KR20100135623A - Electrolyte solution for secondary battery with improved safety and lithium secondary battery containing the same - Google Patents

Abstract

PURPOSE: An electrolyte solution and a lithium secondary battery containing the same are provided to improve safety because SEI film-forming and recovery are enabled. CONSTITUTION: An electrolyte solution for a secondary battery comprises nonaqueous solvent and ionic salt, a first additive forming a protective film on the surface of an anode active material through polymerization reaction during an initial charge-discharging process, a second additive supplementing the protective film of the anode active material through polymerization reaction during a charge-discharging process after the initial process.

Description

Electrolyte for secondary battery with improved safety and lithium secondary battery comprising same {Electrolyte Solution for Secondary Battery with Improved Safety and Lithium Secondary Battery Containing the Same}

The present invention relates to a secondary battery electrolyte with improved safety, and more particularly, in a secondary battery electrolyte including a nonaqueous solvent and an ionic salt, forming a protective film on the surface of the negative electrode active material through a polymerization reaction during an initial charge and discharge process. A first additive and a second additive which supplements the protective film of the negative electrode active material through a polymerization reaction in an initial post-charge / discharge process, wherein the first additive is more reactive than the second additive at an initial charge / discharge potential of the secondary battery. This high, second additive relates to an electrolyte solution for secondary batteries, characterized in that it has superior decomposition resistance than the first additive at the charge and discharge potential of the secondary battery.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is rapidly increasing. Among them, lithium secondary batteries with high energy density and voltage, long cycle life, and low self discharge rate It is commercially used and widely used.

The lithium secondary battery moves from lithium metal oxide used as an anode during initial charging to graphite where lithium ions are used as a cathode, and is 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 generate compounds such as Li ₂ CO ₃ , Li ₂ O, and LiOH. These compounds form a kind of passivation layer on the surface of the graphite cathode, which is called a solid electrolyte interface (SEI) film.

Once formed, the SEI film functions as an ion tunnel to pass only lithium ions. The effect of the ion tunnel solvates lithium ions, and organic solvent molecules having a large molecular weight, such as lithium salts, EC, DMC, or DEC, which move together with lithium ions in the electrolyte are inserted together in the graphite anode. This can prevent the structure of the cathode from collapsing. Once the SEI film is formed, lithium ions again do not react sideways with the graphite cathode or other material, and the amount of charge consumed to form the SEI film has a property of not reversibly reacting upon discharging at an irreversible capacity. Therefore, no further decomposition of the electrolyte occurs and the amount of lithium ions in the electrolyte can be reversibly maintained to maintain stable charge and discharge (J. Power Sources (1994) 51: 79-104). In conclusion, once the SEI film is formed, the amount of lithium ions is reversibly maintained and the battery life characteristics are also improved.

Such SEI membrane is relatively firm under normal conditions that maintain the stability of the electrolyte, that is, a temperature range of -20 to 60 ° C. and a voltage of 4 V or less, and may sufficiently play a role of preventing side reactions between the negative electrode and the electrolyte. However, there is a problem that the durability of the SEI film gradually decreases at high temperature storage in a fully charged state (for example, left at 85 ° C. for 4 days after 100% charge at 4.2V).

That is, 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, Gases such as CH ₄ and C ₃ H ₆ are generated to cause an increase in the battery internal pressure.

The properties of these SEI membranes depend on the type of solvent or additives contained in the electrolyte, and are known as one of the major factors that affect ions and charge transfer, leading to changes in battery performance (see Shoichiro Mori, Chemical). properties of various organic electrolytes for lithium rechargeable batteries, J. Power Source (1997) Vol. 68).

Accordingly, some electrolyte additives that can be used to form an SEI film on the surface of the cathode or to repair some damaged SEI layers during repeated charge and discharge processes are disclosed.

The electrolyte additive for forming and restoring such an SEI layer is consumed in a relatively large amount during the initial formation process, whereas only a small amount of the additive is required for charge / discharge or long term storage. If the amount of the additive added to the battery is small and consumed in the initial formation process, deterioration of the life occurs after charging / discharging or long term storage. However, when the amount of the additive is too large, the surplus additive may react to generate irreversible capacity or decompose to generate gas to deteriorate the stability of the battery.

