JP2006269358A - Porous separator for nonaqueous electrolyte secondary cell and nonaqueous electrolyte secondary cell using above - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous separator for a nonaqueous electrolyte secondary cell with a good heat removal property, and a nonaqueous electrolyte secondary cell using the separator, having a good heat removal property, with less temperature rise at the time of charging and discharging and with high safety. <P>SOLUTION: The porous separator for a nonaqueous electrolyte secondary cell has a thermal conductivity in a thickness direction of 0.5 W/(m K) or more. The nonaqueous electrolyte secondary cell is equipped with a negative electrode and a positive electrode capable of absorbing and releasing lithium, a nonaqueous electrolyte containing a nonaqueous solvent and lithium salt and the above porous separator. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a separator having a heat removal function used in a nonaqueous electrolyte secondary battery, such a separator, a negative electrode and a positive electrode capable of inserting and extracting lithium, a nonaqueous solvent and a lithium salt. The present invention relates to a non-aqueous electrolyte secondary battery that is excellent in heat removal characteristics, includes a non-aqueous electrolyte solution, and has a low temperature rise and high safety.

2. Description of the Related Art Lithium secondary batteries, which are light nonaqueous electrolyte secondary batteries that have a high energy density with the reduction in weight and size of electrical products, are used in a wide range of fields.
Lithium secondary batteries are roughly classified into batteries using a solid electrolyte and batteries using an organic electrolyte.

A lithium secondary battery using an organic electrolyte is typically represented by a positive electrode in which an active material layer containing a positive electrode active material such as a lithium compound typified by lithium cobaltate is formed on a current collector, and graphite. A negative electrode in which an active material layer containing a negative electrode active material such as a carbon material capable of occluding and releasing lithium is formed on a current collector, and an electrolyte such as a lithium salt such as LiPF ₆ is usually aprotic non-ionic. It is mainly composed of a nonaqueous electrolytic solution dissolved in an aqueous organic solvent and a separator made of a polymer porous membrane.

  By the way, the advantage of the lithium secondary battery is that it has a high energy density and can pass a large current, which means that a large amount of heat is generated inside the battery due to ohmic loss due to chemical reaction or current during discharging or charging. To do. Therefore, in a lithium secondary battery, it is important to sufficiently eliminate heat generation during charging / discharging. If heat removal is not performed sufficiently, combustible materials such as a non-aqueous electrolyte and a separator that are constituent materials of the lithium secondary battery may be decomposed to generate smoke and ignition. In addition, when a problem occurs in the charged battery, if the heat removal is insufficient, smoke and fire may still occur. Such heat removal of the battery is a big problem particularly in a battery having a large capacity.

  In recent years, as the capacity and output of batteries have increased, there has been a strong demand for technologies for improving the safety of batteries. Conventionally, in order to improve the safety of the battery, many technologies related to shutdown, shape maintenance at high temperature, etc. have been disclosed for the separator, but these ensure safety when an abnormality such as a temperature rise occurs. In other words, it is only a passive technology, and a separator that can secure the safety by directly and positively suppressing the temperature rise by removing heat inside the battery has not been sufficiently studied.

  Patent Document 1 describes that heat removal is improved by including an electrically insulating inorganic material having excellent thermal conductivity in a separator of a nonaqueous electrolyte battery. The separator needs to have a function of electrically separating the positive electrode and the negative electrode, and electrical insulation is essential as a film. However, if the separator contains an electrically insulating inorganic material having excellent thermal conductivity, The improvement in conductivity is insufficient and the cost is high.

  Patent Document 2 discloses that the solid electrolyte functioning as a separator contains boron nitride, aluminum nitride, non-electroconductive ceramics, etc., thereby increasing the thermal conductivity of the electrolyte layer and improving heat removal. Although disclosed, a battery using a solid electrolyte has a low ionic conductivity as compared with a battery using an organic electrolyte, so that it is difficult to obtain a high output battery. Therefore, with the technique of Patent Document 2, it is difficult to achieve both the heat removal characteristics and the output characteristics of the battery at the required levels, particularly in batteries for automobiles that require a large output.

Therefore, a porous separator for a nonaqueous electrolyte secondary battery excellent in heat removal that can realize a nonaqueous electrolyte secondary battery excellent in stability with little temperature rise during charging and discharging, etc. Was needed.
JP-A-8-255615 JP-A-11-86824

  The present invention has been made in view of the above-described conventional situation, and is excellent in heat removal characteristics using a porous separator for a non-aqueous electrolyte secondary battery excellent in heat removal performance, and this separator, An object of the present invention is to provide a highly safe non-aqueous electrolyte secondary battery with little temperature rise during charging and discharging.

  As a result of diligent research, the present inventors have used a non-aqueous electrolyte solution having an excellent heat removal function by using a porous body having a thermal conductivity in the thickness direction of 0.5 W / (m · K) or more as a separator. The present inventors have found that a secondary battery can be realized and completed the present invention.

  That is, the gist of the present invention is as follows.

