WO2008035806A1 - Electrode assembly and non-aqueous electrolyte battery - Google Patents

Abstract

An electrode assembly having a structure comprising a first microporous membrane separator comprising high-density polyethylene A having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.3 or more per 10,000 carbon atoms and ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 1x106 or more, and a second microporous membrane separator comprising high-density polyethylene B having a terminal vinyl group concentration of 0.2 or less per 10,000 carbon atoms and said ultra-high-molecular-weight polyethylene, which are alternately sandwiched by an anode sheet and a cathode sheet, the difference in a shutdown temperature between said first and second separators being within 5 degrees centigrade.

Description

0ESCRIPTION

ELECTRODE ASSEMBLY AND NON- AQUEOUS ELECTROLYTE BATTERY

FIELD OF THE INVENTION

[0001] The present invention relates to an electrode assembly for a battery having well-balanced safety, discharge capacity and cyclability, and a non-aqueous electrolyte battery comprising such an electrode assembly.

BACKGROUND OF THE INVENTION

[0002] A lithium-ion secondary battery has an electrode assembly comprising an anode and a cathode laminated via a separator. Specific examples of the electrode assembly include a laminate in which planar cathodes and anodes are alternately laminated via separators, a toroidal electrode in which an anode ribbon and a cathode ribbon are wound via separators, etc. Separators are usually constituted by microporous membranes of thermoplastic resins.

[0003] Because the performance of the electrode assembly affecting the properties, productivity and safety of batteries largely depends on the performance of the separator, various proposals have been made to provide combinations of separators with different properties, separators with improved shapes, chemically modified (for example, hydrophilized) separators, etc.

[0004] As an electrode assembly providing good battery longevity, JP 10-199502 A proposes an electrode assembly comprising a fused laminate of a first separator with high tensile strength (for example, nonwoven polyolefin fabric) and a second separator with good storage properties and electrolytic solution absorption (for example, nonwoven fabric of polyethylene terephthalate), the assembly having a structure in which a cathode plate is in contact with the first separator, and an anode plate is in contact with the second separator. [0005] As a secondary battery free from internal short-circuiting due to the detachment of active materials from electrode plates, JP 2000-315489 A proposes a rectangular, non-aqueous-electrolyte, secondary battery comprising alternately laminated planar cathodes and planar anodes, the planar anodes being contained in a separator bag with good shutdown properties, the planar cathodes being contained in a separator bag with good meltdown properties, and each separator bag being made of porous polypropylene or porous polyethylene.

[0006] As a secondary battery with good self-discharge resistance and cyclability, JP 2003-257474 A proposes a nickel-hydrogen secondary battery comprising an electrode assembly constituted by planar anodes and planar cathodes alternately laminated via separators, the separators being constituted by alternately arranged first and second separators, each first separator being a nonwoven polyolefin fabric grafted with a vinyl monomer having a carboxyl group or treated with an acid having a sulfuric group, and each second separator being a nonwoven polyolefin fabric treated with corona discharge or a fluorine gas.

[0007] As a non-aqueous electrolyte battery with good cyclability and resistance to internal short-circuiting between anodes and cathodes, JP 2004-193116 A proposes a non-aqueous electrolyte battery having a toroidal composite structure, in which a first separator (for example, microporous polyethylene membrane having a mode diameter of 0.3 μm or less when measured by mercury intrusion porosimetry) is in contact with an outer surface of a cathode ribbon, and a second separator (for example, microporous polyethylene membrane having a mode diameter of 0.5 μm or more when measured by mercury intrusion porosimetry) is in contact with an inner surface of the cathode ribbon, the first separator being lower in lithium ion permeability than the second separator.

[0008] As a non-aqueous electrolyte battery free from thermal runaway in overcharging or abnormal heating, JP 2005-93077 A proposes a non-aqueous-electrolyte secondary battery comprising a toroidal electrode assembly and a non-aqueous electrolyte, the toroidal electrode assembly being obtained by laminating an anode and a cathode via first and second separators (for example, microporous polyethylene membranes) having different permeability and winding them, the first separator in contact with an outer surface of the cathode having permeability of 180 sec/100 cm³ or more, and the second separator in contact with an inner surface of the cathode having permeability of 120 sec/100 cm³ or less. [0009] As a non-aqueous electrolyte battery free from thermal runaway in overcharging or abnormal heating, JP 2005-93078 A proposes a non-aqueous-electrolyte secondary battery comprising a toroidal electrode assembly and a non-aqueous electrolyte, the toroidal electrode assembly being obtained by laminating an anode and a cathode via first and second separators (for example, microporous polyethylene membranes) having different properties and winding them, the first separator in contact with an outer surface of the cathode having permeability of 400 sec/ 100 cm³ or less, and the second separator in contact with an inner surface of the cathode having a heat shrinkage ratio of 30% or less in TD when measured at 150⁰C for 3 hours.

[0010] However, any batteries comprising the electrode assemblies disclosed in JP 10-199502 A, JP 2000-315489 A, JP 2003-257474 A, JP 2004-193116 A, JP 2005-93077 A and JP 2005-93078 A do not have well-balanced safety, discharge capacity and cyclability. Thus, electrode assemblies providing batteries with well-balanced safety, discharge capacity and cyclability are desired.

OBJECTS OF THE INVENTION

[0011] Accordingly, an object of the present invention is to provide an electrode assembly providing a battery with well-balanced safety, discharge capacity and cyclability.

[0012] Another object of the present invention is to provide a non-aqueous electrolyte battery comprising such an electrode assembly.

DISCLOSURE OF THE INVENTION [0013] As a result of intensive research in view of the above objects, the inventors have found that (1) a composite separator having well-balanced permeability, mechanical strength, heat shrinkage resistance, shutdown properties, meltdown properties and compression resistance can be obtained by combining a first microporous membrane comprising high-density polyethylene A having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.3 or more per 10,000 carbon atoms and ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 1 x 10⁶ or more, and a second polyolefin membrane comprising high-density polyethylene B having the terminal vinyl group concentration of 0.2 or less per 10,000 carbon atoms and the above ultra-high-molecular-weight polyethylene, and that (2) a battery having well-balanced safety, discharge capacity and cyclability can be obtained by using an electrode assembly constituted by laminating an anode sheet and a cathode sheet via two separators of the above microporous membranes alternately. The present invention has been completed based on such findings.

[0014] Thus, the electrode assembly of the present invention has a structure comprising a first microporous membrane separator comprising high-density polyethylene A having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.3 or more per 10,000 carbon atoms and ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 1 x 10⁶ or more, and a second microporous membrane separator comprising high-density polyethylene B having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.2 or less per 10,000 carbon atoms and the above ultra-high-molecular-weight polyethylene, which are alternately sandwiched by an anode sheet and a cathode sheet, the difference in a shutdown temperature between said first and second separators being within 5⁰C.

[0015] In a preferred embodiment of the present invention, the electrode assembly is in a toroidal form. To provide a battery with improved nail penetration safety, it is preferable that the first microporous membrane separator is arranged on an inner surface of said cathode sheet, while said second microporous membrane separator is arranged on an outer surface of said cathode sheet.

[0016] The non-aqueous electrolyte battery of the present invention comprises the above electrode assembly.

BRIEF DESCRIPTION OF THE DRAWING

[0017] Fig. 1 is a partially cross-sectional, perspective view showing one example of cylindrical lithium-ion secondary batteries comprising the electrode assembly of the present invention.

[0018] Fig. 2 is a transverse cross-sectional view showing the battery of Fig. 1.

[0019] Fig. 3 is an enlarged cross-sectional view showing a portion A in Fig. 2.

[0020] Fig. 4(a) is a partial cross-sectional view showing an end portion of an anode sheet in the electrode assembly of Fig. 1.

[0021] Fig. 4(b) is a partial cross-sectional view showing an end portion of a cathode sheet in the electrode assembly of Fig. 1.

