JP2015022832A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery enhanced in durability against high-rate charge and discharge.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a wound electrode body 200; a battery case housing the wound electrode body 200; and a nonaqueous electrolyte filled in the battery case. The wound electrode body 200 is arranged by putting together and winding: a cathode sheet 220 including a long cathode collector 221, and a cathode active material layer 223 on the long cathode collector 221; an anode sheet 240 including a long anode collector 241, and an anode active material layer 243 on the long anode collector; and separator sheets 262 and 264 so that the separator sheets 262 and 264 are interposed between the cathode sheet 220 and the anode sheet 240. The separator sheets 262 and 264 includes end portions 262E and 264E on opposite sides of the separator sheets 262 and 264 in the direction of a winding axis WL of the wound electrode body 200. The porosity A(%) of the end portions 262E and 264E is larger than the porosity B(%) of center portions 262C and 264C except the end portions 262E and 264E (A>B).

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery with improved durability against high-rate charge / discharge.

  In recent years, lithium ion secondary batteries, nickel metal hydride batteries, and other non-aqueous electrolyte secondary batteries have become increasingly important as power sources for mounting on vehicles or personal computers and portable terminals (for example, Patent Documents 1 and 2). In particular, a lithium ion secondary battery that is lightweight and obtains a high energy density is preferably used as a high-output power source mounted on a vehicle. As a typical form of this type of non-aqueous electrolyte secondary battery, a long positive electrode sheet and a long negative electrode sheet are overlapped with a separator sheet interposed therebetween, and this is used. A structure having a wound wound electrode body is known.

JP 2013-037905 A International Publication No. 2011-096069

  By the way, some uses of the non-aqueous electrolyte secondary battery are assumed to be used in a mode in which charging and discharging at a high rate are repeated. A non-aqueous electrolyte secondary battery used as a power source for vehicles is a typical example of a non-aqueous electrolyte secondary battery in which such a usage mode is assumed. However, even though the conventional non-aqueous electrolyte secondary battery has a relatively high durability against a charge / discharge cycle at a low rate, the performance deteriorates in a charge / discharge pattern that repeats high-rate charge / discharge. It has been known that the battery resistance is likely to increase.

  Patent Document 1 describes a technique for improving the cycle life of a secondary battery by disposing a separator composed of a substrate and a porous layer having different pores between a positive electrode and a negative electrode. However, such technology can improve cycle life at a low rate, but repeatedly performs high-rate charge / discharge (for example, rapid charge / discharge at a level required in a lithium ion secondary battery for a vehicle power source). The durability against the pattern could not be improved.

  This invention is made | formed in view of this point, The main objective is to provide the non-aqueous-electrolyte secondary battery with which durability with respect to high-rate charging / discharging was improved.

  The present inventor continuously repeats discharging and charging at a high rate as expected in a non-aqueous electrolyte secondary battery for a vehicle power source in a non-aqueous electrolyte secondary battery having a wound electrode body. We focused on the phenomenon that the internal resistance increased significantly. Then, the influence which the repetition of this high rate charge / discharge has on the nonaqueous electrolyte secondary battery was analyzed in detail.

  As a result, in a non-aqueous electrolyte secondary battery that has been repeatedly charged and discharged at a high rate, the salt concentration of the non-aqueous electrolyte that has permeated into the wound electrode body is uneven (uneven) depending on the location. The non-aqueous electrolyte and a part of the salt move from the central part of the wound electrode body to both ends (open ends) of the wound electrode body, and from both ends (open ends) of the electrode body It has been found that by moving to the outside, the salt concentration at the center in the winding axis direction of the wound electrode body is lower than at both ends (open ends) (the salt concentration is greatly reduced compared to the initial state). It was.

  Thus, when there is a bias in the distribution of the salt concentration of the non-aqueous electrolyte solution, the battery reaction is relatively slow at a portion where the salt concentration is relatively low, so that the high-rate charge / discharge performance as a whole battery is degraded. Further, since the battery reaction concentrates on the portion where the salt concentration is relatively high, the deterioration of the portion is promoted. Any of these events can be a factor that reduces the durability of the non-aqueous electrolyte secondary battery against a charge / discharge pattern (high rate charge / discharge cycle) that repeats high rate charge / discharge.

  Based on this knowledge, the present invention improves the durability of non-aqueous electrolyte secondary batteries against high-rate charge / discharge cycles by the approach of eliminating or mitigating the uneven distribution of salt concentration in the non-aqueous electrolyte. is there.

  That is, the non-aqueous electrolyte secondary battery provided by the present invention includes a positive electrode sheet having a positive electrode active material layer on a long positive electrode current collector, and a negative electrode active material layer on a long negative electrode current collector. A negative electrode sheet, a wound electrode body wound by overlapping a separator sheet interposed between the positive electrode sheet and the negative electrode sheet, a battery case containing the wound electrode body, and a non-injection injected into the battery case A water electrolyte. The nonaqueous electrolytic solution is stored outside the wound electrode body and includes an excess electrolytic solution that is in contact with an end portion in the winding axis direction of the wound electrode body. And in the winding axis direction, the porosity A (%) of the end portions (both end portions) on both sides of the separator sheet is larger than the porosity B (%) of the intermediate portion excluding the end portions (A> B). .

