KR20040088313A - Nonaqueous Electrolyte For Secondary Battery - Google Patents

Abstract

PURPOSE: A nonaqueous electrolyte solution for a secondary battery is provided, to charge/discharge characteristics, lifetime and high rate discharge characteristics at low temperature and to reduce the irreversible reaction and the thickness expansion in high temperature storage by adding a methyl trifluoroalkane sulfonate. CONSTITUTION: The nonaqueous electrolyte solution for a secondary battery comprises 0.1-10 wt% of a methyl trifluoroalkane sulfonate compound represented by the formula as an additive, wherein R is an alkyl or alkenyl group of C1-C5. The secondary battery is a lithium ion secondary battery, an aluminum laminated battery or a lithium polymer battery which uses a positive electrode active material which is a lithium metal oxide selected from the group consisting of LiCoO2, LiMnO2, LiMn2O4, LiNiO2 and LiNixMn_(1-x) O2 or a composite compound (LIM1xM2yO2) and a negative electrode active material which is a crystalline or amorphous carbon or a lithium metal. Preferably the organic solvent used in the electrolyte solution is ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, fluorobenzene or their mixture; and the lithium salt used in the electrolyte solution is at least one selected from the group consisting of LiPF6, LiClO4, LiAsF6 and LiBF4.

Description

Non-aqueous electrolyte for secondary battery {Nonaqueous Electrolyte For Secondary Battery}

In order to improve the performance of the lithium ion secondary battery, in the present invention, the effect of the addition of additives to the electrolyte solution brings a significant improvement in the charge and discharge characteristics of the lithium ion battery, as well as the life characteristics, temperature characteristics, particularly high rate discharge characteristics at low temperatures, irreversible Reducing the reaction results in capacity improvements and significant improvements in OCV, and innovatively reduces thickness expansion in high temperature storage.

As an electrolyte additive for the purpose of suppressing thickness expansion at room temperature or at high temperature (85 ° C.) in order to improve reliability when SET is installed in lithium ion square batteries, aluminum laminated lithium ion secondary batteries, and lithium polymer batteries. It is a feature of the present invention to add 0.1-10 wt% of an alkyl or alkenyl trifluoromethanesulfonate compound as shown in the general formula below to the basic electrolyte composition.

According to the additive effect of the present invention, in the case of a rectangular lithium ion battery, thickness expansion at room temperature and high temperature is reduced, and effects are shown on the life, low temperature, and OCV characteristics.

[General Formula]

(R is an alkyl or alkenyl group having 1 to 5 carbon atoms)

In the super charging of a lithium ion battery, lithium ions from the lithium metal oxide used as the positive electrode move to the carbon (crystalline or amorphous) electrode used as the negative electrode, and are intercalated. Reaction produces Li ₂ CO ₃ , LiO, LiOH, etc. These form a film on the surface of the negative electrode. This film is called SEI (Solid Electrolyte Interface) film.

Lithium ion battery generates (J. Power Sources, 72 (1998) 66 ~ 70) such as CO, CO ₂ , CH ₄ , C ₂ H _6, etc., caused by decomposition of carbonate organic solvents The battery will expand in thickness when charged, and the surface stability film will increase over time when stored at high temperature in a fully charged state (eg, 4 days at 85 ° C after 4.2V 100% charge). It is gradually decayed by the electrochemical and thermal energy, causing continuous side reactions in which the surrounding electrolyte reacts with the exposed new cathode surface. This side reaction causes continuous gas evolution, leading to an increase in the pressure inside the cell. The cause of the increase in the internal pressure can be seen as the decomposition of the surface stabilized film and thus the decomposition of the electrolyte, the experiment to change the SEI formation reaction by adding an additive to the electrolyte to prevent the increase in the internal pressure. For example, Japanese Laid-Open Patent Publication No. 95-176323A discloses a technique for adding CO ₂ to an electrolyte, and Japanese Laid-Open Patent Publication No. 95-320779A discloses a technique for suppressing decomposition of an electrolyte by adding a sulfide compound to the electrolyte. Presented.

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The average discharge voltage of about 3.6 to 3.7 V of a lithium ion battery is one of the biggest advantages of obtaining higher power than other alkaline batteries or Ni-MH and Ni-Cd batteries. 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. Due to these requirements, EC (ethylene carbonate), DMC (dimethyl carbonate), and DEC (diethyl) are required. Carbonate), dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate and the like, and a mixture consisting of fluorobeneze or a combination thereof are used as a solvent. However, the electrolytic solution having such a composition may be a disadvantage in high rate charging and discharging due to the significantly lower ionic conductivity than the aqueous electrolyte used in Ni-MH or Ni-Cd batteries. LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF6 and the like, which are commonly used as the solute of the electrolyte, act as a source of lithium ions in the cell, thereby enabling the operation of a basic lithium ion battery.

