JP2005129481A - Heat resistant lithium battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium battery capable of improving long-term reliability without damaging an electrochemical property like heat resistance or a discharging property even in a severe high temperature environment. <P>SOLUTION: The lithium battery has a positive electrode, a negative electrode, a separator interposed between the electrodes, and nonaqueous electrolyte liquid containing a nonaqueous solvent and electrolyte salt. The nonaqueous solvent contains at least one kind of compound having a boiling point of 200°C or higher, and at least one kind of compound having a boiling point of lower than 200°C out of the compounds shown by general formula (1): X-(O-C<SB>2</SB>H<SB>4</SB>)<SB>n</SB>-O-Y äin the formula, X, Y denote an alkyl group (C1 to C4) independent from each other, n is 1 to 5}. A ratio of the compound shown by general formula (1) to the sum of the total volume in the nonaqueous solvent at 23°C is not less than 95% and not more than 100%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an improvement in an electrolyte of a lithium battery for the purpose of improving heat resistance.

Conventional lithium batteries can be used in a temperature environment up to about 85 ° C, but they are incorporated in automobile electrical components (tire pressure gauges, on-board equipment for automatic toll collection systems, etc.) and factory automation (FA) equipment. The batteries are often exposed to harsh temperature environments exceeding 100 ° C to 150 ° C.
Further, in order to increase production efficiency, a reflow soldering method is used when a battery is incorporated in an electronic device. According to this method, the battery temperature reaches 200 to 260 ° C. for a short time. For this reason, there is a strong demand for a lithium battery with excellent heat resistance reliability that does not swell or deteriorate battery performance even under such severe temperature conditions.

  By the way, as a technique for improving the safety of the lithium secondary battery, a technique using diethylene glycol dimethyl ether or triethylene glycol dimethyl ether as a main solvent of an electrolytic solution has been proposed (for example, see Patent Document 1).

  In addition, as a technology for improving the discharge characteristics of lithium secondary batteries and adding high temperature resistance to the batteries, an electrolytic solution containing butyl diglyme (diethylene glycol dibutyl ether) having a high boiling point as a main solvent is used. A technique using a separator or gasket made of a certain polyphenylene sulfide has been proposed (for example, see Patent Document 2).

  Moreover, the technique which adds carboxylic acid and carboxylic acid ester to a nonaqueous electrolyte is proposed (for example, refer patent document 3, 4).

Japanese Patent Laid-Open No. 1-281677 (page 1-2) JP 2002-298911 A (page 2-3) JP-A-8-321311 (page 1-2) JP-A-9-147910 (page 2-3)

  However, since the battery using the technique described in Patent Document 1 uses a polypropylene separator having a low heat resistance (melting point: about 150 ° C.), the heat resistance is not sufficient. For this reason, this battery cannot be used in the field requiring long-term stability to the above-mentioned temperature of about 150 ° C. or for reflow soldering which is exposed to a temperature of about 200 ° C. at the minimum.

  Moreover, although the battery using the technique of patent document 2 is excellent in heat resistance, since the high viscosity butyl diglyme (diethylene glycol dibutyl ether) is used as the main solvent, the viscosity of a non-aqueous electrolyte is high. For this reason, there exists a problem that the ionic conductivity of electrolyte solution is low and discharge characteristics are bad.

  Moreover, the battery using the technique described in Patent Document 3 reacts with the carboxylic acid ester, the solvent / solute in the nonaqueous electrolytic solution, and lithium in the negative electrode, and a film is formed on the negative electrode surface. Since the reaction with the non-aqueous electrolyte hardly occurs, the storage characteristics are improved. However, this battery has a volume ratio of at least one high dielectric constant solvent selected from the group consisting of ethylene carbonate, propylene carbonate, and butylene carbonate, and 1,2-dimethoxyethane in a ratio of 3: 7 to 7: 3. The high dielectric constant solvent reacts with the negative electrode under high temperature conditions to form a film with high resistance on the surface of the negative electrode. Although this reaction appears conspicuously under high temperature conditions, the amount of the coating film formed becomes excessive because the blending ratio of the high dielectric constant solvent contains a high proportion of 30% by volume or more. Since the internal resistance of the battery is increased by this coating, it is not suitable for the above-described field requiring long-term stability with respect to a temperature of about 150 ° C. or for reflow soldering which is exposed to a temperature of about 200 ° C. at the minimum.

  Moreover, the battery using the technique described in Patent Document 4 contains 1 to 8% by volume of an aliphatic carboxylic acid ester and / or a chain ether in the nonaqueous electrolytic solution, whereby the ionic conductivity of the electrolytic solution is increased. The charge / discharge capacity increases. However, this battery has a volume ratio of at least one cyclic carbonate selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate having high viscosity, and dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. A non-aqueous solvent mixed with 1 is used. For this reason, the same problem as in the above-mentioned Patent Document 3 occurs, and in the field requiring the long-term stability to the above-mentioned temperature of about 150 ° C. or for reflow soldering exposed to a temperature of about 200 ° C. at the minimum. Not suitable.

