KR20130120617A - Electrolyte for lithium secondary battery and lithium secondary battery using the same - Google Patents

Abstract

The present invention relates to an electrolyte for a secondary battery. The electrolyte for a secondary battery according to the present invention includes a non-aqueous solvent, lithium difluoro phosphate, lithiumbis (oxalato) borate, lithium difluoro phosphate, trimethylsilyl group. The lithium secondary battery using the electrolyte for a secondary battery according to the present invention improves electrochemical characteristics and lifetime. [Reference numerals] (AA) Example 1;(BB) Example 2;(CC) Example 3;(DD) Example 4;(EE) Comparative example 1;(FF) Comparative example 2

Description

[0001] The present invention relates to an electrolyte for a secondary battery and a secondary battery containing the same,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a secondary battery and a secondary battery comprising the same, and more particularly, to an electrolyte for a secondary battery including a non-aqueous solvent, a lithium salt and lithium bis (oxalate) borate and lithium difluorophosphate, Battery.

2. Description of the Related Art [0002] In recent years, portable electronic devices have been widely used. As a result, with the rapid miniaturization, weight reduction, and thinning of such portable electronic devices, battery cells that are power sources thereof are small, lightweight and can be charged / discharged for a long time. Development is strongly required.

On the other hand, the secondary battery includes a lead-acid battery, a nickel-cadmium battery, a nickel-hydrogen battery, and a lithium battery according to a negative electrode material or a positive electrode material, and potential or energy density is determined by the intrinsic properties of the electrode material. Among them, lithium secondary batteries are widely used as driving power sources for portable electronic devices because of their high energy density due to their low oxidation-reduction potential and molecular weight.

Lithium secondary batteries are used in various portable electronic devices as a high energy density battery, and they are most sensitive to abuse of the overcharge type in which the normal driving voltage is exceeded during the recharging.

In addition to the stability of the lithium secondary battery, studies on improvement of the battery performance such as excellent charging / discharging performance and improved life span are progressing in accordance with the portability of an electronic device employing a lithium secondary battery. For example, Korean Patent Laid-Open Publication No. 2011-0058507 provides a lithium secondary battery additive having improved performance.

However, it is still required to study the performance improvement and stability of the secondary battery.

Korean Patent Publication No. 2011-0058507

The present invention provides an electrolyte solution for a secondary battery which is stable not only at room temperature but also at a high temperature or a low temperature.

The present invention also provides an electrolyte for a secondary battery having improved battery chemical characteristics and life characteristics, and a secondary battery containing the same.

The present invention provides an electrolyte for a secondary battery and a secondary battery comprising the same.

As a result of deepening research on an electrolyte for a secondary battery, the present applicant found that the lithium secondary battery (oxalate) borate and lithium difluorophosphate are included in the electrolyte for the secondary battery to improve not only the stability of the secondary battery but also the output and cycle life characteristics Thus, the present invention has been filed.

The electrolyte for a secondary battery according to the present invention is characterized by containing a nonaqueous solvent, a lithium salt, lithium bis (oxalate) borate and lithium difluorophosphate.

The lithium bis (oxalato) borate and the lithium difluorophosphate according to an embodiment of the present invention may be contained in an amount of 0.1 to 10% by weight based on the total weight of the electrolytic solution.

The electrolyte according to an embodiment of the present invention may further include at least one salt additive selected from trimethylsilyl borate, trimethylsilyl phosphate, trimethylsilyl phosphite, and trimethylsilyl groups, and the salt additive may be added in an amount of 0.5 To 10% by weight.

Lithium salt, according to one embodiment of the present invention are _{LiPF6, LiClO 4, LiAsF 6,} LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (C 2 F 5 SO _{3) 2, LiN (C 2} F 5 SO 2) 2, LiN (CF 3 SO 2) 2. _{LiN (CxF 2 x +1 SO 2} ) (CyF 2 y +1 SO 2) ( in this example, x, y is a natural number), may be one or more selected from the group consisting of LiCl and LiI, 0.5 ~ 2M secondary battery electrolyte ≪ / RTI >

The non-aqueous solvent according to an embodiment of the present invention may be selected from a linear carbonate solvent, a cyclic carbonate solvent or a mixed solvent thereof. The linear carbonate solvent may include dimethyl carbonate (DMC), diethyl carbonate (DEC) (EPC), dipropyl carbonate (DPC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC) and methyl propyl carbonate (MPC)

The cyclic carbonate solvent may be one or more solvents selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC) and fluorethylene carbonate (FEC).

