JP4918243B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery including a power generation element including a wound positive electrode plate having a mixture layer on both sides.

A nonaqueous electrolyte secondary battery such as a lithium ion battery houses a power generation element in which a positive electrode plate and a negative electrode plate having a mixture layer on both sides are wound in a flat shape via a separator. At that time, since the mixture layer may be cracked by winding, the binding included in the mixture layer on the first peripheral surface (hereinafter referred to as the inner peripheral surface) of both surfaces of the positive electrode plate. A battery has been proposed in which the elastic modulus of the agent is higher than the elastic modulus of the binder contained in the mixture layer on the second surface (hereinafter referred to as the outer peripheral surface) on the back side of the inner peripheral surface (for example, Patent Documents). 1).
JP-A-9-161768

  However, when the elastic modulus on the inner peripheral surface side is higher than the elastic modulus on the outer peripheral surface side, the inner peripheral surface side is less likely to bend than the outer peripheral surface side, and the current collector is coated with the current collector. There is a problem that a large load is applied from the side. FIG. 2 is a cross-sectional view schematically showing the positive electrode plate during winding. In the positive electrode plate 3, a mixture is applied to the inner peripheral surface 3b and the outer peripheral surface 3c of the current collector 3a. When the positive electrode plate 3 is wound in a flat shape, a force is applied from the inner peripheral side mixture layer to the outer peripheral side (arrow A in FIG. 2) in the bent portion. This force is large when the elastic modulus on the inner peripheral side is higher than that on the outer peripheral side, and may break through the current collector 3a. Thus, when the current collector 3a is broken through and the positive electrode plate 3 is broken, there is a problem that the battery capacity is lowered.

  This invention is made | formed in view of such a situation, The tensile fracture elongation of the binder contained in the mixture layer of the 1st surface of the inner peripheral side among both surfaces of a positive electrode plate is the back side of the said 1st surface. Thus, the tensile fracture elongation of the binder contained in the mixture layer on the second surface on the outer peripheral side is greater than 400% and the flexibility of the mixture layer on the first surface is sufficiently high. Another object is to provide a non-aqueous electrolyte secondary battery capable of suppressing breakage of the positive electrode plate.

  Further, according to the present invention, since the mixture layer on the first surface includes a fluorine-based polymer copolymer, the flexibility of the mixture layer on the first surface becomes sufficiently high, and the breakage of the positive electrode plate is suppressed. Another object of the present invention is to provide a non-aqueous electrolyte secondary battery that can be used.

  Further, the present invention provides a non-aqueous electrolyte in which the mixture layer on the second surface contains polyvinylidene fluoride, so that the swelling rate of the mixture layer on the second surface is reduced and swelling of the power generation element can be suppressed. Another object is to provide a secondary battery.

In addition, the present invention provides a nonaqueous electrolyte secondary battery capable of suppressing breakage of the positive electrode plate when the packing density of the mixture layer on the first surface is smaller than the packing density of the mixture layer on the outer peripheral surface. For other purposes.

The present invention also provides a non-aqueous electrolyte secondary battery capable of suppressing the breakage of the positive electrode plate even when the packing density of the mixture layer on the first surface is 3.4 g / cm ³ or more. For other purposes.

  A non-aqueous electrolyte secondary battery according to a first aspect of the present invention is the non-aqueous electrolyte secondary battery having a mixture layer containing a binder on both sides and a power generation element including a wound positive electrode plate. The tensile fracture elongation of the binder contained in the mixture layer on the first surface on the inner circumference side of both surfaces of the plate is the binder contained in the mixture layer on the second surface on the outer circumference side on the back side of the first surface. It is larger than the tensile fracture elongation of the adhesive and is 400% or more.

  The nonaqueous electrolyte secondary battery according to a second aspect of the present invention is characterized in that, in the first aspect, the mixture layer on the first surface includes a fluorine-based polymer copolymer.

  The nonaqueous electrolyte secondary battery according to a third aspect of the present invention is characterized in that, in the first or second aspect, the mixture layer on the second surface contains polyvinylidene fluoride.

