Binder for solid electrolyte-based all-solid-state lithium secondary battery, cathode of all-solid-state lithium secondary battery including the binder, separator of all-solid-state lithium secondary battery including the binder, and solid electrolyte-based all-solid-state lithium secondary battery

Abstract

Provided are a binder for a solid electrolyte-based all-solid-state lithium secondary battery, a positive electrode for an all-solid-state lithium secondary battery including the binder, a separator for an all-solid-state lithium secondary battery including the binder, and a solid electrolyte-based all-solid-state lithium secondary battery. [Solution] The present invention relates to a binder for all-solid-state lithium secondary batteries, in which a carboxyl group of a mercaptocarboxylic acid is grafted to a butadiene polymer, and the molar ratio of the carboxyl group to the butadiene polymer is 0.1 to 30:100; an electrode composite for an all-solid-state lithium secondary battery including the binder; a separator for an all-solid-state lithium secondary battery including the binder; and an all-solid-state lithium secondary battery including the binder. The binder for an all-solid-state lithium secondary battery of the present invention can simultaneously satisfy the stability and solubility of the binder, as well as polarity and adhesive properties, and enables the manufacture of an all-solid-state battery by a solution process based on a wet process, thereby significantly advancing the commercialization of all-solid-state batteries.

Description

[Technical Field]

[0001] The present invention relates to a binder for a solid electrolyte-based all-solid-state lithium secondary battery, an all-solid-state lithium secondary battery cathode including the binder, an all-solid-state lithium secondary battery separator including the binder, and a solid electrolyte-based all-solid-state lithium secondary battery. More particularly, the present invention relates to a binder for a solid electrolyte-based all-solid-state lithium secondary battery that does not react with a sulfide-based solid electrolyte, has solubility in a non-polar solvent (p-xylene), and has adhesive strength greater than or equal to that of a commercial LIB binder (PVDF), an all-solid-state lithium secondary battery cathode including the binder, an all-solid-state lithium secondary battery separator including the binder, and a solid electrolyte-based all-solid-state lithium secondary battery. [Background technology]

[0002] Lithium-ion batteries (LIBs), which have long been used for their high energy and power density and excellent rechargeability, face safety issues due to flammable liquid electrolytes, making them difficult to apply to electric vehicles and energy storage devices. All-solid-state lithium secondary batteries (ASSBs) are attracting attention as a next-generation battery that can solve the stability issues of LIBs by using non-flammable solid electrolytes (SEs). However, despite these advantages, all-solid-state lithium secondary batteries face difficulties in commercialization due to the lack of a scalable battery manufacturing process, as their solid components create interfacial resistance between particles, which is an obstacle to improving primary cell performance.

[0003] As a result, sulfide (Li2SP 2S 5. Li7P3S 11 , Li 10 GeP2S 12 ), oxide (Li4SiO4-Li3PO4, Li 1.5 Al 0.5 Ge 1.5(PO4)3, Li 3x La 2 / 3x Much research has been conducted on inorganic solid electrolytes such as TiO3 and nitrides (Li3N, LiPON), and some sulfide-based solid electrolytes have been confirmed to exhibit ionic conductivity at the same level as organic liquid electrolytes. Among these, argyrodite Li6PS 5X In the case of (X=Cl, Br, I), some of the sulfur atoms are replaced by halogen atoms, improving the ionic conductivity and chemical stability. In particular, Li6PS5Cl has high ionic conductivity (1.3 × 10 at room temperature). 3 S cm -1 ), which has recently attracted much attention due to its low cost and simple manufacturing method. 1 / 3 Co 1 / 3 Mn 1 / 3 Studies on the interfacial stability with O2 and LiMn2O4 electrode materials revealed that the reversible electrochemical behavior of argyrodite also contributes to the capacity retention of all-solid-state lithium secondary batteries, increasing the commercial viability of all-solid-state lithium secondary batteries.

[0004] However, while non-flammable solid electrolytes (SEs) have achieved high ionic conductivity, much research is still needed in terms of battery manufacturing. In particular, sulfide-based solid electrolytes exhibit strong chemical reactivity toward polar solvents, causing conflicts in various battery characteristics and acting as a major obstacle to the commercialization of all-solid-state lithium secondary batteries. All-solid-state lithium secondary batteries are divided into thin-film batteries (<10μm) and bulk-type batteries depending on the thickness of the electrode, and bulk-type batteries for high energy density are further divided into pellet-type batteries based on powder compression and sheet-type batteries based on solution processing depending on the manufacturing process.

[0005] As all-solid-state lithium secondary batteries are mass-produced using a roll-to-roll process, a sheet-type solution process is required to form a thin, uniform solid electrolyte layer, which directly correlates with cell energy density. As a result, binders, an essential element in solution processes, are becoming increasingly important. Binders are particularly important in all-solid-state lithium secondary batteries, where all electrode materials are solid, compared to lithium secondary batteries with liquid electrolytes. In lithium-ion batteries, even if a break occurs between electrode materials due to repeated volume changes in the active material, lithium ions can still move through the liquid electrolyte. However, in all-solid-state lithium secondary batteries, once the electrode material interface is broken due to the weak adhesive strength of the binder, the electrode materials are permanently isolated within the electrode. Therefore, in all-solid-state lithium secondary batteries, the binder serves more than just to connect electrode materials.

[0006] The adhesive properties of polymer binders are determined by the flexibility of the polymer chain and the functional groups. The functional groups of the binder polymer form chemical bonds with the current collector and the surface of the electrode material to exhibit adhesive properties, and the continuous polymer chain structure contributes to improving the electrode's adhesive strength by suppressing the volume expansion of the electrode material. Conventional polymer binders with this polymer structure, such as poly(polyvinylidene fluoride) (PVDF), poly(vinyl alcohol) (PVA), and carboxymethyl cellulose (CMC), exhibit high adhesive strength due to their strong polar functional groups. However, they are soluble only in highly polar solvents (e.g., NMP, water), making them unsuitable for use as binders for all-solid-state lithium secondary batteries.