  As a result of intensive studies based on the above, the present inventors have compared the conventional general technical knowledge that it is preferable to use only a solvent having a high boiling point exceeding the intended heat resistant temperature in the heat resistant battery. 1,2-dimethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, etc., which are low-boiling solvents (boiling point below 200 ° C.), and triethylene glycol dimethyl ether, tetraethylene, which are relatively high boiling solvents (boiling point 200 ° C. or higher) It has been found that by using a mixture of glycol dimethyl ether, diethylene glycol dibutyl ether, or the like, sufficient safety can be secured even in a severe high temperature environment, and discharge characteristics can be greatly improved.

  The present invention has been completed on the basis of the above knowledge, and an object thereof is to provide a lithium battery having excellent heat safety and excellent discharge characteristics.

  The present invention for solving the above-described problems is a lithium battery having a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and a non-aqueous electrolyte solution containing a non-aqueous solvent and an electrolyte salt. Among the compounds represented by the following general formula (1), the nonaqueous solvent has a boiling point of less than 200 ° C. among the compounds represented by the following general formula (1) and at least one compound having a boiling point of 200 ° C. or higher. The total volume ratio at 23 ° C. of the compound represented by the following general formula (1) in the non-aqueous solvent is at least 95% and not more than 100%.

[Chemical formula 1]
_{X- (O-C 2 H 4} ) n -O-Y (1)
(X and Y in the formula are each independently an alkyl group (1 to 4 carbon atoms), and n is 1-5.)

  In the above structure, among the compounds represented by the general formula (1), the compound having a boiling point of less than 200 ° C. contains 1,2-dimethoxyethane, and the total volume of the compound represented by the general formula (1) at 23 ° C. The volume ratio of the compound having a boiling point of less than 200 ° C. is 50% or more and 60% or less.

  In the above structure, among the compounds represented by the general formula (1), the compound having a boiling point of less than 200 ° C. is a compound other than 1,2-dimethoxyethane, and the compound represented by the general formula (1) is 23 ° C. The volume ratio of the compound having a boiling point of less than 200 ° C. in the total volume in may be 50% or more and 90% or less.

  The said structure WHEREIN: The said nonaqueous solvent can be set as the structure which contains cyclic carbonate and / or lactone as a subcomponent.

  In the lithium battery of the present invention, the electrolyte salt may be lithium bis (trifluoromethanesulfonyl) imide or lithium bis (pentafluoroethanesulfonyl) imide.

  In the above configuration, the non-aqueous electrolyte contains one or more compounds selected from the group consisting of carboxylic acid, carboxylic acid ester (excluding lactone), and carboxylic anhydride with respect to 100 parts by mass of the non-aqueous solvent. It can be set as the structure containing 0.01-5 mass parts.

  According to the said structure, the compound whose boiling point is less than 200 degreeC among the compounds represented by the said General formula (1) (henceforth an ether type compound) has comparatively low viscosity. For this reason, when this is included in electrolyte solution, the electroconductivity of lithium ion will improve, the internal resistance value of a battery will become low, and a battery characteristic can be improved.

  However, since the above-mentioned compound has a boiling point of less than 200 ° C., in reflow soldering that is exposed to a temperature of 200 to 260 ° C., a part of the compound volatilizes to increase the internal pressure of the battery and cause the battery to swell. However, according to the said structure, the compound whose boiling point is 200 degreeC or more is contained among the compounds represented by the said General formula (1). Since this compound has high viscosity but is excellent in heat stability, it acts to alleviate an increase in battery internal pressure during reflow soldering by a compound having a boiling point of less than 200 ° C., thereby reducing battery blistering.

  Moreover, the ether compound mentioned above has a very low reactivity with an electrode rather than conventionally used cyclic carbonates, such as ethylene carbonate and propylene carbonate. As a result, a lithium battery having excellent heat safety and excellent discharge characteristics can be realized.

  In addition, all the volume mixing ratios in the present specification are values under conditions of 23 ° C. and 1 atm.

  Here, among the compounds satisfying the general formula (1), the compound having the lowest boiling point is a compound having a configuration in which n is the smallest and the number of X · Y carbon atoms is the smallest. That is, among the compounds satisfying the general formula (1), the compound having the lowest boiling point is 1,2-dimethoxyethane (DME) in which n = 1 and X and Y are both methyl groups. .

  Among the compounds represented by the general formula (1), when a compound having a boiling point of less than 200 ° C. contains 1,2-dimethoxyethane (DME), the critical temperature of the DME is 258 ° C., and the volume mixing ratio is 60 If it is more than volume%, even if a compound having a boiling point of 200 ° C. or higher is mixed, the internal pressure of the battery becomes excessive in normal reflow soldering (200-260 ° C.), and the battery bulge becomes undesirably large. Further, when the volume ratio is less than 50%, the internal resistance value of the battery becomes high, and the effect of improving the battery characteristics is not sufficient. For this reason, it is preferable to restrict within the above range.

