JP4839111B2 - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery that is excellent in resistance to short circuit and heat, is unlikely to suffer a capacity loss due to impact such as dropping, and has a high capacity. <P>SOLUTION: This lithium secondary battery includes: an electrode group composed by winding a strip-like positive electrode and a strip-like negative electrode together with a porous heat-resistant layer interposed therebetween; a non-aqueous electrolyte; and a battery can. The battery has a restricting part for restricting vertical movement of the electrode group. The distance A from the restricting part to the inner bottom face of the battery can and the width B of the negative electrode satisfy the relation: 0. 965&le;B/A&le;0.995. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to a lithium secondary battery having high safety with excellent short-circuit resistance and heat resistance, and more particularly to a technique for preventing a decrease in capacity due to an impact such as dropping.

  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 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 or a substantially elliptical column shape.

  Cylindrical lithium secondary batteries are used, for example, as power sources for power tools and notebook PCs. The cylindrical lithium secondary battery is sealed by caulking the open end of the battery can to a sealing plate. In order to fix the sealing plate in the vicinity of the opening of the battery can, a groove (squeezing part) is provided on the upper side wall of the battery can so that the inner diameter of the battery can is reduced. In Patent Document 1, there is a high capacity design in which the relationship between the width B (38 mm) of the negative electrode and the distance A (39.7 mm) from the throttle portion to the outer bottom surface of the battery can is B / A = 0.957. Proposed.

  A square lithium secondary battery is used as a power source of, for example, a mobile phone and a digital still camera. The prismatic lithium secondary battery is more widely used because it is more easily housed in the device than the cylindrical type. In the case of a rectangular lithium secondary battery, unlike the cylindrical type, the lead connecting the electrode and the terminal is easy to contact the battery can. When a lead having the opposite polarity to the battery can comes into contact with the battery can, a short circuit occurs. Therefore, it is common to provide an insulator (hereinafter referred to as an upper insulator) between the upper part of the electrode group and the lid (sealing plate) of the battery can. In order to further improve short-circuit resistance, it has also been proposed to provide an insulator (hereinafter referred to as a lower insulator) between the lower part of the electrode group and the bottom surface of the battery can (Patent Document 2).

  Usually, the electrode group of the prismatic lithium secondary battery is such that the relationship between the distance A from the lower surface of the upper insulator to the inner bottom surface of the battery can and the width B of the negative electrode satisfies B / A ≦ 0.96. Produced. The larger the B / A, the higher the capacity of the battery. However, if this value is too large, the electrode group tends to be distorted, causing a short circuit in which the positive electrode and the negative electrode are in direct contact. In patent document 2, it has pulled up to B / A = 0.97 by providing the lower insulator used as a cushioning material.

  By the way, when a lithium secondary battery is held in an extremely high temperature environment for a long time, a separator made of a microporous film is likely 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 3). 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. 11-354084 JP 200431263 A Japanese Patent Laid-Open No. 7-220759

  In order to realize a lithium secondary battery with high capacity and excellent short-circuit resistance, it is conceivable to use the proposal of Patent Document 1 or 2 and the proposal of Patent Document 3 in combination. Thereby, the internal short circuit failure is remarkably reduced. However, when the battery is subjected to an impact due to dropping or the like, a significant capacity reduction occurs.

  In view of the above, an object of the present invention is to provide a lithium secondary battery that is excellent in short-circuit resistance and capable of avoiding a decrease in capacity due to dropping and capable of high capacity design.

The present invention relates to a lithium secondary battery including a battery can having a bottom, a side wall, and an upper opening, an electrode group, a nonaqueous electrolyte, and a sealing plate covering the upper opening of the battery can containing the electrode group and the nonaqueous electrolyte. It relates to batteries. 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 interposed therebetween, and the positive electrode includes a positive electrode core material and a positive electrode active material layer supported on the positive electrode core material, the negative electrode is observed including a negative electrode active material layer carried on this and the negative electrode core member, between the positive electrode and the porous heat-resistant layer, or, between the porous heat-resistant layer and the negative electrode, a microporous Having a separator made of a quality film. 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, and includes an insulating filler and a binder. Including an adhesive, 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%. This battery has a restricting portion that restricts the vertical movement of the electrode group. The distance A from the restricting portion to the inner bottom surface of the battery can and the width B of the negative electrode are 0.965 ≦ B / A ≦ 0. .995 is satisfied. When the electrode group is substantially columnar and the battery can is cylindrical, the restricting portion is a groove provided in the upper part of the side wall of the battery can so that the inner diameter of the battery can is reduced, When the electrode group is substantially elliptical columnar and the battery can has a square shape, the battery further includes an insulator provided between the sealing plate and the electrode group, It is the lower surface of an insulator.

  Although the inner bottom surface of the battery can may have slight irregularities, the difference in height between the concave and convex portions is usually 0.05 mm or less and can be ignored. Moreover, the width B of the negative electrode means the length in the short direction of the strip-shaped negative electrode. That is, the width B of the negative electrode corresponds to the maximum height of the electrode portion in the columnar electrode group.

  It is preferable to use an inorganic oxide for the insulating filler. The inorganic oxide preferably contains at least one selected from the group consisting of alumina, silica, magnesia, titania and zirconia.

Na us, to vary the distance A by the depth of the grooves, the distance from the deepest portion of the groove (the most protruding part on the inside of the battery can) to the inner bottom surface of the battery can becomes the distance A.

In the case of a rectangular lithium secondary battery, it is desirable that the distance A from the regulating portion to the inner bottom surface of the battery can and the width B of the negative electrode satisfy 0.975 ≦ B / A ≦ 0.995.

  According to the present invention, it is possible to provide a high-capacity lithium secondary battery that has excellent short-circuit resistance and heat resistance, is less susceptible to capacity reduction due to impact such as dropping, and the like.

  The present invention relates to a lithium secondary battery including a battery can having a bottom, a side wall, and an upper opening, an electrode group, a nonaqueous electrolyte, and a sealing plate covering the upper opening of the battery can containing the electrode group and the nonaqueous electrolyte. It relates to batteries. The electrode group is obtained by winding a belt-like positive electrode and a belt-like negative electrode together with a porous heat-resistant layer interposed therebetween. The positive electrode includes a positive electrode core material and a positive electrode active material layer supported thereon, and the negative electrode includes a negative electrode core material and a negative electrode active material layer supported thereon. The battery of the present invention has a restricting portion that restricts the vertical movement of the electrode group. Here, the distance A from the regulating portion to the inner bottom surface of the battery can and the width B of the negative electrode satisfy 0.965 ≦ B / A ≦ 0.995.

