Passivation behavior of AB5-type hydrogen storage alloys for battery electrode application

Journal of

ALLOYS

ANDC£~M~L~ID5 ELSEVIER

Journal of Alloys and Compounds 221 (1995) 284-290

Passivation behavior of ABs-type hydrogen storage alloys for battery electrode application F. Meli, T. Sakai *, A. Zfittel, L. Schlapbach Institute of Physics, University of Fribourg~ CH-1700 Fribourg, Switzerland

Received 12 September 1994; in final form 26 October 1994

Abstract

In many applications, AB5 type hydrogen storage alloys show passivation behavior, i.e. when fully discharged, metal hydride electrodes show (especially at higher temperatures) a decrease in activity and therefore a decrease in capacity at normal discharge currents for ensuing cycles. Passivation may continue to the point where activity becomes so low that the capacity is no longer accessible. Electrochemical measurements were taken of two different ABs-type alloys, one with manganese and one without manganese (LaNia.aCol.2mlo.4 and LaNi3.4Col.2Alo.3Mno.~).Both alloys showed passivation behavior after remaining in the discharged state. The alloy with manganese showed a stronger tendency to passivation which is in contradiction with earlier observations. Photoelectron spectroscopic analysis together with sputter depth profiling was used to investigate the surface composition of samples which had undergone different surface pretreatments. Surface analysis of electrodes in the passivated state shows a lower content of metallic nickel and a thicker nickel surface oxide film. We attribute the low electrochemical kinetics of the alloys after passivation to the loss of metallic nickel and/or cobalt at the electrode-electrolyte

interface. Keywords: Passivation behavior; Hydrogen storage alloys; Battery electrode application

1. Introduction

The production of cadmium-flee rechargeable batteries is increasing rapidly [1,2]. In these new batteries cadmium is replaced by an electrochemically stable metal hydride. Hydride-forming intermetallic compounds of the LaNi5 family have a high activity and a high charge-discharge rate capability. At present production has concentrated on small cells up to 4 Ah capacity. Some prototypes of larger cells for electric vehicle application have been built [3]. In large prismatic closed cells for electric vehicles (small surface to volume ratio), a considerable increase in temperature can occur at the end of charging and especially during overcharging [4]. This is because the energy supplied to the batteries is no longer stored but is dissipated as heat. The temperature also increases during discharge at high currents owing to joule heating. In high voltage stacks, with many cells in series, passivation of the negative metal hydride electrode can occur owing to cell reversal * Current address: Osaka National Research Institute, Midorigaoka 1-8-31, Osaka 563, Japan.

0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)01464-7

at the end of discharge or to high self-discharge at higher temperatures. This results in a considerable loss in capacity of the battery. Passivation means a decrease in activity and therefore a decrease in capacity at normal discharge currents. This decrease in activity is related to the surface properties of the intermetallic metal hydrides used. Influence of the electrode preparation and the alloy composition on the passivation behavior has been reported by Sakai and coworkers [3,5]. They examined the behavior of metal hydrides at 40 °C with changing alloy composition and conducting material in MH-limited Ni-MH prismatic cells. Significant differences between the electrodes were found when the electrodes were kept in the discharged state, i.e. after releasing hydrogen completely from the electrode. When the MmNi3.~COo.TAlo.8 alloy with high corrosion resistance was used, a 10 wt.% nickel powder-mixed electrode showed a serious capacity decay after a 3 day standing with a potential increase up to 0 Vvs. NiOOH, suggesting oxidation of the nickel surface (passivation). This serious passivation was prevented by mixing 10 wt.% cobalt powder instead of nickel powder, probably because the cobalt had a lower oxidation potential than the nickel.

F. Meli et al. / Journal of Alloys and Compounds 221 (1995) 284-290

Manganese in the alloy also prevented passivation. The MmNia.sCoo.sMno.4Alo.3 alloy which had a high pulverizing rate and high rate of dissolution of manganese from the alloy did not show such passivation even for the nickel powder-mixed electrode, and it showed no potential increase. It would be of interest to investigate the surface composition of alloys with and without manganese in the active and in the passive states to obtain more information about the passivation processes involved. We report electrochemical measurements and X-ray photoelectron spectroscopy (XPS) surface analysis of LaNi3.4Cox.2Alo. 4 and LaNi3.4CoL2Alo.3Mno. 1 after different pretreatments. We chose these two model alloys instead of the usual mischmetal-containing alloys to facilitate XPS analysis.

