Lithium secondary battery

Abstract

To provide an electrolyte solution that can be used even under high-voltage and high-temperature conditions when a positive electrode including a cobalt-free lithium nickel manganese oxide is used.SOLUTION: A lithium secondary battery includes: an electrolyte solution including a nonaqueous organic solvent, a lithium salt, and an additive; a positive electrode including a positive electrode active material; and a negative electrode including a negative electrode active material. The nonaqueous organic solvent contains 5 wt% of ethylene carbonate and less than 5 wt% of ethylene carbonate relative to the total weight of the nonaqueous organic solvent. The additive contains at least one kind of lithium compound selected from lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalate)borate (LiDFOB), lithium difluorophosphate (LiPO2F2), and lithium bisoxalate borate (LiB(C2O4)2:LiBOB). The positive electrode active material incudes cobalt-free lithium nickel manganese oxide.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present description relates to lithium secondary batteries. [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] In particular, as IT devices become increasingly sophisticated, there is a demand for high-capacity batteries. While achieving high capacity through an expansion of the voltage range can increase energy density, there is a problem in that the electrolyte is oxidized in the high-voltage range, deteriorating the performance of the positive electrode.

[0004] In particular, cobalt-free lithium nickel manganese oxides are attracting attention as next-generation positive electrode active materials because they do not contain cobalt in their positive electrode active material composition and are composed primarily of nickel, manganese, etc. Positive electrodes containing these oxides are economical and can achieve high energy density.

[0005] However, when used in a high-voltage environment, positive electrodes containing cobalt-free lithium-nickel-manganese oxides can experience transition metal leaching due to the collapse of the positive electrode structure, which can lead to problems such as gas generation inside the cell and reduced capacity. This transition metal leaching phenomenon tends to intensify in high-temperature environments, and the leached transition metals can precipitate on the surface of the negative electrode, causing side reactions that increase battery resistance and reduce battery life and output characteristics.

[0006] Therefore, when using a positive electrode containing a cobalt-free lithium nickel manganese oxide, an electrolyte that can be used under high voltage and high temperature conditions is required. Summary of the Invention [Problem to be solved by the invention]

[0007] An object of one embodiment is to provide a lithium secondary battery having improved high-voltage and high-temperature characteristics by using a positive electrode containing a cobalt-free lithium nickel manganese-based oxide in combination with an electrolyte that can effectively protect the positive electrode containing a cobalt-free lithium nickel manganese-based oxide, thereby reducing the elution of transition metals under high-voltage and high-temperature conditions and suppressing the collapse of the positive electrode structure. [Means for solving the problem]

[0008] One embodiment of the present invention comprises an electrolyte solution including a non-aqueous organic solvent, a lithium salt, and an additive; a positive electrode including a positive electrode active material; and a negative electrode including a negative electrode active material, The non-aqueous organic solvent contains less than 5 wt % of ethylene carbonate based on the total weight of the non-aqueous organic solvent, the additive contains at least one lithium compound selected from lithium tetrafluoroborate (LiBF), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiPOF), and lithium bis(oxalato)borate (LiB(C0):LiBOB), and the positive electrode active material contains a cobalt-free lithium nickel manganese-based oxide.

[0009] The non-aqueous organic solvent may be composed solely of a chain carbonate.

[0010] The chain carbonate may be represented by the following chemical formula 1: [ka] In the above Chemical Formula 1 R1 and R 2 are each independently a substituted or unsubstituted C1 to C20 alkyl group.

[0011] The non-aqueous organic solvent can be at least two of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and ethyl methyl carbonate (EMC).

[0012] The non-aqueous organic solvent may contain ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of 0:100 to 50:50.

[0013] The additive may be included in an amount of 0.05 to 5.0 parts by weight based on 100 parts by weight of the total electrolyte solution for lithium secondary batteries.

[0014] The additive may be included in an amount of 0.1 to 5.0 parts by weight based on 100 parts by weight of the total electrolyte solution for lithium secondary batteries.

[0015] The electrolyte may further include at least one other additive selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, vinylethylene carbonate (VEC), adiponitrile (AN), succinonitrile (SN), 1,3,6-hexanetricyanide (HTCN), propene sultone (PST), propane sultone (PS), and 2-fluorobiphenyl (2-FBP).

