JP6550159B1 - Separator for secondary battery and method of manufacturing the same - Google Patents

Abstract

Disclosed is a secondary battery separator that is excellent in heat resistance and mechanical strength, has low internal resistance, and can be manufactured by a simple method. A separator for a secondary battery according to the present invention includes a polyolefin-based porous membrane and a plurality of ceramic nanoparticles linearly bonded to cellulose nanofibers laminated on at least one surface of the polyolefin-based porous membrane. And a layer made of a ceramic nanoparticle aggregate supported continuously. [Selection] Figure 1

Description

  The present invention relates to a separator for a secondary battery and a method of manufacturing the same.

  While secondary batteries represented by lithium ion secondary batteries having high energy density are being widely used as power sources for portable electric devices and electric bicycles, they are said to be further increased in capacity and charged / discharged with large current Performance is required. Furthermore, since a secondary battery can not use a water-based electrolyte, a non-aqueous electrolyte (organic electrolyte) having sufficient oxidation-reduction resistance is used, but these non-aqueous electrolytes are flammable. Because of the danger of ignition and so on, secondary batteries are also required to have improved safety.

  For example, a porous film of a polyolefin resin used as a separator for a lithium ion secondary battery (also referred to herein as a "porous film") has its own pores by melting at a temperature around 120 ° C. And shut off the flow of current and ions, and has a shutdown function to suppress the temperature rise of the battery. However, when the temperature rises due to external heat, etc., the battery temperature rises even if the shutdown function works, and for example, when the battery temperature reaches 150 ° C. or more, the porous film shrinks. As a result, the function of isolating the positive and negative electrodes is lost, and an internal short circuit may occur to cause ignition or the like.

  To solve these problems, the internal temperature is reduced by raising the heat shrinkage temperature more than the porous film of polyolefin resin to prevent the occurrence of internal short circuit and to suppress the ignition, and the electrolyte is impregnated to suppress the generation of Joule heat. It is necessary to reduce the resistance value (membrane resistance) of the separator. To address these two problems, separators in which the surface of a polyolefin-based porous film is coated with inorganic fine particles have been developed.

  For example, Patent Document 1 discloses a non-aqueous secondary battery in which a heat-resistant porous layer made of a heat-resistant resin containing an inorganic filler is composed of a polyolefin-based porous thin film laminated on at least one surface. A separator is disclosed. Patent Document 2 discloses a separator in which a mixture of a plurality of inorganic particles and a binder polymer is coated on at least one surface of a heat resistant porous substrate having a melting point of 200 ° C. or higher.

  In addition, separators made of non-woven fabric, which are formed of cellulose fibers (cellulose nanofibers) having a fiber diameter of nm size, have also been developed. However, while the separator formed of cellulose nanofibers is excellent in the separation performance etc., it has a large air permeability. For example, Patent Document 3 discloses a method of obtaining a porous, low-density separator having high airtightness by solvent-substituting water held in a void structure between cellulose fibers and then drying it. There is.

  Furthermore, in Patent Document 4, after making a pellet by making cellulose nanofibers and polyolefin dispersed in water complexed with a twin-screw kneader, a high puncture can be obtained by mixing with paraffin again with a twin-screw kneader. A method for producing a cellulose nanofiber-containing polyolefin microporous stretched film having strength and the like is disclosed.

International Publication No. 2008/156033 JP-A-2010-525542 Japanese Patent Application Laid-Open No. 10-223196 JP, 2013-56958, A

  As in Patent Document 1 or 2, the method of combining the porous resin film and the inorganic fine particles is effective for improving the heat resistance of the separator, but the inorganic fine particles clog the pores of the porous resin film to move ions. There is a problem that the resistance is increased, and further study on thinning of the separator is necessary. Moreover, although the method of patent document 3 or 4 is an effective usage method of the cellulose nanofiber which is excellent in heat resistance and mechanical strength, a manufacturing process is complicated and there exists room for improvement.

  An object of the present invention is to provide a separator for a secondary battery which is excellent in heat resistance and mechanical strength, has a low internal resistance, and can be manufactured by a simple method.

  As a result of intensive investigations, the present inventors conducted a polyolefin-based porous film (polyolefin-based resin) in which a nanoparticle assembly formed by supporting or arranging a plurality of inorganic fine particles in a row by using cellulose nanofibers as a shaft or a substrate. It has been found that by laminating on the surface of the above, a secondary battery separator having excellent heat resistance and mechanical strength and low internal resistance can be obtained, and the present invention has been completed.

That is, the secondary battery separator according to the present invention is
A polyolefin-based porous membrane,
It is characterized in that it comprises: a layer composed of a ceramic nanoparticle aggregate in which a plurality of ceramic nanoparticles are supported linearly and continuously on cellulose nanofibers laminated on at least one surface of the polyolefin-based porous membrane. I assume.

  According to the separator for a secondary battery, a unique feature is that a plurality of ceramic nanoparticles constituting a layer composed of a ceramic nanoparticle aggregate laminated on the surface of a polyolefin-based porous film is supported or induced by cellulose nanofibers. It has an interesting shape. That is, the ceramic nanoparticles are not present at all or almost at the surface of the polyolefin-based porous membrane as single independent particles. Therefore, even if the ceramic nanoparticles are micronized in order to thin the separator, the ceramic nanoparticles do not intrude or clog the pores of the porous membrane, and good ion permeability is maintained.

