Ionic conductivity of substituted Li7TaO6 phases

Solid State lonics 13 (1984) 2 4 9 - 2 5 4 North-Holland, A m s t e r d a m

IONIC CONDUCTWITY E. N O M U R A

OF SUBSTITUTED

* a n d M. G R E E N B L A T T

Li7TaO 6 PHASES

**

Department o f Chemistry, Rutgers, The State University o f New Jersey, New Brunswick, NJ 08903, USA Received 17 October 1983 Revised manuscript received 16 November 1983

Lithium ion conductivity in solid solutions o f Li7Tal-xNbx06. Li7Tal-xBixO6. Li7+xTal-xZrxO 6 and LIF_2xCaxTa06 has been measured as a function of temperature and composition using the complex impedance method. At 200°C the conductlvltles of LiyTao.yNbo.306. Li7Ta O 6Bi 0 406. Li 7 4Ta0 6Zr0 406 and Li6.6Cao.2TaO 6 are 4.3 x 10-~(flcm) -I, 3.0 x 10"~(flcm) -x, 4.0 x [O-b(flcm) -l and 1.6"x lO~(flcm) -l respectively. The r e s u l t s a r e d i s c u s s e d i n r e l a t i o n t o s t r u c t u r a l properties.

1.

INTRODUCTION

There is considerable interest in developing solid lithium ion conductors for utilization in high energy density battery systems. This had led to a search for new solid electrolytes exhibiting high lithium ion conductivity and had stimulated interest in developing a fundamental understanding of ionic transport in solids (1-3), The compounds of lithium hexaoxometallates, formulated as LinM06; n = 6,7 or 8, M - IV, V or VI group element, have a pseudo two dimensional structure and high lithium ion conductivity (4). The structure of LInMO 6 is characterized by o c t a h e d r a l s h e e t s o f C d I 2 - t y p e , b e t w e e n w h i c h 6 Li + a r e i n s e r t e d i n a t e t r a h e d r a l e n v i r o n m e n t as tetr oct Li 6 ( L l a M b ~ c) 06

that for the octahedral lithium ions in the layers (5). Thus by altering the lithium content and/or the number of vacant sites in the layers higher ionic conductivlties might be achieved. We have examined the effect of increased Li + content by partial substitution of tantalum ions with Zr ~+ to yield Li6(Lil+xTal-xZrx[-]l-x)O 6. We have also attempted to look at the effect of decreased LI ÷ content (or increased vacancies) in the layers by studying the conductivltles of samples with substituted divalent cations in Li6(Ltl-2xCaxTaF']l+x)O 6 . Alternatively we hoped to obtain higher lithium conductivities in this system by partial substitution of the tantalum ions with other group V metal ions of larger ionic radii in order to optimize the channel size of the framework structure for lithium diffusion. In this paper we report the results of the lithium conductivity in: Li7Tal_xNbx06. Li7Tal_xBix06, Li7+xTal-xZrxO 6 and Li7_2xCaxTaO 6.

a+b+c = 3 [-]: vacancy Among the lithium hexaoxometallates, Li7TaO 6 (i~e, L i 6 ( L i T a ~ ) O 6) is the best lithium conductor w i t h 0200 " 5 x 10 -~ (flcm) -1 and ORT 4.3 x I0 -8 (4). I t h a s b e e n shown t h a t t h e a c t i v a t i o n energy f o r Lt + d i f f u s i o n i n t h e t e t r a h e d r a l sites bet w e e n the layers is signlflcantly larger than

* **

Present address: Central Laboratory, Yuasa B a t t e r y Co. L t d . T a k a t s u k i O s a k a , J a p a n A u t h o r t o whom c o r r e s p o n d e n c e s h o u l d be addressed.

0 1 6 7 - 2 7 3 8 / 8 4 / $ 0 3 . 0 0 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)

2.