[0007] Therefore, currently reported solution-processed all-solid-state lithium secondary batteries use low-polarity binders that can be dispersed in non-polar or very less polar aprotic solvents such as xylene and toluene, such as butadiene rubber (BR), styrene-butadiene rubber (SBR), styrene-ethylene-butylene-styrene copolymer (SEBS), nitrile-butadiene rubber (NBR), and ethyl cellulose. However, the adhesion of these electrodes is insufficient, and there is also insufficient information about the bonding mechanism within the electrode.

[0008] In addition, all-solid-state lithium secondary batteries replace the existing volatile organic liquid electrolyte with a solid electrolyte, and have advantages in terms of stability as well as high energy density performance. However, the manufacturing process is limited to a dry process with complicated production processes, and commercialization and mass production have not been achieved.

[0009] Meanwhile, unlike existing lithium-ion batteries, all-solid-state batteries do not require a separate polymer separator; instead, a separate solid electrolyte layer plays the role of the separator. Among the methods for creating a solid electrolyte separator layer with a thickness of less than several tens of microns, the most promising method is to create a cast electrolyte layer by casting a slurry containing only a solid electrolyte and a binder onto an all-solid-state battery electrode. While the adhesive strength of the binder is important for a cast electrolyte layer, existing binders have not been able to secure the desired adhesive strength, making them difficult to apply to separators of various sizes and shapes.

[0010] Accordingly, the present inventors first conducted research into "binders for all-solid-state lithium secondary batteries" and explained the adhesive properties and electrochemical properties of all-solid-state lithium secondary batteries depending on the structure and polarity of the binder. As a result, they confirmed that the stability and solubility of the binder and the polarity and adhesive properties are in a trade-off relationship, and that it is necessary to develop a binder for solution-processable all-solid-state lithium secondary batteries with an optimized structure that can simultaneously satisfy the stability and solubility of the binder and the polarity and adhesive properties. Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention provides a binder for a solid electrolyte-based all-solid-state lithium secondary battery that can simultaneously satisfy the stability and solubility of the binder and the polarity and adhesive properties, a positive electrode for an all-solid-state lithium secondary battery including the binder, a separator for an all-solid-state lithium secondary battery including the binder, and a solid electrolyte-based all-solid-state lithium secondary battery. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems, the present invention provides a binder for an all-solid-state lithium secondary battery, in which a chain containing a polar functional group is grafted onto a diene-based polymer, wherein the diene-based polymer is a butadiene polymer or an isoprene polymer, the polar functional group has a strong attraction to an electrode or a solid electrolyte based on a dipole moment, and the molar ratio of the polar functional group to the diene-based polymer is 0.1 to 30.

[0013] In one embodiment of the present invention, the polar functional group is any one selected from the group consisting of a carboxyl group, a hydroxyl group, an ester group, an amine group, and salts thereof.

[0014] In one embodiment of the present invention, the binder comprises a polymer in which one or more compounds selected from the group consisting of mercapto, hydrogen peroxide, peroxy acid, ozone, halogen, silane, and sulfenylhydrogenine are grafted onto a diene-based polymer.

[0015] In one embodiment of the present invention, the binder comprises a diene-based polymer to which a polar functional group of a mercapto compound is grafted, and the mercapto compound is grafted to the diene-based polymer via a thiol group.

[0016] In one embodiment of the present invention, the binder has a polar functional group of a carboxylic acid compound grafted to the diene-based polymer, and the carboxylic acid compound is grafted to the diene-based polymer through an ester group.

[0017] In one embodiment of the present invention, an ester is bonded to another carbon atom adjacent to the carbon atom to which the carboxylic acid compound is grafted of the diene-based polymer.

[0018] In one embodiment of the present invention, the polar functional groups of the binder form hydrogen bonds with OH groups present on the surfaces of the current collector and the active material.

[0019] In one embodiment of the present invention, the binder is dissolved in one or more organic solvents selected from the group consisting of n-hexane, toluene, p-xylene, butyl butyrate, tetrahydrofuran, and diethyl carbonate.

[0020] The present invention also provides an all-solid-state lithium secondary battery electrode composite including the above-described binder, a current collector, an electrode material, and a sulfide-based solid electrolyte.

[0021] In one embodiment of the present invention, the polar functional groups of the binder form hydrogen bonds with OH groups present on the surface of the current collector and the electrode material, and the present invention provides an all-solid-state lithium secondary battery separator including the above-mentioned binder.

[0022] The present invention also provides an all-solid-state lithium secondary battery including the above-described electrode composite or the above-described separator. [Effects of the Invention]

[0023] The present invention provides a binder for all-solid-state batteries that does not react with sulfide-based solid electrolytes, is soluble in non-polar solvents (p-xylene), and has adhesive strength equal to or greater than that of commercial LIB binders (PVDF).

[0024] The binder for an all-solid-state lithium secondary battery of the present invention can simultaneously satisfy binder stability and solubility, polarity, and adhesive properties, thereby enabling the fabrication of an all-solid-state battery using a solution process based on a wet process, significantly accelerating the commercialization of all-solid-state batteries. The binder for an all-solid-state lithium secondary battery of the present invention is expected to be applicable to the wet process currently used in lithium secondary batteries without additional equipment investment. [Brief explanation of the drawings]

[0025] [Figure 1A] 1 shows a schematic diagram of modification of a click binder. [Figure 1B] 1 is a diagram showing the FT-IR results of the binder.

