JP4737949B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  In recent years, electronic devices such as mobile communication devices, notebook computers, palmtop computers, integrated video cameras, portable CD (MD) players, cordless telephones, etc. have been reduced in size and weight. As a power source, a battery having a small size and a large capacity is particularly demanded.

  Examples of batteries that are widely used as power sources for these electronic devices include primary batteries such as alkaline manganese batteries, and secondary batteries such as nickel cadmium batteries and lead storage batteries. Among these, non-aqueous electrolyte secondary batteries that use lithium composite oxide for the positive electrode and carbonaceous materials that can occlude and release lithium ions for the negative electrode are small and light, have high unit cell voltage, and have high energy density. Is attracting attention.

  Non-aqueous electrolytes used in non-aqueous electrolyte secondary batteries have a lower electrical conductivity than aqueous electrolytes, so this secondary battery is larger than secondary batteries that use aqueous electrolytes. Inferior charge / discharge characteristics with current. For this reason, a positive electrode and a negative electrode, which are wound in a spiral shape with a separator or the like interposed therebetween, are used as an electrode group to increase the reaction area of the positive electrode and the negative electrode and compensate for charge / discharge characteristics at a large current. ing. Cylindrical and flat electrode groups are known as spiral electrode groups. Of these, flat electrode groups can reduce the thickness of secondary batteries, but lithium dendrite There is a problem that it tends to precipitate.

  In Patent Document 1, the porosity of the electrode mixture on both sides of the current collector in at least one of the positive electrode and the negative electrode is varied, and the positive electrode and the negative electrode are arranged with the surface having a large porosity of the electrode mixture on the outside. By winding the surface of the electrode mixture with a small porosity inside and winding it through the separator, the concentration of the current density on the negative electrode in the bent part near the core of the flat electrode group is relaxed, It is described that the deposition of metallic lithium on the negative electrode surface is avoided.

In a flat electrode group, the portion where the electrode plate group is laminated in parallel occupies a larger proportion than the bent portion or the curved portion of the electrode plate group. In the part where the groups are stacked in parallel, the reaction area of the positive electrode and the negative electrode is in a relationship of approximately 1: 1. Therefore, as in Patent Document 1, the porosity of the electrode mixture on one side of the current collector and the other If the porosity of the electrode mixture on this surface is made different, current tends to concentrate on the electrode mixture on one side, and overdischarge or overcharge tends to occur, and a long charge / discharge cycle life cannot be obtained.
JP 2000-90981 A

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery having an improved charge / discharge cycle life.

A nonaqueous electrolyte secondary battery according to the present invention includes a flat electrode group in which a positive electrode and a negative electrode are wound in a spiral shape with a separator interposed therebetween, and a nonaqueous electrolyte that is held by the electrode group. In the next battery,
In the separator, the porosity of the portion where the negative electrode is located on the outer peripheral side and opposed to the positive electrode is defined as A (%), and the negative electrode is located on the inner peripheral side and opposed to the positive electrode. a porosity of a portion are upon the B (%), satisfies the following formula (1),
The porosity A is 50% or more and 60% or less, and the porosity B is 35% or more and 45% or less,
Basis weight of the separator 6 g / m ² or more, characterized in der Rukoto 12 g / m ² or less.

        1.11 ≦ (A / B) ≦ 1.71 (1)

  According to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery having an improved charge / discharge cycle life.

  An embodiment of a non-aqueous electrolyte secondary battery according to the present invention will be described with reference to FIGS. FIG. 1 is a perspective view showing a thin nonaqueous electrolyte secondary battery which is an embodiment of the nonaqueous electrolyte secondary battery according to the present invention. FIG. 2 shows the thin nonaqueous electrolyte secondary battery of FIG. FIG. 3 is a partial cutaway perspective view showing a rectangular nonaqueous electrolyte secondary battery which is an embodiment of the nonaqueous electrolyte secondary battery according to the present invention, and FIG. FIG. 5 is a schematic view showing an example of a flat electrode group incorporated in the nonaqueous electrolyte secondary battery of FIGS. 1 to 3, and FIG. 5 is a schematic view for explaining a method for producing the electrode group of FIG. 4.