2. Experimental details The alloys were prepared by arc melting under an argon atmosphere as described previously [6]. The crushed lumps were ground to an average grain size of about 100 /~m. These samples are referred to as "original alloys". For the electrochemical cycle life and passivation measurements, approximately 27 mg active material were mixed with copper powder (Merck p.a.) in the weight ratio 1:3 and pressed to a pellet ( d = 7 mm, p = 5 x 108 Pa). To obtain a copper-free pellet for surface analysis with XPS, about 70 mg active material was pressed into a high void nickel foam and clamped between two fine nickel meshes. The electrodes were electrochemically charge--discharge cycled with a computer-controlled current source in a 6 M KOH electrolyte at 40 °C in an open halfcell. All electrochemical experiments were performed at 40 °C, only the two samples which were surface analyzed after 30 electrochemical cycles were cycled at room temperature. The nickel counter electrode was placed in a separate compartment of the cell. The current was 300 mA g-~ for charge and discharge. The discharge limit was set to - 0.6 V with respect to an Hg/HgO/6 M KOH reference electrode. Additional discharges with a 10 times smaller current (30 mA g-1) were performed every 10th cycle. Pulsed charging and discharging was used to measure equilibrium curves which were converted into "electrochemical PCT curves" as described previously [7]. For surface analysis, electrochemically cycled samples were rinsed with twice distilled water, dried in rough vacuum, pressed to a pellet in air and introduced into the ultrahigh vacuum of the photoelectron spectrometer. The relative concentration of the elements and their chemical state at the surface were analyzed with XPS as described previously [8].

285

3. Results and discussion

3.1. Electrochemical measurements 3.1.1. General properties Electrochemical charge-discharge cycling in open halfcells at 40 °C demonstrates a maximum capacity of 350 mAh g-1 for the LaNi3.4Co12A10.3Mno.l alloy and 320 mAh g-1 for the LaNi3.4Col.2Alo. 4 alloy (Fig. 1). The manganese-containing alloy has a higher capacity • and better high rate discharge ability, which can be seen by the smaller capacity difference between discharges with 300 mA g- 1 and cycles with an additional discharge at 30 mA g-1. The additional capacity obtained with the 30 mA g-1 discharge current increased with increasing cycles, i.e. with increasing capacity loss because the oxidized surface layer disturbs the electrochemical reaction. The internal resistance of the half charged electrodes was measured at 40 °C. It was 3.7 II for the LaNi3.4COl.2Alo.4 alloy and 2.9 f~ for the LaNi3.4CoL2Alo.3Mno.1 alloy. If it is assumed that the reaction resistance contributes to the major part of these values then according to the Butler-Volmer approximation for small overpotentials [9,10], the exchange current densities are 262 mA g-I and 357 mA g-1 respectively. The capacity decrease on cycling was slightly smaller for the alloy without manganese. Self-discharge was measured after at least 20 activation cycles. Both alloys showed the same self-discharge of 19% after 3 days and 45% after 5 days at 40 °C. Electrochemically measured PCT curves for charge and discharge at 40 °C (Fig. 2) confirm the higher capacity of the manganese-containing alloy. The plateau pressures were the same for both alloys. It was well below 1 bar which is suitable for a closed cell application. 400 o~ °

cc'~c~°

~

o

o

'-~ 3 0 0 ¢..-

<

E __~ 2 0 0 U o~ e3 o~

u

100-

O-

r

0

,

,

I

20

'

'

'

I

'

40

'

'

I

60

'

o

with Mn

•

no

Mn

I

80

100

n [cycles]

Fig. 1. Discharge capacity vs. cycle number of LaNia.4Col.2Alo.4 (Q) and LaNi3.4Co,.2Alo.3Mno.t (O) at 40 °C. Charge and discharge currents were 300 mA g - ' . Every 10th cycle an additional discharge with 30 mA g-~ was performed.

