KR101459402B1 - Electrolyte Solution for Secondary Battery with Improved Safety and Lithium Secondary Battery Containing the Same - Google Patents

Abstract

The present invention provides an electrolyte for a secondary battery comprising a non-aqueous solvent and an ionic salt, comprising: a first additive for forming a protective film on a surface of an anode active material through a polymerization reaction during an initial charging / discharging process; Wherein the first additive has a higher polymerization reactivity than the second additive at an initial charge / discharge potential of the secondary battery, and the second additive has a higher charge / discharge potential than the second additive at a charge / discharge potential of the secondary battery. 1 < / RTI > additive. The present invention also provides an electrolyte solution for a secondary battery.

The electrolyte for a secondary battery according to the present invention includes a first additive having a high polymerization reactivity at an initial charge / discharge potential of the secondary battery. When the battery is initially charged and discharged, the SEI film is formed on the surface of the negative electrode by the first additive, If the SEI film is lost due to continuous charge / discharge after the initial charge / discharge, it is possible to form and recover the SEI film during the continuous use of the battery, while preventing the side reaction by the excess additive by the stable second additive under decomposition conditions, The safety is improved, and a high capacity lithium secondary battery can be produced.

Description

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

More particularly, the present invention relates to an electrolyte solution for a secondary battery comprising a nonaqueous solvent and an ionic salt, wherein a protective film is formed on the surface of the negative electrode active material through a polymerization reaction in an initial charge- And a second additive that replenishes the protective film of the negative electrode active material through a polymerization reaction in an initial charging / discharging process after the initial charge / discharge cycle, wherein the first additive has a higher polymerization reactivity than the second additive at an initial charge / discharge potential of the secondary battery, And the second additive is excellent in the resistance to decomposition at the charge / discharge potential of the secondary battery as compared with the first additive.

As technology development and demand for mobile devices are increasing, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life and low self- It has been commercialized and widely used.

The lithium secondary battery moves from lithium metal oxide used as an anode at the time of initial charging to graphite used as a cathode, and inserted between the layers of the graphite electrode. At this time, since lithium is highly reactive, the electrolyte and the carbon constituting the cathode react on the surface of the graphite cathode to produce compounds such as Li ₂ CO ₃ , Li ₂ O, and LiOH. These compounds form a passivation layer on the surface of the graphite anode, which is referred to as a solid electrolyte interface (SEI) film.

Once formed, the SEI film acts as an ion tunnel to pass only lithium ions. Organic solvent molecules, such as lithium salts, EC, DMC or DEC, which have a large molecular weight and move together with lithium ions in the electrolyte solution by solvation of lithium ions by the effect of the ion tunnel, So that it is possible to prevent the structure of the cathode from collapsing. Once the SEI film is formed, lithium ions no longer undergo side reaction with the graphite anode or other material, and the amount of charge consumed in the formation of the SEI film is irreversible and does not react reversibly upon discharging. Therefore, the decomposition of the electrolytic solution no longer occurs, and the amount of lithium ions in the electrolytic solution is reversibly maintained, so that stable charge and discharge can be maintained (J. Power Sources (1994) 51: 79-104). As a result, once the SEI film is formed, the amount of lithium ions is reversibly maintained and the lifetime characteristics of the battery are also improved.

Such an SEI film is relatively firm under the ordinary conditions of maintaining the stability of the electrolyte solution, that is, in the temperature range of -20 to 60 占 폚 and the voltage of 4 V or less, and can sufficiently prevent the side reaction between the anode and the electrolyte. However, there is a problem that the durability of the SEI film is gradually lowered when stored at a high temperature in a fully charged state (for example, left at 85 캜 for four days after 100% charging at 4.2 V).

In other words, when the high-temperature storage in the fully charged state the cathode, while the SEI film gradually collapses and exposed with the lapse of time, so that the surface of the exposed anode reacts with an electrolyte around, causing a side reaction continued to CO, CO _2, CH ₄ , C ₃ H ₆ and the like are generated, resulting in an increase in the cell internal pressure.

