JP2015141814A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery in which the expansion of an electrode body and the buckling are suppressed.SOLUTION: A nonaqueous electrolyte secondary battery comprises: an electrode body formed by winding a positive electrode plate having a positive electrode active material, and a negative electrode plate having a negative electrode active material with separators interposed therebetween. The separators are composed of: a separator A disposed on one of inner and outer peripheral face-sides of the positive electrode plate; and a separator B disposed on the other of the inner and outer peripheral face-sides of the positive electrode plate. The separator A is composed of a microporous film having three or more layers including surface layers including inorganic particles and polyolefin and at least one intermediate layer substantially made of polyolefin. The separator B is composed of a microporous film having three or more layers including surface layers substantially made of polyolefin and at least one intermediate layer including inorganic particles and polyolefin.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery in which expansion associated with charge / discharge is suppressed.

  In recent years, electronic devices such as mobile phones including smartphones, tablet computers, and portable game machines have become widespread. In order to satisfy the demands for higher functionality, smaller size, and lighter weight for these electronic devices, non-aqueous electrolyte secondary batteries are used as their drive power sources.

  A non-aqueous electrolyte secondary battery is manufactured by enclosing an electrode body in which a positive electrode plate and a negative electrode plate are wound or laminated with a separator interposed therebetween, and further injecting a non-aqueous electrolyte. For each active material of the positive electrode and the negative electrode, a material capable of electrochemically inserting and extracting lithium ions is used. A lithium transition metal composite oxide such as lithium cobaltate is generally used as the positive electrode active material, and a carbon material is generally used as the negative electrode active material.

  As a separator for a non-aqueous electrolyte secondary battery, a microporous membrane made of polyolefin is generally used. With the recent increase in demand for higher capacity and higher output for non-aqueous electrolyte secondary batteries, not only active materials but also separators have been actively improved to meet the requirements for battery characteristics. Yes.

  Patent Document 1 discloses a three-layer laminated separator in which the surface layer is made of inorganic particles and polyolefin, and the intermediate layer is made of polyolefin. It is described that such a separator is excellent in the electrolyte impregnation property and mechanical strength, and improves the high-temperature storage characteristics of the battery.

  Patent Document 2 discloses a three-layer separator having a surface layer made of polyolefin and an intermediate layer made of inorganic particles and a thermoplastic resin. It is described that such a separator has no difference in rigidity between its front and back surfaces, so that curling of the separator is prevented and the safety of the battery is improved.

International Publication No. 2006/038532 JP 2012-22911 A

  When the nonaqueous electrolyte secondary battery is charged, lithium ions are occluded in the negative electrode active material, and the electrode body expands. When the active material is filled in the electrode plate at a high density in order to improve the capacity of the nonaqueous electrolyte secondary battery, the amount of expansion per unit volume of the active material layer becomes large. For this reason, the electrode plate may be deformed with the expansion of the active material layer to cause buckling. In a non-aqueous electrolyte secondary battery using a wound electrode body, since the electrode plate length is longer than that of the laminated electrode body, buckling tends to occur. When buckling occurs, not only the effect of the expansion of the active material layer on the expansion of the battery becomes noticeable, but also the active material falls from the current collector or the current collector breaks down, reducing the battery capacity. Sometimes it ends up.

In Patent Documents 1 and 2, no consideration is given to the relationship between the separator and the buckling of the electrode plate and the expansion of the battery accompanying charging and discharging. An object of the present invention is to provide a non-aqueous electrolyte secondary battery in which the buckling of the electrode plate and the expansion of the battery accompanying charging / discharging are suppressed by specifying the configuration of the separator used in the electrode body.

  In order to solve the above problems, a nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode plate having a positive electrode active material and an electrode body in which a negative electrode plate having a negative electrode active material is wound through a separator. The separator includes a separator A disposed on one of the inner peripheral surface side and the outer peripheral surface side of the positive electrode plate, and the inner peripheral surface side and the outer peripheral surface side of the positive electrode plate. The separator A is disposed on the other side, and the separator A is a microporous film of three or more layers, the surface layer including inorganic particles and polyolefin, and at least one intermediate layer substantially consisting of polyolefin. B is characterized in that the surface layer is substantially composed of polyolefin, and at least one intermediate layer is a microporous film of three or more layers including inorganic particles and polyolefin.

