A method for preparing a carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite by hydrothermal reaction

Abstract

The application discloses a method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphite-like carbon nanocomposites through a hydrothermal reaction, and the preparation method is divided into three parts. In the first part, multilayer nitrogen-doped graphite-like carbon (N-GLC) is prepared through a melting and casting method and a high-temperature cracking method. In the second part, the prepared thin-layer N-GLC is prepared through a three-roller grinding machine. In the third part, carbon-coated lithium iron phosphate / nitrogen-doped graphite-like carbon (C@LiFePO4 / N-GLC) is obtained through a hydrothermal method and a high-temperature solid-phase synthesis method. The application can replace expensive graphene, improve the specific capacity of carbon-coated lithium iron phosphate under high power, and has the characteristics of low cost, easy production and easy implementation.

Description

Technical Field

[0001] This invention relates to the field of graphene-like carbon technology, specifically to a method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposites via a hydrothermal reaction. Background Technology

[0002] With the escalating energy crisis and environmental pollution, coupled with the vigorous development of the new energy industry, new energy vehicles have become the preferred alternative to gasoline-powered vehicles. Lithium-ion batteries, in particular, are widely used in electric vehicles and energy storage power stations due to their advantages such as high energy density, long lifespan, low self-discharge rate, and lack of memory effect.

[0003] LiFePO4 has secured a certain market share due to its competitive advantages, such as high cost-effectiveness, long cycle life, excellent thermal stability, and environmental friendliness. However, LiFePO4 has poor conductivity and slightly lower actual capacity, resulting in significant disadvantages in charge-discharge performance at high current densities. Consequently, its actual installation volume in long-range new energy vehicles is lower than that of ternary materials.

[0004] To address the above problems, this invention employs casting and three-roll milling to prepare low-cost nitrogen-doped graphene-like carbon nanosheets (0.5 yuan / g) from urea, replacing expensive graphene (100 yuan / g). The main focus is on constructing a three-dimensional graphene-like conductive network and improving the electrochemical performance of LiFePO4 through carbon coating, thereby improving the electronic conductivity, ion diffusion rate, and various electrode surface properties of LiFePO4. Summary of the Invention

[0005] The purpose of this invention is to solve the technical problems existing in the prior art and to provide a method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials by hydrothermal reaction.

[0006] To achieve the above objectives, the technical solution provided by this invention is: a method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials via hydrothermal reaction. This preparation method comprises three parts: the first part involves preparing multilayer nitrogen-doped graphene-like carbon through a casting method and a high-temperature pyrolysis method; the second part involves exfoliating thin-layer N-GLC using a three-roll mill; and the third part involves obtaining C@LiFePO4 / N-GLC through a hydrothermal method and a high-temperature solid-state synthesis method.

[0007] The method specifically includes the following steps:

[0008] Step (1): Disperse 2g of urea and 0.1g of sodium chloride in 10mL of N,N-dimethylformamide and stir magnetically for 1 hour until completely dissolved to obtain a casting solution. Then add 0.5g of polyethylene glycol to the casting solution and stir to mix evenly to prepare a blended film solution. Coat the blended film solution evenly on copper foil, dry it, immerse it in deionized water to remove NaCl, and finally vacuum dry to prepare porous urea sheets.

[0009] Step (2): Sinter porous urea sheets under a nitrogen-hydrogen mixed atmosphere to prepare multilayer N-GLC;

[0010] Step (3): Use a three-roll mill to peel off the multilayer N-GLC. Mix the multilayer N-GLC with phenolic resin and stir to form a suspension. Grind the suspension in four stages of 12μm-6μm-3μm-1μm on the three-roll mill. Peel 5 times in each stage. Then, centrifuge and wash the ground suspension with ethanol to obtain the thin layer N-GLC after peeling.

[0011] Step (4): Using 60 mL of ethylene glycol as a dispersant, add 0.2 g of thin-layer N-GLC and sonicate at 100 W for 5 minutes to obtain solution 1. Subsequently, add LiOH·H2O and C6H 18 O 24 P6 and FeSO4·7H2O were mixed and ground for 30 minutes in a molar ratio of 3:5:1, and then added to solution 1. The mixture was magnetically stirred at 60°C for 30 minutes to obtain mixture 1.

