JP2012209197A - Multilayer separator and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multilayer separator which can suppress the occurrence of warpage and film peeling.SOLUTION: A multilayer separator 10 includes at least one layer each of a first porous film 12 which contains a heat resistant resin having a melting point or a glass transition point of 150°C or higher and has a porosity of 40% or less by volume and a second porous film 14 which contains a polyolefin resin having a melting point of 150°C or lower; and has a groove 1 having such a shape that a groove width of a bottom part is narrower than a groove width of an opening part on the surface of the first porous film 12.

Description

  The present invention relates to a multilayer separator and a lithium ion secondary battery.

  Conventionally, various porous membranes have been developed and used as filters, electrolytic membranes, separators for nonaqueous solvent batteries, and the like.

  In the field of lithium ion secondary batteries, since active materials with high reactivity are used, various safety devices are provided in the battery or the equipment used. In a lithium ion secondary battery, a separator that separates a positive electrode and a negative electrode as one means for preventing the battery from being heated, ignited, or ruptured due to a short circuit or overcharge of an external circuit. Is being used. In other words, the pores of polyethylene or polypropylene microporous membranes are blocked by heat generated during abnormalities and have a function to stop the battery reaction through the separator, and maintain the shape of the separator even at high temperatures. However, it is required to have a function of preventing direct contact between the positive electrode and the negative electrode.

  In particular, in large capacity lithium ion secondary batteries, for which demand has been increasing in recent years, the capacity is so large that when an internal short circuit occurs, the location generates heat and the internal short circuit is likely to expand. The development of a high-performance separator that can avoid common accidents is eagerly desired.

  Furthermore, a separator of a microporous membrane produced by stretching that is currently widely used does not necessarily have sufficient membrane shape maintaining characteristics, and a separator that has excellent membrane shape maintaining properties even at high temperatures is required.

  In order to solve the problems of the conventional polyolefin microporous membrane, various attempts have been made so far. One of them is a heat-resistant resin having a polyolefin microporous membrane as a base material and a melting point higher than that of polyolefin. A multi-layer separator integrated with a porous membrane made of a coating material has been proposed as a separator that achieves both a function of closing a membrane hole during abnormal heat generation and a membrane shape maintaining property at a high temperature (see Patent Document 1). Specifically, in Patent Document 1, as a battery separator, by using a multilayer separator in which a microporous membrane made of polyolefin and a porous membrane made of a heat resistant resin are combined, polyolefin is generated by heat generated by a micro short circuit. Even when melted, the heat-resistant resin remains unmelted, does not contact the positive electrode and the negative electrode, prevents a secondary short circuit in a large area, and prevents a rapid temperature rise of the battery due to the short circuit. It is disclosed that it can be done.

  In addition, all the multilayer separators disclosed in Patent Document 1 are obtained by coating a heat-resistant resin film on one side of a polyolefin microporous film. General methods for forming a porous layer include a melt extrusion method, a phase separation method, a solvent casting method, and the like. When a porous film is formed by these methods, the heat resistant resin precipitates and solidifies. This increases the density and thus shrinks the volume. For this reason, the single-side coating is not practical because the multilayer separator is severely warped (curled) in order to alleviate this shrinkage.

  Therefore, in order to reduce the warpage of the multilayer separator, a separator in which a porous film made of a heat-resistant layer resin is coated on both sides of a polyethylene microporous film has been proposed (see Patent Document 2).

JP 2006-289657 A Japanese Patent Application No. 2005-209570

  However, in the case of the multilayer separator as disclosed in the above-mentioned Patent Document 2, although the warpage is suppressed because the stress is equally applied on both sides, the stress is not relaxed, so that the film always remains in the peeling direction. There is a problem that contraction stress works and film peeling is likely to occur.

  This invention is made | formed in view of the subject which the said prior art has, and provides the multilayer separator which can suppress generation | occurrence | production of curvature and film | membrane peeling, and a lithium ion secondary battery using the same. Objective.

  As a result of intensive studies to solve the above problems, the present inventors have formed at least one groove on one surface of the multilayer separator, so that the same porous membrane is formed on both surfaces of the multilayer separator. It has been found that it is possible to suppress the occurrence of warping without forming the film, and the present invention has been completed.

  That is, the present invention provides a multilayer separator comprising a laminate of two or more porous membranes and having a groove on at least one surface of the laminate.

  According to the multilayer separator, stress can be dispersed / relaxed at the groove portion formed on the surface of the laminate, the occurrence of warpage can be suppressed, and stress does not remain. Peeling can be suppressed. Further, even when the multilayer separator is bent and used as a constituent member of the battery as in a wound battery, the formation of a groove can reduce the occurrence of wobbling due to the difference between the inner and outer circumferences.

  In the multilayer separator, it is preferable that the groove has a shape in which the groove width at the bottom is narrower than the groove width at the opening in a cross section cut along a plane perpendicular to the extending direction. Further, the shape of the cross section of the groove is more preferably V-shaped or U-shaped. Since the groove has such a cross-sectional shape, the function of the porous film having the groove can be sufficiently maintained while the stress is sufficiently dispersed and relaxed in the groove portion.

