JP2001250587A - Inspection method of electrolytic solution for lithium battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inspection method for quickly and simply determining quality of an electrolytic solution used for a lithium battery by quantifying an amount of hydrofluoric acid contained in the electrolytic solution and the amount of decomposition products contained in a solvent, and to provide the lithium ion battery superior in cycle characteristics. SOLUTION: A radical which is either a 1,1-dyphenyl-2-bicryl hydrazyl or an aroxyl radical is added to the electrolytic solution, and an attenuation quantity of absorption intensity of visible light for a fixed wavelength of the radical is measured. If the total concentration of a radical extinguishing action substance in the electrolytic solution calculated from the attenuation quantity is 1,000 ppm or lower, when converted in terms of hydrofluoric acid, the quality of the electrolytic solution is determined as being satisfactory.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for determining and selecting an electrolyte suitable for a lithium battery.

[0002]

2. Description of the Related Art In recent years, lithium batteries have attracted attention as portable equipment such as mobile phones and small computers, power storage power supplies, and electric vehicle power supplies.

[0003] Lithium batteries occlude and store lithium ions.
It mainly consists of a releasable positive electrode and negative electrode, a separator, and an electrolyte in which a lithium salt is dissolved in a non-aqueous solvent. For the positive electrode,
Layered or spinel oxides such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O _4, and carbon materials are widely and generally used for the negative electrode. As the non-aqueous solvent, non-aqueous solvents such as carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) are widely used. As the lithium salt, a fluorine salt is widely used. The electrolyte may be in the form of a non-aqueous electrolyte, a polymer electrolyte, a gel electrolyte containing a polymer in the electrolyte, or the like.

An electrolyte using a fluorine-based lithium salt is:
If left in the air for a long time, water mixed from the environment causes the fluorine-based lithium salt to react with water to generate free hydrofluoric acid (HF). Free hydrofluoric acid has a corrosive effect on battery components such as a positive electrode active material, a negative electrode active material, an electrolytic solution, a battery case, and a current collector, and adversely affects battery characteristics. As for the molecules of the non-aqueous solvent, cyclic carbonates such as ethylene carbonate and propylene carbonate are hydrolyzed to produce alcohols and diols, which react with fluorine-based lithium salts to produce free hydrofluoric acid. Furthermore, the generated hydrofluoric acid decomposes cyclic carbonates, alcohols, and diols to generate decomposition products.

As a technique for removing impurities such as hydrofluoric acid, alcohols, diols and other decomposition products, for example, JP-A-10-270076 discloses a technique for removing lithium by crystallization or precision distillation. It discloses a method of purifying to a level that can be sufficiently used as a battery electrolyte.

[0006] However, even if the electrolytic solution thus purified is stored in air having a dew point of -50 ° C or higher, it is inevitable to mix water from the environment. In particular, in an environment where the temperature rises, such as in summer, the generation of impurities by the above process is promoted. Therefore, it is desirable that whether or not the impurities contained in the electrolytic solution are kept low enough to be used in a lithium battery is easily determined during the manufacturing process immediately before using the electrolytic solution.

Japanese Patent Application Laid-Open No. 7-35739 discloses that a negative electrode, a positive electrode and a separator made of metallic lithium are placed at 60 ° C.
There has been proposed an electrolytic solution determination method by immersing for 12 hours and examining the discoloration of metallic lithium by a colorimetric method. However, this method lacks quantitative properties and has the disadvantage that it cannot be determined whether the discoloration is due to the generation of hydrofluoric acid, the decomposition product of a solvent, or the moisture.

Therefore, in order to maintain good quality characteristics of a manufactured battery, it is required that impurities in an electrolytic solution can be separated into hydrofluoric acid and a decomposition product of a solvent and can be quantitatively calculated. I have. In particular, it is very important to regulate and control not only the concentration of hydrofluoric acid but also the concentration of decomposition products of the solvent in order to provide a battery having excellent cycle performance.

As a method for analyzing hydrofluoric acid liberated in a non-aqueous electrolyte, an iodine titration method is known. However, since this analysis is performed in an aqueous system, the analyzer is brought into a manufacturing process in a dry air atmosphere. However, there is a disadvantage in that it takes time and effort to transport the measurement sample and lacks convenience. As a method for analyzing the decomposition product of the solvent, a gas chromatography method is known, but hydrofluoric acid and a high-boiling electrolyte mixed in the measurement sample have a bad influence on the main body of the gas chromatograph analyzer. However, there is a problem that it cannot withstand economical use.

