JP2000036325A - Secondary power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an easily manufacturable secondary power supply having a high breakdown voltage and excellent in quick charging/discharging cycle characteristics. SOLUTION: A secondary power supply comprises a positive electrode consisting of a layer 3 chiefly containing activated charcoal and a layer 4 chiefly containing a lithium-containing transition metal oxide to contact the layer 3, a negative electrode 5 confronting the layer 4 of the positive electrode and containing carbon material capable of occluding and separating lithium ions, and an organic electrolytic solution 7 containing lithium salt.

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 and excellent rapid charge / discharge cycle characteristics.

[0002]

2. Description of the Related Art Polarizable electrodes mainly composed of activated carbon are used for both positive and negative electrodes of conventional electric double layer capacitors. 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 an electric double layer capacitor is proportional to the square of the withstand voltage, an organic electrolyte having a high withstand voltage has a higher energy than an aqueous electrolyte. However, even with an electric double layer capacitor using an organic electrolyte, its energy density is one of that of secondary batteries such as lead storage batteries and lithium ion secondary batteries.
/ 10 or less, and further improvement in energy density is required.

On the other hand, Japanese Patent Application Laid-Open No. 64-14882 discloses that an electrode mainly composed of activated carbon is used as a positive electrode, and the [002] plane is 0.338 to 0.356 nm by X-ray diffraction.
There has been proposed a secondary power supply having an upper limit voltage of 3 V using an electrode in which lithium ions are previously stored in a carbon material as a negative electrode. Japanese Patent Application Laid-Open No. H08-107048 proposes a battery using, as a negative electrode, a carbon material which can occlude and desorb lithium ions by absorbing lithium ions in advance by a chemical method or an electrochemical method. Also,
Japanese Patent Application Laid-Open No. 9-55342 proposes a secondary power supply having an upper limit voltage of 4 V and having a negative electrode in which a carbon material capable of absorbing and releasing lithium ions is supported on a porous current collector that does not form an alloy with lithium.

However, in the case of the secondary power supply as described above,
Since the capacity of the negative electrode is much larger than the capacity of the positive electrode and the potential of the negative electrode cannot be made sufficiently low, the voltage of the secondary power supply cannot be increased. On the other hand, there has been proposed a method in which lithium ions are stored in advance in a carbon material capable of storing and releasing lithium ions.

[0006]

A secondary power supply in which a lithium ion is occluded and desorbed into a carbon material capable of absorbing and desorbing lithium ions in advance to form a negative electrode and a positive electrode mainly comprising activated carbon has a high withstand voltage and a high energy Both density and power density can be increased. However, in the case of this secondary power supply,
A process for storing lithium ions in the negative electrode in advance is required.

Accordingly, an object of the present invention is to provide a secondary power supply that can be rapidly charged and discharged, has a high withstand voltage and a high capacity, and is easy to manufacture.

[0008]

According to the present invention, there is provided a positive electrode comprising a layer mainly composed of activated carbon and a layer mainly composed of a transition metal oxide containing lithium in contact with the layer; Provided is a secondary power supply, comprising: a negative electrode containing a carbon material capable of absorbing and desorbing lithium ions facing a layer mainly containing a transition metal oxide; and an organic electrolyte containing a lithium salt. .

In this specification, a negative electrode containing a carbon material capable of absorbing and desorbing lithium ions is joined to a current collector, or a negative electrode layer is formed on the current collector to integrate the negative electrode and the current collector. The thing is called a negative electrode body. A layer obtained by integrating a layer mainly composed of activated carbon with a current collector and contacting and laminating a layer mainly composed of a lithium-containing transition metal oxide to the layer is referred to as a positive electrode body. In addition, both the secondary battery and the electric double layer capacitor are one type of secondary power supply. In this specification, the positive electrode contains activated carbon, and the negative electrode contains a carbon material capable of inserting and extracting lithium ions. The secondary power supply is simply called a secondary power supply.