The properties of such an SEI film depend on the type of the solvent contained in the electrolyte, the characteristics of the additives, etc., and are known to be one of the main factors affecting the performance of the battery by affecting ion and charge transfer (see Shoichiro Mori, Chemical properties of various organic electrolytes for lithium rechargeable batteries, J. Power Source (1997) Vol.

There are some disclosures of electrolyte additives that can be used to form SEI films on the surface of the cathode or to repair some damaged SEI layers during repetitive charge and discharge processes.

The electrolyte additive for the formation and recovery of the SEI layer consumes a relatively large amount in the initial formation process, while only a small amount of additive is required in the subsequent charge / discharge or long-term storage. If the amount of the additive added to the battery is small and the battery is consumed during the initial formation process, the life of the battery may deteriorate during the subsequent charge / discharge or long-term storage. However, when the amount of the additive is too large, the excess additive may react to generate an irreversible capacity or generate gas while decomposing to deteriorate the stability of the battery.

Accordingly, there is a need for a new method that minimizes the problem of inhibiting the high-temperature safety by inducing side reactions of surplus additives, that is, inducing swelling or promoting electrolytic decomposition reaction, while exerting the inherent effect of the additive .

On the other hand, lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as a positive electrode active material of a lithium secondary battery, and lithium-containing manganese oxides such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure, The use of nickel oxide (LiNiO ₂ ) is also considered.

In particular, a lithium nickel oxide such as LiNiO ₂ has a lower discharge capacity than that of the cobalt oxide when it is charged at a low cost of 4.3 V, and the reversible capacity of the doped LiNiO _{2 has} a capacity of LiCoO ₂ (about 165 mAh / g) < / RTI > to about 200 mAh / g. Therefore, in spite of a slightly low average discharge voltage and volumetric density, the compatibilized battery including LiNiO ₂ cathode active material has an improved energy density. Therefore, in order to develop a high capacity battery, Research is actively under way.

However, the LiNiO ₂ cathode active material exhibits an abrupt phase transition of the crystal structure in accordance with the volume change accompanied by the charge / discharge cycle, and when exposed to air and moisture, the chemical resistance of the surface is rapidly deteriorated and excessive gas is generated during storage or cycling There is a problem that practical use is limited.

Thus, a lithium transition metal oxide in which a part of nickel is substituted with another transition metal such as manganese or cobalt has been proposed. These metal-substituted nickel-based lithium-transition metal oxides have an advantage in that they have excellent cycle characteristics and capacity characteristics. However, even in this case, the cycle characteristics are drastically lowered during long-term use, and swelling due to gas generation in the battery, Stability and other problems are not sufficiently solved. Particularly, the lithium nickel transition metal oxide having a high nickel content has a problem that swelling phenomenon of the battery is marked and the high temperature safety is low.

Accordingly, there is a high need for a technique capable of solving the problem of high-temperature safety while using a lithium-nickel-based positive electrode active material suitable for high capacity. Accordingly, a method has been proposed in which vinylene carbonate, vinyl ethylene carbonate, and the like, which are conventionally used as an electrolyte additive for forming an SEI film, are added to lithium cobalt oxide to improve lifetime characteristics and high-temperature safety of the battery.

However, when these materials are used in a battery containing a nickel-based lithium-transition metal oxide as a cathode active material, it has been found that the swelling phenomenon and the deterioration of high-temperature safety become more serious, and practically impossible.

