A foldable integrated negative electrode based on metal nanosheets and a preparation method thereof

Abstract

The application belongs to the field of energy storage devices, and discloses a foldable integrated negative electrode based on metal nanosheets and a preparation method thereof. The application directly obtains a secondary battery negative electrode through simple operation starting from a macroscopic aluminum foil, and comprises the following steps: (1) mixing graphene oxide, a carbon source and a solvent to prepare a separation reagent; (2) spreading the separation reagent on the surface of the aluminum foil; (3) folding and rolling the aluminum foil with the separation reagent spread thereon; (4) repeatedly repeating steps (1)-(3) until the thickness of the aluminum foil is reduced to the nanoscale; and (5) reducing the material obtained in step (4) to obtain a brick-mud structure aluminum metal nanosheet / carbon / graphene integrated negative electrode. From a microscopic perspective, the negative electrode material has a nanoscale thickness, can effectively relieve / contain volume expansion, and from a macroscopic perspective, the typical "brick-mud" structure configuration has excellent mechanical properties and good anti-expansion performance, so that the overall electrode can maintain structural integrity to a certain extent.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of energy storage devices, and particularly relates to a bendable integrated anode based on metal nanosheets and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries have high energy density, high energy efficiency, long cycle life, no memory effect, fast discharge and other advantages, thus having huge market demand in consumer electronics and electric vehicles, grid peak shaving, energy storage power supply, aerospace and other fields. In order to cope with the challenges of lithium ion batteries in performance, cost and environment, the development of electrode materials with abundant reserves, low cost and excellent electrochemical performance is the current research direction.

[0003] Using metal materials as battery anodes, the alloying / de-alloying process of metal anodes and lithium ions can achieve battery reactions, which can obtain high specific capacity and high energy density. Compared with traditional commercial graphite-based anode materials (372 mAh / g), metal anodes have a very obvious advantage in improving battery capacity. Among various metal anodes, aluminum metal not only has a high theoretical capacity but also has the advantages of abundant raw materials and low price, thus attracting widespread attention. In addition, aluminum has excellent electrical conductivity, which can simultaneously serve as the anode active material and the current collector of the battery, which is beneficial to further improve the energy density of the battery, reduce the mass of the battery and reduce the cost. In summary, aluminum anodes have significant advantages and can reduce the overall cost of the battery, and have great commercialization prospects.

[0004] The development of metal aluminum anodes not only improves the anode capacity but also solves the problem of limited lithium resource reserves; at the same time, the aluminum anode provided by the present application is integrated with the current collector, which can further improve the energy density of the device and effectively improve the safety of the lithium ion battery. The specific energy density of this new type of high-efficiency battery system is higher and the cost is lower. However, when aluminum foil is used as an anode sheet, the following problems exist, which leads to the need for further improvement of its cycle performance: (1) During the alloying process of lithium ions and aluminum metal, a huge volume expansion occurs, which causes the electrode to powder and leads to battery capacity decay; (2) The solid electrolyte layer (SEI film) formed by the reaction of metal aluminum and electrolyte at the interface continuously thickens with time, the interfacial impedance continuously increases, the coulombic efficiency decreases, and the battery capacity decays; (3) Due to the continuous change of the volume of the aluminum metal anode during charging and discharging, the SEI film is unstable, and during the deintercalation of lithium, it is continuously generated, broken and regenerated, consuming metal lithium and electrolyte. In Chinese patent publication CN201811561463.X, metal nanosheets are prepared to solve the problem of volume expansion of metal. However, these metal nanosheets need to use a binder and a current collector, and the active material is easy to fall off from the current collector during bending, leading to capacity decay. Therefore, the development of integrated electrode materials becomes particularly important. SUMMARY

[0005] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the primary purpose of the present application is to provide a preparation method of a foldable integrated negative electrode based on metal nanosheets. The present application aims to solve the problem of volume expansion and pulverization of the alloyed aluminum negative electrode during the charging and discharging process, while obtaining an integrated foldable electrode material.

