JPH0558224B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a non-aqueous electrolyte secondary battery, particularly to improvements in its positive electrode.

Conventional technology Until now, alkali metals such as lithium and sodium have been used as negative electrode active materials, and lithium perchlorate and lithium borofluoride have been used as solutes in solvents such as γ-butyrolactone, tetrahydrofuran, propylene carbonate, and dimethoxyethane. Development of secondary batteries using so-called non-aqueous electrolytes in which lithium chloride and the like are dissolved has been progressing.

However, this type of secondary battery has not yet been put into practical use. The reason is that the lifespan of charging and discharging is short.
This is also due to the low charging and discharging efficiency during charging and discharging, and the cause of this property deterioration is mainly a decrease in chemical or physical reversibility during charging and discharging of the positive electrode and negative electrode active materials.

As for positive electrode active materials, oxides and chalcogen compounds having a layered structure or tunnel structure, such as titanium, vanadium, chromium, and molybdenum, have been known so far. Among these, TiS ₂ ,
Chalcogen compounds such as VSe ₂ have excellent reversibility during charging and discharging. However, since chalcogen compounds have a low density, their energy density per volume is low. On the other hand, oxides that have a high density and a metal element with a high oxidation number tend to have a high voltage. Therefore, it is preferable to use an oxide as the positive electrode active material rather than a chalcogen compound from the viewpoint of increasing energy density.

Vanadium oxides such as V ₂ O ₅ and V ₆ O ₁₃ are being considered as positive electrode active materials having high voltage, large charge/discharge capacity, ie, high energy density as described above. Furthermore, in terms of cycle characteristics, these also exhibit good reversibility up to a certain discharge voltage. However, upon discharge to a voltage below this, ie, overdischarge, these oxides transform into an irreversible structure. Even if the battery is charged again after this, the charging and discharging behavior is completely different from that before overdischarge, and in particular, the discharge curve is not flat and decreases monotonically as the battery discharges, which is extremely inconvenient for the battery. Furthermore, the charge/discharge capacity rapidly decreases with cycles when overdischarge is performed. In the case of V ₂ O ₅ , in order to maintain good reversibility, the lower limit of the discharge potential must be 2.4 V with respect to lithium.
In the case of V ₆ O ₁₃ it is 1.7V. When discharging to a potential lower than this, the subsequent charging/discharging behavior changes significantly.

Problems to be Solved by the Invention As described above, a lithium secondary battery using vanadium oxide as a positive electrode active material has good charge/discharge behavior and a high energy density only under the condition of controlling the discharge voltage. However, there was a problem that it deteriorated when over-discharged.

The present invention eliminates these conventional drawbacks, and provides a highly reliable battery with high energy density and excellent charging/discharging behavior with no change in the charging/discharging curve even when overdischarged. The purpose is to provide a non-aqueous electrolyte secondary battery.

Means for Solving the Problems The non-aqueous electrolyte secondary battery of the present invention has a positive electrode active material.
It is characterized by using an oxide represented by xZrO ₂ ·yV ₂ O ₅ (where y/x is 1 to 10).

Effect The above oxide xZrO ₂ yV ₂ O ₅ can be obtained by heat treating a mixture of zirconium oxide (ZrO ₂ ) and vanadium pentoxide (V ₂ O ₅ ) at a temperature of 650°C or higher in the atmosphere. I can do it.

When y/x is smaller than 1, the energy density is small, and when y/x is larger than 10, the discharge curve loses its flatness.

Example The positive electrode active material used as the test electrode was a mixture of ZrO ₂ and V ₂ O ₅ in various ratios and fired at a temperature of 650° C. or higher under atmospheric pressure. As an example, when a mixture of zirconium oxide and vanadium oxide in a molar ratio of 1:1 is baked, ZrV ₂ O ₇ is produced. All tests were conducted with flat batteries.

The above baked product, acetylene black, and tetrafluoroethylene resin were mixed in a weight ratio of 100:10:15. 200 mg of the mixture was molded and crimped into a battery case to which a titanium expanded metal current collector was spot-welded. The diameter of the electrode plate is 17.5mm. Metal lithium with a thickness of 0.38 mm was used as the negative electrode, and it was pressure-bonded to a sealing plate to which a nickel expanded metal current collector was spot-welded.

The electrolyte used was a mixture of propylene carbonate and dimethoxyethane in equal volumes, with lithium perchlorate dissolved at a ratio of 1 mole/mole. To prevent this, a polypropylene nonwoven fabric was used as the separator.

Note that a battery used as a comparative example was also constructed in the same manner as above except for the positive electrode active material.

The battery constructed in this way was charged and discharged at a constant current of 2 mA and a voltage range of 1.2 to 3.8 V.

FIG. 1, FIG. 2, and FIG. 3 respectively show the case where ZrV ₂ O ₇ , which is an example of the present invention, is used as the positive electrode active material.
When using V ₂ O ₅ as a comparative example, and with ZrO ₂
The discharge curves of the first cycle and the second cycle are shown when a mixture with V ₂ O ₅ (molar ratio 1:1) is used. From this, V ₂ O ₅ alone and ZrO ₂
It can be seen that for the mixture of V ₂ O ₅ and V 2 O 5 , the discharge behavior in the first and second cycles is significantly different. That is, when overdischarging is performed, the flatness of the discharge curve disappears, and the discharge capacity also deteriorates significantly.

On the other hand, with ZrV ₂ O ₇ , although there is a slight decrease in discharge capacity in the second cycle compared to the first cycle, the flatness of the discharge curve is good and the charge/discharge behavior is satisfactory. .

FIG. 1 depicts a discharge curve in the second cycle when the y/x value of xZrO ₂ ·yV ₂ O ₅ is changed. From this, it can be seen that when the y/x value is smaller than 1, although the discharge curve is flat, the discharge capacity is small. On the other hand, if the y/x value is greater than 10, the discharge curve will not be flat and will fall monotonically, as shown in FIG. 1, which is inconvenient for the battery. The flatness of the discharge curve is particularly good when y/x is 1 to 5.

Effects of the Invention As described above, according to the present invention, a nonaqueous electrolyte secondary battery with high energy density and excellent charge/discharge behavior with no change in the charge/discharge curve even when overdischarged can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the discharge curves of the first cycle and the second cycle in the non-aqueous electrolyte secondary battery of the example of the present invention, and FIGS. 2 and 3 are diagrams showing the discharge curves in the comparative example, Figure 4 shows V ₂ O ₅ /ZrO ₂
FIG. 3 is a diagram depicting a discharge curve with respect to the ratio (y/x).

Claims (1)

[Claims] 1 A positive electrode, an alkali metal ion conductive non-aqueous electrolyte, and a negative electrode having an alkali metal as an active material are the constituent elements, and the active material of the positive electrode has the formula xZrO ₂ yV ₂ O ₅
A nonaqueous electrolyte secondary battery characterized by being an oxide represented by (where y/x is 1 to 10).