JP6877611B1 - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery capable of achieving both high capacity and high output. SOLUTION: A positive electrode and a negative electrode in which a positive electrode active material layer and a negative electrode active material layer are arranged so as to sandwich a separator, and a lithium salt-containing electrolyte solution are provided, and an electrode density of the negative electrode is 2.5 mAh / cm 2 to 5. It is 0 mAh / cm2, and 30% to 100% of the negative electrode active material contained in the negative electrode active material layer is a negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg, and the average flow diameter of the separator ( A lithium ion secondary battery having a μm) × air permeability (seconds / 100 mL) of 2.0 to 8.0 is provided. [Selection diagram] None

Description

The present invention relates to a lithium ion secondary battery and the like.

Lithium-ion secondary batteries have rapidly become widespread as power sources for portable electronic devices such as mobile phones, notebook computers, video movies, and digital cameras because of their advantages such as high capacity, light weight, and long life. In recent years, in drones and the like, the demand for higher capacity of lithium ion secondary batteries has been increasing, and for example, an electrode density of ^{at least 2.5 mAh / cm 2 is required.} Conventionally, carbon materials such as non-graphitizable carbon or graphite have been used as the negative electrode material of lithium-ion batteries, but the effective capacity of these carbon materials is already saturated in industrial technology and is further increased. It is difficult to increase the capacity.

As a new negative electrode material, it is being studied to use an alloy-based negative electrode, for example, a metal such as silicon (Si) or tin (Sn) or a metalloid. For example, Patent Document 1 exemplifies a negative electrode containing Si, Sn, etc., but does not mention the details thereof, and actually, by specifying the structure and composition of the positive electrode active material, the capacity and output of the battery are increased. It is intended to be converted.

In addition, there is an attempt to increase the output of the battery by using an ion-permeable separator arranged between the positive and negative electrodes. For example, Patent Document 2 states that the air permeability is 100 seconds / 100 mL to 250 seconds / 100 mL, the weight average molecular weight is 250,000 to 500,000, the molecular weight distribution is 7.5 to 12, and the electricity measured under specific conditions. It has been proposed to use a propylene resin microporous film having a resistance value of 2.4 Ω · cm ^{2 or less as a battery separator.} However, Patent Document 2 does not describe the details of the electrode and the relationship between the separator and the electrode at all.

_{Patent Document 3 exemplifies Si, SiO 2} , SnO _2, etc. as the negative electrode material, but in reality, it specializes in the capacity retention rate (that is, high output) of the battery during quick charging, and is naturally used as the negative electrode material. Graphite is used in combination with a separator having a porosity of 45% to 90% and an air permeability of ^{less than 200 seconds / 100 cm 3.}

Japanese Unexamined Patent Publication No. 2014-235855 Japanese Unexamined Patent Publication No. 2013-202944 Japanese Unexamined Patent Publication No. 2005-293950

The battery described in Patent Document 1 still has room for improvement in terms of achieving both high capacity and high output. Further, from Patent Documents 2 or 3, it is difficult to reduce the resistance of the conventional graphite-based negative electrode at an electrode density of 2.5 mAh / cm ² or more, and the air permeability is 30 seconds / 100 cm ³ to 200 seconds / 100 cm ³ It can be seen that the low resistance of the battery cannot be achieved even when used in combination with the low resistance separator of.

In view of the above background technology, an object of the present invention is to provide a lithium ion secondary battery capable of achieving both high capacity and high output.

As a result of diligent studies, the present inventor has found that the above problems can be solved by specifying the combination of the negative electrode active material and the separator in the lithium ion secondary battery, and completed the present invention. Examples of embodiments of the present invention are listed below.
[1]
A positive electrode in which a positive electrode active material layer containing a positive electrode active material is arranged on a positive electrode current collector,
A negative electrode in which a negative electrode active material layer containing a negative electrode active material is arranged on a negative electrode current collector,
With an electrically insulating and porous separator,
An electrolyte solution containing a lithium salt and impregnating the separator, the positive electrode active material layer, and the negative electrode active material layer.
With
The positive electrode and the negative electrode are arranged so that the positive electrode active material layer and the negative electrode active material layer face each other so as to sandwich the separator.
The electrode density of the negative electrode is 2.5 mAh / cm ^{2 to} 5.0 mAh / cm ² .
Of the negative electrode active material contained in the negative electrode active material layer, 30% by weight to 100% by weight is a negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg, and the average flow diameter (μm) of the separator. ) × A lithium ion secondary battery having a value of air permeability (seconds / 100 mL) of 2.0 to 8.0.
[2]
The lithium ion secondary battery according to item 1, wherein the negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg is a silicon (Si) alloy or silicon monoxide (SiO).
[3]
The lithium ion secondary battery according to item 1 or 2, which contains 3% by weight to 30% by weight of polyimide of the negative electrode active material layer.

According to the present invention, both high capacity and high output of a lithium ion secondary battery can be achieved at the same time.

Hereinafter, embodiments for carrying out the present invention (hereinafter, may be abbreviated as “the present embodiment”) will be described in detail, but the present invention is not limited thereto and does not deviate from the gist thereof. Various modifications are possible within the range.

In addition, in this specification, "MD" is an abbreviation of Machine Direction, which means when the separator is continuously formed into a film and in the longitudinal direction of the battery winding body. "TD" is a direction orthogonal to MD, refers to a width direction of a separator, and is an abbreviation for Transverse Direction.

<Lithium-ion secondary battery>
Lithium-ion secondary batteries have the following components:
A positive electrode in which a positive electrode active material layer containing a positive electrode active material is arranged on a positive electrode current collector,
A negative electrode in which a negative electrode active material layer containing a negative electrode active material is arranged on a negative electrode current collector,
With an electrically insulating and porous separator,
An electrolyte solution containing a lithium salt and impregnating the separator, the positive electrode active material layer and the negative electrode active material layer,
To be equipped. In the lithium ion secondary battery, the positive electrode and the negative electrode are arranged so that the positive electrode active material layer and the negative electrode active material layer face each other so as to sandwich the separator. If desired, the lithium ion secondary battery can include elements other than the above-mentioned components, such as an exterior body, a current collector portion on which no electrode active material is formed, lead wires, lead terminals, and the like.

