JP2008004442A - Separator for lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator capable of suppressing reduction in energy density and constituting a lithium secondary battery superior in safety while securing reliability of the battery, and the lithium secondary battery having the separator. <P>SOLUTION: This is the lithium secondary battery which has the separator for the lithium secondary battery that has at least a porous sheet consisting of a woven cloth or a nonwoven cloth, and which has insulating particulates (A), and in which the porous sheet is a laminate of at least two layers or more, and which has at least a negative electrode containing an active material capable of storing and releasing lithium, a positive electrode containing the active material capable of storing and releasing lithium, and the above separator for the lithium secondary battery. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a separator that is inexpensive and excellent in dimensional stability at high temperatures, and a lithium secondary battery that uses the separator and is safe even in a high temperature environment.

  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 the current lithium ion battery, 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 film separator is used, if the battery temperature reaches the shutdown temperature in the case of 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.

  In order to prevent such a short circuit due to heat shrinkage, 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 1 discloses a separator using a wholly aromatic polyamide microporous film, and Patent Document 2 discloses a separator using a polyimide porous film. Patent Document 3 discloses a separator using a polyamide nonwoven fabric, Patent Document 4 discloses a separator based on a nonwoven fabric using aramid fibers, Patent Document 5 discloses a separator using a polypropylene (PP) nonwoven fabric, Patent Document 6 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. On the other hand, nonwoven fabrics using PP fibers or polyester fibers are inexpensive and excellent in dimensional stability at high temperatures. However, since the pore diameter is too large if the nonwoven fabrics remain as they are, for example, at a thickness of 30 μm or less, the positive electrode and the negative electrode Short circuit due to contact and short circuit due to deposition of lithium dendrite cannot be sufficiently prevented.

  There has also been proposed a technique in which a nonwoven fabric made of an inexpensive material is subjected to various processes and used as a separator. For example, Patent Document 7 discloses a method in which polyethylene (PE) fine particles are applied to a PP nonwoven fabric, Patent Document 8 discloses a method in which a polyester nonwoven fabric is coated with wax, and Patent Document 9 discloses a polyester. A method of using a PE microporous film in contact with a non-woven fabric and a PP non-woven fabric is disclosed, and Patent Document 10 discloses a method of using a mixture of inorganic fine particles and organic fine particles in a PP non-woven fabric.

  However, in the technique of mixing inorganic fine particles into a nonwoven fabric, short-circuiting due to lithium dendrite generation cannot be completely prevented unless inorganic fine particles are uniformly and densely filled in the voids of the nonwoven fabric. It is difficult to uniformly charge inorganic fine particles to a substrate having a large property. In addition, when organic fine particles such as PE are used, the same problems as in the case of using inorganic fine particles may occur, and since PE and the like are soft, if the separator is thinned from the viewpoint of particularly increasing the energy density, a hard positive electrode Insulation between the material and the negative electrode material may not be sufficiently maintained, and a short circuit may occur.

JP-A-5-335005 JP 2000-306568 A JP-A-9-259856 Japanese Patent Laid-Open No. 11-40130 JP 2001-291503 A JP 2003-123728 A JP-A-60-136161 Japanese Patent Laid-Open No. 62-283553 JP-A-1-258358 JP 2003-22843 A

  The present invention has been made in view of the above circumstances, and an object thereof is to constitute a lithium secondary battery excellent in safety while suppressing a decrease in energy density as much as possible and ensuring the reliability of the battery. And a lithium secondary battery having the separator.

  The separator for a lithium secondary battery of the present invention that has achieved the above object has at least a porous sheet made of a woven fabric or a non-woven fabric and insulating fine particles (A), and the porous sheet comprises: It is a laminate having at least two or more layers.

  The lithium secondary battery separator can further swell by absorbing fine particles (B) [heat-meltable fine particles (B)] that melt at 80 to 130 ° C. and non-aqueous electrolyte, and You may contain the microparticles | fine-particles (C) [swellable microparticles | fine-particles (C)] which a swelling degree increases with the rise in temperature.

  The lithium secondary battery further comprises at least a negative electrode containing an active material capable of occluding and releasing lithium, a positive electrode containing an active material capable of occluding and releasing lithium, and the lithium secondary battery separator of the present invention. Secondary batteries are also encompassed by the present invention.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium secondary battery excellent in safety | security, and the lithium secondary battery which can comprise this lithium secondary battery, suppressing the fall of energy density as much as possible and ensuring the reliability of a battery. A battery separator can be provided.

  The separator of the present invention is composed of at least a porous sheet made of woven or non-woven fabric and insulating fine particles (A). A porous sheet made of a woven fabric or a non-woven fabric is less likely to cause thermal shrinkage and has good dimensional stability against heat unlike a separator made of a porous film made of a conventionally known polyolefin. Therefore, in the battery having the separator of the present invention, contact between the positive electrode and the negative electrode due to thermal contraction of the separator hardly occurs even when abnormally heated, and good safety can be ensured.

