High power secondary battery

There is provided a secondary battery in which a gasket is melted by internal heat generation to prevent the secondary battery from being short circuited to each other. In one embodiment, a secondary battery includes an electrode assembly having a positive electrode plate, a negative electrode plate and a separator interposed between the positive and negative electrode plates. A can accommodates the electrode assembly and is connected to the negative electrode plate through negative electrode tabs. A cap assembly is positioned at one side of the can and electrically connected to the positive electrode plate through a positive electrode tab. A gasket is positioned between the can and the cap assembly. In the secondary battery, the gasket is formed of a heat resistant material having a predetermined amount of heat-resistant enhancing material added thereto.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. § 119 from an application earlier filed in the Korean Intellectual Property Office on 29 Oct. 2009 and there duly assigned Ser. No. 10-2009-0103467.

BACKGROUND OF THE INVENTION

Field of the Invention

An aspect of the present invention relates to a secondary battery, and more particularly, to a secondary battery in which a gasket remains durable, i.e., maintains its durability, while being melted by internal heat generation to prevent the secondary battery from being short-circuited.

Description of the Related Art

High power lithium secondary batteries used for electrical power tools require large current discharge because of their environmental characteristics. Particularly, a cylindrical high power secondary battery has an electrode tab with a limited width, therefore, a very large amount of heat is generated under the condition of large current discharge.

For example, instantaneous current or surge current that instantaneously flows in the cylindrical high power secondary battery may reach about 100 A. In this case, temperature increases rapidly at an electrode tab in the secondary battery. Therefore, a gasket interposed between a can and a cap assembly is melted, and thus, the can and the cap assembly (or a cap-up of the cap assembly) are short circuited to each other.

SUMMARY OF THE INVENTION

It is therefore one aspect of the present invention to provide an improved secondary battery in which a gasket remains durable while being melted by internal heat, in order to prevent a can and a cap assembly from being short circuited to each other.

It is another aspect of the present invention to provide an improved secondary battery which can improve durability and safety in the environment of large current discharge such as the environment of electrical power tools.

In accordance with an embodiment of the present invention, there is provided a secondary battery including an electrode assembly having a positive electrode plate, a negative electrode plate and a separator interposed between the positive and negative electrode plates; a can for accommodating the electrode assembly, the can being connected to the negative electrode plate through negative electrode tabs; a cap assembly positioned at one side of the can, the cap assembly being connected to the positive electrode plate through a positive electrode tab; and a gasket positioned between the can and the cap assembly. Here, the gasket is formed of a heat resistant material having a predetermined amount of heat-resistant enhancing material added thereto.

In one embodiment, the positive electrode tab is connected to at least a middle portion of the positive electrode plate.

In one embodiment, when the diameter of the can is about 18 mm and the height of the can is about 650 mm, the width of the positive electrode tab is about 3 mm to about 5 mm and the thickness of the positive electrode tab is about 0.1 mm to about 0.2 mm.

In one embodiment, the positive electrode tab is formed of aluminum.

In one embodiment, the negative electrode tabs are connected to both end portions of the negative electrode plate, respectively.

In one embodiment, the heat resistant material includes polymer having a melting point of about 180° C. to about 220° C.

In one embodiment, the heat resistant material includes any one selected from the group consisting of polypropylene, polybutylene terephthalate, polyethylene and polyimide, or mixture thereof.

In one embodiment, the heat-resistant enhancing material includes any one selected from the group consisting of ceramic, glass, glassfiber and combination thereof.

In one embodiment, the heat-resistant enhancing material of about 5 wt % to about 10 wt % is contained in the heat resistant material.

In accordance with embodiments of the present invention, the gasket maintains durable while being melted by internal heat of the secondary battery, so that it is possible to the can and the cap assembly from being short circuited to each other. As is generally explained by the American Society of Mechanical Engineers in “Energy Renewal,” more durable materials will be needed for future (i.e., steam) energy production. In general, the term “durable” means that an element is able to resist wear, decay, etc., well; lasting; enduring. Here, the gasket maintains its original form or deforms slightly while maintaining the integrity of the electrical insulation between the can and the cap-up while being melted by internal heat of the secondary battery.

Further, it is possible to secure heat-resistance safety of the secondary battery under the environment of large current discharge such as the environment of electrical power tools. Furthermore, it is possible to prevent the battery from firing and causing explosion caused by the overcharge of the secondary battery when the can and the cap assembly are short circuited to each other, thereby improving safety of the secondary battery.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1Ais a sectional view of a secondary battery constructed as a comparative example.FIG. 1Bis an exploded perspective view of the secondary battery ofFIG. 1A.FIG. 2is a partially enlarged sectional view of the secondary battery ofFIG. 1A.

