KR101874346B1 - Electrolyte for rechargeable lithium battery and rechargeable lithium battery including same - Google Patents

Abstract

The present invention relates to a nonaqueous electrolyte for a lithium secondary battery and a lithium secondary battery including the nonaqueous electrolyte. The nonaqueous electrolyte improves discharge characteristics at a low temperature of the lithium secondary battery, and increases battery safety, reliability, and lifetime characteristics.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte for a lithium secondary battery, and a lithium secondary battery including the same. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a nonaqueous electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same.

Since a lithium secondary battery, which has recently been spotlighted as a power source for portable electronic devices, uses an organic electrolytic solution, it exhibits a discharge voltage twice as high as that of a battery using an existing alkaline aqueous solution, and as a result, exhibits a high energy density .

The organic electrolytic solution used in the lithium secondary battery is composed of a lithium salt such as LiPF ₆ and an organic solvent. As the organic solvent, it is preferable that the organic solvent has a low reactivity with lithium, a minimized internal resistance to facilitate movement of lithium ions, thermal stability at a wide temperature, high compatibility with a negative electrode active material, and a large amount of lithium salt High dielectric constant is required. Examples of the organic solvent include cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC); Or linear carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC) and the like, and hydrocarbon solvents such as 1,2-dimethoxyethane and diethoxyethane may also be used.

PC in the organic solvent has a low melting point of -49 캜 and exhibits excellent low temperature characteristics, good compatibility with amorphous carbon, and a large dielectric constant, so that a large amount of inorganic lithium salt can be dissolved. However, the PC is highly viscous and, when used with a crystalline carbonaceous anode active material such as graphite, is decomposed while being inserted between the carbon layers of the anode during charging to produce propylene gas and lithium carbonate, There is a problem of increasing irreversible capacity. This irreversible capacity is caused by the structural characteristics of the carbon used primarily, and depends on the degree of reduction of the electrolyte solution at the interface between lithium and carbon and the degree of formation of the electrolyte protective layer formed on the carbon surface. On the other hand, since EC does not react with a graphite anode active material, it can be easily applied to a battery using a crystalline carbon as a cathode, and a large amount of lithium salt can be dissolved due to a large dielectric constant. However, it is disadvantageous in that it has a high viscosity and a high melting point of about 36 캜, which makes it impossible to secure a low-temperature performance.

In addition, linear carbonates such as DMC, DEC and the like have a small viscosity and can easily intercalate between the negative electrode active materials to reduce the irreversible capacity of the battery and reactivity with lithium is small, but the dielectric constant is small and a large amount of lithium salt can not be dissolved There are disadvantages. In particular, DMC has a high electric conductivity and is expected to be used in high current and high voltage batteries, but has a low melting point (4.6 ° C) and low temperature characteristics. In addition, organic solvents such as dimethylformamide and acetonitrile have a large dielectric constant, but are highly reactive with lithium and are hardly practically used.

Therefore, in order to overcome the disadvantages of each electrolyte solvent, recently, a method has been proposed in which DEC having good low-temperature characteristics is added to EC / DEC and various mixtures of one or more organic solvents are used. However, There is a problem in that the characteristics improving effect is not sufficient, the active material decomposition temperature is low and the heat generation amount is large when applied to a battery, so that it is difficult to secure the safety of the battery.

An embodiment of the present invention is to provide a nonaqueous electrolyte for a lithium secondary battery which can improve discharge characteristics at low temperatures of a lithium secondary battery and increase battery safety, reliability, and life characteristics.

Another embodiment of the present invention provides a lithium secondary battery comprising the non-aqueous electrolyte.

The nonaqueous electrolyte for a lithium secondary battery according to an embodiment of the present invention includes a lithium salt; Non-aqueous organic solvent; And a trialkylsilylborate as an additive. The non-aqueous organic solvent includes a low melting point solvent having a melting point of -50 DEG C or less and an ion conductivity of 6 S / cm or more at 25 DEG C.

