Abstract:
An electrode assembly and a rechargeable battery using the same include a positive electrode including a positive current collector and a positive active material on the positive current collector; a negative electrode including a negative current collector and a negative active material on the negative current collector; an outer electrode including an outer current collector and an outer active material on the outer current collector, wherein the outer current collector includes an outer surface facing away from an outer separator and an inner surface facing toward the outer separator, and the outer active material is on both the outer surface and the inner surface, and wherein a thickness of the outer active material is less than at least one of a thickness of the positive active material or a thickness of the negative active material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/261,724, filed on Nov. 16, 2009 in the United States Patent and Trademark Office, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Aspects of embodiments of the present invention relate to an electrode assembly and a rechargeable battery using the same, and more particularly, to an outer electrode of an electrode assembly. 
         [0004]    2. Description of the Related Art 
         [0005]    A rechargeable battery can be recharged and discharged, unlike a primary battery that cannot be recharged. A low-capacity rechargeable battery is typically used for a small portable electronic device such as a mobile phone, a laptop computer, and a camcorder, and a large-capacity rechargeable battery is typically used as an electrical source or a power storage source for driving a motor, such as for a hybrid vehicle. 
         [0006]    A large-capacity, high-power rechargeable battery with high energy density using a non-aqueous electrolyte has been developed, and the rechargeable battery is formed with a large-capacity, high-power rechargeable battery module by coupling a plurality of rechargeable batteries in series or in parallel in order to use it to drive a device, such as a motor for an electric vehicle requiring a large amount of electric power. 
         [0007]    The rechargeable battery may be fabricated in a cylindrical shape, a prismatic shape, a pouch shape, and the like. In a typical rechargeable battery, active material layers are coated on both sides of positive and negative electrodes. 
         [0008]    Outer active material layers of outermost electrodes of a stacked-type rechargeable battery do not participate in charge and discharge reaction and are not needed. 
         [0009]    However, if the unnecessary outer active material layer is removed, electrodes may be bent during compressing after coating. During electrode fabrication, a one-side coating process and assembling process related thereto are added to a general fabrication process. In consideration of the above, an outermost electrode generally includes active material layers coated on both sides of the electrode. However, this may incur an increase of weight and volume, resulting in a decrease of energy density. It also presents a problem of waste of active materials. 
         [0010]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the described technology and, therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY 
       [0011]    According to an aspect of embodiments of the present invention, a rechargeable battery has high energy density. 
         [0012]    According to one embodiment of the present invention, an electrode assembly includes: a positive electrode including a positive current collector and a positive active material on the positive current collector; a negative electrode including a negative current collector and a negative active material on the negative current collector; an outer electrode including an outer current collector and an outer active material on the outer current collector, one of the positive electrode or the negative electrode being between the outer electrode and the other of the positive electrode or the negative electrode; an inner separator between the positive electrode and the negative electrode; and an outer separator between the outer electrode and the one of the positive electrode or the negative electrode, wherein the outer current collector includes an outer surface facing away from the outer separator and an inner surface facing toward the outer separator, and the outer active material is on both the outer surface and the inner surface, and wherein a thickness of the outer active material is less than at least one of a thickness of the positive active material or a thickness of the negative active material. 
         [0013]    In one embodiment, the thickness of the outer active material is about one half of at least one of the thickness of the positive active material or the thickness of the negative active material. 
         [0014]    The outer current collector may include a sheet having a plurality of openings. The sheet may be a mesh-type sheet. 
         [0015]    At least one of the positive current collector or the negative current collector may include a sheet having a plurality of openings. The outer current collector may include a sheet having a plurality of openings larger than the openings of the at least one of the positive current collector or the negative current collector. 
         [0016]    At least one of the positive current collector or the negative current collector may include a sheet that does not have openings. 
         [0017]    In one embodiment, the outer electrode is a first outer electrode, and the electrode assembly further includes a second outer electrode, the positive electrode and the negative electrode being between the first and second outer electrodes. 
         [0018]    A thickness of the outer active material on the outer surface may be different than a thickness of the outer active material on the inner surface. 