  The nonaqueous electrolyte secondary battery according to the present invention can be applied to various forms of nonaqueous electrolyte secondary batteries such as thin and rectangular shapes. An example of a thin and square nonaqueous electrolyte secondary battery is shown in FIGS.

  First, a thin nonaqueous electrolyte secondary battery will be described.

  As shown in FIG. 1, a flat electrode group 2 is accommodated in a container 1 having a rectangular cup shape. The flat electrode group 2 has a structure in which a laminate including a positive electrode 3, a negative electrode 4, and a separator 5 disposed between the positive electrode 3 and the negative electrode 4 is wound so as to form a flat spiral. The nonaqueous electrolyte is held in the electrode group 2. The lid plate 6 is integrated with the container 1. The container 1 and the cover plate 6 are each composed of a laminate film. The laminate film includes an external protective layer 7, an internal protective layer 8 containing a thermoplastic resin, and a metal layer 9 disposed between the external protective layer 7 and the internal protective layer 8. A lid 6 is fixed to the container 1 by heat sealing using a thermoplastic resin of the inner protective layer 8, whereby the electrode group 2 is sealed in the container. A positive electrode tab 10 is electrically connected to the positive electrode 3, and a negative electrode tab 11 is electrically connected to the negative electrode 4, and each is drawn out of the container and serves as a positive electrode terminal and a negative electrode terminal.

  In the thin non-aqueous electrolyte secondary battery illustrated in FIGS. 1 and 2, an example using a cup-shaped container has been described, but the shape of the container is not particularly limited, and can be, for example, a bag shape. .

  Next, the prismatic nonaqueous electrolyte secondary battery will be described.

  As shown in FIG. 3, the electrode group 2 is accommodated in a bottomed rectangular cylindrical container 12 made of metal such as aluminum. In the electrode group 2, a laminate including the positive electrode 3, the separator 5, and the negative electrode 4 is laminated in this order and wound in a flat shape. The spacer 13 having an opening near the center is arranged above the electrode group 2.

  The nonaqueous electrolyte is held in the electrode group 2. A sealing plate 15 having a liquid injection port 14 and having a circular hole opened near the center is laser welded to the opening of the container 12. The liquid injection port 14 is covered with a sealing lid (not shown). The negative electrode terminal 16 is disposed in a circular hole of the sealing plate 15 via a hermetic seal. The negative electrode tab 11 drawn out from the negative electrode 4 is welded to the lower end of the negative electrode terminal 16. On the other hand, a positive electrode tab (not shown) is connected to a container 12 that also serves as a positive electrode terminal.

The electrode group 2 of these secondary batteries will be described with reference to FIGS. As shown in FIG. 5, the positive electrode 3 included in the electrode group 2 includes a positive electrode current collector 3a and a positive electrode active material-containing layer 3b carried on the positive electrode current collector 3a. Maki start portion 3 ₁ of the positive electrode 3 active material-containing layer is in unsupported, the positive electrode active material-containing layer 3b only one side of the seeded end portion 3 _2, the positive electrode current collector 3a constituting the outermost periphery is supported. The point between the sown start portion 3 ₁ and Maki end portion 3 _2, the positive electrode active material-containing layer 3b is carried on both faces of a cathode current collector 3a. On the other hand, the negative electrode 4 includes a negative electrode current collector 4a and a negative electrode active material-containing layer 4b supported on the negative electrode current collector 4a. Maki start portion 4 ₁ of the negative electrode 4 in the active material-containing layer is unsupported, the anode active material-containing layer 4b on one surface of the positive electrode sown start portion 3 ₁ and the facing portion 4 ₂ in the negative electrode current collector 4a is supported . The locations 4 ₂ subsequent negative electrode 4, the negative electrode active material-containing layer 4b is supported on both sides of the negative electrode current collector 4a.

