Abstract:
A resistance-stabilizing additive to an electrolyte for a battery cell in an implantable medical device is presented. At least one resistance-stabilizing additive is selected from a group comprising an electron withdrawing group, an aromatic diacid salt, an inorganic salt, an aliphatic organic acid, an aromatic diacid, and an aromatic monoacid.

Description:
RELATED APPLICATION 
     Attention is directed to U.S. patent application Ser. No. 10/366,214 filed Feb. 13, 2003 entitled “Liquid Electrolyte For An Electrochemical Cell,” published as U.S. Pat. Appl. Pub. No. 2004/0161671 on Aug. 19, 2004, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention generally relates to an electrochemical cell and, more particularly, to an additive in an electrolyte for a battery. 
     BACKGROUND OF THE INVENTION 
     Implantable medical devices (IMDs) detect and treat a variety of medical conditions in patients. IMDs include implantable pulse generators (IPGs) or implantable cardioverter-defibrillators (ICDs) that deliver electrical stimuli to tissue of a patient. ICDs typically comprise, inter alia, a control module, a capacitor, and a battery that are housed in a hermetically sealed container. When therapy is required by a patient, the control module signals the battery to charge the capacitor, which in turn discharges electrical stimuli to tissue of a patient. 
     The battery includes a case, a liner, and an electrode assembly. The liner surrounds the electrode assembly to prevent the electrode assembly from contacting the inside of the case. The electrode assembly comprises an anode and a cathode with a separator therebetween. In the case wall or cover is a fill port or tube that allows introduction of electrolyte into the case. The electrolyte is a medium that facilitates ionic transport and forms a conductive pathway between the anode and cathode. An electrochemical reaction between the electrodes and the electrolyte causes charge to be stored on each electrode. The electrochemical reaction also creates a solid electrolyte interphase (SEI) or passivation film on a surface of an anode such as a lithium anode. The passivation film is ionically conductive and prevents parasitic loss of lithium. However, the passivation film increases internal resistance which reduces the power capability of the battery. It is desirable to reduce internal resistance associated with the passivation film for a battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a cutaway perspective view of an implantable medical device (IMD); 
         FIG. 2  is a cutaway perspective view of a battery in the IMD of  FIG. 1 ; 
         FIG. 3  is an enlarged view of a portion of the battery depicted in  FIG. 2  and designated by line  4 . 
         FIG. 4  is a cross-sectional view of an anode and a passivation film; 
         FIG. 5  is graph that compares discharge and resistance for a conventional and exemplary additive in an electrolyte; 
         FIG. 6  is graph that compares resistance over time for exemplary additives to an electrolyte; 
         FIG. 7  is a flow diagram for forming an electrolyte for a battery; and 
         FIG. 8  is a flow diagram for autoclaving a battery. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers are used in the drawings to identify similar elements. 
     The present invention is directed to an additive for an electrolyte. The additive stabilizes resistance of the battery during storage, thermal processing, and throughout discharge. A resistance-stabilizing additive is defined as one or more chemical compounds, added to an electrolyte, that causes a battery to exhibit low resistance (i.e. generally below 500 ohm centimeter (cm) 2 ) throughout the battery&#39;s useful life. In one embodiment, the additive is characterized by an electron withdrawing group. Exemplary chemical compounds containing electron withdrawing group include 2,2,2,-trifluoroacetamide, and benzoyl acetone. In another embodiment, an organic acid serves as a resistance-stabilizing additive. Exemplary organic acids include benzoic acids, carboxylic acids, malic acid, tetramethylammonium (TMA) hydrogen phthalate and hexafluoroglutaric acid. 
     A battery that includes an exemplary additive may be autoclaved at 125° C. for a half an hour, defined as one cycle, performed three times without adversely affecting the battery. The additives may be used in low, medium, or high capacity batteries. 
       FIG. 1  depicts an implantable medical device (IMD)  10 . IMD  10  includes a case  50 , a control module  52 , a battery  54  (e.g. organic electrolyte battery) and capacitor(s)  56 . Control module  52  controls one or more sensing and/or stimulation processes from IMD  10  via leads (not shown). Battery  54  includes an insulator  58  disposed therearound. Battery  54  charges capacitor(s)  56  and powers control module  52 . 
       FIGS. 2 and 3  depict details of an exemplary organic electrolyte battery  54 . Battery  54  includes a case  70 , an anode  72 , separators  74 , a cathode  76 , a liquid electrolyte  78 , and a feed-through terminal  80 . Cathode  76  is wound in a plurality of turns, with anode  72  interposed between the turns of the cathode winding. Separator  74  insulates anode  72  from cathode  76  windings. Case  70  contains the liquid electrolyte  78  to create a conductive path between anode  72  and cathode  76 . Electrolyte  78 , which includes an additive, serves as a medium for migration of ions between anode  72  and cathode  76  during an electrochemical reaction with these electrodes. 
     Anode  72  is formed of a material selected from Group IA, IIA or IIIB of the periodic table of elements (e.g. lithium, sodium, potassium, etc.), alloys thereof or intermetallic compounds (e.g. Li—Si, Li—B, Li—Si—B etc.). Anode  72  comprises an alkali metal (e.g. lithium, etc.) in metallic or ionic form. 
     Cathode  76  may comprise metal oxides (e.g. vanadium oxide, silver vanadium oxide (SVO), manganese dioxide (MnO 2 ) etc.), carbon monofluoride and hybrids thereof (e.g., CF x +MnO 2 ), combination silver vanadium oxide (CSVO) or other suitable compounds. 
     Electrolyte  78  chemically reacts with anode  72  to form an ionically conductive passivation film  82  on anode  72 , as shown in  FIG. 4 . Electrolyte  78  includes a base liquid electrolyte composition and at least one resistance-stabilizing additive selected from Table 1 presented below. The base electrolyte composition typically comprises 1.0 molar (M) lithium tetrafluoroborate (1-20% by weight), gamma-butyrolactone (50-70% by weight), and 1,2-dimethoxyethane (30-50% by weight). In one embodiment, resistance-stabilizing additives are directed to chemical compounds that include electron withdrawing groups. An exemplary chemical compound with an electron withdrawing group includes 2,2,2-trifluoroacetamide. In another embodiment, the additive is a proton donor such as an organic acid. One type of organic acid is benzoic acid (e.g. 3-hydroxy benzoic acid or 2-4 hydroxy benzoic acid etc.). Every combination of benzoic acid and hydroxyl benzoic acids that exists may be used as a resistance-stabilizing additive composition. Malic acid and tetramethylammonium hydrogen phthalate are other organic acids that may serve as a resistance-stabilizing additive. 
     Tables 1 and 2 list some exemplary resistance-stabilizing additives. In particular, Table 1 ranks each additive as to its effectiveness with a rank of 1 being the highest or best additive and rank 6 being the lowest ranked additive. Table 1 also briefly describes the time period in which battery  54 , which included the specified additive in the electrolyte  78 , exhibited resistance-stabilizing characteristics. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 List of exemplary additive resistance-stabilizing additives 
               
