Lithium secondary battery comprising buffer layer between positive electrode and separator

Abstract

According to one embodiment of the present invention, provided is a lithium secondary battery comprising: an electrode assembly including a positive electrode, a negative electrode, and a separator; and an electrolyte including a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent, wherein the positive electrode and the negative electrode face each other with the separator interposed therebetween, and a buffer layer including an oxide-based solid electrolyte and a protective layer including a reaction product of the oxide-based solid electrolyte and the fluorinated non-solvent are formed between the positive electrode and the separator.

Description

Lithium secondary battery including a buffer layer between the cathode and the separator

[0001] Cross-citation with related application(s)

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2024-0030959, filed March 4, 2024, and Korean Patent Application No. 10-2025-0027002, filed February 28, 2025, the entire contents of which are incorporated herein by reference.

[0003] The present invention relates to a lithium secondary battery including a buffer layer between a cathode and a separator.

[0004] Due to the rapid increase in fossil fuel use, the demand for alternative and clean energy is increasing, and as part of this, the most actively researched field is the field of power generation and storage using electrochemistry.

[0005] A representative example of an electrochemical device that currently utilizes this type of electrochemical energy is the secondary battery, and its application area is gradually expanding.

[0006] Recently, with the increase in 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 been rapidly increasing. Among them, much research has been conducted on lithium secondary batteries that exhibit high energy density and operating potential, long cycle life, and low self-discharge rate, and they have also been commercialized and widely used.

[0007] Furthermore, with growing concern about environmental issues, research is being conducted on electric vehicles and hybrid electric vehicles as alternatives to fossil fuel-powered vehicles like gasoline and diesel, a major source of air pollution. While nickel-metal hydride secondary batteries are primarily used as power sources for these electric and hybrid electric vehicles, research into the use of lithium secondary batteries, which boast high energy density and discharge voltage, is actively underway, and some are nearing commercialization.

[0008] Typically, lithium secondary batteries are structured to have an electrode assembly comprising a positive electrode, a negative electrode, and a porous separator, each impregnated with a non-aqueous liquid electrolyte. Furthermore, the positive electrode is typically manufactured by coating a positive electrode mixture containing a positive electrode active material onto aluminum foil, while the negative electrode is typically manufactured by coating a negative electrode mixture containing a negative electrode active material onto copper foil.

[0009] Typically, the positive electrode active material is a lithium transition metal oxide, and the negative electrode active material is a carbon-based material.

[0010] However, as these liquid electrolyte-based lithium secondary batteries undergo repeated charge and discharge cycles, by-products are generated at the interface of the positive electrode due to the continuous decomposition of the electrolyte, and these by-products lower the output of the electrode, causing a decrease in capacity.

[0011] Additionally, capacity fading is directly related to the overall capacity of the cell and can occur catastrophically when the anode interface degrades, which can be caused by increased cell resistance and overvoltage due to byproducts generated at the anode interface.

[0012] Therefore, in order to continuously increase capacity and extend the lifespan of lithium secondary batteries, it is necessary to improve the stability of the interface between the cathode and the electrolyte.

[0013] The present invention aims to provide a lithium secondary battery capable of continuously expressing capacity and improving lifespan by improving the properties of a separator and suppressing the generation of electrode byproducts due to stabilization of the positive electrode interface.

[0014] According to one embodiment of the present invention,

[0015] An electrode assembly including an anode, a cathode, and a separator, and

[0016] An electrolyte comprising a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent,

[0017] The above anode and the above cathode face each other with the above separator in between,

[0018] A lithium secondary battery is provided, in which a buffer layer including an oxide-based solid electrolyte and a protective layer including a reaction material of the oxide-based solid electrolyte and the fluorinated non-solvent are formed between the positive electrode and the separator.

[0019] Here, the oxide-based solid electrolyte may include LATP represented by the following chemical formula 1.

[0020] [Chemical Formula 1]

[0021] Li 1+x Al x Ti 2-x (PO4)3

[0022] In the above chemical formula 1, 0 < x <0.5.

[0023] More specifically, the oxide-based solid electrolyte comprises Li 1.3 Al 0.3 Ti 1.7 May contain (PO4)3(LATP).

[0024] Additionally, the buffer layer may further include a binder.

[0025] The thickness of the above buffer layer may be 0.1 μm to 10 μm.

[0026] The total ionic conductivity of the above membrane and buffer layer may be 0.1 mS / cm to 5 mS / cm.

[0027] Meanwhile, the protective layer can be formed between the buffer layer and the anode.

[0028] Specifically, the protective layer may have a gradient in which the concentration of a substance derived from the oxide-based solid electrolyte in the reactant decreases from the buffer layer toward the anode in the thickness direction.

[0029] The thickness of the above protective layer may be 0.01 μm to 1 μm.

