CN118402092A - Positive electrode for lithium-sulfur battery and lithium-sulfur battery having high energy density - Google Patents

Abstract

The present invention relates to a positive electrode for a lithium-sulfur battery, which includes a porous carbon material having a low tap density manufactured by centrifugal grinding, thereby achieving high porosity, and forming a thick active material layer for the same carbon amount, and a lithium-sulfur battery including the same. Thus, the positive electrode realizes a lithium-sulfur battery having a high energy density.

Description

Positive electrode for lithium-sulfur battery and lithium-sulfur battery with high energy density

Technical Field

The present invention relates to lithium-sulfur batteries with high energy density.

This application claims the benefit of Korean Patent Application No. 10-2022-0159965 filed on November 25, 2022, Korean Patent Application No. 10-2022-0183586 filed on December 23, 2022, Korean Patent Application No. 10-2022-0183771 filed on December 23, 2022, Korean Patent Application No. 10-2022-0185613 filed on December 27, 2022, Korean Patent Application No. 10-2022-0185614 filed on May 16, 2023 The present invention claims the benefit of and priority to Korean Patent Application No. 10-2023-0063394 filed on May 31, 2023, Korean Patent Application No. 10-2023-0070299 filed on May 31, 2023, Korean Patent Application No. 10-2023-0073163 filed on June 7, 2023, and Korean Patent Application No. 10-2023-0075765 filed on June 13, 2023, the disclosures of which are incorporated herein by reference in their entirety.

Background technique

As energy storage technology has received increasing attention, its application range has expanded to various devices, including mobile phones, tablet computers, laptop computers, cameras, electric vehicles (EVs) and hybrid electric vehicles (HEVs). Therefore, the research and development of electrochemical devices has gradually increased. In this regard, electrochemical devices, especially secondary batteries capable of charging/discharging, including lithium-sulfur batteries, have received widespread attention. In recent years, efforts have been made in the development of batteries to enhance capacity density and specific energy through new designs of electrodes and cells.

Among electrochemical devices, lithium-sulfur (LiS) batteries have attracted much attention as next-generation secondary batteries that can potentially replace lithium-ion batteries due to their high energy density. In these batteries, lithium-sulfur is used as the positive electrode active material. During discharge, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur. In this process, sulfur forms linear-structured lithium polysulfides ( _{Li2S2 , Li2S4} , _{Li2S6 , Li2S8} ) from ring- _{shaped S8} , and the lithium-sulfur battery exhibits _a gradual discharge voltage until the polysulfide (PS) is completely reduced to LiS.

In lithium-sulfur batteries, the use of carbon materials with high specific surface area and high porosity, such as carbon nanotubes, as sulfur carriers can lead to the realization of high energy density and desirable life characteristics. However, additional research is needed to achieve sufficient energy density and life characteristics for commercialization.

Attempts have been made to increase the loading of sulfur, the cathode active material, to improve energy density. However, because sulfur is not conductive, increasing the sulfur content results in decreased reactivity and energy density.

Summary of the invention

technical problem

The present invention aims to provide a lithium-sulfur battery with high load and high energy density.

It should be easily understood that these and other objects and advantages of the present invention can be achieved by the means or methods set forth in the appended claims and their combinations.

Technical solutions

In order to solve the above problems, according to one aspect of the present invention, a positive electrode according to the following embodiment is provided.

A positive electrode according to a first embodiment includes:

Current collector; and

a positive electrode active material layer on at least one surface of the current collector,

The positive electrode active material layer comprises a sulfur-carbon composite material and a binder polymer.

The sulfur-carbon composite material comprises a porous carbon material and a sulfur-based material, and

The ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area of the positive electrode active material layer is 80 to 130 μm/mg.

According to the second embodiment, in the first embodiment, the porosity of the positive electrode active material layer may be 80 volume % or more.

According to the third embodiment, in the first or second embodiment, the amount of elemental sulfur (S) may be 60 wt % or more based on the total weight of the positive electrode active material layer.

According to a fourth embodiment, in any one of the first to third embodiments, the porous carbon material may have an angular particle shape.

According to the fifth embodiment, in any one of the first to fourth embodiments, the porous carbon material may have a particle shape uniformity of 1.3 or less according to the following Formula 1:

[Formula 1]

Particle shape uniformity=[average diameter of the circumscribed circle of the particles]/[average diameter of the inscribed circle of the particles].

According to the sixth embodiment, in any one of the first to fifth embodiments, the porous carbon material may be produced by grinding a raw porous carbon material using a centrifugal mill and sieving the ground porous carbon material through a sieve having a mesh size of 50 μm to 100 μm.

According to the seventh embodiment, in any one of the first to sixth embodiments, the tap density of the porous carbon material may be 0.09 g/cm ³ or less.

According to the eighth embodiment, in any one of the first to seventh embodiments, the porosity of the positive electrode active material layer may be 81 volume % to 85 volume %.

According to a ninth embodiment, in any one of the first to eighth embodiments, the porous carbon material may include a secondary structure formed by aggregation of carbon nanotubes as a primary structure.

According to a tenth embodiment, in any one of the first to ninth embodiments, the tap density of the porous carbon material may be 0.07 g/cm ³ or less.

According to an eleventh embodiment, in any one of the first to tenth embodiments, the amount of elemental sulfur (S) may be 65 wt % to 90 wt % based on the total weight of the positive electrode active material layer.

According to a twelfth embodiment, in any one of the first to eleventh embodiments, the supported amount of sulfur (S) may be 2.9 mg _s / cm ² or more.

According to another aspect of the present invention, a lithium-sulfur battery according to the following embodiment is provided.

A lithium-sulfur battery according to a thirteenth embodiment includes:

The positive electrode according to any one of the first to twelfth embodiments,

negative electrode,

The separator between the positive and negative electrodes, and

Electrolytes.

According to the fourteenth embodiment, in the thirteenth embodiment, the carbon weight per unit area of the positive electrode active material layer and the thickness of the positive electrode active material layer may each be measured after discharging at least once.

According to the fifteenth embodiment, in the thirteenth or fourteenth embodiment, the carbon weight per unit area of the positive electrode active material layer and the thickness of the positive electrode active material layer may each be measured at a state of charge (SOC) of 97% to 100%.

According to a sixteenth embodiment, in any one of the thirteenth to fifteenth embodiments, a weight ratio of the electrolyte to sulfur (S) in the sulfur-carbon composite material (E1/S weight ratio) may be 3.5 g/g or less.

According to a seventeenth embodiment, in any one of the thirteenth to sixteenth embodiments, the energy density of the lithium-sulfur battery may be 400 Wh/kg or more.

Beneficial Effects

According to one aspect of the present invention, a positive electrode for a lithium-sulfur battery having a high loading of a positive electrode active material (sulfur) and a lithium-sulfur battery including the same can be provided. In particular, according to the present invention, a lithium-sulfur battery having a high loading of a positive electrode active material (sulfur) and having improved energy density by maintaining and improving the electrochemical reactivity of sulfur can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the specific capacity evaluation of lithium-sulfur batteries according to Comparative Examples 1, 2 and 4 of the present invention.

FIG. 2 is a graph showing the specific capacity evaluation of the lithium-sulfur batteries of Comparative Example 1 and Examples 1 and 2 according to the present invention.

FIG. 3 is a graph showing relative energy densities of lithium-sulfur batteries according to Examples 1 and 2 and Comparative Examples 1 to 3. FIG.

4a is a scanning electron microscope (SEM) image of a porous carbon material used to measure the uniformity of particle shape of the porous carbon material in Comparative Example 4 of the present invention. The cross arrows on the image respectively represent the major axis and the minor axis used to measure the length.

4b is a SEM image of a porous carbon material used to measure the uniformity of particle shape of the porous carbon material used in Example 1 of the present invention. The cross arrows on the image respectively indicate the major axis and the minor axis used to measure the length.

5 is a SEM image showing that the surface of the porous carbon material of Comparative Example 1 is relatively flat (left) and a SEM image showing that the surface of the porous carbon material of Example 1 is relatively rough (right).

Detailed ways

Hereinafter, the present invention will be described in detail.

When used in this specification, unless the context clearly indicates otherwise, the terms “comprises,” “comprising,” or “having” specify the presence of described elements, but do not preclude the presence or addition of one or more other elements.

Additionally, the terms "about" and "substantially" as used herein are used in the sense of the value or close thereto, given the manufacturing and material tolerances inherent in the circumstances, and are used to prevent unscrupulous infringers from improperly taking advantage of the exact numerical or absolute numerical values set forth in the present invention to aid in understanding the present invention.

