Magnetic pulse welding in medical power manufacturing

A magnetic pulse welding process for joining a current collector to a terminal pin in the construction of electrochemical cells is described. The magnetic pulse welding process utilizes a pulsed direct current and an electrically conductive coil to generate an electro-magnetic force that causes two work pieces to collide with each other and form a bond therebetween. Preferably, the method is used to bond the terminal pin to the cathode current collector. This method of attachment is suitable for either primary or secondary cells, particularly those powering implantable biomedical devices.

FIELD OF THE INVENTION

The present invention relates to the art of electrochemical cells, and more particularly, to an improved method of connecting a current collector to a terminal pin. The present invention is of a magnetic pulse welding method by which the terminal pin is directly connected to the current collector.

PRIOR ART

The recent rapid development in small-sized electronic devices having various shape and size requirements requires comparably small-sized electrochemical cells of different designs that can be easily manufactured and used in these electronic devices. Preferably, the electrochemical cell has a high energy density, and one commonly used cell configuration is a prismatic, case-negative cell design having an intermediate cathode flanked by opposed anode components in contact with the casing and in electrical association with the cathode.

The diverse variety of materials used in the construction of electrochemical cells increases the difficulty of assembling and manufacturing such small intricate devices. It is desirable to build such electrochemical cells with simplified procedures that create an electrochemical cell with a durable and robust construction. Such electrochemical cells require joining various internal components, composed of differing materials, with a strong durable bond. One of these critical connections is that of the terminal pin to the current collector. The terminal pin connects the electrochemical cell's internal current collector to a load such as an implantable medical device.

However, because of the diverse materials with their respective distinct material properties, it is sometimes difficult to join and bond these components together. Typical bonding techniques, such as standard laser and resistance welding practices, are not always ideal in joining components such as terminal pin and current collector materials.

Specifically with respect to the electrochemical cell, joining the terminal pin, typically composed of molybdenum, to that of the current collector, typically composed of aluminum or titanium, has been historically problematic. Previously, intermediate materials and processes have been used to accomplish the joining and bonding of these components made of diverse materials. These intermediate materials and processes add undesirable cost and complexity to the construction of electrochemical cells. Furthermore, such intermediate materials and processes can create brittle bonds that may not be sufficiently robust.

Moreover, the use of laser welding is not ideal. Laser welding typically requires that a cavity be burned into a first material, such as a terminal pin. This cavity is then filled with a second material which creates a metallurgical bond. Such a cavity decreases the cross sectional area of the terminal pin thereby decreasing its strength and possibly creating a brittle bond. Furthermore, laser welding requires exacting precision in bonding the materials together, which adds manufacturing complexity.

In addition, other welding techniques such as resistance welding rely on the application of heat creating a diffused intermetallic bond within a heat affected zone. The creation of such an intermetallic bond through the formation of a heat affected zone, may not be possible given the distinctive compositions of the work pieces. Furthermore, the joining of such materials through the formation of a heat affected zone, may create an undesirable brittle bond.

The present invention provides an improved means of joining dissimilar materials. More specifically, the present welding method enables an improved joining of different materials that are utilized in the manufacture of electrochemical cells. The present invention eliminates the need for intermediate materials as well as the previously described laser welding processes. Such a direct weld procedure reduces cost, complexity and creates a more robust connection. The magnetic pulse welding process of the present invention is fast, simple, easy to control and effective.

SUMMARY OF THE INVENTION

The present invention relates to a method of connecting an electrode current collector, particularly the tab of the current collector, to a terminal pin. Such a configuration forms a direct connection of the terminal pin to the current collector at the tab to provide an electrical connection therebetween. The present invention further relates to a method of connecting the terminal pin to the current collector of different material compositions, geometries and configurations. The present invention is a method of using a magnetic pulse welding process to form a direct connection between the terminal pin and current collector.

In this magnetic pulse welding method, two dissimilar materials, particularly of significantly dissimilar melting temperatures, are joined together in a strong bond. In that respect, the present invention comprises a method by which materials having dissimilar melting temperatures are directly joined by the application of an electro-magnetic force over a short duration of time. Magnetic pulse welding works by generating an electro-magnetic force that physically drives a first material into a second material such that they collide together. The impaction force generated during the collision of the two materials is of enough energy that the two materials become permanently joined together.

A magnetic pulse welding instrument generally comprises a power source, an electrical current switching or pulsing system and a coil structure. The coil, generally comprised of a metallic material, induces a magnetic field about the exterior surface of the coil when an electrical current is applied by the power source. Interaction of the work piece(s) with the magnetic field ultimately induce an electro-magnetic force which enables creation of the weld joint.

In a typical magnetic pulse welding process, work pieces to be joined are first inserted within or are placed adjacent to the coil structure. A pulsating direct electric current is applied to the metallic coil at a relatively high pulsing rate. Application of the pulsing direct electric current to the coil generates a magnetic field flux around the exterior surface of the coil or portion thereof. The magnetic field flux, in the presence of the work piece(s), induces an eddy current within the surface of the work piece(s). The eddy current within the work piece opposes the magnetic field, and a repulsive electro-magnetic force, acting perpendicularly away from the magnetic field, is created. This electro-magnetic force drives the work pieces together at a high rate of speed, thereby creating an impact type weld at the collision site.

