JP2008027839A - Porous membrane with liner, method of manufacturing porous membrane, and method of manufacturing lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous membrane which can be used as a separator for a lithium secondary battery, in which reliability and safety are improved, and which is easy to handle while suppressing reduction of energy density, and to provide its manufacturing method, and a method of manufacturing the lithium secondary battery using the porous membrane as the separator. <P>SOLUTION: This is the porous membrane with a liner of which the heat resistance temperature is at least 150°C or more, and in which the porous membrane containing particulates (A) chemically and electrochemically stable in the lithium secondary battery and containing a binder (B) chemically and electrochemically stable in the lithium secondary battery is formed on the liner having mold releasability. The lithium secondary battery using the porous membrane with the liner as the separator is manufactured via a process of peeling off the porous membrane from the porous membrane with the liner and of laminating a positive electrode and a negative electrode in succession. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a porous membrane with a liner in which a porous membrane suitable for use as a separator of a lithium secondary battery is formed on a liner, a method for producing the same, and a lithium secondary battery using the porous membrane with a liner It is related with the manufacturing method.

  Lithium ion batteries, which are a type of non-aqueous battery, are widely used as power sources for portable devices such as mobile phones and notebook personal computers because of their high energy density. As the performance of portable devices increases, the capacity of lithium ion batteries tends to increase further, and ensuring safety is important.

  In current lithium ion batteries, as a separator interposed between a positive electrode and a negative electrode, for example, a polyolefin-based porous film having a thickness of about 20 to 30 μm is used. In addition, as separator material, the constituent resin of the separator is melted below the thermal runaway temperature of the battery to close the pores, thereby increasing the internal resistance of the battery and improving the safety of the battery in the event of a short circuit. In order to ensure the so-called shutdown effect, polyethylene having a low melting point may be applied.

  By the way, as such a separator, for example, a uniaxially stretched film or a biaxially stretched film is used for increasing the porosity and improving the strength. Since such a separator is supplied as a single film, a certain strength is required in terms of workability and the like, and this is ensured by the above stretching. However, with such a stretched film, the degree of crystallinity has increased, and the shutdown temperature has increased to a temperature close to the thermal runaway temperature of the battery. Therefore, it can be said that the margin for ensuring the safety of the battery is sufficient. hard.

  Further, the film is distorted by the stretching, and there is a problem that when this is exposed to high temperature, shrinkage occurs due to residual stress. The shrinkage temperature is very close to the melting point, ie the shutdown temperature. For this reason, when a polyolefin-based porous membrane separator is used, if the battery temperature reaches the shutdown temperature during abnormal charging or the like, the current must be immediately reduced to prevent the battery temperature from rising. This is because if the pores are not sufficiently closed and the current cannot be reduced immediately, the battery temperature easily rises to the contraction temperature of the separator, and there is a risk of ignition due to an internal short circuit.

  Thus, ensuring both the required strength of the constituent films of the separator and ensuring a sufficiently low shutdown temperature to improve battery safety, and avoiding the problem of heat shrinkage at high temperatures, It is difficult to achieve with a single membrane.

  On the other hand, proposals have been made to improve shrinkage at high temperatures by using a separator as a composite film instead of a single film having a single layer structure as described above. For example, in Patent Documents 1 to 3, a composite film obtained by mixing a polymer having a low melting point (such as polyethylene) and a polymer having a high melting point (such as polypropylene), a film or nonwoven fabric of a polymer having a low melting point, and a high melting point. A technique for forming a separator with a composite film obtained by bonding a polymer film or a nonwoven fabric is disclosed.

  However, since it is difficult to reduce the thickness of such a composite film, there are problems that the energy density is lowered and the manufacturing process is complicated and the manufacturing cost is increased.

  In order to prevent a short circuit due to the above-described thermal contraction, a method using a microporous film or a nonwoven fabric using a heat resistant resin as a separator has been proposed. For example, Patent Document 4 discloses a separator using a microporous film of wholly aromatic polyamide, and Patent Document 5 discloses a separator using a polyimide porous film. Patent Document 6 discloses a separator using a polyamide nonwoven fabric, Patent Document 7 uses a nonwoven fabric as a base material using an aramid fiber, Patent Document 8 uses a polypropylene (PP) nonwoven fabric, and a patent. Document 9 discloses a technique related to a separator using a polyester nonwoven fabric.

  However, a microporous film using a heat-resistant resin such as polyamide or polyimide is excellent in dimensional stability at high temperatures and can be thinned, but is expensive. Nonwoven fabrics using heat-resistant fibers such as polyamide and aramid fibers are also excellent in dimensional stability but are expensive. Furthermore, since the separators using these heat-resistant materials do not have a so-called shutdown characteristic that closes the holes at high temperatures, the safety at the time of abnormalities such as external short-circuits and internal short-circuits where the temperature of the battery suddenly rises is sufficient. It cannot be secured.

  In addition, some proposals have been made in which a microporous film mainly composed of heat-resistant fine particles such as inorganic oxides is used as a separator instead of a resin-based structure. Patent Document 8 discloses a technique in which a porous layer in which heat-resistant fine particles such as inorganic oxides are bound together with a binder is formed on an electrode and used as a separator. Patent Document 9 discloses a technique related to a separator made of a nonwoven fabric and an inorganic filler.

  However, in the method of forming the separator directly on the electrode, since the electrode and the separator are integrated, for example, when the electrode is bent by a wound battery, the separator is cracked together with the electrode. There is a risk of short circuit.

  Also, in a separator using a nonwoven fabric as a base material, the filler must be uniformly filled in the voids of the nonwoven fabric, and particularly when a thin base material is used to make the separator thin, it can be filled uniformly. Have difficulty. Furthermore, when a thin base material is used, the nonwoven fabric has a significantly lower strength than a microporous membrane having the same thickness, which makes it difficult to handle.

  In addition, as a method for forming a porous layer to be applied to a separator, there are some proposals for a method for obtaining an independent film by coating on a substrate and then peeling. Patent Document 10 discloses a method of applying and peeling a polymer solution on a substrate, and Patent Document 11 discloses a method of applying and peeling a porous layer mainly composed of inorganic fine particles on a substrate. Each is shown.

  However, even if the resin film is prepared by the above-described peeling method, the above-described problem of thermal shrinkage of the separator is not completely solved, and the porous layer mainly composed of inorganic fine particles does not thermally shrink. Therefore, there is a problem that it is difficult to reduce the film thickness in terms of strength.

JP-A-5-74436 JP-A-5-251069 JP-A-5-331306 JP-A-5-335005 JP 2000-306568 A JP-A-9-259856 Japanese Patent Laid-Open No. 11-40130 Japanese Patent Laid-Open No. 10-163075 Japanese Patent Laid-Open No. 2003-7279 JP 2003-165863 A JP 2005-276503 A

  The present invention has been made in view of the above circumstances, and can be used as a separator for a lithium secondary battery in which reliability and safety are improved while suppressing a decrease in energy density as much as possible. It is an object of the present invention to provide a porous membrane that is easy to handle, a method for producing the same, and a method for producing a lithium secondary battery using the porous membrane as a separator.

  The porous film of the present invention that has achieved the above object has at least a heat-resistant temperature of 150 ° C. or higher and chemically and electrochemically stable fine particles (A) in a lithium secondary battery, lithium A porous membrane with a liner, characterized in that a porous membrane containing a chemically and electrochemically stable binder (B) in a secondary battery is formed on a liner having releasability. .

  Note that “the heat-resistant temperature is 150 ° C. or higher” in the fine particles (A) means that a substantial dimensional change due to softening or the like does not occur and a chemical composition change does not occur at a temperature of 150 ° C. or higher. This also applies to the fibrous materials described later.

  The method for producing a porous membrane of the present invention comprises at least a heat-resistant temperature of 150 ° C. or higher and chemically and electrochemically stable fine particles (A) in a lithium secondary battery, and a lithium secondary battery. A porous film-forming composition containing a chemically and electrochemically stable binder (B) is applied onto a liner having a releasability and dried to form a porous film with a liner (ie, The porous film with a liner of the present invention is prepared, and the porous film is peeled off from the liner.

  Furthermore, the method for producing a lithium secondary battery according to the present invention includes a step of peeling the porous membrane from the porous membrane with a liner according to the present invention, and following the step, the positive electrode and the negative electrode are interposed therebetween. And laminating as separators to form a laminated body.

  According to the present invention, it is possible to provide a porous film that is excellent in heat resistance, easy to handle, and suitable for a separator for a lithium secondary battery. By using the porous film as a separator, energy density can be reduced. A lithium secondary battery with improved reliability and safety while suppressing the decrease as much as possible can be provided.

