Negative current collector of negative-electrode-free lithium battery and preparation method of negative current collector

Abstract

The preparation method comprises the following steps: mixing graphite with an intercalator and an oxidizing agent to generate an expanded graphite block, and carrying out impurity removal, reduction, slicing and pre-lithiation to obtain the negative current collector. Graphite is selected as a raw material, compared with a metal foil current collector, the lithium affinity of the graphite is high, and generation of dendritic crystals can be effectively inhibited. The expanded graphite block after impurity removal is subjected to reducing gas heat treatment to obtain the graphite elastomer with a three-dimensional structure, and the graphite elastomer has a sponge-like structure, has elasticity, can bear the volume change of battery metal lithium in the repeated deposition process and can maintain the cycle stability of the battery. And the three-dimensional space structure can greatly enhance the metal lithium bearing capability by adjusting the thickness of the current collector. And moreover, the graphite slices are pre-lithiated to form an SEI film, so that the consumption of the graphite current collector on lithium ions is reduced, and the service life and the safety of the non-cathode lithium ion battery can be prolonged and improved.

Description

Technical Field

[0001] The present invention relates to the technology of lithium battery negative electrode field, in particular to a negative electrode current collector of a negative electrode-free lithium battery and a preparation method thereof. Background Art

[0002] Lithium-ion battery is a secondary battery that mainly relies on the movement of lithium ions between the positive electrode and the negative electrode to work. + Insertion and deinsertion back and forth between the two electrodes: During charging, Li + Deintercalation from the positive electrode, through the electrolyte embedded in the negative electrode, the negative electrode is in a lithium-rich state; the opposite is true during discharge. With the development of new energy vehicles, lithium-ion batteries are more and more used in new energy vehicles. At the same time, consumers have higher and higher requirements for the cruising range of new energy vehicles. The cruising range of pure electric vehicles is directly related to the energy density of the power battery they carry. However, traditional lithium-ion batteries have been difficult to meet the demand for lithium battery energy density in the field of new energy vehicles. As a result, the negative-electrode-free lithium battery, which is famous for its high energy density, has once again attracted widespread attention from researchers. Compared with general lithium batteries, the negative-electrode-free lithium battery can achieve the lowest total mass and volume, maximizing the energy density, and the negative-electrode-free lithium battery is mostly made by using metal foil as the current collector. During the battery charging process, lithium ions are removed from the positive electrode and deposited on the surface of the metal foil to form metallic lithium. During the discharge process, these metallic lithiums are formed into lithium ions and embedded back into the positive electrode.

[0003] The negative electrode current collector design of existing negative electrode-free lithium batteries tends to be flat, and the metal foil does not have a strong lithium affinity. When the thickness of lithium deposited on the surface of the negative electrode current collector increases, the current collector cannot effectively inhibit the formation of dendrites; furthermore, the repeated deposition of metal lithium during the battery charging and discharging process causes the battery volume to change, affecting the battery cycle stability. Therefore, it is necessary to propose a new solution to solve the above problems. Summary of the invention

[0004] In view of this, the present invention aims at the deficiencies in the prior art, and its main purpose is to provide a negative electrode current collector for a negative electrode-free lithium battery and a preparation method thereof, which can effectively solve the problems that the negative electrode current collector of the existing negative electrode-free lithium battery does not have a strong lithium affinity and cannot effectively inhibit the formation of dendrites, and the repeated deposition of metallic lithium during the charge and discharge process of the battery causes the battery volume change, affecting the battery cycle stability.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A method for preparing a negative electrode current collector of a negative electrode-free lithium battery comprises the following steps:

[0007] (1) Mix graphite, an intercalating agent, and an oxidizing agent evenly by stirring. The mass ratio of the graphite to the intercalating agent is 1:(8 - 20), and the volume ratio of the intercalating agent to the oxidizing agent is 1:(0.05 - 0.3). Let it stand for 30 - 50 min to obtain an expanded graphite block;

[0008] (2) Place the expanded graphite block obtained in step (1) in a protective atmosphere for heating. The heating temperature is 400 - 600 °C, and the heating time is 10 - 30 min to obtain a purified expanded graphite block, removing residual sulfuric acid and water;

