JP5736914B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium ion battery.

  A non-aqueous electrolyte secondary battery such as a lithium ion battery has a power generation element in which a positive electrode, a negative electrode, and a separator sandwiched between them are stacked and wound in a container, and is disposed between the positive electrode and the negative electrode. In which the electrolyte is held.

  The separator interposed between the positive electrode and the negative electrode of the nonaqueous electrolyte secondary battery has a function of holding the electrolyte in addition to a function of preventing a short circuit due to contact between the positive electrode and the negative electrode. When the electrolyte is held in the separator, the movement of lithium ions between the positive electrode and the negative electrode is facilitated.

  In a non-aqueous electrolyte secondary battery with such a structure, when left at high temperatures or when charge and discharge cycles are repeated, the electrolyte present between the positive electrode and the negative electrode moves to the dead space between the container and the power generation element. There is a problem that the absolute amount of the electrolyte between the electrodes decreases and the durability decreases.

  Patent Document 1 discloses that a portion closer to the container has a lower porosity than a portion far from the container, that is, in a wound type, a portion located on the outer side in the short direction of the electrode group is located on the inner side. A non-aqueous electrolyte secondary battery including a separator having a lower porosity than the above is disclosed. Hereinafter, the porosity of the separator is referred to as porosity.

  However, in the structure of Patent Document 1, the movement of the electrolyte in the width direction of the power generation element can be suppressed, but since the movement of the electrolyte in the longitudinal direction of the power generation element, that is, the winding direction cannot be suppressed, the electrolyte holding capacity is insufficient. There was a problem that the capacity retention rate when left at high temperature was low and the durability was poor.

Japanese Patent Laid-Open No. 2005-35352

  The present invention has been made in view of the above-described conventional problems, and improves the electrolyte retention capacity inside the power generation element, and increases the capacity retention rate when left at high temperatures, thereby improving durability. It is an object to provide an electrolyte secondary battery.

In order to solve the above problems, the present invention provides:
A power generation element in which a positive electrode, a negative electrode, and a separator sandwiched between the positive electrode and the negative electrode are stacked and wound is housed in a container, and an electrolyte is held between the positive electrode and the negative electrode. In the water electrolyte secondary battery, a low porosity region having a porosity smaller than that of the inner peripheral portion is provided on the outer peripheral portion of the separator located on the outer peripheral side of the power generating element.

  The porosity can be changed stepwise from the low porosity region to the inner periphery.

  It is also possible to continuously change the porosity from the low porosity region to the inner periphery.

  It is preferable that the low porosity region is in the outermost peripheral portion of the separator that is not sandwiched between the negative electrode and the positive electrode.

  It is preferable to increase the porosity of a part of the low porosity region compared to other regions.

  The ratio of the difference between the maximum porosity in the inner peripheral portion and the minimum porosity in the low porosity region A to the maximum porosity is preferably 5% or more and 35% or less. More preferably, it is 9 to 31%.

  According to the present invention, since the outer peripheral portion of the separator located on the outer peripheral side of the power generation element has the low porosity region having a lower porosity than the inner peripheral portion, the electrolyte held in the separator between the negative electrode and the positive electrode is wound on the separator. It is suppressed that it moves to a rotation direction and flows into the space between a container and a power generation element. As a result, the electrolyte holding capacity inside the power generation element is improved, and the electrolyte existing outside the power generation element and in the container is greatly reduced. Therefore, the capacity retention rate when left at high temperature is increased, and the durability is improved.

The perspective view of the battery which concerns on this embodiment. The principal part perspective view of the battery which concerns on this embodiment. The expansion perspective view of an electric power generation element. IV-IV sectional view taken on the line of FIG. The figure which shows the change of the porosity of a separator. As a means for changing the porosity of the separator, (a) is an element press, (b) is a winding tension, and (c) is a schematic diagram showing a method using a separator press.

  Embodiments according to the present invention will be described below with reference to the accompanying drawings. In the following description, terms indicating specific directions and positions (for example, terms including “side” and “end”) are used as necessary, but the use of these terms is understood for the invention with reference to the drawings. The technical scope of the present invention is not limited by the meaning of these terms. Further, the following description is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

  FIG. 1 shows a non-aqueous electrolyte secondary battery. As shown in FIG. 2, the non-aqueous electrolyte secondary battery is one in which a power generation element 2 is accommodated in a battery container 1 and sealed with a lid 3. Here, the battery case 1 and the lid 3 constitute an exterior body.

