Composite current collector, pole piece and secondary battery using the same

Abstract

The present invention provides a composite current collector, which comprises a base layer and a conductive layer. The conductive layer is arranged on at least one surface of the base layer. The conductive layer comprises n1 transition layers and n2 metal layers, wherein n1 is a positive integer greater than 1, and n2 is a positive integer greater than 1. In the conductive layer, the metal layers and the transition layers are alternately stacked; the transition layer is composed of a carbon fluoride hydrogen compound, and the chemical formula of the carbon fluoride hydrogen compound is C x F y H z , and x, y and z satisfy: 0.25≤x / (x+y+z)≤0.95, and x≠0, y≠0, z≠0. In the composite current collector provided by the present invention, the provision of the transition layer can have a good protective effect on the metal layer, and promote the improvement of the cycle charge and discharge performance of the electrochemical device using the composite current collector. At the same time, the presence of the transition layer can be conducive to the generation of non-penetrating and small-sized grains in the metal layer, thereby avoiding the formation of a closed loop by the positive and negative current collectors during the acupuncture process of the battery using the composite current collector and the resulting battery thermal runaway, thereby improving the safety performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular, relates to a composite current collector and a pole piece and a secondary battery using the composite current collector. Background Art

[0002] The current collector is one of the indispensable components of lithium-ion batteries. As a structure or part for collecting current, its function is to collect the electrons generated by the electrochemical reaction and conduct them to the external circuit, thereby realizing the process of converting chemical energy into electrical energy. Currently common current collectors include metal foil and composite current collectors. The composite current collector has a multilayer structure with a polymer film as the substrate in the middle and metal layers plated on both sides of the substrate. Compared with traditional metal foil current collectors (aluminum foil or copper foil), composite current collectors have the characteristics of low cost, light weight, and good internal insulation, which is beneficial to improving the energy density and safety of electrochemical devices. It has received widespread attention and application in the new energy industry.

[0003] However, there are two main problems with the composite current collectors that have been put into use in the field of secondary batteries: ① During the battery charge and discharge cycle, the metal layer of the composite current collector used is continuously corroded by the electrolyte, resulting in a significant degradation of the battery performance, which is manifested as poor battery charge and discharge cycle performance; ② The current composite current collector mainly relies on the insulation and flame retardant properties of the intermediate layer, namely the polymer film layer, to improve the safety of the battery. Although this improves the battery safety performance, the improvement is limited. Therefore, in order to further improve the charge and discharge cycle and safety performance of secondary batteries based on composite current collectors, it is necessary to develop a new composite current collector to promote the application and promotion of composite current collectors in secondary batteries. Summary of the invention

[0004] The present invention provides a composite current collector and a pole piece and a secondary battery using the composite current collector. The composite current collector can improve the cycle charge and discharge performance and safety performance of the secondary battery while maintaining good mechanical properties.

[0005] According to a first aspect of the present invention, a composite current collector is provided, the composite current collector comprising a base layer and a conductive layer, at least one surface of the base layer is provided with a conductive layer, the conductive layer comprises n1 transition layers and n2 metal layers, n1 is a positive integer greater than 1, n2 is a positive integer greater than 1, in the conductive layer, the metal layers and the transition layers are alternately stacked; the transition layer is composed of a carbon fluoride compound, the chemical formula of the carbon fluoride compound is C x F y H z , and x, y and z satisfy: 0.25≤x / (x+y+z)≤0.95, and x≠0, y≠0, z≠0.

[0006] In the composite current collector provided in this embodiment, the applicable base layer is not limited, and any base layer that can be used as the current collector can be, for example, a textile layer, a polymer base film, or a polymer base film mixed with conductive particles.

[0007] In the above-mentioned composite current collector, the carbon fluoride compound used to construct the transition layer has sufficient carbon content, so it has good conductivity. At the same time, the carbon fluoride compound also has good corrosion resistance and can generate a certain bonding force with the metal layer. In the composite current collector provided by the present invention, the setting of the transition layer has the following effects: ① The transition layer has good barrier and tolerance to the electrolyte, which can realize layer-by-layer protection of the metal layer, thereby promoting the improvement of the cycle charge and discharge performance of the battery based on the composite copper current collector; ② The transition layer can block the penetration of adjacent metal layer grains, promote the re-nucleation and growth of grains, thereby generating non-penetrating and smaller grains, and the generated metal layer composed of non-penetrating and small grains will produce microcracks after a certain deformation during the battery needle puncture process, and quickly spread to the surrounding area, causing large-scale fracture and fragmentation of the metal layer, thereby realizing the separation of the metal layer and the steel needle, avoiding the positive and negative current collectors from being connected to form a closed loop and the resulting battery thermal runaway, thereby improving the safety performance of the battery; ③ It can strengthen the strain hardening of the composite current collector and reduce local stress concentration, thereby promoting the cracks generated by the deformation of the metal layer of the composite current collector during the battery needle puncture process to be transmitted to the surrounding area, continuously promoting large-scale fracture and fragmentation of the metal layer, avoiding the positive and negative current collectors from being connected to form a closed loop and the resulting battery thermal runaway, thereby improving the safety performance of the battery.

[0008] Preferably, in the chemical formula of the hydrofluorocarbon compound, x, y and z satisfy: 0.3≤x / (x+y+z)≤0.9, 0.05≤y / (x+y+z)≤0.5, 0.05≤z / (x+y+z)≤0.2. The chemical composition of the hydrofluorocarbon compound used to construct the transition layer meets the above characteristics, which can further improve the corrosion resistance of the hydrofluorocarbon compound and its bonding force to the metal layer while maintaining good conductivity, and the flexibility of the transition layer formed by the hydrofluorocarbon compound is also improved.

[0009] In the above scheme: the transition layer can be composed of multiple sub-transition layers; the metal layer can be composed of multiple sub-metal layers; when more than one transition layer is provided on the base layer, the materials of each transition layer can be the same or different; when more than one metal layer is provided on the base layer, the materials of each metal layer can be the same or different.

[0010] Preferably, the composite current collector satisfies at least one of the conditions (a), (b), and (c): (a) the thickness of each transition layer is d1, 2nm≤d1≤50nm; (b) the thickness of the conductive layer is D, 500nm≤D≤2000nm, preferably 800nm≤D≤1800nm; (c) the thickness of the base layer is d3, 1μm≤d3≤10μm.

[0011] When the thickness of each transition layer is d1, 2nm≤d1≤50nm. By controlling the thickness of the transition layer within this range, the safety performance of the composite current collector can be significantly improved by the setting of the transition layer while maintaining good conductivity of the composite current collector, which is reflected in that the electrochemical device using the composite current collector has both good safety performance and cycle performance.

