Lithium secondary battery

Abstract

To provide a lithium secondary battery in which a positive electrode containing lithium nickel manganese oxide is used in combination with an electrolyte solution capable of effectively protecting the positive electrode containing lithium nickel manganese oxide, positive electrode structure collapse is suppressed by reducing transitional metal elution under a high-voltage condition and a high-temperature condition, and accordingly, a high-voltage characteristic and a high-temperature characteristic of the battery are improved.SOLUTION: A lithium secondary battery in one embodiment includes: an electrolyte solution containing a nonaqueous organic solvent, a lithium salt, and an additive; a positive electrode containing a positive electrode active material; and a negative electrode containing a negative electrode active material. The nonaqueous organic solvent contains ethylene carbonate by less than 5 wt% relative to entire weight of the nonaqueous organic solvent. The positive electrode active material contains lithium nickel manganese oxide. The additive contains a compound expressed by Chemical Formula 1. The Chemical Formula 1 is defined in the specification.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This 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, allowing for fast charging. 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 the 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 oxide as a positive electrode active material is a positive electrode active material composed mainly of nickel, manganese, etc., without introducing cobalt into the positive electrode active material composition. Positive electrodes containing this are economical and can achieve high energy density, so it is attracting attention as a next-generation positive electrode active material.

[0005] However, when a positive electrode containing cobalt-free lithium nickel manganese oxide is used in a high-voltage environment, the positive electrode structure collapses, causing transition metal elution, which can lead to problems such as gas generation inside the cell and reduced capacity. This transition metal elution phenomenon tends to become more severe in high-temperature environments, and the eluted 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] Thus, 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] In one embodiment, a lithium secondary battery is provided that uses a positive electrode containing a lithium nickel manganese-based oxide in combination with an electrolyte that can effectively protect the positive electrode containing a lithium nickel manganese-based oxide, thereby reducing the elution of transition metals under high voltage and high temperature conditions, thereby suppressing the collapse of the positive electrode structure, and thereby improving the high voltage and high temperature characteristics of the battery. [Means for solving the problem]

[0008] One embodiment of the present invention includes 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 ethylene carbonate in an amount of less than 5 wt % based on the total weight of the non-aqueous organic solvent; the positive electrode active material includes a lithium nickel manganese oxide, The additive provides a lithium secondary battery, which includes a compound represented by the following Chemical Formula 1: [ka]

[0009] In the above chemical formula 1, L 1 is a substituted or unsubstituted alkylene group having 1 to 3 carbon atoms, R 1 is hydrogen or a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, R 2is a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkenyl group having 2 to 10 carbon atoms, a substituted or unsubstituted alkynyl group having 2 to 10 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 20 carbon atoms.

[0010] The non-aqueous organic solvent may consist solely of a chain carbonate.

[0011] The chain carbonate can be represented by the following chemical formula 2. [ka]

[0012] In the above chemical formula 2, R 3 and R 4 are each independently a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms.

[0013] The non-aqueous organic solvent may be a mixed solvent of 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).

[0014] 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.

[0015] R of Formula 1 2 may be a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms.

[0016] R of Formula 1 2 may be a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms.

[0017] R of Formula 1 2may be a substituted or unsubstituted alkyl group having 1 to 3 carbon atoms.

[0018] The compound represented by Chemical Formula 1 can be represented by the following Chemical Formula 1-1. [ka]

[0019] The compound represented by Chemical Formula 1 may be included in an amount of 0.05 to 5.0 parts by weight based on 100 parts by weight of the total amount (lithium salt + non-aqueous organic solvent) excluding additives in the electrolyte solution for lithium secondary batteries.

[0020] 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), lithium tetrafluoroborate (LiBF), lithium difluorophosphate (LiPOF), and 2-fluorobiphenyl (2-FBP).

