JPH10270005A - Storage battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a storage battery qualified as a safe one as well as such one with a large capacity by providing porous separators between positive and negative electrodes, which hold electrolyte as well as determine the intervals between electrodes and forming electronic conductive films on the surface of these separators. SOLUTION: Although the resistance of a negative electrode 1 in its thickness direction varies depending upon surface directional positions respective resistances are shorted in view of circuit by forming an electronic conductive film M on the surface on the negative electrode 1 side of the separator 2. Therefore, the electric current distribution at the time of charging is uniformalized in surface direction and any local excessive charging is eliminated so that the charging capacity can be increased. For instance, even of lithium ions make their electric deposition in a lithium ion secondary battery, metallic lithium generated on a negative electrode is uniform in surface direction, stays a minute one, electrically dischargeable and becomes safe. Meanwhile, where any electronic conductive film M is formed on the surface of the positive electrode 3 side, the electric current distribution at the time of discharging is similarly uniformalized so that any local excessive discharging is eliminated and the discharging capacity can be also increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a storage battery such as a lithium ion secondary battery or a power storage device such as an electric double layer capacitor.

[0002]

2. Description of the Related Art Generally, an electrochemical battery has a structure including a positive electrode, a negative electrode, and an electrolyte interposed between the two electrodes, which are housed in a battery case. A safety valve is provided in the battery case, and only the safety valve ruptures in the event of an explosion to prevent the entire case from scattering. When the electrolyte is a liquid, a porous separator for preventing a short circuit between the positive and negative electrodes is interposed between the electrodes, and the electrolytic solution penetrates into the pores of the separator. In the case of a solid electrolyte, it also has a function of preventing short-circuiting in place of the separator since it maintains the distance between the electrodes. In each case, the positive electrode and the negative electrode are prepared by mixing a binder and the like with an active material directly involved in the electrochemical reaction or a material that absorbs and releases ions involved in the electrochemical reaction (hereinafter, referred to as a “host substance”). The combined mixture is held on a conductive positive or negative electrode current collector.

For example, in a lithium ion battery, which is expected to be widely used as a power source for small portable electronic devices such as mobile phones and portable personal computers in recent years, a negative electrode containing carbon capable of occluding and releasing lithium ions as a host material has been proposed. A negative electrode plate holding the mixture on the negative electrode current collector and a positive electrode mixture containing a positive electrode active material that reversibly electrochemically reacts with lithium ions, such as a lithium cobalt composite oxide or a lithium nickel composite oxide. A positive electrode plate held by a positive electrode current collector and a separator that holds an electrolyte and is interposed between the negative electrode plate and the positive electrode plate to prevent a short circuit between the two electrodes are provided. The electrolyte is usually made of an aprotic organic solvent in which a lithium salt such as LiClO ₄ or LiPF ₆ is dissolved, but may be a solid electrolyte. However, when the electrolyte is solid, the separator is not essential as described above. As the current collector of the electrode plate, a metal foil such as copper or aluminum is generally used because the current collector itself is required.

In the above-mentioned conventional electrochemical cell, the separator is made of a porous resin film (for example, Japanese Patent Laid-Open No. 5-331306), and when the internal temperature of the cell rises due to an abnormal current, it melts at a predetermined temperature. It was transformed into a non-porous structure, and the battery resistance was cut off due to the increase in its electric resistance to prevent an excessive rise in temperature. This is generally called a separator shutdown function. As another battery safety measure, a protection circuit for preventing overcharge or overdischarge outside the set voltage may be attached to the outer surface of the battery.

On the other hand, in the case of an electric double-layer capacitor used as a low-power DC power supply for backup of ICs and memories, the structure of the electric double-layer capacitor consists of two sheets formed of a material having a large surface area such as activated carbon and a binder. Are disposed facing each other with a porous separator interposed therebetween, and the separator is impregnated with an electrolytic solution.

[0006]

However, regardless of whether it is an electrochemical cell or an electric double layer capacitor, the electrode material is a powder as described above, so that even if it is formed into a plate, the electrode material is not It has irregularities. For this reason, the electric resistance in the thickness direction of the electrode varies depending on the position of the electrode in the surface direction. As a result, the current distribution at the time of charge / discharge differs in the plane direction. For example, when the negative electrode is charged, the portion having a high resistance is fully charged even though the electrode is not fully charged, so that charging is continued, Will be overcharged.
And depending on the type of electrode and electrolyte, overcharging can be done by decomposition of the electrolyte or electrodeposition of lithium metal (for lithium batteries)
Irreversible reaction, etc., and decrease the capacity. Further, since the deposited lithium metal does not discharge, it melts at an abnormally high temperature, moves to the positive electrode, and may explode. The non-uniform current distribution causes local overdischarge even during discharge. Therefore, conventionally, in order to prevent local overcharge and overdischarge, the charge capacity has to be suppressed to 50 to 80% of the theoretical capacity of the negative electrode.