  The separator 5 includes a separator 5A having a high porosity and a separator 5B having a low porosity. Between the porosity A (%) of the separator 5A and the porosity B (%) of the separator 5B, The relationship of the following (1) Formula is materialized.

1.11 ≦ (A / B) ≦ 1.71 (1)
Above the electrode group 2 sandwiches were seeded start portion 4 ₁ of the negative electrode 4 between the separator 5A and the separator 5B, these tabular two Maki core 17a, in the direction of arrow sandwiched between 17b after Maki, superimposed Maki start portion 3 ₁ of the positive electrode 3 in the separator 5A, obtained by winding them into a flat shape.

The innermost electrode group 2, a negative electrode Maki start portion 4 ₁ sandwiched between separators 5A and the separator 5B are located. Cathode Maki start portion 3 ₁ is wound with a slight delay from the negative electrode 4. In the positive and negative electrode starting portions, the positive electrode active material-containing layer 3b and the negative electrode active material-containing layer 4b are opposed to each other via the separator 5A or the separator 5B. Further, the end portion 3 ₂ of the positive electrode 3 is located on the outermost periphery of the electrode group 2.

  The separator 5A is disposed between the positive electrode 3 and the negative electrode 4 facing this on the outer peripheral side. On the other hand, the separator 5B is disposed between the positive electrode 3 and the negative electrode 4 facing this on the inner peripheral side.

The positive electrode tab 10 is welded to the inner peripheral surface of the starting portion 3 ₁ of the positive electrode 3 (that is, the positive electrode current collector 3a). On the other hand, the negative electrode tab 11 is welded to the inner peripheral surface of the starting start portion 4 _{1 of the} negative electrode 4 (that is, the negative electrode current collector 4a). The reinforcing tape 18 for preventing the positive electrode tab 10 from penetrating the separator 5B and contacting the negative electrode current collector 4a covers the surface of the positive electrode tab 10 of the positive electrode current collector 3a. Reinforcing tape 19 for preventing positive electrode tab 10 from penetrating back surface separator 5A and contacting negative electrode 4 is attached to negative electrode current collector 4a on the back surface side of positive electrode tab 10. Moreover, Maki sealing tape 20 is to prevent deviation Maki by fixing the short side of the end portion 3 ₂ Maki positive electrode at the outermost periphery surface of the electrode group 2.

The negative electrode located on the outer peripheral side when viewed from the positive electrode 3 in the R portions at both ends of the electrode group 2 having a flat shape as exemplified in FIG. 4 described above, for example, the R portion surrounded by the straight lines R ₁ and R _2. 4, since the negative electrode 4 has a longer arc than the positive electrode 3, the negative electrode reaction surface is larger than the positive electrode 3 reaction surface. Therefore, for the positive electrode active material-containing layer 3b on the outer periphery side of the positive electrode 3 and the negative electrode active material-containing layer 4b opposite to the positive electrode active material-containing layer 4b, the non-aqueous electrolyte consumption of the negative electrode active material-containing layer 4b is More than electrolyte consumption. As a result of excessive consumption, when the non-aqueous electrolyte of the negative electrode active material-containing layer 4b is depleted, lithium is deposited on the surface, leading to a decrease in charge / discharge cycle life.

  On the other hand, for the negative electrode 4 positioned on the inner peripheral side when viewed from the positive electrode 3, the negative electrode 4 has a shorter arc than the positive electrode 3, and therefore the nonaqueous electrolyte consumption of the negative electrode active material-containing layer 4 b The amount of nonaqueous electrolyte consumption of the layer 3b is hardly increased.

