KR20140067242A - Electrolyte for secondary battery and lithium secondary battery containing the same - Google Patents

Abstract

The present invention relates to an electrolyte for a high-voltage lithium secondary battery and a high-voltage lithium secondary battery including the same. More specifically, the present invention relates to an electrolyte for a high-voltage lithium secondary battery which has excellent high-temperature storage characteristics while reducing the rate of increase in battery thickness by inhibiting the generation of gas to prevent the swelling of the battery because the electrolyte is not oxidized and degraded when kept at high temperatures under high voltages, and which has also excellent discharge characteristics at low temperatures; and to a high-voltage lithium secondary battery including the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a high-voltage lithium secondary battery and a high-voltage lithium secondary battery containing the same, and more particularly, To an electrolyte for a high-voltage lithium secondary battery, which has an excellent high-temperature storage characteristic and an excellent discharge characteristic at a low temperature, and a high-voltage lithium secondary battery comprising the same.

2. Description of the Related Art [0002] In recent years, portable electronic devices have been widely used. As a result, with the rapid miniaturization, light weight, and thinness of portable electronic devices, batteries as a power source thereof are small, lightweight and can be charged / discharged for a long time. Development is strongly required.

Among the currently applied secondary batteries, the lithium secondary battery developed in the early 1990s has advantages such as higher operating voltage and higher energy density than conventional batteries such as Ni-MH, Ni-Cd and sulfuric acid-lead batteries using an aqueous electrolyte solution . However, such a lithium secondary battery has a safety problem such as ignition and explosion due to the use of a non-aqueous electrolyte, and such a problem becomes more serious as the capacity density of the battery is increased.

The nonaqueous electrolyte secondary battery has a serious problem in that the safety of the battery generated during continuous charging is lowered. One of the possible causes of this is fever due to structural collapse of the anode. The working principle of this is as follows. That is, the positive electrode active material of the nonaqueous electrolyte battery is composed of a lithium-containing metal oxide capable of intercalating and deintercalating lithium and / or lithium ions, etc. Such a positive electrode active material is deformed into a thermally unstable structure do. When the battery temperature reaches a critical temperature due to an external physical impact such as high temperature exposure in the overcharged state, oxygen is released from the cathode active material having an unstable structure, and the released oxygen causes an exothermic decomposition reaction with the electrolyte solvent or the like. Particularly, since the combustion of the electrolytic solution is further accelerated by the oxygen released from the anode, such a series exothermic reaction causes ignition and rupture of the battery due to thermal runaway.

In order to control the ignition or explosion due to the temperature rise inside the battery, a method of adding an aromatic compound as a redox shuttle additive in the electrolyte is used. For example, Japanese Patent Publication JP2002-260725 discloses a non-aqueous lithium ion battery capable of preventing an overcharge current and a thermal runaway phenomenon by using an aromatic compound such as biphenyl. In addition, U.S. Patent No. 5,879,834 discloses that aromatic compounds such as biphenyl and 3-chlorothiophene are added in a small amount to electrochemically polymerize in an abnormal overvoltage state to increase the internal resistance to improve the safety of the battery Is described.

However, when using an additive such as biphenyl, the amount of biphenyl is gradually reduced when the battery is decomposed gradually in the charge / discharge process or when the battery is discharged at a high temperature for a long period of time at a relatively high voltage locally at a normal operating voltage There is a problem that safety can not be guaranteed after 300 cycles of charging / discharging, and problems of storage characteristics.

On the other hand, a high voltage battery (4.4V system) has been continuously studied and developed in order to increase the electric charge amount in order to increase the capacity of the battery. Increasing the charging voltage in the same battery system generally increases the charge. However, there arises a problem of safety such as electrolyte decomposition, insufficient space for storing lithium, and danger of electrode potential rise. Therefore, in order to produce a cell operated at a high voltage, the standard reduction potential difference between the anode active material and the cathode active material is likely to be largely maintained, and the entire system is managed by the system so that the electrolyte is not decomposed at this voltage.

Considering this point of the high-voltage battery, when the conventional overcharge preventing agent such as biphenyl (BP) or cyclohexylbenzene (CHB) used in a general lithium ion battery is used, they are decomposed much during normal charge- It can be easily seen that the characteristics of the battery drastically deteriorate even at a high temperature, shortening the life of the battery. When a non-aqueous carbonate-based solvent that is generally used is used as an electrolyte, the electrolyte is charged at a voltage higher than 4.2 V, which is a typical charging potential, and the decomposition reaction of the electrolyte progresses as the charging / There is a problem that it is rapidly deteriorated.

Accordingly, there is a continuing need to develop a method for improving the safety and the capacity at the time of high-temperature storage without deteriorating the life characteristics of the high-voltage battery (4.4V system).