  The multilayer separator includes, as the porous film, a first porous film containing a heat-resistant resin having a melting point or glass transition point of 150 ° C. or higher, and a second resin containing a polyolefin resin having a melting point of 150 ° C. or lower. It is preferable that at least one porous film is included and the groove is formed on the surface of the first porous film.

  The multilayer separator including the first porous film and the second porous film can obtain a film shape maintaining characteristic at a high temperature by the first porous film, and abnormal heat is generated by the second porous film. Since the function of closing the membrane hole at the time can be obtained, the occurrence of an internal short circuit can be sufficiently suppressed. In the multilayer separator, the first porous film containing the heat-resistant resin is likely to shrink in volume during film formation, but the groove is formed on the surface of the first porous film. , The stress is dispersed / relaxed, the occurrence of warpage in the multilayer separator can be sufficiently suppressed, and the film peeling can be sufficiently suppressed. Furthermore, since the stress is dispersed and relaxed by the presence of the groove, it is possible to sufficiently suppress the occurrence of warpage in the multilayer separator without forming the first porous film on both sides of the second porous film. Can do.

  Here, it is preferable that the multilayer separator includes the first porous film as at least one outermost layer of the laminate, and has the groove only in the first porous film which is the outermost layer. Since the multilayer separator has grooves only in the first porous film of the outermost layer, the stress can be sufficiently dispersed and relaxed, and the occurrence of warpage and film peeling can be sufficiently suppressed. In the multilayer separator, it is preferable not to form a groove in the second porous membrane. Moreover, when the 1st porous film which is the outermost layer is formed in both surfaces of the 2nd porous film, a groove | channel will be formed in both 1st porous films.

  In the multilayer separator, the porosity of the first porous film having the groove is preferably 40% by volume or less. When the porosity of the first porous film having grooves is 40% by volume or less, the heat resistance can be further improved, and more excellent film shape maintaining characteristics at high temperatures can be obtained. At this time, since the surface area of the first porous membrane is improved by having a groove, sufficient flow of the electrolyte solution, ease of movement of lithium ions, etc. is ensured even if the porosity is lowered. can do.

  In the multilayer separator, the thickness of the porous film having the groove may be larger than the depth of the groove. In this case, since the groove is cut only halfway in the thickness direction of the porous film, the function of the porous film can be sufficiently ensured. For example, when the porous film having grooves is the first porous film containing a heat-resistant resin, sufficient heat resistance can be ensured.

  The present invention also provides a lithium ion secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator is the multilayer separator of the present invention. In such a lithium ion secondary battery, since the separator is the multilayer separator of the present invention described above, warpage of the separator and film peeling are suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, the multilayer separator which can suppress generation | occurrence | production of curvature and film | membrane peeling, and a lithium ion secondary battery using the same can be provided.

It is a schematic cross section which shows one Embodiment of the multilayer separator of this invention. It is a top view which shows one Embodiment of the multilayer separator of this invention. It is a schematic cross section which shows the cross-sectional shape of the groove | channel in the multilayer separator of this invention. It is a top view which shows the arrangement pattern of the groove | channel in the multilayer separator of this invention. It is a schematic cross section which shows one Embodiment of the lithium ion secondary battery of this invention.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings are not limited to the illustrated ratios.

(Multilayer separator)
FIG. 1 is a schematic cross-sectional view showing an embodiment of the multilayer separator of the present invention, and FIG. 2 is a top view thereof. As shown in FIGS. 1 and 2, the multi-layer separator 10 of the present embodiment has a structure in which two porous films, a first porous film 12 and a second porous film 14, are laminated, Grooves 1 are formed in the first porous film 12. In the present embodiment, the first porous film 12 is a porous film containing a heat resistant resin, and the second porous film 14 is a porous film containing a polyolefin resin. In the multilayer separator 10, the groove 1 has a shape in which the groove width W2 of the bottom 1b is narrower than the groove width W1 of the opening 1a in a cross section (FIG. 1) cut along a plane perpendicular to the extending direction. ing. More specifically, the groove 1 has a V-shaped cross-sectional shape, and the groove width W2 of the bottom 1b is substantially zero. Moreover, the groove | channel 1 is formed in the grid | lattice form seeing from the upper surface, as shown in FIG.

  An example of a cross-sectional shape of the groove 1 is shown in FIGS. 3A to 3E are cross-sectional views taken along a plane perpendicular to the extending direction of the groove 1. The groove 1 shown in FIG. 3A has the same V-shaped cross-sectional shape as that shown in FIG. The groove 1 shown in FIG. 3B has a U-shaped (semicircular or semielliptical) cross-sectional shape. The groove 1 shown in FIGS. 3C and 3D has a rectangular cross-sectional shape. The groove 1 shown in FIG. 3E has a trapezoidal cross-sectional shape.

  The grooves 1 having V-shaped and U-shaped cross-sectional shapes shown in FIGS. 3A and 3B are formed by wall surfaces inclined from the opening 1a toward the bottom 1b. In these cross-sectional shapes, the groove width W2 of the bottom 1b is narrower than the groove width W1 of the opening 1a, and the bottom 1b is in point contact with the second porous film 14, so that the bottom The groove width W2 of 1b is substantially zero. Since the first porous membrane 12 having the groove 1 having such a cross-sectional shape has a large contact area with the second porous membrane 14, the contact area is substantially the same as that without the groove 1. By relaxing the stress by the groove 1, it is possible to suppress the peeling of the first porous film 12 from the second porous film 14, and the effect of the first porous film 12, that is, the high temperature. The effect of improving the film shape maintaining characteristic can be obtained more sufficiently.

