KR20180134152A - Electrolyte of rechargeable lithium battery and rechargeable lithium battery including same - Google Patents

Abstract

The present invention relates to an electrolyte for a lithium secondary battery, and a lithium secondary battery using the same. In particular, the electrolyte comprises a non-aqueous organic solvent, a lithium salt, and an additive represented by one of chemical formulas 1 to 4. In chemical formulas 1 to 4, each substituent group is as described in the detailed description. The electrolyte of the present invention can improve high-temperature storability, and high-temperature swelling characteristics, in particular.

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 [0001]

An electrolyte for a lithium secondary battery, and a lithium secondary battery comprising the same.

Lithium secondary batteries are attracting attention as power sources for various electronic devices because of high discharge voltage and high energy density.

Examples of the positive electrode active material of the lithium secondary battery include LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1-x Co _x O ₂ (0 <x <1), lithium and a transition metal having a structure capable of intercalating lithium ions Oxide is mainly used.

As the anode active material, various types of carbon-based materials including artificial, natural graphite, and hard carbon capable of lithium insertion / desorption are mainly used.

As the electrolyte of the lithium secondary battery, an organic solvent in which a lithium salt is dissolved is used.

Resistance increases when the lithium secondary battery is stored at a high temperature. Therefore, the resistance increase should be suppressed. In particular, when the lithium secondary battery is used for a high output, it is further demanded to suppress the increase in resistance at high temperature storage.

In addition, there is a problem that the voltage drops during charging / discharging of the lithium secondary battery. In order to prevent this, attempts have been made to use succinonitrile as an additive in the electrolyte. However, succinonitrile has to be used excessively to solve the voltage drop problem, In this case, the battery voltage drop problem can be solved, but as the resistance increases, it can not be used for high output. Further, when succinonitrile is used in an excess amount, there is a problem that the cycle life characteristics at room temperature deteriorate.

One embodiment is to provide an electrolyte for a lithium secondary battery capable of suppressing an increase in resistance at high temperature storage and suppressing metal elution and exhibiting excellent battery performance.

Another embodiment provides a lithium secondary battery comprising the electrolyte.

According to one embodiment, there is provided an electrolyte for a lithium secondary battery comprising a non-aqueous organic solvent and an additive represented by one of the following formulas (1) to (4).

[Chemical Formula 1]

(In the formula 1,

Wherein four of R ₁ to R ₆ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN and the others are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)

(2)

(In the formula (2)

R ₇ to R ₁₄ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN, and the others are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)

(3)

(3)

R ₁₅ to R ₂₄ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN, and the remainder are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)

[Chemical Formula 4]

(In the formula 4,

R ₂₆ to R ₃₆ of the net is each independently CN, CH ₂ CN, or C ₂ H ₅ CN, the rest independently represent an alkyl group of the unsubstituted or substituted C1 to C3 each other Im)

The additive may be the compound of formula (1).

The content of the additive may be 0.25 wt% to 1 wt%, and may be 0.5 wt% to 1 wt% based on the total weight of the electrolyte.

Another embodiment of the present invention is a negative electrode comprising a negative electrode active material; A cathode comprising a cathode active material; And a lithium secondary battery comprising the non-aqueous organic solvent, the lithium salt and the electrolyte. The lithium secondary battery may be a cylindrical lithium secondary battery.

The details of other embodiments are included in the detailed description below.

The electrolyte for a lithium secondary battery according to an embodiment of the present invention can improve high-temperature storage characteristics, particularly high-temperature swelling characteristics.

1 is a schematic view of a lithium secondary battery according to an embodiment of the present invention.
2 is a graph showing the CV results of the electrolyte of Comparative Example 2. Fig.
3 is a graph showing the CV results of the electrolyte of Example 1. Fig.
4 is a graph showing the results of LSV (linear sweep voltammetry) of the electrolyte according to Example 2 and Comparative Example 2 at room temperature.
5 is a graph showing the LSV results of the electrolyte according to Example 2 and Comparative Examples 1 and 2 at room temperature.
6 is a graph showing the resistance of the battery using the electrolyte of Examples 1 and 2 and Comparative Example 1 measured at high temperature before storage.
7 is a graph showing the resistance of the battery using the electrolyte of Examples 1 and 2 and Comparative Example 1 measured at high temperature before storage.
8 is a graph showing the cycle life characteristics at room temperature of the battery using the electrolytes of Examples 1 and 2, Comparative Examples 1 and 2, and Reference Examples 1 to 3;

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited thereto, and the present invention is only defined by the scope of the following claims.

One embodiment of the present invention includes a non-aqueous organic solvent; Lithium salts; And an additive represented by one of the following formulas (1) to (4).

