JP2621182B2 - Non-aqueous electrolyte lithium secondary battery - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a non-aqueous electrolyte lithium secondary battery as a power source for portable equipment or a power source for memory backup.

2. Description of the Related Art Non-aqueous electrolyte lithium secondary batteries have higher voltage, higher energy density, better self-discharge resistance and liquid leakage resistance than conventional aqueous batteries, and are highly expected in the future. Although lithium secondary batteries using molybdenum disulfide as a positive electrode have been partly put into practical use at present, they have not reached full-scale commercialization.

The greatest reason is that, for example, there is a large problem that the charge / discharge cycle characteristics are inferior to that of a nickel cadmium battery which is a conventional aqueous battery.

This is a form in which lithium, which is the negative electrode active material of the lithium secondary battery, dissolves in the electrolyte at the time of discharging and precipitates again on the electrode at the time of charging. The so-called dendrite is formed, so that when the next discharge is performed, smooth discharge is not performed, or the dendrite penetrates through the separator and contacts the positive electrode to cause a short circuit phenomenon.

In contrast, JP-A-59-163756 and JP-A-59-163758 disclose an alloy of two or more elements such as cadmium, lead, tin, bismuth, antimony, mercury, and indium, a so-called low melting point fusible alloy. It has been found that lithium can be easily inserted and extracted in a non-aqueous electrolyte, and a non-aqueous electrolyte lithium secondary battery using these alloys as a negative electrode has been proposed.

In other words, in batteries using these alloys as the negative electrode, the lithium occluded inside the alloy is discharged into the electrolyte during discharging, and conversely, when lithium ions of the electrolyte precipitate on the alloy during charging, the alloy is rapidly formed. And diffuses into the alloy and is occluded, so that dendrite does not form on the surface of the alloy negative electrode, thereby exhibiting good charge / discharge cycle characteristics.

Among the above elements, lead, tin, bismuth, and indium, particularly, have a large lithium storage capacity and easily release lithium when the battery is charged and discharged. On the other hand, when a battery is formed by using an alloy obtained by combining these elements as a negative electrode and the charge and discharge are repeated, a large amount of lithium is absorbed. Have. Usually, in order to compensate for this drawback, an alloy is formed by adding an appropriate amount of cadmium, zinc, or the like as an element that plays a role as a kind of binder, although the amount of occluded lithium is small.

In addition, since these alloys have a potential of about 0.4 to 0.6 V with respect to lithium, in order to obtain a high voltage, high energy density lithium secondary battery, the theoretical electric capacity is large and the potential is high. It is necessary to select a positive electrode active material.

As active materials satisfying these requirements, manganese dioxide, chromium oxide, vanadium oxide and the like are known. That is, if the alloy negative electrode and the positive electrode active material are combined, a non-aqueous electrolyte lithium secondary battery having high voltage, high energy density, and excellent charge / discharge cycle characteristics can be obtained.

Problems to be Solved by the Invention On the other hand, in actual use of the battery, it is necessary to consider overdischarge. The same is true for primary batteries,
If the battery is incorporated in the device and used, the battery will continue to discharge forever. Especially when it is used for memory backup, which has been increasing its use recently,
The battery is put into operation from the time it is installed in the device.Discharge continues until the user of the device connects to the AC power supply and starts charging the battery.In extreme cases, the battery is completely discharged. However, the battery voltage may become 0V. Conventional batteries, such as nickel-cadmium batteries, are resistant to this over-discharge use. Even if they are discharged until the battery voltage reaches 0 V, they can be used again as long as they are charged at that time.

In the case of a lithium secondary battery, this overdischarge characteristic is extremely weak, and if the battery is discharged until the voltage becomes 0 V, it does not return to the original state even if charged. This is one of the main reasons why lithium secondary batteries cannot be widely used at present.

Means for Solving the Problems In order to solve this problem, the present invention provides a first active material in which the positive electrode discharges at a high potential and a second active material which discharges at a low potential.
The negative electrode is made of an alloy that absorbs and releases lithium and pressed against lithium or an alloy that has previously absorbed lithium, and the first active material is a solution of the alloy in a non-aqueous electrolyte. Discharge at a higher potential than
Conversely, the second active material discharges at a low potential, and the theoretical amount of chargeable lithium of the negative electrode is larger than the dischargeable amount of electricity of the first active material;
It is intended to provide a non-aqueous electrolyte lithium secondary battery designed to be smaller than the sum of dischargeable electricity amounts of the active material and the second active material.

Operation As described above, when the lithium secondary battery is overdischarged,
The reason for not returning is as follows.

