JP2006004873A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery with excellent charge/discharge cycle characteristics and preservation characteristics. <P>SOLUTION: The nonaqueous electrolyte secondary battery is composed of: an electrode plate group made by winding around a cathode plate 11 and an anode plate 12 through a separator consisting of two or more layers porous resin; nonaqueous electrolyte; and an outer package body housing the electrode plate group and the nonaqueous electrolyte. (1) The separator is made by laminating two or more layers of separators with different coefficients of elasticity in a thickness direction, and (2) a formula of (K<SP>-</SP>/K)>1.1 is satisfied on condition that a coefficient of elasticity of a layer in contact with an anode in a thickness direction is K<SP>-</SP>, and that of a layer with the least coefficient of elasticity in the thickness direction is K. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, the present invention relates to improvement of charge / discharge cycle characteristics of a non-aqueous electrolyte secondary battery.

  In recent years, cordless and portable electronic devices are rapidly advancing, and nonaqueous electrolyte secondary batteries having high voltage and high energy density are being put to practical use as power sources for driving these devices. For the positive electrode of the nonaqueous electrolyte secondary battery, a composite oxide of a transition metal and lithium having a generally high redox potential is used. As the composite oxide, lithium cobaltate, lithium nickelate, lithium manganate, or the like is used, but an oxide containing a plurality of transition metals is also used. A carbon material is generally used for the negative electrode. For the separator that separates the positive electrode and the negative electrode, a microporous film or non-woven fabric made of polyolefin resin is generally used. In addition, there are also films in which a plurality of films having different properties are stacked to have those properties. For example, by combining polyethylene, which has a relatively low melting point of about 120 ° C., and polypropylene, polyimide, etc., which has a high melting point and high mechanical strength, both shutdown performance and short-circuit resistance at abnormally high temperatures can be achieved at a high level. Etc. have been proposed.

  As described above, although there are many proposals for stacking separators having different properties for the purpose of improving safety, there are very few proposals for improving the charge / discharge characteristics.

  The non-aqueous electrolyte secondary battery has a problem that the capacity of the battery gradually decreases when the charge / discharge cycle is repeated. There are various causes of this problem, but the following phenomenon can be considered as a cause of the involvement of the separator. The positive electrode and the negative electrode expand during charging and contract during discharging. This repeated expansion / contraction causes various cycle deterioration factors. For example, an electrolyte solution is extruded during charging from an electrode plate group formed by winding or laminating a positive electrode, a negative electrode, and a separator, and returns to the electrode plate group during discharging. If this is repeated, a portion where the electrolytic solution is insufficient is generated inside the electrode plate group, and charge / discharge characteristics are deteriorated. For this reason, a separator, a positive electrode, and a negative electrode are calculated | required by favorable electrolyte solution absorptivity. Further, a separator having a much smaller elastic modulus in the thickness direction than the positive electrode and the negative electrode repeats contraction / expansion by repeating the expansion / contraction of the positive electrode and the negative electrode. At this time, the electrolytic solution decomposition product on the negative electrode is sucked into the gap inside the separator. Then, a reaction product with the electrolytic solution is newly generated on the negative electrode. By repeating this, the separator is clogged, the lithium ion conductivity is lowered, the charge / discharge state in the electrode plate is uneven, and the charge / discharge characteristics are lowered. In addition, the positive electrode and negative electrode active materials, the conductive agent, and the binder (hereinafter collectively referred to as positive electrode mixture and negative electrode mixture) bite into the separator simultaneously with the electrolytic decomposition product. Will cause clogging.

  Thus, although the expansion / contraction in the thickness direction has a great influence on the charge / discharge characteristics, the physical property value in the thickness direction and the configuration of the electrode plate group have not been studied.

