Positive electrode for lithium-sulfur battery and manufacturing method thereof

Abstract

The present invention relates to a positive electrode for a lithium-sulfur battery and a method for manufacturing the same. The positive electrode for the lithium-sulfur battery includes: a current collector; and a positive electrode active material layer formed on at least one surface of the current collector, wherein the positive electrode active material layer includes a positive electrode active material and a binder, and the positive electrode active material layer has surface properties defined by Sa (the arithmetic average surface roughness of the positive electrode) and Sz (the maximum height roughness of the positive electrode) ((i) 1 μm ≤ Sa ≤ 5 μm, (ii) 10 μm ≤ Sz ≤ 60 μm (where Sa is the average value of the distances from the middle surface of the surface uneven structure of the positive electrode to the highest point and the lowest point of each uneven portion, and Sz refers to the distance from the lowest point to the highest point of the positive electrode)).

Description

Technical Field

[0001] This application claims the priority of Korean Patent Application No. 10-2020-0080757, filed with the Korean Intellectual Property Office on July 1, 2020, and the entire disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a positive electrode for a lithium-sulfur battery and a method for manufacturing the same. Background Art

[0003] As the application range of secondary batteries has expanded not only to small portable electronic devices but also to medium and large electric vehicles (EVs), energy storage systems (ESSs), and electric ships, the demand for lithium secondary batteries with high capacity, high energy density, and long life has increased rapidly.

[0004] Among them, a lithium-sulfur secondary battery is a battery system that uses a sulfur-based material having an S-S bond (sulfur-sulfur bond) as a positive electrode active material and uses lithium metal as a negative electrode active material. Sulfur, which is the main raw material of the positive electrode active material, has the characteristics of being rich in resources, having a low atomic weight, being easy to supply and receive, being inexpensive, being able to reduce the manufacturing cost of the battery, and being non-toxic and environmentally friendly.

[0005] In particular, the theoretical discharge capacity of a lithium-sulfur battery is 1675 mAh / g-sulfur, and theoretically, a high energy storage density of 2600 Wh / kg can be achieved relative to its weight. Therefore, since the theoretical energy density of a lithium-sulfur battery is much higher than the theoretical energy density of other battery systems currently under study (Ni-MH battery: 450 Wh / kg, Li-FeS battery: 480 Wh / kg, Li-MnO2 battery: 1000 Wh / kg, Na-S battery: 800 Wh / kg, lithium-ion battery: 250 Wh / kg), the lithium-sulfur battery has attracted great attention in the medium and large secondary battery market that has been developed so far.

[0006] A lithium-sulfur battery has a positive electrode, a negative electrode, a separator, and an electrolyte as basic components, and the positive electrode corresponds to the main component of the lithium-sulfur battery because the positive electrode active material can have a great influence on the performance of the battery. A positive electrode can be prepared by first adding a binder and a solvent to the positive electrode active material to prepare a positive electrode active material slurry in a fluid form, and then coating the slurry on a current collector and drying it.

[0007] However, when manufacturing the positive electrode for a conventional lithium-sulfur battery, since a positive electrode active material slurry with fluidity (i.e., low solid concentration) is used, there is a problem that after coating by a coating device, the time for the heating and drying processes is long, resulting in a slow manufacturing speed of the positive electrode. To overcome this problem, attempts have been made to use a positive electrode active material slurry with a high solid concentration. However, when the solid concentration of the slurry is high, there are the following problems: due to the lack of fluidity, it is difficult to manufacture the electrode by the existing coating process.

[0008] In addition, as an existing technical literature (paper) related to the preparation of the positive electrode active material slurry, Science Advances, January 3, 2020: Volume 6, Issue 1, eaay2757 discloses that in the case of a method of preparing a slurry by adding water to a mixture formed by mixing a sulfur / carbon composite material and a binder in a dry state, when manufacturing the positive electrode, a network bridging or ribbon bridging is formed between the positive electrode active materials to increase the binding force between the active material particles.

[0009] The above-mentioned existing technical literature discloses that the characteristics of the binding structure between the positive electrode active material particles depend on the method of preparing the slurry, but its limitation is that it does not disclose the specific solid concentration at which the positive electrode active material can further improve the binding force, and it does not disclose at all a method of manufacturing a positive electrode by using a slurry with a high solid concentration and no fluidity.

[0010] Therefore, it is necessary to research and develop a positive electrode for a lithium-sulfur battery and a manufacturing method thereof, which can improve the process efficiency by shortening the drying process of the positive electrode for a lithium-sulfur battery and improving the manufacturing speed, while improving the binding force between the positive electrode active materials, the surface characteristics of the positive electrode, and the capacity and stability of the battery.

[0011] (Non-Patent Document 1) (Paper 1) Science Advances, January 3, 2020: Volume 6, Issue 1, eaay2757. Summary of the Invention

[0012] Technical problem

[0013] To solve the above problems, the inventors of the present invention studied the application of a pressing process using a roll press to the process of manufacturing a positive electrode using a positive electrode active material slurry with a high solid concentration and no fluidity, and thus completed the present invention.

[0014] Accordingly, an object of the present invention is to provide a method for manufacturing a positive electrode for a lithium-sulfur battery, which can reduce the cost of the drying process and increase the production speed by shortening the drying time during the manufacture of the positive electrode. In addition, another object of the present invention is to provide a positive electrode for a lithium-sulfur battery and a method for manufacturing the same, which can improve the adhesion of the positive electrode and make the surface of the positive electrode have small irregularities through bridging between positive electrode active materials with a high solid concentration, thereby improving the surface characteristics and stability of the battery.