The low melting point solvent may be selected from the group consisting of alkyl acetates, alkyl propionates, and mixtures thereof.

The low melting point solvent may be selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, butyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, have.

The low melting point solvent may be contained in an amount of 50% by weight to 70% by weight based on the total weight of the non-aqueous organic solvent.

The trialkylsilylborate may be trimethylsilylborate.

The trialkylsilylborate may be included in an amount of 0.5% by weight to 3% by weight based on the total weight of the nonaqueous electrolyte.

The lithium secondary battery according to another embodiment of the present invention includes the non-aqueous electrolyte.

Other details of the embodiments of the present invention are included in the following detailed description.

The nonaqueous electrolyte for a lithium secondary battery according to the present invention improves discharge characteristics at low temperatures of a lithium secondary battery and can increase battery safety, reliability and life characteristics.

1 is a schematic view schematically showing a structure of a lithium secondary battery according to an embodiment of the present invention.
FIG. 2 is a graph showing the results of observing ionic conductivities at 0 ° C and 25 ° C while adding various low-melting-point solvents to an EC / EMC / DEC mixed solvent containing 1M LiPF ₆ .
3 is a graph showing the results of measurement of the discharge capacity at -20 캜 for the batteries of Example 3 and Comparative Examples 2 and 3. Fig.
4 is a graph showing the results of measurement of discharge capacity at -30 DEG C for the batteries of Example 3 and Comparative Examples 2 and 3. Fig.

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.

Unless otherwise specified in the specification, "alkyl" means a C1 to C7 alkyl group, and specific examples thereof include methyl, ethyl, propyl, isopropyl, pentyl, hexyl, heptyl and the like.

Recently used lithium secondary batteries have been used in a variety of climatic environments. When the battery is used in a low temperature environment, discharge characteristics at an extremely low temperature are required. However, when the electrolyte is frozen under a temperature of -20 ° C or below, the discharge voltage of the battery remarkably drops due to the increase in resistance, .

In contrast, in an embodiment of the present invention, by using a low melting point solvent having a lower melting point and a higher ion conductivity than conventional carbonate-based solvents, it is possible to prevent the electrolyte from freezing when the battery is exposed to an extremely low temperature, The discharge capacity of the battery can be increased at an extremely low temperature and the use of trimethylsilyl borate having excellent ionic conductivity together with the low melting point solvent as a nonaqueous electrolytic solution can form a stable film on the surface of the positive electrode and negative electrode active material, And prevents the drop of the initial discharge voltage at the time of low-temperature discharge and prevents a life-shortening problem that may occur when the low-melting-point solvent is used alone, thereby improving long-term service life characteristics.

That is, the nonaqueous electrolyte for a lithium secondary battery according to an embodiment of the present invention includes a lithium salt; Non-aqueous organic solvent; And a trialkylsilylborate as an additive. The non-aqueous organic solvent includes a low melting point solvent having a melting point of about -50 캜 or lower and an ion conductivity of about 6 S / cm or higher at about 25 캜.

Hereinafter, each component will be examined in detail.

Lithium salt

The lithium salt is dissolved in the non-aqueous organic solvent to act as a source of lithium ions in the battery to enable operation of a basic lithium secondary battery, and a material capable of promoting the movement of lithium ions between the positive electrode and the negative electrode to be. The lithium salt Representative examples are _{LiPF 6, LiBF 4, LiSbF 6} , LiAsF 6, 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, x and y are natural numbers), LiCl, LiI, LiB ( C 2 O 4) 2 ( lithium bis oxalate reyito borate (lithium bis (oxalato) borate; LiBOB) , or a mixture thereof In the present invention, these are included as supporting electrolyte salts.

The lithium salt is preferably used in a concentration range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has an appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance and the lithium ion can effectively move.

