J. Phys. Chem. 1991, 95, 770-774
770
sistent with the previously reported results on the pyridyl ketones.l7-19,25 In 2-4, the TI character is mainly localized on the pyridine ring. In the present magnetophotoselection experiments, the excimer laser (308 nm) generates the Sl(nr*) states of 2-4, in which the direction of the transition moments is perpendicular to the pyridine ring plane (5ax$). Thus, increased signals in the excitation with the ligh: of E 11 B correspond to those from the molecules aligned as 411 Enlargement of the innermost pair observed in the case of E 11 B indicates that Tx can be determined to be the middle sublevel in 2-4. Recently, Schmidt et ale9concluded that pyridine is nonplanar in the TI state owing to the pseudo-Jahn-Teller coupling between 3Bl(nr*)and the close-lying 'A1(7rr*) states. The order of the triplet sublevels was determined as Tz ( Z 11 2-fold symmetry axis), Tx (out-of-plane), and TYfrom the top in energy. Hirota et al.35have obtained the same order of triplet sublevels for 2 as that of pyridine as a result of the time-resolved EPR study using a single crystal. Therefore, an interpretation for the TI state of 2 is depicted as shown in Table I. The order of the triplet sublevels in 2 is different from those of 1 (carbonyl 3n7r* state) and of 4phenylpyridine (pure 'r7r*state).27 The unusual order of the triplet sublevels might be brought by the vibronic mixing between the 3Bl(nr*) and 3A,(rr*) states. In the pure 3nr* state, the out-of-plane sublevel (T,) should be located at the top by analogy with p y r a ~ i n e . ' ~ ,Although ~~ 3 and 4 have a low symmetrical structure, the TIstates may be similarly described because of the
e
(35) Akiyama, S.;Yamauchi, S.;Hirota, N. Symp. Mol. Sfrucf. Jpn. 1988.
(36) Burland, D. M.; Schmidt, J. Mol. Phys. 1971, 22, 19. (37) Nishi. N.; Kinoshita, M.; Nakashima, T.; Shimada, R.; Kanda, Y. Mol. Phys. 1977, 33, 31.
localized character on the pyridine ring. The zfs parameters and spin polarization of the T1states of the present pyridine derivatives are summarized in Table I. Separation between the highest and the lowest sublevels is larger in 2 and 3 than that of pyridine (0.159 ~ m - l ) .In ~ the T1state of 4, the zfs is similar to that of pyridine. The energy separation would directly reflect the change of 3nu* contribution, since the separation depends on the mixing ratio between 3n7r* and '7rr*states in a similar manner to pyrazine derivative.'* Observation of enlargement of the zfs for 4,2, and 3 in this order suggests that n r * character in the T1 states increased with an increase in the electron-withdrawing ability of the substituent. It was also obtained that 7p increases in the same order. Introduction of the electron-withdrawing group at the 4-position raises rr* energy level leading to the enlargement of the n r * contribution in the TI states of the present pyridines. Decreasing of rp with a decrease of nr* character in the TI states might be due to enhancement of the vibronic interaction between 'n7r* and '7rr*states accompanying an increase of the TI So radiationless transition yield. Therefore, it can be proposed that out-of-plane deformation induced by the vibronic coupling may be smaller in the TI states of 2 4 than in the TI state of unsubstituted pyridine.
-
Acknowledgment. We are grateful to Professors J. Higuchi and M. Yagi for the help in the spectral simulation of the triplet TREPR spectra. We also thank Professor S.Yamauchi for helpful discussion. The present work was partially supported by Grants-in-Aid for Developmental Scientific Research No. 63840012 and for Scientific Research No. 01540356 from the Japanese Ministry of Education, Science and Culture. (38) Yamauchi, S.;Miba, K.; Komada, Y.; Hirota, N. J . Phys. Chcm. 1987, 91, 6173.