Creation of the electro-magnetic force is generally described by John Fleming's left-hand rule which states that the interaction of a magnetic field flux (B) and an eddy current (i) creates a electro-magnetic force acting away and perpendicularly from the magnetic field. This electro-magnetic force physically moves a first material, positioned proximal to an energized portion of the coil, into a second material at a high rate of speed. The force of the impaction of the two materials thereby creates a permanent bond therebetween in the order of micro seconds.

Thus, the magnetic pulse welding process is capable of joining metals of dissimilar composition, melting temperature, and/or mechanical properties. Unlike laser and resistance welding, magnetic pulse welding is considered to be a solid state joining process. Direct application of heat, such as through a laser beam or electrical resistance, is not used in magnetic pulse welding. As a result, a heat affected zone at the intersection of two materials is not formed and thus formation of intermetallic bonds is minimized. Bonding during magnetic pulse welding occurs in a fraction of a second, thereby minimizing extensive inter diffusion of materials as is typically the case with other welding techniques. Such intermetallic bonds, particularly those formed within heat affected zones, typically exhibit poor durability and are, therefore, not generally desired for use in an electrochemical cell. Furthermore, because of dissimilarities in composition, not all metals are capable of being joined by laser and resistive welding techniques. In either case, the combination of possible joined materials is limited when utilizing joining techniques such as laser and resistive welding.

In a preferred embodiment, a molybdenum terminal pin is directly joined to an aluminum current collector. Generally, a current collector is in electrical contact with the active material that comprises the anode, the active material that comprises the cathode, or both. The terminal pin may be joined to a single anode or cathode current collector or to multiple anode and cathode current collectors, depending on the specific design and application requirements of the electrochemical cell. The present invention can also be utilized in a variety of rechargeable or non-rechargeable electrochemical cell designs and chemistries. That is in both case negative and case positive designs as the negative terminal. In a case negative design, the anode is connected to the casing as the positive terminal. Alternatively, in a case positive design, the cathode is connected to the casing.

Furthermore, the welding process of the present invention is not limited to the connection of a molybdenum terminal pin to an aluminum current collector. Such a welding process can also be used to directly bond a series of metals of dissimilar melting temperatures, preferably metals in which their respective melting temperatures vary significantly. Furthermore, the welding process of the present invention is not limited to a specific geometry. The material to be welded may be of a plurality of geometries such as, but not limited to, a rectangular form, a curved body or a multi-sided polygon shape.

Thus, the present invention overcomes many inherent difficulties in constructing an electrochemical cell. The present invention increases the cell design capabilities by allowing the terminal pin to directly join to a wide variety of metals of differing melting temperatures that were previously not capable of being bonded together with a resistance or a laser weld process. The present invention reduces manufacturing cost and reduces construction complexity. The present invention also allows for the utilization of different cell chemistries requiring the use of different current collector materials that would not normally allow for a direct connection with the terminal pin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now toFIGS. 1 and 2, there is shown an electrochemical cell10comprising a casing12having spaced-apart front and back walls14and16joined by curved end walls18and20and a curved bottom wall22. The end walls can be curved to provide the casing having an oval cross-section, or they can be generally planar to provide a rectangular or prismatic cross-section. The casing has an opening24provided in a lid26used for filling the casing12with an electrolyte after the cell components have been assembled therein and lid26has been welded to casing12. In its fully assembled condition shown inFIG. 1, a closure means28is hermetically sealed in opening24to close the cell. A terminal pin30is electrically insulated from lid26and casing12by a glass-to metal seal32, as is well known to those skilled in the art.

Cell10comprises an electrode assembly34(FIG. 2) that further comprises anode electrode components36,38and cathode electrode components40,42prevented from contacting each other by a separator membrane44. The anode electrode components36,38are composed of an anode active material supported on an anode current collector46. The cathode electrode components40,42are composed of an active cathode material that is supported on a cathode current collector48. As shown inFIG. 2, the cathode current collector48may comprise a tab50that extends outwardly therefrom.

The embodiment shown inFIG. 1is commonly referred to in the art as a case negative cell. Case negative electrochemical cells are constructed with anode electrode components36,38that are electrically connected to casing12via the anode current collector46while the cathode electrode components40,42are electrically connected to the terminal pin30via the cathode current collector48. Alternatively, a case positive cell design may be constructed by reversing the connections. In other words, terminal pin30is connected to the anode electrode components36,38via the anode current collector46and the cathode electrode components40,42are connected to the casing12via the cathode current collector48.

Both anode current collector46and the cathode current collector48are composed of an electrically conductive material. It should be noted that the electrochemical cell10of the present invention as illustrated inFIGS. 1 and 2can be of either a rechargeable (secondary) or non-rechargeable (primary) chemistry of a case negative or case positive design. The specific geometry and chemistry of the electrochemical cell10can be of a wide variety that meets the requirements of a particular primary and/or secondary cell application.