  However, as described above, a porous sheet made of a woven fabric or a non-woven fabric has a relatively large opening diameter. Therefore, for example, it easily causes generation of lithium dendrite in the battery, or the positive and negative electrodes are in direct contact with each other. In some cases, the battery is short-circuited, and the reliability of the battery is easily impaired.

  The insulating fine particles (A) possessed by the separator are, for example, for filling the voids of the porous sheet to prevent the occurrence of the short circuit. As described above, since the porous sheet is a highly non-uniform substrate, coarse pinholes are likely to occur. In such pinholes, a porous layer mainly composed of insulating fine particles It is thought that. Therefore, the pinhole part is not reinforced by fibers of woven fabric or nonwoven fabric, and the porous layer in such a state is brittle and easily broken, so that powder may fall off when handling a separator such as a slit. The separator tends to crack during the production of the wound electrode body, which tends to decrease the reliability of the battery.

  Therefore, in the present invention, a porous sheet made of woven fabric or non-woven fabric is made into a laminate of two or more layers, so that even if the woven fabric or non-woven fabric whose opening diameter is not strictly controlled, the insulating fine particles The formation of coarse pinholes, which becomes a factor that makes the separator brittle as a structure mainly composed of (A), was suppressed as much as possible. As a result, the flexibility of the separator was secured, the occurrence of a short circuit due to the lithium dendrite was prevented, and the reliability of the battery was improved.

  As described above, in the present invention, for example, in a configuration other than increasing the thickness of the separator, short circuit due to direct contact between the positive and negative electrodes and prevention of short circuit due to lithium dendrite are achieved. Can be made relatively thin, and a decrease in the energy density of a battery using this can be suppressed as much as possible.

The presence or absence of pinholes in the porous sheet according to the separator of the present invention can be measured by, for example, image analysis using a CCD (charge coupled device) camera, mercury porosimeter, bubble point method porosimeter, or the like. For example, the abundance ratio of pinholes having a diameter of 100 μm or more is preferably 1 piece / cm ² or less.

  As described above, a porous sheet having a small pinhole abundance can be obtained by adopting the following configuration.

As the porous sheet, a woven fabric or a non-woven fabric produced by a conventionally known method can be used. Among these, non-woven fabrics are preferred because of the simplicity of the manufacturing method and the need to reduce the thickness described later as much as possible.

  The porous sheet preferably contains at least fibers that do not substantially deform at 150 ° C. as constituent fibers. In the case of such a porous sheet, since the shrinkage hardly occurs at a temperature of 150 ° C. or lower, the safety of the battery at a higher temperature is improved. In addition, “a fiber that does not substantially deform at 150 ° C.” means that a substantial dimensional change due to softening or the like does not occur in the form of a sheet, and specifically, 150 ° C. (or lower). The change in length of the sheet-like material at (temperature), that is, the ratio of shrinkage (shrinkage rate) to the length at room temperature is 5% or less.

  Examples of the material of the constituent fiber of the porous sheet (that is, woven fabric or non-woven fabric) include cellulose, modified cellulose (such as carboxymethyl cellulose), polypropylene (PP), polyester [polyethylene terephthalate (PET), polyethylene naphthalate ( PEN), polybutylene terephthalate (PBT), etc.], polyacrylonitrile (PAN), polyaramid, polyamideimide, polyimide, polyvinyl alcohol (PVA) and other resins; glass, alumina, silica and other inorganic materials (inorganic oxides); etc. Is mentioned. The constituent fibers of the porous sheet may contain one of these materials or two or more. Moreover, the constituent fiber of the porous sheet contains, as necessary, various known additives (for example, an antioxidant in the case of a resin) in addition to the materials described above. It does not matter.

  For example, non-woven fabrics such as paper, PP non-woven fabric, polyester non-woven fabric (PET non-woven fabric, PEN non-woven fabric, PBT non-woven fabric) and PAN non-woven fabric can be exemplified as more preferable among woven fabrics or non-woven fabrics constituting the porous sheet.

  The diameter of the constituent fiber of the porous sheet may be equal to or less than the thickness of the separator, but is preferably 0.01 to 5 μm, for example. If the diameter is too large, the entanglement between the fibrous materials may be insufficient, and the strength of the porous sheet constituted by these, and consequently the strength of the separator may be reduced, making it difficult to handle. On the other hand, if the diameter is too small, the gap of the separator becomes too small, and the ion permeability tends to be lowered, and the load characteristics of the battery may be lowered.

  The porous sheet is a laminate in which two or more layers of woven or non-woven fabrics are laminated, but individual woven or non-woven fabrics (hereinafter sometimes referred to as “material sheets”) to be laminated are the same. The sheet may be a sheet having a different basis weight, thickness, material, or manufacturing method. Further, a woven fabric and a non-woven fabric may be laminated to form a porous sheet.