[0030] Additionally, the protective layer may contain an F element content of 70% to 90% by weight.

[0031] Meanwhile, the lithium salt of the electrolyte may be at least one imide-based lithium salt 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).

[0032] In addition, the fluorinated non-solvent may be at least one 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.

[0033] The above non-aqueous organic solvent may contain an ether-based organic solvent in an amount of 50% by volume or more based on the total volume of the non-aqueous organic solvent.

[0034] The above positive electrode may include lithium iron phosphate represented by the following chemical formula 2 or lithium transition metal oxide represented by the following chemical formula 3 as a positive electrode active material.

[0035] [Chemical Formula 2]

[0036] Li 1+a Fe 1-y M y (PO 4-b )X b

[0037] In the above chemical formula 2, M is at least one selected from Al, Mg, and Ti, and -0.5≤a≤0.5, 0≤y≤0.5, 0≤b≤0.1,

[0038] [Chemical Formula 3]

[0039] Li 1+a’ Ni 1-y’-z’-s Co y’ Mn z’ M' s O 2-b’ X b’

[0040] In the above chemical formula 3, M' is at least one 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’≤0.1이며,

[0041] In the above chemical formulas 2 and 3, X is at least one selected from F, S, and N.

[0042] Furthermore, the negative electrode may include lithium metal as a negative electrode active material.

[0043] Additionally, the separator may be a polyolefin-based substrate.

[0044] Figure 1 is a cross-sectional schematic diagram of a lithium secondary battery according to one embodiment of the present invention.

[0045] Figure 2 is a cross-sectional SEM photograph of the anode after a cycle of Example 1 according to Experimental Example 1.

[0046] Figure 3 is a cross-section of the anode and an elemental analysis photograph after a cycle of Example 1 according to Experimental Example 1.

[0047] Figure 4 is a life characteristics comparison graph according to Experimental Example 2.

[0048] Figure 5 is a life characteristics comparison graph according to Experimental Example 3.

[0049] Hereinafter, the terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, and should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0050] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in their common sense to those of ordinary skill in the art to which the present invention pertains. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless explicitly and specifically defined otherwise.

[0051] The terminology used herein is for the purpose of describing embodiments and is not intended to limit the present invention. In this specification, singular forms also include plural forms, unless specifically stated otherwise. As used herein, the terms "comprises" and / or "comprising" do not exclude the presence or addition of one or more other components in addition to the components mentioned.

[0052] Meanwhile, the terms “consists of” and / or “consisting of” used in the specification mean that other components are not included in amounts greater than trace amounts, i.e., impurities, other than the components mentioned.

[0053] In this specification, "average particle diameter D50" means the particle size at 50% of the volume cumulative particle size distribution of the target particle powder (e.g., positive electrode active material powder, negative electrode active material powder, etc.). The average particle diameter D50 can be measured using a laser diffraction method. For example, the powder of the particles to be measured is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), and then ultrasonic waves of about 28 kHz are irradiated at an output of 60 W, and then a volume cumulative particle size distribution graph is obtained, and then the particle size corresponding to 50% of the volume cumulative amount is measured.

[0054] In this specification, “ionic conductivity” is a value calculated based on the electrochemical impedance measured using an analysis device (VMP3, Bio logic science instrument) at 60°C with an amplitude of 10 mV and a scan range of 500 kHz to 20 MHz.

[0055] In this specification, “elemental analysis” means that when shooting in back scattered electron image mode in which brightness and darkness differ depending on the difference in particle weight, it is possible to distinguish materials from bright and dark areas in the image, and the element content can be analyzed using map spectrum (EDS analysis method).

[0056] In this specification, “porosity” can be obtained by estimating the volume from the area of ​​surface pores confirmed in a randomly sampled range (10 um or more in width and 15 um or more in height) within the measured photograph after measuring the sample surface at a magnification of 2,500 times using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope).

[0057]

[0058] A lithium secondary battery according to one embodiment of the present invention,

[0059] An electrode assembly including an anode, a cathode, and a separator, and

[0060] An electrolyte comprising a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent,

[0061] The above anode and cathode face each other with a separator between them,

[0062] It is characterized in that a buffer layer including an oxide-based solid electrolyte and a protective layer including a reaction material of the oxide-based solid electrolyte and the fluorinated non-solvent are formed between the anode and the separator.

[0063] At this time, in order to explain the structure of the lithium secondary battery, a cross-sectional schematic diagram of the lithium secondary battery according to the present invention is shown in Fig. 1 below.

[0064] Referring to FIG. 1, a lithium secondary battery (100) includes an electrode assembly including a positive electrode (110), a negative electrode (120), and a separator (130), and an electrolyte (140) including a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent.

