WO2012042716A1 - 非水電解質二次電池用セパレータおよびそれを用いた非水電解質二次電池 - Google Patents
非水電解質二次電池用セパレータおよびそれを用いた非水電解質二次電池 Download PDFInfo
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- WO2012042716A1 WO2012042716A1 PCT/JP2011/004082 JP2011004082W WO2012042716A1 WO 2012042716 A1 WO2012042716 A1 WO 2012042716A1 JP 2011004082 W JP2011004082 W JP 2011004082W WO 2012042716 A1 WO2012042716 A1 WO 2012042716A1
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- secondary battery
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
- H01M50/44—Fibrous material
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
- H01M50/411—Organic material
- H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
- H01M50/417—Polyolefins
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
- H01M50/443—Particulate material
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/409—Separators, membranes or diaphragms characterised by the material
- H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
- H01M50/451—Separators, membranes or diaphragms characterised by the material having a layered structure comprising layers of only organic material and layers containing inorganic material
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
- H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
- H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
- H01M50/491—Porosity
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- the present invention relates to an improvement in a separator for a non-aqueous electrolyte secondary battery including a polyolefin porous membrane, and further relates to a non-aqueous electrolyte secondary battery capable of improving a capacity retention rate in a charge / discharge cycle in which rapid charging is performed.
- the battery temperature of a non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery may rise rapidly due to misuse such as an external short circuit or overcharge.
- contrivances have been made to ensure the safety of the battery by a safety mechanism such as a PTC (Positive Temperature Coefficient) element or an SU (Safety Unit) circuit.
- the separator interposed between the electrodes inside the battery is provided with a function (shutdown function) for cutting off current when the battery temperature rises.
- Polyolefin porous membranes are widely used as separators for nonaqueous electrolyte secondary batteries.
- the pores are clogged by the softening of the polyolefin porous membrane. Therefore, the ion conductivity between electrodes lose
- Such a function is called a shutdown function.
- the polyolefin may be damaged due to melting (meltdown), resulting in a short circuit between the positive and negative electrodes.
- non-aqueous electrolyte secondary batteries that require high input / output characteristics are highly demanded for rapid charging.
- it is required to improve the ability of lithium ions to move between electrodes.
- the separator pore size is simply increased in response to such a requirement, an internal short circuit is likely to occur due to the growth of acicular metallic lithium (dendrites).
- dendrites acicular metallic lithium
- the battery capacity is reduced or the separator is clogged.
- the porosity is simply increased, the tensile strength and puncture strength of the separator are reduced, and the handleability is reduced.
- Patent Document 1 proposes manufacturing a separator having a knot extending in the film thickness direction by subjecting a raw material of a fluororesin such as polytetrafluoroethylene to a high temperature of about 300 ° C. Moreover, it is described that the nodule extending in the film thickness direction increases the compressive strength of the separator and suppresses the formation and release of dendrites.
- a fluororesin such as polytetrafluoroethylene
- Patent Document 2 proposes laminating a non-oriented polyolefin resin layer on at least one surface of a biaxially oriented polypropylene porous film. It is described that the handleability of the film can be improved while maintaining high air permeability and porosity.
- Patent Document 3 proposes a separator made of a polyolefin resin containing high molecular weight polyethylene having an average molecular weight of 500,000 or more and polyethylene having an average molecular weight of less than 500,000. It is described that by using such a polyolefin resin, the shutdown temperature can be lowered while maintaining the puncture strength, and further, the shrinkage rate at the time of shutdown can be reduced.
- Patent Document 1 when the fluororesin is used as a raw material and the drawing process is performed at a high temperature, the material cost and the manufacturing cost of the separator are increased. Furthermore, since the fluororesin has high heat resistance, it is difficult to obtain an effective shutdown effect.
- Patent Document 2 it is difficult to control the pore structure of the film only by laminating films having different orientations.
- the battery when the battery is rapidly charged, it is necessary to improve the lithium ion transfer capability between the electrodes. For this purpose, it is necessary to achieve a pore structure suitable for the movement of lithium ions.
- Patent Document 3 using a polyolefin resin containing polyethylene having different average molecular weights and controlling the piercing strength and shutdown temperature of the separator, it is possible to achieve a pore structure suitable for the movement of lithium ions. Have difficulty.
- the present invention has been made in view of the above problems, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery having a good capacity retention rate in a charge / discharge cycle in which rapid charging is performed.
- One aspect of the present invention includes a biaxially oriented polyolefin porous film including an extended-chain crystal and a folded-chain crystal, and the extended-chain crystal and the The folded chain crystal forms a Shish-kebab structure, and the average distance between the adjacent extended chain crystals is 1.5 ⁇ m or more and less than 11 ⁇ m, and between the adjacent folded chain crystals.
- the present invention relates to a separator for a nonaqueous electrolyte secondary battery having an average distance of 0.3 ⁇ m or more and less than 0.9 ⁇ m.
- the adjacent extended chain crystals may be connected by the folded chain crystal.
- Another aspect of the present invention includes the polyolefin porous membrane and a heat resistant porous membrane laminated on the polyolefin porous membrane, and the heat resistant porous membrane has a heat resistance higher than a melting point of the polyolefin.
- the present invention relates to a separator for a non-aqueous electrolyte secondary battery containing a resin.
- Still another aspect of the present invention includes a positive electrode, a negative electrode, an electrode group obtained by winding the separator interposed between the positive electrode and the negative electrode in a spiral shape, and a nonaqueous electrolyte.
- the present invention relates to a water electrolyte secondary battery.
- the pore structure of the separator is suitable for the movement of lithium ions, it is possible to provide a nonaqueous electrolyte secondary battery having a good capacity retention rate in a charge / discharge cycle in which rapid charging is performed. .
- FIG. 1 is a cross-sectional view of a separator for a nonaqueous electrolyte secondary battery having a multilayer structure including a polyolefin porous film and a heat-resistant porous film laminated thereon. It is a partially cutaway perspective view showing an example of the nonaqueous electrolyte secondary battery of the present invention.
- the separator for a non-aqueous electrolyte secondary battery of the present invention includes a biaxially oriented polyolefin porous membrane.
- the polyolefin porous film can be provided with a function of closing and shutting down the pores at an upper limit temperature effective for ensuring safety when the battery is abnormal.
- the biaxially oriented polyolefin porous film has a high degree of orientation, a separator having high mechanical strength is easily obtained.
- the polyolefin porous membrane according to the present invention includes so-called extended chain crystals and folded chain crystals.
- An extended chain crystal is formed by a stretched molecule without being folded. Such crystals are formed when the polyolefin is highly drawn and fiberized.
- the folded chain crystal is a lamellar plate-like crystal, and the molecule is folded at the upper and lower surfaces of the plate-like crystal and is taken into the plate-like crystal again. Since the same molecule repeats such folding several times, the thickness of the plate crystal becomes thinner than the molecular chain length (molecule length).
- the extended chain crystal and the folded chain crystal form a so-called Shish-kebab structure.
- the extended chain crystal is a rod-like crystal oriented in a first direction corresponding to the stretching direction of the molecule.
- the folded chain crystal is a plate crystal whose plane direction is oriented so as to cross the first direction.
- the average angle ⁇ at which the first direction and the second direction intersect is, for example, 30 ° to 150 °, preferably 70 ° to 110 °.
- Plate-like crystals are formed intermittently along the axial direction of the rod-like crystals, like shish kebab. Adjacent rod-like crystals may be connected to each other by plate-like crystals spreading in the radial direction. That is, adjacent extended chain crystals may be connected by a folded chain crystal.
- the average distance between adjacent extended chain crystals is 1.5 ⁇ m or more and less than 11 ⁇ m.
- any more preferable range can be taken within this range.