  The groove 1 having a rectangular cross section shown in FIGS. 3C and 3D is formed by a wall surface perpendicular to the surface of the first porous film 12. The bottom 1b is formed by the surface of the second porous film 14 in FIG. 3C, and is formed by a surface parallel to the surface of the first porous film 12 in FIG. In these cross-sectional shapes, the groove width W1 of the opening 1a and the groove width W2 of the bottom 1b are substantially the same. The first porous film 12 having the groove 1 having a cross-sectional shape shown in FIG. 3C can obtain a sufficient effect of suppressing warpage. On the other hand, the first porous film 12 having the groove 1 having the cross-sectional shape shown in FIG. 3 (d) is covered with the first porous film 12 because the entire surface of the second porous film 14 is covered with the first porous film 12. The effect of the porous film 12, that is, the effect of improving the film shape maintaining characteristics at high temperatures can be obtained more sufficiently.

  The groove | channel 1 which has the trapezoid cross-sectional shape shown in FIG.3 (e) is formed of the wall surface inclined toward the bottom part 1b from the opening part 1a. The bottom portion 1 b is formed by the surface of the second porous film 14. In this cross-sectional shape, the groove width W2 of the bottom 1b is narrower than the groove width W1 of the opening 1a. Since the first porous membrane 12 having the groove 1 having such a cross-sectional shape can increase the contact area with the second porous membrane 14 as compared with that shown in FIG. The effect of the porous film 12, that is, the effect of improving the film shape maintaining characteristics at high temperatures can be obtained more sufficiently.

  Among these cross-sectional shapes, the effects of suppressing the occurrence of warping and film peeling and the effect of improving the film shape maintaining characteristics at high temperatures can be obtained at a high level in a balanced manner. V-shaped and U-shaped shown in FIG.

  The depth D of the groove 1 may be the same as the thickness of the first porous film 12 (FIGS. 3 (a), (b), (c) and (e)), and the first porous film The thickness may be less than 12 (FIG. 3D). As the depth D is larger, the effect of suppressing warpage tends to be improved, and as the depth D is smaller, the film shape maintaining characteristic at high temperature tends to be improved.

  The groove width W1 in the opening 1a of the groove 1 is preferably 500 μm or less, and more preferably 100 μm or less. When the groove width W1 exceeds 500 μm, the film shape maintaining characteristics at high temperatures tend to deteriorate. On the other hand, the groove width W2 at the bottom 1b of the groove 1 is preferably 500 μm or less, and more preferably 100 μm or less. When the groove width W2 exceeds 500 μm, the film shape maintaining characteristic at high temperature tends to be lowered.

  The cross-sectional shape of the groove 1 may be other than those described above. For example, the groove 1 may have a depth D that is less than the thickness of the first porous membrane 12 and a cross-sectional shape that is V-shaped, U-shaped, or trapezoidal. The wall surface of the groove 1 may be any surface such as a flat surface or a curved surface.

  The number of locations where the groove 1 is formed may be any number as long as it is one or more, all the grooves may be connected, or a plurality of grooves may be divided. From the viewpoint of film shape maintenance characteristics at high temperatures, it is preferable that a plurality of grooves are divided and formed, and from the viewpoint of reducing warpage, grooves are uniformly formed throughout the first porous film 12. It is preferable that they are distributed. In addition, since the film shape maintenance characteristic at high temperature is so excellent that the area of the 2nd porous film 14 exposed in the bottom part 1b of the groove | channel 1 is made small, in order to obtain a desired film | membrane shape maintenance characteristic, It is desirable to adjust the cross-sectional shape and arrangement pattern.

  The example of the arrangement pattern of the groove | channel 1 is shown to Fig.4 (a)-(c). 4A to 4C are top views of the multilayer separator 10 as viewed from the first porous membrane 12 side. The arrangement pattern of the grooves 1 shown in FIG. 4A is an arrangement pattern in which the grooves 1 are not formed at the intersections of the grooves 1 in the lattice-like arrangement pattern shown in FIG. Thus, when the groove | channel 1 is discontinuously arrange | positioned, since the 1st porous film 12 turns into a continuous film | membrane, the film shape maintenance characteristic at high temperature can be improved more fully.

  As an example in which the grooves 1 are discontinuously disposed similarly to that illustrated in FIG. 4A, an arrangement pattern illustrated in FIG. The arrangement pattern of the grooves 1 shown in FIG. 4B is such that the extending direction of the grooves 1 is one direction, and the position of the discontinuous portion in the extending direction of the grooves 1 is shifted from the adjacent grooves 1. It is a thing. Even with such an arrangement pattern, the film shape maintaining characteristics at high temperatures can be more sufficiently improved.

  Further, as in the arrangement pattern of the grooves 1 shown in FIG. 4C, the discontinuous first porous films 12 may be alternately arranged while the grooves 1 are continuously arranged.

