JP4839117B2 - Cylindrical lithium secondary battery - Google Patents

Description

  The present invention relates to a cylindrical lithium secondary battery that achieves both safety and battery characteristics.

  Lithium secondary batteries are attracting attention as high-capacity power supplies, mainly portable devices. Furthermore, in recent years, lithium secondary batteries have been attracting attention as a high-output power source mainly in electric vehicles. In general, a chemical battery including a lithium secondary battery has a separator that functions to electrically insulate a positive electrode and a negative electrode and hold an electrolyte. In the case of a lithium secondary battery, a microporous film made of polyolefin (for example, polyethylene, polypropylene, etc.) is mainly used as a separator. The electrode group of the cylindrical lithium secondary battery is formed by winding the positive electrode and the negative electrode together with a separator interposed between them into a cylindrical shape.

  However, when a lithium secondary battery is held in an extremely high temperature environment for a long time, a separator made of a microporous film tends to shrink. When the separator contracts, an internal short circuit in which the positive electrode and the negative electrode are in physical contact may occur. Particularly, in recent years, with the increase in capacity of lithium secondary batteries, separators tend to be thinner. Therefore, prevention of internal short circuit is becoming more important. Once an internal short circuit occurs, the short circuit part may expand due to Joule heat associated with the short circuit current, and the battery may overheat.

Therefore, even if an internal short circuit occurs, it has been proposed to support a porous heat-resistant layer containing an inorganic filler (solid fine particles) and a binder on the electrode active material layer from the viewpoint of suppressing the expansion of the short circuit part. ing. As the inorganic filler, alumina, silica, or the like is used. The porous heat-resistant layer is filled with an inorganic filler, and filler particles are bonded to each other with a relatively small amount of a binder (Patent Document 1). Since the porous heat-resistant layer hardly shrinks even at high temperatures, it has a function of suppressing overheating of the battery when an internal short circuit occurs.
Japanese Patent Laid-Open No. 7-220759

  In recent years, there has been an increasing need to complete charging in a short time in the power source of portable devices. In order to finish charging in a short time, it is necessary to perform high rate charging (for example, 1 hour rate or less). In the high rate charge, compared with the case of the low rate charge (for example, 1.5 hour rate or more), the expansion and contraction of the electrode plate accompanying the charge / discharge are large, and the gas generation becomes remarkable. Therefore, distortion occurs in the electrode group. Since the amount of the binder contained in the porous heat-resistant layer is relatively small, the binding force between the filler particles is small. Therefore, the porous heat-resistant layer may be damaged, and the function of suppressing overheating of the battery during an internal short circuit may be deteriorated.

  An object of the present invention is to provide a cylindrical lithium secondary battery capable of ensuring safety by preventing damage to a porous heat-resistant layer and realizing excellent battery characteristics.

The present invention includes a cylindrical battery can having a bottom, a side wall, and an upper opening, an electrode group, a nonaqueous electrolyte, and a sealing plate that covers the upper opening of the battery can containing the electrode group and the nonaqueous electrolyte. The present invention relates to a lithium secondary battery. The battery can is made of iron or stainless steel. The electrode group is formed by winding a belt-like positive electrode and a belt-like negative electrode together with a porous heat-resistant layer and a separator interposed therebetween. The positive electrode includes a positive electrode core material and a positive electrode active material layer supported on both surfaces thereof, and the negative electrode includes a negative electrode core material and a negative electrode active material layer supported on both surfaces thereof. The positive electrode active material layer includes a lithium-containing transition metal oxide as a positive electrode active material, the negative electrode active material layer includes natural graphite or artificial graphite as a negative electrode active material, and the porous heat-resistant layer has an insulating property. A filler and a binder are included, the amount of the binder is 1 to 10 parts by weight per 100 parts by weight of the insulating filler, and the porosity of the porous heat-resistant layer is 40 to 80%. The thickness A of the porous heat-resistant layer and the thickness B of the side wall of the battery can satisfy 0.005 ≦ A / B ≦ 0.1.

  The thickness A of the porous heat-resistant layer is 2 to 10 μm, the thickness B of the side wall of the battery can is 80 to 300 μm, and the thickness A of the porous heat-resistant layer and the thickness B of the side wall of the battery can are 0.01 ≦ A It is preferable to satisfy /B≦0.05.

