KR102763146B1 - Current collector-free positive electrode for lithium secondary battery, method for preparing the same and lithium secondary battery including the current collector-free positive electrode - Google Patents

Abstract

Disclosed are a collector-free cathode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the collector-free cathode, which reduces the weight of the battery by not using a metal collector for the cathode, thereby improving the energy density, simplifying the process, and reducing the loss of energy density compared to a conventional one when applying a low loading electrode, thereby improving the output, lifespan, and charging performance of the battery. The collector-free cathode for a lithium secondary battery comprises: carbon nanotube paper; and a sulfur-polymer composite layer coated on the carbon nanotube paper, and is characterized in that it does not include a metal collector.

Description

Current collector-free positive electrode for lithium secondary battery, method for preparing the same and lithium secondary battery including the current collector-free positive electrode}

The present invention relates to a current collector-free positive electrode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the current collector-free positive electrode, and more specifically, to a current collector-free positive electrode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the current collector-free positive electrode, which does not use a metal current collector for the positive electrode, thereby reducing the weight of the battery, improving the energy density, simplifying the process, and reducing the loss of energy density compared to a conventional one when applying a low loading electrode, thereby improving the output, lifespan, and charging performance of the battery.

Recently, interest in energy storage technology has been increasing, and as the application fields have expanded to include energy for mobile phones, camcorders, notebook PCs, and even electric vehicles, efforts for research and development of electrochemical devices have been concretized. Electrochemical devices are the field that has received the most attention in this regard, and among them, the development of secondary batteries such as lithium-sulfur batteries that can be recharged and discharged has become the focus of interest, and recently, in order to improve the capacity density and specific energy in the development of such batteries, research and development has been conducted on the design of new electrodes and batteries.

Among these electrochemical devices, lithium-sulfur (Li-S) batteries have high energy density and are attracting attention as next-generation secondary batteries that can replace lithium-ion batteries. In these lithium-sulfur batteries, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur during discharge, during which sulfur forms linear lithium polysulfides (Li ₂ S ₂ , Li ₂ S ₄ , Li ₂ S ₆ , Li ₂ S ₈ ) from ring-shaped S ₈ . These lithium-sulfur batteries are characterized by exhibiting step-by-step discharge voltages until the polysulfide (PS) is completely reduced to Li ₂ S.

FIG. 1 is a cross-sectional schematic diagram of a conventional lithium-sulfur battery. As shown in FIG. 1, the lithium-sulfur battery uses a sulfur-carbon composite (carbon/metal/metal oxide) coated on a metal foil current collector as a positive electrode, and a lithium metal or lithium metal alloy laminated to a metal foil as a negative electrode.

However, since conventional lithium-sulfur batteries use metal foil as a current collector, the energy density of the battery is low, and in order to minimize this problem, a method of increasing the coating amount of electrode material (electrode loading) is being attempted. However, in this case, problems arise in which the output, life, and charging performance of the battery are reduced. Therefore, it is required to present a method that can improve the energy density of the battery by solving the above problems and thereby improving the life, output, and high-rate charging performance of the battery.

Accordingly, the purpose of the present invention is to provide a current collector-free positive electrode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the current collector-free positive electrode, which reduces the weight of the battery by not using a metal current collector for the positive electrode, thereby improving the energy density, simplifying the process, and reducing the loss of energy density compared to a conventional one when applying a low loading electrode, thereby improving the output, lifespan, and charging performance of the battery.

To achieve the above object, the present invention provides a current collector-free positive electrode for a lithium secondary battery, which comprises carbon nanotube paper; and a sulfur-polymer composite layer coated on the carbon nanotube paper, and is characterized in that it does not include a metal current collector.

In addition, the present invention provides a method for manufacturing a current collector-free positive electrode for a lithium secondary battery, comprising the steps of: a) preparing a slurry including sulfur and a polymer; b) pulverizing and dispersing the prepared slurry; and c) coating the pulverized and dispersed slurry on carbon nanotube paper and then drying and heat-treating it.

