Positive electrode binder for lithium secondary battery, lithium secondary battery positive electrode including the same, and lithium secondary battery

Abstract

The present invention discloses a positive electrode binder for a lithium secondary battery, a positive electrode for a lithium secondary battery, and a lithium secondary battery including the same. The positive electrode binder is composed entirely of cationic monomers, thereby capturing polysulfide generated during charging and discharging of the battery and improving the performance of the battery. The positive electrode binder for a lithium secondary battery includes a structural unit derived from a cationic (meth)acrylate monomer containing one or more cations.

Description

Technical Field

[0001] This application claims the benefit of priority of Korean Patent Application No. 10-2020-0100226, filed on Aug. 11, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a cathode binder for a lithium secondary battery, a cathode for a lithium secondary battery, and a lithium secondary battery including the same, and more particularly, to a cathode binder for a lithium secondary battery, a cathode for a lithium secondary battery, and a lithium secondary battery including the same, the cathode binder being composed entirely of cationic monomers, thereby capturing polysulfide generated during charging and discharging of the battery and improving the performance of the battery. Background Art

[0003] As the interest in energy storage technology increases and the application of energy storage technology expands to mobile phones, tablet computers, laptop computers, and video cameras, and further expands to electric vehicles (EVs) and hybrid electric vehicles (HEVs), research and development of electrochemical devices are gradually increasing. Electrochemical devices are the most concerned field in this regard, and among them, the development of secondary batteries such as lithium-sulfur batteries that can be charged and discharged has become the focus of attention. In recent years, in order to increase the energy density when developing such batteries, research and development have been conducted on the design of new electrodes and batteries.

[0004] Among these electrochemical devices, a lithium-sulfur battery (Li-S battery) using lithium metal as the negative electrode and sulfur as the positive electrode has a higher theoretical capacity and energy density (usually about 2500 Wh / kg) than conventional lithium-ion batteries, and since it uses sulfur, which is not only easily available from nature but also inexpensive, as the positive electrode, it is also cost-effective, and thus has attracted much attention as a next-generation secondary battery that can replace lithium-ion batteries. In such a lithium-sulfur battery, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur during the discharge process. At this time, sulfur forms polysulfide (LiPS) having a linear structure from S8 having a cyclic structure. This lithium-sulfur battery is characterized by showing a stepwise discharge voltage before the polysulfide is completely reduced to Li2S.

[0005] However, the biggest obstacle to the commercialization of lithium-sulfur batteries is that in a battery using a sulfur-based compound as the positive electrode active material and an alkali metal such as lithium as the negative electrode active material, a large volume change (˜80%) occurs during charging and discharging, and polysulfide (LiPS, Li2S x ) dissolution and shuttle phenomenon occur during the charging and discharging process. In other words, the biggest problem of lithium-sulfur batteries is that polysulfide generated at the positive electrode during charging and discharging dissolves into the liquid electrolyte, resulting in irreversible capacity reduction and side reactions at the negative electrode.

[0006] More specifically, polysulfide lithium generated when sulfur used as the positive electrode is reduced during discharge has a high solubility and a small size, especially in an ether-based liquid electrolyte. As a result, it can pass through the separator. When it encounters lithium metal used as the negative electrode, it causes side reactions, leading to problems with unstable interfaces. Consequently, the capacity decreases due to the irreversible loss of the positive electrode active material, and the battery life is shortened because sulfur particles deposit on the surface of the lithium metal due to side reactions. Therefore, a technology is needed to prevent polysulfide lithium generated from the positive electrode during battery operation from dissolving into the liquid electrolyte. Summary of the Invention

[0007] [Technical Problem]

[0008] Accordingly, an object of the present invention is to provide a positive electrode binder for a lithium secondary battery, a positive electrode for a lithium secondary battery, and a lithium secondary battery including the same. The positive electrode binder is composed entirely of cationic monomers, thereby capturing polysulfide lithium generated during charging and discharging of the battery and improving the battery performance.

[0009] [Technical Solution]

[0010] To achieve the above object, the present invention provides a positive electrode binder for a lithium secondary battery, which includes a structural unit derived from a cationic (meth)acrylate monomer containing one or more cations.

[0011] In addition, the present invention provides a positive electrode for a lithium secondary battery, which includes the positive electrode binder and a positive electrode active material.

[0012] In addition, the present invention provides a lithium secondary battery, which includes: a positive electrode for a lithium secondary battery; a lithium metal negative electrode; an electrolyte interposed between the positive electrode and the negative electrode; and a separator.