Throughout the specification, "A and/or B" means A or B or both.

The term "composite material" as used herein refers to a material having physically and chemically different phases and more effective functions formed by combining two or more materials.

The term "polysulfide" as used herein is a concept covering "polysulfide ions (S _x ^2- , 1≤x≤8)" and "lithium polysulfide (Li ₂ S _x or LiS _x ^- , 1≤x≤8)".

In the present invention, the "specific surface area" is measured by the Brunauer-Emmett-Teller (BET) method, and specifically, it can be calculated from the nitrogen adsorption amount at liquid nitrogen temperature (77K) using BELSORP mino II of BEL Japan.

In the present invention, “particle size D ₁₀ ” refers to the particle size at 10% of the cumulative volume particle size distribution of particles, “particle size D ₅₀ ” refers to the particle size at 50% of the cumulative volume particle size distribution of particles, and “particle size D ₉₀ ” refers to the particle size at 90% of the cumulative volume particle size distribution of particles.

Each particle size D ₁₀ , D ₅₀ and D ₉₀ can be measured using a laser diffraction method. For example, each particle size D ₁₀ , D ₅₀ and D ₉₀ can be measured by dispersing the target particle powder in a dispersion medium, introducing it into a commercially available laser diffraction particle size measuring instrument (e.g., Microtrac MT 3000), irradiating an ultrasonic wave of about 28kHz with an output of 60W to obtain a cumulative volume particle size distribution diagram, and determining the particle size corresponding to 10%, 50% and 90% of the cumulative volume distribution. That is, for example, the particle size D ₅₀ represents the median or median diameter in the particle size distribution diagram, and it refers to the particle size at 50% point on the cumulative distribution. The particle size represents the particle diameter, and the particle diameter refers to the longest length in the particle.

Unless the context clearly indicates otherwise, the unit “mAh/g _s ” used herein refers to the unit weight capacity of sulfur (S) and can be used interchangeably with mAh/g(s), mAh/gs or any other form of unit expression.

Unless the context clearly indicates otherwise, the unit "mg _s /cm ² " used herein represents the weight per unit area of sulfur (S) and can be used interchangeably with mg(s)/cm ² , mAh/gs or any other form of unit expression.

The term "porosity" as used herein refers to the proportion of voids in a structure to the total volume and is expressed in volume %, and can be used interchangeably with void ratio, porosity degree, etc. Porosity can be measured according to methods known in the art, such as the method specified in ISO 15901:2019.

The present invention provides a positive electrode for use in an electrochemical device and an electrochemical device comprising the same. In the present invention, the electrochemical device may include any device that causes an electrochemical reaction. Specific examples may include any type of primary battery, secondary battery, fuel cell, solar cell or capacitor such as a supercapacitor. In particular, the electrochemical device may be a secondary battery, and the secondary battery may be a lithium ion secondary battery. The lithium ion secondary battery may include, for example, a lithium metal battery, a lithium sulfur battery, an all-solid-state battery, a lithium polymer battery, etc., preferably a lithium sulfur battery.

It is known that conventional lithium-sulfur batteries have the disadvantage of low energy density. In order to solve this disadvantage, carbon materials with high specific surface area and porosity, such as carbon nanotubes, have been used as carriers for sulfur-based materials to achieve relatively high energy density and life characteristics, but commercialization remains challenging. In addition, it is necessary to increase the load of sulfur-based materials in the sulfur-carbon composite material included in the positive electrode to increase the energy density of the lithium-sulfur battery, but because the sulfur-based material has no or almost no conductivity, the higher the load, the lower the reactivity of the positive electrode.

According to one aspect of the present invention, there is provided a positive electrode with improved conductivity, wherein a sulfur-based material such as sulfur (S ₈ ) or a sulfur compound is supported in a porous carbon material with improved shape to increase the loading amount of the sulfur-based material such as sulfur (S ₈ ) or the sulfur compound and ensure conductivity in the positive electrode, and also a lithium-sulfur battery with improved energy density.

The positive electrode according to one aspect of the present invention comprises a current collector and a positive electrode active material layer on at least one surface of the current collector. Specifically, the positive electrode active material layer comprises a sulfur-carbon composite material and a binder polymer. In addition, the sulfur-carbon composite material comprises a porous carbon material and a sulfur-based material.

In one embodiment of the present invention, the porous carbon material may have a particle shape.

In one embodiment of the invention, the porous carbon material may include irregular pores (closed pores) inside the particles and/or irregular pores (open pores) on the surface of the particles. In this case, the average diameter of these pores may be, for example, in the range of 1 to 200 nm, and the porosity may be between 10 and 90% by volume of the total volume of the porous carbon material. For example, the average diameter of these pores may be measured using methods known in the art such as BET measurements based on gas adsorption or mercury intrusion porosimetry.

In one embodiment of the present invention, the porous carbon material has a specific particle size range as described below.

In one embodiment of the present invention, the sulfur-carbon composite material may include a porous carbon material and a sulfur-based material, and the sulfur-based material may be supported and/or coated on all or at least a portion of the pore interior and surface of the porous carbon material.

In one embodiment of the present invention, the positive electrode comprises a porous carbon material as a carrier for an active material, and has a small tap density, thereby having a high porosity. In particular, by loading the active material in the above-mentioned porous carbon material, even when a high content of active material is included, the movement channel of substances such as ions in the positive electrode can be fully ensured, thereby reducing resistance and improving capacity.

Specifically, in the positive electrode, a ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area in the positive electrode active material layer may be 80 to 130 (μm/mg).

In one embodiment of the present invention, the positive electrode may be characterized in that the porosity of the active material layer is 80% by volume or more.

In one embodiment of the present invention, the positive electrode may be characterized in that, based on the total weight of the active material layer, the amount of elemental sulfur (S) is 60 wt % or more, specifically 65 wt % or more. In this case, the elemental sulfur (S) may be derived from a sulfur-based material, in particular, may be derived from inorganic sulfur (S ₈ ) used to manufacture a sulfur-carbon composite material.

In one embodiment of the present invention, the positive electrode may be characterized in that the amount of elemental sulfur (S) is 60 wt % or more, specifically 65 wt % or more, based on the total weight of the active material layer, and the porosity is 80 volume % or more.

That is, according to one aspect of the present invention, the use of the porous carbon material with improved shape can increase the loading amount of the active material in the positive electrode active material and ensure the porosity of the positive electrode.

Sulfur-based materials such as inorganic sulfur (S ₈ ) used as active materials in lithium-sulfur batteries are non-conductors. Therefore, in conventional lithium-sulfur batteries, when the active material (i.e., sulfur-based material) is contained too much in the positive electrode active material layer, the resistance in the positive electrode increases, and the reactivity of the positive electrode decreases. Therefore, it is difficult to contain the sulfur-based material in an amount of 60 wt % or more based on 100 wt % in total of the positive electrode active material layer. Specifically, it is difficult to contain elemental sulfur (S) in an amount of 60 wt % or more based on 100 wt % in total of the positive electrode active material layer.

However, the present invention achieves a ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area of the positive electrode active material layer within a specific range, thereby providing a positive electrode having a larger amount of active material (i.e., sulfur-based material) and a higher active material layer porosity and improved reactivity.

As described above, in the positive electrode according to one aspect of the present invention, the ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area of the positive electrode active material layer is 80 to 130 μm/mg. Here, the "unit area" is 1 cm ² (1 cm×1 cm).

In one embodiment of the present invention, the ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area of the positive electrode active material layer can be calculated by measuring the carbon weight per 1 cm ² of the positive electrode active material layer and measuring the thickness of the positive electrode active material layer.

In the present invention, the carbon weight per unit area of the positive electrode active material layer and the thickness of the positive electrode active material layer can each be measured based on an unused fresh cell immediately after the positive electrode is manufactured without being used for electrochemical reaction. Alternatively, the carbon weight per unit area of the positive electrode active material layer and the thickness of the positive electrode active material layer can each be measured after the electrode is discharged at least once. In this case, it is preferred that the electrode has a capacity retention rate of 97% or more of the initial capacity, but the present invention is not limited thereto.

Specifically, in terms of measurement accuracy, the thickness of the positive electrode active material layer may be preferably measured in a charged state. For example, it may be measured based on a state of charge (SOC) of 97% or more when charged after at least one discharge, for example, SOC 97% to 100%, specifically SOC 100%. Alternatively, the carbon weight per unit area of the positive electrode active material layer and the thickness of the positive electrode active material layer may be measured based on a cycle when the thickness of the positive electrode active material layer is the thinnest.