Therefore, there is a need for a new method for minimizing the problem of impairing high temperature safety by adding an additive in an amount necessary to exert an inherent effect of the additive while causing side reactions of the surplus additive, that is, causing swelling or promoting electrolyte decomposition reaction. .

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

In particular, lithium nickel-based oxides such as LiNiO ₂ have a lower discharge cost than the cobalt-based oxides and have a high discharge capacity when charged to 4.3 V. Thus, the reversible capacity of doped LiNiO ₂ is equivalent to that of LiCoO ₂ (about 165 mAh). / g) in excess of about 200 mAh / g. Thus, despite slightly lower average discharge voltages and volumetric densities, commercialized cells containing LiNiO ₂ positive electrode active materials have improved energy densities, and thus, in order to develop high capacity batteries, Research is active.

However, LiNiO ₂ based positive electrode active material shows a sudden phase transition of the crystal structure according to the volume change accompanying the charge and discharge cycle, the chemical resistance on the surface is rapidly reduced when exposed to air and moisture, and excess gas is generated during storage or cycle There is a problem in that the practical use is limited.

Accordingly, lithium transition metal oxides in which a part of nickel is substituted with other transition metals such as manganese and cobalt have been proposed. Such metal-substituted nickel-based lithium transition metal oxides have an advantage of relatively superior cycle characteristics and capacity characteristics, but even in this case, the cycle characteristics deteriorate rapidly during long-term use, swelling due to gas generation in a battery, and low chemical Problems such as stability have not been sufficiently solved. In particular, lithium nickel-based transition metal oxide having a high nickel content has a problem in that a swelling phenomenon of a battery is severe and low temperature safety is low.

Therefore, there is a high need for a technology capable of solving the high temperature safety problem while using a lithium nickel-based cathode active material suitable for high capacity. Accordingly, a method of improving the life characteristics and high temperature safety of a battery by adding vinylene carbonate, vinyl ethylene carbonate, and the like, which are conventionally used as an electrolyte additive for forming an SEI film, is proposed to lithium cobalt oxide.

However, when these materials are used in a battery containing nickel-based lithium transition metal oxide as a positive electrode active material, there is a problem that the swelling phenomenon and the deterioration of high temperature safety become more serious, and thus it is confirmed that it is practically impossible to apply.

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)

Dicarbonyl compounds represented by (ii)

Disclosed is a nonaqueous electrolyte solution for lithium secondary batteries containing a dicarbonyl compound and vinylene carbonate (VC). In addition, Korean Patent Application Publication No. 2006-030905 includes a composite oxide represented by LiNi _x M _1-xy L _y O ₂ as an active material, and the nonaqueous electrolyte includes a main solvent, a solute, and vinyl ethylene carbonate (VEC). Disclosed is a lithium secondary battery.

However, according to the inventors of the present application, even with these techniques, the improvement of sufficient life characteristics cannot be exhibited, and in particular, the effect of improving high temperature safety such as a swelling phenomenon is extremely small.

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

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.

After extensive research and various experiments, the inventors of the present application form an SEI film in an electrolyte solution through a polymerization reaction during an initial charge and discharge process, and a first additive having a higher polymerization reactivity than a second additive at an initial charge and discharge potential, and In the charging and discharging process after the initial stage, when replenishing the SEI film through a polymerization reaction and simultaneously adding a second additive having better decomposition resistance than the first additive at the charge and discharge potential of the secondary battery, the first additive consumed during initial charge and discharge is Thereby exerting an inherent effect of the additive, which can be consumed to form a protective film of the negative electrode active material during the continuous use of the battery while preventing side reactions by the surplus additive through a second additive that is stable under decomposition conditions, thereby ultimately improving safety and In order to complete the present invention, it was found that a high capacity lithium secondary battery can be manufactured. Reotda.

Accordingly, the present invention is a secondary battery electrolyte comprising a non-aqueous solvent and an ionic salt, the first additive for forming a protective film on the surface of the negative electrode active material through a polymerization reaction during the initial charge and discharge, and polymerization during the charge and discharge process after the initial stage A second additive that replenishes the protective film of the negative electrode active material through the reaction, wherein the first additive has a higher polymerization reactivity than the second additive at an initial charge and discharge potential of the secondary battery, and the second additive has a charge and discharge potential of the secondary battery. It relates to a secondary battery electrolyte, characterized in that the decomposition resistance is superior to the first additive in the.