  In addition, according to the separator for a secondary battery of the present invention, the surface of the polyolefin-based porous film is covered with ceramic nanoparticles and cellulose nanofibers having excellent heat resistance, so that heat resistance of 230 ° C. can be realized. For example, in a lithium ion battery, it has a heat resistance of 190 ° C. or higher, which is the ignition temperature of lithium metal. Furthermore, since the surface of the polyolefin-based porous membrane is covered with cellulose nanofibers having excellent mechanical strength, the separator for a secondary battery of the present invention has low basis weight and low thickness, but also has high tensile strength and the like. Mechanical strength is high.

  Moreover, since the cellulose nanofiber which comprises the separator for secondary batteries of this invention has a function as a binder as described, for example in WO 2013/042720, it needs a binder separately Can be laminated on the surface of the polyolefin-based porous membrane. Therefore, it is possible to increase the ratio of the active material in the secondary battery, and it is possible to realize a secondary battery excellent in energy density. However, this invention does not exclude the case where a separate binder is used, when laminating | stacking the layer which consists of a ceramic nanoparticle assembly on the surface of a polyolefin type porous membrane.

Further, a method of manufacturing a separator for a secondary battery according to the present invention is
Next steps (I) to (III):
(I) preparing a slurry containing a ceramic raw material compound containing at least one metal element and cellulose nanofibers,
(II) subjecting the slurry obtained in the step (I) to a hydrothermal reaction to obtain a ceramic nanoparticle aggregate supported on the cellulose nanofibers;
(III) laminating the ceramic nanoparticle aggregate supported by the cellulose nanofibers obtained in the step (II) on at least one surface of a polyolefin-based porous membrane;
It is characterized by including.

  According to the above method for producing a separator for a secondary battery, a ceramic nanoparticle aggregate supported on cellulose nanofibers can be obtained by a simple method, and such a ceramic nanoparticle aggregate can be formed into a polyolefin-based porous material by a common method. It can be easily laminated on the surface of the sexing membrane. Thereby, the separator for secondary batteries which is excellent in film resistance, heat resistance, and mechanical strength can be provided simply.

  According to the present invention, a secondary battery separator which is excellent in heat resistance and mechanical strength, has a low internal resistance, and can be manufactured by a simple method is realized.

It is a TEM photograph which shows the ceramic nanoparticle assembly which consists of boehmite (AlOOH) obtained by manufacture example 1. FIG.

  Hereinafter, the present invention will be described in detail.

[Construction]
In the separator for a secondary battery according to the present invention, a plurality of ceramic nanoparticles are supported linearly and continuously on cellulose nanofibers laminated on the surface of at least one of a polyolefin-based porous film and a polyolefin-based porous film. And a layer consisting of an assembly of ceramic nanoparticles.

  A ceramic nanoparticle assembly (hereinafter also referred to as “nanoarray”) is a unique structure in which a plurality of ceramic nanoparticles are linearly and continuously supported on cellulose nanofibers (hereinafter also referred to as “CNF”). It takes a shape. The nanoarray means a shape in which one nanoscale structure is arranged and the other nanoscale structure is arranged.

  The nano array in the present invention is, for example, CNF having an average fiber diameter of 50 nm or less, ie, one nanoscale structure, for example, a plurality of ceramic nanoparticles having an average particle size of 30 nm or less, ie, the other nanoscale structure is linear. It has a unique shape, supported in a continuous manner. Here, the "average particle size" refers to the average value of the measured values of the particle sizes (length of the major axis) of several tens of particles in the observation with an electron microscope.

  In the above nano array, since the ceramic nanoparticles are supported on CNF linearly and continuously, it is not necessary to modify the surface of the ceramic nanoparticles or to use a dispersing agent for the purpose of aggregation suppression, and it is further suitably dispersed. Since the state is maintained, it can be easily laminated as a thin layer (thin film) on the surface of the polyolefin-based porous membrane. In addition, since the ceramic nanoparticles are firmly supported by CNF, the nanoarray does not decompose and the ceramic nanoparticles do not behave as single independent particles.

  In the above nano-array, the surface of CNF becomes a nucleation site for heterogeneous nucleation of ceramics, and while the ceramic nanoparticles grow so as to enclose ultra-thin CNF, similarly, they are in contact with adjacent nanoparticles during crystal growth. By continuing the crystal growth, it is considered that the nanoparticles are continuously and firmly supported on the CNF continuously in an orderly manner without unnecessary aggregation, whereby a unique shape is formed.

  The layered portion (layer of the aggregate of ceramic nanoparticles) formed of nanoarrays may be coated on at least one surface of the polyolefin-based porous film as a base material, but it is more preferable to cover both surfaces . By covering both surfaces, not only the problem of curling is eliminated and the handling property is improved, but also the dimensional stability at high temperature is significantly improved, and the durability of the secondary battery can also be improved.

