Abstract:
An encasement for an electrochemical cell and method of making such encasement is discloses. The design of the encasement results in an encasement having an area of high stress located away from the weld zone area of the encasement, where the cover and the case are welded together.

Description:
FIELD 
       [0001]    This disclosure relates to electrochemical cell encasements including covers and cases. 
       BACKGROUND 
       [0002]    The following discussion discloses electrochemical cells and methods of making cells for use in an implantable medical device (IMD) that is very compact, such that IMD can be readily implanted in small spaces within the patient&#39;s anatomy. As such devices get smaller, new challenges in manufacturing of components, such as batteries present themselves. One of the challenges is making battery encasements, for example covers and cases, that can be reliably manufactured to remain sealed during use within a human body. It is known that the welding of certain metals can form areas at the weld that are more susceptible to hydrogen cracking or embrittlement than the base metals. The root of the weld is typically the weakest spot in a welded structure due to stress concentration effect. 
       SUMMARY 
       [0003]    Applicants have discovered a design for an electrochemical cell encasement including a cover and a case in which the welded joints formed at the cover and case interface are created in lower stress areas of the encasement. 
         [0004]    In one embodiment, the battery case is hollow except for one end and has a cylindrical shape and has a circular cross section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is section view of a battery assembly; 
           [0006]      FIG. 2  is close-up section view of a battery cover; 
           [0007]      FIG. 3  is a close-up section view of a battery case; 
           [0008]      FIG. 4  is a close-up section view of a known battery cover; 
           [0009]      FIG. 5  is a close-up section view of a known battery case; 
           [0010]      FIG. 6  is a depiction of the von Mises stress distribution in the proximity of the weld due to internal pressurization of a known design for an electrochemical cell case and cover; 
           [0011]      FIG. 7  is a depiction of the von Mises stress distribution in the proximity of the weld due to internal pressurization of a design for a battery case and cover of the disclosure; 
           [0012]      FIG. 8  is a close-up perspective view of a battery cover of the disclosure; and 
           [0013]      FIG. 9  is a close-up perspective view of a battery case of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    “Anode” and “cathode” are used as these terms are commonly understood in reference to electrochemical cells, for example, batteries and capacitors. 
         [0015]    The electrochemical cells described in this disclosure are useful in compact implantable medical devices (IMDs) that can be implanted within small spaces of an anatomy, such as the vasculature or an organ, for example a chamber of a heart. The same reference numbers are used in multiple figures when referring to the same element of the disclosure. 
         [0016]    Battery assembly  10  for an implantable medical device is shown in  FIG. 1 . In this embodiment, battery assembly  10  includes a generally cylindrical battery case  12  and corresponding battery cover  14 . Battery case  12  and battery cover  14  will be discussed in greater detail below. Within battery case  12  is the anode  16 , cathode  18  and separator  20 . Each of the anode  16 , cathode  18  and separator are generally coaxial with one another. Embedded within the cathode is current collector  22  which is connected to feedthrough pin  24 . The feedthrough pin  24  can be connected to for example, an electronics assembly for an implantable medical device. The battery assembly as depicted provides that in operation, the battery case and cover have a negative potential, also known as “case negative” polarity. The outer surfaces of the battery assembly are designed to be exposed to bodily fluids when implanted. The battery case has a generally tubular shape and is generally circular in cross section. Other shapes in cross section include ovate, elliptical, or any other suitable shape. Of course the battery shape of the battery cover is designed to attach to the battery case as described in this application. 
