Abstract:
A collector, an electrode and a secondary battery adopting the same. The secondary battery includes: an electrode assembly having electrodes obtained by attaching an active material sheet to at least one side of a flat collector with a plurality of through holes formed in a predetermined pattern, and a separator interposed between the electrodes; a case for sealing the electrode assembly; and tabs connected to terminals of the electrode assembly and extended outwards the case, wherein the through holes have a regular polygonal shape and the rib widths among the through holes are equal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a secondary battery, and more particularly, to enhanced collector and electrode structures and a secondary battery adopting the same. 
     2. Description of the Related Art 
     There are various types of rechargeable secondary batteries including a nickel-cadmium (Ni—Cd) battery, a lead acid battery, a nickel metal hydride (Ni-MH) battery, a lithium (Li) ion battery, a Li polymer batter, a metal Li secondary battery and the like. 
     In particular, rechargeable secondary batteries including Li have high energy density to weight ratio compared to the Ni—Cd battery or the Ni-MH battery, so their use is gradually expanding. The Li secondary battery is classified into a Li ion battery using a liquid electrolyte and a Li polymer battery using a polymer solid electrolyte, according to the type of electrolyte. 
     FIG. 1 shows an example of the Li polymer battery. Referring to FIG. 1, the Li polymer battery comprises an electrode assembly  11  in which a positive plate and a negative plate are stacked with a separator interposed therebetween, and a case  12  sealing the electrode assembly  11 . Also, electrode tabs  13  are connected to connection tabs  13   a  of the positive and negative plates, the electrode tabs  13  protruding outside the case  12 . 
     The positive and negative plates are manufactured by laminating a collector formed of copper (Cu) or aluminum (Al) with sheets respectively made of negative and positive active materials. The separator is interposed between the positive and negative plates, a plasticizer is extracted from the electrode assembly including the positive and negative plates and the separator, and then an electrolyte is injected into the empty space from which the plasticizer has been extracted. 
     The collector is formed of expanded metal in order to increase the adhesion with the active material sheets and to increase conductivity by increasing the contact area. 
     However, in manufacturing the electrode through a continuous process, the tensile force to the expanded metal acts in the same direction as that of elongation for manufacturing the expanded metal, so the collector deforms by such elongation. Thus, the expanded metal is not suitable for electrodes produced through continuous mass production systems. Also, the expanded metal is as thick as about 30 μm and the deviation in thickness thereof reaches 10 μm, so it is difficult to control the thickness when attaching the active material sheet. Also, when an electrode plate manufactured by attaching the active material sheet to a collector is cut to a predetermined size, there is burring at the cut part, thereby electrical shorting the positive and negative plates. 
     In order to solve these problems, a punched metal has been used for collectors. However, openings formed in the punched metal are very critical to the properties of the electrode assembly. That is, if the openings of the punched metal are too small, electrical conductivity is improved while smooth extraction of a plasticizer is not guaranteed. Otherwise, the electrical conductivity is reduced even though the plasticizer can be easily extracted. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an object of the present invention to provide a collector, an electrode and a secondary battery adopting the same, in which deformation of the collector, caused by a tensile force during a continuous manufacturing process, can be minimized, electrical conductivity of the collector is good and a plasticizer can be smoothly extracted from the collector. 
     According to an aspect of the object, the present invention provides a collector of a battery, comprising a thin plate member in which a plurality of through holes are formed in a predetermined pattern with a uniform rib width. 
     Preferably, the through holes have a circular or regular polygonal shape. 
     According to another aspect of the object, the present invention provides an electrode of a battery, comprising: a collector formed as a flat member in which a plurality of through holes are formed in a predetermined pattern; and an active sheet attached to at least one side of the collector, wherein assuming that the thickness of the collector is T 1  and the sum of the thicknesses of the collector and the active material sheet is T 2 , T 2 ≦6.32×T 1 . 