  Among the compounds represented by the general formula (1), compounds having a boiling point of less than 200 ° C. other than 1,2-dimethoxyethane have a critical temperature higher than 260 ° C., and 1,2-dimethoxyethane is used as the volume mixing ratio. It can be larger than the case. However, if the volume mixing ratio is more than 90% by volume, even if a compound having a boiling point of 200 ° C. or higher is mixed, the internal pressure of the battery becomes excessive in normal reflow soldering (200-260 ° C.), and the swelling of the battery becomes large. Therefore, it is not preferable. Further, when the volume ratio is less than 50%, the internal resistance value of the battery becomes high, and the effect of improving the battery characteristics is not sufficient. For this reason, it is preferable to restrict within the above range.

In addition, when a cyclic carbonate or lactone is used as a subsidiary component, the subsidiary component has high stability under high temperature conditions, and has a higher relative dielectric constant than the compound represented by the above formula (1), thereby improving cycle characteristics. Works. Accordingly, it is possible to realize a battery that is excellent in battery safety and discharge characteristics in a high-temperature environment and that has high cycle characteristics.
However, these compounds have a problem that, as described above, they react with the negative electrode under a high temperature condition to form a highly resistant film. However, since the volume mixing ratio in the non-aqueous solvent is 5% by volume or less, The harmful effects can be made extremely small.

  In addition, when an imide-based lithium salt is used as the electrolyte salt, these compounds have high electrochemical and thermal stability, so that the electrolytic solution does not deteriorate when exposed to high temperature conditions for reflow. Therefore, with the above configuration, it is possible to provide a battery in which deterioration of discharge characteristics is further suppressed even in a high temperature environment.

  Moreover, when a carboxylic acid, a carboxylic acid ester, or a carboxylic anhydride (hereinafter sometimes referred to as a carboxylic acid) is added to the non-aqueous electrolyte as an additive, the additive reacts with the negative electrode to form a highly conductive film. Form. With this coating, the reaction between the ether compound and the negative electrode under high temperature conditions can be suppressed. For this reason, the increase in internal resistance due to reflow can be suppressed, and the discharge characteristics are further improved.

  The best mode for carrying out the present invention will be described by taking a coin-type lithium secondary battery as an example. FIG. 1 is a cross-sectional view showing the overall configuration of this battery.

(Embodiment)
As shown in FIG. 1, in a battery outer can (positive electrode can) 1, a positive electrode 2 using spinel type lithium manganate as an active material, a negative electrode 3 using a lithium-aluminum alloy as an active material, and an electrode are separated. An electrode body 5 composed of the separator 4 is accommodated. The separator 4 includes at least one compound having a boiling point of 200 ° C. or higher among the compounds represented by the following general formula (1) and a boiling point of 200 among the compounds represented by the following general formula (1). An electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent containing at least one compound having a temperature of less than 0 ° C. and having a total volume ratio of 95% or more and 100% or less of the compound represented by the following general formula (1) Is impregnated. In this battery, the opening of the positive electrode can 2 and the battery sealing can (negative electrode cap) 7 are caulked and sealed through a ring-shaped insulating gasket 6.

[Chemical formula 2]
_{X- (O-C 2 H 4} ) n -O-Y (1)
(X and Y in the formula are each independently an alkyl group (1 to 4 carbon atoms), and n is 1-5.)

  Next, a method for manufacturing a lithium secondary battery according to the present invention will be described.

Preparation of positive electrode Spinel type lithium manganate (LiMn ₂ O ₄ ) as a positive electrode active material, carbon black as a conductive agent, and polyvinylidene fluoride as a binder were mixed at a mass ratio of 94: 5: 1. . This mixture was pressure-molded to obtain a disk-shaped positive electrode pellet having a diameter of 4 mm and a thickness of 0.5 mm. This positive electrode pellet was vacuum-dried (at 250 ° C. for 2 hours) to remove moisture in the pellet, and a positive electrode was produced.

Production of Negative Electrode A stainless steel plate and an aluminum plate were bonded together, and a negative electrode cap made of a clad material was used so that the inner surface was an aluminum plate. A disc-shaped metallic lithium plate having a diameter of 3.5 mm and a thickness of 0.2 mm was pressure-bonded to the surface of the aluminum plate on the inner surface of the negative electrode cap to produce a negative electrode. Since the metal lithium plate pressure-bonded to the surface of the aluminum plate undergoes an alloying reaction due to charge / discharge performed after the battery is sealed, the active material of the negative electrode is a lithium-aluminum alloy.
Preparation of electrolyte solution LiN (CF ₃ SO ₂ ) ₂ as an electrolyte salt was added to a mixed solvent in which 1,2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TeGM) were mixed at a volume ratio of 50:50. It melt | dissolved in the ratio of 1.0M (mol / liter), and produced electrolyte solution.