As described above, when an additive for forming a protective film (hereinafter sometimes referred to as an 'SEI film') of a conventional negative electrode active material is added to an electrolyte, a surplus additive may promote electrolyte decomposition or swelling in a battery. There was a problem of causing side reactions such as inducing, and in particular, a battery containing lithium nickel-based oxide as a positive electrode active material had a problem in which such a phenomenon occurred more seriously.

On the other hand, the secondary battery electrolyte according to the present invention can exhibit the effect of reducing the cycle characteristics and irreversible capacity through the formation of the protective film of the negative electrode active material by the first additive consumed during initial charge and discharge, that is, . In addition, the second additive, which is stable under decomposition conditions, may prevent side reactions due to excess additives, and may form and recover the SEI film when the SEI film is decomposed or lost in the course of continuous charge and discharge.

Therefore, even when the secondary battery electrolyte is applied to a secondary battery using lithium nickel-based oxide as a cathode active material, it does not cause side reactions such as electrolyte decomposition or swelling, and thus ultimately high temperature safety is improved and high capacity lithium secondary The battery can be manufactured.

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

The initial charge and discharge potential is not particularly limited because it may vary depending on the device in which the electrolyte is used. In this regard, the lithium secondary battery performs a formation process during the manufacturing process, wherein the formation process is a process of activating the battery by repeatedly charging and discharging the battery after assembling the battery. It is inserted into the carbon electrode used as a SEI film is formed on the surface of the cathode. In general, such a chemical conversion process is performed by repeating charging and discharging with a constant current or a constant voltage in a range, and may be half charged at 1.0 to 3.8 V or fully charged at 3.8 to 4.5 V. Thus, the initial charge and discharge potential may be a potential of preferably 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 and discharge process.

For example, 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 alone or in combination of two or more thereof. Among them, the first additive may preferably be a vinylidene carbonate compound, and particularly preferably VC or VEC.

Since such a material forms a stable film that suppresses the reduction decomposition of the electrolyte solution on the surface of the cathode, that is, an SEI film, it is possible to suppress or mitigate side reactions such as decomposition of the electrolyte occurring on the surface of the cathode and to reduce lithium ions to the graphite cathode interlayer. Intercalation can be facilitated to reduce the internal resistance of the cell.

However, since the first additive, such as VC or VEC, is not stable under decomposition conditions, there is a problem of causing swelling or promoting electrolyte decomposition reaction while decomposing when included in an excessive amount. Therefore, the first additive is preferably included in an amount consumed completely in the initial charge and discharge conditions.

Accordingly, the surplus of the highly reactive first additive may cause various side reactions in the battery, or may cause the thickness of the SEI film to be excessively thick, thereby increasing the battery internal resistance.

The amount consumed completely in the initial charge and discharge conditions may vary depending on the type and capacity of the battery, and preferably, may be included in an amount of 0.01 to 0.5% by weight based on the total weight of the electrolyte.

On the other hand, the second additive, as defined above, serves to supplement the protective film of the negative electrode active material through a polymerization reaction during the initial charge and discharge process, when the SEI film is decomposed or lost during the continuous charge and discharge process Is used for SEI film formation.

In addition, since the second additive is a material having better degradation resistance than the first additive at the charge and discharge potential of the secondary battery, it may promote decomposition of the electrolyte solution or cause problems such as swelling, which may occur when a large amount of the first additive is added alone. I never do that. The charge and discharge potential of the secondary battery is generally 3.0 ~ 4.5V.

As such, the second additive capable of forming an SEI film having excellent degradation resistance at the charge and discharge potential of the secondary battery may be a siloxane compound including at least one or more unsaturated groups.

In a specific example, the material includes at least one -Si-O-Si- bond and at least one carbon-carbon double bond, or at least one or more -Si-O-Si- bond through a reaction with impurities in an electrolyte solution such as water. It may be a material containing a carbon-carbon double bond. Carbon-carbon double bonds 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 ions. Thus, an SEI film can be formed through crosslinking and lithium ions have excellent mobility.

In addition, the inventors of the present application, in the experiment, when adding the second additive together, even when used in combination with the addition of VC or VEC, electrolyte decomposition decomposition and swelling phenomenon is significantly reduced and battery safety is greatly increased It was confirmed that it can be improved.

In one preferred embodiment, 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 lower alkyl of C _1-10 , and X and Y are each independently lower alkenes of C _1-10 .