[0012] Preferably, 5nm≤d1≤20nm.

[0013] Optionally, the transition layer is prepared by magnetron sputtering or chemical vapor deposition.

[0014] Preferably, the transition layer is prepared by magnetron sputtering, and the related processes for preparing the transition layer include the following settings: graphite with a purity of ≥99.9% is used as a target material, and the target power is set to 1kW to 8kW; the vacuum degree of the vacuum chamber for magnetron sputtering is set to ≤0.1Pa; the gas source for coating includes argon, methane and carbon tetrafluoride, and the gas flow rate is 10mL / min to 500mL / min; the coating time each time is 0.1s to 30s.

[0015] When the thickness of the conductive layer is D, 500nm≤D≤2000nm. Controlling the thickness of the conductive layer within the above range can ensure that the composite current collector maintains good conductivity and that the electrochemical device using the composite current collector has good energy density.

[0016] Preferably, 800 nm ≤ D ≤ 1800 nm; further preferably, 800 nm ≤ D ≤ 1200 nm.

[0017] When the thickness of the base layer is d3, 1 μm≤d3≤10 μm. The preferred thickness range of the base layer is determined based on the difficulty of the preparation process and the cost.

[0018] Preferably, the material constituting the base layer includes at least one of polyethylene terephthalate (PET), polypropylene (PP), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polyethylene (PE), polypropylene, polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyphenylene sulfide (PPS), polyphenylene oxide (PPO), polystyrene (PS), and polyimide (PI).

[0019] Optionally, the base layer is prepared by a melt-extrusion-biaxial stretching method.

[0020] Preferably, the material constituting the metal layer includes at least one of copper, aluminum, copper alloy, and aluminum alloy; the thickness of each metal layer is d2,0<d2≤200nm. By controlling the thickness of the metal layer within this range, the preparation efficiency of the composite current collector is taken into account while ensuring that the setting of the transition layer can significantly improve the safety performance of the current collector. If the metal layer is set too thin, the preparation efficiency of the composite current collector is low. If the metal layer is set too thick, the degree to which the setting of the transition layer improves the safety performance of the composite current collector is reduced.

[0021] Preferably, 50nm≤d2≤150nm.

[0022] Preferably, the composite current collector satisfies conditions (e) and / or (f): (e) the number of metal layers n2 satisfies n2≥2, preferably, 5≤n2≤15; (f) the number of transition layers n1 satisfies n1≥2, preferably, 5≤n1≤15. The metal layers and transition layers are alternately stacked. When the composite current collector satisfies conditions (e) and / or (f), the preparation efficiency of the composite current collector is taken into account while ensuring that the setting of the transition layer can significantly improve the safety performance of the current collector. If the number of metal layers is too many, the preparation efficiency of the composite current collector is low. If the number of metal layers is too few, since the metal layers and transition layers are alternately stacked, the number of transition layers is correspondingly reduced, resulting in a decrease in the degree of improvement in the safety performance of the composite current collector by the setting of the transition layer.

[0023] Optionally, the metal layer is prepared by using one of physical vapor deposition, electroplating, and chemical plating processes, or a combination of multiple processes.

[0024] Preferably, the composite current collector further comprises a protective layer, which is arranged on the surface of the conductive layer, and the material constituting the protective layer comprises at least one of nickel, chromium, nickel-based alloy, copper-based alloy, copper oxide, aluminum oxide, silicon oxide, nickel oxide, chromium oxide, cobalt oxide, graphite, carbon black, copper chromate, copper chromite, carbon nano-quantum dots, carbon nanotubes, carbon nanofibers, graphene, and carbon fluoride. Among them, the carbon fluoride compound suitable for constructing the above-mentioned transition layer is also suitable for constructing the carbon fluoride compound as a protective layer. The provision of the protective layer can better prevent the conductive layer from chemical corrosion or physical damage.

[0025] Optionally, the protective layer is prepared by using one of the processes of physical vapor deposition, chemical vapor deposition, in-situ forming, coating, or a combination of multiple processes.

[0026] When the above-mentioned protective layer is prepared by physical vapor deposition, vacuum evaporation and / or magnetron sputtering are preferably used. When the above-mentioned protective layer is prepared by chemical vapor deposition, atmospheric pressure chemical vapor deposition and / or plasma enhanced chemical vapor deposition are preferably used. When the above-mentioned protective layer is prepared by in-situ molding, a method of in-situ forming a metal oxide passivation layer on the surface of the metal layer is preferably used. When the above-mentioned protective layer is prepared by coating, one of the processes of die coating, blade coating, and extrusion coating or a combination of multiple processes is preferably used.

[0027] Preferably, the thickness of the protective layer is d4, 5nm≤d4≤100nm.

[0028] Preferably, 10nm≤d4≤80nm.

[0029] According to the second aspect of the present invention, a method for preparing a composite current collector as above is provided: the preparation method comprises the following operations: alternately forming a transition layer and a metal layer on at least one surface of a base layer; the transition layer is formed by a magnetron sputtering process and / or a chemical vapor deposition process; the forming conditions of the magnetron sputtering process include: using a carbon-containing target material and setting a target power of 1-8kW; providing a gas containing fluorine and hydrogen elements as a coating gas source and setting a flow rate of the coating gas source to 10-500mL / min; during the coating process, providing a vacuum environment with a vacuum degree ≤0.1Pa and each coating time is 0.1-30s.

[0030] According to a third aspect of the present invention, a pole piece is provided, the pole piece comprising the composite current collector as described above and an active material layer arranged on the surface of the composite current collector.

[0031] According to a fourth aspect of the present invention, a secondary battery is provided, the secondary battery comprising the pole piece as described above. Since the secondary battery uses the composite pole piece provided by the present invention, the secondary battery has both good cycle charge and discharge characteristics and good safety performance. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only embodiments of a part of the present invention, rather than all embodiments.

[0033] Example 1

[0034] In this embodiment, the composite current collector is prepared by magnetron sputtering according to the following steps:

[0035] S1. A biaxially oriented PET film with a thickness of 4.5 μm is used as a base layer, and the base layer is placed in a magnetron sputtering machine.

[0036] S2. A transition layer is deposited on the surface of the base layer. The materials and operating parameters involved in the preparation of the transition layer are set as follows: a graphite target (purity of 99.99%) is used as the target material, and the target power is 2kW; carbon tetrafluoride (CF4), methane (CH4) and argon are used as gas sources, wherein the CF4 flow rate is 180mL / min, the CH4 flow rate is 60mL / min, and the argon flow rate is 50mL / min; the coating vacuum degree is 0.08Pa, the coating time is 1s, and the temperature of the main roller during the coating process is 0°C; the transition layer thus prepared is composed of a carbon fluorine hydrogen compound, and the chemical formula of the carbon fluorine hydrogen compound is C x F y H z , and x, y and z satisfy: x / (x+y+z)=0.6, y / (x+y+z)=0.3, z / (x+y+z)=0.1, and the thickness of the transition layer is 5nm.

