Liquid electrolytes and lithium secondary batteries

Abstract

To provide liquid electrolyte enabling materialization of excellent safety of a battery, and a lithium secondary battery including the liquid electrolyte.SOLUTION: A liquid electrolyte includes lithium salt, non-aqueous organic solvent, and an additive agent. The additive agent includes polymer expressed by the following chemical formula 1. [Chemical Formula 1] In the chemical formula 1, each of R1 to R3 is a dependent alkyl group of hydrogen, fluorine, or 1 to 10 of substituted or unsubstituted carbon, and each of n and m is a mole fraction, and satisfies inequality 50 mol%≤n≤90 mol%, and 10 mol%≤m≤50 mol%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to a liquid electrolyte and a lithium secondary battery containing the same. [Background technology]

[0002] Lithium-ion batteries are rechargeable and have an energy density more than three times higher per unit weight than conventional lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, and nickel-zinc batteries, enabling fast charging. As a result, they have been commercialized for use in laptops, mobile phones, power tools, and electric bicycles, and research and development to further improve energy density is actively underway.

[0003] Such lithium secondary batteries are used by injecting an electrolyte into a battery cell that includes a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium, and a negative electrode containing a negative electrode active material capable of intercalating and deintercalating lithium.

[0004] The electrolytes in currently commercially available lithium-ion batteries use organic carbonate-based solvents to withstand high operating voltages.

[0005] However, organic carbonate solvents are highly volatile and flammable, and have a low flash point. As a result, vaporization at high temperatures can cause the battery to swell, and the polyethylene (PE) separation membrane to melt and shrink, potentially leading to safety problems in lithium secondary batteries.

[0006] To solve the safety problems of lithium secondary batteries, extensive research has been conducted, including separator membranes, current collectors, active materials, and battery management systems (Battery managing system, BMS). However, as long as highly flammable organic solvents are used, it is not easy to solve the problems. In fact, recently, due to the frequent occurrence of ignition accidents in electric vehicles, the safety problems of lithium secondary batteries have been actively discussed. In particular, to solve the fundamental problems, it is necessary to derive a solution method from the perspective of electrolytes.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] One embodiment provides a liquid electrolyte capable of realizing excellent battery safety.

[0009] Another embodiment provides a lithium secondary battery including the above liquid electrolyte.

Means for Solving the Problems

[0010] The liquid electrolyte according to one embodiment of the present invention includes a lithium salt, a non-aqueous organic solvent, and an additive, and the additive includes a polymer represented by the following Chemical Formula 1, providing a liquid electrolyte. [Chemical Formula 1]

Chem.

[0011] A lithium secondary battery according to another embodiment of the present invention includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and the liquid electrolyte. [Effects of the Invention]

[0012] This invention provides a lithium secondary battery with improved safety features that prevents short circuits, overheating, and explosions by blocking ion conduction in the battery when a problem occurs, thereby suppressing the swelling phenomenon caused by the vaporization of the electrolyte and inhibiting the contraction of the separation membrane. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic cross-sectional view showing a lithium secondary battery according to one embodiment. [Figure 2] This is a schematic cross-sectional view showing a lithium secondary battery according to one embodiment. [Figure 3] This is a schematic cross-sectional view showing a lithium secondary battery according to one embodiment. [Figure 4] This is a schematic cross-sectional view showing a lithium secondary battery according to one embodiment. [Figure 5] This graph shows the discharge capacity and Coulomb efficiency of a lithium secondary battery as the charge-discharge cycle progresses at high temperatures. [Figure 6] This is a graph showing the results of a hot-box test of pouch-full cells. [Figure 7] This graph shows the TGA analysis results of a liquid electrolyte relating to one embodiment of the present invention. [Figure 8] This shows the results of measuring the DC resistance of cells at room temperature (25°C) and after maintaining a high temperature (120°C) for one hour. [Modes for carrying out the invention]

[0014] 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 a variety of different forms and is not limited to the embodiments described herein.

[0015] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Unless the context clearly indicates otherwise, singular expressions include plural expressions.

[0016] In this specification, “these combinations” means mixtures of components, laminates, composites, copolymers, alloys, blends, reaction products, etc.

