JP2000100438A - Binder, laminate and lithium secondary battery - Google Patents

Abstract

(57) Abstract: In a lithium secondary battery, the adhesion between a metal current collector and a carbon powder-containing layer is improved, and the durability to charge and discharge is improved. SOLUTION: When a carbon-containing layer containing a carbon powder capable of inserting and extracting lithium ions and a binder is laminated on a conductive thin film (current collector) to produce an electrode of a lithium secondary battery, A cellulose ester having an acyl group having 3 to 18 carbon atoms is used as a binder to improve the adhesion between the conductive thin film and the carbon-containing layer. Examples of the acyl group include an aromatic acyl group, and the acyl group may have an unsaturated bond. The ratio of the binder is about 0.1 to 10 parts by weight based on 100 parts by weight of the carbon powder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses an improved binder (a binder or a binder for an electrode) for binding carbon powder as an electrode material, and improves durability against charge and discharge. And a lithium secondary battery using the same as an electrode.

[0002]

2. Description of the Related Art A lithium secondary battery has advantages such as high energy density and high voltage because it is not necessary to consider the decomposition voltage of water. Therefore, instead of nickel-cadmium secondary batteries, lithium batteries are used as batteries of portable terminals, cordless household electric appliances, notebook personal computers, and the like as mainstream secondary batteries.

On the other hand, as an electrode material for a lithium secondary battery, carbon powder, such as coke, graphite, and organic matter, which is not likely to cause an internal short circuit due to the growth of dendritic lithium in order to improve cycle life. A carbon powder capable of occluding and releasing lithium ions from a body or the like has been proposed.
When an electrode is manufactured using such a carbon material, carbon powder and PVdF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene) or the like as a binder are kneaded with an organic solution to form a slurry. A method of applying on a current collector, drying and solidifying is adopted. In addition, Japanese Patent Laid-Open No. 7-1
Japanese Patent No. 92722 describes that the use of cellulose acetate as a binder for a carbon material can improve a decrease in battery capacity after repeated charge and discharge.

[0004] However, the conventional binder is excellent in the binding property between carbon powders, but is expensive and moreover,
Adhesion with a current collector (usually, a copper current collector is used) is poor. For this reason, the battery capacity gradually decreases due to the stress at the time of forming the electrode or the repetition of charge / discharge and the drop of the carbon powder from the current collector, and the durability against charge / discharge is not sufficient.

[0005]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a binder having excellent adhesion to a conductive thin film, and a laminate in which a conductive thin film and a carbon powder-containing layer are laminated with high adhesion. And a lithium secondary battery using the same.

Another object of the present invention is to provide a binder, a laminate, and a lithium secondary battery using the same, which can further improve the durability against charge / discharge cycles.

[0007]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that a specific cellulose derivative can be used as a binder for carbon powder to form a conductive thin film (such as a metal foil).
The present inventors have found that the adhesiveness with the metal can be improved, and that the durability against molding and charging / discharging cycles can be improved, and the present invention has been completed.

That is, the binder of the present invention is a binder for forming a carbon-containing layer by bonding to a carbon powder on a conductive thin film (such as a metal foil). Contains a cellulose ester having at least an acyl group having 3 to 18 carbon atoms.

The laminate of the present invention is a laminate in which a carbon-containing layer is laminated on at least one surface of a conductive thin film,
The carbon-containing layer contains the binder. This laminate is useful as an electrode of a lithium secondary battery. That is, in the lithium secondary battery of the present invention, at least one of the positive and negative electrodes is stacked on the conductive current collector and at least one surface of the conductive current collector, and is capable of inserting and extracting lithium ions. And a carbon-containing layer containing a carbon powder and a binder, wherein the electrode is formed of the laminate.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The type of carbon powder used in the present invention is not particularly limited, and various carbon materials usable as electrode materials can be used. As the carbon powder, a carbon powder capable of absorbing and releasing lithium ions without the risk of internal short circuit due to the growth of dendritic lithium is desirable. Examples of such carbon powder include carbon materials such as coke, graphite, and carbon black,
Examples include fired bodies of organic substances (pitch, phenol resin, furan resin, polyacrylonitrile, etc.).