In addition, the present invention provides a lithium secondary battery including a current collector-free positive electrode for the lithium secondary battery; an anode; a separator interposed between the positive electrode and the anode; and an electrolyte.

The current collector-free positive electrode for a lithium secondary battery according to the present invention, the manufacturing method thereof, and the lithium secondary battery including the current collector-free positive electrode have the advantages of reducing the weight of the battery by not using a metal current collector for the positive electrode, improving the energy density, simplifying the process, and reducing the loss of energy density compared to conventional ones when applying a low loading electrode, thereby improving the output, lifespan, and charging performance of the battery.

Figure 1 is a cross-sectional schematic diagram of a conventional lithium-sulfur battery.
Figure 2 is an actual image of the CNT paper of the present invention coated with a sulfur-polymer complex.
Figure 3 is an SEM image observing the surface of carbon nanotube paper and the anode manufactured in examples and comparative examples.
Figure 4 is a graph comparing and contrasting the discharge capacity (a) and life characteristics (b) of lithium secondary batteries manufactured according to one embodiment and a comparative example of the present invention.

Hereinafter, the present invention will be described in detail.

A current collector-free positive electrode for a lithium secondary battery according to the present invention comprises carbon nanotube paper; and a sulfur-polymer composite layer coated on the carbon nanotube paper, and is characterized in that it does not include a metal current collector.

Typically, the positive electrode of a lithium secondary battery includes a metal current collector such as aluminum foil, and a process is performed to form an active material by coating and drying a slurry containing a sulfur-carbon complex, a conductive material, and a binder on the metal current collector.

However, since the present invention has a configuration that excludes a metal current collector (current collector free), the present invention enables a free standing electrode configuration with reduced battery weight. Therefore, according to the present invention, not only is the manufacturing process of the battery simplified, but the energy density of the battery is improved, thereby improving the lifespan, output, and high-rate charging performance of the battery. That is, the present invention has a technical feature that can obtain the effect of manufacturing a sulfur-carbon complex by using sulfur and carbon nanotube paper, while also obtaining the effect of omitting the current collector.

The above carbon nanotube paper (CNT Paper) is a sheet-shaped material that can be manufactured by various methods, such as mixing two types of carbon nanotubes (CNTs) or mixing carbon nanotubes and carbon nanofibers (CNFs). In the present invention, it serves as a current collector and a sulfur host (or sulfur carrier), and can be very advantageously used in the present invention that does not use a current collector.

For example, when the carbon nanotube paper is a mixture of two types of carbon nanotubes, one type of carbon nanotube may have a diameter of 5 to 50 nm and a length of 1 to 20 ㎛, and the other type of carbon nanotube may have a diameter of 100 to 500 nm and a length of 10 to 100 ㎛. At this time, the content ratio of the carbon nanotubes having a small diameter and the carbon nanotubes having a large diameter may be 70 to 90 : 10 to 30, preferably 75 to 90 : 10 to 25, and more preferably 80 to 85 : 15 to 20, in terms of weight ratio.

Meanwhile, carbon nanotubes with small diameter and short length can be effective for sulfur loading, and carbon nanotubes with large diameter and long length can be effective for securing conductivity within carbon nanotube paper. Therefore, if the content of carbon nanotubes with large diameter exceeds the above range, the conductivity may increase and the output may improve, but since the specific surface area is small, it may be difficult to load a large amount of sulfur, and therefore it is desirable to appropriately adjust it within the above content ratio.

Next, the sulfur-polymer composite layer located on the surface of the carbon nanotube paper will be described. The sulfur-polymer composite layer (or sulfur-polymer composite) contains sulfur and a specific polymer unique to the present invention as a base, and may further contain, if necessary, a conductive material such as a metal or metal oxide having micropores formed therein, or a conventional positive electrode active material other than conductive carbon, and thus has a composition that is different from that of the sulfur-carbon composite applied to the existing positive electrode.

