Lithium secondary battery containing gel polymer electrolyte

Abstract

A lithium secondary battery including a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and a gel polymer electrolyte, the positive electrode including a positive electrode active material containing a lithium cobalt-based oxide, the gel polymer electrolyte including a polymer and an electrolyte solution, the polymer having three or more functional groups and not including an ether group, the electrolyte solution including a lithium salt and a solvent, and the lithium salt including lithium difluoro(oxalato)borate and LiBF 4 The present invention relates to a lithium secondary battery comprising:

Description

[Technical Field]

[0001] This invention relates to a lithium secondary battery containing a gel polymer electrolyte. [Background technology]

[0002] Lithium-ion batteries are widely used not only as portable power sources for mobile phones, laptops, and digital cameras, but also as medium- and large-scale power sources for power tools, electric bicycles, electric vehicles, and hybrid electric vehicles.

[0003] However, conventional lithium-ion batteries, which are manufactured by injecting liquid electrolytes, have safety issues such as the risk of explosion or fire if subjected to abnormal conditions such as penetration, impact, or pressurization, or if overheated. Therefore, attempts are being made to use semi-solid or solid electrolytes instead of liquid electrolytes to ensure safety. Among these, gel polymer electrolytes (GPEs) are electrolytes that exhibit a gel-like semi-solid state in which a liquid electrolyte is contained in a polymer solid electrolyte. They can be manufactured in the form of a free-standing film and inserted into a battery case, or injected into a battery case in liquid form and then cured into a gel-like electrolyte. Gel polymer electrolytes have the advantage of significantly improved safety compared to liquid electrolytes, but they have the problem of increased resistance and reduced capacity compared to liquid electrolytes. [Overview of the initiative] [Problems that the invention aims to solve]

[0004] This invention provides a lithium secondary battery with improved lifespan characteristics while significantly enhancing safety, by using a gel polymer electrolyte that can be injected into the battery case without viscosity reduction, thereby allowing the use of existing processes. Furthermore, it provides a lithium secondary battery that can operate stably at high voltages without electrolyte decomposition, while using a positive electrode active material containing lithium cobalt oxide. [Means for solving the problem]

[0005] One embodiment of the present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and a gel polymer electrolyte, wherein the positive electrode comprises a positive electrode active material containing a lithium cobalt oxide, the gel polymer electrolyte comprises a polymer and an electrolyte, the polymer having three or more functional groups and not containing an ether group, the electrolyte comprises a lithium salt and a solvent, and the lithium salt comprises lithium difluoro(oxalate) borate and LiBF4. [Effects of the Invention]

[0006] A lithium secondary battery according to one embodiment of the present invention can significantly improve safety and lifespan characteristics by including a gel polymer electrolyte. The gel polymer electrolyte has the advantage of being applicable to existing processes because it can be injected into the battery case without viscosity reduction, and even when lithium cobalt oxide is used as the positive electrode active material, there is no problem of chemical reactions or decomposition at high voltage, so stable battery operation is possible. [Brief explanation of the drawing]

[0007] [Figure 1] This is a schematic cross-sectional view of a lithium secondary battery. [Figure 2] This is a graph of the 1H NMR analysis of the gel polymer electrolyte prepared in Example 1. [Figure 3] This graph shows the battery life characteristics of Example 1 and Comparative Example 1. [Figure 4] This graph shows the battery life characteristics for Example 1, Example 2, and Comparative Example 2. [Figure 5] This graph shows the battery life characteristics for Example 1, Comparative Example 3, and Comparative Example 4. [Figure 6] This graph shows the battery life characteristics for Examples 1, 3, and 4. [Figure 7] This graph shows the viscosity of gel polymer electrolyte compositions according to their polymer content. [Modes for carrying out the invention]

[0008] The following describes specific embodiments in detail so that they can be easily implemented by those with ordinary skill in the art. However, the present invention can be realized in various different forms and is not limited to the embodiments described herein.

[0009] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Unless otherwise clearly stated in the context, singular expressions include plural expressions.

[0010] Here, "these combinations" refers to mixtures of components, laminates, composites, copolymers, alloys, blends, reaction products, etc.

[0011] Here, terms such as “include,” “equip,” or “possess” are intended to specify the existence of a feature, figure, stage, component, or combination thereof, and should be understood not to preemptively exclude the existence or possibility of adding one or more other features, figures, stages, components, or combinations thereof.