F. Mefi et al. / Journal of Alloys and Compounds 221 (1995) 284-290

286 10

oo00

3d delay after discharge

400 •

r-

< •

0

Q.

u

0.1

O •0

•

o

• o

I

J

100

0

o

1

200

u

o 0.01

" ~ °

0

•

00 O 0 0

•

300

E

0

..Q

5d delay a;er discharge

o

I

I

I

charge discharge

0-

I

2 3 4 hydrogen conc. [H/ABs]

5

I

I

10

20

I

30

I

I

40

50

60

n [cycles] 1.0-

10

o

o~ -i-

% % •

.-

0.1 •

• • o o o

o

..-""

• o

0.8-

>

0.7-

N 'E

0.6-

0.5• o

©

I

0

ocooo ooo

o

o

0.01

°°ooooo o~coooo ~

>

..Q t~

0.9-

"l-

It3

1

charge

0.4-

discharge

f

I

I

I

2

3

4

5

6

hydrogen conc. [H/AB5] Fig. 2. Electrochemically measured PCT curves at 40 °C for LaNii4Col.2Alo.4 (top) and LaNii4Co~.2Ale3Mno, (bottom) for charge ( 0 ) and discharge (O).

The manganese-containing alloy showed a smaller hysteresis, however the faster kinetics of this alloy could also partly contribute to this effect. For both alloys, scanning electron micrographs after cycling indicate the same specific surface area. The typical particle size is 5-15 /zm. Most particles have several surface cracks.

3.1.2. Passivation experiments: After approximately 25 and 35 cycles the electrodes were discharged with a current of 300 mA g- t to - 0.6 V vs. Hg/HgO and left at open circuit for 3 and 5 days respectively (Figs. 3 and 4, upper graphs). With the first cycle after the delay of 3 days the manganese containing alloy showed a strongly decreased capacity while the alloy without manganese showed no decrease. After a delay of 5 days both alloys showed a strong capacity decrease on the next cycle. In the ensuing cycles the capacity was slowly regained, which indicates that the capacity decrease was due to kinetic limitations, i.e. passivation, and not due to a corrosion process. The passivated surface recovered to nearly full activity within the next 10-20 cycles. For the manganese-free alloy the rest potential became in the range of -0.87

A ©

I

I

10

20 n

I

I

30

40

after charge after discharge I

50

0

[cycles]

Fig. 3. Passivation behavior of LaNi34Co1.2Ala.~ at 40 °C. The upper graph shows the capacity vs. cycle number and the lower graph shows the rest potentials after charge and after discharge vs. cycle number; ~ potentials after the long delays after discharge; • cycles with additional low current discharge (30 mA g-t).

to -0.85 V vs. Hg/HgO just after discharge to - 0 . 6 V vs. Hg/HgO with 300 mA g-1 (Fig. 3 lower graph). The additional low rate discharge at 30 mA g-1 gave an additional capacity of 20 mAh g - l , increasing the rest potential to around -0.76 V vs. Hg/HgO. When the electrode after discharge with 300 mA g-1 was held for 3 days at the rest potential, it also increased to - 0 . 7 6 V causing a small decrease in capacity (about 30 mAh g-1) for the next cycle. A prominent change was observed for the 5 day delay. The rest potential increased significantly to -0.56 V vs. Hg/HgO causing a rapid decrease in discharge capacity for the next cycle. Almost the same behavior was observed with the manganese-containing alloy (Fig. 4 lower graph) but the rest potential increased more to about -0.84 V vs. Hg/HgO after the 300 mA g-1 discharge which indicates a more complete discharge (lower hydrogen content) due to the faster kinetics of this alloy. A smaller additional capacity of 10 mAh g - 1 was obtained with the 30 mA g-1 discharge which resulted in a rest potential of -0.75 V. The 3 day delay was already sufficient to cause a potential increase to -0.46 V

F. Meli et aL 1 Journal of Alloys and Compounds 221 (1995) 284-290

3d delay after discharge

400 t-" ~" < E

300 -

~

200-

5d delay after discharge

c~

°°~°°• °m~c~°•

o ~

•

oO

/

287

entire sputtering depth, whereas nickel as well as cobalt became metallic in a depth between 4 /~ and 60 /~. For both alloys and all pretreatments cobalt showed only weak concentration changes which resembled the behavior of nickel. The sputter rate for depth profiling was about 4 /~ min-i.