The lithium ion secondary battery according to the present embodiment has a negative electrode density of 2.5 mAh / cm ^{2 to} 5.0 mAh / cm ² , and is 30% by weight of the negative electrode active material contained in the negative electrode active material layer. ~ 100% by weight is a negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg, and the value of the average flow diameter (μm) × air permeability (seconds / 100 mL) of the separator is 2.0 to 8. It is 0.0.

In the present technical field, an electrode density of 2.5 mAh / cm ^{2 to} 5.0 mAh / cm ² is at the level of a high electrode capacitance density, and a negative electrode active material having a theoretical capacitance of 700 Ah / kg to 4000 Ah / kg is a high capacitance negative electrode. It is known to be an active material. Although not bound by theory, in a lithium ion secondary battery, such a high capacity negative electrode has 2.0 ≤ average flow diameter (μm) x air permeability (seconds / 100 mL) ≤ 8.0. When used in combination with a filling separator, lithium ion diffusion is particularly good, which may lead to lower battery resistance, that is, higher output. Further, both high capacity and high output can be achieved only by lowering the air permeability of the separator (that is, lowering the resistance) in the positive electrode / negative electrode facing arrangement in which the positive electrode active material layer and the negative electrode active material layer sandwich the separator. However, it can be achieved by significantly specifying the average flow diameter.

Specifically, the lithium ion secondary battery has a positive electrode plate containing a positive electrode active material capable of storing and releasing lithium ions and the like and a negative electrode plate containing a negative electrode active material capable of storing and releasing lithium ions and the like. It is wound or laminated so as to face each other via a separator for a water electrolyte battery, holds an electrolyte solution, and is housed in an exterior body such as a metal container or a laminate film.

The components of the lithium ion secondary battery will be described below.

<Negative electrode>
The negative electrode includes a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector. The negative electrode active material layer contains the negative electrode active material and can be formed on one side or both sides of the negative electrode current collector. The negative electrode current collector may be provided with terminals in a portion where the negative electrode active material layer is not formed, such as an uncoated part, or a method in which a portion where the negative electrode active material layer is not formed is directly used as a terminal. Has been taken. For the negative electrode current collector, for example, a metal foil such as a copper foil is used. For the negative electrode active material layer, for example, the negative electrode active material and the binder are kneaded and dissolved in a solvent to prepare a negative electrode paste, and the negative electrode paste is used for vapor phase method, liquid phase method, spraying method, firing method, and coating. , Or a combination thereof, can be formed on the negative electrode current collector.

In the present embodiment, the electrode density of the obtained negative electrode is 2.5 mAh / cm ^{2 to} 5.0 mAh / cm ² , and among the negative electrode active materials contained in the negative electrode active material layer, the theoretical capacity is 700 Ah / kg to 4000 Ah. As long as the content of the negative electrode active material at / kg is 30% by weight to 100% by weight, a known negative electrode material may be used in the preparation thereof.

^{The electrode density of the negative electrode is preferably 2.5 mAh / cm 2} or more from the viewpoint of increasing the capacity, ^{and 5.0 mAh / cm 2} or less from the viewpoint of compatibility with the separator according to the present embodiment. From the viewpoint of the compatibility of the viewpoint and the high capacity of the separator according to the present ^embodiment, more preferably ^{2.7mAh / cm 2 ~5.0mAh / cm 2} , 3.0mAh / cm 2 ~5.0mAh It is more preferably / cm ^2.

The content of the negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg is 30% by weight to 100% by weight of the negative electrode active material contained in the negative electrode active material layer, and is compatible with the separator according to the present embodiment. From the viewpoint of properties, it is preferably 50% by weight to 100% by weight, more preferably 70% by weight to 100% by weight.

The negative electrode active material used in the present embodiment may be any material as long as it has a theoretical capacity of 700 Ah / kg to 4000 Ah / kg and can occlude and release lithium ions. From the viewpoint of compatibility with the separator according to the present embodiment, the negative electrode active material is generally preferably an alloy-based negative electrode material rather than a graphite-based material having a theoretical capacity of less than 400 Ah / kg.

The alloy-based negative electrode is a negative electrode material containing at least one of a metal element and a semi-metal element capable of storing and releasing lithium, which is an electrode reactant, as a negative electrode active material and alloying. Refers to the negative electrode contained. By using such a negative electrode material, a high energy density can be obtained. The negative electrode material may be a simple substance, an alloy, or a compound of a metal element or a metalloid element, or may have one or more of these phases in at least a part thereof.

In addition, in this specification, an alloy includes an alloy composed of two or more kinds of metal elements, and also an alloy containing one or more kinds of metal elements and one or more kinds of metalloid elements. Further, the negative electrode active material may contain a non-metal element, and may contain, for example, a binder component such as resin or rubber, various additives, and the like. Some of the structures include solid solutions, eutectic (eutectic mixture), intermetallic compounds, or two or more of them coexisting.

Examples of the metal element or metalloid element constituting the negative electrode material include a metal element or metalloid element capable of forming an alloy with lithium. Specifically, boron, magnesium, aluminum, silicon (Si), sulfur, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, ittrium, zirconium, niobium, molybdenum, palladium, Examples include silver, cadmium, indium, tin, antimony, hafnium, tungsten, platinum, gold, lead, bismuth and gallium. The present invention can also be applied to a composite of the above alloy and carbon, silicon oxide, an amorphous substance or the like as a negative electrode material. Further, the negative electrode active material may be metallic lithium.

Among them, the negative electrode active material is a silicon (Si) alloy and / or monoxide as a negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg from the viewpoint of achieving both high capacity and high output of a lithium ion secondary battery. It is preferable that silicon (SiO) is contained in the range of 30% by weight to 100% by weight.

From the viewpoint of achieving both high capacity and high output of the lithium ion secondary battery, the negative electrode active material layer preferably contains polyimide (PI) in addition to the negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg. More preferably, it contains 3% by weight to 30% by weight of PI based on the weight of the negative electrode active material layer, and more preferably 5% by weight to 20% by weight. If the PI content is less than 3% by weight, the binding property becomes insufficient and the structure of the negative electrode active material layer cannot be maintained. On the other hand, when the PI content exceeds 30% by weight, the flexibility of the electrode is lowered, the handling is deteriorated, the PI inhibits the electron conductivity, and the resistance is increased.

The thickness of the negative electrode active material layer is preferably 5 μm to 60 μm from the viewpoint of compatibility with the separator according to the present embodiment. The thickness of the negative electrode active material layer can be calculated by measuring with a film thickness meter described later or by observing the cross section of the negative electrode microscopically. The thickness of the negative electrode active material layer is determined by optimizing the type and content of the negative electrode active material used, the type and content of the binder, the method of forming the negative electrode active material layer, the texture and bulk density of the negative electrode paste, and the like. It can be adjusted in the range of 5 μm to 60 μm.