[0026] [Figure 2A] 1 is a diagram showing the results of evaluating adhesive properties depending on the degree of modification of a click binder. [Figure 2B] 1 is a graph showing the results of evaluating adhesive properties depending on the content of a click binder. [Figure 2C] 1 is a diagram showing the evaluation results of adhesive properties by grafting functional groups other than carboxylic acid.

[0027] [Figure 3] 1 shows the initial (a) and 80th charge / discharge profiles (b) of a battery incorporating a click binder.

[0028] [Figure 4] 1A is a graph showing the change in absolute discharge capacity per weight of a battery incorporating a click binder; FIG. 1B is a graph showing the change in discharge capacity compared to the first cycle discharge capacity;

[0029] [Figure 5] Schematic diagram of the fabrication of a casting electrolyte layer is shown.

[0030] [Figure 6]1 is a diagram showing evaluation results of adhesive properties of a casting electrolyte layer.

[0031] [Figure 7A] A schematic diagram of epoxy binder modification is shown. [Figure 7B] 1 is a diagram showing the FT-IR results of the binder.

[0032] [Figure 8A] 1 is a diagram showing evaluation results of adhesive properties depending on the degree of maleic acid modification of an epoxy binder. [Figure 8B] 1 is a diagram showing evaluation results of adhesive properties depending on the degree of malonic acid modification of an epoxy binder. [Figure 8C] 1 is a diagram showing evaluation results of adhesive properties depending on the degree of succinic acid modification of an epoxy binder. [Figure 8D] 1 is a diagram showing evaluation results of adhesive properties depending on the degree of adipic acid modification of an epoxy binder.

[0033] [Figure 9] 1A is a graph showing the initial charge / discharge profile of a battery incorporating an epoxy binder, and FIG. 1B is a graph showing the change in absolute discharge capacity per unit weight.

[0034] [Figure 10] 1 is a graph showing the change in discharge capacity over time compared to the first cycle discharge capacity of a battery including an epoxy binder-introduced electrode and a cast electrolyte layer. DETAILED DESCRIPTION OF THE INVENTION

[0035] The present invention will now be described in detail with reference to the accompanying drawings. The following embodiments are provided as examples to fully convey the concept of the present invention to those skilled in the art. Therefore, the present invention is not limited to the following embodiments and may be embodied in other forms. In the drawings, the width, length, thickness, etc. of elements may be exaggerated for convenience. The same reference numerals refer to the same elements throughout the specification.

[0036] In the present invention, a binder for an all-solid-state lithium secondary battery prepared through a click reaction in which a double bond of a diene polymer is activated by a mercapto compound and a polar functional group is grafted is called a "click binder."

[0037] For example, in the present invention, the diene polymer is a butadiene polymer or an isoprene polymer, and a click binder having a molar ratio of polar functional groups to diene compounds in the diene polymer of 5:100 is called a "BRC5 binder."

[0038] In the present invention, the binder for an all-solid-state lithium secondary battery prepared through a click reaction in which double bonds of a diene polymer are activated by hydrogen peroxide, peroxy acid, or ozone, and polar functional groups are grafted is referred to as an "epoxy binder."

[0039] For example, in the present invention, the diene polymer is a butadiene polymer or an isoprene polymer, and the molar ratio of the polar functional group to the diene monomer in the diene polymer is 0.5:100. A binder modified with maleic acid is called a "BRCOOHme0.5 binder."

[0040] The higher the polarity of the binder, the better the adhesion between the active material and the current collector. However, it is difficult to find a solvent-binder combination that is easy to disperse when producing a sulfide-based solid electrolyte slurry. Therefore, it has been difficult to match the polarity between the solvent, binder, and sulfide electrolyte in a mass-scalable electrode manufacturing process.

[0041] To address this issue, the present invention provides a binder for all-solid-state lithium secondary batteries in which chains containing polar functional groups are grafted onto a diene-based polymer. The polar functional groups of the grafted chains bond with the electrode surface and the solid electrolyte with strong attraction based on their dipole moment. In this case, the strong attraction is 0.3D (Debye) to 5D. This has the advantage of improving the bonding properties with the current collector and the solid electrolyte while also providing easy dispersion.

[0042] In one embodiment of the present invention, the butadiene polymer among the diene-based polymers may exist in 1,2-vinyl, 1,4-cis, or 1,4-trans forms depending on the stereostructure of the double bonds, or may be a mixture of these stereostructures. For example, in the case of a butadiene polymer, the content of the butadiene polymer may be 30-40 mol% of 1,4-cis butadiene, 50-60 mol% of 1,4-trans butadiene, and 0-10 mol% of 1,2-vinyl butadiene, or 90-100 mol% of 1,4-cis butadiene and 0-10 mol% of 1,4-trans or 2-vinyl butadiene, but is not limited thereto.

[0043] In one embodiment of the present invention, the polar functional group is any one of a carboxyl group, a hydroxyl group, an ester group, an amine group, and salts thereof, and the polar functional group can form a hydrogen bond with the electrode surface. The molar ratio of the polar functional group compound to the diene polymer monomer is in the range of 0.1 to 30:100.

[0044] In the present invention, the binder is dissolved in an organic solvent such as n-hexane, toluene, p-xylene, butyl butyrate, tetrahydrofuran, diethyl carbonate, or a similar organic solvent. More specifically, one embodiment of the present invention provides a rubber-based binder that can be used to form a slurry of a sulfide-based solid electrolyte without modifying it, while also exhibiting improved binding strength, based on the most commonly used p-xylene solvent. One example of this binder is a rubber-based binder that is grafted with mercaptocarboxylic acid onto a butadiene polymer, a main-chain polymer that is widely used in secondary batteries. By adjusting the molar ratio of mercaptocarboxylic acid to butadiene monomer to 0.1 to 30:100, a binder for all-solid-state batteries was prepared that does not react with the sulfide-based solid electrolyte, has solubility in a nonpolar solvent (p-xylene), and exhibits adhesive strength greater than that of a commercially available LIB binder (PVDF).

