Abstract:
The present invention discloses a device and method pertaining to battery construction. In the preferred embodiment, the shape and construction of the battery is designed to maximize energy density and efficiency, while minimizing volume and related restrictions. Furthermore, the efficient, simplified internal construction of the present invention, using readily available tubes, pins and spacers, renders it safe and reliable for medical applications, and lends to relative ease and cost effectiveness in manufacturing. The utilization of a neutral case further adds to the safety and reliability of the present invention. Also, the strategic positioning of the electrolyte fill hole allows for quick filling. A related battery construction tool and method are also disclosed adding to the overall usefulness and efficiency of the present invention.

Description:
REFERENCE TO PRIOR FILED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application 60/363,455 filed Mar. 11, 2002. 
    
    
     TECHNICAL FIELD 
     This invention relates to a battery and more particularly to a reliable implantable battery having a configuration that maximizes the ratio of active material to internal volume. 
     BACKGROUND 
     Implantable medical devices such as pacemakers, defibrillators, speech processors, left ventricular assist devices (LVAD), and neurostimulators are becoming more and more common and have many unique requirements. Small, round, lightweight devices having flat shapes are desirable for ease of implant and patient comfort. Additionally, these devices have strict safety and reliability requirements. 
     In many of these devices, the batteries make up the majority of the weight and volume. Therefore, batteries are needed of a specific shape to make the best use of the space available in each device. For many of these devices, the batteries themselves should be small, flat, and lightweight. 
     One method of producing flat cells is by using a stacked plate design. As shown in  FIG. 1 , a typical battery stack  10  is constructed such that the positive electrodes  11  have tabs  12  that extend past the edges of the negative electrodes  13  and serve as the current collectors. The positive tabs  12  are collected and connected by a weld or other attachment  14  (shown schematically). Likewise, the negative electrodes  13  have tabs  15  that extend past the edges of the positive electrodes  11  and are collected and connected by a weld or other attachment  16  (shown schematically). The tab design is simple and advantageously has only a small number of cell components. However, the tabs must be of sufficient length to reach each other to connect all the tabs and to insulate and isolate the tabs of the opposite polarity. Using tabs wastes internal volume, or headspace, around the tabs causing the battery volume to be larger or the battery capacity to be smaller than if tabs were not used. Furthermore, because the number of tabs that can be connected together in a weld or electromechanical joint is limited, this construction limits the number of electrodes capable of being stacked together. Overall, using tabs reduces the energy density of the cell. With the search for smaller and smaller packages, especially in medical applications, designers are pressed to fit more into less space, which is a difficult task with the tab design. 
     For stacked plate design batteries, one of the most important design requirements to ensure performance and safety is maintenance of electrode alignment. Proper alignment of the electrodes and separators must include an adequate safety margin for initial assembly and must be maintained for safe operation over the life of the cell. This is especially important for implantable cells because of the effects that a failure may have on the device performance and ultimately on the patient. For tab designs in which all of the terminations are made through a single common point, maintaining proper alignment can be difficult. Cell elements unintentionally may be allowed to rotate, leading to safety concerns such as short circuits or Li plating. While alignment is improved with multi-tab designs, headspace remains an issue, with the multiple tabs taking up significant space in the cell. 
     Another drawback of the typical tab design is that as the electrode layers are cut, the cutting process tends to produce burrs at the corners, particularly at inside corners such as area  17 . These burrs can cause short circuits. 
     Another method of producing flat profile cells is by using a folded-type cell design. In these designs each electrode is layered together in an alternating fold with a separator being combined during each consecutive fold. Each panel of the electrode is connected with a small jumper ribbon of material and serves as the current collector of the entire combination of electrodes. This design is well proven in cell phone applications and has potential for inexpensive construction and manufacture. However, this geometry has size constraints and still has the problem of maintaining alignment of the components during manufacture and usage. The ribbons protrude in a similar manner as the tabs described above, thereby wasting precious space. Because the folded designs do not incorporate alignment features, the relative sizes of the electrodes are used to ensure the safety of the device. The negative electrodes are sized larger than the positive such that under its greatest misalignment, the positive is still covered by the negative. This makes the folded-type battery less space efficient. 