로 표시되는 디카보닐 화합물 또는 (ii)

로 표시되는 디카보닐 화합물과, 비닐렌 카보네이트(VC)를 함유하는 리튬 이차전지용 비수 전해액을 개시하고 있다. 또한, 한국 특허출원공개 제2006-030905호는 LiNi_xM_1-x-yL_yO₂로 표시되는 복합 산화물을 활물질로서 포함하고, 비수전해질이, 주용매, 용질 및 비닐 에틸렌 카보네이트(VEC)를 포함하는 리튬 이차전지를 개시하고 있다. Accordingly, Korean Patent Application Publication No. 2007-089958 discloses (i)

(Ii) a dicarbonyl compound represented by the formula

Discloses a nonaqueous electrolyte solution for a lithium secondary battery containing a dicarbonyl compound represented by the following formula (1) and vinylene carbonate (VC). Korean Patent Application Laid-Open No. 2006-030905 also discloses that a composite oxide represented by LiNi _x M _1-xy L _y O ₂ is included as an active material, and the nonaqueous electrolyte contains a main solvent, a solute and a vinyl ethylene carbonate (VEC) And a lithium secondary battery.

However, according to the inventors of the present application, it has been found that not only the lifetime characteristics can not be sufficiently improved even by these techniques, but the effect of improving the high-temperature safety such as swelling phenomenon is extremely small.

Therefore, when a lithium-nickel-based transition metal oxide is used as a positive electrode active material, there is a high need for a technique for solving the deterioration in cycle characteristics and high-temperature safety.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

The inventors of the present application have conducted intensive research and various experiments, and have found that a first additive having a higher polymerization reactivity than the second additive at an initial charge / discharge potential, When the SEI film is replenished through the polymerization reaction in the initial charging and discharging process and a second additive having a higher resistance to decomposition than the first additive is simultaneously added at the charging / discharging potential of the secondary battery, the first additive consumed at the initial charge / Can be consumed for forming the protective film of the negative electrode active material during continuous use of the battery while preventing the side reaction by the excess additive through the stable second additive under decomposition conditions while exhibiting the effect inherent to the additive, , A lithium rechargeable battery having a high capacity can be produced. To complete the present invention, Reotda.

Accordingly, the present invention provides an electrolyte solution for a secondary battery comprising a non-aqueous solvent and an ionic salt, comprising: a first additive for forming a protective film on the surface of the negative electrode active material through a polymerization reaction during an initial charge- Wherein the first additive has higher polymerization reactivity than the second additive at an initial charge / discharge potential of the secondary battery, and the second additive has a higher charge / discharge potential of the secondary battery than the second additive, Which is superior in resistance to decomposition than the first additive.

As described above, when an additive for forming a protective film of a conventional negative electrode active material (hereinafter, occasionally referred to as an SEI film) is added to an electrolyte solution, the excess additive accelerates decomposition of the electrolyte solution in the battery or causes a swelling phenomenon There is a problem that such a phenomenon occurs more seriously in a battery including a lithium nickel oxide as a cathode active material.

On the other hand, the electrolyte for a secondary battery according to the present invention can exhibit the inherent characteristics of the additive, that is, the cyclic characteristic and the irreversible capacity reduction through formation of the protective film of the negative electrode active material, by the first additive consumed at the time of initial charge- . Further, by the second additive stable under decomposition conditions, the SEI film can be formed and recovered when the SEI film is decomposed or lost in the continuous charge-discharge process, while preventing side reactions due to excess additive.

Accordingly, even when applied to a secondary battery using a lithium nickel oxide as a cathode active material, such an electrolyte for a secondary battery does not cause a side reaction such as electrolyte decomposition or swelling, ultimately improving high-temperature safety, A battery can be manufactured.

In the present invention, the first additive is a material for forming a protective film on the surface of the negative electrode active material through a polymerization reaction in the initial charging / discharging process and is a material having higher polymerization reactivity than the second additive at the initial charge / discharge potential of the secondary battery. It is consumed preferentially by discharging.

