Study of the formation of β-lial alloy by electrochemical techniques in molten LiCl+KCl

J. Electroanal. Chem., 191 (1985) 343-355

343

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

S T U D Y OF T H E F O R M A T I O N OF g-LiAl A L L O Y BY E L E C T R O C H E M I C A L T E C H N I Q U E S I N M O L T E N LiC! + KCi

F. LANTELME Laboratoire d'Electrochimie, Bt.F, U.A. 430, Universitb Pierre et Marie Curie, 4 Place Jussieu, F75230 Paris Cedex 05 (France)

(Received 13th November 1984; in revised form 25th February 1985)

ABSTRACT

The formation of fl-LiAl during electrochemical incorporation of lithium into a-LiAl alloy from molten LiC1+ KCI was investigated by chronoamperometric and chronopotentiometric techniques. It is shown that at the very beginning of the lithium incorporation a supersaturation of the a-phase occurs, the incorporation being controlled by the metal diffusion in the fl-phase. The initial stages of formation of fl-LiA1are explained in terms of a model involving nucleation and three-dimensional growth. In contrast with the model for conventional electrolytical metal deposition the process is controlled in the first moment by the fithium/aluminium interdiffusion in the growing fl-nuclei. However according to the high value of the interdiffusion coefficient the process is rapidly limited by the ohmic resistance of the circuit.

INTRODUCTION The lithium + aluminium alloys are used in experimental molten salt secondary batteries [1]. Their main advantages are to reduce the aggressiveness of lithium metal and to produce a stable potential due to the presence of a two-phase alloy in a large range of concentrations. The t h e r m o d y n a m i c properties of these alloys are now well k n o w n [2]. Their electrochemical behaviour is controlled b y the metal interdiffusion which has already been studied in the single phase alloys [2,3]: the solid solution o f lithium in aluminium (0-9.6 Li a / o : a-phase) and the non-stoichiometric LiA1 alloy ( 4 6 - 5 7 Li a / o : fl-phase). The kinetics of the fl-phase formation has recently been investigated at r o o m temperature in non-aqueous solvents [4], but the mechanism of this phase formation is not yet k n o w n at higher temperatures. We propose here to apply electrochemical pulse techniques to the study of the electrogeneration of the fl-phase f r o m an a-phase alloy in the fused LiCI + KC1 eutectic. 0022-0728/85/$03.30

© 1985 Elsevier Sequoia S.A.

344 EXPERIMENTAL The bath was prepared from pro-analysis reagents prefused in an HCI atmosphere [5a]. The working electrode was prepared from a rod of pure aluminium (Johnson Matthey Gd 1, 20 ppm). Its composition was fixed by potentiostatic deposition of lithium over a long period (about 6 h). Scanning electron micrography showed that during this treatment the electrode surface remained very smooth; this was in agreement with previous observations [5b]; the base a-layer of the electrode can be seen on the micrograph of Fig. 7. The knowledge of the activity coefficient [2] allowed the calculation of the composition of the electrode surface. T h e reference electrode [6] and the counter-electrode were constituted by a two-phase alloy (a + r ) rod; they were electrochemically generated, their composition was about 30 Li a / o ; these electrodes have a very stable potential: 229 mV versus the lithium pool electrode. The fused electrolyte was contained in an iron or Pyrex crucible under an argon atmosphere. Pyrex was not suitable when a lithium pool reference electrode was used; but here the aggressiveness of lithium is considerably reduced; when careful dehydration of the salt and of the atmosphere was performed [5c] no significant attack of the Pyrex vessel occurred even during a three-day experiment. A special electronic device built in the laboratory delivered potentiostatic or galvanostatic pulses with a rise time of 10/ts. The pulse and the electrode response were recorded on a double-trace storage oscilloscope (Tektronix 7023) or on an X - Y recorder. In this latter case a Biomation 805 transient recorder was used as buffer. GENERAL TRENDS For a constant current pulse of high intensity a potential peak generally appears when the potential becomes more negative than the a + fi equilibrium potential. This overpotential is attributed to the energy required to obtain a sufficiently fast nucleation rate. The potential change includes various phenomena such as the charging of the double-layer capacity, the diffusion of electroactive species, the creation and the growth of nuclei [7]. Different parts can be distinguished on the recorded chronopotentiograms (Fig. 1). At the beginning of the chronopotentiograms the potential change is very similar to that observed during the lithium-aluminium interdiffusion in the a-phase [3]. Then the potential reaches the critical region corresponding to the phase transformation a ~ / 3 . At that potential nothing happens except a slight deviation from Sand's law; the potential variation is smaller than that deduced from a pure diffusion process. Indeed a part of the current is now used first for the creation of nuclei and then for their growth. This last part rapidly increases, and at last the current is limited by the resistance of the circuit. In the same way when an abrupt change of potential is imposed a rapid decrease of the current is first observed; it corresponds to the diffusion of lithium atoms in the a-phase (Fig. 2). Then after an induction time the creation of nuclei occurs; a