[Chemical Formula 1]

(In the formula 1,

Wherein four of R ₁ to R ₆ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN and the others are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)

(2)

(In the formula (2)

R ₇ to R ₁₄ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN, and the others are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)

(3)

(3)

R ₁₅ to R ₂₄ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN, and the remainder are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)

[Chemical Formula 4]

(In the formula 4,

R ₂₆ to R ₃₆ of the net is each independently CN, CH ₂ CN, or C ₂ H ₅ CN, the rest independently represent an alkyl group of the unsubstituted or substituted C1 to C3 each other Im)

When a battery using an electrolyte containing an additive according to one embodiment is charged and discharged, the additive is decomposed to form a film having excellent lithium ion conductivity on the surface of the negative electrode, while being adsorbed on the positive electrode at the same time, It is possible to stabilize the unstable structure and suppress the increase in the anode resistance.

As shown in the above Chemical Formulas 1 to 4, the additive according to one embodiment includes four nitrile groups (-CN groups) among the functional groups bonded to the ring. In the case where four nitrile groups are contained, the problem of the elution of the metal from the active material and the current collector, particularly the current collector, which may be generated upon battery charge / discharge can be effectively prevented as compared with the case where two nitrile groups are contained. Can be used in place of succinonitrile. Compared with the case where two nitrile groups are contained, the resistance is low, and the resistance at a high temperature is low, so that high temperature storage characteristics can be effectively improved, especially when the same content is used.

In addition to the nitrile group, the functional group is an alkyl group, particularly a C1 to C3 alkyl group. Since such an alkyl group is a functional group capable of adsorbing a metal well, a metal that can be eluted from the active material or the current collector can be effectively adsorbed at the time of charge / It is possible to suppress problems such as deterioration of safety such as a problem of occurrence of short circuit due to metal elution. When the number of carbon atoms in the alkyl group is more than 3, that is, when the alkyl group is a high alkyl group having a long chain, a film having a hydrophobic property is formed thick and resistance may be increased.

If the functional group other than the nitrile group is hydrogen, the bond with the ring structure tends to be broken, and when the battery including the electrolyte is charged and discharged, a remarkably thick film is formed, which may deteriorate the cycle life characteristics.

The content of the additive may be 0.25 wt% to 1 wt% with respect to the total weight of the electrolyte, and the content of the additive may be 0.5 wt% to 1 wt% with respect to the total weight of the electrolyte. When the content of the additive is within the above range, generation of gas due to decomposition of the electrolytic solution hardly occurs at the time of storing the battery, and gas generation hardly occurs particularly at the time of high temperature storage, thereby exhibiting improved cycle life characteristics. The cycle life high temperature storage characteristics can be further improved. When the content of the additive is less than 0.25 wt%, the effect of using the additive is insufficient. When the content of the additive is more than 1 wt%, the electrolyte is decomposed severely and gas is generated remarkably. The cycle life characteristics may be deteriorated.

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

As the non-aqueous organic solvent, a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based or aprotic solvent may be used.

Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC) EC), propylene carbonate (PC), and butylene carbonate (BC). Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, dimethylacetate, methyl propionate, ethyl propionate, propyl propionate,? -Butyrolactone, decanolide, Lactone, mevalonolactone, caprolactone, and the like may be used.

Examples of the ether solvent include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran. As the ketone solvent, cyclohexanone may be used have.

As the non-protonic solvent, T-CN (T is a linear, branched or cyclic hydrocarbon group having 2 to 20 carbon atoms, A double bond aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes.

The non-aqueous organic solvent may be used alone or in combination. The mixing ratio in the case of mixing one or more of them can be suitably adjusted in accordance with the performance of a desired battery, and this can be widely understood by those skilled in the art.

In the case of the carbonate-based solvent, it is preferable to use a mixture of a cyclic carbonate and a chain carbonate. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of 1: 1 to 1: 9, the performance of the electrolytic solution may be excellent.

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

The aromatic hydrocarbon-based organic solvent may be an aromatic hydrocarbon-based compound represented by the following formula (5).

[Chemical Formula 5]

(Wherein R ₁ to R ₆ are the same or different from each other and selected from the group consisting of hydrogen, halogen, an alkyl group having 1 to 10 carbon atoms, a haloalkyl group, and combinations thereof)

Specific examples of the aromatic hydrocarbon-based organic solvent include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-tri Fluorobenzene, 1,2,4-trifluorobenzene, 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, Examples of the solvent include 2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4- Dichlorotoluene, 2,3-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2, 3-dichlorotoluene, 3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2 , 5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and combinations thereof.

