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

Abstract

An electrolyte for a secondary battery according to an embodiment of the present invention may include a lithium salt, a composite film including an inorganic electrolyte, and a flame-retardant polymer. A 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] The present invention relates to an electrolyte for a secondary battery, a method for preparing the same, and a lithium secondary battery comprising the same. More particularly, the present invention relates to an electrolyte for a secondary battery comprising an inorganic electrolyte, a method for preparing the same, and a lithium secondary battery comprising the same. Background Art

[0002] Secondary batteries are batteries that can be repeatedly charged and discharged. With the development of the information communications and display industries, they are widely used as power sources for portable electronic communication devices such as cameras, mobile phones, and laptop computers (PCs). Furthermore, in recent years, battery packs containing secondary batteries have been developed and used as power sources for environmentally friendly vehicles such as hybrid cars.

[0003] Examples of secondary batteries include lithium secondary batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Among them, lithium secondary batteries have high operating voltage and energy density per unit weight, and are advantageous in terms of charging speed and weight reduction, and therefore are actively being researched and developed.

[0004] Currently, commercial lithium secondary batteries primarily use liquid electrolytes, which can lead to safety issues such as leakage, fire, and explosion due to sudden environmental changes such as temperature fluctuations and external shocks. To address this issue, research is underway to solidify the electrolyte to ensure stability and increase energy density.

[0005] All-solid-state batteries can contain solid electrolytes such as gel polymers, oxides or sulfides, or composite polymers as electrolytes, thereby improving their resistance to fire and explosion caused by external impacts and environmental changes. Summary of the Invention

[0006] (1) Technical issues to be solved

[0007] A technical problem of the present invention is to provide an electrolyte for a secondary battery having improved mechanical properties and high-temperature stability.

[0008] A technical problem of the present invention is to provide a method for preparing an electrolyte for a secondary battery having improved mechanical properties and high-temperature stability.

[0009] A technical problem of the present invention is to provide a lithium secondary battery with improved electrochemical stability.

[0010] (2) Technical solution

[0011] The electrolyte for a secondary battery according to an exemplary embodiment may include: a lithium salt; a composite film including an inorganic electrolyte; and a flame-retardant polymer including a phosphorus-containing functional group. The composite film may include a sintered body of the inorganic electrolyte, and the strength of the composite film measured in a cross-cut test may be ASTM standards 5B to 2B.

[0012] In some embodiments, the inorganic electrolyte may include an oxide-based solid electrolyte.

[0013] In some embodiments, a ratio of the content of the flame retardant polymer to the content of the inorganic electrolyte on a volume basis may be 0.01 to 0.3.

[0014] In some embodiments, the flame retardant polymer may include at least one of a phosphorus-containing functional group and a fluorine atom.

[0015] In some embodiments, the phosphorus-containing functional group may include at least one of a phosphate group, a phosphite group, a phosphonate group, and a phosphazene group.

[0016] In some embodiments, the strength of the composite film measured in a cross-cut test may be ASTM standards 2B to 4B.

[0017] In some embodiments, the composite membrane may include pores, and the porosity of the composite membrane may be 10% to 40%.

[0018] In some embodiments, the composite film may further include a sintered body of an organic binder.

[0019] A lithium secondary battery according to an exemplary embodiment may 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 containing the above-described electrolyte for a secondary battery.

[0020] In the method for preparing a secondary battery electrolyte according to an exemplary embodiment, an inorganic electrolyte, an organic binder, and a solvent may be mixed to prepare a mixed slurry. The mixed slurry may be dried to prepare a primary composite film. The primary composite film may be sintered to prepare a composite film. A flame-retardant polymer may be impregnated into the composite film to prepare a secondary battery electrolyte.

[0021] In some embodiments, the strength of the composite film measured in a cross-cut test may be ASTM standards 5B to 2B.

[0022] In some embodiments, the inorganic electrolyte may include an oxide-based solid electrolyte.

[0023] In some embodiments, the step of impregnating the flame retardant polymer may include impregnating a mixed solution including a flame retardant monomer and an electrolyte in the composite film and curing the mixed solution.

[0024] In some embodiments, the electrolyte may include a lithium salt.

[0025] In some embodiments, the mixed solution may further include a thermal initiator, and curing the mixed solution may include subjecting the mixed solution to a heat treatment.

[0026] In some embodiments, the mixed solution may further include a photoinitiator, and curing the mixed solution may include irradiating the mixed solution with light.

[0027] (3) Beneficial effects

[0028] The electrolyte for a secondary battery prepared according to the exemplary embodiment of the present invention may have improved mechanical properties, thereby improving the stability of the electrolyte layer.

[0029] Furthermore, the electrolyte for a secondary battery prepared according to an exemplary embodiment of the present invention may have self-extinguishing properties, thereby improving high-temperature stability, fire stability, and flame retardancy of the electrolyte layer even when charge / discharge is repeated.

[0030] A lithium secondary battery according to an exemplary embodiment of the present invention may include the electrolyte for a secondary battery, and thus may improve safety at normal and high temperatures, and may improve electrical characteristics.

[0031] The secondary battery electrolyte, preparation method thereof, and lithium secondary battery of the present invention can be widely used in electric vehicles, battery charging stations, and other green technology fields such as solar power generation and wind power generation using batteries. The secondary battery electrolyte, preparation method thereof, and lithium secondary battery of the present invention can be used in eco-friendly electric vehicles and hybrid vehicles, etc., which prevent climate change by suppressing air pollution and greenhouse gas emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a schematic diagram illustrating the structure of an electrolyte for a secondary battery according to an exemplary embodiment.