- One of the preferable ranges is 3.5 ⁇ m or more and less than 7.5 ⁇ m.
- the lower limit of the average distance may be, for example, 3.5 ⁇ m, 5 ⁇ m, or 7.5 ⁇ m.
- the upper limit of the average distance may be, for example, 7.5 ⁇ m, 5 ⁇ m, or 3 ⁇ m in addition to 11 ⁇ m. These upper limits and lower limits can take any combination.
- the average distance between adjacent folded chain crystals (the distance between the thickness centers of the plate crystals) is 0.3 ⁇ m or more and less than 0.9 ⁇ m. However, any more preferable range can be taken within this range.
- One of the preferable ranges is 0.5 ⁇ m or more and less than 0.9 ⁇ m.
- the lower limit of the average distance may be 0.5 ⁇ m or 0.7 ⁇ m in addition to 0.3 ⁇ m.
- the upper limit of the average distance may be 0.7 ⁇ m or 0.5 ⁇ m in addition to 0.9 ⁇ m. These upper limits and lower limits can take any combination.
- the average distance between adjacent extended chain crystals and the average distance between adjacent folded chain crystals are controlled within appropriate ranges. Thereby, the spatial distribution in the fine structure of the polyolefin porous membrane is made uniform. As a result, the liquid retaining ability of the non-aqueous electrolyte by the separator is enhanced, and the ability to move lithium ions that move between the electrodes is enhanced. Therefore, generation and release of dendrites are also suppressed. Therefore, even when charging / discharging cycles for rapid charging are repeated, a high capacity retention rate can be obtained.
- each molecular chain crystal is aligned, and the degree of biaxial orientation is increased. Become very expensive. Therefore, the mechanical strength of the separator is effectively increased. Further, the shrinkage ratio of the polyolefin porous film at a high temperature and the shrinkage ratio at the time of shutdown are reduced, and an internal short circuit can be effectively prevented. As a result, a safer non-aqueous electrolyte secondary battery can be obtained.
- the spatial distribution in the fine structure becomes more uniform as the variation in the distance between adjacent extended chain crystals and the distance between adjacent folded chain crystals is smaller. Therefore, it is considered that effects such as improvement of the liquid retention capacity of the separator, improvement of mechanical strength, and reduction of the shrinkage rate of the entire separator at the time of shutdown can be further enhanced.
- the average distance between adjacent extended chain crystals and the average distance between adjacent folded chain crystals can be measured by observing the microstructure of the polyethylene porous film with an electron microscope at a magnification of 10,000 times. .
- a pair of adjacent extended chain crystals and a pair of folded chain crystals are selected from an electron micrograph.
- crystallization is measured for every 0.5 micrometer in a linear distance along the orientation direction (1st direction) of an extended chain crystal.
- the distance between the center positions of the respective crystals is measured every 0.1 ⁇ m in a linear distance along the orientation direction (second direction) of the plane of the plate crystal.
- the distance between the extended chain crystals may be the distance between the intersections obtained by taking the intersection of the line perpendicular to the first direction and the pair of extended chain crystals.
- the distance between the folded chain crystals may be the distance between the intersections obtained by taking the intersection of the line perpendicular to the second direction and the pair of folded chain crystals.
- a straight line segment PQ is drawn from the point P on the extended chain crystal to the point Q separated by the length L1 in the first direction.
- the line segment PQ has a length L1 (for example, 5 ⁇ m)
- the point P extends to the point R (where the point R is the intersection of the perpendicular of the line segment PQ at the point Q and the extended chain crystal).
- the ratio of the actual length L2 of the chain crystal: L2 / L1 is preferably closer to 1.
- the average value of L2 / L1 obtained when L2 / L1 is measured at 30 points in three fields of view (10 points per field of view) is preferably in the range of 1.1 to 1.5, for example.
- L2 / L1 is obtained, the variation in the distance between adjacent extended chain crystals and the distance between adjacent folded chain crystals is extremely small, and the spatial distribution in the microstructure is extremely uniform. It is considered high.
- the separator of the present invention has the polyolefin porous film as described above and a heat-resistant porous film laminated on the polyolefin porous film.
- the heat resistant porous membrane includes a heat resistant resin having a melting point higher than that of polyolefin.
- the shrinkage rate of the separator at the time of shutting down the polyolefin porous film can be further reduced as compared with the case of the polyolefin porous film alone. Therefore, the safety of the battery can be further improved.
- the temperature at which the polyolefin porous membrane shuts down is, for example, in the range of 120 to 150 ° C. Therefore, the heat resistant resin preferably has a melting point or heat resistance higher than such a temperature. The heat resistance can be evaluated by the heat distortion temperature.
- the shrinkage rate of the separator having a two-layer structure including the polyolefin porous film and the heat-resistant porous film can be reduced to 10% or less when the polyolefin porous film is shut down.
- a more preferable range of such shrinkage is 5 to 8%.
- the shrinkage rate is measured by the TMA method.
- a separator sample having a size of 5 mm ⁇ 14 mm is prepared. This is set in a tensile load tester (TMA apparatus, RIGAKU ThermoPlus 2). The jig clamping allowance was taken up 2 mm each on the top and bottom, and the measurement data size was 5 mm ⁇ 10 mm. In a state where the sample was pulled with a load of 19.6 mN in air, the change in length was measured while changing the sample temperature from room temperature to 250 ° C.
- the Gurley value of the separator of the present invention is, for example, 180 seconds / 100 ml or more and less than 260 seconds / 100 ml. Moreover, the porosity of the separator of this invention is 50% or more and less than 62%, for example. Since the separator has such a Gurley value and porosity, not only is it advantageous for rapid charging, but also variation in battery characteristics is reduced.
- the biaxially oriented structure composed of extended chain crystals and folded chain crystals is formed by flow orientation of a solution of high molecular weight polyolefin or flow orientation of a melt of high molecular weight polyolefin.
- a porous film having a shish kebab structure can be formed by stretching high molecular weight polyethylene at a temperature equal to or higher than the melting point.
- Elongated chain crystals have a high melting point and folded chain crystals have a low melting point.
- the porous film having a shish kebab structure has a structure in which a plurality of such biaxially oriented structures of two types of crystals are stacked.
- FIG. 1 is a schematic diagram showing an example of a pore structure of a separator for a non-aqueous electrolyte secondary battery according to the present invention.
- the extended chain crystals 101 are aligned in the MD direction (machine tensile direction), and the folded chain crystals 102 are aligned in the TD direction (perpendicular to the mechanical tension direction).
- the folded chain crystal 102 the molecular chain crystal is shortened because the molecule is folded.
- the folded chain crystal 102 forms a cross-linked structure that connects the extended chain crystals 101 to each other.
- FIG. 1 schematically shows a plane along the surface direction inside the separator.
- the planes of such a structure are overlapped.
- ionic conduction is achieved by passing lithium ions through the non-aqueous electrolyte held between these molecular chain crystals.
- the average distance between adjacent extended chain crystals 101 is 1.5 ⁇ m or more and less than 11 ⁇ m, and the average distance between adjacent folded chain crystals 102 is 0.3 ⁇ m or more and less than 0.9 ⁇ m.
- a structure in which the molecular chain crystals are aligned in the three-dimensional direction can be obtained. Due to the smooth movement of lithium ions through the non-aqueous electrolyte held in such a fine structure, ion conduction does not stagnate even during rapid charging, and the occurrence of defects such as dendrite growth and release, and even short-circuiting Is greatly suppressed.
- FIG. 2 is a schematic diagram showing another example of the pore structure of the separator for a non-aqueous electrolyte secondary battery of the present invention. Also in the polyolefin porous film of FIG. 2, the extended chain crystal 111 is oriented in the MD direction, and the folded chain crystal 112 is oriented in the TD direction, and has the same structure as FIG. However, the extended chain crystal 111 and the folded chain crystal 112 have more bent portions than in the case of FIG.
