CN100495804C - lithium secondary battery - Google Patents

Abstract

A lithium secondary battery that is excellent in resistance to short-circuits and heat, is unlikely to suffer a capacity loss due to impact such as dropping, and has a high capacity. The lithium secondary battery includes: an electrode assembly including a strip-like positive electrode and a strip-like negative electrode that are wound 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 assembly. 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<=B/A<=0.995.

Description

lithium secondary battery

technical field

The present invention relates to a relatively safe lithium secondary battery having excellent short-circuit resistance and heat resistance. In particular, the present invention relates to a technique for preventing capacity loss of a battery due to impact such as dropping.

Background technique

Lithium secondary batteries have attracted attention as high-capacity power sources for portable devices and others. In addition, lithium secondary batteries have recently attracted increasing attention as high-output power sources for electric vehicles and the like. Chemical batteries such as lithium secondary batteries generally have a separator that electrically insulates the positive electrode from the negative electrode and holds the electrolyte. In lithium secondary batteries, microporous membranes made of polyolefins (such as polyethylene, polypropylene, etc.) are mainly used as separators. An electrode assembly of a lithium secondary battery may be manufactured in a cylindrical or substantially elliptical cylindrical shape by winding a positive electrode and a negative electrode together with a separator interposed between the two electrodes.

Cylindrical lithium secondary batteries are used, for example, as power sources for power tools and notebook computers. A cylindrical lithium secondary battery can be sealed by crimping the open side of its battery case on a sealing plate. In order to fix the sealing plate on the periphery of the top opening of the battery case, the battery case has a groove portion (narrow portion) with a reduced inner diameter in the upper half of its side wall. Patent Document 1 proposes a high-capacity design, in which the width B (38mm) of the negative electrode and the distance A (39.7mm) from the narrow portion to the outer bottom surface of the battery case satisfy the relationship: B/A=0.957.

A prismatic lithium secondary battery is used, for example, as a power source for cellular phones and digital still cameras. Since prismatic lithium secondary batteries are easier to fit into devices than cylindrical ones, they are becoming increasingly popular. In a prismatic lithium secondary battery, a lead wire connecting an electrode and a terminal is easily in contact with a battery case, which is different from a cylindrical lithium secondary battery. A short circuit occurs when a lead of the opposite polarity to the battery case contacts the battery case. Therefore, an insulator (hereinafter referred to as an upper insulator) is usually provided between the upper half of the electrode assembly and the cover (insulation plate) of the battery case. 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 half of the electrode assembly and the bottom of the battery case (Patent Document 2).

The electrode assembly of the prismatic lithium secondary battery is usually manufactured so that the distance A from the lower surface of the upper insulator to the inner bottom surface of the battery case and the width B of the negative electrode satisfy the relationship: B/A≤0.96. The higher the ratio of B/A, the higher the battery capacity. However, if the ratio of B/A is too high, the electrode assembly is sensitive to distortion, which leads to a direct contact between the positive and negative electrodes, i.e., a short circuit. In Patent Document 2, by providing a lower insulator serving as a buffer, the ratio of B/A is set as high as 0.97.

Meanwhile, when a lithium secondary battery is stored for a long time in an extremely high-temperature environment, its separator made of a microporous membrane tends to shrink. If the separator shrinks, the positive and negative electrodes actually touch each other creating an internal short circuit. Considering the recent trend of thinning the separator while increasing the capacity of lithium secondary batteries, preventing internal short circuits has become especially important. Once an internal short circuit occurs, the short circuit will expand due to the Joule heat generated by the short circuit current, resulting in overheating of the battery.

Therefore, in order to suppress the expansion of such a short circuit when an internal short circuit occurs, it has been proposed to form a porous heat-resistant layer containing an inorganic filler (solid fine particles) and a binder on the electrode active material layer. Alumina, silica, and the like can be used as the inorganic filler. Inorganic fillers are filled in the porous heat-resistant layer, where filler particles are bound to each other with a relatively small amount of binder (Patent Document 3). Since the porous heat-resistant layer has shrinkage resistance even at high temperatures, it functions to suppress overheating of the battery under internal short circuits.

Patent Document 1: Japanese Patent Application No. Hei 11-354084.

Patent Document 2: Japanese Patent Application No. 2004-31263.

Patent Document 3: Japanese Patent Application No. Hei 7-220759.

Contents of the invention

The problem to be solved by the present invention

In order to realize a lithium secondary battery having high capacity and excellent short-circuit resistance, the proposal of Patent Document 1 or Patent Document 2 and the proposal of Patent Document 3 may be adopted in combination. This combination significantly reduces internal short circuits, but results in a significant loss of capacity when the battery is subjected to shocks, such as being dropped.

In view of the above-mentioned problems, one of the objects of the present invention is to provide a lithium secondary battery having excellent short-circuit resistance, preventing capacity loss due to dropping, and capable of high-capacity design.

way of solving the problem

The invention relates to a lithium secondary battery, which comprises: a battery case having a bottom, a side wall and a top opening; an electrode assembly; a non-aqueous electrolyte; sealing board. The electrode assembly includes a strip-shaped positive electrode and a strip-shaped negative electrode intertwined with a porous heat-resistant layer interposed between the positive electrode and the negative electrode. The positive electrode includes a positive electrode core member and a positive electrode active material layer coated on the core member, and the negative electrode includes a negative electrode core member and a negative electrode active material layer coated on the core member. The battery has a restriction position that restricts the vertical movement of the electrode assembly, and the distance A from the restriction position to the inner surface of the bottom of the battery case and the width B of the negative electrode satisfy the relationship: 0.965≤B/A≤0.995.

The inner surface of the bottom of the battery case will have slight depressions and protrusions. In this case, however, the difference in height between the recessed portion and the protruding portion is usually not more than 0.05 mm and thus can be ignored. In addition, the width B of the negative electrode refers to the length of the shorter side 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 cylindrical electrode assembly.

A lithium secondary battery according to the present invention has a separator including a microporous membrane between a porous heat-resistant layer and a positive electrode or between a porous heat-resistant layer and a negative electrode.

The porous heat-resistant layer is formed, for example, on the surface of at least one of the positive electrode active material layer and the negative electrode active material layer.

The porous heat-resistant layer includes, for example, insulating fillers and binders. Based on 100 parts by weight of the insulating filler, the content of the binder is preferably 1-10 parts by weight. The porous heat-resistant layer preferably has a porosity of 40 to 80%.

The insulating filler preferably contains an inorganic oxide. The inorganic oxide preferably contains at least one of alumina, silica, magnesia, titania, and zirconia.

When the electrode assembly is substantially cylindrical and the battery case is cylindrical, the restricting portion is preferably a groove of the battery case with a reduced inner diameter provided on the upper half of the side wall of the battery case. When the distance A varies according to the depth of the reduced-diameter groove, the distance from the deepest portion of the groove (the portion that protrudes most toward the center of the battery case) to the inner surface of the bottom of the battery case is the distance A.

A lithium secondary battery according to the present invention has an insulator between an electrode assembly and a sealing plate. In this case, when the electrode assembly is substantially elliptical cylindrical and the battery case is prismatic, the restricting portion is preferably the lower half surface of the insulator. In a prismatic lithium secondary battery, the distance A from the limiting portion to the inner surface of the bottom of the battery case and the width B of the negative electrode preferably satisfy the relationship: 0.975≤B/A≤0.995.

Invention effect

The present invention can provide a lithium secondary battery having excellent short-circuit resistance and heat resistance, avoiding capacity loss due to impact such as dropping, and providing higher capacity.

Description of drawings

Fig. 1 is a schematic cross-sectional view of an example of a cylindrical lithium secondary battery of the present invention; and

Fig. 2 is a schematic cross-sectional view of an example of a prismatic lithium secondary battery of the present invention.

Detailed ways

The invention relates to a lithium secondary battery, which comprises: a battery case having a bottom, a side wall and a top opening; an electrode assembly; a non-aqueous electrolyte; sealing board. The electrode assembly includes a strip-shaped positive electrode and a strip-shaped negative electrode intertwined with a porous heat-resistant layer interposed between the positive electrode and the negative electrode. The positive electrode includes a positive electrode core member and a positive electrode active material layer coated on the core member, and the negative electrode includes a negative electrode core member and a negative electrode active material layer coated on the core member. The battery has a restricting part that restricts the vertical movement of the electrode assembly, and the distance A from the restricting part to the inner surface of the bottom of the battery case and the width B of the negative electrode satisfy the relationship: 0.965≤B/A≤0.995.

The inventors of the present invention have persistently studied an electrode assembly having a porous heat-resistant layer and made the following two discoveries.

First, the electrode assembly with the porous heat-resistant layer has less deformation during the charging/discharging process than the conventional electrode assembly without the porous heat-resistant layer. This may be because the porous heat-resistant layer has a lower surface flatness than the positive electrode, the negative electrode, and the separator, so that the electrodes and the separator do not slide or move.

