Electrolyte for secondary battery, method for preparing same, and lithium secondary battery including same

Abstract

An electrolyte for a lithium secondary battery according to an embodiment of the present invention may include a lithium salt, a composite film including an inorganic electrolyte and an organic binder, and a flame-retardant polymer. A lithium secondary battery according to an embodiment of the present invention may comprise: a positive electrode; the negative electrode is arranged opposite to the positive electrode; and an electrolyte layer that is provided between the positive electrode and the negative electrode and contains an electrolyte for the secondary battery.

Description

Technical Field

[0001] This invention relates to an electrolyte for secondary batteries, a method for preparing the same, and a lithium secondary battery including the electrolyte. More specifically, it relates to an electrolyte for secondary batteries comprising an inorganic electrolyte, a method for preparing the same, and a lithium secondary battery including the electrolyte. Background Technology

[0002] Rechargeable batteries are batteries that can be repeatedly charged and discharged. With the development of the information communication and display industries, rechargeable batteries are widely used as power sources for portable electronic communication devices such as cameras, mobile phones, and laptops (PCs). In addition, in recent years, battery packs including rechargeable batteries have been developed and applied as power sources for environmentally friendly vehicles such as hybrid electric vehicles.

[0003] Examples of secondary batteries include lithium-ion batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Among them, lithium-ion batteries have high operating voltage and energy density per unit weight, and are advantageous for charging speed and lightweight design. Therefore, they are being actively researched and developed.

[0004] Currently, commercially available lithium-ion batteries primarily use liquid electrolytes. Therefore, they are susceptible to safety issues such as leakage, fire, and explosion due to rapid environmental changes, including temperature fluctuations and external impacts. To address this problem, research is underway to solidify the electrolyte to ensure stability and improve energy density.

[0005] All-solid-state batteries can use solid electrolytes such as gel polymers, oxides or sulfides, and composite polymers as electrolytes. This can improve stability against fire and explosion caused by external impacts, changes in the external environment, etc.

[0006] The electrolyte for secondary batteries and its preparation method, as well as the lithium secondary battery of this invention, can be widely used in green technology fields such as electric vehicles, battery charging stations, and other battery-based solar and wind power generation. The electrolyte for secondary batteries and its preparation method, as well as the lithium secondary battery of this invention, can be used in eco-friendly electric vehicles and hybrid vehicles to prevent climate change by suppressing air pollution and greenhouse gas emissions. Summary of the Invention

[0007] (a) Technical problems to be solved

[0008] One technical problem of the present invention is to provide an electrolyte for secondary batteries with improved mechanical properties and high-temperature stability.

[0009] One technical problem of the present application is to provide a method of preparing an electrolyte for a secondary battery having improved mechanical properties and high-temperature stability.

[0010] One technical problem of the present application is to provide a lithium secondary battery having improved electrochemical stability.

[0011] (B) Technical Solution

[0012] The electrolyte for a secondary battery according to an exemplary embodiment can include a lithium salt, a composite film including an inorganic electrolyte and an organic binder, and a flame-retardant compound. The content of the inorganic electrolyte in the total volume of the composite film can be 50 vol% to 95 vol%.

[0013] In some embodiments, the inorganic electrolyte can include an oxide-based solid electrolyte.

[0014] In some embodiments, the content of the organic binder in the total volume of the composite film can be 5 vol% to 50 vol%.

[0015] In some embodiments, the volume ratio of the content of the organic binder to the content of the inorganic electrolyte can be 0.05 to 0.5.

[0016] In some embodiments, the volume ratio of the content of the flame-retardant compound to the content of the inorganic electrolyte can be 0.01 to 0.3.

[0017] In some embodiments, the flame-retardant compound can include at least one of a phosphorus-containing functional group and a fluorine atom.

[0018] In some embodiments, the phosphorus-containing functional group can include at least one of a phosphate group, a phosphite group, a phosphonate group, and a phosphazene group.

[0019] The lithium secondary battery according to an exemplary embodiment can include a positive electrode, a negative electrode disposed opposite the positive electrode, and an electrolyte layer disposed between the positive electrode and the negative electrode and including the above-described electrolyte for a secondary battery.

[0020] The method of preparing an electrolyte for a secondary battery according to an exemplary embodiment can include mixing an inorganic electrolyte, an organic binder, and a solvent to prepare a mixed slurry. The mixed slurry can be dried to prepare a composite film. A flame-retardant polymer can be impregnated in the composite film to prepare an electrolyte for a secondary battery. The volume ratio of the content of the organic binder to the content of the inorganic electrolyte can be 0.01 to 0.5.

[0021] In some embodiments, the inorganic electrolyte may comprise an oxide-based solid electrolyte.

[0022] In some embodiments, the content of the inorganic electrolyte in the total volume of the composite membrane can be from 50% to 95% by volume.

[0023] In some embodiments, the step of impregnating the flame-retardant polymer may include impregnating the composite film with a mixture comprising a flame-retardant monomer and an electrolyte and curing the mixture.

[0024] In some implementations, the electrolyte may contain lithium salts.

[0025] In some embodiments, the mixture may further contain a thermal initiator, and the curing of the mixture may include heat treatment of the mixture.

[0026] In some embodiments, the mixture may further contain a photoinitiator, and curing of the mixture may include irradiating the mixture with light.

[0027] (III) Beneficial Effects

[0028] The electrolyte for secondary batteries prepared according to an exemplary embodiment of the present invention can have improved mechanical properties. Therefore, the stability of the electrolyte layer can be improved, and flame retardancy can be enhanced.

[0029] Furthermore, the electrolyte for secondary batteries prepared according to an exemplary embodiment of the present invention can be self-extinguishing. Therefore, even with repeated charging / discharging of the electrolyte layer, the high-temperature stability and fire resistance of the electrolyte layer can be improved.

[0030] The lithium secondary battery according to an exemplary embodiment of the present invention may include the electrolyte for the secondary battery, thereby improving safety at room temperature and high temperature, and improving electrical characteristics. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the structure of an electrolyte for a secondary battery according to an exemplary embodiment.

[0032] Figure 2 This is a schematic process flow diagram illustrating a method for preparing an electrolyte for a secondary battery according to an exemplary embodiment.

[0033] Figure 3 This is a photograph of the combustion of an electrolyte layer for a secondary battery prepared according to an exemplary embodiment.

