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d. Electrochem. Soc., Vol. 138, No. 3, March 1991 9 The Electrochemical Society, Inc.

n n u m b e r of electrons transferred in electrode

reaction

R universal gas constant, 8.314 J/mol - K

R,., charge-transfer resistance, tl cm 2 R~ solution resistance, fl cm :'~ T temperature, K

v scan rate, V/s

Z overall impedance, l-I cm 2 Z' real part of impedance, fl cm 2 Z" imaginary part of impedance, ~1 cm 2 IZJ m o d u l u s of impedance, II cm 2 G r e e k

[3 s y m m e t r y factor for electrochemical reaction a Warburg coefficient, 1~ cm 2 s -~'~

to angular frequency, radian/s Subscripts

O p e r o x i d e ions R carbonate ions

R E F E R E N C E S

1. A . J . A p p l e b y and S . B . Nicholson, J. Electroanal. Chem., 38, App. 13 (1972).

2. A. J. A p p l e b y and S. B. Nicholson, ibid., 53, 105 (1974). 3. A. J. A p p l e b y and S. B. Nicholson, ibid., 83, 309 (1977). 4. A. J. A p p l e b y and S. B. Nicholson, ibid., 112, 71 (1980). 5. S. H. Lu, Ph.D. Dissertation, Illinois Institute of Tech-

nology, Chicago, IL (1985).

6. I. Uchida, T. Nishina, Y. Mugikura, and K. Itaya, J.

Electroanal. Chem., 206, 229 (1986).

7. I. Uchida, Y. Mugikura, T. Nishina, and K. Itaya, ibid.,

206,

241 (1986).

8. I. Uchida, T. Nishina, Y. Mugikura, and K. Itaya, ibid.,

209, 125 (1986).

9. B . B . Day6, Ph.D. Dissertation, Texas A&M Univer- sity, College Station, TX (1990).

10. T. Nishina, M. Takahashi, and I. Uchida, This Journal,

137, 1112 (1990).

11. R . S . Nicholson and I. Shain, Anal. Chem., 36, 706 (1964).

12. T. Berzins and P. Delahay, J. Am. Chem. Soc., 75, 555 (1953).

13. J. R. Macdonald, J. Electroanal. Chem., 223, 25 (1987). 14. J. R. Macdonald, in " I m p e d a n c e Spectroscopy," J. R.

Macdonald, Editor, p. 1, J o h n Wiley & Sons, New York (1987).

15. J. E. B. Randles, Discuss. F a r a d a y Soc., 1, 11 (1947). 16. B. Ershler, zbid., 1, 269 (1947).

17. J. R. Macdonald and L. D. Potter, Jr., Solid State Ion- ics, 23, 61 (1987).

18. M. Sluyters-Rehbach and J . H . Sluyters, in "Electro- analytical Chemistry," Vol. 4, A. J. Bard. Editor, p. 1, Marcel Dekker, Inc., N e w York (1970).

19. A . J . A p p l e b y and C. Van Drunen, This Journal, 127, 1655 (1980).

20. K. Ramaswami, Ph.D. Dissertation, Illinois Institute of Technology, Chicago, IL (1990).

21. B. Kr. Andersen, Ph.D. Dissertation, The Technical University of Denmark, Lyngby, D e n m a r k (1975).

The Behavior of Zinc Electrode in Alkaline Electrolytes

I. A Kinetic Analysis of Cathodic Deposition

C. Cachet, B. Saidani, and R. Wiart

UPR 15 du CNRS "Physique des Liquides et Electrochimie,'" Laboratoire de l'Universit~ Pierre et Marie Curie, 75252 Paris C~dex 05, France

A B S T R A C T

A model for the cathodic electrode activation is developed so as to account for the polarization curves and i m p e d a n c e plots obtained for zinc deposition in alkaline electrolytes. The reaction pattern involves the two-step discharge of zincate ions through an oxide-containing layer whose ionic and electronic conductivities are potential activated. The sharp elec- trode activation with increasing cathodic polarization is shown to be related to the spreading and thinning of the conduc- tive layer. T h e s e p h e n o m e n a and the concentration of the m o n o v a l e n t intermediate in the layer account for the three time- constants distinguished in the inductive electrode impedance. The growth of granular c o m p a c t deposits, favored by trace lead in the electrolyte, is associated with the exmtenee of a uniformly c o n d u c t i v e layer on the whole electrode surface. T h e presence o f a fluorinated surfactant (FI 110) inhibits the formation of spongy deposits in close c o n n e c t i o n with modifica- tions to both the kinetic parameters of reaehons and the geometrical parameters of the c o n d u c t i v e layer.

Zinc deposition is k n o w n to occur mainly from zincate ions Zn(OH)42- (1-3) but the structure of zincate c o m p l e x e s in supersaturated alkaline electrolytes is not yet com- pletely established. R e c e n t results obtained with concen- trated solutions by laser R a m a n and ~ Z n NMR m e t h o d s have c o n c l u d e d in favor of the symmetrical tetrahedral form Zn(OH)42- (4). The latest data from spectroscopic techniques, E X A F S and neutron diffraction, do not reveal any difference b e t w e e n under- and supersaturated solu- tions, and they have been explained by considering the presence o f waters o f hydration around the Zn(OH)42 ion (5, 6). Thus, the species responsible for supersaturation would be different from the main tetrahydroxozincates by the n u m b e r of water molecules (5).

On the other hand the kinetics and the reaction mechan- ism of the deposition and dissolution of zinc in alkaline so- lutions are still a matter for discussion. A c o n t r o v e r s y ex- ists a b o u t the reaction m e c h a n i s m s which have been derived from the values of reaction orders, Tafel slopes, and e x c h a n g e current densities d e d u c e d from short-time transient m e a s u r e m e n t s (7-14), the quantitative data con- cerning these parameters being spread over a rather wide

range of values. Dirkse and H a m p s o n (7, 8) proposed the following m e c h a n i s m

Zn + OH- ~ Zn OH- [D-I]

Zn O H --, Zn OH + e [D-2] Zn OH + OH -~ Zn(OH)2 + e- [D-3] Zn(OH)2 ~ 2 OH- ~ Zn(OHh 2- [D4] in which the rate-determining step for zinc dissolution is the electrochemical reaction [D-2], in a g r e e m e n t with the e x c h a n g e current density observed to be i n d e p e n d e n t of the zincate concentration. Later on, Dirkse pointed out that the zinc electrode behavior is sensitive to the ionic strength in the electrolyte, and from comparisons at con- stant ionic strength, he confirmed that the rate-deter- m i n i n g step occurs early in the anodic s e q u e n c e (9).