  In the process of containing insulating fine particles (A) in a porous sheet (details of the process will be described later), a liquid composition (paint) in which insulating fine particles (A) are dispersed is applied to the porous sheet. In some cases, the porous sheet may be easily curled in a specific direction due to anisotropy in the thickness direction of the porous sheet, impregnation properties of the liquid composition, and the like. In that case, the porous sheet preferably has a structure in which material sheets (woven fabric or non-woven fabric) having the same configuration are laminated with the front surfaces or the back surfaces facing each other, for example. Therefore, curling properties can be suppressed.

  If the porous sheet is a laminate of two or more layers (two layers, three layers, four layers, etc.), there is no particular limitation on the number of layers, but the separator is made as thin as possible to reduce the energy density of the battery. In terms of increasing the thickness, it is preferable to have two layers.

In addition, the individual woven fabric or nonwoven fabric for constituting the porous sheet is preferably thin from the viewpoint of increasing the energy density of the battery. For example, the thickness is preferably 20 μm or less, and 16 μm. The following is more preferable. In terms of weight per unit area, it is preferably 15 g / m ² or less, and more preferably 10 g / m ² or less.

On the other hand, if the woven fabric or non-woven fabric for forming the porous sheet is too thin, there is a possibility that the strength required for the process of producing slits and wound electrode bodies may not be obtained, and the thickness is 3 μm or more. It is preferably 5 μm or more. Further, in terms of the basis weight, and more preferably preferably at 2 g / ^{m 2} or more and 4g / ^{m 2} or more.

  The porosity of the woven or non-woven fabric for constituting the porous sheet is, for example, 10% or more, more preferably 20% or more, and preferably 90% or less, more preferably 80% or less. . If the porosity of the woven fabric or nonwoven fabric is too small, the ion permeability of the separator may be deteriorated, and if the porosity is too large, the strength of the separator may be insufficient. In addition, the porosity of a woven fabric or a nonwoven fabric can be calculated by a postscript formula (10).

Further, in the woven fabric or nonwoven fabric for forming the porous sheet, the abundance ratio of pinholes having a diameter of 100 μm or more measured by the same method as in the case of the porous sheet is, for example, 1 piece / cm. preferably ² or less, more preferably 0.1 pieces / cm ² or less.

  The thickness of the porous sheet is, for example, 3 μm or more, more preferably 5 μm or more, and 50 μm or less, more preferably 30 μm or less, from the viewpoint of improving the short-circuit prevention effect of the separator and ensuring the strength of the separator. It is desirable.

The basis weight of the porous sheet is preferably 5 g / m ² or more, more preferably 10 g / m ² or more, and 50 g / m ² or less, more preferably 30 g / m ² or less.

  The porosity of the porous sheet is, for example, 20% or more, more preferably 30% or more, and 70% or less, more preferably 60% or less. If the porosity of the porous sheet is too small, the ion permeability of the separator may deteriorate, and if the porosity is too large, the strength of the separator may be insufficient. Note that the porosity of the porous sheet can be calculated by the following formula (10).

  As a method of forming a porous sheet by laminating material sheets that are woven or non-woven fabric, a method in which the material sheets are bonded together using a binder; a method in which the material sheets are bonded together by thermal fusion; And a method of forming a non-woven fabric (porous sheet) by laminating webs as constituent materials of the non-woven fabric at the stage of production. From the viewpoint of thinning the porous sheet, a heat fusion method without using a binder or a method of forming a nonwoven fabric using a laminated web is preferable.

The insulating fine particles (A) according to the present invention may be organic or inorganic fine particles that are electrically insulating (specifically, have a volume resistivity of 10 ¹⁰ Ω · m or more) and can exist stably in the battery. If there is no restriction | limiting in particular, 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; Electrical insulation by surface treatment with non-electrically conductive materials that can constitute the insulating fine particles (A), cross-linked polymer fine particles, and materials constituting heat-resistant polymer fine particles described later] Fine particles having properties 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 crosslinked polymer fine particles [those that do not fall under the following swellable fine particles (C)], 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 insulating fine particles exemplified above, Al ₂ O ₃ (alumina), SiO ₂ (silica), and boehmite are preferable.

  As the size of the insulating fine particles (A), it is sufficient that the particle size at the time of drying is smaller than the thickness of the separator, but preferably has an average particle size of 1/3 to 1/100 of the thickness of the separator. 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 size of the insulating fine particles (A) is too small, the gap between the fine particles (A) becomes small, and the conduction path of ions in the separator becomes long, which may deteriorate the battery characteristics. On the other hand, if the average particle diameter of the insulating fine particles (A) is too large, the gap between the fine particles (A) becomes large, and the short-circuit preventing effect caused by the generation of lithium dendrite may be reduced.