345

-,10 ms.-

or_

~-

-250 r A

[~_~_

-~00 rlA _150 nlA

/ ~ tL~.,,. -

-11R n~A -

_..:

-55 mA

-~,0 r~A

/ Fig. 1. Galvanostatic potential transients for the formation of fl-LiA1 on an a-LiA1 substrate from molten LiC1 + KCl. Electrode surface S = 0.234 cm2; T = 420°C; resistance of the circuit = 0.42 r; equilibrium potential = 0.660 V versus Li electrode; composition Ceq= 0.7 x 10 -3 mol Li cm -3.

r i s i n g c u r r e n t is o b s e r v e d w h i c h is c h a r a c t e r i s t i c of the g r o w t h of nuclei. A s i m i l a r i n t e r p r e t a t i o n has b e e n r e c e n t l y d e d u c e d f r o m the o b s e r v a t i o n s of cyclic v o l t a m m e try [8]. W e p r o p o s e n o w to give a m o r e q u a n t i t a t i v e i n t e r p r e t a t i o n of the o b s e r v e d p h e n o m e n a i n the t r a n s i e n t regime. By u s i n g d i f f e r e n t t i m e scales the a n a l y s e s of the n u c l e a t i o n process a n d o f the g r o w t h of r - n u c l e i are s e p a r a t e d [9]. A s a first a p p r o x i m a t i o n it is c o n s i d e r e d that the e l e c t r o g e n e r a t e d n u c l e i h a v e a spherical s h a p e a n d t h a t their g r o w t h is c o n t r o l l e d b y the m e t a l i n t e r d i f f u s i o n . T h e results of p o t e n t i o s t a t i c a n d g a l v a n o s t a t i c t r a n s i e n t t e c h n i q u e s are d i s c u s s e d below.

.,40ms,-130 r~V E

i/~

-120 ~/

T--

~

,~

-~0- F~-

~

-~V ~V

-

Fig. 2. Potentiostatic current transients for the formation of fl-LiA1 on an a-LiA1 substrate from molten LiC1 + KC1. Electrode surface S = 0.504 cm2; T = 420°C; resistance of the circuit = 0.42 ~; equilibrium potential = 0.330 V versus Li electrode; composition Ceq= 7.4X 10 3 mol Li cm -3.

346

POTENTIOSTATICPULSES Results

Some characteristic chronoamperograms are reported in Fig. 2. The shape of these curves is similar to that observed during the electrocrystallisation of metals such as copper on a vitreous carbon electrode [9]. A very rapid decrease of the current I is first observed. At low overvoltage Cottrell's law is obeyed. The variations of 1 versus t - 1 / 2 are represented in Fig. 3. The linear part observed at short times and low overvoltages is attributed to the lithium diffusion in the a-phase. By using the treatment already developed [10] we can calculate the interdiffusion coefficient D. The surface concentration at the applied potential is deduced from the knowledge of the activity coefficients calculated from a polynomial development [3]. At low overvoltage these interdiffusion coefficients are in good agreement with those previously obtained. This result indicates that the metal interdiffusion does not change very much when it occurs in the supersaturated a-phase. This out-of-equilibrium situation cannot exist for a long time. Indeed as shown in Fig. 3 a rapid departure occurs from Cottrell's law. The electrochemical process is no larger controlled by linear interdiffusion. We attribute this deviation to a nucleation process which corresponds to the appearance of the nuclei of the fl-phase.

]/mA '

120 -130mV

100

\ 80

60

\

40

20

0

g

1'0

t_h/s. £

.v.-

Fig. 3. Plots of I vs. t -1/2 at the indicated overpotentials.Same conditions as Fig. 2.