  The battery case 1 has a rectangular parallelepiped shape whose upper surface is open, and is made of aluminum, an aluminum alloy, or the like.

  In the power generation element 2, a separator 6 made of a porous resin film is disposed between a negative electrode 4 having a negative electrode active material layer provided on a copper foil and a positive electrode 5 having a positive electrode active material layer provided on an aluminum foil. Is. As shown in FIG. 3, these are all belt-like, and can be accommodated in the battery container 1 with the negative electrode 4 and the positive electrode 5 being shifted to the opposite sides in the width direction with respect to the separator 6. It is wound in a flat shape. The negative electrode 4 and the positive electrode 5 have a foil exposed portion where an active material layer is not provided at one end in the width direction. The foil exposed portion is bundled, and the negative electrode current collector 8 and the positive electrode current collector are connected via a clip (not shown). 9 are connected to each other.

  The lid 3 is a metal plate, and a safety valve 10 is provided at the center, and a stopper 11 that closes a liquid injection hole (not shown) is provided at the end. Further, the negative electrode external terminal 12 and the positive electrode external terminal 13 are attached to both sides of the lid 3 so as to be electrically connected to the negative electrode current collector 8 and the positive electrode current collector 9 through the packing 7, respectively. Yes.

  The separator 6 of the power generation element 2 has a low porosity region A having a lower porosity than the inner peripheral portion on the outer peripheral portion of the separator 6 positioned on the outer peripheral side of the power generation element 2.

  For example, as shown in FIG. 5A, the region from the outer peripheral edge to the first turn (first layer) is a low porosity region A, and the porosity gradually increases in the order of region B and region C toward the inner peripheral side. Increase In addition, as shown in FIG. 5B, the porosity of the region B on the inner peripheral side from the low porosity region A from the outer peripheral end to the first turn (first layer) is made larger than that of the low porosity region A, and from the region B. The porosity of the region C on the inner peripheral side can be made smaller than that of the region B. Such a stepwise change in porosity can be easily performed by a winding tension described later. The low porosity region A may be 1 turn or more from the outer peripheral end.

  Further, as shown in FIG. 5 (c), the low porosity region A from the outer peripheral edge to the first turn (first layer) is continuously increased toward the inner peripheral side and further increased toward the inner peripheral side. Thus, the porosity of the regions B and C may be continuously increased. According to this, local movement of the electrolyte in the separator can be suppressed in all respects.

  Furthermore, as shown in FIG. 5 (d), the outermost peripheral region S not sandwiched between the negative electrode and the positive electrode is defined as a low porosity region A, and the inner peripheral region B and region C are increased in porosity. Also good. In this case, as shown in FIG.5 (e), you may make the porosity of the middle part of the area | region S small compared with the porosity of another area | region. According to this, since it is only necessary to change the porosity only at the outermost periphery of the separator, the means for changing the porosity is structurally simple. Further, the outflow of the electrolyte to the outside can be reliably suppressed in the end flow region on the outer peripheral side.

  The ratio of the difference between the maximum porosity (MAX (B, C)) in the inner peripheral areas B and C and the minimum porosity in the low porosity area A to the maximum porosity: (MAX (B, C) -A) / MAX (B, C) × 100 (%) is preferably 1% or more and 35% or less. When the ratio of the difference between the maximum porosity and the minimum porosity to the maximum porosity is 5% or less, the difference in porosity is small, and the improvement of the electrolyte holding capacity is not expected. The function of electrolyte uptake by capillary action to the periphery is diminished. Preferably, it is 2% or more and 30% or less.

  Thus, by having the low-porosity region A having a lower porosity than the inner peripheral portion on the outer peripheral portion of the separator 6 located on the outer peripheral side of the power generation element 2, the separator 6 between the negative electrode 4 and the positive electrode 5 was held. The electrolyte is prevented from moving in the winding direction of the separator 6 and flowing out into the space between the container 1 and the power generation element 2. As a result, the electrolyte holding capacity inside the power generation element 2 is improved, and the electrolyte existing outside the power generation element 2 and in the container 1 is greatly reduced. Therefore, the capacity retention rate when left at high temperature is increased, and the durability is improved.

  As a means for changing the porosity of the separator 6, the inherent porosity can be reduced by applying a physical force to the separator 6 in a heated state. Specifically, there are an element press, a winding tension, and a separator press. Hereinafter, these will be described.

