Characterization of the Materials Comprising the Reactive Interfaces in the Li(Si)∕FeS[sub 2]...

Vol. 128, No. 9 O X I D A T I O N O F H C O O H A N D CI-bOH 1919

Nauk SSSR, 150, 349 (1963). 19. V. E. Kaza r inov and G. Ya. Tysyachnaya , Electro-

khimiya, 8, 731 (1972). 20. A. Wieckowski , J. Electroanal. Chem. InterhaciaI

Electrochem., 78, 229 (1977). 21. M. W. Brei ter , ibid., 14, 407 (1967). 22. T. Biegler, J. Phys. Chem., 72, 1571 (1968). 23. Yu. Vassilyev, V. S. Bagotsky, and O. A. Khazova,

Electrokhimiya, 11, 1505 (1975). 24. G. C. Allen, P. M. Tucker, A. Capon, and R. P a r -

sons, J. Electroanal. Chem. Interhacial Electro-

chem., 50, 335 (1974). 25. H. Angers te in -Kozlowska , B. McDougal, and B. E.

Conway, This Journal, 120, 756 (1973). 26. M. W. Breiter , "Elect rochemical Processes in Fue l

Cells," Spr inger -Ver lag , New York (1969). 27. M. Watanabe and S. Motoo, J. ElectroanaL Chem.

Inter]acial Electrochem., 60, 267 (1975). 28. M. M. Janssen and J. Moolhuysen, Electrochim.

Acta, 21, 801 (1976). 29. B. D. McNicol, R. T. Short , and A. G. Chapman,

J. Chem. Soc., Faraday Trans. 1, 7~, 2735 (1976).

Characterization of the Materials Comprising the Reactive Interfaces in the Li(Si)/FeS

Primary Battery Brad J. Burrow, Ken W. Nebesny, and Neal R. Armstrong

Department ol Chemistry, University of Arizona, Tucson, Arizona 85721

and Rod K. Quinn* and Dale E. Zurawski Sandia National Laboratories, Albuquerque, ~ew Mexico 87185

ABSTRACT

Photomicroscopic, e lectron-microscopic , and surface ana ly t ica l s tudies have been conducted on L i ( S i ) / L i C 1 . KC1/FeS2 e lec t rochemical cells before and af ter discharge, and on ind iv idua l anode and cathode components. Phys ica l and chemical composi t ional changes were observed at both the anode and cathode interface that have significance in the discharge mechanism. Oxygen- r ich species were found at the surface of the FeS2 cathode par t ic les and m i g r a - t ion of sul fur away f rom the cathode dur ing discharge was also observed. The rma l shock of FeS2 single crysta ls confirmed the t endency for sul fur to move away f rom the bu lk wi th the appl ica t ion of hea t to the surface. Ex - posure of the L i (S i ) a l loy to a tmosphere a n d / o r t he rma l shock resu l ted in the fo rmat ion of a l i t h ium-r i ch surface in the carbonate o r oxide form. These findings have significance to ba t t e ry systems that must be s tored for long t ime periods, as wel l as the possible source of res is t ive in terfaces in charge and discharge cycles.

The Li /FeS2 p r i m a r y and L i / F e S secondary ba t - ter ies are the subjec t of in tense research in severa l labora tor ies (1-6). P r i m a r y ba t t e ry appl icat ions r e - quire tha t the anode (Li a l loys) and the cathode (pel - le t ized FeS2) be separa ted by an e l ec t ro ly t e -b inde r (EB) l aye r wi th no membrane separa to r be tween them (1). Stacks of these pel le t ized cells are p laced in series to produce the des i red opera t ing vol tage and the d iam- e ter is var ied to yie ld the desired cur ren t drain. When opera t ing as a p r i m a r y power source, the cells no rma l ly sit in an inact ive s ta te unt i l hea ted to opera t ing t em- pe ra tu res g rea te r than the mel t ing poin t of the e lec- t rolyte , in t imes on the order of seconds (or less) . Opt imizat ion of the opera t ion of these ba t te r ies and deve lopment of design cr i te r ia for fu ture high energy dens i ty bat ter ies , mus t be preceded b y a be t te r unde r - s tanding of the chemical and e lec t rochemical react ions which proceed at each interface. To this end, we have conducted severa l s tudies on intact single cells and on ind iv idua l anode, cathode, and e lec t ro ly te components. A typica l single cell is d ischarged as var iab le currents f rom 0.1 to 1.0A producing a cell vol tage of 1.5-1.9V for per iods f rom minutes to hours using p r e - viously descr ibed techniques (1). These discharges do not no rma l ly consume the ent i re power capaci ty of the ma te r i a l s wi th in each cell.