     In U.S. Pat. No. 6,139,987 to Koo et al., a bipolar battery uses an anode pin and a cathode pin to mount electrodes and contact rings, with anode contact rings fitted into cutouts of the cathode and vice versa, thus obviating the use of tabs. However, it is not clear how the electrodes are actually aligned and electrically and mechanically coupled; proper alignment and coupling are critical to the operation of the battery. Furthermore, the battery is filled with electrolyte through a center hole called an electrolyte injection hole; the electrolyte must flow into the hole and through smaller ports in the injection ring to contact and saturate the electrodes. Moreover, there is no mention of terminals and it is unclear what structures would function as terminals. 
     It is therefore desirable to provide a reliable flat battery having a configuration that eliminates the use of tabs for construction while overcoming other limitations of the prior art. 
     It is also desirable to provide a configuration that can be made into custom shapes for applications that are space-limited, such as implantable medical devices and satellites and other aerospace devices. 
     It is further desirable to provide a safe, compact, space-efficient, high capacity battery. 
     SUMMARY 
     The battery of an embodiment of the present invention uses spacers, preferably in the form of washers on tubes, to connect the electrodes together at exposed areas on the electrode substrate. By having the attachment point interior to the main perimeter of the electrode, this invention maximizes the ratio of active electrode material to internal volume. The electrodes are captured by spacers and are not directly connected to each other. By welding the spacers and substrates together, the electrodes are electrically and mechanically connected. The thicknesses of the spacers are designed to match the thicknesses of the electrodes so that the substrates are not deformed or bent. The substrates maintain a substantially planar form and maintain their spacing from each other. Using this electrode and spacer design, the battery is not constrained by the number of electrodes or by a tab length. The battery may incorporate any number of electrodes and spacers by extending the tube and mounting additional pairs of electrodes and spacers. 
     Furthermore, the battery of an embodiment of the present invention uses an effective three-pin design to maintain alignment of the electrodes and separator layers. 
     Because of the unique construction of the electrode assembly, the shape of the battery may be optimized for each implantable device. 
     The battery of an embodiment of the present invention has a neutral case for safety. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a plan view of a typical stacked battery of the prior art. 
         FIGS. 2–3  are plan views of devices having batteries of the prior art. 
         FIG. 4  is a plan view of the battery of an embodiment of the present invention. 
         FIGS. 5–7  are plan views of three different devices having batteries of various embodiments of the present invention. 
         FIG. 8A  is an exploded view of the battery of an embodiment of the present invention. 
         FIG. 8B  is an exploded view of alternative embodiment of the battery of an embodiment of the present invention. 
         FIGS. 8C  and D are side cross sectional views of the positive and negative feedthroughs of the battery of the embodiment of the  FIG. 8A . 
         FIGS. 8E  and F are side cross sectional views of the positive and negative feedthroughs of the battery of the embodiment of the  FIG. 8B . 
         FIGS. 9–14  illustrate a method for forming the battery stack of an embodiment of the present invention. 
         FIGS. 15–23  illustrate various embodiments of the spacer to substrate configuration. 
         FIGS. 24–26  illustrate various embodiments of the separator of the present invention. 
         FIG. 27  illustrates a shield as used to protect the electrodes and separator during welding. 
         FIGS. 28A and 28B  illustrate methods for connecting the spacers and electrode layers. 
         FIGS. 29–31  illustrate the length of the feedthrough pins for optimum weldability. 
         FIG. 32  illustrates the battery stacks placed on the covers for welding. 
         FIG. 33  is a plan view of the top insulator of an embodiment of the present invention. 
         FIGS. 34–36  are plan views of three embodiments of the bottom and side insulators of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following text describes the preferred mode presently contemplated for carrying out the invention and is not intended to describe all possible modifications and variations consistent with the spirit and purpose of the invention. The scope of the invention should be determined with reference to the claims. As used herein, a “battery” may be a single cell or a series of cells. 
     Shape 
     Prior art batteries typically have some drawbacks when used in circular or ovoid devices. As shown in  FIG. 2 , a cardiac pacemaker  20  is generally circular or ovoid, and its battery  21  is often located on only one side of the device, making the battery side  22  heavier than the other side  23  having electronic components often encased in epoxy. While it may be acceptable to have a lopsided weight in some devices, in other devices, this may be undesirable. As shown in  FIG. 3 , rectangular batteries  31  having all flat edges  32  also have been used in devices  30  having round edges  33 , wasting valuable internal volume  34  in the device. The battery of the present invention is not limited in shape, and we have found that a battery shape shown in  FIGS. 4 and 5  provides weight symmetry, allows components  51  to be placed on either side of the battery  40 , and maximizes the energy density by using otherwise wasted space. Although the scale and aspect ratios are not limited and will vary depending on the application, the dimensions of the battery shown in  FIG. 4  may be, for example, approximately 28 mm long×20 mm wide×5.5 mm thick.  FIG. 6  shows an alternative embodiment of the shape of the battery  61  of the present invention, wherein the device  60  is ovoid. This shape also provides the benefits of the embodiment of  FIG. 4 .  FIG. 7  shows yet another alternative embodiment of the shape of the battery  71  of the present invention, wherein the two flat sides  72  and  73  are of unequal length and the battery  71  is not centered in the device  70 . While this loses some of the weight symmetry provided by the configurations of batteries  40  and  61 , it does provide maximization of energy density and allows components to be placed on either side of the battery  71 . It also allows a larger space for components on one side of the battery than the other, which may be advantageous for some device designs. 