According to such a configuration, movement of the electrolyte solution between the end portion and the surplus electrolyte solution is easier than movement of the electrolyte solution between the end portion and the intermediate portion of the separator sheet, so even if charging and discharging are repeated at a high rate, Bias (unevenness) hardly occurs in the distribution of salt concentration in the wound electrode body. For this reason, the phenomenon in which the internal resistance increases due to the uneven distribution of the salt concentration in the wound electrode body can be mitigated. In this specification, the porosity of the separator sheet is defined as the apparent density of the separator sheet is V ₁ , the mass thereof is W _1, and the true density of the material constituting the separator sheet (actuality of the material including no pores). When (value obtained by dividing mass W ₁ by volume) is ρ ₁ , it is obtained by (1−W ₁ / ρ ₁ V ₁ ) × 100.

  Here, the separator sheet may have a facing portion facing the positive electrode active material layer and a non-facing portion not facing the positive electrode active material layer in the winding axis direction. In this case, for example, the end may be defined as a range from 0.5 mm to 3.5 mm from the non-opposing part and the boundary between the non-opposing part and the opposing part toward the opposing part. Further, the porosity A at the end portion and the porosity B at the intermediate portion may satisfy the relationship of 2.5 ≦ A−B ≦ 7.5. By providing such a difference in porosity (A−B), the uneven distribution of salt concentration can be better resolved. For example, the porosity A at the end may be 40% to 77.5%, and the porosity B at the middle may be 32.5% to 70%. Further, the porosity A of the end portion may exceed 55%, and the porosity B of the intermediate portion may be 55% or less. By providing such a difference between the porosity A and B, the uneven distribution of the salt concentration can be better solved.

In a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the separator sheet has a Gurley value of 83 (sec / 100 cm ³ ) or less at the end and a Gurley value of 90 (sec. / 100 cm ³ ) or more. When the Gurley value is within such a range, the above-described effects can be better exhibited.

FIG. 1 is a diagram illustrating an example of the structure of a lithium ion secondary battery. FIG. 2 is a view showing a wound electrode body of a lithium ion secondary battery. 3 is a cross-sectional view showing a III-III cross section in FIG. 2. FIG. 4 is a cross-sectional view showing a laminated structure of a positive electrode sheet, a negative electrode sheet, and a separator of a wound electrode body for a lithium ion secondary battery. FIG. 5 is a diagram schematically showing a cross section of the cell used in the test example.

  Hereinafter, a non-aqueous secondary battery according to an embodiment of the present invention will be described with reference to the drawings. Here, a structural example of a non-aqueous secondary battery will be described first, and then a non-aqueous secondary battery (lithium ion secondary battery) according to an embodiment of the present invention will be described in detail. In addition, the same code | symbol is attached | subjected suitably to the member and site | part which show | play the same effect | action. Each drawing is schematically drawn and does not necessarily reflect the real thing. Each drawing shows an example only and does not limit the present invention unless otherwise specified. Here, one structural example of a non-aqueous secondary battery and a non-aqueous secondary battery (lithium ion secondary battery) according to an embodiment of the present invention will be described as appropriate based on common drawings. Yes.

  FIG. 1 shows a lithium ion secondary battery 100 according to an embodiment of the present invention. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 200 and a battery case 300. FIG. 2 is a view showing a wound electrode body 200. FIG. 3 shows a III-III cross section in FIG.

  The lithium ion secondary battery 100 is configured in a flat rectangular battery case (that is, an exterior container) 300 as shown in FIG. As shown in FIG. 2, in the lithium ion secondary battery 100, a flat wound electrode body 200 is housed in a battery case 300 together with a non-aqueous electrolyte (not shown).

≪Battery case 300≫
The battery case 300 has a box-shaped (that is, bottomed rectangular parallelepiped) case main body 320 having an opening at one end (corresponding to the upper end in a normal use state of the battery 100), and is attached to the opening. It is comprised from the sealing board (lid body) 340 which consists of a rectangular-shaped plate member which block | closes an opening part.

  The material of the battery case 300 may be the same as that used in the conventional sealed battery, and there is no particular limitation. A battery case 300 mainly composed of a lightweight metal material having good thermal conductivity is preferable. Examples of such a metal material include aluminum, stainless steel, nickel-plated steel, and the like. The battery case 300 (the case main body 320 and the sealing plate 340) according to the present embodiment is made of aluminum or an alloy mainly composed of aluminum.

  As shown in FIG. 1, a positive electrode terminal 420 and a negative electrode terminal 440 for external connection are formed on the sealing plate 340. A thin safety valve 360 is formed between both terminals 420 and 440 of the sealing plate 340 so as to release the internal pressure when the internal pressure of the battery case 300 rises above a predetermined level.

≪Winded electrode body 200 (electrode body) ≫
As shown in FIG. 2, the wound electrode body 200 includes a long sheet-like positive electrode (positive electrode sheet 220) and a long sheet-like negative electrode (negative electrode sheet 240) similar to the positive electrode sheet 220 in total of two sheets. Long sheet separators (separators 262 and 264).