The initial charging / discharging potential is not particularly limited as it may vary depending on the device in which the electrolyte is used. In this regard, the lithium secondary battery performs a formation process in the manufacturing process, and the conversion process is a process of activating the battery by repeating charging and discharging after assembling the battery, wherein lithium ions, And a SEI film is formed on the surface of the negative electrode. Such a conversion process generally proceeds to repeated charge and discharge with a constant current or constant voltage, and can be half-charged at 1.0 to 3.8 V or fully charged at 3.8 to 4.5 V. Thus, the initial charge / discharge potential may preferably be a potential of 2.5 to 4.5 V, more preferably 2.7 to 4.3 V.

The first additive is not particularly limited as long as it is a material capable of forming a protective film, that is, an SEI film, on the surface of the negative electrode active material through a polymerization reaction in the initial charge / discharge process.

For example, it is possible to use vinylene carbonate (VC), vinylene ethylene carbonate (VEC), fluoro-ethylene carbonate, succinic anhydride, lactide, caprolactam, ethylene sulfite, propane sulton PS), propene sultone, vinyl sulfone, derivatives thereof, and halogen substituents. These may be used singly or in combination of two or more. Of these, the first additive may preferably be a vinylidene carbonate-based compound, and particularly preferably VC or VEC.

Since such a material forms a stable film that suppresses the reduction decomposition of the electrolytic solution on the surface of the negative electrode, that is, the SEI film, it is possible to suppress or alleviate side reactions such as electrolyte decomposition and the like occurring on the surface of the negative electrode, Intercalation can be facilitated and the internal resistance of the battery can be reduced.

However, since the first additive such as VC or VEC is not stable under decomposition conditions, if it is contained in an excessive amount, there is a problem of inducing swelling or accelerating decomposition reaction of electrolyte. Therefore, it is preferable that the first additive is included in an amount completely consumed under the initial charge-discharge condition.

Accordingly, the surplus amount of the first additive having high reactivity causes various side reactions in the battery, or the thickness of the SEI film becomes excessively large, which may lead to an increase in internal resistance of the battery.

The amount of completely consumed in the initial charging / discharging conditions may vary depending on the kind and capacity of the battery, and may be 0.01 to 0.5% by weight, based on the total weight of the electrolytic solution.

Meanwhile, as described above, the second additive serves to supplement the protective film of the negative electrode active material through the polymerization reaction in the initial charging and discharging processes, and when the SEI film is decomposed or lost in the continuous charging / discharging process Is used for SEI film formation.

In addition, since the second additive is a substance having better resistance to decomposition than the first additive at the charging / discharging potential of the secondary battery, it is possible to accelerate the decomposition of the electrolyte, which may occur when a large amount of the first additive alone is added, I never do that. The charge / discharge potential of the secondary battery is generally 3.0 to 4.5 V.

As described above, the second additive which is excellent in resistance to decomposition at the charge / discharge potential of the secondary battery and capable of forming the SEI film may preferably be a siloxane-based compound containing at least one unsaturated group.

In a specific example, a material containing at least one of -Si-O-Si-bond and carbon-carbon double bond, or a material containing at least one of -Si-O-Si- Carbon-carbon double bond. The carbon-carbon double bond in the material may preferably be two or more, more preferably two or three.

Such a material includes a functional group having a carbon-carbon double bond and a siloxane capable of conducting lithium ion, and the like. The SEI film can be formed through crosslinking and the lithium ion mobility is excellent.

Moreover, the inventors of the present application have experimentally found that, when the second additive is added together, the electrolyte decomposition phenomenon and the swelling phenomenon are significantly reduced even when VC or VEC is added and used together, It can be improved.

In one preferred example, the second additive may be a compound of formula 1:

X-Si (R ₁ ) (R ₂ ) O-Si (R ₃ ) (R ₄ ) -Y (1)

Wherein R ₁ , R ₂ , R ₃ and R ₄ are each independently hydrogen or C _1-10 lower alkyl and X and Y are each independently a C _1-10 lower alkene.

The term "alkyl" means an aliphatic hydrocarbon substituent. The alkyl substituent may preferably be a saturated alkyl substituent meaning that it does not contain any alkene or alkyne moieties and may be branched, straight chain or cyclic in structure.

