JP2008300300A - Nonaqueous lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery in which various substances which are other than lithium ion and become causes for increasing a deterioration of battery characteristics are prevented from moving between a positive electrode and a negative electrode and as a result, a charge and discharge efficiency can be improved when charge and discharge are repeated and moreover even if charge and discharge are repeated, a decrease in capacity is not caused and a deterioration of rate characteristics is limited. <P>SOLUTION: The nonaqueous lithium ion secondary battery is composed of a positive electrode, a nonaqueous electrolyte solution and a porous separator membrane, and a negative electrode which is electrically insulated from the positive electrode through the porous separator membrane. Between the positive electrode and the negative electrode, there is, on the porous separator membrane, preferably at least one layer of a substantially non-porous lithium ion conductive layer having an air permeability of 10,000 sec/100 cc or more. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a non-aqueous lithium ion secondary battery, and in particular, has a substantially nonporous lithium ion conductive layer property between a positive electrode and a negative electrode, and in particular, a non-aqueous lithium ion secondary battery having excellent rate characteristics. Next battery.

In recent years, non-aqueous lithium ion secondary batteries having high energy density have been widely used as power sources for small portable electronic devices such as mobile phones and notebook personal computers. Such a non-aqueous lithium ion secondary battery is formed by laminating or winding a sheet-like positive and negative electrode and, for example, a polyolefin resin porous film, and charging the battery container made of, for example, a metal can. It is manufactured through a process of injecting an electrolytic solution into a battery container, sealing, and sealing.

However, in recent years, there has been a strong demand for further miniaturization and weight reduction of the above-described small portable electronic devices, and further reduction in thickness and weight have been demanded for lithium ion secondary batteries. Instead of the metal can container, a laminate film type battery container is also used. In addition, positive electrode materials and negative electrode materials have been intensively researched to provide high-capacity and high-power lithium-ion secondary batteries. On the other hand, there is a demand for highly safe lithium-ion secondary batteries. Is growing very much.

Therefore, for example, a proposal has been made to use an incombustible solid electrolyte, an incombustible, heat resistant ionic liquid called a room temperature molten salt, but these have insufficient ion conductivity, In addition, there is a problem that a good interface with the electrode cannot be obtained, and it has not yet been put into practical use.

Furthermore, even through the above-described intensive research, lithium-ion secondary batteries still have a reduced capacity, output characteristics, and safety due to repeated charge and discharge in a room temperature or high temperature atmosphere. There is a problem such as a decrease in performance. There are various causes for this, for example, lithium is deposited on the negative electrode during charging, metal ions are eluted from the positive electrode active material, and the organic solvent oxidized at the positive electrode becomes a cation radical, which is reduced on the negative electrode surface. Metal ions eluted from the positive electrode active material or current collector are deposited on the negative electrode surface, and the electrolytic solution decomposes on the electrode material surface to produce lithium fluoride or the like. As the amount of lithium ions contributing to the capacity decreases, the irreversible capacity increases (for example, Non-Patent Document 1).

Therefore, in order to solve such a problem, for example, at least one surface of the positive electrode active material and the negative electrode active material is covered with a protective film, and the nonaqueous solvent is prevented from being oxidized or reduced on the surface of the active material. Thus, a method for improving the storage stability of the lithium ion secondary battery has been proposed (see Patent Document 1). However, as a result of the study by the present inventors, according to this method, the charge transfer resistance on the surface of the active material is increased, so that the rate characteristics are deteriorated, and the protective film is gradually decomposed during charging and discharging. It was found that there is a problem that long-term characteristics deteriorate.

A method of reducing the initial irreversible capacity by forming a thin film containing a certain sulfide compound or polysulfide compound in a porous film made of polyolefin resin has also been proposed (see Patent Document 2). However, according to this method, the thin film elutes into the organic solvent during repeated charge and discharge, and the inclusions are oxidized at the positive electrode to generate gas, causing the battery to swell. There is a problem that the metal ions and cation radicals eluted from the positive electrode active material cannot be prevented from moving between the electrodes due to non-uniformity, and charge / discharge efficiency is reduced.

