Control of Silver Vanadium Oxide Surface Areas as a Means of Controlling Voltage Delay and RDC Growth in an Electrochemical Cell

Abstract

An electrochemical cell comprising a lithium anode, a cathode comprising a blank cut from a free-standing sheet of a silver vanadium oxide mixture contacted to a current collector. The active material has having a relatively lower surface area and an electrolyte activating the anode and the cathode is described. By optimizing the cathode active material surface area in a SVO-containing cell, the magnitude of the passivating film growth at the solid-electrolyte interphase (SEI) and its relative impermeability to lithium ion diffusion is reduced. Therefore, by using a cathode of an active material, in a range of from about 0.2 m 2 /gram to about 2.6 m 2 /gram, and preferably from about 1.6 m 2 /gram to about 2.4 m 2 /gram, it is possible to eliminate or significantly reduce undesirable irreversible Rdc growth and voltage delay in the cell and to extend its useful life in an implantable medical device.

1 . A method for preparing a cathode for use in an electrochemical cell having the cathode and an anode which is electrochemically oxidized to form metal ions in the cell upon discharge to generate electron flow in an external electrical circuit connected to the cell, wherein the electron flow is generated by intercalation of the metal ions into a cathode active material comprising the cathode and the cell is characterized by an ionically conductive electrolytic solution associated with the anode and the cathode, which comprises:
 a) providing a cathode active material comprising a metallic material, wherein the metallic material comprises from between about 80 weight percent to about 98 weight percent of the cathode active material;   b) mixing the cathode active material with a solvent material to form a paste comprising the cathode active material;   c) subjecting the paste to a first pressing step forming the paste into a cathode sheet which is intercalatable by the metal ions formed by the anode;   d) removing any residual solvent material from the cathode sheet;   e) subjecting the cathode sheet to a forming means that serves to provide at least one cathode plate having a variety of geometric shapes; and   f) laminating at least one of the thusly formed cathode plates on at least one side of a current collector means by subjecting the cathode plate to a second pressing step to form a laminated cathode component as the cathode for use in the electrochemical cell, wherein during the laminating step the cathode active material does not lose its ability to intercalate metal ions formed by the anode.   
     
     
         2 . The method of  claim 1  including providing the paste comprising the cathode material further comprising the addition of binder and conductor materials. 
     
     
         3 . The method of  claim 1  including selecting the solvent material from the group consisting of water and an inert organic material. 
     
     
         4 . The method of  claim 1  including providing the cathode active material in a range from about 0.2 m 2 /gram to about 2.6 m 2 /gram prior to mixing with the solvent material forming the paste. 
     
     
         5 . The method of  claim 1  including forming the paste into the cathode sheet by feeding the paste through a roll mill as the first pressing step. 
     
     
         6 . The method of  claim 5  further including the step of first feeding the paste into a compaction means that serves to provide the cathode active material in a pellet form prior to introduction to the roll mill as the first pressing step. 
     
     
         7 . The method of  claim 1  including selecting the cathode active material from the group consisting of silver vanadium oxide, copper silver vanadium oxide, manganese dioxide, copper vanadium oxide, titanium disulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide, and mixtures thereof. 
     
     
         8 . The method of  claim 1  including providing the cathode sheet having a thickness in the range of from between about 0.0015 inches to about 0.020 inches. 
     
     
         9 . The method of  claim 1  including removing the residual solvent material from the cathode active material by drying. 
     
     
         10 . The method of  claim 1  including providing the cathode having a configuration selected from the group consisting of: SVO/current collector/CF x /current collector/SVO, SVO/current collector/SVO/CF x /SVO/current collector/SVO, and SVO/current collector/CF x , with the SVO facing the lithium anode. 
     
     
         11 . A method of powering an implantable medical device with an electrochemical cell, the cell comprising a lithium anode coupled to a cathode of a cathode active material activated with an electrolyte, comprising the steps of:
 a) providing the anode;   b) providing the cathode of a cathode active material having a surface area from about 0.2 m 2 /gram to about 2.6 m 2 /gram;   c) positioning the anode and the cathode inside a casing with an intermediate separator preventing direct physical contact between them;   activating the anode and the cathode with an electrolyte;   e) connecting a negative terminal and a positive terminal of the cell to the implantable medical device;   f) powering the implantable medical device with the cell; and   g) periodically discharging the cell to deliver at least one pulse of electrical current of significantly greater amplitude than that of a pre-pulse current or open circuit voltage immediately prior to the pulse discharge.   
     
     
         12 . The method of  claim 11  including selecting the cathode active material from the group consisting of silver vanadium oxide, copper silver vanadium oxide, manganese dioxide, copper vanadium oxide, titanium disulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide, fluorinated carbon, and mixtures thereof. 
     
     
         13 . The method of  claim 11  wherein the cathode active material has a surface area from about 1.6 m 2 /gram to about 2.4 m 2 /gram.