As a result of intensive studies, the present inventors have obtained the following two findings regarding an electrode group having a porous heat-resistant layer.
First, the electrode group having a porous heat-resistant layer is less deformed due to charging and discharging than a conventional electrode group having no porous heat-resistant layer. This is probably because the surface smoothness of the porous heat-resistant layer is lower than that of the positive electrode, the negative electrode, and the separator, and the electrode and the separator are less likely to slip or misalign.

  Secondly, when an appropriate deformation of the electrode group does not occur, the electrode group is difficult to be firmly fixed in the battery can. Therefore, when the battery is dropped, displacement of the electrodes in the electrode group may occur, resulting in a decrease in capacity.

  Based on the above findings, in the present invention, the ratio (B / A) of the width B of the negative electrode to the distance A from the regulating portion to the inner bottom surface of the battery can is set larger than in the past. In addition, when the B / A ratio satisfies 0.965 ≦ B / A ≦ 0.995, in particular, the displacement of the electrode in the electrode group with respect to the drop is remarkably suppressed, and the capacity is less likely to decrease.

  When the B / A ratio exceeds 0.96, the electrode group is greatly distorted, so that a short circuit is usually likely to occur. Further, the width of the negative electrode of the lithium secondary battery is usually set larger than that of the positive electrode. Therefore, the deformation of the negative electrode is particularly problematic. However, in the case of the present invention, since the electrode group has a porous heat-resistant layer, even if the end of the negative electrode is slightly deformed near the upper surface or the lower surface of the electrode group, a short circuit is unlikely to occur. Therefore, it is possible to set the B / A ratio to 0.965 or more. According to the present invention, by reducing the width of the negative electrode to the distance A from the regulating portion to the inner bottom surface of the battery can, it is possible to improve the resistance against dropping while achieving high capacity.

  When the B / A ratio is less than 0.965, it is not only difficult to increase the capacity, but also when the battery is dropped, the capacity is likely to decrease due to the displacement of the electrodes in the electrode group. On the other hand, when the B / A ratio exceeds 0.995, the negative electrode is significantly deformed near the upper surface or the lower surface of the electrode group. Therefore, the porous heat-resistant layer is damaged and an internal short circuit easily occurs.

  As long as 0.965 ≦ B / A ≦ 0.995 is satisfied, it is possible to obtain a lithium secondary battery that is less likely to cause an internal short circuit, has a high capacity, and is excellent in resistance to dropping.

  The lithium secondary battery of the present invention may or may not have a separator made of a microporous film. The separator may be provided between the porous heat-resistant layer and the positive electrode, or may be provided between the porous heat-resistant layer and the negative electrode. The separator serves to support a structurally brittle porous heat-resistant layer. Therefore, it is desirable that the battery has a separator from the viewpoint of further improving the resistance to dropping.

  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 securing the function of supporting the porous heat-resistant layer and 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.

  The porous heat-resistant layer preferably contains an insulating filler and a binder. Such a porous heat-resistant layer is obtained by applying a raw material paste containing an insulating filler and a small amount of a binder to a surface of an electrode active material layer or a separator by a method such as a doctor blade or a die coat and drying the paste. It is formed. 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.

  Moreover, what formed the fiber of the high heat resistant resin in the film form can also be used for a 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 higher due to the action of the binder than a film made of fibers of a high heat-resistant resin.

  The thickness of the porous heat-resistant layer is preferably 0.5 to 20 μm, and more preferably 1 to 10 μm. When the thickness of the porous heat-resistant layer is less than 0.5 μm, the effect of suppressing internal short circuit is reduced. On the other hand, when the thickness exceeds 20 μm, the distance between the positive electrode and the negative electrode is excessively widened, so that the output characteristics may be deteriorated.

  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.

  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 more preferably 45 to 65% from the viewpoint of maintaining the mechanical strength and improving the resistance to dropping. Is preferred. Since the porous heat-resistant layer has lower surface smoothness than the positive electrode, the negative electrode, and the separator, slipping (positional displacement) between the electrode and the separator is excessively suppressed. Therefore, the positional deviation of the electrode group tends to occur. On the other hand, by controlling the porosity of the porous heat-resistant layer to 40 to 80% and including an appropriate amount of nonaqueous 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. The effect of setting the porosity to 40 to 80% and the effect of optimizing the B / A ratio are synergistically improved, and the resistance to dropping is further enhanced. When the porosity is less than 40%, the non-aqueous electrolyte does not sufficiently penetrate into the porous heat-resistant layer, so that the electrode group cannot be appropriately expanded. On the other hand, when the porosity exceeds 80%, the mechanical strength of the porous heat-resistant layer decreases.

  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 used for drying 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 an average value of the thicknesses obtained by taking several SEM photographs (for example, 10 locations) of the cross section of the electrode plate. Further, the porosity can be obtained by a mercury porosimeter.

  A cylindrical lithium secondary battery has a columnar (columnar) electrode group having a substantially circular cross section. The cylindrical lithium secondary battery has a cylindrical battery can 100 as shown in FIG. One bottom surface of the cylindrical battery can is opened, and the other bottom surface is closed by a flat bottom portion 110. In the case of a general cylindrical lithium secondary battery, the opening end of the battery can is caulked to the peripheral edge of the sealing plate 120 and the upper opening is sealed. In this case, the restricting portion that restricts the vertical movement of the electrode group is a groove portion 130 provided on the upper side wall of the battery can 100 so that the inner diameter of the battery can 100 becomes smaller. Such a groove 130 also serves to fix the sealing plate 120.