Furthermore, a polymer resin coating layer is provided on a base material having pores through which lithium ions can permeate, and the pore size is made smaller than that of a conventional lithium ion secondary battery separator. There has been proposed a method of suppressing movement between the terminals and thus making it difficult to cause an internal short circuit (see Patent Document 3). However, even with this method, there is a problem that metal ions other than lithium ions and cation radicals cannot be suppressed from moving between electrodes, and charge / discharge efficiency is reduced.
P. Arora et al., Journal of Electrochemical Society, Vol. 145, No. 10, October 1998 JP-A-9-219188 JP 2002-237285 A JP 2004-31084 A

The present invention has been made in order to solve the above-described problems in the conventional lithium ion secondary battery, and reduces deposits on the surface of the negative electrode to increase charge / discharge efficiency and repeat charge / discharge. It is an object of the present invention to provide a non-aqueous lithium ion secondary battery that has a small decrease in capacity, and thus has excellent rate characteristics and little deterioration of rate characteristics.

According to the present invention, in the non-aqueous lithium ion secondary battery comprising a positive electrode, a non-aqueous electrolyte, a porous separator film, and a negative electrode electrically insulated from the positive electrode through the porous separator film, the positive electrode There is provided a non-aqueous lithium ion secondary battery comprising at least one substantially non-porous lithium ion conductive layer between the negative electrode and the negative electrode.

The non-aqueous lithium ion secondary battery according to the present invention has at least one substantially non-porous lithium ion conductive layer between the positive electrode and the negative electrode, and the substantially non-porous lithium ion conductive layer It is possible to suppress movement of various substances other than lithium ions, which cause deterioration of battery characteristics, between the positive and negative electrodes. Thus, according to the non-aqueous lithium ion secondary battery of the present invention, it is possible to reduce reactions other than occlusion and release of lithium ions at the electrode and improve charge and discharge efficiency when charge and discharge are repeated over a long period of time. it can. Further, according to the non-aqueous lithium ion secondary battery of the present invention, the substantially non-porous lithium ion conductive layer also suppresses the formation of precipitates on the surface of the electrode, so that charge and discharge are repeated. However, there is little decrease in the capacity, and therefore the deterioration of the rate characteristic is small.

The non-aqueous lithium ion secondary battery according to the present invention includes a positive electrode capable of occluding and releasing lithium ions, a non-aqueous electrolyte, a porous separator film, and a negative electrode electrically insulated from the positive electrode through the porous separator film. And having at least one substantially non-porous lithium ion conductive layer between the positive electrode and the negative electrode.

In the present invention, the porous separator membrane preferably has pores having an average pore diameter of 0.01 to 5 μm and a porosity in the range of 20 to 95%. The porosity is more preferably in the range of 30 to 90%, and most preferably in the range of 40 to 85%. When the porosity is too low, the ion conduction path is reduced when used as a battery separator, and sufficient battery characteristics cannot be obtained. On the other hand, if the porosity is too high, the strength is insufficient when used as a battery separator, and a thick porous film must be used to obtain the required strength. This is not preferable because the internal resistance of the battery increases.

Furthermore, according to the present invention, the air permeability of the porous separator is usually 1500 seconds / 100 cc.
Or less, preferably 1000 seconds / 100 cc or less. When the air permeability is too high, the ion conductivity is low when used as a battery separator, and sufficient battery characteristics cannot be obtained. The strength of the porous separator film is preferably 1N or more. This is because when the piercing strength is less than 1 N, the substrate may be broken when a surface pressure is applied between the electrodes, thereby causing an internal short circuit.

In the present invention, the porous separator film is not particularly limited as long as it has the above-described characteristics, but if considering solvent resistance and oxidation-reduction resistance, polyolefin such as polyethylene and polypropylene is used. A porous film made of a resin is preferred. However, among them, when heated, the resin melts and the pores are blocked. As a result, the battery can have a so-called shutdown function. Resin films are particularly suitable. Here, the polyethylene resin includes not only a homopolymer of ethylene but also a copolymer of ethylene with an α-olefin such as propylene, butene and hexene.

In addition, according to the present invention, a laminated film of a porous film such as polytetrafluoroethylene or polyimide and the above-mentioned polyolefin resin porous film is also suitably used as a substrate porous film because of its excellent heat resistance. It is done.