  Hereinafter, each porous membrane which comprises the multilayer separator 10 is demonstrated.

  The second porous membrane 14 is a porous membrane (polyolefin microporous membrane) containing a polyolefin resin. The second porous film 14 containing the polyolefin resin may be made of any material as long as it is known, and may be manufactured by any manufacturing method.

  Examples of the polyolefin resin used for the second porous membrane 14 include a crystalline homopolymer or copolymer made by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. Coalescence is mentioned. These homopolymers or copolymers can be used singly or in combination of two or more. The polyolefin resin used for the second porous membrane 14 preferably has a melting point of 150 ° C. or less from the viewpoint of obtaining a good function of closing the membrane pores during abnormal heat generation.

  The second porous membrane 14 usually has a porosity of 30 to 95% by volume, an air permeability at a film thickness of 25 μm of 2000 seconds / 100 cc or less, preferably 800 seconds / 100 cc or less, and an average through-hole diameter of 0. A microporous membrane having mechanical properties of 0.005 to 1 μm, a tensile breaking strength of 80 MPa or more, preferably 100 MPa or more, and a puncture strength of 3000 mN or more, preferably 5500 mN or more is desirable.

  In the present specification, the porosity is a value obtained by dividing the volume of the pore portion of the porous membrane by the total volume of the pore portion and the solid portion of the porous membrane. This porosity can be measured, for example, by a gravimetric method.

  Although the thickness of the 2nd porous membrane 14 is selected suitably, it is 0.1-50 micrometers normally, Preferably it is 1-25 micrometers. If the thickness is less than 0.1 μm, it tends to be difficult to put to practical use due to insufficient mechanical strength of the film, and if it exceeds 50 μm, the effective resistance tends to be too large.

  The first porous film 12 is a porous film containing a heat resistant resin. The first porous membrane 12 is a coating layer for increasing the heat resistance of the multilayer separator 10 and is limited particularly if it is thermally stable under general conditions when using a lithium ion secondary battery. is not.

  In a lithium ion secondary battery, thermal melting of the separator often becomes a problem at about 150 ° C. or higher. Therefore, the first porous film 12 is a porous material having heat resistance particularly in the range of about 150 to 500 ° C. It is preferable to employ a membrane. From the viewpoint of obtaining the first porous film 12 having excellent heat resistance and excellent film shape maintaining characteristics at high temperatures, the heat resistant resin is preferably a resin having a melting point or glass transition point of 150 ° C. or higher.

  The heat-resistant resin (heat-resistant polymer) is not particularly limited, and various known resins can be used. Since the multilayer separator 10 is used as a battery separator for a lithium ion secondary battery, It is desirable that it has an affinity for the electrolyte and at the same time is stable against an electrolytic solution and a battery reaction and has sufficient heat resistance.

  Examples of the heat resistant resin that meets such requirements include polyimide, polyether ether ketone, polyamide, polyamide imide, polyether sulfide, polyether imide, polysulfone, and polyphenylene sulfide. These may be used in the state of a linear polymer, but may be used in the state of a precursor such as a monomer, oligomer, or prepolymer, and the precursor may be post-polymerized by a method such as heating to form a crosslinked product. Among these, polyamideimide is particularly preferable from the viewpoint of film formability. These heat resistant resins may be used alone or in combination of two or more.

  The average pore diameter of the first porous membrane 12 is usually 0.1 μm or more, preferably 0.2 μm or more.

  The porosity of the first porous film 12 is preferably large in order to reduce the total amount of stress generated, and is preferably small from the viewpoint of heat resistance. Note that the first porous film 12 having the grooves 1 can relieve stress due to the presence of the grooves 1 and the surface area is improved, so that the porosity can be made relatively small. The porosity of the first porous membrane 12 having the groove 1 is preferably 40% by volume or less, and more preferably 25 to 35% by volume from the viewpoint of obtaining excellent heat resistance.

  A method for forming the first porous film 12 will be described below. As a method of forming the first porous membrane 12 containing the above heat-resistant resin on at least one surface of the second porous membrane 14, a phase separation method that is a production method generally used for the production method of a separation membrane is used. In addition, the extraction method, the stretching method, the charged particle irradiation method, and the like can be used. However, the second porous film 14 is damaged in the formation process, or the formation of the second porous film 14 is caused by the formation. It is not preferable to inhibit the characteristics. Therefore, the mechanical characteristics and material permeation characteristics of the second porous membrane 14 are not exposed to a temperature exceeding the melting point of the polyolefin resin contained in the second porous membrane 14 and are not accompanied by chemical degradation or radiation degradation. As a method that does not impair the above, for example, a porous method by phase separation of a polymer substance as shown below can be suitably used.

  That is, after at least one surface of the second porous membrane 14 is coated with a heat-resistant resin solution in which a heat-resistant resin is dissolved in a good solvent and phase-separated by contacting with a coagulation liquid containing a poor solvent, By drying, the porous first porous film 12 can be formed.

  Here, the heat-resistant resin solution is usually applied by a conventional casting or coating method such as a roll coater, an air knife coater, a blade coater, a rod coater, a bar coater, a comma coater, a gravure coater, a silk screen coater, or a die coater. And the micro gravure coater method. The groove | channel 1 can be formed by apply | coating so that it may become a desired pattern, when apply | coating a heat resistant resin solution with the said application | coating method.