  In the nonaqueous electrolyte secondary battery according to a fourth aspect of the present invention, in any one of the first to third aspects, the packing density of the mixture layer on the first surface is higher than the packing density of the mixture layer on the second surface. It is small.

The nonaqueous electrolyte secondary battery according to a fifth aspect of the present invention is characterized in that, in any one of the first to fourth aspects, the packing density of the mixture layer on the first surface is 3.4 g / cm ³ or more. .

  In the first invention, since the tensile fracture elongation of the binder contained in the mixture layer on the first surface is larger than the tensile fracture elongation of the binder contained in the mixture layer on the second surface, Flexibility of the first surface mixture layer is higher than that of the surface mixture layer, and at the bent portion of the positive electrode plate, the force applied from the inner periphery side mixture layer to the outer periphery side is reduced, and the positive electrode plate is broken. Can be suppressed. Here, the flexibility of the mixture (binder) has a correlation with the tensile elongation at break.

  In the present invention, since the tensile fracture elongation of the binder contained in the first layer mixture layer is 400% or more, the flexibility of the first layer mixture layer is sufficiently high, and the positive electrode The breakage of the plate can be suppressed. When the tensile fracture elongation of the binder contained in the first layer mixture layer is less than 400%, the flexibility of the first layer mixture layer is not sufficiently high, and the positive electrode plate is broken. There is a case.

  In the second invention, the mixture layer on the first surface contains a fluorine-based polymer copolymer, and the fluorine-based polymer copolymer has high flexibility (high tensile fracture elongation). The flexibility of the surface mixture layer is sufficiently high, and the breakage of the positive electrode plate can be suppressed.

  In the third aspect of the invention, the mixture layer on the second surface contains polyvinylidene fluoride, and the expansion coefficient of polyvinylidene fluoride is small, so that swelling of the power generation element can be suppressed. Therefore, for example, battery swelling under a high temperature environment can be suppressed.

  In the fourth aspect of the invention, since the packing density of the mixture layer on the first surface is smaller than the packing density of the mixture layer on the second surface, the flexibility of the mixture layer on the first surface is reduced. The breakage of the positive electrode plate can be suppressed by making it higher than the flexibility of the agent layer.

In the fifth invention, when the packing density of the mixture layer on the first surface is 3.4 g / cm ³ or more, the flexibility is lowered and the positive electrode plate is easily cut. The flexibility of the one-sided mixture layer can be made higher than the flexibility of the second-sided mixture layer, and the breakage of the positive electrode plate can be suppressed.

  According to the first, fourth, and fifth inventions, in the bent portion of the positive electrode plate, the force applied from the inner peripheral side mixture layer to the outer peripheral side is reduced, and the breakage of the current collector coated with the mixture is suppressed. can do. The battery capacity can be improved by suppressing breakage of the positive electrode plate.

According to the 1st and 2nd invention, the softness | flexibility of the mixture layer of a 1st surface becomes high enough, and can fracture | rupture a positive electrode plate.

According to the third aspect of the invention, the swelling rate of the mixture layer on the second surface is reduced, and swelling of the power generation element can be suppressed. Therefore, battery swelling under a high temperature environment can be suppressed.

Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof.
Example 1
FIG. 1 is a schematic cross-sectional view of a nonaqueous electrolyte secondary battery according to the present invention. A nonaqueous electrolyte secondary battery (hereinafter abbreviated as a battery) 1 includes a positive electrode plate 3 formed by applying a positive electrode mixture to an aluminum current collector and a negative electrode plate 4 formed by applying a negative electrode mixture to a copper current collector. The power generation element 2 wound in a flat shape via the separator 5 and the non-aqueous electrolyte are accommodated in the battery case 6 and have a width of 30 mm, a height of 42 mm, and a thickness of 5 mm. A battery lid 7 provided with a safety valve 8 is attached to the battery case 6 by laser welding, a positive electrode terminal 9 is connected to the positive electrode plate 3 via a positive electrode lead 10, and the negative electrode plate 4 is brought into contact with the inner surface of the battery case 6. Electrically connected.