347

[0/m A

-130 m V

-120mV -110mY

100

_lOOmY

50

pltt / //"s s " ~

~

-60mV

I

F

Fig. 4. Plots of electrocrystallisationcurrents I s = I - Idiff vs. (t "i')1/2 at the indicated overpotentials. The values of ~"are indicated in Table 1. Same conditions as Fig. 2. -

Growth of nuclei At the beginning of the rising part of the chronoamperograms the current is roughly proportional to the square root of the time (Fig. 4) as has been already observed for the metal deposition in fused salt electrolysis [9]. This behaviour is characteristic of the growth of the nuclei at constant nucleus number. Here we suppose that the current is controlled by the metallic interdiffusion in the growing nuclei (see discussion). An attempt at a quantitative explanation is based on the following assumptions: (1) The nuclei have a spherical shape. (2) Their surface concentration is in equilibrium with the adjacent medium. (3) The flux of the electroactive material is given by:

J = -Dac/h

(a)

where Ac represents the concentration difference between the two surfaces (in contact with the electrolyte and with the a-phase), h is the mean distance between the two hemispheres h = (4/3)r, r being the radius of the nuclei and D the interdiffusion coefficient.

348 The model used to calculate J is represented in Fig. (5). J is the flux between the two horizontal sections of the cylinder (surface ~rr 2). The lithium concentration ca at the surface in contact with the electrolyte is deduced from Nernst's law: fc¢ = e x p ( - (~/~ + E ~ q ) n F / R T )

(2)

where f represents the activity coefficient and Eeq the equilibrium potential versus the lithium electrode. The concentration overvoltage ~/c is: ~lc = rl - A / r

(3)

- RcI

7/ is the potential jump; A / r represents the overpotential due to the curvature r of the nucleus [11]; in R e are included the resistance of the circuit (wires, electrodes and bulk of the electrolyte) and the electrolyte resistance R , around the nuclei [12a]: (4)

R, = 1/4Nx r

N = total number of nuclei, X = specific electrical conductivity of the electrolyte. According to these assumptions the current flow through a nucleus of the r-phase is: (5)

i s = nrD/~ ( c a - e~) ~rr 2 / h = 3 n r D B (c¢ - c~) ~rr/4

The conservation of lithium atoms during a radius increase d r of the nucleus gives: (6)

igdt = 4~rrZnrgBdr - 2~rr2nF%dr

The second term on the right represents the quantity of lithium already present in the a-phase (see Fig. 5). Using the expression for ig deduced from the interdiffusion flux (5) and integrating it, gives 3D¢(c B - c~)(t-

r 2 =

~

-

~ - ) / 4 ( 2 ~ - %)

(7)

LiCI + KCI

,xs

-

/ / / Fig. 5. Schemeof the mechanism of growth of a nucleus of fl-LiA1.One important feature of this phase transition is the considerableincrease in the unit cell dimension [15]. (from 0.405 nm for the a-phase to 0.637 nm for the r-phase).

349

It is considered that the growth of nuclei starts at the time ~-; then eqn. (5) becomes:

Ig= ernFN[3/4DB( cB

_

.

,)1/2

c~)] 3/2 ( t - r ) ' / 2 / ( 2 ? B - c~

(8)

This equation indicates that the rising part of the curve I s = f(t) can be represented by the expression:

1~ = p ( t - "c)1/2

(9)

The current Ig is deduced from the experimental current I by substracting the current due to the lithium diffusion in the remaining a-phase (calculated from Cottrell's law with D~ = 8.3 × 10 -1° cm z s-l). The corresponding curves are shown in Fig. 4. The values of p and ~- are calculated for each potential step by a least squares method. The theoretical curves drawn in Fig. 4 (dashed lines) fit correctly the first part of the experimental curves. Now from eqn. (8) it is possible to deduce the values of the number of nuclei N; the value of DB is known from previous experiments [3]; other values deduced from electrochemical transient techniques [2] should be corrected to take into account the displacement of the electrode-electrolyte boundary during the electrolysis [12b]. Moreover, it has been recently suggested [12c] that the increase of D~ on the lithium deficient side of the B-phase can be attributed to structural heterogeneities near the a-fl transition region (grain boundary diffusion, or large values of the partial molal volume of lithium). In the model developed here the interdiffusion occurs in the bulk of the fl-LiA1 crystals and DB --- 10 -6 cm 2 s -1 has been taken as a reasonable value. The concentrations are deduced from the values of the potential (eqn. 2) corrected for the ohmic drop.