[0034] Figure 4This is a photograph of the electrolyte layer for a secondary battery prepared according to an exemplary embodiment after combustion. Detailed Implementation

[0035] According to an exemplary embodiment, a method for preparing an electrolyte for secondary batteries comprising lithium salt, inorganic oxide, organic binder, and flame-retardant polymer is provided, as well as the electrolyte for secondary batteries prepared therefrom. Furthermore, a lithium secondary battery comprising an electrolyte layer containing the electrolyte for secondary batteries is provided.

[0036] The present invention will now be described in detail with reference to the accompanying drawings. However, this is merely an exemplary description, and the present invention is not limited to the specific embodiments described herein.

[0037] In the following description, unless otherwise specifically defined, when describing a layer, film, thin film, region, plate, etc. as being "on" another part, this can include not only the case where it is "directly" "above" another part, but also the case where there are other parts in between.

[0038] When the compound represented by the chemical formula used in this invention has isomers, the compound represented by the chemical formula refers to the representative chemical formula including the isomer.

[0039] In this invention, flame retardancy refers to the ability of a sample to burn when in contact with a flame (ignition source), but to prevent or inhibit itself from generating flame and burning when the flame is removed. The flame retardancy can be evaluated by supplying the sample with a flame of a certain heat from a blowtorch for more than one second to ignite it, and then removing the blowtorch and observing the time required for the flame to extinguish. The shorter the time required for extinguishing, the better the flame retardancy.

[0040] Specifically, the flame-retardant polymer can be prepared as a glass fiber impregnated or self-supporting membrane with a diameter of 19 mm. When a flame of a certain amount of heat is supplied to it with a blowtorch for more than 1 second to ignite it, and then the blowtorch is removed, the polymer can be extinguished within 2 seconds, specifically within 1 second, or more specifically within 0.5 seconds.

[0041] Figure 1 This is a schematic diagram illustrating an electrolyte for a secondary battery according to an exemplary embodiment.

[0042] Reference Figure 1 The electrolyte for secondary batteries (hereinafter, simply referred to as the "electrolyte") may comprise a lithium salt, a composite membrane 105, and a flame-retardant polymer 130. In some embodiments, the electrolyte may comprise a lithium salt, a composite membrane 105, a flame-retardant polymer 130, and an electrolyte solution.

[0043] According to an exemplary embodiment, the composite membrane 105 may comprise an inorganic electrolyte 110 and an organic binder 120. For example, the inorganic electrolyte 110 and the organic binder 120 may be mixed and dispersed within the composite membrane 105, physically contacting or bonded to each other. The composite membrane may be a free-standing membrane.

[0044] The inorganic electrolyte 110 and organic binder 120 of the composite membrane 105 may be unsintered, and the composite membrane 105 may not contain a sintered body of the inorganic electrolyte 110 and / or a sintered body of the organic binder 120.

[0045] The lithium salt can be represented as, for example, Li + X - The anion (X) of the lithium salt - ), can be exemplified by F - Cl - ,Br - 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 - wait.

[0046] In some embodiments, the inorganic electrolyte 110 may be an oxide-based solid electrolyte. For example, an oxide-based solid electrolyte may contain an ionicly conductive compound containing a metal or oxygen. For instance, an oxide-based solid electrolyte may include LLTO-based compounds, LLZO-based compounds, Li...6.4 La3Zr 1.4 Ta 0.6 O 12 LLZTO-based compounds, Li6La2CaTa2O 12 Li6La2ANb2O 12 (A is Ca or Sr), Li2Nd3TeSbO 12 Li3BO 2.5 N 0.5 Li9SiAlO8, LAGP-based compounds, LATP-based compounds, Li 1+x Ti 2-x Al x Si y (PO4)3(0≤x≤1,0≤y≤1), LiAl x Zr 2-x (PO4)3(0≤x≤1,0≤y≤1), LiTi x Zr 2-x (PO4)3 (0≤x≤1, 0≤y≤1), LISICON-based compounds, LIPON-based compounds, perovskite-based compounds, NASICON-based compounds, Al2O3, ZnO2, Ce2O2, TiO2, ZrO2, HfO2, MnO2, MgO, WO2, V2O5 and other metal oxides, etc.

[0047] In one embodiment, the inorganic electrolyte 110 may comprise a lithium-containing oxide-based solid electrolyte. For example, the lithium-containing oxide-based solid electrolyte may include LLTO-based compounds, LLZO-based compounds (e.g., garnet-type LLZO-based compounds), LLZTO-based compounds, NASICON-based compounds, LATP-based compounds, perovskite-based compounds, etc. This can improve the ionic conductivity and mechanical strength of the electrolyte 100, thereby suppressing lithium dendrites and improving high-temperature stability and lifetime characteristics.

[0048] The organic adhesive 120 may contain at least one selected from polyethylene compound-based adhesives, cellulose-based adhesives, acrylic polymer-based adhesives, and copolymer resin adhesives.

[0049] Organic adhesive 120 may include, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), vinylpyrrolidone / vinylacetate (VP / VA) copolymer resin, polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), acrylonitrile-butadiene rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), and polyacrylic acid. acid-based adhesives, poly(3,4-ethylenedioxythiophene) (PEDOT), etc.

[0050] In some embodiments, the organic binder 120 can be the glass transition temperature (T0). g The glass transition temperature (T) is an organic polymer with a temperature range of 100°C to 600°C, 100°C to 500°C, or 100°C to 300°C. In one embodiment, a PVDF-based adhesive may be included. This is achieved by using an organic polymer with the aforementioned glass transition temperature (T). g The organic adhesive 120 can uniformly distribute and fix the inorganic electrolyte 110 even without separate high-temperature heat treatment during the preparation of the electrolyte, thereby stabilizing the electrolyte 100.

[0051] In some embodiments, the flame-retardant polymer 130 may contain a phosphorus functional group. In one embodiment, the phosphorus functional group may contain at least one selected from phosphate ester group, phosphite group, phosphonate group, and phosphazene group.

[0052] For example, the flame-retardant polymer 130 can be a polymer formed by polymerizing or copolymerizing a flame-retardant monomer containing at least one of phosphate ester groups, phosphite ester groups, phosphonate groups, and phosphazene groups. Therefore, side reactions between the flame-retardant polymer 130 and the inorganic electrolyte 110, organic binder 120, and compounds such as lithium salts or electrolytes can be suppressed. Furthermore, by containing phosphorus-containing functional groups, the flame-retardant polymer 130 can block oxygen during combustion and prevent thermal runaway.

[0053] In some embodiments, the flame-retardant polymer 130 may contain fluorine atoms. For example, the flame-retardant polymer 130 may be a polymer formed by polymerizing or copolymerizing flame-retardant monomers containing fluorine atoms. In the event of a fire, the fluorine atoms form free radicals, thereby inhibiting the transfer of the combustion reaction. Therefore, the flame-retardant properties of the electrolyte 100 can be improved by using the flame-retardant polymer 130.