  A schematic cross-sectional view of a porous membrane with a liner of the present invention is shown in FIG. The porous membrane 1 with a liner of the present invention has a heat-resistant temperature of 150 ° C. or higher and is chemically and electrochemically stable fine particles (A) [hereinafter referred to as “heat-resistant fine particles (A)” in a lithium secondary battery. And a porous film 3 containing a chemically and electrochemically stable binder (B) in a lithium secondary battery, a liner 2 having a releasability (so-called release paper or release film) It is formed on the top.

  The heat-resistant fine particles (A) have electrical insulation, are electrochemically stable, and are stable to an electrolyte and a solvent used in a composition for forming a porous film (described later). Further, fine particles having a heat-resistant temperature of 150 ° C. or higher without causing side reactions such as oxidation-reduction in the operating voltage range of the battery may be used. In addition, the term “chemically stable” as used in the present specification means that the electrolyte does not cause a change in form such as dissolution or a chemical structure due to a chemical reaction. "Stable" means that no chemical change occurs during charging and discharging of the battery.

That is, the heat-resistant fine particles (A) are electrically insulating (specifically, a volume resistivity of 10 ¹⁰ Ω · m or more) and are particularly organic or inorganic fine particles that can exist stably in the battery. There is no restriction | limiting, These may be used individually by 1 type and may use 2 or more types together.

Examples of the inorganic fine particles include oxide fine particles such as iron oxide, SiO ₂ , Al ₂ O ₃ , TiO ₂ , BaTiO ₂ , and ZrO; nitride fine particles such as aluminum nitride and silicon nitride; calcium fluoride, barium fluoride, Slightly soluble ionic crystal particles such as barium sulfate; Covalent crystal particles such as silicon and diamond; Clay particles such as montmorillonite; Substances derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica These artificial products; Further, the surface of conductive fine particles such as metal fine particles; oxide fine particles such as SnO ₂ and tin-indium oxide (ITO); carbonaceous fine particles such as carbon black and graphite; Surface treatment with a non-electrically conductive inorganic material that can constitute the heat-resistant fine particles (A), a crosslinked polymer fine particle, a material constituting the heat-resistant fine polymer particle, etc. Fine particles with electrical insulation may be used.

  Organic fine particles (organic powder) include crosslinked polymethyl methacrylate, crosslinked polystyrene, crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, polyimide, melamine resin, phenol resin, benzoguanamine-formaldehyde condensate, etc. Various kinds of crosslinked polymer fine particles [those that do not fall under the following swellable fine particles (D)], heat resistant polymer fine particles such as polypropylene (PP), polysulfone, polyacrylonitrile, polyaramid, polyacetal, thermoplastic polyimide, etc. It can be illustrated. The organic resin (polymer) constituting these organic fine particles is a mixture, modified body, derivative, or copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer) of the materials exemplified above. Polymer) and a crosslinked body (in the case of the above heat-resistant polymer).

Among the fine particles exemplified above, Al ₂ O ₃ (alumina), SiO ₂ (silica), mica, and boehmite are preferable.

  As the size of the heat-resistant fine particles (A), it is sufficient that the particle diameter at the time of drying is smaller than the thickness of the porous film, but it has an average particle diameter of 1/3 to 1/100 of the thickness of the porous film. Specifically, it is desirable that the average particle diameter is 0.01 μm or more, more preferably 0.1 μm or more, and 10 μm or less, more preferably 5 μm or less. If the average particle diameter of the heat-resistant fine particles (A) is too small, the gap between the fine particles (A) becomes small, and thus the ion conduction path in the porous film becomes long, and the battery characteristics may deteriorate. On the other hand, if the average particle diameter of the heat-resistant fine particles (A) is too large, the gap between the fine particles (A) becomes large, and the effect of preventing a short circuit due to the generation of lithium dendrite may be reduced.

  The shape of the heat-resistant fine particles (A) is not particularly limited, and may be a substantially spherical shape or a rugby ball shape, or may be a needle shape or a plate shape. Among these, when a part or all of the heat-resistant fine particles (A) are plate-like, it is particularly preferable because a further improvement in the effect of preventing a short circuit due to lithium dendrite can be expected.

  In the case where the heat-resistant fine particles (A) are plate-like, the aspect ratio (ratio between the maximum length in the plate-like particles and the thickness of the plate-like particles is used from the viewpoint of more effectively exerting the above-mentioned effect due to the plate shape. ) Is preferably 5 or more, and more preferably 10 or more. The aspect ratio of the plate-like heat-resistant fine particles (A) is 100 or less because the specific surface area of the heat-resistant fine particles (A) may become too large if the aspect ratio is too large. Preferably, it is 50 or less.

  When the heat-resistant fine particles (A) are plate-like, the average value of the ratio of the major axis direction length to the minor axis direction length of the flat plate surface is 1 or more, 3 or less, more preferably 2 The following is desirable. If the ratio between the length in the major axis direction and the length in the minor axis direction of the flat surface of the heat-resistant fine particles (A) is too large, the shape of the heat-resistant fine particles (A) approaches a needle shape and the heat-resistant fine particles (A) are plate-like. In some cases, the above-described effect due to the fact is small.

  The average particle size of the heat-resistant fine particles (A) is dispersed in a medium (for example, water) in which the heat-resistant fine particles (A) do not swell using a laser scattering particle size distribution meter (“LA-920” manufactured by HORIBA). The measured number average particle diameter. The average value of the ratio of the long axis direction length to the short axis direction length of the flat plate surface when the heat-resistant fine particles (A) are plate-like is, for example, an image taken by a scanning electron microscope (SEM). Can be obtained by image analysis. Furthermore, the above aspect ratio in the case where the heat-resistant fine particles (A) are plate-like can also be obtained by image analysis of an image taken by SEM.

  Further, as described above, the presence form when the heat-resistant fine particles (A) in the separator are plate-like is preferably such that the flat plate surface is substantially parallel to the surface of the separator, and the average in the vicinity of the surface of the separator. The angle is preferably 30 ° or less. Here, “near the surface” means a region within 10% of the total thickness from the surface of the separator. When the presence form of the plate-like heat-resistant fine particles (A) in the separator is as described above, the heat-resistant fine particles (A) are arranged substantially in parallel when the battery is formed. Since the formation of so-called through holes between the negative electrodes can be prevented, it is possible to more effectively prevent a short circuit caused by lithium dendrite.

  Commercially available products can be used as the plate-like heat-resistant fine particles (A) as described above, for example, “Seraph (trade name)” manufactured by Kinsei Matec Co., Ltd. as alumina, and Dokai Chemical as silica. "Sun Lovely" (trade name) made by the company, "Cerasur" (trade name) made by Kawai Lime Co., as boehmite, "Micro Mica (trade name)" made by Corp Chemical, etc. Is possible.

  The heat-resistant fine particles (A) are desirably 10% by volume or more, more preferably 20% by volume or more, in the total volume of the constituent components (total solid content) of the porous membrane. If it is a porous film containing the heat-resistant fine particles (A) at the volume ratio as described above, the short-circuit prevention function can be made more reliable when it is used in a lithium secondary battery separator. The upper limit of the volume ratio of the heat-resistant fine particles (A) is preferably 80% by volume, for example.

  The porous membrane according to the present invention binds heat-resistant fine particles (A) to each other, or binds organic fine particles (C), swellable fine particles (D), fibrous materials (E), and the like described later. Contains a binder (B).

  As the binder (B), any material may be used as long as it is chemically and electrochemically stable in a lithium secondary battery and can satisfactorily adhere each of the materials constituting the porous film. For example, EVA (With a structural unit derived from vinyl acetate of 20 to 35 mol%), ethylene-acrylate copolymer such as ethylene-ethyl acrylate copolymer, fluorine rubber, SBR, carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC) , Polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), polyurethane, epoxy resin, polyvinylidene fluoride (PVDF), and derivatives thereof. These binders may be used individually by 1 type, and may use 2 or more types together. When these binders (B) are used, they can be used in the form of an emulsion or plastisol dissolved or dispersed in a solvent for a composition for forming a porous film described later.

  In addition, for example, when the organic fine particles (C) and the swellable fine particles (D) described later have adhesiveness alone, these can also serve as the binder (B). Accordingly, the binder (B) includes organic fine particles (C) and swellable fine particles (D) described later that have adhesive properties alone.

  In addition, in the porous membrane, in order to improve the safety when the lithium secondary battery using the separator is overcharged or short-circuited and the inside of the battery is abnormally heated, It is preferable to provide a so-called shutdown function that closes the gap and reduces ion permeability. In order to impart a shutdown function to the porous membrane, it is possible to swell by absorbing organic fine particles (C) that melt at 80 to 130 ° C. or nonaqueous electrolyte, and the degree of swelling is increased by increasing the temperature. It is preferable that the swellable fine particles (D) that increase are contained in the porous membrane. In addition, both the organic fine particles (C) and the swellable fine particles (D) may be added to the porous film, and these composites may be further added.

