Lithium-ion battery

Abstract

The present invention provides a lithium secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, the negative electrode active material layer includes a negative electrode active material, an aqueous binder, and a CMC-Li salt (lithium carboxymethyl cellulose salt), the electrolyte includes a lithium salt and an organic solvent, and the organic solvent includes diethyl carbonate.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0065484 dated May 27, 2022.

[0002] The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery that includes a negative electrode with excellent adhesive properties and has improved capacity retention rate and resistance increase rate. [Background technology]

[0003] With the technological advancements and increasing demand for mobile devices, the demand for rechargeable batteries as an energy source is rapidly increasing. Among these rechargeable batteries, lithium-ion batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0004] Conventional lithium-ion secondary batteries typically use graphite as the negative electrode active material, and charging and discharging occur through repeated insertion and removal of lithium ions from the positive electrode into the negative electrode. While the theoretical capacity of the battery varies depending on the type of electrode active material, a common problem is that the charging and discharging capacity decreases as the cycle progresses.

[0005] This phenomenon is caused by the volume change of the electrodes that occurs as the battery charges and discharges, which separates the electrode active materials or the electrode active materials from the current collector, causing the active materials to cease performing their function. Furthermore, repeated charging and discharging also contributes to the repeated formation and depletion of the SEI (Solid Electrolyte Interface) film on the negative electrode due to by-reaction chemicals.

[0006] Therefore, there is a need for research on binders and electrode materials that can prevent the detachment between electrode active materials or between the electrode active material and the current collector with strong adhesion during the manufacture of the electrode, control the volume expansion of the electrode active material generated during repeated charge and discharge, and improve the structural stability of the electrode and thereby the performance of the battery.

[0007] Since polyvinylidene fluoride (PVDF), a conventional organic solvent-based binder, does not meet the above requirements, recently, aqueous binders such as carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) have been widely used. CMC is used as a thickening agent from the perspective of the electrode slurry, but it is known to impart adhesive properties within the electrode. In particular, CMC-Na salt is known to have excellent electrode adhesion.

[0008] However, when the existing CMC-Na salt is selected as the electrode material, the capacity retention rate after long-term cycling did not reach a satisfactory level. Therefore, in the design of secondary batteries, it is actually necessary to develop a technology that minimizes the consumption of the organic solvent required for the formation of the SEI film and improves the capacity retention rate and the resistance increase rate of the battery with the increase in cycles. Summary of the Invention Problems to be Solved by the Invention

[0009] The present invention is for solving the problems of the prior art as described above, and an object thereof is to provide a lithium secondary battery having excellent resistance characteristics while improving the capacity retention rate after long-term cycling. Means for Solving the Problems

[0010] According to one embodiment of the present invention, a lithium secondary battery is provided comprising a positive electrode, a negative electrode, a separator membrane, and an electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprises a negative electrode active material, an aqueous binder, and a CMC-Li salt (lithium carboxymethyl cellulose salt), and the electrolyte comprises a lithium salt and an organic solvent, the organic solvent comprising diethyl carbonate.

[0011] In one embodiment of the present invention, the organic solvent may contain 10 vol% or more of the diethyl carbonate based on the total volume of the organic solvent.

[0012] In one embodiment of the present invention, the organic solvent is ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate (VC), fluoroethylene carbonate (FEC), dimethyl carbonate (dimethyl It may further contain one or more organic solvents selected from the group consisting of carbonate (DMC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate and ethyl propyl carbonate, dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0013] In one embodiment of the present invention, the organic solvent may further include one or more selected from the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0014] In one embodiment of the present invention, the CMC-Li salt may have 100 or fewer microgels per unit area (5 cm × 5 cm) of a thin film coated with a 1 wt% aqueous solution to a thickness of 200 μm.

[0015] In one embodiment of the present invention, the microgel may have a diameter of 200 μm or less.

[0016] In one embodiment of the present invention, the CMC-Li salt may have a degree of substitution of the hydroxy(-OH) group by the lithium carboxymethyl group (-CH2COOLi) of 0.7 to 1.3.

[0017] In one embodiment of the present invention, the CMC-Li salt may have a weight-average molecular weight (Mw) of 400,000 to 1,500,000.

[0018] In one embodiment of the present invention, the CMC-Li salt may have a solubility of 1.0 to 1.7 by weight at a measurement condition of 23°C.

[0019] In one embodiment of the present invention, the CMC-Li salt may be included in an amount of 10% by weight or less based on the total weight of the negative electrode active material layer.

[0020] In one embodiment of the present invention, the aqueous binder may be SBR (styrene-butadiene rubber).

[0021] In one embodiment of the present invention, the SBR may have a particle size of 90 nm to 500 nm.

[0022] In one embodiment of the present invention, the negative electrode active material may contain one or more carbon-based materials selected from the group consisting of graphite-based carbon, coke-based carbon, and hard carbon.

[0023] In one embodiment of the present invention, the negative electrode active material is Si, SiO

[0028] , , , ,

[0026] , , ,

[0027] , , , (0 < x ≤ 2), Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), and may further contain one or more silicon-based materials selected from the group consisting of.

[0024] In one embodiment of the present invention, the silicon-based material may be contained in an amount of 0.1 to 10 parts by weight based on the total weight of the negative electrode active material.

Advantages of the Invention

[0025] The lithium secondary battery according to the present invention has the effect that the CMC-Li salt contained in the negative electrode complements the lithium that forms the SEI film during battery formation, minimizing the consumption of the lithium salt of the electrolyte and the organic solvent necessary for the formation of the SEI film, and improving the life characteristics and the resistance increase rate.

[0026] In addition, the diethyl carbonate component contained as an organic solvent in the electrolyte has the effect of further doubling such an effect.

