JPH088115B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery.

Structure of Conventional Example and Its Problems Up to now, as a non-aqueous electrolyte secondary battery using an alkali metal such as lithium as a negative electrode, for example, titanium disulfide (TiS ₂ ) and various interlayer compounds are used as a positive electrode active material. As an electrolyte, a battery using an organic electrolyte in which lithium perchlorate (LiCl ₄ ) or the like is dissolved in an organic solvent such as propylene carbonate (hereinafter abbreviated as PC) has been actively developed. However, this type of secondary battery has not yet been put to practical use. The main reason for this is that the life of the number of charging / discharging cycles (cycles) is short, and in particular, the charging / discharging efficiency at the time of charging / discharging on the negative electrode side due to the generation of dendrites is low.

Various methods have been hitherto attempted to improve such drawbacks of the negative electrode. In general, a method has been reported in which the material for the negative electrode current collector is changed to improve the adhesion with Li to be deposited, or an additive for preventing dendrite generation is added to the electrolyte. However, these methods have not been effective enough to completely solve the above problems.

More recently, it has been proposed to use an alloy with lithium as the negative electrode. A lithium-aluminum alloy is well known as an example of this. In this case, although a uniform alloy can be formed, the uniformity disappears when the charge and discharge are repeated, and there is a drawback that the electrode becomes finely divided and disintegrates particularly when the lithium content increases. It has also been proposed to use a solid solution of silver and an alkali metal (JP-A-56-7386). In this case, it is said that there is no disintegration like alloying with aluminum, but the amount of lithium that alloys sufficiently quickly is small, and metallic lithium may precipitate without alloying. To prevent this, the use of porous materials is recommended. Therefore, the charging efficiency of a large current is poor, and in an alloy containing a large amount of lithium, the miniaturization due to charge / discharge is gradually accelerated, and the cycle life is drastically reduced. Other than that, there is an invention using a lithium-mercury alloy (JP-A-57-98978) and an invention using a lithium-lead alloy (JP-A-57-141869). However, in the case of a lithium-mercury alloy, the negative electrode becomes liquid mercury due to discharge, which causes a problem in handling as an electrode plate.
Further, in the case of a lithium-lead alloy, fine pulverization due to charge and discharge of the electrode is more than silver solid solution, and therefore it is considered desirable to set the lead amount in the alloy to about 80 wt%. High energy density batteries cannot be realized.

As described above, as an excellent negative electrode, it has been desired to develop a negative electrode material that has a large amount of alkali metal absorbed and has a large release and occlusion rate and that has a stable electrode shape against repeated charging and discharging. . Also, an alloy material composed of Sn, Bi, Cd, Pb, etc. has been proposed as a negative electrode material. The characteristics of these materials are that they have a high energy density and that they do not undergo pulverization due to repeated charging and discharging. However, although these alloy materials do not become finely pulverized, they have a drawback that there is no other way than to occlude lithium after the battery is constructed because it is extremely hard and has poor workability when occluding lithium.

Aspects of the invention The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery, and an object thereof is to provide a negative electrode of a chargeable / dischargeable battery having high energy density and excellent charge / discharge characteristics and reliability.

TECHNICAL FIELD The present invention relates to a negative electrode when a metal or alloy having an ability to electrochemically occlude or release an alkali metal is used as a negative electrode material. That is,
In order to add mechanical strength to a negative electrode that uses a metal or alloy that has the ability to insert and release an alkali metal, the negative electrode material is pulverized, kneaded with a tetrafluoroethylene resin, and rolled to form a negative electrode. To do.

By dispersing the material that causes the occlusion / release reaction in the fibrous material of the tetrafluoroethylene resin by this, it is intended to prevent pulverization due to charge / discharge and curing of the negative electrode itself after occlusion of the alkali metal. .

Description of Examples As examples of the present invention, the following studies were performed using Al and Sn that easily form an alloy with Li. Al and Sn use commercially available powder, tetrafluoroethylene resin,
A commercially available polyfine powder was used.