(1) A porous separator used in a non-aqueous electrolyte secondary battery, wherein the thermal conductivity in the thickness direction is 0.5 W / (m · K) or more. Porous separator for secondary battery.

(2) The porous separator for a non-aqueous electrolyte secondary battery according to (1), which contains a filler having a thermal conductivity of 30 W / (m · K) or more.

(3) A multilayer porous separator comprising a laminate of at least two layers, wherein at least one layer is an electrical conductive layer having an electrical conductivity in the thickness direction of 10 ⁻¹¹ S / m or more, and the multilayer porous separator The porous separator for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the electrical separator in the thickness direction of the entire porous separator is less than 10 ^-11 S / m.

(4) The porous separator for a non-aqueous electrolyte secondary battery according to (3), wherein the electrically conductive layer contains a carbon material as a filler.

(5) The porous separator for a non-aqueous electrolyte secondary battery according to (3), wherein the electrically conductive layer contains metal powder as a filler.

(6) The porous separator for a non-aqueous electrolyte secondary battery according to claim 3, wherein the electrically conductive layer is made of a metal material.

(7) A non-aqueous electrolyte secondary battery comprising a negative electrode and a positive electrode capable of inserting and extracting lithium, a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt, and a porous separator. A non-aqueous electrolyte secondary battery using the porous separator according to any one of (1) to (6) as the porous separator.

  ADVANTAGE OF THE INVENTION According to this invention, with the porous separator excellent in the heat removal performance, the temperature rise at the time of charge / discharge of a non-aqueous electrolyte secondary battery can be suppressed, and high safety | security can be ensured.

  Hereinafter, embodiments of the present invention will be described in detail. However, the description of the constituent elements described below is an example (representative example) of an embodiment of the present invention, and the present invention does not exceed the gist thereof. The content is not specified.

[Porous separator]
The porous separator of the present invention is characterized in that the thermal conductivity in the thickness direction is 0.5 W / (m · K) or more. Hereinafter, the porous separator of the present invention will be described in detail.

<Thermal conductivity in the thickness direction>
The thickness direction thermal conductivity of the porous separator of the present invention is usually 0.5 W / (m · K) or more, preferably 1.0 W / (m · K) or more, more preferably 2.0 W / (m · K). K) or more. The upper limit of the thermal conductivity in the thickness direction of the porous separator of the present invention is not particularly limited, but is usually 2.5 W / (m · K) or less.

  When the thermal conductivity in the thickness direction of the porous separator is less than 0.5 W / (m · K), the heat removal is insufficient, and the non-aqueous electrolyte secondary battery is a constituent material for the heat generated inside the battery. Combustible materials such as water electrolyte and separator may decompose and cause smoke and fire. In addition, when a problem occurs in the charged battery, smoke and fire may occur similarly.

In the present invention, the thermal conductivity in the thickness direction of the porous separator is a value measured as follows.
(Measurement method of thermal conductivity in thickness direction)
Conforms to JIS A1412. One side of the 25 mm square sample is cooled and maintained at 18 ° C., and the other side is heated and maintained at 42 ° C. The thermal conductivity is obtained from the electric power supplied to the high heat plate necessary for maintaining a constant temperature gradient of 24K in the thickness direction.

<Thickness (film thickness)>
The thickness (film thickness) of the porous separator of the present invention is usually 10 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and usually 200 μm or less, preferably 100 μm or less, more preferably 50 μm or less. If the thickness of the porous separator is less than 10 μm, the mechanical strength becomes too low, which is not preferable. On the other hand, if the thickness of the porous separator exceeds 200 μm, the heat conduction is poor and heat removal cannot be performed sufficiently, which is not preferable. On the other hand, if the thickness of the porous separator is excessively large, the volume ratio of the separator in the battery is increased, the electrode area is relatively reduced, and the battery capacity is decreased.

<Porosity>
The porosity of the porous separator of the present invention is usually 30% or more, preferably 40% or more, more preferably 50% or more, and usually 80% or less, preferably 70% or less, more preferably 60% or less. . If the porosity of the porous separator is less than 30%, the pores are not sufficiently connected, and the ionic conductivity when impregnated with the electrolytic solution is lowered, so that the battery cannot function sufficiently. On the other hand, if the porosity of the porous separator exceeds 80%, the porous separator containing a filler is not preferable because the volume of the thermally conductive matrix is too small to remove heat sufficiently.

<Electric conductivity>
The porous separator must be electrically insulating as a whole in its application. The electrical conductivity in the thickness direction of the entire porous separator is usually less than 10 ⁻¹¹ S / m, preferably 10 ⁻¹² S / m or less, more preferably 10 ⁻¹⁴ S / m or less. If the electrical conductivity of the entire porous separator exceeds this upper limit, there is a possibility of a short circuit in an insulation test or the like during battery assembly, which is not preferable.