Therefore, an ultrahigh molecular weight polyethylene microporous membrane having a weight average molecular weight of 1 million or more and a high porosity polyethylene microporous membrane having a porosity of 45% or more are laminated, and the ultrahigh molecular weight polyethylene is provided on the positive electrode plate side. There is a proposal to improve the high-temperature cycle characteristics by arranging the microporous membrane layer in contact with the high porosity polyethylene microporous membrane layer on the negative electrode plate side (see, for example, Patent Document 1).
JP 2002-15720 A

  However, in this proposal, oxidation of the separator by the positive electrode is suppressed by ultra high molecular weight polyethylene, and clogging of the separator by the electrolytic solution decomposition product at the negative electrode is suppressed by the high porosity separator. Although the present invention is consistent with suppressing the clogging of the separator of the negative electrode film, the present invention has a completely different idea in that the shrinkage / expansion of the separator that causes clogging is controlled. An object of the present invention is to improve cycle characteristics.

  In the present invention, separators having different elastic modulus in the thickness direction are laminated, and a layer having a large elastic modulus in the thickness direction is in contact with the negative electrode, so that the electrolytic solution decomposition product on the negative electrode and the negative electrode are caused by the shrinkage / expansion of the separator This is based on the discovery that absorption of the mixture into the separator can be suppressed.

  The mechanism by which the film and the mixture present on the negative electrode are absorbed by the separator is compressed by the separator when the positive electrode and the negative electrode expand during charging. At this time, at the interface between the negative electrode and the separator, the electrolyte reaction product and the negative electrode mixture on the negative electrode surface are pushed into the porous separator. Thereafter, in the discharged state, the positive electrode and the negative electrode shrink to the original state, so that the separator returns to the original state. At this time, a part of the electrolyte reaction product and the negative electrode mixture pushed into the separator cannot return to the negative electrode due to a frictional force with the wall surface of the hole inside the separator, and remains inside the separator. This amount is considered to increase as the degree of shrinkage in the thickness direction of the separator in contact with the negative electrode increases.

  In view of the above, the present invention provides an electrode plate group obtained by winding a positive electrode plate and a negative electrode plate through a separator composed of two or more layers of porous resin, a non-aqueous electrolyte, and the electrode plate group and the non-aqueous electrode. A non-aqueous electrolyte secondary battery comprising an outer package containing an electrolyte, wherein (1) the separator is formed by laminating two or more separators having different elastic moduli in the thickness direction, and (2) a layer in contact with the negative electrode (K− / K)> 1.1 where K− is the elastic modulus in the thickness direction and K is the elastic modulus in the thickness direction of the layer having the smallest elastic modulus in the thickness direction. The present invention relates to an electrolyte secondary battery.

  According to the present invention, by using two or more laminated separators having different elastic modulus in the thickness direction, a layer having a small elastic modulus in the thickness direction is configured so as not to contact the negative electrode. A water electrolyte secondary battery can be obtained.

  In the present invention, two or more laminated separators having different elastic modulus in the thickness direction are wound with the positive electrode and the negative electrode so that the layer having the smallest elastic modulus in the thickness direction is not in contact with the negative electrode. You may implement | achieve the said structure by overlapping the above single layer separator at the time of winding.

  When the elastic modulus in the thickness direction of the layer in contact with the negative electrode of the laminated separator is K− and the elastic modulus of the layer having the smallest elastic modulus in the thickness direction is K, the ratio K− / K is 1.1 or more. Although the effect of the invention can be obtained, the larger this value, the greater the effect. Preferably it is 1.3 or more.

  Since the layer having the smallest elastic modulus in the thickness direction contracts due to the expansion of the positive electrode and the negative electrode during charging, the porosity is 30% to 80% in order to ensure lithium ion conductivity even when contracted. The thickness is preferably 3 μm to 47 μm. A more preferable thickness is 5 μm to 47 μm.

  If the electrode plate group is configured using three or more laminated separators so that the layer having the smallest elastic modulus in the thickness direction is not in contact with either the negative electrode or the positive electrode, it is more preferable that the positive electrode mixture bites into the separator.

  As a method for producing separators having different elastic moduli in the thickness direction, it is produced by controlling at least one factor such as material, porosity, pore diameter, production method, layer type (microporous membrane, nonwoven fabric, etc.), etc. It is possible, but not particularly limited.

