JP2009187724A - Winding type lithium ion secondary battery - Google Patents

Abstract

Provided is a wound lithium ion secondary battery capable of suppressing a short circuit between positive and negative electrodes even when a direct tab is twisted.
A lithium ion secondary battery has a bottomed cylindrical battery container. An electrode group is accommodated in the battery container. In the electrode group, the positive electrode plate 17 and the negative electrode plate 18 are wound around the shaft core via the separator 16 so that the two electrode plates do not directly contact each other. The positive electrode direct tab 2 extending from the positive electrode plate 17 has an extended end portion joined to the periphery of the flange portion of the positive electrode current collecting ring 4. For the separator 16, a porous polyolefin film having a static friction coefficient of 0.6 or less is used. Even if the positive electrode direct tab 2 is twisted and contacts the separator 16, the positive electrode direct tab 2 slides on the surface of the separator 16.
[Selection] Figure 2

Description

  The present invention relates to a wound-type lithium ion secondary battery, and more particularly, a winding provided with an electrode group in which positive and negative electrodes each having a current collecting direct tab formed at a longitudinal end portion of a metal foil are wound through a separator. The present invention relates to a rotary lithium ion secondary battery.

  A wound lithium ion secondary battery in which positive and negative electrodes are wound through a separator is used in various applications. Above all, taking advantage of the high energy density, it is used in portable devices such as VTR cameras, notebook computers and mobile phones. On the other hand, research and development of large-scale wound lithium ion secondary batteries for high-capacity, high-power, and high-power density are being actively conducted for electric vehicles and power storage. In particular, in the automobile industry, development of electric vehicles using a motor as a power source and hybrid electric vehicles using both an internal combustion engine and a motor is underway in order to deal with environmental problems. The part has already been put to practical use. As such a power source for an electric vehicle, a wound lithium ion secondary battery has attracted attention.

  In a large-sized wound lithium ion secondary battery intended for high output, it is important to reduce the internal resistance of the battery including the current collecting portion in order to improve output characteristics. In order to reduce the resistance of the current collecting portion, it is effective to collect current using a direct tab formed at the end in the longitudinal direction of the metal foil (see, for example, Patent Document 1).

JP2002-134095

  However, when a direct tab is used for current collection, the direct tab may be twisted during winding in the manufacturing process or due to expansion / contraction of the electrode accompanying charging / discharging of the battery. If a certain level of twist is applied to the direct tab, the separator may be pressed and damaged. In this case, there is a possibility that the positive and negative electrodes are short-circuited, leading to a significant deterioration in battery characteristics and a malfunction. Furthermore, the pressure inside the battery rises due to heat generated by the short circuit, and there is a possibility that a high-temperature electrolytic solution or the like is discharged outside the battery.

  An object of the present invention is to provide a wound lithium ion secondary battery that can suppress a short circuit between positive and negative electrodes even if the direct tab is twisted.

  In order to solve the above problems, the present invention provides a wound lithium ion comprising an electrode group in which positive and negative electrodes each having a current collecting direct tab formed on a longitudinal end of a metal foil are wound through a separator. In the secondary battery, the separator has a static friction coefficient of 0.6 or less.

  In the present invention, by setting the coefficient of static friction of the separator to 0.6 or less, even if the direct tab twists and contacts the separator when the positive and negative electrodes are wound or charged / discharged, the direct tab does not touch the surface of the separator. Since the slip separator is prevented from being damaged, a short circuit between the positive and negative electrodes can be suppressed.

  In this case, it is preferable that the static friction coefficient of the separator be 0.39 or less. In addition, the direct tab of the positive electrode may have a part on the metal foil side facing the negative electrode via a separator.

  According to the present invention, by setting the coefficient of static friction of the separator to 0.6 or less, even if the direct tab twists and contacts the separator when the positive and negative electrodes are wound or charged / discharged, the direct tab is separated from the separator. Since the breakage of the separator is suppressed by sliding the surface, an effect that a short circuit between the positive and negative electrodes can be suppressed can be obtained.

  Hereinafter, embodiments of a wound lithium ion secondary battery according to the present invention will be described with reference to the drawings.