Second, when the electrode assembly is not properly deformed, the electrode assembly cannot be firmly fixed in the battery case. Therefore, when such a battery is dropped, the electrodes in its electrode assembly move, resulting in a loss of capacity.

Based on these findings, in the present invention, the ratio of the width B of the negative electrode to the distance A from the restriction to the inner surface of the bottom of the battery case (ratio of B/A) is set to a higher range than the conventional value. When the ratio of B/A satisfies the relationship: 0.965≤B/A≤0.995, the movement of the electrodes in the electrode assembly (especially when the battery is dropped) is obviously suppressed, so that capacity loss does not occur.

If the ratio of B/A exceeds 0.96, the distortion of the electrode assembly becomes large, which often causes a short circuit. In addition, the width of the negative electrode of lithium secondary batteries is usually designed to be larger than the width of the positive electrode, so deformation of the negative electrode is particularly a problem. However, since the electrode assembly in the present invention has a porous heat-resistant layer, even if the edge of the negative electrode is slightly deformed near the upper or lower surface of the electrode assembly, no short circuit will occur. Therefore, the ratio of B/A can be set to 0.965 or higher. According to the present invention, by making the width of the negative electrode close to the distance A from the restricting portion to the inner surface of the bottom of the battery case, high capacity can be achieved while improving drop impact resistance.

If the ratio of B/A is less than 0.965, it is difficult to achieve a high capacity, and in addition, the battery is likely to suffer capacity loss due to movement of electrodes in the electrode assembly when dropped. On the other hand, if the ratio of B/A exceeds 0.995, the negative electrode is significantly deformed near the upper or lower surface of the electrode assembly. Then, the porous heat-resistant layer is damaged, so that internal short circuit is liable to occur.

When 0.965≦B/A≦0.995, a lithium secondary battery free from internal short circuit, high capacity and excellent drop impact resistance can be obtained.

The lithium secondary battery according to the present invention may or may not have a separator including a microporous membrane. The separator is located between the porous heat-resistant layer and the positive electrode or between the porous heat-resistant layer and the negative electrode. The role of the diaphragm is to support the fragile porous heat-resistant layer on the structure. Therefore, in order to further improve the drop impact resistance, the battery preferably has a separator.

The microporous membrane material is preferably polyolefin, and the polyolefin is preferably polyethylene, polypropylene or the like. Microporous membranes comprising both polyethylene and polypropylene can also be used. The thickness of the microporous membrane is preferably 8-20 μm to ensure that it can support the porous heat-resistant layer and maintain a high-capacity design.

The porous heat-resistant layer may be formed only on the surface of the positive electrode active material layer or on the surface of the negative electrode active material layer. Alternatively, the porous heat-resistant layer may be formed simultaneously on the surface of the positive electrode active material layer and the surface of the negative electrode active material layer. However, in order to avoid internal short circuit in a reliable manner, the porous heat-resistant layer is preferably formed on the surface of the negative electrode active material layer designed to have a larger area than the positive electrode active material layer. In addition, the porous heat-resistant layer may be formed on the active material layer on one side of the core member or on the active material layer on both sides of the core member. In addition, the porous heat-resistant layer is preferably adhered on the surface of the active material layer.

The porous heat-resistant layer can be an independent sheet. However, since the flake-shaped porous heat-resistant layer does not have high mechanical strength, it may be difficult to handle. In addition, a porous heat-resistant layer can be adhered to the surface of the separator. However, since the separator shrinks at high temperatures, close attention must be paid to the manufacturing conditions of the porous heat-resistant layer. In order to eliminate such problems, the porous heat-resistant layer is also preferably formed on the surface of the positive electrode active material layer or on the surface of the negative electrode active material layer.

The porous heat-resistant layer preferably contains insulating fillers and binders. The porous heat-resistant layer is formed after coating and drying a raw material slurry containing an insulating filler and a small amount of a binder on the surface of an electrode active material layer or a separator using a doctor blade or a die coater. The raw material slurry is prepared by mixing the insulating filler, binder, and liquid components using, for example, a two-arm kneader.

In addition, the porous heat-resistant layer may be a film formed of fibers of a highly heat-resistant resin. The highly heat-resistant resin is preferably aramid, polyamideimide, or the like. However, a porous heat-resistant layer comprising an insulating filler and a binder has higher structural strength than a film formed of fibers of a high heat-resistant resin due to the action of the binder, and is therefore preferable.

The thickness of the porous heat-resistant layer is preferably 0.5 to 20 μm, and more preferably 1 to 10 μm. If the thickness of the porous heat-resistant layer is less than 0.5 μm, its effect of suppressing internal short circuits decreases. In addition, if the thickness of the porous heat-resistant layer exceeds 20 μm, the distance between the positive electrode and the negative electrode becomes too large, which leads to a decrease in the output characteristics of the battery.

The insulating filler may contain fibers or highly heat-resistant resin beads, but preferably contains inorganic oxides. Since the inorganic oxides are hard, they can keep the distance between the positive electrode and the negative electrode within an appropriate range even when the electrodes expand due to charging/discharging. Among inorganic oxides, such as alumina, silica, magnesia, titania, and zirconia are particularly preferable because they have high electrochemical stability in the working environment of a lithium secondary battery. They can be used alone or in combination of two or more.

In the porous heat-resistant layer comprising an insulating filler and a binder, based on 100 parts by weight of the insulating filler, the content of the binder is preferably 1 to 10 parts by weight, and more preferably 2 to 8 parts by weight, so as to maintain The mechanical strength of the porous heat-resistant layer and its ionic conductivity. Most binders and thickeners inherently swell with electrolytes containing non-aqueous solvents. Therefore, if the content of the binder exceeds 10 parts by weight, the binder may excessively swell to close the micropores of the porous heat-resistant layer, thereby reducing ion conductivity and preventing battery reactions from occurring. On the other hand, if the content of the binder is less than 1 part by weight, the mechanical strength of the porous heat-resistant layer decreases.

The binder used in the porous heat-resistant layer is not particularly limited, but is preferably, for example, polyvinylidene fluoride (hereinafter referred to as PVDF), polytetrafluoroethylene (hereinafter referred to as PTFE), and polyacrylic rubber particles (such as Zeon company's BM-500B (trade name)). It is preferred to use PTFE or BM-500B in combination with a thickener. The thickener is not particularly limited, but is preferably for example carboxymethyl cellulose (hereinafter referred to as CMC), polyethylene oxide (hereinafter referred to as PEO), and modified nitrile rubber (such as BM-720H of Zeon Company (commercial product) name)).

The porosity of the porous heat-resistant layer including the insulating filler and the binder is preferably 40% to 80%, and further preferably 45% to 65% in order to maintain the mechanical strength of the porous heat-resistant layer and improve its drop impact resistance. Since the porous heat-resistant layer has lower surface flatness than the positive electrode, the negative electrode, and the separator, the sliding (movement) of the electrodes and the separator is greatly suppressed. Thus, the electrode assembly is easy to move. However, when the porous heat-resistant layer with a porosity of 40%-80% is immersed in an appropriate amount of electrolyte, the electrode assembly will swell to a suitable degree. Then, the swollen electrode assembly presses against the inner side wall of the battery case. When the effect of 40%-80% porosity and the effect of optimal B/A ratio promote each other, the drop impact resistance is further improved. If the porosity is lower than 40%, the electrolyte solution does not sufficiently infiltrate the porous heat-resistant layer, so that the electrode assembly does not swell to an appropriate degree. On the other hand, if the porosity exceeds 80%, the mechanical strength of the porous heat-resistant layer decreases.

It should be noted that the porosity of the porous heat-resistant layer can be controlled by changing the median particle size of the insulating filler, the content of the binder, and the drying conditions of the raw material slurry. For example, increasing the drying temperature or increasing the flow of hot air for drying can result in a relative increase in porosity. The porosity can be calculated from the following parameters, for example, the thickness of the porous heat-resistant layer, the content of insulating fillers and binders, and the actual specific gravity of insulating fillers and binders. The thickness of the porous heat-resistant layer can be determined by taking SEM photos of several cross-sections (for example, 10 cross-sections) of the electrode and the average thickness of these several cross-sections. Additionally, porosity can be determined using mercury porosimetry.

A cylindrical lithium secondary battery has a cylindrical (cylindrical) electrode assembly with a substantially circular cross section. In addition, the cylindrical lithium secondary battery has a cylindrical battery case 100 as shown in FIG. 1 . The cylindrical battery case is open at one end and closed with a flat bottom 110 at the other end. In the case of a common cylindrical lithium secondary battery, the open side of the battery case is crimped onto the circumference of the sealing plate 120 to seal the top opening thereof. In this case, the restricting portion restricting the vertical movement of the electrode assembly is the groove portion 130 of the battery case 100 having a reduced inner diameter provided on the upper half of the side wall of the battery case 100 . The groove portion 130 also has the function of fixing the sealing plate 120 .