Nucleus creation We have just examined the growth of nuclei. We now study briefly the nucleation process which occurs during the first moments of the potential change. In Fig. 4 it is seen that at the beginning of the pulse the current can be attributed to the nucleation process. The nucleation rate is given by Volmer's equation [11]:

d N / d t = kS exp( - B/~I2.)

(10)

This equation has already been confirmed experimentally for metal deposition in aqueous solutions [13]. Taking account of the relation ~1. = A / r (11) the current deduced from the nucleation rate (eqn. 10) is: 2 kA3 I = 3rrnFS~_3 ( 2 ? p - % ) exp(-B/~12.)

(12)

Moreover, by expressing that this current is used for the creation of N nuclei (radius r = A / y . ) we have:

I

= }~rnFN--T (2?a - c*)

(13)

350 We consider here that the duration of the nucleation process is ( r + t m ) / 2 where t m is the time corresponding to the minimum of the curves I = f(t) (Fig. 2). Then from eqns. (12) and (13) the values of the product k A 3 and B can be calculated by using the results for different values of 7,GALVANOSTATICPULSES As previously the first part of the curves reported in Fig. 1 can be attributed to the metal interdiffusion in the a-phase. Indeed the surface concentration of lithium can be deduced from the potential of the electrode. According to Sand's law this concentration should obey the relation: c = Ceq + ~-ff

(14)

As indicated in Fig. 6 this relation seems to be well obeyed at the beginning of the galvanostatic pulse. The slope of the curves of Fig. 6 gives a value of De = 8.3 × 10-10 cm 2 s-1 which is in good agreement with our previous determination [3]. When the potential of the alloy reaches the equilibrium value of the two-phase system ( a + / 3 ) a part of the current is then used for the creation of nuclei. The mean radius of the nuclei is given by eqn. (11), then eqn. (12) can be used to obtain the values of the product k A 3 and of the constant B.

103(C-Ceq)/(mol Li)crn -3

2

(mA)

.0~5

.lJO

.1'5

t'/21 ~/2 =

Fig. 6. Evolution of the surface concentration of the electrode (Ceq is the surface concentration at equilibrium) during the passage of current of constant intensity versus t 1/2. Same conditions as Fig. 1.

351 After the minimum of the potential, at the time tm, it is considered that the process is under the control of the growth of nuclei. The mechanism is similar to that described for the potentiostatic pulses. However now the potential is not constant and then the concentration ca of lithium at the solid-liquid interface varies and obeys eqn. (2). At time t the conservation of lithium atoms gives: I g ( t - - t m ) = 2 1 r N n F [ 2 ? a ( r 3 - ro3) - c*( r 3 - r03)]/3

(15)

At the time t m it is supposed that N nuclei of radius r0 have been created; this number remains constant. The current Ig can also be deduced from the interdiffusion process as given by eqn. (5): Ig = 3 nFNDI~( et~ - c~ )~rr

(16)

By eliminating Ig from eqns. (15) and (16) one obtains: co-c* r 3 _ 9,8"-'/~ _ n .2 ~~..E____~ - c* ( t - t m ) r - ro3 = 0

(17)

ro is given by eqn. (11) at the minimum of the curves (Fig. 1). Then the value of r is known at a given time t and the number N can be calculated from eqn. (16). As indicated previously we suppose that the N nuclei have been generated during the time interval t m - - t o ; t o is the time at ~/, = 0 , i.e. when the potential of the working electrode is 299 mV versus the lithium electrode. According to eqn. (10) we have: k = N/sf

tm e x p ( - B / ~ I 2, )dt

(19)

to

The above integration is done by Simpson's method; the values of ~/,(t) are deduced from the experimental curves of Fig. 1. Then the value of the constant A can be deduced, the product k A 3 being already known (eqn. 12). DISCUSSION The mechanism of the electronucleation has been previously studied in the case of the electrodeposition of metal [9] when a new metallic phase appears on an inert substrate. We examine here the case of a phase transition which occurs during the metal incorporation. As pointed out above this situation presents some analogy with the metal nucleation. The creation of nuclei of the new phase requires some energy and an increase of the electroactive surface is observed during the growth of the nuclei. However in the phase transition process two other important phenomena are detected. There exists first a supersaturation of the initial phase and then the growth of the nuclei is controlled by the metal interdiffusion into the nuclei. At the beginning of the lithium incorporation a large part of the current is due to the interdiffusion in the substrate. For a galvanostatic pulse the surface concentration obeys Sand's law as long as the lithium concentration remains lower than the

