Crown ether solid electrolytes with mobile halide ions

Solid State lonics 3/4 (1981 ) 389-392 North-Holland Publishing Company

C R O W N E T H E R SOLID E L E C T R O L Y T E S W I T H M O B I L E HALIDE IONS D.S. N E W M A N , Debora H A Z L E T T and K.F. M U C K E R Department of Chemistry. Bowling Green State University. Bowling Green, OH 43403. USA

Complexes were formed between dibenzo-18-crown-6 and lithium halides. These complexes exhibited high anionic conductivity in the solid state at 25°C. The conductivity increased in the order C1 > Br- > 1 .

1. Introduction Macrocyclic polyethers or "crown" ethers have had a profound effect on various aspects of chemistry, ever since Pedersen's classic paper appeared in 1967 [1]. These compounds have the ability to form stable complexes with a wide variety of cations, such as NH~, Na ÷, K ÷, etc., which are relatively soluble in organic solvents. This means that ions which previously could not be used in syntheses because of severe solubility limitations, are now readily available. The general equations describing complex formation between a crown ether and a solvated metal ion may be written as: M "+ . . . n solvent + crown ether ,-~-~crown ether-Mm++ n solvent.

(1)

Although Pedersen studied many different crown ethers, he focused much of his attention on the properties of dibenzo-18-crown-6 (octahydrodibenzo-2,5,8,15,18,21hexaoxacyclooctadecin, DBC) and the complexes it formed with metal halides. These compounds have the general structure shown in fig. 1. A m o n g other things, he found that

"-. i÷I

Fig. 1. Dibenzo-18-crown-6 metal halide complex.

dibenzo-18-crown-6 did not easily complex with lithium salts, presumably because of their high lattice energy and the small radius of Li ÷. Nevertheless, weakly associated complexes were formed and it was our contention that these complexes might exhibit fast cationic conduction in the solid state because of the possibility that the polyether crowns could be made to stack, one upon the other, thereby creating a natural "diffusion pipe" [2,3] into which a cation might enter and, once in, move with ease. We soon discovered, however, that in the solid state the lithium halide salts of dibenzo-18-crown-6 were anionic rather than cationic conductors and this discovery opened up new possibilities for synthesizing interesting solid-state anionic conductors. As far as we could tell, with the exception of private conversations and references to patents, there does not seem to be any literature on these anionic conductors.

2. Experimental Stoichiometrically equivalent quantities of crown ether and alkali metal halide were dissolved in hot methanol and allowed to equilibrate for several hours. The solution concentrations were usually 0.001 M in each component. The methanol was then slowly evaporated leaving a residue of crown e t h e r metal halide salt. The greyish-white solid was then compressed under vacuum at 1800 kg/cm 2 into a cylindrical pellet.

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300

D.S. N e w m a n et al. / Crown ether solid electrolytes with mobile halide ions

T h e c o m p o s i t i o n of each salt was d e t e r m i n e d by m e l t i n g - p o i n t and h a l i d e analysis while the n u m b e r of p h a s e s p r e s e n t was d e t e r m i n e d from p o w d e r X-ray diffraction p a t t e r n s o b t a i n e d with a Phillips X-ray unit using Cu K a r a d i a t i o n . It was unusually difficult to get two diffraction p a t t e r n s of a given c o m p l e x to match. T h e specific c o n d u c t a n c e of each salt was m e a s u r e d with an industrial i n s t r u m e n t ac b r i d g e at a f r e q u e n c y of 1000 Hz using p l a t i n u m e l e c t r o d e s . T h e pellet was held in a d e a d weight c o n d u c t i v i t y cell which also s e r v e d to t h e r m o s t a t the s a m p l e . This cell, which is shown s c h e m a t ically in fig. 2, a l l o w e d us to m a i n t a i n a constant e l e c t r o d e c o n t a c t p r e s s u r e , which could be v a r i e d at will, on the surfaces of each pellet. T h e fraction of the electric c u r r e n t c a r r i e d by e l e c t r o n s was m e a s u r e d with a b l o c k i n g cell using c o p p e r - g r a p h i t e p e l l e t s as the e l e c t r o d e s

[41. Since crown e t h e r s are g e n e r a l l y very p o i s o n o u s , all o p e r a t i o n s w e r e c a r r i e d out in a f u m e h o o d and r u b b e r gloves w e r e always w o r n by the e x p e r i m e n t a l i s t .