As shown inFIG. 2, the cathode current collector48of the present invention generally comprises a screen52, an internal connection tab54in the form of a land that is co-planar with and surrounded by screen52, and an integral external connection tab56. Screen, as pertains to the present invention, is defined herein as a surface on which cathode or anode active material is deposited. A screen may be comprised of a foil having a solid surface or of a mesh or grid having a series of perforations throughout its surface.

Anode current collector46of the present invention, similarly to the cathode current collector48, also generally comprises a screen52, an internal connection tab54in the form of a land that is co-planar with and surrounded by screen52, and an integral external connection tab56.

In a first embodiment of the present invention, external connection tab56, of either an anode current collector46or cathode current collector48, is an outwardly extending continuation of internal tab54. External tab56may not necessarily be coplanar with internal connection tab54and screen52. External tab56may be of an extended, elongated strip of metal such as in a ribbon or coil form, which may not be coplanar with either.

As shown inFIG. 3, weld area58is where two metals meet and are joined together. The weld area58may comprise the location in which two metals of similar or dissimilar composition are joined together. The weld area58may further comprise the location where terminal pin30is joined to a region of the current collector46,48such as the external tab56. Thus, depending on the desired position of terminal pin30in cell10, connection tabs54and56can be of various lengths or shapes to provide additional flexibility in joining the terminal pin30to the current collector48.

For example, if the design of the cell10requires terminal pin30to be positioned closer to or farther away from the center of lid26, the current collector48of the present invention easily accommodates the design changes without having to be changed itself. Terminal pin30is simply joined to a different contact point on either the internal connection tab54or the external connection tab56. Terminal pin30may also be joined directly to the current collector screen52. Of course, there may be cell constructions where it is desirable to connect terminal pin30to multiple locations along the current collector48. Such locations may include but are not limited to, the internal tab54, the external tab56and the current collector screen52. In addition, multiple current collector tabs56may be connected to terminal pin30.

It will be apparent to those skilled in the art that terminal pin30can be directly joined to the current collector48at any contact point along the extent of the internal tab54and the external tab56by using the present magnetic pulse welding procedure. It will also be apparent to those skilled in the art that terminal pin30may be joined at any point along the anode or cathode current collector46,48.

FIG. 3illustrates a case negative embodiment in which cathode current collector48is directly joined to terminal pin30. It is contemplated that anode current collector46could be substituted for cathode current collector48creating a case positive cell design. Such an embodiment is illustrated inFIG. 6in which the terminal pin30is directly joined to either the anode or cathode current collector46,48.

FIGS. 4 and 5illustrate an additional embodiment of joining dissimilar metals in an electrochemical cell10through the use of magnetic pulse welding. In this embodiment, a coupler60is used to join different metals together. As illustrated, the coupler60bridges the two dissimilar materials of the terminal pin30together with the cathode current collector48. The coupler60comprises an inlet that receives the proximal end of the termination pin30while an opposite distal end of the coupler60generally comprises a planar end on to which a current collector tab50may be connected. In this embodiment, the terminal pin30is welded within the inlet portion of the coupler60and the current collector tab50is welded to the planar distal portion of the coupler60. In this case, magnetic pulse welded joints form the respective connections. The coupler60may not only bridge the connection between two dissimilar materials but may also act as an extension between two materials in which there is a gap of space that prohibits direct bonding of the materials.

FIG. 6illustrates an embodiment in which a side edge62of a current collector screen52is joined to the surface of the terminal pin30along vertical axis A-A using the present magnetic pulse welding method. It is contemplated that terminal pin30is not necessarily limited to being joined at the end of side edge62as depicted inFIG. 6. Terminal pin30may be joined distal of side edge62along any portion of the screen52surface. It is further contemplated that the coupler60may be connected external of the cell10. In this case, the coupler60is joined to the distal end of the terminal pin30.

As illustrated inFIG. 6, the joining of terminal pin30to the side edge62may comprise multiple discrete weld areas58or a single weld area58that extends along the entire length of side edge62of either an anode or cathode current collector46,48. Such an anode or cathode current collector46,48may be incorporated with an anode active material or cathode active material of the respective current collector46,48. It is contemplated that the embodiment shown inFIG. 6would be useful in constructing electrochemical cells10of small compact sizes, such as in a “jelly roll” design (not shown). In a “jelly roll” design, a single or multiple current collectors46,48are wound around a central vertical axis A-A of the terminal pin30. The jelly roll design thus enables a small round compact electrochemical design.

In an alternate embodiment, terminal pin30may be connected to multiple anode and cathode current collectors46,48as illustrated inFIG. 7.FIG. 7shows an embodiment illustrating electrode assembly34comprised of multiple cathode electrode components40,42and anode electrode components36,38. As illustrated inFIG. 7, cathode electrode components40,42and anode electrode components36,38are proximate each other in an interleaved, alternating manner.

