JPH1040920A - Film for secondary battery electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery electrode film useful for providing an electrode having an improved dimensional stability against heat, a longer life, and a greater capacity due to a smaller thickness by forming a metallic thin film on the surface of a biaxially oriented film of specific physical properties and made of a prescribed polyester-based resin. SOLUTION: A metallic (such as Al, Ni, etc.) thin film of the thickness preferably 1-1000 nanometers, more preferably 10-1000 nanometers is formed on at least one surface of a biaxially oriented film of the following properties: made of polyester-based resin (such as polyethylene terephthalate, polyethylene 2,6- naphthalate), the thickness is 0.2-30 micrometers, preferably 0.2-20 micrometers, more preferably 0.5-15 micrometers, specially preferably 0.9-10 micrometers, the heat contraction rate at 150 degrees C is 5% or less, preferably 3% or less, and the melting peak temperature by DSC temperature rise measurement is 200 deg.C or higher, preferably 230 deg.C or higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an electrode for a secondary battery and a film used for the same. More specifically, a secondary battery electrode having excellent heat resistance dimensional stability, improved battery capacity by making the electrode material base material thinner, and lighter than conventional products by using a film, and a film used therefor About.

[0002]

2. Description of the Related Art In recent years, with the miniaturization of electric and electronic equipment, there has been a demand for miniaturization of batteries used therein, and with the development of portable electric and electronic equipment, there has been a demand for improved capacity and longer life of batteries. . In many cases, a battery is formed by winding an electrode formed by laminating an electrode agent on a metal foil with a sheet separator interposed therebetween. By making these electrodes and separators thinner, batteries can be made smaller,
In addition, improvement in battery capacity and extension of life are expected.

[0003] However, if the thickness of the metal foil is reduced to meet the above demand, there is a problem that the strength is insufficient depending on the metal. Moreover, since the amount of metal used does not change, the weight of the battery cannot be reduced.

[0004]

One solution to this problem is to use a plastic film which is oriented on a substrate and has strength, and to provide a thin metal film on this surface. Due to the increase in resistance due to the decrease in
Possible temperature increase.

Accordingly, it is an object of the present invention to provide a secondary battery which is excellent in heat resistance, is improved in battery capacity and life by being made thinner, and is lighter than conventional products by using a film as a base material. An object of the present invention is to provide a battery electrode and a film used therefor.

[0006]

According to the present invention, first, according to the present invention, first, a polyester resin having a melting peak temperature of 200 ° C. or higher in a DSC temperature rise measurement, having a thickness of 0.2 mm is used. ~ 30 μm, heat shrinkage at 150 ° C is 5
% Or less, wherein a metal thin film is formed on at least one surface of the biaxially stretched film of at most 1%.

The resin constituting the biaxially stretched film in the present invention is a polyester resin satisfying the following DSC characteristics and having an ester bond in the molecule.

Preferred examples of the acid component constituting the polyester resin include terephthalic acid, isophthalic acid,
Aromatic dicarboxylic acids such as 2,6-naphthalenedicarboxylic acid, aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid and sebacic acid, cyclohexane-1,4
And alicyclic dicarboxylic acids such as dicarboxylic acids. Of these, aromatic dicarboxylic acids are more preferred. Preferred diol components include ethylene glycol, propylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, cyclohexane-1,4-dimethanol, and bisphenol A. Of these, ethylene glycol is more preferred. Further, hydroxycarboxylic acids such as p-hydroxybenzoic acid, m-chloro-p-hydroxybenzoic acid and ω-hydroxycaproic acid can be used, and they can also be in the form of a carbonate.

The polyester resin is preferably a homopolymer, but may be a copolymer. Further, a polymer blend composed of two or more of these homopolymers and / or copolymers may be used.

Of these, polyethylene terephthalate and polyethylene-2,6-naphthalate are particularly preferred.

In the present invention, the polyester resin constituting the biaxially stretched film has a melting peak temperature of 200 ° C. or higher, preferably 230 ° C. or higher in DSC temperature measurement. When the melting peak temperature is lower than 200 ° C., when the battery is used, a short circuit is caused by thermal deformation of the electrode when heat is generated due to internal resistance, which is not preferable.

The film in the present invention must be biaxially stretched, and a uniaxially stretched or unstretched film is not preferable because it causes a variation in capacity due to insufficient thermal dimensional stability.

