JP2002216851A - Manufacturing method of lithium ion secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a lithium ion secondary cell with improved bulging property. SOLUTION: The manufacturing method of the lithium ion secondary sheet cell, enveloped by a sheet-shaped enveloping material, is constituted of a process of charging the assembled cell, a process of removing the gas generated while charging, and a process of selecting non-defective cells through an aging test which leaves the cell for a prescribed period after gas removal in a state of being charged, and further gas removing process is applied to the non-defective cells.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a lithium ion secondary battery, and more particularly, to a method of manufacturing a lithium ion secondary battery capable of improving bulging characteristics (bulge characteristics) when left at high temperatures. About.

[0002]

2. Description of the Related Art In a lithium ion secondary battery, it is known that a chemical reaction occurs at an electrode / electrolyte interface during initial charging, and gas is generated. Lithium-ion secondary batteries are subjected to a degassing step to remove the generated gas, and then to an aging step (aging test). Is dropped, and only non-defective good batteries are shipped as products.

[0003]

However, since it is difficult to completely remove the gas generated in the degassing step, a small amount of gas remains in the battery as a product. For this reason, the lithium ion secondary battery is a so-called sheet battery (particularly a laminated type) using an exterior material other than a metal (aluminum, iron (chrome plating) or the like) can (for example, a sheet-like exterior material such as an aluminum laminated film). ), When a sheet battery as a product is left in a high-temperature (eg, 90 ° C.) atmosphere for a long time, the remaining gas expands and blisters occur. As a result, the positive and negative electrodes are separated and the internal The resistance increased, charging and discharging became unstable, and the battery performance decreased.

The present invention has been made to solve the above problems, and an object of the present invention is to manufacture a lithium ion secondary battery capable of improving blistering characteristics (bulge characteristics) when left at high temperatures. It is to provide a method.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies on the above problems, and as a result, further degassing of non-defective batteries selected by an aging test has resulted in the generation of batteries during the initial charging. It has been found that the exhaust gas can be almost completely removed, and that the bulging property (bulge property) when left at a high temperature can be improved, and the present invention has been completed. That is, the present invention is as follows.

A method for manufacturing a lithium ion secondary sheet battery packaged with a sheet-like package material, comprising the steps of charging the assembled battery, removing gas generated during the charging, and removing the gas. A process for selecting good batteries by an aging test in which the charged batteries are left in a charged state for a predetermined period, and further degassing the good batteries, the method for producing a lithium ion secondary sheet battery .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION There are two types of lithium ion secondary sheet batteries, a stacked type and a wound type. The method for producing a lithium ion secondary sheet battery according to the present invention is particularly advantageously applied to a stacked type. Can be. The laminated type is a type having a structure in which a positive electrode plate 1, a negative electrode plate 2, and a separator (or a solid electrolyte layer) 3 are laminated as shown in a cross-sectional view in FIG. Means). Therefore, a laminated lithium ion secondary sheet battery packaged with a sheet-like package material has a configuration in which the laminated structure 10 is packaged with a sheet-like package material 8 (along with an electrolytic solution when a separator is used). And usually has a plate-like shape.

The present invention is a method for manufacturing a lithium ion secondary sheet battery packaged with a sheet-like packaging material having the above-described structure, and includes a step of charging the battery after assembly, and a step of removing gas generated during the charging. A removing step and a step of selecting a non-defective battery by an aging test in which the battery from which the gas has been removed is left in a charged state for a predetermined period are further performed, and the non-defective battery is further degassed.

The charging is performed under the condition that the battery is charged at a constant current value until a constant voltage is reached, and then charged at a constant voltage in that state, although it varies depending on the size, capacity, shape and the like of the battery. be able to. The gas generated during charging can be removed using, for example, a vacuum pump of a rotary pump type after a constant current discharge until a constant voltage is reached at a constant current value. In addition, the selection of good batteries depends on the batteries that had problems during the aging test (for example,
This can be done by removing the voltage that suddenly drops during this period. In the aging test, the sheet battery from which the gas was removed was charged again, for example, in the same manner as the above-mentioned charging, and left in the charged state, for example, in a 23 ° C. atmosphere for 1 to 2 weeks. Those having a voltage drop of 0.1 V or less are regarded as good products.