<Separator structure>
The structure of the porous separator of the present invention having a thermal conductivity in the thickness direction of 0.5 W / (m · K) or more is not particularly limited. For example, the structure of [1] or [2] below may be taken. it can.
[1] A single-layer separator made of a filler having a high thermal conductivity or containing such a filler (hereinafter sometimes simply referred to as “single-layer separator”).
[2] Multilayer separator having an electrically conductive layer such as a metal mesh (hereinafter, sometimes simply referred to as “multilayer separator”)

First, the case of the single layer separator of [1] will be described.
In the present invention, as a method of setting the thermal conductivity in the thickness direction of the separator, which is a porous body, to 0.5 W / (m · K) or more, a filler having a thermal conductivity of 30 W / (m · K) or more is porous. It can be included in the mass.

  The thermal conductivity of the filler used is usually 30 W / (m · K) or more, preferably 50 W / (m · K) or more, more preferably 70 W / (m · K) or more. The upper limit of the thermal conductivity of the filler is not particularly limited, but is usually 400 W / (m · K) or less. When the thermal conductivity of the filler is less than 30 W / (m · K), the thermal conductivity of a single-layer separator using the filler may not be increased.

  The material of the filler is not particularly limited as long as it satisfies the above-described thermal conductivity, but metal such as aluminum and copper, metal such as alumina, silica, magnesium oxide, zinc oxide, titanium oxide, and beryllium oxide. Carbon materials such as oxides, metal nitrides such as aluminum nitride, boron nitride, silicon nitride, silicon carbide, ketjen black, acetylene black, graphite, activated carbon, etc., or carbon materials such as nanocarbon such as carbon nanotubes, etc. Of these, metal particles such as aluminum and copper, and carbon materials are preferable.

The shape of the filler can be granular, fibrous, scaly, or any other shape.
The suitable range of the size of the filler is appropriately determined depending on the configuration of the single-layer separator, that is, whether it is a single-layer separator made of only the filler described later or a single-layer separator containing the filler in the resin. .

  A plurality of fillers having different materials, shapes, and sizes can be used in combination. For example, a combination in which a small amount of a fibrous or scale-like filler that easily forms percolation is added to a granular filler having high thermal conductivity can be considered. Moreover, even if it is a filler whose heat conductivity is less than 30 W / (m * K), the range which can make the heat conductivity of the thickness direction of the porous separator obtained 0.5 W / (m * K) or more Thus, it can be used together with a filler having a thermal conductivity of 30 W / (m · K) or more.

The single-layer separator of the present invention is
(1) -1 What consists of a filler with high heat conductivity (1) -2 It can be set as two kinds of structures of what contains a filler with high heat conductivity in resin.

(1) -1 Single Layer Separator Made of Filler with High Thermal Conductivity In this case, the single layer separator may be formed of a filler alone, and examples thereof include a sheet made of alumina fibers. Alternatively, the filler may be bonded with a very small amount of binder resin and then treated at a high temperature to carbonize the resin.
In this case, the size of the filler used for the production of the single-layer separator is particularly limited within the range satisfying the aforementioned porosity and the mechanical strength required for the porous separator suitable for the porous separator of the present invention. There is no.

(1) -2 Single-layer separator containing a filler with high thermal conductivity in the resin The filler used in this single-layer separator has an average particle size of usually 0.01 μm or more, preferably 0.05 μm or more. More preferably, it is 0.1 μm or more. Moreover, it is 50 micrometers or less normally, Preferably it is 30 micrometers or less, More preferably, it is 10 micrometers or less. The average particle diameter means the diameter or major axis for granular fillers, the major axis of plate-like parts for scale-like fillers, and the fiber length for fibrous fillers. Point.

If the average particle size of the filler is less than this lower limit, the shear heat generation during the single-layer separator molding becomes too large and the resin deteriorates. In addition, it is difficult to obtain a uniform single-layer separator due to excessive filler aggregation. Further, if the average particle size of the filler exceeds the above upper limit, it is not preferable because breakage is likely to occur in a molding process such as stretching.
In particular, when the average particle size of the filler is 0.3 μm or more and 1 μm or less, it is preferable because the filler easily forms percolation due to secondary aggregation that does not impair the moldability, and the thermal conductivity tends to increase.

  Further, the larger the ratio L / D of the major axis of the filler (fiber length in the case of fibrous filler) (L) and the minor axis (fiber diameter in the case of fibrous filler) (D), the more percolation. This is preferable because the thermal conductivity can be further increased.

  On the other hand, examples of the resin include polyolefin thermoplastic resins such as polyethylene (PE) and polypropylene (PP).

  The filler amount of the single layer separator containing the filler in the resin is usually 7% by volume or more, preferably 10% by volume or more, more preferably 15% by volume or more, and usually 60% by volume or less, preferably with respect to the single layer separator. Is 50% by volume or less, more preferably 40% by volume or less. If the amount of filler is less than this lower limit, the thermal conductivity is insufficient and sufficient heat removal is not obtained, and if the amount exceeds the upper limit, the amount of filler is too large to obtain the strength as a separator.