[0045] In the present invention, the binder includes a polymer grafted with one or more compounds selected from the group consisting of mercapto, hydrogen peroxide, peroxy acid, ozone, halogen, silane, and sulfenylhydrogen, but is not limited to the above as long as it is a compound that modifies the double bond of the diene polymer. The method for modifying the double bond may be, but is not limited to, creating the double bond with epoxy and then modifying it. The halogen compound may include chlorine (Cl), bromine (Br), or iodine (I).

[0046] According to one aspect of the present invention, there is provided a binder for an all-solid-state lithium secondary battery, in which a carboxyl group of a mercaptocarboxylic acid is grafted onto a butadiene polymer.

[0047] In one embodiment of the present invention, the binder is obtained by grafting a carboxyl group of mercaptocarboxylic acid onto a butadiene polymer through a thiol-ene click reaction between a double bond of the butadiene polymer and a thiyl radical formed from mercaptocarboxylic acid.

[0048] The thiol-ene reaction used in the binder synthesis in this invention is a type of click reaction, which forms an alkyl sulfate from a thiol and an alkene using radicals generated by UV irradiation. A click reaction is a bonding reaction that connects two molecules through the interaction between the reactive groups of the two molecules. The click reaction is a well-controlled reaction that operates under mild conditions, is very fast, has a high yield, and is widely used in the synthesis of a variety of materials, including dendritic molecules, crosslinking materials, and modified compounds.

[0049] The epoxy reaction used in the binder synthesis in the present invention is a reaction in which an epoxy group is formed from a double bond using a peracid, for example, generated by the reaction of hydrogen peroxide with a carboxylic acid compound. The epoxy reaction converts a low-reactivity double bond into a reactive epoxy functional group, which can then react with a new compound to obtain a grafted polymer. The epoxy reaction is easy to handle and requires low-cost reactants, making it widely used industrially in the synthesis of various diene-based polymers.

[0050] FIG. 1A shows the synthesis process of a click binder using a click (thiol-ene) reaction between a butadiene polymer and a mercapto compound. The double bond of the butadiene polymer and a thiyl radical formed from mercaptocarboxylic acid undergo a thiol-ene click reaction, resulting in grafting of the carboxyl group of the mercaptocarboxylic acid onto the butadiene polymer.

[0051] In the present invention, butadiene polymers are used as the main chain of the binder because they have excellent mechanical properties and are suitable for binders for all-solid-state batteries that require mechanical properties and elastic properties due to their rubber-like properties.

[0052] In the present invention, mercaptocarboxylic acid is used for grafting the binder since it contains a carboxyl group capable of forming strong hydrogen bonds with the current collector and electrode material as well as a thiol group.

[0053] The polar functional group (e.g., carboxyl group) of the binder synthesized through the click reaction can form strong hydrogen bonds with OH groups present on the surface of the current collector and active material.

[0054] The binder of the present invention, in which the molar ratio of carboxyl groups to butadiene in the butadiene polymer is adjusted to 0.1 to 30:100, has been tested for solubility in seven solvents based on polarity, compatibility with sulfide-based solid electrolytes, adhesive properties when introduced into electrodes, and electrochemical stability, and has been confirmed to be suitable as a binder for all-solid-state lithium secondary batteries.

[0055] The binder of the present invention is dissolved in a sulfide-based solid electrolyte and an organic solvent such as n-hexane, toluene, p-xylene, butyl butyrate, tetrahydrofuran, diethyl carbonate, or an organic solvent having a similar structure to the above.

[0056] The binder of the present invention is compatible with sulfide-based solid electrolytes and has adhesive strength equal to or greater than that of PVDF binders.

[0057] Another aspect of the present invention provides an electrode composite for an all-solid-state lithium secondary battery, comprising the binder, a current collector, an electrode material, and a sulfide-based solid electrolyte. Also, yet another aspect of the present invention provides a separator for an all-solid-state lithium secondary battery, comprising the binder.

[0058] In one embodiment of the present invention, the carboxyl groups of the binder form strong hydrogen bonds with OH groups present on the surface of the current collector and electrode material.

[0059] In a preferred embodiment, the sulfide-based solid electrolyte is argyrodite Li6PS5X (X = Cl, Br, I). Argyrodite Li6PS5X (X = Cl, Br, I) has improved ionic conductivity and chemical stability due to the substitution of some sulfur atoms with halogen atoms. More preferably, Li6PS5Cl (LPSCl) can be used. LPSCl has high ionic conductivity (1.3 x 10 at room temperature). 3 S cm -1 ), low cost, and ease of manufacture further advantage.

[0060] In a preferred embodiment, the electrode material is LiCoO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiMn2O4, etc.

[0061] Another aspect of the present invention provides an all-solid-state lithium secondary battery including the electrode composite or separator. In one embodiment of the present invention, an all-solid-state lithium secondary battery (BRC5 ASSB) incorporating the binder (BRC5), which showed the highest adhesive strength with the NCM711 positive electrode and the lithium (Li) negative electrode, achieved an output of 169.9 mAh g -1 The BR ASSB containing the BR binder (control group) had a 170.7 mAh g -1 After 80 cycles, the BRC5 binder showed a discharge capacity of 81.1% and a capacity retention of 75.4%, respectively. The excellent electrochemical performance of the BRC5 binder is due to the strong adhesive strength of the BRC5 binder, which allows the electrode material interface to form a dense contact without cracks even after repeated cycling.

[0062]

[0063] Hereinafter, a binder for an all-solid-state lithium secondary battery according to the present invention will be described through preferred examples. However, these are presented as preferred examples of the present invention and should not be construed as limiting the present invention in any way. Contents not described herein can be sufficiently inferred by those skilled in the art, so a description thereof will be omitted.