The polymer constituting the above sulfur-polymer composite layer mainly functions as a thickener to improve viscosity, and, if necessary, can maximize the bonding strength between the carbon nanotube paper and sulfur (i.e., also functions as a binder) or can even function as a dispersant. Examples of such polymers include polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polyurethane (PU), polyacrylonitrile (PAN), and mixtures thereof. In addition, there is no particular limitation on the weight average molecular weight (Mw) of the polymer, but, for example, the weight average molecular weight of polyacrylic acid can be from 50,000 to 4,000,000.

Meanwhile, in the sulfur-polymer composite layer, the sulfur may be supported on a support such as a conductive material such as a metal or metal oxide having micropores formed (the method of supporting the sulfur on the support may be a conventional method such as coating). More specifically, the sulfur may be supported on a metal foam support in which pores are formed in a metal such as aluminum (Al) or nickel (Ni), and therefore, a sulfur-polymer composite may be positioned on the surface of the carbon nanotube paper, or a support-polymer composite on which sulfur is supported may be positioned.

Meanwhile, in order to secure high electrical activity of sulfur supported on the carbon nanotube paper or additionally supported on the metal foam support, sulfur should be thinly and uniformly distributed on each surface of the carbon nanotube paper support and the metal foam support, and the surface area and porosity of the support should be well controlled so that electrolyte is sufficiently supplied around the sulfur. The specific surface area of the support such as the carbon nanotube paper and the metal foam may be 10 to 200 m ² /g, the porosity is preferably in the range of 50 to 95%, and the thickness may be 50 to 200 ㎛. The electrode porosity and thickness may be controlled by rolling depending on the porosity after electrode manufacturing.

In addition, the amount of sulfur supported on the support (carbon nanotube paper or metal foam) may be 20 to 90 wt%, preferably 40 to 70 wt%, and more preferably 55 to 65 wt%, based on the total weight of the electrode. Meanwhile, the content ratio of the sulfur and the polymer may be 90 to 99:1 to 10, preferably 95 to 99:1 to 5, in terms of weight ratio. If the content ratio of the sulfur and the polymer is out of the above range, sulfur may not be properly dispersed, making it difficult to manufacture a uniform positive electrode, or the polymer may act as a resistance component, thereby reducing the performance of the battery.

In addition, the sulfur-polymer composite layer may be formed on a portion of the surface of the carbon nanotube paper, but considering the uniformity of the positive electrode surface and the battery energy density when applied to a battery, it is preferable to form it on the entire surface of the carbon nanotube paper.

Meanwhile, the positive electrode may further include a filler, a reaction catalyst, a polysulfide diffusion-preventing material, and a mixture thereof, in addition to the positive electrode active material. The filler is optionally used as a component that suppresses expansion of the electrode, and is not particularly limited as long as it is a fibrous material that does not cause a chemical change in the battery. For example, the filler may be an olefin polymer such as polyethylene or polypropylene, or a fibrous material such as glass fiber or carbon fiber.

The above reaction catalyst is an additionally applicable material for improving the reactivity of sulfur in the battery or for adsorbing polysulfide and preventing its diffusion, and can be used in an amount of 1 to 10 parts by weight based on 100 parts by weight of the total electrode weight, and can be used as an organic or inorganic composite, for example, cobalt (Co) oxide and sulfide, iron (Fe) oxide and sulfide, etc. The polysulfide diffusion prevention material can be used in a particle or film form by mixing or coating it on sulfur and the electrode surface, and there is no particular limitation on the organic or inorganic composite, but for example, the aforementioned cobalt oxide and sulfide, iron oxide and sulfide, etc., polyvinyl alcohol, polyvinyl pyrrolidinone, polyacrylic acid-polybutyl acrylic copolymer, etc. can be used.