More preferably, the film thickness of the porous separator is in the range of 3 to 100 μm. When the thickness of the porous separator is less than 3 μm, the strength is insufficient, and when used as a separator in a battery, the electrodes may contact each other and cause an internal short circuit. On the other hand, when the film thickness of the porous separator exceeds 100 μm, the membrane resistance of the separator increases and the rate characteristics are lowered, which is not preferable.

  In the present invention, any positive electrode active material may be used as long as it is used as a positive electrode of a non-aqueous lithium ion secondary battery. Examples of such a positive electrode active material include lithium cobaltate, spinel type lithium manganate, lithium nickelate, and olivine type lithium iron phosphate.

Any negative electrode active material may be used as long as it is used as the negative electrode of a non-aqueous lithium ion secondary battery. For example, graphite, amorphous carbon, carbon fiber, etc. can be mentioned.

The lithium ion secondary battery according to the present invention has at least one substantially non-porous lithium ion conductive layer between the positive electrode and the negative electrode. In the present invention, “substantially non-porous” means that the air permeability is 10,000 seconds / 100 cc or more, and the air permeability is not so high that the air permeability cannot be measured substantially. means.

The substantially non-porous lithium ion conductive layer is a substance other than lithium ions,
Suppresses movement of various substances that cause deterioration of battery characteristics between the positive and negative electrodes. Here, substances other than lithium ions that cause deterioration of battery characteristics mainly include an active material that is desorbed from the electrode, a transition metal ion that is eluted from the positive electrode active material, and an electrolytic solution oxidized at the positive electrode A decomposition product, for example, an electrolyte cation radical or the like. Here, the transition metal ions eluted from the positive electrode active material are not limited, but mainly ions of cobalt, manganese, nickel, iron, or vanadium.

Thus, according to the non-aqueous lithium ion secondary battery of the present invention, reactions other than occlusion / release of lithium ions at the electrode can be reduced, and charge / discharge efficiency when charge / discharge is repeated for a long time can be improved. Further, the substantially non-porous lithium ion conductive layer also suppresses the formation of precipitates on the surface of the electrode. Therefore, according to the lithium ion secondary battery of the present invention, even when charging and discharging are repeated, The decrease in capacity is small, and therefore the deterioration of rate characteristics is small.

Furthermore, according to the present invention, since the nonaqueous electrolytic solution and the porous separator film are provided, the electrolytic solution is excellent in liquid retention, and lithium ions are sufficiently present on the active material surface even in an electrode generally composed of a powder aggregate. Thus, a non-aqueous lithium ion secondary battery having excellent rate characteristics can be obtained without impairing the original capacity of the electrode active material.

In the present invention, the thickness of the substantially non-porous lithium ion conductive layer is 0.01 to 2.2.
It is preferable to be in the range of μm. When the thickness of the lithium ion conductive layer is smaller than 0.01 μm, sufficiently suppress the movement of substances other than lithium ions that cause deterioration of battery characteristics between the positive electrode and the negative electrode. I can't. On the other hand, when the thickness of the lithium ion conductive layer is larger than 2.2 μm, the resistance of the lithium ion conductive layer is increased and the rate characteristics are deteriorated. In the present invention, the thickness of the substantially nonporous lithium ion conductive layer is more preferably in the range of 0.01 to 2.0 μm, and most preferably in the range of 0.05 to 2.0 μm. is there.

In the present invention, the substantially nonporous lithium ion conductive layer is preferably made of a substantially nonporous polymer solid electrolyte or gel electrolyte.

The polymer solid electrolyte is obtained by containing an electrolyte salt in a solid polymer material as a matrix. As the solid polymer material, polyethylene oxide and derivatives thereof are suitable, but polymer materials such as polyalkylene ether polymers such as polypropylene oxide and polyethylene glycol, polyacrylonitrile, polyphosphazene, and polymethyl methacrylate are also used.

