Patent Application
20080221629
The invention relates to lithium batteries, and more particularly, to lithium batteries for use in implantable medical devices.
A variety of implantable medical devices are used to provide medical therapy to patients. One example of a type of implantable medical devices is a cardiac rhythm management (CRM) device. CRM devices may include, for example, pacemakers and implantable cardioverter defibrillators (ICD). These devices generally function to provide treatment to a patient having a disorder relating to the pacing of the heart, such as bradycardia or tachycardia. For example, a patient having bradycardia may be fitted with a pacemaker, where the pacemaker is configured to monitor the patient's heart rate and to provide an electrical pacing pulse to the cardiac tissue if the heart fails to naturally produce a heart beat in a given time interval. By way of further example, a patient may have an ICD implanted to provide an electrical shock to the patient's heart if the patient experiences atrial fibrillation.
These implantable medical devices require power to operate and this power is typically provided by a battery within the device. Battery performance is a key aspect of the performance of the device. Long battery life is very important because battery replacement generally involves surgery, which is inconvenient for the patient, expensive, and exposes the patient to the risk of complications. Battery reliability is also important because of the critical nature of functions that may be performed by an implanted medical device.
Implantable medical devices commonly use lithium-based batteries. Lithium batteries have a number of advantageous characteristics that make them desirable for use in implantable medical devices. Lithium batteries can be formed into a wide variety of shapes and sizes to efficiently make use of the space available within the implantable device. Lithium batteries also have high power to weight ratios and high power density, which allows for smaller battery size and longer battery life for a given size.
The power output requirements of batteries used in implantable medical devices depend on the type of medical device. For example, batteries used in an ICD have both a low rate requirement and a high rate requirement. The battery must provide continuous low rate current to supply power control electronics and to provide low output therapy in the form of cardiac pacing pulses. The battery must also provide occasional high current pulses to charge high voltage capacitors, which in turn are used to defibrillate the patient's heart. Often more than one pulse is required to defibrillate a patient requiring, thus the battery is capable of delivering multiple high current pulses in a quick succession.
It is important that a lithium battery for use in an implantable medical device be able to satisfy the device power requirements for as long of a period as possible. However, the techniques used to construct the battery can have a significant influence on the performance of the battery and the battery's ability to satisfy the power requirements. For example, in an embodiment of a battery where a lithium sheet is laminated to a current collector, the performance of the battery depends significantly on the quality and durability of the contact between the lithium element and the current collector. Battery performance will be degraded if the contact between the lithium and the current collector is poor. Improved techniques for laminating lithium sheets are needed.
One aspect of the invention relates to a method including the steps of providing a sheet of lithium and a sheet of substrate material and pressing the sheet of lithium and the sheet of substrate material together in a die to form an electrode, where the die has at least one surface that includes a plurality of force concentrating features configured to create regions of relatively higher pressure and regions of relatively lower pressure in at least one of the sheet of lithium and the sheet of substrate material. The method further includes the steps of assembling a battery including the electrode in a battery housing and assembling an implantable medical device including the battery housing.
A further aspect of the invention also relates to a method including the steps of providing a sheet of lithium material and a substrate material and applying force to the sheet of lithium and substrate material to form a plurality of protrusions on at least one of the sheet of lithium and the substrate material. The method further includes the step of pressing at least a portion of the sheet of lithium to the substrate material with sufficient force to at least partially deform the protrusions and form an electrode. The method further includes the steps of assembling a battery including the electrode in a battery housing and assembling an implantable medical device including the battery housing.
Another aspect of the invention relates to a tool for processing a sheet for use in an electrode of a battery. The tool includes a first rigid surface that has a plurality of force concentrating features. This first rigid surface is configured to engage with a surface of the sheet. The tool further includes a second rigid and generally smooth surface that is positioned opposite to the first rigid surface. The tool also includes a mechanism to force the first and second surfaces together against at least the sheet with sufficient force that the plurality of force concentrating features will at least partially deform the sheet.
Yet another aspect of the invention relates to a tool for processing a sheet for use in an electrode of a battery. The tool includes a first rigid surface that has a plurality of protrusions, where the first rigid surface is configured to engage with a surface of the sheet, and a second surface, where the second surface has one or more openings that are configured to provide clearance opposite to the protrusions of the first rigid surface when the first and second surfaces are pressed together. This second surface is configured to engage with at least a portion of the second surface of the sheet. The tool also includes a mechanism to force the first and second surfaces against the sheet with sufficient force to cause the plurality of protrusions to deform the sheet.
