JP2010199083A - Lithium secondary battery - Google Patents

Abstract

A lithium secondary battery having high performance, high energy density and excellent high load characteristics while preventing occurrence of a short circuit in the battery during battery assembly is provided.
A positive electrode layer having at least a lithium-containing metal oxide or metal oxide as an active material, a negative electrode layer having at least an Li metal, a Li alloy, or a carbon material as an active material, and an electrolyte solution and a polymer electrolyte layer therebetween. In the arranged lithium secondary battery, the positive electrode layer and / or the negative electrode layer includes a polymer electrolyte matrix layer containing an electrolytic solution and a polymer, and the ionic conductivity of the polymer electrolyte constituting the polymer electrolyte layer is the polymer electrolyte. The polymer electrolyte comprises a polymer fiber or microporous membrane separator having an air permeability of 1 to 500 sec / cm ³ lower than the ionic conductivity of the polymer electrolyte constituting the matrix layer, and the polymer fiber or microporous The membrane separator is made of polypropylene fiber, polyethylene fiber or polyester fiber, The polymer electrolyte layer is composed of an ion conductive polymer and a polymer fiber or a microporous membrane separator, and a weight ratio of the ion conductive polymer and the polymer fiber or the microporous membrane separator is 91: 9 to 50:50. The above-described problem is solved by a lithium secondary battery.
[Selection figure] None

Description

  The present invention relates to a lithium secondary battery using a polymer electrolyte, and more particularly, uses a polymer electrolyte that reversibly operates at ambient temperature, and particularly constitutes a polymer electrolyte layer disposed between a positive electrode layer and a negative electrode layer. The present invention relates to a lithium secondary battery characterized in that the ionic conductivity of the polymer electrolyte is lower than that constituting the polymer electrolyte matrix layer inside the positive electrode layer and / or the negative electrode layer.

Secondary batteries are often used as power sources for portable devices from the viewpoint of economy. There are various types of secondary batteries, and the most common at present is a nickel-cadmium battery. Recently, a nickel metal hydride battery has become widespread. Further, lithium cobaltate LiCoO ₂ , lithium nickelate LiNiO ₂ , a solid solution Li (Co _1-x Ni _x ) O ₂ , or LiMn ₂ O ₄ having a spinel structure is used for the positive electrode, and Li metal is used for the negative electrode. Lithium secondary batteries that use carbon materials such as Li alloys or graphite, and liquid organic compounds as solvents and lithium compounds as solutes have higher output voltages than nickel-cadmium batteries and nickel metal hydride batteries. Due to its high and high energy density, it is becoming a main power source especially for portable devices and portable information.

As an electrolyte of such a lithium secondary battery, an electrolytic solution in which a lithium salt is dissolved in an organic solvent has been used so far. Since such an electrolyte is flammable, ensuring its safety is a major problem. In response to such problems, it has been found that by replacing the electrolytic solution with a polymer electrolyte, it is possible to suppress liquid leakage and the volatility of the organic solvent, thereby improving safety and reliability.
However, since the battery using this polymer electrolyte has low ionic conductivity in the polymer electrolyte, lithium ions cannot be sufficiently transferred. For this reason, there has been a problem that the discharge capacity, particularly the discharge capacity during high-load discharge, is lowered.

In order to improve the ionic conductivity of the polymer electrolyte, a method of using a gel polymer electrolyte in which an electrolytic solution is added and held in a polymer has been actively developed. This gel polymer electrolyte has an ionic conductivity of 10 ⁻³ S / cm at room temperature, and has a high ionic conductivity comparable to the electrolytic solution. Therefore, although the discharge capacity is increased, the mechanical strength is decreased. For this reason, when assembling the battery, there is a problem that the probability of causing a short circuit in the battery increases.

When lithium (Li) metal is used for the negative electrode, there is known one that suppresses the occurrence of a short circuit by stacking several polymer electrolyte layers for the purpose of reducing a short circuit during charging (see Patent Document 1). Moreover, what suppresses precipitation of dendrites by increasing the ionic conductivity of the polymer electrolyte layer on the negative electrode side is known (see Patent Document 2).
However, Patent Literature 1 and Patent Literature 2 are techniques for suppressing short-circuiting due to lithium dendrite deposition during charging after the battery is assembled. When assembling a battery using a polymer electrolyte, short-circuiting in the battery is also avoided. It is necessary to prevent.

In order to solve the above problems, in a polymer battery composed of a porous membrane separator holding a gel polymer electrolyte, and a positive electrode and a negative electrode, the bubble point of the porous membrane separator is 0.1 to 100 kg / A technique for increasing the discharge capacity while suppressing the occurrence of a short circuit in the battery by using a film having a cm ² and a porosity of the film of 40 to 90% is known ( (See Patent Document 3). The bubble point in this case is a pressure at which bubbles start to emerge from the surface of the sample when the gas pressure is gradually increased after replacing the inside of the sample with ethanol.

