Conductivities and phase equilibria in anhydrous and hydrous LiI-NH4I

Solid State Ionics 5 (1981)403-406 North-HollandPublishingCompany

CONDUCTIVITIES AND PHASE EQUILIBRIA IN ANHYDROUS AND HYDROUS LiI-NH41

P. HARTWIG, K. RUDO, and W. WEPPNER M a x - P l a n c k - l n s t i t u t f~r F e s t k S r p e r f o r s c h u n g D-7000 S t u t t g a r t - 8 0 , F e d e r a l R e p u b l i c o f Germany

Phase equilibria and conductivities of the system LiI-NHwI-HzO were studied by DTA, temperature scanning Guinier-Simon X-ray diffraction, ac-impedance, dcp o l a r i z a t i o n and emf-techniques. A new phase, Liz(NHw)31s, was found which melts at 24300 and shows a low s t r u c t u r a l symmetry. The conduction is predomi n a n t l y i o n i c , o = 3 x lO -6 0-1cm " I at I00°C, with an a c t i v a t i o n enthalpy for aT of 0.93 eV. Traces of water r e a d i l y lead to the formation of L i l - h y d r a t e s . The melting of these compounds produces a d r a s t i c increase in the ionic cond u c t i v i t y of the sample above about 7 0 ° C . LiI.HzO does not show any reaction with NH41. Dispersed 2-phase mixtures of L i l or NH4I with Liz(NH4)31s provide an enhancement in the ionic c o n d u c t i v i t y compared to samples composed of e i t h e r of the pure c o n s t i t u e n t phases.

INTRODUCTION Previous i n v e s t i g a t i o n s h a v e shown high l i t h i u m ion c o n d u c t i v i t i e s in several ternary compounds of the systems Li-N-Hal with Hal : CI, Br, and I [I-4]. The materials are of considerable pract i c a l i n t e r e s t because they are also thermodynamically stable against reaction with elemental l i t h i u m and show high decomposition voltages. In extension of t h i s work we were i n t e r e s t e d in the influence of water, which is p o t e n t i a l l y present through the s t a r t i n g chemicals or tile environment, upon phase e q u i l i b r i u m , thermodynamic and k i n e t i c properties. As p a r t o f t h i s program we have i n i t i a l l y s t u d i e d t h e phase d i a grams and t h e i o n i c conductivities of t h e v a r i ous compounds o f the systems LiI-HzO [5] and L i X - L i O H (X = C I , Br, I ) [ 6 ] . Having nitrogen as an additional component, the lithium nitride iodides have shown a drastic increase in the ionic conductivities at temperatures between a b o u t 70 and 125°C upon e x p o s u r e to humid a t m o s p h e r e s . The c o n d u c t i v i t i e s become higher than in any o t h e r known s o l i d lithium conducting electrolyte w i t h o u t any change o f t h e solid s t a t e becoming obvious. In a similar experimental situation, Sattlegger and Hahn [ 7 , 8 ] have n o t i c e d t h e f o r m a t i o n o f a compound with a simple cubic structure when L i I was n o t carefully dried before preparing the phases o f the system LiI-Li3N. T h i s compound was r e g a r d e d t o be l i t h i u m ammonium iodide which was g i v e n the composition "LiNHwIz". Preliminary e x p e r i ments employing equimolar mixtures of commercial grade NHwI have again shown a large increase in c o n d u c t i v i t y at 70-125°C and apparently seemed to confirm the formation of such a h i g h l y conducting l i t h i u m ammonium iodide under tile i n f l u ence of water. The present paper describes the r e s u l t s of our recent extensive studies of the