The porous heat-resistant layer is preferably supported on at least one surface of two active material layers supported on both surfaces of the core material in at least one of the positive electrode and the negative electrode.
Insulation filler is preferably made of an inorganic oxide.

  When performing high rate charging, a large strain is applied to the electrode group. However, if the side wall of the battery can sufficiently pushes back the electrode group, the porous heat-resistant layer can be prevented from being damaged. In this case, since the porous heat-resistant layer is pressed against the active material layer of the positive electrode or the negative electrode, it is considered that the shape can be maintained.

  However, the porous heat-resistant layer is also required to have a function of holding an electrolyte between the positive electrode and the negative electrode. Therefore, when the porous heat-resistant layer is excessively pressed against the active material layer, the electrolyte is locally depleted in the electrode group. As a result, the battery characteristics are degraded.

  The present invention is based on the above two findings. The present invention proposes that the force by which the side wall of the battery can pushes back the porous heat-resistant layer is controlled within an appropriate range according to the thickness of the porous heat-resistant layer. Thereby, damage to a porous heat-resistant layer can be prevented, and safety at the time of an internal short circuit can be secured. Furthermore, excellent battery characteristics can be realized.

FIG. 1 conceptually shows a part of the cylindrical lithium secondary battery of the present invention.
The positive electrode 13 includes a strip-shaped positive electrode core material 11 and a positive electrode active material layer 12 supported on both surfaces thereof. The negative electrode 16 has a strip-shaped negative electrode core material 14 and a negative electrode active material layer 15 supported on both sides thereof. A porous heat resistant layer 18 is supported on the surface of the negative electrode active material layer 15. The porous heat-resistant layer 18 plays a role of preventing the expansion of the short-circuit portion when an internal short circuit occurs. The positive electrode 13 and the negative electrode 16 are wound together with a strip-shaped separator 17 and a porous heat-resistant layer 18 interposed between them to constitute an electrode group. An exposed portion 14a of the negative electrode core material is disposed on the outermost periphery of the electrode group. The electrode group is accommodated in a cylindrical battery can 19.

  In the present invention, the thickness A of the porous heat-resistant layer and the thickness B of the side wall of the battery can satisfy 0.005 ≦ A / B ≦ 0.1. The porous heat-resistant layer has a function of securing short-circuit resistance (first function) and a function of retaining electrolyte (second function). When the force with which the side wall of the battery can pushes back the electrode group is insufficient, the porous heat-resistant layer is easily damaged during high-rate charging, and the first function is impaired. On the other hand, if the force with which the side wall of the battery can pushes back the electrode group becomes excessive, the porous heat-resistant layer is strongly tightened, so that a sufficient amount of electrolyte cannot be retained. Therefore, the second function is impaired.

  When A / B <0.005, the thickness A of the porous heat-resistant layer is too thin with respect to the thickness B of the side wall of the battery can. When the porous heat-resistant layer is thin, the electrolytic mass retained therein is reduced. In addition, since a large pressure is applied to the electrode group from the side wall of the battery can, the electrolyte tends to be squeezed out from the porous heat-resistant layer. Therefore, the electrolyte is locally depleted in the electrode group, and the battery characteristics are deteriorated. From the viewpoint of optimizing the balance between the first function and the second function, 0.01 ≦ A / B is desirable, and 0.015 ≦ A / B is more desirable.

  In the case of 0.1 <A / B, the thickness A of the porous heat-resistant layer is too thick with respect to the thickness B of the side wall of the battery can. When the porous heat-resistant layer is thick, its flexibility is lowered and the porous heat-resistant layer becomes brittle. Therefore, when the electrode group is deformed by high rate charging, the porous heat-resistant layer tends to collapse. In addition, the strength of the porous heat-resistant layer is insufficient because the side wall of the battery can lacks the force to push back the electrode group. Therefore, the porous heat-resistant layer is easily damaged, and the short circuit resistance of the battery is lowered. From the viewpoint of optimizing the balance between the first function and the second function, A / B ≦ 0.05 is desirable, and A / B ≦ 0.045 is more desirable. From the above, A / B preferably satisfies 0.01 ≦ A / B ≦ 0.05, and particularly preferably satisfies 0.015 ≦ A / B ≦ 0.045.

