Patent Application
20100330405
The present disclosure relates to nonaqueous electrolyte secondary batteries typified by lithium ions secondary batteries, and more particularly to a nonaqueous electrolyte secondary battery with high safety.
Lithium ion secondary batteries, which have been widely used as power sources of mobile electronic equipment in recent years, use carbonaceous materials, for example, allowing insertion and extraction of lithium as active materials for negative electrode plates, and also use complex oxides of transition metals and lithium, such as LiCoO2, as active materials for positive electrode plates, thereby achieving high potential and high discharge capacity. However, with a recent increase in the number of functions of electronic equipment and communication equipment, the capacity of lithium ion secondary batteries needs to be further increased.
Therefore, to increase the capacity of a lithium ion secondary battery, a positive electrode current collector and a negative electrode current collector are coated with active material layers, and then dried, and the resultant structure is compressed by, for example, pressing to a predetermined thickness. With this technique, the active material density is increased, thereby further increasing the capacity.
A nonaqueous electrolyte secondary battery is fabricated by housing, in a battery case, an electrode group formed by stacking or winding a positive electrode plate and a negative electrode plate with a porous insulating layer interposed therebetween, then pouring a nonaqueous electrolyte into the battery case, and then sealing an opening of the battery case with a sealing plate.
With the increase in the capacity, serious consideration needs to be given to safety measures. In particular, when an internal short circuit occurs between the positive electrode plate and the negative electrode plate, a rapid temperature rise of the battery might occur. Therefore, safety enhancement of nonaqueous electrolyte secondary batteries is strongly demanded. Since a temperature rise is very rapid especially in large-size high-power nonaqueous electrolyte secondary batteries, these batteries need measures for safety improvement.
An internal short circuit of a nonaqueous electrolyte secondary battery seems to be caused by breakage or buckling of electrode plates as well as contamination of foreign substances in the battery. Specifically, as illustrated in
More specifically, in winding the electrode plates in a spiral to form the electrode group 30, a tensile stress is applied to the positive electrode plate 24, the negative electrode plate 28, and the porous insulating layer 29. At this time, one of these components exhibiting the lowest degree of extension is broken first. In addition, while the nonaqueous electrolyte secondary battery is charged and discharged, a stress induced by expansion and contraction of the electrode plates is applied to the electrode plates, and repetitive charging and discharging of the battery causes one of the components of the battery exhibiting the lowest degree of extension to be broken first.
For example, as illustrated in
Further, if the positive electrode plate 24 or the negative electrode plate 28 is broken before the porous insulating layer 29 is broken, the broken portion of the electrode plate penetrates the porous insulating layer 29, thereby causing a short circuit between the positive electrode plate 24 and the negative electrode plate 28. This short circuit causes large current to flow, resulting in that the temperature of the nonaqueous electrolyte secondary battery might increase rapidly.
To reduce breakage of the positive electrode plate, Patent Document 1 describes the following technique. Specifically, as illustrated in
PATENT DOCUMENT 1: Japanese Patent Publication No. 2007-103263
With the foregoing technique, however, although breakage of the positive electrode plate caused by a bending stress applied to the positive electrode plate in forming the electrode group is reduced, it is still difficult to reduce breakage or buckling of the electrode plate caused by a stress due to expansion and contraction of the electrode plate during charging and discharging of the nonaqueous electrolyte secondary battery. In addition, in the technique, two types of the positive electrode material mixture slurry need to be respectively prepared for the front and back surfaces of the positive electrode plate and to be applied onto the positive electrode current collector. Accordingly, the process of fabricating the positive electrode plate is complicated.
It is therefore a main object of the present invention to provide a nonaqueous electrolyte secondary battery with high safety in which breakage of a positive electrode plate or buckling of a negative electrode plate occurring during charging and discharging is reduced by reducing a stress induced by expansion and contraction of the negative electrode plate during charging and discharging of the nonaqueous electrolyte secondary battery.
To achieve the object, the present invention employs a configuration in which the degree of extension of the positive electrode plate is increased so as to follow expansion and contraction of the negative electrode plate during charging and discharging such that the degrees of expansion and contraction of the positive electrode plate and the negative electrode plate match each other during charging and discharging.
Specifically, a nonaqueous electrolyte secondary battery in an aspect of the present invention includes an electrode group in which a positive electrode plate including a positive electrode current collector and a positive electrode material mixture layer formed on the positive electrode current collector, and a negative electrode plate including a negative electrode current collector and a negative electrode material mixture layer formed on the negative electrode current collector, are wound or stacked with a separator interposed therebetween, wherein the positive electrode material mixture layer has at least one thin portion extending perpendicularly to a longitudinal direction of the positive electrode plate.