[0064]

[0065] [Example]

[0066] Example 1: Preparation of Click Binder

[0067] Carboxyl groups were grafted onto 1,4-trans 55%, 1,4-cis 36%, 1,2-vinyl 9% polybutadiene through a thiol-ene click reaction (Figure 1A). For grafting the carboxyl groups, mercaptocarboxylic acid was used, which contains a carboxyl group capable of forming strong hydrogen bonds with the current collector and electrode material, along with a thiol group.

[0068] The synthesis mechanism is as follows:

[0069] The photoinitiator benzophenone was photolyzed with UV light to generate radicals. This radical was reacted with mercaptocarboxylic acid to extract the hydrogen atom from the thiol group of the mercaptocarboxylic acid, generating a new thiyl radical. The generated thiyl radical was then reacted with the double bond of polybutadiene, ultimately resulting in the addition of mercaptocarboxyl groups to the polybutadiene in ratios of 1:100, 3:100, 5:100, 10:100, and 15:100. (=mercaptocarboxyl group: butadiene (repeating unit of diene polymer)) The polymer binder was synthesized by grafting with a molar ratio of .

[0070] The degree of grafting of the carboxyl groups of the synthesized binder was qualitatively confirmed by FT-IR. As the molar ratio of mercaptocarboxyl groups used in the reaction increased in FT-IR analysis, the peak at 1710 cm after the click reaction was observed. -1 An increase in the intensity of the peak (corresponding to the carbonyl group peak of carboxylic acid) was observed (Figure 1B). This change in intensity after the click reaction indicates the successful synthesis of a click binder with a controlled degree of grafting of carboxyl groups within the binder.

[0071] To confirm the adhesive strength-enhancing effect of polar functional groups other than carboxyl groups, a polymer binder with grafted alcohol groups was synthesized using mercapto alcohol instead of mercapto carboxylic acid at a molar ratio of 5:100 (= mercapto alcohol group: butadiene) (hereinafter referred to as BRCOH5). The synthesis mechanism was the same as that used to synthesize BRC5 using mercapto carboxylic acid, except for the type of mercapto compound.

[0072]

[0073] Experimental Example 1: Analysis of adhesive properties of binder

[0074] In solid-state batteries, the volume of the active material and solid electrolyte changes during cycling, so the adhesive properties of the binder that connects the electrode materials are a very important factor. To confirm the adhesive properties of the click binder, an electrode containing 2.5 wt% of binder was fabricated and subjected to a 180° peeling test (Figure 2A).

[0075] BRC1, BRC3, BRC5, BRC10, BRC15 and butadiene electrodes were fabricated using p-xylene, which is used in all-solid-state battery slurry, and argyrodite (Li6PS5Cl, hereafter referred to as LPSCl) was used as the sulfide-based solid electrolyte. The composition and average peeling force of each electrode used in the peeling test are the same as in Table 1 below.

[0076]

[0077] [Table 1]

[0078] When the existing commercially available BR binder was used in the cathode composite (NCM711:LPSCl:Super P:BR binder=76:20:1.5:2.5) containing the solid electrolyte used in the actual cell operation, the yield was 1.58gf mm -1 On the other hand, in the case of click binders, even when the amount of carboxylic acid grafted to them was about 1:100 in terms of the molar ratio of carboxylic acid to BR, it was confirmed that the adhesive strength was more than 10 times higher than that of existing BR binders. It was also confirmed that the improvement in adhesive strength of BR binders due to carboxylic acid grafting reached saturation when the grafting amount exceeded 5:100.

[0079] Additionally, due to the high adhesive strength of the developed click binder, it was confirmed that the binder content within the electrode could be minimized (Figure 2B). Although the binder within the electrode contributes to improving processability, it reduces the content of active material within the electrode, which is detrimental to energy density. Therefore, if the binder content within the electrode can be reduced within a range that ensures processability, it can contribute to improving the energy density of the electrode. BRC5 was used as the evaluation binder, and the composition and average peeling force of each electrode used in the peeling test are the same as those in Table 2 below.

[0080] [Table 2]

[0081] As a result of the experiment, it was confirmed that even when the BRC5 content was reduced to 1.0 wt%, excellent adhesive performance of 12.17 gf / mm was achieved. This is more than seven times the performance of the existing BR binder, and it was confirmed that when using a click binder, not only is processability improved by improving the adhesive strength of the electrode, but the electrode energy density can also be improved by reducing the binder content in the electrode.

[0082] Additionally, to confirm the adhesive strength-enhancing effect of polar functional groups other than carboxylic acid, a BRCOH5 binder grafted with alcohol functional groups was synthesized and its adhesive performance was evaluated (Figure 2C). The composition and average peeling force of each electrode used in the peeling test are the same as those in Table 3 below.

[0083] [Table 3]

[0084] As a result of the experiment, it was confirmed that alcohol functional groups can also improve the adhesive properties of binders when grafted to the binder, which indicates that polar functional groups other than carboxylic acid can also contribute to improving the adhesive properties of binders when grafted to the binder.

[0085]

[0086] Experimental Example 2: Analysis of the electrochemical performance of the binder

[0087] The electrochemical performance of ASSBs using the click binder of the present invention was confirmed through ASSBs composed of an NCM711 cathode layer, an LPSCl solid electrolyte layer, and a Li counter electrode. The binders used to fabricate the ASSBs were BRC5 binder, which has the best adhesion, and BR binder for comparison.

[0088] The ASSB was fabricated by sequentially casting a cathode composite layer slurry in p-xylene solvent and a solid electrolyte (SE) slurry onto Al foil, with the composition of each layer being NCM711:LPSCl:super P:BRC5 binder (or BR binder) = 76:20:1.5:2.5. Between the casting of the cathode layer and the electrolyte layer, the cathode layer was vacuum dried at 60°C for 1 hour and at room temperature for 24 hours. Once the cathode composite layer, SE layer, and Li layer were formed, the cells were completed by pressing. The loading amount of 711 active material in the cathode of both cells was 20 mg cm. -2 is.