In one embodiment of the present invention, the ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area of the positive electrode active material layer can be calculated by calculating the weight of carbon loaded per electrode and measuring the thickness of the positive electrode active material layer. Here, the electrode loading can be calculated by the amount of sulfur (S) in the positive electrode according to a known method.

In another embodiment of the present invention, the ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area of the positive electrode active material layer can be calculated by a method of directly analyzing the carbon content per unit area of the positive electrode active material layer and a method of measuring the electrode thickness. Here, the method of directly analyzing the carbon content per unit area may include, for example, known elemental analysis methods, such as ICP-OES analysis, EA analysis, and ICP analysis, but the measurement method is not limited thereto. In addition, the method of measuring the thickness of the electrode may include measuring the thickness of the entire positive electrode using a known thickness gauge such as a thickness gauge of Mitutoyo Co. and subtracting the thickness of the collector to determine the thickness of the positive electrode active material layer, but the measurement method is not limited thereto.

In one embodiment of the present invention, the carbon weight per unit area of the positive electrode active material layer refers to the weight of all carbon, and the all carbon includes: carbon derived from the sulfur-carbon composite material contained in the positive electrode active material layer excluding the collector in the positive electrode to be tested; and carbon derived from the binder and/or conductive material that may be contained in the positive electrode active material layer.

Therefore, in one embodiment of the present invention, when the composition of the positive electrode and the amount of the sulfur-based material in the sulfur-carbon composite material are known, the carbon weight per unit area of the positive electrode active material layer can be calculated from the loading amount.

On the other hand, in lithium-sulfur batteries, the thickness of the positive electrode may vary with repeated charge/discharge cycles of the battery. Therefore, in order to measure the ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area of the positive electrode active material layer, the conditions of the number of charge/discharge cycles and/or SOC may be required. During battery discharge, discharge products are generated inside the porous carbon material structure in the positive electrode active material layer, so it may be difficult to accurately measure the thickness of the positive electrode without excluding the discharge products. Therefore, for example, the thickness of the positive electrode active material layer can be measured based on the state of charge, such as when the SOC is 97% to 100%, preferably when the SOC is 100%.

As described above, the positive electrode in which the ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area of the positive electrode active material layer satisfies 80 to 130 μm/mg can have, for example, elemental sulfur (S) in an amount of 65 wt % or more based on 100 wt % in total of the positive electrode active material layer and a porosity of 80 volume % or more.

In one embodiment of the present invention, the thickness of the positive electrode active material layer and the carbon weight per unit area of the positive electrode active material layer can be, for example, 80 to 120 μm/mg, 80 to 110 μm/mg, 80 to 100 μm/mg, or 83 to 98 μm/mg. In addition, based on the electrode immediately after manufacture (fresh battery), the thickness of the positive electrode active material layer and the carbon weight per unit area of the positive electrode active material layer can be, for example, 90 to 120 μm/mg or 95 to 119 μm/mg.

In one embodiment of the present invention, the sulfur-based material may include but is not limited to any material that provides sulfur (S ₈ ) as an active material for a lithium-sulfur battery. For example, the sulfur-based material includes at least one of sulfur (S ₈ ) or a sulfur compound.

In one embodiment of the present invention, the sulfur-based material may include inorganic sulfur (S ₈ ), Li ₂ S _n (n ≥ 1), an organic sulfur compound such as 2,5-dimercapto-1,3,4-thiadiazole and 1,3,5-trithiocyanic acid, a carbon-sulfur polymer ((C ₂ S _x ) _n , x is 2.5 to 50, n ≥ 2) or a mixture thereof. Specifically, the sulfur-based material may include inorganic sulfur (S ₈ ).

In one embodiment of the present invention, the sulfur-based material can be included in the sulfur-carbon composite material by physical adsorption with the porous carbon material or chemical bonds such as covalent bonds or van der Waals bonds between elemental sulfur (S) and carbon in the porous carbon material. In particular, the sulfur-based material can be chemically bonded to the surface of the porous carbon material to form a composite material.

In the sulfur-carbon composite material according to the present invention, the sulfur-based material may be present in at least one of the interior of the pores of the carbon material or the surface of the carbon material, in which case the sulfur-based material may be present in an area of less than 100%, preferably 1% to 95%, and more preferably 60% to 90% of the interior and surface of the pores of the carbon material. When sulfur (S) is present in the above range on the surface of the carbon material, the maximum effect can be obtained in terms of electron transport area and electrolyte wetting. Specifically, when sulfur is uniformly impregnated on the surface of the carbon material to a small thickness under the area in the above range, the electron transport contact area can be increased during charge and discharge. In the case where sulfur is present in an area corresponding to 100% of the entire surface of the carbon material, the carbon material is completely covered with sulfur, resulting in poor electrolyte wettability and less contact with the conductive material contained in the electrode, thereby being unable to accept electrons and participate in the reaction.

In one embodiment of the present invention, the amount of elemental sulfur (S) may be 60% by weight or more, 65% by weight or more, for example, 60 to 100% by weight, 60 to 90% by weight, 65 to 80% by weight, 65 to 75% by weight, 65 to 70% by weight, or 70 to 75% by weight, based on a total of 100% by weight of the positive electrode active material layer. For example, the amount of elemental sulfur (S) may be 67.2 to 72% by weight, based on a total of 100% by weight of the positive electrode active material layer. When the amount of elemental sulfur (S) is within the above range, the capacity of a battery using the same and the stability of the battery may be improved, but the present invention is not limited thereto.

In one embodiment of the present invention, the porous carbon material may have a shape improvement of the porous carbon material commonly used in lithium-sulfur batteries. Specifically, the porous carbon material may have a particle shape, especially an angular particle shape. For example, the porous carbon material may have a particle shape with prismatic sphericity.

As described below, the porous carbon material can have an improved particle shape by using a centrifugal mill.

5, it was confirmed that the surface of the porous carbon material pulverized by jet milling or the like (left) was relatively flat, while the surface of the porous carbon material ground by a centrifugal mill (right) was relatively rough. It was confirmed that the porous carbon material on the right had an angular particle shape due to the rough surface characteristics.

Specifically, the porous carbon material may include any carbon-based material with pores and conductive properties commonly used in the corresponding technical field. For example, the porous carbon material may include at least one selected from the following: graphite; graphene; carbon black, including Danka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black; carbon nanotubes (CNTs), including single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers, including graphite nanofibers (GNFs), carbon nanofibers (CNFs) and activated carbon fibers (ACFs); graphite, including natural graphite, artificial graphite and expanded graphite; carbon nanorods; carbon nanobelts, carbon nanorods and activated carbon.

In one embodiment of the present invention, the porous carbon material may include carbon nanotubes. Carbon nanotubes are tubes made of carbon connected in a hexagonal shape. According to one embodiment of the present invention, the carbon nanotubes may be single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs) or a combination thereof, depending on the number of carbon atom layers (called "carbon walls") of the carbon nanotubes made. Here, the length of each carbon nanotube is not particularly limited.

In one embodiment of the present invention, the porous carbon material may include carbon nanotubes, specifically multi-walled carbon nanotubes (MWCNTs), to improve sulfur loading, but the present invention is not limited thereto.

In one embodiment of the present invention, the porous carbon material may include a secondary structure formed by aggregation of carbon nanotubes as a primary structure.

In another embodiment of the present invention, the carbon nanotubes may include two or more carbon nanotubes, which are entangled by being in close contact with each other through the cohesive force between them. Specifically, in one embodiment of the present invention, the carbon nanotubes may be provided in the form of a carbon nanotube dispersion in which a single strand is dispersed in a dispersion medium, or in the form of a secondary structure formed by the aggregation of carbon nanotubes of a primary structure.

In this regard, when the porous carbon material includes carbon nanotubes, the carbon nanotubes may include at least one of a bundled secondary structure or an entangled secondary structure.

The bundled secondary structure of carbon nanotubes refers to an aggregate of primary structures (each primary structure is a single strand of carbon nanotubes) arranged in the longitudinal direction of the carbon nanotubes and bonded by cohesive force between carbons, and can be called a bundled CNT.

In one embodiment of the present invention, the carbon nanotubes may comprise, for example, entangled multi-walled carbon nanotubes.

In one embodiment of the present invention, the porous carbon material may have a BET specific surface area of, for example, 150 m ² / g or more. In one embodiment of the present invention, the BET specific surface area of the porous carbon material may be, for example, 150 to 2500 m ² / g, 150 to 2,000 m ² / g, 150 to 1,500 m ² / g, 150 to 1,000 m ² / g, 130 to 300 m ² / g or 170 to 200 m ² / g, but is not limited thereto. In the present invention, the "specific surface area" is measured by the Bruer-Emmett-Teller (BET) method, specifically, it can be calculated from the adsorption amount of nitrogen at liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japan.

