JP2000138074A - Secondary power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve withstand voltage, energy density in high capacity, and reliability of a charge/discharge cycle, by providing a positive electrode containing activated carbon and a lithium containing transition metal oxide, a negative electrode containing the activated carbon, and an organic electrolyte containing quarternary onium salt. SOLUTION: Activated carbon and a lithium containing transition metal oxide, that is a composite oxide of lithium and one or more kinds of transition metals selected from a group composed of V, Fe, Co, Mn, Ni, W, Zn are contained in a positive electrode, or Li, Co, Ni(1-y)O2, or Li2Mn2O4 (where, 0<x<2.0<=y <=1, 0<z<2) is contained. A negative electrode body can be produced similarly to a positive electrode body, mixture of activated carbon powders and a conductive agent is used instead of the mixture of activated carbon powders, the lithium containing transition metal oxide, and a conductive material. In addition, quarternary-onium salt is contained in an organic electrolyte, and its concentration is 0.5-2.5 mol/L, and lithium salt concentration is 0.1-1.5 mol/L.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary power supply having a high withstand voltage, a large capacity, and a high rapid charge / discharge cycle reliability.

[0002]

2. Description of the Related Art Conventionally, as an electrode of an electric double layer capacitor, a polarizable electrode mainly composed of activated carbon has been used for both a positive electrode and a negative electrode. The withstand voltage of the electric double layer capacitor is 1.2 V when an aqueous electrolyte is used, and 2.5 to 3.3 V when an organic electrolyte is used. Since the energy of the electric double layer capacitor is proportional to the square of the voltage, the organic electrolyte having a higher withstand voltage has higher energy than the aqueous electrolyte.

However, even an electric double layer capacitor using an organic electrolyte has an energy density of 1/10 or less of a secondary battery such as a lead storage battery, and further improvement in energy density is required. Increasing the voltage is the most effective way to improve the energy density of the electric double layer capacitor. However, when the voltage is increased, the electrolytic solution is decomposed and the life is extremely shortened. .

As a secondary power source having a high energy density, there is a lithium ion secondary battery or the like. Lithium ion secondary batteries use, for example, LiCoO ₂ or the like as a positive electrode active material,
The negative electrode uses a carbon material capable of inserting and extracting lithium ions, and has a high withstand voltage and a high energy density. However, when the charge / discharge cycle is repeated, the positive electrode active material particularly deteriorates, and there is a problem that the charge / discharge cycle reliability is low.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a secondary power supply having a high withstand voltage, a high capacity, a high energy density, and a high charge / discharge cycle reliability.

[0006]

The present invention is characterized in that it has a positive electrode containing activated carbon and a lithium-containing transition metal oxide, a negative electrode containing activated carbon, and an organic electrolyte containing a quaternary onium salt. To provide a secondary power supply.

[0007] In the present specification, a positive electrode body in which a positive electrode containing activated carbon and a lithium-containing transition metal oxide is joined and integrated with a current collector. The same definition applies to the negative electrode body. Further, both the secondary battery and the electric double layer capacitor are one kind of the secondary power source. In this specification, however, the secondary battery having a specific configuration in which the positive electrode contains activated carbon and a lithium-containing transition metal oxide and the negative electrode contains activated carbon is used. The power supply is simply called a secondary power supply.

The positive electrode contains activated carbon and a transition metal oxide containing lithium, and the negative electrode is an electrode mainly composed of activated carbon.
When an organic electrolyte containing a high-grade onium salt is used as the electrolyte, the positive electrode adsorbs anions in the electrolyte onto the activated carbon and desorbs lithium ions from the lithium-containing transition metal oxide at the positive electrode, and the activated carbon at the negative electrode. Then, lithium ions desorbed from the lithium-containing transition metal oxide and cations in the electrolyte are adsorbed or occluded. In this specification, the term “adsorption” refers to the adsorption of ions to activated carbon due to the formation of an electric double layer at the time of charging, and the term “occlusion” refers to the reaction involving charge transfer at the same time that ions are taken into the electrode. The separation of ions from the activated carbon during discharge is called desorption, and the separation of ions with charge transfer at the same time as the separation of ions is called desorption.

