METHOD FOR PRODUCING A CATHOD OF A LITHIUM-SULFUR BATTERY

Abstract

Method for manufacturing a cathode of a lithium-sulfur secondary battery, comprising: (1) Production of a primary sludge by mixing sulfur, a conductive material, a first solvent and a non-aqueous planar contact binder, (2) Production of a primary composite material by drying the primary sludge and pulverizing the primary sludge, (3) Producing a secondary slurry by mixing the primary composite, the conductive material and a second solvent with an aqueous point contact binder, and (4) Applying the secondary sludge to a cathode plate, wherein the first solvent is one or more selected from the group consisting of N-methylpyrrolidone, acetonitrile, isopropyl ether, benzene, chloroform, n-hexane, methanol, acetone, and toluene, and the non-aqueous planar contact binder is one or more from the group consisting of polyvinyl acetate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polyvinylidene fluoride, polyhexafluoropropylene-polyvinylidene fluoride copolymer, polyethyl acrylate, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, carboxymethylcellulose (CMC), and combinations thereof, wherein the second solvent of step (3) is water, and the aqueous point contact binder is one or more selected from the group consisting of polyvinylpyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), and combinations thereof, and wherein the secondary sludge is produced by dispersing the primary composite material using ultrasonic waves and mixing it with the conductive material, the second solvent and the aqueous point contact binder.

Description

TECHNICAL AREA

[0001] The present invention relates to a method for producing a cathode for a lithium-sulfur battery with improved lifetime characteristics and increased battery capacity. In particular, a sulfur cathode for the lithium-sulfur battery can have two types of binders that differ with respect to solvents and adhesion types. BACKGROUND

[0002] Typical lithium-sulfur batteries have a theoretical energy density of 2600 Wh / kg, which is greater than that of conventional lithium-ion batteries with a theoretical energy density of about 570 Wh / kg and a common value of 120 Wh / kg. However, when the lithium-sulfur battery is discharged, the sulfur in the cathode can melt and evaporate into the electrolyte in the form of a polysulfide (Li₂S₂). xLeakage can occur, potentially destroying the cathode structure and thus reducing battery lifespan. Therefore, the function of a binding agent to maintain a conductive structure can be crucial for the development of lithium-sulfur batteries with these properties, impacting both battery capacity and lifespan.

[0003] In the relevant fields, a binder composition for an electrode has been described, comprising at least one tetracarboxylic acid ester compound, at least one diamine compound, and an organic solvent. Such a composition can exhibit high bonding strength and does not inhibit the formation of a stable interface (SEI) on the surface of an active material.

[0004] Alternatively, a binder composition used for the manufacture of an electrode for a lithium-ion secondary battery was developed, and the composition comprises polymer particles dispersed in an organic medium with a boiling point of 80–350°C at normal pressure. The polymer particles comprise at least one type of structural unit selected from (a) structural units derived from a monoethylene unsaturated carboxylic acid ester monomer, (b) structural units derived from a monoethylene unsaturated carboxylic acid monomer, and (c) structural units derived from a conjugated diene monomer; have a ratio (a) / [(b) + (c)] of 99 / 1–60 / 40 by weight; have a total content of (a), (b) plus (c) of at least 80 wt% based on the total structural units; and are substantially free of structural units of a monoethylene aromatic hydrocarbon monomer.

[0005] Furthermore, an organic binder was also developed, and this organic binder can consist of a polymer with a double bond (i.e., polyolefin rubber with a double bond) and can be cross-linked by vulcanization. The rubber includes, for example, natural rubber and synthetic rubber, and the synthetic rubber includes, for example, styrene-butadiene copolymer, isobutylene-isoprene copolymer (such as butyl rubber), acrylonitrile butadiene rubber (NBR), ethylene propylene diethylene polymer (EPDM), and the like.

[0006] Meanwhile, other examples have provided a cathode composition for a lithium-sulfur secondary battery comprising a vinylidene fluoride-based polymer as the cathode binder. Specifically, it is taught that the vinylidene fluoride-based polymer can be polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, or a copolymer of vinylidene fluoride and tetrafluoroethylene. The composition also includes an organic material incorporating the sulfur and a conductive polymer mixture.