The lower alkyl is, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, ethenyl, propenyl, butenyl, cyclopropyl, cyclobutyl, cyclopentyl .

In the above formula (1), R ₁ to R ₄ are each independently hydrogen or C _1-10 lower alkyl, preferably hydrogen or methyl, more preferably R ₁ to R ₄ may all be methyl .

"Alkene" means a substituent in which at least two carbon atoms are composed of at least one carbon-carbon double bond, wherein X and Y are each independently a C _1-10 lower alkene. Examples of such alkenes are typically ethylene (-CH = CH ₂ ), propylene (-CH = CH-CH ₃ ), butylene (-CH = CH-C ₂ H ₅ , -CH ₂ -CH = CH ₃ , -CH = CH-CH = CH ₂ ), and preferably a structure in which a double bond is located adjacent to Si.

The molecular weight of such a second additive can be appropriately selected within a range preferable for the formation of the SEI film, without forming a polymer through cross-linking in the cell or gelling the electrolyte solution.

The second additive is preferably added in an amount necessary for formation and recovery of the SEI film lost during the charge and discharge of the secondary battery. If the content of the second additive is too high, the irreversible capacity increases relatively. On the other hand, If it is too small, it is difficult to exert a desired effect. In consideration of this, 0.01 to 10% by weight, preferably 0.01 to 5% by weight, based on the total weight of the electrolytic solution may be included. However, considering the problems such as an increase in internal resistance and a decrease in capacity due to the addition of an excessive amount of material, it is preferable that the sum of the first additive and the second additive does not exceed 20% by weight based on the total weight of the electrolytic solution .

Examples of the non-aqueous solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylenecarbonate, dimethyl carbonate, diethyl carbonate, But are not limited to, lactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, Nitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives , Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate and the like can be used. These may be used alone or in combination of two or more, preferably a mixture of linear carbonate and cyclic carbonate may be used.

The ionic salts are preferably may be a salt of lithium, the lithium salt is a material that is readily dissociated dissolved in the non-aqueous solvent, for example, LiCl, LiBr, LiI, LiClO _4, LiBF _4, LiB ₁₀ Cl _10, LiPF _{6, LiCF 3 SO 3, LiCF} 3 CO 2, LiAsF 6, LiSbF 6, LiAlCl 4, CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid Lithium, lithium tetraphenylborate, imide, and the like can be used.

For the purpose of improving the charge-discharge characteristics and the flame retardancy, the electrolytic solution may contain at least one selected from the group consisting of pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride and the like may be added have. In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or carbon dioxide gas may be further added to improve high-temperature storage characteristics.

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

As described above, the electrolyte for a secondary battery according to the present invention simultaneously contains a predetermined first additive and a second additive. The electrolyte contains a sufficient amount of an additive to generate a continuous SEI film in the battery, Side reactions can be minimized. Therefore, from the viewpoint of maximizing the effect of the electrolytic solution, the lithium secondary battery can be preferably used for a secondary battery including a lithium nickel oxide as a cathode active material.

In one preferred embodiment, the secondary battery comprises: (a) a lithium nickel oxide having a Ni content of 30 mol% or more among the transition metal elements capable of intercalating and deintercalating lithium ions as a cathode active material, At least 50% by weight based on the total weight of the positive electrode; And (b) a negative electrode containing amorphous carbon capable of intercalating and deintercalating lithium ions as a negative electrode active material.

That is, when the lithium nickel oxide is included as the cathode active material, the lithium cobalt oxide exhibits an excellent discharge capacity of 20% or more as compared with the case where the lithium cobalt oxide is used as the cathode active material.

The lithium nickel oxide may preferably be a lithium transition metal oxide represented by the following formula (2).

Li _{1 + z} Ni _b Mn _c Me _{1- (b + c)} O ₂ (2)

Wherein Me is at least one element selected from the group consisting of Co, Al, Mg, Ti, Sr, Zn, B, Ca, Cr, Si At least one element selected from the group consisting of Ga, Sn, P, V, Sb, Nb, Ta, Mo, W, Zr, Y and Fe.