[0037] S3. A metal layer is deposited on the surface of the transition layer formed in S2. The materials and operating parameters involved in the preparation of the metal layer are set as follows: a copper target (purity: 99.99%) is used as the target material, and the target power is 12 kW; argon gas is used as the source, the argon gas flow rate is 50 mL / min, the coating vacuum is 0.08 Pa, the coating time is 5 s, and the temperature of the main roller during the coating process is 2°C; the metal layer thus obtained is composed of copper alone, and the thickness of the metal layer is 50 nm.

[0038] S4. Repeat the above S2 and S3 in a cycle until a total of 10 transition layers and 10 metal layers are formed on the two side surfaces of the base layer. On either side surface of the base layer, 10 transition layers and 10 metal layers are alternately stacked to form a conductive layer, and the total thickness of the conductive layer is 550nm, thereby obtaining a composite current collector semi-finished product.

[0039] S5. Place the composite current collector semi-finished product obtained after S4 is completed in a coating device, use a graphene solution with a solid content of 0.10wt.% (the solvent is nitrogen methyl pyrrolidone) as the coating liquid, and then use a die coating process to evenly coat the coating liquid on the surface of the conductive layer on both sides of the composite current collector semi-finished product (the conductive layer on each side has a metal layer as its surface layer), and finally dry it at 70°C to form a protective layer with a thickness of 10nm on the surface of the conductive layer, thereby obtaining the composite current collector of this embodiment, and the total thickness of the composite current collector is 5.62μm.

[0040] Example 2

[0041] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector of this embodiment. The difference from Example 1 is that in S3 of the process of preparing the composite current collector, the coating time of the metal layer is adjusted to 10s, and the thickness of the metal layer obtained is 100nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 1.

[0042] Example 3

[0043] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector of this embodiment. The difference from Example 1 is that in S3 of the process of preparing the composite current collector, the coating time of the metal layer is adjusted to 15s, and the thickness of the metal layer thus prepared is 150nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 1.

[0044] Example 4

[0045] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector of this embodiment. The difference from Example 1 is that in S3 of the process of preparing the composite current collector, the coating time of the metal layer is adjusted to 20s, and the thickness of the metal layer obtained is 200nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 1.

[0046] Example 5

[0047] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector of this embodiment. The difference from Example 2 is that in S2 of the process of preparing the composite current collector, the coating time of the transition layer is adjusted to 0.4s, and the thickness of the transition layer obtained is 2nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 2.

[0048] Example 6

[0049] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector of this embodiment. The difference from Example 2 is that in S2 of the process of preparing the composite current collector, the coating time of the transition layer is adjusted to 4s, and the thickness of the transition layer thus prepared is 20nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 2.

[0050] Example 7

[0051] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector of this embodiment. The difference from Example 2 is that in S2 of the process of preparing the composite current collector, the coating time of the transition layer is adjusted to 10s, and the thickness of the transition layer thus prepared is 50nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 2.

[0052] Example 8

[0053] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector in this embodiment. The difference from Example 2 is that S4 of the process of preparing the composite current collector is adjusted to "repeating the above S2 and S3 in sequence until a total of 3 transition layers and 3 metal layers are formed on the two side surfaces of the base layer, and on either side surface of the base layer, 3 transition layers and 3 metal layers are alternately stacked to form a conductive layer, and the total thickness of the conductive layer is 315nm, thereby obtaining a composite current collector semi-finished product", except for the above differences, the materials used in the process of preparing the composite current collector in this embodiment and other specific operations are strictly consistent with those in Example 2.

[0054] Example 9

[0055] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector in this embodiment. The difference from Example 2 is that S4 of the process of preparing the composite current collector is adjusted to "repeating the above S2 and S3 in sequence until a total of 5 transition layers and 5 metal layers are formed on the two side surfaces of the base layer, and on either side of the base layer, 5 transition layers and 5 metal layers are alternately stacked to form a conductive layer, and the total thickness of the conductive layer is 525nm, thereby obtaining a composite current collector semi-finished product". In addition to the above differences, the materials used in the process of preparing the composite current collector in this embodiment and other specific operations are strictly consistent with those in Example 2.

[0056] Example 10

[0057] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector in this embodiment. The difference from Example 2 is that S4 of the process of preparing the composite current collector is adjusted to "repeating the above S2 and S3 in sequence until a total of 15 transition layers and 15 metal layers are formed on the two side surfaces of the base layer, and on either side of the base layer, 15 transition layers and 15 metal layers are alternately stacked to form a conductive layer, and the total thickness of the conductive layer is 1575nm, thereby obtaining a composite current collector semi-finished product", except for the above differences, the materials used in the process of preparing the composite current collector in this embodiment and other specific operations are strictly consistent with those in Example 2.

[0058] Embodiment 11

[0059] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector of this embodiment. The difference from Example 1 is that S3 of the process of preparing the composite current collector is adjusted to "depositing a metal layer on the surface of the transition layer formed in S2, and the materials and operating parameters involved in the preparation of the metal layer are set as follows: using an aluminum target (purity: 99.99%) as the target material, the target power is 10kW; using argon as the source, the argon flow rate is 50mL / min, the coating vacuum is 0.07Pa, the coating time is 5s, and the temperature of the main roller during the coating process is 2°C", and the metal layer thus obtained is composed of aluminum alone, and the thickness of the metal layer is 50nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector in this embodiment are strictly consistent with those in Example 1.

[0060] Example 12

[0061] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector of this embodiment. The difference from Example 1 is that in S1 of the process of preparing the composite current collector, the film material used as the base layer is replaced with a polypropylene (PP) film with a thickness of 4.5 μm. In addition to the above differences, the materials used in the process of preparing the composite current collector of this embodiment and other specific operations are strictly consistent with those of Example 1.

