Pouch for secondary battery, secondary battery using the same, and apparatus for sealing pouch cases of secondary battery

A pouch for a secondary battery comprising indentations on a sealing area of the pouch is disclosed. At least some of the indentations are filled with the resin layers of the pouch. A secondary battery comprising the pouch, and an apparatus for sealing the pouch are also provided. The indentations in the pouch can take the shape of dots or lines, and prevent the resin layers of the pouch from leaking out of the pouch during heating and pressing of the sealing area of the pouch. Also, when the resin layers collect in the indentations, forming adhesive cores, reliability of the pouch seal is improved despite a reduction in the sealing area of the pouch.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0076134, filed on Sep. 22, 2004, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a secondary battery, and more particularly, to a pouch for containing a secondary battery, to a secondary battery using the pouch, and to an apparatus for sealing the pouch.

BACKGROUND OF THE INVENTION

Due to the wide use of portable electric products such as video cameras, mobile telephones, and portable PCs, secondary batteries, which can be used as the driving power sources of these portable electric products, have become increasingly important. This is because lithium secondary batteries have greater energy concentration per unit weight than other secondary batteries, such as conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and nickel-zinc batteries. In addition, lithium secondary batteries can be rapidly charged. For all these reasons, use of lithium secondary batteries has significantly increased.

Lithium secondary batteries are classified according to the electrolyte used. Lithium ion batteries use liquid electrolytes, and lithium ion polymer batteries use solid polymer electrolytes. Lithium ion polymer batteries can use completely solid polymer electrolytes, which include no liquid electrolyte, or they may use gel type polymer electrolytes, which include some liquid electrolyte.

Lithium ion batteries, which use liquid electrolytes, can be contained in cylindrical or angular metal cans which are sealed by soldering. These can type secondary batteries have limitations on shape, restricting the design of electric products employing these batteries as power sources. Also, reducing the volume of the electric products is difficult with can type secondary batteries.

To address these limitations, pouch type secondary batteries in which two electrodes, a separator, and an electrolyte are contained in a sealed pouch comprising a film, have been developed as alternatives to can type secondary batteries.FIGS. 1 and 1Aillustrate a typical prior art pouch type secondary battery. As shown inFIG. 1A, the pouch of a typical lithium ion polymer battery has a multi-layered structure comprising a polyolefin thermal adhesive layer15having thermal adhesive properties which serves as a sealant, an aluminum metal layer13that serves as a material for maintaining mechanical strength, a water and oxygen barrier layer, a nylon layer11, and a protective layer. The pouch is formed by sequentially laminating these layers. Cast polypropylene (CPP) is commonly used as the polyolefin layer.

As shown inFIG. 1, the pouch comprises a container20having a cavity21and a cover10for covering the cavity21. An electrode assembly30is contained within the cavity21, and comprises an anode31, a cathode35and a separator33. The anode31, cathode35and separator33are laminated and wound to form the electrode assembly30. Electrode tabs37and38protrude from the anode and cathode and tape39is attached to the portion of each tab that lies on the lip23of the container20of the pouch.

Pouch type secondary batteries are flexible and can take a variety of shapes, making it possible to manufacture secondary batteries having the same capacity, but having reduced volume and weight. However, unlike can type secondary batteries, the pouches used in pouch type secondary batteries are soft, making the pouch type secondary batteries weak, and decreasing the reliability of the pouch seal. Therefore, pouch type secondary batteries are mainly used with lithium ion polymer batteries using gel type or completely solid polymer electrolytes, rather than with lithium ion secondary batteries using liquid electrolytes, which may leak.

The electrode assembly is positioned in the pouch of the pouch type secondary battery such that a small pouch can have large capacity, as is needed for secondary batteries. Also, reduction of the area of the lip23surrounding the container20is desired since the lip23is not related to battery capacity or battery function. When the lip23is reduced, electrode assemblies having greater capacity can be contained in the pouch, resulting in secondary batteries having greater capacity. However, reduction of the lip23also reduces the reliability of the seal since the area to which the cover10is sealed to the container20is reduced.