An additional aspect of the invention relates to an implantable medical device including a housing, at least one lead, and circuitry configured to send and receive electrical impulses through the lead. The implantable medical device further includes a lithium battery configured to provide electrical power to the circuitry. The lithium battery includes an electrode having a lamination formed from at least a sheet of lithium and a sheet of a substrate material. The lamination has either a plurality of partially compressed protrusions on the sheet of lithium or a plurality of projections of the sheet of lithium into the surface of the sheet of substrate material.
The invention may be more completely understood by considering the detailed description of various embodiments of the invention that follows in connection with the accompanying drawings.
While the invention may be modified in many ways, specifics have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives following within the scope and spirit of the invention as defined by the claims.
One typical lithium battery type for an implantable medical device is a lithium-manganese dioxide (Li/MnO2) battery. However, other lithium battery chemistries are also usable. Lithium batteries are typically constructed from a number of thin sheets of different materials that are sandwiched together to form a battery assembly. An example embodiment of a lithium battery constructed in this way is shown in
The anode assembly 24 is formed from a sheet of lithium material that constitutes an anode 32 and a material that constitutes a current collector 34. Various embodiments of the application include methods and tools for laminating a sheet of lithium material to a substrate, such as a current collector. Another embodiment of the invention is an implantable medical device including a lithium battery that has an electrode constructed according to the present invention. The current collector may be constructed from a number of different materials. For example, the current collector 34 may be constructed from, among other alternatives, nickel or nickel-based material, stainless steel, aluminum, titanium, or copper. The current collector 34 may comprise a uniform sheet, a wire grid, or other configurations. For convenience of description, the current collector 34 will be referred to below as being constructed from a sheet of nickel or nickel-based material, but it will be recognized that the current collector 34 may alternatively be constructed from any suitable material or configuration without departing from the principles disclosed herein.
The anode assemblies 24 are constructed by pressing together a lithium anode 32 to a nickel current collector 34. The process of pressing the lithium anode 32 to the nickel current collector 34 may also be called lamination.
The elements of repeating arrangement 22 are generally constructed in a shape that is configured to suit the space available in the implantable medical device. Each of the elements of repeating arrangement 22 generally has the same shape. However, one important aspect of repeating arrangement 22 is the need to prevent direct contact between the cathode 28 and the anode 24. If direct contact were to occur, rapid electrical discharge would occur with significant electrical current flowing between the components. This situation could impair the operation of the battery. For this reason, it is important to ensure that the cathode 28 will not contact the anode 24. However, in some embodiments, the cathode 28 is formed from a die cut metal grid that has been coated with a slurry mixture. For these embodiments, the edges of this die cut metal grid can be very sharp and may be able to penetrate the separator 26 or 30, allowing direct contact between the anode 24 and cathode 28. To minimize the likelihood of this happening, the cathode 28 is typically constructed to be slightly larger than the anode 24, to ensure that the cathode 28 edges are not proximate to the anode 24 and therefore not prone to contact the anode 24.
The lithium anode 32 is preferably constructed to be in complete, firm contact with the nickel current collector 34. Operation of the battery requires that electrons transfer between the lithium anode 32 and the nickel current collector 34. However, this can only occur at locations where the anode 32 is actually in contact with the current collector 34. If there are regions of the anode 32 and current collector 34 that are not actually in contact, even though they may be very close to each other, current will either not be able to transfer at such locations or the resistance to current will be high. The presence of locations where current cannot transfer or is subject to high resistance will undesirably limit the performance of the battery. For example, the rate of discharge capabilities of the battery may be degraded. This effect may be especially noticeable near the end of battery life. For at least these reasons, the lithium anode 32 and the nickel current collector 34 are preferably brought together in a fashion that promotes complete, firm contact between them. A lamination process is typically used to create the contact between the lithium anode 32 and the nickel current collector 34.