First, Al and Sn powders each contain 10 wt%
Tetrafluoride ethylene resin was added and kneaded in an amount of 5 wt% with respect to n. When kneaded in this way, a soft rubber-clay-like lump was formed, which was rolled by a roller to fabricate a negative electrode film. In addition, the reason why the amount of tetrafluoroethylene resin is 10 wt% with respect to Al and 5 wt% with respect to Sn is that if it is less than that, the strength is not high when formed into a film, and if it is more than that, the conductivity of the film itself This is due to the marked decrease in sex. Next, as shown in FIG. 1, a trial-made film-shaped negative electrode material 1 having a thickness of about 0.2 mm was pressure-bonded to the Ni expanded metal ² and cut into 10 × 10 mm ² and the Ni ribbon lead 3 was attached to the test electrode. And Also, for comparison, with Al
For Sn, electrode plates made of only metal were manufactured by pressing powder with the same size as the test electrode shown in FIG.

In order to charge and discharge the test electrode as described above, the test electrode 6 and the lithium electrode 7 are put in the H-shaped test cell 5 partitioned by the glass filter 4 as shown in FIG. Lithium oxide (LiClO ₄ ) dissolved propylene carbonate electrolyte 8
A constant current of 1 mA was charged and discharged.

As a result, all of the test electrodes began to change color as the Li ions were occluded and gradually expanded.

Then, especially in the test electrode composed of only Al and Sn metals, the electrode plate itself collapsed when a certain storage amount was exceeded, and so-called pulverization occurred.

FIG. 3 shows the potentials of the test electrodes associated with the absorption of Li ions by the constant current for the Al metal electrode 9, the Sn metal electrode 10, the Al-tetrafluoroethylene resin electrode 11 and the Sn-tetrafluoroethylene resin electrode 12. In the figure showing the change (based on the potential of metallic Li), the potential began to become unstable when the storage capacity exceeded 80 mAh for the Al metal electrode and 100 mAh for the Sn metal electrode, and the electrode plate also collapsed just around this time. I was starting.

However, the test electrode using a tetrafluoroethylene resin as a film-shaped negative electrode had stable potential and expansion of the electrode plate, but the electrode plate itself did not collapse. However, due to the resin content and polarization, the electric capacity became slightly lower. In particular, since Al powder requires more tetrafluoroethylene resin than Sn powder, the amount of Li that can be occluded was smaller than that of Sn electrode plate.

However, when comparing the mechanical strength of the electrode plates, the electrode containing the tetrafluoroethylene resin did not collapse at all in the case of Al and Sn, for example, the lithium ion was stored under the above storage conditions (1 mA). Even if it is occluded with a constant current) and a current is passed until the potential becomes almost 0, it expands as shown in (a) of FIG. 4 and does not collapse, but in the case of a single metal electrode, also in Al Also in Sn, it collapsed and fell off as shown in FIG. 4 (b).

The above is only the first attempt to occlude Li ions in the test electrode, but here, assuming actual charging and discharging,
Using the test cell shown in FIG. 2, charging / discharging of 1 mA was performed at a test electrode potential of 0 V to 1.0 V with respect to the Li reference electrode.
However, regarding the metal electrode, it is known that the electrode plate collapses if it is charged (occluded) to 0 V during the occlusion test, so the charging was stopped before the potential became unstable.