[0012] To clearly represent various layers and regions in the drawings, thicknesses are shown enlarged, and similar parts are given the same drawing reference numerals throughout the specification. When a layer, film, region, plate, or other part is said to be "on top of" or "on" another part, this includes not only when it is "directly on top of" another part, but also when there is another part in between. Conversely, when a part is said to be "directly on top of" another part, it means that there is no other part in between.

[0013] Furthermore, the term "layer" here includes not only the shapes formed on the entire surface when observed in a plan view, but also the shapes formed on a portion of the surface.

[0014] In addition, the average particle size, average size, etc. can be measured by methods widely known to those skilled in the art. For example, they can be measured using a particle size analyzer, or can also be measured from a transmission electron micrograph or a scanning electron micrograph. As another method, the size, etc. can be measured using the dynamic light scattering method, data analysis can be performed to count the number of particles for each particle size range, and then the average particle size value can be obtained by calculation. Unless otherwise defined, the average particle size is the one measured by a particle size analyzer, and the diameter (D 50 ) of the particle with a cumulative volume of 50% by volume in the particle size distribution can be meant.

[0015] Lithium secondary battery In one embodiment, a lithium secondary battery is provided that includes a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and a gel polymer electrolyte. FIG. 1 is a schematic diagram showing a lithium secondary battery according to one embodiment. Referring to FIG. 1, the lithium secondary battery 100 includes a battery cell including a positive electrode 114, a negative electrode 112 positioned opposite to the positive electrode 114, a separator 113 disposed between the positive electrode 114 and the negative electrode 112, and a gel polymer electrolyte (not shown) that impregnates the positive electrode 114, the negative electrode 112, and the separator 113, a battery container 120 that houses the battery cell, and a sealing member 140 that seals the battery container 120.

[0016] Gel polymer electrolyte A gel polymer electrolyte according to one embodiment includes a polymer and an electrolyte solution. The polymer has three or more functional groups and does not contain an ether group. The electrolyte solution includes a lithium salt and a solvent, and the lithium salt includes lithium difluoro(oxalate)borate and LiBF4.

[0017] The gel polymer electrolyte is manufactured by injecting it into a battery case into which an electrode assembly is inserted, and then curing it. This method has the advantage of being applicable to existing manufacturing processes for non-aqueous electrolyte batteries. Furthermore, the gel polymer electrolyte maintains good processability by maintaining low viscosity when injected into the battery case. The gel polymer electrolyte can be easily cured by thermal curing or photocuring, thereby exhibiting a gel-like solid state. In lithium secondary batteries, the gel polymer electrolyte can also be described as containing a crosslinked polymer or a cured polymer and an electrolyte.

[0018] The polymer contained in the gel polymer electrolyte has three or more functional groups, which is advantageous for crosslinking and curing within the battery case, and it can exhibit even better lifespan characteristics compared to batteries using polymers with two or fewer functional groups. Furthermore, the polymer can also be described as a polyfunctional polymer that does not contain ether groups. When lithium cobalt oxide is used as the positive electrode active material, the ether groups in the electrolyte may undergo a chemical reaction and decompose at a high voltage of about 4.4V. On the other hand, the polymer in the gel polymer electrolyte according to one embodiment is a polyfunctional polymer that does not contain ether groups, and even when lithium cobalt oxide is used as the positive electrode active material, it does not decompose or undergo a chemical reaction, enabling stable long-term operation of the lithium secondary battery. The lithium secondary battery according to one embodiment can achieve even better lifespan characteristics compared to batteries using a gel polymer electrolyte containing an ether-based polyfunctional polymer.

[0019] The functional groups in the polymer may be, for example, ester groups (-C(=O)O-), carbonate groups (-OC(=O)O-), or combinations thereof. Thus, the polymer can also be described as an ester-based polyfunctional polymer or a carbonate-based polyfunctional polymer. The gel polymer electrolyte contains a polymer that contains three or more functional groups selected from ester groups and carbonate groups, and does not contain ether groups. This improves processability, ensures the safety of lithium secondary batteries, and dramatically improves their lifespan characteristics.

[0020] The polymer may be derived from a compound containing three or more carbon-carbon double bonds. In other words, the polymer can be understood as a polymer that has been polymerized, crosslinked, or cured from a compound containing three or more carbon-carbon double bonds. A compound containing three or more carbon-carbon double bonds may, for example, be a compound containing three or more acrylic groups (CH2=CH-C(=O)O-). In the polymer, the carbon-carbon double bonds participate in reactions such as polymerization and crosslinking during the curing process of the gel polymer electrolyte in the battery case, and may not be detected in the final battery. In the cured gel polymer electrolyte state, they can be detected as ester groups or carbonate groups.