The biaxially stretched film must have a heat shrinkage at 150 ° C. of 5% or less, more preferably 3%, in order to further improve the above-mentioned thermal dimensional stability.
It is as follows. If the heat shrinkage exceeds 5%, the resulting battery is undesirably short-circuited due to thermal deformation of the electrode when heat is generated due to internal resistance.

When the biaxially stretched film has a Young's modulus of preferably at least 500 kg / mm ² , particularly preferably at least 600 kg / mm ² , the workability of forming a battery by laminating and winding the electrode agent is improved. It is preferable because it becomes good. However, this Young's modulus is represented by the average value of the Young's modulus in the vertical and horizontal directions.

In the biaxially stretched film of the present invention, fine particles having an average particle diameter of preferably 0.05 to 2.0 μm are preferably used as a surface roughening agent within a range not impairing the effects of the present invention. The content can be 0.1 to 5.0% by weight. The fine particles may be internally precipitated particles or externally added particles. Examples of the externally added particles include calcium carbonate, magnesium carbonate, barium carbonate, barium sulfate, calcium phosphate, lithium phosphate, magnesium phosphate, lithium fluoride, aluminum oxide, silicon oxide (silica), titanium oxide, kaolin, talc, and carbon. Black, silicon nitride, boron nitride, crosslinked polymer fine particles (for example, crosslinked polystyrene,
Fine particles such as cross-linked acrylic resin and cross-linked silicone resin)
And the like. These may be used alone,
Also, two or more kinds may be used in combination.

As a method for incorporating such a surface roughening agent, an internal particle precipitation method in which an alkali (earth) metal compound is precipitated as fine particles by adding a phosphorus compound during the production of a polyester resin, An external particle addition method in which an inorganic or organic fine particle inert to a polymer is added in any of the film forming steps.

The biaxially stretched film of the present invention can be produced according to a conventionally known method.
For example, after sufficiently drying the raw material polymer under predetermined conditions,
Well-known melt extrusion equipment (typically extruder)
And melted by heating to a temperature higher than the melting point of the polymer (T _m : ° C.), particularly a temperature of T _m to (T _m +70) ° C. In this extrusion step, the raw material polymer is melt-kneaded so as to be uniform, and the degree of melt-kneading is adjusted.

Next, the melt-kneaded polymer is extruded from a slit-shaped die lip into a sheet and quenched and solidified on a rotary cooling drum to obtain a substantially amorphous unstretched sheet. In this case, in order to enhance the adhesion to the rotary cooling drum and to improve the surface flatness (flatness, smoothness) of the sheet, an electrostatic charge application adhesion method and / or a liquid application adhesion method are preferably employed. The electrostatic charge application adhesion method is a method in which a DC voltage is applied to a linear electrode extending in a direction perpendicular to the flow of a sheet extruded from a die and the surface of the sheet (non-drum side)
This is a method of improving the adhesion between the sheet and the rotary cooling drum by this action. The liquid application adhesion method is a method of improving the adhesion between the sheet and the rotary cooling drum by uniformly applying the liquid to all or a part of the surface of the rotary cooling drum (for example, a portion in contact with both ends of the sheet). It is. In the present invention, both may be used as needed. Further, as a method for producing a substantially amorphous unstretched sheet, an inflation casting method or a casting method may be employed.

The unstretched sheet thus obtained is then stretched biaxially to form a biaxially stretched film. As this stretching method, a sequential biaxial stretching method (tenter method) or a simultaneous biaxial stretching method (tenter method or tube method) can be used. As the stretching conditions in the sequential biaxial stretching method, the unstretched sheet is subjected to a temperature (T _g −10) to (T _g +70) ° C. (where T _g is the highest glass transition temperature) in one direction (longitudinal direction). Or 2 to 6 times, preferably 2.5 times
The film is stretched to 5.5 times, and then in a direction perpendicular to the first step (if the first step is longitudinal, the second step is transverse).
_g to (T _g +70) ° C. 2 to 6 times, preferably 2.
Stretch 5 to 5.5 times. In addition, a method of performing unidirectional stretching in two or more stages can be used, but in such a case, it is desirable that the final stretching ratio is within the above range. Further, after the intermediate heat treatment after the second stage stretching, the film may be stretched again in the same direction as the first stage and / or in the same direction as the second stage. The unstretched sheet may have an area magnification of 6 to 3.
Simultaneous biaxial stretching can be performed so as to be 0 times, preferably 8 to 25 times.