[0065] At this time, the anode (110) and the cathode (120) face each other with a separator (130) in between, and a buffer layer (131) including an oxide-based solid electrolyte and a protective layer (132) including a reaction material in which the oxide-based solid electrolyte and a fluorinated non-solvent react with each other are formed between the anode (110) and the separator (130).

[0066] Here, the protective layer (132) is located between the anode (110) and the buffer layer (131).

[0067] Hereinafter, each component will be described in more detail.

[0068]

[0069] membrane

[0070] The separator is interposed between the positive electrode and the negative electrode, and separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without special restrictions, and in particular, one having low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may also be used.

[0071]

[0072] buffer layer

[0073] The above buffer layer may be formed between the anode and the separator.

[0074] At this time, the buffer layer may be formed separately, formed on the anode, or formed on the separator, but more specifically, it may be formed on the separator for the purposes of ease of manufacturing, reduction of side reactions, and improvement of the properties of the separator.

[0075] Specifically, it can be manufactured by coating a slurry containing the oxide-based solid electrolyte on the anode or the separator and drying it.

[0076] Here, the oxide-based solid electrolyte included in the buffer layer is not limited, and may further include one or more lithium metal oxides or lithium metal phosphates selected from a Nasicon-type solid electrolyte, a Lisicon-type solid electrolyte, a Garnet-type solid electrolyte, a Perovskite-type solid electrolyte, and a LiPON-type solid electrolyte, and more specific examples thereof 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. However, it may include LATP represented by the following chemical formula 1 in detail, and more specifically, Li 1.3 Al 0.3 Ti 1.7 May contain (PO4)3(LATP).

[0077] [Chemical Formula 1]

[0078] Li 1+x Al x Ti 2-x (PO4)3

[0079] In the above chemical formula 1, 0 < x <0.5.

[0080] Specifically, based on the total weight of the oxide-based solid electrolyte, the LATP may be included in an amount of 80 wt% to 100 wt%, and in detail, may be included in an amount of 90 wt% to 100 wt%.

[0081] Since the oxide-based solid electrolyte has excellent atmospheric stability against the intrusion of surrounding oxygen and moisture, it can be expected to have an effect of suppressing byproducts at the anode interface. Among the materials, the LATP has relatively excellent ionic conductivity, and is therefore effective for application to the present invention.

[0082] The average diameter (D50) of the above oxide-based solid electrolyte particles may be 50 nanometers to 10 micrometers, specifically 50 nanometers to 5 micrometers, and more specifically 50 nanometers to 1 micrometer.

[0083] If the particle size is too small outside the above range, agglomeration between particles may occur due to reduced dispersibility. Conversely, if the particle size is too large, large pores are formed by the oxide-based solid electrolyte, which is rather unfavorable in terms of resistance. In other words, if the particle size satisfies the above range, lithium ion conductivity can be increased, resistance can be reduced, and improved lithium secondary battery performance can be achieved.

[0084] Additionally, the buffer layer may further include a binder for binding the oxide-based solid electrolyte to form a solid layer when the oxide-based solid electrolyte is coated and dried on the separator.

[0085] Here, the binder is not limited as long as it does not cause a side reaction with the electrolyte, but in particular, one having a glass transition temperature (Tg) as low as possible can be used, preferably in the range of -200 to 200°C.

[0086] In addition, the binder does not necessarily need to have ion conducting ability, but it is more preferable to use a polymer having ion conducting ability.

[0087] Therefore, it is preferable that the binder have a high dielectric constant as much as possible, and since 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 is preferably 1 or more, specifically, a range of 1.0 to 100 (measurement frequency = 1 kHz), and is particularly preferably 10 or more.

[0088] In addition to the aforementioned functions, the binder may have the characteristic of being gelled when impregnated with a liquid electrolyte, thereby exhibiting a high degree of swelling. In fact, if the binder is a polymer having an excellent electrolyte swelling rate, the electrolyte injected after battery assembly will permeate into the polymer, and the polymer containing the absorbed electrolyte will have electrolyte ion conducting ability. Therefore, if possible, the solubility index should be in the range of 15 to 45 MPa. 1 / 2 Polymers are preferred, with a viscosity of 15 to 25 MPa. 1 / 2 and 30 to 45 MPa 1 / 2 The range is more desirable. The solubility index is 15 MPa. 1 / 2 Less than and 45 MPa 1 / 2 If it exceeds , it becomes difficult to be impregnated (swelled) by a conventional battery liquid electrolyte.

[0089] Examples of such binders include polyvinylidene fluorideco-hexafluoropropylene, polyvinylidene fluoride-cotrichloroethylene, polyacrylate, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyimide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, It may be at least one selected from the group consisting of cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, and polyvinylalcohol.