[0021] The polymer may be derived, for example, from trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, or a combination thereof.

[0022] The polymer may be present in an amount of 1% to 10% by weight relative to 100% by weight of the gel polymer electrolyte, for example, 2% to 8% by weight, 2% to 6% by weight, or 3% to 5% by weight. When this content range is met, a gel-like polymer electrolyte can be effectively produced, and the life characteristics of the lithium secondary battery can be improved.

[0023] In the gel polymer electrolyte, the weight ratio of the polymer to the electrolyte is 1:99 to 10:90, and can be, for example, 2:98 to 8:92, 2:98 to 6:94, or 3:97 to 5:95. When such a weight ratio is satisfied, a gel-like polymer electrolyte can be effectively manufactured, and the life characteristics of the lithium secondary battery can be improved.

[0024] In the gel polymer electrolyte, the electrolyte solution comprises a lithium salt and a solvent. The lithium salt comprises lithium difluoro(oxalate)borate (LiDFOB) and lithium tetrafluoroborate (LiBF4). By including these two types of lithium salts in the electrolyte solution, the gel polymer electrolyte can dramatically improve its lifespan. The gel polymer electrolyte according to one embodiment can achieve superior lifespan characteristics compared to a case in which neither of the two types of lithium salts is included.

[0025] In the electrolyte, LiDFOB and LiBF4 may be present in a molar ratio of 20:80 to 80:20, or for example, 30:70 to 70:30, or 40:60 to 60:40. The presence of the lithium salt in such a molar ratio can improve the lifespan characteristics of the lithium secondary battery.

[0026] The electrolyte may further contain other lithium salts in addition to LiDFOB and LiBF4. These other lithium salts may be, for example, LiPF6, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, lithium bisfluorosulfonylimide (LiFSI), LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, lithium difluorobisoxalate phosphate, LiCl, LiI, lithium bis(oxalate)borate (LiBOB), or combinations thereof.

[0027] The concentration of the lithium salt in the electrolyte is 0.3M to 4M, and may be, for example, 0.6M to 3M, 0.8M to 2.5M, or 1.0M to 2.0M. When the concentration of the lithium salt is within this range, the gel polymer electrolyte can exhibit excellent lithium conductivity, maintain an appropriate concentration when injected into the battery case, and effectively maintain a gel state after curing.

[0028] The solvent in the electrolyte is a solvent commonly used in non-aqueous electrolytes, and may include, for example, a carbonate-based solvent, an ester-based solvent, or a combination thereof. Furthermore, the solvent may not contain an ether-based solvent. When lithium cobalt oxide is used as the positive electrode active material, a problem may arise where ether-based components in the electrolyte decompose in the high-voltage range; therefore, the electrolyte of the gel polymer electrolyte according to one embodiment can be designed to not contain an ether-based solvent.

[0029] The carbonate-based solvent may be dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or a combination thereof.

[0030] The ester solvent may be methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, or a combination thereof.

[0031] When two or more solvents are used as the solvent for the electrolyte, the mixing ratio can be appropriately adjusted according to the desired battery performance. As the carbonate-based solvent, for example, a mixture of cyclic carbonate and linear carbonate can be used, in which case the cyclic carbonate and linear carbonate can be mixed in a volume ratio of approximately 1:1 to approximately 1:9.

[0032] On the other hand, the solvent of the electrolyte may contain fluoroethylene carbonate. In this case, the gel polymer electrolyte can achieve excellent lifespan characteristics while maintaining good processability. In this case, the fluoroethylene carbonate may be included in an amount of 5% to 50% by volume relative to 100% by volume of the solvent, for example, 5% to 40% by volume, 10% to 50% by volume, 15% to 50% by volume, or 20% to 40% by volume. By including it within the above range, the lifespan characteristics of the lithium secondary battery containing the gel polymer electrolyte can be improved.

[0033] The solvent of the electrolyte may further contain ethylene carbonate compounds other than fluoroethylene carbonate, or it may further contain aromatic hydrocarbon organic solvents.

[0034] The ethylene-based carbonate compound may be, for example, difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, or a combination thereof.

[0035] The aforementioned aromatic hydrocarbon solvents include, for example, benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, and tol This could be xylene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, or a combination thereof.

