Solid State Ionics 31 ( 1988) 5-8 North-Holland, Amsterdam

EN TRANSPO r-EiH AN TES

Shiuli GUPTA, S. FATNAIK, S. CHAKLANOBIS Materials Science Program, Indian Inslitute of Technology, Kanpur 208016. India

and

K. SHAH1 Department of Physics, and Materials Science Program, Indian institute of Technology, Kanpur 208016, India

Received 4 August 1987; accepted for publication 14 June 1988

LiBr-Lil mixed crystals and LiBr-AlzOx composites have been studied by means of complex impedance anaiyc-s. ac conductiv- ity, X-ray diffractior,, DTA and SEM techniques. The substitution of wrong size I- ions in LiEr increases the cwductivity and decreases the migration en. rgy of Li-ion vacancies. These results are consistent with tr.ose of the KBr-KI syster#: and earlier predictions. LiBr-AlzOj composites exhibit a sharp increase in the conductivity. The highest conduc?ivity obtaiv,d was z IO- 3 R-’ cm-’ at 302°C for LiBr+ 10 m/o AlzOj.

1. Introduction

The high energy density associated with the lith- ium batteries is now well recognized [ I 1, and there- fore the need for good and stable lithiu electrolytes cannot be overemphasized. The inten- sified and concerted efforts made during the last sev- eral years have led to a number of Li-based solid electrolytes. However, most of these materials a parently do not qualify for batterv applications. must be pointed out here that until recently the on solid electrolyte that has been found suitable for lit ium batteries is LiI, i.e., one of the si liest known ionic solids. The mode batteries which utilize in-site forme electrolvte 2l.P WniWted to have YWl-fnrrmed excep- ,‘_ ___ ‘.r__‘_- X-----------

LiI, viz., LiI-Al,03.

electrolytes which have commercialization.

potential for

Enthused by the success of LiI a osites in low drain solid state ba

number of groups are actively prrrsuing the stu to further impro the ionic conductivity of these classical lithium lides. The conventional d (of aliovalent impurities successful in ~~t~iu~ ha1

Id not be sustained under battery herefore we thought it worthwhile

to explore two other possibilities, namely the en- hancement in conductivity due to (i) substitution of homovalent im urities [ 3-61 an (ii ) dispersion of a second phase particle 17-101.

The starting materials use in :his work are ultra- ined T33m Alfa Products,

wder of 0.05 p

everal hours to obr

were prepared by taking approp and LIP, mixing the tool-o~ghly in an agate mortar and pestle, heating e mixture at 400°C in a seale

6 S. Gupta et al. / LiBr-Lil mixed crystals and LiBr-Ai203 composiies

quartz tube followed by cooling to room tempera- ture. The solid chunk so obtained was reground be- fore pelletization at 6 ton/cm*. The cylindlical pellets so obtained had a diameter of w 11 mm and a thick- ness that varied from 4 to 6 mm. LiBr-Al203 sam- ples were prepared by first pelletizing the appropriately weighed mixture, heating it to 400°C followed by cooling to room temperature. This pro- cess was repeated a few times to ensure the uniform distribution of alumina particles in the LiBr matrix. The SEM studies revealed that the alumina particles are of 0.5 pm size whereas the supplier’s specifica- tion was 0.05 urn. It is possible that during the prep aration of the sample and its subsequent annealing the alumina particles tend to grow in size.

Impedance measurements were carried out using an HP-4 192A low frequency impedance analyser. The temperature of the sample was controlled to within + 1 “C by using a Eurothertn Temperature Con- troller. The X-ray diffraction patterns were recorded at room temperature by using a Rich Seifert ( IsoDebyeFlex 2002) Counter diffractometer that employs Ni filtered CuKa radiation. The scanning speed was 0.6” per minute in 28. The DSC mea- surements were carried out using a DuPont 910 dif- ferential scanning calorimeter along with an R90 TA Programmer and a linear chart recorder.

