordered crystal structure by staged water bath 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 there is an urgent need for a more efficient and controllable modification strategy. Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a lithium iron phosphate cathode material, to overcome the defects of insufficient conductivity and ion diffusion rate of lithium iron phosphate, improve the electrochemical performance of the cathode, 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 object of the present invention is achieved by the following technical solutions:

[0008] On the one hand, the present invention provides a preparation method of a lithium iron phosphate cathode material, comprising the following steps:

[0009] (1) In-situ growth of MIL-88A on the surface of graphene:

[0010] S1 Surface treatment of graphene: Disperse graphene in a mixed solution of concentrated sulfuric acid and concentrated nitric acid, perform ultrasonic treatment, centrifuge and wash until neutral, and then vacuum dry in an oven to obtain surface-treated graphene;

[0011] S2 Disperse the surface-treated graphene in DMF, add ferric chloride hexahydrate and fumaric acid, put them into a reaction kettle, stir evenly for reaction, after the reaction is completed, centrifuge and wash, and dry in an oven to obtain graphene with in-situ grown MIL-88A;

[0012] (2) Carbonization treatment of MIL-88A: Under a nitrogen atmosphere, calcine the graphene with in-situ grown MIL-88A, and naturally cool to room temperature to form a GO-Fe@C three-dimensional conductive framework;

[0013] (3) Embedding lithium iron phosphate in the GO-Fe@C three-dimensional conductive framework: Ball-mill and mix the GO-Fe@C three-dimensional conductive framework with ferrous oxalate, Li2CO3, and NH4H2PO4, and calcine under a nitrogen atmosphere containing hydrogen to obtain the lithium iron phosphate cathode material.

[0014] Since graphene has an extremely high specific surface area, a large amount of MOF can be in-situ grown on the surface of graphene, which greatly improves the binding force between graphene and MOF. After carbonization, MOF and graphene form a three-dimensional conductive framework, greatly improving the conduction efficiency. At the same time, the carbonized MOF still retains its porous structure to accelerate the diffusion of lithium ions, while the graphene framework greatly improves the electron conduction. The two cooperate with each other and synergistically improve the electrochemical performance of the lithium iron phosphate electrode.

[0015] Preferably, in step S1, the volume ratio of concentrated sulfuric acid to concentrated nitric acid is (2-4):1; the drying temperature is 40-50 °C.

[0016] 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).

[0017] Preferably, in step S2, the reaction temperature is 100-120 °C, the time is 3-5 h, and the drying temperature is 40-50 °C.

[0018] Preferably, in step (2), the calcination temperature is 600 - 700 °C, and the calcination time is 1 - 2 h.

[0019] 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), where the mass ratio of the GO - Fe@C three - dimensional conductive framework to ferrous oxalate is 1:(10 - 12).

[0020] Preferably, in step (3), the ball - to - material ratio for ball milling is 1:(3 - 5), the ball milling time is 2 - 4 h, and the ball milling rate is 300 - 400 r / min.

[0021] Preferably, in step (3), in the nitrogen atmosphere containing hydrogen, the volume fraction of hydrogen is 3 - 5%.

[0022] Preferably, in step (3), the calcination temperature is 600 - 700 °C, and the calcination time is 7 - 9 h.

[0023] On the other hand, the present invention provides a lithium iron phosphate cathode material, which is prepared by the above - mentioned preparation method of the lithium iron phosphate cathode material.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] 1. By the technology of in - situ growth of MOF, the binding force between MOF and graphene is improved, the dispersion performance of graphene is improved, and its agglomeration is effectively prevented;

[0026] 2. A three - dimensional network conductive framework is formed by graphene and MOF - derived carbon, greatly improving the electron conduction rate;

[0027] 3. After the iron - based MOF is carbonized, its iron element and ferrous oxalate are used as iron sources together to synthesize lithium iron phosphate in a reducing gas. At the same time, providing iron source through MOF can play a role in fixing lithium iron phosphate, strengthening its tight combination with the three - dimensional conductive network and enhancing its electrochemical performance;

[0028] 4. The 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 cooperate with the high conductivity of graphene to improve the working efficiency of the electrode. Description of the Drawings