A prismatic lithium secondary battery has an electrode assembly having a substantially elliptical cylindrical shape (nearly elliptical cylindrical shape) in cross section. In addition, the prismatic lithium secondary battery has a prismatic (substantially rectangular) battery case 200 as shown in FIG. 2 . The prismatic battery case is open at one end and closed with a flat bottom 210 at the other end. For a common prismatic lithium secondary battery, the top opening of the battery case is sealed by welding the opening edge and the metal sealing plate 220 together. In addition, an insulator 230 (upper insulator) is located between the sealing plate 220 and the electrode assembly to prevent lead wires in the electrode from contacting the battery case 200 . The insulator 230 has electrode lead through holes so that the insulator hardly moves. Therefore, the limiting portion that limits the vertical movement of the electrode assembly is the lower surface of the insulator 230 .

The thickness of the insulator is preferably 2% to 10% of the height of the battery case, so as to ensure its function and reduce dead space.

In a cylindrical lithium secondary battery, the longitudinal section of the groove serving as a restricting part is generally V-shaped or U-shaped due to the limitation of the production method. Therefore, the distance A varies depending on the depth of the groove serving as the restricting portion. In this case, the distance from the deepest part of the groove to the inner surface of the bottom of the battery case is the distance A. In this case, if the ratio of B/A is 0.965 or more, sufficient drop impact resistance can be obtained. However, in consideration of the balance between high capacity and drop impact resistance, it is more preferable that 0.970≦B/A≦0.990 in cylindrical lithium secondary batteries.

On the other hand, in the prismatic lithium secondary battery, the lower surface of the insulator 230 serving as the confinement site is flat. Therefore, in order to obtain excellent drop impact resistance, the ratio of B/A is preferably 0.975 or more. In addition, in consideration of the balance between high capacity and drop impact resistance, 0.975≦B/A≦0990 is more preferable in the prismatic lithium secondary battery.

The positive electrode includes a positive electrode core member and a positive electrode active material layer coated on each side of the core member. The positive electrode core member is in a bar shape suitable for winding and contains Al, Al alloy, or the like. The positive electrode active material layer contains the positive electrode active material as an essential component and may contain optional components such as a conductive agent and a binder if necessary. These materials are not particularly limited, but a preferred cathode active material is a lithium-containing transition metal oxide. Among lithium-containing transition metal oxides, preferred are, for example, lithium cobaltate, modified lithium cobaltate, lithium nickelate, modified lithium nickelate, lithium manganate, and modified lithium manganate.

The negative electrode includes a negative electrode core member and a negative electrode active material layer coated on each side of the core member. The negative electrode core member is in a strip shape suitable for winding and contains Cu, Cu alloy, and the like. The width B of the negative electrode is equal to the width of the negative electrode core member. The negative electrode active material layer contains the negative electrode active material as an essential component and may contain optional components such as a conductive agent and a binder if necessary. These materials are not particularly limited, but preferred negative electrode active materials include various natural graphites, various artificial graphites, silicon-containing composite materials such as silicide, metal lithium, and various alloy materials.

Exemplary binders for positive or negative electrodes include PTFE, PVDF, and styrene-butadiene rubber. Exemplary conductive agents include acetylene black, Ketjen black (registered trademark), and various graphites.

The non-aqueous electrolytic solution preferably contains a non-aqueous solvent and a lithium salt dissolved therein. The lithium salt is not particularly limited, but is preferably, for example, LiPF6 and LiBF4. These lithium salts may be used alone or in combination of two or more. The non-aqueous solvent is not particularly limited, but preferable examples include ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). These nonaqueous solvents may be used alone or in combination of two or more.

The material of the battery case must be electrochemically stable within the working voltage range of the lithium secondary battery. For example, iron or aluminum is preferably used. In addition, the battery case can be plated with nickel or tin.

FIG. 1 is a schematic cross-sectional view of a cylindrical lithium secondary battery of the present invention for demonstration.

A cylindrical electrode assembly was fabricated 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 between the positive and negative electrodes. The separator 103 is located between the porous heat-resistant layer and the positive electrode 101 . However, the separator 103 is unnecessary if the porous heat-resistant layer has a sufficient thickness. The electrode assembly was inserted into the cylindrical battery case 100 . The upper half of the side wall of the battery case 100 has a groove portion 130 whose inner diameter is relatively smaller than other parts. The groove portion 130 is formed after the electrode assembly is put into the battery case 100 . The longitudinal section of the groove portion 130 is U-shaped. After that, the electrolytic solution was injected into the battery case 100 . The top opening of the battery case 100 is sealed by installing the sealing plate 120 on the groove portion 130 and crimping the opening edge of the battery case 100 on the circumference of the sealing plate 120 .

An upper insulator plate 106 and a lower insulator plate 107 of negligible thickness are disposed on top and under the electrode assembly. One end of the positive electrode lead 104 is connected to the core member of the positive electrode 101 , and the other end is connected to the inner terminal 108 a provided on the lower surface of the sealing plate 120 . There is continuity between the inner terminal 108a and the outer positive terminal 108 . One end of a negative electrode lead (not shown) is connected to the core member of the negative electrode 102 , and the other end is connected to the inner bottom surface of the battery case 100 .

Fig. 2 is a schematic cross-sectional view of an exemplary prismatic lithium secondary battery of the present invention.

A nearly elliptical cylindrical electrode assembly is fabricated by winding the positive and negative electrodes together with a separator and a porous heat-resistant layer inserted between the positive and negative electrodes. The electrode assembly 201 is inserted into a substantially rectangular (prismatic) battery case 200 . After the electrode assembly 201 is put into the battery case 200 , an insulator 230 is installed on top of the electrode assembly 201 to prevent a short circuit between the battery case 200 or one end of the positive lead 202 and the negative lead 203 . The insulator 230 is fixed near the opening of the battery case 200 .

The sealing plate 220 has a negative terminal 207 surrounded by an insulating gasket 206 . The negative electrode lead 203 is connected to the negative electrode terminal 207 , and the positive electrode lead 202 is connected to the lower surface of the sealing plate 220 .

The electrolytic solution is injected into the battery case 200 from an injection hole in the sealing plate 220 , and the injection hole is sealed by a sealing plug 209 for welding. The top opening of the battery case 200 is sealed by assembling a sealing plate 220 and welding the opening edge and the sealing plate 220 by laser.

The content of the present invention will be described in detail below with examples.

Example 1

In this embodiment, the cylindrical lithium secondary battery shown in FIG. 1 is explained.

(Battery 1)

(i) Preparation of positive electrode

Using a double-arm kneader, 3 kg of lithium cobaltate, 1 kg of PVDF#1320 (N-methyl-2-pyrrolidone (hereinafter referred to as NMP) solution containing 12 wt % PVDF), 90 g of Kureha Chemical Industry Co., Ltd. Acetylene black and an appropriate amount of NMP were stirred to prepare a positive electrode mixture slurry. The resulting slurry was coated on both sides of a positive electrode core member comprising a 15 μm thick aluminum foil, dried and rolled to form a positive electrode with a positive electrode active material layer. The total thickness of the positive electrode was 160 μm. The positive electrode was cut into strips with a width of 56.5 mm.

(ii) Preparation of negative electrode

Using a double-arm kneader, the BM-400B (water dispersion containing 40wt% modified styrene-butadiene rubber) of 3kg of artificial graphite, 75g of Zeon Company, 30g of CMC, and an appropriate amount of water are stirred to prepare Get out the negative electrode mixture slurry. The resulting slurry was coated on both sides of a negative electrode core member comprising a 10 μm thick copper foil, dried and rolled to form a negative electrode with a negative electrode active material layer. The total thickness of the negative electrode was 180 μm. The negative electrode was cut into strips with a width of 57.5 mm.

(iii) Formation of porous heat-resistant layer

Using a double-arm kneader, 970g of alumina (insulating filler) with a median particle size of 0.3 μm, 375g of Zeon’s BM-720H (NMP solution containing 8wt% modified polyacrylonitrile rubber (viscosity) mixture)), and an appropriate amount of NMP are stirred to prepare a raw material slurry. The resulting raw material slurry was coated on the surface of the negative electrode active material layer, and dried with hot air at 130° C. (flow rate: 1.5 m/min) for 4 minutes, thereby forming a 5 μm thick porous heat-resistant layer. The porosity of each porous heat-resistant layer was 50%. The porosity is calculated based on the following parameters: the thickness of the porous heat-resistant layer determined by taking SEM pictures of its cross-section; the alumina content of the porous heat-resistant layer in a given area by X-ray fluorescence analysis; alumina and adhesion the actual specific gravity of the agent; and the weight ratio between alumina and binder.