The void ratio of the negative electrode is a negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg from the viewpoint of achieving both high capacity and high output of the lithium ion secondary battery and compatibility with the separator according to the present embodiment. The void ratio of the negative electrode when is close to 100% is preferably around 3%, and the void ratio of the negative electrode having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg of about 30% is about 35%. Is preferable.

<Positive electrode>
The positive electrode includes a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector. The positive electrode active material layer contains the positive electrode active material and can be formed on one side or both sides of the positive electrode current collector.

The positive electrode current collector may be provided with terminals in a portion where the positive electrode active material layer is not formed, such as an uncoated part, or a method in which a portion where the positive electrode active material layer is not formed is directly used as a terminal. Has been taken. For the positive electrode current collector, for example, a metal foil such as an aluminum foil is used.

Examples of the positive electrode active material include lithium composite metal oxides such as lithium nickelate, lithium manganate or lithium cobalt oxide, lithium-nickel-cobalt-manganese composite oxides (for example, NCM523), and lithium iron phosphate and the like. Composite metal phosphate and the like can be used.

The positive electrode active material is kneaded with a conductive agent and a binder, applied to a positive electrode current collector as a positive electrode paste, dried, rolled to a predetermined thickness, and then cut to a predetermined size to obtain a positive electrode plate. Here, as the conductive agent, a metal powder stable under a positive electrode potential, for example, carbon black such as acetylene black or a graphite material can be used. Further, as the binder, a material stable under the positive electrode potential, for example, polyvinylidene fluoride, modified acrylic rubber, polytetrafluoroethylene, or the like can be used.

<Separator>
The separator according to the present embodiment is electrically insulating and porous, and is a value obtained by multiplying the average flow rate diameter (μm) and the air permeability (seconds / 100 mL), that is, the average flow rate diameter (μm) × air permeability. The value of degree (seconds / 100 mL) is 2.0 or more and 8.0 or less. The value of average flow rate diameter (μm) x air permeability (seconds / 100 mL) is determined from the viewpoint of achieving both high capacity and high output of the lithium ion secondary battery and the strength of the microporous film constituting the separator. It is preferably 7 or more and 6.5 or less.

The average flow rate diameter of the separator conforms to the half-dry method, can be measured with a palm poromometer, and is preferably in the range of 0.0150 μm to 0.1500 μm from the viewpoint of improving the life characteristics. It is more preferably in the range of 0.0300 μm to 0.0700 μm.

The air permeability of the separator is preferably in the range of 30 seconds / 100 mL to 250 seconds / 100 mL, preferably 50 seconds, from the viewpoint of increasing the output of the lithium ion secondary battery and improving the life characteristics of the battery or the separator. More preferably, it is in the range of / 100 mL to 200 seconds / 100 mL.

The thickness of the separator is preferably in the range of 3 μm to 25 μm from the viewpoint of improving the safety of the lithium ion secondary battery. The lower limit of the thickness of the separator is more preferably 5 μm or more, and further preferably 10 μm or more, from the viewpoint of the strength of the microporous membrane constituting the separator. On the other hand, the upper limit of the thickness of the separator is more preferably 20 μm or less from the viewpoint of increasing the capacity of the battery.

The porosity of the separator is preferably in the range of 30% to 70% from the viewpoint of increasing the output of the lithium ion secondary battery and improving the life characteristics of the battery or the separator. The lower limit of the porosity is more preferably 40% or more from the viewpoint of ion permeability. On the other hand, the upper limit of the porosity is more preferably 60% or less from the viewpoint of the strength of the microporous membrane constituting the separator.

The puncture strength of the separator is preferably in the range of 200 gf to 1200 gf from the viewpoint of improving the safety of the lithium ion secondary battery, and more preferably 200 gf to 600 gf from the viewpoint of heat resistance.

The tensile strength of the separator is preferably 50 MPa or more, more preferably 100 MPa or more for both MD and TD. Further, it is preferable that the tensile strengths of MD and TD do not differ greatly in order to prevent the separator from being torn by protrusions such as an electrode material. The tensile strength ratio (MD / TD) of MD and TD of the separator is preferably in the range of 0.2 to 7.0 in a lithium ion secondary battery provided with an alloy-based negative electrode that expands rapidly during charging. , 0.5 to 5.0, more preferably.

Considering that the polyolefin microporous film is used for a high-capacity lithium ion secondary battery using an alloy-based negative electrode, it may be formed of a microporous film rather than a non-woven fabric or the like in terms of physical strength and safety. preferable. Further, the polyolefin microporous membrane may have a laminated structure. The polyolefin microporous membrane may contain an inorganic substance or the like as long as the effect of the present invention is exhibited. In the case of a laminated structure, an inorganic substance may be contained in the surface layer or the intermediate layer thereof as long as the effect of the present invention is exhibited. Further, from the viewpoint of improving the safety of the lithium ion secondary battery, the separator includes a functional layer such as an inorganic coating layer containing an inorganic component and an adhesive layer containing a thermoplastic resin in addition to the polyolefin microporous membrane. be able to.

From the viewpoint of ensuring the safety of the lithium ion secondary battery, the polyolefin microporous membrane preferably contains 60% by weight or more of the polyolefin component of the polyolefin microporous membrane, such as polyethylene having a melting point of about 135 ° C. The melting point of about 135 ° C. specifically means that the melting point is 130 ° C. to 138 ° C.

The polyolefin microporous membrane has a fuse function that prevents excessive heat generation by blocking the microporous due to thermal melting of the membrane when the battery generates abnormal heat, and when focusing on this fuse function, It can be said that the lower the temperature at which the microporous is closed, that is, the lower the melting point of the main component of the polyolefin microporous membrane, the higher the safety of the battery. On the other hand, if the melting point of the main component of the microporous membrane is too low, the heat resistance of the membrane is lowered, and the separator is ruptured by a slight abnormal heat generation, causing a problem that both electrodes are short-circuited. Therefore, it is preferable to use polyolefin having a melting point of 125 ° C. to 138 ° C. from the viewpoint of the fuse function and the prevention of short circuit.

