JP5611505B2 - Battery separator and lithium secondary battery - Google Patents

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 fill inorganic fine particles on a substrate having high properties. 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.

  There has also been proposed a technique for forming a separator by applying electrochemically stable fine particles such as inorganic fine particles on electrodes (Patent Documents 11 and 12).

  However, if an attempt is made to reduce the thickness of the separator in order to increase the energy density of the battery, the insulation between the positive electrode and the negative electrode cannot be sufficiently maintained, so that a short circuit is likely to occur.

  Moreover, although it is not the structure using a nonwoven fabric, in patent document 13, in order to improve the short circuit resistance of a separator, the technique regarding the separator which mixed the flaky particle | grains is shown.

  However, the technique of Patent Document 13 mainly mixes flaky particles in a normal polyolefin-based separator, and although the short-circuit prevention effect is seen, the problem that the separator contracts has not been solved.

  In addition to these, as techniques for using resin fine particles in battery separators, there are proposals in Patent Documents 14 to 16 and the like in addition to Patent Document 7 and Patent Document 10 described above.

  Among them, the separator disclosed in Patent Document 14 is composed of a mixture of polyolefin-based fine particles and fine particles holding an electrolyte such as polyvinylidene fluoride (PVDF), and exhibits a shutdown characteristic, but these fine particles Is rich in flexibility, and therefore, short circuit resistance is weak and the battery reliability cannot be sufficiently ensured.

  Further, Patent Document 15 discloses a separator formed by forming polymer beads (fine particles) such as polyolefin and acrylic resin into a film shape, but this separator is also short circuit resistant in the same manner as the separator disclosed in Patent Document 14. It is not possible to secure sufficient sex.

  Furthermore, the separator disclosed in Patent Document 16 is also composed of a porous film made of a fluorine-based resin such as highly flexible PDVF and acrylic resin fine particles, which are also disclosed in Patent Document 14 and Patent Document 15. Like the separator, it cannot be said that the short-circuit resistance is sufficient.

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 International Publication No. 97/8863 Pamphlet JP 2000-149906 A JP 2004-288586 A Japanese Patent Laid-Open No. 11-214041 JP 2000-340204 A JP 2004-241135 A

  The present invention has been made in view of the above circumstances, and its purpose is to suppress a decrease in energy density as much as possible, and to ensure the reliability of the battery, while being excellent in safety when abnormally heated. And a lithium secondary battery having the separator.

The battery separator of the present invention that has achieved the above object is a copolymer polyolefin, polypropylene, or polyolefin derivative in which at least a part of the surface of the heat-resistant fine particles (A) is polyethylene, and the structural unit derived from ethylene is 85 mol% or more. , Coated with at least one heat-meltable resin selected from polyolefin wax, petroleum wax and carnauba wax, or at least one swellable resin selected from cross-linked polystyrene, cross-linked acrylic resin and cross-linked fluororesin. It is a porous film containing composite fine particles and a binder, and formed by binding the composite fine particles with the binder.

  The heat-resistant fine particles (A) constituting the composite fine particles (B) are preferably plate-like particles. Examples of the resin constituting the composite fine particles (B) [resin covering at least part of the surface of the heat-resistant fine particles (A)] include, for example, a resin that melts at 80 to 130 ° C. (C)] and at least a resin [swellable resin (D)] that can swell in a non-aqueous electrolyte and whose degree of swelling increases with an increase in temperature.

  Also, a lithium secondary battery comprising 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 battery separator of the present invention is also provided. Are encompassed by the present invention.

  According to the present invention, a lithium secondary battery excellent in safety when abnormally heated while suppressing a decrease in energy density as much as possible and ensuring the reliability of the battery, and the lithium secondary battery are configured. A battery separator can be provided.

  The battery separator of the present invention (hereinafter sometimes abbreviated as “separator”) contains composite fine particles (B) in which at least a part of the surface of the heat-resistant fine particles (A) is coated with a resin as described above. doing.

  Because the heat-resistant fine particles (A) in the composite fine particles (B) can separate the positive electrode and the negative electrode well even if the separator is thinned, the occurrence of a short circuit is prevented while suppressing the decrease in battery energy density as much as possible. In addition, it is possible to suppress the occurrence of a fine short circuit due to the generation of lithium dendrite and to ensure the reliability of the battery.

  Furthermore, by forming the separator using the composite fine particles (B), it is possible to suppress the occurrence of heat shrinkage due to the presence of the heat-resistant fine particles (A) related to the composite fine particles (B). With the resin constituting (B), shutdown can be expressed when the temperature inside the battery becomes high. For this reason, in the battery having the separator of the present invention, even when abnormally heated, contact between the positive electrode and the negative electrode due to thermal contraction of the separator is difficult to occur, and further, ion conduction in the separator is shut off by shutdown, Excellent safety.

  The heat-resistant fine particles (A) constituting the composite fine particles (B) have electrical insulating properties, are electrochemically stable, and are further referred to as a non-aqueous electrolyte (hereinafter simply referred to as “electrolyte”). There is no particular limitation as long as it is stable to the solvent used in the liquid composition used in the production of the separator and does not dissolve in the electrolytic solution at a high temperature. The “high temperature state” as used in the present specification is specifically a temperature of 150 ° C. or higher, and may be any stable particle that does not undergo deformation and chemical composition change in the electrolyte at such a temperature. Further, “electrochemically stable” as used in the present specification means that no chemical change occurs during charging / discharging of the battery.

Specific examples of such heat-resistant fine particles (A) include the following inorganic fine particles and organic fine particles, which may be used alone or in combination of two or more. Examples of the inorganic fine particles (inorganic powder) 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, Slightly soluble ionic crystal particles such as barium fluoride and barium sulfate; Covalent crystal particles such as silicon and diamond; Clay particles such as talc and montmorillonite; Boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite And mineral resource-derived substances such as bentonite or artificial products thereof. Further, the surface of conductive fine particles such as metal fine particles; oxide fine particles such as SnO ₂ and tin-indium oxide (ITO); carbon fine particles such as carbon black and graphite; It may be fine particles that have been made electrically insulating by surface treatment with the above-mentioned materials that constitute the electrically insulating inorganic fine particles or the materials that constitute the organic fine particles described later.