Next, the good battery is further degassed. This degassing can be performed, for example, using a rotary pump type vacuum suction device after discharging again in the same manner as the above-described discharging. With this degassing,
The remaining gas can be almost completely removed.

The positive electrode plate, the negative electrode plate, the separator, the electrolyte, the solid electrolyte layer, and the sheet-like exterior material constituting the laminated lithium ion secondary sheet battery of the present invention include:
Conventionally known ones can be used. Examples of these are shown below.

Each of the positive electrode plate 1 and the negative electrode plate 2 has a layer of a composition containing an active material, a conductive agent, a binder and the like (hereinafter referred to as an active material composition layer 6, 7) is provided. Examples of the positive electrode active material include a Li-Mn-based composite oxide, a Li-Ni-based composite oxide, and a Li-Co-based composite oxide. As the binder for the positive electrode active material, polytetrafluoroethylene, polyvinylidene fluoride, an ethylene-propylene-diene-based polymer, or the like can be used. As the conductive agent, for example, natural or artificial graphites such as fibrous graphite, flaky graphite, and spherical graphite, and conductive carbon black can be used. The amount of the binder is about 1 to 10 parts by weight, and the amount of the conductive agent is 3 to 15 parts by weight, based on 100 parts by weight of the positive electrode active material composition comprising the positive electrode active material, the binder and the conductive agent. The weight is about

As the negative electrode active material, graphites such as fibrous graphite, flaky graphite and spherical graphite, and preferably fibrous mesophase-based graphitized carbon can be used. The same binder as the above-mentioned positive electrode active material can be used. The amount of the negative electrode active material used may be about 80 to 96 parts by weight per 100 parts by weight of the total amount of the negative electrode active material and the binder.

As the current collectors 4 and 5, a foil or a perforated foil formed of a conductive metal can be used, and the thickness may be about 5 μm to 100 μm. As a material of the positive electrode current collector 4, aluminum, stainless steel, or the like is used, and as a material of the negative electrode current collector 5, copper, nickel, silver, stainless steel, or the like is used.

The positive electrode plate 1 and the negative electrode plate 2 can be manufactured according to a known method. For example, the positive electrode plate 1 is prepared by mixing and processing a positive electrode active material, a binder, and a conductive agent, dispersing the mixture in an organic solvent such as N-methylpyrrolidone to form a paste, applying the paste to the positive electrode current collector 4, and drying the paste. Then, it can be obtained by pressing and cutting into an appropriate shape.

Examples of the separator 3 include a separator made of a polyethylene film, a separator made of a polypropylene film, and a separator having a three-layer structure of a polypropylene / polyethylene / polypropylene film.

As a solvent for the electrolytic solution, for example, one or a mixture of two or more selected from ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethoxyethane, diethyl carbonate and dimethoxyethane can be used. .

When a mixed solvent is used as a solvent for the electrolytic solution, it contains at least one selected from diethyl carbonate (DEC) and ethyl methyl carbonate (EMC), and further contains ethylene carbonate (E).
A mixture comprising C), propylene carbonate (PC) and dimethyl carbonate (DMC) is preferred.
The mixing ratio of each component constituting the mixture is at least one type selected from diethyl carbonate and ethyl methyl carbonate, and the mixing ratio is from 25% by volume to 50% by volume.
Is more preferable, and it is more preferable that it is 30% by volume to 35% by volume. In ethylene carbonate, the mixing ratio is preferably from 4% by volume to 20% by volume, and more preferably from 6% by volume to 18% by volume. In propylene carbonate, the mixing ratio is preferably from 3% by volume to 17% by volume, and more preferably from 5% by volume to 15% by volume. In dimethyl carbonate, the mixing ratio is preferably more than 40% by volume and 60% by volume or less, and more preferably 45% by volume to 55% by volume.