[0029] Figure 1 It is the SEM image of lithium iron phosphate in - situ grown in graphene and MOF - derived carbon in Example 1;

[0030] Figure 2 It is the SEM image of lithium iron phosphate in - situ grown in graphene and MOF - derived carbon in Example 2;

[0031] Figure 3 SEM image of lithium iron phosphate grown in-situ in graphene and MOF-derived carbon in Example 3;

[0032] Figure 4 Cyclic voltammetry curves of Examples 1-3 and Comparative Examples 1-3;

[0033] Figure 5 Galvanostatic charge-discharge (GCD) curves of the electrodes of Examples 1-3 and Comparative Examples 1-3 at a current of 0.5 A. Detailed implementation manners

[0034] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0035] In the examples, the mass concentration of concentrated sulfuric acid used is 98%, and the mass concentration of concentrated nitric acid is 70%.

[0036] The graphene used in the examples is of high purity, 98% pure graphene, with a specific surface area of 100-200 m 2 / g.

[0037] Example 1

[0038] The preparation method of the lithium iron phosphate cathode material in this example includes the following steps:

[0039] (1) Grow MIL-88A in-situ on the surface of graphene:

[0040] S1 Treat the surface of graphene;

[0041] Disperse graphene in a mixed solution of concentrated sulfuric acid and concentrated nitric acid with a volume ratio of 2:1 (the mass ratio of graphene to the mixed solution is 1:10), ultrasonically treat it at 40 kHz for 10 min, then stir for 2 h, centrifuge and wash until neutral, and then vacuum dry it in an oven at 40°C;

[0042] S2 Disperse 0.1 part by mass of the surface-treated graphene in 10 parts by mass of DMF, then add 0.1 part by mass of ferric chloride hexahydrate and 0.13 part by mass of fumaric acid (molar ratio of 1:3), put it into a reaction kettle, stir evenly, fully react at 100°C for 5 h, then centrifuge and wash, and dry it in an oven at 40°C for storage for later use;

[0043] (2) Carbonize MIL-88A: Under a nitrogen atmosphere, calcine the graphene with in-situ grown MIL-88A at 600°C for 2 h, and then naturally cool it to room temperature to form a GO-Fe@C three-dimensional conductive framework;

[0044] (3) Embedding lithium iron phosphate into the GO-Fe@C three-dimensional conductive framework: 0.1 part by mass of GO-Fe@C, 1 part by mass of ferrous oxalate, 0.56 part by mass of lithium carbonate, and 0.72 part by mass of ammonium dihydrogen phosphate (molar ratio of Fe:Li:P = 1:1.1:0.9) were ball-milled and mixed at 300 r / min for 4 h, with a ball-to-material ratio of 1:3. Then, they were calcined at 600 °C for 9 h in a nitrogen atmosphere containing 3% (volume ratio) hydrogen. After natural cooling, a high-performance lithium iron phosphate cathode material was obtained.

[0045] Example 2

[0046] The preparation method of the lithium iron phosphate cathode material in this example includes the following steps:

[0047] (1) In-situ growth of MIL-88A on the surface of graphene:

[0048] S1 Treat the surface of graphene;

[0049] Graphene was dispersed in a mixed solution of concentrated sulfuric acid and concentrated nitric acid with a volume ratio of 3:1 (mass ratio of graphene to the mixed solution is 1:10). After ultrasonic treatment at 40 kHz for 10 min, it was stirred for 2 h, centrifuged and washed until neutral, and then vacuum-dried in an oven at 40 °C;

[0050] S2 0.1 part by mass of the surface-treated graphene was dispersed in 10 parts by mass of DMF, and then 0.2 part by mass of ferric chloride hexahydrate and 0.34 part by mass of fumaric acid (molar ratio of 1:4) were added. After being put into a reaction kettle, stirred evenly, and reacted fully at 110 °C for 4 h, it was centrifuged and washed, and dried in an oven at 45 °C for standby;

[0051] (2) Carbonize MIL-88A: In a nitrogen atmosphere, the graphene with in-situ grown MIL-88A 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 framework;

[0052] (3) Embedding lithium iron phosphate into the GO-Fe@C three-dimensional conductive framework: 0.1 part by mass of GO-Fe@C, 1.1 parts by mass of ferrous oxalate, 0.59 part by mass of lithium carbonate, and 0.88 part by mass of ammonium dihydrogen phosphate (molar ratio of Fe:Li:P = 1:1.05:1) were ball-milled and mixed at 350 r / min for 3 h, with a ball-to-material ratio of 1:4. Then, they were calcined at 650 °C for 8 h in a nitrogen atmosphere containing 4% (volume ratio) hydrogen. After natural cooling, a high-performance lithium iron phosphate cathode material was obtained.

