JPH0547384A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To provide a battery with excellent overdischarge resistance at a high temperature in particular by improving a positive electrode in a nonaqueous electrolyte secondary battery using LiCoO2 for the positive electrode and a carbon material for a negative electrode. CONSTITUTION:A Li-containing oxide for controlling potential and an oxide made of Nb, Bi, As, Sb serving as a catalyst poison for suppressing the overdischarge deterioration reaction are added to a positive electrode 1. A composite oxide of Nb, Bi, As, Sb, Li is added to the positive electrode 1, thus the overdischarge deterioration at a high temperature is suppressed.

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, particularly a composite oxide of lithium and cobalt (LiCoO ₂ )
And a carbonaceous material for the negative electrode.

[0002]

2. Description of the Related Art In recent years, portable electronic devices and cordless electronic devices have been rapidly developed, and there is a great demand for a small and lightweight secondary battery having a high energy density as a power source for driving these electronic devices. From this point of view, non-aqueous secondary batteries, particularly lithium secondary batteries, are particularly expected as batteries having high voltage and high energy density.

In particular, recently, a battery system using LiCoO ₂ as a positive electrode active material and a carbon material as a negative electrode has been attracting attention as a high energy density lithium secondary battery. The characteristics of this battery system are that the battery voltage is high (because LiCoO ₂ has a high voltage of 4 V with respect to Li) and that both positive and negative electrodes utilize an intercalation reaction. In particular, since metal Li is not used for the negative electrode, there is no short circuit due to the deposition of dendrite Li and safety and rapid charging can be expected.

Generally, a secondary battery of this kind is basically required to have a high output, a high capacity and a long service life. However, with the recent sophistication of electronic equipment, the equipment is being used. Even when there is no memory, more and more devices consume power for memory backup and control of other control circuits.
That is, if the battery is left attached to the device, the battery continues to be discharged, the capacity is exhausted, and the battery voltage finally reaches 0V. Therefore, the battery is not practical unless it is recovered by being charged again after experiencing such discharge (which is called over-discharge). However, in the case of a lithium secondary battery in which LiCoO ₂ is used for the positive electrode and a carbonaceous material is used for the negative electrode, when such over-discharging is performed, the original capacity is not restored even when charging and discharging again, and the battery capacity is reduced. It had the drawback of becoming low. Then, the over-discharge protection in this cell system, means for adding at Sho 63-228573 and Sho 63-314778 and _{MoO 3, V 2 O 5,} TiO 2 oxide such the positive electrode have been disclosed. But,
In the case of this battery system, the Li source is LiCoO ₂ only,
It was pointed out that a part of Li that contributes to charging / discharging is consumed by the added substance, so that the charging / discharging capacity becomes low and the effect of suppressing the overdischarge deterioration is not so great.
Therefore, as a further improved form thereof, JP-A-2-265
167 discloses a means for adding Li _x MoO ₃ containing Li as a secondary active material to the positive electrode. As a result of a study for confirming the effect of this means, a part of Li, which already contains Li and contributes to charge and discharge, is not consumed by the added substance, and the effect of suppressing over-discharge deterioration is large. there were. However, as a result of further proceeding with this study, although it certainly showed an excellent effect in the vicinity of room temperature (20 ° C. to 30 ° C.), when over-discharging was performed in an environment of high temperature, for example, 60 ° C., charging and discharging were performed again However, there is a drawback that the battery capacity becomes low without recovering the original capacity. Actually, it is considered that it is necessary to take some measures to suppress over-discharge deterioration even in this case, because it can be left at over-discharge while being attached to the device not only near room temperature but also at high temperature. Be done.