However, during thermal fusion of the thermal adhesive polyolefin layer of the cover10to the thermal adhesive polyolefin layer of the container20, a portion of the fused polyolefin layer seeps out of the pouch due to the external pressure applied during fusion. Such an occurrence adversely affects the external appearance of the pouch and reduces the thickness of the fused polyolefin layer, thereby reducing the reliability of the seal. Furthermore, enough of the polyolefin layer may seep out of the pouch during sealing to expose the metal barrier layer underneath the polyolefin layer. This occurrence causes a short circuit between the pouch and the electrode tabs.

SUMMARY OF THE INVENTION

The present invention is directed to a pouch for containing a secondary battery. The pouch prevents the resin layers of the pouch, in particular, the thermal adhesive layers from seeping out of the pouch during sealing by thermal fusion, which includes application of heat and pressure to the pouch. The pouch also improves the reliability of the seal despite a reduction in the sealing area of the pouch, and imparts increased battery capacity.

In another embodiment of the present invention, an apparatus for sealing the pouch is provided. The apparatus prevents at least some of the resin layers of the multi-layered pouch from deviating from their original positions during fusion. This apparatus prevents the metal barrier layer from becoming exposed, thereby preventing short circuits.

According to one embodiment of the present invention, a pouch for a secondary battery comprises a plurality of indentations in at least one of the lip of the container and the edge of the cover. Upon sealing, at least some of the indentations become filled with the resin of the thermal adhesive layers. The indentations can take the shapes of dots or lines and may be formed only on the sealing area or on the entire surface of the container and cover of the pouch, including the sealing area.

The indentations may be arranged on the cover and container of the pouch such that the indentations on the cover intersect the indentations on the container. Alternatively, the indentations on the cover correspond to the indentations on the container so that the resin of thermal adhesive layers forms an adhesive core.

In another embodiment of the present invention, a secondary battery comprises an electrode assembly having positive and negative electrodes separated by a separator. The electrodes and the separator are laminated to form the electrode assembly, which is contained in a pouch comprising two or more layers, forming a bare cell. Indentations are formed on at least one of the container of the pouch and the cover of the pouch and at least some of the indentations become filled with the resin of the thermal adhesive layers upon sealing the pouch.

In one embodiment, the indentations are formed on only one of the cover of the pouch and the container of the pouch, and a thick thermal adhesive layer is formed on the other of the cover and container of the pouch.

According to another embodiment of the present invention, an apparatus for sealing the pouch of a secondary battery comprises a sealing jig or sealing die that applies pressure to the sealing area of the pouch, thereby sealing the pouch. For example, one surface of the sealing jig contacts the top surface of the cover of the pouch, and another surface contacts the bottom surface of the lip of the container. The pouch is sealed by applying heat and pressure to the surfaces of the sealing jig. In one embodiment, the jig has at least one hole in each of its surfaces, such that upon applying pressure to the pouch, air is vented through the hole.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention will now be described in detail with reference to the attached drawings. Throughout the description, like reference numerals are used to identify like components in order to avoid repetitive descriptions of same or similar components.

In one embodiment of the present invention, a pouch100for containing an electrode assembly of a secondary battery comprises a container120and a cover110. The container120comprises a cavity121for containing the electrode assembly and a lip123. The cover110fits over the container120and has a portion that overlaps the lip123of the container. As shown inFIG. 2, indentations117are formed in a sealing area of the pouch100. The sealing area comprises the lip123of the container120as well as the portion of the cover110that overlaps the lip123.