[0089] Figure 3 shows the results of the 0.1C (19.5mA g) test for BRC5 and BR binder ASSBs. -1 ) (range: 2.5–4.3 V vs. Li). The discharge capacities of the BRC5 ASSB and BR ASSB were 170.7 mAh g−1, respectively, in the first cycle. -1 , 169.9mAh g -1 It was confirmed that there was almost no difference in the charge-discharge profile due to overpotential (Figure 3(a)). This indicates that the increase in resistance within the electrode due to the carboxylic acid functional group of BRC5 is minimal, and it can be confirmed that the developed click binder does not have a negative impact on the electrochemical performance of the battery as a resistor. The charge-discharge profile at the 80th cycle confirmed that the discharge capacity of BRC5 ASSB was superior to that of BR ASSB (Figure 3(b)). After 80 cycles, BRC5 ASSB had a discharge capacity of 137.9 mAh g -1 while the BR ASSB showed a discharge capacity of 128.7mAh g after the same number of cycles. -1The discharge capacities of the first and second cycles were 81.1% and 75.4%, respectively, which correspond to capacity retention rates of 81.1% and 75.4%. The change in absolute discharge capacity with cycling (Figure 4(a)) and the change in capacity compared to the first cycle (Figure 4(b)) confirmed that the BRC5 binder has a positive effect on the battery's life characteristics. This result demonstrates the importance of binder adhesive strength, which connects electrode materials during repeated cycling of a casting-based all-solid-state battery.

[0090]

[0091] Experimental Example 3: Casting Electrolyte Layer in Binder

[0092] Unlike existing lithium-ion batteries, solid-state batteries do not require a separate polymer separator; instead, a separate solid electrolyte layer acts as the separator. Of the methods for creating a solid electrolyte separator layer with a thickness of less than several tens of micrometers, the most promising is to create a cast electrolyte layer by casting a slurry containing only a solid electrolyte and binder onto the solid-state battery electrode (Figure 5). The adhesive strength of the binder is important for the cast electrolyte layer, and the developed click binder can be used not only in solid-state battery electrodes but also as a binder for the cast electrolyte layer.

[0093] To confirm the improved adhesive properties of the cast electrolyte layer due to the binder of the present invention, an electrode containing a solid electrolyte was fabricated using a cathode composite composition (NCM711:LPSCl:Super P:BRC5 binder = 76:20:1.5:2.5). Films of 95 wt% LPSCl and 5 wt% LPSCl using p-xylene binder were then fabricated on top of the electrode. For comparison, experimental groups were prepared in which BR and BRC5 were added to the electrode and cast electrolyte layer, respectively, and a 180° peeling test was performed (Figure 6). The experimental groups are referred to as electrode binder and cast electrolyte layer binder, depending on the binder used in the electrode and cast electrolyte layer, respectively. The BR / BR and BRC5 / BR experimental groups, which used the BR binder in the cast electrolyte layer, demonstrated adhesive properties below the minimum measurement value of the 180° peeling test equipment (less than 0.1 gf / mm). This indicates that existing BR binders are unsuitable for use as binders in cast electrolyte layers for solid-state battery electrodes. In contrast, the BR / BRC5 and BRC5 / BRC5 experimental groups, in which BRC5 was used as the cast electrolyte layer, exhibited adhesive strengths of 2.23 gf / mm and 13.03 gf / mm, respectively. This result demonstrates the suitability of the developed BRC5 binder for use as a binder in cast electrolyte layers. The difference in adhesive strength between the BR / BRC5 and BRC5 / BRC5 experimental groups can be interpreted as the adhesive strength of the BR binder used as the electrode binder being weaker than that of the BRC5 used as the cast electrolyte layer binder, meaning that the adhesive properties of the BR / BRC5 experimental group are determined by the BR binder used in the electrode. This means that to ensure the overall mechanical properties of the bilayer film consisting of the electrode and cast electrolyte layer, binders with strong adhesive properties must be incorporated into all layers, demonstrating the versatility of the developed click binder.

[0094]

[0095] Example 2: Preparation of epoxy binder

[0096] In yet another embodiment of the present invention, a diene-based polymer is epoxidized and then reacted with a compound having a polar functional group, such as a carboxyl group, to graft the compound having a polar functional group to the diene-based polymer. While the present invention will be described in detail below through specific examples, the scope of the present invention is not limited thereto. Any polar functional compound that can react with the epoxy intermediate to form a grafted structure falls within the scope of the present invention. Epoxy functional groups were created in 1,4-trans 55%, 1,4-cis 36%, 1,2-vinyl 9% polybutadiene using peracid produced by reacting hydrogen peroxide with a carboxylic acid, and then carboxyl groups were grafted by reaction with a dicarboxylic acid (Figure 7A). For carboxyl group grafting, a dicarboxylic acid was used that contained a carboxyl group capable of forming strong hydrogen bonds with the current collector and electrode material on the opposite side of the carboxyl group that reacts with the epoxy functional group.