Note that the above-described void blocking phenomenon (shutdown function) in the porous film can be evaluated by a Gurley value representing the air permeability of the porous film, for example, when organic fine particles (C) are used. . Specifically, in a porous membrane having a shutdown function, the Gurley value measured at the time of heating by the following measurement method is preferably three times or more before heating (Gurley value measured at room temperature), more preferably 10 times or more. A specific measurement method is to hold the porous membrane at a predetermined temperature for 10 minutes in a thermostatic bath, take it out, slowly cool it, and measure the Gurley value when the temperature is raised to the predetermined temperature. Thereafter, the temperature is increased in increments of 5 ° C., the porous film is held at each temperature for 10 minutes, and then the Gurley value is measured in the same manner. The Gurley value is measured by a method according to JIS P 8117, and is indicated by the number of seconds that 100 ml of air passes through the membrane under a pressure of 0.879 g / mm ² . In addition, when the shutdown completely occurs and the air does not permeate, the Gurley value becomes infinite.

  Moreover, the shutdown phenomenon in the porous film by containing the organic fine particles (C) and the swellable fine particles (D) can also be evaluated by an increase in the internal resistance of the battery. Specifically, the internal resistance value of the battery measured at the time of heating by the measurement method described in Examples below is preferably 5 times or more, more preferably 10 times that before heating (internal resistance value measured at room temperature). It is more than double.

  The organic fine particles (C) have a melting point of 80 to 130 ° C., that is, a melting temperature of 80 to 130 ° C. measured using a differential scanning calorimeter (DSC) in accordance with JIS K 7121. And is electrochemically stable, and is stable to a non-aqueous electrolyte (to be simply referred to as “electrolyte” hereinafter) or a solvent used for a composition for forming a porous film. There is no particular limitation. Specific constituent materials of the organic fine particles (C) include polyethylene (PE), a copolymerized polyolefin having a structural unit derived from ethylene of 85 mol% or more, or a polyolefin derivative (such as chlorinated polyethylene), a polyolefin wax, a petroleum wax, Examples include carnauba wax. Examples of the copolymer polyolefin include an ethylene-vinyl monomer copolymer, more specifically, an ethylene-vinyl acetate copolymer (EVA), an ethylene-methyl acrylate copolymer, or an ethylene-ethyl acrylate copolymer. it can. Moreover, polycycloolefin etc. can also be used. The organic fine particles (B) may have only one kind of these constituent materials, or may have two or more kinds. Among these, PE, polyolefin wax, or EVA having a structural unit derived from ethylene of 85 mol% or more is preferable. Further, the organic fine particles (C) may contain various known additives (for example, antioxidants) added to the resin as necessary in addition to the above-described constituent materials. I do not care.

  Further, for example, a composite of a core-shell structure in which various inorganic fine particles and organic fine particles exemplified above as a specific example of the heat-resistant fine particles (A) are used as a core, and the above-described constituent materials capable of constituting the organic fine particles (C) are used as a shell. Fine particles may be contained in the porous film, and even in the porous film containing such composite fine particles, the shutdown function can be imparted in the same manner as when the organic fine particles (C) are contained.

  A porous film containing organic fine particles (C) that melt at 80 to 130 ° C. [or composite fine particles having the above-mentioned core-shell structure using the constituent material of organic fine particles (C) as a shell] was used as a separator for a lithium secondary battery. In this case, when the separator is exposed to 80 to 130 ° C. (or higher temperature), the organic fine particles (C) (or the constituent resin of the shell portion of the composite fine particles) are melted to block the separator voids. The movement of lithium ions is inhibited, and the rapid discharge reaction at high temperatures is suppressed. Therefore, in this case, the temperature at which the void clogging phenomenon evaluated by the Gurley value described later in the embodiment (the temperature at which the Gurley value becomes three times or more before heating) is the constituent resin (or composite) of the organic fine particles (C). The melting temperature of the constituent resin of the shell part of the fine particles is 130 ° C. or higher.

  The swellable fine particles (D) that can swell by absorbing a non-aqueous electrolyte and whose degree of swelling increases as the temperature rises are high in a battery that uses a porous membrane containing this as a separator. When exposed, the swellable fine particles (D) are transformed into an electrolyte solution in the battery due to the property that the degree of swelling increases with increasing temperature in the swellable fine particles (D) (hereinafter sometimes referred to as “thermal swellability”). Absorbs and swells. At this time, the amount of electrolyte present inside the separator gap becomes so-called “liquid withering” state, and the swollen particles also block the gap inside the separator. Ion conductivity is significantly reduced and the internal resistance of the battery is increased. Therefore, the shutdown function can be imparted to the porous membrane also by containing the swellable fine particles (D).

  As the swellable fine particles (D), the temperature exhibiting the above-mentioned heat swellability is preferably 75 to 125 ° C. If the temperature exhibiting thermal swellability is too high, the thermal runaway reaction of the active material in the battery may not be sufficiently suppressed, and the battery safety improvement effect may not be sufficiently ensured. Moreover, when the temperature which shows heat swelling property is too low, the conductivity of the lithium ion in the battery in a normal use temperature range will become low too much, and it may cause trouble in use of an apparatus. That is, in the porous film of the present invention, since the shutdown temperature is in the range of about 80 to 130 ° C., the temperature at which the swellable fine particles (D) begin to show thermal swellability due to temperature rise is in the range of 75 to 125 ° C. Preferably there is.

Further, as the swellable fine particles (D), those having a degree of swelling B as defined by the following formula measured at 120 ° C. of 1 or more are preferable.
_{B = (V 1 / V 0} ) -1 (1)
[In the above formula (1), V ₀ is the volume of fine particles (cm ³ ) 24 hours after being introduced into the non-aqueous electrolyte at 25 ° C., and V ₁ is introduced into the non-aqueous electrolyte at 25 ° C. 24 hours later, the temperature of the non-aqueous electrolyte is raised to 120 ° C., and the volume of fine particles (cm ³ ) after 1 hour at 120 ° C.]

  The swelling degree of the swellable fine particles (D) having the degree of swelling as described above greatly increases in an environment exceeding the temperature (any one of 75 to 125 ° C.) at which the thermal swellability starts to be exhibited. Therefore, in a battery using a porous membrane containing swellable fine particles (D) having such properties as a separator, when the internal temperature exceeds a specific temperature (for example, the above 75 to 125 ° C.), Since the swellable fine particles (D) further absorb the electrolyte solution in the battery and greatly expand, the conductivity of Li ions is remarkably lowered, so that the safety of the battery can be ensured more reliably. The swelling degree of the swellable fine particles (D) defined by the above formula (1) may cause battery deformation if it becomes too large, and is preferably 10 or less.

  Specific measurement of the swellable fine particles (D) defined by the above formula (1) can be performed by the following method. The swellable fine particles (D) are added to a binder resin solution or emulsion in which the degree of swelling when immersed in an electrolytic solution at room temperature for 24 hours in advance and the degree of swelling defined by the above formula (1) at 120 ° C. is known. A slurry is prepared by mixing, and this is cast on a substrate such as a PET sheet or a glass plate to produce a film, and its mass is measured. Next, the film was immersed in an electrolytic solution at room temperature (25 ° C.) for 24 hours, and the mass was measured. Further, the electrolytic solution was heated to 120 ° C., and the mass after 1 hour was measured at the temperature. The degree of swelling B is calculated by the formulas (2) to (8). [In the following formulas (2) to (8), the volume increase of components other than the electrolyte due to the temperature rise from room temperature to 120 ° C. can be ignored. It is said].

V _i = M _i × W / P _A (2)
V _B = (M ₀ −M _i ) / P _B (3)
_{V C = M 1 / P c} -M 0 / P B (4)
V _V = M _i × (1−W) / P _V (5)
V ₀ = V _i + V _B −V _V × (B _B +1) (6)
V _D = V _V × (B _B +1) (7)
B = [{V ₀ + V _C −V _D × (B _C +1)} / V ₀ ] −1 (8)

Here, in the above formulas (2) to (8),
V _i : volume (cm ³ ) of the swellable fine particles (D) before being immersed in the electrolytic solution,
V ₀ : volume (cm ³ ) of the swellable fine particles (D) after being immersed in the electrolytic solution at room temperature for 24 hours,
V _B : volume of the electrolyte solution (cm ³ ) absorbed in the film after being immersed in the electrolyte solution at room temperature for 24 hours,
V _c : The volume of the electrolytic solution absorbed by the film (cm) during the period from when the electrolytic solution was immersed in the electrolytic solution at room temperature for 24 hours until the electrolytic solution was heated to 120 ° C. and further passed for 1 hour at the above temperature. ³ ),
V _V : volume (cm ³ ) of the binder resin before being immersed in the electrolytic solution,
V _D : volume of the binder resin (cm ³ ) after being immersed in the electrolytic solution at room temperature for 24 hours,
M _i : mass (g) of the film before being immersed in the electrolytic solution,
M ₀ : mass (g) of the film after being immersed in the electrolytic solution at room temperature for 24 hours,
M _l : After immersing in the electrolyte solution at room temperature for 24 hours, the electrolyte solution was heated to 120 ° C., and the mass (g) of the film after 1 hour at the above temperature,
W: Mass ratio of the swellable fine particles (D) in the film before being immersed in the electrolytic solution,
P _A : Specific gravity (g / cm ³ ) of the swellable fine particles (D) before being immersed in the electrolytic solution,
P _B : Specific gravity of electrolyte at room temperature (g / cm ³ ),
P _C: specific gravity of the electrolyte at a predetermined temperature (g / ^cm 3),
P _V : Specific gravity (g / cm ³ ) of the binder resin before being immersed in the electrolytic solution,
B _B : degree of swelling of the binder resin after being immersed in the electrolyte at room temperature for 24 hours,
B _C : Swelling degree of the binder resin at the time of temperature rise defined by the above formula (1).

