KR20030061219A - An electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same - Google Patents

Abstract

PURPOSE: Provided are an electrolyte for lithium secondary battery which is safe when being overcharged, and shows excellent electrochemical property, for example high temperature and life properties, and a lithium secondary battery containing the electrolyte. CONSTITUTION: The electrolyte contains a nonaqueous organic solvent, a lithium salt, and an electrolyte additive, wherein the electrolyte additive is a compound of formula 1. In the formula 1, R1 is C3-C10 cycloalkyl group or substituted cycloalkyl group, R2 is hydrogen, C1-C10 alkyl group, alkoxy group, C6-C10 aryl group, or halogen, m is an integer of 1-6, and n is an integer of 0-6. The compound of formula 1 is cyclohexyl benzene. The electrolyte contains 0.1-10 wt% of the compound of formula 1.

Description

TECHNICAL FIELD The electrolyte for a lithium secondary battery and a lithium secondary battery including the same TECHNICAL FIELD

[Industrial use]

The present invention relates to a lithium secondary battery electrolyte and a lithium secondary battery comprising the same, and more particularly, to a lithium secondary battery electrolyte and a lithium secondary battery comprising the same, which is safe even when overcharged and has excellent electrochemical characteristics such as high temperature and life characteristics. It is about.

[Prior art]

Recently, with the trend toward miniaturization and light weight of portable electronic devices, the need for high performance and high capacity of batteries used as power sources for these devices is increasing. Lithium secondary batteries, which are commercially available and currently used, correspond to the heart of the digital era, which is rapidly being applied to portable phones, notebook computers, camcorders, and the like, which have an average discharge potential of 3.7V, that is, 4C.

In addition to improving battery capacity and performance characteristics, studies are being actively conducted to improve safety such as overcharging characteristics. When the battery is overcharged, depending on the state of charge, lithium is excessively precipitated at the positive electrode, lithium is excessively inserted at the negative electrode, and the positive electrode and negative electrode are thermally unstable, causing rapid exothermic reactions such as decomposition of the organic solvent in the electrolyte and thermal runaway phenomenon. Occurs, a serious problem occurs in the safety of the battery.

In order to solve this problem, the method of adding an aromatic compound as a redox shuttle additive in the electrolyte solution is used. For example, U.S. Patent No. 5,709,968 adds a benzene compound such as 2,4-difluoroanisole to prevent overcharge currents and the resulting thermal runaway phenomenon. It is starting. No. 5,879,834 add a small amount of aromatic compounds such as biphenyl, 3-chlorothiophene, furan and the like to electrochemically polymerize at abnormal overvoltage conditions to increase the internal resistance to improve battery safety. A method is described. These redox shuttle additives act to suppress the overcharge reaction by prematurely raising the temperature inside the battery due to the heat generated by the oxidative heating reaction to quickly and uniformly shut down the pores of the separator. In addition, the polymerization reaction of the additive on the surface of the anode during overcharging also functions to protect the battery by consuming the overcharging current.

However, it is not possible to sufficiently remove the overcharge current by the polymerization reaction of the additive, and the gas swelling phenomenon is intensified due to the decomposition of the oxidation reaction. There is a limit to improvement. The swelling phenomenon refers to a phenomenon in which a central portion of a specific surface is deformed, such as a battery swelling in a specific direction. In addition, these additives have a problem that adversely affect the electrochemical properties of the battery, such as high temperature characteristics or life characteristics of the battery.

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

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide an electrolyte solution for a lithium secondary battery that is safe even when overcharged and has excellent electrochemical characteristics such as high temperature and lifespan characteristics.

Another object of the present invention is to provide a lithium secondary battery including the electrolyte.

1 is a cross-sectional view of a lithium secondary battery.

Figures 2a to 2c is a view showing the cyclic voltagram measurement results of Examples 1 and 2 and Comparative Example 3, respectively.

3A to 3C are diagrams showing current, voltage, and temperature characteristics during overcharging of Example 1, Comparative Examples 1 and 3, respectively.

