Lithium secondary battery containing a buffer layer between the positive electrode and the separator membrane

Abstract

According to one embodiment of the present invention, an electrode assembly comprising a positive electrode, a negative electrode, and a separator membrane, and The electrolyte comprises a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent. The positive electrode and the negative electrode face each other across the separation membrane, A lithium secondary battery is provided, wherein a buffer layer containing an oxide-based solid electrolyte and a protective layer containing a reaction substance between the oxide-based solid electrolyte and the fluorinated non-solvent are formed between the positive electrode and the separation membrane.

Description

[Technical Field]

[0001] [Cross-reference of related applications] This application claims priority rights based on Korean Patent Application No. 10-2024-0030959 dated March 4, 2024, and Korean Patent Application No. 10-2025-0027002 dated February 28, 2025, and all content disclosed in the documents of said Korean Patent Applications is incorporated herein as part of this specification.

[0002] This invention relates to a lithium secondary battery that includes a buffer layer between the positive electrode and a separation membrane. [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 device that utilizes this type of electrochemical energy is the secondary battery, and its range of applications is steadily expanding.

[0005] In recent years, with the increasing technological development and demand for portable devices such as portable computers, mobile phones, and cameras, the demand for secondary batteries as an energy source has rapidly increased. Among these, lithium secondary 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 as alternatives to vehicles that use fossil fuels such as gasoline and diesel vehicles, which are one of the main causes of air pollution. Nickel-metal hydride rechargeable 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-ion rechargeable batteries, which have high energy density and discharge voltage, and some are already at the commercialization stage.

[0007] Generally, lithium secondary batteries have a structure in which a non-aqueous liquid electrolyte is impregnated into an electrode assembly consisting of 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, in lithium secondary batteries with a liquid electrolyte base, the more charge-discharge cycles are repeated, the more the electrolyte decomposes, generating byproducts at the cathode interface. These byproducts reduce the electrode output and cause a decrease in capacity.

[0010] Furthermore, capacitance degradation is directly related to the overall capacitance of the cell, and it occurs fatally when the positive electrode interface degenerates, which can be caused by increased cell resistance and overvoltage due to byproducts generated at the positive electrode interface.

[0011] Therefore, improving the interface stability between the positive electrode and the electrolyte is currently necessary to achieve sustained capacity development and extended lifespan in lithium-ion secondary batteries. [Overview of the project] [Problems that the invention aims to solve]

[0012] The present invention aims to provide a lithium secondary battery that improves the physical properties of the separation membrane while suppressing the generation of electrode by-products due to stabilization of the positive electrode interface, thereby enabling sustained capacity development and extended lifespan. [Means for solving the problem]

[0013] According to one embodiment of the present invention, An electrode assembly including a positive electrode, a negative electrode, and a separator membrane, and The electrolyte comprises a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent. The positive electrode and the negative electrode face each other across the separation membrane, A lithium secondary battery is provided, wherein a buffer layer containing an oxide-based solid electrolyte and a protective layer containing a reaction substance between the oxide-based solid electrolyte and the fluorinated non-solvent are formed between the positive electrode and the separation membrane.

[0014] Here, the oxide-based solid electrolyte may contain LATP represented by the following chemical formula 1. [Chemical formula 1] Li 1+x Al x Ti 2-x (PO4)3 In the above chemical formula 1, 0 <x<0.5である。

[0015] More specifically, the oxide-based solid electrolyte is Li 1.3 Al 0.3 Ti 1.7 (PO4)3(LATP) may be included.

[0016] Furthermore, the buffer layer may further include a binder.

[0017] The thickness of the buffer layer may be 0.1 μm to 10 μm.

[0018] The total ionic conductivity of the separation membrane and buffer layer may be 0.1 mS / cm to 5 mS / cm.

[0019] On the other hand, the protective layer may be formed between the buffer layer and the positive electrode.

[0020] Specifically, the protective layer may have a gradient in the thickness direction such that the concentration of substances originating from the oxide-based solid electrolyte in the reactant decreases as you move from the buffer layer towards the positive electrode.

[0021] The thickness of the protective layer may be 0.01 μm to 1 μm.

[0022] Furthermore, the protective layer may contain element F in an amount of 70% to 90% by weight.

[0023] On the other hand, the lithium salt of the electrolyte may be one or more imide-based lithium salts selected from the group consisting of 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).

[0024] Furthermore, the fluorinated nonsolvent may be one or more selected from the group consisting of TFEE (Ethylene glycol bis(1,1,2,2-tetrafluoroethyl) ether), TTE (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether), DFB (1,2-difluorobenzene), BTFE (bis(2,2,2-trifluoroethyl) ether), OTE (1H,1H,5H-Octafluoropentyl 1,1,2,2-Tetrafluoroethyl ether), and 1,1,2,2-Tetrafluoroethyl 2,2,2-trifluoroethyl ether.

[0025] The non-aqueous organic solvent can contain 50% by volume or more of an ether-based organic solvent based on the total volume of the non-aqueous organic solvent.