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

  When the insulating fine particles (A) are plate-like, the aspect ratio (the ratio of the maximum length in the plate-like particles to the thickness of the plate-like particles) is 5 or more, more preferably 10 or more, and 100 or less. More preferably, it is 50 or less. If the aspect ratio is too small, the above-mentioned effect due to the insulating fine particles (A) being plate-like may be reduced. If the aspect ratio is too large, the specific surface area of the insulating fine particles (A) becomes too large. It can be difficult.

  Further, when the insulating fine particles (A) are plate-like, the average value of the ratio of the long axis direction length to the short 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 insulating fine particles (A) is too large, the shape of the insulating fine particles (A) approaches a needle shape, and the insulating 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 insulating fine particles (A) is dispersed in a medium (for example, water) in which the insulating fine particles (A) do not swell using a laser scattering particle size distribution meter (“LA-920” manufactured by HORIBA). And the number average particle diameter measured. In addition, when the insulating fine particles (A) are plate-like, the average value of the ratio of the long axis direction length to the short axis direction length of the flat plate surface 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 insulating fine particles (A) are plate-like can also be obtained by image analysis of an image photographed by SEM.

  Further, as described above, the presence form when the insulating 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. In the case where the presence form of the plate-like insulating fine particles (A) in the separator is as described above, the insulating fine particles (A) are arranged substantially in parallel when a 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 insulating 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.

  It is desirable that the insulating fine particles (A) be 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 separator. By making the volume ratio of the insulating fine particles (A) in this way, the short-circuit prevention function of the separator can be made more reliable. In addition, it is preferable that the upper limit of the volume ratio of insulating fine particles (A) is 80 volume%, for example.

  As a specific embodiment of the separator of the present invention, a porous sheet which is a laminate of woven fabric or nonwoven fabric (including paper) is used, and insulating fine particles (A) are contained in this sheet. . The porous sheet may have a mode in which the fibrous material (D) is dispersed together with the insulating fine particles (A).

  As a fibrous material (D), the thing similar to the constituent fiber of the woven fabric and nonwoven fabric which are comprised with the raw material of the said illustration can be used, for example. 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 (D) is preferably 10 or more.

  In addition, the separator of the present invention may contain heat-meltable fine particles (B) and swellable fine particles (C) described later, in addition to the insulating fine particles (A) and the fibrous material (D). By using a separator containing the heat-meltable fine particles (B) or swellable fine particles (C), a shutdown function can be imparted to a battery using the separator. Such heat-meltable fine particles (B) and swellable fine particles (C) have electrical insulation properties, are electrochemically stable, and further have insulation properties used in the production of electrolytes and separators. Any fine particles that are stable in the solvent used for the liquid composition containing the fine particles (A) and do not undergo side reactions such as oxidation-reduction in the operating voltage range of the battery may be used. As used herein, “electrochemically stable” means that no chemical change occurs during charging / discharging of the battery.

  The heat-fusible fine particles (B) have a function of inhibiting lithium ion migration and suppressing a rapid discharge reaction at high temperatures by causing the particles to melt at high temperatures to fill the voids of the separator. Specific examples of the heat-meltable fine particles (B) are those that melt at 80 to 130 ° C., that is, a melting temperature of 80 measured using a differential scanning calorimeter (DSC) according to JIS K 7121. Those having a temperature of ˜130 ° C. are preferred. More specifically, polyethylene (PE), copolymerized polyolefin having a structural unit derived from ethylene of 85 mol% or more, or a polyolefin derivative (such as chlorinated polyethylene), polyolefin wax, petroleum wax, carnauba wax, and the like can be given. 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 heat-meltable 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. The heat-meltable fine particles (B) 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.

  The heat-fusible fine particles (B) may be ones that can melt the whole of the fine particles (B) when the battery reaches a high temperature. For example, the heat-fusible fine particles (B) have the insulating fine particles (A) as a core, Fine particles having a core-shell structure in which the above-described heat-meltable resin (resin that melts at 80 to 130 ° C.) that can constitute B) is combined as a shell. Even when such core-shell fine particles are used, when the battery becomes hot, the resin in the shell melts to fill the gaps in the separator, so that the battery can have a good shutdown function.

  The particle diameter of the heat-meltable fine particles (B) is a number average particle diameter measured by the same measurement method as that for the insulating fine particles (A), for example, 0.001 μm or more, more preferably 0.1 μm or more. 15 μm or less, more preferably 1 μm or less is recommended.

  The swellable fine particles (C) can swell by absorbing a non-aqueous electrolyte (hereinafter simply referred to as “electrolyte”) in the battery, and the degree of swelling increases as the temperature rises. It has a function to do. When a lithium secondary battery having a separator containing swellable fine particles (C) is exposed to a high temperature, the swelling degree increases as the temperature of the swellable fine particles (C) increases (hereinafter referred to as “thermal swellability”). Swellable fine particles (C) absorb the electrolyte solution in the battery and swell. 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. Since the ion conductivity is remarkably reduced, a rapid discharge reaction can be suppressed.