According to one embodiment of the present invention, the porous carbon material can be modified in shape by grinding. Specifically, grinding can involve crushing or shearing of particles. For example, particles can be crushed or sheared between two blades. In this case, grinding applies stimulation to the outer surface of the particles to crush or shear the particles. In addition, when grinding particles, the particles can be broken or separated by friction between the rotating blades of the grinder and the particles.

Typically, carbon nanotubes such as single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT) are synthesized using a thermal chemical vapor deposition method or an arc discharge method, and aggregation occurs between individual carbon nanotube particles during the synthesis process. The aggregation of carbon nanotubes can be classified into: physical aggregation, which is the entanglement aggregation of nanotubes as each particle at the μm level; and chemical aggregation, which is the aggregation that occurs by surface attraction (~950meV/nm) such as van der Waals force or intermolecular force at the nm (nanometer) level in the case of, for example, single-walled carbon nanotubes (SWCNT). The aggregation of carbon nanotubes may hinder the formation of a three-dimensional network structure that can improve mechanical strength and conductive properties. Aggregation is a common phenomenon in linear conductive carbon materials such as carbon nanotubes. In this case, the present invention uses a linear conductive carbon material to manufacture an electrode after disintegration by pretreatment such as grinding, thereby satisfying the predetermined tap density range described below and improving the filling rate of the sulfur-based material.

The porous carbon material of a conventional lithium-sulfur battery is used after being pretreated by jet milling. However, because the porous carbon material has low uniformity in particle shape and its particle surface is smooth, the pretreatment of the porous carbon material by jet milling cannot improve the tap density. However, the present invention can use a grinder to pretreat the porous carbon material. Because defects or flaws (e.g., tearing) are generated in each conductive carbon material on the surface of the porous carbon material, and the conductive carbon material is separated or loose, the particle shape uniformity of the porous carbon material can be increased and the surface roughness of the particles can be improved. Accordingly, this can further increase the sulfur load and improve the tap density. However, the present invention is not limited thereto.

In one embodiment of the present invention, the pretreatment for improving the shape of the porous carbon material may further include a process of sieving using a sieve of a predetermined size after the pulverization.

In one embodiment of the present invention, the rotation speed of the mill during the pretreatment of the porous carbon material shape improvement can be 10,000rpm to 25,000rpm or 15,000rpm to 20,000rpm. When the rotation speed of the mill is within the above range, the porous carbon material can be effectively ground. However, the rotation speed of the mill is not limited to the above range, and those skilled in the art can adjust the rotation speed of the mill to grind the porous carbon material to a suitable degree. In a specific embodiment of the present invention, grinding can be carried out using a centrifugal mill such as a Retsch ZM-200 device.

In one embodiment of the present invention, the pretreatment process for improving the shape of the porous carbon material may include centrifugal grinding by grinding at the above speed and screening the particles through a sieve having a size of 50 to 100 μm, 60 to 100 μm or 80 μm. Therefore, the porous carbon material can be improved by grinding to improve the surface roughness of the particles, so that the particle size is small and uniform, and the ratio of the major axis length to the minor axis length converges to less than 1.3, specifically 1.

In one embodiment of the present invention, the particle shape uniformity can be calculated according to the following formula 1.

[Formula 1]

Particle shape uniformity=[average diameter of the circumscribed circle of the particles]/[average diameter of the inscribed circle of the particles].

In the above-mentioned Formula 1, "the circumscribed circle of a particle" refers to the diameter of an imaginary circumscribed circle having the major axis of an arbitrary particle as its diameter, and accordingly refers to the major axis of the particle.

In the above-mentioned Formula 1, "the inscribed circle of the particle" refers to the diameter of an imaginary inscribed circle having the minor axis of an arbitrary particle as its diameter, and accordingly refers to the minor axis of the particle.

The particle shape uniformity can be calculated from the average value of the circumscribed circle diameter and the average value of the inscribed circle diameter of at least 10 particles.

In one embodiment of the present invention, the circumscribed circle diameter and the inscribed circle diameter of the particle may each be measured by analyzing a scanning electron microscope (SEM) image of the porous carbon material.

In one embodiment of the present invention, the porous carbon material may have a controlled particle shape and thus a low tap density through the following pretreatment method.

In one embodiment of the present invention, the porous carbon material can be prepared by grinding the raw porous carbon material using a centrifugal mill and sieving the ground porous carbon material to a target particle size using a sieve. In this case, the mesh size of the sieve can be 2.8 to 4 times the particle size _D50 of the prepared porous carbon material, and can be, for example, 50 μm to 100 μm.

Conventionally, a ball mill, a blade, etc. have been used to pulverize porous carbon materials to control the particle size of the porous carbon materials. However, the conventional pulverization method has a problem in that, since the porous carbon materials are randomly contacted with the balls or blades, porous carbon materials with large particle sizes exist together with porous carbon materials with small particle sizes, resulting in a wide particle size distribution.

The present invention can prepare a porous carbon material by performing a step of grinding the porous carbon material using a centrifugal mill (step 1) and a step of sieving the ground porous carbon material through a sieve (step 2).

In one embodiment of the present invention, step (1) may include grinding by using a centrifugal mill while rotating at an angular velocity of 30 to 125 rad/s. Specifically, the angular velocity may be 30 to 95 rad/s. When the centrifugal grinding speed in step (1) is within the above range, it may be advantageous to finely and uniformly control the particle size of the porous carbon material without increasing the tap density.

In one embodiment of the present invention, the centrifugal mill may include a plurality of rotating teeth, and the porous carbon material may be ground while the rotating teeth are rotating. Specifically, the centrifugal mill may include, for example, 2 to 20, 4 to 18, 6 to 16, 8 to 14, 10 to 14 or 10 to 12 rotating teeth.

In addition, in one embodiment of the present invention, the plurality of rotating teeth may each have a triangular prism shape, and the plurality of rotating teeth may be arranged to face the rotation axis of the centrifugal mill. Specifically, when viewed from the top of the centrifugal mill along the rotation axis of the centrifugal mill, the plurality of rotating teeth may be arranged so that the edges of the triangular prisms intersect at the center of the centrifugal mill.

In one embodiment of the present invention, the plurality of rotating teeth may be made of a material such as stainless steel, titanium, or stainless steel with a protective coating, for example. However, this is not limited thereto.

In one embodiment of the invention, any centrifugal mill with rotating teeth, for example, a Retsch ZM 200 device, can be used in step (1).

In one embodiment of the present invention, centrifugal grinding can be performed at 6,000 to 18,000 rpm, and the particle size of the porous carbon material can be adjusted within the above range. Specifically, centrifugal grinding can be performed using a Retsch ZM 200 device at a speed ranging from 6,000 to 23,000 rpm, particularly at a speed of 6,000 to 18,000 rpm.

In one embodiment of the present invention, considering that the applied force may vary with the size of the centrifugal mill even at the same RPM, the RPM may be adjusted to grind at an angular velocity of 30 to 125 rad/s in consideration of the size of the centrifugal mill according to the following formula:

Angular velocity = (RPM x circumference) / 60 seconds

In the above formula, "circumference" means the distance traveled during one rotation of the rotating tooth.

Step (2) is a step of sieving the porous carbon material after centrifugal grinding in step (1) by sieving.

In one embodiment of the present invention, the screen can be equipped in the centrifugal mill, specifically, can be equipped on the outer edge of the centrifugal mill. Specifically, the screen can be equipped to surround a plurality of rotating teeth in the centrifugal mill.

In one embodiment of the invention, the screen may have a cylindrical shape and be arranged around a plurality of rotating teeth. For example, from a top view of the centrifugal mill, the shortest distance between the plurality of rotating teeth and the screen may be 0.1 to 5 mm or 0.5 to 2 mm, for example 1 mm. The screen may comprise a mesh having trapezoidal and/or circular holes.

In one embodiment of the present invention, when the teeth rotate, the porous carbon material is ground, and under centrifugal force, the porous carbon material with the particle size controlled to the target size immediately passes through a sieve located outside the rim on which the rotating teeth are arranged. Therefore, the problem of further particle size reduction and/or surface damage can be prevented. According to one aspect of the present invention, performing steps (1) and (2) simultaneously can achieve the control of the particle size to the target size and obtain a porous carbon material with a narrow particle size distribution.

In this manner, in one embodiment of the present invention, it may be advantageous to apply centrifugal force to the ground porous carbon material to perform step (2).