  The solvent used for the heat resistant resin solution is appropriately selected according to the properties of the heat resistant resin as shown below. For example, when the heat resistant resin is polyamideimide, the good solvent is not particularly limited, but N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DAMc), N-methyl- Examples include 2-pyrrolidone (NMP). In addition, when moisture exists in the heat resistant resin solution, it is preferable to remove the moisture by treating the heat resistant resin solution with a heated and dehydrated molecular sieve.

  The concentration of the heat-resistant resin in the heat-resistant resin solution is not particularly limited as long as it is a concentration at which a viscosity suitable for film formation is obtained, but is generally based on the total amount of the heat-resistant resin solution. A range of 1 to 20% by mass is suitable.

  In the lithium ion secondary battery separator, in order to make the pore structure of the first porous membrane 12 made of the heat resistant resin appropriate, it is preferable to mix a phase separator in the heat resistant resin solution. The concentration of the phase separation agent is preferably 5 to 50% by mass based on the total amount of the heat resistant resin solution.

  Examples of the phase separation agent include polypropylene glycol, polyethylene glycol, diethylene glycol, triethylene glycol, tripropylene glycol, 1,3-butanediol, 1,4-butanediol, glycerin, polyvinylpyrrolidone, etc. Any material that is soluble in both the good solvent of the resin and the coagulation liquid can be used.

  The coagulation liquid is composed of a mixture of a good solvent and a poor solvent for the heat resistant resin. The proportion of the poor solvent is preferably 30 to 80% by mass based on the total amount of the coagulation liquid. In addition, when a phase separation agent is used in the heat resistant resin solution, the phase separation agent is also added to the coagulation liquid so that the content ratio of the good solvent and the phase separation agent in the heat resistant resin solution is almost equal. It is preferable in the process to add.

  When the heat resistant resin is polyamideimide, the good solvent is the same as the good solvent used for the heat resistant resin solution described above. On the other hand, examples of the poor solvent include alcohols such as methanol and ethanol, benzene, methyl isobutyl ketone, dimethylformamide, water, and the like. Of these, alcohols and water are preferable.

  Further, when the heat-resistant resin is polyether ether ketone, polyamide, polyether sulfone, polyether imide, polysulfone, polyphenylene sulfide, examples of good solvents include cyclohexanone, γ-butyrolactone, ethylene carbonate, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethyl sulfone and the like can be mentioned. On the other hand, examples of the poor solvent include alcohols such as methanol and ethanol.

  Next, the outline of a method for making a porous layer by phase-separating a coating film made of a heat resistant resin solution will be described by taking as an example the case where polyamideimide is used as the heat resistant resin.

  As described above, the coating film after the heat-resistant resin solution is applied onto the second porous film 14 causes the heat-resistant resin to be phase-separated by a method using a coagulating liquid containing a poor solvent. The phase separation is performed by a method in which, for example, a mixed liquid of water and N-methyl-2-pyrrolidone is used as a coagulating liquid and this is brought into contact with the coating film. At this time, the surface porosity can be increased by humidifying the coating film before bringing it into contact with the coagulation liquid. By humidifying the coating film and then immersing it in the coagulation liquid, the solvent removal proceeds after the heat-resistant resin solution is sufficiently phase separated, and the surface layer structure is less likely to become dense. The humidification is preferably performed at a relative humidity of 60 to 100%. The phase-separated coating film is subsequently washed with water and then dried to complete the porous process.

  For a heat-resistant resin other than polyamide-imide, if a solution containing a heat-resistant resin is prepared, applied to the second porous film 14 and phase-separated by performing the same steps as described above, The formed first porous film 12 can be easily formed.

  As mentioned above, although one suitable embodiment of the present invention was described in detail, the present invention is not limited to the above-mentioned embodiment. For example, in the above embodiment, the case where the multilayer separator is composed of two layers of porous films has been shown, but the multilayer separator may be composed of three or more layers of porous films. Examples of such a multilayer separator include a three-layer separator in which the first porous film 12 is formed on both surfaces of the second porous film 14. At this time, it is preferable that the groove | channel 1 is each formed in the 1st porous film 12 of both surfaces. In the case of such a three-layer separator, the occurrence of warpage can be more sufficiently suppressed.

  Moreover, in the said embodiment, although the case where the groove | channel 1 was formed at the time of application | coating of a heat resistant resin solution was demonstrated, you may form the groove | channel 1 after forming the 1st porous membrane 12. FIG. In this case, the depth D of the groove 1 may be larger than the thickness of the first porous film 12. That is, the groove 1 may be formed up to the surface of the second porous film 14 on the first porous film 12 side.

(Lithium ion secondary battery)
FIG. 5 is a schematic cross-sectional view showing the lithium ion secondary battery according to the present embodiment. As shown in FIG. 5, the lithium ion secondary battery 100 is interposed between the positive electrode 20, the negative electrode 30 facing the positive electrode 20, the positive electrode 20 and the negative electrode 30, and the main surface of the positive electrode 20 and the main surface of the negative electrode 30. And a separator 10 in contact with each of the lithium ion secondary batteries. Here, the separator 10 is the multilayer separator 10 mentioned above.