[0016] The cobalt-free lithium nickel manganese oxide may include a lithium composite oxide represented by the following Chemical Formula 3. [Chemical formula 3] Li a Nix Mn y M 1 z M 2 w O 2±b X c In the above Chemical Formula 3, 0.5≦a<1.8, 0≦b≦0.1, 0≦c≦0.1, 0≦w<0.1, 0.6≦x<1.0, 0 <y<0.4、0<z<0.1、w+x+y+z=1であり、 M 1 and M 2 are each independently one or more elements selected from Al, Mg, Ti, Zr, Cr, Sr, V, B, W, Mo, Nb, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

[0017] The lithium composite oxide represented by Chemical Formula 3 can be represented by the following Chemical Formula 3-1. [Chemical formula 3-1] Li a Ni x1 Mn y1 Al z1 M 2 w1 O 2±b X c In the above chemical formula 3-1, 0.5≦a<1.8, 0≦b≦0.1, 0≦c≦0.1, 0≦w1<0.1, 0.6≦x1<1.0, 0 <y1<0.4、0<z1<0.1、w1+x1+y1+z1=1であり、 M 2 are each independently one or more elements selected from Mg, Ti, Zr, Cr, Sr, V, B, W, Mo, Nb, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

[0018] In Formula 3-1, x1 may be 0.6≦x1≦0.79, y1 may be 0.2≦y1≦0.39, and z1 may be 0.01≦z1<0.1.

[0019] The negative electrode active material may include at least one of graphite and a Si composite.

[0020] The lithium secondary battery may have an upper limit charging voltage of 4.35 V or higher. [Effects of the Invention]

[0021] In one embodiment, a cobalt-free lithium nickel manganese oxide-based cathode is combined with an electrolyte capable of effectively protecting the cathode, thereby ensuring the safety of the cathode phase transition even in a high-temperature, high-voltage environment. The decomposition of the electrolyte and side reactions with the electrode are suppressed, reducing gas generation and suppressing an increase in the battery's internal resistance, thereby realizing a lithium secondary battery with improved battery stability and life characteristics. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a schematic diagram illustrating a lithium secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, a lithium secondary battery according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings, which are given by way of example only and are not intended to limit the present invention, which is defined only by the scope of the claims that follow.

[0024] Unless otherwise defined, the term "substituted" used herein means that at least one hydrogen atom in a substituent or compound has been replaced with deuterium, a halogen group, a hydroxyl group, an amino group, a substituted or unsubstituted C1-C30 amine group, a nitro group, a substituted or unsubstituted C1-C40 silyl group, a C1-C30 alkyl group, a C1-C10 alkylsilyl group, a C6-C30 arylsilyl group, a C3-C30 cycloalkyl group, a C3-C30 heterocycloalkyl group, a C6-C30 aryl group, a C2-C30 heteroaryl group, a C1-C20 alkoxy group, a C1-C10 fluoroalkyl group, a cyano group, or a combination thereof.

[0025] In one embodiment of the present invention, "substituted" means that at least one hydrogen atom in a substituent or compound is replaced with deuterium, a halogen group, a C1-C30 alkyl group, a C1-C10 alkylsilyl group, a C6-C30 arylsilyl group, a C3-C30 cycloalkyl group, a C3-C30 heterocycloalkyl group, a C6-C30 aryl group, a C2-C30 heteroaryl group, a C1-C10 fluoroalkyl group, or a cyano group. In another specific embodiment of the present invention, "substituted" means that at least one hydrogen atom in a substituent or compound is replaced with deuterium, a halogen group, a C1-C20 alkyl group, a C6-C30 aryl group, a C1-C10 fluoroalkyl group, or a cyano group. In another specific embodiment of the present invention, "substituted" means that at least one hydrogen atom in a substituent or compound is replaced with deuterium, a halogen group, a C1-C5 alkyl group, a C6-C18 aryl group, a C1-C5 fluoroalkyl group, or a cyano group. In addition, in a specific example of the present invention, "substituted" means that at least one hydrogen atom of a substituent or compound is substituted with deuterium, a cyano group, a halogen group, a methyl group, an ethyl group, a propyl group, a butyl group, a phenyl group, a biphenyl group, a terphenyl group, a trifluoromethyl group, or a naphthyl group.

