JP2000100399A - Manufacture of polymer lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve safety by preventing ruptures upon generation of gases due to overcharge or the like. SOLUTION: This manufacturing method is for a polymer lithium secondary battery, which has a structure where a generating element 1, is housed in an externally attached film 13 with the open periphery thermally fused, and at least one part of a sealing part functioning as a safety valve. Thermal fusion temperature for a sealing zone 14 which functions as a safety valve is made lower than the thermal fusion temperature for a sealing region having no safety valve function.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a polymer lithium secondary battery having a safety valve mechanism.

[0002]

2. Description of the Related Art In recent years, with the development of electronic equipment, there has been a demand for the development of a non-aqueous electrolyte secondary battery that is small, lightweight, has a high energy density, and can be repeatedly charged and discharged. Such a secondary battery includes a negative electrode using lithium or a lithium alloy as an active material, a positive electrode containing an oxide, sulfide, or selenide such as molybdenum, vanadium, titanium, or niobium as an active material, and a nonaqueous electrolyte. There is known a lithium secondary battery including:

In recent years, a negative electrode containing a carbonaceous material that absorbs and releases lithium ions, such as coke, graphite, carbon fiber, a resin fired body, and pyrolytic gas phase carbon, has been used as a negative electrode. The development and commercialization of lithium ion secondary batteries using lithium and lithium manganese oxide-containing batteries are being actively pursued.

[0004] In such a secondary battery, a metal container is used as an exterior material for housing a power generation element.
In recent years, non-aqueous electrolyte secondary batteries using a film material as an exterior material have been studied for the purpose of further reducing the weight and size of the secondary battery. As the film material, a metal foil such as an aluminum foil is provided inside so that exchange of gas such as water vapor and oxygen is not performed inside and outside the film material, and the outside is used to protect the metal foil from physical impact. There is known a composite film in which a resin (for example, polyethylene terephthalate) layer and a heat-fusible resin layer such as an ionomer are disposed inside. The sealing of the power generation element by such a film material is performed by covering the power generation element with the film material so that the heat-fusible resin layer is positioned on the inner surface, and then heat-sealing the opening edge of the film material. This is performed by sealing the opening edges of the film material by thermally fusing the resin layers to each other.

[0005]

However, a non-aqueous electrolyte secondary battery using a film material as an exterior material does not have a safety valve mechanism, so it expands and explodes when gas is generated due to overcharging or the like. There is a problem that there are cases. Rupture may cause damage to electronic devices.

An object of the present invention is to provide a method for manufacturing a polymer lithium secondary battery in which rupture when gas is generated due to overcharge or the like is prevented and safety is improved.

[0007]

A method for manufacturing a polymer lithium secondary battery according to the present invention has a structure in which a power generation element is housed in an exterior film whose opening edge is sealed by heat sealing. A method for manufacturing a polymer lithium secondary battery in which at least one portion of a sealing portion functions as a safety valve, wherein the heat-sealing temperature of the sealing region functioning as the safety valve is the heat of a sealing region having no safety valve function. It is characterized by being lower than the fusing temperature.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of a polymer lithium secondary battery produced by the method according to the present invention will be described below with reference to FIG.

That is, the power generating element 1 of the polymer lithium secondary battery includes a positive electrode 2, a negative electrode 3, and an electrolyte layer 4 disposed between the positive electrode 2 and the negative electrode 3. The positive electrode 2 has a structure in which a positive electrode layer 6 is supported on both surfaces of a current collector 5 made of, for example, a two-dimensional porous body. On the other hand, the negative electrode 3 has a structure in which a negative electrode layer 8 is supported on both surfaces of a current collector 7 made of, for example, a two-dimensional porous body. Strip-shaped positive electrode terminal 9
Is obtained by extending the current collector 5 of each positive electrode 2 in a belt shape. On the other hand, the strip-shaped negative electrode terminal 10 is obtained by extending the current collector 7 of the negative electrode 3 in a strip shape. The positive electrode lead 11 is connected to the two positive terminals 9. Negative electrode lead 12
Is connected to the negative electrode terminal 10. As shown in FIG. 2, such a power generating element 1 is covered with an outer film 13 having a heat-fusible resin disposed on an inner surface thereof in a state where the positive electrode lead 11 and the negative electrode lead 12 extend from the film 13. ing. The opening edge of the film 13 is
Sealed by heat fusion. As shown in FIGS. 2 and 3, the sealing portion (heat-sealing portion) along the longitudinal direction has a region 14 (a region shown by oblique lines in FIGS. 2 and 3) functioning as a safety valve.
Is provided. Area 14 functioning as the safety valve
Is lower than the heat fusion temperature for forming a region having no safety valve function.

