JP4439870B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte secondary battery including a group of electrode plates in which a positive electrode plate and a negative electrode plate are stacked and wound.

  In recent years, with the spread of portable and cordless devices such as mobile phones and notebook computers, the demand for batteries for supplying power to these devices has increased. In particular, there is an increasing demand for secondary batteries that are small and light, have high energy density, and can be repeatedly charged and discharged. Examples of such secondary batteries include non-aqueous electrolyte secondary batteries.

  The non-aqueous electrolyte secondary battery includes an electrode plate group including a positive electrode plate, a negative electrode plate, and a separator, a battery case that houses the electrode plate group, and a non-aqueous electrolyte that is held by the electrode plate group.

  In a wound non-aqueous electrolyte secondary battery in which a positive electrode and a negative electrode are wound and a positive electrode and a negative electrode are alternately stacked to form a plate group, the positive electrode and the negative electrode are separated by a separator. In general, when the temperature of the separator rises due to overcharge, etc., the separator that is a porous polymer resin shuts down (the pores that exist inside and close the surface) and breaks the electrical contact between the bipolar plates. It has a function.

  However, it is preferable for this shutdown to occur at a lower temperature from the viewpoint of safety, but since it is difficult to provide a difference from the meltdown (melting of the separator itself) that occurs when the temperature further increases, In this case, the reliability of safety is reduced.

On the other hand, technical development for improving the safety of non-aqueous electrolyte secondary batteries has been performed. For example, in the electrode plate group having the current collector exposed portion provided on the outermost peripheral portion of the positive electrode plate and the negative electrode plate of the wound type electrode plate group, winding is performed between the portions where the current collector exposed portions overlap each other. It is disclosed that an insulator separator having different specifications from the inside of the structural electrode plate group is provided, and two types of separators having different materials are arranged. (Refer patent document 1.) Thereby, when mechanical pressure is given with respect to the battery at the time of a crash test etc., a collector part can be short-circuited first and the safety | security of a battery can be ensured. Also in the heating test, the separator separating the current collector part melts down, short-circuits the low resistance part, and can be discharged safely.
JP 2000-315489 A

  However, in the invention as described above, when there is no outermost current collecting structure having a winding structure, a short circuit between the current collectors cannot be secured, and a short circuit occurs between the positive and negative electrode mixture parts, and heat is generated. Therefore, it is necessary to take a configuration for suppressing heat generation. In particular, in the heat test, the problem that the reliability of the battery decreases at a temperature where the separator is equal to or higher than the melting point cannot be eliminated.

  In the electrode plate group having a wound structure, facing the positive electrode mixture portion provided on the outer peripheral side of the current collector of the positive electrode plate, facing the negative electrode mixture portion provided on the inner peripheral side of the current collector of the negative electrode plate The amount of the positive electrode mixture portion to be reduced is smaller than that of the horizontal (stacked structure) electrode plate group. On the other hand, in the negative electrode mixture part provided on the outer peripheral part of the current collector of the negative electrode plate, which faces the positive electrode mixture part provided on the inner peripheral part of the current collector of the positive electrode plate, The amount of the agent is larger than that of the horizontal electrode group.

  The cause is (1) the stack of electrode plates is curved, resulting from the difference in curvature radius. Moreover, (2) This is because a metal foil having no hole capable of diffusing lithium (Li) ions between the front surface and the back surface of the current collector is used.

  When the wound nonaqueous electrolyte secondary battery is charged, the negative electrode mixture portion provided on the outer peripheral portion of the current collector of the negative electrode plate causes the opposite positive electrode mixture due to the causes (1) and (2) above. The amount of mixture in the agent part is larger than that in the horizontal electrode plate group.

  In other words, a larger amount of Li ions is inserted in the negative electrode mixture portion provided on the outer peripheral side of the current collector of the negative electrode plate than in the horizontal portion design, and conversely, the negative electrode mixture provided on the inner peripheral side of the current collector of the negative electrode plate. Only a smaller amount of Li ions is inserted into the agent part than in the horizontal part design.