The nonaqueous solvent according to an embodiment of the present invention may have a volume ratio of linear carbonate solvent: cyclic carbonate solvent of 1: 1 to 4: 1.

The present invention also provides a positive electrode; cathode; And a secondary battery including the electrolyte for a secondary battery according to the present invention.

The electrolyte for a secondary battery according to the present invention includes lithium bis (oxalate) borate and lithium difluorophosphate, and the secondary battery including the electrolyte for a secondary battery according to the present invention has excellent stability, decreases internal resistance of the battery, Cycle life characteristics are high.

1 is a graph showing the cycle life characteristics of Examples 1 to 4 and Comparative Examples 1 and 2 as a percentage of the initial capacity.

The present invention relates to a secondary battery electrolyte and a secondary battery including the same, which will be described in detail below. Unless otherwise defined in technical terms and scientific terms used herein, And the description of known functions and configurations which may unnecessarily obscure the gist of the present invention will be omitted in the following description.

The electrolyte for a secondary battery according to the present invention is characterized by containing a nonaqueous solvent, a lithium salt, lithium bis (oxalate) borate and lithium difluorophosphate.

The electrolyte for a secondary battery according to the present invention includes lithium bis (oxalate) borate and lithium difluorophosphate to ensure high stability not only at overcharging or at room temperature, but also at high or low temperature, and excellent performance in output and cycle life I have.

The lithium bis (oxalato) borate and the lithium difluorophosphate according to an embodiment of the present invention may be contained in an amount of 0.1 to 10 wt%, preferably 0.5 to 3 wt%, based on the total weight of the electrolytic solution. If the amount of the additive is less than 0.1 parts by weight, the effect of improving the discharge capacity or output of the lithium secondary battery is insignificant. If the additive is contained in an amount exceeding 10 parts by weight, the characteristics of the lithium secondary battery deteriorate.

The electrolyte according to an embodiment of the present invention may be defined as an ionic material used for protecting a battery when assisting the generation of a solid electrolyte interface (SEI) at the anode surface and overcharging the battery And more preferably, it can be selected from trimethylsilyl borate, trimethylsilyl phosphite, trimethylsilyl phosphite, and trimethylsilyl groups, and when the lithium secondary battery is charged, an electrolyte solution And 0.5 to 10% by weight based on the total weight of the electrolytic solution in order to suppress the occurrence of side reactions between the positive electrode and the negative electrode.

Lithium salt, according to one embodiment of the present invention include, but are not in _{limitation, LiPF6, LiClO 4, LiAsF 6} , LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (C ₂ F ₅ SO ₃ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ . LiN (CxF ₂ x _{+ 1} SO ₂ ) (CyF ₂ y _{+ 1} SO ₂ ) (where x and y are natural numbers), LiCl and LiI, It is preferable that it is contained in the electrolytic solution at a concentration of 2.0 M. If the concentration of the lithium salt is less than 0.5M, the electrical conductivity of the non-aqueous electrolytic solution containing the lithium salt is lowered, so that the performance of the non-aqueous electrolytic solution transferring ions at a high rate between the anode and the cathode of the lithium secondary battery is deteriorated. The viscosity of the non-aqueous electrolytic solution is increased, so that the mobility of the lithium ion is rather reduced and the low-temperature performance is deteriorated.

The non-aqueous solvent according to one embodiment of the present invention is not limited, and a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an aromatic hydrocarbon solvent, or the like can be used. Specifically, as the non-aqueous solvent, a solvent in which one or more selected from the group consisting of an ester solvent, an ether solvent, a ketone solvent, and an aromatic hydrocarbon solvent is mixed with a carbonate solvent can be used.