[0021] The lithium nickel manganese-based oxide may include a cobalt-free lithium composite oxide represented by the following Chemical Formula 4. [Chemical formula 4] Li a Ni x Mn y M 1 z M 2 w O 2±b X c

[0022] In the above chemical formula 4, 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, Sr, V, B, W, Mo, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

[0023] The above-mentioned Chemical Formula 4 can be represented by the following Chemical Formula 4-1. [Chemical formula 4-1] Li a Ni x1 Mn y1 Al z1 M 2 w1 O 2±b X c

[0024] In the above chemical formula 4-1, 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 is one or more elements selected from Mg, Ti, Zr, Sr, V, B, W, Mo, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

[0025] In the above Chemical Formula 4-1, 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.

[0026] The negative electrode active material may include at least one of graphite and a Si composite.

[0027] The lithium secondary battery may have an upper limit charging voltage of 4.35 V or higher. [Effects of the Invention]

[0028] In one embodiment, a positive electrode including a lithium nickel manganese-based oxide is used in combination with an electrolyte capable of effectively protecting the positive electrode including a cobalt-free lithium nickel manganese-based oxide, thereby ensuring phase transition safety of the positive electrode even in a high-temperature, high-voltage environment, and suppressing decomposition of the electrolyte and side reactions with the electrode, thereby reducing gas generation and suppressing an increase in the internal resistance of the battery, thereby realizing a lithium secondary battery with improved battery stability and life characteristics. [Brief explanation of the drawings]

[0029] [Figure 1] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, a lithium secondary battery according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[0031] However, this is presented by way of example only and does not limit the invention, which is defined only by the claims that follow.

[0032] Unless otherwise defined, the term "substituted" as used herein means that at least one hydrogen atom in a substituent or compound is replaced with deuterium, a halogen group, a hydroxyl group, an amino group, a substituted or unsubstituted amine group having 1 to 30 carbon atoms, a nitro group, a substituted or unsubstituted silyl group having 1 to 40 carbon atoms, an alkyl group having 1 to 30 carbon atoms, an alkylsilyl group having 1 to 10 carbon atoms, an arylsilyl group having 6 to 30 carbon atoms, a cycloalkyl group having 3 to 30 carbon atoms, a heterocycloalkyl group having 3 to 30 carbon atoms, an aryl group having 6 to 30 carbon atoms, a heteroaryl group having 2 to 30 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, a fluoroalkyl group having 1 to 10 carbon atoms, a cyano group, or a combination thereof.

[0033] In one embodiment of the present invention, "substituted" means that at least one hydrogen atom in a substituent or compound is substituted with deuterium, a halogen group, an alkyl group having 1 to 30 carbon atoms, an alkylsilyl group having 1 to 10 carbon atoms, an arylsilyl group having 6 to 30 carbon atoms, a cycloalkyl group having 3 to 30 carbon atoms, a heterocycloalkyl group having 3 to 30 carbon atoms, an aryl group having 6 to 30 carbon atoms, a heteroaryl group having 2 to 30 carbon atoms, a fluoroalkyl group having 1 to 10 carbon atoms, 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 substituted with deuterium, a halogen group, an alkyl group having 1 to 20 carbon atoms, an aryl group having 6 to 30 carbon atoms, a fluoroalkyl group having 1 to 10 carbon atoms, 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 substituted with deuterium, a halogen group, an alkyl group having 1 to 5 carbon atoms, an aryl group having 6 to 18 carbon atoms, a fluoroalkyl group having 1 to 5 carbon atoms, or a cyano group. In addition, in a specific example of the present invention, "substituted" means that at least one hydrogen atom in 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.

[0034] 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.

[0035] 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 containing the battery cell, and an encapsulation member 140 sealing the battery container 120.

[0036] The configuration of the lithium secondary battery 100 according to one embodiment of the present invention will be described in more detail below.

[0037] A lithium secondary battery according to one embodiment of the present invention includes an electrolyte, a positive electrode, and a negative electrode.

