A nitrogen-doped carbon nanosheet applied to a lithium-ion battery and a preparation method thereof

Abstract

The present invention provides a nitrogen-doped carbon nanosheet applied to a lithium-ion battery and a preparation method thereof. The carbon nanosheet has the characteristics of being ultrathin (thickness ≤ 5 nm), rich in porous properties, and simple preparation process. The present invention also discloses a preparation method of this carbon nanosheet: firstly, synthesize needle-shaped metal-organic framework material (ZIF-L), disperse ZIF-L in a saturated solution of metal chloride (MCl x ), and completely volatilize the solvent by heating to obtain a white powder (ZIF-L@MCl x ), place ZIF-L@MCl x in a tubular furnace filled with inert gas for high-temperature pyrolysis, and the obtained black product is washed with deionized water and then filtered to obtain the nitrogen-doped carbon nanosheet. The nitrogen-doped carbon nanosheet provided by the present invention has advantages such as high initial Coulomb efficiency, high discharge capacity, and good rate performance as a negative electrode material of a lithium-ion battery.

Description

Technical Field

[0001] The present invention relates to a nitrogen-doped carbon nanosheet applied to a lithium-ion battery and a preparation method thereof, belonging to the technical field of lithium-ion battery science and technology. Background Art

[0002] Lithium-ion batteries have the characteristics of high energy density, pollution-free and low emissions, and are widely used in fields such as portable electronic devices, electric vehicles, and energy storage systems. They are one of the important means for replacing traditional fossil energy and achieving sustainable development. However, lithium-ion batteries face a series of challenges and safety hazards during use, such as capacity attenuation, cycle life, charge and discharge rate, and safety. The specific reasons are as follows: During the charge and discharge process, the lithium-ion negative electrode material will undergo lithium-ion insertion and deinsertion reactions, resulting in volume expansion and contraction of the material. This volume change will cause mechanical stress on the negative electrode material, which may lead to loose contact between particles, structural damage, and exfoliation of powder particles, resulting in a decrease in battery performance and cycle life. Some currently used lithium-ion negative electrode materials, such as graphite, have relatively low specific energy density. This means that the number of lithium ions that can be stored per unit mass of the material is limited, restricting the energy storage capacity of the battery. Therefore, it is urgent to develop new and higher-performance lithium-ion battery negative electrode materials.

[0003] Two-dimensional nanomaterials have advantages such as a high specific surface area, short ion diffusion path, and excellent mechanical stability. They can increase the active surface area, improve the ion diffusion rate, and reduce the mechanical stress caused by volume expansion. By exploring and developing two-dimensional nanomaterials as lithium-ion battery negative electrode materials, the energy density, cycle life, and charge and discharge performance of the battery can be effectively improved. In addition, two-dimensional nanomaterials have adjustable surface chemical properties, and interface engineering can be achieved through surface modification to improve the interface stability and cycle stability of the battery. Therefore, further research on the application of two-dimensional nanomaterials in lithium-ion battery negative electrode materials is of great significance for promoting the development of battery technology, meeting energy demands, and achieving sustainable development.

[0004] Chinese Patent CN 106927447 A discloses a nitrogen-doped carbon nanosheet and a preparation method thereof. Chitosan or its derivative and a solvent are mixed and stirred, and then freeze-dried to obtain a solid aerogel. The solid aerogel is calcined at a high temperature to obtain a solid powder, which is ball-milled to finally obtain a nitrogen-doped carbon nanosheet. Through this method, a nitrogen-doped carbon nanosheet with a stable nitrogen doping amount and a specific surface area greater than 100 m 2N-doped carbon nanosheets per gram. However, during the implementation of this patent, the two-dimensional nanosheets prepared by the ball milling method have a complex process flow and uncontrollable nanosheet thickness. Chinese Patent CN 110416548 A discloses a preparation method and application of a two-dimensional structure of nitrogen-doped porous carbon. By directly pyrolyzing two-dimensional ZIF-8 nanosheets and performing pickling, nitrogen-doped porous carbon with a two-dimensional structure is obtained. The thickness of the nanosheets prepared by this patent is 10 - 20 nm, and from the FESEM images provided by the patent, independent large-sized nanosheets are not formed, and the cross-linking between the sheets is serious, which hinders its charge and discharge performance as an anode material for lithium-ion batteries.