[0036] positive electrode The positive electrode 114 includes a current collector and a positive electrode active material layer located on the current collector, the positive electrode active material layer comprising a positive electrode active material and optionally including a binder and / or conductive material. Here, the current collector may be, for example, aluminum foil, but is not limited thereto.

[0037] The positive electrode active material according to one embodiment contains a lithium cobalt-based oxide. The lithium cobalt-based oxide contains lithium and cobalt, and means an oxide that selectively further contains other elements. The lithium cobalt-based oxide can exhibit excellent efficiency and life characteristics while realizing high capacity, and is economical. The lithium cobalt-based oxide can maintain structural stability without causing a chemical reaction with each other, decomposition, or deterioration during battery driving or under abnormal conditions with the above-described gel polymer electrolyte. When applying the gel polymer electrolyte according to one embodiment while applying a lithium cobalt-based oxide as the positive electrode active material, battery performance such as life characteristics can be maximized.

[0038] The lithium cobalt-based oxide may be represented by, for example, the following Chemical Formula 1. [Chemical Formula 1] Li a Co x M 1 y M 2 1-x-y O2 [[ID=二十]]

[0039] <[ In Chemical Formula 1, 0.9 ≦ a ≦ 1.8, 0.7 ≦ x ≦ 1, 0 ≦ y ≦ 0.3, M 1 and M 2 are each independently Al, B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, or a combination thereof.

[0040] In Chemical Formula 1, 0.8 ≦ x ≦ 1, 0 ≦ y ≦ <0.2>, 0.9 ≦ x ≦ 1, 0 ≦ y ≦ 0.1, 0.92 ≦ x ≦ 1, 0 ≦ y ≦ 0.08, or <0.95> ≦ x ≦ 1, 0 ≦ y ≦ 0.05 may be satisfied.

[0041] The positive electrode active material may further include a coating layer located on the surface of the lithium cobalt-based oxide. This coating layer may contain at least one coating element compound selected from the oxide of the coating element, the hydroxide of the coating element, the oxyhydroxy of the coating element, the oxycarbonate of the coating element, and the hydroxycarbonate of the coating element. The compound forming the coating layer may be amorphous or crystalline. Examples of coating elements included in the coating layer include Al, As, B, Ca, Co, Ga, Ge, K, Mg, Na, Si, Sn, Ti, V, Zr, or combinations thereof. The compound forming the coating layer may be, for example, a lithium-metal oxide, a lithium-metal hydroxide, and / or a lithium-metal carbonate. As an example, the coating layer may include a lithium zirconium oxide. The coating layer formation process can be carried out using a method that does not adversely affect the physical properties of the positive electrode active material, such as spray coating, immersion, dry coating, atomic deposition, or evaporation.

[0042] The average particle size (D50) of the positive electrode active material is 1 μm to 25 μm, and can be, for example, 4 μm to 25 μm, 5 μm to 20 μm, 8 μm to 20 μm, or 10 μm to 18 μm. A positive electrode active material having such a particle size range can be mixed in harmony with other components in the positive electrode active material layer, enabling high capacity and high energy density. The average particle size is measured by a particle size analyzer and refers to the diameter (D50) of particles whose cumulative volume in the particle size distribution is 50 volume%.

[0043] The positive electrode active material may be in the form of secondary particles formed by the aggregation of multiple primary particles, or it may be in the form of a single crystal. Furthermore, the positive electrode active material may be spherical or nearly spherical in shape, or it may be polyhedral or amorphous.

[0044] The positive electrode active material may be present in an amount of 55% to 99.8% by weight relative to the total weight of the positive electrode active material layer, for example, 80% to 90% by weight. When present within this range, the lifespan characteristics can be improved while maximizing the capacity of the lithium secondary battery.

[0045] The binder plays a role in ensuring that the positive electrode active material particles adhere well to each other and that the positive electrode active material adheres well to the current collector. Typical examples of binders include, but are not limited to, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0046] The binder content in the positive electrode active material layer may be approximately 0.1% to 5% by weight relative to the total weight of the positive electrode active material layer.

[0047] The conductive material is used to impart conductivity to the electrodes, and any electronically conductive material that does not cause chemical changes in the battery can be used. Examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metallic materials containing copper, nickel, aluminum, silver, etc., in the form of metal powder or metal fiber; conductive polymers such as polyphenylene derivatives; or conductive materials containing mixtures thereof. The content of the conductive material in the positive electrode active material layer may be 0.1% to 5% by weight relative to the total weight of the positive electrode active material layer.