The biaxially stretched film thus obtained is subjected to a heat treatment, and this treatment is carried out under the condition that the heat shrinkage is 5% or less. In order to obtain this heat shrinkage, T _g 〜 (T _g +140)
The heat treatment is preferably performed at a temperature of 1 ° C. for 1 second to 10 minutes.
At this time, the reaction may be performed under limited contraction or elongation within 20% or under a fixed length, or may be performed in two or more stages.

The thickness of the biaxially stretched film thus obtained is 0.2 to 30 μm, preferably 0.2 to 20 μm,
More preferably, it is 0.5 to 15 μm, particularly preferably 0.9 to 10 μm. If the thickness of the film is too small, it is not preferable because sufficient strength as an electrode agent base material cannot be obtained. On the other hand, if the thickness is too large, the electrode area is insufficient with respect to the battery volume, so that a sufficient battery capacity cannot be obtained, which is not preferable.

The biaxially stretched film of the present invention may be surface-treated for the purpose of improving the adhesion to a metal thin film. The surface processing method is not particularly limited, but preferred examples include application of an easy-adhesive agent layer, corona treatment, and plasma treatment. These surface treatments may be performed during the step of forming the biaxially stretched film, or may be performed in a step different from the film forming step. Among these, it is preferable to carry out during the film forming process.

In order to use the biaxially stretched film of the present invention as a film for a secondary battery electrode, a metal thin film is provided on at least one surface thereof. As the metal forming the thin film, for example, aluminum, nickel, gold, silver,
Although copper, cadmium, etc. are mentioned, it is not particularly limited as long as it is conductive for the purpose of the present invention. The thickness of the metal thin film is usually in the range of 1 to 1000 nm, but is preferably 10 to 1000 nm for the purpose of suppressing the internal resistance heat generation of the battery. Preferable examples of the method for forming the metal thin film include a vacuum deposition method, an electroplating method, and a sputtering method.

The secondary battery electrode film of the present invention can be used as a secondary battery electrode by laminating an electrode agent on the metal thin film surface. As the electrode agent, conventionally known ones, for example, lithium cobaltate, graphite and the like can be used. Furthermore, a secondary battery can be manufactured by a conventionally known method using the secondary battery electrode. For example, when used in a lithium ion secondary battery, a secondary battery electrode film having a copper sputtered film coated with lithium cobaltate is used as a positive electrode, and a secondary battery electrode film having an aluminum evaporated film is made of graphite. The secondary battery can be formed by using the coated material as a negative electrode, winding the resultant with a separator made of a microporous polyethylene film interposed therebetween, and using an organic solvent in which a lithium salt is dissolved as an electrolyte.

[0025]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the invention. In the present invention, various physical properties and characteristics are measured and defined as follows.

(1) Intrinsic viscosity ([η]) A value (unit: dl / g) measured at 25 ° C. using o-chlorophenol as a solvent.

(2) Melting peak temperature (T _m ) 10 mg of the film was set in a thermal analysis system SSC / 5200, DSC5200 manufactured by Seiko Denshi Kogyo Co., Ltd.
Heating at a rate of 20 ° C./min in a nitrogen gas stream,
The endothermic behavior accompanying the melting of the film is analyzed by the first derivative and the second derivative to determine a temperature showing a peak or a shoulder, which is defined as a melting peak temperature (unit: ° C.).

(3) Film Thickness (t) The weight G (g) when the film is sampled in a width W (cm) and a length l (cm), and the density of the film is d.
(G / cm ³ ), it is calculated by the following formula (unit:
μm). The density d is measured using a density gradient tube composed of an aqueous solution of n-heptane to carbon tetrachloride or calcium nitrate.

[0029]

## EQU1 ## t = [g / (W × 1 × d)] × 10000

(4) Heat Shrinkage Rate (s) The film was sampled at 350 mm × 350 mm, and the distance (L ₀ ) between two points at the center was 300 mm.
Attach so that. 10 sheets of the marked films were hung without tension in a hot-air circulation type thermostatic oven set at 150 ° C. After holding for 30 minutes, the film was taken out, and the distance (L) between the scores was measured. The determined heat shrinkage (unit:%) is averaged in each of the vertical and horizontal directions (n = 10), and this is defined as a value.

[0031]

S = [(L ₀ −L) / L ₀ ] × 100

(5) Battery Capacity A lithium ion secondary battery was prepared using the film, and was continuously discharged. The discharge capacity at which the discharge voltage reached 80% was 20%.
Those having a value of 00 mAh or more are regarded as good.

