KR20120011209A - Non-aqueous electrolyte solution for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

PURPOSE: Non-aqueous electrolyte for lithium secondary battery is provided to maintain the storage performance of a lithium secondary battery at high temperatures to be excellent, and to offer improved output at low temperatures. CONSTITUTION: Non-aqueous electrolyte comprises lithium salt, organic solvent, alkylene sulfate in chemical formula 1, an ammonium compound in chemical formula 2, and vinylene carbonate. In chemical formula 1, n is the integer of 2-5. In chemical formula 2, R is an C1-20 alkyl group or aryl group. A Lithium secondary battery comprises a positive electrode consisting of lithium-containing oxide, a negative electrode consisting of carbon materials capable of absorbing and emitting lithium ions, and the non-aqueous liquid electrolyte.

Description

Non-aqueous electrolyte for lithium secondary battery and lithium secondary battery containing same {NON-AQUEOUS ELECTROLYTE SOLUTION FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

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

Recently, with the development of the information and communication industry, electronic devices have become smaller, lighter, thinner and more portable. As a result, the demand for higher energy density of batteries used as power sources for such electronic devices is increasing. Lithium secondary batteries are the batteries that can best meet these demands, and research on these is being actively conducted. The lithium secondary battery is composed of a cathode, an anode, and an electrolyte and a separator that provide a migration path between lithium ions between the cathode and the anode, and are oxidized and reduced when lithium ions are occluded and released from the cathode and the anode. Generate electrical energy.

The nonaqueous electrolyte used in the lithium secondary battery generally contains an electrolyte solvent and an electrolyte salt. However, the electrolyte solvent may decompose at the electrode surface during charge and discharge of the battery, or may be co-intercalated between the carbon material negative electrode layers to collapse the negative electrode structure, thereby inhibiting battery stability.

It is known 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 during initial charging of the battery. However, in general, the SEI film is insufficient to serve as a continuous protective film of the negative electrode, and as a result, when the battery repeats charging and discharging, the lifespan and performance decrease. In particular, the SEI film of the conventional lithium secondary battery is not thermally stable, and when the battery is operated or left at a high temperature, it is susceptible to collapse due to increased electrochemical energy and thermal energy over time. Therefore, under high temperature, the battery performance is further deteriorated, and in particular, gases such as CO ₂ are continuously generated due to collapse of the SEI film, decomposition of the electrolyte, and the like, thereby increasing the internal pressure and thickness of the battery.

In order to solve the above problems, a method of using a vinylene carbonate (VC) or the like as an electrolyte additive capable of forming an SEI film on the surface of the anode has been proposed. However, the vinylene carbonate has excellent storage performance and cycle performance at high temperatures, but has a problem in that the output is decreased at low temperatures.

Recently, the use of lithium secondary batteries has been expanded to various electric vehicles as well as general electronic devices. Accordingly, as the demand for high output batteries is increasing, there is a need for a battery capable of providing excellent output at low temperature as well as high temperature performance. It is gradually emerging.

Accordingly, an object of the present invention is to provide a nonaqueous electrolyte for a lithium secondary battery and a lithium secondary battery having the same, while maintaining excellent high temperature storage performance of the lithium secondary battery while improving the low temperature output.

In order to solve the above problems, according to the present invention, a nonaqueous electrolyte solution for a lithium secondary battery including 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. It is characterized by:

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 electrolyte solution of the present invention, the mixed weight ratio of the alkylene sulfate, the ammonium compound and the vinylene carbonate may be alkylene sulfate: ammonium compound: vinylene carbonate = 1: 0.01 to 1: 1 to 3, but is not limited thereto. It is not.

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

The non-aqueous electrolyte for lithium secondary batteries described above is usefully applied to conventional lithium secondary batteries having a negative electrode and a positive electrode.

As in the present invention, applying a nonaqueous electrolyte to which a predetermined alkylene sulfate, a predetermined ammonium compound, and vinylene carbonate are simultaneously added to a lithium secondary battery maintains high temperature storage characteristics and lowers battery resistance at low temperature Output characteristics can be improved.

Hereinafter, the present invention will be described in detail. The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention in the best way possible. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

According to the present invention, a nonaqueous electrolyte solution for a lithium secondary battery including a lithium salt and an organic solvent includes an alkylene sulfate represented by Formula 1, an ammonium compound represented by Formula 2, and vinylene carbonate:

[Formula 1]

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

[Formula 2]

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

As mentioned above, vinylene carbonate can improve high temperature performance, but is not good at low temperature performance. However, when the alkylene sulfate represented by Formula 1 is used together with vinylene carbonate, the decomposition reaction occurs first of the alkylene sulfate having a relatively high reduction potential compared to the vinylene carbonate. Thereby, the SEI film by a sulfate type compound is formed first, and then the SEI film by vinylene carbonate is formed. Since the film produced by the sulfate-based compound has the advantage of low resistance, it mainly contributes to improving the low temperature output characteristics of the battery, and the film formed by vinylene carbonate (VC) mainly contributes to improving the high temperature characteristics of the battery.