The electrolyte for a lithium secondary battery may further include an ethylene carbonate compound of the following formula (6) to improve battery life.

[Chemical Formula 6]

(Wherein R ₇ and R ₈ are each independently selected from the group consisting of hydrogen, a halogen group, a cyano group (CN), a nitro group (NO ₂ ) and an alkyl group having 1 to 5 fluorinated carbon atoms, and R ₇ and R ₈ is selected from the group consisting of a halogen group, a cyano group (CN), a nitro group (NO ₂ ) and an alkyl group having 1 to 5 fluorinated carbon atoms, provided that R ₇ and R ₈ are not both hydrogen .)

Representative examples of the ethylene carbonate-based compound include diethylene carbonate, diethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, fluoroethylene carbonate, and the like, such as difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, . When such a life improving additive is further used, its amount can be appropriately adjusted.

The lithium salt is dissolved in an organic solvent to act as a source of lithium ions in the cell to enable operation of a basic lithium secondary battery and to promote the movement of lithium ions between the anode and the cathode. The lithium salt Representative examples are _{LiPF 6, LiSbF 6, LiAsF 6} , LiN (SO 2 C 2 F 5) 2, Li (CF 3 SO 2) 2 N, LiN (SO 3 C 2 F 5) 2, LiC 4 F _{9 SO 3, LiClO 4, LiAlO} 2, LiAlCl 4, LiN (C x F 2x + 1 SO 2) (C y F 2y + 1 SO 2) ( where, and x and y are natural numbers, for example from 1 to 20 , One or more selected from the group consisting of LiCl, LiI, and LiB (C ₂ O ₄ ) ₂ (lithium bis (oxalato) borate: LiBOB) The concentration of the lithium salt is preferably within the range of 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has an appropriate conductivity and viscosity, and therefore can exhibit excellent electrolyte performance, Can be moved effectively.

Another embodiment provides a lithium secondary battery comprising the electrolyte, the positive electrode and the negative electrode.

The positive electrode includes a current collector and a positive electrode active material layer formed on the current collector and including a positive electrode active material.

As the positive electrode active material in the positive electrode active material layer, a compound capable of reversibly intercalating and deintercalating lithium (ritied intercalation compound) can be used. Specific examples thereof include cobalt, manganese, nickel, And a composite oxide of lithium and metal selected from a combination of lithium and lithium. As a more specific example, a compound represented by any one of the following formulas may be used. Li _a A _1-b X _b D ₂ (0.90? A? 1.8, 0? B? 0.5); Li _a A _1-b X _b O _{2 -c} D _c (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05); Li _a E _1-b X _b O _{2 -c} D _c (0? B? 0.5, 0? C? 0.05); Li _a E _2-b X _b O _{4 -c} D _c (0? B? 0.5, 0? C? 0.05); Li _a Ni _{1- b c} Co _b X _c D _? (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.5, 0 <?? Li _a Ni _{1- b c} Co _b X _c O _2-α T _α (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 <α <2); Li _a Ni _{1- b c} Co _b X _c O _2-α T ₂ (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 <α <2); Li _a Ni _1-bc Mn _b X _c D _? (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0 <?? Li _a Ni _1-bc Mn _b X _c O _2-α T _α (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 <α <2); Li _a Ni _1-bc Mn _b X _c O _2-α T ₂ (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 (0.90? A? 1.8, 0? B? 0.9, 0? C? 0.5, 0.001? D? 0.1); Li _a Ni _b Co _c Mn _d G _e O ₂ (0.90? A? 1.8, 0? B? 0.9, 0? C? 0.5, 0? D? 0.5, 0.001? E? 0.1); Li _a NiG _b O ₂ (0.90? A? 1.8, 0.001? B? 0.1) Li _a CoG _b O ₂ (0.90? A? 1.8, 0.001? B? 0.1); Li _a Mn _1-b G _b O ₂ (0.90? A? 1.8, 0.001? B? 0.1); Li _a Mn ₂ G _b O ₄ (0.90? A? 1.8, 0.001? B? 0.1); Li _a Mn _1-g G _g PO ₄ (0.90? A? 1.8, 0? G? 0.5); QO ₂ QS ₂ LiQS ₂ V ₂ O ₅ LiV ₂ O ₅ LiZO 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); Li _a FePO ₄ (0.90? A? 1.8)

In the above formula, A is selected from the group consisting of Ni, Co, Mn, and combinations thereof; X is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof; D is selected from the group consisting of O, F, S, P, and combinations thereof; E is selected from the group consisting of Co, Mn, and combinations thereof; T is selected from the group consisting of F, S, P, and combinations thereof; G is selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and combinations thereof; Q is selected from the group consisting of Ti, Mo, Mn, and combinations thereof; Z is selected from the group consisting of Cr, V, Fe, Sc, Y, and combinations thereof; J is selected from the group consisting of V, Cr, Mn, Co, Ni, Cu, and combinations thereof.

