JP2004095197A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery improved in a charge-discharge cycle life at a high temperature. <P>SOLUTION: The nonaqueous electrolyte secondary battery comprises a positive electrode 4, a negative electrode 6, a separator arranged between the positive electrode 4 and the negative electrode 6 having a degree of stretching of at most 7% in a longitudinal direction, and a nonaqueous electrolyte. <P>COPYRIGHT: (C)2004,JPO

Description

【００８５】
表１から明らかなように、実施例１〜５の二次電池は、比較例の二次電池に比べて４５℃の高温での３００サイクル後の容量維持率が高く、かつ８５℃のような高温環境下に貯蔵した際の電圧降下を抑えられることが理解できる。
【００８６】
【発明の効果】
以上詳述したように本発明によれば、高温環境下での充放電サイクル寿命が向上された非水電解質二次電池を提供することができる。
【図面の簡単な説明】
【図１】本発明に係る非水電解質二次電池の一例である円筒形非水電解質二次電池を示す部分断面図。
【図２】図１の円筒形非水電解質二次電池に用いられる電極群の捲き始め端部における正極と負極とセパレータの位置関係を模式的に示した斜視図。
【図３】図１の円筒形非水電解質二次電池に用いられるセパレータを示す平面図。
【符号の説明】
１…容器、
３…電極群、
４…正極、
４ａ…正極集電体、
４ｂ…活物質含有層、
４ｃ…正極タブ、
５…セパレータ、
６…負極、
６ａ…負極集電体、
６ｂ…負極層、
８…封口板。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery.
[0002]
[Prior art]
In recent years, non-aqueous electrolyte batteries using lithium as the negative electrode active material have attracted attention as high energy density batteries, and manganese dioxide (MnO 2) has been used as the positive electrode active material. ₂ ), Fluorocarbon [(CF ₂ ) _n ], Thionyl chloride (SOCl ₂ A primary battery using the above-mentioned method is already frequently used as a backup battery for a power source of a calculator, a clock and a memory.
[0003]
Further, in recent years, with the reduction in size and weight of various electronic devices such as VTRs and communication devices, demand for secondary batteries having a high energy density has increased as a power source for these devices, and lithium secondary batteries using lithium as a negative electrode active material have been required. Research is being actively conducted.
[0004]
In a lithium secondary battery, lithium is used for a negative electrode. Examples of the non-aqueous electrolyte include LiClO 2 in a non-aqueous solvent such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), γ-butyrolactone (γ-BL), and tetrahydrofuran (THF). ₄ , LiBF ₄ , LiAsF ₆ Non-aqueous electrolytes in which lithium salts such as those described above are dissolved, and lithium ion conductive solid electrolytes are used. As the positive electrode active material, mainly TiS ₂ , MoS ₂ , V ₂ O ₅ , V ₆ O _Thirteen , MnO ₂ The use of compounds that undergo a topochemical reaction with lithium has been studied.
[0005]
As a negative electrode incorporated in the above-described lithium secondary battery, for example, coke, resin fired body, carbon fiber, pyrolytic vapor growth carbon, carbon and the like that absorb and release lithium, such as graphite, by using lithium and non-aqueous electrolyte And the deterioration of the negative electrode characteristics due to dendrite deposition have been proposed and are currently in practical use.
[0006]
From the viewpoint of increasing the capacity of a lithium secondary battery, increasing the density of electrodes and reducing the porosity in a battery container are essential conditions for battery design. However, if the design is such that the porosity inside the battery is reduced, the space between the positive electrode and the negative electrode is reduced, so that the heat radiation capability of the battery is reduced. As a result, an abnormal pressure is likely to be applied to the separator, which causes an internal short circuit in a high-temperature environment and causes a problem that the cycle life at a high temperature is shortened.
[0007]
[Problems to be solved by the invention]
An object of the present invention is to provide a non-aqueous electrolyte secondary battery having an improved charge / discharge cycle life at high temperatures.