As the swellable fine particles (C), 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 battery of the present invention, the temperature at which the lithium ion conductivity in the battery is remarkably reduced (so-called shutdown temperature) is preferably in the range of about 80 to 130 ° C. Therefore, the swellable fine particles (C) It is preferable that the temperature at which the thermal swellability starts to be exhibited by the temperature rise is in the range of 75 to 125 ° C.

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

  Specific measurement of the swellable fine particles (C) defined by the above formula (1) can be performed by the following method. Swellable fine particles (C) 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 (C) before being immersed in the electrolytic solution,
V ₀ : volume (cm ³ ) of swellable fine particles (C) after being immersed in an 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 (C) in the film before being immersed in the electrolytic solution,
P _A : specific gravity (g / cm ³ ) of the swellable fine particles (C) before being immersed in the electrolytic solution,
P _B : Specific gravity of electrolyte at room temperature (g / cm ³ ),
P _C: electrolyte specific gravity at ^{120 ℃ (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 (C) is 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 (C) may be those that do not absorb the electrolyte solution (B _R = 0) at room temperature or those that slightly absorb the 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 swellable particles (C), and preferably has satisfied the swelling degree B or B _R, also has an electrically insulating, electrochemically stable, further described below electrolyte, separator There is no particular limitation as long as it is stable to the solvent used in the liquid composition used in the production and does not dissolve in the electrolytic 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 a non-aqueous electrolyte (details of the lithium salt, the type of the organic solvent, the lithium salt concentration, etc. will be described later). ). Therefore, as the swellable fine particles (C), 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 (C) 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 (C) may contain one kind of the above-mentioned constituent materials, or may contain two or more kinds. Furthermore, the swellable fine particles (C) contain, as a constituent component, various known additives (for example, antioxidants) that are added to the resin as necessary, in addition to the above constituent materials. 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 at which it thermally expands. Therefore, in the separator using the swellable fine particles (C) composed of the above-mentioned crosslinked resin, the heat of the swellable fine particles (C) can be obtained through a heating process such as drying of the separator and drying of the electrode group during battery production. Since the swellability 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.

  Further, the swellable fine particles (C) may be entirely composed of the above-exemplified crosslinked resin. For example, the swellable fine particles (C) may be formed using the insulating fine particles (A) as a core. Fine particles having a core-shell structure in which the above-exemplified cross-linked resin exemplified above can be combined as a shell may be used. The swellable fine particles (C) having such a core-shell structure also generate a so-called “liquid drainage” in which the electrolytic solution is absorbed by an increase in the temperature in the battery, and secure a shutdown function that blocks the movement of ions. be able to.

  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 Tg of the swellable fine particles (C) is in the range of about 75 to 125 ° C.

  As the size of the swellable fine particles (C), it is sufficient that the particle size at the time of drying is smaller than the thickness of the separator, but preferably has an average particle size of 1/3 to 1/100 of the thickness of the separator. Specifically, the average particle diameter is preferably 0.1 to 20 μm. If the particle size of the swellable fine particles (C) is too small, the gap between the fine particles becomes small, so that the ion conduction path becomes long and the battery characteristics may deteriorate. Moreover, when the particle size of the swellable fine particles (C) is too large, the gap becomes large, and the short-circuit preventing effect by using the swellable fine particles (C) may be reduced. The average particle size of the swellable fine particles (C) referred to here is obtained by dispersing the fine particles in a non-swelling medium (for example, water) using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA). The measured number average particle diameter.

  The shutdown phenomenon in the separator by containing the heat-meltable fine particles (B) and the swellable fine particles (C) can be evaluated by, for example, 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.

  In the case of the separator containing the heat-meltable fine particles (B) (including composite fine particles having a core-shell structure) that melt at 80 to 130 ° C., the separator is exposed to 80 to 130 ° C. (or higher temperature). The temperature at which the void blockage phenomenon evaluated by the Gurley value described later (the temperature at which the Gurley value becomes 3 times or more that before heating) is the constituent resin of the heat-meltable fine particles (B) (in the case of composite fine particles having a core-shell structure) Is from the melting temperature of the constituent resin of the shell part) to 130 ° C. or less.

  In the separator of the present invention, when the shutdown function is ensured by adding the heat-meltable fine particles (B) or the swellable fine particles (C), the heat-meltable fine particles (B) or the swellable fine particles (C ) Is preferably 20% by volume or more and 80% by volume or less, for example, among all components of the separator. If the content of these fine particles is too small, the shutdown effect due to the inclusion of these may be reduced, and if too large, the content of insulating fine particles (A) in the separator will be reduced. In some cases, the effect of preventing a short circuit due to generation of lithium dendrite may be reduced.

  Also, the fibers constituting the porous sheet are bound to the insulating fine particles (A), the fibrous material (D), and other particles [heat-melting fine particles (B), swellable fine particles (C)]. For the purpose, the separator may contain a binder (E).