  By setting the porosity ratio (A / B) of the separator 5A and the separator 5B to 1.11 or more, the amount of the original nonaqueous electrolyte retained by the negative electrode active material-containing layer 4b facing the outer peripheral side of the positive electrode is increased. Therefore, the negative electrode active material-containing layer 4b can be prevented from depleting the nonaqueous electrolyte. As a result, since lithium electrodeposition can be suppressed, the charge / discharge cycle life can be improved.

  However, if the porosity ratio (A / B) exceeds 1.71, the strength of the separator 5A is reduced, or the nonaqueous electrolyte retention amount of the separator 5B is insufficient, so that a long charge / discharge cycle life can be obtained. Disappear. Therefore, the porosity ratio (A / B) is desirably 1.11 or more and 1.71 or less. A more preferable range of the porosity ratio (A / B) is 1.3 or more and 1.5 or less.

  Desirably, the porosity A of the separator 5A is 50% or more and 60% or less, and the porosity B of the separator 5B is 35% or more and 45% or less. This is due to the reason explained below. When the porosity A of the separator 5A is less than 50%, the nonaqueous electrolyte retention amount of the separator 5A is insufficient even if the porosity ratio (A / B) is 1.11 or more and 1.71 or less. There is a fear. On the other hand, if the porosity A of the separator 5A exceeds 60%, the separator must be thickened to maintain the strength, which may hinder high energy density, and non- This is because the effect of improving water electrolyte retention reaches almost saturation.

  If the porosity B of the separator 5B is less than 35%, the charge / discharge reaction between the positive electrode active material-containing layer 3b and the negative electrode active material-containing layer 4b facing each other with the separator 5B interposed therebetween may not occur smoothly. On the other hand, when the porosity B of the separator 5B exceeds 45%, the porosity A satisfying the porosity ratio (A / B) of 1.11 or more and 1.71 or less becomes less than 50%, or Greater than 60%.

  Therefore, by making the porosity A of the separator 5A to be 50% or more and 60% or less and the porosity B of the separator 5B to be 35% or more and 45% or less, while causing a charge / discharge reaction smoothly, The depletion of the nonaqueous electrolyte in the negative electrode active material-containing layer facing the positive electrode on the outer peripheral side can be suppressed.

The basis weight of the separator 5A and the basis weight of the separator 5B are desirably 6 g / m ² or more and 12 g / m ² or less, respectively. If there is a difference between the basis weight of the separator 5A and the basis weight of the separator 5B, the nonaqueous electrolyte diffusion rate from the separator 5A to the electrode and the nonaqueous electrolyte diffusion rate from the separator 5B to the electrode are different. Even if A / B) is within the range of the formula (1), the nonaqueous electrolyte may be depleted. By setting the basis weight within the above range, the difference in the non-aqueous electrolyte diffusion rate between the separators 5A and 5B can be kept within the allowable range, thereby avoiding depletion of the non-aqueous electrolyte and improving the charge / discharge cycle life. can do.

  As the separators 5A and 5B, a microporous film, a woven fabric, a non-woven fabric, or a laminate of the same material or different materials among these can be used.

  Examples of the material forming the separators 5A and 5B include polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-butene copolymer. As a material for forming the separator, one type or two or more types selected from the types described above can be used. Among these, polyethylene is preferable.

  The thickness of the separators 5A and 5B is preferably 30 μm or less, and more preferably 25 μm or less. Moreover, it is preferable that the lower limit of thickness is 5 micrometers, and a more preferable lower limit is 8 micrometers.

  It is desirable that the flat electrode group satisfies the following formula (2).

0.6 ≦ (L ₂ / L ₁ ) ≦ 0.9 (2)
However, L ₁ is the length of the spiral surface of the electrode group, and L ₂ is the length of the flat portion of the spiral surface of the electrode group. The spiral surface of the electrode group is a surface that intersects perpendicularly to the winding axis of the electrode group. The length of the spiral surface means the longer width of the spiral surface. Moreover, the flat part of a spiral surface means the location where the positive electrode, the negative electrode, and the separator are laminated | stacked substantially parallel. The length of the flat portion is determined by subtracting the length of the R portion of the positive electrode and the negative electrode and the separator are curved in an arc or U-shape from L ₁ both end portions.