The average particle size of the carbon powder is, for example, 0.1 to
200 μm, preferably 1 to 100 μm (for example,
80 μm).

A feature of the present invention is that the binder has 3 carbon atoms.
The use of cellulose esters containing at least one or more acyl groups of about 18 to 18 (or cellulose esters having an aromatic ring). Examples of the acyl group include aliphatic acyl groups (propionyl, butyryl, isobutyryl, t-butyryl, valeryl, isovaleryl, pivaloyl, lauroyl, myristoyl, palmitoyl, stearoyl, and other saturated acyl groups having about 3 to 18 carbon atoms, lactoyl). A hydroxyl group-containing acyl group such as a glyceroyl group; an unsaturated acyl group having about 3 to 18 carbon atoms such as acryloyl, methacryloyl, propioyl, crotonoyl, isocrotonoyl, or oleyl group; a carbocyclic acyl group (benzoyl, naphthoyl, toluoyl) Having an unsaturated bond such as an aromatic acyl group such as carboxybenzoyl or alkoxycarbonylbenzoyl group, a hydroxyl group or an alkoxy group-containing aromatic acyl group such as salicyloyl or anisoyl group, or a cinnamoyl group. That an aromatic acyl group), a heterocyclic acyl group (furoyl, thenoyl,
5- or 6-membered heterocyclic acyl group such as nicotinyl group)
And the like. These acyl groups may be independently introduced into cellulose, or two or more thereof may be introduced into cellulose in combination to form a mixed ester.

Preferred acyl groups are aliphatic acyl groups (particularly acyl groups having about 3 to 10 carbon atoms) or carbocyclic acyl groups (particularly aromatic acyl groups having about 6 to 12 carbon atoms).
Further, in order to further improve the adhesion, the acyl group may be an acyl group having an unsaturated bond (particularly, a polymerizable unsaturated bond) (for example, (meth) acryloyl, crotonoyl,
A cinnamoyl group).

The average degree of substitution of these acyl groups can be selected, for example, from the range of about 0.1 to 3, preferably about 0.5 to 3. The cellulose ester may be a mixed ester having a substituent other than the acyl group (for example, a sulfonic acid residue such as an acetyl group or a tosyl group, or an inorganic acid residue such as a phosphoric acid ester). The average degree of substitution of all the substituents of the cellulose esters is, for example, about 1 to 3, preferably about 1.5 to 3. Further, the viscosity-average degree of polymerization of the cellulose esters is, for example, about 10 to 1,000, preferably about 50 to 900, and more preferably about 100 to 800.

Specific examples of the cellulose ester having an acyl group include, for example, cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, cellulose benzoate, cellulose acetate benzoate, cellulose butyrate benzoate. , Cellulose cinnamate, cellulose acetate cinnamate, cellulose butyrate cinnamate, cellulose phthalate and the like.

When such a cellulose ester is used as a binder of the carbon powder, the adhesion of the carbon-containing layer to the conductive thin film (conductor) can be improved. Although the reason is not clear, the introduction of the substituents imparts flexibility to the binder, stresses on the electrodes during the molding process or the electrode manufacturing process, and the storage and release of lithium ions generated in a charge / discharge cycle, Even if the powder-containing layer and the conductor are deformed due to a change in temperature, the stress is relieved, the peeling and falling off of the carbon-containing layer and the conductor can be prevented, and it is considered that a decrease in adhesion is suppressed. Further, when the acyl group has an unsaturated bond, it is presumed that the unsaturated bond is cross-linked by heating during drying to form a stronger network, and the adhesion to the conductive thin film is further improved. .