Of course, a compound having a coating layer on the surface of the compound may be used, or a compound having a coating layer may be mixed with the compound. The coating layer comprises at least one coating element compound selected from the group consisting of an oxide of the coating element, a hydroxide of the coating element, an oxyhydroxide of the coating element, an oxycarbonate of the coating element and a hydroxycarbonate of the coating element . The compound constituting these coating layers may be amorphous or crystalline. The coating layer may contain Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr or a mixture thereof. The coating layer forming step may be any coating method as long as it can coat the above compound by a method that does not adversely affect physical properties of the cathode active material (for example, spray coating, dipping, etc.) by using these elements, It will be understood by those skilled in the art that a detailed description will be omitted.

In the anode, the content of the cathode active material may be 90 wt% to 98 wt% with respect to the total weight of the cathode active material layer.

In one embodiment, the cathode active material layer may further include a binder and a conductive material. At this time, the content of the binder and the conductive material may be 1 wt% to 5 wt% with respect to the total weight of the cathode active material layer.

The binder serves to adhere the positive electrode active material particles to each other and to adhere the positive electrode active material to the current collector. Typical examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl Polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-acrylonitrile, styrene-butadiene rubber, Butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like, but not limited thereto.

The conductive material is used for imparting conductivity to the electrode. Any conductive material may be used for the battery without causing any chemical change. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black and carbon fiber; Metal powders such as copper, nickel, aluminum, and silver, or metal-based materials such as metal fibers; Conductive polymers such as polyphenylene derivatives; Or a mixture thereof.

The current collector may be an aluminum foil, a nickel foil or a combination thereof, but is not limited thereto.

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

As the negative electrode active material, a material capable of reversibly intercalating / deintercalating lithium ions, a lithium metal, an alloy of lithium metal, a material capable of doping and dedoping lithium, or a transition metal oxide may be used.

Examples of the material capable of reversibly intercalating / deintercalating lithium ions include a carbonaceous material, that is, a carbonaceous anode active material generally used in a lithium secondary battery. Representative examples of the carbon-based negative electrode active material include crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as natural graphite or artificial graphite in the form of amorphous, plate-like, flake, spherical or fibrous type. Examples of the amorphous carbon include soft carbon or hard carbon hard carbon, mesophase pitch carbide, fired coke, and the like.

Examples of the lithium metal alloy include lithium and a group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, May be used.

As the material capable of being doped and dedoped into lithium, Si, SiO _x (0 <x <2) and Si-Q alloy (Q is an alkali metal, an alkali earth metal, a Group 13 element, a Group 14 element, a Group 15 element, group elements, transition metals, rare earth elements and an element selected from the group consisting of, but not Si), Si- carbon composite, Sn, SnO _2, Sn-R (where R is an alkali metal, alkaline earth metal, 13 An element selected from the group consisting of Group IV elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, but not Sn), and Sn-carbon composites. May be mixed with SiO ₂ . The element Q and the element R may be at least one element 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, Pb, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

As the transition metal oxide, lithium titanium oxide may be used.

The negative electrode active material layer includes a negative electrode active material and a binder, and may further include a conductive material.

The content of the negative electrode active material in the negative electrode active material layer may be 95 wt% to 99 wt% with respect to the total weight of the negative electrode active material layer. The content of the binder in the negative electrode active material layer may be 1 wt% to 5 wt% with respect to 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 to 98 wt%, the binder may be used in an amount of 1 to 5 wt%, and the conductive material may be used in an amount of 1 to 5 wt%.

The binder serves to adhere the anode active material particles to each other and to adhere the anode active material to the current collector. As the binder, a water-insoluble binder, a water-soluble binder, or a combination thereof may be used.

Examples of the water-insoluble binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride , Polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

Examples of the water-soluble binder include styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, propylene and olefin copolymers having 2 to 8 carbon atoms, (meth) acrylic acid and (meth) Copolymers or combinations thereof.

When a water-soluble binder is used as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further contained as a thickener. As the cellulose-based compound, carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, alkali metal salts thereof or the like may be used in combination. As the alkali metal, Na, K or Li can be used. The content of the thickener may be 0.1 part by weight to 3 parts by weight based on 100 parts by weight of the negative electrode active material.