  The thickness A of the porous heat-resistant layer is preferably 2 to 10 μm, and more preferably 3 to 8 μm. If the thickness A is too small, the function of improving the short circuit resistance or the function of holding the electrolyte may be insufficient. If the thickness A is too large, the distance between the positive electrode and the negative electrode may be excessively widened, and the output characteristics may be deteriorated.

  The thickness B of the side wall of the battery can is preferably 80 to 300 μm, and more preferably 100 to 250 μm. If the thickness B is too small, it may be difficult to mold the battery can. If the thickness B is too large, it is difficult to increase the energy density of the battery.

  A microporous film is preferably used for the strip-shaped separator. Polyolefin is preferably used as the material of the microporous film, and the polyolefin is preferably polyethylene, polypropylene or the like. A microporous film containing both polyethylene and polypropylene can also be used. The thickness of the microporous film is preferably 8 to 20 μm from the viewpoint of maintaining a high capacity design.

  The porous heat-resistant layer may be provided only on the surface of the positive electrode active material layer, may be provided only on the surface of the negative electrode active material layer, or may be provided on the surface of the positive electrode active material layer and the surface of the negative electrode active material layer. . However, from the viewpoint of reliably avoiding an internal short circuit, it is desirable to provide it on the surface of the negative electrode active material layer designed to have a larger area than the positive electrode active material layer. The porous heat-resistant layer may be provided only on the active material layer on one side of the core material, or may be provided on the active material layer on both sides of the core material. The porous heat-resistant layer is desirably bonded to the surface of the active material layer.

  The porous heat-resistant layer may be an independent sheet. However, the porous heat-resistant layer formed in a sheet shape is not so high in mechanical strength, so that it may be difficult to handle. The porous heat-resistant layer may be provided on the surface of the separator. However, since the separator shrinks at a high temperature, it is necessary to pay close attention to the manufacturing conditions of the porous heat-resistant layer. From the viewpoint of eliminating these concerns, it is desirable to provide a porous heat-resistant layer on the surface of the positive electrode active material layer or the negative electrode active material layer. Since the porous heat-resistant layer has many voids, even if it is formed on the surface of the positive electrode active material layer, the negative electrode active material layer, or the separator, the movement of lithium ions is not hindered. In addition, you may laminate | stack the porous heat resistant layer of the same or different composition.

  The porous heat-resistant layer preferably contains an insulating filler and a binder. Such a porous heat-resistant layer is formed by applying a raw material paste containing an insulating filler and a small amount of a binder to the surface of the electrode active material layer by a method such as doctor blade or die coating, and drying. The The raw material paste is prepared by mixing an insulating filler, a binder, and a liquid component with a twin-type kneader or the like.

  A highly heat-resistant resin fiber formed into a film can be used for the porous heat-resistant layer. For the high heat resistance resin, aramid, polyamideimide, etc. are preferably used. However, a porous heat-resistant layer containing an insulating filler and a binder is preferable because the structural strength is increased by the action of the binder.

  As the insulating filler, fibers or beads of a high heat-resistant resin can be used, but an inorganic oxide is preferably used. Since the inorganic oxide is hard, the interval between the positive electrode and the negative electrode can be maintained within an appropriate range even if the electrode expands with charge / discharge. Among the inorganic oxides, alumina, silica, magnesia, titania, zirconia, and the like are particularly preferable in terms of high electrochemical stability in the usage environment of the lithium secondary battery. These may be used alone or in combination of two or more. As the insulating filler, a high heat-resistant resin such as aramid or polyamideimide can be used. An inorganic oxide and a high heat resistant resin can be used in combination.

  In the porous heat-resistant layer containing an insulating filler and a binder, the amount of the binder is 1 to 100 parts by weight of the insulating filler from the viewpoint of maintaining its mechanical strength and ensuring ionic conductivity. 10 parts by weight is preferable, and 2 to 8 parts by weight is more preferable. Most binders and thickeners have the property of swelling with non-aqueous electrolytes. Therefore, if the amount of the binder exceeds 10 parts by weight, the pores of the porous heat-resistant layer may be blocked due to excessive swelling of the binder, the ion conductivity may be lowered, and the battery reaction may be inhibited. is there. On the other hand, if the amount of the binder is less than 1 part by weight, the mechanical strength of the porous heat-resistant layer may be lowered.