A prismatic lithium secondary battery has a columnar (substantially elliptical columnar) electrode group having a substantially elliptical cross section. Further, the prismatic lithium secondary battery has a prismatic (substantially rectangular parallelepiped) battery can 200 as shown in FIG. One bottom surface of the rectangular battery can is opened, and the other bottom surface is closed by a flat bottom portion 210. In the case of a general prismatic lithium secondary battery, the upper opening of the battery can is sealed by welding the opening end and the metal sealing plate 220. In addition, an insulator (upper insulator) 230 is provided between the sealing plate 220 and the electrode group to prevent contact between the electrode lead and the battery can 200. Since the insulator 230 is provided with a hole through which the electrode lead passes, the insulator hardly moves. Therefore, the restricting portion that restricts the vertical movement of the electrode group is the lower surface of the insulator 230.
In addition, it is preferable to make thickness of an insulator into the range of 2 to 10% of the height of a battery can from the viewpoint of ensuring the function and reducing dead space.

In the case of a cylindrical lithium secondary battery, the longitudinal section of the groove serving as the restricting portion is V-shaped or U-shaped due to the limitations of the construction method. Therefore, the distance A changes depending on the depth of the groove of the restricting portion. In this case, the distance A is the distance from the deepest part of the groove to the inner bottom surface of the battery can. In this case, if the B / A ratio is 0.965 or more, sufficient resistance to dropping can be obtained. However, in the case of a cylindrical lithium secondary battery, 0.970 ≦ B / A ≦ 0.990 is particularly preferable in terms of the balance between high capacity and resistance to dropping.
On the other hand, in the case of a square lithium secondary battery, the lower surface of the insulator serving as the restricting portion is smooth. Therefore, in order to obtain remarkable resistance to dropping, it is desirable that the B / A ratio is 0.975 or more. In the case of a prismatic lithium secondary battery, 0.975 ≦ B / A ≦ 0.990 is particularly preferable in terms of a balance between high capacity and resistance to dropping.

  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 width B of the negative electrode is synonymous with the width of the negative electrode core material. 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-containing 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 or aluminum. The battery can may be plated with nickel or tin.

FIG. 1 is a schematic cross-sectional view of an example of a cylindrical lithium secondary battery of the present invention.
The columnar electrode group is configured by winding the positive electrode 101 and the negative electrode 102 together with the separator 103 and a porous heat-resistant layer (not shown) interposed therebetween. The separator 103 is interposed between the porous heat-resistant layer and the positive electrode 101. However, when the porous heat-resistant layer has a sufficient thickness, the separator 103 is not essential. The electrode group is inserted into a cylindrical battery can 100. A groove portion 130 is provided in the upper portion of the side wall of the battery can 100 so that the inner diameter is smaller than other portions. The groove 130 is formed after the electrode group is accommodated in the battery can 100. The longitudinal section of the groove 130 is U-shaped. Thereafter, a non-aqueous electrolyte is injected into the battery can 100. The sealing plate 120 is disposed on the groove portion 130, and the opening end portion of the battery can 100 is caulked to the peripheral portion of the sealing plate 120, thereby sealing the opening portion of the battery can 100.
Above and below the electrode group, an upper insulating plate 106 and a lower insulating plate 107, each having a negligible thickness, are disposed. One end of the positive electrode lead 104 is connected to the core material of the positive electrode 101, and the other end is connected to the internal terminal 108 a on the lower surface of the sealing plate 120. The internal terminal 108a is electrically connected to the external positive terminal 108. One end of a negative electrode lead (not shown) is connected to the core material of the negative electrode 102, and the other end is connected to the inner bottom surface of the battery can 100.

FIG. 2 is a schematic cross-sectional view of an example of the prismatic lithium secondary battery of the present invention.
The substantially elliptical columnar electrode group 201 is configured by winding a positive electrode and a negative electrode together with a separator and a porous heat-resistant layer interposed therebetween. The electrode group 201 is inserted into a substantially rectangular (rectangular) battery can 200. After the electrode group 201 is accommodated in the battery can 200, an insulator 230 for preventing a short circuit between the battery can 200 or the positive electrode lead 202 and the negative electrode lead 203 is disposed on the upper surface of the electrode group 201. The insulator 230 is fixed near the opening of the battery can 200.
A negative electrode terminal 207 surrounded by an insulating gasket 206 is installed on the sealing plate 220. The negative electrode lead 203 is connected to the negative electrode terminal 207. On the other hand, the positive electrode lead 202 is connected to the lower surface of the sealing plate 220.
The nonaqueous electrolyte is injected into the battery can 200 through the injection hole of the sealing plate 220. Thereafter, the liquid injection hole is closed with a seal 209 by welding. The sealing plate 220 is disposed in the opening of the battery can 200, and the upper end of the battery can 200 is sealed by laser welding the opening end and the sealing plate 220.
Next, the present invention will be specifically described based on examples.

In this embodiment, a cylindrical lithium secondary battery as shown in FIG. 1 will be described.
<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 shape having a width of 56.5 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 strip having a width of 57.5 mm.

(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 by applying hot air of 130 ° C. at a wind speed of 1.5 m / min for 4 minutes to form a porous heat-resistant layer having a thickness of 5 μm. The porosity of the porous heat-resistant layer was 50%. 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) Battery assembly A positive electrode and a negative electrode provided with a porous heat-resistant layer on both sides, a separator made of a polyethylene microporous film having a thickness of 10 μm therebetween (Celgard Co., Ltd., width 60) .7 mm) was interposed to form a cylindrical electrode group.
Subsequently, the electrode group was inserted into an iron cylindrical battery can (inner diameter: 18 mm) plated with nickel. Insulating plates are provided above and below the electrode group, but since these are extremely thin, the thickness is negligible. Thereafter, a groove was formed in the upper part of the side wall of the battery can so that the inner diameter of the battery can was reduced. The longitudinal section of the groove portion was U-shaped, and the depth of the groove portion was 1.5 mm. The distance A from the inner bottom surface of the battery can to the deepest part of the groove was 60.5 mm.
Next, 5.5 g of a nonaqueous electrolyte was injected into the central cavity of the electrode group, and the electrode group was impregnated with the nonaqueous electrolyte. Then, the sealing board was arrange | positioned on the groove part of a battery can, and the opening edge part of the battery can was crimped to the peripheral part of the sealing board. As a result, a cylindrical lithium secondary battery having an inner diameter of 18 mm, a height of 65.0 mm, and a design capacity of 2200 mAh was completed. Ratio of negative electrode width B (57.5 mm) to distance A (60.5 mm): B / A was 0.950.