[0062] Embodiment 13

[0063] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector in this embodiment. The difference from Example 1 is that S2 of the process of preparing the composite current collector is adjusted to "depositing a transition layer on the surface of the base layer, and the materials and operating parameters involved in the preparation of the transition layer are set as follows: using a graphite target (purity of 99.99%) as the target material, the target power is 1.8kW; using carbon tetrafluoride (CF4), methane (CH4) and argon as the gas source, wherein the CF4 flow rate is 200mL / min, the CH4 flow rate is 120mL / min, and the argon flow rate is 50mL / min; the coating vacuum is 0.08Pa, the coating time is 1.2s, and the temperature of the main roller during the coating process is 0°C", and the transition layer thus obtained is composed of a carbon fluoride hydrogen compound, the chemical formula of which is C x F y H z , and x, y and z satisfy: x / (x+y+z)=0.3, y / (x+y+z)=0.5, z / (x+y+z)=0.2. Except for the above differences, the materials and other specific operations used in the process of preparing the composite current collector in this embodiment are strictly consistent with those in Example 1.

[0064] Embodiment 14

[0065] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector in this embodiment. The difference from Example 1 is that S2 of the process of preparing the composite current collector is adjusted to "depositing a transition layer on the surface of the base layer, and the materials and operating parameters involved in the preparation of the transition layer are set as follows: using a graphite target (purity of 99.99%) as the target material, the target power is 2.4kW; using carbon tetrafluoride (CF4), methane (CH4) and argon as the gas source, wherein the CF4 flow rate is 200mL / min, the CH4 flow rate is 120mL / min, and the argon flow rate is 50mL / min; the coating vacuum is 0.08Pa, the coating time is 0.8s, and the temperature of the main roller during the coating process is 0°C", and the transition layer thus obtained is composed of a carbon fluoride hydrogen compound, the chemical formula of which is C x F y H z , and x, y and z satisfy: x / (x+y+z)=0.9, y / (x+y+z)=0.05, z / (x+y+z)=0.05. Except for the above differences, the materials and other specific operations used in the process of preparing the composite current collector in this embodiment are strictly consistent with those in Example 1.

[0066] Embodiment 15

[0067] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector of this embodiment. The difference from Example 1 is that the coating time involved in S3 in the process of preparing the composite current collector is adjusted to 22s, and the thickness of the metal layer obtained is 220nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 1.

[0068] Example 16

[0069] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector of this embodiment. The difference from Example 2 is that in S2 of the process of preparing the composite current collector, the coating time of the transition layer is adjusted to 11s, and the thickness of the transition layer obtained is 55nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 2.

[0070] Embodiment 17

[0071] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector of this embodiment. The difference from Example 2 is that in S2 of the process of preparing the composite current collector, the coating time of the transition layer is adjusted to 0.2s, and the thickness of the transition layer obtained is 1nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector of this embodiment are strictly consistent with those of Example 2.

[0072] Embodiment 18

[0073] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector in this embodiment. The difference from Example 2 is that S4 of the process of preparing the composite current collector is adjusted to "repeating the above S2 and S3 in sequence until a total of 16 transition layers and 16 metal layers are formed on the two side surfaces of the base layer, and on either side surface of the base layer, 16 transition layers and 16 metal layers are alternately stacked to form a conductive layer, and the total thickness of the conductive layer is 1680nm, thereby obtaining a composite current collector semi-finished product", except for the above differences, the materials used in the process of preparing the composite current collector in this embodiment and other specific operations are strictly consistent with those in Example 2.

[0074] Embodiment 19

[0075] This embodiment refers to the method for preparing a composite current collector in Example 2 to complete the preparation of the composite current collector in this embodiment. The difference from Example 2 is that S4 of the process of preparing the composite current collector is adjusted to "repeating the above S2 and S3 in sequence until a total of 2 transition layers and 2 metal layers are formed on the two side surfaces of the base layer, and on either side surface of the base layer, 2 transition layers and 2 metal layers are alternately stacked to form a conductive layer, and the total thickness of the conductive layer is 210nm, thereby obtaining a composite current collector semi-finished product", except for the above differences, the materials used in the process of preparing the composite current collector in this embodiment and other specific operations are strictly consistent with those in Example 2.

[0076] Embodiment 20

[0077] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector in this embodiment. The difference from Example 1 is that S2 of the process of preparing the composite current collector is adjusted to "depositing a transition layer on the surface of the base layer, and the materials and operating parameters involved in the preparation of the transition layer are set as follows: using a graphite target (purity of 99.99%) as the target material, the target power is 1.6kW; using carbon tetrafluoride (CF4), methane (CH4) and argon as the gas source, wherein the CF4 flow rate is 318mL / min, the CH4 flow rate is 132mL / min, and the argon flow rate is 50mL / min; the coating vacuum is 0.08Pa, the coating time is 1.5s, and the temperature of the main roller during the coating process is 0°C", and the transition layer thus obtained is composed of a carbon fluoride hydrogen compound, the chemical formula of which is C x F y H z , and x, y and z satisfy: x / (x+y+z)=0.25, y / (x+y+z)=0.53, z / (x+y+z)=0.22. Except for the above differences, the materials and other specific operations used in the process of preparing the composite current collector in this embodiment are strictly consistent with those in Example 1.

[0078] Embodiment 21

[0079] This embodiment refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector in this embodiment. The difference from Example 1 is that S2 of the process of preparing the composite current collector is adjusted to "depositing a transition layer on the surface of the base layer, and the materials and operating parameters involved in the preparation of the transition layer are set as follows: using a graphite target (purity of 99.99%) as the target material, the target power is 2.6kW; using carbon tetrafluoride (CF4), methane (CH4) and argon as the gas source, wherein the CF4 flow rate is 18mL / min, the CH4 flow rate is 12mL / min, and the argon flow rate is 50mL / min; the coating vacuum is 0.08Pa, the coating time is 0.7s, and the temperature of the main roller during the coating process is 0°C", and the transition layer thus obtained is composed of a carbon fluoride hydrogen compound, the chemical formula of which is C x F y H z , and x, y and z satisfy: x / (x+y+z)=0.95, y / (x+y+z)=0.03, z / (x+y+z)=0.02. Except for the above differences, the materials and other specific operations used in the process of preparing the composite current collector in this embodiment are strictly consistent with those in Example 1.

[0080] Comparative Example 1

[0081] This comparative example uses Example 1 as a control, and adopts a magnetron sputtering method to prepare a composite current collector. The specific steps of preparing the composite current collector in this comparative example are as follows:

[0082] S1. A biaxially oriented PET film with a thickness of 4.5 μm is used as a base layer, and the base layer is placed in a magnetron sputtering machine.

[0083] S2. A metal layer is deposited on the surface of the base layer. The materials and operating parameters involved in the preparation of the metal layer are set as follows: a copper target (purity: 99.99%) is used as the target material, and the target power is 12kW; argon gas is used as the source, the argon flow rate is 50mL / min, the coating vacuum is 0.08Pa, the coating time is 50s, and the temperature of the main roller during the coating process is 2°C; the metal layer thus obtained is composed of copper alone, and the thickness of the metal layer is 500nm, thereby obtaining a composite current collector semi-finished product.