[0013] [Advantageous Effects]

[0014] According to the positive electrode binder for a lithium secondary battery, the positive electrode for a lithium secondary battery, and the lithium secondary battery including the same of the present invention, the present invention has the advantage of being composed entirely of cationic monomers, thereby capturing polysulfide lithium generated during charging and discharging of the battery and improving the battery performance. Description of the Drawings

[0015] Figure 1 and Figure 2 is a reaction route diagram showing a method for preparing a positive electrode binder for a lithium secondary battery according to an embodiment of the present invention.

[0016] Figure 3 is a diagram showing a cationic monomer prepared by a nucleophilic substitution reaction according to an embodiment of the present invention1 Graph of the \(^1H\) NMR analysis results.

[0017] Figure 4 It is a graph showing the cationic monomer prepared by an ion exchange reaction according to an embodiment of the present invention. 13 Graph of the \(^{13}C\) NMR analysis results.

[0018] Figure 5 It is a graph showing the cationic monomer prepared by a nucleophilic substitution reaction according to an embodiment of the present invention. 1 Graph of the \(^1H\) NMR analysis results.

[0019] Figure 6 It is a graph showing the cationic monomer prepared by an ion exchange reaction according to an embodiment of the present invention. 13 Graph of the \(^{13}C\) NMR analysis results.

[0020] Figure 7 It is a graph comparing the discharge capacity and life characteristics of the lithium-sulfur battery according to an embodiment of the present invention and a comparative example.

[0021] Figure 8 It is a capacity-voltage graph of the lithium-sulfur battery according to an embodiment of the present invention.

[0022] Figure 9 It is a capacity-voltage graph of the lithium-sulfur battery in which a conventional binder is applied to the positive electrode. Detailed Description

[0023] As described above, in a lithium secondary battery, particularly a lithium-sulfur battery, polysulfide generated when sulfur used as a positive electrode is reduced during discharge has a high solubility in an ether-based liquid electrolyte, and can pass through a separator due to its small size, and when it comes into contact with lithium metal used as a negative electrode, a side reaction is caused, resulting in problems such as unstable interfaces. As a result, the capacity decreases due to irreversible loss of the positive electrode active material, and the life of the battery is shortened because sulfur particles are deposited on the surface of lithium metal due to side reactions. Therefore, a technique capable of preventing polysulfide generated from the positive electrode during battery operation from dissolving into the liquid electrolyte is required. Therefore, the applicant of the present application invented a positive electrode binder for a lithium secondary battery containing only a cationic monomer, a positive electrode for a lithium secondary battery containing the same, and a lithium secondary battery.

[0024] That is, since lithium polysulfide is mainly composed of lithium cations and polysulfide anions, it can easily interact with polar functional groups. In particular, it is known that the interaction with positively charged functional groups is strong. Therefore, when a positive charge or cation is introduced into the binder, there is an advantage that it can capture lithium polysulfide generated during charging and discharging, thereby improving battery performance. In addition, when using a binder having a crosslinked structure, there is an advantage that it can reduce the volume change occurring during charging and discharging, thereby contributing to maintaining the internal structure of the positive electrode (i.e., providing structural stability).

[0025] Hereinafter, the positive electrode binder for a lithium secondary battery, the positive electrode for a lithium secondary battery containing the same, and the lithium secondary battery according to the present invention will be described in detail.

[0026] Positive electrode binder

[0027] The positive electrode binder for a lithium secondary battery according to the present invention contains only cationic monomers, and specifically may contain a structural unit derived from a cationic (meth)acrylate monomer containing one or more cations. In this case, the cation may be at least one selected from a nitrogen cation, an oxygen cation, and a sulfur cation, but preferably, the cation mainly contains a nitrogen cation having an optimal LiPS adsorption degree or the like. On the other hand, (meth)acrylate means acrylate or methacrylate.

[0028] In a lithium secondary battery (specifically, a lithium-sulfur battery), the positive electrode binder is a component that helps the binding of the positive electrode active material, the conductive material, etc. and the binding to the current collector, and it has been reported that a binder in the form of adding some cationic compounds is used. However, in this case, the problems caused by lithium polysulfide cannot be fundamentally solved. Therefore, the applicant of the present invention has developed a positive electrode binder for a lithium secondary battery in the form completely composed of cationic monomers as in the present invention. In this case, since its cation concentration is very high compared with the existing binder, it can capture and adsorb lithium polysulfide more effectively.

