Patent Publication Number: US-2018040902-A1

Title: Electrode current collector design in a battery

Description:
PRIORITY CLAIM 
     The present application relates to and claims priority from U.S. provisional application Ser. No. 62/371,002, filed Aug. 4, 2016, entitled “Method Of Printing A Conductive Ink Onto A Cathode Surface To Increase Surface Area And Capacitance,” which is hereby expressly incorporated by reference in its entirety to provide continuity of disclosure. 
    
    
     FIELD 
     The present systems and methods relate to the design and method of making batteries for use within implantable medical devices. 
     BACKGROUND 
     Batteries with high energy density and high discharge rate capabilities are desirable for certain applications. This is especially true when the batteries are used in devices where the batteries are difficult to replace and/or recharge, such as in an implantable medical device (IMD). An end-of-life (EOL) indicator for the battery may also be an important feature for this kind of application. 
     The performance of high energy density and high discharge rate batteries used in implantable medical devices (IMD) such as the lithium silver vanadium oxide (SVO) batteries is greatly affected by the cathode current collector material and mechanical design. The collector material needs to be chemically and electrochemically stable against the cathode material and battery electrolyte at the potential at which the cell is designed to operate. The collector design needs to promote good electrode integrity, reduce electrical resistivity and offer high packaging efficiency. 
     SUMMARY 
     Embodiments of a device battery, and methods for fabricating the battery are described herein. 
     In an embodiment, a battery electrode includes a current collector formed from a mesh structure with an opening pattern. The opening pattern does not include any angles less than 90 degrees, and the current collector has a first surface and a second surface opposite the first surface. The electrode also includes a first material layer bonded to the first surface of the current collector, and a second material layer bonded to the second surface of the current collector and to the first material layer through the current collector. 
     In another embodiment, a battery includes an anode, an electrolyte, and a cathode. The cathode includes a current collector, a first material layer and a second material layer as described in the embodiment above. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the devices and methods presented herein. Together with the detailed description, the drawings further serve to explain the principles of, and to enable a person skilled in the relevant art(s) to make and use, the methods and systems presented herein. 
       In the drawings, like reference numbers indicate identical or functionally similar elements. Further, the drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
         FIG. 1  illustrates a common battery configuration. 
         FIG. 2  illustrates a battery configuration, according to an embodiment. 
         FIGS. 3A-3D  are views of various current collector configurations, according to some embodiments. 
         FIG. 4  is a side-view of a cathode, according to an embodiment. 
         FIG. 5  is a graph of equivalent thickness versus percentage of mesh opening. 
         FIG. 6  is a graph of pre-pulse voltage and charge time versus battery capacity for various current collector designs. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of the devices and methods refers to the accompanying drawings that illustrate exemplary embodiments consistent with these devices and methods. Other embodiments are possible, and modifications may be made to the embodiments within the spirit and scope of the methods and systems presented herein. Therefore, the following detailed description is not meant to limit the methods and systems described herein. Rather, the scope of these methods and systems is defined by the appended claims. 
     Exemplary Environment 
     Before describing in detail the design and method of making electrodes of a battery, it is helpful to describe an example environment in which such a battery may be implemented. The battery embodiments described herein may be particularly useful in the environment of an implantable medical device (IMD) such as an implantable cardiac device, e.g., an implantable cardioverter defibrillator (ICD). Examples of such ICDs may be found in U.S. Pat. Nos. 6,327,498 and 6,535,762, each of which is incorporated herein by reference. 
     Battery Design 
     An ICD, such as those described in the patents identified above, requires some form of power source in order to operate. A primary lithium battery may be used to provide a high current output power source. 
       FIG. 1  illustrates an example design for a battery  100 . Battery  100  includes a cathode  102 , an anode  104  separated from the anode via a separator  106 , and some form of electrolyte  108  in contact with anode  104  and cathode  102 . The various battery elements illustrated in  FIG. 1  are provided for representative purposes only and are not intended to limit the structural design of the battery embodiments herein. 
     Separator  106  may be configured such that ions may pass through separator  106  between anode  104  and cathode  102 . An example of separator  106  includes a polyethylene film. Electrolyte  108  may be in liquid form or as a solid or semi-solid polymer in contact with anode  104  and cathode  102 . 