In order to achieve the above object, the present invention is a non-aqueous organic solvent; Lithium salts; And it provides a lithium secondary battery electrolyte comprising an electrolyte additive of the formula (1).

[Formula 1]

(In Formula 1,

R ¹ is a C3-10 cycloalkyl or substituted cycloalkyl group,

R ² is hydrogen, an alkyl group having 1 to 10 carbon atoms, an alkoxy group, an aryl group having 6 to 10 carbon atoms, or halogen,

m is an integer from 1 to 6,

n is an integer from 0 to 6,

m + n is 6 or less.)

This invention also provides the lithium secondary battery containing the said electrolyte solution.

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

The structure of the general non-aqueous lithium secondary battery 1 is as shown in FIG. The battery uses a lithiated intercalation compound as a positive electrode (2) and a negative electrode (4), inserts a separator (6) between the positive electrode (2) and the negative electrode (4) and wound the electrode assembly (8). After forming it is put into the case 10 is manufactured. The top of the battery is sealed with a cap plate 12 and a gasket 14. The cap plate 12 may be provided with a safety vent (16) to prevent the formation of overpressure of the battery. The positive electrode tab 18 and the negative electrode tab 20 are respectively provided on the positive electrode 2 and the negative electrode 4 and the insulators 22 and 24 are inserted to prevent internal short circuit of the battery. The electrolyte 26 is injected before sealing the battery. The injected electrolytic 26 liquid is impregnated into the separator 6.

The lithium secondary battery may have a thermal runaway phenomenon in which the temperature of the battery rapidly rises due to overcharge or short circuit due to a design defect of the battery itself due to misuse or failure of a charger. In particular, during overcharging, excess lithium is released from the positive electrode and precipitated on the surface of the negative electrode, resulting in thermally unstable state of the two electrodes. The exothermic reaction proceeds abruptly by the reaction of oxygen and electrolyte generated by the so-called so-called thermal runaway phenomenon in which the temperature of the battery rises rapidly, leading to the ignition and smoke of the battery exceeding the maximum allowable temperature of the battery.

In the present invention, by using the compound of the formula (1) as an electrolyte additive, it provides an electrolyte that can improve the stability during overcharging of the battery and suppress the swelling phenomenon. Most preferred among the compounds of the formula (1) is cyclohexyl benzene.

[Formula 1]

(In Formula 1,

R ¹ is a cycloalkyl or substituted cycloalkyl group having 3 to 10 carbon atoms,

R ² is hydrogen, an alkyl group having 1 to 10 carbon atoms, an alkoxy group, an aryl group having 6 to 10 carbon atoms, or halogen,

m is an integer from 1 to 6,

n is an integer from 0 to 6,

m + n is 6 or less.)

Most preferred examples of the compound of Formula 1 include cyclohexyl benzene.

The electrolyte additive is added to a non-aqueous organic solvent containing a lithium salt. Lithium salt acts as a source of lithium ions in the battery to enable the operation of the basic lithium secondary battery, the non-aqueous organic solvent serves as a medium to move the ions involved in the electrochemical reaction of the battery.

In the case of the electrolyte additive that has been conventionally used, the standard capacity of the battery is lowered because the oxidation start potential is 4.2-4.5V, and the electrolyte additive is electrochemically unstable in the voltage range which is the normal battery use condition. There was nothing but a problem.

The compound of Chemical Formula 1 used as an electrolyte additive in the present invention rapidly increases the temperature of the electrolyte as the heat is generated by the oxidation reaction at 4.5 V or higher, and thermal runaway due to the heat generated by the oxidation reaction of the electrode material and the electrolyte due to overcharging. The thermal runaway is controlled because the separator is shut down only by the temperature of the electrolyte before it occurs. That is, the present additive has a large calorific value than any other material during oxidation, which is very effective in raising the temperature of the electrolyte. In addition, the electrolyte additive is very stable electrochemically and thermally to remove side effects such as deterioration of battery characteristics such as standard capacity, high rate characteristics, and life characteristics, and thus may be a very useful electrolyte additive.

The electrolyte additive of Chemical Formula 1 is preferably used after being added to a non-aqueous organic solvent.