[0038] The electrolyte solution includes a non-aqueous organic solvent, a lithium salt, and an additive, and the non-aqueous organic solvent includes ethylene carbonate in an amount of less than 5 wt % based on the total weight of the non-aqueous organic solvent, and the additive may include a compound represented by the following Chemical Formula 1: [ka]

[0039] In the above chemical formula 1, L 1 is a substituted or unsubstituted alkylene group having 1 to 3 carbon atoms, R 1 is hydrogen or a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, R 2 is a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkenyl group having 2 to 10 carbon atoms, a substituted or unsubstituted alkynyl group having 2 to 10 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 20 carbon atoms.

[0040] The positive electrode may include a positive electrode active material including a lithium nickel manganese-based oxide.

[0041] In the case of positive electrode active materials containing lithium nickel manganese oxide, especially cobalt-free lithium nickel manganese oxide, structural instability is high under high voltage conditions, causing solvent decomposition and leaching of transition metals, especially Ni.

[0042] The degradation and short circuit caused by the elution of transition metals reduces the life capacity of the battery and causes a sudden increase in resistance.

[0043] However, when the above-mentioned electrolyte is used together, the decrease in battery life capacity and the sudden increase in resistance can be alleviated.

[0044] In particular, by using a positive electrode containing a cobalt-free lithium nickel manganese oxide in an electrolyte containing ethylene carbonate in an amount of less than 5 wt % based on the total weight of the non-aqueous organic solvent, 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.

[0045] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0046] The non-aqueous organic solvent may be a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent.

[0047] 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-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolanes, etc.

[0048] 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.

[0049] For example, the non-aqueous organic solvent may contain ethylene carbonate in an amount of less than 5% by weight based on the total weight of the non-aqueous organic solvent.

[0050] If the content of ethylene carbonate 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, and as a result, Ni is dissolved and deposited on the anode.

[0051] As a specific example, the non-aqueous organic solvent may be composed solely of a chain carbonate, in which case the resistance increase rate during high temperature storage is significantly reduced, thereby achieving excellent high temperature storage characteristics.

[0052] In this specification, the expression "composed solely of a chain carbonate" means that the organic solvent is not mixed with a cyclic carbonate or the like, and is a single organic solvent or a combination of organic solvents that belong to the category of a chain carbonate.

[0053] In one embodiment, the chain carbonate can be represented by the following Chemical Formula 2: [ka]

[0054] In the above chemical formula 2, R 3 and R 4 are each independently a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms.

[0055] As an example, R in Formula 2 3 and R 4 may each independently represent a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, and for example, 3 and R 4 may each independently be a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms.

[0056] In one embodiment, R in Formula 2 3 and R 4may 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.

[0057] For example, the non-aqueous organic solvent according to a specific embodiment may be a mixed solvent of 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).

[0058] 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).

[0059] The non-aqueous organic solvent may contain ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a weight ratio of 0:100 to 50:50.

[0060] It may be more advantageous in terms of improving battery characteristics if the non-aqueous organic solvent contains dimethyl carbonate (DMC) in an amount of more than 50 wt % based on the total weight of the non-aqueous organic solvent.

[0061] For example, the non-aqueous organic solvent may contain ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of 0:100 to 40:60, 0:100 to 30:70, 10:90 to 40:60, or 10:90 to 30:70.

[0062] 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.

[0063] The aromatic hydrocarbon solvent may be an aromatic hydrocarbon compound represented by the following formula 3: [ka]

[0064] In the above chemical formula 3, R 11 ~R 16 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, and combinations thereof.

[0065] 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.

[0066] 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, LiBF4, lithium difluoro(oxalate)borate (LiDFOB), LiPO2F2, LiSbF6, LiAsF6, 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) (wherein x and y are each independently an integer of 1 to 20), LiCl, LiI, and LiB(C2O4)2 (lithium bis(oxalato)borate; LiBOB).

[0067] The lithium salt is preferably used at a concentration 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.