Furthermore, in the above-mentioned conventional electrochemical cell,
Since the speed of heat propagation to the separator and the speed of signal propagation to the protection circuit are slower than the speed of battery temperature rise, the safety valve may rupture before the shutdown function or protection circuit switching function is activated. Was. Although the safety valve is originally provided to explode when the battery is abnormal, it goes without saying that it is preferable that the temperature be lowered without exploding. Therefore, an object of the present invention is to provide a high-capacity and safe power storage device having a configuration different from a conventional separator.

[0008]

In order to achieve the object, a power storage device of the present invention comprises a positive electrode and a negative electrode, and a porous electrode which is located between the electrodes to determine an interval between the electrodes and to hold an electrolyte. In a power storage device including a separator,
An electron conductive film is formed on the surface of the separator. The surface of the separator on which the electron conductive film is formed may be on the positive electrode side, the negative electrode side, or both sides.
However, when it is formed on only one side, the action is slightly different depending on which side it is formed.

The first operation of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view of a power storage device. As described above, the resistance in the thickness direction of the negative electrode 1 differs depending on the position in the surface direction. That is, the values of the resistor R1 and the resistor R2 in the drawing are different. However, in the power storage device of the present invention, the electronically conductive film M formed on the surface of the separator 2 causes the resistance R1 in a circuit.
And the resistor R2 are short-circuited. Therefore, the current distribution during charging is made uniform in the surface direction, local overcharging is eliminated, and the charging capacity can be increased. In a lithium ion secondary battery, even if lithium ions are electrodeposited, the lithium metal generated on the negative electrode by the electrodeposition remains uniform and fine in the plane direction and can be discharged. Therefore, it is safe.

When the electron conductive film M is formed on the surface on the positive electrode side, the current distribution at the time of discharge is similarly made uniform in the plane direction, and local overdischarge is eliminated. Therefore, the discharge capacity can be increased.

Next, the second operation of the present invention will be described. The electron conductive film M is made of metal. Suppose negative electrode 1
It is assumed that an abnormality such as a nail being stuck toward the positive electrode 3 as shown by an arrow from the side has occurred. If the metal film M does not exist, the nail penetrates the negative electrode 1 and short-circuits only when reaching the positive electrode 3. Since the electrode is formed by molding powder as described above, the electrode is not as excellent in thermal conductivity as metal. Therefore, the heat generated by the short circuit stops at that location and becomes locally high. However, in the present invention, since the metal film M exists, a short circuit occurs when the nail reaches the metal film M before reaching the positive electrode 3. And since the metal film M is a metal, the heat generated by the short circuit can be quickly released in the plane direction. In addition, since heat is generated on the separator, the shutdown function is quickly performed. Therefore, it is safe.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The separator is a porous film made of an organic polymer such as polypropylene or polyethylene, and an electrolyte is permeated therein. When a solid electrolyte such as polyvinylidene fluoride or polyacrylonitrile is used as the electrolyte, a separate separator is not required because the electrolyte itself determines the interval between the electrodes and also functions as a separator. However, also in this case, it is effective in terms of the efficiency of the battery reaction if the electrolyte is allowed to penetrate the solid electrolyte.

The electron-conductive film is formed on the surface of the separator by vapor deposition or electroless plating when it is made of metal. When it is made of a conductive polymer, it is formed by dissolving the conductive polymer in a solvent to form a paste, applying the paste on the surface of the separator, and then volatilizing the solvent.

A preferred power storage device is a lithium ion secondary battery. This is because, according to the present invention, electrodeposition of lithium ions is prevented, or even when the electrodeposition is performed, the amount is uniform and the amount is small. Further, the power storage device may be an electric double layer capacitor. Although the extreme surfaces of the electrodes of the electric double layer capacitor also have irregularities, the present invention also prevents overcharging and overdischarging of the electric double layer capacitor.

[0015]

As described above, according to the present invention, the power storage device can be made high-capacity and safe, which is useful as a component of a portable electronic device.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a main part of a power storage device for describing an operation of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Negative electrode 2 Separator 3 Positive electrode M Electron conductive film

Claims (5)

[Claims] 1. A power storage device comprising a positive electrode and a negative electrode, and a porous separator that is located between the electrodes and determines the distance between the electrodes and holds an electrolyte, wherein an electron-conductive film is formed on the surface of the separator. A power storage device, which is formed. 2. The power storage device according to claim 1, wherein said separator is a solid electrolyte. 3. The power storage device according to claim 1, wherein the power storage device is a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. 4. The power storage device according to claim 1, wherein the power storage device is an electric double layer capacitor. 5. The method according to claim 1, wherein said electron conductive film is made of a metal.
The power storage device according to any one of claims 1 to 4.