Composite current collector with elastically deformable thickness, preparation method of composite current collector and lithium ion battery

Abstract

The composite current collector comprises a five-layer composite structure including a copper layer, a polymer film layer, a nanofiber film layer, a polymer film layer and a copper layer which are sequentially arranged, the thickness of the copper layer is 0.2-5 [mu] m, the thickness of the polymer film layer is 0.2-10 [mu] m, the fiber diameter of the nanofiber film layer is 50-2000 nm, and the thickness of the nanofiber film layer is 5-100 [mu] m. The invention also discloses a preparation method of the composite current collector with the elastically deformable thickness. The invention also discloses a lithium ion battery. The composite current collector has the characteristic that the thickness is changed along with the pressure, the performance degradation of a high-specific-energy battery caused by volume expansion of a high-gram-capacity negative electrode can be effectively avoided, the pressure between pole pieces is kept relatively balanced, and good contact is moderately kept, so that the volume expansion and cycle life attenuation of the battery in the charging and discharging process are improved.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and in particular to a composite current collector with elastically deformable thickness, a preparation method thereof, and a lithium ion battery. Background Art

[0002] With the continuous advancement of modern technology, lithium-ion batteries have been widely used in many fields, including electronic products and electric vehicles, due to their advantages such as high energy density, long cycle life, and no memory effect. However, with the continuous improvement of energy density requirements, traditional lithium-ion batteries with graphite as anodes have reached their energy limit. The development of new anodes with higher gram capacity (such as silicon-based anodes and lithium metal anodes) is particularly important. Therefore, high gram capacity anodes, as a higher energy density anode material, have become one of the current research hotspots in the field of lithium-ion batteries.

[0003] However, both silicon-based and lithium metal anodes experience significant volume expansion during charge and discharge, impacting the cycle life and volume expansion of high-energy-density batteries using them as anodes. This problem of low cycle life due to volume expansion is common in high-energy-density lithium-ion battery systems, severely limiting the commercialization of high-capacity silicon-based and lithium metal anode batteries.

[0004] To solve the above problems, existing technologies improve the structural stability of the electrode by modifying the binder, or designing silicon-carbon materials to provide a stable gap structure and reserve space for volume expansion. Although these methods can improve the cycle life of the battery to a certain extent, they still cannot solve the problem of battery volume expansion. Summary of the Invention

[0005] The object of the present invention is to provide a composite current collector with elastically deformable thickness.

[0006] The present invention also provides a method for preparing a composite current collector with elastically deformable thickness.

[0007] The present invention also provides a lithium ion battery.

[0008] The innovation of the present invention lies in the fact that the composite current collector with elastically deformable thickness has the characteristic of changing thickness with the pressure, which can effectively avoid the performance degradation of high-energy-density batteries caused by volume expansion of high-gram capacity negative electrodes, especially the reduction of cycle life. By introducing a porous nanofiber membrane with variable thickness into the current collector, the composite current collector can be compressed when subjected to pressure, thereby reducing the thickness. When the pressure decreases, it can expand, avoiding drastic changes in the internal pressure of the battery, keeping the pressure between the electrodes relatively balanced, and maintaining good contact at a moderate level, thereby improving the battery volume expansion and cycle life attenuation during charging and discharging.

[0009] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is:

[0010] A composite current collector with elastically deformable thickness comprises a five-layer composite structure of a copper layer, a polymer film layer, a nanofiber film layer, a polymer film layer, and a copper layer arranged in sequence, wherein the copper layer has a thickness of 0.2 to 5 μm, the polymer film layer has a thickness of 0.2 to 10 μm, the nanofiber film layer has a fiber diameter of 50 to 2000 nm, and the nanofiber film layer has a thickness of 5 to 100 μm.

[0011] Furthermore, the polymer film layer is polyimide, and the nanofiber film layer is organic nanofiber or organic / composite nanofiber.

[0012] Furthermore, the compression deformation of the composite current collector is 10 to 90%.

[0013] Furthermore, the organic nanofiber is at least one of polyacrylonitrile, polyimide, polyetherimide, PVDF, PTFE, and aramid; and the organic / composite nanofiber is composed of at least one of polyacrylonitrile, polyimide, polyetherimide, PVDF, PTFE, and aramid and an inorganic material or a metal material.

