A graphene oxide / lithium iron phosphate composite material and a preparation method thereof

Abstract

The present invention discloses a graphene oxide / lithium iron phosphate composite material and a preparation method thereof, belonging to the technical field of lithium iron phosphate materials. The method comprises the following steps: S1, preparing a solution, mixing a part of the solution with LiOH·H2O and H3PO4, adjusting the pH, stirring evenly, and then adding FeSO4·7H2O, Na2S2O3 and ascorbic acid, and obtaining a mixed solution after mixing evenly; mixing the remaining solution with a transition metal salt and 2-methylimidazole, heating and stirring, centrifuging and washing to obtain M-ZIF; adding M-ZIF into the mixed solution, heating and stirring, washing and drying to obtain an M-ZIF / S-doped LiFePO4 precursor; S2, preparing thiol-modified graphene oxide; S3, grinding the M-ZIF / S-doped LiFePO4 precursor and the thiol-modified graphene oxide, and performing high-temperature sintering in a nitrogen atmosphere to obtain the graphene oxide / lithium iron phosphate composite material.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium iron phosphate materials, and particularly relates to a graphene oxide / lithium iron phosphate composite material and a preparation method thereof. Background Art

[0002] As an energy storage form, batteries have been widely used in fields such as electric vehicles and hybrid vehicles. However, traditional lead-acid batteries and nickel-metal batteries cannot meet the market demand due to their low energy density and environmental pollution problems. Lithium batteries, due to their environmentally friendly characteristics, are ahead of most other energy storage technologies. Among them, lithium iron phosphate batteries have become one of the most widely used secondary batteries for electric vehicles and energy storage due to their high theoretical capacity (170 mAh / g), long life, relatively high working voltage (3.5 V), high safety, environmental friendliness, and high thermal stability.

[0003] However, the LiFePO4 cathode material also has the following problems: low electronic conductivity (about 10-11S / cm), poor lithium ion diffusion rate, low tap density (3.6g / cm 3 ), and poor rate cycling performance at high currents. These are the main reasons for the poor electrochemical performance of LiFePO4. In order to overcome these obvious disadvantages, many studies have been carried out in the doping and preparation processes, such as heterogeneous doping of LiFePO4, transition metal doping, coating LiFePO4 with conductive polymers, and coating with high electron-conductive materials (mainly carbon). Among them, carbon coating is a simple and effective method to improve the performance of LiFePO4.

[0004] The patent application with the publication number CN 115924899 A discloses a preparation method of a lithium iron phosphate / graphene oxide composite material. This application mixes an iron source with polyamino oxidized cellulose; by limiting the ratio of lithium and iron elements and the particle size of the lithium source, the comprehensive performance of the lithium iron phosphate material in lithium battery applications is improved, and the problem of poor comprehensive performance of the lithium iron phosphate material applied to batteries in the prior art is solved. However, the dispersibility of graphene oxide and lithium iron phosphate needs to be improved to further improve the comprehensive performance of the battery. Summary of the Invention

[0005] The purpose of the present invention is to provide a graphene oxide / lithium iron phosphate composite material and a preparation method thereof to improve the cycle performance of the graphene oxide / lithium iron phosphate composite material applied to batteries.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A preparation method of a graphene oxide / lithium iron phosphate composite material includes the following steps:

[0008] S1. Add deionized water and cetyltrimethylammonium bromide to ethylene glycol and stir evenly to obtain a solution; mix a part of the solution with LiOH·H2O and H3PO4, adjust the pH, stir evenly, then add FeSO4·7H2O, Na2S2O3 and ascorbic acid, and mix evenly to obtain a mixed solution; mix the remaining solution with a transition metal salt and 2-methylimidazole, heat and stir, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat up and stir, wash and dry to obtain an M-ZIF / S-doped LiFePO4 precursor;

[0009] S2. Mix graphene oxide, N-hydroxysuccinimide, N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride and deionized water and stir, add cysteamine hydrochloride, stir, centrifuge and wash to obtain thiol-modified graphene oxide;

[0010] S3. Grind the M-ZIF / S-doped LiFePO4 precursor and thiol-modified graphene oxide, and sinter at high temperature under a nitrogen atmosphere to obtain a graphene oxide / lithium iron phosphate composite material.

[0011] Further, the dosage ratio of the deionized water, cetyltrimethylammonium bromide and ethylene glycol is (30 - 40) mL:(0.5 - 2) g:(60 - 70) mL.