[0026] The positive electrode can contain a lithium iron phosphate represented by the following Chemical Formula 2 or a lithium transition metal oxide represented by the following Chemical Formula 3 as a positive electrode active material. [Chemical Formula 2] Li 1+a Fe 1-y M y (PO 4-b )X b In Chemical Formula 2, M is one or more selected from Al, Mg, and Ti, -0.5 ≦ a ≦ 0.5, 0 ≦ y ≦ 0.5, 0 ≦ b ≦ 0.1, [Chemical Formula 3] Li 1+a’ Ni 1-y’-z’-s Co y’ Mn z’ M’ s O 2-b’ X b’ In Chemical Formula 3, M’ is one or more selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, -0.5 ≦ a’ ≦ 0.5, 0 < y’ < 1, 0 < z’ < 1, 0 ≦ s < 0.2, 0 ≦ b’ ≦ 0.1, In Chemical Formulas 2 and 3, X is one or more selected from F, S, and N.

[0027] Furthermore, the negative electrode can contain lithium metal as a negative electrode active material.

[0028] Also, the separator may be a polyolefin-based substrate.

Brief Description of Drawings

[0029] [Figure 1] It is a cross-sectional schematic view of a lithium secondary battery according to an embodiment of the present invention. [Figure 2] It is a cross-sectional SEM photograph of the positive electrode after cycling of Example 1 according to Experimental Example 1. [Figure 3]This is a photograph of the cross-sectional and elemental analysis of the cathode after cycling in Example 1, according to Experimental Example 1. [Figure 4] This is a graph comparing lifespan characteristics based on Experimental Example 2. [Figure 5] This is a graph comparing lifespan characteristics based on Experimental Example 3. [Modes for carrying out the invention]

[0030] Hereafter, terms and words used in this specification and claims should not be construed to be limited to their ordinary or dictionary meanings, but rather should be interpreted in a sense and concept consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best describe their invention.

[0031] Unless otherwise specified, all terms used herein (including technical and scientific terms) will be used in a sense that can be commonly understood by a person of ordinary skill in the art to which this invention pertains. Furthermore, terms defined in commonly used dictionaries will not 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, singular terms include plural terms unless otherwise specified in the text. The terms “comprises” and / or “comprising” as used in this specification do not preclude the presence or addition of one or more other components in addition to those mentioned.

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

[0034] In this specification, "average particle size D50" refers to the particle size at 50% of the volume-cumulative particle size distribution of the particle powder to be measured (e.g., positive electrode active material powder, negative electrode active material powder, etc.). The average particle size D50 can be measured using the laser diffraction method. For example, after dispersing the powder of the particles to be measured in a dispersion medium, it can be introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), irradiated with ultrasound at approximately 28 kHz at an output of 60 W, and then measured by obtaining a volume-cumulative particle size distribution graph and determining the particle size corresponding to 50% of the volume-cumulative amount.

[0035] In this specification, "ionic conductivity" is a value calculated based on electrochemical impedance measured using an analytical instrument (VMP3, Bio logic science instrument) at 60°C under conditions of amplitude 10mV and scan range 500kHz to 20MHz.

[0036] In this specification, "elemental analysis" refers to the ability to distinguish between substances from the bright and dark areas in a back-scattered electron image, where differences in particle weight result in varying brightness, and to analyze elemental content using map spectrum (EDS analysis).

[0037] In this specification, "porosity" can be determined by measuring the sample surface at 2,500x magnification using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope), and then predicting the volume from the area of ​​surface pores observed in an arbitrarily sampled area (10 μm or more horizontally, 15 μm or more vertically) within the measured photograph.

[0038] A lithium secondary battery according to one embodiment of the present invention is An electrode assembly including a positive electrode, a negative electrode, and a separator membrane, and The electrolyte comprises a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent. The positive electrode and the negative electrode face each other across a separation membrane. The present invention is characterized in that a buffer layer containing an oxide-based solid electrolyte and a protective layer containing a reaction substance between the oxide-based solid electrolyte and the fluorinated non-solvent are formed between the positive electrode and the separation membrane.

[0039] To illustrate the structure of the lithium secondary battery, Figure 1 below shows a schematic cross-sectional view of the lithium secondary battery according to the present invention.

[0040] Referring to Figure 1, the lithium secondary battery 100 includes an electrode assembly comprising a positive electrode 110, a negative electrode 120, and a separation membrane 130, and an electrolyte 140 comprising a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent.

[0041] At this time, the positive electrode 110 and the negative electrode 120 face each other via a separation membrane 130, and between the positive electrode 110 and the separation membrane 130, a buffer layer 131 containing an oxide-based solid electrolyte and a protective layer 132 containing a reactant obtained by the reaction of the oxide-based solid electrolyte and a fluorinated non-solvent are formed.

[0042] Here, the protective layer 132 is located between the positive electrode 110 and the buffer layer 131.

[0043] The following provides a more detailed explanation of each component.

[0044] Separation membrane The separation membrane is interposed between the positive electrode and the negative electrode, separating them and providing a passage for lithium ions to move. Any membrane commonly used as a separation membrane in lithium secondary batteries can be used without particular limitations, but those with low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity are particularly preferred. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof can be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used.