The lithium ions adsorbed or occluded by the negative electrode activated carbon cannot be desorbed or desorbed at the time of discharge, but remain in the activated carbon. Therefore, the open-circuit potential of the negative electrode made of activated carbon is about 3 V with respect to the lithium reference electrode when lithium ions are not adsorbed or occluded, but desorbed or desorbed once lithium ions are adsorbed or occluded. In this case, the voltage is 2.5 V or less, though it depends on the type of the activated carbon.

Thereafter, when the battery is charged again, the quaternary onium ion is adsorbed from about 2.5 V on the negative electrode made of activated carbon. That is, the negative electrode can be charged at a lower potential than the conventional negative electrode mainly composed of activated carbon, and the potential of the negative electrode can be made lower, so that the withstand voltage can be increased.
In addition, in charge and discharge after the second cycle, adsorption and desorption of quaternary onium ions in the negative electrode activated carbon are the main reactions,
The deterioration of the negative electrode due to the charge / discharge cycle is as good as that of the conventional negative electrode made of activated carbon.

In the secondary power supply of the present invention, the positive electrode contains a lithium-containing transition metal oxide, so that the positive electrode capacity is larger than the negative electrode capacity, and the positive electrode potential changes less than the negative electrode potential when charged. As described above, since the negative electrode potential becomes lower, the voltage range in which the decomposition of the electrolytic solution does not occur is wider than that of the electric double layer capacitor, and the withstand voltage can be increased.

The lithium-containing transition metal oxide contained in the positive electrode includes V, Fe, Co, Mn, Ni, W and Zn.
Preferred is a composite oxide of one or more transition metals selected from the group consisting of and lithium. Particularly preferred are Co,
A composite oxide of lithium and at least one selected from the group consisting of Mn and Ni, and Li _x Co _y Ni
_(1-y) O ₂ or Li _z Mn ₂ O ₄ (provided that 0 <x <
2, 0 ≦ y ≦ 1, 0 <z <2. Is preferred. In the present invention, this lithium-containing transition metal oxide is initially a source of lithium ions for lowering the potential of the activated carbon of the negative electrode, and it is considered that the capacity in charge and discharge increases due to the presence of the lithium-containing transition metal oxide. .

In the present invention, the amount of the lithium-containing transition metal oxide in the positive electrode is preferably 5 to 80% by weight. If the amount is less than 5% by weight, the amount of lithium ions desorbed in the initial charging is not sufficient with respect to the amount of lithium ions that the negative electrode can store, and the voltage of the secondary power supply cannot be increased.
If it exceeds 80% by weight, the amount of activated carbon in the positive electrode becomes relatively small, so that the capacity decrease in the charge / discharge cycle becomes large. More preferably, it is 10 to 60% by weight.

The activated carbon contained in the positive electrode and the negative electrode may be the same or different, and have a specific surface area of 300 to 30.
It is preferably 00 m ² / g. The raw material and activation conditions of the activated carbon are not limited. For example, the raw material includes bean, phenol resin, petroleum coke and the like, and the activation method includes a steam activation method and a molten alkali activation method. It is also preferable that the positive electrode and the negative electrode contain conductive carbon black or graphite as a conductive material in order to reduce the resistance. In this case, the conductive material is contained in the positive electrode or the negative electrode in an amount of 0.1 to 20% by weight. Is preferred.

As a method for producing a positive electrode body, for example, a mixture of activated carbon powder, lithium-containing transition metal oxide powder, and carbon black as a conductive material is mixed with polytetrafluoroethylene as a binder, kneaded, and then formed into a sheet. Then, there is a method of fixing the positive electrode to a current collector using a conductive adhesive. Activated carbon powder, lithium-containing transition metal oxide powder, and carbon black as a conductive material are dispersed in a varnish in which polyvinylidene fluoride, polyamideimide, polyimide, or the like is dissolved as a binder, and this liquid is collected by a doctor blade method or the like. It may be obtained by coating on top and drying. The amount of the binder contained in the positive electrode may be 1 depending on the balance between the strength of the positive electrode body and characteristics such as capacity.
Preferably, it is about 20% by weight.

The negative electrode body can be produced in the same manner as the positive electrode body. Instead of the mixture of the activated carbon powder, the lithium-containing transition metal oxide and the conductive material, a mixture of the activated carbon powder and the conductive agent may be used. As for the positive electrode, 1 to 20% by weight is preferable.