  The binder used for the porous heat-resistant layer is not particularly limited, but polyvinylidene fluoride (hereinafter abbreviated as PVDF), polytetrafluoroethylene (hereinafter abbreviated as PTFE), polyacrylic rubber particles (for example, Nippon Zeon ( BM-500B (trade name) manufactured by KK) is preferred. Here, it is preferable to use PTFE and BM-500B in combination with a thickener. The thickener is not particularly limited, but carboxymethyl cellulose (hereinafter abbreviated as CMC), polyethylene oxide (hereinafter abbreviated as PEO), modified acrylonitrile rubber (for example, BM-720H (trade name) manufactured by Nippon Zeon Co., Ltd.) Etc. are preferable.

  The porosity of the porous heat-resistant layer containing the insulating filler and the binder is preferably 40 to 80% and 45 to 65% from the viewpoint of maintaining the mechanical strength and ensuring ionic conductivity. Further preferred. By controlling the porosity of the porous heat-resistant layer to 40 to 80% and including an appropriate amount of electrolyte in the porous heat-resistant layer, the electrode group expands appropriately. Therefore, an electrode group comes to press the inner surface of a battery can moderately. By synergistically producing this effect and the effect obtained by optimizing the B / A ratio, a battery that is particularly excellent in the balance between the first function and the second function can be obtained.

  The porosity of the porous heat-resistant layer can be controlled by changing the median diameter of the insulating filler, changing the amount of the binder, or changing the drying conditions of the raw material paste. For example, if the drying temperature is increased or the amount of hot air is increased, the porosity is relatively increased. The porosity can be obtained by calculation from the thickness of the porous heat-resistant layer, the amounts of the insulating filler and the binder, the true specific gravity of the insulating filler and the binder, and the like. The thickness of the porous heat-resistant layer can be obtained from, for example, an average value of 10 thicknesses obtained by taking several SEM photographs of the cross section of the electrode plate. Further, the porosity can be obtained by a mercury porosimeter.

  The positive electrode includes a positive electrode core material and a positive electrode active material layer supported on both surfaces thereof. The positive electrode core material has a strip shape suitable for winding, and is made of Al, Al alloy or the like. The positive electrode active material layer includes a positive electrode active material as an essential component, and can include a conductive agent, a binder, and the like as optional components. These materials are not particularly limited. However, a lithium-containing transition metal oxide is preferably used for the positive electrode active material. Of the lithium-containing transition metal oxides, lithium cobaltate and modified products thereof, lithium nickelate and modified products thereof, lithium manganate and modified products thereof are preferable.

  The negative electrode includes a negative electrode core material and a negative electrode active material layer carried on both sides thereof. The negative electrode core material has a strip shape suitable for winding and is made of Cu, Cu alloy, or the like. The negative electrode active material layer includes a negative electrode active material as an essential component, and can include a conductive agent, a binder, and the like as optional components. These materials are not particularly limited. However, as the negative electrode active material, various natural graphites, various artificial graphites, silicon-based composite materials such as silicide, lithium metal, various alloy materials, and the like are preferably used.

  As the positive electrode or negative electrode binder, for example, PTFE, PVDF, styrene butadiene rubber, or the like can be used. As the conductive agent, for example, acetylene black, ketjen black (registered trademark), various graphites, and the like can be used.

The non-aqueous electrolyte is preferably a lithium salt dissolved in a non-aqueous solvent. Lithium salt is not particularly limited, LiPF _6, etc. LiBF ₄ are preferred. A lithium salt may be used individually by 1 type, and may be used in combination of 2 or more type. The non-aqueous solvent is not particularly limited, however, for example, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like are preferably used. A non-aqueous solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

  The material of the battery can must be electrochemically stable in the operating voltage range of the lithium secondary battery. For example, it is preferable to use iron, stainless steel, aluminum, or the like. The battery can may be plated with nickel or tin.

  Next, the present invention will be specifically described based on examples.

<Battery 1>
(I) Production of positive electrode 3 kg of lithium cobaltate, 1 kg of PVDF # 1320 (N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) solution containing 12% by weight of PVDF) manufactured by Kureha Chemical Co., Ltd., and acetylene black 90 g and an appropriate amount of NMP were stirred with a double arm kneader to prepare a positive electrode mixture paste. This paste was applied to both sides of a positive electrode core material made of an aluminum foil having a thickness of 15 μm, dried and rolled to form a positive electrode active material layer, whereby a positive electrode having a total thickness of 160 μm was obtained. The positive electrode was cut into a strip having a width of 56 mm.