However, since the film produced by the sulfate-based compound is relatively weak at high temperatures compared to the film produced by vinylene carbonate, it is difficult to sufficiently improve the high temperature characteristics of the battery with only the two additive materials described above. Thus, the present invention used ammonium compounds in combination with the above alkylene sulfates and vinylene carbonates. The present inventors have completed the present invention by using the non-aqueous electrolyte containing the three components described above, and the high temperature performance was maintained much higher than the conventional vinylene carbonate alone, and the low temperature output characteristics were improved.

Specific examples of the alkylene sulfate represented by Formula 1 according to the present invention include, but are not limited to, ethylene sulfate, propylene sulfate, butylene sulfate, or mixtures of two or more thereof, and preferably ethylene sulfate may be used. .

Specific examples of the ammonium compound represented by Formula 2 according to the present invention include, but are not limited to, ammonium benzoate, ammonium methyl benzoate, ammonium acetate, ammonium propionate, or mixtures of two or more thereof. Ammonium benzoate can be used.

The mixed weight ratio of the alkylene sulfate, the ammonium compound and the vinylene carbonate is alkylene sulfate: ammonium compound: vinylene carbonate = 1: 0.01 to 1: 1 to 3 to improve the low temperature output characteristics while maximizing high temperature performance. desirable.

In addition, the ammonium compound according to the present invention has low solubility and is not easily dissolved in a nonaqueous electrolyte solvent. Therefore, in order to improve solubility, it is preferable to select an appropriate organic solvent from among the organic solvents of electrolyte solution, or to adopt an appropriate dissolution method. For example, when both a cyclic carbonate and a linear carbonate are used as the solvent of the electrolyte solution, since the ammonium compound according to the present invention has higher solubility in the cyclic carbonate, when dissolving, the ammonium compound is dissolved in the cyclic carbonate. Preference is given to adding carbonates. In this case, it has a mixed weight ratio of cyclic carbonate: linear carbonate = 10: 90-50: 50, and the content of the ammonium compound is preferably 0.01 part by weight to 1 part by weight with respect to 100 parts by weight of the cyclic carbonate, more preferably 0.05 Parts by weight to 0.1 parts by weight.

In addition, the total content of the alkylene sulfate, ammonium compound and vinylene carbonate may be 1 to 10% by weight based on the total weight of the non-aqueous electrolyte, but is not limited thereto. If the total content of the three-component additive is less than 1% by weight, the film formed by the additive may not secure a sufficient thickness, and thus the reactivity inhibiting effect of the electrolyte solvent may be insufficient, so that the high temperature performance may be lowered. The cell resistance can be increased by increasing.

Lithium salt contained as an electrolyte in the non-aqueous electrolyte of the present invention described above may be used without limitation those conventionally used in the electrolyte for lithium secondary batteries, representative examples of 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 any one or a mixture of two or more thereof Have a back

As the organic solvent included in the nonaqueous electrolyte of the present invention described above, those conventionally used in the lithium secondary battery electrolyte may be used without limitation, and typically, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, di 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 may be representatively used. Can be. In particular, cyclic carbonates of carbonate carbonate (ethylene carbonate, EC) and propylene carbonate (propylene carbonate, PC) of the carbonate-based organic solvent is a high viscosity organic solvent can be used preferably because it has a high dielectric constant to dissociate the lithium salt in the electrolyte. When the cyclic carbonate is mixed with a low viscosity, low dielectric constant linear carbonate such as dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate (EMC) in an appropriate ratio, high electrical conductivity can be obtained. It is possible to make an electrolyte solution having a more preferable use.

The non-aqueous electrolyte solution for a lithium secondary battery of the present invention described above is manufactured into a lithium secondary battery by injecting an electrode structure composed of a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. As the positive electrode, the negative electrode, and the separator constituting the electrode structure, all those conventionally used for manufacturing a lithium secondary battery may be used.

As a specific example, a lithium-containing transition metal oxide may be preferably used as the cathode active material. For example, Li _x CoO ₂ (0.5 <x <1.3), Li _x NiO ₂ (0.5 <x <1.3), and 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 One or a mixture of two or more selected from the group consisting of (0.5 <x <1.3) may be used, but is not limited thereto. In addition to these oxides, sulfides, selenides, and halides may also be used.

Optionally, 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, tin, or the like, into which lithium ions may be occluded and released, may be used. Preferably, a carbon material may be used, and as the carbon material, both low crystalline carbon and high crystalline carbon may be used. Soft crystalline carbon and hard carbon are typical low crystalline carbon, and high crystalline carbon is natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber. High temperature calcined carbon such as (mesophase pitch based carbon fiber), meso-carbon microbeads, Mesophase pitches and petroleum or coal tar pitch derived cokes. In this case, the negative electrode may include a binder, and the binder may include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, Various kinds of binder polymers, such as polymethylmethacrylate, may be used.