[0048] negative electrode In a lithium secondary battery, the negative electrode 112 includes a current collector and a negative electrode active material layer located on the current collector. The negative electrode active material layer includes a negative electrode active material and may further include a binder and / or a conductive material.

[0049] The negative electrode active material includes a substance capable of reversibly inserting / de-inserting lithium ions, lithium metal, an alloy of lithium metal, a substance that can be doped and de-doped with lithium, or a transition metal oxide.

[0050] The material capable of reversibly inserting / deinserting lithium ions is a carbon-based anode active material, which may include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon include graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural graphite or artificial graphite, while examples of amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, and calcined coke.

[0051] As the lithium metal alloy, an alloy of lithium with a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0052] As the substance capable of doping and undoping lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material can be used. Examples of the Si-based negative electrode active material include silicon, silicon-carbon composite, SiOx (0 < x < 2), Si-Q alloy (where Q is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof, excluding Si), and examples of the Sn-based negative electrode active material include Sn, SnO2, Sn-R alloy (where R is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof, excluding Sn). Further, at least one of these can be mixed with SiO2 and used. As the elements Q and R, those 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, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof can be used.

[0053] The silicon-carbon composite may, for example, include a core containing crystalline carbon and silicon particles, and an amorphous carbon coating layer located on the surface of the core. The crystalline carbon may be artificial graphite, natural graphite, or a combination thereof. As the amorphous carbon precursor, coal-based pitch, mesophase pitch, petroleum-based pitch, coal-based oil, petroleum-based medium-grade oil, or polymer resins such as phenolic resin, furan resin, or polyimide resin can be used. In this case, the silicon content may be 10% to 50% by weight relative to the total weight of the silicon-carbon composite. The crystalline carbon content may be 10% to 70% by weight relative to the total weight of the silicon-carbon composite, and the amorphous carbon content may be 20% to 40% by weight relative to the total weight of the silicon-carbon composite. The thickness of the amorphous carbon coating layer may be 5 nm to 100 nm. The average particle size (D50) of the silicon particles may be 10 nm to 20 μm. The average particle size (D50) of the silicon particles is preferably 10 nm to 200 nm. The silicon particles exist in an oxidized form, and in this case, the atomic content ratio of Si:O within the silicon particles, which indicates the degree of oxidation, can be 99:1 to 33:67. The silicon particles are SiO x It is a particle, and in this case, SiO x In this case, the range of x can be greater than 0 and less than 2.

[0054] The Si-based or Sn-based anode active material can be used in combination with a carbon-based anode active material. When using a mixture of the Si-based or Sn-based anode active material and a carbon-based anode active material, the mixing ratio can be 1:99 to 90:10 by weight.

[0055] The content of the negative electrode active material in the negative electrode active material layer may be 95% to 99% by weight relative to the total weight of the negative electrode active material layer.

[0056] In one embodiment, the negative electrode active material layer further contains a binder and may selectively further contain a conductive material. The binder content in the negative electrode active material layer may be 0.1% to 5% by weight relative to the total weight of the negative electrode active material layer. Furthermore, when a conductive material is further included, the negative electrode active material layer may contain 90% to 99.8% by weight of the negative electrode active material, 0.1% to 5% by weight of the binder, and 0.1% to 5% by weight of the conductive material.

[0057] The binder plays a role in ensuring that the negative electrode active material particles adhere well to each other and that the negative electrode active material adheres well to the current collector. As the binder, a non-water-soluble binder, a water-soluble binder, or a combination thereof can be used.

[0058] Examples of the water-insoluble binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, ethylene propylene copolymers, polystyrene, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or combinations thereof.

[0059] Examples of the water-soluble binder include rubber-based binders and polymer resin binders. The rubber-based binder may be selected from styrene-butadiene rubber, acrylate-styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, and combinations thereof. The polymer resin binder may be selected from polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0060] When a water-soluble binder is used as the negative electrode binder, it may further contain a cellulosic compound that can impart viscosity. This cellulosic compound can be a mixture of one or more carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or alkali metal salts thereof. The alkali metal can be Na, K, or Li. The amount of such thickener used may be 0.1 to 3 parts by weight per 100 parts by weight of the negative electrode active material.

[0061] The conductive material is used to impart conductivity to the electrodes, and any electronically conductive material that does not cause chemical changes in the battery can be used. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fiber, carbon nanofiber, and carbon nanotubes; metallic materials containing copper, nickel, aluminum, silver, etc., in the form of metal powder or metal fiber; conductive polymers such as polyphenylene derivatives; or conductive materials containing mixtures thereof.