(Ii) Preparation of negative electrode 3 kg of artificial graphite, 75 g of BM-400B (aqueous dispersion containing 40% by weight of modified styrene butadiene rubber) manufactured by Nippon Zeon Co., Ltd., 30 g of CMC, and an appropriate amount of water, The mixture was stirred with a kneader to prepare a negative electrode mixture paste. This paste was applied to both sides of a negative electrode core material made of a copper foil having a thickness of 10 μm, dried and rolled to form a negative electrode active material layer, whereby a negative electrode having a total thickness of 180 μm was obtained. The negative electrode was cut into a 57 mm wide strip.

(Iii) Formation of porous heat-resistant layer 970 g of alumina (insulating filler) with a median diameter of 0.3 μm and NMP containing 8% by weight of BM-720H (modified polyacrylonitrile rubber (binder)) manufactured by Nippon Zeon Co., Ltd. Solution) 375 g and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a raw material paste. This raw material paste was applied to the surface of the negative electrode active material layer and dried at 120 ° C. under vacuum under reduced pressure for 10 hours to form a porous heat-resistant layer having a thickness of 0.5 μm.
The porosity of the porous heat resistant layer was 48%. The porosity is the thickness of the porous heat-resistant layer determined by cross-sectional SEM imaging, the amount of alumina present in the porous heat-resistant layer of a certain area determined by fluorescent X-ray analysis, the true specific gravity of alumina and the binder, It calculated | required by calculation from the weight ratio of an alumina and a binder.

(Iv) Preparation of non-aqueous electrolyte LiPF in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1: 1: 1 at a concentration of 1 mol / liter. ₆ was dissolved, and further vinylene carbonate equivalent to 3% by weight of the total was added to obtain a non-aqueous electrolyte.

(V) Assembling the battery The battery will be described with reference to FIG.
The positive electrode 5 and the negative electrode 6 provided with a porous heat-resistant layer (not shown) on both sides are combined with a separator 7 made of a polyethylene microporous film having a thickness of 20 μm (A089 manufactured by Celgard Co., Ltd. (trade name) )) To form a cylindrical electrode group.
Insulating plates 8a and 8b were respectively arranged above and below the electrode group, and the electrode group was inserted into an iron cylindrical battery can 1 plated with nickel. The thickness of the side wall of the battery can 1 was 50 μm, and the inner diameter was 18 mm.
One end of a positive electrode lead 5a was connected to the positive electrode 5, and the other end was welded to the lower surface of the sealing plate 2 having a safety valve. One end of a negative electrode lead 6 a was connected to the negative electrode 6, and the other end was welded to the inner bottom surface of the battery can 1. Next, 5.5 g of a non-aqueous electrolyte was injected into the central cavity of the electrode group, and the electrode group was impregnated with the electrolyte.
Thereafter, the opening of the battery can 1 was closed with a sealing plate 2 having a gasket 3 disposed around it, and the opening end of the battery can 1 was caulked to the gasket 3. Thus, a cylindrical lithium secondary battery having a diameter of 18 mm, a height of 65 mm, and a design capacity of 2000 mAh was completed.

<Battery 2>
A cylindrical lithium secondary battery similar to the battery 1 was produced except that the thickness of the side wall of the battery can was 80 μm.

<Battery 3>
A cylindrical lithium secondary battery similar to the battery 1 was produced except that the thickness of the side wall of the battery can was 150 μm.

<Battery 4>
A cylindrical lithium secondary battery similar to the battery 1 was produced except that the thickness of the side wall of the battery can was 300 μm.

<Battery 5>
A cylindrical lithium secondary battery similar to the battery 1 was produced except that the thickness of the side wall of the battery can was 600 μm.

<Battery 6>
A cylindrical lithium secondary battery similar to the battery 1 was produced except that the thickness of the side wall of the battery can was 1000 μm.

<Batteries 7-12>
Cylindrical lithium secondary batteries (batteries 7, 8, 9, 10, 11 and 12) similar to batteries 1, 2, 3, 4, 5 and 6 except that the thickness of the porous heat-resistant layer was 1 μm were produced. did.