[0054] In one embodiment, the flame-retardant polymer 130 may uniformly comprise phosphorus-containing functional groups and fluorine atoms. For example, the flame-retardant polymer 130 may be a polymer polymerized or copolymerized from flame-retardant monomers uniformly comprising phosphorus-containing functional groups and fluorine atoms. For example, the flame-retardant polymer 130 may be a polymer copolymerized from flame-retardant monomers comprising phosphorus-containing functional groups and flame-retardant monomers comprising fluorine atoms. Therefore, the flame retardancy of the electrolyte 100 can be further improved by using the flame-retardant polymer 130.

[0055] In one embodiment, the flame-retardant polymer 130 may comprise monomers, oligomers, polymers, or mixtures thereof.

[0056] In some embodiments, the flame-retardant polymer 130 can be a compound polymerized or copolymerized from a flame-retardant monomer having thermally reactive and / or photoreactive functional groups. For example, the flame-retardant monomer may contain thermally reactive functional groups and can be a compound polymerized by heat. For example, the flame-retardant monomer may contain photoreactive functional groups and can be a compound polymerized by light irradiation. For example, the thermosetting and / or photoreactive functional groups can be (meth)acrylate groups, acrylic groups, ether groups, alcohol groups, alkoxy groups, etc.

[0057] According to an exemplary embodiment, the content of inorganic electrolyte 110 in the total volume of the composite membrane 105 can be from 50% to 95% by volume.

[0058] When the content of inorganic electrolyte 110 in the total volume of composite membrane 105 exceeds 95% by volume, the toughness of the composite membrane may decrease due to the weak particle entanglement force caused by the low content of organic binder 120. Therefore, the mechanical stability of composite membrane 105 may be reduced. Furthermore, the high content of inorganic oxide 110 may make it difficult to manufacture composite membrane 105 in thin film form.

[0059] When the content of inorganic electrolyte 110 in the total volume of composite membrane 105 is less than 50% by volume, the ionic conductivity of composite membrane 105 may decrease. Therefore, the power characteristics and initial capacity efficiency of composite membrane 105 may be reduced. Furthermore, due to the relatively increased content of organic binder 120, in the event of combustion, it may be difficult to maintain the shape of the composite membrane due to the burning of the organic binder 120.

[0060] Within the above-mentioned content range, the flame retardancy and mechanical stability of the composite membrane 105 can be improved, while the power characteristics and initial capacity efficiency can also be improved.

[0061] In some embodiments, the content of inorganic electrolyte 110 in the total volume of composite membrane 105 can be 60% to 95% by volume, or 70% to 95% by volume, 70% to 90% by volume, 75% to 90% by volume, or 75% to 85% by volume. For example, the content of the oxide-based solid electrolyte in the total volume of composite membrane 105 can be within the above-mentioned volume range. Within the above range, the high ionic conductivity of inorganic electrolyte 110 can further improve the ionic conductivity of electrolyte 100. For example, the mechanical properties of electrolyte 100 can be further improved by oxide-based solid electrolytes, etc., and the membrane structure of composite membrane 105 can be maintained without collapse.

[0062] In some embodiments, the content of organic binder 120 in the total volume of composite membrane 105 can be 5% to 50% by volume, 5% to 40% by volume, or 5% to 30% by volume. In one embodiment, the content of organic binder 120 in the total volume of composite membrane 105 can be 10% to 30% by volume, 10% to 25% by volume, or 15% to 25% by volume. Within the above ranges, the inorganic electrolyte 110 inside composite membrane 105 can be uniformly dispersed throughout composite membrane 105, and can stably form the structure of composite membrane 105.

[0063] In one embodiment, the volume ratio of the organic binder 120 to the inorganic electrolyte 110 can be 0.05 to 0.5, 0.053 to 0.5, 0.08 to 0.5, 0.08 to 0.4, 0.08 to 0.3, or 0.1 to 0.3. Within the above ranges, the decrease in ionic conductivity caused by the reduction in the content of inorganic electrolyte 110 can be suppressed, while the stability of the composite membrane 105 structure caused by the increase in the content of organic binder 120 can be improved.

[0064] In some embodiments, the volume ratio of the flame-retardant polymer 130 to the inorganic electrolyte 110 can be 0.01 to 0.3, 0.05 to 0.3, 0.1 to 0.3, 0.1 to 0.25, or 0.15 to 0.25. Within these ranges, the self-extinguishing property of the flame-retardant polymer 130 can improve the ignition stability and high-temperature stability of the electrolyte 100. Furthermore, even if the organic binder 120 contained in the composite membrane 105 ignites, the flame-retardant polymer 130 can maintain the structure of the composite membrane 105.

[0065] Figure 2 This is a process flow diagram illustrating a method for preparing an electrolyte for secondary batteries according to an exemplary embodiment. Hereinafter, refer to... Figure 2 The preparation method of the electrolyte for the above-mentioned secondary battery is explained.

[0066] Reference Figure 2 A mixed slurry can be prepared by mixing inorganic electrolytes, organic binders and solvents (e.g., step S10).

[0067] In some embodiments, the inorganic electrolyte may be an oxide-based solid electrolyte. The oxide-based solid electrolyte may be one of the aforementioned oxide-based solid electrolytes. For example, the oxide-based solid electrolyte may be a lithium-containing oxide-based solid electrolyte.

[0068] The organic adhesive can be any of the aforementioned organic adhesives. For example, the organic adhesive can be a PVB-based adhesive, a PVA-based adhesive, a PVDF-based adhesive, etc.

[0069] In some embodiments, the solvent may be a solvent capable of simultaneously dissolving inorganic electrolytes and organic binders. In one embodiment, the solvent may comprise an organic solvent. The organic solvent may comprise a solvent with at least one functional group selected from alcohols, ketones, amides, esters, ethers, aromatic hydrocarbons, etc. For example, the organic solvent may include 2-propanol, toluene, terpineol, N-methyl-2-pyrrolidone (NMP), etc. The solvent (or organic solvent) may be used alone or in combination of two or more.

[0070] In one embodiment, the solvent may use two organic solvents simultaneously. For example, the solvent may be a mixture of a first solvent and a second solvent that are miscible with each other. The first solvent and the second solvent may each dissolve at least one of the inorganic electrolyte and the organic binder. Therefore, even if either the inorganic electrolyte or the organic binder is insoluble in the first solvent, it may be soluble in the second solvent and mixed.

