Composite ultrathin lithium foil and preparation method and application thereof

Abstract

This invention discloses a composite ultrathin lithium foil, its preparation method, and its applications. It is formed by combining high-oxidation-degree graphene, carbon nanotubes, and molten lithium metal, followed by cooling and rolling. The oxygen-containing functional groups on the graphene react with lithium to form lithiophilic sites. The carbon nanotubes are treated with silicon and zinc acetate to enhance their lithiophilicity, allowing them to be uniformly dispersed in the molten lithium. After cooling, the composite ultrathin lithium foil is rolled using a roller mill. The composite ultrathin lithium foil provided by this invention has high tensile strength and Young's modulus due to the network structure formed by graphene and carbon nanotubes, facilitating large-scale processing and application. Coating it onto the surface of the negative electrode of a lithium-ion battery can pre-lithiate the electrode material, thereby effectively improving the initial coulombic efficiency of the lithium-ion battery. The capacity corresponding to the lithium foil can be controlled by changing the content of graphene and carbon nanotubes to achieve precise pre-lithiation, further improving the performance of lithium-ion batteries to meet the requirements of energy storage applications.

Description

Technical Field

[0001] This invention relates to the field of battery manufacturing technology, and in particular to a composite ultrathin lithium foil, its preparation method, and its application. Background Technology

[0002] The development of new energy sources and the rise of new energy vehicles have placed higher demands on energy conversion and storage. Lithium-ion batteries, primarily lithium-ion batteries, are widely used in intermittent large-scale energy storage, transportation, and mobile electronic devices due to their high energy density, high coulombic efficiency, and low self-discharge rate. A major problem in the development of lithium-ion batteries is the low initial coulombic efficiency, meaning the initial discharge capacity is less than the charging capacity. This is because some lithium ions reach the negative electrode surface during the process and form a solid electrolyte interphase (SEI) film, resulting in irreversible capacity loss. This problem can be solved by pre-lithiating the negative electrode, i.e., introducing an additional lithium source to contribute lithium ions to form the SEI film, thereby improving the initial coulombic efficiency.

[0003] Depending on the method of lithium source replenishment, pre-lithiation can be divided into chemical methods, electrochemical methods, lithium powder methods, and direct lithium metal contact methods. The first three require dispersing the lithium source in organic solvents, which is costly, toxic, harmful, and prone to environmental pollution. In contrast, direct lithium metal contact pre-lithiation is highly efficient and convenient, involving directly coating an extremely thin lithium foil onto the electrode surface, allowing the electrode material to spontaneously intercalate lithium. However, the lithium foil thickness required for pre-lithiation is generally less than 5 μm, resulting in poor mechanical properties and making it difficult to prepare and apply. Moreover, when pure metallic lithium contacts the electrode surface for pre-lithiation, uneven consumption may lead to residues remaining at the inert interface, preventing full utilization. Constructing a network structure within the lithium foil can effectively alleviate these problems. Therefore, lithium secondary batteries urgently require an ultrathin lithium foil with high mechanical properties, controllable areal capacity, and a conductive network structure for negative electrode pre-lithiation treatment.

[0004] Ultrathin lithium foils meeting the above requirements can be prepared by uniformly dispersing carbon materials in lithium. Chinese patent CN111900333B discloses a method for preparing a carbon nanotube film directly composited with molten lithium metal. This method involves preparing a sandwich structure material of carbon nanotube film-lithium foil-carbon nanotube film, and then using temperature gradient control to achieve the wetting and composite of molten lithium metal and carbon nanotube film. Since the two carbon nanotube films already have a relatively high thickness, the final composite material will have a thickness of no less than 100 μm, thus it can only be used as a negative electrode and cannot be applied to pre-lithiation. Furthermore, due to the poor lithium affinity of carbon materials, temperature gradient control alone is insufficient to ensure sufficient lithium wetting. Chinese patent CN111509204B discloses a method for preparing a lithium metal composite negative electrode. This method involves adding lithium and high-viscosity alkanes to molten indium to obtain a melt that is easier to disperse carbon materials in. Carbon materials are then added to the composite and pressed into a sheet as an electrode. Theoretically, this method can reduce the thickness to tens of micrometers for application in pre-lithiation. However, indium is a rare metal with extremely high cost, and high-viscosity alkanes also pose environmental pollution problems. Therefore, it is still necessary to explore an efficient, low-cost, and environmentally friendly method to prepare composite ultrathin lithium foils in which carbon materials are uniformly dispersed in lithium. Summary of the Invention

