Dimensionally stable Li-alloy electrodes for secondary batteries

Solid State Ionics 40/41 (1990) 525-529 North-Holland

DIMENSIONALLY STABLE Li-ALLOY ELECTRODES FOR SECONDARY BATFERIES J.O. BESENHARD, M. HESS and P. KOMENDA Anorganisch-Chemisches Institut, Universitiit Miinster, Wilhelm-Klernm-Strafle 8, D-400 Miinster, Federal Republic of Germany

Dimensionally stable Li-alloycompositeelectrodes for rechargeable organicelectrolytebatteries were made up by compression of a mixture of small particle size Cu- and ~-LiAl-powders.SEM studies demonstrate that the reinforcingCu frameworkis able to sustain the stresses due to the volumechanges of the active LiAIcomponent. The electrochemical polarization behavior of these composite electrodes is only slightlypoorer than that of pure I~-LiA1.

I. Introduction As the use of pure lithium electrodes in organic solvent-based rechargeable batteries is associated with severe problems, above all dendrite growth and corrosion [ 1-5 ], lithium alloy negatives are attractive alternative materials [6-9 ]. For many applications the obvious disadvantages of Li-alloy electrodes compared to lithium electrodes, i.e., lower rate capabilities and lower energy densities, are balanced by their long term storage and cycling behaviour. Moreover, room temperature mobilities of lithium in many of these alloys are quite high [9-16] and the voltage "penalty" for alloy electrodes is only a few hundred mV. The main problem of Li-alloy electrodes are connected with the significant differences in volume between the Li-alloys and the pure basis metals. Therefore, charge/discharge-cycles induce mechanical stresses, cause cracks and finally the materials get crumbly. This is, of course, in particular true for thick electrodes. Thin layers of Li-alloys, corresponding roughly to 50-100 C cm -z [8] may be cycled with high efficiencies. There have been various attempts to avoid or at least to reduce the deterioration of Li-alloy electrodes in organic electrolytes. Most of this work was focussed on AI/13-LiAI electrodes because with regard to energy density, aluminium is a highly attractive basis material for alloy electrodes. On the other hand, aluminium is a very difficult basis material because it is highly reactive ad freshly created 0167-2738/90/$ 03.50 © ElsevierScience Publishers B.V. ( North-Holland )

aluminium surfaces are corroded and passivated in contact with organic electrolytes. Thin-film aluminium-based electrodes have been proposed by several groups of workers [ 17-19 ]. Silicon-rich aluminium electrodes were shown to last longer than high purity aluminium electrodes in cycling experiments in LiC104/propylene carbonate electrolytes. This was due to a reinforcement of the reactive aluminium by silicon crystallites [20,21]; the silicon component only very slowly alloys with lithium at room temperature. A patent was granted to aluminium-copper composite electrodes prepared by vapour phase deposition of thin layers of both metals [22]. The use of basis metals or basis metal alloys which are relatively soft and less reactive versus organic electrolytes than aluminium, e.g. Wood's metal [23 ] or indium [ 24,25 ] also reduces the problems related with crumbling and passivation of Lialloy electrodes during cycling. The same is true for insertion of lithium into graphitic carbons [26,27 ] because in this process the volume changes between charged state (LiC6) and discharged state (pure carbon) are extremely small 9.4%. All-solid lithium electrodes with mixed-conductor matrix, i.e., compact composite electrodes in which a reactant (e.g. LixSi) is finely dispersed in a solid mixed conducting matrix (e.g. LixSn) have been proposed by Huggins [ 28-30 ]. These electrodes show excellent cycling behaviour, however, deep discharge may destroy the conducting matrix. In this paper, we present composite electrodes in which the reactant (15-LiAI) is simply imbedded in

526

Z O. Besenhard et al. / Dimensionally stable Li-alloy electrodes

an inert electronically conductive matrix.

Table 1 Volume changes associated with full capacity cycling of Li-alloys

2. Experimental

Discharged species

Charged species

AI

LiAI

96.78

As

LiAs Li3As

91.63 95.59

Bi

LiBi Li3Bi

75.88 176.51

29.06 15.22

[31 ] [31 ]

C

LiC6

9.35

35.82

[32]

Cd

LiCd3 Li3Cd

17.60 267.71

45.89 15.92

[31 ] [31 ]

In

LiIn

52.29

23.53

[33]

Pb

LiPb Li22Pb5

44.70 233.66

26.43 13.85

[31 ] [31 ]

Sb

Li3Sb

147.14

14.99

[31 ]