EXPERIMENTAL

Li20 was obtained by thermal decomposition of anhydrous Li202 in vacuum at 450°C for 6 hours. Other starting materials were reagent grade Nb205, Ta205, Bi203, CaO and ZrO 2. Mixtures of appropriate composition were throughly mixed in an agate mortar in a He dry box. For example, LiFTa1_xNbxO 6 and Li7Tal_xBixO 6 were prepared from Li20 , Ta205 and Nb205. and Li20 , Ta205 and Bi203. respectively, according to the following equations (6).

E Nomura, M. Greenblatt ~Ionic conductivity o f LiTTaO 6 phases

250

SIGNAL GENERATOR

7Li20+(l-x)Ta205+xNb205 * 2Li7Tal_xNbxO6 7Ll20+(l-x)Ta2Os+xBi203+x02

+ 2Lf7Tal-xBixO 6

The mixtures were pressed into cylindrical pellets 9.5 mm diameter and about 5 ~ thick at 15,000 ibs/in 2. The pressed pellets were transferred to high purity alumina crucibles, were embedded in excess LI20 powder in order to minimize Li20 losses, and were heated at 650°C for [8 hours in alr. After cooling, the pellets were crushed and examined by X-ray powder diffraction using a Norelco diffractometer with Ni filtered copper radiation. Lithium content was determined by atomic absorption speetrophotometrlc methods. Pellet samples for ionic conductivity measurements were prepared by pressing to 6.4 m~ diameter and about 4 ~ thickness at 30,000 ibs/in 2 followed by sinterlng at 950°C for 18 hours in alr and quenching in air. Again, LI20 loss was minimized by covering the pellets with LI20 powder during the sinterlng process. The X-ray diffraction pattern and lithium content of the slntered samples were checked to confirm the Identity and composition of the phases present. Weight loss was not observed after sinterlng. The density of the sintered pellets were about 85% of theoretical value. Both surfaces of the pellets were polished using silicon carbide (#400) paper and sputtered wlth about ibm of gold followed by a coating of silver paint (Englehard #16). The device used for the conductivity measurement is shown in Fig. I. Platinum contact leads were made by spot welding pieces of platinum loll of I0 x I0 r=n2 to platinum wires. Two discs of a-alumina of lO mm diameter and 3 mm thickness were used as insulating spacers. Contact between the sample electrode (sputtered gold and painted silver) and the platinum lead was made by tightening the screw (Fig. I). The

Fig. [ I. 2.

Device

for conductivity measurement

sample Pellet 3. platinum lead a-alumina disc 4. stainless steel dlsc 5. screw

~Rx Fig. 2

A. M. R x.

R~td.

Circuit diagram for AC conductivity measurement

tuned amplifier PS. mixer I. sample Rst d.

phase shifter integrator standard resistance

AC conductivity measurements were made using ionlcally blocking electrodes. The schematic electrical circuit diagram of the AC conductivity measurement is shown in Flg. 2. Lock-ln amplifier (PAR Model 128A) was used as a phase sensitive detector and a Hewlett-Packard Model 200CDR was used as a signal generator. The frequency range used was between 5 Hz and I00 KHz. The measurements were made from room temperature to 250°C in air. We were unable to obtain reproducable results above 250°C for these compounds probably due to lithium losses,

3. RESULTS AND DISCUSSIONS 3.1

LiTTaO 6

Fig. 3 shows the conductivity, o versus I/T of slntered pellets of LI7TaO 6. Different activation energies corresponding to a lower and higher temperature regions are observed. In the higher temperature region, the deviation of ionic conductivity between different pellets is small (which is also a measure of the reliability of the data) and the activation energy is 0.67 eV. In the lower temperature region, the ionic conductivity of different pellets differ more significantly but the actlvatlon energies are the same (0.46 eV). The change in the slope of the o vs I/T plot of Li7TaO 6 probably corresponds to a transition from extrinsic to intrinsic conductivity regions. The variations in the ionic conductivltiea of different samples in the lower temperature region can be explained by differences in defect concentrations or particle packing from sample to sample. Our results are in good agreement wlth Delmas et al (4) except for the slightly higher conductivltles reported by hlm, and the apparent absence of transition in his conductivity plot (Fig. 3).

