Additive for lithium secondary battery, electrolyte for lithium secondary battery containing the same, and lithium secondary battery

Abstract

The present invention provides an additive for lithium secondary batteries, which comprises a core containing polyethylene wax and a shell surrounding the core, the shell comprising a polymer having a melting point of 90°C to 120°C, an electrolyte for lithium secondary batteries containing the same, and a lithium secondary battery.

Description

[Technical Field]

[0001] The present invention relates to an additive for a lithium secondary battery, an electrolyte solution for a lithium secondary battery containing the additive, and a lithium secondary battery. [Background technology]

[0002] Lithium secondary batteries are rechargeable and have an energy density per unit weight that is more than three times higher than conventional lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, and nickel-zinc batteries, and they can be charged quickly. As a result, they have been commercialized for use in laptops, mobile phones, power tools, and electric bicycles, and research and development is actively underway to further improve their energy density.

[0003] Such a lithium secondary battery is used by injecting an electrolyte into a battery cell including a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium, and a negative electrode containing a negative electrode active material capable of intercalating and deintercalating lithium.

[0004] In particular, the electrolyte solution uses an organic solvent in which a lithium salt is dissolved, and such an electrolyte solution is important in determining the stability and performance of a lithium secondary battery.

[0005] LiPF6, the most commonly used lithium salt in electrolytes, has the problem of reacting with the organic solvent in the electrolyte, accelerating the depletion of the solvent and generating a large amount of gas. When LiPF6 decomposes, it produces LiF and PF5, which causes electrolyte depletion in the battery, resulting in deterioration of high-temperature performance and safety vulnerabilities.

[0006] Therefore, there is a demand for an electrolyte solution that does not deteriorate in performance even under high temperature conditions and has improved safety. Summary of the Invention [Problem to be solved by the invention]

[0007] One embodiment provides an additive for lithium secondary batteries with improved thermal stability.

[0008] Another embodiment provides an electrolyte for a lithium secondary battery having improved life characteristics, high-temperature safety, and high-temperature reliability by applying the additive.

[0009] Yet another embodiment provides a lithium secondary battery including the electrolyte solution for lithium secondary batteries. [Means for solving the problem]

[0010] An additive for a lithium secondary battery according to one embodiment includes a core containing a polyethylene wax and a shell surrounding the core, the shell including a polymer having a melting point of 90°C to 120°C.

[0011] The ratio of the thickness of the core to the thickness of the shell may be 1:1 to 4:1.

[0012] The core may have a thickness of 0.1 μm to 2.0 μm, and the shell may have a thickness of 0.025 μm to 0.5 μm.

[0013] The polymer may include poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), polyacrylic acid, polyethylene, poly(methyl methacrylate), polyalkylene oxide, polyalkylene succinate, or a combination thereof.

[0014] The additive may be in the form of a fiber formed using electrospinning.

[0015] According to another embodiment, there is provided an electrolyte solution for a lithium secondary battery, comprising a non-aqueous organic solvent, a lithium salt, and the additive for a lithium secondary battery.

[0016] The additive for lithium secondary batteries may be contained in an amount of 0.1 wt % to 20 wt %, 0.1 wt % to 15 wt %, or 0.1 wt % to 10 wt % relative to the total weight of the electrolyte solution for lithium secondary batteries.

[0017] According to yet another embodiment, there is provided a lithium secondary battery comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and the electrolyte solution. [Effects of the Invention]

[0018] The additive for a lithium secondary battery according to an embodiment has excellent electrolyte impregnation properties, and when applied to an electrolyte, it can maintain battery characteristics without increasing battery resistance.

[0019] In addition, the lithium secondary battery including the additive for lithium secondary batteries according to an embodiment can suppress the occurrence of short circuits due to an increase in resistance above the battery operating temperature, thereby improving the safety of the battery. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a cross-sectional view of an additive according to one embodiment. [Figure 2] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] Although the following detailed description of the embodiments will be made in order to enable those skilled in the art to easily implement the present invention, the actual structure may be realized in various different forms and is not limited to the embodiments described herein.

[0022] In the drawings, the thickness of layers and regions is exaggerated for clarity.

[0023] When a layer, film, region, plate, or other part is said to be "on" another part, this includes not only the case where it is "directly on" the other part, but also the case where there is another part in between. Conversely, when a part is said to be "directly on" another part, it means that there is no other part in between.

[0024] As used herein, "at least one of A, B, or C," "A, B, C, or one of combinations thereof," and "A, B, C, and one of combinations thereof" refer to each and every component and combination thereof (e.g., A, B, A and B, A and C, B and C, or A, B, and C).