<Batteries 2 to 5>
The negative electrode width B was 58.5 mm, 59.2 mm, 60.2 mm or 61.2 mm, the positive electrode width was 57.5 mm, 58.2 mm, 59.2 mm or 60.2 mm, respectively, and the design capacity was 2239 mAh, respectively. , 2266 mAh, 2305 mAh, or 2244 mAh, except that cylindrical lithium secondary batteries 2 to 5 similar to the battery 1 were fabricated. The B / A ratio in each battery was 0.967 (battery 2), 0.979 (battery 3), 0.995 (battery 4) or 1.012 (battery 5), respectively.

<Battery 6>
A cylindrical lithium secondary battery similar to the battery 3 was produced except that the porous heat-resistant layer was formed on the surface of the positive electrode active material layer instead of the negative electrode active material layer.

<Battery 7>
A cylindrical lithium secondary battery similar to the battery 4 was produced, except that the porous heat-resistant layer was formed on the surface of the positive electrode active material layer instead of the negative electrode active material layer.

<Battery 8>
A cylindrical lithium secondary battery similar to the battery 3 was produced except that the porous heat-resistant layer had a thickness of 15 μm and the electrode group was produced without using a separator.

<Battery 9>
A cylindrical lithium secondary battery similar to the battery 3 was produced except that the porous heat-resistant layer of alumina was changed to magnesia having the same median diameter.

<Battery 10>
A cylindrical lithium secondary battery similar to the battery 3 was produced except that the porous heat-resistant layer of alumina was changed to silica having the same median diameter.

<Battery 11>
A cylindrical lithium secondary battery similar to the battery 3 was produced except that the alumina of the porous heat-resistant layer was changed to titania having the same median diameter.

<Battery 12>
A cylindrical lithium secondary battery similar to the battery 3 was produced except that the alumina of the porous heat-resistant layer was changed to zirconia having the same median diameter.

<Battery 13>
A porous heat-resistant layer was formed as follows.
To 1 kg of NMP, 65 g of dried anhydrous calcium chloride was added and heated to 80 ° C. in a reaction vessel to be completely dissolved. After the obtained NMP solution of calcium chloride was returned to room temperature, 32 g of paraphenylenediamine was added and completely dissolved. Thereafter, the reaction vessel was placed in a constant temperature room at 20 ° C., and 58 g of terephthalic acid dichloride was dropped into the NMP solution over 1 hour. Thereafter, the NMP solution was allowed to stand in a constant temperature room at 20 ° C. for 1 hour to advance the polymerization reaction, thereby synthesizing polyparaphenylene terephthalamide (hereinafter abbreviated as PPTA).
After completion of the reaction, the NMP solution (polymerization solution) was replaced from a thermostatic chamber to a vacuum chamber, and degassed by stirring for 30 minutes under reduced pressure. The obtained polymerization solution was further diluted with an NMP solution of calcium chloride to prepare an NMP solution of an aramid resin having a PPTA concentration of 1.4% by weight.

  The obtained NMP solution of aramid resin was applied to one side of the separator with a doctor blade and dried with hot air at 80 ° C. (wind speed 0.5 m / sec). Thereafter, the aramid resin film was sufficiently washed with pure water to remove calcium chloride, and at the same time, micropores were formed in the film and dried. Thus, a porous heat-resistant layer having a thickness of 5 μm was formed on one side of the separator. The porosity of the porous heat resistant layer was 48%. The electrode group was configured such that the porous heat-resistant layer and the positive electrode were in contact with each other. The negative electrode active material layer was not provided with a porous heat-resistant layer. Other than the above, a cylindrical lithium secondary battery was produced in the same manner as the battery 3.

<Battery 14>
A porous heat-resistant layer was formed as follows.
21 g of trimellitic anhydride monochloride and 20 g of diaminodiphenyl ether were added to 1 kg of NMP and mixed at room temperature to prepare an NMP solution of polyamic acid (polyamic acid concentration of 3.9% by weight). The obtained NMP solution of polyamic acid was applied to one side of the separator with a doctor blade. Thereafter, the coating film was dried with hot air of 80 ° C. (wind speed 0.5 m / sec), and at the same time, the polyamic acid was dehydrated and closed to produce polyamideimide. Thus, a porous heat-resistant layer having a thickness of 5 μm was formed on one side of the separator. The porosity of the porous heat-resistant layer was 47%. The electrode group was configured such that the porous heat-resistant layer and the positive electrode were in contact with each other. The negative electrode active material layer was not provided with a porous heat-resistant layer. Other than the above, a cylindrical lithium secondary battery was produced in the same manner as the battery 3.

<Battery 15>
The NMP solution of aramid resin prepared in the same manner as in battery 13 was applied onto a smooth stainless steel (SUS) plate using a doctor blade, and the coating film was dried at 120 ° C. under vacuum under reduced pressure for 10 hours. I let you. Thereafter, the coating film was peeled off from the SUS plate to obtain an independent sheet-like porous heat-resistant layer having a thickness of 15 μm. The porosity of the obtained porous heat-resistant layer was 51%. The positive electrode and the negative electrode were wound with a sheet-like porous heat-resistant layer interposed between them, and the electrode group was configured without using a separator. The negative electrode active material layer was not provided with a porous heat-resistant layer. Other than the above, a cylindrical lithium secondary battery was produced in the same manner as the battery 3.

<Battery 16>
The NMP solution of polyamic acid prepared in the same manner as in the case of the battery 14 was applied onto a smooth stainless steel (SUS) plate using a doctor blade, and the coating film was heated at 80 ° C. with hot air (wind speed 0.5 m / sec). At the same time, the polyamic acid was dehydrated and closed. Thereafter, the coating film was peeled off from the SUS plate to obtain an independent sheet-like porous heat-resistant layer having a thickness of 15 μm. The porosity of the obtained porous heat-resistant layer was 52%. The positive electrode and the negative electrode were wound with a sheet-like porous heat-resistant layer interposed between them, and the electrode group was configured without using a separator. The negative electrode active material layer was not provided with a porous heat-resistant layer. Other than the above, a cylindrical lithium secondary battery was produced in the same manner as the battery 3.