[0022] Fig. 5 is a schematic view showing a method for measuring a shutdown temperature and a meltdown temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] [ 1 ] Composition of separator

[0024] The first separator is formed by a microporous membrane made of a first polyolefin, and the second separator is formed by a microporous membrane made of a second polyolefin.

[0025] (A) First polyolefin

[0026] The first polyolefin is (a) a mixture (polyethylene composition A) of high-density polyethylene A having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.3 or more per 10,000 carbon atoms and ultra-high-molecular-weight polyethylene having a weight-average molecular weight

(Mw) of 1 x 10⁶ or more, or (b) a mixture (polyethylene composition A') of the polyethylene composition A and polyethylene other than the high-density polyethylene A and the ultra-high-molecular-weight polyethylene. [0027] (1) Polyethylene composition A [0028] (i) High-density polyethylene A

[0029] The high-density polyethylene A has a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.3 or more per 10,000 carbon atoms (hereinafter expressed by '710,000 C"). When the terminal vinyl group concentration is less than 0.3/10,000 C, the first separator has poor shutdown properties. The terminal vinyl group concentration of the high-density polyethylene A is preferably 0.4/10,000 C or more, more preferably 0.5/10,000 C or more. [0030] The terminal vinyl group concentration is expressed by the number of terminal vinyl groups per 10,000 carbon atoms in the high-density polyethylene A. The terminal vinyl group concentration is measured by infrared spectroscopy. Specifically, it is measured by (i) forming the sample of the thickness of about 1 mm by heat-pressing a pellet of the high-density polyethylene A, (2) measuring the absorbance A = log(Io/I) at 910 cm^"1, wherein I₀ represents the intensity of transmitted light of a blank cell, and I represents the intensity of transmitted light of a sample cell by a Fourier transform infrared spectrophotometer, and (3) calculating the formula of terminal vinyl group concentration^ 10,000 C) = (1.14 x absorbance A)/ (density (g/cm³) of high-density polyethylene A x thickness (mm) of sample). [0031 ] A weight-average molecular weight (Mw) of the high-density polyethylene A is preferably 1 x 10⁴ to 5 x 10⁵, more preferably 1 x 10⁵ to 5 x 10⁵, most preferably 2 x 10⁵ to 4 x 10³. When the Mw of the high-density polyethylene A is less than 5 x 10⁵, the first separator has a large average pore diameter. A density of the high-density polyethylene A is usually 0.90-0.98 g/cm³, preferably 0.93-0.97 g/cm³, more preferably 0.94-0.96 g/cm³.

[0032] The high-density polyethylene A is not limited to an ethylene homopolymer, but may be a copolymer containing a small amount of other α-olefins. The α-olefins other than ethylene are preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, and styrene. [0033] The high-density polyethylene A can be produced by suspension polymerization, solution polymerization or gas phase polymerization using a chromium-compound-carried catalyst in combination with an organometallic compound disclosed, for example, in JP 1-12777 B. [0034] (ii) Ultra-high-molecular-weight polyethylene

[0035] The ultra-high-molecular-weight polyethylene has Mw of 1 x 10⁶ or more. The ultra-high-molecular-weight polyethylene may be an ethylene homopolymer, or an ethylene- α-olefin copolymer containing a small amount of the other α-olefin. The α-olefin other than ethylene is preferably propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-l, octene-1, vinyl acetate, methyl methacrylate, or styrene. The Mw of the ultra-high-molecular-weight polyethylene is preferably 1 x 10⁶ to 15 x 10⁶, more preferably 1 x 10⁶ to 5 x 10⁶, most preferably 1 x 10⁶ to 3 x 10⁶. [0036] (iii) Formulation and Weight-average molecular weight [0037] The percentage of the ultra-high-molecular- weight polyethylene in the polyethylene composition A is preferably 1 % or more by mass, more preferably 2 to 60% by mass, most preferably 2 to 50% by mass. The Mw of the polyethylene composition A is preferably 1 x 10⁴ to 5 x 10⁶, more preferably 1 x 10⁵ to 4 x 10⁶, most preferably 2 x 10⁵ to 1.5 x 10⁶. When the Mw of the polyethylene composition A is less than 1.5 x 10⁶, the first separator has a large average pore diameter. [0038] (b) Polyethylene composition A'

[0039] The polyethylene composition A' is a mixture of the polyethylene composition A and polyethylene other than the high-density polyethylene A and the ultra-high-molecular-weight polyethylene. The polyethylene composition A may be the same as described above. The other polyethylene than the high-density polyethylene A and the ultra-high-molecular-weight polyethylene is preferably at least one selected from the group consisting of, medium-density polyethylene, branched low-density polyethylene, and linear low-density polyethylene, and its Mw is preferably 1 x 10⁴ to 5 x 10⁵. The other polyethylene than the high-density polyethylene A and the ultra-high-molecular-weight polyethylene is not limited to an ethylene homopolymer, but may be a copolymer containing a small amount of the other α-olefin such as propylene, butene-1, hexene-1, etc. The percentage of the other polyethylene than the high-density polyethylene A and the ultra-high-molecular-weight polyethylene in the polyethylene composition A' is preferably 50% or less by mass, more preferably 20% or less by mass. [0040] (c) Molecular weight distribution Mw/Mn

[0041] Mw/Mn is a measure of a molecular weight distribution; the larger this value, the wider the molecular weight distribution. The Mw/Mn of the first polyolefin is preferably 5-300, more preferably 5-100, most preferably 5-30 in the ultra-high-molecular- weight polyethylene and the other polyethylene. When the Mw/Mn is less than 5, the percentage of a high-molecular-weight component is too high to conduct melt extrusion easily. On the other hand, when the Mw/Mn is more than 300, the percentage of a low-molecular-weight component is too high, resulting in decrease in the strength of the microporous membrane. The Mw/Mn of polyethylene (homopolymer or an ethylene α-olefϊn copolymer) can be properly controlled by a multi-stage polymerization. The multi-stage polymerization method is preferably a two-stage polymerization method comprising forming a high-molecular-weight polymer component in the first stage, and forming a low-molecular-weight polymer component in the second stage. In the case of the polyethylene composition, the larger the Mw/Mn, the larger difference in Mw exists between the ultra-high-molecular-weight polyethylene and the other polyethylene, and vice versa. The Mw/Mn of the polyethylene composition can be properly controlled by the molecular weights and mixing ratios of components. [0042] (2) Other components

[0043] The first polyolefin may contain, in addition to the above components (1), other polyolefins than the first polyolefin, or heat-resistant resins having melting points or glass transition temperatures (Tg) of 170⁰C or higher, in amounts not deteriorating the properties of the separator. [0044] (a) Other Polyolefins

[0045] The other polyolefins than the first polyolefin may be at least one selected from the group consisting of (a) polypropylene, polybutene-1, polypentene-1, poly-4-methylpentene-l, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate, polystyrene and an ethylene α-olefin copolymer, each of which may have Mw of 1 x 10⁴ to 4 x 10⁶, and (b) a polyethylene wax having Mw of 1 x 10³ to 1 x 10⁴. Polypropylene, polybutene-1, polypentene-1, poly-4-methylpentene-l, polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate and polystyrene are not restricted to homopolymers, but may be copolymers containing other α-olefins.

[0046] (b) Heat-resistant resins

[0047] The heat-resistant resins are preferably crystalline resins having melting points of 170⁰C or higher, which may be partially crystalline, and amorphous resins having Tg of 170⁰C or higher. The melting point and Tg are determined by differential scanning calorimetry (DSC) according to JIS K7121. Specific examples of the heat-resistant resins include polyesters such as polybutylene terephthalate (melting point: about 160-230⁰C), polyethylene terephthalate (melting point: about 250-270⁰C), etc., fluororesins, polyamides (melting point: 215-265°C), polyarylene sulfide, polyimides (Tg: 280⁰C or higher), polyamideimides (Tg: 280⁰C), polyether sulfone (Tg: 223°C), polyetheretherketone (melting point: 334°C), polycarbonates (melting point: 220-240⁰C), cellulose acetate (melting point: 220⁰C), cellulose triacetate (melting point: 300⁰C), polysulfone (Tg: 190⁰C), polyetherimide (melting point: 216°C), etc. [0048] (B) Second polyolefin

[0049] The second polyolefin is (a) a mixture (polyethylene composition B) of high-density polyethylene B having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.2 or less per 10,000 carbon atoms and the ultra-high-molecular-weight polyethylene, or (b) a mixture (polyethylene composition B') of the polyethylene composition B and polyethylene other than the high-density polyethylene B and the ultra-high-molecular-weight polyethylene.