3. Results and discussim

3. I. Complex impedance analysis

The complex impedance analysis was attempted in order to extract the dc conductivity of the sam- ples. However in the case of the EiBr-LiI system, the admittance plots (50 Hzrf< 5 MHz) were not sem- icircular; only a tail appeared on the high frequency side. It would appear from these results as if the sem- icircular part would be found at still lower frequen- cies, and that the sampie behaves as if it were equivalent to a resistance-capacitance series com- bination in parallel with another capacitance Fort LiBr-Lil system, therefore, oniy the ac conductivity at various frequencies is studied.

The impedance plots for LiBr-Al&$ samples on the other hand, were quite well behaved as shown io fig. 1 f fm LiBr-20 m/o AlzOl ) at five different tem-

peratures. Thus in this system it was possible ?o ob- tain conductivity values from the complex impedance k ‘ots, although this system also exhibits a tail in the higher frequency region.

3.2. Liblr-Lil mixed crystals

Fig. 2 shows the ionic conductivity (at 5 KHz) as a function of composition x in LiBr, _J, at six dif- ferent temperatures. It is observed that the conduc- tivity of LiBr increases initially with increasing concentration of Lil, passes through a maximum around 30-35 m/o Lii and then starts decreasing until 50-55 m/o LiI. For x> 0.055, the conductivity keeps on increasing with the increasing LiI. The maximum enhancement in the conductivity of LiBr is by a factor of 35 at 385°C which is brought about by the addition of 40 m/o LiI; the absolute magni- tude of the conductivity being 2 x 1 Ow6 R- ’ cm- ‘.

It is instructive to compare the results on the LiBr- LiI system with those of the KBr-KI mixed crystals [ 3 1. The maximum enhancement in KBr-KI is re- ported to be by a factor of 63 at 400°C in KBr-50 m/o KI, as compared to a factor of 35 at 385°C in LiBr-35 m/o LiI. Another contrasting feature in the I iBr-LiI system is that in the log Q versus compo- sition plot, (fig. 2) a minimum occurs around 55 m/o LiI which is absent in KBr-KI system. This es- sentially means that while the conductivity of KI is enhanced by the addition of homovalent impurities like Br-, the effect of addition of Br- to LiI is not appreciable.

We have examined various samples of LiBr-LiI systems by X-ray diffraction at room temperature. It was found that the solubility of LiI in LiBr was quite low m 10 mole% at 25°C. However, it is quite pos- sible that the solubility increases appreciably with increasing temperature. The conductivity results shown in fig. 2 tend to support this view. The con- ductivity isotherms display peaks which shift to higher compositions as temperature increases; the one at the lowest temperature (182°C) shows the peak a? _ 30 mole% Lil while the one at the highest tem- pcrature (385°C) at 40 mole% Lii. It is tempting to associate these peaks at different temperatures with the solubility limits of LiI in LiBr at those temper- atures. It is therfore inferred that the solubility limits of LiI in LiBr, that was limited to 10 mole% at 25°C.

S. Gupta et al. /LiBr-Lil mixed crystals and LiBr-AlJO composites

2 i 6 zR=zees 6 (KA)

Fig. 1. The complex impedance plots for LiBr+ 20 m/o A&O3 at five different temperatures.

has gone upto 30 mole% at 182°C and 40 mole% at 385 ‘C. Thus the region on the left side of the dashed line corresponds to single phase solid solution of LiBr containing LiI, and the region on the right side rep- resents the two-phase mixture. We therefore con- clude that the conductivity of LiI-LiBr increases as long as LiI is soluble in LiBr and begins to decrease beyond the solubility limit, leading to a peak in the

conductivity-composition plot. Further comments on these results must await the detailed phase dia- grams studies on LiBr-LiI system.

The electrical conductivity of LiBr-LiI systems have been investigated in detail as a function of tem- perature. All in all 9 compositions have been stud- ied; pure LiBr and LiI3r containing 10, 20, 30, 40. 50, 60, 70 and 80 m/o LiI, fig. 3 shows the log r~ ver-

-4.5

t 1 VC)

Ll 385 f

Fig. 1. Logarithm of ionic conductivity (5 kHz) versus compo- sition for LiBr-LiI mixed crystals at six different temperatures. The dashed line connecting the peaks separates the LiSr solid solution (LiBr-ss) and the two-phase mixture.