[0033] Figure 2 is a schematic process flow chart for explaining a method for preparing an electrolyte for a secondary battery according to an exemplary embodiment. DETAILED DESCRIPTION

[0034] According to an exemplary embodiment, a method for preparing a secondary battery electrolyte comprising a lithium salt, an inorganic oxide, an organic binder, and a flame retardant polymer and a secondary battery electrolyte prepared thereby are provided. In addition, a lithium secondary battery comprising an electrolyte layer comprising the secondary battery electrolyte is provided.

[0035] Hereinafter, the present invention will 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 exemplarily.

[0036] Hereinafter, in the specification, unless otherwise specifically defined, when describing a layer, membrane, film, region, plate or the like as being "on" another part, this may include not only the case of being "directly" "above" another part, but also the case of having other parts in between.

[0037] When the compound represented by a chemical formula used in the present invention has isomers, the compound represented by the chemical formula refers to a representative chemical formula including the isomers.

[0038] In the present invention, flame retardancy refers to the property of a sample that, while ignited by a flame (ignition source), prevents or inhibits the generation of flames and combustion upon removal of the flame. Flame retardancy can be evaluated by igniting a sample with a constant heat level for at least one second using a blowtorch and then extinguishing the flame after the blowtorch is removed. A shorter extinguishing time indicates superior flame retardancy.

[0039] Specifically, the flame-retardant polymer can be prepared in the form of a glass fiber impregnated or self-supporting film with a diameter of 19 mm. When a blowtorch is used to supply a flame of a certain heat 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, and more specifically within 0.5 seconds.

[0040] Figure 1 is a schematic diagram illustrating an electrolyte for a secondary battery according to an exemplary embodiment.

[0041] Reference Figure 1 The electrolyte for the secondary battery (hereinafter, referred to as "electrolyte") may include a lithium salt, a composite film 105, and a flame retardant polymer 130. In some embodiments, the electrolyte may include a lithium salt, a composite film 105, a flame retardant polymer 130, and an electrolytic solution.

[0042] According to an exemplary embodiment, the composite membrane 105 may include an inorganic electrolyte 110. For example, the inorganic electrolyte 110 may be uniformly dispersed within the composite membrane 105.

[0043] In some embodiments, the composite film 105 may include a sintered body of an inorganic electrolyte 110, specifically, a sintered body of an inorganic electrolyte 110 and an organic binder. For example, the composite film 105 may include a sintered body in which the organic binder is burned out by sintering. Therefore, the composite film 105 can form a free-standing film with sufficient porosity and strength. In addition, the composite film 105 may not include an organic binder, which can reduce the interfacial contact resistance between the electrolyte and the electrode comprising the composite film 105, thereby improving the electrical properties of the secondary battery.

[0044] The lithium salt can be represented by, for example, Li + X - As the anion of the lithium salt (X - ), we can exemplify 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.

[0045] In some embodiments, the inorganic electrolyte 110 may include an oxide-based solid electrolyte. For example, the oxide-based solid electrolyte may include an ion-conductive compound containing a metal or oxygen. For example, the oxide-based solid electrolyte may include an LLTO-based compound, an LLZO-based compound, a 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、LiTi x Zr 2-x (PO4)3(0≤x≤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.

[0046] In one embodiment, the inorganic electrolyte 110 may be 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. As a result, the ionic conductivity and mechanical strength of the electrolyte 100 may be improved, thereby suppressing lithium dendrites and improving high-temperature stability and lifespan characteristics.

[0047] In some embodiments, the flame retardant polymer 130 may include a phosphorus functional group. In one embodiment, the phosphorus functional group may include at least one of a phosphate group, a phosphite group, a phosphonate group, and a phosphazene group. For example, the flame retardant polymer 130 may be a polymer obtained by polymerizing or copolymerizing a flame retardant monomer containing at least one of a phosphate group, a phosphite group, a phosphonate group, and a phosphazene group. Therefore, side reactions of the flame retardant polymer 130 with the inorganic electrolyte 110, the organic binder 120, and compounds such as the lithium salt or the electrolyte can be suppressed. In addition, by including a phosphorus functional group, the flame retardant polymer 130 can block oxygen during combustion and prevent thermal runaway.

[0048] 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 a flame-retardant monomer containing fluorine atoms. In the event of a fire, the fluorine atoms form free radicals, thereby inhibiting the propagation of the combustion reaction. Therefore, the flame-retardant polymer 130 can improve the flame retardancy of the electrolyte 100.

[0049] In one embodiment, the flame-retardant polymer 130 may contain both phosphorus-containing functional groups and fluorine atoms. For example, the flame-retardant polymer 130 may be a polymer formed by polymerizing or copolymerizing a flame-retardant monomer containing a phosphorus-containing functional group and a fluorine atom. For example, the flame-retardant polymer 130 may be a polymer formed by copolymerizing a flame-retardant monomer containing a phosphorus-containing functional group and a flame-retardant monomer containing a fluorine atom. Therefore, the flame-retardant polymer 130 may further improve the flame retardancy of the electrolyte 100.

[0050] In some embodiments, the flame-retardant polymer 130 may be a compound formed by polymerizing or copolymerizing a flame-retardant monomer having a thermo-reactive functional group and / or a photo-reactive functional group. For example, the flame-retardant monomer may contain a thermo-reactive functional group and may be a compound that is polymerized by heat. For example, the flame-retardant monomer may contain a photo-reactive functional group and may be a compound that is polymerized by irradiation with light. For example, the thermosetting functional group and / or the photo-curable functional group may be a (meth)acrylate group, an acrylic acid group, an ether group, an alcohol group, an alkoxy group, or the like.