     Stacking Method 
     As shown in  FIGS. 8A ,  8 C, and  8 D, the stacked battery  40  in an embodiment of the present invention is formed by multiple layers of positive electrodes  81  and negative electrodes  82 , separated by sheets of separator  83 , which preferably comprise polyethylene and/or polypropylene. Each positive electrode  81  has a positive hole  84  through which a positive tube  85  slides, and each negative electrode  82  has a negative hole  86  through which a negative tube  87  slides. Electrodes  81  and  82  comprise substrates,  41   a  and  41   b , respectively, coated on both sides with an active material,  42   a  and  42   b , respectively. Substrates  41   a  and  41   b  may be die cut, laser cut, or the like. To minimize burrs, preferably, the electrode substrate has no sharp inside or outside corners, only rounded. Each positive electrode  81  has an area  88  around hole  84 , and each negative electrode  82  has an area  89  around hole  86 ; these areas have been cleared of active material  42   b  and  42   b , such as by scraping. The separator  83  has cutouts  43  in which tubes  85  and  87  fit. Each negative electrode  82  has a locator hole  44  in which an insulating tube  45  slides. Insulating tube  45  preferably comprises polyperfluoroalkoxyethylene (PFA), polypropylene, polyimide, or a parylene coated metal. Each positive electrode  81  has a locator hole  46  in which an insulating spacer  47  fits. The insulating spacer  47  has a thickness of about 50 to 150% of the thickness of the positive electrode and more preferably about the thickness of the positive electrode. The insulating spacer  47  is slid over insulating tube  45 . The battery stack is housed in a case  48  with a cover  49 . The feedthrough pins  297  and  298  of positive terminal  294  and negative terminal  295 , respectively, are coupled to the positive and negative electrodes  81  and  82 , respectively, as will be described later, and serve as two of three alignment pins in the assembly. Locator pin  296  is the third alignment pin. 
     As shown in  FIG. 9 , the method for stacking begins with a stacking fixture  90 . A stacking plate  91  is mounted on a stacking base  92   a . (The stacking plate  91  is used in a later step to facilitate the removal of the finished stack from the fixture.) Negative tube  87  is mounted on a negative pin  93 . Positive tube  85  is mounted on a positive pin  94 . Insulating tube  45  is mounted on a locator pin  95 . 
     As shown in  FIG. 10 , a negative electrode having only one side of the substrate coated with active material, hereinafter known as a “one-sided negative electrode  101 ”, is mounted with the uncoated side against the fixture such that the negative electrode locator hole  44  slides over the insulating tube  45  and the negative hole  86  slides over the negative tube  87 . Stacking begins with a one-sided negative electrode because the negative electrode surrounds the positive electrode to avoid lithium plating. The negative electrodes are longer and wider than the positive electrodes and have smaller holes and cutouts to ensure that the negative electrodes surround the positive electrodes, as can be seen in  FIG. 8A . The one-sided negative electrode  101  has a cutout  102  such that it avoids contact with the positive tube  85  and positive spacers  284 . Each negative electrode has a cleaned area near the negative hole  86 . The components are dimensioned preferably so that the straight edge of the cleaned area will lie tangent with the negative spacers  121  (shown in  FIG. 12 ). The area is cleaned of active material so that the substrates of the electrodes come into contact with the spacers and so that the active material does not contaminate the stack weld. 
     As shown in  FIG. 11 , a sheet of separator  83  is mounted on the stack such that the separator locator hole  131  engages the insulating tube  45  and the cutouts wrap around the positive and negative tubes. 
