Abstract:
A high discharge rate electrochemical battery structure is disclosed as having a perforated disc electrode stack comprising pluralities of disc electrodes respectively connected to a pair of busbars. Each disc electrode has a tab with two fins which are bent at 90 degrees to the disc electrode. Each fin is inserted between two parts of the corresponding busbar and welded thereto. The design of the busbars and disc electrodes provides for redundancy of electrical contact, high thermal and electrical conductivity and improved resistance to mechanical shock.

Description:
This invention was made with Government support under Contract No. 957991 awarded by the Jet Propulsion Laboratories. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to portable power sources such as batteries and, more particularly, to a structural configuration designed especially for lithium batteries. 
     Description of the Related Art 
     For a number of years there has been a need for storage cells or batteries having high energy density, high-rate discharge capability, long life and an ability to operate under a wide range of temperature extremes. 
     Lithium batteries have thus become of widespread interest of late for their excellent characteristics in all of the above categories when compared to the known magnesium, alkaline and carbon zinc batteries. Just taking temperature performance as an example, at -40 degrees C., a lithium-vanadium pentoxide battery maintains 73% of its room temperature (25 degrees C.) performance and a lithium-sulphur dioxide battery, 60%, while typical magnesium, alkaline and carbon-zinc batteries are virtually useless at that temperature. 
     Shelf-life at elevated temperatures likewise is superior for lithium batteries. At 54 degrees C., the projected shelf life of a lithium battery exceeds 12 months, while that of a mercury battery is only four months. A magnesium battery has a projected shelf life of only seven months at that storage temperature, while carbon-zinc batteries have a projected shelf life of only 1.5 months. 
     Lithium organic cells offer higher energy density, superior cold temperature performance, longer active life and greater cost effectiveness. Thus, clearly, lithium batteries are superior to other known types of batteries and it would be highly desirable to maximize their performance characteristics. 
     A typical prior-art construction of a lithium cell is different from that of mercury and alkaline manganese cells. A lithium foil anode, a separator and a carbonaceous cathode are spirally wound together. This assembly is placed in a steel case, and the anode and cathode are connected with welded tabs to the case and top assembly. Since the electrolyte is non-aqueous, there is no hydrogen gas evolved during discharge. The assembly contains a vent to prevent the build-up of high internal gas pressure resulting from improper use or disposal. 
     Today there is an increasing demand for high rate performance lithium cells and batteries. The low ionic conductivity of electrolyte used in lithium cells provides very little flexibility for improvement in rate capability. The high rate discharge capability can only be created by special design of the electrodes and their interconnections. High rate cell designs can be achieved by increasing the geometrical electrode surface area per unit volume so that the specific current density does not exceed limitations set forth by the conductivity of the electrolyte. One of the most common electrode designs for small cylindrical high rate cells is the wound electrode structure where long strips of anodes, cathodes, and separators are rolled around a common shaft to form a coil which then is placed into a cylindrical container. The wound configuration of electrodes seems to be an easy and simple electrode arrangement which adapts to high volume production. Some of the disadvantages lie in the ohmic losses along the electrodes, which are several feet in length. 
     A more efficient design with lower ohmic losses and heat generation is the disc electrode which can be used for higher discharges than the wound design because of shorter current paths and higher conductivities within the metallic substrates. The novel construction of such an electrode and the associated busbar assembly is among the subject matter of this invention. While disclosed in the context of a lithium battery, it should be understood that the novel features of the invention may find application in other types of batteries as well; for example: mercury, nickel cadmium, and magnesium, to name a few. The structural configuration of the present invention may be utilized in various types of batteries other than lithium to develop more rugged units which are subject to shock, vibration and extreme environmental factors. 
     SUMMARY OF THE INVENTION 
     In brief, one particular arrangement in accordance with the invention comprises a lithium battery construction with a disc-type substrate having one or two tabs which are 180 degrees offset. The round disc substrate is perforated on either side of a non-perforated bridge connecting the tab(s). A solid border surrounds the entire substrate to prevent sharp edges from being exposed. The two half-moon shaped perforated areas are approximately 50% open and allow for electrolyte diffusion from one side of the electrode to the other. A plurality of the disc electrodes are assembled in an electrode stack which consists of many individual electrode pairs respectively connected in parallel with common busbars for anodes and cathodes. 
     In accordance with one particular aspect of the invention, a lithium battery incorporates a busbar electrode contact design which minimizes IR losses and simplifies the assembly process of the multielectrode stacks, while achieving high rate discharge capability. 
     In one particular arrangement in accordance with the invention, the electrode tab is connected to the busbar by fusion welding. The tab is designed to have two fins which are bent inwardly by 90 degrees and each is welded separately to the bus. The fins are designed so that each can carry the maximum current coming from the attached electrodes. Redundancy in contacts will enhance the safety of the cell and assures that all electrodes can be fully discharged to 100% depletion of lithium. 
     In one preferred embodiment, the fins from each electrode are sandwiched between two flat metal bars so that the ends of the substrate fins protrude slightly beyond the edges of the bus. During welding excess tab material is used to fill the small gaps between two adjacent fins and the metal bars. Welding can be successfully performed with TIG (tungsten inert gas) and laser, as known in the art. Both processes were used with sufficient penetration to make good contact between busbar and electrodes. 