Japanese Patent JP2002-260725 U.S. Patent 5,879,834

Disclosure of Invention Technical Problem [8] The present invention has been made to solve the conventional problems, and it is an object of the present invention to provide a lithium secondary battery which is excellent in basic performance such as high rate charge / discharge characteristics and life characteristics, An electrolyte for a high-voltage lithium secondary battery excellent in high-temperature storage characteristics and excellent in discharge characteristics at a low temperature, and a high-voltage lithium secondary battery comprising the same.

In order to achieve the above object,

Lithium salts;

Non-aqueous organic solvent; And

A nitrile ester compound represented by the following formula (1):

[Chemical Formula 1]

In the above formula (1), R is a C1-C5 alkyl group, and n is an integer of 1 to 10.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, R is methyl, ethyl, propyl, butyl or pentyl, and n may be an integer of 1, 2 or 3.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the nitrile ester compound may be selected from a nitrile ester compound having the following structure:

.

.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the nitrile ester compound may be contained in an amount of 1 to 20 wt% based on the total weight of the electrolyte solution.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the electrolyte may be one or more selected from the group consisting of an oxalate borate compound, a carbonate compound substituted with fluorine, a vinylidene carbonate compound, and a sulfinyl group- And may further include two or more additives.

In an electrolyte solution for a high-voltage lithium secondary battery according to an embodiment of the present invention, the electrolyte solution may be lithium difluoroarsalate borate (LiFOB), lithium bis oxalate borate (LiB (C ₂ O ₄ ) ₂ , LiBOB), fluoroethylene (FEC), vinylene carbonate (VC), vinylethylene carbonate (VEC), divinyl sulfone, ethylene sulfite, propylene sulfite, diallyl sulfonate ), Ethane sultone, propane sultone (PS), butane sultone, ethene sultone, butene sultone and propene sultone (PRS).

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolytic solution.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the non-aqueous organic solvent may be selected from a cyclic carbonate solvent, a linear carbonate solvent, and a mixed solvent thereof. The cyclic carbonate may include ethylene carbonate, propylene carbonate, And the linear carbonate may be selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methyl methyl carbonate, ethyl methyl carbonate, methyl ethyl ketone, methyl ethyl ketone, methyl ethyl ketone, Propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate, and mixtures thereof.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the non-aqueous organic solvent may have a mixing volume ratio of linear carbonate solvent: cyclic carbonate solvent of 9: 1 to 5: 5.

In the high-voltage lithium secondary battery electrolyte according to one embodiment of the invention, the lithium salt is _{LiPF 6, LiBF 4, LiClO 4} , LiSbF 6, LiAsF 6, LiN (SO 2 C 2 F 5) 2, LiN (CF 3 SO _{2) 2, LiN (SO 3} C 2 F 5) 2, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiC 6 H 5 SO 3, LiSCN, LiAlO 2, LiAlCl 4, LiN (C x F 2x + 1 SO ₂ ) (C _y F _{2y + 1} SO ₂ ) wherein x and y are natural numbers, LiCl, LiI and LiB (C ₂ O ₄ ) ₂ .

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the lithium salt may be present at a concentration of 0.1 to 2.0 M.

The present invention also provides a high-voltage lithium secondary battery comprising the electrolyte for the high-voltage lithium secondary battery.

The electrolyte for a high-voltage lithium secondary battery according to the present invention comprises a nitrile ester compound having a specific structure in which the number of carbon atoms of an alkylene group existing between a nitrile group and a carbonyl group is changed to two or more, Swelling phenomenon is remarkably improved to exhibit excellent high temperature storage characteristics as well as excellent discharge characteristics even at a low temperature.

In addition, the high-voltage lithium secondary battery including the electrolyte for a high-voltage lithium secondary battery according to the present invention maintains satisfactory basic performances such as high efficiency of charge / discharge characteristics and lifetime characteristics, and the phenomenon that the electrolyte is oxidized / swelling is remarkably improved to exhibit excellent high-temperature storage characteristics, and exhibit excellent discharge characteristics even at low temperatures.

1 - Results of measurement of oxidative decomposition voltage of Example 2 and Comparative Example 1

Hereinafter, the present invention will be described in more detail. Unless otherwise defined, technical terms and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In the following description, And a description of the known function and configuration will be omitted.

The present invention relates to an electrolyte solution for a high-voltage lithium secondary battery, which provides a battery having high-temperature storage characteristics and long life characteristics while securing the stability of the battery in a high-voltage state.

