JP5070659B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery that is highly safe in a high-temperature environment during overcharge, and more specifically, a non-aqueous electrolyte solution using an additive and a separator that improve safety during overcharge. Next battery.
[0002]
[Prior art]
As a method for maintaining the safety of the non-aqueous electrolyte secondary battery in a high temperature environment at the time of overcharging, there are a method using a safety device incorporated roughly and a method for imparting an overcharge resistance property to the power generation element itself. is there. Further, as specific examples of imparting overcharge resistance to the latter power generation element itself, a method for improving the characteristics of the separator, a method for adding an overcharge additive to the electrolyte, and the like have been proposed.
[0003]
2. Description of the Related Art Conventionally, as a configuration for improving safety during overcharging, a configuration using a shutdown function of a separator, which is a function specific to a separator of a nonaqueous electrolyte secondary battery, has been widely used. Normally, the separator plays a role in preventing short circuit between the positive and negative electrodes, but separators using porous polyolefin etc. soften the porous separator when the battery temperature rises significantly due to excess current due to external short circuit etc. By doing so, it has a so-called shutdown function that becomes substantially non-porous and prevents current from flowing.
[0004]
If the battery temperature rises even after the shutdown function, the separator melts and opens a large hole, which may cause a short circuit between the positive electrode and the negative electrode (hereinafter, this phenomenon is referred to as meltdown). It can be said that the higher the temperature at which this meltdown occurs, the higher the safety of the battery. However, in order to reinforce the shutdown function, when the heat melting property is increased, the meltdown temperature is lowered, and the safety is lowered. On the contrary, a separator that satisfies both of these properties is desired.
[0005]
On the other hand, in a configuration in which an overcharge additive is added to the electrolytic solution, various methods for improving safety during overcharge have been proposed. For example, a method of increasing the internal resistance of the battery by polymerizing the additive during overcharge and protecting the battery from overcharge, generating gas during overcharge, and reliably operating an internal electrical cutting device that operates at a predetermined internal pressure There is a method, or a method in which a conductive polymer is generated at the time of overcharge abuse, and a short circuit is generated and discharged inside the battery.
[0006]
Due to recent development competition, non-aqueous electrolyte secondary batteries are strongly demanded to have a higher capacity. Higher capacity has improved performance by improving the active material of the electrode, but the volume of members that do not contribute to the electromotive reaction is reduced, and the substantial active material filled in the limited battery container The capacity is increased by increasing the amount of. For this reason, the positive and negative electrode current collectors and separators tend to be thin. As the separator becomes thinner, the safety against short circuits and the like tends to deteriorate, but since the amount of the active material increases, the demand for safety increases.
[0007]
Therefore, when a battery using a thin separator falls into an overcharged state and becomes hot due to heat generation, the overcharged state is eliminated rather than adopting a method of electrically avoiding the overcharged state. It is effective to adopt the method. Specifically, among the methods described above, the method of protecting the battery from overcharging by increasing the internal resistance of the battery by polymerizing the additive during shutdown of the separator or overcharging of the additive, and overcharging A method of reliably operating an internal electrical cutting device that generates gas and operates at a predetermined internal pressure prevents the overcharge state from continuing by limiting or cutting off the current applied to the battery. On the other hand, the method of automatically discharging by generating a short circuit inside the battery is forcibly discharging inside the battery, and the overcharge state is stopped from the point of releasing the power generation element inside the battery from the overcharge state. It is preferable compared to the above method. In particular, when an additive that generates gas during overcharge is added to a configuration that does not have an internal electrical cutting device that operates at a predetermined internal pressure, such as a prismatic battery, the internal pressure of the battery container increases, which is a safety concern. It is not preferable in terms of sex.