Meanwhile, the current collector-free positive electrode including the above carbon nanotube paper and the sulfur-polymer composite layer may further include a lead (or terminal connection portion) for grounding at one end. The lead or terminal connection portion connected to the positive electrode is, as the name suggests, a member connected to a terminal portion of an electrical or mechanical product that is a target of battery connection. The lead may be connected or bonded to the positive electrode in various ways, and for example, a mechanical compression method such as press compression, an adhesive method using a conductive material, a connection method using a connection member such as a rivet, a welding method using ultrasonic waves, and a welding method using a laser may be applied to bond the lead and the positive electrode.

The material of the above lead may be a typical conductive metal, and examples thereof include nickel (Ni), aluminum (Al), copper (Cu), and iron (Fe). Nickel or copper is preferably used, and nickel, which has superior corrosion resistance compared to copper, is more preferably used. In addition, materials in which nickel is plated on copper, and other materials in which another metal is plated on one of the above-mentioned metals, can also be used as the lead. Meanwhile, a hole or pattern may be formed at one end of the lead that is connected to the anode to improve the bonding strength with the anode. There is no particular limitation on a method for forming a hole or pattern in a lead made of such a metal material, as long as the hole or pattern can be formed without damaging the anode or lead, such as laser irradiation, a mold, or a knife.

Next, a method for manufacturing a current collector-free positive electrode for a lithium secondary battery according to the present invention is described. The method for manufacturing a current collector-free positive electrode for a lithium secondary battery according to the present invention includes the steps of a) manufacturing a slurry containing sulfur and a polymer, b) pulverizing and dispersing the manufactured slurry, and c) coating the pulverized and dispersed slurry on carbon nanotube paper and then drying and heat-treating it.

The method for manufacturing a current collector-free positive electrode for a lithium secondary battery according to the present invention, unlike the positive electrode of a conventional lithium secondary battery, which is manufactured by coating a slurry containing a sulfur-carbon complex, a conductive agent, and a binder on a metal current collector by a liquid phase process or a vapor phase impregnation process and then drying, not only enables the manufacture of a uniform and simple positive electrode by excluding a metal current collector and a conductive agent, but also reduces the weight, thereby improving the energy density of the battery, and further, since a liquid phase process or a vapor phase impregnation process is not used in the manufacture of the positive electrode, it is possible to prevent sulfur from melting or vaporizing and becoming toxic, and further, it has the advantage of easy control of the sulfur content.

In the step a), the polymer may be polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polyurethane (PU), polyacrylonitrile (PAN), metal substituted products thereof, and mixtures thereof as described above, and the content ratio of sulfur and the polymer used in the slurry production may be 90 to 99:1 to 10, preferably 93 to 97:3 to 7, and more preferably about 95:5, as a weight ratio.

Meanwhile, after the step a) is performed, if necessary, DI water (deionized water) or a material having similar properties or functions to the slurry may be added to the slurry so that the solid content ratio in the slurry becomes 15 to 30%, preferably 22 to 27% (if necessary, the material such as DI water may be added from the time of mixing sulfur and the polymer). Meanwhile, if the solid content ratio in the slurry is less than 15%, there is a concern that the slurry may not be uniformly coated on the carbon nanotube paper due to the surface tension of the deionized water, and if it exceeds 30%, the viscosity increases and the slurry may not be uniformly coated on the carbon nanotube paper.

After the slurry is prepared through the above step a), a process of grinding and dispersing the slurry is performed (step b). The grinding may be performed by a conventional grinding method used for grinding slurries, such as ball milling, and the dispersion may be performed at a temperature of 10 to 40° C. for 0.5 to 5 hours.

After the slurry is pulverized and dispersed through the step b), the pulverized/dispersed slurry is coated on carbon nanotube paper and then dried and heat-treated (step c), thereby manufacturing a current collector-free positive electrode for a lithium secondary battery according to the present invention. The coating may be performed by a spray coating method, a doctor blade coating method, or a spin coating method. Among these, considering that the mechanical strength of the carbon nanotube paper, which is a coating substrate, is not high, it may be preferable to use a doctor blade coating method.