(iv) Preparation of electrolyte

LiPF6 was dissolved in a solvent mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1 at a concentration of 1mol/liter, and the This solution was mixed with 3 wt% ethylene carbonate to prepare an electrolytic solution.

(v) Manufacture of batteries

A cylindrical electrode assembly was fabricated by winding the negative and positive electrodes with porous heat-resistant layers formed on both sides with a separator of 10 μm thick polyethylene microporous membrane (available from Celgard K.K., width: 60.7 mm) interposed therebetween .

The electrode assembly was then inserted into a nickel-plated iron cylindrical battery case (inner diameter: 18 mm). It should be noted that the upper and lower insulator plates are located on the top and bottom of the electrode assembly, respectively, and since they are very thin, their thicknesses are negligible. Thereafter, a groove portion is provided in the upper half of the side wall of the battery case where the inner diameter of the battery case is reduced. The longitudinal section of the groove portion was U-shaped, and the depth of the diameter-reduced groove portion was 1.5 μm. The distance A from the inner bottom surface of the battery case to the deepest part of the groove was 60.5 mm.

Next, 5.5 g of electrolytic solution was injected into the hollow portion of the electrode assembly, thereby immersing the electrode assembly in the electrolytic solution. Thereafter, the sealing plate was mounted on the groove portion of the battery case, and the opening side of the battery case was crimped on the circumference of the sealing plate. The manufactured cylindrical lithium secondary battery had an inner diameter of 18 mm, a height of 65.0 mm, and a design capacity of 2200 mAh. The ratio of B/A (the ratio of the width B (57.5 mm) of the negative electrode to the distance A (60.5 mm)) was 0.950.

(Battery 2~5)

Cylindrical lithium secondary batteries 2 to 5 were produced by the same manufacturing method as battery 1, except that the width B of the negative electrode was changed to 58.5mm, 59.2mm, 60.2mm, and 61.2mm respectively, and the width of the positive electrode was changed to 57.5mm respectively , 58.2mm, 59.2mm, and 60.2mm, the design capacity becomes 2239mAh, 2266mAh, 2305mAh, and 2244mAh respectively. The ratios of B/A in the various batteries were 0.967 (Battery 2), 0.979 (Battery 3), 0.995 (Battery 4), and 1.012 (Battery 5).

(Battery 6)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as Battery 3, except that the porous heat-resistant layer was formed on the surface of the positive electrode active material layer instead of the surface of the negative electrode active material layer.

(Battery 7)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as battery 4, except that the porous heat-resistant layer was formed on the surface of the positive electrode active material layer instead of the surface of the negative electrode active material layer.

(Battery 8)

A cylindrical lithium secondary battery was fabricated by the same fabrication method as Battery 3, except that the thickness of the porous heat-resistant layer was changed to 15 μm and the electrode assembly was not fabricated using a separator.

(Battery 9)

A cylindrical lithium secondary battery was produced by the same manufacturing method as Battery 3, except that the aluminum oxide in the porous heat-resistant layer was changed to magnesium oxide having the same median particle size.

(Battery 10)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as battery 3, except that the alumina in the porous heat-resistant layer was changed to silica having the same median particle size.

(Battery 11)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as Battery 3, except that the alumina in the porous heat-resistant layer was changed to titania having the same median particle size.

(Battery 12)

A cylindrical lithium secondary battery was produced by the same manufacturing method as battery 3, except that the alumina in the porous heat-resistant layer was changed to zirconia having the same median particle size.

(Battery 13)

The formation process of the porous heat-resistant layer is as follows.

65 g of dry anhydrous calcium chloride was added to 1 kg of NMP, and the mixture was heated to 80° C. in the reactor to completely dissolve it. The obtained NMP solution of calcium chloride was cooled to room temperature, and then 32 g of p-phenylenediamine was added thereto and completely dissolved. Thereafter, the reactor was placed in a constant temperature room at 20° C., and 58 g of dichloroterephthalic acid was gradually added to the NMP solution within 1 hour. The NMP solution was left in a thermostatic chamber at 20° C. for 1 hour to allow polymerization to proceed, thereby synthesizing poly-p-phenylene terephthalamide (hereinafter referred to as PPTA).

After the reaction was completed, the NMP solution (polymerization solution) was transferred from the thermostatic chamber to a vacuum chamber and stirred under reduced pressure for 30 minutes to degas. The obtained polymerization solution was diluted with an NMP solution of calcium chloride to prepare an NMP solution of an aramid resin containing 1.4 wt% of PPTA.

The resulting NMP solution of aramid resin was applied on one surface of the separator with a doctor blade and dried with hot air at 80°C (flow rate: 0.5 m/sec). The resulting aramid resin film was sufficiently washed with pure water to remove calcium chloride and form microvoids in the film. The film was then dried to form a 5 µm thick porous heat-resistant layer on one surface of the separator. The porosity of the porous heat-resistant layer was 48%. The electrode assembly is fabricated so that the porous heat-resistant layer is in contact with the positive electrode. A porous heat-resistant layer was not formed on the negative electrode active material layer. A cylindrical lithium secondary battery was produced by the same manufacturing method as Battery 3 except for the above differences.

(Battery 14)

The formation process of the porous heat-resistant layer is as follows.

The trichloromellitic anhydride of 21g and the diaminodiphenyl ether of 20g are added in the NMP of 1kg, and they are mixed together at room temperature, prepare the NMP solution of polyamic acid (polyamic acid content: 3.9wt% ). The resulting polyamic acid NMP solution was applied to one surface of the separator with a spatula. The resulting coating film was dried with hot air at 80° C. (flow rate: 0.5 m/sec) to dehydrate and ring the polyamic acid, thereby forming polyamideimide. By this method, a 5 μm thick porous heat-resistant layer was formed on one surface of the separator. The porosity of the porous heat-resistant layer was 47%. The pole assembly is fabricated such that the porous heat-resistant layer is in contact with the positive electrode. A porous heat-resistant layer was not formed on the negative electrode active material layer. A cylindrical lithium secondary battery was produced in the same manufacturing method as Battery 3 except for these differences.

(Battery 15)

The NMP solution of the aramid resin prepared in the same manner as in Battery 13 was coated on a smooth stainless steel (SUS) plate with a doctor blade, and the resulting coating film was dried at 120°C for 10 hours under reduced pressure. The coating film was then separated from the SUS plate to obtain a 15 μm thick porous heat-resistant layer in the form of an independent sheet. The porosity of the porous heat-resistant layer was 51%. An electrode assembly is manufactured by winding a positive electrode and a negative electrode with the sheet-like porous heat-resistant layer interposed therebetween, but excluding the separator. A porous heat-resistant layer was not formed on the negative electrode active material layer. A cylindrical lithium secondary battery was produced by the same manufacturing method as Battery 3 except for the above differences.

(Battery 16)

The NMP solution of polyamic acid prepared in the same manner as in Battery 14 was applied to a smooth stainless steel (SUS) plate with a spatula. The resulting coating was dried with hot air at 80° C. (flow rate: 0.5 m/sec) to dehydrate the polyamic acid to form a ring. The coating film was then separated from the SUS plate to obtain a 15 μm thick porous heat-resistant layer in the form of an independent sheet. The porosity of the porous heat-resistant layer was 52%. An electrode assembly is manufactured by winding a positive electrode and a negative electrode with the sheet-like porous heat-resistant layer interposed therebetween, but excluding the separator. A porous heat-resistant layer was not formed on the negative electrode active material layer. A cylindrical lithium secondary battery was produced by the same manufacturing method as Battery 3 except for the above differences.

(Battery 17)

The formation process of the porous heat-resistant layer is as follows.

A double-arm kneader was used to stir 995 g of alumina (median particle size: 0.3 μm), 62.5 g of Zeon BM-720H, and an appropriate amount of NMP to prepare a raw material slurry. The resulting raw material slurry was coated on the surface of the negative electrode active material layer, and dried with hot air at 130° C. (flow rate: 1.5 m/min) for 4 minutes, thereby forming a 5 μm thick porous heat-resistant layer. The porosity of each porous heat-resistant layer was 61%. A cylindrical lithium secondary battery was produced in the same manufacturing method as Battery 3 except for these differences.

(Battery 18)

The formation process of the porous heat-resistant layer is as follows.

A double-arm kneader was used to stir 990 g of alumina (median particle size: 0.3 μm), 125 g of Zeon BM-720H, and an appropriate amount of NMP to prepare a raw material slurry. The resulting raw material slurry was coated on the surface of the negative electrode active material layer, and dried with hot air at 130° C. (flow rate: 1.5 m/min) for 4 minutes, thereby forming a 5 μm thick porous heat-resistant layer. The porosity of each porous heat-resistant layer was 57%. A cylindrical lithium secondary battery was produced in the same manufacturing method as Battery 3 except for these differences.