JP-A-8-306389 JP-A-8-329983 JP-A-10-284125

However, for the technique of Patent Document 3, the use of the selected porous membrane improves the short circuit in the battery during battery assembly. However, an electrode in which each of the positive electrode and the negative electrode is combined with a polymer electrolyte is used. It is not used, and the contact resistance at the interface between the member constituting the electrode and the polymer electrolyte has not been sufficiently reduced.
An object of the present invention is to provide a lithium secondary battery excellent in high performance, high energy density, and high load characteristics while preventing occurrence of a short circuit in the battery during battery assembly.

  As a result of various investigations to overcome the above problems, the inventors have determined that the ionic conductivity of the polymer electrolyte constituting the polymer electrolyte layer is lower than that of the polymer electrolyte constituting the polymer electrolyte matrix layer inside the electrode. By doing so, it has been found that a lithium secondary battery excellent in high performance, high energy density and high load characteristics can be provided while preventing the occurrence of a short circuit in the battery during battery assembly.

That is, the present invention includes a positive electrode layer using at least a lithium-containing metal oxide or metal oxide as an active material, a negative electrode layer using at least a Li metal, a Li alloy, or a carbon material as an active material, and an electrolyte solution and a polymer electrolyte therebetween. In the lithium secondary battery in which the layers are arranged, the positive electrode layer and / or the negative electrode layer includes a polymer electrolyte matrix layer containing an electrolyte solution and a polymer, and the ionic conductivity of the polymer electrolyte constituting the polymer electrolyte layer is A polymer fiber or microporous membrane separator having an air permeability of 1 to 500 sec / cm ³ lower than the ionic conductivity of the polymer electrolyte constituting the polymer electrolyte matrix layer, and the polymer fiber or Microporous membrane separator is made of polypropylene fiber, polyethylene fiber or polyester fiber And the polymer electrolyte layer is composed of an ion conductive polymer and a polymer fiber or a microporous membrane separator, and the weight ratio of the ion conductive polymer to the polymer fiber or the microporous membrane separator is 91: 9 to 50 A lithium secondary battery is provided.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium secondary battery which has the outstanding discharge capacity at the time of high load discharge can be provided, suppressing the internal short circuit of the battery at the time of an assembly.

  In addition, when the polymer electrolyte layer has a thickness of 150 μm or less, it is further excellent in high-load discharge characteristics, and more electrode active material can be charged into the battery, so that the lithium secondary having a high energy density. Battery can be provided.

  In addition, when the negative electrode active material is a carbon material and the carbon material has amorphous carbon attached to the surface of the graphite particles, it is possible to suppress side reactions with the polymer electrolyte, and the cycle characteristics are excellent. A lithium secondary battery can be provided.

  In addition, when both polymers contained in the polymer electrolyte matrix layer and the polymer electrolyte layer are made of a diacrylate-derived polymer containing a copolymer of polyethylene oxide and polypropylene oxide, they are further excellent in high-load discharge performance and high energy density. The lithium secondary battery which has can be provided.

It is a basic structural view of the battery of the present invention. It is a graph which shows the result of the charging / discharging test of the battery of Examples 4, 5, and 8. It is a graph which shows the cycling characteristics of the battery of Example 4, 6, and 7. FIG.

A lithium secondary battery according to the present invention includes a positive electrode layer having at least a lithium-containing metal oxide or metal oxide as an active material, a negative electrode layer having at least an Li metal, a Li alloy, or a carbon material as an active material, and an electrolyte solution therebetween. And a polymer electrolyte layer. Further, in the lithium secondary battery according to the present invention, the positive electrode layer and / or the negative electrode layer includes a polymer electrolyte matrix layer containing an electrolytic solution and a polymer, and the ionic conductivity of the polymer electrolyte constituting the polymer electrolyte layer is It is characterized by being lower than the ionic conductivity of the polymer electrolyte constituting the polymer electrolyte matrix layer. Furthermore, the lithium secondary battery according to the present invention includes a polymer fiber or a microporous membrane separator in which the polymer electrolyte has an air permeability of 1 to 500 sec / cm ³ . The polymer fiber or microporous membrane separator is made of polypropylene fiber, polyethylene fiber or polyester fiber. Further, the polymer electrolyte layer is composed of an ion conductive polymer and a polymer fiber or a microporous membrane separator, and a weight ratio of the ion conductive polymer and the polymer fiber or the microporous membrane separator is 91: 9 to 50. : 50.