         [0019]    The thickness of the outer active material on one of the outer surface or the inner surface may be between about 1 to about 10 times of the thickness of the outer active material on the other of the outer surface or the inner surface. In one embodiment, the thickness of the outer active material on the one of the outer surface or the inner surface is between about 1 to about 3 times of the thickness of the outer active material on the other of the outer surface or the inner surface. 
         [0020]    According to another embodiment of the present invention, a rechargeable battery includes: an electrode assembly including a positive electrode including a positive current collector and a positive active material on the positive current collector; a negative electrode including a negative current collector and a negative active material on the negative current collector; an outer electrode including an outer current collector and an outer active material on the outer current collector, one of the positive electrode or the negative electrode being between the outer electrode and the other of the positive electrode or the negative electrode; an inner separator between the positive electrode and the negative electrode; and an outer separator between the outer electrode and the one of the positive electrode or the negative electrode, wherein the outer current collector includes an outer surface facing away from the outer separator and an inner surface facing toward the outer separator, and the outer active material is on both the outer surface and the inner surface, and wherein a thickness of the outer active material is less than at least one of a thickness of the positive active material or a thickness of the negative active material; and a case containing the electrode assembly therein. 
         [0021]    According to another embodiment of the present invention, an electrode assembly includes a positive electrode including a positive active material coated on a positive current collector, a negative electrode including a negative active material coated on a negative current collector, and a separator interposed between the positive and negative electrodes. In one embodiment, the outermost electrode disposed on the outermost side among the positive electrodes and the negative electrodes includes a mesh outer current collector and an active material layer having less thickness than a thickness of an active material of a positive electrode or a negative electrode disposed inside. 
         [0022]    According to another embodiment of the present invention, a rechargeable battery includes an electrode assembly including a positive electrode including a positive active material coated on a positive current collector, a negative electrode including a negative active material coated on a negative current collector, and a separator interposed between the positive and negative electrodes; a case housing the electrode assembly; and a terminal electrically connected with the electrode assembly and exposed outside of the case, wherein the outermost electrode disposed on the outermost side of the positive electrode and the negative electrode in the electrode assembly includes a mesh current collector and an active layer formed on the mesh current collector and having a volume less than that of the positive electrode and the negative electrode disposed inside. 
         [0023]    According to another aspect of embodiments of the present invention, an energy density is improved, and the unnecessary raw material waste is prevented or reduced by forming an active material layer having a decreased thickness on the outermost electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The above and other features and advantages of the present invention will become more apparent by describing in detail some exemplary embodiments thereof with reference to the attached drawings. 
           [0025]      FIG. 1  is a cross-sectional view of a rechargeable battery according to an embodiment of the present invention. 
           [0026]      FIG. 2  is a cross-sectional view of an electrode assembly of a rechargeable battery according to another embodiment of the present invention. 
           [0027]      FIG. 3  is a cross-sectional view of an electrode assembly of a rechargeable battery according to another embodiment of the present invention. 
           [0028]      FIG. 4  is a cross-sectional view of an electrode assembly of a rechargeable battery according to another embodiment of the present invention. 
           [0029]      FIG. 5  is a cross-sectional view of an electrode assembly of a rechargeable battery according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF REFERENCE NUMERALS INDICATING SOME ELEMENTS IN THE DRAWINGS 
       [0000]    
       
         
           
               100 : rechargeable battery 
               10 ,  30 ,  40 ,  50 : electrode assembly 
               11 ,  31 ,  41 ,  51 : positive electrode 
               11   a ,  31   a ,  41   a ,  51   a : positive current collector 
               11   b ,  31   b ,  41   b ,  51   b : positive active material layer 
               11   c : positive uncoated region 
               12 ,  32 ,  42 ,  52 : negative electrode 
               12   a ,  32   a ,  42   a ,  52   a : negative current collector 
               12   b ,  32   b ,  42   b ,  52   b : negative active material layer 
               12   c : negative uncoated region 
               13 ,  33 ,  43 ,  53 : separator 
               14 ,  34 ,  44 ,  54 : outermost electrode 
               14   a ,  34   a ,  44   a ,  54   a : outer current collector 
               14   b ,  34   b ,  44   b ,  54   b : outer active material layer 
               20 : case 
               21 : positive terminal 
               22 : negative terminal 
           
         
       
     
       DETAILED DESCRIPTION 
       [0047]    Some exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, embodiments of the present invention may be embodied in different forms and should not be construed as limited to the exemplary embodiments illustrated and set forth herein. Rather, these exemplary embodiments are provided by way of example for understanding of the invention and to convey the scope of the invention to those skilled in the art. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Like reference numerals designate like elements throughout the specification and drawings. 
