Negative electrode current collector and lithium metal battery comprising same

Abstract

The present invention relates to a negative electrode current collector for a lithium metal battery used in a lithium metal battery having a lithium metal as a negative electrode, and a lithium metal battery comprising same. The negative electrode current collector for a lithium metal battery according to the present invention comprises a carbon nanotube fiber formed as a two-dimensional sheet by connecting carbon nanotubes through a network.

Description

Anode current collector and lithium metal battery containing the same

[0001] The present invention relates to a lithium secondary battery, and more particularly, to a negative electrode current collector used in a lithium metal battery having lithium metal as a negative electrode, and a lithium metal battery including the same.

[0002] Lithium metal batteries are batteries whose basic system uses lithium metal as the cathode, replacing the lithium ion intercalation anode of a lithium-ion battery. During charging, lithium ions are deposited on the surface of the cathode current collector or anode composite, reducing them to lithium metal. During discharging, the lithium metal is oxidized back to lithium ions.

[0003] Conventional anode active materials have mainly been utilized or studied, such as graphite anode active materials that undergo a charge / discharge process as lithium ions are inserted or de-inserted into the anode composite, or silicon-like (such as Si, P, or Sn) anode active materials that form an alloy with lithium ions and undergo a charge / discharge process. In this case, the anode active material acts as a lithium ion carrier and does not directly store electrochemical energy, so the weight and volume of the carrier have limitations in reducing the unit capacity of the entire battery. On the other hand, in the case of a lithium metal battery, which is a system that utilizes lithium metal as an anode, the lithium metal itself undergoes a charge / discharge process in the form of precipitation and exfoliation, so it does not require the weight and volume of the carrier, and thus has the advantage of being able to maximize the unit capacity of the entire battery.

[0004] However, when lithium metal undergoes a charge-discharge process on the surface of a conventional negative electrode collector or negative electrode composite, the amount of electrochemical reaction that must occur per unit area during the rapid charge-discharge process increases dramatically. This causes lithium dendrites to grow, which can lead to an increase in lithium metal that is unable to participate in the reaction due to short-circuiting on the lithium metal surface, or can lead to fire accidents due to short-circuiting with the positive electrode.

[0005] In other words, when a copper film is used as a negative electrode collector, lithium metal is directly precipitated from the copper film. In this case, only the area of ​​the copper film can be utilized as an electrochemical surface area, so lithium dendrites can rapidly grow during the charge / discharge process, which can cause short circuits with the positive electrode or lithium peeling during the charge / discharge process, resulting in dead lithium, which can lead to safety issues and reduced lifespan characteristics.

[0006] Meanwhile, recent research has shown that utilizing a carbon-silver composite on a copper film as a lithium metal precipitation-type anode can dramatically improve the lifespan of single-cell lithium metal batteries. However, when applied to multilayer lithium metal batteries, the nano-sized, independent silver particles aggregate with repeated charge / discharge cycles, altering the electrode shape. This can lead to short circuits with the anode or limitations in rapid charge / discharge processes.

[0007] [Prior Art Literature]

[0008] [Patent Document]

[0009] Patent Publication No. 2021-0070822 (June 15, 2021)

[0010] Accordingly, the purpose of the present invention is to provide a negative electrode current collector capable of solving problems caused by lithium dendrites and a lithium metal battery including the same.

[0011] Another object of the present invention is to provide a negative electrode current collector and a lithium metal battery including the same, which can improve the high capacity and lifespan characteristics of a lithium metal battery by implementing low charge exchange resistance per unit area and stable and lightweight characteristics.

[0012] To achieve the above object, the present invention provides a negative electrode current collector for a lithium metal battery comprising a carbon nanotube fiber formed into a two-dimensional sheet by connecting carbon nanotubes in a network.

[0013] The above carbon nanotube fiber has an anisotropic nonwoven form with a certain directionality.

[0014] The above carbon nanotube fibers are bonded directly between the carbon nanotubes through pi-pi interactions.

[0015] The above carbon nanotube has a diameter of 10 nm or less and a length of 100 nm or more.