phase e q u i l i b r i a and k i n e t i c p r o p e r t i e s of the system LiI-NHwI. Since the presence of water was considered to be responsible for the format i o n of such compounds, HzO was also taken i n t o account as a f u r t h e r c o n s t i t u e n t . Another compound in the system LiI-NH~I has been reported by Liang and Epstein [9] which was prepared from an aqueous s o l u t i o n and described to have the composition "Liw(NHw)Is". This mater i a l has shown a c o n d u c t i v i t y of 2 x lOGn-lcm " I a t 25°C. EXPERIMENTAL CONSIDERATIONS Samples f o r the investigation of the anhydrous system were p r e p a r e d from mixtures of lithium iodide (eerac, 99.9~, or Alfa-Ventron, 99+%, n o m i n a l l y a n h y d r o u s ) and ammonium i o d i d e ( M e r c k , 99.5~) with a variation of the composition in s t e p s o f 5 m/o. I t was f o u n d n e c e s s a r y to c a r e fully dry the starting materials beforehand at 300°C and 1 5 0 - 2 0 0 ° C , r e s p e c t i v e l y , in vacuum f o r 2-4 h o u r s . Samples c o n t a i n i n g w a t e r were p r e pared by e m p l o y i n g lithium iodide monohydrate which was p r o d u c e d by e x p o s i n g n o m i n a l l y a n h y d rous LiI or "LiI.3HzO" (Alfa-Ventron, 99+X) to argon with a defined water vapor pressure as described earlier [5]. The powders were mixed i n s i d e a d r y box and s e a l e d u n d e r vacuum i n Duran g l a s s ampoules f o r DTA phase e q u i l i b r i u m studies. Tile samples were either first m e l t e d by h e a t i n g to 300-500°C b e f o r e tile d i f f e r e n t i a l thermal analysis or, for t h e most p a r t , two s u c c e s s i v e h e a t i n g and c o o l ing cycles of the same sample were p e r f o r m e d . The h e a t i n g and c o o l i n g rates were v a r i e d between 2 and l O ° C / m i n . No a t t a c k o f t h e Duran g l a s s was o b s e r v e d i n any e x p e r i m e n t . The c o l o r of the LiI-NH~I samples v a r i e d from w h i t e to

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P. tfartwig et aL / Anhydrous and hydrous l.iI-Ntt 4 I

404

yellowish-brown, iodine.

apparently due

to some excess

Guinier-Simon X - r a y powder d i f f r a c t i o n pattern [ I 0 ] w e r e taken at scanning rates from I to 4 0C/h over tile temperature range between 25 and 260°C in both heating and ooo]ing runs using CuKc~ (& = 1.54056 ~) r a d i a t i o n . The powder mixtures were sealed in Duran Mark tubes under vacuum. Pellets for conductivity, polarization and emfmeasurements were p r e p a r e d by m e l t i n g the v a r i ous m i x t u r e s i n s i d e an argon f i l l e d dry box in mo]ybdenum c r u c i b l e s or Duran glass tubes and p o u r i n g the m e l t i n t o a c y l i n d r i c a l t e l ] o n mould o f 1 cm in d i a m e t e r f o r s o l i d i f i c a t i o n . Alternatively, the powders were heated to a b o u t 400°C in sea]ed g]ass ampou]es, f i n e l y ground and p r e s s e d to p e l l e t s i n s i d e the d r y box, The samp l e thickness was t y p i c a l l y 1-4 mm. In addition, the c o n d u c t i v i t y between two Mo-sheets inserted into s o l i d i f i e d melts inside sealed Duran ampoules was measured. The c o n d u c t i v i t y was determined by the ac-impedance technique using a continuous frequency range from I Hz to 150 kllz, with Mo or Pt foils as i o n i c a l l y blocking e l e c t r i c a l contacts and providing a p u r i f i e d and dry argon atmosphere [1]. The same a r r a n g e m e n t o f the sample sandwiched between two i o n i c a l ] y b]ocking electrodes has been employed in d c - p o l a r i z a t i o n experiments f o r an e s t i m a t i o n o f the e l e c t r o n i c c o n t r i b u t i o n to the t o t a l c o n d u c t i v i t y , V o l t a g e s from 10 to 100 mV were a p p l i e d t o the c e l l . B e s i d e s l i t h i u m i o n s , p r o t o n s have to be c o n s i d ered as the p r e d o m i n a n t l y m o b i l e ionic species ill LiI-NH~I-HzO samples. In order to d i s t i n guish between these p o s s i b i l i t i e s , the transport of a]] other species t h a n l i t h i u m ions across the sample was blocked with the help of a u x i l iary Lil s o l i d l i t h i u m ionic conductors. Tile following galvanic cell