In at least one selected from diethyl carbonate and ethyl methyl carbonate,
When the mixing ratio is less than 25% by volume, the freezing point of the electrolytic solution increases, and particularly at a low temperature of −20 ° C. or lower, the internal resistance of the battery increases, and the charge / discharge cycle characteristics and low-temperature characteristics tend to decrease. Become. On the other hand, the mixing ratio is 50% by volume.
If the temperature exceeds the range, the viscosity of the electrolytic solution increases, the internal resistance of the battery increases, and the charge / discharge cycle characteristics tend to decrease.

In the case of ethylene carbonate, if the above mixing ratio is less than 4% by volume, it is difficult to form a stable film on the surface of the negative electrode plate, and the cycle characteristics tend to deteriorate,
If the mixing ratio exceeds 20% by volume, the viscosity of the electrolyte increases, the internal resistance of the battery increases, and the charge / discharge cycle characteristics tend to decrease.

In propylene carbonate, if the above mixing ratio is less than 3% by volume, the effect of suppressing the increase in impedance due to charge / discharge cycles becomes small, and the cycle characteristics tend to decrease. If it exceeds, the viscosity of the electrolyte increases, the internal resistance of the battery increases, and the charge / discharge cycle characteristics tend to decrease.

In dimethyl carbonate, if the mixing ratio is 40% by volume or less, the viscosity of the electrolytic solution increases, the internal resistance of the battery increases, and the charge / discharge cycle characteristics tend to decrease. If the volume percentage is exceeded, the freezing point of the electrolytic solution increases, and particularly at low temperatures of −20 ° C. or lower, the internal resistance of the battery increases, and the cycle characteristics and low-temperature characteristics tend to decrease.

As the salt (electrolyte) of the electrolytic solution, for example,
LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , L
At least one compound selected from iN (SO ₂ CF ₃ ) ₂ and LiCF ₃ SO ₃ can be used, and more preferably LiPF ₆ and LiBF ₄ . The content of the salt in the electrolytic solution is preferably 0.1 to 2.0 mol / L.
(Moles of salt per liter of solvent), more preferably 0.5 to 1.5 mol / L, most preferably 0.7 to 1.5 mol / L.
1.2 mol / L. When the content is more than 2.0 mol / L, there is a problem that the high-rate characteristics and the low-temperature characteristics are reduced due to an increase in the viscosity of the electrolytic solution.
When the amount is less than mol / L, there is a problem that a sufficient battery capacity cannot be obtained due to a decrease in ionic conductivity, and high-rate characteristics are significantly reduced.

The solid electrolyte layer 3 includes, for example, a layer mainly composed of a salt (electrolyte), a compatible solvent and a fluoropolymer having vinylidene fluoride as a main unit.

The fluoropolymer having vinylidene fluoride as a main unit includes a homopolymer of vinylidene fluoride (polyvinylidene fluoride (PVdF)),
Alternatively, it means a copolymer of vinylidene fluoride and another vinyl monomer having a fluorine atom, and these may be used alone or in combination. Vinyl monomers having a fluorine atom other than the above vinylidene fluoride include hexafluoropropylene (HF
P), chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE) and the like. In particular, copolymers of VdF and HFP and VdF and TFE
Is preferred. The form of the copolymer may be any of a random form and a block form. In the case of a copolymer, the proportion of (unit) of vinylidene fluoride is
Preferably 90-70 mol%, more preferably 85-70 mol%
75 mol%. If this ratio exceeds 90 mol%, even when a solution in which a salt (electrolyte) is dissolved in a compatible solvent is impregnated with a fluoropolymer, the absorption of the solution into the polymer does not proceed sufficiently, and this results in ionic conductivity. , And eventually the resistance of the battery increases, causing a large decrease in the charge / discharge capacity. Also, 70mo
If it is less than 1%, elution of the polymer component occurs due to an increase in the amorphous portion. This causes the long-term reliability of the battery, particularly the cycle characteristics, to be significantly reduced.