[0016] The above carbon nanotube fiber has a form in which carbon nanotubes intersect each other within ±45 degrees with respect to a unidirectional reference line.

[0017] The above negative electrode collector may be composed only of the above carbon nanotube fiber.

[0018] The present invention also provides a lithium metal battery comprising: a positive electrode current collector; a positive electrode active material layer disposed on the positive electrode current collector; a separator disposed on the positive electrode active material layer; an anode current collector disposed on the separator and including a carbon nanotube fiber formed into a two-dimensional sheet by connecting carbon nanotubes in a network; and an electrolyte disposed between the positive electrode current collector and the negative electrode current collector, wherein a lithium metal layer is formed between the separator and the negative electrode current collector during an initial charge process.

[0019] And the lithium metal battery according to the present invention is a non-anode lithium metal battery.

[0020] The negative electrode current collector according to the present invention has carbon nanotubes as a basic structural material, and is formed of carbon nanotube fibers that are strongly bonded to each other by pi-pi interactions without an adhesive material such as a binder, thereby maximizing the surface area where lithium metal charging and discharging can occur, thereby reducing the charge exchange resistance per unit area, and thereby suppressing the growth of lithium dendrites.

[0021] The negative electrode current collector according to the present invention can replace the copper thin film with a two-dimensional sheet based on carbon nanotube fibers having a very low density, or can improve the capacity per unit weight by lowering the density of the composite material, compared to a precipitation-type negative electrode current collector in the form of a copper thin film coated with a conventional copper thin film or a carbon-silver composite material.

[0022] The present invention uses a two-dimensional sheet based on carbon nanotube fibers in the form of anisotropic nonwoven fabric having a constant directionality as a negative electrode current collector, thereby improving the capacity implementation and capacity maintenance characteristics of a lithium metal battery through a wide reaction surface area and excellent interfacial resistance, compared to the case where a conventional copper or carbon-silver composite is used as a precipitation-type negative electrode current collector.

[0023] The negative electrode current collector according to the present invention has a consistent directionality and the carbon nanotubes form a network structure, thereby reinforcing the mechanical properties in the direction in which the carbon nanotubes are arranged. For example, when the manufacture of a lithium metal battery utilizing the negative electrode current collector is performed in a roll-to-roll manner, the carbon nanotubes can be arranged to have a consistent directionality based on the direction in which the negative electrode current collector is advanced.

[0024] And the negative electrode current collector according to the present invention can be used as a lithium precipitation type negative electrode or a current collector for a lithium metal battery.

[0025] Figure 1 is a cross-sectional view showing a non-anode lithium metal battery according to the present invention, and is a cross-sectional view showing the state immediately after assembly.

[0026] Figure 2 is a cross-sectional view showing the non-anode lithium metal battery of Figure 1 after the initial charge / discharge.

[0027] Figure 3 is an SEM image of a negative electrode collector according to Example 1.

[0028] Figure 4 is a graph showing the results of polar Raman spectroscopy analysis of the negative electrode current collector according to Example 1.

[0029] Figure 5 is a graph showing the life characteristics of a non-anode lithium metal battery having an anode current collector according to Comparative Example 1 and Example 1.

[0030] Figure 6 is a drawing showing the morphology of the negative electrode collector according to Comparative Example 1.

[0031] Figure 7 is a drawing showing the morphology of the negative electrode collector according to Example 1.

[0032] This invention is being filed with the support of the National Research and Development Project as follows.

[0033] [National Research and Development Project Supporting This Invention]

[0034] [Project ID] 1425182294

[0035] [Assignment Number] S3310630

[0036] [Ministry Name] Ministry of SMEs and Startups

[0037] [Name of Project Management (Specialist) Agency] Small and Medium Business Technology Information Promotion Agency

[0038] [Research Project Name] Industry-Academia-Research Cooperation Technology Development Project

[0039] [Research Project Title] Development of a Lithium-Carrying System for Energy Storage Devices Using (R)MWCNT Fiber-Based Woven Fabrics

[0040] [Contribution rate] 1 / 1

[0041] [Name of the project performing organization] Korea Electronics Technology Institute

[0042] Research Period: July 1, 2022 - June 30, 2024

[0043] It should be noted that in the following description, only the parts necessary for understanding the embodiments of the present invention are described, and the description of other parts will be omitted to the extent that it does not deviate from the gist of the present invention.