[0008]
[Means for Solving the Problems]
A nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and having a longitudinal elongation at 85 ° C. of 7% or less, and a nonaqueous electrolyte. And characterized in that:
[0009]
Further, the non-aqueous electrolyte secondary battery according to the present invention has a spiral shape by disposing a separator between a positive electrode and a negative electrode including a current collector and an active material-containing layer supported on one or both surfaces of the current collector. A non-aqueous electrolyte secondary battery comprising: a wound electrode group; and a non-aqueous electrolyte held by the electrode group,
The positive electrode has a current collector exposed region at the winding start end, the winding start end of the negative electrode faces the current collector exposed region of the positive electrode via the separator,
The separator has a longitudinal elongation at 85 ° C. of 7% or less.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
A nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and having a longitudinal elongation at 85 ° C. of 7% or less, and a nonaqueous electrolyte. And characterized in that:
[0011]
According to such a secondary battery, an internal short circuit under a high-temperature environment of 45 ° C. or higher can be suppressed, so that the charge / discharge cycle life under a high-temperature environment and high-temperature storage characteristics can be improved. .
[0012]
Hereinafter, the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte will be described.
[0013]
First, the positive electrode and the negative electrode can form an electrode group with a separator interposed therebetween. The electrode group includes, for example, (i) a positive electrode and a negative electrode wound in a flat shape or a spiral shape with a separator interposed therebetween, or (ii) a positive electrode and a negative electrode wound in a spiral shape with a separator interposed therebetween. And then compressing in the radial direction, (iii) bending the positive and negative electrodes one or more times with a separator between them, or (iv) laminating the positive and negative electrodes with a separator between them It is made.
[0014]
The electrode group need not be pressed, but may be pressed to increase the integration strength of the positive electrode, the negative electrode, and the separator. In addition, heating can be performed at the time of pressing.
[0015]
1) Positive electrode
The positive electrode has a structure in which a positive electrode layer containing an active material is supported on one or both sides of a current collector.
[0016]
The positive electrode layer includes a positive electrode active material, a binder, and a conductive agent.
[0017]
As the positive electrode active material, various oxides such as manganese dioxide, lithium manganese composite oxide, lithium-containing nickel oxide, and lithium-containing iron oxide (for example, LiCoO ₂ ), Lithium-containing nickel cobalt oxide (eg, LiNi _0.8 Co _0.2 O ₂ ), Lithium manganese composite oxide (eg, LiMn ₂ O ₄ , LiMnO ₂ ) Is preferable because a high voltage can be obtained.
[0018]
Examples of the conductive agent include acetylene black, carbon black, and graphite.
[0019]
As the binder, for example, polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene copolymer, styrene-butadiene rubber and the like can be used.
[0020]
The mixing ratio of the positive electrode active material, the conductive agent and the binder is desirably in the range of 80 to 95% by weight of the positive electrode active material, 3 to 20% by weight of the conductive agent, and 2 to 7% by weight of the binder.
[0021]
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.
[0022]
For the positive electrode, for example, a positive electrode active material, a conductive agent and a binder are suspended in an appropriate solvent, the suspension is applied to a current collector, dried, and then pressed at a desired pressure 1 to 5 times. It is produced by this.
[0023]
The positive electrode has a packing density of 2.8 g / cm after pressing. ³ Above 3.5g / cm ³ It is desirable that:
[0024]
2) Negative electrode
This negative electrode has a negative electrode current collector and a negative electrode layer supported on one or both surfaces of the negative electrode current collector and containing a negative electrode material and a binder.
[0025]
The negative electrode material is preferably a carbonaceous material that stores and releases lithium ions. As the carbonaceous material, graphite, coke, carbon fiber, graphite material such as spherical carbon or carbonaceous material, thermosetting resin, isotropic pitch, mesophase pitch, mesophase pitch-based carbon fiber or mesophase small sphere, etc. Graphite materials or carbonaceous materials obtained by performing a heat treatment at 3000 ° C. can be used.
[0026]
Preferred among carbonaceous materials are those obtained at a heat treatment temperature of 2000 ° C. or higher and a plane spacing d ₀₀₂ Is a graphite material having a graphite crystal having a size of 0.336 nm or more and 0.34 nm or less.
[0027]
As the binder, for example, polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene copolymer, styrene-butadiene rubber, carboxymethyl cellulose, and the like can be used.
[0028]
The compounding ratio of the negative electrode active material, the conductive agent and the binder is preferably in the range of 80 to 98% by weight of the negative electrode active material, 3 to 30% by weight of the conductive agent, and 1 to 7% by weight of the binder.
[0029]
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 current collector preferably has a thickness of 5 to 20 μm. This is because in this range, the balance between electrode strength and weight reduction can be achieved.
[0030]
The negative electrode is, for example, a negative electrode active material, a conductive agent and a binder are suspended in an appropriate solvent, the suspension is applied to a current collector, dried, and then pressed 1 to 5 times at a desired pressure. It is produced by doing.