Then, a positive electrode made of LiCoO ₂ , a negative electrode made of a carbonaceous material, and a lithium secondary battery made of an organic electrolyte were prototyped, charged and discharged, and then over-discharged by connecting a resistor. Then, the unipolar behavior of each of the positive and negative electrodes at that time is described as Li
As a result of measurement with the reference electrode, as shown in FIG. 4, when the battery voltage reaches 0 V, both positive and negative electrodes are 3.3 V against Li.
It was found that the electric potential was reached nearby. This means that when the positive and negative electrodes become equipotential (battery voltage is 0V), the potential is dominated by the positive electrode. The positive electrode is usually used at a potential around this, and 1 for Li.
Since it is said that reversibility is not impaired even when discharged to near V, it is considered that there is no problem. On the other hand, the negative electrode is usually used at a potential of 1 V or less with respect to Li. That is, it is considered to be a problem that the potential of the negative electrode is maintained at such an extremely noble potential due to overdischarge. For example, in Japanese Patent Laid-Open No. 2-265167, it is pointed out that the negative electrode has such a noble potential, so that the negative electrode current collector is dissolved and deteriorates. Therefore, as a result of investigating the relationship between the potential of the negative electrode and the performance deterioration by the constant potential step method, it was found that
It was found that when the value exceeds 1.0, the capacity characteristic of the negative electrode is significantly deteriorated. Therefore, when the positive and negative electrodes become equipotential (battery voltage is 0V), the equilibrium potential of the positive electrode that controls the potential is maintained at a more base potential (3V or less) to suppress overdischarge deterioration. Is the purpose of JP-A-2-265167,
It was preferable that the auxiliary active material to be added be a Li compound containing Li in advance in order to minimize the loss of Li. By using this means, it was possible to suppress the potential during over-discharging to a low level and suppress the deterioration of the capacity characteristics of the negative electrode, but this effect is effective only near room temperature. For example, in an environment of 60 ° C. When over-discharging was carried out at 1, the deterioration of the capacity characteristics was also recognized after the over-discharging. Therefore,
It cannot be said that the conventional means for controlling the potential alone is sufficient as a countermeasure against the over-discharge at high temperature. Therefore, as a result of a study for investigating the cause of over-discharge deterioration in a high temperature environment, it was found that the electrolyte was decomposed. Further, as a result of examining the decomposition reaction of this electrolytic solution, it was found that the reaction occurred on the surface of the positive electrode, and it was found that this reaction can also occur even if the potential is controlled by the conventional means. Considering that there is no charge transfer between the positive and negative electrodes when 0 V is reached by overdischarge, it is highly possible that this high-temperature overdischarge deterioration is a side reaction that occurs independently on the surface of the positive electrode. Generally, it is said that a Co oxide such as LiCoO ₂ which is an active material is a strong oxidation catalyst for organic substances. In particular, since it is considered that such catalytic reaction activity is promoted in a high temperature environment, a catalytic decomposition reaction of the electrolyte solution, which is an organic substance, occurs as a side reaction during overdischarge at high temperature, which causes deterioration of overdischarge. It is expected to be the cause. Therefore, there is a technical problem that is difficult to recover the secondary battery which is over-discharged especially at a high temperature.

[0006]

The conventional problems to be solved by the present invention are, as mentioned above, high temperatures such as 60 ° C.
The secondary battery, which has been over-discharged under the above environment, cannot maintain the original battery capacity even when it is charged and discharged again, that is, the over-discharge deterioration at high temperature cannot be suppressed.

[0007]

The present invention is a non-aqueous electrolyte secondary battery using a positive electrode mainly composed of LiCoO ₂ and a negative electrode mainly composed of a carbonaceous material. It contains a composite oxide of at least one metal selected from the group of As, Sb and Bi and Li. Further, the positive electrode contains a Li-containing oxide having a discharge potential of 3 V or less with respect to Li, and at least one metal selected from the above group, or a compound of the metal. Particularly, the compound of the above metal is preferably an oxide.

[0008]

The present invention is characterized in that a compound of a metal selected from the group of Nb, As, Sb and Bi is added to the positive electrode, and these compounds suppress the oxidation catalytic activity of Co oxide with respect to organic substances. It acts as a catalyst poison. By using these compounds in addition to actually controlling the potential, it was possible to suppress the over-discharge deterioration at high temperature. From this, it is clear that one cause of over-discharge deterioration under high temperature is the catalytic decomposition reaction of the electrolytic solution, and the means of the present invention for suppressing this reaction is a very effective means. .. Therefore, in order to suppress deterioration due to over-discharge, the positive electrode contains a secondary active material having a discharge potential of 3 V or less with respect to Li, and in order to exert this effect even at high temperature, Nb, As, Sb, Bi It can be said that a means for containing at least one metal selected from the group or a compound of the metal is effective. In particular, when a composite compound of at least one metal selected from the above group and Li is used, control of the potential and suppression of the catalytic decomposition reaction of the electrolytic solution can be achieved at the same time. Further, as a result of further studies, it is preferable that both the side active material and the compound of the above-mentioned metal to be added are oxides, and that the compound oxide is also preferable when forming a compound compound with Li. It is a study of.

[0009]

Embodiments of the present invention and comparative examples will be described below with reference to the drawings.