In addition, the drying can be performed under temperature conditions of 30 to 90°C, preferably 50 to 80°C, and the heat treatment can be performed under a temperature of 100 to 200°C, preferably 130 to 170°C, for 5 to 60 minutes, preferably 20 to 40 minutes.

Meanwhile, the method for manufacturing the above-described collector-free positive electrode may further include, after step c), a step of rolling the positive electrode to adjust the porosity of the manufactured positive electrode, and it is preferable to adjust the porosity of the positive electrode to be 50 to 95%, preferably 55 to 70%.

In addition, the method for manufacturing the current-free positive electrode may further include, after the step c), a step of bonding a lead (terminal connection portion) to one end of the manufactured positive electrode. The lead may be connected or bonded to the positive electrode in various ways, for example, a mechanical bonding method such as press bonding, an adhesion method using a conductive material, a connection method using a connecting member such as a rivet, a welding method using ultrasonic waves, and a welding method using a laser. In the case of the adhesion method using the conductive material, an adhesive containing conductive carbon or conductive metal may be used, and in the case of the press bonding, it may be performed under the condition of pressing with a force of 10 to 100 kg for 1 to 10 seconds. In addition, an electrode cutting process may be added when manufacturing the positive electrode.

Finally, a lithium secondary battery including a current collector-free positive electrode provided by the present invention will be described. The lithium secondary battery including the current collector-free positive electrode includes the current collector-free positive electrode for the lithium secondary battery, the negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and may be exemplified by all lithium-based secondary batteries known in the art, such as a lithium-sulfur battery, a lithium-air battery, and a lithium metal battery, and among these, a lithium-sulfur battery is preferable. In addition to the positive electrode, the remaining negative electrode, separator, and electrolyte applied to the lithium secondary battery may be conventional ones used in the art, and a specific description thereof will be provided later.

A lithium secondary battery including a current collector-free positive electrode of the present invention as described above can be manufactured in various forms, and can be applied to all cell types commonly used in the art, such as a pouch-cell type and a coin-cell type. However, the lithium secondary battery of the present invention is optimized for the pouch-cell type. That is, in the case of the coin-cell type, since the resistance between the upper and lower electrodes is small, there is no difficulty in implementing a general lithium secondary battery as a coin-cell, but this is not the case for the pouch-cell type. However, in the present invention, the existing metal current collector is excluded and replaced with a carbon nanotube paper, thereby securing electrical conductivity even in a plane (length direction or width direction), and thus it can be manufactured in the form of a pouch-cell.

In addition, the present invention fundamentally differs from other existing literatures that mention energy density at the coil-cell level in that it implements a lithium secondary battery (more precisely, a lithium-sulfur battery pouch cell) having a high energy density by appropriately controlling and selecting the pores, thickness, and specific surface area of the carbon nanotube paper and the sulfur-polymer composite, and also easily controlling the amount of sulfur loaded during electrode manufacture.

In addition, the lithium secondary battery of the present invention can be applied to a battery cell used as a power source for a small device, and can also be suitably used as a unit battery of a battery module which is a power source for a medium or large device. In this respect, the present invention also provides a battery module in which two or more of the lithium secondary batteries are electrically connected (in series or in parallel). It goes without saying that the quantity of the lithium secondary batteries included in the battery module can be variously adjusted in consideration of the purpose and capacity of the battery module.

Furthermore, the present invention provides a battery pack electrically connecting the battery modules according to conventional techniques in the related art. The battery module and the battery pack can be used as a power source for one or more medium- to large-sized devices among power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric trucks; electric commercial vehicles; or power storage systems, but are not necessarily limited thereto.

