Abstract:
A battery box sealable against moisture and oxygen ingress and liquid electrolyte egress. The battery box includes a first end, a second end opposite the first end, and sidewalls extending between the first and second ends to define a chamber for housing a number of rechargeable lithium battery cells. The first end and the sidewalls are metallic and have a substantially moisture and oxygen gas impervious joint between them. The second end is of a metal plastic laminate joinable to the sidewalls by a joining medium to form a substantially moisture and oxygen impervious joint therebetween. The second end has electrical connectors extending through it which enable substantially moisture and gas impervious connection to be made between current collectors of the lithium battery cells and the outside of the battery box.

Description:
FIELD OF THE INVENTION 
     This invention relates to the structure and assembly of rechargeable lithium batteries from rechargeable lithium battery cells. 
     BACKGROUND OF THE INVENTION 
     Rechargeable lithium battery cells are generally packaged as individual cell units in either cylindrical metal tubes or foil pouches. Each cell unit contains positive and negative electrodes, electrolyte and positive and negative current collectors. The current collectors must be accessible from outside of the package to enable electrical connection thereto. 
     Because of the materials used in a rechargeable lithium battery cell, the cell must be protected against moisture and oxygen ingress to avoid undesirable reactions. Furthermore, where liquid electrolytes are used, the packaging must also be capable of preventing liquid electrolyte egress. Cell seals are typically bulky and heavy, often representing in excess of 10% of cell weight and volume. 
     In order to meet various reserve capacity and voltage requirements, a plurality of lithium battery cells are typically enclosed in a plastic battery box with appropriate cell interconnections therebetween. The battery boxes primarily provide a structure for housing the battery cells. As the cells are themselves sealed against oxygen and moisture ingress and liquid electrolyte egress, it has not been found necessary or cost efficient in the past to provide a hermetically sealed battery box for lithium-ion cells. 
     In “foil” pouch packaged lithium battery cells, the use of a polymeric coated foil gives rise to a need to adequately ensure that leads from the current collectors don&#39;t contact the foil material and thereby “short-out”. This problem is particularly acute along the outer edges of the foil covering which have usually been cut to size, thereby exposing the electrically conductive foil in close proximity to the leads, creating a need for shielding of the leads in this area. 
     It is an object of the present invention to provide a battery box for rechargeable lithium battery cells which is mechanically strong and hermetically sealable. 
     It is a further object of the present invention to provide a method for combining rechargeable lithium battery cells with a battery box enabling the elimination of foil encapsulation of individual cells and allowing thinner encapsulating materials. 
     SUMMARY OF THE INVENTION 
     A battery box sealable against moisture and oxygen ingress and liquid electrolyte egress. The battery box includes a first end, a second end opposite the first end, and side walls extending between the first and second ends to define a chamber for housing a number of rechargeable lithium battery cells. 
     The first end and the side walls are metallic and have a substantially moisture and oxygen gas impervious joint between them. 
     The second end is of a metal plastic laminate joinable to the side walls by a joining medium to form a substantially moisture and oxygen impervious joint therebetween. The second end has electrical connectors extending through it which enable substantially moisture and gas impervious connection to be made between current collectors of the rechargeable lithium battery cells and the outside of the battery box. 
     A method is provided for assembling a lithium battery from rechargeable lithium battery cell units which have positive and negative electrodes, an electrolyte and positive and negative current collectors. The method comprises the steps of: 
     Stacking a plurality of the cell units in a hermetically sealable box. 
     1. Providing an electrically insulating barrier between adjacent of the cell units and between the cell units and the box. 