[0027] In addition, the lithium secondary battery according to the present invention has the effect of suppressing pinhole formation in the negative electrode and improving the adhesion of the negative electrode by containing a CMC-Li salt with a small number of microgel formations.

Modes for Carrying Out the Invention

[0028] Preferred embodiments of the present invention will now be described in detail. Before that, however, terms and words used herein and in the claims should not be interpreted to be limited to their ordinary or dictionary meanings, but rather to be interpreted as meanings and concepts consistent with the technical spirit of the present invention, based on the principle that an inventor may appropriately define the concepts of terms in order to best describe his own invention.

[0029] Therefore, the embodiments described herein and the configurations illustrated in the drawings represent only one of the most preferred embodiments of the present invention and do not represent the entire technical concept of the present invention. It should be understood that there are various equivalents and modifications that can substitute for them at the time of filing.

[0030] The terms used herein are for illustrative purposes only and are not intended to limit the invention. In this specification, the singular includes the plural unless otherwise specified in the text. The terms “comprises” and / or “comprising” as used in this specification do not preclude the presence or addition of one or more other components beyond those mentioned.

[0031] In this specification, D 50 This refers to the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50 This can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters in size, and can yield highly reproducible and high-resolution results.

[0032] In this specification, "specific surface area" is measured by the BET method, and specifically can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-mini II from BEL Japan.

[0033] In this specification, "weight-average molecular weight (Mw)" refers to the converted value relative to standard polystyrene measured by gel permeation chromatography (GPC). Specifically, the above weight-average molecular weight is a converted value obtained by measuring the value using GPC under the following conditions, and standard polystyrene from the Agilent system was used to prepare the calibration curve.

[0034] <Measurement conditions> Measurement instrument: Agilent GPC (Agilent 1200 series, USA) Column: PL Mixed B (2 concatenated columns) Column temperature: 40℃ Eluent: Tetrahydrofuran Flow rate: 1.0mL / min Concentration: ~1mg / mL (100μL injection)

[0035] This invention provides a lithium secondary battery.

[0036] As a result of diligent research and various experiments, the inventors of this application have confirmed that, as described later, including CMC-Li salt as a binder or thickener in the negative electrode and including diethyl carbonate as an essential component of the organic solvent in the electrolyte increases the capacity retention rate of the secondary battery and reduces the resistance increase rate, thus completing the present invention.

[0037] Accordingly, a lithium secondary battery according to one embodiment of the present invention comprises a positive electrode, a negative electrode, a separator membrane, and an electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprises a negative electrode active material, an aqueous binder, and a CMC-Li salt (lithium carboxymethyl cellulose salt), and the electrolyte comprises a lithium salt and an organic solvent, the organic solvent may include diethyl carbonate.

[0038] The lithium secondary battery of the present invention will be described in detail below.

[0039] <Negative electrode> A negative electrode according to one embodiment of the present invention includes a negative electrode active material layer on at least one surface of a negative electrode current collector. The negative electrode active material layer can be formed by coating the negative electrode current collector with a negative electrode slurry containing a negative electrode active material, an aqueous binder, a CMC-Li salt (lithium carboxymethyl cellulose salt), and a solvent, followed by drying and rolling. The solvent may be an aqueous solvent such as water.

[0040] First, let's explain CMC-Li salts.

[0041] Conventionally, CMC-Na salts containing sodium carboxymethyl groups (-CH2COONa) have been known to have excellent adhesive properties, and CMC-Na salts have been commonly used as binders or thickeners for negative electrodes. While CMC-Na salts do improve the overall performance of secondary batteries, they have not shown satisfactory levels in terms of lifespan and resistance characteristics.

[0042] The negative electrode according to the present invention uses a CMC-Li salt instead of a CMC-Na salt, and since the CMC-Li salt can complement the lithium component that constitutes the SEI film, it minimizes the consumption of lithium salt and organic solvent required for the formation of the SEI film, thereby increasing the capacity retention rate over long cycles of the secondary battery and reducing the resistance increase rate. Furthermore, this effect is doubled when the organic solvent of the electrolyte contains diethyl carbonate.

[0043] In one embodiment of the present invention, the CMC-Li salt is such that the number of microgels per unit area (5 cm × 5 cm) of a thin film coated with a 1 wt% aqueous solution to a thickness of 200 μm may be 100 or less, preferably 90 or less, and more preferably 80 or less.

[0044] The method for measuring the number of microgels described above involves adding CMC-Li salt at a concentration of 1 wt% to deionized water, then spreading the CMC-Li salt solution onto a glass slide to a thickness of 200 μm, and measuring the number of microgels (undissolved material) visible to the naked eye per unit area (5 cm × 5 cm). Specifically, microgels visible to the naked eye are defined as microgels with a diameter of 200 μm or less.

[0045] In the case of conventional CMC-Na salts, the number of microgels exceeds 100 under the same measurement conditions described above, and the greater the number of microgels, the lower the negative electrode adhesion strength may become.

[0046] In one embodiment of the present invention, the CMC-Li salt can be produced by adding LiOH and mono-chloroacetic acid (MCA) to cellulose (single step), or by adding CMC-Na salt to a mixed solution of ethanol and HCl, stirring and reacting to produce H-CMC, and then reacting H-CMC with LiOH to produce CMC-Li salt (multi-step process). Compared to the CMC-Li salt produced by the latter multi-step process, the CMC-Li salt produced by the former single-step process has the advantages of higher purity, superior adhesion improvement effect, and fewer microgels in the dispersion. In particular, the CMC-Li salt produced by the latter multi-step process is even more preferable as the CMC-Li salt of the present invention because the number of microgels measured according to the above-described measurement conditions is 70 or less, more specifically 60 or less.