[0025] Hereinafter, the term "combination" includes mixtures of two or more, mutual substitutions, and stacked structures of two or more.

[0026] Unless otherwise defined herein, particle size can be measured by methods well known to those skilled in the art, such as by using a particle size analyzer or by using optical micrographs taken with a transmission electron microscope or a scanning electron microscope. Alternatively, particle size can be measured using dynamic light scattering, followed by data analysis to count the number of particles in each particle size range, and then calculations can be performed to obtain the average particle size. Unless otherwise defined, particle size refers to the diameter (D50) of particles whose cumulative volume is 50% by volume in the particle size distribution.

[0027] Hereinafter, an additive for a lithium secondary battery according to one embodiment will be described with reference to FIG.

[0028] FIG. 1 is a cross-sectional view of an additive according to one embodiment.

[0029] 1, an additive 1 according to one embodiment includes a core 3 and a shell 5 surrounding the core 3. The core 3 includes a polyethylene wax, and the shell 5 includes a polymer having a melting point of 90°C to 120°C.

[0030] The additive 1 has a structure including a core 3 and a shell 5, and therefore can maintain battery characteristics without increasing battery resistance compared to when the core material is directly introduced into the battery. In addition, since the core 3 includes a safety-enhancing substance, when the shell 5 of the additive melts at high temperatures, the substance is released from the core 3 to the outside, increasing the battery resistance and causing a voltage drop, thereby preventing the occurrence of a short circuit in the battery.

[0031] The ratio of the thickness of the core 3 to the thickness of the shell 5 may be 1:1 to 4:1, for example, 3:2, for example, 2:1, for example, 5:2, or for example, 3:1, but is not limited thereto.

[0032] When the thickness ratio of the core 3 to the shell 5 is within the above range, the time required for melting the shell 5 and releasing the core material can be controlled within an appropriate range, thereby effectively preventing the occurrence of electrode short circuits at high temperatures. Also, when the temperature is not high, the shell 5 is not damaged, preventing an unnecessary increase in battery resistance and a deterioration in battery performance.

[0033] When the additive containing the core 3 and the shell 5 is in the form of a fiber, the "core thickness" refers to the linear length of a line segment extending from the center of the circle that is the cross section of the fiber to a point on the periphery of the core, and the "shell thickness" refers to the linear length between the point where the line segment touches the periphery of the core and the point where the line segment touches the periphery of the shell when the line segment extends from the center of the circle that is the cross section of the fiber to a point on the periphery of the shell.

[0034] When the additive containing the core 3 and the shell 5 is spherical, the "core thickness" refers to the length of a line segment extending from the center of the sphere to a point on the surface of the core, and the "shell thickness" refers to the length between the point where the line segment extends from the center of the sphere to a point on the surface of the shell and the point where the line segment extends from the center of the sphere to a point on the surface of the shell.

[0035] The thickness of the core 3 may be 0.1 μm to 2.0 μm, for example, 0.1 μm or more, 0.15 μm or more, 0.20 μm or more, 0.25 μm or more, 0.30 μm or more, or 0.35 μm or more, and 2.0 μm or less, for example, 1.5 μm or less, 1.4 μm or less, 1.3 μm or less, 1.2 μm or less, 1.1 μm or less, or 1.0 μm or less, but is not limited thereto.

[0036] When the core 3 has a thickness within the above range, the core material is released along with the melting of the shell 5 at the appropriate time, effectively controlling the occurrence of electrode short circuits, while at the same time preventing a decrease in electrolyte impregnation and an unnecessary increase in battery resistance, thereby maintaining battery characteristics.

[0037] The particle size of the polyethylene wax may be 0.05 μm to 1.5 μm, for example, 0.05 μm or more, 0.10 μm or more, 0.15 μm or more, 0.20 μm or more, 0.25 μm or more, or 0.30 μm or more, and 1.5 μm or less, for example, 1.2 μm or less, 1.0 μm or less, 0.8 μm or less, or 0.6 μm or less, but is not limited thereto. The particle size is also called particle size, and refers to the diameter when the polyethylene wax is spherical, or the length of the longest axis when the polyethylene wax is not spherical.

[0038] When the polyethylene wax has a particle size within the above range, it is effectively contained in the core, and when the shell melts at high temperatures, the wax is effectively released from the core, thereby increasing the battery resistance.

[0039] The polyethylene wax preferably has a melting point of 100°C to 140°C, but is not limited thereto.

[0040] The thickness of the shell 5 may be 0.025 μm to 0.5 μm, for example, 0.025 μm or more, 0.05 μm or more, 0.075 μm or more, 0.10 μm or more, 0.125 μm or more, or 0.15 μm or more, and 0.5 μm or less, for example, 0.45 μm or less, 0.40 μm or less, 0.35 μm or less, or 0.30 μm or less, but is not limited thereto.