[0071] In an exemplary embodiment, the volume ratio of the organic binder to the inorganic electrolyte can be 0.05 to 0.5, 0.053 to 0.5, 0.08 to 0.5, 0.08 to 0.45, or 0.08 to 0.4. For example, the inorganic electrolyte and the organic binder can be mixed with a solvent in the above-mentioned range of content ratios.

[0072] When the volume ratio of the organic binder to the inorganic electrolyte is less than 0.01, the composite membrane prepared from the mixed slurry may become more brittle, potentially reducing its mechanical stability. When the volume ratio of the organic binder to the inorganic electrolyte exceeds 0.5, the ionic conductivity of the composite membrane may decrease, potentially reducing its power characteristics and initial efficiency.

[0073] In some embodiments, the volume ratio of the organic binder to the inorganic electrolyte can be 0.08 to 0.3, 0.09 to 0.3, or 0.1 to 0.3. Therefore, the mechanical stability, power characteristics, and initial efficiency of the composite membrane prepared from the slurry containing the inorganic electrolyte and the organic binder can be further improved.

[0074] In some embodiments, the mixed slurry may further contain additives such as plasticizers and dispersants. These additives may be organic additives.

[0075] The plasticizer can be, for example, a plasticizer having a phosphate ester, phthalate, or citrate structure. The phosphate ester can include, for example, triphenyl phosphate (TPP), 4-biphenyl diphenyl phosphate (BDP), and tricresyl phosphate (TCP). The phthalate can include, for example, dimethyl phthalate (DMP), dibutyl phthalate (DBP), dioctyl phthalate (DOP), diphenyl phthalate (DPP), and diethylhexyl phthalate (DEHP). The citrate can include, for example, o-acetyltriethyl citrate (OACTE) and o-acetyltributyl citrate (OACTB).

[0076] The dispersant may include, for example, hydrogenated nitrile butadiene rubber (HNBR), polyvinyl pyrrolidone (PVP), polylactic acid (PLA), polyglycolic acid (PGA), etc.

[0077] According to an exemplary embodiment, a composite film can be manufactured by drying the mixed slurry (e.g., step S20). The mixed slurry can be cast onto a substrate (e.g., a glass substrate and a plastic substrate, etc.) and then dried.

[0078] In some embodiments, the mixed slurry can be dried at 60°C to 400°C, 60°C to 300°C, 80°C to 300°C, 100°C to 300°C, or 120°C to 280°C. Within these temperature ranges, the solvent contained in the mixed slurry is vaporized, thereby drying the mixed slurry. Furthermore, the inorganic electrolyte and organic binder contained in the mixed slurry can be uniformly distributed within the mixed slurry, and the organic binder is physically bonded to the inorganic electrolyte, thereby forming and maintaining the structure of the composite membrane.

[0079] In some embodiments, the mixed slurry can be dried for 30 minutes to 2 hours, 30 minutes to 1.5 hours, or 45 minutes to 1.25 hours. Within these ranges, the solvent inside the mixed slurry can be vaporized and removed.

[0080] In some embodiments, the mixed slurry may not be sintered after drying. Therefore, the organic binder may not be removed, thereby increasing the elastic modulus of the composite membrane. Consequently, the lifetime characteristics of the electrolyte containing the composite membrane can be improved.

[0081] In some embodiments, the content of the inorganic electrolyte in the total volume of the composite membrane can be 50% to 95% by volume, 60% to 95% by volume, or 70% to 95% by volume. Within the above ranges, the inorganic electrolyte can improve the mechanical properties of the composite membrane.

[0082] In one embodiment, the content of the inorganic electrolyte in the total volume of the composite membrane can be 70% to 90% by volume, 75% to 90% by volume, or 75% to 85% by volume. Within the above ranges, the inorganic electrolyte can further improve the mechanical properties of the composite membrane.

[0083] According to an exemplary embodiment, a flame-retardant polymer can be impregnated in the composite film (e.g., step S30). For example, the composite film may have pores formed between the inorganic electrolyte and the organic binder, and the flame-retardant compound can be impregnated in the pores. By impregnating the flame-retardant polymer in the composite film, an electrolyte for secondary batteries can be prepared. For example, the composite film formed on the substrate (e.g., a glass substrate and a plastic substrate, etc.) can be separated from the substrate, and the flame-retardant polymer can be impregnated in the composite film.

[0084] In some embodiments, the flame-retardant polymer can be impregnated in the composite membrane such that the ratio of the volumetric content of the flame-retardant polymer to the content of the inorganic electrolyte contained in the composite membrane can be 0.01 to 0.3, 0.05 to 0.3, 0.1 to 0.3, 0.1 to 0.25, or 0.15 to 0.25. Therefore, the fire stability of the composite membrane can be improved, and the electrical conductivity can be increased while maintaining the structure of the composite membrane at high temperatures.

[0085] According to an exemplary embodiment, a mixture comprising a flame-retardant monomer and an electrolyte can be impregnated in the composite membrane, and the mixture can be cured to impregnate the flame-retardant polymer in the composite membrane.

[0086] In some embodiments, a mixture comprising a flame-retardant monomer and an electrolyte can be impregnated in the composite membrane. For example, a mixture prepared by impregnating the flame-retardant monomer in the electrolyte can be impregnated in the composite membrane. For example, the composite membrane may have pores formed between the inorganic electrolyte and the organic binder, allowing the mixture to permeate into the pores and impregnate the membrane. For example, the composite membrane formed on the substrate (e.g., a glass substrate and a plastic substrate, etc.) can be separated from the substrate and the mixture impregnated in the composite membrane.

[0087] In some embodiments, the volume ratio of the flame-retardant monomer content to the inorganic electrolyte content in the composite membrane and the volume ratio of the flame-retardant polymer content to the inorganic electrolyte content in the composite membrane can be substantially the same. For example, the volume reduction or increase when the flame-retardant monomer is polymerized or copolymerized into the flame-retardant polymer can be less than 0.0001 volume% or less relative to the total volume of the flame-retardant monomer.

[0088] In some embodiments, the flame-retardant monomer may contain a phosphorus-containing functional group. For example, the phosphorus-containing functional group may contain at least one selected from phosphate ester group, phosphite group, phosphonate group, and phosphazene group. Therefore, the flame retardancy of the electrolyte for secondary batteries can be improved.

[0089] In some embodiments, the flame-retardant monomer may contain fluorine atoms. Therefore, the flame retardancy of the electrolyte for the secondary battery can be improved.

