Negative electrode, lithium secondary battery containing it, and battery pack

Abstract

This specification relates to a negative electrode 100 comprising a flame-retardant polymer layer 3 provided on a negative electrode active material layer 2, wherein the flame-retardant polymer layer comprises a flame retardant that is converted to char (later forming an oxygen barrier layer 4) when burned at a temperature of 150°C or higher, a lithium secondary battery containing the same, and a battery pack. The negative electrode according to the above embodiment can ensure the desired battery characteristics under normal conditions while preventing thermal runaway under specific temperature conditions.

Description

[Technical Field]

[0001] This application relates to a negative electrode, a lithium secondary battery including the same, and a battery pack.

[0002] This application claims the benefit as of the filing date of Korean Patent Application No. 10-2023-0140890, filed with the Korean Intellectual Property Office on October 20, 2023, and all its contents are incorporated herein by reference. [Background technology]

[0003] Rechargeable batteries are batteries that can be reused after discharge through recharging. They can be used as an energy source for small devices such as mobile phones, tablet PCs, and vacuum cleaners, and as an energy source for medium-sized or large devices such as personal mobility devices, automobiles, and ESS (Energy Storage Systems) for smart grids.

[0004] In order to enable such a wide range of applications, high performance is also required for secondary batteries. Through numerous studies and developments, lithium secondary batteries, which have high energy density, are easy to process, and can be applied to various electronic devices, are attracting attention.

[0005] However, lithium secondary batteries are not only vulnerable to external shocks, but also prone to internal degradation and frequent fires. These problems can cause significant damage because they lead to heat transfer between numerous battery cells, resulting in thermal runaway at the assembly level, such as battery modules or battery packs connected in series and / or parallel.

[0006] Specifically, if the internal temperature of a lithium secondary battery exceeds a certain temperature, the internal pressure of the cell increases due to the vaporization of the electrolyte, which may cause it to be ejected outside the cell or damage to the separation membrane. If flammable gaseous substances ignite due to the vaporization of the electrolyte or if the separation membrane is damaged, it can cause an internal short circuit, leading to a sustained or chain reaction of inappropriate exothermic reactions due to heating of adjacent cells, which can lead to ignition. This can cause the fire to spread and eventually throughout the entire battery pack. However, lithium ions are highly reactive with water, so attempting to extinguish the fire with water may only cause it to spread further.

[0007] To solve the aforementioned problems, research is needed on materials that can quickly interrupt or suppress the phenomenon of ignition caused by persistent or chain-reaction inappropriate exothermic reactions in lithium-ion batteries. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The inventors have developed a flame-retardant polymer layer for lithium secondary batteries containing a flame retardant that delays ignition above a specific temperature. By applying this flame-retardant polymer layer to the surface of the negative electrode active material layer, they have created a negative electrode that maintains the desired battery characteristics even with a small amount of flame retardant, while simultaneously blocking heat transfer and thermal runaway between cells.

[0009] Specifically, this specification aims to provide a negative electrode comprising a flame-retardant polymer layer containing a flame retardant that is converted to char when burned at a temperature of 150°C or higher, a lithium secondary battery, and a battery pack comprising the same. [Means for solving the problem]

[0010] One embodiment of this specification provides a negative electrode comprising a negative electrode current collector; a negative electrode active material layer provided on the negative electrode current collector; and a flame retardant polymer layer provided on the negative electrode active material layer, wherein the flame retardant polymer layer comprises a flame retardant that converts to char when burned at a temperature of 150°C or higher.

[0011] Other embodiments of the present specification provide a lithium secondary battery including the aforementioned negative electrode; positive electrode; separator interposed between the positive electrode and the negative electrode; and electrolyte.

[0012] Another embodiment of the present specification provides a battery pack including the aforementioned lithium secondary battery as a unit cell.

Advantages of the Invention

[0013] The negative electrode according to one embodiment of the present invention maintains battery characteristics such as a desired long life and low cell resistance under normal conditions, and when burning at a temperature of 150 °C or higher, the flame retardant is converted into char to prevent the inflow of oxygen from the outside and prevent the reaction with the positive electrode, thereby preventing further thermal runaway, delaying the time to thermal runaway, and suppressing gas generation due to ignition.

[0014] Since the negative electrode according to one embodiment of the present invention includes a flame-retardant polymer layer on the negative electrode active material layer, it has the effect of preventing deterioration of the desired battery characteristics by including an amount of flame retardant necessary for preventing thermal runaway.