  In the case of a single-layer separator, since the insulating property as a separator is required in a single layer, the amount of conductive fillers such as carbon and metal particles is limited, but conductive fillers are generally thermally conductive. Therefore, after adding these conductive fillers as much as possible within the range that can maintain the insulation necessary for the separator, the thermal conductivity can be increased by adding non-conductive fillers such as oxides and nitrides. It is preferable to increase it.

  The single-layer separator containing a filler in the resin can be manufactured according to a normal method for manufacturing a resin separator, except that the filler is included.

Next, the case of the multilayer separator [2] will be described.
In this multilayer separator, an electrically conductive layer is provided to increase the thermal conductivity. In other words, since conductive metals and carbon generally have good thermal conductivity, an electrically conductive layer made of such a conductive material is provided, and on the other hand, in order to ensure insulation as a separator, Another electrically insulating layer is laminated on the electrically conductive layer.

  Examples of the electrically conductive layer include those obtained by adding a conductive filler to a resin to the extent that it has conductivity, a layer made of only a conductive filler, or a layer using a conductive porous material such as a metal mesh. .

Therefore, as a structure of a multilayer separator having an electrically conductive layer,
[2] -1 Multilayer separator composed of a laminate of two or more layers having at least an electrically conductive layer and an electrically insulating layer containing a metal filler or carbon filler in the resin. [2] -2 At least a metal mesh or the like A multilayer separator composed of a laminate of two or more layers having an electrically conductive layer composed of a conductive porous body and an electrically insulating layer can be mentioned.

  In the multilayer separator composed of a laminate of two or more layers having at least an electrically conductive layer and an electrically insulating layer containing a metal filler or a carbon filler in a resin, the metal filler or the carbon filler. The electrically conductive layer included in the resin can be formed in the same manner as the resin layer containing the filler in the single-layer separator described above using a conductive filler as a filler.

  In addition, in the multilayer separator composed of a laminate of two or more layers having [2] -2, at least an electrically conductive layer made of a conductive porous material such as a metal mesh, and an electrically insulating layer, the electrically conductive layer is It is composed of porous metal or carbon. Examples of the porous metal include a metal mesh and a punching metal. Examples of porous carbon include punched graphite sheet laminates and carbon fiber aggregates.

The electrical conductivity in the thickness direction of the electrical conductive layer having any of the structures [2] -1 and [2] -2 is usually 10 ⁻¹¹ S / m or more, preferably 10 ⁻⁸ S / m. More preferably, it is 10 ⁻⁵ S / m or more. When the electric conductivity of the electric conductive layer is lower than this lower limit, sufficient thermal conductivity cannot be obtained because percolation is not formed.
The thermal conductivity in the thickness direction of such an electrically conductive layer is usually preferably 100 W / (m · K) or more.

As described above, the porous separator as a whole must be electrically insulating in its application, and the electrical conductivity in the thickness direction of the entire porous separator is usually less than 10 ⁻¹¹ S / m, preferably 10 ⁻¹² S / m or less, more preferably 10 ⁻¹⁴ S / m or less.
Therefore, in the porous separator including the electrical conductive layer having the electrical conductivity as described above, it is necessary to further secure the electrical insulation necessary for the separator by laminating at least one electrical insulation layer. .

Here, the electrical insulating layer can have the same configuration as the layer in the single-layer separator described above, for example,
(i) A layer containing a metal nitride such as boron nitride or aluminum nitride, a metal oxide such as alumina, magnesium oxide or silica, or a non-conductive filler having high thermal conductivity such as silicon carbide.
(ii) Layer containing conductive filler at a concentration that does not have conductivity
(iii) A layer containing both a non-conductive filler and a conductive filler can be used. These layers may be layers composed only of fillers, or may be layers in which fillers are blended with polyolefin-based thermoplastic resins such as polyethylene and polypropylene, as in the single-layer separator described above.

  The thickness of the electric insulating layer varies depending on the electric conductivity of the electric conductive layer to be laminated and the degree of the electric insulating property, but is preferably 1/3 or less, more preferably the total thickness of the separator. Is 1/5 or less, more preferably 1/10 or less. In addition to ensuring the electrical insulation required for the separator, the thinner the electrical insulation layer, the smaller the resistance to heat flow, and this is preferable because a porous separator with excellent heat removal can be obtained.

  Such an electrically conductive layer and an electrically insulating layer are used by sticking each layer together by hot rolling or the like.

[Non-aqueous electrolyte secondary battery]
The porous separator of the present invention described above is used for a non-aqueous electrolyte secondary battery.
Hereinafter, the non-aqueous electrolyte secondary battery of the present invention to which the porous separator of the present invention is applied will be described.

  The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode and a negative electrode capable of occluding and releasing lithium ions, a non-aqueous electrolyte, and a porous separator. The porous separator of the present invention described above is used as the porous separator. It is what has.

<Negative electrode>
As the negative electrode, a material in which an active material layer containing a negative electrode active material and a binder is usually formed on a current collector is used.