  Organic fine particles (organic powder) include crosslinked polymethyl methacrylate, crosslinked polystyrene, polydivinylbenzene, styrene-divinylbenzene copolymer crosslinked product, polyimide, polyurethane, epoxy resin, melamine resin, phenol resin, benzoguanamine-formaldehyde. Various crosslinked polymer fine particles such as condensates (except those corresponding to the swelling resin (D) described later), cellulose and its derivatives, PVDF, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) And fine particles composed of organic resins such as polytetrafluoroethylene and derivatives thereof. 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).

  Further, the heat-resistant fine particles (A) may be particles containing two or more materials (inorganic materials and organic resins) constituting the various inorganic fine particles and organic fine particles exemplified above.

  The average particle diameter of the heat-resistant fine particles (A) is 0.001 μm or more, more preferably 0.1 μm or more, and is preferably 15 μm or less, more preferably 1 μm or less. 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) are not swollen using a laser scattering particle size distribution meter (“LA-920” manufactured by HORIBA). And the number average particle diameter measured.

  The shape of the heat-resistant fine particles (A) may be, for example, a so-called spherical shape, or may be a plate shape or a needle shape. More preferably, it is a plate-like particle. When the heat-resistant fine particles (A) constituting the composite fine particles (B) are plate-like, the composite fine particles (B) [that is, the plate-like heat-resistant fine particles (A) constituting them] in the separator. By causing the flat plate surface to be oriented so as to be substantially parallel to the surface of the separator, the occurrence of a short circuit can be suppressed more favorably. This is because the composite fine particles (B) are oriented as described above so that the composite fine particles (B) overlap with each other on a part of the flat plate surface. It is considered that the hole) is formed in a bent shape instead of a straight line, and thus it is possible to prevent the lithium dendrite from penetrating the separator.

  In order to obtain the effect of orienting the composite fine particles (B) using the plate-like heat-resistant fine particles (A) as described above, the form of the composite fine particles (B) in the separator is as described above. The flat plate surface is preferably substantially parallel to the separator surface. More specifically, the composite fine particles (B) in the vicinity of the separator surface have an average angle of 30 degrees or less between the flat plate surface and the separator surface. [Most preferably, the average angle is 0 degree, that is, the flat plate surface of the composite fine particles (B) in the vicinity of the surface of the separator is parallel to the surface of the separator]. Here, “near the surface” refers to a range of 10% from the surface of the separator to the entire thickness.

  As a form in the case where the heat-resistant fine particles (A) are plate-like particles, for example, 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. Thus, it is desirable that it is 100 or less, more preferably 50 or less. The average value of the ratio of the long axis direction length to the short axis direction length of the flat plate surface of the grain is 0.3 or more, more preferably 0.5 or more, and 3 or less, more preferably 2 or less. It is desirable. When the plate-like heat-resistant fine particles (A) have the above-mentioned aspect ratio or the average value of the ratio of the major axis direction length to the minor axis direction of the flat plate surface, the above-described short-circuit prevention action is more effective. To be demonstrated.

  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 with 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.

More specific examples of the plate-like heat-resistant fine particles (A) include various commercially available products. For example, “Sun Lovely” (SiO ₂ ) manufactured by Dokai Chemical Industries, Ltd. “NST-B1” manufactured by Ishihara Sangyo Co., Ltd. ”Crushed product (TiO ₂ ), Sakai Chemical Industry's plate-like barium sulfate“ H series ”,“ HL series ”, Hayashi Kasei's“ micron white ”(talc), Hayashi Kasei's“ bengel ”(bentonite) ), “BMM” and “BMT” (boehmite) manufactured by Kawai Lime Co., Ltd., “Cerasure BMT-B” [alumina (Al ₂ O ₃ )] manufactured by Kawai Lime Co., “Seraph” (alumina) manufactured by Kinsei Matec Co., Ltd., Yodogawa Minebea's “Yodogawa Mica Z-20” (sericite), Co-op Chemical's “Micro Mica” and “Soshimafu” (Mica) are available. In addition, SiO ₂ , Al ₂ O ₃ , ZrO, and CeO ₂ can be produced by the method disclosed in Japanese Patent Laid-Open No. 2003-206475.

Among the above, a good practice to use a plate-like particles of a compound represented by AlOOH or Al 2 _{O 3} · _H 2 _O as a main component (for example more than 80 wt%), particularly preferably used boehmite .

  The resin for compositing with the heat-resistant fine particles (A) to form the composite fine particles (B) is a heat-meltable resin (C) that melts at 80 to 130 ° C., or at least in a non-aqueous electrolyte. The swellable resin (D) that can be swollen at a temperature and whose degree of swelling increases with an increase in temperature is preferred. Further, the composite fine particles (B) can be constituted by mixing both the hot-melt resin (C) and the swellable resin (D), or can be constituted by using a composite of these two resins. is there. By using composite fine particles (B) composed of the above hot-melt resin (C) or swellable resin (D) in the separator, when the temperature inside the battery becomes high, ion conduction in the separator A so-called shutdown function can be added to shut off.

  The shutdown function in the separator of the present invention can be evaluated by, for example, an increase in resistance due to the temperature of the model cell. That is, a model cell including a positive electrode, a negative electrode, a separator, and an electrolytic solution is prepared, and the model cell is held in a high-temperature bath, and the internal resistance value of the model cell is measured while the temperature is increased at a rate of 5 ° C./min. Then, by measuring the temperature at which the measured internal resistance value is 5 times or more that before heating (resistance value measured at room temperature), this temperature can be evaluated as the shutdown temperature of the separator. In the separator of the present invention, the shutdown temperature evaluated in this way can be set to 80 to 130 ° C., and sufficient ion conductivity is ensured under normal battery use environments, so that the discharge characteristics of the battery can be improved. When the temperature inside the battery rises while being good, the shutdown occurs at a relatively early stage, so that the safety of the battery can be ensured.

  As said heat-meltable resin (C) which can comprise composite microparticles | fine-particles (B), the melting | fusing point is 80-130 degreeC, ie, according to the prescription | regulation of JISK7121, A differential scanning calorimeter (DSC) is used. A resin having a melting temperature of 80 to 130 ° C. measured is preferable. In the case of a separator using composite fine particles (B) obtained using such a heat-meltable resin (C), when the separator is exposed to 80 to 130 ° C. (or higher temperature) in the battery. Further, since the hot-melt resin (C) is melted and the gaps in the separator are closed, the above-described shutdown effect can be ensured more reliably. Therefore, in this case, the shutdown temperature (the temperature at which the internal resistance value becomes 5 times or more before heating) in the separator of the present invention evaluated by the above increase in internal resistance is 130 ° C. above the melting point of the heat-meltable resin (C). It becomes as follows.