     As shown in  FIG. 12 , a negative spacer  121 , which preferably is made from the same material as the negative electrode substrate, is mounted on the negative tube  87 . An insulating spacer  47  is mounted on the insulating tube  45 . A positive electrode  81  is mounted on the stack such that the positive electrode locator hole  46  engages the insulating spacer  47  and the positive hole  84  engages the positive tube  85 . The positive electrode  81  has a cleaned area near the positive hole  84 . The components are dimensioned preferably so that the straight edge of the cleaned area will lie tangent with the positive spacers. The area is cleaned of active material so that the substrates of the electrodes come into contact with the spacers and so that the active material does not contaminate the stack weld. The positive electrode  81  has a cutout  104  such that it avoids the negative tube  87  and spacers  121 . 
     As shown in  FIG. 13 , a sheet of separator  83  is mounted on the stack such that a separator locator hole  131  engages the insulating tube  45  and the cutouts  43  wrap around the positive and negative tubes  85  and  87 , respectively. 
     As shown in  FIG. 14 , a positive spacer  284 , which preferably is made from the same material as the positive electrode substrate, is mounted on the positive tube  85 . A two-sided negative electrode  82  is mounted such that the locator hole  46  slides over the insulating tube  45  and the negative hole  86  slides over the negative tube  87 . The negative electrode  82  has a cutout  102  such that it avoids the positive tube  85  and spacers  284 . The stacking continues in this fashion, repeating the actions illustrated in  FIGS. 10 to 14 , [negative electrode  82  (one-sided negative electrode  101  to start and finish)]-[separator  83 ]-[negative spacer  121 ]-[insulating spacer  47 ]-[positive electrode  81 ]-[separator  83 ]-[positive spacer  284 ]-[negative electrode  82 ]. It should be noted that because the elements between the sheets of separator are simultaneously mounted on the stacking fixture and are not layered, the order of the stacking is critical only for the elements between the separators. The positive spacer  284  and negative electrode  82  are mounted at the same time and then covered by a sheet of separator. Likewise, the negative spacer  121 , insulating spacer  47 , and positive electrode  81  are mounted at the same time and all covered by a sheet of separator  83 . The stacking continues until all positive electrodes  81  have been stacked. Following the last positive electrode, a separator sheet  83  is mounted. Then, a final one-sided negative electrode  101  and negative spacer are mounted. This top electrode is mounted such that the coated side is facing down toward the stack. Depending on the application, any number of electrode layers may be used in this configuration; in certain medical applications, five to sixteen layers are preferred. 
     In an alternative embodiment shown in  FIGS. 8B ,  8 E, and  8 F, an alternative stacking order provides additional safety features while maintaining stacking efficiency. In this embodiment, the stacking order is: negative tube  87 , negative single-sided electrode  101 , negative spacer  121 , positive tube  85 , separator  83 , positive electrode  81 , positive spacer  284 , insulating spacer  47 , separator  83 , negative double-sided electrode  82 , negative spacer  121 , separator  83 , repeating positive electrode through negative spacer, ending with single-sided negative electrode  101  and negative spacer  121 . An advantage of this order is that the spacer and electrode of the same polarity are sandwiched between separator layers. Any errant rotation of the electrode during stacking or otherwise will maintain isolation from the opposite polarity spacers. The separator will maintain the separation. Another advantage is that the spacer face is in more direct contact with the exposed substrate. In the stacking order of  FIG. 8A , the spacer is only in contact with a sector of the substrate not covered by the separator. Greater area in contact will ensure better electrical connection. This ordering still utilizes the volume of the spacer into the cutout volume of the opposite polarity electrode by deforming the separator. Because the separator is so thin, the separator easily deforms around the spacer and into the cutout volume of the electrode. 
     Returning to  FIG. 14 , once the stacking is completed, a face plate  92   b  is aligned and moved toward the base  92   a  and corresponding stacking plate  91 . This sandwiches and compresses the assembled electrodes, and holds them in position so that welding or other means of interconnection can be easily performed. In one embodiment, the base  92   a  has one or more prongs  96   a ,  96   b  that may be inserted into corresponding openings  97   a ,  97   b  to further improve alignment. 
     Stacking Features 
     The electrode stacking method provides alignment of the electrodes. As mentioned above, preferably, the negative electrodes  82  are larger than the positive electrodes  81  by a margin of about 0.5 mm to prevent the lithium ions from plating out, which precludes the practice of aligning the positive electrode edges with the negative electrode edges to position the electrodes. This 0.5-mm border is maintained by the stability of the electrode alignment described below. The negative electrodes are positioned by two tubes and holes, namely the negative tube to the negative hole and locator tube to locator hole. The positive electrodes are positioned by two tubes and holes, namely the positive tube to the positive hole and locator tube to locator hole. Alternatively or additionally, the positive and negative tubes and holes may be noncircular to provide further alignment or to obviate the need for the separate locator tube and hole. 