As a negative electrode active material, a carbonaceous material capable of occluding / releasing lithium such as organic pyrolysate, artificial graphite and natural graphite under various pyrolysis conditions; capable of occluding / releasing lithium such as tin oxide and silicon oxide Metal oxide material; lithium metal; various lithium alloys can be used. These negative electrode active materials may be used individually by 1 type, and may mix and use 2 or more types.
In particular, among the above, a carbonaceous material is preferable as the negative electrode active material.

  The binder is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used during electrode production and other materials used during battery use. Specific examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene / butadiene rubber, isoprene rubber, and butadiene rubber. These may be used individually by 1 type, or may use multiple types together.

  The ratio of the binder in the negative electrode active material layer is such that the lower limit is usually 0.1% by weight or more, preferably 1% by weight or more, more preferably 5% by weight or more, and the upper limit is usually 80% by weight or less. Preferably it is 60 weight% or less, More preferably, it is 40 weight% or less, More preferably, it is 10 weight% or less. If the ratio of the binder is small, the active material cannot be sufficiently retained, so that the negative electrode mechanical strength may be insufficient, and the battery performance such as cycle characteristics may be deteriorated. become.

  In addition, the negative electrode active material layer may contain additives for a normal active material layer such as a thickener.

  The thickener is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used during electrode production and other materials used during battery use. Specific examples thereof include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein. These may be used individually by 1 type, or may use multiple types together.

  Copper, nickel, stainless steel, nickel-plated steel, or the like is used for the negative electrode current collector.

  The negative electrode can be formed by applying a slurry obtained by slurrying the above-described negative electrode active material, a binder, and other additives added as necessary with a solvent, and then drying the current collector.

  As the solvent used for slurrying, an organic solvent that dissolves the binder is usually used. For example, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran and the like are used, but not limited thereto. These may be used individually by 1 type, or may use multiple types together. Moreover, a dispersing agent, a thickener, etc. can be added to water, and an active material can also be slurried with latex, such as SBR.

  Thus, the thickness of the negative electrode active material layer formed is about 10-200 micrometers normally.

  The active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the active material.

<Positive electrode>
As the positive electrode, a material in which an active material layer containing a positive electrode active material and a binder is usually formed on a current collector is used.

  The positive electrode active material is not particularly limited as long as it can electrochemically occlude and release lithium ions. Preferable examples include lithium transition metal composite oxides.

Specific examples of the lithium-transition metal composite oxide, lithium cobalt complex oxides such as LiCoO _2, lithium-nickel composite oxide such as LiNiO _2, include lithium-manganese composite oxides such as LiMnO _2. In these lithium transition metal composite oxides, some of the main transition metal atoms are Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, etc. Replacing with other metals is preferable because it can be stabilized. Any one of these positive electrode active materials may be used alone, or two or more thereof may be used in any combination and ratio.

  The binder is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used during electrode production and other materials used during battery use. Specific examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, EPDM (ethylene-propylene-diene terpolymer), SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), Examples thereof include fluororubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, and nitrocellulose. These may be used individually by 1 type, or may use multiple types together.

  As for the ratio of the binder in the positive electrode active material layer, the lower limit is usually 0.1% by weight or more, preferably 1% by weight or more, more preferably 5% by weight or more, and the upper limit is usually 80% by weight or less, preferably 60% by weight or less, more preferably 40% by weight or less, and still more preferably 10% by weight or less. If the proportion of the binder is small, the active material cannot be sufficiently retained, so that the mechanical strength of the positive electrode is insufficient, and the battery performance such as cycle characteristics may be deteriorated. On the contrary, if the amount is too large, the battery capacity and conductivity are lowered. It will be.

  The positive electrode active material layer usually contains a conductive agent in order to increase conductivity. Examples of the conductive agent include carbonaceous materials such as graphite fine particles such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon fine particles such as needle coke. These may be used individually by 1 type, or may use multiple types together.

  The ratio of the conductive agent in the positive electrode active material layer is such that the lower limit is usually 0.01% by weight or more, preferably 0.1% by weight or more, more preferably 1% by weight or more, and the upper limit is usually 50% by weight or less. , Preferably 30% by weight or less, more preferably 15% by weight or less. If the proportion of the conductive agent is small, the conductivity may be insufficient, and conversely if too large, the battery capacity may be reduced.

  In addition, the positive electrode active material layer can contain additives for a normal active material layer such as a thickener.

  The thickener is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used during electrode production and other materials used during battery use. Specific examples thereof include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein. These may be used individually by 1 type, or may use multiple types together.

  Aluminum, stainless steel, nickel-plated steel or the like is used for the current collector of the positive electrode.

  The positive electrode can be formed by applying a slurry obtained by slurrying the above-described positive electrode active material, a binder, a conductive agent, and other additives added as necessary with a solvent onto a current collector, and drying the positive electrode active material. .

  As the solvent used for slurrying, an organic solvent that dissolves the binder is usually used. For example, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran and the like are used, but not limited thereto. These may be used individually by 1 type, or may use multiple types together. Moreover, a dispersing agent, a thickener, etc. can be added to water, and an active material can also be slurried with latex, such as SBR.