- the distance between a specific folding chain crystal and another folding chain crystal adjacent to the specific folding chain crystal is closer or farther away.
- variation in distance between molecular chain crystals (that is, L2 / L1) is relatively large, and there are portions where lithium ions can move smoothly and portions where movement is difficult. That is, since the conduction of lithium ions may partially stagnate, the effect of suppressing dendrite growth is less than that of the separator of FIG. Since such a tendency becomes conspicuous especially in a charge / discharge cycle in which rapid charging is performed, it is desirable that variation in distance between molecular chain crystals is as small as possible.
- variation in the distance between molecular chain crystals can be directly evaluated from L2 / L1, and can also be indirectly evaluated by the shrinkage rate of a separator, etc.
- the internal structure of the separator can be observed by being exposed by means such as partially peeling the separator or freezing and freezing and slicing the separator. Among these, the method of freezing and slicing is preferable because the internal structure is not easily destroyed.
- polyolefin porous membrane having a structure as shown in FIG. 1 An example of a method for producing a polyolefin porous membrane having a structure as shown in FIG. 1 is shown below.
- the polyolefin constituting the polyolefin porous membrane include polyethylene, polypropylene, ethylene-propylene copolymer and the like. These resins can be used alone or in combination of two or more.
- the first polyethylene having an average molecular weight of 2.5 million or more and the second polyethylene having an average molecular weight of less than 2 million are mixed and used using a stirring device such as a high-speed mixer. Can do.
- the mass ratio of the first polyethylene to the second polyethylene is, for example, 3: 1 to 3: 2.
- a processing aid and a lubricant are added to the mixture, and the mixture is melt-mixed until the whole becomes uniform. Thereafter, the uniform mixture is extruded by an extruder to produce a polyethylene sheet. Next, the polyethylene sheet is stretched at a temperature higher than the melting point of polyethylene (in the range of about 135 ° C. to about 150 ° C.) to form a nonporous film.
- the film is further stretched to cleave between the crystals of the nonporous film.
- Such stretching treatment is performed in a plurality of times.
- a method generally referred to as a roll stretching method the film is passed through a pair of nip rolls having a speed difference, and the film is stretched in the machine axis direction of the roll.
- cracks occur in the amorphous portions between the lamellar crystals of the polyolefin resin, and a biaxially oriented polyolefin porous film is obtained.
- the distance between the extended chain crystals and the distance between the folded chain crystals can be controlled by adjusting the stretching speed, the number of stretching, the rotational speed of the nip roll, and the nip roll pressure.
- the polyolefin porous membrane which has a different Gurley value and a different porosity can be manufactured by changing the raw material and the manufacturing process as appropriate, while having substantially the same shish kebab structure.
- the first polyethylene and the second polyethylene changing the mixing conditions and molding conditions of the mixture of the first polyethylene and the second polyethylene, or changing the stretching conditions of the nonporous film
- control is possible. It is also possible to control the Gurley value and the porosity of the entire separator by changing the state of the heat resistant porous membrane.
- FIG. 3 is a cross-sectional view in the thickness direction of a separator having a two-layer structure having a polyolefin porous film 103 and a heat-resistant porous film 104.
- the heat resistant porous membrane 104 contains a heat resistant resin having a higher melting point or heat resistance than the polyolefin porous membrane. That is, the heat resistant resin is selected such that its melting point or heat distortion temperature is higher than the melting point or heat distortion temperature of the polyolefin contained in the polyolefin porous membrane.
- the heat distortion temperature can be measured as a deflection temperature under load.
- the deflection temperature under load is calculated at a load of 1.82 MPa in a method based on the test method ASTM-D648 of the American Society for Testing Materials.
- the heat distortion temperature of the heat resistant resin is preferably 260 ° C. or higher.
- a heat-resistant resin having a heat distortion temperature of 260 ° C. or higher sufficiently high thermal stability can be exhibited even when the battery temperature rises due to heat storage during overheating (usually about 180 ° C.).
- the upper limit of the heat distortion temperature is not particularly limited, but is about 400 ° C. in view of the separator characteristics and the resin thermal decomposability. The higher the heat distortion temperature, the easier it is to maintain the separator shape when heat shrinkage or meltdown occurs in the polyolefin porous membrane.
- heat-resistant resins include aromatic polyamides such as polyarylate and aramid (fully aromatic polyamides); polyimide resins such as polyimide, polyamideimide, polyetherimide, and polyesterimide; aromatic polyesters such as polyethylene terephthalate; Polyphenylene sulfide; polyether nitrile; polyether ether ketone; polybenzimidazole and the like.
- the heat-resistant resin can be used alone or in combination of two or more.
- aramid, polyimide, polyamideimide, and the like are preferable because they are excellent in heat resistance and ability to retain a nonaqueous electrolyte.
- the heat resistant porous membrane may contain an inorganic filler in order to further increase the heat resistance.
- the inorganic filler include metals or metal oxides such as iron powder and iron oxide; ceramics such as silica, alumina, titania and zeolite; mineral fillers such as talc and mica; carbon fillers such as activated carbon and carbon fibers. Examples thereof include: carbides such as silicon carbide; nitrides such as silicon nitride; glass fibers, glass beads, and glass flakes.
- the form of the inorganic filler is not particularly limited, and may be granular or powdery, fibrous, flaky, massive or the like.
- the inorganic filler can be used alone or in combination of two or more.
- the proportion of the inorganic filler is, for example, 50 to 400 parts by mass, preferably 80 to 300 parts by mass with respect to 100 parts by mass of the heat resistant resin.
- the solution or dispersion of the material of the heat resistant porous membrane may contain an inorganic filler, if necessary, in addition to a heat resistant resin such as aramid, and a pore forming agent such as calcium chloride. It may be included. What is necessary is just to dry as needed, after apply
- a two-layer separator can be produced as follows. First, after casting a polyamic acid solution, the precursor of a heat resistant porous film is produced by stretching. The polyolefin porous film is stacked on the surface of the precursor of the obtained heat-resistant porous film, and heated to a temperature at which the polyolefin porous film does not shut down to integrate them. For such integration, for example, a heat roll is used. Due to the heat from the heat roll, imidization of the polyamic acid proceeds, and the polyamic acid in the precursor of the porous film is converted to polyimide or polyamideimide.
- the polyamic acid may be converted into polyamide or polyamideimide by heating a precursor of the porous film containing polyamic acid before superimposing with the polyolefin porous film.
- the porosity of the heat-resistant porous film can be controlled by changing the stretching conditions.
- Solvents that dissolve or disperse the material of the heat-resistant porous membrane include alcohols such as methanol, ethanol and ethylene glycol (C2-4 alkanol or C2-4 alkanediol); ketones such as acetone; ethers such as diethyl ether and tetrahydrofuran Amides such as dimethylformamide; nitriles such as acetonitrile; sulfoxides such as dimethyl sulfoxide; N-methyl-2-pyrrolidone (NMP) and the like. These solvents can be used alone or in combination of two or more.
- alcohols such as methanol, ethanol and ethylene glycol (C2-4 alkanol or C2-4 alkanediol)
- ketones such as acetone
- ethers such as diethyl ether and tetrahydrofuran Amides such as dimethylformamide
- nitriles such as acetonitrile
- sulfoxides such as dimethyl s
- the thickness of the heat resistant porous membrane is preferably 1 to 16 ⁇ m, more preferably 2 to 10 ⁇ m, from the viewpoint of the balance between safety against internal short circuit and battery capacity.