[0084] S3. Place the composite current collector semi-finished product obtained after S2 is completed in a coating device, use a graphene solution with a solid content of 0.10wt.% (the solvent is nitrogen methyl pyrrolidone) as the coating liquid, and then use a die coating process to evenly coat the coating liquid on the surface of the conductive layer on both sides of the composite current collector semi-finished product (the conductive layer on each side has a metal layer as its surface layer), and finally dry it at 70°C to form a protective layer with a thickness of 10nm on the surface of the conductive layer, thereby obtaining the composite current collector of this comparative example, the total thickness of which is 5.52μm.

[0085] Comparative Example 2

[0086] This comparative example refers to the method for preparing a composite current collector in Example 1 to complete the preparation of the composite current collector of this example. The difference from Example 1 is that S2 of the process of preparing the composite current collector is adjusted to "depositing a carbon layer on the surface of the base layer, and the materials and operating parameters involved in the preparation of the carbon layer are set as follows: using a graphite target (purity of 99.99%) as the target material, the target power is 3kW, the argon gas flow rate is 50mL / min, the coating vacuum is 0.08Pa, the coating time is 1s, and the temperature of the main roller during the coating process is 0°C", and the thickness of the carbon layer obtained is 5nm. In addition to the above differences, the materials and other specific operations used in the process of preparing the composite current collector in this comparative example are strictly consistent with Example 1.

[0087] Preparation Example

[0088] Preparation of Class A Lithium-ion Batteries:

[0089] S1. Preparation of a composite negative electrode sheet. As described above, the metal layer included in the composite current collectors prepared in Examples 1 to 10, 12-21 and Comparative Examples 1 and 2 is made of copper alone, and the composite current collectors are respectively used as negative electrode current collectors; the above materials are taken according to a mass ratio of graphite: conductive carbon (Super P): carbon nanotubes: carboxymethyl cellulose = 96:1.0:0.5:2.5, and the above materials are added to deionized water to prepare a negative electrode slurry with a solid content of 70%; the above negative electrode slurry is coated on the surface of the negative electrode current collector and dried to form a negative electrode active material layer.

[0090] S2. Preparation of conventional positive electrode sheets, using aluminum foil with a thickness of 13 μm as the positive electrode current collector; taking the above materials according to the mass ratio of NCM622: conductive carbon (Super P): carbon nanotubes: polyvinylidene fluoride = 96:1.8:0.5:1.7, adding the above materials to N-methylpyrrolidone to prepare a positive electrode slurry with a solid content of 70%; coating the above positive electrode slurry on the surface of the positive electrode current collector and drying it to form a positive electrode active material layer.

[0091] S3. Selection of the diaphragm: A polyethylene diaphragm (thickness 25 μm) coated with alumina ceramics was used as the diaphragm for assembling lithium-ion batteries.

[0092] S4. Preparation of electrolyte: Propylene carbonate, ethylene carbonate, and ethyl methyl carbonate are mixed in a mass ratio of 1:1:1 to obtain a carbonate solvent, and LiPF6 is added to the carbonate solvent to prepare 1 mol·L -1 LiPF6 carbonate solution is used as the electrolyte of lithium ion battery.

[0093] S5. Assembling lithium-ion batteries. Based on the different types of composite current collectors included, different composite negative electrode sheets are matched with the above-mentioned conventional positive electrode sheets respectively. The above-mentioned conventional positive electrode sheets, isolation membranes, and composite negative electrode sheets are stacked in order to prepare bare cells. The bare cells are placed in the outer packaging shell of the lithium battery, and the electrolyte is injected after drying. After vacuum packaging, standing, formation, shaping and other processes, a Class A lithium-ion battery is obtained.

[0094] Preparation of Class B Lithium-ion Batteries:

[0095] S1. Preparation of composite positive electrode sheet. The metal layer of the composite current collector prepared in Example 11 is made of aluminum. The composite current collector is used as the positive current collector. LiNi 0.6 Mn 0.2 Co 0.2 O2 (NCM622) is used as the positive electrode active material in the positive electrode active material layer.

[0096] S2. Preparation of a conventional negative electrode sheet, using a copper foil with a thickness of 6 μm as a negative electrode current collector, and preparing a negative electrode active material layer using artificial graphite as a negative electrode active material on the surface of the negative electrode current collector.

[0097] S3. The selection of the diaphragm is the same as the diaphragm selected for assembling the Class I lithium-ion battery mentioned above.

[0098] S4. The preparation of the electrolyte is the same as the electrolyte prepared for assembling Class I lithium-ion batteries as described above.

[0099] S5. Assembly of lithium-ion batteries. Based on the different types of composite current collectors included, different composite positive electrode sheets are matched with the above-mentioned conventional negative electrode sheets respectively. The above-mentioned composite positive electrode sheets, isolation membranes, and conventional negative electrode sheets are stacked in order to prepare bare cells. The bare cells are placed in the outer packaging shell of the lithium battery, and the electrolyte is injected after drying. After vacuum packaging, standing, formation, shaping and other processes, a Class B lithium-ion battery is obtained.

[0100] In the preparation operations of the above-mentioned Class A lithium-ion batteries and Class B lithium-ion batteries: the composition of the slurry used to form the negative electrode active material layer on the surfaces of different negative electrode current collectors remains completely the same, and the specific operations for forming the negative electrode active material layer also remain completely the same, thereby keeping the negative electrode active material layer in each lithium-ion battery consistent; the composition of the slurry used to form the positive electrode active material layer on the surfaces of different positive electrode current collectors remains completely the same, and the specific operations for forming the positive electrode active material layer also remain completely the same, thereby keeping the positive electrode active material layer in each lithium-ion battery consistent; except for the differences in the composition of the selected positive and negative electrode sheets, other components used in assembling lithium-ion batteries and related operations are strictly kept consistent.

[0101] Preparation of control lithium-ion battery:

[0102] S1. Preparation of conventional positive electrode sheet, using a traditional aluminum current collector with a thickness of 13 μm (consistent with the traditional aluminum current collector used in Class A lithium-ion batteries) as the positive electrode current collector, and preparing LiNi 0.6 Mn 0.2 Co 0.2 O2 (NCM622) is used as the positive electrode active material in the positive electrode active material layer.

[0103] S2. Preparation of conventional negative electrode sheets, using a traditional copper current collector with a thickness of 6 μm (consistent with the traditional copper current collector used in Class B lithium-ion batteries) as the negative electrode current collector, and preparing a negative electrode active material layer using artificial graphite as the negative electrode active material on the surface of the negative electrode current collector.