Referring toFIGS. 1A and 1B, secondary battery100constructed as the comparative example is configured as a cylindrical lithium secondary battery. Secondary battery100includes an electrode assembly200; a cylindrical can300for accommodating electrode assembly200and an electrolyte; and a cap assembly400coupled to the upper portion of cylindrical can300to seal cylindrical can300.

Electrode assembly200includes a positive electrode plate210formed by coating a positive electrode active material layer on a surface of a positive electrode collector; a negative electrode plate220formed by coating a negative electrode active material layer on a surface of a negative electrode collector; and a separator230positioned between the positive and negative electrode plates210and220to allow positive and negative electrode plates210and220to be electrically isolated from each other. Electrode assembly200is formed by winding positive electrode plate210, negative electrode plate220and separator230in a jelly-roll shape. A positive electrode tab215is provided to electrically connect positive electrode plate210to cap assembly400, and a negative electrode tab225is provided to electrically connect negative electrode plate220to can300.

Insulating plates241and245for preventing contact with cap assembly400or cylindrical can300are formed at the top and bottom of electrode assembly200, respectively.

Cylindrical can300includes a cylindrical side plate310having a constant diameter to form a predetermined space for accommodating the wound electrode assembly200, and a bottom plate320for sealing the bottom of cylindrical side plate310. An opening through which electrode assembly200is inserted into can300is provided at the top of cylindrical side plate310. If negative electrode tab225of the electrode assembly200is joined with bottom plate320of can300, can300serves as a negative electrode. Can300is formed of aluminum (Al), iron (Fe) or alloy thereof.

Can300is provided with a crimping part330bent inward to pressurize an upper portion of cap assembly400. Can300is also provided with a beading part340deeply dug inward to pressurize a lower portion of cap assembly400at the position spaced downward from crimping part330by the distance corresponding to the thickness of cap assembly400.

Cap assembly400includes a safety vent410, a current blocking part420, a secondary protection element480and a cap-up490. Safety vent410, current blocking part420, secondary protection element480and cap-up490are sequentially positioned adjacent to electrode assembly200and stacked with one another. Safety vent410, current blocking part420and secondary protection element480may be integrally formed as one integrated and monolithic entity to decrease the volume and mass of the secondary battery and to reduce contact resistance through a decrease of the contact surface between components. Safety vent410has a groove413.

Safety vent410is formed in a plate shape and provided with a projection411protruding downward from the center portion thereof. Positive electrode tab215is welded to the projection. Safety vent410is formed so that projection411is deformed upward by overpressure generated inside the secondary battery and then separated from positive electrode tab215or so that a weak portion around projection411is broken by the overpressure generated inside the secondary battery.

Current blocking part420is formed so that the center portion430of current clocking part420is broken when the pressure generated inside the secondary battery extremely increases. Current blocking part420is provided with a conductive layer for electrically connecting safety vent410to secondary protection element480via the center portion430of current clocking part420. The conductive layer may be formed of copper (Cu) or copper alloy.

If the temperature inside the secondary battery is extremely increased due to the internal short circuit or overcharge/overdischarge, secondary protection element480is subjected to thermal expansion by the extremely increased temperature. Therefore, secondary protection element480prevents electrical connection between current blocking part420and cap-up480. For example, secondary protection element480may be formed as a positive temperature coefficient (PTC) element having an element layer formed of resin and carbon powder.

Cap-up490becomes a positive electrode terminal exposed to the top of secondary battery100. Cap-up490is positioned at the uppermost portion of cap assembly400. Cap-up490is electrically connected to positive electrode tab215with safety vent410, current blocking part420and secondary protection element480interposed therebetween.

Gasket500is positioned between can300and cap assembly400and allows them to be electrically isolated from each other. Gasket500is formed of polypropylene of which melting point is about 175° C.

In secondary battery100, gasket500is first inserted into can300, and safety vent410, current blocking part420, secondary protection element480and cap-up490are sequentially inserted into can300. Then, cap assembly400is crimped while gasket500is being inserted into can300by bending an upper edge of can300. According to such a configuration, secondary battery100of the comparative example prevents leak of a gas or electrolyte generated therein.

Meanwhile, as illustrated inFIG. 2, when gasket500is melted and deforms, can300and cap-up490are short circuited to each other under the environment of electrical power tools, e.g., an environment of large current discharge in which instantaneous current or surge current of about 100 A flows in secondary battery100. Moreover, if secondary battery100is charged in such a short-circuit state, the firing or explosion of secondary battery100may be caused.