[0090] At this time, the oxide-based solid electrolyte may be included in an amount of 70 wt% to 99 wt%, specifically 80 wt% to 99 wt%, based on the total weight of the buffer layer, and the binder may be included in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 20 wt%.

[0091] If the content of the oxide-based solid electrolyte is too low outside the above range, sufficient reduction intended by the present invention cannot be obtained, and if the content is too high, the content of the binder connecting them is too low, which may result in a decrease in the mechanical properties due to weakened adhesive force between the particles, which is not preferable.

[0092] A buffer layer including the above-described oxide-based solid electrolyte is formed between the anode and the separator. In other words, after the buffer layer is formed on the separator, the electrode assembly can be assembled so that the buffer layer faces the anode. When the buffer layer is formed on the anode, the electrode assembly can be assembled by forming it on the anode and then laminating it with the cathode with the separator interposed therebetween.

[0093] This is also to prevent the problem of side reactions occurring at the anode interface due to the reaction between the anode and the electrolyte, and thus can function as a side reaction prevention layer. In addition, if the oxide-based solid electrolyte comes into direct contact with the anode, especially in the case of a lithium metal anode, it may react with lithium and cause damage to the anode, which may actually deteriorate the cell performance of the lithium secondary battery. Therefore, it should be formed only in the direction facing the anode.

[0094] The thickness of the buffer layer may be 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.

[0095] If it is too thin beyond the above range, not only will the effect of forming the buffer layer not be sufficiently obtained, but it will also not be able to perform the role of preventing shrinkage of the separator, which will lower safety. If it is too thick, resistance may actually increase, which is not desirable.

[0096] Furthermore, the total ionic conductivity of the separator having the buffer layer formed thereon, i.e., the total ionic conductivity of the separator and the buffer layer, can be measured using electrochemical impedance spectroscopy as is known to those skilled in the art, and the ionic conductivity may be from 0.1 mS / cm to 5 mS / cm. Specifically, it may be 0.5 mS / cm or more, 0.7 mS / cm or more, or 1 mS / cm or more, and 4 mS / cm or less.

[0097] If the total ionic conductivity of the separator and the buffer layer is too low outside the above range, the resistance of the lithium secondary battery may increase, thereby deteriorating battery performance. In the case of including the oxide-based solid electrolyte, the ionic conductivity may be within the above range.

[0098] The porosity of the above buffer layer may be 20% to 70% by volume based on the total volume of the buffer layer, specifically 30% to 70% by volume, and more specifically 40% to 60% by volume.

[0099] Outside the above range, if the porosity is too large, the effect of protecting the positive electrode interface is reduced, and if it is too small, the buffer layer is not sufficiently impregnated with electrolyte, which may reduce the mobility of lithium ions, which is not desirable.

[0100]

[0101] protective layer

[0102] As shown in the following drawing 1, and as described above, the protective layer is formed between the buffer layer and the anode.

[0103] This protective layer is not formed through separate manufacturing, but is a layer formed by the reaction between a specific electrolyte described below and the oxide-based solid electrolyte during activation, i.e., at the beginning of the cycle.

[0104] More specifically, it is formed by the reaction of a specific substance included in the electrolyte, specifically a fluorinated non-solvent, and the oxide-based solid electrolyte as described below, and includes such a reaction substance.

[0105] Accordingly, the protective layer may have a gradient in which the concentration of a substance derived from the oxide-based solid electrolyte in the reaction material decreases from the buffer layer toward the anode in the thickness direction.

[0106] Such a protective layer can more effectively suppress the by-product generation reaction at the anode due to electrolyte oxidation at high voltage, can suppress the dissolution of the anode active material, and can also improve the output characteristics by favoring the diffusion of lithium ions at high current density with the above concentration gradient.

[0107] Since the formation of this protective layer is formed by the reaction of a specific substance in the electrolyte with the oxide-based solid electrolyte, the substance formed is determined depending on the type of these substances and the oxide-based solid electrolyte.

[0108] In addition, for this reaction, a fluorinated non-solvent must be included in the electrolyte. Otherwise, if a solvent containing the element F is reacted in the electrolyte solvent, the life characteristics may deteriorate due to electrolyte depletion, which is undesirable.

[0109] The protective layer formed in this manner may contain an F element content of 70 wt% or more, and more specifically, the F element content may be contained in a range of 70 wt% to 90 wt%, and more specifically, 75 wt% to 80 wt%.

[0110] When the fluorine content is within the above range, it can have the same effect as above and act as a protective layer.

[0111] More specifically, the lithium fluoride-based material, more specifically, a lithium fluoride-based polymer having a skeleton of a fluorinated nonsolvent, may be included as a result of a reaction between Li derived from the oxide-based solid electrolyte and F derived from the fluorinated nonsolvent.