[0104] S3. The selection of the diaphragm is the same as the diaphragm selected for assembling the Class I lithium-ion battery mentioned above.

[0105] S4. The preparation of the electrolyte is the same as the electrolyte prepared for assembling Class I lithium-ion batteries as described above.

[0106] S5. Assembly of lithium-ion batteries. Based on the different types of composite current collectors included, different composite positive electrode sheets are matched with the above-mentioned conventional negative electrode sheets respectively. The above-mentioned composite positive electrode sheets, isolation membranes, and conventional negative electrode sheets are stacked in order to prepare bare cells. The bare cells are placed in the outer packaging shell of the lithium battery, and the electrolyte is injected after drying. After vacuum packaging, standing, formation, shaping and other processes, a control lithium-ion battery is obtained.

[0107] The lithium-ion batteries prepared in this preparation example are numbered, as shown in Table 1. In Table 1, the current collectors used to assemble the lithium-ion batteries are shown corresponding to the specific lithium-ion battery numbers.

[0108] Table 1. Lithium-ion batteries prepared in this preparation example

[0109]

[0110]

[0111] Test Case

[0112] 1. Current collector elongation at break test

[0113] (1) Test object

[0114] The composite current collectors prepared in Examples 1-21 and Comparative Examples 1 and 2 were used as test objects.

[0115] (2) Test items and test methods

[0116] Elongation at break test: The elongation at break of the test object is tested in accordance with the national standard GB / T 1040.3-2006.

[0117] 2. Battery performance test

[0118] (1) Test object

[0119] The composite current collector used in the preparation example and the lithium-ion battery prepared therefrom were used as test objects.

[0120] (2) Test items and test methods

[0121] Charge and discharge cycle performance: The lithium-ion battery is charged and discharged 2000 times at a charge and discharge rate of 1C, and the battery capacity retention rate after 2000 cycles of charge and discharge is calculated. The battery capacity retention rate after 2000 cycles of charge and discharge = battery capacity after 2000 cycles of charge and discharge / initial capacity of the battery × 100%.

[0122] Safety performance test: A puncture test is used to verify the safety performance of lithium-ion batteries. Specifically, the lithium-ion battery is placed in a puncture test device. The diameter of the steel needle in the puncture test device is 3mm. The puncture speed is set to 10mm / s. The lithium-ion battery is subjected to a puncture test, wherein 100 repetitions are set for each lithium-ion battery, and each repetition is 1 lithium-ion battery. During the puncture test, if the test object does not explode, catch fire, or smoke, it is marked as "passed", otherwise it is marked as "failed". The number of lithium-ion batteries marked as "passed" and "failed" in the 100 repetitions set for each lithium-ion battery is recorded and counted, and the battery puncture pass rate of each lithium-ion battery is calculated. The battery puncture pass rate of each lithium-ion battery = the number of lithium-ion batteries marked as "passed" / the total number of lithium-ion batteries in the repeated experiments set for the test object × 100%. 3. Result analysis

[0123] The test results of this test example are shown in Table 2. It can be clearly seen from the data shown in Table 2 that in the test example, the battery capacity retention rate and battery puncture pass rate measured by battery D1 and battery D2 are significantly lower. Compared with the above-mentioned lithium-ion batteries, batteries 1 to 21 have higher battery capacity retention rate and higher battery puncture pass rate. In Class A lithium-ion batteries, the negative electrode current collectors used in batteries 1 to 10 and batteries 12 to 21 are all provided with a transition layer composed of a carbon fluorine hydrogen compound. The setting of the transition layer has the following functions: ① The transition layer can tolerate the electrolyte and can adhere to the surface of the metal layer, so as to play a certain barrier role in the contact between the electrolyte and the metal layer, so as to have a good protective effect on the metal layer, so that the cycle charge and discharge performance of batteries 1 to 21 is significantly improved compared with batteries D1 and D2; ② The transition layer can block the penetration of adjacent metal layer grains and promote the re-nucleation and growth of grains, thereby generating non-penetrating and smaller grains. The generated non-penetrating and smaller grains The metal layer composed of small grains will produce micro cracks after a certain deformation during the battery needle puncture process, and quickly spread to the surrounding area, causing large-scale fracture and fragmentation of the metal layer, thereby separating the metal layer from the steel needle, avoiding the positive and negative current collectors from conducting to form a closed loop and the resulting battery thermal runaway, thereby improving the safety performance of the battery; ③ It can strengthen the strain hardening of the composite current collector and reduce local stress concentration, thereby promoting the cracks caused by the deformation of the composite current collector metal layer during the battery needle puncture process to be transmitted to the surrounding area, continuously promoting large-scale fracture and fragmentation of the metal layer, avoiding the positive and negative current collectors from conducting to form a closed loop and the resulting battery thermal runaway, and improving the safety performance of the battery. The above effects are conducive to avoiding the positive and negative current collectors of batteries 1 to 21 from conducting to form a closed loop and the resulting battery thermal runaway, thereby significantly improving the safety performance of batteries 1 to 21 compared to batteries D1 and D2.

