Abstract:
A lithium (Li) polymer battery is provided. The Li polymer battery includes: a positive plate including a positive collector having a plurality of openings and a positive active material layer on at least one surface of the positive collector; a negative plate including a negative collector in a foil form, and a negative active material layer on at least on surface of the negative collector; and a separator between the positive and negative plates, for insulating the positive and negative plates.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lithium (Li) polymer battery, and more particularly, to a Li polymer battery having an improved negative plate structure. 
     2. Description of the Related Art 
     A rechargeable secondary battery is widely used in portable wireless electric apparatus such as camcorders, cellular phones and lap-top computers. There are various 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 battery, a metal Li secondary battery and the like. 
     In particular, the metal Li secondary battery has a driving voltage of 3.6 V, a life span longer than that of the Ni—Cd battery or the Ni—MH battery, and high energy density to weight ratio, so its 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. The Li polymer battery can be embodied into various shapes as opposed to the Li-ion battery, and has merits of high stability and light-weight. 
     FIGS. 1A and 1B shows the structure of a Bellcore Li polymer battery, a type of a Li polymer battery. Referring to FIGS. 1A and 1B a positive plate  11  and a negative plate  12  are stacked with a separator  13  interposed therebetween, each having a multilayered structure. The positive plate  11  consists of a positive collector  11   a  having a plurality of openings h 1 , and positive sheets  11   b  attached to both surfaces of the positive collector  11   a . Also, the negative plate  12  consists of a negative collector  12   a  having a plurality of openings h 2  and negative sheets  12   b  attached to both surfaces of the negative collector  12   a . The positive and negative collectors  11   a  and  12   a  are formed of expanded metal. 
     A manufacturing process of the Bellcore Li polymer battery having the above structure will be described. 
     First, acetone as a solvent, a positive or negative active material, a binder, a conductive material and a plasticizer are mixed to prepare a slurry. Then the slurry is deposited on a polyethylene (PET) base film with a doctor blade to form a thin sheet, the PET base film is removed to produce an electrode sheet of the corresponding active material. Then, the electrode sheets are laminated to both surfaces of each collector at a high temperature by applying pressure, and then cut to a predetermined size. 
     The resulting positive and negative plates  11  and  12  are combined with the separator  13  interposed therebetween, and the stacked structure is further laminated at a high temperature by applying pressure, thereby resulting in a bi-cell structure. Then, the bi-cell structure is soaked in ether to extract the plasticizer. Then, an electrode assembly is formed by stacking 9 of the bi-cells. Then, positive and negative terminals are welded onto the assembly, and electrolyte is infiltrated into the empty space from which the plasticizer has been extracted. Finally, a case is formed around the resulting product. 
     In the conventional Li polymer battery completed by the above method, the positive and negative collectors  11   a  and  12   a  require the openings h 1  and h 2  through which the plasticizer is extracted, so manufacturing the positive and negative collectors  11   a  and  12   a  is complicated. Also, adhesive forces between the positive collector  11   a  and the positive sheets  11   b  and between the negative collector  12   a  and the negative sheets  12   b  become weak due to burr generated during the formation of the openings h 1  and h 2 . In addition, the thicknesses of the positive and negative plates  11  and  12  are nonuniform. 
     Also, when attaching the electrode sheets  11   b  and  12   b  to the positive and negative collectors  11   a  and  12   a , respectively, bubbles may form in the spaces between the sheet and collector due to the openings h 1  and h 2 , thereby lowering conductivity of the positive and negative collectors  11   a  and  12   a.    
     Also, the expanded metal forming the positive and negative collectors  11   a  and  12   a  is supplied in a roll having a width of less than 300 mm. The expanded metal has the property of being extendible in a longitudinal direction by force. Thus, it is difficult to manufacture the positive and negative collectors  11   a  and  12   a  through a continuous process by applying tensile strength. 
     In general, the material of the collectors determine the cost of the battery. Thus, using the expanded metal as a material for the collectors increases the cost of the battery, compared to using punched metal. 
     Also, as the expanded metal is mechanically processed, the metal itself is subject to stress. That is, when the expanded metal is cut in a predetermined size, fine powder of arsenic acid is separated from the collector made of the expanded metal. Also, because the collector made of the expanded metal is in the form of a mesh, a burr is generated due to expansion of strands in the mesh, thereby causing an electrical short circuit. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an object of the present invention to provide a lithium (Li) polymer battery in which a negative collector is a foil without openings, thereby improving the performance of plates and the productivity of the battery. 
     Accordingly, to achieve the above object, there is provided a lithium (Li) polymer battery comprising: a positive plate including a positive collector having a plurality of openings and a positive active material layer on at least one surface of the positive collector; a negative plate including a negative collector in a foil form, and a negative active material layer on at least on surface of the negative collector; and a separator between the positive and negative plates, for insulating the positive and negative plates. 
     Preferably, the negative collector is a copper (Cu) foil. 
    
    
     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: 
     FIGS. 1A and 1B are partial sectional views of an electrode assembly of a conventional lithium (Li) polymer battery; 
     FIG. 2 is an exploded perspective view of a Li polymer battery according to the present invention; and 
     FIGS. 3A and 3B are partial sectional views of an electrode assembly shown in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, a lithium (Li) polymer battery according to the present invention comprises an electrode assembly  20 , a lower case  31  having a receiving portion  32  for holding the electrode assembly  20 , and an upper case  33  which covers the receiving portion  32  to seal the electrode assembly  20 . 
     The electrode assembly  20  is formed by repeatedly stacking a positive plate  21  and a negative plate  22  with a separator  23  interposed therebetween, and positive taps  26   a  and negative taps  24   a  extend from the positive plates  21  and the negative plates  22 , respectively. 