The conductive material is used for imparting conductivity to the electrode. Any conductive material may be used for the battery without causing any chemical change. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, denka black and carbon fiber; Metal powders such as copper, nickel, aluminum, and silver, or metal-based materials such as metal fibers; Conductive polymers such as polyphenylene derivatives; Or a mixture thereof.

The collector may be selected from the group consisting of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foil, a polymer substrate coated with a conductive metal, and a combination thereof.

The positive electrode active material layer and the negative electrode active material layer are formed by mixing an anode active material, a binder and optionally a conductive material in a solvent to prepare an active material composition, and applying the active material composition to a current collector. Since the method of forming the active material layer is well known in the art, detailed description thereof will be omitted herein. As the solvent, N-methylpyrrolidone or the like can be used, but it is not limited thereto. When a water-soluble binder is used for the negative electrode active material layer, water may be used as a solvent used for preparing the negative electrode active material composition.

Further, depending on the type of the lithium secondary battery, a separator may be present between the positive electrode and the negative electrode. The separator may be a polyethylene / polypropylene double layer separator, a polyethylene / polypropylene / polyethylene triple layer separator, a polypropylene / polyethylene / poly It is needless to say that a mixed multilayer film such as a propylene three-layer separator and the like can be used.

The lithium secondary battery according to an exemplary embodiment may be a battery of various shapes such as a cylindrical shape, a square shape, and a pouch shape, but may be suitably cylindrical. 1 is an exploded perspective view of such a cylindrical lithium secondary battery.

1, a lithium secondary battery 1 according to an embodiment includes an electrode assembly wound between a positive electrode 2 and a negative electrode 4 with a separator 3 interposed therebetween, and a case 5, and a sealing member 6 for sealing the case 5. The anode 10, the cathode 20 and the separator 30 may be impregnated with an electrolyte solution (not shown).

Hereinafter, examples and comparative examples of the present invention will be described. The following embodiments are only examples of the present invention, and the present invention is not limited to the following embodiments.

(Example 1)

1.5M LiPF ₆ was added to a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (2: 4: 4 by volume), 0.5 wt% of the additive of the following formula 1a was added to 100 wt% of the mixture, .

[Formula 1a]

(Example 2)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1 except that the content of the additive in the above Formula 1a was changed to 1 wt%.

(Comparative Example 1)

1.5 M LiPF ₆ was added to a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (2: 4: 4 by volume), and 1 wt% succinonitrile was added to 100 wt% of the mixture to prepare an electrolyte for a lithium secondary battery Respectively.

(Comparative Example 2)

1.5 M LiPF ₆ was added to a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (2: 4: 4 by volume) to prepare an electrolyte for a lithium secondary battery.

(Reference Example 1)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that the content of the additive of Formula 1a was changed to 1.5% by weight.

(Reference Example 2)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that the content of the additive of Formula 1a was changed to 2% by weight.

(Reference Example 3)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that the content of the additive in Formula 1a was changed to 3% by weight.

(Comparative Example 3)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that 1 weight% of the additive of the following formula (7) was used instead of 0.5 weight% of the additive of the above formula (1a).

(7)

(Comparative Example 4)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that 1 weight% of the additive of the following Formula 8 was used instead of 0.5 weight% of the additive of the Formula 1a.

[Chemical Formula 8]

(Comparative Example 5)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that 1 weight% of the additive of the following formula (9) was used instead of 0.5 weight% of the additive of the above formula (1a).

[Chemical Formula 9]

(Comparative Example 6)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that 1 wt% of the additive of Chemical Formula 10 was used instead of 0.5 wt% of the additive of Chemical Formula 1a.

[Chemical formula 10]

(Comparative Example 7)

An electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that 1 wt% of the additive of the following Formula 11 was used instead of 0.5 wt% of the additive of the Formula 1a.

(11)

* Evaluation of cyclic voltammetry (CV) characteristics

The cyclic current voltage (scanning speed: 1 mV / sec) of the three electrodes using the electrolyte of Comparative Example 2, graphite working electrode and lithium counter electrode was measured and the results are shown in FIG. 2 . The cyclic current voltage (scan speed: 1 mV / sec) of the three electrodes using the electrolyte, the graphite working electrode and the lithium counter electrode of Example 1 was measured, and the results are shown in Fig.

In Fig. 2 and Fig. 3, cycleX means the number of cycles X times. As shown in FIG. 2 and FIG. 3, the reaction of the electrolyte of Example 1 at a higher potential than that of Comparative Example 2 shows that the reactivity to the negative electrode is increased. In addition, as shown in Fig. 2, the electrolyte of Comparative Example 2 shows a decomposition peak, that is, a reduction peak near 0.4 V, whereas the electrolyte of Example 1 shows a decomposition peak, that is, Is about 2.2V.