≪Positive electrode sheet 220≫
The positive electrode sheet 220 includes a strip-shaped positive electrode current collector 221 and a positive electrode active material layer 223. For the positive electrode current collector 221, for example, a metal foil suitable for the positive electrode can be suitably used. In this embodiment, a strip-shaped aluminum foil having a thickness of about 15 μm is used as the positive electrode current collector 221. An uncoated portion 222 is set along an end portion on one side in the width direction of the positive electrode current collector 221. In the illustrated example, the positive electrode active material layer 223 is held on both surfaces of the positive electrode current collector 221 except for the uncoated portion 222 set on the positive electrode current collector 221. The positive electrode active material layer 223 contains a positive electrode active material, a conductive material, and a binder.

As the positive electrode active material, a material used as a positive electrode active material of a lithium ion secondary battery can be used. Examples of the positive electrode active material include LiNiCoMnO ₂ (lithium nickel cobalt manganese composite oxide), LiNiO ₂ (lithium nickelate), LiCoO ₂ (lithium cobaltate), LiMn ₂ O ₄ (lithium manganate), LiFePO ₄ ( Lithium transition metal oxides such as lithium iron phosphate). Here, LiMn ₂ O ₄ has, for example, a spinel structure. LiNiO ₂ and LiCoO ₂ have a layered rock salt structure. LiFePO ₄ has, for example, an olivine structure. LiFePO ₄ having an olivine structure includes, for example, nanometer order particles. Moreover, LiFePO _{4 having} an olivine structure can be further covered with a carbon film.

  For example, carbon black such as acetylene black (AB) and ketjen black and other powdery carbon materials (such as graphite) can be mixed with the positive electrode active material. In addition to the positive electrode active material and the conductive material, a binder such as polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), or carboxymethyl cellulose (CMC) can be added. A positive electrode mixture (paste) can be prepared by dispersing these in a suitable dispersion medium and kneading. The positive electrode active material layer 223 is formed by applying this positive electrode mixture to the positive electrode current collector 221, drying it, and pressing it to a predetermined thickness.

<< Negative Electrode Sheet 240 >>
As shown in FIG. 2, the negative electrode sheet 240 includes a strip-shaped negative electrode current collector 241 and a negative electrode active material layer 243. For the negative electrode current collector 241, for example, a metal foil suitable for the negative electrode can be suitably used. In this embodiment, a strip-shaped copper foil having a thickness of about 10 μm is used for the negative electrode current collector 241. On one side in the width direction of the negative electrode current collector 241, an uncoated portion 242 is set along the end portion. The negative electrode active material layer 243 is held on both surfaces of the negative electrode current collector 241 except for the uncoated portion 242 set on the negative electrode current collector 241. The negative electrode active material layer 243 contains a negative electrode active material, a thickener, a binder, and the like.

  As the negative electrode active material, one type or two or more types of materials conventionally used in lithium ion secondary batteries can be used without any particular limitation. Preferable examples include carbon-based materials such as graphite carbon and amorphous carbon, lithium transition metal oxides, and lithium transition metal nitrides.

  And like a positive electrode, this negative electrode active material is disperse | distributed to suitable dispersion media with binders, such as a polyvinylidene fluoride (PVDF), a styrene butadiene rubber (SBR), a polytetrafluoroethylene (PTFE), and carboxymethylcellulose (CMC). A negative electrode mixture (paste) can be prepared by kneading. The negative electrode active material layer 243 is formed by applying this negative electrode mixture to the negative electrode current collector 241, drying it, and pressing it to a predetermined thickness.

<< Separator sheets 262, 264 >>
As shown in FIGS. 2 and 3, the separator sheets 262 and 264 are members that separate the positive electrode sheet 220 and the negative electrode sheet 240. In this example, the separator sheets 262 and 264 are made of a strip-shaped sheet material having a predetermined width and having a plurality of minute holes. As a material for the separator sheets 262 and 264, for example, a polyolefin-based resin such as polyethylene (PE) or polypropylene (PP) can be suitably used. The structure of the separator sheets 262 and 264 may be a single layer structure or a multilayer structure. Here, the separator sheets 262 and 264 are made of PE resin. As the PE resin, a homopolymer of ethylene is preferably used. The PE resin is a resin containing 50% by mass or more of a repeating unit derived from ethylene, and is a copolymer obtained by polymerizing an α-olefin copolymerizable with ethylene, or at least copolymerizable with ethylene. It may be a copolymer obtained by polymerizing one kind of monomer. Examples of the α-olefin include propylene. Examples of other monomers include conjugated dienes (for example, butadiene) and acrylic acid.

  The separator sheets 262 and 264 are preferably made of PE having a shutdown temperature of about 120 ° C. to 140 ° C. (typically 125 ° C. to 135 ° C.). The shutdown temperature is sufficiently lower than the heat resistant temperature of the battery (for example, about 200 ° C. or higher). Examples of such PE include polyolefins generally referred to as high-density polyethylene or linear (linear) low-density polyethylene. Alternatively, various types of branched polyethylene having medium density and low density may be used. Moreover, additives, such as various plasticizers and antioxidants, can also be contained as needed.

