JP6299168B2 - Aromatic polyamide porous membrane and battery separator - Google Patents

Description

  The present invention relates to an aromatic polyamide porous membrane, and more particularly to an aromatic polyamide porous membrane that can be suitably used as a separator for an electricity storage device such as a battery.

  Non-aqueous secondary batteries such as lithium ion secondary batteries (LIBs) are widely used mainly for portable devices, and research and development for higher capacity, higher output, and higher safety are now underway. It is being advanced. Accordingly, separators are required to have safety such as heat resistance and short-circuit resistance simultaneously in a high region in addition to ion permeability, electrolytic solution retention, chemical stability, and thinning.

  In addition to the above, applicability to combinations with constituent members other than separators such as positive and negative electrode active materials and electrolytes is also important. For example, in the negative electrode material, instead of a commonly used carbon material, an alloy-based negative electrode that can be expected to have a high capacity, such as silicon-based and tin-based materials, has been studied. It is known that the volume expansion increases as the lithium storage amount increases. Therefore, it is necessary to select a separator having high adaptability to the volume change of the negative electrode.

  As a separator made of a heat resistant material, for example, Patent Documents 1 to 3 disclose a porous film made of an aromatic polyamide (aramid) having excellent heat resistance and chemical stability. Among these, patent document 1 is an example which disclosed the use as a separator of an aramid nonwoven fabric or an aramid paper. Patent Documents 2 and 3 are examples in which an aramid porous membrane obtained by solution casting is disclosed.

JP-A-5-335005 JP 2005-209989 A JP 2010-77335 A

  However, in the nonwoven fabric and the paper-like sheet described in Patent Document 1, because the fiber is folded in the thickness direction, the fiber is easily compressed in the thickness direction, and when compressed, the voids disappear and the resistance increases. There is.

  Further, Patent Document 3 uses a so-called wet method in which an aramid solution is cast and then immersed in a coagulation bath to be deposited. This method tends to coarsen the pores and easily compress in the thickness direction. There is sex. Moreover, since it becomes a hole structure with many partition walls between holes, there exists a possibility that resistance may become large when compressed.

  On the other hand, Patent Document 3 describes a porous film in which the increase in the Gurley permeability is small even when compression is performed in the thickness direction. However, this porous film easily undergoes plastic deformation, and the compression force is unloaded. Therefore, there is a problem in durability when the electrode repeats expansion and contraction after long-term use.

  As described above, there is still room for improvement in adaptation to a lithium ion secondary battery using an electrode having a large volume change such as an alloy-based negative electrode in an aromatic polyamide porous film.

  In view of the above circumstances, the present invention provides a high output and long-term excellent cycle characteristics even when used with an electrode having a large volume change such as an alloy-based negative electrode, and has an excellent heat resistance. An object of the present invention is to provide a porous polyamide porous membrane and a battery separator using the same.

  In order to achieve the above object, the present invention has the following configuration.

(1) is the thickness direction of the compressive deformation modulus 100 to 300 MPa, compressive deformation recovery factor is 70% to 100% der in the thickness direction is, a thickness of 5 to 30 [mu] m, with a porosity of 50-80% Yes, the Gurley air permeability is 1 to 200 seconds / 100 ml, both the longitudinal direction (MD) and the width direction (TD) propylene carbonate liquid uptake are 10 to 100 mm / minute, and the longitudinal direction at 250 ° C. (MD) and none of the heat shrinkage in the width direction (TD) is Ru -0.5～3.0% der, aromatic polyamide porous film.

(2) cell separator obtained by using the aromatic polyamide porous film according to the above (1).

  Since the aromatic polyamide porous membrane of the present invention has a high compressive deformation elastic modulus, blockage of pores due to deformation when compressed in the thickness direction is suppressed. Furthermore, since the deformation recovery rate is high when the compressive force is unloaded, excellent followability is exhibited even when used with an electrode having a large volume change. Therefore, it can be suitably used for a battery separator such as a lithium ion secondary battery.

  When the aromatic polyamide porous membrane of the present invention is used as a separator for a secondary battery, even when an electrode having a large volume change such as an alloy negative electrode is used as a battery electrode, high output and excellent long-term performance are obtained. Cycle characteristics can be obtained.

As the aromatic polyamide used in the present invention, those having a repeating unit represented by the following chemical formula (1) are preferable.
Chemical formula (1):

Each of Ar ₁ and Ar ₂ may be a single group or a plurality of groups and may be a copolymer, but all groups of Ar ₁ and Ar ₂ are represented by the following chemical formula ( 2) and a group selected from the groups represented by chemical formulas (3) to (5).
Chemical formula (2):

Chemical formulas (3) to (5):

In addition, as X, a group selected from —O—, —CO—, —CO ₂ —, —SO ₂ —, —CH ₂ —, —S—, —C (CH ₃ ) ₂ — and the like is used. Can do.

  Here, the chemical formula (2) is a group contributing to the rigidity of the aromatic polyamide, and the chemical formulas (3) to (5) are groups contributing to the flexibility.

As the aromatic polyamide of the present invention, the ratio of the chemical formula (2) is preferably 60 to 85%, more preferably 65 to 80%, in the total of all Ar ₁ and Ar _2. . If the proportion of the chemical formula (2) is less than 60%, the inherent rigidity of the polymer itself is lowered, and the pore structure tends to be coarsened when a porous film is formed. May be less than 100 MPa and not within the scope of the present invention. Moreover, since the obtained porous film tends to have a pore structure with many partition walls between the pores, the resistance may increase when compressed. When the proportion of the chemical formula (2) exceeds 85%, the flexibility of the polymer decreases, and the resulting porous film is likely to undergo plastic deformation and structural destruction when compressed, and the compression deformation recovery rate in the thickness direction. May be less than 70% and not within the scope of the present invention.

Further, a part of hydrogen atoms on the aromatic ring in Ar ₁ and Ar ₂ are halogen groups such as fluorine, bromine and chlorine; nitro groups; cyano groups; alkyl groups such as methyl, ethyl and propyl; methoxy, ethoxy, Those substituted with a substituent such as an alkoxy group such as propoxy are improved in solubility in a solvent and thus can be easily applied to a solution casting method. This is preferable because it reduces the moisture absorption rate. In particular, having an electron-withdrawing substituent such as a halogen group, a nitro group, or a cyano group has excellent electrochemical oxidation resistance in addition to the effects of the above-described substituents. This is preferable because deterioration such as oxidation can be prevented. Among these, a halogen group is more preferable as a substituent, and a chlorine atom is most preferable. Further, hydrogen in the amide bond constituting the polymer may be substituted with a substituent.