[0050] (1) Polyethylene composition B

[0051 ] (i) High-density polyethylene B

[0052] The high-density polyethylene B has a terminal vinyl group concentration

(measured by infrared spectroscopy) of 0.2/10,000 C or less. When the terminal vinyl group concentration is more than 0.2/10,000 C, the second separator has poor mechanical strength and meltdown properties. The terminal vinyl group concentration of the high-density polyethylene B is preferably 0.15/10,000 C or less.

The terminal vinyl group concentration of the high-density polyethylene B is measured by the same method as in the high-density polyethylene A.

[0053] The Mw and density of the high-density polyethylene B may be the same as those of the high-density polyethylene A. When the Mw of the high-density polyethylene B is less than 5 x 10³, the second separator has a large average pore diameter. The high-density polyethylene B is preferably an ethylene homopolymer, but may be a copolymer containing a small amount of the other α-olefϊn. The other α-olefin may be the same as described above.

[0054] The above high-density polyethylene B can be produced by suspension polymerization, solution polymerization or gas phase polymerization using a Ziegler catalyst containing a magnesium compound disclosed, for example, in JP 1-12777 B.

[0055] (ii) Ultra-high-molecular-weight polyethylene

[0056] The ultra-high-molecular-weight polyethylene may be the same as described above. The percentage of the ultra-high-molecular-weight polyethylene in the polyethylene composition B may be the same as that in the polyethylene composition A. The Mw of the polyethylene composition B may be the same as that of the polyethylene composition A.

[0057] (b) Polyethylene composition B'

[0058] The polyethylene composition B' is a mixture of the polyethylene composition B and polyethylene other than the high-density polyethylene B and the ultra-high-molecular-weight polyethylene. The polyethylene composition B may be the same as described above. The other polyethylene than the high-density polyethylene B and the ultra-high-molecular-weight polyethylene may be the same as the other polyethylene than the high-density polyethylene A and the ultra-high-molecular-weight polyethylene in the polyethylene composition A' . The percentage of the other polyethylene than the high-density polyethylene B and the ultra-high-molecular-weight polyethylene is preferably 50% or less by mass, more preferably 20% or less by mass.

[0059] (c) Molecular weight distribution Mw/Mn

[0060] The Mw/Mn of the second polyolefin may be the same as that of the first polyolefϊn.

[0061 ] (2) Other components

[0062] The second polyolefin may contain, in addition to the above components

(1), other polyolefins than the second polyolefin, or heat-resistant resins having melting points or glass transition temperatures (Tg) of 170⁰C or higher, in amounts not deteriorating the properties of the separator. The other polyolefins than the second polyolefin may be the same as the other polyolefins than the first polyolefin.

The heat-resistant resins may be the same as described above.

[0063] [2] Production method of separator

[0064] (A) Production method of first separator

[0065] The method for producing a first separator comprises the steps of (1) melt-blending a first polyolefin and a membrane-forming solvent to prepare a first polyolefin solution, (2) extruding the first polyolefin solution through a die, (3) cooling the extrudate to form a gel-like sheet, (4) stretching the gel-like sheet, (5) removing the membrane-forming solvent from the gel-like sheet, and (6) drying the resultant membrane. The membrane may be slit, if necessary (Slitting step(7)).

After the step (6), a step (8) of stretching the microporous membrane, a heat treatment step (9), a step (10) of cross-linking with ionizing radiations, a hydrophilizing treatment step (11), etc. may be conducted, if necessary.

[0066] (1) Preparation of polyolefin solution [0067] The first polyolefin is melt-blended with a membrane-forming solvent to prepare a first polyolefin solution. The first polyolefin solution may contain various additives such as antioxidants, fine silicate powder (pore-forming agent), etc. in ranges not deteriorating the effects of the present invention, if necessary. [0068] The membrane-forming solvent is preferably liquid at room temperature. The use of a liquid solvent makes it possible to conduct stretching at a relatively high magnification. The liquid solvents may be aliphatic, alicyclic or aromatic hydrocarbons such as nonane, decane, decalin, p-xylene, undecane, dodecane, liquid paraffin, etc., mineral oil distillates having boiling points comparable to those of the above hydrocarbons, and phthalates liquid at room temperature such as dibutyl phthalate, dioctyl phthalate, etc. To obtain a gel-like laminate sheet having a stable liquid solvent content, it is preferable to use non-volatile liquid solvents such as liquid paraffin. A solvent which is miscible with polyethylene in a melt-blended state but solid at room temperature may be mixed with the liquid solvent. Such solid solvent includes stearyl alcohol, ceryl alcohol, paraffin waxes, etc. However, when only a solid solvent is used, uneven stretching, etc. are likely to occur. [0069] The viscosity of the liquid solvent is preferably 30-500 cSt, more preferably 30-200 cSt, at 25°C. When the viscosity at 25°C is less than 30 cSt, the first polyolefin solution is easily foamed, resulting in difficulty in blending. On the other hand, when the viscosity is more than 500 cSt, the removal of the liquid solvent is difficult.

[0070] Though not particularly restricted, the uniform melt-blending of the first polyolefin solution is preferably conducted in a double-screw extruder to prepare a high-concentration polyolefin solution. The membrane-forming solvent may be added before blending, or charged into the double-screw extruder in an intermediate portion during blending, though the latter is more preferable.

[0071] The melt-blending temperature of the polyolefin solution is preferably in a range of the melting point Tm₃ of the first polyolefin + 10⁰C to Tm₃ + 120⁰C. The melting point is measured by differential scanning calorimetry (DSC) according to JIS K7121. Specifically, the melt-blending temperature is preferably 140-250⁰C, more preferably 170-240⁰C, because the above polyethylene compositions A and A' have melting points of about 130-140⁰C.

[0072] A ratio L/D of the screw length L to the screw diameter D in the double-screw extruder is preferably in a range of 20-100, more preferably in a range of 35-70. When L/D is less than 20, melt-blending is insufficient. When L/D is more than 100, the residing time of the polyolefϊn solution in the double-screw extruder is too long. The cylinder of the double-screw extruder preferably has an inner diameter of 40-100 mm.

[0073] The percentage of the first polyolefm is preferably 1-75% by mass, more preferably 20-70% by mass, per 100% by mass of the polyolefm solution. When the polyolefin is less than 1% by mass, the productivity is low. In addition, large swelling or neck-in occurs at the die exit during forming a gel-like sheet, resulting in decrease in the formability and self-supportability of the gel-like sheet. On the other hand, when the polyolefin is more than 75% by mass, the formability of the gel-like sheet is deteriorated. [0074] (2) Extrusion

[0075] The polyolefin solution melt-blended in the extruder is extruded from a die immediately or after pelletized. The die used is usually a sheet-forming die having a rectangular-cross-section orifice, though a double-cylindrical, hollow die, an inflation die lip, etc. may also be used. In the case of the sheet-forming die, the die gap is usually 0.1-5 mm, and it is heated at 140-250⁰C during extrusion. The extrusion speed of the heated solution is preferably 0.2 to 15 m/minute. [0076] (3) Formation of gel-like sheet