[0008]
[Problems to be solved by the invention]
When an aromatic additive is used as an additive in overcharge protection based on each of the above methods, even if the same additive is used, the amount of additive added, relationship with electrolyte salt, non-aqueous solvent and other components, Furthermore, the effect of overcharge protection differs due to the influence of various factors such as the deterioration state. For example, a method of protecting the battery from overcharging by increasing the internal resistance of the battery by polymerizing the additive during overcharging (Japanese Patent No. 3061756), generating gas during overcharging, and operating at a predetermined internal pressure A method of operating an electric cutting device (Japanese Patent No. 3061759) or a method of forcibly discharging a conductive polymer during overcharge abuse (Japanese Patent Laid-Open No. 10-32258) has been proposed. Therefore, by adding an aromatic additive, a conductive polymer is generated at the time of overcharge, and there is a possibility that an action of generating gas or increasing the internal resistance of the battery may occur in preference to the action of ensuring safety by an internal short circuit. In addition, there is a problem in terms of reliability that overcharge protection by forced discharge by the conductive polymer is surely generated.
[0009]
On the other hand, in terms of overcharge protection utilizing the characteristics of the separator, a method using a laminated porous film having a reduced shrinkage rate in the mechanical stretching direction of the separator (Japanese Patent Laid-Open No. 11-123799), using a high molecular weight polyolefin, A method using a separator having an area shrinkage ratio of 30% or less after heat treatment at 105 ° C. (Japanese Patent Laid-Open No. 12-239426) has been proposed. In these methods, nothing is mentioned about the shrinkage rate in the width direction that has been mechanically stretched, and when the battery is exposed to a high temperature environment associated with overcharging, the separator greatly shrinks in the width direction, There is a possibility that the edge of the electrode plate is exposed, and if the shrinkage rate is excessive, a short circuit will occur. Therefore, in the separator prepared by the phase separation opening method instead of the above-described opening method by stretching, a method of using a separator having a shrinkage ratio in the width direction of 25% or less that has been mechanically stretched at a temperature lower than the temperature at which the shutdown function occurs. (Japanese Patent Laid-Open No. 11-322989) has been proposed, but a separator having a small film thickness due to a large separator hole diameter is likely to cause a voltage failure or the like in battery characteristics at room temperature.
[0010]
The present invention solves the above-mentioned conventional problems, and provides a high-capacity non-aqueous electrolyte secondary battery excellent in safety under a high temperature environment by using a combination of a suitable additive and a separator. Objective.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the non-aqueous electrolyte secondary battery of the present invention contains an aromatic additive in the non-aqueous electrolyte, and further applies high temperature and stress assumed to be exposed to the separator during overcharge. Immediately after the shrinkage in the width direction of the machine stretch was 12% 20 % Using a separator in the range of% 28% to 35% And the width direction is held by an insulating member having heat resistance as compared with the separator in a state where the electrode plate group is configured.
[0012]
According to this configuration, the separator prevents the battery internal short circuit due to exposure of the electrode plate at a high temperature, and combines the aromatic additive and the separator having the characteristic parameters described above. An internal short circuit occurs and the overcharged state is eliminated, and it is possible to provide a non-aqueous electrolyte secondary battery with high capacity and excellent reliability.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
A non-aqueous electrolyte secondary battery according to a first invention of the present application is a non-aqueous electrolyte secondary battery including a non-aqueous electrolyte in which an electrolyte salt is dissolved in a positive electrode, a negative electrode, and a non-aqueous solvent, and a non-aqueous electrolyte. Is mixed with an aromatic additive and is 25 kg / cm in the longitudinal direction (hereinafter referred to as MD) mechanically stretched as the separator. ² The contraction rate in the machine-stretched width direction (hereinafter, this direction is referred to as TD) immediately after holding for 15 minutes at a temperature of 120 ° C. in the atmosphere with a tensile load of 12% 20 % Separator is used.
[0014]
As the separator according to the present invention, an electronic insulating microporous thin film having a large ion permeability and an appropriate mechanical strength is used. As the material, it is preferable to use a porous polyolefin such as polypropylene resin, polyethylene resin alone or a laminate or a mixture / combination thereof, from the viewpoint of organic resistance and hydrophobicity and a shutdown function.