In the alternate embodiment shown inFIG. 7, the interleaved electrode assembly34is constructed by alternating cathode electrode components40,42with that of anode electrode components36,38. Each of these cathode electrode components40,42and anode electrode components36,38are incorporated with their respective current collectors46,48with an external tab56(FIG. 3) that extends outside each collector46,48.

As illustrated in the embodiment shown inFIG. 7, a bridge64is formed from external connection tabs56of cathode current collectors48. Bridge64of the illustrated embodiment is comprised of portions of electrically conductive external tabs56of cathode current collectors48that are folded over each other establishing electrical connection therebetween amongst the plurality of cathode current collectors48. The associated anode current collectors46are electrically connected to casing12. Therefore the illustrated embodiment, as shown inFIG. 7, is of a case negative cell design. An alternatively preferred embodiment is of a case positive design. In the contemplated case positive cell design, bridge64may be comprised of portions of anode current collectors46.

As shown inFIG. 7, a lead66is electrically connected to the series of current collector tabs56that comprise bridge64. Lead66is preferably composed of a first metal, most preferably aluminum or titanium. AsFIG. 7illustrates, a portion of the terminal pin30is joined to lead66using the present magnetic pulse welding method, thereby creating an electrical connection between the electrode assembly34and terminal pin30. It is contemplated that terminal pin30may be joined directly to bridge64using the magnetic pulse welding method of the present invention.

The magnetic pulse welding technique may also be used to directly attach the anode and/or cathode current collector46,48to the lid26. As illustrated inFIG. 8, two current collectors46,48are directly attached to a top surface68of the lid26. More specifically, a current collector extension70comprising a land of metal extends from the end of the current collector46,48. As shown, the current collector extension is welded to the top surface68of the lid26thereby creating a hinged relationship between the current collector46,48and the lid26.

FIGS. 9A-9D, illustrate various embodiments of a magnetic pulse welding instrument72. The instrument generally comprises a coil74composed of an electrically conductive material, such as a metal, that is electrically connected to an electrical power source76. In a preferred embodiment, the power source76comprises a capacitor78or plurality of capacitors78that are electrically connected to an electrical ground82. In a preferred embodiment, the capacitor(s)78generate a direct electrical current80that is applied to the coil74. As illustrated, inFIGS. 9A, 9B and 9D, the current80generally flows in the direction towards the capacitor78and away from the ground82.

In addition, the power source76may also comprise a pulse trigger84that enables application of discrete bursts or pulses of direct electrical current to the coil74. Preferably, direct electrical current80is applied to the coil74at a relatively high pulse frequency. In an embodiment, the direct electrical current80can be applied intermittently to the coil74at interval durations lasting between 10 μs to about 100 μs and/or at an oscillating period from about 10 μs to about 50 υs. In a preferred embodiment, direct electrical current80may be applied to the coil74at a frequency rate ranging from about 1 kHz to about 100 kHz.

In addition, it is preferred that the amperage of the direct current being applied to the coil74range from about 1 kA to about 200 kA. It is also preferred that the coil74be designed such that electrical current80travels in one direction therethrough. As shown inFIGS. 9A, 9B, and 9D, direct electrical current80enters the coil74at a first location and exits the coil74at a second location that is different from the first.

The coil74may be constructed in a number of different shapes and geometries. As illustrated inFIGS. 9A and 11A, the coil structure74may comprise a single coil layer. In addition, the coil74shown inFIGS. 9A and 11Ais formed in a shape that is similar to the letter “E”. As illustrated, coil74comprises left and right coil portions86,88and a central coil portion90residing therebetween. Each of the coil portions86,88, and90is connected such that direct electrical current80flows through the coil structure74in one direction. For example, as shown inFIG. 9A, electrical current80is shown flowing into coil leg portion90and flows out of the coil structure74through the left and right coil leg portions86,88. The “E” like structure of the coil structure74shown inFIGS. 9A and 11A, is preferred because it facilitates circumferential flow of the magnetic field100about the coil74. For example, the magnetic field can move in a circumferential direction about an exterior surface of individual leg portions86,88and90.

Alternatively, the coil74may be constructed comprising two layers as illustrated inFIG. 9B. In this construction, the coil74comprises a bottom layer92and a top layer94that oppose each other and are joined together by central portion95. As illustrated, electrical current80flows into the top coil layer94through central portion95and out the bottom coil layer92.

FIG. 9Cillustrates a top view of the two-layer coil construction74shown inFIG. 9B. As shown, the top coil layer94may comprise a “dog bone” shaped construction that aids the flow of the magnetic field around the work pieces. For example, as shown inFIG. 10, the magnetic field100is illustrated as flowing circumferentially about the exterior of central coil portion90. Because of its narrower central coil portion, this coil74embodiment is often referred to as an “H” coil74.