[0031]
The negative electrode has a packing density d of 1.4 g / cm after pressing. ³ 1.6 g / cm ³ It is desirable that
[0032]
As the negative electrode material, in addition to the above-described carbonaceous material that stores and releases lithium ions, a metal that stores and releases lithium, a metal oxide, a metal sulfide, a metal nitride, a lithium metal, or a lithium alloy can be used.
[0033]
Examples of the metal oxide include tin oxide, silicon oxide, lithium titanium oxide, niobium oxide, and tungsten oxide.
[0034]
Examples of the metal sulfide include tin sulfide and titanium sulfide.
[0035]
Examples of the metal nitride include lithium cobalt nitride, lithium iron nitride, and lithium manganese nitride.
[0036]
Examples of the lithium alloy include a lithium aluminum alloy, a lithium tin alloy, a lithium lead alloy, and a lithium silicon alloy.
[0037]
When a material other than carbonaceous material and graphite material that can store and release lithium is used as the negative electrode active material, it is preferable to use, for example, acetylene black, carbon black, graphite, or the like as the conductive agent.
[0038]
3) Separator
The separator is desirably formed from a porous sheet.
[0039]
As the porous sheet, for example, a porous film or a nonwoven fabric can be used. The porous sheet is preferably made of, for example, at least one material selected from polyolefin and cellulose. Examples of the polyolefin include polyethylene and polypropylene. A porous film made of polyethylene, polypropylene, or both is preferable because the safety of the secondary battery can be improved.
[0040]
The separator preferably has a longitudinal stretching ratio at 85 ° C. of 7% or less. Such a separator has excellent resilience under high-temperature conditions, so that it can withstand the pressure applied during high-temperature storage in a charged state, and can prevent minute short circuit and electrode group deformation due to high-temperature storage. Characteristics and cycle life at high temperatures can be improved. However, if the stretching ratio is smaller than 1.5%, the heat shrinkage of the separator during high-temperature storage becomes superior to the stretching ratio, and there is a risk of film breakage and short circuit. Therefore, the stretching ratio at 85 ° C. is desirably 1.5% or more and 7% or less. A more preferred range is 2 to 7%.
[0041]
The separator has a complex elastic modulus in the longitudinal direction at 25 ° C. of 2.7 × 10 ⁸ It is preferably Pa or more. The complex modulus at 25 ° C. is 2.7 × 10 ⁸ When it is less than Pa, the piercing strength and tensile strength of the separator are reduced, so the occurrence rate of internal short circuit under a high temperature environment may increase, the charge / discharge cycle life is shortened, and the voltage drop due to high temperature storage increases. there is a possibility. The complex modulus at 25 ° C. is 2.7 × 10 ⁸ By setting the pressure to Pa or more, crushing and breakage of the separator due to electrode expansion during charging can be reduced, so that high-temperature cycle characteristics and safety can be improved. However, the complex elastic modulus at 25 ° C. is 5 × 10 ⁸ When the pressure exceeds Pa, the separator is hard and hardly deformed, so that when the electrodes undergo a morphological change due to the initial charge, the integrity of the laminated structure in which the electrode group is in close contact is impaired, which may cause coil twisting and the like. Could be. Therefore, the complex elastic modulus at 25 ° C. is 2.7 × 10 ⁸ More than Pa, 5 × 10 ⁸ It is desirable to set it to Pa or less. A more preferred range is 2.7 × 10 ⁸ Pa ~ 4.5 × 10 ⁸ Pa.
[0042]
The complex modulus in the longitudinal direction at 85 ° C. of the separator is 0.6 × 10 ⁸ Desirably, it is Pa or more. Such a separator can reduce deformation such as crushing or breakage due to expansion and contraction of the electrode in a high-temperature environment, so that the charge-discharge cycle life in a high-temperature environment can be further improved. However, the complex elastic modulus at 85 ° C. is 3 × 10 ⁸ If it exceeds Pa, the thermal expansion of the coil generated inside the battery when stored at a high temperature such as 85 ° C. may not be able to be alleviated. Therefore, the complex elastic modulus at 85 ° C. is 0.6 × 10 ⁸ Pa or more, 3 × 10 ⁸ It is desirable to set it to Pa or less. A more preferred range is 0.6 × 10 ⁸ ~ 2 × 10 ⁸ Pa.