When a secondary power supply comprising a conventional positive electrode mainly composed of activated carbon, a negative electrode mainly composed of a carbon material capable of absorbing and desorbing lithium ions and an organic electrolyte containing a lithium salt is charged, the positive electrode reacts with activated carbon. Thus, the anion of the lithium salt of the electrolytic solution is adsorbed, and the negative electrode absorbs lithium ions of the electrolytic solution with respect to the carbon material. However, in this case, since the capacity of the positive electrode is much smaller than the capacity of the negative electrode, a sufficient amount of lithium ions are not occluded in the negative electrode,
The potential of the negative electrode does not become sufficiently low. Therefore, the voltage of the secondary power supply cannot be increased. Therefore, in order to increase the voltage of the secondary power supply, a process of storing lithium ions in the carbon material of the negative electrode in advance by a chemical method or an electrochemical method is required.

On the other hand, the secondary power supply of the present invention is a negative electrode comprising a carbon material capable of inserting and extracting lithium ions through a separator by electrically contacting a layer containing activated carbon with a lithium-containing transition metal oxide. When charged with a small current, lithium ions are desorbed from the lithium-containing transition metal oxide and occluded in the negative electrode, so that the negative electrode potential can be made sufficiently low. Therefore, there is no need to previously store lithium ions in the negative electrode.

In the secondary power supply of the present invention, when charging and discharging with a large current, the adsorption and desorption of ions in the activated carbon is faster than the insertion and extraction of lithium in the lithium-containing transition metal oxide. Therefore, the lithium-containing transition metal oxide in contact with the positive electrode is not involved in charge and discharge at a large current, and the adsorption and desorption of the lithium salt anion to the activated carbon occurs.Therefore, the capacity of the positive electrode is the electric double layer capacity of the activated carbon. It is determined. In addition, since the lithium-containing transition metal oxide does not participate in charge / discharge and does not react in charge / discharge at a large current, deterioration due to the charge / discharge cycle is small.

In the secondary power supply of the present invention, if the lithium-containing transition metal oxide has a function of absorbing lithium ions in the first charge, and absorbing lithium ions in the carbon material capable of absorbing and desorbing lithium ions, it is involved in the charge / discharge cycle. It is not necessary. In particular, in the case of charging / discharging with a large current, it cannot be involved as described above. Therefore, the lithium-containing transition metal oxide only needs to be a compound capable of releasing lithium ions when the potential becomes noble, and can store lithium ions like a compound used for a positive electrode of a normal lithium ion secondary battery. It is not necessary.

In the present invention, the layer mainly composed of a transition metal oxide containing lithium is preferably formed into a sheet using polytetrafluoroethylene as a binder because it can be easily handled. The porosity of the sheet produced at this time is preferably equal to or higher than the porosity of the separator. Specifically, the porosity of the sheet is preferably 40% or more, and more preferably 50 to 80%. If the porosity of this sheet is less than 40%, the resistance during charging and discharging increases, and the output during high-current discharging decreases. The amount of binder in this sheet is 5
It is preferably about 30% by weight.

[0015] The thickness of the lithium-containing transition metal oxide layer is:
If the thickness is too thick, the resistance of the secondary power supply will be high. If the thickness is too thin, the lithium ions cannot be sufficiently occluded in the negative electrode in the first charge. Therefore, it is preferable that the amount of the lithium-containing transition metal oxide capable of desorbing lithium ions is selected to be 0.5 to 1.5 times the amount of the negative electrode carbon material capable of absorbing lithium.

As the lithium-containing transition metal oxide in contact with the layer containing activated carbon, V, Fe, Co, Mn, N
A composite oxide of lithium and one or more transition metals selected from the group consisting of i, W and Zn is preferred. Particularly preferred is 1 selected from the group consisting of Co, Mn and Ni.
Complex oxide of at least one species and lithium, and further 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, the activated carbon contained in the positive electrode preferably has a specific surface area of 800 to 3000 m ² / g. Activated carbon raw materials and activation conditions are not limited,
For example, as raw materials, coconut, phenolic resin, petroleum coke, and the like can be mentioned. As the activation method, a steam activation method,
A molten alkali activation method and the like can be mentioned. In the present invention, steam activated charcoal activated carbon or steam activated phenolic resin activated carbon is preferred. In addition, the layer containing activated carbon preferably contains conductive carbon black or graphite as a conductive material in order to reduce its resistance. 20
% By weight.