[0112] The thickness of such a protective layer may be 0.001 ㎛ to 1 ㎛, and more specifically, it is preferably 0.01 ㎛ or more, 0.05 ㎛ or more, or 0.1 ㎛ or more, and preferably 0.9 ㎛ or less, 0.8 ㎛ or less, 0.7 ㎛ or less, or 0.5 ㎛ or less.

[0113] If it is too thin beyond the above range, the effect of suppressing the dissolution of the positive electrode active material and enhancing lithium ion diffusion due to the formation of the protective layer cannot be obtained, and if it is too thick, the resistance may rather increase, which is not desirable.

[0114] The porosity of the above protective layer may be 30% to 70% by volume based on the total volume of the protective layer, specifically 20% to 60% by volume, and more specifically 30% to 40% by volume.

[0115] Beyond the above range, if the porosity is too large, the effect of protecting the positive electrode interface is reduced, and if it is too small, the mobility of lithium ions may be reduced, which is not desirable.

[0116] When the protective layer is formed, the thickness ratio of the buffer layer and the protective layer may be 1 / 3 to 1 / 100, specifically 1 / 3 to 1 / 50, and more specifically 1 / 5 to 1 / 20.

[0117]

[0118] electrolyte

[0119] Meanwhile, the electrolyte included in the lithium secondary battery according to the present invention must form the protective layer by reacting with the oxide-based solid electrolyte as described above, and therefore includes a fluorinated non-solvent in addition to a lithium salt and a non-aqueous organic solvent.

[0120] Here, the lithium salt is used as a medium for transferring ions within a lithium secondary battery. The lithium salt may be, for example, Li as a cation. + , and the anion is 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 selected from the group consisting of

[0121] Specifically, the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10 , 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) may include a single substance or a mixture of two or more substances selected from the group consisting of, but in terms of excellent stability, imide-based lithium salts, i.e., LiFSI (lithium bis(fluorosulfonyl) imide), LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl) imide), LiN(SO2CF2CF3)2 and LiTFSI (lithium bis(trifluoromethanesulfonyl) It is preferable to include at least one selected from the group consisting of LiN(SO2CF3)2).

[0122] In addition to these, lithium salts commonly used in the electrolyte of lithium secondary batteries can be used without limitation.

[0123] The lithium salt may be appropriately changed within a generally usable range, but in order to obtain an optimal effect of forming a film for preventing corrosion on the electrode surface, it may be included in the electrolyte at a concentration of 0.5 M to 3 M, specifically, at a concentration of 1 M to 2.5 M, and more specifically, at a concentration of 1 M to 2 M. When the concentration of the lithium salt satisfies the above range, the effect of improving the cycle characteristics during high-temperature storage of the lithium secondary battery is sufficient, and the viscosity of the electrolyte is appropriate, so that the electrolyte impregnation property can be improved.

[0124] The above non-aqueous organic solvent is not limited as long as it minimizes decomposition due to oxidation reactions, etc. in the voltage range of the charge / discharge process of a lithium secondary battery, and can exhibit properties together with the material. For example, a carbonate-based organic solvent, an ether-based organic solvent, or an ester-based organic solvent may be used alone or in a mixture of two or more thereof. Specifically, the ether-based organic solvent may be primarily used in consideration of the stability of the fluorinated non-solvent. At this time, the ether-based organic solvent may be included in an amount of about 50 vol% or more, specifically 60 vol%, 70 vol%, or 80 vol% or more, and may be included in an amount of 100 vol% or less, or 90 vol% or less, based on the total non-aqueous organic solvent.

[0125] Additionally, the non-aqueous organic solvent may be included in an amount of 20 wt% to 70 wt% based on the total weight of the electrolyte.

[0126] Among the organic solvents, the carbonate-based organic solvent may include at least one of a cyclic carbonate-based organic solvent and a linear carbonate-based organic solvent. 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), and specifically, may include a mixed solvent of ethylene carbonate having a high dielectric constant and propylene carbonate having a relatively low melting point compared to ethylene carbonate.

[0127] In addition, the linear carbonate-based organic solvent is a solvent having low viscosity and low dielectric constant, and may include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate, and more specifically, may include dimethyl carbonate.

[0128] The above ether organic solvent may be any 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 thereof, but is not limited thereto.

[0129] The above ester organic solvent may include at least one selected from the group consisting of linear ester organic solvents and cyclic ester organic solvents.

[0130] Specific examples of the linear ester organic solvent 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 thereof.

[0131] The above cyclic ester organic solvent may be, as a specific example, 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 thereof, but is not limited thereto.

[0132] Among the above ester solvents, cyclic carbonate compounds are preferably used as high-viscosity organic solvents with high dielectric constants, which facilitate the dissociation of lithium salts in the electrolyte. When low-viscosity, low-dielectric constant linear carbonate compounds and linear ester compounds, such as dimethyl carbonate and diethyl carbonate, are mixed and used in an appropriate ratio with these cyclic carbonate compounds, an electrolyte with high electrical conductivity can be produced, and thus the compounds can be used more preferably.