When a non-aqueous electrolyte secondary battery having such a wound electrode group is exposed to an overcharge mode of 4.2 V or more, a difference occurs in the amount of Li precipitation as described above. Since the negative electrode mixture portion provided on the outer peripheral side of the current collector has a larger Li deposition amount than the negative electrode mixture portion provided on the inner peripheral side of the current collector of the negative electrode plate, thermal instability increases. Thus, a difference appears in the thermal stability inside the battery. Similarly, a difference occurs in the thermal stability of the negative electrode even at 4.2 V or less.

  As described above, there is a difference in the thermal stability in the battery partially, so that a large part appears, so even with the same charge capacity ratio (= charge capacity / battery capacity), the wound electrode is more wound than the horizontal electrode plate group. The plate group is a thermally unstable battery.

  In the prior art, the deterioration of the thermal stability of the curved portion of the wound electrode plate of the nonaqueous electrolyte secondary battery having the wound electrode plate group is not regarded as a problem, and countermeasures to this portion are not Was not.

  In addition, when there is a portion that requires a difference in the Li insertion / extraction reaction per active material weight inside the battery as described above, especially when Li metal is deposited on the negative plate, the negative electrode active material deteriorates. As a result, the cycle characteristics may deteriorate.

  Therefore, in view of the above problems in the present invention, when the battery is in a charged state, the negative electrode mixture portion provided on the outer peripheral side of the current collector of the negative electrode plate into which Li is inserted, Paying attention to the negative electrode mixture part provided on the inner peripheral side of the current collector of the negative electrode plate with less insertion of Li, compared with the negative electrode mixture part provided on the outer peripheral side of the current collector of the negative electrode plate, When an internal short circuit occurs in the battery, a short circuit is generated in the negative electrode mixture portion provided on the inner peripheral side of the current collector of the thermally stable negative electrode plate, thereby suppressing the heat generation of the battery and reducing the battery voltage. The active material can be stabilized by lowering. It is another object of the present invention to provide a highly reliable non-aqueous electrolyte secondary battery that can simultaneously improve cycle characteristics depending on the combination of separators.

A non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode plate and a negative electrode plate laminated together with a separator provided between the positive electrode plate and the negative electrode plate, and a non-aqueous electrolyte secondary battery including a wound electrode plate group. In the battery, the separator uses two separators having different specifications of the first separator and the second separator, only the first separator is disposed on the inner peripheral side of the positive electrode plate, By disposing only the second separator on the outer peripheral side of the positive electrode plate, the first separator and the second separator are alternately stacked in the electrode plate group.

Furthermore, in the nonaqueous electrolyte secondary battery of the present invention, it is preferable that the first separator and the second separator have different specifications of melting point or thermal shrinkage as follows.

  The second separator preferably has a lower melting point than the first separator.

  The second separator preferably has a larger thermal shrinkage than the first separator.

  By disposing as described above, it is opposite to the negative electrode mixture part provided on the inner peripheral side of the current collector of the negative electrode plate, which is more thermally stable than the negative electrode mixture part provided on the outer peripheral side of the current collector of the negative electrode plate By promoting the contact of the positive electrode material mixture portion, the purpose of suppressing heat generation, lowering the battery voltage, and stabilizing the active material can be achieved.

The second separator is preferably thinner than the first separator.

  The second separator preferably has a larger porosity than the first separator.

  The second separator is preferably provided so that the extending direction is perpendicular to the winding direction, and the first separator is preferably provided so that the extending direction is parallel to the winding direction.

  It is preferable that the second separator is provided with a separator produced by uniaxial stretching, and the first separator is provided with a separator produced by multiaxial stretching.

  The second separator preferably has a smaller degree of crosslinking polymerization than the first separator.

  The second separator preferably has a molecular weight smaller than that of the first separator.