[0014] Furthermore, the polyimide in the polymer film layer contains at least one of a hydroxyl group and a carboxyl group.

[0015] Furthermore, the polyimide in the polymer film layer is obtained by cyclizing a polyamic acid formed by condensation polymerization of a dibasic acid anhydride and a diamine; the diamine is composed of functional diamines selected from hydroxyl-containing and carboxyl-containing groups and non-functional diamines, wherein the molar ratio of functional diamines to all diamines is 5 to 80%; the hydroxyl-containing functional diamine is one or more of 5,5'-diamino-2,2'-hydroxy-biphenyl (p-HAB), 2,2'-bis(3-amino-4-hydroxyphenyl)propane (BAHPP), and 3,3'-diamino-4,4'-dihydroxybiphenyl (DADHBP); the carboxyl-containing functional diamine is one or more of 3,5-diaminobenzoic acid (DABA) and 3,5-bis(4-aminophenoxy)benzoic acid (BAPBA).

[0016] A method for preparing a composite current collector with elastically deformable thickness comprises the following steps:

[0017] (1) Synthesis of polyamide solution: any one or more of dibasic acid anhydrides, any one or more of functional diamines, and any one or more of non-functional diamines are selected as raw materials, dissolved in organic solvent A, and then condensed and polymerized to form a polyimide precursor, namely, a polyamic acid solution, and then a silver precursor solution is added to obtain a polyamic acid solution containing silver salt. The polyamic acid solution containing silver salt is diluted with organic solvent A to a desired solid content, and coated to form a polyamic acid / silver salt film containing functional groups, and then dried for 1 to 24 hours to obtain a dried film;

[0018] (2) subjecting the diamine monomer and the dianhydride monomer to a condensation polymerization reaction to obtain a polyamic acid spinning solution;

[0019] (3) depositing the polyamic acid spinning solution on one surface of the dried film obtained in step (1) by electrospinning to obtain a composite film having nanofibers on one side;

[0020] (4) The two composite membranes obtained in step (3) are placed together with one side having the nanofiber membrane, pressure is applied, and the mixture is heated to 300-450° C. in a reducing or inert atmosphere for cyclization and reduction, and the temperature is kept for 0.1-5 h to obtain a polymer membrane layer / nanofiber membrane layer / polymer membrane layer composite structure containing silver on the surface, thereby achieving the first surface metal silvering of the polyimide film surface to form a seed layer, and then cleaned with deionized water to obtain a metal silvered composite membrane;

[0021] (5) The surface of the metal silver composite film obtained in step (4) is treated and immersed in a water-soluble copper salt solution for electroreduction, so that the surface of the film is subjected to secondary surface metal copperization, and finally a five-layer composite structure of copper layer / polymer film layer / nanofiber film layer / polymer film layer / copper layer is formed.

[0022] Furthermore, in step (2), the diamine monomer and the dianhydride monomer are subjected to a condensation polymerization reaction to obtain an initial solution, inorganic particles or metal salts are added to the initial solution, and the mixture is stirred evenly to obtain a polyamic acid spinning solution.

[0023] Furthermore, the silver salt in step (1) is one of silver nitrate, silver fluoride, silver fluoroborate, silver acetate, silver trifluoroacetylacetonate, silver acetylacetonate, or a mixture of two or more thereof; the copper salt in the aqueous solution in step (5) is copper chloride, copper sulfate, or copper nitrate; the molar ratio of the Ag in the silver precursor solution in step (1) to the molar ratio of the dibasic acid anhydride during the synthesis of the polyamic acid solution is 0.04 to 0.75:1, the required solid content is 5 to 20 wt%, and the thickness of the dried film is 1 to 100 μm; and the pressure applied in step (4) is The pressure is 0.01 to 5 MPa; the surface treatment in step (5) is sequentially carried out through three steps: organic solvent B → dilute acid activation → deionized water rinsing; the organic solvent B is at least one of acetone, ethanol, and methanol; the copper ion concentration in the copper salt aqueous solution is 150 to 300 g / L, and the electroreduction time is 1 to 50 min; the organic solvent A is one or more of NN-dimethylformamide (DMF), NN-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO) or N-methylpyrrolidone (NMP).