The present invention relates to a lithium salt; Non-aqueous organic solvent; And a nitrile ester compound represented by the following formula (1): < EMI ID = 1.0 >

[Chemical Formula 1]

In the above formula (1), R is a C1-C5 alkyl group, and n is an integer of 1 to 10.

Generally, in the case of a compound in which a methylene group (CH ₂ ) is introduced between a nitrile group and a carbonyl group (C (= O)), the hydrogen atom of the methylene group existing between the nitrile group and the carbonyl group is easily disappeared, . However, the electrolyte for a high-voltage lithium secondary battery according to the present invention contains a nitrile ester compound having a specific structure in which the number of carbon atoms of the alkylene group existing between the nitrile group and the carbonyl is changed to two or more, Since the existing hydrogen atoms of the alkylene are stably bonded, the side reaction in the battery is suppressed. As a result, the electrolytic solution is oxidized / decomposed at a high voltage to remarkably improve the swelling of the battery, But exhibits excellent discharge characteristics even at low temperatures.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, R in the formula (1) may be selected from methyl, ethyl, propyl, butyl or pentyl and most preferably selected from nitrile ester compounds Can:

.

.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the nitrile ester compound of Formula 1 may be contained in an amount of 1 to 20 wt%, more preferably 1 to 15 wt% %. When the content of the nitrile ester compound of the formula 1 is less than 1% by weight, the effect of suppressing the swelling of the battery during storage at a high temperature or the improvement of the capacity retention rate is insignificant and the discharging capacity of the lithium secondary battery Or output of the lithium secondary battery is insignificant. If the lithium secondary battery is contained in an amount exceeding 20% by weight, the life of the lithium secondary battery deteriorates.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the electrolyte is a lifetime enhancing additive for improving the life of the battery. The electrolyte is an oxalate borate compound, a carbonate compound substituted with fluorine, a vinylidene carbonate compound, Lt; / RTI > group-containing compound.

The oxalate borate compound may be a compound represented by the following formula (2) or lithium bis oxalate borate (LiB (C ₂ O ₄ ) ₂ , LiBOB):

(2)

In Formula 2, R ₁₁ and R ₁₂ are each independently a halogen atom or a halogenated C 1 to C 10 alkyl group.

Specific examples of the oxalate borate type additive include LiB (C ₂ O ₄ ) F ₂ (lithium difluoroarsalate borate, LiFOB) or LiB (C ₂ O ₄ ) ₂ (lithium bis oxalate borate, LiBOB) .

The fluorine-substituted carbonate compound may be fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), fluorodimethyl carbonate (FDMC), fluoroethylmethyl carbonate (FEMC) or a combination thereof.

The vinylidene carbonate-based compound may be vinylene carbonate (VC), vinylethylene carbonate (VEC), or a mixture thereof.

The sulfinyl group (S = O) -containing compound may be sulfone, sulfite, sulfonate and sulphonate (cyclic sulfonate), which may be used singly or in combination. Specifically, the sulfone may be represented by the following formula (3) and may be divinyl sulfone. The sulfite may be represented by the following general formula (4), and may be ethylene sulfite or propylene sulfite. The sulfonate may be represented by the following formula (5) and may be a diallyl sulfonate. Also, non-limiting examples of sulphon include ethane sulphone, propane sulphone, butane sulphone, ethene sulphone, buten sulphone, propene sultone, and the like.

(3)

[Chemical Formula 4]

[Chemical Formula 5]

(Wherein R ₁₃ and R ₁₄ each independently represent a hydrogen atom, a halogen atom, a C 1 -C 10 alkyl group, a C 2 -C 10 alkenyl group, a halogen-substituted C 1 -C 10 alkyl group, or a halogen A substituted C2-C10 alkenyl group).

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the electrolyte is preferably lithium difluoroarsalate borate (LiFOB), lithium bis oxalate borate (LiB (C ₂ O ₄ ) ₂ , LiBOB) (FEC), vinylene carbonate (VC), vinylethylene carbonate (VEC), divinyl sulfone, ethylene sulfite, propylene sulfite, diallyl sulfonate may further comprise additives selected from the group consisting of diallyl sulfonate, ethane sultone, propane sultone (PS), butane sultone, ethene sultone, butene sultone and propene sultone (PRS) have.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, although the content of the additive is not limited, it is preferably 0.1 to 5% by weight based on the total weight of the electrolyte to improve battery life in the secondary battery electrolyte Preferably 0.1 to 3% by weight.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the non-aqueous organic solvent may include a carbonate, an ester, an ether, or a ketone, or a mixed solvent thereof, but may be a cyclic carbonate- And a mixed solvent thereof. It is most preferable to use a mixture of a cyclic carbonate-based solvent and a linear carbonate-based solvent. The cyclic carbonate solvent has a large polarity, which can sufficiently dissociate lithium ions, but has a disadvantage in that the ion conductivity is low due to the high viscosity. Therefore, by using a linear carbonate solvent having a low polarity but a low viscosity in the cyclic carbonate solvent, the characteristics of the lithium secondary battery can be optimized.