A second m e c h a n i s m was proposed by Bockris et al. (10) Zn + OH- ~ Zn OH + e- [B-I]

Zn OH + OH- ~ Zn(OH)2 [B-2[

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J. Electrochem. Soc.,

Vol. 138, No. 3, March 1991 9 The Electrochemical Society, Inc. 679 Zn(OH),~- + OH- ~ Zn(OHh- + e- [B-3]

Zn(OHh- + O H --m Zn(OH)42- [B-4] in which reaction [B-3] is the rate-determining step, in both anodic and cathodic directions. Treating this model by the p s e u d o - e q u i l i b r i u m m e t h o d allowed the authors to account for the reaction orders with respect to OH- and Zn(OH)4 :- and t h e Tafel slopes obtained in both anodic and cathodic directions over an overpotential range of

+_ 100 mY.

More recent m e a s u r e m e n t s of galvanostatic transients carried out at constant ionic strength have been reported (11) to be consistent with the suggested m e c h a n i s m of Bockris et al. Moreover it was shown that the ionic strength has no influence on the kinetics of the zinc elec- trode, at variance with the results of Dirkse.

The p s e u d o - e q u i l i b r i u m m e t h o d was also used to inter- pret the p o t e n t i o d y n a m i c curves of Chang and Prentice (12) who c o n c l u d e d that their results could be explained by the following m o d e l for zinc dissolution

Zn + OH- ~ Zn OH + e- [P-l] Zn OH ~ 2 O H --* Zn(OH)3- + e- [P-2] Zn(OH).~- + OH- ~ Zn(OH)4 z [P-3] in which the rate-determining step is reaction IP-2]. This model can be regarded as a simplified version of the Bockris m o d e l since reaction [P-2] includes reactions [B-2J and [B-3].

Also the kinetics of a m a l g a m a t e d zinc remain an un- solved problem. Various reaction models have been pro- posed (13, 14) and a chemical step between the two electro- chemical reactions has been considered as the rate-determining step (14).

All these kinetic models d e d u c e d from short-time meas- u r e m e n t s present s o m e deficiencies in the sense that they are not sufficient to predict the steady-state behavior of zinc electrodes. F o r e x a m p l e in the case of zinc corrosion, the rate-determining step tbr the anodic reaction obtained from transient m e a s u r e m e n t s has been found to differ from that obtained from steady-state m e a s u r e m e n t s (15). F u r t h e r m o r e , all these m e c h a n i s m s involving a multistep discharge and generally several adsorbed intermediates neither account for the low proper frequencies revealed by ac i m p e d a n c e s p e c t r o s c o p y during zinc deposition (16-20) nor explain the sharp electrode activation observed on steady-state polarization curves in both cathodic and anodic directions (16, 17, 19). C o n s e q u e n t l y it appears that the kinetics of zinc electrode in alkaline electrolytes re- main questionable, particularly regarding the nature of the slow interracial processes taking place at the solid elec- trode surface.

In a recent investigation d e v o t e d to the connection be- t w e e n the m e c h a n i s m of zinc deposition and the deposit morphology, two distinct rate-controlling regimes for the electrode kinetics have been pointed out (19, 20): (i) a low- f r e q u e n c y capacitive feature w h e n irregular deposits were formed and (ii) a low-frequency inductive feature for the growth o f granular and c o m p a c t deposits. At low cathodic overpotentials, it has been shown that the formation of scattered packs of metal on the electrode surface occurs w h e n the discharge of zincate ions is controlled by the ion diffusion through a porous layer which s l o w l y blocks the electrode surface (20). t n situ Raman spectroscopy has c o n c l u d e d that this layer consists of defective-structured ZnO (21), and i m p e d a n c e data are consistent with a de- p e n d e n c e of the mass-transfer coefficient in porous layers upon the cathodic overpotential (20). In the presence of anodically dissolved zinc (ADZ) in electrolytes, a thinner and i n h o m o g e n e o u s porous layer on the electrode would control the growth of spongy deposits (20). Such a m o d e l of mass transfer through a semi-blocking layer existing close to the e q u i l i b r i u m potential offers an outlook de- scribing how the electrode is cathodically or anodically ac- tivated with increasing polarization. That is the reason w h y we decided to reconsider the kinetics of zinc deposi- tion and dissolution taking into account the presence of the interfacial layer.

Recent advances in rechargeable zinc batteries include the use of additives and modified charging methods so as to get a beneficial effect on zinc m o r p h o l o g y (22-28). In par- ticular organic additives can refine the grain size and elimi- nate dendritic growth (29-3 I). Fluorosurfactants have been reported to be efficient inhibitors for the growth of zinc moss and dendrites (19, 32, 33).

With the presence of organic additives in alkaline elec- trolytes, the connections observed between the deposit m o r p h o l o g y and the electrode kinetics are valid again (19). At low current densities, rough deposits grow while an im- portant capacitive feature appears in the low-frequency impedance, thus suggesting again s o m e control of the elec- trode kinetics by the ion diffusion through the interfacial layer. In addition the formation of fine-grained deposits is generally a c c o m p a n i e d by an increase in the charge- transfer resistance and modifications to the rates of inter- facial reactions in alkaline electrolytes (19). ac impedance spectroscopy has already been proved as efficient in eluci- dating the m e c h a n i s m of zinc deposition in acidic electro- lytes, shedding some light on (i) the competition between the hydrogen adsorption and the autocatalytic process of metal discharge (16), (ii) the c o n s e q u e n c e s of this coupling on the formation of spongy, compact, and dendritic de- posits successively with increasing current density (34),

(iii) the modifications to the rates of interfacial reactions induced by the presence of organic additives (31).

In view of the interest in additives both in pracUce and from a fundamental v i e w p o i n t as a parameter able to mod- ify the m e c h a n i s m of interfacial processes, it appears at- tractive to investigate the influence of additives on the ki- netics of the zinc electrode. The present paper is devoted to the kinetics of zinc deposition in alkaline electrolytes and the results, including those obtained in the presence of a surfactant, will be discussed on the basis of a novel model. A second paper will deal with the m e c h a n i s m of zinc dissolution.

Experimental

Electrolytes.--Alkaline electrolytes were made up using "Merck" products of analytical purity and water which was doubly ion-exchanged, passed through a carbon car- tridge and a micropore filter and then twice-distilled in a quartz apparatus.

Most e x p e r i m e n t s were carried out with electrolytes containing either 5M KOH ~ 0.5M ZnO, or 8M KOH + xZnO, w h e r e 1M -< x -<- 2.2M belongs to the domain of supersaturation. Electrolytes were obtained either by dis- solution of ZnO in highly concentrated (ca. 20M) KOH so- lution and s u b s e q u e n t a d e q u a t e dilution, or by anodic dis- solution of pure zinc (Johnson Matthey 99.999%) in KOH solution.

The influence on zinc deposition of an organic additive [surfactant A t o c h e m F1110 corresponding to the chemical formulation C6F,3C~H4 (OC~H4)12OH] was investigated.