As a method for producing a positive electrode body, for example, a mixture of activated carbon powder and conductive carbon black powder is mixed with polytetrafluoroethylene as a binder, kneaded, and then formed into a sheet to form a layer containing activated carbon. Forming
This is fixed to the current collector using a conductive adhesive. Then, the sheet containing the above-mentioned lithium-containing transition metal oxide is obtained by overlapping and contacting the layer containing activated carbon. At this time, the layer containing the activated carbon and the lithium-containing transition metal oxide are preferably applied with an external pressure or the like, so that the better the contact, the lower the resistance of the secondary power supply can be.

As a method for joining the layer containing activated carbon to the current collector, polyvinylidene fluoride as a binder,
Activated carbon powder and conductive carbon black powder may be dispersed in a varnish in which polyamideimide, polyimide, or the like is dissolved, and the resulting liquid may be applied on a current collector by a doctor blade method or the like, and dried to obtain a powder. The amount of the binder contained in the layer containing activated carbon is preferably 1 to 20% by weight in view of the balance between the strength of the positive electrode body and characteristics such as capacity.

The carbon material capable of occluding and releasing lithium ions in the present invention is obtained by X-ray diffraction measurement [002].
It is preferable that the spacing between the surfaces is 0.335 to 0.410 nm. A carbon material having a plane spacing of more than 0.410 nm is liable to be deteriorated in a charge / discharge cycle. Specific examples include petroleum coke, a material obtained by heat-treating mesophase pitch-based carbon material or vapor-grown carbon fiber at 800 to 3000 ° C., natural graphite, artificial graphite, and non-graphitizable carbon material. In the present invention, any of these materials can be preferably used.

When using a non-graphitizable carbon material or a carbon material obtained by treating petroleum coke at a low temperature, the resistance can be reduced by using a vapor-grown carbon mixed with a graphitic carbon material such as a graphitized material. It is preferred. In this case, the weight ratio of the non-graphitizable carbon material or the like to the graphitic carbon material is 9%.
The ratio is preferably 5: 5 to 70:30. If the graphite carbon material is less than 5%, the effect of reducing the resistance cannot be exhibited, and
If it exceeds 0%, the capacity of the negative electrode decreases.

The negative electrode body according to the present invention is formed by kneading polytetrafluoroethylene as a binder and forming the same into a sheet, similarly to the layer containing activated carbon, to form a negative electrode, and bonding the negative electrode to a current collector using a conductive adhesive. Obtainable. Further, polyvinylidene fluoride, polyamideimide or polyimide as a binder, the carbon material is dispersed in a solution obtained by dissolving a resin serving as a binder or a precursor thereof in an organic solvent,
There is also a method in which a current collector is applied and dried. All of these methods are preferred.

In the method obtained by applying the negative electrode layer to the current collector, the solvent for dissolving the resin serving as the binder or the precursor thereof is not limited, but the resin constituting the binder or the precursor thereof can be easily dissolved. N-methyl-2-pyrrolidone (hereinafter referred to as NMP) because it is easily available
Is preferred. Here, a precursor of polyvinylidene fluoride,
The term “polyamideimide precursor” or “polyimide precursor” refers to those which are polymerized by heating to become polyvinylidene fluoride, polyamideimide or polyimide, respectively.

