Polymer binders for hydroxyl group-based or carboxylic acid-based sulfur batteries and sulfur batteries containing the same

Abstract

The present invention relates to a polymer binder for hydroxyl group-based sulfur batteries that can effectively suppress the elution of polysulfide, which is produced by the reduction of sulfur during the discharge process, into the electrolyte, thereby solving the problem of reduced lifespan characteristics of sulfur batteries, a positive electrode for sulfur batteries containing the same, and a sulfur battery. The polymer binder for sulfur batteries according to one embodiment of the present invention comprises a water-soluble polymer and a polymer of the formula M(OH) n The present invention is characterized by being produced by a chemical reaction with a hydroxyl group-based crosslinking agent represented by the formula (wherein M is a metal or quasimetallic, and n is an integer from 1 to 5). Furthermore, the present invention relates to a polymer binder for carboxylic acid-based sulfur batteries that can effectively suppress the elution of polysulfide, which is produced when sulfur is reduced during the discharge process, into the electrolyte, thereby solving the problem of reduced lifespan characteristics of sulfur batteries, a positive electrode for sulfur batteries containing the same, and a sulfur battery. The polymer binder for sulfur batteries according to one embodiment of the present invention is a crosslinked water-soluble polymer binder produced by a chemical reaction between a water-soluble polymer and a carboxylic acid-based crosslinking agent.

Description

[Technical Field]

[0001] The present invention relates to a binder applicable to sulfur batteries containing sulfur as a positive electrode active material, such as lithium-sulfur batteries or sodium-sulfur batteries. More specifically, it relates to a hydroxyl group-based or carboxylic acid-based polymer binder for sulfur batteries that can effectively suppress the elution of polysulfide into the electrolyte when sulfur is reduced during the discharge process, thereby solving the problem of reduced lifespan characteristics of sulfur batteries, as well as a positive electrode for sulfur batteries and a sulfur battery containing the same. [Background technology]

[0002] Rechargeable batteries are used as a power source for many electronic devices. With the development of numerous stand-alone electronic devices, the range and importance of rechargeable batteries are increasing. In recent years, with the accelerated development of electric vehicles and energy storage, interest in high-capacity rechargeable batteries has grown.

[0003] However, conventional lithium-ion batteries have many problems for use in electric vehicles and smart grid power storage applications. Firstly, lithium-ion batteries are expensive, and the cost of the secondary battery can account for more than 70% of the purchase price of an electric vehicle. Secondly, even when lithium-ion batteries are used in electric vehicles, it is difficult to achieve high energy densities of 260 Wh / kg or more. Even with advances in research and development, there are technical limitations to achieving an energy density of 300 Wh / kg with lithium-ion batteries containing conventional electrode materials.

[0004] Therefore, to solve these problems, a new battery system is needed that can achieve low cost and high energy density, and one promising candidate is a sulfur battery, which uses sulfur as the main material of the positive electrode active material, such as lithium-sulfur batteries or sodium-sulfur batteries.

[0005] Sulfur batteries can achieve a high energy density of 2600 Wh / kg per unit weight, which is about seven times that of lithium-ion batteries. Sulfur itself has advantages such as having a small atomic weight, being abundant and inexpensive, being non-toxic, and being environmentally friendly. For these reasons, it is being re-evaluated and attracting attention as a next-generation secondary battery.

[0006] However, sulfur batteries also have drawbacks, most notably the large volume expansion (approximately 80%) that occurs during the charge-discharge process, and the leaching of polysulfides into the electrolyte when sulfur is reduced during the discharge process. The latter, in particular, not only causes a large amount of active material weight loss but also leads to significant self-discharge, ultimately reducing the lifespan of the sulfur battery. Therefore, for sulfur batteries to be put into practical use, it is essential to solve the polysulfide leaching problem as a top priority.

[0007] One way to solve these problems is to develop a novel binder that not only binds the conductive material and the active material (sulfur) within the positive electrode, but also suppresses volume expansion and polysulfide elution. However, conventional binders such as polyvinylidene fluoride (PVdF) are limited to binding the conductive material and the active material (sulfur), and have limitations in fulfilling all of the aforementioned roles. [Overview of the project] [Problems that the invention aims to solve]

[0008] The problem that the present invention aims to solve is to provide a polymer binder for carboxylic acid-based sulfur batteries, a positive electrode for sulfur batteries containing the same, and a sulfur battery, which can be applied to sulfur batteries containing sulfur as a positive electrode active material, and which, in addition to playing a role in bonding the conductive material and the active material sulfur, suppress volume expansion and effectively suppress the elution of polysulfide generated when sulfur is reduced during the discharge process into the electrolyte, thereby eliminating weight loss of the active material and solving the problem of reduced lifespan characteristics of sulfur batteries. [Means for solving the problem]

[0009] To solve the above technical problems, the present invention includes a crosslinked water-soluble polymer binder, which is applied to a sulfur battery containing sulfur as a positive electrode active material, and is characterized by being produced by a chemical reaction between a water-soluble polymer and a carboxylic acid-based crosslinking agent.

[0010] The water-soluble polymer according to one embodiment of the present invention is characterized by being selected from the group consisting of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and combinations thereof.

[0011] The carboxylic acid-based crosslinking agent according to one embodiment of the present invention is tartaric acid (C4H6O6), fumaric acid (C4H4O4), succinimide (C4H5NO2), malic acid (C4H6O5), adipic acid (C6H 10 It is characterized by being selected from the group consisting of 04), malonic acid (C3H4O4), acetylsalicylic acid (C9H8O4), levulinic acid (C5H8O3), and combinations thereof.