[0124] Comparing the test results of batteries 1, 13, 14, 20, and 21 in this test example, compared with batteries 20 and 21, batteries 1, 13, and 14 have higher battery capacity retention rates and higher battery needle penetration rates, and have better cycle charge and discharge performance and safety. Among the above test objects, the difference lies in the chemical composition of the carbon fluorine and hydrogen compounds that constitute the composite current collector transition layer. The carbon content, fluorine content, and hydrogen content in the carbon fluorine and hydrogen compounds will affect the properties of the transition layer, which is reflected in the fact that the lithium-ion batteries using these composite current collectors have certain differences in cycle charge and discharge performance and safety performance. Among them, as the carbon content of the carbon fluorohydrogen compound in the transition layer increases, the conductivity of the composite current collector tends to increase while the flexibility tends to decrease. When the carbon content of the carbon fluorohydrogen compound constituting the transition layer satisfies 0.3≤x / (x+y+z)≤0.9, the transition layer can have both good conductivity and flexibility. Relative to the above-mentioned preferred carbon content range of the carbon fluorohydrogen compound, when the carbon content of the carbon fluorohydrogen compound used to constitute the transition layer is relatively low, the conductivity of the transition layer will deteriorate to a certain extent, thereby resulting in a decrease in the conductivity of the composite current collector and a decrease in the cycle performance of the battery. When the carbon content of the carbon fluorohydrogen compound used to constitute the transition layer is relatively high, the flexibility of the transition layer decreases, and relatively speaking, the possibility of defects in the transition layer increases, resulting in a decrease in the cycle performance of the battery. The fluorine content and hydrogen content in the carbon fluoride compound have a comprehensive impact on the corrosion resistance and polarity of the transition layer. When the fluorine content of the carbon fluoride compound satisfies 0.05≤y / (x+y+z)≤0.5 and the hydrogen content satisfies 0.05≤z / (x+y+z)≤0.2, the transition layer can have excellent corrosion resistance and the polarity of the transition layer can be controlled within the preferred range so that the transition layer can be stably compounded with the metal layer. Compared with the carbon fluoride compound whose fluorine content and hydrogen content can reach the above preferred range, when the fluorine content of the carbon fluoride compound is low or the hydrogen content is high, the corrosion resistance of the transition layer is reduced, resulting in a decrease in the cycle performance of the battery. When the fluorine content of the carbon fluoride compound is high or the hydrogen content is low, the polarity of the transition layer is reduced, resulting in a decrease in the bonding force between the transition layer and the metal layer, resulting in a decrease in the cycle performance and safety performance of the battery. In general, with regard to the carbon fluoride compound constituting the transition layer, the chemical composition C of the carbon fluoride compound is made x F y H z Satisfying the conditions of 0.3≤x / (x+y+z)≤0.9, 0.05≤y / (x+y+z)≤0.5, and 0.05≤z / (x+y+z)≤0.2 can enable the transition layer to have better comprehensive performance. On the basis of ensuring that the transition layer has good conductivity and flexibility, the corrosion resistance of the transition layer and its bonding force to the metal layer can be further improved, so that the lithium-ion battery using the composite current collector provided with the transition layer can show excellent cycle charge and discharge performance and safety.

[0125] The difference between the composite current collectors prepared in Examples 2, 5 to 7, 16, and 17 is that the thickness of the transition layer is different. By comparing the test results of lithium-ion batteries (batteries 2, 5 to 7, 16, and 17) using the above-mentioned composite current collectors as negative electrode current collectors, it can be seen that: as the thickness of the transition layer of the applied composite current collector increases, the cycle capacity retention rate of the lithium-ion battery shows a trend of first increasing and then decreasing. On the other hand, the battery puncture pass rate measured by the lithium-ion battery shows a trend of first increasing significantly and then tending to remain stable. The reason for the above-mentioned variation in test results may be that a transition layer is provided based on the application of lithium-ion batteries: increasing the thickness of the transition layer can improve the barrier and tolerance of the transition layer to the electrolyte, thereby promoting the improvement of the cycle charge and discharge performance of the battery based on the composite copper current collector, but when the thickness of the transition layer exceeds a certain range, the conductivity of the conductive layer decreases, causing the cycle charge and discharge performance of the lithium-ion battery using the composite current collector to deteriorate; on the other hand, increasing the thickness of the transition layer can promote large-scale fission of the conductive layer during the puncture process, thereby making it easier for the conductive layer to disconnect from the steel needle, which is reflected in the significant improvement of the battery puncture pass rate measured by the test object, that is, the safety performance of the lithium-ion battery is significantly improved, but when the thickness of the transition layer exceeds a certain range, further increasing the thickness of the transition layer is difficult to significantly improve the battery puncture pass rate, and the safety performance of the lithium-ion battery is basically stable. Based on the test results analysis of batteries 2, 5 to 7, 16, and 17, and by comprehensively considering the cyclic charge and discharge performance and safety performance of lithium-ion batteries, the preferred value range of the transition layer thickness d1 is 2 nm to 50 nm, and the more preferred value range is 5 nm to 20 nm.

[0126] The difference between the composite current collectors prepared in Examples 1 to 4 and 15 is the different thickness of the metal layer. By comparing the test results of the test objects (batteries 1 to 4 and 15) using the above composite current collectors as negative current collectors, it can be seen that as the thickness of the metal layer of the composite current collector increases, the capacity retention rate and battery puncture pass rate measured by the lithium-ion battery both show a trend of first increasing and then decreasing. The reason for the change in the above test results may be that the conductive layer of the composite current collector used in the lithium-ion battery is composed of an alternating stack of metal layers and transition layers. The change in the thickness of the metal layer will affect the distribution of the grain size in the metal layer along its thickness direction, and then affect the fracture behavior of the metal layer during the puncture test, causing the safety performance of the lithium-ion battery to change. At the same time, the change in the grain size along the thickness direction of the metal layer will also affect the conduction resistance of electrons during the charge and discharge process of the lithium-ion battery, thereby affecting the cycle charge and discharge performance of the battery. Based on the test results analysis of batteries 1 to 4 and 15, by comprehensively considering the cyclic charge and discharge performance and safety performance of lithium-ion batteries, the thickness of the metal layer d2 should preferably be controlled within a value range of no more than 200 nm, and the more preferred value range of d2 is 50 nm to 150 nm.

[0127] In the process of preparing the composite current collectors in Examples 2, 8 to 10, 18 and 19, the difference is that the number of transition layers n1 and the number of metal layers n2 are different. The composite current collectors prepared in the above examples are respectively applied to the preparation of batteries 2, 8 to 10, 18 and 19. By comparing the test results of batteries 2, 8 to 10, 18 and 19, it can be seen that: with the increase of the number of metal layers and transition layers set in the composite current collector, the thickness of the composite current collector increases, and the capacity retention rate and battery needle penetration rate measured by the lithium-ion battery both show a change pattern of first increasing significantly and then tending to remain stable. The reason for the above test results may be that by increasing the number of transition layers and metal layers and increasing the multi-layer alternating structure of transition layers and metal layers, on the one hand, the protection of the transition layer to the metal layer can be strengthened, and the inhibition of the electrolyte corrosion of the metal layer can be strengthened, thereby promoting the improvement of the battery cycle charge and discharge performance. On the other hand, it can promote the conductive layer of the lithium-ion battery to be more prone to large-scale fission during the needle puncture test, so that the conductive layer is easier to disconnect from the steel needle, thereby promoting the safety performance of the lithium-ion battery. However, when the number of transition layers and metal layers exceeds 15 layers, continuing to increase the number of layers of the two will make it difficult to significantly improve the performance of lithium-ion batteries based on such composite current collectors. However, the increase in the number of transition layers and metal layers also increases the production process and preparation cost of the composite current collector. Combined with the above results analysis, considering the cycle charge and discharge performance, safety performance, and preparation efficiency and cost of the lithium-ion battery based on such composite current collectors, the number of transition layers and metal layers is preferably 5 to 15 layers.