FIG. 1 is a vertical cross section of a coin-type battery used to demonstrate the battery characteristics of the embodiment of the present invention and the comparative example.
In FIG. 1, the positive electrode 1 is a mixture of an active material with carbon powder (5% by weight based on the active material) which is a conductive material, and tetrafluoroethylene resin powder (7% by weight based on the active material) which is a binder. The titanium net 3 is fixed by spot welding to the inside of the positive electrode case 2 and is press-molded. Also,
The negative electrode 4 is a mixture of carbonaceous material powder which is an active material and polyacrylic acid type resin powder (5% by weight based on the carbonaceous material) which is a binder, and is spot-welded inside the sealing plate 5. It is press-molded on the stainless steel net 6 fixed by. Then, these were sealed together with a polypropylene separator 7 and an electrolytic solution 8 through a polypropylene gasket 9 to obtain a completed battery having a diameter of 20 mm and a height of 1.6 mm. The electrolytic solution used was 1 mol of lithium perchlorate dissolved in a mixed solvent of propylene carbonate and ethylene carbonate. This battery is in a discharged state immediately after trial manufacture and starts from charging.

(Comparative Example 1) The curve shown by the broken line in FIG. 5 is a constant current charge / discharge of 2 mA in the case of the battery of Comparative Example 1 (battery using only LiCoO _{2 for the} positive electrode), and the charge end voltage was 4 It is the charging / discharging voltage characteristic of the 10th cycle when the voltage is set to .1 V and the discharge end voltage to 3.0 V. In the case of this battery, the average discharge voltage was 3.7V. The curve shown by the broken line in FIG. 6 shows the discharge capacity-cycle characteristics when this charging / discharging is repeated. As is clear from FIG. 6, the discharge capacity is maintained at 90% or more of the initial value even after 100 cycles, and the cycle reversibility is excellent. Therefore, first, at room temperature (20 ° C)
The degree of deterioration of the battery performance due to over-discharge in the above was investigated. Over-discharging means that after 10 cycles of charging and discharging under the above conditions, the battery is taken out in a discharged state, discharged with a resistance of 50Ω, and after reaching 0 V, it is left for 10 days with the resistance connected. Is. After this over-discharging was experienced at the 10th cycle, charging and discharging were performed again, and as a result, the charge-discharge voltage characteristics were reduced in capacity by nearly 20%. Then, even when the cycle was repeated, the capacity remained low as shown by the solid line in FIG. Therefore,
It was found that the capacity characteristics of this battery deteriorated by experiencing over-discharge at room temperature.

(Comparative Example 2) Next, the potential of the positive electrode is controlled.
Comparative Example 2 in which a secondary active material is added to suppress over-discharge deterioration
The applied battery is shown below. LiCoO in the positive electrode₂In addition to
By LiMn₂O _FourUsing an active material in which 5 mol% of
A coin-type battery similar to the above was prototyped. And Comparative Example 1
Perform a charge-discharge test including over-discharge at room temperature under the same conditions as above.
It was. The curve shown by the broken line in FIG.
The characteristics of the charge and discharge voltage of the fourth cell are those of Comparative Example 1 (breakdown of FIG. 5).
You can see that it is almost the same as the line). Also, the discharge capacity
The quantity-cycle characteristics are also shown in the comparative example as shown by the broken line in FIG.
The characteristics of the battery No. 1 (dashed line in Fig. 6) are almost the same.
I understand. Therefore, in the positive electrode, LiMn₂O_FourAdd
Is almost unfavorable for at least normal charge / discharge characteristics.
It is not considered to have any impact. Then this
Batteries undergo the same over discharge as above, and then charge and discharge again.
As a result, as shown by the solid line in FIG.
It was only about 2%. And cycle further
Even after returning, as shown by the solid line in Fig. 8, the subsequent cycle
The characteristics were excellent. As described above, LiMn
₂O_FourSuppresses over-discharge deterioration by adding
It became clear that there is an effect. Investigate this cause
As a result, the unipolar behavior of the positive and negative electrodes was measured, and as a result, it reached 0V.
Position where the potential of positive and negative electrodes at time is less than 3.0V against Li
It was found that the deterioration of the negative electrode capacity characteristics
It is speculated that this was suppressed.