The fluoropolymer having vinylidene fluoride as a main unit has a carboxyl group (—COO).
H) a vinyl group having a functional group consisting of a sulfonic acid group (—SO ₂ OH), a carboxylic acid ester group (—COOR), an amide group (—CONH ₂ ), or a phosphoric acid group (—PO (OH) ₂ ). A polymer of a monomer may be grafted (the substituent R in the carboxylic acid ester group (—COOR) is a lower alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, and a butyl group). . When the fluoropolymer is in a polymer form in which the polymer containing such a functional group is grafted, the adhesion of the solid electrolyte layer to the positive electrode or the negative electrode is improved, and the resistance between the electrodes is reduced, so that the polymer has good performance. A battery is obtained. As the vinyl monomer having a functional group, a monomer composed of a compound having a carbon number of 4 or less excluding the functional group is preferable. As the carboxyl group-containing monomer, those having one carboxyl group such as acrylic acid, methacrylic acid, crotonic acid, vinyl acetic acid, and allyl acetic acid, and those having two or more carboxyl groups such as itaconic acid and maleic acid can also be used. It is. As the sulfonic acid group-containing monomer, styrene sulfonic acid, vinyl sulfonic acid and the like are preferable. As the carboxylic acid ester group-containing monomer, methyl acrylate, butyl acrylate, and the like are preferable. Acrylamide and the like are preferable as the amide group-containing monomer.
Phosphoric acid group-containing monomers include triphenyl phosphate,
Tricresyl phosphate and the like are preferred. Most preferred of these are acrylic acid or methacrylic acid.

The degree of grafting can be determined by several factors, but most importantly, the length of time the activated substrate is in contact with the graft monomer,
The degree of preactivation of the substrate by radiation, the extent to which the grafted monomeric material can penetrate the substrate, and the temperature at which the substrate and monomer are in contact. For example, when the graft monomer is an acid, the degree of grafting can be monitored by sampling a solution containing the monomer, titrating against a base, and measuring the concentration of the remaining monomer. The degree of grafting is preferably 2 to 20% of the final weight, particularly preferably 3 to 1%.
It is 2%, particularly preferably 5 to 10%. The grafting may be performed by a method of activating (generating free radicals) the polymer substrate by light irradiation or heat.

The fluoropolymer having vinylidene fluoride as the main unit preferably has a melt flow index at 230 ° C. and 10 kg of 1.0 g / 10 min or less, more preferably 0.7 to 0.2 g / 10 min. preferable. The melt flow index is
It is a value measured by the method described in standard ASTM D1238. When the melt flow index is 1.0 g / 10 min or less, the solid electrolyte layer exhibits more excellent mechanical strength, and the conductivity at room temperature is further improved.

As the salt (electrolyte) used for the solid electrolyte layer, those mentioned above as the salt of the electrolytic solution can be used.

As the compatible solvent used for the solid electrolyte layer, those listed above as the solvent for the electrolytic solution can be used.

The solid electrolyte layer may further contain a plasticizer (eg, tetraethylene glycol dimethyl ether, N-methylpyrrolidone, etc.), ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, etc., in addition to the above components. The plasticizer acts to prevent crystallization of the compatible solvent at the desired operating temperature and to ensure sufficient electrical conductivity.