From these results, since the electrolyte of Comparative Example 2 reacts at a potential lower than that of ethylene carbonate at which the reduction peak appears at 0.5 V, the electrolyte of Example 1 reacts at a high potential while ethylene carbonate is first reduced, It can be understood that it is reduced prior to ethylene carbonate. In this way, decomposition of ethylene carbonate can be suppressed when ethylene carbonate is reduced earlier than that of ethylene carbonate. This shows that the peak at about 0.5 V due to ethylene carbonate decomposition is higher than that of Comparative Example 2 (Fig. 2), Example 1 Can be clearly seen from the result of very reduced

As described above, it is possible to prevent lithium migration due to depletion of the solvent of the electrolyte due to the decomposition of the ethylene carbonate, and to prevent the formation of a film due to the ethylene carbonate which can act as a resistor and degrade cycle life characteristics and output characteristics.

From these results, it can be seen that the lithium secondary battery using the electrolyte of Example 2 can form a coating on the negative electrode at the initial charging / discharging time, and the lithium insertion / removal resistance of the coating is small.

* Linear Sweep Voltammetry (LSV)

The anodic polarization of the electrolyte according to Example 2 and Comparative Example 2 was measured by using the linear principle preliminary method in order to evaluate the decomposition of oxidized electrodes at 25 ° C. The results are shown in FIG. . In the measurement, a Pt electrode was used as a working electrode, and a three electrode electrochemical cell using Li as a counter electrode and a reference electrode was used. At this time, the scan was performed at a speed of 1 mV / sec from 3V to 7V.

As shown in Fig. 4, the electrolyte of Example 2 showed no peak, while the electrolyte of Comparative Example 2 showed a peak between about 5.8 V and about 6.3 V. As a result, the electrolyte of Example 2 exhibited oxidative decomposition It can be seen that the reaction does not occur, and that the oxidation stability is improved by adding the compound of formula (1a).

In order to evaluate the decomposition of the oxidized electrodes at 25 ° C of the electrolytes according to Example 2 and Comparative Examples 1 and 2, anodic polarization was measured using a linear main prior method, and the results are shown in FIG. 5 Respectively. A three electrode electrochemical cell using Li as a counter electrode and a reference electrode was used as a working electrode and a counter electrode. At this time, the scan was performed at a speed of 1 mV / sec from 3V to 7V.

5, since the peak height of the electrolyte according to Example 2 is much lower than that of Comparative Examples 1 and 2, the addition of the additive of Formula 1a It can be seen that Cu elution can be effectively suppressed as compared with the case of using (Example 2), (Comparative Example 2) or using succinonitrile (Comparative Example 1).

* Battery manufacturing

LiNi _1/3 Co _1/3 Al _1/3 O ₂ 96 wt% of the positive electrode active material, ₂ wt% of the Ketjen black conductive material and 2 wt% of polyvinylidene fluoride were mixed in a N-methylpyrrolidone solvent to prepare a positive electrode active material slurry . The positive electrode active material slurry was coated on an aluminum foil, dried and rolled to prepare a positive electrode.

A negative electrode active material slurry was prepared by mixing 96% by weight of graphite anode active material, 2% by weight of Ketjenblack conductive material and 2% by weight of polyvinylidene fluoride in N-methylpyrrolidone solvent. The negative active material slurry was coated on a copper foil, dried and rolled to prepare a negative electrode.

A 18650 lithium secondary battery was produced by a conventional method using the prepared positive electrode, negative electrode, and electrolytes prepared in Examples 1 and 2, Comparative Examples 1 to 7, and Reference Examples 1 to 3. At this time, 5.8 g of electrolyte main liquid amount was used.

* Measurement of electrochemical impedance spectroscopy (EIS)

The lithium secondary batteries using the electrolytic solution of the electrolytic solutions of Examples 1 and 2 and Comparative Example 1 were charged to 4.2 V and 1 C, respectively, at SOC (State of Charge) of 100% (charging and discharging at 4.2 V, The battery was charged up to 100% charge capacity when the total charge capacity was set to 100%), and stored at 60 占 폚 for 28 days. Impedance before storage and after storage were measured and the results are shown in Figs. 6 Respectively.

As shown in FIG. 6, there is almost no difference in the impedances of Examples 1 and 2 and Comparative Example 1 before the high-temperature storage, that is, the charge transfer resistance, but as shown in FIG. 7, Is significantly lower than that of Comparative Example 1.