The electrolyte of such a lithium ion battery is usually stable at a temperature range of -20 ° C to 60 ° C and must maintain stable characteristics even at a voltage of 4V region.

Lithium ion derived from lithium metal oxide selected from LiCoO2, LiMnO2, LiMn2O4, LiNi _x Mn _(1-x) O2, LiNiO2, or a composite lithium metal oxide thereof used as a positive electrode in a supercharged lithium ion battery is used as a negative electrode It moves to carbon (crystalline or amorphous) or lithium metal electrode and is intercalated into carbon of negative electrode. At this time, lithium is highly reactive and reacts with carbon negative electrode to make Li ₂ CO ₃ , LiO, LiOH, etc. on the surface of negative electrode. It forms a film. This film is called SEI (Solid Electrolyte Interface) film. The SEI film prevents the reaction between lithium ions and carbon negative electrodes or other materials during charge and discharge after initial formation. In addition, it acts as an ion tunnel to pass only lithium ions.

The effect of this ion tunnel is to dissolve lithium ions, and organic solvents (e.g., EC, DMC, DEC, etc.) having a large molecular weight that move together are cointercalated together in the carbon negative electrode and the carbon negative electrode Prevents the structure of Once this film is formed, the lithium ions do not react sideways with the carbon anode or other materials, thereby reversibly maintaining the amount of lithium ions. As such, the role of SEI film in lithium secondary batteries is very important.

That is, the carbon material of the negative electrode reacts with the electrolyte during supercharge to form a passivation layer on the surface of the cathode, thereby maintaining stable charge and discharge without further decomposition of the electrolyte. (J. Power Sources, 51 (1994) 79-104) At this time, the amount of charge consumed to form the surface stabilization layer on the cathode surface is irreversible capacity, and has the characteristic of not reversibly reacting upon discharge. For this reason, the lithium ion battery may maintain a stable cycle life after the super charge reaction without exhibiting any irreversible surface stabilization layer formation reaction.

However, in the thin rectangular battery, (J. Power Sources, 72 (1998) 66-70) gas generation such as CO, CO ₂ , CH ₄ , C ₂ H ₆ generated by decomposition of carbonate-based organic solvents in the above-mentioned SEI formation reactions The battery's thickness expands during charging, and the passivation film increases over time when the battery is stored at high temperature in a fully charged state (eg, 4 days at 85 ° C after 4.2V 100% charge). It gradually collapses, causing continuous side reactions in which the surrounding electrolyte reacts with the exposed new cathode surface. At this time, the main gases generated are CO, CO ₂ , CH ₄ , C ₂ H _6, etc. depending on the type of carbonate and the cathode active material used (J. Power Sources, 72 (1998) 66-70) . Due to gas evolution, the pressure inside the battery increases.

In this reaction, in the case of a thin lithium ion battery, a square battery, an aluminum laminated battery, and a lithium polymer battery, the thickness of the battery increases, which causes the problem of making the SET installation itself difficult.

In the present invention, in order to suppress the decomposition of the organic solvent at the time of standing at high temperature after full charge, to suppress the thickness expansion of the rectangular lithium battery, aluminum laminated battery and lithium polymer battery, which is a thin lithium ion battery, Or 0.1% to 10% of an alkenyl trifluoromethanesulfonate compound is added to the basic electrolyte solution.

[General Formula]

(Wherein R is an alkyl or alkenyl group of 1 to 5)

Example 1

A lithium ion secondary battery was manufactured by the following procedure.

The positive electrode was mixed with LiCoO ₂ (C-10 having an average particle diameter of 10 μm), a binder of PVDF (Polyvinylidene Fluoride, trade name KFW-1300), and carbon (Super P) as a conductive material in a predetermined weight ratio. A positive electrode slurry was prepared by dispersing with -pyrrolidone (N-methyl-2-pyrrolydone). The slurry was coated on aluminum foil having a thickness of 15 μm, dried, and rolled to prepare a positive electrode.

As the negative electrode, N-methyl-2-pyrrolydinone (N-methyl-2-pyrrolydinone) is used by mixing crystalline artificial graphite and PVDF (Polyvinylidene Fluoride, trade name KFW-1100) as a negative active material at a predetermined weight ratio. It was dispersed to prepare a negative electrode slurry. The slurry was coated on a copper foil having a thickness of 12 μm, dried, and rolled to prepare a negative electrode.