[0012] The water-soluble polymer according to one embodiment of the present invention is polyvinyl alcohol (PVA), and the carboxylic acid-based crosslinking agent is tartaric acid (C4H6O6).

[0013] The content of the carboxylic acid-based crosslinking agent according to one embodiment of the present invention is characterized in that it is 1 to 20% by weight relative to the total weight of the water-soluble polymer and the carboxylic acid-based crosslinking agent.

[0014] On the other hand, the present invention includes a positive electrode for a sulfur battery containing sulfur as a positive electrode active material, comprising a positive electrode active material layer comprising a positive electrode active material and a crosslinked water-soluble polymer binder according to the above-described embodiment. Furthermore, the present invention further includes a lithium-sulfur battery or a sodium-sulfur battery comprising the above-described positive electrode for a sulfur battery.

[0015] Furthermore, in order to achieve the above technical objectives, the present invention relates to a binder applicable to a sulfur battery containing sulfur as a positive electrode active material, comprising a water-soluble polymer and a hydroxyl group having the formula M(OH) n The present invention contains a crosslinked water-soluble polymer binder characterized by being produced by a chemical reaction with a compound represented by (where M is a metal or quasimetallic, and n is an integer from 1 to 5), i.e., a hydroxyl group crosslinking agent.

[0016] A water-soluble polymer according to one embodiment of the present invention is The material is characterized by being selected from the group consisting of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and combinations thereof.

[0017] In a hydroxyl group-based crosslinking agent according to one embodiment of the present invention, M is selected from the group consisting of sodium (Na), potassium (K), copper (Cu), cobalt (Co), calcium (Ca), nickel (Ni), boron (B), zirconium (Zr), aluminum (Al), silicon (Si), lithium (Li), radium (Ra), tantalum (Ta), tungsten (W), molybdenum (Mo), vanadium (V), manganese (Mn), and combinations thereof.

[0018] The water-soluble polymer according to one embodiment of the present invention is polyvinyl alcohol (PVA), and the hydroxy group-based crosslinking agent is boric acid.

[0019] The content of the hydroxy group-based crosslinking agent according to one embodiment of the present invention is 0.1 to 1% by weight based on the total weight of the water-soluble polymer and the hydroxy group-based crosslinking agent.

[0020] On the other hand, the present invention relates to a positive electrode of a sulfur battery containing sulfur as a positive electrode active material, A positive electrode for a sulfur battery including a positive electrode active material layer containing a positive electrode active material and the crosslinked water-soluble polymer binder according to one embodiment of the present invention described above. Further, a lithium-sulfur battery or a sodium-sulfur battery including the positive electrode for a sulfur battery is further included.

Effects of the Invention

[0021] According to the present invention as described above, the polymer binder according to one embodiment of the present invention, in addition to binding the conductive material and sulfur as the active material in the electrode, is a novel concept binder capable of suppressing volume expansion and suppressing the elution of polysulfides, eliminating the weight loss of the active material, and having the effect of solving the problem of deterioration of the life characteristics of the sulfur battery.

[0022] In particular, the polymer binder according to one embodiment of the present invention is a crosslinked water-soluble polymer binder produced by a chemical reaction between a water-soluble polymer and a carboxylic acid-based crosslinking agent, and has an attractive force with a functional group present in polysulfide and chemically binds to polysulfide, thereby effectively suppressing the elution of polysulfide into the electrolyte as a result.

[0023] The effects of the present invention are not limited to those described above, and are clearly understood by those skilled in the art from the description of the entire specification, and also include other effects not explicitly mentioned.

Brief Description of the Drawings

[0024] [Figure 1] It is a schematic diagram showing the case where a crosslinked water-soluble polymer binder (PVA-BA) according to an embodiment of the present invention is used as a binder in a lithium-sulfur battery. [Figure 2] It is a graph of the FT-IR results according to Experimental Example 1. [Figure 3] It is a graph of the mechanical strength results according to Experimental Example 2. [Figure 4] It is a graph of the swelling degree and solubility experiment results according to Experimental Example 3. [Figure 5] It is a graph of the polysulfide adsorption experiment results according to Experimental Example 4. [Figure 6] It is a graph of the polysulfide adsorption experiment results according to Experimental Example 4. [Figure 7] It is a graph of the test results of the cycle characteristics of the battery according to Experimental Example 5 (charge-discharge rate 0.2C rate). [Figure 8] It is a FE-SEM result photograph of observing the electrode surface shape according to Experimental Example 6. [[ID=​​​​​​​​​​​​​​​​​​​

[0025] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The advantages and features of the present invention, as well as methods for achieving them, will become apparent by referring to the embodiments described in detail below together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and can be carried out in a variety of different forms, and these embodiments are provided merely to complete the disclosure of the present invention and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined solely by the claims. Throughout the specification, the same reference numerals refer to the same components.

[0026] Unless otherwise specified, all terms used herein (including technical and scientific terms) are used in a sense that can be commonly understood by those skilled in the art in which the present invention pertains. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless explicitly specified otherwise. The terms used herein are for the purpose of describing embodiments and are not intended to limit the present invention. In this specification, singular forms include plural forms unless otherwise specified in the context.

