KR100412527B1 - A non-aqueous electrolyte and a lithium secondary battery comprising the same - Google Patents

Abstract

The electrolyte solution for lithium secondary batteries of the present invention is a lithium salt; Non-aqueous organic solvents; And vinyl ester compounds of the formula:

[Formula 1]

Wherein R ₁ , R ₂ and R ₃ are hydrogen, primary, secondary or tertiary alkyl groups, alkenyl groups or aryl groups.

The vinyl ester compound added to the electrolyte of the present invention is decomposed before the carbonate organic solvent at the time of initial charge to form an SEI film, thereby inhibiting the decomposition of the carbonate organic solvent. The lithium secondary battery containing the electrolyte solution of the present invention is excellent in safety and reliability.

Description

A non-aqueous electrolyte and a lithium secondary battery comprising the same {A NON-AQUEOUS ELECTROLYTE AND A LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

Field of invention

The present invention relates to a non-aqueous electrolyte and a lithium secondary battery comprising the same, and more particularly, a non-aqueous electrolyte and a lithium secondary comprising the same, which can prevent the battery from expanding when it is stored at room temperature and at a high temperature after charging. It relates to a battery.

Prior art

Recently, with the development of the high-tech electronic industry, it is possible to reduce the weight and weight of electronic equipment, thereby increasing the use of portable electronic devices. As a power source for such portable electronic devices, the necessity of a battery having a high energy density has been increased, and research on lithium secondary batteries has been actively conducted.

Lithium metal oxide is used as a positive electrode active material of a lithium secondary battery, and lithium metal, a lithium alloy, (crystalline or amorphous) carbon or a carbon composite material is used as a negative electrode active material. The active material is applied to a current collector with a suitable thickness and length, or the active material itself is coated in a film shape to be wound or laminated with a separator, which is an insulator, to form an electrode group, and then placed in a can or a similar container, and then injected with an electrolyte solution. A secondary battery is manufactured.

The average discharge voltage of the lithium secondary battery is about 3.6 to 3.7 V, and high power can be obtained as compared with other alkaline batteries, Ni-MH batteries, Ni-Cd batteries and the like. However, in order to achieve such a high driving voltage, an electrochemically stable electrolyte composition is required in the charge and discharge voltage range of 0 to 4.2 V. For this reason, a mixture of non-aqueous carbonate solvents such as ethylene carbonate, dimethyl carbonate and diethyl carbonate is used as the electrolyte solution. However, the electrolytic solution having such a composition has a problem in that the battery characteristics are lowered at high rate charge / discharge compared to the aqueous electrolytic solution used in Ni-MH batteries or Ni-Cd batteries.

During the initial charging of the lithium secondary battery, lithium ions from the lithium metal oxide as the positive electrode move to the carbon electrode as the negative electrode and are intercalated with carbon. At this time, lithium is highly reactive and reacts with the carbon electrode to generate Li ₂ CO ₃ , LiO, LiOH and the like to form a film on the surface of the negative electrode. Such a coating is called a solid electrolyte interface (SEI) film. The SEI film formed at the beginning of charging prevents the reaction of lithium ions with a carbon anode or other material during charging and discharging. It also acts as an ion tunnel, allowing only lithium ions to pass through. The ion tunnel serves to prevent the organic solvents of a large molecular weight electrolyte which solvates lithium ions and move together and are co-intercalated with the carbon anode to decay the structure of the carbon anode. Therefore, once the SEI film is formed, lithium ions do not react sideways with the carbon negative electrode or other materials again, so that the amount of lithium ions is reversibly maintained. In other words, the carbon of the cathode reacts with the electrolyte at the initial stage of charging to form a passivation layer such as an SEI film on the surface of the cathode to maintain stable charge and discharge without further decomposition of the electrolyte ( J. Power Sources , 51 (1994), 79-104. For this reason, the lithium secondary battery can maintain a stable cycle life after showing no reaction of forming an irreversible passivation layer after the initial charging reaction.