(+) M o / L i l L i l / s a m p l e / L i l / M o ( - )

I

was employed in a transference experiment. Another experimental setup was designed to attempt to t r a n s f e r protons across the samples by blocking a l l other species with tile help of the galvanic c e l l (+) Ar, Hz/sample/Pt ( - ) The sample was tube.

melted

into

II a U-shaped

glass

RESULTS The d i f f e r e n t i a l thermal analysis of the anhydrous system LiI-NHwI has c l e a r l y shown that only one intermediate phase e x i s t s . The composition is r i c h e r in NH~I t h a n in L i I . The purest s i n gle phase X-ray pattern was observed f o r a r a t i o

of 3:2 U'Liz(NH~)31s"). The compound is h y g r o scopic, dissolves in the presence o f a i r and decomposes slowly at 200°C. The Guinier-Simon p a t t e r n shows a large number o f diffuse }ines i n d i c a t i n g a low s t r u c t u r a l symmetry of tl~is compound. B e s i d e s t h e s e lines o n l y the p a t t e r n s of NH41 or L i l were v i s i b ] e for careful]y dried m a t e r i a ] s f o r any i n v e s t i g a t e d c o m p o s i t i o n . Tire reported cubic phase [7,8] was o n l y observed when w a t e r was p r e s e n t . In tile case o f tile presence of t r a c e s of water, tile X - r a y p a t t e r n of tile c u b i c p h a s e was s u p e r i m p o s e d on t h a t of the anhydrous l i t h i u m ammonium iodide. The additional l i n e s due to tile c u b i c m a t e r i a l d i s appeared upon h e a t i n g a t about 120°C. A comparison with e a r l i e r s t r u c t u r a l i n v e s t i g a tions of LiI.HzO I l l ] w h i c h showed a cubic perovskite t y p e structure with the space group Pm3m-Oh I and the lattice parameter a : 4.296 R (Z = I) indicates that the presently o b s e r v e d cubic pattern is due to l i t h i u m iodide monohydrate, The disappearance of the lines of tile cubic phase corresponds to the melting of L i I . H z O a t 128°C a n d / o r to the e u t e c t i c t e m p e r a ture of L i l - L i l . H z O (125°C) [5]. Mixtures of LiI.HzO and NH41 provide o n l y the pattern of both these s t a r t i n g materials between room t e m p e r a t u r e and the melting point of Lil.HzO (128°C). Crystallization from s o l u t i o n s o f mixtures o f LiI.HzO and Nil41 in e t h a n o l and acetone has merely provided the same o r i g i n a l compounds. Accordingly, no reaction occurs between these compounds. D i f f e r e n t i a l thermal a n a l y s i s has shown in many cases w i t h low r e p r o d u c i b i l i t y small thermal peaks a t about 70-80°C f o r samples at the L i I rich side and a t about 120-130°C at the NH~I-rich side o f the system LiI-NII~I. This observation is presumably a consequence of small amounts of water which may s t i l l h a v e remained in the sample due to the d i f f i c u l t y to remove the water completely from ammonium iodide w i t h out decomposition and may be related to the eutectic temperatures of 66°C for LiI.3HzO-LiI.2HzO, 77°C f o r Lil.2HzO-LiI.HzO, and 125°0 f o r L i I . H z O - L i l . Small amounts of the triand dihydrates of L i I may be formed by picking up the traces of water from the ammonium iodide. No changes of the X-ray pattern were observed f o r the same materials in the indicated temperature range where the small thermal peaks occurred, Lithium ammonium iodide melts at 243oc, The a c - c o n d u c t i v i t y measurements may be i n t e r p r e t e d in terms o f s i m p l e e q u i v a l e n t Debye c i r c u i t s and the i n t e r s e c t i o n o f the s t r a i g h t line w i t h the real axis i n complex plane p l o t s is t a k e n as tile r e s i s t a n c e o f the sample. The c o n d u c t i v i t y o f the new l i t h i u m ammonium iodide phase is presented in Figure I. An Arrhenius behavior is observed f o r the e n t i r e investigated temperature range from 20-243°C f o r

