KR101538485B1 - Non-aqueous electrolyte solution for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery containing the same. According to the present invention, a non-aqueous electrolyte solution for a lithium secondary battery comprising a lithium salt and an organic solvent includes an alkylene sulfate having a specific structure, an ammonium compound having a specific structure, and a vinylene carbonate. The nonaqueous electrolyte solution for a lithium secondary battery of the present invention can simultaneously improve low temperature output and high temperature storage performance.

Description

TECHNICAL FIELD [0001] The present invention relates to a non-aqueous electrolyte for a lithium secondary battery, and a lithium secondary battery including the same. BACKGROUND OF THE INVENTION [0001]

The present invention relates to a nonaqueous electrolyte for a lithium secondary battery and a lithium secondary battery containing the same. More particularly, the present invention relates to a nonaqueous electrolyte having excellent low temperature output and high temperature storage performance and a lithium secondary battery containing the same.

BACKGROUND ART [0002] With the development of the information communication industry in recent years, electronic devices are becoming smaller, lighter, thinner, and portable. As a result, there is a growing demand for higher energy density of batteries used as power sources for such electronic devices. Lithium secondary batteries are the ones that can best meet these demands, and researches on them are actively under way. The lithium secondary battery includes a cathode, an anode, and an electrolyte that provides a pathway for lithium ions between the cathode and the anode, and a separator. The lithium secondary battery is formed by oxidation and reduction reactions when lithium ions are intercalated and deintercalated in the cathodes and the anodes. Generate electrical energy.

The non-aqueous electrolyte used in the lithium secondary battery generally includes an electrolyte solvent and an electrolyte salt. However, the electrolyte solvent may be decomposed on the surface of the electrode during charging and discharging of the cell, or may be co-intercalated between the carbonaceous negative electrode layers to collapse the negative electrode structure, thereby deteriorating the stability of the battery.

It has been found that the above problems can be solved by a solid electrolyte interface (SEI) film formed on the surface of the negative electrode by reduction of the electrolyte solvent upon initial charging of the cell. However, in general, the SEI film is insufficient to serve as a continuous protective film of the negative electrode. Eventually, when the battery is repeatedly charged and discharged, the lifetime and the performance are lowered. In particular, the SEI film of a conventional lithium secondary battery is not thermally stable, and therefore, when the battery is operated or left at a high temperature, it is liable to be collapsed due to increased electrochemical energy and thermal energy with time. Therefore, the battery performance is further lowered at a high temperature. Particularly, the breakdown of the SEI film, the decomposition of the electrolyte, and the like cause continuous gas such as CO ₂ to increase the internal pressure and thickness of the battery.

In order to solve the above-mentioned problems, a method of using vinylene carbonate (VC) as an electrolyte additive capable of forming an SEI film on a surface of a negative electrode has been proposed. However, in the case of vinylene carbonate, the storage performance and cycle performance at high temperatures are excellent, but the output is lowered at low temperatures.

In recent years, lithium secondary batteries have been widely used not only in general electronic devices but also in electric vehicles. As a result, the demand for high output batteries has been increasing. Therefore, there is a need for batteries capable of providing excellent output at low temperature as well as high temperature performance It is gradually emerging.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery having the same, wherein the low temperature output is improved while maintaining high-temperature storage performance of the lithium secondary battery.

According to an aspect of the present invention, there is provided a nonaqueous electrolyte solution for a lithium secondary battery comprising a lithium salt and an organic solvent, wherein the nonaqueous electrolyte solution comprises an alkylene sulfate represented by the following formula 1, an ammonium compound represented by the following formula 2, and vinylene carbonate . ≪ / RTI >

In Formula 1, n is an integer of 2 to 5,

In Formula 2, R is an alkyl group or an aryl group having 1 to 20 carbon atoms.

In the electrolyte solution of the present invention, the alkylene sulfate is preferably ethylene sulfate, and the ammonium compound is preferably ammonium benzoate.

In the electrolytic solution of the present invention, the mixing ratio by weight of the alkylene sulfate, the ammonium compound and the vinylene carbonate may be 1: 0.01 to 1: 1 to 3: alkylene sulfate: ammonium compound: vinylene carbonate, It is not.

In the electrolytic solution of the present invention, the total content of the alkylene sulfate, the ammonium compound and the vinylene carbonate may be 1 to 10% by weight based on the total weight of the nonaqueous electrolyte solution, but is not limited thereto.

The nonaqueous electrolyte solution for a lithium secondary battery described above is usefully applied to a conventional lithium secondary battery having a negative electrode and a positive electrode.