     The advantages of this design are that it allows a high rate of discharge, has excellent electrical and thermal conductivity, good contact reliability with contact redundancy provided by two welded tabs, shock resistance, volumetric efficient design, and it allows for TIG and laser welding. 
    
    
     DESCRIPTION OF THE DRAWING 
     A better understanding of the present invention may be realized from a consideration of the following detailed description, taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 shows a battery in accordance with the present invention in longitudinal section; 
     FIG. 2 is an enlargement of Detail 2 of FIG. 1 showing structural details thereof; 
     FIG. 3A is an elevational view of a busbar support for the battery of FIG. 1; 
     FIG. 3B shows the busbar support of FIG. 3A in cross-section with an associated electrode partially broken away; 
     FIG. 4 shows an enlargement of the laser-beam weld connecting the electrode (cathode or anode) to the busbars in the battery of FIG. 1; 
     FIG. 5 shows the laser-beam weld of the portion 5--5 of FIG. 1 in further detail; 
     FIG. 6 shows a detailed cross-section of the feedthrough pin structure designated 6--6 in the battery of FIG. 1; 
     FIG. 7 shows the electrodes of the battery of FIG. 1 in plan view; 
     FIG. 8 shows one of the electrodes of FIG. 7 in greater detail; 
     FIG. 9 is an oblique view of a stack of the electrodes of FIG. 7; and 
     FIG. 10 is a further, oblique view of one of the electrodes of FIG. 7. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in greater detail by reference to the drawing figures. FIG. 1 shows a lithium battery as a preferred embodiment of the present invention. However, it should be understood that this is for purposes of detailed description only and is not to be interpreted as limiting the present invention to application in a lithium battery or any one particular type of battery. 
     In the longitudinal sectional view of FIG. 1, a battery 2 is surmounted by a cover 1. In the center of the cover is a vent 13. Connections are made to the battery by means of feedthrough pins 10 made of Alloy 52. These pins are insulated by means of the feedthrough glass 11 (Corning 9013). The feedthrough pins are held in the cover 1 by means of the feedthrough body 12. The feedthrough pins 10, insulator glass 11 and feedthrough body 12 are shown in cross-section detail in FIG. 6 (Detail 6--6 in FIG. 1). 
     The feedthrough pins 10 are electrically and mechanically connected to a pair of busbar tabs 3, respectively connected to the busbars 7 and 8, one set being provided on each side of the battery. The busbars are described in greater detail in relation to FIGS. 4, 7 and 9 below. There are, in accordance with the present invention, two types of busbars for each of the positive and negative poles of the battery--one being an inside busbar 8 (provided in pairs) and the other being an outside busbar 7. The busbar tabs and the inside and outside busbars are all made of nickel 200. 
     The battery case 2 has a liner 9 made of Tefzel. A reinforcing disc 19 is provided at the bottom of the battery case. Corresponding pluralities of carbon cathodes 15 and lithium anodes 16 are in a stacked configuration 50 between assemblies of end plate insulator 5, also made of Tefzel, and end plate 4 mounted respectively at upper and lower ends of the stack 50. Electrolyte 20, having the chemical composition SOCl 2 , (thionyl chloride) for the solvent and LiAlCl 4  (lithium aluminum tetrachloride) for the salt (in the case of a lithium-thionyl chloride battery), fills the space within the battery case 2 surrounding the cathodes, anodes and busbars. Each anode 16 has a substrate 6. Each cathode 15 has a substrate 14. Between each cathode and anode pair there is a porous glass mat separator, 17 or 18. 
     A hold-down assembly (not shown) holds down the stack of anodes 16, cathodes 15, and separators 17 and 18, stacked in an alternating arrangement (FIG. 2) of cathode 15, separator 17, anode 16, separator 18, cathode 15, etc., throughout the volume of the electrode stack 50 between end plate insulators 5. A plan view of the stacked electrodes is shown in relation to the electrodes and busbars in the case of the battery 2 in FIG. 7. 
     FIGS. 3A and 3B show the busbar support for the inside and outside busbars 7 and 8 of FIG. 1. The busbar support is shown in cross-section in FIG. 3B. Seen from the inside of the battery shown in FIG. 1, the busbar support appears as in FIG. 3A. 
     A plan view of the busbar support and how it attaches to the busbars and disc electrodes is shown in FIG. 3B. The busbar support 26 has an outer, cylindrical surface 26a which contacts the inner surface of case liner 9 (see FIG. 1). On the inner, flat face of busbar support 26 there are two L-shaped flanges 26b. These two flanges create a slot into which fit outer busbar 7 and inner busbars 8. Electrode fin 36 (FIG. 8) is sandwiched between inner and outer busbars 7 and 8. The disc electrode of the present invention, in partial plan view, is shown in FIG. 3B, corresponding to the anode 16 and cathode 15, respectively. 