oo

3.2.1. Original and cycled samples L)

100-

O-

I

I

I

I

I

10

20

30

40

50

60

n [cycles] 1,0-

0

0.9-

~

eoooco

'-ro) -r"

cA >

0.8-

>

0.7-

'E

0.6-

Fig. 5 shows the Ni, Co and La atomic concentrations of the alloys LaNia.4foz.2Alo. 4 and LaNia.4COl.2AIo.3Mno. 1 as a function of sputtered depth. The plots are for the alloys in the "original" state and for alloy powders which were electrochemically charge-discharge cycled 30 times (at room temperature). Both "original" samples showed the same strong lanthanum enrichment at the surface. At a sputtered depth of approximately 16/1,, the nickel to lanthanum ratio for both "original alloys" was only 0.6 (see Fig. 6). 4.0~

3.5-

tO

3.0-

0

0.50.4-

A 0 I

I

10

20

I

I

30 40 n [cycles]

2.0

I

50

60

Fig. 4. Passivation behavior of LaNi3.4col.2Alo.aMno.l at 40 °C. The upper graph shows the capacity vs. cycle number and the lower graph shows the rest potentials after charge and discharge; potentials after the long delays after discharge; • cycles with additional low current discharge (300 and 30 mA g-I).

followed by a significant lowering of the discharge capacity for the next cycle. The rest potentials after discharge and delay (i.e. in the passivated state) increased to about - 0.5 V vs. Hg/HgO. At this potential nickel is already oxidized according to [11]: Ni + 2OH- ~

2.5-

after charge after discharge

z

1.5

.~,

2.5

._o

2.0

E°

1.5

-D- No Mn

--o- with Mn --=-- No Mn cycled -4- with Mn cycled

1.0 o

0.5-

Ni(OH)2 + 2e2,5-

at -0.758 V vs. Hg/HgO at 25 °C With ensuing cycles following passivation, the rest potentials after discharge remained at a lower level owing to the incomplete discharge in the passivated state as shown in Figs. 3 and 4.

3.2. Surface analysis Using XPS we analyzed the surface of powder pellets of the two alloys in the "original" state, after 30 electrochemical cycles performed at room temperature and also after 30 electrochemical cycles with an additional delay after the last discharge for 3 days at open circuit (performed at 40 °C). For all analyzed samples the lanthanum was oxidized throughout the

t,J -~=

2.0-

l

1.5-

¢~

1"0 t 0.5 0.0

I

0

I

I

I

I

50

1O0

150

200

approximate depth [A] Fig. 5. XPS sputter depth profiles which show the Ni, Co and La atomic concentrations for LaNi~.4Co1.2AIo., (17 I1) and for LaNi3.,Cot.2Alc.3Mno.t (O • ) . The open symbols are for the "original alloy" and the closed symbols are for alloy powders which have been electrochemically charge-discharge cycled 30 times (at room temperature).

288

F. Meli et al. / Journal of Alloys and Compounds 221 (1995) 284-290

The increased nickel content and the thinner nickel oxide of the manganese-containing alloy correlate with its better kinetics, i.e. measured higher rate capability and higher exchange current density. The nickel oxide thickness was in the same range as found earlier for other cycled alloys, i.e. LaNi4.TA10.3(10 /~), LaNis(20 ~) and LaNi4.sSi0.5(4 /~) [12,13]. After cycling some lanthanum was dissolved into the electrolyte and a nickel-rich layer was left over with the remaining nickel probably in the form of superparamagnetic clusters [14].

3.02.5, 2.0, .o E

1.5,

z

--o-0.5,

after cycling:

" "

Iloy

~ Mn in alloy .-A- no Mn in alloy

0.0, '

'

'

0

'

I

'

'

'

'

50

I

'

'

'

'

100

I

'

'

'

150

'

200

approximate depth [A] Fig. 6. Ni to La ratio derived from XPS sputter depth profiles after hydrogen gas activation ("original alloy") and after 30 charge-discharge cycles (at room temperature).

IMg~l i

INi 2P,~I

3.2.2. Passivated samples Figs. 8 and 9 show the XPS sputter depth profiles for La, Ni and Co of LaNia.4COl.2Alo. 4 and LaNi3.4Col.2ml0.3Mn0.1 samples after undergoing 30 charge-discharge cycles and an additional delay after discharge at open circuit for 3 days (40 °C) in the nickel foam type electrodes. The results of corresponding cycling experiments with the copper-containing elec-

sputter 4-

4 rain 1 min

a)

t~ ----0

3-

0 rain

©

-(3-- Ni La Co

E

o 03

o¢

2-

.0

.< t-

E

1

--°----'--~-------

D

G.)