<Battery 17>
A porous heat-resistant layer was formed as follows.
995 g of alumina having a median diameter of 0.3 μm, 62.5 g of BM-720H manufactured by Nippon Zeon Co., Ltd., 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 by applying hot air of 130 ° C. at a wind speed of 1.5 m / min for 4 minutes to form a porous heat-resistant layer having a thickness of 5 μm. The porosity of the porous heat resistant layer was 61%. Otherwise, the cylindrical lithium secondary battery was fabricated in the same manner as the battery 3.

<Battery 18>
A porous heat-resistant layer was formed as follows.
A raw material paste was prepared by stirring 990 g of alumina having a median diameter of 0.3 μm, 125 g of BM-720H manufactured by Nippon Zeon Co., Ltd., and an appropriate amount of NMP with a double-arm kneader. This raw material paste was applied to the surface of the negative electrode active material layer and dried by applying hot air of 130 ° C. at a wind speed of 1.5 m / min for 4 minutes to form a porous heat-resistant layer having a thickness of 5 μm. The porosity of the porous heat-resistant layer was 57%. Otherwise, the cylindrical lithium secondary battery was fabricated in the same manner as the battery 3.

<Battery 19>
A porous heat-resistant layer was formed as follows.
900 g of alumina having a median diameter of 0.3 μm, 1250 g of BM-720H manufactured by Nippon Zeon Co., Ltd., 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 by applying hot air of 130 ° C. at a wind speed of 1.5 m / min for 4 minutes to form a porous heat-resistant layer having a thickness of 5 μm. The porosity of the porous heat-resistant layer was 42%. Otherwise, the cylindrical lithium secondary battery was fabricated in the same manner as the battery 3.

<Battery 20>
A porous heat-resistant layer was formed as follows.
800 g of alumina having a median diameter of 0.3 μm, 2500 g of BM-720H manufactured by Nippon Zeon Co., Ltd., 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 by applying hot air at 130 ° C. for 4 minutes at a wind speed of 1.5 m / min to form a porous heat-resistant layer having a thickness of 5 μm. The porosity of the porous heat-resistant layer was 35%. Otherwise, the cylindrical lithium secondary battery was fabricated in the same manner as the battery 3.

<Batteries 21 to 25>
After applying the raw material paste for the porous heat-resistant layer, the speed of the hot air during drying was 0.5 m / min, 1 m / min, 2 m / min, 5 m / min, or 8 m / min, respectively. Similarly, cylindrical lithium secondary batteries 21 to 25 were produced. The porosity of the porous heat-resistant layer in each battery was 30% (battery 21), 42% (battery 22), 60% (battery 23), 78% (battery 24) or 89% (battery 25), respectively. .

<Battery 26>
A cylindrical lithium secondary battery similar to the battery 1 was produced except that the thickness of the separator was 15 μm and the porous heat-resistant layer was not provided.

<Battery 27>
A cylindrical lithium secondary battery similar to the battery 2 was produced except that the thickness of the separator was 15 μm and the porous heat-resistant layer was not provided.

<Battery 28>
A cylindrical lithium secondary battery similar to the battery 3 was produced except that the thickness of the separator was 15 μm and the porous heat-resistant layer was not provided.

<Battery 29>
A cylindrical lithium secondary battery similar to the battery 4 was produced except that the thickness of the separator was 15 μm and the porous heat-resistant layer was not provided.

<Battery 30>
A cylindrical lithium secondary battery similar to the battery 5 was produced except that the thickness of the separator was 15 μm and the porous heat-resistant layer was not provided.

  Each battery was conditioned and discharged twice and then stored for 7 days in a 45 ° C. environment. Then, the following evaluation was performed. The characteristics, battery design and evaluation results of the porous heat-resistant layer are shown in Tables 1, 2 and 3, respectively.

(Internal short circuit inspection)
100 batteries were prepared and charged under the following conditions in an environment of 20 ° C., and the open circuit voltage was measured. Thereafter, the battery was left in a 45 ° C. environment for 10 days, and the open circuit voltage was measured again. A battery having an open circuit voltage difference of 0.3 V or more before and after being left in a 45 ° C. environment was regarded as an internal short circuit battery. Table 3 shows the incidence of internal short-circuit batteries.
Constant current charging: Charging current value 1500mA / end-of-charge voltage 4.2V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA

(Drop test)
The battery that passed the internal short-circuit test was charged and discharged under the following conditions in a 20 ° C. environment to determine the discharge capacity.
Constant current charging: Charging current value 1500mA / 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 2200mA / discharge end voltage 3V
Thereafter, the battery was dropped 30 times from a height of 16 cm in a 20 ° C. environment, and then charged and discharged under the above-described conditions to determine the discharge capacity. The ratio of the discharge capacity after the drop test to the discharge capacity before the drop test was determined as a percentage. The obtained values are shown in Table 3 as “drop resistance”.

(Internal short circuit inspection after drop test)
The battery after the drop test was subjected to the same internal short circuit inspection as before the drop test. The results are shown in Table 3 as “occurrence rate of internal short circuit after dropping”.

(High output characteristics)
Each battery was charged and discharged under the following conditions in an environment of 20 ° C. to determine the discharge capacity.
Constant current charging: Charging current value 1500mA / 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 440 mA / discharge end voltage 3 V
Constant current charging: Charging current value 1500mA / 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 4400 mA / discharge end voltage 3V
The ratio of the capacity at 4400 mA discharge to the capacity at 440 mA discharge was determined as a percentage. The obtained values are shown in Table 3 as “high output characteristics”.

(Nail penetration test)
Each battery was charged to a final voltage of 4.35 V at a charging current value of 2200 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. Table 3 shows the temperature reached after 90 seconds.

  The battery 1 in which the width B of the negative electrode is too small with respect to the distance A from the groove portion serving as the restriction portion to the inner bottom surface of the battery can has a small capacity density and a low drop resistance. When the battery 1 after the drop test was disassembled, it was found that the electrode in the electrode group was misaligned.

  It can be seen that, although the battery 1 escaped the internal short circuit due to the action of the porous heat-resistant layer, the capacity decreased due to the decrease in the effective area where the positive electrode and the negative electrode face each other. The electrode group that has become difficult to be deformed by providing the porous heat-resistant layer is difficult to be firmly fixed inside the battery can. Therefore, it is considered that the electrode in the electrode group is unwound when repeated dropping.