  Thus, the thickness of the positive electrode active material layer formed is about 10-200 micrometers normally.

  The active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the active material.

<Non-aqueous electrolyte>
(Non-aqueous solvent)
As the non-aqueous solvent of the electrolyte used in the non-aqueous electrolyte secondary battery of the present invention, any known solvent can be used as the solvent for the non-aqueous electrolyte secondary battery. For example, cyclic carbonates such as alkylene carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate (preferably alkylene carbonates having 3 to 5 carbon atoms); dialkyls such as dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, and ethyl methyl carbonate Chain carbonates such as carbonates (preferably dialkyl carbonates having an alkyl group having 1 to 4 carbon atoms); cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; chain ethers such as dimethoxyethane and dimethoxymethane; γ-butyrolactone, γ -Cyclic carboxylic acid esters such as valerolactone; and chain carboxylic acid esters such as methyl acetate, methyl propionate, and ethyl propionate. These may be used alone or in combination of two or more.

  Among the above exemplified solvents, a mixed non-aqueous solvent in which a cyclic carbonate and a chain carbonate are mixed is preferable from the viewpoint of improving overall battery performance such as charge / discharge characteristics and battery life. The mixed non-aqueous solvent contains cyclic carbonate and chain carbonate in an amount of 15% by volume or more of the whole non-aqueous solvent, and the total of these volumes is 70% by volume or more of the whole non-aqueous solvent. It is preferable to do.

  As the cyclic carbonate used in the mixed non-aqueous solvent in which the cyclic carbonate and the chain carbonate are mixed, an alkylene carbonate having 2 to 4 carbon atoms in the alkylene group is preferable. Specific examples thereof include ethylene carbonate, propylene carbonate, butylene carbonate and the like. Of these, ethylene carbonate and propylene carbonate are preferable.

  Moreover, as the chain carbonate used in the mixed non-aqueous solvent obtained by mixing the above cyclic carbonate and chain carbonate, a dialkyl carbonate having an alkyl group having 1 to 4 carbon atoms is preferable. Specific examples thereof include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate and the like. Of these, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are preferable.

  Each of these cyclic carbonates and chain carbonates may be used independently, or a plurality of types may be used in any combination and ratio.

  The ratio of the cyclic carbonate in the mixed non-aqueous solvent is 15% by volume or more, particularly 20 to 50% by volume, and the ratio of the chain carbonate is 30% by volume or more, particularly 40 to 80% by volume. The content ratio is preferably cyclic carbonate: chain carbonate = 1: 1 to 4 (volume ratio).

  Furthermore, the mixed non-aqueous solvent may contain a solvent other than the cyclic carbonate and the chain carbonate as long as the battery performance of the manufactured lithium battery is not deteriorated. The ratio of the solvent other than the cyclic carbonate and the chain carbonate in the mixed non-aqueous solvent is usually 30% by volume or less, preferably 10% by volume or less.

(Lithium salt)
Arbitrary things can be used as lithium salt which is a solute of nonaqueous system electrolyte. For example, inorganic lithium salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ ; LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C _{4 F 9 SO 2), LiC} (CF 3 SO 2) 3, LiPF 4 (CF 3) 2, LiPF 4 (C 2 F 5) 2, LiPF 4 (CF 3 SO 2) 2, LiPF 4 (C 2 F ₅ SO ₂ ) ₂ , LiBF ₂ (CF ₃ ) ₂ , LiBF ₂ (C ₂ F ₅ ) ₂ , LiBF ₂ (CF ₃ SO ₂ ) ₂ , LiBF ₂ (C ₂ F ₅ SO ₂ ) _2, etc. Examples include lithium salts. Of these, fluorine-containing lithium salts such as LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiN (C ₂ F ₅ SO ₂ ) ₂ , particularly LiPF ₆ and LiBF ₄ are preferable. In addition, about lithium salt, 1 type may be used independently and 2 or more types may be used together.

  The lower limit of the concentration of these lithium salts in the non-aqueous electrolyte is usually 0.5 mol / l or more, especially 0.75 mol / l or more, and the upper limit is usually 2 mol / l or less, especially 1.5 mol / l. l or less. When the concentration of the lithium salt exceeds this upper limit value, the viscosity of the nonaqueous electrolytic solution increases and the electrical conductivity also decreases. Moreover, since electrical conductivity will become low if less than this lower limit, it is preferable to prepare a non-aqueous electrolyte within the said concentration range.

(Film forming agent)
The non-aqueous electrolyte solution can contain a film forming agent.

  As the film forming agent, carbonate compounds having an ethylenically unsaturated bond such as vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, trifluoropropylene carbonate, phenylethylene carbonate and erythritan carbonate, succinic anhydride, glutaric anhydride, Carboxylic anhydrides such as maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride, etc. It is done. In particular, vinylene carbonate, vinyl ethylene carbonate, and succinic anhydride are preferable as the film forming agent from the viewpoint of good cycle characteristic improvement effect and temperature dependency of film resistance, and vinylene can be formed because particularly good quality film can be formed. More preferably, carbonate is used. In addition, these film formation agents may be used individually by 1 type, and may mix and use 2 or more types.