When (L ₂ / L ₁ ) exceeds 0.9, since the ratio of the R portion in the electrode group is low, the cycle life can be improved even if the porosity ratio (A / B) of the separator is specified. It may not be possible. On the other hand, if (L ₂ / L ₁ ) is less than 0.6, there is a possibility that the non-aqueous electrolyte consumption bias cannot be corrected by the definition of the porosity ratio (A / B) of the separator. A more preferable range of (L ₂ / L ₁ ) is 0.7 or more and 0.8 or less.

  The electrode group 2 is manufactured by winding the positive electrode 3 and the negative electrode 4 in a flat shape with the separator 5 interposed as described above. It is possible to press to increase the integrated strength. It is also possible to apply heating during pressing.

  The electrode group 2 can contain an adhesive polymer in order to increase the integrated strength of the positive electrode 3, the negative electrode 4 and the separator 5. Examples of the adhesive polymer include polyacrylonitrile (PAN), polyacrylate (PMMA), polyvinylidene fluoride (PVdF), polyvinyl chloride (PVC), and polyethylene oxide (PEO).

  In FIGS. 4 to 5 described above, the example in which the outermost periphery of the electrode group is the positive electrode has been described. Further, the outermost periphery of the electrode group can be a separator.

  Hereinafter, the positive electrode 3, the negative electrode 4, and the nonaqueous electrolyte will be described.

1) Positive electrode 3
The positive electrode includes a positive electrode current collector 3a and an active material containing layer 3b supported on one or both surfaces of the current collector 3a.

Examples of the positive electrode active material include various metal oxides such as manganese dioxide, lithium manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel cobalt oxide, lithium-containing iron oxide, and lithium. Examples thereof include vanadium oxide and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Among them, when a lithium-containing cobalt oxide (for example, LiCoO ₂ ), a lithium-containing nickel cobalt oxide (for example, LiNi _0.8 Co _0.2 O ₂ ), or a lithium manganese composite oxide (for example, LiMn ₂ O ₄ , LiMnO ₂ ) is used. This is preferable because a high voltage can be obtained. As the positive electrode active material, one kind of oxide may be used alone, or two or more kinds of oxides may be mixed and used.

  The active material-containing layer can contain a binder. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethersulfone, ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). be able to.

  The active material-containing layer may contain a conductive agent. Examples of the conductive agent include acetylene black, carbon black, and graphite.

  As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, aluminum, stainless steel, or nickel.

  The positive electrode is produced, for example, by suspending a positive electrode active material, a conductive agent and a binder in an appropriate solvent, applying the suspension to a current collector, drying and pressing.

2) Negative electrode 4
The negative electrode 4 includes a negative electrode current collector 4a and an active material containing layer 4b supported on one or both surfaces of the current collector 4a.

As the negative electrode active material, a material that absorbs and releases lithium ions is used. Examples of the negative electrode active material include graphite materials, carbonaceous materials such as graphite, coke, carbon fiber, spherical carbon, pyrolytic vapor phase carbonaceous material, and fired resin; thermosetting resin, isotropic pitch, mesophase Graphite material obtained by subjecting pitch-based carbon, mesophase pitch-based carbon fiber, mesophase microspheres, etc. (especially mesophase pitch-based carbon fiber is preferable because of its high capacity and charge / discharge cycle characteristics) to 500-3000 ° C Or a carbonaceous material; a chalcogen compound such as titanium disulfide, molybdenum disulfide, or niobium selenide; a light metal such as aluminum, an aluminum alloy, a magnesium alloy, lithium, or a lithium alloy; Among them, it is preferable to use a graphite material having a graphite crystal having a (002) plane spacing _d002 of 0.34 nm or less. A nonaqueous electrolyte secondary battery provided with a negative electrode containing such a graphite material as a negative electrode active material can greatly improve battery capacity and large current discharge characteristics. More preferably, the surface interval _d002 is 0.337 nm or less.