[0051] In some embodiments, the volume-based ratio of the flame-retardant polymer 130 content to the inorganic electrolyte 110 content 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 the above ranges, the self-extinguishing property of the flame-retardant polymer 130 can improve the fire stability and high-temperature stability of the electrolyte 100. In addition, even if the organic binder 120 included in the composite film 105 catches fire, the flame-retardant polymer 130 can maintain the structure of the composite film 105.

[0052] In some embodiments, the composite membrane 105 may be formed with pores 150. For example, the pores 150 may be formed in a sintered body of the inorganic electrolyte 110 and the organic binder where the organic binder is burned out and removed. Thus, the flame-retardant polymer 130 may be located in the pores 150 of the composite membrane 105.

[0053] In some embodiments, the porosity of the composite membrane 105 may be 10% to 40%, 10% to 35%, 15% to 30%, 20% to 35%, or 10% to 25%. Within the above ranges, the flame-retardant polymer 130 may be located within the composite membrane 105 at the above ratio while maintaining the structure of the composite membrane 105. Therefore, the ionic conductivity and flame retardancy of the electrolyte 100 may be improved, and the electrical resistance may be reduced.

[0054] According to an exemplary embodiment, the strength of the composite film 105 measured in a cross-cut test may be ASTM standards 2B to 5B.

[0055] ASTM standard 5B indicates the case where no surface was peeled off in the cross-cut test. ASTM standard 4B indicates the case where the surface peeled off in the cross-cut test accounts for more than 0% and less than 5% of the total area. ASTM standard 3B indicates the case where the surface peeled off in the cross-cut test accounts for more than 5% and less than 15% of the total area. ASTM standard 2B indicates the case where the surface peeled off in the cross-cut test accounts for more than 15% and less than 35% of the total area. ASTM standard 1B indicates the case where the surface peeled off in the cross-cut test accounts for more than 35% and less than 65% of the total area. ASTM standard 0B indicates the case where the surface peeled off in the cross-cut test accounts for more than 65% of the total area.

[0056] When the strength of the composite film 105 measured in the cross-cut test is lower than ASTM Standard 2B, the electrolyte including the composite film 105 may be fractured and cracked, thereby possibly reducing electrical / chemical stability.

[0057] When the strength of the composite film 105 measured in the cross-cut test is 2B to 5B according to ASTM standards, the electrical / chemical stability and mechanical properties of the composite film 105 may be improved.

[0058] In some embodiments, the strength of the composite film 105 measured in the cross-cut test can be 2B to 5B or 3B to 5B according to ASTM standards. Within this range, the electrical / chemical stability and mechanical properties of the composite film 105 can be further improved. For example, when the strength of the composite film 105 measured in the cross-cut test is 4B or less according to ASTM standards, specifically 2B to 4B, 3B to 4B, or 4B, the brittleness of the composite film 105 can be appropriately maintained, thereby reducing damage to the composite film 105 caused by external impacts, etc., and improving processability.

[0059] Figure 2 1 is a process flow chart for explaining a method for preparing an electrolyte for a secondary battery according to an exemplary embodiment. Figure 2 , a method for preparing the above-mentioned secondary battery electrolyte is described.

[0060] Reference Figure 2 , a mixed slurry may be prepared by mixing an inorganic electrolyte, an organic binder, and a solvent (eg, step S10 ).

[0061] In some embodiments, the inorganic electrolyte may be an oxide-based solid electrolyte. The oxide-based solid electrolyte may be the above-mentioned oxide-based solid electrolyte. For example, the oxide-based solid electrolyte may be an oxide-based solid electrolyte containing lithium.

[0062] The organic binder may include at least one selected from the group consisting of a polyvinyl compound-based binder, a cellulose-based binder, an acrylic polymer-based binder, and a copolymer resin binder.

[0063] The organic binder may include, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), carboxymethylcellulose (CMC), vinylpyrrolidone / vinylacetate (VP / VA) copolymer resin, polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), acrylonitrile-butadiene rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), polyacrylic acid (PAN ... acid)-based adhesives, poly(3,4-ethylenedioxythiophene) (PEDOT), and the like.

[0064] In some embodiments, the organic binder may have a glass transition temperature (T g ) is an organic polymer having a temperature of 100° C. to 600° C., 100° C. to 500° C., or 100° C. to 300° C. In one embodiment, a PVDF-based binder may be included. By using a binder having the above-mentioned glass transition temperature (Tg) range, in the preparation of the electrolyte, the organic binder can uniformly distribute the inorganic electrolyte, etc. even without performing a separate heat treatment at a high temperature.

[0065] In some embodiments, the solvent can use a solvent that can dissolve both the inorganic electrolyte and the organic binder. In one embodiment, the solvent can include an organic solvent. The organic solvent can include a solvent having at least one functional group selected from alcohol, ketone, amide, ester, ether, aromatic hydrocarbons, etc. For example, the organic solvent can include 2-propanol, toluene, terpineol, N-methyl-2-pyrrolidone (NMP), etc. The solvent (or organic solvent) can be used alone or in combination of two or more.

[0066] 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 among the above solvents. The first solvent and the second solvent can dissolve at least one of an inorganic electrolyte and an organic binder, respectively. Therefore, even if either the inorganic electrolyte or the organic binder is insoluble in the first solvent, they can be dissolved in the second solvent and mixed.

[0067] In some embodiments, the mixed slurry may further include additives such as plasticizers and dispersants. The additives may be organic additives.

[0068] The plasticizer can be, for example, a plasticizer having a phosphate, phthalate, and citrate structure. The phosphate 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-acetyl tributyl citrate (OACTB).