         [0048]      FIG. 1  is a cross-sectional view of a rechargeable battery according to an embodiment of the present invention, and  FIG. 2  is a cross-sectional view of an electrode assembly of a rechargeable battery according to another embodiment of the present invention. 
         [0049]    Referring to  FIG. 1 , a rechargeable battery  100  includes an electrode assembly  10  for performing charge and discharge, and a case  20  housing the electrode assembly  10 . 
         [0050]    The case  20 , in one embodiment, forms an external housing for the rechargeable battery  100  and provides a space for housing, or containing, the electrode assembly  10  therein. For example, the case  20  may be formed in a pouch shape of film or in a cuboid prismatic shape having an opening for receiving the electrode assembly  10  (e.g., an electrode assembly having a shape corresponding to the cuboid case). In one embodiment, the case  20  may be formed of a metal, such as aluminum, aluminum alloy, nickel-plated steel, or the like, or alternatively, as a pouch of laminate film, or of any other suitable material. In one embodiment, a cap assembly  25  is mounted to the opening of the case  20 , thereby sealing the case  20 . 
         [0051]    In one embodiment, a positive terminal  21  and a negative terminal  22  are electrically connected to the electrode assembly  10 , and the positive terminal  21  and the negative terminal  22  are protruded outside the case  20 . The positive terminal  21  and the negative terminal  22 , in one embodiment, are fixed by the cap assembly  25 . 
         [0052]    In one embodiment, the positive terminal  21  is electrically connected to a positive uncoated region  11   c  of the electrode assembly  10 , and the negative terminal  22  is electrically connected to a negative uncoated region  12   c  of the electrode assembly  10 . Further, in one embodiment, the positive terminal  21  and the negative terminal  22  protrude outside the case  20 , a sealing member  23  is mounted between the positive terminal  21  and the case  20 , and another sealing member  23  is mounted between the negative terminal  22  and the case  20 . The positive terminal  21  and the negative terminal  22  may protrude in the same direction without limitation, or may protrude in opposite directions from each other. 
         [0053]    With reference to  FIG. 2 , the electrode assembly  10  includes a positive electrode  11 , a negative electrode  12 , and a separator  13  disposed between the positive electrode  11  and the negative electrode  12 . Further, in one embodiment, the electrode assembly  10  has a structure in which a plurality of positive electrodes  11  and negative electrodes  12  are laminated together in a stacked structure with a separator  13  between each positive electrode  11  and negative electrode  12 . 
         [0054]    The positive electrode  11 , in one embodiment, has a structure including positive active material layers  11   b  formed on both surfaces of a positive current collector  11   a . The positive current collector  11   a , in one embodiment, is formed of aluminum, stainless steel, or the like, or any other suitable material, and formed in a mesh square sheet, or any other suitable shape, having a plurality of openings or pores formed therein. The openings, in one embodiment, are diamond-shaped and have a length along a long diagonal line of 0.8 to 1.6 mm and a width along a short diagonal line of 0.4 to 1.2 mm. In one embodiment, the length along the long diagonal line is about 1.2 mm and the width along the short diagonal line is about 0.8 mm. However, embodiments of the present invention are not limited thereto. That is, in other embodiments, the openings may have any other suitable shape and size. The positive active material layer  11   b , in one embodiment, includes LiCoO 2 , LiMnO 2 , LiFePO 4 , LiNiO 2 , LiMn 2 O 4 , or the like, a conductive agent, a binder, or the like. The positive active material layer  11   b , in one embodiment, is coated on the positive current collector  11   a , or in other embodiments, may be attached thereon using a lamination method or any other suitable method. 
         [0055]    In one embodiment, a positive uncoated region  11   c  where the positive current collector  11   a  is exposed, rather than the positive active material layer  11   b  being formed thereon, is formed on the positive electrode  11 . 