[0005] To address the above problems, this invention provides a method for preparing composite ultrathin lithium foil, which involves melt-blending lithiophilic carbon nanotubes, graphene with high oxidation degree, and metallic lithium, followed by roll pressing. The specific steps are as follows:

[0006] (1) Take 100-200 mg of carbon nanotubes and 200-400 mg of zinc acetate powder and disperse them in 10-20 ml of deionized water. Stir for 1-6 h, then dry at 100-130 °C. Place them in a tube furnace and heat to 300-400 °C under the protection of inert gas. Keep warm for 3-12 h. After cooling, collect the lithophile-treated carbon nanotubes.

[0007] (2) 400 mg of metallic lithium, 50–100 mg of highly oxidized graphene, and 50–100 mg of lithophilized carbon nanotubes were placed in a closed container under argon protection and heated until the metallic lithium melted. The mixture was stirred and mixed for 1–6 hours to obtain a homogeneous mixture. The oxygen content of the highly oxidized graphene was 30%–40%, and the Id / Ig ratio in the Raman spectrum was greater than 1.

[0008] (3) Under argon protection, the mixture obtained in step (2) is cooled to room temperature, pre-pressed into a sheet, and then rolled into a composite ultrathin lithium foil by an electric roller mill under double coating of high-strength plastic film or metal foil.

[0009] Furthermore, the heating temperature in step (2) is 400-500℃ and the stirring speed is 200-5000rpm.

[0010] Further, the high-strength plastic film described in step (3) is made of one or more of the following materials: polypropylene, polyacrylonitrile, polyethylene, polyethylene oxide, polypropylene oxide, polyvinyl chloride, polyvinylidene fluoride, polytetrafluoroethylene, polycaprolactone, and polyethylene terephthalate. The metal foil is made of one or more of the following materials: titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, technetium, ruthenium, tantalum, tungsten, rhenium, and osmium. The thickness of the high-strength plastic film or metal foil is 10–500 μm. The rolling speed of the electric roller mill is 10–50 mm / s. -1 The resulting composite ultrathin lithium foil has a thickness of 30–50 μm.

[0011] The main components of the obtained composite ultrathin lithium foil are graphene, carbon nanotubes, and metallic lithium. The carbon nanotubes undergo lithiophilic treatment, and the graphene, with its high oxidation degree, reacts with lithium to enhance its lithiophilicity. This allows the two carbon materials to be uniformly dispersed in the metallic lithium, forming a network structure. This composite ultrathin lithium foil can be coated onto the surface of the negative electrode to pre-lithiate it, thereby improving the initial coulombic efficiency of the lithium secondary battery.

[0012] This invention also provides an application of composite ultrathin lithium foil in batteries, wherein the composite ultrathin lithium foil is rolled and bonded to the negative electrode material by an electric roller mill, and then assembled with the positive electrode, separator and electrolyte, for use in one of lithium-ion batteries, lithium metal batteries, lithium-sulfur batteries and lithium-air batteries.

[0013] Furthermore, the negative electrode material is selected from lithium metal, graphite negative electrode, soft carbon negative electrode, hard carbon negative electrode, silicon-carbon negative electrode, graphite-silicon suboxide negative electrode, nano silicon negative electrode, silicon suboxide negative electrode, and tin-based negative electrode.