[0069] The dispersant may include, for example, hydrogenated nitrile butadiene rubber (HNBR), polyvinyl pyrrolidone (PVP), polylactic acid (PLA), polyglycolic acid (PGA), and the like.

[0070] According to an exemplary embodiment, the primary composite film may be manufactured by drying the mixed slurry (eg, step S20 ), or by casting the mixed slurry on a substrate (eg, a glass substrate, a plastic substrate, etc.) and drying the mixed slurry.

[0071] 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 the above temperature range, the solvent contained in the mixed slurry is vaporized, thereby drying the mixed slurry. In addition, the inorganic electrolyte and organic binder contained in the mixed slurry can be uniformly distributed inside the mixed slurry.

[0072] In some embodiments, the mixed slurry may be dried for 30 minutes to 2 hours, 30 minutes to 1.5 hours, or 45 minutes to 1.25 hours. Within the above ranges, the solvent inside the mixed slurry may be vaporized and removed.

[0073] In an exemplary embodiment, the primary composite film may be sintered to prepare a composite film (eg, step S30 ).

[0074] For example, the primary composite membrane comprising the inorganic electrolyte and the organic binder can be sintered to burn out the organic matter, including the organic binder, contained in the primary composite membrane. Thus, a composite membrane can be prepared in which the organic matter is removed from the primary composite membrane and the residual inorganic electrolyte is present. Furthermore, pores can be formed at the locations where the organic matter, including the organic binder, was present.

[0075] In some embodiments, the sintering may be photo-sintering using photothermal effects. For example, the sintering may be performed using a method that utilizes the energy of a xenon lamp to perform sintering through a photothermal effect.

[0076] In some embodiments, the photosintering can be performed for 1 second to 10 minutes, 10 seconds to 10 minutes, or 10 seconds to 8 minutes. Within the above range, sufficient photothermal effect can be generated to perform sintering.

[0077] In some embodiments, the photo-sintering can be performed at 600°C to 1100°C, 700°C to 1100°C, 800°C to 1100°C, or 800°C to 1000°C. The sintering temperature is a value that converts the energy applied to the surface into a temperature using a simulation of the device. Within the above temperature range, organic matter including the organic binder can be oxidized and decomposed, thereby being removed from the primary composite membrane. In addition, over-sintering can be avoided, thereby improving the strength of the composite membrane and improving the stability of the electrolyte layer including the composite membrane.

[0078] In some embodiments, the strength of the composite film prepared by the above method measured in a cross-cut test may be the above-mentioned ASTM standard strength.

[0079] According to an exemplary embodiment, a flame retardant polymer may be impregnated into the composite film (eg, step S40 ).

[0080] The composite membrane can be a porous composite membrane having pores formed at a position where organic matter is removed. By using the porous composite membrane, the flame retardant polymer can be located in the pores of the porous composite membrane. For example, the flame retardant polymer can be impregnated in the pores of the porous composite 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 flame retardant polymer can be impregnated in the composite membrane.

[0081] In some embodiments, the flame-retardant polymer may be impregnated into the composite membrane such that the ratio of the volume-based content of the flame-retardant polymer to the volume-based content of the inorganic electrolyte in the composite membrane may 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. Thus, the fire stability of the composite membrane can be improved, and the electrical conductivity can be improved while maintaining the structure of the composite membrane at high temperatures.

[0082] According to an exemplary embodiment, a mixed solution including a flame retardant monomer and an electrolyte may be impregnated into the composite film, and the mixed solution may be cured, thereby impregnating the flame retardant polymer including a phosphorus-containing functional group into the composite film.

[0083] In some embodiments, a mixed solution comprising a flame retardant monomer and an electrolyte can be impregnated into the composite film. For example, a mixed solution prepared by immersing the flame retardant monomer in the electrolyte can be impregnated into the composite film. For example, the composite film can have pores formed between the inorganic electrolyte and the organic binder, and the mixed solution can penetrate into the pores and be impregnated. For example, the composite film formed on the substrate (e.g., a glass substrate, a plastic substrate, etc.) can be separated from the substrate and the mixed solution can be impregnated into the composite film.

[0084] In some embodiments, the volume ratio of the flame retardant monomer to the inorganic electrolyte contained in the composite film and the volume ratio of the flame retardant polymer to the inorganic electrolyte contained in the composite film can be substantially the same. For example, the volume reduced or increased when the flame retardant monomer is polymerized or copolymerized into the flame retardant polymer can be less than 0.0001 volume % or less than 0.00001 volume % relative to the total volume of the flame retardant monomer.

[0085] In some embodiments, the flame-retardant monomer may include a phosphorus-containing functional group. For example, the phosphorus-containing functional group may include at least one of a phosphate group, a phosphite group, a phosphonate group, and a phosphazene group. Thus, the flame retardancy of the secondary battery electrolyte may be improved.

[0086] In some embodiments, the flame retardant monomer may be a flame retardant monomer containing fluorine atoms, thereby improving the flame retardancy of the secondary battery electrolyte.

[0087] In one embodiment, the flame retardant monomer may include both a phosphorus-containing functional group and a fluorine atom, thereby further improving the flame retardancy of the secondary battery electrolyte.

[0088] In some embodiments, the flame retardant monomer may include a thermoreactive functional group and / or a photoreactive functional group. For example, the flame retardant monomer may include a thermoreactive functional group and may be a compound that polymerizes by heat. For example, the flame retardant monomer may include a photoreactive functional group and may be a compound that polymerizes by irradiation with light. For example, the thermosetting functional group and / or photocurable functional group may include a (meth)acrylate group, an acrylic acid group, an ether group, an alcohol group, an alkoxy group, and the like.