In addition, as the separator, conventional porous polymer films conventionally used as separators, for example, polyolefins such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer, etc. The porous polymer film made of the polymer may be used alone or by laminating them, or a conventional porous nonwoven fabric, for example, a non-woven fabric made of high melting point glass fiber, 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 cylindrical, square, pouch type, or coin type 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 more completely explain the present invention to those skilled in the art.

Example  One

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

In addition, artificial graphite was used as the negative electrode active material, and LiMnO ₂ was used as the positive electrode active material, and the laminate was laminated together with the polyethylene film separator to manufacture a laminated lithium secondary battery in a conventional manner.

Comparative example  One

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

Comparative example  2

An electrolyte 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 to the electrolyte.

Comparative example  3

An electrolyte 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 to the electrolyte.

Experimental Example  1: low temperature output measurement

Each of the batteries prepared in Example 1 and Comparative Examples 1 to 3 was applied at a constant power of 50, 55, 60, 65, 70 W at -30 ° C. for 5 seconds to measure discharge output, respectively. It is 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 output of the battery of Example 1 of the present invention shows about 20 to 30% superior performance compared to 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 of Comparative Example 2 is far behind in Example 1 of the present invention.

Experimental Example  2: high temperature storage performance measurement

Each battery prepared in Example 1 and Comparative Examples 1 to 4 was stored for 100 days in a temperature environment of 55 ℃.

The batteries were charged at a constant current of 1C = 15A, and after the battery voltage became 4.2V, the first charge was performed at a constant voltage of 4.2V until the charging current value became 50 mA. The discharged battery was discharged until the battery voltage reached 3V at a constant current of 1C for the first charged battery. Capacity retention ratio was calculated by comparing the discharge capacity after 50 days high temperature storage compared to the initial discharge capacity using the method described above.

In addition, the resistance increase rate was calculated by comparing the resistance after 50 days high temperature storage compared to the initial resistance.

Capacity retention rate (%) % Increase in resistance 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 is about 20% or more superior to Comparative Example 1 and Comparative Example 2, except that Comparative Example 3 is equivalent to Example 1 It has a capacity retention rate, but in terms of resistance increase rate, it can be seen that the battery of Example 1 is far superior to all the batteries of Comparative Examples 1 to 3.

Claims (10)

In the nonaqueous electrolyte solution for lithium secondary batteries containing a lithium salt and an organic solvent,
The non-aqueous electrolyte non-aqueous electrolyte lithium secondary battery, characterized in that it comprises an alkylene sulfate represented by the formula (1), an ammonium compound represented by the formula (2) and vinylene carbonate:
[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 of claim 1,
A mixed weight ratio of the alkylene sulfate, the ammonium compound and the vinylene carbonate is an alkylene sulfate: ammonium compound: vinylene carbonate = 1: 0.01 to 1: 1, characterized in that the non-aqueous electrolyte for lithium secondary batteries. The method of claim 1,
The total content of the alkylene sulfate, ammonium compound and vinylene carbonate is 1 to 10% by weight based on the total weight of the non-aqueous electrolyte, non-aqueous electrolyte for lithium secondary batteries. The method of claim 1,
The organic solvent comprises a cyclic carbonate and a linear carbonate, and the cyclic carbonate: linear carbonate = 10: 90 ~ 50: 50, characterized in that the non-aqueous electrolyte solution for a lithium secondary battery. The method of claim 4, wherein
The ammonium compound is a non-aqueous electrolyte lithium secondary battery, characterized in that 0.01 to 1 parts by weight based on 100 parts by weight of the cyclic carbonate. The method of 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 ) A non-aqueous electrolyte solution for lithium secondary batteries, characterized in that any one selected from the group consisting of ₃ or a mixture of two or more thereof. The method of claim 1,
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 of claim 1,
The ammonium compound is any one selected from the group consisting of ammonium benzoate, ammonium methyl benzoate, ammonium acetate, ammonium propionate or a mixture of two or more thereof. The method of claim 1,
The organic solvent is propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, dipropyl carbonate, dimethylsulfuroxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, gamma-butyrolactone and tetra Non-aqueous electrolyte solution for lithium secondary batteries, characterized in that any one or a mixture of two or more selected from the group consisting of hydrofuran. A lithium secondary battery comprising a positive electrode made of a lithium-containing oxide, a negative electrode made of a carbon material capable of occluding and releasing lithium ions, and a nonaqueous electrolyte solution,
The nonaqueous electrolyte is a lithium secondary battery, characterized in that the nonaqueous electrolyte for lithium secondary battery of any one of claims 1 to 9.