* Electrochemical Society A c t i v e M e m b e r . Key words: fused sa l t s , e lec t rocte , e l e c t r o n spectroscopy, passi-

ration.

In this paper , we r epor t on the correlation of elec- t ron microscopic, photomicroscopic, and var ious elec- t ron spectroscopic surface analysis da ta to the s tudy of the L i (S i ) a l l o y / e l e c t r o l y t e - b i n d e r in terface and the F e S J e l e c t r o l y t e - b i n d e r in terface f rom single cells and ba t t e ry discharges. Migra t ion of components f rom both anode and cathode dur ing hea t ing and discharge is noted. Surface analysis of discrete anode and ca thode components and of s tandard mate r ia l s confirm many of these migra t ion processes.

Experimental Section The mate r ia l s used for s ingle cell tes t ing were

typica l ba t t e ry composit ions (4). The anode pe l le t was made from an L i (S i ) a l loy of 40 or 45 weight percent (w/o) Li. S t a n d a r d par t ic le size was 76-422 ~m (--40 to +200 mesh) y ie ld ing a typical pe l le t dens i ty of 1.0 g / c m 3.

The e l ec t ro ly te -b inde r (EB) pellet , used to separa te the anode and cathode, was composed of an LiC1 �9 KC1 eutectic (45/55 w/o ) and a MgO binder (Magl i te S, Merck Chemical Company, San Francisco, Cal i - forn ia) . The finely d iv ided MgO acts to immobi l ize the e lec t ro ly te when mel ted bu t does not reac t wit~ e i ther the anode or cathode. The mix tu re was composed of 70 w / o e lec t ro ly te and 30 w / o b inder and fo rmed to a densi ty of about 1.7 g / c m 3.

The cathode powder was 64 w / o FeS2, 16 w / o LiC1 �9 KC1 eutectic e lectrolyte , and 20 w/o EB. The EB used

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1920 J. Electrochem. Soc.: E L E C T R O C H E M I C A L SCIENCE A N D T E C H N O L O G Y S e p t e m b e r 198I

in the cathode was 12 w/o SiO2 binder and 88 w/o LiC1. KC1 eutectic. FeS2 was r ead i ly avai lab le as g ranu la r i ron pyr i te obta ined from Matheson, Coleman, and Bell Manufac tur ing Company, Norwood, Ohio, wi th a par t ic le size less than 295 ;~m (50 mesh) . Pel lets were formed to a dens i ty of 2.6 g / cm ~. To assemble a cell, the pel le ts were s tacked a round

a center ing post and sandwiched be tween two stainless steel disks used as cur ren t collectors. The assembled cells were then placed be tween prehea ted pla tens to mel t the e lectrolyte , and discharged for various t imes and at various t empera tu res and cur ren t densi t ies on a s tandard single cell tes te r (6).

For microscopic studies, d ischarged cells were b roken th rough the center and the exposed interface was smoothed with i n c r e a s i n g l y finer sandpaper . Optical examina t ion before and af ter sanding showed that the in tegr i ty of the diffused mate r i a l was preserved. Cells f rom any given series of discharges were assembled as a uni t and examined s imul taneous ly under both the opt ical and the scanning electron microscopes. The interfaces of pa r t i cu la r in teres t in this s tudy involve the e lec t rochemical ly ac t ive L i (S i ) anode and FeS2 cathode wi th LiC1. KC1 e lec t ro ly te and MgO b inder (al l 1 m m thickness) .

Al l expe r imen ta l appara tus except the Auger elec- t ron and x - r a y photoelect ron spect rometers was lo- ca ted in a "d ry room" faci l i ty wi th an a tmosphere con- ta ining less than 300 ppm water . This unique fea ture a l lows cells to be prepared , discharged, and micro- scopical ly examined in a r e l a t i ve ly wa t e r - f r e e en- v i ronment .

The scanning e lect ron microscope is an In te rna t iona l Scientific Ins t ruments Model Super I I I - A with a resolut ion of 70A. Samples were examined under a 100 #A beam cur ren t wi th a 30 keV accelera t ing voltage. The e lect ron probe microanalys is (EM) using energy dispers ive x - r a y spectroscopy (XRF) was per formed wi th a Pr inceton Gamma-Tech XCEL-1000 analyzer . The analyzer used a l i t h ium-dr i f t ed silicon detector wi th a measured resolut ion of 153 eV and 1000 counts / sec at 5.9 keV. For the e lementa l analysis, samples were examined under 3000>< magnificat ion exposing a sur - face area of 0.03 m m 2. The spect ra were collected for 30 sec f rom five different spots on each pe l le t and then averaged. To quant i fy the amount of a given element, window in tegra ls were used to measure x - r a y counts; the number of counts being propor t iona l to the amount of an element. The x - r ays emi t ted by the e lement wi th in a given energy range or window were first counted. The number of x - r a y s in a background window were s imul taneous ly counted; the background window being selected to have the same wid th and close to the same energy as the e lementa l window but not containing any x - r a y s f rom the element. The e lementa l x - r a y s were then d iv ided by the background x - r a y s resu l t ing in a unit less rat io (S /S o and Fe /Feo) . The rat io reflects an atomic in tens i ty uncorrec ted for the fluorescence atomic sensit ivi ty.