[0010]

In view of the above-mentioned problems relating to a method for determining an electrolyte suitable for use in a lithium battery,
Lithium ion with excellent cycle characteristics by quantifying the content of hydrofluoric acid in the electrolyte and the content of decomposition products of the solvent, and providing an inspection method for quickly and easily discriminating the quality of the electrolyte used for the lithium battery. It is intended to provide a battery.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to add a radical to an electrolytic solution and measure the attenuation of visible light absorption intensity at a specific wavelength of the radical. A method for inspecting an electrolyte for a lithium battery, which determines the quality of the electrolyte. Further, the present invention is the method for testing an electrolyte for a lithium battery, wherein the radical is any of 1,1-diphenyl-2-picrylhydrazyl and alloxyl radical. The present invention also provides a method for testing a lithium battery electrolyte, which is determined to be non-defective when the total concentration of the radical-scavenging substance in the electrolyte calculated from the attenuation is 1000 ppm or less in terms of hydrofluoric acid. In addition, the concentration of the radical-scavenging substance in the electrolytic solution calculated from the attenuation amount is less than 100 ppm in terms of hydrofluoric acid in less than 10 seconds from the start of the measurement, and is 10 seconds or more and 10 minutes or less after the start of the measurement. This is an inspection method for a lithium battery electrolyte solution that is determined to be non-defective when the quantitative value is less than 900 ppm in terms of hydrofluoric acid.

In other words, the present inventors have determined the amount of impurities in these electrolytic solutions because free hydrofluoric acid and decomposition products of the solvent produced by leaving the electrolytic solution for a long period of time adversely affect the cycle characteristics. We studied the method for quick and simple operation. As a result, free hydrofluoric acid becomes
1,1-diphenyl-2-picrylhydrazyl represented by the formula:

[0013]

Embedded image Or an alkoxyl radical represented by (Chemical Formula 2)

Embedded image While reacting very sensitively with such radicals, it has been found that the decomposition product of the solvent reacts slowly with radicals such as the aforementioned 1,1-diphenyl-2-picrylhydrazyl and alkoxyl radicals. Reached.

The inspection method of the present invention is characterized in that radicals are added to an electrolytic solution and the decay behavior of the visible light absorption intensity inherent to the radicals is tracked over time, so that the hydrofluoric acid reacts extremely sharply and reacts slowly. And the decomposition product of the solvent to be separated by the difference in the reaction time, and the amount of each can be measured. Both hydrofluoric acid and the decomposition product of the solvent are substances having a function of eliminating radicals, but the radical eliminating speed of hydrofluoric acid is very fast and does not require 10 seconds. The rate of radical disappearance of the decomposition product of the solvent is slow and requires 10 seconds or more, but is completed within 10 minutes.

Thus, the radical-eliminating substance which reacts within 10 seconds is derived from hydrofluoric acid, and the radical-eliminating substance which requires more than 10 seconds to react is derived from the decomposition product of the solvent. So, it is possible to separate.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be illustrated below, but the present invention is not limited to these embodiments.

A battery suitable for using the electrolyte to be tested by the above-described testing method is mainly composed of a positive electrode and a negative electrode capable of inserting and extracting lithium ions, a separator, and a non-aqueous electrolyte in which a fluorine-based lithium salt is dissolved. Lithium battery.

As the positive electrode, LiCoO ₂ , LiN
In addition to iO ₂ , LiMn ₂ O _4, etc., LiCoO ₂ , LiNiO
₂ , an oxide in which Co, Ni, and Mn sites of LiMn ₂ O ₄ are partially substituted with another element can be used. As the other elements, Li, B, V, Al, Ni,
Elements such as Co, Mg, Cr, and Tb can be suitably used.

The carbon material used for the negative electrode active material may be any carbon material capable of occluding and releasing lithium. In particular, the plane spacing (d ₀₀₂ ) estimated by the X-ray diffraction method is 0.3354.
~ 0.3369 nm, crystal size in the C-axis direction (Lc)
Is preferably 20 nm or more.

As the separator, an insulating thin film having excellent ion permeability and mechanical strength can be used. Olefin polymers such as polypropylene and polyethylene are preferably used because of their resistance to organic solvents and hydrophobicity. The pore size of the separator is in a range generally used for a battery, and is, for example, 0.01 to 1 μm. Also,
The thickness is also in the range generally used for batteries, and is, for example, 5 to 40 μm.