         [0056]    The negative electrode  12 , in one embodiment, has a structure including negative active material layers  12   b  formed on both surfaces of a negative current collector  12   a . The negative current collector  12   a , in one embodiment, is formed of copper, stainless steel, aluminum, or the like, or any other suitable material, and formed in a mesh square sheet, or any other suitable shape, having a plurality of openings or pores formed therein. The openings may be configured as described above with respect to the openings of the positive current collector  11   a . The negative active material layer  12   b , in one embodiment, includes Li 4 Ti 5 O 12  or a carbon-based active material, a conductive agent, a binder, or the like. In one embodiment, a negative uncoated region  12   c  where the negative current collector  12   a  is exposed, rather than the negative active material layer  12   b  being formed thereon, is formed on the negative electrode  12 . The negative active material layer  12   b , in one embodiment, is coated on the negative current collector  12   a , or in other embodiments, may be attached thereon using a lamination method or any other suitable method. 
         [0057]    The separator  13 , in one embodiment, is formed of a porous material for insulating between the positive electrode  11  and the negative electrode  12  and providing a passage for ions. 
         [0058]    In one embodiment, as shown in  FIG. 2 , a plurality of positive electrodes  11  and negative electrodes  12  are stacked and laminated with a separator  13  between each pair of a positive electrode  11  and an adjacent negative electrode  12 . Further, an outermost electrode  14  is disposed on the outermost side of electrode assembly  10 . According to one embodiment, the outermost electrode  14  is disposed adjacent to the negative electrode  12  with the separator  13  therebetween (see  FIG. 2 ), so the outermost electrode  14  acts as a positive electrode. However, the present invention is not limited thereto and, in other embodiments, the outermost electrode  14  may be formed to act as a negative electrode. 
         [0059]    The outermost electrode  14 , in one embodiment, includes an outer current collector  14   a  and outer active material layers  14   b  formed on both surfaces of the outer current collector  14   a . Further, in one embodiment, the outer active material layer  14   b  of the outermost electrode  14  has a thickness that is less than that of the positive active material layer  11   b . In one embodiment, the outer current collector  14   a  and the outer active material layers  14   b  of the outermost electrode  14  are respectively formed of the same material as in the positive current collector  11   a  and the positive active material layer  11   b . In one embodiment, the outer current collector  14   a  includes a mesh sheet having a plurality of openings that are larger in size than openings of the positive current collector  11   a . The outer active material layer  14   b , in one embodiment, has a thickness of about one half of the thickness of the positive active material layer  11   b . In other words, the amount of the outer active material layer  14   b , in one embodiment, is the amount capable of reacting with the negative active material layer  12   b  disposed on one surface of the negative electrode  12  disposed adjacent the outermost electrode  14  with the separator  13  interposed therebetween, so as to prevent or reduce unnecessary cost associated with a thickness of an outer active material layer exceeding an amount capable of reacting. In one embodiment, the thickness of the positive active material layer  11   b , and also a thickness of the negative active material layer  12   b , is 240 to 440 μm, and further, in one embodiment, is 320 to 340 μm. Also, in one embodiment, the thickness of the outer active material layer  14   b  is 120 to 220 μm, and further, in one embodiment, is 160 to 170 μm. However, embodiments of the present invention are not limited to the above-described thicknesses and, in other embodiments, the positive active layer  11   b , the negative active layer  12   b , and the outer active layer  14   b  may have any other suitable thicknesses. 
         [0060]    Further, in the above-described embodiment, when the outer active material layer  14   b  of outermost electrode  14  has a thickness of about one half of the thickness of the positive active material layer  11   b  of the positive electrode  11 , the amount of the outer active material layer  14   b  reacting with the negative electrode  12  disposed closely inside the outermost electrode  14  correspond to each other, so the volume is not increased unnecessarily. 
         [0061]    In addition, in one embodiment when the outer current collector  14   a  of the outermost electrode  14  is formed of a mesh structure, the outer active material layers  14   b  disposed on both surfaces of the outer current collector  14   a  may participate in the charge and discharge reaction. In addition, in the above-described embodiment, because the outer active material layers  14   b  are formed on both surfaces of the outer current collector  14   a , distortion of the outermost electrode  14  is prevented or reduced. 