[0045] Buffer layer The buffer layer may be formed between the positive electrode and the separation membrane.

[0046] In this case, the buffer layer may be formed separately, on the positive electrode, or on the separation membrane. More specifically, it may be formed on the separation membrane for purposes such as ease of manufacturing, reduction of side reactions, and improvement of the separation membrane's physical properties.

[0047] Specifically, the product can be manufactured by coating the cathode or the separation membrane with a slurry containing the oxide-based solid electrolyte and then drying it.

[0048] Here, the oxide-based solid electrolyte contained in the buffer layer is not limited and 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 include one or more selected from the group consisting of LATP (lithium aluminum titanium phosphate), 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. More specifically, it may include LATP represented by the following chemical formula 1, and more specifically, Li 1.3 Al 0.3 Ti 1.7 (PO4)3(LATP) may be included. [Chemical formula 1] Li 1+x Al x Ti 2-x (PO4)3 In the above chemical formula 1, 0 <x<0.5である。

[0049] Specifically, based on the total weight of the oxide-based solid electrolyte, the LATP may be present in an amount of 80% to 100% by weight, and more specifically, in an amount of 90% to 100% by weight.

[0050] Oxide-based solid electrolytes exhibit excellent atmospheric stability against ambient oxygen and moisture intrusion, and are therefore expected to suppress by-products at the cathode interface. Among these materials, LATP is particularly effective for application of the present invention due to its relatively superior ionic conductivity.

[0051] The average diameter (D50) of the oxide-based solid electrolyte particles is 50 nanometers to 10 micrometers, more specifically 50 nanometers to 5 micrometers, and even more specifically 50 nanometers to 1 micrometer.

[0052] If the range is exceeded and is too small, interparticle aggregation occurs due to decreased dispersibility. Conversely, if it is too large, the oxide-based solid electrolyte forms large pores, which is detrimental in terms of resistance. In other words, when the range is met, lithium-ion conductivity is increased, resistance is reduced, and improved lithium secondary battery performance can be achieved.

[0053] Furthermore, the buffer layer may further contain a binder for binding the oxide-based solid electrolyte to form a solid layer when the oxide-based solid electrolyte is coated onto the separation membrane and dried.

[0054] Here, the binder is not limited as long as it does not undergo a side reaction with the electrolyte, and in particular, a binder with the lowest possible glass transition temperature (Tg) can be used, preferably in the range of -200 to 200°C.

[0055] 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.

[0056] Therefore, it is preferable that the binder has the highest possible 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.

[0057] In addition to the functions described above, the binder may have the characteristic of gelling upon liquid electrolyte impregnation, resulting in a high degree of electrolyte impregnation. In fact, if the binder is a polymer with excellent electrolyte impregnation, the electrolyte injected after battery assembly will permeate the polymer, and the polymer holding the absorbed electrolyte will acquire electrolyte ion conductivity. Therefore, the solubility index should be as high as possible, between 15 and 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 of 15 MPa is even more preferable. 1 / 2 Less than and 45 MPa 1 / 2 If the value exceeds a certain level, it becomes difficult for the material to be impregnated (swelled) by a typical liquid electrolyte used in batteries.

[0058] Examples of such binders include polyvinylidene fluorideco-hexafluoropropylene, polyvinylidene fluoridecotrichloroethylene, polyacrylate, polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyimide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate. It is one or more substances selected from the group consisting of propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxylmethylcellulose, and polyvinyl alcohol.

[0059] At this time, the oxide-based solid electrolyte may be present in an amount of 70% to 99% by weight, more specifically 80% to 99% by weight, based on the total weight of the buffer layer, and the binder may be present in an amount of 1% to 30% by weight, more specifically 1% to 20% by weight.

[0060] If the oxide-based solid electrolyte content exceeds the aforementioned range and is too low, it is not possible to obtain a sufficient reduced product as intended by this invention. If the content is too high, the content of the binder connecting them is too low, which is undesirable because it weakens the adhesive strength between these particles and reduces their mechanical properties.

[0061] Such a buffer layer containing the oxide-based solid electrolyte is formed between the positive electrode and the separation membrane. In other words, after the buffer layer is formed on the separation membrane, the electrode assembly is assembled so that the buffer layer faces the positive electrode. If the buffer layer is formed on the positive electrode, the electrode assembly is assembled by forming it on the positive electrode and then stacking it with the negative electrode via the separation membrane.

[0062] This also serves to prevent the occurrence of side reactions at the positive electrode interface due to the reaction between the positive electrode and the electrolyte, thereby acting as a side reaction prevention layer. Furthermore, if the oxide-based solid electrolyte is in direct contact with the negative electrode, especially in the case of a lithium metal negative electrode, it may react with lithium, causing damage to the negative electrode and actually reducing the cell performance of the lithium secondary battery. Therefore, it should only be formed in the direction facing the positive electrode.