That is, the molar fraction of nickel is 30 to 90% and contains Mn and a predetermined metal element (Me). Such a cathode active material has an advantage in that it has a higher capacity than lithium cobalt oxide and is superior in structural stability to lithium nickel oxide (LiNiO ₂ ) in which the content of Ni in the transition metal elements is 100 mol%.

In one preferred example, the metal-doped nickel-based cathode active material may be a so-called three-component material in which Me is Co. If the content of the Ni element in the transition metal is too small, it is difficult to expect a high capacity. Conversely, if too large, the crystal structure may be distorted or collapsed. In consideration of this, the metal-doped nickel-based active material preferably has an excess nickel composition relative to manganese and cobalt in the case where b in the formula (2) is 0.4 to 0.7, that is, the molar fraction of nickel is 40 to 70% desirable.

Such a lithium-transition metal oxide having an excess nickel composition has an advantage of high capacity and operating potential, but has a problem in high-temperature characteristics. However, in the secondary battery according to the present invention, since the electrolyte contains a predetermined additive, the problem can be minimized.

The cathode active material may be a lithium nickel oxide represented by the formula (2) alone or, in some cases, a cathode active material capable of intercalating and deintercalating lithium ions.

Examples thereof include a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + y} Mn _2-y O ₄ (where y is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ and LiMnO ₂ ; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; Formula _{LiNi 1-y M y O 2} ( where, the M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, y = 0.01 ~ 0.3 Im) Ni site type lithium nickel oxide which is represented by; Formula _{LiMn 2-y M y O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, y = 0.01 ~ 0.1 Im) or Li ₂ Mn ₃ MO ₈ (where, M = Fe, Co, Ni, Cu, or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

However, the lithium nickel oxide represented by Formula 2 has a high discharge capacity, and is preferably contained in an amount of at least 50 wt% or more, more preferably 80 to 100 wt% .

The lithium secondary battery according to the present invention comprises, for example, an anode, a cathode, a separator, and an electrolyte solution containing a lithium salt.

The positive electrode is prepared, for example, by applying a mixture of a positive electrode active material, a conductive material and a binder on a positive electrode current collector, followed by drying, and if necessary, a filler is further added. The negative electrode is also manufactured by applying and drying the negative electrode material on the negative electrode collector, and if necessary, may further include the above-described components.

The separation membrane is interposed between the cathode and the anode, and an insulating thin film having high ion permeability and mechanical strength is used.

The collector, the electrode active material, the conductive material, the binder, the filler, the separator, the electrolyte, and the lithium salt are well known in the art, and a detailed description thereof will be omitted herein.

The lithium secondary battery according to the present invention can be manufactured by a conventional method known in the art. In the lithium secondary battery according to the present invention, the structure of the positive electrode, the negative electrode and the separator is not particularly limited. For example, each of the sheets may be formed into a cylindrical shape by a winding type or a stacking type, Or inserted into a case of a pouch type.

Hereinafter, the present invention will be described in further detail with reference to the following examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[Example 1]

1-1. Preparation of electrolytic solution

0.2% by weight of VC and 0.3% by weight of a siloxane derivative represented by the general formula (2) containing methyl substituents and two ethylene groups were added to a 1.0 M electrolytic solution obtained by dissolving LiPF ₆ in EC to prepare an electrolytic solution for a lithium secondary battery Respectively.

1-2. Cathode manufacturing

The cathode had a composition of 93 wt% of a carbon active material (MCMB10-28 of Osaka Gas Co., Ltd.) and 7 wt% of polyvinylidene difluoride (PVDF, Kynar 761 from Elf Atochem) NMP) in a mixer (Mixer of Ika) for 2 hours, then coated on a copper foil current collector and dried at 130 ° C.