  As the separator sheets 262 and 264, a uniaxially or biaxially stretched porous resin sheet can be suitably used. Among these, a porous resin sheet uniaxially stretched in the longitudinal direction (MD direction: Machine Direction) is particularly preferable because it has an appropriate strength and has little heat shrinkage in the width direction. For example, when a separator sheet having such a uniaxially stretched resin sheet in the longitudinal direction is used, thermal contraction in the longitudinal direction can also be suppressed in an aspect wound with the positive electrode sheet and the negative electrode sheet. Therefore, the porous resin sheet uniaxially stretched in the longitudinal direction is particularly suitable as a material for the separator sheet constituting the wound electrode body.

  The thickness of the separator sheets 262 and 264 is preferably about 10 μm to 30 μm, and more preferably about 10 μm to 25 μm. If the separator sheets 262 and 264 are too thick, the ion conductivity of the separator sheets 262 and 264 may be reduced. On the other hand, if the thickness of the separator sheets 262 and 264 is too small, there is a possibility that a film breakage may occur. The thicknesses of the separator sheets 262 and 264 can be obtained by image analysis of images taken by the SEM.

In the example illustrated in FIG. 2, the separator sheets 262 and 264 are configured by sheet-like members. Separator sheets 262 and 264 may have a layer of particles having insulating properties on the surface thereof. Here, as the particles having insulating properties, inorganic fillers having insulating properties (for example, fillers such as metal oxides and metal hydroxides) or resin particles having insulating properties (for example, particles such as polyethylene and polypropylene). ). In the lithium ion secondary battery 100, the heat resistant porous layer 266 (FIG. 3) containing an inorganic filler is formed on the surface of the separator sheets 262 and 264 on the side facing the positive electrode active material layer 223. The inorganic filler is preferably one that has heat resistance and is electrochemically stable within the battery use range. Preferred examples include alumina (Al ₂ O ₃ ), alumina hydrate (eg boehmite (Al ₂ O ₃ .H ₂ O)), magnesium hydroxide (Mg (OH) ₂ ), magnesium carbonate (MgCO ₃ ), and the like. Inorganic metal compounds are exemplified. One or more of these inorganic metal compound materials can be used. The average particle size of the inorganic filler is preferably about 1 μm to 10 μm.

  In this example, as shown in FIGS. 2 and 3, the width b1 of the negative electrode active material layer 243 is slightly wider than the width a1 of the positive electrode active material layer 223. Furthermore, the widths c1 and c2 of the separator sheets 262 and 264 are slightly wider than the width b1 of the negative electrode active material layer 243 (c1, c2> b1> a1).

≪Electrolyte (nonaqueous electrolyte) ≫
As the electrolytic solution (non-aqueous electrolytic solution), the same non-aqueous electrolytic solution conventionally used for lithium ion secondary batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,3-dioxolane, and the like. One kind or two or more kinds selected from the group can be used. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _{3 and the} like. Lithium salts can be used. As an example, a nonaqueous electrolytic solution in which LiPF ₆ is contained in a mixed solvent of ethylene carbonate and diethyl carbonate (for example, a mass ratio of 1: 1) at a concentration of about 1 mol / L can be given.

  The nonaqueous electrolytic solution is impregnated in the wound electrode body 200 and stored outside the wound electrode body 200, and the winding electrode body 200 has an end (opening end) 200A in the winding axis direction. (Refer FIG. 2) and the excess electrolyte solution which touches. The impregnated electrolytic solution and the surplus electrolytic solution can be moved back and forth through the open end 200A (see FIG. 2) of the wound electrode body 200.

  Hereinafter, the separator sheets 262 and 264 of the lithium ion secondary battery 100 according to an embodiment of the present invention will be described in more detail. As shown in FIGS. 1 to 3, the lithium ion secondary battery 100 includes a positive electrode sheet 220 including a positive electrode active material layer 223 on a long positive electrode current collector 221, and a long negative electrode current collector 241. A negative electrode sheet 240 including a negative electrode active material layer 243 and a wound electrode body 200 obtained by winding and winding separator sheets 262 and 264 interposed between the positive electrode sheet 220 and the negative electrode sheet 240 are provided. In addition, the battery case 300 in which the wound electrode body 200 is accommodated and the nonaqueous electrolyte injected into the battery case 300 are provided. The nonaqueous electrolytic solution is stored outside the wound electrode body 200, and includes a surplus electrolytic solution that contacts the end portion 200A (see FIG. 2) of the wound electrode body 200 in the winding axis direction.

  In the lithium ion secondary battery 100 including the wound electrode body 200 as shown in FIGS. 1 to 3, the present inventor discharges at a high rate as assumed in a lithium ion secondary battery for a vehicle power source. We focused on the phenomenon that the internal resistance rises significantly when charging is repeated continuously. Therefore, the effect of repeated high-rate charge / discharge on the lithium ion secondary battery was analyzed in detail.

  As a result, in the lithium ion secondary battery that repeats the high rate, the non-aqueous electrolyte solution that has permeated the wound electrode body 200 has a non-aqueous electrolyte concentration that is uneven (uneven) depending on the location. As a result, a part of the non-aqueous electrolyte and the lithium salt move from the center part in the winding axis direction of the wound electrode body 200 to both end parts (opening end parts) and from both end parts to the outside of the electrode body. Thus, it has been found that the lithium salt concentration in the central portion of the wound electrode body 200 in the winding axis direction is lower than both end portions (the lithium salt concentration is greatly reduced compared to the initial state).