[0006] Another purpose of the present application is to provide a foldable integrated negative electrode based on metal nanosheets prepared by the above-mentioned method.

[0007] Still another purpose of the present application is to provide the application of the above-mentioned foldable integrated negative electrode based on metal nanosheets in secondary batteries. The present application starts from macroscopic metal foils, and through simple operations, directly obtains a secondary battery negative electrode, which can be directly used for battery assembly.

[0008] The purposes of the present application are achieved by the following solutions:

[0009] A preparation method of a foldable integrated negative electrode based on metal nanosheets, comprising the following steps:

[0010] (1) Isolation reagent preparation: uniformly mix graphene oxide, carbon source, and solvent to prepare an isolation reagent;

[0011] (2) Uniformly spread a layer of isolation reagent on the surface of the metal foil;

[0012] (3) Fold and roll the metal foil with the spread isolation reagent;

[0013] (4) Repeat steps (1) to (3) until the thickness of the metal foil is reduced to the nanoscale;

[0014] (5) Perform high-temperature annealing treatment on the material obtained in step (4) to obtain the desired brick structure metal nanosheet / carbon / graphene integrated negative electrode.

[0015] The graphene oxide in step (1) is obtained by graphite intercalation oxidation, and has a sheet size of 10-50 μm and a thickness of 0.35-10 nm (corresponding to 1-30 layers of graphene);

[0016] The carbon source in step (1) is one or more of polyacrylonitrile (PAN), phenolic resin, polystyrene, polymethyl methacrylate, asphalt, and other organic materials;

[0017] The solvent in step (1) is one or more of N,N-dimethylformamide (DMF), tetrahydrofuran, gasoline, cyclohexane, benzene, and other organic solvents.

[0018] The mass ratio of the graphene oxide, the carbon source and the solvent in step (1) is (0.5-15):(45-80):(15-45);

[0019] The thickness of the metal foil in step (2) is 50-200 mu m, and the thickness of the isolation agent coated on the metal foil is 10-50 mu m; the metal foil is preferably an aluminum foil;

[0020] The rolling in step (3) refers to rolling to half the thickness of the metal foil after folding;

[0021] The high-temperature annealing condition in step (5) refers to a temperature of 400-1000 DEG C, a treatment time of 20-120 minutes, a reducing gas of hydrogen, and an inert gas of argon; the flow rate of the reducing gas is 10-200 sccm, and the flow rate of the inert gas is 100-400 sccm.

[0022] A foldable integrated negative electrode based on metal nanosheets prepared by the above method;

[0023] Application of the foldable integrated negative electrode based on metal nanosheets in a secondary battery;

[0024] The secondary battery comprises a positive electrode current collector, a positive electrode, a brick structure metal nanosheet / carbon / graphene integrated negative electrode, an electrolyte and a separator.

[0025] The positive electrode material comprises a lithium ion embedded positive electrode compound material (such as lithium cobaltate, lithium iron phosphate, nickel cobalt manganese ternary material, etc.), an anion intercalation type layered positive electrode material (such as flake graphite, mesocarbon microbeads, molybdenum disulfide, etc.), an organic positive electrode material (such as a metal and titanium cyanide complex, etc.) which has an oxidation-reduction reaction with anions, etc.

[0026] The electrolyte of the secondary battery comprises a liquid electrolyte, a gel electrolyte and a solid electrolyte.