[0068] R of Formula 1 2 may be a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms.

[0069] R of Formula 1 2 may be a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms.

[0070] R of Formula 1 2 may be a substituted or unsubstituted alkyl group having 1 to 3 carbon atoms.

[0071] The compound represented by Chemical Formula 1 can be represented by the following Chemical Formula 1-1. [ka]

[0072] The compound represented by Chemical Formula 1 may be included in an amount of 0.05 to 5.0 parts by weight based on 100 parts by weight of the total amount (lithium salt + non-aqueous organic solvent) excluding additives in the electrolyte solution for lithium secondary batteries.

[0073] For example, the compound represented by Chemical Formula 1 may be included in an amount of 0.05 to 3.0 parts by weight based on 100 parts by weight of the total amount (lithium salt + non-aqueous organic solvent) excluding additives in the electrolyte solution for lithium secondary batteries.

[0074] For example, the compound represented by Chemical Formula 1 may be included in an amount of 0.1 to 3.0 parts by weight, 0.3 to 3.0 parts by weight, 0.5 to 3.0 parts by weight, or 0.5 to 2.0 parts by weight, based on 100 parts by weight of the total amount (lithium salt + non-aqueous organic solvent) excluding additives in the electrolyte solution for lithium secondary batteries.

[0075] When the content of the compound represented by Chemical Formula 1 is within the above range, an increase in resistance at high temperatures can be prevented, thereby realizing a lithium secondary battery with improved life and output characteristics.

[0076] 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), succinic acid nitrile (SN), 1,3,6-hexanetricyanide (HTCN), propene sultone (PST), propane sultone (PS), lithium tetrafluoroborate (LiBF), lithium difluorophosphate (LiPOF), and 2-fluorobiphenyl (2-FBP).

[0077] By further including the other additives, the life span can be further improved or gas generated from the positive and negative electrodes during high temperature storage can be effectively controlled.

[0078] 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 amount (lithium salt + non-aqueous organic solvent) of the electrolyte solution for lithium secondary batteries excluding additives.

[0079] When the content of other additives is within the above range, the increase in film resistance can be minimized, thereby improving battery performance.

[0080] 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.

[0081] The positive electrode active material may include a lithium nickel manganese-based oxide.

[0082] For example, the lithium nickel manganese-based oxide may include at least one of cobalt-free lithium composite oxides represented by the following Chemical Formula 4: [Chemical formula 4] Li a Ni x Mn y M 1 z M 2 w O 2±b X c

[0083] In the above chemical formula 4, 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, Sr, V, B, W, Mo, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

[0084] In this specification, the cobalt-free lithium composite oxide as a positive electrode active material means a positive electrode active material that does not contain cobalt in its composition and is composed mainly of nickel, manganese, and the like.

[0085] 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 forming the 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. Since this method is well understood by those skilled in the art, detailed explanations will be omitted.

[0086] For example, the formula 4 can be represented by the following formula 4-1. [Chemical formula 4-1] Li a Ni x1 Mn y1 Al z1 M 2 w1 O 2±b X c

[0087] In the above chemical formula 4-1, 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 is one or more elements selected from Mg, Ti, Zr, Sr, V, B, W, Mo, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

[0088] In one embodiment, in the formula 4-1, 0.6≦x1≦0.9, 0.1≦y1<0.4, and 0 <z1<0.1であってもよく、0.6≦x1≦0.8、0.2≦y1<0.4、および0<z1<0.1であってもよい。

[0089] For example, in the above chemical formula 4-1, 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.

[0090] 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.

[0091] In one embodiment of the present invention, the positive electrode active material layer may include a binder, and the content of the binder may be 1 wt % to 5 wt % based on the total weight of the positive electrode active material layer.

[0092] The binder serves to firmly adhere the positive electrode active material particles to each other and to firmly adhere the positive electrode active material 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.

[0093] The positive electrode current collector may be made of Al, but is not limited thereto.