On the other hand, the gel electrolyte is obtained by holding a non-aqueous solvent and an electrolyte salt dissolved in the polymer network structure as a gel, and may be either a physical crosslinked gel or a chemically crosslinked gel. As the physical crosslinking gel, for example, one having polyacrylonitrile and its derivatives or a polyvinylidene fluoride-6-fluorinated propylene copolymer as a skeleton having a network structure is used. On the other hand, as the chemical cross-linking gel, a copolymer of methyl methacrylate and a carboxylic acid, or a cross-linkage obtained by adding a cross-linking agent to polyvinyl alcohol or the like as a skeleton having a network structure is used. Moreover, what swollen the crosslinked body of the polyether multi-component copolymer is also used. Examples of the non-aqueous solvent include cyclic esters such as ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone, ethers such as tetrahydrofuran and dimethoxyethane, and chain structures such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Esters can be used alone or as a mixture of two or more.

In the polymer solid electrolyte or gel electrolyte, as the electrolyte salt, for example, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, or the like is preferably used. These electrolyte salts are usually used in the range of 1 to 50% by weight in the polymer solid electrolyte or gel electrolyte.

However, in the present invention, the polymer salt forming the polymer solid electrolyte, the non-aqueous solvent forming the gel, and the lithium salt used as the electrolyte salt are not limited to the above examples.
The electrolyte salt is appropriately determined according to the type and amount of the solvent used.

According to the present invention, it is preferable that both the polymer solid electrolyte and the gel electrolyte described above include a polar part. Here, the part having polarity preferably means a part containing an oxygen atom, a nitrogen atom, a sulfur atom or a phosphorus atom, for example, a hydroxyl group, a carboxyl group, a ketone group, an ether group, an ester group, an amine group, a thioether. Specific examples include groups and phosphine groups. However, in this invention, the site | part which has polarity is not limited to the said illustration.

Since both the solid polymer electrolyte and the gel electrolyte are small in density and flexible, by using them as the substantially non-porous lithium ion conductive layer, they are lightweight and practical in shape. An excellent non-aqueous lithium ion secondary battery can be obtained. In particular, as described above, when using a polymer solid electrolyte containing a polar part, dissociation of lithium ions in the polymer solid electrolyte is promoted, and when using a gel electrolyte containing a polar part Since the swellability with respect to the non-aqueous solvent is improved, the ion conductivity of the lithium ion conductive layer is improved, and a non-aqueous lithium ion secondary battery having excellent rate characteristics can be obtained.

Furthermore, compared to lithium ions, transition metal ions and electrolyte radicals are coordinated to polar sites in solid polymer electrolytes and gel electrolytes compared to lithium ions. The mobility in the lithium ion conductive layer is low, and movement between electrodes due to concentration diffusion, thermal diffusion, and the like is further suppressed.

According to the present invention, the substantially non-porous lithium ion conductive layer as described above, preferably the solid polymer electrolyte or the gel electrolyte is supported on at least one of the positive electrode, the porous separator film, and the negative electrode. Preferably, it is carried on a porous separator membrane. When the solid polymer electrolyte or gel electrolyte is supported on the porous separator membrane, they may be supported on one surface of the porous separator membrane or on both surfaces.

Since the surface of the porous separator film is smoother than the surface of the positive electrode or the negative electrode, it is possible to carry a lithium ion conductive layer that is made thinner than the case of carrying it on the positive electrode or the negative electrode. It is possible to obtain a non-aqueous lithium ion secondary battery in which deposits on the negative electrode surface are reduced, charging / discharging efficiency is high, capacity deterioration is small even when charging / discharging is repeated, and deterioration of rate characteristics is suppressed. it can.

Thus, in order to support the solid polymer electrolyte on the porous separator membrane, for example, the solid polymer electrolyte is dissolved in an appropriate organic solvent such as acetone, ethyl acetate, butyl acetate to form a solution. Casting or spraying the solution on the surface of the porous film that forms the base material of the porous separator membrane, or impregnating the porous film in the solution, followed by drying to remove the organic solvent used, A solid polymer electrolyte layer may be formed and laminated on the surface of the porous film. As another method, a solid polymer electrolyte may be formed into a film by melt extrusion, and this film may be bonded to a substrate porous film by thermal lamination or the like.

In the case of a gel electrolyte, a polymer layer for forming a network structure in the gel electrolyte is formed on the surface of the porous film by the same method as described above, laminated, and then charged into a battery container. When the electrolytic solution is injected and incorporated in the battery, the polymer layer on the surface of the porous film swells with the electrolytic solution containing the electrolyte salt to form a gel electrolyte. In this way, the gel electrolyte can be supported on the porous separator membrane during battery assembly.