[0029] More specifically, the cationic (meth)acrylate monomer may contain any one or more of a cationic monomer represented by Formula 1 and a cationic monomer having a crosslinked form represented by Formula 2, and preferably may contain a cationic monomer having a crosslinked form represented by Formula 2 above. It may be more preferably to contain both the cationic monomer represented by Formula 1 and the cationic monomer having a crosslinked form represented by Formula 2 at the same time:

[0030] [Formula 1]

[0031]

[0032] wherein R1, R2, and R3 are each independently hydrogen or an alkyl group having 1 to 4 carbon atoms, preferably each is a methyl group, X is a halogen group (F, Cl, Br, or I) or bis(trifluoromethanesulfonyl)imide (TFSI), and m and n are each independently an integer from 0 to 4,

[0033] [Formula 2]

[0034]

[0035] wherein R4, R5, R6, R7, R8, and R9 are each independently hydrogen or an alkyl group having 1 to 4 carbon atoms, preferably each is a methyl group, X is a halogen group or bis(trifluoromethanesulfonyl)imide (TFSI), o and q are each independently an integer from 0 to 4, and p is an integer from 0 to 8. In this case, p can preferably be an integer from 2 to 4.

[0036] On the other hand, when the cationic (meth)acrylate monomer contains the cationic monomer represented by Formula 1 and the cationic monomer having a crosslinked form represented by Formula 2, the cationic monomer represented by Formula 1 and the cationic monomer having a crosslinked form represented by Formula 2 can be contained in a weight ratio of 0.5 to 2:2 to 0.5.

[0037] In addition, the positive electrode binder can contain a solvent. In this case, relative to the total weight of the binder, the positive electrode binder can contain 5 to 15% by weight of a structural unit derived from a cationic (meth)acrylate monomer containing more than one cation and 85 to 95% by weight of a solvent. In addition, within the limit that does not impair the object of the present invention, the positive electrode binder can also contain conventional additives contained in conventional positive electrode binders for lithium secondary batteries.

[0038] Examples of the cationic monomer represented by Formula 1 can include, but are not limited to, the cationic monomer represented by the following Formula 1a and the cationic monomer represented by the following Formula 1b:

[0039] [Formula 1a]

[0040]

[0041] [Formula 1b]

[0042]

[0043] In addition, examples of the cationic monomer represented by Formula 2 can include, but are not limited to, the cationic monomer having a crosslinked form represented by the following Formula 2a and the cationic monomer having a crosslinked form represented by the following Formula 2b:

[0044] [Formula 2a]

[0045]

[0046] [Formula 2b]

[0047]

[0048] On the other hand, as described above, the positive electrode binder for a lithium secondary battery of the present invention may contain at least one of a cationic monomer represented by Formula 1 and a cationic monomer having a crosslinked form represented by Formula 2. For example, the positive electrode binder for a lithium secondary battery of the present invention may contain a cationic monomer represented by Formula 1a and a cationic monomer represented by Formula 2a, may contain a cationic monomer represented by Formula 1b and a cationic monomer represented by Formula 2a, and may contain all cationic monomers represented by Formula 1a, 1b, 2a, 2b, etc. As long as at least one cationic monomer of the present invention is included, the cationic monomers may be used alone or in combination without any particular limitation.

[0049] The above positive electrode binder for a lithium secondary battery of the present invention is prepared as follows: in the presence of a solvent, a (meth)acrylate compound containing at least one of nitrogen, oxygen, and sulfur is subjected to a nucleophilic substitution reaction (S N 2 reaction) with a haloalkyl compound to produce a cationic monomer containing a halogen anion as a counter ion of the cation. After the reaction is completed, a step of additionally removing the solvent and washing the reaction product to purify unreacted substances may be performed (in the case of this preparation method, a cationic monomer represented by Formula 1a or a cationic monomer represented by Formula 2a may be prepared).

[0050] Examples of the (meth)acrylate compound containing at least one of nitrogen, oxygen, and sulfur may be 2-(dimethylamino)ethyl methacrylate, 2-(dimethylamino)ethyl 1-vinylimidazole acrylate, 2-(hydroxymethyl)-2-(dimethylamino)ethyl acrylate, etc., but any (meth)acrylate compound containing at least one of nitrogen, oxygen, and sulfur may be used without any particular limitation. In addition, examples of the haloalkyl compound may be common haloalkyl compounds such as iodoethane and 1,4-dibromobutane.

[0051] Relative to 1 equivalent of the (meth)acrylate compound containing at least one of nitrogen, oxygen, and sulfur, the amount of the haloalkyl compound may be 0.5 to 1 equivalent. In addition, the nucleophilic substitution reaction (S N 2 reaction) may be carried out at 30 to 70 °C for 12 to 24 hours, the solvent may be removed by a method commonly used in the art such as a rotary evaporator method, and the washing may be carried out using an organic solvent such as ether.