In step (2), the porous carbon material centrifugally ground in step (1) is transferred to a sieve and sieved by the sieve. Steps (1) and (2) may be performed in a continuous process.

The particle size of the porous carbon material can be controlled by controlling the mesh size of the sieve used in step (2). The mesh size of the sieve can be 2.8 to 4 times the target particle size _D50 of the porous carbon material (2.8≤mesh size/target _D50≤4 ). When the mesh size of the sieve is limited as described above, the desired _D50 particle size of the porous carbon material can be achieved, and a porous carbon material with a narrow particle size distribution can be obtained.

In one embodiment of the present invention, the target particle size _D50 of the porous carbon material can be, for example, the particle size _D50 of the porous carbon material manufactured according to one aspect of the present invention, and can be, for example, in the range of 10μm to 100μm, 5μm to 90μm, 10μm to 80μm, 15μm to 70μm, 20μm to 60μm, 10μm to 50μm, 15μm to 40μm or 20μm to 40μm.

In a lithium-sulfur battery according to one embodiment of the present invention, the porous carbon material preferably has a tap density of, for example, 0.1 g/cm ³ or less or less than 0.1 g/cm ³ when measured after a container containing the porous carbon material is tapped 1000 times. For example, in one embodiment of the present invention, the tap density of the porous carbon material may be 0.09 g/cm ³ or less. Specifically, the tap density of the porous carbon material may be 0.07 g/cm ³ or less. More specifically, the tap density of the porous carbon material may be 0.02 g/cm ³ to 0.09 g/cm ³ , 0.05 g/cm ³ to 0.09 g/cm ³ , or 0.05 g/cm ³ to 0.07 g/cm ^3. When the tap density of the porous carbon material is within the above range, the load of the sulfur-based material in the porous carbon material can be increased, the porosity of the positive electrode can be improved, and a lithium-sulfur battery with high energy density and high positive electrode reactivity can be provided, but the present invention is not limited thereto.

In the present invention, the tap density may be measured according to ASTM B527-06, and may be measured using TAP-2S (LOGAN).

According to one embodiment of the present invention, the porous carbon material with improved shape may include spherical particles having pores on the surface, wherein the particle shape uniformity according to the following formula 1 is 1.3 or less. For example, the particle shape uniformity may be 1 to 1.3, 1 to 1.2, or 1 to 1.1.

[Formula 1]

Particle shape uniformity = [average diameter of the circumscribed circle of the particle] / [average diameter of the inscribed circle of the particle]

In the present invention, the uniformity of particle shape can be expressed as the ratio of the length of the major axis to the length of the minor axis of the particle, that is, the length ratio of the diameter of the circumscribed circle of the particle to the diameter of the inscribed circle of the particle. In this case, as the uniformity is higher, the value can be closer to "1", and as the uniformity is lower, the value can be farther away from 1.

In one embodiment of the present invention, "major axis" can represent the longest length of a particle, preferably, it can be measured as the length of the diameter of the circumscribed circle of the particle. In addition, "minor axis" can represent the shortest length of a particle, preferably, it can be measured as the length of the diameter of the inscribed circle of the particle.

In the present invention, the major axis length (diameter of the circumscribed circle) and the minor axis length (diameter of the inscribed circle) of each particle can be measured using an image analyzer such as SEM or a transmission electron microscope (TEM), or by a known particle size measurement method.

In one embodiment of the present invention, particle shape uniformity can be measured by using a scanning electron microscope (S-4800, Hitachi High-Technologies Corporation) to directly shoot the sheet above the particles distributed and fixed in a parallel arrangement, and use Azokun (Asahi Kasei Engineering Corporation) to analyze the image. In this case, the particle shape uniformity of at least 5 particles, at least 10 particles, such as 10 to 1,000 particles, 100 to 1,000 particles, 10 to 500 particles or 100 to 500 particles or 10 particles can be measured, and the average value of the measured particle shape uniformity can be used as the particle shape uniformity of the particles. For example, the shape uniformity of 300 particles can be measured, and the average value of the measured particle shape uniformity can be the shape uniformity of all particles. However, the number of particles used to measure the particle shape uniformity is not limited to the above range, and those skilled in the art can select an appropriate number of particles.

Figures 4a and 4b show SEM images (Jeol Inc, 1,000 times magnification) for measuring the particle shape uniformity of the porous carbon material according to one embodiment of the present invention. According to one embodiment of the present invention, the particle shape uniformity can be calculated by measuring the ratio of the major axis length to the minor axis length of at least 5 particles on the porous carbon material image and calculating the average value.

In one embodiment of the present invention, the particle size _D50 of the shape-modified porous carbon material can be, for example, 120 μm or less, such as 100 μm or less, 90 μm or less, or 80 μm or less. When the particle size of the porous carbon material satisfies the above range, the uniformity of the electrode surface can be increased, and the cycle life of the battery can be improved.

In another embodiment of the present invention, the shape-modified porous carbon material may have a width factor (BF) of 7 or less. The width factor (BF) as described in the present invention may represent a ratio of particle size distribution of particle size D ₉₀ to particle size D ₁₀ in the porous carbon material, and may be calculated as a value of [particle size D ₉₀ /particle size D ₁₀ ].

In the present invention, “particle size D ₁₀ ” refers to the particle size at 10% of the cumulative volume particle size distribution of particles, “particle size D ₅₀ ” refers to the particle size at 50% of the cumulative volume particle size distribution of particles, and “particle size D ₉₀ ” refers to the particle size at 90% of the cumulative volume particle size distribution of particles.

Each of _D10 , _D50 and _D90 can be measured using a laser diffraction method. For example, each of _D10 , _D50 and _D90 can be measured by dispersing a powder of the particles to be measured in a dispersion medium, introducing into a commercially available laser diffraction particle size measuring instrument (e.g., Microtrac MT3000), irradiating with an ultrasonic wave of about 28 kHz at an output of 60 W to obtain a cumulative volume particle size distribution diagram, and determining the particle sizes corresponding to 10%, 50% and 90% of the cumulative volume distribution, respectively.

Using the above-mentioned porous carbon material, the positive electrode according to one aspect of the present invention may have a porosity of 80 volume % or more, but the mechanism of the present invention is not limited thereto.

In one embodiment of the present invention, the porosity of the positive electrode may be, for example, 80% to 90% by volume, specifically 80% to 85% by volume, 81% to 85% by volume, or 82% to 83% by volume. When the porosity of the positive electrode is within the above range, the reactivity of the positive electrode may be improved, but the present invention is not limited thereto.

In the present invention, the porosity of the positive electrode can be measured, for example, by a commonly used Hg porosimeter, for example, a mercury porosimeter (Micromeritics AUTOPORE V) can be used to measure. In addition, the porosity of the positive electrode can be measured by the Bruer-Emmett-Teller (BET) measurement method using a commonly used adsorbed gas such as nitrogen, for example, BELSORP series analyzers such as mini II of BEL Japan can be used to measure, but the present invention is not limited to this. The porosity measured by the above method may refer to the total volume of the pores in the positive electrode. In addition, the porosity can be measured by calculation from the true density of the constituent materials of the positive electrode, the apparent density of the manufactured positive electrode, and the thickness of the positive electrode. Specifically, the porosity can be calculated as the value of [(true density-apparent density)/true density]×100(%) of the positive electrode.

Hereinafter, the configuration of the positive electrode of the present invention will be described in more detail.

The sulfur-carbon composite material can be formed by simply mixing the sulfur material with the carbon material, or coating or loading it in a core-shell structure. The coating of the core-shell structure may include coating any of the sulfur material and the carbon material on another material, such as covering the surface of the carbon material with sulfur or vice versa. In addition, the loading may include filling the sulfur material in the carbon material, especially in the pores of the carbon material. The sulfur-carbon composite material can be provided in any form that satisfies the content ratio of the above-mentioned sulfur and carbon materials, and the present invention is not limited thereto.

The method for manufacturing the sulfur-carbon composite material according to the present invention is not limited to a specific method, and the sulfur-carbon composite material can be manufactured by a composite material forming method known in the art, the method comprising: (S1) mixing a porous carbon material with a sulfur-based material; and (S2) forming a composite material.

The mixing step (S1) can be performed by using a stirrer commonly used in the art to improve mixing between the sulfur-based material and the porous carbon material. In this case, the mixing time and speed can be selectively adjusted according to the amount and conditions of the raw materials.