         [0062]      FIG. 3  is a cross-sectional view of an electrode assembly of a rechargeable battery according to another embodiment of the present invention. 
         [0063]    Referring to  FIG. 3 , the electrode assembly  30  according to one exemplary embodiment includes a positive electrode  31 , a negative electrode  32 , a separator  33  disposed between the positive electrode  31  and the negative electrode  32 , and an outermost electrode  34  disposed on an outermost side of the electrode assembly  30 . 
         [0064]    According to one embodiment, a plurality of positive electrodes  31  and negative electrodes  32  are alternately stacked and laminated with a separator  33  interposed between each pair of adjacent positive and negative electrodes  31 ,  32 , and two outermost electrodes  34  are disposed on the outermost sides of the electrode assembly  30 . The outermost electrode  34  according to one embodiment, as shown in  FIG. 3 , is disposed adjacent to the negative electrode  32  with the separator  33  therebetween, and thus, the outermost electrode  34  acts as a positive electrode. 
         [0065]    The positive electrode  31 , in one embodiment, includes a positive current collector  31   a  and positive active material layers  31   b  formed on both surfaces of the positive current collector  31   a . The positive current collector  31   a , in one embodiment, is formed of a quadrangle flat sheet having no openings. The negative electrode  32 , in one embodiment, includes a negative current collector  32   a  and negative active material layers  32   b  formed on both surfaces of the negative current collector  32   a . The negative current collector  32   a , in one embodiment, is formed of a quadrangle flat sheet having no openings. 
         [0066]    The outermost electrode  34 , in one embodiment, includes an outer current collector  34   a  and outer active material layers  34   b  formed on both surfaces of the outer current collector  34   a , and the outer current collector  34   a  is formed of a mesh flat sheet having a plurality of openings. The outer active material layers  34   b  disposed on both surfaces of the outermost electrode  34  have a thickness less than a thickness of the positive active material layer  31   b . Further, in one embodiment, the outer active material layer  34   b  has a thickness of about one half of the thickness of the positive active material layer  31   b  of the positive electrode. 
         [0067]    According to the above-described embodiment, the positive electrode  31  and the negative electrode  32  disposed inside the outermost electrode  34  include a general current collector (e.g., a sheet that does not have openings) instead of a mesh current collector, but the outermost electrode  34  includes a mesh current collector. When the outermost electrode  34  is configured having the mesh current collector, the outer active material layers  34   b  disposed on both surfaces of the outer current collector  34   a  may participate in the charge and discharge so that the outer active material layer  34   b  may have a thin thickness. 
         [0068]      FIG. 4  is a cross-sectional view of an electrode assembly of a rechargeable battery according to another embodiment of the present invention. 
         [0069]    Referring to  FIG. 4 , an electrode assembly  40  according to one exemplary embodiment includes a positive electrode  41 , a negative electrode  42 , a separator  43  disposed between the positive electrode  41  and the negative electrode  42 , and an outermost electrode  44  disposed on an outermost side of the electrode assembly  40 . 
         [0070]    In one embodiment, a plurality of positive electrodes  41  and negative electrodes  42  are alternately stacked and laminated with a separator  43  between each pair of adjacent positive and negative electrodes  41 ,  42 , and two outermost electrodes  44  are disposed on the outermost sides of the electrode assembly  40 . The outermost electrode  44  according to one embodiment, as shown in  FIG. 4 , is disposed adjacent to the negative electrode  42  with a separator  43  therebetween, and the outermost electrode  44  acts as a positive electrode. 
         [0071]    The positive electrode  41 , in one embodiment, includes a positive current collector  41   a  having a mesh structure and positive active material layers  41   b  formed on both surfaces of the positive current collector  41   a . The negative electrode  42 , in one embodiment, includes a negative current collector  42   a  having a mesh structure and negative active material layers  42   b  formed on both surfaces of the negative current collector  42   a.    
         [0072]    The outermost electrode  44 , in one embodiment, includes an outer current collector  44   a  and outer active material layers  44   b  formed on both surfaces of the outer current collector  44   a , and the outer current collector  44   a  is formed of a mesh flat sheet having a plurality of openings. 