The cyclic carbonate solvent may be selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, fluorethylene carbonate, and mixtures thereof. The linear carbonate solvent may be selected from the group consisting of dimethyl carbonate, Diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate, and mixtures thereof.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the non-aqueous organic solvent is a mixed solvent of a cyclic carbonate solvent and a linear carbonate solvent, and the mixing ratio of the linear carbonate solvent: cyclic carbonate solvent is 1: 9: 1, preferably in a volume ratio of 1.5: 1 to 4: 1.

In the electrolyte for a high-voltage lithium secondary battery according to an embodiment of the present invention, the lithium salt is not limited to LiPF ₆ , LiBF ₄ , LiClO ₄ , LiSbF ₆ , LiAsF ₆ , LiN (SO ₂ C ₂ F ₅ ) ₂ , _{LiN (CF 3 SO 2) 2} , LiN (SO 3 C 2 F 5) 2, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiC 6 H 5 SO 3, LiSCN, LiAlO 2, LiAlCl 4, LiN (C _{x F 2x +1 SO 2) (} C y F 2y +1 SO 2) ( where, x and y are natural numbers), LiCl, LiI, and LiB (C ₂ O _4), or both is selected from the group consisting of ₂ Or more.

The concentration of the lithium salt is preferably in the range of 0.1 to 2.0 M, and more preferably in the range of 0.7 to 1.6 M. If the concentration of the lithium salt is less than 0.1 M, the conductivity of the electrolyte is lowered to deteriorate the performance of the electrolyte. If the concentration exceeds 2.0 M, the viscosity of the electrolyte increases and the lithium ion mobility decreases. The lithium salt acts as a source of lithium ions in the battery, thereby enabling operation of a basic lithium secondary battery.

The electrolyte for a high-voltage lithium secondary battery of the present invention is stable in a temperature range of -20 ° C to 60 ° C and maintains electrochemically stable characteristics even in a voltage range of 4.4V. Therefore, all lithium secondary batteries such as a lithium ion battery and a lithium polymer battery Can be applied.

The present invention also provides a high-voltage lithium secondary battery comprising the electrolyte for the high-voltage lithium secondary battery.

Non-limiting examples of the secondary battery include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, and a lithium ion polymer secondary battery.

The high-voltage lithium secondary battery manufactured from the electrolyte for a high-voltage lithium secondary battery according to the present invention exhibits a low-temperature discharge efficiency of 70% or more and a high-temperature storage efficiency of 75% or more, and the thickness increase rate of the battery is 4 to 14% .

The high-voltage lithium secondary battery of the present invention includes a positive electrode and a negative electrode.

The positive electrode includes a positive electrode active material capable of intercalating and deintercalating lithium ions, and the positive electrode active material is preferably a composite metal oxide of lithium and at least one selected from cobalt, manganese, and nickel. In addition to these metals, the employment rate of the metals may be varied. In addition to these metals, Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Sr, V, and a rare earth element. Specific examples of the positive electrode active material may include a compound represented by any one of the following formulas:

Li _a A _{1 - b} B _b D ₂ wherein, in the formula, 0.90? A? 1.8, and 0? B? 0.5; Li _a E _{1 - b} B _b O _{2 - c} D _{c where} 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05; LiE _{2 - b} B _b O _{4 - c} D _{c where} 0? B? 0.5, 0? C? 0.05; Li _a Ni _{1 -b- c} Co _b B _c D _? Wherein, in the formula, 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0 <? Li _a Ni _{1 - b - c} Co _b B _c O _{2 - ?} F _{? Wherein the} 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0 <? Li _a Ni _{1- b c} Co _b B _c O _2-α F ₂ wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 <α <2; Li _a Ni _1-bc Mn _b B _c D _? Wherein, in the formula, 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0 <? Li _a Ni _{1 -bc} Mn _b B _c O _{2 -?} F _? Wherein, in the formula, 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0 <? Li _a Ni _{1 -bc} Mn _b B _c O _2-α F ₂ wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 <α <2; Li _a Ni _b E _c G _d O ₂ wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, and 0.001 ≤ d ≤ 0.1; Li _a Ni _b Co _c Mn _d GeO ₂ wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, and 0.001 ≤ e ≤ 0.1; Li _a NiG _b O ₂ (in the above formula, 0.90? A? 1.8, and 0.001? B? 0.1); Li _a CoG _b O ₂ wherein, in the above formula, 0.90? A? 1.8, and 0.001? B? 0.1; Li _a MnG _b O ₂ (in the above formula, 0.90? A? 1.8, 0.001? B? 0.1); Li _a Mn ₂ G _b O ₄ wherein, in the above formula, 0.90? A? 1.8, and 0.001? B? 0.1; QO _2; QS ₂ ; LiQS ₂ ; V ₂ O ₅ ; LiV ₂ O ₅ ; LiIO ₂ ; LiNiVO _4; Li _(3-f) J ₂ (PO ₄ ) ₃ (0? F? 2); Li _(3-f) Fe ₂ (PO ₄ ) ₃ (0? F? 2); And LiFePO _4.