  The binder (E) is electrochemically stable and stable with respect to the electrolytic solution, and further comprises fibers constituting the porous sheet, insulating fine particles (A), heat-melting fine particles (B), swellable fine particles ( C), any fibrous material (D) or the like may be used, for example, EVA (with 20 to 35 mol% of a structural unit derived from vinyl acetate), ethylene-ethyl acrylate copolymer, etc. Ethylene-acrylate copolymer, fluorine rubber, SBR, carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), polyurethane, epoxy resin, polyfluoride And vinylidene chloride (PVDF) and its derivatives. These binders may be used individually by 1 type, and may use 2 or more types together. When these binders (E) are used, they can be used in the form of an emulsion or plastisol that is dissolved or dispersed in a solvent of a liquid composition for forming a separator described later.

  For example, when the heat-meltable fine particles (B) and the swellable fine particles (C) have adhesiveness alone, they can also serve as the binder (E).

  The thickness of the separator is, for example, 3 μm or more, more preferably 5 μm or more, from the viewpoint of further improving the battery short-circuit prevention effect, ensuring the separator strength and improving the handleability while further improving the energy density of the battery. Thus, it is preferably 50 μm or less, more preferably 30 μm or less.

In addition, the porosity of the separator is, for example, 20% or more, more preferably 30% or more, and preferably 70% or less, more preferably 60% or less in a dry state. If the porosity of the separator is too small, the ion permeability may be reduced, and if the porosity is too large, the strength of the separator may be insufficient. The porosity of the separator: P (%) can be calculated by obtaining the sum of each component i from the thickness of the separator, the mass per area, and the density of the constituent components using the following formula (10).
_{P = Σ a i ρ i /} (m / t) (10)
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 separator (g / cm ² ), t: thickness of separator (cm).

The separator of the present invention is carried out by a method according to JIS P 8117, and the Gurley value indicated by the number of seconds that 100 ml of air passes through the membrane under a pressure of 0.879 g / mm ² is 10 to 300 sec. Is desirable. If the air permeability is too high, the ion permeability is reduced, whereas if it is too low, the strength of the separator may be reduced. Further, the strength of the separator is desirably 50 g or more in terms of piercing strength using a needle having a diameter of 1 mm. If the piercing strength is too low, a short circuit may occur due to the breakthrough of the separator when lithium dendrite crystals are generated.

  As a manufacturing method of the separator of this invention, the following method is employable, for example. A production method of applying a liquid composition (slurry or the like) containing insulating fine particles (A) to a porous sheet composed of a woven fabric or a laminate of two or more layers, and then drying at a predetermined temperature. It is.

The liquid composition for forming the separator of the present invention comprises insulating fine particles (A), and if necessary, hot-melt fine particles (B), swellable fine particles (C), fibrous materials (D), binders (E) and the like, and these are dispersed in a solvent (including a dispersion medium, the same applies hereinafter) [the binder (E) may be dissolved]. The solvent used in the liquid composition can uniformly disperse the insulating fine particles (A), the heat-meltable fine particles (B), the swellable fine particles (C), and the fibrous material (D), and the binder (E). Any one can be used as long as it can be dissolved or dispersed uniformly. Examples thereof include aromatic hydrocarbons such as toluene; furans such as tetrahydrofuran; ketones such as methyl ethyl ketone and methyl isobutyl ketone; N-methyl-2-methylpyrrolidone; Organic solvents are preferred. 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 addition, when the binder (E) is water-soluble and is used as an emulsion, water may be used as a solvent. In this case, alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, etc.) The interfacial tension can also be controlled by appropriately adding.

  In the liquid composition, the solid content containing the insulating fine particles (A), the heat-meltable fine particles (B), the swellable fine particles (C), the fibrous material (D), and the binder (E) is, for example, 10 to 40. It is preferable to set it as the mass%.

  In the separator of the present invention, it is preferable that part or all of the insulating fine particles (A) are present in the voids of the porous sheet. In addition, fine particles other than the insulating fine particles (A) [hot-melt fine particles (B), swellable fine particles (C)] and fibrous materials (D) are partially or entirely made of a porous sheet. More preferably, the structure is present in the voids. By adopting such a structure, the effect of using fine particles other than the insulating fine particles (A) and the fibrous material (D) [for example, if it is a heat-meltable fine particle (B) or a swellable fine particle (C) , The shutdown effect] is more effectively exhibited. In order to make the insulating fine particles (A), the heat-meltable fine particles (B), the swellable fine particles (C), and the fibrous material (D) exist in the voids of the porous sheet, for example, the above-described liquid composition is used. After impregnating the porous sheet, a process such as drying after removing a surplus liquid composition through a certain gap may be used.

