JP2000260415A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having excellent regenerative (charge) characteristics, no short-circuiting and insured safety, by properly setting the thickness and the porosity of a separator between a positive electrode and a negative electrode. SOLUTION: In a nonaqueous electrolyte secondary battery 10 formed by dipping an electrode 14 formed by laminating a positive electrode 16 containing a metal oxide capable of occluding and discharging lithium in its positive electrode active material and a negative electrode 18 containing the carbon oxide capable of occluding and discharging lithium in its negative electrode active material with the intervention of a separator 20 in lithium ion conductive and nonaqueous organic electrolyte, the thickness of the separator 20 is set in a range of 10 μm-30 μm, and the porosity of the separator 20 is set in a range of 30-65%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery in which a positive electrode and a negative electrode are laminated via a separator. The present invention relates to a non-aqueous electrolyte secondary battery that can be immersed in an organic electrolyte solution and obtain a battery output by repeating charge and discharge.

[0002]

2. Description of the Related Art Conventionally, as a non-aqueous electrolyte secondary battery of this type, for example, lithium cobalt oxide (LiCo
O ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), or the like is used as a typical example of a lithium ion secondary battery using the positive electrode as an active material.

In this lithium ion secondary battery, a positive electrode containing a metal oxide capable of storing and releasing lithium in a positive electrode active material and a negative electrode containing a carbonaceous material capable of storing and releasing lithium in a negative electrode active material form a separator. The electrode body laminated via the immersion is immersed in a lithium ion conductive non-aqueous organic electrolyte. By repeating charge and discharge between the two electrodes, discharge is caused by movement of lithium ions in the electrolyte to the positive electrode, and charge is performed by movement to the negative electrode. By repeating such charge and discharge, a secondary battery is obtained. The characteristic of is exhibited.

As the required characteristics of such a secondary battery, not only safety in use of the battery, but also output capacity and charge / discharge cycle characteristics have been focused on. Under such circumstances, for example, Japanese Patent Application Laid-Open No. 9-92257 discloses a lithium ion nonaqueous electrolyte secondary battery in which a separator interposed between a positive electrode and a negative electrode has a thickness of 35 μm to 45 μm.
μm, and the porosity of the separator is 40% to 55%.
% Is proposed. This aims at avoiding a short circuit between the electrodes and obtaining a discharge characteristic of a high load.

[0005]

However, charging and discharging as a secondary battery, such as an electric vehicle, particularly an electric vehicle that uses both the power output of a gasoline engine and the electric output of a secondary battery as in a hybrid vehicle, When both of these characteristics are required more than conventional secondary batteries, there is a problem that the regenerative (charging) characteristics are particularly insufficient.

Accordingly, the present inventor has conducted various studies and found that it is necessary to further reduce the thickness of the separator interposed between the positive electrode and the negative electrode, thereby improving the regenerative characteristics. FIG. 2 shows a relationship between current (A) and voltage (V) characteristics for evaluating regenerative characteristics.

In this figure, the regenerative characteristics can be represented by the O (A) vertical axis straight line, the cut-off voltage (shown by a dotted line), and the area of the portion surrounded by the current / voltage straight line. Therefore, when the thickness of the separator is reduced, the position of the voltage (Vo) at the contact of the current / voltage straight line at the time of the current O (A) does not change.
This is the idea that the slope of the current / voltage straight line is reduced and the regenerative characteristics are improved.

The problem to be solved by the present invention is to provide an excellent regenerative (charging) property by properly selecting the thickness and porosity of a separator interposed between a positive electrode and a negative electrode.
Moreover, it is an object of the present invention to provide a highly safe nonaqueous electrolyte secondary battery in which a short circuit between electrodes is avoided.

[0009]

According to the present invention, there is provided an electrode assembly in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween in a non-aqueous electrolyte. In the non-aqueous electrolyte secondary battery, the thickness of the separator is in the range of 10 μm to 30 μm, preferably 15 μm to 30 μm.
The gist is that the separator has a range of not less than 25 μm and not more than 25 μm, and the porosity of the separator is in a range of 30% to 65%.

When the thickness of the separator is small, short-circuiting between the electrodes is apt to occur. Conversely, when the thickness of the separator is large, the volumes of the active materials of the positive electrode and the negative electrode are relatively reduced, and the overall charge / discharge capacity is increased. Is undesirably reduced. When the porosity of the separator is small, the resistance increases, and the rate characteristics deteriorate. On the contrary, when the porosity of the separator is large, a short circuit between the electrodes easily occurs.