Brief Description of the Drawings

[0015] [Figure 1] It is a schematic diagram of a negative electrode according to the prior art. [Figure 2] It is a schematic diagram of a negative electrode according to an embodiment of the present invention. [Figure 3] It is a schematic diagram of a negative electrode according to an embodiment of the present invention. [Figure 4] It is a schematic diagram of a lithium secondary battery including a negative electrode according to an embodiment of the present invention. [Figure 5] It is a schematic diagram of a battery pack including a lithium secondary battery according to an embodiment of the present invention. [Figure 6] It is a schematic diagram of a moving means including a battery pack according to an embodiment of the present invention.

Modes for Carrying Out the Invention

[0016] Before describing the present invention, let us first define some terms.

[0017] In this specification, "p~q" can mean the range "p or greater and q or less".

[0018] In this specification, when a part is described as "containing" or "having" a component, unless otherwise defined, it may mean that it may include other components rather than excluding them.

[0019] In this specification, singular expressions include plural expressions unless otherwise defined.

[0020] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as they would be generally understood by a person of ordinary skill in the art to which this invention pertains. Terms as defined in commonly used dictionaries should be interpreted as having the meaning consistent with their meaning in the context of the relevant art, and not as ideal or overly formal unless explicitly defined herein.

[0021] The present invention will be described in detail below with reference to the drawings so that a person with ordinary skill in the art to which the present invention pertains can easily implement it. However, the present invention can be embodied in various different forms and is not limited to the following description.

[0022] <Negative electrode> One embodiment of this specification provides a negative electrode comprising a flame-retardant polymer layer on a negative electrode active material layer provided on a negative electrode current collector, wherein the flame-retardant polymer layer comprises a flame retardant that is converted to char when burned at a temperature of 150°C or higher.

[0023] Referring to Figures 2 and 3, this is a negative electrode 100 according to an embodiment of the present invention, in which a negative electrode active material layer 2 is provided on a negative electrode current collector 1, a flame-retardant polymer layer 3 is provided on the negative electrode active material layer 2, and the flame-retardant polymer layer 3 is converted to char when burned at a temperature of 150°C or higher to form an oxygen barrier layer 4 which forms a film, thereby blocking oxygen 5 flowing in from the outside. This prevents a sustained or chain reaction of exothermic reactions caused by oxygen, and prevents thermal runaway due to heat generation. On the other hand, Figure 1 is a diagram of a conventional negative electrode, in which the negative electrode active material layer 2 is provided on a negative electrode current collector 1, and it is difficult to block oxygen 5 flowing in from the outside.

[0024] The aforementioned exothermic reaction occurs when the cell temperature rises due to abnormal conditions such as physical shock outside the cell or cell aging. This triggers internal reactions between the cell's components, such as the positive electrode, negative electrode, SEI (solid electrolyte interface) layer, and electrolyte, leading to self-heating. Furthermore, if this self-heating becomes severe, the cell may ignite and thermal runaway may occur. In particular, the negative electrode is highly reactive when lithium is charged, and when it comes into contact with the positive electrode, the spontaneous reaction causes serious internal heat generation, which can lead to cell ignition and thermal runaway.

[0025] The negative electrode according to this specification uses a specific flame retardant, and when ignition occurs due to initial self-heating, the flame-retardant polymer layer is converted to char at a specific temperature (150°C) or higher, sealing the pores. This prevents direct contact between the positive and negative electrode active material layers, efficiently blocking sustained heat transfer and oxygen supply inside and outside the negative electrode, and effectively weakening the level of ignition.

[0026] Referring to Figure 4(a), if ignition does not occur, the flame-retardant polymer layer 3 has a porous structure with pores 6. However, referring to Figure 4(b), if ignition occurs, the flame retardant in the flame-retardant polymer layer is converted to char at a specific temperature (150°C) or higher, forming an oxygen barrier layer 4, which can cause pore clogging 7 and interrupt the chain reaction of ignition.

[0027] In this specification, the negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treatments with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used.

[0028] In this specification, the negative electrode current collector may generally have a thickness of 1 μm to 100 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the negative electrode active material. Furthermore, the negative electrode current collector may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0029] In this specification, the term "flame retardant" may also be referred to as "flame retardant substance," and refers to a flame retardant substance contained in a flame retardant polymer layer provided on the surface of the negative electrode active material layer, which ensures flame retardancy by blocking heat transfer pathways and suppressing the generation of decomposition gases.