[0041] When the shell 5 has a thickness within the above range, the occurrence of electrode short circuits can be effectively controlled by timely melting of the shell 5 and release of the core material, and at the same time, the battery resistance is not unnecessarily increased, thereby maintaining the battery characteristics.

[0042] In one embodiment, the shell 5 may include a polymer having a melting point of 90° C. to 120° C. For example, the melting point of the polymer may be 90° C. or higher, such as 95° C. or higher, or 100° C. or higher, or may be 120° C. or lower, such as 115° C. or lower, or 110° C. or lower. For example, the polymer may be a thermoplastic resin.

[0043] Since the melting point of the polymer contained in the shell 5 is within the above range, the shell 5 is maintained stably within the operating temperature range during charging and discharging of the battery, thereby not increasing the resistance of the battery, and the shell 5 melts appropriately at high temperatures of 100°C or higher, allowing the safety-enhancing material in the core 3 to be released in a timely manner, thereby effectively preventing the occurrence of short circuits in the battery.

[0044] By way of example, the thermoplastic resin may be poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), polyacrylic acid, polyethylene, poly(methyl methacrylate), polyalkylene oxide, polyalkylene succinate, or a combination thereof, such as, but not limited to, poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), polyalkylene oxide, polyalkylene succinate, or a combination thereof.

[0045] The polyalkylene oxide may be polyethylene oxide, polypropylene oxide, polybutylene oxide, polypentylene oxide, polyhexylene oxide, polyheptylene oxide, etc., such as polyethylene oxide, polypropylene oxide, or polybutylene oxide, etc., such as polyethylene oxide, polypropylene oxide, etc., but is not limited thereto.

[0046] The polyalkylene succinate may be polyethylene succinate, polypropylene succinate, polybutylene succinate, polypentylene succinate, polyhexylene succinate, polyheptylene succinate, or polyoxyethylene succinate, for example, polyethylene succinate, polypropylene succinate, or polybutylene succinate, for example, polybutylene succinate, but is not limited to these.

[0047] The additive 1 may be in the form of a fiber formed by electrospinning. When the additive is in the form of a fiber, the core material can be effectively dissolved at high temperatures, thereby effectively preventing battery fires. In addition to the fiber form, the additive 1 may be in the form of an amorphous, plate-like, spherical, or other shape as long as it has a structure including a core 3 and a shell 5 surrounding the core 3, but is not limited thereto.

[0048] When preparing the core-shell structured additive 1, the electrospinning process can be carried out by a known process taking into consideration the melting temperatures of the core material and the thermoplastic resin.

[0049] The polyethylene wax used in the electrospinning process may be in any form as long as it is contained in the core 3, such as, but not limited to, particles, plates, flakes, and combinations thereof.

[0050] The electrolyte for a lithium secondary battery according to another embodiment includes a non-aqueous organic solvent, a lithium salt, and the additive 1.

[0051] The additive 1 may be included in an amount of 0.1 wt % to 20 wt %, 0.1 wt % to 15 wt %, or 0.1 wt % to 10 wt % based on the total weight of the electrolyte for lithium secondary batteries. For example, the additive may be included in an amount of 0.1 wt % or more, e.g., 0.2 wt % or more, 0.3 wt % or more, 0.4 wt % or more, 0.5 wt % or more, 0.6 wt % or more, 0.7 wt % or more, 0.8 wt % or more, 0.9 wt % or more, or 1.0 wt % or more, and 15.0 wt % or less, e.g., 14.0 wt % or less, 13.0 wt % or less, 12.0 wt % or less, 11.0 wt % or less, 10.0 wt % or less, or 9.0 wt % or less based on the total weight of the electrolyte for lithium secondary batteries, but is not limited thereto.

[0052] When the content of Additive 1 is within the above range, the battery resistance does not increase at the battery operating temperature, allowing the battery characteristics to be maintained, and the battery resistance increases above the battery operating temperature, thereby realizing a lithium secondary battery with improved safety.

[0053] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can migrate, and may be a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent.

[0054] Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, decanolide, mevalonolactone, and caprolactone. Examples of the ether solvent include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran. The ketone solvent may be cyclohexanone, etc. The alcohol solvent may be ethyl alcohol, isopropyl alcohol, etc. The aprotic solvent may be nitriles such as R-CN (R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolanes, etc.

[0055] The non-aqueous organic solvents may be used alone or in combination of two or more thereof. When two or more thereof are used in combination, the mixing ratio may be appropriately adjusted depending on the desired battery performance, which should be widely understood by those skilled in the art.