[0009] (3) Transfer the purified expanded graphite block obtained in step (2) to an atmosphere of a mixed gas for heat treatment. This mixed gas is composed of an inert gas and a reducing gas, and the volume ratio of the inert gas to the reducing gas is (70 - 90):(10 - 30). The heat treatment temperature is 800 - 1200 °C, and the heat treatment time is 30 - 60 min to obtain a three-dimensional structured graphite elastomer, and reduce the oxidized part in the purified expanded graphite block, enhancing electrical conductivity and mechanical strength;

[0010] (4) Perform slicing on the three-dimensional structured graphite elastomer obtained in step (3) to obtain graphite slices;

[0011] (5) Immerse the graphite slices obtained in step (4), biphenyl, and lithium metal together in 2-methyltetrahydrofuran. The mass ratio of the graphite slices to biphenyl and lithium metal is 1:(1 - 10):(0.8 - 5). After soaking for 3 - 10 min, wash with 2-methyltetrahydrofuran and dry to obtain a negative electrode current collector.

[0012] As a preferred embodiment, in step (1), the graphite is flake graphite.

[0013] As a preferred embodiment, in step (1), the intercalating agent is concentrated sulfuric acid.

[0014] As a preferred embodiment, in step (1), the oxidizing agent is hydrogen peroxide.

[0015] As a preferred embodiment, in step (1), the expanded graphite block is assembled by winding multiple expanded graphites.

[0016] As a preferred embodiment, in step (2), the protective atmosphere is an argon atmosphere.

[0017] As a preferred embodiment, in step (3), the inert gas is argon.

[0018] As a preferred embodiment, in step (3), the reducing gas is hydrogen.

[0019] As a preferred solution, in the step (4), the thickness of the graphite slice is 1-5 mm.

[0020] A negative electrode current collector of a lithium metal-free battery is prepared by the method for preparing a negative electrode current collector of a lithium metal-free battery described above.

[0021] Compared with the prior art, the present invention has obvious advantages and beneficial effects. Specifically, it can be seen from the above technical solutions that:

[0022] By using graphite as the raw material, compared with the metal foil current collector, graphite has a strong lithium affinity and can effectively inhibit the generation of dendrites. The expanded graphite block after impurity removal is subjected to heat treatment with a reducing gas to obtain a three-dimensional graphite elastomer, which has a sponge-like structure, is elastic, can withstand the volume change during the repeated deposition of metallic lithium in the battery, and maintains the cycle stability of the battery. Moreover, the three-dimensional space structure can significantly enhance the ability to carry metallic lithium by adjusting the thickness of the current collector. In addition, the graphite slice forms a SEI film after prelithiation, reducing the consumption of lithium ions by the graphite current collector itself, and can enhance the lifespan and safety of the lithium metal-free battery.

[0023] To more clearly illustrate the structural features and functions of the present invention, the present invention will be described in detail below with reference to the drawings and specific embodiments: Description of the Drawings

[0024] Figure 1 is the AC impedance spectrogram of the negative electrode current collectors prepared in Example 1 and Comparative Example 3 of the present invention;

[0025] Figure 2 is the cycle curve of Example 1, Comparative Example 1, and Comparative Example 2 of the present invention. Detailed Embodiments

[0026] The present invention discloses a method for preparing a negative electrode current collector of a lithium metal-free battery, which includes the following steps:

[0027] (1) Mix flake graphite, concentrated sulfuric acid, and hydrogen peroxide evenly by stirring. The mass ratio of the flake graphite to the concentrated sulfuric acid is 1:(8-20), and the volume ratio of the concentrated sulfuric acid to the hydrogen peroxide is 1:(0.05-0.3). Let it stand for 30-50 min to obtain an expanded graphite block, which is assembled by winding multiple expanded graphites;

[0028] (2) Place the expanded graphite block obtained in step (1) in an argon atmosphere for heating. The heating temperature is 400-600 °C, and the heating time is 10-30 min to obtain an expanded graphite block after impurity removal, removing residual sulfuric acid and water;