In the above formula, A is Ni, Co, Mn or a combination thereof; B is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element or a combination thereof; D is O, F, S, P, or a combination thereof; E is Co, Mn or a combination thereof; F is F, S, P or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V or a combination thereof; Q is Ti, Mo, Mn or a combination thereof; I is Cr, V, Fe, Sc, Y or a combination thereof; J may be V, Cr, Mn, Co, Ni, Cu or a combination thereof.

The negative electrode includes a negative electrode active material capable of intercalating and deintercalating lithium ions. Examples of the negative electrode active material include carbon materials such as crystalline carbon, amorphous carbon, carbon composite and carbon fiber, lithium metal, an alloy of lithium and other elements . Examples of the amorphous carbon include hard carbon, coke, mesocarbon microbead (MCMB) calcined at 1500 ° C. or lower, and mesophase pitch-based carbon fiber (MPCF). The crystalline carbon is a graphite-based material, specifically natural graphite, graphitized coke, graphitized MCMB, and graphitized MPCF. Preferably, the carbonaceous material is a material having an interplanar distance d002 of 3.35 to 3.38 Å and a crystallite size (Lc) of at least 20 nm by X-ray diffraction. Other elements constituting the alloy with lithium may be aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium.

The anode or the cathode can be produced by dispersing an electrode active material, a binder and a conductive material, and if necessary, a thickening agent in a solvent to prepare an electrode slurry composition, and applying the slurry composition to an electrode current collector. As the positive electrode current collector, aluminum or an aluminum alloy may be commonly used, and copper or a copper alloy may be used as the negative electrode current collector. The anode current collector and the anode current collector may be in the form of a foil or a mesh.

The binder is a substance that acts as a paste for the active material, mutual adhesion of the active material, adhesion to the current collector, buffering effect on the expansion and contraction of the active material, and the like. Examples of the binder include polyvinylidene fluoride (PVdF), polyhexafluoro (PVdF / HFP)), poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, polyvinyl ether, poly (methyl methacrylate) Acrylate), polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, styrene-butadiene rubber, and acrylonitrile-butadiene rubber. The content of the binder is 0.1 to 30% by weight, preferably 1 to 10% by weight, based on the electrode active material. If the content of the binder is too small, the adhesive force between the electrode active material and the current collector is insufficient. If the content of the binder is too large, the adhesive force is improved but the content of the electrode active material is reduced accordingly.

The conductive material is used for imparting conductivity to the electrode. Any conductive material may be used without causing any chemical change in the battery. Examples of the conductive material include a graphite conductive agent, a carbon black conductive agent, a metal or metal compound system And at least one selected from the group consisting of a conductive agent may be used. Examples of the graphite based conductive agent include artificial graphite and natural graphite. Examples of the carbon black conductive agent include acetylene black, ketjen black, denkablack, thermal black, channel black and the like. Examples of metal or metal compound conductive agents include perovskite such as tin, tin oxide, tin phosphate (SnPO ₄ ), titanium oxide, potassium titanate, LaSrCoO ₃ , LaSrMnO ₃ , There is material. However, the present invention is not limited to the conductive agents listed above.

The content of the conductive agent is preferably 0.1 to 10% by weight based on the electrode active material. When the content of the conductive agent is less than 0.1% by weight, the electrochemical characteristics are deteriorated, and when it exceeds 10% by weight, the energy density per weight is decreased.

The thickening agent is not particularly limited as long as it can serve to control the viscosity of the active material slurry. For example, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and the like can be used.

As the solvent in which the electrode active material, the binder, the conductive material and the like are dispersed, a non-aqueous solvent or an aqueous solvent is used. Examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N, N-dimethylaminopropylamine, ethylene oxide and tetrahydrofuran.