For this reason, when the thickness of the separator is reduced in the range of 5 μm to 30 μm, the porosity thereof is also reduced in the range of 35% to 65%, and conversely, when the thickness of the separator is increased, the porosity thereof is reduced. Should be higher.
Thereby, a short circuit between the electrodes is avoided, and excellent regenerative characteristics and output characteristics are obtained by suppressing the resistance. The pore diameter of the separator is desirably 0.01 μm or more and 0.4 μm or less. If it is less than 0.01 μm, the internal resistance is large and efficient charging and discharging is not easy. Also,
If it is larger than 0.4 μm, a short circuit between electrodes is likely to occur.

The present invention is particularly applied to a lithium ion non-aqueous electrolyte secondary battery, and the gist of the present invention is that a metal oxide capable of occluding and releasing lithium, etc. A positive electrode including a positive electrode active material and a negative electrode including a carbonaceous material capable of inserting and extracting lithium as a negative electrode active material are laminated with a separator interposed therebetween, to form a lithium ion conductive non-aqueous organic material. In a non-aqueous electrolyte secondary battery immersed in an electrolytic solution, the separator has a thickness in a range of 10 μm to 30 μm and a porosity of the separator in a range of 30% to 65%.

In this case, a transition metal oxide containing Li or a conductive polymer doped with Li can be used as the positive electrode active material. Specifically, Li _x MO _z (M is C
o, Mn, Ni, V, and at least one Fe-based transition metal), and a conductive polymer such as polyaniline, polypyrrole, and polyacene doped with Li ions. In the case of a Li-containing transition metal oxide, it is preferable to add an appropriate amount of a conductive material to the positive electrode active material to impart conductivity to the positive electrode layer. For example, one or more of carbonaceous powders such as carbon black, acetylene black, and graphite can be used as a mixture. These substances are kneaded with an organic binder, dissolved in an appropriate solvent as necessary, applied as a positive electrode paste composition on the surface of the metal current collector and dried, and pressed as necessary to obtain a positive electrode.

As the negative electrode active material, for example, a metal, a carbonaceous material, a Li-containing transition metal oxide, or a conductive polymer doped with Li ions can be used. Specifically, metal Li, an alloy of Li with another metal such as Al, Zn, Sn, graphite capable of inserting and extracting Li, a carbonaceous material such as amorphous carbon, Li _x MO _z (M is Co, M
(including at least one transition metal such as n, Ni, V, and Fe), and conductive polymers such as polyaniline, polypyrrole, and polyacene doped with Li ions. In the case of a powder such as a carbonaceous material, an organic binder is mixed into the powder, dissolved in an appropriate solvent if necessary, and then applied as a negative electrode paste composition to the surface of the metal current collector and dried, and pressed as necessary. A negative electrode is used.

As the separator sandwiched between the positive electrode and the negative electrode, a porous film such as polyethylene or polypropylene, a nonwoven fabric or a woven fabric is used, and the separator is sandwiched between the positive electrode and the negative electrode to form an electrode body. Then, this electrode body is sealed in a battery case together with the non-aqueous electrolyte to complete a lithium secondary battery.

As the non-aqueous electrolyte, a solution obtained by dissolving a lithium salt, which is usually an electrolyte, in an organic solvent is used. As the electrolyte salt, LiPF ₆ , LiA ₅ F ₆ , LiSb
F ₆ , LiBF ₄ , LiClO ₄ , LiI, LiBr,
LiCl, LiAlCl, LiHF ₂ , LiSCN, L
ISO ₃ CF _2, and the like. Of these,
LiPF ₆ , LiBF ₄ and LiClO ₄ are preferred.

As a solvent for dissolving the electrolyte,
Organic solvents having a relatively high dielectric constant are preferably used, for example, cyclic carbonates such as ethylene carbonate and propylene carbonate, acyclic carbonates such as dimethyl carbonate and ethyl methyl carbonate, tetrahydrafuran and 2-methyltetrahydrofuran And the like, lactones such as γ-butyl lactone, sulfur compounds such as sulfolane, and nitriles such as acetonitrile. Among them, cyclic carbonates such as ethylene carbonate and propylene carbonate, and acyclic carbonates such as dimethyl carbonate and ethyl methyl carbonate are used.

Also in this lithium ion non-aqueous electrolyte secondary battery, the thickness of the separator is 5 μm to 30 μm.
When thinned in the range of μm, the porosity is also 35% to 65%.
%, The porosity is preferably high when the thickness of the separator is large. This avoids a short circuit between the electrodes, suppresses a rise in resistance, and maintains a high rate characteristic.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a schematic configuration of the lithium secondary battery according to the present embodiment. In this lithium secondary battery 10, an electrode body 14 as a charge / discharge element is housed in a battery case (battery case) 12 having a bottomed cylindrical shape.
The surroundings are filled with a non-aqueous organic electrolyte.