The fluorinated lithium salts include LiPF ₆ , LiBF ₄ , and LiAs exhibiting high lithium ion conductivity.
F ₆ , LiOSO _c CF ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (C
_{n F 2 n + 1 SO 2} ) (C m F 2m + 1 SO 2) (n = 1~2, m = 1~4) and the like are used. These fluorinated lithium salts are generally contained in a non-aqueous solvent in an amount of 0.1M to 3.0M, preferably 0.5M to 3.0M.
Use it after dissolving it to a concentration of 2.0M.

The non-aqueous solvent is preferably used in combination with a high dielectric constant solvent and a low viscosity solvent. Examples of the high dielectric constant solvent include ethylene carbonate (EC),
Preferable examples include cyclic carbonates such as propylene carbonate (PC). These high dielectric constant solvents may be used alone or in combination of two or more. As the low-viscosity solvent, for example, dimethyl carbonate (DMC), methyl ethyl carbonate (M
Preferred are chain carbonates such as EC) and dimethyl carbonate (DMC), and lactones such as γ-butyrolactone. These low viscosity solvents may be used alone or in combination of two or more.

(Example 1) Ethylene carbonate and diethylene carbonate which were sufficiently purified were mixed in a volume ratio of 1: 1.
After dissolving sufficiently purified LiPF ₆ at a concentration of 1 mol / l, an electrolytic solution was prepared in which the solution was allowed to stand in dry air at a dew point of −60 ° C. for 12 hours. An alkoxyl radical represented by (Chemical Formula 2) is added to the electrolytic solution,
The amount of the radical scavenger for up to 10 seconds and the amount of the radical scavenger for 10 seconds to 10 minutes were quantified.

Details of the analysis method are described in Mukai, K .; Oka, W .; Wata
nabe, K.; Egawa, Y.; Nagaoka, S. Kinetic Study of Free-
Radical-Scavenging Action of Flavonoids in Homogen
eousand Aqueous Trion X-100 Micellar Solutions. J.
Phys. Chem A. 101, 1997, 3746-3753. Specifically, an ultraviolet / visible light absorption spectrometer (Shimadzu UV-2100S) is set with an atmosphere-adjustable cell, and the cell is set to an inert gas atmosphere such as nitrogen or argon. An electrolytic solution diluted with ethylene carbonate is installed, and an ethylene carbonate solution of an alkoxyl radical is installed in a cell on the reference side. The cell is adjusted to a temperature range of 25 ° C. ± 0.5 ° C. The detector of the analyzer is equipped with a stopped flow spectrometer (Unisoku Model RS-
450), and the absorption peak intensity attributed to the radical species of the alkoxyl radical at λ = 580 nm is 100
By tracking over time at time intervals of １００100,000 ms, preferably 100 ms, the change in concentration and amount were calculated.

The hydrofluoric acid and the alkoxyl radical are 1
Since the reaction is on a mole to 1 mole basis, the molar amount of the radical scavenging substance can be calculated from the attenuated molar amount of the alkoxyl radical. As a result, the concentration of the radical-eliminating agent was 10 ppm for 10 seconds, and 100 ppm for 10 seconds to 10 minutes.

Using this electrolyte, a coin-type lithium battery having a capacity of 20 mAh and a diameter of 20 mm and a thickness of 1.6 mm as shown in FIG. 3 was prepared. The positive electrode 1 is made of LiCoO ₂
The powder, acetylene black and polytetrafluoroethylene powder were mixed at a weight ratio of 85: 10: 5, and toluene was added and kneaded sufficiently. This was formed into a sheet having a thickness of 0.8 mm by a roller press. Next, this was punched out into a circle having a diameter of 16 mm,
After drying for 5 hours, a positive electrode 1 was obtained. The positive electrode 1 has a positive electrode current collector 6
The positive electrode can 4 was pressed and used. As the negative electrode active material, artificial graphite (average particle size: 6 μm, spacing (d ₀₀₂ ) by X-ray diffraction method: 0.337 nm, crystal size (Lc) in the C-axis direction (Lc): 55 nm) and polytetrafluoroethylene powder in a weight ratio The mixture was mixed at 95: 5, and toluene was added and kneaded well. This was formed into a sheet having a thickness of 0.1 mm by a roller press. Next, this was punched into a circle having a diameter of 16 mm, and dried at 200 ° C. under reduced pressure for 15 hours to obtain a negative electrode 2. The negative electrode 2 was used by being pressed against a negative electrode can 5 provided with a negative electrode current collector 7. As the separator 3, a polypropylene microporous film was used. 8 is a sealing material.