Examples of the ester solvent include butyrolactone (BL), decanolide, valerolactone, mevalolactone, caprolactone, methyl acetate, methyl propionic acid, ethyl propionic acid, n- ethyl acetate, n-propyl acetate or butyl acetate, and examples of the ether solvent include dibutyl ether and the like. Examples of the aromatic hydrocarbon solvent include benzene, fluorobenzene, toluene, fluorotoluene, trifluorotoluene, xylene, and the like.

The linear carbonate solvent may be selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl (meth) acrylate (EC), propylene carbonate (EPC), dipropyl carbonate (DPC), methyl ethyl carbonate (MEC), and methyl propyl carbonate (MPC), and the cyclic carbonate solvent may be ethylene carbonate PC), butylene carbonate (BC), vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

In addition, various organic solvents that can be used in the lithium secondary battery may be used without limitation, and two or more selected organic solvents may be used in combination.

For example, when a solvent in which a carbonate-based solvent and an aromatic hydrocarbon-based solvent are mixed is used, the non-aqueous solvent according to an embodiment of the present invention may contain a carbonate-based solvent and an aromatic hydrocarbon-based solvent in a volume ratio of 1: 1 to 30: .

More preferably, the non-aqueous solvent according to an embodiment of the present invention may be used by mixing at least one solvent selected from a cyclic carbonate solvent and at least one solvent selected from a linear carbonate solvent.

The cyclic carbonate solvent has a large polarity, which can sufficiently dissociate lithium ions, but has a disadvantage in that the ion conductivity is low due to the high viscosity. Therefore, the characteristics of the lithium secondary battery can be optimized by mixing the cyclic carbonate solvent with a linear carbonate solvent having a low polarity but a low viscosity.

The volume ratio of the linear carbonate solvent to the cyclic carbonate solvent may be 1: 1 to 4: 1, preferably 1: 1 to 2: 1, more preferably 1: The volume ratio of ethyl methyl carbonate: dimethyl carbonate: ethylene carbonate may be 0.5: 1: 0.5 to 1: 1.

The present invention also provides a positive electrode; cathode; And a secondary battery including the electrolyte for a secondary battery according to the present invention.

An anode according to an embodiment of the present invention includes a current collector and a cathode active material layer formed on the current collector. The current collector may be made of aluminum, but is not limited thereto. The positive electrode active material layer may include a positive electrode active material, a binder, and a conductive material, and examples of the positive electrode active material include LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO _2, or LiNi ₁ -xyCoxMyO ₂ (0? X? y? 1, 0? x + y? 1, and M is Al, Sr, Mg or La), or lithium intercalation compounds such as lithium chalcogenide compounds. Any material that can be used as a positive electrode active material in a secondary battery can be used.

The negative electrode according to an embodiment of the present invention includes a current collector and a negative electrode active material layer formed on the current collector. Metal such as copper may be used as the collector, and carbon, carbon composite lithium metal, lithium alloy, carbon-metal complex, etc. may be used as the negative electrode active material. Materials can be used.

One or both of the positive electrode and the negative electrode according to an embodiment of the present invention may further include a binder and a conductive material.

The binder according to one embodiment of the present invention is not limited as long as it can be used by those of ordinary skill in the art. Examples of the binder include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, Polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylate, polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, Styrene-butadiene rubber, epoxy resin, nylon, and the like can be used, but it is not limited thereto.

The conductive material is used for imparting conductivity to the electrode. Any conductive material may be used without causing any chemical change in the battery. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, , Carbon-based materials such as carbon fibers; Metal materials such as metal powder or metal fibers such as copper, nickel, aluminum and silver; Conductive polymer materials such as polyphenylene derivatives; Or a conductive material containing a mixture thereof.

The present invention will be described in detail with reference to the following examples. However, the following examples are merely examples of the present invention, and the claims of the present invention are not limited thereto.