Electrodes a n d cell.--All electrolytes were d e o x y g e n a t e d with argon bubbling during electrolysis in polyethylene cells, and they were maintained at 25~

During zinc deposition the a n o d e was generally a plati- num-gauze cylinder. A zinc cylinder anode (Johnson Matthey 99.999%) was used to generate anodically dis- solved zinc (ADZ) in some particular cases.

The w o r k i n g electrode was a rotating disk electrode (0.5 cm diam) m a d e of zinc (Johnson Matthey 99.999%) that was polished with e m e r y paper grit 1200 before electrol- ysis. The lateral wall of electrodes was insulated with ep- oxide resin (Buehler). In most experiments, the electrode rotation speed 12 was sufficient (2500 rpm) to avoid any in- fluence of the transport of zinc species on the steady-state polarization curves.

Electrode potentials were referred to either a saturated calomel electrode (SCE) used with two c o m p a r t m e n t s sep- arated by fritted glass or a HgO/Hg electrode in 5M KOH solution assembled with a capillary as separator and whose potential is -0.2V against SCE. Due to the stray ca- pacitances and resistances of separators in reference elec- trodes, a phase shift was observed in ac i m p e d a n c e meas- u r e m e n t s at frequencies higher than 10 kHZ; it was

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680

J. Electrochem. Soc.,

Vol. 138, No. 3, March 1991 9 The Electrochemical Society, Inc.

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Fig. 1. Steady-state polarization curves for zinc deposition from vari- ous electrolytes; curve 1, 5M KOH + O.5M ZnO; curve 2, 8M KOH + 2.2M ZnO; curve 3, 8M KOH + 1.3M ZnO. Electrode rotation ~! = 2 , 5 0 0 rpm.

eliminated by using a p l a t i n u m wire paralleled with the reference electrode through a 0.1 FF capacitance.

Experimental methods.--Steady-state polarization curves were corrected for ohmic drop and they were ob- tained potentiostatical]y except for the vertical region of curves which was analyzed u n d e r galvanostatic control. After formation of an initial deposit at 50 m A c m -2 for 20 rain, the c u r r e n t was stepped and the steady-state polar- ization curves were obtained after stabilization of the elec- trode potential, which needed a time close to 5 rain. ac im- p e d a n c e m e a s u r e m e n t s were performed using a frequency response analyzer (Solartron 1174) controlled by an Apple microcomputer. The surface morphology of zinc elec- trodes was e x a m i n e d by s c a n n i n g electron microscopy (SEM).

Results

Electrode polarization and deposit morphology.-

Various polarization curves obtained u n d e r steady-state conditions from various electrolytes are represented in Fig. 1. The e q u i l i b r i u m potential was -1.345V vs. HgO for curve 1 a n d - 1.37V vs. HgO for curves 2 and 3, this latter value c o r r e s p o n d i n g to the saturated electrolyte (8M KOH + 0.SM ZnO) which is t h e r m o d y n a m i c a l l y stable. At low cathodic polarizations, the c u r r e n t remains small d u e to the electrode blocking by the interracial layer, as al-

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ready reported (20). With increasing cathodic overpoten- tial, a sharp electrode activation is observed on all curves in Fig. 1. Along the vertical branches corresponding to such an electrode activation, granular compact deposits are formed. As shown in Fig. 2, the deposit morphology changes with increasing c u r r e n t density from irregular- shaped grains (Fig. 2a) to more regular hexagonal crystal- lites (Fig. 2b) producing a slightly bright electrode surface.

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) unless CC License in place (see abstract).

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address. Redistribution subject to ECS terms of use (see

130.203.136.75

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J. Electrochem. Soc.,

Vol. 138, No. 3, March 1991 9 The Electrochemical Society, Inc. 681 > E 9 - ~ 10 0 0

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Fig. 5. Cathodic overpotential dependence of the product of the charge transfer resistance R, and current density i for two electrolytes: curve 1, SM KOH + 0.5M ZnO; curve 2, SM KOH + 0.1M ZnO. The straight dashed line corresponds to the relationship Rj = [111.

Electrode impedance.--Also along the vertical branches of the polarization curves, a low frequency inductive im- pedance appears with two or three time constants, accord- ing to the electrolyte c o n t e n t (17) and the presence of anodically dissolved zinc (ADZ) or surfactant (19). Two ex- amples of i m p e d a n c e plots are exhibited in Fig. 3. At cur- rent densities lower than 50 m A c m 2 (plot A), three time constants can be distinguished in the inductive loop. Above this limit, a capacitive loop appears between two in- ductive loops (plot B) all the more as the current density increases. It is noteworthy that the first inductive loop ob- served with decreasing frequencies has a proper fre- q u e n c y which markedly increases with raising current density, even though the current activation lies over a very small potential range. The other time constants of the low- frequency i m p e d a n c e do not depend u p o n the current density.

It seems that the small capacitive loop apparent in plot B in Fig. 3 reflects the diffusion of zincate species in the elec- trolyte. As exhibited in Fig. 4 this loop becomes larger with decreasing electrode rotation speed fl, and its proper frequency is an increasing function of fl (see plots C and C') in agreement with a diffusion control. At the lowest ro-

tation speed (plot C') a typical diffusion loop appears as the c o n s e q u e n c e of ion diffusion. This diffusion process likely occurs not only in the electrolyte b u t also through a thin a n d i n h o m o g e n e o u s layer of oxidation products, since a spongy deposit is formed u n d e r such conditions.

In all i m p e d a n c e plots obtained d u r i n g the growth of compact zinc deposits, the high-frequency loop corre- sponds to the double layer capacitance (about 100 ~F cm-5) in parallel with the charge-transfer resistance Rt. The overpotential d e p e n d e n c e of the product of the charge- transfer resistance R, and current density i is depicted in Fig. 5 for two electrolytes. These curves do not conform to the classical relationship R,i = hi valid at low polarization for a reversible charge transfer (35, 36) and the relatively low values of the product indicate that the charge-transfer resistance decreases i n s t a n t a n e o u s l y with increasing over- potential. Such a decreased charge-transfer resistance sug- gests the presence of an interfacial conductive layer through which the zincate ion discharge takes place and whose electrical conductivity is potential activated. Fur- thermore, it can be seen on curve 1 that the transition cor- responding to the sharp electrode activation generates a steep variation of the Rti product. With a dilute electrolyte the transition is observed to occur more progressively (curve 2).

Influence of a zinc anode.--With the use of a soluble zinc anode, the polarization of the zinc cathode progressively decreases with time d u r i n g metal deposition u n d e r galvan- ostatic control, a n d the faster this polarization drift the lower the c u r r e n t density. In Fig. 6, the time d e p e n d e n c e of the polarization curve is illustrated, with the correlative change in the electrode impedance. At the b e g i n n i n g of deposition a low frequency inductive impedance is ob- served (plot A) with a double layer capacitance of 60 ~.F cm-2 in agreement with the growth of a granular compact deposit. After 4 h of deposition, the diffusion loop in plot B characterizes the formation of a spongy deposit (Fig. 7) whose large surface area is consistent with the capacitance (20 m F cm 2) deduced from the high frequency capacitive feature.