(Constitution)
As shown in FIG. 1, the wound lithium ion secondary battery 20 of the present embodiment includes an electrode group 6 in which strip-like positive and negative electrode plates are wound in a spiral shape with a separator interposed therebetween. The electrode group 6 is housed in an iron battery container 7 having a bottomed cylindrical shape and nickel plating.

  On the upper side of the electrode group 6, an aluminum positive electrode current collecting ring 4 for collecting the electric potential from the positive electrode plate is disposed on a substantially extended line of the hollow shaft core 1. The positive electrode current collecting ring 4 is fixed to the upper end portion of the shaft core 1. The edge part of the positive electrode direct tab 2 extended from the positive electrode plate is joined to the peripheral edge of the collar part integrally protruding from the periphery of the positive electrode current collecting ring 4 by ultrasonic welding. A disc-shaped battery lid serving as a positive electrode external terminal is disposed above the positive electrode current collecting ring 4. The battery lid includes an aluminum lid case 12, a lid cap 13, a valve retainer 14 that keeps airtightness, and a cleavage valve 11 that is cleaved by an increase in internal pressure, and these are laminated to form a peripheral edge of the lid case 12. It is assembled by caulking and fixing. One end of two positive electrode lead plates 9 formed by stacking a plurality of aluminum ribbons is fixed to the upper portion of the positive electrode current collecting ring 4, and another one is fixed to the lower surface of the lid case 12. One end of the book is welded. The other ends of the two positive electrode lead plates 9 are connected by welding.

  On the other hand, a copper negative electrode current collecting ring 5 for collecting a potential from the negative electrode plate is disposed below the electrode group 6. The outer peripheral surface of the lower end portion of the shaft core 1 is fixed to the inner peripheral surface of the negative electrode current collecting ring 5. The edge of the negative electrode direct tab 3 extended from the negative electrode plate is welded to the outer peripheral edge of the negative electrode current collecting ring 5. A copper negative electrode lead plate 8 for electrical conduction is welded to the lower portion of the negative electrode current collecting ring 5, and the negative electrode lead plate 8 is welded to the inner bottom portion of the battery container 7. In this example, the dimensions of the battery container 7 are set to a height of 108.0 mm, an outer diameter of 40 mm, and an inner diameter of 39 mm.

The battery lid is caulked and fixed to the upper part of the battery container 7 via an insulating and heat resistant EPDM resin gasket 10. For this reason, the inside of the lithium ion secondary battery 20 is sealed. Further, a non-aqueous electrolyte solution (not shown) is injected into the battery container 7. As the non-aqueous electrolyte, a solution obtained by dissolving 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) as a lithium salt in a mixed solvent of ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 1 is used. .

  As shown in FIG. 2A, in the electrode group 6, the positive electrode plate 17 and the negative electrode plate 18 are wound through the separator 16 so that the two electrode plates do not directly contact each other. At this time, the positive electrode direct tab 2 and the negative electrode direct tab 3 are wound so as to be positioned on opposite end surfaces of the electrode group 6, respectively. The winding direction is set to the left when viewed from the positive electrode direct tab 2 side. As shown in FIG. 3A, the positive electrode direct tab 2 extends from the positive electrode plate at each position from the inner periphery to the outer periphery at the end face of the wound electrode group 6, and each extended end portion is a positive electrode collector. The electric ring 4 is gathered in the vicinity of the periphery of the collar part and joined to the periphery of the collar part (see also FIG. 1). In this example, the positive electrode direct tab 2 is joined to four locations on the periphery of the flange portion of the positive electrode current collecting ring 4 (the same applies to the negative electrode direct tab 3).

  Further, as shown in FIG. 2A, the separator 16 is disposed so that the end portions thereof are located outside the end portions of the positive electrode plate 17 and the negative electrode plate 18 (on the positive electrode current collecting ring 4 side). The negative electrode plate 18 is disposed such that the end portion thereof is located so as to protrude from the end portion of the positive electrode plate 17. Therefore, the positive electrode direct tab 2 has a part on the positive electrode plate 17 side (see reference numeral 2 a), that is, the vicinity of the aluminum foil (metal foil) side constituting the positive electrode plate 17 faces the negative electrode plate 18 through the separator 16. is doing. As the separator 16, one having a separator-separator static friction coefficient of 0.6 or less measured by overlapping two separators 16 and measuring according to Japanese Industrial Standard (JIS-K7125) is used. In this example, the separator 16 has a width of 90.5 mm and a thickness of 16 μm. For example, a porous polyolefin film typified by a polyethylene film is used.