The high-voltage lithium secondary battery of the present invention may include a separator which prevents a short circuit between the positive electrode and the negative electrode and provides a moving path of lithium ions. Examples of the separator include polypropylene, polyethylene, polyethylene / polypropylene, polyethylene / / Polyolefin-based polymer membranes such as polyethylene, polypropylene / polyethylene / polypropylene, or multi-membranes thereof, microporous films, woven fabrics and nonwoven fabrics. Further, a film coated with a resin having excellent stability may be used for the porous polyolefin film.

The high-voltage lithium secondary battery of the present invention may have other shapes such as a cylindrical shape, a pouch shape, and the like.

Hereinafter, examples and comparative examples of the present invention will be described. However, the following examples are only a preferred embodiment of the present invention, and the present invention is not limited to the following examples. The electrolyte solution can be formed by dissolving the lithium salt in the basic solvent so that the lithium salt such as LiPF ₆ becomes 1 mole (1M) concentration in order to obtain the lithium ion concentration of 1 mole (1M) .

[Production Example 1] Synthesis of 3-cyanopropionic acid methyl ester (hereinafter referred to as "PHE16")

Pyridine (57 g) and glutamic acid (50 g) were dissolved in methanol (250 mL), and then trichlorosilicic acid (67 g) was added slowly at 0 ° C. After stirring at room temperature for 12 hours, sodium hydrogen sulfite was added to remove the remaining trichlorosuccinic acid and the white solid was removed by filtration. After filtration, sulfuric acid (10 mL) was added to the filtrate, the temperature was added to the reactor to which the reflux condenser was connected, and the mixture was stirred at 70 ° C for 5 hours. After the temperature was lowered to room temperature, the methanol solvent was removed by distillation under reduced pressure, the product was extracted with water and diethyl ether, and the diethyl ether solvent was removed by distillation under reduced pressure. After adding magnesium sulfate and drying, 3-cyanopropionic acid methyl ester (23 g) was obtained by distillation under reduced pressure.

¹ H NMR (CDCl ₃ , 500 MHz)? 3.64 (s, 3H), 2.57 (m, 4H)

[Comparative Preparation Example 1] Synthesis of cyanoacetic acid ethyl ester (hereinafter referred to as "PHE13")

A mixture of cyanoacetic acid (51 g), sodium hydroxide (24 g), brominated ethane (327 g) and tetrabutylammonium bromide (19 g) was added to a chlorobenzene and water (2/1, And the temperature was raised, and the mixture was stirred at 100 DEG C for 4 hours. After the reaction was completed, the reaction solution was cooled to room temperature and the aqueous layer was separated and extracted with t-butyl methyl ether. Magnesium sulfate was added and dried, and then purified by distillation to obtain purified cyanoacetic acid ethyl ester (44 g).

_{^{1 H NMR (CDCl 3, 500}} MHz) δ 4.18 (q, 2H), 3.44 (s, 2H), 1.25 (t, 3H)

[Comparative Preparation Example 2] Synthesis of cyanoacetic acid methyl ester (hereinafter referred to as 'PHE14')

A mixed solution of t-butyl methyl ether and water (2/1 by volume) mixed solution (450 g) was added to a mixed solution of 300 g of methane chloride, 51 g of cyanoacetic acid, 24 g of sodium hydroxide and 19 g of tetrabutylammonium bromide. mL), the temperature was added, and the mixture was stirred at 100 DEG C for 4 hours. After the reaction was completed, the reaction solution was cooled to room temperature and the aqueous layer was separated and extracted with t-butyl methyl ether. Sodium sulfate was added thereto, followed by drying and distillation under reduced pressure to obtain purified cyanoacetic acid methyl ester (40 g).

¹ H NMR (CDCl ₃ , 500 MHz)? 3.82 (s, 3H), 3.52 (s, 2H)

[Example 1-6 and Comparative Example 1-3]

The electrolyte solution was prepared by dissolving LiPF ₆ in a mixed solvent of ethylene carbonate (EC): ethyl methyl carbonate (EMC) in a volume ratio of 3: 7 to prepare a 1.0 M solution. The resulting solution was dissolved in a basic electrolyte (1M LiPF ₆ , EC / EMC = 3 : 7) were further added with the components listed in Table 1 below.

A battery to which the non-aqueous electrolytic solution was applied was prepared as follows.

LiNiCoMnO ₂ and LiMn ₂ O ₄ were mixed as a cathode active material at a weight ratio of 1: 1, and polyvinylidene fluoride (PVdF) as a binder and carbon as a conductive agent were mixed at a weight ratio of 92: 4: 4, Methyl-2-pyrrolidone to prepare a positive electrode slurry. This slurry was coated on an aluminum foil having a thickness of 20 mu m, followed by drying and rolling to prepare a positive electrode. Styrene-butadiene rubber as a binder and carboxymethylcellulose as a thickener were mixed in a weight ratio of 96: 2: 2 and then dispersed in water to prepare a negative electrode active material slurry. This slurry was coated on a copper foil having a thickness of 15 mu m, followed by drying and rolling to prepare a negative electrode.