[0047] In CMC-Li salts, the fewer microgels measured by the method described above, the more effectively aggregation of the negative electrode active material and conductive material can be prevented during the manufacturing process of the negative electrode, thereby suppressing the formation of pinholes in the manufactured negative electrode active material layer. This has the effect of suppressing the deterioration of battery performance caused by the formation of pinholes.

[0048] In one embodiment of the present invention, the CMC-Li salt may have a degree of substitution of the hydroxy(-OH) group by the lithium carboxymethyl group (-CH2COOLi) of 0.7 to 1.3, preferably 0.8 to 1.2.

[0049] In one embodiment of the present invention, the CMC-Li salt may have a weight-average molecular weight (Mw) of 400,000 to 1,500,000, preferably 450,000 to 1,350,000, and more preferably 500,000 to 1,200,000.

[0050] In one embodiment of the present invention, the CMC-Li salt may have a solubility of 1.0 to 1.7, preferably 1.1 to 1.6, and more preferably 1.2 to 1.5, based on the weight measured at 23°C. The solubility may be a percentage of the weight of the CMC-Li salt relative to the total weight of deionized water and the CMC-Li salt.

[0051] When using a CMC-Li salt that satisfies all of the above-mentioned degree of substitution, weight-average molecular weight, and solubility ranges, the CMC-Li salt in the negative electrode can be uniformly dispersed without agglomerating, improving adhesion within the negative electrode and potentially significantly improving the overall performance of the secondary battery, such as capacity and lifespan characteristics.

[0052] In one embodiment of the present invention, the pH of the CMC-Li salt may be 6.5 to 8.0, preferably 6.7 to 7.9, and more preferably 6.9 to 7.85.

[0053] The viscosity of the above CMC-Li salt can range from 2,000 cps to 12,000 cps under measurement conditions of 23°C and a B-type LV viscometer at 12 rpm.

[0054] The above-mentioned CMC-Li salt may be included in an amount of 10% by weight or less, preferably 0.3 to 5% by weight, and more preferably 0.5 to 1.5% by weight, based on the total weight of the negative electrode active material layer. When the CMC-Li salt is included in the above-mentioned amount, excellent rapid charging characteristics and energy density can be obtained.

[0055] The above aqueous binder can be SBR (styrene-butadiene rubber). Since SBR can be dispersed in water in the form of an emulsion as an aqueous binder, an organic solvent does not need to be used, the adhesive force is strong, and accordingly the content of the binder can be reduced and the content of the negative electrode active material can be increased, which is advantageous for increasing the capacity of the lithium secondary battery. In particular, when using the above CMC-Li salt together with SBR, the ratio of the negative electrode active material per unit volume of the negative electrode active material layer can be further increased. Therefore, while high capacity is possible, the electrode processability is improved due to an increase in the solid content of the negative electrode slurry, and there is an effect that the swelling phenomenon is improved.

[0056] When using the above CMC-Li salt as a thickener and using SBR having a particle size within a certain numerical range, the performance of the secondary battery can be further improved. In one specific example, the above SBR can have a particle size of 90 nm to 500 nm, and more specifically, can be 100 nm to 400 nm.

[0057] In one embodiment of the present invention, the CMC-Li salt and the aqueous binder can be contained in an amount of 1 to 30% by weight based on the total weight of the negative electrode active material layer.

[0058] Also, in one specific example, the content ratio of the above CMC-Li salt and the aqueous binder can be 1:5 to 2:1 by weight, more specifically, can be 1:3 to 3:2, and even more specifically, can be 2:3 to 1:1.

[0059] The above negative electrode active material can be used without limitation to the negative electrode active materials generally used in lithium secondary batteries. For example, carbon-based materials such as non-graphitizable carbon and graphite-based carbon; Li x Fe2O3 (0 ≦ x ≦ 1), Li x WO2 (0 ≦ x ≦ 1), Sn x Me 1-x Me’ y O zMetal composite oxides such as (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogens; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8), lithium metal, lithium alloys, silicon-based alloys, tin-based alloys, metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5, conductive polymers such as polyacetylene, Li-Co-Ni-based materials, etc. can be used.

[0060] However, not all negative electrode active materials show the same effect, and among the above negative electrode active materials, when a carbon-based material is included, the most excellent effect can be shown. Specifically, as the carbon-based material, both low-crystalline carbon and high-crystalline carbon can be used. As the high-crystalline carbon, natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and calcined carbon such as petroleum or coal tar pitch-derived cokes can be used. Specifically, graphite-based negative electrode active materials such as natural graphite and artificial graphite are more preferable in that reversible insertion and desorption of lithium ions are possible while maintaining their structural and electrical properties.

[0061] The above natural graphite can have a specific surface area of 1.5 m 2 / g to 8 m 2 / g, specifically 2.1 m 2 / g to 4.5 m 2 / g, more specifically 2.5 m 2 / g to 4 m 2It may have a specific surface area of ​​ / g. When the above-mentioned natural graphite has a specific surface area within the above range, it is possible to impart adhesive force to the negative electrode with a specific surface area above a certain level, while preventing an excessive increase in the initial irreversible capacity during charging and discharging due to the specific surface area.

[0062] Furthermore, the above-mentioned natural graphite may have a tap density of 0.9 g / cc to 1.3 g / cc, specifically 0.92 g / cc to 1.2 g / cc, and more specifically 0.95 g / cc to 1.15 g / cc. When the above-mentioned natural graphite has a tap density within the above range, it may have suitable negative electrode adhesion and excellent energy density.

[0063] In one embodiment of the present invention, the artificial graphite is 0.4m 2 / g~5.0m 2 It may have a specific surface area of ​​0.5 m² / g, specifically 0.5 m². 2 / g~4.0m 2 / g, more specifically 0.6m 2 / g~3.0m 2 It may have a specific surface area of ​​ / g. If the above artificial graphite has a specific surface area within the above range, it is possible to prevent an increase in the initial irreversible capacity during charging and discharging.