[0043]
The complex modulus of elasticity of the separator used in the present invention can be measured by a complex impedance measuring method (Complex Dynamic Modulus) using a rheographic piezo manufactured by TOYO Seiki (machine No. 111100602) or a measuring device having a performance equivalent to or similar to that of 3 Hz. Is applied and measured.
[0044]
The stretching ratio in the longitudinal direction at 85 ° C. of the separator is obtained when measuring the complex elastic modulus in the longitudinal direction at 85 ° C. of the separator. The load applied to the separator in the measurement of the complex modulus in the longitudinal direction at 85 ° C. is desirably 100 g.
[0045]
The thickness of the separator is desirably 30 μm or less. A more preferred range is 5 to 30 μm, and a still more preferred range is 10 to 25 μm.
[0046]
The separator preferably has a heat shrinkage of 20% or less when the separator is present at 120 ° C. for one hour. More preferably, the heat shrinkage is 15% or less.
[0047]
The porosity of the separator is preferably in the range of 30 to 70%. A more preferred range of porosity is 35-70%.
[0048]
The separator has an air permeability of 500 seconds / 100 cm. ³ The following is preferred. Air permeability is 100cm ³ Means the time (seconds) required for the air to drop onto the porous sheet. A more preferred range is 30 seconds / 100 cm ³ ~ 500sec / 100cm ³ The more preferable range is 50 seconds / 100 cm. ³ ~ 150 sec / 100cm ³ It is.
[0049]
Further, it is preferable that the end of the separator along the short direction extends 0.25 mm to 2 mm as compared with the end of the negative electrode along the short direction.
[0050]
4) Non-aqueous electrolyte
As the non-aqueous electrolyte, those having a liquid, gel or solid (polymer solid electrolyte) form can be used.
[0051]
The liquid non-aqueous electrolyte (non-aqueous electrolyte) is obtained, for example, by dissolving an electrolyte (for example, lithium salt) in a non-aqueous solvent. The gel nonaqueous electrolyte contains a nonaqueous electrolyte and a polymer material that holds the nonaqueous electrolyte. Examples of the polymer material include polyvinylidene fluoride, polyacrylonitrile, polyethylene oxide, polyvinyl chloride, polyacrylate, and polyvinylidene fluoride hexafluoropropylene.
[0052]
As the non-aqueous solvent, a known non-aqueous solvent can be used as a solvent for the non-aqueous electrolyte secondary battery, and is not particularly limited.Ethylene carbonate, a melting point lower than that of the ethylene carbonate, and a donor number of 18 It is preferable to use a non-aqueous solvent mainly composed of a mixed solvent with one or more of the following non-aqueous solvents (hereinafter, referred to as a second solvent). Such a non-aqueous solvent is advantageous in that it is stable with respect to the material constituting the negative electrode, hardly causes reductive decomposition or oxidative decomposition of the electrolyte, and has high conductivity.
[0053]
Examples of the second solvent include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl propionate, methyl propionate, propylene carbonate, γ-butyrolactone, acetonitrile, ethyl acetate, toluene, xylene, methyl acetate, and the like. Among them, a chain carbonate is preferable. The second solvent can be used alone or in the form of a mixture of two or more.
[0054]
The viscosity of the mixed solvent at 25 ° C. is preferably 28 mp or less. It is preferable that the compounding amount of the ethylene carbonate in the mixed solvent is 10 to 80% by volume. If the ratio is out of this range, the conductivity may be reduced or the solvent may be decomposed, and the charge / discharge efficiency may be reduced. A more preferable blending amount of the ethylene carbonate is 20 to 75% by volume ratio. By increasing the amount of ethylene carbonate in the non-aqueous solvent to 20% by volume, solvation of ethylene carbonate with lithium ions is facilitated, so that the effect of suppressing the decomposition of the solvent can be improved.
[0055]
A more preferable composition of the mixed solvent is a mixed solvent of EC and MEC, EC and PC and MEC, EC and MEC and DMC, EC, MEC, PC and DEC, and the volume ratio of MEC is 30 to 80%. preferable. Thus, the conductivity can be improved by setting the volume ratio of MEC to 30 to 80%, more preferably 40 to 70%. On the other hand, from the viewpoint of suppressing the reductive decomposition reaction of the solvent, using an electrolytic solution in which carbon dioxide gas is dissolved is effective in improving the capacity and cycle life.