[0089] In one embodiment, the flame retardant monomer may simultaneously contain at least one of a phosphorus-containing functional group, a thermoreactive functional group, and a photoreactive functional group. In one embodiment, the flame retardant monomer may simultaneously contain at least one of a phosphorus-containing functional group, a fluorine atom, a thermoreactive functional group, and a photoreactive functional group.

[0090] In some embodiments, the electrolyte may include a thermal initiator and / or photoinitiator for inducing thermal curing and / or photocuring of the flame retardant monomer. In one embodiment, the content of the thermal initiator and / or photoinitiator may be 0.5 to 2 parts by weight relative to 100 parts by weight of the flame retardant monomer contained in each electrolyte composition.

[0091] For example, the thermal initiator may include an azo compound such as 2,2'-azobis(2-cyanobutane), 2,2'-azobis(methylbutyronitrile), 2,2'-azobisisobutyronitrile (AIBN), azobisdimethyl-valeronitrile (AMVN), or a peroxide compound such as benzoyl peroxide, acetyl peroxide, dilauroyl peroxide, di-tert-butyl peroxide, cumyl peroxide, and hydrogen peroxide.

[0092] 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-trimethylbenzoyl-diphenyl phosphine oxide, and other acylphosphine and α-aminoketone.

[0093] In some embodiments, the electrolyte may include a lithium salt. The lithium salt may include the above-mentioned lithium salts. This can improve the ionic conductivity of the electrolyte for the secondary battery.

[0094] In some embodiments, the electrolyte may include an organic solvent. For example, the organic solvent may be a carbonate-based organic solvent such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), dipropyl carbonate (DPC), and vinylene carbonate (VC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, cyclopentane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran. These may be used alone or in combination of two or more.

[0095] In one embodiment, the organic solvent may be a carbonate-based organic solvent. Thus, the electrochemical stability and chemical stability of the electrolyte for a secondary battery can be improved.

[0096] In one embodiment, the electrolyte may further include additives, such as cyclic carbonate-based compounds, fluorine-substituted cyclic carbonate-based compounds, sultone-based compounds, cyclic sulfate-based compounds, cyclic sulfite-based compounds, phosphate-based compounds, and borate-based compounds.

[0097] The cyclic carbonate-based compound may include vinylene carbonate, vinylethylene carbonate (VEC), and the like.

[0098] The fluorine-substituted cyclic carbonate-based compound may include fluoroethylene carbonate (FEC) and the like.

[0099] The sultone-based compound may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, and the like.

[0100] The cyclic sulfate-based compound may include 1,2-ethylene sulfate, 1,2-propylene sulfate, and the like.

[0101] The cyclic sulfite-based compound may include ethylene sulfite, butylene sulfite, and the like.

[0102] The phosphate-based compound may include lithium difluoro bis-oxalato phosphate, lithium difluoro phosphate, and the like.

[0103] The borate-based compound may include lithium bis(oxalate) borate, and the like.

[0104] In some embodiments, the mixed solution impregnated in the composite membrane may be solidified, thereby preparing an electrolyte for a secondary battery.

[0105] In some embodiments, the flame retardant monomer may be polymerized or copolymerized while the mixed solution is being solidified. Therefore, the electrolyte for a secondary battery may include a flame retardant polymer.

[0106] In some embodiments, curing the mixed solution may be performed by heat treating the mixed solution. For example, by heat curing the mixed solution, the flame retardant monomer in the composite film is polymerized or copolymerized into a flame retardant polymer.

[0107] In some embodiments, the temperature of the heat treatment may 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 may be performed at a temperature increase rate of 1° C. to 10° C. / min, 2° C. / min to 10° C. / min, or 3° C. / min to 10° C. / min from room temperature (e.g., 25° C.).

[0108] In some embodiments, the heat treatment can be performed for 30 minutes to 2 hours, 30 minutes to 1.5 hours, or 45 minutes to 1.25 hours.

[0109] Within the above temperature and time ranges, the flame retardant monomers in the mixed liquid can be fully polymerized or copolymerized.

[0110] In some embodiments, the mixed solution can be cured by irradiating the mixed solution with light. For example, by subjecting the mixed solution to light curing, the flame-retardant monomer within the composite film can be polymerized or copolymerized into a flame-retardant polymer. Therefore, the polymerization or copolymerization reaction of the flame-retardant monomer can be carried out at a relatively low temperature, thereby preventing damage to the composite film caused by high-temperature heat treatment.

[0111] In some embodiments, the UV curing process for the photopolymerization may use a wavelength of 250 nm to 400 nm and 800 mW / cm 2 Up to 1100mW / cm 2 In one embodiment, the UV curing process can be carried out for 5 seconds to 20 seconds. Within the above wavelength and intensity range, the flame retardant monomers in the mixed solution can be fully polymerized or copolymerized.

[0112] 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 and containing the above-described electrolyte.

[0113] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on at least one side of the positive electrode current collector.

[0114] The positive electrode current collector may include stainless steel, nickel, aluminum, titanium, or alloys thereof. The positive electrode current collector may also include aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. The thickness of the positive electrode current collector may be, for example, 10 μm to 50 μm, but is not limited thereto.

[0115] The positive active material may include a compound that can reversibly intercalate and deintercalate lithium ions.

[0116] According to an exemplary embodiment, the positive active material may include lithium-nickel metal oxide. The lithium-nickel metal oxide may further include at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0117] In some embodiments, the positive active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following Chemical Formula 1.

[0118] [Chemical Formula 1]

[0119] Li x Ni a M b O 2+z

[0120] In Chemical Formula 1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and −0.5≤z≤0.1 may be satisfied. As described above, M may include Co, Mn, and / or Al.