[0015] Technical solution

[0016] According to a first aspect of the present invention, there is provided a positive electrode for a lithium-sulfur battery, the positive electrode for a lithium-sulfur battery comprising: a current collector; and a positive electrode active material layer formed on at least one surface of the current collector, wherein the positive electrode active material layer comprises a positive electrode active material and a binder, and the positive electrode active material layer has surface properties defined by S a (arithmetic mean surface roughness of the positive electrode) and S z (maximum height roughness of the positive electrode): (i) 1 μm ≤ S a ≤ 5 μm, (ii) 10 μm ≤ S z ≤ 60 μm (where S a is the average value of the distances from the middle surface of the surface irregularity structure of the positive electrode to the highest point and the lowest point of each irregularity portion, and S z refers to the distance from the lowest point to the highest point of the positive electrode).

[0017] In one embodiment of the present invention, the adhesion between the current collector and the positive electrode active material layer may be 6.5 gf / cm to 9.5 gf / cm.

[0018] In one embodiment of the present invention, the positive electrode may have a porosity of 50% to 80%.

[0019] In one embodiment of the present invention, the sulfur loading of the positive electrode may be 1 mAh / cm 2 to 10 mAh / cm 2 .

[0020] In one embodiment of the present invention, the positive electrode active material may comprise one selected from the following: elemental sulfur, sulfur compounds, sulfur-carbon composites, and combinations thereof.

[0021] According to a second aspect of the present invention, there is provided a method for manufacturing a positive electrode for a lithium-sulfur battery, the method comprising the following steps:

[0022] (1) Mixing a sulfur-carbon composite with a binder, and then adding water to prepare a positive electrode active material slurry;

[0023] (2) Place the positive electrode active material slurry on a current collector, and then cover it with a release film to prepare a structure in the order of current collector - slurry - release film;

[0024] (3) Compress the structure using a roll press;

[0025] (4) Remove the release film from the compressed structure to prepare a positive electrode; and

[0026] (5) Dry the positive electrode,

[0027] wherein in step (1), the solid concentration of the positive electrode active material slurry is 50 wt% to 70 wt%.

[0028] In one embodiment of the present invention, step (1) may be a step of mixing a sulfur - carbon composite material and a binder in a ratio of 90:10 to 98.5:1.5.

[0029] In one embodiment of the present invention, in step (3), the roll gap of the roll press may be 100 μm to 200 μm.

[0030] In one embodiment of the present invention, step (5) is a step of drying by heating, and the drying temperature may be 40°C to 90°C.

[0031] According to the third aspect of the present invention, the present invention provides a lithium - sulfur battery comprising the above - mentioned positive electrode.

[0032] Beneficial effect

[0033] The positive electrode for a lithium - sulfur battery according to the present invention has small unevenness due to its excellent surface properties, thereby being able to prevent uneven growth of the negative electrode and the formation of dendrites, and having excellent adhesion as an effect due to the bridge - type structure between positive electrode active materials.

[0034] In addition, the method for manufacturing a positive electrode for a lithium - sulfur battery according to the present invention has the following advantages: shortening the drying time of the positive electrode during the process, thereby reducing the cost of the drying process in manufacturing the positive electrode and improving the production speed of the positive electrode. Brief Description of the Drawings

[0035] Figure 1 Shows a schematic diagram of the manufacturing process of the positive electrodes of Examples 1 and 2 of the present invention.

[0036] Figure 2 Is a photograph of the positive electrode active material slurry with a high solid content prepared in Example 1 of the present invention.

[0037] Figure 3It is a graph showing the change in the drying rate measured during the production of the positive electrodes of Example 1 according to the present invention and Comparative Examples 1 and 2 with respect to the drying time.

[0038] Figure 4 It is a graph showing the arithmetic mean surface roughness (S a ) of the positive electrodes of Examples 1 to 2 according to the present invention and Comparative Examples 1 and 3 to 5.

[0039] Figure 5 It is a graph showing the maximum height roughness (S z ) of the positive electrodes of Examples 1 to 2 according to the present invention and Comparative Examples 1 and 3 to 5.

[0040] Figure 6 It is a graph showing the measurement results of the adhesion force of the positive electrodes of Example 1 according to the present invention and Comparative Examples 1 and 3.

[0041] Figure 7 It is an SEM image of the surface of the positive electrode of Example 1 according to the present invention.

[0042] Figure 8 It is a graph showing the initial discharge capacity of the lithium-sulfur batteries of Example 3 and Comparative Example 6 according to the present invention. Detailed Description

[0043] The embodiments provided according to the present invention can all be realized through the following description. It should be understood that the following description should be regarded as describing the preferred embodiments of the present invention, and the present invention is not necessarily limited thereto.

[0044] Positive electrode for lithium-sulfur battery

[0045] The present invention provides a positive electrode for a lithium-sulfur battery, which includes: a current collector; and a positive electrode active material layer formed on at least one surface of the current collector, wherein the positive electrode active material layer includes a positive electrode active material and a binder, and the positive electrode active material layer has surface properties defined by the following S a (arithmetic mean surface roughness of the positive electrode) and S z (maximum height roughness of the positive electrode):

[0046] (i) 1 μm ≤ S a ≤ 5 μm

[0047] (ii) 10 μm ≤ S z ≤ 60 μm

[0048] (wherein S a is the average of the distances from the middle surface of the surface uneven structure of the positive electrode to the highest point and the lowest point of each uneven portion, and S z refers to the distance from the lowest point to the highest point of the positive electrode).