[0056]
The main impurities present in the mixed solvent include water and organic peroxides (eg, glycols, alcohols, carboxylic acids). Each of the impurities may affect the cycle life and the capacity. In addition, self-discharge during storage at a high temperature (60 ° C. or higher) may increase. For this reason, in a non-aqueous electrolyte containing a non-aqueous solvent, the impurities are preferably reduced as much as possible. Specifically, the water content is preferably 50 ppm or less, and the organic peroxide content is preferably 1000 ppm or less.
[0057]
Examples of the electrolyte include lithium perchlorate and lithium hexafluorophosphate (LiPF ₆ ), Lithium tetrafluoroborate (LiBF ₄ ), Lithium arsenic hexafluoride (LiAsF) ₆ ), Lithium trifluorometasulfonate (LiCF ₃ SO ₃ ), Lithium bistrifluoromethylsulfonylimide [LiN (CF ₃ SO ₂ ) ₂ ] And other lithium salts. Among them, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ It is preferable to use
[0058]
The amount of the electrolyte dissolved in the non-aqueous solvent is desirably in the range of 0.5 to 2 mol / L.
[0059]
A cylindrical non-aqueous electrolyte secondary battery, which is an example of the non-aqueous electrolyte secondary battery according to the present invention, will be described with reference to FIGS.
[0060]
FIG. 1 is a partial cross-sectional view showing a cylindrical non-aqueous electrolyte secondary battery which is an example of the non-aqueous electrolyte secondary battery according to the present invention. FIG. 2 is used for the cylindrical non-aqueous electrolyte secondary battery shown in FIG. FIG. 3 is a perspective view schematically showing a positional relationship between a positive electrode, a negative electrode, and a separator at a winding start end of the electrode group to be formed. FIG. 3 is a plan view showing a separator used in the cylindrical nonaqueous electrolyte secondary battery in FIG. It is.
[0061]
For example, a cylindrical container 1 having a bottom made of stainless steel has an insulator 2 disposed at the bottom. The electrode group 3 is housed in the container 1. The electrode group 3 is manufactured by spirally winding a positive electrode 4 and a negative electrode 6 with a separator 5 interposed therebetween. In this spiral electrode group, as shown in FIG. 2, the positive electrode 4 is supported on both surfaces of the current collector 4a except for a strip-shaped positive electrode current collector 4a and a winding start end of the current collector 4a. And a strip-like positive electrode tab 4c welded to the winding start end of the current collector 4a. Further, the negative electrode 6 includes a strip-shaped negative electrode current collector 6a and a negative electrode layer 6b carried on both surfaces of the negative electrode current collector 6a. The winding start end of the negative electrode 6 faces the current collector 4 a exposed region of the positive electrode 4 via the separator 5. With such a configuration, since the winding start end of the negative electrode no longer faces the positive electrode active material layer 4b of the positive electrode 4, current concentration on the negative electrode end can be reduced, and the internal short-circuit occurrence rate of the secondary battery can be reduced. Can be further reduced, and the charge / discharge cycle life can be further improved. Further, the positive electrode current collector 4a is preferably formed from aluminum. Since the positive electrode current collector 4a made of aluminum is stable with respect to the positive electrode potential and has excellent electric conductivity, it can contribute to the improvement of the rate characteristics and cycle characteristics of the battery. Further, it is desirable that the positive electrode tab 4c be welded to the current collector exposed area at the end of the winding start of the positive electrode 4. Thereby, the uncoated portion of the positive electrode can be shortened.
[0062]
A non-aqueous electrolyte is contained in the container 1. The insulating paper 7 having a central portion opened is disposed above the electrode group 3 in the container 1. The insulating sealing plate 8 is arranged in the upper opening of the container 1, and the sealing plate 8 is fixed to the container 1 by caulking the vicinity of the upper opening inward. The positive electrode terminal 9 is fitted in the center of the insulating sealing plate 8. The positive electrode tab 4c is connected to the positive electrode terminal 9. The negative electrode 6 is connected to the container 1 serving as a negative terminal via a negative lead (not shown).
[0063]
In the cylindrical non-aqueous electrolyte secondary battery having the structure shown in FIGS. 1 and 2 described above, the winding start end of the negative electrode faces the current collector exposed region of the positive electrode via the separator. Although the current concentration at the winding start end can be suppressed, an internal short circuit easily occurs in a high-temperature environment, so that the cycle life is inferior. In particular, when the density of the secondary battery is increased to increase the capacity, the cycle life is significantly reduced.