T (“Cl 400

-501 , 300 200 100

I I I

[LiBr~lOmol*LLil]

-6.0 -

7 E ”

7 E -7.o- L

s

b

E

-8.O-

I

-90 t

_I

0 LB”, 1 CIKHZ

0 &TN, 0 5KHI

Ob:eV, O.tKHz

I I I I I L

1.5 2.0 2.5’ 1

103/T (Pi-‘)

0

Fig. 3. Logarithm of ionic conductivity versus inverse of temper- ature (K- ’ ) for one of the nine compositions studied in the LiBr- LiI system. viz. LiBrf 10 m/o LiI, at five different frequencies.

8 S. Cupta el al. / LiBr-LiI .,rixed crystals and LiBr-AlJO C~ -+tes

sus 1 03/T plot for one of these samples (LiBr+ 10 m/o LiI) at five different frequencies. The intrinsic and extrinsic regions of conduction are evident. The activation energy in the low temperature (extrinsic) region is almost the same 0.41-O-42 eV for all fre- quencies. This must be identified as the energy of migration of cation vacancies (h, ). The slope of the log 0 versus 1 /T plot in the intrinsic region is 1.27 eV which yields the energy of formation of defects, St,=2 ( 1.27-0.42) = 1.70 eV. It has been observed tha? there is no significant change in the formation energy of defects due to the addition of up to 40 m/o LiI. Beyond this, there is a decrease in h, values with iucreasing concentration of LiI as expected. On the other hand the migration energy decreases monotonically over the entire composition range. Once again there are certain contradictions in be- haviour between the LiBr-LiI and KBr-W systems. The enthalpy of formation of defects (h,) in LiBr does not decrease at all with the addition of upto 40 m/o LiI. The addrtion of 40 m/o KI in KBr lowers the activation energy by 0.18 eV or h 12%.

3.3. LiBr - .412Q3 composites Ac~owle~ge~ent

The conductivity composition plot for LiBr-A1203 s tkat the conductivitf rises

tered around 10 m/o A1203 and then decreases. The highest conductivity obtained is w 10m3 a-’ cm-’ at 302°C for LiI + 10 m/o Alz03 which is about 6 orders of magnitude higher than that of pure LiBr. At 84”C, the enhancement in conductivity is by about 4 orders of magnitude. These results may be com- pared with those of LiI-A1203 system which had been studied extensively [ 7 1. The highest enhancement in this system is reported to be by about 2-3 orders of magnitude. Thus the dispersion of A1203 in LiBr is more effective than that in LiI.

Examination of LiBr-A1,03 by scanning electron microscopy (SEM ) technique indicated that the dis- tribution of A&O3 particles was not very uniform, and that the particle size (0.5 urn) varied from that stipulated by manufacturers (’ .05 pm ). It is possi- ble that during sample preparations and annealing, the size of AlzOs particles increased, Efforts are being made towards the inclusion of still finer A1203 par- ticles in LiBr in order to further enhance the

t :? . conduc,;lvity.

This research was sponsored by the Department of Science and Technology. Government of India un- der the Project 3 ( 3 !/83-S?’

e ferences

[ I ] K. Shahi, J.B. Wape.er and B.B. Owens, in: Lithium batter- ies, ed. .It:>n-Psul Gabano (Academic Press, New York, 1983 ) p. 4ti7.

I2 ] J.R. Akridge and H. Vourlis, Solid State lonics D 8/ I9 ( 1986) 1082.

[3] M. Shahi and J.B. Wagner Jr., 9. Phys. Cbcm. Solids 43 (1982)713;44(1983)89.

[4]0.Johannesen and ~.~c~e~vy~.~~ys. Chem. Solids 47 (1986) 265.

____ _-2x, *.__: “_L..

I-7 I C.C. Eiang. 3. Ebectrochem. SOC. 120 ( 1973 ) I 289. ~~]~.Jowa~dJ.~.Wa~er JF.,J.Electra;hem.Soc. 1% (1979)

163.

[9]K.Shahiand3.8.WagnerJr..J.Solidtr;tate Chcm,42(1982)

llO]ko andkar and J.B. Wagner Jr. SoPld S-Z&Z lonics 20 f P9#6) 247.