The organic electrolyte in the present invention contains a quaternary onium salt as a solute, and its cation is R ¹ R ² R ³
R ⁴ N ⁺ or R ¹ R ² R ³ R ⁴ P ⁺ (R ¹ , R ² , R
³ and R ⁴ are each independently an alkyl group having 1 to 6 carbon atoms. ) And the anions are PF ₆ ⁻ , BF ₄ ⁻ , ClO ₄ ⁻ ,
N (CF ₃ SO ₂ ) ₂ ⁻ , CF ₃ SO ₃ ⁻ , C (SO ₂ CF
_{3) 3} ^-, AsF ₆ ^- is preferably one or more selected from the group consisting of ^- and SbF _6. 4th in organic electrolyte
The concentration of the onium salt is preferably 0.5 to 2.5 mol / L, particularly preferably 1.0 to 2.0 mol / L.

Preferably, the electrolyte contains a lithium salt. The anion of the lithium salt is P
F ₆ ⁻ , BF ₄ ⁻ , ClO ₄ ⁻ , N (CF ₃ SO ₂ ) ₂ ⁻ , CF
^{_{3 SO 3 -, C (SO}} 2 CF 3) 3 -, AsF 6 - and SbF
₆ ^- 1 or more is preferably selected from the group consisting of. When the lithium salt is contained in the electrolytic solution, the amount of the lithium-containing transition metal oxide of the positive electrode, which is a source of lithium ions for lowering the potential of the activated carbon of the negative electrode, can be reduced. The concentration of the lithium salt in the electrolyte is 0.1
To 1.5 mol / L, particularly preferably 0.5 to 1.3 mol / L.

The solvent of the electrolytic solution is preferably at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane and dimethoxyethane.

[0020]

EXAMPLES Next, the present invention will be described more specifically with reference to Examples (Examples 1 to 6) and Comparative Examples (Example 7), but the present invention is not limited thereto. The production and measurement of the cells in Examples 1 to 7 were all performed in an argon glove box having a dew point of −60 ° C. or less.

Example 1 40% by weight of activated carbon having a specific surface area of 2000 m ² / g, 40% by weight of LiCoO ₂ , 10% by weight of conductive carbon black, and a binder obtained by a steam activation method using a phenol resin as a raw material A mixture comprising 10% by weight of polytetrafluoroethylene was kneaded with ethanol, rolled, and then heated to 200 ° C.
For 2 hours to obtain an electrode sheet. This sheet was bonded to an aluminum foil using a conductive adhesive having polyamideimide as a binder, and heat-treated at 300 ° C. for 2 hours under reduced pressure to obtain a positive electrode body. The electrode area was 24 cm ² , and the thickness of the electrode sheet was 150 μm.

50% by weight of activated carbon having a specific surface area of 2000 m ² / g obtained by a steam activation method using a phenol resin as a raw material, 10% by weight of conductive carbon black, and 10% by weight of polytetrafluoroethylene as a binder. The mixture was kneaded with ethanol, and an electrode sheet was prepared in the same manner as in the positive electrode. This sheet was bonded to an aluminum foil using a conductive adhesive having polyamideimide as a binder in the same manner as the positive electrode.
For 2 hours to obtain a negative electrode body. Electrode area is 24cm
^2. The thickness of the electrode sheet was 150 μm.

The above-mentioned positive electrode body and the above-mentioned negative electrode body are placed with their electrode surfaces facing each other via a polypropylene non-woven fabric separator.
A 4 cm ² element was fabricated. A solution obtained by dissolving 1 mol / L of (C ₂ H ₅ ) ₃ (CH ₃ ) NBF ₄ and 1 mol / L of LiBF ₄ in propylene carbonate was used as an electrolytic solution, and the element was sufficiently impregnated with the electrolytic solution. Got power. This secondary power supply was initially charged at 3.5V for 24 hours and then discharged to 1V. Then 3.5V to 1V
The initial capacity was measured in the range up to. Thereafter, a charge / discharge cycle was performed at a charge / discharge current of 240 mA in a range from 3.5 V to 1 V, the capacity after 1000 cycles was measured, and the capacity change rate was calculated. Table 1 shows the results.