(Battery 19)

The formation process of the porous heat-resistant layer is as follows. A double-arm kneader was used to stir 900 g of alumina (median particle size: 0.3 μm), 1250 g of Zeon BM-720H, and an appropriate amount of NMP to prepare a raw material slurry. The resulting raw material slurry was coated on the surface of the negative electrode active material layer, and dried with hot air at 130° C. (flow rate: 1.5 m/min) for 4 minutes, thereby forming a 5 μm thick porous heat-resistant layer. The porosity of each porous heat-resistant layer was 42%. A cylindrical lithium secondary battery was produced in the same manufacturing method as Battery 3 except for these differences.

(Battery 20)

The formation process of the porous heat-resistant layer is as follows.

A double-arm kneader was used to stir 800 g of alumina (median particle size: 0.3 μm), 2500 g of Zeon BM-720H, and an appropriate amount of NMP to prepare a raw material slurry. The resulting raw material slurry was coated on the surface of the negative electrode active material layer, and dried with hot air at 130° C. (flow rate: 1.5 m/min) for 4 minutes, thereby forming a 5 μm thick porous heat-resistant layer. The porosity of each porous heat-resistant layer was 35%. A cylindrical lithium secondary battery was produced in the same manufacturing method as Battery 3 except for these differences.

(Battery 21～25)

Cylindrical lithium secondary batteries 21 to 25 were produced in the same manufacturing method as battery 3, except that the flow rate of the hot air used to dry the coated raw material slurry to form a porous heat-resistant layer was changed to 0.5m respectively. /min, 1m/min, 2m/min, 5m/min, and 8m/min. The porosity of the porous heat-resistant layer of each battery was 30% (Battery 21), 42% (Battery 22), 60% (Battery 23), 78% (Battery 24), and 89% (Battery 25).

(Battery 26)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as Battery 1 except that the thickness of the separator was changed to 15 μm and no porous heat-resistant layer was formed.

(Battery 27)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as that of Battery 2, except that the thickness of the separator was changed to 15 μm and no porous heat-resistant layer was formed.

(Battery 28)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as Battery 3, except that the thickness of the separator was changed to 15 μm and no porous heat-resistant layer was formed.

(Battery 29)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as Battery 4, except that the thickness of the separator was changed to 15 μm and no porous heat-resistant layer was formed.

(Battery 30)

A cylindrical lithium secondary battery was fabricated by the same manufacturing method as Battery 5, except that the thickness of the separator was changed to 15 μm and no porous heat-resistant layer was formed.

Each battery was precharged and discharged twice, and stored at 45°C for 7 days. Afterwards, they are evaluated as follows. Table 1, Table 2, and Table 3 summarize the characteristics of the porous heat-resistant layer, battery design, and evaluation results, respectively.

(internal short circuit check)

100 samples of each battery are manufactured. These sample batteries were discharged under the following conditions in an environment of 20° C., and their open circuit voltages were measured. Thereafter, these batteries were preserved for 10 days in an environment of 45° C. and their open circuit voltages were measured again. When the difference between the open circuit voltage of the battery before and after storage at 45°C is 0.3V or more, it can be determined that this type of battery is in an internal short circuit state. The occurrence rate of internal short circuit is shown in Table 3.

Constant current charging: charging current 1500mA/charging termination voltage 4.2V;

Constant voltage charging: charging voltage 4.2V/charging termination current 100mA.

(drop test)

The batteries subjected to the internal short-circuit inspection were charged and discharged under the following conditions in an environment of 20° C., and their discharge capacities were obtained.

Constant current charging: charging current 1500mA/charging termination voltage 4.2V;

Constant voltage charging: charging voltage 4.2V/charging termination current 100mA;

Constant current discharge: discharge current 2200mA/discharge termination voltage 3V.

Thereafter, these batteries were repeatedly dropped from a height of 16 cm 30 times in an environment of 20°C, and then charged and discharged under the above-mentioned conditions, and their discharge capacities were obtained. The percentage of the discharge capacity after the drop test relative to the discharge capacity before the drop test was thus obtained. The results of drop impact resistance are shown in Table 3.

(Internal short circuit inspection after drop test)

After the drop test, use the same method as before the drop test to check the internal short circuit of the battery. Table 3 shows the results of the occurrence rate of internal short circuits after dropping.

(high output characteristics)

Various batteries were charged and discharged under the following conditions in an environment of 20° C., and their discharge capacities were obtained.

Constant current charging: charging current 1500mA/charging termination voltage 4.2V;

Constant voltage charging: charging voltage 4.2V/charging termination current 100mA;

Constant current discharge: discharge current 440mA/discharge termination voltage 3V;

Constant current charging: charging current 1500mA/charging termination voltage 4.2V;

Constant voltage charging: charging voltage 4.2V/charging termination current 100mA;

Constant current discharge: discharge current 4400mA/discharge termination voltage 3V.

The percentage of the battery's discharge capacity at 4400 mA relative to its discharge capacity at 440 mA can then be obtained. Table 3 shows the high output characteristic results.

(nail puncture test)

Various batteries were charged to a cut-off voltage of 4.35V at a charging current of 2200mA. In an environment of 20°C, an iron nail (2.7 mm in diameter) was driven into the side walls of various charged batteries at a speed of 5 mm/sec, and the temperature of the batteries was measured with a thermocouple attached to the side walls of the batteries. The battery temperature after 90 seconds is shown in Table 3.

Table 1

Battery Porous heat-resistant layer (μm) The location of the porous heat-resistant layer Diaphragm (μm) filler Binder content (wt%) Porosity(%) 1 5 negative electrode 10 Aluminum oxide 3 50 2 5 negative electrode 10 Aluminum oxide 3 50 3 5 negative electrode 10 Aluminum oxide 3 50 4 5 negative electrode 10 Aluminum oxide 3 50 5 5 negative electrode 10 Aluminum oxide 3 50 6 5 positive electrode 10 Aluminum oxide 3 50 7 5 positive electrode 10 Aluminum oxide 3 50 8 15 negative electrode none Aluminum oxide 3 50 9 5 negative electrode 10 magnesium oxide 3 50 10 5 negative electrode 10 silica 3 50 11 5 negative electrode 10 Titanium dioxide 3 50 12 5 negative electrode 10 Zirconia 3 50 13 5 diaphragm 10 Aramid 48 14 5 diaphragm 10 polyamideimide 47 15 15 individual flakes none Aramid 51 16 15 individual flakes none polyamideimide 52 17 5 negative electrode 10 Aluminum oxide 0.5 61 18 5 negative electrode 10 Aluminum oxide 1 57 19 5 negative electrode 10 Aluminum oxide 10 42 20 5 negative electrode 10 Aluminum oxide 20 35 twenty one 5 negative electrode 10 Aluminum oxide 3 30 twenty two 5 negative electrode 10 Aluminum oxide 3 42 twenty three 5 negative electrode 10 Aluminum oxide 3 60 twenty four 5 negative electrode 10 Aluminum oxide 3 78 25 5 negative electrode 10 Aluminum oxide 3 89 26 none 15 27 none 15 28 none 15 29 none 15 30 none 15

Table 2

Battery Negative electrode width B(mm) Positive electrode width (mm) Design capacity (mAh) B/A 1 57.5 56.5 2200 0.950 2 58.5 57.5 2239 0.967 3 59.2 58.2 2266 0.979 4 60.2 59.2 2305 0.995 5 61.2 60.2 2344 1.012 6 59.2 58.2 2266 0.979 7 60.2 59.2 2305 0.995 8 59.2 58.2 2266 0.979 9 59.2 58.2 2266 0.979 10 59.2 58.2 2266 0.979 11 59.2 58.2 2266 0.979 12 59.2 58.2 2266 0.979 13 59.2 58.2 2266 0.979 14 59.2 58.2 2266 0.979 15 59.2 58.2 2266 0.979 16 59.2 58.2 2266 0.979 17 59.2 58.2 2266 0.979 18 59.2 58.2 2266 0.979 19 59.2 58.2 2266 0.979 20 59.2 58.2 2266 0.979 twenty one 59.2 58.2 2266 0.979 twenty two 59.2 58.2 2266 0.979 twenty three 59.2 58.2 2266 0.979 twenty four 59.2 58.2 2266 0.979 25 59.2 58.2 2266 0.979 26 57.5 56.5 2200 0.950 27 58.5 57.5 2239 0.967 28 59.2 58.2 2266 0.979 29 60.2 59.2 2305 0.995 30 61.2 60.2 2344 1.012

table 3

Battery Short circuit occurrence rate (%) Drop impact resistance (%) Occurrence rate of short circuit after falling (%) High output characteristics (%) Battery temperature after nailing (°C) 1 0 93.0 2 90.3 86 2 0 99.8 1 91.4 85 3 0 99.9 0 90.6 84 4 0 99.7 0 92.3 86 5 twenty four 99.8 2 91.9 83 6 2 100.0 0 90.2 83 7 14 99.5 9 88.7 90 8 3 99.7 19 86.6 83 9 0 99.7 0 88.9 84 10 0 99.8 0 88.7 86 11 0 99.9 2 88.6 81 12 0 99.9 0 88.9 86 13 0 99.7 8 89.2 86 14 0 99.6 9 89.5 82 15 3 99.5 16 90.1. 91 16 1 99.8 14 90.6 92 17 0 99.8 11 90.1 94 18 0 99.8 0 88.9 89 19 0 99.8 2 83.8 80 20 0 99.9 0 79.5 80 twenty one 0 95.7 0 82.4 83 twenty two 0 98.6 0 87.9 84 twenty three 0 99.8 0 89.0 88 twenty four 0 99.7 6 90.5 85 25 0 100.0 10 93.4 90 26 0 99.7 0 88.1 128 27 15 99.9 0 88.2 124 28 twenty two 99.8 0 87.9 126 29 30 99.8 0 89.1 130 30 46 99.8 0 88.8 124