Example 1

LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder and carbon as a conductive agent were mixed at a weight ratio of 92/4/4 and dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry Respectively. This slurry was coated on an aluminum foil having a thickness of 15 mu m to a thickness of 140 mu m, followed by drying and rolling to prepare a positive electrode.

Crystalline artificial graphite as a negative electrode active material and PVDF (polyvinylidene fluoride) as a binder were mixed at a weight ratio of 92: 8 and then dispersed in N-methyl-2-pyrrolidone to prepare a negative electrode slurry. This slurry was coated on a copper foil having a thickness of 12 μm to a thickness of 100 μm, followed by drying and rolling to prepare a negative electrode.

The positive electrode and the negative electrode thus prepared were wound and compressed using a polyethylene separator having a thickness of 14 μm and placed in a Pouch having a thickness of 30 mm 48 mm 6 mm.

1% by weight of lithium bis (oxalate) borate was added to a mixed non-aqueous solvent of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate (EC: EMC: DMC) = 1: 1: 1 in which LiPF ₆ was dissolved at 1.15 M as a secondary battery electrolyte. And 1% by weight of lithium difluorophosphate were added to prepare an electrolyte solution for a secondary battery, which was then injected into the above-prepared pouch to prepare a secondary battery.

[Example 2]

A secondary battery was produced in the same manner as in Example 1 except that the electrolyte solution for a secondary battery further contained 2% by weight of trimethylsilylborate (TMSB) as a salt additive.

[Example 3]

A secondary battery was prepared in the same manner as in Example 1 except that the electrolyte solution for a secondary battery contained 2% by weight of trimethylsilyl phosphate (TMSPa) as a salt additive.

Example 4

A secondary battery was prepared in the same manner as in Example 1 except that the electrolyte solution for a secondary battery further contained 2% by weight of trimethylsilyl phosphite (TMSPi) as a salt additive.

Comparative Example 1

A secondary battery was produced in the same manner as in Example 1, except that lithium bis (oxalate) borate and lithium difluorophosphate were not added to the electrolyte for the secondary battery of Example 1.

Comparative Example 2

A secondary battery was produced in the same manner as in Example 1 except that lithium difluorophosphate was not added to the electrolyte for the secondary battery of Example 1.

[Evaluation of physical properties 1: Evaluation of discharge capacity at room temperature (25 캜)] [

The secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were charged at 0.2 C to 4.2 V and discharged at 2.75 V and 0.2 C, respectively.

Discharge capacity at room temperature (mAh / g) Improvement rate Example 1 921 And 5% Example 2 924 Gt; of 5.3% < / RTI > Example 3 940 And 7.2% Example 4 930 Gt; 6% < / RTI > Comparative Example 1 855 - Comparative Example 2 877 -

As shown in Table 1, it can be seen that the discharge capacities at room temperature of the secondary batteries of Examples 1 to 4 are improved as compared with Comparative Examples 1 and 2.

[Evaluation of physical properties 2: Evaluation of discharge capacity at high temperature (60 占 폚) or low temperature (-20 占 폚)

 Each of the secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 and 2 was charged at 0.2 C to 4.2 V, discharged at 2.75 V and 0.2 C, charged again to 4.2 V at 0.5 C, and then charged in a high- The results are shown in Table 2 below.

60 ° C -20 ° C High discharge capacity (mAh / g) Improvement rate (% relative to Comparative Example 2) High discharge capacity (mAh / g) Improvement rate (% relative to Comparative Example 2) Example 1 830 6.4% 482  8.8% Example 2 836 7.1% 485  9.5% Example 3 845 8.3% 490  10.6% Example 4 840 7.6% 483  9.0% Comparative Example 1 726 - 410 - Comparative Example 2 780 - 443 -

As shown in Table 2, it can be seen that the discharge capacities of the secondary batteries of Examples 1 to 4 at high temperature and low temperature are superior to those of Comparative Examples 1 to 2.

[Evaluation of physical properties 3: evaluation of cycle output at room temperature (25 占 폚) or low temperature (-30 占 폚)

 Each of the secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 and 2 was charged at 0.2 C to 4.2 V and discharged at 2.75 V and 0.2 C, After the 1C charge, the output of the secondary battery was measured by DC-IR and is shown in Table 3 below.