[0077] The polyolefin solution extruded from the die is cooled to form a gel-like sheet. Cooling is preferably conducted at least to a gelation temperature at a speed of 50°C/minute or more. Cooling is preferably conducted to 25⁰C or lower. Such cooling sets the micro-phases of the polyolefin separated by the membrane-forming solvent. Generally, the lower cooling speed provides the gel-like sheet with larger pseudo-cell units, resulting in a coarser higher-order structure. On the other hand, the higher cooling speed results in denser cell units. The cooling speed less than 50°C/minute leads to increased crystallinity, making it unlikely to provide the gel-like sheet with suitable stretchability. Usable as the cooling method are a method of bringing the extrudate into direct contact with a cooling medium such as cooling air, cooling water, etc., a method of bringing the extrudate into contact with rolls cooled by a cooling medium, etc. [0078] (4) Stretching of gel-like sheet

[0079] The resultant gel-like sheet is stretched in at least one direction. The gel-like sheet can be uniformly stretched because it contains the membrane-forming solvent. The gel-like sheet is stretched to a predetermined magnification after heated, by a tenter method, a roll method, an inflation method or a combination thereof. The stretching may be conducted monoaxially or biaxially, though the biaxial stretching is preferable. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching or multi-stage stretching (for instance, a combination of the simultaneous biaxial stretching and the sequential stretching) may be used, though the simultaneous biaxial stretching is preferable. [0080] The stretching magnification is preferably 2 folds or more, more preferably 3-30 folds in the monoaxial stretching. In the biaxial stretching, the stretching magnification is preferably 3 folds or more in any direction (preferably 9 folds or more, more preferably 16 folds or more, most preferably 25 or more in area magnification). With the area magnification of 9 folds or more, the pin puncture strength of the microporous membrane is improved. When the area magnification is more than 400 folds, stretching apparatuses, stretching operations, etc. are restricted. [0081] The stretching temperature is preferably the melting point Tm₃ of the first polyolefin + 1O⁰C or lower, more preferably in a range of the crystal dispersion temperature Tcd_a of the first polyolefin or higher and lower than Tm₃. When the stretching temperature is higher than the Tm₃ + 10⁰C, the first polyolefin is molten, failing to orient molecular chains by stretching. When the stretching temperature is lower than the Tcd_a, the first polyolefin is so insufficiently softened that the microporous membrane is easily broken by stretching, failing to achieve high-magnification stretching.

[0082] The polyethylene compositions A and A' have Tm₃ of about 130-140⁰C and Tcd_a of about 90-100⁰C. Tcd_a is determined from the temperature characteristics of the dynamic viscoelasticity of the polyethylene resin measured according to ASTM D 4065. Accordingly, the stretching temperature is 90-140⁰C, preferably 100-130⁰C. [0083] Such stretching causes cleavage between polyethylene lamellas, making the polyethylene phases finer and forming large numbers of fibrils. The fibrils form a three-dimensional network structure (three-dimensionally irregularly connected network structure). The stretching improves the mechanical strength of the microporous membrane and expands its pores, making the microporous membrane particularly suitable for battery separators.

[0084] Depending on the desired properties, stretching may be conducted with a temperature distribution in a thickness direction to provide the microporous polyolefin membrane with excellent mechanical strength. The details of this method are described in Japanese Patent 3347854. [0085] (5) Removal of membrane-forming solvent

[0086] For the purpose of removing (washing away) the membrane-forming solvent, a washing solvent is used. Because the first polyolefin phase is separated from a membrane-forming solvent phase, the removal of the membrane-forming solvent provides a porous membrane. The removal (washing away) of the membrane-forming solvent can be conducted by using known washing solvents. The washing solvents include volatile solvents, such as saturated hydrocarbons such as pentane, hexane, heptane, etc., chlorinated hydrocarbons such as methylene chloride, carbon tetrachloride, etc., ethers such as diethyl ether, dioxane, etc., ketones such as methyl ethyl ketone, etc., linear fluorocarbons such as trifluoroethane, C₆Fi₄, C₇Fi₆, etc., cyclic hydrofluorocarbons such as CsH₃F₇, etc., hydrofluoroethers such as C₄F₉OCH₃, C₄F₉OC₂H₅, etc., perfluoroethers such as C₄F₉OCF₃, C₄F₉OC₂F₅, etc. [0087] The washing of the microporous membrane after stretching can be conducted by immersion in the washing solvent and/or the showering of the washing solvent. The washing solvent used is preferably 300-30,000 parts by mass per 100 parts by mass of the stretched membrane. The washing temperature is usually

15-30⁰C, and if necessary, heating may be conducted during washing. The heating temperature during washing is preferably 80⁰C or lower. Washing with the washing solvent is preferably conducted until the amount of the remaining membrane-forming solvent becomes less than 1% by mass of that added.

[0088] (6) Drying

[0089] The microporous polyolefin membrane obtained by stretching and the removal of the membrane-forming solvent is dried by a heat-drying method, a wind-drying method, etc. The drying temperature is preferably equal to or lower than the above Tcd_a, particularly 5°C or more lower than the Tcd_a. Drying is conducted until the remaining washing solvent becomes preferably 5% or less by mass, more preferably 3% or less by mass, per 100% by mass (on a dry basis) of the microporous membrane. Insufficient drying undesirably leads to decrease in the porosity of the microporous membrane by a subsequent heat treatment, resulting in poor permeability.

[0090] (7) Slitting of microporous membrane

[0091] The dried microporous membrane is slit. A known slitter can be used.

The width of the separator, if necessary, can be properly controlled. The width of the separator is preferably 5 to 200 mm.

[0092] (8) Stretching of microporous membrane

[0093] The dried microporous membrane may be stretched (re-stretched) at least monoaxially. The re-stretching may be conducted by a tenter method, etc. while heating the microporous membrane likewise above. The re-stretching may be monoaxial or biaxial. In the case of the biaxial stretching, simultaneous biaxial stretching or sequential stretching may be used, though the simultaneous biaxial stretching is preferable. Incidentally, because the re-stretching is usually conducted on the microporous membrane in a long sheet form, which is obtained from the stretched gel-like sheet, the directions of longitudinal and transverse in the re-stretching is the same as those in the stretching of the gel-like sheet. This is true in other production methods.

[0094] The stretching temperature of the microporous membrane is preferably the melting point Tm₃ of the first polyolefin or lower, more preferably in a range from the

Tcd_a to the Tm₃. The polyethylene compositions A and A' have Tm₃ of about

130-140⁰C. Accordingly, the stretching temperature is 90-135⁰C, preferably

95-130⁰C.

[0095] The monoaxial re-stretching magnification is 1.1-1.8 folds. In the case of monoaxial stretching, it is 1.1-1.8 folds in a longitudinal or transverse direction. In the case of biaxial stretching, it is 1.1-1.8 folds in both longitudinal and transverse directions, which may be the same or different in the longitudinal and transverse directions, though the same re-stretching magnification is preferable in both directions.

[0096] When the stretching magnification of the microporous membrane is less than 1.1 folds, the resultant membrane has insufficient permeability, electrolytic solution absorption and compression resistance. When this magnification is more than 1.8 folds, too fine fibrils are formed, and the heat shrinkage resistance and the electrolytic solution absorption are reduced.

[0097] (9) Heat treatment

[0098] The dried microporous membrane is preferably heat-treated to stabilize crystals for uniform lamellas. The heat treatment may comprise heat-setting and/or annealing. The heat-setting is preferably conducted by a tenter method or a roll method. The heat-setting temperature is preferably in a range from the above crystal dispersion temperature Tcd_a to the melting point Tm₃, more preferably in a range of the stretching temperature of the microporous membrane ±5°C, most preferably in a range of the stretching temperature of the microporous membrane

±3°C. [0099] The annealing is a heat treatment with no load applied to the microporous membrane, and may be conducted by using a heating chamber with a belt conveyer or an air-floating-type heating chamber. The annealing may be conducted continuously after heat-setting with the tenter slackened. The annealing temperature is preferably the melting point Tm₃ or lower. Such annealing provides the microporous membrane with high permeability and strength.

[0100] (10) Cross-linking of microporous membrane

[0101] The microporous membrane may be cross-linked by ionizing radiation rays such as α-rays, β-rays, γ-rays, electron beams, etc. In the case of irradiating electron beams, the amount of electron beams is preferably 0.1-100 Mrad, and the accelerating voltage is preferably 100-300 kV. The cross-linking treatment elevates the meltdown temperature of the microporous polyolefin membrane.

