Abstract:
A lithium ion battery configured to yield a high energy density output by minimizing head space, i.e., wasted interior volume, within the battery case and/or by reducing electrical energy losses internal to the battery.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application 60/199,893 filed Apr. 26, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a lithium ion battery construction for producing a high energy density output. 
     BACKGROUND OF THE INVENTION 
     Various applications, such as in hybrid electric vehicles (HEV), require batteries exhibiting a high energy density output. “Energy density” refers to the ratio of the energy available from a battery to the weight or volume of the battery. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a lithium ion battery, and fabrication method therefor, configured to yield a high energy density output by minimizing head space, i.e., wasted interior volume, within the battery case and/or by reducing electrical energy losses internal to the battery. 
     A battery in accordance with the present invention includes a metal case comprised of a thin peripheral wall, e.g., cylindrical, surrounding an interior volume. In accordance with a preferred embodiment, the interior volume is substantially fully occupied by an electrode assembly comprised of a positive electrode strip, a negative electrode strip, and separator strips, superimposed on one another and helically wound to form a so called “jelly roll”. The positive electrode strip is formed of a metal substrate or foil, e.g., aluminum, having positive active material formed on both faces thereof. The negative electrode strip is formed of a metal substrate or foil, e.g., copper, having negative active material formed on both faces thereof. 
     In accordance with the invention, a set of multiple negative tabs extend from spaced locations along the negative electrode substrate toward the upper end of the case peripheral wall. A set of multiple positive spaced tabs extend from the positive electrode substrate toward the lower end of the case peripheral wall. 
     In accordance with a preferred embodiment of the invention, the negative tabs are electrically connected to a current collector, i.e., a metal ring, mounted within the interior volume adjacent to the upper end of the case peripheral wall. The metal ring is insulated from the peripheral wall, preferably by a concentric dielectric ring. 
     In further accordance with the preferred embodiment, the positive tabs are electrically connected to the peripheral wall by first folding them over the lower wall edge. A conductive end cap is mounted against the peripheral wall lower edge to pinch and electrically connect the positive tabs therebetween. The end cap is then sealed to the case peripheral wall, e.g., by laser welding, to completely seal the battery lower end and electrically connect the positive tabs to the case peripheral wall. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 comprises a cutaway side view of a battery in accordance with the present invention; 
     FIG. 2 comprises an enlarged sectional view depicting multiple layers of an electrode assembly or “jelly roll” contained within the battery case of FIG. 1; 
     FIG. 3 depicts a portion of an electrode strip employed in the jelly roll of FIG. 2 showing the electrode substrate, the active material thereon, and multiple tabs extending outward from the substrate; 
     FIG. 4 is an enlarged cutaway side view of the upper end of the battery of FIG. 1 showing multiple tabs extending from the jelly roll assembly and electrically connected to a metal ring insulated from the battery case peripheral wall by a dielectric ring; 
     FIG. 5 is an isometric view showing the upper end of the battery similar to FIG. 4 but with the end cap displaced; and 
     FIG. 6 is an isometric view showing the lower end of the battery having multiple tabs extending from the jelly roll and folding over the peripheral wall bottom edge for capture by an end cap which is then sealed to the peripheral wall bottom edge. 
    
    
     DETAILED DESCRIPTION 
     The following text describes a presently contemplated preferred embodiment for practicing the invention. The description of the preferred embodiment should not be interpreted as a limitation on the scope of the invention which is defined by the accompanying claims. 
     FIG. 1 illustrates the preferred construction of a battery in accordance with the present invention for producing a high energy density output. The battery is comprised of a case  12  including a peripheral wall  14  formed of a thin sheet of metal, e.g., aluminum. The peripheral metal wall  14  preferably, but not necessarily, forms a cylindrical tube, having an open upper end  16  and open lower end  18 . The peripheral wall  14  defines an interior volume  20  which is to be essentially fully occupied by an electrode assembly  22 , preferably configured as a so called “jelly roll”, in accordance with the assembly procedure to be described hereinafter. 
     The jelly roll assembly  22  is comprised of a negative electrode strip, a positive electrode strip, and separator strips, superimposed and helically wound together. More particularly, attention is directed to FIG. 2 which illustrates a cross sectional portion of the jelly roll assembly  22 . It can be seen that the assembly is comprised of a first polarity (e.g., negative) electrode strip  30 , a second polarity (e.g., positive) electrode strip  32  and separator strips  34 . The negative electrode strip  30  is comprised of an elongate metal substrate  40 , e.g., copperfoil, having negative active material  42  deposited on both faces of the foil substrate  40 . Similarly, the positive electrode strip  32  is comprised of an elongate metal substrate  44 , e.g., aluminum foil, having positive active material  46  deposited on both faces thereof. Dielectric separator strips, e.g., polyethylene, separate adjacent electrode strip layers. 
