Electrode assembly with separated support tapes in secondary battery

Separated support tapes are attached to an electrode assembly which includes a negative electrode plate, a positive electrode plate, and a separator for performing charge and discharge operations. The separated support tapes are attached to a lower portion of the electrode assembly so as to allow the electrode assembly to be easily inserted into a can, while reducing the possibility of fire in a secondary battery caused by a hard short when the secondary battery is compressed due to external force applied thereto, preventing battery performance from being deteriorated by allowing electrolyte to sufficiently flow into a lower portion of the electrode assembly, and reducing a manufacturing cost of the secondary battery by minimizing the amount of separated support tape to be used.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from my application JELLY ROLL WITH SEPARATE SUPPORT TAPE IN SECONDARY BATTERY filed with the Korean Industrial Property Office on 15 Oct. 2003 and there duly assigned Serial No. 2003-71949.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a secondary battery and, more particularly, to separated support tapes having an improved structure and attached to a lower portion of an electrode assembly in order to allow the electrode assembly to be easily inserted into a can of a prismatic type secondary battery.

2. Related Art

A secondary battery has an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator. The electrode assembly is accommodated in a can made from aluminum or an aluminum alloy, and an electrolyte is then injected into the can. In this state, the can is sealed, thereby forming the secondary battery. An electrode terminal insulated from the can is provided at an upper portion of the secondary battery in order to form a cathode or an anode of the secondary battery. If the electrode terminal is an anode, the can serves as a cathode, and if the electrode terminal is a cathode, the can serves as an anode. Safety devices, such as a PTC (positive temperature coefficient) device, a thermal fuse, and a PCM (protective circuit module), are connected to the secondary battery. Such safety devices are electrically connected to the anode and the cathode of the secondary battery in order to prevent the secondary battery from being damaged or broken by shutting off current applied to the secondary battery if the voltage of the secondary battery is suddenly increased due to overcharge or over-discharge. The secondary battery equipped with the safety devices is accommodated in a case, thereby forming a secondary battery pack.

A secondary battery includes an electrode assembly, a negative electrode tap, a positive electrode tap, and a single support tape. The electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator interposed between the positive electrode plate and the negative electrode plate, and wound in the form of a jelly roll. The negative electrode tap electrically connected to the negative electrode plate and the positive electrode tap electrically connected to the positive electrode plate are upwardly withdrawn from an upper portion of the electrode assembly. The electrode assembly wound in the form of a jelly roll is accommodated in a can having a hexahedral shape and made of metal, such as aluminum.

A single support tape having a predetermined length and a predetermined width is attached to a lower portion of the electrode assembly in order to allow the electrode assembly to be easily inserted into the can when the electrode assembly in the form of a jelly roll is inserted into the can. The single support tape completely surrounds the lower portion of the electrode assembly except for predetermined marginal regions of about 1 to 2 mm formed at both lower sides of the electrode assembly.

However, when performing a safety test on the secondary battery while accommodating the electrode assembly in a can, the electrode assembly being provided at its lower portion with the single support tape, the following problems may occur:

Firstly, when performing a battery safety test by pressing both lateral portions of the battery in a widthwise direction thereof in order to evaluate the possibility of firing in the battery, a middle portion of the battery is bent as the battery is compressed, so that the battery is folded. At this point, due to provision of the single support tape supporting the lower portion of the electrode assembly, a wrinkle may occur at a middle portion of a lower half of the electrode assembly. Thus, the possibility of fire in the battery due to a hard short may increase.

Secondly, if the signal support tape strongly grips the lower portion of the electrode assembly, electrolyte cannot sufficiently flow into the lower portion of the electrode assembly, thereby deteriorating performance and/or battery capacity of the battery.

Thirdly, since the single support tape substantially covers the entire area of the lower portion of the electrode assembly, a large amount of single support tape is necessary, resulting in increased manufacturing cost of the battery. Moreover, the single support tape is necessary only to allow the electrode assembly to be easily inserted into the can, and the single support tape may be released from the electrode assembly after the electrode assembly has been inserted into the can if the signal support tape is weakly attached to the lower portion of the electrode assembly. Thus, such excessive usage of the single support tape must be reconsidered.

SUMMARY OF THE INVENTION

The present invention has been developed in order to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an electrode assembly capable of reducing the possibility of fire in a secondary battery caused by a hard short when the secondary battery is compressed due to external force applied thereto, capable of preventing battery performance from being deteriorated by allowing electrolyte to sufficiently flow into a lower portion of the electrode assembly, and capable of reducing the manufacturing cost of the secondary battery by minimizing the amount of separated support tape to be used.