E. Nomura, M. Greenblatt /Ionic conductivity o/LiT Ta06 phases 300

200

I00

I

i

I

10-2 ~ iO- 3

50 °C

AFTER "~X~/ DELM~L$ ETAL

Compound Li7TaO6 t Li7NbO6 t Li7BiO6 t LI7Ta 0 6Nb 0 406 * Li7Ta0~6Bi0~406*

3.2

I

I

2.0

J

2.5

30

IO00/T

(K -I)

"

3.5

The temperature dependence o f the conductivity of Li7TaO 6 ( O , ~ , ~ represents different Li7TaO 6 samples)

Li7TaI_xM~O6(MV-Nb,BI)

The X-ray diffraction patterns of the solid solutions prepared were identical to that of Li7TaO 6 except for shifts in 20. The lattice parameters calculated for a hexagonal unit cell increased with increasing values of x for Li7Tal-xNbxO 6 and for Li7Tal-xBixO 6 in the region O~x~0.40 as shown in Fig. 4. Cell parameters of selected phases are given in Table I • The lattice parameters of Li7Tao.6Nbo.406 are larger than those of either of the component compounds, Li7TaO 6 and

I I

I

I

15.501

iO_3

5.39 5.40 5.50 5.42 5.44

15.11 15.12 15.45 15.17 15.23

200 1150

llO0

50"C

I

~ I

IO_Z

Lattice p a r a m e t e r s o f Li7Tal_xNbxO6(~) and Li7Tal_xBixO6(~)

2.0 2.2 2 4

2£

2~

-

3.0

IO~T (K-I)

X Fig. 4

c(A)

LI7N~6, while the lattice parameters of LI7T~.6BIo.406 are inte~edlate between those of LiTTaO 6 and Li78106. The bismuth substituted phases appear to obey Vegard's law while the LiTTaL_xNb~6 system does not. The e f f e c t i v e ionic radii of Ta(V) and BI(V) in octahedral coordination are 0.64A and 0.76A respectively (8), hence the observed smooth increase in cell volume of LiyTal-xBixO 6 with increasing values of x. For Ta(V) and ~ ( V ) in octahedral environments the effective ionic radii are the same (8) (the cell parameters within experimental error are also the same), nevertheless, ~ observe a slight increase in cell volume up to ~ 0 . 4 in Li7Tal_xNbxO 6 (Fig. 4), beyond which the cell volume appears to decrease. This effect ~ y ~ due to d l f f e ~ ences in T a - ~ interactions vs Ta-Ta and N ~ N b interactions via oxygens in this structure. The temperature dependence of the ionic conductivities of LiTTal-xNbxO 6 phases are shown in Fig. 5 The ionic conductivitles at 200°C increase from I.I x lO-~ (flcm)-I at x = 0.0 up to a ~ x l m u m of 4.3 x lO-~ (ficm)-I for x = 0.3 and then decrease as x is increased

iI

5.40~.-~l

a(A)

• From ref. 7 • Estimated error in the lattice parameters is ±0.01A.

,o-t ,o-1 Fig. 3

Lattice Parameters of Ll7Ta1_xNbxO 6 and Li7Tal_xBixO 6 compounds.

I

~'~ 10-4 ~ ,

10-81

Table I.

251

Fig. 5

The temperature dependence of the c o ~ ductivity o f Ll7Tal_xNbxO6 phases

252

E. Nomura, Ill. Greenblatt /Ionic conductivity o[ LIT TaO6 phases

10-:3

;~00 150

I00

1

O"C

,oo

0o:lc

1 0 _ 4, \i~

I

Y E io-s b

10-6 10-7 2.0 2.2 2.4 2.6 2.8 3.0

IO-Z 2.0 22 2.4 2.6 2.8 3.0 IOS/T (K-')