[0007] However, the techniques described above may not be sufficient to provide a desired level of adhesion strength, charging and discharging efficiency, stability and consistency in a production process, so that the physical properties of a battery requiring high efficiency and stability, such as a car battery, are met.

[0008] Therefore, the present invention was made to provide a continuous production process for a cathode including a binder, which constitutes a cathode of a lithium-sulfur battery and which can be characterized by stable discharge electricality in a high-capacity lithium-sulfur battery. The described binder can also provide high adhesion strength with a small quantity, thereby increasing the energy density of the battery.

[0009] The foregoing information disclosed in this background section is intended only to enhance the understanding of the background of the invention and may therefore contain information that does not constitute any prior art already known to a person skilled in the art in this country.

[0010] US 2003 / 0073000 A1 discloses a positive active material comprising a sulfur compound, a conductivity adhering to the sulfur compound, and a binder containing at least one polymer to bind the conductivity to the sulfur compound.

[0011] WO 2011 / 148357 A1 relates to composite materials, their manufacture and use in electrical cells, wherein the composite material comprises a reaction product of at least one organic polymer, sulfur and carbon in a polymorphic form. SUMMARY OF THE INVENTION

[0012] The present invention can provide a technical solution to the problems described above in the relevant field.

[0013] In a described but not claimed aspect, a cathode composition of a lithium-sulfur secondary battery is provided, comprising: sulfur, a conductive material, a non-aqueous planar contact binder and an aqueous spot contact binder.

[0014] In certain described but unclaimed embodiments, the sulfur may be a sulfur particle, and the conductive material may be a conductive particle. In particular, the planar contact with the sulfur particles or the conductive material particles may occur in a planar phase, and the point contact includes or may include the sulfur particles or the conductive material particles in a point phase.

[0015] In a described but unclaimed exemplary embodiment, the conductive material can be the cathode composition, but is not limited to one or more selected from the group consisting of graphite, Super C, vapor-grown carbon fibers, Ketjen black, Denka black, acetylene black, carbon black, carbon nanotubes, multi-walled carbon nanotubes, ordered mesoporous carbon and combinations thereof.

[0016] In a described but unclaimed exemplary embodiment, the non-aqueous planar contact binder can be the cathode composition, but is not limited to one or more selected from the group consisting of polyvinyl acetate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polyvinylidene fluoride, polyhexafluoropropylene-polyvinylidene fluoride copolymer, polyethyl acrylate, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, carboxymethylcellulose (CMC) and combinations thereof.

[0017] In a described but unclaimed exemplary embodiment, the aqueous point contact binder can be of the cathode composition, but is not limited to one or more selected from the group consisting of polyvinylpyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR), carboxymethylcellulose and combinations thereof.

[0018] In a described but unclaimed exemplary embodiment, the non-aqueous planar contact binder of the cathode composition can be closer to the sulfur particles than the aqueous point contact binder.

[0019] In a described but unclaimed exemplary embodiment, the cathode composition may comprise: the sulfur in an amount of about 40 to 85 wt.%, the conductive material in an amount of about 10 to 50 wt.%, the non-aqueous planar contact binder in an amount of about 2 to 25 wt.%, and the aqueous point contact binder in an amount of about 2 to 25 wt.%, based on the total weight of the cathode composition.

[0020] In another aspect, the present invention provides a method for manufacturing a cathode of a lithium-sulfur secondary battery, comprising: Production of a primary sludge by mixing sulfur, a conductive material, a first solvent and a non-aqueous planar contact binder, Production of a primary composite material by drying and pulverizing the primary sludge, Producing a secondary slurry by mixing the primary composite, the conductive material and a second solvent with an aqueous point contact binder, and

[0021] Application of the secondary sludge to a cathode plate, wherein the first solvent used is one or more selected from the group consisting of N-methylpyrrolidone, acetonitrile, isopropyl ether, benzene, chloroform, n-hexane, methanol, acetone and toluene, and the non-aqueous planar contact binder is one or more selected from the group consisting of polyvinyl acetate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polyvinylidene fluoride, polyhexafluoropropylene-polyvinylidene fluoride copolymer, polyethyl acrylate, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, carboxymethylcellulose (CMC) and combinations thereof, and wherein the second solvent used in step (3) is water, and the aqueous point contact binder is one or more selected from the group consisting of polyvinylpyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR),Carboxymethylcellulose (CMC) and combinations thereof.