Also, swellable microparticles (D) is the swelling degree B _R at room temperature (25 ° C.) which is defined by the following equation (9) is preferably 0 or more and 1 or less.
B _R = (V ₀ / V _i ) -1 (9)
In the above formula (9), V ₀ and V _i are the same as those described for the above formulas (2) to (8).

That is, the swellable fine particles (D) may be those that do not absorb the electrolyte solution (B _R = 0) at room temperature or those that absorb some electrolyte solution. In the range (for example, 70 ° C. or less), the absorption amount of the electrolytic solution does not change so much regardless of the temperature, and thus the degree of swelling does not change so much. It only needs to increase.

The swelling particles (D), and preferably has satisfied the swelling degree B or B _R, also has an electrically insulating, electrochemically stable, further described below electrolyte, porous There is no particular limitation as long as it is stable in the solvent used in the composition for forming a film and is not soluble in the electrolyte solution at a high temperature.

  In the known lithium secondary battery, for example, a solution in which a lithium salt is dissolved in an organic solvent is used as the non-aqueous electrolyte (details such as the type of lithium salt and the organic solvent, the lithium salt concentration are described later). Described in the description of the lithium secondary battery). Therefore, as the swellable fine particles (D), in the organic solvent solution of the lithium salt, when the temperature reaches any of 75 to 125 ° C., the above-mentioned thermal swellability starts to be exhibited. Those that can swell so that B satisfies the above values are recommended.

  Specific examples of the constituent material of the swellable fine particles (D) include a crosslinked resin. Specific examples include polystyrene (PS), acrylic resin, polyalkylene oxide, fluorine resin, styrene butadiene rubber (SBR) and the like, and crosslinked products of these derivatives; urea resin; polyurethane; The swellable fine particles (D) may be composed only of these constituent materials, and may be composed of inorganic fine particles or organic fine particles that are stable with respect to the electrolytic solution [for example, the above-exemplified inorganic fine particles corresponding to the heat-resistant fine particles (A) Or a fine particle having a core-shell structure in which the above-described constituent materials are combined as a shell. Further, the swellable fine particles (D) may contain the above-mentioned constituent materials alone or in combination of two or more. Furthermore, the swellable fine particles (D) contain, as a constituent component, various known additives (for example, antioxidants) that are added to the resin as necessary in addition to the constituent materials described above. It doesn't matter.

  In the above-mentioned resin cross-linked body, even if it expands due to a temperature rise once, the volume change accompanying the temperature change is reversible such that it shrinks again by lowering the temperature. In a so-called dry state that does not contain water, it is stable up to a temperature higher than the temperature of thermal expansion. Therefore, in the porous film using the swellable fine particles (D) composed of the above crosslinked resin, the swellability can be obtained even through a heating process such as drying during the formation of the porous film or drying of the electrode group during battery production. Since the heat swellability of the fine particles (D) is not impaired, handling in such a heating process becomes easy. Furthermore, by having the above reversibility, even if the shutdown function is activated once due to the temperature rise, it can be made to function as a separator again when safety is ensured by the temperature drop in the battery. is there.

  Among the above-described constituent materials, crosslinked PS, crosslinked acrylic resin [for example, crosslinked polymethyl methacrylate (PMMA)] and crosslinked fluororesin [for example, crosslinked polyvinylidene fluoride (PVDF)] are preferable, and crosslinked PMMA is particularly preferable.

  Although the details of the mechanism by which the fine particles of these resin crosslinked bodies swell due to temperature rise are not clear, for example, in crosslinked PMMA, the glass transition point (Tg) of PMMA, which is the main component of the particles, is around 100 ° C. It is conceivable that the cross-linked PMMA particles become flexible near Tg and absorb more electrolyte and swell. Therefore, it is considered desirable that the glass transition point of the swellable fine particles (D) is in the range of about 75 to 125 ° C.

  As the size of the organic fine particles (C) and the swellable fine particles (D), the particle size at the time of drying may be smaller than the thickness of the porous film, but it is 1/3 to 1/100 of the thickness of the porous film. It is preferable that it has an average particle diameter, and specifically, it is preferable that an average particle diameter is 0.1-20 micrometers. If the particle size of these fine particles is too small, the gap between the fine particles becomes small, and thus the ion conduction path becomes long, and the battery characteristics may be deteriorated. On the other hand, when the particle size is too large, the gap becomes large, and the effect of preventing the short circuit by using the organic fine particles (C) and the swellable fine particles (D) may be reduced. In addition, the average particle diameter of the particle | grain here is the number average particle | grains which disperse | distributed the particle | grains to the medium (for example, water) which does not swell using a laser scattering particle size distribution analyzer (for example, "LA-920" by HORIBA). Is the diameter.

  In the porous film, when the shutdown function is ensured by adding organic fine particles (C) or swellable fine particles (D), the organic fine particles (C) or the swellable fine particles (D) in the porous film The content is preferably 10% by volume or more and 80% by volume or less in the total volume of the constituent components of the porous membrane, for example. If the content of these fine particles is too small, the shutdown effect due to the inclusion of these may be small, and if too large, the content of the heat-resistant fine particles (A) in the porous film will decrease. For example, the effect of preventing a short circuit due to generation of lithium dendrite may be reduced.

  The porous membrane according to the present invention may contain a fibrous material (E) as a reinforcing material for increasing the strength. As the fibrous material (E), if it has electrical insulation, is electrochemically stable, and is stable to the electrolyte solution described in detail below and the solvent used in the composition for forming a porous film, Although there is no particular limitation, the heat resistant temperature is preferably 150 ° C. or higher.

  The “fibrous material” in the present specification means that having an aspect ratio [length in the longitudinal direction / width (diameter) in a direction perpendicular to the longitudinal direction] of 4 or more. The aspect ratio of the fibrous material (E) is preferably 10 or more.

  Specific constituent materials of the fibrous material (E) include, for example, cellulose, modified cellulose (such as carboxymethyl cellulose), polypropylene (PP), polyester [polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene. Terephthalate (PBT), etc.], polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polyaramid, polyamideimide, polyimide and other resins; glass, alumina, silica and other inorganic materials (inorganic oxides); and the like. The fibrous material (E) may contain one of these constituent materials, or may contain two or more. In addition to the above-described constituent materials, the fibrous material (E) contains various known additives (for example, an antioxidant in the case of a resin) as necessary. It doesn't matter.

  The diameter of the fibrous material (E) may be equal to or less than the thickness of the porous membrane, but is preferably 0.01 to 5 μm, for example. If the diameter is too large, when using a sheet-like material formed by gathering a large number of fibers described later, the entanglement between the fibrous materials is insufficient, the strength of the sheet-like material composed of these, In some cases, the strength of the porous membrane is reduced, making it difficult to handle. On the other hand, if the diameter is too small, the voids in the porous membrane tend to be too small and the ion permeability tends to decrease, and the load characteristics may be deteriorated in a battery using this as a separator.

  The presence state of the fibrous material (E) in the porous membrane is, for example, preferably such that the angle of the major axis (long axis) with respect to the porous membrane surface is 30 ° or less on average, 20 ° The following is more preferable.

  In the porous membrane containing the fibrous material (E), a large number of fibrous materials (E) are aggregated to form a sheet-like material only by these, for example, woven fabric, non-woven fabric, paper A porous film having a structure containing the heat-resistant fine particles (A) and the like may be used in the sheet, and the fibrous material (E) and the heat-resistant fine particles (A) are uniformly dispersed. It is good also as the porous membrane of the structure contained in the form. Or it can also be set as the structure which combined both above-mentioned structures.

  In the porous membrane containing the fibrous material (E), part or all of the heat-resistant fine particles (A) are fibrous from the viewpoint of more effectively exerting the function of the heat-resistant fine particles (A). It is preferable to have a structure that exists in the voids of the sheet-like material composed of the material (E). Also, when using organic fine particles (C) and swellable fine particles (D), some or all of these fine particles are present in the voids of the sheet-like material composed of the fibrous material (E). The structure is preferable because the functions of the fine particles can be more effectively exhibited.