[0012] Step (5): Quickly pour mixture 1 into the reaction vessel to carry out a hydrothermal reaction. After the reaction is completed, cool it to room temperature to obtain sample 1.

[0013] Step (6): Centrifuge sample 1 to remove the supernatant, and then wash it 3 times with anhydrous ethanol to obtain sample 2;

[0014] Step (7): Add sample 2 and 0.0948g urea to 100mL anhydrous ethanol, ultrasonically disperse for 1 hour, then dry by blowing air, and then vacuum dry at 80℃ for 10 hours or more to obtain sample 3. Take out the dried sample 3 and grind it in a mortar for 30 minutes to make the powder uniform.

[0015] Step (8): Place the ground sample 3 into a tube furnace for calcination at a heating rate of 2℃ / min. Calcinate at 700℃ for 6 hours. The protective gas is a hydrogen-argon mixture. After natural cooling, take out the sample to obtain the LiFePO4@C / N-GLC composite material.

[0016] Preferably, in step (1), the optimal mass ratio of Urea, NaCl, and PEG is: Urea:NaCl:PEG = 20:1:5.

[0017] Preferably, the sintering temperature in step (2) is 700-900℃ and the sintering time is 1-3h.

[0018] Preferably, in step (3), the mass ratio of multilayer N-GLC to phenolic resin is 1:20.

[0019] Preferably, the mass ratio of thin-layer N-GLC to LiOH·H2O in step (4) is 1:20.

[0020] Preferably, in step (4), increasing the power and time of ultrasonic dispersion ensures uniform ultrasonic dispersion of the graphene-like carbon structure.

[0021] Preferably, in step (5), the hydrothermal reaction temperature is 160-200℃ and the hydrothermal time is 4-5 hours.

[0022] Preferably, the centrifuge parameters in step (6) are 5000-7000 rpm and the centrifugation time is 3 minutes or more.

[0023] Preferably, in step (7), the mass of urea accounts for 2% of the lithium iron phosphate, the forced air drying is carried out at 60-80℃ for more than 2 hours, and the air pressure during vacuum drying must be lower than -1.0MPa.

[0024] Preferably, in step (8), a hydrogen-argon mixture is introduced into the tubular furnace for 20-40 minutes before calcination, and the hydrogen-argon mixture is continuously introduced during the calcination process to ensure the inertness of the reaction environment and prevent oxidation. The calcination temperature is 600-750℃ and the calcination time is 5-7 hours.

[0025] Beneficial effects of this invention:

[0026] This invention employs a carbon coating surface treatment that does not affect the crystal structure of lithium iron phosphate (LiFePO4). Urea can be prepared into nitrogen-doped graphene-like carbon through solution casting and three-roll milling, which can replace expensive graphene.

[0027] Furthermore, nitrogen-doped graphene carbon not only possesses a layered structure but also has active sites introduced by nitrogen doping. This nitrogen-doped graphene carbon has a cost as low as 0.5 yuan / g, and can be used to construct low-cost carbon-coated lithium iron phosphate (C@LiFePO4) three-dimensional conductive network structures, thereby promoting the application of lithium iron phosphate cathode materials in high-power batteries.

[0028] This invention is characterized by low cost, easy production, and easy implementation. Attached Figure Description

[0029] The accompanying drawings, which are provided to further illustrate the invention and constitute a part of this invention, are illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention.

[0030] Figure 1 This is the XRD pattern of Embodiment 1 in this invention;

[0031] Figure 2 This is a physical image of Embodiment 1 of the present invention;

[0032] Figure 3 This is a graph showing the electrochemical performance test results of Example 1 in this invention. Detailed Implementation

[0033] This section will describe in detail specific embodiments of the present invention. Preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the drawings is to supplement the textual description with graphics, so that people can intuitively and vividly understand each technical feature and overall technical solution of the present invention, but they should not be construed as limiting the scope of protection of the present invention.

