JP2007066667A - Large-capacity secondary battery excellent in safety in internal short circuit and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a large-capacity secondary battery which is safe even when an internal short circuit occurs; and to provide its manufacturing method. <P>SOLUTION: This secondary battery has a flat shape where the area of the largest surface is not smaller than 50 cm<SP>2</SP>and volume energy density is not smaller than 400 Wh/L. A value obtained by dividing the current capacity of the battery by the volume of a negative electrode collector of a short circuit part is not greater than 30 mAh/mm<SP>3</SP>. This manufacturing method is used for manufacturing the secondary battery having the flat shape where the area of the largest surface is not smaller than 50 cm<SP>2</SP>and volume energy density is not smaller than 400 Wh/L. The safety in the internal short circuit is enhanced by setting the volume of the negative electrode collector with respect to the current capacity of the battery not smaller than a predetermined value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-capacity secondary battery that is safe even when an internal short circuit occurs, and a method for manufacturing the same, and more specifically, a flat plate-shaped large-capacity secondary battery that is safe even when an internal short circuit has a surface area of a certain value or more. The present invention relates to a battery and a manufacturing method thereof.

In recent years, along with the reduction in size and weight of electronic devices, it is not possible to secure a sufficient battery space in the devices. Therefore, there is a demand for a battery with a high degree of freedom in shape as well as a demand for a large capacity. For example, in a portable product such as a notebook computer, a plurality of thin lithium secondary batteries are arranged on the back side of the liquid crystal screen (see the upper diagram of FIG. 1). However, when the assembled battery is placed on the back side of the liquid crystal screen, there is a problem that a temperature difference occurs between the batteries due to the difference in the battery position from the cold cathode tube for ensuring the brightness of the liquid crystal screen. The difference in battery temperature has a great effect on the cycle characteristics of each battery and causes variations in battery life (see the lower diagram of FIG. 1).
Therefore, it is expected to realize a large capacity flat battery as shown in FIG. However, the large capacity battery as shown in FIG. 2 has a problem in terms of safety because the current flowing through the internal short circuit portion is extremely large, and the battery is easily ruptured and ignited.

  By the way, there are UL (Underwriters Laboratories Inc.) in the United States and battery industry associations as standards for battery safety in Japan, and it is generally said that there is no problem if the UL test is passed. However, the flat battery as shown in FIG. 2 has failed to pass the UL collision test.

  In order to pass the collision test, by constructing a battery (Patent Document 1) having an electrode having a current collector that is electrically separated into a plurality of parts and a case made of a material such as stainless steel, It can be considered that the battery has improved impact resistance and corrosion resistance (Patent Document 2). However, since the number of parts increases the cost of the battery, a means for passing the collision test without increasing the cost has been demanded.

JP-A-10-172574 JP-A-5-74423

When the battery temperature rises, it reaches a dangerous state such as smoke, fire, or explosion, so it is desirable to keep the battery temperature below a certain level in order to improve safety. Among them, lithium secondary batteries have a large discharge capacity, and when the battery temperature rises for some reason, the battery self-heats as a result, and the battery temperature further rises, so a countermeasure against heat is indispensable. Therefore, prior to the present invention, the inventors have noted that the risk associated with heat generation of the battery is related to the value of the surface area / volume, that is, the surface area = heat radiation area, and the volume = electric capacity stored in the battery. In addition, by setting the volume surface area ratio (surface area / volume) to 0.8 or more, it is possible to provide a safe large-capacity battery excellent in heat dissipation (Japanese Patent Application No. 2004-295105). However, it has not been able to cope with the problem of heat generation during an internal short circuit caused by an external impact such as a collision.
An object of the present invention is to provide a high-capacity secondary battery that is safe even when an internal short circuit occurs, and a method for manufacturing the same.

  The reason why the battery ruptures and ignites in the collision test of the secondary battery is that an internal short circuit occurs in the battery at the time of the collision, so that current concentrates in the short circuit part, generates heat, and decomposition of the electrolytic solution occurs. Fig. 3 shows the results of DSC (differential scanning calorimetry) measurements of the positive and negative electrodes of the lithium secondary battery in the charged state. The negative electrode can observe heat generation due to the SEI reaction at about 120 ° C (Reference (1)). (See Reference 2), and heat generation with oxygen generation can be observed at a temperature exceeding about 200 ° C. in the positive electrode (see reference (2)). In other words, ignition in a lithium secondary battery causes a negative electrode SEI (Solid electrolyte interface) reaction due to heat generation and the battery temperature rises (about 120 ° C), thereby causing an oxygen liberation reaction at the positive electrode and further raising the battery temperature ( Finally, the free oxygen and the substance in the electrolyte react and lead to ignition. Therefore, if the negative electrode heat generation temperature can be suppressed, the SEI decomposition temperature is not reached, and a safe battery that does not rupture or ignite even in the case of a short circuit is obtained.