I 0 3 / T (K -' ) Fig. 6

The temperature dependence of the conductivity of Li7Tal_xBixO 6 phases

further (Fig. 5). The activation energy of conduction is 0.63 eV for Li7Ta0.7Nbo.306, somewhat smaller than the 0,67 eV observed for LiTTaO 6 • The ionic conductivlties of Li7Tal_xBixO 6 phases are shown in Fig. 6. The ionic conductlvities increase to 3.0 x i0 -~ (flcm)-I at 200"C at x = 0.4. The activation energy for conduction is 0.60 eV in Li7Ta0.TBi0.306. In the Li7Tal_xNbxO 6 and LiTTal_xBixO 6 phases the observed increase in ionic conductivity and decrease in activation energy at x = 0.3 and 0.4 respectively compared to LiTTaO 6 is most likely due to optimizing the channel size lithium diffusion at these compositions.

for

3.3

Li7+xTal_xZrxO 6

The powder X-ray diffraction pattern of Li7+xTal-xZrxO 6 is similar to that of LiTTaO 6. The lattice parameters increased with increasing values of x; a - 5.44A and c = 15.25A were found for Li7.4Tao.6Zr0.406. The ionic conductlvlties of Li7+xTal_xZrxO 6 are shown in Fig. 7. The change in the slope of o vs I/T curves of Ll7,1Ta0.9Zr0.106 corresponds most likely again to the transition from extrinsic to intrinsic mechanism of conductivity. The activation energies are 0.50 eV and 0.60 eV in the lower and higher temperature regions respectively. A similar change in the slope of the o vs I/T plot of Li7.3Ta0.7Zr0.306 is observed at a significantly higher temperature, suggesting that the increased conductivity observed in the Li7+xTal_xZrxO 6 phases is due to the larger concentration of mobile Li + ions. Li7.4Ta0.6Zro.406 is in the region of extrinsic conductivity for the entire range of temperature measured.

Fig. 7

3.4

The temperature dependence of conductivity of Li7+xTal_xZrxO 6 phases

Li7_2xCaxTaO 6

The powder X-ray diffraction pattern of Li7-2xCaxTaO 6 for 0.05~x40.5 is similar to that of Li7TaO 6. The lattice parameter ~ increases linearly with increasing values of x from x=0.0 to x - 0.2, and remains constant beyond x=0.2 (Fig. 8). The cell parameter S is unchanged for all values of x (Fig. 8). This indicates that the formula Li6(Lil_2xCaxTa l+x)06 represents correctly, what actually occurs - the substitution of calcium for two lithium ions in the octahedral sites. The larger effective ionic radius of Ca2+(l.O0~) ref. 8 compared with LI+(0.76A) accounts for the increase in the a parameter with increasing x. Since Ca substitution appears to occur preferentlally for the lithium in the layers, the c parameter (perpendicular to the octahedral la~ers) is not affected by increasing calcium substitution. However, the ~ parameter would also be expected to increase for x>0.2. We do not detect any excess CaO (or any other phase) in the X-ray pattern of Li7-2xCaxTaO 6 up to x = 0.5 (i.e. Ll6(Cao.5Ta~l.5)O6). We have found that a stoichlometric starting mixture of Li20 , CaO

5'501 •

115.50 ®

~

A I 1,5.10 0 0.2 0.4 0,6 0.8 1.0 X

Fig. 8

Lattice Parameters of Li7_2xCaxTaO 6

Ois_a; Q i s

c

E. Nomura, M. Greenblart / Ionic conductivity o f LiT TaO6 phases I0-~,

200

150

I00

I

I

I

iO °

the Li + conductivity increases with x, with a maximum of 1.6 x 10 -4 (flcm)-I at 200°C for x = 0.2 (E a = 0.52 eV). The activation energy decreases, and the conductivity i n c r e a s e s with increasing number of vacancies, and Improved channel size, and is optimized at x = 0.2. Further decrease in the number of mobile lithium ions leads to a decrease in conductivity (Fig. 9).

10-4 i

E

0

cl

253

is - s

b

4.