The Auger e lec t ron spectroscopy (AES) da ta were taken wi th a Phys ica l Electronics Auger sys tem wi th a single-pass, cy l indr ica l m i r ro r analyzer . For a typical spectrum, the e lec t ron gun was opera ted at 2 kV and 5-20 ~A and a beam d iamete r of ca. 0.1 m m was main - tained. The pressure was main ta ined at 5 X 10 - l ~ Torr. The cells were b roken in a glove box in high pur i ty argon and t rans fe r red quickly to the high vacuum chamber. Careful microscopic examina t ion in- d ica ted no visible signs of contaminat ion. This observa- t ion is suppor ted b y the fact tha t l i t t le or no carbon was observed in the AES spectra. Carbon peaks are typica l of most contaminat ion.

X - r a y photoelectron spectroscopy (XPS) da ta were taken f rom a GCA McPherson ESCA 36 Photoelec t ron Spec t romete r using a 127 ~ electrostat ic deflector as an

electron analyzer . The x - r a y source emi t ted MgKa radia t ion (1253.6 eV). The spec t rometer was in te r - faced to a PDP 8/e minicomputer , which provided a summat ion average of the spect ra as wel l as d igi ta l control of the analyzer . For all the spec t ra obtained, pressure was main ta ined in the 10-~-10 - s Tor t range.

Results and Discussion

Photomicroscopic and electron microscopic studies.- Figure 1 illustrates a typical voltage-time dependence of a single cell d ischarged at 100 m A / c m 2 and 520~ Curve (a) is the total cell potent ia l whi le curve (b) is the anode- to - re fe rence potent ia l and curve (c) is the ca thode- to - re fe rence potent ial . F r o m these data, the fol lowing reproducib le features are noted. A posi- t ive vol tage excursion is observed dur ing the first min- ute (seconds) of discharge fol lowed by a nea r ly con- s tant vol tage response of 1.5-1.6V for the next severa l minutes. A decl ine in opera t ing vo l tage is observed af ter about 20 min for anodes of this composit ion (i.e., approx ima te ly 45 w / o ) . Smal l vol tage changes observed dur ing discharge have been shown to be due to composi t ional changes of the l i th ium al loy (3, 25). Polar iza t ion of the cathode is responsible for the s teadi ly decl ining potent ia l a f te r about 30 min of discharge. Final ly , the sharp decline in cell vol tage occurs when the anode can no longer suppor t the cur ren t drawn, i.e., at about 85% of theore t ica l Li capacity. However , ba t ter ies bu i l t wi th t.hese cells and tested a t this cur ren t densi ty typ ica l ly exper ience end of l ife due to the in te rna l t empera tu re cooling to <352~ no rma l ly expending less than 50% of the l i t h ium capacity.

Significant composi t ional changes at the anode/eIec- trolyte in terface and at the ca thode /e lec t ro ly te in te r - face were indica ted by s tudy of the photomicrographs of cross-sectioned, d ischarged single cells. F igure 2 shows the photomicrographs of three single cells which were, (a) held at open circui t for 2 min, (b) discharged at 0.25A for 20 min, and (c) d ischarged at 0.25A for 80 rain - - a l l at 520~

Severa l features were evident in al l photomicroscopy studies. These are summar ized as follows: (i) a significant change in color and morpho logy was observed for the anode at the e lec t ro ly te in ter face fol lowing discharge; (ii) the FeS2 par t ic les which comprise the cathode mate r i a l undergo an appa ren t phase t r ans fo r - mat ion at thei r outer per imeter . The t rans format ion is dependent upon the ex ten t of d ischarge of the single cell and the t empera tu re of the discharge; (iii) a new phase appears as a l aye r in the e l ec t ro ly t e -b inde r (EB) region fol lowing discharge. The thickness of this

2 .5 , i J i I I i I i i

2.0

> 1.5

h

~ 0

(b)

10 20 30 40 50 60 70 80 90 IO0

TIME (rain)

Fig. 1. Discharge curves of a typical Li(Si)-FeS2 single cell. Current Drain ~ 100 mA/cm 2, temperature ~_ 520~ Curve (a) cell potential vs. reference as a function of discharge time; curve (b) anode potential vs. reference; curve (c) cathode potential vs.

reference.