The step (S2) of forming a composite material can be carried out by a method commonly used in the art. The present invention is not limited to a specific composite material forming method. A commonly used method, such as a dry method, or a wet method such as spraying can be used. For example, the mixture of sulfur and carbon material obtained after mixing can be ground by ball milling and placed in an oven at 120°C to 160°C for 20 minutes to 1 hour to uniformly coat the molten sulfur on the interior of the first carbon material and on the surface of the first carbon material.

In one embodiment of the present invention, the sulfur-carbon composite material may be manufactured by sequentially mixing a porous carbon material and a sulfur-based material and heat-treating the mixture of the porous carbon material and the sulfur-based material at 120° C., but the manufacturing method is not limited thereto.

In one embodiment of the present invention, based on 100 wt % of the sulfur-carbon composite material, the sulfur (S) content of the sulfur-carbon composite material may be 60 wt % or more, 70 wt % or more, or 75 wt % or more. For example, based on 100 wt % of the sulfur-carbon composite material, the sulfur (S) content of the sulfur-carbon composite material may be 60 wt % to 99 wt %, 65 wt % to 99 wt %, 70 wt % to 99 wt %, 75 wt % to 90 wt %, 70 wt % to 85 wt %, 70 wt % to 80 wt %, or 70 wt % to 75 wt %.

In one embodiment of the present invention, the positive electrode active material layer may further include a binder polymer in addition to the sulfur-carbon composite material. In addition, if necessary, the positive electrode active material layer may further include a conductive material in addition to the positive electrode active material and the binder resin. In this case, in one embodiment of the present invention, based on 100% by weight of the positive electrode active material layer, the content of the positive electrode active material, specifically the sulfur-carbon composite material, in the positive electrode active material layer may be 70% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more.

In one embodiment of the present invention, the loading amount of the active material of the positive electrode, specifically the loading amount of sulfur (S), can be 2.9 mg _s / cm ² or more, specifically 3.1 mg _s / cm ² or more, but the present invention is not limited thereto. According to one aspect of the present invention, the above-mentioned characteristics of the porous carbon material and the positive electrode can be conducive to realizing a high-load electrode.

In one embodiment of the present invention, when the loading amount of the positive electrode is converted into capacity, the loading amount can be, for example, 3.5 mAh/cm ² or more, specifically 3.5 mAh/cm ² to 10 mAh/cm ² , more specifically 3.5 mAh/cm ² to 5 mAh/cm ² or 3.5 mAh/cm ² to 4.5 mAh/cm ² , but the present invention is not limited thereto.

The binder polymer can play a role in attaching the positive active material particles to each other and improving the bonding strength between the positive active material and the positive current collector, and specific examples may include at least one of the following: polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene rubber (EPDM rubber), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber or its various copolymers. Based on the total weight of the positive active material layer, the content of the binder resin may be 1 to 30% by weight, preferably 1 to 20% by weight, and more preferably 1 to 10% by weight.

Conductive materials can be used to provide conductivity to the electrode, and can include, but are not limited to, any conductive material that has the ability to conduct electrons without causing any chemical changes in the corresponding battery. Specific examples may include at least one of the following: graphite such as natural graphite or artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotubes; metal powders or metal fibers of copper, nickel, aluminum, and silver; conductive whiskers of zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. When a conductive material is used, the content of the conductive material can generally be 1 to 30% by weight, preferably 1 to 20% by weight, and more preferably 1 to 10% by weight, based on the total weight of the positive electrode active material layer.

In one embodiment of the present invention, the positive electrode collector may include various types of positive electrode collectors used in the corresponding technical field. For example, the positive electrode collector may include: stainless steel, aluminum, nickel, titanium, sintered carbon; or aluminum or stainless steel treated with carbon, nickel, titanium or silver on the surface. The thickness of the positive electrode collector may generally be 3 μm to 500 μm, and may have fine concave-convex on the surface to increase the adhesion strength of the positive electrode active material. The positive electrode collector may be in various forms such as films, sheets, foils, nets, porous bodies, foams and non-woven fabrics.

According to another aspect of the present invention, a lithium-sulfur battery comprising the positive electrode is provided.

Specifically, the lithium-sulfur battery comprises a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is the positive electrode mentioned above.

A unit including a positive electrode, a negative electrode, and a separator may be referred to as an electrode assembly, and for example, the electrode assembly may be formed into a stacked or stacked/folded structure including a negative electrode and a positive electrode stacked with a separator interposed between the negative electrode and the positive electrode; or a jelly-roll structure including a rolled negative electrode, a positive electrode, and a separator. In addition, in the jelly-roll structure, an additional separator may be placed on the outside to prevent contact between the negative electrode and the positive electrode.

The negative electrode may include: a negative electrode current collector; and a negative electrode active material layer on at least one surface of the negative electrode current collector, the negative electrode active material layer may include a negative electrode active material, and if necessary, further include a conductive material and/or a binder.

The current collector, active material, conductive material, and binder of the negative electrode may include current collectors, active materials, conductive materials, and binders commonly used in lithium-sulfur batteries, and the present invention is not limited to a specific type.

The separator is disposed between the negative electrode and the positive electrode in the electrode assembly. The separator can separate the negative electrode from the positive electrode and provide a movement channel for lithium ions, and can include but is not limited to any type of separator commonly used in lithium secondary batteries.

The electrolyte may include any type of electrolyte usable in a lithium-sulfur battery, such as an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte, but is not limited thereto.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent may include an ether solvent to improve the charge/discharge performance of the battery. As a non-aqueous solvent, the ether solvent may include at least one of the following: a cyclic ether (e.g., 1,3-dioxolane, tetrahydrofuran, tetrahydropyran, etc.), a linear ether compound (e.g., 1,2-dimethoxyethane, etc.) or a low-viscosity fluorinated ether (e.g., 1H, 1H, 2'H, 3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl trifluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether, 1H, 1H, 2'H-perfluorodipropyl ether).

In one embodiment of the present invention, the organic solvent may include a mixture of 2-methylfuran and dimethoxyethane, for example, a mixture of 2-methylfuran and dimethoxyethane in a volume ratio (volume/volume) of 1:9 to 5:5, but is not limited thereto.

The lithium salt may include, but is not limited to, any compound used in a lithium secondary battery to provide lithium ions. Specifically, the lithium salt may include LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlO _{4 , LiAlCl 4 ,} LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) ₂ , LiCl, LiI or LiB(C ₂ O ₄ ) ₂ . The concentration of the lithium salt may be in the range of 0.1 to 5.0 M, preferably in the range of 0.1 to 3.0 M. When the concentration of the lithium salt is included in the above range, the electrolyte may have suitable conductivity and viscosity and exhibit excellent electrolyte properties, thereby effectively transmitting lithium ions.

In addition to the above-mentioned electrolyte constituents, the electrolyte may further contain additives to improve the life characteristics of the battery, prevent the capacity decay of the battery and improve the discharge capacity of the battery. For example, the additives may include at least one of the following: LiNO ₃ , halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted Oxazolidinone, N,N-substituted imidazoline, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum chloride, but not limited thereto. The content of the additive may be 0.1 wt % to 10 wt %, preferably 0.1 wt % to 5 wt %, based on the total weight of the electrolyte. In one embodiment of the present invention, the additive may include LiNO ₃ .

On the other hand, in one embodiment of the present invention, in the lithium-sulfur battery of the present invention, the total weight of the electrolyte to the total weight of the elemental sulfur (S) in the positive electrode, specifically the total weight of the elemental sulfur (S) contained in the sulfur-carbon composite material in the positive electrode (El/S) may be preferably, for example, 3.5 g/g or less, for example, 3.3 g/g or less, 3.2 g/g or less, or 3.0 g/g or less. In addition, in the lithium-sulfur battery, the total weight of the electrolyte to the total weight of the elemental sulfur (S) in the positive electrode (El/S) may be 2.0 to 3.0 g/g, for example, 2.3 g/g. Using the sulfur-carbon composite material according to the present invention, a lithium-sulfur battery having the above-mentioned El/S ratio range can be realized, thereby improving energy density. However, a lithium-sulfur battery using the sulfur-carbon composite material may have an El/S ratio higher than the above-mentioned range, and the present invention is not limited thereto.

The lithium-sulfur battery is not limited to a specific shape and may be in various shapes such as a cylindrical shape, a stacked shape, and a coin shape.

In addition, the present invention provides a battery module including a lithium-sulfur battery as a unit cell. The battery module can be used as a power source for medium and large-sized devices requiring high temperature stability, long cycle life characteristics, and high capacity characteristics.

According to one embodiment of the present invention, the sulfur (S) discharge capacity per unit weight of the lithium-sulfur battery using the above-mentioned positive electrode may be 1,000 mAh/gs or more, further 1,100 mAh/gs, but the present invention is not limited thereto.

