JP2001023697A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the crystal breakdown of WO3 in over discharge, to lengthen the charge/discharge cycle life, and to enhance over discharge characteristics in a nonaqueous electrolyte secondary battery containing WO3. SOLUTION: The nonaqueous electrolyte secondary battery has a positive electrode 5 containing tungsten trioxide (WO3) and a compound capable of doping/undoping lithium, a negative electrode 6, and a nonaqueous electrolyte, and as the compound, a material having a reduction potential of average 1 V or higher vs. metal lithium when discharged at a current density of 0.1 mA/cm2 is used. The compound for composing the positive electrode 5 contains at least either one of lithium titanium oxide (Li4/3Ti5/3O4) having spinel structure and niobium pentoxide (Nb2O5), and the mixing ratio in weight of the compound to tungsten trioxide (WO3) composing the positive electrode 5 is 50-200 to 100 of tungsten trioxide.

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 used as a DC power supply for a portable terminal, a memory backup power supply, and the like.

[0002]

2. Description of the Related Art With the rapid development of technologies in the field of electronics in recent years, the miniaturization of electronic devices has been progressing. The power supply for these devices is small, lightweight and high energy
The demand for a battery having a high density is increasing, and a lithium secondary battery using lithium for a negative electrode has attracted attention and is being studied worldwide. The trend of research so far has been to study 4V-class cathodes such as lithium cobaltate, spinel manganate, and lithium nickelate, as well as 3V-class cathodes such as lithium-containing manganese oxide and vanadium pentoxide. Have been put to practical use.

However, recently, in response to a demand to reduce power consumption of equipment, the operating voltage of a semiconductor device has been reduced, and a battery with a low charge / discharge voltage has been demanded. 0.5-1.5 V class tungsten trioxide (hereinafter referred to as WO ₃ ), niobium pentoxide (hereinafter referred to as Nb)
₂ O ₅ ), lithium titanium oxide having a spinel structure (hereinafter, Li _4/3 Ti _5/3 O ₄ ) and the like have been studied.

In the case of tungsten trioxide in particular, electrochemical magazine No. As shown in 9.599 (1981), the charge / discharge curve has a slope type. In a battery using only tungsten trioxide as a positive electrode and a lithium aluminum alloy as a negative electrode, the discharge curve shows an inclined type, as in the case described above. This battery has a voltage of 3
Although it is set to V, it can also be used as a 2.5V or 2V class battery. For this reason, it is possible to set a voltage corresponding to a device to be used, and the application as a positive electrode material constituting a nonaqueous electrolyte battery is promising.

[0005]

As a result of various studies conducted by the inventors of the present invention, it has been found that a battery using WO ₃ as a positive electrode is very weak against a charge / discharge cycle having a deep discharge depth and an overdischarge. all right. This is because when WO ₃ becomes lower than about 1 V with respect to metallic lithium, the crystal is gradually destroyed.

An object of the present invention is to prevent the crystal breakage and to improve overdischarge characteristics and charge / discharge cycle life.

[0007]

In order to achieve the above object, a non-aqueous electrolyte secondary battery of the present invention comprises a positive electrode, a negative electrode, a non-aqueous electrolyte prepared by mixing WO ₃ with a compound capable of doping and undoping lithium. The compound is characterized in that the compound has an average reduction potential of 1 V or more when discharged at a current density of 0.1 mA / cm ² with respect to lithium metal.

As the compound, Nb ₂ O ₅ , which operates at substantially 2 V or less with respect to the potential of metallic lithium and has excellent reversibility of charge and discharge, or Li which can be charged and discharged near 1.5 V, _4/3 Ti _5/3 O ₄ is preferred.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The invention according to claim 1 of the present invention relates to a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte containing WO ₃ and a compound capable of doping and undoping lithium. Wherein the compound has an average reduction potential of 1 V or more when discharged at a current density of 0.1 mA / cm ² with respect to lithium metal.

[0010] The battery having the above structure comprises a compound and WO.
_It can be used even at a low charging voltage of about 3V to 1.5V. Further, the reduction voltage of WO ₃ does not drop below 1V. For this reason, it is excellent in overdischarge characteristics of 1 V or less, and even when discharging is performed until the voltage reaches 1 V or less, W
It is possible to repeat charging and discharging in a stable state without destruction of the O ₃ crystal.

According to a second aspect of the present invention, the compound constituting the positive electrode comprises at least one of Li _4/3 Ti _5/3 O ₄ and Nb ₂ O ₅ . According to this, even if the potential drops due to discharge, Li _4/3 Ti mixed with WO _3
5/3 O ₄ and, because it is held at a potential of Nb ₂ O _5, WO ₃
Over discharge characteristics without the potential of
It has the effect of having excellent charge / discharge cycle characteristics.