[0026] 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, into cylindrical, prismatic, coin, pouch, and other 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.

[0027] Here, a cylindrical lithium secondary battery will be described as an example of a lithium secondary battery. Fig. 1 is a schematic diagram illustrating the structure of a lithium secondary battery according to one embodiment. Referring to Fig. 1, a lithium secondary battery 100 according to one 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 housing the battery cell, and an encapsulation member 140 sealing the battery container 120.

[0028] The configuration of the lithium secondary battery 100 according to one embodiment of the present invention will be described in more detail below.

[0029] A lithium secondary battery according to one embodiment of the present invention includes an electrolyte, a positive electrode, and a negative electrode.

[0030] The electrolyte solution includes a non-aqueous organic solvent, a lithium salt, and an additive.

[0031] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can migrate.

[0032] The non-aqueous organic solvent may be a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent.

[0033] Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (EMC), 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 bond-oriented ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolanes, etc.

[0034] The non-aqueous organic solvent 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 is well understood by those skilled in the art.

[0035] For example, the non-aqueous organic solvent may contain less than 5% by weight of ethylene carbonate based on the total weight of the non-aqueous organic solvent.

[0036] If the content of ethylene carbonate exceeds the above range and is 5 wt % or more based on the total weight of the non-aqueous organic solvent, the activity of Ni increases during high voltage operation, and the oxidation state of Ni tends to be reduced from tetravalent to divalent. As a result, ethylene carbonate, which has low oxidation stability, is oxidized and decomposed, resulting in Ni elution and deposition on the anode.

[0037] As a specific example, the non-aqueous organic solvent may be composed solely of a chain carbonate, which significantly reduces the rate of increase in resistance during high-temperature storage, thereby achieving excellent high-temperature storage characteristics.

[0038] In this specification, the term "composed only of chain carbonate" means that the non-aqueous organic solvent belonging to the category of chain carbonate is not mixed with a cyclic carbonate or the like, and is contained alone or in combination.

[0039] In one embodiment, the chain carbonate may be represented by the following Chemical Formula 1: [ka] In the above Chemical Formula 1 R 1 and R 2 are each independently a substituted or unsubstituted C1 to C20 alkyl group.

[0040] As an example, R 1 and R 2 may each independently be a substituted or unsubstituted C1 to C10 alkyl group, for example, 1 and R 2 may each independently be a substituted or unsubstituted C1 to C5 alkyl group.

[0041] In one embodiment, R in Formula 1 1 and R 2may each independently be a substituted or unsubstituted methyl group, a substituted or unsubstituted ethyl group, a substituted or unsubstituted n-propyl group, a substituted or unsubstituted n-butyl group, a substituted or unsubstituted n-pentyl group, a substituted or unsubstituted iso-butyl group, or a substituted or unsubstituted neo-pentyl group.

[0042] For example, the non-aqueous organic solvent according to one specific embodiment may be at least two of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and ethyl methyl carbonate (EMC).

[0043] The non-aqueous organic solvent according to a most specific embodiment may be a mixed solvent of dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC).

[0044] The non-aqueous organic solvent may contain ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of 10:90 to 50:50.

[0045] The non-aqueous organic solvent may be more advantageous in terms of improving battery characteristics if it contains more than 50% by volume of dimethyl carbonate (DMC) based on the total volume of the non-aqueous organic solvent.

[0046] For example, the non-aqueous organic solvent may contain ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of 10:90 to 40:60, or 10:90 to 30:70.

[0047] 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.

[0048] The aromatic hydrocarbon solvent may be an aromatic hydrocarbon compound represented by the following Chemical Formula 2. [ka] In the above formula 2, R 9 ~R 14 are the same or different and are selected from the group consisting of hydrogen, halogen, alkyl groups having 1 to 10 carbon atoms, haloalkyl groups having 1 to 10 carbon atoms, and combinations thereof.

[0049] 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, fluorobenzene, fluoroisopropyl ether ... and combinations thereof.

[0050] The lithium salt is dissolved in a non-aqueous organic solvent and serves as a lithium ion source in the battery, enabling basic lithium secondary battery operation and promoting the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of such lithium salts include LiPF6, LiSbF6, LiAsF6, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide: LiFSI), LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, and LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (wherein x and y are integers of 1 to 20), LiCl, and LiI.