According to one embodiment of the present invention, a lithium-sulfur battery using the above-mentioned positive electrode may have an energy density of 350Wh/kg or more, but the present invention is not limited thereto. Specifically, the energy density of the lithium-sulfur battery may be 400Wh/kg or more or 430Wh/kg or more, for example, the energy density of the lithium-sulfur battery may be 350Wh/kg to 500Wh/kg or 430Wh/kg to 480Wh/kg, but the higher the energy density of the lithium-sulfur battery, the better the performance of the battery, and thus the upper limit of the energy density is not particularly limited.

In one embodiment of the present invention, the lithium-sulfur battery can be used in small devices such as mobile phones and medium-to-large devices, and examples of the medium-to-large devices may include: power tools; electric vehicles, including electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV); electric two-wheeled vehicles, including electric bicycles, electric scooters; electric golf carts; energy storage systems that work using power generated by electric motors, but are not limited to these.

In one embodiment of the present invention, the lithium-sulfur battery may be a pouch-type, coin-type, or cylindrical lithium-sulfur battery, but the type of the lithium-sulfur battery is not limited thereto.

Hereinafter, the present invention will be described in more detail by way of examples, but the following examples are provided by way of illustration, and the scope of the present invention is not limited thereto.

<Manufacturing of lithium-sulfur batteries>

Example 1

Preparation of porous carbon materials

Aggregates of multi-walled carbon nanotubes (Cnano, MWCNT, tap density 0.14 g/cm ³ , particle shape uniformity 1.52) were prepared. Subsequently, the aggregates were ground using a centrifugal mill (Retsch, ZM-200) at 18,000 rpm (angular velocity 94.2 rad/s) and passed through a sieve with a mesh size of 80 μm to prepare a porous carbon material with improved shape.

The tap density of the improved porous carbon material was 0.07 g/cm ³ , and the particle shape uniformity was measured to be 1.07.

In this case, the tap density was measured after tapping the container containing the porous carbon material 1,000 times, and the particle shape uniformity represented the ratio of the average diameter of the circumscribed circle to the average diameter of the inscribed circle of 5 particles through the SEM image ( FIG. 4 b ) of the porous carbon material, and was calculated as the value of [average diameter of the circumscribed circle (major axis)/average diameter of the inscribed circle (minor axis)] (measurement 1: 1.01, measurement 2: 1.06, measurement 3: 1.02, measurement 4: 1.18, measurement 5: 1.07).

Manufacturing of sulfur-carbon composite materials

The shape-improved porous carbon material obtained as described above was uniformly mixed with sulfur (S ₈ ) at a weight ratio of 30:70 (CNT:S ₈ ). Subsequently, heat treatment was performed in an oven at 155° C. for 30 minutes to load sulfur in the porous carbon material, thereby manufacturing a sulfur-carbon composite material.

Cathode Manufacturing

The sulfur-carbon composite material obtained as described above and polyacrylic acid as a binder polymer were added to water and mixed together to prepare a positive electrode slurry. In this case, the weight ratio of the sulfur-carbon composite material to the binder polymer was 96:4. The solid content in the slurry was 27% by weight.

The slurry was coated on an aluminum foil (thickness: 20 μm) using a Mathis coater and dried at 50° C. for 24 hours, followed by roll pressing to manufacture a positive electrode. The porosity of the manufactured positive electrode active material layer was 83% by volume, and the loading amount of the active material was 3.1 mg/cm ² . In this case, the porosity was calculated as a percentage of a value obtained by subtracting the density (apparent density) of the positive electrode active material layer excluding the current collector in the manufactured positive electrode from the true density of the constituent material of the positive electrode active material layer and dividing by the true density.

Porosity (volume %) = [(true density - apparent density) / true density] × 100

Manufacturing of lithium-sulfur batteries

For the negative electrode, a 45 μm thick lithium metal foil was prepared, and for the electrolyte, ₃ wt% LiNO3 and 0.75 M LiFSI were dissolved in a mixed organic solvent of 2-methylfuran and dimethoxyethane in a volume ratio of 3:7 to prepare a mixed solution.

The positive electrode and the negative electrode manufactured and prepared as described above were placed with a polyethylene separator having a thickness of 16 μm and a porosity of 46 volume % therebetween, and an electrolyte was injected in an amount 2.3 times the weight of sulfur (S) in the sulfur-carbon composite material for the positive electrode, thereby manufacturing a lithium-sulfur battery (El/S=2.3 g/g).

Example 2

A lithium sulfur battery was manufactured by the same method as Example 1 except that the weight ratio of porous carbon material to sulfur was changed to 25:75 (CNT:S ₈ ) when manufacturing the sulfur-carbon composite material. In this case, the porosity of the manufactured positive electrode was 82 vol % and the loading amount of the active material was 3.1 mg/cm ² .

Comparative Example 1

Preparation of porous carbon materials

Aggregates of multi-walled carbon nanotubes (Cnano, MWCNT, tap density 0.14 g/cm ³ , particle shape uniformity 1.52) were prepared. Subsequently, the aggregates were jet milled to prepare carbon nanotube aggregates with a tap density of 0.1 g/cm ³ and a particle shape uniformity of 1.44.

Manufacturing of sulfur-carbon composite materials

The porous carbon material obtained as described above was uniformly mixed with sulfur at a weight ratio of 30:70 (CNT:S ₈ ). Subsequently, heat treatment was performed in an oven at 155° C. for 30 minutes to load sulfur in the porous carbon material, thereby manufacturing a sulfur-carbon composite material.

Cathode and battery manufacturing

A lithium-sulfur battery was manufactured by the same method as in Example 1 except that the sulfur-carbon composite material manufactured as described above was used. In this case, the porosity of the manufactured positive electrode was 79 volume %, and the loading amount of the active material was 3.1 mg/cm ² .

Comparative Example 2

A lithium sulfur battery was manufactured by the same method as Comparative Example 1 except that the weight ratio of porous carbon material to sulfur was changed to 25:75 (CNT:S ₈ ) when manufacturing the sulfur-carbon composite material. In this case, the porosity of the manufactured positive electrode was 78 vol % and the loading amount of the active material was 3.1 mg/cm ² .

Comparative Example 3

A lithium sulfur battery was manufactured by the same method as Comparative Example 1 except that the weight ratio of porous carbon material to sulfur was changed to 35:65 (CNT:S ₈ ) when manufacturing the sulfur-carbon composite material. In this case, the porosity of the manufactured positive electrode was 80 vol % and the loading amount of the active material was 3.1 mg/cm ² .

Comparative Example 4

Preparation of porous carbon materials

Carbon nanotube aggregates with a tap density of 0.2 g/cm ³ and a particle shape uniformity of 1.44 were prepared without pretreatment (grinding).

Figure 4a shows an SEM image of a porous carbon material, and the particle shape uniformity was calculated as the value of [average diameter of the circumscribed circle (long axis)/average diameter of the inscribed circle (short axis)] of 5 particles on the image. (Measurement 1: 2.06, Measurement 2: 1.26, Measurement 3: 1.37, Measurement 4: 1.36, Measurement 5: 1.16)

Manufacturing of sulfur-carbon composite materials

The porous carbon material obtained as described above was uniformly mixed with sulfur (S ₈ ) at a weight ratio of 25:75 (CNT:S ₈ ). Subsequently, heat treatment was performed in an oven at 155° C. for 30 minutes to load sulfur in the porous carbon material, thereby manufacturing a sulfur-carbon composite material.

Cathode and battery manufacturing

Subsequently, a lithium-sulfur battery was manufactured by the same method as Comparative Example 1 except that the sulfur-carbon composite material manufactured as described above was used. In this case, the porosity of the manufactured positive electrode was 77 volume %, and the loading amount of the active material was 3.1 mg/cm ² .

<Evaluation of Physical Properties of Lithium-Sulfur Batteries>

The characteristics of the lithium-sulfur batteries prepared above are summarized in Tables 1 and 2 below.

Tap density

According to ASTM B527 standard method, after the porous carbon material for manufacturing the sulfur-carbon composite material is placed in a test container and tapped 1,000 times using a tapping device, the ratio of the mass (mass, M) (g) of the porous carbon material to its volume (volume, V) (cm ³ ) is measured to measure the tap density.

Tap density (TD) = [M/V]

Particle shape uniformity

After obtaining a 1,000-fold magnification SEM image (S-4800, Hitachi High-Technologies Corporation) of the porous carbon material used to manufacture the above sulfur-carbon composite material, the major axis length and minor axis length of 5 sulfur-carbon composite materials on the SEM image were measured, and the particle shape uniformity was measured. At this time, the major axis is equal to the diameter of the imaginary circumscribed circle of the particle, and the minor axis is equal to the diameter of the imaginary inscribed circle of the particle.