[0090] In one embodiment, the flame-retardant monomer may simultaneously contain phosphorus-containing functional groups and fluorine atoms. Therefore, the flame retardancy of the electrolyte for the secondary battery can be further improved.

[0091] In some embodiments, the flame-retardant monomer may contain thermally reactive functional groups and / or photoreactive functional groups. For example, the flame-retardant monomer may contain thermally reactive functional groups and may be a compound polymerized by heat. For example, the flame-retardant monomer may contain photoreactive functional groups and may be a compound polymerized by light irradiation. For example, the thermosetting and / or photoreactive functional groups may use (meth)acrylate groups, acrylate groups, ether groups, alcohol groups, alkoxy groups, etc.

[0092] In one embodiment, the flame-retardant monomer may simultaneously contain at least one of phosphorus-containing functional groups, thermally reactive functional groups, and photoreactive functional groups. In another embodiment, the flame-retardant monomer may simultaneously contain at least one of phosphorus-containing functional groups, fluorine atoms, thermally reactive functional groups, and photoreactive functional groups.

[0093] In some embodiments, the electrolyte may contain a thermal initiator and / or a photoinitiator for inducing the thermosetting and / or photosetting of the flame-retardant monomer. In one embodiment, the content of the thermal initiator and / or photoinitiator may be from 0.5 parts by weight to 2 parts by weight relative to 100 parts by weight of the flame-retardant monomer contained in each electrolyte composition.

[0094] For example, the thermal initiator may include azo compounds such as 2,2-azobis(2-cyanobutane), 2,2-azobis(methylbutyronitrile), 2,2'-azobisisobutyronitrile (AIBN), and azobisdimethyl-valeronitrile (AMVN), or peroxide compounds such as benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, cumyl peroxide, and hydrogen peroxide.

[0095] For example, the photoinitiator may include 2-hydroxy-2-methyl-1-phenylpropane-1-one (HMPP), benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenylglyoxylate, benzyl dimethyl ketal, 2,4,6-trimethyl-benzoyl-trimethyl phosphine oxide, and other acylphosphines and α-aminoketones.

[0096] In some embodiments, the electrolyte may contain a lithium salt. The lithium salt may include the lithium salt described above. This can improve the ionic conductivity of the electrolyte for secondary batteries.

[0097] In some embodiments, the electrolyte may contain an organic solvent. For example, the organic solvent may be carbonate-based organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methyl propyl carbonate (MPC), dipropyl carbonate (DPC), and vinylene carbonate (VC), as well as dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran. These may be used alone or in combination of two or more.

[0098] In one embodiment, the organic solvent may be a carbonate-based organic solvent. Therefore, the electrochemical and chemical stability of the electrolyte for secondary batteries can be improved.

[0099] In one embodiment, the electrolyte may further comprise additives. The additives may include, for example, cyclic carbonate compounds, fluorinated cyclic carbonate compounds, sulopentalide compounds, cyclic sulfate compounds, cyclic sulfite compounds, phosphate compounds, and borate compounds.

[0100] The cyclic carbonate-based compound may include vinylene carbonate, ethylene carbonate (VEC), etc.

[0101] The fluorine-substituted cyclic carbonate compounds may include fluoroethylene carbonate (FEC), etc.

[0102] The sulfonyl compounds may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0103] The cyclic sulfate-based compounds may include 1,2-ethylene sulfate, 1,2-propylene sulfate, etc.

[0104] The cyclic sulfite-based compounds may include ethylene sulfite, butylene sulfite, etc.

[0105] The phosphate-based compounds may include lithium difluorobis-oxalato phosphate, lithium difluorophosphate, etc.

[0106] The borate-based compounds may include lithium bis(oxalate)borate, etc.

[0107] In some embodiments, the mixture impregnated in the composite membrane can be cured. Therefore, an electrolyte for secondary batteries can be prepared.

[0108] In some embodiments, the flame-retardant monomers can be polymerized or copolymerized while the second mixture is being cured. Therefore, the electrolyte for secondary batteries can contain flame-retardant polymers.

[0109] In some embodiments, the curing of the second mixture can be achieved by heat treatment. For example, by heat curing the second mixture, the flame-retardant monomers inside the composite film are polymerized or copolymerized into flame-retardant compounds.

[0110] In some embodiments, the heat treatment can be 40°C to 160°C, 60°C to 160°C, 80°C to 160°C, 80°C to 140°C, or 80°C to 120°C. In one embodiment, the heat treatment can be performed by increasing the temperature from room temperature (e.g., 25°C) at a rate of 1°C / min to 10°C / min, 2°C / min to 10°C / min, or 3°C / min to 10°C / min.

[0111] In some implementations, the heat treatment may be carried out for 30 minutes to 2 hours, 30 minutes to 1.5 hours, or 45 minutes to 1.25 hours.

[0112] Within the aforementioned temperature and time range, the flame-retardant monomers within the second mixture can be fully polymerized or copolymerized.

[0113] In some embodiments, the curing of the second mixture can be achieved by light irradiation. For example, the flame-retardant monomers inside the composite film can be polymerized or copolymerized into a flame-retardant polymer by photocuring the second mixture. Therefore, the polymerization or copolymerization of the flame-retardant monomers can be carried out at relatively low temperatures. This prevents damage to the composite film from high-temperature heat treatment.

[0114] In some embodiments, the UV curing process used for the photopolymerization can use wavelengths from 250 nm to 400 nm and 800 mW / cm². 2 Up to 1100mW / cm 2 The light intensity is used. In one embodiment, the UV curing process can be performed for 5 to 20 seconds. Within the aforementioned wavelength and intensity range, the flame-retardant monomers within the second mixture can be fully polymerized or copolymerized.

[0115] A secondary battery according to an exemplary embodiment may include: a positive electrode; a negative electrode disposed opposite to the positive electrode; and an electrolyte layer disposed between the positive electrode and the negative electrode.

[0116] The positive electrode may include a positive electrode current collector and a layer of positive electrode active material disposed on at least one side of the positive electrode current collector.

[0117] The positive electrode current collector may include stainless steel, nickel, aluminum, titanium, or alloys thereof. It may also include aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. The positive electrode current collector may be, for example, from 10 μm to 50 μm, but is not limited thereto.

[0118] The positive electrode active material may contain compounds that can reversibly insert and deintercalate lithium ions.

[0119] According to an exemplary embodiment, the positive electrode active material may comprise a lithium-nickel metal oxide. The lithium-nickel metal oxide may further comprise at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0120] In some embodiments, the positive electrode active material or the lithium-nickel metal oxide may comprise a layered structure or a crystal structure represented by the following chemical formula 1.