[0017] In this specification, terms such as “includes,” “equip,” or “have” are intended to specify the presence of implemented features, figures, stages, components, or combinations thereof, and should be understood not to preemptively exclude the presence or possibility of adding one or more other features, figures, stages, components, or combinations thereof.

[0018] Unless otherwise defined herein, particle size may refer to average particle size. Furthermore, average particle size refers to the average particle size (D50), which means the diameter of the particle whose cumulative volume in the particle size distribution is 50% by volume. Average particle size (D50) can be measured by methods widely known to those skilled in the art, such as using a particle size analyzer, or by using a transmission electron microscope or scanning electron microscope. Alternatively, it can be measured using a dynamic light-scattering device, and after data analysis to determine the number of particles for each particle size range, the average particle size (D50) can be calculated. Alternatively, it can be measured using the laser diffraction method. When measuring using the laser diffraction method, more specifically, the particles to be measured are dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT3000), and after irradiating with ultrasound at approximately 28 kHz at an output of 60 W, the average particle size (D50) based on the 50% particle size distribution in the analyzer can be calculated.

[0019] 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.

[0020] Furthermore, in this specification, the term "layer" includes not only the shapes formed on the entire surface when observed in a plan view, but also shapes formed on a part of the surface.

[0021] In this specification, "or" is not interpreted as having an exclusive meaning; for example, "A or B" is interpreted as including A, B, A+B, etc.

[0022] The term "metal" is interpreted as encompassing general metals, transition metals, and metalloids.

[0023] In this specification, "substituted" means that, unless otherwise defined, a substituent or at least one hydrogen atom in a compound is substituted with deuterium, a halogen group, a hydroxyl group, an amino group, a substituted or unsubstituted C1-C30 amine group, a nitro group, a substituted or unsubstituted C1-C40 silyl group, a C1-C30 alkyl group, a C1-C10 alkylsilyl group, a C6-C30 arylsilyl group, a C3-C30 cycloalkyl group, a C3-C30 heterocycloalkyl group, a C6-C30 aryl group, a C2-C30 heteroaryl group, a C1-C20 alkoxy group, a C1-C10 fluoroalkyl group, a cyano group, or a combination thereof.

[0024] In one example of the present invention, "substitution" means that at least one hydrogen atom in the substituent or compound is substituted with deuterium, a halogen group, a C1-C30 alkyl group, a C1-C10 alkylsilyl group, a C6-C30 arylsilyl group, a C3-C30 cycloalkyl group, a C3-C30 heterocycloalkyl group, a C6-C30 aryl group, a C2-C30 heteroaryl group, a C1-C10 fluoroalkyl group, or a cyano group. Furthermore, in a specific example of the present invention, "substitution" means that at least one hydrogen atom in the substituent or compound is substituted with deuterium, a halogen group, a C1-C20 alkyl group, a C6-C30 aryl group, a C1-C10 fluoroalkyl group, or a cyano group. Furthermore, in a specific example of the present invention, "substitution" means that at least one hydrogen atom in the substituent or compound is substituted with deuterium, a halogen group, a C1-C5 alkyl group, a C6-C18 aryl group, a C1-C5 fluoroalkyl group, or a cyano group. Furthermore, in a specific example of the present invention, "substitution" means that a substituent or at least one hydrogen in the compound is substituted with deuterium, a cyano group, a halogen group, a methyl group, an ethyl group, a propyl group, a butyl group, a phenyl group, a biphenyl group, a terphenyl group, a trifluoromethyl group, or a naphthyl group.

[0025] Lithium secondary batteries can be classified into cylindrical, prismatic, pouch-type, coin-type, and other types depending on their form. Figures 1 to 4 are schematic diagrams showing a lithium secondary battery according to one embodiment, where Figure 1 is circular, Figure 2 is prismatic, and Figures 3 and 4 are pouch-type batteries. Referring to Figures 1 to 4, the lithium secondary battery 100 may include an electrode assembly 40 with a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 in which the electrode assembly 40 is housed. The positive electrode 10, negative electrode 20, and separator 30 may be impregnated with an electrolyte (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the case 50, as shown in Figure 1. Also, in Figure 2, the lithium secondary battery 100 may include a positive electrode lead tap 11 and a positive electrode terminal 12, a negative electrode lead tap 21 and a negative electrode terminal 22. As shown in Figures 3 and 4, the lithium secondary battery 100 may include electrode taps 70, namely a positive electrode tap 71 and a negative electrode tap 72, which serve as electrical pathways for guiding the current formed in the electrode assembly 40 to the outside.