The binder obtained as described above is cured by heating and is excellent in chemical resistance, mechanical properties and dimensional stability. The temperature of the heat treatment is preferably 200 ° C. or higher. If the temperature is 200 ° C. or higher, even if it is a polyamideimide precursor or a polyimide precursor, it is usually polymerized to be a polyamideimide or a polyimide, respectively. The atmosphere for the heat treatment is preferably an inert atmosphere such as nitrogen or argon, or a reduced pressure of 1 torr or less. Polyamide imide or polyimide has resistance to the organic electrolyte used in the present invention, and is sufficiently resistant to high temperature heating of about 300 ° C. or heating under reduced pressure to remove water from the negative electrode.

In the present invention, if an adhesive layer made of polyamideimide or polyimide is interposed between the negative electrode and the current collector, the adhesive force between the negative electrode and the current collector becomes stronger. In this case, a varnish obtained by previously dissolving polyamideimide, polyimide or their precursors in a solvent on a current collector is applied by a coating method such as a doctor blade method, and dried to form an adhesive layer. A negative electrode is formed. Also, if a conductive material such as copper and graphite is dispersed in a varnish forming an adhesive layer,
This is preferable because the contact resistance between the negative electrode and the current collector can be reduced.
The varnish containing the conductive material can also be interposed as a conductive adhesive between the layer containing the activated carbon and the current collector when the layer containing the activated carbon is formed into a sheet.

The lithium salt contained in the organic electrolyte according to the present invention includes LiPF ₆ , LiBF ₄ , LiClO ₄ , L
iN (SO ₂ CF ₃ ) ₂ , CF ₃ SO ₃ Li, LiC
_{(SO 2 CF 3) 3,} LiAsF 6 and one or more selected from the group consisting of LiSbF ₆ are preferred. The solvent preferably contains 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. Electrolyte solutions composed of these lithium salts and solvents have high withstand voltage and high electric conductivity. The concentration of the lithium salt in the electrolyte is 0.1 to 2.5 mol / L,
Furthermore, 0.5 to 2 mol / L is preferable.

[0027]

EXAMPLES Next, the present invention will be described more specifically with reference to Examples (Examples 1 to 4) and Comparative Examples (Example 5), but the present invention is not limited thereto.

The cell of the example was manufactured as shown in FIG. The production and measurement of the cells in Examples 1 to 5 were all performed in an argon glove box having a dew point of −60 ° C. or less.

Example 1 80% 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 consisting of wt% was kneaded with ethanol, kneaded, rolled, and vacuum dried at 200 ° C. for 2 hours to obtain a layer 3 containing activated carbon. This layer 3 was bonded to a positive electrode current collector 1 made of aluminum foil using a conductive adhesive having a polyamideimide resin as a binder, and heat-treated at 300 ° C. for 2 hours under reduced pressure to obtain a positive electrode body. The layer made of activated carbon had an area of 24 cm ² and a thickness of 180 μm.

Next, a petroleum coke-based carbon material was
A carbon material capable of occluding and desorbing lithium ions was obtained by heat treatment at ℃. The plane interval of the [002] plane of the carbon material by X-ray diffraction was 0.340 nm. 80% by weight of this carbon material and 30% of vapor grown carbon as a conductive agent
A mixture composed of 10% by weight of the material treated at 00 ° C. and 10% by weight of polytetrafluoroethylene was kneaded by adding ethanol, rolled, and then vacuum-dried at 200 ° C. for 2 hours to obtain a negative electrode 5. This negative electrode 5 was joined to a current collector 2 made of copper foil using a conductive adhesive having a polyamideimide resin as a binder, and heat-treated at 300 ° C. for 2 hours under reduced pressure.
A negative electrode body was obtained. The electrode area was 24 cm ² , and the thickness of the electrode sheet was 100 μm.

A mixture consisting of 90% by weight of LiCoO ₂ and 10% by weight of polytetrafluoroethylene is kneaded by adding ethanol, formed into a sheet, rolled, and formed into a porosity of 60%.
%, And a layer 4 containing a lithium-containing transition metal oxide having a thickness of 100 μm was obtained. This layer 4 was brought into contact with the above-mentioned layer 3 containing activated carbon, and was opposed to the above-mentioned negative electrode body via a polypropylene separator 6 having a porosity of 60%, thereby producing a secondary power supply element. The device was sufficiently immersed in an electrolyte solution 7 composed of a propylene carbonate solution containing 1 mol / L LiBF ₄ and then charged to 4.2 V at a constant current of 5 mA. 240
A charge / discharge cycle test was performed at a charge / discharge current of mA in a range from 4.2 V to 3 V, and a capacity change rate after 1000 cycles was calculated. Table 1 shows the results.