25 ℃ -30 ℃ Initial capacity (mAh) Output (W) Initial capacity (mAh Output (W) Example 1 921 90.4 921 70.7 Example 2 924 94.2 924 75.6 Example 3 940 95.6 940 78.2 Example 4 930 90.6 930 73.3 Comparative Example 1 855 85.4 855 57.8 Comparative Example 2 877 87.0 877 62.3

As shown in Table 3, it can be seen that the powers of Examples 1 to 4 were improved by 3.9%, 8.3%, 9.8% and 4.1%, respectively, at room temperature (25 ° C) The output of Examples 1 to 4 was improved by 13.5%, 21.3%, 25.5% and 17.6%, respectively, as compared with Comparative Example 2.

[Property evaluation 4: Evaluation of cycle life characteristics]

Each of the secondary batteries manufactured in Examples 1 to 4 and Comparative Examples 1 and 2 was charged at 0.2 C to 4.2 V, discharged at 2.75 V and 0.2 C, charged at 1 C to 4.2 V, and discharged at 2 C. The percentages by volume are shown in Table 4 below and shown graphically in FIG.

percentage(%) Improvement rate (% compared to Comparative Example 2) Example 1 93.3  10.0% Example 2 93.6  10.4% Example 3 94.8  11.8% Example 4 93.3  10.2% Comparative Example 1 79.9 - Comparative Example 2 84.8 -

As shown in Table 4 and FIG. 1, even when the electrolyte for a secondary battery of Examples 1 to 4 of the present invention had 500 cycles, the cycle capacity ratio with respect to the initial capacity was maintained at 90% or more.

As shown in the evaluations 1 to 4, the discharge capacity of the secondary battery electrolyte according to the present invention, the discharge capacity at a low temperature or a high temperature, and the output at room temperature or low temperature are improved, It can be seen that the device has a high lifetime characteristic even in a high cycle.

Claims (10)

A non-aqueous solvent, lithium salt and lithium bis (oxalato) borate and lithium difluorophosphate, the electrolyte solution for a secondary battery comprising a. The method of claim 1,
Lithium bis (oxalato) borate and lithium difluorophosphate is a secondary battery electrolyte, characterized in that contained in 0.1 to 10% by weight relative to the total weight of the electrolyte. The method of claim 1,
The electrolyte solution further comprises at least one salt additive selected from a compound containing trimethyl silyl borate, trimethyl silyl phosphate, trimethyl silyl phosphite and trimethyl silyl group. The method of claim 3,
Wherein the salt additive is contained in an amount of 0.5 to 10% by weight based on the total weight of the electrolytic solution. The method of claim 1,
Lithium salt is _{LiPF6, LiClO 4, LiAsF 6,} LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (C 2 F 5 SO 3) 2, LiN (C 2 _{F 5 SO 2) 2, LiN} (CF 3 SO 2) 2. Wherein the electrolyte is at least one selected from the group consisting of LiN (CxF ₂ x _{1 +} SO ₂ ) (CyF ₂ y _{+ 1} SO ₂ ) (where x and y are natural numbers), LiCl and LiI. 6. The method of claim 5,
Lithium salt is a secondary battery electrolyte, characterized in that present in a concentration of 0.5 ~ 2M. The method of claim 1,
Wherein the non-aqueous solvent is selected from a linear carbonate-based solvent, a cyclic carbonate-based solvent or a mixed solvent thereof. 8. The method of claim 7,
The linear carbonate solvent is at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, ethyl propyl carbonate, dipropyl carbonate, ethyl methyl carbonate and methyl propyl carbonate,
Wherein the cyclic carbonate solvent is at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate and fluoroethylene carbonate. 8. The method of claim 7,
Wherein the nonaqueous solvent has a volume ratio of linear carbonate solvent: cyclic carbonate solvent of 1: 1 to 4: 1. anode; cathode; And a secondary battery electrolyte according to any one of claims 1 to 9.

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