In the battery 1 in which the width B of the negative electrode is too small relative to the distance A from the groove portion (restricted portion) to the inner bottom surface of the battery case, the capacity density is low, and the drop impact resistance is low. After the drop test, the battery 1 was disassembled for observation, and it was found that the electrodes of the wound electrode assembly moved.

The battery 1 did not suffer from internal short circuit due to the effect of the porous heat-resistant layer, but the reduction in the effective area (the area where the positive and negative electrodes face each other) resulted in loss of its capacity. With a porous heat-resistant layer, its electrode assembly can resist deformation, so it cannot be firmly fixed on the inside of the battery case. The electrodes of the wound electrode assembly may thus move when dropped repeatedly.

On the other hand, in the battery 5 in which the negative electrode width B is too large relative to the distance A from the groove portion to the inner bottom surface of the battery case, the short-circuit resistance is low. A sample of battery 5 determined to be in an internal short-circuit state was disassembled for observation. It was found that the porous heat-resistant layer on the surface of the negative electrode was damaged in the upper half of the electrode assembly. It was also found that the septum cracked.

Batteries 2 to 4 having a B/A ratio of 0.965 to 0.995 had high short-circuit resistance and improved drop impact resistance. The battery of the present invention is equipped with a porous heat-resistant layer in addition to the separator. Therefore, even when the negative electrode is wider than the positive electrode and the upper half of the electrode assembly is slightly deformed, the double-layer structure consisting of the porous heat-resistant layer and the separator insulates the deformed part. In addition, due to the high ratio of B/A, the electrode assembly is firmly sandwiched between the groove portion and the inner bottom surface of the battery case. The battery's drop impact resistance may thus be improved.

In the batteries 26 to 30 having no porous heat-resistant layer, the drop impact resistance was excellent regardless of the position of the restricting portion. It is believed that the electrode assembly without the porous heat-resistant layer deforms to an appropriate degree so that it is firmly held within the battery case. Even when these batteries are dropped, electrode movement of the wound electrode assembly, which causes capacity loss, may thus be suppressed. However, these batteries 26-30 significantly overheated in the nail puncture test. In addition, in the batteries 27 to 29 having the same restriction positions as the batteries 2 to 4, the short-circuit resistance is low. It is believed that the negative electrodes of these batteries 27-29 were slightly deformed. However, since these batteries do not have a porous heat-resistant layer, internal short circuits cannot be prevented when the separator cracks due to deformation of the negative electrode.

In Batteries 6 and 7, a porous heat-resistant layer was formed on the surface of the positive electrode active material layer. Of these two batteries, battery 7 with a wider negative electrode showed relatively low short-circuit resistance. This may be because the porous heat-resistant layer is formed on the surface of the cathode active material layer which is narrower than the anode active material layer, so that the anode surface is in contact with the upper edge of the cathode at the upper half of the electrode assembly.

In battery 8 without a separator, its short-circuit resistance after being dropped was slightly lower. Its porous heat-resistant layer is structurally more fragile than the diaphragm. Then, the porous heat-resistant layer is partially damaged by the impact of the drop, so that a short circuit occurs.

In Batteries 13 and 14 having a porous heat-resistant layer made of a heat-resistant resin on the surface of the separator, the short-circuit resistance after dropping was slightly lower. The mechanical strength of a porous heat-resistant layer made of a heat-resistant resin is lower than that of a porous heat-resistant layer comprising an insulating filler and a binder. A short circuit may thus occur due to the impact of the drop.

In Batteries 15 and 16 having separate porous heat-resistant layer sheets and no separator, their short-circuit resistance after dropping was lower than that of Batteries 13 and 14 after being dropped. This stems from the fact that the porous heat-resistant layer made of heat-resistant resin is poor in strength and the strength of the porous heat-resistant layer is not improved by the absence of the separator.

In Battery 17 containing 0.5% by weight of the binder in the porous heat-resistant layer, the short-circuit resistance after dropping was slightly lower. This may be because the low content of binder weakens the adhesion of filler particles, resulting in poor mechanical strength of the porous heat-resistant layer.

On the other hand, in the battery 20 containing 20 wt% of the binder, the high output characteristics were low. This may be because the excess binder reduces the porosity of the porous heat-resistant layer, and in addition, the excess binder swells with the electrolyte, which closes the micropores of the porous heat-resistant layer and reduces the ionic conductivity. On the other hand, in Batteries 18 to 19 having a binder content of 1 wt % to 10 wt %, both the short circuit resistance and high output characteristics were excellent.

In the battery 21 with a porous heat-resistant layer having a porosity of 30% obtained by controlling the drying conditions, the drop impact resistance was slightly lower. The reason for this may be as follows. Due to the lower porosity, the porous heat-resistant layer does not fully infiltrate the electrolyte, so the swelling of the electrode assembly is less. Therefore, the electrode assembly cannot be prevented from moving when dropped. In Battery 25 having a porous heat-resistant layer with a porosity of 89%, the short-circuit resistance after being dropped was slightly low. This may be due to the poor mechanical strength of the porous heat-resistant layer.

On the other hand, in the batteries 22 to 24 with porous heat-resistant layers having a porosity of 40% to 80%, both drop impact resistance and short circuit resistance after drop were excellent. The reason for this may be as follows. Due to the optimized porosity, the mechanical strength of the porous heat-resistant layer is maintained. In addition, the porous heat-resistant layer swells to a suitable degree with the electrolyte. As a result, movement of the electrode assembly is prevented.

Example 2

In this embodiment, the prismatic lithium secondary battery shown in FIG. 2 is explained.

(battery 31)

An electrode assembly was produced in the same manufacturing method as in Example 1 except for the following differences. The total thickness of the positive electrode becomes 150 μm, and the width of the positive electrode becomes 42.7 mm. The total thickness of the negative electrode became 150 μm, and the width of the negative electrode became 43.7 mm. The width of the diaphragm becomes 47 mm. The shape of the electrode assembly becomes an elliptical cylinder.

The resulting electrode assembly was inserted into an aluminum prismatic battery case of 49 mm high (bottom thickness: 0.5 mm), 34 mm wide, and 5.2 mm thick. After installing a 1.5 mm thick insulator on top of the electrode assembly, 2.5 g of the same electrolyte as in Example 1 was injected into the battery case. The distance A from the inner bottom surface of the battery case to the lower surface of the insulator was 46.0 mm. It should be noted that the lower half of the electrode assembly is insulated from the battery case by an insulating sheet, but since the insulating sheet is very thin, its thickness is negligible.

Thereafter, a 1.0 mm thick rectangular sealing plate was mounted on the top opening of the battery case, and the opening side of the battery case and the periphery of the sealing plate were welded together by laser. The manufactured prismatic lithium secondary battery has a height of 50 mm, a width of 34 mm, a thickness of 5.2 mm, and a design capacity of 950 mAh. The ratio of B/A (the ratio of the negative electrode width B (43.7 mm) to the distance A (46.0 mm)) was 0.95.

(Battery 32～35)

The prismatic lithium secondary batteries 32-35 were manufactured by the same manufacturing method as the battery 31, except that the width B of the negative electrode was changed to 44.6mm, 45mm, 45.7mm, and 46.5mm respectively, and the width of the positive electrode was changed to 43.6mm, 44mm, 44.7mm, and 45.5mm, the design capacity becomes 970mAh, 979mAh, 994mAh, and 1012mAh respectively. The ratios of B/A in the various batteries were 0.970 (Battery 32), 0.978 (Battery 33), 0.993 (Battery 34), and 1.011 (Battery 35).

(batteries 36 and 37)

The prismatic lithium secondary batteries 36 and 37 were made in the same manufacturing method as the batteries 33 and 34 respectively, except that the porous heat-resistant layer was formed on the surface of the positive electrode active material layer and not on the surface of the negative electrode active material layer .