  The active material-containing layer can contain a binder. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like. Can be used.

  As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, copper, stainless steel, or nickel.

  The negative electrode is prepared by, for example, kneading a negative electrode active material and a binder in the presence of a solvent, applying the obtained suspension to a current collector, drying it, and then pressing it once at a desired pressure or 2 It is produced by multi-stage pressing ˜5 times.

3) Nonaqueous electrolyte A nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte (for example, lithium salt) dissolved in the nonaqueous solvent. The form of the non-aqueous electrolyte can be liquid (non-aqueous electrolyte), gel or solid.

  Examples of the non-aqueous solvent include γ-butyrolactone, ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,3-dioxolane, methylsulfolane. , Acetonitrile, propylnitrile, anisole, acetate, propionate, halogenated aromatic hydrocarbon, vinylene carbonate, vinyl ethylene carbonate, phenyl ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, γ-valerolactone, propionic acid Methyl, ethyl propionate, 2-methylfuran, furan, thiophene, catechol carbonate, ethylene sulfite, 12-crown -4, tetraethylene glycol dimethyl ether or the like can be used, and two or more kinds may be mixed and used.

  The non-aqueous solvent preferably contains a surfactant such as trioctyl phosphate (TOP) in order to improve the wettability with the separator. The addition amount of the surfactant is preferably 3% or less, and more preferably in the range of 0.1 to 1%.

Examples of the electrolyte dissolved in the nonaqueous solvent include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), and lithium arsenic hexafluoride. (LiAsF ₆ ), lithium trifluorometasulfonate (LiCF ₃ SO ₃ ), lithium bistrifluoromethylsulfonylimide (LiN (CF ₃ SO ₂ ) ₂ ), lithium bispentafluoroethylsulfonylimide (LiN (C ₂ F ₅ SO ₂ ) Lithium salts such as ₂ ) can be mentioned. The type of electrolyte used can be one type or two or more types.

  The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2.5 mol / L. A more preferable range is 1 to 2 mol / L.

  The amount of the nonaqueous electrolyte is preferably 0.2 to 0.6 g per 100 mAh of battery unit capacity. A more preferable range of the nonaqueous electrolytic mass is 0.25 to 0.55 g / 100 mAh.

[Example]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Example 1)
<Preparation of positive electrode>
First, 90% by weight of lithium cobalt oxide (Li _x CoO ₂ ; X is 0 <X ≦ 1) powder, 5% by weight of acetylene black, and 5% by weight of polyvinylidene fluoride (PVdF) dimethylformamide (DMF) solution was added and mixed to prepare a slurry. The slurry is applied to both surfaces of a current collector made of an aluminum foil having a thickness of 15 μm except for the specific portion shown in FIG. 5 described above, and then dried and pressed to have the structure shown in FIG. 5 described above. A positive electrode was produced.

<Production of negative electrode>
As a carbonaceous material, 95% by weight of powder of mesophase pitch-based carbon fiber (having a (002) plane spacing (d ₀₀₂ ) of 0.336 nm determined by powder X-ray diffraction) heat-treated at 3000 ° C. and carboxymethyl cellulose (CMC) ) And 1.5% by weight of styrene-butadiene rubber (SBR) were mixed in the presence of water to prepare a slurry. A negative electrode having the structure shown in FIG. 5 is applied by applying the slurry to both surfaces of a current collector made of a copper foil having a thickness of 12 μm, excluding the specific part shown in FIG. 5, drying, and pressing. Was made.

The surface spacing d ₀₀₂ of (002) plane of the carbonaceous material were respectively determined by the powder X-ray diffraction spectrum half width midpoint method. At this time, scattering correction such as Lorentz scattering was not performed.