  In addition, an insulation coating is applied to the entire circumference of the collar portion peripheral surface of the electrode group 6 and the positive electrode current collecting ring 4. For the insulation coating, an adhesive tape in which a hexamethacrylate adhesive is applied to one side of a polyimide base material is used. The pressure-sensitive adhesive tape is wound one or more times from the collar surface to the outer circumferential surface of the electrode group 6. By adjusting the lengths of the positive electrode plate 17, the negative electrode plate 18, and the separator 16, the diameter of the electrode group 6 is set to 38 ± 0.1 mm.

  The negative electrode plate 18 constituting the electrode group 6 has a rolled copper foil (metal foil) having a thickness of 10 μm as a negative electrode current collector. A negative electrode mixture containing amorphous carbon powder capable of occluding and releasing lithium ions as a negative electrode active material is coated on both surfaces of the rolled copper foil. In the negative electrode mixture, for example, 10 parts by weight of polyvinylidene fluoride (hereinafter abbreviated as PVDF) as a binder (binder) is blended with 90 parts by weight of amorphous carbon powder. When applying the negative electrode mixture to the rolled copper foil, a dispersion solvent N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) is used. An uncoated portion of a negative electrode mixture having a width of 30 mm is formed at the end in the longitudinal direction of the rolled copper foil. The uncoated part is notched in a comb shape (rectangular shape), and the negative electrode direct tab 3 is formed in the notch remaining part. The interval between the adjacent negative electrode direct tabs 3 is set to 50 mm, and the width of the negative electrode direct tabs 3 is set to 5 mm. In other words, the negative electrode direct tab (remaining notch) 3 extends from the end of the rolled copper foil in a direction intersecting with the longitudinal direction of the rolled copper foil. The negative electrode plate 18 is pressed by a heatable roll press so that the thickness becomes 70 μm after drying, and the width is cut to 88 mm.

  On the other hand, the positive electrode plate 17 has an aluminum foil (metal foil) having a thickness of 20 μm as a positive electrode current collector. A positive electrode mixture containing a lithium transition metal double oxide such as lithium manganate as a positive electrode active material is applied to both surfaces of the aluminum foil. In the positive electrode mixture, for example, 10 parts by weight of the flake graphite as a conductive material and 5 parts by weight of PVDF as a binder are blended with 100 parts by weight of the lithium transition metal double oxide. When applying the positive electrode mixture to the aluminum foil, a dispersion solvent NMP is used. An uncoated portion of the positive electrode mixture having a width of 30 mm is formed at the end portion in the longitudinal direction of the aluminum foil, similarly to the negative electrode plate 18, and the positive electrode direct tab 2 is formed. The interval between adjacent positive electrode direct tabs 2 is set to 50 mm, and the width of the positive electrode direct tabs 2 is set to 5 mm. In other words, the positive electrode direct tab (remaining notch) 2 extends from the end of the aluminum foil in a direction intersecting with the longitudinal direction of the aluminum foil. After drying, the positive electrode plate 17 is pressed in the same manner as the negative electrode plate 18 so as to have a thickness of 90 μm, and the width is cut to 84 mm.

(Battery assembly)
The lithium ion secondary battery 20 is assembled in the following procedure. That is, the positive electrode plate 17 and the negative electrode plate 18 are wound around the shaft core 1 by the winding device through the separator 16 to produce the electrode group 6. At this time, a part of the positive electrode direct tab 2 faces the negative electrode plate 18 through the separator 16. After the positive electrode direct tab 2 and the negative electrode direct tab 3 extending from both end faces of the electrode group 6 are welded to the positive electrode current collecting ring 4 and the negative electrode current collecting ring 5, respectively, the electrode group 6 is inserted into the battery container 7. . A negative electrode lead plate 8 pre-welded to the negative electrode current collecting ring 5 is welded to the inner bottom portion of the battery container 7. In order to crimp and fix the battery lid at a position 9 mm below the upper end of the battery container 7, a stepped portion is formed to form a stepped portion. After connecting the positive electrode current collecting ring 4 and the battery lid with the positive electrode lead plate 9, a nonaqueous electrolytic solution is injected into the battery container 7 from the hollow portion of the shaft core 1 to infiltrate the electrode group 6 into the nonaqueous electrolytic solution. . Then, the assembly of the lithium ion secondary battery 20 is completed by crimping and fixing the gasket 10 and the battery lid on the stepped portion.