* Direct current internal resistance (DCIR) increase rate

Lithium secondary batteries using the electrolytes of Examples 1 and 2, Comparative Examples 1 and 2, and Reference Examples 1 to 3 were stored at 60 ° C. in an oven at room temperature (25 ° C.) for 30 days The DC resistance (DC-IR) was measured under the following conditions.

The battery was charged to a voltage of 50% of a state of charge (state of charge) at a current of 0.5 C in one cycle (a state where the battery was charged so as to have a 50% charge capacity when the total charge capacity of the battery was set to 100%), After stopping for 10 minutes,

0.5 C for 30 seconds, then stopped for 30 seconds, charged at a constant current of 0.5 C for 30 seconds, stopped for 10 minutes,

1.0 C for 30 seconds, then stopped for 30 seconds, charged at 0.5 C for 1 minute with a constant current, stopped for 10 minutes,

2.0 C for 30 seconds, then stopped for 30 seconds, charged at 0.5 C for 2 minutes with a constant current, stopped for 10 minutes,

3.0 C for 30 seconds, then stopped for 30 seconds, charged at 0.5 C for 2 minutes with a constant current, and stopped for 10 minutes.

The average voltage drop value for each C-rate for 30 seconds is the DC voltage value (measured by V = IR).

Table 1 shows the DC resistance values measured after the initial 28 days. The resistance increase rate over time after storage at high temperature was calculated according to the following formula 1 and is shown together in the following Table 1. [

<Formula 1>

Resistance increase rate = [(DC resistance after high temperature storage - initial resistance before high temperature storage) / initial resistance before high temperature storage] X 100

Additive type and content DC resistance value (mohm) DCIR growth rate (%) Early After 28 days at 60 ° C Example 1 Formula Ia, 0.5 wt% 19.14 21.69 13 Example 2 Formula 1a, 1 wt% 19.45 22.68 17 Reference Example 1 (1a), 1.5 wt% 19.75 24.29 23 Reference Example 2 Formula 1a, 2 wt% 20.01 - - Reference Example 3 (1a), 3 weight% 21.00 - -

In Table 1, "-" "indicates that excessive amounts of gas are generated in the case of Reference Examples 2 and 3 in which the additives are used in an excessive amount of 2% by weight and 3% by weight, the battery is short-circuited and DCIR can not be measured .

As shown in Table 1, the DCIR increase rates of Examples 1 and 2 are 13% and 17%, while Comparative Example 1 is 31%, which is remarkably high.

In the case of Reference Example 1 in which the compound of Formula 1a was used in an amount of 1.5 wt%, it was found that the thickness increase rate was as high as 23%. As described above, in Reference Examples 2 and 3, , The thickness was excessively increased, and the battery was short-circuited and the thickness could not be measured. From these results, it can be seen that the high-temperature storage characteristics of Examples 1 and 2 are greatly improved as compared with Comparative Example 1 and Reference Examples 1 to 3.

In addition, the DCIR of the lithium secondary battery using the electrolytes of Comparative Examples 3 to 7 was measured under the above conditions, and the DC resistance value and the resistance increase rate measured after the elapse of the initial 30 days were calculated according to the above formula 1, The results of Example 2 are shown in Table 2 below for comparison.

Additive type and content
DC resistance value (V) DCIR growth rate (%)
Early After 30 days at 60 ° C Example 2 Formula 1a, 1 wt% 19.45 22.68 17 Comparative Example 3 Formula 7, 1 wt% 19.4 23.28 20 Comparative Example 4 Formula 8, 1 wt% 20.1 24.32 21 Comparative Example 5 Formula 9, 1 wt% 20.5 25.215 23 Comparative Example 6 10, 1 wt% 19.4 23.08 19 Comparative Example 7 Formula 11, 1 wt% 21.2 26.28 24

As shown in Table 2 above, it can be seen that the DCIR increase rate of the secondary battery using the electrolyte of Example 2 using the additive of Formula 1a of the three-ring ring having four CN groups is the lowest. On the other hand, Comparative Examples 3 and 5 using the additive of the CN group 2-bonded 3-ring ring or the 5-bonded 6-ring group, Comparative Example 4 using the additive of the heterocyclic ring even though the CN group is combined with 4, The DCIR increase rate is increased in Comparative Example 6 in which the three-membered ring is bonded, in Comparative Example 6 in which the remaining functional group is hydrogen, and in Comparative Example 7 in which the alkyl group of the remaining functional group is a C4 alkyl group in the three-membered ring having four CN groups.