Non-aqueous Organic solvent

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

In one embodiment of the present invention, the non-aqueous organic solvent includes a low melting point solvent having a melting point of about -50 占 폚 or lower and an ion conductivity of about 6 S / cm or higher at about 25 占 폚. Preferably, the low melting point solvent has a melting point of about -120 캜 to -50 캜 and an ion conductivity of about 6 S / cm to about 11 S / cm at about 25 캜. By having the above melting point and ionic conductivity, it does not freeze even at a low temperature and exhibits improved lithium ion conductivity, so that it can improve the low temperature discharge capacity when applied to an electrolyte of a lithium secondary battery.

The low melting point solvent may be selected from the group consisting of alkyl acetates, alkyl propionates, and mixtures thereof. Specific examples thereof include methyl acetate, ethyl acetate, propyl acetate, butyl acetate, dimethyl acetate, methyl propionate , Ethyl propionate, propyl propionate, butyl propionate, and mixtures thereof. Alkyl in the alkyl acetate and alkyl propionate is C1 to C15 alkyl. Preferably, ethyl acetate, ethyl propionate, or a mixture thereof can be used.

The low melting point solvent is preferably included in an amount of about 10% by weight to about 70% by weight based on the total weight of the non-aqueous organic solvent. When it is included in the above-mentioned content range, it is possible to exhibit an effect of improving low-temperature characteristics.

The non-aqueous organic solvent used in one embodiment of the present invention may be used in combination with the above-described compound in the form of a carbonate, ester, ether, ketone, alcohol or aprotic organic solvent used as a nonaqueous organic solvent in a lithium secondary battery Solvent.

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) can be used. As the ester solvent, γ-butyrolactone, decanolide, valerolactone, mevalonolactone ), Caprolactone, etc. 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 alcoholic solvent, ethyl alcohol, isopropyl alcohol and the like can be used. As the aprotic solvent, R-CN (R is a C2 to C20 linear, branched or cyclic hydrocarbon group, A linking aromatic ring or an ether linkage); Amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes.

The non-aqueous organic solvent may be used alone or in admixture of one or more. If the non-aqueous organic solvent is used in combination, the mixing ratio may be appropriately adjusted according to the desired cell performance. .

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 about 1: 1 to about 1: 9, the performance of the electrolytic solution may be excellent.

The non-aqueous organic solvent may further include the 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 about 1: 1 to about 30: 1.

The aromatic hydrocarbon-based organic solvent may be an aromatic hydrocarbon-based compound represented by the following formula (1).

[Formula 1]

In Formula 1, R ₁ to R ₆ are each independently hydrogen, halogen, a C1 to C10 alkyl group, a C1 to C10 haloalkyl group, or a combination thereof.

The aromatic hydrocarbon-based organic solvent is selected from the group consisting of benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3- , 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4 - triiodobenzene, toluene, fluorotoluene, 1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene, 1,2,4 - trichlorotoluene, iodotoluene, 1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotol Ene, 1,2,3-tree-iodo toluene, 1,2,4-iodo toluene, xylene, or may be a combination thereof.

The non-aqueous electrolyte may further include vinylene carbonate or an ethylene carbonate-based compound represented by the following formula (2) to improve battery life.

(2)

Wherein R ₇ and R ₈ are each independently hydrogen, a halogen group, a cyano group (CN), a nitro group (NO ₂ ) or a C1 to C5 fluoroalkyl group, and at least one of R ₇ and R ₈ Is a halogen group, a cyano group (CN), a nitro group (NO ₂ ) or a C1 to C5 fluoroalkyl group.

Representative examples of the ethylene carbonate-based compound include, for example, difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, fluoroethylene carbonate, . When the vinylene carbonate or the ethylene carbonate compound is further used, the amount of the vinylene carbonate or the ethylene carbonate compound can be appropriately controlled to improve the life.

additive

In one embodiment of the present invention, the nonaqueous electrolyte comprises a trialkylsilylborate as an additive for increasing ionic conductivity.

In the trialkylsilylborate, the alkyl may be C1 to C7 alkyl, and specific examples thereof include trimethylsilyl borate and the like.