     In order to maintain the 0.5-mm border at the locator hole, an insulating spacer is used. This insulating spacer engages the positive electrode at its locator hole and the locator tube. The electrodes are aligned initially by the tubes and subsequently by the feedthrough pins and locator pin. The feedthrough pins thus function as both the alignment anchors and as the current collectors. This design compactly provides terminal connections and alignment anchors. All the edges of the positive coated material are within the perimeter of the coated negative material. 
     The stacking of electrodes provides mechanical attachment of the electrodes. The positive spacers  284  capture and connect the positive substrate  41   a  of the positive electrodes. Likewise, the negative spacers capture and connect the negative substrate of the negative electrodes. Preferably after stacking is complete, the positive spacers  284  are welded together and the negative spacers are welded together. Alternatively, the welding may take place in steps throughout the stacking process as one or more electrode layers are added to the stack. To weld, preferably, a laser  283  is directed at the edge of the stack of spacers and substrates, as shown in  FIG. 28A , preferably in a low moisture, low oxygen environment. The laser welds thus produced serve as the mechanical and electrical connections for the electrodes. Alternatively, resistance welding or other attachment means known in the art may be used. 
     The positive and negative spacers are mechanical spacers. As seen in  FIGS. 8C and 8E , the positive spacer  284  is designed to fit within the negative cutout and maintain the spacing between the positive electrodes; therefore, in the preferred embodiment, the positive spacer thickness is the thickness of the negative electrode plus the thickness of the positive electrode without its substrate Oust the active material on both sides of the substrate). The negative spacer is designed to fit within the positive cutout and maintain the spacing between the negative electrodes; therefore, in the preferred embodiment, the negative spacer thickness is the thickness of the positive electrode plus twice the thickness of the negative electrode without its substrate. The spacers maintain the spacing of the electrodes so that they are not deformed and remain substantially parallel to each other. 
     In another alternative, but not preferred, embodiment, instead of having one positive electrode between adjacent positive spacers and one negative electrode between adjacent negative spacers, there could be more than one positive electrode between adjacent positive spacers and more than one negative electrode between adjacent negative spacers; for example, three electrodes between adjacent spacers. In that case, the thickness of the spacers would be increased to compensate for the reduced number of spacers. 
     In another embodiment, the positive electrode, negative electrode, or both may comprise a structure other than a substrate with active material thereon. For example, the battery may be a primary battery with a lithium metal anode with or without a substrate. As another example, one or both electrodes may comprise a foamed metal impregnated with active material. In both of these examples, there would be no need to remove an active material layer from the surface of the electrode, and the thickness of the spacers would only be that of the opposite polarity electrode. To facilitate welding an electrode having a foamed metal substrate, active material may be removed from the region to be welded, such as by the method taught in U.S. Pat. No. 5,314,544 to Oweis, which is hereby incorporated by reference in its entirety. The region to be welded may be compressed to further facilitate welding; again, the spacers would be dimensioned accordingly, as can be appreciated by one of ordinary skill in the art. 
     The spacers provide the majority of the weld material. The substrates of the electrodes provide little material to weld together. Without the spacers, the substrates would be difficult to weld. Another method of attachment would be needed. 
     As shown in  FIG. 28A , the positive tube  85  has a positive end feature  282  that has the same diameter as the positive spacer  284 . Likewise, the negative tube has a negative end feature that has the same diameter as the negative spacer. These features allow the ends of the tubes to act as spacers and capture and connect the electrode substrates  41   a  together. During the welding, this positive end feature  282  of the positive tube  85  is welded to the positive electrode that is in direct contact with it as well as to the adjoining positive spacer, and the negative end feature of the negative tube is welded to the one-sided negative electrode that is in direct contact with it as well as to the adjoining negative spacer. Because the spacers and electrodes of each polarity are connected to each other, and the end features are connected to the adjoining electrode and spacer, the end features are thereby connected to all of the electrodes. The thickness of the end feature is preferably the same thickness or thinner than that of the standard spacer. An end feature with the thickness same as a spacer allows for a more substantial weld, but increases the overall stack thickness without increasing energy, thereby decreasing energy density. A thinner end feature allows for an overall thinner stack, but allows for a less substantial weld. 