Only one type of polyolefin may be used, or a plurality of types may be used. The polyolefin used in the present embodiment is not particularly limited, and examples thereof include polyethylene, polypropylene, poly-4-methyl-1-pentene, and the like, and one or more of them can be used, but they are excellent in stretchability during film formation. It is preferable to use one or more types of polyethylene.

As the polyethylene, homopolymer polyethylene having a density of 0.940 g / cm ³ or more or copolymer polyethylene having an α-olefin comonomer content of 2 mol% or less is preferable, and homopolymer polyethylene is more preferable. The type of α-olefin comonomer is not particularly limited. The viscosity average molecular weight (Mv) of polyethylene is preferably in the range of 100,000 to 4,000,000 from the viewpoint of improving the safety of the lithium ion secondary battery, and is preferably in the range of 100,000 to 2,000. It is more preferably in the range of 000, and even more preferably in the range of 200,000 to 1,000,000.

The polyethylene polymerization catalyst is not particularly limited and may be any of Ziegler-type catalyst, Phillips-type catalyst, Kaminsky-type catalyst and the like. As a method for polymerizing polyethylene, there are one-step polymerization, two-step polymerization, or more multi-step polymerization, and any method of polyethylene can be used, but polyethylene obtained by one-step polymerization is preferable. Various polyolefins can be blended without impairing the film-forming property and within the range that does not deviate from the requirements of the present invention. For example, low melting point polyethylene having a high content of α-olefin comonomer for improving pore blocking characteristics, polypropylene and poly-4-methyl-1-pentene for improving heat resistance, and the like can be blended.

In this embodiment, as polypropylene (PP), for example, homopolypropylene, ethylene propylene random copolymer, ethylene propylene block copolymer, or the like can be used. Of these, homopolypropylene is preferable. In the case of the copolymer, the ethylene content is preferably 1.0% by weight or less from the viewpoint that the crystallinity of polypropylene does not decrease and the permeability of the microporous membrane does not decrease. The viscosity average molecular weight (Mv) of polypropylene is preferably 100,000 or more from the viewpoint of mechanical strength of the obtained polyolefin microporous film, and preferably less than 1,000,000 from the viewpoint of moldability. The Mv of PP is more preferably in the range of 100,000 to 800,000, still more preferably in the range of 100,000 to 600,000.

When polyolefin is used in the separator according to the present embodiment, the viscosity average molecular weight (Mv) of the separator is preferably 100,000 or more. Although the reason is not clear, it is preferable that the Mv of the separator, which is the final product, is 100,000 or more, which has better cycle characteristics and is preferable from the viewpoint of film strength. Although there is no particular upper limit of Mv as a separator, it is preferably 4,000,000 or less, more preferably 2,000,000 or less, and even more preferably 1,000,000 or less from the viewpoint of film forming property. Here, the viscosity average molecular weight of the separator refers to the viscosity average molecular weight of the polymer material constituting the polyolefin microporous membrane which has been subjected to steps such as stretching, extraction, and heat fixation to become the final separator.

When polyolefin is used in the separator according to the present embodiment, the molecular weight distribution Mw / Mn as the separator is preferably 3 or more and less than 15, from the viewpoint of porosity, and more preferably 3 or more and 10 or less. Generally, in film processing, the wider the molecular weight distribution of the film constituent material is, the better the stretchability is, and the narrower the molecular weight distribution is, the better the pore opening property. It is preferable that the molecular weight distribution is not too wide for a microporous membrane separator which is used together with an alloy-based negative electrode having severe expansion and contraction and has a risk of crushing of the surface layer, because it is required to have excellent pore-opening property. Here, Mw / Mn as a separator refers to Mw / Mn of a polymer material constituting a polyolefin microporous film which has been subjected to steps such as stretching, extraction, and heat fixation to become a final separator.

Inorganic substances, antioxidants, lubricants, and other improvers can be added to the separator as long as the performance is not impaired. The amount of these additions is preferably 40% by weight or less, more preferably more than 0% by weight and 20% by weight or less with respect to the resin component.

The separator includes (a) a step of melt-kneading the resin and the plasticizer, (b) a step of extruding the melt into a sheet and cooling and solidifying it, (c) a step of stretching the sheet in at least one axial direction, and (d). It can be suitably produced by a method for producing a separator, which comprises a step of extracting a plasticizer and (e) a step of heat-fixing.

The method for producing the separator will be described below. As long as the obtained separator satisfies 2.0 ≤ average flow rate diameter (μm) x air permeability (seconds / 100 mL) ≤ 8.0, the polymer type, solvent type, and extrusion The method, stretching method, extraction method, pore opening method, heat fixing / heat treatment method, and the like are not limited in any way.

In the method for producing a separator, the above-mentioned polyolefin is preferably used. If necessary, the polyolefin includes phenolic, phosphorus, sulfur and other antioxidants, metal soaps such as calcium stearate or zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, and antifogging agents. , Known additives such as coloring pigments can be mixed and used.

When the separator is a microporous film, (a) resin and plasticizer, (antioxidant and inorganic powder if necessary) are melt-kneaded (melt-kneading step), and (b) melt is sheeted. A step of extruding into a shape and cooling and solidifying (casting step), (c) a step of stretching at least in the uniaxial direction (stretching step), (d) a step of extracting a plasticizer and optionally an inorganic powder (extraction step), ( e) It can be obtained from the heat fixing step (heat fixing step).

The order and number of these steps are not particularly limited, but the following three types are preferable.
1. 1. (A) step-> (b) step-> (c) step-> (d) step-> (e) step 2. (A) step-> (b) step-> (c) step-> (d) step-> (c) step-> (e) step 3. The above 1 or 2 is more preferable than the step (a) → step (b) → step (d) → step (c) → step (e).

(A) Melt-kneading step The concentration of the antioxidant shall be 0.2% by weight (hereinafter sometimes referred to as "wt%") or more with respect to the total amount of the raw material resin from the viewpoint of preventing molecular deterioration. Is preferable, and from the viewpoint of economic efficiency, it is preferably 3 wt% or less, more preferably 0.4 wt% to 3 wt%, and even more preferably 0.5 wt% to 2 wt%. The raw material resin refers to a resin (preferably polyolefin) mixed with other organic materials (excluding plasticizers described later) and inorganic materials.