[0102] (11) Hydrophilizing treatment

[0103] The microporous membrane may be subjected to a hydrophilizing treatment (treatment of imparting hydrophilic property). The hydrophilizing treatment may be a monomer-grafting treatment, a surfactant treatment, a corona-discharging treatment, etc. The monomer- grafting treatment is preferably conducted after the cross-linking treatment.

[0104] In the case of the surfactant treatment, any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants may be used, and the nonionic surfactants are preferred. The microporous membrane is dipped in a solution of a surfactant in water or a lower alcohol such as methanol, ethanol, isopropyl alcohol, etc., or coated with the surfactant solution by a doctor blade method.

[0105] (B) Production method of second separator

[0106] The method for producing a second separator may be the same as the method for producing a first separator except for using the second polyolefin. In the step (1), the melt-blending temperature of the second polyolefin solution is preferably in a range of the melting point Tni_b of the second polyolefin + 10⁰C to Tm_b + 12O⁰C. In the step (4), the stretching temperature is preferably the above Tm_b + 10⁰C or lower, more preferably in a range of the crystal dispersion temperature Tcdb of the second polyolefin or higher and lower than Tm_b. In the step (6), the drying temperature is preferably equal to or lower than the above Tcdb, particularly 5⁰C or more lower than Tcd_b. In the step (8), the stretching temperature of the microporous membrane is preferably the above Tm_b or lower, more preferably in a range from the above Tcd_b to Tm_b. In the step (9), the heat-setting temperature is preferably in a range from the above Tcd_b to Tm_b, more preferably in a range of the stretching temperature of the microporous membrane ±5⁰C, most preferably in a range of the stretching temperature of the microporous membrane ±3⁰C. The annealing temperature is preferably the above Tm_b or lower. The polyethylene compositions B and B' have Tm_b of about 130-140⁰C and Tcd_b of about 90-100⁰C. [0107] In the method for producing a second separator, to improve meltdown properties further, a porous coating layer of a fluororesin (polyvinylidene difluoride, polytetrafluoroethylene, etc), polypropylene, polyamides, polyarylene sulfide, etc., may be formed on at least one surface of microporous polyethylene membrane. [0108] [3] Electrode Sheet

[0109] The anode sheet comprises a current collector, and an anodic active material layer, and the cathode sheet comprises a current collector, and a cathodic active material layer. Known materials may be used for the current collector, the anodic active material and the cathodic active material depending on the types of the secondary battery (lithium-ion secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc). Explanations will be made below on the anode sheet and the cathode sheet in the lithium-ion secondary batteries, and their production method. [0110] (A) Anode sheet

[0111] Specific examples of materials for the current collector of the anode sheet include metallic foils of aluminum, copper, nickel, stainless steel, titanium, etc., and an aluminum foil is preferable. The anodic active material is a material that can absorb and discharge lithium ions. Specific examples of the anodic active material may be inorganic compounds such as transition metal oxides, composite oxides of lithium and transition metals (lithium composite oxides), transition metal sulfides, etc.

The transition metals may be V, Mn, Fe, Co, Ni, etc. Preferred examples of the lithium composite oxides are laminar lithium composite oxides containing at least one transition metal. Specific examples of laminar lithium composite oxides include lithium nickelate, lithium cobaltate, lithium manganate, LiMn₂O₄,

Li(Ni-Mn-Co)O₂, etc.

[0112] The anode sheet is produced by coating a paste of the anodic active material, a binder, and a solvent on the current collector, drying, and pressing. The paste preferably contains conductive additives, which may be flake graphite, carbon black, etc. The binder may be fluoro-compounds such as polyvinylidene difluoride, polytetrafluoroethylene, etc. The solvent may be, for instance,

N-methyl-2-pyrrolidone, etc. Though not particularly restricted, coating method may be, for instance, a doctor blade method, etc.

[0113] The anodic active material layer is formed on one or both surfaces of the current collector, depending on the desired structure of the electrode assembly.

Though not restricted, the thickness of the current collector is preferably 5 to 60 μm, more preferably 8 to 40 μm, and the thickness of the anodic active material layer is preferably 20 to 300 μm, more preferably 40 to 150 μm, per one side of the anode sheet.

[0114] (B) Cathode sheet

[0115] Materials for the current collector of the anode sheet may be the same metallic foils as described above, and a copper foil is preferable. Specific examples of the cathodic active material may be carbonaceous materials such as mesophase carbon microbeads, natural graphite, artificial graphite, cokes, carbon black, etc.

Another examples may be Si, Sn, Si-C, etc.

[0116] The cathode sheet is produced by coating a paste of the cathodic active material, a binder, conductive additives and a solvent on the current collector, drying, and pressing. The binder, the conductive additives and the solvent may be the same as in the anode sheet. The binder for the cathode sheet may be an aqueous dispersion of rubber (styrene-butadiene rubber, etc). Though not particularly restricted, the coating method may be a doctor blade method, etc. [0117] The cathodic active material layer is formed on one or both surfaces of the current collector, depending on the desired structure of the electrode assembly. The thickness of the current collector of the cathode sheet may be the same as in the current collector of the anode sheet. The thickness of the cathodic active material layer may be the same as in the anodic active material layer. The current collectors in the anode sheet and the cathode sheet may not have the same thickness. The anodic active material layer and the cathodic active material layer may not have the same thickness.

[0118] [4] Structure of electrode assembly

[0119] Figs. 1 to 4 show an example of a cylindrical lithium-ion secondary battery comprising the electrode assembly of the present invention. This battery has a toroidal electrode assembly 1 comprising a second separator 11, a cathode sheet 13, a first separator 10, and an anode sheet 12. To provide the battery with improved nail penetration safety, the toroidal electrode assembly 1 is preferably wound such that the second separator 11 is arranged on an outer surface of the cathode sheet 13, while the first separator 10 is arranged on the inner surface of the cathode sheet 13. In this example, the second separator 11 is arranged on inside surface of the toroidal electrode assembly 1, as shown in Fig 2.

[0120] In this example shown in Fig 3, the anodic active material layer 12b is formed on both surfaces of the current collector 12a, and the cathodic active material layer 13b is formed on both surfaces of the current collector 13a,. As shown in Figs. 2 and 4, an anode lead 20 connects an end portion of the anode sheet 12 to a battery lid 27, and an cathode lead 21 connects an end portion of the cathode sheet 13 to a battery can 23. [0121] When the toroidal electrode assembly 1 is used in a rectangular lithium-ion secondary battery, the assembly 1 is wound to a long cylinder. In the case of a lithium-ion secondary battery having a laminate electrode structure comprising alternately arranged planar anode 12 and cathode 13, the first and second separators 10, 11 are alternately sandwiched by the planar anode 12 and the cathode 13. [0122] The electrode assembly 1 comprises (a) the first separator 10 comprising the high-density polyethylene A to have excellent shutdown properties, and (b) the second separator 11 comprising the high-density polyethylene B to have excellent mechanical strength and meltdown properties. Moreover, the first and second separators 10, 11 are also excellent in permeability, heat shrinkage resistance, and compression resistance. The difference in a shutdown temperature between the first and second separators 10, 11 is within 5⁰C. When this difference is more than 5°C, the battery has low safety. The shutdown temperature of the first and second separators 10, 11 is preferably 125⁰C or higher to obtain excellent battery safety. [0123] [5] Non-aqueous electrolyte battery

[0124] A lithium-ion secondary battery has the above electrode assembly 1. The first and second separators 10, 11 contain an electrolytic solution. [0125] The electrolytic solution is obtained by dissolving a lithium salt in an organic solvent. The lithium salts may be LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂),, Li₂B₁₀Cl₁₀, LiN(C₂F₅SO₂)₂, LiPF₄(CF₃),, LiPF₃(C₂F₅)₃, lower aliphatic carboxylates of lithium, LiAlCU, etc. The lithium salts may be used alone or in combination. The organic solvents may be organic solvents having high boiling points and high dielectric constants such as ethylene carbonate, propylene carbonate, ethylmethyl carbonate, γ-butyrolactone, etc.; organic solvents having low boiling points and low viscosity such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, diethyl carbonate, etc. These organic solvents may be used alone or in combination. Because the organic solvents having high dielectric constants have high viscosity, while those having low viscosity have low dielectric constants, their mixtures are preferably used.