  In the separator, in the case where the insulating fine particles (A) are plate-like, in order to enhance the orientation and to more effectively exert the above-described effect by using the plate-like insulating fine particles (A). In the sheet-like material impregnated with the liquid composition, it is only necessary to give a share to the liquid composition. For example, in the manufacturing method described above, by impregnating the liquid composition described above as a method for causing the insulating fine particles (A) and the like to exist in the voids of the sheet-like material, a method of passing a certain gap is used. It is possible to share the liquid composition, and the orientation of the plate-like insulating fine particles (A) can be enhanced. Furthermore, the orientation of the plate-like insulating fine particles (A) can also be improved by drying to remove the solvent of the liquid composition.

  The separator of the present invention is not limited to the structure shown above. For example, the insulating fine particles (A) may not be present independently of each other, and may be mutually or porous. Part of the fibers constituting the adhesive sheet may be fused.

  If the lithium secondary battery of this invention has the separator of this invention, there is no restriction | limiting in particular about another structure and structure, A conventionally well-known structure and structure can be employ | adopted.

  Examples of the form of the lithium secondary battery include a cylindrical 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.

The positive electrode is not particularly limited as long as it is a positive electrode used in a conventionally known lithium secondary battery, that is, a positive electrode containing an active material capable of occluding and releasing lithium. 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 such as LiMn ₂ O ₄ Manganese 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. The positive electrode mixture is formed into a molded body (ie, a current collector as a core material). In other words, a positive electrode mixture layer) or the like can be used.

  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, that is, a negative electrode containing an active material capable of occluding and releasing Li. 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 obtained 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 a molded body (negative electrode mixture layer) using a current collector as a core material And those having a negative electrode agent layer such as those made of the above-mentioned various alloys and lithium metal foils 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.

  As with the lead portion on the negative electrode side, the negative electrode layer (including a layer having a negative electrode active material and a negative electrode mixture layer) is usually formed on a part of the current collector during the preparation of the negative electrode. Without leaving the exposed portion of the current collector, it is provided as a lead portion. However, the lead portion on the negative electrode side 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 laminate in which the above positive electrode and the above negative electrode are laminated via the separator of the present invention, or a wound electrode body in which this 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, vinylene carbonates, 1,3-propane sultone, diphenyl disulfide, cyclohexane, biphenyl, fluorobenzene, t- for the purpose of improving safety, charge / discharge cycleability, and high-temperature storage properties of these electrolytes. 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.

  The lithium secondary battery of the present invention can be applied to the same uses as various uses 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
<Preparation of separator>
As insulating fine particles (A), alumina (average particle size: 0.3 μm): 1000 g, water: 800 g, isopropyl alcohol (IPA): 200 g, and binder (F) as PVB: 375 g are put in a container, and 1 by a three-one motor. The mixture was dispersed by stirring for a time to obtain a uniform slurry. A two-layer nonwoven fabric (porous sheet, thickness 23 μm, basis weight 10 g / m ²⁾ obtained by stacking two layers of PET nonwoven fabric having a thickness of 12 μm and a basis weight of 5 g / m ² in this slurry and forming two layers by thermal calender press treatment. The slurry was applied by pulling up, and then passed through a gap having a predetermined interval, and then dried to obtain a separator having a thickness of 25 μm. In addition, when the cross section of this separator was observed by SEM, it has confirmed that the insulating fine particles (A) existed in the space | gap of a two-layer nonwoven fabric.

<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 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 were spirally wound through the separator to obtain a wound electrode body. The wound electrode body is crushed into a flat shape, loaded into a laminate film outer package made by Dai Nippon Printing Co., Ltd., and electrolyte solution (LiPF ₆ is added to a solvent in which ethylene carbonate and ethyl methyl carbonate are mixed at a volume ratio of 1: 2). A solution dissolved at a concentration of 1.2 mol / l) was injected and vacuum sealed to produce a lithium secondary battery.

Example 2
In the same slurry as in Example 1, further, as a swellable fine particle (C), crosslinked PMMA fine particles [“Gantz Pearl (trade name)” manufactured by Ganz Kasei Co., Ltd., average particle size: 2 μm, B _R = 0.8, B = 1.8 ]: 250 g was added and dispersed by stirring for 3 hours with a three-one motor to obtain a uniform slurry. The same two-layer nonwoven fabric as used in Example 1 was passed through this slurry, and the slurry was applied by pulling up, then passed through a gap having a predetermined interval, and then dried to obtain a separator having a thickness of 25 μm. Got. When the cross section of this separator was observed with an SEM, it was confirmed that insulating fine particles (A) and swellable fine particles (C) were present in the voids of the two-layer nonwoven fabric.

  A lithium secondary battery was produced in the same manner as in Example 1 except that the separator produced in the above procedure was used.