In the polymer lithium secondary battery having such a structure, when a gas is generated due to, for example, overcharging and the internal pressure of the film 13 increases, as shown in FIG. Since the adhesive is peeled off and the opening is opened, and the gas in the film 13 can escape to the outside from this portion, the rupture of the film 13 can be avoided.

As the positive electrode, the negative electrode, the electrolyte layer and the package film of the polymer lithium secondary battery, for example, those described below can be used.

(Positive Electrode 1) The positive electrode 1 has a structure in which a positive electrode layer 6 containing a positive electrode active material, a non-aqueous electrolyte and a binder for holding the electrolyte is laminated on both surfaces of a current collector 5.

Examples of the positive electrode active material include various oxides (eg, lithium manganese composite oxide such as LiMn ₂ O ₄ , manganese dioxide, lithium-containing nickel oxide such as LiNiO _2, and lithium-containing cobalt oxide such as LiCoO ₂ , for example). Oxide, lithium-containing nickel-cobalt oxide, lithium-containing amorphous vanadium pentoxide and the like, and chalcogen compounds (for example, titanium disulfide and molybdenum disulfide). Among them, it is preferable to use a lithium manganese composite oxide, a lithium-containing cobalt oxide, and a lithium-containing nickel oxide.

The non-aqueous electrolyte is prepared by dissolving an electrolyte in a non-aqueous solvent.

Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and dimethyl carbonate (D
MC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone (γ-
BL), sulfolane, acetonitrile, 1,2-dimethoxyethane, 1,3-dimethoxypropane, dimethyl ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran and the like. The non-aqueous solvents may be used alone or as a mixture of two or more.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ) and lithium hexafluorophosphate (L
iPF _6), boric tetrafluoride lithium (LiBF _4), lithium hexafluoroarsenate (LiAsF _6), lithium salts such as lithium trifluoromethane sulfonate (LiCF ₃ SO ₃₎ may be mentioned.

The amount of the electrolyte dissolved in the non-aqueous solvent is desirably 0.2 mol / l to 2 mol / l.

The binder has a property of retaining a non-aqueous electrolyte. Examples of such a binder include a polyethylene oxide derivative, a polypropylene oxide derivative, a polymer containing the derivative, polytetrafluoropropylene, a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP), and polyvinylidene fluoride ( PVdF) or the like can be used. Among them, a VdF-HFP copolymer is preferred.

The positive electrode may include a conductive material from the viewpoint of improving conductivity. Examples of the conductive material include artificial graphite, carbon black (eg, acetylene black), nickel powder, and the like.

As the two-dimensional porous body as the current collector, a mesh made of aluminum or an aluminum alloy, an expanded metal, a punched metal, or the like can be used. The current collector of the positive electrode is not limited to such a two-dimensional porous body, and for example, an aluminum foil can be used. When using aluminum foil as the current collector,
The positive electrode layer is laminated on only one side of the current collector.

As the positive electrode terminal, a current collector extending in a band shape as described with reference to FIG. 1 can be used. A band-shaped aluminum foil is connected to the current collector, and this is connected to the positive electrode. It may be used as a terminal.

The positive electrode lead can be formed, for example, from an aluminum foil.

(Negative Electrode 3) In the negative electrode 3, a negative electrode layer 8 including a negative electrode active material, a non-aqueous electrolyte, and a binder holding the electrolyte is supported on a current collector 7 such as a two-dimensional porous body. Having a structure.

Examples of the negative electrode active material include carbonaceous materials that occlude and release lithium ions. Such carbonaceous materials include, for example, those obtained by firing organic polymer compounds (eg, phenolic resin, polyacrylonitrile, cellulose, etc.), those obtained by firing coke and mesophase pitch, and those made by artificial graphite. And carbonaceous materials represented by natural graphite and the like. Among them, it is preferable to use a carbonaceous material obtained by firing the mesophase pitch at a temperature of 500 ° C. to 3000 ° C. in an inert gas atmosphere such as an argon gas or a nitrogen gas under normal pressure or reduced pressure.

As the non-aqueous electrolyte, those similar to those described above for the positive electrode are used.

The binder has a property of retaining a non-aqueous electrolyte. As such a binder, a polymer of the same type as that described in the above-described positive electrode can be used, and among them, a VdF-HFP copolymer is preferable.

For the two-dimensional porous body as the current collector, a mesh made of copper or a copper alloy, an expanded metal, a punched metal, or the like can be used. Further, the current collector of the negative electrode is not limited to such a two-dimensional porous body, and for example, a copper foil can be used.