[0053] Example 3

[0054] The preparation method of the lithium iron phosphate cathode material in this example includes the following steps:

[0055] (1) In-situ growth of MIL-88A on the surface of graphene:

[0056] S1 Treat the surface of graphene;

[0057] Disperse graphene in a mixed solution of concentrated sulfuric acid and concentrated nitric acid with a volume ratio of 4:1 (the mass ratio of graphene to the mixed solution is 1:10), ultrasonically treat it at 40 kHz for 10 min, then stir for 2 h, centrifuge and wash until neutral, and then vacuum dry it in an oven at 50 °C;

[0058] S2 Disperse 0.1 part by mass of the surface-treated graphene in 10 parts by mass of DMF, then add 0.3 part by mass of ferric chloride hexahydrate and 0.64 part by mass of fumaric acid (molar ratio of 1:5), put it into a reaction kettle, stir evenly, fully react at 120 °C for 3 h, then centrifuge and wash, and dry it in an oven at 50 °C for standby;

[0059] (2) Carbonize MIL-88A: Under a nitrogen atmosphere, calcine the graphene with in-situ grown MIL-88A at 700 °C for 1 h, and then naturally cool it to room temperature to form a GO-Fe@C three-dimensional conductive framework;

[0060] (3) Embed lithium iron phosphate in the GO-Fe@C three-dimensional conductive framework: Mix 0.1 part by mass of GO-Fe@C, 1.2 parts by mass of ferrous oxalate, 0.62 part 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) by ball milling at 400 r / min for 2 h, with a ball-to-material ratio of 1:5, and calcine it at 700 °C for 7 h under a nitrogen atmosphere containing 5% (volume fraction) hydrogen. After natural cooling, a high-performance lithium iron phosphate cathode material is obtained.

[0061] SEM images of lithium iron phosphate in-situ grown in graphene and MOF-derived carbon in Examples 1-3 are as Figures 1-3 shown. It can be seen that the lithium iron phosphate embedded and grown in the conductive network of graphene and MIL-88A-derived carbon 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 the conductive networks of graphene and MIL-88A-derived carbon. This porous structure combined with the three-dimensional conductive framework significantly increases the specific surface area and lithium ion diffusion rate of lithium iron phosphate, improving its electrochemical performance.

[0062] Comparative Example 1

[0063] Only embed lithium iron phosphate in graphene and cancel the relevant operations of MOF.

[0064] The preparation method of the lithium iron phosphate cathode material is specifically as follows: 0.1 part by mass of graphene, 1 part by mass of ferrous oxalate, 0.56 part by mass of lithium carbonate, and 0.72 part 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 h, with a ball-to-material ratio of 1:3. Then, it is calcined at 600 °C for 9 h in a nitrogen atmosphere containing 3% (volume ratio) of hydrogen to obtain the lithium iron phosphate cathode material.

[0065] Comparative Example 2

[0066] Cancel the operation of in-situ growth of MOF on the graphene surface and change it to: directly synthesize MOF and embed it into lithium iron phosphate after carbonization.

[0067] The preparation method of the lithium iron phosphate cathode material includes the following steps:

[0068] (1) Add 0.1 part by mass of ferric chloride hexahydrate and 0.13 part by mass of fumaric acid (molar ratio of 1:3) to 10 parts by mass of DMF, put it into a reaction kettle, stir evenly, react fully at 100 °C for 5 h, then centrifuge and wash, and dry in an oven at 40 °C to obtain MIL-88A;

[0069] (2) Carbonize MIL-88A: Under a nitrogen atmosphere, calcine MIL-88A at 600 °C for 2 h, and then naturally cool to room temperature to form MIL-88A-derived carbon;

[0070] (3) Composite modification of lithium iron phosphate with graphene and MIL-88A: 0.073 part by mass of graphene, 0.027 part by mass of MIL-88A-derived carbon, 1 part by mass of ferrous oxalate, 0.56 part by mass of lithium carbonate, and 0.72 part 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 h, with a ball-to-material ratio of 1:3. Then, it is calcined at 600 °C for 9 h in a nitrogen atmosphere containing 3% (volume ratio) of hydrogen, and after natural cooling, the lithium iron phosphate cathode material is obtained.