[0034] This invention discloses a method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposites via hydrothermal reaction. The preparation method comprises three parts: the first part is to prepare multilayer nitrogen-doped graphene-like carbon through a casting method and a high-temperature pyrolysis method; the second part is to prepare thin-layer N-GLC by exfoliation using a three-roll mill; and the third part is to obtain C@LiFePO4 / N-GLC through a hydrothermal method and a high-temperature solid-state synthesis method.

[0035] The first part consists of four steps: the first step is to disperse urea and sodium chloride in N,N-dimethylformamide (DMF) and magnetically stir to obtain a casting solution; the second step is to add polyethylene glycol to the casting solution, stir and mix it evenly to prepare a blended film solution; the third step is to uniformly coat the blended film solution onto copper foil, dry it, immerse it in deionized water to remove sodium chloride, and then vacuum dry it; the fourth step is to sinter the porous urea sheets in a nitrogen-hydrogen mixed atmosphere to prepare a multilayer N-GLC.

[0036] The second part consists of three steps: the first step is to mix and stir the multilayer N-GLC with phenolic resin to form a suspension; the second step is to use a three-roll mill to peel off the suspension; the third step is to use ethanol to centrifuge and wash the suspension to obtain the thin layer N-GLC after peeling.

[0037] The third part consists of three steps: The first step is to combine N-GLC, LiOH·H2O, and C6H 18 O 24P6 and FeSO4·7H2O are dissolved in ethylene glycol; the second step is to carry out a hydrothermal reaction to obtain a precipitated precursor; the third step is to introduce a hydrogen-argon mixed gas and place the precursor powder and urea into a tube furnace for calcination.

[0038] The method specifically includes the following steps:

[0039] Step (1): Disperse 2g of urea and 0.1g of sodium chloride in 10mL of N,N-dimethylformamide and stir magnetically for 1 hour until completely dissolved to obtain a casting solution. Then add 0.5g of polyethylene glycol to the casting solution and stir to mix evenly to prepare a blended film solution. Coat the casting solution evenly on copper foil, dry it, immerse it in deionized water to remove NaCl, and finally vacuum dry to prepare porous urea sheets.

[0040] Step (2): Sinter porous urea sheets under a nitrogen-hydrogen mixed atmosphere to prepare multilayer N-GLC;

[0041] Step (3): Use a three-roll mill to peel off the multilayer N-GLC. Mix the multilayer N-GLC with phenolic resin and stir to form a suspension. Grind the suspension in four stages of 12μm-6μm-3μm-1μm on the three-roll mill. Peel 5 times in each stage. Then, centrifuge and wash the ground suspension with ethanol to obtain the thin layer N-GLC after peeling.

[0042] Step (4): Using 60 mL of ethylene glycol as a dispersant, add 0.2 g of thin-layer N-GLC and sonicate at 100 W for 5 minutes to obtain solution 1. Subsequently, add LiOH·H2O and C6H 18 O 24 P6 and FeSO4·7H2O were mixed and ground for 30 minutes in a molar ratio of 3:5:1, and then added to solution 1. The mixture was magnetically stirred at 60°C for 30 minutes to obtain mixture 1.

[0043] Step (5): Quickly pour mixture 1 into the reaction vessel to carry out a hydrothermal reaction. After the reaction is completed, cool it to room temperature to obtain sample 1.

[0044] Step (6): Centrifuge sample 1 to remove the supernatant, and then wash it 3 times with anhydrous ethanol to obtain sample 2;

[0045] Step (7): Add sample 2 and 0.0948g urea to 100mL anhydrous ethanol, ultrasonically disperse for 1 hour, then dry by blowing air, and then vacuum dry at 80℃ for 10 hours or more to obtain sample 3. Take out the dried sample 3 and grind it in a mortar for 30 minutes to make the powder uniform.

[0046] Step (8): Place the ground sample 3 into a tube furnace for calcination at a heating rate of 2℃ / min. Calcinate at 700℃ for 6 hours using a hydrogen-argon mixture as the protective gas. After natural cooling, remove the sample to obtain the C@LiFePO4 / N-GLC composite material. (Grind sample 3 for a period of time to ensure uniform powder. Store the ground sample 3 in a bag to maintain the stability of the material. Appropriate means (such as stirring, vibration, etc.) can be used to disperse the powder to ensure that large lumps do not form.)