  If there is a bias in the distribution of the lithium salt concentration of the non-aqueous electrolyte in this way, the battery reaction becomes relatively slow at the portion where the lithium salt concentration is relatively low, so the high rate charge / discharge performance as a whole battery decreases. To do. Further, since the battery reaction concentrates on a portion where the lithium salt concentration is relatively high, deterioration of the portion is promoted. Any of these events can be a factor of reducing the durability of the lithium ion secondary battery against a charge / discharge pattern (high rate charge / discharge cycle) that repeats high rate charge / discharge.

  The present invention proposes a novel structure that can alleviate the uneven distribution of the lithium salt concentration in the wound electrode body 200 based on such estimation.

  FIG. 4 shows an embodiment of the present invention, in which the negative electrode sheet 240, the separator sheets 262 and 264, and the positive electrode sheet 220 that are overlapped in the wound electrode body 200 are wound in the winding axis direction (for example, the separator sheet 262, The cross section cut | disconnected by the width direction of H.264 is typically shown. In FIG. 4, unlike the actual case, the sheets 220, 262, 240, and 264 are shown with a gap. In this embodiment, as shown in FIG. 4, the separator sheets 262 and 264 are wider than the positive electrode active material layer 223. In this case, the separator sheets 262 and 264 are opposed to the positive electrode active material layer 223 and the positive electrode active material layer 223 in the direction of the winding axis WL. Portions 262b and 264b (hereinafter referred to as non-positive electrode facing portions) that are not provided. The non-positive electrode facing portions 262b and 264b are provided on both edges of the separator sheets 262 and 264.

  In the lithium ion secondary battery 100 proposed here, the separator sheets 262 and 264 have the porosity of the end portions 262E and 264E on both sides of the separator sheets 262 and 264 in the direction of the winding axis WL of the wound electrode body 200. A (%) is larger than the porosity B (%) of the intermediate portions 262C and 264C excluding the end portions 262E and 264E (A> B).

<< End portions 262E, 264E of separator sheets 262, 264 >>
Here, the end portions 262E, 264E of the separator sheets 262, 264 are non-positive electrode facing portions 262b, 264b and boundaries 262P, 264P between the non-positive electrode facing portions 262b, 264b and the positive electrode facing portions 262a, 264a, respectively. It is good to prescribe | regulate as a range of 0.5 mm or more (preferably 1 mm or more) to the 262b, 264b side. In other words, the width W of the portion facing the positive electrode active material layer 223 in the end portions 262E and 264E of the separator sheets 262 and 264 may be 0.5 mm or more (preferably 1 mm or more). Further, the end portions 262E and 264E are formed from the non-positive electrode facing portions 262b and 264b and the boundaries 262P and 264P between the non-positive electrode facing portions 262b and 264b and the positive electrode facing portions 262a and 264a toward the non-positive electrode facing portions 262b and 264b, respectively. It may be defined as a range within 5 mm (preferably within 3 mm, more preferably within 2.5 mm). In other words, the width W of the portion facing the positive electrode active material layer 223 in the end portions 262E and 264E of the separator sheets 262 and 264 may be 3.5 mm or less (preferably 3 mm or less).

  For example, the end portions 262E and 264E of the separator sheets 262 and 264 are arranged such that the edges 262L of the separator sheets 262 and 264 with respect to the width from the edges 262L and 264L to the center of the separator sheets 262 and 264 in the winding axis WL direction. H.264L may be specified in a region of 3% to 20% (preferably 5% to 15%). Further, the end portions 262E and 264E may be defined by an area of 5 mm or more and 10 mm or less (preferably 6 mm or more and 8 mm or less) from the edges 262L and 264L of the separator sheets 262 and 264 in the direction of the winding axis WL.

≪Porosity≫
The porosity A of the end portions 262E and 264E may be larger than the porosity B of the intermediate portions 262C and 264C excluding the end portions 262E and 264E. For example, the porosity A of the end portions 262E and 264E is suitably about 40% or more (for example, 40% ≦ A ≦ 77.5%), and more than about 55% (for example, 55% <A ≦ 77). 0.5%) is preferred. Further, the porosity B of the intermediate portions 262C and 264C excluding the end portions 262E and 264E may be smaller than the porosity A of the end portions 262E and 264E. For example, the porosity B of the intermediate portions 262C and 264C is suitably about 32.5% or more (for example, 32.5% ≦ B ≦ 70%), preferably 55% or less (for example, 32.5%). ≦ B ≦ 55%). For example, the porosity A of the end portions 262E and 264E and the porosity B of the intermediate portions 262C and 264C preferably satisfy the relationship of 2.5% ≦ A−B ≦ 7.5%, and 5% ≦ More preferably, the relationship of AB ≦ 7.5% is satisfied.