- the heat-resistant porous film has a relatively low porosity and ionic conductivity, but by setting the thickness to 20 ⁇ m or less, it is easy to suppress an excessive increase in impedance and suppress a decrease in charge / discharge characteristics. can do.
- the porosity of the heat resistant porous membrane is, for example, 20 to 70%, preferably 25 to 65%, from the viewpoint of sufficiently securing the movement of lithium ions.
- the Gurley value is preferably 180 seconds / 100 ml or more and less than 260 seconds / 100 ml, and the porosity is preferably 50% or more and less than 62%.
- a more preferable range of the Gurley value is 180 seconds / 100 ml or more and less than 250 seconds / 100 ml, and a more preferable range of the porosity is 55% or more and less than 62%.
- These separator physical property values are physical property values of the entire separator, and in the case of a separator having a laminated structure as shown in FIG. 4, are the physical property values of the entire separator including the heat resistant porous film.
- the thickness of the entire separator can be selected from the range of, for example, 5 to 35 ⁇ m, preferably 10 to 30 ⁇ m, and may be 12 to 20 ⁇ m.
- the ratio of the thickness Tpo of the polyolefin porous membrane to the thickness Tht of the heat resistant porous membrane is preferably 4-9.
- the separator may contain a conventional additive (such as an antioxidant).
- a conventional additive such as an antioxidant
- the oxidation resistance of the polyolefin porous membrane can be enhanced by adding an antioxidant to the surface layer of the polyolefin porous membrane.
- an antioxidant include at least one selected from the group consisting of a phenolic antioxidant, a phosphoric acid antioxidant, and a sulfur antioxidant. You may use together a phenolic antioxidant, a phosphoric acid type antioxidant, or a sulfur type antioxidant. Sulfur-based antioxidants are highly compatible with polyolefins. Therefore, it is suitable for a polyolefin porous membrane.
- phenolic antioxidants examples include 2,6-di-t-butyl-p-cresol, 2,6-di-t-butyl-4-ethylphenol, triethylene glycol-bis [3- (3- Examples thereof include hindered phenol compounds such as t-butyl-5-methyl-4-hydroxyphenyl) propionate] and n-octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate.
- sulfur-based antioxidant examples include dilauryl thiodipropionate, distearyl thiodipropionate, and dimyristyl thiodipropionate.
- phosphoric acid antioxidant tris (2,4-di-t-butylphenyl) phosphite is preferable.
- FIG. 4 is a partially cutaway perspective view of a cylindrical lithium ion secondary battery according to an embodiment of the present invention.
- the lithium ion secondary battery of FIG. 4 includes an electrode group 14 in which a strip-like positive electrode 5 and a strip-like negative electrode 6 are wound through a separator 7.
- the electrode group 14 is housed in a bottomed cylindrical metal battery case 1 together with a nonaqueous electrolyte (not shown).
- the positive electrode 5 includes a positive electrode current collector made of a metal foil and a positive electrode active material layer disposed on the surface thereof.
- the negative electrode 6 includes a negative electrode current collector made of a metal foil and a negative electrode active material layer disposed on the surface thereof.
- the positive electrode lead 5a is electrically connected to the positive electrode 5.
- a negative electrode lead 6 a is electrically connected to the negative electrode 6.
- the electrode group 14 is housed in the battery case 1 together with the lower insulating plate 9 with the positive electrode lead 5 a led out.
- the end of the positive electrode lead 5 a is welded to the sealing plate 2.
- the sealing plate 2 includes a positive external terminal 12 and a safety mechanism for a PTC element and an explosion-proof valve (not shown).
- the lower insulating plate 9 is interposed between the bottom surface of the electrode group 14 and the negative electrode lead 6 a led out downward from the electrode group 14.
- the end of the negative electrode lead 6 a is welded to the inner bottom surface of the battery case 1.
- An upper insulating ring (not shown) is placed on the upper surface of the electrode group 14.
- An annular step is formed along the side surface of the battery case 1 above the upper insulating ring.
- the electrode group 14 is fixed in the battery case 1 by the stepped portion.
- a predetermined amount of nonaqueous electrolyte is injected into the battery case 1.
- the positive electrode lead 5 a is bent and accommodated in the battery case 1.
- a sealing plate 2 having a gasket 13 at its peripheral edge is placed.
- a sealed cylindrical lithium ion secondary battery is obtained by caulking the opening end of the battery case 1 inward.
- the electrode group 14 includes a positive electrode 5, a separator 7, a negative electrode 6, and another separator 7 that are stacked in this order and wound in a spiral shape using a core (not shown). It is produced by extracting.
- the components of the electrode group 14 (the positive electrode 5, the negative electrode 6, and the separator 7) are overlapped with the end portions of the two separators 7 protruding from the end portions in the longitudinal direction of the positive electrode 5 and the negative electrode 6. In a state where the protruding end portion of the separator 7 is sandwiched between a pair of cores, the constituent elements of the electrode group 14 are wound. Only a few separators 7 may be wound for the first few turns (for example, the first to third turns).
- the separator of the present invention is particularly useful when a high-capacity electrode group is produced by winding with high tension together with a positive electrode or negative electrode with a large amount of positive electrode active material or negative electrode active material.
- a high-capacity electrode group is produced by winding with high tension together with a positive electrode or negative electrode with a large amount of positive electrode active material or negative electrode active material.
- the probability of occurrence of dendrites and the degree of polarization usually tend to increase when rapidly charged.
- the separator of the present invention when the separator of the present invention is used, the ability of lithium ions to move between electrodes is enhanced, so that the probability of dendrite generation and the degree of polarization are unlikely to increase.
- a high-capacity battery has a capacity density (a value obtained by dividing the nominal capacity of the battery by the mass of the battery), for example, 44000 mAh / kg or more, and further 51000 mAh / kg or more.
- the upper limit of the capacity density is about 75000 mAh / kg.
- the 18650 type high capacity cylindrical battery has a nominal capacity of 2000 mAh or more, preferably 2300 mAh or more, and the use of the separator is suitable.
- the wound electrode group may be a flat electrode group having an oval end surface perpendicular to the winding axis.
- the charging method of the non-aqueous electrolyte secondary battery is not particularly limited, but a method of performing constant current charging until reaching a predetermined battery voltage Vx and then performing constant voltage charging at the battery voltage Vx is preferable.
- the current value gradually decays. Therefore, constant voltage charging is terminated when a predetermined current value is reached or when a predetermined time has elapsed.
- the current value at a time rate of 1 C corresponds to a current value for charging or discharging a capacity corresponding to the nominal capacity in one hour.
- the positive electrode includes, for example, a sheet-like positive electrode current collector and a positive electrode active material layer disposed on the surface of the positive electrode current collector.
- a positive electrode current collector for example, a metal foil formed of aluminum, aluminum alloy, stainless steel, titanium, titanium alloy, or the like can be used.
- the material of the positive electrode current collector can be appropriately selected in consideration of workability, practical strength, adhesion to the positive electrode active material layer, electronic conductivity, corrosion resistance, and the like.
- the thickness of the positive electrode current collector is, for example, 1 to 100 ⁇ m, preferably 10 to 50 ⁇ m.
- the positive electrode active material layer may contain a conductive agent, a binder, a thickener, and the like as optional components in addition to the positive electrode active material.
- a positive electrode active material for example, a lithium-containing transition metal compound that accepts lithium ions as a guest can be used.
- a composite metal oxide of at least one metal selected from cobalt, manganese, nickel, chromium, iron and vanadium and lithium more specifically, LiCoO 2 , LiMn 2 O 4 , LiNiO 2 , LiCo x Ni 1 ⁇ xO 2 (0 ⁇ x ⁇ 1), LiCo y M 1-y O 2 (0.6 ⁇ y ⁇ 1), LiNi z M 1-z O 2 (0.6 ⁇ z ⁇ 1), LiCrO 2 , Examples include ⁇ LiFeO 2 and LiVO 2 .