The term "alkyl" refers to an aliphatic hydrocarbon substituent. Alkyl substituents may be saturated alkyl substituents, preferably meaning they do not contain any alkenes or alkyne moieties, which may be of branched, straight chain or cyclic structure.

Examples of the lower alkyl include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, ethenyl, propenyl, butenyl, cyclopropyl, cyclobutyl, cyclopentyl and the like. .

In Formula 1, R ₁ -R ₄ are each independently hydrogen or lower alkyl of C _1-10 , preferably hydrogen or methyl, and more preferably, R ₁ -R ₄ may be all methyl. .

In Formula 1, X and Y are each independently C _1-10 lower alkenes, and “alkene” refers to a substituent including at least two carbon atoms having at least one carbon-carbon double bond. 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- CH ₃ , —CH═CH—CH═CH ₂ ), and the like, and preferably have a structure in which a double bond is located adjacent to Si.

The molecular weight of this second additive can be appropriately selected in a range suitable for formation of an SEI film without forming a polymer through crosslinking or gelling the electrolyte in a battery.

The second additive is preferably added in a content necessary for formation and recovery of the SEI film lost during the charge and discharge of the secondary battery, and when its content is too high, the irreversible capacity is increased, and conversely, If it is too small, it is difficult to achieve the desired effect. In consideration of this, it is preferably included in 0.01 to 10% by weight, more preferably 0.01 to 5% by weight based on the total weight of the electrolyte. However, in consideration of problems such as an increase in internal resistance and a decrease in capacity due to the addition of excess material, the sum of the first additive and the second additive is preferably not more than 20% by weight based on the total weight of the electrolyte. .

As said non-aqueous solvent, for example, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyl Low lactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxolon, aceto Nitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative Aprotic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyroionate and ethyl propionate can be used. These may be used alone or in combination of two or more, preferably a mixture of linear carbonates and cyclic carbonates may be used.

The ion salt may be preferably a lithium salt, and the lithium salt is a material that is easily dissociated in the nonaqueous 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 phenyl borate, imide and the like can be used.

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, Nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride and the like may be added. have. In some cases, in order to impart nonflammability, a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics.

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

As described above, the secondary battery electrolyte according to the present invention includes a predetermined first additive and a second additive at the same time, including a sufficient amount of additives to produce a continuous SEI film in the battery, but by the surplus additive Side reactions can be minimized. Therefore, in view of maximizing the effect of the electrolyte, the lithium secondary battery may be preferably used in a secondary battery including lithium nickel-based oxide as a cathode active material.

In one preferred embodiment, the secondary battery (a) can store and release lithium ions as a positive electrode active material, lithium nickel-based oxide having a Ni content of 30 mol% or more of the transition metal elements based on the total weight of the positive electrode active material The anode comprises at least 50% by weight; And (b) a negative electrode including amorphous carbon capable of occluding and releasing lithium ions as a negative electrode active material.

That is, when the lithium nickel-based oxide is included as the positive electrode active material, the discharge capacity is 20% or more superior to the case of using the lithium cobalt-based oxide as the positive electrode active material. Thus, the high capacity characteristics can be sufficiently exhibited, and thus it can be used more effectively.

The lithium nickel-based oxide may be preferably a lithium transition metal oxide of Formula 2 below.

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

In the above formula, -0.5≤z≤0.5, 0.3≤b≤0.9, 0.1≤c≤0.8, b + c <1, Me is 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 mole fraction of nickel is 30 to 90%, and contains Mn and a predetermined metal element (Me). Such a positive electrode active material has an advantage in that its structural stability is superior to lithium nickel oxide (LiNiO ₂ ), which has a higher capacity than lithium cobalt oxide and has a content of Ni of 100 mol% in transition metal elements.

In one preferred embodiment, the metal doped nickel-based positive electrode active material may be a so-called three-component material Me is Co. In addition, when the content of the Ni element in the transition metal is too small, it is difficult to expect a high capacity, on the contrary, too much may cause distortion or collapse of the crystal structure. In consideration of this, the metal-doped nickel-based active material is particularly preferably a composition of nickel excess relative to manganese and cobalt, when b is 0.4 to 0.7 in formula (2), that is, the mole fraction of nickel is 40 to 70%. desirable.

Such a lithium transition metal oxide having an excessive 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 a predetermined additive is included in the electrolyte, there is an advantage that such a problem can be minimized.