             
          
           
               
                   
                 Chemical 
                 Exemplary additive 
                   
                   
               
               
                 Rank 
                 class 
                 compound 
                 Chemical Structure 
                 Notes 
               
               
                   
               
               
                 3 
                 Aromatic diacid salts 
                 Tetramethyl- ammonium (TMA) hydrogen phthalate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited excellent resistance- stabilizing characteristic during storage Battery exhibited good to neutral resistance- stabilizing characteristic during discharge 
               
               
                   
               
               
                 6 
                 Inorganic acid salts 
                 Tetrabutyl- ammonium (TBA) hydrogen sulfate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited good resistance- stabilizing characteristic during storage Battery exhibited neutral resistance- stabilizing characteristic during discharge 
               
               
                   
               
               
                 5 
                 Aliphatic organic acids 
                 Phosphonoacetic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited excellent resistance- stabilizing characteristic during storage Battery exhibited good to neutral resistance- stabilizing characteristic during discharge 
               
               
                   
               
               
                 1 
                 (*) 
                 2,2,2- Trifluoroacetamide 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited excellent resistance- stabilizing characteristic during storage and discharge 
               
               
                   
               
               
                   
                 (*) 
                 Trifluoromethyl vinyl acetate 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited very good resistance- stabilizing characteristic during discharge 
               
               
                   
               
               
                 4 
                 Aromatic diacids 
                 Phthalic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited good resistance- stabilizing characteristic during storage and discharge 
               
               
                   
               
               
                   
                 (*) 
                 Benzoylacetone 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited good resistance- stabilizing characteristic during storage and discharge 
               
               
                   
               
               
                   
                 (*) 
                 Benzoyltrifluoro- acetone 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited good resistance- stabilizing characteristic during storage and discharge 
               
               
                   
               
               
                 2 
                 Aromatic mono- acids 
                 Benzoic acid 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 Battery exhibited excellent resistance- stabilizing characteristic during storage and discharge 
               
               
                   
               
               
                 (*) These compounds include a chemical structure that is characterized by one or more electron-withdrawing groups (e.g. —CF 3 , —C 6 H 5  located one or two carbon atoms from a double-bonded oxygen atom (i.e. a ketone group)). Additionally, the listed additives may be added to the base electrolyte composition in the range of about 0.001 M to 0.5 M. 
               