  Specific examples of the heat-meltable resin (C) include polyethylene (PE), copolymerized polyolefin having a structural unit derived from ethylene of 85 mol% or more, polypropylene (PP), polyolefin derivative (chlorinated polyethylene, chlorinated polypropylene, etc.) Polyolefin wax, petroleum wax, carnauba wax and the like. 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. For the heat-meltable resin (C), each of the exemplified resins may be used alone or in combination of two or more. Among these, PE, polyolefin wax, or EVA having a structural unit derived from ethylene of 85 mol% or more is preferable. Moreover, the heat-meltable resin (C) may contain various known additives (for example, antioxidants) added to the resin as necessary.

  The swellable resin (D) that can constitute the composite fine particles (B) is a resin that can swell in a non-aqueous electrolyte and increases the degree of swelling as the temperature rises. The swelling resin (D) has a property that the degree of swelling increases with an increase in temperature in the swelling resin (D) when a battery including a separator having the composite fine particles (B) using the swelling resin (D) is exposed to a high temperature. (Hereinafter, sometimes referred to as “thermal swellability”), the swellable resin (D) absorbs the electrolyte in the battery 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. Since the ion conductivity is remarkably reduced, a rapid discharge reaction can be suppressed.

  The swellable resin (D) is preferably one having a temperature of 75 to 125 ° C. showing the above-mentioned heat swellability. 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 separator of the present invention, the temperature at which the lithium ion conductivity in the battery is remarkably reduced (so-called shutdown temperature) is desirably in the range of about 80 to 130 ° C. as described above. The temperature at which the resin (D) starts to exhibit thermal swellability due to temperature rise is preferably in the range of 75 to 125 ° C.

  Moreover, as swelling resin (D), the thing whose swelling degree B defined by a following formula measured at 120 degreeC is 1.0 or more is preferable.

_{B = (V 1 / V 0} ) -1 (1)
[In the above formula (1), V ₀ is the volume of the resin (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 nonaqueous electrolyte is raised to 120 ° C., and the volume (cm ³ ) of the resin after 1 hour at 120 ° C. is meant. ]
The swellable resin (D) having the above degree of swelling greatly increases its degree of swelling in an environment exceeding the above temperature (any temperature of 75 to 125 ° C.) that starts to show thermal swellability. Therefore, in the battery using the separator having the composite fine particles (B) containing the swellable resin (D) having such properties, when the internal temperature exceeds a specific temperature (for example, 75 to 125 ° C.). Thus, the swelling resin (D) further absorbs the electrolyte solution in the battery and expands greatly, so that the lithium ion conductivity is remarkably reduced, so that the safety of the battery can be ensured more reliably. Become. The swelling degree of the swellable resin (D) 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 resin (D) defined by the above formula (1) can be performed by the following method. A swellable resin (D) is 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 base material such as a polyethylene terephthalate (PET) sheet or 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 resin (D) before being immersed in the electrolytic solution,
V ₀ : volume (cm ³ ) of the swellable resin (D) after being immersed in the electrolyte 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 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 resin (D) in the film before being immersed in the electrolytic solution,
P _A : specific gravity (g / cm ³ ) of the swellable resin (D) 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).

Moreover, swelling resin (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 resin (D) may be one that does not absorb the electrolyte solution (B _R = 0) at room temperature or one that absorbs 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 resin (D), 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 containing the composite fine particles (B) used during 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 resin (D), in the organic solvent solution of the lithium salt, when it reaches any temperature of 75 to 125 ° C., it begins to show the above thermal swellability, and preferably the degree of swelling in the solution Those that can swell so that B satisfies the above values are recommended.

  Further, in the present invention, even if the resin itself has a property of being dissolved in an organic solvent such as an electrolytic solution, it is organic as a result of being combined with the heat-resistant fine particles (A) by chemical bonding by a chemical method described later. Resins that are stable to solvents can also be used as the swellable resin (D). The swelling degree of the swellable resin (D) in such a case can be determined by the same method as described above using the composite particles.

  Specific examples of the swellable resin (D) include polystyrene (PS), acrylic resin, polyalkylene oxide, fluororesin, styrene butadiene rubber (SBR), and derivatives and cross-linked products thereof; urea resin; polyurethane; Can be mentioned.

  For the production of the composite fine particles (B), the swellable resin (D) exemplified above may be used alone or in combination of two or more. Further, the swellable resin (D) may contain various known additives (for example, antioxidants) added to the resin as necessary.

  For example, in the case where the swellable resin (D) is a crosslinked resin exemplified above, the volume change accompanying the temperature change is reversible, such that even if it expands due to a temperature rise, it shrinks again by lowering the temperature, Moreover, these resin crosslinked bodies are stable up to a temperature higher than the temperature of thermal expansion in a so-called dry state containing no electrolytic solution. Therefore, in the separator having the composite fine particles (B) obtained by using the above-mentioned crosslinked resin as the swellable resin (D), the swellable resin can be obtained through a heating process such as drying of the separator or drying of the electrode group during battery production. Since the thermal swellability of (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.

  Examples of the swellable resin (D) include cross-linked PS, cross-linked acrylic resin [for example, cross-linked polymethyl methacrylate (PMMA)], and cross-linked fluororesin [for example, cross-linked polyvinylidene fluoride (PVDF)] among the various resins exemplified above. Preferably cross-linked PMMA is particularly preferred.

  Although the details of the mechanism by which these swellable resins (D) 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 the Tg of PMMA, and absorb and swell more electrolytic solution. Therefore, it is considered desirable that the Tg of the swellable resin (D) is in the range of about 75 to 125 ° C.

  As the structure of the composite fine particles (B), at least a part of the surface of the heat-resistant fine particles (A) [for example, 50 to 100 area% in the total area of the surface of the heat-resistant fine particles (A)] is coated with a resin. There is no particular limitation. For example, a core-shell structure in which the heat-resistant fine particles (A) are the core and the resin is the shell, a sea-island structure in which islands made of the heat-resistant fine particles (A) are included in the sea of the resin, and one surface of the heat-resistant fine particles (A). Any structure such as a partially covered structure in which a portion is coated with a resin can be employed. Among these structures, it is preferable to have a structure (form) that retains the form of the heat-resistant fine particles (A) as much as possible. In particular, when the heat-resistant fine particles (A) are plate-like particles, this is used. In order to further draw out the characteristics due to the above and to ensure the above-described short-circuit prevention effect by orienting the flat plate surface substantially parallel to the separator surface, the composite fine particles (B) also have a plate-like form. Recommended.