The inventor said that in flat plate large-capacity batteries, as the area of the short-circuited portion becomes narrower, the current concentration at the time of the short-circuit becomes local, so the ease of rupture / ignition correlates with the area of the short-circuited portion. Thought. However, when a prototype was made and verified, the ease of rupture and ignition was not necessarily proportional to the area of the short circuit.
Therefore, the inventor considered that the degree of heat dissipation differs depending on the thickness of the current collector. Eventually, by producing prototypes of several hundred types of batteries, the battery that ruptures and ignites, when the volume of the negative electrode current collector of the short-circuited part with respect to the current capacity of the battery is above a certain level, The present inventors have found that no rupture / ignition occurs and have come to the present invention.

The first invention is a flat-plate-type secondary battery having an area of the largest surface of 50 cm ² or more and a volume energy density of 400 Wh / L or more, and satisfies the following formula 1 as a safety standard. This is a featured secondary battery.
The second invention is characterized in that, in the first invention, the area of the widest surface is 100 cm ² or more.
According to a third invention, in the first or second invention, the electrolyte layer is a non-fluid lithium secondary battery.
A fourth invention is a method of manufacturing a secondary battery having an area of the widest surface of 50 cm ² or more and a volume energy density of 400 Wh / L or more, wherein the volume of the negative electrode current collector relative to the current capacity of the battery is This is a method for manufacturing a flat-plate-shaped secondary battery that increases the safety at the time of an internal short circuit by setting it to a predetermined value or more.
A fifth invention is characterized in that, in the fourth invention, the volume of the negative electrode current collector with respect to the current capacity of the battery satisfies the following expression (2).

  According to the present invention, it is possible to provide a high-capacity secondary battery that is safe even when an internal short circuit occurs without increasing the number of parts such as a battery active material, a current collector, a separator, an electrolyte, a battery case, and a lead. Become.

As one embodiment of the battery of the present invention, a single cell made of a positive electrode, a negative electrode, and a separator, which is a plate-shaped unit battery element, is laminated and enclosed in a polymer-metal composite laminate film bag. FIG. 4 shows an example of a single cell structure. The current collector is an aluminum foil, and an improved transition metal lithium oxide (LiNi _1-xy Co _x (Met) _y O ₂ etc. Is a positive electrode made by impregnating a gel electrolyte with a transition metal selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, and V or a lanthanoid metal) A separator made of a porous polymer film on the lower side, a negative electrode made of a surface-modified graphite impregnated with a gel electrolyte, and a copper foil on the lower side. Although the positive electrode is positioned as a battery element on the upper side in the drawing, the negative electrode may be positioned.
In addition, this invention can be applied also to the winding type battery which wound the electrode group on a thin plate in the shape of a spiral, and can also be applied to the lithium secondary battery which does not gelatinize electrolyte solution.

  The battery of the present invention is configured by stacking a plurality of single cells in a housing. The housing is a container in which a container (for example, a pouch) is constituted by a polymer-metal composite laminate film, and a battery stacked inside can be vacuum-sealed. Encapsulation is performed mainly by heat-sealing polyolefin films. By using the laminate container, the external potential becomes neutral, and the safety at the time of crash can be further increased.

  In the present invention, the unit battery element in which the positive electrode plate, the separator, and the negative electrode plate are laminated is configured in the same manner as the conventional unit battery element. For example, the positive electrode plate is formed by applying and drying the above-described positive electrode active material on one side of the reaction part of the positive electrode current collector, and the negative electrode plate is formed by applying and drying the negative electrode active material as described above on both sides of the reaction part of the negative electrode current collector. Thus, the separator can be exemplified by a polyolefin porous film. Further, a positive electrode current collector is formed on the positive electrode plate, and a negative electrode current collector is formed on the negative electrode plate, and these are joined to the positive electrode terminal lead and the negative electrode terminal lead by ultrasonic welding or the like. This joining may be performed by resistance welding. However, the unit cell element of the present invention is not limited to these.