  By the way, the positive electrode is applied, for example, to one side or both sides of the current collector with a slurry-like mixture in which a positive electrode active material, a binder and, if necessary, a conductive agent, a thickener and the like are kneaded and dispersed in a solvent, An active material layer is formed by drying and rolling, and a positive electrode lead is welded to a plain portion of a current collector without an active material layer.

  As the current collector of the positive electrode, an aluminum foil or a foil having a thickness of 10 μm to 60 μm subjected to lath processing or etching treatment is preferable.

Although it does not specifically limit as a positive electrode active material, For example, the lithium containing transition metal compound which can accept a lithium ion as a guest is used. For example, cobalt, manganese, nickel, chromium, complex metal oxide of at least one metal and lithium selected from iron and _{vanadium, LiCoO 2, LiMnO 2, LiNiO} 2, LiCo x Ni (1-x) O 2 (0 <X <1), LiCrO ₂ , αLiFeO ₂ , LiVO _{2 and the} like are preferable.

  The binder is not particularly limited as long as it can be dissolved or dispersed in a solvent. For example, fluorine binder, acrylic rubber, modified acrylic rubber, styrene-butadiene rubber (SBR), acrylic Polymers, vinyl polymers and the like can be used alone or in combination of two or more. As the fluorine-based binder, for example, polyvinylidene fluoride, a copolymer of vinylidene fluoride and propylene hexafluoride, or a dispersion of a polytetrafluoroethylene resin is preferable.

  As the conductive agent that can be added as necessary, acetylene black, graphite, carbon fiber, etc. are preferably used alone or in combination of two or more, and as the thickener, ethylene-vinyl alcohol copolymer, carboxy is used. Methyl cellulose, methyl cellulose and the like are preferable.

  As the solvent, a solvent in which the binder can be dissolved or dispersed is suitable. When using a binder that can be dissolved or dispersed in an organic solvent, N-methyl-2-pyrrolidone, N, N-dimethylformamide, tetrahydrofuran, dimethylacetamide, dimethylsulfoxide, hexamethylsulfuramide, tetramethylurea, acetone It is preferable to use an organic solvent such as methyl ethyl ketone alone or as a mixture thereof, and when using a water-soluble binder, water or warm water is preferable.

  In addition, various dispersants, surfactants, stabilizers, and the like can be added as necessary when the slurry mixture is kneaded and dispersed.

  The coating method is not particularly limited, and the slurry-like mixture kneaded and dispersed as described above, for example, slit die coater, reverse roll coater, lip coater, blade coater, knife coater, gravure coater. It can be easily applied to the current collector using a dip coater or the like. As the drying method, drying close to natural drying is preferable, but considering the productivity, it is preferable to dry the mixture at a temperature of 70 ° C. to 300 ° C. for 1 minute to 5 hours.

  Rolling is preferably carried out several times at a linear pressure of 1000 kg / cm to 2000 kg / cm or by changing the linear pressure until a predetermined thickness is reached by a roll press.

  The negative electrode is, for example, coated on one surface of a current collector with a negative electrode active material, a binder and, if necessary, a slurry-like mixture in which a conductive additive is kneaded and dispersed in an organic solvent, and dried. After coating and drying on the other side, the active material layer is formed by rolling, and the negative electrode lead is welded to the plain portion of the current collector without the active material layer.

  As the negative electrode current collector, a copper foil, a foil having a thickness of 5 μm to 50 μm subjected to lath processing or etching treatment is preferable.

  Although it does not specifically limit as a negative electrode active material, For example, it obtains by baking the carbon material obtained by baking organic polymer compounds (Phenol resin, polyacrylonitrile, cellulose, etc.), coke, and pitch. Carbon material, artificial graphite, natural graphite or the like can be used. As the shape, a spherical shape, a scale shape or a lump shape can be used.