  By arranging in this way, a negative electrode mixture portion provided on the inner peripheral side of the current collector of the negative electrode plate, which is more thermally stable than a negative electrode mixture portion provided on the inner peripheral side of the current collector of the negative electrode plate, By facilitating a short circuit between the positive electrode mixture portions facing each other, the purpose of suppressing heat generation and lowering the battery voltage and stabilizing the active material can be achieved without adversely affecting the cycle characteristics.

As described above, the nonaqueous electrolyte secondary battery of the present invention can suppress heat generation caused by a short circuit inside the battery during the heating test. In addition, cycle characteristics can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  As an example of the nonaqueous electrolyte secondary battery, FIG. 1 shows a longitudinal sectional view of a schematic sectional view of a cylindrical lithium secondary battery, and FIG. 2 shows a transverse sectional view thereof. Moreover, the schematic sectional drawing which expanded the curved part of the electrode-plate laminated part is shown in FIG. Thus, the nonaqueous electrolyte secondary battery of the present invention is not limited to the cylindrical type, and the electrode plate portions that overlap in the wound electrode plate group are curved as shown in FIG. If it does, it is applied and it is not limited to the shape of a battery.

  In FIG. 1, the nonaqueous electrolyte secondary battery uses two types of separators, each having different specifications, a second separator 7 and a first separator 8 sandwiching a positive electrode plate 5 and a negative electrode plate 6. . The electrode plate group 4 is laminated in the order of the second separator 7, the positive electrode plate 5, the first separator 8, and the negative electrode plate 6, and is wound in a spiral shape with the negative electrode plate 6 being provided so as to be provided inside. Has been created. The electrode plate group 4 is sandwiched between the upper insulating plate 9 and the lower insulating plate 10 and accommodated in the battery case 1 together with the electrolytic solution. From the electrode plate group 4, the positive electrode lead 5 d is electrically connected to the sealing plate 2, and the negative electrode lead 6 d is electrically connected to the case 1. The case 1 and the sealing plate 2 are sealed with an insulating packing 3.

For the separator having two different specifications, for example, those having different molecular structures such as polyethylene and polypropylene are applied, and even those having the same molecular structure are preferably different in melting point, heat shrinkage, and shutdown temperature. Factors that determine these specifications include the degree of stretching of the separator, the stretching direction, the number of stretching axes, the degree of crosslinking polymerization, the molecular weight, and the molecular structure.

  As a result of verification in the present invention, it has been found that a combination of separators having the following two types of specifications is highly effective.

1) The second separator 7 is a separator having a melting point lower than that of the first separator 8.

2) The second separator 7 is a separator having a larger thermal shrinkage than the first separator 8.

In 1) to 2), in order to make a difference in the specifications of the separator, there is a method of making a difference in the degree of stretching, thickness, stretching direction, number of stretching axes, degree of crosslinking polymerization, molecular weight, and molecular structure of the separator.

Also, in 1) to 2) , as shown in FIG. 3 , the current collector of the negative electrode plate 6c that is more thermally stable than the negative electrode mixture portion 6b provided on the outer peripheral side of the current collector of the negative electrode plate 6c. By urging the contact of the positive electrode mixture portion facing the negative electrode mixture portion 6a provided on the peripheral side, the heat generation behavior at the time of short circuit can be suppressed. Further, when limited to 1) to 3), the first By increasing the Li diffusion resistance of the separator 8, the reaction resistance increases between the positive electrode facing the negative electrode mixture portion 6 b, so that the Li insertion reaction into the negative electrode is suppressed, and the negative electrode mixture portion 6 a and the negative electrode mixture are suppressed. This has the effect of adjusting the reaction rate of the Li insertion / release reaction of the part 6b, and the cycle characteristics can also be improved.

  The material used for the separator is not particularly limited as long as it is not electrically conductive, and the material is polyethylene, polypropylene, polyester, polyamide, polyolefin, polyacrylate, polymethacrylate, polysulfone, polycarbonate, polytetra Fluoroethylene or the like is used.