[0022] In an exemplary embodiment, the conductive material of the method may be, but is not limited to, one or more selected from the group consisting of graphite, Super C, vapor-phase grown carbon fibers, Ketjen black, Denka black, acetylene black, carbon black, carbon nanotubes, multi-walled carbon nanotubes, ordered mesoporous carbon and combinations thereof.

[0023] In an exemplary embodiment, the secondary sludge of the process may comprise: the sulfur in an amount of 40 to 85 wt.%, the conductive material in an amount of 10 to 50 wt.%, the non-aqueous planar contact binder in an amount of 2 to 25 wt.%, and the aqueous spot contact binder in an amount of 2 to 25 wt.%, based on the total weight of the secondary sludge.

[0024] According to the present invention, the secondary sludge is produced by dispersing the primary composite material using ultrasonic waves and mixing the primary composite material with the conductive material, the second solvent and the aqueous point contact binder.

[0025] In one exemplary embodiment, the secondary sludge is applied to a cathode plate.

[0026] Other aspects and preferred embodiments of the invention are discussed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The foregoing and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof, which are illustrated by the accompanying drawings, which serve only for illustration and are therefore in no way intended to limit the present invention, and wherein: Fig.Figure 1 schematically shows an exemplary binder that can create a non-aqueous planar contact between conventional cathode binders for a lithium-sulfur battery; Fig. Figure 2 schematically shows a binder that can create an aqueous point contact between conventional cathode binders for a lithium-sulfur battery; Fig. Figure 3 schematically shows an exemplary pattern (left) in which two types of binders according to an exemplary embodiment of the present invention can come into contact with an active cathode material of a lithium-sulfur battery; and an exemplary pattern (right) in which two types of binders can create a point contact or a planar contact according to an exemplary embodiment of the present invention; and Fig.Figure 4 is an exemplary diagram showing discharge curves of samples 1 and 2, as described in the example according to an exemplary embodiment of the present invention.

[0028] It is understood that the accompanying drawings are not necessarily to scale and show a somewhat simplified representation of various preferred features that illustrate the basic principles of the invention. The specific design features of the present invention, as disclosed herein, such as, among others, specific dimensions, orientations, locations, and shapes, are partly determined by the intended application and usage environment.

[0029] In the figures, the reference numbers represent identical or equivalent parts of the present invention across some of the figures in the drawings. DETAILED DESCRIPTION

[0030] In a described but not claimed aspect, the present invention provides a cathode composition of a lithium-sulfur secondary battery, which may comprise: sulfur, a conductive material, a non-aqueous planar contact binder, and an aqueous point contact binder.

[0031] In certain described but unclaimed embodiments, the sulfur can be a sulfur particle, and the conductive material can be a conductive material particle. In particular, the planar contact with the sulfur particles or the conductive material particles can occur in a planar phase, and the point contact with the sulfur particles or the conductive material particles can occur in a point phase.

[0032] In a described but unclaimed preferred aspect, a cathode composition of a lithium-sulfur secondary battery is provided, comprising: sulfur; a conductive material; a non-aqueous planar contact binder; and an aqueous spot contact binder, wherein a planar contact with the sulfur or the conductive material is made in or comprises a planar phase, and a spot contact with the sulfur or the conductive material is made in or comprises a spot phase. In a further aspect of the invention, the present invention provides a method for producing a cathode of a lithium-sulfur secondary battery, comprising the following: Production of a primary sludge by mixing sulfur, a conductive material, a first solvent and a non-aqueous planar contact binder, Production of a primary composite material by drying and pulverizing the primary sludge, Producing a secondary slurry by mixing the primary composite, the conductive material and a second solvent with an aqueous point contact binder, and Applying the secondary sludge to a cathode plate. The following section provides detailed reference to the various exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings and which are described below. Although the invention is described using exemplary embodiments, it is understood that the present description is not intended to limit the invention to these exemplary embodiments. As used here, the terms "lithium-sulfur battery", "lithium-sulfur cell", "cell", "battery" and the like refer to a lithium-sulfur secondary battery unless otherwise specified. Furthermore, the term "PVdF" refers to polyvinylidene fluoride, and the term "SBR" refers to styrene-butadiene rubber. In general, the binder that makes up the cathode of a lithium-sulfur battery can be divided into two types: a non-aqueous planar contact binder and an aqueous point contact binder, based on the solvent used here and the adhesion type.