  The aromatic polyamide porous membrane of the present invention preferably has a compressive deformation modulus in the thickness direction of 100 to 300 MPa. More preferably, it is 130-250 MPa, More preferably, it is 150-200 MPa. By setting the compressive deformation modulus in the thickness direction within the above range, when used as a battery separator, even if a load that compresses in the thickness direction of the membrane is applied due to the expansion of the electrode, the hole is crushed and blocked by the deformation. Therefore, good ionic conduction can be maintained. Further, even when the separator is thinned, it is possible to prevent the positive and negative electrodes from being short-circuited due to being crushed. In particular, in a lithium ion secondary battery, when a high capacity alloy-based negative electrode such as a silicon-based or tin-based material is used as a negative electrode material, if the lithium storage amount increases due to charging, a significant volume expansion may be exhibited. It preferably has a compressive deformation modulus. Furthermore, in addition to the expansion of the electrode, a load that compresses in the thickness direction may be applied in the manufacturing process, in the step of winding together with the positive and negative electrodes, and in the step of compressing it after winding. If the compressive deformation modulus in the thickness direction is less than 100 MPa, the holes may be crushed by the expansion of the electrode or a compression load in the manufacturing process, and a sufficient output may not be obtained when used as a battery separator. In addition, when the conduction path of lithium ions is limited by crushing the holes, dendritic lithium metal that is one of the causes of a micro short circuit may be easily grown. Furthermore, when the separator is thinned, the positive and negative electrodes may come into contact with each other and short circuit may be easily caused. When the compressive deformation elastic modulus in the thickness direction exceeds 300 MPa, structural resistance of the electrode may be induced because the resistance against expansion of the electrode is too strong. In order to set the compressive deformation modulus in the thickness direction within the above range, it is preferable to control the pore structure by setting the polymer structure of the aromatic polyamide as described above and further setting the stretching conditions as described below.

  The aromatic polyamide porous membrane of the present invention preferably has a compression deformation recovery rate in the thickness direction of 70 to 100%. More preferably, it is 80-100%, More preferably, it is 90-100%. By setting the compression deformation recovery rate in the thickness direction within the above range, when used as a battery separator, even if the electrode expands and contracts due to charging and discharging, it follows the volume change and continues for a long time. Excellent cycle characteristics can be obtained. In particular, in a lithium ion secondary battery, when a high capacity alloy negative electrode such as silicon or tin is used as the negative electrode material, the volume change during charging / discharging may be significant. It is preferable to have. When the compressive deformation recovery rate in the thickness direction is less than 70%, the electrode cannot follow up when the electrode repeats expansion and contraction after long-term use, resulting in liquid drainage or poor resistance to contact with the electrode. However, the output may decrease. In order to bring the compression deformation recovery rate in the thickness direction within the above range, the polymer structure of the aromatic polyamide is set as described above, and the orientation of the aromatic polyamide molecular chain is controlled by setting the stretching conditions as described below. It is preferable to do.

  The aromatic polyamide porous membrane of the present invention preferably has a porosity of 50 to 80%. More preferably, it is 60 to 70%. When the porosity is less than 50%, when used as a battery separator, the amount of electrolyte solution is small, and sufficient solvent molecules are available for lithium ions to solvate when rapidly charged and discharged. Cannot be compensated for and may cause polarization. Even if the amount of deformation in the thickness direction is very small, vacancies that are ion conduction paths are likely to be blocked, and a sufficient output may not be obtained. Furthermore, when charging / discharging is repeated, performance degradation may occur due to liquid drainage. If the porosity exceeds 80%, the compressive deformation modulus may be less than 100 MPa and may not fall within the scope of the present invention. Further, when used as a battery separator, a short circuit between the electrodes may easily occur. In order to make the porosity within the above range, it is preferable that the formulation of the membrane forming stock solution and the production conditions of the porous membrane are within the ranges described below.

  The aromatic polyamide porous membrane of the present invention preferably has a Gurley permeability of 1 to 200 seconds / 100 ml. More preferably, it is 5 to 100 seconds / 100 ml. If the Gurley air permeability is less than 1 second / 100 ml, the strength decreases, and the film may be broken during processing, or a short circuit between the electrodes may easily occur when used as a battery separator. When the Gurley air permeability is greater than 200 seconds / 100 ml, the resistance is large, and the output is likely to decrease when used as a battery separator. In order to make the Gurley air permeability within the above range, it is preferable that the formulation of the membrane forming stock solution and the production conditions of the porous membrane are as described below.

  The aromatic polyamide porous membrane of the present invention preferably has a propylene carbonate liquid uptake of 10 to 100 mm / min in the longitudinal direction (MD) and the width direction (TD). More preferably, it is 15-100 mm / min. If the liquid wicking property is less than 10 mm / min, when used as a battery separator, when the electrolyte is partially insufficient, the averaging is not performed quickly, and the battery output and cycle characteristics are degraded due to liquid drainage. There is. It is preferable that the liquid sucking property is large, and the upper limit is not particularly defined. However, in the case of a porous film, the limit is generally about 100 mm / min. In order to make the wicking property within the above range, the formulation of the membrane forming solution and the manufacturing conditions of the porous membrane are as described below, and there are few partition walls between the pores, and the pore structure allows the liquid to easily penetrate into the membrane surface. Is preferred.

  The thickness of the aromatic polyamide porous membrane of the present invention is preferably 5 to 30 μm, and more preferably 10 to 30 μm. More preferably, it is 15-25 micrometers. When the thickness is less than 5 μm, the strength is low, and the film may be broken during processing, or when used as a separator, self-discharge may be greatly reduced, cycle characteristics may be lowered, or the withstand voltage may be low and the electrodes may be short-circuited. When it exceeds 30 μm, when used as a separator, the output may decrease due to an increase in internal resistance, or the thickness of the active material layer that can be incorporated in the battery may become thin, and the capacity per volume may decrease. The thickness of the porous membrane is determined by the polymer structure of the aromatic polyamide, the degree of polymerization, the concentration of the film-forming stock solution, the viscosity of the film-forming stock solution, the additive in the film-forming stock solution, the casting thickness, the porosity condition, the wet bath temperature, and the heat treatment temperature. It can be controlled by various conditions such as temperature and stretching conditions.