Vol. 128, No. 9 REACTIVE INTERFACES 1921

Fig. 2. Photomicrographs of the cross sections of three single cells which were (a) held at open circuit for 2 min at 520~ (b) discharged at 0.25A for 2 min, and (c) discharged at 0.2SA for 80 mln. The anode layer is at the left, the EB layer at the center, and the cathode layer is at the right in each photograph.

layer and its proximity to the anode is a complex function of the discharge current, temperature, and voltage. This layer can be seen in the center of Fig. 2 (b) and (c). From these photomicrographic studies, it was apparent that stoichiometric changes were occur- ring at each interface which may have significance

for the performance of these and other lithium-metal sulfide batteries.

Electron microscopic Studies of similarly discharged cells were conducted to further study the changes noted above. Figure 3 shows three photomicrographs and the corresponding x-ray fluorescence dot maps

Fig. 3. Scanning electron micrographs and sulfur (K~) x-ray fluorescence dot maps of cross sections of three single cells which were ,discharged at 1.0A for 20 min at (a) 440~ (b) 520~ and (c) 600~ Again the anode, EB layer, and cathode are displayed from left to right. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.129.182.74Downloaded on 2014-09-12 to IP


1922 J. F, lectrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y September 1981

for sulfur (Ks radiat ion excited at 25 keY) as a function of discharge temperature , f rom 440 ~ to 600~ The new phase in the EB layer is t ransformed from a discrete band to more dispersed l inear regions of new mater ia l as the discharge t empera tu re increases. The sulfur density maps show the movemen t of sulfur f rom the cathode region into the EB layer. Iron (Fe, Ka) was also detected by the XRF method, in the same region as sulfur. Tables I and II give more informat ion on the stoichiometric changes undergone at the cathode /EB interface. Table I shows the re la t ive increase of both sulfur and iron in the EB layer as a function of discharge time. Table II shows the re la t ive increase in each e lement wi th increasing discharge temperature . In all cases the concentrations of sulfur and iron show increases of a factor of two to four by increasing ei ther t ime or tempera ture of discharge. TEM and x - r a y analysis of the portions of the su l fur - r ich phase in the EB layer of cells discharged under high current drain have indicated the presence of l i th ium sulfide and an Fe containing phase.

Fur ther studies of compositional changes at the anode and ca thode/e lec t ro ly te interfaces were carried out using Auger electron spectroscopy (AES) . AES spectra were recorded on a po in t -by-po in t basis as a function of la teral position on nondischarged and discharged single cells and then compared with electron micrographs of s imilar regions of the single cell. The computat ion of re la t ive atomic ratios for certain ele- ments as a function of distance is shown in Fig. 4. We have recorded the sensi t iv i ty-corrected intensi ty of each e lement as a re la t ive atomic ratio wi th respect to

Table I. Relative sulfur (Ks) and iron (Ks) x-ray fluorescence (XRF) intensities in the EB layer of a single cell, discharged at

0.25A, 520~ for the times shown

Time (min) S/S ~ Fe/Fe ~

2 1.3 0.94 10 3.4 1.0 20 6.3 1.2 40 7.2 1.4 8J 9.8 2.7

Table II. Relative sulfur (Kc~) and iron (Ks) XRF intensities in the EB layer of single cells discharged at 1.0A for 20 min at the

temperatures shown

Temp (~ S/S ~ Fe/Fe ~

440 1.2 0.98 520 6.9 1.2 600 8~4 3.1

chlorine. Analysis of the data in this fashion removed many of the uncertaint ies in the relat ionship be tween intensi ty of the Auger signal and real concentrat ion of the e lement (7). Chlorine was an ubiquitous com- ponent of each single cell. The cathode pellets are a mix ture of FeS2 and the LiC1. KCI eutectic, the EB layer is composed main ly of that same eutectic, and chlorine is found throughout the anodes of c e l l s h e a t e d to high temperatures. Fluctuat ions of the e lements noted below are not due to large changes in the chlo-

Fig. 4. Scanning electron micrographs and AES/line scans for the relative atomic ratios of sulfur, iron, and silicon in freshly fractured single cells which were (a) held at open circuit for 2 rain at 520~ and (b) discharged at 0.1A for 20 min at 520~



VoZ. 128, No. 9 R E A C T I V E I N T E R F A C E S 1923

r ine AES in tens i ty as this in tens i ty r ema ined reason- ably constant .