[0015]
Further, the additive according to the present invention is an additive made of an aromatic compound, and preferable ones include biphenyl, furan, thiophene and derivatives thereof. Specifically, biphenyl, pyrrole, N-methylpyrrole, thiophene, furan, indole, 3-chlorothiophene, 3-bromothiophene, 3-fluorothiophene, 1,2 dimethoxybenzene, 1-methyl-3-pyridinium tetra There is a fluorobaud rate. Among these, a compound that polymerizes at a voltage that is higher than the maximum operating charging voltage of the battery and less than the overcharge voltage at which the battery becomes dangerous is suitable. From the viewpoint of the polymerization potential and stability under battery operation, biphenyl is suitable. , Furan, indole and 3-chlorothiophene are particularly preferred.
[0016]
The positive electrode in the present invention has a conventionally known structure, but a mixture comprising a lithium-containing composite oxide such as lithium cobaltate, lithium nickelate, and manganese spinel as an active material and a mixture of a conductive agent and a binder is a current collector. It is made by coating.
[0017]
In the negative electrode of the present invention, carbon such as natural graphite and artificial graphite is used as a main active material, but in addition, various metals such as aluminum, various alloys mainly composed of aluminum, tin oxide and the like are used. There are conventionally known materials such as oxides and metal nitrides, and a mixture obtained by mixing a conductive agent and a binder is applied to a current collector in the same manner as the positive electrode.
[0018]
Nonaqueous electrolytes (hereinafter referred to as electrolyte solutions) include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) as nonaqueous solvents. A mixture of two or more chain carbonates such as The electrolyte salt is LiPF ₆ And LiBF _Four Conventionally known lithium salts can be used.
[0019]
The battery of the present invention causes an internal short circuit due to the aromatic additive and the separator in a high temperature environment at the time of overcharging, and the mechanism of this internal short circuit is as follows.
[0020]
When the battery voltage exceeds the maximum operating voltage during overcharging, the aromatic additive polymerizes and grows on the surface of the positive electrode to form a polymer layer having a high resistance value. When the voltage further increases, the polymer layer becomes a conductive polymer. When the overcharge state continues, the conductive polymer grows inside the separator pores, and finally penetrates to reach the negative electrode, causing an internal short circuit. As a result, in the battery using the separator and aromatic additive according to the present invention, the conductive polymer is formed from a relatively early stage of the overcharge process, and the charge depth of the positive electrode and the negative electrode is excessively more than necessary. Since it does not reach, the danger at the time of overcharge can be suppressed.
[0021]
In the above-described mechanism, when the aromatic additive is not added or the addition amount is less than 0.1% by weight, the overcharged state becomes serious before the conductive polymer is sufficiently grown. It will progress. In addition, when the amount of aromatic additive added is large, for example, when it is larger than 10% by weight, the effect of suppressing the overcharge state can be exhibited. is there.
[0022]
Furthermore, as a result of intensive studies by the present inventors, it has been found that a suitable value exists for the TD shrinkage rate of the separator. Conventionally, regarding the TD shrinkage, the separator characteristics in the normal temperature range have been studied. However, when the battery falls into an overcharged state, each component is exposed to a high temperature environment due to the heat generated by the battery itself, so that the separator is required to have characteristics different from those in the normal temperature range. Under such a background, the present inventors have obtained knowledge that it is necessary to evaluate in a thermally and physically added state assumed in a high temperature environment, and keep it in a predetermined condition. As a result, it was concluded that a separator having the following characteristics was suitable for the battery of the present invention.
[0023]
Accordingly, processing conditions applied in advance to the separator will be described. The temperature raising mechanism in the overcharged state of the battery is complicated, and it is difficult to accurately simulate the state of stress applied to the separator in the electrode plate group. In particular, it is important to consider that the state of thermal shrinkage of the separator changes due to the stress applied to the separator at the same high temperature environment temperature. Therefore, the present inventors set the separator in the MD as 25 kg / cm as the state of the separator as a highly reproducible processing condition. ² It has been found that a state in which a tensile load of 1 is applied is preferable. Normally, when producing a spirally wound electrode plate group, the separator is wound with a certain amount of tension. That is, it is arranged in the wound electrode plate group in a state where a tensile load is applied to the MD.