However, as a result of the battery being subjected to the same over-discharging as described above in an environment of 60 ° C. and then charged and discharged again, the capacity deterioration was about 25% as shown by the broken line in FIG. However, as is clear from comparison with FIG. 7, the voltage characteristics in particular change significantly (polarization increases). The characteristics did not recover even after repeated cycles, and the capacity remained low. In particular, it is expected that the cause of such large polarization is that some side reaction (which is considered to be decomposition of the electrolytic solution) occurs during overdischarge at high temperature, and this inhibits the electrode reaction. .. Although LiMn ₂ O ₄ was used as the sub-active material in Comparative Example 2, other Li-containing composite oxides, for example, those composed of Mo, V, Fe or the like and Li, and oxides other than Li-containing composite sulfide or the like were used. I also examined the thing. As a result, all showed the effect of controlling the electric potential to some extent, and were effective in suppressing the over-discharge deterioration at room temperature.
However, most of the above Li-containing composite compounds have high temperatures (6
It deteriorated when over-discharged in an environment of 0 ° C. Therefore,
It was considered that the means for controlling the potential alone was not sufficient to suppress the over-discharge deterioration at high temperature.

(Example 1) Next, the potential of the positive electrode is controlled.
Comparative Example 2 in which a secondary active material is added to suppress over-discharge deterioration
Regarding the battery of the present invention, which is a further improvement of the applied battery,
An example of is shown. LiCoO for the positive electrode₂In addition to LiM
n₂O_FourOf the active material used in Comparative Example 2 containing 5 mol% of
And further Nb₂O_FiveUsing an active material containing 3 mol% of
It was Then, a coin-type battery was prototyped using this active material. Next
Under the same conditions as in Comparative Example 2 under an environment of 60 ° C.
After being charged and discharged again, the result of Fig. 2
As shown by the solid line, the capacity characteristics are those of the battery of Comparative Example 2.
It was superior to (dotted line in FIG. 2). further
After that, the cycle was repeated, but the cycle was reversible.
Is excellent and has the characteristics and characteristics of a battery that does not experience over discharge.
I got something that didn't change much. From this result,
Nb with almost no effect on characteristics₂O_FiveIs hot
It is clear that it has the effect of suppressing over-discharge deterioration at
It Especially Nb₂O _FiveAddition occurs during overdischarge at high temperature.
Assuming that this side reaction is due to the oxidation catalyst reaction of the positive electrode,
It was performed in anticipation of acting as a catalyst poison for this reaction.
It was. Thus, Nb₂O_FiveBy using
Therefore, the idea of catalyst poison
By applying it, it is possible to suppress over-discharge deterioration at high temperature.
It is considered to be one. Therefore, there is a possibility as a catalyst poison.
Bi, As, Sb and other metals or oxidation of these metals
As a result of examining the effect of addition,
It showed good results for thermal overdischarge. However, a similar effect is expected.
Wait, Nb, Bi, As, Sb and other sulfides and selenization
We also examined the addition of substances, but did not add over-discharge deterioration.
Although smaller than the oxide, is the effect smaller than the oxide?
It was. Especially when sulfide or selenide is used,
It was found that sulfur and selenium components were eluted in the
It was In addition, as a side active material that controls the potential of the positive electrode,
In Example 1, LiMn₂O_FourWas used, but Li₂MnO_Four，
Li₃VO_Four, Li₂SnO₃Other oxides such as
As a result of examining also, the same effect is shown in any case.
I understood. However, it does not contain Li that controls the potential of the positive electrode.
If sulfide or selenide is used as a secondary active material,
The effect of warm over-discharging was excellent, but that of high-temperature over-discharging
In the same manner as above, when sulfur or selenium is added to the electrolyte,
The component was eluted and its effect was small. So Li
CoO₂Active material for controlling the potential applied to the catalyst and catalyst
The results of further studies on both of the poisonous additives
As a result, in order to suppress the over discharge deterioration at high temperature,
In any case, due to the chemical stability of
I found it desirable.

Next, the amount of the auxiliary active material added to LiCoO ₂ for controlling the potential and the additive amount of the catalyst poison were examined. As a result, the auxiliary active material (composite oxide containing Li) added for controlling the potential should be at least 3 mol% or more with respect to LiCoO ₂ , and the additive (oxide) that becomes a catalyst poison should be at least 2 mol% or more. I found out.