F igure 4a shows the Fe (LMM), S (LMM), and Si (LMM) line profiles for a cell hea ted to 520~ bu t not discharged. There was some var ia t ion in the i ron and sulfur re la t ive atomic rat ios in the cathode layer . Con- centra t ions of i ron and sul fur were also de tec ted a t the EB interface. I t is wor th not ing tha t the re la t ive atomic rat io for silicon at the anode /EB interface increases as a funct ion of dis tance away from tha t in - terface. Since the Li/C1 rat io r ema ined constant in this same region, we conclude tha t some deple t ion of s i l i - con occurred at the anode /EB interface. The lower ing of the silicon content was not due to wet t ing of the L i (S i ) al loy by the LiC1/KC1 eutectic. These resul t s are cor robora ted by o ther surface analysis studies p r e - sented below. F igure 4b shows s imi la r da ta for a single cell d ischarged at 520~ for 20 min at 1.0A. The l ine profiles for this cell c lear ly show the migra t ion of both su l fur and i ron away f rom the ca thode /EB interface. This observat ion was made severa l t imes on different locations along the p lane of the single cell, thus i t is c lear ly not an ar t i fac t of the f rac ture procedure . These resul ts confirm the presence of sul fur and of i ron in the EB layer as seen by e lect ron microprobe techniques. F igure 4b also confirms the deplet ion of silicon from the anode /EB interface. I t is not ye t c lea r whe the r this deple t ion is d i rec t ly re la ted to the morphologica l changes observed in the photomicrographs of the anodes which have been heav i ly discharged.

Surface analysis of individual anode and cathode components.--The discharge behav ior of the l i t h i u m / s i l icon- i ron disulfide p r i m a r y ba t te r ies is obviously compl ica ted by a va r i e ty of morphologica l and composi t ional changes. X - r a y photoelect ron spectroscopy (XPS) and AES, wi th i o n - b e a m depth profiling, were conducted on the separa te anode and cathode mater ia l s to help in the e lucidat ion of these changes. X P S / A E S studies of the iron disull~de cathode materials.--The XPS da ta ob ta ined for the powdered and single crys ta l i ron sulfides and re la ted s tandard ma te - r ia ls are shown in Fig. 5 and 6. Of pa r t i cu la r in teres t is the compar ison of the surface composit ions of the var ious FeS~ mater ia ls . F igu re 5 shows changes in the

o=S/Fe FeS 2 SINGLE CRYSTAL

v=O/Fe

2.0

1.0'

\ kN

%% \ %\ / ,~lk,,. -e

\ / "11

V\V/ v%

100 200 300 ~I 0 1000 5000

SPUTTER DISTANCE (~,) Fig. 6. AES/deptb-profile plot of FeS2 single crystal surface

before and after electron beam annealing.

Fe(2p, 1/2, 3/2) peaks of a sample which was syn- thesized in our l abora to ry and a commercia l FeS2 sample. La rge r amounts of oxygen are p resen t on the surfaces of the commercia l samples. Examina t ion of the sulfur (2s) or (2p) peaks in Fig. 6 indicates tha t the sulfur exists in the form of SO42- anions (perhaps SO8- anions) in addi t ion to the sulfide form (8) on the commercia l FeS2. F u r t h e r evidence of the different surface fea tures of the two FeS2 samples lies in the existence of energy- loss peaks of the F e ( 2 p ) t rans i t ions of the commercia l sample. These peaks indicate the probable presence of F e ( I I I ) on the surface ma te r i a l (9). I t is possible tha t oxygen is bonded d i rec t ly to i ron in both the FeO and Fe203 forms, as wel l as in i ron sulfates on the commercia l FeS2. The presence of oxygen- r i ch mate r ia l s in the cathode m a y lead to a posit ive vol tage excursion in the in i t ia l s tages of p r i m a r y cell discharge (1, 26). This vol tage excurs ion (as shown in Fig. 1) is min imized b y using cathode mater ia l s tha t are oxygen-f ree , and b y discharging in an oxygen- f r ee a tmosphere (22).

(a) FeS ~ ~ ( s y n . ) Fe (21)) i 7~ ~ S (21))

725.95 (b) FeO ,%.~ ,~ ,~

725.3 711 .C5 (C) FeS04 ~ ~

(d) FeS2 "T " w"'rt ~ .,,,~. (comm.) ,~o~ ~ , ~

BINDING ENERGY (eV)

Fig. 5 ESCA spectra of FeS2 cathode and standard materials, Fe(2p), S(2p), and O(ls) spec- tral regions. Spectra (a) FeS2 prepared standard; spectra (b) FeO standard, specta (c) FeSO4 standard, and spectra (d) FeS~ commercial sample.