  The thickness of the porous membrane according to the present invention is desirably 3 μm or more, more preferably 5 μm or more, and 50 μm or less, more preferably 30 μm or less. If the porous film is too thin, the effect of suppressing a fine short circuit due to dendrites is not sufficient, and it becomes difficult to ensure the reliability of the battery. On the other hand, if it is too thick, the energy density of the battery tends to be small.

The porosity of the porous membrane is preferably 15% or more, more preferably 20% or more, and 70% or less, more preferably 60% or less. If the porosity of the porous membrane is too small, the ion permeability may be reduced, and if the porosity is too large, the strength of the porous membrane may be insufficient. The porosity of the porous membrane: P (%) can be calculated by calculating the sum of each component i from the thickness of the porous membrane, the mass per area, and the density of the constituent components using the following equation.
_{P = Σ a i ρ i /} (m / t)
Here, in the above formula, a _i : ratio of component i expressed by mass%, ρ _i : density of component i (g / cm ³ ), m: mass per unit area of the porous membrane (g / cm ²⁾ ), T: Thickness (cm) of the porous membrane.

  Furthermore, the air permeability of the porous film indicated by the Gurley value measured by the above method is desirably 30 to 300 sec. If the air permeability is too high, the ion permeability is reduced, whereas if it is too low, the strength of the porous membrane may be reduced. Further, the strength of the porous membrane is desirably 50 g or more in terms of piercing strength using a needle having a diameter of 1 mm. In a battery using a porous film having a puncture strength that is too small as a separator, when a dendrite crystal of lithium is generated, there is a possibility that a short circuit occurs due to the piercing of the separator.

  As the method for producing a porous membrane according to the present invention, for example, the following methods (I) and (II) can be employed. In the method (I), a porous film-forming composition (slurry, paste, etc.) containing heat-resistant fine particles (A) and a binder (B) is applied on a liner having releasability, and then predetermined. It is the manufacturing method dried at the temperature of.

  The composition for forming a porous film comprises a heat-resistant fine particle (A) and a binder (B), and if necessary, an organic fine particle (C), a swellable fine particle (D), a fibrous material (E) and the like. And these are dispersed in a solvent (including a dispersion medium, hereinafter the same) [the binder (B) may be dissolved]. The solvent used in the composition for forming a porous film can uniformly disperse the heat-resistant fine particles (A), the organic fine particles (C), the swellable fine particles (D), and the fibrous material (E), and the binder (B However, organic solvents such as aromatic hydrocarbons such as toluene; furans such as tetrahydrofuran; ketones such as methyl ethyl ketone and methyl isobutyl ketone; are suitable. In addition, for the purpose of controlling the interfacial tension, alcohols (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate may be appropriately added to these solvents. In the case where the binder (B) is used as an emulsion, water may be used as a solvent. In this case, alcohols (methyl alcohol, ethyl alcohol, ethylene glycol, etc.) may be added as appropriate to control the interfacial tension. it can.

  In the composition for forming a porous film, the solid content including the heat-resistant fine particles (A), the binder (B), the organic fine particles (C), the swellable fine particles (D), and the fibrous material (E) is, for example, 10 to 10. It is desirable to set it as 80 mass%, and it is still more desirable to set it as 20-70 mass%.

  (II) The method for producing a porous membrane is the same porous membrane forming composition as used in the method (I) described above, which is applied to or impregnated into a sheet-like material comprising a fibrous material (E), In this method, the film is dried on a liner having a releasability and dried at a predetermined temperature before drying.

  The “sheet-like material” referred to in the method (II) corresponds to a sheet-like material (nonwoven fabric or the like) composed of the fibrous material (E). Specifically, a porous sheet such as a non-woven fabric having a structure in which at least one of the above-exemplified materials is included as a constituent component and the fibrous materials are entangled with each other can be used. More specifically, non-woven fabrics such as paper, PP non-woven fabric, and polyester non-woven fabric (PET non-woven fabric, PEN non-woven fabric, PBT non-woven fabric, etc.) can be exemplified.

  In the case of producing a porous membrane by the method (II), as described above, a part of the heat-resistant fine particles (A) in the voids of the sheet-like material formed of the fibrous material (E) or It is preferable to have a structure in which all are present. Further, when organic fine particles (C) and swellable fine particles (D) are used, a part or all of these fine particles are within the voids of the sheet-like material. It is preferable to have a structure existing in In order to allow the heat-resistant fine particles (A), the organic fine particles (C), and the swellable fine particles (D) to exist in the voids of the sheet-like material, for example, after impregnating the sheet-like material with the composition for forming a porous film A process such as drying after removing the excess porous film-forming composition through a certain gap may be used.

  Further, when plate-like particles are used as the heat-resistant fine particles (A) in the porous film, in order to increase the orientation, in the sheet-like material impregnated with the porous film-forming composition, Just share it. For example, in the production method of (II), after impregnating the porous film-forming composition described above as a method for causing the heat-resistant fine particles (A) and the like to exist in the voids of the sheet-like material, a certain amount is obtained. It is possible to share the composition by the method of passing through the gap, and the orientation of the heat-resistant fine particles (A) can be improved. In either of the production method (I) and the production method (II), after applying the porous film-forming composition on the liner, the magnetic field is removed before drying to remove the solvent in the composition. It is also possible to orient the plate-like heat-resistant fine particles (A) by applying.

  The liner used in the porous membrane with a liner of the present invention is stable with respect to the composition for forming a porous membrane, is composed of a material having heat resistance at a predetermined drying temperature, and has an appropriate release property. If it has, there will be no restriction | limiting in particular, When employ | adopting any of the method of (I) and the method of (II) as a manufacturing method of a porous membrane, the same liner can be used.

  Specifically, it is possible to use a substrate whose surface has been subjected to a release treatment with a release agent or the like. More specifically, as the substrate, a resin film such as a polyester film, a polyolefin film, a polyimide film, or a polyamide film; a paper; a paper coated with a resin; and the like can be used, and a polyester film is particularly preferable. Moreover, as a mold release agent, conventionally known mold release agents such as fluorine and silicon can be used.

  The releasability of the liner is determined by winding the porous film together with the positive electrode and the negative electrode during battery production without peeling the liner and porous film in the winding process and slitting process after the porous film is formed. In the peeling process between the liner and the porous film immediately before the winding process, the porous film is easily peeled off from the liner without breaking, and the porous film does not remain unnecessarily on the liner. It is preferable to adjust appropriately according to the selection of the type of the release agent and the coating amount. In addition, when the base material itself is provided with the above-described releasability in advance, it can be used as a liner without using a release agent.

  The porous membrane according to the present invention can be widely used as a separator for non-aqueous electrolyte batteries regardless of whether it is a primary battery or a secondary battery. Hereinafter, a secondary battery which is a particularly main application of the porous membrane according to the present invention will be described.

  The battery (lithium secondary battery) according to the present invention is not particularly limited as long as it has the porous film according to the present invention as a separator, and a conventionally known configuration and structure can be adopted.

  Examples of the form of the battery include a tubular shape (such as a rectangular tube shape or a cylindrical shape) using a steel can or an aluminum can as an outer can. Moreover, it can also be set as the soft package battery which used the laminated film which vapor-deposited the metal as an exterior body.

If it is a positive electrode used for a conventionally well-known lithium secondary battery as a positive electrode, there will be no restriction | limiting in particular. For example, as an active material, lithium-containing transition metal oxide represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, etc.); lithium manganese such as LiMn ₂ O ₄ Oxide; LiMn _x M _(1-x) O _{2 in} which part of Mn of LiMn ₂ O ₄ is substituted with another element; olivine type LiMPO ₄ (M: Co, Ni, Mn, Fe); LiMn _0.5 Ni _0.5 O ₂ ; Li _{(1 + a)} Mn _x Ni _y Co _(1-xy) O ₂ (−0.1 <a <0.1, 0 <x <0.5, 0 <y <0. 5); can be applied, and a known conductive additive (carbon material such as carbon black) or a binder such as polyvinylidene fluoride (PVDF) is appropriately added to these positive electrode active materials. Finishing the positive electrode mixture into a molded body using the current collector as the core material Or the like can be used things.

  As the current collector of the positive electrode, a metal foil such as aluminum, a punching metal, a net, an expanded metal, or the like can be used. Usually, an aluminum foil having a thickness of 10 to 30 μm is preferably used.

  The lead portion on the positive electrode side is normally provided by leaving the exposed portion of the current collector without forming the positive electrode mixture layer on a part of the current collector and forming the lead portion at the time of producing the positive electrode. However, the lead portion is not necessarily integrated with the current collector from the beginning, and may be provided by connecting an aluminum foil or the like to the current collector later.