[0029] (3) Transfer the impurity-removed expanded graphite block obtained in step (2) to an atmosphere of a mixed gas composed of argon and hydrogen. The volume ratio of argon to hydrogen is (70 - 90):(10 - 30). The heat treatment temperature is 800 - 1200 °C, and the heat treatment time is 30 - 60 min to obtain a three-dimensional graphite elastomer. At the same time, the oxidized part in the impurity-removed expanded graphite block can be reduced, enhancing the conductivity and mechanical strength. Moreover, when the three-dimensional graphite elastomer is compressed, the elastic deformation amount can exceed 60%, and it has good resilience, which can absorb the volume change caused by the repeated deposition of metallic lithium, significantly enhancing the cycle life of the lithium metal-free anode lithium-ion battery.

[0030] (4) Slice the three-dimensional graphite elastomer obtained in step (3) to obtain graphite slices with a thickness of 1 - 5 mm.

[0031] (5) Immerse the graphite slices obtained in step (4), biphenyl, and lithium metal in 2-methyltetrahydrofuran together. The mass ratio of the graphite slices, biphenyl, and lithium metal is 1:(1 - 10):(0.8 - 5). After soaking for 3 - 10 min, wash with 2-methyltetrahydrofuran and dry to obtain a negative electrode current collector.

[0032] The present invention also discloses a negative electrode current collector for a lithium metal-free anode lithium-ion battery, which is prepared by the preparation method of the negative electrode current collector for the lithium metal-free anode lithium-ion battery described above.

[0033] The following will be described in detail with multiple examples and comparative examples.

[0034] Example 1

[0035] (1) Mix natural graphite, concentrated sulfuric acid, and hydrogen peroxide evenly by stirring. The mass ratio of natural graphite to concentrated sulfuric acid is 1:10, and the volume ratio of concentrated sulfuric acid to hydrogen peroxide is 1:0.1. Let it stand for 30 min to obtain an expanded graphite block composed of multiple expanded graphite wound and assembled.

[0036] (2) Place the expanded graphite block obtained in step (1) in an argon atmosphere for heating. The heating temperature is 460 °C, and the heating time is 30 min to obtain an impurity-removed expanded graphite block.

[0037] (3) Transfer the impurity-removed expanded graphite block obtained in step (2) to an atmosphere of a mixed gas composed of argon and hydrogen. The volume ratio of argon to hydrogen is 90:10. The heat treatment temperature is 1000 °C, and the heat treatment time is 30 min to obtain a three-dimensional graphite elastomer.

[0038] (4) Slice the three-dimensional graphite elastomer obtained in step (3) to obtain graphite slices, and the thickness of the graphite slices is 1.5 mm;

[0039] (5) Immerse the graphite slices obtained in step (4) together with biphenyl and lithium metal in 2-methyltetrahydrofuran. The mass ratio of the graphite slices, biphenyl, and lithium metal is 1:1:0.8. After soaking for 5 min, wash with 2-methyltetrahydrofuran and dry to obtain the negative electrode current collector.

[0040] Example 2

[0041] (1) Mix flake graphite, concentrated sulfuric acid, and hydrogen peroxide evenly by stirring. The mass ratio of the flake graphite to the concentrated sulfuric acid is 1:8, and the volume ratio of the concentrated sulfuric acid to the hydrogen peroxide is 1:0.1. Let it stand for 50 min to obtain an expanded graphite block, which is assembled by winding multiple expanded graphites;

[0042] (2) Place the expanded graphite block obtained in step (1) in an argon atmosphere for heating. The heating temperature is 400 °C and the heating time is 20 min to obtain a purified expanded graphite block;

[0043] (3) Transfer the purified expanded graphite block obtained in step (2) to an atmosphere of a mixed gas for heat treatment. This mixed gas is composed of argon and hydrogen, and the volume ratio of argon to hydrogen is 70:30. The heat treatment temperature is 800 °C and the heat treatment time is 60 min to obtain a three-dimensional graphite elastomer;

[0044] (4) Slice the three-dimensional graphite elastomer obtained in step (3) to obtain graphite slices, and the thickness of the graphite slices is 1 mm;

[0045] (5) Immerse the graphite slices obtained in step (4) together with biphenyl and lithium metal in 2-methyltetrahydrofuran. The mass ratio of the graphite slices, biphenyl, and lithium metal is 1:2:5. After soaking for 3 min, wash with 2-methyltetrahydrofuran and dry to obtain the negative electrode current collector.