[Example 2] LiMn instead of LiCoO _2
A positive electrode body was obtained in the same manner as in Example 1 except that ₂ O ₄ was used.
A secondary power supply was obtained in the same manner as in Example 1 except that this positive electrode body was used. Evaluation was performed in the same manner as in Example 1 using this secondary power supply. Table 1 shows the results.

Example 3 LiNi is used instead of LiCoO ₂
A positive electrode body was obtained in the same manner as in Example 1 except that O ₂ was used. A secondary power supply was obtained in the same manner as in Example 1 except that this positive electrode body was used. Evaluation was performed in the same manner as in Example 1 using this secondary power supply. Table 1 shows the results.

Example 4 LiCoO ₂ is replaced with LiCo
_A positive electrode body was obtained in the same manner as in Example 1 except that _0.2 Ni _0.8 O ₂ was used. A secondary power supply was obtained in the same manner as in Example 1 except that this positive electrode body was used. Evaluation was performed in the same manner as in Example 1 using this secondary power supply. Table 1 shows the results.

[Example 5] Instead of activated carbon obtained by a steam activation method using a phenol resin as a raw material, activated carbon having a specific surface area of 500 m ² / g obtained by a molten KOH activation method using petroleum coke as a raw material was used for the positive electrode and the negative electrode. A positive electrode body and a negative electrode body were obtained in the same manner as in Example 1 except for the difference. A secondary power supply was obtained in the same manner as in Example 1 except that the positive electrode body and the negative electrode body were used. Evaluation was performed in the same manner as in Example 1 using this secondary power supply. Table 1 shows the results.

[Example 6] Instead of activated carbon obtained by a steam activation method using a phenolic resin as a raw material, specific surface area 17 obtained by a steam activation method using a palm as a raw material.
Example 1 except that 00 m ² / g activated carbon was used for the positive and negative electrodes
In the same manner as in the above, a positive electrode body and a negative electrode body were obtained. A secondary power supply was obtained in the same manner as in Example 1 except that the positive electrode body and the negative electrode body were used.
Evaluation was performed in the same manner as in Example 1 using this secondary power supply.
Table 1 shows the results.

Example 7 Example 1 except that LiCoO ₂ was not added to the positive electrode and the content of activated carbon in the mixture was 80% by weight.
In the same manner as in the above, a positive electrode body was obtained. A secondary power supply was obtained in the same manner as in Example 1 except that this positive electrode body was used. Evaluation was performed in the same manner as in Example 1 using this secondary power supply. Table 1 shows the results.

[0030]

[Table 1]

[0031]

The secondary power supply of the present invention has a high withstand voltage and a large capacity. When charging and discharging are first performed in the secondary power supply of the present invention, lithium ions remain in the negative electrode activated carbon, and the open circuit potential of the negative electrode can be made low. Therefore, both the conventional positive electrode and the negative electrode can use the negative electrode in a wider range at the time of charging than the electric double layer capacitor made of activated carbon, so that the withstand voltage can be improved.

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Claims (6)

[Claims] 1. A secondary power supply comprising: a positive electrode containing activated carbon and a lithium-containing transition metal oxide; a negative electrode containing activated carbon; and an organic electrolyte containing a quaternary onium salt. 2. The method according to claim 1, wherein the lithium-containing transition metal oxide is V, F
2. A composite oxide of at least one selected from the group consisting of e, Co, Mn, Ni, W and Zn with lithium.
The secondary power supply according to the above. 3. The method according to claim 1, wherein the lithium-containing transition metal oxide is Li _x C
o _y Ni _1-y O ₂ or Li _z Mn ₂ O ₄ (where 0 <
x <2, 0 ≦ y ≦ 1, 0 <z <2. 2. The secondary power supply according to claim 1, wherein 4. A lithium-containing transition metal oxide containing 5
The secondary power source according to claim 1, wherein the secondary power source is contained in an amount of about 80% by weight. 5. The method according to claim 1, wherein the organic electrolyte contains a lithium salt.
The secondary power supply according to 2, 3, or 4. 6. The organic electrolyte according to claim 1, wherein the lithium salt is 0.1-1.
5 mol / L, quaternary onium salt is 0.5 to 2.5 mo
The secondary power supply according to claim 5, wherein 1 / L is included.