[0094] The negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector and containing a negative electrode active material.

[0095] The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and undoping lithium, or a transition metal oxide.

[0096] As the material capable of reversibly intercalating / deintercalating lithium ions, carbon materials can be used, and any of the carbon-based negative electrode active materials generally used in lithium secondary batteries can be used. Representative examples thereof include crystalline carbon, amorphous carbon, or both. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite. Examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, fired coke, and the like.

[0097] As the alloy of 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.

[0098] Examples of the material 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 11 (where R 11 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), and the like. At least one of these can also be used in combination with SiO2.

[0099] The elements Q and R11 Examples of such materials that can be used include 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.

[0100] Examples of the transition metal oxide include vanadium oxide, lithium vanadium oxide, or lithium titanium oxide.

[0101] In a specific embodiment, the negative electrode active material can include at least one of graphite and a Si composite.

[0102] The Si composite includes a core containing Si-based particles and an amorphous carbon coating layer. For example, the Si-based particles can include one or more of silicon particles, Si-C composites, SiO x (0 < x ≦ 2), and Si alloys.

[0103] 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 may be 10 nm to 200 nm.

[0104] In this specification, the average particle size may be the particle size (D50) at 50% as the volume ratio in the cumulative size-distribution curve.

[0105] 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 interruption of the conductive path due to particle crushing during charge and discharge can be prevented.

[0106] The core containing the Si-based particles additionally contains amorphous carbon, and in this case, the central portion does not contain amorphous carbon, and amorphous carbon is present only in the surface portion of the Si composite.

[0107] In this case, the surface portion means the region from the outermost surface of the center to the outermost surface of the Si composite.

[0108] Furthermore, the Si-based particles are present substantially uniformly throughout the Si composite, that is, at a substantially uniform concentration in the center and the surface.

[0109] The amorphous carbon may be soft carbon, hard carbon, mesophase pitch charcoal, calcined coke, or a combination thereof.

[0110] For example, the Si—C composite can include silicon particles and crystalline carbon.

[0111] The silicon particles may be included in an amount of 1 to 60 wt % of the total weight of the Si—C composite, for example, 3 to 60 wt %.

[0112] The crystalline carbon may be, for example, graphite, and more specifically, natural graphite, artificial graphite, or a combination thereof.

[0113] The crystalline carbon may have an average particle size of 5 μm to 30 μm.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] The content of the negative electrode active material in the negative electrode active material layer may be 95 wt % to 99 wt % based on the total weight of the negative electrode active material layer.

[0118] 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 binder in an amount of 1 wt % to 5 wt %, and the conductive material in an amount of 1 wt % to 5 wt %.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] The conductive material is used to impart conductivity to the electrodes, and any material can be used as long as it does not cause a chemical change in the constructed battery and is electronically conductive. 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 mixtures thereof.

[0124] The negative electrode current collector may be made of a material 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.

[0125] Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such separators can be made of polyethylene, polypropylene, polyvinylidene fluoride, or multilayer films of two or more of these. Of course, mixed multilayer films 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.

[0126] The lithium secondary battery may have an upper limit charging voltage of 4.35 V or more. For example, the upper limit charging voltage may be 4.35 V to 4.55 V.

[0127] Examples of the present invention and comparative examples are described below. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples. [Example]

[0128] Fabrication of lithium secondary batteries Example 1 LiNi as the positive electrode active material 0.75 Mn 0.23 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.

[0129] 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.

[0130] A mixture of artificial graphite and Si 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, and carboxymethyl cellulose were mixed in a weight ratio of 98:1:1, respectively, and dispersed in distilled water to prepare a negative electrode active material slurry.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] 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: 1 part by weight of a compound represented by the following chemical formula 1-1 / 10 parts by weight of fluoroethylene carbonate (FEC) / 0.5 parts by weight of succinonitrile (SN) [ka] (However, in the above 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.)

[0135] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Formula 1-1 was not added to the electrolyte composition.