Here, the porosity of the separator sheets 262 and 264 is determined by setting the apparent volume of the separator sheets 262 and 264 to V ₁ , the mass of the separator sheets 262 and 264 to W _1, and the true density of the materials constituting the separator sheets 262 and 264 It is assumed that (1-W ₁ / ρ ₁ V ₁ ) × 100, where ρ ₁ is a value obtained by dividing the mass W ₁ by the actual volume of the material not included. The porosity of the separator sheets 262 and 264 can be arbitrarily controlled, for example, by changing the heat (heating temperature) applied when the separator sheet is stretched during production. For example, the end portions 262E, 264E and the intermediate portion 262C are changed by changing the heating temperature between the end portions 262E, 264E and the intermediate portions 262C, 264C of the separator sheets 262, 264 at the time of stretching (for example, heating only the intermediate portions 262C, 264C). Separator sheets 262 and 264 having different porosities from H.264C can be obtained.

  According to the knowledge of the present inventor, the portion having a high porosity in the separator sheets 262 and 264 has a lower resistance to the movement of the electrolytic solution (and hence the lithium salt) than the portion having a low porosity. Therefore, in the separator sheets 262 and 264, the portion with high porosity tends to move the electrolyte more easily than the portion with low porosity.

  In the lithium ion secondary battery 100 proposed here, the porosity A of the end portions 262E and 264E on both sides of the separator sheets 262 and 264 in the direction of the winding axis WL of the wound electrode body 200 is the intermediate portion 262C, It is greater than the porosity B of 264C (A> B). For this reason, the movement of the electrolytic solution between the end portions 262E, 264E and the surplus electrolytic solution becomes easier than the movement of the electrolytic solution between the end portions 262E, 264E and the intermediate portions 262C, 264C. That is, in one embodiment of the present invention, the movement of the electrolyte solution between the end portions 262E, 264E and the surplus electrolyte solution is easier than the movement of the electrolyte solution between the end portions 262E, 264E and the intermediate portions 262C, 264C. Thus, even if high-rate charge / discharge is repeated, the change in the lithium salt concentration at the end (opening end) in the winding axis direction of the wound electrode body 200 is suppressed, and the distribution of the lithium salt concentration in the wound electrode body 200 Is less likely to be biased. For this reason, the phenomenon that the internal resistance increases due to the uneven distribution of the lithium salt concentration in the wound electrode body 200 can be mitigated.

≪Gurley value≫
The Gurley values of the end portions 262E and 264E of the separator sheets 262 and 264 are preferably smaller than the Gurley values of the intermediate portions 262C and 264C. For example, the Gurley value of the end portions 262E and 264E is suitably less than about 90 (sec / 100 cm ³ ), for example, 40 (sec / 100 cm ³ ) to 80 (sec / 100 cm ³ ), preferably 80 ( sec / 100 cm ³ ) or less, and more preferably 70 (sec / 100 cm ³ ) or less. The Gurley values of the intermediate portions 262C and 264C excluding the end portions 262E and 264E are preferably larger than the Gurley values of the end portions 262E and 264E. For example, the Gurley value of the intermediate portions 262C and 264C is suitably about 90 (sec / 100 cm ³ ) or more, and preferably 100 (sec / 100 cm ³ ) or more. According to such a configuration, the uneven distribution of the lithium salt concentration can be better resolved. In this specification, the Gurley values of the separator sheets 262 and 264 are measured in accordance with JIS L 1096: 2010 “Fabric and knitted fabric test method”.

≪Test example≫
The inventor has experimentally evaluated the operational effects of the separator sheets 262 and 264. Here, the evaluation cell was composed of a laminate cell (lithium ion secondary battery) shown in the schematic cross-sectional view of FIG. The evaluation cell includes a positive electrode 220 having a positive electrode active material layer 223 formed on one side of a positive electrode current collector 221, a negative electrode 240 having a negative electrode active material layer 243 formed on one side of a negative electrode current collector 241, and a positive electrode active material. A separator 262 interposed between the layer 223 and the negative electrode active material layer 243 is provided. The width of the separator 262 is wider than the width of the positive electrode active material layer 54. Therefore, the separator 262 has a positive electrode facing portion 262a facing the positive electrode active material layer 223 and a non-positive electrode facing portion 262b not facing the positive electrode active material layer 223 in the width direction. In the evaluation cell, a constant load is applied to the stacking direction of the positive electrode 220, the separator 262, and the negative electrode 240. Due to this load, the load applied to the inside of the wound electrode body 200 during battery operation is reproduced. In FIG. 5, the actual thickness of each member 220, 240, 262 is not reflected. The laminate bag and each lead terminal are omitted as appropriate.

In the positive electrode 220, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ powder was used as the positive electrode active material particles contained in the positive electrode active material layer 223, PVDF was used as the binder, and acetylene black (AB) was used as the conductive material. Here, as a mixture for forming the positive electrode active material layer 223, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , PVDF, and AB in a mass ratio, LiNi _1/3 A paste was prepared in which Mn _1/3 Co _1/3 O ₂ : PVDF: AB = 93: 3: 4 and NMP was mixed as a dispersion solvent. And this paste was apply | coated to the strip | belt shape on the aluminum foil (thickness 15 micrometers) as the positive electrode electrical power collector 221, it was made to dry, and it rolled by the roll press, and formed the positive electrode 220.

  In the negative electrode 240, natural graphite powder was used as negative electrode active material particles contained in the negative electrode active material layer 243, SBR was used as a binder, and CMC was used as a thickener. Here, as a mixture for forming the negative electrode active material layer 243, graphite, SBR, and CMC are contained in a mass ratio of graphite: SBR: CMC = 98: 1: 1, and water is a dispersion solvent. A mixed paste was prepared. And this paste was apply | coated to the strip | belt shape on copper foil (thickness 10 micrometers) as the negative electrode collector 241, it was made to dry, and the negative electrode 240 was formed by performing the rolling by roll press.