In this way, by using a polymer electrolyte having a conductivity lower than that of the polymer electrolyte contained in the positive electrode layer and / or the negative electrode layer in the polymer electrolyte layer, the mechanical strength of the electrolyte layer is increased. Sometimes the incidence of short circuits in the battery decreases.
Further, the ionic conductivity of the polymer electrolyte constituting the polymer electrolyte layer is preferably 0.1 mS / cm or more at room temperature. When it is lower than 0.1 mS / cm, the battery performance is remarkably deteriorated, which is not preferable. More preferably, it is 1-1000 mS / cm. On the other hand, the ionic conductivity of the polymer electrolyte contained in the positive electrode layer and / or the negative electrode layer is preferably 1.4 times or more that of the polymer electrolyte constituting the polymer electrolyte layer, and is 1.4 to 4.1 times that of the polymer electrolyte. It is more preferable that

As the polymer electrolyte layer, a gel polymer electrolyte layer holding a non-aqueous electrolyte solution is used, and the electrolyte layer includes polymer fibers or a microporous membrane separator.
In the case of the polymer electrolyte layer, the ionic conductivity is lower than that of the electrolytic solution. Therefore, in order to improve the ionic conductivity, a gel-like polymer electrolyte in which a nonaqueous electrolytic solution is held in the polymer matrix is used. The capacity, particularly the discharge capacity during high load discharge can be increased. However, since the gel polymer electrolyte has low mechanical strength, the occurrence rate of short circuit during battery assembly is high. Therefore, by holding the gel polymer electrolyte in the polymer fiber or microporous membrane separator, it is possible to increase the discharge capacity, particularly during high-load discharge, and to suppress the occurrence of short circuits during battery assembly. It is preferable because it is possible.

  The weight ratio of the ion conductive polymer (polymer electrolyte) constituting the polymer matrix layer to the lithium salt-containing nonaqueous electrolytic solution is preferably more than 2:98. That is, when the weight ratio of the ion conductive polymer is smaller than 2, the non-aqueous electrolyte cannot be sufficiently retained, and the non-aqueous electrolyte may ooze out from the gel polymer electrolyte.

  Polypropylene fiber, polyethylene fiber, or polyester fiber can be used as the polymer fiber, and a polyolefin-based microporous membrane can be used as the microporous membrane separator. These are preferable because they are chemically stable with respect to the non-aqueous electrolyte. The thickness of the polymer fiber and the microporous membrane separator is preferably 150 μm or less, more preferably 20 μm or less, for the same reason as described above.

The negative electrode active material of the lithium secondary battery is a carbon material, and in particular, it is preferable that non-stylish carbon is attached to the surface of the graphite particles.
Since the negative electrode active material is a carbon material that utilizes a lithium ion storage-release process, there is no precipitation of dendritic lithium during charging, battery short-circuiting is drastically reduced, and safety is improved. In particular, when the negative electrode active material is graphite particles with amorphous carbon attached to the surface, it is possible to prevent decomposition of the electrolyte layer made of an ion-conductive polymer, so there is no leakage and thus long-term reliability. Will improve.

  The polymer electrolyte layer or the polymer electrolyte inside the electrode comprises an ion conductive polymer and a Li salt, and may optionally contain polymer fibers or a microporous membrane separator. Furthermore, an excellent polymer electrolyte can be obtained by modifying the polymer. For example, a polymer having a comb-like structure obtained by copolymerization or blending with another polymer or graft polymerization of another polymer on the main skeleton of the polymer may be used, but is not particularly limited. Examples of the ion conductive polymer include polyethylene oxide, polypropylene oxide, a mixture of polyethylene oxide and polypropylene oxide, a copolymer of ethylene oxide and propylene oxide, and a polymer of a PEO-based polymer containing an alkylene oxide as a constituent component and a vinyl group at a terminal group. , An alkylene oxide-acrylonitrile copolymer, a diacrylate, a copolymer having a triacrylate-based polyfunctional group, and the like.

The Li salt is preferably at least one of LiBF ₄ , LiPF ₆ , LiClO ₄ , LiCF ₃ SO ₃ or LiN (CF ₃ SO ₂ ) ₂ , but is not limited thereto.
The ion conductive polymer can be used as a gel by containing an organic solvent and a Li salt. Examples of the organic solvent include cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC); cyclic esters such as γ-butyrolactone (GBL); chain esters such as methyl propionate and ethyl propionate; diethyl carbonate ( DEC), chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (EMC); ethers such as tetrahydrofuran or derivatives thereof, 1,3-dioxane, 1,2-dimethoxyethane, methyl diglyme; acetonitrile, Nitriles such as benzonitrile; dioxolane or a derivative thereof; sulfolane or a derivative thereof alone or a mixture of two or more thereof. However, it is not limited to these. Further, the blending ratio and blending method are not limited.

  In particular, it is a gel containing an organic electrolytic solution in which a Li salt is dissolved in a mixed organic solvent in which one or more solvents selected from PC, GBL, EMC, DMC or DEC are mixed with a solvent composed of EC. This is preferable because there is little decomposition at the negative electrode using a graphite-based carbon material with attached crystalline carbon as an active material.