[0012] Further, the dosage ratio of the LiOH·H2O, H3PO4, FeSO4·7H2O, Na2S2O3, ascorbic acid and part of the solution is (1.2 - 1.5) g:(0.9 - 1.2) g:(2.7 - 3) g:(0.1 - 0.3) g:(0.01 - 0.03) g:(60 - 70) mL.

[0013] Further, the pH is adjusted to 6.5 - 7.

[0014] Further, the dosage ratio of the transition metal salt, 2-methylimidazole and the remaining solution is (0.1 - 0.2) g:(1 - 2.5) g:(30 - 40) mL.

[0015] Further, the transition metal salt is one or a combination of Co(NO3)2, Fe(NO3)3, Zn(NO3)2 and NiSO4.

[0016] Further, the heating and stirring is carried out at 40 - 70 °C for 4 - 8 h.

[0017] Further, the heating-up stirring is carried out at a rate of 5 - 10 °C / min to 180 - 200 °C and then heated for 8 - 10 h.

[0018] Further, the high-temperature sintering is carried out by heating to 250-350°C at a rate of 5-10°C / min, calcining for 1.5-2.5 h, and then heating to 750-800°C at a rate of 5-10°C / min and calcining for 6-8 h.

[0019] Further, the dosage ratio of graphene oxide, N-hydroxysuccinimide, N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride, deionized water, and cysteamine hydrochloride is (0.1-0.5) g:(0.3-0.6) g:(0.3-0.9) g:(60-90) mL:(0.05-0.08) g.

[0020] A graphene oxide / lithium iron phosphate composite material is prepared by the preparation method of the above-mentioned graphene oxide / lithium iron phosphate composite material.

[0021] Advantages of the present invention:

[0022] (1) The preparation method of a graphene oxide / lithium iron phosphate composite material provided by the present invention uses graphene oxide and lithium iron phosphate materials for compounding. Since graphene oxide has excellent electrical conductivity, it can significantly improve the overall electrical conductivity of the composite material; in addition, the two-dimensional sheet structure of graphene oxide can disperse lithium iron phosphate particles, prevent their agglomeration, improve the utilization rate of active substances, and improve the electrochemical performance of the battery.

[0023] (2) The present invention uses M-ZIF / S for co-doping lithium iron phosphate. M-ZIF is a MOF structure formed by a transition element and 2-methylimidazole. Due to its porosity and open framework structure, it has a high specific surface area, which is beneficial to increasing the contact area between the electrode material and the electrolyte, thereby improving the ion transport efficiency and the rate of electrochemical reaction. The doped sulfur atoms replace the positions of some oxygen atoms or phosphorus atoms, promoting the diffusion rate of lithium ions inside the material. In addition, it can make the particles more uniform and finer, and the distribution is more uniform. The synergistic effect of the anions (S) and cationic groups (M-ZIF) in M-ZIF / S can effectively reduce the energy barrier of electron energy band transition, improve the electronic conductivity, and enhance the capacity and cycle life of the battery.

[0024] (3) The present invention uses cysteamine hydrochloride to carry out mercapto modification on graphene oxide, which can not only provide more active sites on the electrode surface to promote the occurrence of electrochemical reactions; in addition, the mercapto group will adsorb with the transition metal in M-ZIF, which is beneficial to the uniform wrapping of lithium iron phosphate by graphene oxide, improve the dispersibility of graphene oxide, and thus improve the electrochemical performance of the composite material. Specific embodiments

[0025] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0026] Example 1

[0027] This embodiment provides a graphene oxide / lithium iron phosphate composite material, which is prepared by the following steps:

[0028] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, and then add 2.8 g of FeSO4·7H2O, 0.2 g of Na2S2O3, and 0.02 g of ascorbic acid. After mixing evenly, a mixed solution is obtained; mix 30 mL of the solution with 0.15 g of Co(NO3)2 and 1.5 g of 2-methylimidazole, mix well, stir at 60 °C for 8 h, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat it to 180 °C at a rate of 5 °C / min and heat for 8 h, wash it alternately with ethanol and deionized water three times, and dry it at 60 °C to obtain an M-ZIF / S-doped LiFePO4 precursor;