[0027] The terms "comprises" and / or "comprising" as used in this specification do not exclude the presence or addition of one or more other components, steps, operations, and / or elements described.

[0028] Throughout this specification, the term "sulfur battery" refers to a battery that uses sulfur as the main material of the positive electrode active material, such as lithium-sulfur batteries or sodium-sulfur batteries. More specifically, it may include any battery that uses a material selected from the group consisting of the element sulfur, sulfur-containing compounds having a sulfur-sulfur combination, sulfur-carbon composites, and combinations thereof as the main material of the positive electrode active material.

[0029] As used herein, "polysulfide" refers to polysulfides produced as intermediate products when sulfur is reduced during the discharge process of a sulfur battery. In one embodiment, in the case of a lithium-sulfur battery, this may be lithium polysulfide (also referred to as "LiPS") represented by the chemical formulas Li2S8, Li2S6, Li2S4, or Li2S2.

[0030] The present invention relates to a polymer binder for sulfur batteries, a positive electrode for sulfur batteries containing sulfur as a positive electrode active material, and a sulfur battery, which can solve the problems of weight loss of the active material and deterioration of the life characteristics of sulfur batteries by not only bonding the conductive material with the active material sulfur, but also suppressing volume expansion and effectively suppressing the elution of polysulfide, which is produced when sulfur is reduced during the discharge process, into the electrolyte. The present invention will be described in more detail below.

[0031] A polymer binder according to one embodiment of the present invention comprises a water-soluble polymer and a polymer of formula M(OH) n It is characterized by being a crosslinked water-soluble polymer binder produced by a chemical reaction with a hydroxyl group crosslinking agent represented by the formula. In the above formula, M is a metal or quasimetallic, and n is an integer from 1 to 5.

[0032] The water-soluble polymer can be selected from the group consisting of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and combinations thereof.

[0033] The formula M(OH) of the aforementioned hydroxyl group crosslinking agent nIn this, M can be specifically selected from the group consisting of sodium (Na), potassium (K), copper (Cu), cobalt (Co), calcium (Ca), nickel (Ni), boron (B), zirconium (Zr), aluminum (Al), silicon (Si), lithium (Li), radium (Ra), tantalum (Ta), tungsten (W), molybdenum (Mo), vanadium (V), manganese (Mn), and combinations thereof. More specifically, the hydroxyl group crosslinking agents include sodium hydroxide (NaOH), potassium hydroxide (KOH), copper hydroxide (Cu(OH)2), cobalt hydroxide (Co(OH)2 or Co(OH)3), calcium hydroxide (Ca(OH)2), nickel hydroxide (Ni(OH)2), boric acid (B(OH)3), zirconium hydroxide (Zr(OH)4), aluminum hydroxide (Al(OH)3), silica, lithium hydroxide (LiOH), radium hydroxide (Ra(OH)2), tantalum hydroxide (Ta(OH)5), tungsten hydroxide (W(OH)4), and molybdenum hydroxide. It may be one or more of the following: hydroxide (Mo(OH)4), vanadium hydroxide (V(OH)5), and manganese hydroxide (Mn(OH)2).

[0034] The cross-linked water-soluble polymer binder of the present invention, produced by cross-linking such a water-soluble polymer with a hydroxyl group cross-linking agent through a chemical reaction, not only suppresses the elution of polysulfide into the electrolyte by physical means, but also effectively suppresses the elution of polysulfide into the electrolyte by chemical means, as it has an attractive force with the functional groups present on the polysulfide and chemically bonds with the polysulfide.

[0035] In fact, Figure 1 is a schematic diagram showing the case in a lithium-sulfur battery where a cross-linked water-soluble polymer binder (PVA-BA) according to one embodiment of the present invention is used as the binder for the positive electrode. As shown in Figure 1, the polymer binder (PVA-BA) according to the present invention has an attractive force with the functional groups present in lithium polysulfide (LiPS) and is chemically bonded to lithium polysulfide (LiPS). This effectively suppresses the elution of lithium polysulfide (LiPS) into the electrolyte, and as a result, the weight loss of sulfur, which is the active material, can be resolved.

[0036] On the other hand, when producing a crosslinked water-soluble polymer binder by a chemical reaction between a water-soluble polymer and a hydroxyl group crosslinking agent according to one embodiment of the present invention, it is extremely important to optimize the content of the hydroxyl group crosslinking agent.

[0037] This is because, when a small amount of hydroxyl group crosslinking agent is added, the attractive force with polysulfide weakens, making it difficult to effectively suppress the elution of polysulfide. On the other hand, when added in excess, defects are also introduced, making the ester bond more susceptible to cleavage during repeated charge-discharge processes, which may cause cracks in the electrode.

[0038] Therefore, in order to effectively suppress the elution of polysulfide, maintain the mechanical strength of the electrode without a decrease in the electrode's mechanical strength, and ensure that the electrode remains crack-free even after repeated charging and discharging, it is necessary to optimize the content of the hydroxyl group crosslinking agent. In one embodiment of the present invention, the content of the hydroxyl group crosslinking agent may be 0.1 to 1.5% by weight, more preferably 0.2 to 1.0% by weight, and most preferably 0.4 to 0.7% by weight, relative to the total weight of the water-soluble polymer and the hydroxyl group crosslinking agent. The content of the crosslinking agent can be controlled within the above range, and the hydroxyl group (OH) in the crosslinking agent molecule - The more of these factors there are, the more likely it is that a similar effect can be achieved with a relatively small amount, while the fewer these factors there are, the more a relatively large amount is required.

