Lithium secondary battery with excellent thermal safety and method for manufacturing the same

Abstract

According to one embodiment of the present invention, a lithium secondary battery is provided, comprising a positive electrode including a positive electrode current collector and a positive electrode active material layer formed on one or both sides of the positive electrode current collector, a negative electrode including a negative electrode current collector and a negative electrode active material layer formed on one or both sides of the negative electrode current collector, a substrate and a separation membrane including an oxide-based solid electrolyte layer formed on one or both sides of the substrate and containing lithium aluminum titanium phosphate (LATP), wherein the negative electrode active material layer and the oxide-based solid electrolyte layer face each other, an inorganic layer containing reduced LATP and SEI substances is formed at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer, and the atom content of F in the inorganic layer is 7 to 15 atomic percent based on all atoms present in the inorganic layer.

Description

[Technical Field]

[0001] Cross-reference of related applications This application claims priority rights based on Korean Patent Application No. 10-2023-0174493 dated December 5, 2023, Korean Patent Application No. 10-2023-0169616 dated November 29, 2023, and Korean Patent Application No. 10-2024-0166418 dated November 20, 2024, and all content disclosed in the documents of said Korean patent applications is incorporated herein as part of this specification.

[0002] The present invention relates to a lithium secondary battery having excellent thermal safety and a method for manufacturing the same. [Background technology]

[0003] The rapid increase in fossil fuel use has led to a growing demand for alternative and clean energy sources, and one of the most actively researched areas in this field is electrochemical power generation and energy storage.

[0004] Currently, a typical example of an electrochemical element that uses this type of electrochemical energy is the secondary battery, and its range of applications is steadily expanding.

[0005] Recently, with the increasing technological development and demand for portable devices such as portable computers, mobile phones, and cameras, the demand for rechargeable batteries as an energy source has rapidly increased. Among these, lithium-ion batteries, which exhibit high energy density and operating potential, long cycle life, and low self-discharge rate, have been the subject of much research, and have been commercialized and are now widely used.

[0006] Furthermore, as concern for environmental issues grows, much research is being conducted on electric vehicles and hybrid electric vehicles that can replace vehicles that use fossil fuels, such as gasoline and diesel vehicles, which are one of the main causes of air pollution. Nickel-metal hydride secondary batteries are mainly used as the power source for such electric vehicles and hybrid electric vehicles, but research is actively being conducted on the use of lithium secondary batteries, which have high energy density and discharge voltage, and some are already in the commercialization stage.

[0007] Generally, lithium secondary batteries have a structure in which a non-aqueous electrolyte is impregnated into an electrode assembly that includes a positive electrode, a negative electrode, and a porous separator membrane. Generally, the positive electrode is manufactured by coating aluminum foil with a positive electrode mixture containing a positive electrode active material, and the negative electrode is manufactured by coating copper foil with a negative electrode mixture containing a negative electrode active material.

[0008] Typically, the positive electrode active material is a lithium transition metal oxide, and the negative electrode active material is a carbon-based material.

[0009] However, with the recent growth of equipment sectors requiring high-capacity batteries, such as electric vehicles and hybrid electric vehicles, the energy density level required for lithium secondary batteries has been steadily increasing. Therefore, attempts are being made to use negative electrodes containing Si, which has a high theoretical capacity, as the negative electrode active material.

[0010] However, while silicon-based materials containing silicon have attracted attention due to their extremely high energy density, they require improvement because the irreversible phase formation of lithium silicate reduces Coulomb efficiency, and the explosive reactions that occur during thermal runaway result in poor thermal safety.

[0011] At this time, the Coulomb efficiency can be resolved to some extent by compensating for irreversible reactions through the prelithiation process of Si-based materials. However, the increase in resistance is reduced and the output characteristics can be improved only if the SEI formed at this time is of good quality, and thermal safety remains a problem. [Overview of the project] [Problems that the invention aims to solve]

[0012] Therefore, the present invention aims to provide a lithium secondary battery with improved Coulomb efficiency and thermal safety by controlling the SEI formation mode. [Means for solving the problem]

[0013] According to one embodiment of the present invention, A positive electrode comprising a positive electrode current collector and a positive electrode active material layer formed on one or both sides of the positive electrode current collector, A negative electrode comprising a negative electrode current collector and a negative electrode active material layer formed on one or both sides of the negative electrode current collector, and The separation membrane comprises a substrate and an oxide-based solid electrolyte layer containing lithium aluminum titanium phosphate (LATP) formed on one or both sides of the substrate. The negative electrode active material layer and the oxide-based solid electrolyte layer face each other. An inorganic layer containing the reduced LATP and SEI material is formed at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer. A lithium secondary battery is provided, wherein the atomic content of F in the inorganic layer is 7 to 15 atomic percent based on all atoms present in the inorganic layer.

[0014] Here, the inorganic layer may fill some or all of the voids in the oxide-based solid electrolyte layer, or form a separate layer at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer, or be included in all of these forms.

[0015] In this case, more specifically, the inorganic layer can be configured to fill the voids in the oxide-based solid electrolyte layer with a volume of 10% to 100% based on the total volume of the voids, forming a separate layer with a thickness of 5 nm to 100 nm.

[0016] More specifically, the LATP reduced product is formed in such a way that 50% or more by weight fills the voids in the oxide-based solid electrolyte layer along the surface of the LATP particles. The SEI material is formed in a space charge region where the concentration of Li+ is locally high, together with the LATP reduced product, or on the surface of the LATP reduced product, filling the voids in the oxide-based solid electrolyte layer, or on the surface of the oxide-based solid electrolyte layer, in the form of a separate layer or all of these forms.

[0017] The reduced product of LATP, which is one component of the inorganic layer, may include lithified-LATP formed by a spontaneous lithiumization reaction represented by the following reaction formula 1. [Reaction Equation 1] Li 1.3 Al 0.3 Ti 1.7 (PO4)3 → Li3Al 0.3 Ti 1.7 (PO4)3

[0018] Furthermore, the SEI substance, which is another component of the inorganic layer, may contain LiF, and may further contain one or more substances selected from the group consisting of Li2CO3 and Li2O.

[0019] Furthermore, the inorganic layer can be composed of LATP reduced products and SEI substances.

[0020] On the other hand, the oxide-based solid electrolyte layer can be formed on both sides of the substrate.

[0021] The substrate may be a polyolefin substrate, and the oxide-based solid electrolyte layer can be composed of an oxide-based solid electrolyte containing lithium aluminum titanium phosphate (LATP) and a binder.

[0022] Here, the oxide-based solid electrolyte layer may be formed on one surface of the base material with a thickness of 0.1 μm to 20 μm, respectively.

[0023] On the other hand, the positive electrode active material layer may contain a lithium transition metal oxide represented by the following Chemical Formula 1 as a positive electrode active material. [Chemical Formula 1] Li 1+x Ni a Co b Mn c M 1-(a+b+c) O2 In the above formula, M is one or more selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, 0 ≦ x ≦ 0.5, 0 < a < 1, 0 < b < 1, 0 < c < 1.

[0024] Further, the negative electrode active material layer may contain 80% by weight or more of pre-lithiated silicon oxide as a negative electrode active material based on the total weight of the negative electrode active material.

[0025] Furthermore, the lithium secondary battery may further include an electrolyte containing LiFSI as a lithium salt and fluoroethylene carbonate (FEC) as an electrolyte solvent.

[0026] According to another embodiment of the present invention, Forming an oxide-based solid electrolyte layer containing lithium aluminum titanium phosphate (LATP) on one or both surfaces of a base material to produce a separator; Forming a positive electrode active material layer on one or both surfaces of a positive electrode current collector to produce a positive electrode, and forming a negative electrode active material layer on one or both surfaces of a negative electrode current collector to produce a negative electrode; Manufacturing an electrode assembly by interposing the separator between the positive electrode and the negative electrode such that the oxide-based solid electrolyte layer and the negative electrode active material layer face each other; Manufacturing a secondary battery by incorporating the electrode assembly into a secondary battery case; The steps of positioning the secondary battery between a plurality of pressure plates and applying pressure to the secondary battery; and A method for manufacturing a lithium secondary battery is provided, which includes the step of activating the secondary battery while applying pressure to it.