(Embodiment 2) Next, according to the present invention, it is expected that the effects of both a side active material for controlling the potential added to LiCoO ₂ and an additive which becomes a catalyst poison will be exhibited at the same time. Li ₃ Nb which is a complex oxide of Li and Nb
A study was conducted to add O ₄ . The active material is LiCoO ₂
Was used to which 5 mol% of Li ₃ NbO ₄ was added. Then, using this active material, a coin-type battery similar to that of the above-described Example 1 was manufactured. Next, under the same conditions as in Example 1 above, after experiencing over-discharge in an environment of 60 ° C., charging / discharging was performed again. Fig. 3 shows the charge / discharge voltage characteristics of this battery. The characteristics before over-discharging (broken line in Fig. 3)
And a characteristic (solid line) after undergoing over-discharge at the above-mentioned high temperature, and also in the case of this battery, similarly to the battery of the present invention of Example 1, the performance deterioration due to the over-discharge at the high-temperature is extremely small. It was excellent. After that, the cycle was repeated, but the cycle reversibility was excellent, and the characteristics which were almost the same as the characteristics of the battery that did not experience overdischarge were obtained. Therefore, it is clear that even in the case of this battery, the effects of both potential control and catalyst poison can be exhibited at the same time with almost no effect on other characteristics.
In Example 2, although the result concerning the composite oxide of Li and Nb was described, as a result of examining the case of using Bi, As or Sb instead of Nb, it was found that almost the same effect was obtained.

Next, the amount of the above composite oxide added to LiCoO ₂ was examined. As a result, it was found that in any of the above-mentioned composite oxides, a sufficient effect can be obtained by adding at least 3 mol% or more to LiCoO ₂ . Therefore, the amount of addition is smaller than that of the battery of the present invention described in Example 1, and this means is considered to be a more effective means.

[0018]

As is apparent from the above description, by applying the present invention, even if the battery is over-discharged while it is still attached to the device, especially when it is over-discharged at a high temperature, the battery can be charged again. Since the performance is restored, it is possible to provide a non-aqueous electrolyte battery which is extremely advantageous in practical use.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of coin type batteries used in Examples of the present invention and Comparative Examples.

FIG. 2 is a diagram showing a comparison of charge / discharge voltage characteristics of the batteries of Example 1 and Comparative Example 2 of the present invention.

FIG. 3 is a diagram showing charge / discharge voltage characteristics of the battery of Example 2 of the present invention.

FIG. 4 is a diagram showing unipolar behavior during positive and negative electrode overdischarge.

5 is a diagram showing charge / discharge voltage characteristics of the battery of Comparative Example 1. FIG.

FIG. 6 is a diagram showing capacity-cycle characteristics of the battery.

7 is a diagram showing charge / discharge voltage characteristics of the battery of Comparative Example 2. FIG.

FIG. 8 is a diagram showing capacity-cycle characteristics of the battery.

[Explanation of symbols]

 1 Positive electrode 2 Positive electrode case 3 Titanium net 4 Negative electrode 5 Sealing plate 6 Stainless steel net 7 Separator 8 Electrolyte 9 Gasket

Claims (7)

[Claims] 1. A non-aqueous electrolyte secondary battery using a positive electrode mainly composed of a composite oxide of lithium and cobalt and a negative electrode mainly composed of a carbonaceous material, wherein the positive electrode is Nb, As, S.
A non-aqueous electrolyte secondary battery containing a composite oxide of at least one metal selected from the group of b and Bi and lithium. 2. The composite oxide of at least one metal selected from the group of Nb, As, Sb and Bi contained in the positive electrode and lithium has a lower limit of 3 mol% with respect to LiCoO ₂ . Non-aqueous electrolyte secondary battery. 3. A non-aqueous electrolyte secondary battery using a positive electrode mainly composed of a composite oxide of lithium and cobalt and a negative electrode mainly composed of a carbonaceous material, wherein the positive electrode is 3 relative to Li.
Non-aqueous electrolyte containing a lithium-containing compound having a discharge potential of V or less as a secondary active material and containing at least one metal selected from the group of Nb, As, Sb and Bi or a compound of the metal as an additive. Secondary battery. 4. The non-aqueous electrolyte secondary battery according to claim 3, wherein the secondary active material contained in the positive electrode is a lithium-containing oxide. 5. The additive contained in the positive electrode is an oxide.
Alternatively, the non-aqueous electrolyte secondary battery described in 4. 6. The non-aqueous electrolyte secondary battery according to claim 3, 4 or 5, wherein the lower limit of the sub-active material contained in the positive electrode is 3 mol% with respect to LiCoO ₂ . 7. The non-aqueous electrolyte secondary battery according to claim 3, 4, 5 or 6, wherein the lower limit of the additive contained in the positive electrode is 2 mol% with respect to LiCoO ₂ .