As the cathode active material, lithium nickel-based oxides represented by Formula 2 may be used alone, or in some cases, a cathode active material capable of occluding and releasing lithium ions may be mixed.

Examples thereof include layered compounds such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ), or compounds 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 , LiMnO _2, and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O ₇ and the like; Ni-site-type lithium nickel oxide represented by the formula LiNi _1-y M _y O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and y = 0.01 to 0.3; Formula LiMn _2-y M _y O ₂ , wherein M = Co, Ni, Fe, Cr, Zn or Ta and y = 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , where M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

However, the lithium nickel-based oxide represented by the formula (2) shows a high discharge capacity, it is preferably contained in an amount of at least 50% by weight or more based on the total cathode active material, more preferably from 80 to 100% by weight May be included.

The lithium secondary battery according to the present invention is composed of, for example, a positive electrode, a negative electrode, a separator, a lithium salt-containing electrolyte solution, and the like.

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

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

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

The lithium secondary battery according to the present invention can be produced by conventional methods known in the art. In addition, the structure of the positive electrode, the negative electrode, and the separator in the lithium secondary battery according to the present invention is not particularly limited, and, for example, each of these sheets in a winding type (winding type) or stacking (stacking type) cylindrical, rectangular Or it may be a form inserted into the case of the pouch type.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example 1

1-1. Preparation of Electrolyte

To 1.0M electrolyte solution in which LiPF ₆ was dissolved in EC, 0.2 wt% of VC and 0.3 wt% of siloxane derivative according to Formula 2 including methyl substituents and two ethylene groups were added as electrolyte additive to prepare a lithium secondary battery electrolyte. It was.

1-2. Preparation of Cathode

The negative electrode was composed of 93% by weight of a carbon active material (MCMB10-28 from Osaka Gas) and 7% by weight of polyvinylidene difluoride (PVDF, Kynar 761 from Elf Atochem), a solvent of N-methyl-2-pyrrolidone ( NMP) was mixed in a mixer (mixer manufactured by Ika) for 2 hours, coated on a copper foil current collector, and dried at 130 ° C.

1-3. Manufacture of anode

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% PVDF (Kynar 761) as a binder, and 3 wt% acetylene black as a conductive material ( NMP) was mixed in a mixer (mixer manufactured by Ika) for 2 hours, and 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, Inc.) was disposed between the negative electrode and the positive electrode prepared in this way and wound in a cylindrical shape to assemble a battery of 18650 standard, and then a lithium secondary battery was prepared by injecting electrolyte.

The battery thus produced was charged to 4.2 V at 1 mA and discharged at a termination voltage of 2.5 V at 1 mA of current. After such initial charging and discharging, five cycles of charging and discharging, charging at a current of 10 mA and an upper limit of 4.2 V and discharging at a 2.5 V termination voltage, were performed. Then, after storing for one week and two weeks at a temperature of 65 DEG C in a state of charge of five cycles, respectively, discharge was performed under the same conditions to specify the capacity.

[Example 2]

An electrolyte was prepared by adding 0.3 wt% of the siloxane derivative according to Formula 2 including methyl substituents and one ethylene group instead of the siloxane derivative according to Formula 2 including methyl substituents and two ethylene groups as an electrolyte additive. Except that, a lithium secondary battery was manufactured 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 the electrolyte additive, one of the methyl substituents was substituted with hydrogen and 0.3 wt% of the siloxane derivative according to formula (2) containing two ethylene groups was added. A lithium secondary battery was manufactured in the same manner as in Example 1, except that an electrolyte solution was prepared.

 Example 4

A lithium secondary battery was manufactured in the same manner as in Example 1, except that VC was added in an amount of 0.01% by weight instead of 0.2% by weight as an electrolyte additive.

Example 5

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the cathode was manufactured using LiNi _0.8 Co _0.1 Mn _0.1 O ₂ as the cathode active material.

Example 6

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the cathode was manufactured using LiNi _0.5 Co _0.2 Mn _0.3 O ₂ as the cathode active material.

Example 7

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 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 manufactured in the same manner as in Example 1, except that no electrolyte additive was added.

Comparative Example 2

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

Comparative Example 3

A lithium secondary battery was manufactured in the same manner as in Example 1, except that only 0.5 wt% of a siloxane derivative according to Formula 2 including methyl substituents and two ethylene groups as an electrolyte additive was added to prepare an electrolyte. It was.