  The ratio of the heat-resistant fine particles (A) in the composite fine particles (B) is a volume ratio, for example, preferably 50% or more, more preferably 60% or more, and 90% or less. It is preferable.

  The composite fine particles (B) have an average particle size measured by the same method as that for the heat-resistant fine particles (A), for example, 0.002 μm or more, more preferably 0.2 μm or more, and 15 μm or less. Preferably it is 3 μm or less.

  As a method for producing the composite fine particles (B) [a method of combining the heat-resistant fine particles (A) and the resin], for example, a conventionally known method of physically combining the heat-resistant fine particles (A) and the resin, A chemical method for combining the heat-resistant fine particles (A) and the resin by bonding can be used.

  As a physical method, a method in which heat-resistant fine particles (A) and resin powder are combined by mechanical alloying; a slurry in which heat-resistant fine particles (A) are dispersed in a dispersion medium, and a resin is dispersed in the dispersion medium. A method of coating the surface of the heat-resistant fine particles (A) with a resin by spray drying using the slurry and the solution dissolved in the solvent; dipping and dispersing the heat-resistant fine particles (A) in the resin solution; And a method of coating the heat-resistant fine particles (A) with a resin after drying. Moreover, as a chemical method, the method of carrying out the graft polymerization of the resin molecule on the surface of heat-resistant fine particle (A), and coating the surface of heat-resistant fine particle (A) with resin by this etc. is mentioned.

  Among the various methods described above, the mechanical alloying method and the spray drying method are preferable. Commercially available devices such as a planetary ball mill, mechanofusion (manufactured by Hosokawa Micron), and a hybridizer (manufactured by Nara Machinery Co., Ltd.) can be used as the device used for mechanical alloying.

  In the separator of the present invention, by using the composite fine particles (B), it is possible to improve the particle filling property when forming the separator, and to provide a more uniform separator while providing a shutdown function. Is possible.

  The separator of the present invention may be in the form of an independent film or may have a structure integrated with electrodes (positive electrode and / or negative electrode). In the case of the form of an independent membrane, it is preferable to use the fibrous material (E) as a reinforcing material in order to ensure necessary strength.

  The fibrous material (E) is electrically insulative, electrochemically stable, and further contains an electrolyte solution described in detail below and a liquid containing composite fine particles (B) used in the production of the separator. As long as the solvent used in the composition is stable, there is no particular limitation, but those having characteristics that do not substantially deform at 150 ° C. are preferable. 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.

  Although the diameter of a fibrous material (E) should just be below the thickness of a separator, it is preferable that it is 0.01-5 micrometers, for example. When the diameter is too large, for example, when the fibrous material (E) is used to form a sheet-like material and used as a separator, the entanglement between the fibrous materials is insufficient, and the strength and elongation of the sheet-like material are reduced. In this case, 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 state of the presence of the fibrous material (E) in the separator (sheet-like material) is, for example, preferably that the angle of the long axis (axis in the longitudinal direction) with respect to the separator surface is 30 ° or less on average, 20 More preferably, it is not more than 0 °.

  When these fibrous materials (E) are used, a large number of the fibrous materials (E) are aggregated to form a sheet-like material by these alone, such as woven fabric, nonwoven fabric, paper, etc. A separator having a configuration in which the composite fine particles (B) are contained in the sheet may be used, or the fibrous material (E) and the composite fine particles (B) are contained in a uniformly dispersed form. The separator may be configured as described above. Moreover, it can also be set as the structure which united both above-mentioned structures. Note that the separators having the above-described configurations can be integrated with electrodes as well as independent films.

  In addition to the composite fine particles (B) and the fibrous material (E), the separator of the present invention comprises heat-resistant fine particles (A), a heat-meltable resin (C), and a swellable resin (D). It may be contained in a form other than B). As long as the hot-melt resin (C) and the swellable resin (D) hold the electrolytic solution and ensure ionic conductivity, prevent short circuit and prevent the original function of the separator having a shutdown function, Any form may be used. Specifically, it may be present in the form of fine particles or in a solid form between the composite fine particles (B), or in the form of a flat shape, a fiber shape, or a porous film shape. In any case, it is desirable that the composite fine particles (A) are 30% by volume or more, more preferably 40% by volume or more, in the total volume of the constituent components of the separator. The upper limit of the volume ratio of the composite fine particles (B) is preferably 80% by volume, for example. If it deviates from the said range, it will become difficult to set it as the structure which provides a shutdown characteristic, preventing a short circuit as a separator.

  Also, the composite fine particles (B) or the above-mentioned fibrous material (E), heat-resistant fine particles (A), heat-meltable resin (C), and swellable resin (D) are bound to each other or to each other. For this purpose, the separator may contain a binder (F). As the binder (F), it is electrochemically stable and stable with respect to the electrolytic solution, and further, composite fine particles (B), fibrous materials (E), heat-resistant fine particles (A), heat-meltable resin (C), Any swellable resin (D) or the like may be used as long as it can adhere well. For example, ethylene (e.g., EVA having a structural unit derived from vinyl acetate of 20 to 35 mol%), ethylene-ethyl acrylate copolymer or the like Acrylate copolymers, various rubbers and derivatives thereof [styrene-butadiene rubber (SBR), fluorine rubber, urethane rubber, ethylene-propylene-diene rubber (EPDM), etc.], cellulose derivatives [carboxymethylcellulose (CMC), hydroxyethylcellulose, hydroxypropyl Cellulose etc.], polyvinyl alcohol (PVA), polyvinyl butyral (PVB) Polyvinylpyrrolidone (PVP), polyurethane, epoxy resins, PVDF, include such PVDF-HFP copolymer, it is also possible to use them alone or may be used in combination of two or more. When these binders (F) are used, they can be used in the form of an emulsion dissolved or dispersed in a solvent of a liquid composition for forming a separator described later.