(Cellulose nanofibers: CNF)
CNFs that constitute nanoarrays are skeletal components that occupy about 50% of all plant cell walls, and are lightweight high-strength fibers that can be obtained by disentanglement of plant fibers that constitute such cell walls to nanosize etc. It also has good dispersibility in water.

  The average fiber diameter of CNF is, for example, 50 nm or less, preferably 20 nm or less, and more preferably 10 nm or less. The lower limit value is not particularly limited, but is usually 1 nm or more.

  The average length of CNF is preferably 1 μm to 500 μm from the viewpoint of efficiently laminating on the surface of the polyolefin porous membrane, and from the viewpoint of effectively laminating on the surface of the polyolefin porous membrane as a binder, More preferably, it is 5 micrometers-200 micrometers, More preferably, it is 10 micrometers-100 micrometers.

  The viscosity at 25 ° C. of a 10 mass% concentration CNF slurry using the above CNF is preferably 30000 mPa · s to 200000 mPa · s, more preferably 40000 mPa · s to 150,000 mPa · s, still more preferably 45000 mPa · s It is-120000 mPa · s.

(Ceramics nanoparticles)
The ceramic nanoparticles that constitute the nanoarray include at least one metal element and can form nano-sized particles, as long as they have electrical insulating properties, and are particularly limited. I will not. Examples of such ceramic nanoparticles include particles of oxides, hydroxides, carbonates, phosphates of metal elements, and composites thereof. Among them, dehydration reaction is an endothermic reaction when heated. Hydroxide or hydrous oxide having the effect of suppressing temperature rise of the battery is preferable.

Specifically, SiO ₂ , TiO ₂ , ZnO ₂ , SnO ₂ , Al ₂ O ₃ , MnO, MnO ₂ , NiO, Eu ₂ O ₃ , Y ₂ O ₃ , Nb ₂ O as constituent materials of ceramic nanoparticles. ₃ , Nb ₂ O ₅ , InO, ZnO, Fe ₂ O ₃ , Fe ₃ O ₄ , Co ₃ O ₄ , Co ₂ O ₄ , ZrO ₂ , CeO ₂ , VO ₂ , V ₂ O ₅ , AlOOH (boehmite), Y ₃ Al ₅ O ₁₂ BaTiO ₃ , In ₂ O ₃ -SnO ₂ , MgO-Al ₂ O ₃ -SiO ₂ , SiO ₂ -Al ₂ O ₃ , SiO ₂ -TiO ₂ , SiO ₂ -ZrO ₂ , TiO ₂ -ZrO ₂ , SiO _{2 2 -Al 2 O 3 -ZrO 2,} Al (OH) 3, Mg (OH) 2, Ca (OH) 2, Ni (OH) 2, Mn (OH) 2, Fe (OH) 2, Zn (O _{) 2, H 3 BO 3,} Cr 2 O 3 · nH 2 O, CaO · SiO 2 · nH 2 O, CaO · Al 2 O 3 · nH 2 O, CaCO 3, MgCO 3, BaCO 3, Na 2 CO 3 , K ₂ CO ₃ , AlPO ₄ , Ca ₃ (PO ₄ ) _{2 and the} like. Among these, AlOOH, Al (OH) ₃ , Mg (OH) ₂ , Ca (OH) ₂ , Ni (OH) ₂ , H ₃ BO ₃ , Cr ₂ O ₃ .nH ₂ O, CaO.SiO _2. nH ₂ O, CaO.Al ₂ O ₃ .nH ₂ O is preferable, and Al (OH) ₃ , Mg (OH) ₂ , or AlOOH having high chemical stability is more preferable, which has a low dehydration temperature.

Furthermore, the viewpoint that Al (OH) ₃ , Mg (OH) ₂ and AlOOH have the effect of protecting the positive electrode from hydrofluoric acid present in the non-aqueous secondary battery and improving the durability of the secondary battery. Are also preferred. That is, in the non-aqueous secondary battery, hydrofluoric acid corrodes the positive electrode active material to cause deterioration in durability, but AlOOH has a function of adsorbing and coprecipitating hydrofluoric acid. Therefore, if a separator containing AlOOH is used, the concentration of hydrofluoric acid in the electrolyte can be maintained at a low level, and the durability of the secondary battery can be improved.

  The ceramic nanoparticles that constitute the nanoarray consist of microcrystalline particles. Specifically, the average particle size of such ceramic nanoparticles is preferably 30 nm or less, more preferably 20 nm or less. The lower limit value is not particularly limited, but is usually 3 nm or more. Thus, ceramic nanoparticles are very small particles, and together with the above-described very thin CNF of average fiber diameter, can form a very thin nanoarray of axial diameter. For this reason, even if such a nanoarray is laminated on the surface of a polyolefin-based porous film, it is possible to make the obtained separator for a secondary battery of the present invention a thin film.

  Examples of the crystal habit of the ceramic nanoparticles (the outer shape of the crystal) include plate-like, needle-like, hexahedron and column-like. Among them, hexahedral particles extended in the axial direction of CNF are preferable from the viewpoint of strong support with CNF and a reduction in the amount of binder required for lamination to a polyolefin-based porous film.