The polymer fiber or microporous membrane separator added to the electrolyte layer is a non-woven fabric having physical properties with an air permeability of 1 to 500 sec / cm ³ . Air permeability is low, is not sufficiently obtained ionic conductivity than 1 sec / cm ^3, high mechanical strength is not sufficient than 500 sec / cm ^3, it may be likely to cause a short circuit of the battery. Furthermore, the weight ratio of the ion conductive polymer and the polymer fiber or the microporous membrane separator constituting the electrolyte layer is in the range of 91: 9 to 50:50. When the weight ratio of the ion conductive polymer is higher than 91, sufficient mechanical strength may not be obtained, and when it is lower than 50, ionic conductivity may not be sufficiently obtained.

  The polymer electrolyte matrix layer inside the positive electrode layer and / or the negative electrode layer, and the polymer electrolyte layer disposed between them are impregnated with these electrolyte precursors in a liquid state in these electrodes, polymer fibers or microporous membrane separators. It can be produced by crosslinking with actinic rays or heat, or by removing the solvent. The electrolyte layer disposed between the positive electrode layer and the negative electrode layer does not have to have a single layer structure, and may have a multilayer structure in the electrolyte layer. Further, the surface of the electrolyte layer may be treated in order to prevent diffusion of the solvent between the positive electrode layer and the electrolyte layer or between the negative electrode layer and the electrolyte layer and to improve the adhesion at each electrolyte layer interface.

  In addition, it is also important to use a polymerization initiator when crosslinking is necessary. Particularly in the crosslinking method by ultraviolet rays or heating, it is preferable to add a polymerization initiator of several percent or less. As the polymerization initiator, commercially available products such as 2,2-dimethoxy-2-phenylacetophenone (DMPA) and benzoyl peroxide (BPO) can be used. The wavelength of ultraviolet light is suitably 250 to 365 nm.

  If water is contained in the electrolyte solvent, a side reaction between the water and the solvent will occur during charge / discharge of the battery, causing problems such as reduction in efficiency and cycle life of the battery itself and generation of gas. Sometimes. For this reason, it is necessary to reduce the water content of the electrolyte solvent as much as possible. Therefore, in some cases, the electrolyte solvent may be dehydrated using molecular sieve, alkali metal, alkaline earth metal, or activated aluminum. The amount of water is 1000 ppm or less, preferably 100 ppm or less.

In order to constitute the electrode of the lithium secondary battery of the present invention, a transition metal oxide or lithium-containing metal oxide powder is used as a positive electrode active material, and a conductive agent, a binder, and in some cases, a solid electrolyte. It is formed by mixing. Examples of the transition metal oxide include vanadium oxide V ₂ O ₅ and chromium oxide Cr ₃ O ₈ . Examples of the lithium-containing metal oxide include cobalt lithium oxide (Li _x CoO ₂ : 0 <x <2), nickel lithium acid (Li _x NiO ₂ : 0 <x <2), and nickel oxide cobalt cobalt composite oxide (Li _x (Ni _1-y Co _y ) O ₂ : 0 <x <2, 0 <y <1), manganese lithium acid (Li _x Mn ₂ O ₄ : 0 <x <2, Li _x MnO ₂ : 0 <x < 2), lithium vanadium LiV ₂ O ₅ , LiVO ₂ , tungsten lithium oxide LiWO ₃ , molybdenum lithium oxide LiMoO _{3 and the} like.

  As the conductive agent, carbon materials such as acetylene black and graphite powder, metal powder, and conductive ceramics can be used. Fluorine polymers such as polytetrafluoroethylene and polyvinylidene fluoride, and polyolefin polymers such as polyethylene and polypropylene can be used as the binder. These mixing ratios can be 1 to 50 parts by weight of the conductive agent and 1 to 30 parts by weight of the binder with respect to 100 parts by weight of the transition metal oxide or lithium-containing metal oxide. When the amount of the conductive agent is less than 1 part by weight, the resistance or polarization of the electrode increases, and the capacity as the electrode decreases, so that a practical lithium secondary battery cannot be constructed. On the other hand, if the amount of the conductive agent is more than 50 parts by weight, the amount of transition metal oxide or lithium-containing metal oxide in the electrode is decreased, so that the capacity is undesirably reduced. When the amount of the binder is less than 1 part by weight, the binding ability is lost and the electrode cannot be configured. On the other hand, if the binder is larger than 30 parts by weight, the resistance or polarization of the electrode is increased, and the amount of transition metal oxide or lithium-containing metal oxide in the electrode is decreased, so that the capacity is reduced and is not practical. .