[0044] The terms and words used in this specification and claims described below should not be interpreted as limited to their conventional or dictionary meanings, but should be interpreted with meanings and concepts that conform to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term in order to explain his own invention in the best way. Therefore, it should be understood that the embodiments described in this specification and the configurations illustrated in the drawings are merely preferred embodiments of the present invention and do not represent all of the technical idea of ​​the present invention, and that there may be various equivalents and modified examples that can replace them at the time of this application.

[0045] Hereinafter, embodiments of the present invention will be described in more detail with reference to the attached drawings.

[0046] Here, the term 'non-cathode lithium metal battery' refers to a lithium metal battery that initially (immediately after assembly) has a cathode without lithium metal, and in which a lithium metal layer is formed on the cathode current collector through initial charge and discharge.

[0047] Fig. 1 is a cross-sectional view showing a non-anode lithium metal battery according to the present invention, showing the state immediately after assembly. Fig. 2 is a cross-sectional view showing the non-anode lithium metal battery of Fig. 1 after the initial charge / discharge.

[0048] Referring to FIG. 1, a non-anode lithium metal battery (100) according to the present invention includes a positive electrode current collector (10), a positive electrode active material layer (20) disposed on the positive electrode current collector (10), a separator (30) disposed on the positive electrode active material layer (20), an anode current collector (40) disposed on the separator (30), and an electrolyte (not shown) disposed between the positive electrode current collector (10) and the negative electrode current collector (40). Here, the negative electrode current collector (40) includes a carbon nanotube fiber formed into a two-dimensional sheet by connecting carbon nanotubes in a network.

[0049] As shown in FIG. 1, the non-anode lithium metal battery (100) according to the present invention does not have a lithium metal layer immediately after assembly.

[0050] As shown in FIG. 2, in the non-anode lithium metal battery (100) according to the present invention, after initial charge and discharge, a lithium metal layer (50) is formed by precipitation between the separator (30) and the anode current collector (40).

[0051] The non-cathode lithium metal battery (100) according to the present invention will be described in detail as follows.

[0052] Electron movement occurs through an electrochemical reaction of the positive electrode active material contained in the positive electrode active material layer (20) in the positive electrode current collector (10). The positive electrode current collector (10) may be a heat-resistant metal, such as iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. The positive electrode current collector may be aluminum or stainless steel. The upper surface of the positive electrode current collector (10) may also be roughened to improve the adhesive strength with the positive electrode active material layer (20).

[0053] The cathode active material layer (20) may contain a main cathode active material and a sacrificial cathode active material. The main cathode active material may be a lithium-transition metal oxide or a lithium-transition metal phosphate, which may be a material containing 1 mol of lithium per mol. The lithium-transition metal oxide may be a composite oxide of lithium and at least one transition metal selected from the group consisting of cobalt, manganese, nickel, and aluminum. For example, the lithium-transition metal oxide may be Li(Ni 1-x-y Co x Mn y )O2(0≤x≤1, 0≤y≤1, 0≤x+y≤1), Li(Ni 1-x-y Co x Al y )O2(0≤x≤1, 0 <y≤1, 0<x+y≤1), 또는 Li(Ni 1-x-y Co x Mn y )2O4(0≤x≤1, 0≤y≤1, 0≤x+y≤1). The lithium-transition metal phosphate may be a complex phosphate of lithium and at least one transition metal selected from the group consisting of iron, cobalt, and nickel. For example, the lithium-transition metal phosphate may be, for example, Li(Ni 1-x-y Co x Fe y )PO4(0≤x≤1, 0≤y≤1, 0≤x+y≤1).

[0054] The positive electrode active material layer (20) may further include a polymer binder and / or a conductive material. The polymer binder may include a fluororesin such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluorethylene, vinylidene fluoride copolymer, hexafluoropropylene, a polyolefin resin such as polyethylene, polypropylene, and the like, and a cellulose such as carboxymethyl cellulose.