[Comparative Example 4]

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the cathode was manufactured using LiCoO ₂ as the cathode active material.

[Comparative Example 5]

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the cathode was manufactured using LiNiO ₂ as the cathode active material.

[Comparative Example 6]

A lithium secondary battery was manufactured in the same manner as in Example 1, except that only 0.5 wt% of VEC was added as an electrolyte additive to prepare an electrolyte.

[Experimental Example] Life time characteristic measurement and high temperature storage characteristic measurement

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

As a result of the experiment, the batteries of the examples according to the present invention were found to have an excellent effect on the life characteristics and high temperature storage characteristics compared to the batteries of the comparative example.

Although described above with reference to embodiments of the present invention, those skilled in the art will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

As described above, the present invention has a high polymerization reactivity at an initial charge / discharge potential in a secondary battery electrolyte, and an excellent decomposition resistance at a charge / discharge potential, and a SEI film formed at an initial charge / discharge process after the initial charge and discharge potential. By including the second additive at the same time, it is possible to minimize problems such as accelerated electrolyte decomposition and swelling that occur when the conventional additive is added in an amount necessary for forming and restoring the SEI film, thereby improving the high temperature safety and cycle characteristics of the battery. In addition, when the electrolytic solution is applied to a battery containing a high nickel content of lithium transition metal oxide as a positive electrode active material, the effect of the addition can be further doubled, and ultimately, a high capacity battery can be provided.

Claims (14)

A secondary battery electrolyte comprising a nonaqueous solvent and an ionic salt, and 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 and discharge process, and a negative electrode through a polymerization reaction during an initial charge and discharge process. And a second additive supplementing a protective film of the active material, wherein the first additive has a higher polymerization reactivity than the second additive at an initial charge and discharge potential of the secondary battery, and the second additive is a first additive at the charge and discharge potential of the secondary battery. Electrolytic solution for secondary batteries, characterized by more excellent decomposition resistance. The electrolyte solution for a secondary battery according to claim 1, wherein the charge and discharge potential of the secondary battery is 2.5 to 4.5 V. The method of claim 1, wherein the first additive is a secondary battery electrolyte, characterized in that it is contained in an amount consumed completely in the initial charge and discharge conditions. According to claim 3, wherein the first additive is a secondary battery electrolyte, characterized in that contained in 0.01 to 0.5% by weight based on the total weight of the electrolyte. The electrolyte solution for a secondary battery according to claim 1, wherein the first additive is a vinylidene carbonate compound. The electrolyte of claim 5, wherein the vinylidene carbonate compound is VC (vinylene carbonate) or VEC (vinylene ethylene carbonate). The electrolyte solution for a secondary battery of claim 1, wherein the second additive is a siloxane compound including at least one unsaturated group. The method of claim 7, wherein the second additive is a secondary battery electrolyte, characterized in that the 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 lower alkyl of C _1-10 , and X and Y are each independently lower alkenes of C _1-10 . The secondary battery electrolyte of claim 1, wherein the nonaqueous solvent is a mixture of linear carbonate and cyclic carbonate, and the ionic salt is a lithium salt. A lithium secondary battery comprising the secondary battery electrolyte according to claim 1. The method of claim 10, wherein the secondary battery (a) can store and release lithium ions as a positive electrode active material, lithium nickel-based oxide having a Ni content of 30 mol% or more of the transition metal elements based on the total weight of the positive electrode active material The anode comprises at least 50% by weight; And (b) a negative electrode including amorphous carbon capable of occluding and releasing lithium ions as the negative electrode active material. The lithium secondary battery according to claim 11, wherein the lithium nickel-based oxide is a lithium transition metal oxide of the following general formula (2).                        Li_{1 + z}Ni_bMn_cMe_{1- (b + c)}O₂ (2) In the above formula, -0.5≤z≤0.5, 0.3≤b≤0.9, 0.1≤c≤0.8, b + c <1, Me is 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. 13. The lithium secondary battery of claim 12, wherein the lithium nickel oxide of Formula 2 has Me of Co and 0.4 ≦ b ≦ 0.7. The lithium secondary battery of claim 11, wherein the lithium nickel-based oxide comprises 80 to 100 wt% based on the total weight of the positive electrode active material.