In the present invention, Li _{4/3 is} used as the compound.
Although Ti _5/3 O ₄ and Nb ₂ O ₅ have been described as examples, doping and undoping of lithium is possible, and the reduction potential (discharge potential) is 0.1 mA / cm with respect to metallic lithium. If the substance has a potential of 1 V or more on average at the current density of ² ,
It is considered that a similar effect can be obtained.

According to a third aspect of the present invention, the mixing ratio of WO ₃ and the compound constituting the positive electrode is specified, and the mixing ratio of the two is from 100 to 50 for WO ₃ in weight ratio. The range is 200.

When the weight ratio of the compound to WO ₃ 100 is less than 50, the ratio of the compound in the positive electrode is relatively reduced, and accordingly, the capacity for a potential of 1 V or less is also reduced. Therefore, the effect of preventing the potential of WO _{3 from} dropping to 1 V or less is reduced. Further, when the weight ratio of the compound exceeds 200, the ratio of WO _{3 in} the positive electrode decreases, and when the potential of WO ₃ reaches 1 V or less, the crystal is immediately destroyed and it is impossible to charge and discharge. Become.

According to the present invention, the electric capacity of the negative electrode is set smaller than the electric capacity of the positive electrode, and the battery capacity is regulated to the negative electrode. By regulating the battery capacity by the negative electrode capacity, the discharge is converged by improving the potential of the negative electrode, and has the effect of preventing the potential of WO _{3 from} dropping to 1 V or less.

When the positive electrode is composed of WO ₃ and a compound, the potential of WO ₃ can be prevented from dropping to 1 V or less. However, when the electric charge is discharged beyond the electric capacity of the positive electrode, the potential of WO ₃ eventually drops to 1 V or less. Therefore, while the potential of the mixture is maintained, the discharge is converged by improving the potential of the negative electrode. Thereby, even when the battery voltage becomes 0 V, it is possible to stabilize the battery in a state where the negative electrode potential has risen to the potential of the mixture of 1 V or more. According to this configuration, even when the battery is in a deep discharge depth or in an overdischarged state, stable charging and discharging can be performed without destroying WO ₃ crystals.

According to a sixth aspect of the present invention, the negative electrode comprises LiAl
Use an alloy. Thereby, generation of dentite at the time of charge and discharge is suppressed, and stable charge and discharge can be performed. As the negative electrode combined with the positive electrode of the present invention, in addition to an Al alloy capable of inserting and extracting lithium, Si, an Si alloy, or a metal oxide which can be charged and discharged at a low potential is preferable. ₂ , LiFeO ₂ , SiO, SnO and the like can be used. Among these, those having stable characteristics particularly at a high potential are preferable, and an Al alloy that absorbs lithium, WO ₂ , and SiO are most suitable.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A specific example of the present invention will be described below with reference to FIGS. The embodiments described below are only examples embodying the present invention, and do not limit the technical scope of the present invention.

Example 1 A positive electrode containing WO ₃ and LiA
FIG. 1 is a cross-sectional view illustrating the configuration of a coin-type nonaqueous electrolyte secondary battery according to the present embodiment including a negative electrode using an l-alloy. 1 is a case also serving as a positive terminal, 2 is a sealing plate also serving as a negative terminal,
Reference numeral 3 denotes a polypropylene gasket for insulating the case from the sealing plate, and reference numeral 4 denotes a separator made of a polypropylene nonwoven fabric.

Reference numeral 5 denotes a positive electrode, and the positive electrode active material is WO ₃ :
Nb ₂ O ₅ was adjusted to 100: 50 and the active material was 90 wt.
%, 5 wt% of carbon black as a conductive material, and 5 wt% of a fluorine resin as a binder were kneaded and molded into pellets having a thickness of about 0.6 mm and a diameter of 12.0 mm. About 30m in electric capacity
Ah. And it vacuum-dried at 200 degreeC, and performed the dehydration process, and it was set as the positive electrode.

Reference numeral 6 denotes a negative electrode, which contains L at 5 wt% of lithium.
An iA alloy was used and the diameter was 15 mm. The capacity ratio was positive electrode: negative electrode = 1: 0.9. The electrolytic solution is a mixture of ethylene carbonate (hereinafter, referred to as EC) and 1,2-dimethoxyethane (hereinafter, referred to as DME) in an equal volume of a mixed solvent of LiN.
(CF ₃ SO ₂ ) ₂ dissolved at a rate of 1 mol / l was used. The size of the battery is 20 mm in diameter and 2.0 mm in thickness. This is called battery A. Further, the positive electrode active material was WO ₃ : Nb ₂ O ₅ = 50: 50, and the positive electrode active material was W.
O ₃ : Nb ₂ O ₅ = 33: 67, and the batteries B and C have the same configuration as the battery A in other configurations, respectively.

(Example 2) Next, in the positive electrode active material, W
The ratio of O ₃ to Li _4/3 Ti _5/3 O ₄ is 100: 5
The batteries were adjusted to 0, 100: 100, and 100: 200, and the other configurations were the same as the battery A.
E and F.