The method for forming the solid electrolyte layer is not particularly limited. For example, (a) a fluoropolymer as a polymer substrate is formed into a film by a known molding method such as extrusion molding or blow molding, or a coating liquid of a fluoropolymer is prepared, and the coating liquid is prepared in advance for release. A film is formed by applying the composition on the surface of a substrate, heating and drying the coated film, and peeling the film from the substrate for release.After that, the obtained film is treated with a salt compatible solvent. A method of forming a solid electrolyte layer by immersing in a solution dissolved in a solution (including a case of immersing in a solution together with a positive electrode and a negative electrode in a battery manufacturing process), (b) a method in which a salt and a compatible solvent are used. Dissolve in a suitable solvent, further add a fluoropolymer, dissolve the fluoropolymer while heating if necessary, apply this solution to the surface of the substrate for peeling to form a coating film, Increase the temperature of the membrane in stages Dried and evaporated all the solvent, a method of separating the solid electrolyte layer from the release base material,
(C) forming a coating film of a solution in which the salt, the compatible solvent and the fluoropolymer are dissolved directly on at least one surface of the plate-shaped positive electrode and / or negative electrode, and evaporating the solvent to form a solid Examples of the method include a method of forming an electrolyte layer.

As the solvent in the above methods (b) and (c), for example, tetrahydrofuran (TH
F), dimethylacetamide, dimethylformamide and the like are used. In the method (b), the release substrate on which the coating film is formed may be dried by passing through a heating chamber having a different temperature.

Although the thickness of the solid electrolyte layer varies depending on the size of the positive electrode and the negative electrode, it is generally 1 μm to 50 μm.
m, preferably selected from the range of 10 μm to 30 μm.

In the present invention, the sheet-like exterior material refers to a material in which a resin layer is laminated on at least one surface of a metal foil. Examples of the metal foil include aluminum, copper, and iron. Examples of the resin include thermoplastic resins such as polyester and polypropylene. As long as it has such a thermoplastic resin layer, it can be sealed only by externally covering the laminated structure (stack) and heat-welding the periphery thereof,
The battery manufacturing operation is simple. The thickness of the metal foil is preferably 10 to 100 μm, more preferably 20 to 70 μm
And the thickness of the resin layer is preferably 5 to 100 μm,
More preferably 10 to 50 μm, and the total thickness is preferably 20 to 200 μm, more preferably 4
0 to 150 μm.

The stacked sheet battery of the present invention comprises a positive electrode plate, a negative electrode plate, and a separator or solid electrolyte layer having a rectangular shape substantially the same size as the positive electrode plate and the negative electrode plate. By producing a laminated structure in which one or more units in which a separator or a solid electrolyte layer is sandwiched are repeated, and the laminated structure is accommodated in a sheet-like exterior material (when a separator is used, together with an electrolytic solution) Can be made. Here, each of the positive electrode plate and the negative electrode plate is obtained by forming an active material layer on at least one surface of a rectangular plate-shaped current collector by the above-described method.

[0037]

The present invention will be described more specifically with reference to the following examples and comparative examples.

(Preparation of Stacked Sheet Battery) <Preparation of Positive Electrode> 90 parts by weight of lithium cobaltate granules as a positive electrode active material, 5 parts by weight of polyvinylidene fluoride as a binder, and artificial graphite 5 as a conductive agent Parts by weight and 70 parts by weight of N-methylpyrrolidone were mixed to form a slurry. This slurry is applied on both sides of an aluminum foil (thickness: 20 μm) as a positive electrode current collector, dried, and then subjected to a rolling treatment (rolling temperature: 25 ° C., rolling ratio: 30%) to give 20 mg / cm ² per side of the aluminum foil. Positive electrode plate having a positive electrode active material composition layer (thickness of 70 μm) (width 30 mm,
(Length 50 mm).

<Preparation of Negative Electrode Plate> 95 parts by weight of graphitized carbon fiber, 5 parts by weight of polyvinylidene fluoride and N
-60 parts by weight of methylpyrrolidone were mixed to form a slurry. This slurry is used as a negative electrode current collector in a copper foil (thickness 14).
μm), dried and then rolled (rolling temperature 100 ° C., rolling rate 20%) to give 1
0 mg / cm ² negative electrode active material composition layer (75 μm thickness)
A negative electrode plate having a width of 32 mm and a length of 52 mm was prepared.