Therefore it appears that the growth of compact deposits is destabilized by the presence of anodically dissolved zinc

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Fig. 7. Deposit morphology observed at point B in Fig. 6

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(ADZ) in the electrolyte. The influence of these species can be c o m p a r e d with that of nitrates which favor the forma- tion of spongy deposits and the incorporation of zinc oxide into zinc deposits (37, 38). The destabilizing influence of A D Z species is only transitory since it disappears after a time-lag (3 or 4 h) s u b s e q u e n t to their formation in the electrolyte. T h e n after this time-lag, using a platinum a n o d e in the same electrolyte leads back to both the growth of a c o m p a c t deposit and a low-frequency in- d u c t i v e impedance.

Influence of lead in the electrolyte.--Because of the pres- e n c e of lead (5 x 10 s) in the zinc o x i d e used for the electro- lyte preparation, a small a m o u n t of lead is present in the electrolytes obtained fi'om ZnO. Therefore the zinc de- posits contain s o m e lead inclusions, mainly as the current density for zinc deposition is lower since lead is deposited at the limiting diffusion rate (20).

After a pre-electrolysis during a sufficiently long time so as to e x h a u s t 99% of the initially dissolved lead in the elec- trolyte, it is noticed that the growth of granular and com- pact deposits b e c o m e s unstable after deposition for 2 h with a c o n c o m i t a n t decrease in the overpotential and a c o n c o m i t a n t c h a n g e of the i n d u c t i v e low-frequency im- p e d a n c e into a capacitive one. This t i m e - d e p e n d e n c e of both the i m p e d a n c e plots and the electrode polarization observed in the a b s e n c e of lead ions is similar to that pro- d u c e d by the presence of anodically dissolved zinc de- picted in Fig. 6. C o n s e q u e n t l y it appears that the presence of a small a m o u n t of lead ions in the electrolyte is neces- sary to stabilize the fbrmation of granular deposits over a long scale of time. S u c h a situation confirms that lead in- hibits the irregular growth of zinc deposits in favor of com- pact deposits (39-42). H o w e v e r an addition higher than 10 4M lead generates again the irregular growth of de- posits, probably due to the propensity of lead to form den- drites.

Influence qf an organic addi~ive.--It is k n o w n that the presence of the surfactant F l l l 0 in alkaline zincate elec- trolytes inhibits zinc deposition by increasing the elec- t r o d e polarization (19). As an e x a m p l e it is shown in Fig. 8 that the cathodic shift of the activation branch of the polar- ization c u r v e is so m u c h higher as the additive concentra- tion increases. It is noticeable that the potential shift is lessened in electrolytes m o r e concentrated in KOH and ZnO: for e x a m p l e this shift is only 40 m V with 10-'~M F1110 in the electrolyte 8M KOH + 1M ZnO.

The inhibiting effect of the additive also appears on im- p e d a n c e data. Within the activation domain of polarization curves, the size of inductive loops is modified with the presence of F1110, as exemplified in Fig. 9. As c o m p a r e d with Fig. 3 (A) relative to the additive-free electrolyte, the size of the m e d i u m i n d u c t i v e loop is m u c h bigger on plots (P) and (Q) in Fig. 9 and the lowest-frequencies loop is m u c h smaller. In addition, the f r e q u e n c y proper to the first inductive loop o b s e r v e d with decreasing frequencies is less d e p e n d e n t upon cd with 10-SM F l l l 0 and becomes

close to 10 Hz and cd i n d e p e n d e n t with 10 :~M F l l l 0 . S u c h modifications to i m p e d a n c e spectra indicate that the addi- tive modifies the rates of the interracial processes involved in the process of zinc deposition. Moreover these modifi- cations correspond to a lessened activation of the rate of charge transfer with increasing polarization, as evident from Fig. 10 which reveals an increase in the Rti p r o d u c t with the presence of the additive.

Along the vertical curves 2 and 3 in Fig. 8, granular zinc deposits were obtained, e v e n w h e n using a zinc anode. So it appears that the deleterious effect of the ADZ species which generate spongy zinc electrodeposits is eliminated in the presence o f F1110, and this additive can be regarded as a possible r e m e d y for the problem of maintaining a good c o m p a c t n e s s of zinc deposits w h e n the zinc electrode undergoes successive dischargings and rechargings in a secondary battery. H o w e v e r the additive adsorption coupled with the transport of organic molecules results in a cone-shaped deposit with spirals c o n n e c t e d with the hy- d r o d y n a m i c flow. Similar h y d r o d y n a m i c effects have been already reported with surfactants (17, 19, 43), the deposit protuberances being capable of generating some eddies which result in local adsorbed inhibitor excesses, or dis- charging ion excesses, in the streamlines of the hydrody- namic flow.

E

(9

ql

E

H I

Re/.CO.cm 2"

30

13o

_,X'_

('M

E

u l

a

40O

J Re/-C)-c m2

0.2

12

Fig. 9. Complex plane impedance plots at points P and Q in Fig. 8. Frequencies in Hz.

) unless CC License in place (see abstract).

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address. Redistribution subject to ECS terms of use (see

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J. Electrochem. Soc.,

Vol. 138, No. 3, March 1991 9 The Electrochemical Society, Inc. 683

1

I

I

0

25

S0

75

s

2

Fig. 10. Influence of the surfactant F1110 on the

R,i

product. Same conditions as for Fig. 8.

Model for Zinc Deposition

A t l o w c a t h o d i c p o l a r i z a t i o n s w h e r e t h e c u r r e n t r e m a i n s small, it has b e e n s h o w n t h a t t h e d i s c h a r g e o f z i n c a t e ions is c o n t r o l l e d by t h e ion d i f f u s i o n t h r o u g h t h e d e f e c t s of a p o r o u s layer w h i c h b l o c k s t h e e l e c t r o d e (20). T h e p r e s e n t m o d e l c o n c e r n s t h e e l e c t r o d e a c t i v a t i o n w h i c h t a k e s p l a c e s h a r p l y in a c u r r e n t d o m a i n w h e r e n o i n f l u e n c e o f mass- t r a n s p o r t effects is o b s e r v e d on polarization c u r v e s and w h e r e g r a n u l a r c o m p a c t d e p o s i t s are f o r m e d . T h e r e f o r e no d i f f u s i o n p r o c e s s has to be c o n s i d e r e d in t h e c a t h o d i c acti- v a t i o n d o m a i n . T h e c a t h o d e b e h a v i o r c a n b e e x p l a i n e d by c o n s i d e r i n g t h a t t h e d i s c h a r g e of z i n c a t e ions i n v o l v e s t h e t w o suc- c e s s i v e c h a r g e - t r a n s f e r r e a c t i o n s a d m i t t e d in t h e literature (7-14) Zn" + e- -m Zn' [I] Z n ' + e -mZn [II]

where Zn '* denotes the discharging zincate species Zn(OH)42 and the monovalent intermediate Zn r is proba- bly hydroxylated such as the compounds ZnOH or Zn(OH)., involved in the reaction models proposed by Dirkse, Bockris, and Prentice. The mechanism proposed in the present paper does not consider any likely chemical step associated with a variation in the hydroxylation de- gree, thereby assumed to be fast in comparison with the two electrochemical steps [I] and Ill].