[0133] The above fluorinated non-solvent may be a material that reacts with the oxide-based solid electrolyte to form a protective layer, thereby exhibiting effects of reducing the dissolution of positive electrode active material, suppressing the formation of interfacial side reactions, and improving lithium ion mobility at high current density. For example, the fluorinated non-solvent may be at least one 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 specifically, may be TTE.

[0134] The above-mentioned fluorinated non-solvent may be included in an amount of 70 wt% or less, specifically 40 wt% to 70 wt%, based on the total weight of the electrolyte. If the content of the above-mentioned substance exceeds 70 wt%, the lithium salt may not be sufficiently dissolved in the electrolyte, resulting in precipitation of the lithium salt. Consequently, side reactions that reduce the lifespan or resistance characteristics of the lithium secondary battery may occur.

[0135]

[0136] anode

[0137] The above positive electrode may have a structure 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.

[0138] Here, the positive electrode current collector may be any conductive material that does not induce chemical changes in the battery, and is not particularly limited. For example, the current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like.

[0139] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector to enhance adhesion to the positive electrode active material layer. The positive electrode current collector may be used in various forms, such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabrics.

[0140] The above positive electrode active material layer includes a positive electrode active material and may include a conductive material, a binder, and other materials as needed.

[0141] The above positive electrode active material is not limited to a compound capable of reversible intercalation and deintercalation of lithium, but specifically may include a lithium iron phosphate represented by the following chemical formula 2 or a lithium transition metal oxide represented by the following chemical formula 3.

[0142] [Chemical Formula 2]

[0143] Li 1+a Fe 1-y M y (PO 4-b )X b

[0144] In the above chemical formula 2, M is at least one selected from Al, Mg, and Ti, and -0.5≤a≤0.5, 0≤y≤0.5, 0≤b≤0.1,

[0145] [Chemical Formula 3]

[0146] Li 1+a’ Ni 1-y’-z’-s Co y’ Mn z’ M' s O 2-b’ X b’

[0147] In the above chemical formula 3, M' is at least one 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’≤0.1이며,

[0148] In the above chemical formulas 2 and 3, X is at least one selected from F, S, and N.

[0149] These lithium iron phosphates or lithium transition metal oxides may be included in an amount of 80 wt% or more, specifically 90 wt% to 100 wt%, based on the total weight of the positive electrode active material.

[0150] In addition, the positive electrode active material is a lithium metal oxide, such as a lithium-manganese oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt oxide (e.g., LiCoO2, etc.), a lithium-nickel oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese oxide (e.g., 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 oxides (e.g., Li 1+x3 Ni 1-Y1 Co Y1 O2(where, -0.5≤x3≤0.5, 0 <Y1<1) 등), 리튬-망간-코발트계 산화물(예를 들면, 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 oxides (e.g., Li 1+a2 (Ni p1 Coq1 Mn r1 )O4(wherein, -0.5≤a2≤0.5, 0<p1<2, 0<q1<2, 0<r1<2, p1+q1+r1=2), etc.), and one or more compounds among these may be included.

[0151] The above positive electrode active material may be included in an amount of 60 to 98 wt%, preferably 80 to 98 wt%, and more preferably 90 to 98 wt%, based on the total weight of the positive electrode active material layer.

[0152] The conductive agent is a component for further improving the conductivity of the positive electrode active material, and the conductive agent is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and for example, carbon powder such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon powder; conductive powder such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used.

[0153] The above-mentioned conductive material may be included in an amount of 0.1 to 20 wt%, specifically 0.5 to 10 wt%, and more specifically 0.5 to 5 wt%, based on the total weight of the positive electrode active material layer.

[0154] The above binder is a component that assists in 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, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0155] Typically, the binder may be included in an amount of 0.5 to 20 wt%, specifically 0.5 to 10 wt%, and more specifically 0.5 to 5 wt%, based on the total weight of the positive electrode active material layer.

[0156] In addition, the above-mentioned other materials may further include, for example, fillers as components that suppress expansion. The fillers are not particularly limited as long as they can suppress expansion of the electrode without causing chemical changes in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fiber and carbon fiber; and the like.

[0157]

[0158] cathode

[0159] The above negative electrode may have a structure 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.

[0160] Here, the negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used.

[0161] The above-mentioned negative electrode collector may typically have a thickness of 3 μm to 500 μm, and like the positive electrode collector, it may be provided with fine irregularities on the surface of the negative electrode collector to enhance the bonding strength of the negative electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric.

[0162] The above negative electrode active material layer includes a negative electrode active material and, if necessary, may include a conductive material, a binder, and other materials as described in the positive electrode.

