Abstract:
An alkali metal electrochemical cell capable of delivering high current pulses without exhibiting voltage delay, rapidly recovering its open circuit voltage and having high current capacity, is described. The stated benefits are realized by conditioning the cell heated at an elevated temperature for an extended period of time.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an alkali metal electrochemical cell, and more particularly, to a lithium cell conditioned for current pulse discharge applications. 
     2. Description of the Prior Art 
     It is well known that the anode surface film, known as solid-electrolyte interphase (SEI), plays a very important role in the discharge performance of either a primary or a secondary alkali metal electrochemical cell, and in particular, a lithium cell. A good SEI benefits cell performance, including high discharge capacity, long useful life, better rechargeability for secondary cells and little or no voltage delay during current pulse applications. Since the formation of a surface film is unavoidable for alkali metal, and in particular, lithium metal anodes, and for lithium intercalated carbon anodes, due to their low potential and high reactivity towards organic electrolytes, much effort has been focused on modification of the chemical composition and morphology of the anode surface film. 
     The ideal anode surface film should be electrically insulating and ionically conducting. While most alkali metal, and in particular lithium electrochemical, systems meet the first requirement, the second requirement is difficult to achieve. As a result, impedance builds up inside the cell due to this surface layer formation which often results in reduced discharge voltage and reduced cell capacity. In the case of a cell subjected to current pulse discharge, voltage delay and voltage drop may occur as a result of the anode surface film and limit the effectiveness of the cell under these conditions, i.e., decrease the current pulse discharge capacity of the cell. 
     Among the known techniques for reducing voltage delay under current pulse discharge conditions are the inclusion of various voltage delay reducing additives in the electrolyte. U.S. Pat. No. 4,658,753 to Fleischer describes adding to the electrolyte a substance from the group of alkyl-2-cyanoacrylates and acrylic and substituted acrylic ester polymers for the purpose of minimization of voltage delay in lithium oxyhalide cells. 
     The present invention does not relate to modifying the electrochemical system by the inclusion of a voltage delay reducing additive into the electrolyte. Instead, the alkali metal cell is subjected to elevated temperature to heat condition the cell. After heat conditioning, the cell is dischargeable under current pulse applications with reduced or no appreciable voltage delay. 
     SUMMARY OF THE INVENTION 
     It is known that voltage delay is typically exhibited in an alkali metal/mixed metal oxide cell, such as a lithium/silver vanadium oxide cell, that has been depleted of approximately 40% to 70% of its capacity and is subjected to current pulse discharge applications. The present invention is directed to improvements in the pulse discharge performance of an alkali metal electrochemical cell, and more particularly, a primary lithium, solid cathode electrochemical cell, by subjecting the cell to elevated temperature to heat condition the cell. The thusly conditioned cell exhibits reduced and in some cases no voltage delay under current pulse discharge usage, which is an unexpected result. While the relevant mechanism is not fully understood at this time, it is believed that the extended thermal expose of an electrochemical cell according to the present invention provides the SEI with enhanced ionic conductivity, which benefits pulse discharge application. The concept of this invention is likewise applicable to secondary alkali metal electrochemical cells. 
     These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and to the appended drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The electrochemical cell according to the present invention includes an anode electrode selected from Group IA of the Periodic Table of Elements, including lithium, sodium, potassium, etc., and their alloys and intermetallic compounds including, for example Li-Si, Li-B and Li-Si-B alloys and intermetallic compounds. The preferred anode comprises lithium, and the more preferred anode comprises a lithium alloy, the preferred lithium alloy being lithium-aluminum with the aluminum comprising from between about 0% to about 50% by weight of the alloy. The greater the amount of aluminum present by weight in the alloy the lower the energy density of the cell. 
     The form of the anode may vary, but preferably the anode is a thin metal sheet or foil of the anode metal, pressed or rolled on a metallic anode current collector, i.e., preferably comprising nickel, to form an anode component. In the exemplary cell of the present invention, the anode component has an extended tab or lead of the same material as the anode current collector, i.e., preferably nickel, integrally formed therewith, such as by welding and contacted by a weld to a cell case of conductive metal in a case-negative electrical configuration. Alternatively, the anode may be formed in some other geometry, such as a bobbin shape, cylinder or pellet to allow an alternate low surface area cell design. 
     The cathode electrode may comprise a metal element, a metal oxide, a mixed metal oxide, a metal sulfide or carbonaceous compounds, and combinations thereof. Suitable cathode active materials include silver vanadium oxide (SVO), copper vanadium oxide, copper silver vanadium oxide (CSVO), manganese dioxide, titanium disulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide, carbon and fluorinated carbon, and mixtures thereof. The solid cathode exhibits excellent thermal stability and is generally safer and less reactive than a non-solid cathode. 