  As shown in FIG. 6A, the element press is a method of pressing the power generating element 2 in which a negative electrode, a positive electrode, and a separator are stacked and wound in a stacking direction by pressing devices 14a and 14b. By this element press, the porosity of the outer peripheral separator to which the press load is directly applied can be made smaller than the porosity of the inner peripheral side.

  As shown in FIG. 6 (b), the winding tension is supplied to the core 15 in a process in which the negative electrode 4 and the positive electrode 5 are sandwiched and wound while the left-handed separator 6a and the right-handed separator 6b are wound around the core 15. This is a method in which the left-handed separator 6a and the right-handed separator 6b are pressed by the pressing rollers 16a and 16b to adjust their tension. By increasing the tension of the separators 6a and 6b by the pressing rollers 16a and 16b from the start of winding of the left-handed separator 6a and the right-handed separator 6b through the intermediate winding to the end of winding, the porosity on the outer peripheral side of the separator is changed to the porosity on the inner peripheral side. Can be made smaller.

  As shown in FIG. 6 (c), the separator press cuts the separator 6 of the power generating element 2 in which the negative electrode 4, the positive electrode 5, and the separator 6 are stacked and wound, and then before the separator 6 is stopped with tape, And only the outermost separator 6 in which the negative electrode 5 is not present is pressed while being heated by heating rollers 17a and 17b having a polygonal cross section such as a triangle or a square. In addition, it is preferable that the corner | angular part of a polygon is chamfered. By pressing the outermost separator 6, the porosity on the outer peripheral side of the separator 6 can be made smaller than the porosity on the inner peripheral side. In addition, it is also possible to use a roller having a circular cross section by appropriately adjusting the time, pressure, and temperature of the separator press.

  In order to confirm the effect of the present invention, the capacity retention rates of the examples of the present invention in which the porosity was changed on the outer peripheral side, the middle, and the inner peripheral side and the comparative examples were compared.

<Preparation of positive electrode>
86% by mass of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as a positive electrode active material, 6% by mass of acetylene black as a conductive additive, and 8% by mass of polyvinylidene fluoride (PVdF) as a binder % And a positive electrode mixture paste containing N-methylpyrrolidone as a solvent was prepared. This positive electrode mixture paste was applied to both surfaces of an aluminum current collector foil having a thickness of 20 μm and vacuum-dried, and then compression molded with a roll press to obtain a positive electrode.

<Preparation of positive electrode>
As the negative electrode active material, hard carbon having an interlayer distance d ₀₀₂ = 0.379 nm and an average particle diameter d ₅₀ = 9 μm was used. A negative electrode mixture paste containing 95% by mass of hard carbon as the negative electrode active material, 5% by mass of PVdF, and N-methylpyrrolidone as the solvent was prepared. This negative electrode mixture paste was applied to both sides of a 10 μm thick copper current collector foil and vacuum-dried, followed by compression molding with a roll press to obtain a positive electrode.

<Preparation of electrolyte>
0.8 mol of lithium hexafluorophosphate (LiPF ₆ ) was added to a mixed solution in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:20:50. The solution was dissolved so as to be / l and 0.2 mass% of 1,3-propene sultone was added.

<Preparation of separator>
As the separator, a polyolefin microporous film having a width of 150 mm and a thickness of 0.025 mm was used.

<Production of battery>
The power generation element was produced by winding the negative electrode and the positive electrode in a flat shape with a separator interposed therebetween. This power generation element was welded to a lid on which a current collector was previously assembled, and stored in an aluminum battery container. After the battery container and the lid were laser welded, an electrolyte solution was injected from the injection hole, and the injection hole was sealed and welded to produce a nonaqueous electrolyte secondary battery having a predetermined battery capacity.