     In an alternative embodiment, instead of assembling the spacers and electrode substrates on tubes  85  and  87  while on pins  95  and  93  of stacking fixture  90  ( FIG. 9 ), the spacers and electrodes may be assembled directly onto the pins  95  and  93  of stacking fixture  90 . This embodiment is beneficial because it removes the requirement for tubes and decreases the number of parts necessary to make the battery, thus decreasing costs. 
     In another alternative embodiment, the positive and negative tubes do not have to have an end feature as discussed earlier. Also, the first and last substrate layers do not have to be sandwiched between spacers. Instead, these layers can be connected to spacers by resistance or ultrasonic welding prior to stacking. Then, the spacer-electrode assembly can be stacked in the manner described above. This embodiment decreases the number of spacers necessary for the stack and also slightly decreases the stack height, yielding a slightly higher energy density. 
     Spacer-Electrode Connection 
     The tube-spacer-electrode scheme for connecting the layers of the battery together necessitates that the area of the electrode near the spacer be cleaned of active material, leaving essentially a metal substrate. This positive electrode substrate is preferably aluminum or stainless steel, and the negative electrode substrate is preferably titanium, copper, or stainless steel. The cleaned area of the electrode is preferably at least the area of the spacer, but an extra surrounding area also generally is cleaned. For example, a 0.25-mm border may be cleaned where the spacers would be placed on the negative electrodes, and a 1-mm border may be cleaned where the spacers would be placed on the positive electrodes. 
     The rest of the configuration near the weld can vary to a large extent. As shown in  FIG. 15 , the preferred embodiment has the edge  151  of the substrate  41  tangent to the spacer edge  152 . As shown in  FIG. 16 , the holes in the electrode may be positioned a greater distance from the edge than the radius of the spacers; in that case, a cutout  153  is provided near the spacers to accommodate this and allow for the substrate  41  to maintain tangency with the spacer edge  152 . Alternatively, as shown in  FIG. 17 , the spacers may be larger in diameter; however, this decreases the capacity of the battery by wasting space due to the extended cleaned area and subsequent removal of active materials. As another alternative, as shown in  FIGS. 18–20 , the spacers may be a different shape, such as oval, rectangle, or unique shapes, for example, having protrusions  201  ( FIG. 20 ) that help in protecting the separator from the weld. When laser welding, the laser beam and plume reflect with the reflected angle equal to the incident angle. Thus, it can be advantageous to have a flat surface to weld, such as those shown in  FIGS. 19–21 . A flat surface can be achieved simply by cutting a washer to have a flat surface on one side, as shown in  FIG. 21A . However, round spacers are currently preferred for their radial symmetry, which eliminates alignment issues that are present for different shaped spacers. Also, round spacers are easier to manufacture, and thus cheaper. As an alternative, the C-shaped spacer with keyed tube or pin of  FIG. 21B  provides a flat surface for aligning with the electrode and welding, with a way of orienting the spacer. The spacer could be provided with other keyed shapes for orientation. 
     In preferred embodiments shown in  FIGS. 15–21 , the edge of the substrate is tangent to the spacer edge. Alternatively, as shown in  FIG. 22 , substrate  41  may overhang beyond the spacer, with the extra substrate providing more filler material for the weld. As another alternative weld configuration, shown in  FIG. 23 , the edge of the spacer sticks out from substrate  41  slightly so that it is not tangent to the edge. This may trap the laser beam in between the spacers, thus producing a better weld. Having more filler material is beneficial to avoid creating gaps between the spacers. Due to tolerances, tangency cannot be guaranteed. In a preferred embodiment the spacer is tangent to the substrate edge or is slightly recessed from it, as in  FIG. 22 . 
     The separator of the battery is an interlayer between layers of positive and negative electrodes. In a preferred embodiment, the separator is captured between the positive spacers on one side and the negative spacers on the other, and is also aligned using a small hole where the separator slides onto the locator tube. Thus, the separator cannot translate or swivel. As shown in  FIG. 24 , the cutouts of the separator located where it slides near the tubes are preferably flared so that the exposed area of the spacers is maximized. The angle at which the separator flares is chosen such that the separator  83  covers the corners of the electrode  241  with an extra border of 0.5 mm. The flaring of the separator cutout enables more flexibility in determining the depth at which the spacers can be placed from the edge of the battery electrodes. As shown in  FIG. 25 , if the separator cutout were not flared, the gap would be much smaller, so that it would be much more difficult, if not impossible to perform the laser welds without damaging the separator  83 . In another preferred embodiment, shown in  FIG. 26 , the separator  83  has a cutout that allows a wider gap to the depth of the tube, where there is a notch  261  that locates the separator on the tube. 