  In the aromatic polyamide porous membrane of the present invention, it is preferable that both the heat shrinkage in the longitudinal direction (MD) and the width direction (TD) at 250 ° C. are −0.5 to 3.0%, and −0 More preferably, it is 5 to 1.0%. When the thermal shrinkage rate exceeds 3.0%, a short circuit may occur at the end of the battery due to the shrinkage of the separator during abnormal heat generation of the battery. In order to make the heat shrinkage within the above range, it is preferable that the polymer structure of the aromatic polyamide is as described above and the logarithmic viscosity is high within the range described below. Moreover, it is preferable to perform extending | stretching and heat processing on the conditions mentioned later.

  The aromatic polyamide porous membrane of the present invention preferably has a stress at break in at least one direction of 30 MPa or more. When the stress at break is less than 30 MPa, the film may be broken due to high tension and fluctuation in tension during processing, and productivity may be reduced. From the viewpoint of improving productivity, the stress at break is more preferably 40 MPa or more, and further preferably 50 MPa or more. The upper limit is not particularly defined, but generally about 1 GPa is the limit if it is a porous film. In order to make the stress at break within the above range, it is preferable that the polymer structure of the aromatic polyamide is as described above and the logarithmic viscosity is high within the range described below. Moreover, it is preferable to perform extending | stretching and heat processing on the conditions mentioned later.

  In the aromatic polyamide porous membrane of the present invention, the elongation at break in the longitudinal direction (MD) and the width direction (TD) is preferably 10% or more. Since the elongation is high, film breakage in the processing step can be reduced, and processing at high speed becomes possible. Further, when used as a battery separator, the battery can follow the expansion and contraction of the electrode during charge and discharge without breaking, and the durability and safety of the battery can be ensured. Since workability, durability and safety are further improved, the breaking elongation is more preferably 20% or more, and further preferably 30% or more. The upper limit is not particularly defined, but generally about 200% is the limit for porous membranes. In order to make the elongation at break within the above range, it is preferable that the polymer structure of the aromatic polyamide is as described above and the logarithmic viscosity is high within the range described below. Moreover, it is preferable to perform extending | stretching and heat processing on the conditions mentioned later.

  The aromatic polyamide porous membrane of the present invention preferably has a puncture strength on the front and back surfaces of 80 N / mm or more. The puncture strength here refers to the maximum load when the needle is pierced perpendicularly to the measurement surface of the porous membrane at a speed of 300 mm / min using a needle having a radius of curvature of 1.5 mm at the tip. The value divided by. When the puncture strength is less than 80 N / mm, when used as a battery, it may lead to a film breakage due to precipitated lithium metal or foreign matter mixed in the manufacturing process or a short circuit between the positive and negative electrodes. The piercing strength is more preferably 150 N / mm or more, and further preferably 200 N / mm or more. The upper limit is not particularly defined, but if it is an aromatic polyamide porous membrane having a porosity of 50 to 80%, the upper limit is generally about 500 N / mm. In order to make the puncture strength within the above range, it is preferable that the polymer structure of the aromatic polyamide is as described above and the logarithmic viscosity is high within the range described below. Moreover, it is preferable to perform extending | stretching and heat processing on the conditions mentioned later.

  Next, the manufacturing method of the aromatic polyamide porous membrane of this invention is demonstrated below. First, when an aromatic polyamide is polymerized using, for example, acid dichloride and diamine as raw materials, a solution in an aprotic organic polar solvent such as N-methylpyrrolidone, N, N-dimethylacetamide, dimethylformamide, dimethylsulfoxide, etc. A method of synthesis by polymerization, a method of synthesis by interfacial polymerization using an aqueous medium, or the like can be employed. Solution polymerization in an aprotic organic polar solvent is preferable because the molecular weight of the polymer can be easily controlled.

  In the case of solution polymerization, in order to obtain a polymer having a high molecular weight, the water content of the solvent used for the polymerization is preferably 500 ppm or less (mass basis, the same applies hereinafter), more preferably 200 ppm or less. If both the acid dichloride and diamine used are used in equal amounts, an ultra-high molecular weight polymer may be formed. Therefore, the molar ratio should be adjusted so that one is 95.0-99.5 mol% of the other. Is preferred. In addition, the polymerization reaction of the aromatic polyamide is exothermic, but if the temperature of the polymerization system rises, side reaction may occur and the degree of polymerization may not be sufficiently increased. It is preferable to cool. The temperature of the solution during polymerization is more preferably 30 ° C. or lower. Furthermore, when acid dichloride and diamine are used as raw materials, hydrogen chloride is produced as a by-product in the polymerization reaction, but when neutralizing this, an inorganic neutralizing agent such as lithium carbonate, calcium carbonate, calcium hydroxide, Alternatively, an organic neutralizer such as ethylene oxide, propylene oxide, ammonia, triethylamine, triethanolamine, diethanolamine may be used.

In order to obtain the aromatic polyamide porous membrane of the present invention, the logarithmic viscosity (η _inh ) of the aromatic polyamide polymer is preferably 1.8 to 3.5 dl / g, and 2.2 to 3.0 dl / g. More preferably, it is g. When the logarithmic viscosity is less than 1.8 dl / g, the bond strength between the chains due to the entanglement of the polymer molecular chains decreases, so mechanical properties such as toughness and strength may decrease, and the thermal shrinkage rate may increase. . When the logarithmic viscosity exceeds 3.5 dl / g, the solubility in a solvent may be reduced, or aromatic polyamide molecules may aggregate and it may be difficult to form a porous membrane.

  Next, a film-forming stock solution used in the process for producing the aromatic polyamide porous membrane of the present invention (hereinafter sometimes simply referred to as a film-forming stock solution) will be described.