[0033] In Fig.Figure 1 shows an exemplary non-aqueous planar contact binder. The non-aqueous planar contact binder can offer advantages. For example, the slurry can exhibit improved dispersibility and stability in a non-aqueous solvent. In particular, because PVdF exhibits lithium-ion conductivity when swollen in an electrolyte, the slurry can be easily mixed, generating a high voltage during discharge. However, the use of the non-aqueous solvent can necessitate high temperatures and a long drying time. Furthermore, a large quantity of the binder may be required to achieve a specific degree of adhesion, thus reducing the energy density of a cell and complicating a continuous drying process.

[0034] In Fig.Figure 2 shows an exemplary aqueous spot-contact binder. The aqueous spot-contact binder can also offer advantages. It dries easily and, due to its low boiling point, can be used in a continuous or intermittent production process for lithium-sulfur battery electrodes. Furthermore, since a small amount of highly adhesive binder can be used, the energy density of a cell can be increased. However, the large particle size of the binder, such as several tens of nanometers, can cause high electrochemical resistance; and because dispersion of a hydrophilic active material can be difficult, the dispersibility and stability of the slurry can decrease, leading to a drop in battery voltage due to internal electrode resistance during discharge.

[0035] Consequently, the present invention, as described in Fig.Figure 3 illustrates a process that utilizes both types of binders: the non-aqueous planar contact binder and the aqueous spot contact binder. The non-aqueous planar contact binder can be used on the part closest to the sulfur, enabling high voltage transfer during discharge, while the aqueous spot contact binder can be used on the other part, ensuring high adhesion strength. Furthermore, due to the aqueous binder in the electrode coating, the drying conditions can be moderate and light, thus providing a cathode composition for a lithium-sulfur secondary battery where two types of binders can be applied for continuous or intermittent coating.

[0036] In particular, a cathode composition of a lithium-sulfur secondary battery is described but not claimed, which may comprise the following: sulfur, a conductive material, a non-aqueous planar contact binder, and an aqueous spot contact binder. In certain embodiments, the sulfur may be a sulfur particle, and the conductive material may be a conductive material particle. In particular, the planar contact comprises or may comprise the sulfur particle or the conductive material particle in a planar phase, and the spot contact comprises or may comprise the sulfur particle or the conductive material particle in a spot phase.

[0037] The conductive material may be selected from, but is not limited to, the group consisting of graphite, Super C (TIMCAL), vapor-grown carbon fibers, Ketjen black, Denka black, acetylene black, carbon black, carbon nanotubes, multi-walled carbon nanotubes, ordered mesoporous carbon and combinations thereof.

[0038] The non-aqueous planar contact binder can be selected from the group consisting of polyvinyl acetate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polyvinylidene fluoride, polyhexafluoropropylene-polyvinylidene fluoride copolymer, polyethylene acrylate, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, carboxymethylcellulose (CMC), and combinations thereof, or in particular polyvinylpyrrolidone. Polyvinylpyrrolidone, for example, can be used as a non-aqueous planar contact binder because it exhibits a significantly higher ionic conductivity than other binders when swollen in a cell electrolyte.

[0039] The aqueous point-contact binder can be selected from the group consisting of polyvinylpyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), and combinations thereof, or in particular styrene-butadiene rubber (SBR). SBR, for example, can be used as an aqueous point-contact binder because it can exhibit significantly high adhesion strength even in small quantities.