That is, when the battery is over-discharged, the discharge voltage decreases, and eventually the voltage of the battery becomes 0 V, which indicates that usually either the capacity of the positive electrode or the capacity of the negative electrode is exhausted. When the capacity of the positive electrode is exhausted, that is, when considering the case of the positive electrode capacity regulation, as described above, as a positive electrode active material of a lithium secondary battery, an inorganic compound such as manganese dioxide, chromium oxide, or vanadium oxide is used. When these are discharged below a certain potential, the crystal structure changes. Therefore, even if the battery is charged, it does not return to the original state.

On the other hand, when the capacity of the negative electrode is exhausted, that is, in the case of regulating the negative electrode capacity, the lithium occluded in the alloy negative electrode is dissolved as an ion in the electrolyte with discharge,
When all of the lithium is consumed, the potential of the negative electrode increases, and a dissolution reaction of the alloy negative electrode starts to occur, and eventually all of the lithium is dissolved in the electrolyte. Therefore, since there is no alloy that should absorb lithium even when the battery is charged next time, the battery is not charged. The above is the reason why the lithium secondary battery cannot be overdischarged.

As a method of overcoming this drawback, in the case of a battery with positive electrode capacity regulation, even if the capacity of the positive electrode active material is exhausted, some measure is taken to raise the potential of the positive electrode to a potential higher than the potential at which the crystal structure of the active material changes. You should keep it.

On the other hand, in the case of regulating the capacity of the negative electrode, even if the capacity of lithium pressed or occluded on the alloy negative electrode is exhausted, the potential of the negative electrode may be kept below the potential of the alloy by some means. Although it is theoretically as described above, considering the case where the capacity of the positive electrode is regulated as a practical problem, the above-mentioned positive electrode active materials usually change their crystal structures in the range of 1.0 to 2.0 V with respect to lithium. Therefore, in order to prevent the potential of the positive electrode from becoming lower than that, the discharge potential of the negative electrode must be discharged at a potential or more that causes a change in the crystal structure of each positive electrode active material. Is too low to be a high voltage, high energy density battery. Therefore, in the case of a lithium secondary battery, the capacity of the negative electrode is regulated, and when the capacity of the negative electrode is exhausted, it is necessary to consider that the potential of the negative electrode is kept below the melting potential of the alloy in the nonaqueous electrolyte by using some means. Must.

Table 1 shows the dissolution potentials of lead, bismuth, tin, indium, cadmium, and zinc, which are considered as constituent elements of the alloy of the negative electrode, with respect to lithium in a nonaqueous electrolyte.

Manganese dioxide considered as the above positive electrode active material,
Chromium oxide and vanadium oxide both have a discharge potential of 3 V or more in a non-aqueous electrolyte. Therefore, when a battery with a negative electrode capacity regulation is configured, except for a part of bismuth or zinc, the metal elements shown in Table 1 Will be dissolved.

Therefore, as it is, a secondary battery that can be overdischarged cannot be realized.

In the present invention, the above-mentioned positive electrode active material (first
Together with a second active material that discharges at a potential lower than the dissolution potential of the various metal elements shown in Table 1.

That is, although the battery as a whole is a battery with a regulated negative electrode capacity, the positive electrode first discharges the first active material at a potential of 3 V or more at the positive electrode, and starts discharging the second active material when the capacity is exhausted. . If the battery is designed so that the capacity of the lithium that has been occluded or pressed into the alloy of the negative electrode during the discharge of the second active material is exhausted, the potential of the negative electrode eventually becomes When the discharge potential of the second active material reaches
The battery voltage becomes 0 V, and no more current flows,
The discharge of the battery will end. At that time, as described above, the second active material is discharged at a potential lower than the dissolution potential of various alloying elements. Therefore, these alloying elements must be kept at that potential without being dissolved. become.

That is, an ideal nonaqueous electrolyte lithium secondary battery having excellent overdischarge resistance can be obtained.

Note that molybdenum oxide (Mo) was used as the second active material.
O ₂ ), molybdenum trioxide, molybdenum disulfide, molybdenum trisulfide, titanium disulfide, titanium oxide, tungsten oxide, niobium disulfide, niobium trioxide (Nb ₂ O ₅ ), niobium selenide (NbSe ₃ ), etc. . All of these are active materials that discharge at a potential of 2.4 V or less with respect to lithium, and meet the above conditions.

EXAMPLES (Example 1) Manganese dioxide as a first positive electrode active material and titanium disulfide as a second positive electrode active material were mixed at a weight ratio of 2: 1.
The aqueous dispersion of this mixture, the copolymer of carbon black as the conductive material and the copolymer of ethylene tetrafluoride and propylene hexafluoride as the binder was respectively 100: 5 by weight ratio (however, the aqueous dispersion is a solid content conversion). After mixing at a ratio of 10 and drying, the mixture was pressed into a disk having a diameter of 15 mm and a thickness of 0.5 mm to obtain a positive electrode. Using this positive electrode, the flat battery shown in FIG. 1 was assembled. As the negative electrode, one obtained by melting lead, indium, and cadmium at a weight ratio of 75: 5: 20, respectively, and pressing lithium on an alloy was used.