  The nano-array is preferably an aggregate of ceramic nanoparticles having a uniform particle size and shape, but may be an aggregate of ceramic nanoparticles having different particle sizes and shapes, and the chemical composition is different 2 It may be an aggregate of species or more of ceramic nanoparticles.

(Polyolefin-based porous membrane)
The polyolefin-based porous film constituting the separator for a secondary battery according to the present invention is made of a polyolefin, has a large number of micropores inside, and has a structure in which these micropores are connected. Is a membrane that allows passage of gas or liquid to the surface of the Examples of the polyolefin include polyethylene, polypropylene, polymethylpentene, and combinations thereof. Particularly preferable is polyethylene, but high density polyethylene, a mixture of high density polyethylene and ultrahigh molecular weight polyethylene, and the like are preferable as this polyethylene.

  The polyolefin-based porous membrane preferably has a porosity of 20% to 60%. If the porosity is less than 20%, the membrane resistance when used as a secondary battery separator becomes too high, which is not preferable because it reduces the output of the secondary battery. In addition, when the porosity exceeds 60%, the deterioration of the shutdown characteristics becomes remarkable, which is not preferable.

  The air permeability of the polyolefin porous membrane specified in JIS P 8117 "Paper and paperboard-Air permeability and air resistance test method (intermediate region)-Gurley method" is 10 seconds / 100 cc per unit thickness.・ Μm or more is preferable. When the air permeability per unit thickness is lower than 10 seconds / 100 cc · μm, a polyolefin-based porous membrane is formed at the interface between the nano-array layered portion (layer comprising ceramic nanoparticle aggregate) and the polyolefin-based porous membrane It is not preferable because clogging may occur to cause a marked increase in the membrane resistance or a marked decrease in the shutdown characteristics.

  In the separator for a secondary battery of the present invention, the thickness of the polyolefin-based porous film is preferably 10 μm or more. When the film thickness of the polyolefin-based porous thin film is smaller than 10 μm, mechanical strength such as tensile strength and piercing strength is not sufficient, which is not preferable.

(Separator for secondary battery)
The film thickness of the separator for a secondary battery of the present invention, that is, the total thickness including the layer composed of the ceramic nanoparticle assembly and the polyolefin-based porous film is preferably 45 μm or less, and more preferably 40 μm or less. When the film thickness of a separator exceeds 45 micrometers, since the output characteristic of the secondary battery to which this is applied falls, it is unpreferable.

  That is, the thickness of the layer composed of the ceramic nanoparticle aggregate to be laminated on the polyolefin-based porous membrane is preferably 35 μm or less, and more preferably 30 μm or less. More specifically, when a layer composed of a ceramic nanoparticle assembly is laminated on one side of a polyolefin-based porous film, the thickness of the layer composed of such a ceramic nanoparticle assembly is preferably 5 μm to 35 μm, more preferably Is 5 μm to 25 μm, and when the layer of the ceramic nanoparticle assembly is laminated on both sides of the polyolefin-based porous film, the thickness of the layer of the ceramic nanoparticle assembly is preferably 5 μm to 17 And more preferably 5 μm to 10 μm.

  The secondary battery separator of the present invention preferably has a heat shrinkage of 10% or less when heated at 105 ° C. for 30 minutes. When the thermal contraction rate exceeds 10% under the heating condition, the heat generation caused by the micro short circuit may expand the ruptured membrane area of the separator, and a large current may flow.

  The following method can be employed for measuring the thermal contraction rate of the secondary battery separator.

The secondary battery separator is punched out to a diameter of 17 mm, the diameter is measured at 10 points with a caliper, and the area before heat treatment is calculated from the average value. Next, the separator fragments are allowed to stand in a thermostat at 105 ° C. for 30 minutes with no substantial tension applied, and then cooled to room temperature (eg 25 ° C.), and the diameter is measured at 10 points with a caliper The area after heat treatment is calculated from the average value. And the thermal contraction rate of the separator for secondary batteries is computed by following formula (1). The measurement of this thermal contraction rate was performed to the separator of 5 different secondary batteries prepared simultaneously, the average value was calculated | required, and it was set as the thermal contraction rate.
Thermal contraction rate = {(area of secondary battery separator before heating) − (area of secondary battery separator after heating)} / (area of secondary battery separator before heating) * 100% (1)

In the separator for a secondary battery of the present invention, the membrane resistance is preferably 5 Ω · cm ² or less, more preferably 4 Ω · cm ² or less. Here, the film resistance is a value according to an alternating current impedance method measured at 20 ° C. using an electrolytic solution in which 1 M of LiPF ₆ is dissolved in a mixed solvent of propylene carbonate and ethylene carbonate (volume ratio 1: 1). When the film resistance is larger than 5 Ω · cm ^2, it becomes difficult to obtain good rate characteristics and cycle characteristics in the case of a secondary battery.

  The puncture strength of the separator for a secondary battery according to the present invention is preferably 300 g or more, more preferably 400 g or more. Here, the piercing strength is the maximum piercing when a needle needle (a semicircular needle with a diameter of 1.0 mm and a tip radius of 0.5 mm) is penetrated through a separator for a secondary battery under a piercing speed of 100 mm / min. It is a load.