[0024]
On the other hand, regarding the temperature condition, it is necessary to consider the time required for the overcharged state to be eliminated by the conductive polymer after the battery reaches the overcharged state and the overcharged state. It was found that it is preferable to hold at medium temperature of 120 ° C. for 15 minutes. Here, the time of 15 minutes means that the separator has no change in the TD shrinkage at that temperature, that is, a time sufficient for reaching the saturation, and a time longer than this may be sufficient, and the TD shrinkage reaches the saturation. If so, a time shorter than this may be used, but 15 minutes is preferable as the retention time with high reproducibility.
[0025]
Thus, the separator according to the present invention has a separator of 25 kg / cm in MD. ² It is preferable to measure the TD shrinkage immediately after holding for 15 minutes at a temperature of 120 ° C. in the state of applying a tensile load (hereinafter, this treatment is referred to as heat treatment). Ku, Yoshi When the aromatic additive is selected from the group consisting of biphenyl, furan, indole and 3-chlorothiophene, the TD shrinkage rate is 12% to 20%, which is a very excellent effect.
[0026]
When the TD shrinkage ratio after heat treatment is less than 12%, the puncture strength is weak in manufacturing the separator, and the insulation failure of the battery increases. On the other hand, if the TD shrinkage after the heat treatment exceeds 25%, an internal short circuit occurs due to the electrode plate exposure at a high overcharge temperature, and the short circuit current increases, resulting in a dangerous situation. Accordingly, the TD shrinkage after heat treatment is preferably 12% to 25%.
[0027]
Next, the non-aqueous electrolyte secondary battery according to the second invention of the present application is a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a non-aqueous electrolyte in which an electrolyte salt is dissolved in a non-aqueous solvent, and a separator, The non-aqueous electrolyte is a mixture of aromatic additives, and 25 kg / cm of MD is used as the separator. ² TD and shrinkage ratio immediately after holding for 15 minutes at a temperature of 120 ° C. in the state of applying a tensile load of 28% to 35% In the electrode plate group in which the separator is further disposed between the positive electrode and the negative electrode, the TD of the separator is held by an insulating member having higher heat resistance than the separator.
[0028]
According to this configuration, when exposed to a high temperature environment associated with overcharging, an internal short circuit occurs due to the aromatic additive and the separator as in the first invention of the present application.
[0029]
The separator according to the second invention has a larger TD shrinkage rate than the battery according to the first invention described above, and the separator MD is 25 kg / cm. ² The TD shrinkage ratio is from 26% to 40% after heat treatment at 120 ° C. for 15 minutes in the air with a tensile load applied. For this reason, when a battery falls into an overcharge state, the contraction degree of a separator becomes large compared with the battery which concerns on 1st invention. However, since the TD of the separator of the electrode plate group that is laminated or wound through the separator is fixed with an insulating film having higher heat resistance than the separator, the electrode is caused by the shrinkage of the separator at a high temperature during overcharge. It is possible to reliably prevent the plate exposure and the occurrence of a short circuit inside the battery due to this. In this way, by combining the aromatic additive and the separator according to the second invention, at the time of overcharging, the polymerized conductive polymer penetrates the separator and causes an internal short circuit to eliminate the overcharged state. Therefore, it is possible to provide a non-aqueous electrolyte secondary battery with high capacity and excellent reliability. In the case where the aromatic additive is selected from the group consisting of biphenyl, furan, indole and 3-chlorothiophene, the TD shrinkage after heat treatment is 28% to 35%, showing particularly excellent effects. .
[0030]
When the shrinkage after heat treatment is less than 26%, it is not necessary to fix the TD of the separator of the electrode plate group. When the TD shrinkage after heat treatment exceeds 40%, the separator is damaged under overcharge and locally inside. A short circuit occurs and the short circuit current increases, resulting in a dangerous situation. Therefore, the TD shrinkage after heat treatment is preferably 26% to 40%.