<Change of porosity>
When producing the power generation element, the porosity of the separator was variously changed to produce examples and comparative examples.
In Examples 1 to 5, an element press was used as the porosity changing means. In Examples 1 to 3, the temperature during element pressing was changed to 60 ° C., 70 ° C., and 80 ° C., and the time was set to 10 seconds. In Examples 4 and 5, the temperature during element pressing was 70 ° C., and the time was changed to 5 seconds and 20 seconds. If the temperature at the time of element pressing is increased, the porosity A at the outer peripheral portion can be lowered, and the porosity A can be lowered even if the time is lengthened.
In Examples 6 to 8, winding tension was used as the porosity changing means. In Example 6, the tension (g) of the outer peripheral portion, intermediate portion, and inner peripheral portion of the separator during winding was changed to 800, 600, and 400. In Example 7, the tension (g) of the outer peripheral portion, intermediate portion, and inner peripheral portion of the separator during winding was changed to 1200, 600, and 400. In Example 8, the tension (g) of the outer peripheral portion, intermediate portion, and inner peripheral portion of the separator during winding was changed to 1200, 800, and 400. In Example 9, the tension (g) of the outer peripheral portion, intermediate portion, and inner peripheral portion of the separator during winding was changed to 1200, 400, and 1200. When the tension is increased, the porosity A of the outer peripheral portion can be decreased.
In Examples 10 to 12, a separator press was used as the porosity changing means. In Examples 10 to 12, the pressing force during separator pressing was constant (5 Mpa), the temperature was changed to 60 ° C., 70 ° C., and 80 ° C., and the times were 20 seconds, 15 seconds, and 10 seconds, respectively. When the temperature at the time of the separator press is increased, the porosity A of the outer peripheral portion can be decreased.
As a comparative example, a power generation element that does not change porosity was produced.

<Measurement of porosity>
The porosity of Example 1-12 and the comparative example was changed by changing the porosity as described above, and each region was cut into a predetermined size to prepare a measurement sample, which was a method based on ASTM-D-1622. It was measured. The porosity can also be obtained as a percentage of the actual weight with respect to the weight obtained from the thickness, width, length and specific gravity of the separator.

<Measurement of capacity retention>
The battery capacity retention rate after the power generation element produced as described above was stored for 120 days at a high temperature of 60 ° C. in an 80% state of charge (SOC) was measured by the following method.
Specifically, the batteries of Examples 1-12 and Comparative Examples were charged to 4.2 V at a constant current of 1 CA, and then constant voltage charged at 4.2 V so that the total charging time was 3 hours. Thereafter, the battery was discharged to 2.4 V at a constant current of 1 CA. The discharge capacity at this time was defined as the discharge capacity before high-temperature storage. In addition, 1CA is a current value when discharging a fully charged battery in one hour.
Next, the batteries of Examples 1-12 and Comparative Examples were charged to a voltage corresponding to SOC 80% at a constant current of 1 CA, and then charged at a constant voltage such that the total charging time was 3 hours. Then, the test which preserve | saves in a 60 degreeC thermostat was performed for 120 days.
After that, the battery is charged to 4.2 V at a constant current of 1 CA at room temperature, and then charged at a constant voltage of 4.2 V so that the total charge amount is 3 hours. Discharged until. The storage capacity after storage was calculated by dividing the discharge capacity after storage by the discharge capacity before storage, which was defined as the discharge capacity after storage.

Table 1 shows the measurement results of the porosity and capacity retention ratio of Examples 1-12 and Comparative Examples.

As apparent from Table 1, the ratio of the difference between the maximum porosity and the minimum porosity to the maximum porosity was 9 to 31%.
The capacity retention rates of Examples 1-12 in which the porosity of the outer peripheral portion was smaller than that of the inner peripheral portion were all 80% or more, and it was confirmed that the porosity was higher than the retention rate of 72% of the comparative example. It was.

  In addition, this invention is not limited to the structure described in the said embodiment, A various change is possible. For example, the above embodiment is a wound flat type, but can also be applied to a wound cylindrical type and laminated flat type non-aqueous electrolyte secondary battery.

  The present invention can be applied to various batteries such as lead-acid batteries in addition to lithium ion batteries.

DESCRIPTION OF SYMBOLS 1 Battery container 2 Power generation element 3 Lid 4 Negative electrode 5 Positive electrode 6 Separator

Claims (2)

A power generation element in which a positive electrode, a negative electrode, and a separator sandwiched between the positive electrode and the negative electrode are stacked and wound is housed in a container, and an electrolyte is held between the positive electrode and the negative electrode. in aqueous electrolyte secondary batteries, it has a small low porosity region porosity than the inner periphery to the outer periphery of the separator positioned on the outer peripheral side of the power generating element,
The low porosity region is at the outermost peripheral portion of the separator not sandwiched between the negative electrode and the positive electrode,
A nonaqueous electrolyte secondary battery characterized in that a part of the porosity in the middle of the low porosity region is made smaller than in other regions . 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of a difference between the maximum porosity in the inner peripheral portion and the minimum porosity in the low porosity region A to the maximum porosity is 5% or more and 35% or less. .

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