[0024] A lithium-ion battery comprises a composite current collector with elastically deformable thickness as a negative electrode current collector.

[0025] The beneficial effects of the present invention are:

[0026] 1. The characteristic of the composite current collector in the present invention that the thickness changes with pressure can effectively inhibit the volume expansion of the battery, thereby improving the battery cycle life and other electrical properties.

[0027] 2. The present invention adopts a high-temperature resistant polymer to prepare the intermediate nanofiber membrane layer. Since the nanofiber membrane has a high porosity and good elastic deformation, it is the main part of the thickness change of the entire composite current collector, and a stable internal support structure is built, which significantly improves the elastic deformation of the composite current collector, thereby providing the possibility for the thickness of the entire composite current collector to be variable. The variable thickness structure of the composite current collector can prevent problems such as changes in electrode contact caused by volume changes of the battery cell during the charging and discharging process, thereby ensuring the integrity and stability of the battery structure and extending the cycle life of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a structural schematic diagram of the present invention. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings.

[0030] Example 1: A composite current collector with elastically deformable thickness, comprising a five-layer composite structure of a copper layer, a polymer film layer, a nanofiber film layer, a polymer film layer, and a copper layer arranged in sequence, wherein the copper layer has a thickness of 0.2 μm, the polymer film layer has a thickness of 0.2 μm, the nanofiber film layer has a fiber diameter of 50 nm, and the nanofiber film layer has a thickness of 5 μm.

[0031] The compression deformation of the composite current collector is 10%.

[0032] The polymer film layer is polyimide, the nanofiber film layer is organic nanofiber, and the organic nanofiber is polyacrylonitrile;

[0033] The polyimide in the polymer film layer contains at least one of a hydroxyl group and a carboxyl group.

[0034] The polyimide in the polymer film layer is obtained by cyclizing a polyamic acid formed by condensation polymerization of a dibasic acid anhydride and a diamine; the diamine is composed of functional diamines selected from hydroxyl-containing and carboxyl-containing groups and non-functional diamines, wherein the molar ratio of the functional diamine to all diamines is 5%; the hydroxyl-containing functional diamine is 5,5'-diamino-2,2'-hydroxy-biphenyl (p-HAB); and the carboxyl-containing functional diamine is 3,5-diaminobenzoic acid (DABA).

[0035] Example 2: A composite current collector with elastically deformable thickness, comprising a five-layer composite structure of a copper layer, a polymer film layer, a nanofiber film layer, a polymer film layer, and a copper layer arranged in sequence, wherein the copper layer has a thickness of 3 μm, the polymer film layer has a thickness of 5 μm, the nanofiber film layer has a fiber diameter of 1000 nm, and the nanofiber film layer has a thickness of 50 μm.

[0036] The compression deformation of the composite current collector is 50%.

[0037] The polymer film layer is polyimide, the nanofiber film layer is organic nanofiber, and the organic nanofiber is polyimide.

[0038] The polyimide in the polymer film layer contains at least one of a hydroxyl group and a carboxyl group.

[0039] The polyimide in the polymer film layer is obtained by cyclizing a polyamic acid formed by condensation polymerization of a dibasic acid anhydride and a diamine; the diamine is composed of functional diamines selected from hydroxyl-containing and carboxyl-containing groups and non-functional diamines, wherein the molar ratio of the functional diamines to all diamines is 40%; the hydroxyl-containing functional diamine is 2,2'-bis(3-amino-4-hydroxyphenyl)propane (BAHPP); and the carboxyl-containing functional diamine is 3,5-bis(4-aminophenoxy)benzoic acid (BAPBA).

[0040] Example 3: A composite current collector with elastically deformable thickness, comprising a five-layer composite structure of a copper layer, a polymer film layer, a nanofiber film layer, a polymer film layer, and a copper layer arranged in sequence, wherein the copper layer has a thickness of 5 μm, the polymer film layer has a thickness of 10 μm, the nanofiber film layer has a fiber diameter of 2000 nm, and the nanofiber film layer has a thickness of 100 μm.