The trialkylsilylborate is preferably contained in an amount of about 0.1 wt% to about 5 wt%, and preferably about 0.1 wt% to about 3 wt%, based on the total weight of the nonaqueous electrolyte. When included in the above content range, excellent ion conductivity can be exhibited.

The nonaqueous electrolyte according to the present invention has the above-described structure, thereby improving the discharge characteristics at low temperature of the lithium secondary battery, and increasing the battery safety, reliability, and life characteristics.

According to another embodiment of the present invention, there is provided a lithium secondary battery comprising the non-aqueous electrolyte.

The lithium secondary battery can be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery depending on the type of the separator and electrolyte used. The lithium secondary battery can be classified into a cylindrical shape, a square shape, a coin shape, Depending on the size, it can be divided into bulk type and thin type. The structure and the manufacturing method of these cells are well known in the art, and detailed description thereof will be omitted.

1 is an exploded perspective view of a lithium secondary battery according to one embodiment. 1, the lithium secondary battery 100 has a cylindrical shape and includes a cathode 112, a cathode 114, a separator 113 disposed between the cathode 112 and the anode 114, An electrolyte 114 (not shown), an electrolyte (not shown) impregnated into the separator 113, a battery container 120, and a sealing member 140 for sealing the battery container 120. The lithium secondary battery 100 is constructed by laminating a cathode 112, an anode 114 and a separator 113 in this order and then winding them in a spiral wound state in the battery container 120.

The negative electrode 112 includes a current collector and a negative electrode active material layer formed on the current collector, and the negative electrode active material layer includes a negative electrode active material.

The negative electrode active material includes 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.

As a material capable of reversibly intercalating / deintercalating lithium ions, any carbonaceous anode active material commonly used in lithium ion secondary batteries can be used as the carbonaceous material. Typical examples thereof 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 (soft carbon) Or hard carbon, mesophase pitch carbide, fired coke, and the like.

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

As the material capable of doping and dedoping lithium, Si, SiO _x (0 <x <2), Si-X ₁ alloy (X ₁ is an alkali metal, an alkali earth metal, a group 13 to 16 group element, , A rare earth element or a combination thereof and not Si), Sn, SnO ₂ and Sn-X ₂ alloy (X ₂ is an alkali metal, an alkali earth metal, a Group 13 to 16 Group element, a transition metal, And Sn is not used), and at least one of them may be mixed with SiO ₂ . The specific elements of X ₁ and X ₂ include Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, As, Sb, Bi, S, Se, Te, Po, or a combination thereof.

Examples of the transition metal oxide include vanadium oxide, lithium vanadium oxide, and the like.

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

The binder serves to adhere the anode active material particles to each other and to adhere the anode active material to the current collector. Typical examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, Such as polyvinyl chloride, polyvinyl fluoride, polymers comprising ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, Styrene-butadiene rubber, epoxy resin, nylon, and the like may be used, but the present invention is not limited thereto.

The conductive material is used for imparting conductivity to the electrode. Any material can be used as long as it does not cause any chemical change in the battery. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, , Carbon-based materials such as carbon fibers; 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 may be used.

The current collector may be a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foil, a copper foil, a polymer substrate coated with a conductive metal, or a combination thereof.

The anode 114 includes a current collector and a cathode active material layer formed on the current collector.