[0056] In addition, in the case of the carbonate-based solvent, it is preferable to use a mixture of cyclic carbonate and chain carbonate in a volume ratio of 1:1 to 1:9, which results in excellent electrolyte performance.

[0057] The non-aqueous organic solvent may further include an aromatic hydrocarbon-based organic solvent in addition to the carbonate-based solvent, and the carbonate-based solvent and the aromatic hydrocarbon-based solvent may be mixed in a volume ratio of 1:1 to 30:1.

[0058] The aromatic hydrocarbon solvent may be an aromatic hydrocarbon compound represented by the following Chemical Formula 1: [ka] ...chemical formula 1

[0059] In the above chemical formula 1, R 201 ~R 206 are the same or different and may be hydrogen, halogen, an alkyl group having 1 to 10 carbon atoms, a haloalkyl group, or a combination thereof.

[0060] Specific examples of the aromatic hydrocarbon solvent include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, and combinations thereof.

[0061] The electrolyte may further include vinylene carbonate, vinylethylene carbonate, or an ethylene carbonate-based compound of the following Formula 2 as a life-enhancing additive to improve battery life. [ka] ...Chemical formula 2

[0062] In the above chemical formula 2, R 207 and R 208 are the same or different and can be selected from hydrogen, a halogen group, a cyano group (CN), a nitro group (NO2), or a fluorinated alkyl group having 1 to 5 carbon atoms.

[0063] Representative examples of the ethylene carbonate compound include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, fluoroethylene carbonate, etc. When such a life-improving additive is further used, the amount used can be appropriately adjusted.

[0064] The lithium salt is dissolved in a non-aqueous organic solvent and serves as a source of lithium ions in the battery, enabling basic operation of a lithium secondary battery, and promoting the movement of lithium ions between the positive electrode and the negative electrode.

[0065] Representative examples of the lithium salt include LiPF6, LiBF4, LiDFOP, LiDFOB, LiPO2F2, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide: LiFSI), LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (where x and y are natural numbers, for example, integers of 1 to 20), LiCl, LiI, or LiB(C2O4)2 (lithium bis(oxalato)borate: LiBOB).

[0066] The lithium salt concentration is preferably within the range of 0.1 M to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0067] The additive and the electrolyte can be applied to a lithium secondary battery.

[0068] Hereinafter, a lithium secondary battery according to an embodiment will be described with reference to FIG.

[0069] The lithium secondary battery 100 according to one embodiment includes a positive electrode 114 including a positive electrode active material, a negative electrode 112 including a negative electrode active material, and the electrolyte solution.

[0070] Lithium secondary batteries can be classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries depending on the type of separator and electrolyte used, and into cylindrical, prismatic, coin, and pouch types depending on the shape, and into bulk and thin film types depending on the size. The structures and manufacturing methods of these batteries are widely known in the art, so detailed description will be omitted.

[0071] Here, a cylindrical lithium secondary battery will be described as an example of a lithium secondary battery. Fig. 2 is a diagram schematically illustrating the structure of a lithium secondary battery according to an embodiment. Referring to Fig. 2, a lithium secondary battery 100 according to an embodiment includes a battery cell including a positive electrode 114, a negative electrode 112 facing the positive electrode 114, a separator 113 disposed between the positive electrode 114 and the negative electrode 112, and an electrolyte (not shown) impregnating the positive electrode 114, the negative electrode 112, and the separator 113, a battery container 120 containing the battery cell, and an encapsulating member 140 sealing the battery container 120. The positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material.

[0072] The positive electrode active material may be a compound capable of reversibly intercalating and deintercalating lithium (lithiated intercalation compound). Specifically, at least one of composite oxides of lithium and metals including cobalt, manganese, nickel, or a combination thereof may be used.

[0073] The composite oxide may be a composite oxide in which a portion of the metal is replaced with a metal other than the metal. The composite oxide may be a phosphate compound, such as at least one selected from LiFePO4, LiCoPO4, and LiMnPO4. The composite oxide may also have a coating layer on its surface, or a mixture of the composite oxide and a composite oxide with a coating layer may be used. The coating layer may include at least one coating element compound selected from oxides, hydroxides, oxyhydroxides, oxycarbonates, and hydroxycarbonates of the coating element. The compound forming the coating layer may be amorphous or crystalline. The coating element contained in the coating layer may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating layer formation process may be performed using any coating method (e.g., spray coating, dipping, etc.) that does not adversely affect the physical properties of the positive electrode active material using such elements in the compound, and detailed description thereof will be omitted as it is well understood by those skilled in the art.