[0121] [Chemical Formula 1]

[0122] Li x Ni a M b O 2+2

[0123] In chemical formula 1, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1. As mentioned above, M can contain Co, Mn, and / or Al.

[0124] The chemical structure represented by Formula 1 indicates the bonding relationships contained in the layered or crystalline structure of the positive electrode active material, and does not exclude additional elements. For example, M may contain Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element of the positive electrode active material. Formula 1 is provided to represent the bonding relationships of the main active elements, and it should be understood that Formula 1 includes the introduction and substitution of additional elements.

[0125] In one embodiment, in addition to the primary active element, auxiliary elements may be further included to enhance the chemical stability of the positive electrode active material or the layered structure / crystal structure. These auxiliary elements may be incorporated into the layered structure / crystal to form a bond, and this should be understood to also include the chemical structures represented by Formula 1.

[0126] The auxiliary element may include at least one of, for example, Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element may serve as an auxiliary active element, along with Co or Mn, to contribute to the capacity / power activity of the positive electrode active material; for example, Al.

[0127] For example, the positive electrode active material or the lithium-nickel metal oxide may contain a layered structure or a crystal structure represented by the following chemical formula 1-1.

[0128] [Chemical Formula 1-1]

[0129] Li x Ni a M1 b1 M2 b2 O 2+z

[0130] In chemical formula 1-1, M1 may contain Co, Mn, and / or Al. M2 may contain the aforementioned auxiliary elements. In chemical formula 1-1, the following conditions may be met: 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, -0.5≤z≤0.1.

[0131] The positive electrode active material may further include coating elements or doping elements. For example, elements that are substantially the same as or similar to the auxiliary elements described above can be used as coating elements or doping elements. For example, one or more combinations of the elements described above can be used as coating elements or doping elements.

[0132] The coating element or doping element may exist on the surface of the lithium nickel metal oxide particles or penetrate through the surface of the lithium nickel metal composite oxide particles and be contained in the bonding structure represented by chemical formula 1 or chemical formula 1-1.

[0133] The positive electrode active material may contain nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, NCM-based lithium oxide with increased nickel content can be used.

[0134] Ni can be provided as a transition metal related to the power and capacity of lithium secondary batteries. Therefore, as described above, by using a high-content (High-Ni) composition for the positive electrode active material, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0135] However, with increasing Ni content, the long-term storage stability and lifetime stability of the cathode or secondary battery may relatively decrease, and side reactions with the electrolyte may also increase. However, according to an exemplary embodiment, conductivity can be maintained by including Co, while lifetime stability and capacity retention characteristics can be improved by including Mn.

[0136] The Ni content in the NCM-based lithium oxide (e.g., the mole fraction of nickel in the total moles of nickel, cobalt, and manganese) can be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the Ni content can be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0137] In some embodiments, the positive electrode active material may further comprise lithium cobalt oxide-based active material, lithium manganese oxide-based active material, lithium nickel oxide-based active material, or lithium iron phosphate (LFP)-based active material (e.g., LiFePO4).

[0138] In some embodiments, the positive electrode active material may comprise, for example, a manganese-rich (Mn-rich) based active material, a lithium-rich layered oxide (LLO) / over lithiated oxide (OLO) based active material, or a cobalt-less based active material having a chemical structure or crystal structure represented by the following chemical formula 2.

[0139] [Chemical Formula 2]

[0140] p[Li₂MnO₃]·(1-p)[Li q JO2]

[0141] In Chemical Formula 2, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may include at least one element selected from Mn, Ni, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0142] In some embodiments, the positive electrode active material may also be a sodium-based active material or a potassium-based active material. The sodium-based active material may include a layered structure or a crystal structure in which Li in Chemical Formula 1, Chemical Formula 1-1, and / or Chemical Formula 2 is replaced by Na and / or K.

[0143] In some embodiments, the positive electrode active material may also be a calcium-based active material. The calcium-based active material may include, for example, a calcium-cobalt active material and a calcium-phosphate active material.

[0144] For example, the positive electrode active material may be mixed in a solvent to prepare a positive electrode paste. The positive electrode paste may be coated on a positive electrode current collector and then dried and calendered to form a positive electrode active material layer. The coating process may be performed by methods such as gravure coating, slot die coating, multi-layer simultaneous die coating, imprinting, doctor blade coating, dip coating, bar coating, casting, etc., and is not limited thereto. The positive electrode active material layer may further include an adhesive and may optionally further include an electrolyte, a conductive material, a thickening agent, etc. [[ID=D10]]

[0145] Examples of the solvent used in the manufacture of the positive electrode active material layer may include N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMA), dimethylaminopropylamine (DMAPA), ethylene oxide (EO), tetrahydrofuran (THF), etc.

[0146] In one embodiment, the electrolyte included in the positive electrode active material layer may be the electrolyte for the secondary battery described above. In one embodiment, the electrolyte included in the positive electrode active material layer may be the inorganic electrolyte described above, but the electrolyte included in the positive electrode active material layer may be the same as or different from the inorganic electrolyte included in the composite film. For example, the secondary battery may be provided as an all-solid battery including the electrolyte or the inorganic electrolyte.

[0147] The adhesive may contain at least one selected from polyethylene compound-based adhesives, cellulose-based adhesives, acrylic polymer-based adhesives, and copolymer resin adhesives.

[0148] The adhesive may include, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), ethylene pyrrolidone / vinyl acetate (VP / VA) copolymer resin, polyvinylidene fluoride (PVDF), vinylidene fluoride-co-hexafluoropropylene copolymer (poly(vinylidene fluoride-co-hexafluoropropylene)), polyacrylonitrile, polymethyl methacrylate, nitrile rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc. In one embodiment, the positive electrode adhesive may be a PVDF-based adhesive.

[0149] The conductive material can be added to enhance the conductivity of the positive electrode active material layer and / or the mobility of lithium ions or electrons. For example, the conductive material may include carbon-based conductive materials such as graphite, carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes, vapor-grown carbon fiber (VGCF), and carbon fiber, and / or metal-based conductive materials including perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3, but is not limited to these.

[0150] The positive electrode active material layer may further contain thickeners and / or dispersants. As one embodiment, the positive electrode active material layer may contain thickeners such as carboxymethyl cellulose (CMC).

[0151] The negative electrode may include a negative electrode current collector and a layer of negative electrode active material disposed on at least one side of the negative electrode current collector.

[0152] The positive electrode current collector may include, for example, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, etc. For example, the negative electrode current collector may be 10 μm to 50 μm, but is not limited thereto.