1-3. Manufacture of anode

A solution of N-methyl-2-pyrrolidone (N-methyl-2-pyrrolidone) as a solvent in a composition of 94 wt% LiNi _0.53 Co _0.2 Mn _0.27 O ₂ as a positive electrode active material, 3 wt% of PVDF (Kynar 761) NMP) in a mixer (Mixer of Ika) for 2 hours, then coated on an aluminum foil current collector and dried at 130 ° C.

1-4. Manufacture of batteries

A separator (celgard 2400, manufactured by Hoechst Celanese Co.) was placed between the anode and the cathode, and the battery was wound into a cylindrical shape. A battery of the 18650 standard was assembled, and an electrolyte was injected to prepare a lithium secondary battery.

The battery thus prepared was charged to 4.2 V at 1 mA and discharged at a final voltage of 2.5 V at a current of 1 mA. After such initial charging and discharging, charging and discharging were performed at a current of 10 mA and an upper limit voltage of 4.2 V and discharging at a 2.5 V termination voltage for 5 cycles. Thereafter, the cells were stored for 1 week and 2 weeks at a temperature of 65 ° C in a charged state of 5 cycles, respectively, and discharge was carried out under the same conditions to determine the capacity.

[Example 2]

That an electrolytic solution was prepared by adding 0.3% by weight of a siloxane derivative according to Formula 2 containing methyl substituents and one ethylene group instead of a siloxane derivative according to Formula 2 containing methyl substituents and two ethylene groups as an electrolyte additive , A lithium secondary battery was produced in the same manner as in Example 1. [

[Example 3]

Instead of the siloxane derivative according to formula (2) containing methyl substituents and two ethylene groups as an electrolyte additive, 0.3% by weight of the siloxane derivative according to formula (2) wherein one of the methyl substituents is substituted with hydrogen and two ethylene groups are added A lithium secondary battery was produced in the same manner as in Example 1, except that an electrolyte solution was prepared.

 [Example 4]

A lithium secondary battery was prepared in the same manner as in Example 1, except that VC was added as an electrolyte additive in an amount of 0.01 wt% instead of 0.2 wt% to prepare an electrolytic solution.

[Example 5]

A lithium secondary battery was produced in the same manner as in Example 1, except that LiNi _0.8 Co _0.1 Mn _0.1 O ₂ was used as the positive electrode active material to prepare a positive electrode.

[Example 6]

A lithium secondary battery was produced in the same manner as in Example 1, except that LiNi _0.5 Co _0.2 Mn _0.3 O ₂ was used as the positive electrode active material to prepare a positive electrode.

[Example 7]

A lithium secondary battery was prepared in the same manner as in Example 1, except that a 1.0 M electrolyte solution in which LiPF ₆ was dissolved in EC / EMC (1: 2) was used.

[Comparative Example 1]

A lithium secondary battery was produced in the same manner as in Example 1, except that the electrolyte additive was not added.

[Comparative Example 2]

A lithium secondary battery was prepared in the same manner as in Example 1, except that VC was added as an electrolyte additive in an amount of 0.5 wt% to prepare an electrolytic solution.

[Comparative Example 3]

A lithium secondary battery was produced in the same manner as in Example 1, except that only 0.5% by weight of the siloxane derivative represented by the general formula (2) containing methyl substituents and two ethylene groups was added as an electrolyte additive to prepare an electrolytic solution. Respectively.

[Comparative Example 4]

A lithium secondary battery was prepared in the same manner as in Example 1, except that LiCoO ₂ was used as a positive electrode active material to prepare a positive electrode.

[Comparative Example 5]

A lithium secondary battery was produced in the same manner as in Example 1, except that LiNiO ₂ was used as a positive electrode active material to prepare a positive electrode.

[Comparative Example 6]

A lithium secondary battery was prepared in the same manner as in Example 1, except that VEC alone as an electrolyte additive was added in an amount of 0.5 wt% to prepare an electrolytic solution.