[0136] 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 20 wt % based on the total weight of the non-aqueous organic solvent in the electrolyte composition.

[0137] Examples 2 to 4 Lithium secondary batteries were manufactured in the same manner as in Example 1, except that the compound represented by Formula 1-1 was added in amounts of 0.5 parts by weight, 1.5 parts by weight, and 2 parts by weight, respectively.

[0138] Comparative Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that 1 part by weight of a compound represented by the following formula a was added instead of the compound represented by formula 1-1 in the electrolyte composition. [ka]

[0139] 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.

[0140] Comparative Examples 4 to 6 Lithium secondary batteries were fabricated in the same manner as in Example 1 and Comparative Examples 1 and 2, except that the positive electrode active material was changed to LiCoO2.

[0141] Comparative Examples 7 to 9 The positive electrode active material is LiNi 0.5 Co 0.2 Al 0.3Lithium secondary batteries were fabricated in the same manner as in Example 1 and Comparative Examples 1 and 2, except that O2 was used.

[0142] Comparative Examples 10 to 12 The positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1 Lithium secondary batteries were fabricated in the same manner as in Example 1 and Comparative Examples 1 and 2, except that O2 was used.

[0143] Examples 6, 7 and Comparative Example 13 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), 40:60 (Example 7), and 70:30 (Comparative Example 13), respectively.

[0144] The compositions are as shown in Table 1 below.

[0145] [Table 1A] [Table 1B]

[0146] Evaluation 1: High temperature storage characteristics evaluation For the lithium secondary batteries fabricated in Examples 1 to 7 and Comparative Examples 1 to 13, the initial DC resistance (DCIR) was measured using ΔV / ΔI (change in voltage / change in current) values. The maximum energy state inside the battery was defined as a fully charged state (SOC 100%), and the batteries were stored at high temperature (60°C) for 30 days, after which the DC resistance was measured. The DCIR increase rate (%) was calculated using the following equation 1, and the results are shown in Tables 2, 3, and 6. [Formula 1] DCIR increase rate = {(DCIR after 30 days) / (initial DCIR)} x 100%

[0147] Evaluation 2: High temperature life characteristics evaluation The lithium secondary batteries fabricated in Example 1 and Comparative Examples 1 to 3 were charged and discharged once at 0.2 C to measure the charge and discharge capacity. Furthermore, the lithium secondary batteries produced in Example 1 and Comparative Examples 1 to 3 were charged at an upper charge voltage limit of 4.35 V, and then discharged at a constant current of 0.2 C to 2.5 V to measure the initial discharge capacity. The discharge capacity was measured again by performing 200 charge / discharge cycles under the conditions of 0.33 C charge (CC / CV, 4.35 V, 0.025 C cut-off) / 1.0 C discharge (CC, 2.5 V cut-off) and 45° C. The ratio of the discharge capacity to the initial discharge capacity was recorded as the capacity recovery rate (%, recovery) in Table 5 below, and the recovery rate increase rate relative to Comparative Example 2 was calculated using the following Equation 2, and the results are shown in Table 5 below. [Formula 2] Increase rate of capacity recovery rate relative to Comparative Example 2={(capacity recovery rate) / (capacity recovery rate of Comparative Example 2)}*100

[0148] Evaluation 3: Measurement of gas generation rate after high temperature storage The lithium secondary batteries according to Example 1, Example 5, 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 amount of gas generated and the rate of increase calculated by the following formula 3 are shown in Table 4 below. [Formula 3] Increase rate = {(gas generation amount after 7 days) / (gas generation amount on day 1)}*100

[0149] [Table 2]

[0150] [Table 3]

[0151] [Table 4]

[0152] [Table 5]

[0153] [Table 6]

[0154] Referring to Tables 2 to 6, it can be seen that the DC-IR increase rate is reduced and the high-temperature storage characteristics and high-temperature charge / discharge characteristics are all improved when the electrolyte solution according to the present invention is combined with a cobalt-free positive electrode active material.