[0026] The following describes an electrolyte according to one embodiment.

[0027] The electrolyte according to one embodiment of the present invention is a liquid electrolyte comprising a lithium salt, a non-aqueous organic solvent, and an additive, the additive may include a polymer represented by the following chemical formula 1. [Chemical formula 1] [ka] In chemical formula 1, R 1 ~R 3 Each of these is independently hydrogen, fluorine, or a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms. n and m are mole fractions, where 50 mol% ≤ n ≤ 90 mol% and 10 mol% ≤ m ≤ 50 mol%.

[0028] The polymer represented by chemical formula 1 gels through crystallization in the liquid electrolyte when exposed to high temperatures above 100°C. This not only suppresses the vaporization of the electrolyte and the resulting expansion of the battery, but also blocks ion and electron conduction in the battery. Through a mechanism that physically fixes the PE separation membrane, which melts and shrinks at approximately 140°C, it prevents direct contact between the positive and negative electrodes, i.e., prevents the rapid release of energy due to a short circuit, thereby suppressing fire and explosion and improving the safety of the battery.

[0029] In particular, the polymer represented by chemical formula 1 is dispersed in a liquid electrolyte before problems such as high temperature or impact occur. Since the electrolyte containing the polymer represented by chemical formula 1 is a liquid electrolyte, its composition in the electrolyte phase differs from that of a solid electrolyte formed by the polymerization of polymers together with a polymerization initiator.

[0030] In the case of a solid electrolyte, the electrolyte exists in a solid state within the battery, resulting in low ionic conductivity, low output, and a short lifespan. This should result in a significant difference in performance compared to batteries using the liquid electrolyte according to the present invention.

[0031] For example, n and m may be such that n:m is 80mol%:20mol%~60mol%:40mol% or 80mol%:20mol%~70mol%:30mol%.

[0032] The higher the relative content of vinylidene fluoride structural units within the polymer compared to the content of trifluoroethylene structural units, the more effectively the safety of the battery is improved.

[0033] The molecular weight of the polymer may range from 100,000 g / mol to 400,000 g / mol.

[0034] For example, the molecular weight of a polymer may range from 200,000 g / mol to 400,000 g / mol.

[0035] The polymer may be a particulate phase having an average particle size of 10 nm to 1 μm.

[0036] For example, the average particle size of the polymer may be 10 nm to 900 nm, 10 nm to 800 nm, 10 nm to 700 nm, or 10 nm to 600 nm.

[0037] For example, the average particle size of the polymer may be 30 nm to 1 μm, 50 nm to 1 μm, 100 nm to 1 μm, 100 nm to 900 nm, 100 nm to 800 nm, 100 nm to 700 nm, or 100 nm to 600 nm.

[0038] The polymer may be in particulate form, and if the average particle size is within the above range, the precipitation of the polymer in the electrolyte can be mitigated, thereby reducing the volume decrease due to repeated cycles.

[0039] As an example, R in chemical formula 1 1 ~R 3 Each of these may independently be hydrogen, or a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms.

[0040] For example, the polymer may be poly(vinylidene fluoride-co-trifluoroethylene).

[0041] The polymer may gel at a temperature of 100°C to 120°C.

[0042] For example, the polymer may be included in an amount of 0.1 to 15 parts by weight per 100 parts by weight of the total liquid electrolyte.

[0043] Specifically, the polymer may be included in an amount of 0.5 to 15 parts by weight, for example, 1 to 15 parts by weight, or 5 to 15 parts by weight, per 100 parts by weight of the total liquid electrolyte.

[0044] When the polymer content is within the above range, it is possible to realize a lithium secondary battery with improved safety by preventing resistance increase at high temperatures while ensuring lifespan and output characteristics.

[0045] Non-aqueous organic solvents act as a medium through which ions involved in the electrochemical reactions of batteries can move.

[0046] As non-aqueous organic solvents, carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvents can be used.