  The content of the film forming agent in the non-aqueous electrolyte is 0.01% by weight or more, preferably 0.1% by weight or more, more preferably 0.3% by weight or more, and 10% by weight or less, preferably 8%. % By weight or less, more preferably 7% by weight or less. If the content of the film forming agent is below the lower limit of the above range, it is difficult to obtain the effect of improving the cycle characteristics of the battery. On the other hand, if the content exceeds the upper limit, the rate characteristics at low temperatures may be lowered.

(Other additives)
In addition to the non-aqueous solvent, lithium salt and film forming agent, the non-aqueous electrolyte solution used in the present invention includes other useful components as necessary, for example, conventionally known overcharge inhibitors, dehydrating agents, deoxidizing agents, Various additives such as a positive electrode protective agent may be contained.

[Battery configuration]
The non-aqueous electrolyte secondary battery of the present invention is manufactured by assembling the above-described positive electrode, negative electrode, non-aqueous electrolyte, and separator into an appropriate shape. Furthermore, other components such as an outer case can be used as necessary.

  The battery shape is not particularly limited, and can be appropriately selected from various commonly used shapes according to the application. Examples of commonly used shapes include a cylinder type with a sheet electrode and separator spiral, an inside-out cylinder type with a combination of pellet electrode and separator, a coin type with a stack of pellet electrode and separator, and a sheet Examples include a laminate type in which electrodes and separators are laminated. The method for assembling the battery is not particularly limited, and can be appropriately selected from various commonly used methods according to the shape of the target battery.

  The general embodiment of the non-aqueous electrolyte secondary battery of the present invention has been described above, but the non-aqueous electrolyte secondary battery of the present invention is not limited to the above-described embodiment and does not exceed the gist thereof. As long as it is possible, various modifications can be made.

  EXAMPLES Next, although an Example demonstrates this invention further more concretely, this invention is not limited to description of a following example, unless the summary is exceeded.

  In the following, battery components and batteries other than the separator are obtained as follows.

<Preparation of non-aqueous electrolyte>
Purified ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 in a dry argon atmosphere to prepare a mixed solvent. In this mixed solvent, sufficiently dried LiPF ₆ was dissolved at a rate of 1.0 mol / l, and then vinylene carbonate was mixed so that the concentration in the non-aqueous electrolyte was 2% by weight. A non-aqueous electrolyte was used.

<Preparation of positive electrode>
LiCoO ₂ was used as the positive electrode active material, and 6 parts by weight of carbon black and 9 parts by weight of polyvinylidene fluoride (trade name “KF-1000” manufactured by Kureha Chemical Co., Ltd.) were added to 85 parts by weight of LiCoO ₂ and mixed. Disperse with 2-pyrrolidone to form a slurry. This was uniformly applied to one side of a 20 μm-thick aluminum foil as a positive electrode current collector, dried, and then pressed by a press machine so that the density of the positive electrode active material layer was 3.0 g / cm ^3. It was.

<Preparation of negative electrode>
Using natural graphite powder as a negative electrode active material, 94 parts by weight of natural graphite powder was mixed with 6 parts by weight of polyvinylidene fluoride, and dispersed with N-methyl-2-pyrrolidone to form a slurry. This was uniformly applied to one side of a 18 μm-thick copper foil as a negative electrode current collector, dried, and then pressed by a pressing machine so that the density of the negative electrode active material layer was 1.5 g / cm ^3. did.

<Battery assembly>
The negative electrode plate and the positive electrode plate produced as described above were rolled up together with the separator, and the outermost periphery was stopped with a tape to obtain a spiral electrode body. The electrode body was inserted into the stainless steel battery case formed into a cylindrical shape from the opening. After that, the negative electrode lead connected to the negative electrode of the electrode body is welded to the inner bottom portion of the battery case, and the positive electrode lead connected to the positive electrode of the electrode body is increased to a predetermined level or more when the gas pressure inside the battery rises. Welded with the bottom of the working current interrupter. In addition, an explosion-proof valve and a current interrupt device were attached to the bottom of the sealing plate. And after inject | pouring the said electrolyte solution, the battery case was sealed with the sealing board and the insulating gasket made from a polypropylene at an opening part, and it was set as the lithium secondary battery.