In the battery of the present invention, the negative electrode structure is composed of a negative electrode current collector and a negative electrode active material, and a negative electrode current collector having a thickness such that the current concentration parameter represented by the following formula 3 is less than ³⁰ mAh / mm ³ is used. It is characterized by doing. Here, the current capacity of the battery is the current capacity of the entire battery element housed in the battery case, and in the case of a stacked battery, is the current capacity of the bare cell. The negative electrode current collector volume of the short-circuited portion is a value obtained by multiplying the area of the portion subjected to external impact by the thickness of the negative electrode current collector.
The current concentration parameter can be used as an index as to whether or not a burst or ignition occurs when an internal short circuit occurs in the battery.

The present invention has a remarkable effect in a large-capacity battery having the largest flat plate area of 50 cm ² or more, preferably 100 cm ² or more and a volume energy density of 400 Wh / L or more. This is because the risk of rupture / ignition at the time of internal short circuit increases as the capacity increases. At this time, as disclosed in Japanese Patent Application No. 2004-295105, the volume surface area ratio (surface area / volume) is preferably 0.8 or more. This is because the danger associated with the heat generation of the battery is related to the value of (surface area = heat radiation area) / (volume = battery capacity).
The volume energy density (Wh / L) is a value obtained by dividing the energy that can be stored in the battery by the volume of the battery. The capacity of the unit cell that does not include the battery case and the current extraction lead may be 400 Wh / L or more. The assumed battery thickness is 10 mm or less, preferably 5 mm or less.

  Hereinafter, details of the present invention will be described with reference to examples, but the present invention is not limited to the examples.

Collision tests (UL1642 compliant) were conducted on 240 patterns of batteries with different negative electrode current collector thickness, vertical dimensions, and horizontal dimensions to verify the presence or absence of rupture and ignition. In Examples 1 to 80, the vertical dimension of the negative electrode current collector is 50 mm, Examples 81 to 160 are 100 mm, and Examples 161 to 240 are 150 mm. The thickness of each battery is about 1.5-5 mm.
In addition, the numerical value of the electric current value shown in the following examples shall mean the total electric current value unless there is a special description.

[Example 1]
(1) Production of Negative Electrode A slurry made of surface-modified graphite and polyvinylidene fluoride (PVdF) was applied on a 6 μm thick copper foil so that one side was 2.1 mAh / cm ² and dried. After adjusting this to a predetermined thickness, the length and width were cut to the dimensions shown in Table 1.

(2) Fabrication of positive electrode LiNi _1-xy Co _x (Met) _y O ₂ (Met is Al, Cr, Mn, Fe, Mg, La, Ce, etc.) so that it becomes 2.05 mAh / cm ² on a 15 μm thick aluminum foil. A slurry made of PVdF and a conductive additive (which is one or more elements of transition metals or lanthanoid metals selected from Sr and V) was applied and dried. After adjusting this to a predetermined thickness, the length and width were cut to the dimensions shown in Table 1.

(3) Manufacture of separator It cut to the same length and width dimension (Table 1) as the collector of the porous film made from polyethylene of 16 micrometers in thickness.

(4) Production of cell A unit cell having a negative electrode (single-sided electrode), a separator, and a positive electrode (single-sided electrode) as one unit was produced. This unit cell was sufficiently impregnated with an electrolytic solution, and then heated to obtain a polymer unit cell. After 14 sets of these were laminated, current extraction leads were attached and housed in an aluminum laminate case to produce a 1435 mAh battery.

(5) Initial charge / discharge
i) Charging The battery was constant-current charged to 4.2 V at 287 mA, which is 1/5 of the battery capacity (hereinafter referred to as 1/5 C), and constant voltage charged at 4.2 V. The charge termination condition was 8 hours from the start of charging.
ii) Discharge
A constant current was discharged to 3.0V at 1 / 5C. In this initial charge / discharge test, a battery having a capacity of 1435 mAh or more was confirmed as a good battery, and a collision test was conducted.