P. Hartwig et al. I Anhydrous and hydrous LiI-NH 4 1

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A d r - v o l t a g e o f 100 mV was a p p l i e d t o c e l l i f o r 23 days at a temperature o f 12000. A steady state current o f the o r d e r o f 2 x IO "$ A cm "z was o b s e r v e d o v e r the e n t i r e time p e r i o d w i t h o u t any indication of a breakdown. This result clearly indicates the movement o f l i t h i u m ions in L i z ( N H ~ ) 3 I s . Application o f d c - v o l t a g e s up to 0.3 V to c e l l I I did not show any transport of protons across the dry samples e v e n a f t e r extended periods of time. Oonductivity measurements o f the samples containing dispersed m i x t u r e s of 2 solid phases, i.e., lithium iodide or ammonium i o d i d e and l i t h i u m ammonium i o d i d e have shown h i g h e r v a l u e s for the conductivity than for either single phase sample o f the c o n s t i t u e n t compounds. A maximum enhancement by a factor o f about 10 c o u l d be o b s e r v e d depending on the individual sample and p a r a m e t e r s such as g r a i n s i z e s and temperature. A l s o , the a c t i v a t i o n enthalpy for eT v a r i e s s l i g h t l y w i t h i n the range from 0.95 to 1.1 eV.

i

61'8

2

2~2

24

26

J 1 • J 2'8 5 3 2 34 -- '03/T [K-'] T I°cI - - 280 240 2 0 0

Figure I: Semilogarithmic representation of the product of the c o n d u c t i v i t y and absolute temperature of Liz(NHw)31s as a function of tlme inverse absolute temperature. The various symbols indicate the r e s u l t s of experiments with d i f f e r e n t l y prepared samples. Melting occurs at 243°C.

the product of the c o n d u c t i v i t y and absolute temperature, The a c t i v a t i o n enthalpy is 0.93 eV. The increase at higher temperatures is due to melting of the sample. The p a r t i a l conduct i v i t y due to electrons and holes was estimated [12I from the d c - p o l a r i z a t i o n experiments to be at least 4 orders of magnitude s m a l l e r than the total (ionic) conductivity a t any t e m p e r a t u r e . Equilibration of the c u r r e n t s is very slo~, indicating low d i f f u s i o n coefficients of the e l e c t r o n i c m i n o r i t y charge c a r r i e r s .

160

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The c o n d u c t i v i t i e s of various water containing samples are shown in Figure 2 as a f u n c t i o n of the temperature. The equimolar mixture of LiI.HzO and NH~I has a c o n d u c t i v i t y which is of the same order of magnitude as for LiI.HzO [5] w i t h i n the l i m i t s of e r r o r . The sharp increase in c o n d u c t i v i t y is observed at time melting p o i n t of the l i t h i u m iodide monohydrate. Also, at the i n i t i a l heating a d r a s t i c increase is observed at a temperature in the range from 70-8000. This effect is typically less s h a r p and less r e p r o d u c i b l e f o r subsequent h e a t i n g and c o o l i n g runs. This temperature range agrees with the melting p o i n t of LiI.2HzO which may h a v e been formed due to a surplus of water in the sample. Similar observations w e r e made for other LiI-HzO-NH~I mixtures.