[0153] The negative electrode active material layer may include a negative electrode active material. The negative electrode active material may use a material that can adsorb and desorb lithium ions. For example, the negative electrode active material may use carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, carbon fibers, etc.; lithium metal; lithium alloys; silicon (Si)-containing substances or tin (Sn)-containing substances, etc.

[0154] Examples of the amorphous carbon may include hard carbon, soft carbon, coke, mesocarbon microbead (MCMB), mesophase pitch-based carbon fiber (MPCF), etc.

[0155] Examples of the crystalline carbon may include graphite-based carbons such as natural graphite, artificial graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc.

[0156] The lithium metal may include pure lithium metal or lithium metal formed with a protective layer for inhibiting dendrite growth, etc. In one embodiment, a lithium metal-containing layer deposited or coated on the negative electrode current collector may be used as the negative electrode active material layer. In one embodiment, a lithium thin film layer may also be used as the negative electrode active material layer.

[0157] Elements included in the lithium alloy may include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium, etc.

[0158] The silicon-containing substance may provide further increased capacity characteristics. The silicon-containing substance may include Si, SiO x (0 < x < 2), metal-doped SiO x (0 < x < 2), silicon-carbon composites, etc. The metal may include lithium and / or magnesium, and metal-doped SiO x (0 < x < 2) may include metal silicate.

[0159] For example, the negative electrode active material may be mixed in a solvent to prepare a negative electrode slurry. After the negative electrode slurry is coated / deposited on the negative electrode current collector, it is dried and calendered to manufacture the negative electrode active material layer. The coating process may be carried out by a method substantially the same as the manufacturing method of the positive electrode active material layer. The negative electrode active material layer may further include a binder, and may optionally further include an electrolyte, a conductive material, a thickener, etc.

[0160] In some embodiments, the negative electrode may further include a negative electrode active material layer in the form of lithium metal formed by a deposition / coating process.

[0161] Examples of solvents that can be used for the negative electrode active material layer include water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol, tert-butanol, etc.

[0162] In one embodiment, the electrolyte contained in the negative electrode active material layer can be the electrolyte used in the secondary battery described above. In another embodiment, the electrolyte contained in the negative electrode active material layer can be the inorganic electrolyte described above, but the electrolyte contained in the negative electrode active material layer can be the same as or different from the inorganic electrolyte contained in the composite membrane. For example, the secondary battery can be provided as an all-solid-state battery containing the electrolyte described above or an inorganic electrolyte.

[0163] The aforementioned substances, which can be used in the manufacture of the positive electrode, can be used as the adhesive, conductive material, and thickener.

[0164] In some implementations, the negative electrode adhesive may be a styrene-butadiene rubber-based adhesive, carboxymethyl cellulose, polyacrylic acid-based adhesive, poly(3,4-ethylenedioxythiophene) (PEDOT)-based adhesive, etc.

[0165] According to an exemplary embodiment, an electrolyte layer may be disposed between the positive electrode and the negative electrode. The electrolyte layer may be a solid electrolyte layer. For example, the solid electrolyte layer may be an electrolyte layer containing the aforementioned electrolyte.

[0166] According to an exemplary embodiment, the battery cell is defined by a positive electrode, a negative electrode, and a solid electrolyte layer, and an electrode assembly can be formed by stacking multiple said battery cells. For example, the electrode assembly can be formed by winding, lamination, folding, etc.

[0167] For example, tabs (positive and negative tabs) may protrude from the positive and negative current collectors and extend to one side of the housing, respectively. The tabs may be fused to said side of the housing and connected to electrode leads (positive and negative leads) extending to or exposed outside the housing.

[0168] For example, pouch-shaped shells, prismatic shells, cylindrical shells, coin-shaped shells, etc. can be used.

[0169] The embodiments of the present invention will be further described below with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are only for illustrating the present invention and are not intended to limit the scope of the claims. Various changes and modifications can be made to the embodiments within the scope of the present invention and its technical concept, which is obvious to those skilled in the art, and such variations and modifications naturally fall within the scope of the claims.

[0170] Examples and Comparative Examples

[0171] Example 1

[0172] Preparation of electrolytes for secondary batteries

[0173] LLZTO(Li) will be used as an inorganic electrolyte. 6.4 La3Zr 1.4 Ta 0.6 O 12 A mixed slurry is prepared by mixing polyvinyl butyral (PVB) as an organic binder in a volume ratio of 4:1 with a mixed solvent in which propylene carbonate (PC) and dimethyl sulfoxide (DMSO) are mixed in a mixing ratio of 1:1.

[0174] The mixed slurry is cast onto a polyethylene terephthalate (PET) film and then dried at 250°C for 1 hour to form a composite film.

[0175] The inorganic electrolyte content in the total volume of the composite membrane is 80% by volume.

[0176] A mixture was prepared by mixing a compound represented by the following chemical formula 3, which serves as a flame-retardant monomer, with a 1.0 M LiTFSi solution (a mixed solvent of EC / EMC with a volume ratio of 25:75).

[0177] The composite membrane is separated from the PET membrane, and the mixture is impregnated into the composite membrane at a volume ratio of 10:1. The volume ratio of the flame-retardant monomer to the inorganic electrolyte is 0.2.

[0178] [Chemical Formula 3]

[0179]

[0180] Subsequently, the second mixture is heated at 5°C / min and thermally cured at 80°C for 1 hour to prepare an electrolyte for secondary batteries.

[0181] Examples 2 to 9

[0182] The electrolyte for secondary batteries was prepared using the same method as in Example 1, except that the volume ratio of the content of the inorganic material, the content of the flame-retardant monomer, and the content of the inorganic electrolyte in the total volume of the composite membrane was changed as shown in Table 1 below.

[0183] Comparative Example 1

[0184] The electrolyte for secondary batteries was prepared using the same method as in Example 1, except that the mixture did not contain flame-retardant monomers in the preparation of the electrolyte for secondary batteries.

[0185] Comparative Examples 2 to 7

[0186] The electrolyte for secondary batteries was prepared using the same method as in Example 1, except that the volume ratio of the content of the inorganic material, the content of the flame-retardant monomer, and the content of the inorganic electrolyte in the total volume of the composite membrane was changed as shown in Table 1 below.

[0187] [Table 1]

[0188]

[0189] Experimental Example 1: Evaluation of Electrolytes

[0190] (1) Evaluation of cohesion

[0191] The cohesive force between inorganic electrolyte particles contained in the electrolytes for secondary batteries prepared according to the above embodiments and comparative examples was measured.