[0055] The conductive material may be at least one selected from the group consisting of carbon black, carbon black (CB), conducting graphite, ethylene black, and carbon nanotubes (CNT).

[0056] The separator (30) separates the negative electrode current collector (40) and the positive electrode active material layer (20) and provides a passage for lithium ions to move. Any separator generally used in lithium secondary batteries can be used without any particular restrictions. It is preferable that the separator have low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity. For example, the separator (30) may include polyethylene, polypropylene, or a copolymer of polyethylene and polypropylene, and a multilayer film of two or more layers thereof may be used.

[0057] The electrolyte may be a liquid electrolyte. The liquid electrolyte may be a non-aqueous electrolyte solution. The non-aqueous electrolyte solution comprises an electrolyte, which is a lithium salt, and a medium, wherein the lithium salt may be lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium trifluoromethanesulfonate (LiCF3SO3), lithium hexafluoroacetate (LiAsF6), or lithium trifluoromethanesulfonylimide (Li(CF3SO2)2N). The medium may include ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, fluoroethylene carbonate, acrylonitrile, γ-caprolactone, or a combination of two or more thereof. For example, the medium may be a combination of dimethyl carbonate and fluoroethylene carbonate. The liquid electrolyte may further contain an additive in addition to the lithium salt and medium. The additive may be LiNO3.

[0058] And the negative electrode current collector (40) is formed of carbon nanotube fibers having high conductivity without causing chemical changes in the lithium metal battery (100). That is, in the present invention, by applying a two-dimensional sheet based on carbon nanotube fibers in the form of anisotropic nonwoven fabric having a certain directionality as the negative electrode current collector (40) as a non-negative current collector on which lithium metal is precipitated, the capacity implementation and capacity maintenance characteristics of the lithium metal battery (100) can be improved through a wide reaction surface area and excellent interfacial resistance compared to the case where a conventional copper or carbon-silver composite is used as a precipitation-type non-negative current collector.

[0059] In the case of existing negative electrode current collectors, they are composed of a copper thin film or a precipitation-type negative electrode current collector in the form of a copper thin film coated with a carbon-silver composite material, and have problems such as lithium dendrite growth during the process of lithium metal precipitation or deterioration of life characteristics and capacity implementation characteristics due to changes in electrode shape.

[0060] In contrast, the negative electrode current collector (40) according to the present invention utilizes a floating catalyst chemical vaporization deposition method based on a direct spinning method to manufacture a two-dimensional sheet in the form of an anisotropic nonwoven fabric using carbon nanotube fibers composed of 100% carbon nanotubes, and applies this as a precipitation-type non-negative electrode current collector to a lithium metal battery, thereby solving the problem caused by the growth of existing lithium dendrites.

[0061] The carbon nanotube fibers constituting the negative electrode current collector (40) according to the present invention have an anisotropic nonwoven fabric form with a certain directionality. Here, since the carbon nanotube fibers are directly bonded to each other through pi-pi interactions, the negative electrode current collector (40) can omit the use of an adhesive material such as a binder.

[0062] Because the negative electrode current collector (40) can be composed only of carbon nanotube fibers, the surface area where lithium metal charging and discharging can occur is maximized, thereby reducing the charge exchange resistance per unit area and suppressing the growth of lithium dendrites.

[0063] And the negative electrode current collector (40) is made of a conventional copper thin film, or a precipitation-type negative electrode current collector in the form of a copper thin film coated with a carbon-silver composite material, or a two-dimensional sheet based on carbon nanotube fibers with a very low density compared to a precipitation-type negative electrode, so that the copper thin film can be replaced or the density of the composite material can be lowered to improve the implementation of capacity per unit weight.

[0064] The negative electrode current collector (40) according to the present invention has a certain directionality and the carbon nanotubes form a network structure, thereby reinforcing the mechanical properties in the direction in which the carbon nanotubes are arranged. For example, when the manufacture of a lithium metal battery (100) utilizing the negative electrode current collector (40) is performed in a roll-to-roll manner, the carbon nanotubes can be arranged to have a certain directionality based on the direction in which the negative electrode current collector (40) is advanced.