[0039] Preferred manufacturing examples and embodiments of the present invention will be described in detail below. Prior to this, terms or words used herein and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather in a manner and concept consistent with the technical idea of ​​the present invention, based on the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0040] Therefore, the configurations shown in the manufacturing examples and embodiments described herein are merely the most preferred manufacturing examples and embodiments of the present invention and do not represent the entire technical concept of the present invention. It should be understood that various equivalents and variations may exist that can substitute for them at the time of filing.

[0041] In the following figures, diagrams, and graphs, B or BA refers to boric acid.

[0042] Manufacturing Example 1: Production of Polymer Binder Solution JPEG2026514296000002.jpg54134

[0043] As shown in the figure above, polymer binder solutions containing the cross-linked water-soluble polymer binders of Examples 1 to 3 were produced by cross-linking polyvinyl alcohol (PVA) and boric acid (B(OH)3). The specific production method is as follows.

[0044] First, 6 g of PVA was added to 90 mL of distilled water and stirred at 90°C to dissolve completely, thereby preparing a PVA solution. Similarly, the crosslinking agent (boric acid) was dissolved in 90 mL of distilled water at room temperature with varying amounts as described below, to prepare a boric acid solution (BA solution). Then, the BA solution was added dropwise to the PVA solution while stirring at 45°C and pH 7 for more than 10 hours to prepare a PVA-BA solution.

[0045] Examples 1 to 3 were manufactured under identical conditions, with the only difference being the boric acid content, as shown in Table 1 below. The boric acid content in Table 1 represents the percentage of the total weight of polyvinyl alcohol and boric acid that boric acid accounts for. On the other hand, a comparative example was prepared using only PVA without the addition of boric acid.

[0046] Table 1 JPEG2026514296000003.jpg34151

[0047] Manufacturing Example 2: Manufacturing of Polymer Binder Film The comparative example and Examples 1 to 3 produced in Production Example 1 were coated onto a glass substrate to a thickness of 0.4 to 0.5 mm and then dried at 60°C for 6 hours. After that, any remaining unreacted substances and impurities were removed using distilled water, and the film was heated at 110°C for 2 hours to produce a polymer binder film.

[0048] Manufacturing Example 3: Electrode Manufacturing The comparative example and Examples 1 to 3 manufactured in Manufacturing Example 1 were used as binders to produce electrodes. The specific method for manufacturing the electrodes is as follows.

[0049] First, acetylene carbon black and sulfur were mixed in a weight ratio of 1:3 to produce the active material. Next, the active material, binder, and conductive material were mixed in a weight ratio of 80:10:10 to produce the mixture (electrode material). After that, the produced mixture (electrode material) was coated onto aluminum (Al) foil and then dried to produce the electrode.

[0050] Manufacturing Example 4: Battery Manufacturing A coin cell was manufactured using the electrode, separator (Celgard 2400), electrolyte, and lithium foil (thickness 150 μm) produced in Manufacturing Example 3 above. Here, as the electrolyte, tetraethylene glycol dimethyl ether (TEGDME), 1,3-dioxolane (1,3-DOL), and 1,2-dimethoxyethane (1,2-DME) (1:1, v / v) were used, in which 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 2 wt% lithium nitrate (LiNO3) were dissolved.

[0051] The following experiments were conducted using the materials manufactured as described above.

[0052] Experimental Example 1: FT-IR analysis to confirm cross-linking Using the polymer film produced in Production Example 2 as a sample, FT-IR analysis was performed to confirm whether the crosslinking between PVA and boric acid was properly carried out. Figure 2 is a graph showing the results of the FT-IR spectrum. For reference, the black line in Figure 2 is the FT-IR spectrum of boric acid.

[0053] As shown in Figure 2, 660 cm -1 The peak (OBO) located at 1270cm -1 The peak (BOC) located at [location] was observed to increase in intensity and shift to the left as the amount of boric acid added increased. Furthermore, it was observed at approximately 3000–3570 cm. -1The broad peak located at [location] was observed to disappear as the amount of boric acid added increased. This indicates that crosslinking was successfully formed within the polymer binders according to Examples 1 to 3.

[0054] Experimental Example 2: Mechanical Strength Evaluation The polymer film produced in the above-mentioned Production Example 2 was used as a sample, and its mechanical strength (breaking strength) was measured. Figure 3 is a graph showing the results of the mechanical strength test.

[0055] As shown in Figure 3, it was confirmed that Examples 1 to 3 had higher mechanical strength than the comparative example. This means that the polymer binder according to the present invention can suppress electrode cracking caused by volume change (~80%) that occurs when lithium and sulfur react during charging and discharging, and maintain electrochemical properties. On the other hand, Example 2, which has a boric acid content of 0.44% by weight, was confirmed to have the highest mechanical strength.

[0056] Experimental Example 3: Evaluation of Swelling Degree and Solubility The polymer film produced in Production Example 2 was cut into 2.5 × 2.5 cm pieces to make test specimens, and the degree of swelling and solubility were measured. Specifically, the degree of swelling and solubility were measured according to the following formulas.

[0057] Equation (1) - Degree of Swelling = {(Sb-Sa) / Sa} × 100

[0058] Formula (2)-Solubility={(Sa-Sc) / Sa}×100

[0059] In equations (1) and (2) above, Sa is the initial weight of the specimen, Sb is the weight measured after removing the specimen from the electrolyte and holding it at room temperature for 24 hours, and Sc is the weight measured after removing the specimen from the electrolyte and holding it at room temperature for 24 hours and drying it completely.