352 transition phase concentration (c~,i = 9.6 x 10 -3 mol cm-3). As shown in Fig. 6 the experimental curves are in good agreement with Sand's law (eqn. 14) with D~ = 8.3 x 10 -1° cm 2 s -1. In the vicinity of the phase transition region some deviation appears: a part of the current is now used by the nucleation process. The lithium interdiffusion continues with a decreasing current. A supersaturation of the a-phase appears; the higher the current the higher is the maximum lithium concentration; for a current i = 1.07 A c m - 2 the lithium concentration in the a-phase reaches the value (?max 12.4 × 10 -3 mol cm -3. Likewise when an abrupt change of the concentration is carried out, Cottrell's law is obeyed for potential jumps lower than the phase transition potential. In Fig. 3 are reported the curves I = f(t-1/2) deduced from the chronoamperograms of Fig. 2. In this case a part of the current is controlled by the nucleation process. At low overpotential the curves are nearly linear although an important contribution arises from the creation of nuclei. At ~/= 70 mV about 50% of the current can be attributed to the nucleation during the first part (decreasing current) of the curves. However the most significant effect comes from the growth of nuclei which consumes an important part of the electrical current. The time dependence of the electrochemical response brings some indications about the mechanism of the nucleus growth. Indeed different phenomena can be invoked to explain the experimental results: kinetics of the electrochemical reaction, diffusion of electroactive species in the electrolyte [14], ohmic drop in the electrolyte around the nuclei [15], and metal interdiffusion. The ion diffusion around the nuclei has often been considered during the metal deposition [9,14]. However here the lithium is a constituent of the electrolyte; its concentration is very high (Cci+ = 1.7 X 10 - 2 tool cm -3) and the contribution of the migration process is not negligible. When ionic diffusion in the electrolyte is the limiting step the current around the nuclei of radius r is given by [7]: =

Ig = 2,1rrNFOLi+CLi+ [1 -- e x p ( - ~cnF/RT ) ]

(20)

For example at ~/= 100 mV and t = 1.5t m we observe (Fig. 2) a current I = 68 mA. The overpotential corrected for the ohmic drop (R c = 0.42 ~2) is ~/c= 71 mV. According to the results reported in Table 1 we have: N = 9.6 × 108 and r = 4.9 x 10-6cm; at 420°C the value of DLi+ is 2.5 X 10 5 c m 2 S-1 [16]. Equation (20) gives Ig = 850 A which is far greater than the experimental results. It is clear that the lithium ion flux does not control the current. The electrolyte resistance R n around the nuclei is given by eqn. (4). At ~/= 100 mV it is found R n = 3.5 X 10 -6 ~ (with X = 1.38 ~2-1 cm -1 at 420°C [17]). This resistance is always negligible with respect to the resistance of the circuit (0.42 ~). The growth of nuclei in the new phase is controlled b y the interdiffusion process. F r o m N M R results [18] it has been considered that fl-LiA1 is a fairly ionic compound. AI is reduced and Li ÷ is incorporated for charge compensation. The part of the nucleus which is in contact with the electrolyte is a lithium rich region. The aluminium arrives from the a - f l interface. We suppose that the two surfaces are in equilibrium with the adjacent medium. The mean path of the interdiffusing metals

353 TABLE 1 Results deduced from potentiostatic experiments. Electrode surface S = 0.504 cm2; resistance of the circuit R c = 0.4 f~; equilibrium potential Eeq = 0.330 V vs. Li; composition Ceq = 7.4X10 -3 mol Li cm 3. Values of the nucleation constants (eqns. 8, 12, 13): B = 2.9 x 10 -3 V2; kA 3 = 1.9 X 10 -9 nuclei c m s - l ; A = 1 6 X 1 0 - 8 c m ; k = 4 . 5 × 1 0 1 1 nucleicm 2 s-1 Potential