Production of Battery A separator made of a non-woven fabric made of polyphenylene sulfide (PPS) was placed on the negative electrode, and the electrolytic solution was poured into the separator. Then, the said positive electrode was mounted on the separator, and also the stainless steel positive electrode can was covered on it. The positive electrode can and the negative electrode cap were caulked and sealed through an insulating gasket made of polyetheretherketone to produce a lithium secondary battery having a battery diameter (diameter) of 6 mm and a thickness of 1.4 mm. PPS and polyether ether ketone are resins having high heat resistance (melting point: PPS, about 280 ° C .; polyether ether ketone, about 340 ° C.).

(Examples 1-44, Comparative Examples 1-9)
As shown in Tables 1 to 3 below, a battery was fabricated in the same manner as in the above embodiment except that the type of nonaqueous solvent, the mixing ratio, the type of additive, the amount added, and the type of electrolyte salt were changed. did.

The compound names abbreviated in Tables 1 to 3 are as follows.
DME: 1,2-dimethoxyethane DGM: diethylene glycol dimethyl ether DGE: diethylene glycol diethyl ether TGM: triethylene glycol dimethyl ether DGB: diethylene glycol dibutyl ether TeGM: tetraethylene glycol dimethyl ether PC: propylene carbonate EC: ethylene carbonate However, in Examples 33-44 The electrolyte salt concentration is 0.75M, and all others are 1.0M.

  In order to examine the relationship between the long-term safety, reflow resistance, and discharge characteristics after reflow, the solvent composition of the non-aqueous electrolyte, and additives in the high-temperature environment of the battery produced above, Examples 1-44 and Comparative Example 1 The following experiments 1 to 4 were performed using the batteries of ˜9.

[Experiment 1]
Using the batteries of Comparative Examples 1 to 6, the relationship between the reflow resistance, the internal resistance after reflow, and the main solvent of the electrolytic solution was examined.

<Reflow resistance test>
Each battery is placed in a reflow oven set so that the surface temperature of the battery is 230 seconds when the temperature is 150 ° C. or higher, 90 seconds when the temperature is 200 ° C. or higher, and 40 seconds (maximum 260 ° C.) when the temperature is 250 ° C. or higher. After charging, the change in the overall battery height was examined for each battery.

<Measurement of internal resistance>
The 1 kHz AC internal resistance value of each battery after the reflow resistance test was measured.

  The result of Experiment 1 is shown in FIG.

The boiling point of each solvent is as shown below.
1,2-dimethoxyethane 85 ° C
Diethylene glycol dimethyl ether 162 ° C
Diethylene glycol diethyl ether 185 ° C
Triethylene glycol dimethyl ether 216 ° C
Diethylene glycol dibutyl ether 256 ° C
Tetraethylene glycol dimethyl ether 275 ° C

  From FIG. 2, it was found that the higher the boiling point of the main solvent, the smaller the swelling of the battery in the reflow resistance test and the higher the internal resistance value. From the results shown in this figure, it was found that a battery having both a low internal resistance value possessed by a compound having a boiling point of less than 200 ° C. and a low battery bulge possessed by a compound having a boiling point of 200 ° C. or higher was preferable.

  In FIG. 2, the data of Comparative Example 1 using 1,2-dimethoxyethane (DME) having a boiling point of 85 ° C. is not shown, but the battery bursts due to reflow, and the internal resistance The value and battery blister could not be measured.

[Experiment 2]
Using the batteries of Examples 1 to 17 and Comparative Examples 1, 2, 6, and 7, the compound having a boiling point of 200 ° C. or higher in the mixed solvent of the electrolytic solution, the composition of the compound having a boiling point of less than 200 ° C., and after the reflow resistance test The relationship between the battery swelling, internal resistance (IR), discharge capacity, high rate (50 μA) discharge capacity, and low temperature (−20 ° C.) discharge capacity was examined. However, these tests were not performed on batteries that had burst in the reflow resistance test.

  The reflow resistance test and the internal resistance value were measured in the same manner as in Experiment 1 above, and the discharge capacity, high rate discharge capacity, and low temperature discharge capacity were measured under the following conditions.

<Measurement of discharge capacity>
Each battery after the reflow resistance test was charged at a constant voltage of 3.0 V for 30 hours, then fixed resistance discharge of 500 kΩ was performed, and the discharge capacity until the battery voltage reached 2.0 V was measured.

<Measurement of high-rate discharge capacity>
Each battery after the reflow resistance test was charged at a constant voltage of 3.0 V for 30 hours, and then high-rate discharge of 50 μA was performed, and the discharge capacity until the battery voltage reached 2.0 V was measured.

<Measurement of low-temperature discharge capacity>
Each battery after the reflow resistance test was charged at a constant voltage of 3.0 V for 30 hours, then a fixed resistance discharge of 500 kΩ was performed at −20 ° C., and the discharge capacity until the battery voltage reached 2.0 V was measured. did.

  The results of Experiment 2 are shown in Table 4 below.