The cavity121of the container120may be formed by pressing a multi-layer film. A gas chamber may be formed on the side of the cavity for removing gas generated by initial aging, charging and discharging. The indentations117in the sealing area may be formed separately from the formation of the cavity121. Alternatively, the indentations117may be formed simultaneously with the cavity121by pressing, thereby reducing processing time and cost. The indentations117may take any suitable shape. For example, the indentations may be dot shaped, as shown inFIG. 5, or they may comprise linear grooves, as shown inFIGS. 2,4and6.

As shown inFIGS. 2A and 2B, a secondary battery according to one embodiment of the present invention comprises an electrode assembly130contained within a pouch comprising a container120and a cover110. The electrode assembly130comprises an anode131and a cathode135, each of which is formed by filling a grid with an electrode active material or coating a thin film with the electrode active material. The anode131, the cathode135and a separator133positioned between the anode and cathode, are laminated and wound to form a jelly-roll structure. Except for lithium ion polymer batteries using completely solid polymer electrolytes, the jelly-roll type separator of the electrode assembly is impregnated with the electrolyte. Lithium ion polymer batteries using completely solid electrolytes do not require this additional electrolyte impregnation because the separators in those batteries include the electrolyte. The electrode assembly130is contained in the container121of the pouch.

Electrode tabs137and138are attached to the anode131and cathode135, respectively, and are spaced apart from each other by a predetermined distance. The electrode tabs electrically connect the electrode assembly to an outside source. Additionally, thermal adhesion tapes139may be attached to the portions of the electrode tabs37and38that overlap the lip123of the container120of the pouch. These tapes139enable better adhesion of the electrode tabs139to the pouch.

In one embodiment of the present embodiment, the pouch comprises three layers. However, it is understood that more layers may be provided. Also, the layers of the pouch may comprise materials different from those specifically described. For example, the cast polypropylene (CPP) thermal adhesive layer115may alternatively comprise a material selected from the group consisting of propylene chloride, polyethylene, ethylene propylene copolymers, copolymers of polyethylene and acrylic acid, and copolymers of polypropylene and acrylic acid.

The pouch comprises a nylon layer111, a cast polypropylene layer115and an aluminum layer113. The thickness of the entire pouch ranges from about 40 to about 120 micrometers. The nylon layer111and the CPP layer115each have a thickness ranging from about 10 to about 40 micrometers. The thickness of the aluminum layer113ranges from about 20 to about 100 micrometers.

In one embodiment, as shown inFIG. 3B, the depth of each indentation217is about 100 micrometers and generally corresponds to the thickness of the pouch. In another embodiment, as shown inFIG. 3A, the indentations317are formed only in the thermal adhesive layer315and each indentation317is about 10 to about 40 micrometers deep, generally corresponding to the thickness of the thermal adhesive layer315.

Each indentation can have any suitable cross-sectional shape, for example a semi-circle or triangle, so that the indentations are easily formed. The width of each indentation may be equal to its depth. The depths of the indentations can be controlled by controlling the distance between the indentations and the temperature and pressure during thermal fusion. In addition, the temperature and pressure during thermal fusion can be controlled by controlling the distance between the indentations and the depth of the indentations. Controlling these variables is necessary to ensure that the thermal adhesive layer does not seep out of the pouch.

FIG. 4is a perspective view of a sealed bare cell of a secondary battery400according to one embodiment of the present invention. As shown, the indentations417are linear in shape and are formed only on the sealing area of the pouch, i.e., the indentations417are formed on the lip423of the container of the pouch and on that portion of the cover of the pouch that overlaps the lip423. In this embodiment, each indentation417on the cover of the pouch corresponds in position to the position of a indentation on the lip423of the container.

In sealing the pouch, a majority of the indentations417become filled with the portions of the thermal adhesive layer that deviates from its original position during application of heat and pressure. The thermal adhesive layers within the indentations are then cooled and hardened to form a hard seal.

FIG. 5is a perspective view of a sealed bare cell of a secondary battery500according to another embodiment of the present invention. In this embodiment, the indentations517are dot shaped and are formed by embossing. To provide space for the thermal adhesive layer, the pitch of a dot shaped indentation517is greater than the pitch of a linear indentation417.