[0046] Example 3

[0047] (1) Mix flake graphite, concentrated sulfuric acid, and hydrogen peroxide evenly by stirring. The mass ratio of the flake graphite to the concentrated sulfuric acid is 1:12, and the volume ratio of the concentrated sulfuric acid to the hydrogen peroxide is 1:0.05. Let it stand for 35 min to obtain an expanded graphite block, which is assembled by winding multiple expanded graphites;

[0048] (2) Place the expanded graphite block obtained in step (1) in an argon atmosphere for heating. The heating temperature is 600 °C and the heating time is 13 min to obtain a purified expanded graphite block;

[0049] (3) Transfer the purified expanded graphite block obtained in step (2) to an atmosphere of a mixed gas composed of argon and hydrogen, with the volume ratio of argon to hydrogen being 85:15. Conduct heat treatment at a temperature of 1200 °C for 45 minutes to obtain a three-dimensional graphite elastomer.

[0050] (4) Slice the three-dimensional graphite elastomer obtained in step (3) to obtain graphite slices with a thickness of 2 mm.

[0051] (5) Immerse the graphite slices obtained in step (4) together with biphenyl and lithium metal in 2-methyltetrahydrofuran. The mass ratio of the graphite slices to biphenyl and lithium metal is 1:3:4. After soaking for 3 - 10 minutes, wash with 2-methyltetrahydrofuran and dry to obtain a negative electrode current collector.

[0052] Example 4

[0053] (1) Mix natural flake graphite, concentrated sulfuric acid, and hydrogen peroxide evenly by stirring. The mass ratio of natural flake graphite to concentrated sulfuric acid is 1:10, and the volume ratio of concentrated sulfuric acid to hydrogen peroxide is 1:0.3. Let it stand for 45 minutes to obtain an expanded graphite block composed of multiple expanded graphite coils assembled together.

[0054] (2) Heat the expanded graphite block obtained in step (1) in an argon atmosphere at a temperature of 550 °C for 10 minutes to obtain a purified expanded graphite block.

[0055] (3) Transfer the purified expanded graphite block obtained in step (2) to an atmosphere of a mixed gas composed of argon and hydrogen, with the volume ratio of argon to hydrogen being 75:25. Conduct heat treatment at a temperature of 900 °C for 38 minutes to obtain a three-dimensional graphite elastomer.

[0056] (4) Slice the three-dimensional graphite elastomer obtained in step (3) to obtain graphite slices with a thickness of 2 mm.

[0057] (5) Immerse the graphite slices obtained in step (4) together with biphenyl and lithium metal in 2-methyltetrahydrofuran. The mass ratio of the graphite slices to biphenyl and lithium metal is 1:10:4. After soaking for 8 minutes, wash with 2-methyltetrahydrofuran and dry to obtain a negative electrode current collector.

[0058] Example 5

[0059] (1) Mix the flake graphite with concentrated sulfuric acid and hydrogen peroxide evenly by stirring. The mass ratio of the flake graphite to the concentrated sulfuric acid is 1:20, and the volume ratio of the concentrated sulfuric acid to the hydrogen peroxide is 1:0.3. Let it stand for 30 - 50 min to obtain an expanded graphite block, which is assembled by winding multiple expanded graphites;

[0060] (2) Place the expanded graphite block obtained in step (1) in an argon atmosphere for heating. The heating temperature is 480 °C and the heating time is 18 min to obtain an expanded graphite block after impurity removal;

[0061] (3) Transfer the expanded graphite block after impurity removal obtained in step (2) to an atmosphere of a mixed gas for heat treatment. This mixed gas is composed of argon and hydrogen, and the volume ratio of argon to hydrogen is 90:10. The heat treatment temperature is 1000 °C and the heat treatment time is 45 min to obtain a three-dimensional graphite elastomer;

[0062] (4) Slice the three-dimensional graphite elastomer obtained in step (3) to obtain graphite slices, and the thickness of the graphite slices is 5 mm;

[0063] (5) Immerse the graphite slices obtained in step (4) together with biphenyl and lithium metal in 2-methyltetrahydrofuran. The mass ratio of the graphite slices to biphenyl and lithium metal is 1:6:3. After soaking for 10 min, wash with 2-methyltetrahydrofuran and dry to obtain a negative electrode current collector.