The above cellulose esters may be used in combination with other binders, if necessary. Examples of such a binder include a cellulose derivative (eg, cellulose acetate), an ester of a sugar analog (eg, acetylglucosamine, acetylgalactosamine, acetylcholine), a fluororesin [PVdF (polyvinylidene fluoride), and PTFE].
(Polytetrafluoroethylene), PCTFE (polychlorotrifluoroethylene), PVF (polyvinyl fluoride), PFA (tetrafluoroethylene-perfluoroalkylvinyl ether copolymer), PETFE
(Tetrafluoroethylene-ethylene copolymer), etc.], and fluorinated graphite.

The binder may further contain various additives such as plasticizers, stabilizers (antioxidants, ultraviolet absorbers, heat stabilizers such as alkaline earth metal compounds, etc.), flame retardants, and antistatic agents. , An adhesion improver (such as a coupling agent), a crosslinking agent, and a polymerization initiator.

Such a binder (binder) can improve the adhesion to a conductive thin film (or a sheet-like conductor), and can be used for an electrode using a carbon material (for example, an electrode for a lithium battery, particularly an electrode for a lithium secondary battery). ) Are useful as binders or binders.

The laminate of the present invention has a structure in which carbon powder is closely adhered or bonded to a conductive thin film (such as a metal foil) with a binder. Together with the carbon-containing layer. An organic or inorganic conductive thin film (sheet-like or plate-like conductor) on which a carbon powder-containing layer is laminated, which acts as a current collector when a battery is formed, facilitates charge transfer, and has high moldability. Conductors (current collectors), particularly conductive metal thin films (metal current collectors such as copper foil and aluminum foil) are desirable. The thickness of the conductive thin film is not particularly limited, and can be selected, for example, from a range of about 0.01 to 1 mm.

The proportion of the binder is 0.1 to 10 parts by weight, preferably 1 to 8 parts by weight, more preferably about 2 to 8 parts by weight, based on 100 parts by weight of the carbon powder. If the amount of the binder is too small, the binding force between the carbon powders and the adhesion to the current collector are reduced. If the amount is too large, the charge / discharge capacity is reduced due to the decrease in the amount of the carbon powder, and the durability to the charge / discharge cycle is reduced. The property is likely to decrease.

The carbon-containing layer may be formed on at least one surface of the conductive thin film (planar conductor). Usually, the carbon-containing layer can be formed on both surfaces of the conductor. The thickness of each carbon-containing layer is not particularly limited, and is, for example, 1 to 300 μm, preferably 5 to 300 μm.
Can be selected from the range of about 200 to 200 μm.
It is about 0 μm.

The laminate of the conductive thin film (plane conductor) and the carbon-containing layer is prepared by a conventional method, for example, by preparing a coating agent containing carbon powder and a binder (for example, an organic solvent-containing coating agent). ,
This coating agent can be obtained by applying it on a conductor and drying it. The coating agent can be prepared by mixing an organic solvent and a binder, adding carbon powder, kneading the mixture, and dispersing the carbon powder. Note that the surface of the conductive thin film may be subjected to cleaning treatment, surface treatment (acid treatment, or the like).

Such a laminate is useful as an electrode (eg, a negative electrode) of a battery (particularly, a lithium secondary battery).
In this lithium secondary battery, at least one of the positive and negative electrodes is formed of an electrode in which carbon powder capable of inserting and extracting lithium ions is adhered to a metal current collector with a binder.

FIG. 1 is a schematic sectional view showing an example of a lithium secondary battery. This battery comprises a positive electrode 1 and a negative electrode 2 facing each other, a separator 3 interposed between and separated from these two electrodes, a negative electrode lead 4 connected to the negative electrode 2, a positive electrode 1
, A positive electrode lead 5, a positive electrode external terminal 6, a negative electrode external terminal 7, and the like. The positive electrode 1 and the negative electrode 2
Is housed in a case in a state of being spirally (or spirally) wound through a separator 3 in which a non-aqueous electrolyte is injected or impregnated. The positive electrode 1 is connected to a positive electrode external terminal 6 via a positive electrode lead 5, and the negative electrode 2 is connected to a negative electrode external terminal 7 via a negative electrode lead 4. The chemical energy generated inside the battery is transferred to the outside as electric energy. Can be removed.