[0040] Meanwhile, the non-aqueous planar contact binder can exist closer to sulfur particles than the aqueous point contact binder, due to the greater ionic conductivity when the non-aqueous binder can be swollen in an electrolyte, and an increase in the discharge voltage.

[0041] Furthermore, the composition according to the invention can comprise the following: sulfur in an amount of about 40 to 85 wt.%, the conductive material in an amount of about 10 to 50 wt.%, the non-aqueous planar contact binder in an amount of about 2 to 25 wt.%, and the aqueous point contact binder in an amount of about 2 to 25 wt.%, based on the total weight of the cathode composition. Due to its moderate drying requirements compared to those of a conventional binder, the composition according to the invention can also be subjected to a continuous coating process. At the same time, the electrochemical resistance during charging and discharging can be reduced, thereby generating a stable voltage curve of about 2.0 V or higher.

[0042] In contrast, the present invention provides a method for manufacturing a cathode of a lithium-sulfur secondary battery according to the invention, which comprises: Production of a primary sludge by mixing sulfur, a conductive material, a first solvent and a non-aqueous planar contact binder, Production of a primary composite material by drying and pulverizing the primary sludge, Producing a secondary slurry by mixing the primary composite, the conductive material and the solvent with an aqueous point contact binder, and

[0043] Applying the secondary sludge to a cathode plate.

[0044] The first solvent is one or more selected from the group consisting of N-methylpyrrolidone, acetonitrile, i-propyl ether, benzene, chloroform, n-hexane, methanol, acetone, and toluene, and the non-aqueous planar contact binder is selected from the group consisting of polyvinyl acetate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polyvinylidene fluoride, polyhexafluoropropylene-polyvinylidene fluoride copolymer, polyethyl acrylate, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, carboxymethylcellulose (CMC), and combinations thereof.

[0045] The second solvent is water, and the aqueous point contact binder is selected from the group consisting of polyvinylpyrrolidone, polytetrafluoroethylene, styrene butadiene rubber (SBR), and carboxymethylcellulose (CMC), or in particular styrene butadiene rubber (SBR).

[0046] Meanwhile, the conductive material may be selected from the group consisting of graphite, Super C (TIMCAL), vapor-grown carbon fibers, Ketjen black, Denka black, acetylene black, carbon black, carbon nanotubes, multi-walled carbon nanotubes, ordered mesoporous carbon and combinations thereof, but is not limited to these.

[0047] Furthermore, the secondary sludge may include: the sulfur in an amount of approximately 40 to 85 wt.%, the conductive material in an amount of approximately 10 to 50 wt.%, the non-aqueous planar contact binder in an amount of approximately 2 to 25 wt.%, and the aqueous point contact binder in an amount of approximately 2 to 25 wt.%, based on the total weight of the secondary sludge composition.

[0048] The secondary slurry, on the other hand, is produced by dispersing the primary composite material using ultrasonic waves and mixing the primary composite material with the conductive material, the second solvent, and the aqueous point-contact binder. This step can offer an advantage in that the primary composite material is dispersed more uniformly in the aqueous solvent.

[0049] In particular, in the manufacturing process of a cathode plate according to an exemplary embodiment of the present invention, the application of the secondary sludge to the cathode plate can be carried out continuously. In other words, the manufacturing process can be carried out continuously without interruption. When manufacturing a cathode for a lithium-sulfur battery, the cathode can usually be dried at a temperature of about 100°C or lower due to the low melting point of sulfur, in contrast to the manufacturing of conventional lithium-ion batteries.If the cathode for a lithium-sulfur battery is produced in facilities for conventional lithium-ion batteries and NMP is used as a solvent, the NMP solvent will not dry sufficiently due to such a low drying temperature, potentially requiring the production facilities to be shut down to evaporate the solvent. However, if the aqueous binder according to exemplary embodiments of the present invention is used, the drying and production of the cathode can be carried out without such a shutdown of the production facilities. EXAMPLES

[0050] The following examples illustrate the invention and are not intended to limit it.