  In the present invention, one of the separator A and the separator B may be disposed in the electrode body so as to be on the inner peripheral surface side of the positive electrode plate, and the other may be disposed on the outer peripheral surface side of the positive electrode plate.

  The content of inorganic particles contained in the surface layer of the separator A and the intermediate layer of the separator B is preferably 5 to 40% by mass with respect to the total mass of each layer.

  As the separator A, it is preferable to use a microporous film having three or more layers in which a layer containing inorganic particles and polyolefin is disposed on the surface layer, but a single-layer microporous film containing inorganic particles and polyolefin may be used. it can. In this case, the content of the inorganic particles in the separator is preferably 5 to 40% by mass with respect to the total mass of the separator.

  As the separator B, it is preferable to use a three or more microporous film in which a layer made of polyolefin is disposed on the surface layer, but a single layer microporous film made of polyolefin can also be used.

  According to the present invention, since the buckling of the electrode plate accompanying the charge / discharge cycle is suppressed, the expansion of the battery is also suppressed. Moreover, since the active material is prevented from falling off from the current collector and the current collector is prevented from being broken, a rapid capacity drop associated with the charge / discharge cycle is prevented.

FIG. 1 is a perspective view showing a rectangular nonaqueous electrolyte secondary battery used in Examples and Comparative Examples cut in the vertical direction.

  Hereinafter, embodiments for carrying out the present invention will be described. However, it is not intended to limit the present invention to the following embodiments.

  As the positive electrode active material used in the present invention, any material that has been conventionally used as a positive electrode active material of a non-aqueous electrolyte secondary battery can be used without limitation. Specific examples include lithium transition metal composite oxides such as lithium cobaltate, lithium nickelate and lithium manganate capable of reversibly occluding and releasing lithium.

As a negative electrode active material used for this invention, if it is a material conventionally used as a negative electrode active material for nonaqueous electrolyte secondary batteries, it can be used without limitation. Specific examples include carbon materials such as graphite, silicon, tin, and titanium oxide.

  In the present invention, the inorganic particles contained in the separator are preferably insulating inorganic particles, and more preferably metal oxide particles. Examples of the metal oxide include silicon, aluminum, titanium, and magnesium oxides.

  In the present invention, examples of the polyolefin contained in the separator include polyethylene and polypropylene, and these can be used in combination.

  As the non-aqueous electrolyte used in the present invention, a non-aqueous solvent in which a lithium salt as an electrolyte salt is dissolved can be used. Moreover, solid electrolytes, such as a polymer which has lithium ion conductivity, can also be used.

  As the non-aqueous solvent used in the present invention, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferable. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and fluoroethylene carbonate. Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, and methyl butyl carbonate. From the viewpoint of the viscosity and ionic conductivity of the solvent, the cyclic carbonate and the chain carbonate are preferably used in a volume ratio of 5:95 to 50:50.

Examples of the lithium salt used in the present invention include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ( _{C 4 F 9 SO2), LiC} (CF 3 SO 2) 3, LiC (C 2 F 5 SO 2) 3, LiAsF 6, LiClO 4, Li 2 B 10 Cl 10, Li 2 B 12 Cl 12 and mixtures thereof Etc. The concentration of the lithium salt in the non-aqueous solvent is preferably 0.5 to 2.0 mol / L.

  Further, it is preferable to add at least one of vinylene carbonate, cyclohexylbenzene, and dinitrile compound to the nonaqueous electrolyte. By including these compounds in a range of 0.1% by mass or more and 5.0% by mass or less with respect to the total mass of the nonaqueous electrolyte, the cycle characteristics and storage characteristics of the nonaqueous electrolyte secondary battery are obtained. In addition, battery characteristics such as safety during overcharging are improved.