[0051] 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.

[0052] The additive can include at least one lithium compound selected from lithium tetrafluoroborate (LiBF), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiPOF), and lithium bisoxalatoborate (LiB(C0):LiBOB). The additive may be included in an amount of 0.05 to 5.0 parts by weight based on 100 parts by weight of the total electrolyte solution for lithium secondary batteries. For example, the additive may be included in an amount of 0.1 to 5.0 parts by weight based on 100 parts by weight of the total electrolyte solution for lithium secondary batteries.

[0053] The non-aqueous organic solvent contains less than 5 wt % of ethylene carbonate, and the electrolyte containing the additives described above can be used to alleviate the deterioration of battery life characteristics and the sudden increase in resistance.

[0054] In particular, by using a positive electrode containing a cobalt-free lithium nickel manganese oxide described below in combination, the elution of transition metals under high voltage and high temperature conditions can be effectively reduced, thereby suppressing the collapse of the positive electrode structure and improving the high voltage and high temperature characteristics of the battery.

[0055] Meanwhile, the electrolyte may additionally contain at least one other additive selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, vinylethylene carbonate (VEC), adiponitrile (AN), succinonitrile (SN), 1,3,6-hexanetricyanide (HTCN), propene sultone (PST), propane sultone (PS), and 2-fluorobiphenyl (2-FBP).

[0056] By further including the other additives, the life span can be further improved and gas generated at the positive and negative electrodes during high temperature storage can be effectively controlled.

[0057] The other additives may be included in an amount of 0.2 to 20 parts by weight, specifically 0.2 to 15 parts by weight, for example, 0.2 to 10 parts by weight, based on 100 parts by weight of the total electrolyte solution for lithium secondary batteries.

[0058] When the content of other additives is within the above range, the increase in film resistance can be minimized, thereby contributing to improved battery performance.

[0059] The positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material.

[0060] The positive electrode active material may include a cobalt-free lithium nickel manganese-based oxide.

[0061] In this specification, the cobalt-free lithium nickel manganese oxide as a positive electrode active material refers to a positive electrode active material that does not contain cobalt in its composition and is composed mainly of nickel, manganese, etc.

[0062] For example, the cobalt-free lithium nickel manganese oxide may include at least one lithium composite oxide represented by the following Chemical Formula 3: [Chemical formula 3] Li a Ni x Mn y M 1 z M 2 w O 2±b X c In the above Chemical Formula 3, 0.5≦a<1.8, 0≦b≦0.1, 0≦c≦0.1, 0≦w<0.1, 0.6≦x<1.0, 0 <y<0.4、0<z<0.1、w+x+y+z=1であり、 M 1 and M 2 are each independently one or more elements selected from Al, Mg, Ti, Zr, Cr, Sr, V, B, W, Mo, Nb, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

[0063] Of course, the lithium composite oxide may have a coating layer on its surface, or the lithium composite oxide may be mixed with a compound having a coating layer. The coating layer may include at least one coating element compound selected from the group consisting of oxides of the coating elements, hydroxides of the coating elements, oxyhydroxides of the coating elements, oxycarbonates of the coating elements, and hydroxycarbonates of the coating elements. The compounds forming the coating layer may be amorphous or crystalline. The coating element contained in the coating layer may be Mg, Al, 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, as this is well understood by those skilled in the art and will not be described in detail.

[0064] In the case of a positive electrode active material containing the cobalt-free lithium nickel manganese oxide, the structural instability is strong under high voltage conditions, which causes solvent decomposition and leaching of transition metals, especially Ni.

[0065] The elution of such transition metals leads to deterioration and short circuiting, which reduces the lifespan of the battery and causes a sudden increase in resistance.

[0066] However, when the above-mentioned electrolyte is used together, the decrease in the life capacity of the battery and the sudden increase in resistance can be alleviated.

[0067] In particular, by using a positive electrode containing a cobalt-free lithium nickel manganese oxide in an electrolyte containing less than 5 wt % ethylene carbonate, it is possible to effectively reduce the elution of transition metals under high voltage and high temperature conditions, thereby suppressing the collapse of the positive electrode structure and improving the high voltage and high temperature characteristics of the battery.