With this configuration, the degrees of expansion and contraction of the positive electrode plate and the negative electrode plate match each other during charging and discharging. Accordingly, a stress due to a difference in the degree of expansion and contraction between the positive electrode plate and the negative electrode plate during charging and discharging can be reduced, resulting in reducing breakage or buckling of the electrode plates.
In another aspect of the present invention, the thin portion of the positive electrode material mixture layer is preferably located on at least an inner surface of the positive electrode current collector.
In another aspect of the present invention, the thin portion of the positive electrode material mixture layer is preferably located on each surface of the positive electrode current collector, and the thin portion located on an inner surface of the positive electrode current collector and the thin portion located on an outer surface of the positive electrode current collector preferably have phases which are shifted from each other.
In another aspect of the present invention, the thin portion of the positive electrode material mixture layer is preferably located on each surface of the positive electrode current collector, and the thin portion located on an inner surface of the positive electrode current collector preferably has a width lager than a width of the thin portion located on an outer surface of the positive electrode current collector. The positive electrode material mixture layer may include multiple ones of the at least one thin portion, and widths of the thin portions may decrease from a winding center to a winding end of the electrode group.
In another aspect of the present invention, the positive electrode material mixture layer preferably includes multiple ones of the at least one thin portion, the positive electrode material mixture layer is preferably formed on each surface of the positive electrode current collector, and a distance between each adjacent ones of the thin portions located on an inner surface of the positive electrode current collector is smaller than a distance between each adjacent ones of the thin portions located on an outer surface of the positive electrode current collector. The positive electrode material mixture layer may include multiple ones of the at least one thin portion, and a distance between each adjacent ones of the thin portions may increase from a winding center to a winding end of the electrode group.
In another aspect of the present invention, the thin portion of the positive electrode material mixture layer is preferably located at least at a position where the positive electrode material mixture layer has a small radius of curvature near a winding center of the electrode group.
In another aspect of the present invention, instead of the thin portion, the positive electrode material mixture layer preferably has at least one low-density active material portion extending perpendicularly to the longitudinal direction of the positive electrode plate.
According to the present invention, the degrees of expansion and contraction of the positive electrode plate and the negative electrode plate match each other during charging and discharging. Accordingly, a stress due to a difference in the degree of expansion and contraction between the positive electrode plate and the negative electrode plate during charging and discharging can be reduced, thereby reducing breakage or buckling of the electrode plates. As a result, a nonaqueous electrolyte secondary battery with high safety in which an internal short circuit due to the foregoing problems is reduced.
Embodiments of the present invention will be described in detail hereinafter with reference to the drawings. It should be noted that the present invention is not limited to the following embodiments. Various changes and modifications may be made without departing from the scope of the present invention, and the following embodiments may be combined as necessary.
As illustrated in
As illustrated in
The number and the shape (e.g., thickness, width, and pitch) of the thin portions 3a and 3b provided in parts of the positive electrode material mixture layers 2a and 2b are not specifically limited, and may be determined according to the degree of extension of the negative electrode plate 8 to be used.
Referring now to
The thin portions 3a and 3b can be formed in a series of processes of forming the positive electrode material mixture layers on the surfaces of the positive electrode current collector 1. Specifically, in coating the surface of the positive electrode current collector 1 with positive electrode material mixture slurry by using a die coater, the pressure in a manifold of a die is adjusted to be negative, and the amount of the positive electrode material mixture slurry to be discharged from an end of the die is adjusted, thereby forming thin portions 3a and 3b thinner than the other portions of the positive electrode material mixture layers 2a and 2b. At this time, after adjusting the pressure in the manifold of the die to a negative value, the pressure is released so as to adjust the timing at which the positive electrode material mixture slurry is discharged again, thereby forming thin portions 3a and 3b having a uniform width along the direction perpendicular to the longitudinal direction of the positive electrode plate 4.
Thereafter, the positive electrode material mixture slurry is dried, and then the positive electrode material mixture layers 2a and 2b are pressed to thicknesses not smaller than those of the thin portions 3a and 3b. Subsequently, the positive electrode current collector 1 is subjected to a slitter process to have a predetermined width and a predetermined length, thereby obtaining a positive electrode plate 4 in the shape of a long strip.