  These mixtures are pressure-bonded to a current collector or dissolved in a solvent such as N-methyl-2-pyrrolidone to form a slurry, which is applied to the current collector and dried, followed by an organic electrolyte and an ion conductive polymer precursor. A positive electrode can be formed by impregnating a mixture of a body and an initiator and solidifying by light irradiation or heat. Alternatively, the positive electrode material, the conductive material, the organic electrolyte, and the initiator may be mixed and solidified by light irradiation or heat. As the current collector, a conductive material such as a metal foil, a metal mesh, or a metal nonwoven fabric can be used.

The negative electrode in the non-aqueous secondary battery of the present invention is a lithium alloy such as metallic lithium or lithium aluminum, or a substance capable of inserting / extracting lithium ions, for example, a conductive polymer such as polyacetylene, polythiophene, or polyparaphenylene, Pyrolytic carbon, pyrolytic carbon obtained by gas phase decomposition in the presence of catalyst, carbon baked from pitch, coke, tar, etc., carbon obtained by calcination of polymers such as cellulose, phenol resin, natural graphite, artificial A graphite material such as graphite or expanded graphite, a substance such as WO ₂ or MoO ₂ that can insert and desorb lithium ions, or a composite thereof can be used. Among them, a carbon material in which amorphous carbon is attached to the surface of graphite particles is preferable.

  These mixtures are pressure-bonded to a current collector or dissolved in a solvent such as N-methyl-2-pyrrolidone to form a slurry, which is applied to the current collector and dried, followed by an organic electrolyte and an ion conductive polymer precursor. A negative electrode can be formed by impregnating a mixture of a body and an initiator and solidifying by light irradiation or heat. Alternatively, these mixtures may be mixed with an organic electrolyte and a precursor of an ion conductive polymer, and solidified by light irradiation or heat. As the current collector, a conductive material such as a metal foil, a metal mesh, or a metal nonwoven fabric can be used.

  The non-aqueous secondary battery according to the present invention is configured by joining the positive electrode layer and the current collector, and the negative electrode layer and the current collector to the external electrodes, respectively, and interposing the electrolyte layer therebetween. The shape of the secondary battery of the present invention is not particularly limited, and examples thereof include a cylindrical shape, a button shape, a square shape, and a sheet shape, but are not limited thereto. Examples of the exterior material include metals and Al laminated resin films. These battery manufacturing steps are preferably performed in an inert atmosphere such as argon or in dry air in order to prevent moisture from entering.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.
The precursor of the ion conductive polymer (polymer electrolyte precursor) used in all of the following examples and comparative examples is a diacrylate monomer containing a copolymer of polyethylene oxide and polypropylene oxide. The precursor was crosslinked by irradiation with ultraviolet rays having a maximum output wavelength of 365 nm, and DMPA was used as a polymerization initiator at 0.1% by weight of the precursor.

  First, FIG. 1 shows a basic structure of a battery manufactured according to the present invention. DESCRIPTION OF SYMBOLS 1 is an electrode terminal, 2 is a polymer electrolyte layer whose ion conductivity is lower than the polymer electrolyte contained in the positive electrode layer side and the negative electrode layer side, 3 is positive electrode material and polymer electrolyte, 4 is positive electrode current collector, 5 is negative electrode current collector The body 6 is a negative electrode material and a polymer electrolyte, and 7 is an exterior material made of an Al laminate resin film for shielding the battery from the outside air.

-Preparation of polymer electrolyte layer A method for preparing a polymer electrolyte layer is described below.
Polymer electrolyte layer 1
To a quartz glass substrate having an area of 10 cm ² , LiPF ₆ dissolved in a diacrylate monomer containing a copolymer of polyethylene oxide and polypropylene oxide, which is a precursor of a polymer electrolyte, so as to be 4.5% by weight. After casting, a spacer having a thickness of 50 μm was bitten, a quartz glass substrate was placed thereon and fixed, and irradiated with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. The thickness of the obtained polymer electrolyte layer was 50 μm.

Polymer electrolyte layer 2
First, an electrolytic solution in which LiPF ₆ was dissolved in a mixed solvent of EC and GBL (EC content 35% by weight) so as to be 13% by weight was prepared, and the electrolytic solution and a precursor of polymer electrolyte, polyethylene oxide and polypropylene A diacrylate monomer containing an oxide copolymer was prepared at a weight ratio of 90:10. The mixed solution was cast on a quartz glass substrate having an area of 10 cm ² , a spacer having a thickness of 50 μm was cast, the quartz glass substrate was placed thereon and fixed, and irradiated with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. The resulting gel polymer electrolyte layer had a thickness of 50 μm.

Polymer electrolyte layer 3
First, an electrolytic solution in which LiPF ₄ is dissolved in a mixed solvent of EC and PC (PC content 35% by weight) so as to be 13% by weight is prepared, and the electrolytic solution and a polymer oxide precursor, polyethylene oxide and polypropylene are prepared. A diacrylate monomer containing an oxide copolymer was prepared at a weight ratio of 90:10. The mixed solution was cast on a quartz glass substrate having an area of 10 cm ² , a spacer having a thickness of 50 μm was cast, the quartz glass substrate was placed thereon and fixed, and irradiated with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. The resulting gel polymer electrolyte layer had a thickness of 50 μm.