[0052] On the other hand, when the amount of the haloalkyl compound is 0.5 equivalents relative to 1 equivalent of the (meth)acrylate compound, the cationic monomer can be prepared in a crosslinked form (in this case, the cationic monomer represented by Formula 2a or the cationic monomer represented by Formula 2b can be prepared).

[0053] On the other hand, in order to perform ion exchange with bis(trifluoromethanesulfonyl)imide (TFSI) anions that have fewer side reactions during battery operation and are more easily dissociated from cations, after the reaction is terminated, it may further include adding a lithium salt (LiTFSI) to the cationic monomer containing a halogen anion to initiate an ion exchange reaction, thereby preparing a step of a cationic monomer containing bis(trifluoromethanesulfonyl)imide (TFSI) anions as counterions of the cation. In addition, if necessary, it may further include a step of purifying unreacted substances (according to this preparation method, the cationic monomer represented by Formula 1b or the cationic monomer represented by Formula 2b can be prepared, and if the amount of the haloalkyl compound is 0.5 equivalents relative to 1 equivalent of the (meth)acrylate compound, only the cationic monomer represented by Formula 2b can be prepared).

[0054] The ion exchange reaction can be carried out at room temperature for 12 to 24 hours, and the purification after the ion exchange reaction can be carried out by dissolving the product precipitated after the ion exchange reaction in an organic solvent such as tetrahydrofuran and then precipitating it in purified water (deionized (DI) water).

[0055] On the other hand, Figure 1 and Figure 2 is a reaction roadmap showing a method for manufacturing a positive electrode binder for a lithium secondary battery according to an embodiment of the present invention. It can be seen from Figure 1 that the cationic monomer represented by Formula 1a and the cationic monomer represented by Formula 1b are sequentially prepared, and it can be seen from Figure 2 that the cationic monomer represented by Formula 2a and the cationic monomer represented by Formula 2b are sequentially prepared.

[0056] Positive electrode for lithium secondary battery

[0057] The positive electrode for a lithium secondary battery according to the present invention includes the above positive electrode binder and a positive electrode active material.

[0058] This positive electrode for a lithium secondary battery of the present invention can exhibit excellent battery performance due to the electrostatic attraction between the cations contained in the positive electrode binder and polysulfide, and in particular, improves the positive electrode containing a PVDF binder commonly used in lithium secondary batteries such as conventional lithium-sulfur batteries.

[0059] That is, if the positive electrode of a lithium-sulfur battery contains the positive electrode binder of the present invention, the volume change of sulfur during charging and discharging of the lithium-sulfur battery can be suppressed. In particular, it can improve the battery performance by capturing and adsorbing polysulfide lithium to prevent the polysulfide lithium generated in the positive electrode during battery operation from dissolving into the liquid electrolyte.

[0060] In the positive electrode for a lithium secondary battery, based on 100 parts by weight of the total weight of the positive electrode, the content of the positive electrode binder can be 1 to 50 parts by weight, preferably 3 to 15 parts by weight. If the content of the binder is less than 1 part by weight, the adhesion between the positive electrode active material and the current collector may be insufficient or the ability to retain polysulfide lithium may decrease. If the content of the binder exceeds 50 parts by weight, the adhesion is improved, but the content of the positive electrode active material is reduced by such an amount, thereby reducing the capacity of the battery.

[0061] The positive electrode active material preferably contains sulfur (S) atoms, and more preferably can be a sulfur-carbon composite material. Considering that since the electrical conductivity of sulfur is about 5.0×10 -14 S / cm, which is close to that of a non-conductor, it is not easy for an electrochemical reaction to occur at the electrode. Moreover, considering that since the overvoltage is very large, the actual discharge capacity and voltage are much lower than the theoretical values. Therefore, a carbon material with conductivity is used together (that is, a structure in which sulfur is loaded in the pores of the carbon material).

[0062] The sulfur contained in such a sulfur-carbon composite material can be at least one selected from the following: inorganic sulfur (S8), Li2S n (n≥1), organic sulfur compounds, and carbon-sulfur polymers [(C2S x ) n : x = 2.5 to 50, n≥2], and inorganic sulfur (S8) can be preferably applied. In addition, the carbon material constituting the sulfur-carbon composite material can be applied without any particular limitation as long as it has a porous structure or a high specific surface area and is commonly used in the art. For example, the carbon material with a porous structure can be but not limited to at least one selected from the following: graphite; graphene; carbon blacks such as Degussa black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); and activated carbon, and its shape can be used without limitation as long as it is in the form of a ball, rod, needle, plate, tube, or block and is commonly used in lithium secondary batteries.