  The secondary battery to which the secondary battery separator is applied is not particularly limited as long as it has the positive electrode, the negative electrode, the electrolytic solution, and the separator as essential components.

[Production method]
The separator for a secondary battery according to the present invention comprises the following steps (I) to (III):
(I) preparing a slurry containing a ceramic raw material compound containing at least one metal element and cellulose nanofibers,
(II) subjecting the slurry obtained in the step (I) to a hydrothermal reaction to obtain an aggregate of ceramic nanoparticles supported on cellulose nanofibers,
(III) obtaining the ceramic nanoparticle aggregate supported on cellulose nanofibers obtained in the step (II) by at least one surface of a polyolefin-based porous membrane it can.

  By using this manufacturing method, a useful secondary battery separator can be formed by a simple method.

(Step (I))
Step (I) is a step of preparing a slurry containing a ceramic raw material compound containing at least one metal element and cellulose nanofibers. In step (I), first, a ceramic raw material compound containing at least one metal element, water, and cellulose nanofibers are mixed to obtain a slurry A.

  Specifically as a ceramic raw material compound, metal compounds, such as an aluminum compound, a magnesium compound, a calcium compound, a nickel compound, a chromium compound, a silicon compound, or a boron compound, are mentioned, for example. Among them, sulfates, nitrates, carbonates, acetates, oxalates, oxides, hydroxides, halides and the like of the above metal elements can be suitably used.

  Water is used when preparing the slurry A by mixing these ceramic raw material compounds and cellulose nanofibers. The amount of water used is preferably 10 moles to 300 moles with respect to 1 mole of the metal element of the ceramic material compound from the viewpoint of solubility or dispersibility of each raw material, ease of stirring, efficiency of hydrothermal reaction, etc. Furthermore, 50 mol-200 mol are preferable. In addition, the content of the cellulose nanofibers in the slurry A is preferably 0.1 parts by mass to 20 parts by mass, and more preferably 0.1 parts by mass to 10 parts by mass with respect to 100 parts by mass of water in the slurry A It is a department.

  The mixing ratio of the ceramic raw material compound to the cellulose nanofibers is 0.1 parts by mass to 10000 parts by mass of the ceramic raw material compound, and more preferably 1 part by mass to 1000 parts by mass with respect to 100 parts by mass of the cellulose nanofibers. Part, particularly preferably 5 parts by mass to 200 parts by mass. When the mixing amount of the ceramic raw material compound with respect to the cellulose nanofibers is extremely large, surplus metal components which are not nanoarrayed in the hydrothermal reaction in the next step (II) are combined to form large diameter crystals and contaminants in the nanoarray It becomes. When a nano-array containing such contaminants is successively laminated on the surface of a polyolefin-based porous film in step (III) to form a separator for a secondary battery, such contaminants become electrical resistance and discharge characteristics deteriorate. Alternatively, pores with large pore sizes may be generated around the contaminants to cause a short circuit.

  In the step (I), next, an alkaline solution is added to the above-mentioned slurry A to make a slurry B, and a metal component dissolved or dispersed in the slurry B is made a metal hydroxide by a neutralization reaction. In order to add the alkaline solution, it is preferable to add a sufficient amount to keep the pH of the slurry B at 25 ° C. at 10-14. As such an alkaline solution, for example, an aqueous solution of sodium hydroxide, potassium hydroxide, sodium carbonate, ammonia and the like can be used, and among them, it is preferable to use sodium hydroxide, sodium carbonate or a mixed solution thereof.

  The slurry B is preferably stirred to allow the neutralization reaction to proceed from the viewpoint of generating a metal hydroxide well. The temperature of the slurry B during the neutralization reaction is preferably 5 ° C. or more, more preferably 10 ° C. to 60 ° C. Moreover, 5 minutes-120 minutes are preferable, and, as for the stirring time of the slurry B, 30 minutes-60 minutes are more preferable.

(Step (II))
The step (II) is a step of subjecting the obtained slurry B to a hydrothermal reaction to obtain a nanoarray (a layer composed of a ceramic nanoparticle assembly).

  The temperature during the hydrothermal reaction is preferably 100 ° C. or more, and more preferably 190 ° C. or less from the viewpoint of suppressing the thermal decomposition of the cellulose nanofibers. Moreover, 130 degreeC-180 degreeC are more preferable, and, as for the said temperature, 140 degreeC-160 degreeC are especially preferable. The hydrothermal reaction is preferably carried out in a pressure-resistant vessel, and when carried out at 190 ° C. or less, the pressure at this time is preferably 1.2 MPa or less, and when the hydration reaction is carried out at 100 ° C. to 190 ° C., 0. It is preferably 1 MPa to 1.2 MPa, and preferably 130 MPa to 0.9 MPa at 130 ° C. to 180 ° C., and preferably 0.3 MPa to 0.6 MPa at 140 ° C. to 160 ° C. The hydrothermal reaction time is preferably 0.5 hours to 24 hours, and more preferably 0.5 hours to 15 hours. When the hydrothermal reaction time exceeds 24 hours, there is a possibility that the diameter of the ceramic nanoparticles constituting the nanoarray becomes excessively large.