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In the following, the physical properties and battery characteristics of the porous separator film and the gel fraction of the crosslinked methyl methacrylate / 4-hydroxybutyl acrylate copolymer supported on the composite separator were evaluated or measured as follows.

(Thickness of porous separator film)
It was determined based on a measurement with a 1/10000 mm thickness gauge and a 10,000 times scanning electronic microscopic photograph of the cross section of the porous separator membrane (porous film).

(Porosity of porous separator membrane)
Weight W (g) per unit area S (cm ² ) of porous separator membrane, average thickness t (cm
) And the density d (g / cm ³ ) of the resin constituting the porous film.
Porosity (%) = (1− (100 W / S / t / d)) × 100

(Air permeability of porous separator membrane)
It calculated | required based on JISP8117.

(Puncture strength)
A piercing test was performed using a compression test kit KES-G5 manufactured by Kato Tech Co., Ltd. The maximum load was read from the load displacement curve obtained by the measurement, and the puncture strength was obtained. A needle having a diameter of 1.0 mm and a radius of curvature of the tip of 0.5 mm was used at a speed of 2 cm / second.

(Gel fraction)
After measuring the weight of the composite separator carrying the copolymer of weight W ₀ , this was shaken in ethyl acetate for 24 hours to elute the uncrosslinked copolymer. The composite separator is taken out from ethyl acetate and dried, and then its weight is measured to determine the weight W of the copolymer (crosslinked copolymer) remaining on the composite separator. It was determined by (W / W ₀ ) × 100.

Reference example 1
(Preparation of electrode sheet)
Lithium cobalt oxide (cell seed C-10 manufactured by Nippon Chemical Industry Co., Ltd.) 8 as a positive electrode active material
5 parts by weight and 10 parts by weight of acetylene black (Denka Black manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive additive and 5 parts by weight of vinylidene fluoride resin (KF Polymer L1120 manufactured by Kureha Chemical Industry Co., Ltd.) as a binder are mixed. Then, this was made into a slurry using N-methyl-2-pyrrolidone so as to have a solid content concentration of 15% by weight.

  This slurry was applied to an aluminum foil (current collector) having a thickness of 20 μm to a thickness of 200 μm, dried at 80 ° C. for 1 hour, and then at 120 ° C. for 2 hours, and then pressed by a roll press, so that the thickness of the active material layer was A 100 μm sheet was obtained, cut into a 42 mm square, and used as a positive electrode sheet.

Also, mesocarbon microbeads (MC gas manufactured by Osaka Gas Chemical Co., Ltd.), which is a negative electrode active material
MB6-28) 80 parts by weight, 10 parts by weight of acetylene black (Denka Black manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary agent, and vinylidene fluoride resin (KF Polymer L1120 manufactured by Kureha Chemical Industry Co., Ltd.) 10 as a binder Part by weight was mixed, and this was made into a slurry using N-methyl-2-pyrrolidone so as to have a solid concentration of 15% by weight.

  This slurry was applied to a copper foil (current collector) having a thickness of 20 μm to a thickness of 200 μm, dried at 80 ° C. for 1 hour, dried at 120 ° C. for 2 hours, and then pressed by a roll press to form an active material layer. A negative electrode sheet having a thickness of 100 μm was obtained, and this was cut into a 44 mm square to be used as a negative electrode sheet.

Reference example 2
(Production of control battery)
A porous film made of polyethylene resin having a thickness of 16 μm, a porosity of 40%, an air permeability of 300 seconds / 100 cc, and a piercing strength of 3.0 N was prepared as a porous separator film (the same applies hereinafter). The negative electrode sheet obtained in Reference Example 1, the porous separator film, and the positive electrode sheet obtained in Reference Example 1 were laminated in this order. An electrolytic solution comprising an ethylene carbonate / diethyl carbonate (1/1 by weight) mixed solvent in which lithium hexafluorophosphate is dissolved in the package at a concentration of 1.0 mol / L after the laminate is prepared in an aluminum laminate package. Then, the package was sealed, and the lithium ion secondary battery A was assembled as a control battery.