[0063] The sulfur-carbon composite material can have a particle size of 10 to 50 μm. If the particle size of the sulfur-carbon composite material is less than 10 μm, there is a problem that the resistance between particles increases and overvoltage is generated in the electrode of the lithium-sulfur battery. If the particle size exceeds 50 μm, the surface area per unit weight decreases, so the wetting area with the electrolyte and the reaction sites with lithium ions in the electrode decrease, and the amount of electron transfer relative to the size of the composite material decreases, which may delay the reaction. As a result, the discharge capacity of the battery may decrease.

[0064] On the other hand, the positive electrode for a lithium secondary battery may further contain a conductive material.

[0065] There is no particular limitation on the conductive material as long as it does not cause side reactions in the internal environment of the lithium secondary battery and has excellent conductivity without causing chemical changes in the battery. The conductive material can generally be graphite or conductive carbon, and for example, it can be but is not limited to one selected from the following: graphite such as natural graphite or artificial graphite; carbon black-based materials such as carbon black, acetylene black, Ketjen black, Denka black, thermal cracking carbon black, channel carbon black, furnace black, lamp black, and Sumitomo black; carbon-based materials with a crystal structure of graphene or graphite; conductive fibers such as carbon fibers and metal fibers; fluorinated carbon; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive oxides such as titanium oxide; conductive polymers such as polyphenylene derivatives; and mixtures of two or more thereof.

[0066] Based on the total weight of the positive electrode material containing the positive electrode active material of 100 parts by weight, the addition amount of the conductive material is generally 0.5 to 50 parts by weight, preferably 1 to 30 parts by weight. If the content of the conductive material is too low, that is, if it is less than 0.5 parts by weight, it is difficult to obtain the effect of improving conductivity, or the electrochemical characteristics of the battery may deteriorate. If the content of the conductive material exceeds 50 parts by weight, that is, if it is too much, the amount of the positive electrode active material is relatively small, and thus the capacity and energy density may decrease. There is no particular limitation on the method of incorporating the conductive material into the positive electrode material, and conventional methods known in the relevant field such as coating on the positive electrode active material can be used. In addition, if necessary, adding a second coating having conductivity to the positive electrode active material can replace the addition of the conductive material as described above.

[0067] A filler can be selectively added to the positive electrode of the present invention as a component for suppressing the swelling of the positive electrode. There is no particular limitation on such a filler as long as it can suppress the swelling of the electrode without causing chemical changes in the battery, and examples thereof can include: olefin polymers such as polyethylene and polypropylene; fiber materials such as glass fibers and carbon fibers.

[0068] The positive current collector can be, but is not limited to: platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO2), FTO (F-doped SnO2), or their alloys; or aluminum (Al) or stainless steel with a surface treated with carbon (C), nickel (Ni), titanium (Ti), silver (Ag), etc. The shape of the positive current collector can be in the form of foil, film, sheet, stamping, porous body, foam, etc.

[0069] Lithium secondary battery

[0070] In addition, the present invention provides a lithium secondary battery, which includes a positive electrode for a lithium secondary battery, a lithium metal negative electrode, an electrolyte disposed between the positive electrode and the negative electrode, and a separator, and the lithium secondary battery is preferably a lithium-sulfur battery.

[0071] Generally, a lithium secondary battery is composed of the following components: a positive electrode, which is composed of a positive electrode material and a current collector; a negative electrode, which is composed of a negative electrode material and a current collector; and a separator, which blocks the electrical contact between the positive electrode and the negative electrode and allows the movement of lithium ions, and the lithium secondary battery includes an electrolyte solution that conducts lithium ions by being impregnated therein. The negative electrode can be manufactured according to conventional methods known in the art. For example, a negative electrode active material, a conductive material, a binder, and, if necessary, a filler, etc. are dispersed and mixed in a dispersion medium (solvent) to form a slurry, and the slurry can be coated on the negative current collector, and then dried and roll-pressed to prepare the negative electrode.

[0072] The negative electrode active material can be lithium metal or a lithium alloy (such as an alloy of lithium and a metal such as aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium). The negative current collector can be, but is not limited to: platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), copper (Cu), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO2), FTO (F-doped SnO2), or their alloys; or copper (Cu) or stainless steel with a surface treated with carbon (C), nickel (Ni), titanium (Ti), silver (Ag), etc. The shape of the negative current collector can be in the form of foil, film, sheet, stamping, porous body, foam, etc.