     FIG. 4 shows in greater detail the attachment of the electrode substrate of the instant invention to the busbars. The two portions 36 of the anode or cathode are each sandwiched between the outside busbar 7 and one of the inside busbars 8. The fin portions 36 are then laser-beam welded; alternatively TIG (tungsten inert gas) welding may be used to attach the fin portions to the busbars. 
     These fin portions are shown in greater detail in FIG. 10. Fins 36 are formed by making a 90 degree bend (36a) in electrode substrate 38. Since there are two fin portions 36 for each disc electrode 33, there is redundancy of contact between the electrode and the busbars. Each fin portion 36 is designed to carry the maximum current flow from the attached electrode. This redundancy of contact and current path enhances the safety of the cell and assures that all anodes can be discharged to 100% depletion of lithium. 
     A further aspect of the instant invention is the configuration of the tabs of the electrodes 16 and 15 which protrude slightly beyond the outer edge of busbars 7 and 8. This excess tab material is used to fill in the small gaps between two adjacent electrode fins and the metal busbars during the welding process. 
     Turning now to FIGS. 5 and 6, further details of the feedthrough pins are disclosed. FIG. 5, corresponding to Detail 5--5 in FIG. 1, shows the busbar tab 3 as it attaches to feedthrough pin 10. The feedthrough pin is laser-beam welded to the busbar tab. FIG. 6 shows the feedthrough pin 10 as it attaches to the cover 1 of FIG. 1 (Detail 6--6 of FIG. 1). Feedthrough pin 10 is insulated by feedthrough glass 11 (Corning 9013) from feedthrough body 12 which is made of 304L-series stainless steel. Feedthrough body 12 is in turn welded to the cover 1 by laser-beam welding. 
     FIG. 7 shows the cathodes and anodes (referred to generically as &#34;electrodes&#34;) and separators in plan view in relation to the battery of FIG. 1. In this arrangement, the anode 28 is made slightly smaller in diameter than the cathode 27. A separator 30 is slightly larger than the cathode. Porous glass mat is used as the separator material. The anode is made of a nickel 200 substrate with an active layer of lithium on either side. The cathode also has a nickel 200 substrate with an active layer of carbon on either side. The exception is the end electrode at the bottom of the electrode stack shown in FIG. 1. Only the side facing the remainder of the stack is &#34;active&#34; on the end electrode, i.e., has a carbon or lithium layer atop the nickel substrate. The liner is shown at 29, and the busbars and fin connection at 31. 
     As may be seen in FIG. 7, the cathodes, anodes and separators have a straight margin to the lateral sides of FIG. 7, to allow for the busbars and fins. Thus they measure somewhat less in the lateral direction than the diameter of the circle from which they are cut. The end plate 4 (FIG. 1) is preferably laser-beam welded to a plurality of stack straps (not shown) which are provided to hold the stack 50 of electrodes and separators together. 
     Turning now to FIG. 8, the disc electrode 33 of the battery of FIG. 1 is shown in greater detail. The round electrode substrate is perforated at 35 on either side of a central non-perforated bridge 38 connected to the tab 36a. A solid border 34 surrounds the entire disc electrode to prevent sharp edges from being exposed. The two half-moon shaped perforated areas 37 are approximately 50% open and allow for electrolyte diffusion from one side of the electrode to the other. 
     The disc electrode and busbar assembly of the present invention are shown together in FIG. 9. The disc electrodes 39 are placed in a stack 50 of interspersed anodes and cathodes commonly connected to the respective busbar assemblies comprising components 41 and 42. The electrode tab having fins 40 is connected to the busbars 41 and 42 by inserting the fin 40 between busbar components 41 and 42 and welding the busbars and fins together by using laser-beam or TIG welding. The busbar assembly having the components 41 and 42 is then welded to the busbar tab 3 of FIG. 1 which in turn is connected to the feedthrough pin 10 for collection of the discharge current at the terminal. The sets of anodes and cathodes are connected to their respective busbars and feedthrough pins in this manner. 
     An important feature of the invention is the tab design. The tab 36a has two fins 36. The tab 36a with the fins 36 is bent 90 degrees with respect to the disc electrode plane 33. This 90 degree bend is shown more clearly in FIG. 10. There, the tab 36a having two fins 36 is shown having a 90 degree bend with respect to the disc electrode 33. The unperforated bridge portion 38 is shown in FIG. 10, as is the unperforated border 34. Also shown in FIG. 10 is the unperforated area 37 surrounding individual perforations 35. Full circles are used for the perforations, which may be made, for example, by either the well-known photo-etching method or by punching. Other shapes, such as square, rectangular, elliptical, etc. may be used as desired. In the preferred embodiment depicted in FIG. 10, for example, the individual perforations are 0.060 inches in diameter and are laid out in the rhombic pattern shown in the figure. 
     Although there have been described hereinabove various specific arrangements of an improved disc electrode and busbar in a lithium battery in accordance with the invention for the purpose of illustrating the manner in which the invention may be used to advantage, it will be appreciated that the invention is not limited thereto. Accordingly, any and all modifications, variations or equivalent arrangements which may occur to those skilled in the art should be considered to be within the scope of the invention as defined in the annexed claims.