  As the binder and the thickener that can be added as necessary, the same binder and thickener as the positive electrode can be used, and as the conductive assistant, the same conductive agent as the positive electrode can be used. it can.

  A non-aqueous electrolyte can be prepared by dissolving a solute in a non-aqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-dichloroethane, 1,3-dimethoxypropane, 4- Methyl-2-pentanone, 1,4-dioxane, acetonitrile, propionitrile, butyronitrile, valeronitrile, benzonitrile, sulfolane, 3-methyl-sulfolane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylformamide, dimethylsulfoxide, dimethylformamide, Trimethyl phosphate, triethyl phosphate and the like can be used. These non-aqueous solvents can be used alone or as a mixed solvent of two or more.

As the solute contained in the nonaqueous electrolyte, for example, a lithium salt having a strong electron withdrawing property is used. For example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) _{3 and the} like can be mentioned. It is done. These solutes may be used alone or in combination of two or more. These solutes are preferably dissolved in the non-aqueous solvent at a concentration of 0.5 to 1.5 M (mol / liter).

  The non-aqueous electrolyte is preferably added with 0.01 to 5.0% by weight of at least one selected from vinylene carbonate, which is a cyclic compound having a C═C unsaturated bond, and derivatives thereof as an additive. Since vinylene carbonate forms a dense film that suppresses decomposition of the non-aqueous electrolyte on the negative electrode, it has the effect of further improving the charge / discharge cycle characteristics of the battery.

Example 1
A non-aqueous electrolyte secondary battery having a cylindrical shape 18650 (diameter 18 mm, height 65 mm) as shown in FIG. 1 was produced.

(I) Production of positive electrode 100 parts by weight of LiCoO ₂ as a positive electrode active material, 3.5 parts by weight of carbon black as a conductive agent, and a dispersion of polytetrafluoroethylene as a binder (solid content 60% by weight) %) And 7 parts by weight of an aqueous solution of carboxymethyl cellulose (solid content 1% by weight) as a thickener were kneaded and dispersed to obtain a paste-like positive electrode mixture. This positive electrode mixture was applied to both sides of a current collector made of aluminum foil having a thickness of 20 μm, rolled, dried in dry air (dew point: −50 ° C. or lower) at 200 ° C. for 5 hours, and then cut into predetermined dimensions. Thus, the positive electrode 11 was obtained. An aluminum lead 14 was connected to the positive electrode 11.

(Ii) Production of negative electrode 100 parts by weight of artificial graphite as a negative electrode active material, 5 parts by weight of a dispersion of styrene butadiene rubber (solid content 48% by weight) as a binder, and carboxymethylcellulose as a thickener 140 parts by weight of an aqueous solution (solid content: 1% by weight) was kneaded and dispersed to obtain a negative electrode mixture. This negative electrode mixture was applied to both sides of a current collector made of copper foil having a thickness of 14 μm, rolled, dried in dry air (dew point: −50 ° C. or lower) at 110 ° C. for 5 hours, and then cut into predetermined dimensions. Thus, the negative electrode 12 was obtained. A nickel lead 15 was connected to the negative electrode 12.

(Iii) Preparation of non-aqueous electrolyte LiPF ₆ was dissolved at a ratio of 1.2 mol / liter in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1: 2, and 2.0 parts by weight of vinylene carbonate was added. Thus, a non-aqueous electrolyte was obtained.

(Iv) Composition of separator 42% porosity composed of polypropylene resin, 35N / m ² thickness modulus in the thickness direction, 10μm thick layer, 42% porosity composed of polyethylene resin, 20N modulus in the thickness direction A two-layer separator 13 having a layer of m ² and a thickness of 10 μm was used.

  For measuring the elastic modulus in the thickness direction, an RTC-1150A test apparatus manufactured by Orientec Co., Ltd. was used. The compression test terminal was 30 mm in diameter, and an appropriate number of samples were stacked for testing. The elastic modulus in the thickness direction was determined from the strain-stress curve obtained in the test.