[0068] As an example, the lithium composite oxide that can be represented by the chemical formula 3 may be represented by the following chemical formula 3-1. [Chemical formula 3-1] Li a Ni x1 Mn y1 Al z1 M 2 w1 O 2±b X c In the chemical formula 3-1, 0.5 ≦ a < 1.8, 0 ≦ b ≦ 0.1, 0 ≦ c ≦ 0.1, 0 ≦ w1 < 0.1, 0.6 ≦ x1 < 1.0, 0 < y1 < 0.4, 0 < z1 < 0.1, and w1 + x1 + y1 + z1 = 1, M 2 is one or more elements independently selected from Mg, Ti, Zr, Cr, Sr, V, B, W, Mo, Nb, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

[0069] In one embodiment, in the chemical formula 3-1, 0.6 ≦ x1 ≦ 0.9, 0.1 ≦ y1 ≦ 0.4, and 0 ≦ z1 ≦ 0.1 may be satisfied, and 0.6 ≦ x1 ≦ 0.8, 0.2 ≦ y1 < 0.4, and 0 < z1 < 0.1 may be satisfied.

[0070] For example, x1 in the chemical formula 3-1 may be 0.6 ≦ x1 ≦ 0.79, y1 may be 0.2 ≦ y1 ≦ 0.39, and z1 may be 0.01 ≦ z1 < 0.1.

[0071] The content of the positive electrode active material may be 90% to 98% by weight based on the total weight of the positive electrode active material layer.

[0072] In one embodiment of the present invention, the positive electrode active material layer may contain a binder. At this time, the content of the binder may be 1% to 5% by weight based on the total weight of the positive electrode active material layer.

[0073] The binder serves to favorably adhere positive electrode active material particles to each other and to favorably adhere the positive electrode active material to a 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.

[0074] The positive electrode current collector may be made of, but is not limited to, Al foil.

[0075] 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.

[0076] The negative electrode active material includes a material capable of reversibly inserting / extracting lithium ions, lithium metal, a lithium metal alloy, a material capable of being doped with and dedoped from lithium, or a transition metal oxide.

[0077] The material capable of reversibly inserting / extracting lithium ions may be a carbon material, and any carbon-based negative electrode active material commonly used in lithium secondary batteries may be used, 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.

[0078] As the alloy of the lithium metal, an alloy of lithium and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0079] Examples of the substance capable of doping and undoping lithium include Si, Si-C composite, SiOx (0 < x < 2), Si-Q alloy (where Q is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element excluding Si, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof), Sn, SnO2, Sn-R (where R is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element excluding Sn, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof), etc. Also, at least one of these can be mixed with SiO2 and used.

[0080] As the elements Q and R, those selected from the group consisting of 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.

[0081] Examples of the transition metal oxide include vanadium oxide, lithium vanadium oxide, or lithium titanium oxide.

[0082] In a specific embodiment, the negative electrode active material can include at least one of graphite and a Si composite.

[0083] The Si composite includes a core containing Si-based particles and an amorphous carbon coating layer. For example, the Si-based particles are silicon particles, Si-C composite, SiO xIt can contain one or more of (0 < x ≤ 2) and Si alloy.

[0084] As an example, the core containing the Si-based particles contains voids at the center, the radius of the center corresponds to 30% to 50% of the radius of the Si composite, the average particle size of the Si composite is 5 μm to 20 μm, and the average particle size of the Si-based particles can be 10 nm to 200 nm.

[0085] In this specification, the average particle size can be the particle size (D50) at 50% in volume ratio in the cumulative size-distribution curve.

[0086] When the average particle size of the Si-based particles is within the above range, the volume expansion generated during charge and discharge can be suppressed, and the disconnection of the conductive path due to particle crushing during charge and discharge can be prevented.

[0087] The core containing the Si-based particles additionally contains amorphous carbon. At this time, the center portion does not contain amorphous carbon, and the amorphous carbon can exist only on the surface portion of the Si composite.

[0088] At this time, the surface portion means the region from the outermost surface of the center portion to the outermost surface of the Si composite.

[0089] Also, the Si-based particles are substantially uniformly contained in the Si composite as a whole, that is, they can exist at a substantially uniform concentration in the center portion and the surface portion.

[0090] The amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbide, calcined coke, or a combination thereof.

[0091] For example, the Si-C composite can contain silicon particles and crystalline carbon.