[0062] As the negative electrode current collector, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with a conductive metal, and combinations thereof can be selected.

[0063] Separator The separator 113 separates the positive electrode 114 and the negative electrode 112 and provides a passage for lithium ions to move. Any separator commonly used in lithium-ion batteries can be used. In other words, a separator with low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity can be used. For example, it can be selected from glass fiber, polyester, Teflon®, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof, and can be in the form of a nonwoven or woven fabric. For example, polyolefin polymer separators such as polyethylene and polypropylene are mainly used in lithium-ion batteries. Coated separators containing ceramic components or polymer materials can also be used to ensure heat resistance or mechanical strength, and can be selectively used in a single-layer or multi-layer structure.

[0064] On the other hand, in one embodiment, a method for manufacturing a lithium secondary battery is provided, which includes preparing a polymer precursor having three or more functional groups containing carbon-carbon double bonds and not containing ether groups; preparing an electrolyte containing a lithium salt containing lithium difluoro(oxalate) borate and LiBF4 and a solvent; mixing the polymer precursor and the electrolyte to prepare a gel polymer electrolyte composition; inserting an electrode assembly including a positive electrode containing a positive electrode active material containing a lithium cobalt oxide, a separator, and a negative electrode into a battery case; injecting the gel polymer electrolyte composition into the battery case; and curing the gel polymer electrolyte composition to obtain a lithium secondary battery containing a gel polymer electrolyte.

[0065] The details of each component are as described above. The gel polymer electrolyte composition may be cured by, for example, thermal curing, which can be carried out at, for example, 50°C to 200°C for 10 minutes to 5 hours.

[0066] The following describes examples and comparative examples of the present invention. The following examples are merely illustrative of the present invention and are not limited to the following examples.

[0067] Example 1 1. Preparation of gel polymer electrolyte composition Trimethylolpropane trimethacrylate (TMPTMA) was prepared as a polymer precursor. As the electrolyte component, lithium difluoro(oxalate) borate (LiDFOB) and LiBF4 were dissolved at a molar concentration of 0.6 M each in a solvent prepared by mixing fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a 1:2 volume ratio (total 1.2 M).

[0068] A gel polymer electrolyte composition was prepared by mixing the prepared electrolyte solution and polymer precursor in a weight ratio of 97:3.

[0069] 2. Battery manufacturing Li 1.02 Co 0.9823 Al 0.0127 Mg 0.005 A cathode active material layer composition was prepared by mixing 96% by weight of O2 cathode active material, 2% by weight of polyvinylidene fluoride binder, 2% by weight of carbon nanotube conductive material, and N-methylpyrrolidone solvent in a mixer. This composition was then coated with aluminum foil, dried, and rolled to produce the cathode.

[0070] An electrode assembly was prepared by placing a polyethylene-polypropylene multilayer separator between the prepared positive electrode and the lithium metal counter electrode. This electrode assembly was inserted into a pouch-type battery case, and the prepared gel polymer electrolyte composition was injected into the case. The electrolyte composition was then cured by heat treatment at 70°C for 3 hours to produce a half-cell in which the gel polymer electrolyte was formed.

[0071] Evaluation Example 1: 1 1H NMR evaluation For the gel polymer electrolyte after curing in Example 1 1¹H NMR analysis was performed, and the results are shown in Figure 2. Referring to Figure 2, it can be confirmed that the carbon-carbon double bond peak, indicated by the dotted circle, disappeared. This indicates that all of the carbon-carbon double bonds in the polymer precursor participated in the reaction during the curing process. In other words, it was determined that 100% of the polymer precursor in the electrolyte composition was converted into a type of cross-linked polymer.

[0072] Comparative Example 1 (Ether-based polymer) A gel polymer electrolyte composition and a battery were prepared in the same manner as in Example 1, except that ethoxylated trimethylolpropane triacrylate (ETPTA), a trifunctional compound containing an ether group, was used as the polymer precursor for the gel polymer electrolyte composition.

[0073] Evaluation Example 2: Lifetime Characteristics Evaluation The batteries manufactured in Example 1 and Comparative Example 1 were charged at 25°C with a constant current of 0.1C to an upper voltage limit of 4.25V, and then discharged with a constant current of 0.1C to a discharge termination voltage of 3.5V to perform initial charge and discharge. Subsequently, charging to 0.5C and discharging to 0.5C were repeated 200 times in the voltage range of 3.5V to 4.25V. The discharge capacity was measured during the 200 cycles, and the capacity retention rate, which is the ratio of the discharge capacity in each cycle to the initial discharge capacity, was evaluated. The results are shown in Figure 3.