<Separator>
A microporous polyethylene film having the porosity and basis weight shown in Table 1 below was prepared as a separator A having a high porosity. In addition, a microporous polyethylene film having the porosity and basis weight shown in Table 1 below was prepared as a separator B having a low porosity. The ratio (A / B) of the porosity A (%) of the separator A and the porosity B (%) of the separator B is shown in Table 1 below.

  The porosity of the separators A and B was measured by the method described below.

That is, a 10 cm × 10 cm size sample is cut from each separator, and the following equation (3) is used from the volume V (cm ³ ) and weight W (g) of the sample and the density ρ (g / cm ³ ) of the separator constituent material. Calculated. In the present example, the density of polyethylene was defined as density ρ.

Porosity A, B (%) = {V− (W / ρ)} / V × 100 (3)
<Production of electrode group>
A positive electrode tab made of a strip-shaped aluminum foil (thickness: 100 μm) is ultrasonically welded to the current collector at the beginning of the positive electrode, and a negative electrode tab made of a strip-shaped nickel foil (thickness: 100 μm) is used as the current collector at the start of the negative electrode Were ultrasonically welded. Next, a negative electrode is sandwiched between a separator A having a high porosity and a separator B having a low porosity, and these are sandwiched between two flat cores, and then the positive electrode is stacked on the separator A. These were wound into a flat shape and then subjected to a heat press to obtain a flat electrode group having the structure shown in FIG.

The obtained flat electrode group was cut in a direction perpendicular to the winding axis, and the spiral surface of the electrode group was photographed. In the obtained image of the spiral surface, the length L ₁ of the spiral surface and the length L ₂ of the flat portion were measured, and (L ₂ / L ₁ ) was calculated. The results are shown in Table 1 below.

A laminate film having a thickness of 100 μm, in which both sides of an aluminum foil are covered with polyethylene, is formed into a rectangular cup shape by a press, and the same as that used in the calculation of (L ₂ / L ₁ ) in the obtained container. The electrode group produced by various methods was stored.

  Next, the electrode group in the container was vacuum dried at 80 ° C. for 12 hours to remove moisture contained in the electrode group and the laminate film.

<Preparation of non-aqueous electrolyte>
Ethylene carbonate (EC), γ-butyrolactone (GBL), and orthochlorotoluene (o-CT; oxidation potential 4.8 V) are mixed so that the weight ratio (EC: GBL: o-CT) is 35: 60: 5 Thus, a non-aqueous solvent was prepared. Lithium tetrafluoroborate (LiBF ₄ ) was dissolved in the obtained non-aqueous solvent so that its concentration was 1.5 mol / L to prepare a non-aqueous electrolyte.

  After injecting the non-aqueous electrolyte into the electrode group in the container so that the amount per battery capacity 1Ah is 4.8 g and sealing by heat sealing, the structure shown in FIGS. A nonaqueous electrolyte secondary battery having a thickness of 3.6 mm, a width of 35 mm, a height of 62 mm, and a nominal capacity of 0.65 Ah was assembled.

  The following treatment was applied to the non-aqueous electrolyte secondary battery as an initial charge / discharge process. First, constant current / constant voltage charging was performed for 15 hours at room temperature to 4.2 V at 0.2C. Then, it discharged to 3.0V at 0.2C at room temperature, and manufactured the nonaqueous electrolyte secondary battery.

  Here, 1C is a current value necessary for discharging the nominal capacity (Ah) in one hour. Therefore, 0.2 C is a current value necessary for discharging the nominal capacity (Ah) in 5 hours.

(Examples 2-7 and Comparative Examples 1-5)
The porosity of the separator A, the porosity of the separator B, the porosity ratio (A / B), the basis weight of the separators A and B, and the ratio of the flat portion on the spiral surface of the electrode group (L ₂ / L ₁ ) A non-aqueous electrolyte secondary battery having the same configuration as described in Example 1 was manufactured except that the settings were made as shown in Table 1 below. The thicknesses of the separators A and B used in Examples 1 to 7 and Comparative Examples 1 to 5, respectively, were in the range of 20 to 25 μm.