The electrode assembly 14 has a structure in which a positive electrode 16 and a negative electrode 18 are overlapped via a separator 20 and wound around a core rod 22. In this case, the positive electrode 16 uses lithium manganate (LiMn ₂ O ₄ ) as a positive electrode active material, uses graphite as a conductive agent, and mixes 5% by weight of polyvinylidene fluoride as a binder with N-methylpyrrolidone. Was added as a dispersant to prepare a positive electrode mixture paste. The paste was applied to both sides of a core material made of aluminum foil, dried and rolled to obtain a positive electrode 16. At this time, the density of the positive electrode was 2.67 g / cm ³ , and the thickness of the A1 foil was 15
μm, and the total thickness after coating the positive electrode active material was 127 μm.

The negative electrode 18 is prepared by mixing spherical graphite as a negative electrode active material, 5% by weight of polyvinylidene fluoride as a binder, and adding N-methylporolidone as a dispersant to form a negative electrode mixture paste. did. The paste was applied to both sides of a core material made of copper foil, dried and rolled to obtain a negative electrode 18. At this time, the negative electrode density was 1.25 g / cm ³ , the thickness of the copper foil was 10 μm, and the total thickness after applying the negative electrode active material was 125.
μm.

As the separator 20, a porous film made of polypropylene, which was cut more widely than the positive electrode 16 and the negative electrode 18, was used. Various thicknesses of the separator were used in the range of 5 μm to 40 μm as shown in the results of various experiments described below. Various porosity was used within the range of 25% to 70%.

Then, a separator 20 is interposed between the positive electrode 16 and the negative electrode 18 and the whole is spirally wound to form the electrode assembly 1.
4 and accommodated in the battery case 12 to accommodate the negative electrode lead 2
4 is spot-welded to the inner bottom surface of the battery case 12, a non-aqueous electrolyte is injected into the battery case 12, the battery cover 26 and the positive electrode lead 28 are spot-welded, and then the battery cover 26 is attached to the battery case 12. Then, the upper part of the battery case 12 was sealed by caulking. As the non-aqueous electrolyte, ethylene carbonate (EC) and diethylene carbonate (DEC) are mixed at a volume ratio of 1: 1 and lithium hexafluorophosphate (L) is mixed.
iPF ₆ ) dissolved at 1 mol / l is used.

Next, a charge / discharge test was performed using the battery obtained by the above method. Initial charge / discharge was performed at an ambient temperature of 20 ° C and 0.2
After a constant current of mA / cm ² was passed to charge the battery to an upper limit voltage of 4.2 V, the battery was discharged to a lower limit voltage of 3.0 V under a constant current of 0.2 mA / cm ² . After the first charge / discharge, the reference capacity of the battery was measured. The second and subsequent charge / discharge conditions were as follows: an ambient temperature of 20 ° C. and a constant current of 1 mA / cm ² .
Charged to an upper limit voltage (cutoff voltage) of 4.2 V, an ambient temperature of 20 ° C., a constant current of 1 mA / cm ² , and a lower limit voltage of 3.0
The cycle of discharging to V was performed 5 times. The fifth (sixth) discharge capacity was defined as a reference capacity 1C.

Next, the charge density of each sample was measured.
That is, the battery was charged at a constant current of 1 mA / cm ² until it reached 80% of the reference capacity 1C (SOC 80%), and after a pause of 1 hour, measurement of the closed circuit voltage and current value was started. For the measurement of the closed circuit voltage and the current value, first, the battery was discharged at a current value of C / 3 for 10 seconds, then, after resting for 5 minutes, charged at a current value of C / 3 for 10 seconds. Next, after a pause of 5 minutes, charging / discharging at a current value of 1 C for 10 seconds was performed with a pause of 5 minutes.

Further, after that, charging and discharging were performed with a current value of 3C, 6C, 9C, and 12C with a pause of 5 minutes. Then, at the completion of charging and discharging for each 10 seconds, the closed circuit voltage and current value were measured, and a regression line was obtained from each measured value to obtain an IV diagram. As described with reference to FIG. 2 described above, the regenerative power, that is, the charging power of the battery, can be expressed by the area surrounded by the O (A) straight line, the cutoff voltage straight line, and the current / voltage straight line. Divide by battery weight,
The regeneration density was calculated. Further, a regenerative current value was determined from the intersection of the cutoff voltage and the current / voltage straight line. Table 1 below summarizes the test results. Note that the current value indicates “− (minus)” for the charge.

[0027]

[Table 1]

Table 1 shows the test results obtained by selecting the thickness (μm) of the separator in multiple steps. In this table, the IV slope (Ω), the regenerative density (W / kg), and the regenerative current (A) are all shown as average values. The porosity (%) is set to 25 to 70%.