[0064]
Here, the densified cylindrical non-aqueous electrolyte secondary battery satisfies the following expression (2) when the cylindrical non-aqueous electrolyte secondary battery is fully charged under the following conditions.
[0065]
Full charge condition: Charge at room temperature for 12 hours at a current value of 0.2 C and a constant voltage of 4.2 V.
[0066]
1.0 <(R ₁ / R ₂ ) ≦ 1.2 (2)
More preferably, 1.0 <(R ₁ / R ₂ ) ≦ 1.1.
[0067]
R ₁ Is the maximum diameter of the electrode group in the fully charged state, and R ₂ Is the internal diameter of the container.
[0068]
In such a cylindrical non-aqueous electrolyte secondary battery, by setting the stretching ratio of the separator 5 at 85 ° C. in the longitudinal direction (in this case, equal to the winding direction) at 7 ° C. to 7% or less, as shown in FIG. Deformation such as crushing or breakage of the separator due to expansion and contraction of the electrode due to charge and discharge under the environment can be reduced, so the occurrence rate of internal short circuit is reduced, and the charge and discharge cycle life and high temperature storage characteristics are improved. be able to. In particular, when the above-mentioned expression (2) is satisfied, a cylindrical non-aqueous electrolyte secondary battery having a high capacity, a long charge / discharge cycle life, and excellent high-temperature storage characteristics can be realized.
[0069]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0070]
(Example 1)
<Preparation of positive electrode>
First, lithium cobalt oxide (Li _x CoO ₂ X is 0 <X ≦ 1), 91% by weight of powder, 3% by weight of acetylene black, 3% by weight of graphite, and 3% by weight of polyvinylidene fluoride (PVdF) as a binder. And N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. The slurry was applied to both sides of a current collector made of aluminum foil having a thickness of 15 μm except for one end on the short side, dried, and pressed to obtain an electrode density of 3.2 g / cm. ³ Was produced.
[0071]
<Preparation of negative electrode>
Mesophase pitch-based carbon fiber heat-treated at 3000 ° C. (fiber diameter: 8 μm, average fiber length: 20 μm, aspect ratio: 0.4, plane spacing (d) determined by powder X-ray diffraction (d) ₀₀₂ ) Is 0.3360 nm and the specific surface area by the BET method is 1 m ² / G) as a carbonaceous material. A slurry was prepared by mixing 93% by weight of this carbonaceous material, 7% by weight of polyvinylidene fluoride (PVdF) as a binder, and N-methyl-2-pyrrolidone (NMP). The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm, dried, and pressed to obtain an electrode density of 1.45 g / cm. ³ Was produced.
[0072]
<Separator>
A separator made of a polyethylene porous film having a thickness of 25 μm, a heat shrinkage of 20% at 120 ° C. for 1 hour, and a porosity of 50% was prepared.
[0073]
With respect to this separator, a measurement frequency of 3 Hz is applied using a piezo (machine No. 111100602) made by TOYO Seiki Co., Ltd. by a complex impedance measurement method (Complex Dynamic Modulus), and the MD direction (longitudinal direction, in this case, equal to the winding direction). ) Was measured at 25 ° C. and 85 ° C., and the results are shown in Table 1 below. In the measurement of the complex elastic modulus at 85 ° C., the magnitude of the load was set to 100 g.
[0074]
In addition, the measurement of the longitudinal stretching ratio at 85 ° C. of the separator was performed simultaneously with the measurement of the longitudinal complex modulus at 85 ° C. The results are shown in Table 1 below.
[0075]
<Preparation of non-aqueous electrolyte>
Lithium hexafluoroborate (LiPF) was used as a mixed solvent (mixing volume ratio 1: 2) of ethylene carbonate (EC) and methyl ethyl carbonate (MEC). ₆ ) Was dissolved at a concentration of 1 mol / L to prepare a non-aqueous electrolyte.
[0076]
<Preparation of electrode group>
A strip-shaped positive electrode tab was welded to a current collector exposed area at one end (a winding start end) of the positive electrode in a short direction, and a strip-shaped negative electrode lead was welded to the negative electrode current collector. Next, the positive electrode and the negative electrode are interposed with the separator therebetween, and the winding start end of the negative electrode is spirally wound with the separator facing the current collector exposed region of the positive electrode with the separator interposed therebetween. Was prepared.