  For example, as described above, the separator of the present invention contains composite fine particles (B) in the sheet-like material composed of the fibrous material (E) [further, if necessary, heat-resistant fine particles (A), heat A meltable resin (C), a swellable resin (D) and the like] can be used. In this case, the sheet-like material composed of the fibrous material (E) hardly undergoes thermal shrinkage, such as a separator made of a conventionally known polyolefin porous film, and has good dimensional stability against heat. Therefore, even when the battery is abnormally heated, a short circuit due to thermal contraction of the separator can be prevented well.

  Moreover, the separator of this invention can also be made into the form of the porous film | membrane in which many composite microparticles | fine-particles (B) are bind | concluded with a binder (F) other than the above forms, for example. . Also in this case, the heat-resistant fine particles (A), the heat-meltable resin (C), the swellable resin (D), the fibrous material (E) and the like are bound together with the composite fine particles (B) by the binder (F). May be. In the separator of such a form, because it is produced by a production method [a production method having a step of applying a liquid composition containing composite fine particles (B), etc. to a substrate and drying it] such that no strain remains in the separator, Since the dimensional stability against heat is good as in the case of the separator including the sheet-like material composed of the fibrous material (E), the separator is also thermally contracted even during abnormal heating of the battery. Short circuit can be prevented well.

  In the separator of the present invention having each of the above forms, for example, the thermal shrinkage rate at 150 ° C. can be less than 5%. For example, even when the inside of the battery reaches about 150 ° C., the separator shrinks almost. Therefore, a short circuit due to contact between the positive and negative electrodes can be prevented, and the safety of the battery at high temperatures can be improved. The “150 ° C. heat shrinkage rate” of the separator refers to the size of the separator before putting the separator in a thermostatic bath, raising the temperature to 150 ° C. and leaving it for 30 minutes, and then putting it in the thermostatic bath. The reduction ratio of the dimension calculated | required by this is expressed in percentage.

  The thickness of the separator is 3 μm or more, more preferably 5 μm or more, and is preferably 50 μm or less, more preferably 30 μm or less. If the separator is too thin, the short-circuit preventing effect may be reduced, and the separator may have insufficient strength and may be difficult to handle. On the other hand, if the separator is too thick, the energy density of the battery tends to be small.

  Further, the porosity of the separator is desirably 20% or more, more preferably 30% or more, and 70% or less, more preferably 60% or less in a dry state. If the porosity of the separator is too small, the ion permeability (ion conductivity) 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 calculating 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.

P = Σa _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 separator (g / cm ² ), t: thickness of separator (cm).

Further, the air permeability of the separator, which is carried out by a method according to JIS P 8117 and indicated by the Gurley value indicated by the number of seconds in which 100 ml of air passes through the membrane under a pressure of 0.879 g / mm ² , is 10 to 10. It is preferable that it is 300 sec. 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. In the case of an independent membrane, the strength of the separator is preferably 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 the method for producing the separator of the present invention, for example, the following methods (I), (II) and (III) can be employed. The method (I) is a production method in which a liquid composition (slurry or the like) containing composite fine particles (B) is applied or impregnated on an ion-permeable sheet-like material and then dried at a predetermined temperature.

The “sheet-like material” in the method (I) corresponds to a sheet-like material (various, woven fabric, non-woven fabric, etc.) 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, polyester non-woven fabric (PET non-woven fabric, PEN non-woven fabric, PBT non-woven fabric, etc.), PAN non-woven fabric, and PVA non-woven fabric can be exemplified.

As thickness of the said sheet-like material, it is preferable that it is 20 micrometers or less, for example, and it is more preferable that it is 16 micrometers or less. The basis weight is preferably 15 g / m ² or less, more preferably 10 g / m ² or less. Further, the lower limit of the thickness is preferably 5 μm, and the lower limit of the basis weight is preferably 2 g / m ² .

  As a method for producing a nonwoven fabric corresponding to the sheet-like material, conventionally known methods such as wet, dry, melt blow, spun bond, and charge melt spinning can be used.

  The liquid composition for forming the separator of the present invention comprises composite fine particles (B), heat-resistant fine particles (A), heat-meltable resins (C), swellable resins (D), binders as necessary. (F) and the like, which are dispersed or dissolved in a solvent (including a dispersion medium, the same applies hereinafter). The solvent used in the liquid composition can uniformly disperse the composite fine particles (B), the heat-resistant fine particles (A), and the like, and can also contain the binder (F), the hot-melt resin (C), and the swellable resin (D). 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 as long as they can be dissolved or dispersed uniformly. 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 (F) is water-soluble or used as an emulsion, water may be used as a solvent. In this case, alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, etc.) are appropriately used. In addition, the interfacial tension can be controlled.

  In the liquid composition, the solid content including the composite fine particles (B), the heat-resistant fine particles (A), the hot-melt resin (C), the swellable resin (D), and the binder (F) is, for example, 10 to 40 mass. % Is preferable.

  In addition, when a hot-melt resin (C) and a swellable resin (D) have adhesiveness independently, these can also serve as a binder (F).

  When the sheet-like material composed of the particles is composed of a fibrous material such as paper, PP nonwoven fabric, and polyester nonwoven fabric, and the opening diameter of the gap is relatively large (for example, In the case where the opening diameter of the gap is 5 μm or more), this tends to cause a short circuit of the battery. Therefore, in this case, it is preferable to have a structure in which a part or all of the composite fine particles (B) and further the heat-resistant fine particles (A) exist in the voids of the sheet-like material. Further, part or all of the heat-meltable resin (C) and the swellable resin (D) may be present in the voids of the sheet-like material, and these resins (C) and (D) are fine particles. When it has a form, it is particularly preferable to take such a structure. In order to make the composite fine particles (B) etc. exist in the voids of the sheet-like material, for example, after impregnating the above-mentioned liquid composition into the sheet-like material, the excess liquid composition is removed through a certain gap. Then, a process such as drying may be used. When the heat-resistant fine particles (A) used for the composite fine particles (B) are plate-like particles, or when the heat-resistant fine particles (A) used separately from the composite fine particles (B) are plate-like particles, these plate-like particles are used. In order to enhance the orientation of the particles and to make the function work effectively, a method of applying shear or a magnetic field to the liquid composition in the substrate impregnated with the liquid composition may be used. For example, as described above, the liquid composition can be sheared by impregnating the liquid composition into a sheet and then passing through a certain gap.

