Ionic conductivity of alkali metal chloroaluminates

Volume 58A, number 4

PHYSICS LETI’ERS

6 September 1976

IONIC CONDUCTIVITY OF ALKALI METAL CHLOROALUMINATES W. WEPPNER and R.A. HUGGINS Stanford University, Stanford, California 94305, USA Received 16 July 1976 The electrical conductivity of three alkali metal chloroaluminates has been investigated from room temperature 7 and 3.2 X i0~ f2~cmt ,and to above the melting point. Ionic conductivities at 25°Caxe 1.2 X 10—6, 3.5 X iO— activation enthalpies are 0.47, 0.46 and 0.53 eV for LiA1C1 4, NaA1C14 and KA1C14.

In a previous investigation [1] large values of ionic conductivity at low temperatures have been measured in liquid and solid LiALC14. Because of the general interest in solid alkali metal electrolytes for many technical applications, as well as for various thermodynamic and kinetic measurements for scientific purposes, this work has been extended to the study of other alkali metal chioroaluminates. Recently some highly conductive Na~and K~solid electrolytes have been discovered by other authors [2, 3] , but in most cases they are not of practical use because of low chemical stability and difficulties in preparation. The formation of ternary compounds between aluminum chloride and alkali metal chlorides in a 1:1 molar ratio was first observed by Kendall et al. [4] in 1923, but the properties of these compounds have not yet been studied extensively. According to the proposed phase diagrams of the systems LiCl-A1C13 [4— 6], NaC1-A1Q3 [4,7—11] and KQ-AlCl3 [7—9],the compound MAid4 (M = Li, Na or K) is the only known existing intermediate phase in all cases. A remarkable characteristic of these systems is their very low eutectic temperatures; 104 [4] (80 [61), 107 [11] (93 [12]) and 128 [8, 9] (114 [12] )°Cfor the Li, Na and K systems, respectively, at compositions of 60— 70 mb Ala3. Ternary systems with a second alkali metal chloride or a small amount of another chloride, can have even lower melting points [12—14]. In the case of a mixture of 2.6 mb Baa2, 33.8 mb NaCL, and 63.6 mb AlCl3 melting temperatures as low as 50°Chave been observed [14] Of the reported melting points for the ternary compounds, the values 146 [15], 153 [11] and 256 [13, 15] °Cfor LiA1C14, NaC1C14 and KA1Q4, respectively, seem to be the most reliable. .

In the molten state, the chloroaluminates have shown high electrical conductivities, above 0.1 ~ cm—’ [16—20] They have been considered as electrolytes for use in high energy and power density secondary batteries to operate at elevated temperatures. They also have been dissolved in organic and inorganic solvents, and used as nonaqueous electrolytes at room temperature. In the solid state, two-phase NaA1C14-A1C13 mixtures have shown decreasing electrical conductivities with excess of Ala3, and the electrical conductivity has been found to increase by a factor of 200 on the addition of a small amount of CaSO4 [16]. The structures of the solid alkali metal chloroaluminates consist of positively charged alkali metal ions and A1Q~ions, with four formula units per elementary cell. The chlorine atoms are located tetrahedrally around the aluminum atoms, and linked together in three dimensions by the alkali metal ions located in the spaces in between [15,21]. A full structural analysis has as yet only been worked out for NaA1C14 [211, and all the A1C14- tetrahedra evidently are arranged with one face nearly parallel to the (001) plane. They point downward along the c axis in the (010) plane and upward in the (020) plane. The chlorine atoms thus form pairs of layers perpendicular to the c-axis, with the tetrahedrally coordinated aluminum ions, as well as the sodium ions lying in a layer in between. The distances between the sodium ions and their seven nearest chlorine ion neighbors are between 2.79 and 3.29 A; these are larger than the sum of the normal sodium and chlorine ionic radii. The X-ray data for KA1C14 were interpreted in terms of a structure similar to that of AgMnO4 [15]. In the case of LiAIC14, infrared data have been ex.