[Example 2] LiMn instead of LiCoO _2
A secondary power device was obtained in the same manner as in Example 1, except that a layer containing a lithium-containing transition metal oxide was formed using ₂ O ₄ .
Using this device, charging was performed in the same manner as in Example 1, and a charge / discharge cycle test was performed to calculate a capacity change rate. Table 1 shows the results.

[Example 3] LiNi instead of LiCoO ₂
A secondary power supply device was obtained in the same manner as in Example 1, except that a layer containing a lithium-containing transition metal oxide was formed using O ₂ . Using this device, charging was performed in the same manner as in Example 1, and a charge / discharge cycle test was performed to calculate a capacity change rate. Table 1 shows the results.

Example 4 LiCoO ₂ was replaced with LiCo
_A secondary power supply device was obtained in the same manner as in Example 1, except that a layer containing a lithium-containing transition metal oxide was formed using _0.2 Ni _0.8 O ₂ . Using this element, charging was performed in the same manner as in Example 1,
A charge / discharge cycle test was performed to calculate a capacity change rate. Table 1 shows the results.

Example 5 A secondary power supply device was obtained in the same manner as in Example 1, except that the layer containing activated carbon was opposed to the negative electrode body via a separator without using the layer containing LiCoO ₂ .
Using this device, charging was performed in the same manner as in Example 1, and a charge / discharge cycle test was performed to calculate a capacity change rate. Table 1 shows the results.

[0036]

[Table 1]

[0037]

The secondary power supply according to the present invention has a high withstand voltage and excellent rapid charge / discharge cycle characteristics. In addition, occlusion of lithium ions into the negative electrode carbon material at the time of manufacturing the secondary power supply does not need to be performed by a chemical method or an electrochemical method in advance, and can be performed by charging after the secondary power supply is manufactured. It is easy to make a secondary power supply.

[Brief description of the drawings]

FIG. 1 is a sectional view illustrating a secondary power supply element according to an embodiment.

[Explanation of symbols]

 1: positive electrode current collector 2: negative electrode current collector 3: layer containing activated carbon 4: layer containing lithium-containing transition metal oxide 5: negative electrode 6: separator 7: electrolytic solution

Continued on the front page F-term (reference) 5H003 AA01 AA04 AA08 BA00 BB01 BB05 BB11 BC06 BD00 BD02 5H014 AA02 AA06 BB11 EE01 EE08 EE10 HH00 HH06 5H029 AJ02 AJ05 AJ14 AK03 AK08 AK18 DJ04J04 AM05

Claims (6)

[Claims] 1. A positive electrode comprising a layer mainly composed of activated carbon and a layer mainly composed of a lithium-containing transition metal oxide in contact with the layer, and a lithium-containing transition metal oxide of the positive electrode mainly comprising a separator interposed therebetween. A secondary power supply, comprising: a negative electrode containing a carbon material capable of absorbing and desorbing lithium ions facing a layer to be formed, and an organic electrolyte containing a lithium salt. 2. The secondary power source according to claim 1, wherein the layer mainly composed of a transition metal oxide containing lithium is a sheet containing polytetrafluoroethylene as a binder. 3. 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.
Or the secondary power supply according to 2. 4. 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. The secondary power supply according to claim 1 or 2, wherein 5. The secondary power supply according to claim 1, wherein the activated carbon is steam activated activated palm-based activated carbon or steam activated activated phenolic resin activated carbon. 6. The carbon material of the negative electrode, wherein a plane interval of a [002] plane is 0.335 to 0.410 nm.
The secondary power supply according to 3, 4, or 5.