[0014] Compared with the prior art, the present invention has the following significant features and beneficial effects:

[0015] (1) Highly oxidized graphene consists of complete graphene regions and defect regions with oxygen-containing functional groups. The graphene regions can react with lithium metal to form LiC. x In this compound, oxygen-containing functional groups can also react with lithium to form LiOH and -COOLi, effectively promoting the binding of graphene with lithium. This efficient dispersion and binding allows graphene to be randomly oriented in lithium. Therefore, although the introduced new phase and valence bonds form a low-conductivity interface layer, the overall structure is not continuous. At the same time, the introduction of carbon nanotubes can construct a graphene-carbon nanotube conductive network, so that the composite lithium foil still has high conductivity and does not significantly hinder electron transfer during the battery reaction process. After the lithophilization treatment of carbon nanotubes, the active sites on their surface can also be better anchored with lithium, which is beneficial to their dispersion in lithium.

[0016] (2) The low cohesive energy of carbon nanotubes makes them easier to curl up and aggregate in lithium, and even after lithiophilization, it is difficult to form a uniform conductive network. Graphene, on the other hand, is easier to disperse uniformly in lithium, allowing the curled carbon nanotubes to be connected in series on a two-dimensional plane, effectively improving the uniformity of the structure. At the same time, carbon nanotubes anchor the graphene, preventing the slippage of graphene sheets. The full combination of both with lithium effectively improves the mechanical properties of lithium foil, preventing the wrinkling or even breakage of single lithium metal foil at low thicknesses, thus enabling the preparation of self-supporting films with a thickness of less than 50 μm for pre-lithiation of battery anodes.

[0017] (3) The conductive network structure formed by graphene / carbon nanotubes can transfer charge more effectively, preventing the situation where a single lithium foil remains at the inert interface due to non-uniform lithiation and cannot be fully utilized.

[0018] (4) Graphene and carbon nanotubes can regulate the surface electric field during lithium deposition / stripping, promote uniform lithium deposition, and inhibit lithium dendrite formation, thereby improving the cycle stability of the battery.

[0019] (5) All preparation steps involved in this invention can be completed efficiently using industrial-grade equipment, and the entire preparation and use process does not require the use of organic solvents or pre-assembly of batteries. This enables efficient, safe, and environmentally friendly large-scale production to meet the needs of commercial batteries. Attached Figure Description

[0020] Figure 1 Photograph of the composite ultrathin lithium foil prepared in Example 1 of this invention.

[0021] Figure 2 SEM image of the composite ultrathin lithium foil prepared in Example 1 of this invention.

[0022] Figure 3 The image shown is the XRD pattern of the graphene / carbon nanotube composite ultrathin lithium foil prepared in Example 1 of this invention.

[0023] Figure 4 This is a comparison chart of the mechanical properties of the composite ultrathin lithium foil prepared in Example 2 of the present invention and the pure lithium foil prepared in Comparative Example 1.

[0024] Figure 5 The first charge-discharge curves of the button batteries assembled in Examples 1, 3, and 2 of this invention are shown below.

[0025] Figure 6 Nyquist diagram of the button cell assembled in Embodiment 3 and Comparative Example 2 of the present invention. Detailed Implementation

[0026] The present invention will now be further described with reference to the accompanying drawings and embodiments.

[0027] Example 1

[0028] (1) Lithophilization of carbon nanotubes: 200 mg carbon nanotubes and 400 mg zinc acetate powder were dissolved in 20 ml of deionized water and stirred at 1000 rpm for 6 h. Afterwards, the mixture was dried at 130 °C and placed in a tube furnace under inert gas protection at 10 °C for 1 min. -1 The temperature was increased to 400℃ at a certain rate, held for 3 hours, and then collected after cooling for later use.

[0029] (2) Preparation of graphene / carbon nanotube / lithium composite material: In an argon-filled glove box, 100 mg of highly oxidized graphene, 100 mg of lithiophilically treated carbon nanotube powder, and 400 mg of lithium metal sheets / foils were placed in a closed container equipped with a stirrer. The container was heated to 400 °C and stirred at 200 rpm for 6 h to uniformly disperse the graphene and carbon nanotubes in molten lithium. The oxygen content of the highly oxidized graphene was 30%, and the Id / Ig ratio in the Raman spectrum was 1.2.