As is clear from Table 1, the thinner the separator, the smaller the negative slope of IV, the higher the regenerative density (W / kg) based on the battery weight, and the higher the regenerative current. The value (A) indicates a high value. This means that the thinner the separator, the better the regenerative characteristics. The thickness of the separator of 30 μm is a limit for maintaining the regenerative current characteristics, and it is impossible to increase the thickness of the separator any more.

On the other hand, as can be seen from Table 1, the separator thickness is 10 μm, the porosity is 40%, and the short ratio is 1.5%.
A short circuit between the electrodes has occurred. To avoid this, the porosity can be reduced a little to about 35%,
Then, there is a concern that the resistance will increase and the rate characteristics will decrease. Probably separator thickness 10μm,
A porosity of 30% seems to be the limit.

Next, Table 2 shows the test results obtained by further selecting the porosity (%) in multiple steps. The thickness (μm) of the separator is uniformly 25 μm, and the porosity (%) is variously selected within a range of 30% to 70%.
The IV slope (Ω), regenerative density (W / kg), and regenerative current (A) are all shown as average values.

[0032]

[Table 2]

As is clear from Table 2, when the thickness of the separator is fixed at 25 μm, the porosity is 30% to 30%.
In the range of 65%, no short-circuit between the electrodes occurred at all (short ratio 0%), but when the porosity was 70%, a short-circuit between the electrodes with a short ratio of about 2.0% occurred. Therefore, in order to avoid the short circuit between the electrodes, it is necessary to keep the porosity of the separator to about 65%. In this case, if the thickness of the separator is slightly increased, the porosity can be slightly increased. However, since the regenerative current value (A) is decreased by increasing the thickness of the separator, the thickness (μm) of the separator and the pore size are reduced. Adjustment with the rate (%) is necessary.

The present invention is not limited to the above-described embodiment at all, and various modifications can be made without departing from the gist of the present invention. For example, in the above embodiment, a lithium ion secondary battery using a lithium manganese oxide (LiMn ₂ O ₄ ) positive electrode active material has been described, but other lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide Various types of L including a lithium ion secondary battery using a positive electrode active material such as (LiNi ₂ O ₄ )
The invention can be applied to an i-ion secondary battery, and further to a laminated nonaqueous electrolyte secondary battery composed of a positive electrode / anode / a nonaqueous electrolyte / separator. In essence, the present invention provides a secondary battery in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween to cause a charge / discharge reaction by immersing the electrode body in a non-aqueous electrolytic solution. At the same time, in order to improve the ion conductivity between the positive electrode and the negative electrode, the thickness of the separator and the porosity thereof were optimized.

[0035]

According to the non-aqueous electrolyte secondary battery of the present invention, the thickness of the separator interposed between the positive electrode and the negative electrode is 10 μm to 10 μm.
The electrode spacing is reduced to a minimum of 30 μm, and the porosity of the separator is selected within a range of 30% to 65%. Is also avoided. Therefore, according to the non-aqueous electrolyte battery of the present invention, since it has excellent regenerative (charging) characteristics and high safety without inter-electrode short-circuit, electric vehicles including information communication devices such as personal computers and mobile phones. It is highly expected to be applied to power sources (especially hybrid vehicles).

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

FIG. 2 is a current-voltage relationship diagram for explaining a regenerative (charging) characteristic which is an object of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Lithium secondary battery 14 Electrode body 16 Positive electrode 18 Negative electrode 20 Separator

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor: Yoichi Koyama 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central R & D Laboratories Co., Ltd. 41, Yokomichi, Toyota Central Research Laboratory, Inc. (72) Inventor Toshihiko Inoue 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Kiwa Adachi 1-1-1, Showacho Kariya City, Aichi Prefecture F term in DENSO CORPORATION (reference) 4F071 AA15 AA20 AH12 BC01 BC17 4F074 AA17 AA24 CE02 DA02 DA03 DA23 DA49 5H021 BB12 HH02 HH03 5H029 AJ02 AJ12 AK03 AK16 AL03 AL07 AL08 AL12 AL16 AM03 H04 DJ05J04

Claims (2)

[Claims] In a non-aqueous electrolyte secondary battery in which an electrode body in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween is immersed in a non-aqueous electrolyte, the separator has a thickness of 10 μm to 30 μm.
m and the porosity of the separator is 30%
A non-aqueous electrolyte secondary battery characterized by being in the range of 65% to 65%. 2. A positive electrode containing a metal oxide capable of occluding and releasing lithium in a positive electrode active material and a negative electrode containing a carbonaceous material capable of occluding and releasing lithium in a negative electrode active material with a separator interposed therebetween. In a non-aqueous electrolyte secondary battery in which the electrode assembly thus immersed is immersed in a lithium ion conductive non-aqueous organic electrolytic solution, the separator has a thickness of 10 μm to 30 μm.
μm and a porosity of the separator of 30 μm.
% To 65%.