[0049] In this specification, S of the positive electrode active material layer a (arithmetic mean surface roughness of the positive electrode) and S z (maximum height roughness of the positive electrode) can be measured based on the ISO 25178 (Geometric Product Specifications (GPS)-Surface texture: areal) standard.

[0050] According to the above standard, S a (arithmetic mean surface roughness of the positive electrode) can be defined as follows:

[0051] [Equation 1]

[0052]

[0053] (In Equation 1, S a is the arithmetic mean surface roughness of the positive electrode, A is the measurement area, and z(x, y) is the height profile.)

[0054] According to the above standard, S z (maximum height roughness of the positive electrode) can be defined as follows:

[0055] [Equation 2]

[0056] S z = |Max[z(x, y)]| + |Min[z(x, y)]|

[0057] (In Equation 2, S z is the maximum height roughness of the positive electrode, and z(x, y) is the height profile.)

[0058] S of the positive electrode active material layer a (arithmetic mean surface roughness of the positive electrode) can be 1 μm or more, 1.5 μm or more, 2 μm or more, 2.5 μm or more, 3 μm or more, 3.5 μm or more, or 4 μm or more, and can be 5 μm or less, 4.9 μm or less, 4.8 μm or less, 4.7 μm or less, 4.6 μm or less, or 4.5 μm or less. If S a is less than 1 μm, the surface area becomes too small, which may cause a problem of reduced reactivity of the positive electrode active material. On the other hand, if S a exceeds 5 μm, unevenness of the positive electrode is significantly formed, which may cause uneven growth of the negative electrode and may cause stability-related problems such as internal short circuit and ignition of the battery due to the formation of dendrites.

[0059] S of the positive electrode active material layer z(The maximum height roughness of the positive electrode) can be 10 μm or more, 15 μm or more, 20 μm or more, 25 μm or more, 30 μm or more, 35 μm or more, 40 μm or more, 45 μm or more, or 50 μm or more, and can be 60 μm or less, 59 μm or less, 58 μm or less, 57 μm or less, 56 μm or less, or 55 μm or less. If S z is less than 10 μm, the surface area becomes too small, which may cause problems such as a decrease in the reactivity of the positive electrode active material. On the other hand, if S z exceeds 60 μm, unevenness of the positive electrode is significantly formed, which may cause uneven growth of the negative electrode and may cause stability-related problems such as internal short circuit and fire of the battery due to the formation of dendrites.

[0060] The adhesion force between the current collector and the positive electrode active material layer can be 6.5 gf / cm or more, 7 gf / cm or more, or 7.5 gf / cm or more, and can be 9.5 gf / cm or less, 9 gf / cm or less, 8.5 gf / cm, or 8 gf / cm or less. Referring to Figure 7 the SEM image, if the above range is satisfied, the binder does not completely dissolve between the positive electrode active material slurries to form a structure in the form of a bridge between the particles of the positive electrode active material, thereby improving the binding force between the positive electrode active materials and having excellent positive electrode adhesion.

[0061] The porosity of the positive electrode can be 50% or more, 55% or more, or 60% or more, and can be 80% or less, 75% or less, or 70% or less. If the porosity of the positive electrode is less than 50%, the mass transfer resistance increases when applied to the battery, which may reduce the capacity of the electrode. If the porosity of the positive electrode exceeds 80%, there may be a problem of excessive reduction in the energy density per unit volume of the battery.

[0062] The positive electrode for a lithium-sulfur battery includes: a current collector; and a positive electrode active material layer formed on at least one surface of the current collector.

[0063] The current collector supports the positive electrode active material and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, as the positive electrode current collector, copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon; copper or stainless steel whose surface is treated with carbon, nickel, silver, etc.; aluminum-cadmium alloy, etc. can be used.

[0064] The current collector can enhance the binding force with the positive electrode active material by having fine unevenness on its surface, and can be formed in various forms such as a film, sheet, foil, sieve, net, porous body, foam, or non-woven fabric.

[0065] The positive electrode active material layer includes a positive electrode active material and a binder.

[0066] The positive electrode active material may include one selected from the following: elemental sulfur, sulfur compounds, sulfur-carbon composites, and combinations thereof, and preferably includes a sulfur-carbon composite.

[0067] When sulfur is contained in the positive electrode active material, since sulfur itself is not conductive, it can be used in combination with a conductive material such as a carbon material. Therefore, sulfur can be included in the form of a sulfur-carbon composite.

[0068] The sulfur loading amount in the positive electrode can be 1 mAh / cm 2 to 10 mAh / cm 2 , preferably 2 mAh / cm 2 to 8 mAh / cm 2 , more preferably 3 mAh / cm 2 to 6 mAh / cm 2 .

[0069] The carbon contained in the sulfur-carbon composite is a porous carbon material, which provides a framework capable of uniformly and stably fixing sulfur and compensates for the low conductivity of sulfur to enable smooth progress of the electrochemical reaction.

[0070] The porous carbon material can generally be manufactured by carbonizing various carbon material precursors. The porous carbon material may contain non-uniform pores therein, the average diameter of the pores being in the range of 1 nm to 200 nm, and the porosity may be in the range of 10% to 90% of the total volume of the porous carbon material. If the average diameter of the pores is less than the above range, the pore size is only at the molecular level and it is impossible to impregnate with sulfur. On the contrary, if the average diameter of the pores exceeds the above range, the mechanical strength of the porous carbon material is weakened, which is not preferred for application to the manufacturing process of the electrode.