[0092] The silicon particles may be included in an amount of 1 wt % to 60 wt % of the total weight of the Si—C composite, for example, 3 wt % to 60 wt %.

[0093] The crystalline carbon may be, for example, graphite, and more specifically, may be natural graphite, artificial graphite, or a combination thereof.

[0094] The average particle size of the crystalline carbon may be 5 μm to 30 μm.

[0095] When the negative electrode active material includes both graphite and a Si composite, the graphite and the Si composite may be included in the form of a mixture, and in this case, the graphite and the Si composite may be included in a weight ratio of 99:1 to 50:50.

[0096] More specifically, the graphite and the Si composite may be contained in a weight ratio of 97:3 to 80:20, or 95:5 to 80:20.

[0097] The amorphous carbon precursor may be coal-based pitch, mesophase pitch, petroleum-based pitch, coal-based oil, petroleum-based heavy oil, or a polymer resin such as a phenol resin, a furan resin, or a polyimide resin.

[0098] 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.

[0099] In one embodiment of the present invention, 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 conductive material is further included, the negative electrode active material may be used in an amount of 90 wt % to 98 wt %, the conductive material may be used in an amount of 1 wt % to 5 wt %, and the conductive material may be used in an amount of 1 wt % to 5 wt %.

[0100] The binder serves to effectively adhere the negative electrode 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.

[0101] The non-water-soluble binder may be polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0102] 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.

[0103] 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.

[0104] The conductive material is used to impart conductivity to the electrodes, and any electron-conductive material that does not undergo chemical change in the battery that is constructed 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 of these.

[0105] 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.

[0106] Depending on the type of lithium secondary battery, a separator may be present between the positive electrode and the negative electrode. Examples of such separators include polyethylene separators, polypropylene separators, polyvinylidene fluoride, and multilayer separators of two or more of these. Of course, mixed multilayer separators such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator can also be used.

[0107] The lithium secondary battery may have an upper limit charging voltage of 4.35 V or more, for example, 4.35 V to 4.55 V.

[0108] Examples of the present invention and comparative examples are described below. These examples are merely examples of the present invention, and the present invention is not limited to these examples.

[0109] Fabrication of lithium secondary batteries Example 1 LiNi as the positive electrode active material 0.75 Mn0.23 Al 0.02 O2, polyvinylidene fluoride as a binder, and acetylene black as a conductive material were mixed in a weight ratio of 96:3:1 and dispersed in N-methylpyrrolidone to prepare a positive electrode active material slurry.

[0110] The positive electrode active material slurry was coated on an aluminum foil having a thickness of 15 μm, dried at 100° C., and then pressed to prepare a positive electrode.

[0111] A mixture of artificial graphite and Si composite in a weight ratio of 93:7 was used as the negative active material, and the negative active material was mixed with styrene-butadiene rubber binder and carboxymethyl cellulose in a weight ratio of 98:1:1, respectively, and dispersed in distilled water to prepare a negative active material slurry.

[0112] The Si composite used had a core containing artificial graphite and silicon particles, and the surface of the core was coated with coal-based pitch.

[0113] 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.

[0114] The prepared positive and negative electrodes were assembled with a 10 μm thick polyethylene separator to prepare an electrode assembly, and an electrolyte was injected to prepare a lithium secondary battery.

[0115] The electrolyte composition is as follows: (Electrolyte composition) Lithium salt: LiPF61.5M Non-aqueous organic solvent: ethyl methyl carbonate: dimethyl carbonate (EMC:DMC = 20:80 volume ratio) Additive: LiPO2F2 1 part by weight (However, in the electrolyte composition, "parts by weight" refers to the relative weight of the additive to 100 parts by weight of the entire electrolyte (lithium salt + non-aqueous organic solvent) excluding the additive.)

[0116] Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that LiPO2F2 in the additive composition was changed to LiBF4.

[0117] Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that LiPO2F2 in the additive composition was changed to LiDFOB.

[0118] Example 4 A lithium secondary battery was manufactured in the same manner as in Example 1, except that LiPO2F2 in the additive composition was changed to LiBOB.

[0119] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1 using the additive composition except that LiPO2F2 was not used.

[0120] Comparative Example 2 A lithium secondary battery was fabricated in the same manner as in Example 1, except that ethylene carbonate was added in an amount of 5 wt % based on the total weight of the non-aqueous organic solvent in the electrolyte composition.