  On the other hand, the battery 5 in which the width B of the negative electrode is too large with respect to the distance A from the groove portion to the inner bottom surface of the battery can has low short-circuit resistance. When the battery 5 that was regarded as an internal short circuit was disassembled, the porous heat-resistant layer provided on the surface of the negative electrode was broken above the electrode group. It was also found that the separator was torn.

  Batteries 2 to 4 having a B / A ratio of 0.965 to 0.995 have high short-circuit resistance and improved drop resistance. The battery of the present invention is provided with a porous heat-resistant layer in addition to the separator. Therefore, even if the negative electrode provided larger than the positive electrode is slightly deformed at the upper part of the electrode group, the insulation of the deformed portion is ensured by the double structure of the separator and the porous heat-resistant layer. Moreover, since the B / A ratio is large, the electrode group can be firmly held between the groove portion and the inner bottom surface of the battery can, and thus it is considered that the drop resistance is improved.

  The batteries 26 to 30 not provided with the porous heat-resistant layer were excellent in drop resistance regardless of the position of the restricting portion. The electrode group not having the porous heat-resistant layer is considered to be firmly fixed in the battery can because it deforms moderately. Thereby, even when the battery is dropped, it is considered that the winding deviation of the electrode group causing the capacity reduction is suppressed. However, the batteries 26 to 30 were overheated during the nail penetration test. Moreover, the batteries 27-29 which provided the control part in the same position as the batteries 2-4 fell in short circuit resistance. In the batteries 27 to 29, it is considered that slight deformation of the negative electrode occurs. However, since there is no porous heat-resistant layer, it is considered that an internal short circuit could not be prevented when the separator was damaged due to deformation of the negative electrode.

  Of the batteries 6 and 7 in which the porous heat-resistant layer was formed on the surface of the positive electrode active material, the battery 7 with the longer negative electrode width was somewhat low in short circuit resistance. This is presumably because the porous heat-resistant layer was provided on the surface of the positive electrode active material layer narrower than the negative electrode active material layer, so that the surface of the negative electrode and the upper end of the positive electrode were in contact with each other at the upper part of the electrode group.

  The battery 8 which did not use the separator had somewhat low short circuit resistance after dropping. The porous heat-resistant layer is structurally fragile compared to the separator. Therefore, it is considered that the porous heat-resistant layer was partially destroyed by the impact of the drop and a short circuit occurred.

  Batteries 13 and 14 provided with a porous heat-resistant layer made of a heat-resistant resin on the surface of the separator had somewhat lower short-circuit resistance after dropping. A porous heat-resistant layer made of a heat-resistant resin has a lower mechanical strength than a porous heat-resistant layer containing an insulating filler and a binder. Therefore, it is considered that a short circuit occurred due to the impact of the drop.

  Batteries 15 and 16 using an independent sheet-like porous heat-resistant layer and not using a separator had lower short-circuit resistance after dropping than batteries 13 and 14. This is related to the low strength of the porous heat-resistant layer made of a heat-resistant resin and the absence of the effect of improving the strength of the porous heat-resistant layer by the separator.

  The battery 17 in which the content of the binder contained in the porous heat-resistant layer was 0.5% by weight was somewhat low in short-circuit resistance after dropping. This is presumably because the adhesive strength between the filler particles was weakened due to the low content of the binder, and the mechanical strength of the porous heat-resistant layer was lowered.

  On the other hand, the battery 20 in which the binder content was 20% by weight had low high output characteristics. This is because the porosity of the porous heat-resistant layer is lowered by the excess binder, and the excess binder is swollen by the non-aqueous electrolyte, and the pores of the porous heat-resistant layer are blocked, and the ion conduction. This is thought to be due to a decline in sex. On the other hand, in the batteries 18 to 19 having a binder content of 1 to 10% by weight, both short-circuit resistance and high output characteristics were good.

  By controlling the drying conditions, the battery 21 in which the porosity of the porous heat-resistant layer was 30% had somewhat lower drop resistance. This is because the pores of the porous heat-resistant layer are small, and the non-aqueous electrolyte cannot be sufficiently permeated into the porous heat-resistant layer, and the expansion of the electrode group is small. Therefore, it is considered that the movement of the electrode group due to the drop could not be prevented. The battery 25 in which the porosity of the porous heat-resistant layer was 89% had somewhat low short circuit resistance after dropping. This is considered because the mechanical strength of the porous heat-resistant layer is low.

  On the other hand, the batteries 22 to 24 having a porosity of the porous heat-resistant layer of 40 to 80% both had good drop resistance and short-circuit resistance after dropping. It is considered that the mechanical strength of the porous heat-resistant layer was maintained because the porosity was optimized. Further, it is considered that the movement of the electrode group was prevented because the porous heat-resistant layer was appropriately expanded with the nonaqueous electrolyte.

In this example, a prismatic lithium secondary battery as shown in FIG. 2 will be described.
<Battery 31>
The total thickness of the positive electrode was changed to 150 μm, the width of the positive electrode was changed to 42.7 mm, the total thickness of the negative electrode was changed to 150 μm, the width of the negative electrode was changed to 43.7 mm, the width of the separator was changed to 47 mm, and the shape was changed to an elliptic cylinder An electrode group was produced in the same manner as in Example 1 except that.
The obtained electrode group was inserted into an aluminum square battery can having a height of 49 mm (bottom thickness of 0.5 mm), a width of 34 mm, and a thickness of 5.2 mm. After disposing an insulator having a thickness of 1.5 mm on the electrode group, 2.5 g of the same nonaqueous electrolyte as in Example 1 was injected into the battery can. The distance A from the inner bottom surface of the battery can to the lower surface of the insulator was 46.0 mm. The lower part of the electrode group is insulated from the battery can by an insulating sheet. Since the insulating sheet is extremely thin, its thickness is negligible.
Thereafter, a rectangular sealing plate having a thickness of 1.0 mm was placed on the opening of the battery can, and the opening end and the peripheral edge of the sealing plate were laser-welded. As a result, a prismatic lithium secondary battery having a height of 50 mm, a width of 34 mm, a thickness of 5.2 mm, and a design capacity of 950 mAh was completed. Ratio of negative electrode width B (43.7 mm) to distance A (46.0 mm): B / A was 0.95.