[0077]
The above-mentioned electrode group and the above-mentioned electrolytic solution were respectively housed in a stainless steel bottomed cylindrical container having a thickness of 25 μm to assemble the above-mentioned cylindrical lithium ion secondary battery having a battery can outer shape of 18 mm and a height of 65 mm shown in FIG. Was.
[0078]
The lithium ion secondary battery was subjected to a constant voltage charging at 4.2 C for 12 hours at 4.2 V as an initial charging step to produce a lithium ion secondary battery.
[0079]
Further, regarding this lithium ion secondary battery, (R ₁ / R ₂ ) Was measured, and the results are shown in Table 1.
[0080]
(Examples 2 to 4)
A configuration similar to that described in Example 1 described above, except that the complex elastic modulus at 25 ° C. and 85 ° C. and the stretching ratio at 85 ° C. in the MD direction of the separator are set to the values shown in Table 1 below. Was manufactured.
[0081]
(Example 5)
(R) in equation (2) described above. ₁ / R ₂ ) Was manufactured as shown in Table 1 below, except that the lithium ion secondary battery having the same configuration as that described in Example 1 was manufactured.
[0082]
(Comparative example)
Except for setting the complex elastic modulus at 25 ° C. and 85 ° C. in the MD direction (longitudinal direction) and the stretching rate at 85 ° C. of the separator to the values shown in Table 1 below, the same as described in Example 1 described above was used. A lithium ion secondary battery having the same configuration as that of was manufactured.
[0083]
The obtained secondary batteries of Examples 1 to 5 and Comparative Example were charged at a charging current of 1000 mAh to 4.2 V over 3 hours, and then subjected to a charging / discharging cycle test of discharging to 3 V at 1000 mAh at an ambient temperature of 45 ° C. Then, the capacity retention rate after 300 cycles (the discharge capacity at the first cycle was defined as 100%) was measured. The capacity in the first cycle and the capacity retention after 300 cycles are shown in Table 1 below. In addition, a high-temperature storage test in which the battery was stored at 85 ° C. for 24 hours in a 4.2 V charged state was performed, and a battery having a voltage lower than 4 V was regarded as NG.
[0084]
[Table 1]

[0085]
As is clear from Table 1, the secondary batteries of Examples 1 to 5 have a higher capacity retention after 300 cycles at a high temperature of 45 ° C. than the secondary batteries of Comparative Example, and have a capacity of 85 ° C. It can be understood that the voltage drop when stored in a high temperature environment can be suppressed.
[0086]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to provide a nonaqueous electrolyte secondary battery having an improved charge / discharge cycle life under a high temperature environment.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view showing a cylindrical non-aqueous electrolyte secondary battery as an example of a non-aqueous electrolyte secondary battery according to the present invention.
FIG. 2 is a perspective view schematically showing a positional relationship between a positive electrode, a negative electrode, and a separator at a winding start end of an electrode group used in the cylindrical non-aqueous electrolyte secondary battery of FIG.
FIG. 3 is a plan view showing a separator used in the cylindrical non-aqueous electrolyte secondary battery of FIG. 1;
[Explanation of symbols]
1 ... container,
3 ... electrode group,
4 ... Positive electrode,
4a: positive electrode current collector,
4b ... active material containing layer,
4c: positive electrode tab,
5 ... separator,
6 ... negative electrode,
6a: negative electrode current collector,
6b: negative electrode layer,
8. Sealing plate.

Claims (4)

A non-aqueous electrolyte comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and having a longitudinal elongation at 85 ° C. of 7% or less, and a non-aqueous electrolyte. Secondary battery. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the separator has a complex elastic modulus in the longitudinal direction at 25 ° C. of 2.7 × 10 ⁸ Pa or more. The non-aqueous electrolyte secondary battery according to claim 1, wherein the separator has a complex elastic modulus in a longitudinal direction at 85 ° C. of 0.6 × 10 ⁸ Pa or more. 4. An electrode group wound spirally with a separator disposed between a positive electrode and a negative electrode including a current collector and an active material-containing layer supported on one or both surfaces of the current collector, and held by the electrode group A non-aqueous electrolyte secondary battery comprising:
The positive electrode has a current collector exposed region at the winding start end, the winding start end of the negative electrode faces the current collector exposed region of the positive electrode via the separator,
The nonaqueous electrolyte secondary battery, wherein the separator has a stretch ratio in the longitudinal direction at 85 ° C. of 7% or less.

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