  The lithium ion secondary battery 100 mainly includes a power generation element 40, a case 50 that houses the power generation element 40 in a sealed state, and a pair of leads 60 and 62 connected to the power generation element 40.

  In the power generation element 40, a pair of positive electrodes 20 and negative electrodes 30 are arranged to face each other with the multilayer separator 10 interposed therebetween. The positive electrode 20 is obtained by providing a positive electrode active material layer 24 on a plate-like (film-like) positive electrode current collector 22. The negative electrode 30 is obtained by providing a negative electrode active material layer 34 on a plate-like (film-like) negative electrode current collector 32. The main surface of the positive electrode active material layer 24 and the main surface of the negative electrode active material layer 34 are in contact with the main surface of the multilayer separator 10. In the present embodiment, the first porous film 12 and the positive electrode active material layer 24 of the multilayer separator 10 are in contact with each other, and the second porous film 14 and the negative electrode active material layer 34 are in contact with each other. The direction of the separator 10 may be reversed. Leads 60 and 62 are connected to the ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

  Hereinafter, the positive electrode 20 and the negative electrode 30 are collectively referred to as electrodes 20 and 30, and the positive electrode current collector 22 and the negative electrode current collector 32 are collectively referred to as current collectors 22 and 32. The positive electrode active material layer 24 and the negative electrode The active material layer 34 may be collectively referred to as the active material layers 24 and 34.

  First, the electrodes 20 and 30 will be specifically described.

  The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

The positive electrode active material layer 24 is mainly composed of a positive electrode active material, a binder, and a conductive auxiliary agent in an amount as necessary. The positive electrode active material includes insertion and extraction of lithium ions, desorption and insertion of lithium ions (intercalation), or doping and dedoping of lithium ions and a counter anion (for example, ClO ₄ ⁻ ) of the lithium ions. It is not particularly limited as long as it can be reversibly advanced, and a positive electrode active material used in a known lithium ion secondary battery can be used. For example, a lithium containing metal oxide is mentioned. Examples of the lithium-containing metal oxide include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a general formula: LiNi _x Co _y Mn _z O ₂ ( x + y + z = 1), a composite metal oxide, a lithium vanadium compound (LiV ₂ O ₅ ), an olivine-type LiMPO ₄ (wherein M represents Co, Ni, Mn or Fe), lithium titanate (Li ₄ Ti ₅ O _12), and the like.

  The binder binds the active materials to each other and binds the active material to the current collector 22. The binder is not particularly limited as long as the above-described bonding is possible. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) ) And the like.

  In addition to the above, as the binder, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFPPTFE-based) Fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride Ride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber It may be used vinylidene fluoride-based fluorine rubbers such as rubber (VDF-CTFE-based fluorine rubber).

  In addition to the above, for example, polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, cellulose, styrene / butadiene rubber, isoprene rubber, butadiene rubber, ethylene / propylene rubber, and the like may be used as the binder. Also, thermoplastic elastomeric polymers such as styrene / butadiene / styrene block copolymers, hydrogenated products thereof, styrene / ethylene / butadiene / styrene copolymers, styrene / isoprene / styrene block copolymers, and hydrogenated products thereof. May be used. Further, syndiotactic 1,2-polybutadiene, ethylene / vinyl acetate copolymer, propylene / α-olefin (carbon number 2 to 12) copolymer may be used.

  Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also exhibits the function of the conductive assistant particles, it is not necessary to add the conductive assistant.

As the ion-conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) A crosslinked polymer of a polyether compound, polyepichlorohydrin, polyphosphazene, polysiloxane, polyvinylpyrrolidone, polyvinylidene carbonate, polyacrylonitrile, etc.) monomers, and LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCl, LiBr, Examples thereof include a composite of a lithium salt such as Li (CF ₃ SO ₂ ) ₂ N, LiN (C ₂ F ₅ SO ₂ ) ₂ or an alkali metal salt mainly composed of lithium. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

  The conductive auxiliary agent is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 24, and a known conductive auxiliary agent can be used. Examples thereof include carbon materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO.

  The content of the binder in the positive electrode active material layer 24 is not particularly limited, but is preferably 1% by mass to 15% by mass based on the sum of the masses of the active material, the conductive additive, and the binder, and is 3% by mass. More preferably, it is 10 mass%. By setting the content of the active material and the binder in the above range, the obtained electrode active material layer 24 can suppress the tendency that the amount of the binder is too small to form a strong active material layer. In addition, the amount of the binder that does not contribute to the electric capacity increases, and the tendency that it is difficult to obtain a sufficient volume energy density can be suppressed.

  The content of the conductive additive in the positive electrode active material layer 24 is not particularly limited, but when added, it is usually preferably 0.5% by mass to 20% by mass with respect to the active material, and 1% by mass to It is more preferable to set it as 12 mass%.