[0155] In addition, it can be seen that the lithium secondary batteries according to the examples exhibited a significant reduction in the amount of gas generated after storage at high temperatures.

[0156] 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]

[0157] 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 comprising a positive electrode active material; and a negative electrode including a negative electrode active material; The non-aqueous organic solvent is composed of a chain carbonate and ethylene carbonate, which is contained in an amount of less than 5 wt % or is not contained in the non-aqueous organic solvent based on the total weight of the non-aqueous organic solvent; the chain carbonate is a mixed solvent of 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); the positive electrode active material includes a lithium nickel manganese oxide, The lithium nickel manganese oxide is a cobalt-free lithium composite oxide represented by the following chemical formula 4: [Chemical formula 4] Li a Ni x Mny M 1 z M 2 w O 2±b X c In the above chemical formula 4, 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, Sr, V, B, W, Mo, Si, Ba, Ca, Ce, Cr, Fe, and Nb; X is one or more elements selected from S, F, P, and Cl; The additive for a lithium secondary battery includes a compound represented by the following Chemical Formula 1: 【Chemical 1】 In the above chemical formula 1, L １ is a substituted or unsubstituted alkylene group having 1 to 3 carbon atoms, R １ is hydrogen or a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, R ２ is a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted alkenyl group having 2 to 10 carbon atoms, a substituted or unsubstituted alkynyl group having 2 to 10 carbon atoms, or a substituted or unsubstituted aryl group having 6 to 20 carbon atoms.

2. 2. The lithium secondary battery according to claim 1, wherein the non-aqueous organic solvent consists solely of the chain carbonate.

3. 2. The lithium secondary battery according to claim 1, wherein the non-aqueous organic solvent is a mixed solvent of ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of 10:90 to 50:

50.

4. R of Formula 1 ２ 2. The lithium secondary battery according to claim 1, wherein is a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms.

5. R of Formula 1 ２ 2. The lithium secondary battery according to claim 1, wherein is a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms.

6. R of Formula 1 ２ 2. The lithium secondary battery according to claim 1, wherein is a substituted or unsubstituted alkyl group having 1 to 3 carbon atoms.

7. The compound represented by Chemical Formula 1 is represented by the following Chemical Formula 1-1: 【Chemistry 3】 。

8. 2. The lithium secondary battery of claim 1, wherein the compound represented by Chemical Formula 1 is included in an amount of 0.05 to 5.0 parts by weight based on 100 parts by weight of the total amount (lithium salt + non-aqueous organic solvent) of the electrolyte solution for a lithium secondary battery excluding the additive.

9. The electrolyte may be selected from 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), succinic acid nitrile (SN), 1,3,6-hexanetricyanide (HTCN), propene sultone (PST), propane sultone (PS), lithium tetrafluoroborate (LiBF ４ ), lithium difluorophosphate (LiPO ２ F ２ 2. The lithium secondary battery according to claim 1, further comprising at least one other additive selected from the group consisting of 2-fluorobiphenyl (2-FBP), ... and 2-fluorobiphenyl (2-FBP).

10. The lithium secondary battery according to claim 1, wherein the chemical formula 4 is represented by the following chemical formula 4-1: [Chemical formula 4-1] Li ａ Ni ｘ１ Mn ｙ１ Al ｚ１ M ２ ｗ１ O ２±ｂ X ｃ In the above chemical formula 4-1, 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 selected from Mg, Ti, Zr, Sr, V, B, W, Mo, Si, Ba, Ca, Ce, Cr, Fe, and Nb, and X is one or more elements selected from S, F, P, and Cl.

11. 11. The lithium secondary battery according to claim 10, wherein, in Chemical Formula 4-1, x1 is 0.6≦x1≦0.79, y1 is 0.2≦y1≦0.39, and z1 is 0.01≦z1<0.

1.

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

13. 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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