[0060] Based on this, considering the meaning of the swelling and solubility values, a low swelling degree means that even when the polymer binder is exposed to the electrolyte for a long time, the electrolyte did not penetrate between the polymer binders. A low solubility means that the polymer binder did not dissolve much even when exposed to the electrolyte. As a result, low values ​​for both of these indicate that crosslinking bonds were well formed within the polymer binder, and further suggest that it can well accommodate the volume change (~80%) that occurs when lithium and sulfur react during charging and discharging.

[0061] Figure 4 is a graph showing the measured swelling and solubility. As shown in Figure 4, the swelling and solubility of Examples 1 to 3 were all lower than that of the comparative example, and it was confirmed that Example 2, which had a boric acid content of 0.44% by weight, showed the lowest swelling and solubility.

[0062] Experimental Example 4: Evaluation of Polysulfide Elution Inhibition Lithium polysulfide (Li2S6) was mixed with Example 2, which was manufactured in Manufacturing Example 1, and the degree of adsorption of the polymer binder according to Example 2 to the lithium polysulfide was evaluated. Figure 5 is a graph showing the results of X-ray photoelectron spectroscopy (XPS) for Experimental Example 4. In Figure 5, the black line shows the XPS results for Example 2 alone, and the red line shows the XPS results after mixing Example 2 with lithium polysulfide and adsorption.

[0063] As shown in Figure 5, the red line shows peaks for S2s, S2p, and Li1s that were not observed in the black line, indicating that lithium polysulfide was adsorbed. Furthermore, Figure 6 is a magnified graph of the Li1s region in Figure 5, and as shown in Figure 6, the formation of Li-O, Li-S, and Li-B peaks can be confirmed, further indicating that lithium polysulfide was definitely adsorbed.

[0064] Experiment Example 5: Evaluation of Battery Cycle Characteristics The electrochemical properties of the battery manufactured in Manufacturing Example 4, i.e., the cycle characteristics of the battery under high-speed charge and discharge conditions, were evaluated. Figure 7 is a graph of the results evaluated at a charge and discharge rate of 0.2C. A 100mA class charge and discharge device was used, and the evaluation temperature was 25°C.

[0065] As shown in Figure 7, compared to a comparative example using PVA alone as a binder, the batteries using Examples 1 to 3, in which PVA and boric acid are cross-linked as binders, were found to exhibit more stable and superior cycle characteristics over 300 cycles. Among these, Example 2, with a boric acid content of 0.44% by weight, was found to exhibit the most excellent cycle characteristics.

[0066] Experimental Example 6: Observation of electrode surface shape after cycling The surface shape of the electrodes manufactured in Manufacturing Example 3 was compared and observed before and after the charge-discharge cycle to evaluate how well the polymer binder held the electrodes and suppressed crack formation. 300 charge-discharge cycles were performed at a rate of 0.2C. Figure 8 shows field emission scanning electron microscope (FE-SEM) images of the electrode surface shape observed before and after the charge-discharge cycle, where (a) is the comparative example, (b) is Example 1, (c) is Example 2, and (d) is Example 3, each using the polymer binder.

[0067] As shown in Figure 8, in the comparative example using PVA alone as a binder, significant crack formation was observed after charging and discharging. In contrast, in the electrodes using Examples 1 to 3, in which PVA and boric acid were cross-linked as binders, crack formation was significantly reduced even after charging and discharging. This indicates that the cross-linking of PVA and boric acid not only suppresses the elution of lithium polysulfide but also suppresses the occurrence of electrode cracks due to volume changes (~80%) caused by the reaction of lithium and sulfur during charging and discharging. In particular, Example 2, with a boric acid content of 0.44% by weight, showed the fewest cracks and the most stable surface shape.

[0068] When such a cross-linked water-soluble polymer binder of the present invention is applied to a sulfur battery containing sulfur as a positive electrode active material, such as a lithium-sulfur battery or a sodium-sulfur battery, in addition to its role as a basic binder that binds the conductive material to the active material sulfur, it effectively suppresses the elution of polysulfide into the electrolyte, thereby eliminating weight loss of the active material, and also effectively suppresses the occurrence of electrode cracks by readily accepting the volume change caused by the reaction between lithium and sulfur during charging and discharging. As a result, the sulfur battery can exhibit stable and high cycle characteristics.

[0069] Furthermore, throughout this specification, the term "sulfur battery" refers to a battery that uses sulfur as the main material of the positive electrode active material, such as lithium-sulfur batteries or sodium-sulfur batteries. More specifically, it can include any battery that uses a material selected from the group consisting of sulfur element, sulfur-sulfur compounds having sulfur-sulfur bonds, sulfur-carbon composites, and combinations thereof as the main material of the positive electrode active material.

[0070] As used herein, "polysulfide" refers to polysulfides produced as intermediate products when sulfur is reduced during the discharge process of a sulfur battery. In one embodiment, in the case of a lithium-sulfur battery, this may be lithium polysulfide (also referred to as "LiPS") represented by the chemical formulas Li2S8, Li2S6, Li2S4, or Li2S2.