(Battery 38)

A prismatic lithium secondary battery was fabricated by the same fabrication method as battery 33, except that the thickness of the porous heat-resistant layer was changed to 15 μm and the electrode assembly was not fabricated using a separator.

(Battery 39～42)

The prismatic lithium secondary batteries 39-42 were produced by the same manufacturing method as the battery 33, except that the aluminum oxide in the porous heat-resistant layer was changed to magnesium oxide, silicon dioxide, and titanium dioxide with the same median particle size, respectively. , and zirconia. (Battery 43～50)

Prismatic lithium secondary batteries 43-50 were manufactured by the same manufacturing method as battery 33, except that the same porous heat-resistant layers as batteries 13-20 in Example 1 were used respectively.

(Battery 51～55)

The prismatic lithium secondary batteries 51 to 55 were produced in the same manufacturing method as the battery 33, except that the flow rate of the hot air used to dry the coated raw material slurry to form the porous heat-resistant layer was changed to 0.5 m/ min, 1m/min, 2m/min, 5m/min, and 8m/min. The porosity of the porous heat-resistant layer of each battery was 30% (Battery 51), 42% (Battery 52), 60% (Battery 53), 78% (Battery 54), or 89% (Battery 55).

(Battery 56~60)

Prismatic lithium secondary batteries 56 to 60 were produced in the same manufacturing method as batteries 31 to 35, respectively, except that the thickness of the separator was changed to 15 μm and the porous heat-resistant layer was not provided.

Each battery was precharged and discharged twice, and then stored at 45°C for 7 days. Afterwards, they are evaluated as follows. Table 4, Table 5, and Table 6 summarize the characteristics of the porous heat-resistant layer, battery design, and evaluation results, respectively.

(internal short circuit check)

These batteries were checked for internal short circuit in the same manner as in Example 1, except that they were charged under the following conditions. The results are shown in Table 6.

Constant current charging: charging current 665mA/charging termination voltage 4.2V;

Constant voltage charging: charging voltage 4.2V/charging termination current 100mA.

(drop test)

The drop impact resistance of these batteries was evaluated in the same manner as in Example 1, except that they were charged and discharged under the following conditions. The results are shown in Table 6.

Constant current charging: charging current 665mA/charging termination voltage 4.2V;

Constant voltage charging: charging voltage 4.2V/charging termination current 100mA;

Constant current discharge: discharge current 950mA/discharge termination voltage 3V.

(Inspection of internal short circuit after drop test) After the drop test, these batteries were inspected for internal short circuit in the same manner as before the drop test. Table 6 shows the results of the occurrence rate of short circuits after the drop.

(high output characteristics)

In an environment of 20°C, various batteries were discharged and charged under the following conditions, and their discharge capacities were obtained.

Constant current charging: charging current 665mA/charging termination voltage 4.2V;

Constant voltage charging: charging voltage 4.2V/charging termination current 100mA;

Constant current discharge: discharge current 190mA/discharge termination voltage 3V;

Constant current charging: charging current 665mA/charging termination voltage 4.2V;

Constant voltage charging: charging voltage 4.2V/charging termination current 100mA;

Constant current discharge: discharge current 1900mA/discharge termination voltage 3V.

The percentage of the battery's discharge capacity at 1900 mA relative to its discharge capacity at 190 mA can then be obtained. See Table 6 for the high output characteristic results.

(nail puncture test)

Various cells were charged to a cut-off voltage of 4.35V at a charge current of 950mA. A nail was driven into the battery by the same method as in Example 1, and the temperature of the battery after 90 seconds was evaluated. The results are shown in Table 6.

Table 4

Battery Porous heat-resistant layer (μm) The location of the porous heat-resistant layer Diaphragm (μm) filler Binder content (wt%) Porosity(%) 31 5 negative electrode 10 Aluminum oxide 3 50 32 5 negative electrode 10 Aluminum oxide 3 50 33 5 negative electrode 10 Aluminum oxide 3 50 34 5 negative electrode 10 Aluminum oxide 3 50 35 5 negative electrode 10 Aluminum oxide 3 50 36 5 positive electrode 10 Aluminum oxide 3 50 37 5 positive electrode 10 Aluminum oxide 3 50 38 15 negative electrode none Aluminum oxide 3 50 39 5 negative electrode 10 magnesium oxide 3 50 40 5 negative electrode 10 silica 3 50 41 5 negative electrode 10 Titanium dioxide 3 50 42 5 negative electrode 10 Zirconia 3 50 43 5 diaphragm 10 Aramid 48 44 5 diaphragm 10 polyamideimide 47 45 15 individual slices none Aramid 51 46 15 individual slices none polyamideimide 52 47 5 negative electrode 10 Aluminum oxide 0.5 61 48 5 negative electrode 10 Aluminum oxide 1 57 49 5 negative electrode 10 Aluminum oxide 10 42 50 5 negative electrode 10 Aluminum oxide 20 35 51 5 negative electrode 10 Aluminum oxide 3 30 52 5 negative electrode 10 Aluminum oxide 3 42 53 5 negative electrode 10 Aluminum oxide 3 60 54 5 negative electrode 10 Aluminum oxide 3 78 55 5 negative electrode 10 Aluminum oxide 3 89 56 none 15 57 none 15 58 none 15 59 none 15 60 none 15

table 5

Battery Negative electrode width B(mm) Positive electrode width (mm) Design capacity (mAh) B/A 31 43.7 42.7 950 0.950 32 44.6 43.6 970 0.970 33 45 44 979 0.978 34 45.7 44.7 994 0.993 35 46.5 45.5 1012 1.011 36 45 44 979 0.978 37 45.7 44.7 994 0.993 38 45 44 979 0.978 39 45 44 979 0.978 40 45 44 979 0.978 41 45 44 979 0.978 42 45 44 979 0.978 43 45 44 979 0.978 44 45 44 979 0.978 45 45 44 979 0.978 46 45 44 979 0.978 47 45 44 979 0.978 48 45 44 979 0.978 49 45 44 979 0.978 50 45 44 979 0.978 51 45 44 979 0.978 52 45 44 979 0.978 53 45 44 979 0.978 54 45 44 979 0.978 55 45 44 979 0.978 56 43.7 42.7 950 0.950 57 44.6 43.6 970 0.970 58 45 44 979 0.978 59 45.7 44.7 994 0.993 60 46.5 45.5 1012 1.011

Table 6

Battery Short circuit occurrence rate (%) Drop impact resistance (%) Occurrence rate of short circuit after drop (%) High output characteristics (%) Battery temperature after nail piercing (°C) 31 0 93.3 1 90.4 83 32 0 98.0 0 91.2 86 33 0 99.9 0 90.1 80 34 0 99.7 0 89.4 82 35 20 98.8 0 88.9 83 36 4 99.8 3 90.2 86 37 11 99.7 12 91.3 86 38 2 99.8 13 87.8 93 39 0 99.7 0 89.1 86 40 0 99.9 0 90.1 86 41 0 99.8 0 88.9 82 42 0 99.8 0 90.4 84 43 0 99.6 10 88.8 82 44 0 99.7 11 90.1 84 45 0 99.8 12 91.2 85 46 0 99.9 11 90.7 82 47 0 99.9 8 90.4 87 48 0 99.8 0 91.4 82 49 0 99.7 0 85.3 81 50 0 99.9 0 80.3 78 51 0 96.0 0 87.7 83 52 0 99.8 0 88.2 88 53 0 100.0 0 89.4 81 54 0 99.7 0 90.0 85 55 2 99.8 14 92.1 94 56 0 99.7 0 88.4 126 57 15 99.9 0 88.9 123 58 20 99.8 0 90.2 131 59 28 99.8 0 89.1 124 60 48 99.8 5 90.1 130

In the battery 31 in which the width B of the negative electrode is too small relative to the distance A from the lower surface of the insulator (restriction portion) to the inner bottom surface of the battery case, the capacity density is small, and the drop impact resistance is low. After the drop test, the battery 31 was disassembled for observation, and it was found that the electrodes of the wound electrode assembly moved.

The battery 31 has no internal short circuit due to the effect of the porous heat-resistant layer, but the reduction in the effective area (the area where the positive and negative electrodes face each other) results in loss of its capacity. Due to the porous heat-resistant layer, the electrode assembly is resistant to deformation, so it cannot be firmly fixed on the inside of the battery case. The electrodes of the wound electrode assembly may thus move when dropped repeatedly.

On the other hand, in the battery 35 in which the width B of the negative electrode is too large relative to the distance A from the lower surface of the insulator to the inner bottom surface of the battery case, the short-circuit resistance is low. A sample of the battery 35 determined to be in the internal short-circuit state was disassembled for observation. It was found that the porous heat-resistant layer on the surface of the negative electrode was damaged in the upper half of the electrode assembly. It was also found that the septum cracked.