[0128] Table 2. Statistics of battery performance test results of the lithium-ion batteries tested in this test case

[0129]

[0130]

[0131] Among the lithium-ion batteries tested, the difference between battery 1, battery 13, battery 14, battery 20, battery 21 and battery D2 lies in the material composition of the transition layer included in the composite current collector used in the lithium-ion battery, wherein the transition layer provided in the composite current collector used in battery 1 (corresponding to Example 1), battery 13 (corresponding to Example 13), battery 14 (corresponding to Example 14), battery 20 (corresponding to Example 20), and battery 21 (corresponding to Example 21) is composed of carbon fluorine hydrogen compounds, while the transition layer provided in the composite current collector used in battery D2 (corresponding to Example 2) is composed of only carbon. In order to fully reflect the influence of the material composition of the transition layer on the performance of the composite current collector and the lithium-ion battery, the composite current collector used in the above lithium-ion battery is further tested for elongation at break, and the test results are shown in Table 3. From the data shown in Table 3, it can be seen that the measured elongation at break corresponding to the composite current collector prepared in Comparative Example 2 is significantly lower than that of other composite current collectors tested. This shows that compared with the transition layer structure composed of carbon fluorine compounds, the application of a transition layer composed of a single carbon substance will significantly reduce the elongation at break of the composite current collector, thereby significantly deteriorating the mechanical properties of the composite current collector. Although the test results presented in Table 2 show that in the battery capacity retention rate test of the lithium-ion battery carried out in this test example, the battery capacity retention rate of battery D2 is higher than that of battery 13, battery 14, battery 20, and battery 21, however, considering the elongation at break test results of this test example, when the carbon content of the transition layer increases, the elongation at break of the composite current collector decreases, and the mechanical properties of the composite current collector deteriorate. Therefore, in the process of actual application, as the number of cycles of the lithium-ion battery gradually increases, the external stress on the composite current collector accumulates, making the mechanical properties of the composite fluid have an increasingly obvious effect on the structural stability of the composite current collector and the cycle performance of the lithium-ion battery. As a result, as the number of cycles increases, the deterioration of the cycle stability of battery D2 will become increasingly obvious, which is reflected in the degradation of the battery performance of battery D2.

[0132] Table 3. Statistics of elongation at break test results of composite current collectors

[0133]

[0134]

[0135] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solutions of the present invention can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present invention.

Claims

1. A composite current collector, characterized in that: The composite current collector comprises a base layer and a conductive layer, wherein the conductive layer is disposed on at least one surface of the base layer, wherein the conductive layer comprises n1 transition layers and n2 metal layers, wherein n1 is a positive integer greater than 1, and n2 is a positive integer greater than 1, and in the conductive layer, the metal layers and the transition layers are alternately stacked; The transition layer is made of a carbon fluoride hydrogen compound, the chemical formula of the carbon fluoride hydrogen compound is CxFyHz, and x, y and z satisfy: 0.25≤x / (x+y+z)≤0.95, and x≠0, y≠0, z≠0; The composite current collector preparation method comprises the following operations: alternately forming the transition layer and the metal layer on at least one surface of the base layer, and first forming the transition layer and then forming the metal layer; Wherein, the transition layer is formed by a magnetron sputtering process; The molding conditions of the magnetron sputtering molding process include: using a carbon-containing target material and setting the target power to 1-8kW; providing a gas containing fluorine and hydrogen elements as a coating gas source and setting the flow rate of the coating gas source to 10-500mL / min; during the coating process, providing a vacuum environment with a vacuum degree of ≤0.1Pa, and each coating time is 0.1-30s.

2. The composite current collector according to claim 1, characterized in that: In the chemical formula of the hydrofluorocarbon compound, x, y and z satisfy: 0.3≤x / (x+y+z)≤0.9, 0.05≤y / (x+y+z)≤0.5, 0.05≤z / (x+y+z)≤0.

2.

3. The composite current collector according to claim 1, characterized in that , the composite current collector satisfies at least one of the conditions (a), (b), and (c): (A) The thickness of each transition layer is d1, 2nm≤d1≤50nm; (b) the thickness of the conductive layer is D, 500nm≤D≤2000nm; (c) The thickness of the base layer is d3, 1 μm≤d3≤10 μm.

4. The composite current collector according to claim 3, characterized in that , the composite current collector satisfies 800nm≤D≤1800nm.

5. The composite current collector according to claim 1, characterized in that: The material constituting the metal layer includes at least one of copper, aluminum, copper alloy and aluminum alloy; the thickness of each metal layer is d2,0<d2≤200nm.

6. The composite current collector according to claim 5, characterized in that: 50nm≤d2≤150nm.

7. The composite current collector according to claim 5, characterized in that: The composite current collector satisfies conditions (e) and / or (f): (e) The number of layers n2 of the metal layer satisfies 5≤n2≤15; (f) The number n1 of the transition layer satisfies 5≤n1≤15.

8. The composite current collector according to claim 1, characterized in that: The material constituting the base layer includes at least one of polyethylene terephthalate, polypropylene, polybutylene terephthalate, polyethylene naphthalate, polyethylene, polypropylene, polyvinyl chloride, polyvinylidene fluoride, polytetrafluoroethylene, polyphenylene sulfide, polyphenylene oxide, polystyrene, and polyimide.

9. The composite current collector according to any one of claims 1 to 8, characterized in that: The composite current collector also includes a protective layer, which is arranged on the surface of the conductive layer. The material constituting the protective layer includes at least one of nickel, chromium, nickel-based alloys, copper-based alloys, copper oxide, aluminum oxide, silicon oxide, nickel oxide, chromium oxide, cobalt oxide, graphite, carbon black, copper chromate, copper chromite, carbon nano-quantum dots, carbon nanotubes, carbon nanofibers, graphene, and carbon fluorocarbons.

10. The composite current collector according to claim 9, characterized in that: The thickness of the protective layer is d4, 5nm≤d4≤100nm.

11. A method for preparing the composite current collector according to any one of claims 1 to 10, characterized in that: The preparation method comprises the following operations: alternately forming the transition layer and the metal layer on at least one surface of the base layer, and first forming the transition layer and then forming the metal layer; Wherein, the transition layer is formed by a magnetron sputtering process; The molding conditions of the magnetron sputtering molding process include: using a carbon-containing target material and setting the target power to 1-8kW; providing a gas containing fluorine and hydrogen elements as a coating gas source and setting the flow rate of the coating gas source to 10-500mL / min; during the coating process, providing a vacuum environment with a vacuum degree of ≤0.1Pa, and each coating time is 0.1-30s.

12. A pole piece, characterized in that: The pole piece comprises the composite current collector according to any one of claims 1 to 10 and an active material layer arranged on the surface of the composite current collector.

13. A secondary battery, characterized in that: The secondary battery comprises the electrode sheet as claimed in claim 12.

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