[0064] Furthermore, the above-mentioned artificial graphite may have a tap density of 0.7 g / cc to 1.1 g / cc, specifically 0.75 g / cc to 1.05 g / cc, and more specifically 0.8 g / cc to 1.0 g / cc. When the above-mentioned artificial graphite has a tap density within the above range, the negative electrode active material may have an excellent energy density.

[0065] In one embodiment of the present invention, the graphite-based negative electrode active material may have a degree of spheroidization of 0.8 or more, preferably 0.85 to 1, and more preferably 0.9 to 1.

[0066] The higher the degree of spheroidization of the graphite-based negative electrode active material, the better the binder adheres to the surface of the negative electrode active material, allowing for smooth dispersion without clumping or aggregation, thus improving the overall adhesion strength of the negative electrode.

[0067] The sphericity described above can be a value obtained by dividing the circumference of a circle with the same area as the projected image by the perimeter of the projected image when the graphite-based negative electrode active material is projected, and specifically, it can be represented by the following Mathematical Formula 1. The sphericity can be measured using a particle shape analyzer, for example, a particle shape analyzer such as Sysmex FPIA3000 manufactured by Malvern.

[0068] [Mathematical Formula 1] Sphericity = Circumference of a circle with the same area as the image of the graphite-based active material / Perimeter of the projected image

[0069] In a preferred embodiment, in addition to the carbon-based material, the negative electrode active material may further contain a silicon-based material. The silicon-based material is one or more materials selected from the group consisting of Si, SiO x (0 < x ≦ 2), Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si). The element Y can be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0070] The silicon-based material may be contained in an amount of 0.1 to 10 parts by weight, preferably 0.5 to 8 parts by weight, more preferably 1 to 6 parts by weight, based on the total weight of the negative electrode active material.

[0071] The particles of the silicon-based material may have an average particle size (D 50 ) of 1 μm to 30 μm, specifically, an average particle size (D 50 ) of 3 μm to 20 μm, more specifically, 4 μm to 10 μm can be obtained. The average particle size (D 50If the average particle size (D) is too small, the side reaction with the electrolyte will increase, which may reduce the lifespan performance. 50 If the size is too large, the volume expansion during charging and discharging will be large, which can cause particle cracking and thus reduce the battery life. Therefore, when the silicon-based material particles satisfy the above range, the side reactions with the electrolyte and the volume expansion of the silicon-based material particles can be maintained to an appropriate extent, and the battery containing them can exhibit excellent battery life characteristics.

[0072] On the other hand, the negative electrode active material layer may further selectively contain conductive materials and / or fillers and other additives in addition to the CMC-Li salt, aqueous binder and negative electrode active material.

[0073] The conductive material mentioned above is not particularly limited as long as it is conductive without inducing a chemical change in the battery, and may include, for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0074] The conductive material described above may typically be included in an amount of 1 to 30% by weight, based on the total weight of the first negative electrode active material layer or the second negative electrode active material layer.

[0075] The above-mentioned filler is used selectively as a component to suppress electrode expansion and is not particularly limited as long as it is a fibrous material that does not induce chemical changes in the battery. For example, olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers may be used.

[0076] The above-mentioned negative electrode current collector can be any metal that has high conductivity and to which an electrode slurry containing the negative electrode active material can easily adhere, and that is non-reactive within the battery voltage range. Specifically, copper, stainless steel, aluminum, nickel, titanium, calcined carbon; stainless steel surface-treated with carbon, nickel, titanium, or silver; aluminum-cadmium alloy; non-conductive polymer surface-treated with a conductive material; or conductive polymer can be used. Furthermore, fine irregularities can be formed on the surface to strengthen the bonding force of the negative electrode active material, and it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0077] The thickness of the negative electrode current collector in this invention may be the same in all cases within the range of 3 μm to 200 μm, but may have different values ​​depending on the circumstances.

[0078] In the negative electrode of the present invention, in which a negative electrode active material layer is formed on the negative electrode current collector described above, the loading amount of the negative electrode active material layer is 10 mg / cm³. 2 ~20 mg / cm³ 2 It could be 14 mg / cm³. 2 ~16 mg / cm³ 2 This is possible. In the case of conventional negative electrode mixtures containing CMC-Na salt, the maximum loading amount that can obtain industrially usable charging characteristics is approximately 13 mg / cm³. 2 The following was the case, but in the case of the negative electrode containing the CMC-Li salt of the present invention, the maximum was 16 mg / cm³. 2 It exhibits excellent capacitance retention and resistance characteristics even with a large load.

[0079] The above-mentioned negative electrode active material layer may have a thickness of 200 μm or less, specifically a thickness of 20 μm to 180 μm, and more specifically, a thickness of 30 μm to 150 μm.

[0080] <Electrolyte> In one embodiment of the present invention, the electrolyte may be a lithium salt-containing non-aqueous electrolyte, and the lithium salt-containing non-aqueous electrolyte may contain a non-aqueous organic solvent and a lithium salt.

[0081] In one embodiment of the present invention, the non-aqueous organic solvent may contain diethyl carbonate as an essential component.

[0082] In a preferred embodiment of the present invention, the organic solvent may contain 10 vol% or more, preferably 15 vol% or more, and more preferably 20 vol% or more of diethyl carbonate based on the total volume of the organic solvent.