Furthermore, as illustrated inFIG. 9D, the coil74may be comprised of a one-piece cylindrical construction. As illustrated inFIG. 9D, the coil74is constructed similar to that of a cylindrical tube having an elongated perimeter wall and a lumen extending therewithin such that working pieces may be positioned within the cylindrical coil74. As shown, direct electrical current80enters the coil at a first location96and travels through the coil74until it reaches a second coil location98at which the current80exists the coil74.

FIG. 10illustrates a magnified view of the principle of magnetic pulse welding. As shown inFIG. 10, movement of the direct electrical current80within the electrically conductive coil74induces a magnetic field100about the exterior of the coil74. The magnetic field100travels circumferentially about the exterior surface of a section of the coil74(FIGS. 9A to 9D) in a clockwise direction. In the presence of a first metallic work piece106, positioned within or adjacent the coil74, the magnetic field100in turn induces an eddy current102which propagates through the surface of the work piece106. Interaction of the eddy current102within the magnetic field100further induces an electro-magnetic force104that acts in a direction perpendicular to the magnetic field100. More specifically, the electro-magnetic force104acts in a direction that is away and perpendicular from the magnetic field100.

When an intermittent direct electrical current80is applied to the coil74, a magnetic field flux (B) is generated that penetrates through the work piece106adjacent, and most proximate, the energized coil74. Interaction of the metallic work piece106with the magnetic field flux (B), further creates an eddy current102that travels through the surface of the electrically conductive work piece positioned proximate the energized coil74.

Creation of the electro-magnetic force104is in accordance with John Fleming's left-hand rule, which states that when electrical current flows in an electrically conductive material, and an external magnetic field is applied across the flow, the electrically conductive material experiences a force perpendicular to both the magnetic field and the direction of the electrical current. Thus, an electro-magnetic force104(F), proportional to the eddy current102(I) within the surface of the work piece106and the magnetic field flux (B) about the work piece, is generated (F=I×B). As shown inFIG. 10, the electro-magnetic force104acts perpendicular and away from the magnetic field100that flows about the exterior of the coil74.

The phenomena occurring during magnetic pulse welding is generally given by the following equations:

Where ∇ equals the divergence of a tensor field, i equals the eddy current density, K equals the electrical conductivity, μ equals the magnetic permeability, Bois the magnetic flux density at the lower surface of the work piece proximal to the coil, Biis the magnetic flux density at the upper surface of the work piece proximal to the coil, t is the thickness of the first metal, F equals the magnitude of the electro-magnetic force and ω is the angular frequency of the changing magnetic field.

As shown inFIG. 10, it is the electro-magnetic force104that physically moves at least a portion of the work piece106, proximate the coil surface, in a direction away from the coil surface. Furthermore, it is this electro-magnetic force104that physically moves at least a portion of the work piece106, proximate the coil74, into another second work piece108, thus causing a collision between the two work pieces and thereby creating a weld therebetween.

FIGS. 11A, 11B and 11Cillustrate embodiments in which work pieces106,108may be positioned within the magnetic force welding instrument72. In one embodiment, illustrated inFIG. 11A, a first metal106and a second metal108are positioned adjacent an energized first coil surface110. More specifically, the exterior surface of the first metal106is positioned such that it may be in physical contact with the first coil surface110. The second metal108is positioned adjacent the first metal106. The first metal106is positioned proximate the first coil surface110and the second metal108is positioned distal the first coil surface110. In addition, the first metal106or “flyer metal”, is positioned such that it is capable of moving. On the other hand, the second metal108or “parent metal”, is positioned such that it is stationary and not capable of moving within the weld fixture.

Similarly,FIG. 11Billustrates a two-layer coil embodiment in which the first and second metals106,108are positioned adjacent the top coil portion94. More specifically, the first metal106is positioned in physical contact with the energized first coil surface110. The second metal is positioned adjacent the first metal106and distal of the first coil surface110.

In an additional embodiment, as shown inFIG. 11C, the first and second metals106,108, may be positioned between electrically energized top and bottom opposing portions94,92. In this embodiment, both the top and bottom coil portions94,92are electrically energized, thus providing two opposing magnetic fields100with respect to the first and second metals106,108. In addition, eddy currents102are induced in each of the first and second metals106,108. Therefore, top and bottom electro-magnetic forces104are formed which provide forces with which to move the opposing metals106,108such that they come together and collide, thereby creating a weld joint therebetween.

Furthermore, in an embodiment illustrated inFIGS. 11A, 11B, 11C, and 11Da gap of space112preferably resides between adjacent first and second metals106,108. This gap of space112, which may range from about 0.5 μm to about 5 cm, provides a space for the first metal106to accelerate within before impacting the opposing exterior surface of the second metal108. In a preferred embodiment, at least one insulator body114may be positioned between the adjacent exterior surfaces of the first and second metals106,108. The insulator body114is preferred because the material does not interact with the magnetic field100that is generated. Furthermore, if desired, an insulation layer (not shown) may be positioned between the first metal106and the coil surface110. In addition, a fixture clamp116may also be used to secure the work pieces in place at their appropriate positions to produce a proper magnetic pulse weld connection. The fixture clamp116may be positioned above and/or below the coil74.