[0030] In this specification, combustion means an exothermic reaction in which a substance rapidly combines with oxygen while producing heat or a flame. In particular, combustion can mean a reaction that generates heat at a high temperature of 150°C or higher, or a reaction that occurs when an electrolyte vaporizes and ignites, producing heat or a flame, and rapidly reacting with oxygen flowing in from an external source.

[0031] In this specification, char refers to a substance that forms a thermally stable layer through condensation reactions such as dehydration of a flame retardant or a flame retardant composition containing a polymer upon combustion, and can play a role in preventing contact with external substances (especially oxygen) and suppressing combustion.

[0032] In one embodiment of this specification, the amount of flame retardant can be adjusted depending on the composition of the negative electrode active material, the amount of negative electrode active material loaded, etc., but assuming that the flame retardant polymer layer is 5 μm thick, the ratio of the flame retardant to the total amount of negative electrode active material may be about 1% to 10%.

[0033] In one embodiment of this specification, the flame retardant may include a phosphorus-based flame retardant.

[0034] In this specification, phosphorus-based flame retardants may mean substances containing phosphate ester compounds.

[0035] In one embodiment of this specification, the phosphorus-based flame retardant may include one or more selected from the group consisting of polyphosphates, phosphonates, phosphinates, phosphine oxides, and phosphazenes, but is not particularly limited as long as it corresponds to a phosphorus-based flame retardant that is a phosphate ester compound.

[0036] If necessary, the phosphorus-based flame retardant may mean a single substance or a mixture of two or more substances.

[0037] According to the above embodiment, phosphorus-based flame retardants readily form a char film at high temperatures of 150°C or higher.

[0038] In one embodiment of this specification, the flame-retardant polymer layer may further contain a flame-retardant resin.

[0039] In one embodiment of this specification, the flame-retardant resin may contain oxygen bonds and aromatic groups in its main chain.

[0040] In one embodiment of this specification, the flame-retardant resin may be one or more selected from the group consisting of polycarbonate resins, polyphenyl ether resins, and phenolic resins, but is not particularly limited as long as it is a flame-retardant resin containing oxygen bonds and aromatic groups in its main chain.

[0041] If necessary, the flame-retardant resin may mean a single substance or a mixture of two or more substances.

[0042] In this specification, the flame-retardant resin may be blended with the phosphorus-based flame retardant described above.

[0043] According to the above embodiment, the flame-retardant resin is used together with a phosphorus-based flame retardant to further ensure flame retardancy and to facilitate char formation during high-temperature combustion.

[0044] In one embodiment of this specification, the weight ratio of the flame retardant to the flame retardant resin is not particularly limited, but may be blended in a weight ratio of, for example, 1:0.1 to 1:2.

[0045] In one embodiment of this specification, the flame-retardant polymer layer may further contain a binder resin.

[0046] In this specification, the binder resin may be a material known in the art, as long as it does not depart from the scope of the present invention, and may include, for example, polyacrylamide.

[0047] In one embodiment of this specification, the thickness of the flame-retardant polymer layer may be 2 μm or more and 30 μm or less.

[0048] The aforementioned unit "μm" can mean micrometer.

[0049] In one embodiment of this specification, the negative electrode active material layer may include at least one of a silicon-based active material and a carbon-based active material.

[0050] In one embodiment of this specification, the negative electrode active material layer may include a carbon-based active material.

[0051] The carbon-based active material prevents swelling due to repeated charging and discharging on the negative electrode or side of the lithium secondary battery of the present invention, and can contribute to improving excellent cycle characteristics or battery life performance.

[0052] In one embodiment of this specification, the negative electrode active material layer may include a silicon-based active material.

[0053] Generally, silicon-based active materials are known to have a capacity more than 10 times higher than carbon-based active materials. Thus, when applying silicon-based active materials to the negative electrode, an electrode with a high level of energy density can be realized even with a thin thickness.

[0054] In one embodiment of the present specification, the silicon-based active material may include one or more selected from the group consisting of SiOx (x = 0), SiOx (0 < x < 2), Si / C composite, and Si alloy.

[0055] On the other hand, in the case of SiO2, since it does not react with lithium ions and cannot store lithium, it is preferable that x is within the above range. The silicon-based active material may be a Si / C composite composed of a composite of Si and C or Si.

[0056] If necessary, the silicon-based active material can mean a single substance of one kind or a mixed substance in which two or more kinds are combined.