[0071] The shape of the porous carbon material is in the form of a sphere, rod, needle, plate, tube, or block, and can be used without limitation as long as it is generally used in lithium-sulfur batteries.

[0072] The porous carbon material may have a porous structure or a high specific surface area, and may be any of the porous carbon materials conventionally used in the art. For example, the porous carbon material may be, but is not limited to, at least one selected from the following: graphite; graphene; carbon blacks such as Degussa black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); and graphites such as natural graphite, artificial graphite, expanded graphite, and activated carbon.

[0073] The present invention has no particular limitation on the preparation method of the sulfur-carbon composite material, and a method commonly used in the art can be adopted.

[0074] In addition to the positive electrode active material, the positive electrode may further include at least one additive selected from the following: transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, and alloys of these elements and sulfur.

[0075] The transition metal elements may include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, Hg, etc., and the Group IIIA elements may include Al, Ga, In, Tl, etc., and the Group IVA elements may include Ge, Sn, Pb, etc.

[0076] The binder holds the positive electrode active material in the current collector and organically connects between the positive electrode active materials to further improve the binding force between them, and as the binder, any binder known in the art can be used.

[0077] For example, the binder may include one selected from the following: polyvinylidene fluoride, polytetrafluoroethylene, polyvinylpyrrolidone, styrene-butadiene rubber, nitrile rubber, styrene-isoprene rubber, carboxymethyl cellulose, and combinations thereof.

[0078] In addition, lithium carboxymethyl cellulose (Li-CMC) in which lithium ions replace hydrogen ions in carboxymethyl cellulose may be included as the binder. Carboxymethyl cellulose (CMC) has a risk of explosion during battery operation due to the generation of hydrogen gas by the hydrogen ions contained in the carboxyl group. In the case of sodium carboxymethyl cellulose (Na-CMC) in which sodium ions (Na + ) replace hydrogen ions (H + ), the life characteristics may deteriorate due to the sodium ions, while when using Li-CMC as the binder, relatively excellent stability and improved life characteristics can be exhibited.

[0079] The positive electrode active material layer may further include a conductive material.

[0080] The conductive material is a material that is used as a path for electrons to move from the current collector to the positive electrode active material by electrically connecting the electrolyte and the positive electrode active material. The conductive material can be used without limitation as long as it has conductivity.

[0081] For example, as the conductive material, the following substances can be used alone or in combination: graphite such as natural graphite or artificial graphite; carbon black such as Super-P, Degussa black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon derivatives such as carbon nanotubes and fullerenes; conductive fibers such as carbon fibers and metal fibers; fluorocarbons; metal powders such as powders of aluminum and nickel; or conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0082] The positive electrode active material paste for forming the positive electrode active material layer may further contain a solvent.

[0083] As the solvent, a solvent capable of uniformly dispersing the positive electrode active material, the conductive material, and the binder is used. Such a solvent is an aqueous solvent, and most preferably water. In this case, the water may be distilled water or deionized water. However, it is not necessarily limited thereto, and if necessary, a lower alcohol that can be easily mixed with water can be used. Examples of the lower alcohol include methanol, ethanol, propanol, isopropanol, and butanol. Preferably, they can be used in combination with water.

[0084] Method for manufacturing a positive electrode for a lithium-sulfur battery

[0085] The method for manufacturing a positive electrode for a lithium-sulfur battery according to the present invention includes the following steps:

[0086] (1) Mixing a sulfur-carbon composite material with a binder, and then adding water to prepare a positive electrode active material paste;

[0087] (2) Placing the positive electrode active material paste on a current collector, and then covering it with a release film to prepare a structure in the order of current collector - paste - release film;

[0088] (3) Compressing the structure using a roll press;

[0089] (4) Removing the release film from the compressed structure to prepare a positive electrode; and

[0090] (5) Drying the positive electrode,

[0091] wherein in step (1), the solid concentration of the positive electrode active material paste is 50 to 70% by weight.

[0092] The method for manufacturing a positive electrode for a lithium-sulfur battery includes step (1): mixing a sulfur-carbon composite material with a binder, and then adding water to prepare a positive electrode active material slurry. Step (1) may be a step of mixing the sulfur-carbon composite material and the binder in a ratio of 90:10 to 98.5:1.5, preferably 93:7 to 98:2, more preferably 96:4 to 97.5:2.5. If the ratio is less than the above range, the positive electrode active material slurry will not detach from the release film, which may cause problems in manufacturing the positive electrode. On the other hand, if the ratio exceeds the above range, there may be a problem that the positive electrode active material layer detaches from the current collector as the binder content decreases.

[0093] In the step of preparing the positive electrode active material slurry in step (1), a step of adding a conductive material to the mixture of the sulfur-carbon composite material and the binder may also be included. Specifically, the positive electrode active material slurry can be prepared by mixing the sulfur-carbon composite material, the binder, and the conductive material and adding water.

[0094] A conductive material is a material that serves as a path for electrons to move from the current collector to the positive electrode active material by electrically connecting the electrolyte and the positive electrode active material. Any conductive material can be used without limitation as long as it has conductivity.

[0095] For example, as the conductive material, the following substances can be used alone or in combination: graphite such as natural graphite or artificial graphite; carbon black such as Super-P, Degussa black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon derivatives such as carbon nanotubes and fullerenes; conductive fibers such as carbon fibers and metal fibers; fluorocarbons; metal powders such as powders of aluminum and nickel; or conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0096] In this specification, the solid concentration is defined as the mass fraction of solid materials other than the solvent in the slurry mixture.