[0074] The positive electrode active material may be, for example, one or more lithium composite oxides represented by the following chemical formula 3.

[0075] [Chemical formula 3] Li x M 1 y M 2 z M 3 1-y-z O2 In the above chemical formula 3, 0.5≦x≦1.8, 0 <y≦1、0≦z≦1、0≦y+z≦1、M 1 , M 2 , and M 3may each independently be any one selected from metals such as Ni, Co, Mn, Al, Sr, Mg, or La, and combinations thereof.

[0076] In one embodiment, the positive electrode active material is LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi a Mn b Co c O2(a+b+c=1), LiNi a Mn b Co c Al d O2(a+b+c+d=1) or LiNi e Co f Al g O2(e+f+g=1).

[0077] For example, the LiNi a Mn b Co c O2(a+b+c=1), LiNi a Mn b Co c Al d O2(a+b+c+d=1), and LiNi e Co f Al g The positive electrode active material selected from O2 (e+f+g=1) may be a high nickel (high Ni) based positive electrode active material.

[0078] For example, the LiNi a Mn b Co c O2 (a+b+c=1) and LiNi a Mn b Co c Al d In the case of O2 (a+b+c+d=1), the nickel content may be 60% or more (a≧0.6), more specifically 80% or more (a≧0.8).

[0079] For example, the LiNi e Co f Al gIn the case of O2 (e+f+g=1), the nickel content may be 60% or more (e≧0.6), more specifically 80% or more (e≧0.8).

[0080] The content of the positive electrode active material may be 90% by weight to 98% by weight based on the total weight of the positive electrode active material layer.

[0081] The positive electrode active material layer may optionally include a conductive material and a binder, and the content of the conductive material and the binder may be 1.0 wt % to 5.0 wt % respectively based on the total weight of the positive electrode active material layer.

[0082] The conductive material is used to impart conductivity to the positive electrode, and any material that is electron-conductive and does not cause a chemical change in the constructed battery can be used. Examples of conductive materials that can be used include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber; metal-based materials such as metal powder or metal fiber of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and conductive materials containing mixtures thereof.

[0083] The binder serves to firmly adhere the positive electrode active material particles to each other and to the current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0084] The positive electrode current collector may be made of Al, but is not limited thereto.

[0085] The negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector and including a negative electrode active material.

[0086] The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, a lithium metal alloy, a material capable of being doped with and dedoped from lithium, or a transition metal oxide.

[0087] The material capable of reversibly intercalating / deintercalating lithium ions may be any carbonaceous negative electrode active material commonly used in lithium secondary batteries, representative examples of which include crystalline carbon, amorphous carbon, or a combination of these. Examples of crystalline carbon include amorphous, plate-like, flake-like, spherical, or fibrous graphite, such as natural graphite or artificial graphite. Examples of amorphous carbon include soft carbon, hard carbon, mesophase pitch carbide, and calcined coke.

[0088] As the lithium metal alloy, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0089] Examples of substances that can be doped and undoped with lithium include Si, Si-C composites, SiOx (0 < x < 2), Si-Q alloys (where Q is an element selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements, group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO2, Sn-R (where R is an element selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements, group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), etc. Also, at least one of these can be mixed with SiO2 and used.

[0090] As the elements Q and R, those selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof can be used.

[0091] Examples of the transition metal oxides include vanadium oxides, lithium vanadium oxides, or lithium titanium oxides, etc.

[0092] In a specific embodiment, the negative electrode active material may be a Si-C composite containing a Si-based active material and a carbon-based active material.

[0093] In the Si-C composite, the average particle size of the Si-based active material may be 50 nm to 200 nm. When the average particle size of the Si-based active material is within the above range, volume expansion generated during charge and discharge can be suppressed, and the interruption of the conductive path due to particle crushing during charge and discharge can be prevented.

[0094] The Si-based active material may be contained in an amount of 1 to 60% by weight, for example, 3 to 60% by weight, based on the total weight of the Si-C composite.

[0095] In another specific embodiment, the negative electrode active material may further include crystalline carbon in addition to the Si—C composite.

[0096] When the negative electrode active material includes both a Si-C composite and crystalline carbon, the Si-C composite and crystalline carbon may be included in the form of a mixture, and in this case, the Si-C composite and crystalline carbon may be included in a weight ratio of 1:99 to 50:50. More specifically, the Si-C composite and crystalline carbon may be included in a weight ratio of 5:95 to 20:80.

[0097] The crystalline carbon may include, for example, graphite, and more specifically, may include natural graphite, artificial graphite, or a mixture thereof.