The laminate of the present invention can be used as at least one of the positive electrode and the negative electrode. When using the laminate as a positive electrode, as the negative electrode active material of the counter electrode, Li metal, metal chalcogen compound, and the like can be used,
When the laminate is used as a negative electrode, a metal oxide (LiCoO ₂ , LiNiO ₂ , L
iMnO ₂ , LiMn ₂ O ₄ ), metal chalcogen compounds having a layered structure, conductive polymers, and the like can be used. An electrode such as a positive electrode is not limited to the conductor (such as a conductive metal thin film), but may be a composition containing a conductive agent (graphite, graphite fluoride, acetylene black, manganese dioxide, etc.), an active material, and a binder. Or by applying the composition on the conductor.

As the electrolyte, a lithium salt such as LiBF ₄ , LiClF ₄ , LiPF ₆ , LiCl
O _4, etc. can be used as the aprotic solvent of the electrolytic solution, for example, .gamma.-butyrolactone, ethylene carbonate, propylene carbonate, dimethyl carbonate,
Diethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolan, 4-methyloxolan and the like can be used. As the separator, an ion-permeable membrane, for example, an olefin-based porous membrane such as a polypropylene microporous membrane can be used.

[0028] The shape and size of the battery of the present invention are particularly limited.
For example, cylindrical, flat, square, button type
It can be applied to non-aqueous batteries of various shapes.

[0029]

According to the present invention, since a specific binder is used, it is resistant to stress during molding and the adhesion between the carbon powder-containing layer and the current collector may be reduced even if charging and discharging are repeated. ,
The carbon powder can be prevented from dropping from the current collector, and the durability against repeated charging and discharging can be improved. Therefore, stable battery characteristics can be maintained for a long period of time.

[0030]

The present invention will be described below in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 [Preparation of laminate having carbon powder-containing layer] 2.0 parts by weight of cellulose cinnamate and N-methylpyrrolidone
And 0 parts by weight to prepare a binder solution. A graphite powder (crystal structure: d ₀₀₂ = 0.335) was added to the binder solution.
(Lc> 100 nm), 38.0 parts by weight were added and kneaded to prepare a slurry, and this slurry was applied to both surfaces of a copper foil (about 35 μm thick) by a doctor blade method,
By vacuum drying at a temperature of 2 ° C. for 2 hours, a laminate having a carbon powder-containing layer (thickness: about 100 μm) was obtained.

Example 2 [Preparation of laminate having carbon powder-containing layer] A laminate having a carbon powder-containing layer was prepared in the same manner as in Example 1, except that cellulose benzoate was used as a binder.

Comparative Example 1 A laminate having a carbon powder-containing layer was produced in the same manner as in Example 1 except that PVdF was used as a binder in the production of a laminate having a carbon powder-containing layer.

[Evaluation of Adhesion of Laminate] With respect to the laminates obtained in Examples and Comparative Examples, the adhesion strength between the carbon powder-containing layer and the metal foil was measured as follows.

After sticking a mending tape (manufactured by Sumitomo 3M Limited) on the surface of the carbon powder-containing layer, the tape was pulled vertically using a rheometer (manufactured by Fudo Industry Co., Ltd.) to contain the carbon powder. The tensile strength when the layer peeled off at the interface with the metal foil was measured.

The results are shown in Table 1. As shown in Table 1,
The laminate of the present invention has higher adhesion strength than the comparative example.