The multi-step discharge of zincate ions can be recon- ciled with the presence of a surface layer whose properties are potential dependent, considering that reactions Ill and [II] take place through a layer of mixed conductivity and composition ZnzO, (with x > y, corresponding to an excess of Zn atoms in a ZnO matrix), as schematized in Fig. I I. On the conductive fraction 0 of the electrode, reaction II] is as- sumed to occur at the distance L from the metal layer in- terface (X ~ o), whereas reaction [Ill leading to the metal atom will take place at X = o. In comparison with thc liter- ature, the originality of our model mainly consists in con- sidering the potential dependence of the geometrical prop- erties of the conductive layer as connected with the long time-constants observed in the inductive impedance (44, 45).

Assuming a low-field activation of the two reversible consecutive steps [I] and [II] taking place through the layer of thickness L, their rates are, respectively

v, = K, (Zn")tnl/L [1] v2 = K2 (Zn') Inl/L [2] w h e r e ",1 is t h e c a t h o d i c o v e r p o t e n t i a l , (Zn") is t h e c o n c e n - t r a t i o n o f z i n c a t e s p e c i e s at X = L, e q u a l to t h e v a l u e in t h e b u l k e l e c t r o l y t e w h e n t h e r e is no d i f f u s i o n control, (Zn') is t h c m e a n i n t e r m e d i a t e c o n c e n t r a t i o n w i t h i n t h e c o n d u c - t i v e layer, t h e r a t e - c o n s t a n t s K, a n d K2 are p r o p o r t i o n a l to

X

1-e

L z J- z nl

e-lz hi!_. Z n

O

\

Fig. 11. Interphase model for zinc deposition; reactions taking place through the conductive layer.

'Insulating laver"

t h e e l e c t r o n and ion c o n d u c t i v i t i e s o f t h e layer, respec- tively.

F o r [~11 > [~1"[ c o r r e s p o n d i n g to t h e t h r e s h o l d E* of p o t e n - tial E w h e r e t h e e l e c t r o d e a c t i v a t i o n starts, t h e coeff• K, a n d K2 are c o n s i d e r e d as p o t e n t i a l a c t i v a t e d

K, = K,* exp (b, In - n*l) [3]

K~ = K2* e x p (b2 In - n*[) [4] w h e r e K~* and K2* d e n o t e rate-constants, b, and b2 are t h e a c t i v a t i o n coefficients. T h e n t h e c u r r e n t d e n s i t y i g e n e r a t e d by r e a c t i o n s [I] and [II] is g i v e n by i = r e ]~ [Kl* (Zn n) e x p (51 In - ~1"[) L + K2* (Zn') exp (b2 In - n*])] [5] P r o v i d e d t h a t t h e v a r i a t i o n s of t h e p a r a m e t e r s L a n d (Zn ~) a r e w e l l s e p a r a t e d in time, it can be c o n s i d e r e d t h a t t h e layer t h i c k n e s s L is c o n s t a n t d u r i n g t h e s h o r t - t i m e do- m a i n w h e r e t h e v a r i a t i o n of (Zn ~) t a k e s place. T h e n t h e c o n c e n t r a t i o n (Zn x) results f r o m t h e m a s s b a l a n c e

d[L

(Zn')] d(Zn') - - - L = v , - v 2 [ 6 ] d t d t T h e g e o m e t r i c a l p a r a m e t e r s e a n d L of t h e c o n d u c t i v e layer are g i v e n by Eq. [7 ] a n d [8], r e s p e c t i v e l y

d L d t - K - K ' L - K 5 [1 - e x p ( - b 5 In - ~l*l)] [7] dO d t - (1 - e) K.~* e x p (b3 [~/- n*[) - 0 K4 [8] w h e r e t h e s p r e a d i n g a n d t h e t h i n n i n g of t h e c o n d u c t i v e layer are a s s u m e d to o c c u r for In[ > bl*l a n d t h e n follow po- tential a c t i v a t i o n s g o v e r n e d by t h e a c t i v a t i o n coefficients b3 a n d bs, r e s p e c t i v e l y .

In Eq. [7], K a n d K ' L d e n o t e t h e rates of t h e slow c h e m i - cal r e a c t i o n s w h i c h e n s u r e t h e r e n e w a l of t h e c o n d u c t i v e layer d u r i n g t h e d e p o s i t g r o w t h , e . g . , t h e d i s p r o p o r - t i o n a t i o n K 2 Z n O H K ' a n d / o r t h e p r e c i p i t a t i o n

Z n + Z n O + H 2 0

K Zn(OH)42 ~- K ' Z n O + H20 + 2 O H -

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address. Redistribution subject to ECS terms of use (see

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So as to be a s s o c i a t e d w i t h l o n g t i m e - c o n s t a n t s , t h e rates o f t h e s e c h e m i c a l r e a c t i o n s h a v e to be m u c h l o w e r t h a n t h o s e o f r e a c t i o n s [I] a n d [II] w h i c h e x p r e s s t h e r e l a t i v e l y fast r e d u c t i o n o f z i n c a t e i o n s to zinc a t o m s . T h e last t e r m in Eq. [7] e x p r e s s e s t h e r a t e o f b r e a k d o w n o f t h e l a y e r (e.g., b y p e r c o l a t i o n o f m e t a l islands) w h i c h is s h a r p l y p o t e n t i a l a c t i v a t e d for I~J > In*l.

A l t h o u g h t h e r e is no d i r e c t e x p e r i m e n t a l d a t a a b o u t t h e layer c o n d u c t i v i t y , it c a n b e s u p p o s e d t h a t i n h o m o g e n e i - ties in t h e c o m p o s i t i o n , t h e t h i c k n e s s or t h e s t r u c t u r e of t h e s u r f a c e l a y e r i n d u c e i n h o m o g e n e i t i e s in t h e l a y e r con- d u c t i v i t y . F o r m a l l y d i v i d i n g t h e e l e c t r o d e s u r f a c e into a c o n d u c t i v e f r a c t i o n 0 a n d a n o n c o n d u c t i n g fraction (1-0), Eq. [8] reflects t h e b a l a n c e b e t w e e n t h e c o n d u c t i v e and in- s u l a t i n g f r a c t i o n s d u e to t h e p e r m a n e n t r e n e w a l o f t h e layer, a n d K~* a n d K, d e n o t e t h e rate c o n s t a n t s for t h e s e c h a n g e s in c o n d u c t i v i t y .