[0098] The crystalline carbon may have an average particle size of 5 μm to 30 μm.

[0099] Herein, the average particle size may be the particle size at 50% by volume (D50) on a cumulative size-distribution curve.

[0100] The Si-C composite may further include a shell surrounding the surface of the Si-C composite, and the shell may include amorphous carbon. The amorphous carbon may include soft carbon, hard carbon, mesophase pitch carbide, calcined coke, or a mixture thereof.

[0101] The amorphous carbon may be contained in an amount of 1 to 50 parts by weight, for example, 5 to 50 parts by weight, or 10 to 50 parts by weight, relative to 100 parts by weight of the carbon-based active material.

[0102] The content of the negative electrode active material in the negative electrode active material layer may be 95% by weight to 99% by weight based on the total weight of the negative electrode active material layer.

[0103] In one embodiment, the negative electrode active material layer includes a binder and may optionally further include a conductive material. The content of the binder in the negative electrode active material layer may be 1 wt% to 5 wt% based on the total weight of the negative electrode active material layer. When the negative electrode active material layer further includes a conductive material, the negative electrode active material may be 90 wt% to 98 wt%, the binder may be 1 wt% to 5 wt%, and the conductive material may be 1 wt% to 5 wt%.

[0104] The binder serves to firmly adhere the negative active material particles to each other and to the current collector. The binder may be a water-insoluble binder, a water-soluble binder, or a combination thereof.

[0105] The water-insoluble binder may be polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0106] The water-soluble binder may be a rubber-based binder or a polymer resin binder. The rubber-based binder may be selected from styrene-butadiene rubber, acrylated styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, and combinations thereof. The polymer resin binder may be selected from polytetrafluoroethylene, ethylene-propylene copolymer, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, ethylene-propylene-diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0107] When a water-soluble binder is used as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further included as a thickener. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li. The amount of the thickener used may be 0.1 to 3 parts by weight per 100 parts by weight of the negative electrode active material.

[0108] The conductive material is used to impart conductivity to the electrode, and any material that is electron-conductive and does not cause a chemical change in the constructed battery can be used. Examples of conductive materials that can be used include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber; metal-based materials such as metal powder or metal fiber of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and conductive materials containing mixtures thereof.

[0109] The negative electrode current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0110] Depending on the type of lithium secondary battery, a separator may be present between the positive electrode and the negative electrode, and the separator may be a porous substrate or a composite porous substrate.

[0111] The porous substrate is a substrate containing voids through which lithium ions can move. The porous substrate may be, for example, polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof, such as a mixed multilayer film of polyethylene / polypropylene two-layer separator, polyethylene / polypropylene / polyethylene three-layer separator, or polypropylene / polyethylene / polypropylene three-layer separator.

[0112] The composite porous substrate may include a porous substrate and a functional layer disposed on the porous substrate. The functional layer may be, for example, at least one of a heat-resistant layer and an adhesive layer, allowing for additional functionality. For example, the heat-resistant layer may include a heat-resistant resin and, optionally, a filler. The adhesive layer may include an adhesive resin and, optionally, a filler. The filler may be an organic filler or an inorganic filler.

[0113] The additive for a lithium secondary battery according to an embodiment may be included in the electrolyte as described above, and may also be applied to a current collector, an electrode tab, a separator, etc. of a lithium secondary battery.

[0114] When the additive is applied to a current collector or an electrode tab, the additive may be dispersed in a suitable solvent and coated as a coating solution on the uncoated portion of the current collector or the electrode tab. When the additive is applied to a separator, the additive may be included in the components of the separator, or a coating solution in which the additive is dispersed in a suitable solvent may be coated on at least one surface of the separator.

[0115] Hereinafter, the embodiments of the present invention will be described in more detail with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. [Example]

[0116] Additive manufacturing Synthesis Example 1 Polymer solutions containing 10 wt% polyethylene wax (particle size 0.5 μm) and 10 wt% poly(vinylidene fluoride hexafluoropropylene) (PVDF-HFP) were prepared and electrospun to produce fiber-shaped additives with a core-to-shell thickness ratio of 1:1.

[0117] Synthesis Example 2 Polymer solutions containing 20 wt% polyethylene wax (particle size 0.5 μm) and 10 wt% poly(vinylidene fluoride hexafluoropropylene) (PVDF-HFP) were prepared and electrospun to produce fiber-shaped additives with a core-to-shell thickness ratio of 2:1.

[0118] Synthesis Example 3 Polymer solutions containing 40 wt% polyethylene wax (particle size 0.5 μm) and 10 wt% poly(vinylidene fluoride hexafluoropropylene) (PVDF-HFP) were prepared and electrospun to produce fiber-shaped additives with a core-to-shell thickness ratio of 4:1.