[0097] The solid concentration of the positive electrode active material slurry prepared by mixing the sulfur-carbon composite material and the binder and then adding water can be 50 wt% or more, 52 wt% or more, 54 wt% or more, 56 wt% or more, or 58 wt% or more, and can be 70 wt% or less, 68 wt% or less, 66 wt% or less, 64 wt% or less, or 62 wt% or less. If the solid concentration is less than 50 wt%, the drying time will be prolonged, and thus the production speed of the positive electrode will be reduced. On the other hand, if the solid concentration exceeds 70 wt%, the adhesion of the positive electrode active material slurry will be reduced, making it difficult to manufacture the positive electrode.

[0098] The method for manufacturing a positive electrode for a lithium-sulfur battery includes step (2): placing the positive electrode active material slurry on a current collector, and then covering it with a release film to prepare a structure in the order of current collector - slurry - release film.

[0099] The release film is used to prevent the positive electrode active material slurry from adhering to the roller during the roll pressing process for compression and not detaching, and it can be a release film made of polyethylene terephthalate (PET), polypropylene (PP), high-density polyethylene (HDPE), or low-density polyethylene (LDPE). Preferably, it can be a release film formed by coating an organosilicon or fluorine-based hydrophobic material on a polymer material, but it is not particularly limited thereto.

[0100] The method for manufacturing a positive electrode for a lithium-sulfur battery includes step (3): compressing the structure using a roll press.

[0101] By passing the structure having the order of current collector - slurry - release film through a roll press, the positive electrode active material slurry with a high solid concentration is compressed by the rolls to form a positive electrode with a constant thickness. The roll press can be composed of two parallel rolls, and the load of the manufactured positive electrode can be controlled by adjusting the gap between the rolls, i.e., the roll gap.

[0102] In the compression step using a roll press, the roll gap can be 100 μm or more, 115 μm or more, 130 μm or more, or 145 μm or more, and can be 200 μm or less, 185 μm or less, 170 μm or less, or 155 μm or less. If the roll gap is less than 100 μm, there may be a problem that a positive electrode with a uniform thickness cannot be formed because the positive electrode active material slurry contains positive electrode active materials with particle sizes larger than the roll gap. On the other hand, if the roll gap exceeds 200 μm, the phenomenon of cracking and detachment on the surface of the positive electrode may be aggravated because the positive electrode has too high a load.

[0103] The method for manufacturing a positive electrode for a lithium-sulfur battery includes step (4): removing the release film from the compressed structure to prepare a positive electrode.

[0104] Through the compression process using a roll press, the positive electrode active material slurry is spread to a uniform thickness to form a positive electrode. Therefore, it is necessary to remove the release film before the final drying process. When drying without removing the release film, the heating and drying time increase, which may increase the cost of the drying process and reduce the production speed of the positive electrode manufacturing process.

[0105] The method for manufacturing a positive electrode for a lithium-sulfur battery includes step (5): drying the positive electrode.

[0106] Step (5) is a step of drying by heating the moisture contained in the positive electrode active material slurry, preferably carried out in an oven. By drying, the moisture contained in the positive electrode active material slurry is removed, and finally the positive electrode can be manufactured.

[0107] The drying temperature in step (5) can be 40 °C or higher, 45 °C or higher, or 50 °C or higher, and can be 90 °C or lower, 85 °C or lower, 80 °C or lower. If the drying temperature is lower than 40 °C, since the drying time of the positive electrode is prolonged, it is difficult to achieve the effect of reducing the cost of the drying process and increasing the production speed of the positive electrode as the drying time is shortened. On the other hand, if the drying temperature exceeds 90 °C, there may be a problem that sulfur in the positive electrode active material sublimes, thereby reducing the load of the positive electrode.

[0108] Lithium-sulfur battery

[0109] The lithium-sulfur battery according to the present invention may include: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte.

[0110] The positive electrode is as described above in this specification.

[0111] The negative electrode may include: a negative electrode current collector; and a negative electrode active material layer coated on one or both surfaces of the negative electrode current collector. Alternatively, the negative electrode may be a lithium metal plate.

[0112] The negative electrode current collector is used to support the negative electrode active material layer and is not particularly limited as long as it has high conductivity and does not cause any chemical changes in the battery. The negative electrode current collector may be selected from: copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, their alloys, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver. The alloy may be an aluminum-cadmium alloy. In addition, sintered carbon, a non-conductive polymer surface-treated with a conductive material, or a conductive polymer may be used. Generally, a copper foil is used as the negative electrode current collector.

[0113] In addition, regarding its shape, various forms can be used, such as a film, sheet, foil, net, porous body, foam, or non-woven fabric with / without fine concavities and convexities formed on its surface.

[0114] In addition to the negative electrode active material, the negative electrode active material layer may further include a conductive material, an adhesive, etc. At this time, the conductive material and the adhesive are as described above.

[0115] The negative electrode active material may include: a material capable of reversibly inserting or extracting lithium ions (Li + ) ; a material capable of reacting with lithium ions to reversibly form a lithium-containing compound; lithium metal; or a lithium alloy.

[0116] A material capable of reversibly inserting or extracting lithium ions (Li+ ) The material can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. Materials that can react with lithium ions (Li + ) to reversibly form a lithium-containing compound can be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy can be, for example, an alloy of lithium (Li) and a metal selected from the following: sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0117] Preferably, the negative electrode active material can be lithium metal. Specifically, it can have the form of a lithium metal thin film or lithium metal powder.