T h e layer i n h o m o g e n e i t i e s c a n be r e l a t e d to t h e ad- s o r p t i o n o f an o r g a n i c a d d i t i v e w h i c h locally i n h i b i t s t h e r e a c t i o n s for b o t h t h e zinc d e p o s i t i o n a n d t h e f o r m a t i o n o f t h e c o n d u c t i v e layer. In t h a t case, 0 m i g h t c o r r e s p o n d to t h e e l e c t r o d e r e g i o n w h e r e zinc spirals h a v e b e e n ob- s e r v e d , all t h e m o r e as this r e g i o n s t r e t c h e s w i t h increas- ing c u r r e n t d e n s i t y .

S t e a d y s t a t e o f the e l e c t r o d e . - - T h e s t e a d y - s t a t e s o l u t i o n s o f Eq. [6]-[8] are, for d ( Z n t ) / d t = d L / d t = dO/dt = o, g i v e n by t h e f o l l o w i n g v a l u e s o f p a r a m e t e r s K,* (Zn") (Zn') - e x p [(b, - 52)In - "11"1] [9] g2* K - Ks[1 - e x p ( - b 5 In - n'I)] L = [lO] K ' K3* o - [ 1 1 1 Ka* + K4 e x p ( - b a [~ - n*l) T h e n t h e s t e a d y - s t a t e c u r r e n t d e n s i t y is 2F 0 [hi K,* (Zn H) i - e x p (b~ In - n*l) [12] L E l e c t r o d e i m p e d a n c e . - - A s a l r e a d y r e p o r t e d (44), an in- d u c t i v e faradaic i m p e d a n c e can be p r e d i c t e d f r o m Eq. [5-8] w i t h t h r e e r e l a x a t i o n p r o c e s s e s c o r r e s p o n d i n g to t h e pa- r a m e t e r s (Zn~), 0, a n d L. L i n e a r i z i n g Eq. [5] gives t h e faradaic i m p e d a n c e Zr ai oi ~.0.~ A(ZnI) Zr-I - + a ~ {Znl),0,L a(Zn') A ]hi Oi ,,(z bL AO ai ,.(znh. AL + - - + - - ao ~lnl oL , Alnl [13] T h e first t e r m o f t h e r i g h t - h a n d s i d e c o r r e s p o n d s to t h e c h a r g e - t r a n s f e r r e s i s t a n c e R t s u c h as ( b , + b 2 ) Rt-' = 2FOK, (ZnH)L -' 1 + Inl [14] 2

According to Eq. [12] and [14]

Inl

Rti - [15] bl + b2

1 + - - I n l

2 in a g r e e m e n t w i t h t h e e x p e r i m e n t a l r e s u l t Rti < [nl.

The other partial derivatives in Eq. [13] are given by, re- spectively [16] a(~/aZn') ,~.o,L i 2(Zn I) o i i ~ n.(Znl),o- L [171 ,)~ =_i [18] ~l,(Znl).l. 0 F o r s m a l l s i n u s o i d a l p e r t u r b a t i o n s o f p o t e n t i a l w i t h angu- lar f r e q u e n c y o~, d i f f e r e n t i a t i n g Eq. [6]-[8] gives t h e pertur- b a t i o n s of p a r a m e t e r s (Znl), 0, a n d L, r e s p e c t i v e l y A(Zn') (Zn') (b, - b2) h h l 1 + j ~ vt

[19]

AL Ksb5 e x p (-b~ 1~1 - n'J) A[~I K ' (1 + jolT2) [20] AO baO (1 - O) - - - [ 2 1 ] ,hind 1 + j~o ~a w h e r e t h e t i m e c o n s t a n t s are, r e s p e c t i v e l y L a 9 , - [22] /<2* [nl exp (b2 In - ~*I) ~. = K ' ' [23] va = [K4 + Ka* e x p (b3 ]n - n*])]-' [24] It is n o t e w o r t h y t h a n Eq. [19] has b e e n o b t a i n e d by dif- f e r e n t i a t i n g Eq. [6] w h e n t h e p a r a m e t e r L is c o n s i d e r e d as c o n s t a n t . T h a t is in c o n f o r m i t y w i t h t h e e x p e r i m e n t a l re- sults s h o w i n g t h a t t h e t i m e c o n s t a n t s % a n d x2 are v e r y dif- f e r e n t f r o m o n e a n o t h e r . T h e total e l e c t r o d e i m p e d a n c e Z is c a l c u l a t e d by con- s i d e r i n g t h e faradaic i m p e d a n c e Zf in parallel w i t h t h e d o u b l e l a y e r c a p a c i t a n c e Ca Z -l = Zf-' + jCd~ [25] R e s u l t s o f s i m u l a t i o n . - - T w o e x a m p l e s of polarization c u r v e s a n d i m p e d a n c e plots s i m u l a t e d f r o m t h e p r e s e n t m o d e l a n d t h e p a r a m e t e r s listed in T a b l e I a r e p r e s e n t e d in Fig. 12. T h e s e p a r a m e t e r s h a v e b e e n c h o s e n b y u s i n g a trial and e r r o r p r o c e d u r e so as to fit t h e e q u a t i o n s w i t h t h e e x p e r i m e n t a l data. T h i s p r o c e d u r e d o e s n o t p r o v e t h a t t h e sets o f v a l u e s for p a r a m e t e r s g i v e n in T a b l e I a r e u n i q u e , and t h e p r e s e n t m o d e l o n l y c l a i m s to d e m o n s t r a t e t h a t t h e t w o - s t e p d i s c h a r g e o f z i n c a t e ions t h r o u g h a c o n d u c t i v e layer is a b l e to a c c o u n t for t h e e x p e r i m e n t a l polarization c u r v e s a n d c o m p l e x p l a n e i m p e d a n c e plots. C u r v e 1 in Fig. 12 r e p r o d u c e s t h e e x p e r i m e n t a l r e s u l t s o b t a i n e d w i t h t h e e l e c t r o l y t e 5M K O H + 0.5M ZnO. O n d i a g r a m (C) t h e i n d u c t i v e l o o p o b s e r v e d a r o u n d 20 Hz reflects t h e relax- ation o f (Zn z) a n d t h a t close to 6 m H z is d u e to t h e s l o w t h i n n i n g o f t h e c o n d u c t i v e layer; as s h o w n in Fig. 13a, t h e t h i c k n e s s L s h a r p l y r e d u c e s f r o m 600 to 10A w i t h increas- ing o v e r p o t e n t i a l o v e r 3 inV. I n t h e c a s e of t h i s a d d i t i v e - free e l e c t r o l y t e , t h e interfacial l a y e r is c o n s i d e r e d to b e en-