  Hereinafter, an example of a square nonaqueous electrolyte secondary battery as a mode for carrying out the present invention will be described in detail with reference to FIG.

Example 1
(Preparation of positive electrode plate)
Mix so that 95 parts by mass of lithium cobaltate (LiCoO ₂ ) as a positive electrode active material, 2.5 parts by mass of acetylene black as a conductive agent, and 2.5 parts by mass of polyvinylidene fluoride as a binder. The mixture was kneaded so as to be uniformly dispersed in N-methylpyrrolidone as a dispersion medium to prepare a positive electrode mixture slurry. This positive electrode mixture slurry was applied to both surfaces of an aluminum positive electrode core having a thickness of 15 μm by a doctor blade method and dried to form a positive electrode active material layer on both surfaces of the positive electrode core. Then, the positive electrode active material layer was rolled with a rolling roller and cut into a predetermined size to produce a positive electrode plate 11.

(Preparation of negative electrode plate)
Graphite as the negative electrode active material is mixed in 95 parts by mass, styrene butadiene rubber as the binder is mixed in 2 parts by mass, and carboxymethyl cellulose as the thickener is mixed in 3 parts by mass. This mixture is mixed with water as a dispersion medium. A negative electrode mixture slurry was prepared by kneading so as to be uniformly dispersed in the slurry. This negative electrode mixture slurry was applied to both surfaces of a copper negative electrode core having a thickness of 10 μm by a doctor blade method and dried to form a negative electrode active material layer on both surfaces of the negative electrode core. Furthermore, the negative electrode active material layer was rolled with a rolling roller, and cut into a predetermined size to produce a negative electrode plate 12.

(Preparation of separator a)
A separator a composed of a three-layer microporous film in which the surface layer is an inorganic particle-containing layer containing inorganic particles and polyolefin, and the intermediate layer is a polyolefin layer composed of polyolefin was produced as follows.

  The raw material of the inorganic particle-containing layer was prepared by mixing and stirring together with liquid paraffin as a plasticizer so that silica as inorganic particles was 15 parts by mass and polyethylene as polyolefin was 85 parts by mass. And the polyethylene as polyolefin was mixed and stirred with the liquid paraffin as a plasticizer, and the raw material of the polyolefin layer was prepared.

  The raw material of each layer was molded into a sheet by a co-extrusion method so that the inorganic particle-containing layers were disposed on both surfaces of the polyolefin layer. Furthermore, the liquid paraffin was removed from this sheet | seat, and the separator a was produced by extending | stretching a sheet | seat. The thickness of one side of the inorganic particle-containing layer that becomes the surface layer of the separator a is 2 μm, the thickness of the polyolefin layer that becomes the intermediate layer is 14 μm, and the total thickness is 18 μm.

(Preparation of separator b)
A separator b made of a three-layer microporous membrane in which the surface layer was a polyolefin layer made of polyolefin and the intermediate layer was an inorganic particle-containing layer containing inorganic particles and polyolefin was produced as follows.

  Polyethylene as a polyolefin was mixed and stirred together with liquid paraffin as a plasticizer to prepare a raw material for the polyolefin layer. And it mixed and stirred with the liquid paraffin as a plasticizer so that the silica as an inorganic particle might be 15 mass parts and the polyethylene as a polyolefin might be 85 mass parts, and the raw material of the inorganic particle content layer was prepared.

  Next, the raw material of each layer was shape | molded by the coextrusion method so that the polyolefin layer may be arrange | positioned on the surface of the both sides of an inorganic particle content layer. Furthermore, the liquid paraffin was removed from this sheet | seat, and the separator b was produced by extending | stretching a sheet | seat. The thickness of one side of the polyolefin layer serving as the surface layer of the separator b is 2 μm, the thickness of the inorganic particle-containing layer serving as the intermediate layer is 14 μm, and the total thickness is 18 μm.

