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 FeS2 with 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 Li2FeS2were 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.
0022-365419 112095-0774$02.50/0
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.
0 1991 American Chemical Society
Intermediates Formed in the Reduction of FeS2
--
+ 6Li+ + 6e- 2Li2FeS2+ Li2S2+ FeO Li2S2+ FeO Li2FeS2 3Li2FeS2 + 6Li+ + 6e3Fe0 + 6Li2S
3FeS2
-
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 115 (3) (4) (5)
An objective of the present work was to attempt to identify the nature of the intermediates formed in the room-temperature cell. Previous studies of FeS2 cathodes in this system using powder X-ray diffraction techniques had not proved particularly informative. It was therefore decided to use s7FeMhbauer spectroscopy as a probe. Initially, measurements were made in situ in partially discharged cells. However, it was quickly recognized that lowtemperature Mijssbauer spectra were essential in attempting to unravel the nature of the reduction products. This required removing the cathode from cells in order to record their Mossbauer spectra. The possible formation of Li2FeS2and Li3Fe2S4as products in the reduction required that samples of these compounds be prepared and characterized. For the compound Li2FeS2,single crystals were obtained and the X-ray crystal structure dete~mined.~ The s7Fe M h b a u e r spectra at room temperature and 4.2 K were also measured. The preparation of Li3Fe2S4did not yield single crystals suitable for an X-ray structure determination. The X-ray Powder diffraction pattern has been recorded, together with the 'Fe Mijssbauer spectrum at room temperature and 4.2 K. In earlier work the compound Na3Fe2S, had been studied: and, for comparison purposes, this compound was prepared, its X-ray powder diffraction pattern was measured, and its 57FeMossbauer spectra were recorded.
Experimental Section Even-layered (=12 mg/cm2) FeS2 cathodes were prepared by spreading Transvaal pyrite (99% pyrite) from a binder slurry (1.5% EPDM (Royalene 512, Uniroyal Rubber Co.) in cyclohexane) onto an aluminum substrate. The cathodes were evaporated to dryness, and the pyrite mass was determined to fO.1 mg. The final content of binder in the cathodes was ca. 0.5%. Cathodes were compressed between steel rollers to ca. 70% of pyrite density. Galvanic cells were assembled from 2cm2 disks cut from the cathodes. Cathode disks were pressure wetted (ca. 400 psi of argon) with 1 M LiClO,/propylene carbonate electrolyte. Lithium perchlorate (J. T. Baker) was dried overnight under vacuum at 120 OC. Propylene carbonate (Aldrich Chemical Co.) was distilled under reduced pressure (0.7 mmHg) and had a water content of