As the non-aqueous organic solvent, carbonate, ester, ether or ketone may be used. The carbonate may be dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC) ethylene carbonate (EC), Propylene carbonate (PC), butylene carbonate (BC), and the like may be used, and the ester may be n-methyl acetate, n-ethyl acetate, n-propyl acetate, or the like. Carbonate solvent in the non-aqueous organic solvent. In this case, it is preferable to use a mixture of cyclic carbonate and chain carbonate.

In addition, the electrolyte solution of the present invention may further include an aromatic organic solvent in the carbonate solvent. As the aromatic organic solvent, an aromatic compound represented by Chemical Formula 2 may be used.

[Formula 2]

(In Formula 2,

R ³ is halogen or an alkyl group having 1 to 10 carbon atoms,

q is an integer from 0 to 6.

Specific examples of the benzene organic solvent include benzene, fluorobenzene, toluene, trifluorotoluene, xylene and the like.

The lithium salt may be LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiN (CxF _{2x + 1} SO ₂ ) (CyF _{2y + 1} SO ₂ ) (where x and y are natural numbers), LiCl, and LiI can be used by mixing one or two or more selected from the group consisting of Do.

The amount of the compound of Formula 1 of the present invention is preferably 0.1 to 50% by weight based on the electrolyte. If the amount of the compound of Formula 1 is less than 0.1% by weight with respect to the electrolyte, the effect of adding the additive is insufficient to prevent thermal runaway phenomenon due to overcharging of the battery, and the amount of the compound of Formula 1 is 50% by weight of the electrolyte If it exceeds%, there is a problem such as a decrease in high rate characteristics is not preferable.

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

As the positive electrode active material of the lithium secondary battery, all of transition metal oxides or lithium chalcogenide compounds that are commonly used may be used, and representative examples thereof include metal oxides such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiMn ₂ O ₄ . Can be used.

Also, Lithium metal or a commonly used carbonaceous material may be used as the negative electrode active material, and representative examples thereof may include crystalline graphite having good dislocation flatness and relatively good reversibility in charge and discharge processes.

The lithium secondary battery of the present invention is excellent in safety without deteriorating electrochemical characteristics such as discharge capacity, efficiency, and cycle life. That is, safety can be ensured under conditions such as overcharge, external short circuit, compression, and overcharge penetration.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only one preferred embodiment of the present invention and the present invention is not limited to the following examples.

(Example 1)

Ethylene carbonate (EC): ethyl methyl carbonate (EMC): flopyrene carbonate (PC): 1.15M LiPF ₆ as an electrolyte salt in a mixed solvent in which fluorobenzene (FB) was mixed in a volume ratio of 30: 55: 5: 10. It was added to prepare an electrolyte solution. Cyclohexylbenzene was added to the electrolyte by 3% by weight based on the electrolyte. A slurry was prepared by dissolving LiCoO ₂ , a conductive material (super P), and a binder polyvinylidene fluoride (PVDF) as a positive electrode active material in N-methylpyrrolidone (NMP) at a weight ratio of 94: 3: 3. The slurry was coated on aluminum foil, dried, and rolled into a roll press to prepare a positive electrode plate having a thickness of 0.147 mm. Graphite (Petoca Co., Ltd.) and binder (PVDF), which are negative electrode active materials, were dissolved in NMP to prepare a slurry. The slurry was applied to a copper current collector, dried, and rolled in a roll press to prepare a negative electrode plate having a thickness of 0.178 mm. A rectangular battery was prepared by inserting a separator having a thickness of 0.025 mm made of a porous film of polyethylene (PE) between the positive electrode plate and the negative electrode plate and injecting an electrolyte solution.

(Example 2)

Except for adding 5% by weight of cyclohexylbenzene to the electrolyte solution it was carried out in the same manner as in Example 1.

(Example 3)

Except for adding 7% by weight of cyclohexyl benzene to the electrolyte solution it was carried out in the same manner as in Example 1.

(Comparative Example 1)

The same procedure as in Example 1 was conducted except that no cyclohexylbenzene was added.