     As shown in  FIG. 27 , a shield  270  is used to cover the electrodes and separator as much as possible to prevent the weld energy from burning the edges of the separator. Directing the laser beam very near to the center of the gap between the spacer and substrate and using a shield to protect the layers of the battery avoids reflections of the laser beam onto the separator or electrodes of the battery. The shield is preferably made of metal and has very thin appendages  271  designed with the same angling as the flared cutout of the separator. The shields for the negative and positive connections are different according to the geometry and dimensions of each side. The appendages  271  span from above the stacked battery to the spacers, and are at least 0.10 mm away from the separator. If the shield were to touch the separator, it could melt it from the heat generated by the laser conducted through the shield. In addition to protecting the separator from any damage such as burning or melting, the shield helps to protect the stack from foreign particles, which may be from the plume generated by the laser. 
     As shown in  FIG. 28B , the top of the tube may be swaged over the top spacer to strengthen the connection of the spacer and electrode layers. Swaging is preferably performed prior to laser welding the stack, but may be performed instead of the laser welding. A swaging tool  280  has a special tip  281  to deform the top of the tube, pushing it out over the top spacer. This keeps the stack together mechanically. With the top of the tube pushed out, the top spacer is held tightly on the tube. Also, extra tube material above the top spacer pushes down on the spacers, keeping them from coming off. This swaging process is very similar to riveting. 
     Tube-Feedthrough Pin Connection 
     As described above, the electrodes are preferably laser welded to the corresponding spacers and tubes to form a battery stack. The tubes of the battery stack are slipped onto the feedthrough pins of the feedthroughs in the cover  49 . As shown in  FIGS. 29–31 , laser welds  293  are used to connect the feedthrough pins to the ends of the tubes, as follows. As shown in  FIG. 29 , the positive feedthrough pin  290  is approximately the same length as the positive tube  85  or up to 1 mm longer ( FIG. 31 ). A pin that is too long can create a lot of splatter of the weld. A fixture is used to cover the electrodes to protect them from splatter of welds. Preferably the feedthrough pin is not more than about 0.5 mm shorter than the tube ( FIG. 30 ) to facilitate welding. As shown in  FIG. 32 , one or more laser welds  293  are delivered to the positive feedthrough pin  297  and the positive tube  85 . The laser welds  293  are made along the border between the feedthrough pin and the tube so that the laser melts together some of the tube material with the feedthrough pin material. Five welds  293  generally ensure a good weld between the tube and feedthrough pin materials. The negative feedthrough pin and negative tube are welded in the same way. 
     In an alternative embodiment, instead of assembling the spacers and electrode substrates on tubes  85  and  87  while on pins  95  and  93  of stacking fixture  90 , the spacers and electrodes may be assembled directly onto the feedthrough pins. This embodiment eliminates the steps of welding the tubes to the stack and removing the tubes from the stacking fixture  90 . 
     In another alternative embodiment that has elements of the previous two described, the spacers and electrodes are assembled and welded onto tubes, which are on the feedthrough pins instead of on stacking fixture pins  95  and  93 . The feedthrough pins may be welded to the tubes before or after completion of the electrode assembly. 
     Neutral Case 
     The case of the battery is preferably neutral; therefore, any unintended contact of the feedthrough pin to the case will not cause a short circuit. The electrode stack is insulated on all sides, including top, bottom, and sides, making the battery safer. 
     As shown in  FIGS. 33 and 34 , in a first embodiment of the insulation, a top insulator  330  and bottom insulator  340  of Kapton® polyimide film, of thickness 0.001″ to 0.005″ insulates the stack. While this film is preferably as thin as possible for space considerations, about 0.002″ is preferable for manufacturability. Top insulator  330  has three holes for the two feedthrough pins and the locator pin. Top insulator  330  is mounted on the cover assembly over the feedthrough pins and locator pin. Then the stack is mounted on the pins. The pins are welded to the tube. After the tube of the stack is welded to the pins of the cover assembly, the stack can be inserted into the case. 
