Abstract:
A primary electrochemical cell having an oxidizable active anode material, a cathode current collector, and an electrolytic solution. The electrolytic solution consists essentially of liquid cathode material, an electrolyte solute for imparting conductivity, and iodine monochloride for catalyzing the electroreduction of the liquid cathode material. In specific embodiments the anode material was lithium, the liquid cathode material was thionyl chloride or sulfuryl chloride and the electrolyte solute was lithium tetrachloroaluminate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to application Ser. No. 538,465, filed Oct. 3, 1983, by Keith A. Klinedinst and William D. K. Clark entitled &#34;Electrochemical Cell&#34;; application Ser. No. 538,464, filed Oct. 3, 1983, by Keith A. Klinedinst and William D. K. Clark entitled &#34;Primary Electrochemical Cell&#34;; and application Ser. No. 809,747 filed Nov. 8, 1985 by Keith A. Klinedinst and William D. K. Clark entitled &#34;Primary Electrochemical Cell,&#34; which application is a continuation-in-part of application Ser. No. 538,464. 
     BACKGROUND OF THE INVENTION 
     This invention relates to electrochemical cells. More particularly, it is concerned with primary electrochemical cells having an oxidizable active anode material, a cathode current collector, and an electrolytic solution comprising a reducible liquid cathode material and an electrolyte solute dissolved therein. 
     A particularly effective class of primary electrochemical cells which employs soluble or liquid cathode materials, as opposed to the more conventional solid cathode cells, has undergone rapid development in recent years. In these cells, the active cathode material is usually a fluid solvent for an electrolyte solute which provides conductivity. The active anode of the cell is usually lithium or other highly electropositive metal. During discharge the solvent is electrochemically reduced on a cathode current collector. 
     One particular type of electrochemical cell of the foregoing class which contains a lithium anode employs a reducible liquid cathode of an oxyhalide, specifically thionyl chloride or sulfuryl chloride. Typically the electrolyte solute dissolved in the oxyhalide solvent is lithium tetrachloroaluminate. Lithium/oxyhalide electrochemical cells have proven to have outstanding weight and volume energy density, long shelf life, and unusually high power density when compared with other cells previously available. 
     SUMMARY OF THE INVENTION 
     An electrochemical cell in accordance with the present invention which provides improved output voltage and output capacity comprises an oxidizable anode material, a cathode current collector, and an electrolytic solution in contact with the anode material and the cathode current collector. The electrolytic solution consists essentially of reducible liquid cathode material, an electrolyte solute dissolved in the reducible liquid cathode material for imparting conductivity to the electrolytic solution, and iodine monochloride for catalyzing the electroreduction of the liquid cathode material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows curves of discharge characteristics of electrochemical cells employing a reducible liquid cathode material of thionyl chloride with and without a catalyst of iodine monochloride under one set of discharge conditions; 
     FIG. 2 shows curves of discharge characteristics of electrochemical cells employing a reducible liquid cathode material of thionyl chloride with and without a catalyst of iodine monochloride under another set of discharge conditions; 
     FIG. 3 shows curves of discharge characteristics of electrochemical cells employing a reducible liquid cathode material of sulfuryl chloride with and without a catalyst of iodine monochloride under one set of discharge conditions; and 
     FIG. 4 shows curves of discharge characteristics of electrochemical cells employing a reducible liquid cathode material of sulfuryl chloride with and without a catalyst of iodine monochloride under another set of discharge conditions. 
     For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following discussion and appended claims in connection with the above-described drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Electrochemical cells in accordance with the present invention employ an anode, a cathode current collector, and an electrolytic solution which is in contact with the anode and cathode current collector. The anode and cathode current collector are separated from each other as by a thin porous layer of insulating material. The electrolytic solution comprises a fluid, reducible solvent cathode material with an electrolyte solute and a catalyst of iodine monochloride dissolved therein. 
     The anode is an oxidizable material and is preferably lithium metal. Other oxidizable materials which may be employed in electrochemical cells of this type include other alkali metals and also alkaline earth metals. The electrolytic solution comprises a solvent of a reducible liquid cathode material. Among the cathode materials found useful are fluid oxyhalides, fluid non-metallic oxides, fluid non-metallic halides, fluid metallic halides, and mixtures thereof. The oxyhalides thionyl chloride (SOCl 2 ) and sulfuryl chloride (SOC 2  Cl 2 ) are preferred liquid cathode materials. 
     The electrolyte solute of the electrolytic solution may be LiAlCl 4 , LiAlBr 4 , LiBC1 4 , LiBF 4 , LiAsF 6 , LiSbC1 6 , Li 2  SnCl 6 , or Li 2  TiCl 6 . The electrolyte solute may also be a Lewis acid such as AlCl 3 , SnCl 4 , TiCl 4 , SbC 5 , or BCl 3 , or a combination of a Lewis acid with a Lewis base such as LiCl, LiBr, or LiF. The molarity of the solute in the solution is usually from about 0.5 to about 2.5M. In lithium/oxyhalide cells the molarity of the solute is preferably from about 1.5 to about 2.0M. 
     In accordance with the present invention, the electrolytic solution also includes an electroreducing catalyst of iodine monochloride (ICl). 
     The following examples are for the purpose of further illustrating and explaining the present invention, and are not to be taken as limiting the scope thereof. 
     EXAMPLE I 
     Test electrochemical cells were constructed with polished vitreous carbon working electrodes and with lithium counter and reference electrodes. The cells contained an electrolytic solution of 1.0M lithium tetrachloroaluminate (LiAlCl 4 ) in thionyl chloride (SOCl 2 ). (A quantity of POCl 3  was added as a cosolvent. This cosolvent does not interfere with the operation of the cell since it is reduced below 2.0 V upon the vitreous carbon working electrode.) Cells of this type were discharged at ambient temperature with constant 3.2 mA/cm 2  current densities both with and without the addition of 0.1M of ICl catalyst to the oxyhalide electrolyte. The resulting discharge characteristics are listed in Table I. A 510 mV increase in average load voltage and a 435% increase in discharge capacity were achieved by the addition of 0.1M ICl to the SOCl 2  electrolyte. 
     