- M is at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb and B.
- M is preferably at least one selected from Mg and Al.
- the positive electrode active material can be used alone or in combination of two or more.
- the binder is not particularly limited as long as it can be dissolved or dispersed in the dispersion medium by kneading.
- the binder include fluororesins, rubbers, acrylic polymers or vinyl polymers (monomers or copolymers of monomers such as acrylic monomers such as methyl acrylate and acrylonitrile, vinyl monomers such as vinyl acetate), and the like.
- the fluororesin include polyvinylidene fluoride, a copolymer of vinylidene fluoride and propylene hexafluoride, and polytetrafluoroethylene.
- rubbers include acrylic rubber, modified acrylonitrile rubber, and styrene butadiene rubber (SBR). You may use a binder individually or in combination of 2 or more types.
- the binder may be used in the form of a dispersion dispersed in a dispersion medium.
- Examples of the conductive agent include acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black and other carbon blacks; various graphites such as natural graphite and artificial graphite; conductive fibers such as carbon fibers and metal fibers Can be used.
- a thickener may be used as necessary.
- the thickener include ethylene-vinyl alcohol copolymers, cellulose derivatives (carboxymethyl cellulose, methyl cellulose, etc.) and the like.
- the dispersion medium is not particularly limited as long as the binder can be dissolved or dispersed, and either an organic solvent or water (including warm water) can be used depending on the affinity of the binder for the dispersion medium.
- the organic solvent include N-methyl-2-pyrrolidone; ethers such as tetrahydrofuran; ketones such as acetone, methyl ethyl ketone and cyclohexanone; amides such as N, N-dimethylformamide and dimethylacetamide; sulfoxides such as dimethyl sulfoxide; Examples include tetramethylurea. You may use a dispersion medium individually or in combination of 2 or more types.
- the positive electrode active material layer is prepared by preparing a slurry-like mixture in which a positive electrode active material and, if necessary, a binder, a conductive agent and / or a thickener are kneaded and dispersed together with a dispersion medium. It can be formed by attaching to a current collector. Specifically, the positive electrode active material layer can be formed by applying a mixture to the surface of the positive electrode current collector by a known coating method, drying, and rolling if necessary. Part of the positive electrode current collector is formed with a portion where the surface of the current collector is exposed without forming the positive electrode active material layer, and the positive electrode lead is welded to the exposed portion.
- the positive electrode is preferably superior in flexibility.
- the mixture can be applied using a known coater, for example, a slit die coater, a reverse roll coater, a lip coater, a blade coater, a knife coater, a gravure coater, or a dip coater. Drying after coating is preferably performed under conditions close to natural drying, but may be dried at a temperature range of 70 ° C. to 200 ° C. for 10 minutes to 5 hours in consideration of productivity.
- the rolling of the active material layer can be performed by, for example, using a roll press machine and repeating the rolling several times under a linear pressure of 1000 to 2000 kgf / cm (19.6 kN / cm) until a predetermined thickness is reached. it can. If necessary, the linear pressure may be changed and rolled.
- the positive electrode active material layer can be formed on one side or both sides of the positive electrode current collector.
- the active material density in the positive electrode active material layer is 3 to 4 g / ml, preferably 3.4 to 3.9 g / ml or 3.5 to 3.7 g / ml when a lithium-containing transition metal compound is used as the active material. is there.
- the thickness of the positive electrode is, for example, 70 to 250 ⁇ m, preferably 100 to 210 ⁇ m.
- the negative electrode includes, for example, a sheet-like negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector.
- a metal foil formed of copper, copper alloy, nickel, nickel alloy, stainless steel, aluminum, aluminum alloy, or the like can be used.
- the negative electrode current collector is preferably a copper foil or a metal foil made of a copper alloy in consideration of processability, practical strength, adhesion to the negative electrode active material layer, electronic conductivity, and the like.
- the form of the current collector is not particularly limited, and may be, for example, a rolled foil, an electrolytic foil, a perforated foil, an expanded material, a lath material, or the like.
- the thickness of the negative electrode current collector is, for example, 1 to 100 ⁇ m, preferably 2 to 50 ⁇ m.
- the negative electrode active material layer may contain a conductive agent, a binder, a thickener and the like in addition to the negative electrode active material.
- a material having a graphite type crystal structure capable of reversibly occluding and releasing lithium ions such as natural graphite, spherical or fibrous artificial graphite, non-graphitizable carbon (hard carbon), easy graphite Examples thereof include carbon materials such as carbonizable carbon (soft carbon).
- a carbon material having a graphite-type crystal structure in which a lattice spacing (002) interval (d002) is 0.3350 to 0.3400 nm is preferable.
- silicon; silicon-containing compounds such as silicide; lithium alloys containing at least one selected from tin, aluminum, zinc, and magnesium, and various alloy materials can also be used.
- Examples of the silicon-containing compound include silicon oxide SiO ⁇ (0.05 ⁇ ⁇ 1.95). ⁇ is preferably 0.1 to 1.8, more preferably 0.15 to 1.6. In the silicon oxide, a part of silicon may be substituted with one or more elements. Examples of such elements include B, Mg, Ni, Co, Ca, Fe, Mn, Zn, C, N, and Sn.
- the binder As the binder, the conductive agent, the thickener, and the dispersion medium, those exemplified for the positive electrode can be used.
- the negative electrode active material layer is not limited to the coating using a binder or the like, but can be formed by a known method.
- the negative electrode active material may be formed by depositing on the current collector surface by a vapor phase method such as a vacuum deposition method, a sputtering method, or an ion plating method.
- a slurry-like mixture containing a negative electrode active material, a binder, and a conductive material as necessary may be formed by the same method as that for the positive electrode active material layer.
- the negative electrode active material layer may be formed on one side of the negative electrode current collector or on both sides.
- the active material density is 1.3 to 2 g / ml, preferably 1.4 to 1.9 g / ml, more preferably 1 .5 to 1.8 g / ml.
- the thickness of the negative electrode is, for example, 100 to 250 ⁇ m, preferably 110 to 210 ⁇ m.
- a flexible negative electrode is preferred.
- Non-aqueous electrolyte The nonaqueous electrolyte is prepared by dissolving a lithium salt in a nonaqueous solvent.
- the non-aqueous solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; chain carbonates such as dimethyl carbonate and diethyl carbonate; lactones such as ⁇ -butyrolactone; halogenated alkanes such as 1,2-dichloroethane; Alkoxyalkanes such as 1,2-dimethoxyethane and 1,3-dimethoxypropane; ketones such as 4-methyl-2-pentanone; ethers such as 1,4-dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; acetonitrile, propionitrile Nitriles such as butyronitrile, valeronitrile and benzonitrile; sulfolane, 3-methyl-sulfolane; amide
- lithium salts having a strong electron-withdrawing property such as LiPF 6 , LiBF 4 , LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , LiN (SO 2 CF 3 ) 2 , LiN (SO 2 C 2 F 5 ). 2 and LiC (SO 2 CF 3 ) 3 .
- a lithium salt can be used individually or in combination of 2 or more types.
- the concentration of the lithium salt in the nonaqueous electrolyte is, for example, 0.5 to 1.5M, preferably 0.7 to 1.2M.
- An additive may be appropriately added to the nonaqueous electrolyte.
- vinylene carbonate (VC), cyclohexylbenzene (CHB), and modified products thereof may be used.
- VC vinylene carbonate
- CHB cyclohexylbenzene
- modified products thereof may be used.