[0163] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium, and examples thereof 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 alloy, Sn alloy, or Al alloy; metallic oxides capable of doping and dedoping 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, and any one of these or a mixture of two or more thereof may be used. Specifically, the negative electrode may include lithium metal as the negative electrode active material, and more specifically, may be composed solely of lithium metal.

[0164] Alternatively, the negative electrode may be composed solely of lithium metal without a negative electrode current collector.

[0165] However, a structure including a separate current collector in addition to the lithium metal used as the negative electrode active material layer is more preferable in terms of stability and structural aspects.

[0166] The negative electrode comprising the lithium metal as a negative electrode active material can be formed by physically bonding, rolling, or depositing the negative electrode current collector. The deposition method can use an electrical deposition method or a chemical vapor deposition method.

[0167] Here, the lithium metal may include an alloy that partially contains, in addition to lithium (Li), one metal selected from the group consisting of nickel (Ni), tin (Sn), copper (Cu), and indium (In).

[0168]

[0169] Hereinafter, examples will be described to demonstrate that a lithium secondary battery according to one embodiment of the present invention exhibits improved effects.

[0170]

[0171] <Example 1>

[0172] A membrane laminate was manufactured by forming a buffer layer (thickness: 3 μm) on one side of a polyolefin material substrate (thickness: 15 μm).

[0173] Here, the buffer layer is Li 1.3 Al 0.3 Ti 1.7 (PO4)3: A slurry was prepared by mixing an acrylic copolymer (CSB130, Toyoink) in a weight ratio of 95:5 and dispersing it in acetone, and coating the slurry on one side of the polyolefin material substrate and drying it.

[0174] 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.

[0175] A 15 micrometer thick aluminum (Al) metal film was prepared as a positive electrode current collector, and a positive electrode slurry was prepared by dispersing LiFePO4 / C as a positive electrode active material, carbon black as a conductive material, and PVDF as a binder in a weight ratio of 96 1:3 in an NMP solvent on one surface of the aluminum metal film, and coating, drying, and rolling to a thickness of 60 micrometers to prepare a positive electrode.

[0176] An electrode assembly was manufactured by interposing the separator laminate between the positive electrode and a lithium foil (thickness: 45 μm) as the negative electrode, with the buffer layer facing the positive electrode, and the electrode assembly was manufactured by dissolving LiFSI in a 1M:1.2M ratio of LiFSI and ethylene glycol dimethyl ether solvent, and adding TTE to make 70% of the total weight of the electrolyte, and then placing the electrode assembly in a case to manufacture a coin half-cell.

[0177]

[0178] <Example 2>

[0179] A coin half-cell was manufactured as in Example 1, except that an electrolyte in which LiFSI was dissolved in a FEC: DMC: OTE solvent of 16:64:20 (v / v) to a molal concentration of 2.5 m was used as the electrolyte composition in Example 1.

[0180]

[0181] <Comparative Example 1>

[0182] A coin half-cell was manufactured in the same manner as in Example 1, except that a polyolefin material substrate (thickness: 16 micrometers) was prepared instead of the membrane laminate in Example 1 and used therein.

[0183]

[0184] <Comparative Example 2>

[0185] A coin half-cell was manufactured in the same manner as in Example 1, except that the electrode assembly was manufactured by interposing the buffer layer so that it faced the cathode in the membrane laminate manufactured in Example 1.

[0186]

[0187] <Comparative Example 3>

[0188] In the above Example 2, a coin half-cell was manufactured as in Example 2 using an electrolyte in which 1 wt% of the additive VC was added to a solvent EC: DEC = 1:1 (v / v) in which 1 M LiPF6 was dissolved as the electrolyte composition.

[0189]

[0190] <Comparative Example 4>

[0191] A coin half-cell was manufactured as in Example 2, except that an electrolyte containing 1 wt% of additives VC and 10 wt% of FEC was used in a solvent EC: DEC = 1:1 (v / v) in which 1 M LiPF6 was dissolved as the electrolyte composition in Example 2.

[0192]

[0193] Experimental Example 1

[0194] The coin half-cells manufactured in Examples 1 and 2 above were aged at 25°C for 12 hours, charged at the same temperature at a constant current of 0.1C until 3.7 V, and activated for 2 cycles by completely discharging at a constant current of 0.1C at 25°C. Afterwards, the process of charging at a constant current of 0.5C at 25°C until 3.7 V and discharging at 0.5C was repeated until the initial discharge capacity reached 70%.

[0195] And, after the cycle of Example 1, a cross-sectional SEM photograph of the anode was taken, and the results are shown in Fig. 2. In addition, EDS analysis was performed on the cross-section of the anode to analyze the surface elements, and the results are shown in Fig. 3 below.