[0071] The present invention is applicable to a sulfur battery containing sulfur as a positive electrode active material, such as a lithium-sulfur battery or a sodium-sulfur battery. In addition to the role of binding a conductive material and sulfur which is an active material, it suppresses volume expansion and effectively suppresses the elution of polysulfide generated by the reduction of sulfur into the electrolyte during the discharge process, thereby eliminating the weight loss of the active material and solving the problem of deterioration of the life characteristics of the sulfur battery. The present invention relates to a polymer binder for a sulfur battery, a positive electrode for a sulfur battery containing the same, and a sulfur battery. Hereinafter, the present invention will be described more specifically.

[0072] The polymer binder according to an embodiment of the present invention is characterized in that it is a crosslinked water-soluble polymer binder produced by a chemical reaction between a water-soluble polymer and a carboxylic acid crosslinking agent.

[0073] The water-soluble polymer may be selected from the group consisting of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and combinations thereof.

[0074] The carboxylic acid crosslinking agent may be selected from the group consisting of tartaric acid (C4H6O6), fumaric acid (C4H4O4), succinimide (C4H5NO2), malic acid (C4H6O5), adipic acid (C6H 10 O4), malonic acid (C3H4O4), acetylsalicylic acid (C9H8O4), levulinic acid (C5H8O3), and combinations thereof.

[0075] The cross-linked water-soluble polymer binder of the present invention, produced by cross-linking such a water-soluble polymer with a carboxylic acid-based cross-linking agent through a chemical reaction, not only suppresses the elution of polysulfide into the electrolyte by physical means, but also effectively suppresses the elution of polysulfide into the electrolyte by chemical means by chemical means, as it has an attractive force with the functional groups present on the polysulfide and chemically bonds with the polysulfide.

[0076] In fact, Figure 1 is a schematic diagram comparing the case in a lithium-sulfur battery where PVA alone is used as the positive electrode binder and the case where a cross-linked water-soluble polymer binder (PVA-TA) according to one embodiment of the present invention is used. As shown in Figure 1, unlike PVA alone, the polymer binder (PVA-TA) according to the present invention has an attractive force with the functional groups present in lithium polysulfide (LiPS) and is chemically bonded to lithium polysulfide (LiPS). This effectively suppresses the elution of lithium polysulfide (LiPS) into the electrolyte, and as a result, weight loss of sulfur, which is the active material, can be eliminated.

[0077] On the other hand, when producing a crosslinked water-soluble polymer binder by a chemical reaction between a water-soluble polymer and a carboxylic acid-based crosslinking agent according to one embodiment of the present invention, it is extremely important to optimize the content of the carboxylic acid-based crosslinking agent.

[0078] This is because, when a small amount of carboxylic acid-based crosslinking agent is added, the attractive force with polysulfide weakens, making it difficult to effectively suppress the elution of polysulfide. On the other hand, when added in excess, defects are also introduced, making the ester bond more susceptible to cleavage during repeated charge-discharge processes, which may lead to cracks in the electrode.

[0079] Therefore, in order to effectively suppress the elution of polysulfide, maintain the mechanical strength of the electrode without reducing it, and ensure that the electrode remains crack-free even after repeated charging and discharging, it is necessary to optimize the content of the carboxylic acid-based crosslinking agent. In one embodiment of the present invention, the content of the carboxylic acid-based crosslinking agent may be 1 to 20% by weight, more preferably 5 to 15% by weight, and most preferably 7 to 13% by weight, relative to the total weight of the water-soluble polymer and the carboxylic acid-based crosslinking agent. The content of the crosslinking agent can be controlled within the above range, and the carboxyl group (COOH) in the crosslinking agent molecule is important. - The more of these factors there are, the more likely it is that a similar effect can be achieved with a relatively small amount, while the fewer these factors there are, the more a relatively large amount is required.

[0080] Preferred manufacturing examples and embodiments of the present invention will be described in detail below. Prior to this, terms or words used herein and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather in a manner and concept consistent with the technical idea of ​​the present invention, based on the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0081] Therefore, the configurations shown in the manufacturing examples and embodiments described herein are merely the most preferred manufacturing examples and embodiments of the present invention and do not represent the entire technical concept of the present invention. It should be understood that various equivalents and variations may exist that can substitute for them at the time of filing.

[0082] In the following figures, diagrams, and graphs, T or TA refers to tartaric acid.

[0083] Manufacturing Example 1': Production of Polymer Binder Solution JPEG2026514296000004.jpg72130

[0084] As shown in the figure above, polymer binder solutions containing the cross-linked water-soluble polymer binders of Examples 1' to 3' were prepared by cross-linking polyvinyl alcohol (PVA) and tartaric acid (C4H6O6). The specific manufacturing method is as follows.

[0085] First, PVA was added to distilled water at a concentration of 20% by weight and stirred at 90°C until completely dissolved. Then, tartaric acid was added to the PVA solution and stirred at 40°C and pH 6-7 for more than 12 hours.

[0086] Examples 1' to 3' were manufactured under identical conditions, with the only difference being the tartaric acid content, as shown in Table 2 below. The tartaric acid content in Table 1 represents the percentage of tartaric acid's weight relative to the total weight of polyvinyl alcohol and tartaric acid. On the other hand, a comparative example was prepared using only PVA without the addition of tartaric acid.

[0087] Table 2 JPEG2026514296000005.jpg31152

[0088] Manufacturing Example 2': Manufacturing of Polymer Binder Film Comparative Example' and Examples 1' to 3', manufactured in Manufacturing Example 1', were coated onto a glass substrate to a thickness of 0.4 to 0.5 mm and then dried at 60°C for 6 hours. Subsequently, any remaining unreacted substances and impurities were removed using distilled water, and the film was heated at 110°C for 2 hours to produce a polymer binder film.