In a preferred embodiment, the work piece that is positioned proximate the first energized coil surface110, such as the first metal106, may comprises an electrical conductivity that is greater than the second work piece, such as the second metal108. This is because positioning the work piece with the greater electrical conductivity proximate the energized coil surface110, generally increases strength of the eddy current102therewithin and thus, generally increases the electro-magnetic force104.

In yet a further embodiment, shown inFIG. 11D, the first and second metals106,108may be positioned within a coil74that is cylindrically shaped. In this embodiment, the work pieces106,108are positioned within the coil such that the coil74circumferentially surrounds them. As shown, first metal106is positioned most proximate an interior surface118of the cylindrically shaped coil74.

In a preferred embodiment, this coil configuration is designed for work pieces having a curved or round cross-section. For example, an outer tubular metal may be joined to a second tube or an elongated body having a smaller diameter. Similarly to the previous embodiments, a dielectric body114may be positioned between the surfaces of the first and second metals106,108thus creating a gap of space112therebetween.

Just as with the other welding embodiments previously discussed, when the power supply76is energized, the applied direct electrical current80travels through the coil74, generating the magnetic field100circumferentially about an exterior space of the coil74. The interaction of the magnetic field100with the work piece thus creates flow of eddy current102within the work piece which in turn, induces the electro-magnetic force104acting perpendicularly and away from the magnetic field100. The electro-magnetic force104acts on the first metal106, thereby, physically moving the first metal106and accelerating it across the gap of space112. After having traveled across the gap of space112, the first metal106collides with the surface of the second metal108creating a bond therebetween.

For purposes of illustration, the first metal106is referred to herein as the work piece that is positioned most proximal to the first coil surface110. The second metal108is referred to as the work piece positioned adjacent the first metal106and distal from the first coil surface110in comparison to the position of the first metal106within the pulse welding fixture. However, in operation, the position of the first and second metals106,108may be reversed.

In addition, the term “work piece” is defined herein, as a metal material that is acted upon during the pulse welding process. A work piece may comprise the first or second metal106,108, such as that of at least a portion of a component in an electrochemical cell10that is positioned within a fixture of the magnetic pulse welding instrument72.

In addition, the work piece that moves and physically impacts another work piece is often referred to as a “flyer metal”. The work piece that is impacted is generally referred to as the “parent metal”. As defined herein, the term “flyer metal” is the portion of metal that physically moves and impacts another metal. Generally, a first metal106positioned most proximal an energized portion of coil74, impacts a second metal108. However, the second metal108may be the “flyer” metal if it is positioned proximal an energized coil surface. The term “parent metal” is defined herein as the portion of metal that remains stationary throughout the magnetic pulse welding process. The parent metal is impacted by the “flyer” metal during the magnetic pulse welding process.

FIGS. 12A-12Eillustrate various embodiments of materials comprised of different geometries that can be joined together utilizing the magnetic pulse welding technique of the present invention.FIG. 12Aillustrates an embodiment of the magnetic pulse weld connection of a first metal106having a curved cross-section, such as a rod or terminal pin30, to that of a second metal108, having a rectangular cross-section, such as a tab50or current collector46,48. As shown in the embodiment illustrated inFIG. 12A, the weld area58extends across a width of the planar surface of the second metal108.

FIG. 12Billustrates a top view of the magnetic pulse weld connection shown inFIG. 12A. As shown the second metal108comprising the rectangular cross-section, conforms to the curved exterior surface106A of the first metal106comprising the curved cross-section. The planar surface108A of the second metal forms a weld area58, which deforms from its originally planar shape to follow the contour of the exterior surface of the curved first metal106.

FIGS. 12C and 12Dillustrate an embodiment of the direct magnetic pulse weld connection between first and second metals106,108having respective planar surfaces. For example, this embodiment may represent the connection between the tab50and the current collector46,48. More specifically, a magnetic pulse weld connection between a first end of tab50to a side edge of a perimeter of a current collector46,48. As shown inFIG. 12C, the weld area58extends across the width of second metal108.FIG. 12Dillustrates a cross-sectional view taken from the side of the weld connection shown inFIG. 12C. As shown, the weld connection is formed at a portion of the interface between the exterior surfaces of the first and second metals106,108.

FIG. 12Eillustrates a cross-sectional view taken from the side of first metal106comprising a tubular body and having a frusto-conical end joined, such as an end of a coupler60, to a second metal108having a curved cross-section, such as a terminal pin30, using the magnetic pulse welding process of the present invention. These embodiments shown inFIGS. 12A-12Eillustrate the wide variety of geometries in which metals of similar as well as dissimilar material compositions can be directly welded together.