[0074] Referring to Figure 3, in Comparative Example 1, which used an ether-based polymer, the capacity retention rate decreased from the beginning of the cycle, reaching a level of 70% after 200 cycles. In contrast, in Example 1, a capacity retention rate of 90% was obtained after 200 cycles, confirming that it exhibits superior life characteristics.

[0075] Example 2 (Moisture Change) A gel polymer electrolyte composition and a battery were prepared in the same manner as in Example 1, except that pentaerythritol tetraacrylate (PETTA), a tetrafunctional compound, was used as the polymer precursor for the gel polymer electrolyte composition, and the electrolyte and polymer composition were mixed in a ratio of 98 parts by weight.

[0076] Comparative example 2 (bifunctional polymer) A gel polymer electrolyte composition and a battery were prepared in the same manner as in Example 1, except that polycaprolactone diacrylate, a bifunctional compound, was used as the polymer precursor for the gel polymer electrolyte composition, and the electrolyte and polymer composition were mixed in a ratio of 96:4 parts by weight.

[0077] Evaluation Example 3: Lifetime Characteristics Evaluation The batteries prepared in Example 1, Example 2, and Comparative Example 2 underwent initial charging, discharging, and cycling in the same manner as in Evaluation Example 2. The discharge capacity was measured over 200 cycles, and the capacity retention rate, which is the ratio of the discharge capacity in each cycle to the initial discharge capacity, was evaluated. The results are shown in Figure 4.

[0078] Referring to Figure 4, in the case of Comparative Example 2, the capacity decreased from around 50 cycles, and the capacity retention rate at 200 cycles was very low at around 50%, whereas in Examples 1 and 2, good capacity retention rates were observed. This confirms that, compared to Comparative Example 2, in which a bifunctional polymer was applied to the gel polymer electrolyte, Examples 1 and 2, which applied a trifunctional or higher polymer, showed an improvement in lifespan characteristics.

[0079] Comparative Example 3 (Electrolyte Change) A gel polymer electrolyte composition and a battery were prepared in the same manner as in Example 1, except that the electrolyte component of the gel polymer electrolyte composition was prepared by adding 5 parts by volume of fluoroethylene carbonate to 100 parts by volume of a solvent containing a mixture of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:5:2, adding 3 parts by volume of vinylene carbonate, and dissolving LiPF6 lithium salt at a molar concentration of 1.15 M.

[0080] Comparative Example 4 (Electrolyte Change) A gel polymer electrolyte composition and a battery were prepared in the same manner as in Example 1, except that the electrolyte component of the gel polymer electrolyte composition was prepared by dissolving LiTFSI lithium salt at a molar concentration of 0.8 M and LiDFOB lithium salt at a molar concentration of 0.2 M in a solvent containing a mixture of ethyl methyl carbonate and fluoroethylene carbonate in a volume ratio of 3:1.

[0081] Evaluation Example 4: Lifetime Characteristics Evaluation The batteries prepared in Example 1, Comparative Example 3, and Comparative Example 4 underwent initial charge-discharge and cycling in the same manner as in Evaluation Example 2. Approximately 80 cycles were performed, and the ratio of the discharge capacity in each cycle to the initial discharge capacity was evaluated as the capacity retention rate. The results are shown in Figure 5.

[0082] Referring to Figure 5, Example 1 showed a capacity retention rate of over 90%, demonstrating excellent lifespan characteristics. In contrast, Comparative Example 3, which had a different electrolyte composition, experienced a sharp drop in capacity after 20 cycles. Furthermore, Comparative Example 4, which used a different electrolyte composition, saw a sharp decrease in capacity before reaching 10 cycles, indicating significantly poor lifespan characteristics. This confirmed that it is unsuitable for gel polymer electrolyte components.

[0083] Example 3 (More polymer content ratio changed) A gel polymer electrolyte composition and a battery were prepared in the same manner as in Example 1, except that the electrolyte and polymer precursor were mixed in a weight ratio of 99:1.

[0084] Example 4 (More polymer content ratio changed) A gel polymer electrolyte composition and a battery were prepared in the same manner as in Example 1, except that the electrolyte and polymer precursor were mixed in a weight ratio of 95:5.