As the antioxidant, a phenolic antioxidant which is a primary antioxidant is preferable. Specifically, 2,6-di-t-butyl-4-methylphenol, pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) probioneto], Examples thereof include octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate. It should be noted that a secondary antioxidant can also be used in combination. Specifically, phosphorus-based antioxidants such as tris (2,4-di-t-butylphenyl) phosphite and tetrakis (2,4-di-t-butylphenyl) -4,4-biphenylene-diphosphonite. Examples thereof include sulfur-based antioxidants such as dilauryl-thio-dipropionate.

A plasticizer refers to a non-volatile solvent that can form a uniform solution above its melting point when mixed with a raw material resin. For example, liquid paraffin, hydrocarbons such as paraffin wax, di-2-ethylhexylphthalate (DOP), diisodecylphthalate, diheptylphthalate and the like can be mentioned. Of these, liquid paraffin is preferable.

The concentration of the raw material resin with respect to the total amount of the raw material resin and the plasticizer to be melt-kneaded is preferably 15 wt% to 60 wt%, more preferably 20 wt% to 50 wt% from the viewpoint of film permeability and film-forming property.

Polymers other than polyolefins and other materials can also be blended within a range that does not impair the film-forming property and does not impair the effects of the present invention. Further, if necessary, metal soaps such as calcium stearate and zinc stearate, and known additives such as ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments also impair film forming properties. It is possible to mix without impairing the effects of the present invention.

Examples of the melt-kneading method include a method of mixing with a Henschel mixer, a ribbon blender, a tumbler blender, or the like, and then melt-kneading with a screw extruder such as a single-screw extruder or a twin-screw extruder, a kneader, or a Banbury mixer. As a method of melt-kneading, it is preferable to use an extruder capable of continuous operation. A twin-screw extruder is more preferable from the viewpoint of kneadability. The plasticizer may be mixed with the raw material resin by the above Henschel mixer or the like. Alternatively, it may be directly fed to the extruder during melt kneading.

Further, the melt-kneading is preferably carried out in an atmosphere of an inert gas such as nitrogen from the viewpoint of preventing deterioration of the resin due to heat.

The temperature at the time of melt-kneading is preferably 140 ° C. or higher, more preferably 160 ° C. or higher, and even more preferably 180 ° C. or higher from the viewpoint of dispersibility. The temperature during melt-kneading is preferably 300 ° C. or lower, more preferably 280 ° C. or lower, and even more preferably 260 ° C. or lower from the viewpoint of preventing molecular deterioration.

(B) Casting Step As a method of forming the kneaded product obtained in the melt-kneading step into a sheet, a method of solidifying the melt by cooling can be mentioned. Examples of the cooling method include a method of directly contacting with a cooling medium such as cold air or cooling water, a method of contacting with a roll or a press machine cooled by a refrigerant, and the like. A method of contacting with a roll or a press machine cooled by a refrigerant is preferable in terms of excellent thickness control.

(C) Stretching Step Examples of the stretching method include sequential biaxial stretching by combining a roll stretching machine and a tenter, simultaneous biaxial stretching by simultaneous biaxial tenter and inflation molding, and the like. Above all, from the viewpoint of high strength, simultaneous biaxial stretching is preferable, and simultaneous biaxial stretching by a simultaneous biaxial tenter is more preferable.

The stretched surface magnification is preferably 20 times or more from the viewpoint of improving strength, and is preferably 100 times or less in order to prevent an increase in heat shrinkage stress due to excessive stretching. It is more preferably 28 to 60 times, and even more preferably 30 to 50 times.

The stretching temperature can be selected with reference to the raw material resin composition and / or the concentration. The stretching temperature is preferably in the range of the melting point of the kneaded resin obtained in the previous step (melting point −30 ° C.) to the melting point of the kneaded resin obtained in the previous step from the viewpoint of preventing fracture due to excessive stretching stress. The melting point of the kneaded resin obtained in the previous step is determined by a differential scanning calorimetry (DSC) measurement method.

(D) Extraction step It is desirable that the extraction solvent is a poor solvent for the polyolefin constituting the film, a good solvent for the plasticizer, and a boiling point lower than the melting point of the polyolefin constituting the film. .. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, hydrofluoroether and hydrofluorocarbon. Examples thereof include non-chlorine halogenated solvents, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, and ketones such as acetone and methyl ethyl ketone. It is appropriately selected from these and used alone or in combination. Of these, methylene chloride and methyl ethyl ketone are preferable. As a method for extracting the plasticizer, the sheet obtained in the casting step or the stretching step may be extracted by immersing it in these extraction solvents or showering, and then sufficiently dried.

Further, when the inorganic powder is added in the step (a), it is possible to extract a part or all of the inorganic powder with a solvent in which the inorganic powder is soluble.

(E) Heat fixing step The heat fixing is carried out by a tenter, a roll stretching machine or the like in a predetermined temperature atmosphere by combining low magnification stretching and relaxation operation.

The processing temperature of the heat fixing step is in the range of the molded product obtained in the previous step (melting point + 5 ° C.) to the molded product obtained in the previous step (melting point −10 ° C.). This temperature range is preferable for controlling the average flow rate diameter (μm) and air permeability (seconds / 100 mL), which are the parameters of the present invention, within an appropriate range. Here, the melting point of the molded product obtained in the previous step is determined by a differential scanning calorimetry (DSC) measurement method.

For example, when the melting point of the molded product obtained in the previous step is 134 ° C., the processing temperature in the heat fixing step is 124 ° C. or higher and 139 ° C. or lower.

The draw ratio in low-magnification stretching is preferably 1.1 times to 2.5 times, more preferably 1.2 times to 2.0 times, still more preferably 1. It is 3 times to 1.7 times. Excessive stretching is not preferable because it increases the possibility of film breakage.

The relaxation operation is an operation of slightly restoring the MD and / or TD dimensions of the film. The relaxation ratio with respect to the film size after low-magnification stretching is preferably 0.9 times or less, more preferably 0.85 times or less, still more preferably 0.8 times or less from the viewpoint of reducing heat shrinkage. Excessive mitigation is not desirable as it reduces productivity.

Other processes (functional layer forming process, winding process, slit process, surface modification process)
The method for producing the separator can optionally include other steps such as a functional layer forming step, a winding step, a slit step and the like. The functional layer forming step is preferably performed from the viewpoint of improving the safety of the lithium ion secondary battery, and specifically, the microporous film formed by the steps (a) to (e) is inorganic. The component-containing slurry can be applied and dried to form an inorganic coating layer, or the thermoplastic resin-containing slurry or latex can be applied and dried to form an adhesive layer.