[0064] Example 6

[0065] (1) Mix the flake graphite with concentrated sulfuric acid and hydrogen peroxide evenly by stirring. The mass ratio of the flake graphite to the concentrated sulfuric acid is 1:16, and the volume ratio of the concentrated sulfuric acid to the hydrogen peroxide is 1:0.25. Let it stand for 50 min to obtain an expanded graphite block, which is assembled by winding multiple expanded graphites;

[0066] (2) Place the expanded graphite block obtained in step (1) in an argon atmosphere for heating. The heating temperature is 500 °C and the heating time is 25 min to obtain an expanded graphite block after impurity removal;

[0067] (3) Transfer the expanded graphite block after impurity removal obtained in step (2) to an atmosphere of a mixed gas for heat treatment. This mixed gas is composed of argon and hydrogen, and the volume ratio of argon to hydrogen is 90:10. The heat treatment temperature is 950 °C and the heat treatment time is 50 min to obtain a three-dimensional graphite elastomer;

[0068] (4) Slice the three-dimensional graphite elastomer obtained in step (3) to obtain graphite slices, and the thickness of the graphite slices is 3 mm;

[0069] (5) Immerse the graphite slices obtained in step (4) together with biphenyl and lithium metal in 2-methyltetrahydrofuran. The mass ratio of the graphite slices, biphenyl, and lithium metal is 1:7:0.8. After soaking for 10 min, wash with 2-methyltetrahydrofuran and dry to obtain the negative electrode current collector.

[0070] Comparative Example 1

[0071] Comparative Example 1 is a copper foil current collector with a thickness of 0.2 mm.

[0072] Comparative Example 2

[0073] Comparative Example 2 is graphite paper, which has a planar structure and a thickness of 1.5 mm.

[0074] Comparative Example 3

[0075] Compared with Example 1, Comparative Example 3 lacks step (5), and the remaining steps are the same as those in Example 1.

[0076] Perform performance tests on the above-mentioned multiple examples and multiple comparative examples. The test results are shown in Table 1. Among them, the test method is as follows: Use lithium iron phosphate material as the positive electrode, and use the above-mentioned multiple examples and multiple comparative examples as the negative electrode current collector to construct a non-aqueous button battery. The reversible capacity is calculated based on the mass of the active material of lithium iron phosphate.

[0077]

[0078] Table 1

[0079] It can be clearly seen from the above data that the negative electrode current collector obtained by the preparation method of the present invention not only has a high initial Coulombic efficiency, but also has excellent cycle performance. At a current rate of 1C, after 200 cycles, the lithium-free battery constructed can still maintain a high capacity retention rate, reaching 80.2%. Comparing Example 1 with Comparative Examples 1-3, since Comparative Examples 1-2 have a planar structure and cannot effectively accommodate the deposited metallic lithium, they cannot absorb the volume changes caused by the repeated deposition of metallic lithium. At a current rate of 1C, after 200 cycles, the capacity retention rates are only 43.3% and 65.6%, far inferior to Example 1; moreover, because Comparative Example 2 is graphite paper and has a relatively strong affinity for metallic lithium, its capacity retention rate is higher than that of Comparative Example 1; however, when the graphite paper of Comparative Example 2 is used as the current collector, a part of lithium is consumed during the first charge and discharge to form a SEI film. Therefore, the initial Coulombic efficiency of Comparative Example 2 is lower than that of Comparative Example 1. In addition, comparing Example 1 with Comparative Example 3, since a SEI film is formed on the surface of the negative electrode current collector prepared in Example 1, the consumption of lithium ions by the current collector itself is reduced, while no SEI film is formed on the surface of the negative electrode current collector of Comparative Example 3, resulting in excessive consumption of lithium by the negative electrode current collector of Comparative Example 3 during the first charge and discharge process, leading to a significant decrease in the initial Coulombic efficiency of the lithium-free battery constructed in Comparative Example 3 compared to that in Example 1, from 94.6% to 75.3%. At the same time, observe and analyze Figure 1 , which is the AC impedance spectrogram of the negative electrode current collectors prepared in Example 1 and Comparative Example 3. It can be seen that the pre-lithiation effect on the surface of the negative electrode current collector in Example 1 is obvious, and an obvious SEI film characteristic spectrogram appears, further demonstrating this view; and, observe Figure 2 , which is the cycle curve graph of Example 1, Comparative Example 1, and Comparative Example 2. It can be clearly seen from the graph that the lithium-free battery constructed using the negative electrode current collector prepared in Example 1 has better capacity, Coulombic efficiency, and cycle stability than Comparative Examples 1 and 2, and has been greatly improved compared with the traditional technology.