Positive electrode plate _{5, Li x M y N 1-} y O 2 ( wherein, x represents, 1.10 ≧ x ≧ 0.98, and y satisfies 1 ≧ y ≧ 0. Further, M, N is , M ≠ N, and at least one of Co, Ni, Mn, Cr, Fe, Mg, Al, and Zn.), A positive electrode mixture made of Al or an Al alloy. A positive electrode plate obtained by coating on a current collector is preferred.

  The negative electrode plate 6 is a negative electrode current collector made of Cu, Ni, or a Cu—Ni alloy, and a negative electrode mixture containing a negative electrode active material made of carbon, graphite, alloy, or metal oxide capable of charging and discharging Li. A negative electrode plate obtained by coating on the body is preferred.

  In the above battery, the separator is melted in the heat resistance test or overcharge test, the internal short circuit occurs between the positive and negative electrode plates, the battery self-heat is generated, and the battery temperature is prevented from rising. Between the negative electrode mixture portion 6a provided inside the negative electrode plate 6 with the negative electrode current collector 6c interposed therebetween, facing the positive electrode mixture portion 5b provided outside the positive electrode plate 5 with the current collector 5c interposed therebetween. Before the short circuit between the negative electrode mixture part 6b and the positive electrode mixture part 5a which is more unstable than the negative electrode mixture part 6a occurs, the battery heat generation is suppressed, the battery voltage is lowered, and the active material is reduced. Can be stabilized.

  Another object of the present invention is to provide a highly reliable non-aqueous electrolyte secondary battery, which may improve cycle characteristics at the same time.

  Next, specific examples of the present invention will be described.

Example 1
A cylindrical lithium secondary battery having the same configuration as that shown in FIGS. 1 to 3 in the embodiment was produced as follows.

  The positive electrode plate is composed of a lithium composite oxide LiCoO2, a conductive agent acetylene black, and a binder PVdF in a weight ratio on the positive electrode mixture portions 5a and 5b on both surfaces of a positive electrode current collector 5c made of an Al foil having a thickness of 20 μm. After applying a mixture of 100: 3: 3, it was dried and rolled to produce a positive electrode plate 5 having a thickness of 170 μm.

The negative electrode plate is made of a negative electrode mixture portion 6 on both surfaces of a negative electrode current collector 6c made of Cu foil having a thickness of 20 μm.
A carbon material as a negative electrode active material mixed with a binder rubber resin was applied to a and 6b, followed by drying and rolling to prepare a negative electrode plate 6 having a thickness of 170 μm.

  For the production of the separator, polyethylene was melt-coated at 200 ° C. from a die and led to a cooling roll to obtain a 40 μm polyethylene film. In the stretching process, the film was stretched to 230% in a heated air circulation oven at 130 ° C. The porous film obtained by stretching was heat-set with a roll heated to 115 ° C. to obtain a 25 μm porous polyethylene film. The porosity was 40% from the measurement. The separator thus obtained was used as the first separator 8. The second separator 7 is a porous film obtained by stretching the unstretched polyethylene film used as a raw material in the first separator to 270% and then heat-fixing it with a roll heated at 150 ° C. to 20 μm. A polyethylene film separator was used. The porosity was 40% from the measurement.

Using the positive electrode plate 5, the negative electrode plate 6, the second separator 7, and the first separator 8, a wound electrode plate group is produced, and power generation is made of a nonaqueous electrolyte held by the electrode plate group. The part was housed in a stainless steel case 1 and a terminal made of a sealing plate 2 was attached to produce a battery. A cylindrical lithium secondary battery with a battery size of 65 mm in height and a diameter of 18 mm was produced, and the capacity was 1800 mAh. At this time, an EC: EMC mixed solvent (volume ratio of 1: 1) containing 1 mol / l of LiPF ₆ was used as the non-aqueous electrolyte.

  About the specification of the used separator, the porosity and the heat shrinkage were measured by the following methods.