FIG. 13illustrates a magnified embodiment of a weld zone or collision impact zone120formed within the weld area58. As shown, the impaction of the first metal106with that of the second metal108creates a boundary line122, which delineates the two metals106,108. However, unlike other welding techniques, such as laser welding or electrical resistance welding, the fraction of a second from coil energization to work piece impact created by the magnetic pulse welding, does not allow for the creation of a heat affected zone (HAZ) as is typical with respect to the other welding techniques. As a result of a minimized or eliminated heat affected zone, extensive inter-metallic bonds are not formed. Formation of inter-metallic bonds is generally not desired as they tend to form a relatively brittle interface. Therefore, by minimizing the formation of inter-metallic bonds within the weld zone, the joined metals are generally more durable and robust.

It is noted however, that although the example of the boundary line122illustrated inFIG. 13, is shown to be similar to a sinusoidal wave, the boundary line122, may have a different appearance. For example, the boundary line122may have an alternate wave appearance, or may appear similar to a straight line, or may have a meandering line appearance. In either case, the impaction zone120generally comprises a boundary line122which delineates the first metal106from the second metal108.

Metallic bonds formed by magnetic pulse welding are generally characterized as having an increased surface hardness as compared to their initial non-welded surfaces. This is because the higher hardness at the interface can generally result from the intense plastic deformation that occurs due to the high velocity collision or, alternatively, to a fine grain microstructure which may form during rapid solidification of the welded interface at the impact zone118. For example, velocities of the first106or “flying” metal may range from about 75 m/s to as much as 150 m/s or greater. The resulting impaction force may range from about 0.5 GPa to about 1.0 GPa depending on the composition of the first and second metals106,108as well as the strength of the magnetic field generated.

In a preferred embodiment, the first and second metals106,108may be of a different composition having dissimilar properties, or alternatively, they may be composed of a similar composition. Examples of materials that may be joined together using the magnetic pulse welding process may comprise aluminum, molybdenum, titanium, nickel, steel, stainless steel, niobium, copper, gold, silver, palladium, molybdenum, tantalum, tungsten, and combinations thereof.

A first metal106, for example, comprising an aluminum current collector46,48, may have a lower melting temperature than that of a second metal108, for example, a molybdenum terminal pin30. Furthermore, the first and second metals106,108may have a difference in melting temperature that is greater than 125° C., more preferably greater than 250° C. and most preferably greater than 500° C. Examples of first metals include, but are not limited to, aluminum (melting temperature 660° C.), titanium (melting temperature 1,725° C.), nickel (melting temperature 1,453° C.), steel (melting temperature 1,130° C.), stainless steel (melting temperature 1,353° C.), niobium (melting temperature 2,468° C.), copper (melting temperature 1,083° C.), gold (melting temperature 1,064° C.), silver (melting temperature 961° C.), palladium (melting temperature 1,554° C.), and combinations thereof. Examples of second metals include, but are not limited to, molybdenum (melting temperature 2,617° C.), tantalum (melting temperature 2,996° C.), tungsten (melting temperature 3,410° C.), and combinations thereof. It is contemplated that any or a combination of first metals106may be joined together with a second metal108as described in the present invention. It is preferred that the current collectors46,48including the tab50of the cathode current collector48be composed of a first metal106and that the terminal pin30be composed of a second metal108.

As previously mentioned, the present invention is applicable to either primary or secondary electrochemical cells. A primary electrochemical cell that possesses sufficient energy density and discharge capacity for the rigorous requirements of implantable medical devices comprises a lithium anode or its alloys, for example, Li—Si, Li—Al, Li—B and Li—Si—B. The form of the anode may vary, but preferably it is of a thin sheet or foil pressed or rolled on a metallic anode current collector46.

The cathode of a primary cell is of electrically conductive material, preferably a solid material. The solid cathode may comprise a metal element, a metal oxide, a mixed metal oxide and a metal sulfide, and combinations thereof. A preferred cathode active material is selected from the group consisting of silver vanadium oxide, copper silver vanadium oxide, manganese dioxide, cobalt nickel, nickel oxide, copper oxide, copper sulfide, iron sulfide, iron disulfide, titanium disulfide, copper vanadium oxide, and mixtures thereof.

Before fabrication into an electrode for incorporation into an electrochemical cell10, the cathode active material is mixed with a binder material such as a powdered fluoro-polymer, more preferably powdered polytetrafluoroethylene or powdered polyvinylidene fluoride present at about 1 to about 5 weight percent of the cathode mixture. Further, up to about 10 weight percent of a conductive diluent is preferably added to the cathode mixture to improve conductivity. Suitable materials for this purpose include acetylene black, carbon black and/or graphite or a metallic powder such as powdered nickel, aluminum, titanium and stainless steel. The preferred cathode active mixture thus includes a powdered fluoro-polymer binder present at about 3 weight percent, a conductive diluent present at about 3 weight percent and about 94 weight percent of the cathode active material.

The cathode component40,42may be prepared by rolling, spreading or pressing the cathode active mixture onto a suitable cathode current collector48. Cathodes prepared as described are preferably in the form of a strip wound with a corresponding strip of anode material in a structure similar to a “jellyroll” or a flat-folded electrode stack, such as that illustrated inFIG. 6.