CONCLUSION

10-6 The ionic conductlvities of substituted Li7TaO 6 phases, within a specific range of composition characteristic o f the substitutional species, appear to be somewhat superior than the parent compound. It appears that optimizing the channel size for Li + diffusion has about the same effect on the conductivity as increasing the Li + concentration, as observed in Li7Tao.6Bi0.406 and LiT.4Tao.6Zr0.406 in Table II. Surprislngly, increasing the concentration of vacancies (Li6.6Cao.2Ta06, Table II) has a much smaller effect on the conductivity than might have been expected given that LI8MIVo 6 compounds with no vacancies in the octahedral layers (LI6[LI2M06]) are an order of m a g n i t u d e p o o r e r conductors than LITMVO6 phases (LI6[LIM[~06]) with octahedral vacancies. In Table II, we summarize conductivity data for the best Li solid electrolytes. It may be concluded that solid solutions of lithium hexaoxometallates are also members of good lithium ion conducting solid electrolytes.

0.2

05

I0-~

2.0

2.2

2.4

2.6

0 0.05 2.8

3.0

I 0 3 / T ( K -' ) Fig. 9

The temperature dependence of the conductivity of Li7_2xCaxTaO 6

and Ta205 made up to yield Li6(Lio.4Cao.3Ta~]l.3)O6 before heat treatment contains detectable levels o f CaO in the powder X-ray diffraction pattern. Thus if solid solutions of Li6(Lil-2xCaxTa)O 6 formed only in the range 0.05gx~0.2 as the a cell parameter variation might suggest, the excess CaO and other impurities should be detectable. The fact that a single phase LiTTaO 6 like pattern is obtained for the whole range of x studied indicates continuous solid solution formation. The observed variation in the a cell parameter for x > 0.2 may be the net result of two equal, but opposing effects: increase in a with x due to the larger ionic size of Ca 2+, a~d decrease in a with x due to the increase in the number of vacancies. Apparently, beyond some specific concentration of vacancies (>1.2) the lattice tends to contract in the layer. The variation of the ionic conductivity of Li7-2xCaxTaO 6 as a function of x shown in Fig. 9 is similar to the ~ parameter variation;

Table II Conductivity data for Li compounds. Compounds °RT (flcm~-I LI-B-AI203 1.3 x lO -4 Lil4Zn(Ge04) 4 (LISICON) LI4B7012CIo.68Br0.32 Li3.75Si0.75P0.2504 Li4.4Si0.6A10.404 Li4.6Alo.6Si0.404 LI7TaO 6 Li7Ta0.7Nbo.306 LiyTao.6Bio.406 Li7.4Ta0.6Zr0.406 Li6.6Cao.2TaO 6

5.4 x I0 -7. 4.8 x 10-7* 2.8 x I0 -7. 4.3 3.7 7.3 7.3 3.4 6.2

x x x x x x

I0 -8. 10 - 8 * 10-8* 10 -8* lO -7. 10-8*

ACKNOWLEDGEMENT This work was supported in part by the Office of Naval Research, Contract NOOOI4-82-K-0317.

0200 (ficm)-I 2.0 x lO -~ 1.7 6.5 1,0 7.6 7.7 5.0 I.I 4.3 3.0 4.0 1.6

x x x x x x x x x x x

10 -4 I0 -b lO-3 I0 -b lO-5 I0-~ I0 -~ I0 -~ 10 - ~ 10 -4 10-4

Activation Enersy (eV) 0.19 0.36 0.50 0.53 0.60 0.58 0.68 0.66 0.67 0.63 0.60 0.52 0.52

Temp. -100 180 50 50 30 70 85 25 80 80 50 50 25

Range (°C) - 180 - 800 - 300 - 230 230 - 230 - 230 - 500 - 230 - 230 - 230 - 230 - 230

Ref. 9 i0 1 1 1 1 4 this work this work this work thls work this work

E. Nomura, M. Greenblatt /Ionic conductivity of LiI TaO6 phases

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