<Assembly of Stacked Sheet Battery> The positive electrode plate and the negative electrode plate produced above, and a rectangular film of polyvinylidene fluoride (PVdF) (thickness: 25 μm)
m, width 34 mm, length 54 mm), the positive electrode plate, the PVdF rectangular film, and the negative electrode plate are stacked so that the unit in which the PVdF rectangular film is sandwiched between the positive electrode plate and the negative electrode plate is repeated eight times, thereby forming a laminated structure. The body was made.

Next, LiPF ₆ was added to a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a ratio of 50 vol%: 50 vol% at a concentration of 1.0 mol / mol.
The laminated structure was immersed in a solution of L (concentration after preparation) for 1 hour to gel a rectangular PVdF film.

Next, the laminated structure obtained by gelling the PVdF rectangular film was housed in an Al-laminated film obtained by laminating a thermoplastic resin as an exterior material on one side or both sides, to obtain a laminated sheet battery. Completed.

Example 1 A group (100) of the above-prepared stacked sheet batteries was used at a constant current of 300 mA.
The battery was charged until the voltage reached 2 V, and further charged at a constant voltage of 4.2 V for 2 hours. Thereafter, constant current discharge was performed at a constant current of 300 mA until the voltage reached 2.75 V. Next, gas generated during charging was removed using a rotary pump type vacuum suction device. Subsequently, the stacked sheet battery from which the gas was removed was charged under the same conditions as described above. In this charged state, 1
The battery was left to stand for a week, and during this period, the laminated sheet battery whose voltage dropped to less than 4.1 V was removed as a defective product, and only a good laminated sheet battery was selected. The non-defective laminated sheet battery was discharged under the same conditions as described above. Furthermore, the discharged non-defective laminated sheet battery was degassed using a rotary pump type vacuum suction device.

Comparative Example 1 A discharged non-defective laminated sheet battery was treated in the same manner as in Example 1 except that no further degassing was performed.

Example 1 (Evaluation of Battery Characteristics / Bulge Test)
The length (mm) of one of the non-defective laminated sheet batteries described in Comparative Example 1 in the thickness direction after standing for 1 hour in an atmosphere of 90 ° C. was measured. Table 1 shows the results.

[0046]

[Table 1]

From the results shown in Table 1, it can be seen that the stacked sheet battery of Example 1 was 1-fold less in a 90 ° C. atmosphere than the stacked sheet battery of Comparative Example 1 in which the degassing step was not performed after the sorting step.
It can be seen that the swelling after standing for a long time was remarkably small, and the swelling property (bulge property) when left at high temperature was remarkably improved.

[0048]

According to the present invention, the gas generated during the first charge can be almost completely removed by further degassing the good batteries selected by the aging test, and It is possible to provide a method for manufacturing a lithium ion secondary battery capable of improving blister characteristics (bulge characteristics) when left at a high temperature.

[Brief description of the drawings]

FIG. 1 is a sectional view of a stacked lithium ion secondary sheet battery according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode plate 2 negative electrode plate 3 separator or solid electrolyte layer 4 positive electrode current collector 5 negative electrode current collector 6 positive electrode active material composition layer 7 negative electrode active material composition layer 8 sheet-shaped exterior material 10 laminated structure 20 laminated lithium ion Secondary sheet battery

Continued on the front page. (72) Inventor Toshiki 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. Itami Works (72) Inventor Shogo Tanno 4-3-1 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. Inside the Itami Works (72) Inventor Mitsuhiro Marumoto 4-3 Ikejiri, Itami City, Hyogo Mitsubishi Electric Cable Co., Ltd.Itami Works F-term (reference) BB02 BB03 FF31

Claims (1)

[Claims] 1. A method for manufacturing a lithium ion secondary sheet battery packaged with a sheet-like package material, comprising: charging a battery after assembly; removing gas generated during the charging; A step of selecting non-defective batteries by an aging test in which the removed battery is left in a charged state for a predetermined period of time, further degassing the non-defective batteries, Production method.