[0121] The chemical structure represented by Chemical Formula 1 represents the bonding relationship contained in the layered structure or crystal structure of the positive active material, and does not exclude additional elements. For example, M may include Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element (main active element) of the positive active material. Chemical Formula 1 is provided to represent the bonding relationship of the main active elements, and it should be understood that Chemical Formula 1 is a formula including the introduction and substitution of additional elements.

[0122] In one embodiment, in addition to the main active element, an auxiliary element for enhancing the chemical stability of the positive electrode active material or the layered structure / crystal structure may be further included. The auxiliary element may be mixed into the layered structure / crystal to form a bond, and it should be understood that this case is also included in the chemical structure represented by Chemical Formula 1.

[0123] The auxiliary element may include, for example, at least one of 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 that contributes to the capacity / power activity of the positive electrode active material together with Co or Mn, such as Al.

[0124] For example, the positive active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following Chemical Formula 1-1.

[0125] [Chemical Formula 1-1]

[0126] Li x Ni a M1 b1 M2 b2 O 2+z

[0127] In Chemical Formula 1-1, M1 may include Co, Mn, and / or Al. M2 may include the auxiliary elements described above. In Chemical Formula 1-1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, and -0.5≤z≤0.1 may be sufficient.

[0128] The positive electrode active material may further include a coating element or a doping element. For example, an element substantially identical to or similar to the auxiliary element may be used as the coating element or the doping element. For example, one or a combination of two or more of the above elements may be used as the coating element or the doping element.

[0129] The coating element or the doping element may be present on the surface of the lithium nickel metal oxide particle or permeate through the surface of the lithium nickel metal oxide particle and be included in the bonding structure represented by the Chemical Formula 1 or the Chemical Formula 1-1.

[0130] The positive electrode active material may include a nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, an NCM-based lithium oxide with an increased nickel content may be used.

[0131] 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-nickel (High-Ni)) composition for the positive electrode active material, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0132] However, as the Ni content increases, the long-term storage stability and lifespan stability of the positive electrode or secondary battery may be relatively reduced, and side reactions with the electrolyte may also increase. However, according to exemplary embodiments, the conductivity can be maintained by including Co, while the lifespan stability and capacity retention characteristics can be improved by including Mn.

[0133] 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) may be greater than 0.6, greater than 0.7, or greater than 0.8. In some embodiments, the Ni content may 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.

[0134] In some embodiments, the positive electrode active material may further include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP)-based active material (eg, LiFePO 4 ).

[0135] In some embodiments, the positive electrode active material may include, 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.

[0136] [Chemical Formula 2]

[0137] p[Li2MnO3]·(1-p)[Li q JO2]

[0138] 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.

[0139] 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 or potassium-based active material may include a layered structure or 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.

[0140] 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.

[0141] 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 manufacture a positive electrode active material layer. The coating process may be performed by methods such as gravure coating, slot die coating, multilayer 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 a binder and may optionally further include an electrolyte, a conductive material, a thickening agent, etc.

[0142] Solvents used in the manufacture of the positive electrode active material layer may include, for example, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMA), dimethylaminopropylamine (DMAPA), ethylene oxide (EO), tetrahydrofuran (THF), etc.

[0143] In one embodiment, the electrolyte included in the positive electrode active material layer may be the electrolyte for the secondary battery described above, but the electrolyte included in the positive electrode active material layer may be the same as or different from the inorganic electrolyte of the composite film. In one embodiment, the electrolyte included in the positive electrode active material layer may be the above-mentioned inorganic electrolyte. For example, the secondary battery may be provided as an all-solid battery including the electrolyte or inorganic electrolyte.

[0144] The binder may include at least one selected from the group consisting of a polyvinyl compound-based binder, a cellulose-based binder, an acrylic polymer-based binder, and a copolymer resin binder.

[0145] The binder may include, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), carboxymethyl cellulose (CMC), vinyl pyrrolidone / vinyl acetate (VP / VA) copolymer resin, polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (poly(vinylidenefluoride-co-hexafluoropropylene)), polyacrylonitrile, polymethyl methacrylate, acrylonitrile-butadiene rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc.

[0146] The conductive material may 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), carbon fiber, and / or metal-based conductive materials such as perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3, but is not limited thereto.

[0147] The positive electrode active material layer may further include a thickener and / or a dispersant, etc. As an embodiment, the positive electrode active material layer may include a thickener such as carboxymethyl cellulose (CMC).

[0148] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector.

[0149] The positive electrode current collector may include, for example, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, etc. For example, the thickness of the negative electrode current collector may be 10 μm to 50 μm, but is not limited thereto.

[0150] The negative electrode active material layer may include a negative electrode active material. The negative electrode active material may be a material that can adsorb and desorb lithium ions. For example, the negative electrode active material may be a carbon-based material such as crystalline carbon, amorphous carbon, carbon composites, or carbon fibers; lithium metal; a lithium alloy; a silicon (Si)-containing material; or a tin (Sn)-containing material.

[0151] Examples of the amorphous carbon may include hard carbon, soft carbon, coke, mesocarbon microbead (MCMB), mesophase pitch-based carbon fiber (MPCF), etc.

[0152] Examples of the crystalline carbon may include graphite-based carbons such as natural graphite, artificial graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc.

[0153] The lithium metal may include pure lithium metal or lithium metal formed with a protective layer for suppressing dendrite growth, etc. In one embodiment, the 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, the lithium thin film layer may also be used as the negative electrode active material layer.

[0154] Examples of the elements included in the lithium alloy may include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium, etc.