  Next, the operation of the lithium ion secondary battery 20 of the present embodiment will be described focusing on the operation of the separator 16.

  The lithium ion secondary battery 20 of this embodiment includes an electrode group 6 in which a positive electrode plate 17 on which a positive electrode direct tab 2 is formed and a negative electrode plate 18 on which a negative electrode direct tab 3 is formed are wound through a separator 16. ing. The positive electrode direct tab 2 and the negative electrode direct tab 3 are produced when the electrode group 6 is produced at the time of battery assembly, or when the positive electrode plate 17 and the negative electrode plate 18 expand and contract as the battery is charged and discharged. Will be twisted. That is, as shown in FIG. 3B, when the electrode group 6 is viewed from the positive electrode current collecting ring 4 side, it turns clockwise with respect to the electrode group 6 during battery assembly or charge / discharge (in the direction of arrow A in the figure). When the force is applied, the positive electrode direct tab 2 is twisted. At this time, the positive electrode direct tab 2 contacts the separator 16 at an angle rather than at a right angle to press the separator 16 (in the direction of arrow B in the figure). Then, as shown in FIG. 2B, the positive electrode direct tab 2 deforms the end portions of the separator 16 and the negative electrode plate 18.

  In such a case, in the conventional wound type lithium ion secondary battery, when the positive electrode direct tab is twisted more than a certain amount, the metal positive electrode direct tab may damage the resin film separator. When the separator is broken, a short circuit occurs between the positive electrode plate and the negative electrode plate in the electrode group, resulting in a decrease in battery performance and, consequently, a battery function stop, which impairs battery reliability.

  Here, how to apply a force when the direct tab contacts the separator obliquely will be described. In general, when an object is pressed obliquely against a plane with a constant force, the force applied from the object to the plane can be decomposed into an orthogonal component orthogonal to the plane and a horizontal component along the plane. When the force of the horizontal component becomes larger than the product of the force of the vertical component and the static friction coefficient of the plane (vertical component force × static friction coefficient), the object slides on the plane. In other words, when the coefficient of static friction of the separator is reduced, the surface of the separator is easily slid even if the direct tab contacts the separator obliquely.

  In this embodiment, a porous polyolefin film having a static friction coefficient of 0.6 or less is used as the separator 16. Therefore, even when the positive electrode plate 17 and the negative electrode plate 18 are wound or when the battery is charged / discharged, even if the positive electrode direct tab 2 or the negative electrode direct tab 3 is twisted and contacts the separator 16, the positive electrode direct tab 2 or negative electrode direct The tab 3 slides on the surface of the separator 16. Thereby, since the breakage | damage of the separator 16 is suppressed, the short circuit between the positive electrode plate 17 and the negative electrode plate 18 can be suppressed, and the reliability of a battery can be ensured.

  Further, in the present embodiment, the end of the negative electrode plate 18 is disposed so as to protrude from the end of the positive electrode plate 17, and a part of the positive electrode direct tab 2 on the positive electrode plate 17 side is interposed via the separator 16. It faces the negative electrode plate 18 (see reference numeral 2a in FIG. 2A). Since the positive electrode direct tab 2 is formed in the uncoated portion of the positive electrode mixture, a gap is formed between the positive electrode direct tab 2 and the separator 16. Even if the positive electrode direct tab 2 is twisted by the gap, the force pressing the separator 16 can be reduced. Thereby, since the breakage | damage of the separator 16 is suppressed, the short circuit between the positive electrode plate 17 and the negative electrode plate 18 can be suppressed, and a battery function can be ensured.