  The negative electrode current collector 32 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

The negative electrode active material layer 34 is mainly composed of a negative electrode active material, a binder, and a conductive auxiliary agent in an amount as necessary. The negative electrode active material reversibly advances the insertion and removal of lithium ions, the desorption and insertion of lithium ions, or the doping and dedoping of lithium ions and counter anions of the lithium ions (for example, ClO ₄ ⁻ ). If it can be made, it will not specifically limit, The negative electrode active material used for the well-known lithium ion secondary battery can be used. For example, it can be combined with natural graphite, artificial graphite, mesocarbon microbeads, mesocarbon fiber (MCF), coke, glassy carbon, carbon materials such as organic compound fired bodies, lithium such as Al, Si, Sn, etc. _metal, amorphous compounds mainly SiO 2, oxides of SnO ₂ or the like, lithium titanate _{(Li 4 Ti 5 O 12)} , and the like.

  As the binder and the conductive additive, the same materials as those used for the positive electrode 20 described above can be used. Moreover, what is necessary is just to employ | adopt content similar to content in the positive electrode 20 mentioned above also about content of a binder and a conductive support agent.

  The electrodes 20 and 30 can be produced by a commonly used method. For example, it can manufacture by apply | coating the coating material containing an active material, a binder, a solvent, and a conductive support agent on a collector, and removing the solvent in the coating material apply | coated on the collector.

  As the solvent, for example, N-methyl-2-pyrrolidone, N, N-dimethylformamide and the like can be used.

  There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method.

  The method for removing the solvent in the paint applied on the current collectors 22 and 32 is not particularly limited, and the current collectors 22 and 32 applied with the paint are dried, for example, in an atmosphere of 80 ° C. to 150 ° C. Just do it.

  Then, the electrodes on which the active material layers 24 and 34 are formed in this way may then be pressed by a roll press device or the like as necessary. The linear pressure of the roll press can be, for example, 10 to 50 kgf / cm.

  The electrodes 20 and 30 can be manufactured through the above steps.

  Next, other components of the lithium ion secondary battery 100 will be described.

The electrolyte is contained in the positive electrode active material layer 24, the negative electrode active material layer 34, and the multilayer separator 10. The electrolyte is not particularly limited, and, for example, in the present embodiment, an electrolyte solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, and the withstand voltage during charging is limited to a low level. As the electrolyte solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. Examples of the lithium salt include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN Salts such as (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , LiBOB can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Moreover, as an organic solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, etc. are mentioned preferably, for example. These may be used alone or in combination of two or more at any ratio.

  The case 50 seals the power generation element 40 and the electrolyte solution therein. The case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 5, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

  The leads 60 and 62 are made of a conductive material such as aluminum.

  Then, the leads 60 and 62 are respectively welded to the positive electrode current collector 22 and the negative electrode current collector 32 by a known method, and a multilayer is formed between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30. What is necessary is just to insert in the case 50 with electrolyte solution in the state which pinched | interposed the separator 10, and seals the entrance of case 50. FIG.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
Polyamide imide resin solution (non-volatile content: 30% by mass, solvent: NMP, glass transition temperature: 283 ° C.) 50 parts by mass, polyethylene glycol (number average molecular weight Mn = 1000) 20 parts by mass and NMP 30 parts by mass were added. The mixture was mixed uniformly at room temperature to prepare a heat resistant resin solution.

  The obtained heat-resistant resin solution was placed on one side of a 20 μm-thick polyethylene microporous membrane (second porous membrane, porosity: 40%, shutdown temperature: 134 ° C.) with 450 μm × 450 μm eyes (grooves). The cross-sectional shape of the groove is shown using a screen printer in which the lattice-like pattern shown in FIG. 2 having a size of the enclosed coating film) and a gap of 50 μm (groove width of the groove opening) is formed. It was applied so as to have the rectangular shape shown in 3 (c) to form a coating film having a thickness of 28 μm. This coating film is immersed in a coagulation liquid consisting of 50 parts by mass of water and 50 parts by mass of NMP together with a polyethylene microporous film for 3 minutes, then washed with ion-exchanged water and then dried with hot air at 60 ° C. for 30 minutes to make the coating porous. Then, a heat-resistant porous film (first porous film) having a thickness of 4 μm and having grooves in a lattice shape was formed on the surface of the polyethylene microporous film. Thereby, a multilayer separator was obtained. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

(Example 2)
A multilayer separator was produced in the same manner as in Example 1 except that a screen printer having a grid-like pattern formed of 1450 μm × 1450 μm eyes and a 50 μm gap was used. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

(Comparative Example 1)
A heat-resistant resin solution having a thickness of 4 μm without grooves was obtained in the same manner as in Example 1 except that the heat-resistant resin solution used in Example 1 was applied to one side of the polyethylene microporous film used in Example 1 using a bar coater. A multilayer separator having a porous membrane formed on the surface of a polyethylene microporous membrane was produced. The obtained multi-layer separator was significantly warped and difficult to handle.