Polymer electrolyte layer 4
First, an electrolytic solution in which LiPF ₄ is dissolved in a mixed solvent of EC and PC (PC content 35% by weight) so as to be 13% by weight is prepared, and the electrolytic solution and a polymer oxide precursor, polyethylene oxide and polypropylene are prepared. A diacrylate monomer containing an oxide copolymer was prepared at a weight ratio of 95: 5. The mixed solution was cast on a quartz glass substrate having an area of 10 cm ² , a spacer having a thickness of 50 μm was cast, the quartz glass substrate was placed thereon and fixed, and irradiated with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. The resulting gel polymer electrolyte layer had a thickness of 50 μm.

Polymer electrolyte layer 5
First, an electrolytic solution in which LiPF ₄ is dissolved in a mixed solvent of EC and PC (PC content 35% by weight) to 13% by weight is prepared, and a copolymer of polyethylene oxide and polypropylene oxide, which is a precursor of the polymer electrolyte. Was prepared so that the weight ratio of acrylate monomer containing a copolymer of polyethylene oxide and polypropylene oxide was 75:25.
A mixture of the electrolyte and polymer electrolyte precursor was prepared at a weight ratio of 95: 5. The mixed solution was cast on a quartz glass substrate having an area of 10 cm ² , a spacer having a thickness of 50 μm was cast, the quartz glass substrate was placed thereon and fixed, and irradiated with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. The resulting gel polymer electrolyte layer had a thickness of 50 μm.

Table 1 shows the ionic conductivity at 20 ° C. of the five types of electrolyte layers.
The ionic conductivity was measured as follows.
The impedance was measured using Li for the electrode. The resistance value was measured from the colle-coll plot at 20 ° C. with an impedance analyzer, and the ionic conductivity was determined.

In the following examples and comparative examples, electrolytes having the above-described compositions were used.
Example 1
An artificial graphite powder having (d ₀₀₂ ) = 0.337 nm by X-ray wide angle diffraction method, (Lc) = 100 nm, (La) = 100 nm, and a specific surface area by BET method of 10 m ² / g, is polyfluorinated as a binder. The paste obtained by mixing 9% by weight of vinylidene (PVDF), adding N-methyl-2-pyrrolidone (NMP), and mixing and dissolving the resultant was coated on a rolled copper foil having a thickness of 20 μm. Obtained. The electrode area was 9 cm ² and the thickness was 85 μm.

A paste obtained by mixing 7% by weight of PVDF as a binder and 5% by weight of acetylene black having an average particle diameter of 2 μm as a conductive material with LiCoO ₂ powder having an average particle diameter of 7 μm, adding NMP, and mixing and dissolving. Was coated on a rolled aluminum foil having a thickness of 20 μm, and after drying and pressing, a positive electrode was obtained. The electrode area was 9 cm ² and the thickness was 80 μm.

The polymer electrolyte layer has the same composition as that of the polymer electrolyte layer 1, cast the electrolyte precursor solution on a quartz glass substrate having an area of 10 cm ² , and a spacer having a thickness of 20 μm is placed on the quartz glass substrate. The sample was fixed and irradiated with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. The thickness of the obtained polymer electrolyte layer was 20 μm.
The positive electrode layer having the same composition as that of the polymer electrolyte layer 2 was used. The positive electrode was allowed to stand for 5 minutes under reduced pressure, the polymer electrolyte precursor solution was poured, and the mixture was further allowed to stand for 15 minutes. Thereafter, ultraviolet rays were irradiated for 2 minutes at an intensity of 38 mw / cm ² . The obtained positive electrode layer contained a gel polymer electrolyte and had a thickness of 80 μm.

The negative electrode layer having the same composition as that of the polymer electrolyte 3 was used. The negative electrode was allowed to stand for 5 minutes under reduced pressure, the polymer electrolyte precursor solution was poured, and the mixture was further allowed to stand for 15 minutes. Thereafter, ultraviolet rays were irradiated for 2 minutes at an intensity of 38 mw / cm ² . The obtained negative electrode layer contained a gel polymer electrolyte and had a thickness of 85 μm.
Thus obtained positive electrode layer, polymer electrolyte layer, and negative electrode layer were sequentially bonded to produce a battery.