As the negative electrode terminal, a current collector extending in a band shape as described with reference to FIG. 1 can be used. A band-like copper foil is connected to the current collector, and this is connected to the negative electrode. It may be used as a terminal.

The negative electrode lead can be formed of, for example, a copper foil.

(Electrolyte Layer) This electrolyte layer contains a non-aqueous electrolyte and a binder for holding the electrolyte.

As the non-aqueous electrolyte, those similar to those described above for the positive electrode are used.

The binder has a property of retaining a non-aqueous electrolyte. As such a binder, a polymer of the same type as that described in the above-described positive electrode can be used, and among them, a VdF-HFP copolymer is preferable.

From the viewpoint of further improving the strength, the electrolyte layer may contain organic particles or inorganic particles such as silicon oxide powder.

4) Outer Film 13 The outer film 13 has a heat-fusible resin disposed on the inner surface. The heat-fusible resin preferably has non-aqueous electrolyte resistance, for example, ionomer,
Modified polyolefins (eg, low density polyethylene,
High density polyethylene, polypropylene). Among them, ionomers are preferred.

It is preferable that the exterior film 13 has a function of blocking water vapor. Examples of the exterior film 13 having such a function include a film in which a metal thin film is interposed. The metal thin film can be formed from, for example, aluminum, copper, and nickel.

As the exterior film having a heat sealing resin disposed on the inner surface and having a function of blocking water vapor, specifically, polyethylene (PE) / polyethylene laminated from the inner surface (seal surface) side to the outer surface Terephthalate (PE
T) / Al foil / PET multilayer film; PE / Nylon / Al foil / PET multilayer film; Ionomer / Ni
A multilayer film of foil / PE / PET; a multilayer film of ethylene vinyl acetate (EVA) / PE / Al foil / PET; a multilayer film of ionomer / PET / Al foil / PET can be used. Here, P on the sealing surface side
Films other than E, ionomer, and EVA have moisture resistance, air resistance, and chemical resistance.

The opening edge of the above-mentioned exterior film is sealed by heat fusion. At least one of the sealing portions functions as a safety valve. The heat-sealing temperature for forming the sealing region functioning as the safety valve is lower than the heat-sealing temperature for forming the sealing region without the safety valve function.

The heat-sealing temperature of the sealing region having the safety valve function is preferably not lower than the melting point of the heat-fusible resin on the inner surface of the exterior film and not higher than the melting point plus 10 ° C. This is due to the following reasons. When the heat-sealing temperature is lower than the melting point of the heat-fusible resin, the adhesiveness between the exterior films is not good, so that the sealing property of the exterior film is reduced, and the liquid leakage during storage or during use is reduced. Or charge / discharge characteristics may be degraded. On the other hand, if the heat fusion temperature is higher than the temperature obtained by adding 10 ° C. to the melting point of the heat fusion resin, the adhesiveness between the exterior films becomes high, so that gas is generated due to overcharging and the internal pressure increases. In this case, it may be difficult to prevent the polymer lithium secondary battery from exploding.

The heat-sealing temperature of the sealing region which does not function as a safety valve is preferably 30 to 50 ° C. higher than the melting point of the heat-fusible resin on the inner surface of the exterior film.

In FIGS. 1 to 4 described above, a region having a safety valve function is formed in the sealing portion along the longitudinal direction of the film, but the region is provided anywhere except the vicinity of the positive electrode lead and the negative electrode lead. Is also good. For example, it may be formed on the side where the lead is not fixed among the end sides orthogonal to the longitudinal direction of the film.

Although the number of regions having the safety valve function is one in FIGS. 1 to 4 described above, two or more regions may be formed.

In FIGS. 1 to 4 described above, the positive and negative electrode leads extend from the same end of the exterior film, but the negative electrode lead extends from an end different from the end where the positive lead extends. It may be extended.

As described in detail above, the manufacturing method according to the present invention has a structure in which a power generation element is housed in an exterior film whose opening edge is sealed by heat sealing, and at least one of the sealing portions A method for manufacturing a polymer lithium secondary battery in which one portion functions as a safety valve, wherein a heat fusion temperature of a sealing region functioning as the safety valve is higher than a heat fusion temperature of a sealing region having no safety valve function. It is characterized by being low. According to such a manufacturing method, at least one region having low sealing strength can be provided in the sealing portion of the exterior film. As a result, gas is generated from the power generation element due to overcharging or the like, and when the internal pressure of the film accommodating the power generation element rises, the above-described exterior film in the region where the sealing strength is low is peeled off and opened, and this opening is opened. Since the gas in the film can escape to the outside from the portion, the rupture of the secondary battery can be prevented.