I

0 i

i

~

i '

~

il

:!. i :

100

4 min 0 rain

.... I .... I .... t .... I .... 890 885 880 875 870 865 Binding Energy [eV] Fig. 7. X-ray photoelectron core level spectra showing the Ni 2p peaks after different sputtering times, (a) for the "original alloys", (b) and (c) for alloy powders which have been electrochemically charge-discharge cycled 30 times at room temperature; (b)

I

I

I

200 300 400 approximate depth [A]

I

500

600

Fig. 8. XPS sputter depth profiles for La, Ni and Co of LaNi3.4Cot.xAlo.3 after 30 charge-discharge cycles and thereafter an additional delay after discharge at open circuit for 3 days (40 °C).

4 - O - Ni - 0 - La --C}-- Co

'- 3

2

LaNia.4foL2Al~.aMno.i, (c) LaNi3.4foL2Alo.4.

After electrochemical cycling, nickel was enriched in a subsurface layer. The nickel to lanthanum ratio for the cycled alloys at a sputtered depth of approximately 16 /~, was 1.6 and 2.4 without and with manganese respectively (Fig. 6). Furthermore, the nickel oxide surface layer was thinner on the manganese-containing alloy, 4 / ~ compared with 16/~ (Fig. 7, 10) being the prominent difference between the two different alloys.

0

I

I

0

100

I

I

I

200 300 400 approximate depth [A]

I

I

500

600

Fig. 9. XPS sputter depth profiles for La, Ni and Co of LaNi3.4Col.2A10.4Mno,i after 30 charge-discharge cycles and thereafter an additional delay after discharge at open circuit for 3 days (40 °C).

289

F. Meli et al. / Journal of Alloys and Compounds 221 (1995) 284-290

100-

~

50-

U

0

I

0

20

I

I

40 60 approx, depth [A]

I

80

100

Fig. 10. Content of metallic nickel as percentage of the total nickel (oxidized plus metallic) as a function of the sputtered depth for LaNi3.4Cot.2AIo.4 and LaNi3.4cot.2Alo.3Mno.t each not cycled, cycled and "passivated", i.e. after 30 electrochemical cycles and thereafter a 3 day delay after discharge (40 *C).

trodes indicate that the alloy with manganese showed considerable passivation while the alloy without manganese showed no passivation. The rest potentials after the 3 days delay for the copper-free compacted eleco trodes for the XPS analysis, however, were -0.731 V for the alloy without manganese and -0.659 V vs. Hg/ HgO for the alloy containing manganese. So for both alloys nickel is expected to be oxidized at the surface and both samples should be passivated to some extent. Surface analysis revealed for both alloys a significant potential-induced enrichment of nickel oxide and a much lower lanthanum oxide content down to a depth of 15 /~ (Fig. 8 no manganese, and Fig. 9 with manganese). In the depth range 20-400 A the manganesecontaining alloy had more lanthanum oxide which led to a lower total nickel content than the alloy without manganese. The thickness of the nickel oxide (Fig. 10) increased strongly to more than 60/~ for both alloys. The nickel oxide layer was thickest for the passivated samples, followed by the "original" samples, and the most thin for the cycled samples. The cobalt oxide layer thickness was the same for each sample as the nickel oxide layer thickness. Cobalt oxides are reported to have a high electrical conductivity [15] and, furthermore, they are soluble in concentrated KOH solutions [4]. Therefore, the electrode passivation was probably caused by the surface oxidation of nickel.

4. Discussion

The small amount of manganese in the manganesecontaining alloy induced significant changes in the alloy properties. The manganese increased the capacity and the rate capability (better kinetics). XPS analysis of the alloys after cycling revealed that the manganese-

containing alloy had a higher content of metallic nickel at the surface which works as a good electric conductor and catalyst. Keeping electrodes in the discharged state at 40 °C for a few days results in a passivation of the alloy surface which reduces the capacity considerably for the ensuing cycles. XPS depth profiles showed that both alloys have a lower content of metallic nickel and a thicker nickel oxide film at the surface after passivation. The cobalt oxide layer thickness was the same for each sample as the nickel oxide layer thickness. The electrochemical experiments demonstrate that passivation occurs only if the rest potential after discharge rises above -0.758 V. Therefore, the passivation is due to the following reaction [11]: Ni + 2 O H -