However, there is a problem that gas is generated inside the battery due to decomposition of the carbonate-based organic solvent during the SEI film formation reaction ( J. Power Sources , 72 (1998), 66-70). Such gas may be H ₂ , CO, CO ₂ , CH ₄ , C ₂ H ₆ , C ₃ H ₈ , C ₃ H _6, or the like, depending on the type of the non-aqueous organic solvent and the negative electrode active material. Due to the gas generation inside the battery, the thickness of the battery is expanded during charging. In addition, as time passes during high temperature storage after charging, the passivation layer gradually collapses due to the increased electrochemical and thermal energy, thereby continuously causing side reactions in which the exposed cathode surface and the surrounding electrolyte react. At this time, gas is continuously generated to increase the pressure inside the battery. The increase in the internal pressure causes a phenomenon in which the center portion of the specific surface of the battery is deformed, such as the rectangular battery and the lithium polymer battery (PLI) bulging in a specific direction. As a result, a local difference occurs in the adhesion between the electrode plates in the electrode group of the battery, thereby degrading performance and safety of the battery and making it difficult to install the lithium secondary battery.

As a method for solving the above problems, there is a method for improving the safety of a secondary battery including a nonaqueous electrolyte by installing a vent or a current breaker for ejecting the internal electrolyte when the internal pressure rises above a certain level. However, this method has a problem of causing a risk of malfunction due to the internal pressure rise.

In addition, a method of changing an SEI formation reaction by injecting an additive into an electrolyte solution to suppress an increase in internal pressure is known. For example, Japanese Patent Laid-Open No. 9-73918 discloses a method for improving the high temperature storage property of a battery by adding 1% or less of a diphenyl picrylhydrazyl compound, and Japanese Patent Laid-Open No. 8-321312 No. 1 discloses a method of improving the service life performance and long-term shelf life by using a compound of 1 to 20% N-butyl amines in an electrolytic solution. Japanese Patent Application Laid-Open No. 8-64238 discloses 3 × 10 ^-4 to 3 × 10. A method of improving the shelf life of a battery by adding ^-3 moles of calcium salt is disclosed, and Japanese Patent Laid-Open No. 6-333596 discloses a method of improving the shelf life of a battery by adding an azo compound to suppress the reaction between the electrolyte and the negative electrode. A method is disclosed. In addition, Japanese Patent Laid-Open No. 7-176323 discloses a method of adding CO ₂ to an electrolyte, and Japanese Patent Laid-Open No. 7-320779 describes a method of suppressing decomposition of an electrolyte by adding a sulfide compound to the electrolyte. .

In order to improve the storage and stability of the battery as described above, a method of inducing a proper film formation on the surface of a negative electrode such as an SEI film by adding a small amount of organic or inorganic material is used. However, the added compound, depending on its intrinsic electrochemical properties, interacts with carbon, which is a cathode during initial charge and discharge, to form a film that is decomposed or unstable. As a result, ion mobility in the electrons decreases and gas is generated in the battery. Rather, by increasing the internal pressure, there is a problem of worsening the storage and stability, life performance and capacity of the battery.

An object of the present invention is to provide a non-aqueous electrolyte solution containing a vinyl ester compound that can suppress the generation of gas inside the battery due to decomposition of the carbonate-based organic solvent during the initial charge.

Another object of the present invention is to provide a lithium secondary battery with little change in the thickness of the battery at room temperature charging and high temperature storage after charging.

1 is a graph showing cycle life characteristics of a lithium secondary battery including an electrolyte prepared according to Examples and Comparative Examples of the present invention.

In order to achieve the above object of the present invention, the present invention is a lithium salt; Non-aqueous organic solvents; And it provides a lithium secondary battery electrolyte comprising a vinyl ester compound of the formula (1):

[Formula 1]

Wherein R ₁ , R ₂ and R ₃ are hydrogen, primary, secondary or tertiary alkyl groups, alkenyl groups or aryl groups.

The invention also the electrolyte; A positive electrode comprising a compound capable of intercalation / deintercalation of lithium ions as a positive electrode active material; And a negative electrode including carbon, a carbon composite, a lithium metal, or a lithium alloy as a negative electrode active material.