  The polymer solution after polymerization may be used as it is for the film-forming stock solution, or it may be used after being isolated and then redissolved in an inorganic solvent such as the above-mentioned aprotic organic polar solvent or sulfuric acid. . The method for isolating the aromatic polyamide is not particularly limited, but the solvent and neutralized salt are extracted into water by introducing the polymerized aromatic polyamide solution into a large amount of water, and only the precipitated aromatic polyamide is removed. The method of drying after isolate | separating is mentioned. Further, a metal salt or the like may be added as a dissolution aid during re-dissolution. The metal salt is preferably an alkali metal or alkaline earth metal halide dissolved in an aprotic organic polar solvent, such as lithium chloride, lithium bromide, sodium chloride, sodium bromide, potassium chloride, potassium bromide, etc. Is mentioned.

  The content of the aromatic polyamide in 100% by mass of the film-forming stock solution is preferably 5 to 20% by mass, more preferably 9 to 15% by mass. If the content of the aromatic polyamide in the film-forming stock solution is less than 5% by mass, the compression deformation elastic modulus and the compression deformation recovery rate in the thickness direction may not fall within the scope of the present invention due to the coarsening of the pore structure. In addition, mechanical properties such as toughness and strength may decrease, and the thermal shrinkage rate may increase. When the content of the aromatic polyamide in the film-forming stock solution exceeds 20% by mass, the aromatic polyamide polymers tend to agglomerate during the production of the porous membrane, and the porosity and the Gurley permeability are in accordance with the present invention. May not be within range.

  It is preferable to mix a hydrophilic polymer with the film-forming stock solution for the purpose of improving pore forming ability. The hydrophilic polymer to be mixed is preferably 1 to 10% by mass, more preferably 1 to 6% by mass with respect to 100% by mass of the film-forming stock solution. When the content of the hydrophilic polymer in the film-forming stock solution is less than 1% by mass, aromatic polyamide molecules may aggregate in the process of forming the porous film, making it difficult to form the porous film. . When the content exceeds 10% by mass, the pore structure may be coarsened or the strength may be reduced in the resulting porous membrane. In addition, the residual amount of the hydrophilic polymer in the porous film eventually increases, and heat resistance and rigidity may be reduced, and the elution of the hydrophilic polymer into the electrolytic solution may occur.

  The hydrophilic polymer is a polymer containing at least one substituent selected from the group consisting of a polar substituent, particularly a hydroxyl group, an acyl group, and an amino group, among polymers that are soluble in an aprotic organic polar solvent. Preferably there is. Examples of such a polymer include polyvinyl pyrrolidone (hereinafter sometimes referred to as PVP), polyethylene glycol, polyvinyl alcohol, polyacrylamide, polyacrylic acid, polyethyleneimine, and the like. It is most preferable to use PVP having good compatibility with the aromatic polyamide. The weight average molecular weight of PVP is preferably 500,000 to 3,000,000. When the weight average molecular weight is less than 500,000, when low molecular weight PVP remains in the porous membrane, the heat resistance of the porous membrane is reduced or the PVP is dissolved in the electrolyte when used as a battery separator. There is a risk of doing. When the weight average molecular weight exceeds 3 million, it may be difficult to form a porous film because the solution viscosity of the film forming stock solution becomes too high. The hydrophilic polymer may be put into the aromatic polyamide solution after polymerization or the re-dissolved aromatic polyamide solution, or may be put into an aprotic organic polar solvent together with the isolated aromatic polyamide and kneaded.

  In order to improve the processability by forming protrusions on the surface of the resulting porous membrane to improve the coefficient of static friction and improve the workability, inorganic or organic particles may be added to the stock solution.

  As for the solution viscosity of the film-forming stock solution, the value measured at 30 ° C. and 10 rpm using a B-type viscometer is preferably 100 to 800 Pa · s. More preferably, it is 200-600 Pa.s. When the solution viscosity is less than 100 Pa · s, the compression deformation elastic modulus and the compression deformation recovery rate in the thickness direction may not fall within the scope of the present invention due to the coarsening of the pore structure. In addition, mechanical properties such as toughness and strength may decrease, and the thermal shrinkage rate may increase. If the solution viscosity exceeds 800 Pa · s, it may be difficult to form a porous membrane.

  A porous membrane is produced by a so-called solution casting method using the membrane-forming stock solution prepared as described above. Typical examples of the method for producing a porous film by solution casting include a wet method and a precipitation method. However, in a wet method using a coagulation bath, the pores formed in a coarse shape or a thickness direction are formed. In some cases, non-uniformity may occur or partition walls are likely to be formed between the holes. For this reason, in order to obtain the porous film of the present invention, it is preferable to form the film by a deposition method in which the pore structure can be controlled finely and uniformly.

  When producing a porous film by the precipitation method, first, the film-forming stock solution is cast (cast) on a support using a die or a die coater to obtain a cast film of the film-forming stock solution, Precipitate to obtain a porous film. The material for the support is not particularly limited, and examples thereof include resins such as stainless steel, glass, and polyethylene terephthalate (PET). As a method for precipitating the polymer from the cast membrane, a method for precipitating the polymer by absorbing the cast membrane in a temperature-controlled humidity atmosphere, a method for reducing the solubility of the polymer by cooling the cast membrane and causing phase separation or precipitation. And a method of depositing a polymer by spraying mist water on a cast film. In the cooling method, it takes time to deposit the polymer, and the pore shape is likely to be nonuniform, and the productivity may be lowered. On the other hand, in the method of spraying mist-like water, a dense layer may be formed on the surface. For these reasons, the method of absorbing moisture into the cast film under a temperature-controlled humidity atmosphere is preferable because the supply rate and amount of water can be arbitrarily controlled and a homogeneous porous structure can be formed in a short time.

In the production process of the porous membrane of the present invention, the volumetric absolute humidity of the temperature-controlled humidity atmosphere is preferably 10 to 180 g / m ³ . More preferably, it is 30-100 g / m < ³ >, More preferably, it is 40-90 g / m < ³ >. Moreover, within the range which satisfy | fills this absolute humidity, it is preferable that the temperature of atmosphere is 20-70 degreeC and a relative humidity shall be 60-95% RH. More preferably, the temperature of the atmosphere is 30 to 60 ° C., and the relative humidity is 70 to 90% RH. The treatment time in the temperature-controlled humidity atmosphere is preferably 0.5 to 5 minutes, and more preferably 0.5 to 3 minutes.