FIG. 3is a partial sectional view of a secondary battery constructed as an embodiment of the present invention.FIG. 4is a partially enlarged sectional view of the secondary battery ofFIG. 3.

Referring toFIG. 3, secondary battery100A of this embodiment includes a cylindrical lithium secondary battery. Secondary battery100A includes an electrode assembly200a, a cylindrical can300for accommodating electrode assembly200aand an electrolyte, and a cap assembly400aconnected to the top of can300to seal can300. The electrolyte includes a nonaqueous electrolyte. The electrolyte may be a solid polymer containing lithium salt or a liquid polymer in which the lithium salt is dissociated in an organic solvent.

Electrode assembly200aincludes a positive electrode plate210, a negative electrode plate220and a separator230interposed between positive and negative electrode plates210and220. A positive electrode tab215aelectrically connects positive electrode plate210to the cap assembly400a. A negative electrode tab (not shown, see225ofFIG. 1A) electrically connects negative electrode plate220to can300.

Cap assembly400aincludes a conductive plate410a, a support plate450and a cap-up490a. The positive electrode tab215amay be connected to the conductive plate410aby a method such as welding. Support plate450supports cap assembly400aand electrically connects conductive plate410ato cap-up490a. Support plate450may be entirely conductive or may merely have a conductive layer formed at a portion thereof. Cap-up490aserves as a positive electrode terminal exposed to the exterior of cap assembly400a. Cap-up490amay be adhered closely to an upper portion of support plate450by a bent edge portion of support plate450. Conductive plate410aand/or cap-up490amay be integrally formed as one integrated and monolithic entity with support plate450.

Cap assembly400aof this embodiment is formed considering the environment of large current discharge such as environment of electrical power tools. For example, cap assembly400ahas a structure having a safety element such as PTC omitted therefrom. Here, the safety element is easily short-circuited in large current discharge, therefore, may cause a problem when the secondary battery is used.

A gasket600is positioned between can300and cap assembly400a, and allows can300and cap assembly400a to be electrically isolated from each other. Particularly, as illustrated inFIG. 4, gasket600of this embodiment protrudes convexly between the cap assembly and electrode assembly and contacts the cap assembly and electrode assembly, and is supported between the cap assembly and electrode assembly by following a convex shape of a beading portion while contacting a beading portion of the can.

Heat resistant material610may include any one selected from the group consisting of polypropylene (PP), polybutylene terephthalate (PBT), polyethylene and polyimide, or mixture thereof.

Heat-resistant enhancing material620is formed of a material having a higher melting point than that of the heat resistant material610. For example, heat-resistant enhancing material620may include any one of ceramic, glass, glassfiber and combination thereof.

Gasket600may be formed by mixing heat-resistant enhancing material620of about 5 wt % to about 10 wt % with heat resistant material610in the melted or liquid state and then performing a hot injection molding with respect to the mixture. For example, heat-resistant enhancing material620may include superplastic ceramic or nano-glass that is a nano-particle material. By using the ceramic or glass nano-particle material, the heat resistant temperature of heat resistant material610may be increased by about 10° C. to about 20° C. However, there may be a slight difference depending on the kind of heat resistant material610.

If heat-resistant enhancing material620having an amount less than 5 wt % is mixed with the heat resistant material610, the heat-resistant enhancing effect of the gasket600may be disadvantageously insignificant. If the heat-resistant enhancing material620having an amount greater than 10 wt % is mixed with the heat resistant material610, the elasticity of the gasket600may be disadvantageously lowered.

In accordance with this embodiment, when the reference temperature at which the gasket of the secondary battery is durable in the large current discharge or high power test is about 180° C. or about 230° C., a gasket using polypropylene (PP) with a melting point of about 175° C. as a main element is durable at the reference temperature of about 180° C., and a gasket using a material such as polybutylene terephthalate (PBT) with a melting point of about 220° C. as a main element is also durable at the test temperature of about 230° C. As described above, in accordance with this embodiment, the melting point of gasket600is increased by using heat-resistant enhancing material620, so that it is possible to prevent can300and cap assembly400afrom being short circuited due to melting of the gasket600under the environment of large current discharge. The heat resistant material includes polymer having a melting point of about 180° C. to about 220° C.

FIG. 5Ais a plan view illustrating a positive electrode plate and a positive electrode tab, applicable to the secondary battery ofFIG. 3.