     Preferably, the cathode active material comprises a mixed metal oxide formed by chemical addition, reaction or otherwise intimate contact or by a thermal spray coating process of various metal sulfides, metal oxides or metal oxide/elemental metal combinations. The materials thereby produced contain metals and oxides of Groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the Periodic Table of Elements, which includes the noble metals and/or their oxide compounds. 
     By way of illustration, and in no way intended to be limiting, an exemplary cathode active material comprises silver vanadium oxide having the general formula Ag x  V 2  O y  in any one of its many phases, i.e. β-phase silver vanadium oxide having in the general formula x=0.35 and y=5.18, γ-phase silver vanadium oxide having in the general formula x=0.74 and y=5.37 and ε-phase silver vanadium oxide having in the general formula x=1.0 and y=5.5, and combination and mixtures of phases thereof. 
     Such cathode active materials may be pressed into a cathode pellet with the aid of a suitable binder material such as a fluoro-resin powder, preferably polytetrafluoroethylene (PTFE) powder, and a material having electronic conduction characteristics such as graphite powder, acetylene black powder and carbon black powder. In some cases, no binder material or electronic conductor material is required to provide a similarly suitable cathode body. In some cases, the cathode active materials may also be prepared by rolling, spreading or pressing a mixture of the materials mentioned above onto a suitable current collector. Cathodes prepared as described above may be in the form of one or more plates operatively associated with at least one or more plates of anode material, or the cathode may be in the form of a strip wound with a corresponding strip of anode material in a structure similar to a &#34;jellyroll&#34;. 
     The cell of the present invention includes a separator to provide physical separation between the anode and cathode active electrodes. The separator is of electrically insulative material to prevent an internal electrical short circuit between the electrodes, and the separator material also is chemically unreactive with the anode and cathode active materials and both chemically unreactive with and insoluble in the electrolyte. In addition, the separator material has a degree of porosity sufficient to allow flow therethrough of the electrolyte during the electrochemical reaction of the cell. Illustrative separator materials include non-woven glass, polypropylene, polyethylene, glass fiber material, ceramics, polytetrafluoroethylene membrane commercially available under the designations ZITEX (Chemplast Inc.), polypropylene membrane, commercially available under the designation CELGARD (Celanese Plastic Company Inc.) and DEXIGLAS (C. H. Dexter, Div., Dexter Corp.) 
     The form of the separator typically is a sheet which is placed between the anode and cathode electrodes and in a manner preventing physical contact therebetween. Such is the case when the anode is folded in a serpentine-like structure with a plurality of cathode plates disposed intermediate the anode folds and received in a cell casing or when the electrode combination is rolled or otherwise formed into a cylindrical &#34;jellyroll&#34; configuration. 
     The electrochemical cell of the present invention further includes a nonaqueous, ionically conductive electrolyte which serves as a medium for migration of ions between the anode and the cathode during the electrochemical reactions of the cell. The electrochemical reaction at the cathode involves conversion of ions in atomic or molecular forms which migrate from the anode to the cathode. Thus, nonaqueous electrolytes suitable for the present invention are substantially inert to the anode and cathode materials, and they exhibit those physical properties necessary for ionic transport namely, low viscosity, low surface tension and wettability. 
     A suitable electrolyte has an inorganic, ionically conductive salt dissolved in a nonaqueous solvent, and more preferably, the electrolyte includes an ionizable alkali metal salt dissolved in a mixture of aprotic organic solvents comprising a low viscosity solvent and a high permittivity solvent. The inorganic, ionically conductive salt serves as the vehicle for migration of the anode ions to intercalate into the cathode active material, and has the general formula MM&#39;F 6  wherein M is an alkali metal similar to the alkali metal comprising the anode and M&#39; is an element selected from the group consisting of phosphorous, arsenic and antimony. Examples of salts yielding M&#39;F 6  are: hexafluorophosphate (PF 6 ), hexafluoroarsenate (AsF 6 ) and hexafluoroantimonate (SbF 6 ). Alternatively, the corresponding sodium or potassium salts may be used. 
     Preferably the electrolyte comprises at least one ion-forming alkali metal salt of hexafluorophosphate, hexafluoroarsenate or hexafluoroantimonate dissolved in a suitable organic solvent wherein the ion-forming alkali metal is similar to the alkali metal comprising the anode. Thus, in the case of an anode comprising lithium, the alkali metal salt comprises lithium hexafluorophosphate, lithium hexafluoroarsenate or lithium hexafluoroantimonate dissolved in a suitable solvent mixture. Other inorganic salts useful with the present invention include LiBF 4 , LiCIO 4  and LiCF 3  SO 3 , and mixtures thereof. The cathode active material is preferably the transition mixed metal oxide AgV 2  O 5 .5 (SVO). 