Nucleation

Time at

Number of

At t = t m

At t = 1.5t m

step, ~/mV

time, r/ms

minimum current, tin/ms

nuclei, 10 -9 N

radius of nuclei, 106 r ° / c m

radius of nuclei, 106 r / c m

mean interdiffusion path, 104 8 / c m

surface coverage /%

60 70 80 90 100 110 120 130

146 58 26 18.8 14.2 10.0 7.5 6.1

172 75 41 24 18.3 13.7 11.2 9.6

0.16 0.26 0.31 0.65 0.96 1.0 1.1 1.2

9 7 6 4.5 4 3.7 3.6 3.4

11.2 8.7 7.6 5.4 4.9 4.6 4.5 4.3

7.2 4.7 3.5 2.7 2.3 2 1.8 1.7

6.5 6.2 5.6 6 7.2 7 6.7 7

= ( 2 D B t ) a/2 at the mean time t = 1.5t m is reported in Table 1. This distance is far greater than the dimensions of the nuclei which justify the linear dependence of the flow rate (eqn. 5). Some simplifying assumptions have been introduced to reach an analytical treatment of the electrocrystallisation process. The formation and the growth of nuclei have been artificially separated; however this is suggested for example by the curves of Fig. 4 where an abrupt change of the current is clearly visible at the end of the nucleation period. At that time it is observed that the radius of the created nuclei is roughly proportional to the thickness (2 Oa~') a/2 o f the penetration layer of the lithium atoms in the a-phase. Unfortunately it was not possible to observe the deposit during its formation. In Fig. 7 we give an electron scanning micrograph of the electrode after a potential pulse. Although the electrode was removed from the bath under potentiostatic conditions a restructuring of the deposit probably occurred. The mechanism of the electrocrystallisation studied here is different from that observed previously for the pure metal deposition where the diffusion of electroactive ions was the limiting step. In this latter case we have shown [9] that the duration of the nucleation was limited by the overlap of the ionic depletion zone around the nuclei (screening zone) [7,19]. This is not the case here. Indeed the depletion of electroactive species around the nuclei is negligible. The end of the nucleation period is correlated to the potential change due to the ohmic drop when the interdiffusion flux through the increasing amounts of/3-phase becomes very large; after a long time the lithium incorporation is controlled by the resistance of the circuit. In conclusion the model developed here interprets correctly the experimental observations. The main features of the chronopotentiograms and of the chronoamperograms are explained by the growth of nuclei which is controlled by the

354

Fig. 7. Scanning electron micrograph of nuclei of ]3-LiA1 on substrate of a-LiA1 after a potentiostatic pulse at ~/= 60 mV. Starting equilibrium potential E~q = 330 mV vs. Li electrode. On this photograph about 500 nuclei are identified. The nucleus density is 108 nuclei cm -2 which can be compared to the corresponding result of Table 1:3.2 × 108 nuclei cm-2 are found for ~ = 60 mV. m e t a l i n t e r d i f f u s i o n . T o d r a w s o m e q u a n t i t a t i v e i n t e r p r e t a t i o n s it h a s b e e n s u p p o s e d t h a t t h e n u c l e i h a v e a s p h e r i c a l s h a p e w h i c h is p r o b a b l y c o r r e c t o n l y a t t h e v e r y b e g i n n i n g o f t h e i r f o r m a t i o n . R a p i d l y t h e e l e c t r o c h e m i c a l p r o c e s s is u n d e r t h e control of the ohmic drop due to the resistance of the circuit. Volmer's classical law h a s b e e n u s e d to d e s c r i b e t h e n u c l e a t i o n p r o c e s s [11]. H o w e v e r as p o i n t e d o u t i n a n o t h e r p a p e r [9] t h e s u r f a c e p r o p e r t i e s o f t h e s u b s t r a t e c a n p l a y a n i m p o r t a n t r o l e in the nucleus formation which may explain the large difference observed between t h e v a l u e s o f t h e k i n e t i c c o n s t a n t k; f o r e x a m p l e t h e r e s u l t s r e p o r t e d i n T a b l e 2 a r e

TABLE 2 Results deduced from galvanostatic experiments. Electrode surface S = 0.234 cm2; resistance R = 0.42 f~; equilibrium potential Ecq = 0.660 V vs. Li; composition c = 0.7 X 10 -3 mol Li cm-3. Values of the nucleation constants (eqns. 12, 16, 19): B = 2.9X10 -3 V2?kA 3 = 7.5X10 -9 nuclei cm s - l ; A = 2.6X 10-8cm; k = 4 . 1 x l 0 1 4 nuclei cm -2 s -1 Intensity of the current,

