JP2012216460A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of preventing contact between a positive electrode and a negative electrode due to contraction of a separator at high temperature.SOLUTION: The nonaqueous electrolyte secondary battery is configured that a power generator element 2 which is made by laminating and wounding a positive electrode 5, a negative electrode 4, and a separator 6 sandwiched therebetween is housed in a case and an electrolyte is held between the positive electrode 5 and the negative electrode 4, and includes at least two regions (A, B) having lower porosity than a center part in an end part in a width direction of the separator 6.

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 preventing a short circuit due to contact between the positive electrode and the negative electrode in addition to a function of holding the electrolyte. The separator is provided so as to protrude outward from the facing portion of the positive electrode and the negative electrode so that the positive electrode and the negative electrode do not contact even when contracted at a high temperature. When this protrusion margin is large, there is a problem that the width of the separator increases and the energy density decreases.

  In addition, in non-aqueous electrolyte secondary batteries, 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 was a problem that the absolute amount of the electrolyte decreased and the durability decreased.

  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.

  However, although the structure of Patent Document 1 is intended to improve the electrolyte holding function by the separator, when the separator contracts at a high temperature, the positive electrode and the negative electrode are exposed from the end portion in the width direction of the separator and are short-circuited. There is a fear.

  Further, in Patent Document 2, in a stacked nonaqueous electrolyte secondary battery, a low porosity portion having a lower porosity than a portion other than the peripheral portion is present in the peripheral portion of the separator as viewed from the stacking direction. Are disclosed.

  However, in view of the fact that the short circuit between the active material layers of the positive electrode and the negative electrode is concentrated on the peripheral portion, the structure of Patent Document 2 is such that the porosity of the peripheral portion of the separator is made smaller than other portions. It prevents the conductive particles from falling off from the peripheral edge of the active material layer to prevent a short circuit between the active material layers of the positive electrode and the negative electrode. No consideration is given to the short circuit between the positive electrode and the negative electrode due to the high temperature shrinkage of the separator. . Hereinafter, the porosity of the separator is referred to as porosity.

Japanese Patent Laid-Open No. 2005-35352 JP 2008-140551 A

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery that can prevent contact between the positive electrode and the negative electrode when the separator is shrunk at a high temperature.

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, the separator has at least two regions having a porosity smaller than that of the central portion at the end portion in the width direction.

  In at least two regions at the end in the width direction of the separator, the porosity can be changed stepwise from the end toward the center.

Of the at least two regions at the end in the width direction of the separator, the region with the smaller porosity can be formed on the end side in the width direction with respect to the opposing portion of the positive electrode and the negative electrode.
Or the porosity of the area | region of an edge part side can be made larger than the porosity of the area | region of the center part side in the at least 2 area | region of the width direction edge part of the said separator.

  In at least two regions at the end in the width direction of the separator, the porosity may be continuously changed from the end toward the center.

  It is preferable that the difference of the smaller porosity of the two regions at the end portion with respect to the porosity of the central region is 20 to 30%. More preferably, it is 23 to 25%.

  According to the present invention, the separator has at least two regions having a porosity smaller than that of the central portion at the width direction end portion, and therefore, the separator has a low shrinkage rate at a high temperature and protrudes outward from the facing portion of the positive electrode and the negative electrode. The cost can be reduced and the energy density can be increased.

  Moreover, it is suppressed that the electrolyte hold | maintained at the separator moves to the outer side of the width direction of a separator, and flows out into the space between a container and an electric 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.

  Further, in general, when the battery is used in a high temperature environment, the electrolyte in the separator is decomposed to generate gas, and a gas pool is generated, so that the electrolyte in the power generation element is insufficient and the battery capacity may be reduced. In the present invention, since the porosity at both ends of the separator is small, a capillary phenomenon is likely to occur, and the electrolyte in the dead space between the container and both ends of the power generation element is supplied to the inside of the separator by this capillary phenomenon. As a result, the capacity retention rate 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 expanded view which shows the arrangement | positioning relationship of a separator, a positive electrode, and a negative electrode. The figure which shows the change of the porosity of a separator. The schematic of the means to change the porosity of a separator.