[0121] Comparative Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 1 part by weight of fluoroethylene carbonate (FEC) was added instead of LiPO2F2 in the electrolyte composition.

[0122] Comparative Examples 4 to 9 Lithium secondary batteries were fabricated in the same manner as in Examples 1 to 4 and Comparative Examples 1 and 2, except that the positive electrode active material was changed to LiCoO2.

[0123] Comparative Examples 10 to 15 The positive electrode active material is LiNi0.5 Co 0.2 Al 0.3 Except for changing to O2, lithium secondary batteries were fabricated in the same manner as in Examples 1 to 4 and Comparative Examples 1 and 2.

[0124] Comparative Examples 16 to 21 The positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1 Except for changing to O2, lithium secondary batteries were fabricated in the same manner as in Examples 1 to 4 and Comparative Examples 1 and 2.

[0125] Example 5 A lithium secondary battery was fabricated in the same manner as in Example 1, except that the non-aqueous organic solvent was changed to 100% dimethyl carbonate by volume.

[0126] Examples 6 to 8 Lithium secondary batteries were fabricated in the same manner as in Example 1, except that the mixing ratio of ethyl methyl carbonate and dimethyl carbonate was changed to a volume ratio of 30:70 (Example 6), a volume ratio of 40:60 (Example 7), and a volume ratio of 70:30 (Example 8), respectively.

[0127] Evaluation 1: Evaluation of high-temperature storage characteristics For the lithium secondary batteries fabricated in Examples 1 to 8 and Comparative Examples 1 to 21, the initial DC resistance (DCIR) was measured using the ΔV / ΔI (change in voltage / change in current) value, and then the maximum energy state inside the battery was set to a fully charged state (SOC 100%). After storing the batteries in this state at high temperature (60°C) for 30 days, the DC resistance was measured and the DCIR increase rate (%) was calculated using the following formula 1. The results are shown in Tables 1, 2, and 5 below. [Formula 1] DCIR increase rate = {(DCIR after 30 days) / (initial DCIR)} x 100%

[0128] Evaluation 2: Evaluation of high temperature life characteristics The lithium secondary batteries fabricated in Examples 1 to 4 and Comparative Examples 1 to 3 were charged and discharged once at 0.2 C to measure the charge and discharge capacity.

[0129] Furthermore, the lithium secondary batteries produced in Examples 1 to 4 and Comparative Examples 1 to 3 were charged at the upper limit charge voltage (4.4 V), and then discharged at a constant current of 0.2 C to 2.5 V to measure the initial discharge capacity.

[0130] The discharge capacity was measured again by performing 200 charge / discharge cycles at 0.33 C charge (CC / CV, 4.4 V, 0.025 C cutoff) / 1.0 C discharge (CC, 2.5 V cutoff) at 45°C. The discharge capacity ratio to the initial discharge capacity is shown in Table 4 below as the capacity recovery rate (%).

[0131] Evaluation 3: Measurement of gas generation amount after high temperature storage The lithium secondary batteries manufactured in Examples 1 to 4 and Comparative Examples 1 to 3 were left at 60°C for 7 days, and the amount of gas generated (ml) on the first and seventh days was measured using a refinery gas analyzer (RGA). The results are shown in Table 3 below.

[0132] [Table 1]

[0133] [Table 2]

[0134] [Table 3]

[0135] [Table 4]

[0136] [Table 5]

[0137] Tables 1, 2, and 5 show that the DC-IR increase rate is reduced and high-temperature storage characteristics are improved in compositions that combine the electrolyte according to the present invention with a cobalt-free (Co-free) cathode active material. The DC-IR increase rate after high-temperature storage is expected to be higher and battery life characteristics are expected to be reduced in electrolyte compositions that do not contain the additive according to the present invention (Comparative Example 1), contain other additives (Comparative Example 3), contain 5 wt% or more of EC (Comparative Example 2), or contain other cathode active materials (Comparative Examples 4 to 21).

[0138] It was confirmed that the composition of the non-aqueous organic solvent exhibited better effects when the volume ratio of EMC:DMC was 0:100 to 50:50.

[0139] Referring to Table 3, it can be seen that the amount of gas generated was significantly reduced after high temperature storage.