A further issue exists with respect to the contact between the anode 32 and the current collector 34. Namely, lithium has a tendency to form a layer that consists of lithium byproducts on its surface. Not wanting to be bound by theory, it is believed that lithium reacts with certain materials that it comes in contact with, such as gases within air, materials in processing and manufacturing equipment, and materials in storage or handling equipment. For example, it is believed that lithium reacts with atmospheric N2 and CO2 to form byproducts on its surface. When the lithium anode 32 is pressed against the current collector 34, a layer of lithium byproducts can inhibit the flow of electrons from the anode 32 to the current collector 34. It is therefore desired that the contact between the lithium anode 32 and the current collector 34 be directly against a fresh lithium surface without having byproducts at the contact surface.
A lamination of anode 32 and current collector 34 is typically created by applying pressure to the lithium anode 32 and the nickel current collector 34 to force them together. An issue arises, however, because lithium is very malleable. As pressure is applied to the lithium anode 32, it has a tendency to extrude and expand outward. This extrusion outward can cause the lithium anode 32 to approach, or equal or exceed, the size of the cathode 28 that is constructed to be nominally larger than the lithium anode 32. In some cases, the amount of pressure that can be applied to the lithium anode 32 without causing excessive extrusion is not sufficient to properly laminate the lithium anode 32 to the current collector 34. If the pressure applied to the lithium anode 32 is increased in an attempt to better laminate it to the current collector 34, the lithium may extrude outward excessively, possibly to a size that would make it more difficult to align the cathode 28 and anode 32 to avoid direct contact, or to a size that would allow direct contact between the cathode 28 and anode 32 regardless of the alignment of the cathode 28 and anode 32 after the battery is assembled. Because higher pressure lamination tends to cause increased extrusion of the anode and consequently reduced accuracy in the anode size, simply increasing the lamination pressure would increase the likelihood of a short circuit.
A die assembly configured for advantageously laminating an anode 32 to a current collector 34 is depicted in
In one embodiment, pressure concentrating features 78 include a plurality of depressions 80 in material support surface 76. These depressions 80 may take any of a number of shapes and configurations.
In yet another embodiment, pressure concentrating features 78 include a plurality of embosses or protrusions. These protrusions may take any of a number of shapes and configurations.
An alternative embodiment of a die base 172 having pressure concentrating features 178 on a material support surface 176 is shown in
In other embodiments, pressure concentrating features 178 are a series of intersecting ridges 142 in material support surface as shown in
One process of forming an anode assembly from an anode 32 and a nickel current collector 34 will now be discussed with reference to
Generally, the lithium anode 32 will be in contact with, or facing, the material support surface 76 of die base 72, and the nickel current collector 34 will be in contact with, or facing, the die pressure plate 74. This process will be described in detail, though it is also possible for the current collector 34 to be positioned in contact with, or facing, the material support surface 76 of the die base 72.
In some embodiments, a film may be provided between the die base 72 and lithium anode 32 and between die pressure plate 74 and current collector 34 to minimize sticking of the elements of anode assembly 24 to die assembly 70. In one embodiment, the contact surface 88 of die pressure plate 74 is uniform and generally flat. In another embodiment, contact surface 88 has a plurality of openings that are configured to be subjected to a vacuum. These vacuum openings may be useful for holding the current collector 34 in position with respect to the die pressure plate 74 while the components are being prepared to be laminated. Other surface characteristics or profiles of pressure plate 74 are also usable.
Once the cathode 32 and current collector 34 are positioned within die assembly 70 as shown in
The presence of high pressure zones cause localized pressures within the lithium anode 32 that are greater than the pressures that would be imparted by a standard flat die for the same die assembly force. These greater localized pressures promote complete, firm contact between the lithium anode 32 and the current collector 34, particularly within the high pressure zones. Furthermore, because the high pressure zones exist only in discrete regions of the anode 32 and current collector 34, with the balance of the surface area being under significantly lower pressure, the lithium that extrudes outward under pressure from the high pressure zones will tend to be received by, and expand into, the relief areas defined by the pressure concentrating features 78 such as depressions 80 or grooves 82 or by the space between pressure concentrating features 78 such as protrusions 140 or ridges 142. These relief areas tend to minimize the possibility of the lithium extruding outward under pressure beyond the edges of the lithium sheet. This in turn limits the problem of the anode 32 being prone to contact with the cathode 28.
