A method for preparing a cellulose-based flexible current collector

Abstract

This invention discloses a method for preparing a cellulose-based flexible current collector. Copper powder and ammonium persulfate are sequentially added to an aqueous solution of an alkali metal hydroxide, and the mixture is stirred thoroughly until the copper powder is completely dissolved. A cellulose film is immersed in the resulting solution, removed, washed with deionized water, and then freeze-dried. The resulting film is then immersed in an aqueous solution of borohydride, removed, washed with deionized water, freeze-dried, and pressed to obtain the cellulose-based flexible current collector. The resulting nanocellulose / copper film contains a large number of uniformly distributed copper metal nanoparticles. The tensile strength of this film can reach 211.4 MPa, and the conductivity can reach 324.7 S / cm. The electrochemical performance of a battery assembled using this film as the current collector and silicon / carbon composite material as the active material is significantly better than that of a battery using commercial copper foil as the current collector.

Description

Technical Field

[0001] This invention relates to new materials technology, and in particular to a method for preparing a flexible current collector based on cellulose. Background Technology

[0002] Wearable devices have received widespread attention in recent years. According to forecasts from Borui Industry Research Institute, the global market size for smart wearable devices will grow at an annual rate of 10.6% from 2023 to 2029, reaching US$171.73 billion in 2029. Generally, wearable devices require energy storage devices. To suit the characteristics of wearables, energy storage devices should be flexible. Therefore, research on flexible energy storage devices is increasing daily. Traditional lithium-ion batteries typically use aluminum foil and copper foil as current collectors for the positive and negative electrodes, respectively; currently, zinc-ion batteries, which are receiving much attention, generally use titanium foil, stainless steel foil, or stainless steel mesh as current collectors for the positive electrode, and zinc foil as both the current collector and active material for the negative electrode. However, these metal foils have problems such as high density and creases when bent or deformed. Furthermore, smooth metal foils with poor flexibility limit battery performance, especially when using electrode materials with large volume changes during charging and discharging (such as silicon anode materials in lithium-ion batteries).

[0003] Due to their advantages such as good flexibility, low density, and high conductivity, carbon-based current collectors such as carbon cloth, graphene foam, and carbon nanotube membranes have been extensively reported in the literature for constructing flexible energy storage devices. However, these carbon-based current collectors have shortcomings such as poor mechanical properties and insufficient conductivity. In invention patent ZL201910268621.0, longitudinally cut wood slabs were used to construct highly conductive and flexible nickel-plated wood chips with a conductivity of 1150 S / m and a tensile strength of 21.9 MPa. However, the above method has the following problems: (1) it is extremely difficult to achieve the low thickness required for practical current collectors in wood slabs; (2) due to the size limitations of the wood itself, it is impossible to produce large-sized current collectors; (3) generally, better performance can only be obtained along the vascular bundle direction. Summary of the Invention

[0004] Purpose of the invention: The technical problem to be solved by the present invention is to provide a method for preparing a flexible current collector based on cellulose with high conductivity and excellent mechanical properties, which is in contrast to the shortcomings of the prior art.

[0005] Technical solution: The preparation method of the cellulose-based flexible current collector of the present invention includes the following steps:

[0006] (1) Add copper powder and ammonium persulfate sequentially to a 3-10M alkali metal hydroxide aqueous solution and stir thoroughly until the copper powder is completely dissolved;

[0007] (2) Immerse the cellulose film in the solution obtained in step (1) for 1 to 30 days, then remove it, wash it with deionized water, and freeze-dry it.

[0008] (3) The film obtained in step (2) is immersed in a 0.2-0.6M borohydride aqueous solution for 10-60 minutes, then removed, washed with deionized water, freeze-dried, and then pressed at 50-200 bar for 0.5-10 minutes to obtain a cellulose-based flexible current collector.

[0009] Further, in step (1), the ratio of copper powder to aqueous hydroxide solution is 1g: 40-160mL. In step (1), the ratio of ammonium persulfate to aqueous hydroxide solution is 0.15g: 40-160mL. In step (1), the alkali metal hydroxide is any one or a combination of LiOH, NaOH, or KOH. In step (2), the cellulose membrane is any one or a combination of nanocellulose membrane, cotton cloth, cellulose filter membrane, and cellulose paper. In step (3), the borohydride salt is any one or a combination of sodium borohydride or lithium borohydride.