[0140] Referring to Table 4, it can be seen that the DC-IR increase rate is reduced and high-temperature charge-discharge characteristics are improved in the composition in which the electrolyte solution according to the present invention and the Co-free positive electrode active material are combined.

[0141] Although the preferred embodiment of the present invention has been described above, the present invention is not limited thereto, and various modifications can be made within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and it is to be understood that these modifications also fall within the scope of the present invention. [Explanation of symbols]

[0142] 100: Lithium secondary battery 112: Negative electrode 113: Separator 114: Positive electrode 120:Battery container 140: Enclosure material

Claims

1. an electrolyte solution comprising a non-aqueous organic solvent, a lithium salt, and an additive; a positive electrode including a positive electrode active material; and a negative electrode including a negative electrode active material; The additive is lithium tetrafluoroborate (LiBF ４ ), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiPO ２ F ２ ), and lithium bis(oxalato)borate (LiB(C) ２ O ４ ) ２ : LiBOB), The positive electrode active material is a lithium secondary battery containing a cobalt-free lithium nickel manganese oxide, The non-aqueous organic solvent does not contain ethylene carbonate, The non-aqueous organic solvent comprises ethyl methyl carbonate and dimethyl carbonate, and the volume ratio of ethyl methyl carbonate to dimethyl carbonate is 10:90 to 30:70; the positive electrode active material includes a cobalt-free lithium nickel manganese oxide; The cobalt-free lithium nickel manganese oxide includes a lithium composite oxide represented by the following chemical formula 3: [Chemical formula 3] Li a Ni x Mny M 1 z M 2 w O 2±b X c (In the above Chemical Formula 3, 0.5≦a<1.8, 0≦b≦0.1, 0≦c≦0.1, 0≦w<0.1, 0.6≦x<1.0, 0<y<0.4, 0<z<0.1, and w+x+y+z=1; M 1 and M 2 are each independently one or more elements selected from Al, Mg, Ti, Zr, Cr, Sr, V, B, W, Mo, Nb, Si, Ba, Ca, Ce, Cr, Fe and Nb, and X is one or more elements selected from S, F, P and Cl).

2. The lithium secondary battery described in claim 1, wherein the non-aqueous organic solvent further contains at least one of diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC).

3. 2. The lithium secondary battery according to claim 1, wherein the lithium compound is contained in an amount of 0.05 to 5.0 parts by weight based on 100 parts by weight of the total electrolyte solution for the lithium secondary battery.

4. 2. The lithium secondary battery according to claim 1, wherein the lithium compound is contained in an amount of 0.1 to 5.0 parts by weight based on 100 parts by weight of the total electrolyte solution for the lithium secondary battery.

5. 2. The lithium secondary battery according to claim 1, wherein the electrolyte solution further contains at least one other additive selected from the group consisting of vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, vinylethylene carbonate (VEC), adiponitrile (AN), succinonitrile (SN), 1,3,6-hexanetricyanide (HTCN), propene sultone (PST), propane sultone (PS), and 2-fluorobiphenyl (2-FBP).

6. The lithium secondary battery according to claim 1, wherein the lithium composite oxide represented by Chemical Formula 3 is represented by the following Chemical Formula 3-1: [Chemical formula 3-1] Li ａ Ni ｘ１ Mn ｙ１ Al ｚ１ M ２ ｗ１ O ２±ｂ X ｃ (In the above chemical formula 3-1, 0.5≦a<1.8, 0≦b≦0.1, 0≦c≦0.1, 0≦w1<0.1, 0.6≦x1<1.0, 0<y1<0.4, 0<z1<0.1, and w1+x1+y1+z1=1, M ２ are each independently one or more elements selected from Mg, Ti, Zr, Cr, Sr, V, B, W, Mo, Nb, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

7. 7. The lithium secondary battery according to claim 6, wherein in Chemical Formula 3-1, x1 satisfies 0.6≦x1≦0.79, y1 satisfies 0.2≦y1≦0.39, and z1 satisfies 0.01≦z1<0.

1.

8. The lithium secondary battery according to claim 1 , wherein the negative electrode active material comprises at least one of graphite and a Si composite.

9. 2. The lithium secondary battery according to claim 1, wherein the upper limit charging voltage of the lithium secondary battery is 4.35 V or higher.

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