[0047] The C@LiFePO4 / N-GLC composite material prepared by this invention can replace expensive graphene and improve the specific capacity of carbon-coated lithium iron phosphate at high power.

[0048] Example 1

[0049] (1) Urea flakes were prepared by melting and casting: Urea and sodium chloride were dispersed in DMF and magnetically stirred for 1 hour until completely dissolved; PEG was added to the casting solution and stirred and mixed evenly to prepare a blended film solution; the blended film solution was evenly coated on copper foil, dried and then immersed in deionized water to remove NaCl; porous urea flakes were prepared by vacuum drying.

[0050] (2) Preparation of multilayer nitrogen-doped graphene carbon nanosheets: Multilayer N-GLCs were prepared by sintering porous urea sheets at 800℃ for 2h in a nitrogen-hydrogen mixed atmosphere.

[0051] (3) Use a three-roll mill to peel off multilayer N-GLC and prepare thin-layer N-GLC: Mix multilayer N-GLC with phenolic resin and stir to form a suspension; mill the mixture with a three-roll mill; centrifuge and wash the suspension with ethanol to obtain thin-layer N-GLC after peeling.

[0052] (4) Hydrothermal preparation of LiFePO4 / N-GLC: N-GLC, LiOH·H2O, C6H 18 O 24 P6 and FeSO4·7H2O were dissolved and dispersed uniformly in ethylene glycol; the solution was placed in a hydrothermal reactor at 180℃ for 4 hours, and centrifuged to obtain the precipitate precursor;

[0053] (5) Preparation of C@LiFePO4 / N-GLC by high temperature solid phase synthesis: The precursor powder and urea were ultrasonically mixed evenly in anhydrous ethanol, and after vacuum drying, the mixture was heated in a tube furnace at 700℃ for 6h (5% H2 + Ar) to obtain the final product C@LiFePO4 / N-GLC.

[0054] (6) Assemble half-cells: Prepare C@LiFePO4 / N-GLC slurry; coat it on aluminum foil; prepare electrode sheets; prepare half-cells in a glove box;

[0055] Performance testing: The phase characteristics of C@LiFePO4 / N-GLC were characterized, and the electrochemical performance of the half-cell was tested.

[0056] The XRD pattern of the product in this embodiment is shown below. Figure 1 .

[0057] See the physical image of the product in this embodiment. Figure 2 .

[0058] The electrochemical performance graph of the product in this embodiment is shown below. Figure 3 .

[0059] This invention addresses the shortcomings of lithium iron phosphate (LiFePO4) batteries, such as low capacity and slow charge / discharge rates, by adding low-cost conductive graphene-like carbon and carbon coating to improve its electrochemical performance. To overcome the poor conductivity of LiFePO4 cathode materials, this invention employs carbon coating as a surface treatment, which does not affect the crystal structure of LiFePO4. Urea can be prepared into nitrogen-doped graphene-like carbon through solution casting and three-roll milling, replacing expensive graphene. Nitrogen-doped graphene-like carbon not only possesses a layered structure but also has active sites introduced by nitrogen doping. The cost of this nitrogen-doped graphene-like carbon is as low as 0.5 yuan / g, and it can be used to construct a low-cost carbon-coated three-dimensional conductive network structure for LiFePO4 (C@LiFePO4), thereby promoting the application of LiFePO4 cathode materials in high-power batteries. This invention is characterized by low cost, ease of production, and ease of implementation.

[0060] Without causing conflict, those skilled in the art can freely combine and use the above-mentioned additional technical features.

[0061] The above description is only a preferred embodiment of the present invention. Any technical solution that achieves the purpose of the present invention by essentially the same means is within the protection scope of the present invention.