As described in the present invention, when a nonaqueous electrolytic solution to which a predetermined alkylene sulfate, a predetermined ammonium compound and vinylene carbonate are simultaneously added is applied to a lithium secondary battery, excellent storage characteristics at high temperatures are maintained, battery resistance at low temperatures is lowered, The output characteristics can be improved.

Hereinafter, the present invention will be described in detail. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

According to the present invention, a non-aqueous electrolyte solution for a lithium secondary battery comprising a lithium salt and an organic solvent includes an alkylene sulfate represented by the following formula (1), an ammonium compound represented by the following formula (2), and vinylene carbonate:

[Chemical Formula 1]

In Formula 1, n is an integer of 2 to 5,

(2)

In Formula 2, R is an alkyl group or an aryl group having 1 to 20 carbon atoms.

As described above, vinylene carbonate can improve the high-temperature performance, but does not have excellent low-temperature performance. However, when the alkylene sulfate represented by the above formula (1) is used together with vinylene carbonate, the alkylene sulfate having relatively high reduction potential is decomposed earlier than vinylene carbonate. As a result, the SEI film formed by the sulfate compound is formed first, and the SEI film formed by the vinylene carbonate is formed next. The coating formed by the sulfate compound has an advantage of reducing the resistance, and thus contributes mainly to improving the low temperature output characteristics of the battery. The coating formed by the vinylene carbonate (VC) mainly contributes to improving the high temperature characteristics of the battery.

However, since the film formed by the sulfate compound is relatively weak at a higher temperature than the film formed by the vinylene carbonate, it is difficult to sufficiently improve the high-temperature characteristics of the battery with only the above-mentioned two additive materials. Therefore, the present invention uses an ammonium compound together with the above alkylene sulfate and vinylene carbonate. The inventors of the present invention confirmed that the use of a non-aqueous electrolyte containing the above-mentioned three components kept the high temperature performance at a much higher level than the case of using only vinylene carbonate and improved the low-temperature output characteristics, and completed the present invention.

Specific examples of the alkylene sulfate represented by the formula (1) according to the present invention include ethylene sulfate, propylene sulfate, butylene sulfate, or a mixture of two or more thereof, but not limited thereto. Ethylene sulfate can be used .

Specific examples of the ammonium compound represented by Formula 2 according to the present invention include ammonium benzoate, ammonium methylbenzoate, ammonium acetate, ammonium propionate, or a mixture of two or more thereof, but are not limited thereto, May use ammonium benzoate.

The mixing ratio by weight of the alkylene sulfate, the ammonium compound and the vinylene carbonate is 1: 0.01 to 1: 1 to 3: alkylene sulfate: ammonium compound: vinylene carbonate to maximize the high temperature performance while improving the low temperature output characteristics desirable.

In addition, the ammonium compound according to the present invention has low solubility and is not easy to dissolve in the non-aqueous electrolyte solvent. Therefore, in order to increase the solubility, it is preferable to select an appropriate organic solvent in the organic solvent of the electrolytic solution or adopt an appropriate dissolving method. For example, when both the cyclic carbonate and the linear carbonate are used as a solvent for the electrolytic solution, the ammonium compound according to the present invention has a higher solubility in the cyclic carbonate. Therefore, when dissolved, the ammonium compound is dissolved in the cyclic carbonate, It is preferable to add a carbonate. In this case, the mixing ratio of the cyclic carbonate to the linear carbonate is 10:90 to 50:50, and the content of the ammonium compound is preferably 0.01 to 1 part by weight, more preferably 0.05 to 1 part by weight based on 100 parts by weight of the cyclic carbonate. Parts by weight to 0.1 parts by weight.

The total amount of the alkylene sulfate, the ammonium compound, and the vinylene carbonate may be 1 to 10% by weight based on the total weight of the nonaqueous electrolyte solution, but is not limited thereto. If the total content of the three-component additive is less than 1% by weight, the coating formed by the additive does not secure a sufficient thickness, and the effect of inhibiting the reactivity of the electrolyte solvent is insufficient, so that the high temperature performance may be deteriorated. And the cell resistance can be increased.