  The negative electrode is not particularly limited as long as it is a negative electrode used in a conventionally known lithium secondary battery. For example, carbon that can occlude and release lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers as active materials One type or a mixture of two or more types of system materials is used. In addition, elements such as Si, Sn, Ge, Bi, Sb, In and their alloys, lithium-containing nitrides, oxides and other compounds that can be charged and discharged at a low voltage close to lithium metal, or lithium metals and lithium / aluminum alloys Can also be used as a negative electrode active material. A negative electrode mixture prepared by appropriately adding a conductive additive (carbon material such as carbon black) or a binder such as PVDF to these negative electrode active materials and using a current collector as a core material is used. In addition, the above-described various alloys and lithium metal foils may be used alone or formed on a current collector.

  When a current collector is used for the negative electrode, a copper or nickel foil, a punching metal, a net, an expanded metal, or the like can be used as the current collector, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is preferably 5 μm.

  Similarly to the lead portion on the positive electrode side, the negative electrode lead portion is usually exposed to the current collector without forming a negative electrode agent layer (a layer having a negative electrode active material) on a part of the current collector during negative electrode fabrication. It is provided by leaving a part and using it as a lead part. However, the negative electrode side lead portion is not necessarily integrated with the current collector from the beginning, and may be provided by connecting a copper foil or the like to the current collector later.

  The electrode can be used in the form of a laminated electrode body in which the above positive electrode and the above negative electrode are laminated via the porous film according to the present invention, or a wound electrode body in which the electrode is wound.

As described above, a solution obtained by dissolving a lithium salt in an organic solvent is used as the nonaqueous electrolytic solution. The lithium salt is not particularly limited as long as it dissociates in a solvent to form Li ⁺ ions and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, inorganic lithium salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ ; LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group]; it can.

  The organic solvent used in the electrolytic solution is not particularly limited as long as it dissolves the above lithium salt and does not cause side reactions such as decomposition in the voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; Chain ethers such as ethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; nitriles such as acetonitrile, propionitrile and methoxypropionitrile; Sulfites such as ethylene glycol sulfite; and the like, and these may be used alone. , It may be used in combination of two or more thereof. In order to obtain a battery with better characteristics, it is desirable to use a combination that can obtain high conductivity, such as a mixed solvent of ethylene carbonate and chain carbonate. In addition, for the purpose of improving the safety, charge / discharge cycleability, and high-temperature storage properties of these electrolytes, vinylene carbonates, 1,3-propane sultone, diphenyl disulfide, cyclohexane, biphenyl, fluorobenzene, t- Additives such as butylbenzene can also be added as appropriate.

  The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  In addition, instead of the above organic solvent, melting at room temperature such as ethyl-methylimidazolium trifluoromethylsulfonium imide, heptyl-trimethylammonium trifluoromethylsulfonium imide, pyridinium trifluoromethylsulfonium imide, guanidinium trifluoromethylsulfonium imide A salt can also be used.

  Furthermore, a polymer material that contains the above non-aqueous electrolyte and gels may be added, and the non-aqueous electrolyte may be gelled and used for a battery. Polymer materials for making the non-aqueous electrolyte into a gel include PVDF, PVDF-HFP, PAN, polyethylene oxide, polypropylene oxide, ethylene oxide-propylene oxide copolymer, crosslink having ethylene oxide chain in the main chain or side chain Known host polymers capable of forming a gel electrolyte, such as polymers and cross-linked poly (meth) acrylates, can be mentioned.

  Examples of the method for producing the lithium secondary battery include the following method (i) or (ii).

  In the method (i), a positive electrode in which a positive electrode mixture layer is formed on one side or both sides of a current collector, and a negative electrode in which a negative electrode layer is formed on one side or both sides of the current collector are cut into a predetermined size. The laminated secondary electrode body is laminated through the porous membrane according to the present invention, and the laminated electrode body is inserted into an outer can or a laminated film outer body, injected with an electrolytic solution, and sealed to be a lithium secondary battery. It is a method.

  In the method (i), a lamination step of laminating the positive electrode and the negative electrode through the porous membrane is provided following the peeling step of peeling the porous membrane from the liner-attached porous membrane. That is, by providing a peeling step for peeling the porous membrane from the liner immediately before the laminating step, the porous membrane reinforces most steps of lithium secondary battery production with a liner or electrode (positive electrode and negative electrode). Therefore, even a porous membrane that does not have the strength necessary for handling as an independent membrane can be used. Therefore, for example, if it is a range which can prevent a short circuit enough when it is set as a battery, a porous membrane can be made thin and the energy density of a battery can be raised.

  The liner-attached porous membrane may be cut into a predetermined size in advance, or may be cut simultaneously with the lamination with the electrode. Alternatively, a so-called transfer method can be used in which an adhesive layer is provided in advance on either the electrode (positive electrode or negative electrode) or the porous film, and the porous film is peeled off from the liner while adhering the porous film to the electrode. . Furthermore, when a battery is formed using a gel electrolyte or a solid electrolyte, a so-called bipolar structure electrode in which a positive electrode mixture layer and a negative electrode layer are provided on each surface of the current collector may be used.

  In the method (ii), following the stacking step in which the positive electrode, the porous film, and the negative electrode in the method (i) are stacked to form a stacked body (laminated electrode body), the stacked body is wound in a spiral shape. And a method for manufacturing a lithium secondary battery having a winding step of forming a wound electrode body. In the method (ii), a positive electrode and a negative electrode that have been cut into strips in advance are stacked via a porous film immediately after being peeled from the liner-attached porous film, and then the stacked body is wound into a spiral shape, Cut to a predetermined length. The wound electrode body obtained by the winding process is inserted into the exterior can or the laminate film exterior body, and sealed after injecting the electrolyte, similarly to the laminated electrode body in the manufacturing method (i). Thus, a lithium secondary battery is obtained.

  In either the method (i) or the method (ii), the lithium secondary battery after the electrolyte solution is injected and sealed may be subjected to precharging, aging, or the like according to a conventional method.

  The lithium secondary battery according to the present invention can be applied to the same uses as those in which conventionally known lithium secondary batteries are used.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

Example 1
1000 g of alumina (Al ₂ O ₃ ) fine particles (average particle size 0.3 μm) as heat-resistant fine particles (A), 800 g of water, 200 g of isopropyl alcohol (IPA), and PVB [manufactured by Sekisui Chemical Co., Ltd. "SREC KX-5 (trade name)"] 1000 g was put in a container, and stirred and dispersed with a three-one motor for 1 hour to obtain a uniform slurry (a composition for forming a porous film). This slurry was applied onto a silicone-coated polyester film (thickness 25 μm) using a die coater and then dried to obtain a porous membrane with a liner having a thickness of 20 μm.

For the porous membrane according to Example 1, the alumina volume content calculated with the specific gravity of alumina being 4.0 g / cm ³ and the specific gravity of PVB being 1.1 g / cm ³ is 77.5%.

Example 2
A porous membrane with a liner having a thickness of 15 μm was prepared in the same manner as in Example 1 except that the heat-resistant fine particles (A) were changed to plate-like boehmite (average particle size 1 μm, aspect ratio 10).

For the porous membrane according to Example 2, the volume content of the plate boehmite calculated as the specific gravity of the plate boehmite is 3.0 g / cm ³ and the specific gravity of PVB is 1.1 g / cm ³ is 82.1%. .

Example 3
As a binder (B), 500 g of SBR latex [“TRD-2001 (trade name)” manufactured by JSR Co., Ltd.] and 30 g of CMC (“2200” manufactured by Daicel Chemical Co., Ltd.) and 4000 g of water are placed in a container and room temperature until uniformly dissolved. Was stirred. Further, 2500 g of an aqueous dispersion of cross-linked PMMA (average particle size 0.3 μm, B _R = 0.5, B = 4) was added as swellable fine particles (D), and the mixture was stirred with a disper at 2800 rpm for 1 hour. Dispersed. To this, 3000 g of plate-like alumina (average particle size 2 μm, aspect ratio 25) as heat-resistant fine particles (A) was added, and stirred with a disper at 2800 rpm for 3 hours to obtain a uniform slurry (composition for forming a porous film) ). This slurry was applied onto a silicone-coated polyester film in the same manner as in Example 1 and dried to obtain a porous membrane with a liner having a thickness of 25 μm.

For the porous membrane according to Example 3, the specific gravity of plate-like alumina was 4.0 g / cm ³ , the specific gravity of SBR was 0.97 g / cm ³ , the specific gravity of CMC was 1.6 g / cm ³ , and the specific gravity of crosslinked PMMA was The volume content of the plate-like alumina calculated as 1.2 g / cm ³ is 41.5%.

Example 4
300 g of EVA (a structural unit derived from vinyl acetate of 15 mol%) as a binder and 6000 g of toluene were placed in a container and stirred at room temperature until evenly dissolved. Further, 1000 g of PE powder (average particle size 5 μm, melting point 105 ° C.) was added in four portions as organic fine particles (B), and dispersed with a disper at 2800 rpm for 1 hour. Thereafter, 4000 g of alumina fine particles (average particle size 0.4 μm) as heat-resistant fine particles (A) are added and stirred with a disperser at 2800 rpm for 3 hours to produce a uniform slurry (a composition for forming a porous film). did. This slurry was applied onto a silicone-coated polyester film in the same manner as in Example 1 and dried to obtain a porous membrane with a liner having a thickness of 25 μm.