[0047] As carbonate-based solvents, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC) can be used. As ester-based solvents, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, decanolide, mevalonolactone, and caprolactone can be used. As ether-based solvents, dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran can be used. Furthermore, cyclohexanone can be used as a ketone solvent. Ethyl alcohol and isopropyl alcohol can be used as alcohol solvents, and R can be used as an aprotic solvent. 1 -CN(R 1 Nitriles such as nitriles (which are linear, branched, or cyclic hydrocarbon groups having 2 to 20 carbon atoms and may include double-bonded aromatic rings or ether bonds), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes can be used.

[0048] Non-aqueous organic solvents can be used individually or in combination of one or more, and the mixing ratio when used in combination can be appropriately adjusted according to the desired battery performance, which should be widely understood by those working in this field.

[0049] A non-aqueous organic solvent according to one embodiment of the present invention may contain a cyclic carbonate and a linear carbonate (cyclic carbonate: linear carbonate) in a volume ratio of 10:90 vol% to 50:50 vol%.

[0050] Specifically, the non-aqueous organic solvent may contain cyclic carbonates and linear carbonates in a volume ratio of 10:90 vol% to 40:60 vol%, for example, 10:90 vol% to 30:70 vol% or 10:90 vol% to 25:75 vol%.

[0051] In the aforementioned non-aqueous organic solvent, above a certain temperature, the polymers in the liquid electrolyte gel, increasing the viscosity of the electrolyte. As the resistance increases, the gelled polymers act as insulators, blocking ion and electron conduction in the battery, thus ensuring the safety of the battery.

[0052] The liquid electrolyte may further contain at least one other additive from among vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, vinylethylene carbonate (VEC), adiponitrile (AN), succinate nitrile (SN), 1,3,6-hexanetricyanide (HTCN), propensultone (PST), propanesultone (PS), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), and 2-fluorobiphenyl (2-FBP).

[0053] By further containing other additives, the lifespan can be further improved, or the gases generated from the positive electrode and the negative electrode during high-temperature storage can be effectively controlled.

[0054] The other additives may be contained in a content of 0.2 to 20 parts by weight, specifically 0.2 to 15 parts by weight, for example 0.2 to 10 parts by weight, based on 100 parts by weight of the whole electrolyte for lithium secondary batteries.

[0055] When the content of the other additives is within the above range, it can minimize the increase in film resistance and contribute to the improvement of battery performance.

[0056] The lithium salt is dissolved in an organic solvent, acts as a source of lithium ions in the battery to enable the basic operation of the lithium secondary battery, and plays a role in promoting the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI)), LiC4F9SO3, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2)(where x and y are integers from 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFOB), lithium bis(oxalato)borate (LiBOB), and can include one or more selected therefrom.

[0057] Another embodiment of the present invention provides a lithium secondary battery including a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and the aforementioned liquid electrolyte.

[0058] The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material and can optionally further contain a binder and / or a conductive material.

[0059] As the positive electrode active material, a lithium-intercalated compound capable of reversible intercalation and deintercalation can be used. Specifically, one or more composite oxides of lithium with metals selected from cobalt, manganese, nickel, and combinations thereof can be used.

[0060] The composite oxide may be a lithium transition metal composite oxide, and specific examples include lithium nickel oxides, lithium cobalt oxides, lithium manganese oxides, lithium iron phosphate compounds, cobalt-free nickel-manganese oxides, or combinations thereof.

[0061] As an example, a compound represented by any one of the following chemical formulas can be used: Li a A 1-b X b O 2-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li a Mn 2-b X b O 4-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li a Ni 1-b-c Co b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2);Li a Ni 1-b-c Mn b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2);Li a Ni b Co c L 1 d G eO2(0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5, 0≦e≦0.1);Li a NiG b O2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a CoG b O2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn 1-b G b O2(0.90≦a≦1.8, 0.001b≦0.1);Li a Mn2G b O4(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn 1-g G g PO4(0.90≦a≦1.8, 0≦g≦0.5);Li (3-f) Fe2(PO4)3(0≦f≦2);Li a FePO4 (0.90 ≤ a ≤ 1.8).

[0062] In the above chemical formula, A is Ni, Co, Mn, or a combination thereof; X is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements, or a combination thereof; D is O, F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; L 1 These are Mn, Al, or a combination of these.