     2. Making any desired electrical connections between selected of the positive and negative current collectors. 
     3. Providing electrical continuity between selected of the current collectors and an exterior of the battery box. 
     4. Hermetically sealing the battery box. 
    
    
     DESCRIPTION OF DRAWINGS 
     Preferred embodiments of the invention are described below with reference to the accompanying drawings in which: 
     FIG. 1 is a perspective view, partially cut away of a prior art rechargeable lithium battery cell; 
     FIG. 2 is an exploded perspective view of a rechargeable lithium battery according to the present invention; 
     FIG. 3 is a top plan view of a second end of a battery box according to the present invention; 
     FIG. 4 is a bottom plan view of a second end of a battery box according to the present invention; 
     FIG. 5 is a section on line  5 — 5  of FIG. 3, showing only the sectioned surface. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A rechargeable lithium battery according to the present invention is generally illustrated by reference  10  in FIG.  2 . The battery  10  comprises three rechargeable lithium battery cells  12  stacked in a battery box  14  having a first end  16 , a second end  18  and side walls  20 . Although three lithium-ion cells are shown, the actual number may vary depending on the application. 
     FIG. 1 shows a typical foil wrapped lithium battery cell  12 . The cell contains a positive electrode  22 , a negative electrode  24 , an electrolyte  26  and positive and negative “leads” or “current collectors”,  28  and  30  respectively, enclosed in a poach  32 . The leads  28  and  30  provide electrical communication between tile positive and negative electrodes  22  and  24  respectively and the outside of The pouch  32 . 
     Rechargeable lithium batteries usually include a lithium-ion containing negative electrode, but may instead have a lithium metal or lithium negative alloy. Nevertheless, insofar as the teachings of this specification would apply to either a lithium metal or a lithium-ion containing negative electrode, the expressions “lithium battery cell” and “lithium battery” are intended to cover both variants. 
     As discussed in the background above, it is necessary to prevent moisture and oxygen contact with the inside of the lithium battery cell. In the past, this has been achieved on a cell by cell basis by encapsulating each lithium-ion cell in a substantially moisture and oxygen impervious, i.e. “hermetically sealed” shell. For example, in the case of the prior art lithium battery cell  12  in FIG. 1, the pouch  32  may be made from a suitable metal foil with a plastic laminate coating on at least the inside surface and preferably on both surfaces. Aluminum is the usually selected metal foil, however, copper may also be used or any other metal that can be formed to a suitable thickness and which exhibits good moisture and oxygen impermeability. 
     Unfortunately, a foil pouch requires a relatively thick foil to be sufficiently moisture and gas impermeable and the resulting foil package can represent up to 10% of cell weight and volume. Furthermore, the foil is prone to breaking in the corners if one tries to cram the lithium battery cell  12  into a tightly fitting container. 
     According to the present invention, the battery box  14  performs the function of sealing against moisture and oxygen ingress, unlike prior art battery boxes which basically acted as a holder for individually sealed cells. This allows the use of thinner and lighter pouches  32  and the possibility of non-metallic pouches  32 , for example, of a polymer film. In the case of non-liquid electrolytes, packaging can basically be eliminated in favour of merely electrically isolating the individual lithium battery cells  12  from each other and the battery box  14 . An advantage of non-metallic pouches is the avoidance of the risk of a short circuit between the current collectors or leads  28  and  30  and exposed metal foil along the edge of the pouch  32 . 
     Where liquid electrolytes  26  are used, the pouch  32  merely has to be substantially impervious to electrolyte egress, which may be a lesser requirement to meet than oxygen or moisture ingress. The pouch  32  may therefore be much thinner and also less prone to leaking when “scrunched”  20  into a compact box than would be the case with a typical foil pouch. For the sake of clarity, it should be understood that by “moisture”, it is intended herein to refer primarily to water, particularly in vapour form. 
     The battery box  10  has a first end  16  and side walls  20  of a metallic material such as tin. The first end  16  may be integral with the side walls  20 , however, it may be more convenient for assembly to have a separate first end  16  joinable to the side walls  20  by any suitable means, such as soldering or possibly a polymeric adhesive. 
     The side walls  20  illustrated are in a generally rectangular configuration. This is not a requirement, but merely suits the shape of the lithium battery cells  12  illustrated. Other shapes may be selected as required. 