For porous film according to Example 4, the volume content of the calculated alumina specific gravity of alumina specific gravity of 4.0 g ^/ cm 3, EVA specific gravity of 0.94 g / ^cm 3, PE as 1.0 g / cm ³ Is 43.1%.

Example 5
To the slurry prepared in Example 1, 100 g of alumina fiber (average fiber diameter 3 μm, aspect ratio 30,000) was further added as a fibrous material (E) and stirred at room temperature until uniform. The slurry (composition for forming a porous membrane) thus obtained was applied onto a silicone-coated polyester film and dried in the same manner as in Example 1, and the porous membrane with a liner having a porous membrane thickness of 20 μm. A membrane was obtained.

For the porous membrane according to Example 5, the alumina fine particles were calculated with a specific gravity of alumina fine particles of 4.0 g / cm ³ , a specific gravity of PVB of 1.1 g / cm ³ , and a specific gravity of alumina fibers of 4.0 g / cm ³ . The volume content is 71.9%.

Example 6
The slurry (composition for forming a porous film) prepared in Example 1 is passed through a PP meltblown nonwoven fabric having a thickness of 15 μm, and the slurry is applied by pulling up and then scraped through gaps having a predetermined interval and dried. Before the above, the same silicone-coated polyester film as used in Example 1 was overlaid and dried to obtain a porous membrane with a liner having a thickness of 20 μm.

For the porous membrane according to Example 6, the specific gravity of alumina was calculated as 4.0 g / cm ³ , the specific gravity of PVB was 1.1 g / cm ³ , and the specific gravity of PP related to the PP nonwoven fabric was calculated as 0.9 g / cm ³ . The volume content of alumina is 38.8%.

  The following evaluation was performed about the porous membrane which concerns on the porous membrane with a liner of Examples 1-6. The results are shown in Table 1.

<Heating characteristics of porous membrane>
The porous membrane was peeled from the liner-attached porous membranes of Examples 1 to 6, and left in a thermostatic bath at 150 ° C. for 30 minutes, and the shrinkage rate of the porous membrane was measured. Further, as Comparative Example 1, a PE microporous membrane (thickness 20 μm), which is a conventionally known separator, is also left in a thermostatic bath at 150 ° C. for 30 minutes, and the same measurement as that of the porous membrane according to Examples 1 to 6 is performed. went.

  The shrinkage rate was measured as follows. A porous membrane piece cut out to 4 cm × 4 cm is sandwiched between two stainless plates fixed with clips, left in a thermostatic bath at 150 ° C. for 30 minutes, taken out, and the length of each porous membrane piece is measured. The percentage of decrease in length compared with the length before the test was taken as the shrinkage rate.

  Moreover, the change of the Gurley value accompanying a temperature rise was measured on the following conditions about the porous membrane which concerns on Example 4, and the microporous membrane of the comparative example 1. FIG. After measuring the Gurley value by the above measurement method at room temperature, hold the porous membrane at 80 ° C. for 10 minutes in a thermostatic bath, take it out, slowly cool it, and measure the Gurley value when the temperature is raised to 80 ° C. did. Thereafter, the temperature was increased to 150 ° C. in increments of 5 ° C., the porous membrane was held at each temperature for 10 minutes, and then the Gurley value was measured in the same manner. From the above measurement, the temperature (rising temperature) at which the Gurley value became three times or more that before heating was determined.

  Table 1 shows the following. In the microporous membrane of Comparative Example 1 corresponding to the conventional product, the thermal shrinkage rate is large. In the battery using this as the separator, the separator shrinks until the internal temperature reaches 150 ° C. There is a risk of short circuit due to contact of the negative electrode. On the other hand, in the porous membranes according to Examples 1 to 6, almost no heat shrinkage was observed, and substantially no deformation occurred at the visual level. Therefore, in a battery using these as a separator, even when the internal temperature reaches 150 ° C., the contact between the positive electrode and the negative electrode is sufficiently hindered by the separator, and the occurrence of a short circuit can be prevented. Furthermore, in the porous membrane of Example 4, the Gurley value (air permeability) is increased three times or more in the temperature range of 100 to 130 ° C. after the heating, compared to the heating in the thermostat. Therefore, in the battery using the porous membrane of Example 4 as the separator, the flow of current is hindered by the decrease in the ion permeability of the separator in the process of increasing the internal temperature, and the reliability and safety are ensured. obtain. On the other hand, in the microporous film of Comparative Example 1, the temperature at which the Gurley value starts to increase is high, and the shutdown response is inferior.

Example 7
<Preparation of positive electrode>
LiCoO _{2 as a} positive electrode active material: 80 parts by mass, acetylene black as a conductive additive: 10 parts by mass, and PVDF as a binder: 5 parts by mass uniformly using N-methyl-2-pyrrolidone (NMP) as a solvent It mixed so that positive electrode mixture containing paste might be prepared. This paste was applied to both or one side of a 15 μm thick aluminum foil serving as a current collector, dried, and then calendered to give a total thickness of 150 μm (with a positive electrode mixture layer on both sides) or The thickness of the positive electrode mixture layer is adjusted to 82.5 μm (one side provided with a positive electrode mixture layer), cut to have a width of 48 mm and a length of 88 mm, and the positive electrode aluminum foil is exposed. Tabs were added to the parts.

<Production of negative electrode>
A negative electrode mixture-containing paste was prepared by mixing 90 parts by mass of graphite as a negative electrode active material and 5 parts by mass of PVDF as a binder so as to be uniform using NMP as a solvent. This negative electrode mixture-containing paste is applied to both sides of a 10 μm thick collector made of copper foil, dried, and then calendered to adjust the thickness of the negative electrode mixture layer so that the total thickness is 142 μm. And it cut | disconnected so that it might become width 50mm and length 90mm, and also tab-attached to the exposed part of the copper foil of this negative electrode.

<Battery assembly>
The positive electrode and the negative electrode obtained as described above were laminated through a porous film peeled from the liner-coated porous film produced in Example 1, to produce a laminated electrode body. A positive electrode having a positive electrode mixture layer provided on only one side of the current collector is disposed on both outer sides of the laminated electrode body, and the positive electrode mixture layer and the negative electrode agent layer each have four layers in the laminated electrode body. The layers were laminated as follows. Further, in the method of laminating the positive electrode and the negative electrode via the porous membrane, as shown in FIG. 2, the liner-attached porous membrane 1 slit into a width of 53 mm and wound up in a roll shape is used. Immediately after the porous membrane 3 was peeled off, the positive electrode 4 and the negative electrode 5 were laminated and cut every 93 mm in length. In FIG. 2, 6 is a positive electrode tab and 7 is a negative electrode tab.

The laminated electrode body produced as described above was loaded in a laminate film packaging material, and an electrolyte (LiPF ₆ was added to a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 2 at 1.2 mol / liter. The solution was dissolved at a concentration of 1) and vacuum sealed to produce a lithium secondary battery. The electrode terminal was taken out by laminating positive and negative electrode tabs, welding the electrode terminal, and taking out the terminal from the exterior material.

  FIG. 3 and FIG. 4 show schematic views of the fabricated lithium secondary battery. FIG. 3 is a plan view of the lithium secondary battery 100, and a portion where the laminated electrode body 20 exists in the exterior material 10 (and a part of the positive electrode terminal 8 and the negative electrode terminal 9) is indicated by a dotted line. 4 is a cross-sectional view taken along line AA in FIG.

  As shown in FIG. 4, in the laminated electrode body loaded in the exterior material 10 of the lithium secondary battery 100, the positive electrode in which the positive electrode mixture layer 12 is formed on one side of the positive electrode current collector 11 is disposed on both outer sides. A negative electrode in which a negative electrode agent layer 14 is formed on both surfaces of a negative electrode current collector 13 is disposed inside a battery of these positive electrodes via a separator made of a porous film 3. On the inner side, a positive electrode in which a positive electrode mixture layer 12 is formed on both surfaces of the positive electrode current collector 11 is disposed via a separator made of a porous film 3. And the positive electrode tab 6 of each positive electrode is laminated | stacked, and is connected to the positive electrode terminal 8, and the said positive electrode terminal 8 is pulled out to the exterior material exterior. Moreover, the negative electrode tab 7 of each negative electrode is laminated | stacked and connected to the negative electrode terminal 9, and the said negative electrode terminal 9 is pulled out to the exterior material exterior.

Examples 8-12
Lithium secondary batteries were produced in the same manner as in Example 7 except that the porous membrane with liner in Examples 2 to 6 was used.