Si

Li2Si Li4Si

175.12 322.57

15.15 11.64

[31] [31 ]

Sn

Li22Sn5

676.31

28.75

[31 ]

Zn

LiZn4 LiZn

13-LiA1/Cu composite materials were made by compression of an intimate mixture of ca. 80 wt% milled 13-LiA1 (Kawecki Berylco Industries, Li : AI : Fe = 48.52 : 51.44 : 0.04 at%, typical particle size 5-30 lam) and ca. 20 wt% oxide-free Cu-"dust" (Heraeus, Hanau, typical particle size 2-5 p.m) at ca. 20 MPa. In some cases the compression was followed by a sintering process (3-10 min at ca. 1000 ° C, argon atmosphere) in stainless steel containers which were inserted in quartz ampoules. Electrodes were made up by pressing (at ca. 20 MPa) either the original mixture or the pulverized sintered material on Cu-grids (ca. 20 mg/cm 2). Impedance studies were performed in the galvanostatic mode, using a Solartron 1174 frequency response analyser. SEM micrographs were taken of unsputtered samples with a JEOL JSM-840A microscope supplied with a vacuum gate.

Vol. increase (%)

11.33 70.64

Molar vol. of Li (cm 3 )

Ref.

19.67

[31 ] [31]

24.88 8.47

40.81 17.30

[31 ] [31 ]

[31 ] [ 31 ]

3. Results and discussion Despite the corrosion and passivation problems associated with aluminium and especially freshly created aluminium surfaces in contact with organic electrolyte solutions, we have concentrated our work on 13-LiAI. This was because Li-alloys of most of the chemically more stable basis metals could hardly meet the energy density standards set by competing secondary battery systems like advanced N i / C d batteries. In this context it is also of importance that the full capacity of Li-rich alloys like Li3Bi or Li22Pb5 can hardly be used in dimensionally stabilized dec-' trodes because of the drastic changes in volume (see table 1 ).

3.1. Microscopic studies A most striking feature of Cu/[3-LiAI pressed powder electrodes is their golden shiny appearance, which remains practically unchanged even after a series of cycles (e.g. 75 cycles at ia=ic=0.5 A / c m 2, ta= tc=45

min) and also after a full capacity discharge in LiC104/propylene carbonate (PC) electrolytes. In this respect there is no obvious difference between electrodes made from the original or from the sintered powder mixture. Therefore, the sintering process does not seem to be absolutely necessary, provided that oxide-free Cu-powder has been used. On the other hand, the sintering process causes the consumption of all the copper by formation of various A1-Cu phases [ 34 ] and so decreases the amount of electrochemically active aluminium. Moreover, the room temperature mobility of lithium in 13-LiA1 is significantly lowered by Cu-contaminations in the basis aluminium [ 7 ]. Therefore, further studies were made with the unsintered Cu/~-LiAI mixture. Judging from X-ray diffraction, the pressing process (20 MPa at room temperature) does not lead to any formation of A1-Cu phases. Unfortunately, it is difficult to visualize the shiny appearance of cycled Cu/I3-LiAI electrodes in pho-

J.O. Besenhard et al. / Dimensionally stable Li-alloy electrodes

a)

: ,

10,pm

527

tographs. Figs. l a,b shows SEM pictures of electrodes before and after cycling. There is no obvious difference in panicle size and distribution and by evidence of element mapping there has been no transport of Cu or AI. By contrast, cycling of pure 13-LiA1 in LiC104/PC creates large and deep cracks, corresponding to the enormous changes in volume (see table 1). Fig. 2 shows a SEM photograph of a pure 13-LiA1 electrode which was exposed to the same cycling conditions as the Cu/fI-LiAl composite electrode presented in fig. lb. Starting the cycling not with [~-LiA1but with high purity A1 ("Raffinal", 99.998%, Vereinigte Aluminium Werke) in the long run leads to a topography very similar to that shown in fig. 2.

3.2. Polarization and impedance studies An obvious objection against an ionically blocking framework structure in mixed conductor electrodes is hindrance of solid state ionic transport. There is, indeed, some extra polarization due to the copper framework, however, this effect is surprisingly small. Simple current ramp polarization studies in the O-

b)

'

~

lOj~m

Fig. 1. SEM pictures of Cu/I~-LiAI (20/80 wt%) pressed powder electrodes; (a) before, (b) after 30 cycles in 0.5 M LiCIOJPC, ia=i¢=0.2 mA/cm 2, & = & = 4 5 min; pictures taken in the charged state, Cu-areas are shown in bright.