[0027] Here, the multiple pressure plates may be fastened together with multiple pressure bolts to apply pressure, and specifically, the pressure bolts may pressurize the secondary battery with a fastening strength of 6 kgf·cm or more, and more specifically, they may pressurize the secondary battery with a fastening strength in the range of 10 to 20 kgf·cm.

[0028] Furthermore, the activation step may be carried out under temperature conditions of 40°C to 60°C. [Brief explanation of the drawing]

[0029] [Figure 1] This is a schematic diagram illustrating the formation of an inorganic layer at the interface between the negative electrode and the separation membrane according to one embodiment of the present invention. [Figure 2] This is a top view of a pressurizing device applied to the activation step in a method for manufacturing a lithium secondary battery according to another embodiment of the present invention. [Figure 3] Figure 2 is a cross-sectional view of the pressurizing device. [Figure 4] This is an SEM image of the separation membrane surface obtained by disassembling a lithium secondary battery manufactured according to Experimental Example 1. [Figure 5] This is an XRD graph of the separation membrane obtained by disassembling a lithium secondary battery manufactured according to Experimental Example 1. [Figure 6] This is a graph of Si component analysis based on the coating layer depth of the negative electrode, obtained by disassembling a lithium secondary battery manufactured according to Experimental Example 2. [Figure 7] This is a component analysis graph based on the coating layer depth of the negative electrode, obtained by disassembling a lithium secondary battery manufactured according to Experimental Example 2. [Figure 8]This is a graph of the component analysis based on the coating layer depth of the separation membrane obtained by disassembling a lithium secondary battery manufactured according to Experimental Example 3. [Figure 9] This is a DSC evaluation graph of the lithium secondary battery manufactured using Experimental Example 4. [Figure 10] This is a calorimeter evaluation graph of a lithium secondary battery manufactured according to Experimental Example 5. [Figure 11] This is a DC-IR evaluation graph of the lithium secondary battery manufactured according to Experimental Example 6. [Figure 12] This is a comparative graph of the discharge rate characteristics of lithium secondary batteries manufactured according to Experimental Example 6. [Figure 13] This is a DC-IR evaluation graph of the lithium secondary battery manufactured according to Experimental Example 7. [Figure 14] This is a comparative graph of the discharge rate characteristics of lithium secondary batteries manufactured according to Experimental Example 7. [Modes for carrying out the invention]

[0030] Hereafter, terms and words used in this specification and in the claims should not be interpreted in a manner limited to their ordinary or lexicographical meanings, but rather in a manner consistent with the technical idea of ​​the present invention, based on the principle that an inventor may appropriately define the concept of a term in order to best describe their invention.

[0031] Unless otherwise defined, all terms used herein (including technical and scientific terms) are intended to be used in a way that is commonly understood by those with ordinary skill in the art to which the present invention pertains. Furthermore, terms defined in commonly used dictionaries are not intended to be interpreted ideally or excessively unless explicitly defined otherwise.

[0032] The terms used herein are for illustrative purposes only and are not intended to limit the invention. In this specification, the singular form includes the plural form unless otherwise specified. The terms “comprises” and / or “comprising” as used in this specification do not preclude the presence or addition of one or more other components beyond those mentioned.

[0033] On the other hand, the terms "consists of" and / or "consisting of" used in the specification mean that, in addition to the mentioned components, other components are not present in amounts greater than trace amounts, i.e., only as impurities.

[0034] Furthermore, the dimensions and thicknesses of the components shown in the drawings are arbitrary for illustrative purposes and are not necessarily limited to those shown in the present invention. The thicknesses are enlarged in the drawings to clearly represent the various layers and regions. Also, for illustrative purposes, the thicknesses of some layers and regions are exaggerated in the drawings.

[0035] Furthermore, when we say that a layer, membrane, region, plate, or other part is "on top of" or "on top of" another part, this includes not only the case where it is "directly on top of" another part, but also the case where there is another part in between. Conversely, when we say that one part is "directly on top of" another part, it means that there is no other part in between. Also, being "on top of" or "on top of" a reference part means being located above or below the reference part, and does not necessarily mean being located "on top of" or "on top of" in the opposite direction of gravity.

[0036] Furthermore, throughout the specification, when we refer to "on a plane," it means when the subject is viewed from above, and when we refer to "on a cross-section," it means when the cross-section obtained by cutting the subject perpendicularly is viewed from the side.

[0037] Lithium-ion battery A lithium secondary battery according to one embodiment of the present invention is A positive electrode comprising a positive electrode current collector and a positive electrode active material layer formed on one or both sides of the positive electrode current collector, A negative electrode comprising a negative electrode current collector and a negative electrode active material layer formed on one or both sides of the negative electrode current collector, and The separation membrane comprises a substrate and an oxide-based solid electrolyte layer containing lithium aluminum titanium phosphate (LATP) formed on one or both sides of the substrate. The negative electrode active material layer and the oxide-based solid electrolyte layer face each other. An inorganic layer containing the reduced LATP and SEI material is formed at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer. The inorganic layer is characterized by containing 7 to 15 atomic percent of F, based on all atoms present in the inorganic layer.

[0038] positive electrode The positive electrode may have a structure that includes a positive electrode current collector and a positive electrode active material layer formed on one or both sides of the positive electrode current collector.

[0039] Here, the positive electrode current collector is not particularly limited as long as it is conductive without inducing a chemical change in the battery. For example, the current collector can be stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc.

[0040] The positive electrode current collector can have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance adhesion to the positive electrode active material layer. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0041] The positive electrode active material layer comprises a positive electrode active material and may optionally include conductive materials, binders, and other additives.

[0042] The positive electrode active material is not limited as long as it is a compound capable of reversible intercalation and deintercalation of lithium. Specifically, it can include a lithium metal oxide containing one or more metals such as cobalt, manganese, nickel, or aluminum and lithium. More specifically, the positive electrode active material can include a lithium transition metal oxide represented by the following Chemical Formula 1. [Chemical Formula 1] Li 1+x Ni a Co b Mn c M 1-(a+b+c) O2 In the above formula, M is one or more selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, 0 ≦ x ≦ 0.5, 0 < a < 1, 0 < b < 1, 0 < c < 1.

[0043] In addition, the positive electrode active material includes lithium-manganese-based oxides (for example, LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (for example, LiCoO2, etc.), lithium-nickel-based oxides (for example, LiNiO2, etc.), lithium-nickel-manganese-based oxides (for example, Li 1+x’ Ni 1-Y Mn Y O2 (where -0.5 ≦ x' ≦ 0.5, 0 < Y < 1), Li 1+x’’ Mn 2-Z Ni Z O4 (where -0.5 ≦ x'' ≦ 0.5, 0 < Z < 2), etc.), lithium-nickel-cobalt-based oxides (for example, Li 1+x’’’ Ni 1-Y1 Co Y1 O2 (where -0.5 ≦ x''' ≦ 0.5, 0 < Y1 < 1), etc.), lithium-manganese-cobalt-based oxides (for example, Li 1+x’’’’ Co 1-Y2 Mn Y2 O2 (where -0.5 ≦ x'''' ≦ 0.5, 0 < Y2 < 1), Li 1+x’’’’’ Mn 2-Z1 Co Z1O4 (where -0.5 ≤ x'''''' ≤ 0.5, 0 < Z1 < 2, etc.), lithium-nickel-manganese-cobalt-based oxide (e.g., Li 1+a1 (Ni p Co q Mn r )O2 (where -0.5 ≤ a1 ≤ 0.5, 0 < p < 1, 0 < q < 1, 0 < r < 1, p + q + r = 1) or Li 1+a2 (Ni p1 Co q1 Mn r1 )O4 (where -0.5 ≤ a2 ≤ 0.5, 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, p1 + q1 + r1 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li 1+a3 (Ni p2 Co q2 Mn r2 M s2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and a3, p2, q2, r2, and s2 are atomic fractions of independent elements, respectively, -0.5 ≤ a3 ≤ 0.5, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, p2 + q2 + r2 + s2 = 1), etc.), lithium iron phosphate (e.g., Li 1+a4 Fe 1-p3 M p3 (PO 4-b4 )X b4 (where M is one or more selected from Al, Mg, and Ti, X is one or more selected from F, S, and N, -0.5 ≤ a4 ≤ 0.5, 0 ≤ p3 ≤ 0.5, ≤ b4 ≤ 0.1), etc., and one or two or more of these compounds can be included.