[0085] Evaluation Example 5: Lifetime Characteristics Evaluation Initial charge-discharge and cycling were performed on the batteries manufactured in Examples 1, 3, and 4 using the same method as in Evaluation Example 2. Approximately 120 cycles were performed, and the ratio of the discharge capacity in each cycle to the initial discharge capacity was evaluated as the capacity retention rate. The results are shown in Figure 6.

[0086] Referring to Figure 6, Example 3, which contains 1% by weight of polymer, showed a capacity retention rate of 80% after 120 cycles, while Example 1, with a polymer content of 3% by weight, and Example 4, with a polymer content of 5% by weight, showed a capacity retention rate of 90%, confirming that they achieved a higher level of lifespan characteristics compared to the other comparative examples.

[0087] Evaluation Example 6: Viscosity Evaluation of Gel Polymer Compositions For each of the following cases—Example 3, where the polymer precursor content is 1% by weight; Example 1, where the polymer precursor content is 3% by weight; Example 4, where the polymer precursor content is 5% by weight; and Reference Example 1, where only the electrolyte is applied without polymer—the viscosity of the gel polymer electrolyte composition was measured before injection into the battery case, and the results are shown in Figure 7.

[0088] Here, viscosity was measured using a Brookfield viscometer with a 62-spindle speed at 5 rpm. The reference solvent for viscosity measurement was mineral oil (KS1000 & 5000) as a viscometer calibration standard solution.

[0089] Referring to Figure 7, it was confirmed that the conductivity of the gel polymer electrolyte composition actually decreases as the polymer content increases from 0% to 5%. When the viscosity of the gel polymer electrolyte composition increases relative to the electrolyte due to the polymer component, it can be difficult to pour it into the battery case, which can cause problems in the process. However, the gel polymer electrolyte composition according to one embodiment exhibits a viscosity even lower than the viscosity of the contained electrolyte (0% polymer), ensuring good processability without any problems during the pouring process, and offering the advantage of being able to directly apply the manufacturing process of existing non-aqueous electrolyte batteries.

[0090] Although preferred embodiments have been described in detail above, the scope of the present invention is not limited thereto. Furthermore, various modifications and improvements by those skilled in the art that utilize the basic concepts defined in the claims also fall within the scope of the present invention. [Explanation of symbols]

[0091] 100: Lithium-ion rechargeable battery 112: Negative electrode 113: Separator 114: Positive electrode 120:Battery container 140: Encapsulation material

Claims

1. A lithium secondary battery comprising a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and a gel polymer electrolyte, The positive electrode comprises a positive electrode active material containing a lithium cobalt-based oxide. The gel polymer electrolyte comprises a polymer and an electrolyte solution. The aforementioned polymer has three or more functional groups and does not contain an ether group. The electrolyte comprises a lithium salt and a solvent, wherein the lithium salt is lithium difluoro(oxalate) borate and LiBF ４ Includes, In the polymer of the gel polymer electrolyte, the functional group is an ester group, a carbonate group, or a combination thereof. In the electrolyte of the gel polymer electrolyte, the solvent comprises fluoroethylene carbonate. A lithium secondary battery in which the fluoroethylene carbonate is contained in an amount of 5% to 50% by volume relative to 100% by volume of the solvent.

2. The lithium secondary battery according to claim 1, wherein the polymer in the gel polymer electrolyte is derived from trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, or a combination thereof.

3. The lithium secondary battery according to claim 1, wherein the weight ratio of the polymer to the electrolyte in the gel polymer electrolyte is 1:99 to 10:

90.

4. The lithium secondary battery according to claim 1, wherein the concentration of the lithium salt in the electrolyte of the gel polymer electrolyte is 0.3 M to 4 M.

5. The lithium difluoro(oxalate) borate and the LiBF in the electrolyte of the gel polymer electrolyte. ４ The lithium secondary battery according to claim 1, wherein the molar ratio is 20:80 to 80:

20.

6. The lithium secondary battery according to claim 1, wherein the solvent in the electrolyte of the gel polymer electrolyte is a carbonate-based solvent, an ester-based solvent, or a combination thereof.

7. The lithium cobalt-based oxide in the positive electrode is represented by the following chemical formula 1, according to claim 1: [Chemical formula 1] Li ａ Co ｘ M １ ｙ M ２ １－ｘ－ｙ O ２ In the above chemical formula 1, 0.9 ≤ a ≤ 1.8, 0.7 ≤ x ≤ 1, 0 ≤ y ≤ 0.3, M １ and M ２ Each of these is independently Al, B, Ba, Ca, Ce, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, or a combination thereof.

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