[0010] Beneficial Effects: Compared with existing technologies, this invention has the following significant advantages: Through the coordination of copper ions with hydroxyl groups in cellulose, copper ions are fully, uniformly, and firmly loaded onto the interior and surface of the cellulose film, and then transformed in situ into metallic copper nanoparticles during the subsequent reduction process. This results in a cellulose-based flexible current collector that simultaneously possesses high conductivity and excellent mechanical properties. The obtained product overcomes the shortcomings of carbon-based current collectors such as poor mechanical properties and insufficient conductivity, including carbon cloth, graphene foam, and carbon nanotube films, and avoids problems such as difficulty in reducing the thickness of nickel-plated wood chips, size limitations, and performance anisotropy. Attached Figure Description

[0011] Figure 1 A photograph of the nanocellulose / copper film obtained in Example 1;

[0012] Figure 2 The XRD patterns of the nanocellulose film used in Example 1 and the obtained nanocellulose / copper film are shown.

[0013] Figure 3 The image shows a SEM image of the nanocellulose / copper film obtained in Example 1.

[0014] Figure 4 The tensile stress-strain curve of the nanocellulose / copper film obtained in Example 1 is shown.

[0015] Figure 5 The rate performance of the battery assembled using the nanocellulose / copper thin film obtained in Example 1 as the current collector;

[0016] Figure 6 The cycle performance of the battery assembled using the nanocellulose / copper thin film obtained in Example 1 as the current collector is shown.

[0017] Figure 7 The rate performance of the battery assembled using commercial copper foil as the current collector in Comparative Example 1 is shown.

[0018] Figure 8 The cycle performance of the battery assembled using commercial copper foil as the current collector in Comparative Example 1 is shown. Detailed Implementation

[0019] Example 1

[0020] (1) Add 2g of copper powder and 0.3g of ammonium persulfate to 160mL of 6.25M sodium hydroxide aqueous solution and stir thoroughly until the copper powder is completely dissolved;

[0021] (2) The nanocellulose film was immersed in the solution obtained in step (1), and after 14 days it was taken out, washed with deionized water, and then freeze-dried.

[0022] (3) The film obtained in step (2) is immersed in a 0.42M lithium borohydride aqueous solution. After 30 minutes, it is taken out, washed with deionized water, freeze-dried, and then pressed at 100 bar for 1 minute to obtain a flexible conductive nanocellulose / copper film.

[0023] Battery Assembly and Testing: A silicon / carbon composite material (Kelode), lithium-ionized polyacrylic acid binder, and carbon nanotube conductive agent were coated onto the surface of the nanocellulose / copper film obtained in this embodiment at a mass ratio of 70:15:15. The film was then vacuum-dried at 80°C for 12 hours and cut into 12mm diameter discs as working electrodes. Using these working electrodes, along with a lithium foil counter electrode and a 1M lithium hexafluorophosphate electrolyte (solvent consisting of ethyl carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 1:1:1), a coin cell was assembled. Constant current charge-discharge testing was performed using a LAND CT2001A battery testing system, with a voltage range of 0.005-1.5V.

[0024] Figure 1 The image shows a photograph of the nanocellulose / copper film obtained in this embodiment, demonstrating the film's flexibility.

[0025] Figure 2 The XRD patterns of the nanocellulose film used in this embodiment and the obtained nanocellulose / copper film show that a large amount of copper metal was successfully deposited on the nanocellulose film.

[0026] Figure 3The SEM image of the nanocellulose / copper film obtained in this embodiment shows that the copper metal particles deposited on the nanocellulose film are very uniform and the particle size is in the nanometer range. Copper ions can coordinate with the hydroxyl groups in cellulose, thereby being fully, uniformly and firmly loaded into the interior and surface of the cellulose film, and then transformed in situ into metallic copper nanoparticles during the subsequent reduction process.

[0027] Figure 4 The tensile stress-strain curve of the nanocellulose / copper film obtained in this embodiment shows that its tensile strength and Young's modulus can reach 211.4 MPa and 7.4 GPa, respectively. The above strength is significantly higher than that of nickel-plated wood chips (21.9 MPa, along the vascular bundle direction, ZL201910268621.0), about 44 times that of carbon cloth (WOS1002, CeTech, Taiwan, China), and more than 1000 times that of graphene foam (Adv. Funct. Mater., 2015, 25: 3916-3924).

[0028] The conductivity of the nanocellulose / copper film obtained in this embodiment is 324.7 S / cm, which is more than 28 times that of nickel-plated wood chips and graphene foam, and more than 17 times that of carbon cloth.