                       TABLE I______________________________________Li/SOCl.sub.2 Discharge Characteristics at 3.2 mA/cm.sup.2vsCatalyst ConcentrationICl(Moles/l) E.sub.avg (Volts)                   Capacity (mC/cm.sup.2)______________________________________0.00         2.73        340.10         3.24       182______________________________________ 
    
     EXAMPLE II 
     Cells identical to those described in Example I were discharged at constant 6.4 mA/cm 2  rates to yield the discharge characteristics listed in Table II. Addition of the electrocatalyst ICl to the SOCl 2  electrolyte resulted in a 550 mV increase in average load voltage and a 185% increase in discharge capacity. 
     
                       TABLE II______________________________________Li/SOCl.sub.2 Discharge Characteristics at 6.4 mA/cm.sup.2vsCatalyst ConcentrationICl(Moles/l) E.sub.avg (Volts)                   Capacity (mC/cm.sup.2)______________________________________0.00         2.68       25.40.10         3.23       72.8______________________________________ 
    
     EXAMPLE III 
     Test electrochemical cells were constructed with polished vitreous carbon working electrodes and with lithium counter and reference electrodes. The cells contained an electrolytic solution of 1.0M lithium tetrachloroaluminate (LiAlCl 4 ) in sulfuryl chloride (SOP 2  Cl 2 ) (A quantity of POCl 3  was added as a cosolvent. This cosolvent does not interfere with the operation of the cell since it is reduced below 2.0 V upon the vitreous carbon working electrode.) Cells of this type were discharged at ambient temperature with constant 3.2 mA/cm 2  current densities both with and without the addition of 0.1M of ICl catalyst to the oxyhalide electrolyte. The resulting discharge characteristics are listed in Table III. A 50 mV increase in average load voltage and a 11-fold increase in discharge capacity were achieved by the addition of 0.1M ICl to the SO 2  Cl 2  electrolyte. 
     
                       TABLE III______________________________________Li/SO.sub.2 Cl.sub.2 Discharge Characteristics at 3.2 mA/cm.sup.2vsCatalyst ConcentrationICl(Moles/l) E.sub.avg (Volts)                   Capacity (mC/cm.sup.2)______________________________________0.00         3.20        450.10         3.25       549______________________________________ 
    
     EXAMPLE IV 
     Cells identical to those in Example III were discharged at constant 6.4 mA/cm 2  rates. The resulting discharge characteristics are listed in Table IV. A 260 mV increase in average load voltage and a 450% increase in discharge capacity resulted from the addition of the ICl electrocatalyst. 
     
                       TABLE IV______________________________________Li/SO.sub.2 Cl.sub.2 Discharge Characteristics at 6.4 mA/cm.sup.2vsCatalyst ConcentrationICl(Moles/l) E.sub.avg (Volts)                   Capacity (mC/cm.sup.2)______________________________________0.00         3.00        420.10         3.26       230______________________________________ 
    
     EXAMPLE V 
     Cells identical to those described in Example III were discharged at constant 19.2 mA/cm 2  rates. The resulting discharge characteristics are listedin Table V. A 330 mV increase in average load voltage and a 445% increase in discharge capacity resulted from the addition of the ICl electrocatalyst. 
     