(6) Crash test (UL1642 compliant)
i) Lateral collision
A fully charged battery was prepared by charging at a constant current to 4.2V at 1 / 5C and charging at a constant voltage until it was reduced to 93mA, which was 1 / 15C current (hereinafter referred to as 1 / 15C) at 4.2V. As shown in Fig. 5, after placing a fully charged battery on a flat surface, place a round bar with a diameter of 15.8mm parallel to the electrode surface of the battery and at a right angle to the upper terminal direction of the battery, approximately at the center of the battery. A 9.1kgf heavy object was dropped on the round bar (lateral collision).
Here, a portion crushed by a round bar having a diameter of 15.8 mm becomes a short-circuit portion. 11mAh / mm ^{3 in} Table 1 is the current concentration parameter at the time of lateral collision calculated based on the above formula 1. The single cell capacity (103mAh) is divided by the volume (100mm x 6μm x 15.8mm). It is calculated.
As a result of testing the collision at n = 2, the battery of Example 1 was not ruptured or ignited.

ii) Longitudinal collision A new fully charged battery is prepared and, as shown in FIG. 6, after placing the fully charged battery on a flat surface, a round bar with a diameter of 15.8 mm is parallel to the electrode surface of the battery and The battery was placed almost in the center of the battery parallel to the upper terminal direction (rotated 90 degrees from FIG. 5), and a heavy material of 9.1 kgf was dropped on the round bar (longitudinal collision).
Here, a portion crushed by a round bar having a diameter of 15.8 mm becomes a short-circuit portion. 22mAh / mm ^{3 in} Table 1 is the current concentration parameter at the time of vertical collision calculated based on the above formula 1. The single cell capacity (103mAh) is divided by the volume (vertical 50mm × 6μm × 15.8mm). It is calculated.
As a result of testing the collision at n = 5, the battery of Example 1 was not ruptured or ignited.

[Examples 2 to 5]
A battery shown in Table 1 in which the battery in Example 1 and the positive electrode and negative electrode current collectors in different lateral sizes were produced, and a collision test was performed in the same procedure as in Example 1.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of conducting a longitudinal collision test at n = 2, all batteries were ruptured and ignited.
The current concentration parameter in the lateral direction of the batteries of Examples 2 to 5 was less than ³⁰ mAh / mm ³ , but the current concentration parameter in the vertical direction was 30 mAh / mm ³ or more.

[Examples 6 to 10]
A battery having the negative electrode current collector (copper foil) thickness of 8 μm and having the specifications shown in Table 2 was produced in the same procedure as in Example 1, and a collision test was performed.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing a longitudinal collision test with n = 2, no bursting / ignition occurred in Examples 6 and 7, but bursting / ignition occurred in Examples 8-10.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 11 to 15]
A battery having the negative electrode current collector (copper foil) thickness of 10 μm and having the specifications shown in Table 3 was produced in the same procedure as in Example 1, and a collision test was performed.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing a longitudinal collision test with n = 2, no rupture / ignition occurred in Examples 11 to 13, but rupture / ignition occurred in Examples 14 and 15.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 16 to 20]
A battery having the negative electrode current collector (copper foil) thickness of 12 μm and having the specifications shown in Table 4 was produced in the same procedure as in Example 1 and subjected to a collision test.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing a longitudinal collision test at n = 2, no rupture / ignition occurred in Examples 16 to 19, but rupture / ignition occurred in Example 20.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 21 to 25]
(1) Production of negative electrode A battery having the specifications shown in Table 5 in the same procedure as in Example 1, except that the negative electrode (copper foil) is 2.63 mAh / cm ² on one side and the positive electrode (aluminum foil) is 2.48 mAh / cm ^2. Was manufactured and a collision test was performed.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing the collision test in the lateral direction at n = 2, no rupture / ignition occurred in Example 21, but rupture / ignition occurred in Examples 22-25.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 26 to 30]
A battery having the negative electrode current collector (copper foil) thickness of 8 μm and having the specifications shown in Table 6 was produced in the same procedure as in Example 1 and subjected to a collision test.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing a longitudinal collision test at n = 2, no burst / ignition occurred in Examples 26 and 27, but burst / ignition occurred in Examples 28-30.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 31 to 35]
A battery having the negative electrode current collector (copper foil) thickness of 10 μm and having the specifications shown in Table 7 was produced in the same procedure as in Example 1 and subjected to a collision test.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing a longitudinal collision test at n = 2, no rupture / ignition occurred in Examples 31 and 32, but rupture / ignition occurred in Examples 33 to 35.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 36 to 40]
A battery having the negative electrode current collector (copper foil) thickness of 12 μm and having the specifications shown in Table 8 was produced in the same procedure as in Example 1 and subjected to a collision test.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing a longitudinal collision test at n = 2, no burst / ignition occurred in Examples 36 to 38, but burst / ignition occurred in Examples 39 and 40.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 41 to 60]
A battery having the specifications shown in Table 9 was prepared in the same procedure as in Example 1, except that the negative electrode (copper foil) was 3.17 mAh / cm ² on one side and the positive electrode (aluminum foil) was 3.02 mAh / cm ^2. Went.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing a longitudinal collision test at n = 2, no rupture / ignition occurred in Examples 46, 51, 52, 56, and 57, but Examples 41 to 45, 47 to 50, 53 to 55, For 58-60, rupture and ignition occurred.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 61 to 80]
A battery with the specifications shown in Table 10 was prepared in the same procedure as in Example 1, except that the negative electrode (copper foil) was 3.73 mAh / cm ² on one side and the positive electrode (aluminum foil) was 3.55 mAh / cm ^2. Went.
As a result of performing a lateral collision test with n = 2, no rupture / ignition occurred in all the batteries.
As a result of performing the longitudinal collision test at n = 2, no rupture / ignition occurred in Examples 66, 71, 76, and 77, but Examples 61 to 65, 67 to 70, 72 to 75, and 78 to For 80, rupture and ignition occurred.
The current concentration parameter of the batteries that burst and ignited was ³⁰ mAh / mm ³ or more, and the current concentration parameters of the other batteries were less than ³⁰ mAh / mm ³ .