[0063] The thickness of the buffer layer is 0.1 μm to 10 μm, and more specifically, it is preferably 0.5 μm or more, 1 μm or more, or 2 μm or more, and preferably 8 μm or less, 6 μm or less, 5 μm or less, or 4 μm or less.

[0064] If the thickness exceeds the aforementioned range and is too thin, not only will the effect of forming the buffer layer not be fully obtained, but it will also fail to perform its role in preventing separation membrane shrinkage, thus reducing safety. Conversely, if the thickness is too high, the resistance may increase, which is undesirable.

[0065] Furthermore, the total ionic conductivity of the separation membrane on which the buffer layer is formed, i.e., the total ionic conductivity of the separation membrane and the buffer layer, is measured using electrochemical impedance spectroscopy as is commonly known to technicians in the art, and the ionic conductivity is between 0.1 mS / cm and 5 mS / cm. More specifically, it can be 0.5 mS / cm or higher, 0.7 mS / cm or higher, or 1 mS / cm or higher, and 4 mS / cm or lower.

[0066] If the total ionic conductivity of the separation membrane and buffer layer is too low beyond the aforementioned range, the resistance of the lithium secondary battery will actually increase, reducing its performance. However, if the oxide-based solid electrolyte is included, it is possible to have an ionic conductivity within the aforementioned range.

[0067] The porosity of the buffer layer is 20% to 70% by volume, more specifically 30% to 70% by volume, and even more specifically 40% to 60% by volume, based on the total volume of the buffer layer.

[0068] If the porosity is too large beyond the aforementioned range, the protective effect of the positive electrode interface decreases, and if it is too small, the electrolyte does not sufficiently impregnate the buffer layer, which is undesirable because it reduces the mobility of lithium ions.

[0069] protective layer As shown in Figure 1 below, and as explained above, the protective layer is formed between the buffer layer and the positive electrode.

[0070] Such a protective layer is not formed by a separate manufacturing process, but rather is formed when the specific electrolyte described below and the oxide-based solid electrolyte are activated, i.e., react at the beginning of the cycle.

[0071] More specifically, as described below, the electrolyte contains specific substances, more particularly those formed by the reaction between a fluorinated non-solvent and the oxide-based solid electrolyte, and includes such reactants.

[0072] Therefore, the protective layer may have a gradient in which the concentration of substances originating from the oxide-based solid electrolyte in the reactant decreases as you move from the buffer layer towards the positive electrode, with respect to the thickness direction.

[0073] Such a protective layer can more effectively suppress the byproduct generation reaction at the positive electrode due to electrolyte oxidation at high voltage, can suppress the dissolution of the positive electrode active material, and can also improve output characteristics by prioritizing lithium ion diffusion at high current densities with the aforementioned concentration gradient.

[0074] Since such a protective layer is formed by the reaction of a specific substance within the electrolyte with the oxide-based solid electrolyte, the substance formed is determined according to the type of these substances and the oxide-based solid electrolyte.

[0075] Furthermore, the aforementioned fluorinated non-solvent must be included in the electrolyte for such a reaction to occur. Otherwise, when a solvent containing element F is reacted in the electrolyte solvent, electrolyte depletion occurs, which is undesirable as it reduces the lifespan characteristics.

[0076] The protective layer formed in this manner contains 70% by weight or more of element F, more specifically 70% to 90% by weight of element F, and more specifically 75% to 80% by weight.

[0077] When the fluorine content is within the aforementioned range, it can exhibit such effects and act as a protective layer.

[0078] More specifically, the material may include a lithium fluoride-based substance, or more precisely, a lithium fluoride-based polymer having a fluorinated non-solvent skeleton, formed by the reaction of Li derived from the oxide-based solid electrolyte and F derived from the fluorinated non-solvent.

[0079] The thickness of such a protective layer is 0.001 μm to 1 μm, and more specifically, it is preferably 0.01 μm or more, 0.05 μm or more, or 0.1 μm or more, and preferably 0.9 μm or less, 0.8 μm or less, 0.7 μm or less, or 0.5 μm or less.

[0080] If the thickness exceeds the aforementioned range and is too thin, the effects of suppressing the dissolution of the positive electrode active material and promoting lithium ion diffusion due to the formation of the protective layer cannot be obtained. If it is too thick, the resistance may actually increase, which is undesirable.

[0081] The porosity of the protective layer is 30% to 70% by volume, based on the total volume of the protective layer, more specifically 20% to 60% by volume, and even more specifically 30% to 40% by volume.

[0082] If the porosity exceeds the aforementioned range and is too large, the protective effect on the positive electrode interface decreases, and if it is too small, the mobility of lithium ions decreases, which is undesirable.

[0083] When the protective layer is formed, the thickness ratio of the buffer layer to the protective layer is 1 / 3 to 1 / 100, more specifically 1 / 3 to 1 / 50, and more specifically 1 / 5 to 1 / 20.

[0084] electrolyte On the other hand, the electrolyte contained in the lithium secondary battery according to the present invention must react with the oxide-based solid electrolyte to form the protective layer, as described above. Therefore, in addition to the lithium salt and non-aqueous organic solvent, it contains a fluorinated non-solvent.