Production Example 1
Polyethylene oxide (manufactured by Aldrich, average molecular weight 900,000) is dissolved in acetonitrile at a concentration of 10% by weight, and lithium hexafluorophosphate is dissolved in this solution so that the ratio of lithium atoms to ether oxygen in the polyethylene oxide is 0.05. In addition, polymer solution 1 was prepared.

Production Example 2
10 g of poly (vinylidene fluoride / hexafluoropropylene) copolymer (Kynar 2801 manufactured by Elf Atchem) was dissolved in 90 g of N-methyl-2-pyrrolidone to prepare a polymer solution 2 having a concentration of 10% by weight.

Production Example 3
In a 500 mL three-necked flask equipped with a reflux condenser, 95 g of methyl methacrylate,
4 g of 4-hydroxybutyl acrylate, 67 g of ethyl acetate and 0.2 g of N, N′-azobisisobutyronitrile were added, and after stirring and mixing for 30 minutes while introducing nitrogen gas, radical polymerization was performed at a temperature of 64 ° C. I did it.

When about 1 hour passed, radical polymerization proceeded and the viscosity of the reaction mixture began to rise.
The polymerization was continued for 8 hours as it was, then cooled to about 40 ° C., 0.1 g of N, N′-azobisisobutyronitrile was added again, and the mixture was heated again to 70 ° C., and further post-polymerized for 8 hours. Was done. Then, it cooled to about 40 degreeC, ethyl acetate 166g was added, and it stirred and mixed until the whole became uniform, and the ethyl acetate solution of the methylmethacrylate / 4-hydroxybutyl acrylate copolymer was obtained.

20 g of an ethyl acetate solution of methyl methacrylate / 4-hydroxybutyl acrylate copolymer thus obtained and 0.06 g of a benzoyl peroxide cross-linking agent were dissolved in 3.94 g of ethyl acetate to obtain a polymer solution 3 having a concentration of 25% by weight. Was prepared.

Example 1
Acetonitrile was added to the polymer solution 1 and stirred in an inert gas atmosphere at room temperature to obtain a uniform polymer solution having a concentration of 8.3%. The polymer solution thus obtained was applied to one side of the porous separator film with a wire bar (# 20) and then dried by heating at 50 ° C. to volatilize acetonitrile. Thus, a composite separator was obtained in which a solid polymer electrolyte layer having a thickness of 0.5 μm was supported as a substantially nonporous lithium ion conductive layer on one surface of the porous separator membrane.

Next, the base separator resin film of the composite separator faces the negative electrode sheet obtained in Reference Example 1, and the polymer solid electrolyte on the base resin film faces the positive electrode sheet obtained in Reference Example 1. Thus, a laminated body was obtained. This laminate was placed in an aluminum laminate package, and then an electrolytic solution composed of a mixed solvent of ethylene carbonate / diethyl carbonate (weight ratio 1/1) in which lithium hexafluorophosphate was dissolved at a concentration of 1.0 mol / L was poured. Then, the package was sealed, and a lithium ion secondary battery B was assembled.

Example 2
Add N-methyl-2-pyrrolidone to polymer solution 2 and stir at room temperature to a concentration of 8.3%.
A homogeneous polymer solution was obtained. After apply | coating the obtained solution to the single side | surface of a porous separator film | membrane with a wire bar (# 20), it heat-dried at 60 degreeC and volatilized N-methyl-2- pyrrolidone. Thus, a composite separator having a poly (vinylidene fluoride / hexafluoropropylene) copolymer layer having a thickness of 0.5 μm supported on one surface of the porous separator film was obtained.

Next, the base separator resin film of the composite separator faces the negative electrode sheet obtained in Reference Example 1, and the copolymer layer on the base resin film faces the positive electrode sheet obtained in Reference Example 1. Thus, a laminate was obtained. After this laminate was prepared in an aluminum laminate package, an electrolytic solution comprising an ethylene carbonate / diethyl carbonate (weight ratio 1/1) mixed solvent in which lithium hexafluorophosphate was dissolved at a concentration of 1.0 mol / L was poured. Then, the package was sealed, and a lithium ion secondary battery C was assembled.