         [0017]    Useful battery materials include a case and cover made of an electrically conductive material such as alpha-beta and beta titanium alloys such as Ti-6Al4V or Ti-15Mo, respectively, stainless steels, titanium, for example grade 1, or any other grade, an anode of for example, lithium metal, a cathode of for example, a hybrid mixture of carbon monofluoride (CF x ) and silver vanadium oxide (CSVO)and may further contain carbon black or polytetrafluoroethylene (PTFE), or both and the separator  20  can include porous polypropylene film, such as that provided by Celgard, LLC of Charlotte, N.C. (e.g., CELGARD 2500, CELGARD 4560, and the like). The battery assembly also includes a liquid electrolyte (not shown) for facilitating ionic transport and forming a conductive pathway between the anode  16  and the cathode  18 . The feedthrough pin and the current collector are made from an electrically conductive material such as titanium, platinum, niobium, molybdenum, alloys of titanium, stainless steel, or alloys of any of these. The feedthrough pin is normally a solid unitary component. 
         [0018]      FIGS. 2 and 3  depict battery cover  14  and battery case  12  in close-up. Battery cover  14  includes a fillport  26  for adding electrolyte and an opening  28  for the feedthrough pin  24 . Battery case  12  has an open end  13  and a closed end  9  and a cylindrical or tubular portion  7 . After electrolyte is added through the fillport, the fillport is sealed. Width  30  of the battery case  12  cooperates with internal width  32  of battery cover  32  to form a robust fit. When battery cover  14  is fitted onto the open end  13  of the battery case  12 , battery cover  14  is welded to the open end  13  of the case  12 . The battery cover and the battery case are centered about an axis  15 . 
         [0019]      FIGS. 8 and 9  depict the battery cover and case depicted in  FIGS. 2 and 3  in perspective views. 
         [0020]    In this embodiment, the battery cover  14  includes a divot or indentation  17  for identification of the exterior surface of the battery cover due to its relatively small size. For example, the outside diameter of the battery case and the battery cover can range from about 2 mm to about 7.5 mm and can be any diameter between about 2 mm and about 7.5 mm. The length of the battery assembly can range from about 8 mm to about 90 mm and can be any length between about 8 mm and about 90 mm. 
         [0021]    When the battery cover  14  is mated to battery case  12 , inside surface  34  of flange  36  abuts outside surface  38  of lip  40  and bottom surface  42  of flange  36  abuts ledge  44  of the battery case  12 . Lip  40  fits or protrudes within the groove  43  in battery cover  14 . Groove  43  has a substantially rectangular section  45  adjacent to a substantially triangular section  47  and within the groove. Lip  40  fits within substantially rectangular section  45  and may or may not extend to contact or abut the top surface  58  of groove. The outside surface  35  of groove  43  corresponds to the inside surface of flange  34 . Substantially triangular section  47  has a shape substantially that of a right triangle, for example, a 30-60-90 right triangle. 
         [0022]    In this embodiment, the width A of the lip  40  is a width that extends from the diameter of case inner surface  49  to a point between case outer surface diameter  51 . Case outer surface diameter  51  is greater than case inner surface diameter and case inner surface diameter is less than case outer surface diameter. Ledge  44  has a width F that extends substantially horizontally from the outside surface  38  of lip  40  to the case outer surface diameter  51 . 
         [0023]    Battery cover  14  has a width B, top surface  52 , bottom surface  54  and outside surface  56 . Groove  43  has a depth C that substantially corresponds to the height D of the lip  40  of the case. The width A of the lip  40  substantially corresponds to the width E of the substantially rectangular section  45  of the groove. The overall width H of the groove  43  as measured at the bottom surface  58  of cover is the sum of width H of the substantially rectangular section  45  of groove and the width I of the base of the substantially triangular section  47 . The width G of flange  36  substantially corresponds to the width F of ledge  44 . The width G of flange  36  extends from the outside surface  56  of cover  14  to the inside surface of flange  34 , which is also the outside surface  35  of groove  43 . 
         [0024]    In this embodiment, the lip, flange, ledge, bottom surface and groove are annular (see  FIGS. 8 and 9 ). Of course, the shape of these elements, and others described in this application, are dependent upon the overall shape of the battery case and the battery cover. 
