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J. Electroanal. Chem., 94 (1978) 77--81 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

Preliminary n o t e

CYCLING BEHAVIOUR AND CORROSION OF Li--Al ELECTRODES IN ORGANIC ELECTROLYTES

J.O. BESENHARD

Institute of Inorganic Chemistry, Technical University of Munich, Liehtenbergstr. 4, D-8046 Garching (F.R.G.)

(Received 16th August 1978)

INTRODUCTION

Secondary Li-A1 alloy electrodes

charging A1 + x L i + + xe- --= ~ LixA1

~discharging

have already found some application as negatives in molten salt l i thium batteries, such as the Li--AI/FeS2 cell in LiC1/KC1 eutectic [1]. The main ad- vantage of these alloy electrodes over pure Li is that they are solid at operating temperatures of about 700 K; the disadvantage is a voltage "pena l ty" of about 0.3 V, corresponding to the formation energy of the alloy.

Formation of the intermetallic compound LiA1 at room temperature by electroplating of Li on A1 from propylene carbonate electrolytes was already demonstrated by Dey [2].

There are two main problems of metallic Li secondary electrodes in organic electrolytes: (i) deposition of Li from organic Li ÷ electrolytes yields very dendritic deposits, and (ii) the anodic stripping efficiency of electro- deposited Li, which typically is about 80% after immediate stripping, is very poor after prolonged wet stand in the electrolyte solution [3].

To minimize the dendrite problem, several attempts were made to replace Li by Li--A1 alloys [4--7], where Li-rich alloys were preferred [6,7] . A considerably improved wet stand behaviour of cathodically formed Li--A1 alloys as compared to Li deposited on inert substrates was already reported earlier [8,9]. In this paper, the dependence of the stripping efficiency on cycle number wet stand t ime and kind of electrolyte is discussed. The results are encouraging and practical secondary room temperature cells with Li--A1 negative, characterized by low self discharge rate, should be feasible.

EXPERIMENTAL

Propylene carbonate (PC) was fractionated in vacuum and stored over a 4 A molecular sieve, dimethyl sulfoxide (DMSO) was refluxed over CaHz and fractionated in vacuum. LiC104 and LiI were vacuum dried at 450 K for 60 h.

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The typical H20 content of 0.5 M electrolytes was about 30 ppm and was determined by gas chromatography of the solvents and Karl-Fischer ti tration of LiC104 and LiI.

RESULTS AND DISCUSSION

Due to its small size, t ransport o f Li ÷ in solid host materials is surprisingly fast and in general characterized by a low activation energy. Nevertheless, the rate of the cathodic Li--A1 alloy formation is limited by Li ÷ transport in the solid phase, as is demonstrated by comparison with the Li deposition on a Cu substrate, allowing much higher current densities in the same electrolyte (Figs. l a and b). There is a typical overvoltage at the beginning of the alloy forma- tion (Fig. la) , which decreases only slightly after prolonged cycling and seems to be much less pronounced in a hard A1--Cu-4 alloy (Fig. lc) .

In fact, incorporation of Li in AI--Cu-4 and in pure A1 starts at about the same potential vs. Li, but the reversible potential of the formed Li--A1 alloy is more negative in the case of A1--Cu-4. We attr ibute this behaviour to hindered Li transport in the A1--Cu-4 alloy, which causes the formation of Li-rich alloys with more negative potential on the electrode surface.

Typically, Li transport is fast in " so f t " host lattices, such as Mg [10], A1 [7], Tl, Sn and Pb. On the other hand, materials with " o p e n " structural ele- ments, such as the layer type materials Sb and Bi [11--13] show high mobil- ities for incorporated Li.

Galvanostatic cycling of Li--A1 electrodes obtained by cathodic incorpora- tion of Li in A1 yields only poor Li recoveries in the 1st cycle, as much of the

E u

E 10

-5

-10

-:15

(Q I •

-250 250

I

÷

75O }-250 0 25O -250 0 25O 75O

E / V v s L i / L i +

Fig. 1. Cyclic voltammograms in 0.5 M LiI/PC, v = 5 mV s -I . (a) AI 99.9; (b) Cu; (c) AI--Cu-4.