(Comparative Example 1)
The polymer electrolyte layer has the same composition as the polymer electrolyte layer 4, the electrolyte precursor solution is cast on a quartz glass substrate having an area of 10 cm ² , a spacer having a thickness of 20 μm is cast, and the quartz glass substrate is placed thereon. The gel-like polymer electrolyte layer was prepared by irradiating with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. A battery was fabricated in the same manner as in Example 1 except that this polymer electrolyte layer was used.
When 100 batteries of Example 1 and Comparative Example 1 were assembled and short-circuit checked, the battery of Example 1 had 0 short circuits and the battery of Comparative Example 1 had 5 short circuits. From the above, it was found that even if the ionic conductivity of the electrolyte layer is high, a short circuit during assembly occurs if the mechanical strength of the polymer electrolyte layer is not sufficient.

(Example 2)
A positive electrode layer and a negative electrode layer were prepared in the same manner as in Example 1 except that the positive electrode layer and the negative electrode layer had the same composition as the polymer electrolyte layer 4. The positive electrode layer and the negative electrode layer each contain a gel polymer electrolyte.
A polymer electrolyte layer having the same composition as the polymer electrolyte layer 3 is used, and the polymer electrolyte precursor solution is immersed in a non-woven fabric made of polyester having an air permeability of 380 sec / cm ³ , an area of 10 cm ² , and a thickness of 20 μm. Was allowed to stand for 15 minutes under reduced pressure in order to penetrate into the nonwoven fabric. The gel polymer electrolyte layer was obtained by irradiating with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. At this time, the weight ratio of the polymer electrolyte to the polymer fiber was 90:10.
Thus obtained positive electrode layer, polymer electrolyte layer, and negative electrode layer were sequentially bonded to produce a battery.

(Example 3)
The polymer electrolyte layer has the same composition as the polymer electrolyte layer 3, the electrolyte precursor solution is cast on a quartz glass substrate having an area of 10 cm ² , a spacer having a thickness of 20 μm is placed, and the quartz glass substrate is placed thereon. The gel-like polymer electrolyte layer was prepared by irradiating with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. A battery was produced in the same manner as in Example 2 except that this polymer electrolyte layer was used.
When 100 of each of the batteries of Examples 2 and 3 were assembled and a short circuit check was performed, the battery of Example 2 had 0 short circuits and the battery of Example 3 had 3 short circuits. From the above, it was found that even if the ionic conductivity of the electrolyte layer is high, a short circuit during assembly occurs if the mechanical strength of the polymer electrolyte layer is not sufficient. In particular, it has been found that when a gel polymer electrolyte is used, it is more preferable to add polymer fibers or a microporous membrane separator to the electrolyte layer.

Example 4
A positive electrode layer and a negative electrode layer were prepared in the same manner as in Example 1 except that the positive electrode layer and the negative electrode layer had the same composition as the polymer electrolyte layer 4.
A polymer electrolyte layer having the same composition as the polymer electrolyte layer 3 is used, and the polymer electrolyte precursor solution is immersed in a non-woven fabric made of polyester having an air permeability of 380 sec / cm ³ , an area of 10 cm ² , and a thickness of 20 μm. Was allowed to stand for 15 minutes under reduced pressure in order to penetrate into the nonwoven fabric. Then, ultraviolet rays were irradiated for 2 minutes at an intensity of 38 mW / cm ² to prepare a gel polymer electrolyte layer. At this time, the weight ratio of the polymer electrolyte to the polymer fiber was 90:10.
Thus obtained positive electrode layer, polymer electrolyte layer, and negative electrode layer were sequentially bonded to produce a battery.

(Example 5)
Example 4 except that a polyester nonwoven fabric having an air permeability of 500 sec / cm ³ , an area of 10 cm ² , and a thickness of 150 μm is used for the polymer electrolyte layer, and the weight ratio of the polymer electrolyte to the polymer fibers is 95: 5. A battery was similarly prepared.

(Example 6)
Surface having negative electrode active material (d ₀₀₂ ) = 0.336 nm by X-ray wide angle diffraction method, (Lc) = 100 nm, (La) = 97 nm, specific surface area by BET method of 2 m ² / g, and average particle diameter of 10 μm A battery was fabricated in the same manner as in Example 4 except that amorphous graphite powder was used.

(Example 7 (reference example))
A battery was fabricated in the same manner as in Example 6 except that the positive electrode layer and the negative electrode layer had the same composition as the polymer electrolyte layer 5.

(Example 8)
A polyester electrolyte non-woven fabric having an air permeability of 500 sec / cm ³ , an area of 10 cm ² and a thickness of 160 μm is used as the polymer electrolyte layer, and the weight ratio of the polymer electrolyte to the polymer fibers is 95: 5. A battery was similarly prepared.
The batteries of Examples 4, 5, and 8 were charged at a constant current of 2.3 mA until the battery voltage reached 4.1 V, and after reaching 4.1 V, the batteries were charged at a constant voltage for a charging time of 12 hours. Discharge was performed at a constant current of 2.3 mA, 5 mA, 10 mA, and 20 mA until the battery voltage reached 2.75 V, respectively. The results of the charge / discharge test under these conditions are shown in FIG.
As a result of this test, it was found that when the thickness of the polymer fiber or the microporous membrane separator in the electrolyte layer was less than 150 μm, the performance during high-load discharge was improved, and preferably 20 μm or less. From these, it can be seen that a lithium secondary battery having a high energy density and excellent load characteristics can be obtained.