         [0073]    According to one embodiment, the outer active material layers  44   b  disposed on both surfaces of the outermost electrode  44  have a thickness less than a thickness of the positive active material layer  41   a  disposed on both surfaces of the positive electrode  41 . Further, in one embodiment, the outer active material layer  44   b  of the outermost electrode  44  has a thickness of about one half of the thickness of the positive active material layer  41   b  of the positive electrode  41 . 
         [0074]    According to one embodiment, where a thickness of an outer active material layer  44   ba  disposed between the outer current collector  44   a  and the separator  43  is D 1 , and a thickness of an outer active material layer  44   bb  disposed on the outer side of outer current collector  44   a  is D 2 , in the outermost electrode  44 , D 1 /D 2  is 0.1 to 10. In other words, the active material layers  44   ba  and  44   bb  disposed on both surfaces of the outer current collector  44   a  in the outermost electrode  44  may be formed to have the same thickness, or one of the active material layers  44   ba ,  44   bb  may be up to about 10 times thicker than the other one of the active material layers  44   ba ,  44   bb . In one embodiment, D 1 /D 2  is ⅓ to 3. Further, in one embodiment, D 1 /D 2  is 0.5 to 2. 
         [0075]      FIG. 5  is a cross-sectional view of an electrode assembly of a rechargeable battery according to another embodiment of the present invention. 
         [0076]    Referring to  FIG. 5 , the electrode assembly  50  according to one exemplary embodiment includes a positive electrode  51 , a negative electrode  52 , a separator  53  disposed between the positive electrode  51  and the negative electrode  52 , and an outermost electrode  54  disposed on an outermost side of the electrode assembly  50 . 
         [0077]    In one embodiment, a plurality of positive electrodes  51  and negative electrodes  52  are alternately stacked and laminated with a separator  53  between each pair of adjacent positive and negative electrodes  51 ,  52 , and two outermost electrodes  54  are disposed on the outermost sides of the electrode assembly  50 . The outermost electrode  54  according to one embodiment, as shown in  FIG. 5 , is disposed adjacent to the negative electrode  52  with a separator  53  therebetween, and thus, the outermost electrode  54  acts as a positive electrode. 
         [0078]    The positive electrode  51 , in one embodiment, includes a positive current collector  51   a  and positive active material layers  51   b  formed on both surfaces of the positive current collector  51   a . The positive current collector  51   a , in one embodiment, is formed of a quadrangle flat sheet having no openings. The negative electrode  52 , in one embodiment, includes a negative current collector  52   a  and negative active material layers  52   b  formed on both surfaces of the negative current collector  52   a . The negative current collector  52   a , in one embodiment, is formed of a quadrangle flat sheet having no openings. 
         [0079]    The outermost electrode  54 , in one embodiment, includes an outer current collector  54   a  and outer active material layers  54   b  formed on both surfaces of the outer current collector  54   a , and the outer current collector  54   a  is formed of a mesh flat sheet having a plurality of openings. According to one embodiment, the outer active material layers  54   b  disposed on both surfaces of the outermost electrode  54  have a thickness less than a thickness of the positive active material layer  51   b . Further, in one embodiment, the outer active material layer  54   b  has a thickness of about one half of the thickness of the positive active material layer  51   b  of the positive electrode. 
         [0080]    According to one embodiment, where a thickness of an outer active material layer  54   ba  disposed between the outer current collector  54   a  and the separator  53  is D 3 , and a thickness of an outer active material layer  54   bb  disposed on the outer side of outer current collector  54   a  is D 4 , in the outermost electrode  54 , D 3 /D 4  is 0.1 to 10. In other words, the active material layers  54   ba  and  54   bb  disposed on both surfaces of the outer current collector  54   a  in the outermost electrode  54  may be formed to have the same thickness, or one of the active material layers  54   ba ,  54   bb  may be up to about 10 times thicker than the other one of the active material layers  54   ba ,  54   bb . In one embodiment, D 3 /D 4  is ⅓ to 3. Further, in one embodiment, D 3 /D 4  is 0.5 to 2. 
         [0081]    While this disclosure has been described in connection with what is presently considered to be some exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.