[0030] (3) Preparation of graphene / carbon nanotube composite ultrathin lithium foil: After cooling the above graphene / carbon nanotube / lithium in an argon atmosphere, the composite material is pre-pressed into a sheet with a thickness of less than 1 mm. Both sides of the sheet are coated with high-strength polytetrafluoroethylene film and rolled by an electric roller mill at a rolling speed of 10 mm / s. -1 The material is rolled five times at the same thickness, and then rolled multiple times with the roller spacing decreasing by 10% each time. This process continues until a composite ultrathin lithium foil with a thickness of 30μm is produced.

[0031] The appearance of the resulting composite ultrathin lithium foil is as follows: Figure 1 As shown, this is a silver thin film, with a thickness of approximately 30 μm as measured by a thickness gauge. Due to its excellent mechanical properties, it can be used for large-scale preparation of large-size, low-thickness samples. SEM characterization confirmed that its thickness is approximately 30 μm and it consists of grains with a size of approximately 10 μm. Figure 2 At higher magnifications, the grains appear to be lithium-coated graphene sheets, while carbon nanotubes are interspersed on the outer surface, connecting the different grains. This unique structure suppresses the formation of continuous low-conductivity interfaces while improving mechanical properties. The distinct peaks near 20° in XRD confirm the presence of low-crystallinity carbon materials. Figure 3 Meanwhile, the characteristic peaks of Li and LiOH are also very obvious, the latter indicating the reaction of Li with the functional groups on the graphene surface, which improves the overall binding.

[0032] Example 2

[0033] Other conditions were the same as in Example 1, except that the thickness of the composite lithium foil was changed to 50 μm for comparison with the mechanical properties of pure lithium foil.

[0034] from Figure 4 It can be observed that, compared to pure lithium foil (tensile strength 1.43 MPa), the tensile strength of composite ultrathin lithium foil is increased to 5.28 MPa, while the elastic modulus is also improved.

[0035] Example 3

[0036] The composite ultrathin lithium foil obtained in Example 1 was used in a lithium-ion battery. The specific steps are as follows:

[0037] (1) Preparation of negative electrode: Silicon powder, styrene-butadiene rubber, sodium carboxymethyl cellulose and carbon black were dissolved in deionized water at a mass ratio of 6:1:1:2 and stirred at 1000 rpm for 6 hours to form a uniform slurry. The slurry was coated on a copper foil with a thickness of 20 μm and dried to obtain an unlithiated negative electrode.

[0038] (2) Pre-lithiation of the negative electrode: The negative electrode obtained in step (1) is bonded to the composite ultrathin lithium foil obtained in Example 1, and rolled by an electric roller mill under the protection of a high-strength polytetrafluoroethylene film, at a speed of 10 mm / s. -1 The two materials are rolled five times at a rolling speed to ensure they are fully combined.

[0039] (3) Preparation of positive electrode: Lithium iron phosphate, polyvinylidene fluoride and carbon black are dissolved in N-methylpyrrolidone and stirred at 1000 rpm for 6 hours to form a uniform slurry. The slurry is coated on an aluminum foil with a thickness of 20 μm and dried to obtain the positive electrode.

[0040] (4) Battery assembly: The pre-lithiated negative electrode, PP / PE / PP composite separator and positive electrode are assembled into a button battery. The electrolyte is a 1M lithium hexafluorophosphate solution with a volume ratio of ethylene carbonate and diethyl carbonate of 1:1.

[0041] Figure 5 The first charge-discharge processes of batteries pre-lithiated with graphene / carbon nanotube composite ultrathin lithium foil and those without pre-lithiation were demonstrated. Compared to the un-lithiated silicon anode battery, the pre-lithiated battery showed a significant improvement in initial coulombic efficiency. Furthermore, the pre-lithiation effect could be altered by controlling the composition and thickness of the graphene / carbon nanotube composite ultrathin lithium foil. Based on a stable battery system, precise pre-lithiation achieving 100% initial coulombic efficiency can be achieved. Moreover… Figure 6 EIS analysis showed that the introduction of the composite lithium foil did not have a significant impact on the equivalent series resistance of the system, and even reduced the charge transfer resistance. This is because the initial reaction interface changed from silicon to composite lithium foil, which effectively promoted electron and ion transfer.