  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 at least two regions having a porosity smaller than that of the central portion at the end in the width direction.
For example, as shown in FIG. 5, assuming that the two regions at the end in the width direction of the separator 6 are A and B, and the region at the center is C, as shown in FIG. The porosity of B can be made smaller than the porosity of the central region C, and the porosity of the regions A and B can be changed stepwise toward the central region C.
Moreover, as shown in FIG.6 (b), you may make the porosity of the area | region A of the edge part side (endmost part) larger than the porosity of the area | region B of the center part side.
Further, as shown in FIG. 6C, the porosity of the region A and the region B may be continuously changed toward the region C in the central portion.
Of the two regions A and B at the end, the region with the smaller porosity is preferably located on the end side in the width direction with respect to the opposing portion of the positive electrode and the negative electrode. With this configuration, a sufficient electrolyte is held in the facing portion between the positive electrode and the negative electrode, so that the capacity retention rate when left at high temperature is increased and durability is improved.

  It is preferable that the difference of the smaller porosity of two area | regions (A, B) of the width direction edge part with respect to the porosity of the center area | region (C) is 20 to 30%. When the difference in porosity at the end with respect to the porosity at the center is 20% or less, the difference in porosity is small, and no improvement in the electrolyte retention capacity is expected, and when it is 30% or more, the capillary from the end to the center The function of electrolyte uptake due to the phenomenon is diminished. Preferably, it is 23 to 25%.

  Thus, by having at least two regions A and B having a smaller porosity than the central region C at the end in the width direction of the separator, the shrinkage rate at a high temperature is smaller than that of the separator having a constant porosity, The protrusion margin from the edge of the opposing part of a positive electrode and a negative electrode to the outside can be made small, and high energy density can be achieved.

  Moreover, the electrolyte held by the separator 6 between the negative electrode 4 and the positive electrode 5 moves to the outer side in the width direction of the separator 6 by making the porosity at the end in the width direction of the separator 6 smaller than the porosity at the center. Outflow into the space between 1 and the power generation element 2 is suppressed. 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.

  Furthermore, in general, when the battery is used in a high temperature environment, the electrolyte in the separator is decomposed to generate gas, resulting in gas accumulation, etc., resulting in a shortage of electrolyte in the power generation element, which may reduce the battery capacity. In the present invention, since the porosity at both ends of the separator is small, a capillary phenomenon is likely to occur. Due to this capillary phenomenon, the electrolyte in the dead space between the container 1 and both ends of the power generation element 2 is separated from the separator 6. As a result of being supplied to the inside, the capacity retention rate is increased and the durability is improved.

  As a means for changing the porosity of the separator 6, it is possible to reduce the inherent porosity by applying a physical force to the separator 6 in a heated state before the winding (hereinafter referred to as pre-heating press). Can do.

  FIG. 7 shows an outline of this pre-heating press. The separator 6 is passed through a press equipped with rollers 16a and 16b having a polygonal shape such as a triangle or square, and the two areas A and B at the end of the separator 6 are pressed by the surfaces of the rollers 16a and 16b for a predetermined time. . In addition, it is preferable that the corner | angular part of a polygon is chamfered. Thereby, as shown in FIG. 8, the porosity of two area | regions A and B of the width direction edge part of separator 6a, 6b can be made smaller than the porosity of the area | region C of the center part. The separator 6 previously pressed in this way is taken up together with the positive electrode and the negative electrode by a winding machine. In the process of supplying the separator to the winding machine, if a sufficient time for the pre-heating press as described above can be secured, a press machine including the rollers 16a and 16b as described above is used as a part of the winding machine. It may be incorporated. It is also possible to use a roller having a circular cross section by appropriately adjusting the time, pressure, and temperature of the preheating press.

  In order to confirm the effect of the present invention, the high temperature shrinkage rate and the capacity retention rate were compared in the example of the present invention in which the porosity was changed in the two regions A and B at the end in the width direction and the region C in the center and the comparative example. .