[0029] S2. Mix 0.3 g of graphene oxide, 0.4 g of N-hydroxysuccinimide, 0.6 g of N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride, and 80 mL of deionized water and stir. Add 0.06 g of cysteamine hydrochloride, stir at room temperature for 6 h, centrifuge, and wash alternately with ethanol and deionized water three times to obtain thiol-modified graphene oxide;

[0030] S3. Grind the M-ZIF / S-doped LiFePO4 precursor and thiol-modified graphene oxide, with a grinding particle size of 140 mesh. Under a nitrogen atmosphere, heat it to 300 °C at a rate of 10 °C / min and calcine for 2 h, and then heat it to 800 °C at a rate of 10 °C / min and calcine for 8 h to obtain a graphene oxide / lithium iron phosphate composite material.

[0031] Example 2

[0032] Compared with Example 1, the difference in this embodiment is that the dosages of Co(NO3)2 and 2-methylimidazole are reduced. The specific implementation steps of S1 are as follows:

[0033] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.2 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; mix 30 mL of the solution with 0.1 g of Co(NO3)2 and 1 g of 2-methylimidazole, stir at 60 °C for 8 h after mixing evenly, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat to 180 °C at a rate of 5 °C / min and heat for 8 h, wash alternately with ethanol and deionized water three times, and dry at 60 °C to obtain the M-ZIF / S-doped LiFePO4 precursor;

[0034] The remaining raw materials and the preparation process are the same as those in Example 1.

[0035] Example 3

[0036] Compared with Example 1, the difference in this example is that the dosages of Co(NO3)2 and 2-methylimidazole are increased. The specific implementation steps of S1 are as follows:

[0037] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.2 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; mix 30 mL of the solution with 0.2 g of Co(NO3)2 and 2.5 g of 2-methylimidazole, stir at 60 °C for 8 h after mixing evenly, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat to 180 °C at a rate of 5 °C / min and heat for 8 h, wash alternately with ethanol and deionized water three times, and dry at 60 °C to obtain the M-ZIF / S-doped LiFePO4 precursor;

[0038] The remaining raw materials and the preparation process are the same as those in Example 1.

[0039] Example 4

[0040] Compared with Example 1, the difference in this example is that "Co(NO3)2" is replaced by "Zn(NO3)2". The specific implementation steps of S1 are as follows:

[0041] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.2 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; mix 30 mL of the solution with 0.15 g of Zn(NO3)2 and 1.5 g of 2-methylimidazole, mix well and stir at 60 °C for 8 h, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat to 180 °C at a rate of 5 °C / min for 8 h, wash alternately with ethanol and deionized water three times, and dry at 60 °C to obtain an M-ZIF / S-doped LiFePO4 precursor;

[0042] The remaining raw materials and the preparation process are the same as those in Example 1.

[0043] Example 5

[0044] Compared with Example 1, the difference in this example is that the amount of Na2S2O3 is increased. The specific implementation steps of S1 are as follows:

[0045] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.3 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; mix 30 mL of the solution with 0.15 g of Co(NO3)2 and 1.5 g of 2-methylimidazole, mix well and stir at 60 °C for 8 h, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat to 180 °C at a rate of 5 °C / min for 8 h, wash alternately with ethanol and deionized water three times, and dry at 60 °C to obtain an M-ZIF / S-doped LiFePO4 precursor;

[0046] The remaining raw materials and the preparation process are the same as those in Example 1.

[0047] Example 6

[0048] Compared with Example 1, the difference in this example is that the amount of Na2S2O3 is decreased. The specific implementation steps of S1 are as follows:

[0049] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.1 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; mix 30 mL of the solution with 0.15 g of Co(NO3)2 and 1.5 g of 2-methylimidazole, stir at 60 °C for 8 h after mixing evenly, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat to 180 °C at a rate of 5 °C / min and heat for 8 h, wash three times alternately with ethanol and deionized water, and dry at 60 °C to obtain the M-ZIF / S-doped LiFePO4 precursor;

[0050] The remaining raw materials and the preparation process are the same as those in Example 1.

[0051] Example 7

[0052] Compared with Example 1, the difference in this example is that while reducing the dosages of Co(NO3)2 and 2-methylimidazole, the dosage of Na2S2O3 is increased. The specific implementation steps of S1 are as follows:

[0053] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.3 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; mix 30 mL of the solution with 0.1 g of Co(NO3)2 and 1 g of 2-methylimidazole, stir at 60 °C for 8 h after mixing evenly, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat to 180 °C at a rate of 5 °C / min and heat for 8 h, wash three times alternately with ethanol and deionized water, and dry at 60 °C to obtain the M-ZIF / S-doped LiFePO4 precursor;

[0054] The remaining raw materials and the preparation process are the same as those in Example 1.