Example 3
As a binder (E), SBR latex: 100 g and CMC: 30 g and water: 4000 g were put in a container and stirred at room temperature until evenly dissolved. Furthermore, plate-like boehmite (average particle diameter: 1 μm, aspect ratio: 10): 1000 g was added as insulating fine particles (A), and dispersed by stirring with a disper at 2800 rpm for 1 hour. To this, 1000 g of PE emulsion (average particle size: 1 μm, melting point: 125 ° C.) was added as hot-melt fine particles (B), and the mixture was stirred with a disper at 2800 rpm for 3 hours to obtain a uniform slurry. Using an applicator, this slurry was applied by sliding onto a PET two-layer nonwoven fabric (porous sheet) with a gap of 50 μm, a thickness of 18 μm, and a basis weight of 8 g / m ² , and dried to obtain a thickness of A 20 μm separator was obtained. When the cross section of this separator was observed by SEM, it was confirmed that the flat surface of the insulating fine particles (A) in the vicinity of the surface was oriented within 30 ° on the average with respect to the surface of the separator.

  A lithium secondary battery was produced in the same manner as in Example 1 except that the separator produced in the above procedure was used.

Comparative Example 1
A separator was produced in the same manner as in Example 1 except that a PP non-woven fabric having a thickness of 23 μm and a basis weight of 10 g / m ² was used instead of the porous sheet. A lithium secondary battery was produced in the same manner as in Example 1 except that this separator was used.

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

  The batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were subjected to the following electrochemical evaluation (measurement of discharge capacity ratio) and safety test. The results are shown in Table 1.

<Discharge capacity ratio>
For the batteries of Examples 1 to 3 and Comparative Examples 1 and 2, charging by constant current charging at 1C (up to 4.2V) and constant voltage charging at 4.2V (total time of constant current charging and constant voltage charging 2) .5 hours), the process of discharging to 3.0 V at 1 C is repeated as one cycle, and the charge / discharge is repeated, and the ratio of the discharge capacity to the charge capacity for 10 cycles after the second cycle excluding the first charge / discharge cycle. That is, the average value of the discharge capacity ratio (%) was determined. The discharge capacity ratio was determined as an average value of 10 batteries.

<Reliability test>
The batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were left in a high temperature layer of 150 ° C., the time until the function of the battery was lost was measured, and a reliability test at a high temperature was performed.

  Moreover, about each battery of Example 2, 3 and the comparative example 2, the shutdown temperature was also measured. The internal resistance of the batteries was measured while raising the temperature of these batteries from room temperature at a rate of 2 ° C. per minute in a thermostatic bath, and the point at which the internal resistance increased 5 times was taken as the shutdown temperature.

  As can be seen from Table 1, in the lithium secondary batteries of Examples 1 to 3, the discharge capacity ratio is good and at a practical level, the occurrence of short circuits due to lithium dendrite is suppressed, and the reliability is excellent. . On the other hand, the battery of Comparative Example 1 has a low discharge capacity ratio. Further, in the lithium secondary batteries of Examples 1 to 3, when left at 150 ° C., the time until the function of the battery is lost is longer than that of the battery of Comparative Example 2 corresponding to the conventional battery, and is reliable. Had improved. Furthermore, it can be said that the shutdown temperature of the lithium secondary batteries of Examples 2 and 3 to which shutdown characteristics are imparted is lower than that of the battery of Comparative Example 2, and safety is improved.

Claims (9)

  A lithium secondary battery comprising at least a porous sheet made of woven or non-woven fabric and insulating fine particles (A), wherein the porous sheet is a laminate of at least two layers Separator.   The separator for a lithium secondary battery according to claim 1, wherein the porous sheet contains at least a fiber that does not substantially deform at 150 ° C.   The separator for a lithium secondary battery according to claim 1 or 2, wherein a part or all of the insulating fine particles (A) are present in the voids of the porous sheet.   The lithium secondary battery separator according to any one of claims 1 to 3, wherein the insulating fine particles (A) are at least one kind of fine particles selected from the group consisting of alumina, silica and boehmite.   The separator for a lithium secondary battery according to any one of claims 1 to 4, wherein a part or all of the insulating fine particles (A) are plate-shaped fine particles.   The separator for a lithium secondary battery according to any one of claims 1 to 5, further comprising fine particles (B) that melt at 80 to 130 ° C.   The lithium secondary battery according to any one of claims 1 to 6, further comprising fine particles (C) capable of absorbing the nonaqueous electrolytic solution to swell and increasing the degree of swelling as the temperature rises. Separator. The lithium secondary battery separator according to claim 7, wherein the fine particles (C) have a swelling degree B defined by the following formula of 1 or more.
_{B = (V 1 / V 0} ) -1
[In the above formula, V ₀ is the volume of fine particles (cm ³ ) 24 hours after being introduced into the nonaqueous electrolyte at 25 ° C., and V ₁ is 24 after being introduced into the nonaqueous electrolyte at 25 ° C. The temperature of the non-aqueous electrolyte is raised to 120 ° C. after a period of time, and the volume of fine particles (cm ³ ) after 1 hour at the above temperature is meant. ] It has at least a negative electrode containing an active material capable of occluding and releasing lithium, a positive electrode containing an active material capable of occluding and releasing lithium, and a separator for a lithium secondary battery according to any one of claims 1 to 8. A featured lithium secondary battery.