  In the present embodiment, an example in which the static friction coefficient is set to 0.6 or less is shown as the separator 16, but the static friction coefficient is set to 0 in consideration of ensuring the prevention of breakage of the separator 16 and improving the battery reliability. .39 or less is preferable. Moreover, in this embodiment, although the polyethylene film was illustrated as a material of a separator, this invention is not limited to this, What kind of material can be used if a static friction coefficient is 0.6 or less. . An example of such a material is a polyolefin film such as a polypropylene film.

  Further, in this embodiment, the positive electrode direct tab 2 and the negative electrode direct tab 3 are formed by cutting the uncoated portion of the positive electrode mixture of the aluminum foil and the uncoated portion of the negative electrode mixture of the rolled copper foil into a comb shape, respectively. Although an example of forming is shown, the present invention is not limited to this, and may be triangular, for example. Of course, there is no limitation on the width and interval of the positive electrode direct tab 2 and the negative electrode direct tab 3.

  Furthermore, in the present embodiment, the cylindrical lithium ion secondary battery 20 is illustrated, but the present invention is not limited to the battery shape, and a wound lithium ion in which the positive electrode plate and the negative electrode plate are wound. It can be applied to a secondary battery. As a battery shape other than the cylindrical shape, for example, it can be applied to a polygonal battery such as a square shape.

  Furthermore, in the present embodiment, lithium manganate is exemplified as the positive electrode active material and amorphous carbon is exemplified as the negative electrode active material, but the present invention is not limited to these. As the positive electrode active material, for example, lithium transition metal double oxides such as lithium cobalt double oxide typified by lithium cobaltate and lithium nickel double oxide typified by lithium nickelate can be used. Alternatively, a lithium transition metal double oxide in which a part of a transition metal element such as lithium or manganese in the crystal is substituted or doped with a transition metal element such as Fe, Co, Ni, Cr, Al, or Mg may be used. Further, there is no particular limitation on the crystal structure, and the crystal structure may be any of spinel type, layered type, and olivine type. On the other hand, as a negative electrode active material, the carbon material normally used for a lithium ion secondary battery can be used. Examples of the negative electrode active material other than the present embodiment include natural graphite and various graphite materials such as artificial graphite, coke, and the like, and the particle shape is also particularly limited, such as scaly, spherical, fibrous, or massive. Is not to be done. Moreover, there is no restriction | limiting in particular about a electrically conductive material, a binder, etc., You may use the material normally used for a lithium ion secondary battery.

  Next, examples of the lithium ion secondary battery 20 manufactured according to the present embodiment will be described. A comparative example prepared for comparison is also shown.

  As shown in Table 1 below, in this example, six types of polyolefin film separators having different static friction coefficients were used. The static friction coefficient is obtained by overlapping two identical separators and measuring the static friction coefficient between the separator and the separator by a method according to Japanese Industrial Standard (JIS-K7125). The static friction coefficient of each separator was 0.28 for separator 1, 0.32 for separator 2, 0.39 for separator 3, 0.55 for separator 4, 0.60 for separator 5, and 0.78 for separator 6. It was.

(Battery production)
Ten electrode groups 6 each using each separator were prepared for each separator. After welding the positive electrode direct tab 2 and the negative electrode direct tab 3 respectively extending from both end faces of the electrode group 6 to the positive electrode current collecting ring 4 and the negative electrode current collecting ring 5, ten electrode groups 6 prepared for each separator. For five of them, the positive electrode current collector ring 4 was fixed and the electrode group 6 was rotated 45 ° clockwise as viewed from the positive electrode current collector ring side, whereby the positive electrode direct tab 2 was twisted. A lithium ion secondary battery was produced using the obtained electrode group 6.

(Test evaluation 1)
The produced battery was charged to 4.1 V with a current value of 0.1 C, and then left in a thermostat at 25 ° C. for 3 days. The battery voltage after standing was measured, and the number of batteries whose voltage dropped to 4.0 V or less was counted. The percentage of the battery that caused a voltage drop was calculated as a percentage when the twist was applied to each separator and when it was not added. Table 2 shows the obtained ratio of the voltage drop battery.