Kinetics and Mechanism of the Oxidation of Thlocyanate by Iodate' Reuben H. Simoyi, Morningstar Manyonda, Jonathan Masere, Michael Mtambo, Ignatius Ncube, Hemant Patel, Department of Chemistry, University of Zimbabwe, Box MPl67, Mount Pleasant, Zimbabwe
Irving R. Epstein, and Kenneth Kustin* Department of Chemistry, Brandeis University, Box 91 10, Waltham, Massachusetts 02254 (Received: June 5, 1990; In Final Form: August 2, 1990)
-
A mechanistic study has been performed on the complex reaction between thiocyanate and iodate in acidic medium. A 14-step
mechanism is proposed in which the first step of the oxidation is IO3- + SCN- + 2H+ HOSCN + HI02 The HOSCN is then rapidly oxidized by either HIOz or Iz to give SO?-via HO'SCN. The transient formation of I2 in excess thiocyanate is attributed to a competition between the Dushman reaction, IO3- + 51- + 6H+ 312+ 3H20, which is strongly catalyzed by acid, and slower, acid-inhibited reactions, which consume iodine. The consumption of iodine by thiocyanate proceeds via the rapid formation of the 12SCN-complex, which later hydrolyzes to give HOSCN. Other observed reaction dynamics have been explained through the same proposed mechanism. Computer simulation using this mechanism gives good agreement with experiment.
-
(1) Systematic Design of Chemical Oscillators. 66. For part 65, see: Lengyel, 1.; RBbai, G.; Eptein, I. R.J . Am. Chcm. Soc. 1990, 112, 4606. (2) NagypBI, 1.; Bazsa, G.; Eptein, 1. R. J . Am. Chem. SOC.1986, 108, 3635. (3) RBbai, G.; Beck. M. T.J . Chem. Soc., Dalfon Truns. 1985, 1669.
0022-3654/91/2095-0770$02.50/0
(4) Orbin, M.; Eptein, I. R. J . Am. Chcm. Soc. 1985, 107, 2302. (5) RBbai, G.;Beck, M. T.; Kustin, K.; Eptein, 1. R. J. Phys. Chcm. 1989, 93, 2853. (6) Orbin, M.; Epstein, I. R. J . Am. Chcm. Soc. 1989. 111. 2891. (7) OrbBn, M.; Eptein, 1. R. J . Am. Chcm. Soc. 1987, 109, 101.
0 1991 American Chemical Society
Oxidation of Thiocyanate by Iodate
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 771
often accompanied by free radical mechanisms! autoinhibition? and autocatalysis.1° To address the need for more information about the reactions that give rise to complex dynamics, we have investigated several component reactions involved in complex sulfur systems. An example is the oscillatory bromate-thiourea reaction in a stirred tank reactor (CSTR)" in which the reaction of bromine with thioureaI0 was implicated in the oscillatory behavior. Recently we have concentrated our studies on the oxidation of the thiocyanate ion. The reactions of thiocyanate in a CSTR with chlorite,I2bromate," and hydrogen peroxide5 all show oscillatory behavior. These three oscillators are not well understood, because the component reactions, particularly those involving sulfurcontaining species, have not been fully characterized. Our first study9 involved the oxidation of thiocyanate by a mild oxidant, iodine. Despite its apparent simplicity, the oxidation of thiocyanate by iodine is quite complex, with strong autoinhibition by the products I- and H+. Thiocyanate ion is a pseudohalide ion that behaves very much like the iodide ion.14 The bromate-thiocyanate" and chloritethiocyanateI2oscillators, for example, can be thought of as derived by substituting thiocyanate for iodide in the corresponding bromate-iodideI5 and chlorite-iodideI6 oscillators. Thiocyanate can form thiocyanogen, (SCN)2,14which is analogous to 12. The oxidation of thiocyanate proceeds via HOSCN, an analogue of
005
~01.17
08
1
I
Time
We have previously reported the observation of complex reaction dynamics in the iodate-thiocyanate reaction.'* In this paper we present a study of the mechanism of this reaction, which expands the 9-reaction network of the earlier study and which accounts for the observed complex behavior.