- an additive that acts when the lithium ion secondary battery is overcharged for example, terphenyl, cyclohexylbenzene, diphenyl ether, or the like may be used.
- the additives may be used alone or in combination of two or more.
- the ratio of these additives is not particularly limited, but is, for example, about 0.05 to 10% by mass with respect to the non-aqueous electrolyte.
- Battery cases include cylindrical and square cases with an open top, and the material is aluminum alloy containing a small amount of metals such as manganese and copper, inexpensive nickel-plated steel from the viewpoint of pressure strength A spear or the like is preferable.
- the separator of the present invention can be used in a high capacity battery such as a 18650 type cylindrical battery.
- Example 1 (1) Preparation of positive electrode 5 An appropriate amount of N-methyl-2-pyrrolidone, 100 parts by mass of lithium cobaltate as a positive electrode active material, 2 parts by mass of acetylene black as a conductive agent, and polyvinylidene fluoride resin as a binder 3 parts by mass was added and kneaded to prepare a slurry mixture. This slurry was continuously applied to both surfaces of a strip-shaped aluminum foil (thickness: 15 ⁇ m) which is a positive electrode current collector. However, an exposed portion of the aluminum foil for connecting the positive electrode lead 5a was left.
- the dried coating film of the mixture was rolled together with the positive electrode current collector at a linear pressure of 1000 kgf / cm (9.8 kN / cm) 2 to 3 times, the thickness of the coating film was adjusted to 180 ⁇ m, and the positive electrode active material layer It was. Then, the positive electrode 5 was obtained by cut
- the active material density of the positive electrode active material layer was 3.6 g / ml.
- the positive electrode lead 5a made of aluminum was ultrasonically welded to the exposed portion of the aluminum foil to which the mixture was not applied.
- An insulating tape made of polypropylene resin was attached to the ultrasonic welded portion so as to cover the positive electrode lead 5a.
- the negative electrode current collector for 30 minutes is rolled together with the negative electrode current collector at a linear pressure of 110 kgf / cm (1.08 kN / cm) 2 to 3 times to adjust the thickness of the coating film to 174 ⁇ m.
- a negative electrode active material layer was obtained.
- the negative electrode 6 was obtained by cut
- the active material density of the negative electrode active material layer was 1.6 g / ml.
- the negative electrode lead 6a made of nickel was resistance-welded to the exposed portion of the copper foil to which the mixture was not applied.
- An insulation tape made of polypropylene resin was attached to the resistance welded portion so as to cover the negative electrode lead 6a.
- a polyethylene having an average molecular weight of 3 million and a polyethylene having an average molecular weight of 1 million were prepared at a mass ratio of 2: 1 and mixed using a high-speed mixer stirring device. Further, 0.3% by mass of diorganopolysiloxane as a processing aid and 0.15% by mass of stearic acid as a lubricant were added and uniformly melt mixed. And the extrusion molding was performed with the extruder and the polyethylene sheet was produced. Next, the polyethylene sheet was stretched at 145 ° C. to obtain a nonporous film.
- the nonporous film obtained by the above process was passed through a pair of nip rolls, and the film was stretched in the machine axis direction of the rolls.
- a plurality of different products (polyethylene porous film) were prepared by changing the stretching speed, the number of stretching, the rotational speed of the nip roll, and the nip roll pressure.
- the thickness of the polyethylene porous film was 20 ⁇ m.
- the produced polyethylene porous film was observed with an electron microscope at a magnification of 10,000 times, and the distances between the extended chain crystals and the folded chain crystals that could be confirmed within one field of view were measured.
- a pair of adjacent extended chain crystals and a pair of folded chain crystals are selected from an electron micrograph, and the extended chain crystals are each 0.5 ⁇ m (a linear distance along the molecular chain orientation direction).
- the distance between the center positions of each crystal was measured every 0.1 ⁇ m (linear distance along the plane direction of the plate-like crystal). 50 points were measured for each crystal in one visual field, and a total of 150 points were measured in three visual fields, and an average value was calculated.
- separators made of polyethylene porous membrane were classified into 12 types as shown in Table 1.
- the palator was cut into a long sheet size with a width of 60.9 mm and used for the production of an electrode group.
- Electrode Group 14 The positive electrode 5 and the negative electrode 6 were wound in a vortex shape with the separator 7 interposed therebetween to form the electrode group 14. After winding, the separator was cut, the clamping between the pair of cores was loosened, and the cores were extracted from the electrode group. In the electrode group, the length of the separator was 700 to 720 mm.
- the upper insulating ring was placed on the upper surface of the electrode group 14 accommodated in the battery case 1, and an annular step was formed on the upper part of the battery case 1 above.
- the sealing plate 2 was laser welded to the positive electrode lead 5a led out above the battery case 1, and then a nonaqueous electrolyte was injected into the battery case.
- the non-aqueous electrolyte was prepared by dissolving LiPF 6 in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio 2: 1) to a concentration of 1.0M and adding cyclohexylbenzene to 0%. It was prepared by adding 5% by mass.
- EC ethylene carbonate
- EMC ethyl methyl carbonate
- the positive electrode lead 5a was bent and accommodated in the battery case 1, and the sealing plate 2 provided with the gasket 13 at the peripheral edge portion was placed on the stepped portion.
- the cylindrical lithium ion secondary battery was produced by crimping the opening edge part of the battery case 1 inward, and sealing.
- the produced battery is a 18650 type having a diameter of 18.1 mm and a height of 65.0 mm, and has a nominal capacity of 2600 mAh.
- Ten batteries were prepared for each separator classification, and a total of 120 batteries were manufactured.
- the produced battery was evaluated for charge / discharge characteristics.
- the charge / discharge test was performed in a constant temperature bath at 25 ° C. Charging was carried out with rapid charging and a charging rate set to 0.8C or equivalent. The discharge rate was 1C. The discharge capacity was measured for each cycle. This charging / discharging was performed 500 cycles, and the average capacity retention rate with respect to the initial capacity of the battery after 500 cycles was calculated.
- the evaluation results are shown in Table 1.
- sample No. 1, 2, 3, 10, 11, and 12 correspond to comparative examples.
- Example 2 A battery was fabricated in the same manner as in Example 1 except that the separator described below was used.
- a polyethylene porous membrane having an average distance between extended chain crystals of 3.0 ⁇ m or more and less than 5.0 ⁇ m and an average value of distance between folded chain crystals of 0.5 ⁇ m or more and less than 0.7 ⁇ m (No. 1 in Table 1).
- Various structures equivalent to 6) were produced.
- the thickness of the polyethylene porous film was 20 ⁇ m or 17 ⁇ m.
- No. 1 in Table 1 was used for the polyethylene porous membrane having a thickness of 20 ⁇ m.
- the production process conditions were changed as appropriate so that the finished Gurley value and porosity were changed while having a structure equivalent to 6, and several types of polyethylene porous membranes were produced.
- an aramid NMP solution was applied to one surface of a polyethylene porous film having a thickness of 17 ⁇ m.
- a separator having a two-layer structure having a polyethylene porous film and a heat-resistant porous film was obtained.
- the separator having a two-layer structure was adjusted to have a total thickness of 20 ⁇ m.
- the conditions of the manufacturing process were changed as appropriate.
- Several types of polyethylene porous membranes having a structure equivalent to 6 were prepared. Then, as necessary, the finished Gurley value and porosity were changed by appropriately changing the state of the aramid applied.
- a separator having only a polyethylene porous film having a thickness of 20 ⁇ m is a separator A
- a separator having a two-layer structure having a polyethylene porous film and a heat-resistant porous film is a separator B.