     The bottom insulator  340  is cut such that it covers the bottom and sides of the case while leaving an opening around the fill hole  370 . The bottom insulator may be cut in different shapes to achieve the same goal. In a first embodiment, the bottom insulator  340  has two flaps  341  and  342 , one on each side of a center portion  343 , which are folded such that the center portion  343  covers the bottom of the case and the flaps cover the side walls of the case and curve around the ends of the case. The cutout exposes the fill hole  370 . The cutout edges preferably maintain a 0.5-mm border from the edge of the fill hole  370  to keep the electrolyte from being pulled into the seal location due to capillary action, contaminating it and disrupting final laser weld sealing. 
     As shown in  FIG. 35 , in a second embodiment of the insulation, the bottom insulator  350  has two flaps  351  and  352  on one side of a center portion  353  that are folded such that the center portion  353  covers the bottom of the case and the two flaps cover the side walls of the case and curve around the ends of the case and overlap along the other straight side wall. The cutout exposes the fill hole  370 . Again, the cutout edges preferably are at least 2 mm from the center of the fill hole  370  to keep the electrolyte from being pulled into the seal location, contaminating it and disrupting final laser weld sealing. 
     As shown in  FIG. 36 , in a third embodiment of the insulation, the bottom insulator  360  has one flap  361  on one side of a center portion  363  folded such that the center portion  363  covers the bottom of the case and the flap covers the side wall and curves around the ends of the case. The flaps end such that an opening is left for the fill hole. Again, the cutout edges preferably are at least 2 mm from the center of the fill hole  370  to keep the electrolyte from being pulled into the seal location, contaminating it and disrupting final laser weld sealing. 
     The cover assembly with the stack is inserted into the case with the bottom insulator covering the bottom and sides of the case. The case is laser welded to the cover. The electrodes are held stationary by the tube and pin welds. The bottom insulator insulates the sides of the case from contact with any electrodes that may inadvertently break free from the stack. 
     Another embodiment that provides a neutral case is a parylene-coated case, cover, and fill plug. Parylene is a generic name for a unique series of polymers based on paraxylene. Parylene has excellent properties for use in a battery. Parylene resists chemical attack and is insoluble in organic solvents. Parylene has a dielectric strength of 5000 volts/mil. Parylene can be coated via vapor deposition onto the underside of the cover and the side walls and bottom of the case and bottom side of the fill plug. The parylene can be coated on the order of 0.0005″ thickness and achieve excellent electrical, mechanical, and chemical properties for the battery. The welded stack can be directly mounted onto the parylene-coated cover. The stack is then inserted into the parylene-coated case. Because the parylene obviates the need for the Kapton layers, more volume is available in which to add more electrode layers and hence, greater discharge capacity. 
     Filling 
     After the cover is welded to the case, the battery is filled with electrolyte through the fill hole  370  located on the side of the case  48 , as shown in  FIG. 8A . The side filling exposes the electrolyte to the edges of the stack through which the electrolyte is absorbed into the active materials on the electrodes. By introducing the electrolyte to the edges of the stack rather than to the flat surface of the top or bottom electrode, the electrolyte can easily fill all of the layers of the electrode stack, decreasing filling time. Alternatively, but not preferably, the fill hole may be located on the bottom of the case or the case cover. 
     Because filling requires space in the cell in which to inject the electrolyte for absorption into the electrode materials, replacing the polyimide sheet insulation with parylene assists in the electrolyte filling process by providing increased space for easier and faster filling. 
     After filling, a fill plug (not shown) is mounted and laser welded to seal the battery. 
     As an alternative to filling the battery with electrolyte through the fill hole  370 , a solid polymer electrolyte may be interleaved between the positive and negative electrode layers. The stacked design of the present invention lends itself particularly well to use with a polymer electrolyte. Polymer electrolyte can be die cut to the desired shape, is flexible, and can be housed in a flexible bag without the danger of leaking. 
     The specific implementations disclosed above are by way of example and for enabling persons skilled in the art to implement the invention only. We have made every effort to describe all the embodiments we have foreseen. There may be embodiments that are unforeseeable and which are insubstantially different. We have further made every effort to describe the invention, including the best mode of practicing it. Furthermore, various aspects of the invention may be used in other applications than those for which they were specifically described herein. For example, the stacked electrode configuration having spacers for connections may be used in capacitors as well as batteries. Any omission of any variation of the invention disclosed is not intended to dedicate such variation to the public, and all unforeseen, insubstantial variations are intended to be covered by the claims appended hereto. Accordingly, the invention is not to be limited except by the appended claims and legal equivalents.