Further, the obtained separator may be wound as a master roll, and if necessary, the master roll may be slit to form a slit roll.

If necessary, surface treatments such as electron beam irradiation, plasma irradiation, ion beam irradiation, surfactant coating, and chemical modification may be applied to the separator to the extent that the effects of the present invention are not impaired. It is possible. Further, norbornene resin or the like may be added in advance as a chemical cross-linking agent.

<Electrolyte solution>
The electrolyte solution generally contains a non-aqueous solvent and a metal salt such as a lithium salt, a sodium salt, or a calcium salt dissolved therein. As the non-aqueous solvent, a cyclic carbonate ester, a chain carbonate ester, a cyclic carboxylic acid ester and the like are used. Examples of lithium salts include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiAsF ₆ , and lower aliphatic carboxylics. Examples thereof include lithium acid, LiCl, LiBr, LiI, borates, imide salts and the like.

The electrolyte solution according to the present embodiment is preferably impregnated in the separator, the positive electrode active material layer, and the negative electrode active material layer from the viewpoint of achieving both high capacity and high output of the lithium ion secondary battery.

<Exterior body>
As the exterior body, a metal can, a laminated packaging material, or the like can be used. The metal can is preferably made of aluminum or an aluminum alloy. As the laminated packaging material, a resin film; a film obtained by laminating a metal foil and a resin film or the like is preferable.

The exterior body can incorporate an electrode laminate or an electrode winding body including a laminated structure of positive electrode / separator / negative electrode, and then the electrolyte solution can be injected and sealed through the opening of the exterior body. ..

Unless otherwise specified, the above-mentioned measurement methods for various parameters are measured according to the measurement methods in the examples described later.

Next, the present embodiment will be described more specifically with reference to Examples and Comparative Examples, but the present Embodiment is not limited to the following Examples as long as the gist of the present embodiment is not exceeded.

Unless otherwise specified, the above-mentioned various parameters were measured at room temperature of 23 ± 2 ° C. and humidity of 42 ± 2% based on the following test method.

<Separator>
Viscosity average molecular weight (Mv)
Based on ASTM-D4020, the ultimate viscosity [η] at 135 ° C. in a decalin solvent is determined. The Mv of polyethylene was calculated by the following formula.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
For polypropylene, Μv was calculated by the following formula.
[Η] = 1.10 × 10 ^-4 Mv ^0.80
In the following examples, examples using polyethylene and polypropylene are illustrated, but in each of the examples, since 90% or more of the resin constituting the separator is polyethylene, the viscosity average molecular weight of the separator in the examples is , Calculated by the above polyethylene formula.

Molecular weight distribution (Mw / Mn)
The molecular weight distribution Mw / Mn of the polymer material constituting the microporous membrane obtained in Examples and Comparative Examples was calculated by gel permeation chromatography (GPC) measurement. The device uses Waters' trademark, ALC / GPC 150-C type, and Toso Co., Ltd.'s trademark, TSK gel GMH6-HT's two 60 cm columns and Showa Denko's, trademark, AT- One 807 / S column is used in series and contains 1,2,4 containing 10 ppm pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate]. -Measurement was performed at 140 ° C. using trichlorobenzene as a mobile phase solvent.

Using a calibration line prepared using commercially available monodisperse polystyrene with a known molecular weight as a standard substance, 0.43 (polyethylene Q factor / polystyrene Q factor = 17) was added to the polystyrene-equivalent molecular weight distribution data of each sample obtained. By multiplying by .7 / 41.3.), Polystyrene-equivalent molecular weight distribution data was obtained.

Film thickness (μm)
The measurement was performed at room temperature of 23 ± 2 ° C. using a micro thickener manufactured by Toyo Seiki Co., Ltd., KBM ™.

Porosity (%)
A 10 cm x 10 cm square sample is cut from a microporous membrane, its volume (cm ³ ) and mass (g) are determined, and from these and the membrane density (density of the materials that make up the membrane) (g / cm ³ ), the following equation is obtained. Was calculated using.
Porosity = (volume-mass / membrane density) / volume x 100

Air permeability (seconds / 100 mL)
The air permeability is defined as the air permeability resistance measured by a Garley type air permeability meter (manufactured by Toyo Seiki Co., Ltd., GB2 (trademark)) in accordance with JIS P-8117.

Average flow diameter (μm)
The average flow rate diameter (μm) was measured using a palm pollometer (Porous Materials, Inc.: CFP-1500AE) in accordance with the half-dry method. Perfluoropolyester (trade name "Galwick", surface tension γ = 15.6 dyn / cm) manufactured by the same company was used for the immersion liquid. For the dry curve and the wet curve, the applied pressure and the amount of air permeation were measured, and from the pressure PHD (Pa) at which 1/2 of the obtained dry curve and the wet curve intersect, the average flow diameter was calculated by the following equation. Obtain dHD (μm).
dHD = 2860 x γ / PHD

Puncture strength (N / μm)
A microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm using a KES-G5 ™ handy compression tester manufactured by Kato Tech. Next, the central portion of the fixed microporous membrane was subjected to a puncture test with a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec in an atmosphere of 23 ± 2 ° C. to obtain a maximum piercing load (N). Asked.

Tensile strength (MPa), tensile elongation (%)
According to JIS K7127, MD and TD samples (shape; width 10 mm x length 100 mm) were measured using a tensile tester manufactured by Shimadzu Corporation, Autograph AG-A type (trademark). The sample used had a chuck distance of 50 mm, and tape (manufactured by Nitto Denko Packaging System Co., Ltd., trade name: N.29) was attached to one side of both ends (25 mm each) of the sample. Further, in order to prevent the sample from slipping during the test, a fluororubber having a thickness of 1 mm was attached to the inside of the chuck of the tensile tester.
The tensile elongation (%) was determined by dividing the elongation amount (mm) leading to fracture by the inter-chuck distance (50 mm) and multiplying by 100.
The tensile strength (MPa) was determined by dividing the strength at break by the sample cross-sectional area before the test. The measurement was carried out at a temperature of 23 ± 2 ° C., a chuck pressure of 0.30 MPa, and a tensile speed of 200 mm / min (for a sample in which a distance between chucks of 50 mm cannot be secured, a strain rate of 400% / min).