[0083] Generally, lithium-ion batteries charge and discharge by repeatedly inserting (intercalating) and deintercalating (deintercalating) lithium ions from the lithium metal oxide positive electrode to the graphite negative electrode. During this process, lithium is highly reactive and reacts with the carbon electrode to produce Li2CO3, LiO, LiOH, etc., forming a film on the surface of the negative electrode. This film is called a solid electrolyte interface (SEI) film, and the SEI film formed in the initial stages of charging prevents the reaction between lithium ions and the carbon negative electrode or other substances during charging and discharging. It also acts as an ion tunnel, allowing only lithium ions to pass through. This ion tunnel prevents the organic solvent of the electrolyte, which has a large molecular weight and moves along with the lithium ions, from co-intercalating with the carbon negative electrode and causing its structure to collapse.

[0084] The lithium secondary battery of the present invention contains the above-mentioned CMC-Li salt in the negative electrode and has the effect of minimizing the consumption of organic solvent in the electrolyte used for the formation of the SEI film by supplementing the lithium that constitutes the SEI film formed in the initial stage of activation charging (formation). In order to double this effect, the present invention may contain diethyl carbonate (DEC) as a non-aqueous organic solvent, thereby further improving the cycle characteristics and resistance characteristics. In this case, the content of the above-mentioned diethyl carbonate may be 10 vol% or more, more preferably 15 vol% or more, and more preferably 20 vol% or more, based on the total volume of the organic solvent.

[0085] The electrolyte of the present invention, in addition to diethyl carbonate (DEC), also contains ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate (VC), fluoroethylene carbonate (FEC), and dimethyl carbonate (dimethyl The mixture may further contain one or more organic solvents selected from the group consisting of ethylene carbonate (DMC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate and ethyl propyl carbonate, dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, preferably one or more organic solvents selected from the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0086] The lithium salts mentioned above are substances that are readily soluble in the non-aqueous electrolyte, such as LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10 LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium 4-phenylborate, imide, etc. may be used.

[0087] Furthermore, the electrolyte may contain, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, in order to improve charge-discharge characteristics and flame retardancy. In some cases, halogen-containing solvents such as carbon tetrachloride and trifluoroethylene may be added to impart nonflammability, and carbon dioxide may be added to improve high-temperature storage characteristics, and FEC (fluoroethylene carbonate) and PRS (propene sultone) may be added.

[0088] In one specific example, lithium salts such as LiPF6, LiClO4, LiBF4, and LiN(SO2CF3)2 can be added to a mixed solvent of a highly dielectric solvent, such as a cyclic carbonate of EC or PC, and a low-viscosity solvent, such as a linear carbonate of DEC, DMC, or EMC, to produce a lithium salt-containing non-aqueous electrolyte.

[0089] <Positive electrode> In one embodiment of the present invention, the positive electrode may be manufactured by forming a positive electrode active material layer on a positive electrode current collector. The positive electrode active material layer may be formed by coating a positive electrode slurry containing a positive electrode active material, a binder, a conductive material, and a solvent onto a positive electrode current collector, followed by drying and rolling.

[0090] The positive electrode current collector described above is not particularly limited as long as it is conductive without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., can be used. The positive electrode current collector can also typically have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0091] The above positive electrode active material is not particularly limited, and any compound known in the art that allows for reversible intercalation and deintercalation of lithium can be used without limitation. Specifically, the above positive electrode active material is a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x Lithium manganese oxides such as O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, Cu2V2O; chemical formula LiNi 1-x M x Ni-site type lithium nickel oxide represented as O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 to 0.3); chemical formula LiMn 2-x M xLithium manganese composite oxide represented as O2 (where M = Co, Ni, Fe, Cr, Zn, or Ta, and x = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu, or Zn); LiNi x Mn 2-x This may include, but is not limited to, lithium manganese complex oxides with a spinel structure represented as O4; LiMn2O4 in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion; disulfide compounds; lithium iron phosphate compounds represented as LiFePO4; disulfide compounds; Fe2(MoO4)3, etc.

[0092] However, since improving energy density is important for batteries to which the present invention is applied, the positive electrode active material may include a lithium transition metal oxide with a high Ni content represented by the following chemical formula 1.

[0093] Li 1+x Ni a Co b Mn c M 1-(a+b+c) O 2-y A y (1)

[0094] In the above formula, M is at least one selected from the group consisting of Cu, Ti, Mg, Al, Pt, and Zr. A is an oxygen-substituted halogen, The following conditions apply: 0≦x≦0.5, 0.8≦a≦1, 0≦b≦0.2, 0≦c≦0.2, 0.9≦a+b+c≦1, and 0≦y≦0.001.

[0095] More specifically, the above a can be 0.88 ≤ a < 1.

[0096] Furthermore, lithium transition metal oxide represented by the above chemical formula 1 can be used in combination with other active materials.

[0097] The above-mentioned positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder, along with the positive electrode active material described above.

[0098] The above-mentioned positive electrode conductive material is used to impart conductivity to the electrode and can be carbon black, graphite, carbon fiber, carbon nanotubes, metal powder, conductive metal oxide, organic conductive material, etc. Commercially available conductive materials currently include acetylene black (products from Chevron Chemical Company or Gulf Oil Company, etc.), Ketjen Black EC (products from Armak Company), Vulcan XC-72 (products from Cabot Company), and Super P (products from MMM). Of these, carbon nanotubes, carbon nanofibers, and carbon black are preferred as conductive materials in the present invention, with carbon nanotubes being the most preferred. The conductive network of carbon nanotubes can mitigate the binder lifting phenomenon during the drying process of the positive electrode slurry, making it the most preferred conductive material to be included in the positive electrode of the present invention.