[0071] Comparative Example 3

[0072] Cancel the MOF-related operation, generate lithium iron phosphate, and add 1% (percentage by mass of the total mass of the prepared lithium iron phosphate material) of glucose and calcine together.

[0073] The preparation method of the lithium iron phosphate cathode 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 h, and the ball-to-material ratio is 1:3. Then, it is calcined at 600 °C for 9 h in a nitrogen atmosphere containing 3% (volume ratio) of hydrogen to obtain the lithium iron phosphate cathode material.

[0074] The lithium iron phosphate electrode materials prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to electrochemical performance tests.

[0075] Preparation of lithium iron phosphate electrode sheets:

[0076] S1 Dissolve PVDF in NMP to prepare a 5 wt.% PVDF solution;

[0077] S2 Mix the prepared lithium iron phosphate cathode material and the PVDF solution at a mass ratio of 9:1 and stir evenly;

[0078] S3 Use a coater to coat the slurry on the aluminum foil with a thickness of 60 μm and place it in an oven at 70 °C for drying;

[0079] S4 Cut the dried aluminum foil into 12 mm circular pieces.

[0080] Battery assembly:

[0081] Use a CR2032 type battery case, use a lithium sheet as the negative electrode material, use polypropylene as the separator, and 1 mol / L LiPF6 as the electrolyte to assemble a button cell. Perform electrochemical performance tests on it in a two-electrode system at room temperature, and the test results are shown in Table 1 and Figures 4-5 as shown.

[0082] Table 1 Electrochemical test results

[0083]

[0084] As can be seen from Table 1 and Figure 5 it can be known that: the specific capacity of the lithium iron phosphate cathode material prepared by the method of the present invention can reach up to more than 160 mAh / g, which is much higher than that of Comparative Examples 1-3; from Figure 4It can be seen that the polarization potential differences of Examples 1-3 are much smaller than those of Comparative Examples 1-3, and their peak shapes are sharper and the current densities are higher, indicating that the insertion and extraction of lithium ions and the redox reaction of iron ions during charge and discharge in Examples 1-3 are easier to proceed, and the electrochemical performance of the electrode is better. This is because the present invention improves the conductivity and ion diffusion performance of lithium iron phosphate by constructing a three-dimensional conductive network, in-situ growth of MOF on the surface of graphene, and the synergistic effect of the porous structure of MOF porous carbon derivatives and graphene. A three-dimensional conductive network is constructed by the combination of graphene and MIL-88A-derived carbon, significantly improving the conductivity of lithium iron phosphate. The in-situ growth of MIL-88A on the surface of graphene enhances the binding strength between graphene and MIL-88A, prevents the structure from being damaged during subsequent processes and charge and discharge, and improves the stability of the three-dimensional conductive network. Graphene provides high conductivity to accelerate the passage of electrons, and the porous structure of the carbonized derivative of MIL-88A provides a lithium ion diffusion channel to accelerate lithium ion diffusion. 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; wherein the calcination temperature is 600-700°C and the calcination time is 1-2h; (3) Embedding lithium iron phosphate in the GO-Fe@C three-dimensional conductive skeleton: ball-milling the GO-Fe@C three-dimensional conductive skeleton with ferrous oxalate, Li2CO3, and NH4H2PO4, and calcining them in a nitrogen atmosphere containing hydrogen to obtain a lithium iron phosphate positive electrode material; In step (3), ferrous oxalate, Li2CO3, and NH4H2PO4 are added according to 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); 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; In step (3), in the nitrogen atmosphere containing hydrogen, the volume proportion of hydrogen is 3-5%.

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 (3), the calcination temperature is 600-700°C and the calcination time is 7-9h.

6. 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 5.

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