  Next, the deposited aromatic polyamide porous membrane is peeled off from the support or from the support and introduced into a wet bath to remove the solvent, the hydrophilic polymer not taken in, and additives such as inorganic salts. Do. The bath composition is not particularly limited, but it is preferable to use water or an organic solvent / water mixed system from the viewpoint of economy and ease of handling. Further, the wet bath may contain an inorganic salt. The wet bath temperature is preferably 20 ° C. or higher because the solvent and the like can be efficiently removed. Although the upper limit of the bath temperature is not particularly defined, it is efficient to suppress the temperature to 90 ° C. in consideration of the generation of bubbles due to water evaporation or boiling. The introduction time is preferably 1 to 20 minutes. Furthermore, you may extend | stretch in the longitudinal direction (MD) and width direction (TD) of a porous membrane in a wet bath.

  Next, the porous film that has been desolvated is subjected to heat treatment using a tenter or the like. At this time, after drying the porous film in the longitudinal direction (MD) and the width direction (TD) before drying the moisture from the water-containing porous film, the glass transition temperature of the aromatic polyamide is exceeded. Heat treatment is preferably performed at a temperature. By performing stretching, the deformation elastic modulus against compression in the thickness direction can be improved. Furthermore, since the pore path of the porous membrane spreads in the in-plane direction and the liquid sucking property is improved, it is possible to suppress a decrease in battery output and cycle characteristics due to liquid drainage when used as a battery separator. On the other hand, when the molecular chain of the polymer is oriented in the plane direction by stretching, the deformation recovery rate with respect to compression in the thickness direction may decrease. Therefore, in the present invention, a high deformation recovery rate can be obtained with respect to compression in the thickness direction by performing stretching in a water-containing state and suppressing the orientation of the aromatic polyamide molecular chain in the stretching direction. The water content in the porous membrane at the completion of stretching is preferably 20 parts by mass or more, more preferably 50 parts by mass or more with respect to 100 parts by mass of the aromatic polyamide. Moreover, you may complete extending | stretching in the above-mentioned wet bath. When the moisture content in the porous membrane is less than 20 parts by mass with respect to 100 parts by mass of the aromatic polyamide, the orientation of the aromatic polyamide molecular chain in the stretching direction becomes stronger, so that the compression deformation recovery rate in the thickness direction May be less than 70% and may not fall within the scope of the present invention. Furthermore, elongation and heat resistance may be reduced. The draw ratio to MD and TD is preferably 1.05 to 1.50 times in both directions, and more preferably 1.10 to 1.30 times. When the draw ratio is less than 1.05 times, the compressive deformation modulus in the thickness direction is less than 100 MPa, which may not be within the scope of the present invention. Further, when the liquid sucking property is less than 10 mm / min, it may not fall within the scope of the present invention, and when used as a battery separator, battery output and cycle characteristics may be deteriorated due to liquid drainage. When the draw ratio exceeds 1.50 times, the porous membrane may be easily broken during stretching, and the elongation and heat resistance may be reduced. The stretching ratio to MD and TD may be the same or different as long as both are within the above range. Here, the draw ratio of the present invention is a value calculated on the basis of the cast length and cast width.

In addition, when the heat treatment temperature after stretching is T and the glass transition temperature of the aromatic polyamide is T _g , by setting T _g + 10 ≦ T ≦ T _g +40, the orientation of the aromatic polyamide molecular chain in the stretching direction can be increased. It is preferable because it is relaxed and the compression deformation recovery rate becomes high. When T <T _g +10, the orientation of the aromatic polyamide molecular chain in the stretching direction is hardly relaxed, and the compression deformation recovery rate in the thickness direction is less than 70 MPa, which may not fall within the scope of the present invention. When T> T _g +40, mechanical properties such as elongation at break may be deteriorated due to polymer decomposition or the like.

  Since the aromatic polyamide porous membrane of the present invention has a high compressive deformation elastic modulus, blockage of pores due to deformation when compressed in the thickness direction is suppressed. Furthermore, since the deformation recovery rate is high when the compressive force is unloaded, excellent followability is exhibited even when used with an electrode having a large volume change. Therefore, it can be suitably used for various battery separators.

  As an example of a battery using the aromatic polyamide porous membrane of the present invention as a separator, a lithium ion secondary battery can be cited. Particularly, when an alloy-based negative electrode active material having a high capacity and a large volume change is used as a negative electrode material. , Good battery performance can be exhibited. Examples of the alloy-based negative electrode active material include silicon oxide, silicon alloy, tin alloy, and lithium alloy. These may be used as a mixture of two or more thereof, or may be used as a mixture with a carbon-based active material such as graphite. May be.

  A battery using the aromatic polyamide porous membrane of the present invention as a separator provides high output and excellent long-term cycle characteristics even when an electrode with a large volume change such as an alloy-based negative electrode is used as the electrode. It is done. Furthermore, since the aromatic polyamide porous membrane of the present invention has excellent heat resistance, the obtained secondary battery can be used even at high temperatures. Therefore, the secondary battery using the aromatic polyamide porous membrane of the present invention as a separator is used for traffic such as small electronic devices, electric vehicles (EV), hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV) and the like. It can be suitably used as a power source for large-scale industrial equipment such as engines and industrial cranes. Further, it can be suitably used as a power storage device for leveling power or a smart grid in a solar cell, a wind power generator, or the like.

[Methods for measuring physical properties and methods for evaluating effects]
The physical properties were measured in the examples according to the following method.

(1) Compressive deformation elastic modulus in thickness direction It was measured under the following conditions by a nanoindentation method (continuous stiffness measurement method).

Measuring device: Micro hardness tester Nano Indenter XP
(MTS Systems)
Working indenter: Diamond regular triangular pyramid indenter Measurement atmosphere: room temperature, in air The sample was cut into 1 cm square, and the Poisson's ratio of the sample was 0.4. Here, the measurement was carried out on a composite (PET / sample / PET three-layer structure) in which a sample was sandwiched between polyethylene terephthalate (PET) films having a thickness of 75 μm, and composites obtained from data up to an indentation depth of 0.5 μm. The elastic modulus of a single sample was calculated using the elastic modulus of the body. In addition, the compression deformation elastic modulus in the said conditions of the used PET film single-piece | unit was 3.4 GPa.

(2) Compressive deformation recovery rate in the thickness direction The displacement in the thickness direction was measured under repeated loading and unloading in the thickness direction of the sample under the following conditions.