[0097] The synthesis mechanism is as follows:

[0098] Polybutadiene, hydrogen peroxide, and carboxylic acid were dissolved in chloroform and stirred at 40°C to generate epoxy functional groups. Reaction of the hydrogen peroxide with carboxylic acid produced peracid, which then reacted with the double bond of polybutadiene to ultimately synthesize polymer binders in which epoxy was grafted to polybutadiene at molar ratios of 0.5:100, 1:100, 1.5:100, 2:100, and 2.5:100 (epoxy:polybutadiene). The synthesized epoxy-only modified binders were referred to as BREPCx, based on the molar ratio of epoxy functional groups to the diene polymer used. For example, a binder in which the molar ratio of epoxy functional groups to the diene polymer was modified to 5:100 was referred to as "BREPC5 binder." The synthesized BREPC and a dicarboxylic acid compound were dissolved in tetrahydrofuran in a molar ratio of 1:10 (epoxy:dicarboxylic acid) and stirred at 60°C to synthesize a carboxylic acid-grafted polymer binder. The carboxylic acid on one side of the dicarboxylic acid compound reacted with the epoxy to graft the dicarboxylic acid compound onto the polybutadiene, while the carboxylic acid on the other side remained unreacted, providing a polar functional group that clung to the polybutadiene and provided strong binding strength to the current collector and electrode materials. To ensure that the carboxylic acid on one side of the dicarboxylic acid remained unreacted, the dicarboxylic acid compound was over-reacted with the epoxy in a molar ratio of 1:10. The dicarboxylic acid compounds used were maleic acid, malonic acid, succinic acid, and adipic acid. The synthesized epoxy binder is called BRCOOHxY (x = me (maleic acid), m (malonic acid), s (succinic acid), a (adipic acid), Y = molar ratio of polar functional groups relative to the diene polymer) depending on the type of dicarboxylic acid compound used and the molar ratio of polar functional groups in the diene polymer relative to the diene polymer. For example, a binder in which malonic acid is used as the dicarboxylic acid compound and the molar ratio of polar functional groups in the diene polymer relative to the diene polymer is modified to 1:100 is called a "BRCOOHm1 binder."

[0099] The degree of grafting of the carboxyl groups of the synthesized binder was qualitatively confirmed through FT-IR. FT-IR analysis showed that the 1700 cm -1 An increase in the peak intensity was observed (Figure 7B). This change in intensity after the epoxy reaction and dicarboxylic acid compound reaction indicates the successful synthesis of an epoxy binder with carboxyl groups grafted into the binder.

[0100]

[0101] Experimental Example 2: Analysis of adhesive properties of binder

[0102] In solid-state batteries, the volume of the active material and solid electrolyte changes during cycling, so the adhesive properties of the binder that connects the electrode materials are a very important factor. To confirm the adhesive properties of the epoxy binder, we fabricated electrodes containing 2.5 wt% of the binder and performed a 180° peeling test (Figures 8A, 8B, 8C, and 8D).

[0103] BRCOOHme0.5, BRCOOHm1,1.5,2,2.5, BRCOOHs1,1.5,2, BRCOOHa1,1.5,2,2.5 electrodes were fabricated using p-xylene, which is used in all-solid-state battery slurry, and argyrodite (Li6PS5Cl, hereafter referred to as LPSCl) was used as the sulfide-based solid electrolyte. The composition and average peeling force of each electrode used in the peeling test are the same as in Table 4 below.

[0104]

[0105] [Table 4]

[0106] In the case of epoxy binders, even when the molar ratio of carboxylic acid to BR was around 0.5:100, the adhesive strength was more than seven times higher than that of conventional BR binders. The adhesive strength improvement of BR binders due to carboxylic acid grafting was confirmed to saturate when the grafting ratio reached 1:100 or higher. This can be interpreted as the adhesive strength improvement saturating even with a relatively small modification of functional groups within the binder, because the epoxy functional groups were converted to alcohol functional groups during the carboxylic acid grafting modification, resulting in the grafting of additional alcohol functional groups other than carboxylic acid. Since the electrochemical resistance of binders is weakly correlated with the content of polar functional groups, improving adhesive strength through such small amounts of functional group modification is believed to be beneficial in terms of the resistance within the electrode where the binder is incorporated.

[0107]

[0108] Experimental Example 2: Analysis of the electrochemical performance of the binder

[0109] The electrochemical performance of ASSBs using the click binder of the present invention was confirmed through ASSBs composed of an NCM711 cathode layer, an LPSCl solid electrolyte layer, and a Li counter electrode. The binders used to fabricate the ASSBs were BRCOOHa2 and BRCOOH2.5.

[0110] The ASSB was fabricated by sequentially casting a cathode composite layer slurry in p-xylene solvent and a solid electrolyte (SE) slurry onto Al foil, with the composition of each layer being NCM711:LPSCl:super P:BRCOOHa2 binder (or BRCOOH2.5 binder) = 76:20:1.5:2.5. Between the casting of the cathode layer and the electrolyte layer, the cathode layer was vacuum dried at 60°C for 1 hour and at room temperature for 24 hours. Once the cathode composite layer, SE layer, and Li layer were formed, the cells were completed by pressing. The loading amount of 711 active material in the cathode of both cells was 20 mg cm.-2 is.

[0111] Figure 9(a) shows the results of the 0.1C (19.5 mA g) analysis of BRCOOHa2 and BRCOOHa2.5 binder ASSBs. -1 ) (range: 2.5–4.3 V vs. Li). The discharge capacities of both the BRCCOOHa2 ASSB and the BRCOOHa2.5 ASSB were 169.9 mAh g -1 It was confirmed that there was almost no difference in the charge-discharge profile due to overpotential (Fig. 9(a)). This indicates that the increase in the internal resistance of the electrode due to the increase in the content of carboxylic acid functional groups from BRCOOHa2 to BRCOOHa2.5 is minimal, and the first cycle discharge capacity (169.9 mAh g) was significantly higher than that of the BR ASSB. -1 ) and the difference in charge / discharge profile is minimal, confirming that the developed epoxy binder does not negatively affect the electrochemical performance of the battery as a resistance. After 30 cycles, the BRCOOHa2 ASSB achieved 155.3 mAh g -1 and the BRCOOH2.5 ASSB after the same number of cycles had a discharge capacity of 150.2mAh g -1 The BR ASSB showed a discharge capacity of 151.4 mAh g after 30 cycles, which corresponds to a capacity retention rate of 91.4% and 88.4%, respectively. -1 This corresponds to a capacity retention rate of 88.7%. Such excellent cycle characteristics of the epoxy binder demonstrate the importance of binder adhesive strength that connects electrode materials during repeated cycling of cast-based all-solid-state batteries.