[0042] Example 4

[0043] (1) Lithophilization of carbon nanotubes: 100 mg of carbon nanotubes and 200 mg of zinc acetate powder were dissolved in 10 ml of deionized water and stirred at 1000 rpm for 1 h. Afterwards, the mixture was dried at 100 °C and placed in a tube furnace under inert gas protection at 10 °C for 1 min. -1 The temperature was increased to 300℃ at a certain rate, held for 12 hours, and then collected after cooling for later use.

[0044] (2) Preparation of graphene / carbon nanotube / lithium composite material: In an argon-filled glove box, 50 mg of highly oxidized graphene, 50 mg of lithiophilically treated carbon nanotube powder, and 400 mg of lithium metal sheets / foils were placed in a closed container equipped with a stirrer. The container was heated to 500 °C and stirred at 5000 rpm for 1 h to uniformly disperse the graphene and carbon nanotubes in molten lithium. The oxygen content of the highly oxidized graphene was 40%, and the Id / Ig ratio in the Raman spectrum was 1.37.

[0045] (3) Preparation of graphene / carbon nanotube composite ultrathin lithium foil: After cooling the above graphene / carbon nanotube / lithium in an argon atmosphere, the composite material is pre-pressed into a sheet with a thickness of less than 1 mm. Both sides of the sheet are coated with high-strength polytetrafluoroethylene film and rolled by an electric roller mill at a rolling speed of 50 mm / s. -1 The material is rolled three times at the same thickness, and then rolled multiple times with the roller spacing decreasing by 10% each time. This process continues until a composite ultrathin lithium foil with a thickness of 50μm is produced.

[0046] The resulting composite ultrathin lithium foil has a tensile strength of 9.76 MPa, exhibiting good mechanical properties.

[0047] Comparative Example 1

[0048] Pure lithium is pre-pressed into sheets less than 1 mm thick, then coated on both sides with high-strength plastic film, and rolled using an electric double-roll mill at a rolling speed of 10 mm / s. -1 The material is rolled five times at the same thickness, and then rolled multiple times with the roller spacing decreasing by 10% each time. This process continues until a pure lithium foil with a thickness of 50 μm is produced, which is then used for mechanical property testing.

[0049] Comparative Example 2

[0050] Assemble the same type of battery without pre-lithiation treatment.

[0051] 1. Preparation of negative electrode: Silicon powder, styrene-butadiene rubber, sodium carboxymethyl cellulose and carbon black were dissolved in deionized water at a mass ratio of 6:1:1:2 and stirred at 1000 rpm for 6 hours to form a uniform slurry. The slurry was coated on a copper foil with a thickness of 20 μm and dried to obtain an unlithiated negative electrode.

[0052] 3. Preparation of the positive electrode: Lithium iron phosphate, polyvinylidene fluoride and carbon black were dissolved in N-methylpyrrolidone and stirred at 1000 rpm for 6 hours to form a uniform slurry. The slurry was then coated onto an aluminum foil with a thickness of 20 μm and dried to obtain the positive electrode.

[0053] 4. Battery assembly: Assemble the above-mentioned unlithiated negative electrode, PP / PE / PP composite separator, and positive electrode into a button cell. The electrolyte is a 1M lithium hexafluorophosphate solution with a volume ratio of ethylene carbonate and diethyl carbonate of 1:1.