[0029] Figure 5 The rate performance of the battery assembled using the nanocellulose / copper film obtained in this embodiment as the current collector is shown. The discharge specific capacity of the battery in the last cycle at current densities of 0.2, 0.5, 1, 2, and 5 A / g were 1087.9, 998.7, 887.8, 799.6, and 725.2 mAh / g, respectively. When the current density recovered to 0.2 A / g, the specific capacity of the battery recovered to 1061.6 mAh / g in the 55th cycle. The above electrochemical performance is significantly better than that of the battery in Comparative Example 1 that used commercial copper foil as the current collector.

[0030] Figure 6 The cycling performance of the battery assembled using the nanocellulose / copper film obtained in this embodiment as the current collector is shown. After 1000 cycles at a current density of 1 A / g, the battery has a discharge specific capacity of 693.1 mAh / g and a capacity retention rate (relative to the first cycle) of 77.1%, which is significantly higher than that of the battery in Comparative Example 1 that uses commercial copper foil as the current collector.

[0031] Example 2

[0032] (1) Add 2g of copper powder and 0.35g of ammonium persulfate to 100mL of 10M sodium hydroxide aqueous solution and stir thoroughly until the copper powder is completely dissolved;

[0033] (2) Soak the cotton cloth in the solution obtained in step (1), take it out after 7 days, wash it with deionized water, and then freeze dry it;

[0034] (3) The film obtained in step (2) is immersed in a 0.26M lithium borohydride aqueous solution. After 30 minutes, it is taken out, washed with deionized water, freeze-dried, and then pressed at 100 bar for 1 minute to obtain a flexible conductive cotton / copper film.

[0035] The conductivity of the cotton / copper film obtained in this embodiment is 262.2 S / cm, as shown by IV testing.

[0036] Example 3

[0037] (1) Add 2g of copper powder and 0.3g of ammonium persulfate to 320mL of 3M potassium hydroxide aqueous solution and stir thoroughly until the copper powder is completely dissolved;

[0038] (2) Immerse the cellulose filter membrane in the solution obtained in step (1), take it out after 3 days, wash it with deionized water, and then freeze-dry it;

[0039] (3) The membrane obtained in step (2) is immersed in a 0.6M sodium borohydride aqueous solution. After 10 minutes, it is taken out, washed with deionized water, freeze-dried, and then pressed at 50 bar for 10 minutes to obtain a flexible conductive cellulose filter paper / copper membrane.

[0040] Comparative Example 1

[0041] Commercial copper foil was used to replace the nanocellulose / copper film obtained in Example 1 as the current collector, and the cells were assembled into batteries under the same conditions for testing.

[0042] Figure 7 This example demonstrates the rate performance of the battery assembled using commercial copper foil as the current collector. The specific discharge capacities of the battery in the last cycle at current densities of 0.2, 0.5, 1, 2, and 5 A / g were 1073, 960.9, 837.4, 736.2, and 593.5 mAh / g, respectively. When the current density recovered to 0.2 A / g, the specific capacity of the battery recovered to 1033.7 mAh / g in the 55th cycle.

[0043] Figure 8 This embodiment presents the cycle performance of the battery assembled using commercial copper foil as the current collector. After 1000 cycles at a current density of 1 A / g, the battery has a discharge specific capacity of 429.7 mAh / g and a capacity retention (relative to the first cycle) of 47.8%.

Claims

1. A method of making a flexible current collector based on cellulose, characterized in that, The process comprises the following steps: (1) Copper powder and ammonium persulfate are sequentially added to 3-10 M aqueous alkali hydroxide solution, and stirred thoroughly until the copper powder is completely dissolved; (2) Cellulose film is soaked in the solution obtained in step (1), taken out after 1-30 days, washed with deionized water, and then freeze-dried; (3) The film obtained in step (2) is soaked in 0.2-0.6 M aqueous borohydride salt solution, taken out after 10-60 min, washed with deionized water, freeze-dried, and then pressed at 50-200 bar for 0.5-10 min to obtain a cellulose-based flexible current collector.

2. The method of making a flexible cellulose-based current collector according to claim 1, characterized in that: The ratio of copper powder to aqueous hydroxide solution in step (1) is 1 g: 40-160 mL.

3. The method of making a flexible cellulose-based current collector of claim 1, wherein: The ratio of ammonium persulfate to aqueous hydroxide solution in step (1) is 0.15 g: 40-160 mL.

4. The method of making a cellulose-based flexible current collector of claim 1, wherein: The alkali hydroxide in step (1) is any one or a combination of multiple of LiOH, NaOH or KOH.

5. The method of making a cellulose-based flexible current collector of claim 1, wherein: The borohydride salt in step (3) is any one or a combination of multiple of sodium borohydride or lithium borohydride.

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