         [0025]    The battery cover can be mated to the battery case by mating the lip  40  of the case within the groove  43  of the cover. When the cover and the case are mated together, the bottom surface of the flange abuts the surface of the ledge and forms a joint  60 , referring to  FIG. 1 . The battery cover  14  may be welded to the battery case  12  at the joint by known means for example, laser welding, either continuous or pulse. 
         [0026]    Applicants have discovered that the welding of the battery cap and battery case disclosed in this application provides a weld zone having lower stress than a known design, discussed in more detail below. 
         [0027]    A known design for a substantially cylindrical battery case  100  and cover  102  is shown in  FIGS. 4 and 5 . In the design shown in  FIGS. 4 and 5 , cover portion width  104  is designed to cooperatively fit inside the internal case width  106 . When fitted together, cover joint surface  108  and case joint surface  110  would abut and then be welded together at the seam formed by the two abutted surfaces. In this design, Applicants determined that high material stress was located within the weld zone. This phenomenon is shown in  FIG. 6 . 
         [0028]      FIG. 6  shows a depiction of cover  100  mated to case  102  of the known design depicted in  FIGS. 4 and 5 . Weld zone  112  is shown covering the resulting weld joint  114  and surrounding area within a radius. Through finite element analysis (FEA), the highest stress found in the known design was found in localized area  116  when the battery assembly is loaded with an internal pressure. This localized area  116  of stress lies within or overlaps in the weld zone  112 . 
         [0029]      FIG. 7  shows a depiction of battery cover  14  mated to battery case  12  of the design of the disclosure. Weld zone  48  is shown covering the resulting weld joint  50  and surrounding area within a radius. Through the same finite element analysis as described above, the highest stress found in the design disclosed in  FIGS. 2 and 3  was found in localized area  52 . As can be seen, localized area  52  is outside of the weld zone  48  and is located where the material is less susceptible to cracking. Another advantage of the disclosed design is that the maximum stress is reduced because of less stress concentration. 
         [0030]    In one aspect the disclosure provides a method including forming a case for an electrochemical cell, the case including a case outer surface having a diameter or x or y dimension, a case inner surface having a diameter or x or y dimension less than the diameter or x or y dimension of the case outer surface, a closed end, and an open end, the open end of the case having a lip and a ledge, the lip having a height and a width, the width of the lip extending from the case inner surface to a point between the diameter or x or y dimension of the inner and outer case surfaces, the ledge having a surface extending from the lip to the case outer surface, and forming a cover for attaching to the open end of the case, the cover including a cover outer diameter or x or y dimension, a groove extending inwardly from a bottom surface of the cover, a flange having a bottom surface and the flange having a width extending from the outer surface of the cover to the outer surface of the groove, the groove having a substantially rectangular section and may have a substantially triangular section, the rectangular section having an opening for receiving the lip of the case and the bottom surface of the flange formed to abut the surface of the ledge. 
         [0031]    In another aspect, the disclosure provides an apparatus including a case for an electrochemical cell, the case including a case outer surface having a diameter or an x or y dimension, a case inner surface having a diameter or x or y dimension less than the diameter of the case outer surface, a closed end, and an open end, the open end of the case having a lip and a ledge, the lip having a height and a width, the width of the lip extending from the case inner surface to a point between the diameter of the inner and outer case surfaces, the ledge having a surface extending from the lip to the case outer surface and a cover for attaching to the open end of the case, the cover including a cover outer diameter or x or y dimension, a groove extending inwardly from a bottom surface of the cover, a flange having a bottom surface and the flange having a width extending from the outer surface of the cover to the outer surface of the groove, the groove having a substantially rectangular section, the rectangular section having an opening for receiving the lip of the case and the bottom surface of the flange formed to abut the surface of the ledge. 
         [0032]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. Of note, the system components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Moreover, while certain embodiments or figures described herein may illustrate features not expressly indicated on other figures or embodiments, it is understood that the features and components of the system and devices disclosed herein are not necessarily exclusive of each other and may be included in a variety of different combinations or configurations without departing from the scope and spirit of the invention. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.