[0073] The separator is disposed between the positive electrode and the negative electrode, preventing short circuits therebetween and serving as a path for lithium ions. As the separator, olefin polymers such as polyethylene and polypropylene, glass fibers, etc. can be used in the form of sheets, multilayers, microporous membranes, woven fabrics, non-woven fabrics, etc., but the present invention is not limited thereto. On the other hand, if a solid electrolyte such as a polymer (e.g., an organic solid electrolyte, an inorganic solid electrolyte, etc.) is used as the electrolyte, the solid electrolyte can also act as a separator. Specifically, an insulating film with high ion permeability and mechanical strength is used. The pore diameter of the separator is generally in the range of 0.01 to 10 μm, and the thickness can generally be in the range of 5 to 300 μm.

[0074] As the electrolyte for the non-aqueous electrolyte (non-aqueous organic solvent), carbonates, esters, ethers, or ketones can be used alone or in combination of two or more thereof, but are not limited thereto. For example, aprotic organic solvents such as dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, methyl n-acetate, ethyl n-acetate, n-propyl acetate, triphosphate, dibutyl ether, N-methyl-2-pyrrolidone, 1,2-dimethoxyethane, tetrahydrofuran, tetrahydrofuran derivatives such as 2-methyltetrahydrofuran, dimethyl sulfoxide, formamide, dimethylformamide, dioxolane and its derivatives, acetonitrile, nitromethane, methyl formate, methyl acetate, trimethoxymethane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, methyl propionate, ethyl propionate, etc. can be used, but are not limited thereto.

[0075] A lithium salt can be added to the electrolyte (so-called non-aqueous electrolyte containing a lithium salt). The lithium salt can include but is not limited to lithium salts known to be advantageously soluble in non-aqueous electrolytes, such as LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiPF3(CF2CF3)3, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, or lithium imide, etc. In order to improve charge-discharge characteristics, flame retardancy, etc., the (non-aqueous) electrolyte can also contain pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (poly)ethylene glycol dimethyl ethers, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted Oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. If necessary, halogen-containing solvents such as carbon tetrachloride and trifluoroethylene can be further added to impart nonflammability, and carbon dioxide gas can also be added to improve the high-temperature storage characteristics.

[0076] The lithium secondary battery of the present invention can be manufactured by conventional methods in the art. For example, a lithium secondary battery can be manufactured by inserting a porous separator between a positive electrode and a negative electrode and introducing a non-aqueous electrolyte. The lithium secondary battery according to the present invention can be applied not only to a battery cell used as a power source for a small device, but also particularly suitable as a unit cell of a battery module used as a power source for a medium and large device. In this regard, the present invention also provides a battery module in which at least two lithium secondary batteries are electrically connected (in series or in parallel). Needless to say, considering the use and capacity of the battery module, the number of lithium secondary batteries included in the battery module can be variously adjusted.

[0077] In addition, the present invention provides a battery pack in which the battery modules are electrically connected according to conventional techniques in the art. The battery module and the battery pack can be used as a power source for at least one of the following medium and large devices: power tools; electric vehicles, including electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV); electric trucks; electric commercial vehicles; or power storage systems, but the present invention is not limited thereto.

[0078] Preferred Embodiments

[0079] Hereinafter, preferred examples of the present invention will be described to facilitate understanding of the present invention. However, it will be apparent to those skilled in the art that the following examples are only for illustrating the present invention, and various changes and variations can be made within the scope and spirit of the present invention, and such changes and variations are within the scope of the claims.

[0080] In the following examples, cationic monomers represented by Formulas 1a, 1b, 2a, and 2b were prepared, and for convenience, they will be described again below:

[0081]

[0082] [Example 1] Preparation of positive electrode binder for lithium secondary battery

[0083] First, 2-(dimethylamino)ethyl methacrylate was placed in an acetonitrile solvent, stirred at 45 °C, and then iodoethane was added dropwise, and the reaction (nucleophilic substitution reaction (S NThe reaction ((nucleophilic substitution reaction (S N 2 reaction)) was carried out at 45 °C for 18 hours. After the reaction was completed, the solvent was removed using a rotary evaporator, and the reaction product was washed with diethyl ether to purify the unreacted substances, thereby preparing a cationic monomer represented by Formula 1a.

[0084] Then, LiTFSI and the prepared cationic monomer of Formula 1a were separately dissolved in purified water (DI water), and then the solution containing LiTFSI was dropped into the solution containing the cationic monomer of Formula 1a, and then stirred at room temperature for 18 hours (ion exchange reaction), dissolved in MC, and extracted with purified water to purify the unreacted substances, thereby preparing a cationic monomer (positive electrode binder) represented by Formula 1b.

[0085] [Example 2] Preparation of positive electrode binder for lithium secondary battery

[0086] First, 2-(dimethylamino)ethyl methacrylate was placed in an acetonitrile solvent and stirred at 50 °C, and then 0.5 equivalent of 1,4-dibromobutane was dropped into it, and the reaction (nucleophilic substitution reaction (S N 2 reaction)) was carried out at 50 °C for 18 hours. After the reaction was completed, the solvent was removed using a rotary evaporator, and the reaction product was washed with diethyl ether to purify the unreacted substances, thereby preparing a cationic monomer in a crosslinked form represented by Formula 2a.