[0080] The above is only a preferred embodiment of the present invention, and does not impose any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and decorations made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a negative current collector of a lithium battery without a negative electrode, characterized in that: It includes the following steps: (1) Mix graphite, an intercalating agent, and an oxidizing agent evenly by stirring. The mass ratio of graphite to the intercalating agent is 1:(8 - 20), and the volume ratio of the intercalating agent to the oxidizing agent is 1:(0.05 - 0.3). Let it stand for 30 - 50 min to obtain an expanded graphite block; (2) Place the expanded graphite block obtained in step (1) in a protective atmosphere for heating. The heating temperature is 400 - 600 °C, and the heating time is 10 - 30 min to obtain a purified expanded graphite block; (3) Transfer the purified expanded graphite block obtained in step (2) to an atmosphere of a mixed gas for heat treatment. This mixed gas is composed of an inert gas and a reducing gas. The volume ratio of the inert gas to the reducing gas is (70 - 90):(10 - 30). The heat treatment temperature is 800 - 1200 °C, and the heat treatment time is 30 - 60 min to obtain a three-dimensional structured graphite elastomer; (4) Perform slicing on the three-dimensional structured graphite elastomer obtained in step (3) to obtain graphite slices; (5) Immerse the graphite slices obtained in step (4), biphenyl, and lithium metal together in 2-methyltetrahydrofuran. The mass ratio of the graphite slices, biphenyl, and lithium metal is 1:(1 - 10):(0.8 - 5). After soaking for 3 - 10 min, wash with 2-methyltetrahydrofuran and dry to obtain a negative electrode current collector; 2. The method for preparing the negative electrode current collector of the lithium metal battery without a negative electrode according to claim 1, wherein: In step (1), the graphite is flake graphite.

3. The preparation method of the negative electrode current collector of the lithium metal battery without a negative electrode according to claim 1, wherein: In step (1), the intercalating agent is concentrated sulfuric acid.

4. The method for preparing the negative electrode current collector of the lithium metal battery without a negative electrode according to claim 1, wherein: In step (1), the oxidizing agent is hydrogen peroxide.

5. The preparation method of the negative current collector of the lithium metal battery without a negative electrode according to claim 1, wherein: In step (1), the expanded graphite block is assembled by winding multiple expanded graphites.

6. The preparation method of the negative current collector of the lithium metal battery without a negative electrode according to claim 1, wherein: In step (2), the protective atmosphere is an argon atmosphere.

7. The preparation method of the negative current collector of the lithium metal battery without a negative electrode according to claim 1, wherein: In step (3), the inert gas is argon.

8. The preparation method of the negative current collector of the lithium metal battery without a negative electrode according to claim 1, wherein: In step (3), the reducing gas is hydrogen.

9. The method for preparing the negative electrode current collector of the lithium metal battery without a negative electrode according to claim 1, wherein: In step (4), the thickness of the graphite slice is 1 - 5 mm.

10. The negative current collector of a lithium battery without a negative electrode, characterized in that: It is prepared by the method for preparing a negative electrode current collector of a non-negative electrode lithium battery according to any one of the preceding claims 1 - 9.

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