(Example 2) Ethylene carbonate and diethylene carbonate which were sufficiently purified were mixed in a volume ratio of 1: 1.
And dissolved sufficiently purified LiPF ₆ at a concentration of 1 mol / l, and then dried at 25 ° C in dry air at a dew point of -60 ° C.
An electrolytic solution left for a day was prepared.

An alkoxyl radical was added to the electrolytic solution, and the amount of the radical scavenger for 10 seconds and the amount of the radical scavenger for 10 seconds to 10 minutes were quantified in the same manner as in Example 1. As a result, the concentration of the radical-eliminating agent was 10 ppm for 10 seconds, and 200 ppm for 10 seconds to 10 minutes.

A coin-type lithium battery was prepared in the same manner as in Example 1 except that the above-mentioned electrolyte was used as the electrolyte.

Example 3 A sufficiently purified mixture of ethylene carbonate and diethylene carbonate in a volume ratio of 1: 1
, And dissolved sufficiently purified LiPF ₆ at a concentration of 1 mol / l, and then dried at 60 ° C in dry air with a dew point of -60 ° C.
An electrolyte left for 0 days was prepared.

To this electrolyte solution, an alkoxyl radical was added, and the amount of the radical scavenger for 10 seconds and the amount of the radical scavenger for 10 seconds to 10 minutes were quantified in the same manner as in Battery 1 of the present invention. As a result, the radical scavenging substance was 100 ppm by 10 seconds, and from 10 seconds to 1 ppm.
The amount of the radical-scavenging substance up to 0 minutes was 800 ppm.

A coin-type lithium battery was prepared in the same manner as in Example 1 except that the above-mentioned electrolyte was used as the electrolyte.

Example 4 A sufficiently purified mixture of ethylene carbonate and diethylene carbonate in a volume ratio of 1: 1
After sufficiently dissolving LiPF ₆ at a concentration of 1 mol / l, the mixture was allowed to stand in dry air at a dew point of −30 ° C. for 5 hours to intentionally absorb moisture.
An electrolyte solution left at 7 ° C. for 7 days was prepared.

An alkoxyl radical was added to this electrolytic solution, and the amount of the radical scavenger for 10 seconds and the amount of the radical scavenger from 10 seconds to 10 minutes were quantified in the same manner as in Example 1. As a result, the radical-scavenging substance was 200 ppm for 10 seconds and the amount of the radical-scavenging substance was 500 ppm for 10 seconds to 10 minutes.

A coin-type lithium battery was prepared in the same manner as in Example 1 except that the above-mentioned electrolyte was used as the electrolyte.

Comparative Example 1 A sufficiently purified ethylene carbonate and diethylene carbonate were mixed in a volume ratio of 1: 1.
, And dissolved sufficiently purified LiPF ₆ at a concentration of 1 mol / l, and then dried at 60 ° C in dry air at a dew point of -60 ° C.
An electrolyte left for 0 days was prepared.

An alkoxyl radical was added to this electrolytic solution, and the amount of the radical scavenger for 10 seconds and the amount of the radical scavenger from 10 seconds to 10 minutes were quantified in the same manner as in Example 1. As a result, the amount of the radical-eliminating substance was 10 ppm from 10 seconds to 10 minutes, and the amount of the radical-eliminating substance was 1000 ppm from 10 seconds to 10 minutes.

A coin-type lithium battery was prepared in the same manner as in Example 1 except that the above-mentioned electrolyte was used as the electrolyte.

(Comparative Example 2) Ethylene carbonate and diethylene carbonate, which were sufficiently purified, were mixed in a volume ratio of 1: 1.
, And dissolved sufficiently purified LiPF ₆ at a concentration of 1 mol / l, and then dried at 60 ° C in dry air with a dew point of -60 ° C.
An electrolyte left for 00 days was prepared.

An alkoxyl radical was added to this electrolytic solution, and the amount of the radical scavenging substance for 10 seconds and the amount of the radical scavenging substance for 10 seconds to 10 minutes were quantified in the same manner as in Example 1. As a result, the concentration of the radical-scavenging agent for 10 seconds was 100 ppm, and the concentration of the radical-scavenging agent for 10 seconds to 10 minutes was 2000 ppm.
Met.