[0054] Comparative Example 3

[0055] Preparation steps (1) and (3) are the same as in Example 2, and step (2) is as follows:

[0056] In an argon-filled glove box, 100 mg of highly oxidized graphene, 100 mg of lithophile-treated carbon nanotube powder, and 400 mg of lithium metal sheets / foil were placed in a closed container equipped with a stirrer. The container was heated to 400 °C and stirred at 200 rpm for 6 hours to ensure uniform dispersion of the graphene and carbon nanotubes in molten lithium. The highly oxidized graphene had an oxygen content of 10% and an Id / Ig ratio of 0.7 in its Raman spectrum.

[0057] The tensile strength of the obtained composite lithium foil was only 1.68 MPa, far lower than that of the composite ultrathin lithium foil in Example 2, and its ductility was also poor. This is because there are insufficient oxygen-containing functional groups on the graphene, resulting in insufficient lithiophilic sites and poor dispersion of graphene in molten lithium, which leads to local phase separation and a significant reduction in strength.

[0058] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described above. For those skilled in the art, any changes, improvements, or substitutions made to achieve the purpose of the invention without departing from the concept of the present invention are all within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a composite ultrathin lithium foil, characterized in that, Includes the following steps: (1) Take 100~200 mg of carbon nanotubes and 200~400 mg of zinc acetate powder and disperse them in 10~20 ml of deionized water. Stir for 1~6 h, then dry at 100~130 ℃, put them in a tube furnace, heat to 300~400 ℃ under the protection of inert gas, keep warm for 3~12 h, and collect the lithophile-treated carbon nanotubes after cooling. (2) Place 400 mg of lithium metal, 50-100 mg of high-oxidation graphene, and 50-100 mg of lithophile-treated carbon nanotubes in a closed container protected by argon gas, heat until the lithium metal melts, and stir and mix for 1-6 h to obtain a homogeneous mixture; wherein the oxygen content of the high-oxidation graphene is 30%-40%, and the Id / Ig ratio in the Raman spectrum is greater than 1; (3) Under argon protection, the mixture obtained in step (2) is cooled to room temperature, pre-pressed into a sheet, and then rolled into a composite ultrathin lithium foil by an electric roller mill under double coating of high-strength plastic film or metal foil.

2. The method for preparing the composite ultrathin lithium foil according to claim 1, characterized in that, The heating temperature in step (2) is 400~500 ℃.

3. The method for preparing the composite ultrathin lithium foil according to claim 1, characterized in that, The stirring speed in step (2) is 200~5000 rpm.

4. The method for preparing the composite ultrathin lithium foil according to claim 1, characterized in that, The high-strength plastic film described in step (3) is made of one or more of the following materials: polypropylene, polyacrylonitrile, polyethylene, polyethylene oxide, polypropylene oxide, polyvinyl chloride, polyvinylidene fluoride, polytetrafluoroethylene, polycaprolactone, and polyethylene terephthalate.

5. The method for preparing the composite ultrathin lithium foil according to claim 1, characterized in that, The metal foil mentioned in step (3) is made of one or more of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, technetium, ruthenium, tantalum, tungsten, rhenium, and osmium.

6. The method for preparing the composite ultrathin lithium foil according to claim 1, characterized in that, The thickness of the high-strength plastic film or metal foil mentioned in step (3) is 10~500 μm.

7. The method for preparing the composite ultrathin lithium foil according to claim 1, characterized in that, The rolling speed of the electric double-roll mill in step (3) is 10~50 mm / s. -1 .

8. The method for preparing the composite ultrathin lithium foil according to claim 1, characterized in that, The thickness of the composite ultrathin lithium foil described in step (3) is 30~50 μm.

9. An application of a composite ultrathin lithium foil in batteries, characterized in that, The composite ultrathin lithium foil is prepared by the method described in claim 1; the composite ultrathin lithium foil and the negative electrode material are rolled and bonded together by an electric roller mill, and then assembled with the positive electrode, separator and electrolyte for use in a lithium-ion battery or a lithium metal battery.

10. The application according to claim 9, characterized in that, The anode material is selected from lithium metal, graphite anode, soft carbon anode, hard carbon anode, silicon-carbon anode, graphite-silicon suboxide anode, nano-silicon anode, silicon suboxide anode, and tin-based anode.

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