[0031]
In the battery described above, if the thickness of the separator is 21 μm or more, it is not only disadvantageous in terms of battery characteristics such as high capacity of the battery and high rate discharge, but also an internal short circuit is less likely to occur. On the other hand, when the thickness is 7 μm or less, even if an internal short circuit occurs reliably in a high temperature environment during overcharging, the battery becomes dangerous due to its thinness. Therefore, the thickness of the separator is preferably 8 to 20 μm.
[0032]
【Example】
Next, specific examples of the present invention will be described using examples.
[0033]
First, separators having various characteristics described below were manufactured under different conditions.
[0034]
<Preparation of separator>
In this example, a separator (a polyethylene (PE) film) was produced.
[0035]
First, a PE film was manufactured by the method described below. 40 parts by weight of high density polyethylene (average molecular weight 320,000) and 60 parts by weight of liquid paraffin were melt kneaded in a twin screw extruder. A polymer gel sheet was produced by extrusion casting from a coat hanger die onto a cooling roll. The thickness was 1.8 mm at this point. This polymer gel sheet was stretched before extraction by 7 × 7 times at 122 ° C. using a simultaneous biaxial stretching machine. Thereafter, the liquid paraffin was extracted and removed by dipping in methylene chloride. Further, using a tenter, the film was stretched twice to TD at 125 ° C., and then heat treated while relaxing TD stretching by 17%. A PE film having a thickness of 16% was produced as a separator A by the above-described steps.
[0036]
Separator A is cut into a rectangle of 120 mm for MD and 50 mm for TD, and is placed in a thermostatic layer set at a temperature of 120 ° C. in the atmosphere, with a weight of 200 g on MD, 25 kg / cm ² Was set under the condition of applying a tensile load of and held for 15 minutes. (Hereinafter, it is called MD heat processing.) After MD heat processing, TD dimension was measured in the laboratory controlled to 23 degreeC, and the shrinkage rate was computed. The shrinkage rate was 14%.
[0037]
TD shrinkage rate (%) = {(TD length before heat treatment−TD length after heat treatment) / TD length before heat treatment} × 100
Next, separator B having the same thickness as separator A and a different total pore volume was manufactured.
[0038]
40 parts by weight of high density polyethylene (average molecular weight 400,000) and 60 parts by weight of liquid paraffin were melt-kneaded in a twin screw extruder. A polymer gel sheet was produced by extrusion casting from a coat hanger die onto a cooling roll. The thickness was 1.8 mm at this point. This polymer gel sheet was stretched before extraction by 7 × 4 times at 130 ° C. using a simultaneous biaxial stretching machine. Thereafter, the liquid paraffin was extracted and removed by dipping in methylene chloride. Furthermore, after extending | stretching 3 times to TD at 130 degreeC using the tenter, it heat-processed, extending | stretching TD 17%. Through the steps described above, a PE film having a thickness of 16% was produced as separator B.
[0039]
Separator B was cut into a rectangle of 120 mm in MD and 50 mm in TD and subjected to MD heat treatment. After the MD heat treatment, the TD dimension was measured in a laboratory adjusted to 23 ° C., and the shrinkage rate was calculated. The shrinkage rate was 12%.
[0040]
Hereinafter, by changing the molecular weight and stretching conditions of polyethylene in the same manner as separator A or separator B, 24 types of separators A to Y consisting of TD shrinkage ratio and thickness after heat treatment as shown in (Table 1) A separator was prepared.
[0041]
[Table 1]
[0042]
<Production of battery>
In order to evaluate the temperature change at the time of overcharging of the battery of the present invention, a square battery described below was produced.
[0043]
FIG. 1 is a structural diagram (partially sectional view) of a prismatic battery according to Examples 1 to 34 of the present invention.
[0044]
In FIG. 1, a nonaqueous electrolyte secondary battery 1 is wound with a positive electrode 2, a negative electrode 3, and a separator 4, and is built in a case 5 together with an electrolyte solution (not shown) in which an electrolyte salt is dissolved in a nonaqueous solvent. And is sealed with a sealing plate 6.
[0045]
In a general cylindrical battery, a safety element such as a safety valve or a PTC element is incorporated in the sealing plate, but in the battery of the embodiment, no safety mechanism is incorporated in the sealing plate 6 because it is square. .