1924 J. Electrochem. Soc.: E L E C T R O C H E M I C A L

An AES s tudy was per formed on 1 m m slices of single crystals of na tu ra l FeS2 (not pol ished) to obtain a comparison of surface composit ion with the XPS data, a depth profile of the crystal , and informat ion on the movement of species through the la t t ice at the high opera t ing t empera tu res no rma l ly found in the ba t t e ry dur ing discharge. Single c rys ta l FeS2 was selected for s tudy as a ma t t e r of convenience. I t is known that the discharge behavior of single cells using single crys ta l FeS2 cathodes is ve ry s imi lar in its in i t ia l stages to the behavior observed using pel le t ized FeS2 par t ic les (6). The surface composi t ional changes observed at the FeS2 single crys ta l surface can be ex t rapo la ted to those observed in the smal le r part icles .

The AES surface spect ra of the FeS2 surface showed advent i t ious ly adsorbed carbon and oxygen and no other impuri t ies . Brief ion spu t te r ing removed these adsorbed mate r ia l s and lef t a surface wi th an S / F e ra t io of 0.63. This number is corrected for ionizat ion cross section, but does not reflect the t rue s to ichiometry because of the uncer ta in t ies in such numbers for both the ma jo r sulfur and iron Auger t ransi t ions. An elec- t ron beam hea te r was used to t he rma l ly shock the FeS2 samples while st i l l in u l t r ah igh vacuum. F igure 6 shows the resul ts of an A E S / d e p t h profile, before and af ter t r ea tmen t of the FeS2 surface wi th a 2W electron beam for 30 sec. Fol lowing the appl ica t ion of the the rmal shock, a large increase in the S / F e rat io (--~ 2 . 0 ) w a s observed, consistent wi th the migra t ion of sulfur away from the FeS2 matr ix . Fo l lowing several thousand seconds of ion sput ter ing, the or iginal surface composit ion was achieved.

SEM/EM analysis of the t he rma l ly shocked FeS2 samples confirmed the morphologica l and chemical changes indica ted by the AES studies. A dis t inct separa t ion of phases was indica ted fol lowing the rmal shock. X - r a y fluorescence point analysis indica ted a su l fur - r ich nonhomogeneous l aye r on the surface of the crystal (ex tending to depths of up to 1/~m), and a su l fur - deficient l ayer under lying. X - r a y diffraction of the under ly ing layer showed it to be ma in ly FeS0.9 (py r - rho t i te ) . The format ion of this phase by s imple ap - pl icat ion of heat is discussed be low wi th regard to composi t ional changes in the p r i m a r y ba t te ry .

Fu r the r studies have shown tha t prolonged heat ing (grea te r than 1 min) under the condit ions in Fig. 6

SCIENCE AND T E C H N O L O G Y September 1981

fur ther increased the S / F e rat io on the single c rys ta l surface. Electron beam heat ing of the sample unde r condit ions sufficient to cause desorpt ion of sulfur f rom the surface while at about 10 -9 Tor t caused a decrease of the surface S / F e rat io back to levels tha t were seen on the unhea ted surface. These exper iments are descr ibed in more detai l e lsewhere (14).

XPS/AES studies of anode materials.--XPS spectra were obtained f rom 40 w/o L i (S i ) a l loy and 45 w/o Li (Si) alloy, and l i th ium/s i l icon alloys which had been exposed to atmosphere. The carbon ( l s ) spectra [as in Fig. 7 (c ) ] indica ted the presence of two forms of carbon on the surface of the L i (S i ) alloy. This was the case for al l l i th ium meta l samples examined. The la rger peak at lower b inding energy [charge shif t corrected to 284.4 eV (7)] was a t t r ibu ted to hydro - carbon contaminat ion of the XPS vacuum system. Comparison of the C ( l s ) peaks of the al loy to the Li2CO3 s tandard indica ted tha t the res t of the al loy surface carbon was p robab ly present as the carbonate species. As discussed below, this con taminant was not observed in the AES exper iments conducted in a c leaner vacuum. It does imply, however , tha t the ca r - bonate will be a con taminant on samples exposed to normal ba t t e ry assembly atmospheres .

In a t t empt ing to record XPS spect ra of Li (Si) a l loy samples which had been exposed to a tmosphere for long t imes (10-24 hr) we discovered an in te res t ing technique for examining a f resh ly exposed L i (S i ) a l loy surface. Spect ra shown in Fig. 7(a) show the C ( l s ) , S i (2s) , and L i ( l s ) regions for surfaces which were a tmospher ica l ly exposed for 10 hr, loaded into the XPS system, and then had the f resh L i /S i su r - face exposed jus t pr ior to analysis b y physical r e - moval of the passive layer which forms on these m a t e - rials. F igure 7(b) spect ra were obtained for the same spectra l regions on a sample which had been f resh- f rac tured under argon and then a tmospher ica l ly ex - posed for 1-5 min pr io r to loading in the spect rometer . Spect ra i l lus t ra ted in Fig. 7 (c) are for a sample f rac- tu red and loaded in a tmosphere . Comparison of Fig. 7 (a ) , (b) , and (c) indica ted that carbonate bu i ldup on the al loy surface is slow at 10-~ Torr, bu t s t i l l apprec iable over the 24 hr requ i red to achieve ope ra t - ing vacuum. It appears tha t the first few monolayers