As the cathode active material, a compound capable of reversibly intercalating and deintercalating lithium (a lithiated intercalation compound) can be used. Concretely, it is possible to use at least one of cobalt, manganese, nickel, or a composite oxide of a metal and lithium in combination thereof. Specific examples thereof include compounds represented by one of the following formulas: Li _a A _{1 - b} R _b D ₂ wherein, in the formula, 0.90? a? 1.8 and 0? b? 0.5; Li _a E _{1 - b} R _b O _{2 - c} D _c , wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05; LiE _{2 - b} R _b O _{4 - c} D _{c where} 0? B? 0.5, 0? C? 0.05; Li _a Ni _{1 -b- c} Co _b R _c D _{α where} 0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, and 0 <α ≦ 2; Li _a Ni _{1 -b- c} Co _b R _c O _{2 - ?} Z _{? Where} 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05 and 0 <? Li _a Ni _{1 -b- c} Co _b R _c O _{2 - ?} Z ₂ wherein 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, and 0 <? Li _a Ni _1-bc Mn _b R _c D _? Wherein, in the formula, 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05 and 0 <? Li _a Ni _{1 -b- c} Mn _b R _c O _{2 - ?} Z _{? Where} 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05 and 0 <? Li _a Ni _{1 -b- c} Mn _b R _c O _{2 - ?} Z ₂ wherein 0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05 and 0 <? 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 (in the above formula, 0.90? A? 1.8 and 0.001? B? 0.1); Li _a MnG _b O ₂ wherein, in the above formula, 0.90? A? 1.8 and 0.001? B? 0.1; Li _a Mn ₂ G _b O ₄ (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 ; LiTO 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; R 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; Z 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; T is Cr, V, Fe, Sc, Y or a combination thereof; J is V, Cr, Mn, Co, Ni, Cu or a combination 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 may comprise, as a coating element compound, an oxide, a hydroxide of a coating element, an oxyhydroxide of a coating element, an oxycarbonate of a coating element, or a hydroxycarbonate of a 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 carried out by any of coating methods such as spray coating, dipping, and the like without adversely affecting the physical properties of the cathode active material by using these elements in the above compound. It is a content that can be well understood by people engaged in the field, so detailed explanation will be omitted.

The cathode active material layer also includes a binder and a conductive material.

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 can be used without causing any chemical change in the battery. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, Metal powders such as black, carbon fiber, copper, nickel, aluminum, and silver, metal fibers, and the like, and conductive materials such as polyphenylene derivatives may be used alone or in combination.

The current collector may be aluminum (Al), but the present invention is not limited thereto.

The negative electrode 112 and the positive electrode 114 are prepared by mixing an active material, a conductive material and a binder in a solvent to prepare an active material composition, and applying the composition to a current collector. The method of manufacturing the electrode is well known in the art, and therefore, a detailed description thereof will be omitted herein. As the solvent, N-methylpyrrolidone or the like can be used, but it is not limited thereto.

The electrolyte is as described above.

The separator 113 may exist between the anode 114 and the cathode 112 depending on the type of the lithium secondary battery. The separator 113 may be made of polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more thereof. The separator may be a polyethylene / polypropylene double layer separator, a polyethylene / polypropylene / polyethylene triple layer separator, a polypropylene / polyethylene / Polypropylene three-layer separator, or the like can be used.

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  One)

A mixture of LiCoO ₂ and Li (Ni _1/3 Co _1/3 Mn _1/3 ) O ₂ (weight ratio: 7: 3) as a positive electrode active material was mixed with polyvinylidene fluoride (PVDF) Were mixed in a weight ratio of 92: 4: 4, and N-methyl-2-pyrrolidone was added to prepare a positive electrode slurry. The prepared positive electrode slurry was applied to an aluminum foil having a thickness of 20 mu m as a current collector, followed by drying in a vacuum oven and rolling to prepare a positive electrode.

Crystalline artificial graphite as a negative electrode active material and PVDF as a binder were mixed at a weight ratio of 92: 8, and then N-methyl-2-pyrrolidinone was added and dispersed to prepare an anode slurry. The prepared negative electrode slurry was applied to a copper foil having a thickness of 15 탆 as a current collector, dried in a vacuum oven and rolled to prepare a negative electrode.

An electrode assembly was prepared using the porous separator made of polyethylene having a thickness of 25 mu m as a separator between the positive electrode and the negative electrode prepared above. The electrode assembly was wound, compressed, and inserted into a square can. 10% by weight of ethyl acetate (EA) and 1% by weight of trimethylsilylborate (TMSB) were added to a mixed solvent of ethylene carbonate (EC) / diethyl carbonate (DEC) (mixing volume ratio = 30: And then LiPF ₆ was dissolved in 1M, and then a non-aqueous electrolyte was injected thereinto, followed by sealing to prepare a lithium secondary battery.