     Low viscosity solvents include tetrahydrofuran (THF), methyl acetate (MA), diglyme, triglyme, tetraglyme, dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME) and mixtures thereof, and high permittivity solvents include cyclic carbonates, cyclic esters and cyclic amides such as propylene carbonate (PC), ethylene carbonate (EC), acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-butyrolactone (GBL) and N-methyl-pyrrolidinone (NMP) and mixtures thereof. In the present invention, the anode is lithium metal and the preferred electrolyte is 1.0M to 1.4M LiAsF 6  dissolved in an aprotic solvent mixture comprising a 50/50 mixture (by volume) of propylene carbonate (PC) and dimethoxyethane (DME). 
     The assembly of the cell described herein is preferably in the form of a plurality of cathode plates, each pressed onto an expanded metal current collector and encapsulated in a separator envelope, and an anode in the form of a strip encapsulated in a separator envelope and wrapped accordion style around the cathode plates. This electrochemical system is inserted into a prismatic cell housing that may comprise materials such as stainless steel, mild steel, nickel-plated mild steel, titanium or aluminum, but not limited thereto, so long as the metallic material is compatible for use with components of the cell. 
     The cell header comprises a metallic lid with a first hole to accommodate a glass-to-metal seal/terminal pin feedthrough and a second hole for electrolyte filling. The glass used is of a corrosion resistant type having from between about 0% to about 50% by weight silicon such as CABAL 12, TA 23, FUSITE 425 or FUSITE 435. The positive terminal pin feedthrough preferably comprises titanium although molybdenum, aluminum, nickel alloy, or stainless steel can also be used. The cell header comprises elements having compatibility with the other components of the electrochemical cell and is resistant to corrosion. The cathode lead is welded to the positive terminal pin in the glass-to-metal seal and the header is welded to the case containing the electrode stack. The cell is thereafter filled with the electrolyte solution described hereinabove and hermetically sealed such as by close-welding a stainless steel ball over the fill hole, but not limited thereto. This above assembly describes a case-negative cell which is the preferred construction of the exemplary cell of the present invention. As is well known to those skilled in the art, the exemplary electrochemical system of the present invention can also be constructed in a case-positive configuration. 
     In accordance with the present invention, after assembly of the cell is completed, the cell is heated at an elevated temperature for a period of time so that the cell is dischargeable under current pulse applications without exhibiting voltage delay. According to a preferred mode of the present invention, which will be described in detail presently, the cell is heated at a temperature of at least about 75° C. for at least 14 days. 
    
    
     The following examples describe the manner and process of an electrochemical cell according to the present invention, and set forth the best mode contemplated by the inventors of carrying out the invention, but are not construed as limiting. 
     EXAMPLE I 
     Tests cells were constructed having a lithium anode and an electronically conductive cathode comprising silver vanadium oxide, along with an organic electrolyte. The cell design utilized a cathode in the form of a sheet consisting of a mixture of 94% of SVO cathode active material, by weight, along with 3% of a fluoro-resin powder such as polytetrafluoroethylene, 2% graphite, and 1% black carbon. The mixture was pressed onto an expanded metal current collector, and encapsulated in a polypropylene film laminate separator. The anode of each cell was a strip of metallic lithium, also encapsulated in a polypropylene film laminate separator. The anode was wrapped accordion style around the cathode plates. Both electrodes were inserted into a prismatic housing that was filled with a 1 molar solution of lithium hexafluoroarsenate (LiAsF 6  ) dissolved in an equal volume mixture of propylene carbonate (PC) and 1,2-dimethoxyethane (DME) as the electrolyte. The test cells were hermetically sealed. 
     To study the effects of various high temperature exposures on the performance of lithium/silver vanadium oxide (Li/SVO) cells, a test matrix of 132 cells, constructed as described above and consisting of eleven test groups with twelve cells per group, was built. Elevated temperature conditioning consisted of subjecting the various cells to temperatures of either 50° C., 65° C. or 80° C. (±5° C.) for either 7, 14 or 21 continuous days. An additional test was performed in which some cells were exposed for a total of 14 days to four cycles of 3.5 days at 65° C. and 3.5 days at room temperature. One group was designated a control group and did not undergo heat conditioning. 
     All cells were subjected to dimensional analysis and helium leak tests both prior to and immediately following their respective elevated temperature exposure. Prior to discharge, all cells were subjected to a standard burn-in, which consisted of a 2.49 KΩ load for 17 hours, followed by an open circuit &#34;rest&#34; period and a single pulse train of four, 2.0 amp pulses; 10 seconds on and 15 seconds open circuit per pulse at one week after elevated temperature conditioning. The discharge regimes used were either Accelerated Pulse Test (APT), 1 year ADD, 3 year ADD or 5 year ADD. Each group of 12 cells was divided into groups of three cells evenly distributed among the four discharge regimes. The test matrix of 132 cells is shown in Table 1. 