(6)
1
Q
Experimental Section Materials. The following analytical grade chemicals were used without further purification: potassium iodate, potassium thiocyanate, sodium perchlorate (Fisher), and perchloric acid, 72% (Protea Medical Services). The perchloric acid was standardized with sodium hydroxide, which in turn had been standardized with phthalic acid (E. Merck). The potassium iodate was standardized by addition of excess acidified potassium iodide (British Drug Houses), followed by titration of the liberated iodine against sodium thiosulfate with freshly prepared starch as the indicator. The thiocyanate was standardized by titrating with silver nitrate, using ferric ammonium sulfate as the indicator.19 Both the potassium iodate and the potassium thiocyanate were found to be very stable and could maintain their titres over long periods of time. Methods. The reaction was followed by monitoring the absorbance of I2 and 1; at their experimentally measured isosbestic point of 474 nm on a Techtron UV635 and on a Pye Unicam SPl750 UV/vis spectrophotometer. We also monitored the pH with a Philips PW9449 pH meter. Specific-ion electrodes are affected by both iodide and thiocyanate ions and thus could not give unambiguous readings for either ion. The reactions were carried out at 25 f 0.1 "C and 0.2 M ionic strength (NaC104). A Hi-Tech SF-3L stopped-flow spectrophotometer was used to (8) Hashimoto, S.;Sunamoto, J. Bull. Chem. Soc. Jpn. 1966, 39, 1207. (9) Simoyi, R. H.; Epstein, 1. R.; Kustin, K. J . Phys. Chem. 1989, 93, 2792. (IO) Simoyi, R. H.; Eptein, 1. R. J . Phys. Chem. 1987, 91, 5124. (1 1) Simoyi, R. H. J. Phys. Chem. 1986, 90, 2802. (12) Alamgir, M.; Epstein, 1. R. J . Phys. Chem. 1985, 89, 3611. (13) Simoyi, R. H. J. Phys. Chem. 1987.91, 1557. (14) Huhwy, J. E. Inorganic Chemistry, 2nd ed.; Harper and Row: New York. 1978:. rDDr 678-679. (15) Alamgir. M.; De Kepper, P.; OrMn, M.; Eptein, 1. R. J. Am. Chem. S a . 1983, 103, 2641. (16) Datw, C. E.; OrMn, M.; Dc Kepper, P.; Eplein, I. R. J . Am. Chem. sa.1982, 104, 504. (17) Wilson, 1. R.; Harris, G. M. J . Am. Chem. Soc. 1%1, 83, 286. (18) Simoyi, R. H.; Epstein, 1. R.; Kustin, K. J. Phys. Chem. 1989, 93, 1689. (19) VOgel, A. I. Textbook of Quantifative Inorganic Analysis, 3rd ed.; Wiley: New York, 1961; p 265. I
Time Is1
Figure 1. Absorbance traces (A = 460 nm) at the 12/I; isusbcstic point. (a) Excess thiocyanate (R = 20), showing induction period and transient formation of iodine, reaching a peak after about 85 s. [SCN-Io 0.02 M; [I0;lO = 0,001 M; [H+l0= 0.01 M. (b) Excess iodate ( R = 0.25), showing Iz remaining at the end of the reaction. [SCN-Io = 0.005 M; [I0;lO = 0.02 M; [H+] = 0.05 M. (c) At the same [SCN-Io and [IO 2 (Figure 2a). When R < 1, the relationship between [Izlmaxand [H+Iois less clear, because at low acid concentrations the absorbance traces are of type c. The induction period in the type a curves varies as 1 /[H'loZ (Figure 2b). After the peak iodine concentration has been reached, the rate of consumption of I2 is inversely proportional to [H+Io. Thus in high acid concentrations there is a rapid formation of Iz followed by its slow consumption, while at lower acid concentrations I2 builds up more slowly but is consumed more rapidly. In high acid conditions, it sometima took days for all the iodine to be consumed with R >> 2.