- Classify PE porous film having only a polyethylene porous film having a thickness of 20 ⁇ m
- An aramid NMP solution was prepared as follows. First, in a reaction tank, a predetermined amount of dry anhydrous calcium chloride was added to an appropriate amount of NMP and heated to be completely dissolved. After returning this calcium chloride-added NMP solution to room temperature, a predetermined amount of paraphenylenediamine (PPD) was added and completely dissolved. Next, terephthalic acid dichloride (TPC) was added dropwise to the PPD-added solution, and polyparaphenylene terephthalamide (PPTA) was synthesized by a polymerization reaction. The polymerization solution after completion of the reaction was deaerated by stirring for 30 minutes under reduced pressure. The obtained polymerization solution was further appropriately diluted with a calcium chloride-added NMP solution to prepare an aramid NMP solution.
- TPC terephthalic acid dichloride
- PPTA polyparaphenylene terephthalamide
- the Gurley value and porosity of the produced separator were measured, and separator A and separator B were classified into 11 types (Nos. 13 to 23) as shown in Table 2 according to the measured values. Ten batteries were prepared for each separator classification, and a total of 220 batteries were prepared.
- the separator's Gurley value air permeability was measured in accordance with JIS-P8117.
- a digital type Oken type air permeability tester manufactured by Asahi Seiko Co., Ltd.
- the separator was set in the apparatus, and the time required for 100 ml of air to pass through was measured. Measurements were taken five times at different locations, and the average value was calculated.
- the porosity of the separator was measured as follows. In the case of a monolayer film of a polyethylene porous film, five test pieces having an area of 6.0 cm ⁇ 6.0 cm were cut out. The thickness and mass of these test pieces were measured, and the mass w (g / cm 3 ) per volume was calculated from the average value of 5 pieces. Then, using the specific gravity s1 (g / cm 3 ) of polyethylene, the porosity V (%) was calculated from 100 ⁇ (1- (w / s1)).
- the separator of the present invention and the non-aqueous electrolyte secondary battery using the separator are excellent in cycle characteristics and safety.
- the separator of the present invention can be applied to various non-aqueous electrolyte secondary batteries, but is particularly suitable for a high-capacity non-aqueous electrolyte secondary battery in which rapid charging is performed.
Abstract
Description
特許文献1では、ポリテトラフルオロエチレンのようなフッ素樹脂の原料を約300℃の高温で延伸処理することにより、膜厚方向に伸びる結節を有するセパレータを製造することが提案されている。また、膜厚方向に伸びる結節により、セパレータの圧縮強度が高められるとともに、デンドライトの生成や遊離が抑制されると記載されている。
隣接する前記伸びきり鎖結晶間は、前記折りたたみ鎖結晶によって連結されていてもよい。
収縮率は、TMA法で測定する。まず、サイズ5mm×14mmのセパレータのサンプルを準備する。これを引っ張り加重試験機(TMA装置、RIGAKU ThermoPlus 2)内にセットする。上下に治具挟み代を、それぞれ2mmずつ取り、測定時の資料寸法は5mm×10mmとした。空気中で、19.6mNの荷重でサンプルを引っ張った状態で、室温から250℃まで、10℃/分の昇温速度でサンプル温度を変化させながら、その長さの変化を測定した。ポリオレフィン多孔膜のシャットダウン温度の手前からサンプルの収縮が始まる。サンプルは、シャットダウン温度を越えたあたりで、最大収縮を示す。その後、サンプルは再び初期長さ(10mm)に近づいていく。初期長さ(10mm)に対する、最大収縮の際の長さの減少割合を収縮率として算出する。
本発明のセパレータにおける伸びきり鎖結晶と折りたたみ鎖結晶による二軸配向構造は、高分子量ポリオレフィンの溶液の流動配向や、高分子量ポリオレフィンの融液の流動配向により形成される。例えば、高分子量ポリエチレンを、融点以上の温度で延伸することにより、シシカバブ構造を有する多孔膜を形成することができる。伸びきり鎖結晶は、高い融点を有し、折りたたみ鎖結晶は低い融点を有する。シシカバブ構造を有する多孔膜は、このような2種類の結晶による二軸配向構造が複数層重なったような構造を有する。
ポリオレフィン多孔膜を構成するポリオレフィンとしては、ポリエチレン、ポリプロピレン、エチレン-プロピレン共重合体などが例示できる。これらの樹脂は、単独で又は二種以上を組み合わせて使用できる。例えば、均一混合と高密度化の観点から、平均分子量250万以上の第1ポリエチレンと、平均分子量200万未満の第2ポリエチレンとを、高速ミキサーなどの攪拌装置を用いて混合して使用することができる。第1ポリエチレンと第2ポリエチレンとの質量比は、例えば3:1~3:2である。
図3は、ポリオレフィン多孔膜103と耐熱性多孔膜104とを有する2層構造のセパレータの厚さ方向の断面図である。
まず、ポリアミド酸溶液を流延した後、延伸することにより、耐熱性多孔膜の前駆体を作製する。得られた耐熱性多孔膜の前駆体の表面に、ポリオレフィン多孔膜を重ね、ポリオレフィン多孔膜がシャットダウンしない程度の温度に加熱し、両者を一体化させる。このような一体化には、例えば熱ロールが用いられる。熱ロールからの熱により、ポリアミド酸のイミド化が進行し、多孔膜の前駆体中のポリアミド酸は、ポリイミドまたはポリアミドイミドに変換される。必要により、ポリオレフィン多孔膜と重ね合わせる前に、ポリアミド酸を含む多孔膜の前駆体を加熱して、ポリアミド酸をポリアミドまたはポリアミドイミドに変換してもよい。このような方法では、延伸条件を変化させることにより、耐熱性多孔膜の空隙率を制御することができる。