Among the compounds represented by the following formula (1), a compound having a boiling point of less than 200 ° C. (low boiling solvent) and a compound having a boiling point of 200 ° C. or more among compounds represented by the following formula (1) (high boiling solvent) In Examples 1 to 9 using a non-aqueous electrolyte in which LiN (CF ₃ SO ₂ ) ₂ was dissolved in a solvent mixed at a volume mixing ratio of 1: 1 (23 ° C.), the internal resistance value was 446Ω or less, the battery Batteries with excellent characteristics were obtained with a bulge of 0.045 mm or less, a discharge capacity of 2.61 mAh or more, a high rate discharge capacity of 1.55 mAh or more, and a low temperature discharge capacity of 0.33 mAh or more.

On the other hand, in Comparative Examples 1 and 2 using a nonaqueous electrolyte in which LiN (CF ₃ SO ₂ ) ₂ was dissolved only in a low-boiling solvent, the battery blister was 0.061 mm or burst, and the high-boiling solvent Comparative Examples 6 and 7 using a non-aqueous electrolyte in which only LiN (CF ₃ SO ₂ ) ₂ was dissolved were found to have low battery characteristics with a high rate discharge capacity of 1.39 mAh or less and a low temperature discharge capacity of 0.08 mAh or less. It was.

[Chemical formula 3]
_{X- (O-C 2 H 4} ) n -O-Y (1)
(X and Y in the formula are each independently an alkyl group (1 to 4 carbon atoms), and n is 1-5.)

  This is considered as follows. Low boiling solvents have high chemical stability and relatively low viscosity. For this reason, when this is included in electrolyte solution, the internal resistance value of a battery will become low and a battery characteristic can be improved. However, since the above-described compound has a boiling point of less than 200 ° C., in the reflow resistance test that is temporarily exposed to a temperature of 260 ° C., the internal pressure of the battery is increased, which causes the battery to blow up. However, the high boiling point solvent contained in the mixed solvent is excellent in stability to heat although it has high viscosity. For this reason, it acts so as to alleviate the rise in the internal pressure of the battery during the reflow resistance test with the low boiling point solvent, thereby reducing the swelling of the battery. For this reason, the lithium battery which has the outstanding heat-resistant safety | security and the outstanding discharge characteristic was obtained.

  On the other hand, when only the low boiling point solvent is used, the battery is largely swollen because there is no action to alleviate the increase in the internal pressure of the battery during the reflow resistance test due to the mixing of the high boiling point solvent, and the boiling point is as low as 85 ° C. In Comparative Example 1 using dimethoxyethane (DME), it is considered that the battery has been ruptured. On the other hand, when only a high-boiling solvent is used, the viscosity of the high-boiling solvent itself is high, so the conductivity of lithium ions in the electrolyte is low, and battery characteristics are particularly low under high-rate discharge and low-temperature discharge conditions. Conceivable.

Moreover, from the results of Examples 2 and 10-12 in which the solvent composition was the same and the type of the electrolyte salt was changed, imide electrolyte salts (LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO _{2) were obtained.} In Examples 2 and 10 using ₂ ), the low-temperature (−20 ° C.) discharge capacity was 0.91 mAh or more, whereas perfluoroacid electrolyte salts (LiPF ₆ , LiBF ₄ ) were used. In Examples 11 and 12, it was found that the low-temperature discharge capacity was as extremely low as 0.03 mAh.

  This is considered as follows. The imide electrolyte salt has high thermal stability, and the electrolytic solution does not deteriorate even after the reflow resistance test. On the other hand, perfluoroacid-based electrolyte salts have low thermal stability, and the electrolyte solution is significantly degraded by a reflow resistance test. Since the deterioration of the electrolytic solution significantly affects the discharge capacity at low temperatures, it is considered that the results shown in Table 4 were obtained.

  The electrolyte salt was the same, 1,2-dimethoxyethane (DME) was used as the low boiling solvent, tetraethylene glycol dimethyl ether (TeGM) was used as the high boiling solvent, and the mixing ratio of the low boiling solvent and the high boiling solvent was changed. From the results of Examples 1, 13, and 14, in the batteries (Examples 1 and 13) in which the volume mixing ratio of DME was in the range of 50 to 60%, the swelling of the batteries was as small as 0.050 mm or less, and the battery characteristics were excellent. On the other hand, in Example 14 in which the volume mixing ratio of DME was 70%, it was found that the swelling of the battery was as large as 0.150 mm, and the battery characteristics were greatly deteriorated.

  This is considered as follows. Since 1,2-dimethoxyethane (DME) has a critical temperature of 258 ° C., the internal pressure of the battery is remarkably increased in a reflow resistance test that temporarily reaches a temperature of 260 ° C. When the volume mixing ratio of DME is 60% or less, the blistering of the battery is reduced to 0.050 mm or less due to the action of mitigating the increase in the internal pressure of the battery due to the mixed high boiling point solvent (tetraethylene glycol dimethyl ether in the examples). Suppressed. On the other hand, when the volume mixing ratio of DME is more than 60%, the effect of alleviating the increase in the internal pressure of the battery due to the mixed high boiling point solvent becomes small, so that the battery bulge becomes as large as 0.150 mm. In addition, it is considered that the adhesion of the active material is lowered by the swelling of the battery, and the battery characteristics are deteriorated.