  As shown in Table 2, when no twist was applied, no voltage drop occurred in all lithium ion secondary batteries, regardless of which separator was used. However, when a twist is applied, a voltage drop does not occur in the lithium ion secondary battery using the separators 1 to 5 having a static friction coefficient of 0.6 or less, whereas a separator having a static friction coefficient of 0.78. In the lithium ion secondary battery using 6, voltage drop occurred at a rate of 40%. When the lithium ion secondary battery in which the voltage drop occurred was disassembled and the cause of the voltage drop was examined, occurrence of a short circuit due to the positive electrode direct tab was confirmed in all the lithium ion secondary batteries in which the voltage drop occurred. From this, it was found that the short circuit between the positive electrode plate 17 and the negative electrode plate 18 by the positive electrode direct tab 2 can be suppressed by using a separator having a static friction coefficient of 0.6 or less.

(Test evaluation 2)
After measuring the battery voltage in Test Evaluation 1, each lithium ion secondary battery was discharged to 2.7 V and disassembled. The separator of the part facing the inner side of the tenth positive electrode direct tab from the end of the positive electrode plate was taken out for each battery. The separator taken out was embedded in an ultraviolet curable resin and cured by irradiating with ultraviolet rays, and then the portion where the positive electrode direct tab was in contact was cut in a direction perpendicular to the positive electrode direct tab, and the cross section was observed. The number of batteries with a scratch depth of 8 μm or more was counted. The percentage of scratches with a depth of 8 μm or more due to the positive electrode direct tab was determined as a percentage depending on whether or not twist was added for each separator. The obtained scratch ratio is shown in Table 3 below.

  As shown in Table 3, when no twist was applied, no cracks having a depth of 8 μm or more were generated in all lithium ion secondary batteries, regardless of which separator was used. However, when twist is applied, scratches with a depth of 8 μm or more are observed, and the ratio is 20% for the lithium ion secondary battery using the separator 4 having a static friction coefficient of 0.55, and the static friction coefficient is 0. It was 40% for the lithium ion secondary battery using the separator 5 of .60, and 80% for the lithium ion secondary battery using the separator 6 having a static friction coefficient of 0.78. In contrast, in the lithium ion secondary battery using the separators 1 to 3 having a static friction coefficient of 0.39 or less, generation of scratches having a depth of 8 μm or more was not recognized. From this, it has been clarified that a lithium ion secondary battery having further safety and reliability can be obtained by using a separator having a static friction coefficient of 0.39 or less.

  The present invention provides a wound type lithium ion secondary battery that can suppress a short circuit between positive and negative electrodes even if the direct tab is twisted, and thus contributes to the manufacture and sale of a wound type lithium ion secondary battery. So it has industrial applicability.

It is sectional drawing of the winding type lithium ion secondary battery of embodiment which concerns on this invention. It is sectional drawing which shows a part by the side of the positive electrode current collection ring of the electrode group which comprises the winding type lithium ion secondary battery of embodiment, (A) is the state which the twist does not generate | occur | produce in an electrode group, (B) Indicates a state in which the electrode group is twisted. It is explanatory drawing which shows typically the direction where the force by the generation | occurrence | production of a twist is seen when the electrode group which comprises a winding type lithium ion secondary battery is seen from the positive electrode current collection ring side, (A) is a twist in an electrode group. A state in which no twist occurs, (B) shows a state in which the electrode group is twisted.

Explanation of symbols

2 Positive electrode direct tab 2a Part of positive electrode direct tab (part of metal foil side)
3 Negative electrode direct tab 6 Electrode group 16 Separator 17 Positive electrode plate 18 Negative electrode plate 20 Winding type lithium ion secondary battery

Claims (3)

  In a wound-type lithium ion secondary battery having an electrode group in which positive and negative electrodes each having a current collecting direct tab formed on a longitudinal end portion of a metal foil are wound through a separator, the static friction coefficient of the separator is 0. A wound lithium ion secondary battery, characterized in that it is 6 or less.   The wound lithium ion secondary battery according to claim 1, wherein the static friction coefficient is 0.39 or less.   2. The wound lithium ion secondary battery according to claim 1, wherein a part of the metal foil side of the direct tab of the positive electrode faces the negative electrode through the separator.

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