A film separator made of polyethylene (PE) having a thickness of 25 탆 was stacked between the prepared electrodes to form a cell using a pouch having a size of 8 mm × 270 mm × 185 mm in size , And the above non-aqueous electrolyte solution was injected to prepare a 25Ah-class lithium secondary battery for EV.

The performance of the 25Ah battery for EV thus manufactured was evaluated as follows. The evaluation items are as follows.

* Evaluation Items *

1. Capacity recovery rate after 30 days at 60 ° C (high temperature storage efficiency): After charging for 3 hours at 4.4 V and 12.5 A CC-CV at room temperature, after leaving for 30 days at 60 ° C, The available capacity (%) versus initial capacity was measured.

2. Thickness increase rate after 60 days at 60 ° C: After charging for 3 hours at 4.4 V and 12.5 A CC-CV at room temperature, the thickness of the battery was measured as A at 30 ° C. at 60 ° C. and atmospheric exposure using a sealed thermostat When the thickness of the left-handed cell is denoted by B, the increase rate of the thickness is calculated as shown in Equation 1 below.

[Formula 1]

(B-A) / A * 100 (%)

3. Normal temperature life: After charging for 3 hours at 4.4V and 25A CC-CV at room temperature, discharge 300 times to 2.7V with current of 2.7V and 25A. At this time, the first discharge capacity was denoted by C, and the 300th discharge capacity was divided by the first discharge capacity to calculate the capacity retention rate during the lifetime.

4. -20 ℃ 1C Discharge: After charging for 3 hours at 12.5A and 4.4V CC-CV at room temperature, leave for 4 hours at -20 ℃, discharge to CC with current of 25A, The available capacity (%) was measured.

Electrolyte composition After 30 days at 60 ° C Capacity retention rate during lifetime -20 ℃ discharge
Volume Capacity recovery rate Thickness increase rate Example 1 Basic electrolyte + PHE16 5% 70% 14% 73% 72% Example 2 Basic electrolyte + PHE16 10% 78% 10% 78% 75% Example 3 Basic electrolyte + PHE16 15% 73% 8% 64% 78% Example 4 Basic electrolyte + PHE16-2 10% 82% 9% 85% 77% Example 5 Basic electrolyte + PHE16 10% + LiBOB 1wt% 84% 12% 81% 79% Example 6 Basic electrolyte + PHE16 10% + VC 1wt% 90% 10% 92% 76% Example 7 Basic electrolyte + PHE16 10% + VC 1wt% + PS 1wt% 92% 3% 93% 75% Comparative Example 1 Basic electrolyte 37% 30% 20% 55% Comparative Example 2 Basic electrolyte + PHE13 10% 12% 85% - 58% Comparative Example 3 Basic electrolyte + PHE14 10% 8% 90% - 52% Basic electrolyte: 1M LiPF ₆ , EC / EMC = 3: 7
PHE16: The compound of Preparation Example 1
PHE16-2: 4-cyanobutanoic acid methyl ester
PHE13: Compound of Comparative Preparation Example 1
PHE14: Compound of Comparative Preparation Example 2
LiBOB: Lithium-bis (Oxalato) Borate
VC: Vinylene carbonate
PS: 1,3-propane sultone

As described above, the high-voltage lithium secondary battery including the electrolyte for a high-voltage lithium secondary battery according to the present invention has a low-temperature discharge efficiency of 72% or more and a high-temperature storage efficiency of 70% or more. Also, it was confirmed that the thickness increase rate of the battery was very low at a high temperature of 3 to 14% when left for a long time, and it was confirmed that the capacity retention rate during the life span was more than 64% (Examples 1 to 7). On the other hand, Comparative Examples 1 to 3 exhibited a low temperature discharge efficiency of 58% or less and a high temperature storage efficiency of 37% or less, and the cell thickness increase rate was 30 to 90% at a high temperature for a long period of time. The retention ratios were 20% for Comparative Example 1, but not for Comparative Examples 2 and 3.

In order to measure the oxidative decomposition voltage of Example 2 and Comparative Example 1, a Pt electrode was used as a working electrode, a counter electrode was used as a reference electrode, and an LSV (Linear Sweep Voltametry) was measured. As a result, the electrolytic solution of Example 2 (basic electrolyte + PHE16 10%) was added with 10% of PHE16 compared with the basic electrolyte of Comparative Example 1, and the oxidation potential of the electrolyte was increased, (Fig. 1).