<Batteries 32-35>
The negative electrode width B was 44.6 mm, 45 mm, 45.7 mm or 46.5 mm, the positive electrode width was 43.6 mm, 44 mm, 44.7 mm or 45.5 mm, respectively, and the design capacities were 970 mAh, 979 mAh, 994 mAh, respectively. Or the square-shaped lithium secondary batteries 32-35 similar to the battery 31 were produced except having set it as 1012 mAh. The B / A ratio in each battery was 0.970 (battery 32), 0.978 (battery 33), 0.993 (battery 34), or 1.011 (battery 35), respectively.

<< Batteries 36 and 37 >>
Square lithium secondary batteries 36 and 37 similar to the batteries 33 and 34 were produced, respectively, except that the porous heat-resistant layer was formed on the surface of the positive electrode active material layer.

<Battery 38>
A square lithium secondary battery similar to the battery 33 was fabricated except that the porous heat-resistant layer had a thickness of 15 μm and the electrode group was fabricated without using a separator.

<Batteries 39 to 42>
Square lithium secondary batteries 39 to 42 similar to the battery 33 were prepared, respectively, except that the alumina of the porous heat-resistant layer was changed to magnesia, silica, titania or zirconia having the same median diameter.

<< Battery 43-50 >>
Square lithium secondary batteries 43 to 50 similar to those of Example 33 were produced, respectively, except that the same porous heat-resistant layer as that of batteries 13 to 20 of Example 1 was used.

<Batteries 51 to 55>
After applying the raw material paste for the porous heat-resistant layer, the speed of the hot air during drying was 0.5 m / min, 1 m / min, 2 m / min, 5 m / min, or 8 m / min, respectively. Similarly, rectangular lithium secondary batteries 51 to 55 were produced. The porosity of the porous heat-resistant layer in each battery was 30% (battery 51), 42% (battery 52), 60% (battery 53), 78% (battery 54), or 89% (battery 55), respectively. .

<< Battery 56-60 >>
Square lithium secondary batteries 56 to 60 similar to the batteries 31 to 35 were prepared, respectively, except that the separator thickness was 15 μm and the porous heat-resistant layer was not provided.

  Each battery was conditioned and discharged twice and then stored for 7 days in a 45 ° C. environment. Then, the following evaluation was performed. Tables 4, 5 and 6 show the characteristics of the porous heat-resistant layer, the battery design and the evaluation results, respectively.

(Internal short circuit inspection)
Short-circuit resistance was evaluated in the same manner as in Example 1 except that charging was performed under the following conditions. The results are shown in Table 6.
Constant current charging: Charging current value 665 mA / end-of-charge voltage 4.2 V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA

(Drop test)
“Fall resistance” was evaluated in the same manner as in Example 1 except that charging / discharging was performed under the following conditions. The results are shown in Table 6.
Constant current charging: Charging current value 665 mA / end-of-charge voltage 4.2 V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current value 950 mA / discharge end voltage 3 V

(Internal short circuit inspection after drop test)
The battery after the drop test was subjected to the same internal short circuit inspection as before the drop test. The results are shown in Table 6 as “Short-circuit occurrence rate after dropping”.

(High output characteristics)
Each battery was charged and discharged under the following conditions in an environment of 20 ° C. to determine the discharge capacity.
Constant current charging: Charging current value 665 mA / end-of-charge voltage 4.2 V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current value 190 mA / discharge end voltage 3 V
Constant current charging: Charging current value 665 mA / end-of-charge voltage 4.2 V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current value 1900 mA / discharge end voltage 3 V
The ratio of the capacity at 1900 mA discharge to the capacity at 190 mA discharge was determined as a percentage. The obtained values are shown in Table 6 as “high output characteristics”.

(Nail penetration test)
Each battery was evaluated for the temperature reached after 90 seconds after nail penetration in the same manner as in Example 1 except that the battery was charged to a final voltage of 4.35 V at a charging current value of 950 mA. The results are shown in Table 6.

  The battery 31 in which the width B of the negative electrode is too small with respect to the distance A from the lower surface of the insulator, which is the restricting portion, to the inner bottom surface of the battery can has a small capacity density and a low drop resistance. When the battery 31 after the drop test was disassembled, it was found that the winding deviation of the electrodes in the electrode group occurred.

  It can be seen that the capacity of the battery 31 decreased due to the decrease in the effective area where the positive electrode and the negative electrode face each other, although the internal heat short circuit was avoided by the action of the porous heat-resistant layer. The electrode group that has become difficult to deform due to the provision of the porous heat-resistant layer is difficult to be firmly fixed inside the battery can. Therefore, it is considered that the electrode group is unwound when repeated dropping.

  On the other hand, the battery 35 in which the negative electrode width B is too large with respect to the distance A from the lower surface of the insulator to the inner bottom surface of the battery can has low short-circuit resistance. When the battery 35 that was regarded as an internal short circuit was disassembled, the porous heat-resistant layer provided on the surface of the negative electrode was broken above the electrode group. It was also found that the separator was torn.

  Batteries 33 to 34 having a B / A ratio of 0.975 to 0.995 have high short-circuit resistance and improved drop resistance. The battery of the present invention is provided with a porous heat-resistant layer in addition to the separator. Therefore, even if the negative electrode provided larger than the positive electrode is slightly deformed at the upper part of the electrode group, the insulation of the deformed portion is ensured by the double structure of the separator and the porous heat-resistant layer. In addition, since the B / A ratio is large, the electrode group can be firmly held between the lower surface of the insulator and the inner bottom surface of the battery can, and thus it is considered that the drop resistance is improved.

  However, the battery 32 having a B / A ratio of 0.965 to 0.975 has a slightly lower drop resistance than the cylindrical battery 2 (Example 1) having the same B / A ratio. It was. In the case of a cylindrical battery, the groove as the restricting portion has a gentle V-shaped or U-shaped cross section. Therefore, the upper end portion of the electrode group receives pressure from the inclination of the groove portion. On the other hand, in the case of a square battery, since the lower surface of the insulator, which is a restricting portion, is a flat surface, there is no inclination like a groove portion. This is considered to have caused a difference in the range of the more effective B / A ratio.