Measuring device: Micro compression tester MCTW-500 (manufactured by Shimadzu Corporation)
Working indenter: Diamond flat indenter (indenter diameter 500 μm)
Load mode: repeated load / unload
(Load load: 10 mN, unloading load: 0.5 mN)
Loading speed: 0892 mN / sec Number of repetitions: 50 times Measurement atmosphere: room temperature, in air Samples cut into 1 cm square were used. Here, measurement was carried out on a composite (PET / sample / PET three-layer structure) in which a sample was sandwiched between polyethylene terephthalate (PET) films having a thickness of 75 μm, and the displacement of the PET film was subtracted from the displacement of the obtained composite. Thus, the displacement of the single sample was calculated.

  The compression deformation recovery rate was calculated from the following equation using the maximum displacement during loading and the unloading displacement in each load / unload cycle.

Compression deformation recovery rate (%) = (maximum displacement−displacement at unloading) × 100 / maximum displacement (3) Porosity The thickness and mass of a 100 mm square sample are measured, and the apparent density (bulk) of the porous membrane is measured. density) was determined _{d 1.} From this and the true density d _{0 of the} polymer, the porosity was calculated using the following equation.

Porosity (%) = (1−d ₁ / d ₀ ) × 100
(4) Gurley air permeability A B-type Gurley densometer (manufactured by Yasuda Seiki Seisakusho) was used to measure the Gurley air permeability of the porous film according to the method defined in JIS-P8117 (1998). The porous membrane of the sample is clamped to a circular hole with a diameter of 28.6 mm and an area of 645 mm ² , and the air inside the cylinder is passed from the test hole to the outside of the cylinder by the inner cylinder (inner cylinder mass 567 g), and 100 ml of air passes The Gurley air permeability was determined by measuring the time to perform.

(5) Uptake property of propylene carbonate solution Measurement was performed using the method defined in JIS-P8141 (2004) except that the test solution was changed from water to propylene carbonate and the measurement time was changed from 10 minutes to 1 minute.

(6) Thickness The thickness of the porous membrane was measured using a constant pressure thickness measuring instrument FFA-1 (manufactured by Ozaki Seisakusho). The probe diameter is 5 mm and the measurement load is 1.25 N. The thickness was arbitrarily measured at 10 points on a 100 mm square porous membrane sample, and the average value was obtained.

(7) Thermal contraction rate at 250 ° C. The porous membrane of the sample was cut into a strip shape having a width of 10 mm and a length of 120 mm, and the long side was taken as the measurement direction. Marked from both ends of the long side portion approximately 10 mm, the distance between the marks was L _1. 250 ° C. 10 min in a hot air oven, a distance between the mark after the heat treatment with no over substantially tension and L _2, was calculated thermal shrinkage by the following formula. The film was measured five times in the longitudinal direction and in the width direction, and the average value was obtained.

Thermal contraction rate (%) = ((L ₁ −L ₂ ) / L ₁ ) × 100
(8) Logarithmic viscosity (η _inh )
The polymer was dissolved at a concentration of 0.5 g / dl in N-methylpyrrolidone (NMP) to which 2.5% by mass of lithium bromide (LiBr) was added, and the flow-down time was 30 ° C. using an Ubbelohde viscometer. Was measured. The flow time of blank NMP in which the polymer was not dissolved was measured in the same manner, and the logarithmic viscosity (η _inh ) was calculated using the following equation.

η _inh (dl / g) = [ln (t / t ₀ )] / 0.5
t ₀ : Blank flow time (seconds)
t: Sample flow time (seconds)
(9) Glass transition temperature of aromatic polyamide The glass transition temperature was measured by the temperature modulation differential scanning calorimetry (temperature modulation DSC) method under the following conditions. As the measurement sample, one obtained by isolating aromatic polyamide alone from the solution after polymerization was used.

Measuring device: Q100 (manufactured by TA Instruments)
Data processing device: Universal Analysis 2000
(Manufactured by TA Instruments)
Measurement atmosphere: Nitrogen flow (50 mL / min)
Temperature / calorie calibration: High purity indium Temperature range: 25-350 ° C
Temperature increase rate: 2 ° C / min Sample amount: Approximately 5mg
(10) Battery Evaluation As described below, a coin-type lithium ion secondary battery was prepared, and two types of evaluations A and B were performed. Each evaluation was carried out with 10 cells.

・ Positive electrode A mixture of 90 parts by mass of lithium iron phosphate (LiFePO ₄ ) as an active material, 5 parts by mass of acetylene black as a conductive additive and 5 parts by mass of aromatic polyamide as a binder in N-methylpyrrolidone (NMP) The positive electrode paste was prepared by dispersing in the above. Here, the aromatic polyamide used for the binder is a mixture of 2-chloro-1,4-phenylenediamine / 4,4′-diaminodiphenyl ether = 85/15 (mol%) as the diamine component, and 2-as the acid dichloride component. A random copolymer polymerized using chloroterephthaloyl chloride was used. The obtained paste was applied onto a 20 μm thick aluminum foil as a current collector, dried at 120 ° C. for 15 minutes, and then compression molded. This was punched into a circle with a diameter of 13 mm to obtain a positive electrode.

-Negative electrode An N-methylpyrrolidone mixture containing 85 parts by mass of silicon monoxide (SiO) as an active material, 5 parts by mass of acetylene black as a conductive additive, and 10 parts by mass of an aromatic polyamide obtained in the same manner as the positive electrode as a binder. A negative electrode paste was prepared by dispersing in (NMP). The obtained paste was applied onto a 15 μm-thick stainless steel foil (SUS304) as a current collector and dried at 120 ° C. for 15 minutes. This was punched into a circle having a diameter of 14.5 mm to obtain a negative electrode. The obtained negative electrode was pre-doped with lithium foil for the purpose of compensating for the initial irreversible capacity.

Electrolyte Solution used was a solution in which LiBF ₄ was dissolved at 1.5 mol / L in a mixed solvent having a volume ratio of propylene carbonate (PC) / γ-butyrolactone (GBL) = 30/70.

-Assembly The said negative electrode was left still on the spacer of the welding-sealed sealing board equipped with the gasket so that a negative electrode agent might turn up, and the said electrolyte solution was inject | poured from the top. On top of that, a porous film of a sample (a circle having a diameter of 17 mm) was left as a separator, and an electrolytic solution was injected from above the separator. Next, the positive electrode was allowed to stand with the positive electrode agent facing down, and the case was allowed to stand. This was sealed with a caulking machine to produce a coin-type battery having a diameter of 20 mm and a thickness of 3.2 mm.