<< Batteries 13-18 >>
Cylindrical lithium secondary batteries (batteries 13, 14, 15, 16, 17, and 18) similar to the batteries 1, 2, 3, 4, 5, and 6 except that the thickness of the porous heat-resistant layer was 2 μm were produced. did.

<Batteries 19 to 24>
Cylindrical lithium secondary batteries (batteries 19, 20, 21, 22, 23, and 24) similar to batteries 1, 2, 3, 4, 5, and 6 except that the thickness of the porous heat-resistant layer was set to 3 μm did.

<< Battery 25-32 >>
Cylindrical lithium secondary battery similar to battery 1 except that the thickness of the porous heat-resistant layer is 4 μm and the thickness of the side wall of the battery can is 50 μm, 80 μm, 150 μm, 200 μm, 300 μm, 500 μm, 600 μm and 1000 μm, respectively Batteries 25, 26, 27, 28, 29, 30, 31 and 32) were produced.

<Batteries 33 to 40>
Cylindrical lithium secondary battery (same as battery 1) except that the thickness of the porous heat-resistant layer is 7 μm and the thickness of the side wall of the battery can is 50 μm, 80 μm, 150 μm, 200 μm, 300 μm, 500 μm, 600 μm and 1000 μm, respectively Batteries 33, 34, 35, 36, 37, 38, 39 and 40) were produced.

<Batteries 41 to 48>
Cylindrical lithium secondary battery (same as battery 1) except that the thickness of the porous heat-resistant layer is 10 μm and the thickness of the side wall of the battery can is 50 μm, 80 μm, 150 μm, 200 μm, 300 μm, 500 μm, 600 μm and 1000 μm. Batteries 41, 42, 43, 44, 45, 46, 47 and 48) were produced.

<Battery 49-54>
Cylindrical lithium secondary batteries (batteries 49, 50, 51, 52, 53 and 54) similar to the batteries 1, 2, 3, 4, 5 and 6 except that the thickness of the porous heat-resistant layer was 20 μm were produced. did.
In batteries 2 to 54, the porosity of the porous heat-resistant layer was 46 to 49%.

[Evaluation]
Each battery was conditioned and discharged twice and then stored for 7 days in a 45 ° C. environment. Then, the following evaluation was performed. Table 1 shows the thickness A of the porous heat-resistant layer, the thickness B of the side wall of the battery can, and the evaluation results.

(Nail penetration test)
Each battery was charged to a final voltage of 4.35 V or 4.45 V at a charging current value of 2000 mA. Under a 20 ° C. environment, an iron nail having a diameter of 2.7 mm was pierced at a speed of 5 mm / second on the side surface of the charged battery, and the battery temperature was measured with a thermocouple attached to the side surface of the battery. The temperature reached after 90 seconds was determined.

(Cycle life test)
Charging / discharging was repeated 500 cycles under the following condition (1) or (2) in a 20 ° C. environment. The ratio (capacity maintenance ratio) of the discharge capacity at the 500th cycle to the initial discharge capacity was obtained as a percentage.
Condition (1)
Constant current charging: Charging current value 1400mA / end-of-charge voltage 4.2V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current value 2000 mA / discharge end voltage 3 V

Condition (2)
Constant current charging: Charging current value 1400mA / end-of-charge voltage 4.2V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current value 4000 mA / discharge end voltage 3 V

  Batteries 3-6, 10-12, 17, 18, 24, and 32 having a ratio (A / B) of the thickness A (μm) of the porous heat-resistant layer to the thickness B (μm) of the side wall of the battery can is less than 0.005. The deterioration of the cycle life characteristics was remarkable. This result is related to the fact that the thickness of the porous heat-resistant layer is relatively thin with respect to the battery can. The thin porous heat-resistant layer has a small electrolytic mass that can be held, and the electrolyte is easily squeezed out by the pressure from the side wall of the battery can. Therefore, it is considered that the electrolyte in the electrode group has been exhausted.