図4は、本発明の一実施形態に係る円筒型リチウムイオン二次電池の一部切り欠き斜視図である。図4のリチウムイオン二次電池は、帯状の正極5と、帯状の負極6とがセパレータ7を介して捲回された電極群14を備えている。電極群14は、非水電解質(図示せず)とともに有底円筒型の金属製の電池ケース1に収容されている。正極5は、金属箔からなる正極集電体とその表面に配した正極活物質層とを備えている。負極6は、金属箔からなる負極集電体とその表面に配した負極活物質層とを備えている。
本発明のセパレータを用いた非水電解質二次電池の場合、一般的な非水電解質二次電池に比べて、高い電流値で定電流充電を行うことが可能である。例えば、0.7Cを超える時間率、さらには0.8Cを超える時間率の電流値で、急速充電することが可能である。なお、1Cの時間率の電流値は、1時間で公称容量分の容量を充電または放電する電流値に対応する。
(正極)
正極は、例えば、シート状の正極集電体と、正極集電体の表面に配した正極活物質層とから構成される。正極集電体としては、例えば、アルミニウム、アルミニウム合金、ステンレス鋼、チタン、チタン合金などで形成された金属箔などが使用できる。正極集電体の材料は、加工性、実用強度、正極活物質層との密着性、電子伝導性、耐食性などを考慮して適宜選択できる。正極集電体の厚さは、例えば、1~100μm、好ましくは10~50μmである。
負極は、例えば、シート状の負極集電体と、負極集電体の表面に配した負極活物質層とから構成される。負極集電体としては、例えば、銅、銅合金、ニッケル、ニッケル合金、ステンレス鋼、アルミニウム、アルミニウム合金などで形成された金属箔などが使用できる。負極集電体は、加工性、実用強度、負極活物質層との密着性、電子伝導性などを考慮して、銅箔、銅合金からなる金属箔などが好ましい。集電体の形態は特に制限されず、例えば、圧延箔、電解箔などであってもよく、孔開き箔、エキスパンド材、ラス材などであってもよい。負極集電体の厚さは、例えば、1~100μm、好ましくは2~50μmである。
柔軟性を有する負極が好ましい。
非水電解質は、非水溶媒にリチウム塩を溶解することにより調製される。非水溶媒としては、例えば、エチレンカーボネート、プロピレンカーボネート、ブチレンカーボネートなどの環状カーボネート;ジメチルカーボネート、ジエチルカーボネートなどの鎖状カーボネート;γ-ブチロラクトンなどのラクトン;1,2-ジクロロエタンなどのハロゲン化アルカン;1,2-ジメトキシエタン、1,3-ジメトキシプロパンなどのアルコキシアルカン;4-メチル-2-ペンタノンなどのケトン;1,4-ジオキサン、テトラヒドロフラン、2-メチルテトラヒドロフランなどのエーテル;アセトニトリル、プロピオニトリル、ブチロニトリル、バレロニトリル、ベンゾニトリルなどのニトリル;スルホラン、3-メチル-スルホラン;ジメチルホルムアミドなどのアミド;ジメチルスルホキシドなどのスルホキシド;リン酸トリメチル、リン酸トリエチルなどのリン酸アルキルエステルなどが例示できる。非水溶媒は、単独で又は二種以上組み合わせて使用できる。
ここでは、図4を参照して説明したのと同様の円筒型リチウムイオン二次電池を作製した。
(1)正極5の作製
適量のN-メチル-2-ピロリドンに、正極活物質としてコバルト酸リチウムを100質量部、導電剤としてアセチレンブラックを2質量部、及び結着剤としてポリフッ化ビニリデン樹脂を3質量部加えて混練し、スラリー状の合剤を調製した。このスラリーを、正極集電体である帯状のアルミニウム箔(厚さ15μm)の両面に連続して塗布した。ただし、正極リード5aを接続するためのアルミニウム箔の露出部を残した。乾燥後の合剤の塗膜を正極集電体とともに線圧1000kgf/cm(9.8kN/cm)で、2~3回圧延し、塗膜の厚さを180μmに調整し、正極活物質層とした。その後、両面に正極活物質層を有する集電体を、幅57mm、長さ620mmのサイズに裁断することにより、正極5を得た。正極活物質層の活物質密度は、3.6g/mlであった。
適量の水に、負極活物質としてリチウムを吸蔵及び放出可能な鱗片状黒鉛を100質量部、結着剤としてスチレンブタジエンゴム(SBR)の水性ディスパージョンを固形分として1質量部、増粘剤としてカルボキシメチルセルロースナトリウムを1質量部加えて混練し、スラリー状の合剤を調製した。このスラリーを、負極集電体である帯状の銅箔(厚さ10μm)の両面に連続して塗布した。ただし、負極リード6aを接続するための銅箔の露出部を残した。110℃で30分間乾燥した後の合剤の塗膜を負極集電体とともに線圧110kgf/cm(1.08kN/cm)で、2~3回圧延し、塗膜の厚さを174μmに調整し、負極活物質層とした。その後、両面に負極活物質層を有する集電体を、幅59mm、長さ645mmのサイズに裁断することにより、負極6を得た。負極活物質層の活物質密度は、1.6g/mlであった。
平均分子量が300万であるポリエチレンと平均分子量が100万であるポリエチレンを質量比2:1で準備して、高速ミキサー攪拌装置を用いて混合した。さらに、加工助剤としてジオルガノポリシロキサンを0.3質量%、潤滑剤としてステアリン酸を0.15質量%加えて、均一に溶融混合した。そして、押出機により押し出し成形を行い、ポリエチレンシートを作製した。次に、ポリエチレンシートを145℃で延伸して、無孔質フィルムを得た。
平均値に応じてポリエチレン多孔膜からなるセパレータを表1に示すように12種類に分類した。パレータは幅60.9mmの長尺シートサイズにカットし、電極群の作製に供した。
なお、No.4、9のセパレータでは、30ポイントで測定されたL2/L1の平均値は1.1~1.5の範囲内であり、No.5、8のセパレータでは1.07~1.40の範囲内であり、No.6、7のセパレータでは1.05~1.35の範囲内であった。
正極5と負極6とを、これらの間に、セパレータ7を介在させて、渦捲状に捲回して電極群14を構成した。捲回後、セパレータを裁断し、一対の捲芯による挟持を緩め、電極群から捲芯を抜き取った。なお、電極群において、セパレータの長さは、700~720mmであった。
ニッケルメッキした鋼鈑(肉厚0.20mm)から、プレス成型により作製した金属製の円筒型電池ケース(直径17.8mm、総高64.8mm)1内に、電極群14及び下部絶縁板9を収納した。このとき、下部絶縁板9は、電極群14の底面と電極群14から下方に導出された負極リード6aとの間に挟持させた。負極リード6aは、電池ケース1の内底面と抵抗溶接した。
作製した電池は、直径18.1mm、高さ65.0mmの18650型で、公称容量2600mAhである。電池は、セパレータの分類毎に、それぞれ10個ずつ作製し、全部で120個作製した。
充放電試験は、25℃の恒温槽中で行った。充電は、急速充電とし、充電レートを0.8C相当に設定して行った。また、放電レートは、1C相当とした。放電容量はサイクル毎に測定した。この充放電を500サイクル行い、500サイクル経過した電池の、初期容量に対する平均容量維持率を算出した。評価結果を表1に示す。
なお、表1において、サンプルNo.1、2、3、10、11および12は、比較例に相当する。
以下に述べるセパレータを用いる以外は、実施例1と同様に電池を作製した。
2 封口板
5 正極
5a 正極リード
6 負極
6a 負極リード
7 セパレータ
9 下部絶縁板
12 正極外部端子
13 ガスケット
14 電極群
101、111 伸びきり鎖結晶
102、112 折りたたみ鎖結晶
103 ポリオレフィン多孔膜
104 耐熱性樹脂多孔膜
Claims (8)
- 伸びきり鎖結晶と、折りたたみ鎖結晶と、を含む二軸配向性のポリオレフィン多孔膜を含み、
前記伸びきり鎖結晶と前記折りたたみ鎖結晶とが、シシカバブ構造を形成しており、
隣接する前記伸びきり鎖結晶間の平均距離が、1.5μm以上、11μm未満であり、
隣接する前記折りたたみ鎖結晶間の平均距離が、0.3μm以上、0.9μm未満である、非水電解質二次電池用セパレータ。 - さらに、前記ポリオレフィン多孔膜に積層された耐熱性多孔膜を有し、前記耐熱性多孔膜は、前記ポリオレフィン多孔膜の融点よりも高い融点もしくは耐熱性を有する樹脂を含む、請求項1記載の非水電解質二次電池用セパレータ。
- 前記ポリオレフィン多孔膜が、異常時に細孔を閉塞するシャットダウン機能を有し、
シャットダウン時の収縮率が10%以下である、請求項2記載の非水電解質二次電池用セパレータ。 - 前記耐熱性多孔膜が、前記樹脂100質量部に対して、50~400質量部の無機フィラーを含む、請求項1~3のいずれか1項に記載の非水電解質二次電池用セパレータ。
- ガーレー値が180秒/100ml以上、260秒/100ml未満であり、空隙率が50%以上、62%未満である、請求項1~4のいずれか1項に記載の非水電解質二次電池用セパレータ。
- 前記ポリオレフィン多孔膜が、平均分子量250万以上の第1ポリエチレンと、平均分子量200万未満の第2ポリエチレンとの混合物を含む、請求項1~5のいずれか1項に記載の非水電解質二次電池用セパレータ。
- 正極と、負極と、前記正極と前記負極との間に介在する請求項1~6のいずれか1項に記載のセパレータとを含む電極群、並びに非水電解質を備える、非水電解質二次電池。
- 44000mAh/kg以上の容量密度を有する、請求項7記載の非水電解質二次電池。
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