The manufacturing method of (II) of the separator of the present invention is such that the liquid composition further contains a fibrous material (E), which is coated on a substrate such as a film or a metal foil, and dried at a predetermined temperature. And then peeling from the substrate. The liquid composition used in the method (II) is the same as the liquid composition used in the method (I) except that it is essential to contain the fibrous material (E). The solid content concentration including the state (E) is preferably 10 to 40% by mass, for example. The separator obtained by the method (II) also has a structure in which some or all of the composite fine particles (B) are present in the voids of the sheet-like material formed of the fibrous material (E). Is desirable.

The method for producing (III) of the separator of the present invention includes, for example, the above-mentioned composite fine particles (B), heat-resistant fine particles (A), heat-meltable resin (C), swellable resin (D) fine particles or fibrous materials A liquid composition such as a slurry dispersed in water or a suitable solvent is prepared using a binder (F) as necessary in (E), etc., and a blade coater, roll coater, die coater, spray coater is prepared. In this method, the liquid composition is applied onto an electrode (positive electrode or negative electrode) using a conventionally known coating apparatus such as, and then dried. Thereby, the separator of the structure integrated with the electrode can be obtained. As the liquid composition, for example, the same liquid composition as described for the production method of (I) or (II) can be used.

  In addition, the separator of this invention is not limited to said structure. For example, the composite fine particles (B) do not have to be present independently of each other, and some of them may be fused to each other or to the fibrous material (E).

  The separator produced by the methods (I) to (III) is preferably subjected to a heat treatment after drying to remove volatile components such as moisture and solvent (dispersion medium) contained therein. By removing these volatile components, it is possible to provide a lithium secondary battery with excellent long-term reliability because deterioration of battery characteristics when charging and discharging are repeated in the lithium secondary battery can be suppressed. The residual amount of moisture and solvent is preferably 100 ppm or less with respect to the separator.

  The temperature of the heat treatment is set to a temperature lower than the shutdown temperature of the separator when the hot-melt resin (C) is included in the separator. When heat treatment is performed at a temperature equal to or higher than the shutdown temperature, the pores of the separator are blocked, so that the lithium secondary battery using such a separator is inferior in characteristics. In the case of a separator that does not contain a heat-meltable resin (C) in the separator and ensures the provision of shutdown characteristics with the swellable resin (D), as described above, the characteristics of the separator even if heat treatment is performed in a dry state. The heat treatment temperature is not particularly limited as long as the temperature is lower than the thermal decomposition temperature of the resin.

  The specific heat treatment temperature is, for example, 70 to 140 ° C., and the heat treatment time is, for example, 1 hour or more, more preferably 3 hours or more, and 72 hours or less, more preferably 24 hours or less. It is desirable to do. Such heat treatment can be performed, for example, in a warm air circulation type thermostatic bath. Moreover, you may dry under reduced pressure using a vacuum dryer as needed.

  The separator of the present invention can be widely used as a separator for nonaqueous electrolyte batteries regardless of whether it is a primary battery or a secondary battery. Hereinafter, a secondary battery, which is the main application of the separator of the present invention, will be described.

  The battery (lithium secondary battery) of the present invention is not particularly limited as long as it has the separator of the present invention, 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.

The positive electrode is not particularly limited as long as it is a positive electrode used in a conventionally known nonaqueous electrolyte battery (lithium secondary battery). 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. Finished positive electrode mixture into a molded body with the current collector as the core material 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 nonaqueous electrolyte battery (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 laminate in which the above positive electrode and the above negative electrode are laminated via the separator of the present invention, or an electrode wound 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.

<Manufacture of composite fine particles>
Production Example 1
Silica (“FS-3DC (trade name)” manufactured by Denki Kagaku Kogyo Co., Ltd., average particle size 3 μm) 100 g as heat-resistant fine particles (A), and cross-linked PMMA (manufactured by Ganz Kasei Co., average particle size) as swellable resin (D) 0.4 μm, B _R = 0.5, B = 4] 25 g were used, and these were treated for 10 minutes using a planetary ball mill to produce composite fine particles (B-1). With respect to the obtained composite fine particles (B-1), the volume content of silica calculated with a specific gravity of silica of 2.2 g / cm ³ and a specific gravity of crosslinked PMMA of 1.2 g / cm ³ is 68.6%.

Production Example 2
Plate-like boehmite [BMM (trade name) manufactured by Kawai Lime Co., Ltd., average particle size 1 μm, aspect ratio 10] 100 g as heat-resistant fine particles (A), and cross-linked PMMA (manufactured by Ganz Kasei Co., Ltd.) Using an average particle diameter of 0.4 μm, B _R = 0.5, B = 4] 20 g, and using Mechanofusion (manufactured by Hosokawa Micron Corporation), these were treated for 5 minutes to obtain composite fine particles (B-2). Produced. With respect to the obtained composite fine particles (B-2), the volume content of the plate boehmite calculated with a specific gravity of the plate boehmite of 3.0 g / cm ³ and a specific gravity of the crosslinked PMMA of 1.2 g / cm ³ was 66.7. %.

Production Example 3
100 g of plate-like alumina [“Seraph 02025 (trade name)” manufactured by Kinsei Matec Co., Ltd., average particle diameter of 2 μm, aspect ratio of 25] as heat-resistant fine particles (A) was dispersed in 200 g of water to prepare a slurry. The above slurry and an emulsion containing PE as the heat-meltable resin (C) [60 Chemipearl W-900 (trade name) manufactured by Mitsui Chemicals, average particle size 0.6 μm] are combined using a spray dryer. To produce composite fine particles (B-3). About the obtained composite fine particles (B-3), the volume content of the plate-like alumina calculated by setting the specific gravity of plate-like alumina to 4.0 g / cm ³ and the specific gravity of PE to 1.0 g / cm ³ is 51.0%. It is.

Production Example 4
100 g of alumina (Al ₂ O ₃ ) fine particles [“SUMIKORANDAN AA1.5 (trade name)” manufactured by Sumitomo Chemical Co., Ltd.], which is a heat-resistant fine particle (A), are added to a silane coupling agent (3-methacryloxypropyltriethoxysilane). After adding 1 g to 100 g of water in which 1 g was dissolved and stirring at room temperature for 1 hour, filtration and drying were performed to prepare alumina having a methacryl group on the surface. Alumina prepared above is dispersed in toluene, 50 g of methyl methacrylate monomer is dropped into this under an inert gas stream, and polymerization is carried out at 60 ° C. for 4 hours using azoisobutyronitrile (AIBN) as a polymerization initiator. The product was filtered, washed, and dried to produce composite fine particles (B-4) in which PMMA chains were grafted on the surface of the alumina fine particles. With respect to the composite fine particles (B-4), the weight ratio of alumina to PMMA was measured by thermogravimetric analysis (TGA). As a result, alumina / PMMA = 10/1. Moreover, about the obtained composite microparticles | fine-particles (B-4), the volume content rate of the alumina calculated with the specific gravity of alumina being 4.0 g / cm ³ and the specific gravity of PMMA being 1.2 g / cm ³ is 75.0%. .