[0155] The silicon-containing material may provide further increased capacity characteristics. The silicon-containing material may include Si, SiO x (0 < x < 2), metal-doped SiO x (0 < x < 2), silicon-carbon composite, etc. The metal may include lithium and / or magnesium, and the metal-doped SiO x (0 < x < 2) may include metal silicate.

[0156] 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 may be 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 contain a binder, and may optionally further contain an electrolyte, a conductive material, a thickener, etc.

[0157] 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.

[0158] Examples of the solvent for the negative electrode active material layer may include, for example, water, pure water, deionized water, distilled water, ethanol, isopropyl alcohol, methanol, acetone, n-propanol, tert-butanol, etc.

[0159] In one embodiment, the electrolyte contained in the negative electrode active material layer may be the aforementioned electrolyte for a secondary battery. In one embodiment, the electrolyte contained in the negative electrode active material layer may be the aforementioned inorganic electrolyte, but the electrolyte contained in the negative electrode active material layer may be the same as or different from the inorganic electrolyte of the composite membrane. For example, the secondary battery may be provided as an all-solid-state battery containing the aforementioned electrolyte or an inorganic electrolyte.

[0160] As the binder, the conductive material, and the thickener, the above-mentioned substances that can be used when producing the positive electrode can be used.

[0161] In some embodiments, the negative electrode binder may use a styrene-butadiene rubber-based binder, carboxymethyl cellulose, a polyacrylic acid-based binder, a polyethylenedioxythiophene (PEDOT)-based binder, or the like.

[0162] According to an exemplary embodiment, an electrolyte layer may be provided 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 comprising the above-mentioned electrolyte.

[0163] According to an exemplary embodiment, a 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 a plurality of the battery cells. For example, the electrode assembly can be formed by winding, lamination, folding, etc.

[0164] For example, the tabs (positive and negative) may protrude from the positive and negative current collectors, respectively, and extend to one side of the housing. The tabs may be fused to the one side of the housing and connected to electrode leads (positive and negative leads) extending to or exposed outside the housing.

[0165] For example, a pouch-type case, a prismatic case, a cylindrical case, a coin-type case, etc. may be used.

[0166] The following further describes the embodiments of the present invention with reference to specific experimental examples. The examples and comparative examples included in the experimental examples are intended only to illustrate the present invention and are not intended to limit the scope of the claims. Various changes and modifications to the embodiments may be made within the scope and technical concept of the present invention, which will be apparent to those skilled in the art, and such variations and modifications are naturally within the scope of the claims.

[0167] Examples and Comparative Examples

[0168] Example 1

[0169] Preparation of electrolytes for secondary batteries

[0170] LLZTO (Li 6.4 La3Zr 1.4 Ta 0.6 O 12 ) and polyvinyl butyral (PVB) as an organic binder were mixed in a volume ratio of 4:1 with a mixed solvent in which propylene carbonate (PC) and dimethyl sulfoxide (DMSO) were mixed in a mixing ratio of 1:1 to prepare a mixed slurry.

[0171] The mixed slurry was cast on a polyethylene terephthalate (PET) film and then dried at 80° C. for 1 hour to form a primary composite film.

[0172] The primary composite film was sintered at 1000° C. for 1 minute to prepare a composite film. The sintering was performed by irradiating the primary composite film with a xenon lamp to perform photothermal sintering.

[0173] A compound represented by the following Chemical Formula 3, which is a flame retardant monomer, was mixed with a 1.0 M LiTFSi solution (EC / EMC mixed solvent with a volume ratio of 25:75) to prepare a mixed solution.

[0174] The composite film was separated from the PET film, and the mixed solution was impregnated into the composite film at a volume ratio of 10:1. The ratio of the content of the flame retardant monomer to the content of the inorganic electrolyte on a volume basis was 0.2.

[0175] [Chemical Formula 3]

[0176]

[0177] Subsequently, the mixed solution was heated at 5° C. / min and thermally cured at 80° C. for 1 hour to prepare an electrolyte for a secondary battery.

[0178] Example 2 to Example 7

[0179] The electrolyte for secondary batteries was prepared by the same method as in Example 1, except that the sintering temperature, porosity, and volume ratio of the content of the flame retardant monomer to the content of the inorganic electrolyte of the primary composite membrane were changed to those shown in Table 1 below.

[0180] Comparative Examples 1 to 5

[0181] The electrolytes for secondary batteries of Comparative Examples 1 to 4 were prepared by the same method as in Example 1, except that, when preparing the electrolytes for secondary batteries, the sintering temperature, porosity, and volume ratio of the content of the flame retardant monomer and the content of the inorganic electrolyte of the primary composite membrane were changed to those shown in Table 1 below.

[0182] Furthermore, an electrolyte for a secondary battery of Comparative Example 5 was prepared by the same method as in Example 1, except that the primary composite film was not sintered.

[0183] Experimental Example 1

[0184] (1) Measurement of cross-cut strength

[0185] The cross-cut strength is measured in accordance with KS M ISO:2013 (Test methods for adhesion of paints and varnishes).

[0186] Specifically, the mixed slurries of the above examples and comparative examples were cast on a copper foil support, dried at 80°C for 1 hour, and sintered at 1000°C for 1 minute to prepare samples. The sintering was performed by photothermal sintering using a xenon lamp.

[0187] The sample was completely dried at 80° C. for 3 hours and then cross-hatched (grid-like scratches) at intervals of 1 mm.

[0188] After rubbing the cross-section with a brush or writing brush, apply tape and rub with an eraser to ensure complete adhesion. Within 90 seconds, grasp one end of the tape and pull it at an angle of approximately 180° to remove it. The cross-cut strength is measured by measuring the area removed relative to the total surface area of ​​the cross-cut.