However, when actually trying to charge and discharge, the metal electrode collapsed and the capacity decreased, and eventually the Al electrode fell off by the 4th to 7th cycles, and the Sn electrode also by the 10th to 15th cycles. All have fallen out. Then, some of the fragments generated by the dropping of the electrode plate were accumulated in the bottom of the cell and others were fine powder floating in the electrolyte. The substances produced by this loss are Li-Al or Li-
Sn alloy is an active substance, and if it floats and comes into contact with the positive electrode, it causes a so-called internal short circuit in the battery. Therefore, it can be said that it is almost impossible to use the metal single electrode from the viewpoint of the secondary battery. Next, the test electrode of the present invention produced by mixing a metal with a tetrafluoroethylene resin was subjected to the above charge and discharge up to 100 cycles, but the electrode plate did not collapse at all, and the electric capacity was slightly small, As shown in FIG. 5, the charge / discharge characteristics from the initial stage to the 100th cycle were very stable. Further, in this test, the Li electrode as the counter electrode remarkably generates dendrites, and therefore the electrode plate and the electrolytic solution were periodically (every 5 cycles) replaced on the Li electrode side of the H-type cell.

Next, an example in which the present invention is applied to an alloy electrode instead of a metal alone will be shown below.

Among the alloy electrodes that have recently been reported to be excellent negative electrode materials, we examined Sn-Cd alloys with 85% Sn and 15% Cd. Using this alloy, for example, a test electrode as shown in Fig. 1 was made as a prototype, and it was tried with a simple metal electrode such as Sn or Al.
When I charge and discharge Li ions, it expands without adding tetrafluoroethylene resin, and it never collapses. This is an unprecedented advantage of this type of alloy. However, this type of alloy electrode has the property that, due to the absorption of Li ions, the original flexibility of the alloy itself is not lost, and the electrode plate becomes hard and brittle. For example, if an alloy plate is made to absorb Li ions and then hit with a hammer, it will be shattered if it does not crack even if it does not absorb Li. The property that such an electrode becomes hard does not affect the performance of a battery when it is normally used.However, for example, the battery may be impacted from the outside, and it is safe or strict in reliability. After all, I do not want it. So, Sn-Cd alloy of Sn85%, Cd15% was made into fine powder,
By mixing with a tetrafluoroethylene resin, an electrode plate was prototyped in the same manner as in the above study. Further, with respect to the Sn-Cd alloy, the amount of tetrafluoroethylene resin was 5 wt% was suitable. In order to compare the electrode plate containing the tetrafluoroethylene resin thus prototyped with the alloy-only electrode plate, a button-type battery was prototyped as follows using each electrode plate. Further, in this study, since the reliability was examined, fluorinated graphite for a primary battery was used for the positive electrode. As shown in FIG. 6, the button-type battery for study comprises a fluorinated graphite positive electrode 13, a test electrode 14 of the present invention, and a polypropylene separator 15, and a gasket.
1MLiC via sealing plate 17 and battery case 18 via
A completed battery was obtained by sealing with propylene carbonate electrolyte 19 in which lO ₄ was dissolved. In addition, since the test electrode does not contain Li as an active material as it is, the test electrode 20 is pre-bonded to the sealing plate 21 before sealing the battery as shown in FIG.
Lithium ions were occluded by bringing them into contact with metallic lithium 23 in an electrolyte 22 containing Li ions.

When the button-type battery prototyped in this way was discharged at a constant current of 2 mA, it was compared with the discharge characteristic curve 24 of the battery only of the alloy not containing tetrafluoroethylene resin as shown in FIG. Discharge characteristic curve of batteries containing ethylene resin 25
Had slightly larger polarization, and the discharge capacity (1.5V termination) was also smaller. However, the discharge characteristics of all the batteries containing only alloy do not necessarily become like the curve 24 of FIG. 8, and at some 30%, some curves 26 shown by broken lines in FIG. It showed unusual characteristics as typified. Therefore, when disassembling a defective battery that showed abnormal discharge, the alloy electrode was cracked,
The finely broken pieces were also carried to the positive electrode side. As a result of the investigation, it was found that the cause of this defect was that the pressure applied in the caulking process at the time of sealing breaks the test electrode. Therefore, when a drop battery test (dropping on a steel plate from a height of 1 m), which is a battery composed only of alloy electrodes, was performed after the prototype of the battery, the defective rate of only the sealing was 30.
%, The defect rate increased to 75% when the drop test was added. As described above, it has been found that the electrode plate made of only the alloy has excellent characteristics, but has a drawback of being weak against impact. However, in the test electrode of the present invention to which the tetrafluoroethylene resin was applied, all the prototyped batteries had the discharge curves shown in FIG.
It exhibited a property equal to 24, and no defect occurred even when the drop test was performed. As described above, from the viewpoint of electrode plate strength, it can be said that an electrode plate having flexibility like the negative electrode of the present invention is desired.