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6 September 1976

plained by a possible distortion of the AlCl~tetrahedra [15]. The lattice of NaAlCl4 is orthorhombic, whereas those93°. of LiA1C14 and KA1C14 are monoclinic, with ~3about The alkali metal chloroaluminates have been prepared by reacting A1C1 3 (puriss., ~ 99%, iron and water free, Fluka AG) with LiC1 (> 99.3% Baker Chemical Company), NaCl (Fluka, puriss. p.a.) or KC1 (Fluka, puriss. p.a.), respectively. The individual chiorides were finely powdered, mixed in the appropriate equal molar ratio and then heated to about 30°C above the melting point of the salts, using Pyrex glass tubes and an electric furnace in a helium filled dry box. Without further treatment the melts were slightly brownish in color. Any water present was removed by keeping the salts in the molten state at least overnight. Other impurities were removed by displacement through pure alkali metals. Any.aluminum metal that might also be formed in this way has no disturbing influence because it is practically insoluble in the molten chloroaluniinates [22].and Shifts in the ratiochloride betweenarethe alkali metal chlorides the aluminum compensated by a small amount of precipitation. After this treatment, all the melts became colorless, transparent and had very low viscosities. The solid chloroaluminates were prepared by decanting the cleaned melt into a Pyrex tube containing two parallel molybdenum sheets about 1 cm X 5 cm in dimension and 0.4 cm apart, and cooling in situ at a very slow rate. The molybdenum electrodes were kept in position by teflon holders. It was observed in the present experiments that, as mentioned in the literature [23] , molybdenum and teflon appear to be resistant to attack by the alkali metal chioroaluminates for appreciable periods of time. The solidified melts were always nearly transparent and partially white because of many thin cracks. Earlier work [1] showed that use of this cell arrangement is advantageous compared to the use of the standard polycrystalline pellet technique. Electrical conductivity measurements were carried out in a helium atmosphere dry box, using a General Radio ac bridge with an external sine function generator in the frequency range between 50 Hz and 100 kHz. The values of specific conductivity were derived from the geometrical dimensions of the cell and complex impedance (reactance versus series resistance) plane plots interpreted in terms of the commonly observed electrical equivalent circuit for solid electrolyte-block246

1 2501 1 2 .~

0

150 I

100 I

80I

50

30 I 20 I

cr

I

o

-

—

~ a’

Li Al Cl4 2

Eo rO.47eV

-

-3 N Al Cl E0

—

0.46 eV

—

K Al C 14 E0

0.53 eV

-

6l 8

-

25 —

/

3

.4

10 [K I Fig. 1. Semiogarithmic presentation of the electrical bulk conductivity of lithium-, sodium- and potassium chloroaluminate times the absolute teml,erature as a function of the inverse absolute temperature. I T x

ing electrode systems [24] In order to evaluate any electronic contribution to the total conductivity, dc polarization measurements were made by applying constant voltages to the cell by use of a potentiostat. The current was determined from the voltage drop across a large ohmic resistance between the counter and the reference electrode. The results of the ac conductivity, measurements for the compounds LiA1C1 4, NaAlCl4, and KAIC14 are shown in fig. 1. The product of the specific conductivities times the absolute temperature are semilogarithmicaliy plotted versus the inverse temperature from ambient temperature to well above the melting points. Measured electrical conductivity data obtained upon both heating and cooling were in good agreement. No aging processes, i.e., changes of the electrical properties were observed during several cycles. The deviations from a sharp change in the conductivities at the melting points of the salts are probably due to small inhomogeneities in the sample temperature profile. The .

Volume 58A, number 4

PHYSICS LETTERS

6 September 1976

‘specific electric conductivities at 25°Cdecrease appreciably with increasing ionic radius of the alkali metal ions. The values found are 1.2 X 10—6, 3.5 X iO~ and 3.2 X l0~ ~ cm~for LIA1C14, NaA1Q4 and

good electronic conductors, integration of eq. (2) between the two electrodes of a sample with length L, making use of eq. (1), yields

KA1C14, respectively. The activation enthalpies for aT apparently differ very little among the three cornpounds, and are given by 0.47, 0.46, and 0.53 eV, respectively. The chloroaluminates became molten at temperatures that are in good agreement with those reported in the literature and mentioned above. The showed the expected high conductivities in melts the range of 10—1 to 1 £2~cm~,aboutlO~to l0~times higher than in the solid state. The activation enthalpies in the liquid state are smaller than 0.1 eV. From the shape of the impedance and admittance plane plots in the common interpretation [24] it is possible to conclude that the investigated chloroalummates are predominately ionic conductors. In order to verify this information and to get more quantitative values on the electronic transference numbers, dc polarization measurements have been carried out. Because the molybdenum electrodes are not able to deliver ions to the sample, if a dc voltage is applied in steady state, the electrical current is only due to the transport of electrons and holes. Voltages between 20

lelL/E, (3) where E is the applied voltage. On the other hand,if d/.Le ~ dIzM+, i.e., in the case of low electronic concentrations but large disorder in the lattice of the mobile ions, integration of eq. (2) under the assumption 2e,hof= ideal 14,h +dilute kT insolutions ce,h, givesfor the electronic species, / Ge + Gh = q(c~u~ + ChUh)

and 400 mV, well below the thermodynamically expected decompositionvoltages, have been applied to such a cell. If the ionic current is suppressed, the graclient of the electrochemical potential ~= .t + qçb of the ions becomes zero, so that 1M~

dI

q

“ ‘

d’~ (1’ 0 and q are the chemical potential of the M~ ions (related to one particle), the electrostatic potential, and the elementary charge respectively. Under this condition, by the substitution of the electric potential according to eq. (1) in the general expression for the flux of the electrons and holes, the total electrical current density is given by /dji dpM+\ iel_(CeUe+ChUh)(~) (2) =

—

X

X

where Ce,h, Ue,h and ~e are the concentration (partides/cm3), the electrical mobility and the chemical potential of the electrons and holes, respectively, If ~ ~° d1.L~,i.e., the concentration of the electronic species is practically constant, as expected in