Melting points of raw materials and other resin samples Measured using DSC60 manufactured by Shimadzu Corporation. In the case of a sheet, it was punched into a circle with a diameter of 5 mm, and several sheets were stacked to make about 3 mg, which was used as a measurement sample. This was spread on an aluminum open sample pan having a diameter of 5 mm, a clamping cover was placed on the pan, and the sample sealer was used to fix the inside of the aluminum pan. The temperature was measured from 30 ° C. to 180 ° C. at a heating rate of 10 ° C./min under a nitrogen atmosphere, and the temperature at which the melting endothermic curve was maximized was taken as the melting point of the raw material and other resin samples.

<Electrode>
Thickness (μm)
The thickness of the electrode and the current collector was measured at room temperature of 23 ± 2 ° C. using a micro thickener manufactured by Toyo Seiki Co., Ltd., KBM ™.
The thickness of the positive electrode active material layer and the negative electrode active material layer was calculated according to the following formula.
Thickness of electrode active material layer = Thickness of electrode − Thickness of current collector The thickness of current collector is measured by measuring the thickness of the part where the electrode active material layer is not formed, such as the uncoated part of the electrode. The value is set to the value.

Void ratio of electrode active material layer The part where the electrode active material layer of the electrode is formed and the part where the electrode active material layer of the electrode is not formed are punched to a diameter of 5 cm, respectively, and the film thickness (μm) and mass (g) are determined. It was measured. The volume of the electrode active material layer (cm ³ ) was calculated from the thickness (μm) of the electrode active material layer and the electrode area (cm ^2). The porosity was calculated from them and the true density (g / cm ³ ) of the materials constituting the electrode active material layer using the following equation.
Void ratio = (Volume of electrode active material layer- (Mass of electrode-Mass of current collector) / True density of materials constituting the electrode active material layer) / Volume of electrode active material layer x 100
If the diameter is 5 cm and it cannot be punched out, measure it with the appropriate dimensions possible.

Battery performance evaluation

Preparation of positive electrode LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM523) 92% by weight as active material, acetylene black (AB) 4% by weight as conductive aid, and polyvinylidene fluoride (PVdF) 4 as binder A slurry was prepared by dispersing% by weight in N-methylpyrrolidone (NMP) using Philmix (manufactured by PRIMIX). This slurry was applied to one side of a 20 μm-thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression-molded with a roll press to form a positive electrode. At this time, the electrode density was 3.0 mAh / cm ² and the porosity was 34%. The obtained positive electrode was cut into a length of 60 mm and a width of 30 mm to prepare a positive electrode 1. At that time, 10 mm from one end in the vertical direction of the positive electrode was cut so as to be an uncoated portion, and the remaining 50 mm was cut so as to be a positive electrode active material layer forming portion. The positive electrode 2 and the positive electrode 3 were prepared so as to have the electrode density and the porosity shown in Table 1 in the same manner as in the production of the positive electrode 1 except for the amount of the slurry applied and the conditions of compression molding.

Preparation of Negative Electrode 80% by weight of silicon (Si) metal powder as active material, 5% by weight of acetylene black (AB) as conductive aid, and 15% by weight of polyimide (PI) as binder in N-methylpyrrolidone (NMP). A slurry was prepared by dispersing using a fill mix (manufactured by PRIMIX Corporation). This slurry was applied to one side of a copper foil having a thickness of 20 μm as a negative electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression-molded with a roll press to form a negative electrode. At this time, the electrode density was 3.3 mAh / cm ² , and the porosity was 2%. The obtained negative electrode was cut into a length of 62 mm and a width of 32 mm to prepare a negative electrode 1. At that time, 10 mm from one end in the vertical direction of the negative electrode was cut so as to be an uncoated portion, and the remaining 52 mm was cut so as to be a negative electrode active material layer forming portion. Similarly, a paste is prepared with the composition of the negative electrode active material layer shown in Table 2, and the electrode density shown in Table 3 is obtained by changing the coating amount of the slurry, and the voids shown in Table 3 are changed by changing the conditions of the roll press machine. Negative electrodes 2 to 9 were prepared so as to have a porosity.

_{Preparation of non-aqueous electrolyte solution In an argon box, 1% by weight of vinylene carbonate and LiPF 6} as a solute were concentrated in a mixed solvent of ethylene carbonate (EC): diethyl carbonate (DEC) = 1: 1 (volume ratio). A non-aqueous electrolyte solution was prepared by dissolving each of them so as to have a concentration of 1 M.

Battery assembly The positive electrode terminal (0.15 mm aluminum, Hosen Co., Ltd.) is attached to the uncoated part of the positive electrode, and the negative electrode terminal (0.15 mm nickel, Hosen Co., Ltd.) is attached to the uncoated part of the negative electrode. )) Was welded. The separator was cut into a length of 64 mm and a width of 34 mm.
Next, the negative electrode, the separator, and the positive electrode were stacked in this order to prepare a laminated structure. At this time, the surface on which the positive electrode active material layer of the positive electrode is formed and the surface on which the negative electrode active material layer of the negative electrode is formed face each other. Further, the positive electrode to which the positive electrode terminal was joined and the negative electrode to which the negative electrode terminal was joined were installed so as not to come into direct contact with each other including the terminal portion.
Prepare an exterior body in which a laminated film (EL408PH, manufactured by Dai Nippon Printing Co., Ltd.) is formed in a bag shape, insert the laminated structure, and place the laminated structure at a position where a part of the terminals protrudes from the exterior body. It was fixed to the laminate film with an imide tape (No. 360A # 25 × 8 mm × 20 m) cut to 8 mm.
Then, it was dried under vacuum at 65 ° C. for 12 hours. Next, the above-mentioned non-aqueous electrolytic solution was injected into the exterior body in the argon box, and the terminal was sealed so as to protrude from the exterior body. The amount (ml) of the non-aqueous electrolyte solution injected into the container was the total amount of voids (ml) between the electrode and the separator. The amount of voids was calculated from the porosity or porosity (%), the area (cm ² ), and the film thickness (μm).