reaction IO; + I- + 2H+ 8 HOI + H102 HOI + SCN- s HOSCN + IHOSCN + HI02 + H20 SO:- + 4H+ + CN- + IHOI + I- + H+ S I2 + H20 12 + CN- F? ICN + II2 + SCN- s ISCN + IISCN + H2O S HOSCN + I- + H+ ICN + H+ + I- s HCN + 12 H+ + CN- z? HCN HOSCN + 212 + 3Hz0 SO-: + 41-+ 6H+ + HCN
-
-
I, + I- z? I,IO; + SCN- + 2H+ * HOSCN + H102 HI02 + 1- + H+ s 2HOI 2HI02 s HOI + IO3- Ht
+
Effect of Thiocyanate. When R > 2, the maximum iodine absorbance also varies linearly with [SCN-Io. Figure 3a shows data from two series of experiments at two different acid concentrations. The rate of consumption of iodine after the maximum is directly proportional to [SCN-Io. Effect of Iodate. As shown in Figure 4a, the effect of iodate on [121max is similar to that of thiocyanate. On the other hand, has little discernible effect on the rate of consumption of iodine after the maximum. The induction period is inversely proportional to the iodate concentration (Figure 4b). Mechanism We propose a mechanism made up of 14 steps, RI-Rl4 (Table I). Except for a few reactions that are composites, the kinetic parameters for most of these reactions are known. The rate laws and the rate constants used in this study are summarized in Table 11.
Reaction R l . This is the first step in the oxidation of iodide by iodate in acidic media. Its rate depends strongly on the acid concentration. Reaction R1 is normally the rate-determining step in the DushmanZ0and related reactions. This step will make no significant contribution at the beginning of the reaction but will (20) Liebhafsky, H.A,; Roe, G. M. fnt. J . Chem. Kine?. 1971, I f ,
693.
Oxidation of Thiocyanate by Iodate
TABLE II: Rate Constants and Rate Laws Used in Numerical Simulationsa
no. RI R2 R3 R4 RS R6 Rl R8 R9 RIO RI 1 R12 R13 R14 a Ail
vi 1.44 X 103[I0,-][I-][H']2 3.67 x 1051HO111SCN-l 5 X 1O6[HOSCNj[H10;] 3 X 1012[HOI][I-][H+] 5 X 105[12][CN-] 6 X IO'[IJ[SCN-] 1.05 X 101[ISCN] 5 X 103[lCN][H'][I-] 5 X 10'2[H'][CN-] 1 X 106[HOSCN][12]2 6.2 X 109[12][1-] 4.5 X 105[103-][SCN-][H+]Z 2 X 10'o[H102][1-][H'] 6 X 105[H102]2
, 0 05-
A
v, 5 X 10-2[HOI][H102] 2 x 101lHOSCNIII-l __ -
=O 2m21
IO[ICN] [I-] 8.5[ISCN] [I-] 2 X I03[HOSCN][I-][Ht] 8.8 X I03[HCN][12] 1 X IO'[HCN] =O 8.5 X 106[13-] 1 X 10-3[HOSCN][HI02]
9.0 X 10'[HOI]2 5 X 101[HOI][I03-][H']
concentration units in M, time in s.