     Each of anode  104  and cathode  102  may include some active material bonded to a current collector (see  FIG. 2 ). The active materials take part in the electrochemical reaction to produce the current, while the current collectors are conductive materials that provide a low-resistance path for the current to flow. For example, anode  104  may include a lithium foil bonded to a current collector, while cathode  102  may include some metal oxide material (such as silver vanadium oxide) mixed with other additives (such as carbon black or graphite) and a binder material (such as polyvinylidene difluoride (PVDF) or polytetrafluoroethylene (PTFE)) and bonded to a current collector. These types of materials may be used to make a lithium battery. 
     The current from battery  100  is typically delivered to a load  110 . The size of load  110  affects the amount of current that flows between anode  104  and cathode  102 . 
       FIG. 2  illustrates another example design for a battery  200 , according to an embodiment. Battery  200  includes a stacked structure of alternating cathode material  202  and anode material  204 , separated by a separator  206 . Each layer of cathode material  202  is bonded to a cathode current collector  208   a,  while each layer of anode material  204  is bonded to an anode current collector  208   b.  The stacked layers are enclosed within a housing  210 . Although not explicitly shown in  FIG. 2 , an electrolyte would also exist around cathode material  202  and anode material  204  to facilitate the ion transport between the anode and cathode materials. The electrolyte may be a polymer or liquid electrolyte as would be understood to one skilled in the art. Examples of the electrolyte include lithium bis-trifluoromethanesulfonimide (LiTFSI) in propylene carbonate/dimethoxyethane or Lithium hexafluoroarsenate (LiAsF 6 ) in propylene carbonate/dimethoxyethane. The stacked combination of cathode material  202  and cathode current collector  208   a  constitutes a cathode  102  of battery  200  while the stacked combination of anode material  204  and anode current collector  208   b  constitutes an anode  104  of battery  200 . 
     Cathode current collectors  208   a  may be electrically connected together to form the positive terminal of battery  200  (cathode), while anode current collectors  208   b  may be connected together to form the negative terminal of battery  200  (anode). In one embodiment, anode material  204  comprises a lithium foil, and cathode material  202  comprises a metal oxide material. Separator  206  may be polyethylene. A typical battery  200  for use in an ICD using lithium anode material  204  and silver vanadium oxide cathode material  202  has an operating open circuit voltage (OCV) between 3.25 and 2.35 V with a cathode capacity of 315 mAh/g, for example. 
       FIG. 3A  illustrates a current collector  208  formed from a mesh structure  302 . Current collector  208  also includes a tab  301  that makes conductive contact with current collector  208  and provides a structure for electrical connections to be made. In one example, tab  301  is welded to current collector  208 . 
     Mesh structure  302  allows for material layers to be placed on either side of current collector  208  and to be bonded both to the mesh structure, and to each other through the openings of the mesh structure. Current collector  208  may be used as part of either an anode or cathode of a battery depending on the composition of the material layers bound to current collector  208 . In this current collector design, mesh structure  302  has a diamond-like repeating pattern as illustrated in the blown up portion of the figure. The use of the diamond pattern may result in about 47% of the surface area of mesh structure  302  being open, for example. However, the diamond pattern includes sharp angles (i.e., angles less than 90 degrees.) These acute angles can create a narrow path for the material layers to protrude through the openings in mesh structure  302  and bond to each other causing incomplete filling of the openings through current collector  208 , thus raising the overall resistance of the electrode. 
       FIG. 3B  illustrates another current collector  208  having a mesh structure  304 , according to an embodiment. Mesh structure  304  includes an opening pattern (i.e., a pattern of openings such as a honeycomb pattern) that does not include any angles less than 90 degrees. It should be understood that the opening pattern having angles all equal to or greater than 90 degrees does apply to those patterns directly along edges of current collector  208 , as these patterns along the edges are often cut off at angles that may form acute corners. In one example, mesh structure  304  includes a repeating hexagonal pattern as illustrated in the blown up portion. The hexagonal pattern may result in about 57% of the surface area of mesh structure  304  being open, for example. By using a pattern that does not include any acute angles, the material layers bound to either side of mesh structure  304  can bond together more easily through the openings in mesh structure  304 , thus strengthening the integrity of the electrode. Additionally, the higher opening percentage (i.e., the percentage of the surface area of the mesh structure that is represented by open space as compared to solid material) across the surface area of mesh structure  304  reduces the weight and volume of current collector  208 . The reduced weight/volume may increase the total cell packing efficiency of the battery. 
     The repeating hexagonal pattern of mesh structure  304  may include hexagons that have a width between about 0.030 inches and 0.040 inches. Other shapes such rectangles, squares, pentagons, octagons, circles, or ovals may be used as well with similar dimensions. 