  Also, the electrolyte salt is the same, the solvent other than 1,2-dimethoxyethane (DME) is used as the low boiling solvent, tetraethylene glycol dimethyl ether (TeGM) is used as the high boiling solvent, and the low boiling solvent and the high boiling solvent are mixed. From the results of Examples 2 to 5 and 15 to 17 in which only the ratio was changed, the batteries having low volume boiling point solvent mixing ratios in the range of 50 to 90% (Examples 2 to 5, 16, and 17) exhibited low temperature characteristics. While excellent results of 0.33 mAh or more were obtained, Example 15 in which the volume mixing ratio of the low boiling point solvent was 30% was found to have a low low temperature characteristic of 0.11 mAh.

  This is considered as follows. Since the low boiling point solvent contains a compound having a boiling point and a critical temperature higher than 1,2-dimethoxyethane (DME), the degree of increase in the internal pressure of the battery in the reflow resistance test that temporarily reaches a temperature of 260 ° C. It is smaller than when only DME is used as the low boiling point solvent. For this reason, even if the volume mixing ratio of the low boiling point solvent is 90%, the swelling of the battery is prevented by the action of mitigating the increase in the internal pressure of the battery due to the mixed high boiling point solvent (tetraethylene glycol dimethyl ether in the embodiment). It was suppressed to 0.036 mm or less. On the other hand, if the volume mixing ratio of the low-boiling solvent is less than 50%, it is considered that the viscosity of the electrolytic solution is increased because the mixed high-boiling solvent is excessive, and the discharge capacity is lowered particularly under low temperature conditions. It is done.

[Experiment 3]
Using the batteries of Examples 2, 18 to 21, and Comparative Examples 8 and 9, the compound having a boiling point of 200 ° C. or higher in the mixed solvent of the electrolytic solution, the composition of the compound having a boiling point of less than 200 ° C., and the battery after the reflow resistance test The relationship between battery blistering, internal resistance, discharge capacity, high rate discharge capacity, low temperature discharge capacity, and cycle characteristics was investigated.

  The reflow resistance test, internal resistance value, discharge capacity, high-rate discharge capacity, and low-temperature discharge capacity were measured in the same manner as in Experiment 1 or 2, and the cycle characteristics were measured under the following conditions.

<Measurement of cycle characteristics>
Each battery after the reflow resistance test is charged for 30 hours at a constant voltage of 3.0 V, discharged until the battery voltage reaches 2.0 V with a fixed resistance of 500 kΩ, and the discharge capacity is 50% of the discharge capacity of the first cycle. The number of cycles to be measured was measured.

  The results of Experiment 3 are shown in Table 5 below.

  From Table 5, it was found that as the additive amount of the secondary component (cyclic carbonate) increases, the cycle characteristics become better and the internal resistance increases, the high-rate discharge capacity, and the low-temperature discharge capacity tend to decrease. Here, when the additive amount of the subcomponent is 5% by volume or less, an increase in internal resistance, a high rate discharge capacity and a low temperature discharge capacity are minimized, and a battery having good cycle characteristics is obtained. It was found that when the amount of addition was more than 5% by volume, the battery characteristics were greatly deteriorated such that the internal resistance was 1733Ω or more, the high-rate discharge capacity was 0.11 mAh or less, and the low-temperature discharge capacity was 0.05 mAh or less.

  This is considered as follows. The cyclic carbonate used as an auxiliary component has high temperature stability and a higher relative dielectric constant than the ether compound as the main solvent, and acts to improve cycle characteristics. However, cyclic carbonate is highly reactive with the negative electrode and forms a highly resistive coating on the negative electrode surface. For this reason, since an internal resistance value increases, a battery characteristic will fall. When the amount of cyclic carbonate added is 5% by volume or less, it is preferable because an increase in internal resistance, a decrease in high-rate discharge capacity and a low-temperature discharge capacity can be minimized, and a battery having good cycle characteristics can be obtained.

[Experiment 4]
Using the batteries of Examples 2, 18, 22 to 32, the type and amount of additives, battery blistering after reflow resistance test, internal resistance, discharge capacity, high rate discharge capacity, low temperature discharge capacity, and cycle The relationship with characteristics was investigated. The measurement of the reflow resistance test, internal resistance value, discharge capacity, high rate discharge capacity, and low temperature discharge capacity is the same as in Experiment 1 or 2 above.

  From Table 6 above, it was found that as the amount of ethyl formate increased, the internal resistance decreased and the swelling of the battery increased (Examples 22 to 26). Here, when the additive amount is 5% by mass or less, an increase in battery swelling is minimized, and a good battery with low internal resistance is obtained. The additive amount is less than 5% by mass. When the amount was large, it was found that the battery characteristics were greatly deteriorated such that the high rate discharge capacity was 0.75 mAh or less and the low temperature discharge capacity was 0.65 mAh or less.