  The resulting hydrothermal reaction product is a nanoarray consisting of cellulose nanofibers and metal oxides, or metal oxides having cellulose nanofibers and water of crystallization, and after filtration, it is washed with water and dried to obtain a single product. It can be released. When such a nanoarray is washed with water, it is preferable to use 5 parts by mass to 100 parts by mass of water per 1 part by mass of the nanoarray.

  After washing, in order to remove the water contained in the obtained nanoarray, a drying process is applied as needed. As a drying means, lyophilization, vacuum drying, hot air drying is used, and lyophilization or hot air drying is preferable. When hot air drying is performed, the temperature of the hot air may be 50 ° C. to 230 ° C., preferably 70 ° C. to 200 ° C., from the viewpoint of suppressing the thermal decomposition of the cellulose nanofibers. Moreover, the processing time which drying requires is suitably set according to temperature.

  Also, from the viewpoint of being able to reduce the aggregation of the nanoarrays, after washing of the nanoarrays, drying may not be performed, and repulping may be performed to prepare a slurry containing the nanoarrays of a desired concentration. The concentration of the nanoarray in the slurry is preferably 1% by mass to 40% by mass, and more preferably, from the viewpoint of handling at the time of lamination to the surface of the polyolefin porous membrane performed in the next step (III). It is 5% by mass to 30% by mass, and more preferably 10% by mass to 30% by mass.

  From the viewpoint of obtaining a nano-array having high dispersibility by enhancing the dispersibility of the cellulose nanofibers, the slurry of the cellulose nanofibers is previously subjected to hydrothermal treatment in step (I), and then the ceramic raw material compound is added in step (II). The slurry may be added and subjected to an additional (second time) hydrothermal reaction to obtain a nanoarray.

(Step (III))
Step (III) is a step of laminating the nanoarray obtained by step (II) on at least one surface of the polyolefin-based porous membrane.

  As described above, since the nanoarray also has a binder effect, in this step (III), the nanoarray or nanoarray slurry is stirred and mixed in a solvent consisting of water and an organic solvent (eg, ethanol, ethylene glycol) The slurry C may be used to laminate the nanoarray on the surface of the polyolefin-based porous membrane.

  A binder may be mixed with the nano-array slurry C. As a binder, a carboxymethylcellulose is mentioned, for example. By using this binder, the nano-array can be more firmly laminated on the surface of the polyolefin-based porous membrane. At this time, the mixing amount of the binder mixed in the nano-array slurry C is preferably 0.1 parts by mass to 5 parts by mass, more preferably 0.5 parts by mass with respect to 100 parts by mass of the nano-array in the slurry C. It is part to 2 parts by mass.

  A general coating method such as a knife coater, a die coater, a gravure coater, a Mayer bar coater, a reverse roll coater, or a gravure coater can be used for laminating the nanoarray on the surface of the polyolefin-based porous film. In addition, as a method of laminating the nano array on both sides of the polyolefin-based porous membrane, the polyolefin-based porous membrane is passed between two opposing Meyer bars, a reverse roll, a die, etc. The method of working is preferred. In addition, when laminating | stacking a nano array on both surfaces of a polyolefin-type porous membrane, you may make a polyolefin-type porous membrane immerse in the nano array slurry C.

  The drying method of the nano array slurry C after the coating treatment is preferably vacuum drying, isothermal drying or hot air drying at a temperature of 40 ° C. or lower, particularly from the viewpoint of preventing curling during drying in single-sided coated products. Vacuum drying is more preferred.

  EXAMPLES Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples. In addition, the film thickness of each secondary battery separator shown to the following Example and a comparative example is a measured value by a film thickness meter (made by Mitutoyo, ID-C112XB).

[Production example 1: Nano-array made of boehmite (AlOOH)]
3.15 g of aluminum sulfate Al ₂ (SO ₄ ) ₃ .16H ₂ O, 3.89 g of cellulose nanofibers (manufactured by Daicel Finechem, KY 100 G, water content 90 mass%), and 55 mL of water are mixed for 60 minutes to prepare a slurry A1 Made. To the obtained slurry A1, 12.0 g of a 10% by mass aqueous solution of NaOH was added and mixed for 5 minutes to prepare a slurry B1.

  The slurry B1 was charged into an autoclave and subjected to a hydrothermal reaction at 140 ° C. and 0.36 MPa for 1 hour. The resulting hydrothermal reaction product is allowed to cool, then filtered and washed with water to obtain cake C1 (moisture content: 37%) of a nanoarray of boehmite (AlOOH) (average particle diameter of boehmite: 9 nm) Obtained. The TEM observation image (JEM-ARM200F by Nippon Denshi Co., Ltd. make) of the nano array which consists of boehmite obtained by hot-air-drying the said cake C1 at 80 degreeC is shown in FIG.

[Production Example 2: Boehmite Particles]
A sol C2 (water content: 23%) of boehmite (average particle diameter: 9 nm) was obtained in the same manner as in Production Example 1 except that cellulose nanofibers were not used.