The heat-sealing temperature of the sealing region functioning as the safety valve is equal to or higher than the melting point of the heat-fusible resin used for heat-sealing the opening edge portion and equal to or lower than the temperature obtained by adding 10 ° C. to the melting point. By maintaining the sealing property of the exterior film in a favorable state, that is, while avoiding liquid leakage during high-temperature storage, the secondary battery bursts when the internal pressure increases due to overcharging or the like. Can be prevented.

[0045]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments according to the present invention will be described below in detail with reference to the drawings.

(Examples 1 to 5) <Preparation of positive electrode not impregnated with nonaqueous electrolyte> Lithium-manganese composite oxide having a composition formula of LiMn ₂ O ₄ as an active material, carbon black, and vinylidene as a binder Fluoride-hexafluoropropylene (VdF-HF
A paste was prepared by mixing the copolymer powder of P) and dibutyl phthalate (DBP) in acetone. The obtained paste was applied on a PET film to prepare a positive electrode sheet not impregnated with a non-aqueous electrolyte. The obtained positive electrode sheet was heat-pressed on both surfaces of a current collector made of expanded metal made of aluminum with a hot roll to produce a non-aqueous electrolyte-unimpregnated positive electrode.

<Preparation of Negative Electrode Unimpregnated with Nonaqueous Electrolyte> Mesophase pitch carbon fiber as an active material, VdF-HFP copolymer powder as a binder, and dibutyl phthalate (DBP) were mixed in acetone, Was prepared. The obtained paste was applied on a PET film to produce a non-aqueous electrolyte-unimpregnated negative electrode sheet. The obtained negative electrode sheet was heat-pressed with heat rolls on both surfaces of a current collector made of a copper expanded metal to produce a non-aqueous electrolyte-unimpregnated negative electrode.

<Preparation of electrolyte layer not impregnated with non-aqueous electrolyte> Silicon oxide powder, a copolymer powder of VdF-HFP as a binder, and dibutyl phthalate (DBP) were mixed in acetone to form a paste. . Put the obtained paste into PET
An electrolyte layer not impregnated with a non-aqueous electrolyte was prepared by coating on a film, forming a sheet, and cutting.

<Preparation of Nonaqueous Electrolyte> A nonaqueous electrolyte was prepared by dissolving LiPF ₆ as an electrolyte in a nonaqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed.

<Battery assembly> A positive electrode not impregnated with a non-aqueous electrolyte was
Sheets, one non-aqueous electrolyte impregnated negative electrode and two non-aqueous electrolyte non-impregnated electrolyte layers were prepared, and these were laminated so that the electrolyte layer was interposed between the positive electrode and the negative electrode. And heated and pressed by a heated rigid roll.

From the obtained laminate, the plasticizer in the laminate was removed by solvent extraction and dried to obtain a non-aqueous electrolyte-impregnated power generating element.

A composite film having a polyethylene terephthalate (PET) layer, an aluminum layer, and an ionomer resin layer laminated in this order was prepared as an exterior film. The exterior film was folded in two so that the ionomer resin layer was positioned inside, and the exterior film covered the non-aqueous electrolyte-unimpregnated power generating element. Except for one portion serving as a nonaqueous electrolyte injection port, the opening edge of the exterior film was heat-fused with a fusion width of 5 mm. At this time, the region over the length of 5 mm near the center of the opening edge along the longitudinal direction was set to the temperature shown in Table 1 below. The other part of the edge of the opening was set at 130 ° C., which is 40 ° C. higher than the melting point of the ionomer resin (90 ° C.). Subsequently, a non-aqueous electrolytic solution having the above composition was injected from an opening edge that was not heat-sealed, and the opening edge was heat-sealed at a fusion width of 5 mm and a fusion temperature of 130 ° C. as described above. 1 to
It has the structure shown in FIG. 3 and has a width of 55 mm and a length of 80 m
m of the polymer lithium secondary battery was manufactured.

(Comparative Example) An exterior film similar to that described in Examples 1 to 5 was folded in two so that the ionomer resin layer was located inside, and this Example 1 The same power generating element not impregnated with the non-aqueous electrolyte as described in 5 was coated. 5 m fusion width except for the opening edge of the exterior film, which is a non-aqueous electrolyte injection port
m and the fusion temperature was set to 130 ° C. to perform thermal fusion. Subsequently, a non-aqueous electrolyte having the same composition as that described in Examples 1 to 5 was injected from the edge of the opening that was not heat-sealed, and the opening edge was fused with a welding width of 5 mm and a fusion temperature. To 130
A polymer lithium secondary battery having a width of 55 mm and a length of 80 mm having the structure shown in FIGS.