Examples of the lithium salt include LiPF _{6 ,} LiBF _{4 ,} LiSbF _{6 ,} LiAsF _{6 ,} LiClO _{4 ,} LiBF _{4 ,} LiBF _{4 ,} LiBF _{4 , 4, LiN (C 2 F 5} SO 2) 2, LiN (CF 3 SO 2) 2, CF 3 SO 3 Li and LiC (CF ₃ SO ₂₎ any one selected from the group consisting of _3, or a mixture of two or more of these Have a back

Examples of the organic solvent included in the non-aqueous electrolyte of the present invention include those commonly used in an electrolyte for a lithium secondary battery, and examples thereof include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, Any one selected from the group consisting of propyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, gamma-butyrolactone and tetrahydrofuran, or a mixture of two or more thereof . In particular, ethylene carbonate (EC) and propylene carbonate (PC), which are cyclic carbonates in the carbonate-based organic solvent, are highly viscous organic solvents and can be preferably used since they have high permittivity to dissociate lithium salts in the electrolyte And a low viscosity and low dielectric constant linear carbonate such as dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate (EMC) are mixed with the cyclic carbonate in an appropriate ratio, the high electric conductivity The electrolyte solution can be more preferably used.

The nonaqueous electrolyte solution for a lithium secondary battery of the present invention described above is manufactured as a lithium secondary battery by injecting it into an electrode structure composed of a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The positive electrode, the negative electrode and the separator constituting the electrode structure may be any of those conventionally used in the production of lithium secondary batteries.

As the cathode active material, a lithium-containing transition metal oxide may be preferably used. For example, Li _x CoO ₂ (0.5 <x <1.3), Li _x NiO ₂ (0.5 <x <1.3), Li _x MnO ₂ (0.5 <x <1.3), Li x Mn 2 O 4 (0.5 <x <1.3), Li x (Ni a Co b Mn c) O 2 (0.5 <x <1.3, 0 <a <1, 0 <b <1, 0 <c <1 , a + b + c = 1), Li x Ni 1 -y Co y O 2 (0.5 <x <1.3, 0 <y <1), Li x Co 1 -y Mn y _{O 2 (0.5 <x <1.3} , 0≤y <1), Li x Ni 1 -y Mn y O 2 (0.5 <x <1.3, O≤y <1), Li x (Ni a Co b Mn c) _{O 4 (0.5 <x <1.3} , 0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), Li x Mn 2 -z Ni z O 4 (0.5 <x <1.3, 0 <z <2 ), Li x Mn 2 -z Co z O 4 (0.5 <x <1.3, 0 <z <2), Li x CoPO 4 (0.5 <x <1.3) and Li _x FePO ₄ (0.5 < x < 1.3), or a mixture of two or more thereof. However, the present invention is not limited thereto. In addition to these oxides, sulfide, selenide and halide may also be used.

Alternatively, the cathode active material of the present invention may be coated or doped with a metal oxide such as alumina (Al ₂ O ₃ ) on its surface.

As the negative electrode active material, a carbon material, lithium metal, silicon, or tin, in which lithium ions can be occluded and released, can be used. Preferably, carbon materials can be used, and carbon materials such as low-crystalline carbon and highly-crystalline carbon can be used. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber high temperature sintered carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes. The negative electrode may include a binder. Examples of the binder include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, Various kinds of binder polymers such as polymethylmethacrylate may be used.

As the separator, a conventional porous polymer film conventionally used as a separator, for example, a polyolefin such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer and an ethylene / methacrylate copolymer A porous polymer film made of a high molecular weight polymer may be used alone or in a laminated manner, or a nonwoven fabric made of a conventional porous nonwoven fabric such as a glass fiber having a high melting point, a polyethylene terephthalate fiber or the like may be used. It is not.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a square shape, a pouch shape, a coin shape, or the like using a can.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example  One

1M LiPF ₆ solution having a composition of ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 3: 7 (v / v%) was used as the electrolyte solution. To the electrolyte solution, 2% by weight of vinylene carbonate (VC) 1% by weight of ethylene sulfate and 0.03% by weight of ammonium benzoate were added.

Further, artificial graphite was used as a negative electrode active material, LiMnO ₂ was used as a positive electrode active material, and a polyethylene film separator was laminated thereon, and a laminate type lithium secondary battery was produced by a conventional method.

Comparative Example  One

An electrolytic solution and a battery were prepared in the same manner as in Example 1, except that only 2% by weight of vinylene carbonate (VC) was added to the electrolytic solution as an additive.

Comparative Example  2

An electrolytic solution and a battery were prepared in the same manner as in Example 1, except that 2% by weight of vinylene carbonate (VC) and 1% by weight of ethylene sulfate were added as an additive to the electrolytic solution.

Comparative Example  3

An electrolytic solution and a battery were prepared in the same manner as in Example 1, except that 2% by weight of vinylene carbonate (VC) and 0.03% by weight of ammonium benzoate were added as an additive to the electrolytic solution.