[0089] Manufacturing Example 3': Electrode Manufacturing Electrodes were manufactured using Comparative Example ' and Examples 1' to 3', respectively, which were produced in Manufacturing Example 1', as binders. The specific method for manufacturing the electrodes is as follows.

[0090] First, acetylene carbon black and sulfur were mixed in a weight ratio of 1:3 to produce the active material. Next, the active material, binder, and conductive material were mixed in a weight ratio of 80:10:10 to produce the mixture (electrode material). After that, the mixture (electrode material) was coated onto aluminum (Al) foil and then dried to produce the electrode.

[0091] Manufacturing Example 4': Battery Manufacturing A coin cell was manufactured using the electrode, separator (Celgard 2400), electrolyte, and lithium foil (thickness 150 μm) manufactured in the above manufacturing example 3'. Here, as the electrolyte, tetraethylene glycol dimethyl ether (TEGDME), 1,3-dioxolane (1,3-DOL), and 1,2-dimethoxyethane (1,2-DME) (1:1, v / v) in which 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 2 wt% lithium nitrate (LiNO3) were dissolved were used.

[0092] The following experiments were conducted using the materials manufactured as described above.

[0093] Experimental Example 1': FT-IR analysis to confirm cross-linking Using the polymer film produced in Production Example 2' as a sample, FT-IR analysis was performed to confirm whether the crosslinking between PVA and tartaric acid was properly carried out. Figure 10 is a graph showing the results of the FT-IR spectrum. For reference, the red line (labeled TA) in Figure 10 is the FT-IR spectrum of tartaric acid.

[0094] As shown in Figure 10, 1750 cm -1 The peak located at (-C=O) was hardly observed in the comparative example, but appeared in Examples 1' to 3', confirming that the peak increased with increasing tartaric acid content. Also, at 3200-3500 cm⁻¹ -1The peak (-OH) located at [location] showed broadening in Examples 1' to 3'. This confirmed that crosslinking was successfully formed within the polymer binders in Examples 1' to 3'.

[0095] Experimental Example 2': Evaluation of swelling degree and solubility The polymer film produced in Production Example 2' was cut into 2.5 × 2.5 cm pieces to prepare test specimens, and the degree of swelling and solubility were measured. Specifically, the degree of swelling and solubility were measured according to the following formulas.

[0096] Equation (1) - Degree of Swelling = {(Sb-Sa) / Sa} × 100

[0097] Formula (2)-Solubility={(Sa-Sc) / Sa}×100

[0098] In equations (1) and (2) above, Sa is the initial weight of the specimen, Sb is the weight measured after removing the specimen from the electrolyte and holding it at room temperature for 24 hours, and Sc is the weight measured after removing the specimen from the electrolyte and holding it at room temperature for 24 hours and drying it completely.

[0099] Based on this, considering the meaning of the swelling and solubility values, a low swelling degree means that even when the polymer binder is exposed to the electrolyte for a long time, the electrolyte did not penetrate between the polymer binders. A low solubility means that the polymer binder did not dissolve much even when exposed to the electrolyte. As a result, low values ​​for both of these indicate that crosslinking bonds were well formed within the polymer binder, and further suggest that it can well accommodate the volume change (~80%) that occurs when lithium and sulfur react during charging and discharging.

[0100] Figure 11 is a graph showing the measured swelling and solubility. As shown in Figure 11, the swelling and solubility of Examples 1' to 3' were all lower than that of Comparative Example ', and it was confirmed that Example 2', which had a tartaric acid content of 10% by weight, showed the lowest swelling and solubility.

[0101] Experimental Example 3': Evaluation of Battery Cycle Characteristics The electrochemical properties of the battery manufactured in manufacturing example 4', i.e., the cycle characteristics of the battery under high-speed charge-discharge conditions, were evaluated. Figure 12 is a graph of the results evaluated at a charge-discharge rate of 0.5C, and Figure 13 is a graph of the results evaluated at a charge-discharge rate of 1.0C. In both Figure 12 and Figure 13, a 100mA class charge-discharge device was used, and the evaluation temperature was 25°C.

[0102] As shown in Figure 12, compared to a comparative example using only PVA as a binder, the batteries using Examples 1' to 3', which cross-link PVA and tartaric acid, were confirmed to exhibit more stable and superior cycle characteristics over 300 cycles. Among these, Example 2', with a tartaric acid content of 10% by weight, was confirmed to exhibit the best cycle characteristics. In Figure 13, where the charge-discharge speed was varied, Example 2' was also confirmed to exhibit the best cycle characteristics.

[0103] Experimental Example 4': Observation of electrode surface shape after cycle The surface morphology of the electrodes manufactured in the aforementioned Manufacturing Example 3' was compared and observed before and after the charge-discharge cycle to evaluate how well the polymer binder held the electrodes and suppressed crack formation. 400 charge-discharge cycles were performed at a rate of 0.5C. Figure 14 shows field emission scanning electron microscope (FE-SEM) images of the electrode surface morphology observed before and after the charge-discharge cycle, where (a) is Comparative Example ', (b) is Example 1', (c) is Example 2', and (d) is Example 3', each using different polymer binders.