The obtained secondary batteries of Examples 1 to 5 and Comparative Example were charged to 4.2 V at a constant current of 0.2 C.
After repeating three cycles of initial charge / discharge of discharging to 2.7 V at a constant current of 0.2 C, a sealing test (liquid leakage test) and a safety test were performed.

In the liquid leakage test, after storing each of 20 charged secondary batteries under the following conditions (i) or (ii), the appearance of the secondary batteries is visually observed to determine whether or not there is liquid leakage. And the results are shown in Table 1 below.

(I) Stored in an atmosphere at a temperature of 45 ° C. and a humidity of 93% RH for 10 days. (Ii) Stored in an atmosphere at a temperature of 60 ° C. for 10 days.
An overcharge test was performed at a constant current of 0 mA and a maximum voltage of 15 V, gas was forcibly generated in the exterior film, and the presence or absence of rupture was examined. Further, for the secondary batteries of Examples 1 to 5, the time required until the films in the region over the length of 5 mm near the center of the sealing portion along the longitudinal direction were peeled off, that is, the time required for the safety valve to operate. Is measured (average value),
These results are also shown in Table 1 below.

[0057]

[Table 1]

As is apparent from Table 1, the secondary batteries of Examples 1 to 5 in which one portion of the sealing portion functions as a safety valve by making the heat sealing temperature of the opening edge of the exterior film different are high temperature. Alternatively, it can be seen that there is little liquid leakage when stored in a high-temperature and high-humidity atmosphere, and it is possible to avoid bursting during overcharge. Further, in the secondary battery of Example 5 in which the heat sealing temperature of the sealing region functioning as the safety valve is 105 ° C., the first example in which the heat sealing temperature of the sealing region functioning as the safety valve is 85 to 100 ° C. It can be seen that the time until the safety valve operates is slightly longer than that of the secondary batteries of Nos. 1 to 4.

On the other hand, the secondary battery of the comparative example in which the temperature at which the opening edge of the exterior film was thermally fused and the safety valve function region was not formed in the sealing portion was stored in a high-temperature or high-temperature and high-humidity atmosphere. It can be seen that there is no liquid leakage, but rupture occurs during overcharge.

In the above-described embodiment, a polymer lithium secondary battery including a unit cell having a five-layer structure in which a positive electrode, an electrolyte layer, a negative electrode, an electrolyte layer, and a positive electrode are stacked in this order has been described as an example. Is not limited to such a five-layer structure. For example, a positive electrode, a negative electrode, and an electrolyte layer may be used one by one to form a three-layer structure.

[0061]

As described in detail above, according to the present invention, it has excellent sealing properties, can prevent rupture when the internal pressure rises due to overcharging, etc., and improves safety. And a method for producing a polymer lithium secondary battery.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a polymer lithium secondary battery manufactured by a method according to the present invention.

FIG. 2 is a plan view showing the polymer lithium secondary battery of FIG.

FIG. 3 is a side view showing the polymer lithium secondary battery of FIG. 1;

FIG. 4 is a side view showing a state in which a safety valve of the polymer lithium secondary battery of FIG. 1 is operated.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Power generation element, 11 ... Positive electrode lead, 12 ... Negative electrode lead, 13 ... Outer film, 14 ... Sealing part which functions as a safety valve.

Continued on the front page F term (reference) 5H011 AA10 AA13 CC02 CC06 CC10 DD13 GG01 HH02 JJ12 KK04 5H012 AA03 BB04 DD01 EE01 FF08 GG01 JJ02 JJ06 JJ10 5H029 AJ12 AK02 AK03 AK05 AL06 AL07 AM02 DJ03 AM04 DJ05

Claims (4)

[Claims] 1. A polymer lithium secondary battery having a structure in which a power generation element is housed in an exterior film whose opening edge is sealed by heat sealing, and at least one of the sealing portions functions as a safety valve. The method for manufacturing a polymer lithium secondary battery, wherein the heat sealing temperature of the sealing region functioning as the safety valve is lower than the heat sealing temperature of the sealing region having no safety valve function. Production method. 2. The heat-sealing temperature of the sealing region functioning as the safety valve is equal to or higher than the melting point of the heat-fusible resin used for heat-sealing the opening edge, and is added to the melting point by 10 ° C. 2. The method for producing a polymer lithium secondary battery according to claim 1, wherein the temperature is not higher than the temperature. 3. The method according to claim 1, wherein the heat-fusible resin used for heat-sealing the opening edge is an ionomer resin. 4. The method for manufacturing a polymer lithium secondary battery according to claim 1, wherein the exterior film has a function of blocking water vapor.