increase in importance as iodide is formed. Reaction R2. The nucleophilic attack of thiocyanate on HOI was postulated in an earlier study of the oxidation of thiocyanate by iodine.g It is a bimolecular process with a rate constant 13.67 X lo7 M-I s-l. It is the first step in the pH-dependent pathway for the oxidation of thiocyanate by iodine. In nonradical pathways, the oxidation of thiocyanate proceeds through HOSCN. Reaction R3. This is a composite of at least three consecutive processes in which HOSCN is oxidized to sulfate and cyanide. This oxidation should be easy and fast. Reaction R4. This is a fast reaction that has been studied by temperature-jump techniques.21 It has an equilibrium that lies overwhelmingly to the left at neutral or acidic pH. Reaction R5. This rapid reaction consumes iodine molecules as soon as they are formed. It is, however, strongly inhibited by acid, because acid reduces the available CN- via reaction R9. Reaction R6. This reaction has been suggestedg as the first step in the pH-independent path for the oxidation of thiocyanate by iodine. It proceeds via the rapid formation of 12SCN-,22which then dissociates to give ISCN and I-.23 Reaction R7. This reaction is analogous to the reverse of reaction R4. ISCN is an interhalogen that should be very similar in properties to 12. Reaction R8. In high acid environments, we expect ICN to decompose to yield HCN.24 This reaction helps to explain the oligooscillatory behavior observed in excess iodate conditions.I6 Reaction R9. This fast protolytic reaction is significant at pHs lower than the pK, of H C N (9.21). Reacrion RIO. This reaction is a composite of at least three fast steps, with no single step being rate determining. Reaction RI I . The formation of 13- is very important in the autoinhibition observed in the 12-SCN- reaction? Triiodide ion is relatively unreactive toward thiocyanate. The equilibrium constant of its formation from I2 and I- is about 770 M25in our media. Reacrion R12. This is one of the most important reactions in the mechanism. It is analogous to reaction R1 with thiocyanate substituting for iodide. Its inclusion is suggested by the observation that the length of the induction period preceding the formation of iodine is inversely proportional to [I03-]o[H+]02. Reactions R13 and R14. These are rapid reactions, often invoked in simulations of oscillators in acidic iodate solution. They prevent [HI02] from becoming too high. (21) (a) Eigen, M.; Kustin, K. J . Am. Chem. SOC.1962, 84, 1355. (b) Palmer, D. A.; van Eldik, R. Inorg. Chem. 1986, 25, 928. (22) Orszagh, I.; Bazsa, G.; Beck, M. T. Inorg. Chim. Acra 1972,6,271. (23) Briot, G. T.; Smith, R. H. Ausr. J . Chem. 1973, 26, 1863. (24) Griffith. R. 0.;McKeown, A. Tram. Faraday Soc. 1935, 31, 868. (25) Turner, D. H.; Flynn, G. W.; Sutin, N.; Beitz, J. V. J . Am. Chem. Soc. 1972, 94, 1554. (26) (a) Edelson, D.; Noyes, R. M. J . Phys. Chcm. 1979, 83, 212. (b) Noyes, R. M.; Furrow, S. D. J . Am. Chem. SOC.1982, 104, 43. (c) De Keppcr, P.;Epstein, 1. R. J . Am. Chem. Soc. 1982, 104, 49.
a l
L
d
\
I 0
I 200
100 Time
Figure 5. Experimental (-) conditions of Figure la.
300
(s)
and simulated (A) absorbances under the
Computer Simulations The iodate-thiocyanate reaction was simulated by using the 14-step mechanism of Table I with the kinetic parameters and rate laws in Table 11. Three different software packages were used: the NAG (National Algorithms Group) Gear routine, the Livermore Solver of Ordinary Differential Equations ( B O D E ) package, and the IMSL Gear. All calculations were carried out in double precision with tolerances of 10-5-106. The three integrators gave nearly identical results. Agreement between experiments and calculations was generally good. Figure 5 shows a typical example. The rate constants used in the simulations were taken from the literature, except for reactions R3, R7, R8, R10, and R12. Of these reactions, we found the simulations insensitive to the rate constants of R3 and R10 (the composite reactions) so long as they were high enough so as not to be rate determining in the oxidation of thiocyanate. The pseudointerhalogen ISCN