[Separator evaluation]
The porosity was determined using a mercury porosimeter manufactured by Yuasa Ionics. The thermal shrinkage is performed by measuring the width of the separator shrunk in the width direction after heating at 130 ° C. by sandwiching a separator cut into 5 cm × 5 cm with a glass plate and applying a load of 100 g, and comparing it with the following formula before and after heating. Calculated.

Thermal shrinkage rate (%) = width after shrinkage (cm) / 5 cm × 100
This battery was evaluated as follows.

[Battery evaluation]
(1) Heat test A battery charged to an open circuit voltage of 4.2 V is installed in a thermostat set at 130 ° C, the battery temperature reaches 130 ° C, and whether or not the battery temperature overshoots 130 ° C or more. The thermal stability of the battery was measured by measuring the time from the overshoot until the temperature was stabilized at 130 ° C. The battery temperature reached a maximum of 138 ° C. and the overshoot time was 13 min.

(2) Cycle test In a 20 ° C environment, a battery designed with a capacity of 1800 mAh before the cycle test is prepared, the cycle test is performed at a discharge voltage of 3.0 V up to a charge voltage of 4.2 V, and the discharge capacity after 50 cycles is measured. did. The charging / discharging current at this time was 1.8A. The discharge capacity at 50 cycles was 1710 mAh.

Example 2
As the first separator 8, a porous polyethylene film separator having a thickness of 25 μm and a porosity of 40% used in the first separator 8 of Example 1 was used. For the second separator 7, the unstretched polyethylene film used in Example 1 was stretched to 260% in the stretching step, and then heat-set with a roll heated at 150 ° C. to obtain 25 μm. A porous polyethylene film separator was used. The porosity was 50% from the measurement. A battery was produced in the same manner as in Example 1 except that the porous polyethylene film separator was used as the second separator 7.

  In the heating test, the maximum temperature was 139 ° C. and the overshoot time was 17 minutes. In the cycle test, it was 1705 mAh.

Example 3
A porous polyethylene film separator having a thickness of 25 μm and a porosity of 40% used in the first separator 8 of Example 1 was used as the second separator 7. For the second separator 7, polypropylene was melt coated from a die at 200 ° C. and led to a cooling roll to obtain a 40 μm polyethylene film. In the stretching process, the film was stretched to 230% in a heated air circulation oven at 150 ° C. The porous film obtained by stretching was heat-set with a roll heated to 125 ° C. to obtain a 25 μm porous polypropylene film separator. From the measurement, the porosity was 40%.

  A porous polyethylene film separator having a thickness of 25 μm and a porosity of 40% used in Example 1 was used for the second separator 7, and the same porous polypropylene film separator as that of Example 1 was used for the first separator 7. A battery was produced by the method.

  In the heating test, the maximum temperature was 133 ° C. and the overshoot time was 9 minutes. In the cycle test, it was 1650 mAh.

Example 4
A porous polyethylene film separator having a thickness of 25 μm and a porosity of 40% used in the first separator 8 of Example 1 was used as the second separator 7. The first separator 8 was a three-layer separator (Celgard 2300) in which polyethylene was sandwiched between polypropylene (PP). The PP / PE / PP three-layer separator having a thickness of 25 μm was used. The porosity was 40% from the measurement. A battery was produced in the same manner as in Example 3 except that the above three-layer separator was used as the second separator 7.

  In the heating test, the maximum temperature was 132 ° C. and the overshoot time was 5 minutes. In the cycle test, it was 1650 mAh.

Example 5
As the first separator 8, the porous polyethylene film separator 8 a having a thickness of 25 μm and a porosity of 40% used in the first separator 8 of Example 1 was used. For the second separator 7, the unstretched polyethylene film used as the raw material of the separator used in the first separator 8 of Example 1 is stretched to 260% in the stretching step, and then further perpendicular to the stretching. The film was stretched 120% in the direction (battery electrode plate width direction). Then, the porous polyethylene film separator obtained by heat-fixing with the roll heated at 150 degreeC to 25 micrometers was used. The porosity was 40% from the measurement. A battery was produced in the same manner as in Example 1 except that the porous polyethylene film separator was used as the second separator 7.