[0057] In one embodiment of the present specification, the silicon-based active material includes one or more selected from the group consisting of SiOx (x = 0) and SiOx (0 < x < 2), and provides a negative electrode composition containing 70 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material.

[0058] In one embodiment of the present specification, the silicon-based active material may contain 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material, and may contain 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0059] The silicon-based active material according to this specification contains 70 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material, and when compared with a silicon-based active material using a SiOx (0 < x < 2) system as the main substance, its theoretical capacity can be realized to be much higher than that of the silicon-based active material of this specification.

[0060] In one embodiment of this specification, the silicon-based active material may use pure silicon (Si) as the silicon-based active material. Using pure silicon (Si) as the silicon-based active material means, as described above, when the silicon-based active material is based on 100 parts by weight in total, it can mean including pure Si particles (SiOx (x = 0)) that are not combined with other particles or elements within the above range.

[0061] In one embodiment of this specification, the negative electrode active material layer may further include a conductive material and a binder.

[0062] In this specification, the conductive material and the binder contained in the negative electrode active material can be referred to as a negative electrode conductive material and a negative electrode binder, respectively.

[0063] The negative electrode conductive material is used to impart conductivity to the electrode, and as long as it does not cause a chemical change on the side surface of the battery and has electronic conductivity, it can be used without special restrictions. However, the negative electrode conductive material is applied to the negative electrode and has a completely different configuration from the positive electrode conductive material applied to the positive electrode. That is, in the case of the negative electrode conductive material, it plays a role in capturing the contacts between silicon-based active materials with a very large volume expansion of the electrode during charging and discharging, while the positive electrode conductive material plays a role in imparting partial conductivity while playing a role of a buffer for buffering during rolling, so the negative electrode conductive material and the positive electrode conductive material have different configurations and roles.

[0064] Specific examples of the negative electrode conductive material may include one or more selected from the group consisting of a dot-shaped conductive material, a planar conductive material, and a linear conductive material.

[0065] Specifically, the point-like conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, conductive fibers, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives. Preferably, it may contain carbon black and / or artificial graphite in that it exhibits high conductivity and excellent dispersibility.

[0066] The planar conductive material can improve conductivity by increasing surface contact between silicon particles within the negative electrode, and at the same time suppress the disruption of the conductive path due to volume expansion. The planar conductive material can be described as a plate-type conductive material or a bulk-type conductive material. An example of the planar conductive material may include at least one selected from the group consisting of plate graphite, graphene, graphene oxide, and graphite flakes, and plate graphite is preferred.

[0067] The linear conductive material may be a carbon nanotube. The carbon nanotube may be a bundle-type carbon nanotube. The bundle-type carbon nanotube may contain multiple carbon nanotube units. Specifically, unless otherwise specified, "bundle type" here refers to a bundle or rope-like secondary shape in which multiple carbon nanotube monomers are arranged side by side or intertwined with substantially the same orientation along the longitudinal direction of the carbon nanotube monomers. The carbon nanotube unit has a graphite sheet that is cylindrical in shape with a nanoscale diameter, and sp 2It has a bonded structure. Depending on the angle and structure in which the graphite surface is wound, it can exhibit conductive or semiconductor properties. Compared to entangled type carbon nanotubes, the bundled carbon nanotubes can be dispersed more uniformly during anode manufacturing, smoothly forming a conductive network within the anode and improving the conductivity of the anode.

[0068] In one embodiment of this specification, the negative electrode conductive material is provided in an amount of 0.1 parts by weight or more and 40 parts by weight or less, based on 100 parts by weight of the negative electrode composition.

[0069] In another embodiment, the negative electrode conductive material may contain 0.1 parts by weight or more and 40 parts by weight or less, preferably 0.2 parts by weight or more and 30 parts by weight or less, more preferably 0.4 parts by weight or more and 25 parts by weight or less, and most preferably 0.4 parts by weight or more and 10 parts by weight or less, based on 100 parts by weight of the negative electrode composition.

[0070] The anode binder plays a role in improving adhesion between anode active material particles and adhesion between the anode active material and the anode current collector. Depending on whether it dissolves well in an aqueous solvent such as water, it can be classified into an aqueous binder and a non-aqueous binder (organic binder). Specific examples of the anode binder include carboxymethylcellulose (CMC) binders, styrene-butadiene rubber (SBR) binders, polyacrylic acid (PAA) binders, polyacrylamide (PAM) binders, polyacrylonitrile (PAN) binders, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), etc., or various copolymers derived from these may be used, or a mixture of two or more selected from these may be used.