As the separator 262, a porous film made of a single layer of polyethylene (PE) was used. The separator 262 was formed with a heat resistant porous layer 266 on the surface facing the positive electrode 220. The heat resistant porous layer 266 includes alumina particles as an inorganic filler. As the electrolytic solution, ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate were mixed at a volume ratio of 3: 4: 3, and 1.1 mol of LiPF ₆ was dissolved. The rated capacity when such a battery is charged to 4.2 V is approximately 40 mAh.

<< Evaluation test battery samples 1 to 7 >>
Hereinafter, a plurality of samples 1 to 7 of the battery for evaluation test will be described. As shown in FIG. 5, the samples 1 to 7 of the battery for evaluation test have different porosities at the end portions 262E on both sides of the separator 262 and the intermediate portion 262C excluding the end portions 262E. Further, the Gurley value is different between the end portion 262E and the intermediate portion 262C excluding the end portion 262E. In Samples 1 to 7, the width W of the portion facing the positive electrode active material layer 223 in the end portion 262E of the separator 262 was set to 0.5 mm.

<Sample 1>
In Sample 1, the end portion 262E had a porosity of 57.5%, and the intermediate portion 262C had a porosity of 55%. The Gurley value of the end portion 262E was 83 (sec / 100 cm ³ ), and the Gurley value of the intermediate portion 262C was 90 (sec / 100 cm ³ ).

<Sample 2>
In sample 2, the porosity of the end portion 262E of the separator 262 was 60%, and the porosity of the intermediate portion 262C was 55%. The Gurley value of the end portion 262E was 80 (sec / 100 cm ³ ), and the Gurley value of the intermediate portion 262C was 90 (sec / 100 cm ³ ).

<Sample 3>
In sample 3, the porosity of the end portion 262E of the separator 262 was 62.5%, and the porosity of the intermediate portion 262C was 55%. The Gurley value of the end portion 262E was 70 (sec / 100 cm ³ ), and the Gurley value of the intermediate portion 262C was 90 (sec / 100 cm ³ ).

<Sample 4>
In sample 4, the porosity of the end portion 262E of the separator 262 was 65%, and the porosity of the intermediate portion 262C was 55%. Further, the Gurley value of the end portion 262E was 55 (sec / 100 cm ³ ), and the Gurley value of the intermediate portion 262C was 90 (sec / 100 cm ³ ).

<Sample 5>
In sample 5, the porosity of the end portion 262E of the separator 262 was 67.5%, and the porosity of the intermediate portion 262C was 55%. Further, the Gurley value of the end portion 262E was 40 (sec / 100 cm ³ ), and the Gurley value of the intermediate portion 262C was 90 (sec / 100 cm ³ ).

<Sample 6>
In Sample 6, in the separator 262, the porosity of the end portion 262E was 55%, and the porosity of the intermediate portion 262C was 55%. That is, in sample 6, there was no difference in porosity between the end 262E and the intermediate portion 262C. The Gurley value of the end portion 262E was 89 (sec / 100 cm ³ ), and the Gurley value of the intermediate portion 262C was 90 (sec / 100 cm ³ ).

<Sample 7>
In Sample 7, the porosity of the end portion 262E was 65%, and the porosity of the intermediate portion 262C was 65%. That is, in sample 7, the separator 262 provided no difference in porosity between the end portion 262E and the intermediate portion 262C. The Gurley value of the end portion 262E was 55 (sec / 100 cm ³ ), and the Gurley value of the intermediate portion 262C was 55 (sec / 100 cm ³ ).

<< Evaluation of battery for evaluation test >>
A charge / discharge pattern that repeats high-rate charge / discharge was applied to each of the batteries of Samples 1 to 7 obtained as described above, and a charge / discharge cycle test was performed. Specifically, in a room temperature (about 25 ° C.) environment, a high-rate charge / discharge cycle in which charging is performed to 4.2 V by 2 C constant current charging and discharging to 3.0 V by 0.4 C constant current discharging is performed. Repeated continuously. Then, the rate of increase in resistance was calculated from the IV resistance (the initial resistance of the battery) before the charge / discharge cycle test and the IV resistance after the charge / discharge cycle test. Here, the IV resistance before and after the charging / discharging cycle was measured by measuring the current obtained by charging the battery at SOC 60% and performing charging treatment at 1C, 3C, and 5C for 10 seconds respectively in an environment of 25 ° C. The voltage drop value ΔV, which is a value obtained by subtracting the voltage value at the time of 10 seconds from the initial voltage value, is plotted on the vertical axis, and the value is obtained from the slope. The IV resistance increase rate is obtained by (IV resistance after charge / discharge cycle test / IV resistance before charge / discharge cycle test) × 100. The results are shown in Table 1.