[Example 1: A separator for a secondary battery in which a nano-array of boehmite is thinly laminated on one side of a polyethylene separator]
A slurry was prepared by mixing 2.8 g of the cake C1 obtained in Production Example 1 with 50 mL of a solvent in which ethanol and water were mixed at a mass ratio of 2: 1, and stirring for 5 minutes while maintaining the temperature of 25 ° C. There was obtained a 2.5% by mass slurry D1.

  After coating the slurry D1 with a roll coater on one side of a commercially available polyethylene separator (double scope, COD-20-B) with a thickness of 20 μm, vacuum drying at 40 ° C. is carried out to obtain boehmite having a layer thickness of 8 μm. Thus, a secondary battery separator E1 (entire thickness: 28 μm) in which the nano array is laminated on one side was obtained.

[Example 2: A separator for a secondary battery in which a nano-array of boehmite is thickly laminated on one side of a polyethylene separator]
A separator for a secondary battery in which a nanoarray of boehmite having a layer thickness of 24 μm is laminated on one side in the same manner as in Example 1 except that the thickness of the slurry D1 coated on one side of the polyethylene separator is changed by adjusting a roll coater. E2 (total thickness: 44 μm) was obtained.

[Example 3: Separator for a secondary battery in which nanoarrays of boehmite are laminated on both sides of a polyethylene separator]
A nanoarray made of boehmite is laminated in the same manner as in Example 1 on the side of the separator E1 for a secondary battery obtained in Example 1 on which the nanoarray is not laminated, and the nanoarray made of boehmite with a layer thickness of 8 μm A separator E3 (total thickness: 36 .mu.m) for the secondary battery was laminated on both sides.

[Comparative Example 1: Separator for secondary battery in which boehmite particles are laminated on both sides of polyethylene separator]
A slurry in which 2.5 g of the sol C2 of boehmite obtained in Preparation Example 2 was mixed with 50 mL of a solvent in which ethanol and water were mixed at a mass ratio of 2: 1 was stirred for 5 minutes while maintaining the temperature of 25 ° C. The slurry D2 having a solid content of 2.5% by mass was obtained.

  After coating the slurry D2 with a roll coater on one side of a commercially available polyethylene separator (COD-20-B) having a thickness of 20 μm, vacuum drying at 40 ° C. is performed to obtain a separator in which boehmite particles are laminated on one side. After that, the secondary battery separator F1 in which boehmite particles having a layer thickness of 6.5 μm were laminated on both sides by laminating the boehmite particles on the side on which the boehmite particles are not laminated according to the same procedure (overall thickness: 33 μm) was obtained.

[Comparative Example 2: Separator for secondary battery in which boehmite particles and CNF are laminated on both sides of polyethylene separator]
1.7 g of the sol C2 of boehmite obtained in Preparation Example 2 and 3.89 g of cellulose nanofibers (CNF) (KY-100G) were mixed in 50 mL of a solvent in which ethanol and water were mixed at a mass ratio of 2: 1. The slurry was stirred for 5 minutes while maintaining the temperature at 25 ° C. to obtain a slurry D3 with a solid content of 2.5% by mass.

  A slurry D3 is coated on one side of a commercially available polyethylene separator (COD-20-B) having a thickness of 20 μm by a roll coater and then vacuum dried at 40 ° C. to obtain a separator in which boehmite particles and CNF are laminated on one side. Then, the boehmite particles and CNF are laminated in the same manner on the side where the boehmite particles and CNF are not laminated, and a mixed layer of boehmite particles and CNF with a layer thickness of 7.5 μm is laminated on both sides. A secondary battery separator F2 (total thickness: 35 μm) was obtained.

Comparative Example 3 A Separator for a Secondary Battery in which a Nano-Array Made of Boehmite is Very Thickly Laminated on One Side of a Polyethylene Separator
A separator for a secondary battery in which a nanoarray of 39 μm thick boehmite was laminated on one side in the same manner as in Example 1 except that the thickness of the slurry D1 coated on one side of the polyethylene separator was changed by adjusting the roll coater. F3 (total thickness: 59 μm) was obtained.

Reference Example 1: Separator for Secondary Battery Composed of Polyolefin-Based Porous Membrane Only
A commercially available polyethylene separator (manufactured by Double Scope, COD-20-B) was adopted as the secondary battery separator F4 of the reference example (film thickness 20 μm).

«Evaluation of heat shrinkage rate»
The heat shrinkage ratio when heated at 105 ° C. for 30 minutes for the secondary battery separators (E1 to E3 and F1 to F4) of all the examples, comparative examples and reference examples is shown by the above method and the formula (1). I asked. The obtained results are shown in Table 1.

«Measurement of membrane resistance»
In the separators (E1 to E3 and F1 to F4) for secondary batteries of all the Examples, Comparative Examples and Reference Examples, 1 M of LiPF ₆ was dissolved in a solvent in which propylene carbonate and ethylene carbonate were mixed at a volume ratio of 1: 1. The film resistance was measured by an alternating current impedance method measured at 20 ° C. using an electrolyte.

  Specifically, a 3 cm × 3 cm secondary battery separator (E1 to E3, F1 to F4) was sandwiched between two 2 cm × 2 cm aluminum foils on which lead tabs were attached so that the aluminum foil did not short circuit. After that, the secondary battery separators (E1 to E3, F1 to F4) impregnated with the above-mentioned electrolytic solution were sealed under reduced pressure in an aluminum laminate pack so that the tabs would come out, to obtain a test cell.