( Example  2 to 18 and Comparative Example  1 to 3)

A battery was produced in the same manner as in Example 1, except that a non-aqueous electrolyte prepared by mixing in the amounts and compositions as shown in Table 1 below was used.

Solvent (volume ratio) Low melting point solvent
(weight%) LiPF ₆
(M) TMSB
(weight%) Example 1 EC / DEC = 30:60 EA 10 1.0 One Example 2 EC / DEC = 30:40 EA 30 1.0 One Example 3 EC / DEC = 30:20 EA 50 1.0 One Example 4 EC = 30 EA 70 1.0 One Example 5 EC / DEC = 30:60 EP 10 1.0 One Example 6 EC / DEC = 30:40 EP 30 1.0 One Example 7 EC / DEC = 30:20 EP 50 1.0 One Example 8 EC / DEC = 30 EP 70 1.0 One Example 9 EC / DEC = 30:20 EA 50 1.0 0.1 Example 10 EC / DEC = 30:20 EA 50 1.0 0.5 Example 11 EC / DEC = 30:20 EA 50 1.0 2 Example 12 EC / DEC = 30:20 EA 50 1.0 3 Example 13 EC / DEC = 30:20 MA 50 1.0 One Example 14 EC / DEC = 30:20 PA 50 1.0 One Example 15 EC / DEC = 30:20 BA 50 1.0 One Example 16 EC / DEC = 30:20 MP 50 1.0 One Example 17 EC / DEC = 30:20 PP 50 1.0 One Example 18 EC / DEC = 30:20 BP 50 1.0 One Comparative Example 1 EC / DEC / EMC = 30:20:50 - 1.0 - Comparative Example 2 EC / DEC / EMC = 30: 20: 50 - 1.0 One Comparative Example 3 EC / DEC = 30:20 EA 50 1.0 -

In Table 1, EC is ethylene carbonate, DEC is diethyl carbonate, EMC is ethyl methyl carbonate, EA is ethyl acetate, EP is ethyl propionate, MA is methyl acetate, PA is propyl acetate, BA is butyl acetate, Butyl propionate, PP means propyl propionate, and TMSB means trimethyl silyl borate.

The melting points of the solvents used in the above Examples and Comparative Examples are shown in Table 2 and FIG.

menstruum Melting point (℃) EC (ethyl carbonate) 36 PC (propyl carbonate) -48 DMC (dimethyl carbonate) 3 EMC (ethyl methyl carbonate) -15 DEC (methyl ethyl carbonate) -43 EA (ethyl acetate) -84 MA (methyl acetate) -94 PA (propyl acetate) -93 BA (butyl acetate) -78 EP (ethyl propionate) -73 MP (methyl propionate) -87 PP (propyl propionate) -76 BP (butyl propionate) -74

FIG. 2 shows the results obtained by adding various low-melting-point solvents to the EC / EMC / DEC mixed solvent (mixing volume ratio = 3: 5: 2) containing 1M LiPF ₆ in the contents shown in Table 3 And the ion conductivity (S / cm) at 0 占 폚 and 25 占 폚.

Electrolyte LiPF ₆ EC / EMC / DEC mixed solvent
(Mixing volume ratio) additive
(weight%) REF 1M 3: 5: 2 - MA1 1M 3: 5: 2 10 MA2 1M 3: 5: 2 20 EA1 1M 3: 5: 2 10 EA2 1M 3: 5: 2 20 PA1 1M 3: 5: 2 10 PA2 1M 3: 5: 2 20 MP1 1M 3: 5: 2 10 MP2 1M 3: 5: 2 20 EP1 1M 3: 5: 2 10 EP2 1M 3: 5: 2 20 PP1 1M 3: 5: 2 10 PP2 1M 3: 5: 2 20 BP1 1M 3: 5: 2 10 BP2 1M 3: 5: 2 20

( Test Example )