[0063] As an example, the positive electrode active material may be a high-nickel positive electrode active material in which the nickel content relative to 100 mol% of the metal excluding lithium is 80 mol% or more, 85 mol% or more, 90 mol% or more, 91 mol% or more, or 94 mol% or more and 99 mol% or less in a lithium transition metal composite oxide. High-nickel positive electrode active materials can achieve high capacity and can be applied to high-capacity, high-density lithium secondary batteries.

[0064] The content of the positive electrode active material is 90% to 99.9% by weight relative to 100% by weight of the positive electrode active material layer, and the content of the binder and conductive material may be 0.1% to 5% by weight, respectively, relative to 100% by weight of the positive electrode active material layer.

[0065] 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 positive electrode 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, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, and nylon.

[0066] Conductive materials are used to impart conductivity to electrodes, and any electronically conductive material that does not cause chemical changes can be used in the battery that is constructed. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fibers, carbon nanofibers, and carbon nanotubes; metallic materials in the form of metal powders or metal fibers containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0067] Al can be used as the positive electrode current collector, but is not limited to this.

[0068] The negative electrode comprises a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, and optionally further comprising a binder and / or conductive material.

[0069] The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and undoping lithium, or a transition metal oxide.

[0070] Examples of the material capable of reversibly intercalating / deintercalating lithium ions include carbon-based negative electrode active materials, which can include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, and examples of amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0071] As the alloy of lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0072] As the material capable of doping and undoping lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material can be used. The Si-based negative electrode active material is silicon, a silicon-carbon composite, SiO x (0 < x < 2), a Si-Q alloy (Q is selected from an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element (excluding Si), a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof), or a combination thereof. The Sn-based negative electrode active material may be Sn, SnO2, a Sn-based alloy, or a combination thereof.

[0073] A silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in the form of silicon particles and amorphous carbon coated on the surface of the silicon particles. For example, it may include secondary particles (core) in which primary silicon particles are granulated and an amorphous carbon coating layer (shell) located on the surface of these secondary particles. Amorphous carbon may also be located between the primary silicon particles, for example, the primary silicon particles may be coated with amorphous carbon. The secondary particles may be dispersed in the amorphous carbon matrix.

[0074] The silicon-carbon composite may further contain crystalline carbon. For example, the silicon-carbon composite may include a core containing crystalline carbon and silicon particles and an amorphous carbon coating layer located on the surface of this core.

[0075] Si-based or Sn-based anode active materials can be used in combination with carbon-based anode active materials.

[0076] For example, the negative electrode active material layer may contain 90% to 99% by weight of the negative electrode active material, 0.5% to 5% by weight of the binder, and 0% to 5% by weight of the conductive material.

[0077] 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. Non-aqueous binders, aqueous binders, dry binders, or combinations thereof can be used as binders.

[0078] Examples of non-aqueous binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or combinations thereof.

[0079] The water-based binder may be selected from styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0080] When using an aqueous binder as the negative electrode binder, it may further contain a cellulose series compound that can impart viscosity. This cellulose series compound can be a mixture of one or more carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or their alkali metal salts. Na, K, or Li can be used as the alkali metal.

[0081] The dry binder is a polymeric substance that can be formed into fibers, and may be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0082] Conductive materials are used to impart conductivity to electrodes, and any electronically conductive material that does not cause chemical changes can be used in the battery that is constructed. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fibers, carbon nanofibers, and carbon nanotubes; metallic materials in the form of metal powders or metal fibers containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0083] As the negative electrode current collector, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrates coated with conductive metal, and combinations thereof can be selected.

[0084] Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such separators can be made of polyethylene, polypropylene, polyvinylidene fluoride, or multilayer films of two or more layers of these materials. Mixed multilayer films such as polyethylene / polypropylene two-layer separators, polyethylene / polypropylene / polyethylene three-layer separators, and polypropylene / polyethylene / polypropylene three-layer separators can also be used.

[0085] The separator may include a porous substrate and a coating layer containing organic, inorganic, or a combination thereof located on one or both sides of the porous substrate.

[0086] The porous substrate may be a polymer film formed from one polymer selected from polyethylene, polyolefins such as polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyetherketone, polyaryletherketone, polyetherimide, polyamideimide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon®, and polytetrafluoroethylene, or from a copolymer or mixture of two or more of these polymers.