[0065] The carbon nanotubes included in the carbon nanotube fiber may have a diameter of 10 nm or less and a length of 100 nm or more.

[0066] The carbon nanotube fiber may have a shape in which the carbon nanotubes intersect each other within ±60 degrees with respect to a unidirectional reference line, and preferably may have a shape in which the carbon nanotubes intersect each other within ±45 degrees.

[0067] The negative electrode current collector (40) according to the present invention may be composed solely of carbon nanotube fibers. Alternatively, the negative electrode current collector (40) according to the present invention may include conductive nanoparticles added to carbon nanotube fibers. The conductive nanoparticles include metals or carbon materials. The carbon materials may include carbon black, carbon nanotubes, graphene, and the like.

[0068] Meanwhile, although FIGS. 1 and 2 disclose examples in which the negative electrode current collector is used as a lithium precipitation-type non-negative electrode, the present invention is not limited thereto. For example, the negative electrode current collector according to the present invention can be used as a current collector for a lithium secondary battery.

[0069]

[0070] [Examples and Comparative Examples]

[0071] In order to confirm the electrochemical characteristics of the negative electrode collector according to the present invention, negative electrode collectors according to examples and comparative examples and lithium metal batteries based on the negative electrode collectors were manufactured as follows.

[0072] A copper thin film was used as the negative electrode collector according to Comparative Example 1.

[0073] The negative electrode current collector according to Example 1 was manufactured as a carbon nanotube fiber composed of 100% carbon nanotubes by a floating catalyst chemical vapor deposition method.

[0074] As shown in Fig. 3, the manufactured carbon nanotube fiber is a two-dimensional sheet in the form of an anisotropic nonwoven fabric with a specific directionality using nano-sized carbon nanotubes as the basic unit material. Here, Fig. 3 is an SEM image of the negative electrode current collector according to Example 1.

[0075] It was confirmed through polarized Raman spectroscopy, as shown in Fig. 4 and Table 1, that carbon nanotubes within the carbon nanotube fiber have a certain directionality and are bonded to adjacent carbon nanotubes through pi-pi interactions.

[0076] Here, Fig. 4 is a graph showing the polar Raman spectroscopy results of the negative electrode collector according to Example 1.

[0077]

[0078] Based on the negative electrode collector according to Comparative Example 1 and Example 1, a battery was manufactured as follows, and lithium metal precipitation and stripping experiments were performed.

[0079] A lithium / copper-based half-cell was manufactured according to Comparative Example 1. That is, the cell according to Comparative Example 1 has a structure in which a lithium metal layer is formed on a copper thin film.

[0080] A half-cell based on lithium / copper+CNF was manufactured according to Example 1. Here, CNF refers to the carbon nanotube fiber according to Example 1. 'Copper+CNF' indicates a structure in which CNF according to Example 1 is formed on a copper thin film. That is, the cell according to Example 1 has a structure in which CNF is interposed between a lithium metal layer and a copper thin film, compared to the cell according to Comparative Example 1.

[0081] And as an electrolyte, both Comparative Example 1 and Example 1 used an electrolyte having the following composition.

[0082] Electrolyte: 1M LiPF6EC (ethylene carbonate): DEC (Diethyl carbonate) = 3:7 (v / v) + 10wt% FEC

[0083] Figure 5 is a graph showing the life characteristics of a non-anode lithium metal battery having a cathode current collector according to Comparative Example 1 and Example 1. Here, the life characteristics are measured at a 1C driving current (1 mA / cm 2 ) constant current charge / discharge, cutoff -1 mAh / cm 2 Or, the evaluation was performed under driving conditions of -1 V to 1 V.

[0084] Referring to Fig. 5, it can be confirmed that Example 1 exhibits better life characteristics than Comparative Example 1.