[0085] Here, the lithium salt is used as a medium for transferring ions within a lithium secondary battery. The lithium salt is, for example, Li as a cation. + It includes, and as an anion, F - Cl - , Br - , I - NO3 - , N(CN)2 - BF4 -ClO4 - B 10 Cl 10 - AlCl4 - AlO2 - PF6 - CF3SO3 - 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 - At least one of the following groups is selected.

[0086] Specifically, the lithium salts are LiCl, LiBr, LiI, LiBF4, LiClO4, and LiB 10 Cl 10Lithium lithium salts can include a single substance or a mixture of two or more 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 due to their superior stability, imide-based lithium salts, namely LiFSI (Lithium bis(fluorosulfonyl)imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl)imide), LiN(SO2CF2CF3)2, and LiTFSI (lithium It is preferable that the mixture contains one or more selected from the group consisting of bis(trifluoromethanesulfonyl)imide and LiN(SO2CF3)2).

[0087] In addition to these, lithium salts commonly used as electrolytes in lithium secondary batteries can be used without restriction.

[0088] The lithium salt can be appropriately changed within a range that is normally usable, but in order to obtain the optimal effect of forming a protective film to prevent corrosion on the electrode surface, it is included in the electrolyte at a concentration of 0.5 M to 3 M, more specifically, 1 M to 2.5 M, and more specifically, 1 M to 2 M. When the concentration of the lithium salt meets 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.

[0089] The non-aqueous organic solvent is not limited as long as it minimizes decomposition by oxidation reactions within the voltage range of the lithium secondary battery's charge-discharge process and can exhibit its properties together with the substance. For example, carbonate-based organic solvents, ether-based organic solvents, or ester-based organic solvents can be used individually or in mixtures of two or more. More specifically, considering the stability of the fluorinated non-solvent, the ether-based organic solvent can be mainly used. In this case, the ether-based organic solvent is present in an amount of approximately 50% by volume or more, more specifically 60%, 70%, or 80% by volume or more, and 100% or less by volume or 90% or less, based on the total non-aqueous organic solvent.

[0090] Furthermore, the non-aqueous organic solvent is included in an amount of 20% to 70% by weight based on the total weight of the electrolyte.

[0091] 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). Specifically, it may include a mixed solvent of ethylene carbonate having a high dielectric constant and propylene carbonate having a relatively lower melting point compared to ethylene carbonate.

[0092] Furthermore, the linear carbonate-based organic solvent 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, as a solvent having low viscosity and low dielectric constant, and more specifically, it may contain dimethyl carbonate.

[0093] 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 to these.

[0094] 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.

[0095] The linear ester-based organic solvents typically include, but are not limited to, 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.

[0096] The cyclic ester organic solvent may, but is not limited to, 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.

[0097] 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, when such cyclic carbonate compounds are mixed with low-viscosity, low-dielectric-constant linear carbonate compounds and linear ester compounds, such as dimethyl carbonate and diethyl carbonate, in appropriate proportions, an electrolyte with high electrical conductivity can be produced, and this mixture is even more preferable.

[0098] The fluorinated non-solvent is a substance that reacts with the oxide-based solid electrolyte to form a protective layer, thereby exhibiting effects such as reduced elution of the positive electrode active material, suppression of interfacial side reactions, and improved lithium ion mobility at high current densities. For example, the fluorinated non-solvent is one or more selected from the group consisting of TFEE (Ethylene glycol bis(1,1,2,2-tetrafluoroethyl) ether), TTE (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether), DFB (1,2-difluorobenzene), BTFE (bis(2,2,2-trifluoroethyl) ether), OTE (1H,1H,5H-Octafluoropentyl 1,1,2,2-Tetrafluoroethyl ether), and 1,1,2,2-Tetrafluoroethyl 2,2,2-trifluoroethyl ether, and more specifically, TTE.

[0099] The fluorinated non-solvent is included in an amount of 70% by weight or less, specifically 40% to 70% by weight, based on the total weight of the electrolyte. If the content of the substance exceeds 70% by weight, the lithium salt may not be sufficiently dissolved in the electrolyte, and precipitation of the lithium salt may occur. This may result in a side reaction that reduces the lifespan or resistance characteristics of the lithium secondary battery.

[0100] 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.

[0101] Here, the positive electrode current collector is not particularly limited as long as it does not induce a chemical change in the battery and is conductive. 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.

[0102] The positive electrode current collector has 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 various forms such as film, sheet, foil, net, porous material, foam, and nonwoven fabric.

[0103] The positive electrode active material layer includes a positive electrode active material and may optionally include a conductive material, a binder, and other substances.