In FIG. 1, reference numeral 1 denotes a sealing plate made of nickel-plated stainless steel, on which an alloy negative electrode 2 is spot-welded on its inner surface, and lithium 3 is pressed thereon. Reference numeral 4 denotes a polypropylene separator impregnated with an electrolyte obtained by dissolving lithium perchlorate in a solvent composed of 1,3-dioxolane at a ratio of 1 mol / mol. Reference numeral 5 denotes the disc-shaped positive electrode, and a titanium current collector 7 spot-welded to a stainless steel case 6.
It is crimped on. 8 is a polypropylene gasket,
The dimensions of the completed battery are 20mm in diameter and 1.6mm in thickness. The theoretical electric capacity of manganese dioxide as the first active material in the positive electrode was 24 mAh, the theoretical electric capacity of titanium disulfide as the second active material was 8 mAh, and the theoretical electric capacity of lithium pressed on the alloy negative electrode was 40 mAh. . The battery was discharged at 20 ° C with a current of 1 mA to 2 V and then charged with a current of 1 mA to 3.4 V three times, and then a hole was made in the side of the battery. When lithium was used as a reference electrode in the same electrolyte used and similarly discharged at 20 ° C. and a current of 1 mA,
FIG. 2 shows the behavior of the positive electrode and the negative electrode, and the battery voltage.

As is clear from FIG. 2, manganese dioxide, which is the first active material of the positive electrode, first discharges, and about 19 hours later, titanium disulfide, which is the second active material, starts discharging. While titanium disulfide discharges lithium from about 2.2V, the capacity of the negative electrode lithium is exhausted, and the potential of the negative electrode rapidly rises to reach 1.9V. Therefore, at this point, the battery voltage becomes 0V. If the discharge is further continued, the potentials of the positive and negative electrodes are reversed. However, in actual use, since the discharge is generally performed with a constant resistance, the discharge ends when the voltage of the battery becomes 0V. For this reason, even if the battery is left as it is, the positive electrode is in a state where the capacity of the second active material remains, and does not change at all, and the negative electrode also has a melting potential of lead, indium, and cadmium, which are constituent elements of the alloy. Since it is kept at 1.9V, which is much lower, there is no change. In this case, the fact that the amount of lithium charged in the negative electrode is 40 mAh and the actual amount of discharge electricity is 24 mAh indicates that the amount of manganese dioxide in the alloy negative electrode and the positive electrode that is taken out and does not come out is 16 mAh. ing. Next, after leaving this battery for one month in this state, it was charged with a current of 1 mA at 20 ° C. until the battery voltage reached 3.4 V, and discharged again at 1 mA. Was. This shows that in this battery system, even if the battery is overdischarged until the voltage becomes 0 V, no change occurs in the system itself.

Example 2 Chromium oxide (Cr ₂ O ₅ ) as a first active material and molybdenum disulfide as a second active material were mixed at a weight ratio of 5: 2, and this mixture and carbon as a conductive material were mixed. The aqueous dispersion of the copolymer of black and ethylene tetrafluoride / hexafluoropropylene as the binder is mixed in a weight ratio of 100: 5: 10 (however, the aqueous dispersion is a solid content conversion). After drying, a positive electrode is formed by pressing into a disk having a diameter of 15 mm and a thickness of 0.5 mm. Using this positive electrode, the flat battery shown in FIG. 1 was assembled in the same manner as in Example 1. The negative electrode is
50:25:25 lead, bismuth and cadmium in weight ratio
, And an alloy was prepared by pressing lithium on an alloy. The structure of the battery was exactly the same as that of Example 1, except that propylene carbonate and 1,3-
A mixture in which dioxolane was mixed at a volume ratio of 1: 1 and lithium perchlorate was dissolved at a ratio of 1 mol / l was used.
The theoretical electric capacity of chromium oxide as the first active material in the positive electrode was 24 mAh, the theoretical electric capacity of molybdenum disulfide as the second active material was 5.5 mAh, and the theoretical electric capacity of lithium pressed on the alloy negative electrode was Is 38 mAh.