Further, due to a problem in battery production, the alloy electrode becomes harder as Li is occluded, so there is an inconvenience that Li must be occluded after the battery is constructed.However, the negative electrode of the present invention is just after occluding Li. Since it is a flexible material such as rubber clay, it has excellent workability and can solve the above inconvenience.

It has been conventionally known that, in an alkaline storage battery using a zinc negative electrode, powdered zinc is bound with a resin to form an electrode plate. The purpose in this case was to increase the surface area of zinc to enable high-rate charge / discharge, and to reduce the generation of dendrites by utilizing the fact that the true area is significantly larger than the apparent area. However, when deep charge / discharge is performed, most of the negative electrode zinc is dissolved, so that only the resin remains in the negative electrode and zinc deposition is not uniform during charging, and dendrite generation is remarkable. However, in the present invention, since the absorption and desorption of lithium are utilized, the amount of lithium occluded in the negative electrode only changes even when deep charging and discharging are performed, and Al or Sn, Sn-Cd alloy which is the negative electrode material is changed. Does not change, and charge and discharge always proceed uniformly on the negative electrode surface, and dendrites do not occur. Therefore, the idea of the zinc electrode cannot be applied to the electrode of the present invention.

EFFECTS OF THE INVENTION As described above, the present invention can not only provide a battery having high energy density and excellent reliability, but also significantly improve workability in manufacturing.

[Brief description of drawings]

FIG. 1 is an external view of a test electrode in one embodiment of the present invention, FIG. 2 is a configuration diagram of the H-type test cell, and FIG. 3 is a potential change characteristic diagram during charging (storage) of the test electrode. FIG. 4 is a diagram showing the state of the test electrode after absorption of Li, FIG. 5 is a charge / discharge curve of the negative electrode in one embodiment of the present invention, and FIG. 6 is a cross-sectional view of a coin battery of one embodiment of the present invention. FIG. 7 shows the negative electrode of the coin type battery.
FIG. 8 is a discharge curve of the coin-type battery showing the state of occluding Li. 1 …… Negative electrode material, 2 …… Ni expanded metal, 3 …… Ni
Ribbon lead, 4 ... Glass filter, 5 ... Test cell, 6 ... Test electrode, 7 ... Lithium electrode, 8 ... Propylene carbonate electrolyte, 9 ... Al metal plate, 10 ... Sn metal plate, 11 …
… Al-tetrafluoroethylene resin electrode, 12 …… Sn-tetrafluoroethylene resin electrode, 13 …… positive electrode, 14,20 …… test electrode, 15 ……
Separator, 16 …… Gasket, 17,21 …… Seal plate, 18
…… Battery case, 19,22 …… Electrolyte, 23 …… Metallic lithium.

Claims (4)

[Claims] 1. A positive electrode, an electrolyte containing an alkali metal ion, and a negative electrode whose negative electrode material is a metal or an alloy that occludes or releases an alkali metal ion during charge and discharge, and the negative electrode comprises: A non-aqueous electrolyte secondary battery comprising the pulverized negative electrode material and a tetrafluoroethylene resin. 2. The negative electrode material is at least one metal selected from Sn, Al, Mg, Pb and In.
The non-aqueous electrolyte secondary battery according to the item. 3. The negative electrode material is an alloy comprising at least two metal elements selected from Sn, Bi, Pb, Cd, In, Sb, Zn and Ag. A non-aqueous electrolyte secondary battery according to claim 1. 4. The non-aqueous electrolyte secondary battery according to claim 1, 2, or 3, wherein the alkali metal is lithium.