     
                       TABLE 1______________________________________Group A:    7 days exposure at 50° C.Group B:    7 days exposure at 65° C.Group C:    7 days exposure at 80° C.Group D:   14 days exposure at 50° C.Group E:   14 days exposure at 65° C.Group F:   14 days exposure at 80° C.Group G:   21 days exposure at 50° C.Group H:   21 days exposure at 65° C.Group I:   21 days exposure at 80° C.Group J:   4 cycles of 3.5 days at 65° C., 3.5 days      at room temperatureGroup K:   Controls cells (No elevated temperature      exposure)______________________________________ 
    
     All cells passed helium leak tests both before and after thermal conditioning, and no significant dimensional changes were noted for any cell following thermal exposure. The Accelerated Pulse Test consisted of a pulse train of four, 2.0 amp pulses; 10 seconds on and 15 seconds open circuit per pulse every 30 minutes (no background load). The APT cell capacities are summarized in Table 2. Regression analysis was used to determine if either temperature or exposure time had a significant effect on cell capacity. Results of the regression analysis to 2.0, 1.7 and 1.5 volts indicated that temperature did have a statistically significant effect on capacity. On the other hand, the regression analysis results also indicated that exposure time did not have a significant effect on capacity. 
     
                                           TABLE 2__________________________________________________________________________ACCELERATED PULSE TEST (APT) DISCHARGE RESULTSSERIAL TEST EXPOSURE              TEST TEMP                     CAPACITY (mahrs)NUMBER GROUP      TIME (DAYS)              (°C.)                     @ 2.0V                          @ 1.7V                               @ 1.5V__________________________________________________________________________035172 A     7      50     1521 1749 1840028876 A     7      50     1536 1758 1847024789 A     7      50     1413 1649 1744024438 B     7      65     1488 1694 1774024439 B     7      65     1469 1683 1772029222 B     7      65     1533 1742 1828029294 C     7      80     1513 1708 1787029790 C     7      80     1536 1737 1822028879 C     7      80     1528 1729 1824035181 D    14      50     1540 1746 1832028930 D    14      50     1516 1734 1823034466 D    14      50     1513 1736 1813024431 E    14      65     1502 1687 1768036433 E    14      65     1488 1659 1728024861 E    14      65     1498 1675 1739035226 F    14      80     1491 1702 1789025029 F    14      80     1494 1687 1764035161 F    14      80     1507 1700 1788024405 G    21      50     1462 1674 1757030781 G    21      50     1481 1709 1796024272 G    21      50     1484 1697 1785024270 H    21      65     1498 1694 1778035099 H    21      65     1550 1760 1837035174 H    21      65     1565 1767 1849024265 I    21      80     1474 1669 1760024280 I    21      80     1468 1651 1737035197 I    21      80     1491 1719 1809028864 J    3.5 on/3.5 off              65     1525 1740 1832024303 J    3.5 on/3.5 off              65     1499 1702 1795035223 J    3.5 on/3.5 off              65     1500 1730 1815035247 K    N/A     room temp                     1643 1838 1892028886 K    N/A     room temp                     1552 1761 1845028925 K    N/A     room temp                     1538 1732 1812__________________________________________________________________________ 
    
     Continuous Versus Cyclic Exposure: 
     Cell groups E and J were both exposed to 65° C. for a total of 14 days. However, group E was exposed for 14 continuous days while group J was exposed in 4 cycles of 3.5 days each (with a 3.5 day &#34;rest&#34; between cycles). A Student&#39;s t-test was performed on the capacity data for groups E and J to determine if continuous versus cyclic thermal exposure resulted in significantly different cell capacities. Results of these t-tests on APT capacities to 2.0, 1.7 and 1.5 volts indicated that the average capacities of the continuously exposed cells was typically 1% to 4% lower than that of the cyclically exposed cells. While this difference is deemed statistically significant, the resulting difference in capacity is not considered meaningful. Thus, no meaningful effects on APT cell capacity were seen over the range of storage temperatures and times tested and no meaningful difference was seen in the capacities of cyclically exposed versus continuously exposed cells. 
     Destructive Analysis: 
     All thirty-three cells which completed APT discharge were destructively analyzed. The range of exposure temperatures and durations tested resulted in no internal cell anomalies. No adverse effects were seen on the cell internal components over the range of storage temperatures and times tested. 
     EXAMPLE II 
     Analysis of 1 Year ADD Cell Performance: 
     One year ADD consisted of a 17.4 KΩ background load and a pulse train of four, 2.0 amp pulses; 10 seconds on and 15 seconds open circuit per pulse, six times a year. An analysis of variance (anova) was performed on the cell discharge data at the 6, 8, 10 and 12 month pulse trains. The results are shown in Tables 3, 4, 5 and 6, respectively. The particular pulse train voltages shown are the last voltage recorded during the first pulse of the train (P1 Last), the minimum voltage recorded during the first pulse of the pulse train (P1 Min.), the minimum voltage recorded during the fourth pulse of the pulse train (P4 Min.) and the voltage delay. Voltage delay occurs when the pulse minimum voltage is less than the last pulse voltage. An anova was also performed on the end-of-life capacity at P4 Min. =1500 my. The results are shown in Table 7 including the maximum beginning-of-life thickness of the cell case (max. BOL thickness) and maximum end-of-life thickness of the cell case (max. EOL thickness). The average of the selected data is also shown in the various tables. A 95% confidence level was chosen for all statistical analyses. 
     Six Month Pulse Train: 
     A summary of the six month discharge data for those cells subjected to the 1 year ADD is found in Table 3. No statistically significant difference was noted between the performance of the thermally exposed cells (groups A to J) and the control cells (group K) for the P1 last, P1 min., P4 min. and voltage delay readings. 
     Eight Month Pulse Train 
     A summary of the eight month discharge data for those cells subjected to the 1 year ADD is found in Table 4. No statistically significant difference was noted between the performance of the thermally exposured cells (groups A to J) and the control cells (group K) for the P1 Last, P1 Min., P4 Min. and voltage delay readings. 
     Ten Month Pulse Train: 
     A summary of the ten month discharge data for those cells subjected to the 1 year ADD is found in Table 5. 
     P1 Last.: Cell groups C (80° C., 7 days), F (80° C., 14 days), H(65° C., 21 days), I (80° C., 21 days) and J (65° C., cycled) indicated statistically significant differences in their P1 Last readings when compared to the control group. The average P1 Last voltage for groups C, F, H, I and J ranged from 10 to 11 percent (186 to 209 mv) higher than the average P1 Last voltage of the control group. In general, the P1 Last voltage tended to increase with increased exposure temperature. Exposure time had no significant effect on P1 Last voltage. 
     P1 Min.: Cell groups C (80° C., 7 days), F (80° C., 14 days), H (65° C., 21 days), I (80° C, 21 days) and J (65° C., cycled) showed statistically significant differences in their P1 Min. readings when compared to the control group. The average P1 Min. voltage for groups C, F, H, I and J ranged from 22 to 25 percent (380 to 419 mv) higher than the average P1 Min. voltage of the control group. In general, the P1 Min. voltage tended to increase with increasing exposure temperature. Exposure time had no significant effect on P1 Min. voltage. 
     P4 Min.: Cell groups C (80° C., 7 days), F (80° C., 14 days), H (65° C., 21 days) and J (65° C., cycled) showed statistically significant differences in their P4 Min. readings compared to the control group. The average P4 Min. voltage for groups C, F, H and J ranged from 7 to 9 percent (141 to 173 mv) higher than the average P4 Min. voltage of the control group. In general, the P4 Min. voltage tended to increase with increasing exposure temperature. Exposure time had no significant effect on P4 Min. voltage. 
     Voltage Delay: Cell groups C (80° C., 7 days), F (80° C., 14 days), H (65° C., 21 days), I (80° C., 21 days) and J (65° C., cycled) showed statistically significant differences in their voltage delay readings when compared to the control cells (Group K). As previously mentioned, an alkali metal/mixed metal oxide cell such as a lithium/silver vanadium oxide cell that has been depleted of approximately 40% to 70% of its capacity typically exhibits voltage delay when subjected to current pulse discharge conditions. Thus, the lack of significant voltage delay in the thermally conditioned cells, i.e., cell groups C, F, H, I and J, is an unexpected result of the thermal conditioning of the present invention. 
     Twelve Month Pulse Train: 
     A summary of the twelve month discharge data for those cells subjected to 1 year ADD is found in Table 6. 
     P1 Last: No statistically significant difference was noted between the performance of the thermally exposured cells (groups A to J) and the control cells (group K). 
     P1 Min.: Only cell group I (80° C., 21 days) showed statistically significant difference when compared to the control group. Group I exhibited a mean P1 Min voltage which was 302 mv (18%) higher than that of the control group. 
     P4 Min.: No statistically significant difference was noted between the performance of the thermal exposure cells (groups A to J) and the control cells (group K). Voltage Delay: No significant voltage delay was evident for cell group F (80° C., 14 days) and I (80° C., 21 days). However, voltage delay for groups C (80° C., 7 days), H (65° C., 21 days) and J (65° C., cycled) was evident at the 12 month pulse train. Again, the absence of voltage delay in cell group F and I is an unobvious result of the thermal conditioning of the present invention. 
     No statistically significant difference was noted between the discharge performance of the thermally exposured cells (groups A to J) and the control cells (group K). 
     End-of-Life Capacity: 
     A summary of the end-of-life capacity data is found in Table 9 with no statistically significant difference between the end-of-life capacity of the thermally exposured cells (groups A to J) and the control cells (group K) at a 95% confidence level. 
     Continuous Versus Cyclic Exposure: 
     Cell groups E and J were both exposed to 65° C. for a total of 14 days. However, group E was exposed for 14 continuous days while group J was exposed in 4 cycles of 3.5 days each (with a 3.5 day &#34;rest&#34; between cycles). No statistically significant difference was noted between the discharge performance of groups E and J at a 95% confidence level. 
     Dimensional Characteristics: 
     Maximum thickness measurements for each cell are shown in Table 7. No statistically significant dimensional changes were noted for any cell following thermal exposure. Following 1 year ADD discharge, the total swelling (max. EOL thickness--max. BOL thickness) ranged from 0,013 to 0.056 inches, with an average of 0.037 inches. No trend based on exposure temperature or exposure time was evident. 
     
                                           TABLE 3__________________________________________________________________________1 YEAR ADD DISCHARGE RESULTS (6 MONTH PULSE TRAIN)SERIAL TEST TEMP          TIME P1 LAST                      P1 MIN.                             P4 MIN.                                    VOLTAGE DELAYNUMBER GROUP      (°C.)          (DAYS)               (mv)                  (avg.)                      (mv)                         (avg.)                             (mv)                                (avg.)                                    (mv) (avg.)__________________________________________________________________________020685 A    50   7   2258                  2275                      2244                         2251                             2239                                2258                                    14   24035173     50   7   2272   2214   2258   58028901     50   7   2296   2296   2278    0035186 B    65   7   2282                  2275                      2230                         2204                             2264                                2260                                    52   72024592     65   7   2263   2173   2247   90035170     65   7   2281   2208   2268   73024892 C    80   7   2259                  2269                      2110                         2149                             2245                                2253                                    149  120035220     80   7   2278   2163   2261   115023491     80   7   2271   2175   2252   96023532 D    50  14   2266                  2265                      2236                         2216                             2246                                2247                                    30   49020675     50  14   2269   2250   2253   19024876     50  14   2259   2162   2243   97035177 E    65  14   2281                  2280                      2205                         2209                             2269                                2264                                    76   71035199     65  14   2288   2225   2271   63020668     65  14   2272   2197   2252   75023599 F    80  14   2293                  2286                      2293                         2286                             2265                                2266                                     0    0035166     80  14   2280   2280   2262    0031135     80  14   2286   2285   2272    1035182 G    50  21   2279                  2280                      2208                         2220                             2268                                2268                                    71   61035198     50  21   2275   2230   2263   45035178     50  21   2287   2221   2272   66029351 H    65  21   2275                  2299                      2187                         2239                             2263                                2279                                    88   60030779     65  21   2283   2191   2272   92035356     65  21   2338   2338   2301    0028867 I    80  21   2284                  2265                      2284                         2265                             2250                                2238                                     0    0020692     80  21   2254   2254   2232    0024830     80  21   2257   2257   2232    0024264 J    65  cycled               2292                  2302                      2292                         2302                             2273                                2279                                     0    0024335     65  cycled               2313   2313   2285    0024308     65  cycled               2301   2300   2279    1035175 K    control          control               2274                  2275                      2209                         2238                             2259                                2261                                    65   37035206     control          control               2283   2245   2268   38024378     control          control               2268   2260   2255    8__________________________________________________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________1 YEAR ADD DISCHARGE RESULTS (8 MONTH PULSE TRAIN)SERIAL TEST TEMP          TIME P1 LAST                      P1 MIN.                             P4 MIN.                                    VOLTAGE DELAYNUMBER GROUP      (°C.)          (DAYS)               (mv)                  (avg.)                      (mv)                         (avg.)                             (mv)                                (avg.)                                    (mv) (avg.)__________________________________________________________________________020685 A    50   7   2155                  2150                      2114                         2090                             2169                                2179                                    41   61035173     50   7   2118   2075   2171   43028901     50   7   2178   2080   2197   98035186 B    65   7   2153                  2146                      2147                         2140                             2184                                2180                                     6    6024592     65   7   2139   2138   2170    1035170     65   7   2146   2136   2186   10024892 C    80   7   2144                  2162                      2130                         2131                             2172                                2182                                    14   30035220     80   7   2164   2147   2189   17023491     80   7   2177   2117   2186   60023532 D    50  14   2153                  2147                      2118                         2101                             2172                                2171                                    35   46020675     50  14   2170   2125   2183   45024876     50  14   2117   2059   2158   58035177 E    65  14   2133                  2160                      2116                         2135                             2162                                2181                                    17   25035199     65  14   2166   2165   2193    1020668     65  14   2180   2123   2187   57023599 F    80  14   2206                  2196                      2125                         2129                             2200                                2197                                    81   67035166     80  14   2188   2135   2192   53031135     80  14   2194   2126   2199   68035182 G    50  21   2140                  2140                      2110                         2115                             2187                                2185                                    30   26035198     50  21   2135   2111   2179   24035178     50  21   2146   2123   2190   23029351 H    65  21   2167                  2186                      2128                         2119                             