<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 Example 1, the porosity of the two regions A and B at the end in the width direction is made smaller than the porosity of the region C at the center by the pre-heating press, and the porosity of the region A at the end is set to the center It was made smaller than the porosity of region B. Region A had a width of 4 mm and a force of 5 MPa was applied at 60 ° C. for 30 seconds, and region B had a width of 4 mm and a force of 5 MPa was applied at 40 ° C. for 30 seconds.
In Example 2, the porosity of the two regions A and B at the end in the width direction is made smaller than the porosity of the region C at the central portion by the preheating press, and the porosity of the region A at the end portion is set to the central portion side. It was made larger than the porosity of region B. Region A had a width of 4 mm and a force of 5 MPa was applied at 25 ° C. for 30 seconds, and region B had a width of 4 mm and a force of 5 MPa was applied at 38 ° C. for 30 seconds.
As Comparative Example 1, a power generation element that does not change porosity was produced.
In Comparative Example 2, the porosity of one region A at the end in the width direction was made smaller than the porosity of the region C in the central portion by the pre-heating press, and the region B was made the same as the porosity of the region C in the central portion. Region A had a width of 4 mm, a force of 5 MPa was applied at 65 ° C. for 30 seconds, and regions B and C were not pre-warmed.

<Measurement of porosity>
The porosities of Examples 1 and 2 and Comparative Examples 1 and 2 were measured by a method in accordance with ASTM-D-1622 by preparing a measurement sample by cutting each region into a predetermined dimension after pre-heating press. did. 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 shrinkage at high temperature>
Cut the separator into a predetermined size (for example, 100 mm x 100 mm) and support it without applying tension (hang it with something like a clothespin or fix a part of the separator on a flat table with tape). Expose to a 120 ° C oven for a certain period of time. The size in the width direction of the separator after taking out from the oven is measured, and compared with the value before putting in the oven, the shrinkage ratio is defined as the shrinkage rate at high temperature.

<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 and 2 and Comparative Examples 1 and 2 were charged to 4.2 V at a constant current of 1 CA, and then the total charging time was set to 3 hours at 4.2 V. After voltage charging, 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 and 2 and Comparative Examples 1 and 2 were charged to a voltage corresponding to SOC 80% at a constant current of 1 CA, and then the total charging time was 3 hours at that voltage. After charging at a constant voltage, a test for storage in a constant temperature bath at 60 ° C. 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 so that the total charge amount is 3 hours at 4.2 V, and then 2.4 V at a constant current of 1 CA. 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 porosity of Examples 1 and 2 and Comparative Examples 1 and 2, and the measurement results of shrinkage at high temperature and capacity retention.

As is clear from Table 1, the difference in the porosity of the smaller end portion (A, B) with respect to the porosity of the central watershed (C) was 23 to 25%.
The capacity retention ratios of Examples 1 and 2 having two regions A and B having lower porosity than the center region C at the width direction end are all 90% or more, and the center region at the width direction end. It was confirmed that although the retention rate of Comparative Example 2 having one region A having a porosity smaller than C was not significantly different from the retention rate of 89%, the porosity was higher than the constant retention rate of 83% of Comparative Example 1.

  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 (6)

  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. The nonaqueous electrolyte secondary battery according to claim 1, wherein the separator has at least two regions having a porosity smaller than that of a central portion at an end in a width direction of the separator.   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the porosity is changed stepwise from the end portion toward the center portion in at least two regions of the width direction end portion of the separator.   3. The low-porosity region of at least two regions of the separator in the width direction end portion is formed closer to the end portion in the width direction than the facing portion of the positive electrode and the negative electrode. Non-aqueous electrolyte secondary battery.   3. The nonaqueous electrolyte secondary battery according to claim 2, wherein in at least two regions at the end portion in the width direction of the separator, the porosity of the region on the end portion side is larger than the porosity of the region on the center portion side. .   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the porosity is continuously changed from the end portion toward the center portion in at least two regions of the width direction end portion of the separator.   6. The non-aqueous electrolyte according to claim 1, wherein a difference in porosity between the two regions at the end portion with respect to the porosity at the central region is 20 to 30%. Next battery.

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