should hydrolyze in the manner of 12, Br2, and C12(reaction R7). We chose the forward hydrolysis rate constant of ISCN higher than that of I2 (10.5 vs 2.2), based on a recent study2' of the hydrolysis of the interhalogen ICl. Because of its polarity, which makes it more susceptible to attack by water, IC1 hydrolyzes much faster than either C12or 12. Our higher hydrolysis rate constant for ISCN can be justified by similar arguments. Reaction R8 only contributes at the end of the reaction in excess iodate conditions. It is responsible for the observed second increase in [I2] (ref 18, Figure 2). Except for reactions R3 and R10, all reactions were treated as reversible. Discussion Our proposed 14-step mechanism can explain all the observed reaction dynamics. The dependence of the induction time on 1/[H+Io2and l/[IO3-lO can be linked to reaction R12. At the beginning of the reaction, R12 is more important than R1, because [SCN-Io >> [I-lO. As [I-] builds up from reactions R2 and R3, reaction R1 becomes competitive. Excess Thiocyanate Conditions. The reaction commence with R12, producing HOSCN and HIO,, which can produce I- via reaction R3. Reaction R1 then produces HOI and HI02. Thiocyanate (R2) and iodide (R4) now compete for HOI. Reaction R4 is pH dependent, and at high [H'] it rapidly produces 12, which accumulates. Normally, R4 is much faster than R2. If all the thiocyanate were oxidized via R2, we would expect the iodate-thiocyanate reaction to behave like the bromate-iodide reactionZ6in that I2 would be formed quantitatively before it began to disappear. We know, however, from Figure 2a that [I2], is proportional to [H'],. This limitation on the formation of I2 is explained by noting that R4,which is accelerated by acid and competes with R2, is much faster than any other reaction in the solution except for the protolytic reactions. Reaction R7, which is inhibited by acid, is the most important reaction in the pH-dependent pathway in the oxidation of thio(27) Wang, Y. L.; Nagy, J. C.; Margerum, D. W. J . Am. Chem. Soc. 1989, 1 1 1 , 7838. (28) Simoyi, R. H.; Masvikeni, P.; Sikosana, A. J . Phys. Chem. 1986.90, 4126.
J. Phys. Chem. 1991, 95, 774-779
714
cyanate by iodine. After all the IO3- is exhausted, production of HOI ceases, and consequently production of I, via R4 halts. The consumption of 1, then proceeds via R5 and R6 + R7. Since SCN- is in excess, all the iodine will be consumed, giving rise to traces of [I,] (proportional to absorbance) against time as shown in Figure la. Figure 2b shows that [I2lmaX(proportional to maximum absorbance) is also proportional to [SCN-lo. The fact that [I2lmXis consistently greater at high [H+Iocan be explained by reaction R7, which is hindered by [H+]. Excess Iodate Conditions. If R is small, thiocyanate becomes depleted instead of iodate. When the thiocyanate disappears, reaction R1 quantitatively consumes all the I-, leading to IO3- + 51-
+ 6H+
-
312 + 3 H 2 0
After [IZlmax is reached, the consumption of iodine is mainly through reaction R5. The cleavage of the S-C bond in SCNshould occur just before the formation of Sod2SCN-
-
HOSCN
-
H02SCN
-
H03SCN
-+
S042-
CN-
+ H+
The cyanide thus released will then react with 1, at the end of the oxidation chain. If this cleavage occurred earlier, e.g., at HOSCN, then the transient formation of I2 in excess thiocyanate might never have been observed.
General Considerations. The transition from Figure 1b (going through [I2lmpx)to Figure IC (monotonic increase in [I2]) was achieved by decreasing [H+] while keeping [SCN-] and [IO,-] unchanged. This observation implies that reactions that produce iodine must be promoted by acid, while reactions that consume iodine are either unaffected or retarded by acid. Further evidence is also furnished by the observed increase in [I2]- with [H+]. In our mechanism we have ignored potential polymerization reactions of the various sulfur species. We have also not considered possible disproportionations of the type ZHOSCN
-
HOiSCN
+ H+ + SCN-
As long as these reactions occur after the formation of HOSCN (which is our rate-determining step), they will be kinetically inconsequential.