[0126] In assembling the battery, the anode sheet 12, the cathode sheet 13, and the first and second separators 10, 11 are impregnated with the electrolytic solution, so that the separators 10, 11 (microporous membranes) are provided with ion permeability. The impregnation treatment is usually conducted by immersing the electrode assembly 1 in the electrolytic solution at room temperature. A cylindrical lithium-ion secondary battery can be produced by introducing the toroidal electrode assembly 1 (see Figs. 1 to 4) into a battery can 23 having a insulation plate 22 at the bottom, injecting the electrolytic solution into the battery can 23, covering the electrode assembly 1 with a insulation plate 22, caulking a battery lid 27 to the battery can 23 via a gasket 28. The battery lid 27 has a current interrupt device 24, a bent plate 25 and a PTC device 26, thereby having an anode terminal function.

[0127] A non-aqueous electrolyte battery comprising the electrolyte battery of the present invention has a discharge capacity of 1 ,500 mAh or more, and a capacity recovery ratio of 60% or more, the preferred capacity recovery ratio is 75% or more.

[0128] Though the electrode assembly and the non-aqueous electrolyte battery of the present invention have been explained taking a lithium-ion secondary battery for example, the electrode assembly of the present invention is not limited to thereto, but may be used in other types of non-aqueous electrolyte batteries. The electrode assembly and the non-aqueous electrolyte battery of the present invention are not restricted to the above examples, but any modifications may be added thereto unless deviating from the scope of the present invention.

[0129] The present invention will be explained in more detail referring to

Examples below without intention of restricting the scope of the present invention.

[0130] Example 1

[0131] ( 1 ) Preparation of first separator

[0132] Dry-blended were 100 parts by mass of a polyethylene composition A comprising 20% by mass of ultra-high-molecular-weight polyethylene (UHMWPE) having a weight-average molecular weight (Mw) of 2.0 x 10⁶ and a molecular weight distribution Mw/Mn of 8, and 80% by mass of high-density polyethylene A (HDPE-A) having a terminal vinyl group concentration of 0.6/10,000 C, Mw of 3.5 x 10³ and Mw/Mn of 13.5, and 0.2 parts by mass of tetrakis[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate] methane as an antioxidant. The terminal vinyl group concentration of HDPE-A was determined by measuring absorbance A = log(Io/I) at 910 cm^"1, wherein I₀ represents the intensity of transmitted light of a blank cell, and I represents the intensity of transmitted light of a sample cell, by a Fourier transform infrared spectrophotometer (FREEXACT-II, available from Horiba, Ltd), and calculating the formula of terminal vinyl group concentration^ 10,000 C) = (1.14 x absorbance A) / (density (g/cm³) of high-density polyethylene A x thickness (mm) of the sample). The polyethylene composition A had Mw of 6.8 x 10⁵, Mw/Mn of 21.5, Tm_a of 134°C, and Tcd_a of 100⁰C. [0133] The Mw and Mw/Mn of UHMWPE, HDPE and polyethylene composition were measured by gel permeation chromatography (GPC) under the following conditions:

Measurement apparatus: GPC-150C available from Waters Corporation,

Column: Shodex UT806M available from Showa Denko K.K.,

Column temperature: 135°C,

Solvent (mobile phase): o-dichlorobenzene,

Solvent flow rate: 1.0 ml/minute,

Sample concentration: 0.1% by weight (dissolved at 135°C for 1 hour),

Injected amount: 500 μl,

Detector: Differential Refractometer available from Waters Corp., and

Calibration curve: Produced from a calibration curve of a single-dispersion, standard polystyrene sample using a predetermined conversion constant. [0134] 30 parts by mass of the resultant mixture was charged into a strong-blending double-screw extruder having an inner diameter of 58 mm and L/D of 52.5, and 70 parts by mass of liquid paraffin [50 cst (40⁰C)] was supplied to the double-screw extruder via a side feeder. Melt-blending was conducted at 230⁰C and 250 rpm to prepare a polyethylene solution. This polyethylene solution was extruded from a T-die mounted to a tip end of the double-screw extruder, and drawn and cooled by cooling rolls controlled at 5°C while reeling up, to form a gel-like sheet. Using a tenter-stretching machine, the gel-like sheet was simultaneously biaxially stretched at 115°C, such that the stretching magnification was 5 folds in both longitudinal (MD) and transverse (TD) directions.

[0135] The stretched gel-like sheet was immersed in a washing bath of methylene chloride controlled at 25°C, and washed with the vibration of 100 rpm for 3 minutes to remove liquid paraffin. The resultant membrane was air-cooled at room temperature. The dried membrane was heat-set at 124°C for 30 seconds while being fixed to the tenter-stretching machine, to produce a microporous polyethylene membrane. The resultant microporous polyethylene membrane was slit to produce a first separator 10 (see Fig. l) having a width of 60 mm. [0136] (2) Preparation of second separator

[0137] 100 parts by mass of a polyethylene composition B comprising 30% by mass of the above UHMWPE and 70% by mass of high-density polyethylene B (HDPE-B) having a terminal vinyl group concentration of 0.1/10,000 C, Mw of 3.0 x 10⁵ and Mw/Mn of 8.6 was dry-blended with 0.2 parts by mass of the above antioxidant. The polyethylene composition B had a melting point Tm_b of 135°C, a crystal dispersion temperature Tcd_b of 100⁰C, Mw of 8.1 x 10⁵, and Mw/Mn of 17.2. A second separator 11 of a microporous polyethylene membrane having a width of 60 mm was produced in the same manner as above, except that this polyethylene composition B was used, and that the heat-setting temperature of the microporous membrane was 127°C. [0138] (3) Production of anode sheet

[0139] 92.7 parts by mass of composite lithium-cobalt oxide (LiCoO₂), 4.2 parts by mass of acetylene black, and 3.1 parts by mass of polyvinylidene difluoride (PVDF) were added to N-methyl-2-pyrrolidone, and mixed by stirring for 1 hour to prepare an anodic active material paste.

[0140] The anodic active material paste was applied to both surfaces of an aluminum foil current collector having a thickness of 20 μm by a doctor blade method to form uniform-thickness layers. The layers were dried to produce an anode sheet 12 having anodic active material layers on both surfaces of the current collector.

[0141] (4) Production of cathode sheet

[0142] 88 parts by mass of mesophase carbon microbeads, 10 parts by mass of acetylene black, and 2 parts by mass of PVDF were added to N-methyl-2-pyrrolidone, and mixed to prepare an cathodic active material paste. The cathodic active material paste was applied to both surfaces of a copper foil current collector having a thickness of 10 μm by a doctor blade method to form uniform-thickness layers. The layers were dried to produce a cathode sheet 13 having cathodic active material layers on both surfaces of the current collector.

[0143] (5) Production of electrode assembly

[0144] The 60-mm-wide second separator 11, the 57-mm-wide cathode sheet 13, the 60-mm-wide first separator 10, and the 55-mm-wide anode sheet 12 were laminated in this order to produce an electrode assembly 1. The electrode assembly

1 was wound such that the second separator 11 was arranged on the outer surface of the cathode sheet 13.

[0145] (6) Preparation of electrolytic solution

[0146] 1 mol/liter Of LiPF₆ was added to a mixed solvent of ethylene carbonate

(EC) and ethyl methyl carbonate (EMC) at a volume ratio of 40/60 to prepare an electrolytic solution.