Referring toFIG. 5A, positive electrode plate210aincludes a positive electrode collector211formed of a metal foil with excellent conductivity, e.g., aluminum (Al) foil, and positive electrode active material layers212aand212bcoated on both surfaces of the positive electrode collector211. Positive electrode active material layer212aand212bmay be formed to be spaced apart from each other at a predetermined interval.

In this embodiment, a positive electrode non-coating portion214cmay be formed at a middle portion of positive electrode plate210a. Positive electrode non-coating portions214aand214bmay be formed at both end portions of positive electrode plate210a. Here, a positive electrode active material is not formed at the positive electrode non-coating portions. Meanwhile, both ends portions of positive electrode plate210amay be omitted in the structure in which a positive electrode tab215ais connected to the middle portion of positive electrode plate210a.

In this embodiment, at least one positive electrode tab215ais joined with positive electrode non-coating portion214cpositioned at the middle portion of positive electrode plate210aby a method such as welding so as to be suitable for the environment of large current discharge. Positive electrode tab215ais formed of an aluminum (Al) material. In order to connect positive electrode tab215ato cap assembly400a, positive electrode tab215aextends upward from positive electrode plate210awith a certain length.

In this case, positive electrode tab215ais formed to have a width W of about 3 mm to about 5 mm and a thickness (not shown) of about 0.1 mm to about 0.2 mm. The dimension of positive electrode tab215ais considered when the cylindrical can (sec cylindrical can300ofFIGS. 1A and 1B) has a diameter D of about 18 mm and a height H of about 65 mm. Here, the width and thickness of positive electrode tab215aare about two times greater than the width (about 1 mm to about 3 mm) and thickness (about 0.05 mm to about 0.1 mm) of positive electrode tab215illustrated inFIG. 1Ashowing the comparative example. If positive electrode tab215ahaving such a dimension is connected to the middle portion of positive electrode plate210a, the secondary battery can be appropriately used in the environment of large current discharge such as the environment of electrical power tools.

Meanwhile, if the width W of positive electrode tab215aexceeds about 5 mm or the thickness of positive electrode tab215aexceeds about 0.2 mm, the electrode assembly wound while having positive electrode tab215connected thereto has a roughly triangular section. Therefore, such dimension is not preferable for the cylindrical secondary battery. When the size or diameter of the cylindrical can is increased, the width W and thickness of the positive electrode tab215amay be formed larger in correspondence with the increase of the size of cylindrical can300.

FIG. 5Bis another plan view illustrating a negative electrode plate and a negative electrode tab, applicable to the secondary battery ofFIG. 3.

Referring toFIG. 5B, negative electrode plate220aincludes a negative electrode collector221formed of a conductive metal foil, e.g., copper (Cu) or nickel (Ni) foil, and a negative electrode active material layer222coated on both surface of negative electrode collector221. Negative electrode non-coating portions224aand224bare formed at both end portions of negative electrode plate220a. Here, a negative electrode active material layer is not formed at negative electrode non-coating portions224aand224b.

Negative electrode tabs225aand225bare connected to negative electrode non-coating portions224aand224bpositioned at both end portions of the negative electrode plate220a, respectively. Negative electrode tabs225aand225bmay be formed of a nickel (Ni) material and extends to one side, e.g., an upside from electrode assembly200with a certain length. Alternatively, negative electrode tabs225aand225bmay be formed to protrude from the negative electrode plate220ato the downside opposite to the upside or the extending direction of the negative electrode collector221.

If at least two negative electrode tabs225aand225bare connected to both end portions of the negative electrode plate220a, the secondary battery can be appropriately used in the environment of large current/high power discharge such as the environment of electrical power tools.

In accordance with these embodiments, in the environment of applications such as electrical power tools, the gasket advantageously maintains its original form or alternatively deforms slightly while maintaining the integrity of the electrical insulation between the can and the cap-up and thereby assuring electrical insulation when the gasket is melted by the rapid increase in the internal temperature, so that the gasket may prevent the can and the cap-up from becoming short circuited to each other by remaining durable under high internal temperature.

In the environment of large current discharge while using such as a power tool, it is necessary to increase a width and thickness of the positive electrode tab in order to discharge a large amount of current easily and/or rapidly. Therefore, the positive electrode tab of the secondary battery is not burned to break with a rapid increase of internal temperature in the environment in which a large amount of current is discharged and a safety part is not functioning normally. When the positive electrode tab is not burned to break; however, the gasket will be melted due to a high internal temperature. As a result, there is a possibility of igniting a fire or exploding of the secondary battery in the case of melting of gasket. In accordance with this present invention, the gasket will be not melted in the environment of large current discharge using such as a power tool. Therefore, it is possible to prevent the secondary battery from igniting a fire or exploding.