[0005] The present invention develops a two-dimensional nanomaterial with adjustable surface chemical properties and realizes interface engineering through surface modification to improve the interface stability and cycling stability of the battery. We prepared ZIF-L@MCl x precursor as the precursor-derived nitrogen-doped carbon nanosheets. The preparation process of this catalyst is simple, with low cost, and has a high specific capacity, which is conducive to promoting the practical application of lithium-ion batteries. Summary of the Invention

[0006] The object of the present invention is to provide a nitrogen-doped carbon nanosheet applied to lithium-ion batteries and its preparation method. Using the surface modification technology of two-dimensional metal-organic nanomaterials, first synthesize needle-like metal-organic framework material (ZIF-L), disperse ZIF-L in a saturated solution of metal chloride (MCl x , where M refers to one of the metals Na, Ca, Mg, Li, K, and x refers to the valence state of the M metal, and the value range of x is 1 - 2), and completely volatilize the solvent by heating to obtain a white powder (ZIF-L@MCl x ), place ZIF-L@MCl x in a tubular furnace filled with inert gas for high-temperature pyrolysis, and the obtained black product is filtered after being washed with deionized water to obtain nitrogen-doped carbon nanosheets.

[0007] The object of the present invention is achieved through the following solutions:

[0008] A preparation method of nitrogen-doped carbon nanosheets, comprising the following steps:

[0009] (1) Preparation of needle-like ZIF-L: Dissolve appropriate amounts of 2-methylimidazole and zinc salt in deionized water respectively, mix the above two solutions under stirring at room temperature, stir at a low speed for a certain time and then filter through a sintered glass funnel, and further dry in a vacuum drying oven. The obtained white powder is needle-like ZIF-L;

[0010] (2) ZIF-L@MCl xPreparation of precursor: Take ZIF-L and an appropriate amount of metal chloride MCl x Disperse them in deionized water and ultrasonically form a suspension; place the above liquid in an oil bath and heat it. After the metal chloride is completely recrystallized, use a spatula to collect and grind the solid white crystals to obtain ZIF-L@MCl x precursor;

[0011] (3) Preparation of nitrogen-doped carbon nanosheets: Place the ZIF-L@MCl x precursor in a high-temperature tube furnace filled with inert gas, set a certain heating time and heating temperature, and wash the obtained black powder with deionized water and then filter it. The final obtained product is the nitrogen-doped carbon nanosheets.

[0012] Furthermore, in step (1), the zinc salt is one or more of zinc chloride (ZnCl2), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), zinc sulfate (ZnSO4), zinc oxalate (ZnC2O4), zinc acetate (Zn(CH3COO)2), and zinc carbonate (ZnCO3).

[0013] Furthermore, in step (1), the molar ratio of 2-methylimidazole to the zinc salt is 2-10:1. Further preferably, the molar ratio of 2-methylimidazole to the zinc salt is 4-8:1.

[0014] Furthermore, in step (1), the stirring time is 3-12 h, and the stirring temperature is room temperature.

[0015] Furthermore, in step (1), the vacuum drying time is 12-48 h, and the vacuum drying temperature is 60-80 °C.

[0016] Furthermore, in step (2), the metal chloride is one or more of sodium chloride (NaCl), calcium chloride (CaCl2), magnesium chloride (MgCl2), lithium chloride (LiCl), and potassium chloride (KCl).

[0017] Furthermore, in step (2), the mass ratio of the metal chloride to ZIF-L is 20-80:1.

[0018] Furthermore, in step (2), the ultrasonic time is 1-5 h.

[0019] Furthermore, in step (2), the heating temperature is 50-90 °C.

[0020] Furthermore, in step (3), the inert gas is one of Ar, He, and N2.

[0021] Furthermore, in step (3), the heating time is 1-5 h, and the heating temperature is 900-1200 °C.

[0022] The preparation scheme of the present invention is that the nitrogen-doped carbon nanosheets have an ultra-thin feature with a thickness ≤ 5 nm.

[0023] In the preparation scheme of the present invention, the metal chloride plays a role in chemical exfoliation during the high-temperature carbonization of ZIF-L.

[0024] In the preparation scheme of the present invention, the temperature program setting of the high-temperature carbonization directly affects the exfoliation effect of ZIF-L. A high temperature of 900 - 1200 °C is beneficial to the pyrolysis of ZIF-L, and the generated small molecules serve as raw materials for the formation of nitrogen-doped carbon nanosheets.

[0025] The preparation scheme of the present invention is that a kind of nitrogen-doped carbon nanosheet is ZIF-L@MCl x The precursor is prepared by pyrolysis.

[0026] The present invention also provides the application of the described nitrogen-doped carbon nanosheets as the anode material of a lithium-ion battery.