The positive electrode material mixture slurry is obtained by mixing and dispersing a positive electrode active material, a conductive agent, and a binder in a dispersion medium, and the resultant mixture is kneaded, while being adjusted to have an optimum viscosity for application onto the positive electrode current collector 1.
Examples of the positive electrode active material include complex oxides such as lithium cobaltate, denatured lithium cobaltate (e.g., a substance in which aluminium or magnesium is dissolved in lithium cobaltate), lithium nickelate, denatured lithium nickelate (e.g., a substance in which nickel partially substitutes for cobalt), lithium manganate, and denatured lithium manganate.
As the conductive agent, carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and various types of graphite may be used solely or a two or more of these materials may be used in combination, for example.
Examples of the binder include polyvinylidene fluoride (PVdF), denatured polyvinylidene fluoride, polytetrafluoroethylene (PTFE), and rubber particle binder containing acrylate units. Acrylate monomer to which a reactive functional group is introduced or acrylate oligomer may be mixed in the binder.
The negative electrode plate 8 can be fabricated with a general technique as described below.
First, a negative electrode active material and a binder are mixed and dispersed in a dispersion medium, and the resultant mixture is kneaded while being adjusted to have an optimum viscosity for application onto the negative electrode current collector 5, thereby obtaining negative electrode mixture material slurry.
Examples of the negative electrode active material include various types of natural graphite, artificial graphite, silicon-based composite materials such as silicide, and various types of alloy composition materials.
Examples of the binder include polyvinylidene fluoride, and denatured polyvinylidene fluoride. To enhance lithium ion acceptability, styrene-butadiene rubber particles (SBR), denatured SBR, and cellulose-based resin such as carboxymethyl cellulose (CMC) are also preferably used or a material obtained by adding a small amount of such materials to the styrene-butadiene rubber particles or the denatured styrene-butadiene rubber particles is preferably used.
The obtained negative electrode mixture material slurry is applied onto the surface of the negative electrode current collector 5, is dried, and then is pressed to a predetermined thickness, thereby forming a negative electrode material mixture layer. Thereafter, the negative electrode material mixture layer is subjected to a slitter process to have a predetermined width and a predetermined length, thereby obtaining a negative electrode plate 8 in the shape of a long strip.
As illustrated in
In winding the positive electrode plate 4 and the negative electrode plate 8 with the separator 9 interposed therebetween to form an electrode group, a difference in curvature causes a tensile stress to be applied to an outer negative electrode material mixture layer 6a of the negative electrode plate 8, and also causes a compressive stress to an inner negative electrode material mixture layer 6b of the negative electrode plate 8.
Under this situation, as illustrated in
Further, in the electrode group formed by winding the positive electrode plate 4 and the negative electrode plate 8 with the separator 9 interposed therebetween, the curvatures of the electrode plates gradually decrease from the winding center to the winding end of the electrode group. Accordingly, the tensile stress applied to the outer negative electrode material mixture layer 6a and the compressive stress applied to the inner negative electrode material mixture layer 6b described above gradually decrease.
In view of this phenomenon, as illustrated in
Advantages obtained by adjusting the widths of the thin portions 3a and 3b as illustrated in
Specifically, as illustrated in
In addition, as illustrated in
In the first embodiment, the thin portions 3a and 3b are provided in parts of the positive electrode material mixture layers 2a and 2b so that a stress to the positive electrode plate 4 induced by expansion and contraction of the negative electrode plate 8 is reduced. On the other hand, in the second embodiment, a portion having a low density of an active material (hereinafter referred to as a low-density active material portion) is provided in parts of the positive electrode material mixture layers 2a and 2b, thereby obtaining similar advantages to those obtained in the first embodiment.
Specifically, since expansion and contraction of the negative electrode plate 8 are caused by insertion and extraction of lithium in/from the negative electrode active material layer, the presence of low-density active material portions are provided in parts of the positive electrode material mixture layers 2a and 2b of the positive electrode plate 4 facing the negative electrode plate 8 can locally reduce the amount of insertion and extraction of lithium in/from the negative electrode active material layer. In this manner, expansion and contraction of the negative electrode plate 8 can be reduced, thereby reducing a stress to the positive electrode plate 4 induced by expansion and contraction of the negative electrode plate 8.