(Production of electrode body)
The positive electrode plate 11 and the negative electrode plate 12 produced as described above are wound together with the positive electrode plate inner peripheral surface side separator 13 and the positive electrode outer peripheral surface side separator 14 and pressed to obtain a flat electrode body. 15 was produced. The separator a was used for the positive electrode plate inner peripheral surface side separator 13, and the separator b was used for the positive electrode outer peripheral surface side separator 14.

(Preparation of non-aqueous electrolyte)
A nonaqueous solvent used for a nonaqueous electrolyte by mixing ethylene carbonate (EC), ethylmethyl carbonate (EMC) and diethyl carbonate (DEC) at a volume ratio of 20:50:30 (25 ° C., 1 atm). Was prepared. In this non-aqueous solvent, lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte salt was dissolved at 0.9 mol / L to prepare a non-aqueous electrolyte.

(Preparation of non-aqueous electrolyte secondary battery)
The flat electrode body 15 produced as described above was inserted into the outer case 16, and the insulating spacer 21 was disposed on the electrode body 15. After the sealing plate 17 was welded to the opening of the outer case 16, a nonaqueous electrolyte was injected from the liquid injection hole 22, and the liquid injection hole 22 was sealed to seal the inside of the battery. The outer case 16 is electrically connected to the positive electrode plate 11, and the outer case 16 and the sealing plate 17 function as positive terminals. A negative electrode terminal 19 is attached to the sealing plate 17 via an insulator 18, and the negative electrode terminal 19 is electrically connected to a negative electrode lead 20 led out from the negative electrode plate 12. The external size of the nonaqueous electrolyte secondary battery 10 is 4.3 mm thick × 34 mm high × 43 mm wide, and the design capacity is 840 mAh.

(Example 2)
A nonaqueous electrolyte secondary battery according to Example 2 was produced in the same manner as in Example 1 except that the arrangement of the separators a and b was changed.

(Comparative Example 1)
A nonaqueous electrolyte secondary battery according to Comparative Example 1 was produced in the same manner as in Example 1 except that a single-layer separator made only of polyethylene was used instead of the separators a and b.

(Comparative Example 2)
A nonaqueous electrolyte secondary battery according to Comparative Example 2 was produced in the same manner as in Example 1 except that the separator a was used instead of the separator b.

(Comparative Example 3)
A nonaqueous electrolyte secondary battery according to Comparative Example 3 was produced in the same manner as in Example 1 except that the separator b was used instead of the separator a.

(First charge)
The non-aqueous electrolyte secondary batteries according to Examples and Comparative Examples produced as described above were charged at 25 ° C. with a constant current of 1 It (= 840 mA) until the battery voltage reached 4.35 V, and then 4. The battery was charged at a constant voltage of 35 V until reaching 17 mA.

(Discharge-charge cycle)
The battery after the first charge was repeatedly discharged and charged under the following conditions for 500 cycles. Discharging was performed at a constant current of 1 It until the battery voltage reached 3.0V. Charging was performed under the same conditions as the first charging. The rest time after discharging and after charging was 10 minutes.

(Measurement of battery thickness)
The thickness of the nonaqueous electrolyte secondary battery according to the example and the comparative example after the battery assembly, after the initial charge and after the cycle was measured using a micrometer. And according to the following formula | equation, the battery expansion rate after the first charge and the battery expansion rate after the cycle were calculated. The results are summarized in Table 1.

Battery expansion rate after initial charge (%) = ((Battery thickness after initial charge−Battery thickness after assembly) / Battery thickness after assembly) × 100
Battery expansion rate after cycle (%) = ((Battery thickness after cycle−Battery thickness after assembly) / Battery thickness after assembly) × 100

  From Table 1, it can be seen that in both Examples 1 and 2, the battery thickness expansion rate after 500 cycles is suppressed to be lower than that in the comparative example. Since the same effect is obtained in Examples 1 and 2, one of separators a and b is arranged on the inner peripheral surface side of the positive electrode plate in the electrode body, and the other is on the outer peripheral surface side of the positive electrode plate. It can be seen that it is only necessary to be placed in.