[0027] The electrochemical test is mainly carried out by assembling a CR-2032 type button battery lithium-ion half-cell with the nitrogen-doped carbon nanosheets as the working electrode (the separator is Whatman GF / D glass fiber separator, and the electrolyte is 1 M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate (EC) and dimethyl carbonate (DEC) with a volume ratio of 1:1), and then carrying out relevant electrochemical tests. The assembled battery is tested after standing for 24 hours. The constant current charge and discharge cycle characteristics of the battery are tested with a multi-channel battery test system (Neware BTS-610), and the voltage test window is 0.02 - 3.0 V. The cyclic voltammogram curve of the battery is tested with an electrochemical workstation (Chenhua CHI-660C / D), the tested voltage scanning range is 0.02 - 3.0 V, and the scanning speed is 0.1 mV s -1 .

[0028] The technical scheme of the present invention has the following advantages and beneficial effects:

[0029] (1) Currently, the preparation of two-dimensional nanomaterials on the market is mainly through conventional mechanical exfoliation methods. Usually, transparent tape is bonded to the surface of the material, and an intermediate material is introduced to enhance the adhesion between the substrate and the target material, so as to exfoliate large-sized two-dimensional nanomaterials. However, this method has great limitations and is only applicable to interlayer materials based on van der Waals forces, and is ineffective for materials with strong electron coupling effects. The nitrogen-doped carbon nanosheets prepared by the present invention are different from the conventional mechanical exfoliation method. They are formed by chemical exfoliation reactions of metal chlorides during pyrolysis, and have a good breaking effect on strong intermolecular or ionic bonds. Through this method, two-dimensional metal framework materials can be effectively exfoliated into nanosheets, and there is no need to remove the intermediate material through complex means during the treatment process, which greatly improves the production efficiency and economy of two-dimensional materials.

[0030] (2) The nitrogen-doped carbon nanosheets prepared by the present invention as the anode material of lithium-ion batteries have the advantages of high initial Coulomb efficiency, high discharge capacity and good rate performance in application. Through charge-discharge tests, it can be obtained that the specific capacity of this material is close to 400 mAh / g, and after 20 cycles, the charge-discharge efficiency can still remain 100%, exceeding the current commercial graphite carbon materials.

[0031] (3) The equipment for the preparation method of nitrogen-doped carbon nanosheets proposed by the present invention is simple, the reaction conditions are mild, the process is easy to control, there is no need to add toxic and harmful reagents, and it is convenient for the industrialization and engineering application of the catalyst. Description of the Drawings

[0032] Figure 1 Scanning electron microscope (SEM) image of ZIF-L prepared in Example 1.

[0033] Figure 2 SEM image of the nitrogen-doped carbon nanosheets prepared in Example 1.

[0034] Figure 3 Transmission electron microscope (TEM) image of the nitrogen-doped carbon nanosheets prepared in Example 1.

[0035] Figure 4 X-ray diffraction (XRD) image of ZIF-L prepared in Example 1.

[0036] Figure 5 XRD image of the nitrogen-doped carbon nanosheets prepared in Example 1.

[0037] Figure 6 X-ray photoelectron spectroscopy (XPS) image of the nitrogen-doped carbon nanosheets prepared in Example 1.

[0038] Figure 7Thickness data of the nitrogen-doped carbon nanosheets prepared in Example 1.

[0039] Figure 8 Half-cell performance graph of the lithium-ion battery obtained with the nitrogen-doped carbon nanosheets prepared in Example 1 as the anode material. Detailed implementation manners

[0040] Example 1

[0041] A preparation method of nitrogen-doped carbon nanosheets applied to lithium-ion batteries, comprising the following steps:

[0042] (1) Preparation of needle-like ZIF-L: Dissolve 2-methylimidazole (16 mmol, 1.314 g) and zinc nitrate hexahydrate (2 mmol, 0.595 g) in 40 ml of deionized water respectively. Mix the above two solutions under stirring at room temperature, stir at a low speed of 400 rpm / min for 8 h, then filter through a sintered glass funnel, and further dry in a vacuum drying oven at 80 °C for 12 h. The obtained white powder is needle-like ZIF-L.

[0043] (2) Preparation of ZIF-L@NaCl precursor: Take 1 g of ZIF-L and 20 g of NaCl, disperse them in 50 ml of deionized water, and ultrasonicate for 1 h to form a suspension; heat the above liquid in an oil bath to 80 °C. After the metal chloride is completely recrystallized, collect and grind the solid white crystalline substance with a spatula to obtain the ZIF-L@NaCl precursor.