Fig. 7. XPS C(ls), Si(2s), and Li(ls) spectra of 40 w/o Li/Si alloys exposed to atmosphere for various times: (a) the C(ls), Si(2s), and Li(ls) spectra of freshly exposed Li/Si alloy; (b) the C(ls), Si(2s), and Li(ls) spectra of Li/Si alloy exposed momentarily to atmosphere; (c) the C(ls), Si(2s), and Li(ls) spectra of Li/Si alloy fractured and loaded under a normal atmosphere.

C (ls) Si (2s) Li (ls)

I I I 91 284 277

I L I I 158 151 144 61 53

BINDING ENERGY (eV)

I

45



Vo}. 128, No. 9 REACTIVE INTERFACES 1925

were added very quickly (less than a few minutes). Subsequent buildup probably follows an isotherm dependence as observed for other active metals.

The Si(2s) spectrum in Fig. 7(c) showed that no detectable silicon was present on the strongly passivated alloys. Since the escape depth of the Si (2s) electron is about 20-35A in most solids, this lack of a silicon signal implies a silicon-depleted layer of at least 50-100A. (12, 27). The freshly exposed surfaces showed higher concentrations of silicon. Aging studies discussed below indicate the segregation of l i thium and silicon during passivation of the surface.

The L i ( l s ) spectrum of the freshly exposed Li(Si) surface is shown in Fig. 7(a). The pristine nature of this surface was confirmed by the appearance of only one L i ( l s ) peak due to l i thium metal (13). On those samples which have been par t ia l ly passivated, two L i ( l s ) peaks are observed [Fig. 7(b) and (c)]. One might expect that only ionic lithium, with a higher L i ( l s ) binding energy, would be observed on these samples. Previous studies have shown the reduction of ionic l i thium compounds to metallic lithium by the x - ray source (11). Two forms of lithium are always observed in an XPS spectrum of any li thium salt. Figures 7(b) and (c) show an ionic l i thium whose concentration roughly correlates to the extent of passivation. Further studies of the kinetics of x - ray photoreduction of these compounds and the kinetics of surface passivation are underway (14).

In studies conducted thus far, no evidence of the formation of LisN on the Li(Si) surface has been observed. N( l s ) signal intensities were below detectable limits. Although nitrogen will react with lithium at room temperature (15), the reaction is too slow to effectively compete with reactions of the metal with CO2, H20, and 02. Furthermore, LisN decomposes in the presence of H20 to form Li20 and NI-I8 (16).

Figure 8 shows the AES spectrum of a typical Li /Si alloy (45 w/o) . The surface of this material [Fig. 8(a)] showed only lithium, oxygen, and traces of carbon and silicon in the spectrum. The low levels of silicon are consistent with the XPS data; the absence of carbon is l ikely due to the fact that these materials were freshly fractured under high puri ty argon and loaded into the AES, UHV system directly. An expanded AES spec- t rum indicated that the precise energy position and intensity of the l i thium Auger transitions are strongly dependent upon chemical form and matr ix (17-21), but our data clearly indicate no metallic lithium on the surface (21). Lithium does not possess sufficient core electrons to undergo an intra-atomic, Auger ioni- zation. The detected electron must be donated by neighboring atoms; thus, several Auger transitions are possible, depending on the energy levels of the electrons in the neighboring atoms (18). These AES spectra show transitions consistent with l i thium in a fully oxidized form (17).

Following extensive ion sputtering, a larger silicon peak appeared [Si/Li : 0.26, Fig. 8(b)] . The alloy stoichiometry was still not achieved even after prolonged sputtering. We feel that the heating caused by the incident ion beam is also responsible for initiating migration of the l i thium away from the bulk alloy (see below). The continued presence of oxygen on the extensively sputtered surface is puzzling. The bulk composition of the alloy does not indicate appreciable oxygen present (22). When the sample in Fig. 8 was exposed to laboratory atmosphere for about 10 rain, and then reanalyzed, the spectrum in Fig. 8(c) was obtained. Note the disappearance of silicon in favor of increased l i thium and oxygen intensities. This again demonstrates the effect of atmospheric passivation on these alloys.