* High temperature storage characteristics

The lithium secondary batteries of Examples 1 and 2, Comparative Example 1 and Reference Examples 1 and 2 were charged and discharged once at 0.2 C to measure charge / discharge capacities (before high temperature storage). The measured discharge capacity is shown in the following Table 3 as the initial capacity. A lithium secondary battery using the electrolytes of Examples 1 and 2, Comparative Example 1, and Reference Examples 1 and 2 was charged at a SOC of 100% (100% charged at 100% ), Stored at 85 ° C for 5 hours, and then subjected to initial charge and discharge at 0.5C. The charge / discharge characteristics at this time are referred to as capacity retention, and the charge / discharge capacity was measured. The discharge capacity ratio measured with respect to the initial capacity is shown in Table 3 below as the capacity retention rate.

Subsequently, the battery subjected to the initial charge and discharge at 0.5 C was subjected to a single cycle at 0.2 C, and the charge and discharge characteristics at this time are referred to as recovery characteristics. Generally, the high temperature storage characteristics mean recovery characteristics. The charge / discharge capacity at this time is measured, and the discharge capacity ratio to the initial capacity is shown in the following Table 3 as the capacity recovery ratio.

Type and content of additive compounds Initial capacity (mAh / g) Capacity retention rate (%) Capacity recovery rate (%) Comparative Example 1 Succinonitrile, 1 wt% 3092.8 86.77 94.32 Example 1 Formula Ia, 0.5 wt% 3089.0 88.63 95.12 Example 2 Formula 1a, 1 wt% 3089.6 88.14 94.54 Reference Example 1 (1a), 1.5 wt% 3080 86.4 93.0 Reference Example 2 Formula 1a, 2 wt% 3069 - - Reference Example 3 (1a), 3 weight% 3036 - -

In Table 3, in the case of Reference Examples 2 and 3 in which the additives were used in an excessive amount of 2% by weight and 3% by weight, the battery was short-circuited with too much gas and the capacity retention rate and the capacity recovery rate It can not be done.

As shown in Table 3, it can be seen that the capacity retention and the capacity recovery rate of the lithium secondary battery using the electrolytes of Examples 1 and 2 are superior to those of Comparative Example 1 using succinonitrile. In Reference Example 1, which used an excessive amount of the compound of Formula 1a, the capacity retention rate and the capacity recovery rate were lower than that of Example 1. As described above, in the case of Reference Examples 2 and 3, The battery was short-circuited and the capacity retention rate and the capacity recovery rate could not be measured. From these results, it can be seen that the characteristics of the high-temperature battery of Examples 1 and 2 were greatly improved as compared with Comparative Example 1 and Reference Examples 1 to 3.

* At room temperature cycle life characteristics

A lithium secondary battery using the electrolytes of Examples 1 and 2, Comparative Example 1 and Reference Examples 1 and 2 was charged and discharged 300 times at a room temperature (25) at 1.3 C, and the discharge capacity was measured. The capacity ratio in each cycle to the one-time discharge capacity was determined. The results are shown in FIG. As shown in Fig. 8, the cycle life characteristics at room temperature in Examples 1 and 2 were superior to those in Comparative Example 1 using succinonitrile, and in Reference Examples 1 to 3 in which the compound of formula (1a) Life characteristics.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. As will be understood by those skilled in the art. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

Claims (6)

Non-aqueous organic solvent;
Lithium salts; And
And an additive represented by one of the following formulas (1) to (4)
Electrolyte for lithium secondary battery.
[Chemical Formula 1]

(In the formula 1,
Wherein four of R ₁ to R ₆ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN and the others are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)
(2)

(In the formula (2)
R ₇ to R ₁₄ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN, and the others are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)
(3)

(3)
R ₁₅ to R ₂₄ are independently of each other CN, CH ₂ CN or C ₂ H ₅ CN, and the remainder are independently of each other a substituted or unsubstituted C1 to C3 alkyl group)
[Chemical Formula 4]

(In the formula 4,
R ₂₆ to R ₃₆ of the net is each independently CN, CH ₂ CN, or C ₂ H ₅ CN, the rest independently represent an alkyl group of the unsubstituted or substituted C1 to C3 each other Im) The method according to claim 1,
Wherein the content of the additive is 0.25 wt% to 1 wt% with respect to the total weight of the electrolyte. The method according to claim 1,
Wherein the content of the additive is 0.5% by weight to 1% by weight based on the total weight of the electrolyte. The method according to claim 1,
Wherein the additive is a compound represented by Formula 1 above. A negative electrode comprising a negative electrode active material;
A cathode comprising a cathode active material; And
The electrolyte of any one of claims 1 to 4
&Lt; / RTI &gt; 6. The method of claim 5,
The lithium secondary battery is a cylindrical lithium secondary battery.

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