Table I. Sets of parameters (1) and (2) for the simulation of curves (1) and (2) in Fig. 12, respectively

Parameter Unit (I) (2) Kl* (Zn H) cm-t s q V-' 2.4 x 10 '2 1.32 x I0 iz bt V I 80 20 K2* cmZs ' V l 2.1 x 10 -9 7.10 -'~ b2 V-! 20 3 K3* s-' - - 0.628 ba V- ' - - 400 K4 s-t 0 0.628 Ks cm s-' 2.18 x 10 -a 1.49 x I0 -~ bs V I 1500 675 K cm s i 2.2 x 10 -a 1.5 x 10 -v K' s q 3.7 x 10 -3 7.10 2

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address. Redistribution subject to ECS terms of use (see

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685

i

7 5 _

E

o

u

(9

bE

5 0 _

o

6k

"E

" ~

E

,

400

2 0 0

0

- 6 0 0

o ~

,

,

,

l l m 2 0.8 1.6 2.4 Re (Z)/_D... crn

25 _

R.(Z)

1~

0 6 0 0

c-/o

;

D

2

x

E/mV

H g O

0,

I

I

I

/ /

x

i x I

i

-]1363

-'1366

4'14 O0

--'I403

',

I

I I1]-[ " r l * l / m v

1

I

!

l'tlctl*t/mY

0

1

2

3

0

1

2

3

Fig. 12. Polarization curves and complex plane impedance plots at points C and D calculated with the sets of parameters (1) and (2) given in Table I, respectively. The simulated curves (1) and (2) correspond to the experimental data obtained with the electrolyte 5M KOH + O.SM ZnO (o) and with lO-SM F1110 in the same electrolyte (x), respectively.

tirely conductive (0 remains equal to unity since K4 = 0, Fig. 13b). It is noteworthy that such a uniformity in the layer conductivity is probably connected with the benefi- cial influence of trace lead in the electrolyte to favor the growth of compact deposits on the whole electrode surface.

Curves 2 in Fig. 12 and 13 are relative to the situation ob- served with the presence of 10-SM Fl110 in the electrolyte. I n addition to the potential d e p e n d e n c e s of (Zn x) and L, the spreading of the conductive fraction of the layer with in- creasing overpotential generates the large inductive loop apparent around 2 Hz on the diagram (D). Such an increase in 0 with overpotential plays a major role in the electrode activation, and it might correspond to the progressive spreading of the cone-shaped deposit over the entire sur- face of the disk electrode. Consequently, it appears that not only the additive modifies the kinetic parameters (K~*, Ks*, b~, b~) of the charge-transfer reactions, b u t it also

changes the geometrical parameters (L, 0) of the interfacial layer.

I n agreement with Eq. [22] the layer t h i n n i n g also ex- plains why the proper frequency f~ = 1/2 ~ %, ascribed to the Zn z concentration in the layer, markedly raises with in- creasing cd, although the potential varies over a very small range, as depicted in Fig. 14. This behavior is particularly clear for the additive-free electrolyte.

Moreover the values of the R t i product calculated from Eq. [15] account for the experimental data given in Fig. 10. With the additive-free electrolyte for which b~ = 80 V -~ and b2 = 20 V ~, the electrode activation takes place for hl = 17 mV, thus giving R t i = 9.2 mV. With the presence of 10-r'M F l l I 0 , I~l = 56my, bl = 20V -l, and b2 = 3 V L" thereby R t i = 34 mV. These calculated values agree witl~ the experimental solid lines in Fig. 10.

The impedance spectra measured at the b e g i n n i n g of zinc deposition in the presence of ADZ species (plot A in

.--I 600 , o o

(oi

:I,L

0.6 20 0.4 0.2 0 0 1 2

31,.,-,_,,.,*l/mVqq

0 I 2 3'""'*l/mVn'l'-fl Fig. 13. Calculated potential dependences of (a) the thickness L and (b) the coverage 0 of the conductive layer, under the conditions of curves (1) and (2) in Fig. 12, respectively.

400 _

30O _

0

2OO

1 0 0 _

o o o

0

20 40 60 80 .LImA.cm-2

Fig. 14. Current dependence of the frequency fl for the (Zn I) relax- ation. The calculated curves (1) and (2) and the experimental points correspond to the same parameters and conditions as for Fig. 12.

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686

Fig. 6) reveal three inductive loops like those obtained with the surfactant F1110, thus suggesting a similar behav- ior of the electrodes in both situations.

However, wit.h the presence of 10-3M F l l l 0 , the fre- q u e n c y f, is observed to remain current i n d e p e n d e n t , and thereby the p r e s e n t model based on the potential depend- ence of the thickness of the interfacial layer does not apply any more. I n this case the lowest-frequencies inductive loop might result from a delayed activation of the conduc- tivity of a thin layer likely connected with the slow addi- tive desorption.

Conclusions

On the basis of e x p e r i m e n t a l data obtained by electro- chemical i m p e d a n c e spectroscopy, it is confirmed that the discharge of zincate ions in alkaline electrolytes implies two successive charge-transfer reactions, possibly coupled to relatively fast chemical steps. It is also shown that the electrode steady state is controlled by the presence of an interfacial layer whose geometrical properties and conduc- tivity d e p e n d u p o n the electrode polarization.

The zinc electrode is quasi-blocked by an oxide contain- ing layer near the e q u i l i b r i u m potential. The sharp elec- trode activation, taking place over a very small range of ca- thodic potential, can be considered as the c o m m o n result of the low-field activation of the charge-transfer reactions through the layer which acquires a mixed conductivity, spreads, and becomes thinner. A model has b e e n proposed to a c c o u n t for the three time-constants distinguished in the inductive electrode i m p e d a n c e with decreasing fre- q u e n c y as, respectively, d u e to the relaxations of (i) the concentration of the m o n o v a l e n t intermediate Zn ~, (it) the coverage, and (iii) the thickness of the conductive layer, the long time-constants being connected with the slow variation of the geometrical properties.

From the SEM e x a m i n a t i o n of electrodes, it is con- cluded that to obtain granular and compact deposits, a uni- formly conductive layer should be m a i n t a i n e d over the whole electrode surface, and the presence of trace lead in the electrolyte appears to be beneficial from this stand- point.

Using a zinc anode generates ADZ species in the electro- lyte and triggers the formation of spongy deposits. This problem can be remedied with the addition of a fluori- nated surfactant F l l I 0 which stabilizes the growth of granular deposits. The influence of this additive on the electrode morphology is shown to be connected with changes in both the kinetic coefficients of the interfacial reactions and the geometrical parameters of the conduc- tive layer.