[0087] The organic material may include polyvinylidene fluoride polymers or (meth)acrylic polymers.

[0088] Inorganic materials may include, but are not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0089] Organic and inorganic materials can exist mixed together in a single coating layer, or they can exist in a form where a coating layer containing organic materials and a coating layer containing inorganic materials are stacked on top of each other.

[0090] Examples and comparative examples of the present invention are described below. However, the following examples are merely one embodiment of the present invention, and the present invention is not limited to these examples. [Examples]

[0091] (Example: Fabrication of a lithium secondary battery) (Example 1) LiNi 0.8 Co 0.1 Mn 0.1 A cathode active material slurry was prepared by mixing O2, polyvinylidene fluoride as a binder, and acetylene black as a conductive material in a weight ratio of 97.5:1:1.5, and dispersing the mixture in N-methylpyrrolidone.

[0092] A cathode active material slurry was coated onto a 15 μm thick aluminum (Al) foil, dried at 100°C, and then rolled (pressed) to produce the cathode.

[0093] Artificial graphite was used as the negative electrode active material. The negative electrode active material was mixed with styrene-butadiene rubber binder and carboxymethylcellulose in a weight ratio of 98:1:1, and the mixture was dispersed in distilled water to prepare a negative electrode active material slurry.

[0094] The negative electrode active material slurry was coated onto a 10 μm thick copper (Cu) foil, dried at 100°C, and then rolled (pressed) to produce the negative electrode.

[0095] An electrode assembly was fabricated by assembling the fabricated positive and negative electrodes with a 10 μm thick polyethylene separator, and a lithium secondary battery was manufactured by injecting a liquid electrolyte.

[0096] The composition of the liquid electrolyte is as follows: (liquid electrolyte composition) Lithium salt: LiPF61.15M Non-aqueous organic solvent: ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate (volume ratio of EC:EMC:DMC = 20:40:40) Additives: (1) Polymer: Solvene(R)200 / P200 (Mw=200,000, n:m=80:20; Solvay) 15 parts by weight, (2) 1 part by weight of vinylene carbonate (VC) (However, in the electrolyte composition, "parts by weight" refers to the relative weight of the additive to the total weight of the electrolyte (lithium salt + non-aqueous organic solvent) excluding the additive.)

[0097] (Example 2) A lithium secondary battery was fabricated in the same manner as in Example 1, except that Solvene(R)200 / P400 (Mw=400,000, n:m=80:20; Solvay) was used instead of Solvene(R)200 / P200.

[0098] (Example 3) A lithium secondary battery was fabricated in the same manner as in Example 1, except that Solvene(R)250 / P400 (Mw=400,000, n:m=75:25; Solvay) was used instead of Solvene(R)200 / P200.

[0099] (Examples 4 and 5) A lithium secondary battery was prepared in the same manner as in Example 1, except that the polymer content was changed to 5 parts by weight and 10 parts by weight, respectively, to produce the liquid electrolyte.

[0100] (Comparative Example 1) A lithium secondary battery was fabricated in the same manner as in Example 1, except that polymers were not used.

[0101] (Comparative Example 2) A lithium secondary battery was prepared in the same manner as in Example 1, except that a composition containing 1 g of polymer (Solvene(R)200 / P200) and 0.5 g of polymerization initiator (AIBN) was added to 95.5 g of a non-aqueous organic solvent (ethylene carbonate:ethyl methyl carbonate:dimethyl carbonate = 20:40:40 volume ratio) in which 1.15 M LiPF6 and 1 part by weight of vinylene carbonate (VC) were dissolved, and the mixture was heated at 60°C for 24 hours to form a gel polymer electrolyte.

[0102] (Evaluation 1: High-temperature (45°C) life characteristics evaluation) The lithium secondary batteries prepared in Examples 1, 4, 5, and Comparative Example 1 were subjected to 3 charge-discharge cycles at 1C, followed by 50 cycles of charging at 1.5C and discharging at 1C to evaluate the battery's discharge capacity and Coulomb efficiency. The evaluation results are shown in Figure 5. The voltage range used was 3.0V to 4.2V (discharge up to 3.0V and charge up to 4.2V), and the evaluation temperature was 45°C.

[0103] Figure 5 is a graph showing the discharge capacity and Coulomb efficiency of a lithium secondary battery as the charge-discharge cycle progresses at high temperatures.

[0104] Referring to Figure 5, it can be confirmed that the liquid electrolyte containing the polymer according to the present invention maintains discharge capacity and Coulomb efficiency at an equivalent or higher level compared to liquid electrolytes without the polymer. From these evaluation results, it can be predicted that the battery's lifespan characteristics will be maintained at an excellent level without deterioration.

[0105] (Rating 2: Hotbox rating) After the chemical conversion cycle of full pouch cells from Examples 1-5 and Comparative Example 1, they were charged to SOC50 (3.94V), exposed to 130°C silicone oil for 5 minutes, and the degree of volume expansion was compared. The evaluation results are shown in Figure 6.

[0106] Figure 6 is a graph showing the results of the hot-box test of pouch-full cells.

[0107] Referring to Figure 6, it can be seen that in the pouch cell without polymer (Comparative Example 1), the pouch cell ruptured due to volume expansion caused by the gasification of the electrolyte, but in the pouch cell containing polymer (Example), the degree of volume expansion was mitigated because the gasification of the electrolyte was suppressed. In other words, it can be seen that thermal safety is improved even in high-temperature storage environments when a liquid electrolyte containing polymer is used.

[0108] (Rating 3: TGA (Thermo Gravimetric Analysis) rating) To measure the weight change of a substance due to temperature changes, TGA analysis was performed on Example 1, Comparative Example 1, and Comparative Example 2. The evaluation results are shown in Figure 7.

[0109] Figure 7 is a graph showing the TGA analysis results of a liquid electrolyte according to one embodiment of the present invention.

[0110] Referring to Figure 7, in the case of the liquid electrolyte according to Example 1, the TGA analysis results show that the temperature at which the mass at room temperature is halved by vaporization and thermal decomposition is 124°C, which is 42°C higher than the temperature at which the mass of the liquid electrolyte (without polymer) according to Comparative Example 1 is halved (82°C). This indicates that when a liquid electrolyte containing dissolved polymer is introduced, the organic solvent participates in the dissolution of the polymer, causing an increase in the boiling point due to van der Waals forces between the polymer and the solvent. This delays the vaporization of the electrolyte at high temperatures, thereby delaying the expansion of the battery and the explosion caused by the increase in internal pressure.

[0111] On the other hand, when the polymer itself was evaluated, no mass halving occurred. In Comparative Example 2, where a gel polymer electrolyte was applied, it was confirmed that the effects of delayed vaporization of the electrolyte, battery expansion, and explosion delay due to increased internal pressure, which are the objectives of the present invention, could not be expected.

[0112] (Evaluation 4: High-temperature DC-IR resistance (DC-IR) characteristic evaluation) For the lithium secondary batteries prepared in Example 1 and Comparative Example 1, the initial DC resistance (DCIR) was measured using the ΔV / ΔI (voltage change / current change) value. Then, the maximum energy state inside the battery was set to a fully charged state (SOC 100%), and the DC resistance was measured at room temperature (25°C) in this state. After reaching a high temperature (120°C) and maintaining it for 1 hour, the DC resistance of the cells was measured. The evaluation results are shown in Figure 8.

[0113] Figure 8 shows the results of measuring the DC resistance of cells that were maintained for one hour after reaching room temperature (25°C) and high temperature (120°C).

[0114] Referring to Figure 8, it can be predicted that when the battery with the liquid electrolyte according to Example 1 is heated to 120°C, the resistance increases significantly due to the gelation of the electrolyte. As a result, the gelled electrolyte acts as an insulator between the positive and negative electrodes, reducing the risk of rapid energy release, explosion, and fire due to a short circuit between the positive and negative electrodes.

[0115] (Evaluation 5: DC-IR resistance characteristics after high-temperature storage) For the lithium secondary batteries prepared according to Example 1 and Comparative Example 2, the initial DC-IR (DC-IR) was measured using the ΔV / ΔI (voltage change / current change) value. Then, the maximum energy state inside the battery was set to a fully charged state (SOC 100%), and the DC-IR was measured at room temperature (25°C) in this state. The DC-IR was then measured after being left at a high temperature (60°C) for 5 days. The evaluation results are shown in Table 1.

[0116] [Table 1]

[0117] Referring to Table 1, during gel polymer formation (Comparative Example 2), resistance increase already occurs at 50°C to 60°C when the initiator is activated, and the high-temperature resistance value of the cell is higher than that of the Example. In addition, a further resistance increase occurs due to the sustained crosslinking reaction, resulting in a significantly higher resistance increase rate compared to the Example.

[0118] On the other hand, since the polymer in the liquid electrolyte in the examples gels at temperatures of 100°C to 120°C, the increase in resistance due to gelation hardly occurs at 60°C, so the resistance when left standing at 60°C is relatively low.

[0119] In the present invention, the liquid electrolyte containing the polymer additive undergoes gelation when the polymer crystallizes at high temperatures. The gelled liquid electrolyte forms a solid film that acts as an insulator, blocking ion conduction and electrical conduction between the positive and negative electrodes, thereby suppressing battery ignition and explosion.

[0120] Furthermore, the gelled liquid electrolyte can suppress high-temperature shrinkage of the separation membrane by physically bonding the positive electrode, separation membrane, and negative electrode, thereby preventing short circuits between the positive and negative electrodes.

[0121] While preferred embodiments of the present invention have been described above, the present invention is not limited thereto. It can be implemented in various ways within the scope of the claims, the detailed description of the invention, and the attached drawings, and these also naturally fall within the scope of the present invention. [Explanation of Symbols]

[0122] 100: Lithium-ion rechargeable battery 10: Positive electrode 11: Positive lead tap 12: Positive terminal 20: Negative electrode 21: Negative lead tap 22: Negative terminal 30: Separator 40: Electrode assembly 50: Case 60: Sealing member 70: Electrode Tap 71: Positive Tap 72: Negative Tap

Claims

1. It contains a lithium salt, a non-aqueous organic solvent containing three types of carbonate-based solvents, and additives. The three types of carbonate-based solvents mentioned above include cyclic carbonates and linear carbonates. The aforementioned additive is a liquid electrolyte containing a polymer represented by the following chemical formula 1: [Chemical formula 1] 【Chemistry 1】 In the aforementioned chemical formula 1, R １ ~R ３ Each of these is independently hydrogen, fluorine, or a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms. n and m are mole fractions, respectively, where 50 mol% ≤ n ≤ 90 mol% and 10 mol% ≤ m ≤ 50 mol%.

2. The liquid electrolyte according to claim 1, wherein n and m are such that n:m is 80 mol%:20 mol% to 60 mol%:40 mol%.

3. The liquid electrolyte according to claim 1, wherein the molecular weight of the polymer is 100,000 g / mol to 400,000 g / mol.

4. The liquid electrolyte according to claim 1, wherein the polymer is a particle phase having an average particle size of 10 nm to 1 μm.

5. R of the above chemical formula 1 １ ~R ３ The liquid electrolyte according to claim 1, wherein each is independently hydrogen or a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms.

6. The liquid electrolyte according to claim 1, wherein the polymer is poly(vinylidene fluoride-co-trifluoroethylene).

7. The liquid electrolyte according to claim 1, wherein the polymer gels at a temperature of 100°C to 120°C.

8. The liquid electrolyte according to claim 1, wherein the polymer is contained in an amount of 0.1 to 15 parts by weight per 100 parts by weight of the total liquid electrolyte.

9. The liquid electrolyte according to claim 1, wherein the non-aqueous organic solvent contains the cyclic carbonate and the linear carbonate in a volume ratio of 10:90 vol% to 50:50 vol%.

10. The liquid electrolytes include vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, vinylethylene carbonate (VEC), adiponitrile (AN), succinate nitrile (SN), 1,3,6-hexanetricyanide (HTCN), propensultone (PST), propanesultone (PS), and lithium tetrafluoroborate (LiBF). ４ ), lithium difluorophosphate (LiPO ２ F ２ The liquid electrolyte according to claim 1, further comprising at least one other additive from among ), and 2-fluorobiphenyl (2-FBP).

11. Positive electrode containing positive electrode active material, A negative electrode containing a negative electrode active material, and A lithium secondary battery comprising the liquid electrolyte described in any one of claims 1 to 10.

Images