(Example 3)
A multilayer separator was produced in the same manner as in Example 1 except that a screen printer having a grid pattern formed of 450 μm × 450 μm eyes and a 150 μm gap was used. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

Example 4
A multilayer separator was produced in the same manner as in Example 1 except that the thickness of the coating film applied to the heat-resistant resin solution with a screen printer was 38 μm. The thickness of the thermally porous film (first porous film) after drying was 6 μm. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

(Example 5)
The heat-resistant resin solution used in Example 1 was applied to one side of the polyethylene microporous film used in Example 1 using a bar coater to form a coating film having a thickness of 12 μm and having no grooves. Then, in the same manner as in Example 1, a heat resistant resin solution was applied using a screen printer on which a grid-like pattern composed of 450 μm × 450 μm eyes and a 50 μm gap was formed, and a porous treatment was performed. A multilayer separator was produced. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

(Example 6)
A screen printer in which the heat-resistant resin solution used in Example 1 is formed on one side of the polyethylene microporous membrane used in Example 1 with a grid pattern formed of 450 μm × 450 μm eyes and 50 μm gaps. A multilayer multi-separator was produced in the same manner as in Example 1 except that the coating was applied so that the cross-sectional shape of the groove was V-shaped as shown in FIG. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

(Example 7)
The heat-resistant resin solution used in Example 1 is shown in FIG. 4 (a) in a lattice-like pattern consisting of 450 μm × 450 μm eyes and a 50 μm gap on one side of the polyethylene microporous membrane used in Example 1. As shown in FIG. 3A, the cross-sectional shape of the groove is shown in FIG. 3A using a screen printer in which a pattern in which gaps are discontinuously arranged so as not to intersect with each other (non-continuous pattern) is formed. A multilayer separator was produced in the same manner as in Example 1 except that it was applied so as to form a mold. The obtained multilayer separator was suppressed in warpage and excellent in handling properties.

(Comparative Example 2)
The polyethylene microporous membrane used in Example 1 is dipped in the heat-resistant resin solution used in Example 1 and passed through a Mayer bar with a clearance of 80 μm, and then the porous treatment is performed in the same manner as in Example 1. Thus, a multilayer separator in which a heat-resistant porous film having a thickness of 4 μm was formed on both surfaces of the polyethylene microporous film was produced. The obtained multi-layer separator had no warping and excellent handling properties, but the film was peeled off only by applying a slight force, and its practicality was insufficient.

<Measurement of thermal shrinkage>
The multilayer separators obtained in Examples and Comparative Examples were cut into 2.5 cm × 7.5 cm to obtain test pieces. The test piece was sandwiched between two slide glasses, heated to each test temperature (100 ° C., 140 ° C., 180 ° C.) with a clip applied so that the test piece did not curl, and held for 10 minutes. The test piece after heating is collected and the following formula:
{(Test piece area before heating−Test piece area after heating) / Test piece area before heating} × 100
Was used to calculate the heat shrinkage rate (%).

<Measurement of warpage>
The multilayer separators obtained in Examples and Comparative Examples were cut into 5.0 cm × 5.0 cm to obtain test pieces. This test piece was allowed to stand on a smooth surface, and a portion having the largest warp height was measured using a clearance gauge to obtain a warp amount.

<Evaluation of membrane state>
About the multilayer separator obtained by the Example and the comparative example, the presence or absence of film | membrane peeling was evaluated. Those in which film peeling was not confirmed were regarded as good.

  Table 1 below collectively shows information and evaluation results regarding the multilayer separators produced in the examples and comparative examples.

  According to the present invention, it is possible to provide a multilayer separator suitable for a lithium ion secondary battery, which is excellent in reliability with little film peeling and suppressed warping, and a lithium ion secondary battery using the same. Therefore, the present invention greatly contributes to the industry.

  DESCRIPTION OF SYMBOLS 1 ... Groove, 10 ... Multi-layer separator, 12 ... 1st porous membrane, 14 ... 2nd porous membrane, 20 ... Positive electrode, 22 ... Positive electrode collector, 24 ... Positive electrode active material layer, 30 ... Negative electrode, 32 DESCRIPTION OF SYMBOLS ... Negative electrode collector, 34 ... Negative electrode active material layer, 40 ... Power generation element, 50 ... Case, 52 ... Metal foil, 54 ... Polymer film, 60, 62 ... Lead, 100 ... Lithium ion secondary battery.

Claims (8)

  A multilayer separator comprising a laminate of two or more porous membranes and having a groove on at least one surface of the laminate.   The multilayer separator according to claim 1, wherein the groove has a shape in which a groove width at a bottom portion is narrower than a groove width at an opening portion in a cross section cut along a plane perpendicular to the extending direction.   The multilayer separator according to claim 2, wherein the shape of the cross section of the groove is V-shaped or U-shaped.   As the porous film, a first porous film containing a heat-resistant resin having a melting point or glass transition point of 150 ° C. or higher, and a second porous film containing a polyolefin resin having a melting point of 150 ° C. or lower, The multilayer separator according to claim 1, wherein at least one layer is included and the groove is provided on a surface of the first porous film.   5. The multilayer separator according to claim 4, wherein the multilayer porous body includes the first porous film as at least one outermost layer of the laminate, and the groove is formed only in the first porous film that is the outermost layer.   The multilayer separator according to claim 4 or 5, wherein the porosity of the first porous film having the groove is 40% by volume or less.   The multilayer separator according to claim 1, wherein a thickness of the porous film having the groove is larger than a depth of the groove. A lithium ion secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator,
The lithium ion secondary battery whose said separator is the multilayer separator as described in any one of Claims 1-7.

Images