Acknowledgments

We gratefully acknowledge the financial support of CNRS, AFME, a n d E D F u n d e r contract A.R.C. "Zinc Elec- trode" No. 3296 of PIRSEM. We are grateful to ATOCHEM for providing surfactant F1110.

Manuscript s u b m i t t e d March 12, 1990; revised manu- script received Aug. 25, 1990. This was Paper No. 2 pre- sented at the Hollywood, FL, Meeting of the Society, Oct. 15-20, 1989.

C. N. R. S. assisted in meeting the publication costs of this article.

LIST OF SYMBOLS

(b,), = 1-3, 5 activation coefficients with potential, V-I

Ca double layer capacitance, F cm -~

E potential, V

F Faraday's constant, 96,500 C/g-equiv-

alent

i c u r r e n t density, A cm 2

Im imaginary part of the electrode imped-

ance Z

j

x/-~

K and K~ rate constants, cm s

K', K3*, and K4 rate constants, s - '

K,, Ks, Kl*, a n d Kz* rate constants, cm 2 s - ' V-' L layer thickness, cm Re R,

t

v~ and v: Z Zr Greek symbols to 0

real part of the electrode impedance Z charge-transfer resistance, l! cm 2 time, s reaction rates, s ' cm-2 electrode impedance, II cm: faradaic impedance, ll cm 2 overpotential, V

angular frequency, rad 9 s-' coverage, dimensionless time-constants, s

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) unless CC License in place (see abstract).

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address. Redistribution subject to ECS terms of use (see

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(10)

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A Rechargeable Lithium Battery Employing Cobalt Chevrel-

Phase Compound as the Cathode

Shinichiro Yamaguchi, Takashi Uchida, and Masataka Wakihara*

Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan

A B S T R A C T

The single-phase region of cobalt Chevrel-phase c o m p o u n d (C%MoGSs .{CoCP) was determined by x-ray diffraction analysis. The n o n s t o i c h i o m e t r i c range of CoCP was very narrow and the only CoCP with y = 1.6, 8 - z = 7.7 could be pre- pared as a single phase. The CoCP was evaluated as a cathode for lithium secondary, batteries. 1M LiC104 in PC was used as an electrolyte. The discharge properties and discharge-charge cycling properties were measured galvanostatically u n d e r constant current densities from 0.1 to 2.0 mA/cm 2. The cell exhibited good discharge performance; for e x a m p l e w h e n the cell was discharged u n d e r a cd = 0.l mA/cm 2, 4.8 Li/Co~ 6Mo6S7-,. were incorporated betbre the cell voltage fell down to 1.0V (energy density: 277 Wh/kg). Also a rechargeability of more than 200 cycles was observed at cd = 0.5 mA/cm 2. T h e c u r v e of OCV with varying Li content in the CoCP was very flat and near 2.1 V. The x-ray analysis of lithium incorporated cobalt Chevrel phase, Li~CoCP, was two sets of hexagonal lattice parameters showing the existence of two types of Chevrel phases (having different lattice parameters) coexisting m a wide range of 0 < x < 4.5.

Factors d e t e r m i n i n g the practicality of secondary elec- trochemical cells based on lithium intercalation systems include cell voltage, energy density, and cycle life. Re- cently, a large n u m b e r of chalcogenides with three-dimen- sional framework,

e.g.,

m o l y b d e n u m cluster sulfides, MxMo6Ss, the so called Chevrel-phase compound, has been e x a m i n e d as a cathode material for room t e m p e r a t u r e sec- ondary organic electrolyte lithium batteries (1-6). Chevrel- phase c o m p o u n d s have large e m p t y cavities for the uptake of guest metal species such as L i . Previously we deter- m i n e d the single-phase region of Cu,Mo6S8 2 (CuCP) (7), Fe,Mo6Ss_., (FeCP) (8), Ni~jMo6S8 (NiCP) (9), and e x a m i n e d the cathode properties of these nonstoichiometric Chev- rel-phase c o m p o u n d s for use in lithium secondary cells. We observed that the intercalation of lithium ions into C u C P is a c c o m p a n i e d by copper deposition after several discharge-charge cycles as pointed out by Takeda

et al.

(4) and Uchida

et al.

(5). Accordingly, CuCP behaves as al- most pure Mo6SA in such cells. Tarascon

et al.

(10) recently have reported the cathode properties of silver Chevrel- phase AgMo6S8 (AgCP) in which the silver ions also depos- ited as metallic silver with lithium intercalation, and the Li/AgCP cell actually worked as Li/Mo6S8 cell after several cycles. However, the deposition of copper or silver gives rise to lattice distortion a m o n g Mo~S8 clusters, and this di- minishes their discharge cycle life (6). Previously, we ob- served no metal deposition in the cathode both of F e C P (8) and NiCP (9) systems e v e n after deep discharge-charge cy- cles. Thus, the cell p e r f o r m a n c e strongly depends on the third metallic e l e m e n t in the Chevrel-phase compound. Since cobalt is in the same VIII group as Fe and Ni, cobalt Chevrel-phase may behave as a good cathode material for lithium secondary cell.

In the present study, the single-phase region of CoCP was determined, and discharge-charge behavior of Li/ CoCP cells is described.

Experimental

CoCP with the c o m p o s i t i o n Co~Mo~S~ z was prepared by m i x i n g the ternary e l e m e n t s in desired ratios. The m i x e d

* Electrochemical Society Active Member.

elements were sealed in an evacuated quartz ampul. At first, the ampul was heated to 1000~ for 24 h followed by rapid quenching. Then the sample was resealed and heated to 1000~ for 48 h followed by quenching. The phase identification of the products was carried out by powder x-ray diffractometry using CuK~ radiation ob- tained from a high-power x-ray generator (50 kV, 100 mA: Rotaflex RV-200, Rigaku Company) and the product was identified by J C P D S card No. 30-450. We have reason to believe that the initial composition is that of the final prod-

2.0

1.9

1.8

1.7

1.6

1.5

1.4

1.3

1.2

I I I I I I I I n D

I

I

I

7.3 7.4. 7.5

I

,

(2)

I

I

I a

a

a

(e)

v l

...,} a

/ v / o I

\o

o

/

I

\

/

I

\

I

A \

s

I

I

I

I

I

7.6

7.'7 7.8 7.g 8.0

8 - z

Fig. 1. Single-phase region of Co,Mo~S6., at 1000~ (1) I , Co~o658-, (CoCP): (2) L J, CoCP + CoMo254; (3) ~ , CoCP +~ CoMo2S, + MoS2: (4) ~ , CoCP + MoS2: (5) 9 CoCP + Mo253 4- MoS2; (6) V, CoCP + Mo.

) unless CC License in place (see abstract).

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address. Redistribution subject to ECS terms of use (see

130.203.136.75

) unless CC License in place (see abstract).

References

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