3. Results and discussion X-ray diffraction p a t t e r n s w e r e o b t a i n e d for each p o l y c r y s t a l l i n e s a m p l e of c r o w n e t h e r - a l kali h a l i d e c o m p l e x . T h e p o w d e r p a t t e r n for the L i F c o m p l e x c o n t a i n e d t h r e e sets of lines. O n e set was from the neat crown ether, o n e set was f r o m L i F a n d a third set was p r e s u m a b l y f r o m the crown e t h e r - L i F c o m p l e x a l t h o u g h this was n e v e r e s t a b l i s h e d with c e r t a i n t y . F r o m the in-

I

"--LEAD WEIGHT

I

"--ALUMINUM CAP

.r---1 JACKET

I

SOLID ELECTROLYTE

I"

aASE

tensity of the lines we e s t i m a t e d the third phase was p r e s e n t in relatively low c o n c e n t r a t i o n . T h e r e f o r e , the m e a s u r e d c o n d u c t i v i t y w o u l d be that of a m i x t u r e r a t h e r than a single c o m p o u n d . Surprisingly, the m e l t i n g p o i n t is r a t h e r sharp. T h e p o w d e r p a t t e r n s for the lithium b r o m i d e and i o d i d e c o m p l e x e s a n d for the p o t a s s i u m c h l o r i d e c o m p l e x i n d i c a t e d that only o n e p h a s e was p r e s e n t in each of these systems. T h e powd e r p a t t e r n s for the crown e t h e r - L i C 1 system gave two sets of lines. O n e set of lines was for a new c o m p o u n d , p r e s u m a b l y the c o m p l e x , and the s e c o n d set was for u n c o m p l e x e d LiCI which we were n e v e r able to c o m p l e t e l y r e m o v e . F r o m the intensity of the lines from the LiCI, we e s t i m a t e this p h a s e to be p r e s e n t in low conc e n t r a t i o n . T h e n u m b e r of p h a s e s f o u n d and the melting p o i n t of each c o m p l e x is listed in table 1. O u r inability to isolate p u r e d i b e n z o - 1 8 c r o w n - 6 - L i F or LiCI is consistent with P e d e r sen's results. T h e d i a m e t e r of the "'hole'" in the crown is b e t w e e n 2.6 and 3.2 A [11 w h e r e a s the best e s t i m a t e for the d i a m e t e r of a Li + ion is 1.72 A [6]. T h e r e f o r e , the c o m b i n a t i o n of a loose fit and a high lattice e n e r g y will m a k e it difficult for a stable 1 : 1 c o m p l e x to form with these salts. H o w e v e r , it it not perfectly clear that the hole in the crown will not shrink, e i t h e r b e c a u s e of the a t t r a c t i o n of the Li + ion for the o x y g e n s or the c h a n g i n g of the p o s i t i o n s of the crown c a r b o n s from "exo'" to " e n d o ' " in the p r e s e n c e of a "'guest" ion. U n f o r t u n a t e l y , we have not b e e n able to find single-crystal X-ray diffraction p a t t e r n s for any of the d i b e n z o - 1 8 c r o w n - 6 - 1 i t h i u m salts. T h e a v e r a g e specific c o n d u c t a n c e , K, of each Table 1 Melting points of and n u m b e r of phases in, crown ether complexes Complex

Solid phases

Melting point (°C)

LiF -LiCI -LiBr -Lil

3 1+? l 1

162-63 159 149-52 146 157

1

Fig. 2. Dead weight conductivity cell.

D.S. N e w m a n et al. / Crown ether solid electrolytes with mobile halide ions Table 2 Specific c o n d u c t a n c e a n d f r a c t i o n of t h e c u r r e n t c a r r i e d b y electrons Complex

K( ~ cm)-t

% electronic conductance

-LiF -LiC1 -LiBr -LiI -KC1

8.1 × 4.4x 1.6 × 5.5 x < 10

0.001 0.07 0.16 _

10 5 10 4 10 .4 10 -6 7

391

02 n

cnmple×

CLI

+

Anode: Cqthode:

6

~ Cu

tt

2El-

+

C.tl~CIJC12

+ 2e

Eli + I/2 r)2 + 2e----r~uo (2Li + + 2e + 1/2 0 2 ----Li2(])

Fig. 3. E l e c t r o l y s i s e x p e r i m e n t .