A coin-type lithium battery was prepared in the same manner as in Example 1 except that the above-mentioned electrolyte was used as the electrolyte.

Using the batteries prepared in Examples 1 to 4 and Comparative Examples 1 and 2, a charge / discharge test was performed. The charge end voltage was 4.2 V, the discharge end voltage was 3.0 V, and both the charge and discharge currents were 1 mA. The test temperature was 25 ° C., and constant current charging and discharging were performed. As the charge / discharge cycle life, the number of cycles when the discharge capacity was reduced to 80% of the initial value was measured.

[0043]

[Table 1]

When comparing the test results of the battery of the present invention and the comparative battery, the concentration (a) of the radical scavenging substance from 10 seconds to 10 minutes was less than 1000 ppm in terms of hydrofluoric acid, and the radical scavenging substance was 10 seconds. It is understood that when the concentration (b) of the active substance is less than 200 ppm in terms of hydrofluoric acid, a high cycle life is exhibited. Also, (a) +
It can be seen that when the concentration of (b) is 1000 ppm or less, a high cycle life is exhibited.

When various non-aqueous solvents, such as ethylene carbonate, which have been sufficiently purified, are each stored at a high temperature for a long period of time without containing a lithium salt, and then inspected by the same inspection method as in Example 1, the radical disappears. No active substance has been identified.

Although an alkoxyl radical was used in the above example, a similar test was conducted using 1,1-diphenyl-2-picrylhydrazyl. It was confirmed that the value showed no change. Among these radicals,
1,1-Diphenyl-2-picrylhydrazyl is very preferably used because it is stable in air in a solid state and has excellent handling properties. In addition, it is also preferable to use a radical having a low self-decay in the electrolyte solvent instead of these.

As the positive / negative electrode active material, LiCo is used.
Examples have been given of lithium secondary batteries using O ₂ and artificial graphite. However, the effects of the present invention have been confirmed when other positive electrode materials and negative electrode materials are used.

Although a stopped flow spectrometer is used in the embodiments, any device having a function of measuring the visible absorption over time may be used. Further, the determination can be made sufficiently visually without using a spectrophotometer. That is, after adding a trace amount of a radical substance as a reagent to the electrolytic solution to be measured, the quality of the electrolytic solution can be determined by visually observing a change in color. Suitable for. Since the quality control step is simplified by incorporating this inspection step into the manufacturing process, a high-quality lithium ion battery having excellent cycle characteristics can be stably manufactured at low cost.

[0049]

As described above, the present invention quantifies the content of hydrofluoric acid and the content of decomposition products of a solvent in an electrolytic solution to quickly and easily determine the quality of an electrolytic solution used in a lithium battery. An inspection method for discriminating is provided, and a lithium ion battery having excellent cycle characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view of a battery according to an embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 positive electrode 2 negative electrode 3 separator 4 positive electrode can 5 negative electrode can 6 positive electrode current collector 7 negative electrode current collector

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Yufu, 2-3-1, Furube-cho, Takatsuki-shi, Osaka F-term in Yuasa Corporation 5H029 AJ05 AJ07 AJ13 AK03 AL06 AL07 AM03 AM04 AM05 AM07 BJ03 BJ12 CJ28 HJ10 5H030 AA01 AA10 AS03 AS08 AS11 FF11 FF52

Claims (4)

[Claims] 1. A method for inspecting an electrolyte for a lithium battery, which determines the quality of the electrolyte by adding a radical to the electrolyte and measuring the attenuation of visible light absorption intensity of a specific wavelength of the radical. 2. The method according to claim 1, wherein said radical is 1,1-diphenyl-2.
2. The method for testing an electrolyte for a lithium battery according to claim 1, wherein the method is one of picrylhydrazyl and alloxyl radical. 3. The total concentration of the radical-scavenging substance in the electrolytic solution calculated from the attenuation amount is 1000 in terms of hydrofluoric acid.
Claim 1 which is judged to be non-defective when it is less than ppm.
Or the method for testing a lithium battery electrolyte according to any one of the above items. 4. The method according to claim 1, wherein the concentration of the radical-scavenging substance in the electrolytic solution calculated from the attenuation is less than 200 ppm in terms of hydrofluoric acid in less than 10 seconds from the start of the measurement, and 10 seconds or more after the start of the measurement. The method for testing an electrolyte for a lithium battery according to any one of claims 1 to 3, wherein the non-defective product is determined as having a quantitative value of less than 1000 ppm in terms of hydrofluoric acid in minutes or less.