Hereinafter, the present invention will be described in more detail.

The electrolyte solution of the present invention is a lithium salt; Non-aqueous organic solvents; And vinyl ester compounds of the formula (1).

The lithium salt used as the solute of the electrolyte is lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), Lithium hexafluoroarsenate (LiAsF ₆ ), lithium bis (trifluoromethyl) sulfonyl imide (LiN (CF ₃ SO ₂ ) ₂ ), lithium bis (perfluoroethyl) sulfonyl imide ( One or a mixture of two or more of LiN (C ₂ F ₅ SO ₂ ) ₂ ) may be used. At this time, the concentration of the salt is preferably used in the range of 0.7 to 2.0M. If the salt concentration is less than 0.7M, the conductivity of the electrolyte is lowered, the performance of the electrolyte is lowered. If the salt is more than 2.0M, the viscosity of the electrolyte is increased, thereby reducing the mobility of lithium ions. These lithium salts act as a source of lithium ions in the battery to enable operation of the basic lithium secondary battery.

As the non-aqueous organic solvent, an organic solvent such as cyclic or chain carbonate may be used, or two or more may be used in combination. In the present invention, it is preferable to use the cyclic carbonate and the chain carbonate in a volume ratio of 1: 1 to 1: 9. The performance of the electrolytic solution is desirable when mixed in the above volume ratio.

Specific examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and specific examples of the chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), and di Propyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC) and the like.

In addition, the electrolyte solution of the present invention may further include a benzene organic solvent in the carbonate solvent. The benzene organic solvents include benzene, fluorobenzene, toluene, trifluorotoluene, xylene and the like. The composition of the electrolyte solution containing the benzene organic solvent is preferably a volume ratio of 1: 1 to 10: 1 carbonate solvent / benzene solvent. The performance of the electrolytic solution is desirable when mixed in the above volume ratio.

A non-aqueous electrolyte solution of the present invention is prepared by adding a vinyl ester compound of Formula 1 to a non-aqueous organic solvent containing lithium salt:

[Formula 1]

Wherein R _1, R ₂ and R ₃ is hydrogen, primary, secondary or a tertiary alkyl group, an alkenyl group or an aryl group, the double hydrogen, an alkyl group of _{C 1 ~C 11, C 2 ~C} 11 alkenyl group, or is an aryl group of C ₆ ~C ₁₄ are preferred.

Among the vinyl ester compounds represented by the formula (1), vinyl acetate, vinyl benzoate, vinyl propionate and the like can be preferably used.

The vinyl ester compound is preferably added in an amount of 0.1 to 10% by weight, and more preferably 0.1 to 5% by weight based on the electrolyte. When the amount of the vinyl ester compound used is less than 0.1% by weight, it is difficult to expect the effect of suppressing gas generation inside the battery, and when the amount of the vinyl ester compound exceeds 10% by weight, the initial charge and discharge efficiency and the life performance of the battery decrease with increasing use. The problem arises.

The vinyl ester compound decomposes earlier than the carbonate organic solvent during initial charging to react with lithium ions to form an SEI film, thereby inhibiting decomposition of the carbonate organic solvent. Therefore, since the generation of gas due to decomposition of the carbonate-based organic solvent during the initial charging can be suppressed, it is possible to prevent the thickness of the rectangular battery or the lithium polymer battery from expanding during normal temperature charging or high temperature storage after charging.

The electrolyte of the lithium secondary battery of the present invention is usually stable in the temperature range of -20 ~ 60 ℃ to maintain a stable characteristic even at a voltage of 4V to improve the safety and reliability of the lithium secondary battery. The electrolyte solution of the present invention can be applied to all lithium secondary batteries such as lithium ion batteries and lithium polymer batteries.

In the present invention, the positive electrode active material of the lithium secondary battery is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , or LiNi _1-xy Co _x M _y O ₂ (0 ≦ x ≦ 1, 0 ≦ _y ≦ 1, 0 ≤ x + y ≤ 1, M is a compound capable of intercalation / deintercalation of lithium ions, such as lithium metal oxides such as metals such as Al, Sr, Mg, La, etc. or lithium chalcogenide compounds As the negative electrode active material, crystalline or amorphous carbon, carbon composite, lithium metal, or lithium alloy is used.