[0147] (7) Assembling of battery

[0148] An anode lead 20 was attached to an end portion of the anode sheet 12 in the toroidal electrode assembly 1, and a cathode lead 21 was attached to an end portion of the cathode sheet 13 in the toroidal electrode assembly 1. With the toroidal electrode assembly 1 placed in a battery can 23 having an insulation plate 22 at the bottom, the anode lead 20 was connected to a battery lid 27, and the cathode lead 21 was connected to a battery can 23. After the electrolytic solution prepared in the above step (6) was injected into the battery can 23, the toroidal electrode assembly 1 was covered with the insulation plate 22, and the battery lid 27 was caulked to the battery can 23 via the gasket 28 to produce a 18650-type, cylindrical lithium-ion secondary battery having a diameter of 18 mm and a height of 65 mm. [0149] Example 2

[0150] A first separator 10 was produced in the same manner as in Example 1, except that the concentration of a polyethylene solution was 25% by mass, that the stretching temperature was 117°C, and that annealing was conducted at 125°C to make the transverse length 0.9 folds. The first separator 10 was used to produce a lithium-ion secondary battery in the same manner as in Example 1. [0151] Example 3

[0152] 100 parts by mass of a polyethylene composition A comprising 30% by mass of the above UHMWPE and 70% by mass of the above HDPE-A was dry-blended with 0.2 parts by mass of the above antioxidant. The polyethylene composition A had a melting point Tm_a of 134°C, a crystal dispersion temperature Tcd_a of 100⁰C, Mw of 8.4 x 10⁵, and MwMn of 23.8. A first separator 10 was produced in the same manner as in Example 1 , except that this polyethylene composition A was used, that the stretching temperature was 114°C, that the microporous membrane was stretched at 123°C to 1.1 folds in transverse direction by a tenter-stretching machine, and that the heat-setting temperature of the microporous membrane was 123⁰C. A lithium-ion secondary battery was produced using this first separator 10 in the same manner as in Example 1. [0153] Example 4

[0154] 100 parts by mass of a polyethylene composition A comprising 5% by mass of the above UHMWPE and 95% by mass of the above HDPE-A was dry-blended with 0.2 parts by mass of the above antioxidant. The polyethylene composition A had a melting point Tm₃ of 133⁰C, a crystal dispersion temperature Tcd_a of 100⁰C, Mw of 4.3 x 10⁵, and Mw/Mn of 15.9. A first separator 10 was produced in the same manner as in Example 1 , except that this polyethylene composition A was used, the concentration of a polyethylene solution was 35% by mass, that the stretching temperature was 116°C, that the microporous membrane was stretched at 126°C to 1.3 folds in transverse direction by a tenter-stretching machine, and that the heat-setting temperature of the microporous membrane was 126⁰C. A lithium-ion secondary battery was produced using this first separator 10 in the same manner as in Example 1. [0155] Example 5

[0156] A second separator 11 was produced in the same manner as in Example 1, except that the concentration of a polyethylene solution was 25% by mass, that the microporous membrane was stretched at 127°C to 1.1 folds in transverse direction by a tenter-stretching machine, and that annealing was conducted at 127°C such that the length in a transverse direction became 0.9 folds. A lithium-ion secondary battery was produced using this second separator 11 in the same manner as in Example 1. [0157] Example 6

[0158] A first separator 10 was produced in the same manner as in Example 2, and a second separator 11 was produced in the same manner as in Example 5. A lithium-ion secondary battery was produced using these first and second separators 10, 11 in the same manner as in Example 1. [0159] Example 7

[0160] 100 parts by mass of a polyethylene composition A comprising 5% by mass of the above UHMWPE and 95% by mass of the above HDPE-B was dry-blended with 0.2 parts by mass of the above antioxidant. The polyethylene composition B had a melting point Tm_b of 134⁰C, a crystal dispersion temperature Tcdb of 100⁰C, Mw of 3.8 x 10⁵, and Mw/Mn of 10.6. A second separator 11 was produced in the same manner as in Example 1 , except that this polyethylene composition B was used, that the stretching temperature was 117°C, that the microporous membrane was stretched at 130⁰C to 1.4 folds in transverse direction by a tenter-stretching machine, and that the heat-setting temperature of the microporous membrane was 130⁰C. A lithium-ion secondary battery was produced using this second separator 11 in the same manner as in Example 1.

[0161] Example 8

[0162] A first separator 10 was produced in the same manner as in Example 1, except that the stretching temperature was 114°C. A second separator 11 was produced in the same manner as in Example 1 , except that the stretching temperature was 113°C. A lithium-ion secondary battery was produced using these first and second separators 10, 11 in the same manner as in Example 1.

[0163] Comparative Example 1

[0164] A lithium-ion secondary battery was produced in the same manner as in

Example 1, except that the first separator 10, the cathode sheet 13, the first separator

10, and the anode sheet 12 were laminated in this order to form an electrode assembly.

[0165] Comparative Example 2

[0166] A lithium-ion secondary battery was produced in the same manner as in

Example 1, except that the second separatorll, the cathode sheet 13, the second separatorl l, and the anode sheet 12 were laminated in this order to form an electrode assembly.

[0167] Comparative Example 3

[0168] A separator was produced in the same manner as the second separator 11 in

Example 1 , except that only the above HDPE-B was used, that the concentration of a polyethylene solution was 40% by mass, that the stretching temperature was 114⁰C, that the microporous membrane was stretched at 128°C to 1.4 folds in transverse direction by a tenter-stretching machine, and that the heat-setting temperature of the microporous membrane was 128°C. A lithium-ion secondary battery was produced in the same manner as in Example 1 , except that this separator was used instead of the second separator 11.

[0169] Comparative Example 4 [0170] 100 parts by mass of a polyethylene composition B comprising 3% by mass of the above UHMWPE and 97% by mass of the above HDPE-B was dry-blended with 0.2 parts by mass of the above antioxidant. The polyethylene composition B had a melting point Tπi_b of 135⁰C, a crystal dispersion temperature

Tcd_b of 100⁰C, Mw of 3.5 x 10³, and Mw/Mn of 9.8. A second separator 11 was produced in the same manner as in Example 1 , except that this polyethylene composition B was used, that the concentration of a polyethylene solution was 40% by mass, that the stretching temperature was 119°C, that the microporous membrane was stretched at 130⁰C to 1.4 folds in transverse direction by a tenter-stretching machine, and that the heat-setting temperature of the microporous membrane was

130⁰C. A lithium-ion secondary battery was produced using this second separator

11 in the same manner as in Example 1.

[0171] Comparative Example 5

[0172] A lithium-ion secondary battery was produced in the same manner as in

Example 1, except that the first separator 10, the cathode sheet 13, the second separator 11, and the anode sheet 12 were laminated in this order to form an electrode assembly, and that the first separator 10 was arranged on an outer surface of the cathode sheet 13.

[0173] The properties of the microporous polyethylene membranes obtained in

Examples 1-8 and Comparative Examples 1-5 were measured by the following methods. The results are shown in Table 1.

[0174] ( 1 ) Average thickness (μm)

[0175] The thickness of each microporous polyethylene membrane was measured by a contact thickness meter at 5-mm longitudinal intervals over the width of 30 cm, and averaged.

[0176] (2) Air permeability (sec/ 100 cm³/20 μm)

[0177] Air permeability Pj measured on each microporous membrane having a thickness T₁ according to JIS P8117 was converted to air permeability P₂ at a thickness of 20 μm by the equation of P₂ = (Pi x 20)/Ti. [0178] (3) Porosity (%)

[0179] Measured by a weight method.

[0180] (4) Pin puncture strength (mN/20 μm)

[0181] The maximum load was measured, when each microporous membrane having a thickness of Ti was pricked with a needle of 1 mm in diameter with a spherical end surface (radius R of curvature: 0.5 mm) at a speed of 2 mm/second.

The measured maximum load Lj was converted to the maximum load L₂ at a thickness of 20 μm by the equation of L₂ = (Li x 20)/Ti, and used as pin puncture strength.