~ Ni(OH)2 + 2e at -0.758 V vs. Hg/HgO

The smaller electrocatalytic activity and surface conductivity of the alloy grains due to oxidation of the nickel caused the lower discharge capacity for the next cycle. Once the surface is passivated, it does not recover so easily to the original state because of the poor conductivity of Ni(OH)2. For cobalt the corresponding reaction is [11] Co + 2 O H -

, Co(OH)2 + 2eat -0.828 V vs. Hg/HgO

This reaction occurs earlier during discharge and causes no passivation. Cobalt oxides are reported to have a high electrical conductivity [15] and, furthermore, they are soluble in concentrated KOH solutions [4]. After a delay of 3 days after discharge, LaNi3.4Col.2Alo.4 showed no passivation while LaNi3.4Col.2Alo.3Mno.1 was passivated. Although the measured self-discharge was the same for both alloys, the potential while remaining in the discharged state increased faster to more positive values for the manganese-containing alloy. The higher rate capability of the manganese-containing alloy results in a smaller hydrogen content of the alloy after the discharge with 300 mA g- 1 and therefore in a faster potential increase and nickel oxidation. This would explain higher passivation rate of the manganese-containing alloy with faster kinetics. It was reported previously that the alloy MmNi3.sCo0.5Alo.3Mno.4 showed higher resistance to passivation than the alloy without manganese MmNi3.sCoo.7Alo.8 [3,5]. In this case the manganesecontaining alloy was able to keep the hydrogen potential after keeping the electrode in the discharged state for more than 3 days, while the alloy without manganese could not keep the potential, causing passivation. The low cobalt and the high manganese content in MmNi3.sCoo.5Alo.3Mno.4 would facilitate pulverisation

290

F. Meli et al. / Journal of Alloys and Compounds 221 (1995) 284-290

leading to a continuous oxidation of rare earth elements, aluminum, manganese and cobalt from the alloy, preventing the surface oxidation of nickel. In this work, the alloy LaNi3.4Col.2Alo.aMno. 1 has a much higher cobalt content and much lower manganese content, i.e. much higher corrosion resistance, not giving enough effect to prevent the oxidation of nickel.

Acknowledgement We gratefully acknowledge financial support by the Swiss Department of Energy (BEW).

References [1] T. Sakai, H. Yoshinaga, H. Miyamura, N. Kuriyama and H. Ishikawa, J. Alloys Comp., 180 (1992) 37. [2] (3. Sandrock, S. Suda and L. Schlapbaeh, Applications. In L. Schlapbach (ed.), Hydrogen/n IntermetaUic Compounds 11, Topics in Applied Physics, Vol. 67, Springer, Berlin, 1992, Chapter 5.

[3] T. Sakai, H. Miyamura, N. Kuriyama, 1. Uehara, M. Muta, A. Takagi, U. Kajiyama, K. Kinoshita and F. Isogai, Z Alloys Comp., 192 (1993) 158. [4] T. Sakai, M. Muta, H. Miyamura, N. Kuriyama and H. Ishikawa, Electrochem. Soc. Proc., 93--8 (1993) 240. [5] T. Sakai, H. Miyamura, N. Kuriyama, H. Ishikawa and I. Uehara, Z. Phys. Chem. N. F., 183 (1994) 333. [6] T. Sakai, A. Takagi, K. Kinoshita, N. Kuriyama, H. Miyamura and H. Ishikawa, J. Less-Common Met., 172-174 (1991) 1185. [7] A. Zfittel, F. Meli and L. Schlapbaeh, Z. Phys. Chem. N. F., 183 (1994) 355. [8] F. Meli, A. Ziittel and L. Schlapbach, J. Alloys Comp., 202 (1993) 81. [9] C. Hamann and W. Vielstich, Elektrochemie I, taschentext, Verlag Chemie, Weinheim, 1981. [10] A. Ziittel, F. Meli and L. Schlapbach, J. Alloys Comp., 206 (1994) 31. [11] Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1980, 60th edn. [12] F. Meli and L. Schlapbach, J. Less-Common Met., 172-174 (1991) 1252. [13] F. Meli, A. Ziittel and L. Schlapbach, J. Alloys Comp., 190 (1992) 17. [14] L. Schlapbach, J. Phys. F, 10 (1980) 2477. [15] M. Oshitani, H. Yufu, K. Takashirna, S. Tsuji and Y. Matsumara, J. Electrochem. Soc., 136 (1989) 590.