[0118] There is no particular limitation on the method for forming the negative electrode active material, and methods for forming layers or films commonly used in the art can be used. For example, methods such as compression, coating, and deposition can be used. In addition, the negative electrode of the present invention also includes the case where a lithium metal thin film is formed on a metal plate by initial charging after assembling the battery under the condition that there is no lithium thin film in the current collector.

[0119] There is no particular limitation on the electrolyte as long as it is a non-aqueous solvent that serves as a medium through which ions participating in the electrochemical reaction of the battery can move. For example, the solvent can be a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, or an aprotic solvent. Examples of carbonate solvents can specifically include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC), etc. Examples of ester solvents can specifically include methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethyl ethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, methylvalerolactone, or caprolactone, etc. Examples of ether solvents can specifically include diethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, or polyethylene glycol dimethyl ether, etc. Examples of ketone solvents can specifically include cyclohexanone, etc. Examples of alcohol solvents can specifically include ethanol or isopropanol, etc. Examples of aprotic solvents can specifically include: nitriles such as acetonitrile; amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane (DOL); or sulfolane. The non-aqueous organic solvent can be used alone or in combination of one or more. When used in combination of one or more, the mixing ratio can be appropriately adjusted according to the desired performance of the battery. Specifically, a mixture with a volume ratio of 1:1 of 1,3-dioxolane and dimethoxyethane may be preferred.

[0120] According to the manufacturing process and required performance of the final product, the electrolyte can be injected at an appropriate stage during the manufacturing process of the lithium-sulfur battery. That is, it can be injected before assembling the lithium-sulfur battery or at the final stage of assembly.

[0121] A conventional separator can be inserted between the positive electrode and the negative electrode. The separator is a physical separator having the function of physically separating the electrodes and can be used without particular limitation as long as it is used as a conventional separator. In particular, a separator with low resistance to ion movement in the electrolyte and excellent impregnation ability for the electrolyte is preferred.

[0122] In addition, the separator can enable the transmission of lithium ions between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. The separator can be made of a porous, non-conductive or insulating material. The separator can be used without any particular limitation as long as it is used as a separator in the lithium-sulfur battery. The separator can be an independent component, such as a film or coating added to the positive electrode and / or the negative electrode.

[0123] The separator can be made of a porous substrate. Any porous substrate can be used as long as it is a porous substrate commonly used in lithium-sulfur batteries. The porous polymer film can be used alone or in the form of a laminate. For example, a non-woven fabric or a polyolefin porous film made of glass fibers with a high melting point or polyethylene terephthalate fibers, etc. can be used, but it is not limited thereto.

[0124] In the present invention, there is no particular limitation on the material of the porous substrate, and any material can be used as long as it is a porous substrate commonly used in electrochemical devices. For example, the porous substrate can include one or more materials selected from the following: polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyether ether ketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzobis oxazole) and polyarylate.

[0125] There is no particular limitation on the thickness of the porous substrate, but it can be 1 μm to 100 μm, preferably 5 μm to 50 μm. Although the thickness range of the porous substrate is not particularly limited to the above range, if the thickness is much thinner than the above lower limit, the mechanical properties deteriorate and thus the separator may be easily damaged during battery use.

[0126] There are also no particular limitations on the average diameter and porosity of the pores present in the porous substrate, but they can be 0.1 μm to 50 μm and 10% to 95% respectively.

[0127] In the case of the lithium-sulfur battery according to the present invention, in addition to the winding process as a general process, a lamination or stacking and folding process of the separator and the electrode can also be performed.

[0128] There is no particular limitation on the shape of the lithium-sulfur battery, and it can have various shapes such as cylindrical, laminated, and coin-shaped.

[0129] Examples

[0130] Hereinafter, preferred examples are given to help understand the present invention, but the following examples are provided only to make the present invention easier to understand, and the present invention is not limited thereto.

[0131] Manufacture of positive electrode for lithium-sulfur battery: Examples 1 to 2 and Comparative Examples 1 to 5

[0132] Example 1

[0133] 97 wt% of a sulfur-carbon composite material (S:C = 70:30 (weight ratio)) and 3 wt% of Li-CMC (lithium-carboxymethyl cellulose) as a binder in solid powder form were mixed, and then water was added thereto to prepare a positive electrode active material slurry having a solid concentration of 60 wt%.

[0134] The prepared positive electrode active material slurry was placed on one side of an aluminum current collector, and then a PET release film (Mitsubishi, MRF-125, thickness 125 μm) was covered on the slurry to prepare a structure having the order of current collector-slurry-release film. Then, the structure was put into a roll press, and compression treatment was performed while adjusting the roll gap to 120 μm.

[0135] After compression, the release film was removed from the structure, and the structure was put into an oven and heated and dried at a temperature of 50 °C to manufacture a positive electrode having a porosity of 63% and a sulfur loading of 3.65 mAh / cm 2 of.

[0136] Example 2

[0137] A sulfur-carbon composite material (S:C = 75:25), carbon fiber as a conductive material, and Li-CMC (lithium-carboxymethyl cellulose) as a binder in solid powder form were mixed at a weight ratio of 92:5:3, and water was added thereto to prepare a positive electrode active material slurry having a solid concentration of 55 wt%.