[Experimental Example] Measurement of life characteristics and measurement of high-temperature storage characteristics

The battery cells manufactured in each of the above-described Examples and Comparative Examples were measured for the change in thickness during 500 cycles and the capacity of the charged state of the battery, respectively. Further, in Examples 1 to 3 and Comparative Example 1 The cells prepared in ~ 2 were stored at 60 ° C for 4 weeks and then the capacity was measured.

As a result of the tests, it was confirmed that the batteries of the examples according to the present invention exert superior effects in terms of the life characteristics and the high temperature storage characteristics as compared with the batteries of the comparative examples.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

INDUSTRIAL APPLICABILITY As described above, the present invention provides an electrolyte for a secondary battery, which has a high polymerization reactivity at an initial charge-discharge potential and has a high resistance to decomposition at a charge / discharge potential and a SEI film By including the second additive at the same time, it is possible to minimize problems such as accelerated decomposition of electrolytic solution and swelling phenomenon which are caused when conventional additive is added in an amount required for formation and recovery of the SEI film, thereby improving the high temperature safety and cycle characteristics of the battery When such an electrolyte is applied to a battery containing a nickel-rich lithium transition metal oxide as a cathode active material, the effect of addition can be doubled, and ultimately, a battery with a high capacity can be provided.

Claims (14)

An electrolyte for a secondary battery comprising a non-aqueous solvent and an ionic salt, a lithium secondary battery including a positive electrode and a negative electrode, (a) a first additive for forming a protective film on the surface of the negative electrode active material through a polymerization reaction in an initial charge / discharge process, and a second additive for supplementing a protective film of the negative active material through a polymerization reaction in an initial charge- Wherein the first additive is a material having higher polymerization reactivity than the second additive at an initial charge / discharge potential of the secondary battery, and is contained in an amount of 0.01 to 0.5% by weight based on the total weight of the electrolyte so as to be completely consumed under initial charge- 2 additive is a material excellent in resistance to decomposition at a charge / discharge potential of the secondary battery and higher than that of the first additive, and is contained in an amount of 0.01 to 10% by weight based on the total weight of the electrolyte; (b) a lithium nickel oxide of the following formula (2) capable of intercalating and deintercalating lithium ions and having a Ni content of 50 mol% or more among the transition metal elements is 80 to 100 Weight% Included Bipolar: Li _{1 + z} Ni _b Mn _c Me _{1- (b + c)} O ₂ (2) In the above formula, Me, Co, Al, Mg, Ti, Sr, Zn, B, Ca, Cr, Si At least one element selected from the group consisting of Ga, Sn, P, V, Sb, Nb, Ta, Mo, W, Zr, Y and Fe; (c) a negative electrode comprising amorphous carbon capable of intercalating and deintercalating lithium ions as a negative electrode active material; And a lithium secondary battery. The lithium secondary battery according to claim 1, wherein the charge / discharge potential of the secondary battery is 2.5 to 4.5 V. delete delete The lithium secondary battery according to claim 1, wherein the first additive is a vinylene carbonate compound. The lithium secondary battery according to claim 5, wherein the vinylene carbonate compound is vinylene carbonate (VC) or vinylene ethylene carbonate (VEC). The lithium secondary battery according to claim 1, wherein the second additive is a siloxane-based compound containing at least one unsaturated group. The lithium secondary battery according to claim 7, wherein the second additive is a compound represented by the following general formula (1) X-Si (R ₁ ) (R ₂ ) O-Si (R ₃ ) (R ₄ ) -Y (1) Wherein R ₁ , R ₂ , R ₃ and R ₄ are each independently hydrogen or C _1-10 lower alkyl and X and Y are each independently a C _1-10 lower alkene. The lithium secondary battery according to claim 1, wherein the non-aqueous solvent is a mixture of a linear carbonate and a cyclic carbonate, and the ionic salt is a lithium salt. delete delete delete delete delete