(Comparative Example 2)

The same procedure as in Example 1 was carried out except that 5% by weight of 2,4-diluoroanisole was added instead of cyclohexylbenzene as an electrolyte additive.

(Comparative Example 3)

Except for adding 5% by weight of biphenyl (biphenyl) instead of cyclohexylbenzene as an electrolyte solution was carried out in the same manner as in Example 1.

When the charge and discharge of the rectangular batteries of Examples 1 to 3 and Comparative Examples 1 to 3 for 7 hours at 0.2C, the conversion efficiency, standard capacity (mAh), high rate capacity (mAh) at 2C, at 1C and 12V The overcharge test and the life retention rate after 300 cycles were evaluated in Table 1 below.

TABLE 1

Chemical Efficiency (%) Standard capacity (mAh) High capacity capacity (mAh) Overcharge (1C, 12V) Life retention rate (%) Example 1 95 970 930 shut down 85 Example 2 95 972 925 shut down 84 Example 3 95 970 920 shut down 83 Comparative Example 1 95 960 920 Fire (paragraph) 80 Comparative Example 2 93 968 910 Fire (paragraph) 82 Comparative Example 3 93 948 900 Fire (paragraph) 70

As shown in Table 1, Examples 1 to 3 were found to have higher chemical conversion efficiency, standard capacity, and lifetime retention than Comparative Examples 1 to 3, and were found to be safer even when overcharged.

2A to 2C show cyclic voltammograms of the batteries of Examples 1 and 2 and Comparative Example 2, respectively. The cyclic voltagram measurement was performed using a tripolar cell, and lithium metal was used as a reference electrode, and a platinum electrode was used as a counter electrode and a working electrode. The cyclic voltagram was measured at a potential of about 2.75 V to 5.0 V at a scanning speed of 10 mV / s.

The battery of Example 1 exhibited an oxidation initiation potential of 4.8V and the battery of Comparative Example 2 exhibited an oxidation potential of 4.5V. The increase in the oxidation initiation potential indicates that the material participating in the redox reaction is stable, and it can be seen from the above results that the battery of Example 1 is more stable.

The battery of Example 1 and Comparative Examples 1 and 3 were overcharged with a voltage of 12V at 1C to measure the change in voltage and temperature of the battery over time, and the results are shown in FIGS. 3A to 3C. As shown in FIG. 3A, the battery of Example 1 was found to increase in temperature after 20 minutes after overcharging. This may be because cyclohexylbenzene consumed an overcharge current. The temperature of the battery gradually increased and the voltage remained stable without dropping at 12V. On the contrary, in Comparative Examples 1 and 3, the temperature of the battery rapidly increased, and the voltage rose to 12V and then dropped to 0V.

Thus, when this additive is added to the electrolyte solution and applied to the square cell, the current is consumed by the oxidation reaction of the additive after about 20 excess after overcharging, and the temperature of the electrolyte solution itself is increased by the reaction heat to suppress threatening thermal runaway reaction. It can be seen that the safety of the battery is greatly improved by shutting down the separator safely.

The lithium secondary battery including the electrolyte of the present invention is safe even when overcharged, and has excellent electrochemical characteristics such as high temperature and life characteristics.

Claims (4)

Non-aqueous organic solvents; Lithium salts; And electrolyte additives, The electrolyte additive is a lithium secondary battery electrolyte solution of the formula (1). [Formula 1] (In Formula 1, R ¹ is a cycloalkyl or substituted cycloalkyl group having 3 to 10 carbon atoms, R ² is hydrogen, an alkyl group having 1 to 10 carbon atoms, an alkoxy group, an aryl group having 6 to 10 carbon atoms, or halogen, m is an integer from 1 to 6, n is an integer from 0 to 6, m + n is 6 or less.) The electrolyte of claim 1, wherein the compound of Formula 1 is cyclohexylbenzene. According to claim 1, wherein the content of the compound of Formula 1 is 0.1 to 10% by weight of the electrolyte for lithium secondary battery electrolyte. The lithium secondary battery containing the electrolyte solution for lithium secondary batteries in any one of Claims 1-3.