(6) Battery Life The prepared battery is subjected to repeated charge / discharge tests under the conditions of 100 ° C., and a battery having a deterioration cycle of 500 times or more until a short circuit occurs in 10% of the total number of batteries is determined to be good. .

Example 1 A polyethylene-2,6-naphthalate (T _m = 267 ° C.) having an [η] of 0.65 was sufficiently dried, melt-extruded from a slit die, and then subjected to an electrostatic charge printing and adhesion method. It was brought into close contact with a rotating cooling drum having a surface temperature of 15 ° C. and solidified rapidly to obtain an unstretched sheet.

The unstretched sheet was successively biaxially stretched at a stretching temperature of 140 ° C. at a stretching ratio of 3.7 times in the longitudinal direction and 4.0 times in the transverse direction. After cooling once, a constant-length heat treatment at 240 ° C. was performed to obtain a biaxially stretched film having a thickness of 7.0 μm. The heat shrinkage of the obtained biaxially stretched film was as shown in Table 1.

A positive electrode material obtained by applying lithium cobalt oxide on both surfaces of the biaxially stretched film and copper, and a negative electrode material obtained by depositing aluminum on both surfaces of the biaxially stretched film and applying graphite. Prepared, wound with a separator consisting of a polyethylene microporous membrane interposed between the two, and dissolved lithium hexafluorophosphate in a mixed solvent of ethylene carbonate / diethyl carbonate / ethyl acetate and used as an electrolyte for lithium ion secondary Battery was created. The capacity and life of the obtained battery were as shown in Table 1.

Comparative Example 1 As a raw material of a biaxially stretched film, a polyethylene terephthalate / isophthalate copolymer (T _m = 0.65) in which an isophthalic acid component was copolymerized at a rate of 12 mol% and [η] was 0.65. 193 ° C.)
At a stretching temperature of 60 ° C., 3.7 times in the longitudinal direction and 4.0 times in the lateral direction.
A film was formed and a battery was prepared in the same manner as in Example 1 except that the film was formed by successively biaxially stretching the film at a double magnification and then performing a constant-length heat treatment at 120 ° C. to form a film.

The capacity and life of the obtained battery were as shown in Table 1. This battery is likely to cause a short circuit because the melting peak temperature of the base film is less than 200 ° C.
The service life was short.

[Comparative Example 2] The thickness of the biaxially stretched film was 4
A film was formed and a battery was prepared in the same manner as in Example 1 except that the thickness was set to 0 μm.

The capacity and life of the obtained lithium ion secondary battery were as shown in Table 1. In this battery, the battery capacity was insufficient because the film serving as the electrode material base was too thick.

Comparative Example 3 As a raw material of a biaxially stretched film, polyethylene terephthalate (T) having [η] of 0.65 was used.
_m = 258 ° C.) and successively biaxially stretched at a stretching temperature of 120 ° C. at a draw ratio of 3.7 times in the machine direction and 4.0 times in the transverse direction, and then subjected to a constant-length heat treatment. A film was formed and a battery was prepared in the same manner as in Example 1 except that the film was formed.

The capacity and life of this battery were as shown in Table 1. In this battery, the thermal shrinkage of the film was too large, so that a short circuit was likely to occur, and the life was short.

[0043]

[Table 1]

[0044]

As described above, in the secondary battery prepared using the film for a secondary battery electrode of the present invention, the battery capacity is improved by making the electrode material substrate thinner, and the melting peak as the electrode material substrate is improved. By using a biaxially stretched film having a high temperature and a low heat shrinkage, a short circuit due to thermal deformation of the electrode material substrate is prevented, and the battery life is excellent.

A secondary battery prepared using the film for a secondary battery electrode of the present invention has a lighter weight and a higher weight energy density than a conventional product using a metal foil as an electrode material base. It has become.

Claims (3)

[Claims] 1. A biaxially stretched film comprising a polyester resin having a melting peak temperature of 200 ° C. or more in DSC temperature rise measurement, a thickness of 0.2 to 30 μm, and a heat shrinkage at 150 ° C. of 5% or less. A film for a secondary battery electrode, wherein a metal thin film is formed on at least one surface of the above. 2. A secondary battery electrode obtained by laminating an electrode agent on a metal thin film surface of the secondary battery electrode film according to claim 1. 3. A secondary battery incorporating the secondary battery electrode according to claim 2.