[0182] (5) Heat shrinkage ratio (%)

[0183] The shrinkage ratios of each (triple-layer) microporous membrane in both longitudinal and transverse directions were measured three times when exposed to

105⁰C for 8 hours, and averaged to determine the heat shrinkage ratio.

[0184] (6) Pore radius

[0185] Measured by mercury intrusion porosimetry.

[0186] (7) Shutdown temperature (⁰C)

[0187] As shown in Fig. 5, a test piece TP having a size of 3 mm and 10 mm in the stretching directions MD and TD, respectively, is cut out of a microporous polyethylene membrane 100. Using a thermomechanical analyzer (TMA/SS6000 available from Seiko Instruments Inc.), the test piece TP is heated from room temperature at a speed of 5°C/minute, with its upper end 100a gripped by a holder 3 and a weight 4 of 2 g attached to its lower end 100b. A temperature at a point of inflection observed near the melting point was defined as shutdown temperature.

[0188] (8) Meltdown temperature (⁰C)

[0189] Using the above thermomechanical analyzer, a test piece TP of 10 mm

(TD) and 3 mm (MD) was heated from room temperature at a speed of 5°C/minute under a load of 2 g according to the method shown in Fig. 5. The temperature at which the test piece TP was ruptured by melting was used as "meltdown temperature." [0190] (9) Air permeability after heat compression (sec/100 cm³/20 μm)

[0191] A microporous membrane sample was sandwiched by a pair of highly flat plates, and heat-compressed by a press machine under a pressure of 2.2 MPa (22 kgf/cm²) at 90⁰C for 5 minutes. The heat-compressed microporous membrane having a thickness of Ti' was measured with respect to air permeability Pi' according to JIS P8117. The measured air permeability Pi ' was converted to air permeability

P₂' at a thickness of 20 μm by the equation of P₂' = (Pi' x 2O)Ay.

[0192] The properties of the batteries obtained in Examples 1-8 and Comparative

Examples 1-5 were measured by the following methods. The results are shown in

Table 1.

[0193] (10) Discharge capacity

[0194] The battery was charged to 4.2 V at constant current of 900 mA by using a charge/discharge tester, further charged until charging current decreased to 10 mA at a constant voltage of 4.2 V, and then discharged down to 3.0 V at a constant current of 260 mA, at 25°C. The measured initial capacity was used as discharge capacity

(mAh).

[0195] (11) Overcharge test

[0196] The battery was charged to 5 V at a constant current of 800 mA (quantity of electricity: 0.5 C) by using a charge/discharge tester at 25⁰C to examine whether or not smoking and ignition occurred.

[0197] (12) Nail penetration test

[0198] The battery was charged to 4.2 V at a constant current of 900 mA by using a charge/discharge tester, further charged until charging current decreased to 10 mA at a constant voltage of 4.2V, at 25°C. A stainless steel nail of 3 mm in diameter heated to 25°C pricked the charged battery kept at 25⁰C, at a center in a radial direction at a speed of 9 mm/second, to examine whether or not smoking and ignition occurred.

[0199] (13) Capacity recovery ratio (cyclability) after cyclability test

[0200] The battery was charged to 4.2 V at a constant current of 1,600 mA (quantity of electricity: 1 C) by using a charge/discharge tester, further charged until charging current decreased to 10 mA at a constant voltage of 4.2 V, and then discharged down to 3.0 V at a constant current of 1,600 mA (quantity of electricity: 1 C), to examine discharge capacity (initial capacity) at 25°C. This charge/discharge operation was repeated 100 times (cyclability test). The discharge capacity was measured again by the same method to determine a capacity recovery ratio (%) after the cyclability test. The capacity recovery ratio (%) of the battery was determined by the following equation:

Capacity recovery ratio (%) = [(capacity after cyclability test)/(initial capacity)] x 100.

[0201] Table 1

[0202] Table 1 (Continued)

[0203] Table 1 (Continued)

[0204] Table 1 (Continued)

[0205] Table 1 (Continued)

[0206] Table 1 (Continued)

[0207] Table 1 (Continued)

[0208] Note: (1) Mw represents weight-average molecular weight.

[0209] (2) Mw/Mn represents a molecular weight distribution which is determined by weight-average molecular weight / number-average molecular weight.

[0210] (3) The number of terminal vinyl groups per 10,000 carbon atoms (the number/10,000 C).

[0211] (4) Tm represents the melting point of the polyethylene composition.

[0212] (5) Ted represents the crystal dispersion temperature of the polyethylene composition.

[0213] (6) The concentration of the first polyethylene solution and the concentration of the second polyolefin solution.

[0214] (7) MD represents a longitudinal direction, and TD represents a transverse direction.

[0215] (8) Laminate structure from outside, in which (I) represents the first separator, (II) represents the second separator, "cathode" represents the cathode sheet, and "anode" represents the anode sheet.

[0216] (9) Although the separator composed of HDPE-B in Comparative

Example 3 is not categorized in the first or second separator, it is listed in the column of "Second separator."

[0217] As is clear from Table 1, the electrode assemblies of Examples 1 to 8 comprised the first and second separators having well-balanced permeability, mechanical strength, heat shrinkage resistance, shutdown properties, meltdown properties and compression resistance. Particularly, the first separator had excellent shutdown properties, and the second separator had excellent mechanical strength and meltdown properties. Therefore, the resultant battery was provided with excellent safety, discharge capacity and cyclability.

[0218] The battery of Comparative Example 1 having two first separators containing HDPE-A without the second separator containing HDPE-B was smoked in the nail penetration test, indicating that it was poorer in safety than those of Examples 1 to 8. The battery of Comparative Example 2 having two second separators containing HDPE-B without the first separator containing HDPE-A was smoked in the overcharge test, indicating that it was poorer in safety than those of Examples 1 to 8. The battery of Comparative Example 3 having one first separator and the other separator made only of HDPE-B without containing UHMWPE was smoked in the nail penetration test, indicating that it was poorer in safety than those of Examples 1 to 8. The battery of Comparative Example 4 having an electrode assembly comprising the first and second separators, whose difference in a shutdown temperature was more than 5⁰C, was smoked in the nail penetration test and the overcharge test, indicating that it was poorer in safety than those of Examples 1 to 8. The battery of Comparative Example 5 having a electrode assembly comprising the first separator on an outer surface of said cathode sheet and the second separator on an inner surface of said cathode sheet was smoked in the nail penetration test, indicating that it was poorer in safety than those of Examples 1 to 8.

EFFECT OF THE INVENTION

[0219] A battery having well-balanced safety, discharge capacity and cyclability can be obtained by using the electrode assembly of the present invention. This electrode assembly is suitable for non-aqueous electrolyte batteries.

Claims

C L A I M S

1. An electrode assembly having a structure comprising a first microporous membrane separator comprising high-density polyethylene A having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.3 or more per 10,000 carbon atoms and ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 1 x 10⁶ or more, and a second microporous membrane separator comprising high-density polyethylene B having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.2 or less per 10,000 carbon atoms and said ultra-high-molecular-weight polyethylene, which are alternately sandwiched by an anode sheet and a cathode sheet, the difference in a shutdown temperature between said first and second separators being within 5⁰C.

2. The electrode assembly according to claim 1, wherein it is in a toroidal form.

3. The electrode assembly according to claim 2, wherein said first separator is arranged on an inner surface of said cathode sheet, while said second separator is arranged on an outer surface of said cathode sheet.

4. A non-aqueous electrolyte battery having a discharge capacity of 1 ,500 mAh or more and a capacity recovery ratio of 60% or more, which comprises an electrode assembly having a structure comprising a first microporous membrane separator comprising high-density polyethylene A having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.3 or more per 10,000 carbon atoms and ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 1 x 10⁶ or more, and a second microporous membrane separator comprising high-density polyethylene B having a terminal vinyl group concentration (measured by infrared spectroscopy) of 0.2 or less per 10,000 carbon atoms and said ultra-high-molecular-weight polyethylene, which are alternately sandwiched by an anode sheet and a cathode sheet, the difference in a shutdown temperature between said first and second separators being within 5⁰C.