After repeating the cycle of discharging this battery at 20 ° C. at a current of 1 mA to 2 V at 20 ° C. and then charging it at a current of 1 mA up to 3.4 V in the same manner as in Example 1, a hole was made in the side of the battery. FIG. 3 shows the behavior of the positive and negative electrodes and the voltage of the battery when lithium was used as the reference electrode in the same electrolyte used for the battery in a dry air atmosphere and the battery was also discharged at 20 ° C. and a current of 1 mA.

As can be seen from FIG. 3, as in the case of FIG. 2, chromium oxide, which is the first active material of the positive electrode, first discharges, and after about 19 hours, molybdenum disulfide, which is the second active material, discharges. To start. Molybdenum disulfide also starts discharging at about 2.2 V, as in the case of titanium disulfide. In the meantime, the capacity of lithium on the negative electrode is exhausted, and the potential of the negative electrode rises, reaches 2.0 V, which is the discharge potential of the positive electrode at that time, the battery voltage becomes 0 V, and the discharge ends. As in Example 1, this potential is much lower than the dissolution potential of the constituent elements of the alloy negative electrode, so that no change occurs in the battery itself. Subsequently, after the battery was left for one month, the battery was charged again at 20 ° C. with a current of 1 mA until the battery voltage reached 3.4 V, and discharged under the same conditions. The same behavior as in FIG. 3 was exhibited. The same pattern was repeated several times, but the behavior was almost unchanged.

Advantageous Effects of the Invention As is apparent from the above, according to the present invention, the first active material and the second active material appropriate for the positive electrode are selected, and the discharge capacity of the second active material is maintained. By designing a battery so that the dischargeable electric capacity of lithium pressed or occluded on the alloy negative electrode is exhausted, it is possible to provide a non-aqueous electrolyte lithium secondary battery capable of overdischarging the battery voltage to 0 V. The effect is obtained. At the same time, in the present invention, the selection of the second active material of the positive electrode is an important point, and in order to make the battery over-dischargeable any number of times, the active material itself must have reversibility of charge and discharge. No. In addition, it goes without saying that this active material is not limited to those described in the examples, but covers all the materials satisfying the above conditions.

[Brief description of the drawings]

FIG. 1 is a sectional view of a flat battery used in an embodiment of the present invention,
2 and 3 are diagrams showing overdischarge characteristics of the battery of the present invention. 1 ... sealing plate, 2 ... alloy negative electrode, 3 ... lithium, 4 ...
… Separator, 5… positive electrode, 6… case, 7… current collector, 8… gasket.

──────────────────────────────────────────────────続 き Continuing on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H01M 4/58 H01M 4/58 (72) Inventor Yukio Nishikawa 1006 Kazuma Kazuma Kadoma Matsushita Electric Industrial Co., Ltd. In-company (72) Inventor Toru Matsui 1006 Kazuma Kadoma, Matsushita Electric Industrial Co., Ltd. (56) References JP-A-59-165372 (JP, A) JP-A-62-27455 (JP, A)

Claims (3)

(57) [Claims] 1. A positive electrode comprising a chargeable and dischargeable first active material and a second active material, and an alloy which occludes lithium during charging and releases lithium during discharging is press-bonded with lithium or preliminarily stores lithium. A negative electrode made of an alloy
In a battery provided with a non-aqueous electrolyte, the first active material discharges at a higher potential than the potential at which the alloy dissolves in the non-aqueous electrolyte, and the second active material discharges at a lower potential. And the amount of dischargeable electricity of lithium press-bonded to the alloy or inserted into the alloy is the first.
A non-aqueous electrolyte lithium secondary battery characterized by being larger than the dischargeable amount of electricity of the active material and smaller than the sum of the dischargeable amounts of electricity of the first active material and the second active material. . 2. The method according to claim 1, wherein the first active material of the positive electrode is manganese dioxide (Mn).
O ₂ ), chromium oxide (CrOx), vanadium oxide (VO
x), wherein the second active material is molybdenum oxide (MoO ₂ ), molybdenum trioxide (MoO ₃ ),
Molybdenum disulfide (MoS ₂ ), molybdenum trisulfide (Mo
S ₃ ), titanium disulfide (TiS ₂ ), titanium oxide (TiO ₂ ), tungsten oxide (WO ₃ ), niobium disulfide (NbS ₂ ),
Niobium oxide (Nb ₂ O ₅ ), niobium selenide (NbS
The non-aqueous electrolyte lithium secondary battery in the range first claim of e ₃₎ is a type chosen from the group consisting of claims. 3. The alloy for absorbing and releasing lithium of the negative electrode is tin (Sn), lead (Pb), bismuth (Bi), indium (I
n) at least one selected from the group consisting of
3. The non-aqueous electrolyte lithium secondary battery according to claim 1, wherein the non-aqueous electrolyte lithium battery comprises n) and one selected from the group consisting of cadmium (Cd).