In particular, when comparing Example 2 with Comparative Examples 2 and 3, it was found that the high temperature storage efficiency (Example 2: 78%, Comparative Example 2: 12%, Comparative Example 3: (Example 2: 10%, Comparative Example 2: 85%, Comparative Example 3: 90%), low temperature discharge efficiency (Example 2: 75%, Comparative Example 2 : 58%, Comparative Example 3: 52%). This difference is attributed to the structural characteristics of the nitrile ester compound added to the basic electrolytic solution. The nitrile ester compound of the present invention has a structure in which the number of carbon atoms of the alkylene group existing between the nitrile group and the carbonyl is 2 or more, Or cyanoacetic acid methyl ester, which is a compound in which a methylene group (CH ₂ ) is introduced between a nitrile group of 3 or carbonyl (C (= O)). The nitrile ester compounds of Comparative Examples 2 and 3 could easily fall off the hydrogen atoms of the methylene groups present between the nitrile group and the carbonyl group to cause side reactions in the cell, resulting in a low temperature storage efficiency and a low temperature discharge efficiency, The increase rate of the thickness of the battery is increased.

However, the electrolyte for a high-voltage lithium secondary battery according to the present invention contains a nitrile ester compound having a structure in which the number of carbon atoms of an alkylene group existing between the nitrile group and the carbonyl is 2 or more, Since the hydrogen atoms are stably bonded, the side reaction in the battery is suppressed. As a result, the electrolytic solution is oxidized / decomposed at a high voltage to remarkably improve the swelling of the battery, thereby exhibiting excellent high temperature storage characteristics, Lt; / RTI &gt;

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. And various modifications may be made to the invention. Therefore, modifications of the embodiments of the present invention will not depart from the scope of the present invention.

Claims (13)

Lithium salts;
Non-aqueous organic solvent; And
An electrolyte solution for a lithium secondary battery comprising a nitrile ester compound represented by the following formula:
[Chemical Formula 1]

In the above formula (1), R is a C1-C5 alkyl group, and n is an integer of 1 to 10. The method according to claim 1,
Wherein R is methyl, ethyl, propyl, butyl or pentyl, and n is an integer of 1, 2 or 3, 3. The method of claim 2,
Wherein the nitrile ester compound is selected from a nitrile ester compound having the following structure:

. The method according to claim 1,
Wherein the nitrile ester compound is contained in an amount of 1 to 20 wt% based on the total weight of the electrolytic solution. The method according to claim 1,
Wherein the electrolytic solution further comprises at least one additive selected from the group consisting of an oxalate borate compound, a carbonate compound substituted with fluorine, a vinylidene carbonate compound, and a sulfinyl group-containing compound. 6. The method of claim 5,
The electrolytic solution may be at least one selected from the group consisting of lithium difluoroarsalate borate (LiFOB), lithium bis oxalate borate (LiB (C ₂ O ₄ ) ₂ , LiBOB), fluoroethylene carbonate (FEC), vinylene carbonate (VEC), divinyl sulfone, ethylene sulfite, propylene sulfite, diallyl sulfonate, ethane sultone, propane sultone (PS), butane An electrolyte solution for a lithium secondary battery, further comprising an additive selected from the group consisting of butane sultone, ethene sultone, butene sultone and propene sultone (PRS). 6. The method of claim 5,
Wherein the additive is contained in an amount of 0.1 to 5.0% by weight based on the total weight of the electrolytic solution. The method according to claim 1,
Wherein the non-aqueous organic solvent is selected from cyclic carbonate-based solvents, linear carbonate-based solvents, and mixed solvents thereof. 9. The method of claim 8,
Wherein the cyclic carbonate is selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, vinylethylene carbonate, fluorethylene carbonate and mixtures thereof, and the linear carbonate is selected from the group consisting of dimethyl carbonate, diethyl carbonate, Wherein the electrolyte is selected from the group consisting of carbonates, ethyl methyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate, and mixtures thereof. 9. The method of claim 8,
Wherein the non-aqueous organic solvent has a mixing volume ratio of linear carbonate solvent: cyclic carbonate solvent of 1: 1 to 9: 1. The method according to claim 1,
Wherein the lithium salt is selected from the group consisting of LiPF ₆ , LiBF ₄ , LiClO ₄ , LiSbF ₆ , LiAsF ₆ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiN (SO ₃ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC ₆ H ₅ SO ₃ , LiSCN, LiAlO ₂ , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) , x and y are natural numbers), LiCl, LiI, and LiB (C ₂ O ₄ ) ₂ . The method according to claim 1,
Wherein the lithium salt is present in a concentration of 0.1 to 2.0 M. A lithium secondary battery comprising an electrolyte solution for a lithium secondary battery according to any one of claims 1 to 12.

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