                       TABLE V______________________________________Li/SO.sub.2 Cl.sub.2 Discharge Characteristics at 19.2 mA/cm.sup.2vsCatalyst Concentration                     CapacityICl(Moles/l)   E.sub.avg (Volts)                     (mC/cm.sup.2)______________________________________0.00           2.94       17.30.10           3.27       94.5______________________________________ 
    
     EXAMPLE VI 
     Cells identical to those in Example III were discharged at constant 32.0 mA/cm 2  rates. The resulting discharged characteristics are listed in Table VI. Addition of the ICl electrocatalyst to the SO 2  Cl 2  electrolyte resulted in a 290 mV increase in average load voltage and a 235% increase in discharge capacity. 
     
                       TABLE VI______________________________________Li/SO.sub.2 Cl.sub.2 Discharge Characteristics at 32.0 mA/cm.sup.2vsCatalyst Concentration                     CapacityICl(Moles/l)   E.sub.avg (Volts)                     (mC/cm.sup.2)______________________________________0.00           2.70       13.30.10           2.99       44.8______________________________________ 
    
     EXAMPLE VII 
     Li/SOCl 2  cells were constructed with PTFE-bonded Shawinigan acetylene black cathodes (5 cm 2  ×1 mm) and with 1.0M LiAlCl 4  in SOCl 2  as electrolyte. To the electrolyte was added iodine monochloride to catalyze the electroreduction of SOCl 2 , the iodine monochloride concentration ranging between 0.0M and 0.1M. These cells were discharged at ambient temperature through 20 ohm loads to yield the average load voltages, current densities, and discharge capacities listed in Table VII. As shown, the overvoltage for SOCl 2  reduction was reduced by 160 mV and the discharge capacity was increased by 40% by the addition of 0.10M ICl to the SOCl 2  electrolyte. Discharge curves for cells without the ICl catalyst and with 0.10M ICl are shown in FIG. 1. 
     
                       TABLE VII______________________________________Li/SOCl.sub.2 Cell Constant Load Discharge Characteristicsat Ambient Temperature vs ICl Catalyst Concentration                            CapacityICl(Moles/l)    E.sub.avg (Volts)                I.sub.avg (mA/cm.sup.2)                            (mAhr/cm.sup.2)______________________________________0.00     2.89        28.9        18.90.05     2.93        29.3        24.10.10     3.05        30.5        26.1______________________________________ 
    
     EXAMPLE VIII 
     Li/SOCl 2  cells identical to those Example VII with ICl concentration ranging between 0.00M and 0.20M were discharged through 57 ohm loads. The resulting discharge characteristics are listed in Table VIII. The discharge curves obtained with and without the addition of 0.05M ICl to the SOCl 2  electrolyte are compared in FIG. 2. The addition of 0.05M ICl catalyst produced a 170 mV increase in average load voltage and a 40% increase in discharge capacity. 
     
                       TABLE VIII______________________________________Li/SOCl.sub.2 Cell Constant Load Discharge CharacteristicsAt Ambient Temperature vs ICl Catalyst Concentration                            CapacityICl(Moles/l)    E.sub.avg (Volts)                I.sub.avg (mA/cm.sup.2)                            (mAhr/cm.sup.2)______________________________________0.00     3.02        10.6        34.60.05     3.19        11.2        48.70.10     3.22        11.3        44.60.20     3.29        11.6        41.5______________________________________ 
    
     EXAMPLE IX 
     Li/SO 2  Cl 2  cells were constructed with PTFE-bonded i Shawinigan acetylene black cathodes (5 cm 2  ×1 mm) and with 1.0M LiAlCl 4  in SO 2  Cl 2  as electrolyte. To the electrolyte was added iodine monochloride to catalyze the electroreduction of SO 2  Cl 2 , the iodine monochloride concentration ranging between 0.0M and 0.10M. These cells were discharged at ambient temperature through 20 ohm loads to yield the average load voltages, current densities, and discharge capacities listed in Table IX. As shown, the overvoltage for SO 2  Cl 2  reduction was reduced by 430 mV and the discharge capacity was increased by 65% by the addition of 0.10M ICl to the SO 2  Cl 2  electrolyte. Discharge curves for cells without the ICl catalyst and with 0.10M ICl are shown in FIG. 3. 
     