  Although the cause of the effect of the present invention is not clear, it is presumed that the adhesiveness between the separator and the electrode plate changes depending on the presence or absence of inorganic particles in the surface layer of the separator. In other words, when an electrode body is manufactured using a separator having high adhesion to the electrode plate, the separator tries to maintain the shape of the original electrode body, but stress due to expansion and contraction of the electrode plate cannot be relieved. When the stress exceeds a certain value, the electrode plate is locally deformed and buckling occurs. When an electrode body is manufactured using a separator having low adhesion to the electrode plate, the separator relieves stress due to expansion and contraction of the electrode plate, but retains the shape change of the electrode body due to electrode plate expansion and contraction. I can't. That is, when two types of separators having different adhesion to the electrode plate are used as in Examples 1 and 2, the electrode body shape holding action and the stress relaxation action due to the electrode plate expansion and contraction are mutually related. As a result of the action, it is presumed that the effects of the present invention as described above were achieved.

  In the above embodiment, a three-layer separator is used. However, it is considered that the effect of the present invention is exhibited due to surface physical properties such as adhesion between the separator and the electrode plate. Therefore, whether it is a single-layer separator or a separator having more than three layers, the surface layer of one separator is an inorganic particle-containing layer and the surface layer of the other separator is a polyolefin layer. It is thought that an effect is produced. The polyolefin layer preferably contains no inorganic particles, but may contain an amount of inorganic particles that does not affect the adhesion to the electrode plate.

  Since the present invention can effectively suppress the swelling of the nonaqueous electrolyte secondary battery, the industrial applicability is great.

DESCRIPTION OF SYMBOLS 10 Nonaqueous electrolyte secondary battery 11 Positive electrode plate 12 Negative electrode plate 13 Positive electrode plate inner peripheral surface side separator 14 Positive electrode plate outer peripheral surface side separator 15 Electrode body 16 Exterior case 17 Sealing plate 18 Insulator 19 Negative electrode terminal 20 Negative electrode lead 21 Insulating spacer 22 Injection hole

Claims (6)

A non-aqueous electrolyte secondary battery comprising an electrode body in which a positive electrode plate having a positive electrode active material and a negative electrode plate having a negative electrode active material are wound through a separator,
The separator comprises a separator A disposed on one of the inner peripheral surface side and the outer peripheral surface side of the positive electrode plate, and a separator B disposed on the other of the inner peripheral surface side and the outer peripheral surface side of the positive electrode plate,
The separator A is a microporous film having three or more layers, the surface layer containing inorganic particles and polyolefin, and at least one intermediate layer substantially consisting of polyolefin,
The separator B is a microporous film having three or more layers, the surface layer being substantially made of polyolefin, and at least one intermediate layer containing inorganic particles and polyolefin.
Non-aqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1,
The surface layer contains inorganic particles and polyolefin, and at least one intermediate layer is replaced with three or more microporous membranes substantially composed of polyolefin,
The separator A is a single-layer microporous film containing inorganic particles and polyolefin,
Non-aqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1,
The surface layer is substantially composed of polyolefin, and at least one intermediate layer is replaced with three or more microporous membranes containing inorganic particles and polyolefin,
The separator B is a single-layer microporous film substantially made of polyolefin,
Non-aqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 1,
The surface layer contains inorganic particles and polyolefin, and at least one intermediate layer is replaced with three or more microporous membranes substantially composed of polyolefin,
The separator A is a single-layer microporous film containing inorganic particles and polyolefin,
The surface layer is substantially composed of polyolefin, and at least one intermediate layer is replaced with three or more microporous membranes containing inorganic particles and polyolefin,
The separator B is a single-layer microporous film substantially made of polyolefin,
Non-aqueous electrolyte secondary battery.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the inorganic particles in each surface layer containing the inorganic particles of the separator A is 5 to 40% by mass.   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the inorganic particles in the intermediate layer containing the inorganic particles of the separator B is 5 to 40 mass%.

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