[0104] The positive electrode active material is not limited to any compound that allows for reversible intercalation and deintercalation of lithium, but specifically may include lithium iron phosphorus oxide represented by chemical formula 2 or lithium transition metal oxide represented by chemical formula 3. [Chemical formula 2] Li 1+a Fe 1-y M y (PO 4-b )X b In the above chemical formula 2, M is one or more selected from Al, Mg, and Ti, and -0.5 ≤ a ≤ 0.5, 0 ≤ y ≤ 0.5, 0 ≤ b ≤ 0.1. [Chemical formula 3] Li 1+a’ Ni 1-y’-z’-s Co y’ Mn z’ M' s O 2-b’ X b’ In the chemical formula 3, M’ is one or more selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, -0.5 ≦ a’ ≦ 0.5, 0 < y’ < 1, 0 < z’ < 1, 0 ≦ s < 0.2, 0 ≦ b’ ≦ 0.1, In the chemical formulas 2 and 3, X is one or more selected from F, S, and N.

[0105] Such lithium iron phosphate or lithium transition metal oxide is contained at 80% by weight or more, specifically 90% to 100% by weight, based on the total weight of the positive electrode active material.

[0106] In addition, the positive electrode active material includes lithium-manganese-based oxides (such as LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (such as LiCoO2, etc.), lithium-nickel-based oxides (such as LiNiO2, etc.), lithium-nickel-manganese-based oxides (such as Li 1+x1 Ni 1-y1 Mn y1 O2 (where -0.5 ≦ x1 ≦ 0.5, 0 < y1 < 1), Li 1+x2 Mn 2-z2 Ni z2 O4 (where -0.5 ≦ x2 ≦ 0.5, 0 < z2 < 2, etc.), lithium-nickel-cobalt-based oxides (such as Li 1+x3 Ni 1-Y1 Co Y1 O2 (where -0.5 ≦ x3 ≦ 0.5, 0 < Y1 < 1, etc.), lithium-manganese-cobalt-based oxides (such as Li 1+x4 Co 1-Y2 Mn Y2 O2 (where -0.5 ≦ x4 ≦ 0.5, 0 < Y2 < 1), Li 1+x5 Mn 2-Z1 Co Z1 O4 (where -0.5 ≦ x5 ≦ 0.5, 0 < Z1 < 2, etc.), lithium-nickel-manganese-cobalt-based oxides (such as 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.), and one or more of these compounds can be included.

[0107] The positive electrode active material is contained at 60 to 98% by weight, preferably 80 to 98% by weight, more preferably 90 to 98% by weight based on the total weight of the positive electrode active material layer.

[0108] The conductive material is a component for further improving the conductivity of the positive electrode active material. Such a conductive material is not particularly limited as long as it has conductivity without inducing chemical changes 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 crystal structure; conductive fibers such as carbon fibers and metal fibers; carbon fluoride powders; 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; conductive materials such as polyphenylene derivatives, etc. can be used.

[0109] The conductive material is contained at 0.1 to 20% by weight, specifically 0.5 to 10% by weight, more specifically 0.5 to 5% by weight based on the total weight of the positive electrode active material layer.

[0110] The binder is a component that helps bind the conductive material, the positive electrode active material, and the positive electrode current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polyethylene, polypropylene, ethylene - propylene - diene monomer, sulfonated ethylene - propylene - diene monomer, styrene - butadiene rubber, fluorine rubber, and various copolymers thereof.

[0111] Typically, the binder is included in an amount of 0.5 to 20% by weight, more specifically 0.5 to 10% by weight, and more specifically 0.5 to 5% by weight, based on the total weight of the positive electrode active material layer.

[0112] Furthermore, the aforementioned other substances may further include, for example, fillers as components that suppress expansion. The filler is not particularly limited as long as it can suppress the expansion of the electrodes without inducing a chemical change in the battery, and for example, orifine polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers; etc. can be used.

[0113] negative electrode The negative electrode may have a structure that includes a negative electrode current collector and a negative electrode active material layer formed on one or both sides of the negative electrode current collector.

[0114] Here, the negative electrode current collector is not particularly limited as long as it does not induce a chemical change in the battery and has high conductivity. 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 alloy can be used.

[0115] The negative electrode current collector typically has 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 various forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0116] The negative electrode active material layer comprises a negative electrode active material and may optionally include conductive materials, binders, and other materials as described for the positive electrode.

[0117] The negative electrode active material can be a compound capable of reversible intercalation and deintercalation of lithium, and examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metallic oxides capable of doping and dedoping with lithium such as SiOβ (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compounds and carbonaceous materials such as Si-C composites or Sn-C composites. One or more of these can be used, but more specifically, the negative electrode can contain lithium metal as the negative electrode active material, and more specifically, it can consist solely of lithium metal.

[0118] Alternatively, the negative electrode may consist only of lithium metal without a negative electrode current collector.

[0119] However, a structure that includes a separate current collector in addition to the lithium metal used as the negative electrode active material layer is even more preferable in terms of stability and structural integrity.

[0120] The negative electrode containing the lithium metal as the negative electrode active material may be formed on the negative electrode current collector by physically joining, rolling, or vapor deposition. The vapor deposition method can be either electrodeposition or chemical vapor deposition.

[0121] Here, the lithium metal may be an alloy that contains, in addition to lithium (Li), a metal selected from the group consisting of nickel (Ni), tin (Sn), copper (Cu), and indium (In).