DC-IR measurement As the initial charge / discharge of the assembled battery, a cycle of charging to a voltage of 4.2 V with a current amount of 0.05 C and discharging to a voltage of 2 V with a current amount of 0.05 C is performed in a constant temperature bath at 25 ° C. 3 I went there times. Next, the battery was charged to half the capacity (SOC50) of the discharge capacity of the third cycle, and charged and discharged with currents of 0.5C, 0.33C, 1C, and 2C for 10 seconds each. At this time, the horizontal axis was plotted with the amount of current and the vertical axis was plotted with the amount of voltage drop during discharge, and DC-IR (Ω) was obtained from the slope.
The value of 1C was calculated from the following formula.
1C = theoretical capacity of positive electrode active material mAh / g × content of positive electrode active material in positive electrode active material%% × mass of positive electrode active material layer g / 100

Negative electrode density In an argon box, a lithium metal with a thickness of 0.5 mm is cut into a length of 64 mm and a width of 34 mm, and a negative electrode terminal (0.15 mm nickel, Hosen Co., Ltd.) is located 10 mm from one end in the vertical direction. ) Was lightly pressed with a finger and crimped. Similar to the above-mentioned "battery assembly", the negative electrode with a terminal, the separator, and the lithium metal with a terminal were laminated to prepare a laminated structure, which was fixed to the inside of the exterior body with a polyimide tape. At that time, the drying, the injection of the electrolytic solution, and the sealing were performed in the same manner as in the above-mentioned "battery assembly".
The temperature was 25 ° C., the current value was 0.05C, the voltage range was 0.05V to 2.0V, and charging and discharging were performed three times.
The negative electrode density mAh / cm ² was calculated based on the following formula.
Negative electrode density mAh / cm ² = 3rd charge capacity mAh / (5.2 cm x 3.2 cm)
The value of 1C was calculated from the following formula.
For example, when two types of active substances A and B are mixed,
1C = ((Theoretical capacity of negative electrode active material A mAh / g × Content rate of active material A in the negative electrode active material layer) + (Theoretical capacity of negative electrode active material B mAh / g × Active material in the negative electrode active material layer) B content%)) × Mass of negative electrode active material layer g / 100

Positive Electrode Density Similar to the above "battery assembly" to "negative electrode density", a laminated structure is produced by stacking the positive electrode with terminals, the separator, and the lithium metal with terminals in an argon box to produce an exterior body. It was fixed inside with polyimide tape. In the same manner as in the above "battery assembly" to "negative electrode density", drying, injection of electrolytic solution, and sealing were performed.
The temperature was 25 ° C., the current value was 0.05 C, the voltage range was 2.5 V to 4.3 V, and charging and discharging were performed three times.
The positive electrode density was calculated based on the following formula.
Positive electrode density mAh / cm ² = 3rd charge capacity mAh / (5.0 cm x 3.0 cm)
The value of 1C was calculated in the same manner as for the negative electrode.

Separator film formation example Mv 250,000 homopolymer polyethylene is 47.5% by weight, Mv 700,000 homopolymer polyethylene is 47.5% by weight, and Mv 400,000 homopolymer polypropylene is 5% by weight. , Dry blended using a tumbler blender. To 99% by weight of the obtained pure polymer mixture, 1% by weight of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] was added as an antioxidant, and the tumbler was again added. A mixture such as a polymer was obtained by dry blending using a blender. The obtained mixture such as polymer was replaced with nitrogen and then supplied to a twin-screw extruder by a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C., 7.59 × ^10-5 m ² / sec) was injected into the extruder cylinder by a plunger pump.

The feeder and pump were adjusted so that the ratio of the amount of liquid paraffin to the total mixture extruded by melt-kneading was 67% by weight. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 330 rpm, and a discharge rate of 300 kg / h. Subsequently, the melt-kneaded product was extruded and cast on a cooling roll controlled to have a surface temperature of 25 ° C. via a T-die to obtain a gel sheet having a thickness of 1720 μm. Next, it was guided to a simultaneous biaxial tenter stretching machine to perform biaxial stretching. The set stretching conditions are an MD magnification of 7.0 times, a TD magnification of 6.5 times, and a set temperature of 122 ° C. Next, the drawn product was led to a methylene chloride tank and sufficiently immersed in methylene chloride to extract and remove liquid paraffin, and then methylene chloride was dried and removed. Next, it was guided to a TD tenter, stretched 2.0 times in the TD direction, and then heat-fixed at a relaxation rate of 0.90 times and a temperature of 128 ° C. Various physical properties were evaluated according to the above method.

By changing the molecular weight of polyethylene, the amount of inorganic substances added, the amount of discharge, the thickness of the gel sheet, the stretching ratio and the temperature based on this film forming example, the air permeability (seconds) and the average flow rate diameter shown in Table 4 below are changed. A plurality of microporous membranes having (μm) were prepared and used as separators.

Examples 1 to 17, Comparative Examples 1 to 6
The positive electrodes 1 to 3 obtained by the above "preparation of positive electrode" and
Negative electrodes 1 to 9 obtained by the above "preparation of negative electrode" and
Various microporous membranes as separators having the air permeability (seconds) and average flow rate diameter (μm) shown in Table 4 produced based on the above "Example of Separator Membrane Formation", and
A battery was formed according to the above-mentioned "battery assembly" using the non-aqueous electrolytic solution obtained by the above-mentioned "preparation of non-aqueous electrolytic solution", and further evaluated by the above-mentioned "DC-IR measurement". The evaluation results are shown in Table 5.

From Table 5, when the present embodiment is followed, the negative electrode density is as ^{high as 2.5 mAh / cm 2} or more, and the DC-IR is as high as 3.0 or less, resulting in high capacity and high output. Can be compatible.

Claims (3)

A positive electrode in which a positive electrode active material layer containing a positive electrode active material is arranged on a positive electrode current collector,
A negative electrode in which a negative electrode active material layer containing a negative electrode active material is arranged on a negative electrode current collector,
With an electrically insulating and porous separator,
An electrolyte solution containing a lithium salt and impregnating the separator, the positive electrode active material layer, and the negative electrode active material layer.
With
The positive electrode and the negative electrode are arranged so that the positive electrode active material layer and the negative electrode active material layer face each other so as to sandwich the separator.
The electrode density of the negative electrode is 2.5 mAh / cm ^{2 to} 5.0 mAh / cm ² .
Of the negative electrode active material contained in the negative electrode active material layer, 30% by weight to 100% by weight is a negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg, and the average flow diameter (μm) of the separator. ) × A lithium ion secondary battery having a value of air permeability (seconds / 100 mL) of 2.0 to 8.0. The lithium ion secondary battery according to claim 1, wherein the negative electrode active material having a theoretical capacity of 700 Ah / kg to 4000 Ah / kg is a silicon (Si) alloy or silicon monoxide (SiO). The lithium ion secondary battery according to claim 1 or 2, which contains 3% by weight to 30% by weight of polyimide of the negative electrode active material layer.