In order to prevent internal short circuit conditions, the cathode electrode40,42is separated from the anode electrode36,38by the separator membrane44. The separator membrane44is preferably made of a fabric woven from fluoropolymeric fibers including polyvinylidine fluoride, polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethylene used either alone or laminated with a fluoropolymeric microporous film, non-woven glass, polypropylene, polyethylene, glass fiber materials, ceramics, polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.) and a membrane commercially available under the designation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.).

A primary electrochemical cell includes a nonaqueous, ionically conductive electrolyte having an inorganic, ionically conductive salt dissolved in a nonaqueous solvent and, more preferably, a lithium salt dissolved in a mixture of a low viscosity solvent and a high permittivity solvent. The salt serves as the vehicle for migration of the anode ions to intercalate or react with the cathode active material and suitable salts include LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4, LiO2, LiAlCl4, LiGaCl4, LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN, LiO3SCF3, LiC6F5SO3, LiO2CCF3, LiSO6F, LiB(C6H)4, LiCF3SO3, and mixtures thereof.

Suitable low viscosity solvents include esters, linear and cyclic ethers and dialkyl carbonates such as tetrahydrofuran (THF), methyl acetate (MA), diglyme, triglyme, tetraglyme, dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1-ethoxy,2-methoxyethane (EME), ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate, dipropyl carbonate, and mixtures thereof. High permittivity solvents include cyclic carbonates, cyclic esters and cyclic amides such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, acetonitrile, dimethyl sulfoxide, dimethyl, formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactone (GBL), N-methyl-pyrrolidinone (NMP), and mixtures thereof. The preferred electrolyte for a lithium primary cell is 0.8M to 1.5M LiAsF6or LiPF6dissolved in a 50:50 mixture, by volume, of PC as the preferred high permittivity solvent and DME as the preferred low viscosity solvent.

By way of example, in an illustrative case negative primary cell, the active material of cathode body is silver vanadium oxide as described in U.S. Pat. Nos. 4,310,609 and 4,391,729 to Liang et al., or copper silver vanadium oxide as described in U.S. Pat. Nos. 5,472,810 and 5,516,340 to Takeuchi et al., all assigned to the assignee of the present invention, the disclosures of which are hereby incorporated by reference.

In secondary electrochemical systems, the anode electrode42,44comprises a material capable of intercalating and de-intercalating the alkali metal, and preferably lithium. A carbonaceous anode comprising any of the various forms of carbon (e.g., coke, graphite, acetylene black, carbon black, glassy carbon, etc.), which are capable of reversibly retaining the lithium species, is preferred. Graphite is particularly preferred due to its relatively high lithium-retention capacity. Regardless of the form of the carbon, fibers of the carbonaceous material are particularly advantageous because they have excellent mechanical properties that permit them to be fabricated into rigid electrodes capable of withstanding degradation during repeated charge/discharge cycling.

The cathode electrode40,42of a secondary cell preferably comprises a lithiated material that is stable in air and readily handled. Examples of such air-stable lithiated cathode materials include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese. The more preferred oxides include LiNiO2, LiMn2O4, LiCoO2, LiCo0.92Sn0.08O2and LiCo1-xNixO2.

The lithiated active material is preferably mixed with a conductive additive selected from acetylene black, carbon black, graphite, and powdered metals of nickel, aluminum, titanium and stainless steel. The electrode further comprises a fluoro-resin binder, preferably in a powder form, such as PTFE, PVDF, ETFE, polyamides and polyimides, and mixtures thereof. The current collector46,48is selected from stainless steel, titanium, tantalum, platinum, gold, aluminum, cobalt nickel alloys, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium- and molybdenum-containing alloys.

Suitable secondary electrochemical systems are comprised of nonaqueous electrolytes of an inorganic salt dissolved in a nonaqueous solvent and more preferably an alkali metal salt dissolved in a quaternary mixture of organic carbonate solvents comprising dialkyl (non-cyclic) carbonates selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC), and mixtures thereof, and at least one cyclic carbonate selected from propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC) and vinylene carbonate (VC), and mixtures thereof. Organic carbonates are generally used in the electrolyte solvent system for such battery chemistries because they exhibit high oxidative stability toward cathode materials and good kinetic stability toward anode materials.

A preferred material for the casing is titanium although stainless steel, mild steel, nickel-plated mild steel and aluminum are also suitable. The casing header comprises a metallic lid having an opening to accommodate the glass-to-metal seal/terminal pin feedthrough for the cathode electrode. The anode electrode or counter electrode is preferably connected to the case or the lid. An additional opening is provided for electrolyte filling. The casing header comprises elements having compatibility with the other components of the electrochemical cell and is resistant to corrosion. The cell is thereafter filled with the electrolyte solution described hereinabove and hermetically sealed such as by close-welding a titanium plug over the fill hole, but not limited thereto.

Now, it is therefore apparent that the present invention has many features among which are reduced manufacturing cost and construction complexity. While embodiments of the present invention have been described in detail, that is for the purpose of illustration, not limitation.