[0119] Synthesis Example 4 A fiber-shaped additive was prepared in the same manner as in Synthesis Example 1, except that electrospinning was performed so that the core and shell thickness ratio was 1:2.

[0120] Comparative synthesis example 1 A fiber-shaped additive was prepared in the same manner as in Synthesis Example 1, except that a polymer solution containing polyethylene glycol (PEG) was used instead of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP).

[0121] Comparative synthesis example 2 A fiber-shaped additive was prepared in the same manner as in Synthesis Example 1, except that a polymer solution containing polypropylene (PP) was used instead of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP).

[0122] Lithium secondary battery manufacturing Example 1 LiNi as the positive electrode active material 0.88 Co 0.07 Al 0.05 O2, polyvinylidene fluoride as a binder, and acetylene black as a conductive material were mixed in a weight ratio of 96:2:2 and dispersed in N-methylpyrrolidone to prepare a positive electrode active material slurry.

[0123] The positive electrode active material slurry was coated on an aluminum foil having a thickness of 14 μm, dried at 110° C., and then pressed to prepare a positive electrode.

[0124] A mixture of artificial graphite and Si-C composite in a weight ratio of 93:7 was used as the negative electrode active material. The negative electrode active material, styrene-butadiene rubber binder as a binder, and carboxymethyl cellulose as a thickener were mixed in a weight ratio of 97:1:2 and dispersed in distilled water to prepare a negative electrode active material slurry.

[0125] The Si-C composite has a core containing artificial graphite and silicon particles, and the surface of the core is coated with coal-based pitch.

[0126] The negative electrode active material slurry was coated on a copper foil having a thickness of 10 μm, dried at 100° C., and then pressed to prepare a negative electrode.

[0127] The prepared positive and negative electrodes were assembled with a 25 μm thick polyethylene separator to prepare an electrode assembly, and an electrolyte solution was injected to prepare a lithium secondary battery. The composition of the electrolyte solution is as follows:

[0128] (Electrolyte composition) Salt: LiPF61.3M Solvent: ethylene carbonate (EC): propylene carbonate (PC): ethyl propionate (EP): propyl propionate (PP) = 15:15:25:45 (volume ratio) Additives: 3 parts by weight of fluoroethylene carbonate, 1 part by weight of SN, 15 parts by weight of the additive produced in Synthesis Example 1 (However, in the composition of the electrolyte solution, "parts by weight" refers to the relative weight of the additive with respect to 100 parts by weight of the entire electrolyte solution (lithium salt + non-aqueous organic solvent).)

[0129] Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 15 parts by weight of the additive prepared in Synthesis Example 2 was used instead of the additive prepared in Synthesis Example 1.

[0130] Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 15 parts by weight of the additive prepared in Synthesis Example 3 was used instead of the additive prepared in Synthesis Example 1.

[0131] Example 4 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 30 parts by weight of the additive prepared in Synthesis Example 4 was used instead of the additive prepared in Synthesis Example 1.

[0132] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the additive prepared in Synthesis Example 1 was not used.

[0133] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 2 parts by weight of polyethylene wax was used instead of the additive prepared in Synthesis Example 1.

[0134] Comparative Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 10 parts by weight of polyethylene wax was used instead of the additive prepared in Synthesis Example 1.

[0135] Comparative Example 4 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 15 parts by weight of the additive prepared in Comparative Synthesis Example 1 was used instead of the additive prepared in Synthesis Example 1.

[0136] Comparative Example 5 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 15 parts by weight of the additive prepared in Comparative Synthesis Example 2 was used instead of the additive prepared in Synthesis Example 1.

[0137] Evaluation 1: High temperature life evaluation The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 5 were charged at a constant current of 0.5 C at 45°C until the voltage reached 4.4 V, and then cut off at a current of 0.05 C in constant voltage mode while maintaining 4.4 V. Next, the batteries were discharged at a constant current of 0.5 C until the voltage reached 3.0 V. This process was repeated 100 times. As a result of the charge / discharge experiment, the capacity retention rate after 100 cycles calculated using the following formula 1 is shown in Table 1 below.

[0138] [Formula 1] Capacity retention rate at 100 cycles [%] = [50 cycle discharge capacity / 1 cycle discharge capacity] x 100

[0139] [Table 1]

[0140] Referring to Table 1 above, the lithium secondary batteries according to Examples 1 to 3 all have excellent high-temperature life characteristics of 85% or more after 100 cycles. It can be seen that the lithium secondary batteries according to Examples 1 to 3 have excellent high-temperature life characteristics compared to the lithium secondary batteries according to Comparative Examples 2 to 5.