[0192] Specifically, the force required to disengage inorganic electrolyte particles at a depth of 10 μm from the surface of the electrolyte for the secondary battery is measured as the cohesive force using a surface and interface cutting analysis system (SAICAS).

[0193] (2) Flame retardancy evaluation - combustion evaluation

[0194] The electrolyte for the secondary battery prepared according to the above embodiments and comparative examples was burned for 5 minutes to evaluate whether it burned.

[0195] The following is an evaluation of whether it burns.

[0196] ○: Combustion area less than 5%

[0197] △: The burning area is 5% to 20%.

[0198] ×: The burning area exceeds 20%, or the composite membrane cannot maintain its shape during combustion.

[0199] The evaluation results are shown in Table 2 below.

[0200] [Table 2]

[0201] Category Evaluation of the aggregation force (F H , N) Evaluation of combustion Example 1 0.08 ○ Example 2 0.04 △ Example 3 0.02 △ Example 4 0.08 △ Example 5 0.08 ○ Example 6 0.02 △ Example 7 0.02 △ Example 8 0.04 △ Example 9 0.04 △ Comparative Example 1 0.04 × Comparative Example 2 - × Comparative Example 3 - × Comparative Example 4 - × Comparative Example 5 - × Comparative Example 6 - × Comparative Example 7 - ×

[0202] Referring to Table 2, in embodiments where the content of inorganic electrolyte in the total volume of the composite membrane is 50% to 95% by volume, the organic binder can maintain the shape of the composite membrane while ensuring sufficient contact between the inorganic electrolytes. Therefore, it can have an interparticle cohesion of more than 0.01N while having a burning area of ​​less than 20% during combustion, exhibiting excellent flame retardancy.

[0203] Figure 3 These are combustion photographs from a combustion evaluation of an exemplary implementation scheme. Specifically, Figure 3 This is a photograph of the combustion of the electrolyte used in the secondary battery in the combustion evaluation of Example 1.

[0204] Figure 4 These are photographs of the electrolyte for secondary batteries after a combustion evaluation in an exemplary embodiment. Specifically, Figure 4 This is a photograph of the electrolyte for secondary batteries after the combustion evaluation in Example 1.

[0205] Reference Figure 3 and Figure 4 In Example 1, where the volume ratio of inorganic electrolyte to organic binder was adjusted to 4:1 and the volume ratio of flame-retardant monomer content to inorganic electrolyte content was adjusted to 0.2, the composite film maintained its shape even after combustion evaluation.

[0206] In comparative examples where the flame-retardant polymer does not contain flame-retardant monomers or can be formed by the polymerization of flame-retardant monomers, or where the volume of inorganic electrolyte in the total volume of the composite membrane is adjusted to less than 50% by volume or greater than 95% by volume, the burning area after combustion exceeds 20%.

[0207] In Comparative Example 1, which does not contain flame-retardant monomers or is a flame-retardant polymer that can be formed by the polymerization of flame-retardant monomers, the burning area exceeds 20% after the combustion evaluation.

[0208] In Comparative Examples 2, 4, and 5, where the volume of inorganic electrolyte in the total volume of the composite membrane was reduced to less than 50% by volume, the aggregation force could not be evaluated because no contact was generated between the inorganic electrolyte particles. Furthermore, after combustion evaluation, the combustion of the organic binder caused voids to appear between the inorganic electrolytes, resulting in the disappearance of interparticle contact force and a combustion area exceeding 20%.

[0209] In Comparative Examples 3, 6, and 7, where the content of inorganic oxides was increased to over 95% by volume, a composite membrane in thin film form could not be formed due to the high content of inorganic oxides. Furthermore, it was difficult to maintain the thin film morphology of the composite membrane, and the burned area after combustion exceeded 20%.

Claims

1. An electrolyte for a secondary battery, comprising: Lithium salts; Composite membranes comprising inorganic electrolytes and organic binders; and Flame-retardant polymer electrolyte, in, The content of the inorganic electrolyte in the total volume of the composite membrane is 50% to 95% by volume.

2. The electrolyte for a secondary battery according to claim 1, wherein, The inorganic electrolyte includes an oxide-based solid electrolyte.

3. The electrolyte for a secondary battery according to claim 1, wherein, The content of the organic adhesive in the total volume of the composite film is 5% to 50% by volume.

4. The electrolyte for a secondary battery according to claim 1, wherein, The volume ratio of the content of the organic binder to the content of the inorganic electrolyte is 0.05 to 0.

5.

5. The electrolyte for a secondary battery according to claim 1, wherein, The volume ratio of the flame-retardant compound content to the inorganic electrolyte content is 0.01 to 0.

3.

6. The electrolyte for a secondary battery according to claim 1, wherein, The flame-retardant compound contains at least one of phosphorus-containing functional groups and fluorine atoms.

7. The electrolyte for a secondary battery according to claim 6, wherein, The phosphorus-containing functional group includes at least one of phosphate ester group, phosphite group, phosphonate group and phosphazene group.

8. A lithium secondary battery, comprising: positive electrode; The negative electrode is disposed opposite to the positive electrode; and An electrolyte layer is disposed between the positive electrode and the negative electrode, and includes the electrolyte for a secondary battery as described in claim 1.

9. A method for preparing an electrolyte for a secondary battery, comprising the following steps: Inorganic electrolytes, organic binders, and solvents are mixed to prepare a mixed slurry; The mixed slurry is dried to prepare a composite membrane; as well as Flame-retardant polymer electrolyte is impregnated in the composite membrane. The volume ratio of the content of the organic binder to the content of the inorganic electrolyte is 0.01 to 0.

5.

10. The method for preparing the electrolyte for a secondary battery according to claim 9, wherein, The inorganic electrolyte includes an oxide-based solid electrolyte.

11. The method for preparing the electrolyte for a secondary battery according to claim 9, wherein, The content of the inorganic electrolyte in the total volume of the composite membrane is 50% to 95% by volume.

12. The method for preparing the electrolyte for a secondary battery according to claim 9, wherein, The step of impregnating the flame-retardant polymer electrolyte includes impregnating the composite membrane with a mixture comprising a flame-retardant monomer and an electrolyte and curing the mixture.

13. The method for preparing the electrolyte for a secondary battery according to claim 12, wherein, The electrolyte contains lithium salt.

14. The method for preparing the electrolyte for a secondary battery according to claim 12, wherein, The mixture further contains a thermal initiator. The curing of the mixture includes heat treatment of the mixture.

15. The method for preparing the electrolyte for a secondary battery according to claim 12, wherein, The mixture further contains a photoinitiator. The curing of the mixture includes irradiating the mixture with light.

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