The batteries manufactured in Examples and Comparative Examples were evaluated for low temperature capacity at -20 占 폚 and -30 占 폚, and life characteristics after 500 cycles at room temperature, and the results are shown in Table 4 below. The low-temperature capacities of Comparative Examples 2 and 3 and Example 3 at -20 캜 and -30 캜 are shown in Figs. 3 and 4, respectively. Specifically, the batteries manufactured in the above-mentioned Examples and Comparative Examples were activated by a charge / discharge cycle and then charged under a CC-CV condition at a current of 400 mA at a charging voltage of 4.35 V, For 2 hours, and after freezing, discharging was performed up to 2.75 V at 1 C to confirm the discharge capacity.

The lifespan characteristics of the battery were 500 cycles at room temperature (24 to 25 ° C) with one cycle of charging at 1 C, 4.2 V, or 4.35 V charging voltage for 2 hours 30 minutes under CC-CV conditions and then discharging at 1 C 3.2 V cut- After the cycle was repeated, the ratio of the discharge capacity after 500 cycles to the initial discharge capacity was evaluated as the life characteristics.

-20 ℃ discharge capacity
(%) -30 ℃ discharge capacity
(%) Life characteristics after 500 cycles at room temperature
(%) Example 1 42 7 88 Example 2 47 9 88 Example 3 50 15 87 Example 4 53 14 82 Example 5 38 5 89 Example 6 40 7 87 Example 7 45 12 85 Example 8 47 15 85 Example 9 45 2 83 Example 10 43 8 84 Example 11 48 17 84 Example 12 51 15 81 Example 13 45 10 85 Example 14 44 10 82 Example 15 44 7 80 Example 16 43 7 83 Example 17 40 5 80 Example 18 41 5 80 Comparative Example 1 12 0 88 Comparative Example 2 15 0 89 Comparative Example 3 40 0 80

As shown in Table 4 and FIG. 3 and FIG. 4, the batteries of Examples 1 to 18 according to the present invention exhibited excellent discharge capacities at a low temperature as compared with the batteries of Comparative Examples 1 to 3, Respectively.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, And falls within the scope of the invention.

100: Lithium secondary battery
112: cathode
113: Separator
114: anode
120: Battery container
140: sealing member

Claims (4)

Lithium salts;
Non-aqueous organic solvent; And
A nonaqueous electrolyte for a lithium secondary battery comprising an additive,
The non-aqueous organic solvent includes a low melting point solvent having a melting point of -50 DEG C or less and an ion conductivity of 6 S / cm or more at 25 DEG C,
The low melting point solvent is contained in an amount of 50% by weight to 70% by weight based on the total weight of the non-aqueous organic solvent,
The low melting point solvent is selected from the group consisting of ethyl acetate, ethyl propionate, and mixtures thereof,
Wherein the additive is trialkylsilylborate,
The trialkylsilylborate is contained in an amount of 0.5% by weight to 3% by weight based on the total weight of the nonaqueous electrolyte,
Wherein the lithium secondary battery comprises a negative electrode including a carbon-based negative active material.
The method according to claim 1,
Wherein the trialkylsilylborate is trimethylsilylborate.
A negative electrode comprising a carbon-based negative electrode active material;
anode; And
A lithium secondary battery comprising a non-aqueous electrolyte,
The non-
Lithium salts;
Non-aqueous organic solvent; And
&Lt; / RTI &gt;
Wherein the non-aqueous organic solvent comprises a low melting point solvent having a melting point of -50 DEG C or less and an ion conductivity of 6 S / cm or more at 25 DEG C, and the low melting point solvent is 50 wt% 70% by weight,
The low melting point solvent is selected from the group consisting of ethyl acetate, ethyl propionate, and mixtures thereof,
Wherein the additive is trialkylsilylborate,
And the non-aqueous electrolyte comprises the trialkylsilylborate in an amount of 0.5% by weight to 3% by weight based on the total weight of the nonaqueous electrolyte.
Lithium secondary battery.
The method of claim 3,
Wherein the trialkylsilylborate is trimethylsilylborate.

Images