6Ommx 34mmx 5.2mm aluminum laminated (Aluminum) by winding using the positive and negative electrode and the separator (Cellgard, 2320) made of PP (polypropylene) / PE (polyethylene) / PP (polypropylene) material having a thickness of 20 μm. laminated) The cells were assembled into pouches. Herein, an electrolytic solution was prepared such that LiPF ₆ was 1.0 M in a solvent of EC: DMC: EMC = 1: 1: 1 composition.

Methyltrifluoromethanesulfonate (henceforth MTFMS) was added to this electrolyte at the ratio of 1.0% by weight, and the battery was produced.

The fabricated battery was charged 7.0 hours under CC (Constant current) / CV (Constant vlotage) conditions with a current of 200 mA, discharged to 2.75 V again with a current of 200 mA, and then vacuumed gas generated during charging and discharging. Charged at 4.2V charging voltage for 3 hours at a current of 500mA. After full charge, the thickness of the battery and the thickness change measured for 4 hours and 96 hours while leaving the battery in an 85 ° C. high temperature chamber for 4 days are shown in [Table 1], and the capacity recovery performance after 4 days is shown in [Table 2]. It was.

Example 2

Except adding MTFMS to the electrolyte in a weight ratio of 2.0%, a battery was manufactured by the same process as in Example 1, and the thickness change and the volume recovery performance of the battery were measured through the same conditions and procedures as in Example 1 [Table 1] and It is shown in [Table 2].

Example 3

A battery was manufactured by the same process as in Example 1, except that MTFMS was added to the electrolyte in a weight ratio of 5.0%, and the thickness change and the volume recovery performance of the battery were measured through the same conditions and procedures as in Example 1. [Table 1] And [Table 2].

[Comparative Example]

Except not adding MTFMS to the electrolyte, a battery was manufactured by the same process as in Example 1, and the thickness change and the volume recovery performance of the battery were measured under the same conditions and procedures as in Example 1, [Table 1] and [Table] 2].

As a result of this experiment, by adding Methyl Trifluoromethanesulfonate to the electrolyte, the thickness expansion at high temperature after full charge was significantly reduced than the comparative example.

To evaluate the capacity retention characteristics of this battery, discharge experiments were made after leaving the battery at 85'C for 4 days after charging. The results are shown in [Table 2].

By adding MTFMS to the electrolytic solution, the capacity storage characteristics at high temperatures were improved over the comparative examples without addition.

Low temperature characteristics of the batteries prepared by the above Examples and Comparative Examples are shown in Table 3 below.

As a result of this experiment, the low temperature characteristics were improved by adding Methyl Trifluoromethanesulfonate to the electrolyte than the comparative example without addition.

The OCV change of the cells prepared by the above Examples and Comparative Examples when left for one week is shown in Table 4 below.

By adding methyltrifluoromethanesulfonate to the electrolytic solution, the OCV characteristics were almost equivalent to those of the comparative example without addition.

The life characteristics of the battery are shown in the following [Table 5].

By adding methyltrifluoromethanesulfonate to the electrolytic solution, the lifespan characteristics are equivalent to those of the comparative example without addition.

In the present invention, in the case of a thin lithium ion battery, a rectangular battery, an aluminum laminated battery, and a lithium polymer battery, methyltrifluoromethanesulfonate is added to an electrolyte solution at 0.1 to 10%, and then left at a high temperature of 85 ° C after full charge. As a result, the thickness expansion was significantly improved from 27.9% to less than 4.5% after 4 hours, and from 65.7% to less than 18.6% after 96 hours.

The capacity storage characteristics at high temperature after full charge showed an equivalent performance or higher than when no additives were added, and the low temperature discharge characteristics, OCV characteristics, and lifetime characteristics showed higher than or equal performances when no additives were added.

Claims (3)

Lithium metal oxide or a composite compound (LiM1 _x M2 _y O ₂ ) selected from LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _x Mn _(1-x) O ₂ is used as the cathode active material. In a lithium ion secondary battery and a lithium polymer battery using crystalline or amorphous carbon or lithium metal, the compound of the following general formula is added as an electrolyte solution additive to a basic use electrolyte composition in a non-aqueous electrolyte solution for lithium batteries. [General Formula] Wherein R is alkyl or alkenyl having 1 to 5 carbon atoms The method of claim 1, The organic solvent used in the composition of the electrolyte is ethylene carbonate. Propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, fluorobenzene, or a mixture thereof. The method of claim 1, The lithium salt used in the electrolyte composition is at least one selected from the group consisting of LiPF6, LiClO4, LiAsF6 and LiBF4 non-aqueous electrolyte for lithium batteries.