[0055] Example 8

[0056] Compared with Example 1, the difference in this example is that while increasing the dosages of Co(NO3)2 and 2-methylimidazole, the dosage of Na2S2O3 is reduced. The specific implementation steps of S1 are as follows:

[0057] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.1 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; mix 30 mL of the solution with 0.2 g of Co(NO3)2 and 2.5 g of 2-methylimidazole, mix well and stir at 60 °C for 8 h, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat it to 180 °C at a rate of 5 °C / min and heat for 8 h, wash it three times alternately with ethanol and deionized water, and dry it at 60 °C to obtain the M-ZIF / S-doped LiFePO4 precursor;

[0058] The remaining raw materials and the preparation process are the same as those in Example 1.

[0059] Example 9

[0060] Compared with Example 1, this example is different in that the amount of cysteamine hydrochloride is reduced. The specific implementation steps of S2 are as follows:

[0061] S2. Mix and stir 0.3 g of graphene oxide, 0.4 g of N-hydroxysuccinimide, 0.6 g of N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride and 80 mL of deionized water, add 0.06 g of cysteamine hydrochloride, stir at room temperature for 6 h, centrifuge, and wash three times alternately with ethanol and deionized water to obtain thiol-modified graphene oxide;

[0062] The remaining raw materials and the preparation process are the same as those in Example 1.

[0063] Comparative Example 1

[0064] Compared with Example 1, this comparative example is different in that Na2S2O3 is not added. The specific implementation steps are as follows:

[0065] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; mix 30 mL of the solution with 0.15 g of Co(NO3)2 and 1.5 g of 2-methylimidazole, mix evenly, stir at 60 °C for 8 h, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat it to 180 °C at a rate of 5 °C / min for 8 h, wash it alternately with ethanol and deionized water three times, and dry it at 60 °C to obtain an M-ZIF / S-doped LiFePO4 precursor;

[0066] The remaining raw materials and the preparation process are the same as those in Example 1.

[0067] Comparative Example 2

[0068] Compared with Example 1, the difference in this comparative example is that Co(NO3)2 and 2-methylimidazole are not added. The specific implementation steps are as follows:

[0069] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.2 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; stir at 60 °C for 8 h, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat it to 180 °C at a rate of 5 °C / min for 8 h, wash it alternately with ethanol and deionized water three times, and dry it at 60 °C to obtain an M-ZIF / S-doped LiFePO4 precursor;

[0070] The remaining raw materials and the preparation process are the same as those in Example 1.

[0071] Comparative Example 3

[0072] Compared with Example 1, the difference in this comparative example is that Co(NO3)2, 2-methylimidazole and Na2S2O3 are not added at the same time. The specific implementation steps are as follows:

[0073] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; stir at 60 °C for 8 h, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat it to 180 °C at a rate of 5 °C / min and heat for 8 h, wash it three times alternately with ethanol and deionized water, and dry it at 60 °C to obtain an M-ZIF / S-doped LiFePO4 precursor;

[0074] The remaining raw materials and the preparation process are the same as those in Example 1.

[0075] Comparative Example 4

[0076] The difference between this comparative example and Example 1 is that graphene oxide is not subjected to thiol modification. The specific implementation steps are as follows:

[0077] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution; mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.2 g of Na2S2O3 and 0.02 g of ascorbic acid, and mix evenly to obtain a mixed solution; add 0.15 g of Co(NO3)2 and 1.5 g of 2-methylimidazole to 30 mL of the solution, mix evenly and stir at 60 °C for 8 h, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat it to 180 °C at a rate of 5 °C / min and heat for 8 h, wash it three times alternately with ethanol and deionized water, and dry it at 60 °C to obtain an M-ZIF / S-doped LiFePO4 precursor;

[0078] S2. Grind the M-ZIF / S-doped LiFePO4 precursor and graphene oxide, with a grinding particle size of 140 mesh, calcine it at a rate of 10 °C / min to 300 °C for 2 h in a nitrogen atmosphere, and then calcine it at a rate of 10 °C / min to 800 °C for 8 h to obtain a graphene oxide / lithium iron phosphate composite material.