Batteries 33 to 34 having a B/A ratio of 0.975 to 0.995 had high short-circuit resistance and improved drop impact resistance. The battery of the invention is equipped with a porous heat-resistant layer in addition to the diaphragm. Therefore, even when the negative electrode is wider than the positive electrode and the upper half of the electrode assembly is slightly deformed, the double-layer structure consisting of the porous heat-resistant layer and the separator insulates the deformed part. In addition, due to the high ratio of B/A, the electrode assembly is firmly sandwiched between the surface of the lower half of the insulator and the inner bottom surface of the battery case. The battery's drop impact resistance may thus be improved.

However, in the battery 32 having a B/A ratio of 0.965 to 0.975, the drop impact resistance was slightly lower than that of the cylindrical battery 2 (Example 1) having the same ratio range of B/A. In the case of a cylindrical battery, the cross section of its groove portion (restricted portion) is approximately V-shaped or U-shaped. Therefore, the top of the electrode assembly is pressed by the slope of the groove portion. On the other hand, in the case of a prismatic lithium secondary battery, the surface of the lower half of the insulator (restriction portion) is flat, so there is no slope like the groove portion. Various batteries may therefore have the above-mentioned differences in the range of more effective B/A ratios.

In the batteries 56 to 60 having no porous heat-resistant layer, the drop impact resistance was excellent regardless of the position of the restricting portion. It is believed that since the electrode assembly without the porous heat-resistant layer is deformed to an appropriate degree, it is held firmly within the battery case. Even when these batteries are dropped, electrode movement of the wound electrode assembly, which causes capacity loss, may thus be suppressed. However, these batteries 56-60 significantly overheated in the nail puncture test. In addition, in the batteries 57 to 59 having the same positions of the restriction parts as the batteries 32 to 34, the short-circuit resistance is low. It is believed that the negative electrodes of these cells 57-59 were slightly deformed. However, since these batteries do not have a porous heat-resistant layer, internal short circuits cannot be prevented when the separator cracks due to deformation of the negative electrode.

In batteries 36 and 37, a porous heat-resistant layer was formed on the surface of the positive electrode active material layer. Among the two batteries, the battery 37 with a wider negative electrode showed relatively lower short-circuit resistance. This may be because the porous heat-resistant layer is formed on the surface of the positive electrode active material layer which is narrower than the negative electrode active material layer, so that the negative electrode surface is in contact with the upper half edge of the positive electrode at the upper half of the electrode assembly.

In the battery 38 without a separator, the short-circuit resistance after dropping was slightly lower. The porous heat-resistant layer is structurally more fragile than the diaphragm. Then, the porous heat-resistant layer is partially damaged by the impact of the drop, so that a short circuit occurs.

In batteries 43 and 44 having a porous heat-resistant layer made of a heat-resistant resin on the surface of the separator, the short-circuit resistance after dropping was slightly lower. The mechanical strength of a porous heat-resistant layer made of a heat-resistant resin is lower than that of a porous heat-resistant layer comprising an insulating filler and a binder. Then, a short circuit occurs by the drop impact action.

In Batteries 45 and 46 having separate porous heat-resistant layer sheets and no separator, their short-circuit resistance after being dropped was lower than that of Batteries 43 and 44 after being dropped. This stems from the fact that the porous heat-resistant layer made of heat-resistant resin is poor in strength and the strength of the porous heat-resistant layer is not improved by the absence of the separator.

In Battery 47 containing 0.5% by weight of the binder in the porous heat-resistant layer, the short-circuit resistance after dropping was slightly lower. This may be because the low content of binder weakens the adhesion of filler particles, resulting in poor mechanical strength of the porous heat-resistant layer.

On the other hand, in the battery 50 containing 20 wt% of the binder, the high output characteristics were slightly lower. This may be because the excess binder reduces the porosity of the porous heat-resistant layer, and in addition, the excess binder swells with the electrolyte, which closes the micropores of the porous heat-resistant layer and reduces the ionic conductivity. On the other hand, in Batteries 48 to 49 having a binder content of 1 wt % to 10 wt %, both the short circuit resistance and high output characteristics were excellent.

In the battery 51 with a porous heat-resistant layer having a porosity of 30% obtained by controlling the drying conditions, the drop impact resistance was slightly lower. The reason for this may be as follows. Due to the low porosity, the porous heat-resistant layer does not fully infiltrate the electrolyte, so the swelling of the electrode assembly is small. Therefore, the electrode assembly cannot be prevented from moving when dropped. In the battery 55 having a porous heat-resistant layer with a porosity of 89%, the short-circuit resistance after being dropped was slightly lower. This may be due to the poor mechanical strength of the porous heat-resistant layer.

On the other hand, in the batteries 52 to 54 with porous heat-resistant layers having a porosity of 40% to 80%, both drop impact resistance and short circuit resistance after drop were excellent. The reason for this may be as follows. Due to the optimized porosity, the mechanical strength of the porous heat-resistant layer is maintained. In addition, the porous heat-resistant layer swells to a suitable degree with the electrolyte. Movement of the electrode assembly is thus prevented.

Industrial Applicability

Since the lithium secondary battery of the present invention has excellent short-circuit resistance and heat resistance and high safety and does not have capacity loss due to impact such as dropping, it can be used as a power source of any portable device such as a personal digital assistant and a portable battery. Electronic equipment. The lithium secondary battery of the present invention can be used, for example, as a power source for small electric storage devices for household use, two-wheeled vehicles, electric vehicles, and dual-purpose electric vehicles, and the use of the present invention is not particularly limited.

Claims (13)

1. A lithium secondary battery, comprising: a battery case having a bottom, a side wall and an opening at the top; an electrode assembly; a non-aqueous electrolyte; and covering the battery case for containing the electrode assembly and the electrolyte The top opening of the sealing plate, It is characterized in that the electrode assembly includes a strip-shaped positive electrode and a strip-shaped negative electrode intertwined with a porous heat-resistant layer inserted between the positive electrode and the negative electrode, and the positive electrode includes a positive electrode core part and a coating coated on both sides of the positive electrode core part. a positive electrode active material layer, the negative electrode comprising a negative electrode core part and a negative electrode active material layer coated on both sides of the negative electrode core part, The battery has a restricting part that restricts the vertical movement of the electrode assembly, and the distance A from the restricting part to the inner bottom surface of the battery case and the width B of the negative electrode satisfy the relationship: 0.965≤B/A≤0.995 , The porous heat-resistant layer contains insulating fillers and binders, and The porous heat-resistant layer has a porosity of 40% to 80% and a thickness of 0.5 to 20 μm. 2. The lithium secondary battery according to claim 1, further comprising a separator comprising a microporous membrane, the separator being inserted between the porous heat-resistant layer and the positive electrode or between the porous heat-resistant layer and the negative electrode. 3. The lithium secondary battery according to claim 1, wherein the porous heat-resistant layer is formed on a surface of at least one of the positive electrode active material layer and the negative electrode active material layer. 4. The lithium secondary battery according to claim 1, wherein the insulating filler comprises an inorganic oxide. 5. The lithium secondary battery according to claim 4, wherein the inorganic oxide comprises at least one selected from alumina, silica, magnesia, titania, and zirconia. 6 . The lithium secondary battery according to claim 1 , wherein, based on 100 parts by weight of the insulating filler, the amount of the binder is 1-10 parts by weight. 7. The lithium secondary battery according to claim 1, wherein the electrode assembly is cylindrical, the battery case is cylindrical, and the restricting part is located on the upper part of the side wall of the battery case to reduce the inner diameter. The groove portion of the battery case is small. 8. The lithium secondary battery as claimed in claim 1, further comprising an insulator between the electrode assembly and the sealing plate, wherein the electrode assembly is a cylinder with an elliptical cross-section, and the The battery case has a prism shape, and the limiting part is the lower surface of the insulator. 9. The lithium secondary battery according to claim 7, characterized in that, the distance A from the restricting part to the inner surface of the bottom of the battery case and the width B of the negative electrode satisfy the relationship: 0.970≤ B/A≤0.990. 10. The lithium secondary battery according to claim 8, characterized in that, the distance A from the restricting part to the inner surface of the bottom of the battery case and the width B of the negative electrode satisfy the relationship: 0.975≤ B/A≤0.990. 11. The lithium secondary battery according to claim 8, characterized in that, the distance A from the restricting part to the inner surface of the bottom of the battery case and the width B of the negative electrode satisfy the relationship: 0.975≤ B/A≤0.995. 12 . The lithium secondary battery according to claim 1 , wherein, based on 100 parts by weight of the insulating filler, the amount of the binder is 2-8 parts by weight. 13. The lithium secondary battery according to claim 1, wherein the porous heat-resistant layer has a porosity of 40%-65%.

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