[0196] Referring to FIG. 2, it can be confirmed that the positive electrode surface characteristics change after cycling of the coin half-cell according to the present invention.

[0197] Specifically, it is formed from the lower layer of Fig. 2 as an anode-protective film-buffer layer. That is, it can be confirmed that the anode according to the embodiment has a buffer layer (upper layer in the cross-section) and a protective film (black indicated area between the upper protective film and the lower anode) formed with a thickness of 1㎛ or less, and referring to Fig. 3, it can be confirmed that the content of the F element is very high in the protective film portion.

[0198] Accordingly, it can be confirmed that in the case of the present invention, the generation of by-products at the anode interface is suppressed.

[0199]

[0200] Experimental Example 2

[0201] Using the coin half-cells of Example 1, Comparative Examples 1 and 2, the same cycle as Experimental Example 1 was repeated until the initial discharge capacity reached 70%, and the life maintenance rate of the nth discharge capacity compared to the 1st discharge capacity was measured, and the results are shown in Figure 4 below.

[0202] Referring to Fig. 4, it can be confirmed that the present invention exhibits excellent life characteristics.

[0203]

[0204] Experimental Example 3

[0205] Using the coin half-cells of the above Example 2, Comparative Examples 3 and 4, the same cycle as Experimental Example 1 was repeated until the initial discharge capacity reached 70%, and the life maintenance rate of the nth discharge capacity compared to the 1st discharge capacity was measured, and the results are shown in Figure 5 below.

[0206] Referring to FIG. 5, it can be confirmed that the present invention exhibits excellent life characteristics.

[0207]

[0208] Anyone with ordinary skill in the art to which the present invention pertains will be able to make various applications and modifications within the scope of the present invention based on the above contents.

[0209] The lithium secondary battery according to the present invention includes a buffer layer including an oxide-based solid electrolyte between a cathode and a separator, and a protective layer including a reaction material formed by a reaction between the oxide-based solid electrolyte and the electrolyte between the cathode and the separator, thereby improving the properties of the separator, suppressing the generation of by-products at the cathode interface, and preventing an increase in resistance and overvoltage, thereby having the effect of improving continuous capacity expression and lifespan.

Claims

1. An electrode assembly including a positive electrode, a negative electrode, and a separator, and An electrolyte comprising a lithium salt, a non-aqueous organic solvent, and a fluorinated non-solvent, The above anode and cathode face each other with the separator between them, A lithium secondary battery, wherein a buffer layer including an oxide-based solid electrolyte and a protective layer including a reaction material of the oxide-based solid electrolyte and the fluorinated non-solvent are formed between the positive electrode and the separator.

2. In paragraph 1, The above oxide-based solid electrolyte is a lithium secondary battery including 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.

3. In paragraph 2, The above oxide-based solid electrolyte is Li 1.3 Al 0.3 Ti 1.7 A lithium secondary battery containing (PO4)3(LATP).

4. In paragraph 1, A lithium secondary battery wherein the buffer layer further comprises a binder.

5. In paragraph 1, A lithium secondary battery wherein the thickness of the buffer layer is 0.1㎛ to 10㎛.

6. In paragraph 1, A lithium secondary battery wherein the ionic conductivity of the separator and the buffer layer is 0.5 mS / cm to 5 mS / cm.

7. In paragraph 1, A lithium secondary battery wherein the protective layer is formed between the buffer layer and the positive electrode.

8. In paragraph 1, A lithium secondary battery in which the protective layer has a gradient in which the concentration of a substance derived from the oxide-based solid electrolyte in the reactant decreases from the buffer layer toward the positive electrode in the thickness direction.

9. In paragraph 1, A lithium secondary battery wherein the thickness of the protective layer is 0.01㎛ to 1㎛ or less.

10. In paragraph 1, A lithium secondary battery, wherein the protective layer contains 70 to 90 wt% of the F element.

11. In paragraph 1, A lithium secondary battery in which the lithium salt of the electrolyte is at least one imide-based lithium salt 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).

12. In paragraph 1, A lithium secondary battery, wherein the above fluorinated non-solvent is at least one 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.

13. In paragraph 1, A lithium secondary battery, wherein the non-aqueous organic solvent contains an ether-based organic solvent in an amount of 50% by volume or more based on the total volume of the non-aqueous organic solvent.

14. In paragraph 1, The above positive electrode is a lithium secondary battery including lithium iron phosphate represented by the following chemical formula 2 or 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 the above chemical formula 2, M is at least one 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’ Mr z’ M' s O 2-b’ X b’ In the above chemical formula 3, M' is at least one 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’≤0.1이며, In the above chemical formulas 2 and 3, X is at least one selected from F, S, and N.

15. In paragraph 1, The above negative electrode is a lithium secondary battery containing lithium metal as a negative electrode active material.

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