Below, a description is provided of the negative electrode, separator, and electrolyte applied to the lithium secondary battery according to the present invention.

cathode

Anything that can absorb and release lithium ions can be used as the negative electrode, and examples thereof include metal materials such as lithium metal and lithium alloys, and carbon materials such as low-crystalline carbon and high-crystalline carbon. Representative examples of low-crystalline carbon include soft carbon and hard carbon, and examples of high-crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and petroleum or coal-based coke.

High-temperature calcined carbons such as coal tar pitch derived cokes are representative examples. In addition, alloy series containing silicon and oxides such as Li4Ti5O12 are also well-known cathodes.

At this time, the cathode may include a binder, and various types of binder polymers may be used as the binder, such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyacrylonitrile, polymethylmethacrylate, and styrene-butadiene rubber (SBR).

The above negative electrode may optionally further include a negative electrode current collector for supporting the negative electrode active layer including the negative electrode active material and the binder. The above negative electrode current collector may be specifically selected from the group consisting of copper, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and an aluminum-cadmium alloy may be used as the alloy. In addition, calcined carbon, a non-conductive polymer surface-treated with a conductive agent, or a conductive polymer, etc. may be used.

The above binder plays a role in pasting the negative active material, mutual adhesion between the active materials, adhesion between the active material and the current collector, and a buffering effect for expansion and contraction of the active material. Specifically, the binder is the same as that described above for the binder of the positive electrode. In addition, the negative electrode may be lithium metal or a lithium alloy. As a non-limiting example, the negative electrode may be a thin film of lithium metal, or may be an alloy of lithium and one or more metals selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn.

Membrane

The separator is interposed between the positive and negative electrodes to prevent short circuits between them and to provide a passage for lithium ions to move. As the separator, an olefin polymer such as polyethylene or polypropylene, glass fiber, etc. can be used in the form of a sheet, a multi-membrane, a microporous film, a woven fabric, a non-woven fabric, etc., but is not necessarily limited thereto. Meanwhile, when a solid electrolyte such as a polymer (e.g., an organic solid electrolyte, an inorganic solid electrolyte, etc.) is used as the electrolyte, the solid electrolyte can also function as a separator. Specifically, a thin insulating film having high ion permeability and mechanical strength is used. Meanwhile, the separator has pores through which lithium ions move, and the diameter of the pores can be 0.01 to 10 ㎛, and the thickness can be 5 to 300 ㎛, but is not limited thereto.

Electrolyte

The electrolyte contains solvents and lithium salts, and may further contain additives as needed. As the solvent, a typical non-aqueous solvent that acts as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. Examples of the non-aqueous solvent include carbonate solvents, ester solvents, ether solvents, ketone solvents, alcohol solvents, and aprotic solvents.

More specific examples include the carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); the ester solvents, such as methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methylpropionate, ethylpropionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, and caprolactone; and the ether solvents, such as diethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, Examples of the non-aqueous solvents include dimethoxyethane, diethoxyethane, diglyme, triglyme, tetraglyme, tetrahydrofuran, 2-methyltetrahydrofuran, and polyethylene glycol dimethyl ether. In addition, examples of the ketone solvents include cyclohexanone, examples of the alcohol solvents include ethyl alcohol and isopropyl alcohol, and examples of the aprotic solvents include nitriles such as acetonitrile, amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane (DOL), and sulfolane. The non-aqueous solvents such as the above can be used alone or in mixtures of two or more, and the mixing ratio when two or more are mixed can be appropriately controlled depending on the performance of the intended battery, and an example is a solvent in which 1,3-dioxolane and dimethoxyethane are mixed in a volume ratio of 1:1.

Hereinafter, preferred examples are presented to help understand the present invention, but the following examples are only illustrative of the present invention, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope and technical idea of the present invention, and it is also natural that such changes and modifications fall within the scope of the appended patent claims.