70

rnpLiI 2HzO

I

I 18

2

22

24

26

28

3

---~O~lt

32

34

{K-']

Figure 2: Semilogarithmic r e p r e s e n t a t i o n o f the p r o d u c t o f the c o n d u c t i v i t y and a b s o l u t e t e m p e r ature of an e q u i m o l a r mixture of N H 4 1 and L i I . H z O as a f u n c t i o n o f the inverse absolute temperature. The d o t t e d l i n e indicates the r e s u l t s o f f r e s h samples upon the f i r s t heating. This conductivity i s a p p a r e n t l y due t o some s u r plus of water and the f o r m a t i o n of Lil.2HzO. The v a l u e s drop t o the s o l i d l i n e in the c o u r s e of days. The c o n d u c t i v i t i e s of L i I - m o n o - and d i h y d r a t e a r e shown f o r c o m p a r i s o n .

406

P. Hartwig et al. / Anhydrous and hydrous LiI-NI-I 4 1

DISCUSSION Structural, phase equilibrium and conductivity measurements of LiI-NH41 and related systems are highly sensitive to water and require the exclusion of even traces of water, Lithium iodide hydrates are readily formed. The Debye-Scherrer X-ray pattern observed by Sattlegger and Hahn [ 7 , 8 ] and by a n a l y s i s o f t h e main l i n e s r e l a t e d to "LiN11412" may be considered as a consequence of this hydration e f f e c t due to the lack of proper experimental techniques, The X - r a y p a t t e r n of t h e c u b i c phase i s , b o t h w i t h r e g a r d to the positions and t h e intensities of t h e l i n e s , predominantly determined by the iodine atoms and is more v i s i b l e than the large number of diffuse lines of the lithium ammonium iodide on the Guinier f i l m . The discrepancy between the r a t i o of s t a r t i n g materials of mixtures of L i l and NH~I in the chemical analysis might be explained in terms of the presence of w a t e r in ammonium iodide. The material prepared by Liang and Epstein ("Li~(NH~)Is" [ 9 ] ) , also includes the influence of water since the preparation w e n t through aqueous solutions of L i I and NH41. The X-ray pattern of the material prepared according to the indicated procedure shows neither the pattern of the cubic LiI.HzO phase nor of Liz(NH4)31s. Lithium ammonium iodide is a lithium ion conductor of modest conductivity. This material and i t s mixtures with L i l or NH~I are also use{u] as a r i g i d system to provide continuous paths of highly conducting molten e l e c t r o l y t e s , LiI.xHzO (x = 1 , 2 , 3 ) , along external and i n t e r g r a n u I a r surfaces above about 7O°C. Lithium ammonium iodide also indicates a higher lithium ionic conductivity when a dispersed second phase of another lithium ion conductor such as L i l is added. I t is demonstrated that tile enhancement effect of the ionic conductivity is not restricted to a d i s p e r s e d i n e r t phase such as i n t h e case of LiI-AlzO3 but also mixtures of two dispersed ionic conductors may show t h e same 'effect,

ACKNOWLEDGEMENT The a u t h o r s g r a t e f u l l y a c k n o w l e d g e many h e l p f u l discussions with Professors R.A. Huggins, Stanford University, and A. Rabenau, Max-Planck-lnstitut fur FestkBrperforschun9, Stuttgart. REFERENCES [I]

Hartwig, P., Neppner, N. and Nichelhaus, Mat. Res. Bull. 14 (1979) 493.

W.,

[2]

Hartwig, P., Neppner, N., Nichelhaus, W and Rabenau, A., Sol. State Commun. 30 (1979) 601.

[3]

Hartwig, P., Heppner, N., Wichelhaus, W. and Rabenau, A. in Fast Ion Transport in Solids, Electrodes and Electrolytes (P. Vashishta, J.N. Mundy, and G.K. Shenoy, eds.) NorthHolland, New York, (1979) 487.

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H a r t w i g , P . , Weppner, H . , H i c h e l h a u s , W., and Rabenau, A. Angew. Chem. 92 (1980) 72.

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H a r t w i g , P . , Rabenau, A . , and Meppner, J. Less. Comm. M e t a l s , in p r e s s .

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Sattlegger, H. and Hahn, (1964) 534.

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S a t t l e g g e r , H. D i s s e r t a t i o n , Germany, 1964.

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L i a n g , C.C. 3,513,027,

and J. E p s t e i n , 19 Hay 1970.

[10]

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A.,

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