1,5

1.0 ,... 0,5 x

0-

I

~ 2,0

1,5 1,0 0,5

i [ m A c m -2]

lOj~m, ] Fig. 2. SEM picture of I~-LiAI after 30 cycles in 0.5 M L i C I O J PC, i, = i¢ = 0.2 mA/cm 2, t, = t¢= 45 min; picture taken the charged state.

Fig. 3. Current ramp polarization studies of Cu/13-LiAI (20/80 wt%) composite electrodes and ~LiA1 electrodes prepared in situ (charge 8 C / c m 2) from high purity A1 in 0.5 M LiC1OJPC, v = 0 . 0 2 mA/s.

528

J.O. Besenhard et al. / Dimensionally stable Li-alloy electrodes

200

. . . .

~,

I

200

. . . .

i

. . . .

i

. . . .

30 kHz

O3

tO0 + 18kHz

03 (b

I00

I

i

i

i

i

~Z

200

. . . .

I

100

++

. . . .

I

O.IHz

+

+ ++

0

b

I

200 400 real resistance [ ~ ]

~, c~

+

30kHz

0.1Hz

o l l l l

++

t2.SkHz

.m ~ t~

O -100

++++i

i

÷

600

•~_ -lOO

~,

. . . .

i

'

200

.

.

.

.

i

'2/00 . . . .

L 400 real resistance [ ~ ]

.

.

.

.

I

.

.

.

.

I00

i + ++ 15kHz

12,5kHz

+ 12,SkHz +~+ !

30kHz

+ ++

30kHz

+÷

.~

d .E - t o o

i

0

i

I

i

i

600

i

i

I

200 400 real resistance [ ~ ]

i

i

i

i

600 =

.E -loo

i

0

+

O,1H i

~

i

I

i

i

i

i

[

200 400 real resistance [~1

i

~

i

i

600 ,.

Fig. 4. Impedance spectra of Cu/13-LiAI (20/80 wt%) composite electrodes and in situ formed pure ~LiAI electrodes in 0.5 M LiCIOJ PC, spectra taken in the galvanostatic mode after 90 min predischarge: (a) in situ formed electrode, C= 50 C/cm 2, ia=0.2 mA/cm2; (b) composite electrode, ia=0.2 mA/cm2; (c) in situ formed electrode, C= 50 C/cm 2, i.= 1 mA/cm2; (d) composite electrode, &= 1 mA/ c m 2.

18 m A / c m 2 range did not show drastic differences between Cu/13-LiA1 ( 2 0 / 8 0 wt%) composite electrodes and 13-LiAI electrodes prepared in situ from high purity A1 (see fig. 3). Impedance studies were more discriminating between the two types o f 13-LiA1 electrodes. In fig. 4a,b,c,d impedance spectra taken in the galvanostatic mode after a 90 min pre-discharge are compared. At 0.2 m A / c m 2 the differences in the spectra are not too striking, taking into account that in situ prepared electrodes have the advantage o f a consider-' able surface roughness over the smooth composite electrodes. The center o f the RC-semicircles are more displaced below the real axis in the case of composite electrodes. This is related with a dispersion of the relaxation time and may be due to inhomogeneities o f the electrode [35 ]. At 1 m A / c m 2 the impedance spectra o f Cu/13-LiA1 composite electrodes differ from those o f in situ formed I3-LiAI electrodes insofar as they show a

curved Warburg branch at low frequencies. This behaviour is typical for diffusion limitation in pores [2Ol.

4. Conclusion and outlook

Composite electrodes made by compression o f high purity small particle size copper powder and 13-LiAI powder are dimensionally stable during cycling in organic electrolytes. Prolonged cycling experiments performed so far indicate that in the long run electrolyte solution may penetrate into the composite structure, because o f the dimensional stability, a surface sealing of composite electrodes with polymers retarding solvent penetration or with Li ÷ conducting glasses seems to be possible. Applications are hoped for in the fields o f thin layer "painted" electrodes and large size macroporous electrodes made up e.g. o f beads o f the composite material.

J.O. Besenhard et al. / Dimensionally stable Li-alloy electrodes

Acknowledgement Financial support of Deutsche Forschungsgemeinschaft, Fonds der Chemischen kraft GmbH

Industrie and Silber-

is g r a t e f u l l y a c k n o w l e d g e d .