The existence of high pressure zones has the further advantage of causing microextrusion at the surface of the lithium anode 32. As mentioned, the high pressure zones create areas of higher pressure than exists at the relief areas, and this pressure differential tends to cause the lithium anode 32 material to extrude into the relief areas, such as depressions 80 or grooves 82 or the space between features such as protrusions 140 or ridges 142. This extrusion causes material on the face of lithium anode 32 to be translated laterally, and this lateral translation causes breaks, fissures, and smearing in the surface of the lithium anode 32. These breaks, fissures, and smears provide a fresh lithium surface, which has generally not reacted to form byproducts, to be exposed to the current collector 34. This fresh surface makes more effective and durable electrical contact with the nickel current collector 34 as compared with a surface that includes a layer of byproducts. In some cases, the existence of high pressure zones can cause portions of the lithium anode 32 to project into the surface of the nickel current collector 34.
Now referring to
Many embodiments of second die assembly 92 are usable. One embodiment is depicted in
Structured anode assembly 99 is placed within second die assembly 92 and force is applied to the structured anode assembly 99 through base 94 and pressure plate 95. The amount of force applied is generally chosen to be sufficient to at least partially deform and flatten any features on the lithium anode 32 that extend away from the primary plane of the anode 32. Such features may extend away from the primary plane of the anode 32 as a consequence of the uneven application of pressure to the anode 32 through the high pressure zones corresponding to the pressure concentrating features 78. By at least partially deforming and flattening any features that extend from the lithium anode 32, the surface of the lithium anode may be transformed to a surface that is either substantially flat or that has a plurality of partially compressed protrusions or depressions. Applying force to the surface of anode 32 to produce a flatter overall surface of the anode assembly 24 may have several advantages, including denser packaging of the battery elements.
A previously structured surface that has had force applied to at least partially deform and flatten features on the surface may be identified by a number of means, such as through microscopic evaluation of the surface or a cross-section through the lithium sheet for characteristic indications of material flow, folding, or compression. In addition, the surface of a lithium anode 32 that has had compressed protrusions or depressions and has been used in a battery for at least a period of time can show regions of preferential lithium usage, where these regions of greater lithium usage indicate areas that have been in more complete contact with the current collector, such as at compressed protrusions or depressions. Similarly, a surface of the current collector may also be analyzed to detect the presence of indentations or other surface features representative of the existence of regions of high pressure with the lithium sheet. An atomic force microscope or scanning electron microscope can be used to detect these features, for example.
Now referring to
In one embodiment, protrusions 108 are characterized by ridges that extend laterally across substantially the entire cylinder width and have a square or rectangular or triangular cross section and a height relative to the cylinder of 0.0004 inches (0.010 mm). In other embodiments, protrusions have a height relative to the cylinder of 0.0001 to 0.005 inches (0.0025 to 0.127 mm). In another embodiment, protrusions 108 are characterized by a series of conical protrusions having a height relative to the cylinder of 0.0001 to 0.005 inches (0.0025 to 0.127 mm). The protrusions 108 may have profiles similar to the ridges 142 of
In operation, first roller 102 is driven, such as by a motor, causing a sheet of lithium 106 to be pulled into the nip formed between first roller 102 and second roller 104. The driving action of lithium sheet 106 causes second roller 104 to rotate. However, the thickness of lithium sheet 106 is generally larger than the shortest distance between first and second rollers 102, 104. Consequently, the protrusions 108 on first roller create corresponding pressure concentrating features 298, such as depressions 110 and protrusions 124, on lithium sheet 106 after it has been pulled through the nip of the rollers 102, 104. The shape of the pressure concentrating features 298 will generally correspond to the shape of the protrusions, such that, for example, a series of protrusions 108 having square or rectangular cross sections will produce depressions 110 having a square cross section. Alternatively, a series of protrusions having conical cross sections will produce depressions having a conical cross section.