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® ' S _L-- i I I I

'°Sl , I , I

2;. i m I I

I , I 1 0 i 2 0

O . 1 2

'oS '°°S i __z ______j I , I I I , 2 0 1 2 0 1 2 0 1 2

Fig.2. Galvanostatic cycling in 0.5 M LiI/PC, i = 1 mA cm -~. (a) A1 99.9, 1st, 10th and 250th cycle; (b) Al--Cu-4, 1st, 10th and 100th cycle.

Li is retained in the host material. After a few cycles, however, this "retention capacity" is filled up and the current efficiency is practically 100% (Fig. 2a), i.e. exceeds even that of Li on a Li substrate [14]. Results with the A1--Cu-4 alloy show a much higher "retention capacity" in the first cycle (Fig. 2b) and, as expected, higher polarization losses.

Permanent cycling of A1 in Li ÷ electrolytes causes severe roughening and finally disintegration of the electrode. During this process, particles of Li--A1 and A1 lose contact with the substrate. On cycling excessive A1, a loss of Li--A1 as well as decomposition of solvent decrease the current efficiency; the loss of A1 can be detected by re-weighing the A1 substrate. For unprotected A1 rods, cycled at 1 mA cm -2 in 0.5 M LiC104/PC and 0.5 M LiI/PC we found the A1 loss per cycle to be 5--10% of the reactive A1 (assuming formation of the 1:1 LiA1 ~-phase). As the current efficiencies are considerably better, a good deal of the electrode material must be lost as A1. This agrees with X-ray investiga- tions of the electrode "sludge" which predominantly consisted of A1. There- fore, it is expected that disintegration of closely packed electrodes which do not allow "crumbling off" is considerably lower. High rate Li-incorporation into A1 results in Li.rich Li--A1 phases and finally in deposition of metallic Li on the surface. In this case, the cycling behaviour is similar to that of metallic Li on an inert substrate, i.e. the electrode is passivated by decomposition products and the cycling efficiency decreases dramatically.

An outstanding property of the cathodically formed Li--A1 is the corrosion stability in organic electrolytes. In Fig. 3a, 1st cycle Li recoveries after storage in LiI/PC (identical values were obtained in LiC1OJPC) are presented and compared with results on a Cu substrate (Fig. 3b). Due to the "retention capacity" of A1, 1st cycle Li recoveries are lower than those for the following cycles, however, 1st cycles were compared for reasons of experimental convenience.

The surprising stability of the Li--A1 alloy in PC electrolytes is not a result of the +0.3 V shift in the standard potential compared to Li/Li ÷. There are many examples where Li alloys or Li intercalation compounds (e.g. Li- amalgam [15], Li3As [11] and Li intercalated in graphite [16], which all have standard potentials of about +1 V vs. Li/Li ÷) show even higher reactivity vs. PC than metallic Li.

80

t_

90

80

70

60

50

o

° o o O0 0 0 0 0

O0 0 0 0 0 0 0

r~ 0

m {3

m [] {:3 [3 []

n [3

D

I [ I I ,

wet stond t i m e / d

90i

8o

L-

?O

60

50

b V

V V

V

V

V

V

~7

V

V

[ I 0 2'5 5

we t s tond t i m e / d

Fig. 3. Li recovery after wet stand in 0.5 M LiI/PC. (a) Li incorporated in AI 99.9, (o) charging and discharging at 1 mA cm -2, charge density 2700 mAs cm-2; (o) charging and discharging at 5 mA crn -2, charge density 13500 mAs cm -2. (b) Li deposited on Cu, (V) charging and discharging at 1 mA cm -2, charge density 2700 mAs cm-L

In fact, PC is not only thermodynamical ly instable vs. Li or Li-alloys, but it can be reduced at many metallic materials at about +1 V vs. Li/Li ÷ and at porous electrodes this reduction current frequently exceeds 1 mA cm -2, related to the geom. area [17]. In view of this the stability of Li--A1 is even more surprising when the large real surface area of the cathodically formed Li--A1 alloy (a bright A1 surface appears nearly black after incorporation of Li) is considered. A corrosion current of Li--Al in the order of only 10-2 pA cm-2 geom. surface results when the total of the efficiency losses presented in Fig. 3a are counted as "chemical corrosion", although a part of the losses will only be due to passivation.