[0071] In one embodiment of this specification, a negative electrode composition is provided in which the weight-average molecular weight of the negative electrode binder is 100,000 g / mol or more and 1,000,000 g / mol or less.

[0072] In this specification, weight-average molecular weight (Mw) and number-average molecular weight (Mn) are polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using commercially available monodisperse polystyrene polymers of various degrees of polymerization (standard samples) as standard substances. In this specification, molecular weight refers to weight-average molecular weight unless otherwise specified.

[0073] By satisfying the aforementioned weight-average molecular weight range, the material will exhibit excellent mechanical strength, high intermolecular interaction, and superior electrode bonding strength. Furthermore, when this range is satisfied, the viscosity of the binder can be selected within an appropriate range, and when this is used to manufacture the negative electrode, it will exhibit excellent electrode coating properties.

[0074] In one embodiment of this specification, the negative electrode binder is provided in an amount of 1 to 20 parts by weight, based on 100 parts by weight of the negative electrode composition.

[0075] In one embodiment of this specification, the negative electrode binder may contain 20 parts by weight or less, preferably 15 parts by weight or less, based on 100 parts by weight of the negative electrode composition, and may also contain 1 part by weight or more, 5 parts by weight or more, or 10 parts by weight or more.

[0076] As described above, when it includes a negative electrode active material, a negative electrode conductive material, and a negative electrode binder, it can be called a negative electrode composition.

[0077] In some cases, a solvent may be added to the negative electrode composition, which can be referred to as a negative electrode slurry. The solvent used here can be referred to as a solvent for forming the negative electrode slurry.

[0078] The solvent for forming the negative electrode slurry can be N-methyl-2-pyrrolidone (NMP), water, or the like, but is not limited to these.

[0079] In this specification, the negative electrode means that the negative electrode has undergone a series of electrode processes, including a coating step of applying a negative electrode slurry to at least one surface of the negative electrode current collector, a pressing step of pressing it to a certain thickness with a roll press, and a slitting step of cutting it according to the electrode specifications.

[0080] In one embodiment of this specification, the negative electrode may be formed by coating one or both sides of a current collector with a negative electrode slurry containing the negative electrode composition.

[0081] In one embodiment of this specification, the solid content of the negative electrode slurry may be 5% or more and 40% or less.

[0082] In another embodiment, the solid content of the negative electrode slurry may be in the range of 5% to 40%, preferably 7% to 35%, and more preferably 10% to 30%.

[0083] The solid content of the negative electrode slurry can mean the amount of negative electrode composition contained in the negative electrode slurry, and can mean the amount of negative electrode composition based on 100 parts by weight of the negative electrode slurry.

[0084] When the solid content of the negative electrode slurry satisfies the aforementioned range, the viscosity during negative electrode active material layer formation is appropriate, minimizing the clumping phenomenon of particles in the negative electrode composition and enabling efficient formation of the negative electrode active material layer.

[0085] Further details regarding the aforementioned negative electrode are subject to the standards known in the industry.

[0086] <Lithium-ion secondary battery> According to one embodiment of this specification, a lithium secondary battery is provided that includes, in addition to the aforementioned negative electrode, a positive electrode, a separation membrane interposed between the negative electrode and the positive electrode, and an electrolyte.

[0087] In this specification, the positive electrode may include a positive electrode current collector and a positive electrode active material layer containing positive electrode active material on at least one surface of the positive electrode current collector.

[0088] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive, but for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector may also have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0089] In one embodiment of this specification, the thickness of the positive electrode current collector layer may be 1 μm or more and 100 μm or less, and the thickness of the positive electrode active material layer may be 20 μm or more and 500 μm or less. However, the thickness can be varied in various ways depending on the type and application of the positive electrode used, and is not limited thereto.

[0090] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2Ni-site type lithium nickel oxide represented as O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01 ≤ c2 ≤ 0.3); chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxides represented as O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 0.01 ≤ c3 ≤ 0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4 in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion, etc., are examples, but are not limited to these. The positive electrode may be Li-Metal.

[0091] The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder, along with the positive electrode active material. When the positive electrode active material, positive electrode conductive material, and positive electrode binder are included, it is referred to as a positive electrode composition, and when the positive electrode composition further includes a solvent for forming a positive electrode slurry, it can be referred to as a positive electrode slurry.