In this lithium ion secondary battery, the poly (vinylidene fluoride / hexafluoropropylene) copolymer layer swells with an electrolytic solution containing a lithium salt and functions as a gel electrolyte, that is, a lithium ion conductive layer.

Example 3
Ethyl acetate was added to the polymer solution 3 and stirred at room temperature to obtain a uniform polymer solution having a concentration of 8.3%. The obtained solution was applied to one side of the porous separator film with a wire bar (# 20), and then heated and dried at 60 ° C. to volatilize ethyl acetate, and then this was placed in a 90 ° C. incubator for 168 hours. The methyl methacrylate / 4-hydroxybutyl acrylate copolymer was cross-linked. Thus, a composite separator in which a cross-linked copolymer layer having a thickness of 0.5 μm was supported on one surface of the porous separator membrane was obtained. This crosslinked copolymer had a gel fraction of 58.3%.

Next, the composite separator faces the negative electrode sheet obtained in Reference Example 1 with the base resin film, and the crosslinked copolymer layer on the base resin film becomes the positive electrode sheet obtained in Reference Example 1. The laminate was obtained by laminating so as to face each other. After this laminate was prepared in an aluminum laminate package, an electrolytic solution comprising an ethylene carbonate / diethyl carbonate (weight ratio 1/1) mixed solvent in which lithium hexafluorophosphate was dissolved at a concentration of 1.0 mol / L was poured. Then, the package was sealed, and a lithium ion secondary battery D was assembled.

In this lithium ion secondary battery, the crosslinked copolymer layer functions as a gel electrolyte, that is, a lithium ion conductive layer, by swelling with an electrolyte containing a lithium salt.

As shown in FIG. 1, a scanning electron micrograph of the surface of the crosslinked copolymer layer carried on the composite separator is substantially nonporous.

Example 4
In Example 3, the crosslinked copolymer layer carried on the base resin film of the composite separator was opposed to the negative electrode sheet obtained in Reference Example 1, and the base resin film was obtained in Reference Example 1. A lithium ion secondary battery E was assembled in the same manner as in Example 3, except that the composite separator was laminated with the positive and negative electrode sheets so as to face the positive electrode sheet to obtain a laminate. This lithium ion secondary battery E is the same as the lithium ion secondary battery D except that the crosslinked copolymer layer is located on the negative electrode sheet side.

Comparative Example 2
In Example 3, N-methyl-2-pyrrolidone was added to the polymer solution 3 so that it might become 10 weight part, and the uniform polymer solution with a density | concentration of 8.3% was prepared. After applying this to the porous separator film in the same manner as in Example 3, the ethyl acetate and N-methyl-2-pyrrolidone were volatilized, and then this was put into a 90 ° C. incubator for 168 hours. The methacrylate / 4-hydroxybutyl acrylate copolymer was cross-linked, and thus a cross-linked copolymer layer having a thickness of 0.5 μm was supported on one surface of the porous separator film to obtain a composite separator. The crosslinked copolymer had a gel fraction of 57.9%. A lithium ion secondary battery F was produced in the same manner as in Example 3 except that this composite separator was used.

In this composite separator, ethyl acetate and N-methyl-2 having different boiling points are used as solvents.
-The polymer solution consisting of pyrrolidone was applied to the porous separator film, the solvent was volatilized, and the copolymer was crosslinked by heating, so that the surface of the crosslinked copolymer layer was scanned with a scanning electron microscope. As shown in FIG. 2, the photograph has holes of about 1 to 5 μm.

The laminate seal type lithium ion secondary batteries A to F obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were charged and discharged three times at a rate of 0.2 CmA under a constant temperature (25 ° C.). The third discharge capacity was determined as the initial discharge capacity. The third charge / discharge efficiency was determined as the initial charge / discharge efficiency. Thereafter, the battery was charged at 0.2 CmA, then discharged at 2 CmA to obtain a 2 CmA discharge capacity, and the initial rate characteristics were evaluated by the percentage (%) of the 2 CmA discharge capacity with respect to the initial discharge capacity.