[0141] Evaluation 2: Heat exposure evaluation The lithium secondary batteries according to Examples 1 to 4 and Comparative Examples 1 to 5 were subjected to 0.5C / 4.4V 0.05C cut-off charging, and then heat exposure evaluation was carried out.

[0142] The lithium secondary batteries according to Examples 1 to 4 and Comparative Examples 1 to 5 were placed in a chamber, and the temperature was then increased from room temperature to 136°C at a rate of 5±2°C per minute. The temperature was maintained for about 1 hour while observing the changes in the lithium secondary batteries. This process was repeated twice, and the results are shown in Table 2 below.

[0143] [Table 2]

[0144] In Table 2, "NG" means that thermal runaway was observed at the exposure temperature, "OK" means that a sudden voltage drop was observed instead of thermal runaway at the exposure temperature, and (-) means that thermal exposure evaluation was not performed.

[0145] The lithium secondary battery according to Comparative Example 1 exhibited thermal runaway when exposed to high temperatures, resulting in poor battery safety. This is interpreted as being due to the fact that the lithium secondary battery according to Comparative Example 1 does not contain polyethylene wax and therefore is unable to exhibit the battery safety benefits of polyethylene wax.

[0146] The lithium secondary battery according to Comparative Example 4 exhibited a thermal runaway phenomenon at 134°C. This is interpreted as the shell of the additive contained in the lithium secondary battery according to Comparative Example 4 being easily melted by polyethylene glycol (PEG), preliminarily releasing the core material, resulting in the thermal runaway phenomenon being observed above 134°C.

[0147] In the above evaluation, thermal runaway was observed in the lithium secondary battery according to Comparative Example 5. This is interpreted as being because the shell of the additive contained in the lithium secondary battery according to Comparative Example 5 was not melted in a timely manner due to polypropylene (PP), and the core material was not released, which meant that safety could not be sufficiently improved.

[0148] In contrast, when exposed to high temperatures, the lithium secondary batteries according to Examples 1 to 4 experience a sudden rise in battery resistance and a sudden drop in battery voltage due to the melting of the shell of the additive according to one embodiment and the release of the polyethylene wax contained in the core. Therefore, it can be seen that the lithium secondary batteries according to Examples 1 to 4 all have superior battery stability compared to the lithium secondary batteries according to Comparative Examples 1 to 5.

[0149] Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited to these, and various modifications and improvements made by those skilled in the art using the basic concept of the invention defined in the claims also fall within the scope of the present invention. [Explanation of symbols]

[0150] 100 Lithium secondary battery 112 Negative electrode 113 Separator 114 Positive electrode 120 Battery container 140 Enclosure material

Claims

1. a core comprising a polyethylene wax, and a shell surrounding the core; The additive for lithium secondary batteries, wherein the shell comprises a polymer having a melting point of 90°C to 120°C.

2. 2. The additive for lithium secondary batteries according to claim 1, wherein the ratio of the thickness of the core to the thickness of the shell is 1:1 to 4:

1.

3. The additive for lithium secondary batteries according to claim 1, wherein the core has a thickness of 0.1 μm to 2.0 μm, and the shell has a thickness of 0.025 μm to 0.5 μm.

4. 2. The additive for lithium secondary batteries according to claim 1, wherein the polymer comprises poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), polyacrylic acid, polyethylene, poly(methyl methacrylate), polyalkylene oxide, polyalkylene succinate, or a combination thereof.

5. The additive for a lithium secondary battery according to claim 1 , wherein the additive is in the form of a fiber formed by electrospinning.

6. non-aqueous organic solvents, lithium salts, and An electrolyte for a lithium secondary battery, comprising the additive for a lithium secondary battery according to any one of claims 1 to 5.

7. 7. The electrolyte solution for a lithium secondary battery according to claim 6, wherein the additive for a lithium secondary battery is contained in an amount of 0.1 wt % to 20 wt % based on the total weight of the electrolyte solution for a lithium secondary battery.

8. 7. The electrolyte solution for a lithium secondary battery according to claim 6, wherein the additive for a lithium secondary battery is contained in an amount of 0.1 wt % to 15 wt % based on the total weight of the electrolyte solution for a lithium secondary battery.

9. 7. The electrolyte solution for a lithium secondary battery according to claim 6, wherein the additive for a lithium secondary battery is contained in an amount of 0.1 wt % to 10 wt % based on the total weight of the electrolyte solution for a lithium secondary battery.

10. a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and A lithium secondary battery comprising the electrolyte solution according to claim 6.

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