             
          
         
       
     
     Table 2 lists exemplary additive compositions that are mixed with the base electrolyte composition to produce effective resistance-stabilization in battery  54 . Effective additive compositions are based upon additives that exhibit superior resistance-stabilizing characteristics either at the beginning of life (BOL) or at the end of life (EOL) of battery  54 . In one embodiment, an additive composition comprises a first additive that exhibits substantially superior resistance-stabilizing characteristics at the BOL whereas a second additive exhibits substantially superior resistance-stabilizing characteristics at the EOL. In another embodiment, a first resistance-stabilizing additive exhibits a substantially superior resistance-stabilizing characteristics at the BOL whereas a second resistance-stabilizing additive exhibits average resistance-stabilizing characteristics at the EOL. In still yet another embodiment, a first resistance-stabilizing additive exhibits substantially superior resistance-stabilizing characteristics at the EOL whereas a second resistance-stabilizing additive exhibits average resistance-stabilizing characteristics at the BOL. Generally, each additive is combined with the electrolyte  78  through dissolution or other suitable means. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Exemplary resistance-stabilizing composition additives 
               
             
          
           
               
                   
                 Additive compositions 
                 Quantity of each additive 
               
               
                   
                   
               
               
                   
                 TMA hydrogen phthalate + 
                 About 0.001 M to about 0.5M 
               
               
                   
                 2,2,2-Trifluoroacetamide 
               
               
                   
                 TMA hydrogen phthalate + 
                 About 0.001 M to about 0.5M 
               
               
                   
                 Trifluoromethyl vinyl acetate 
               
               
                   
                 TMA hydrogen phthalate + 
                 About 0.001 M to about 0.5M 
               
               
                   
                 Acetone 
               
               
                   
                 TMA hydrogen phthalate + 
                 About 0.001 M to about 
               
               
                   
                 Xylitol 
                 0.05M 
               
               
                   
                 Phosphonoacetic acid + 
                 About 0.001 M to about 0.5M 
               
               
                   
                 2,2,2-Trifluoroacetamide 
               
               
                   
                 Phosphonoacetic acid + 
                 About 0.001 M to about 0.5M 
               
               
                   
                 Trifluoromethyl vinyl acetate 
               
               
                   
                 Phosphonoacetic acid + 
                 About 0.001 M to about 0.5M 
               
               
                   
                 Acetone 
               
               
                   
                 Phosphonoacetic acid + 
                 About 0.001 M to about 0.5M 
               
               
                   
                 Xylitol 
               
               
                   
                   
               
             
          
         
       
     
       FIGS. 5-6  graphically depict the resistance-stabilizing superiority of electrolyte  78  over a control electrolyte  88 . Electrolyte  78  includes 2,2,2-trifluoroacetamide as the resistance-stabilizing additive and the base electrolyte composition previously described. Control electrolyte  88  is the base electrolyte composition without any additive. Passivation layer  82  initially possesses similar discharge to passivation layer formed by control electrolyte  88 . However, later in the discharge (e.g. about 0.90 ampere·hour(Ah)), the passivation layer formed by control electrolyte  88  exhibits resistance that substantially increases. In contrast, electrolyte  78  that includes the additive causes battery  54  to exhibit resistance that remains substantially below the resistance of control electrolyte  88  late in discharge. For example, electrolyte  78  results in battery  54  having 30 ohms lower resistance than control electrolyte  88 , as show in  FIG. 5 . 
     If the resistance increases in the area between 1 and 1.2 Ah of the curve and IMD  10  records the voltage after a high current event (e.g. telemetry event etc.), a recommended replacement time (RRT) signal may be generated. Preferably, desirable resistance is kept low as long as possible to increase efficiency of battery  54 . 
       FIG. 7  depicts a method for forming a resistance-stabilizing additive composition. At operation  200 , a first resistance stabilizing additive is selected. At operation  210 , the first resistance stabilizing additive is combined with a second resistance stabilizing additive to create a resistance stabilizing composition. 
       FIG. 8  depicts a method for autoclaving battery cell  54 . Battery cell  54  is inserted into a chamber of an autoclave at operation  300 . Battery cell  54  includes an electrolyte and a first resistance-stabilizing additive combined with the electrolyte. At block  310 , heat is applied to the chamber of the autoclave. Generally, the autoclaving process occurs at a temperature of 125° C. for a half an hour per cycle. The autoclave cycle is repeated at least three times. After three cycles of autoclaving, battery cell  54  adequately operates. 
     The following patent application is incorporated by reference in its entirety. Co-pending U.S. patent application Ser. No. 11/343,323 (U.S. Pat. App. Pub. No. 2007/0176151), entitled “ELECTROLYTE ADDITIVE FOR PERFORMANCE STABILITY OF BATTERIES”, filed by Kevin Chen, Donald Merritt and Craig Schmidt on Jan. 31, 2006, and assigned to the same Assignee of the present invention, describes resistance-stabilizing additives for electrolyte. 
     Although various embodiments of the invention have been described and illustrated with reference to specific embodiments thereof, it is not intended that the invention be limited to such illustrative embodiments. For example, while an additive composition is described as a combination of two additives, it may also include two or more additives selected from Table 1. The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.