[0092] The positive electrode conductive material is used to impart conductivity to the electrode and can be used in any battery without particular limitations as long as it does not cause chemical changes and has electronic conductivity. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these materials alone or a mixture of two or more can be used.

[0093] The positive electrode binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder include polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers derived therefrom, or mixtures of two or more selected therefrom.

[0094] When the aforementioned solvent refers to a solvent for forming a positive electrode slurry, it may, but is not limited to, N-methyl-2-pyrrolidone (NMP) or water.

[0095] In this specification, the positive electrode means one that has undergone a series of electrode processes, including a coating step of applying a positive electrode composition to at least one surface of a positive electrode current collector, a pressing step of pressing it to a certain thickness with a roll press, and a slitting step of cutting it according to electrode specifications.

[0096] Further details regarding the aforementioned positive electrode are subject to the standards known in the industry.

[0097] In this specification, the electrolyte refers to a substance that can be used in the manufacture of a lithium secondary battery, and includes, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes.

[0098] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0099] Specific examples of the non-aqueous organic solvent include aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate, etc.

[0100] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, have a high dielectric constant as high-viscosity organic solvents and can preferably be used because they can dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, at an appropriate ratio, an electrolyte having high electrical conductivity can be produced and can be more preferably used.

[0101] The metal salt can be a lithium salt, and the lithium salt is a substance that is easily soluble in the non-aqueous electrolyte. For example, as the anion of the lithium salt, F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3- , (CF3SO2)2N - , (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - You may use one or more selected from the group consisting of the following:

[0102] In addition to the components of the electrolyte, the electrolyte may further contain one or more additives for the purpose of improving the battery's lifespan, suppressing the decrease in battery capacity, and improving the battery's discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0103] In this specification, the separation membrane separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any membrane commonly used as a separation membrane in lithium secondary batteries is generally acceptable. In particular, the separation membrane is preferred to have low resistance to ion movement in the electrolyte while exhibiting excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as those made from polyolefin polymers including ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof, may be used. Alternatively, ordinary porous nonwoven fabrics, such as those made from high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, coated separation membranes containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and may be selectively used in single-layer or multi-layer structures.

[0104] In this specification, a lithium secondary battery may be a concept that includes an electrode assembly comprising a positive electrode, a negative electrode, and a separator membrane, and a battery case that receives an electrolyte.

[0105] <Battery Pack> The battery pack described herein is shown in Figure 5.

[0106] One embodiment of this specification provides a battery module and / or battery pack 300 that includes the lithium secondary battery 200 as a unit cell.

[0107] Since the battery module and / or battery pack includes the lithium secondary battery 200, the above-mentioned provisions regarding lithium secondary batteries can be applied as is.

[0108] In this specification, the battery pack 300 may have a structure in which a lithium secondary battery 200 is included in the pack housing 201.

[0109] According to Figure 5, the lithium secondary battery 200 may be replaced with coin-shaped, pouch-shaped, or rectangular batteries, as needed, in addition to the cylindrical shape shown.

[0110] Depending on the circumstances, the battery pack described herein may include one or more battery module units.

[0111] As shown in Figure 6, the battery pack 300 can be used as a power source for medium to large devices selected as needed from a group consisting of means of transport 400 such as electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles, and energy storage systems (EES).

[0112] [Examples] The following are preferred embodiments to aid in understanding the present invention. However, these embodiments are for illustrative purposes only, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of this description and the technical concept, and such variations and modifications naturally fall within the scope of the appended claims.

[0113] Manufacturing example: Manufacturing of negative electrodes Manufacturing example 1. A negative electrode slurry was prepared by adding polyacrylamide, which was used as the negative electrode active material, first conductive material, second conductive material, and binder in a weight ratio of 90:4.6:0.4:5 to distilled water as the solvent for forming the negative electrode slurry (solid content concentration 28% by weight).

[0114] Specifically, the first conductive material is a plate-shaped graphite (specific surface area: 17 m²). 2 The second conductive material was SWCNT (single-walled carbon nanotube), with a particle size of 3.5 μm (D50) and a weight of 3.5 μm.

[0115] As for the specific mixing method, the first conductive material, binder, and water were dispersed using a homomixer at 2500 rpm for 30 minutes, then the second conductive material was added and dispersed for 10 minutes, and then the silicon-based active material was added as the negative electrode active material, and dispersed at 2500 rpm for 30 minutes to produce the negative electrode slurry.

[0116] As the negative electrode current collector layer, 950 mg / 50 cm of the negative electrode slurry is applied to the cross-section of a copper current collector (thickness: 15 μm). 2 The material was coated with the specified loading amount, rolled (roll press), and dried in a vacuum oven at 130°C for 10 hours to form a negative electrode active material layer (thickness: 95 μm), which was used as the negative electrode (negative electrode thickness: 110 μm, negative electrode porosity: 30.0%).

[0117] A solution was prepared by diluting a flame retardant (polyphosphate) and flame retardant resin (polycarbonate resin) / binder (polyacrylamide) in a weight ratio of 1:0.5:1 in distilled water to a concentration of 10 wt%.

[0118] A diluted solution was coated onto the manufactured negative electrode and dried in a 60°C oven for 10 hours to produce a negative electrode containing a flame-retardant polymer layer (thickness of the flame-retardant polymer layer: 5 μm).

[0119] Manufacturing example 2. The negative electrode was manufactured in the same manner as in Manufacturing Example 1, except that the weight ratio of the flame retardant and flame retardant resin was changed to 1:0.3.

[0120] Manufacturing example 3. The negative electrode was manufactured in the same manner as in Manufacturing Example 1, except that the weight ratio of the flame retardant and flame retardant resin was changed to 1:1.2.

[0121] Manufacturing example 4. The negative electrode was manufactured in the same manner as in Manufacturing Example 1, except that the weight ratio of the flame retardant and flame retardant resin was changed to 1:1.5.

[0122] Manufacturing example 5. The negative electrode was manufactured in the same manner as in Manufacturing Example 1, except that the thickness of the flame-retardant polymer layer was changed to 10 μm.

[0123] Manufacturing example 6. The negative electrode was manufactured in the same manner as in Manufacturing Example 1, except that the thickness of the flame-retardant polymer layer was changed to 20 μm.

[0124] Comparative manufacturing example 1. The negative electrode was manufactured in the same manner as in Manufacturing Example 1, except that it did not contain a flame-retardant polymer layer.

[0125] Comparative manufacturing example 2. The negative electrode was manufactured in the same manner as in Manufacturing Example 1, except that a styrene-based flame-retardant resin was used.

[0126] Comparative manufacturing example 3. The negative electrode was manufactured in the same manner as in Manufacturing Example 1, except that it did not contain a flame retardant.

[0127] Examples: Manufacturing of lithium secondary batteries <Examples 1-6 and Comparative Examples 1-3> LiNi 0.6 Co 0.2 Mn 0.2 A cathode slurry was prepared by adding O2 (average particle size (D50): 15 μm), carbon black (product name: Super C65, manufacturer: Timcal) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 97:1.5:1.5 to N-methyl-2-pyrrolidone (NMP) as a solvent for cathode slurry formation (solid content concentration 78% by weight).

[0128] As the positive electrode current collector, an aluminum current collector (thickness: 12 μm) is coated on both sides with the positive electrode slurry at a rate of 537 mg / 25 cm². 2 The material was coated with the specified loading amount, rolled (roll press), and dried in a vacuum oven at 130°C for 10 hours to form a positive electrode active material layer (thickness: 65 μm), thereby manufacturing the positive electrode (positive electrode thickness: 77 μm, porosity: 26%).

[0129] A polyethylene separation membrane was interposed between the positive electrode and the negative electrodes of the above-mentioned production examples 1-6 and comparative production examples 1-3. A lithium secondary battery was manufactured by injecting an electrolyte prepared by mixing fluoroethylene carbonate (FEC) and diethyl carbonate (DMC) in a volume ratio of 10:90 as an organic solvent, adding vinylene carbonate at 3% by weight relative to the total weight of the electrolyte, and adding LiPF6 as a lithium salt at a concentration of 1M.

[0130] <Example of experiment> Experimental Example 1: Evaluation of Thermal Runaway Time (Evaluation of Ignition Duration) The lithium secondary batteries manufactured in Examples 1-6 and Comparative Examples 1-3 were evaluated for their ignition duration. The time required for ignition to occur on the negative electrode surface and for self-extinguishing was measured, and the evaluation results are shown in Table 1 below.