[0046]
In the positive electrode 2, 10% by weight of carbon powder as a conductive agent and 5% by weight of polyvinylidene fluoride resin (PVdF resin) as a binder are mixed with 85% by weight of lithium cobaltate powder, and these are dispersed in dehydrated NMP. A slurry was prepared, applied to a positive electrode current collector made of aluminum foil, dried and rolled.
[0047]
The negative electrode 3 uses artificial graphite powder as a negative electrode active material, and 95% by weight of this is mixed with 5% by weight of a PVdF resin as a binder, and these are dispersed in dehydrated NMP to produce a slurry. A positive electrode current collector made of foil was applied, dried and rolled.
[0048]
As the separator 4, 13 types of separators A to M shown in Table 1 above were used.
[0049]
In addition, the electrolyte includes LiPF in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1: 1. ₆ 1 mol / liter dissolved was used. The amount of the electrolytic solution is about 2.5 ml.
[0050]
The produced square battery has a width of 30 mm, a height of 48 mm, and a thickness of 5 mm. The design capacity of this size, which is usually marketed, is 700 mAh, and the thickness of the separator 4 is generally 25 to 27 μm. The battery of this example had a design capacity of 750 mAh having a higher capacity. For this reason, when the thickness of the separator 4 is larger than 25 μm, the wound electrode plate group cannot be reliably inserted into the case.
[0051]
FIG. 2 shows a structural diagram (partially sectional view) of a prismatic battery according to Examples 35 to 68 of the present invention.
[0052]
In FIG. 2, a non-aqueous electrolyte secondary battery 1 is wound with a positive electrode 2, a negative electrode 3, and a separator 4, and is built in a case 5 together with an electrolyte solution (not shown) in which an electrolyte salt is dissolved in a non-aqueous solvent. And is sealed with a sealing plate 6. Although the shape is different from the sealing plate 6 shown in FIG. 1, the function is the same. Except that the separator 4 is fixed with TD by an adhesive tape 7 made of polypropylene (PP), it is the same as the battery fabrication of Examples 1 to 34.
[0053]
As the separator 4, 12 kinds of separators N to Y shown in the above-mentioned (Table 1) were used.
[0054]
<Example>
Separator A was assembled into a battery together with 3.8 ml of an electrolyte containing 5% by weight of biphenyl. This battery is referred to as the battery of Example 1. Hereinafter, 13 types of separators A to M shown in (Table 1), aromatic additives and their addition amounts are combined as shown in (Table 2) and (Table 3), and Examples 2 to 33 are combined. Assemble the battery. Further, the positive and negative electrode plates were wound through the separator N. The separator TD was fixed with PP tape, and the battery was assembled together with 2.5 ml of an electrolyte containing 5% by weight of biphenyl. This battery is referred to as battery of Example 34. Furthermore, 12 types of separators N to Y shown in (Table 1), a fragrance additive and the amount of addition thereof are combined as shown in (Table 2) and (Table 3), and Examples 35 to 67 are combined. Assemble the battery.
[0055]
<Comparative example>
The separator A was used in the same manner as in Example 1, and the battery was assembled together with an electrolyte solution to which no aromatic additive was added. This is referred to as Comparative Example 1. Further, Comparative Examples 2 to 7 were prepared by combining 13 types of separators A to M shown in (Table 1), aromatic additives, and addition amounts thereof shown in (Table 2) and (Table 3). Assemble the battery. Further, the separator N was used in the same manner as in Example 34, and a battery was assembled together with an electrolyte solution to which no aromatic additive was added. This is designated as Comparative Example 8. Furthermore, 12 types of separators N to Y shown in (Table 1), aroma additives and their addition amounts in combinations shown in (Table 2) and (Table 3), Comparative Examples 9 to 14 I built a battery.
[0056]
<Battery evaluation>
A total of 83 of these batteries were evaluated by the method described below.