Thereafter, charging at 1 CmA and discharging at 1 CmA was taken as one cycle, and 300 cycles of charging and discharging were repeated. The capacity maintenance rate was evaluated as a percentage (%) of the discharge capacity at the 300th cycle with respect to the discharge capacity at the 1st cycle where charging / discharging of 1 CmA was started. Moreover, the average charge / discharge efficiency of 300 cycles was evaluated as the average efficiency. Thereafter, the battery was charged at 0.2 CmA, discharged at 0.2 CmA, and the rate characteristics after repeated charge and discharge were evaluated at a percentage (%) of 0.2 CmA with respect to the 1 CmA discharge capacity at the 300th cycle. The results are shown in Table 1.

  From the results shown in Table 1, the lithium ion secondary battery according to the present invention has little deterioration in initial discharge capacity, initial charge / discharge efficiency and initial rate characteristics, and has high capacity and high output. In addition, since the charge / discharge efficiency during the cycle test is improved, a lithium ion secondary battery with little deterioration in capacity and excellent long-term reliability can be obtained even after repeated charge / discharge.

Examples 5-11
In Example 3, ethyl acetate was added to polymer solution 3 and stirred at room temperature, and the polymer concentrations were 2.1%, 3.2%, 4.3%, 8.4%, 16%, 22% and A 25% homogeneous polymer solution was obtained.

Each polymer solution thus obtained was applied to one side of the porous separator film with a wire bar (# 20), and then heated and dried at 60 ° C. to volatilize ethyl acetate. 168 hours in a thermostat at 0 ° C. to crosslink the copolymer, and thus thicknesses of 0.010 μm, 0.050 μm, 0.10 μm, 0.50 μm, 1.0 μm, 2.0 μm and 2 on one side, respectively. A composite separator carrying a 2 μm cross-linked copolymer layer was obtained. In these composite separators, the gel fractions of the crosslinked copolymers were 48.3%, 50.2%, 64.4%, 58.3%, 67.7%, 55.0% and 44, respectively. .5%. Using these composite separators, lithium ion secondary batteries G to M were assembled in the same manner as in Example 3. However, the lithium ion secondary battery D of Example 3 and the lithium ion secondary battery J of Example 8 are the same.

About the said lithium ion secondary battery G-M, the battery characteristic was evaluated by the same method as the above. The results are shown in Table 2.

As is apparent from the results shown in Table 2, according to the present invention, the thickness of the lithium ion conductive layer supported on the porous separator membrane is substantially non-porous in the range of 0.01 to 2.2 μm, Preferably, when it can be in the range of 0.01 to 2.0 μm, most preferably in the range of 0.05 to 2.0 μm, it has a high capacity and excellent rate characteristics. In addition, it is possible to obtain a non-aqueous lithium ion secondary battery that is high in capacity, has little capacity deterioration, and has suppressed deterioration of rate characteristics.

2 is a scanning electron micrograph of the surface of a crosslinked polymer layer as an example of a substantially non-porous lithium ion conductive layer supported on a porous separator membrane. 4 is a scanning electron micrograph of the surface of a porous crosslinked polymer layer as a comparative example supported on a porous separator membrane.

Claims (5)

In a non-aqueous lithium ion secondary battery comprising a positive electrode, a non-aqueous electrolyte, a porous separator film, and a negative electrode electrically insulated from the positive electrode through the porous separator film, the non-aqueous lithium ion secondary battery is provided between the positive electrode and the negative electrode. A non-aqueous lithium ion secondary battery comprising at least one substantially nonporous lithium ion conductive layer.   The non-aqueous lithium ion secondary battery according to claim 1, wherein the substantially nonporous lithium ion conductive layer has a permeability of 10,000 seconds / 100 cc or more. The nonaqueous lithium ion secondary battery according to claim 1 or 2, wherein the substantially nonporous lithium ion conductive layer is a polymer solid electrolyte or a gel electrolyte. The substantially non-porous lithium ion conductive layer is supported on a porous separator.
To 4. The non-aqueous lithium ion secondary battery according to any one of 3 to 4. The porous separator membrane has a thickness in the range of 3 to 100 μm, and the substantially nonporous lithium ion conductive layer has a total thickness in the range of 0.01 to 2.2 μm. 4. The nonaqueous lithium ion secondary battery according to any one of 4 above.