Acknowledgment. We acknowledge the Director of the University of Zimbabwe Computer Center, Prof. J. G. Sheppard, for many helpful discussions on the computer simulation studies. We thank Prof. Dale Margerum for insightful discussions of the kinetics of interhalogen reactions. This work was supported by Research Grant CHE-8800169 from the National Science Foundation, Research Grant 2.9999.102789 from the University of Zimbabwe Research Board, and a Fulbright Fellowship to R.H.S.
An 57FeM-bauer Study of the Intermediates Formed in the Reduction of FeS, In the Li/FeS, Battery System C.H.W.Jones,* P. E. Kovacs, R. D. Sharma, and R. S. McMillant Department of Chemistry. Simon Fraser University, Burnaby, B.C., Canada V5A IS6 (Received: June 25, 1990)
57FeMbsbauer spectroscopy has been used to study the intermediates formed at the FeS2cathode during room-temperature discharge in the LilLiC104-propylene carbonatelFeS2 cell. Mijssbauer spectra recorded at 4.2 K provided evidence for the formation of Li3Fe2S4and another lithiated phase, similar to but not exactly the same as Li2FeS2. Small particles of superparamagnetic iron were also formed early in the discharge. When LiAsFs was the electrolyte, no intermediates were detectable and rapid reduction to small particles of iron occurred. Chemical lithiation of FeS2with n-BuLi in hexane produced a mixture of reduction products similar to that observed for the LilLiCI04-propylene carbonatelFeS, cell.
Introduction In an earlier paper' we described the results of an 57Fe Massbauer study of fully discharged cathodes in the room-temperature LilLiAsF6-propylene carbonatelFeS, battery system. Evidence was presented that the final reduction product formed in this cell is very small particles of iron which exhibit superparamagnetism. Those experiments led to an estimate of the particle size in the reduced cathode. Another important aspect of this study was to use 57Fe Mksbauer spectroscopy to attempt to identify the intermediates formed during discharge, and the findings of this aspect of the work are reported here. Tomczuk et a1.2 studied the high-temperature Li/FeS2 battery system in which a LiC1-KCI eutectic molten salt was the electrolyte. They proposed that LijFeaS4,Li2+,Fel-,Sz ( x 0.2), Fel-$, and Li2FeS2 were all formed as intermediates. The solid solution phase Li2+xFel-$2 decomposes upon cooling to yield Li2.33F%.q7S2 and Li2FeS2. The room-temperature system, employing LiCIO, in propylene carbonate and 1,2-dimethoxyethane as an electrolyte, has been studied by Iwakara using ESCA, 'National Research Council, Ottawa, Ontario, Canada.
scanning electron microscopy, and ion microprobe analysi~.~ They proposed a simple twestep reduction mechanism involving Li2FeS2 as an intermediate. FeS2
+ 2Li+ + 2e-
+
Li2FeS2 2Li+
+ 2e-
-
Li2FeS2
2Li2S
+ Feo
(1)
(2)
Clark and co-workers studied the same room-temperature system by performing chemical analysis on cathodes removed from partially discharged cathode^.^ They concluded that an intermediate Li,FeS2 was formed and that x = 1.5 gave the best fit to the analytical data. However, the presence of this intermediate did not provide a complete explanation of all the data, and another proposal made was ( 1 ) Jones, C. H.W.; Kovacs, P. E.; Sharma, R. D.; McMillan, R. D. J. Phys. Chem. 1990, 94, 832. (2) Tomczuk, 2.;Tani, B.; Otto, N . C.; Roche, M.F.: Vissers. D. R. J. Electrochem. Soc. 1982, 129, 925. (3) Iwakura, C.; Isobe, N.; Tsmura, H.Electrochim. Acta lW)3,28,277. (4) Nardi, J. C.; Clark, M.B.; Evans, W. P. Abstracts of Papers. Symposium on Electric Power Sources in Horological and Microtechnical Products, Mulhause, France, 1981; Extended Abstract, 48.
0022-365419 112095-0774$02.50/0 0 1991 American Chemical Society