     Assuming that the size of the through holes is M and the sum of the thicknesses of the collector and the active material sheet is T 2 , preferably, 3.75T 2 ≦M≦18.75T 2 . 
     Assuming that the pitch between the through holes is P and the sum of the thicknesses of the collector and the active material sheet is T 2 , preferably, 1.2M≦P≦1.6M. 
     According to still another aspect of the object, the present invention provides a secondary battery comprising: an electrode assembly having electrodes obtained by attaching an active material sheet to at least one side of a flat collector with a plurality of through holes formed in a predetermined pattern, and a separator interposed between the electrodes; a case for sealing the electrode assembly; and tabs connected to terminals of the electrode assembly and extended outwards the case, wherein the through holes have a regular polygonal shape and the rib widths among the through holes are equal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
     FIG. 1 is an exploded perspective view of an example of a conventional secondary battery; 
     FIG. 2 is an exploded perspective view of a secondary battery according to the present invention; 
     FIG. 3 is a partially exploded perspective view of the electrode assembly of FIG. 2; 
     FIG. 4 is a graph showing the relationship between the thickness of the electrode and the resistance according to the present invention; 
     FIG. 5 is a graph showing the relationship between the size of through holes formed in a collector and the resistance according to the present invention; 
     FIG. 6 is a graph showing the relationship between the size of the through holes and the amount of extracted plasticizer varying with the time; 
     FIG. 7 is a graph showing the relationship between the pitch of the through holes and the resistance according to the present invention; and 
     FIG. 8 is a graph showing the relationship between the pitch of the through holes and the amount of extracted plasticizer according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, which shows a collector and a secondary battery adopting the collector according to the present invention, an electrode assembly  100  is formed by alternating a positive electrode  110  and a negative electrode  120  with a separator  130  interposed therebetween. The positive electrode  110  and the negative electrode  120  are connected to electrode tabs  101  and  102 , respectively, by conductive connection members  102   a  and  101   a . Also, the electrode assembly  100  is sealed by the case  140 , and terminals of the electrode tabs  101  and  102  are drawn out of a case  140 . 
     In the secondary battery having the above structure, the positive and negative electrodes  110  and  120  are obtained by attaching an active material sheet  115  containing an active material and an additive to the collector  111  as shown in FIG.  3 . After the positive and negative electrodes  110  and  120  are combined with the separator  130  interposed therebetween, a plasticizer is extracted from the active material sheet  115  and the separator  130 , and then an electrolyte is injected into the empty space of the resulting electrode assembly from which the plasticizer has been extracted. 
     The collector  111  is a thin plate formed of copper (Cu) or aluminum (Al), having a plurality of circular or polygonal through holes  112 . The surface of the thin plate is coated with zinc (Zn) and carbon (C) so as to improve the conductivity thereof. Also, the width of a rib  113  separating the adjacent through holes  112  and  112 ′ is uniform over the entire collector  111 . Preferably, the through holes  112  are made in the form of a regular polygon, e.g., rectangle, triangle, or hexagon. 
     FIG. 4 shows the performance of the electrode according to the thickness T 2  (see FIG. 3) of the electrodes  110  and  120 , and the thickness T 1  (see FIG. 3) of the collector  111 . Here, the thicknesses of the used punched metal was 15 μm and 25 μm. 
     The resistance of the electrodes was measured by increasing the thickness of an active material sheet attached to each collector. As a result, in the case of the electrode adopting the collector with a thickness T 1  of 15 μm, the resistance of the battery is 120 mΩ with an electrode thickness T 2  of 102 mm, which is not suitable to be used as a battery. Also, in the case of the electrode adopting the collector with a thickness T 1  of 25 μm, the resistance of the battery reaches 120 mΩ with the electrode thickness T 2  of 158 mm, which cannot be used for a battery. 
     That is, it is clear that the thickness of the electrode T 2  is approximately 7 times the thickness T 1  of the collector. 
     As a result of accurate calculation based on the above, it is preferable that the thickness T 1  of the collector has the following relationship (1) with the thickness T 2  of electrode. 
     