-Finish charge / discharge The produced coin-type battery was charged in a 20 ° C atmosphere at a constant current of 0.2C until the battery voltage reached 3.6V, and after the charge, it was aged at 20 ° C for 96 hours. Thereafter, discharging was performed at a constant current of 0.2 C until the battery voltage reached 2.5V.

  The battery evaluation of the following a and b was implemented using the coin-type battery which finished charging / discharging. For the evaluation, an average of three excellent results among the results obtained from five coin-type batteries was used. If evaluation of any of a and b is either A, B or C, it can be said that the battery is practically excellent. More preferably, any evaluation of a and b is A or B, and more preferably any evaluation of a and b is A.

a. Output characteristics All the evaluations were performed in an atmosphere of 120 ° C. The produced coin-type battery was charged at a constant current of 0.2 C until the battery voltage reached 3.6 V, and then discharged at a constant current of 0.2 C until the battery voltage reached 2.5 V. The discharge capacity at this time was a discharge capacity at 0.2C. Next, the battery was charged at a constant current of 0.2 C until the battery voltage reached 3.6 V, and then discharged at a constant current of 5 C until the battery voltage reached 2.5 V. The discharge capacity at this time was taken as the discharge capacity at 5C. From these results, the capacity retention rate was calculated using the following formula, and A to E were evaluated according to the following criteria.

Capacity maintenance ratio (%) = (discharge capacity at 5 C) / (discharge capacity at 0.2 C) × 100
A: 60% or more B: 50% or more and less than 60% C: 40% or more and less than 50% D: Less than 40% E: Cannot be measured due to a short circuit.

b. Cycle characteristics All the evaluations were performed in an atmosphere of 120 ° C. The manufactured coin-type battery was charged at a constant current of 5C until the battery voltage reached 3.6V, and then discharged at a constant current of 5C until the battery voltage reached 2.5V. This charging / discharging was made into 1 cycle, and charging / discharging of 100 cycles was performed in total. From the discharge capacity at the first cycle and the discharge capacity at the 100th cycle, the capacity retention rate was calculated using the following equation, and A to E were evaluated according to the following criteria.

Capacity retention rate (%) = (discharge capacity at the 100th cycle) / (discharge capacity at the first cycle) × 100
A: 80% or more B: 70% or more and less than 80% C: 60% or more and less than 70% D: Less than 60% E: Cannot be measured due to short circuit.

  Hereinafter, the present invention will be described more specifically based on examples, but the present invention is not limited thereto.

  Table 1 shows the production conditions of the porous membrane and the evaluation results of the obtained porous membrane in the examples and comparative examples.

Example 1
To dehydrated N-methylpyrrolidone (NMP, manufactured by Mitsubishi Chemical Corporation), 2-chloro-1,4-phenylenediamine (manufactured by Nippon Kayaku Co., Ltd.) corresponding to 20 mol% and 80 mol% corresponding to the total amount of diamine 4,4′-diaminodiphenyl ether (manufactured by Tokyo Chemical Industry Co., Ltd.) was dissolved in a nitrogen stream and cooled to 30 ° C. or lower. Then, 2-chloroterephthaloyl chloride (manufactured by Nippon Light Metal Co., Ltd.) corresponding to 99 mol% was added to the total amount of diamine while maintaining the system at 30 ° C. or lower under a nitrogen stream, and the total amount was added Thereafter, the mixture was stirred for about 2 hours to polymerize the aromatic polyamide. The obtained polymerization solution was neutralized with 97 mol% lithium carbonate (Honjo Chemical Co., Ltd.) and 6 mol% diethanolamine (Tokyo Kasei Co., Ltd.) with respect to the total amount of acid chloride to obtain an aromatic polyamide solution. It was. The obtained aromatic polyamide had a logarithmic viscosity η _inh of 2.5 dl / g and a glass transition temperature of 255 ° C.

  Next, polyvinylpyrrolidone (PVP, BASF K90), RO water, and NMP for dilution are added to the obtained aromatic polyamide solution so as to have the following composition, followed by stirring at 60 ° C. for 2 hours. A film forming stock solution was obtained. The final content of each component with respect to 100% by mass of the film-forming stock solution is 10% by mass of aromatic polyamide, 5% by mass of PVP, and 10% by mass of water, and the remaining 75% by mass is contained in NMP and the polymerization stock solution. Japanese salt (lithium chloride, diethanolamine hydrochloride).

  This film-forming stock solution can be applied in the form of a film from a base to a stainless steel (SUS316) belt, which is a support, and the coating film can be peeled off from the belt in temperature-controlled air at a temperature of 50 ° C. and a relative humidity of 85% RH. Processed until Next, the coating film was peeled from the belt and introduced into a 30 ° C. water bath to extract the solvent, PVP, and the like, and at the same time, stretched 1.20 times in the longitudinal direction (MD). Subsequently, the obtained porous film in a water-containing state was introduced into a tenter chamber having a temperature of 150 ° C., and stretched 1.20 times in the width direction (TD) at a constant length. The water content in the porous membrane at the completion of stretching was 80 parts by mass with respect to 100 parts by mass of the aromatic polyamide. Thereafter, heat treatment was performed for 1 minute in a tenter chamber having a constant length and a constant width and a temperature of 280 ° C. to obtain a porous film.

(Example 2)
Porous in the same manner as in Example 1 except that 2-chloro-1,4-phenylenediamine is 30 mol% with respect to the total amount of diamine, and 4,4′-diaminodiphenyl ether is 70 mol% with respect to the total amount of diamine. A membrane was obtained.

(Example 3)
Porous in the same manner as in Example 1 except that 2-chloro-1,4-phenylenediamine is 50 mol% with respect to the total amount of diamine and 4,4′-diaminodiphenyl ether is 50 mol% with respect to the total amount of diamine. A membrane was obtained.

Example 4
Porous in the same manner as in Example 1 except that 2-chloro-1,4-phenylenediamine is 70 mol% with respect to the total amount of diamine, and 4,4′-diaminodiphenyl ether is 30 mol% with respect to the total amount of diamine. A membrane was obtained.