[0112]

[0113] Experimental Example 3: Casting Electrolyte Layer in Binder

[0114] Unlike existing lithium-ion batteries, solid-state batteries do not require a separate polymer separator; instead, a separate solid electrolyte layer acts as the separator. Of the methods for creating a solid electrolyte separator layer with a thickness of less than several tens of micrometers, the most promising is to create a cast electrolyte layer by casting a slurry containing only a solid electrolyte and binder onto the solid-state battery electrode (Figure 5). The adhesive strength of the binder is important for a cast electrolyte layer, and the developed epoxy binder, like the click binder, can be used not only in solid-state battery electrodes but also as a binder for the cast electrolyte layer.

[0115] To confirm the effect of the cast electrolyte layer containing the epoxy binder developed in this invention on the electrochemical performance, an electrode with a cathode composite composition containing a solid electrolyte (NCM711:LPSCl:SuperP:BRCOOHs1 binder = 76:20:1.5:2.5) was fabricated. An LPSCl film with a p-xylene-based binder composition of 97.5 wt% LPSCl and 2.5 wt% LPSCl was then fabricated. This was then combined with an anode composite composition containing a solid electrolyte (Gr(graphite):LPSCl:BRCOOHs1 binder = 71:26.5:2.5) to fabricate an ASSB containing only the cast electrode and electrolyte layer. The loading of the 711 active material in the positive electrode of the cell was 20 mg cm. -2 The capacity ratio of the positive electrode to the negative electrode is 1:1.2.

[0116] Figure 10 shows the 0.1 C (19.5 mA g) of ASSBs containing BRCOOH binder. -1 ) (range: 2.5 to 4.3 V vs. Li) shows the constant current charge / discharge profile. -1The battery exhibited a discharge capacity of 1000 mAh. This performance is similar to the discharge capacity of a lithium metal half-cell containing a powder solid electrolyte separator layer several hundred microns thick, indicating that the inclusion of a thin solid electrolyte separator layer containing the developed epoxy binder does not significantly adversely affect electrochemical performance. Furthermore, due to the inclusion of a thin solid electrolyte separator layer, the battery's volumetric energy density was improved compared to half-cells containing existing powder solid electrolyte separator layers. This demonstrates the need for the binder technology of the present invention to fabricate thin solid electrolyte separator layers in order to reach an industrially viable volumetric energy density.

[0117]

[0118] Although the preferred embodiments of the present invention have been illustrated and described above, it goes without saying that the present invention is not limited to the specific embodiments described above, and that various modifications may be made by those skilled in the art without departing from the gist of the present invention as claimed in the claims. However, such modifications should not be understood separately from the technical ideas and perspectives of the present invention.

Claims

1. A binder for an all-solid-state lithium secondary battery, in which a chain containing a polar functional group is grafted onto a diene polymer, the diene polymer is a homopolymer of butadiene or a homopolymer of isoprene that does not contain a repeating unit derived from an aromatic compound, the polar functional group has a strong attraction to the electrode or solid electrolyte based on a dipole moment; a ratio (%) of the number of moles of polar functional groups to the number of moles of monomers in the diene polymer of 0.1 to 30;

2. 2. The binder for an all-solid-state lithium secondary battery according to claim 1, wherein the polar functional group is any one selected from the group consisting of a carboxyl group, a hydroxyl group, an ester group, an amine group, and salts thereof.

3. 2. The binder for an all-solid-state lithium secondary battery according to claim 1, wherein the binder comprises a polymer in which the diene-based polymer is grafted with one or more compounds selected from the group consisting of mercapto, hydrogen peroxide, peroxy acid, ozone, halogen, silane, and sulfenylhydrogen.

4. The binder for an all-solid-state lithium secondary battery according to claim 3 , wherein the binder is obtained by grafting a polar functional group of a mercapto compound onto the diene-based polymer.

5. The binder for an all-solid-state lithium secondary battery according to claim 4 , wherein the mercapto compound is grafted to the diene-based polymer via a thiol group.

6. The binder for an all-solid-state lithium secondary battery according to claim 1 , wherein the binder is obtained by grafting a polar functional group of a carboxylic acid compound onto the diene-based polymer.

7. The binder for an all-solid-state lithium secondary battery according to claim 6 , wherein the carboxylic acid compound is grafted to the diene-based polymer through an ester group.

8. 8. The binder for an all-solid-state lithium secondary battery according to claim 7, wherein the carboxylic acid compound has a hydroxyl group bonded to another carbon adjacent to the carbon of the grafted diene-based polymer.

9. 2. The binder for an all-solid-state lithium secondary battery according to claim 1, wherein the polar functional group of the binder forms a hydrogen bond with an OH group present on the surface of the current collector and the active material.

10. 2. The binder for an all-solid-state lithium secondary battery according to claim 1, wherein the binder is dissolved in one or more organic solvents selected from the group consisting of n-hexane, toluene, p-xylene, butyl butyrate, tetrahydrofuran, and diethyl carbonate.

11. An all-solid-state lithium secondary battery electrode composite comprising the binder according to claim 1 , a current collector, an electrode material, and a sulfide-based solid electrolyte.

12. The electrode composite for an all-solid-state lithium secondary battery according to claim 11, wherein the polar functional group of the binder forms a hydrogen bond with an OH group present on the surface of the current collector and the electrode material.

13. An all-solid-state lithium secondary battery comprising the electrode composite according to claim 12.

14. A separator for an all-solid-state lithium secondary battery, comprising the binder according to any one of claims 1 to 10.

15. An all-solid-state lithium secondary battery comprising the separator according to claim 14.

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