[0087] Then, LiTFSI and the prepared cationic monomer of Formula 2a were separately dissolved in purified water (DI water), and then the solution containing LiTFSI was dropped into the solution containing the cationic monomer of Formula 2a, and then stirred at room temperature for 18 hours (ion exchange reaction), and then the precipitated reaction product was dissolved in THF and precipitated 3 times in purified water to purify the unreacted substances, thereby preparing a cationic monomer in a crosslinked form represented by Formula 2b (positive electrode binder).

[0088] [Experimental Example 1] Chemical Structure Analysis of Positive Electrode Binder (Cationic Monomer)

[0089] NMR analysis was performed to confirm whether the cationic monomers prepared in Examples 1 and 2 were synthesized normally. Figure 3 And Figure 5 are the graphs showing the 1 1H NMR analysis results of the cationic monomer prepared by the nucleophilic substitution reaction according to the embodiments of the present invention, and Figure 4 and Figure 6 are the graphs showing the 13 13C NMR analysis results of the cationic monomer prepared by the ion exchange reaction according to the embodiments of the present invention. Figure 3 is the 1The figure of 1H NMR analysis Figure 4 is of the "cationic monomer represented by Formula 1b" prepared in Example 1 13 The figure of 13C NMR analysis Figure 5 is of the "cationic monomer with a crosslinked form represented by Formula 2a" prepared in Example 2 1 The figure of 1H NMR analysis, and Figure 6 is of the "cationic monomer with a crosslinked form represented by Formula 2b" prepared in Example 2 13 The figure of 13C NMR analysis

[0090] First, as a result of the NMR analysis to confirm whether the cationic monomer prepared in Example 1 was synthesized normally, as Figure 3 shown, the chemical structures of the cationic monomers (Formula 1a) prepared by nucleophilic substitution reaction were all shown, and by Figure 4 confirmation, the CF3 group contained in the TFSI anion was observed after ion exchange, thereby confirming that the cationic monomer (Formula 1b) prepared by ion exchange reaction was also synthesized normally

[0091] Next, as a result of the NMR analysis to confirm whether the cationic monomer prepared in Example 2 was synthesized normally, as Figure 5 shown, the chemical structures of the cationic monomers (Formula 2a) prepared by nucleophilic substitution reaction were all shown, and by Figure 6 confirmation, the CF3 group contained in the TFSI anion was observed after ion exchange, thereby confirming that the cationic monomer (Formula 2b) prepared by ion exchange reaction was also synthesized normally

[0092] [Example 3] Preparation of positive electrode for lithium secondary battery

[0093] A pre - binder solution was prepared by dissolving the positive electrode binder prepared in Example 1, the positive electrode binder prepared in Example 2, and 3 mol% of V - 65 (manufacturer: Wako Chemical) as a thermal initiator in NMP solvent. (※V - 65: 2,2'-azobis(2,4 - dimethylvaleronitrile). It is a thermal initiator with a lower starting temperature than AIBN, and because the sulfur positive electrode has the characteristic that heat treatment cannot be carried out at high temperatures, so the V - 65 is used as the initiator)

[0094] Then, the positive electrode active material (Ketjenblack: sulfur = 3:7 (weight%)), the conductive material (Super P), and the binder material were mixed at a mass ratio of 7:2:1 to prepare a positive electrode paste. Finally, the prepared positive electrode paste was coated on aluminum foil with a doctor blade and dried in a vacuum oven at 60 °C for 12 hours to prepare a positive electrode for a lithium secondary battery

[0095] [Comparative Example 1] Preparation of positive electrode for lithium secondary battery

[0096] A positive electrode paste was prepared by dissolving PVDF in an NMP solvent to prepare an adhesive solution, and then mixing a positive electrode active material (Ketjen black:sulfur = 3:7 (wt%)), a conductive material (Super P), and an adhesive material in a mass ratio of 7:2:1. Finally, the prepared positive electrode paste was coated on an aluminum foil with a doctor blade and dried in a vacuum oven at 60 °C for 12 hours to prepare a positive electrode for a lithium secondary battery.

[0097] [Example 4, Comparative Example 2] Preparation of lithium-sulfur battery

[0098] The positive electrodes prepared in Example 3 and Comparative Example 1 were placed facing a lithium metal negative electrode, and then a Celgard separator was inserted between the positive electrode and the negative electrode. Then, an electrolyte prepared by dissolving LiTFSI and LiNO3 in a DOL / DME solvent at concentrations of 1 M and 0.2 M, respectively, was injected into the case to prepare a coin cell type lithium-sulfur battery.