[Examples 81 to 160]
Except that the longitudinal dimension (L) of the negative electrode (copper foil) was 100 mm, batteries having the specifications shown in Tables 11 to 14 were prepared in the same procedure as in Examples 1 to 80, and a collision test was performed. Examples 1 to 80 correspond to 80 + n examples, respectively.
As a result of performing a lateral collision test with n = 2 and a longitudinal collision test with n = 2, the current concentration parameter of the battery that had ruptured or ignited was ³⁰ mAh / mm ³ or more. current concentration parameter could be confirmed to be less than 30 mAh / mm ^3.

[Examples 161 to 240]
Except that the longitudinal dimension (L) of the negative electrode (copper foil) was 150 mm, batteries having the specifications shown in Tables 15 to 18 were produced in the same procedure as in Examples 1 to 80, and a collision test was performed. Examples 1 to 80 correspond to 160 + n examples, respectively.
As a result of performing a lateral collision test with n = 2 and a longitudinal collision test with n = 2, the current concentration parameter of the battery that had ruptured or ignited was ³⁰ mAh / mm ³ or more. current concentration parameter could be confirmed to be less than 30 mAh / mm ^3.

[Summary]
In the batteries of Examples 1 to 240, the batteries having a current concentration parameter of less than ³⁰ mAh / mm ³ in the collision test did not cause the battery temperature to exceed 130 ° C. even after the collision, and neither rupture nor ignition occurred. Batteries with a current concentration parameter of 30 mAh / mm ³ or more burst and ignited over 130 ° C in about 2 seconds after the collision.

It is explanatory drawing at the time of arrange | positioning multiple lithium secondary batteries on the back side of a liquid crystal screen. It is explanatory drawing at the time of arrange | positioning a large format lithium secondary battery in the back side of a liquid crystal screen. It is explanatory drawing of the ignition mechanism in a secondary battery to lithium. It is the figure which showed an example of the single cell structure of the battery which concerns on this invention. It is explanatory drawing of the embodiment of the horizontal direction of a collision test (UL1642). It is explanatory drawing of the embodiment of the vertical direction of a collision test (UL1642).

Claims (5)

A planar secondary battery having the widest surface area of 50 cm ² or more and a volume energy density of 400 Wh / L or more,
A secondary battery characterized by satisfying the following formula 1 as a safety standard.
The secondary battery according to claim 1, wherein the area of the widest surface is 100 cm ² or more.   The secondary battery according to claim 1 or 2, wherein the electrolyte layer is a non-fluid lithium secondary battery. A method for manufacturing a flat-plate-shaped secondary battery having an area of the widest surface of 50 cm ² or more and a volume energy density of 400 Wh / L or more,
A method for manufacturing a secondary battery, wherein the volume of the negative electrode current collector with respect to the current capacity of the battery is set to a predetermined value or more to increase safety during an internal short circuit. The method for producing a secondary battery according to claim 4, wherein the volume of the negative electrode current collector with respect to the battery current capacity satisfies the following formula 2.