     The positive taps  26   a  and the negative taps  24   a  form a positive tap bundle  26  and a negative tap bundle  24  respectively, which are connected to a positive terminal  25  and a negative terminal  27  respectively. The positive terminal  25  and the negative terminal  27  are drawn out of the case when the electrode assembly  20  is put in the receiving portion  32  of the lower case  31  and the receiving portion  32  is then covered by the upper case  33 . 
     Referring to FIGS. 3A and 3B the positive plate  21  consists of a positive collector  21   a  having a plurality of openings H and a positive active material layer  21   b  on at least one surface of the positive collector  21   a . Also, the negative plate  22  consists of a negative collector  22   a  in a foil form and a negative active material layer  22   b  on at least one surface of the negative collector  22   a . The positive plate  21  is combined with the negative plate  22  with the separator  23  interposed therebetween in order to insulate the plates  21  and  22 , forming a cell. Then, such a cell is repeatedly stacked to form the electrode assembly  20 . 
     According to the present invention, preferably, the negative collector  22   a  is a copper (Cu) foil without openings, and the positive collector  21   a  is expanded metal or punched metal containing aluminum (Al), and has a plurality of openings H. 
     The positive plate  21  and the negative plate  22  are obtained by depositing a positive active material slurry and a negative active material slurry on both surfaces of positive collector  21   a  and negative collector  22   a , respectively. The negative active material slurry is prepared by dissolving (10% total volume) polyvinylidene fluoride (PVDF) as a binder in N-methyl-2-pyrrolidone (NMP) and adding (18% total volume) plasticizer, (2% total volume) carbon black as a conductive material and (70% total volume) carbon as a negative active material to the solution, and then stirring the mixture until it reaches a viscosity of 20,000-30,000 centipoise (cps). 
     Also, the positive active material slurry is prepared by dissolving (10% total volume) PVDF as a binder in acetone, adding (15-18% total volume) plasticizer, (2-5% total volume) carbon black as a conductive material and (70% total volume) LiCoO 2  as a positive active material to the solution, and then stirring the mixture until it reaches a viscosity of 20,000 cps. 
     Preferably, the prepared negative and positive active material slurries are directly applied to both surfaces of the respective negative and positive collectors  22   a  and  21   a  using a doctor blade to form coatings. 
     According to another preferred embodiment, a positive active material slurry is prepared by dissolving (10% total volume) PVDF as a binder in acetone, adding (15-18% total volume) plasticizer, (2-5% total volume) carbon black as a conductive material and (70% total volume) LiCoO 2  as a positive active material to the solution, and then stirring the mixture until it reaches a viscosity of 20,000 cps. 
     Also, a negative active material slurry is prepared by dissolving (10% total volume) PVDF as a binder in acetone, adding (15-18% total volume) plasticizer, (2-5% total volume) carbon black as a conductive material and (70% total volume) carbon as a negative active material to the solution, and then stirring the mixture until it reaches a viscosity of 20,000 cps. 
     Then, the prepared positive and negative active material slurries are made into thin sheets, e.g., using a doctor blade and then the resulting sheets are attached to both surfaces of the respective positive and negative collectors  21   a  and  22   a.    
     The amount of the materials contained in the positive and negative active material slurries can be varied without limitation to the described embodiments. 
     According to the present invention, the Cu foil without openings is adopted as the negative collector  22   a , so that electrical conductivity is improved, adhesive force during the laminating on the negative collector  22   a  with the active material layer  22   b  is strong, and the thickness of the negative plate is uniform. Also, generation of a burr or bubbles between the conventional negative collector and negative sheet, caused by the openings, can be prevented. 
     In addition, even though the negative collector  22   a  has no openings, the plasticizer contained in the negative plate  22  and the separator  23  can be extracted through the openings H of the positive collector  21   a  in the direction indicated by the arrows in FIG.  3 B. 
     Such effect of the present invention can be understood more fully through the following experiments. 
     EXPERIMENTAL EXAMPLE 1 
     The amount of plasticizer extracted from the conventional battery adopting the negative collector  2 l a  which is made of a expanded Cu and has openings was compared with that of the battery adopting Cu foil as the negative collector  22   a  a according to the present invention. Here, each electrode assembly had 9 bi-cells, and (15% total volume) the plasticizer was used for the preparation of the bi-cells. 
     As a result, the amount of plasticizer extracted from the conventional battery was approximately 3.4801 g, and that from the battery according to the present invention was approximately 3.4526 g, which were nearly the same. That is, it can be understood that the amount of plasticizer extracted from the battery is not decreased even though the Cu foil without openings is adopted as a negative collector. 
     EXPERIMENTAL EXAMPLE 2 
     A positive sheet having a thickness of approximately 80 μm and a negative sheet having a thickness of approximately 120 μm were respectively attached to the positive collector  21   a  of Al expanded metal, and the negative collector  22   a  formed of a Cu foil having a thickness of 20 μm, to obtain a battery according to the present invention. Then, the resistance of the battery was measured and compared with that of the conventional battery. 
     As a result, the resistance of the battery according to the present invention was 50-60 mΩ while that of the conventional battery was 80-100 mΩ. That is, it can be understood that the electrical conductivity of the Li polymer battery according to the present invention increases as the contact area between the electrode negative sheet and the negative collector increases. 
     Also, the adhesive force of the negative plate was increased to 18 gf/mm 2  or more compared to 10 gf/mm 2  of the conventional negative plate. As a result, the life span of the battery was increased by approximately 7% or more with respect to the conventional battery, and the energy density of the battery increased by 5-10%. 
     In addition, because a foil having a comparatively large tensile strength is used as a negative collector, manufacturing through continuous processing is possible. Also, the negative active material slurry can be applied directly to the negative collector, so that manufacture of the plate is simplified.