Example 1
1000 g of composite fine particles (B-1) prepared in Production Example 1 above as composite fine particles (B), 800 g of water, 200 g of isopropyl alcohol (IPA), and PVB [S-LEK KX-5 (Sekisui Chemical Co., Ltd.) as binder (F) 375 g of product name) ”] was put into a container and dispersed by stirring for 1 hour with a three-one motor to obtain a uniform slurry. A non-woven fabric made of PP having a thickness of 15 μm (manufactured by Nippon Kogyo Paper Co., Ltd.) was passed through this slurry, and the slurry was applied by pulling and then dried to obtain a separator having a thickness of 20 μm.

The separator of Example 1, the composite fine particles (B-1) a specific gravity of 1.89 g / cm ^3, the specific gravity of the binder 1.1 g / ^cm 3, the specific gravity of PP according to the PP non-woven fabric 0.9 g / cm ³ The volume content of the composite fine particles (B-1) calculated as follows is 28.5%.

Example 2
A separator was produced in the same manner as in Example 1 except that the composite fine particles (B-2) produced in Production Example 2 were used as the composite fine particles (B).

For the separator of Example 2, the specific gravity of the composite fine particles (B-2) is 2.4 g / cm ³ , the specific gravity of the binder is 1.1 g / cm ³ , and the specific gravity of PP according to the PP nonwoven fabric is 0.9 g / cm ^3. As a result, the volume content of the composite fine particles (B-2) is 27.7%.

Example 3
A separator was produced in the same manner as in Example 1 except that the composite fine particles (B-3) produced in Production Example 3 were used as the composite fine particles (B).

The separator of Example 3, the composite fine particles (B-3) a specific gravity of 3.53 g / cm ^3, the specific gravity of the binder 1.1 g / ^cm 3, the specific gravity of PP according to the PP non-woven fabric 0.9 g / cm ³ As a result, the volume content of the composite fine particles (B-3) is 28.1%.

Example 4
A slurry containing the composite fine particles and the binder (F) was prepared in the same manner as in Example 1 except that the composite fine particles (B-4) prepared in Production Example 4 were used as the composite fine particles (B). To this slurry, 300 g of an emulsion containing PE as a heat-meltable resin (C) [“Chemipearl W-700 (trade name)” manufactured by Mitsui Chemicals, average particle diameter of 1 μm] was added, and a slurry containing plate-like silica [ 100 g of “Sun Lovely LFS (trade name)” manufactured by Dokai Chemical Co., Ltd.] was added and stirred uniformly to prepare a slurry. This slurry was applied to a non-woven fabric made of PET [made by Freudenberg, thickness 15 μm] using a reverse roll coater and dried to obtain a separator having a thickness of 16 μm.

For the separator of Example 4, the specific gravity of the composite fine particles (B-4) was 3.3 g / cm ³ , the specific gravity of the binder was 1.1 g / cm ³ , the specific gravity of PE was 1.0 g / cm ³ , and the specific gravity of silica was The volume content of the composite fine particles (B-4) calculated as 2.2 g / cm ³ and the specific gravity of PET according to the PET nonwoven fabric is 1.38 g / cm ³ is 29.0%.

Comparative Example 1
As heat-resistant fine particles (A), 1000 g of alumina (Al ₂ O ₃ ) fine particles used in Production Example 4, 800 g of water, 200 g of IPA, and PVB as a binder (F) [“SREC KX-5 (trade name) manufactured by Sekisui Chemical Co., Ltd.” ] 375 g was put into a container and dispersed by stirring for 1 hour with a three-one motor to obtain a uniform slurry. To this slurry, 800 g of an emulsion containing PE as a heat-meltable resin (C) [“Chemical Pearl W-700 (trade name)” manufactured by Mitsui Chemicals, average particle size: 1 μm] was added, and PP having a thickness of 15 μm was added thereto. A non-woven fabric (manufactured by Nippon Kogyo Paper Co., Ltd.) was passed through, 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 20 μm.

<Heating characteristics of separator>
The separators of Examples 1 to 4 were left in a constant temperature bath at 150 ° C. for 30 minutes, and the shrinkage rate of the separators was measured. Further, as a separator for comparison, the separator of Comparative Example 1 and a 20 μm-thick PE microporous membrane (Comparative Example 2) were also left in a thermostatic bath at 150 ° C. for 30 minutes, and were the same as the separators of Examples 1 to 4. Was measured. The results are shown in Table 1.

  The shrinkage rate was measured as follows. A separator piece cut out to 4 cm × 4 cm was 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 separator piece was measured. The rate of decrease in length compared to the length was taken as the shrinkage rate.

Moreover, the shutdown temperature was calculated | required with the following method. Each separator piece cut to a size of 4cm x 4cm is sandwiched between two stainless plates with terminals, inserted into a bag of aluminum laminate film, injected with non-aqueous electrolyte, and then the tip of the terminal is put into the bag The bag was sealed in a state where it was taken out of the bag to prepare a test sample. Here, as the non-aqueous electrolyte, a solution in which LiPF ₆ was dissolved at a concentration of 1.2 mol / l in a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 2 was used. The sample was placed in a thermostatic bath, and the resistance value when a 1 kHz alternating current was applied to the terminal with a contact resistance meter [3560 AC milliohm HiTester (trade name)] manufactured by HIOKI was measured from room temperature to 1 per minute. The temperature was raised at a rate of 0 ° C. and heated, and the temperature change of the internal resistance was determined. The temperature at which the resistance value was 10 times or more the value at room temperature was taken as the shutdown temperature of the separator.

  Table 1 shows the following. The separator of Comparative Example 2 corresponding to the conventional product has a large heat shrinkage rate. In the battery using this, the separator shrinks until the internal temperature reaches 150 ° C., and the positive electrode and the negative electrode are in contact with each other. There is a risk of short circuit. On the other hand, in the separators of Examples 1 to 4, almost no thermal shrinkage was observed, and substantially no deformation occurred at the visual level. Therefore, in the battery using these, even if the internal temperature reaches 150 ° C., the separator can sufficiently prevent the contact between the positive electrode and the negative electrode, thereby preventing the occurrence of a short circuit.