As an alternative to the die assembly 100, the die assembly 70 of
Yet another alternative embodiment of a die assembly for imparting pressure concentrating features onto a lithium sheet or piece alone is shown in
Another embodiment of a die assembly 200 for preprocessing a lithium sheet or piece is illustrated in
After a sheet or piece of lithium has been processed in a die assembly such as die assembly 72, 100, 126, 172 or 200 to form a structured surface, it is further processed in a customary manner to form an anode assembly. If a web or sheet of lithium was processed to form a structured surface, then the web or sheet of lithium is die-punched to form a lithium piece of the desired shape and profile for a battery. As illustrated in
The preceding description of the various aspects of the invention has been limited for clarity and ease of description to describing processing of the lithium anode 32 and features associated with the lithium anode 32. However, it will be appreciated that the invention is also fully applicable to processing of the current collector 34 and to features associated with current collector 34. Any processing step that has been described with reference to anode 32 is equally applicable to current collector 34 and is expected to produce similar benefits. Likewise, any feature that has been described with reference to anode 32 is equally applicable to current collector 34 and is expected to produce similar benefits. By way of example but without limitation, pressure concentrating features may be formed on current collector 34 instead of on anode 32.
Additional details and examples related to the components of a battery for an implantable medical device will now be described, with reference to
The separator 26 is designed to allow ionic communication between anode assembly 24 and cathode 28 while preventing electrical contact between the two components. The separator 26 may be constructed from any of a number of materials. For example, in various embodiments, separator 26 is constructed from a microporous polypropylene membrane, a microporous polyethylene membrane, or a layer of polyethylene laminated between two layers of polypropylene. In one embodiment, a separator including polypropylene provides relatively high strength and toughness while the relatively low melt temperature of the polyethylene is advantageous in the event of a short circuit. During a short circuit, the elevated cell temperature causes the pores of separator 26 to melt together or “shut down,” which in turn reduces Li ion transport and causes the cell to cool down safely.
Many embodiments of lithium anode 32 and current collector 34 are usable. In various usable embodiments, the lithium anode 32 is about 0.001 to 0.05 inches (0.025 to 1.27 mm) thick, and in some embodiments, about 0.002 inch (0.05 mm) thick or 0.003 inch (0.08 mm) thick or 0.005 inch (0.127 mm) thick. In various usable embodiments, nickel current collector 34 is about 0.0001 to 0.005 inches (0.0025 to 0.127 mm) thick, and in some embodiments about 0.001 inch (0.025 mm) thick or 0.002 inch (0.05 mm) thick.
The anode assembly 24 also includes a tab 40 (as shown in
In various embodiments, the cathode includes at least one metal oxide, metal sulphide, metal selenide, metal halide or metal oxyhalide compound or their corresponding lithiated forms. The cathode 28 may include manganese, vanadium, silver, molybdenum, tungsten, cobalt, nickel, or chromium. The cathode may also include a main group compound such as carbon monofluoride or iodine. Other compositions of the cathode are within the scope of this disclosure. In one example, the cathode 28 is formed from a mixture that includes manganese dioxide (MnO2) active material. In one embodiment, cathode 28 includes a substrate that is coated with slurry composed of an active material, which is then dried, calendared, and die cut to a final shape. According to various other embodiments, the cathode is made from compressed powder, dough and/or slurry.
In one embodiment, the cathode substrate is constructed from tabs 42 (as shown in
The edges of the cathode 28 contain a sheared metal material, which is quite sharp and can easily puncture the plastic separator 26, 30. For this reason, the cathodes 28 are preferably constructed to be larger than the anode assemblies 24. Preferably, the cathodes 28 are larger than the anode assemblies 24 by at least an amount that represents the size of the sheared material region, plus the expected variability in size, plus a margin of safety. In various example embodiments, cathodes 28 are constructed to be 0.005 to 0.1 inches (0.12 to 2.5 mm) larger than anode assemblies 24, and about 0.02 inches (0.5 mm) larger than anode assemblies.
A plurality of repeating arrangements 22, and possibly one additional cathode 38, are assembled together into a stack 44. After stack 44 is assembled, the cathode tabs 42 which were welded to the cathode are laser welded to each other to create a parallel interconnect configuration, thereby forming a cathode stack. Furthermore, the anode tabs 40 are welded together to create a similar parallel interconnect configuration on the anode side, forming an anode stack. A stainless steel case 38 is provided to contain stack 44, the case 38 being formed from two halves 37, 39 that are configured to mate together to contain stack 44.
In the embodiment shown in
The case 38 may also include a fill opening 58, as shown in
Referring now to
The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.