The positive electrode plate 3 is composed of 95% by weight of lithium cobalt composite oxide (LiCoO ₂ ) as a positive electrode active material and a copolymer of vinylidene fluoride and hexafluoropropylene (P (VDF− HFP), fluorine polymer copolymer) 3% by weight, or 3% by weight of polyvinylidene fluoride (PVDF) having a tensile elongation at breakage of 250% as a binder and 2% by weight of acetylene black as a conductive agent N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture thus prepared to prepare a paste, which was then applied to a 20 μm thick aluminum foil current collector and dried.

However, the power generation element 2 is wound with the positive electrode plate 3 positioned on the inner side of the negative electrode plate 4, and the positive electrode mixture is the inner peripheral surface (first surface) of the two of the front and back surfaces of the positive electrode plate 3. In this case, P (VDF-HFP) having a tensile fracture elongation of 400% is used as the binder, and PVDF having a tensile fracture elongation of 250% is used as the binder on the outer peripheral surface (second surface). That is, the positive electrode mixture on the inner peripheral surface of the positive electrode plate 3 and the positive electrode mixture on the outer peripheral surface have different binders. The packing density of the positive electrode mixture layer on the inner peripheral surface of the positive electrode plate 3 is smaller than the packing density of the positive electrode mixture layer on the outer peripheral surface, and the packing density of the positive electrode mixture layer on the inner peripheral surface is 3.4 g / cm. ³ or more.

  Here, the tensile fracture elongation is measured in accordance with ASTM D638. More specifically, a No. V (type V) dumbbell (test piece) is produced from a 2.0 mm ± 0.4 mm thick sheet adjusted at 23 ° C. using a punching blade. Then, the measurement is performed at an initial length of 40 mm and a test speed of 50 mm / min in a room adjusted to 25 ° C. The measurement is performed with five test pieces, and an average value is obtained.

  The negative electrode plate 4 was prepared by adding NMP to a negative electrode mixture obtained by mixing 90% by weight of graphite (graphite) and 10% by weight of PVDF as a binder, and then adjusting the paste to a 10 μm thick copper. It was manufactured by applying and drying on both sides of the foil current collector.

A polyethylene microporous membrane was used for the separator 5, and an electrolytic solution in which 1 mol / l LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 5: 5 was used for the electrolytic solution.

(Example 2)
As a binder on the inner peripheral surface of the positive electrode plate 3, a copolymer of vinylidene fluoride and tetrafluoroethylene (P (VDF-TFE), fluorine-based polymer copolymer) having a tensile elongation at break of 440% is used. A battery was produced in the same manner as in Example 1 except that PVDF having a tensile elongation at break of 280% was used as the surface binder.

(Example 3)
As a binder on the inner peripheral surface of the positive electrode plate 3, a copolymer of vinylidene fluoride and chlorotrifluoroethylene having a tensile elongation at break of 470% (P (VDF-CTFE), a fluorine-based polymer copolymer) is used. A battery was fabricated in the same manner as in Example 1 except that PVDF having a tensile fracture elongation of 280% was used as the binder on the outer peripheral surface.

Example 4
As a binder for the inner peripheral surface of the positive electrode plate 3, a copolymer of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene having a tensile elongation at break of 480% (P (VDF-TFE-HFP), fluorine-based polymer copolymer) Polymer) and PVDF having a tensile fracture elongation of 280% as a binder on the outer peripheral surface, and a battery similar to that of Example 1 was manufactured.

(Comparative Example 1)
The PVDF having a tensile fracture elongation of 250% is used as the binder on the inner peripheral surface of the positive electrode plate 3, and the PVDF having a tensile fracture elongation of 250% is used as the binder on the outer peripheral surface. A battery was produced.

(Comparative Example 2)
PVDF with a tensile fracture elongation of 280% is used as the binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-HFP) with a tensile fracture elongation of 420% is used as the binder on the outer peripheral surface. A battery similar to that of Example 1 was produced.

(Comparative Example 3)
P (VDF-HFP) having a tensile fracture elongation of 420% is used as a binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-HFP) having a tensile fracture elongation of 420% is used as a binder on the outer peripheral surface. Otherwise, a battery similar to that of Example 1 was produced.