—

Ge + Uh = q(ce Ue + ChUh) =

—

—

~ =-~-

r

I E “

[exp~—~)

i—i —

1]

(4)

for the sum of the maximum electronic conductivities over the whole sample. ‘and “represent the left and right hand phase boundary, respectively. The voltage E times q, which is n’~),has been substituted into eq. (4) for because in this case the electrostatic potential gradient vanishes. The analysis of the dc data according to eqs. (3) and (4) resulted in transference numbers of the electronic species < 10—2, <3 X i0~ and <4 X 10—2 for LiA1C1 4, NaAlCl4 and K AIC14, respectively, measured at several temperatures. Therefore, the investigated chioroaluminates are practically pure ionic conductors. Because of their size, the A1C14 ions are assumed to be relatively immobile. The conductivity may, therefore, be attributed to the motion of the Compared to simple, binary chlorides, those alkali alkali metal ions. metal tetrachloroaluminates have conductivities that are higher by several orders of magnitude. Compared to the recently discovered sodium and potassium solid electrolytes [2, 31 and the beta aluminas, however, the conductivities are still low. Thus far we have not yet investigated the possibility of increasing the conductivity by adding dopants, as has been successfully done with materials with closely related structures [25]. These experiments show that, in contrast to most

(14 (j4 14) —

—

previous work, attention should be also given to chlorides with polyanions as possible candidates for use as alkali metal ion-conducting solid electrolytes. This work was supported by the Advanced 247

Volume 58A, number 4

PHYSICS LETTERS

Research Projects Agency through the Office of Naval Research under Contract N00014-67-A-0112-0075. One of us (WW) also gratefully acknowledges the grant of a NATO Scholarship through the German Academic Exchange Service (DAAD).

References (1] W. Weppner and R.A. Huggins, J. Electrochem. Soc., to be published. . [2] J. Singer, H. Kautz, W. Fiedler and J. Fordyce, NASA Technical Memorandum TM X-71753, presented at Amer. Ceram. Soc., Washington, D.C., May, 1975. [3] J.B. Goodenough, H.Y-P. Hong and J.A. Kafalas, Mat. Res. Bull. 11(1976) 203. [4] J. Kendall, E.D. Crittenden and H.K. Miller, J. Amer. Chem. Soc. 45 (1923) 963. [5] E.M. Levin, C.R.Robbins and H.F. McMurdie, Phase diagrams for ceramists, 1969 Supplement, The Amer. Ceram. Soc., Inc., Columbus, Ohio (1969). [6] A.I. Morozov, V.G. Kuznetsov and S.l. Maksimova, Russ. J. Inorg. Chem. 16 (1971) 1773. [7] E.M. Levin, C.R. Robbins and H. F. MeMurdie, Phase diagrams for ceramists, The Amer. Chem. Soc., Columbus, Ohio (1964). [8] U.I. Shvartsman, Zh. Fix. Khim. 14 (1940) 254. [9] U.I. Shvartsman, Zap. Inst. Khim. Akad. Nauk Ukr. RSR 7 (1940) 3.

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[101 A. Chr6tien and E. Lous, Compt. Rend. Akad. Sci. paris 217 (1943) 451. (11] E.M. Levin, J.F. Kinney, R.D. Wells ahd J.T. Benedict, J. Res. Nat. Bur. Stand. 78(A) (1974) 505. (12] L. Wasilewski, A. Kaczorowski and M. Dynkin, Przem. Chem. 18 (1934) 608. [13] W. Fischer and A.L. Simon, Z. Anoig. Allgem. Chem. 306 (1960) 1. [14] M.A. Kuvakin, L.I. Talanova and A.L Kulikova, Russ. J. Inorg. Chem. 18 (1973) 602. [15] K.N. Semenenko, V.N. Surov and N.S. Kedrova, Russ. J. Inorg. Chem. 14 (1969) 481. [16] V.A. Plotnikov and P.T. Kalita, Zh. Russ. Fix. Khim. Obshch. 62 (1930) 2195. [17] Y. Yamaguti and S. Sisido, J. Chem. Soc. (Japan) 62 (1941) 304. (18) H. Grothe, Z. Elektrochem. 53 (1949) 362. [19) R. Midorikawa, J. Electrochem. Soc. (Japan) 23(1955) 72, 127, 310. [20] C.R. Boston, L.F. Grantham and S.J. Yosim, J. Electrothem. Soc. 117 (1970) 28. (21] N.C. Baenziger, Acta Cryst. 4 (1951) 216. (22] V.N. Storozhenko, Russ. J. Phys. Chem. 48 (1974) 1010. (23] G.L. Groshev and Z.I. Yurlova, Tr. Khim. Khim. Tekhnol. 7 (1964) 391. [24] I.D. Raistrick and R.A. Huggins, in: Proc. Symp. Workshop Adv. Batt. Res. and Design, Argonne Nat. Lab. (1976), p. B-277. [25] I.D. Raistrick, Y-W. Hu and R.A. Huggins, to be published in Mat. Res. Bull.