To approximate the conditions seen by the freshly exposed anode surface in the high temperature battery

(a)

N'(E)

(b)

(c)

Si C

' i / ~ 0

0

Li

0

0 20 40 60 80 100 120

(e~

100 300 500

KINETIC ENERGY (eV)

Fig. 8. Auger spectra and ion-beam depth profiling of 45 w/o Li(Si) alloy. (a) Surface scan; (b) surface scan after approximately 5000A had been ion-beam sputtered, and (c) surface scan after exposure to the atmosphere for 10 min.

environment, a series of experiments was conducted where an ion-sputtered alloy was subjected to thermal shock using an electron-bombardment heater. The alloy surface was ion sputtered until appreciable silicon was present [Si/Li = 0.14, similar to Fig. 8(c)] . The surface was then subjected to E-beam heating of lW for 1 min and the silicon dropped to below detection limits. Clearly, heating of the sample caused migration of the lithium away from the silicon as was the case for atmospheric passivation. Diffusion of Li into p- type single crystals of silicon at high temperatures has been reported previously (23), supporting this hy- pothesis.

Discussion

The microscopic examination of the cathode and anode materials before and after discharge leads to the observations that (i) the FeS2 particle undergoes a chemical and physical change at its surface which likely leads to the migration of a sulfur-rich layer into the EB region of the single-cell; and (ii) there is a color change and a morphological change in the Li (Si) anode during discharge which is l ikely due to a chemical compositional change. This compositional change is possibly due to the migration of lithium away from the anode bulk, as shown by AES studies. The AES/XPS experiments on the individual components show that (i) there is a significant concentration of oxygen-rich species on the surface on the FeS2 cathode; (ii) that thermal shock of the cathode material leads to a migration of sulfur away from the bulk and leaves a substoichiometric iron sulfide behind; (iii) a passive oxide/carbonate film forms on the Li(Si) surface following exposure to atmosphere; and ( iv) that atmospheric passivation and/or thermal shock of the anode material l eave an anode surface that is rich in ionic lithium_



1926 J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY September 1981

Oxide-rich forms of sulfur and iron on the cathode surface may be responsible for the initial voltage excursion seen in the cathode during single cell discharge. The voltage excursion correlates with the particle size used in the cathode pellet (6). Smaller cathode par- ticles lead to a larger voltage excursion which is consistent with their higher oxide content. Depletion of the oxide-rich layer leads eventually to a more constant discharge voltage.

Thermal or electrochemical migration of sulfur away from the cathode material is not unexpected, especially at the operating temperature of the thermal battery system. Thermal loss of sulfur can cause serious prob- lems in the poisoning of current collectors and un- wanted reaction directly with the anode material. In the event that recharging is desired, this loss of volatile sulfur will result in lower charge/discharge capacity. The migration of sulfide from the cathode can be understood in terms of the migration of all anions toward that anode (24). Reasons for the migration of iron are less obvious and may be due to the formation of a nonstoichiometric iron, sulfur, potassium, lithium complex. An X-phase (Li2FeS2) and a J-phase (LiK6Fe24S26C1) have been identified in discharged Li/FeS2 batteries following cooling (4). Countering the movement of sulfide is the precipitation of a lithium- sulfur salt in the EB layer. Any precipitation of this type means the removal of current carrying ions from the electrolyte and an addition to the internal cell resistance if the precipitate has low solubility at the operating temperature of the battery.

Movement of lithium at the anode surface can lead to increased resistance toward charge transfer when this migration occurs with the formation of a passive oxide or carbonate layer. Loss of lithium from the anode interface has also been observed through the reduction of the adjacent KC1 to metallic potassium, which can volatilize to another part of the sealed battery (1). This type of lithium movement would result in decreased discharge capacity at the anode surface. The kinetics of migration processes at the anode/EB interface are currently under investigation (14).

Further experiments will study the significance of each of these complications in individual half-cells. At the high temperatures of cell operation, the resistance to current flow represented by the reactions noted above may not be restrictive to the near opti- mum performance of these batteries. If storage of the battery is required before use (especially in adverse environments), or if extended use is required, such as long discharge or several charge and discharge cycles, then these interracial reactions will be important.

Acknowledgment This work was supported by the U.S. Department of

Energy (DOE) under contract DE-ACO4-76-DP00789.

Manuscript submitted Aug. 5, 1980; revised manuscript received March 26, 1981.

Any discussion of this paper will appear in a Discus- sion Section to be published in the June 1982 JOURNAL.

All discussions for the June 1982 Discussion Section should be submitted by Feb. 1, 1982.

Publication costs of this article were assisted by Sandia National Laboratories.

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Characterization of the Materials Comprising the Reactive Interfaces in the Li(Si)∕FeS[sub 2]...

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