In addition, the batteries of Examples 4, 6, and 7 were each charged at a constant current of 2.3 mA until the battery voltage reached 4.1 V, and after reaching 4.1 V, the batteries were charged at a constant voltage for 12 hours. The battery was discharged at a constant current of 2.3 mA until the battery voltage reached 2.75V. The cycle characteristics were evaluated under these charge / discharge conditions. The result is shown in FIG.
From this result, it was found that the use of surface amorphous graphite powder as the negative electrode active material can suppress the side reaction with the polymer electrolyte layer, and thus the cycle characteristics are improved. It can also be seen that mixing the polymer electrolyte ion-conductive polymer precursor in the positive electrode layer and the negative electrode layer improves the reaction rate during UV irradiation, thereby improving the cycle characteristics.

Example 9
The Li—Al alloy foil was press-bonded onto a 50 μm thick Ni mesh to obtain a sheet negative electrode. This electrode was cut out to 31 × 31 mm, and a Ni current collecting tab was welded to form a negative electrode. The thickness was 100 μm.
100 parts by weight of manganese dioxide powder, 10 parts by weight of acetylene black as a conductive agent, and 7 parts by weight of PVDF as a binder were dry-mixed and stirred and mixed in NMP to obtain a slurry. This slurry was coated on a 20 μm thick Al foil, NMP was removed by drying, and then pressed to obtain a sheet-like positive electrode. This electrode was cut out to 30 × 30 mm, and an Al current collecting tab was welded to obtain a positive electrode. The thickness was 70 μm.
A polymer electrolyte layer, a positive electrode layer, and a battery were produced in the same manner as in Example 2.

(Example 10 (reference example))
A polymer electrolyte layer having the same composition as the polymer electrolyte layer 3 is used, an electrolyte precursor solution is cast on a quartz glass substrate having an area of 10 cm ² , a spacer having a thickness of 20 μm is placed, and a quartz glass substrate is placed thereon. The gel-like polymer electrolyte layer was prepared by irradiating with ultraviolet rays at an intensity of 38 mW / cm ² for 2 minutes. A battery was fabricated in the same manner as in Example 7 except that this polymer electrolyte layer was used.
When 100 batteries of each of Examples 9 and 10 were assembled and a short circuit check was performed, the battery of Example 9 had 0 short circuits and the battery of Example 10 had 4 short circuits. From the above, it has been found that when Li metal is used for the negative electrode, the short circuit during assembly cannot be suppressed unless the strength of the polymer electrolyte layer is sufficient.

DESCRIPTION OF SYMBOLS 1 Electrode terminal 2 Polymer electrolyte layer 3 Positive electrode material and polymer electrolyte 4 Positive electrode collector 5 Negative electrode collector 6 Negative electrode material and polymer electrolyte 7 Exterior material

Claims (4)

Lithium in which a positive electrode layer having at least a lithium-containing metal oxide or metal oxide as an active material, a negative electrode layer having at least an Li metal, a Li alloy, or a carbon material as an active material, and an electrolyte and a polymer electrolyte layer disposed therebetween In the secondary battery, the positive electrode layer and / or the negative electrode layer includes a polymer electrolyte matrix layer containing an electrolytic solution and a polymer, and the ionic conductivity of the polymer electrolyte constituting the polymer electrolyte layer constitutes the polymer electrolyte matrix layer The polymer electrolyte includes a polymer fiber or microporous membrane separator having an air permeability of 1 to 500 sec / cm ³ lower than the ionic conductivity of the polymer electrolyte, and the polymer fiber or microporous membrane separator is polypropylene. Fiber, polyethylene fiber or polyester fiber, and said polymer The electrolyte layer is composed of an ion conductive polymer and a polymer fiber or a microporous membrane separator, and a weight ratio of the ion conductive polymer and the polymer fiber or the microporous membrane separator is 91: 9 to 50:50. Rechargeable lithium battery.   The lithium secondary battery according to claim 1, wherein the polymer electrolyte layer has a thickness of 150 μm or less.   The lithium secondary battery according to claim 1 or 2, wherein the negative electrode active material is a carbon material, and the carbon material is obtained by attaching amorphous carbon to the surface of the graphite particles.   The both polymer contained in the said polymer electrolyte matrix layer and the said polymer electrolyte layer consists of a diacrylate origin polymer containing the copolymer of a polyethylene oxide and a polypropylene oxide, It is any one of Claims 1-3 Lithium secondary battery.

Images