[0138] Except for the method of preparing the positive electrode active material slurry, the same method as in Example 1 was performed to manufacture a positive electrode having a porosity of 63% and a sulfur loading of 3.4 mAh / cm 2 of.

[0139] Comparative Example 1

[0140] A sulfur-carbon composite material (S:C = 70:30 by weight) and an aqueous solution with a concentration of 2 wt% of Li-CMC (lithium carboxymethyl cellulose) as a binder were mixed so that the weight ratio of the solid content of the sulfur-carbon composite material to Li-CMC was 97:3, and then water was added thereto to prepare a positive electrode active material slurry with a solid content of 30 wt%.

[0141] Thereafter, except for coating the positive electrode active material slurry on one side of an aluminum current collector using a doctor blade coater (Mathis Switzerland, SV-M) and drying it in an oven at 50 °C, the same method as in Example 1 was carried out to manufacture a positive electrode for a lithium-sulfur battery. At this time, the porosity of the manufactured positive electrode was 78%.

[0142] Comparative Example 2

[0143] A positive electrode for a lithium-sulfur battery was manufactured in the same manner as in Example 1, except that drying was carried out by heating without removing the release film.

[0144] Comparative Example 3

[0145] A positive electrode for a lithium-sulfur battery was manufactured by putting the positive electrode manufactured in Comparative Example 1 into a roll press and performing an additional compression process while adjusting the roll gap to 50 μm. At this time, the porosity of the manufactured positive electrode was 63%.

[0146] Comparative Example 4

[0147] A sulfur-carbon composite material (S:C = 75:25 by weight), carbon fiber as a conductive material, and an aqueous solution with a concentration of 2 wt% of Li-CMC (lithium carboxymethyl cellulose) as a binder were mixed so that the weight ratio of the solid content of the sulfur-carbon composite material, carbon fiber, and Li-CMC was 92:5:3, and then water was added thereto to prepare a positive electrode active material slurry with a solid concentration of 30 wt%.

[0148] Except for the method of preparing the positive electrode active material slurry, the same method as in Comparative Example 1 was carried out to manufacture a positive electrode with a porosity of 69% and a sulfur loading of 3.4 mAh / cm 2 2.

[0149] Comparative Example 5

[0150] A positive electrode for a lithium-sulfur battery was manufactured by putting the positive electrode manufactured in Comparative Example 4 into a roll press and performing an additional compression process while adjusting the roll gap to 50 μm. At this time, the porosity of the manufactured positive electrode was 63%.

[0151] Manufacture of lithium-sulfur battery: Example 3 and Comparative Example 6

[0152] Example 3

[0153] The positive electrode prepared in Example 2 and a lithium metal negative electrode with a thickness of 45 μm were placed opposite to each other, and then a polyethylene separator with a thickness of 20 μm and a porosity of 46% was inserted between the positive electrode and the negative electrode.

[0154] In addition, as the electrolyte, while using 1,3-dioxolane (DOL) / dimethoxyethane (DME) (1:1, volume / volume) as the solvent, 1 M of LiTFSI and 3 wt% of lithium nitrate (LiNO3) were injected to fabricate a coin cell type lithium-sulfur battery.

[0155] Comparative Example 6

[0156] A coin cell type lithium-sulfur battery was prepared in the same manner as in Example 3, except that the positive electrode fabricated in Comparative Example 5 was used.

[0157] Experimental Example 1: Measurement of drying time when manufacturing a positive electrode for a lithium-sulfur battery

[0158] When fabricating the positive electrodes of Example 1 and Comparative Examples 1 and 2, the drying time (minutes) and drying rate (%) required to remove the solvent from the positive electrode active material slurry were measured, and the results are shown in Figure 3 .

[0159] Specifically, in the case of the drying rate, the following formula was used to measure and calculate the change in the mass of the residual moisture over time:

[0160] Drying rate (%) = (1 - ((mass of residual moisture) / (mass of initial moisture))) × 100

[0161] As Figure 3 shown, for Example 1, more than 95% of the solvent contained in the slurry was removed within 35 minutes, while it took 60 minutes for Comparative Example 1 to achieve the same drying rate. In addition, it was found that Comparative Example 2, where drying was carried out with the release film covered, had a relatively slower drying rate compared to Example 1 and Comparative Example 1.

[0162] From the above results, it was confirmed that in the case of Example 1, where the positive electrode was fabricated by using a slurry with a solid concentration of 50 wt% to 70 wt% and applying a roll pressing process, the drying time was significantly shortened compared to Comparative Examples 1 and 2.

[0163] Experimental Example 2: Measurement of surface characteristics of a positive electrode for a lithium-sulfur battery

[0164] For the positive electrodes prepared in Examples 1 to 2 and Comparative Examples 1 and 3 to 5, a measuring instrument (KEYENCE, VK-X150) was used to measure the surface roughness. Thus, the S a(Arithmetic mean surface roughness (arithmetic mean height) of the positive electrode: the average value of the distances from the middle surface of the uneven structure on the positive electrode surface to the highest and lowest points of each uneven part) and S z (Maximum height roughness (maximum height) of the positive electrode: the distance from the lowest point to the highest point of the positive electrode), and the results are shown in Table 1 below and Figure 4 and 5 in.