[0085] The morphology of the negative electrode according to Comparative Example 1 and Example 1 was confirmed, and the results are as shown in FIGS. 6 and 7. Here, FIG. 6 is a drawing showing the morphology of the negative electrode current collector according to Comparative Example 1. And FIG. 7 is a drawing showing the morphology of the negative electrode current collector according to Example 1. Here, in order to confirm the difference in the morphology of the negative electrode according to lithium deposition, lithium was applied to the negative electrode current collector at 2 mAh / cm. 2 It was transferred to .

[0086] Referring to Figure 6, in the case of Comparative Example 1, it was confirmed that lithium in the shape of a dendrite was formed.

[0087] On the other hand, referring to Fig. 7, it was confirmed that lithium dendrites were not formed in the case of Example 1.

[0088] In this way, the negative electrode current collector according to Example 1 has carbon nanotubes as its basic structural material, and is formed of carbon nanotube fibers that are strongly bonded to each other by pi-pi interactions without an adhesive material such as a binder, thereby maximizing the surface area where lithium metal charging and discharging can occur, thereby reducing the charge exchange resistance per unit area, and thereby suppressing the growth of lithium dendrites.

[0089] And, the negative electrode collector according to Example 1 can improve the capacity per unit weight by replacing the copper thin film with a two-dimensional sheet based on carbon nanotube fibers having a very low density compared to the negative electrode collector of the copper thin film as in Comparative Example 1 or by lowering the density of the composite material.

[0090] Meanwhile, the embodiments disclosed in this specification and drawings are merely specific examples to aid understanding and are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that other modifications based on the technical concepts of the present invention are possible in addition to the embodiments disclosed herein.

[0091] [Explanation of symbols]

[0092] 10: Positive current collector

[0093] 20: Positive active material layer

[0094] 30: Membrane

[0095] 40: Negative current collector

[0096] 50: Lithium metal layer

[0097] 100: Lithium metal battery

Claims

1. A negative electrode current collector for a lithium metal battery comprising carbon nanotube fibers formed into a two-dimensional sheet by connecting carbon nanotubes in a network.

2. In paragraph 1, A negative electrode current collector for a lithium metal battery, characterized in that the carbon nanotube fibers have an anisotropic nonwoven fabric form with a certain directionality.

3. In paragraph 2, A negative electrode current collector for a lithium metal battery, characterized in that the carbon nanotube fibers are directly bonded to each other by pi-pi interactions.

4. In paragraph 2, A negative electrode current collector for a lithium metal battery, characterized in that the carbon nanotube has a diameter of 10 nm or less and a length of 100 nm or more.

5. In paragraph 2, The above carbon nanotube fiber is a negative electrode current collector for a lithium metal battery, characterized in that the carbon nanotubes have a form in which they intersect each other within ±45 degrees with respect to a unidirectional reference line.

6. In paragraph 1, A negative electrode current collector for a lithium metal battery, characterized in that the negative electrode current collector is composed only of the carbon nanotube fiber.

7. Bipolar collector; A cathode active material layer disposed on the above cathode current collector; A separator disposed on the positive electrode active material layer; A negative electrode current collector disposed on the above separator and including carbon nanotube fibers formed into a two-dimensional sheet by connecting carbon nanotubes in a network; and An electrolyte disposed between the positive electrode current collector and the negative electrode current collector; A lithium metal battery characterized in that a lithium metal layer is formed between the separator and the negative electrode current collector during the initial charging process.

8. In paragraph 7, A lithium metal battery, characterized in that the carbon nanotube fibers have an anisotropic nonwoven fabric form with a certain directionality.

9. In paragraph 8, A lithium metal battery, characterized in that the carbon nanotube fibers are directly bonded to each other by pi-pi interactions.

10. In paragraph 8, A lithium metal battery, wherein the carbon nanotube fibers have a form in which the carbon nanotubes intersect each other within ±45 degrees with respect to a unidirectional reference line.

11. In paragraph 7, A lithium metal battery characterized in that the negative electrode current collector is composed only of the carbon nanotube fibers.

12. In paragraph 7, A lithium metal battery, characterized in that the above lithium metal battery is a non-anode lithium metal battery.

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