[0131] [Table 1]

[0132] As can be seen from Table 1, when the ignition duration of Examples 1-6 and Comparative Examples 1-3 was examined, it was confirmed that the ignition duration of the comparative examples was inferior to that of the examples. In Example 1, where the flame retardant / flame retardant resin ratio was 1:0.5, the time required for self-extinguishing was the shortest at 2.0 seconds, and the ignition duration of Examples 2-4 was also short, with a maximum of 3.7 seconds. This confirmed that the flame retardant polymer layer suppressed the ignition duration and substantially prevented ignition.

[0133] Experimental Example 2: Lifespan Evaluation For the lithium secondary batteries of Examples 1-6 and Comparative Examples 1-3, the lifespan was evaluated by capacity retention rate using an electrochemical charger / discharger.

[0134] In-situ cycle testing was performed on lithium secondary batteries at 4.2-3.0V 1C / 0.5C. During the test, the batteries were charged / discharged at 0.33C / 0.33C (4.2-3.0V) every 50 cycles, and the capacity retention rate was calculated using the following formula.

[0135] Capacity retention rate (%) = {(Discharge capacity in the Nth cycle) / (Discharge capacity in the first cycle)} × 100

[0136] [Table 2]

[0137] As can be seen in Table 2, when we examined the capacity retention rate by cycle for Examples 1-6 and Comparative Examples 1-3, we confirmed that introducing a flame-retardant polymer layer to the negative electrode surface did not significantly affect the life evaluation results.

[0138] Experimental Example 3: Evaluation of Cell Resistance In Experimental Example 2, during testing, the capacitance retention rate was measured by charging / discharging at 0.33C / 0.33C (4.2-3.0V) every 50 cycles. Then, the resistance was measured by discharging with a 2.5C pulse using SOC50, and the resistance increase rate was compared and analyzed.

[0139] Regarding the measurement and evaluation of the aforementioned resistance increase rate, data was calculated for 200 cycles, and the results are shown in Table 3 below.

[0140] [Table 3]

[0141] As can be seen in Table 3, when we examined the resistance increase rate over cycles for Examples 1-6 and Comparative Examples 1-3, we confirmed that the resistance increase rate for Examples 1-6 after 200 cycles was not significantly higher than that of Comparative Examples 1 and 3, which lacked a flame-retardant polymer coating layer or flame retardant. This confirmed that introducing a flame-retardant polymer layer, as in Examples 1-6, does not affect the operation of the cell. [Explanation of Symbols]

[0142] 1...Negative electrode current collector 2...Negative electrode active material layer 3. Flame-retardant polymer layer 4. Oxygen barrier layer 5 ···Oxygen 6 ···Stomata 7. Blocked stomata 100...Negative electrode 200 ···Lithium rechargeable battery 201 ···Housing 300... Battery pack 400 ··Means of transportation

Claims

1. A negative electrode comprising: a negative electrode current collector; a negative electrode active material layer provided on the negative electrode current collector; and a flame-retardant polymer layer provided on the negative electrode active material layer, The flame-retardant polymer layer is a negative electrode containing a flame retardant that is converted to char when burned at temperatures of 150°C or higher.

2. The negative electrode according to claim 1, wherein the flame retardant comprises a phosphorus-based flame retardant.

3. The negative electrode according to claim 2, wherein the phosphorus-based flame retardant comprises one or more selected from the group consisting of polyphosphate, phosphonate, phosphinate, phosphine oxide, and phosphazene.

4. The anode according to claim 1, wherein the flame-retardant polymer layer further comprises a flame-retardant resin.

5. The anode according to claim 4, wherein the flame-retardant resin is one or more selected from the group consisting of polycarbonate resin, polyphenyl ether resin, and phenolic resin.

6. The negative electrode according to claim 1, wherein the thickness of the flame-retardant polymer layer is 2 μm or more and 30 μm or less.

7. The negative electrode according to claim 1, wherein the negative electrode active material layer comprises at least one of a silicon-based active material and a carbon-based active material.

8. The negative electrode according to claim 7, wherein the silicon-based active material comprises one or more selected from the group consisting of SiOx (x=0), SiOx (0<x<2), Si / C composite material, and Si alloy.

9. The negative electrode according to claim 7, wherein the negative electrode active material layer further comprises a conductive material and a binder.

10. The negative electrode according to any one of claims 1 to 9; Positive electrode; A separation membrane interposed between the positive and negative electrodes; and electrolyte Lithium-ion batteries, including lithium-ion batteries.

11. A battery pack comprising a lithium secondary battery as described in claim 10 as a unit cell.

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