[0044] (3) Preparation of nitrogen-doped carbon nanosheets: Place 5 g of the ZIF-L@NaCl precursor in a high-temperature tube furnace filled with argon, set a certain heating time (4 h) and heating temperature (1200 °C). The obtained black powder is washed with deionized water and then filtered. The final obtained product is the nitrogen-doped carbon nanosheets.

[0045] Figure 1 Scanning electron microscope (SEM) image of the ZIF-L prepared in Example 1. This material is needle-like, with a thickness of about 20 - 50 nm.

[0046] Figure 2 and Figure 3 SEM image and TEM image of the nitrogen-doped carbon nanosheets prepared in Example 1 respectively. The nitrogen-doped carbon nanosheets have an irregular flaky feature, a thickness of about 5 nm, and wrinkles are distributed on the surface.

[0047] Figure 4 X-ray diffraction (XRD) pattern of the ZIF-L prepared in Example 1, showing highly regular crystal structure characteristics.

[0048] Figure 5XRD pattern of the nitrogen-doped carbon nanosheets prepared in Example 1. It can be seen that the material is mainly an amorphous carbon structure.

[0049] Figure 6 X-ray photoelectron spectroscopy (XPS) pattern of the nitrogen-doped carbon nanosheets prepared in Example 1. It can be seen that the main components of the material are carbon and nitrogen elements.

[0050] Figure 7 Thickness data of the nitrogen-doped carbon nanosheets prepared in Example 1. The average thickness is about 5 nm.

[0051] Application of the nitrogen-doped carbon nanosheets prepared in Example 1 as the anode material of a lithium-ion battery. The specific operation steps are as follows:

[0052] Assemble a CR-2032 type button lithium-ion half-cell with the nitrogen-doped carbon nanosheets as the working electrode (the separator is Whatman GF / D glass fiber separator, and the electrolyte is 1 M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate (EC) and dimethyl carbonate (DEC) with a volume ratio of 1:1), and then conduct relevant electrochemical tests. The assembled battery is tested after standing for 24 hours. The constant current charge-discharge cycle characteristics of the battery are tested using a multi-channel battery test system (Neware BTS-610), and the voltage test window is 0.02 - 3.0 V. The cyclic voltammogram of the battery is tested using an electrochemical workstation (Chenhua CHI-660C / D), the test voltage scan range is 0.02 - 3.0 V, and the scan rate is 0.1 mV s -1 。

[0053] Figure 8 Half-cell performance graph of the lithium-ion battery obtained with the nitrogen-doped carbon nanosheets prepared in Example 1 as the anode material. It can be seen that the specific capacity per gram of the nitrogen-doped carbon nanosheets reaches 385 mAh / g, and the charge-discharge efficiency can still be close to 100% after 20 cycles.

[0054] Example 2

[0055] A preparation method of nitrogen-doped carbon nanosheets applied to lithium-ion batteries, including the following steps:

[0056] (1) Preparation of needle-like ZIF-L: Dissolve 2-methylimidazole (16 mmol, 1.314 g) and zinc nitrate hexahydrate (2 mmol, 0.595 g) in 40 ml of deionized water respectively. Mix the above two solutions under stirring at room temperature, stir at a low speed of 400 rpm / min for 8 h, then filter through a sintered glass funnel, and further dry in a vacuum drying oven at 80 °C for 12 h. The obtained white powder is needle-like ZIF-L.

[0057] (2) Preparation of ZIF-L@MgCl2 precursor: Take 1 g of ZIF-L and 20 g of MgCl2 and disperse them in 50 ml of deionized water, and ultrasonicate for 1 h to form a suspension; place the above liquid in an oil bath and heat it to 80 °C. After the metal chloride is completely recrystallized, use a spatula to collect and grind the solid white crystals to obtain the ZIF-L@MgCl2 precursor.

[0058] (3) Preparation of nitrogen-doped carbon nanosheets: Place 5 g of the ZIF-L@MgCl2 precursor in a high-temperature tube furnace filled with argon, set a certain heating time (4 h) and heating temperature (1000 °C). The obtained black powder is washed with deionized water and then filtered. The final product obtained is the nitrogen-doped carbon nanosheets.