                       TABLE IX______________________________________Li/SO.sub.2 Cl.sub.2 Cell Constant Load Discharge CharacteristicsAt Ambient Temperature vs ICl Catalyst Concentration                            CapacityICl(Moles/l)    E.sub.avg (Volts)                I.sub.avg (mA/cm.sup.2)                            (mAhr/cm.sup.2)______________________________________0.00     2.71        27.1        15.30.05     3.09        30.9        22.60.10     3.14        31.4        25.4______________________________________ 
    
     EXAMPLE X 
     Li/SO 2  Cl 2  cells identical to those described in Example IX with ICl concentrations ranging between 0.00M and 0.10M were discharged through 57 ohm loads. The resulting discharge characteristics are tabulated in Table X. The discharge curves obtained with and without the addition of 0.05M ICl to the SO 2  Cl 2  electrolyte are compared in FIG. 4. As shown, a 150 mV increase in average load voltage and a 70% increase in discharge capacity resulted from the addition of 0.05 M ICl to the SO 2  Cl 2  electrolyte. 
     
                       TABLE X______________________________________Li/SO.sub.2 Cl.sub.2 Cell Constant Load Discharge CharacteristicsAt Ambient Temperature vs ICl Catalyst Concentration                            CapacityICl(Moles/l)    E.sub.avg (Volts)                I.sub.avg (mA/cm.sup.2)                            (mAhr/cm.sup.2)______________________________________0.000    3.02        10.6        27.60.010    3.17        11.1        47.00.025    3.24        11.4        43.90.05     3.26        11.4        38.70.10     3.26        11.4        34.3______________________________________ 
    
     As shown by the foregoing examples, iodine monochloride is an effective oxyhalide electroreduction catalyst. It has been found particularly effective at concentrations between 0.01M and 0.20M. 
     The mechanism by which small quantities of iodine monochloride catalyze the electroreduction of oxyhalides has not been determined with certainty. It is known that I -   is readily converted to Cl -   by reaction with SOCl 2  (via the formation of the relatively unstable thionyl iodide). 
     
         4I.sup.- +2SOCl.sub.2 →S+SO.sub.2 +2I.sub.2 +4Cl.sup.-(1) 
    
     As disclosed in applicant&#39;s aforementioned application Ser. No. 809,747, small quantities of I 2  have been found to be an effective catalyst for the electroreduction of SOCl 2 . The following reactions have been proposed to explain this catalysis. 
     Electrochemical Reaction: 
     
         2I.sub.2 (adsorbed)+4e.sup.- →4I.sup.-              (2) 
    
     Chemical Reaction: 
     
         4I.sup.- +2SOCl.sub.2 →S+SO.sub.2 +2I.sub.2 +4Cl.sup.-(3) 
    
     Overall Reaction: 
     
         2SOCl.sub.2 +4e.sup.- →S+SO.sub.2 +4Cl.sup.-        (4) 
    
     Assuming that, on a carbon electrode, the electroreduction of iodine monochloride occurs more rapidly and at higher potentials than the electoreduction of thionyl chloride, the following series of reactions constitute a possible mechanism by which iodine monochloride may catalyze the reduction of thionyl chloride on a carbon electrode. 
     Electrochemical Reactions: 
     
         2ICl(adsorbed)+2e.sup.- 2I°+2Cl.sup.-               (5) 
    
     
         2I°+2e.sup.- →2I.sup.-                       (6) 
    
     Chemical Reaction: 
     
         2I.sup.- +SOCl.sub.2 →1/2S+1/2SO.sub.2 +I.sub.2 +2Cl.sup.-(7) 
    
     Overall Reaction: 
     
         2ICl+SOCl.sub.2 +4e.sup.- →1/2S+1/2SO.sub.2 +I.sub.2 +4Cl.sup.-(8) 
    
     These reactions would be followed by similar reaction involving molecular iodine (reactions (2) through (4) above). The net result is that thionyl chloride is reduced at potentials governed by the I 2  /I -   redox couple. 
     By analogy, assuming that the electroreduction of iodine monochloride occurs more rapidly and at higher potentials than the electroreduction of sulfuryl chloride, the following series of reactions constitute a possible mechanism by which iodine monochloride may catalyze the reduction of sulfuryl chloride on a carbon electrode. 
     Eletrochemical Reaction: 
     
         2ICl(adsorbed)+2e.sup.- →2I° (adsorbed)+2Cl.sup.-(9) 
    
     Chemical Reaction: 
     
         2I° (adsorbed)+SO.sub.2 Cl.sub.2 →SO.sub.2 +2ICl(adsorbed) (10) 
    
     Overall Reaction 
     
         SO.sub.2 Cl.sub.2 +2e.sup.- →SO.sub.2 +2Cl.sup.-    (11) 
    
     While there have been shown and described what are considered preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims.