Ronald A. Guidotti and Frederick W. Reinhardt. Battery Development Division ..... A. R. Baldwin was the lead engineer responsible for the LL TB. This ll'ur!:. l\'i1S.
LITHIATION OF FeS2 FOR USE IN THERMAL BATTERIES
Ronald A. Guidotti and Frederick W. Reinhardt Battery Development Division, Sandia National Laboratories Albuquerque, NM 87185-5800 ABSTRACT
The inability of the standard Sandia catholyte to meet the discharge requirements of a 60-min, Li(Si)/FeS::: long-life thermal battery (LL TB) necessitated the development of a new catholyte based on lithiated FeS;. Lithiation of FcS:..... bv. Li:S, - Li allovs, and Li...,O ...... \vas examined under various processing conditions and the resulting catholytes were tested in a half-size, 5-ccll battery. The best overall results were achieved with Li:;O-lithiated catholytes. The optimum compositions were successfully tested in the full-size LL TB. The excessive, initial voltage transient normally observed was totally climina ted and ..-.., the pulse performance ncar the end of life was dramatically improved.
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INTRODCCTIO~
Thermal b:lttcrics based on the Li(Si) FeS:. electrochemical system ha'e been successfully dc,cloped at Sandia National Laboratories in a variety of sizes. for ,·arious current densities and periods of operation for weapons applications. The requirements posed by the long-life thermal battery (LLTBJ. howner. pro\ed to be a challenge. It was not possible to reliably meet the requirements of a 60-min activated life using the standard catholytc a'ailablc at the time. As a consequence, it was necessary to develop altcrnati>c catholytcs for this application. Catholytcs based on lithiated FeS::: were shown to ha,·c grca t promise. This report describes the results of an intensive screening study of lithiatcd catholytcs for the LL TB, using a 5-cell battery as the test \Chicle. The bulk of the effort was directed to Li:,S. Li-Al and Li-Si alloys, and Li:,O as lithiation agents. The effect of heJ.ting conditions and extent of lithiation (Li FeS:. mole ratio) upon bJ.ttery performJncc were evaluated. The Li content and particle size of the Li alloys were also examined to a limited extent. The best lithiatcd mixes were then tested in the full-size (10-ccll) LL TB. Historical Background
LL TB Test Profile - The load profile used for testing of the 10-ccll LL TB is shown in Figure I. Various resistive loads arc superimposed upon a constant-current background during dischJrge. The initial background current or 50 rnA (-2.5 mA/em2) is stepped to 275 rnA (-13.5 mA;'cm2) after 260 s. The battery is then subjected to a heavy· pulse (5.7 ohms) at 300 sand moderate pu'lses (10 ohms) at 550 and 3580 s. The latter pulse, which occurs 20 s before the end of life. is critical. Once the electrolyte begins to freeze, the resulting rapid increase in internal resistance can result in failure of the battery to meet the pulse requirements.
Standard Catholyte - The normal FeS.., catholyte consists of 64%* FeS, (v.:ith a particle size of -230+325 mesh) and 36% electrol~'te-binder (EB) mix (88% LiCI-KCI eutectic/1.2% fumed SiO:::). The performance of this catholytc in the LL TB in static tests is shown in Figure 2 for the ambient-temperature extremes which the battery can expect to experience. The large voltage "spike" observed at the beginning of discharge is characteristic of FcS;--cspecially for unpurificd material which contains oxidized-iron compounds. The original peak-voltage specification for the LL TB was 20V, but it was necessary to increase this to 24V to allow for the spike. For hot activation conditions (4QOC). difficulties \Vcrc sometimes encountered with decomposition of FcS2 when the batteries overheated, so that discharge to the FeS plateau took place. This reduced the tolerance for the minimum voltage at the 3580 s pulse to unacceptable levels. Since the performance of the normal catholyte was margin::ll, it was necessary to consider alternate catholytes for this application. Lithiatcd Cathol\'tc - From a theoretical perspective. N. Godshall predicted that the initial voltage spike could be remo,ed if the acti,·ity of Li in the catholytc was fixed (made invariant) before discharge (1..2). This can be accomplished by starting in the 3-phasc. Li..,S-FcS..,-FcS field of the Li-Fc-S phase diagram. Preliminary tests in which FcS.., was lithiatcd with Li2S showed th::lt this w:1s indeed effective in climin:1ting the initi:1l voltage spike (2.3). As a result of Godsh:1ll's initial success. :1n intcnsi'e effort w:1s mounted at S:1ndia to screen a number of promising lithiation agents and processes for prcp:Hation of catholytcs suitJ.blc for usc with the LL TB.
EXPERI\IE:"'TAL \la terials The FcS.., used for the lithiatcd catholytcs was -425 mesh (step techniques were comparable, subsequent catholytc mixes lithiatcd with Li"S were prepared by the !-step method. to expedite the screening studies. High levels of lithiation (such as the Li. FeS::; mole ratio of 0.28. abo\C) reduce the cfrectivc cathode capacity. It was desirable to reduce the extent or lithiation to the lowest level possible while at the same time minimizing the \·oltagc transient. The effect of degree of lithiation upon battery performance is summarized in Figures 5a and 5b for !-step mixes. While the voltage transient was effectively removed at a Li FeS" ratio of -0.10. there was still a small. residual voltage "hump" at the beginning of the discharge trace. This \Oltage hump was essentially removed at a Li, FcS::; mole ratio of -0.15. According to work performed at Argonne 0ational Laboratory. the tic triangles in the Li-Fc-S ternary phase diagram arc such that it may not be necessary to add FeS to the FcS" and Li2S to achieve a fixed Li activity (5). This was corroborated by the battery-discharge data for lithiatcd mixes of comparable extent of lithiation as the above mixes. but without added FcS. The voltage-time traces were very similar l'or the two types or catholytcs. Not having to add FcS during lithiation further simplified processing. While Li::;S is an effective lithiation agent. it is a noxious material with which to work. As a consequence. alternate lithiation agents were explored.
Lithiation by Li Alloys Addition of anodic materials (e.g., Li alloys) to the catholyte should have the same *Registered trademark of Johns·Mansville Corp.
effect as pre-discharging the FcS:; once the catholytc is heated above the melting point of the LiCl-KCl eutectic (352°C). The final catholyte composition should be equivalent to that obtained by reaction of FcS2 with FcS and Li2S, assuming the elemental Si that remains (after complete consumption of the Li in the alloy) takes no part in the electrochemical reaction and no substantial decomposition of FcS2 occurs. The EB mix used in catholytes lithiatcd with Li alloys was the MgO-based material, since these alloys arc reactive toward the Si02 in the Si02·bascd mix. Only the !-step process was examined for lithiation by Li alloys. Since Li-Si alloy (44% Li) is used as a standard anode material, it was examined first. The effect of the degree of lithiation upon performance is shown in Figures 6a and 6b for catholytes made with -230+325 mesh Li-Si alloy. The plateau portions of the discharge traces were Ycry similar (Figure 6a). with some spread in the voltage data ncar the end of life. As expected. the acti\·ated lives were shorter for the mixes that had been more heavily lithiatcd. Varying the fusion time from 2 to 8 h had no significant effect upon catholytc performance for the flat portion of the discharge curve. The most satisfactory degree of lithiation with a minimum voltage hump w3s achieved at a Li;'FcS2 mole ratio of 0.15 (Figure 6b). When coarser Li-Si alloy of the same composition as above was used for lithiation. the performance of the corresponding cathodes w3s essenti3lly the s3me as when the finer Li-Si alloy was used. Comparable results for the same extent of lithiation were also obtained for 3 Li-Si 3lloy cont3ining 35% Li and for 3 Li-...\1 3lloy cont3ining 20% Li. Because of potenti3l thermal-man3gement problems in large-scale processing of lithiatccl catholytes using Li alloys. Li:;O was examined :1s an altern::JtiYe lithiation agent. Lithiation IJy Li20
The usc of Li-,0 for lithiation of FeS2 avoids the exothermic reactions associated with Li alloys ::tnd the noxious nature of Li2S. It must be emphasized. howc\er. that the discharge mechanism is changed when Li:;O is involved. Instead of the tcrn:Hy Li-Fc-S system. one is now de::tling with the quatern::try Li-Fc-S-0 system. As a result. the composition of the 4-componcnt c3tholyte must be within a 4-phasc region to fix the Li activity and thus avoid the initial \Olt::tge spike. According to work by Asel3gc and Hellstrom. the relevant 4-phasc region for the purposes of this study 1s the FeS2-Li3Fe:;.S4-LiFe50s-Li:;.S04 region (6). Since Li20 is potentially reactive with the high-surface-area SiO:; in the Si02-based EB. only the 1'v1g0-bascd EB was used for these catholyte mixes. Both the !-step as well as the 2-step process were examined for lithiation by Li:;O. The addition of FeS to the starting mixture was explored 3S well. The best results were obtained with the 1-step process without addition of FeS to the catholyte prior to fusion at 4000C under argon. No substantial differences in performance were observed when the fusion time w::ts varied from 0 h (i.e., unfused) to 16 h. A fusion time of 8 h was chosen as a standard, however, to be consistent with the heating conditions used for optimum lithiation with Li2S and with Li alloys. As noted previously, the fusion improved the pelletization properties of the ca tho! yte. The performance of Li:;O-lithiated catholytes is shown as a function of extent of lithiation in Figures 7a and 7b. Based on the observed performance, a Li/FeS2 mole ratio of 0.16 was chosen as being optimum for lithiation of FcS2 by Li20.
The relative performance of· the lithia ted catholytes in 5-cell batteries is shown in Figure 8 for the three lithiation agents at approximately the same degree of Iithiation of Fes 2 . The best overall performance was obtained with the Li20-Iithiated catholyte. To simulate impure FeS2, 3% Fe203 was added to the three types of catholytes prior to fusion. Each of the lithiation agents compensated for the added Fe203, by removal of the voltage spike that would have been pronounced in the absence of lithiation.
Verification Tests To verify the relative performance observed in 5-cell batteries, 20 additional 5-cell batteries (10 at 74oc and 10 at -540C) were tested for each of the three types of lithiated mixes. These batteries used Min-K sleeves rather than the ceramic-blanket wrap and, as a result, had activated lives that were almost twice those of the earlier batteries. The optimized compositions were used for catholytes lithiated with Li-Si alloy (44% Li) and Li20 (Li/FeS2=0.15 and 0.16, respectively). The Li/FcS2 mole ratio for the Li2S-lithiatcd catholyte was 0.10, rather than the optimized value of 0.15.* The results arc summarized in Table !.
Table l.
Comparison of performance data for heavily-insulated 5-cell Li(Si)/FeS2 batteries built with lithiated catholytes.
Lith. Agent
LijFeS2, m/m
Activ. Temp., oc
Peak Volt., v
Act. Life to 7.5V, min
Li2S Li -Si Li20
0.103 0.154 0.164
74 74 74
9.787 9.791 9.796
28.02 (0.52)* 25.60 (0.45) 28.09 (0.85)
Li2S Li -Si Li20
0.103 0.154 0.164
-54 -54 -54
9.507 9.529 9.527
14.94 (0.32) 14.83 (0.36) 16.18 (0.31)
* Standard deviation for 10 tests; Min-K insulation used. For the cold batteries, the peak voltages for the eatholytes lithiated with Li-Si alloy and Li..,O were comparable and slightlv higher than those for the Li..,S-Iithiated mix. The activated life w::ts greatest, howe~ver: for ~the Li:O-Iithi::ttcd catholytc.- For the hot batteries. the peak volt:1ges were comparable for the three mixes. The activ::ttcd life w:1s still gre:1test !'or the Li:O-Iithi::lted mix. ::tlthough there was more spre:1d in the d:1t:1 rel:1tive to the cold b:1tteries. \Vhen a number of the promising cJtholytcs were tested in the full-size LL TB. the result:1nt data paralleled the trends obsGrvcd in the smaller 5-cell batteries. *At the time these tests were initiated, a Li/Fes 2 mole ratio of 0.10 was the minimum acceptable for the extent of lithiation of Fes 2 by Li 2 s. Subsequent tests with a Li/FeSz mole ratio of 0.15, however, showed this to be preferred. The impact upon relative performance for the three different lithiation agents is not significant for the purposes of this study.
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A typical 'oltage-time trace for the LL TB using the Li20-Iithiated mix made \Vith -325+--125 mesh FeS2 is shown in Figure 9 . along v:ith the corresponding trace for the unlithiated c::nholyte . "hich used -230+325 mesh FeS2.* The lithiated mix performed substJntially better ::Jnd more than met Jll of the LL TB discharge requirements.
CONCLUSIONS
A 5-cell battery was successfully used to screen a large number of lithiated catholytes for replacement of the inadequate standard catholyte in the LL TB using the Li(Si)/FeS2 electrochemical system. Li::.S . Li-Si and Li-AI alloys, and Li20 were all effective in remo' ing the initi::Jl \Oltage spike which was detrimental to battery performance. The optimum Li FeS: mole ratio was 0.15 for Li2S. 0.15 for the Li-Si alloy. and 0.16 for Li2S . respectively. Li-Si and Li-Al alloys were equally effective for lithiation of FeS:. . and were not greJtly influenced by particle size or moderate changes in alloy composition. The best results were obtained with the !-step lithiation process. where the ingredients were simply blended together in a single step and fused at 400°C for 8 h under an inert gas; the fusion time was not critical for dfective Iithiation. While unfused lithiated catholytcs functioned adequately, the fusion step was still included in the processing, as it resulted in a more uniform catholyte with better pelletizing properties. The preferred choice for FeS-. lithiation w:~s Li-.0, as c:~tholytes Iithi:-tted with Li:.O performed the best o'erall. In addition. Li20 did not possess the noxious qualities of Li:S :1nd did not cause the exothermic reactions :1ssociated with lithiation of FeS2 by Li alloys. The performance of the Li:;O-lithiated c:-ttholytes in the LL TBs paralleled that for the 5-cell batteries. The LL TB was able to exceed all or the discharge requirements when the Li-.0-lilhiatcd catholyte (Li, FeS::. mole ratio=O.l6) was used.
ACKNOWLEDGi\1ENTS
The authors wish to acknowledge the assistance of P. G. Neiswander in generating the data for the LL TB. N. A. Godshall was responsible in developing the lithiation concept. without which this work would not have been possible. T. L. Aselage and E. E. Hellstrom pro,·ided helpful suggestions :~nd advice regarding the interpretation of the Li-Fe-S-0 phase diagram. A. R. Baldwin was the lead engineer responsible for the LL TB. This ll'ur!:. l\'i1S perfvrmeJ at SnnJin Xi11ivnal Lahvrntvrin. supported h1· tlze U.S. Department vf Energ.1· under contract number DE-ACO.f-76DP00789.
*Data were not available for performance in the LLTB of unlithiated catholytes made with either ·325+425 or -425 mesh Fes 2 . However, 5·cell-battery data were available for unlithiated catholytes made with ·230+325 and -325+425 mesh FeS2. They showed comparable performance under test conditions similar to that for the LLTB . It is felt that the observed differences in performance noted in Figure 9 are due to lithiation and not to particle-size effects.
REFERENCES
1. N. A. Godshall, "A New Technique for Improving Voltage Regulation 1n Li/FcS2 Thermal Batteries," J2nd Power Sources Symposium. Cherry Hill, NJ, June 9-12, 1986. 2. N. A. Godshall, "Multi-phase Cathode Composition for Voltage Spike Elimination and Extended Discharge Life of Li/FeS2 Thermal Batteries," Serial No. 872,718, June 10, 1986. 3. R. A. Guidotti, "Improved Methods for Achieving the Equilibrium Number of Phases in Mixtures Suitable for Usc in Battery Electrodes, E.G., for Lithiating FeS2," Serial No. 872,728, June 10, 1986. 4. R. J. Antepenko, R. L. Poole, and W. W. Welbon, "Size Reduction and Purification of Iron Disulfide," GEPP-TM-499 (St. Petersburg, FL: General General Electric Co., Neutron Devices Dept., April 1980). 5. Z. Tomczuk, B. Tani, N. C. Otto, M. F. Roche, and D. R. Vissers. "Phase Relationships in Positive Electrodes of High Temperature Li-AI/LiCI-KCl/FeS:: Cells," J. Electrochem. Soc .. 129, 925 (1982). 6. T. L. Asclage and E. E. Hellstrom, ":'-.fulticomponent Ph:1se Di:1gr:1ms for B:1ttery Applications: II. Applic:1tion to Oxygen Impurities in the Li(Si)/FeS2 Battery Cathode," to be published in J. Electruchem. Soc.
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