[0041] The compression deformation of the composite current collector is 90%.

[0042] The polymer film layer is polyimide, and the nanofiber film layer is organic / composite nanofiber; the organic / composite nanofiber is composed of polyacrylonitrile and inorganic materials.

[0043] The polyimide in the polymer film layer contains at least one of a hydroxyl group and a carboxyl group.

[0044] The polyimide in the polymer film layer is obtained by cyclizing a polyamic acid formed by condensation polymerization of a dibasic acid anhydride and a diamine; the diamine is composed of functional diamines selected from hydroxyl-containing and carboxyl-containing groups and non-functional diamines, wherein the molar ratio of functional diamines to all diamines is 80%; the hydroxyl-containing functional diamine is 3,3'-diamino-4,4'-dihydroxybiphenyl (DADHBP); and the carboxyl-containing functional diamine is 3,5-diaminobenzoic acid (DABA).

[0045] Example 4: Referring to Example 1, the nanofiber membrane layer is organic nanofiber or organic / composite nanofiber, the organic nanofiber is polyacrylonitrile, polyimide, polyetherimide, PVDF, PTFE, aramid; the organic / composite nanofiber is composed of at least one of polyacrylonitrile, polyimide, polyetherimide, PVDF, PTFE, aramid and inorganic material or metal material.

[0046] The hydroxyl-containing functional diamine is one or more of 5,5'-diamino-2,2'-hydroxy-biphenyl (p-HAB), 2,2'-bis(3-amino-4-hydroxyphenyl)propane (BAHPP), and 3,3'-diamino-4,4'-dihydroxybiphenyl (DADHBP); the carboxyl-containing functional diamine is one or more of 3,5-diaminobenzoic acid (DABA) and 3,5-bis(4-aminophenoxy)benzoic acid (BAPBA).

[0047] Example 5: A method for preparing a composite current collector with elastically deformable thickness, comprising the following steps:

[0048] (1) Synthesis of polyamide solution: The dibasic acid anhydride, functional diamine and non-functional diamine of Example 1 are selected as raw materials, dissolved in organic solvent A and polymerized by condensation to form a polyimide precursor, namely a polyamic acid solution, and then a silver precursor solution is added to obtain a polyamic acid solution containing silver salt, wherein the silver salt is silver nitrate; the polyamic acid solution containing silver salt is diluted with organic solvent A to a solid content of 5 wt%, coated to form a polyamic acid / silver salt film containing functional groups, and dried for 1 hour to obtain a dried film; the molar ratio of Ag in the silver precursor solution to the molar ratio of the dibasic acid anhydride during the synthesis of the polyamic acid solution is 0.04:1, and the thickness of the dried film is 1 μm; the organic solvent A is NN-dimethylformamide (DMF);

[0049] (2) subjecting the diamine monomer and the dianhydride monomer to a condensation polymerization reaction to obtain a polyamic acid spinning solution;

[0050] (3) depositing the polyamic acid spinning solution on one surface of the dried film obtained in step (1) by electrospinning to obtain a composite film having nanofibers on one side;

[0051] (4) The two composite membranes obtained in step (3) are pressed together with one side having the nanofiber membrane, and pressure is applied at a pressure of 0.01 MPa; the two composite membranes are heated to 300° C. in a reducing atmosphere for cyclization and reduction, and the temperature is kept for 0.1 h to obtain a polymer membrane layer / nanofiber membrane layer / polymer membrane layer composite structure containing silver on the surface, thereby achieving the first surface metal silvering of the polyimide film surface to form a seed layer, and then cleaned with deionized water to obtain a metal silvered composite membrane;

[0052] (5) The surface of the metal silver composite film obtained in step (4) is treated and then immersed in a water-soluble copper salt solution, and electro-reduced for 1 minute to achieve secondary surface copperization of the film surface, and finally form a five-layer composite structure of copper layer / polymer film layer / nanofiber film layer / polymer film layer / copper layer; the aqueous copper salt is copper chloride, and the copper ion concentration in the aqueous copper salt is 150g / L. The surface treatment is carried out in three steps in sequence: organic solvent B → dilute acid activation → deionized water rinsing; organic solvent B is acetone.

[0053] Example 6: A method for preparing a composite current collector with elastically deformable thickness, comprising the following steps:

[0054] (1) Synthesis of polyamide solution: The dibasic acid anhydride, functional diamine and non-functional diamine of Example 2 are selected as raw materials, dissolved in organic solvent A and polymerized by condensation to form a polyimide precursor, namely a polyamic acid solution, and then a silver precursor solution is added to obtain a polyamic acid solution containing silver salt, wherein the silver salt is silver fluoride; the polyamic acid solution containing silver salt is diluted with organic solvent A to a solid content of 10 wt%, coated to form a polyamic acid / silver salt film containing functional groups, and dried for 10 hours to obtain a dried film; the molar ratio of Ag in the silver precursor solution to the molar ratio of the dibasic acid anhydride during the synthesis of the polyamic acid solution is 0.3:1, and the thickness of the dried film is 50 μm; the organic solvent A is NN-dimethylacetamide (DMAc);

[0055] (2) condensing and polymerizing the diamine monomer and the dianhydride monomer to obtain an initial solution, adding inorganic particles to the initial solution, and stirring uniformly to obtain a polyamic acid spinning solution;

[0056] (3) depositing the polyamic acid spinning solution on one surface of the dried film obtained in step (1) by electrospinning to obtain a composite film having nanofibers on one side;

[0057] (4) The two composite membranes obtained in step (3) are pressed together with one side having the nanofiber membrane, and pressure is applied at a pressure of 2 MPa; the two composite membranes are heated to 400° C. in an inert atmosphere for cyclization and reduction, and the heat is maintained for 2 h to obtain a polymer membrane layer / nanofiber membrane layer / polymer membrane layer composite structure containing silver on the surface, thereby achieving the first surface metal silvering of the polyimide film surface to form a seed layer, and then cleaned with deionized water to obtain a metal silvered composite membrane;

[0058] (5) The surface of the metal silver composite film obtained in step (4) is treated and immersed in a water-soluble copper salt solution, and electro-reduced for 30 minutes to achieve secondary surface copperization of the film surface, and finally form a five-layer composite structure of copper layer / polymer film layer / nanofiber film layer / polymer film layer / copper layer; the aqueous copper salt is copper sulfate, and the copper ion concentration in the aqueous copper salt is 200g / L. The surface treatment is carried out in three steps: organic solvent B → dilute acid activation → deionized water rinsing; organic solvent B is ethanol.

[0059] Example 7: A method for preparing a composite current collector with elastically deformable thickness, comprising the following steps:

[0060] (1) Synthesis of polyamide solution: The dibasic acid anhydride, functional diamine and non-functional diamine of Example 3 are selected as raw materials, dissolved in organic solvent A and polymerized by condensation to form a polyimide precursor, namely a polyamic acid solution, and then a silver precursor solution is added to obtain a polyamic acid solution containing silver salt, wherein the silver salt is silver fluoroborate; the polyamic acid solution containing silver salt is diluted with organic solvent A to a solid content of 20 wt%, coated to form a polyamic acid / silver salt film containing functional groups, and dried for 24 hours to obtain a dried film; the molar ratio of Ag in the silver precursor solution to the molar ratio of the dibasic acid anhydride during the synthesis of the polyamic acid solution is 0.75:1, and the thickness of the dried film is 100 μm; the organic solvent A is dimethyl sulfoxide (DMSO);

[0061] (2) condensing and polymerizing the diamine monomer and the dianhydride monomer to obtain an initial solution, adding a metal salt to the initial solution, and stirring uniformly to obtain a polyamic acid spinning solution;

[0062] (3) depositing the polyamic acid spinning solution on one surface of the dried film obtained in step (1) by electrospinning to obtain a composite film having nanofibers on one side;

[0063] (4) The two composite membranes obtained in step (3) are pressed together with one side having the nanofiber membrane, and pressure is applied at a pressure of 5 MPa; the two composite membranes are heated to 450° C. in a reducing atmosphere for cyclization and reduction, and the temperature is kept for 5 h to obtain a polymer membrane layer / nanofiber membrane layer / polymer membrane layer composite structure containing silver on the surface, thereby achieving the first surface metal silvering of the polyimide film surface to form a seed layer, and then cleaned with deionized water to obtain a metal silvered composite membrane;

[0064] (5) The surface of the metal silver composite film obtained in step (4) is treated and immersed in a water-soluble copper salt solution, and electro-reduced for 50 minutes to achieve secondary surface copperization of the film surface, and finally form a five-layer composite structure of copper layer / polymer film layer / nanofiber film layer / polymer film layer / copper layer; the aqueous copper salt is copper nitrate, and the copper ion concentration in the aqueous copper salt is 300g / L. The surface treatment is carried out in three steps: organic solvent B → dilute acid activation → deionized water rinsing; the organic solvent B is methanol.

[0065] Example 8: Referring to Example 5,

[0066] The silver salt is one or a mixture of two or more of silver nitrate, silver fluoride, silver fluoroborate, silver acetate, silver trifluoroacetylacetonate, and silver acetylacetonate; the organic solvent A is one or more of NN-dimethylformamide (DMF), NN-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP); and the organic solvent B is at least one of acetone, ethanol, and methanol.

[0067] Example 9: A lithium-ion battery comprising a composite current collector with elastically deformable thickness as a negative electrode current collector.

[0068] Comparative Example 1: Steps (2) and (3) of Example 5 were omitted, and the dried films were directly pressed together and pressure was applied, while other conditions remained unchanged.

[0069] Comparative Example 2: No silver salt was added to step (1) of Example 5, and other conditions remained unchanged.

[0070] Comparative Example 3: Instead of adding the functional diamine containing a carboxyl group to the functional diamine in step (1) of Example 5, a diamine containing a hydroxyl group is added, and other conditions remain unchanged.

[0071] The negative electrode current collectors of Examples 5, 6, and 7 and Comparative Examples 1, 2, and 3 were used to assemble 10 positive and 11 negative soft-pack batteries, which were injected with liquid. The batteries were charged and discharged at 0.1C for the first formation charge, with a charging cut-off voltage of 4.3V. The batteries were then switched to constant voltage charging until the current dropped to 0.05C and allowed to stand for 30 minutes. The battery was then discharged at 0.1C with a cut-off voltage of 2.5V. The battery capacity after constant capacity was 3.8Ah. The batteries were cycled at 1C / 1C under a pressure of 1MPa with a voltage range of 2.8 to 4.3V. The test temperature was room temperature. After 1000 cycles, the battery capacity retention rate and the battery thickness expansion rate were measured.

[0072] The experimental results are shown in Table 1.

[0073] Table 1

[0074]

[0075]

[0076] The embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

Claims

1. A composite current collector with elastically deformable thickness, characterized in that: The invention comprises a five-layer composite structure of a copper layer, a polymer film layer, a nanofiber film layer, a polymer film layer, and a copper layer arranged in sequence. The thickness of the copper layer is 0.2-5 μm, the thickness of the polymer film layer is 0.2-10 μm, the fiber diameter of the nanofiber film layer is 50-2000 nm, and the thickness of the nanofiber film layer is 5-100 μm.

2. The composite current collector with elastically deformable thickness according to claim 1, characterized in that: The polymer film layer is polyimide, and the nanofiber film layer is organic nanofiber or organic / composite nanofiber.

3. The composite current collector with elastically deformable thickness according to claim 1, characterized in that: The composite current collector has a compression deformation of 10-90%.

4. The composite current collector with elastically deformable thickness according to claim 1, characterized in that: The organic nanofiber is at least one of polyacrylonitrile, polyimide, polyetherimide, PVDF, PTFE, and aramid; the organic / composite nanofiber is composed of at least one of polyacrylonitrile, polyimide, polyetherimide, PVDF, PTFE, and aramid and an inorganic material or a metal material.

5. The composite current collector with elastically deformable thickness according to claim 2, characterized in that: The polyimide in the polymer film layer contains at least one of a hydroxyl group and a carboxyl group.