To accomplish the above object, there is provided an electrode assembly comprising: a negative electrode plate, a positive electrode plate, and a separator for performing charge and discharge operations; and separated support tapes attached to a lower portion of the electrode assembly so as to allow the electrode assembly to be easily inserted into a can.

According to an exemplary embodiment of the present invention, the separated support tapes are symmetrically attached to the lower portion of the electrode assembly and about a center portion of the electrode assembly while being spaced apart from each other by a predetermined distance.

In addition, the separated support tapes can be asymmetrically attached to the lower portion of the electrode assembly and about a center portion of the electrode assembly while being spaced apart from each other by a predetermined distance.

According to the exemplary embodiment of the present invention, at least two separated support tapes are employed, each separated support tape being attached to a respective side of the lower portion of the electrode assembly.

The total length of the separated support tapes preferably is in a range of about ¼ to ¾ of the total width of the electrode assembly.

According to the exemplary embodiment of the present invention, a first side end of the separated support tape is spaced from a longitudinal center line of the electrode assembly by more than 10% of a total width of the electrode assembly, and a second side end of the separated support tape is spaced from one side end of the electrode assembly by more than 10% of the total width of the electrode assembly.

The separated support tape preferably has a height in a range of about 3 to 8 mm when the separated support tape is attached to the electrode assembly.

According to the exemplary embodiment of the present invention, the separated support tape preferably has a thickness of about 20 to 60 μm.

The separated support tape preferably has a tensile strength of at least 2 kgf/cm2.

According to the exemplary embodiment of the present invention, the separated support tape preferably has an elongation of at least 50%.

The separated support tape is preferably made of material selected from the group consisting of polyimide, polyethylene, and polypropylene.

The separated support tape preferably has an adhesive strength of at least 20 g/mm, acryl-based adhesive is preferably coated on one surface of the separated support tape, and the separated support tape is preferably attached to the electrode assembly with pressure of 0.1 to 0.4 Mpa.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an exemplary embodiment of the present invention will be described with reference to accompanying drawings.

FIG. 1is perspective view showing an electrode assembly in the form of a jelly roll having a single support tape for allowing the electrode assembly to be easily inserted into a can.

Referring toFIG. 1, the secondary battery includes an electrode assembly11, a negative electrode tap12, a positive electrode tap13, and a single support tape14. Although not specifically shown inFIG. 1, the electrode assembly11includes a positive electrode plate, a negative electrode plate, and a separator interposed between the positive electrode plate and the negative electrode plate, and wound in the form of a jelly roll. The negative electrode tap12electrically connected to a negative electrode plate and the positive electrode tap13electrically connected to a positive electrode plate are upwardly withdrawn from an upper portion of the electrode assembly11. The electrode assembly wound in the form of a jelly roll is accommodated in a can (not shown) having a hexahedral shape and made of metal, such as aluminum.

A single support tape14having a predetermined length and a predetermined width is attached to a lower portion of the electrode assembly11in order to allow the electrode assembly11to be easily inserted into the can when the electrode assembly11in the form of a jelly roll is inserted into the can. As shown inFIG. 1, the single support tape14completely surrounds the lower portion of the electrode assembly11except for predetermined marginal regions of about 1 to 2 mm formed at both lower sides of the electrode assembly11.

FIG. 2is a perspective view showing an electrode assembly having separated support tapes according to an exemplary embodiment of the present invention.

As shown inFIG. 2, the present invention includes an electrode assembly21, a negative electrode tap22, a positive electrode tap23and separated support tapes24.

In order to fabricate the electrode assembly21, active materials are coated on both surfaces of a collector. The collector is then dried and is subjected to a roll pressing process such that the collector is cut into a positive electrode plate and a negative electrode plate. In this state, the positive electrode plate and the negative electrode are wound after interposing a separator therebetween, thereby forming the electrode assembly21. That is, after alternately aligning the positive electrode plate and the negative electrode plate about the separator, the positive electrode plate, the separator and the negative electrode plate are wound in a spiral pattern by using a winding mandrel having an oval shape, thereby forming the electrode assembly21. The negative electrode plate is then further wound one more time at an outermost portion of the electrode assembly21, and the electrode assembly21is sealed by means of tape. After forming the oval-shaped electrode assembly21in the form of a jelly roll, a pressing process is carried out with respect to the oval-shaped electrode assembly21in order to insert the oval-shaped electrode assembly21into a prismatic type can (not shown). The width of the positive electrode plate is preferably smaller than the width of the negative electrode plate, and the width of the negative electrode plate is preferably smaller than a width of the separator.