References [ 1 ] V.R. Koch, J. Power Sources 6 ( 1981 ) 357. [2] M. Hughes, N.A. Hampson and S.A.G.R. Karunathilaka, J. Power Sources 12 (1984) 83. [ 3 ] R.V. Moshtev, J. Power Sources 11 (1984) 93. [4] M. Garreau, J. Thevenin and B. Molandou, in: Lithium batteries, ed. A.N. Dey, Proc. Electrochem. Soc. 84 (1984) 28. [ 5 ] J.O. Besenhard, J. Giirtler, P. Komenda and A. Paxinos, J. Power Sources 20 (1987) 253. [6]B.M.L. Rao, R.W. Francis and H.W. Christopher, J. Electrochem. Soc. 124 (1977) 1490. [7] J.O. Besenhard, J. Electroanal. Chem. 94 (1978) 81. [8] M. Garreau, J. Tevenin and M. Fekir, J. Power Sources 9 (1983) 235. [9] J. Wang, I.D. Raistrick and R.A. Huggins, J. Electrochem. Soc. 133 (1986)457. [ I I ] A . Anani, S. Crouch-Baker and R.A. Huggins, J. Electrochem. Soc. 134 ( 1987 ) 3088. [12]A. Anani, S. Crouch-Baker and R.A. Huggins, Proc. Electrochem. Soc. 87 (1987) 365. [ 13 ] J.O. Besenhard and H.P. Fritz, Electrochim. Acta 20 ( 1975 ) 513. [ 14] A.S. Baranski and W.R. Fawcett, J. Electrochem. Soc. 129 (1982) 901. [15] Y. Gernov, P. Zlatiliova and R.V. Mosthev, J. Power Sources, 12 (1984) 145. [ 16 ] Y. Geronov, P. Zlatilova and G. Staikov, J. Power Sources, 12 (1984) 255.

529

[ 17] J.R. Owen, W.C. Maskell, B.C.H. Steele, T. Sten Nielsen and O. Toil Soerensen, Solid State Ionics 13 (1984) 329. [18] W.C. Maskell and J.R. Owen, J. Electrochem. Soc. 132 (1985) 1602. [ 19 ] P. Zlatilova, I. Balkanov and Y. Geronov, J. Power Sources, 24 (1988) 71. [20] J.O. Besenhard, H.P. Fritz, E. Wudy, K. Kietz and H. Meyer, J. Power Sources 14 (1985) 193. [21 ] J.O. Besenhard, P. Komenda, A. Paxinos, E. Wudy and M. Josowicz, Solid State Ionics, 18/19 (1986) 823. [22] F. Kita, K. Yoshimatsu and K. Kajita, Jap. Kokai 63 132 64 A2; Chem. Abstr. 108 (1988) 170814j. [23]Y. Matsuda, in: 32nd Proc. Int. Power Sources Symp. (1988) 124. [24] N. Yasuro, T. Kazuo and F. Sanehiro, Jap. Kokai 61 193 360; Chem. Abstr. 106 (1987) 70349b. [25 ] U. Satoshi, S. Shinji, A. Toru and F. Minoru, Jap. Kokai 61 168 872; Chem. Abstr. 106 (1987) 53235u. [26] R. Kanno, Y. Takeda, T. lchikawa, K. Nakanishi and O. Yamamoto, J. Power Sources, 26 (1989) 535. [ 27] M. Mohri, N. Yanagisawa, Y. Tajima, H. Tanaka, T. Mitate, S. Nakjima, M. Yoshida, Y. Yoshimoto, T. Suzuki and H. Wada, J. Power Sources 26 (1989) 545. [28 ] B.A. Boukamp, G.C. Lesh and R.A. Huggins, J. Electrochem. Soc. 128 (1981) 725. [29]A. Anani, S. Crouch-Baker and R.A. Huggins, J. Electrochem. Soc. 135 (1988)2103. [30] R.A. Huggins, J. Power Sources 26 (1989) 109. [31]T. Landolt-B6rnstein, Structure data of elements and intermetallic phases, Vol. 6 (Springer, Berlin, 1971 ). [32] D. Billaud, E. McRae and A. Hrrold, Mat. Res. Bull. 14 (1979) 857. [33] E. Zintl and G. Bauer, Z. Physik. Chem. (B) 20 (1933) 245. [34] J.O. Besenhard, J. GiJrtler and P. Komenda, in: chemical physics of intercalation, eds. A.P. Legrand and S. Flandrois (Plenum, New York, 1987) p. 469. [35]J. Ross Macdonald and W.B. Johnson, in: Impedance Spectroscopy, ed. J. Ross Macdonald (Wiley, New York, 1987) p. 1.