(Comparative Example 4)
PVDF with a tensile fracture elongation of 280% is used as the binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-TFE) with a tensile fracture elongation of 440% is used as the binder on the outer peripheral surface. A battery similar to that of Example 1 was produced.

(Comparative Example 5)
P (VDF-TFE) having a tensile fracture elongation of 440% is used as a binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-TFE) having a tensile fracture elongation of 440% is used as a binder on the outer peripheral surface. Otherwise, a battery similar to that of Example 1 was produced.

(Comparative Example 6)
PVDF with a tensile fracture elongation of 280% is used as the binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-CTFE) with a tensile fracture elongation of 470% is used as the binder on the outer peripheral surface. A battery similar to that of Example 1 was produced.

(Comparative Example 7)
PVDF having a tensile fracture elongation of 280% is used as the binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-TFE-HFP) having a tensile fracture elongation of 480% is used as the binder on the outer peripheral surface. Produced the same battery as in Example 1.

(Comparative Example 8)
P (VDF-TFE-HFP) having a tensile fracture elongation of 480% is used as a binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-TFE-) having a tensile fracture elongation of 480% is used as a binder on the outer peripheral surface. A battery was manufactured in the same manner as in Example 1 except that HFP) was used.

(Comparative Example 9)
P (VDF-HFP) having a tensile fracture elongation of 420% is used as the binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-TFE) having a tensile fracture elongation of 440% is used as the binder on the outer peripheral surface. Otherwise, a battery similar to that of Example 1 was produced.

(Comparative Example 10)
P (VDF-TFE) having a tensile fracture elongation of 440% is used as a binder on the inner peripheral surface of the positive electrode plate 3, and P (VDF-TFE-HFP) having a tensile fracture elongation of 480% is used as a binder on the outer peripheral surface. Otherwise, a battery similar to that of Example 1 was produced.

(Comparative Example 11)
P (VDF-HFP) with a tensile fracture elongation of 350% is used as the binder on the inner peripheral surface of the positive electrode plate 3, and PVDF with a tensile fracture elongation of 280% is used as the binder on the outer peripheral surface. A battery similar to that of Example 1 was produced.

(Comparative Example 12)
P (VDF-TFE) with a tensile fracture elongation of 370% is used as the binder on the inner peripheral surface of the positive electrode plate 3, and PVDF with a tensile fracture elongation of 280% is used as the binder on the outer peripheral surface. A battery similar to that of Example 1 was produced.

  The flexibility of the positive electrode plate 3 was examined for the batteries of the above-described examples and comparative examples. Moreover, the initial discharge capacity and the discharge capacity after being left in a high temperature environment were measured. Regarding the flexibility of the positive electrode plate 3, in the power generating element 2 wound with the positive electrode plate 3 inside, the presence or absence of cutting (cracking) of the positive electrode plate 3 at the innermost peripheral portion where the angle of bending was the largest was confirmed. The number of tests is 10 for each example and each comparative example, and it is considered that there are no cracks (◯), 1-2 cracks (Δ), 3 or more cracks (×, defective cutting). It was.

  The initial discharge capacity is as follows: in a room temperature (20 ° C.) atmosphere, after charging for 2.5 hours at a constant current and a constant voltage of a charging current of 680 mA and a charging voltage of 4.20 V, a discharge current of 680 mA and a final voltage of 2.75 V It measured by discharging. The number of tests was 3 for each example and each comparative example, and the average value of 3 was determined. In addition, the discharge capacity after being left in a high temperature environment is 80 ° C. after charging the battery whose initial discharge capacity has been measured with a constant current and a constant voltage of 680 mA and a charge voltage of 4.20 V at room temperature for 2.5 hours. The sample was allowed to stand for 48 hours in a constant temperature bath and then discharged at 25 ° C. with a constant current of 680 mA to a final voltage of 2.75 V. Then, the ratio (%) of the discharge capacity (room temperature) after leaving the high temperature environment with respect to the initial discharge capacity (room temperature) is calculated, and the calculated ratio is set as the retention rate, and the high temperature environment storage characteristics are evaluated with this retention rate (O ○, Δ, ×). The test results are shown in Table 1.