The positive electrode plate is formed by coating slurry, such as positive electrode active materials, binders or conductive agents, on both surfaces of a metal collector. For instance, the positive electrode plate is formed by uniformly coating a mixture (slurry or paste) consisting of a positive electrode material of LiNixCoyO2, a conductive agent of super-P, and a binder of polyvinylidenefluoride (PVDF), which are mixed with an inorganic solvent of N-methyl-2-pyrrolidone (NMP), on both surfaces of a collector made of aluminum foil. In this state, the collector is subjected to a drying process in order to remove the inorganic solvent, and the collector is then subjected to a roll pressing process so that a positive electrode plate having a thickness of about 0.147 mm is achieved.

The negative electrode plate is formed by coating slurry, such as positive electrode active materials, binders or conductive agents, on both surfaces of a metal collector. For instance, the negative electrode plate is formed by uniformly coating a mixture (slurry or paste) consisting of a positive electrode material of MCF and a binder of polyvinylidenefluoride (PVDF), which are mixed with an inorganic solvent of N-methyl-2-pyrrolidone (NMP), on both surfaces of a collector made of copper foil. In this state, the collector is subjected to a drying process in order to remove the inorganic solvent, and the collector is then subjected to a roll pressing process so that a negative electrode plate having a thickness of about 0.173 mm is achieved.

The separator is formed by using a polyethylene porous film having a thickness of about 0.025 mm, and which has a stability and rarely reacts with inorganic solvent.

The collector for forming the positive electrode plate and the negative electrode plate can be made of any materials as long as the materials are not subject to chemical change. In general, the collector is made of aluminum, copper, stainless, nickel, titan, or plastic carbon. In addition, surfaces of the collector can be coated with carbon, nickel, titan or silver. In a case of a lithium ion secondary battery, the positive electrode plate is made of aluminum foil and the negative electrode plate is made of copper foil considering such factors as oxidation resistant property, flexibility of the electrode, and cost. The thickness of the copper foil or aluminum foil is about 1 to 30 μm, preferably about 5 to 20 μm. However, the present invention is not limited to any particular thickness of the copper foil or aluminum foil.

Similar to the negative electrode plate, the negative electrode tap22is made of copper or nickel. The negative electrode tap22made of copper preferably has a width of about 1 to 10 mm and a thickness of about 5 to 50000 μm.

Similar to the positive electrode plate, the positive electrode tap23is made of aluminum or nickel. The positive electrode tap23made of aluminum preferably has a width of about 1 to 10 mm and a thickness of about 5 to 50000 μm.

The separated support tapes24surround a lower portion of the electrode assembly21up to a predetermined level or height. Each of the separated support tapes24is separately attached to a respective side portion of the electrode assembly21and about a center of the electrode assembly21. AlthoughFIG. 2shows two separated support tapes24, which are separately attached to respective side portions of the electrode assembly21and about the center of the electrode assembly21, it is also possible to provide four separated support tapes. In this case, two support tapes24are attached to each side portion of the electrode assembly21. Herein, the separated support tapes24are not provided in predetermined areas of the lower portion of the electrode assembly21adjacent to a longitudinal center line of the electrode assembly21. Although it is more preferable that the electrode assembly21have no separated support tapes24, in this case, the lower portion of the electrode assembly21may be damaged when the electrode assembly21is inserted into the can. Thus, it is preferable to set the total length of the separated support tapes24to ¼ to ¾ of the width of the electrode assembly21so that the separated support tapes24can sufficiently support the lower portion of the electrode assembly21. If the total length of the separated support tapes24is less than ¼ of the width of the electrode assembly21, the separated support tapes24cannot sufficiently support the electrode assembly21. In addition, since the function of the separated support tapes24is to simply grip the lower portion of the electrode assembly21when the electrode assembly21is inserted into the can, it is not necessary for the total length of the separated support tapes24to be identical to the width of the electrode assembly21. Specifically, it is not necessary that the total length of the separated support tapes24be more than ¾ of the width of the electrode assembly21.

The separated support tapes24are preferably made of a material having a superior elongation property. Preferably, the separated support tapes24are made of a material having an elongation of at least 50%. If the elongation of the separated support tapes24is less than 50%, it is difficult for the separated support tapes24to closely grip the electrode assembly21. In addition, the separated support tapes24must have a tensile strength of at least 2 kgf/cm2. If the separated support tapes24have a tensile strength of less than 2 kgf/cm2, the separated support tapes24cannot sufficiently support the electrode assembly21. Preferably, the separated support tapes24are made of polyimide, polyethylene or polypropylene, which possess superior endurance against the electrolyte of the secondary battery.

In addition, acryl-based adhesive, which rarely reacts with the electrolyte of a secondary battery, and which has an adhesive force of at least 20 g/mm, is preferably coated on a surface of the separated support tapes24. If the adhesive force of the acryl-based adhesive is less than 20 g/mm, the separated support tapes24cannot sufficiently support the electrode assembly21.