       
           T   2 ≦6.32× T   1   (1) 
       
     
     In addition, the smaller the size of the through holes  112  formed in the collector  111 , the larger the contact area between the collector  111  and the active material sheet  115  is, which increases conductivity. However, as the amount of the plasticizer extracted decreases, the amount of electrolyte injected into the electrode assembly decreases. Also, the size of the through holes  112  affects the tensile force of the collector and electrode. 
     In order to investigate the relationship between the size M of the through holes  112  and the conductivity of an electrode, the size M of the through holes formed in the collector  111  of an electrode with a thickness of 80 μm was varied to 0.1 mm, 0.3 mm, 0.7 mm, 1.5 mm and 2.0 mm, and then the resistance of the electrodes were measured. 
     As a result, as shown in FIG. 5, when the size M of the through hole was 0.1 mm, the conductivity of the electrode was excellent with a resistance of approximately  40  mΩ, and resistance of the electrode adopting the collector having through holes of 2.0 mm was 153 mΩ which exceeds the battery operational resistance of 120 mΩ. 
     FIG. 6 shows the amount of extracted plasticizer according to the size M of the through holes  112  in the collector  111 . As shown in FIG. 6, in the cases where the size M of the through hole  112  was 0.3 mm, 0.7 mm, 0.5 mm and 2.0 mm, a plasticizer of 90% or more was extracted within 10 minutes. Meanwhile, in the case where the size M of the through hole was 0.1 mm, the amount of plasticizer extracted reaches 90% or more after 50 minutes. 
     As can be understood from the above results, in the case of the collector  111  having through holes of 0.1 mm, the conductivity of the electrode is excellent while the amount of plasticizer extracted is small. On the other hand, in the case of the collector having through holes of 2 mm, a large amount of plasticizer is extracted while the conductivity is low. The same result as above is obtained in an electrode having a different thickness. Thus, preferably, the size of the through holes  112  in the collector  111  is in a range of 0.2˜2 mm. The size M of the through holes  112  and the thickness T 2  of the electrode have the following relationship (2). 
     
       
         3.7 T   2 ≦M≦18.75 T   2   (2) 
       
     
     The relationship between the distance between each center of the through holes  112 , that is, a pitch P, and the size M of the through holes is graphically expressed in FIGS. 7 and 8. To obtain the results, collectors each having pitches corresponding to 1.1, 1.2, 1.3, 1.5, 1.6 and 1.7 times the size M of the through holes were manufactured, and the amount of plasticizer extracted and the electrical conductivity were measured. 
     As a result, the resistance of the collector with the pitch of 1.1 M reached 150 mΩ, which exceeds the battery&#39;s critical resistance of 120 mΩ. When the pitch is in a range of 1.1 M to 1.6 M, the amount of extracted plasticizer was in a normal range as 92% (in 30 minutes). However, in the case where the pitch is 1.7 M, the amount of plasticizer extracted in 30 minutes was 87%. That is, it can be understood that it is preferable that the pitch P of the through hole  112  has a relationship (3) with the size M of the through holes. 
     
       
         1.2M≦P≦1.6M  (3) 
       
     
     In the collector according to the present invention and a secondary battery adopting the collector, the collector  111  is flat, thin, and has a uniform thickness. Through holes  112  having a polygonal shape are located at a uniform interval, thereby improving the electrical conductivity between the active material sheet and the collector. 
     Also, the pitch P of the through hole  112  is in a predetermined range, so a uniform electron transfer path is guaranteed and the plasticizer is smoothly extracted. Further, part of the active material sheet enters each through hole  112 , thereby reinforcing the adhesion between the collector and the active material sheet. 
     In particular, because the thickness T 2  of the electrode reaches six times the thickness T 1  of the collector  111 , and the size M of the through hole  112  is in a range expressed by the relationship (2), generation of cavities is prevented, which occurs due to insufficient insertion of active material into the through holes when the active material sheet is attached to the collector. 
     According to the present invention, since a plurality of through holes  112  are present in the collector  111 , continuous production is possible without deformation which is caused even by a small tensile force in the conventional case of using expanded metal. That is, the collector according to the present invention does not have through holes formed through expansion, but adopts a flat member in which a plurality of through holes are already punched in a predetermined shape. Thus, deformation by a small tensile force can be prevented. 
     According to the experimental results, the adhesion between the collector according to the present invention and the active material sheet was 15 kgf/mm which is a 50% or more improvement from that (10 kgf/mm) of the conventional collector. Also, the energy density of the battery was improved by 5˜10% or more. In addition, it is easy to extract the plasticizer from and inject the electrolyte into the electrode assembly. 
     While the present invention has been illustrated and described with reference to specific embodiments, further modifications and alterations within the spirit and scope of this invention as defined by the appended claims will become evident to those skilled in the art. For example, the collector according to the present invention can be applied to a wound electrode assembly and a corrugated electrode assembly.