Experimental Example  1: Low temperature output measurement

Each of the batteries manufactured in Example 1 and Comparative Examples 1 to 3 was subjected to a constant output of 50, 55, 60, 65 and 70 W for 5 seconds at -30 캜 to measure the discharge output, respectively. Are shown in Table 1.

Low temperature (-30 ℃) Output (W / Ah) Example 1 4.8 Comparative Example 1 4.0 Comparative Example 2 4.9 Comparative Example 3 3.7

As shown in Table 1, it can be seen that the low temperature power of the battery of Example 1 of the present invention is about 20 to 30% superior to that of Comparative Example 1 and Comparative Example 3. However, although the battery of Comparative Example 2 using only ethylene sulfate and vinylene carbonate as an additive has a low temperature output characteristic equivalent to that of the present invention, it can be confirmed that the battery at the high temperature performance described below does not achieve the first embodiment of the present invention.

Experimental Example  2: High-temperature storage performance measurement

Each of the cells prepared in Example 1 and Comparative Examples 1 to 4 was stored at a temperature of 55 캜 for 100 days.

The batteries were charged at a constant current of 1 C = 15 A. After the voltage of the battery reached 4.2 V, the first charge was performed until the charge current reached 50 mA at a constant voltage of 4.2 V. The discharge was carried out for the battery which was charged for the first time until the battery voltage reached 3 V at a constant current of 1 C to determine the discharge capacity. The capacity retention rate was determined by comparing the discharge capacity after 50 days storage at a high temperature with respect to the initial discharge capacity using the above-described method.

Also, the rate of resistance increase was obtained by comparing the resistance after 50 days of high temperature storage compared to the initial resistance.

Capacity retention rate (%) Rate of resistance increase (%) Example 1 78 25 Comparative Example 1 62 33 Comparative Example 2 69 34 Comparative Example 3 78 46

As shown in Table 2, the capacity retention rate at the high-temperature storage of the battery of Example 1 of the present invention was about 20% or more superior to that of Comparative Example 1 and Comparative Example 2, It can be confirmed that the battery of Example 1 exhibits far superior performance to all the batteries of Comparative Examples 1 to 3 in terms of the resistance increase rate.

Claims (10)

A nonaqueous electrolyte solution for a lithium secondary battery comprising a lithium salt and an organic solvent,
Wherein the nonaqueous electrolyte solution comprises an alkylene sulfate represented by the following formula (1), an ammonium compound represented by the following formula (2), and a vinylene carbonate:
[Chemical Formula 1]

In Formula 1, n is an integer of 2 to 5,
(2)

In Formula 2, R is an alkyl group or an aryl group having 1 to 20 carbon atoms. The method according to claim 1,
Wherein the weight ratio of the alkylene sulfate, the ammonium compound, and the vinylene carbonate is 1: 0.01 to 1: 1 to 3: alkylene sulfate: ammonium compound: vinylene carbonate. The method according to claim 1,
Wherein the total amount of the alkylene sulfate, the ammonium compound and the vinylene carbonate is 1 to 10% by weight based on the total weight of the non-aqueous electrolyte. The method according to claim 1,
Wherein the organic solvent comprises a cyclic carbonate and a linear carbonate, and the cyclic carbonate: linear carbonate has a mixing weight ratio of 10:90 to 50:50. 5. The method of claim 4,
Wherein the ammonium compound is used in an amount of 0.01 to 1 part by weight based on 100 parts by weight of the cyclic carbonate. The method according to claim 1,
The lithium salt is _{LiPF 6, LiBF 4, LiSbF 6} , LiAsF 6, LiClO 4, LiN (C 2 F 5 SO 2) 2, LiN (CF 3 SO 2) 2, CF 3 SO 3 Li and LiC (CF ₃ SO ₂ ) ₃ , or a mixture of two or more thereof. 2. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, The method according to claim 1,
Wherein the alkylene sulfate is any one selected from the group consisting of ethylene sulfate, propylene sulfate, and butylene sulfate, or a mixture of two or more thereof. The method according to claim 1,
Wherein the ammonium compound is any one selected from the group consisting of ammonium benzoate, ammonium methyl benzoate, ammonium acetate, and ammonium propionate, or a mixture of two or more thereof. The method according to claim 1,
The organic solvent may be at least one selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, Hydrofluoric acid, and hydrofluoric acid, or a mixture of two or more thereof. A lithium secondary battery comprising a positive electrode made of a lithium-containing oxide, a negative electrode made of a carbon material capable of absorbing and desorbing lithium ions, and a non-aqueous electrolyte,
The lithium secondary battery according to any one of claims 1 to 9, wherein the non-aqueous electrolyte is a non-aqueous electrolyte for a lithium secondary battery.