[0044] Among these, from the viewpoint of being able to enhance the capacity characteristics and stability of the battery, the lithium metal oxide is LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (e.g., Li(Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2)O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, and Li(Ni 0.8 Mn 0.1 Co 0.1 )O2 etc.), lithium nickel cobalt aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2 etc.), or lithium nickel manganese cobalt aluminum oxide (e.g., Li(Ni 0.86 Co 0.05 Mn 0.07 Al 0.02 It may be Li(O2), lithium iron phosphorus oxide (e.g., LiFePO4), and one or more of these can be used as a mixture of two or more. 0.86 Co 0.05 Mn 0.07 Al 0.02 ) May contain O2.

[0045] The positive electrode active material may be present in an amount of 60 to 98% by weight, preferably 80 to 98% by weight, and more preferably 90 to 98% by weight, based on the total weight of the positive electrode active material layer.

[0046] The conductive material is a component for further improving the conductivity of the positive electrode active material, and such a conductive material is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powders such as natural graphite, artificial graphite, or graphite with a highly developed crystalline structure; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon powder; conductive powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used.

[0047] The conductive material may be included in an amount of 0.1 to 20% by weight, more specifically 0.5 to 10% by weight, or more specifically 0.5 to 5% by weight, based on the total weight of the positive electrode active material layer.

[0048] The binder is a component that assists in the bonding between the conductive material, the positive electrode active material, and the positive electrode current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0049] Typically, the binder may be included in an amount of 0.5 to 20% by weight, more specifically 0.5 to 10% by weight, or more specifically 0.5 to 5% by weight, based on the total weight of the positive electrode active material layer.

[0050] Furthermore, the aforementioned other additives may include, for example, fillers as components that suppress expansion. The fillers are not particularly limited as long as they can suppress the expansion of the electrodes without inducing chemical changes in the battery, and for example, olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers; etc. can be used.

[0051] negative electrode The negative electrode, like the positive electrode, has a structure in which a negative electrode active material layer is formed on one or both sides of the negative electrode current collector.

[0052] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used.

[0053] The negative electrode current collector can typically have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0054] The negative electrode active material may include conductive materials, binders, and other additives as described above, together with the negative electrode active material.

[0055] In the present invention, the negative electrode active material may contain 80% by weight or more of pre-lithified silicon oxide, which can exhibit high energy density and other properties, based on the total weight of the negative electrode active material.

[0056] As mentioned above, the aforementioned silicon oxide is a material that has problems with thermal safety due to the formation of an irreversible lithium silicate phase which reduces Coulomb efficiency and the explosive reaction that occurs during thermal runaway. Previously, it was only used in mixtures with carbon-based materials at a concentration of 10% by weight or less, more specifically 5% by weight or less, based on the total weight of the negative electrode active material.

[0057] However, according to the present invention, the Coulomb efficiency is resolved by compensating for irreversible reactions through the prelithiation process of Si-based materials, and a thick layer of inorganic excess SEI with a high F content can be formed through the reduced LATP described later, thereby improving thermal safety, and can contain 80% to 100% by weight, more specifically 90% to 100% by weight, and even more specifically 100% by weight.

[0058] On the other hand, the silicon oxide can be represented in detail by the following chemical formula 2. [Chemical formula 2] SiOx Here, x is 0 <x<2である。

[0059] More specifically, the silicon oxide may be SiO2 or SiO, and more specifically, SiO.

[0060] Here, SiO is the total composition of a mixture of Si and SiO2, and the x value is determined by the mixing ratio of Si and SiO2.

[0061] Such silicon oxide may be included as a negative electrode active material in a pre-lithified state, and the pre-lithification can be carried out by contacting Li with the silicon oxide powder, for example, by a physical-electronic method.

[0062] On the other hand, the negative electrode active material may be one or more carbon-based materials, Si-based materials, or Li selected from the group consisting of silicon oxide, graphite, amorphous hard carbon, low-crystalline soft carbon, carbon black, acetylene black, Ketjen black, Super P, graphene, and fibrous carbon, in addition to silicon oxide. x Fe2O3 (0 ≤ x ≤ 1), Li x WO2(0≦x≦1), Sn x Me 1-x Me' y O z(Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8), etc. metal composite oxides; lithium metal; lithium alloy; silicon-based alloy; tin-based alloy; metal oxides such as SiO, SiO2, SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-based materials; titanium oxides; lithium titanium oxides, etc. can be further included, but it is not limited to only these as long as they are known in the art.

[0063] Separation membrane The separation membrane has a structure including a base material and an oxide-based solid electrolyte layer formed on one or both surfaces of the base material and containing lithium aluminum titanium phosphate (LATP).

[0064] Here, the base material can be used without special restrictions as long as it is usually used as a separation membrane base material in a lithium secondary battery, and it is particularly preferable that it has low resistance to ion movement of the electrolyte while having excellent electrolyte moisture retention ability.

[0065] For example, the base material can be a polyolefin-based base material such as a polyethylene homopolymer, a polypropylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, and a porous polymer film or a laminate structure of two or more layers thereof can be used. Also, a normal porous non-woven fabric, for example, a non-woven fabric made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. can be used as the separation membrane, but specifically, it may also be a polyolefin base material.

[0066] An oxide-based solid electrolyte layer is formed on one or both surfaces of the base material.

[0067] In this case, the oxide-based solid electrolyte layer may contain an oxide-based solid electrolyte and binder containing lithium aluminum titanium phosphate (LATP).

[0068] In addition to lithium aluminum titanium phosphate, the oxide-based solid electrolyte may further include one or more lithium metal oxides or lithium metal phosphoroxides selected from Nasicon-type solid electrolytes, Lisicon-type solid electrolytes, Garnet-type solid electrolytes, Perovskite-type solid electrolytes, and LiPON-type solid electrolytes. More specifically, it may further include one or more compounds selected from the group consisting of LAGP (lithium aluminum germanium phosphate) compounds, LLZO (lithium lanthanum zirconium oxide) compounds, LLZTO (lithium lanthanum zirconium tantalum oxide) compounds, LLTO (lithium lanthanum titanium oxide) compounds, LSTP (lithium silicon titanium phosphate) compounds, and LGPO (lithium germanium phosphate) compounds, but more specifically, it can consist of lithium aluminum titanium phosphate.

[0069] Such oxide-based solid electrolytes may be included in an amount of 70% to 99% by weight, or more specifically, 80% to 99% by weight, based on the total weight of the oxide-based solid electrolyte layer.

[0070] If the content deviates from the aforementioned range and is excessively low, a sufficient reduced product as intended by this application can be obtained. However, if the content is excessively high, the content of the binder connecting them is excessively low, which is undesirable because it can lead to a decrease in mechanical properties due to weakened adhesion between these particles.

[0071] The average diameter (D50) of the oxide-based solid electrolyte particles may be 50 nanometers to 10 micrometers, more specifically 50 nanometers to 5 micrometers, or even more specifically 50 nanometers to 1 micrometer.

[0072] If the value deviates from the aforementioned range and is excessively small, interparticle aggregation may occur due to decreased dispersibility. Conversely, if the value is excessively large, the oxide-based solid electrolyte will form large pores, which is detrimental in terms of resistance. In other words, when the value is within the aforementioned range, lithium-ion conductivity can be increased, resistance can be reduced, and improved secondary battery performance can be achieved.