(V) Assembly of battery The following operation was performed in dry air. First, the positive electrode 11 and the negative electrode 12 were laminated with the separator 13 disposed between them, and wound to obtain a cylindrical electrode plate group. At this time, the layer having a large elastic modulus in the thickness direction was configured to be in contact with the negative electrode (K− / K = 1.75). The obtained electrode plate group was accommodated in a cylindrical iron battery case 18 having a nickel plating inside. An upper insulating plate 16 and a lower insulating plate 17 are arranged above and below the electrode plate group, respectively. The negative nickel lead 15 was connected to the inner bottom surface of the case 18, a groove was formed in the upper part of the case 18, and then the positive aluminum lead 14 was welded to the metal part of the sealing body 19. Further, after injecting a non-aqueous electrolyte into the case 18, the sealing body 19 having the positive electrode terminal 20 is applied to the opening of the case 18, and the opening end of the case 18 is caulked to the upper peripheral edge of the sealing body 19 to seal the battery. did. In this way, a cylindrical battery An (n = 1 to 5) as shown in FIG. 1 was completed. The battery capacity of the battery An was 2000 mAh.

(Vi) Battery evaluation [Charge / discharge cycle characteristics]
The battery An (n = 1 to 5) that has undergone the finishing process is discharged at a constant current of 2000 mA under a 20 ° C. environment until the voltage between terminals reaches 3 V, and is charged for 2 hours at an upper limit voltage of 4.2 V and an upper limit current of 2000 mA. The charge / discharge cycle was repeated 500 cycles. The discharge capacity at the first cycle is C1, the discharge capacity at the 500th cycle is C2, and the capacity retention ratio X1 is X (%) = (C2 / C1) × 100.
Determined by The average value of the results is shown in Table 1.

<< Comparative Example 1 >>
A battery R1n (n) is formed in the same manner as in Example 1 except that a single-layer separator made of a polyethylene resin-made porosity of 42%, a thickness direction elastic modulus K = 20 N / m ² , and a thickness of 20 μm is used. = 1 to 5). Battery R1n was evaluated in the same manner as in Example 1. The average value of the results is shown in Table 1.

Example 2
Porosity 42% made of polyethylene resin, elastic modulus 20N / m ² in the thickness direction, 10 μm thick layer, porosity made of polyethylene resin 36%, elastic modulus 23 N / m ² in the thickness direction, thickness 10 μm In the same manner as in Example 1, except that a layer having a large elastic modulus in the thickness direction was configured to be in contact with the negative electrode (K− / K = 1.15), a battery Bn ( n = 1-5) were produced. Battery Bn was evaluated in the same manner as in Example 1. The average value of the results is shown in Table 1.

<< Comparative Example 2 >>
A battery R2n (n) is formed in the same manner as in Example 1 except that the same separator as in Example 2 is used and a layer having a small elastic modulus in the thickness direction is configured to be in contact with the negative electrode (K− / K = 1.00). = 1 to 5). Battery R2n was evaluated in the same manner as in Example 1. The average value of the results is shown in Table 1.

Example 3
Porosity 50% of polyethylene resin, elastic modulus 17N / m ² in the thickness direction, a layer of thickness 10 [mu] m, porosity 36% of polyethylene resin, the thickness direction modulus 23N / m ^2, a thickness of 10 [mu] m In the same manner as in Example 1, except that a two-layer separator composed of the above layer was used and a layer having a large elastic modulus in the thickness direction was configured to be in contact with the negative electrode (K− / K = 1.35), the battery Cn ( n = 1-5) were produced. The battery Cn was evaluated in the same manner as in Example 1. The average value of the results is shown in Table 1.

<< Comparative Example 3 >>
A battery R3n (n) is formed in the same manner as in Example 3, except that the same separator as in Example 3 is used and a layer having a small elastic modulus in the thickness direction is configured to be in contact with the negative electrode (K− / K = 1.00). = 1 to 5). Battery R3n was evaluated in the same manner as in Example 1. The average value of the results is shown in Table 1.