  This is considered as follows. Carboxylic acid, carboxylic acid ester, and carboxylic anhydrides (carboxylic acids) used as additives form a highly conductive film on the surface of the negative electrode, and react with ethers or propylene carbonate, which are main solvents, with the negative electrode. Since it can be suppressed, it acts to reduce the internal resistance. However, since carboxylic acids react with the manganese compound contained in the positive electrode by reflow and decompose to generate gas, the internal pressure of the battery increases and the battery is swollen. When the amount of carboxylic acid added is 0.01 to 5% by mass, a battery with minimal internal swelling and low internal resistance can be obtained.

[Experiment 5]
Using the batteries of Examples 33 to 44, the relationship between the additive, the pulse discharge characteristics before reflow, the internal resistance value of the battery after the reflow resistance test, and the pulse discharge characteristics was examined. The reflow resistance test and the measurement of the internal resistance value are the same as in Experiment 1 above.

<Pulse discharge test>
Pulse discharge was performed on a 3.6 kΩ fixed resistor for 0.29 seconds, and the lowest voltage at that time was defined as the pulse discharge voltage.

  From Table 7, in Examples 34 to 44 to which an additive (carboxylic acid) was added, the pulse discharge voltage after reflow was 1.63 to 2.05 (V), and the internal resistance was 283 to 415 (Ω). It was found to be far superior to 1.44 (V) and 920 (Ω) of Example 33 in which no agent was added.

  This is considered as follows. The carboxylic acid ester used as an additive can form a highly conductive film on the surface of the negative electrode and suppress the reaction between ethers, which are main solvents, and propylene carbonate, and the negative electrode. It works to make it smaller. It is considered that the pulse discharge characteristics are improved by the reduction of the internal resistance.

  Moreover, from the comparison of Examples 39, 43, and 44 in which the amount of n-butyl formate as an additive was changed, the pulse discharge voltage after reflowing was within the range of 0.01 to 5% by mass. It was found that there was no significant difference from 1.98 to 2.05V. Therefore, if the additive amount is in the range of 0.01 to 5% by mass, the pulse discharge voltage after reflow is sufficiently improved.

  From Experiment 4 and Experiment 5, the reason is not clear, but it can be seen that the effect of improving battery characteristics by adding carboxylic acids is remarkably exhibited when the blending ratio of the low boiling point solvent is high.

[Other matters]
(1) Carboxylic acids such as formic acid, acetic acid, propionic acid, oxalic acid, maleic acid, benzoic acid, phthalic acid, metaphthalic acid, terephthalic acid, methyl formate, ethyl formate, n-propyl formate, isopropyl formate Carboxylic acid esters (excluding lactone) such as n-butyl formate, isobutyl formate, amyl formate, isoamyl formate, methyl acetate, ethyl acetate and methyl propionate, and carboxylic anhydrides such as acetic anhydride and phthalic anhydride Similar effects can be obtained.

However, if the amount of carboxylic acids added to the electrolyte is large, exposure to high-temperature conditions such as reflow may cause the carboxylic acids to react with the manganese compound contained in the positive electrode, generating gas and causing the battery to swell. There is. For this reason, it is preferable that the addition amount of the said carboxylic acids is 0.01-5 mass parts with respect to 100 mass parts of electrolyte solution.
In order to sufficiently obtain the effects of carboxylic acids, the blending ratio of the low boiling point solvent is preferably 50% or more, more preferably 60% or more, and further preferably 70% or more.

  (2) Although ethylene carbonate and propylene carbonate are used as accessory components in the above examples, other cyclic carbonates such as butylene carbonate and vinylene carbonate, and lactones such as γ-butyrolactone may be used. Moreover, you may add these mixtures.

  (3) Moreover, since this invention is applicable if it is a lithium battery, the application object is not limited to the lithium secondary battery described in the above embodiment, and the same excellent effect is also obtained in the lithium primary battery. can get.

(4) When the present invention is applied to a lithium secondary battery, it is preferable to use spinel type lithium manganate (LiMn ₂ O ₄ ) as the positive electrode active material because it is inexpensive and has high thermal stability. . However, other lithium-containing transition metal oxides such as lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), and lithium-containing iron oxide (LiFeO ₂ ) may be used, and a mixture thereof. It may be. Moreover, the lithium containing transition metal oxide which has another metal element in a crystal lattice may be sufficient.
As the negative electrode active material, it is preferable to use lithium metal, a lithium alloy, a metal alloyed with lithium, or the like.

  (5) In addition, when lithium metal or lithium alloy is used for the negative electrode, as the positive electrode active material, a metal oxide such as manganese dioxide or niobium pentoxide that does not contain lithium and absorbs and releases lithium ions can be used alone. Or boron oxide can be used.