  On the other hand, in the batteries 33, 41, 42, 49, 50 and 51 having an A / B ratio exceeding 0.1, overheating in the nail penetration test was significant. When these batteries were disassembled, the porous heat-resistant layer was uniformly removed not only at the place where the nail was pierced. This result is related to the thickness of the porous heat-resistant layer being relatively thick with respect to the battery can. Since the thick porous heat-resistant layer becomes brittle, it tends to collapse when the electrode group is deformed during high rate charging. Furthermore, since the side wall of the battery can is thin, the force to push back the electrode group is also fragile. Therefore, it is considered that the porous heat-resistant layer was damaged.

  Regardless of the thickness of the side wall of the battery can, the batteries 1 to 12 exhibited a significant decrease in cycle life characteristics under severe charge / discharge conditions (2) in which discharge was performed at 4000 mA. Therefore, if the porous heat-resistant layer is 1 μm or less, the effect of the present invention is considered to be too thin. However, in the case of the condition (1), a relatively good value is obtained even when the porous heat-resistant layer is 1 μm or less.

  Regardless of the thickness of the side wall of the battery can, the batteries 49 to 54 had a significant decrease in cycle life characteristics under the condition (2). In addition, overheating in a nail penetration test of a battery charged to 4.45 V was relatively significant. Therefore, when the porous heat-resistant layer is 20 μm or more, it is considered that the thickness of the porous heat-resistant layer is too thick and the effect of the present invention is reduced.

  Overall, when the battery can was too thick (for example, more than 300 μm), the cycle life characteristics in the condition (2) tended to be reduced. Moreover, when the battery can was too thin (for example, 50 μm), overheating in the nail penetration test of the battery charged to 4.45 V was observed.

  Since the cylindrical lithium secondary battery of the present invention has excellent short-circuit resistance, high safety, and excellent high rate discharge characteristics, it can be used in all portable devices (for example, portable information terminals and portable electronic devices). Etc.) can be used as a power source. However, the use of the cylindrical lithium secondary battery of the present invention is not particularly limited, and can be used as a power source for a small household electric power storage device, a motorcycle, an electric vehicle, a hybrid electric vehicle, and the like.

It is a partial section conceptual diagram of the cylindrical lithium secondary battery of the present invention. It is a longitudinal cross-sectional view of the cylindrical lithium secondary battery which concerns on the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 19 Battery can 2 Sealing plate 3 Gasket 5, 13 Positive electrode 5a Positive electrode lead 6, 16 Negative electrode 6a Negative electrode lead 7, 17 Separator 8a, 8b Insulating plate 11 Positive electrode core material 12 Positive electrode active material layer 14 Negative electrode core material 14a Negative electrode core material Exposed portion 15 Negative electrode active material layer 18 Porous heat resistant layer

Claims (4)

Lithium including a cylindrical battery can having a bottom, a side wall, and an upper opening, an electrode group, a non-aqueous electrolyte, and a sealing plate covering the upper opening of the battery can containing the electrode group and the non-aqueous electrolyte A secondary battery,
The material of the battery can is iron or stainless steel,
The electrode group is formed by winding a strip-shaped positive electrode and a strip-shaped negative electrode together with a porous heat-resistant layer and a separator interposed therebetween, and the positive electrode comprises a positive electrode core material and a positive electrode active material carried on both surfaces thereof A negative electrode core material and a negative electrode active material layer carried on both sides thereof,
The positive electrode active material layer includes a lithium-containing transition metal oxide as a positive electrode active material, the negative electrode active material layer includes natural graphite or artificial graphite as a negative electrode active material,
The porous heat-resistant layer includes an insulating filler and a binder, and the amount of the binder is 1 to 10 parts by weight per 100 parts by weight of the insulating filler, and the porosity of the porous heat-resistant layer Is 40-80%,
A cylindrical lithium secondary battery in which a thickness A of the porous heat-resistant layer and a thickness B of a side wall of the battery can satisfy 0.005 ≦ A / B ≦ 0.1.   2. The thickness A of the porous heat-resistant layer is 2 to 10 μm, the thickness B of the side wall of the battery can is 80 to 300 μm, and 0.01 ≦ A / B ≦ 0.05. Cylindrical lithium secondary battery.   The porous heat-resistant layer is supported on at least one surface of two active material layers supported on both surfaces of the core material in at least one of the positive electrode and the negative electrode. Cylindrical lithium secondary battery. The insulating filler is made of an inorganic oxide, cylindrical lithium secondary battery according to claim 1, wherein.

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