[0027] Compared with the prior art, the application has the following advantages and beneficial effects:

[0028] (1) The integrated negative electrode based on two-dimensional aluminum nanosheets, pyrolytic carbon, and graphene, constructed by this invention, has the following characteristics: Microscopically, the active material metal nanosheets in the integrated negative electrode of aluminum nanosheet / carbon / graphene in the brick-and-mortar structure have a nanometer-scale thickness (typically less than 5 nm), which can effectively alleviate / accommodate volume expansion. At the same time, its rolled and wrinkled structure in the planar direction can provide external free space for volume expansion, effectively alleviating structural stress. Secondly, graphene oxide itself has an ultra-large specific surface area and good film-forming properties, which can bond the amorphous carbon precursor and tin nanosheets together during the rolling process to form a robust integrated structure; then, through high-temperature carbonization, the graphene oxide is reduced to highly conductive graphene, thereby obtaining an integrated negative electrode material with good conductivity. Thirdly, the amorphous carbon links the metal nanosheets and graphene together in the form of gravel, thereby achieving a robust brick-and-mortar structure. On the other hand, from a macroscopic perspective, the typical "brick-and-mortar" structure configuration constructed from metal nanosheets and carbon / graphene has excellent mechanical properties and good resistance to expansion, thus ensuring that the electrode as a whole maintains structural integrity to a certain extent.

[0029] (2) The integrated negative electrode prepared by this invention has the following characteristics: First, the integrated electrode has a simple process: the lithium-ion battery negative electrode containing metal nanosheets is obtained directly from macroscopic metal foil without the need to mix it with binders, conductive agents, etc. and coat it onto the metal current collector; Second, the composite is uniform: the metal nanosheet / carbon composite material is obtained directly from macroscopic metal foil through rolling and pyrolysis carbonization, which reduces the oxidation of metal nanosheets during the separation process and avoids the problem of uneven composite caused by mixing metal nanosheets with carbon source; Third, the performance is excellent: the synergistic effect of nano-thick aluminum metal and in-situ generated carbon can combine the advantages of both, solve the volume expansion problem structurally and form a stable SEI film, thereby giving the battery good cycle stability; Fourth, the integrated electrode has bendable properties and can be used to manufacture bendable batteries. Attached Figure Description

[0030] Figure 1 A schematic diagram of the integrated negative electrode of aluminum metal nanosheets / carbon / graphene with brick-mud structure: (1) is a brick structure composed of aluminum metal nanosheets, and (2) is a mud structure composed of graphene and carbon. Detailed Implementation

[0031] The present invention will be further described in detail below with reference to embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0032] The reagents used in the examples are commercially available unless otherwise specified.

[0033] Example 1

[0034] The graphene oxide, polyacrylonitrile, DMF were mixed at 5:80:15 to obtain a separation reagent; the separation reagent was uniformly spread on the surface of a metal foil, the thickness of the aluminum foil was 50 μm, and the thickness of the separation reagent coated on the aluminum foil was 20 μm; the aluminum foil with the separation reagent spread thereon was folded in half and rolled to half the thickness of the folded aluminum foil; the spread-folding-rolling step was repeated 12 times; the material rolled 12 times was reduced at 450 degrees Celsius for 60 minutes to obtain an aluminum metal negative electrode foil / current collector, which is the desired brick structure metal nanosheet / carbon / graphene integrated negative electrode.

[0035] A battery was assembled using the aluminum metal negative electrode foil / current collector as the negative electrode, LiPF6 / EC:DEC (4:6) (5 wt.% FEC was added as an additive) as the electrolyte, and lithium iron phosphate as the positive electrode. The electrochemical performance is shown in Tables 1 and 2.

[0036] Examples 2-6

[0037] Examples 2-6 and Example 1 are the same in the steps of the surface modification preparation process of the aluminum metal negative electrode foil / current collector, except that different polymers are selected.

[0038]

[0039] Examples 7-10

[0040] Examples 7-10 and Example 1 are the same in the steps of the surface modification preparation process of the aluminum metal negative electrode foil / current collector, except that the ratio of the separation layer is different.

[0041]

[0042]

[0043] Examples 11-14

[0044] Examples 11-14 and Example 1 are the same in the steps of the surface modification preparation process of the aluminum metal negative electrode foil / current collector, except that the number of rolling times is different.