Comparative Example 2
A lithium secondary battery was produced in the same manner as in Example 7 except that the PE microporous membrane of Comparative Example 1 was used as a separator.

  The lithium secondary batteries according to Examples 7 to 12 and Comparative Example 2 were subjected to electrochemical evaluation (discharge capacity ratio) and reliability evaluation. The results are shown in Table 2.

<Discharge capacity ratio>
Regarding the lithium secondary batteries according to Examples 7 to 12 and Comparative Example 2, constant current charging at 0.2 C (up to 4.2 V) and charging by constant voltage charging at 4.2 V (constant current charging and constant voltage charging) Thereafter, discharge was performed at 0.2 C or 2 C up to 3.0 V, and a discharge capacity ratio (load characteristic) with respect to 0.2 C at 2 C was obtained.

<Reliability evaluation>
The lithium secondary batteries according to Examples 7 to 12 and Comparative Example 2 were left in a constant temperature bath at 150 ° C., and the time until short circuit was measured.

  As can be seen from Table 2, in the lithium secondary batteries according to Examples 7 to 12, the discharge capacity ratio was relatively good and at a practical level. Moreover, the lithium secondary battery which concerns on Examples 7-12 has the long time to a short circuit compared with the lithium secondary battery which concerns on the comparative example 2, and has improved reliability and safety | security.

Example 13
<Preparation of positive electrode>
LiCoO _{2 as a} positive electrode active material: 80 parts by mass, acetylene black as a conductive additive: 10 parts by mass, and PVDF as a binder: 5 parts by mass are mixed so as to be uniform using NMP as a solvent. An agent-containing paste was prepared. This paste was intermittently applied to both sides of an aluminum foil having a thickness of 15 μm as a current collector so that the active material application length was 280 mm on the front surface and 210 m on the back surface, dried, and then subjected to a calendering treatment to obtain a total thickness. The thickness of the positive electrode mixture layer was adjusted to 150 μm, and the positive electrode mixture layer was cut to a width of 43 mm to produce a positive electrode having a length of 280 mm and a width of 43 mm. Further, the exposed portion of the aluminum foil of the positive electrode was tabbed.

<Production of negative electrode>
A negative electrode mixture-containing paste was prepared by mixing 90 parts by mass of graphite as a negative electrode active material and 5 parts by mass of PVDF as a binder so as to be uniform using NMP as a solvent. This negative electrode mixture-containing paste was intermittently applied on both sides of a 10 μm-thick current collector made of copper foil so that the active material application length was 290 mm on the front surface and 230 mm on the back surface, dried, and then subjected to calendar treatment. The thickness of the negative electrode mixture layer was adjusted so that the total thickness was 142 μm, and the negative electrode mixture layer was cut to have a width of 45 mm to produce a negative electrode having a length of 290 mm and a width of 45 mm. Further, a tab was attached to the exposed portion of the copper foil of the negative electrode.

<Battery assembly>
The positive electrode and the negative electrode obtained as described above are laminated through the porous film peeled from the liner-coated porous film produced in Example 3, and this is wound in a spiral shape to form a wound electrode body. did. The wound electrode body is crushed into a flat shape and loaded into an aluminum outer can, and an electrolyte (LiPF ₆ is added to a solvent in which ethylene carbonate and ethyl methyl carbonate are mixed at a volume ratio of 1: 2 at a ratio of 1.2 mol / liter. 1) was injected and sealed to prepare a lithium secondary battery.

  In addition, the process of laminating | stacking a porous film with a positive electrode and a negative electrode was performed as showing the schematic of FIG. That is, two liner-coated porous membranes 1 wound up in a roll shape, and positive electrode 4 and negative electrode 5 wound up in a roll shape are prepared. The porous film 3 was peeled, and immediately thereafter, the positive electrode 4 and the negative electrode 5 drawn from the roll and the peeled porous film 3 were laminated. The porous film 3 peeled from the liner-attached porous film 1 drawn from one roll was interposed between the positive electrode 4 and the negative electrode 5 and peeled from the liner-attached porous film 1 drawn from the other roll. The porous membrane 3 was superposed on the surface of the positive electrode 4 opposite to the negative electrode 5 side. And after laminating | stacking the porous membrane 3, the positive electrode 4, and the negative electrode 5, it wound around the spiral shape.

Comparative Example 3
A lithium secondary battery was produced in the same manner as in Example 13 except that the PE microporous membrane of Comparative Example 1 was used as the separator instead of the porous membrane.

  The lithium secondary batteries according to Example 13 and Comparative Example 3 were evaluated as follows. The results are shown in Table 3.

<Discharge capacity ratio>
About the lithium secondary battery which concerns on Example 13 and Comparative Example 3, the discharge capacity ratio was calculated | required on the same conditions as the battery which concerns on Example 7, etc.

<Reliability test>
The batteries according to Example 13 and Comparative Example 3 were left in a constant temperature layer at 150 ° C., and the time until the battery function was lost was measured to evaluate the reliability and safety at high temperatures.

<Shutdown temperature>
The batteries according to Example 13 and Comparative Example 3 were placed in a thermostatic chamber, and the internal resistance of the battery was measured while increasing the temperature from room temperature at a rate of 2 ° C. per minute. The temperature at which the internal resistance increased 5 times was determined as the shutdown temperature. It was.

  As can be seen from Table 3, the lithium secondary battery according to Example 13 has a longer time until the function of the battery is lost after being left at 150 ° C. than the battery according to Comparative Example 3.・ Excellent safety. It can also be seen that the battery according to Example 13 has good shutdown characteristics.

It is a schematic sectional drawing of the porous membrane with a liner of this invention. 9 is a conceptual diagram for explaining a manufacturing process of a lithium secondary battery in Example 7. FIG. 1 is a schematic plan view of a lithium secondary battery according to an example. FIG. 4 is a schematic cross-sectional view taken along line AA in FIG. 3. 14 is a conceptual diagram for explaining a manufacturing process of a lithium secondary battery in Example 13. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Porous membrane with liner 2 Liner 3 Porous membrane 4 Positive electrode 5 Negative electrode

Claims (12)

  At least a heat-resistant temperature of 150 ° C. or higher and chemically and electrochemically stable fine particles (A) in the lithium secondary battery, and a chemically and electrochemically stable binder ( A porous membrane with a liner is formed on a liner having releasability.   The porous membrane with a liner according to claim 1, wherein the porous membrane further contains organic fine particles having a melting point of 80 to 130 ° C.   The porous membrane with a liner according to claim 1 or 2, further comprising fine particles that can swell by absorbing a non-aqueous electrolyte and that increase in degree of swelling as the temperature rises. The liner according to claim 3, wherein the fine particles which can swell by absorbing the nonaqueous electrolytic solution and whose degree of swelling increases with an increase in temperature have a degree of swelling B defined by the following formula of 1 or more. With porous membrane.
_{B = (V 1 / V 0} ) -1
[In the above formula, V ₀ is the volume of particles (cm ³ ) 24 hours after being introduced into the non-aqueous electrolyte at 25 ° C., and V ₁ is 24 after being introduced into the non-aqueous electrolyte at 25 ° C. The temperature of the non-aqueous electrolyte is raised to 120 ° C. after time, and the volume (cm ³ ) of the particles after 1 hour at 120 ° C. is meant. ]   The liner-attached porous membrane according to any one of claims 1 to 4, wherein the fine particles (A) are inorganic oxide fine particles.   The porous membrane with a liner according to any one of claims 1 to 5, wherein the fine particles (A) are plate-like particles.   The fine film (A) is boehmite, and the porous membrane with a liner according to any one of claims 1 to 6.   The porous membrane with a liner according to any one of claims 1 to 7, wherein the porous membrane further contains a fibrous material having a heat resistant temperature of 150 ° C or higher.   The fibrous material having a heat-resistant temperature of 150 ° C. or higher is composed of at least one material selected from the group consisting of cellulose, cellulose modified material, polypropylene, polyester, polyaramid, polyamideimide, polyimide, glass, alumina, and silica. The porous membrane with a liner according to claim 1, which is contained.   At least a heat-resistant temperature of 150 ° C. or higher and chemically and electrochemically stable fine particles (A) in the lithium secondary battery, and a chemically and electrochemically stable binder ( B) is applied to a liner having a releasability and dried to produce a porous film with a liner, and the porous film is peeled off from the liner. A method for producing a porous film, which is characterized. A method for producing a lithium secondary battery having a separator comprising a positive electrode, a negative electrode, and a porous membrane,
A step of peeling the porous membrane from the liner-attached porous membrane according to any one of claims 1 to 9, and subsequently to the step, the positive electrode and the negative electrode are interposed between the porous membrane as a separator. A method for producing a lithium secondary battery, comprising: a step of superimposing to form a laminated body. The manufacturing method of the lithium secondary battery of Claim 11 which has the process of winding the laminated body of a positive electrode, a porous membrane, and a negative electrode in a spiral shape.

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