[0057]
The design capacity of the battery is 750 mA. First, after charging to 4.2 V at a constant current of 380 mA, a charge / discharge cycle of discharging to 3.0 V at a constant current of 380 mA was repeated 10 cycles. The discharge capacity at the 10th cycle was defined as the initial capacity of each battery. The initial capacity of all 83 batteries satisfied the design capacity. Moreover, charging / discharging was performed in a 20 degreeC thermostat. After that, each battery was charged up to 4.2V at a constant current of 380 mA, and further an overcharge test was conducted for 3 hours at a constant current of 750 mA. The surface temperature of the battery in this process was measured, Evaluated. These results are also shown in (Table 2) and (Table 3).
[0058]
[Table 2]
[0059]
[Table 3]
[0060]
As apparent from (Table 2), in the batteries of Examples 1 to 33, the abnormal temperature increase was suppressed despite the fact that the separator was thin. In all the batteries of the examples, current was flowing in a state where voltage was applied, and a minute internal short circuit occurred in the separator. On the other hand, all the batteries of the comparative examples had abnormally high temperatures.
[0061]
Although all the electrode plate dimensions used and the room temperature dimensions of the separators were the same, there were some batteries that caused abnormal temperature rise and others that did not occur like the batteries in the examples and comparative examples, as described above. This is because the shrinkage rate is different.
[0062]
Examples 1 to 33 All the additives (biphenyl, furan, indole, 3-chloro-thiophene) were added in 2.5% by weight and in 5% by weight, and there was no difference in effect.
[0065]
From Example 34 66 In this battery, the abnormal temperature rise was suppressed despite the thin separator. In all the batteries of the examples, current was flowing with voltage applied, and a minute internal short circuit occurred in the separator. On the other hand, all of the batteries of the comparative examples had an abnormal temperature increase.
[0066]
Although all the electrode plate dimensions and separators used had the same room temperature dimensions, there were batteries that did not rise abnormally and batteries that did not occur like the batteries of the examples and comparative examples. This is because the TD of the separator is fixed.
[0067]
From Example 34 66 For all the additives (biphenyl, furan, indole, 3-chloro-thiophene), the addition amount was 2.5% by weight and 5% by weight, and there was no difference in effect.
[0068]
【Effect of the invention】
As described above, according to the present invention, it is possible to improve the safety of the nonaqueous electrolyte secondary battery under high temperature conditions despite the use of a thin separator.
[Brief description of the drawings]
FIG. 1 is a schematic view (cross-sectional view) of a prismatic battery used in an example of the present invention.
FIG. 2 is a conceptual diagram (partial cross-sectional view) of a prismatic battery used in an example of the present invention.
[Explanation of symbols]
1 Nonaqueous electrolyte secondary battery
2 Positive electrode
3 Negative electrode
4 Separator
5 cases
6 Sealing plate
7 TD fixing member

Claims (2)

A nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a nonaqueous electrolyte in which an electrolyte salt is dissolved in a nonaqueous solvent, and a separator,
The non-aqueous electrolyte is a mixture of at least one aromatic additive selected from biphenyl, furan, indole and 3-chlorothiophene, and the separator is made of a porous polyolefin and further mechanically stretched. in after holding for 15 minutes in air at a temperature of 120 ° C. in a state that gave a longitudinal tensile load of 25 kg / cm ^2, mechanical stretched width direction of shrinkage has 12% to 20%, wherein A non-aqueous electrolyte secondary battery, wherein the separator has a thickness of 8 to 20 μm . A nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a nonaqueous electrolyte in which an electrolyte salt is dissolved in a nonaqueous solvent, and a separator,
The non-aqueous electrolyte is a mixture of at least one aromatic additive selected from biphenyl, furan, indole and 3-chlorothiophene, and the separator is made of a porous polyolefin and further mechanically stretched. After holding for 15 minutes at a temperature of 120 ° C. in the atmosphere with a tensile load of 25 kg / cm ^{2 in} the longitudinal direction, the contraction rate in the width direction of the machine stretch is 28% to 35 %, and The separator is held in an electrode group arranged between a positive electrode and a negative electrode by a mechanically stretched width direction by an insulating member having a higher heat resistance compared to the separator, and the separator has a thickness of 8 To 20 μm, a non-aqueous electrolyte secondary battery.