Example 1: _Al 2 _{O 3} containing PE film / graphite containing PE film laminated porous separator]
A composition of 30 parts by weight of polyethylene (PE) having a weight average molecular weight of 700,000, 70 parts by weight of paraffin wax and alumina (Al ₂ O ₃ ) particles having an average particle diameter of 0.2 μm were melt-kneaded at a volume fraction of 70/30. Created a sheet. The sheet was biaxially stretched at 120 ° C., further immersed in isopropyl alcohol (IPA) at 60 ° C. to extract and dry paraffin wax, and a PE film containing Al ₂ O ₃ with a porosity of 35% and a thickness of 5 μm. Got.
Similarly, a composition was prepared by melting and kneading 30 parts by weight of PE having a weight average molecular weight of 700,000, 70 parts by weight of paraffin wax, and graphite particles having an average particle diameter of 5 μm at a volume fraction of 70/30. The sheet is biaxially stretched at 120 ° C., further immersed in IPA at 60 ° C. to extract and dry paraffin wax, and the porosity is 36%, the thickness is 20 μm, and the electric conductivity in the thickness direction is 6 × 10 ^−4. An S / m graphite-containing PE film was obtained. These two films were stacked and integrated by hot rolling at 80 ° C. to obtain a separator.
The laminated separator had a thermal conductivity in the thickness direction of 0.9 W / (m · K), and an electrical conductivity in the thickness direction of 3 × 10 ⁻¹⁴ S / m.
Using this separator, a battery was prepared by the above-described method using an Al ₂ O ₃ -containing PE film on the positive electrode side and a graphite-containing PE film on the negative electrode side.
After the battery was fully charged at 4.2 V, a nail penetration test was performed, but the battery did not smoke.

Example 2: _Al 2 _{O 3} containing PE film / Al mesh laminated porous separator]
In Example 1, instead of the graphite-containing PE film, a separator was prepared in the same manner except that an aluminum (Al) mesh having a thickness of 60 μm and an electric conductivity of 1.5 × 10 ⁷ S / m in the thickness direction was laminated. .
This laminated separator had a thermal conductivity of 4.5 W / (m · K) and an electric conductivity in the thickness direction of 8 × 10 ⁻¹⁴ S / m.
Using this separator, a battery was prepared in the same manner as in Example 1. The battery was fully charged at 4.2 V and then subjected to a nail penetration test. However, the battery did not emit smoke.

[Comparative Example 1: Al ₂ O ₃ containing PE film single layer porous separator]
In analogy to the procedure of manufacturing _Al 2 _{O 3} containing PE film in Example 1, 35% porosity, was obtained _Al 2 _{O 3} containing PE film having a thickness of 25 [mu] m.
The thermal conductivity in the thickness direction of this film was 0.4 W / (m · K).
Using only this Al ₂ O ₃ -containing PE film as a separator, a battery was prepared in the same manner as in Example 1. When the battery was fully charged at 4.2 V and a nail penetration test was performed, the battery smoked. .

[Comparative Example 2: BaSO ₄ -containing PE film single layer porous separator]
In the procedure for producing the Al ₂ O ₃ -containing PE film in Example 1, the same procedure was followed except that barium sulfate (BaSO ₄ ) particles having an average particle size of 0.6 μm were used instead of Al ₂ O ₃ particles. A BaSO ₄ -containing PE film having a porosity of 36% and a thickness of 26 μm was obtained.
The thermal conductivity in the thickness direction of this film was 0.2 W / (m · K).
Using only this BaSO ₄ -containing PE film as a separator, a battery was prepared in the same manner as in Example 1. When the battery was fully charged at 4.2 V and a nail penetration test was performed, the battery smoked.

  The non-aqueous electrolyte secondary battery of the present invention comprising the porous separator for a non-aqueous electrolyte secondary battery of the present invention having excellent heat removal characteristics is less likely to increase in temperature due to heat generation during charging and discharging, etc. It is excellent and can be used for various known applications. Specific examples include notebook computers, pen input computers, mobile computers, electronic book players, mobile phones, mobile faxes, mobile copy, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidiscs, and transceivers. , Electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, lighting fixtures, toys, gaming devices, small devices such as watches, strobes, cameras, and large devices such as electric vehicles and hybrid vehicles However, the present invention is not limited to these examples.

Claims (7)

  A porous separator used for a non-aqueous electrolyte secondary battery, wherein the thermal conductivity in the thickness direction is 0.5 W / (m · K) or more, for a non-aqueous electrolyte secondary battery Porous separator.   2. The porous separator for a non-aqueous electrolyte secondary battery according to claim 1, comprising a filler having a thermal conductivity of 30 W / (m · K) or more. A multilayer porous separator comprising a laminate of at least two layers, wherein at least one layer is an electrical conductive layer having an electrical conductivity in the thickness direction of 10 ^-11 S / m or more, and the entire multilayer porous separator The porous separator for a non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized in that the electrical conductivity in the thickness direction is less than 10 ^-11 S / m.   The porous separator for a nonaqueous electrolyte secondary battery according to claim 3, wherein the electrically conductive layer contains a carbon material as a filler.   The porous separator for a nonaqueous electrolyte secondary battery according to claim 3, wherein the electrically conductive layer contains metal powder as a filler.   The porous separator for a non-aqueous electrolyte secondary battery according to claim 3, wherein the electrically conductive layer is made of a metal material.   A non-aqueous electrolyte secondary battery comprising a negative electrode and a positive electrode capable of occluding and releasing lithium, a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt, and a porous separator. A non-aqueous electrolyte secondary battery using the porous separator according to claim 1 as a porous separator.