     The lithium battery cells  12  may be interconnected by interconnecting leads  34  in series or parallel as required. Principal output leads  36  connect the non-interconnected current collectors of opposite polarity on the first and last lithium battery cells  12  to electrical connectors  40  extending through the second end  18  of the battery box  14 . Further output leads  38  connect the interconnecting leads  34  to further electrical connectors  40  extending through the second end  18 . It is not necessary to make all of the cell interconnections internally as separate electrical connectors  40  for charge or discharge purposes may be provided for each positive and negative current collector,  28  and  30  respectively, and cell interconnections could then be made externally if desired. 
     The second end  18  of the battery box  14  must be sealably connectable to the side walls  20  while providing separate electrical connections through to the outside of the box at electrical connectors  40 . 
     From the standpoint of oxygen and moisture impermeability, a metallic second end  18  would be preferred, however, this would require a suitably insulating sealing arrangement to provide discrete electrical connectors  40 . It has been found that a suitable degree of moisture and air impermeability is obtained from a copper/thermoset/copper printed circuit board with suitable modifications in the region of the electrical connectors  40  as described below. 
     Reference is now made to FIGS. 3,  4  and  5  which illustrate the second end  18  in detail. FIG. 3 illustrates an inner face  42  which has an inner foil layer  44 , typically of copper, laminated over a non-conductive substantially rigid substrate  46  such as a typical thermoset plastic printed circuit board substrate. The inner foil layer  44  extends substantially over the entire inner face  42 , but for circular breaches  48  extending about the electrical connectors  40  to create discrete electrically isolated connector areas  50 . 
     An insulating coating  52 , such as a plastic laminate, extends over the inner foil layer  44  leaving an exposed edge  54  extending about the perimeter of the inner face  42  and exposed areas which act as the electrical connectors  40 . The exposed edge  54  enables a solder connection to be made between the second end  18  and the side walls  20 . 
     To avoid oxygen and moisture migration through the substrate  46  at the breaches  48 , a substantially moisture and oxygen impervious overlay  56  is placed over the breaches  48 . The overlay  56  may be of an adhesive foil and may have a non-conductive polymeric coating facing into the battery box  14 . Each overlay  56  has a circular opening  58  extending through it and spaced apart from the associated electrical connector  40  to expose the connector  40  and avoid electrical conductivity therebetween. 
     The electrical connectors  40  may be trough plated holes extending through he substrate  46  between the adjacent portions of the inner foil layer  44  and au outer foil layer  64 . The electrical connectors  40  provide electrical continuity between the conductive layers inside and the outside of the battery box  14 . As such, any desired electrical connections could be made directly to the electrical connectors  40 . Nevertheless, it generally proves more convenient to provide electrical terminals along an edge of the strip, such as battery terminals  60  in FIGS. 3 and 4 which may also be through plated holes. The battery terminals  60  are electrically connected to respective of the electrical connectors  40  by conductor strips  62 . The conductor strips  62  can be formed in the outer foil layer  64  on an outer face of the second end  18  by etching away portions of the outer foil layer  64 , leaving discrete conductor strips  62 . 
     An outer insulating coating  66 , such as a plastic laminate, may be applied over the outer foil layer  64  to avoid undesirable electrical contact with the outer foil layer  64 . 
     The battery box  14  may optionally contain electronic circuitry, such as schematically illustrated at reference  68  in FIG. 2, connected to the lithium battery cells  12  to monitor the battery charging and discharging process. Alternatively, the electronic circuitry  68  may be externally connected to the battery terminals  60  and not necessarily part of the battery  10 . 
     A sealable port  70  may be provided to enable evacuation and sealing or backfilling (wholly or partly) with an appropriate potting material. The sealable port  70  may be a through plated opening sealable by solder as with the electrical connectors  40 . 
     The above description is intended in an illustrative rather than a restrictive sense. Variations to the specific components and methodology described may be apparent to persons skilled in the relevant art or arts. Insofar as such variations are within the spirit and scope of any of the claims set out below, they are to be considered as covered by those claims.