[0189] The cross-cut strength was evaluated as follows.

[0190] The area of ​​detachment relative to the total area is 0%: ISO 0 (ASTM 5B)

[0191] The area of ​​detachment relative to the total area is more than 0% and less than 5%: ISO 1 (ASTM 4B)

[0192] The area of ​​detachment relative to the total area is more than 5% and less than 15%: ISO 2 (ASTM 3B)

[0193] The area of ​​detachment relative to the total area is more than 15% and less than 35%: ISO 3 (ASTM 2B)

[0194] The area of ​​detachment relative to the total area is more than 35% and less than 65%: ISO 4 (ASTM 1B)

[0195] The area of ​​detachment relative to the total area is more than 65%: ISO 5 (ASTM 0B)

[0196] (2) Porosity measurement

[0197] The porosity of the composite membranes according to the above examples and comparative examples was measured. The porosity was measured by scanning electron microscopy (SEM) microstructure analysis.

[0198] The evaluation results are shown in Table 1 below.

[0199] [Table 1]

[0200]

[0201] Experimental Example 2: Combustion Evaluation

[0202] The electrolyte layer for a secondary battery prepared according to the above-described example and the separator and electrolyte layer impregnated with the electrolytic solution prepared according to the comparative example were burned for 5 minutes to evaluate whether or not they burned.

[0203] The presence or absence of combustion was evaluated as follows.

[0204] ○: Burning area is less than 5%

[0205] △: Burning area is 5% to 20%

[0206] ×: Burned area exceeds 20%

[0207] The evaluation results are shown in Table 2 below.

[0208] [Table 2]

[0209]

[0210] The electrolytes for secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 4 were formed into self-supporting composite films as sintered bodies of inorganic electrolytes by sintering a primary composite film.

[0211] The porosity and cross-cut strength of the secondary battery electrolyte of Comparative Example 5, which did not produce a sintered primary composite membrane, could not be measured. Therefore, only the secondary battery electrolytes of Examples 1 to 7 and Comparative Examples 1 to 4 were evaluated for composite membrane porosity, cross-cut strength, and combustion.

[0212] Referring to Tables 1 and 2, in Examples having cross-cut strengths of ASTM standards 2B to 5B, in the combustion evaluation, the burned area was 20% or less relative to the total surface area.

[0213] In the comparative examples having a cross-cut strength lower than ASTM standard 2B (0B and 1B), the burned area exceeded 20% relative to the total surface area in the combustion evaluation.

[0214] In Comparative Examples 2 and 3, in which the cross-cut strength was lower than ASTM Standard 2B and the volume ratio of the flame retardant monomer content to the inorganic electrolyte content was reduced to less than 0.01, the burning area in the combustion evaluation increased within different porosity ranges.

[0215] In Comparative Example 4, where the cross-cut strength was lower than ASTM Standard 2B and the volume ratio of the flame retardant monomer content to the inorganic electrolyte content was increased to more than 0.3, it was difficult to maintain the shape of the composite film, and the burned area increased in the combustion evaluation.

Claims

1. An electrolyte for a secondary battery, comprising: lithium salts; A composite membrane comprising a sintered body of an inorganic electrolyte; and flame retardant polymers, in, The strength of the composite film measured in the cross-cut test is ASTM standard 2B to 5B.

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

3. The electrolyte for secondary batteries according to claim 1, wherein The ratio of the content of the flame retardant polymer to the content of the inorganic electrolyte is 0.01 to 0.3 on a volume basis.

4. The electrolyte for secondary batteries according to claim 1, wherein The flame retardant polymer includes at least one of a phosphorus-containing functional group and a fluorine atom.

5. The electrolyte for secondary batteries according to claim 4, wherein The phosphorus-containing functional group includes at least one of a phosphate group, a phosphite group, a phosphonate group, and a phosphazene group.

6. The electrolyte for secondary batteries according to claim 1, wherein The strength of the composite film measured in the cross-cut test is ASTM standard 2B to 4B.

7. The electrolyte for secondary batteries according to claim 1, wherein The composite membrane includes pores, and the porosity of the composite membrane is 10% to 40%.

8. The electrolyte for secondary batteries according to claim 1, wherein The composite film further comprises a sintered body of an organic binder.

9. A lithium secondary battery comprising: positive electrode; a negative electrode, the negative electrode being arranged opposite to the positive electrode; and An electrolyte layer is provided between the positive electrode and the negative electrode and contains the electrolyte for a secondary battery according to claim 1 .

10. A method for preparing an electrolyte for a secondary battery, comprising the following steps: mixing an inorganic electrolyte, an organic binder, and a solvent to prepare a mixed slurry; drying the mixed slurry to prepare a primary composite membrane; sintering the primary composite membrane to prepare a composite membrane; as well as The composite film is impregnated with a flame retardant polymer.

11. The method for preparing a secondary battery electrolyte according to claim 10, wherein: The strength of the composite film measured in the cross-cut test is ASTM standard 5B to 2B.

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

13. The method for preparing a secondary battery electrolyte according to claim 10, wherein: The step of impregnating the flame retardant polymer includes impregnating a mixture containing a flame retardant monomer and an electrolyte into the composite film and curing the mixture.

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

15. The method for preparing a secondary battery electrolyte according to claim 13, wherein: The mixed solution further comprises a thermal initiator, The solidification of the mixed liquid includes heat treating the mixed liquid.

16. The method for preparing a secondary battery electrolyte according to claim 13, wherein: The mixed solution further comprises a photoinitiator, The solidification of the mixed liquid includes irradiating the mixed liquid with light.

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