First, as illustrated in
Next, as illustrated in
This embodiment differs from the first embodiment in that the positive electrode material mixture layers 2a and 2b are pressed to thicknesses not smaller than those of the thin portions 3a and 3b in the first embodiment, whereas the positive electrode material mixture layers 2a and 2b are pressed to thicknesses smaller than those of the thin portions 3a and 3b in the second embodiment. Accordingly, in the second embodiment, the surface of each of the positive electrode material mixture layers 2a and 2b is plane. Therefore, in this embodiment, the diameter of an electrode group formed by winding the positive electrode plate 4 and a negative electrode plate 8 with a separator 9 interposed therebetween can be smaller than that of the electrode group of the first embodiment.
The number and the shape (e.g., thickness, width, and pitch) of the low-density active material portions 7a and 7b provided in parts of the positive electrode material mixture layers 2a and 2b are not specifically limited, and may be determined according to the degree of expansion and contraction of the negative electrode plate 8 to be used.
Referring now to
Alternatively, as illustrated in
It should be recognized that the foregoing description has been set forth for purposes of preferred embodiments of the present invention, and is not intended to limit the scope of the invention, and various changes and modifications may be made. For example, in the above embodiments, the electrode group is formed by winding the positive electrode plate and the negative electrode plate with the separator interposed therebetween. Alternatively, the electrode group may be formed by stacking the positive electrode plate and the negative electrode plate with the separator interposed therebetween.
Configurations and advantages of the present invention will be further described hereinafter based on examples. However, it should be noted that the present invention is not limited to the following examples.
First, 100 parts, by weight, of lithium cobaltate as an active material, 2 parts, by weight, of acetylene black as a conductive agent, and 2 parts, by weight, of polyvinylidene fluoride as a binder were stirred and kneaded with an appropriate amount of n-methyl-2-pyrrolidone, thereby producing positive electrode material mixture slurry.
Next, as illustrated in
Thereafter, the positive electrode plate 4 was pressed to a total thickness of 165 μm, thereby allowing each of the positive electrode material mixture layers 2a and 2b to have a thickness of 75 μm. Subsequently, the resultant positive electrode plate 4 was subjected to a slitter process to have a predetermined width, thereby obtaining a positive electrode plate 4.
On the other hand, 100 parts, by weight, of artificial graphite as a negative electrode active material, 2.5 parts, by weight, (1 part, by weight, in terms of the solid content of a binder) of a styrene-butadiene rubber particle dispersing element (solid content: 40 parts, by weight) as a binder, and 1 part, by weight, of carboxymethyl cellulose as a thickener were stirred with an appropriate amount of water, thereby producing negative electrode material mixture slurry.
Next, the negative electrode mixture material slurry was applied onto a negative electrode current collector 5 made of copper foil (with a Cu purity of 99.9%) having a thickness of 10 μm, and the resultant structure was dried. In this manner, a negative electrode plate 8 in which negative electrode material mixture layers 6a and 6b each had a thickness of 110 μm was obtained. Subsequently, this negative electrode plate 8 was pressed to have a total thickness of 180 μm, and then was subjected to a slitter process to have a predetermined width, thereby obtaining a negative electrode plate 8.
The positive electrode plate 4 and negative electrode plate 8 thus obtained were wound in a spiral with a separator 9 of a polyethylene microporous film with a thickness of 20 μm interposed therebetween, thereby forming an electrode group 10. This electrode group 10 was housed in a closed-end cylindrical battery case 11, and then a nonaqueous electrolyte in which 1 M of LiPF6 and 3 parts, by weight, of VC were dissolved in a predetermined amount of an EC, DMC, and MEC mixture solvent was poured in this battery case 11. Thereafter, an opening of the battery case 11 was sealed with the sealing plate 15, thereby obtaining a cylindrical lithium ion secondary battery 17 illustrated in
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In each of the first to thirteenth examples, 100 lithium ion secondary batteries 17 were fabricated, and 500 cycles of charging and discharging were performed. However, no cycle deterioration occurred. Further, 20 batteries were taken from the 100 lithium ion secondary batteries 17 subjected to 500 cycles of charging and discharging, and electrode groups 10 of these 20 batteries were disassembled. Then, no failures such as lithium precipitation, breakage and buckling of electrode palates, and peeling-off of electrode mixture layers were observed.
The present invention is useful for batteries for use as power sources of mobile equipment which needs to have its capacity increased with an increase in the number of functions of electronic equipment and communication equipment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-162909 | Jun 2008 | JP | national |
| 2008-165564 | Jun 2008 | JP | national |
| 2008-202818 | Aug 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/002769 | 6/18/2009 | WO | 00 | 8/5/2010 |