[Example 1] Manufacturing of a current-free positive electrode

First, a slurry was prepared by mixing sulfur and polyacrylic acid at a weight ratio of 95:5, and then deionized water (DI water) was added to the slurry so that the solid content ratio of the slurry became 25%. Next, the slurry including the DI water was pulverized by a ball-milling method and dispersed at a temperature of 25°C for 1 hour. Subsequently, the pulverized and dispersed slurry was coated on the surface of carbon nanotube paper as shown in FIG. 2 (FIG. 2 is an actual image of the CNT paper of the present invention coated with a sulfur-polymer composite) (at this time, the coating thickness was adjusted so that the sulfur content became 61.6 wt% with respect to the total weight of the positive electrode), dried at a temperature of 50 to 80°C, and heat-treated at 155°C for 30 minutes, thereby producing a positive electrode for a lithium secondary battery excluding a metal current collector. Finally, the above-mentioned manufactured anode was rolled to set the porosity to 65%.

[Comparative Example 1] Manufacturing of a conventional anode

A slurry containing a sulfur-carbon (CNT) composite having a sulfur content of 70 wt%, Denkablack (conductive material), and styrene-butadiene rubber/carboxymethyl cellulose (binder, weight ratio 7:3) in a weight ratio of 88:5:7 was coated on aluminum foil by a doctor blade coating method to manufacture a cathode for a conventional lithium secondary battery (sulfur content in the cathode: 61.6 wt%), and then the manufactured cathode was rolled to set the porosity to 65%.

[Comparative Example 2] Manufacturing of a conventional anode

After sprinkling sulfur powder on the surface of carbon nanotube paper, a heat treatment was performed at 155°C for 30 minutes to manufacture a conventional positive electrode for lithium secondary batteries (sulfur content in the positive electrode: 61.6 wt%). Thereafter, the manufactured positive electrode was rolled to set the porosity to 65%.

[Experimental Example 1] Evaluation of Surface Uniformity of Positive Electrode for Lithium Secondary Battery

In order to evaluate the surface uniformity of the positive electrodes for lithium secondary batteries manufactured in Example 1 and Comparative Example 2, the surface of each positive electrode was observed using a scanning electron microscope (SEM). Fig. 3 is an SEM image observing the surface of the carbon nanotube paper and the positive electrodes manufactured in the Examples and Comparative Examples. The left figure of Fig. 3 is an image observed at a magnification of 1,000 times, and the right figure is an image observed at a magnification of 5,000 times.

As illustrated in FIG. 3, it was confirmed that the anode of Example 1, in which a sulfur-polymer composite layer was formed on the surface of the carbon nanotube paper (i.e., sulfur was coated in the form of a slurry), had a more uniform surface without agglomeration compared to the anode of Comparative Example 2, in which sulfur was scattered on the surface of the carbon nanotube paper.

[Example 2, Comparative Examples 3-4] Manufacturing of Lithium Secondary Battery

After preparing the positive electrode and lithium metal negative electrode manufactured in each of Example 1, Comparative Examples 1 and 2, a porous polyethylene separator was interposed between the positive electrode and the negative electrode to manufacture an electrode assembly, and after positioning the electrode assembly inside a case, an electrolyte was injected in an amount of about 2.2 times the weight of sulfur to manufacture a pouch-type lithium secondary battery having 0.45 Ah (450 Wh/kg).

[Experimental Example 2] Evaluation of discharge capacity and life characteristics of lithium secondary batteries

The lithium secondary batteries manufactured in Example 2, Comparative Examples 3 and 4 were discharged at 0.33 mA/cm ² @ 30oC to evaluate the initial discharge capacity and life characteristics of the batteries. Fig. 4 is a graph comparing and contrasting the discharge capacity (a) and life characteristics (b) of lithium secondary batteries manufactured according to one Example and Comparative Example of the present invention.

As a result of evaluating the discharge capacity and life characteristics of the lithium secondary batteries manufactured in the above Example 2, Comparative Examples 3 and 4, it was found that the battery of Comparative Example 4, which applied the cathode of Comparative Example 2 with poor surface uniformity, had a poor discharge capacity compared to the batteries of Example 2 or Comparative Example 3, as shown in FIG. 4(a), and it was confirmed through FIG. 4(b) that the discharge capacity decreased significantly when the C-rate increased.