Number of nuclei, 10 9 N

I/mA

Time at maximum overvoltage, tm/mS

At t = t m radius of nuclei, 106r0/cm

At t = 1.5 t m radius of nuclei, 106r/cm

surface coverage /%

40 65 90 115 150 200 250

26 12 6.2 4.3 3.3 2.3 1.8

2.5 7.8 9.0 11.8 25 26 112

1.4 1.0 0.8 0.75 0.65 0.69 0.54

2.8 1.7 1.5 1.4 0.97 1.0 0.65

6.2 7.0 6.1 6.8 7.8 9.6 15.0

-

355 o b t a i n e d f r o m a n e a r l y p u r e a l u m i n i u m e l e c t r o d e w h e r e a s t h o s e of T a b l e 1 c o m e f r o m a m o r e l i t h i u m rich e l e c t r o d e ; it s e e m s t h a t the l i t h i u m e n r i c h m e n t r e d u c e s t h e n u c l e a t i o n rate; h o w e v e r s o m e a d d i t i o n a l e x p e r i m e n t s a r e r e q u i r e d to r e a c h a m o r e definitive conclusion. ACKNOWLEDGEMENTS This work was supported by C.N.R.S. (France) under contract ATP PIRSEM. T h e a u t h o r wishes to t h a n k Dr. M a u r i n f r o m L a b o r a t o i r e d e P h y s i q u e des L i q u i d e s et E l e c t r o c h i m i e , U n i v e r s i t 6 P i e r r e et M a r i e C u r i e , for f r u i t f u l d i s c u s s i o n s a n d for his h e l p in p e r f o r m i n g s c a n n i n g e l e c t r o n m i c r o g r a p h s . REFERENCES 1 2 3 4 5

6 7 8 9 10 11

12 13 14 15 16 17 18 19

A.K. Fischer and D.R. Vissers, J. Electrochem. Soc., 130 (1983) 5. C.J. Wen, B.A. Boukamp, R.A. Huggins and W. Weppner, J. Electrochem. Soc., 126 (1979) 2258. F. Lantelme, Y. lwadate, Y. Shi and M. Chemla, J. Electroanal. Chem., 187 (1985) 229. Y. Geronov, P. Zlatilova and G. Staikov, Electrochim. Acta, 29 (1984) 551. (a) F. Lantelme, J.P. Hanselin and M. Chemla, Electrochim. Aeta, 22 (1977) 1113; (b) M. Lassouani, Th6se de 36me cycle, Universit6 P. et M. Curie, Paris, 1984; (c) F. Lantelme, D. Inman and D.G. Lovering in R.G. Gale and D.G. Lovering (Eds.), Molten Salt Techniques, Vol. 2, Plenum Press, New York, 1984, p. 162. L. Redey and D.R. Vissers, J. Electrochem. Soc., 128 (1981) 2703. A.N. Baraboshkin, V.A. Isaev and V.N. Chebotin, Sov. Electrochem., 17 (1981) 397. Y.S. Fung, D. Inman and S.H. White, J. Appl. Electrochem., 12 (1982) 669. F. Lantelme and J. Chevalet, J. Electroanal. Chem., 121 (1981) 311. F. Lantelme and M. Chemla, Z. Naturforsch., 38A (1983) 106. E. Budevski, G. Staikov and V. Bostanov in J. Bourdon (Ed.), Growth and Properties of Metal Clusters, Elsevier, Amsterdam, 1980, p. 85; T. Erdey-Gruz and Volmer, Z. Phys. Chem., 157 (1931) 165. (a) W. Lorenz, Z. Phys. Chem., 202 (1953) 275; (b) K.B. Oldham and D.O. Raleigh, J. Electrochem. Soc., 118 (1971) 252; (c) F. Lantelme, J. Electroanal. Chem., to be published. R. Kaischew and B. Mutaftschiew, Z. Phys. Chem., 204 (1955) 334. G.J. Hills, D.R. Shiffrin and J. Thompson, Electrochim. Acta, 19 (1974) 657. G. Staikov, P.D. Yankulov, K. Mindjov, B. Aladjov and E. Budevski, Electrochim. Acta, 29 (1984) 661. F. Lantelme and P. Turq, Z. Naturforsch., 39A (1984) 162. E.R. Van Artsdalen and I.S. Yaffe, J. Phys. Chem., 59 (1955) 118. J.R. Willhite, N. Karnezos, P. Cristea and J.O. Britain, J. Phys. Chem. Solids, 37 (1976) 1073. B.R. Scharifker and J. Mostany, J. Electroanal. Chem., 177 (1984) 13.