[0099] The BET specific surface area of ​​the above carbon nanotube is 100m². 2 / g~1000m 2 / g, 150m 2 / g~800m 2 / g, 150m 2 / g~500m 2 / g, 150m 2 / g~300m 2 / g or 150m 2 / g~200m 2 It could be / g

[0100] The above-mentioned positive electrode conductive material may be included in the positive electrode active material layer in an amount of 0.1 to 30% by weight, more specifically 0.1 to 10% by weight, and more specifically 0.5 to 5% by weight.

[0101] The above-mentioned positive electrode binder can be any commonly used binder polymer without limitation. For example, a variety of binder polymers such as polyvinylidene fluoride-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) can be used.

[0102] The above-mentioned positive electrode binder may be included in the positive electrode active material layer in an amount of 0.1 to 30% by weight, more specifically 0.1 to 10% by weight, and more specifically 0.5 to 5% by weight.

[0103] <Separation membrane> In one embodiment of the present invention, the separation membrane can be any porous substrate commonly used as a separation membrane in lithium secondary batteries. For example, a polyolefin-based porous membrane or nonwoven fabric can be used, but is not particularly limited thereto. In particular, a material that has low resistance to ion movement of the electrolyte while having excellent electrolyte moisture absorption capacity is preferred.

[0104] Examples of the above-mentioned polyolefin-based porous membranes include membranes formed from polyethylene such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, as well as polyolefin polymers such as polypropylene, polybutylene, and polypentene, either individually or as mixtures thereof.

[0105] The above-mentioned nonwoven fabrics include, in addition to polyolefin-based nonwoven fabrics, nonwoven fabrics formed from polymers such as polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalene, either individually or as mixtures thereof. The structure of the nonwoven fabric may be a spunbond nonwoven fabric or a meltblown nonwoven fabric composed of long fibers.

[0106] The thickness of the porous substrate described above is not particularly limited, but may be between 5 μm and 50 μm. The pore size and pore density present in the porous substrate are also not particularly limited, but may be between 0.01 μm and 50 μm, and between 10% and 95%, respectively.

[0107] On the other hand, in order to improve the mechanical strength of the separation membrane composed of the porous substrate and to suppress short circuits between the positive and negative electrodes, the porous substrate may further include a porous coating layer containing inorganic particles and a binder polymer on at least one surface.

[0108] The lithium secondary battery of the present invention can be manufactured by forming an electrode assembly by placing a separation membrane between the positive electrode and the negative electrode, and then injecting an electrolyte after placing the electrode assembly in a cylindrical or rectangular battery case. Alternatively, the electrode assemblies can be stacked, impregnated with an electrolyte, and the resulting product can be placed in a battery case and sealed.

[0109] The above-mentioned battery case may be one that is commonly used in this field, and there are no restrictions on its external shape depending on the battery's application. For example, it may be cylindrical, rectangular, pouch-type, or coin-type, using a can.

[0110] The following description will refer to embodiments of the present invention, but this is for the purpose of making the invention easier to understand and does not limit the scope of the invention thereto.

[0111] The lithium secondary battery according to the present invention exhibits excellent resistance characteristics, discharge capacity, output characteristics, and capacity retention rate stably, making it useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in energy storage systems (ESS) and electric vehicles such as hybrid electric vehicles (HEVs).

[0112] The following description will refer to embodiments of the present invention, but this is for the purpose of making the invention easier to understand and does not limit the scope of the invention thereto.

[0113] <Example 1> Preparation of CMC-Li salt CMC-Na salt was added to a mixed solution of ethanol and HCl (85:15, v / v), stirred and reacted at 35°C for 2 hours, and then thoroughly washed with a mixed solution of ethanol and distilled water (85:15, v / v) to produce H-CMC. H-CMC was added to a mixed solution of ethanol and distilled water (90:10, v / v) containing 7 wt% LiOH, and stirred and reacted at 50°C for 2 hours. After the reaction was complete, the solution was neutralized with acetic acid (pH 7), and then thoroughly washed with a mixed solution of ethanol and distilled water (85:15, v / v) to produce CMC-Li salt. At this time, the CMC-Li salt had a degree of substitution of the hydroxyl (-OH) group by the lithium carboxymethyl group (-CH2COOLi) of 1.0, a weight-average molecular weight (Mw) of 700,000, a viscosity of 2,200 cps, and a solubility of 1.3.

[0114] Manufacturing of negative electrodes The CMC-Li salt produced above, SBR with a particle size of 200 nm as an aqueous binder, and natural graphite as the negative electrode active material were mixed in a ratio of CMC-Li salt:SBR:natural graphite = 1:1:98 by weight, and water was added as a solvent to produce a negative electrode slurry. The loading amount was 10 mg / cm³. 2 The negative electrode was manufactured by applying the negative electrode slurry to the copper current collector and then drying it in a vacuum oven at 120°C for more than two hours.

[0115] Manufacturing of rechargeable batteries The above-mentioned negative electrode was used as the negative electrode, and the positive electrode was prepared by applying the positive electrode slurry to aluminum foil and then drying it in a vacuum oven at 120°C for more than 2 hours. At this time, the positive electrode slurry was Li[Ni 0.6 Mn 0.2 Co 0.2 A lithium secondary battery was prepared by dispersing 6.7 parts by weight of O29, 1.3 parts by weight of graphite (which functions as a conductive material), and 2.0 parts by weight of polyvinylidene fluoride (PVDF) (which functions as a binder) in an NMP solvent. The electrolyte was prepared by dissolving LiPF6 at a concentration of 1 M in an organic solvent with a volume ratio of 30 vol% / 20 vol% / 50 vol% of diethyl carbonate (DEC) / ethylene carbonate (EC) / ethyl methyl carbonate (EMC).

[0116] <Examples 2 to 5> CMC-Li salt was produced from CMC-Na salt using the same method as in Example 1 above, but the degree of substitution of the hydroxy(-OH) group by the lithium carboxymethyl group (-CH2COOLi), the weight-average molecular weight (Mw), and the solubility were changed as shown in Table 1 below.