FIG. 6is a perspective view of a sealed bare cell of a secondary battery600according to yet another embodiment of the present invention. In contrast withFIGS. 4 and 5, the indentations617in this embodiment are linear in shape and are formed on the entire cover of the pouch, including the sealing area. In addition, the indentations615may be formed either on the entire surface of the container, including the cavity, or on only the lip of the container. According to this embodiment, processing of the secondary battery is simplified since the indentations need not be formed separately. However, it is understood that the surplus thermal adhesive layer fills only the indentations in the sealing area.

FIG. 7is a partial cross-sectional view of a sealed sealing area of a pouch according to another embodiment of the present invention. The indentations are filled with the thermal adhesive layer715when the cover of the pouch is sealed to the lip of the container, creating adhesive cores753.

As shown inFIG. 7, pressure is applied to the indentations during thermal fusion causing surplus thermal adhesive layer715to fill into the indentations. The indentations in the lip of the container of the pouch and the indentations in the cover of the pouch correspond in position, thereby creating adhesive cores753upon cooling and hardening the thermal adhesive layers715within the indentations.

FIG. 8is a partial cross-sectional view of a sealing area of a pouch positioned within a sealing apparatus according to one embodiment of the present invention. The sealing apparatus may comprise a jig or a die into which the sealing area of the pouch is inserted. The structures of sealing jigs and sealing dies is known.

Referring toFIG. 8, an electrode assembly (not shown) is positioned in the cavity of the pouch and the cavity is covered by the cover of the pouch. The sealing area is then positioned between first and second plates850aand850bof a sealing apparatus. The sealing apparatus is then used to heat and press the sealing area of the pouch, thereby sealing the pouch. Although not shown, the sealing apparatus includes a heater for heating the first and second plates850aand850bof the apparatus. The sealing apparatus also includes means, such as a vice, for pressing the first and second plates850aand850bof the apparatus together.

In one embodiment, at least one, and preferably a plurality of indentations851are formed on the areas of the first and second plates850aand850bwhich contact the sealing area of the pouch. Upon pressing, the nylon layers811, metal barrier layers813and thermal adhesive layers815partially melt, become flexible and seep into the indentations851in the plates850aand850bof the sealing apparatus. As a result, portions of the pouch will be thicker than the rest of the layer. In particular, when the thermal adhesive layers815collect in the indentations851of the plates850aand850b,adhesive cores are created, thereby enforcing the pouch seal. The sealed pouches may have the external appearance shown in eitherFIG. 4orFIG. 5.

In another embodiment, shown inFIG. 9, indentations951are formed on the areas of the first and second plates950aand950bwhich contact the sealing area of the pouch. In addition, holes953located generally in the area of the indentations951are provided in the plates950aand950b.Upon pressing, the air compressed in the indentations951can vent through the holes953, preventing pressure in the indentations951from increasing, and enabling easy collection of the resin of the thermal adhesive layer915in the indentations951.

According to one embodiment of the present invention, the thermal adhesive layers are formed in the indentations upon fusion and pressing of the thermal adhesive layers during sealing. This construction prevents the fused thermal adhesive layers from leaking out of the pouch, thereby improving adhesion. According to another embodiment of the present invention, the plates of the sealing apparatus comprise indentations for collecting the fused layers during sealing. This construction prevents the partially fused resins from leaking out of the pouch and prevents deterioration of the outer appearance of the pouch. Also, the partially collected fused resin improves the pouch seal. Accordingly, the pouches of the present invention improve the reliability of the pouch seal despite a reduction in the sealing area of the pouch, thereby increasing battery capacity.

Exemplary embodiments of the present invention have been described for illustrative purposes only. Those skilled in the art will appreciate that various modifications, additions and substitutions can be made without departing from the spirit and scope of the invention as disclosed in the accompanying claims.