The slurry containing the active material is applied to a current collector of a thin plate with a suitable thickness and length, or the active material itself is applied in a film shape to be wound or laminated with a separator which is an insulator to form an electrode group, and then placed in a can or a similar container. Thereafter, a non-aqueous electrolyte solution of the present invention is injected to manufacture a lithium secondary battery. As the separator, a resin film such as polyethylene or polypropylene or a double layer film of polyethylene / polypropylene may be used.

The following presents a preferred embodiment to aid the understanding of the present invention. However, the following examples are merely provided to more easily understand the present invention, and the present invention is not limited to the following examples.

(Example 1)

1M LiPF ₆ was added to a non-aqueous organic solvent mixed with ethylene carbonate / dimethyl carbonate (EC / DMC) 1/1, and 2% by weight of vinyl acetate was added to the electrolyte as a vinyl ester compound to prepare an electrolyte. .

(Example 2)

An electrolyte solution was prepared in the same manner as in Example 1, except that vinyl benzoate was added instead of vinyl acetate.

(Example 3)

An electrolyte was prepared in the same manner as in Example 1, except that vinyl propionate was added instead of vinyl acetate.

(Comparative Example 1)

An electrolyte was prepared in the same manner as in Example 1, except that the vinyl ester compound was not added.

Decomposition Voltage Measurement

The decomposition voltages of the electrolyte solutions of Examples 1 to 3 and Comparative Example 1 were measured by cyclic voltametry, and are shown in Table 1 below.

Resolution voltage (V) Example 1 0.8 Example 2 1.2 Example 3 0.9 Comparative Example 1 0.5

Cycle voltage measurement conditions are as follows:

Working electrode: MCF, reference electrode: Li-metal, counter electrode: Li-metal

Voltage range: 3V to 0V, Scan Rate: 0.1 mV / s.

The electrolyte solution of Examples 1 to 3 to which the vinyl ester compound is added has a higher decomposition voltage than the electrolyte solution of Comparative Example 1 to which the compound is not added, and thus is decomposed first during initial charging, and the SEI film formation reaction occurs at the decomposition voltage. At this time, since the formed SEI film prevents decomposition of the carbonate-based organic solvent, gas generation due to decomposition of these organic solvents can be suppressed. As a result, the internal pressure of the battery can be reduced, thereby reducing the thickness of the battery after full charge.

Fabrication of Lithium Secondary Battery

LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride (PVdF) as a binder and carbon as a conductive agent were mixed in a weight ratio of 92: 4: 4, and then dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. It was. The slurry was coated on an aluminum foil having a thickness of 20 μm, dried, and rolled to prepare a positive electrode. Crystalline artificial graphite as a negative electrode active material and PVdF as a binder were mixed in a weight ratio of 92: 8, and then dispersed in N-methyl-2-pyrrolidone to prepare a negative electrode slurry. The slurry was coated on a copper foil having a thickness of 15 μm, dried, and rolled to prepare a negative electrode. The prepared electrodes were wound and compressed using a polyethylene separator having a thickness of 25 μm, placed in a rectangular can of 30 mm × 48 mm × 6 mm, and then the electrolyte solutions of Examples 1 to 3 and Comparative Example 1 were injected. Square cells were prepared.

Battery thickness change after charging

The lithium secondary batteries prepared by injecting the electrolyte solutions of Examples 1 to 3 and Comparative Example 1 were charged with a current of 170 mA and a charging voltage of 4.2 V under constant current-constant voltage (CC-CV) conditions, and then left for 1 hour. It discharged to 2.75V with 170 mA of current, and left for 1 hour. This process was repeated three times and then charged with 4.2V charging voltage for 2 hours and 30 minutes at a current of 425mA. After the initial cell assembly, the thickness increase rate after the charge to the thickness of the battery was measured and the results are shown in Table 2 below.