[0099] [Experimental Example 2] Evaluation of discharge capacity and life characteristics of lithium secondary battery

[0100] For the lithium-sulfur batteries manufactured in Example 4 and Comparative Example 2, the discharge capacity and life characteristics were evaluated by setting the current density to a 0.2-2C rate. Figure 7 is a graph comparing the discharge capacity and life characteristics of the lithium-sulfur batteries according to the examples and comparative examples of the present invention, Figure 8 is a capacity-voltage graph of the lithium-sulfur battery according to an example of the present invention, and Figure 9 is a capacity-voltage graph of the lithium-sulfur battery in which a conventional adhesive is applied to the positive electrode.

[0101] As a result of evaluating the discharge capacity and life characteristics of the lithium-sulfur batteries manufactured in Example 4 and Comparative Example 2 as described above, as Figure 7 shown, compared with the lithium-sulfur battery of Comparative Example 2 in which the conventional adhesive PVDF is used as the positive electrode adhesive, the lithium-sulfur battery of Example 4 in which the cationic monomer is used as the positive electrode adhesive shows more excellent discharge capacity and life characteristics.

[0102] Furthermore, by showing the Figure 8 and Figure 9It was confirmed that in the lithium-sulfur battery of Example 4 in which a cationic monomer was used as the positive electrode binder, the range of 2.1 V to 1.8 V where the reduction reaction from Li2S4 to Li2S occurred was longer. From this, it can be seen that when the cationic monomer of the present invention is used as the positive electrode binder, polysulfide lithium is fixed inside the positive electrode and an additional reaction occurs.

Claims

1. A cathode binder for a lithium secondary battery, the cathode binder comprising a structural unit derived from a cationic (meth)acrylate monomer containing one or more cations, wherein the cationic (meth)acrylate monomer comprises a cationic monomer having a crosslinked form represented by Formula 2: [Formula 2] wherein R4, R5, R6, R7, R8, and R9 are each independently hydrogen or an alkyl group having 1 to 4 carbon atoms, X is a halogen group or bis(trifluoromethanesulfonyl)imide (TFSI), o and q are each independently an integer from 0 to 4, and p is an integer from 0 to 8.

2. The cathode binder for a lithium secondary battery according to claim 1, wherein the cationic (meth)acrylate monomer further comprises a cationic monomer represented by Formula 1: [Formula 1] wherein R1, R2, and R3 are each independently hydrogen or an alkyl group having 1 to 4 carbon atoms, X is a halogen group or bis(trifluoromethanesulfonyl)imide (TFSI), and m and n are each independently an integer from 0 to 4.

3. The cathode binder for a lithium secondary battery according to claim 2, wherein the cationic (meth)acrylate monomer comprises the cationic monomer represented by Formula 1 and the cationic monomer having a crosslinked form represented by Formula 2 in a weight ratio of 0.5 to 2:2 to 0.

5.

4. The cathode binder for a lithium secondary battery according to claim 1, wherein, relative to the total weight of the binder, the cathode binder comprises: 5% to 15% by weight of a structural unit derived from a cationic (meth)acrylate monomer containing at least one cation, and 85 to 95% by weight of a solvent.

5. The cathode binder for a lithium secondary battery according to claim 2, wherein the cationic monomer represented by Formula 1 is a cationic monomer represented by Formula 1a or a cationic monomer represented by Formula 1b: [Formula 1a] [Formula 1b] 6. The cathode binder for a lithium secondary battery according to claim 1, wherein the cationic monomer having a crosslinked form represented by Formula 2 is a cationic monomer having a crosslinked form represented by Formula 2a or a cationic monomer having a crosslinked form represented by Formula 2b: [Formula 2a] [Formula 2b] 7. A cathode for a lithium secondary battery, the cathode comprising: the cathode binder according to claim 1; and a cathode active material.

8. The cathode for a lithium secondary battery according to claim 7, wherein the cathode active material is a sulfur-carbon composite material.

9. The cathode for a lithium secondary battery according to claim 7, wherein, relative to 100 parts by weight of the total weight of the cathode, the content of the cathode binder is 1 part by weight to 50 parts by weight.

10. A lithium secondary battery, the lithium secondary battery comprising: the cathode for a lithium secondary battery according to claim 7; a lithium metal anode; an electrolyte interposed between the cathode and the anode; and a separator.

11. The lithium secondary battery according to claim 10, wherein the lithium secondary battery is a lithium-sulfur battery.

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