In principle, this extremely low self-discharge rate of the Li--A1 alloy may be due either to a high overvoltage for the PC reduction on Li--A1 or to a protect ion by passivation. Because of the presence of reactive trace impurities and the reactivity of the polar solvents themselves we do not believe in the existence of a "clean" Li--A1 surface in any organic electrolyte. There are, however, obviously no passivating layers with high resistivity as there is no voltage delay ("Spannungssack") at the beginning of the reoxidation of the Li--A1 alloy. Moreover, protect ion by Li2CO3 films formed by reduction of PC, as it is assumed for metallic Li in PC electrolytes [18] is not an essential reason for the stability of the Li--Al alloy, since only slightly lower Li recoveries after wet stand were observed in DMSO electrolytes where the stability of deposits of metallic Li was extremely poor.

So far there are no indications for any indispensable effect of solvent or solute on the stability of the Li--A1 alloy in organic electrolytes. On the other hand, Li alloys of Hg, As and also Mg lose their capacity very fast on wet stand in organic electrolytes, i.e. general advantages of Li alloying (e.g.

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improved contact of Li with the substrate) cannot explain the extraordinary properties of Li--A1. Therefore we assume A1 to be an essential component of the corrosion protection.

H20 contaminations up to 100 ppm did not affect seriously the wet stand behaviour of Li--A1 and even after addition of 500--1000 ppm H20, Li recoveries of stored Li--A1 electrodes were much better than those of Li on inert substrates stored in electrolytes with less than 50 ppm H20.

These findings suggest gel-like solvated polymeric A13. compounds, analogue to AI(OH)3 condensates such as (A1OOH)n, on the Li--A1 surface. Those films would protect from further solvent attack and allow Li ÷ ion exchange and transport. Polymers of this type would be restricted to more than two-valent host metal cations which, in addition, cannot be reduced by the bulk electrode.

None of the above-mentioned metals with the exception of A1 would fulfill these requirements. The gel model is also supported by significantly decreasing Li recovery with increasing solute concentration. This might be explained as a "salting out" of the protective gel, although impurities and trace water introduced with the solute would produce a similar effect. Work is in progress to characterize reaction products on excessively stored Li--A1 electrodes.

A C K N O W L E D G E M E N T S

The author is grateful to Prof. Dr. H.P. Fritz for kindly supporting this work and to Prof. Dr. G. Winkhaus, Vereinigte Aluminium Werke, for sup- plying various A1 samples.

R E F E R E N C E S

1 C.A. Melendres, J. Electrochem. Soc., 124 (1977) 650. 2 A.N. Dey, J. Electrochem. Soc., 118 (1971) 1547. 3 R, Selim and P. Bro, J. Electrochem. Soc., 121 (1974) 1457. 4 Get. Often. 1 935 943 (1970). 5 Get. Often. 2 262 660 (1973). 6 U.S. Pat. 4 002 492 (1977). 7 B.M.L. Rao, R.W. Francis and H~A. Ch~_stopher, J. Electrochem. Soc., 124 (1977) 1490. 8 J.O. Besenhard, J. Electroanal. Chem., 78 (1977) 189. 9 J .R.v . Beek and P.J. Rommers, l l t h In ternat ional Power Sources Symposium, Brighton, 1978.

Abstr. 87. 10 MoM. Nichoison, J. Electrochem. Soc., 121 (1974) 734. 11 J.O. Besenhard and H.P. Fritz, Electrochim. Acta, 20 (1975) 513. 12 W. Weppner and R.A. Huggins, J. Electrochem. Soc., 124 (1977) 1569. 13 W. Weppner and R.A. Hugglns, J. Solid State Chem., 22 (1977) 297. 14 R.D. Rauh, T.F. Reise and S.B~ Brummer, J. E l ec t ~chem. Soe., 125 (1978) 186. 15 F.P. Dousek, J. Jansta and J. P~ha, J. Electroanal. Chem., 46 (1978) 281. 16 J.O. Besenhard and H.P. Fritz, J. Electroanal. Chem., 53 (1974) 281. 17 G. Eichinger, J. Electroanal. Chem., 74 (1976) 183. 18 J.O. Besenhard and G. Eichinger, J. Electroanal. Chem., 68 (1976) 1.