2192                                2204                                    39   67030779     65  21   2178   2116   2201   62035356     65  21   2214   2114   2220   100028867 I    80  21   2181                  2174                      2129                         2125                             2177                                2169                                    52   49020692     80  21   2169   2121   2164   48024830     80  21   2171   2124   2166   47024264 J    65  cycled               2196                  2201                      2133                         2129                             2209                                2213                                    63   72024335     65  cycled               2211   2112   2219   99024308     65  cycled               2197   2142   2212   55035175 K    control          control               2104                  2127                      2012                         2061                             2160                                2172                                    92   66035206     control          control               2124   2059   2174   65024378     control          control               2153   2113   2181   40__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________1 YEAR ADD DISCHARGE RESULTS (10 MONTH PULSE TRAIN)SERIAL TEST TEMP          TIME P1 LAST                      P1 MIN.                             P4 MIN.                                    VOLTAGE DELAYNUMBER GROUP      (°C.)          (DAYS)               (mv)                  (avg.)                      (mv)                         (avg.)                             (mv)                                (avg.)                                    (mv) (avg.)__________________________________________________________________________020685 A    50   7   2009                  1968                      1817                         1750                             2013                                2000                                    192  218035173     50   7   1892   1676   1949   216028901     50   7   2003   1758   2037   245035186 B    65   7   2017                  2025                      1829                         1844                             2046                                2052                                    188  182024592     65   7   2061   1932   2075   129035170     65   7   1998   1770   2036   228024892 C    80   7   2100                   2111*                      2099                          2105*                             2096                                 2109*                                     1     6*035220     80   7   2104   2087   2111    17023491     80   7   2129   2129   2119    0023532 D    50  14   2021                  2002                      1837                         1798                             2028                                2018                                    184  204020675     50  14   2043   1891   2051   152024876     50  14   1942   1666   1974   276035177 E    65  14   1946                  2041                      1722                         1930                             1967                                2053                                    224  111035199     65  14   2063   1967   2085    96020668     65  14   2114   2102   2108    12023599 F    80  14   2136                   2121*                      2136                          2121*                             2131                                 2121*                                     0     0*035166     80  14   2112   2112   2115    0031135     80  14   2115   2115   2118    0035182 G    50  21   1947                  1948                      1704                         1701                             1999                                1997                                    243  247035198     50  21   1940   1687   1986   253035178     50  21   1957   1711   2005   246029351 H    65  21   2104                   2107*                      2087                          2087*                             2114                                 2120*                                     17   20*030779     65  21   2090   2048   2106    42035356     65  21   2127   2127   2140    0028867 I    80  21   2108                   2111*                      2108                          2111*                             2098                                2097                                     0     0*020692     80  21   2112   2112   2097    0024830     80  21   2112   2112   2095    0024264 J    65  cycled               2122                   2130*                      2112                          2126*                             2135                                 2141*                                     10    4*024335     65  cycled               2131   2130   2145    1024308     65  cycled               2137   2135   2142    2035175 K    control          control               1863                  1921                      1664                         1707                             1929                                1968                                    199  214035206     control          control               1890   1700   1944   190024378     control          control               2009   1757   2032   252__________________________________________________________________________ *Indicates statistically significant difference (compared to control grou @ 95% confidence level) 
    
     
                                           TABLE 6__________________________________________________________________________1 YEAR ADD DISCHARGE RESULTS (12 MONTH PULSE TRAIN)SERIAL TEST TEMP          TIME P1 LAST                      P1 MIN.                             P4 MIN.                                    VOLTAGE DELAYNUMBER GROUP      (°C.)          (DAYS)               (mv)                  (avg.)                      (mv)                         (avg.)                             (mv)                                (avg.)                                    (mv) (avg.)__________________________________________________________________________020685 A    50   7   1841                  1851                      1726                         1719                             1822                                1844                                    115  132035173     50   7   1828   1698   1826   130028901     50   7   1885   1734   1883   151035186 B    65   7   1850                  1858                      1628                         1642                             1863                                1870                                    222  216024592     65   7   1884   1668   1889   216035170     65   7   1841   1631   1858   210024892 C    80   7   1874                  1921                      1627                         1735                             1855                                1904                                    247  186035220     80   7   1898   1658   1899   240023491     80   7   1992   1921   1958    71023532 D    50  14   1884                  1860                      1750                         1718                             1874                                1847                                    134  142020675     50  14   1868   1728   1859   140024876     50  14   1827   1676   1808   151035177 E    65  14   1754                  1851                      