[0117] Subsequently, a negative electrode and a lithium secondary battery containing the above-mentioned CMC-Li salt were manufactured using the same method as in Example 1.

[0118] <Examples 6 to 17> The CMC-Li salt was prepared using the same method as in Example 1. The degree of substitution of the hydroxyl (-OH) group by the lithium carboxymethyl group (-CH2COOLi), molecular weight (Mn), and solubility of the CMC-Li salt thus prepared were the same as those of Example 1.

[0119] Subsequently, a negative electrode and lithium secondary battery containing the above-mentioned CMC-Li salt were manufactured in the same manner as in Example 1, except that the composition of the organic solvent in the electrolyte was changed as shown in Table 1.

[0120] <Examples 18-23> CMC-Li salt was produced from CMC-Na salt using the same method as in Example 1 above, but the degree of substitution of the hydroxy(-OH) group by the lithium carboxymethyl group (-CH2COOLi), the weight-average molecular weight (Mw), and the solubility were changed as shown in Table 1 below.

[0121] Subsequently, a negative electrode and a lithium secondary battery containing the above-mentioned CMC-Li salt were manufactured using the same method as in Example 1.

[0122] <Example 24> A CMC-Li salt was prepared by adding LiOH and mono-chloroacetic acid (MCA) to cellulose, with a degree of substitution of hydroxyl (-OH) groups by lithium carboxymethyl groups (-CH2COOLi) of 1.0, a weight-average molecular weight (Mw) of 700,000, a viscosity of 2,200 cps, and a solubility of 1.3.

[0123] Subsequently, a negative electrode and a lithium secondary battery containing the above-mentioned CMC-Li salt were manufactured using the same method as in Example 1.

[0124] <Comparative Example 1> A CMC-Na salt was prepared with a degree of substitution of hydroxy(-OH) groups of 1.0, a weight-average molecular weight (Mw) of 700,000, and a solubility of 1.3.

[0125] In Example 1 described above, the negative electrode and lithium secondary battery were manufactured in the same manner as in Example 1, except that CMC-Na salt was used instead of CMC-Li salt during the manufacturing of the negative electrode.

[0126] <Comparative Example 2 to Comparative Example 4> A negative electrode was manufactured using the same CMC-Na salt as in Comparative Example 1 and the same method as in Comparative Example 1. Subsequently, a lithium secondary battery was manufactured using the same method as in Comparative Example 1, except that the composition of the organic solvent in the electrolyte was changed as shown in Table 1.

[0127] <Comparative Example 5 to Comparative Example 7> The CMC-Li salt was prepared using the same method as in Example 1. The degree of substitution of the hydroxyl (-OH) group by the lithium carboxymethyl group (-CH2COOLi), molecular weight (Mn), and solubility of the CMC-Li salt thus prepared were the same as those of Example 1.

[0128] Subsequently, a negative electrode and lithium secondary battery containing the above-mentioned CMC-Li salt were manufactured in the same manner as in Example 1, except that the composition of the organic solvent in the electrolyte was changed as shown in Table 1.

[0129] [Table 1]

[0130] Experimental Example 1: Measurement of the number of undissolved particles in CMC-Li salt and CMC-Na salt The CMC-Li salt from Example 1 was added to deionized water at a concentration of 1 wt% and mixed / stirred at a speed of 10 rpm for 10 minutes. After coating a glass plate with the above CMC-Li salt solution to a thickness of 200 μm, the number of microgels (undissolved material) with a diameter of 200 μm or less that were visible to the naked eye was measured in a unit area of ​​5 cm × 5 cm.

[0131] The number of microgels was measured for Examples 2 to 24 and Comparative Examples 1 to 7 using the same method as described above, and the results are shown in Table 2.

[0132] Experimental Example 2: Evaluation of Adhesion Strength Each of the negative electrodes produced by Examples 1 to 24 and Comparative Examples 1 to 7 was cut to a specific size (20 mm x 150 mm).

[0133] The cut negative electrode was attached to a glass slide measuring 25 mm in width and 75 mm in length, facing the negative electrode active material layer, using double-sided tape in the longitudinal direction. In other words, the glass slide was attached to a region corresponding to half of the negative electrode's longitudinal direction. After that, a roller was rotated 10 times to ensure uniform adhesion of the double-sided tape, and an evaluation sample was prepared.

[0134] Next, the slide glass portion of the evaluation sample is fixed to the sample stage of a Universal Testing Machine (UTM) (LS5, AMETEK), and half of the negative electrode, to which the slide glass does not adhere, is connected to a load cell equipped with the UTM. The load applied to the load cell is measured while moving it 50 mm at a speed of 100 mm / min with a 90° force. At this time, the average value of the load measured in the section from 20 mm to 40 mm of the travel distance is calculated. This is repeated a total of 5 times, and the average value is evaluated as the negative electrode adhesion strength (gf / 20 mm) of each sample.

[0135] Experiment Example 3: Initial Performance Evaluation Each secondary battery manufactured according to Examples 1 to 24 and Comparative Examples 1 to 7 was formed with a current of 200 mA (0.1 C-rate), then stored at 60°C for one day, after which the gas inside the battery was removed (degassing process). Subsequently, it was charged at a rate of 0.33 C-rate (0.33 C constant current charging, 4.25 V constant voltage charging, and 0.005 C cut-off) and discharged at a rate of 0.33 C-rate (0.33 C constant current discharge, 3.0 V cut-off), and the discharge capacity and the negative electrode efficiency (Coulomb efficiency) at this time were measured. The discharge capacity was calculated based on the weight of the negative electrode active material, and the results are shown in Table 2 below.

[0136] Experimental Example 4: Evaluation of Lifetime Characteristics by Long-Term Cycles The secondary batteries manufactured according to Examples 1 to 24 and Comparative Examples 1 to 7 were subjected to formation with a current of 200 mA (0.1 C-rate), then stored at 60°C for one day, and the gas inside the battery was removed (degassing process). Subsequently, the batteries were charged at a rate of 0.33 C-rate (0.33 C constant current charging, 4.25 V constant voltage charging, 0.005 C cut-off) and discharged at a rate of 0.33 C-rate (0.33 C constant current discharge, 3.0 V cut-off). This process was repeated 300 times, and the capacity retention rate was calculated by substituting the results into Equation 1 below. The results are shown in Table 2 below.

[0137] Equation (1): Capacity retention rate (%) = (Discharge capacity after 300 cycles / Initial discharge capacity) × 100

[0138] [Table 2]

[0139] Referring to Tables 1 and 2 above, it was shown that the CMC-Li salts in Examples 1 to 19, 21, and 23 to 24 produced significantly fewer microgels compared to the CMC-Na salts in Comparative Examples 1 to 4. As a result, it is expected that negative electrodes using CMC-Li salt as a thickening agent will suppress the formation of pinholes compared to negative electrodes using CMC-Na salt as a thickening agent, thereby preventing the degradation of battery performance that can be caused by pinholes.

[0140] Furthermore, the lithium secondary batteries according to the embodiments of the present invention (Examples 1 to 24) were shown to have superior initial performance and capacity retention after 300 cycles compared to comparative lithium secondary batteries (Comparative Examples 1 to 4) in which CMC-Na salt was used as a thickening agent for the negative electrode. This is thought to be due to the fact that the CMC-Li salt minimized the consumption of lithium salt and organic solvent in the electrolyte when forming the solid electrolyte interface layer (SEI Layer) within the negative electrode.

[0141] Furthermore, while the lithium secondary batteries of Examples 1 to 24, which contain diethyl carbonate (DEC) as the organic solvent of the electrolyte, were shown to have superior initial performance and capacity retention compared to the lithium secondary batteries of Comparative Examples 5 to 7, which do not contain diethyl carbonate as the organic solvent of the electrolyte, it is clear that including diethyl carbonate as the organic solvent of the electrolyte is preferable in order to double the effects of the present invention.

[0142] On the other hand, in Examples 18 to 23, where the degree of substitution of the CMC-Li salt falls outside the range of 0.7 to 1.3, the weight-average molecular weight (Mw) of the CMC-Li salt falls outside the range of 400,000 to 1,500,000, or the solubility of the CMC-Li salt falls outside the range of 1.0 to 1.7, it can be confirmed that the number of microgels is large, or that the initial performance or capacity retention rate of the battery is inferior to that of Examples 1 to 17. Therefore, in the present invention, it is preferable that the CMC-Li salt has a degree of substitution in the range of 0.7 to 1.3, a weight-average molecular weight in the range of 400,000 to 1,500,000, and a solubility in the range of 1.0 to 1.7.

[0143] Furthermore, it was observed that the CMC-Li salt of Example 24 had an even greater number of microgels than the CMC-Li salts of Examples 1 to 17, and the negative electrode of Example 24 was found to have lower adhesive strength compared to the negative electrodes of Examples 1 to 17. Therefore, it is considered more preferable to select a CMC-Li salt manufactured in a multi-stage process for the present invention.

Claims

1. A lithium secondary battery comprising a positive electrode, a negative electrode, a separator membrane, and an electrolyte, The aforementioned negative electrode is The negative electrode current collector and the negative electrode active material layer formed on at least one surface of the negative electrode current collector are included. The negative electrode active material layer comprises a negative electrode active material, an aqueous binder, and a CMC-Li salt. The electrolyte comprises a lithium salt and an organic solvent. The aforementioned organic solvent comprises diethyl carbonate, The CMC-Li salt is characterized by having 100 or fewer microgels per unit area (5 cm × 5 cm) of a thin film coated with a 1 wt% aqueous solution to a thickness of 200 μm. The CMC-Li salt has a degree of substitution of the hydroxyl (-OH) group by a lithium carboxymethyl group (-CH2COOLi) of 0.7 to 1.

3. The CMC-Li salt has a weight-average molecular weight (Mw) of 400,000 to 1,500,000. The CMC-Li salt is a lithium secondary battery in which the solubility is 1.0 to 1.7 by weight at a measurement condition of 23°C.

2. The lithium secondary battery according to claim 1, wherein the organic solvent contains 10 vol% or more of the diethyl carbonate based on the total volume of the organic solvent.

3. The lithium secondary battery according to claim 1, further comprising one or more organic solvents selected from the group consisting of ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, fluoroethylene carbonate, dimethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate, dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

4. The lithium secondary battery according to claim 1, wherein the organic solvent further comprises one or more selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate.

5. The lithium secondary battery according to claim 1, wherein the CMC-Li salt is contained in an amount of 10% by weight or less based on the total weight of the negative electrode active material layer.

6. The lithium secondary battery according to claim 1, wherein the aqueous binder is an SBR.

7. The lithium secondary battery according to claim 1, wherein the negative electrode active material comprises one or more carbon-based materials selected from the group consisting of graphite-based carbon, coke-based carbon, and hard carbon.

8. The negative electrode active material is Si, SiO ｘ The lithium secondary battery according to claim 7, further comprising one or more silicon-based materials selected from the group consisting of (0 < x ≤ 2) and Si-Y alloys (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements and combinations thereof, and is not Si).

9. The lithium secondary battery according to claim 8, wherein the silicon-based material is included in an amount of 0.1 to 10 parts by weight relative to the total weight of the negative electrode active material.

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