Example 5
<Production of negative electrode>
A negative electrode active material-containing paste is prepared by mixing 95 parts by mass of graphite as a negative electrode active material and 5 parts by mass of PVDF as a binder with N-methyl-2-pyrrolidone (NMP) as a solvent in a uniform manner. Prepared. This negative electrode mixture-containing paste was intermittently applied to both sides of a 10 μm thick current collector made of copper foil so that the active material application length was 320 mm on the front surface and 260 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 330 mm and a width of 45 mm. Further, a tab was welded to the exposed portion of the copper foil of the negative electrode to form a lead portion.

<Preparation of positive electrode>
The positive electrode active material LiCoO ₂ : 85 parts by mass, the conductive auxiliary agent acetylene black: 10 parts by mass, and the binder PVDF: 5 parts by mass are mixed uniformly using NMP as a solvent. An agent-containing paste was prepared. This paste was intermittently applied on both sides of a 15 μm thick aluminum foil serving as a current collector so that the active material application length was 319 to 320 mm on the front surface and 258 to 260 mm on the back surface, dried, and then subjected to calendar treatment. The thickness of the positive electrode mixture layer was adjusted so that the total thickness was 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 330 mm and a width of 43 mm. Further, a tab was welded to the exposed portion of the aluminum foil of the positive electrode to form a lead portion.

<Battery assembly>
The positive electrode and the negative electrode obtained as described above were spirally wound through the separator of Example 1 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.

Examples 6-8, Comparative Examples 3-4
A lithium secondary battery was produced in the same manner as in Example 1 except that the separator was changed to those in Examples 2 to 4 or Comparative Examples 1 and 2.

Example 9
The same slurry as prepared in Example 1 was applied on the same negative electrode as that prepared in Example 5 using a die coater so that the thickness when dried was 15 μm, and dried. An integrated product was obtained. The same positive electrode as that produced in Example 5 was layered on the separator side of the obtained integrated negative electrode and separator, and wound into a spiral shape to obtain a wound electrode body. A lithium secondary battery was produced in the same manner as in Example 5 except that this wound electrode body was used.

Example 10
The same slurry as prepared in Example 1 was applied onto the same positive electrode as that prepared in Example 5 using a die coater so that the thickness when dried was 10 μm, and dried. An integrated product was obtained. On the separator side of the obtained positive electrode and separator integrated product, the same negative electrode as that produced in Example 5 was layered and wound into a spiral shape to obtain a wound electrode body. A lithium secondary battery was produced in the same manner as in Example 5 except that this wound electrode body was used.

  The nonaqueous electrolyte batteries of Examples 5 to 10 and Comparative Examples 3 to 4 were subjected to electrochemical evaluation (discharge capacity ratio measurement) and safety evaluation. The results are shown in Table 2.

<Discharge capacity ratio>
About the batteries of Examples 5 to 10 or Comparative Examples 3 to 4, constant current charging at 0.2C (up to 4.2V) and charging by constant voltage charging at 4.2V (total of constant current charging and constant voltage charging) After 15 hours), the battery was discharged at 0.2 C up to 3.0 V, and charge / discharge efficiency (ratio of discharge capacity to charge capacity) was determined. In addition, charging / discharging efficiency was calculated | required as an average value of 10 batteries, respectively.

<Safety evaluation>
The batteries of Examples 5 to 10 or Comparative Examples 3 to 4 are allowed to stand in a constant temperature bath at 150 ° C. for 1 hour until the temperature of the battery rises by 30 ° C. or more from the temperature in the constant temperature bath (abnormal temperature rise). Time was measured to evaluate safety at high temperatures.

  As can be seen from Table 2, in the batteries of Examples 5 to 10, it was difficult to cause an internal short circuit, and high charge / discharge efficiency was obtained. On the other hand, since the battery of Comparative Example 3 caused an internal short circuit, the charge / discharge efficiency was lower than that of the battery of Example. Moreover, in the batteries of Examples 5 to 10, it was found that abnormal temperature rise hardly occurred when held at a high temperature, and the safety was improved as compared with the battery of Comparative Example 4 which is a conventionally known battery.

Claims (11)

At least a part of the surface of the heat-resistant fine particles is selected from polyethylene, a copolymerized polyolefin having a structural unit derived from ethylene of 85 mol% or more, polypropylene, a polyolefin derivative, a polyolefin wax, a petroleum wax, and a carnauba wax. Containing composite fine particles coated with a meltable resin, and a binder different from the heat meltable resin ,
A porous film formed by binding the composite fine particles with the binder,
The battery separator, wherein a ratio of the heat-resistant fine particles in the composite fine particles is 50% or more by volume.   The battery separator according to claim 1, wherein the heat-resistant fine particles are inorganic fine particles.   The battery separator according to claim 2, wherein at least a part of the heat-resistant fine particles is boehmite.   The battery separator according to any one of claims 1 to 3, which has a thermal shrinkage at 150 ° C of less than 5%.   Furthermore, the separator for batteries in any one of Claims 1-4 containing a fibrous material.   The battery separator according to claim 5, wherein a part or all of the composite fine particles are present in a void of a sheet-like material composed of a fibrous material.   The battery separator according to any one of claims 1 to 6, wherein the resin covering at least a part of the surface of the heat-resistant fine particles is a heat-meltable resin that melts at 80 to 130 ° C. Composite particles formed by coating at least a part of the surface of the heat-resistant fine particles with a swellable resin that can swell at least in a non-aqueous electrolyte and increase the degree of swelling as the temperature rises, and the swellable resin Containing a different binder, a porous film formed by binding the composite fine particles with the binder,
The battery separator is characterized in that the swellable resin is at least one selected from cross-linked polystyrene, cross-linked acrylic resin and cross-linked fluororesin. The battery separator according to claim 8, wherein the swelling resin has a swelling degree B defined by the following formula of 1.0 or more.
_{B = (V 1 / V 0} ) -1
[In the above formula, V ₀ is the volume of the resin (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 resin after 1 hour at 120 ° C. 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 battery separator according to any one of claims 1 to 9. Lithium secondary battery.   The lithium secondary battery according to claim 10, wherein the battery separator is integrated with at least one of the positive electrode and the negative electrode.