     Jelly roll assemblies as thus far described are well known in the art, as are suitable positive and negative active materials and substrates. The present invention is directed to a preferred construction for maximizing the energy density output from a battery utilizing a jelly roll assembly of known materials. In accordance with a first aspect of the invention, as schematically represented in FIG. 2, multiple positive and negative tabs extend in opposite directions from the electrode strip substrates and are respectively electrically connected to the case and a negative end cap. For example, note that FIG. 2 depicts tabs  50  and  52  extending upwardly from the negative substrate  40  and connected together by a current collector  54 . Similarly, FIG. 2 schematically depicts multiple tabs  56  and  58  extending downwardly from positive electrode substrate  44  for connection to a current collector  60 . The respective current collectors  54  and  60  can be externally connected to provide power to a load represented at  62 . 
     Attention is now directed to FIG. 3 which illustrates a portion of an electrode strip in accordance with the invention for use in the jelly roll assembly  22 . The electrode strip of FIG. 3 is depicted as comprising a negative metal substrate  40  having spaced multiple tabs  50 ,  52  etc electrically connected thereto and extending outwardly therefrom. The tabs can be formed integral with the elongate metal foil forming the substrate  40  or can be separate metal members connected to the foil  40  as by welding. FIG. 3 also depicts the negative active material  42  affixed to the face of the substrate  40 . The positive electrode strip is constructed similarly to the negative electrode strip illustrated in FIG. 3 except that its tabs  56 ,  58  extend in a direction opposite to the direction of tabs  50  and  52 . 
     Attention is now directed to FIG. 4 which illustrates the upper end of the battery case  12  in greater detail. Note that the peripheral wall  14  defines an inwardly extending annular shoulder  68  proximate to its upper end. Prior to inserting the electrode assembly  22  into the case  12 , a dielectric ring, e.g., PFA,  70  is inserted into the interior case volume from the peripheral wall open lower end  18 . The dielectric ring  70  defines a shoulder  72  which is seated flush against the annular shoulder  68 . More specifically, the dielectric ring  70  is dimensioned so that its outer diameter  74  is press-fit against the inner diameter  76  at the upper end of the peripheral wall  14 . The press-fit between the dielectric ring  70  and the inner surface of the peripheral wall  14  forms a seal preventing any leakage therepast. 
     After the dielectric ring  70  is installed at the upper end of the peripheral wall  14 , a metal ring  78 , e.g., nickel, is inserted from the open lower end  18  of the peripheral wall and concentrically seated against the dielectric ring  70 . More particularly, the outer surface  80  of ring  78  is press fit and seals against the inner surface  81  of dielectric ring  70 . More over, shoulder  82  of metal ring  78  seats against shoulder  84  of dielectric ring  70 . Note that the metal ring  78  defines an inwardly extending flange  86  having an upper surface  88 . 
     After the metal ring  78  and dielectric ring  70  have been installed at the upper end of the peripheral wall  14 , the aforedescribed jelly roll assembly  22  is inserted into the case  12  from the open lower end  18 . The upwardly extending negative tabs  50 ,  52  are formed to extend through a central opening  90  in annular flange  86 . The negative tabs  50 ,  52  are then electrically secured, as by welding, to the ring surface  88 . 
     The positive tabs  56 ,  58 , etc. extend outwardly form the lower end  18  of case  12 . The lower end  18  terminates at a bottom wall edge  94 . The positive tabs  56 ,  58  etc. are folded over the edge  94  as depicted in FIG.  6 . Thereafter, an end cap  96  is mounted on the edge  94 , pinching the positive tabs therebetween. The end cap  96  is preferably aluminum and is welded to the bottom edge  94  thereby electrically connecting the positive tabs to the case peripheral wall  14 . Protruding tab material is then trimmed to the end cap circumference. The end cap  96  is preferably formed with an annular shoulder  98  which bears against the bottom edge  94  of the wall  14  enabling the ready formation of a leak free seal by various techniques, such as laser welding. 
     After the lower open end of the casing  12  is sealed by the end cap  96 , electrolyte can be inserted into the battery via the aforementioned central opening  90  in metal ring flange  86 . A large end cap  100  defining an annular shoulder  102  is sealed against annular shoulder  104  on metal ring  78 . The end cap  100  defines a central opening  106 , which is then sealed by minor end cap  108 . 
     From the foregoing, it should now be recognized that a battery construction has been described herein which minimizes the amount of wasted space within the battery case and insures that substantially the entire interior volume sealed with the case is available and can be used for accommodating an electrode assembly. Wasted interior space is minimized by directly connecting a plurality of first polarity tabs extending from the electrode assembly to a current collection ring adjacent the upper end of the case. A plurality of second polarity tabs is directly connected to a lower edge of the case thereby assuring that substantially the entire volume is available for accommodating the electrode assembly. This construction yields a high energy density output which is further enhanced by the utilization of multiple tabs which reduces the internal resistance of the battery. 
     While the invention has been described with reference to a specific preferred embodiment, it should be recognized that numerous modifications and variations will occur to those skilled in the art without departing from the spirit and scope of the invention defined by the appended claims.