1621                         1686                             1719                                1833                                    133  165035199     65  14   1862   1672   1878   190020668     65  14   1937   1764   1902   173023599 F    80  14   2051                  1999                      2051                         1962                             2024                                1978                                     0    37035166     80  14   1963   1893   1951    70031135     80  14   1982   1941   1960    41035182 G    50  21   1842                  1844                      1658                         1672                             1859                                1857                                    184  171035198     50  21   1841   1670   1851   171035178     50  21   1848   1689   1862   159029351 H    65  21   1929                  1933                      1720                         1735                             1919                                1928                                    209  198030779     65  21   1903   1657   1898   246035356     65  21   1966   1828   1968   138028867 I    80  21   2024                  2019                      2024                          2019*                             1977                                1975                                     0    0020692     80  21   2016   2016   1972    0024830     80  21   2018   2018   1975    0024264 J    65  cycled               1984                  1991                      1762                         1772                             1983                                1988                                    222  219024335     65  cycled               1994   1779   1996   215024308     65  cycled               1994   1774   1986   220035175 K    control          control               1818                  1858                      1695                         1717                             1826                                1863                                    123  141035206     control          control               1845   1704   1853   141024378     control          control               1912   1752   1909   160__________________________________________________________________________ *Indicates statistically significant difference (compared to control grou @ 95% confidence level) 
    
     
                                           TABLE 7__________________________________________________________________________1 YEAR ADD END-OF-LIFE CAPACITY &amp; THICKNESSSERIAL TEST TEMP          TIME CAPACITY @ p4MIN = 1500 mv                                MAX. BOL THICKNESS                                             MAX. EOL THICKNESSNUMBER GROUP      (°C.)          (DAYS)               (mAhr) (group avg.)                                (Inches)                                      (group avg.)                                             (Inches)                                                   (group__________________________________________________________________________                                                   avg.)020685 A    50   7   1732   1765      0.348 0.350  0.385 0.384035173     50   7   1765             0.346        0.388028901     50   7   1797             0.357        0.380035186 B    65   7   1845   1814      0.349 0.349  0.388 0.390024592     65   7   1779             0.347        0.391035170     65   7   1818             0.351        0.392024892 C    80   7   1734   1771      0.349 0.349  0.390 0.386035220     80   7   1814             0.348        0.388023491     80   7   1765             0.349        0.381023532 D    50  14   1767   1752      0.350 0.349  0.389 0.386020675     50  14   1756             0.349        0.376024876     50  14   1733             0.348        0.394035177 E    65  14   1683   1750      0.351 0.350  0.407 0.392035199     65  14   1817             0.350        0.389020668     65  14   1750             0.348        0.380023599 F    80  14   1785   1793      0.349 0.349  0.378 0.379035166     80  14   1833             0.347        0.380031135     80  14   1761             0.350        0.378035182 G    50  21   1828   1810      0.346 0.345  0.396 0.390035198     50  21   1787             0.345        0.385035178     50  21   1816             0.345        0.388029351 H    65  21   1782   1811      0.354 0.353  0.399 0.388030779     65  21   1788             0.349        0.395035356     65  21   1862             0.357        0.370028867 I    80  21   1759   1735      0.350 0.348  0.378 0.375020692     80  21   1716             0.348        0.376024830     80  21   1731             0.346        0.370024264 J    65  cycled               1834   1825      0.347 0.346  N/A   N/A024335     65  cycled               1823             0.346        N/A024308     65  cycled               1818             0.346        N/A035175 K    control          control               1764   1798      0.347 0.347  0.392 0.385035206     control          control               1809             0.347        0.387024378     control          control               1821             0.347        0.377__________________________________________________________________________ 
    
     Summary: 
     No statistically significant differences were noted between the discharge performance of the elevated temperature cells and the control cells at either the 6 or 8 month pulse trains. 
     At the 10 month pulse train, cell groups C (80° C., 7 day), F (80° C., 14 day), H (65° C., 21 day), I (80° C., 21 day) and J (65° C., cycled) showed statistically significant differences in terms of their pulse voltage when compared to the control cells (group K). All of these thermally exposed cell groups exhibited higher voltages than the control group. In general, the cell performance (at 10 months) tended to improve with increasing exposure temperature. Exposure time had no significant effect on cell performance. Thus, no meaningful differences were noted between the performance of the elevated temperature cells and the control cells at the 12 month pulse train. 
     End-of-Life Capacity: 
     There was no statistically significant difference between the end-of-life capacity of the thermally exposed cell (Groups A-J) and the control cells (Group K) at a 95% confidence level. 
     Dimensional Characteristics: 
     Total call swelling (max. EOL thickness--max. BOL thickness) ranged from 0.013 to 0.056 inches, with an average of 0.037 inches. No trend based on exposure temperature or exposure time was evident. 
     Voltage Delay: 
     The control cells, (Group K), 50° C. exposure cells (groups A, D and G), 65° C. exposure cells (groups B, E and H) and the 80° C., 7 days exposure cells (group C) displayed reasonably typical voltage delay (&gt;100 mv) at either the ten or twelve month pulse train (or in some cases at both the ten and twelve month pulse trains.) Group F (80° C., 14 days exposure) cells, while exhibiting larger than expected voltage delay at the first pulse train (269 to 394 mv), exhibited no statistically significant voltage delay throughout their remaining life. Group I (80° C., 21 day exposure) cells exhibited no statistically significant voltage delay at any time during discharge. 
     At this time, the three year ADD and five year ADD tests have not been completed. 
     It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the appended claims.