[0122] The following description will refer to an example to demonstrate that the lithium secondary battery according to one embodiment of the present invention exhibits improved effects.

[0123] <Example 1> A separation membrane laminate was manufactured by forming a buffer layer (thickness: 3 μm) on one surface of a polyolefin substrate (thickness: 15 micrometers).

[0124] Here, the buffer layer is Li 1.3 Al 0.3 Ti 1.7 (PO4)3:acrylic copolymer (CSB130, Toyoink Co., Ltd.) was mixed in a 95:5 ratio by weight and dispersed in acetone to produce a slurry. This slurry was then coated onto one surface of the polyolefin substrate and dried to produce the product.

[0125] The porosity of the buffer layer was 48.5%, and the total ionic conductivity of the polyolefin substrate and the buffer layer was 0.81 mS / cm.

[0126] A 15-micrometer thick aluminum (Al) metal thin film was prepared as the positive electrode current collector. A positive electrode slurry was manufactured by dispersing LiFePO4 / C as the positive electrode active material, carbon black as the conductive material, and PVDF as the binder in an NMP solvent in a weight ratio of 961:3 on one side of the aluminum metal thin film, coating the slurry to a thickness of 60 micrometers, drying it, and rolling it to produce the positive electrode.

[0127] An electrode assembly was manufactured by interposing the separation film laminate between the positive electrode and a lithium foil (thickness: 45 μm) as the negative electrode, with the buffer layer facing the positive electrode. This electrode assembly was then placed in a case along with an electrolyte prepared by dissolving LiFSI in a 1 M:1.2 M ratio of LiFSI to ethylene glycol dimethyl ether solvent, and adding TTE to make up 70% of the total weight of the electrolyte.

[0128] <Example 2> A coin half-cell was manufactured in the same manner as in Example 1, except that the electrolyte used was one in which LiFSI was dissolved in FEC:DMC:OTE solvent in a ratio of 16:64:20 (v / v) at a molar concentration of 2.5 m.

[0129] <Comparative Example 1> A coin half-cell was manufactured in the same manner as in Example 1, except that a polyolefin substrate (thickness: 16 micrometers) was prepared and used instead of the separation membrane laminate in Example 1.

[0130] <Comparative Example 2> A coin half-cell was manufactured in the same manner as in Example 1, except that the buffer layer was interposed in the separation membrane laminate manufactured in Example 1 so as to face the negative electrode when manufacturing the electrode assembly.

[0131] <Comparative Example 3> Coin half-cells were manufactured in the same manner as in Example 2, using an electrolyte composition of 1 M LiPF6 dissolved in a solvent EC:DEC = 1:1 (v / v) with 1 wt% of the additive VC added.

[0132] <Comparative Example 4> Coin half-cells were manufactured in the same manner as in Example 2, except that the electrolyte composition used was an electrolyte prepared by adding 1 wt% of VC and 10 wt% of FEC to a solvent EC:DEC=1:1 (v / v) in which 1 M LiPF6 was dissolved.

[0133] <Experimental Example 1> The coin half-cells manufactured in Examples 1 and 2 were aged at 25°C for 12 hours, charged at the same temperature with a constant current of 0.1C until they reached 3.7V, and then fully discharged at 25°C with a constant current of 0.1C for two cycles of activation. After that, they were charged at 25°C with a constant current of 0.5C until they reached 3.7V, and the process of discharging at 0.5C was repeated until the initial discharge capacity reached 70%.

[0134] Then, a cross-sectional SEM image of the cathode after cycling in Example 1 was taken, and the results are shown in Figure 2. In addition, EDS analysis was performed on the cathode cross-section to analyze the surface elements, and the results are shown in Figure 3 below.

[0135] Referring to Figure 2, it can be confirmed that the positive electrode surface characteristics of the coin half-cell according to the present invention change after cycling.

[0136] Specifically, the cathode is formed from the bottom layer in Figure 2, consisting of a positive electrode, a protective film, and a buffer layer. That is, in the positive electrode according to the example, it can be confirmed that the buffer layer (upper layer in cross-section) and the protective film (black indicated area between the upper protective film and the lower positive electrode) are formed with a thickness of 1 μm or less, and referring to Figure 3, it can be confirmed that the fluorine element content is very high in the protective film portion.

[0137] Therefore, it can be confirmed that, in the case of the present invention, the generation of by-products at the positive electrode interface is suppressed.

[0138] <Experimental Example 2> Using the coin half-cells of Example 1, Comparative Examples 1 and 2, the cycle was repeated as in Experimental Example 1 until the discharge capacity reached 70% of the initial discharge capacity. The lifetime retention rate of the nth discharge capacity relative to the first discharge capacity was measured, and the results are shown in Figure 4 below.

[0139] Referring to Figure 4, it can be confirmed that the present invention exhibits excellent lifespan characteristics.

[0140] <Experimental Example 3> Using the coin half-cells of Example 2, Comparative Examples 3 and 4, the cycle was repeated as in Experimental Example 1 until the discharge capacity reached 70% of the initial discharge capacity. The lifetime retention rate of the nth discharge capacity relative to the first discharge capacity was measured, and the results are shown in Figure 5 below.