       FIG. 3C  illustrates another current collector  208  having a mesh structure  306 , according to an embodiment. Mesh structure  306  includes larger openings than mesh structure  304 , and may have a total percentage opening of about 65% across the surface area of mesh structure  306 , for example. Mesh structure  306  may also include a repeating hexagonal pattern as illustrated in the blown up portion of the figure. The repeating hexagonal pattern of mesh structure  306  may include hexagons that have a width between about 0.045 inches and 0.055 inches. Other shapes such as rectangles, squares, pentagons, octagons, circles, or ovals may be used as well with similar dimensions. 
       FIG. 3D  illustrates another current collector  208  having a mesh structure  308 , according to an embodiment. Mesh structure  308  includes larger openings than mesh structure  304  or mesh structure  306 , and may have a total percentage opening of about 70% across the surface area of mesh structure  308 , for example. Mesh structure  308  may also include a repeating hexagonal pattern as illustrated in the blown up portion of the figure. The repeating hexagonal pattern of mesh structure  308  may include hexagons that have a width between about 0.060 inches and 0.070 inches. Other shapes such as rectangles, squares, pentagons, octagons, circles, or ovals may be used as well with similar dimensions. 
     According to an embodiment, current collector  208  and its associated mesh structure  304 / 306 / 308  are machined, cast, stamped, forged, or otherwise formed from a metal such as aluminum, stainless steel, or titanium, to name a few example materials. A conductive coating, such as carbon coating, may also be applied to the surface of mesh structure  304 / 306 / 308  to further promote binding strength and conductivity. Current collector  208  may have a total thickness between about 0.001 inches and 0.005 inches, for example. 
       FIG. 4  illustrates an example side view of cathode  102  that includes current collector  208  flanked on both sides by cathode material layer  202   a  and cathode material layer  202   b,  according to an embodiment. Tab  301  also makes electrical contact with current collector  208 . Cathode material layer  202   a  and cathode material layer  202   b  may be substantially the same material. Cathode material layer  202   a  bonds to a first surface of current collector  208  (i.e., the first surface of the mesh structure), and cathode material layer  202   b  bonds to a second surface of the current collector  208  (i.e., the second surface of the mesh structure, opposite the first surface of the mesh structure). Cathode material layers  202   a  and  202   b  also bond to each other through the openings of the mesh structure, according to an embodiment. 
     Each of cathode material layer  202   a  and cathode material layer  202   b  may include a polytetrafluoroethylene (PTFE) binder with particles of silver vanadium oxide (SVO). In one example, each of cathode material layer  202   a  and  202   b  includes about 3% of PTFE, 94% SVO, and 2% of carbon black, and 1% graphite to promote better conductivity. 
       FIG. 5  is a graph showing the effects of the mesh structure thickness based on the total percentage of openings across a surface area of the mesh structure. As can be seen in the graph, a higher mesh opening percentage yields a lower overall solid mesh volume added to the pressed electrode and a lower equivalent mesh thickness. This occurs because having a higher opening percentage allows for more of the material layers to be pressed into the openings and bond across the mesh structure. Thus, a greater volume of the material can fill between the openings of the mesh structure, and the overall thickness of the electrode is reduced. 
       FIG. 6  is a graph showing various electrical properties of a battery made with different current collector designs compared to the depth of discharge (DOD) of the battery. To perform the testing, experimental batteries were built using different current collector designs for the SVO-based cathodes. The battery cells were tested following a three month 72 C ADD life test protocol, which involves fully discharging the cell with a high discharge rate at an elevated temperature of between 70 degrees Celsius and 75 degrees Celsius. 
     As can be seen from the graph in  FIG. 6 , the pre-pulse voltage (read along the left side of the y-axis) of the battery cells using the four different current collector designs remains roughly the same across the lifetime of the cells (up to about 80% of total discharge). Thus, the change in current collector design has no adverse effect on the pre-pulse voltage. 
       FIG. 6  also illustrates that the charge time (read along the right side of the y-axis) of the different cells is faster when using a higher opening percentage across the cathode current collector. The difference in charge time is more noticeable as the battery cell becomes more discharged. 
     Conclusion 
     Exemplary embodiments of the present systems and methods have been presented. The systems and methods are not limited to these examples. These examples are presented herein for purposes of illustration, and not limitation. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the systems and methods herein. 
     Further, the purpose of the Abstract provided herein is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is not intended to be limiting as to the scope of the present system and method in any way.