Example 3 Next, WO 3 was used as the positive electrode active material.
₃ , Nb ₂ O ₅ , and Li _4/3 Ti _5/3 O ₄ , respectively, and their weight ratios are 100: 25: 25, 100:
The batteries were adjusted to 50:50 and 100: 100: 100, and the other configurations were the same as the battery A.
H and I.

In the present embodiment, the weight ratios of the two substances to be mixed are equal, but the weight ratios of the substances to be mixed are not necessarily equal, and the weight ratio of the mixture is set to WO _3.
It may be adjusted to 50 to 200 with respect to 100.

In addition, lithium doping and undoping are possible, and the reduction potential (discharge potential) is 0.1 mA / cm ^{2 at} a current density of 0.1 mA / cm ² with respect to metallic lithium. It is possible to mix two or more of these substances as long as the substance has a weight ratio of the mixture of 50 to 200 as described above.
If it is adjusted to, it can be similarly implemented.

Comparative Example 1 Subsequently, the positive electrode active material was WO ₃
In other configurations, the battery under the same conditions as the battery A is denoted by J.

(Comparative Example 2) Further, the capacity ratio of the positive electrode to the negative electrode was set to 1: 1.2, and the battery having the same configuration as the battery A in other configurations was designated as K.

In the above description, E is used as the electrolytic solution.
LiN (CF ₃ SO ₂ ) in a mixed solvent of equal volume of C and DME
_{Although a} solution in which ₂ was dissolved at a ratio of 1 mol / l was used, a general electrolytic solution used in a non-aqueous electrolyte secondary battery can be similarly implemented.

The batteries A to K were subjected to a charge / discharge cycle test and an overdischarge test.

The charge / discharge cycle test was performed at 20 ° C.
At a constant current of mA, the charge upper limit cut voltage was set to 2.5 V, the discharge lower limit cut voltage was set to 0.5 V, and charge / discharge was set to 5 V.
The cycle was repeated. The electric capacity retention rate after 5 cycles when the initial capacity was 100% was measured. The results are shown in (Table 1).

[0031]

[Table 1]

[0032] maintains the discharge capacity of about 90% or more after the battery A~I in 5 cycles products of the present invention, compared with the battery J, Nb ₂ O ₅ to WO _3, Li _4/3 Ti _5/3 Mixing O ₄ has a considerable effect on improving the charge / discharge cycle characteristics. Also, as you can see when comparing with Battery K,
The regulation of the negative electrode is also effective in improving the charge / discharge cycle characteristics.

Next, an overdischarge test at a high temperature of 60 ° C. will be described. In this test, a 3 kΩ load was connected and 2
It is kept for 0 days. When the battery is continuously discharged with a load of 3 kΩ, the terminal voltage of the battery becomes close to 0 V in about one day.
After being kept as it is for 20 days, it was charged at 2.5 V, and its recovery capacity was calculated with the initial capacity being 100%. The measurement results are shown in (Table 2).

[0034]

[Table 2]

Similar to the results of the charge / discharge cycle test, the batteries A to I have about 7 days even after 20 days of overdischarge at a high temperature of 60 ° C.
It can be seen that the recovery capacity of 5% or more is maintained, and the overdischarge characteristics are improved as compared with the batteries J and K.

[0036]

As described above, according to the present invention, WO ₃ has a potential of 1 V or more in metallic lithium and a compound having reversibility of charge and discharge, that is, Li _4/3 Ti _5/3 O ₄ , Nb ₂ O
By mixing at least one of the _five
WO ₃ crystal breakage can be prevented, and the charge / discharge cycle life and overdischarge characteristics can be improved. Further, by restricting the negative electrode, the overdischarge characteristics are further stabilized, and the charge / discharge cycle characteristics are more effectively improved.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a coin-type battery according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Case 2 Sealing plate 3 Gasket 4 Separator 5 Positive electrode 6 Negative electrode

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference)

Claims (5)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte containing tungsten trioxide (WO ₃ ) and a compound capable of doping and undoping lithium. 0.1 mA /
A non-aqueous electrolyte secondary battery having an average reduction potential of 1 V or more when discharged at a current density of cm ² . 2. The compound constituting the positive electrode contains at least one of lithium titanium oxide having a spinel structure (Li _4/3 Ti _5/3 O ₄ ) and niobium pentoxide (Nb ₂ O ₅ ). 2. The non-aqueous electrolyte secondary battery according to 1. _3. The mixing ratio of the tungsten trioxide (WO ₃ ) and the compound constituting the positive electrode is such that the weight ratio of tungsten trioxide is 100 and the compound is 50 to 20.
3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the value is in the range of 0. 4. The negative electrode capacity is set to be smaller than the positive electrode capacity, and the battery capacity is regulated to the negative electrode.
4. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode is a LiAl alloy.