[0141] Referring to Figure 5, it can be confirmed that the present invention exhibits excellent life characteristics.

[0142] Anyone with ordinary skill in the field to which this invention belongs will be able to make various applications and modifications within the scope of this invention based on the above content. [Industrial applicability]

[0143] The lithium secondary battery according to the present invention includes a buffer layer containing an oxide-based solid electrolyte between the positive electrode and the separation membrane, and a protective layer containing a reactant formed by the reaction between the oxide-based solid electrolyte and the electrolyte between the positive electrode and the separation membrane. This improves the physical properties of the separation membrane, suppresses the generation of by-products at the positive electrode interface, prevents increases in resistance and overvoltage, and improves sustained capacity development and lifespan.

Claims

1. An electrode assembly including a positive electrode, a negative electrode, and a separator membrane, and The electrolyte comprises a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent. The positive electrode and the negative electrode face each other across the separation membrane, A lithium secondary battery comprising a buffer layer containing an oxide-based solid electrolyte and a protective layer containing a reaction substance between the oxide-based solid electrolyte and the separation membrane.

2. The lithium secondary battery according to claim 1, wherein the oxide-based solid electrolyte comprises LATP represented by the following chemical formula 1: [Chemical formula 1] Li １＋ｘ Al ｘ Today ２－ｘ (PO ４ ) ３ In the above chemical formula 1, 0 < x < 0.

5.

3. The oxide-based solid electrolyte is Li １．３ Al ０．３ Ti １．７ (PO ４ ) ３ The lithium secondary battery according to claim 2, which contains (LATP).

4. The lithium secondary battery according to claim 1, wherein the buffer layer further comprises a binder.

5. The lithium secondary battery according to claim 1, wherein the thickness of the buffer layer is 0.1 μm to 10 μm.

6. The lithium secondary battery according to claim 1, wherein the ionic conductivity of the separation membrane and the buffer layer is 0.5 mS / cm to 5 mS / cm.

7. The lithium secondary battery according to claim 1, wherein the protective layer is provided between the buffer layer and the positive electrode.

8. The lithium secondary battery according to claim 1, wherein the protective layer has a gradient such that the concentration of the substance originating from the oxide-based solid electrolyte in the reactant decreases as it moves from the buffer layer to the positive electrode, with respect to the thickness direction.

9. The lithium secondary battery according to claim 1, wherein the thickness of the protective layer is 0.01 μm to 1 μm or less.

10. The lithium secondary battery according to claim 1, wherein the protective layer contains 70% to 90% by weight of element F.

11. The lithium salt of the electrolyte is LiFSI (Lithium bis(fluorosulfonyl)imide), LiN(SO4). ２ F) ２ ), LiBETI (lithium bis(perfluoroethanesulfonyl)imide), LiN(SO ２ CF ２ CF ３ ) ２ and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiN(SO ２ CF ３ ) ２ The lithium secondary battery according to claim 1, which is one or more imide-based lithium salts selected from the group consisting of ).

12. The fluorinated non-solvents are TFEE (Ethylene glycol bis(1,1,2,2-tetrafluoroethylene)ethyl), TTE (1,1,2,2-tetrafluoroethylene-2,2,3,3-tetrafluoropropylethylene), DFB (1,2-difluorobenzone), BTFE (bis(2,2,2-trifluoroethylene)ethyl), and OTE (1H,1H,5H-Octafluoropentyl1,1,2,2-tetrafluoroethylene). The lithium secondary battery according to claim 1, wherein one or more are selected from the group consisting of ether, and 1,1,2,2-tetrafluoroethylene(2,2,2-trifluoroethylene(ether)).

13. The lithium secondary battery according to claim 1, wherein the non-aqueous organic solvent contains 50% by volume or more of an ether-based organic solvent based on the total volume of the non-aqueous organic solvent.

14. The lithium secondary battery according to claim 1, wherein the positive electrode comprises lithium iron phosphorus oxide represented by the following chemical formula 2 or lithium transition metal oxide represented by the following chemical formula 3 as the positive electrode active material: [Chemical formula 2] Li １＋ａ Fe １－ｙ M ｙ (PO ４－ｂ )X ｂ In the above chemical formula 2, M is one or more selected from Al, Mg, and Ti, and -0.5 ≤ a ≤ 0.5, 0 ≤ y ≤ 0.5, 0 ≤ b ≤ 0.

1. [Chemical formula 3] Li １＋ａ’ Ni １－ｙ’－ｚ’－ｓ Co ｙ’ Mn ｚ’ M' ｓ O ２－ｂ’ X ｂ’ In the above chemical formula 3, M' is one or more selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and -0.5 ≤ a' ≤ 0.5, 0 < y' < 1, 0 < z' < 1, 0 ≤ s < 0.2, 0 ≤ b' ≤ 0.

1. In the chemical formulas 2 and 3, X is one or more selected from F, S, and N.

15. The lithium secondary battery according to claim 1, wherein the negative electrode contains lithium metal as the negative electrode active material.

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