complex salt at 25°C, and the percentage of the current carried by electrons are listed in table 2. Two separate pellets were used to obtain the conductivity of each complex. One pellet was approximately twice the thickness of the other. Constant pressure was applied to the electrodes and the interfacial resistance was corrected for by means of the equation: Rsalt = (RI] +

Rs,) - (RI2+ Rs2),

where R I~ and R~2 are the interfacial resistances, assumed to be equal, in the two measurements. Omitting the - L i F system from consideration because it is a mixture rather than a pure compound, the lithium-halide complexes conduct in the order C l - > Br > I indicating the principal charge carrier is the anion. In addition to being the best conductor, the chloride complex has the lowest electronic contribution to its conductivity. To establish with even greater certainty that the principal charge carrier is the halide ion, a rough estimate of the C1- ion's transference number was obtained from the crown ether-LiC1 complex by using a modification of T u b a n d r s electrolysis technique. A dc was passed through the salt which was held between copper electrodes, and the amount and nature of the deposited material was obtained by analyzing the electrodes. We assumed the electronic current to be zero. CuCl2 formed at the anode in proportion to the number of coulombs of electricity that passed through the salt and CuO formed at the cathode, together with a trace of Li (presumably as Li20). The experiment and the likely electrode reactions are shown schematically in fig. 3. Since the number of equivalents of C1- that deposited was nearly

equal to the number of equivalents of electricity that flowed, whereas only a trace of Li deposited, we assume the C1 transference number is nearly one. These data further support our contention that the halide ion is the principal charge carrier. Assuming this to be so, the dibenzo-18-crown-6-LiC1 complex is among the best room-temperature Ci- ion conductors known and the bromide-ion complex is far and away the best Br- ion conductor we have come across. The mechanism of ion transport in these systems is not yet well understood, but there is some unusual behavior occurring, and some inferences can be drawn as to how the ions move. By using the change in the complexes' U V spectrum, Pedersen found that the binding strength of the ions to the dibenzo-18-crown-6 polyether cavity is in the order K + > Na + > Rb + > Cr + > Li + [1]. Shizuka et al. [5] found that in methanol solution the internal quenching of a photochemically exicted state of the crownether increased in the reverse order, namely, blank > Li + > Cs + > Rb + > Na + > K +. Since the internal quenching is thought to be due to conformational changes of the molecule, this means the K + complex is the most rigid and the Li ÷ complex the least rigid. This same ordering was observed at 77 K in a solid matrix as well. The fact that the KCI complex should be a better C1- ion conductor than the LiC1 complex based on both reduced coulombic attraction between the ions and the absence of a second, presumably insulating, phase and it is the worst conductor of all, suggests something about the mechanism of ion transport.

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D.S. Newman et al. / Crown ether solid electrolytes with mobile halide ions

We think Pedersen's and Shizuka's data taken together with ours implies that the unusually high halide ion conductivity of the Li complexes is due to a highly defective lattice which arises because the crown complex solidifies in a variety of different conformations. If the salts were intrinsic fast ion conductors, in which the conductivity came about as a result of the crystal's structure, as is the case in RbAg415 for example, then the KCI complex would almost have to be the best conductor and our X-ray diffraction patterns would be clearer. We think the defects in the lithium complexes arise because, depending on the microscopic history of the complex, the Li ÷ causes the crown to vary its configuration and consequently, the halide its orientation. The I- ion is simply too large to take advantage of the defects whereas the CIand Br ion can move into defects in the lattice structure. Presently, experiments are underway to test the temperature dependence of the conductivity and preliminary results indicate an extremely steep rise in K versus T I which may mean Li +

ions are beginning to contribute to the conductivity.

Acknowledgement The authors wish to thank the Faculty Research Committee of Bowling Green State University for providing the funds for this work. We also wish to thank Laszlo Kescses for constructing and contributing to the design of the conductivity cell.

References [1] C.J. Pedersen, J. Am. Chem. Soc. 89 (1967) 7017. [2] D.S. Newman, C. Frank, R.W. Matlack, S. Twining and V. Krishnan, Electrochim. Acta 22 (1977) 811. [3] D.S. Newman, Electrochim. Acta 24 (1979) 789. [4] C. Wagner, 7th C I T C F Meeting, Lindau (1955) pp. 361375. [5] H. Shizuka, K. Takada and T. Morita, J. Phys. Chem. 84 (1980) 994. [6] M.F.C. Ladd, Theoret. Chim. Acta 12 ([968;) 333.