[0073] The aforementioned average diameter (D50) represents the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. D50 can be measured, for example, using the laser diffraction method. This laser diffraction method can generally measure particle sizes ranging from submicron to several millimeters, and can yield highly reproducible and high-resolution results.

[0074] On the other hand, the binder, which is another component of the oxide-based solid electrolyte layer, is not limited as long as it does not undergo a side reaction with the electrolyte, but in particular, a binder with the lowest possible glass transition temperature (Tg) can be used, preferably in the range of -200 to 200°C.

[0075] Furthermore, while the binder does not necessarily need to have ion-conducting ability, it is even more preferable to use a polymer that does have ion-conducting ability.

[0076] Therefore, it is preferable that the binder has a high dielectric constant, as the degree of salt dissociation in the electrolyte actually depends on the dielectric constant of the electrolyte solvent. The higher the dielectric constant of the polymer, the better the degree of salt dissociation in the electrolyte. The dielectric constant of the polymer can be 1 or higher, more specifically in the range of 1.0 to 100 (measurement frequency = 1 kHz), and is particularly preferably 10 or higher.

[0077] In addition to the functions described above, the binder may have the characteristic of gelling during liquid electrolyte impregnation, exhibiting a high electrolyte impregnation rate (degree of swelling). In fact, if the binder is a polymer with excellent electrolyte impregnation rate, the electrolyte injected after battery assembly will permeate the polymer, and the polymer holding the absorbed electrolyte will acquire electrolyte ion conductivity. Therefore, if possible, the solubility index should be 15-45 MPa. 1 / 2 A polymer is preferred, and the pressure is 15-25 MPa. 1 / 2 and 30-45 MPa 1 / 2 A range is even more preferable. Solubility index of 15 MPa 1 / 2 Less than and 45 MPa 1 / 2 If the value exceeds a certain limit, it becomes difficult for the liquid electrolyte used in batteries to swell.

[0078] Examples of such binders include polyvinylidene fluoride co-hexafluoropropylene, polyvinylidene fluoride cotrichloroethylene, polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyimide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate. It may be one or more selected from the group consisting of propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxyl methyl cellulose, and polyvinyl alcohol.

[0079] More specifically, the oxide-based solid electrolyte layer of the present invention aims to have the LATP contained in such an oxide-based solid electrolyte layer face the negative electrode active material layer of the negative electrode and form a reduced product of LATP at the interface between them. Therefore, it is preferable that the oxide-based solid electrolyte layer containing LATP and a binder are composed of such an oxide-based solid electrolyte layer and a binder, and it is preferable that no other substances are included as they may interfere with the reduction of LATP.

[0080] Therefore, the binder may be included in an amount of 1% to 30% by weight, more specifically 1% to 20% by weight, based on the total weight of the oxide-based solid electrolyte layer.

[0081] Such an oxide-based solid electrolyte layer can be formed on one or both sides of the substrate, but it is more preferable to form it on both sides because it exhibits even better overpotential reduction. Of course, if the oxide-based solid electrolyte layer is formed on only one side, the electrode assembly must be manufactured so that the oxide-based solid electrolyte layer faces the negative electrode active material layer of the negative electrode in order to form the LATP reduced product intended in this application.

[0082] In this case, the oxide-based solid electrolyte layer can be formed on one surface of the substrate with a thickness of 0.1 μm to 20 μm, and more specifically, with a thickness of 1 μm to 10 μm.

[0083] If the thickness exceeds the aforementioned range and is excessively thin, the intended effect of LATP reduction may not be fully achieved, and if it is excessively thick, the resistance may actually increase, which is undesirable.

[0084] On the other hand, the total thickness of the separation membrane, which includes the substrate and the oxide-based solid electrolyte layer, may be 5 micrometers to 50 micrometers, more specifically 5 micrometers to 40 micrometers, and even more specifically 10 micrometers to 30 micrometers. When the thickness of the separation membrane satisfies the above range, it is possible to effectively prevent short circuits between the positive and negative electrodes while minimizing the resistance of the lithium secondary battery. As a result, it is possible to prevent a decrease in the energy density of the lithium secondary battery and improve its lifespan characteristics.

[0085] On one hand, whether the oxide-based solid electrolyte layer is formed on one side or both sides, the oxide-based solid electrolyte layer will face the negative electrode active material layer of the negative electrode. As a result, the reduced LATP is formed at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer. Specifically, it is formed while lithium ions move during the charge and discharge of the lithium secondary battery, particularly during the activation process.

[0086] Here, the reduced product of LATP can include lithiated-LATP formed by the spontaneous lithiation reaction in which LATP is represented by the following Reaction Formula 1. [Reaction Formula 1] Li 1.3 Al 0.3 Ti 1.7 (PO4)3 → Li3Al 0.3 Ti 1.7 (PO4)3

[0087] Also, at the interface between the negative electrode and the separator, lithium ions reacted with the electrolyte during the charge and discharge of the lithium secondary battery, particularly during the activation process, form SEI substances.

[0088] Therefore, an inorganic layer containing the reduced product of LATP and SEI substances is formed at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer. Specifically, the inorganic layer is composed of the LATP reduced product and SEI substances.

[0089] At this time, the SEI substance can contain LiF, and can further contain one or more substances selected from the group consisting of Li2CO3 and Li2O. In other words, the inorganic layer may contain lithiated-LATP and LiF, and may further contain one or more selected from the group consisting of Li2CO3 and Li2O in addition to these.

[0090] Such an inorganic layer is formed by the bonding of LATP and lithium ions in the oxide-based solid electrolyte layer and the reaction of the electrolyte with lithium ions, so it is mainly formed on the surface of the oxide-based solid electrolyte layer.

[0091] In this case, since the oxide-based solid electrolyte layer contains voids inside, the inorganic layer may be formed in a manner that fills some or all of the voids in the oxide-based solid electrolyte layer, or it may be formed as a separate layer at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer, or it may be formed in all of these forms, i.e., it may be included in the voids or form a layer.

[0092] When the inorganic layer fills the voids in the oxide-based solid electrolyte layer, it can be filled to 10% by volume or 100% by volume, more specifically to 30% by volume or 100% by volume, and even more specifically to 50% by volume or 80% by volume.

[0093] Furthermore, if the inorganic layer is formed as a separate layer at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer, such a layer may have a thickness of 5 nm to 100 nm, more specifically 5 nm to 50 nm, and even more specifically 10 nm to 50 nm.

[0094] More specifically, as the LATP in the oxide-based solid electrolyte layer comes into contact with the pre-lithified silicon oxide, it begins to form reduced products. As a result, 50% or more of the LATP reduced products, based on weight, fill the voids in the oxide-based solid electrolyte layer along the surface of the LATP particles. During charging, such as the activation process of the lithium secondary battery, the surface of the separation membrane has a space charge region with a locally high concentration of Li+ ions, and SEI material is formed in this space charge region by a side reaction between Li ions and the electrolyte.

[0095] Therefore, the SEI material may be formed in the region together with the LATP reduced product, or on the surface of the LATP reduced product in a manner that fills the voids in the oxide-based solid electrolyte layer, or as a separate layer on the surface of the oxide-based solid electrolyte layer, or in all of these forms.

[0096] To further explain the formation of such an inorganic layer, Figure 1 shows a schematic diagram of the interface where the separation membrane and the negative electrode of this invention come into contact.

[0097] Referring to Figure 1, the separation membrane 110 has a structure that includes a substrate 111 and an oxide-based solid electrolyte layer 112 containing LATP formed on one surface of the substrate 111. This oxide-based solid electrolyte layer 112 is in direct contact with the negative electrode active material layer 121 formed on the negative electrode current collector 122 of the negative electrode 120. When LATP and Li ions come into contact at these interfaces, a spontaneous reaction occurs to form a reduced product 131 containing lithium-LATP, which is formed by the reduction of LATP. At this time, more than 50% by weight of the reduced product is formed along the surface of the LATP particles, filling the voids in the oxide-based solid electrolyte layer 112. Subsequently, the oxide solid electrolyte layer has voids and a space charge region 132 on its surface where the concentration of Li+ is locally high. In this region 132, SEI substances are formed through side reactions of the electrolyte. Together with or separately from the LATP reduced product, these substances fill the remaining voids in the oxide solid electrolyte layer and are also formed as a separate layer with thickness (t), thus forming an additional inorganic layer 132 on the surface of the oxide solid electrolyte layer 112.

[0098] On the other hand, in order to achieve the effects intended by the present invention, the inorganic layer may contain 7 to 15 atomic percent of F atoms based on all atoms present in the inorganic layer, more specifically 8 to 15 atomic percent, and even more specifically 10 to 13 atomic percent.

[0099] When deviating from the above range and having a low F atom content, an inorganic excess, that is, a thick layer of an organic component with low interatomic binding energy and low lithium ion conductivity which is not an inorganic rich SEI layer is formed, and an SEI layer that is easily decomposed is formed. Therefore, the thermal safety becomes unstable, the resistance increases, which is not preferable. On the contrary, when the F atom content is excessively high, since such a substance containing the F element must be added to the electrolyte in excess, side reactions are likely to occur inside the electrolyte, the viscosity of the electrolytic solution increases, and there is a problem that the conductivity of the lithium ions moving in the electrolytic solution becomes low, which is not preferable.

[0100] In addition, as described above, since the inorganic layer also grows in the pores of the oxide-based solid electrolyte layer and at the interface between this and the negative electrode, its thickness is formed even thicker than that of a general SEI layer, so it is more effective in improving thermal safety.

[0101] electrolyte On the other hand, the lithium secondary battery can further contain an electrolyte.

[0102] [[ID=十三]] Here, the electrolyte is a lithium non-aqueous electrolyte and can contain a lithium salt and a non-aqueous organic solvent.

[0103] The lithium salt is used as a mediator for transmitting ions in the lithium secondary battery. The lithium salt contains, for example, Li as a cation + and, as an anion, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO2 - , PF6 - , CFSO3

[0014] , CH3CO2 - , CF3CO2- AsF6 - SbF6 - CH3SO3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - BF2C2O4 - BC4O8 - PF4C2O4 - PF2C4O8 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - , C4F9SO3 - CF3CF2SO3 - CF3CF2(CF3)2CO - (CF3SO2) 2CH - CF3(CF2)7SO3 - , and SCN - It may include at least one selected from the group consisting of the following.

[0104] Specifically, the lithium salts are LiCl, LiBr, LiI, LiBF4, LiClO4, and LiB 10 Cl 10It may contain a single substance or a mixture of two or more substances selected from the group consisting of LiAlCl4, LiAlO2, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (Lithium bis(fluorosulfonyl) imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl) imide, LiN(SO2CF2CF3)2, and LiTFSI (lithium bis(trifluoromethanesulfonyl) imide, LiN(SO2CF3)2), but more specifically, it must contain Li[N(SO2F)2], i.e., LiFSI, which can increase the F atom content in the inorganic layer, thereby reducing thermal decomposition and improving thermal safety.

[0105] In addition to these, lithium salts commonly used as electrolytes in lithium secondary batteries can be used without restriction.

[0106] The lithium salt can be appropriately changed within a normally usable range, but in order to obtain the optimal effect of forming a protective film to prevent corrosion on the electrode surface, it may be included in the electrolyte at a concentration of 0.5 M to 3 M, more specifically, 1 M to 2.5 M, and even more specifically, 1 M to 2 M. When the concentration of the lithium salt satisfies the above range, the effect of improving the cycle characteristics of the lithium secondary battery during high-temperature storage is sufficient, the viscosity of the electrolyte is appropriate, and the electrolyte impregnation is improved.

[0107] The aforementioned non-aqueous organic solvent is not limited as long as it minimizes decomposition due to oxidation reactions during the charging and discharging process of the lithium secondary battery and can exhibit the desired properties together with the additive. For example, carbonate-based organic solvents, ether-based organic solvents, or ester-based organic solvents can be used individually or in combination of two or more types, and in particular, carbonate-based organic solvents can be used.

[0108] Of the aforementioned organic solvents, the carbonate-based organic solvent may include at least one of cyclic carbonate-based organic solvents and linear carbonate-based organic solvents. Specifically, the cyclic carbonate-based organic solvent may include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and fluoroethylene carbonate (FEC). More specifically, by including fluoroethylene carbonate (FEC), the F atom content in the inorganic layer can be increased to achieve the effect intended by this application.

[0109] Furthermore, the linear carbonate-based organic solvent is a solvent having low viscosity and low dielectric constant, and may contain at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, and more specifically may contain diethyl carbonate.

[0110] The ether-based organic solvent may be one selected from the group consisting of ethylene glycol dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, and ethyl propyl ether, or a mixture of two or more of these, but is not limited thereto.

[0111] The ester-based organic solvent mentioned above is at least one selected from the group consisting of linear ester-based organic solvents and cyclic ester-based organic solvents.

[0112] The linear ester-based organic solvents mentioned above can typically be any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, or a mixture of two or more of these, but are not limited to these.

[0113] The cyclic ester organic solvent may, but is not limited to, be any one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more of these.

[0114] Among the ester solvents mentioned above, cyclic carbonate compounds are preferred because they are high-viscosity organic solvents with high dielectric constants that effectively dissociate lithium salts in the electrolyte. Furthermore, by mixing such cyclic carbonate compounds with low-viscosity, low-dielectric-constant linear carbonate compounds and linear ester compounds such as dimethyl carbonate and diethyl carbonate in appropriate ratios, a gel-type electrolyte with high electrical conductivity can be produced, making it even more preferable to use such a mixture.

[0115] Furthermore, the lithium non-aqueous electrolyte may further contain functional additives, which may be included to prevent negative electrode collapse from being induced in high-power environments, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and swelling improvement effects during high-temperature storage.

[0116] Specifically, the functional additive may include, as a typical example, one or more functional additives selected from the group consisting of sultone compounds, sulfite compounds, sulfone compounds, sulfate compounds, halogen-substituted carbonate compounds, nitrile compounds, cyclic carbonate compounds, phosphate compounds, borate compounds, and lithium salt compounds.

[0117] The sultone compound mentioned above includes at least one compound selected from the group consisting of 1,3-propanesultone (PS), 1,4-butanesultone, ethensultone, 1,3-propensultone (PRS), 1,4-butensultone, and 1-methyl-1,3-propensultone, and may be included in an amount of 0.3% to 5% by weight, specifically 1% to 5% by weight, based on the total weight of the gel-type electrolyte. If the content of the sultone compound in the gel-type electrolyte exceeds 5% by weight, an excessively thick film may be formed on the electrode surface, causing increased resistance and output degradation. The resistance due to the excessive amount of additive may also increase, potentially degrading the output characteristics.

[0118] The aforementioned sulfite-based compounds include one or more compounds selected from the group consisting of ethylene sulfite, methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethyl ethylene sulfite, 4,5-diethyl ethylene sulfite, propylene sulfite, 4,5-dimethylpropylene sulfite, 4,5-diethylpropylene sulfite, 4,6-dimethylpropylene sulfite, 4,6-diethylpropylene sulfite, and 1,3-butylene glycol sulfite, and may be included in an amount of 3% by weight or less based on the total weight of the gel-type electrolyte.

[0119] The sulfone compound may be one or more compounds selected from the group consisting of divinyl sulfone, dimethyl sulfone, diethyl sulfone, methyl ethyl sulfone, and methyl vinyl sulfone, and may be included in an amount of 3% by weight or less based on the total weight of the gel-type electrolyte.

[0120] The aforementioned sulfate-based compounds include ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS), and may be present in an amount of 3% by weight or less based on the total weight of the gel-type electrolyte.

[0121] Furthermore, the halogen-substituted carbonate compound may be fluoroethylene carbonate (FEC), and may be included in an amount of 5% by weight or less based on the total weight of the gel-type electrolyte. If the content of the halogen-substituted carbonate compound in the gel-type electrolyte exceeds 5% by weight, the cell swelling performance may deteriorate.

[0122] Furthermore, the nitrile compounds mentioned above include at least one compound selected from the group consisting of succinonitrile, adiponitrile (Adn), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonile, cyclohexanecarbonile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0123] The cyclic carbonate compound may be vinylene carbonate (VC) or vinylethylene carbonate, and may be included in an amount of 3% by weight or less based on the total weight of the gel-type electrolyte. If the content of the cyclic carbonate compound in the gel-type electrolyte exceeds 3% by weight, the cell swelling suppression performance may deteriorate.

[0124] The phosphate compound mentioned above includes one or more compounds selected from the group consisting of lithium difluoro(bisoxalato) phosphate, lithium difluorophosphate, tetramethyltrimethylsilyl phosphate, trimethylsilyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, and tris(trifluoroethyl) phosphate, and may be present in an amount of 3% by weight or less based on the total weight of the gel-type electrolyte.

[0125] The borate compound mentioned above may include lithium oxalyl difluoroborate, and may be present in an amount of 3% by weight or less based on the total weight of the gel-type electrolyte.

[0126] The lithium salt compound is a compound different from the lithium salt contained in the lithium non-aqueous electrolyte, and includes one or more compounds selected from the group consisting of LiPO2F2, LiODFB, LiBOB (lithium bisoxalate borate (LiB(C2O4)2) and LiBF4), and can be included in an amount of 3% by weight or less based on the total weight of the gel-type electrolyte.

[0127] The functional additives may be a mixture of two or more types, present in an amount of 20% by weight or less, specifically 0.1% to 10% by weight, based on the total weight of the lithium non-aqueous electrolyte. If the content of the functional additives exceeds 20% by weight, excessive side reactions may occur in the lithium non-aqueous electrolyte during battery charging and discharging. In particular, if they are not sufficiently decomposed at high temperatures, unreacted substances or precipitated substances may remain in the lithium non-aqueous electrolyte at room temperature. This may cause side reactions that reduce the lifespan or resistance characteristics of the lithium metal battery.

[0128] Manufacturing method of lithium secondary batteries On the other hand, a method for manufacturing a lithium secondary battery according to another embodiment of the present invention is provided.

[0129] Specifically, the method for manufacturing the lithium secondary battery is: A step of producing a separation membrane by forming an oxide-based solid electrolyte layer containing lithium aluminum titanium phosphate (LATP) on one or both sides of a substrate; A step of manufacturing a positive electrode by forming a positive electrode active material layer on one or both sides of a positive electrode current collector, and a step of manufacturing a negative electrode by forming a negative electrode active material layer on one or both sides of a negative electrode current collector; A step of manufacturing an electrode assembly by interposing the separation membrane between the positive electrode and the negative electrode so that the oxide-based solid electrolyte layer and the negative electrode active material layer face each other; A step in manufacturing a secondary battery by incorporating the electrode assembly into a secondary battery case; The steps of positioning the secondary battery between a plurality of pressure plates and applying pressure to the secondary battery; and The method is characterized by including the step of activating the secondary battery while applying pressure to it.

[0130] Figures 2 and 3 schematically show a top view and a cross-sectional view, respectively, of a pressurizing device applied to the manufacturing method of such a lithium secondary battery.

[0131] The lithium secondary battery according to the present invention is manufactured by producing a positive electrode, a negative electrode, and a separator membrane as described above, stacking them so that the oxide-based solid electrolyte layer of the separator membrane faces the negative electrode active material layer of the negative electrode to produce an electrode assembly, and then incorporating these together with the electrolyte into a secondary battery case.

[0132] Subsequently, referring to Figures 2 and 3, pressure is applied to the lithium secondary battery 200 in the vertical or horizontal directions with plate-shaped pressure plates 210 positioned above and below or to the left and right of the lithium secondary battery 200. In the case of Figures 2 and 3, a predetermined pressure (F) is applied simultaneously from the top and bottom of the lithium secondary battery 200 with the pressure plates 210 acting as an intermediate medium. This allows for much more uniform pressure application than when pressure is applied directly to the lithium secondary battery 200 without an intermediate medium, or when pressure is applied to only one of the top and bottom surfaces.

[0133] Alternatively, it is also possible to fix one side of the lithium secondary battery 200 to a support and apply pressure only to the opposite side. For example, a fluid pressure plate 210 continuously applies pressure to a stationary support (i.e., the support and the pressure plate are in complete contact), an external force is applied to the pressure plate 210 to temporarily create a predetermined space, the lithium secondary battery 200 is interposed, and then the external force applied to the pressure plate 210 is removed. In this case, a uniform pressure (F) can be applied to both sides of the lithium secondary battery 200 in a very easy manner.

[0134] Here, multiple pressure plates 210 are fastened together with multiple pressure bolts 220, allowing pressure to be applied to the lithium secondary battery 200. Applying pressure (F) by fastening with pressure bolts 220 in this way has the advantage of allowing pressure to be applied to a precise location.

[0135] For example, the applied pressure (F) can be expressed as the fastening torque strength of the pressure plate 210, and the pressure bolt 220 can be pressurized with a fastening strength of 6 kgf·cm or more, more specifically, with a fastening strength of 10 to 20 kgf·cm.

[0136] When applying pressure with a fastening strength of less than 6 kgf·cm, effective SEI formation can be difficult.

[0137] On the other hand, when two or more lithium secondary batteries are stacked and pressure is applied simultaneously from the top and bottom using a pressure plate as an intermediate medium, pressure can be applied uniformly to multiple lithium secondary batteries. Furthermore, a multi-layer pressure application method is also possible, in which lithium secondary batteries are interposed one by one between multiple pressure plates before applying pressure.

[0138] Next, the lithium secondary battery can be activated while pressure is applied to it.

[0139] Here, the activation process may include a charge-discharge process and can be carried out at temperatures between 25°C and 60°C, more specifically, at temperatures between 40°C and 60°C.

[0140] Specifically, in the activation process of the lithium secondary battery manufacturing process, an inorganic layer containing the reduced LATP and SEI substances is formed at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer. By applying a specific range of pressure to the lithium secondary battery during the activation stage, the formation mode of the inorganic layer containing the reduced LATP and SEI substances can be controlled with good quality.

[0141] In the following, an example will be used to illustrate how a lithium secondary battery according to one embodiment of the present invention exhibits improved effects. [Examples]

[0142] <Example 1> (LATP) A separation membrane was manufactured by forming oxide-based solid electrolyte layers (thickness: 2 μm) on both sides of a polyolefin substrate (polyethylene, thickness: 9 micrometers).

[0143] Here, the oxide-based solid electrolyte layer is Li 1.3 Al 0.3 Ti 1.7 (PO4)3: An electrolyte slurry was prepared by mixing an acrylic copolymer (CSB130, Toyoink Co., Ltd.) in a ratio of 95:5 by weight and dispersing it in acetone. This slurry was then coated onto one surface of the polyolefin substrate and dried to produce the product.

[0144] <Comparative example 1> (CCS) A separation membrane was manufactured by forming an organic-inorganic mixed layer (thickness: 20 μm) on both sides of a polyolefin substrate (polyethylene, thickness: 9 micrometers).

[0145] Here, the organic-inorganic mixed layer was produced by mixing Al2O3:CSB130 in a ratio of 95:5 by weight, dispersing it in NMP, coating one surface of the polyolefin substrate with the resulting organic-inorganic slurry, and drying it.

[0146] <Experimental Example 1> First, surface SEM images of the separation membrane produced in Example 1 were taken and shown in Figure 4, and XRD analysis was performed, with the graph shown in Figure 5.

[0147] Subsequently, a 15-micrometer thick aluminum (Al) metal thin film is prepared as the positive electrode current collector, and Li(Ni) is used as the positive electrode active material on one side of the aluminum metal thin film. 0.86 Co 0.05 Mn 0.07 Al 0.02A cathode slurry was produced by dispersing carbon nanotubes (O2) as a conductive material and PVDF as a binder in an NMP solvent in a weight ratio of 96:1:3, and then coating, drying, and rolling the slurry to a thickness of 60 micrometers to manufacture the cathode.

[0148] A copper (Cu) metal thin film with a thickness of 8 micrometers was prepared as the negative electrode current collector. A negative electrode slurry was manufactured by dispersing prelithiated SiO (SiO) as the negative electrode active material, carbon black as the conductive material, and SBR and CMC as binders in an NMP solvent in a weight ratio of 80:10:9.5:0.5 on one side of the copper metal thin film, coating it to a thickness of 100 micrometers, drying it, and rolling it to produce the negative electrode.

[0149] An electrode assembly was fabricated by interposing the separation membrane produced in Example 1 between the positive electrode and the negative electrode, and positioning the oxide-based solid electrolyte layer or organic-inorganic mixed layer facing the negative electrode. An electrolyte was then injected into the assembly, prepared by dissolving 1.0 M of LIFSI and 0.5 M of LiPF6 in a non-aqueous organic solvent having a composition of fluoroethylene carbonate (FEC):diethyl carbonate (DEC) = 10:90 (volume ratio), to produce a 100 mA pouch-type bicell.

[0150] Each of the aforementioned bicells was subjected to a pressure jig at 25°C with a fastening strength of 12 kgf·cm, and charged with a constant current of 0.1C until it reached 4.20V. This process was repeated three times: aging at 25°C for 24 hours, aging at 60°C for 20 hours, and then completely discharging with a constant current of 0.33C.

[0151] After disassembling the bicells that had completed the charge and discharge, SEM images of the separation membrane surface were taken and shown in Figure 4 along with the SEM image of the separation membrane before charge and discharge. XRD analysis was performed by irradiating the separation membrane surface with X-rays at angles of 0 to 50 degrees and comparing the diffraction patterns obtained with the peak positions of LATP and lithium-ionized LATP in the ICDD, and the results are shown in Figure 5.

[0152] Referring to Figures 4 and 5, it can be confirmed that after charging and discharging, Li-reduced products are formed on the surface of the separation membrane according to this invention, filling the voids in the oxide-based solid electrolyte layer.

[0153] <Experimental Example 2> Bicells were manufactured and charged / discharged using the separation membrane from Example 1 and Comparative Example 1 in the same manner as in Experimental Example 1.

[0154] Subsequently, after disassembling the bicell, elemental quantitative analysis in the depth direction was performed by etching the separated film specimen with an XPS-equipped sputtering gun on the negative electrode surface (spot size: 200 μm, sputtering gun energy: 1000 eV, etching rate: 1.08 nm / sec). The elemental content was analyzed in this way to examine the SEI material composition and its thickness, and the results are shown in Figures 6 and 7. The atomic content of F is shown in Table 1.

[0155] Specifically, Figure 6 shows the results of analyzing the Si element content on the negative electrode surface. From this, the thickness of the inorganic layer was examined, and Figure 7 shows the results of analyzing the elemental content of Li, F, C, and O.

[0156] Referring to Figure 6, it can be seen that the anode of the bicell using the separation membrane of Comparative Example 1 has a relatively thin inorganic layer because the Si content increases rapidly in a short period of time, while the anode of the bicell using the separation membrane of Example 1 of the present application shows a slower Si exposure, resulting in the formation of a thicker inorganic layer.

[0157] On the other hand, referring to Figure 7 and Table 1 below, it can be seen that the negative electrode surface of the bicell using the separation membrane of Example 1 had a high content of Li, F, and O, and a low content of C, indicating that a large amount of SEI substances, such as LiF and Li2O, were formed.

[0158] [Table 1]

[0159] <Experimental Example 3> The elemental content of the separation membrane from Example 1 was analyzed by performing XPS profile analysis as in Experimental Example 2, and the results are shown in Figure 8.

[0160] Subsequently, using the same method as in Experimental Example 1, a bicell was manufactured and charged / discharged using the separation membrane from Example 1.

[0161] After charging and discharging, the bicells were decomposed, and the elemental content was analyzed by XPS profile analysis on the separation membrane surface. The results, along with the analysis data of the separation membrane before charging and discharging, are shown in Figure 8. The atomic content of F is shown in Table 2.

[0162] Referring to Figure 8 and Table 2, it can be seen that the F content on the surface of the separation membrane increased rapidly after charging and discharging, indicating that the F corresponding to the SEI component increased due to charging and discharging, and therefore an inorganic layer with an excess of inorganic material was formed.

[0163] [Table 2]

[0164] <Experimental Example 4> Bicells were manufactured and charged / discharged using the separation membrane from Example 1 and Comparative Example 1 in the same manner as in Experimental Example 1. The fully discharged bicells were disassembled, the electrolyte was washed with the negative electrode, and then DSC evaluation was performed.

[0165] The aforementioned DSC evaluation measured the heat flow rate at which exothermic or endothermic reactions occurred when a fully discharged electrode was heated at a heating rate of 10°C / min in the 25-250°C range.

[0166] Furthermore, using the same method as in Experimental Example 1, bicells were manufactured using the separation membrane of Example 1 and the separation membrane of Comparative Example 1, and charged and discharged. After being fully charged again under CC-CV conditions of 0.1C and 4.35V, they were disassembled and the DSC evaluation was performed on the negative electrode.

[0167] The results are shown in Figure 9.

[0168] Referring to Figure 9, it can be seen that, under fully charged conditions and with electrolyte present, the main peak of the negative electrode of the bicell containing the separation membrane according to Example 1 of the present application was wider and a split was observed compared to the case of Comparative Example 1 containing the separation membrane. Furthermore, under complete discharge and electrolyte washing conditions, a high-temperature shift in the onset temperature was observed at the negative electrode of the bicell containing the separation membrane according to Example 1 of the present application, indicating that the separation membrane according to the present application offers superior thermal safety.

[0169] <Experimental Example 5> Under the same conditions as in Experimental Example 4, a calorimeter evaluation was performed to measure the total heat generated (water temperature) of the exothermic reaction when a spark was applied to a substance in an Autoclave pressurized in a 30 bar O2 atmosphere relative to the negative electrode to induce forced ignition. The results are shown in Figure 10.

[0170] Referring to Figure 10, it can be confirmed that under fully charged conditions with electrolyte present, the heat generation of the negative electrode of the bicell containing the separation membrane in Example 1 decreased by approximately 5.7% (4224 → 3985 Cal / g) compared to the negative electrode of the bicell containing the separation membrane in Comparative Example 1. Under completely discharged conditions with electrolyte washing, it can be confirmed that the heat generation decreased by approximately 8.6% (3075 → 2812 cal / g).

[0171] This will further demonstrate that using the separation membrane described in this invention offers superior thermal safety.

[0172] <Experimental Example 6> Bicells were prepared using the separation membrane from Example 1 and the separation membrane from Comparative Example 1 in the same manner as in Experimental Example 1.

[0173] The manufactured bicells were charged at 45°C and the pressurized jigs were subjected to fastening strengths of 6 kgf·cm ('LATP6'), 12 kgf·cm ('LATP12'), and 18 kgf·cm ('LATP18'), respectively. The batteries were then charged with a constant current of 0.1C until the voltage reached 4.20V, aged for 24 hours at 25°C, aged for 20 hours at 60°C, and then completely discharged with a constant current of 0.33C. This process was repeated three times.

[0174] Subsequently, the DCIR was measured when the charge state of the SOC50 was set and a 2.5C discharge current was applied for 0.1, 10, and 30 seconds, respectively. The results are shown in Figure 11.

[0175] Furthermore, the discharge rate characteristics were evaluated and the results are shown in Figure 12.

[0176] The aforementioned discharge rate characteristics were determined by performing a CC / CV (cut when CV current reaches 0.05C) discharge with a bicell at 0.2C, and performing three discharges each at 0.2C, 0.33C, 0.5C, 1C, 2C, and 3C. The capacity was checked on the third discharge for each, and the rate at which the discharge capacity was maintained at a high rate relative to the discharge capacity at 0.2C was confirmed.

[0177] Referring to Figures 11 and 12, it can be seen that when using the separation membrane according to the present invention, the resistance is reduced, and that the rate of reduction is even greater when the applied pressure is increased. On the other hand, in terms of capacity, it can be seen that there is little to no significant difference in performance, or that it is superior.

[0178] <Experimental Example 7> The positive electrode was prepared in the same manner as in Experimental Example 1, and the negative electrode was manufactured in the same way except that the negative electrode active material used was a mixture of graphite and SiO in a weight ratio of 94.5:5.5.

[0179] Furthermore, using the separation membranes produced in Example 1 and Comparative Example 1, electrode assemblies were manufactured with the oxide-based solid electrolyte layer or organic-inorganic mixed layer facing the negative electrode. An electrolyte consisting of 1M LiPF6 dissolved in a carbonate solvent with EC:EMC = 3:7 (vol%) was used, and an 840mA pouch-type battery cell was manufactured by stacking the positive and negative electrodes three times each in a stack cell manufacturing process.

[0180] The manufactured bicells were charged to 4.20V with a constant current of 0.1C while the pressurized jig was pressurized at 45°C and 12kgf·cm, respectively, and then aged for 24 hours at 25°C and 20 hours at 60°C. This process was repeated three times, followed by complete discharge with a constant current of 0.33C.

[0181] The DCIR was measured by setting the charge state of the SOC50 and discharging it under 2.5C conditions, and the results are shown in Figure 13.

[0182] Furthermore, the discharge rate characteristics were evaluated and the results are shown in Figure 14.

[0183] The method for evaluating the discharge rate characteristics is as described in Experimental Example 6.

[0184] Referring to Figures 13 and 14, it can be seen that when using the separation membrane according to the present invention, the resistance is reduced, and that the rate of reduction is even greater when the applied pressure is increased. On the other hand, in terms of capacity, it can be seen that there is little to no significant difference in performance, or that it is superior.

[0185] Anyone with ordinary skill in the field to which this invention belongs can make various applications and modifications within the scope of this invention based on the above description. [Industrial applicability]

[0186] The lithium secondary battery according to the present invention has the effect of improving the thermal safety of the lithium secondary battery by forming an oxide-based solid electrolyte layer containing LATP on a separation membrane substrate and bringing it into contact with the negative electrode active material layer, thereby forming a thick layer of SEI with a high F atom content and an excess of inorganic materials at the interface between these two layers, so that LATP reduced products are formed together with SEI material.

[0187] Furthermore, by controlling the formation mode of SEI with such a high fluorine atom content in inorganic materials, battery performance can be improved.

Claims

1. A positive electrode comprising a positive electrode current collector and a positive electrode active material layer formed on one or both sides of the positive electrode current collector, A negative electrode comprising a negative electrode current collector and a negative electrode active material layer formed on one or both sides of the negative electrode current collector, and The separation membrane comprises a substrate and an oxide-based solid electrolyte layer containing lithium aluminum titanium phosphate (LATP) formed on one or both sides of the substrate. The negative electrode active material layer and the oxide-based solid electrolyte layer face each other. An inorganic layer containing the reduced product of LATP and the SEI substance is formed at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer. A lithium secondary battery wherein the atomic content of F in the inorganic layer is 7 to 15 atomic percent based on all atoms present in the inorganic layer.

2. The lithium secondary battery according to claim 1, wherein the inorganic layer fills some or all of the voids in the oxide-based solid electrolyte layer, forms a separate layer at the interface between the oxide-based solid electrolyte layer and the negative electrode active material layer, or is included in all of these forms.

3. The lithium secondary battery according to claim 2, wherein the inorganic layer is formed to fill the voids of the oxide-based solid electrolyte layer at a volume of 10% to 100% based on the total volume of the voids, and to form a separate layer with a thickness of 5 nm to 100 nm.

4. The reduced LATP is formed in such a manner that 50% or more by weight fills the voids in the oxide-based solid electrolyte layer along the surface of the LATP particles. The lithium secondary battery according to any one of claims 1 to 3, wherein the SEI material is formed in a form that fills the voids of the oxide-based solid electrolyte layer together with the reduced LATP in a space charge region where the concentration of Li+ is locally high, or on the surface of the reduced LATP, or is formed on the surface of the oxide-based solid electrolyte layer in a separate layer or in all of these forms.

5. The lithium secondary battery according to claim 1, wherein the reduced product of LATP includes lithified-LATP formed by a spontaneous lithiation reaction represented by the following reaction formula 1 of LATP. [Reaction Equation 1] Li １．３ Al ０．３ Ti １．７ (PO ４ ) ３ →Li ３ Al ０．３ Ti １．７ (PO ４ ) ３

6. The lithium secondary battery according to claim 1, wherein the SEI substance includes LiF.

7. The aforementioned SEI substance is Li ２ CO ３ , and Li ２ The lithium secondary battery according to claim 6, further comprising one or more substances selected from the group consisting of O.

8. The lithium secondary battery according to claim 1, wherein the inorganic layer is composed of a LATP reduced product and an SEI substance.

9. The lithium secondary battery according to claim 1, wherein the oxide-based solid electrolyte layer is formed on both sides of the substrate.

10. The lithium secondary battery according to claim 1, wherein the substrate is a polyolefin substrate.

11. The lithium secondary battery according to claim 1, wherein the oxide-based solid electrolyte layer is composed of an oxide-based solid electrolyte containing lithium aluminum titanium phosphate (LATP) and a binder.

12. The lithium secondary battery according to claim 1, wherein the oxide-based solid electrolyte layer is formed on one surface of the substrate with a thickness of 0.1 μm to 20 μm.

13. The lithium secondary battery according to claim 1, wherein the positive electrode active material layer contains a lithium transition metal oxide represented by the following chemical formula 1 as the positive electrode active material: [Chemical formula 1] Li １＋ｘ Ni ａ Co ｂ Mn ｃ M １－（ａ＋ｂ＋ｃ） O ２ In the above formula, M is one or more elements selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo. 0 ≤ x ≤ 0.5, 0 < a < 1, 0 < b < 1, 0 < c < 1.

14. The lithium secondary battery according to claim 1, wherein the negative electrode active material layer contains 80% by weight or more of pre-lithified silicon oxide as the negative electrode active material, based on the total weight of the negative electrode active material.

15. The lithium secondary battery according to claim 1, wherein the lithium secondary battery further comprises an electrolyte containing LiFSI as a lithium salt and fluoroethylene carbonate (FEC) as an electrolyte solvent.

16. A method for manufacturing a lithium secondary battery according to claim 1, A step of producing a separation membrane by forming an oxide-based solid electrolyte layer containing lithium aluminum titanium phosphate (LATP) on one or both sides of a substrate; A step of manufacturing a positive electrode by forming a positive electrode active material layer on one or both sides of a positive electrode current collector, and a step of manufacturing a negative electrode by forming a negative electrode active material layer on one or both sides of a negative electrode current collector; A step of manufacturing an electrode assembly by interposing the separation membrane between the positive electrode and the negative electrode so that the oxide-based solid electrolyte layer and the negative electrode active material layer face each other; A step in manufacturing a secondary battery by incorporating the electrode assembly into a secondary battery case; The steps of positioning the secondary battery between a plurality of pressure plates and applying pressure to the secondary battery; and A method for manufacturing a lithium secondary battery, comprising the step of activating the secondary battery while applying pressure to the secondary battery.

17. The method for manufacturing a lithium secondary battery according to claim 16, wherein the plurality of pressure plates are fastened together with a plurality of pressure bolts to apply pressure.

18. The method for manufacturing a lithium secondary battery according to claim 17, wherein the pressurizing bolt pressurizes the secondary battery with a fastening strength of 6 kgf·cm or more.

19. The method for manufacturing a lithium secondary battery according to claim 17, wherein the pressurizing bolt pressurizes the secondary battery with a fastening strength in the range of 10 to 20 kgf·cm.

20. The method for manufacturing a lithium secondary battery according to claim 16, wherein the activation step is performed under temperature conditions of 40°C to 60°C.

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