[0059] Example 3

[0060] A preparation method of nitrogen-doped carbon nanosheets applied to lithium-ion batteries, comprising the following steps:

[0061] (1) Preparation of needle-like ZIF-L: Dissolve 2-methylimidazole (16 mmol, 1.314 g) and zinc nitrate hexahydrate (2 mmol, 0.595 g) in 40 ml of deionized water respectively. Mix the above two solutions under stirring at room temperature, stir at a low speed of 400 rpm / min for 8 h, and then filter through a sintered glass funnel. Further dry in a vacuum drying oven at 80 °C for 12 h. The obtained white powder is the needle-like ZIF-L.

[0062] (2) Preparation of ZIF-L@NaCl / LiCl precursor: Take 1 g of ZIF-L, 20 g of NaCl and 20 g of LiCl and disperse them in 50 ml of deionized water, and ultrasonicate for 1 h to form a suspension; place the above liquid in an oil bath and heat it to 80 °C. After the metal chloride is completely recrystallized, use a spatula to collect and grind the solid white crystals to obtain the ZIF-L@NaCl / LiCl precursor.

[0063] (3) Preparation of nitrogen-doped carbon nanosheets: Place 5 g of the ZIF-L@NaCl / LiCl precursor in a high-temperature tube furnace filled with inert gas, set a certain heating time (4 h) and heating temperature (1000 °C). The obtained black powder is washed with deionized water and then filtered. The final product obtained is the nitrogen-doped carbon nanosheets.

[0064] The above embodiments are only the preferred embodiments of the present invention, which are only used to explain the present invention, rather than limiting the present invention. Any changes, substitutions, modifications, etc. made by those skilled in the art without departing from the spirit and essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A preparation method of nitrogen-doped carbon nanosheets, characterized in that, It includes the following steps: (1) Preparation of needle-like ZIF-L: Dissolve 2-methylimidazole and zinc salt in deionized water respectively, mix the above two solutions under stirring at room temperature, after stirring and reacting, filter by a sintered glass funnel, and dry under vacuum. The obtained white powder is needle-like ZIF-L; (2)ZIF-L@MCl x Precursor preparation: Take ZIF-L and metal chloride MCl x Disperse them in deionized water and ultrasonically form a suspension; Heat the above liquid in an oil bath. After the metal chloride is completely recrystallized, use a spatula to collect and grind the solid white crystals to obtain ZIF-L@MCl x Precursor; In metal chloride MCl x M refers to one of the metals Na, Ca, Mg, Li, K, and x refers to the valence state corresponding to the M metal, and the value range of x is 1 to 2; (3) Preparation of nitrogen-doped carbon nanosheets: ZIF-L@MCl x The precursor was placed in a high-temperature tube furnace filled with inert gas and heated. The obtained black powder was washed with deionized water and then filtered. The final product was the nitrogen-doped carbon nanosheets.

2. The preparation method of the nitrogen-doped carbon nanosheets according to claim 1, wherein, In step (1), the zinc salt is one or more of zinc chloride, zinc nitrate hexahydrate, zinc sulfate, zinc oxalate, zinc acetate, and zinc carbonate; the molar ratio of 2-methylimidazole to zinc salt is 2-10:

1.

3. The preparation method of the nitrogen-doped carbon nanosheets according to claim 1, characterized in that, In step (1), the stirring time is 3-12 h, and the stirring temperature is room temperature; the vacuum drying time is 12-48 h, and the vacuum drying temperature is 60-80 °C.

4. The preparation method of the nitrogen-doped carbon nanosheets according to claim 1, characterized in that, In step (2), the metal chloride is one or more of sodium chloride, calcium chloride, magnesium chloride, lithium chloride, and potassium chloride, and MCl x represents the corresponding metal chloride.

5. The preparation method of the nitrogen-doped carbon nanosheets according to claim 1, wherein, In step (2), the mass ratio of metal chloride to ZIF-L is 20-80:

1.

6. The preparation method of the nitrogen-doped carbon nanosheets according to claim 1, characterized in that, In step (2), the ultrasonic time is 1-5 h; the heating temperature is 50-90 °C.

7. The preparation method of the nitrogen-doped carbon nanosheets according to claim 1, wherein, In step (3), the inert gas is one of Ar, He, and N2.

8. The preparation method of the nitrogen-doped carbon nanosheets according to claim 1, characterized in that, In step (3), the heating time is 1-5 h, and the heating temperature is 900-1200 °C.

9. A nitrogen-doped carbon nanosheet prepared by the preparation method according to any one of claims 1-8, characterized in that, The nitrogen-doped carbon nanosheets have an ultrathin feature with a thickness ≤ 5 nm.

10. Application of the nitrogen-doped carbon nanosheets as claimed in claim 9 as a battery anode material in a lithium-ion battery.

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