[0045]

[0046] Example 15

[0047] Example 15 is the same as the surface modification preparation process of the aluminum metal negative electrode foil / current collector in Example 1, except that after rolling, it is not subjected to reduction treatment, but the rolled material is peeled off to obtain aluminum metal nanosheets, and then the nanosheets are mixed with conductive carbon black and PVDF binder at a ratio of 8:1:1 to coat the copper foil current collector to prepare an electrode.

[0048]

[0049] Examples 2-6 are compared with Example 1, and the difference is that different polymers are selected. As can be seen from Table 1, the cycle performance and coulombic efficiency have great differences.

[0050] Examples 7-10 are compared with Example 1, and the difference is that the ratio of the isolation layer is different. From Examples 7-10 and Example 1, it can be seen that the material with a suitable ratio of the isolation layer exhibits more excellent cycle performance.

[0051] Examples 11-14 are different in the number of rolling times. In general, the material with a suitable number of rolling times exhibits more excellent cycle performance.

[0052] Example 15 is a turning experiment and battery cycle performance test using a commonly used coating method to prepare an electrode. In general, the integrated electrode exhibits more excellent cycle performance.

[0053] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement methods, and all shall be included in the protection scope of the present application.

Claims

1. A method for preparing a bendable integrated negative electrode based on metal nanosheets, characterized in that... Includes the following steps: (1) Preparation of isolation reagent: The isolation reagent is prepared by uniformly mixing graphene oxide, carbon source and solvent; (2) Spread a layer of separating agent evenly on the surface of the metal foil; (3) Fold the metal foil with the isolation reagent spread on it in half and roll it; (4) Repeat steps (2) to (3) until the thickness of the metal foil is reduced to the nanometer level; (5) The material obtained in step (4) is subjected to high-temperature annealing to obtain the desired brick-and-mortar structure metal nanosheet / carbon / graphene integrated anode. In step (1), the mass ratio of graphene oxide, carbon source, and solvent is (0.5~15):(45~80):(15~45); the carbon source is one or more of polyacrylonitrile, phenolic resin, polystyrene, polymethyl methacrylate, and asphalt; the thickness of the metal foil is 50-200 μm. The high-temperature annealing conditions mentioned in step (5) refer to a temperature of 400-1000℃ and a processing time of 20-120 minutes.

2. The method for preparing a bendable integrated negative electrode based on metal nanosheets according to claim 1, characterized in that: The graphene oxide mentioned in step (1) is obtained by graphite intercalation oxidation, with a sheet size of 10-50 μm and a thickness of 0.35-10 nm, corresponding to 1-30 graphene layers.

3. The method for preparing a bendable integrated negative electrode based on metal nanosheets according to claim 1, characterized in that: The solvent mentioned in step (1) is one or more of N,N-dimethylformamide, ethanol, toluene, anisole, and THF.

4. The method for preparing a bendable integrated negative electrode based on metal nanosheets according to claim 1, characterized in that: In step (2), the thickness of the isolation reagent coated on the metal foil is 10-50 μm; the metal foil is aluminum foil.

5. The method for preparing a bendable integrated negative electrode based on metal nanosheets according to claim 1, characterized in that: The rolling process mentioned in step (3) refers to rolling to half the thickness of the folded metal foil.

6. The method for preparing a bendable integrated negative electrode based on metal nanosheets according to claim 1, characterized in that: The reducing gas in the high-temperature annealing conditions described in step (5) is hydrogen, and the inert gas is argon; the flow rate of the reducing gas is 10-200 sccm, and the flow rate of the inert gas is 100-400 sccm.

7. A bendable integrated negative electrode based on metal nanosheets prepared by the method according to any one of claims 1 to 6.

8. The application of a flexible integrated negative electrode based on metal nanosheets as described in claim 7 in a secondary battery.

9. The application of a bendable integrated negative electrode based on metal nanosheets as described in claim 8 in a secondary battery, characterized in that: The secondary battery includes a positive current collector, a positive electrode, a brick-and-mortar structure metal nanosheet / carbon / graphene integrated negative electrode, an electrolyte, and a separator.

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