  In the heat resistance test, the maximum temperature was 140 ° C. and the overshoot time was 19 min. In the cycle test, it was 1680 mAh.

<< Comparative Example 1 >>
As the second separator 7 and the first separator 8, a porous polyethylene film separator having a thickness of 25 μm and a porosity of 40% used in the first separator 8 of Example 1 was used. A battery was produced in the same manner as in Example 1 with the separator 8 having the same specifications.

  In the heat resistance test, the maximum temperature was 142 ° C. and the overshoot time was 65 minutes. In the cycle test, it was 1650 mAh.

Next, Tables 1 and 2 summarize the evaluation results of the separator specifications, the heating test, and the cycle test for each of the lithium secondary batteries of the above Examples and Comparative Examples.

As shown in Table 2, Example 3 and Comparative Example 1 are first compared for the maximum temperature and overshoot time in the heating test . By using polypropylene having a melting point higher than that of polyethylene used for the second separator 7 for the first separator 8, the short-circuit due to the melt-down of the second separator 7 can be accelerated, so that the negative electrode mixture portion 6 b and the positive electrode Since the negative electrode mixture portion 6a and the positive electrode mixture portion 5b can be short-circuited earlier than the mixture portion 5a and discharge can be performed, heat generation can be suppressed as in Example 1 without deteriorating cycle characteristics.

  Next, Example 4 and Comparative Example 1 are compared. By using a PP / PE / PP three-layer separator in which polypropylene having a higher melting point than polyethylene used for the second separator 7 is laminated on the first separator 8, a short circuit due to meltdown of the second separator 7 is accelerated. Therefore, the negative electrode mixture portion 6a and the positive electrode mixture portion 5b are first short-circuited from the negative electrode mixture portion 6b and the positive electrode mixture portion 5a, and discharge can be performed. It was possible to suppress heat generation in the same manner as

  Next, Example 5 and Comparative Example 1 are compared. By using as the second separator 7 a separator having a characteristic that uses a separator having a larger thermal contraction rate in the width direction than that of the first separator 8, the negative electrode mixture portion 6 b and the positive electrode mixture portion 5 a are used for battery heating. The short-circuit area between the mixture portion 6a and the positive electrode mixture portion 5b is increased, the discharge at this portion can be increased, and heat generation can be suppressed as in Example 1 without deteriorating cycle characteristics. .

  The nonaqueous electrolyte secondary battery of the present invention is useful as a portable power source having excellent safety and a long life.

The longitudinal cross-sectional view of an example of the battery concerning embodiment of this invention 1 is a cross-sectional view of an example of a battery according to an embodiment of the present invention. The enlarged view of the curved part of the electrode-plate lamination | stacking part of the electrode plate group of the battery concerning embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Case 2 Sealing plate 3 Insulation packing 4 Electrode plate group 5 Positive electrode plate 5a, 5b Positive electrode mixture part 5c Positive electrode collector 5d Positive electrode lead 6 Negative electrode plate 6a, 6b Negative electrode mixture part 6c Negative electrode collector 6d Negative electrode lead 7 1st 2 separator 8 first separator 9 upper insulating plate 10 lower insulating plate

Claims (1)

In a nonaqueous electrolyte secondary battery comprising a positive electrode plate and a negative electrode plate laminated together with a separator provided between the positive electrode plate and the negative electrode plate, and a wound electrode plate group,
The separator uses two separators having different specifications for the first separator and the second separator,
By disposing only the first separator on the inner peripheral side of the positive electrode plate and disposing only the second separator on the outer peripheral side of the positive electrode plate, the first separator and the second separator are disposed in the electrode plate group. The separators are alternately stacked , and the second separator has a lower melting point than the first separator, or the second separator has a larger thermal shrinkage than the first separator. Next battery.