[0037]

[Table 1]

Example 3 [Preparation of Battery] [Preparation of Positive Electrode] 90 parts by weight of LiCoO ₂ as a positive electrode active material, 5 parts by weight of artificial graphite as a conductive agent, and 5 parts by weight of polyvinylidene fluoride as a binder Is mixed with a 5% by weight N-methylpyrrolidone solution containing, to prepare a slurry. The slurry is applied to both surfaces of an aluminum foil as a positive electrode current collector by a doctor blade method,
A positive electrode was produced by vacuum drying at 2 ° C. for 2 hours.

[Preparation of Negative Electrode] The laminate prepared in Example 1 was used as a negative electrode.

[Preparation of Electrolyte] LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate in the same amount (volume ratio: 1: 1) to prepare an electrolyte of 1 mol / liter.

[Assembly of Battery] Using these positive and negative electrodes and the electrolyte, an AA size (AA) having the structure shown in FIG.
Battery) was assembled. Note that an ion-permeable polypropylene microporous membrane was used as a separator.

Example 4 [Preparation of Battery] A battery was prepared in the same manner as in Example 3 except that the laminate prepared in Example 2 was used as a negative electrode.

Comparative Example 2 Example 3 was repeated except that the laminate of Comparative Example 1 was used as a negative electrode.
In the same manner as in the above, a battery was produced.

[Durability of Each Battery for Charge and Discharge Cycles] The batteries of Examples 3 and 4 and Comparative Example 2
After charging to 4.2 V at mA, a charge / discharge cycle test was performed in which the cycle of discharging at 200 mA to a discharge end voltage of 2.75 V was one cycle, and the charge / discharge cycle characteristics of each battery were examined.

Table 2 shows the discharge capacity at the first cycle and the discharge capacity at the 500th cycle of each battery.

[0046]

[Table 2]

As shown in Table 2, the battery of the present invention has a smaller decrease in discharge capacity with the progress of the charge / discharge cycle than the battery of the comparative example. This means that by using a cellulose ester having a specific acyl group as a binder, the adhesion between the carbon powder-containing layer and the metal foil (a copper current collector) is improved, and the charge / discharge cycle proceeds. It is presumed that the carbon powder-containing layer was prevented from falling off from the copper current collector.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a battery manufactured in an example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Positive electrode 2 ... Negative electrode 3 ... Separator 4 ... Negative electrode lead  5 Positive electrode lead 6 ... Positive external terminal 7 ... Negative external terminal

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (reference) // B32B 7/02 104 B32B 7/02 104 F term (reference) 4F100 AA37B AB17 AB33 AJ06B AS00B AT00C BA02 BA03 BA10A BA10C DE01B GB41 JG01A JK06 JL00 JM02A YY00B 4J040 BA041 FA241 GA06 HA026 LA06 LA09 MA01 NA19 5H029 AJ11 AK03 AK05 AK06 AK07 AK16 AL04 AL06 AL07 AL12 AM03 AM04 AM05 AM07 BJ12 DJ07 DJ08 EJ11 HJ01

Claims (6)

[Claims] 1. A binder for forming a carbon-containing layer by bonding to carbon powder on a conductive thin film, wherein the binder contains a cellulose ester having an acyl group having at least 3 to 18 carbon atoms. Characteristic binder. 2. An acyl group having an unsaturated bond.
The binder as described in the above. 3. The binder according to claim 1, wherein the cellulose ester has an aromatic acyl group. 4. A laminate in which a carbon-containing layer is laminated on at least one surface of a conductive thin film, wherein the carbon-containing layer comprises the binder according to any one of claims 1 to 3. Laminate containing. 5. The laminate according to claim 4, wherein the carbon-containing layer contains 0.1 to 10 parts by weight of a binder based on 100 parts by weight of the carbon powder. 6. At least one of the positive and negative electrodes is bonded to a conductive current collector and carbon powder that is laminated on at least one surface of the conductive current collector and that can store and release lithium ions. And a carbon-containing layer containing an agent. 6. The lithium secondary battery according to claim 4, wherein the electrode is formed of the laminate according to claim 4 or 5.