Example 4
A porosity of 42% made of polypropylene resin, an elastic modulus in the thickness direction of 35 N / m ² and a thickness of 6 μm, a porosity of 42% made of polyethylene resin, an elastic modulus in the thickness direction of 20 N / m ² , and a thickness of 8 μm A battery was fabricated in the same manner as in Example 1 except that a 20 μm thick three-layer separator (K− / K = 1.75) formed by laminating the layers in order of polypropylene layer / polyethylene layer / polypropylene layer was used. Dn (n = 1-5) was produced. The battery Dn was evaluated in the same manner as in Example 1. The average value of the results is shown in Table 1.

  As is apparent from Table 1, it can be seen that the cycle characteristics of the example are improved with respect to the comparative example 1. From the comparison between Comparative Example 2 and Example 2 and the comparison between Comparative Example 3 and Example 3, even if the same separator is used, if the elastic modulus in the thickness direction of the separator in contact with the negative electrode is small, the cycle characteristics are not improved, rather It is falling. From these facts, it can be seen that the larger the K− / K ratio, the better the cycle characteristics. This is because the electrolytic solution decomposition product and the negative electrode mixture on the negative electrode are absorbed in the separator due to the shrinkage and expansion of the separator during charging and discharging, and the negative electrode is in contact with the negative electrode by increasing the K− / K ratio. This is presumed to be an effect of suppressing the shrinkage / expansion of the separator. Further, Example 1 and Example 4 have the same K− / K ratio, but Example 4 has better cycle characteristics. This is presumably because the separator in contact with the positive electrode also has a high elastic modulus in the thickness direction, so that the positive electrode mixture is prevented from biting into the separator.

  ADVANTAGE OF THE INVENTION According to this invention, the nonaqueous electrolyte secondary battery excellent in the charge / discharge cycle characteristic can be obtained, and it is useful as a drive power source for portable equipment.

A longitudinal sectional view of an example of the nonaqueous electrolyte secondary battery of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Positive electrode 12 Negative electrode 13 Separator 14 Positive electrode lead 15 Negative electrode lead 16 Upper insulating plate 17 Lower insulating plate 18 Battery case 19 Sealing body 20 Positive electrode terminal

Claims (8)

An electrode plate group formed by winding a positive electrode plate and a negative electrode plate through a separator composed of two or more layers of porous resin, a non-aqueous electrolyte, and a non-aqueous electrolyte comprising an outer package housing the electrode plate group and the non-aqueous electrolyte. A water electrolyte secondary battery, wherein (1) the separator is formed by laminating two or more separators having different elastic modulus in the thickness direction, and (2) the elastic modulus in the thickness direction of the layer in contact with the negative electrode is K −, A non-aqueous electrolyte secondary battery in which (K− / K)> 1.1, where K is the elastic modulus of the layer having the smallest elastic modulus in the thickness direction. The non-aqueous electrolyte secondary battery according to claim 1, wherein the separator in which two or more layers having different elastic moduli in the thickness direction are laminated is manufactured by changing a material. The non-aqueous electrolyte secondary battery according to claim 1, wherein the separator in which two or more layers having different elastic moduli in the thickness direction are laminated is produced by changing the porosity. The non-aqueous electrolyte secondary battery in which the ratio of elastic modulus in the thickness direction according to claim 1 is (K- / K)> 1.3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the total thickness is 5 μm to 50 μm, and the thickness of the layer having the smallest elastic modulus in the thickness direction is 3 μm to 47 μm. The nonaqueous electrolyte secondary battery according to claim 1, wherein the porosity of the layer having the smallest elastic modulus in the thickness direction is 30% to 80%. The nonaqueous electrolyte secondary battery according to claim 1, wherein the separator is made of a polyolefin resin. The separator in which layers having different elastic moduli in the thickness direction are laminated, and is formed by laminating three or more separators, and the layer having the smallest elastic modulus in the thickness direction is configured not to contact the negative electrode and the positive electrode. Non-aqueous electrolyte secondary battery.

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