After preparing similar test cells having two or three separators between the aluminum foils similarly, these cells are placed in a constant temperature bath at 20 ° C., and resistance is measured by an alternating current impedance method with an amplitude of 10 mV and a frequency of 100 kHz. It was measured. The slope obtained by linear approximation of the plot of the resistance value with respect to the number of separators was multiplied by the electrode area (4 cm ² ) to obtain the film resistance (Ω · cm ² ) per one separator.

  The obtained results are shown in Table 1.

«Measurement of puncture strength»
The puncture strength of each of the secondary battery separators (E1 to E3 and F1 to F4) of all the examples, comparative examples and reference examples was measured by a puncture strength tester (ZTS manufactured by Imada Co., Ltd.).

  The obtained results are shown in Table 1.

<< Evaluation of charge / discharge characteristics of secondary battery >>
Lithium ion secondary batteries were manufactured using the secondary battery separators (E1 to E3 and F1 to F4) of all the examples, comparative examples, and reference examples as separators.

Specifically, first, 30 ml of water and CNF (manufactured by Daicel Finechem, Celish FD 200 L, water content 20 mass%) were added to lithium olivine type manganese iron phosphate (LiMn _0.7 Fe _0.3 PO ₄ ) manufactured by a hydrothermal method. Then, the resultant was pulverized and mixed in a ball mill, and fired at 700 ° C. for 1 hour in a nitrogen atmosphere to obtain a positive electrode active material. Next, this positive electrode active material, ketjen black, and polyvinylidene fluoride were mixed at a blending ratio of 90: 5: 5 in mass ratio, N-methyl-2-pyrrolidone was added thereto, and the mixture was sufficiently kneaded to prepare a positive electrode slurry. . Then, using a coater, the positive electrode slurry was applied to a current collector made of an aluminum foil with a thickness of 20 μm, and vacuum drying was performed at 80 ° C. for 12 hours. Then, it punched out in disk shape of (phi) 14 mm, and pressed for 2 minutes at 16 MPa using a hand press, and it was set as the positive electrode.

As a negative electrode, lithium foil punched out to φ 15 mm was used. The electrolytic solution volume of ethylene carbonate and ethyl methyl carbonate ratio of 1: 1 solvent mixture in a ratio of used was LiPF ₆ was dissolved at a concentration of 1 mol / L.

  As the separators, the secondary battery separators (E1 to E3 and F1 to F4) of all the examples, comparative examples and reference examples were used.

  These battery parts were incorporated and stored in an atmosphere having a dew point of −50 ° C. or less by a conventional method to obtain a coin-type secondary battery (CR-2032).

  The charge / discharge characteristics at constant current density were evaluated using the obtained secondary battery. Specifically, the measurement was performed using a discharge capacity measurement device (HJ-1001SD8 manufactured by Hokuto Denko Corporation) in an environment of a temperature of 30 ° C. The charging conditions at this time were constant current charging with a current of 0.2 CA (17 mA / g) and a voltage of 4.5 V, and the discharging conditions were constant current discharging with a current of 3 CA and a final voltage of 2.0 V.

  The obtained results are shown in Table 1.

  From Table 1, it can be seen that the secondary battery separators (E1 to E3) obtained in Examples 1 to 3 have a small thermal contraction rate and a small membrane resistance, and exhibit high puncture strength. Moreover, it turns out that the secondary battery containing these each secondary battery separators (E1-E3) is excellent in the charge / discharge characteristic.

Claims (6)

A polyolefin-based porous membrane,
It is characterized in that it comprises: a layer composed of a ceramic nanoparticle aggregate in which a plurality of ceramic nanoparticles are supported linearly and continuously on cellulose nanofibers laminated on at least one surface of the polyolefin-based porous membrane. To be a separator for secondary batteries.   The separator for a secondary battery according to claim 1, wherein an average particle diameter of the ceramic nanoparticles is 30 nm or less.   The separator for a secondary battery according to claim 1, wherein an average fiber diameter of the cellulose nanofibers is 50 nm or less.   The thickness of the layer made of the ceramic nanoparticle aggregate is 5 μm or more on one side, and the total thickness including the layer made of the ceramic nanoparticle aggregate and the polyolefin-based porous film is 45 μm or less. The separator for a secondary battery according to any one of claims 1 to 3, which is assumed to be. The said ceramic nanoparticle contains 1 type, or 2 or more types selected from Al (OH) ₃ , Mg (OH) ₂ , and AlOOH, It is characterized by the above-mentioned. Secondary battery separator. Next steps (I) to (III):
(I) preparing a slurry containing a ceramic raw material compound containing at least one metal element and cellulose nanofibers,
(II) subjecting the slurry obtained in the step (I) to a hydrothermal reaction to obtain a ceramic nanoparticle aggregate supported on the cellulose nanofibers;
(III) A step of laminating the ceramic nanoparticle aggregate supported by the cellulose nanofibers obtained in the step (II) on at least one surface of a polyolefin-based porous membrane , And a method of manufacturing a separator for a secondary battery.