[0165] Table 1:

[0166] <![CDATA[S a (μm)]]> <![CDATA[S z (μm)]]> Example 1 4.03 54.8 Example 2 2.463 45.05 Comparative Example 1 10.13 94.8 Comparative Example 3 6.34 85.9 Comparative Example 4 10.42 100.2 Comparative Example 5 5.39 61.53

[0167] As shown in Table 1, it can be confirmed that in Examples 1 and 2 where the positive electrode is manufactured by using a slurry with a high solid concentration of 50 wt% to 70 wt% and applying a roll pressing process, compared with Comparative Example 1 or Comparative Examples 3 to 5 where the positive electrode is prepared using a slurry with a low solid concentration, the surface roughness is smaller, the unevenness is smaller, and the surface is flatter.

[0168] From this, it is found that in the case of the positive electrode manufactured by Examples 1 and 2, the surface unevenness is relatively small, and the uneven growth of the negative electrode or the formation of dendrites can be prevented, thereby improving the stability of the battery.

[0169] Experimental Example 3: Measurement of adhesion of a positive electrode for a lithium-sulfur battery

[0170] The positive electrodes prepared in Example 1 and Comparative Examples 1 and 3 were cut into a size of 100 mm × 20 mm, and then adhered to a glass slide using a double-sided tape (3M 9070) with an electrode surface, and a laminator (GMP, PHOTONEX-SYNC325) was used to prepare samples for the peel test.

[0171] A 90° peel test was performed using a universal testing machine (AMETEK, LS1) capable of measuring the adhesion force on the peel test sample, so as to measure the applied peel resistance (gf / cm), and then the adhesion force of each electrode was calculated, and the results are shown in Table 2 below and Figure 6 in.

[0172] Table 2:

[0173] Example 1 Comparative Example 1 Comparative Example 3 Adhesion (gf / cm) 7.83 1.27 5.98

[0174] In addition, an image of the positive electrode active material of Example 1 taken using a scanning electron microscope (SEM) (JEOL, JSM-7200F) is shown in Figure 7 in.

[0175] It was confirmed from the above results that when the positive electrode active material slurry had a high solid concentration of 50 wt% to 70 wt% as in Example 1, the slurry was not completely dissolved in the binder, so it had a bridge structure between the active materials, thereby improving the adhesion of the positive electrode.

[0176] Experimental Example 4: Measurement of initial discharge capacity of a lithium-sulfur battery

[0177] For the lithium-sulfur batteries fabricated in Example 3 and Comparative Example 6, the initial discharge capacity from 2.5 V to 1.8 V was measured using a charge / discharge measurement instrument (PESC 05-0.01, PNE Solution, Korea), and the results are shown in Figure 8 the following.

[0178] From Figure 8 the results, it was confirmed that, compared with the lithium-sulfur battery using a slurry with a low solid content to fabricate the positive electrode as described in Comparative Example 6, in the case of the lithium-sulfur battery including a positive electrode fabricated by using a slurry with a high solid concentration of 50 wt% to 70 wt% and applying a roll pressing process as described in Example 3, although the cost of the drying process was reduced and the production speed of the positive electrode was increased due to a significantly shortened drying time during the positive electrode fabrication process, it showed the same level of initial discharge capacity.

[0179] Simple modifications or variations of the present invention are within the scope of the present invention, and the specific protection scope of the present invention is subject to the appended claims.

Claims

1. A positive electrode for a lithium-sulfur battery, the positive electrode for the lithium-sulfur battery comprising: A current collector; and A positive electrode active material layer formed on at least one surface of the current collector, wherein the positive electrode active material layer comprises a positive electrode active material and a binder, and The positive electrode active material layer has surface properties defined by the arithmetic mean surface roughness S of the positive electrode a and the maximum height roughness S of the positive electrode z where S a is the average of the distances from the midplane of the surface uneven structure of the positive electrode to the highest and lowest points of each uneven portion, and S z refers to the distance from the lowest point to the highest point of the positive electrode: (i) 1μm ≤ S a ≤ 2.463μm (ii) 10 μm ≤ S z ≤ 45.05 μm.

2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the adhesion force between the current collector and the positive electrode active material layer is 6.5 gf / cm to 9.5 gf / cm.

3. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the porosity of the positive electrode is 50% to 80%.

4. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the sulfur loading of the positive electrode is 1 mAh / cm 2 to 10 mAh / cm 2 .

5. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the positive electrode active material includes at least one selected from the following: elemental sulfur, sulfur compounds, sulfur-carbon composites, and combinations thereof.

6. A method for manufacturing the positive electrode for a lithium-sulfur battery according to any one of claims 1 to 5, the method comprising the following steps: (1) Mixing a sulfur-carbon composite with a binder, and then adding water to prepare a positive electrode active material slurry; (2) Placing the positive electrode active material slurry on a current collector, and then covering it with a release film to prepare a structure in the order of current collector - slurry - release film; (3) Compressing the structure using a roll press; (4) Removing the release film from the compressed structure to prepare a positive electrode; and (5) Drying the positive electrode, wherein in step (1), the solid concentration of the positive electrode active material slurry is 50% by weight to 70% by weight.

7. The method for manufacturing a positive electrode for a lithium-sulfur battery according to claim 6, wherein step (1) is a step of mixing the sulfur-carbon composite and the binder in a ratio of 90:10 to 98.5:1.

5.

8. The method for manufacturing a positive electrode for a lithium-sulfur battery according to claim 6, wherein in step (3), the roll gap of the roll press is 100 μm to 200 μm.

9. The method for manufacturing a positive electrode for a lithium-sulfur battery according to claim 6, wherein step (5) is a step of drying by heating, and the drying temperature is 40°C to 90°C.

10. A lithium-sulfur battery, the lithium-sulfur battery comprising the positive electrode according to any one of claims 1 to 5.

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