  As shown in Table 1, Comparative Examples 1, 2, 4, 6, and 7 using PVDF as the binder on the inner peripheral surface of the positive electrode plate 3 have a small tensile fracture elongation of PVDF and low flexibility. As in the prior art (FIG. 2), a large force is applied to the current collector (3a) from the inner peripheral surface (3b) side, and all (10) positive electrode plates 3 are cut (marked with x in the table). Moreover, since the positive electrode plate is cut, the initial discharge capacity is small. On the other hand, other comparative examples and examples using a fluorine-based polymer copolymer as the binder on the inner peripheral surface of the positive electrode plate 3 have higher tensile fracture elongation and higher flexibility than PVDF. The force applied to the current collector (3a) from the surface (3b) side is small, and the positive electrode plate 3 is not cut (circle mark in the table), or the positive electrode is broken as in Comparative Examples 11 and 12 There are only a few plates (two or less) (△ mark in the table). Therefore, it is preferable to use a fluorine-based polymer copolymer having a large tensile fracture elongation as the binder on the inner peripheral surface of the positive electrode plate 3. As shown in Comparative Examples 11 and 12, even when a fluorine-based polymer copolymer is used as the binder on the inner peripheral surface of the positive electrode plate 3, the tensile fracture elongation is slightly less than 400%. Since the positive electrode plate 3 is cut, the tensile elongation at break is set to 400% or more.

Further, as shown in Comparative Examples 3 and 9, even when a fluorine-based polymer copolymer is used as the binder on the inner peripheral surface of the positive electrode plate 3, the fluorine-based polymer copolymer is used as the binder on the outer peripheral surface. When is used, since the flexibility of both sides is high, the swell of the power generation element 2 becomes large, and the high temperature storage characteristics are deteriorated. On the other hand, as shown in Examples 1 to 4, when a fluoropolymer is used as the binder on the inner peripheral surface of the positive electrode plate 3 and PVDF is used as the binder on the outer peripheral surface, the outer peripheral surface is used. The flexibility on the side becomes lower than that on the inner peripheral surface side, the swelling of the power generation element 2 is suppressed, and the high temperature storage property is good. When a fluorine-based polymer copolymer is used for the positive electrode mixture on the inner peripheral surface of the positive electrode plate 3 and PVDF is used for the outer periphery, the positive electrode plate 3 is not cut, and the high-temperature storage property is also good. Note that the above-described effects of the present invention are more apparent when the positive electrode mixture has a high packing density (eg, 3.4 g / cm ³ or more).

It is a schematic sectional drawing of the nonaqueous electrolyte secondary battery which concerns on this invention. It is sectional drawing which shows the positive electrode plate at the time of winding typically.

DESCRIPTION OF SYMBOLS 1 Nonaqueous electrolyte secondary battery 2 Power generation element 3 Positive electrode plate 4 Negative electrode plate 5 Separator 6 Battery case 7 Battery cover 8 Safety valve 9 Positive electrode terminal 10 Positive electrode lead

Claims (5)

In a non-aqueous electrolyte secondary battery having a mixture layer containing a binder on both sides and a power generation element including a wound positive electrode plate,
The tensile fracture elongation of the binder contained in the mixture layer on the first surface on the inner circumference side of both surfaces of the positive electrode plate is contained in the mixture layer on the second surface on the outer circumference side on the back side of the first surface. non-aqueous electrolyte secondary battery, characterized by being much larger than the tensile break elongation of the binder, and is 400% or more. The mixture layer of the first surface, the non-aqueous electrolyte secondary battery according to claim 1 Symbol mounting, characterized in that it comprises a fluoropolymer copolymer. The mixture layer of the second surface, the non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized in that it comprises a polyvinylidene fluoride. The packing density of the mixture layer of the first surface, the non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, characterized in that less than the packing density of the mixture layer of the second surface. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4 , wherein a packing density of the mixture layer on the first surface is 3.4 g / cm ³ or more.

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