6. The composite current collector with elastically deformable thickness according to claim 1, characterized in that: The polyimide in the polymer film layer is obtained by cyclizing a polyamic acid formed by condensation polymerization of a dibasic acid anhydride and a diamine; the diamine is composed of functional diamines selected from hydroxyl-containing and carboxyl-containing groups and non-functional diamines, wherein the molar ratio of functional diamines to all diamines is 5 to 80%; the hydroxyl-containing functional diamine is one or more of 5,5'-diamino-2,2'-hydroxy-biphenyl (p-HAB), 2,2'-bis(3-amino-4-hydroxyphenyl)propane (BAHPP), and 3,3'-diamino-4,4'-dihydroxybiphenyl (DADHBP); the carboxyl-containing functional diamine is one or more of 3,5-diaminobenzoic acid (DABA) and 3,5-bis(4-aminophenoxy)benzoic acid (BAPBA).

7. A method for preparing a composite current collector with elastically deformable thickness according to any one of claims 1 to 6, characterized in that: The following steps are involved: (1) Synthesis of polyamide solution: any one or more of dibasic acid anhydrides, any one or more of functional diamines, and any one or more of non-functional diamines are selected as raw materials, dissolved in organic solvent A, and then polymerized by condensation to form a polyimide precursor - polyamic acid solution, and then a silver precursor solution is added to obtain a polyamic acid solution containing silver salt, and the polyamic acid solution containing silver salt is diluted with organic solvent A to a desired solid content, and coated to form a polyamic acid / silver salt film containing functional groups, and dried for 1 to 24 hours to obtain a dried film; (2) subjecting the diamine monomer and the dianhydride monomer to a condensation polymerization reaction to obtain a polyamic acid spinning solution; (3) depositing the polyamic acid spinning solution on one surface of the dried film obtained in step (1) by electrospinning to obtain a composite film having nanofibers on one side; (4) The two composite membranes obtained in step (3) are placed together with one side of the nanofiber membrane, pressure is applied, and the two composite membranes are heated to 300-450° C. in a reducing or inert atmosphere for cyclization and reduction, and the temperature is kept for 0.1-5 h to obtain a polymer membrane layer / nanofiber membrane layer / polymer membrane layer composite structure containing silver on the surface, thereby achieving the first surface metal silvering of the polyimide film surface to form a seed layer, and then cleaned with deionized water to obtain a metal silver composite membrane; (5) The surface of the metal silver composite film obtained in step (4) is treated and immersed in a water-soluble copper salt solution for electroreduction, so that the surface of the film is subjected to secondary surface metal copperization, and finally a five-layer composite structure of copper layer / polymer film layer / nanofiber film layer / polymer film layer / copper layer is formed.

8. The method for preparing a composite current collector with elastically deformable thickness according to claim 7, characterized in that: In the step (2), a diamine monomer and a dianhydride monomer are subjected to a condensation polymerization reaction to obtain an initial solution, inorganic particles or metal salts are added to the initial solution, and the mixture is stirred evenly to obtain a polyamic acid spinning solution.

9. The method for preparing a composite current collector with elastically deformable thickness according to claim 7, characterized in that: The silver salt in step (1) is one of silver nitrate, silver fluoride, silver fluoroborate, silver acetate, silver trifluoroacetylacetonate, silver acetylacetonate, or a mixture of two or more thereof; the copper salt in the aqueous solution in step (5) is copper chloride, copper sulfate, or copper nitrate; the molar ratio of the Ag in the silver precursor solution in step (1) to the molar ratio of the dibasic acid anhydride during the synthesis of the polyamic acid solution is 0.04-0.75:1, the required solid content is 5-20wt%, and the thickness of the dried film is 1-100μm; the pressure when applying pressure in step (4) is 0.04-0.75:

1. The surface treatment in step (5) is carried out in three steps: organic solvent B → dilute acid activation → deionized water rinsing; the organic solvent B is at least one of acetone, ethanol, and methanol; the copper ion concentration in the aqueous copper salt is 150-300 g / L, and the electroreduction time is 1-50 min; the organic solvent A is one or more of NN-dimethylformamide (DMF), NN-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), or N-methylpyrrolidone (NMP).

10. A lithium ion battery, characterized in that: A composite current collector with elastically deformable thickness as claimed in any one of claims 1 to 9 is used as a negative electrode current collector.

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