Claims

1. A method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials via hydrothermal reaction, characterized in that: The preparation method consists of three parts: the first part is to prepare multilayer nitrogen-doped graphene-like carbon by melting and casting and high-temperature pyrolysis; the second part is to prepare thin-layer N-GLC by exfoliation using a three-roll mill; and the third part is to obtain C@LiFePO4 / N-GLC by hydrothermal method and high-temperature solid-state synthesis. The method specifically includes the following steps: Step (1): Disperse 2g of urea and 0.1g of sodium chloride in 10mL of N,N-dimethylformamide and stir magnetically for 1 hour until completely dissolved to obtain a casting solution. Then add 0.5g of polyethylene glycol to the casting solution and stir to mix evenly to prepare a blended film solution. Coat the blended film solution evenly on copper foil, dry it, immerse it in deionized water to remove NaCl, and finally vacuum dry to prepare porous urea sheets. Step (2): Sinter porous urea sheets in a nitrogen-hydrogen mixed atmosphere to prepare multilayer N-GLC; Step (3): Use a three-roll mill to peel off the multilayer N-GLC. Mix the multilayer N-GLC with phenolic resin and stir to form a suspension. Grind the suspension in four stages of 12μm-6μm-3μm-1μm on the three-roll mill. Peel 5 times in each stage. Then, centrifuge and wash the ground suspension with ethanol to obtain the thin layer N-GLC after peeling. Step (4): Using 60 mL of ethylene glycol as a dispersant, add 0.2 g of thin-layer N-GLC and sonicate at 100 W for 5 minutes to obtain solution 1. Subsequently, add LiOH·H2O and C6H 18 O 24 P6 and FeSO4·7H2O were mixed and ground for 30 minutes in a molar ratio of 3:5:1, and then added to solution 1. The mixture was magnetically stirred at 60°C for 30 minutes to obtain mixture 1. Step (5): Quickly pour mixture 1 into the reaction vessel to carry out a hydrothermal reaction. After the reaction is completed, cool it to room temperature to obtain sample 1. Step (6): Centrifuge sample 1 to remove the supernatant, and then wash it 3 times with anhydrous ethanol to obtain sample 2; Step (7): Add sample 2 and 0.0948g urea to 100mL anhydrous ethanol, ultrasonically disperse for 1 hour, then dry with a forced air, and then vacuum dry at 80℃ for 10 hours or more to obtain sample 3. Take out the dried sample 3 and grind it with a mortar for 30 minutes to make the powder uniform. Step (8): Place the ground sample 3 into a tube furnace for calcination at a heating rate of 2℃ / min. Calcinate at 700℃ for 6 hours. The protective gas is a hydrogen-argon mixture. After natural cooling, take out the sample to obtain the C@LiFePO4 / N-GLC composite material.

2. The method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials by hydrothermal reaction according to claim 1, characterized in that: In step (2), the sintering temperature is 700-900℃ and the sintering time is 1-3h.

3. The method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials by hydrothermal reaction according to claim 1, characterized in that: In step (3), the mass ratio of multilayer N-GLC to phenolic resin is 1:

20.

4. The method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials by hydrothermal reaction according to claim 1, characterized in that: In step (4), the mass ratio of thin-layer N-GLC to LiOH·H2O is 1:

20.

5. The method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials by hydrothermal reaction according to claim 1, characterized in that: In step (5), the hydrothermal reaction temperature is 160-200℃ and the hydrothermal time is 4-5 hours.

6. The method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials by hydrothermal reaction according to claim 1, characterized in that: The centrifuge parameters in step (6) are 5000-7000 rpm and the centrifugation time is 3 minutes or more.

7. The method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials by hydrothermal reaction according to claim 1, characterized in that: In step (7), the mass of urea accounts for 2% of the lithium iron phosphate. The forced-air drying is carried out at 60-80℃ for more than 2 hours. The air pressure must be lower than -1.0MPa during vacuum drying.

8. The method for preparing carbon-coated lithium iron phosphate / nitrogen-doped graphene-like carbon nanocomposite materials by hydrothermal reaction according to claim 1, characterized in that: In step (8), a hydrogen-argon mixture is introduced into the tubular furnace for 20-40 minutes before calcination. During the calcination process, the hydrogen-argon mixture is continuously introduced to ensure the inertness of the reaction environment and prevent oxidation. The calcination temperature is 600-750℃ and the calcination time is 5-7 hours.

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