Battery thickness change after full charge (%) Example 1 6.2 Example 2 8.7 Example 3 7.4 Comparative Example 1 9.4

It can be seen that the lithium secondary battery including the electrolyte solutions of Examples 1 to 3 to which the vinyl ester compound was added has a much reduced thickness expansion at room temperature charging as compared to Comparative Example 1 in which the vinyl ester compound is not added.

Battery Life Performance Measurement

The lithium secondary batteries prepared using the electrolyte solutions of Examples 1 to 3 and Comparative Example 1 were charged for 2 hours and 30 minutes to 800 mA and 4.2 V charged voltage under CC-CV conditions, and 800 mA and 2.75 V under CC conditions. Discharged to cut-off voltage. The charge / discharge was repeated 300 times to measure the capacity decrease according to the cycle, and the result is shown in FIG. 1. As shown in FIG. 1, the battery using the electrolyte solution of Comparative Example 1 was discharged during the charge / discharge cycle. On the other hand, the battery using the electrolyte solutions of Examples 1 to 3 shows a small decrease in capacity, while the remarkably decreases.

The vinyl ester compound added to the electrolyte of the present invention is decomposed before the carbonate organic solvent at the time of initial charge to form an SEI film, thereby inhibiting the decomposition of the carbonate organic solvent. Accordingly, the lithium secondary battery to which the electrolyte solution of the present invention is applied reduces the internal pressure of the battery by suppressing generation of gas due to decomposition of the carbonate-based organic solvent during initial charging, and expands the thickness of the battery during normal temperature charging and high temperature storage after charging. It is prevented and also has excellent life characteristics.

Simple modifications or changes of the present invention can be easily carried out by those skilled in the art, and all such modifications or changes can be seen to be included in the scope of the present invention.

Claims (13)

Lithium salts; An organic solvent in which a carbonate solvent and at least one benzene solvent selected from the group consisting of benzene, fluorobenzene, toluene, trifluorotoluene, and xylene are mixed in a volume ratio of 1: 1 to 10: 1; And A non-aqueous electrolyte solution for a lithium secondary battery comprising a vinyl ester compound of Formula 1 below: [Formula 1] Wherein R ₁ , R ₂ and R ₃ are hydrogen, a primary, secondary or tertiary alkyl group, an alkenyl group or an aryl group. The method of claim 1, wherein the lithium salt is lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ) , Lithium hexafluoroarsenate (LiAsF ₆ ), and lithium bis (trifluoromethyl) sulfonyl imide (LiN (CF ₃ SO ₂ ) ₂ ), lithium bis (perfluoroethyl) sulfonyl imide ( LiN (C ₂ F ₅ SO ₂ ) ₂ ) A non-aqueous electrolyte solution for a lithium secondary battery which is at least one selected from the group consisting of. The non-aqueous electrolyte solution for lithium secondary batteries according to claim 1, wherein the lithium salt has a concentration of 0.7 to 2.0 M. The nonaqueous electrolyte of claim 1, wherein the nonaqueous organic solvent is a mixed solvent of a cyclic carbonate and a chain carbonate. The non-aqueous electrolyte solution for lithium secondary batteries according to claim 4, wherein the cyclic carbonate is at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and mixtures thereof. The method of claim 4, wherein the chain carbonate is dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC And at least one selected from the group consisting of a mixture thereof. The non-aqueous electrolyte of claim 4, wherein the cyclic carbonate and the chain carbonate are mixed in a volume ratio of 1: 1 to 1: 9. delete delete delete The non-aqueous electrolyte solution for lithium secondary batteries according to claim 1, wherein the compound represented by Chemical Formula 1 is at least one compound selected from the group consisting of vinyl acetate, vinyl benzoate, and vinyl propionate. Non-aqueous electrolyte according to any one of claims 1 to 7 and 11; A positive electrode including a compound capable of intercalation / deintercalation of lithium ions as a positive electrode active material; And Anode containing carbon, carbon composite, lithium metal, or lithium alloy as anode active material The lithium secondary battery which consists of. The lithium secondary battery according to claim 12, wherein the battery is a lithium ion battery or a lithium polymer battery.