(6) When the present invention is applied to a lithium primary battery, manganese dioxide, graphite fluoride, iron disulfide, iron sulfide, etc. can be used as the positive electrode active material. The use of manganese is preferred.
Moreover, as a negative electrode active material, it is preferable to use lithium metal, a lithium alloy, or the like.

  (7) As the electrolyte salt, it is preferable to use an imide-based lithium salt from the viewpoint of thermal stability, but a small amount of other lithium salt may be included.

  (8) In addition, since the battery of the present invention can be used for a long time in a severe high temperature environment near 150 ° C., the material of the separator has a high heat resistance temperature (melting point / decomposition temperature) exceeding 150 ° C. Is preferable, higher than the melting temperature of reflow solder (185 ° C), more preferably higher than the minimum temperature during reflow (200 ° C), more preferably higher than the maximum temperature during reflow (260 ° C) Most preferably, it is high.

  In addition to the polyphenylene sulfide and the polyether ether ketone, the separator material having the above-described material may be a heat-resistant resin such as polyether ketone, polybutylene terephthalate, or cellulose, or a filler such as glass fiber added to the resin material. Furthermore, the resin etc. which improved the heat-resistant temperature are mention | raise | lifted.

  (9) In the above embodiment, a caulking sealing method using a gasket is used to seal the opening of the battery outer can, but instead of a sealing method by laser irradiation, a heat resistant resin is used. A method of thermally welding a sealing member to be formed may be used.

  When sealing a battery with a gasket or heat-resistant resin, the material must satisfy the same heat-resistant temperature condition as the separator material from the viewpoint of heat-resistant reliability (prevention of leakage etc.) of the battery. desirable.

  As described above, according to the present invention, a lithium battery that can be used safely over a long period of time in a high temperature environment of about 100 ° C. to 150 ° C. and that has little deterioration in discharge performance even under such a high temperature environment is realized. Can do. Such a battery of the present invention is excellent in heat-resistant safety and discharge characteristics. Therefore, it is possible to apply a reflow soldering method in which a high temperature of about 200 ° C. to 260 ° C. is applied in a very short time of about 100 seconds for mounting. Even in this case, the reflow heat does not cause the battery to swell greatly or deteriorate the battery performance.

It is sectional drawing which shows typically the coin-type lithium secondary battery which concerns on this invention. It is a graph showing the correlation with the boiling point of an ether type compound, the swelling of a battery, and an internal resistance value.

Explanation of symbols

1 Battery outer can (positive electrode can)
2 Positive electrode 3 Negative electrode 4 Separator 5 Electrode body 6 Insulating gasket 7 Battery sealing can (negative electrode cap)

Claims (6)

In a lithium battery having a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and a nonaqueous electrolytic solution containing a nonaqueous solvent and an electrolyte salt,
Among the compounds represented by the following general formula (1), the non-aqueous solvent is at least one compound having a boiling point of 200 ° C. or higher,
Including at least one compound having a boiling point of less than 200 ° C. among the compounds represented by the following general formula (1),
The total volume ratio at 23 ° C. of the compound represented by the following general formula (1) in the non-aqueous solvent is 95% or more and 100% or less.
The lithium battery characterized by the above-mentioned.
_{X- (O-C 2 H 4} ) n -O-Y (1)
(X and Y in the formula are each independently an alkyl group (1 to 4 carbon atoms), and n is 1-5.) The lithium battery according to claim 1,
Among the compounds represented by the general formula (1), a compound having a boiling point of less than 200 ° C. includes 1,2-dimethoxyethane, and the boiling point occupies the total volume at 23 ° C. of the compound represented by the general formula (1). Is a volume fraction of a compound having a temperature of less than 200 ° C. of 50% or more and 60% or less,
The lithium battery characterized by the above-mentioned. The lithium battery according to claim 1,
Among the compounds represented by the general formula (1), the compound having a boiling point of less than 200 ° C. is a compound other than 1,2-dimethoxyethane, and the total volume of the compound represented by the general formula (1) at 23 ° C. The volume fraction of the compound having a boiling point of less than 200 ° C. is 50% or more and 90% or less,
The lithium battery characterized by the above-mentioned. The lithium battery according to claim 1, 2 or 3,
The non-aqueous solvent contains 5% by volume or less of a cyclic carbonate and / or lactone at 23 ° C. as an accessory component.
A lithium battery as described above. The lithium battery according to claim 1, 2, 3 or 4,
The electrolyte salt is lithium bis (trifluoromethanesulfonyl) imide or lithium bis (pentafluoroethanesulfonyl) imide.
A lithium battery as described above. The lithium battery according to claim 1, 2, 3, 4 or 5,
The nonaqueous electrolyte contains 0.01 to 5 of one or more compounds selected from the group consisting of carboxylic acid, carboxylic acid ester (excluding lactone), and carboxylic anhydride with respect to 100 parts by mass of the nonaqueous solvent. Including mass parts,
The lithium battery characterized by the above-mentioned.

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