[0079] The remaining raw materials and the preparation process are the same as those in Example 1.

[0080] Comparative Example 5

[0081] The difference between this comparative example and Example 1 is that graphene oxide is not added. The specific implementation steps are as follows:

[0082] S1. Add 40 mL of deionized water and 1 g of cetyltrimethylammonium bromide to 60 mL of ethylene glycol and stir evenly to obtain a solution. Mix 70 mL of the solution with 1.3 g of LiOH·H2O and 1 g of H3PO4, adjust the pH to 7, stir evenly, then add 2.8 g of FeSO4·7H2O, 0.2 g of Na2S2O3 and 0.02 g of ascorbic acid. After mixing evenly, a mixed solution is obtained. Mix 30 mL of the solution with 0.15 g of Co(NO3)2 and 1.5 g of 2-methylimidazole, stir at 60 °C for 8 h after mixing evenly, centrifuge and wash to obtain M-ZIF. Add M-ZIF to the mixed solution, heat it to 180 °C at a rate of 5 °C / min and heat for 8 h, wash it three times alternately with ethanol and deionized water, and dry it at 60 °C to obtain the M-ZIF / S-doped LiFePO4 precursor;

[0083] S2. Grind the M-ZIF / S-doped LiFePO4 precursor, with a grinding particle size of 140 mesh, heat it to 300 °C at a rate of 10 °C / min in a nitrogen atmosphere and calcine for 2 h, then heat it to 800 °C at a rate of 10 °C / min and calcine for 8 h to obtain the lithium iron phosphate composite material.

[0084] The remaining raw materials and the preparation process are the same as those in Example 1.

[0085] Performance Test

[0086] Use the graphene oxide / lithium iron phosphate composite materials obtained in Examples 1 - 9 and Comparative Examples 1 - 5 as raw materials for the positive electrode active material of the coin cell, prepare the coin cell, and conduct capacity testing and cycle testing on the prepared coin cell according to SJ / T 11797—2022 respectively; the test results are shown in Table 1:

[0087]

[0088] By methods such as heterogeneous doping, transition metal doping, conductive polymer coating and carbon coating, the electrochemical performance of the positive electrode material can be effectively improved. Among them, carbon coating is a simple and effective improvement method. As a single-atom layer carbon material with a two-dimensional honeycomb lattice structure, graphene has a high proportion of sp 2 coordinated carbon, showing higher conductivity than disordered or sp 3 coordinated carbon. Its high specific surface area provides sufficient conductive space for LiFePO4 / C, significantly improving the electronic conductivity. In addition, through the introduction of additional free electrons or holes, transition metal doping can increase the electronic conductivity of the material, lower the energy barrier for lithium ion diffusion, thereby increasing the migration rate of lithium ions and improving the rate performance.

[0089] As can be seen from Table 1, compared with Example 1, the differences in Examples 2-3 lie in that the dosages of Co(NO3)2 and 2-methylimidazole affect the content of M-ZIF in the material. Too little content affects the ion transport efficiency and the rate of electrochemical reaction. Due to its porosity and open framework structure, too much content will affect the overall stability of the composite material. Compared with Example 1, the difference in Example 4 is only the replacement of transition metal ions. As catalytic active centers, transition metal ions can provide abundant active sites, which helps to improve the efficiency of electrochemical reactions. Compared with Example 1, in Examples 5-6, the doped sulfur atoms promote the diffusion rate of lithium ions inside the material. In addition, it can make the particles more uniform and finer, and the distribution is more uniform. Too little doping will correspondingly reduce the electrochemical performance of the material, but too much sulfur doping will lead to a decrease in the conductivity of the material. Compared with Example 1, the differences in Examples 7-8 lie in the adjustment of the ratio of M-ZIF and sulfur source within a reasonable range. From the results, composite materials with good performance can be obtained. Compared with Example 1, a decrease in the dosage of cysteamine hydrochloride in Example 9 will affect the adsorption between graphene oxide and lithium iron phosphate.