(Example 5)
As the diamine for obtaining the aromatic polyamide, 2-chloro-1,4-phenylenediamine corresponding to 50 mol% and 3,4′-diaminodiphenyl ether corresponding to 50 mol% (manufactured by Tokyo Chemical Industry Co., Ltd.) The porous membrane was obtained in the same manner as in Example 1 except that.

(Example 6)
A porous membrane was obtained in the same manner as in Example 3 except that the draw ratios in the longitudinal direction (MD) and the width direction (TD) were each 1.05 times.

(Example 7)
A porous film was obtained in the same manner as in Example 3 except that the temperature of the heat treatment applied at a constant length and constant width after completion of stretching was 230 ° C.

(Example 8)
The final content of each component with respect to 100% by mass of the film-forming stock solution is 12% by mass of aromatic polyamide, 4% by mass of PVP, 10% by mass of water, NMP and neutralized salts (lithium chloride, diethanolamine hydrochloride contained in the polymerization stock solution). ) A porous membrane was obtained in the same manner as in Example 3 except that the content was 74% by mass.

Example 9
The final content of each component with respect to 100% by mass of the film-forming stock solution is 8% by mass of aromatic polyamide, 2% by mass of PVP, 10% by mass of water, NMP, and neutralized salts (lithium chloride, diethanolamine hydrochloride contained in the polymerization stock solution). ) A porous membrane was obtained in the same manner as in Example 3 except that the content was 80% by mass.

(Examples 10 and 11)
A porous membrane-forming stock solution was obtained in the same manner as in Example 3. When this film-forming stock solution is applied in the form of a film from a die onto a belt as a support, the thickness of the porous film is adjusted as shown in Table 1 by adjusting the coating thickness. Thus, a porous membrane was obtained.

(Comparative Example 1)
Monomers for obtaining an aromatic polyamide were 1,3-phenylenediamine (manufactured by Tokyo Chemical Industry Co., Ltd.) corresponding to 50 mol% with respect to the total amount of diamine, 4,4′-diaminodiphenyl ether corresponding to 50 mol%, and 99 A porous membrane was obtained in the same manner as in Example 1 except that isophthaloyl chloride (manufactured by Tokyo Chemical Industry Co., Ltd.) corresponding to mol% was used.

(Comparative Example 2)
As monomers for obtaining the aromatic polyamide, 1,4-phenylenediamine (manufactured by Tokyo Chemical Industry Co., Ltd.) corresponding to 50 mol% with respect to the total amount of diamine, 4,4′-diaminodiphenyl ether corresponding to 50 mol%, and 99 A porous membrane was obtained in the same manner as in Example 1 except that isophthaloyl chloride corresponding to mol% was used.

(Comparative Example 3)
Porous in the same manner as in Example 1 except that 2-chloro-1,4-phenylenediamine is 10 mol% with respect to the total amount of diamine and 4,4′-diaminodiphenyl ether is 90 mol% with respect to the total amount of diamine. A membrane was obtained.

(Comparative Example 4)
In the same manner as in Example 1 except that 2-chloro-1,4-phenylenediamine is 80 mol% with respect to the total amount of diamine, and 4,4′-diaminodiphenyl ether is 20 mol% with respect to the total amount of diamine. A polymerization stock solution of an aromatic polyamide was obtained.

  Next, in order to remove the neutralized salt in the polymerization stock solution, this solution was put into a mixer together with water, and the polymer was precipitated and taken out while stirring. The polymer taken out was washed with water and dried under reduced pressure at 120 ° C. for 24 hours to isolate the aromatic polyamide.

  Using the isolated aromatic polyamide, a film-forming stock solution was prepared so that the aromatic polyamide was 10% by mass, PVP 5% by mass, water 5% by mass, NMP 80% by mass, and the porous membrane was prepared in the same manner as in Example 1. Obtained.

(Comparative Example 5)
A porous membrane was obtained in the same manner as in Example 3 except that the stretching ratio in the longitudinal direction (MD) and the width direction (TD) was 0.90.

(Comparative Example 6)
In a water bath, a water-containing porous material was obtained in the same manner as in Example 3 until it was stretched 1.20 times in the longitudinal direction (MD) with a constant width. Next, the obtained film is introduced into a tenter chamber having a constant length and a constant temperature of 150 ° C., and moisture is dried until the water content in the porous film becomes 10 parts by mass with respect to 100 parts by mass of the aromatic polyamide. I let you. Thereafter, the film was introduced into a tenter chamber having a temperature of 200 ° C., and stretched 1.20 times in the width direction (TD) at a constant length. The water content in the porous membrane at the completion of stretching was 0 part by mass with respect to 100 parts by mass of the aromatic polyamide. Finally, heat treatment was performed for 1 minute in a tenter chamber having a constant length and a constant width and a temperature of 280 ° C. to obtain a porous film.

(Comparative Examples 7 and 8)
A porous membrane-forming stock solution was obtained in the same manner as in Comparative Example 3. When this film-forming stock solution is applied in the form of a film from the die onto the support belt, the thickness of the porous film is adjusted as shown in Table 1 by adjusting the coating thickness. Thus, a porous membrane was obtained.

  Since the aromatic polyamide porous membrane of the present invention has a high compressive deformation elastic modulus, blockage of pores due to deformation when compressed in the thickness direction is suppressed. Furthermore, since the deformation recovery rate is high when the compressive force is unloaded, excellent followability is exhibited even when used with an electrode having a large volume change. Therefore, it can be suitably used for a battery separator such as a lithium ion secondary battery. When the aromatic polyamide porous membrane of the present invention is used as a separator for a secondary battery, even when an electrode having a large volume change such as an alloy-based negative electrode is used as the electrode, a high output and an excellent cycle over a long period of time Characteristics are obtained.

Claims (2)

A compression deformation modulus in the thickness direction 100 to 300 MPa, is 70% to 100% der compressive deformation recovery factor in the thickness direction is, a thickness of 5 to 30 [mu] m, a porosity of 50-80%, the Gurley The air permeability is 1 to 200 seconds / 100 ml, both the longitudinal direction (MD) and the width direction (TD) propylene carbonate liquid uptake are 10 to 100 mm / minute, and the longitudinal direction (MD) at 250 ° C. and width both of the heat shrinkage direction (TD) is Ru -0.5～3.0% der, aromatic polyamide porous film. A battery separator using the aromatic polyamide porous membrane according to claim 1 .