[0051] The secondary sludges of samples 1 and 2 were prepared according to the compositions described in Table 1 below. The procedure for preparing the secondary sludge was described as follows: (1) Production of a primary sludge by mixing sulfur, a conductive material, a first solvent and a non-aqueous planar contact binder, (2) Producing a primary composite material by drying the primary sludge and pulverizing the primary sludge, and (3) Producing a secondary slurry by mixing the primary composite, the conductive material and a second solvent with an aqueous point contact binder.

[0052] The sulfur used in the samples was in particle form. [Table 1] Sample # sulfur Conductive material Non-aqueous planar contact binder Aqueous point contact binder Sulfur particles in a size of 5 µm or less VGCF (vapor-phase grown carbon fibers) PVDF SBR 1 71% by weight 23 wt.% 0 wt.% 6 wt.% 2 71% by weight 23 wt.% 3 wt.% 3 wt.%

[0053] The first solvent for dissolving and dispersing the non-aqueous planar contact binder was NMP, and the second solvent for dissolving and dispersing the aqueous point contact binder was distilled water.

[0054] If the sample contained only PVdF, NMP (N-methylpyrrolidone), which has a high boiling point, was used as a solvent. However, this was required for the drying process at approximately 100°C for about 30 minutes, which was unsuitable for a continuous coating procedure. Therefore, this sample was excluded from the subsequent experiment.

[0055] If the sample contained only SBR (sample #1), its drying condition was approximately 70°C for 3 minutes, which allowed for a continuous coating process. However, due to the large particle size of the binder, a significant degree of electrochemical resistance was generated during charging and discharging of a battery.

[0056] When the sample contained PVdF as a non-aqueous planar contact binder and SBR as an aqueous spot contact binder, a continuous coating process was applied due to the use of an aqueous solvent during the coating process. Simultaneously, the electrochemical resistance generated during charging and discharging of a battery was reduced, resulting in a stable voltage curve. In summary, the processability of an electrode coating was improved, and the energy density of a cell was increased.

[0057] The primary discharge curve for each sample is in Fig. 4 shown.

[0058] The invention has been described in detail with reference to its preferred embodiments.

Claims

[1] Method for manufacturing a cathode of a lithium-sulfur secondary battery, comprising: (1) Production of a primary sludge by mixing sulfur, a conductive material, a first solvent and a non-aqueous planar contact binder, (2) Production of a primary composite material by drying the primary sludge and pulverizing the primary sludge, (3) Producing a secondary slurry by mixing the primary composite, the conductive material and a second solvent with an aqueous point contact binder, and (4) Applying the secondary sludge to a cathode plate, wherein the first solvent is one or more selected from the group consisting of N-methylpyrrolidone, acetonitrile, isopropyl ether, benzene, chloroform, n-hexane, methanol, acetone, and toluene, and the non-aqueous planar contact binder is one or more from the group consisting of polyvinyl acetate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polyvinylidene fluoride, polyhexafluoropropylene-polyvinylidene fluoride copolymer, polyethyl acrylate, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, carboxymethylcellulose (CMC), and combinations thereof, wherein the second solvent of step (3) is water, and the aqueous point contact binder is one or more selected from the group consisting of polyvinylpyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), and combinations thereof, and wherein the secondary sludge is produced by dispersing the primary composite material using ultrasonic waves and mixing it with the conductive material, the second solvent and the aqueous point contact binder. [2] Method according to claim 1, wherein the conductive material is one or more selected from the group consisting of graphite, Super C, vapor-grown carbon fibers, Ketjen black, Denka black, acetylene black, carbon black, carbon nanotubes, multi-walled carbon nanotubes, ordered mesoporous carbon and combinations thereof. [3] The method of claim 1, wherein the secondary sludge comprises: the sulfur in an amount of 40 to 85 wt.%, the conductive material in an amount of 10 to 50 wt.%, the non-aqueous planar contact binder in an amount of 2 to 25 wt.%, and the aqueous spot contact binder in an amount of 2 to 25 wt.%, based on the total weight of the secondary sludge. [4] Method according to claim 1, wherein the application of the secondary sludge to the cathode plate is carried out continuously.

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