  As shown in Table 1, in Samples 1 to 5, the porosity of the end portion 262E of the separator 262 is larger than the porosity of the intermediate portion 262C. In Samples 1 to 5, the difference in porosity (A−B) between the porosity A of the end portion 262E and the porosity B of the intermediate portion 262C is set to 2.5% to 7.5%. . On the other hand, in samples 6 and 7, the separator 262 has no difference in porosity between the end portion 262E and the intermediate portion 262C. In samples 1 to 5 where the porosity of the end portion 262E is larger than the porosity of the intermediate portion 262C, the resistance increase rate after the charge / discharge cycle test is low and good compared to the samples 6 and 7 where the porosity is not different. Durability performance was shown. In the case of the battery used here, the resistance increase rate was improved by 25% or more by setting the difference in porosity (AB) to 2.5% or more. On the other hand, a separator having a porosity difference (A-B) exceeding 10% is not preferable because the physical property value (for example, tensile strength) tends to decrease in addition to the effect of improving the high-rate durability. . Also, the amount of voltage drop due to self-discharge tends to increase. From these points, the difference in porosity (A−B) is preferably 10% or less, and more preferably 7.5% or less (Samples 1 to 5).

<< Battery Samples 8-16 for Evaluation Test >>
As shown in FIG. 5, each of the samples 8 to 16 of the battery for evaluation test has a width W of 1 mm and 1.5 mm of the portion facing the positive electrode active material layer 223 in the end portion 262E of the separator 262. Batteries for evaluation tests were constructed by changing to 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, and 5 mm. The configuration was the same as Sample 1 except that the width W was different. And the resistance increase rate after a charging / discharging cycle test was evaluated. The results are shown in Table 2.

  As shown in Table 2, the resistance increase rate tended to decrease as the width W of the portion facing the positive electrode active material layer 223 in the end portion 262E of the separator 262 increased. If the width W is too large, the inflow path of the electrolyte does not change from the conventional one in which the porosity of the entire separator is increased, so that the effect of relaxing the change in the lithium salt concentration at the end 262E is not sufficiently exhibited. Presumed to have led to an increase in resistance. From the viewpoint of suppressing the resistance increase, the width W of the portion facing the positive electrode active material layer 223 in the end portion 262E of the separator 262 is preferably 0.5 mm or more and 3.5 mm or less (Samples 1 and 8). To 13), more preferably 0.5 mm to 2.5 mm (samples 1, 8 to 11), and particularly preferably 0.5 mm to 1.5 mm (samples 1, 8, and 9).

  Although the lithium ion secondary battery according to one embodiment of the present invention has been described above, the secondary battery according to the present invention is not limited to any of the above-described embodiments, and various modifications can be made. For example, a cylindrical battery is known as another battery form. A cylindrical battery is a battery in which a wound electrode body is accommodated in a cylindrical battery case.

  Further, as described above, the present invention can contribute to improvement in durability against high rate charge / discharge of a secondary battery (for example, a lithium ion secondary battery). For this reason, this invention is suitable for the secondary battery for vehicle drive power supplies, such as a drive battery of a hybrid vehicle and an electric vehicle. That is, the secondary battery can be suitably used as a power source for driving a vehicle that drives a motor (electric motor) of a vehicle such as an automobile. The power source for driving the vehicle may be an assembled battery in which a plurality of secondary batteries are combined.

100 Lithium ion secondary battery 200 Winding electrode body 220 Positive electrode sheet 221 Positive electrode current collector 223 Positive electrode active material layer 240 Negative electrode sheet 241 Negative electrode current collector 243 Negative electrode active material layer 262, 264 Separator sheets 262C, 264C Intermediate portions 262E, 264E End 266 Heat-resistant porous layer 300 Battery case

Claims (6)

A positive electrode sheet having a positive electrode active material layer on a long positive electrode current collector, a negative electrode sheet having a negative electrode active material layer on a long negative electrode current collector, and interposed between the positive electrode sheet and the negative electrode sheet A wound electrode body wound with a separator sheet to be wound,
A battery case containing the wound electrode body;
A non-aqueous electrolyte injected into the battery case,
The non-aqueous electrolyte is stored outside the wound electrode body, and includes an excess electrolyte solution in contact with an end portion in the winding axis direction of the wound electrode body,
Non-aqueous electrolysis in which the porosity A (%) at both end portions of the separator sheet is larger than the porosity B (%) at the intermediate portion excluding the end portion (A> B) in the winding axis direction Liquid secondary battery.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the porosity A of the end portion and the porosity B of the intermediate portion satisfy a relationship of 2.5 ≦ A−B ≦ 7.5. The separator sheet, in the winding axis direction,
A facing portion facing the positive electrode active material layer;
A non-opposing portion not facing the positive electrode active material layer,
The end portion is defined as a range from 0.5 mm to 3.5 mm from the non-opposing part and a boundary between the non-opposing part and the opposing part toward the opposing part. Non-aqueous electrolyte secondary battery. The porosity A of the end is 40% to 77.5%;
The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the porosity B of the intermediate portion is 32.5% to 70%.   The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the porosity A of the end portion is more than 55% and the porosity B of the intermediate portion is 55% or less. . The separator sheet is
The Gurley value at the end is 83 (sec / 100 cm ³ ) or less,
The non-aqueous electrolyte secondary battery according to claim 1, wherein a Gurley value in the intermediate portion is 90 (sec / 100 cm ³ ) or more.

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