  The obtained secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 5 were subjected to constant current and constant voltage charging with a charging end voltage of 4.2 V for 3 hours at a charging rate of 1.0 C, and then 1.0 C. The charge / discharge cycle for discharging to 3.0 V was repeated at 20 ° C., the discharge capacity at the 500th cycle was measured, the discharge capacity at the first cycle was taken as 100%, the capacity retention rate at 500 cycles was calculated, and the result was Shown in Table 1 below.

  As is clear from Table 1, the secondary batteries of Examples 1 to 7 including the electrode group in which the porosity ratio (A / B) of the separators A and B is 1.11 or more and 1.71 or less are: It can be understood that the capacity retention ratio after 500 cycles is high as compared with the secondary batteries of Comparative Examples 1 to 5 in which the porosity ratio (A / B) is out of the above range.

Among them, the secondary batteries of Examples 1 to 5 in which (L ₂ / L ₁ ) is 0.6 or more and 0.9 or less are those in Example 7 in which (L ₂ / L ₁ ) is out of the above range. Compared to the secondary battery, the capacity retention rate after 500 cycles was high.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The perspective view which shows the thin nonaqueous electrolyte secondary battery which is one Embodiment of the nonaqueous electrolyte secondary battery concerning this invention. The fragmentary sectional view which cut | disconnected the thin nonaqueous electrolyte secondary battery of FIG. 1 along the II-II line. The partial notch perspective view which shows the square nonaqueous electrolyte secondary battery which is one Embodiment of the nonaqueous electrolyte secondary battery which concerns on this invention. The schematic diagram which shows an example of the flat type electrode group integrated in the nonaqueous electrolyte secondary battery of FIGS. FIG. 5 is a schematic diagram for explaining a manufacturing method of the electrode group in FIG. 4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Container, 2 ... Electrode group, 3 ... Positive electrode, 3a ... Positive electrode collector, 3b ... Active material containing layer, 4 ... Negative electrode, 4a ... Negative electrode collector, 4b ... Active material containing layer, 5A, 5B ... Separator , 6 ... lid, 7 ... resin layer, 8 ... thermoplastic resin layer, 9 ... metal layer, 10 ... positive electrode tab, 11 ... negative electrode tab, 18, 19 ... reinforcing tape, 20 ... Maki sealing tape, L ₂ ... electrode The length of the flat portion of the group, L ₁ ... the length of the spiral surface of the electrode group.

Claims (4)

In a non-aqueous electrolyte secondary battery comprising a flat electrode group in which a positive electrode and a negative electrode are spirally wound via a separator, and a non-aqueous electrolyte held in the electrode group,
In the separator, the porosity of the portion where the negative electrode is located on the outer peripheral side and opposed to the positive electrode is defined as A (%), and the negative electrode is located on the inner peripheral side and opposed to the positive electrode. the porosity of a portion are upon the B (%), satisfies the following formula (1),
The porosity A is 50% or more and 60% or less, and the porosity B is 35% or more and 45% or less,
The basis weight of the separator 6 g / m ² or more, a non-aqueous electrolyte secondary battery, characterized der Rukoto 12 g / m ² or less.
1.11 ≦ (A / B) ≦ 1.71 (1) The non-aqueous electrolyte secondary battery according to claim 1, wherein the (A / B) is 1.3 or more and 1.5 or less . The non-aqueous electrolyte secondary battery according to claim 1, wherein the electrode group satisfies the following expression (2).
0.6 ≦ (L ₂ / L ₁ ) ≦ 0.9 (2)
However, L ₁ is the length of the spiral surface of the electrode group, and L ₂ is the length of the flat portion of the spiral surface. The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous electrolyte contains a surfactant.

Images