Preferably, the separated support tapes24have a thickness of about 20 to 60 μm. If the separated support tapes24have a thickness less than 20 μm, the separated support tapes24cannot sufficiently support the electrode assembly21. In contrast, if the separated support tapes24have a thickness of more than 60 μm, the thickness of the electrode assembly21may be increased.

In addition, preferably, the separated support tapes24have a height of about 3 to 8 mm when the separated support tapes24are attached to the electrode assembly21. If the height of the separated support tapes24is less than 3 mm, a contact surface between the separated support tapes24and the electrode assembly21is too small, so that the separated support tapes24cannot be securely attached to the electrode assembly21. In addition, if the height of the separated support tapes24exceeds 8 mm, not only is the thickness of the electrode assembly21increased, but also the amount of the separated support tapes24is unnecessarily increased.

In addition, it is preferable to protect the lower portion of the electrode assembly21from external pressure when the electrode assembly21has been inserted into the can. Thus, the separated support tapes24are preferably released from the lower portion of the electrode assembly21when the electrode assembly21has been inserted into the can. In the case wherein the separated support tapes24cannot be released from the lower portion of the electrode assembly21, the influence of the separated support tapes24exerted on the lower portion of the electrode assembly21must be minimized. To this end, the pressure of a drum may be lowered when attaching the separated support tapes24to the electrode assembly21. That is, the pressure applied by the drum, which attaches the separated support tapes24to the electrode assembly21, is set in a range of about 0.1 to 0.4 Mpa. If the pressure of the drum is less than 0.1 Mpa, the separated support tapes24cannot be attached to the electrode assembly21.

FIG. 3is a front sectional view showing separated support tapes attached to a lower portion of an electrode assembly according to an exemplary embodiment of the present invention.

Referring toFIG. 3, two separated support tapes24are attached to respective sides of the lower portion of the electrode assembly21while being spaced apart from each other by a predetermined distance about a longitudinal center line CL of the electrode assembly21. On the assumption that the total width of the electrode assembly21is “a”, that the distance between one side end of the electrode assembly21and one side end of the separated support tapes24is “b”, and that the distance between a longitudinal center line CL of the electrode assembly21and the other side end of the separated support tapes24is “c”, a compression test for the secondary battery was carried out while varying the width of the separated support tapes24. As a result, various possibilities of fire for the secondary battery are represented as shown in following experimental examples 1 to 4. The compression test was carried out by applying compression force of 13 KN to the charged secondary battery for two seconds while laterally aligning a charged secondary battery in parallel to jigs gripping the charged secondary battery. In addition, after releasing the secondary battery from the jigs, the possibility of fire in the secondary battery was determined.

Experimental Example 1

When the compression test was carried out under the condition of b/a=0 and c/a=0, a possibility of fire for the secondary battery was about 30%.

Experimental Example 2

When the compression test was carried out under the condition of b/a=0.1 and c/a=0, the rate of fire in the secondary battery was about 20%.

Experimental Example 3

When the compression test was carried out under the condition of b/a=0 and c/a=0.1, the rate of fire in the secondary battery was about 20%.

Experimental Example 4

When the compression test was carried out under the condition of b/a=0.1 and c/a=0.1, the rate of fire in the secondary battery was about 5%.

As understood from the above experimental examples, if b/a and c/a are at least 0.1, the rate of fire in the secondary battery is significantly reduced. Accordingly, it is preferable to place the separated support tapes24away from the longitudinal center line CL of the electrode assembly21by at least or more than 10% of the total width of the electrode assembly21. In addition, it is preferable to place one side end of the separated support tapes24away from one side end of the electrode assembly21by at least or more than 10% of the total width of the electrode assembly21. In consideration of the possibility of insertion failure occurring when inserting the electrode assembly21into the can, the other side end of the separated support tapes24must be spaced from the longitudinal center line CL of the electrode assembly21by at least or more than 10% of the total width of the electrode assembly21. In this case, it does not matter that one side end of the separated support tapes24is spaced from one side end of the electrode assembly21by 1 to 2 mm.

As described above, the electrode assembly having the separated support tape according to the present invention reduces the possibility of fire in the secondary battery caused by a hard short when the secondary battery is collapsed due to external force applied thereto, prevents battery performance from being deteriorated by allowing the electrolyte to flow sufficiently into the lower portion of the electrode assembly, and reduces manufacturing cost of the secondary battery by minimizing the amount of separated support tape to be used.

While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment and the drawings, but, on the contrary, it is intended to cover various modifications and variations within the spirit and scope of the appended claims.