[0090] Combining the data of Comparative Examples 1-3, Examples 7-8 and Example 1, it can be seen that the synergistic effect of anions (S) and cationic groups (M-ZIF) in M-ZIF / S can effectively reduce the energy barrier of electron energy band transition, thereby improving the electron conductivity and enhancing the capacity and cycle life of the battery. Comparing Comparative Example 4, Example 9 and Example 1, it can be seen that mercapto-modified graphene oxide can not only promote the occurrence of electrochemical reactions. In addition, the mercapto group will adsorb with the transition metal in M-ZIF, enhancing the stability of the composite material. Compared with Example 1, in Comparative Example 5, after graphene oxide is not added, the capacity and cycle life of the battery of the composite material both show a downward trend.

[0091] In summary, a graphene oxide / lithium iron phosphate composite material and a preparation method thereof provided by the present invention have a relatively high battery capacity and cycle stability, and have good application prospects in the technical field of lithium iron phosphate materials.

[0092] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variation thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0093] Although embodiments of the present invention have been shown and described, those of ordinary skill in the art will understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a graphene oxide / lithium iron phosphate composite material, characterized in that: The following steps are involved: S1. Prepare a solution, mix part of the solution with LiOH·H2O and H3PO4, adjust the pH, stir evenly, then add FeSO4·7H2O, Na2S2O3 and ascorbic acid, mix evenly to obtain a mixed solution; mix the remaining solution with a transition metal salt and 2-methylimidazole, heat and stir, centrifuge and wash to obtain M-ZIF; add M-ZIF to the mixed solution, heat and stir, wash and dry to obtain a M-ZIF / S-doped LiFePO4 precursor; S2, preparing thiol-modified graphene oxide; S3, grinding the M-ZIF / S-doped LiFePO4 precursor and the thiol-modified graphene oxide, and sintering them at high temperature in a nitrogen atmosphere to obtain a graphene oxide / lithium iron phosphate composite material; The solution is deionized water and hexadecyltrimethylammonium bromide added to ethylene glycol and stirred evenly; The M-ZIF is a MOF structure formed by transition metal and 2-methylimidazole.

2. The method for preparing a graphene oxide / lithium iron phosphate composite material according to claim 1, characterized in that: The dosage ratio of LiOH·H2O, H3PO4, FeSO4·7H2O, Na2S2O3, ascorbic acid and partial solution is (1.2-1.5) g: (0.9-1.2) g: (2.7-3) g: (0.1-0.3) g: (0.01-0.03) g: (60-70) mL; The pH was adjusted to 6.5-7.

3. The method for preparing a graphene oxide / lithium iron phosphate composite material according to claim 1, characterized in that: The ratio of the transition metal salt, 2-methylimidazole and the remaining solution is (0.1-0.2) g: (1-2.5) g: (30-40) mL; The transition metal salt is one or a combination of Co(NO3)2, Fe(NO3)3, Zn(NO3)2 and NiSO4.

4. The method for preparing a graphene oxide / lithium iron phosphate composite material according to claim 1, characterized in that: The heating stirring is stirring at 40-70°C for 4-8h; the heating stirring is heating to 180-200°C at a rate of 5-10°C / min, and then heating for 8-10h.

5. The method for preparing a graphene oxide / lithium iron phosphate composite material according to claim 1, characterized in that: The high temperature sintering is to heat up to 250-350°C at a rate of 5-10°C / min, calcine for 1.5-2.5h, and then heat up to 750-800°C at a rate of 5-10°C / min, and calcine for 6-8h.

6. The method for preparing a graphene oxide / lithium iron phosphate composite material according to claim 1, characterized in that: The usage ratio of the deionized water, hexadecyltrimethylammonium bromide and ethylene glycol is (30-40) mL: (0.5-2) g: (60-70) mL.

7. The method for preparing a graphene oxide / lithium iron phosphate composite material according to claim 1, characterized in that: The thiol-modified graphene oxide is prepared by the following steps: Graphene oxide, N-hydroxysuccinimide, N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride and deionized water are mixed and stirred, cysteamine hydrochloride is added, stirred, and centrifuged and washed to obtain thiol-modified graphene oxide.

8. The method for preparing a graphene oxide / lithium iron phosphate composite material according to claim 7, characterized in that: The usage ratio of the graphene oxide, N-hydroxysuccinimide, N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride, deionized water and cysteamine hydrochloride is (0.1-0.5) g: (0.3-0.6) g: (0.3-0.9) g: (60-90) mL: (0.05-0.08) g.

9. A graphene oxide / lithium iron phosphate composite material, characterized in that: The method is prepared according to any one of claims 1 to 8.

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