Particle shape uniformity = [(average diameter of the circumscribed circle of the particles)/(average diameter of the inscribed circle of the particles)]

Sulfur (S ₈ ) content in the positive electrode active material layer

The mass of sulfur (S ₈ ) relative to the total weight of the positive electrode active material layer was calculated from the mass of sulfur (S ₈ ) used to prepare the sulfur-carbon composite material and the mass of the sulfur-carbon composite material used to prepare the positive electrode active material layer.

Porosity

For the positive electrode prepared as described above, the porosity was calculated as a percentage of a value obtained by subtracting the density of the positive electrode active material layer excluding the current collector in the manufactured positive electrode from the true density of the material constituting the positive electrode active material layer (apparent density) and dividing by the true density.

Porosity (volume %) = [(true density - apparent density) / true density] × 100

Ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area

Each measurement is based on a ratio immediately after manufacture and a ratio of the battery during operation in which the battery is repeatedly charged and discharged.

First, for a fresh battery immediately after manufacture, the thickness of the positive electrode is measured using a thickness gauge (Mitutoyo Co.), and the thickness of the positive electrode active material layer is measured by subtracting the thickness of the current collector from the measured thickness. In addition, the carbon weight per unit area (1 cm ² ) of the positive electrode active material layer is measured by calculating the carbon weight in the positive electrode active material layer from the composition of the positive electrode used in the manufacturing step and dividing by the active material loading amount (mg/cm ² ). In this case, the calculated value is cross-verified by measuring the carbon weight according to the following method. After removing the current collector from the positive electrode and then performing ICP-OES (inductively coupled plasma optical emission spectrometry) analysis on the positive electrode active material layer using an elemental analyzer, the carbon weight relative to the total weight of the active material layer is obtained and divided by the area (cm ² ) of the positive electrode active material layer to be measured, thereby obtaining the carbon weight per unit area (1 cm ² ) of the active material layer (mg/cm ² ).

For a battery in operation, in a cycle in which the thickness of the positive electrode active material layer represents the minimum thickness during operation that maintains a capacity of 90% or more compared to the initial capacity of the battery, after measuring the thickness of the positive electrode active material layer in a fully charged (SOC 100%) state, the ratio of the thickness of the positive electrode active material layer to the carbon weight per unit area measured above is calculated.

Table 1

Table 2

<Evaluation of specific capacity of lithium-sulfur batteries>

FIG. 1 is a performance evaluation graph of lithium-sulfur batteries according to Comparative Examples 1, 2, and 4, and FIG. 2 is a performance evaluation graph of lithium-sulfur batteries according to Examples 1 and 2 and Comparative Example 1.

Each lithium-sulfur battery manufactured in the examples and comparative examples was discharged at 0.1C (C rate) at 25°C in CC mode (constant current mode) to 1.8V, and charged at 0.1C constant current to 2.5V, and then the discharge capacity was measured. The discharge capacity is measured as the discharge capacity per unit sulfur (S) content in the positive electrode (specific capacity, mAh/g(s)).

1, it was confirmed that in a lithium-sulfur battery using a conventional porous carbon material having a high tap density and low particle shape uniformity, as the sulfur content increased, the reactivity decreased and the discharge capacity decreased (Comparative Examples 1 and 2). In addition, it was confirmed that as the density of the porous carbon material increased, the reactivity decreased and the discharge capacity decreased (Comparative Examples 1 and 4).

2, it was confirmed that a lithium-sulfur battery using a porous carbon material having a low tap density and high particle shape uniformity by shape modification has improved reactivity compared to a lithium-sulfur battery using a porous carbon material having the same sulfur content but having a high tap density and low particle shape uniformity (Example 1 and Comparative Example 1). In addition, it was confirmed that when a surface-modified porous carbon material was used, the sulfur load increased, but the reactivity was maintained (Examples 1 and 2).

<Energy Density Comparison>

The energy density was calculated by measuring the capacity of the lithium-sulfur batteries according to Examples 1 and 2 and Comparative Examples 1 to 3, multiplying the measured capacity by the voltage and dividing by the weight of each lithium-sulfur battery, and the results are shown in the following Table 3 and FIG. 3 .

3 is a graph showing relative energy densities of lithium-sulfur batteries according to Examples 1 and 2 and Comparative Examples 1 to 3 and the energy density of Comparative Example 1. FIG.

table 3

Energy density (Wh/kg) Comparative Example 1 425 Comparative Example 2 418 Comparative Example 3 425 Example 1 446 Example 2 458

In the case of Comparative Examples 1 to 3, that is, lithium-sulfur batteries conventionally using porous carbon materials having high tap density and low particle shape uniformity, it was confirmed that as the sulfur content increased, the reactivity decreased and the discharge capacity decreased, resulting in low energy density (Comparative Examples 1 and 2). It was confirmed that Comparative Example 3 had high reactivity due to low sulfur loading, but had low energy density due to a small amount of positive electrode active material.

On the other hand, it was confirmed that the lithium-sulfur battery according to Example 1 had improved reactivity and discharge capacity and the energy density increased accordingly, and Example 2 had a higher sulfur content than that of Example 1 without reducing the discharge capacity, thereby increasing the energy density.

Although the present invention has been described above with respect to a limited number of embodiments and drawings, the present invention is not limited thereto, and it will be apparent to those skilled in the art that various changes and modifications may be made thereto within the scope of the technical aspects of the present invention and the appended claims and their equivalents.

Claims (17)

1. A positive electrode, the positive electrode comprising:

A current collector; and

A positive electrode active material layer on at least one surface of the current collector,

Wherein the positive electrode active material layer comprises a sulfur-carbon composite material and a binder polymer,

Wherein the sulfur-carbon composite material comprises a porous carbon material and a sulfur-based material, and

Wherein a ratio of a thickness of the positive electrode active material layer to a weight of carbon per unit area of the positive electrode active material layer is 80 μm/mg to 130 μm/mg.

2. The positive electrode according to claim 1, wherein the positive electrode active material layer has a porosity of 80% by volume or more.

3. The positive electrode according to claim 1, wherein the amount of elemental sulfur (S) is 60 wt% or more based on the total weight of the positive electrode active material layer.

4. The positive electrode according to claim 1, wherein the porous carbon material has an angular particle shape.

5. The positive electrode according to claim 1, wherein the porous carbon material has a particle shape uniformity according to the following formula 1 of 1.3 or less:

[ formula 1]

Particle shape uniformity= [ average diameter of circumscribed circle of particle ]/[ average diameter of inscribed circle of particle ].

6. The positive electrode according to claim 1, wherein the porous carbon material is produced by grinding a raw material porous carbon material using a centrifugal mill and sieving the ground porous carbon material by a sieve having a mesh size of 50 μm to 100 μm.

7. The positive electrode according to claim 1, wherein the porous carbon material has a tap density of 0.09g/cm ³ or less.

8. The positive electrode according to claim 1, wherein the positive electrode active material layer has a porosity of 81 to 85% by volume.

9. The positive electrode according to claim 1, wherein the porous carbon material contains a secondary structure formed by aggregation of carbon nanotubes as a primary structure.

10. The positive electrode according to claim 1, wherein the porous carbon material has a tap density of 0.07g/cm ³ or less.

11. The positive electrode according to claim 1, wherein the amount of elemental sulfur (S) is 65 to 90 wt% based on the total weight of the positive electrode active material layer.

12. The positive electrode according to claim 1, wherein a sulfur (S) loading amount is 2.9mg _s/cm² or more.

13. A lithium sulfur battery, the lithium sulfur battery comprising:

The positive electrode of any one of claims 1 to 12;

A negative electrode;

a separator between the positive electrode and the negative electrode; and

An electrolyte.

14. The lithium sulfur battery of claim 13, wherein the carbon weight per unit area of the positive electrode active material layer and the thickness of the positive electrode active material layer are each measured after at least one discharge.

15. The lithium sulfur battery according to claim 13, wherein the carbon weight per unit area of the positive electrode active material layer and the thickness of the positive electrode active material layer are each measured at a state of charge (SOC) of 97% to 100%.

16. The lithium sulfur battery of claim 14, wherein a weight ratio (El/S weight ratio) of the electrolyte to the sulfur (S) in the sulfur-carbon composite is 3.5g/g or less.

17. The lithium sulfur battery of claim 13 wherein the lithium sulfur battery has an energy density of 400Wh/kg or more.