  The batteries 56 to 60 that did not have the porous heat-resistant layer were excellent in drop resistance regardless of the position of the restricting portion. The electrode group not having the porous heat-resistant layer is considered to be firmly fixed in the battery can because it is appropriately deformed. Thereby, even when the battery is dropped, it is considered that the winding deviation of the electrode group causing the capacity reduction is suppressed. However, the batteries 56-60 were overheated during the nail penetration test. In addition, the batteries 57 to 59 provided with the restriction portions at the same positions as the batteries 32 to 34 have a short circuit resistance. In the batteries 57 to 59, it is considered that a slight deformation of the negative electrode occurs. However, since there is no porous heat-resistant layer, it is considered that an internal short circuit could not be prevented when the separator was damaged due to deformation of the negative electrode.

  Of the batteries 36 and 37 in which the porous heat-resistant layer is formed on the surface of the positive electrode active material layer, the battery 37 with the longer negative electrode width was somewhat low in short circuit resistance. This is presumably because the porous heat-resistant layer was provided on the surface of the positive electrode active material layer narrower than the negative electrode active material layer, so that the surface of the negative electrode and the upper end of the positive electrode were in contact with each other at the upper part of the electrode group.

  The battery 38 that did not use a separator had somewhat low short circuit resistance after dropping. The porous heat-resistant layer is structurally fragile compared to the separator. Therefore, it is considered that the porous heat-resistant layer was partially destroyed by the impact of the drop and a short circuit occurred.

  The batteries 43 and 44 provided with a porous heat-resistant layer made of a heat-resistant resin on the surface of the separator had somewhat low short circuit resistance after dropping. A porous heat-resistant layer made of a heat-resistant resin has a lower mechanical strength than a porous heat-resistant layer containing an insulating filler and a binder. Therefore, it is considered that a short circuit occurred due to the impact of the drop.

  Batteries 45 and 46 using a porous heat-resistant layer of an independent sheet and not using a separator had lower short circuit resistance after dropping than batteries 43 and 44. This is related to the low strength of the porous heat-resistant layer made of a heat-resistant resin and the absence of the effect of improving the strength of the porous heat-resistant layer by the separator.

  The battery 47 in which the content of the binder contained in the porous heat-resistant layer was 0.5% by weight was somewhat low in short-circuit resistance after dropping. This is presumably because the adhesive strength between the filler particles was weakened due to the low content of the binder, and the mechanical strength was lowered.

  On the other hand, the battery 50 in which the binder content was 20% by weight had somewhat low high output characteristics. This is because the porosity of the porous heat-resistant layer is lowered by the excess binder, and the excess binder is swollen by the non-aqueous electrolyte, and the pores of the porous heat-resistant layer are blocked, and the ion conduction. This is thought to be due to a decline in sex. On the other hand, the batteries 48 to 49 having a binder content of 1 to 10% by weight both had good short circuit resistance and high output characteristics.

  By controlling the drying conditions, the battery 51 in which the porosity of the porous heat-resistant layer was 30% had a somewhat low drop resistance. This is because the pores of the porous heat-resistant layer are small, and the non-aqueous electrolyte cannot be sufficiently permeated into the porous heat-resistant layer, and the expansion of the electrode group is small. Therefore, it is considered that the movement of the electrode group due to the drop could not be prevented. The battery 55 in which the porosity of the porous heat-resistant layer was 89% had somewhat low short circuit resistance after dropping. This is considered because the mechanical strength of the porous heat-resistant layer is low.

  On the other hand, the batteries 52 to 54 in which the porosity of the porous heat-resistant layer was 40 to 80% both had good drop resistance and short-circuit resistance after dropping. Since the porosity is optimized, the mechanical strength is maintained, and furthermore, the porous heat-resistant layer is appropriately swollen with a non-aqueous electrolyte, so that it is considered that the movement of the electrode group is prevented.

  The lithium secondary battery of the present invention is excellent in short-circuit resistance and heat resistance, has high safety, and is less susceptible to capacity reduction due to impact such as dropping. It can be used as a power source for electronic equipment. However, the use of the lithium secondary battery of the present invention is not particularly limited, and the lithium secondary battery 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 cross-sectional schematic diagram of an example of the cylindrical lithium secondary battery of this invention. It is a cross-sectional schematic diagram of an example of the square lithium secondary battery of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Cylindrical battery can 101 Positive electrode 102 Negative electrode 103 Separator 104 Positive electrode lead 106 Upper insulating board 107 Lower insulating board 108 External positive electrode terminal 108a Internal terminal 110 Bottom part 120 Sealing board 130 Groove part

200 Square battery can 201 Electrode group 202 Positive electrode lead 203 Negative electrode lead 206 Insulating gasket 207 Negative electrode terminal 209 Seal 210 Bottom 220 Sealing plate 230 Insulator

Claims (4)

A lithium secondary battery comprising: a 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. Because
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 interposed therebetween, and the positive electrode comprises a positive electrode core material and a positive electrode active material layer carried on both surfaces thereof. The negative electrode includes a negative electrode core material and negative electrode active material layers supported on both sides thereof, and is between the porous heat-resistant layer and the positive electrode, or between the porous heat-resistant layer and the negative electrode. Having a separator made of a microporous film,
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, and includes an insulating filler and a binder. Including a binder, 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 battery has a restricting portion that restricts the vertical movement of the electrode group, and a distance A from the restricting portion to an inner bottom surface of the battery can and a width B of the negative electrode are 0.965 ≦ B. /A≦0.995 meet the,
When the electrode group is substantially columnar and the battery can is cylindrical, the restricting portion is a groove provided at the upper portion of the side wall of the battery can so that the inner diameter of the battery can is reduced,
When the electrode group is substantially elliptical columnar and the battery can is a square shape, the battery further includes an insulator provided between the sealing plate and the electrode group, A lithium secondary battery , which is a lower surface of the insulator . It said insulating filler comprises an inorganic oxide, a lithium secondary battery according to claim 1, wherein. The lithium secondary battery according to claim 2 , wherein the inorganic oxide includes at least one selected from the group consisting of alumina, silica, magnesia, titania, and zirconia. When the electrode group has a substantially elliptical column shape and the battery can has a square shape, the distance A from the regulating portion to the inner bottom surface of the battery can and the width B of the negative electrode are 0.975 ≦ B / satisfy a ≦ 0.995, the lithium secondary battery of claim 1, wherein.

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