[0104] As shown in Figure 14, in electrodes using a comparative example with PVA alone as a binder, significant crack formation was observed after charging and discharging. In contrast, in electrodes using Examples 1' to 3', in which PVA and tartaric acid were cross-linked as binders, crack formation was significantly reduced even after charging and discharging. This indicates that cross-linking of PVA and tartaric acid not only suppresses the elution of lithium polysulfide but also suppresses the occurrence of electrode cracks due to volume changes (~80%) caused by the reaction of lithium and sulfur during charging and discharging.

[0105] When such a cross-linked water-soluble polymer binder of the present invention is applied to a sulfur battery containing sulfur as a positive electrode active material, such as a lithium-sulfur battery or a sodium-sulfur battery, in addition to its role as a basic binder that binds the conductive material to the active material sulfur, it effectively suppresses the elution of polysulfide into the electrolyte, thereby eliminating weight loss of the active material, and also effectively suppresses the occurrence of electrode cracks by readily accepting the volume change caused by the reaction between lithium and sulfur during charging and discharging. As a result, the sulfur battery can exhibit stable and high cycle characteristics.

[0106] While embodiments of the present invention have been described above, those with ordinary skill in the art to which the present invention pertains will understand that the invention can be easily modified into other specific forms without altering the technical idea or essential features of the present invention. Therefore, the embodiments described above should be understood to be illustrative and not limiting in all respects.

Claims

1. In a binder applied to a sulfur battery containing sulfur as the positive electrode active material, Water-soluble polymer and formula M(OH) ｎ A crosslinked water-soluble polymer binder characterized by being produced by a chemical reaction with a hydroxyl group-based crosslinking agent represented by the formula (wherein M is a metal or quasimetallic, and n is an integer from 1 to 5).

2. The crosslinked water-soluble polymer binder according to claim 1, characterized in that the water-soluble polymer is selected from the group consisting of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and combinations thereof.

3. The above formula M(OH) ｎ The crosslinked water-soluble polymer binder according to claim 1, wherein M is selected from the group consisting of sodium (Na), potassium (K), copper (Cu), cobalt (Co), calcium (Ca), nickel (Ni), boron (B), zirconium (Zr), aluminum (Al), silicon (Si), lithium (Li), radium (Ra), tantalum (Ta), tungsten (W), molybdenum (Mo), vanadium (V), manganese (Mn), and combinations thereof.

4. The crosslinked water-soluble polymer binder according to claim 1, characterized in that the water-soluble polymer is polyvinyl alcohol (PVA) and the hydroxyl group crosslinking agent is boric acid.

5. The crosslinked water-soluble polymer binder according to claim 1, characterized in that the content of the hydroxyl group crosslinking agent is 0.1 to 1% by weight relative to the total weight of the water-soluble polymer and the hydroxyl group crosslinking agent.

6. In the positive electrode of a sulfur battery containing sulfur as the positive electrode active material, Positive electrode active material and, A positive electrode for a sulfur battery comprising a positive electrode active material layer containing a crosslinked water-soluble polymer binder according to any one of claims 1 to 5.

7. In a sulfur battery containing sulfur as the positive electrode active material, A lithium-sulfur battery comprising the positive electrode described in claim 6.

8. In a battery containing sulfur as the positive electrode active material, A sodium-sulfur battery comprising the positive electrode described in claim 6.

9. In a binder applied to a sulfur battery containing sulfur as the positive electrode active material, A cross-linked water-soluble polymer binder characterized by being produced by a chemical reaction between a water-soluble polymer and a carboxylic acid-based cross-linking agent.

10. The crosslinked water-soluble polymer binder according to claim 9, characterized in that the water-soluble polymer is selected from the group consisting of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and combinations thereof.

11. The carboxylic acid crosslinking agent is tartaric acid (Tartaric acid, C ４ H ６ O ６ ), fumaric acid (Fumaric acid, C ４ H ４ O ４ ), succinimide (Succinimide, C ４ H ５ NO ２ ), malic acid (Malic acid, C ４ H ６ O ５ ), adipic acid (Adipic acid, C ６ H １０ O ４ ), malonic acid (Malonic acid, C ３ H ４ O ４ ), acetylsalicylic acid (Acetylsalicylic acid, C ９ H ８ O ４ ), levulinic acid (Levulinic acid, C ５ H ８ O ３ ), and is characterized by being selected from the group consisting of combinations thereof. The crosslinked water-soluble polymer binder according to claim 9.

12. The water-soluble polymer is polyvinyl alcohol (PVA), and the carboxylic acid-based crosslinking agent is tartaric acid (C). ４ H ６ O ６ The crosslinked water-soluble polymer binder according to claim 9, characterized in that it is ( ).

13. The crosslinked water-soluble polymer binder according to claim 9, characterized in that the content of the carboxylic acid-based crosslinking agent is 1 to 20% by weight relative to the total weight of the water-soluble polymer and the carboxylic acid-based crosslinking agent.

14. In the positive electrode of a sulfur battery containing sulfur as the positive electrode active material, Positive electrode active material and, A positive electrode for a sulfur battery comprising a positive electrode active material layer containing a crosslinked water-soluble polymer binder according to any one of claims 9 to 13.

15. In a sulfur battery containing sulfur as the positive electrode active material, A lithium-sulfur battery comprising the positive electrode described in claim 14.

16. In a battery containing sulfur as the positive electrode active material, A sodium-sulfur battery comprising the positive electrode described in claim 14.

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