On the other hand, it was confirmed through Fig. 4a that the battery of Example 2, which applied the positive electrode of Example 1 with excellent surface uniformity, implemented a discharge capacity similar to that of the battery (Comparative Example 3) that applied the conventional positive electrode using a sulfur-carbon composite and a slurry process, and through Fig. 4b that the life of the battery was very excellent compared to both Comparative Examples 3 and 4.

Claims (14)

Carbon nanotube paper; and
It comprises a sulfur-polymer composite layer coated on the above carbon nanotube paper,
The above carbon nanotube paper is a mixture of two types of carbon nanotubes, one type of carbon nanotube having a diameter of 5 to 50 nm and a length of 1 to 20 ㎛, and the other type of carbon nanotube having a diameter of 100 to 500 nm and a length of 10 to 100 ㎛.
The content ratio of the above small diameter carbon nanotubes and the above large diameter carbon nanotubes is 70 to 90:10 to 30 in weight ratio,
A current collector-free positive electrode for a lithium secondary battery, characterized in that it does not contain a metal current collector. A current collector-free positive electrode for a lithium secondary battery, characterized in that in claim 1, the polymer is selected from the group consisting of polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polyurethane (PU), polyacrylonitrile (PAN), and mixtures thereof. A current collector-free positive electrode for a lithium secondary battery, characterized in that in claim 1, the sulfur is supported on a metal foam support to form a composite layer. A current collector-free positive electrode for a lithium secondary battery, characterized in that the content ratio of sulfur and polymer in claim 1 is 90 to 99:1 to 10 in terms of weight ratio. delete delete A current collector-free positive electrode for a lithium secondary battery, characterized in that in claim 1, the amount of sulfur loaded on the carbon nanotube paper is 20 to 90 wt% with respect to the total weight of the electrode. a) a step of preparing a slurry containing sulfur and a polymer;
b) a step of grinding and dispersing the manufactured slurry; and
c) a step of coating the pulverized and dispersed slurry on carbon nanotube paper and then drying and heat treating it;
The above carbon nanotube paper is a mixture of two types of carbon nanotubes, one type of carbon nanotube having a diameter of 5 to 50 nm and a length of 1 to 20 ㎛, and the other type of carbon nanotube having a diameter of 100 to 500 nm and a length of 10 to 100 ㎛.
A method for manufacturing a current collector-free positive electrode for a lithium secondary battery, characterized in that the content ratio of the carbon nanotubes having a small diameter and the carbon nanotubes having a large diameter is 70 to 90:10 to 30 in weight ratio. A method for manufacturing a current collector-free positive electrode for a lithium secondary battery, characterized in that in claim 8, the coating in step c) is performed by a coating method selected from the group consisting of a spray coating method, a doctor blade coating method, and a spin coating method. A method for manufacturing a current collector-free positive electrode for a lithium secondary battery, characterized in that in claim 8, the drying in step c) is performed under a temperature condition of 30 to 90° C., and the heat treatment is performed under a temperature of 100 to 200° C. for 5 to 60 minutes. A method for manufacturing a current collector-free positive electrode for a lithium secondary battery, characterized in that, after step c), the method further comprises a step of rolling the positive electrode to adjust the porosity of the manufactured positive electrode. A method for manufacturing a current collector-free positive electrode for a lithium secondary battery, characterized in that the porosity of the positive electrode is adjusted to 50 to 95% through the rolling according to claim 11. A lithium secondary battery comprising: a current collector-free positive electrode according to any one of claims 1 to 4 and 7; an anode; a separator interposed between the positive electrode and the anode; and an electrolyte. A lithium secondary battery according to claim 13, characterized in that the lithium secondary battery is a lithium-sulfur battery.